Offshore Turbine Applications

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HSEHealth & Safety

Executive

Offshore gas turbines (and majordriven equipment) integrity and

inspection guidance notes

Prepared by ESR Technology Ltd for the

Health and Safety Executive 2006

RESEARCH REPORT 430



HSEHealth & Safety

Executive

Offshore gas turbines (and majordriven equipment) integrity and

inspection guidance notes

Martin Wall, Richard Lee & Simon FrostESR Technology Ltd

551.11 Harwell International Business CentreHarwell

OxfordshireOX11 0QJ

Gas turbines are widely used offshore for a variety of purposes including power generation,compression, pumping and water injection. Relatively little information is included in safety cases, forexample only the manufacture, model, power rating (MW), fuel types, and installation drawingsshowing the location of the turbines. Some descriptive text may be included on the power generationpackage, back-up generators and arrangements for power transmission to satellite platforms.Information on integrity management and maintenance is limited or at a high level.

This Inspection Guidance Note provides a more detailed assessment of gas turbines (GTs) andmajor driven equipment installed on UK offshore installations, focussing on integrity and maintenanceissues. This complements the advice in HSE Guidance Note PM84, recently re-issued, coveringcontrol of risks for gas turbines used in power generation and HSE Research Report RR076 whichprovides general guidance on rotating equipment including turbines. The applications, systems andcomponents of offshore gas turbines are reviewed. Guidance is given on the integrity issues andmaintenance typical for different systems. Summaries are given of database information on theturbines installed on UK installations together with recent incident and accident data. Recentexperience and anecdotal information from operators is also reviewed. The inspection guidance noteis principally designed to provide information for HSE inspectors in safety assessments, incidentinvestigations and prior to site visits. The note may also be of interest to manufacturers, suppliersand operators of gas turbines (GTs) used offshore.

This report and the work it describes was co-funded by the Health and Safety Executive (HSE) andthe EU’s Fifth Framework Programme of Research. Its contents, including any opinions and/orconclusions expressed, are those of the authors alone and do not necessarily reflect HSE policy.

HSE BOOKS



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© Crown copyright 2006

First published 2006

All rights reserved. No part of this publication may bereproduced, stored in a retrieval system, or transmitted in

any form or by any means (electronic, mechanical,photocopying, recording or otherwise) without the priorwritten permission of the copyright owner.

Applications for reproduction should be made in writing to:Licensing Division, Her Majesty's Stationery Office,St Clements House, 2-16 Colegate, Norwich NR3 1BQor by e-mail to [email protected]



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Acknowledgements

The authors would like to thank the HSE inspectors, turbine suppliers, operators and others whohave contributed to this report and allowed pictures and other information to be reproduced. In

particular we would like to thank the following HSE staff for their contribution: Prem Dua the project technical officer, Jim MacFarlane for advice on rotating equipment issues, Tom Gudginfor his valuable comments on electrical issues and control systems, Stan Cutts for advice in thecontext of the KP3 initiative, Danny Shuter for handling project issues and HSE inspectors whoattended project seminars at Aberdeen, Bootle, Norwich and London for their comments. Rainer Kurz from Solar is thanked specifically for allowing us to use some of the images andintroductory information from his IGTI 2004 paper. This project was initiated by the HSEResearch Strategy Unit. The authors of HSE Research Report RR076 on rotating equipment arethanked for providing a starting point for present project.



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Foreword

This report covers the inspection and integrity of gas turbines (GTs) and major drivenequipment (compressors, pumps, alternators). The focus is on offshore applications including

floating installations and FPSOs. The work is directly relevant to HSE’s Key Programme 3(KP3) initiative.

The report is intended principally as an information source for HSE inspectors in safetyassessments, incident investigations and prior to site visits. The note may also be of interest tomanufacturers, suppliers and operators of gas turbines (GTs) used offshore.

The areas covered include: what can go wrong, typical inspection and maintenance, what isdone differently offshore, relevant, codes and standards, hazards and safety concerns, good and

best practice, summary of incident and accident data (RIDDOR, DO), a review of the mainsystems and components and how they work. A summary is given of advice in other HSEdocuments including PM84 and RR076.

Specific areas covered include: the basics of gas turbines, applications offshore, packagingconcepts, electrical and control systems, major driven equipment, GTs on UK installations,safety codes and regulations (including environmental), hazards and failure modes, maintenanceand inspection, operational issues and recent trends.

Section 1 provides an introduction and advice on use of the information in the reportSection 2 gives an introduction to gas turbines, the types of gas turbines that are used offshore,

packaging concepts and their applications.Section 3 summarises the main applications offshoreSection 4 describes offshore turbine packages in more detailSection 5 summarises the integrity, safety and maintenance issues for major driven equipment

building on the information in RR076Section 6 addresses the associated electrical systems.Section 7 focuses on control systems a main safety consideration and recent developmentsincluding synchronisation and corrected parameter controlSection 8 summarises the turbines installed in the UK sector.Section 9 covers safety cases, codes and regulations.Section 10 looks at degradation and failure modes including an analysis of incident, accidentdangerous occurrence and reliability data. Summary tables are given by system and component.Section 11 looks at maintenance and inspection practice in-service and at overhaul.Section 12 looks at operational issues including hazards, start-up and shutdown, surge

prevention, risk assessment and hazard management.Section 13 reviews recent trends in gas turbines including dry low emissions (DLE), micro-turbines, waste heat recovery systems and combine cycle gas turbines.Section 14 gives operational support guidance based on the principles developed in RR076.Section 15 gives examples of good and best practice with applicable guidance and regulationsand references listed in Sections 16 and 17 respectively.

Supplementary information is included in a number of Appendices. Appendix 1 gives a currentlist of UK installations and Appendix 2 describes what would be included in a typical

procurement package technical specification for gas turbines for a UK offshore installation. Appendix 3 reproduces HSE guidance note PM84 on gas turbines, Appendix 4 summarises themain turbine suppliers for UK installations derived from an analysis of DTI emissions data andother sources. The specifications for gas turbines used in the UK sector are summarised in

Appendix 5. The key systems and components are described in more detail in Appendix 6 .



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CONTENTS LIST

Foreword iv

1 Introduction to Inspection Guidance Notes 1

1.1 BACKGROUND 11.2 MAP OF GUIDANCE PROCESS 1

1.3 APPLICATION OF GUIDANCE NOTES 2

2 Basics of Gas turbines 3

2.1 INTRODUCTION 3

2.2 SYSTEMS AND COMPONENTS 4

2.3 HOW A GAS TURBINE WORKS 5

2.4 WORKING CYCLE 62.5 PRESSURE, VOLUME AND TEMPERATURE 7

2.6 CHANGES IN VELOCITY AND PRESSURE 72.7 GAS TURBINES OFFSHORE 8

2.8 TYPES OF GAS TURBINE 9

2.9 PACKAGING CONCEPTS 92.10 TURBINE PACKAGES 102.11 DESIGN FACTORS 122.12 TURBINE CONFIGURATION 122.13 DRIVEN EQUIPMENT 13

2.14 OFFSHORE ENCLOSURES 142.15 GAS TURBINE GT CYCLES 142.16 FUELS 15

3 Applications Offshore 17

3.1 POWER GENERATION 173.2 GAS GATHERING 183.3 GAS LIFT 19

3.4 WATERFLOOD 193.5 EXPORT COMPRESSION 20

4 Offshore Packages 21

4.1 MODULAR TURBINE PACKAGES 214.2 DESIGN OPTIONS 224.3 FPSO TURBINE PACKAGES 22

5 Major Driven Equipment 25

5.1 ALTERNATORS 26

5.2 COMPRESSORS 27

Applications 29Package Elements 29Package Configuration 30Hazards 30PM84 Guidance 31Components 32

5.3 PUMPS 34



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6 Electrical Systems 35

6.1 ELECTRICAL SYSTEMS 35

6.2 ELECTRICAL SYSTEMS GUIDANCE 366.3 ELECTROMAGNETIC RADIATION 376.4 MAINTENANCE OF ELECTRICAL SYSTEMS 37

7 Control Systems 41

7.1 PM84 GUIDANCE ON CONTROL SYSTEMS 437.2 RECENT DEVELOPMENTS IN CONTROL SYSTEMS 43

Corrected parameter control 43Control Synchronisation 44Triple Modular Redundant TMR Control Systems 45Redundant Network Control 46Standard Control System 46Software Architecture for a Standard control system 47

8 Gas Turbines on UK Installations 49

8.1 PACKAGERS 50

8.2 SUPPLIERS 50

9 Safety Cases, Codes and Regulations 53

9.1 RELEVANT UK INSTALLATIONS 539.2 INFORMATION FROM SAFETY CASES 539.3 HSE GUIDANCE NOTE PM84 539.4 DESIGN CODES 54

9.5 EMISSION REGULATIONS 559.6 ELECTRICAL REGULATIONS 55

9.7 LEGAL REQUIREMENTS 56

10 Hazards and Failure Modes 59

10.1 WHAT CAN GO WRONG 5910.2 FAILURE MECHANISMS AND ANALYSIS 59

Creep 59Thermo-mechanical fatigue 60High-cycle fatigue 60Metallurgical embrittlement 60Environmental attack 60Foreign body damage 60Manufacture or repair 60Failure analysis 60Materials 61 Air Compressors 62Combustors 62Turbines 63

10.3 PM84 ADVICE ON MECHANICAL FAILURES 6310.4 ANECDOTAL INFORMATION 6410.5 ACCIDENT, INCIDENT AND DANGEROUS OCCURRENCE DATA 65

Data extracted 65 Analysis of Data 65

10.6 IMIA INDUSTRIAL GAS TURBINE MEMBERS FAILURE STATISTICS 69



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10.7 RELIABILITY DATA FOR GAS TURBINES 7010.8 SUMMARY TABLES BY SYSTEM AND COMPONENT 7010.9 OTHER HAZARDS 80

11 Maintenance and Inspection 81

11.1 OVERVIEW 81

11.2 INSPECTION & REPAIR 82

Refurbishment of Gas Turbine Components 82Evaluation of damage 83Disassembly 84Dimensional checking 84Non-destructive testing (NDT) 84Metallurgical Examination 85Defining of workscope 85Processes 85Nozzle and Vanes 85Buckets and Blades 85Quality records 86

11.3 MAINTENANCE GUIDANCE 86

Fuels 88Water (or steam) Injection 90Cyclic Effects 90Rotor 90

11.4 DISASSEMBLY INSPECTIONS 94

Combustion Inspection 94Hot-Gas-Path Inspection 94

11.5 MAJOR INSPECTION 9611.6 TURBINE BORE INSPECTIONS 97

11.7 CLEANING 9711.8 SUMMARY BY SYSTEM AND COMPONENT 99

12 Operational Issues 105

12.1 HAZARDS 10512.2 START-UP AND SHUT-DOWN 105

12.3 SURGE PREVENTION 106

12.4 RECYCLE FACILITY 107

12.5 CONTROL SYSTEMS 108

12.6 VIBRATION MONITORING 10812.7 FIRE DETECTION REQUIREMENTS 109

12.8 PRECAUTIONS AGAINST FIRE109

12.9 RISK ASSESSMENT FOR ROUTINE ACTIVITIES 111

12.10 ACCESS 112

12.11 HAZARD MANAGEMENT IN HOT-SPOTS 11212.12 PRECAUTIONS AGAINST EXPLOSION 11312.13 VENTILATION 114

12.14 FUEL SUPPLY SYSTEMS 116

12.15 GAS FUEL 117

12.16 ADDITIONAL EXPLOSION PRECAUTIONS FOR LIQUID FUELS AND OILS 11712.17 EMERGENCY PROCEDURES 11812.18 AIR AND GAS SEALS 118

12.19 CHANGEOVER IN DUEL FUEL SYSTEMS 118



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13 Recent Trends 119

13.1 MICROTURBINE DEVELOPMENT 119

13.2 DRY LOW EMISSIONS (DLE) 11913.3 STEAM INJECTION FOR EMISSION REDUCTION AND POWER OUTPUT 12013.4 WASTE HEAT RECOVERY UNITS 12013.5 COMBINED CYCLE GAS TURBINES 120

14 Operational Support Guidance 123

15 Examples of good and Best practice 127

16 List of Applicable Guidance and Regulations 131

17 References 133

APPENDICES

Appendix 1 List of UK installations A1

Appendix 2 Typical procurement package technical specification A2

Appendix 3 HSE guidance note PM84 on gas turbines A3

Appendix 4 Gas turbine suppliers and summary for UK installations A4

Appendix 5 Specification of turbines used in UK sector A5

Appendix 6 Key systems and components A6



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1 INTRODUCTION TO INSPECTION GUIDANCE NOTES

This Inspection Guidance Note provides a detailed assessment of gas turbines (GTs) and major driven equipment installed on UK offshore installations, covering inspection, integrity andmaintenance issues. This complements the advice in HSE Guidance Note PM841, recently re-issued, covering control of risks for gas turbines used in power generation. The report is alsocomplementary to HSE Research Report RR0762,which provides more general advice onmachinery and rotating equipment including GTs. The applications, systems and components of offshore gas turbines are reviewed. Guidance is given on the integrity issues and maintenancetypical for different systems. Summaries are given of database information on the turbinesinstalled on UK installations together with recent incident and accident data. Recent experienceand anecdotal information from operators is also included. The guidance note is aimed atmanufacturers, suppliers and operators of gas turbines (GTs) used offshore as well as to provideguidance to HSE inspectors in safety assessments, incident investigations and prior to site visits.

1.1 BACKGROUND

Gas turbines are widely used offshore for a variety of purposes including power generation,compression, pumping and water injection, often in remote locations. GTS are commonly duelfuelled, to run on fuel taken from the production process in normal operation or alternatively ondiesel. Electrical power can also be generated to run other systems on the offshore installation.GTs offshore are typically from 1 to 50MW and may be modified aero-engines or industrial.Aeroderivative designs are increasingly used, particularly for the gas-generator. Lightweightindustrial designs for offshore use are also available.

Relatively little information is included in safety cases, for example only the manufacture,

model, ISO power rating (MW), fuel types, and installation drawings showing the location of the turbines. Some descriptive text may be included on the power generation package, back-upgenerators and arrangements for power transmission to satellite platforms. Information onintegrity management and maintenance is limited or at a high level. This document is intendedto provide more detailed information.

1.2 MAP OF GUIDANCE PROCESS

The guidance note is broken down into a number of discrete sections. Section 1 provides anintroduction and advice on use of the information in the report. Section 2 gives an introductionto gas turbines, the types of gas turbines that are used offshore, packaging concepts and their

applications. The main applications offshore and offshore turbine packages are coveredspecifically in Sections 3 and 4. The integrity, safety and maintenance issues for major drivenequipment is summarised in section 5, building on the information in RR076.

Sections 6 and 7 address the associated electrical and control systems, a main safetyconsideration. Recent developments including synchronisation and corrected parameter controlare included. Section 8 summarises the turbines installed in the UK sector, Section 9 coverssafety cases, codes and regulations and Section 10 looks at degradation and failure modesincluding an analysis of incident, accident dangerous occurrence and reliability data. Summarytables are given by system and component. Section 11 looks at maintenance and inspection

practice in-service and at overhaul. Operational issues including hazards, start-up and shutdown,surge prevention, risk assessment and hazard management are covered in Section 12. Recent

trends in gas turbines including dry low emissions (DLE), micro-turbines, waste heat recovery



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systems and combine cycle gas turbines are reviewed in Section 13. Section 14 givesoperational support guidance based on the principles developed in RR076 with examples of good and best practice in Section 15. Applicable guidance and regulations and references listedin Sections 16 and 17 respectively.

Supplementary information is included in a number of Appendices. Appendix 1 gives a currentlist of UK installations and Appendix 2 describes what would be included in a typical

procurement package technical specification for gas turbines for a UK offshore installation.Appendix 3 reproduces HSE guidance note PM84 on gas turbines, Appendix 4 summarises themain turbine suppliers for UK installations derived from an analysis of DTI emissions data andother sources. The specifications for gas turbines used in the UK sector are summarised inAppendix 5. Appendix 6 describes the key systems and components.

1.3 APPLICATION OF GUIDANCE NOTES

The guidance notes are intended to provide advice to HSE inspectors prior to site visits, inaccident investigations and in evaluation of safety cases. The report may also be of interest toother parties including dutyholders, users, manufacturers, suppliers and operators.



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2 BASICS OF GAS TURBINES

2.1 INTRODUCTION

A gas turbine (GT) converts fuel into mechanical output power to drive equipment including pumps, compressors, generators, blowers and fans. Gas turbines are widely used in the oil andgas industry in production, midstream and downstream applications with around 300-400installed on both fixed and mobile UK offshore installations. A typical gas turbine containsthree main systems: the compressor , the combustor – otherwise referred to as gas-generator or core engine and the power turbine. These main systems are illustrated schematically in Figure

1. A cross section through an Alstom GTX100 industrial turbine is shown in Figure 2 and for anAvon aeroderivative gas turbine in Figure 3. The gas generator itself for this latter turbinedesign is shown in Figure 4.

Figure 1 The main systems in a gas turbine used for power generation:compressor, gas generator or combustor and power turbine. Courtesy Solar 5

Figure 2 Alstom GTX100 turbine with cross section through GTX100 gas turbineshowing compressor, combustion system and power turbine and bearing

arrangements. Courtesy Alstom

A gas turbine is a complex component operating at high speeds and high temperatures. This puts demanding conditions on the materials and components, which need to perform in theseenvironments and maintain tight dimensional tolerances. To function a turbine needs a number

of ancillary and support systems. Provision has to be made for air-intake, fuel input, starting andignition, dispersion of exhaust gases, as well as cooling, lubrication of bearings and sealing.



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This total system forms the turbine package. Packaging concepts are described in more detail inSection 2.10.

Figure 3 Rolls Royce Avon gas generator with RT48 Power Turbine

2.2 SYSTEMS AND COMPONENTS

The gas turbine itself contains three main components:

x Compressor (AC) Compresses the air before combustion and expansion through theturbine

x Gas generator (GG) including combustor and gas turbine (GT). Ignition of air and fuelmixture to give a smooth stream of uniformly heated gas into the power turbine

x Power turbine (PT) The power turbine has the task of providing the power to drive thecompressor and accessories and, in the case of driven equipment of providing shaft

power for power generation, or driving the compressor or pump. It does this byextracting energy from the hot gases released from the combustion system andexpanding them to a lower pressure and temperature.

Other key systems within the package include the fuel system either natural gas or liquid(pumped), the bearing lube oil system including tank and filters, pumps (main, pre/post,

backup), the starter (usually either pneumatic, hydraulic or a variable speed ac motor), coolingsystems, controls (on-skid, off-skid), driven equipment and the seal gas system (compressors).

There is other ancillary equipment external to the turbine package. This includes: the enclosureand fire protection, the acoustic housing, the inlet system including air-filter (self-cleaning,

barrier, inertial) and silencer, the exhaust system including silencer and the exhaust stack, a lube



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oil cooler (water, air), the motor control centre, switchgear, neutral ground resistor and inletfogger/cooler.

A detailed description of each of the main systems and individual components is given in

Reference

3

and Appendix 6.

.

Figure 4 Avon gas generator. Courtesy Rolls Royce

2.3 HOW A GAS TURBINE WORKS

The gas turbine is a heat engine using air as a working fluid to provide thrust (Figure 5). Toachieve this the air passing through the engine has to be accelerated. This means that thevelocity or kinetic energy of the air is increased. To obtain this increase the pressure energy isfirst of all increased followed by the addition of heat energy before final conversion back tokinetic energy in the form of a high velocity jet efflux. A good description of the principles,design and detail of gas turbine engines can be found in References 4 and 5.

The working cycle of the gas turbine is similar to that of the four-stroke piston engine. In thegas-turbine engine, combustion occurs at a constant pressure, whereas in the piston engine itoccurs at a constant volume. In each case there is air-intake, compression, combustion and

exhaust. These processes are intermittent in the case of a piston engine, whereas in a gasturbine they occur continuously giving a much greater power output for the size of engine.

The pressure of the air does not rise during combustion due to the continuous action of theturbine engine and the fact the combustion chamber is not an enclosed space. The volume doesincrease. This process is known as heating at constant pressure. The lack of pressurefluctuations allows the use of low octane fuels and light fabricated combustion chambers, incontrast to the piston engine.



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Air Intake Æ Compression Æ CombustionÆ Exhaust

Figure 5 Cross section through a gas-turbine showing the continuous process of air-

intake, compression, combustion and exhaust in an aeroderivative design. CourtesyRolls Royce.

2.4 WORKING CYCLE

The working cycle upon which the gas turbine functions is represented by the cycle shown onthe pressure volume diagram in Figure 6 below. Point A represents air at atmospheric pressurethat is compressed in the air compressor stage along the line AB. From B to C heat is added tothe air in the gas generator by introducing and burning fuel at constant pressure, therebyconsiderably increasing the volume of air. Pressure losses in the combustion chambers areindicated by the drop between B and C. From C to D the gases resulting from combustion

expand through the power turbine and exhaust back to the flare. During this part of the cycle,some of the energy in the expanding gases is turned into mechanical power by the turbine;which can be used for power generation or to drive mechanical equipment such as compressorsor pumps.

Pressure

Volume

B

C

A D

Combustionheat energy added

Expansionthrough turbine

and nozzle

Compression pressure energy added

Ambient Air

Figure 6 The working cycle for a gas-turbine engine



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2.5 PRESSURE, VOLUME AND TEMPERATURE

The higher the temperature of combustion the greater is the expansion of the gases, because the

gas turbine is essentially a heat engine. The gas entry temperature following combustion mustnot exceed design limits or safe operating limits for materials in the turbine assembly.

The use of air-cooled blades and thermal barrier coatings in the turbine assembly permits ahigher gas temperature and consequently a higher thermal efficiency. During the working cycleof the turbine engine, the airflow or working fluid receives and gives up heat, so producingchanges in its pressure, volume and temperature. These changes as they occur are closely relatedthrough the relationships that apply in Boyle’s and Charles’ Laws.

Consequently, the product of the pressure and the volume of the air at the various stages in theworking cycle is proportional to the absolute temperature of the air at those stages. Thisrelationship applies for whatever means are used to change the state of the air. For example,

whether energy is added by combustion or by compression, or is extracted by the turbine, theheat change is directly proportional to the work added or taken away. It is the change in themomentum of the air that provides the thrust on the turbine. Local decelerations of airflow arealso required, as for instance, in the combustion chambers to provide a low velocity zone for theflame to burn.

There are three stages in the turbine working cycle during which these changes occur. Duringcompression, work is done to increase the pressure and decrease the volume of the air. Thisgives a corresponding rise in the temperature. During combustion, fuel is added to the air and

burnt to increase the temperature, there is a corresponding increase in volume whilst the pressure remains almost constant. During expansion, work is taken from the gas stream by theturbine assembly, there is a decrease in temperature and pressure with a corresponding increase

in volume.

2.6 CHANGES IN VELOCITY AND PRESSURE

The path of the air through a gas turbine varies according to the design. Changes in the velocityand pressure of air are consequent from aerodynamic and energy requirements. For example,during compression a rise in the pressure of the air is required and not an increase in its velocity.After the air has been heated and its internal energy increased by combustion, an increase in thevelocity of the gases is necessary to force the turbine to rotate.

Changes in the temperature and pressure of the air can be traced through an turbine by using an

airflow diagram. With the airflow being continuous, volume changes are shown up as changesin velocity. The efficiency with which these changes are made will determine to what extent thedesired relations between the pressure, volume and temperature are attained. In an efficientcompressor, higher pressure will be generated for a given work input and for a giventemperature rise of the air. Conversely, the more efficient the use of the expanding gas by theturbine, the greater the output of work for a given drop of pressure in the gas.

When air is compressed or expanded at 100 per cent efficiency, the process is called adiabatic.An adiabatic change means there are no energy losses in the process, for example by friction,conduction or turbulence. It is obviously impossible to achieve this efficiency in practice. 90

per cent is a good adiabatic efficiency for the compressor and turbine.



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Changes in velocity and pressure within the turbine stages are effected by means of the size andshape of the ducts through which the air passes on its way through the turbine. Where aconversion from velocity (kinetic) energy to pressure is required, the passages are divergent inshape. Conversely, where it is required to convert the energy stored in the combustion gases to

velocity energy, a convergent passage or nozzle is used.

The design of the passages and nozzles is of great importance. Their good design will affect theefficiency with which the energy changes are effected. Any interference with the smooth airflowcreates a loss in efficiency and could result in component failure due to vibration caused byeddies or turbulence of the airflow.

Figure 7 A gas-turbine driving a generator: 1 Fresh air, 2 compressor, 3 combustionchamber, 4 Burners, 5 frame cylinder, 6 turbine, 7 gas turbine exhaust gas, 8

Generator. Courtesy SWRI 3

2.7 GAS TURBINES OFFSHORE

Gas turbine packages offshore often differ to those used in other applications because of thedifferent drivers 3. Optimum size and high power to weight ratio are key factors offshore, aswell as availability, reliability and ruggedness. Efficiency has traditionally not been so critical

because of the availability of fuel. The increasing requirement for low emissions has madecombustion efficiency an important factor. A decision is needed on whether to go for largeturbines with appropriate back-up or a smaller number of lower power turbines for specificapplications. Most suppliers have different gas turbine products for the oil and gas market. Arecent trend has been towards low-emission turbines driven by recent environmental legislation

(SI 2005 No 925 The Greenhouse Gas Emission Trading Scheme Regulations, see Section 9.5).Some of these issues are also relevant onshore.



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Table 1 Main drivers for turbines used in the oil and gas sector. Compared to driversfor normal industrial applications

Oil & Gas Requirements I ndustrial Power Generation Requirements

Availability / Reliability –

Ruggedness

High Power/Weight ratio

Efficiency not Critical

Cost of Electricity

Efficiency

Cost of Operations and Maintenance

2.8 TYPES OF GAS TURBINE

There are two main types of gas turbine: industrial and aero-derivative. Aeroderivative GTs area development from aircraft engines and differ in a number of respects to industrial turbines:they are usually lighter than industrial engines, often have power turbines (PTs) manufactured

by a different manufacturer and have all anti-friction bearings in the gas producer. There is anincreasing trend to use aeroderivative gas turbines offshore in the UK, at least in terms of thegas generator (see Section 8).

This distinction is no longer so clear. It is common practice now to include an aeroderivativegas generator (GG) with a conventional power turbine (PT) such as in the GE PGT series.Industrial GTs for offshore use such as those produced by Solar have moved on in simplicityand design and increasingly mirror aeroderivative designs in size and weight. It is common

practice for turbine suppliers to match their power turbine with a standard aero-derivative gasgenerator, for example the LM2500 from GE utilises a Rolls Royce RB211. Industrial heavyduty gas turbines are referred to as Type H by the American Petroleum Institute API. Modular or aero-derivative gas turbines, are designated Type G.

Coincidentally aero-derivatives usually offer higher efficiency and faster start-up, particularlyfor larger engines. Major maintenance of aero-derivatives and smaller industrial gas turbines isusually off-site (sometimes with engine exchange). For larger industrial gas turbines major maintenance is usually on-site. In the past industrial gas turbines were preferred toaeroderivative gas turbines in process applications and in mechanical drive applications where awide range (70% to 100%) speed control was required.

Aeroderivative GTs offer advantages in offshore or oil field applications where allowable massand available space are limited. The reliability and availability of the specific gas turbine arekey criteria in selection. Aero-derivative gas turbines traditionally have required premium gasand liquid fuels. If the gas turbine fuel available is a crude oil, residual fuel oil, very lean gas,refinery mix gas or a gas that is subject to changes then an industrial gas turbines may haveadvantages. Fuel control is an important factor in low emission or DLE turbines.

2.9 PACKAGING CONCEPTS

Gas turbines for offshore installations are normally provided as part of a turbine package

developing a rated power at a rated speed and mounted on a single skid (Figure 8) and are not



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normally custom-built to meet the user's particular power requirements. API RP 11 PGT givesgeneral requirements and limitations in applying these standard turbine designs.

Packaging offers several advantages. It offers a fully integrated system that can be plugged in to

the installation. It facilitates a modular approach where the same modular systems can be usedin different applications; but configured to fit the fuel and exhaust requirements of the specificinstallation. It combines systems that have been developed and shown to work together. It issimpler to get safety case approval from regulatory bodies where similar packages have already

been used on other installations.

Figure 8 Typical gas turbine package offshore installation. Courtesy Solar

2.10 TURBINE PACKAGES

The systems that would usually be included as part of a gas turbine package are illustrated below inFigure 9. These include:

x Air compressor (AC),

x Gas generator (GG) including combustor and gas turbine (GT),x Power turbine (PT),

x Fuel system either natural gas or liquid (pumped),

x Bearing lube oil system including tank and filters, pumps (main, pre/post, backup),

x Starter (usually either pneumatic, hydraulic or variable speed ac motor),

x Controls (on-skid, off-skid),

x Driven equipment

x Seal gas system (compressors).

There are requirements for other ancillary equipment external to the turbine package. Thisincludes: the enclosure and fire protection, the inlet system including air-filter (self-cleaning,

barrier or inertial) and silencer, the exhaust system including the exhaust stack and silencer, a



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lubricating oil cooler (water, air), the motor control center, switchgear, neutral ground resistor and inlet fogger/cooler. The layout of these systems is illustrated in Figure 10 below.

x

Figure 9 Cross section showing the typical systems included as part of aturbine package. Courtesy Solar/SwRI

Figure 10 Schematic showing the systems typically included outside the turbinepackage. Courtesy SWRI3



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2.11 DESIGN FACTORS

Factors that needed to be considered in designing turbines offshore include: low weight anddimensions, minimising vibration, resistance to saltwater, resistance to pitch and roll

particularly in floating installations. The use of 3-point mounting is common to isolate the GTfrom deck movements.

Issues in procurement are considered in Appendix 2. The main basis for procurement isnormally API 616. A range of other factors need to be considered dependent on the installation.These may include:

x Operating requirements

x Spares inventory

x Type selection – aeroderivative or industrial, one or two shaft

x Site environment and fuel considerations

x Power requirements

x Installation – cranes, safe access, lay down areas, mounting, enclosures, auxiliaryequipment

x Noise levels- limits, support information, general requirements

x Oil tank vents

x Materials – specification,temperature, corrosion and environment resistance, coatings,certification

x Starting drives - gas expansion starters, hydraulic motors, diesel engines

x Foundations, baseplates and mountings

x Controls and instrumentation

x Inlet system – intake location, new configurations, material, leak prevention, joints andmovement allowances

x Air intake gridsx Air compressor cleaning

x Exhaust system – Exhaust emission, height, proximity to process equipment, rainingress, maintenance access, recirculation

x Combustion air filtration – requirements, anti-icing, shutters

x Fire protection – ventilation dampers, extinguishing systems, enclosure surveillance

x Acoustic enclosures – accessibility, ventilation, area classification

x Fuels and fuel systems – fuel selection, gas fuels and systems, liquid fuels and systems,dual fuel systems, power augmentation

x Inspection and tests – general, combustion tests, complete unit or string tests

Best practice in procurement is often included in the operators design and engineering practices.These advise on the above issues and other factors such as: definitions of vital, non-essentialand non-essential services, how these impact on selection, gas turbine enclosure ventilation,mounting and foundation requirements, exhaust stack rain-catcher requirements and key issuesfor gas turbine washing systems. Diagrams of typical installation arrangements may beincluded.

2.12 TURBINE CONFIGURATION

Gas turbines offshore are normally installed in an n+1 configuration with the additional unit providing spare capacity in case of shutdown. The number of turbines is typically 3 or 5

offshore, depending on sparing requirements and the power needed. Using a number of smaller turbines gives more flexibility if there is frequent need for turning on and off capacity. Whether



13

to go for large or smaller engines depends on the flexibility required. With smaller engines thereare more start-ups and shutdowns. There is a trade-off between size and maintainability whererequirements exist to reduce topside weight. Standards for gas turbines give limited flexibility,for example A36 steel is defined for the baseplate. Increasingly turbine suppliers such as Solar,

Rolls Royce and MAN use a modular approach, with application selectors to assist in theselection of modules, filters, and other ancillary components. In offshore applications there is atrend to use increasingly lighter materials for the casings

2.13 DRIVEN EQUIPMENT

Gas turbines are used in a number of functions offshore including oil field power generation;gas gathering; enhanced oil recovery including gas lift, gas injection and waterflood; exportcompression; gas plants and gas transport in pipelines. This is an efficient use of gas or liquidfuel which is naturally produced on most oil installations. Typically the GT would drive acompressor or pump, normally with gas fuel. Turbines normally duel-fuel with natural gas as

primary. The secondary diesel is used in emergency situations e.g well shut down and in

bringing systems up. Then gas is used for fuel.

(a) (b)

(c)

Figure 11 Examples of equipment driven by a gas turbine and other methods: (a)centrifugal compressor driven by 2-shaft gas turbine, (b) centrifugal compressor driven by variable speed electric motor (c) reciprocating compressor driven by gas

motor. Courtesy Solar



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In this guidance note the compressors, pumps and other equipment that supplies these functionsare referred to as driven equipment where they are mechanically driven by the turbine itself either directly or indirectly. In more recent installations there is a trend to use gas turbines

primarily for power generation with other equipment driven electrically particularly for satelliteor remote installations. Equipment may also be driven by gas motors.

There can be considerable variation in the size and ratings for a gas turbine

x Available Output Power Range: 20 kW- 250MW (25 hp – 350,000 hp)

x Smaller units (60MW or below) are typically used offshore.

x Typical Gas Turbine Simple Cycle Efficiency: 25- 35%

x Output Speed Range: 3000 - 25000 rpm

x Fuels: Natural Gas, Liquid Fuels or duel fuel

2.14 OFFSHORE ENCLOSURES

To mitigate the risk in case of turbine failure and to reduce noise it is common but not universalto house offshore GTs in enclosures. Offshore gas turbines may be subject to salt spray. Toavoid corrosion damage stainless steel is normally used for the enclosures, bolts and hardware.

In smaller installations and FPSOs there can be advantages not to enclose the gas turbine. Thiseliminates safety risks associated with access to enclosed spaces, reduces the risk of gas or hydrocarbon build up and simplifies ventilation requirements. Gas turbines generally operatesmoothly provided a uniform supply of air, fuel and environmental conditions are maintained,this may be more difficult to achieve if the GT is not enclosed.

Gas turbines emit a noise level which is higher than that normally permitted and acousticenclosures are invariably required. Particular precautions are required for the enclosure, inwhich high temperatures may prevail and flammable vapour may be present.

The acoustic enclosure may include the gas turbine, its auxiliaries and driven equipment, or itmay have separate compartments for each of these individual units. The nature of theinstallation, the type of driven equipment and the composition of any flammable vapour whichcould be released within the enclosure will generally dictate whether the enclosure shall becontinuous or shall have separate compartments.

Noise control requirements and ergonomics require the use of off-base mounted turbine

enclosures to provide more space for maintenance and better control of noise emission insteadof the type of enclosures formerly used which were close-fitted and mounted on the turbine

baseplate). The enclosure is often fitted with strategically located lifting beams on which a chain block can be fitted for minor maintenance activities.

2.15 GAS TURBINE GT CYCLES

The generation of electricity by a GT is implemented by several different systems. The simplecycle only generates electricity. In combined heat and power (CHP) plants and with waste heatrevovery systems (WHRU) the residual heat in the engine exhaust is used for a variety of

purposes ranging from industrial process heating to domestic hot water. Combined cycle gasturbine (CCGT) plant uses the residual heat to raise steam, which drives a steam turbine



15

producing further electricity. CHP, WHR and CCGT are increasingly used in offshoreapplications. More information on these is included in Section 13.

2.16 FUELS

A variety of fuels can be used by a gas turbine. While natural gas is the preferred fuel for mostUK plants, liquefied petroleum gas (LPG), refinery gas, gas oil, diesel and naphtha may be usedas main, alternate, standby or startup fuels. Hydrogen and biogas derivatives are alsoincreasingly being used and fuel can include waste streams produced on-site. Aero-derivativeand low emission turbines have more precise fuel requirements. Fuels are covered in Paragraph5 of HSE Guidance Notes PM 84 and also in Paragraphs 48 to 53.

The choice is dependent on commercial and environmental considerations. Each type of fuel hasits own particular hazards arising from its physical and chemical properties. Offshore the fuelwould come from the production process with diesel backup used for startup and productionshutdown.

The characteristics of the intended fuel(s) would be stated in the data/requisition sheets.Manufacturers are required to confirm the suitability of the intended fuel(s) and to support thiswith evidence of prior experience with fuels of similar quality and composition, see ASTM D2880. The Manufacturer would also advise on any treatment needed for the intended fuel(s) torender it suitable for the proposed application. It also needs to be verified that the smokeemission of the intended fuel is within local regulations.

In marginal cases, it would be investigated whether identical fuels have been used by other operators and any specific design requirements determined, especially in relation to traceelements. Gas turbine hot parts are particularly sensitive to alkaline metals such as sodium and

potassium. Other elements may have additional restrictions due to environmental emission

limits and the general corrosion requirements of downstream systems. Fuels containing heavymetals may require additional fuel treatment systems. Manufacturers have comprehensiveguides to suitable fuels including advice on the permissible level of contaminants andconcentration of corrosive agents which can be tolerated in a particular fuel. This advice would

be followed in reaching agreement with the gas turbine manufacturer on acceptable levels andconcentrations for the intended fuel(s). Fuel composition is usually normalised using the Wobbe

Index and evaluated for all operating conditions, including start up



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3 APPLICATIONS OFFSHORE

Gas turbines are used in the gas and petroleum industries to provide pumping and gascompression facilities, often in remote locations such as a pipeline. In this case the GT may runon fuel taken from the pipeline. Electrical power can also be generated if required, for instanceon an oil production platform. GTdriven plant can be utilised for local or national power-generation requirements. Turbines up to about 50 megawatts (MW) may be either industrial or modified aero-engines, while larger industrial units up to about 330 MW are purpose-built.

Applications offshore include power generation, gas injection, gas lift, waterflood and exportcompression 3. A distinction can be made between upstream, midstream and downstreamapplications. In this context production facilities are upstream with pipelines and transportaion

being midstream. The specific applications where gas turbines are used offshore are summarised below.

Upstream applications of gas turbines in the oil and gas industry include the following:

x Self-Generation- Power generation to meet needs of oil field or platform

x Enhanced Oil Recovery (EOR)- Advanced technologies to improve oil recovery

x Gas Lift - Injecting gas into the production well to help lift the oil

x Waterflood - Injection of water into the reservoir to increase reservoir pressure andimprove production

x Gas Re-injection- Re-injection of natural gas into the reservoir to increase thereservoir pressure

x Export Compression- Initial boosting of natural gas pressure from field into pipeline(a.k.a. header compression)

x Gas Gathering - Collecting natural gas from multiple wells

x Gas Plant and Gas Boost - Processing of gas to pipeline quality; i.e., removal of sulphur, water and CO. components

x Gas Storage/Withdrawal - Injecting of gas into underground structure for later use:summer storage, winter withdrawal

In midstream applications gas turbines may be used for:

x Pipeline Compression - Compression stations on pipeline to "pump" natural gas;typically 800-1200 psi compression

x Oil Pipeline Pumping - Pumping of crude or refined oil.

Gas turbines are also used in downstream applications including refineries. These are not

covered in the context of this inspection guidance note.

3.1 POWER GENERATION

The primary application of gas turbines offshore is in power generation. The turbine will provide direct drive to an alternator to generate power for the installation. It is normal to have atleast two GTs on main platforms with an emergency generator as back-up. Satellite and remoteor unmanned platforms are commonly provided with power from the main installation viaumbilicals rather than having their own gas turbines.

There will be different power generation requirements for floaters/semi-submersibles, fixed leg

platforms and onshore. This will depend on electrical requirements and fuel gas availability



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The typical gas turbine size in this application is 1 MW - 30MW. The number and configurationof turbines depends on the flexibility and redundancy needed and to allow for future platformupgrades.

Figure 12 Array of three gas turbines being used for power generation offshoreon an FPSO. Courtesy Solar

3.2 GAS GATHERING

Gas gathering is used to collect natural gas from several wells. Modern offshore installationsmay produce from 50 or more wells. In gas gathering a turbine of typically 3MW - 20MW

would typically be used



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Figure 13 3 Body Compressor Skid for Gas Gathering Application. Courtesy Solar

3.3 GAS LIFT

Gas-lift helps Increase crude oil production by injecting natural gas into the oil well. Reductionin oil density and aeration helps oil flow. Gas is separated and re-injected. A typical gas turbinesize of 3MW-20MW would be used in this application.

3.4 WATERFLOOD

Waterflood is another method of enhanced Oil Recovery. A gas turbine drives a centrifugalwater pump (usually with gearbox). The pressure is usually up to 600bar. Pump cavitation must

be avoided. The typical gas turbine size in this application is: 1 MW-15MW

Figure 14 Schematic illustrating water flooding for enhanced oil recovery. CourtesySolar.



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3.5 EXPORT COMPRESSION

Export compression is used to boost the gas pressure to flow gas to plant or pipeline. Thetypical gas turbine size in this application would be ~ 3MW-30MW with the larger turbines being used in pipeline export.

Figure 15 Gas turbine being used in export compression. Courtesy Solar



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4 OFFSHORE PACKAGES

4.1 MODULAR TURBINE PACKAGES

In oil and gas and other sectors, turbine suppliers are increasingly offering modular turbine packages 6 for both aero-derivative and industrial gas turbines. These offer advantages in termsof short installation time, smaller package size, ease of maintenance, achieving regulatoryapproval and reduced cost. Such packages can be tailored with a wide range of options to fit therequirements of an individual oil and gas installation. Such packages typically include threemodules; a turbine module, a compressor module and an air-intake module.

Figure 16 Example of modular approach for aero-derivative gas generator maintenance. Courtesy Rolls Royce

Within these units smaller modules may be included to facilitate replacement, substitution or maintenance (Figure 16). The following systems can vary:

x Starter

x Lube oil

x Fuel



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x Air-Intake

x Exhaust

For example, to fit single level or multi-level installations and routes to flare, the option of axial

or radial exhaust configurations offers flexibility.

The modules can be pre-installed and packages have a common frame size. The drivers for amodular approach are short installation time and lower total cost. An additional benefit is theshort turbine change-out time. The sequence for a modular system would be as follows: shutdown, disconnect combustion system, disconnect air-intake module, turbine ready for transport.The time required for installation could typically be as follows6 :

x Installation of Foundation and Generator module 2h

x Turbine module and air-intake module 4h

x Total installation ½ day

x 14 days to start-up

Modular systems also allow a short turbine change-out time. The typical sequence of operationsmay be as follows :

x Step 1 Shutdown and disconnect combustion system

x Step 2 Disconnect air intake module

x Step 3 Remove turbine ready for transport

4.2 DESIGN OPTIONS

An important option is provision for axial or radial exhaust . This gives flexibility in layout.

Radial exhausts are excellent for multilayered systems with the silencer above. Axial exhaustsallow direct link to a waste heat recovery units (WHRU) and heavier equipment to be installedon the top deck. Approximately 50% of offshore installations have a WHRU, usually a glycolcleaner. The axial v radial exhaust option in Solar Titan 130 and Taurus 170 gives layoutflexibility To aid installation the two exhaust options may be configured to have the same widthand external dimensions. For example in the Titan 130 turbine both exhaust modules are 3.12mwide and 14.22m long. In the axial system there is an additional 4.22m to the silencer, with a13.1m vertical rise to the silencer for the radial exhaust. More information on design optionsand procurement is given in Appendix 2.

4.3 FPSO TURBINE PACKAGES

Deepwater installations are an area of growth in the oil and gas sector with over 150 newFloating production Systems (FPSs) due to be installed Worldwide in the next 5 years. FloatingProduction Storage and Offloading (FPSO) vessels are the most significant, followed byTension Leg Platforms (TLPs) and other options such as semi-submersibles. Worldwideapproximately 10-15% of gas turbine packages are on floating installations. For example, Solar currently have nearly 300 turbine packages on FPSs of a total of 2375 turbine packagesoffshore

3. These are mainly Taurus 70 or Titan 130 turbines. For power generation an FPSO

will typically have one or more gas turbines, usually including waste heat recovery (WHRU)sytems.

The key design drivers are low topside weight, limited space and resistance to changing weather conditions. These impose specific requirements on the turbine package. A typical turbine



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package can weigh 110 tons. A one ton reduction in topside weight on a floating installation can produce savings of $10,000 in cost.

Location of turbines on FPSOs will depend on the installation. On the Trenergy FPSO3, Solar

turbines are installed in the middle of the vessel. Turbines used on FPSOs can be industrial or aeroderivative. It is understood that Solar turbines installed on FPSOs up to Jun 2004 were allindustrial 3.

Special mounting procedures are needed on FPSOs to allow for pitch and roll. Baffles are usedto stop oil movement, scavenge pumps are used on the drains of engine bearings to ensure oil isalways flowing, a 3-point mounting is used verified by finite-element analysis. A singlemounting in front with two back mountings – gives the maximum flexibility on loading.Multiple base plates are generally used as this is less costly and allows scavenging for spare

parts. On offshore platforms and floating production and FPSOs the design of the machinerymodules can be significantly simplified if the gas turbine driving train baseplate design is rigidand supported on a three-point mount. Alignment of the driving train is then unaffected by

platform movements. Installation of the driving train on a steel structure allows tuning to avoidvibration transmission.

Normally offshore the gas turbine train would allow for continuous operation under a tilt angleof maximum 3 degrees. A structural analysis would be performed to achieve the requiredstiffness of the baseplate, together with stress analysis of connecting pipe work and cables toensure that no distortion will occur.

For FPSOs the maximum tilt angle can be substantially larger than 3 degrees. The actual staticand dynamic displacement requirements for these applications would be specified separately. Asa guide, turbine-driven generator sets in essential services must be capable of normal operationup to and including the maximum angles specified, while generator sets in non-essential

services and mechanical-drive packages and compressor sets in process services would becapable of surviving, but not necessarily capable of operating, at these maximum angles.



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5 MAJOR DRIVEN EQUIPMENT

There are two options for equipment driven by gas turbines, either to provide power directlyfrom the turbine, known as single shaft , or to drive indirectly with the driven equipment on aseparate shaft, known as two-shaft . Mechanical Drive comprises a packaged gas turbine androtating equipment driven by it. The base frame will be a common single unit. For larger or onshore units this is often of two or more segments bolted together. The gas turbine may be of twodistinct types as below:

Single shaft gas turbine

In a single-shaft gas turbine the Power Turbine (PT) and Gas Generator Turbine (GGT) arecombined mechanically on to a single shaft. A single shaft turbine has all internal parts rotatingat the same speed. This gives simplicity, but requires the driven equipment to be started andoperated at the same time as the turbine core. The main use is for electric power generation.This configuration is used in fixed speed applications (in a range: 90%-100% full speed). For

example to produce generator drive via gearbox (1500 rpm - 50 hz, 1800 rpm - 60 hz).

Figure 17 Single shaft turbine with shaft coupling. Courtesy Solar/SwRI

Two-Shaft Gas Turbine (no Shaft Coupling)

A two-shaft gas turbine has no mechanical connection between the power turbine and the hot

gas generator, thus permitting the power turbine to rotate on its shaft independently of the hotgas generator. In a two shaft gas turbine the Power Turbine (PT) is independently supported onits own shaft and bearings. This allows variable speed applications (typically in range 25%-100% full speed). This configuration is used for compressor, pump and blower applications.Two Shaft turbines permit the core engine to be started without spinning the driven equipment,This configuration is applicable to mechanical drive packages.



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Figure 18 Two-Shaft Gas Turbine (no Shaft Coupling). Courtesy Solar.

Single and two-shaft gas turbines can be used across the full power range: from 0-100% fullload , however efficiency will be low and emissions high at loads below 60%.

5.1 ALTERNATORS

The term power generation package refers to a packaged gas turbine and alternator on acommon base. Power generation is the most common application of gas turbines offshore. Theturbine package is intended for fixed speed operation for electricity generation. The gas turbinewill have matched power turbine. A load gearbox is used to match turbine and alternator shaft

speeds. Detailed information on the safety and risk issues associated with alternators can befound in HSE report RR0762 covering inspection guidance on rotating equipment. Thealternator is directly driven and mounted on the cold inlet end of the shaft before thecompressor.



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Figure 19 Typhoon gas turbine power generation package. Courtesy EGT

Figure 20 Cross section through Typhoon gas turbine power generation package

5.2 COMPRESSORS

The second most common application of gas turbines offshore is in gas compression. A

compressor package is an enclosed gas turbine with one or two gas compressors co-axially on



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the end of the output shaft. All turbine elements are mounted to a common baseframe. Anoffshore compressor package will typically be provided as a single lift module to givesimplified installation and transportation to the platform. This module includes all systems,exhaust and waste heat recovery unit (WHRU). In addition to the GT and the driven

compressor the package would include:

x A sub-base providing added stiffness for gas turbine and compressor skids

x 3-point mounts to give isolation from twisting and vibration

x An inclinometer to give alarm and shutdown at high list, trim, pitch, roll angles

x Baffles to provide a continued supply of lube oil at inclined operation

x A scavenging pump to give a forced supply of lube oil at inclined operation

Figure 21 Single lift gas turbine compression modules. Courtesy Solar, Rolls Royce

Figure 22 Typical offshore gas-turbine compressor package



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Applications

Gas Compressors are used to increase the pressure of a process gas, in order to drive it into a pipeline system to an onshore process plant, to use on the producing well as gas lift, to re-inject

gas for reservoir pressure maintenance or for use as a fuel gas. Centrifugal compressors are preferred for high mass flow systems because of their simplicity and reliability compared withscrew or reciprocating compressors. In order to achieve the required pressure ratio, severalcompression stages may be required, in one or more casings. Each compression stage is carriedout by a rotor in a matching diffuser. Mechanically linked compressors, working together withdrive and support equipment, may be regarded as a single system for design and safety

purposes. More detailed information on compressors can be found in HSE Report RR076.

Package Elements

An offshore gas turbine compressor package used to compress hydrocarbon gas typicallycomprises a twin shaft aero-derivative gas turbine driving a barrel casing centrifugal

compressor. The package would also include the control system & ancillary equipment. The package is mounted on a 3-point mounting skid baseplate. It is normal to enclose the gas turbineis enclosed in an acoustic enclosure with its own fire & gas system. Ancillary equipment andsystems will include:

x Inlet Air System & Filter

x Fuel System

x Exhaust Duct

x Lubricating Oil System

x Compressor Dry Gas Seals & Support System

x Drive Gearbox ( if required )

x Auxiliary Gearboxx Shaft Couplings

x Cooling System

x Piping Systems

x Condition Monitoring

Figure 23 Offshore gas turbine driven compression package. Courtesy Solar.



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Package Configuration

Figure 24 below shows the typical configuration for an offshore gas turbine compressor package.

Figure 24 Process Schematic Diagram - Gas Turbine Driven Gas CompressionSystem. Courtesy RR0762

Hazards

The major hazards have been evaluated in RR076 2 and relate to the inventory of flammable gasthat can be released if there is an equipment failure. Hazard assessment must relate to thecomplete package and not just the compressor body.

The injury risk from a mechanical failure is relatively low, as the robust casing will retain parts.Hot / moving parts may still cause injury local to the machine. Most compressors have gas sealson moving drive shafts or piston rods. These are safety critical items when handling hazardousmaterials. The gas turbine is dependent on various ancillary systems for safe operation,operating procedures and control system must ensure that these are operational prior to turbinestart, and at all times during operation. Hot surfaces will be fitted with heat shields or thermalinsulation. These must be in place for operator safety.

Multi-stage centrifugal gas compressors contain high speed moving parts within a robust casing.Mechanical failure can result in severe internal damage but this is not likely to pose a directhazard to people who are not close to the equipment. The greatest potential threat is theuncontrolled release of a flammable hydrocarbon gas, particularly if the gas is then able to forman explosive mixture within a relatively enclosed space.



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The risk is reduced by ensuring that compressors are competently operated and maintained, andthat protective systems are regularly tested and in good order. The overall system design should

provide suitable remote isolations, knockout pots and adequate vent routes. Control system

issues are covered in detail in Section 7.

A limited number of safety issues can arise from inclusion of a gearbox within a machine package. The most serious are: the potential for accidental or failure engagement of auxiliarydrives, used to rotate the compressor at low speed, leading to massive overspeed and usualdisintegration of the drive; bursting of the gear wheels (design or manufacturing flaws); firesdue to leakage of lubricating oil.

Misalignment of the main drive coupling , even within its tolerance limits, puts increased loadson adjacent shaft bearings. It also reduces the service life of the coupling, as flexible elementsare subjected to greater strains. Coupling lubrication (where required) and inspections must be

proactively maintained as the coupling has significant mass and has the potential to become a

dangerous missile if it fails. Loss of drive is not normally a safety-related incident; specialdesign requirements apply if drive continuity is critical. More information of flexible couplingscan be found in Reference 2.

PM84 Guidance

Paragraph 58 of HSE Guidance Note PM84 notes that those concerned with the supply andoperation of gas compressor stations used in UK should be aware that the foreword to BS EN12583: 2000 Gas supply systems - compressor stations - functional requirements contains thefollowing proviso:`In the UK the national safety body, the Health and Safety Executive (HSE) (see CR 13737), has

required additional precautions at gas turbine driven plant, eg compressors, combined heat and

power (CHP) and combined cycle gas turbine (CCGT), in order to comply with the general provisions of the Health and Safety at Work etc Act (HSWA). These additional precautions are

contained in HSE Guidance (Control of safety risks at gas turbines used for power generation)'.

Surge in driven compressors

Surge, which is the flow reversal within the compressor, accompanied by high fluctuating loadon the compressor bearings, has to be avoided to protect the compressor. Surge avoidance incentrifugal compressors driven by a two stage GT has been reviewed and modelled by Kurz7..

The possible operating points of a centrifugal gas compressor are limited by maximum andminimum operating speed, maximum available power, choke flow, and stability (surge) limit.The usual method for surge avoidance (“anti-surge-control”) consists of a recycle loop that can

be activated by a fast acting valve (“anti-surge valve”) when the control system detects that thecompressor approaches its surge limit. Typical control systems use suction and discharge

pressure

If the surge margin reaches a preset value (often 10%), the anti-surge valve starts to open,thereby reducing the pressure ratio of the compressor and increasing the flow through thecompressor. The situation is complicated by the fact that the surge valve also has to be capableof precisely controlling flow. Additionally, some manufacturers place limits on how far intochoke (or overload) they allow their compressors to operate. A safety critical situation can ariseupon emergency shutdown (ESD) if manufacturer’s surge prevention measures are not properlyadhered to.



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Here, the fuel supply to the gas turbine driver is cut off instantly, thus letting the power turbineand the driven compressor coast down on their own inertia . Because the head-making capabilityof the compressor is reduced by the square of its running speed, while the pressure ratio acrossthe machine is imposed by the upstream and downstream piping sys-tem, the compressor would

surge if the surge valve cannot provide fast relief of the pressure. The deceleration of thecompressor as a result of inertia and dissipation are decisive factors. The speed at which the pressure can be relieved of the pressure not only depends on the reaction time of the valve, butalso on the time constants imposed by the piping system. The transient behavior of the pipingsystem depends largely on the volumes of gas enclosed by the various components of the pipingsystem, which may include, besides the piping itself, various scrubbers, knockout drums, andcoolers. Models allow simulation of such upset situations and avoid their occurrence in servicemodel to simulate shutdown events and define simpler rules that help with proper sizing of upstream and downstream piping systems, as well as the necessary control elements.

Normal practice is to a 5% margin control to the surge limit and protect against the possibilityof surge by use of a recycle valve and operating within turbine suppliers safe operating limits.

These precautions mitigate against the possibility of surge on emergency shutdown (ESD).

Components

Acoustic enclosure

The acoustic enclosure for an aero-derivative gas turbine is normally close fitting, and fitted outwith ventilation and Fire & Gas Detection Systems. The internal space is tightly packed, makingaccess to internal components quite difficult. A problem on one component has the potential toaffect adjacent components or systems, either by release of material, vibration or over-heating.It may be necessary to remove a component to work on that component or to gain access toadjacent components.

Figure 25 Typhoon mechanical drive package. Courtesy EGT Acoustic enclosure



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Baseframe

The baseframe needs to be sufficiently rigid to maintain machine alignment, despite movementof the supporting structure or vessel. The 3-point mounting system normally used eliminates thetransmission of twisting forces to and from the baseframe.

In order to save space, and the weight of additional bases, as many as possible of the ancillarysystems e.g. lubrication oil system, seal gas support system, are built into the main baseframe.The control panel may be built on to the end of the baseframe (which is convenient for pre-wiring) or mounted separately (which permits control panels for separate machines to begrouped together).

Gas Turbine

The configuration for a compression package is identical to turbines in other drivenapplications. The turbine will have a fuel manifold wrapped around the middle of the machine,with multiple combustor fuel feeds. Flexible connections will link to the inlet and exhaust ducts.

The gas turbine is typically centre-line mounted from the baseframe. This ensures internalalignment while permitting thermal expansion of the machine. The main drive shaft will be atthe hot or exhaust end for a mechanical drive package and fitted with a flexible coupling. Asimilar configuration is used for any auxiliary drive shafts.

Figure 26 Cross section through Typhoon gas turbine mechanical drive package.Courtesy EGT

Any mechanical failure of the turbine, or an explosion within the acoustic enclosure, coulddisrupt fuel pipework, with the potential for a significant release. Missiles, in the form of ejected compressor blades or other high-speed components, may be thrown in a mainly radialdirection, with the potential to damage people or critical systems at some distance from the

turbine.



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Gas Compressor

The gas compressor and drive gearbox (if fitted) are normally outside the acoustic enclosure,they may still be very closely packed with service pipework & cable trunking. Good designshould permit ready access to compressor bearings, instruments and drive couplings. The air

inlet housing is located separate from the turbine next to the external cladding of the processarea. A multi-stage barrel type centrifugal gas compressor is centre-line mounted on anextension of the common base-frame, ensuring shaft alignment. Where two compressors arerequired to achieve the required pressure ratio, the second compressor is likely to be driven fromthe first compressor shaft, by a mechanical gearbox. All shafts require alignment within thetolerances of the shaft couplings.

Process Pipework

Process pipework is connected to the barrel casings, usually by flanged connections. Fullywelded assembly is also possible. Thermal expansion of process pipework must be allowed for

by good pipe support and flexibility design; bellows are not preferred. Dependent on operating

temperatures the compressor casing and pipework may be lagged. The centre-line supportsystem must not be lagged, as it has to remain at ambient temperatures, so far as is possible.

Gearbox and Auxiliary gearbox

The drive gearbox included within the machine package allows the manufacturer to optimiseoperating speeds of the gas turbine driver and centrifugal compressor separately. The technicaldisadvantages of additional skid length, equipment complexity, and weight are offset by the

benefits for the design of compressor and turbine. Gas turbine drive packages will include anauxiliary gearbox, normally integral to the cold end of the machine. This provides the necessarylinkage for turbine starting, and mechanical drives where required for oil or fuel pumps.

Main Drive Coupling The use of flexible couplings within a machine package is essential to provide the necessarydegrees of freedom to enable the machine elements to be aligned, and compensate for anyflexibility inherent in the installation skid.

5.3 PUMPS

Pump packages have a similar configuration to that shown for compressors. Normally thesewill require a smaller turbine. Detailed guidance on the safety risks associated with turbinedriven pumps can be found in HSE inspection guidance document RR076 2. Pumps offer asuitable application for use of micro-turbines.



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6 ELECTRICAL SYSTEMS

6.1 ELECTRICAL SYSTEMS

Gas turbines contain a number of electrical systems associated with control, start-up, anciliarysystems and system monitoring. These include ignition, governing, controls andinstrumentation systems, fuel pumps, inlet guide vane (IGV) controls for variable stators,lubrication pumps and monitoring systems for speed, torque, thrust and pressure. There will beassociated electrical systems for driven equipment. Some of this equipment will be mounted as

part of the GT skid with separate systems for example in the control room. The major use of gasturbines offshore is for power generation using an alternator driven by the turbine. Thealternator has it’s own electrical and electromagnetic concerns2.

The associated risks do not differ significantly to electrical systems on other large mechanicalequipment installed offshore. Specific risk factors for gas turbines are:

x the potential for gas leakage from the gas turbine and exhaust systems;

x the potential for leak of fuel, seal oil or hydraulic oil;

x the high temperatures, particularly in the combustor, transition and turbine.

In the presence of flammable substances, electrical equipment can be the source of ignition dueto sparking or high temperature surfaces integral to the electrical equipment. It is important thatelectrical equipment is correctly selected, used and maintained in hazardous areas where there isthe potential for flammable substances to be present. Control of gas turbine operation andemissions requires use of sensors and monitoring devices often exposed to high temperaturesand environmental attack; this places special requirements on the materials used in such sensorsand monitoring devices and the associated electrical systems.

PM84 provides specific guidance on gas turbines including electrical issues. Specific electricalissues covered include:

x Compliance of technical plant to UK and international standards.

x Electrical protection systems to avoid overload

x Enclosures and hazardous area classification

x Site safety rules and operational procedures

x Requirements for risk assessment

x Identification and labelling systems and the positioning of labels and notices onswitchgear, transformers, control gear and plant

x Legal requirements particularly commissioning and work on live electricalsystems

x Electromagnetic radiation and protection measures for live conductors magneticfield risk and corona discharge

x Practices not covered by existing safety rules and operating procedures such aslive brush changing in relation to the exciter system of the alternator (not usedoffshore).

Note that live brush changing is not carried out offshore due to the hazards this would incur.There are few situations that can be envisaged where it is not possible to shut down the exciter system and do in a safe manner. Regulation 16 of the Electricity at Work regulations wouldapply



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Electromagnetic radiation from close proximity to live conductors is covered by NationalRadiological Protection Board guidance 32 and discussed in more detail in PM84.

6.2 ELECTRICAL SYSTEMS GUIDANCE

Electrical issues are covered in Paragraphs 69 to 78 of PM84. When the initial tenders for new power generation are drawn up, care should be given to the consideration of the technicalspecifications for the electrical plant, equipment, installations and systems to be provided. It isessential to establish that what is to be provided and installed will comply with the relevanthealth and safety legislation in the United Kingdom and relevant national or internationalstandards. The electrical protection system should minimise the risk of potentially damagingoverload situations (that may result in catastrophic drive-train failure). Note that no UK offshoreinstallations synchronise with the grid system.

Hazardous area classification should be carried out for all plant items and pipework containingflammable substances such as fuel or oils, whether in an enclosure or otherwise. It should becarried out in accordance with relevant regulations (The Dangerous Substances and ExplosiveAtmospheres Regulations 2002), the associated Approved Codes of Practice L10121, L13422 andL13823 and current recognised standards such as BS EN 60079-10 1996 Normally, enclosureswould be expected to be classified zone 2. In some cases it may be possible to justify thereduction of zone sizes by making a conservative allowance for the effects of the ventilation inaccordance with relevant standards andguidance. This must take into account the extent of flammable areas from CFD predictions as described at paragraphs 35-41 above. Zoned areasmay be safe when the plant has shut down, if the fuel and other flammables are adequatelyisolated, as described in paragraph 50 of PM84, and sufficiently de-pressurised. Additional

guidance relating to area classification for natural gas is given in IGE/SR/25.20 All electricalequipment should be checked to confirm it is suitable for the area classification.

HSE Offshore Division Operations Note ON58 issued in January 2003 provides a short guidefor the offshore industry on the Dangerous Substances and Explosive Atmospheres Regulations2002 DSEAR. HSE Offshore Division Operations Notes 59 and 63 issued in January andDecember 2003 respectively provide relevant guidance on the Equipment and Protective

Systems intended for use in Potentially Explosive Atmospheres Regulations 1996 EPS .

Before plant is taken into use, site safety rules and operational procedures should be carefullymatched to the original specifications for the electrical installation, to avoid misunderstandings

by the operators. Specific agreements between users/purchasers and manufacturers as required

by some relevant standards, need to be checked to ensure full compliance.

Risk assessment should be carried out on all the electrical systems for the plant. The assessmentshould also include all the risks arising during system verification and commissioning tests. Theuser will be responsible for ensuring that the suppliers of electrical systems provide sufficientinformation to describe the safe use of their equipment.

Identification and labelling systems and the positioning of labels and notices on switchgear,transformers, control gear and plant have been used in the UK which differ from those normallyused. It is essential that employees are fully conversant with alternative identification andlabelling systems and that labels, notices and instructions are clearly displayed. Where this is a

potential problem, systems will have to be replaced with more familiar ones or further trainingwill be needed.



37

People carrying out commissioning or live work must be familiar with the plant and systems to be commissioned. They must be trained in using a permit-towork system as described in theregulations referred to in paragraph 87 of PM84. They must also consider the effects the work

could have on other people and plant. Adequate documentation and drawings must be availableat handover and final documentation must be completed as soon as practicable followingcompletion of commissioning. For the purpose of commissioning activities, inhibits andoverrides may need to be temporarily installed in order to prove the system controls. If this isthe case, a log should be maintained to ensure that they are removed and the systems reinstated,

prior to the equipment being made fully operational.

Existing safety rules and operating procedures may not address the requirements of the plant. Itmay be necessary to confirm before taking operational responsibility that the rules, proceduresand all equipment, including where necessary personal protective equipment, are in place. Staff will also have to be familiar and practised in these matters.

If electrical apparatus is located outside, then some environmental protection will be needed for the appropriate Ingress Protection (IP) code.

6.3 ELECTROMAGNETIC RADIATION

Electromagnetic radiation hazards are covered by Paragraphs 76 to 78 of PM84. Employersshould use the guidance published by the National Radiological Protection Board32 whenassessing whether there is a risk to health.

Current flows greater than a few hundred amps are capable of producing a significant magneticfield risk at a distance of less than one metre. Bare HV conductors may lead to people beingexposed to electric fields which exceed the NRPB investigation levels of 12 kV/m. On GT plant

the HV conductors are normally phase segregated and insulated, which will prevent coronadischarge. The only exception is the conductors from the transformer bushing to the bankingcompound where a visible corona may be present.

If the measured field strengths exceed the investigation level, more detailed investigation should be carried out to determine the induced currents arising from potential exposures. These should be compared with the published basic restrictions and, if necessary, preventative measurestaken. Such measures could include limiting the proximity at which people may approach liveconductors. Restricting the duration of exposure is not an acceptable control strategy. In thiscase suitable barriers and signs shall be in place to warn of the potential for danger.

6.4 MAINTENANCE OF ELECTRICAL SYSTEMS

Maintenance of electrical systems in hazardous areas is a specialised area and covered by anumber of standards and regulations. These include:

BS EN 60079-17: 2003 Electrical apparatus for explosive gas atmospheres. Part 17: inspectionand maintenance of electrical installations in hazardous areas (other than mines).

IEC 60079 –17 Recommendations for inspections and maintenance of electricalinstallations in hazardous areas (other than mines).

IEC 60079-19 Repair and overhaul for apparatus used in explosive atmospheres(other than mines or explosives)



38

BS 5345 Code of practice for the selection, installation and maintenance of electrical apparatus for use in potentially explosive atmospheres (other than mining applications or explosives processing and manufacture).

Other IEC guidance in regard to flameproof enclosures, increased safety, intrinsic safety, protection is also relevant. Electrical apparatus and hazardous areas has been reviewed byGarside13. Other relevant regulations are listed in Section 16.

Specific advice and information on relevant electrical codes and regulations is given in the HSEguidance on Explosive Atmospheres – Classification of hazardous areas (Zoning) and selectionof Equipment www.hse.gov.uk/comah/sragtech/techmeasareaclas.htm.

Inspection schedules for different equipment type and locations are given in the Tables in BSEN 60079-17: 2003.

Gas turbines present some specific concerns in regard to electrical equipment. There is the risk

of ignition in the event of a leak of gas, fuel or lubricating oil. Gas turbine components andcasings get extremely hot during operation, particularly in the hot-gas-path and combustionsystem. Any on-skid electrical equipment must be suitably protected and enclosed. Particular consideration is needed for sensors, wiring and other electrical equipment associated withcontrol and monitoring systems. HSE guidance note PM84 highlights specific concerns inregard to electrical and control systems in gas turbines. See Section 12 and Appendix 3.

A typical summary of points to look for at routine or periodic inspection of electrical systems13

may include:

x Apparatus Tag number

x Cable identifications correct to loop diagram/hook-up drawing

x Apparatus has no unauthorised modifications

x Any rectification work noted at previous inspections has been carried out suitably

x Earth connections secure

x No undue corrosion (especially on flanges for Ex d)

x Cable entries tight

x No degradation of required IP rating

x No broken covers or fan cowls

x No build up of dirt on cooling fins (especially on motors)

x Electrical connections tight (especially Ex e and Ex n)

x Correct lamp ratings

x No changes to area classification. (If so, the type of protection or its apparatus group or

T-class may not be suitable.)x No damage to associated cables

x No damage to apparatus

x Covers/lids correctly secured

x Apparatus mounting firm and acceptable

x Filters clean and free from dirt and debris

x Breathing and draining devices clean and free from dirt and debris

x Cable supports OK

x No external obstructions to flamepaths (Ex d)

x No excessive grease on flamepaths (Ex d)

x No hard setting compound on flamepaths (Ex d)

x No unauthorised gaskets (especially Ex d)



39

x Gaskets of correct type

x No excessive grease on bearings

x No signs of excessive temperatures (e.g. brittle or burnt insulation)

x No cracks to ceramic feedthroughs or insulators (especially Ex d, Ex e, and Ex de)

x No obvious changes to surrounding processes which could affect area classificationx No signs of leakage of filling medium (especially Ex o and Ex q)

x Filling medium at required level (especially Ex o and Ex q)

x No signs of leakage from stopper boxes or stopper glands (especially Ex d)

x Electrical aspects remain secure: especially earth loop tests, and especially for Ex ia/ib.Compare value with that previously noted

x No dirt or obstructions to fan covers

x No signs of excessive vibration

x Surveillance circuits functioning correctly (especially Ex p)

x Correct associated apparatus installed (Ex ia/ib)

x Conduit seals satisfactory at passage between non-hazardous area and hazardous area

x Cable identifications correct, and no changes to wiring



41

7 CONTROL SYSTEMS

Gas turbines are sophisticated pieces of equipment and synchronisation of the key systems(compressor, gas turbine, power turbine) is crucial to ensure smooth operation and avoid surgeand other operational issues. This is undertaken using the control system. The control systemwill also control a range of other functions including start-up, monitoring, fuel flow andignition, lubrication, emergency shutdown ESD. The current trend is to use distributed control

systems, both on and off the skid, with separate modules, actuators and sensors to controlindividual functions. Turbines are generally replaced after 6-7 years. The control system would

be updated more regularly, typically every 3 years.

The operation of all parts of the system may be affected by temperature, environment, air inputand other factors. The different systems must respond in a synchronised way to operationalchanges, changes in loading, start-up and shutdown. This synchronisation is crucial inemergency shutdowns.

(a) (b) (c)

Figure 27 Modern gas turbine control system components: 9a) GE control system

(Type IV). Images courtesy GE Power Systems, Woodward.

Machines within a package need to be integrated to function as a complete system. Controlsystems are designed to provide this essential control and protection for the machine elements.This requires key logical interlocks between the main control system, the turbine control and thecompressor control. These will provide for start / run permits and sequence control, e.g. ControlRoom authorisation for turbine start. These logical signals must be of high integrity as theycannot be bypassed or ignored. There will then be numerical (possibly a mixture of analogueand digital) signals controlling e.g. compressor load, turbine set speed, and for data logging.



42

On some of these signals it may be permissible to operate with manual over-ride, for exampleduring load changes. Alternately, the system may be intended to operate purely in fullyautomatic mode. This will require increased sophistication such as speed ramping, critical speedavoidance, operating temperature bands, load and speed matching during duty changes. More

guidance on these is provided in HSE Research Report RR076

2

(a) (b)

Figure 28 Control system metering and actuating components.: (a) gas fuel meter, (b)Water injection valve fuel system. Courtesy Woodward

It is normal to test as much as possible of the Package onshore before shipping. It is not possibleto fully test and tune the control system prior to commissioning. Computer models may be usedto test the interlocks, and to some degree the load control. The greater the degree of automation,the greater the demands on the commissioning team, who must set up and prove the system,

knowing that in normal operation load changes will be done without close manual supervision.Control software must be rigorously checked, subject to strict version and change recording andcontrol. Pre-programmed cards can be fitted to the wrong machine; they may be physicallyidentical ( to Model and Serial Number ) but carry different instructions.

The control system will manage the following systems necessary in normal operation and start-up and shutdown. Most of the control will be managed remotely (off-skid) via a separatecontrol room. Some systems and actuators are necessarily mounted on-skid.

x Dedicated PLC

x Emergency shutdown system ESD

x Temperature and vibration monitors

x Overspeed\ monitors and tripsx Fuel isolation and vent valves

x Lubrication control systems

x Ignition and flame control systems

Separation of safety related functions (e.g. ESD, safety interlocks) or plant protection functionsfrom the GT operational control functions is not always possible, but is recommended wherever reasonably practicable. Such separation usually results in a smaller and less complex safetysystem, which in turn minimises the chance of design, implementation or maintenance errors inthe critical safety related functions. In addition, separation enables design features that providesecurity against misuse, independence against failures in the operational control system and

avoidance of common mode faults. Where safety related and operational control functions do



43

overlap, their design should be such as to ensure that any change made to the operationalcontrols does not reduce the integrity of the safety related functions.

General guidance on control systems and how control system failures can lead to accidents is

given in ‘Out of Control’ , HSG238 2

nd

Edition, HSE, 2003. The booklet discusses the technicalcauses of control system failure by describing actual case studies and highlights the importanceof adopting a systematic approach throughout the system lifecycle with particular emphasis onthe specification phase. The booklet summarises the lifecycle approach toelectrical/electronic/programmable electronic safety-related systems contained in BS EN 61508.

7.1 PM84 GUIDANCE ON CONTROL SYSTEMS

Control systems are covered in Paragraphs 46 and 47 of PM84.For those hazards identified byrisk assessment and which are addressed by precautions inherent within the GT control package,safety-related systems should be identified, specified, implemented, tested and maintained inaccordance with the principles of BS EN 61508 or IEC 61511 as appropriate. Interfaces between

the GT and site control systems should be checked to avoid mismatch and subsequent failure.Strict controls should be in place to prevent unauthorized access to safety related systems. Suchsystems may include, for example, the GT purge cycle, flame detection, fuel isolation,ventilation detection, fixed fire protection, engine trip, and gas detector alarm/trip settings.

A mechanism for control of software changes is recommended as part of the overallmanagement of the software. This should also include copies of the software being held atsecure locations and procedures being in place to audit and confirm that the copies are all to thesame revision. Any changes to the hardware/software of safety-related control systems should

be accompanied by an impact assessment to determine what effect such changes will have onthe safety integrity of the control system. Any adverse effects identified by the assessment willrequire the design of the control system to be revisited, and possibly modified, to restore the

safety integrity to its original level. BS EN 61508 describes a mechanism for this process. Anychanges to the safety-related control system should be documented, including the reasons for the change, relevant technical details, the impact assessment, the design review, and anychanges to the operating/maintenance regime. The asset owner or custodian should sign off allrelevant documentation.

7.2 RECENT DEVELOPMENTS IN CONTROL SYSTEMS

Corrected parameter control

The main current method of control for gas turbines is corrected parameter control based on an

analysis of the results from the monitoring system and sensors. In most cases the turbine iscontrolled by monitoring the exhaust temperature and then varying fuel input. Control is morecomplex for mechanical drive GTs, as they need to run at a variable power range (typically 15-100% of full power).

The basis for turbine control is exhaust control curves. The control system pulls back fuel if themonitored exhaust temperature is too high. Turbine performance will be influenced by other factors such as inlet filter fouling and humidity. Inlet filter fouling also induces under-firingreducing efficiency. The humidity effect correction is a function of the water in the system.

The controller uses a baseload calibration curve with additional terms:

T= F(PR comp) +'T (NLP) + '(Tin) +' (Pexp)….



44

The equation is additive allowing corrections to be removed if they are not relevant.

PT Speed

Time

Speed

Exhaust

Temperature

Figure 29 schematic illustrating the effect of the control system on power turbine (PT)speed as the monitored exhaust temperature goes up.

Recent developments in control systems have produced performance improvements8. For example GE use a 9% correction to exhaust temperature (of 1000 ˚C) in their new control logiccompared to the old. This gave benefits in steady fuel flow and saved 0.5MW compared to theold T48 controls. A number of measures are implemented to ensure the control system remains

failsafe: the lowest reading are taken, the turbine trips if a sensor is lost, and a limit is set on

exhaust temperature.

Refinements in the new GE CPC Virtual T48 Control system8 include a special control logic,fewer sensors, and more accessible sensors in the exhaust outlet, which do not disturb the gas

path. A 0.5MW saving was reported with a constant firing temperature TFire , maximising power and environmental benefit. Power turbine LPT speeds are obtained from a model. The controlsystem is independent of site tuning but it is necessary to measure humidity and include this inthe fault logic.

A current trend is the use of control synchronisation and triple modular redundant (TMR)Control systems 10 9. The application of such control systems to LM2500 aeroderivative andLM2X Marine gas turbines is discussed in Reference10. These refinements increase redundancy

but add complexity as there are more things to look at. This balance needs to be evaluated todetermine whether the use of TMR and control synchronisation is justified for a given gasturbine.

Control Synchronisation

Control synchronisation is an improvement in control system particularly relevant to aero-derivative gas turbines. For aero-derivative GTs response time is critical. A 5-10ms responsetime is sufficient to prevent overspeed and the consequent system damage that might occur tothe GT. Industrial GTs are more tolerant.

In a synchronous control system the clock controlling the monitoring of the turbine (exhaust

temperature etc.) is synchronised with the clock controlling the application of any changes to



45

turbine operation (fuel input etc.). By synchronising the time of measurement and feeding to thecontrol system this gives much better accuracy and timing in the response.

Application

Software

RAM Input/Output

Synchronous

Figure 30 schematic showing the elements of a synchronous control system andsynchronization of the monitoring and application clocks

Most current controls and/or PCCS are asynchronous and have two separate clocks onecontrolling the monitoring system and one the implementation. This results in jitter or variabledelays in response of the system to any changes that are monitored by the sensors. The sensor goes to an analogue signal, is converted to a digital signal, gets processed and then is redirected

back to an analogue output. Resampling gives more performance improvements. However,delays can build up with time, for example 30ms delay on the 4th sampling. Aero-derivative gasturbines are affected by only 10ms delay.

In a synchronous system, the software is in line with hardware. There are two clocks, one themaster, and the two are in synchronisation. The delay of approximately 10ms is fixed andknown even with re-sampling and can be compensated for in the control software.

The synchronisation of monitoring and turbine system response can have important safetyapplications. In one incident reported to HSE on an FPSO one PLC in an asynchronous systemwent dead, the other did not know and shutdown all power to the FPSO. The FPSO required

ballasting and power for dynamic positioning to stay on station. The situation degraded and theFPSO listed. It is important to be aware of such pitfalls and the potential for knock-on failures.

Triple Modular Redundant TMR Control Systems

The Triple Modular Redundant (TMR) control system is a new development with triplicate processor units (CPUs) and triplicate input/output. To implement a change 2 out of 3 must

agree. In the event of a loss of a CPU the control system can still function.



46

CPU

CPU

CPU

Figure 31 Schematic illustrating a Triple Modular Redundant TMR control system.Two out of three must agree. The control system can continue to operate if one CPU

fails.

TMR is adapted from aerospace technology. In aerospace it is necessary to keep systemsoperational with multiple sensor loss. TMR also guards against software bugs. Today’s

processors are markedly superior.

Redundant Network Control

A simpler less costly alternative to TMR is Redundant Network Control. Examples are given of for LM2000 and LM2500 GTs by Woodward in Reference 9. Redundant marine gas turbines areused on cruise ships including the QE2 and the Princess for power generation for electric marine

propulsion. In redundant network control, Master/slave backup and a Synchronous interface(I/O) improves performance, reduces delays and eliminates random timing. TMR costly andcomplex Redundant CPU is more cost effective than TMR.

Standard Control System

The Operator has a lot of say how equipment is run10. The control system is typically optimisedwith the Operator on their machines. Gas turbines typically have a 15-30 year lifespan

depending on the use, maintenance overhaul schemes, parts available, upgrades andmodifications.

There is a trend to develop standard control systems10. Digital control systems typically have a

5-10 year lifespan, typically one third of turbine life. Changes in performance, environmentalcondition, upgrades and operational data acquisition can require an upgrade to the controlsystem. Control manufacturers have sought to develop a standard product that could work for final two thirds of GT life. The need for flexibility makes it difficult to do this at the designstage. The main advantages of a standard system would be:

x Low cost and off-the shelf

x No re-engineering

x Quicker installation and commissioning (important for refits).



47

x Allow to complete within 1 week outage

x Improved product support

x Built in high speed datalogging capability

This would require a high speed control interface (I/O) for core signal, hardware, software andcommunications applicable to single-shaft or dual shaft GTs. A typical control system would

perform four main control functions:

x Gas fuel actuator

x Liquid fuel actuator

x NOx actuator

x Power augmentation actuator

The performance requirements include fast synchronous behaviour and repeatability for dynamic performance. There will always be some gas turbines that wont fit and require a

bespoke system. The intention would be have a standard control system applicable to themajority of market.

Software Architecture for a Standard control system

TurbineFuel-ControlCore

Hardware InterfaceComm, ALM, SD

Custom

details

Figure 32 Schematic illustrating the software architecture for a standard controlsystem10

The intention from a software perspective would be to have a Core turbine control module, thatdoes not require modification each time each time, a hardware interface and some customdetails, as illustrated in Figure 32 above. There would be separate engineering control for thethree software sections.

Gas turbine control testing is typically carried out using simulators. This allows the user to testout in advance that everything works OK, debug and test. The control system would then just

plug in. For example, Woodward10 have developed the NETSIM PC simulation that couples aGas Turbine Control Application Programme (GAP) software with turbine models. This isillustrated in Figure 33 below. The runtime software allows system checkout. The following

features are included:



48

x Trending and on-line capture

x Datalogging for high speed events

x Data event buffering (10ms resolution)

x Simulation same or better than the requirements for an aeroderivative GT

PC SystemEmulation

HardwareEmulation

TargetHardware

GTC ApplicationProgramme

GAP

NETSIMTurbine

Simulator

TurbineEmulation

Turbine

Figure 33 Schematic diagram of control simulation software modules. CourtesyWoodward10

A standard control system of this type has been evaluated by Woodward10 on 2 units: a RollsRoyce Avon GT in August 2003 and a GTC250 at Mykonos in Greece. The models were run totest options on deceleration and load drop. The simulation showed The CPU was using lessthan 15% capacity.



49

8 GAS TURBINES ON UK INSTALLATIONS

An analysis of DTI emissions data at April 2004 showed 273 gas turbines installed oninstallations in the UK sector. The emission regulations are relatively new and there may beadditional turbines which have not yet been reported. A summary of the installations in the UK sector and the number of gas turbines installed on each can be found in Appendix 1.Increasingly new or satellite plants or remote unmanned plants are added to existing fields. Inmany cases these will not have their own gas-turbines for power generation with power beingsupplied by umbilical from adjacent platforms.

Aeroderivative gas-turbines are increasingly favoured offshore because of the requirements for low weight, simple changeout and ease of maintenance. For example the aeroderivative GE LMseries are now favoured over GE’s older industrial Frame series. Industrial gas turbines havealso evolved for offshore use to give compact modular systems utilising modern aerospace rotor technology and are very different to the earlier bulkier technology commonly used in onshore

power generation. The Solar range of gas turbines used offshore (Saturn, Centaur, Taurus, Marsand Titan are all industrial gas turbines, but offer compact, light weight accessible designseasily incorporated in offshore skids3.

Figure 34 Summary of gas turbine suppliers for UK installations. Source analysis of DTI emissions data April 2004

It is common for modern turbines to include modules from another manufacturer. Themanufacturers own Power Turbine (PT) may be used with a gas generator from another supplier. Examples are the GE LM Series which uses Rolls Royce RB211 and Avon gasgenerators. Similarly, some Dresser Rand gas turbines now utilise GE LM Series gasgenerators. Detailed performance specifications for gas turbines worldwide are compiled

annually by Gas Turbine World Journal38.



50

8.1 PACKAGERS

In most cases the complete turbine package will be supplied by the supplier of the turbine.There are exceptions. In some cases the turbine supplier may included turbine equipment fromanother supplier because of the particular design requirements for the installation.

There are a number of independent engineering contractors who may supply the package. Therehave been increasing trends to modularisation and standardisation of packages. This is preferred

by operators because it simplifies the approval process if similar packages have been installed previously. It also simplifies operation and maintenance if all turbines have a similar configuration. Due to the package approach the customer has little direct influence over design,although user groups have been set up to address common issues.

8.2 SUPPLIERS

There has been much consolidation of GT suppliers in recent years with smaller supplier beingacquired by the main players to give a portfolio of products covering different sectors. For example Siemens Westinghouse aquired Alstom’s gas turbine interests including those for offshore application. Alstom had previously aquired European gas turbines (EGT), formerlyRuston.

There are four main players currently worldwide in the oil and gas sector as shown belowinTable 2. These comprise Rolls-Royce, Siemens Westinghouse, GE and Solar. As well as themodel type and power rating the model number usually includes additional numbers and lettersto indicate the version and upgrades that have been applied to the turbine. Turbine suppliers donot upgrade without good reason. Upgrades are usually based on service experience with design

changes to eliminate degradation or operation problems encountered in service.

Figure 35 More detailed breakdown of gas turbine suppliers for UK installations.Source analysis of DTI emissions data April 2004



51

It is worth noting that many current designs such as the Avon and RB211 turbines have evolvedover a lifetime of 20 years or more, though component and system technology has significantlyadvanced.

Other suppliers will often use gas turbine systems from the main suppliers. For exampleDresser Rand match the RB211 gas turbines to their own power turbines (PT), John Brown andThomassen are European manufacturing partners for GE industrial turbines but may give their own designation to the model. Hitachi-Toshiba similarly are Japanese manufacturing partnersfor GE. GE match their power turbines (PT) to the Rolls Royce RB211 and Avon gasgenerators to give the LM series of aeroderivative gas turbines. EGT (formerly Ruston) was alicensee to GE for full manufacture of Frame series rotor assemblies for other GE associates.

A more detailed breakdown of gas turbines on offshore installations in the UK sector byindividual manufacturers is shown in Figure 35. The information in these tables has beengathered from a number of sources including DTI emissions data at April 2004. Analysis of emissions data shows there are currently more than 270 gas turbines on UK installations.

This list is dominated by the same suppliers and models as the Worldwide list with Rolls-Royceand Siemens-Westinghause, the biggest players accounting for approximately a quarter each of UK offshore turbines. A more detailed listing can be found in Appendix 2.

Table 2 Major Oil and Gas Market Players UK and Worldwide (Below 30 MW)

Company % UK Models UK Models Worldwide

Siemens-Westinghouse(Alstom, Ruston, EGT)

46% Tornado G8000/8004Alstom Ruston

TB3000/4500/5000PGT10

8MW14MW

10MW

TyphoonTornado

CyclonePGT10

5MW8MW

13MW14MW

Rolls-Royce (Avon, Coberra,RB211)

27% Avon 1534/1535RB211Coberra 2000/6000Olympus GT SK30

15MW30MW20MW35MW

AvonRB211

15MW30MW

General Electric Oil & Gas: 12% GE Frame 5 5MWGE-1201/1401A-CLM2500+ 25MW

LM5000/6000

5MW10-15MW25MW

40-50MW

GE5 5 MWGE 10 10 MWLM 1600 16 MW

LM 2500 25 MW

5MW10MW16MW

25MW

Solar Turbines 11% Saturn 20Centaur GSC 40/50Mars 90/100Taurus 60

1MW3-4MW8-10MW6MW

Saturn 20 1 MWCentaur 40/50 3-4 MWTaurus 60/70 5-7 MWMars 90/100 8-10 MWTitan 130 13 MW

1MW3-4MW5-7MW8-10MW13MW

Other 12% ABB GT35Dresser KG2MTU V16Pratt & Witney ST18

1-2MW12MW2MW



5 2

R B 2 1 1

A v o n

C o b e r r a R

o l l s R o y c e

w w w . r o l l s r o y c e . c o m

S a t u r n

C e n t a u r

T a u r u s

M a r s

T i t a n

S o l a r

T y p e t i t l e h e r e

G E I n d u s t r i a l T u r b i n e s

N u o v o P i g n e o n e

J o h n B r o w n

T h o m a s s e n

H i t a c h i - T o s h i b a

M a n u f a c t u r i n g P a r t n e r s

G e n e r a l E l e c t r i c

A l s t o m P o w e r T u r b i n e s

w w w . a l s t o m . c o m

E u r o p e a n G a s T u r b i n e s E G T

( f o r m e r l y R u s t o n )

D e m a g D e l e v a l

S i e m

e n s W e s t i n g h o u s e

w w w . i n d u s t r i a l . t u r b i n e s . s i e m e n s . c o m

A B B

A l l i s o n

D r e s s e r

R

a n d , K o n g s b e r g

M A N

P r a t t & W i t n e y

O t h e r

O

f f s h o r e G a s T u r b i n e

S u p p l i e r s

F i g u r e 3 6 S u m m a r y o f m a i n o f f s h o r e g a s t u r b i n e s u

p p l i e r s , s u b s i d i a r y c o m p a n i e s a n d m a n u f a c t u r i n g a s s o c i a t e s



53

9 SAFETY CASES, CODES AND REGULATIONS

9.1 RELEVANT UK INSTALLATIONS

A summary of the installations in the UK sector and the number of gas turbines installed oneach can be found in Appendix 1. Increasingly new or satellite plants or remote unmanned

plants are added to existing fields. In many cases these will not have their own gas-turbines for power generation with power being supplied by umbilical from adjacent platforms.

9.2 INFORMATION FROM SAFETY CASES

Relatively little information on gas turbines can be found in safety cases. In most cases this islimited to the number of turbines, their function, power rating (MW) and their locationincluding plan drawings. In some cases the make of the turbines and Tag Numbers may also beincluded. A number of safety cases were examined during the project to look at the differencesin the information supplied by different operators.

9.3 HSE GUIDANCE NOTE PM84

HSE guidance note PM841 on gas turbines has recently been updated. This is not industryspecific and provides succinct advice on key safety issues for gas turbines. The document wasdrawn up by a working group which included HSE, offshore operators and turbine suppliers.PM84 provides guidance but is not mandatory on offshore operators. PM84 is reproduced infull in Appendix 3 of this report. The areas covered by PM84 include:

x Fuels

x Hazards

x Risk assessment

x Precautions against fire

x Precautions against explosion

x Requirements to satisfy ATEX directive

x Ventilation

x Control systems

x Fuel supply systems

x Gas Fuel

x Additional explosion precautions for liquid fuels and oils

x Gas compressor stations

x Emergency procedures

x Mechanical failuresx Electrical issues

x Electromagnetic radiation

x Legal requirements

PM84 provides a very useful port of call for inspectors for definitive guidance on these keyissues. A summary of the main guidance from PM84 on each of these issues is included in therelevant section of this report.



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9.4 DESIGN CODES

Procurement and design of gas turbines for operation in the UK sector is usually based on theAmerican API design codes. These are well developed and include standard forms that can

provide the basis for procurement. For gas turbine applications in the oil & gas sector, API 616is the foundation for most purchase specifications. Operators are reluctant to vary from standard package specifications because of the additional regulatory approval that may be required. For similar reasons the turbines used on a given installation for a given function, such as power generation, are usually likely to be of very similar specification.

The codes give some flexibility, for example; API 616 Foreword states: "EquipmentManufacturers, in particular, are encouraged to suggest alternatives to those specified when suchapproaches achieve improved energy effectiveness and reduce total life costs without sacrificeof safety and reliability."

The following codes mostly affect the packaging:

x API 616 - Gas Turbines

x API 617 - Compressors

x API 614 - Lube Oil System

x API 670 - Machinery Protection

x API 613 - Continuous Duty Gear

x API 677 - Auxiliary Drive Gear

x API 671 - Flexible Couplings

In addition there are codes governing testing and operation:

x ASME PTC-22 Gas Turbine Testingx ASME PTC-10 Compressor Testing

x ASME B133 - Gas Turbines

API 616 and ASME PTC-22 are the only two principal gas turbine specific codes for oil & gasapplications. API 670, 614, 613, etc. are more generic codes. The codes cover most aspects of the gas turbine package and often form the main basis for procurement. The informationincludes:

x Definitions - ISO Rating, Normal Operating Point, Maximum Continuous Speed, TripSpeed, etc; mechanical integrity - blade natural frequencies, vibration levels, balancingrequirements, alarms and shutdowns;

x Design requirements and features - materials, welding, accessories, controls,instrumentation, inlet/exhaust systems, fuel systems; inspection, testing, and preparationfor shipment; and

x Minimum testing, inspection and certification documentation requirements.

API 616 does not cover government local codes & regulations



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9.5 EMISSION REGULATIONS

Emission levels on offshore installations are controlled by the EU Emissions Trading SchemeRegulations 2005. This is implemented in the UK through Statutory Instrument 2005 No. 925

The Greenhouse Gas Emissions Trading Scheme Regulations 2005. Permits for operations areissued by the Licensing and Consents Unit in the DTI Offshore Environment and

Decommissioning, Environmental Management Team. The EMT also maintain a database onemissions equipment installed offshore. Gas turbines are included within these current emissionregulations. Permit applications and guidance can be found at the DTI Oil and Gas,environment home page http://www.og.dti.gov.uk/environment/euetsr.htm

9.6 ELECTRICAL REGULATIONS

Gas turbines contain electrical systems associated with operation, monitoring and control. Thehazards associated with electrical equipment on offshore installations are well recognised. Tominimise the risk of spark ignition, fire or explosion equipment must meet the regulations on

hazardous area classification and control of ignition sources24, 22

This requires equipment to besuitable for the zone in which it is installed. Guidance on the application of these regulations inthe context of gas turbines can be found in PM84.

An international standard BS EN 60079/10 explains the basic principles of area classificationfor gases and vapours. From 1 July 2003 DSEAR requires that new equipment and protectivesystems used in a hazardous area must be selected on the basis of the requirements set out in theDTIs equipment and protective systems for use in potentially explosive atmospheres regulations1996 (as amended) (EPS). A detailed listing of applicable standards can be found in Reference18. HSE Offshore Operations Note ON59 and ON63 give a guide to the EPS regulations for Offshore application.

Note that FPSOs and jack-ups are excluded from the EPS regulations, see paragraph 5 of ON59.This states that “EPS applies to fixed offshore installations but not to seagoing vessels and

mobile offshore units together with equipment on board such vessels or units”. EPS thereforeexcludes FPSOs and floating production platforms (FPPs), as well as MODUs, flotels and other mobile units.t

Electrical systems represent a safety hazard because of the risk of spark ignition, fire or explosion. For this reason there are many regulations governing the use of electrical equipmentoffshore. Specific precautions are required to prevent electrical equipment being a flame sourcein hazardous areas, for example the use of flameproof electrical equipment and enclosures. Thismay be supplemented by other risk-reduction measures including dilution ventilation, explosionrelief and explosion suppression.

It is exceptional for gas turbines and enclosures to be installed in a hazardous area. PM84recommends their installation in Zone 1 areas (see definitions of zones in BS EN 60079-17should be avoided. If installation is contemplated in Zone 2 areas, expert specialist adviceshould be sought. HSE guidance document PM84 gives advice on precautions that should beconsidered, including source of combustion air and ventilation air, fast-acting gas detectors,engine exhaust forced ventilation, pressure detection and interlocks, access during operation, air loss, BS EN standards for hazardous areas Relevant regulations are the Dangerous Substances

and Explosive Atmospheres Regulations 200222

and associated Approved Codes of Practice,21,22

.HSE Offshore Operations Note ON58 gives a short guide for operators to the DSEAR regulations.



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There are very specific requirements in BS EN 60079-17:2003 relating to use and maintenanceof electrical equipment for use in Zone 1 and 2 areas. More information on the selection,installation, use and maintenance of electrical systems for potentially hazardous atmospherescan be found in References 27 and 28.

9.7 LEGAL REQUIREMENTS

Legal requirements are covered in Paragraphs 79 to 91 of PM84.

The Health and Safety at Work etc Act 1974 requires that an employer ensure the health safetyand welfare at work of all employees and people affected by such work activities. Dutiesinclude the provision and maintenance of plant and systems of work that are, so far as isreasonably practicable, safe and without risk to health.

The Management of Health and Safety at Work Regulations 1999 (MHSWR) require a risk

assessment to be carried out to identify and implement any necessary preventative and protective measures.

The Provision and Use of Work Equipment Regulations 1998 complement MHSWR. The risk assessment will help identify all the protective and preventative measures that have to be takenin order to select suitable work equipment and safeguard dangerous parts or features of thatequipment.

The Confined Spaces Regulations 1997, together with the associated Approved Code of Practice,22 define a confined space. An acoustic enclosure around a GT is likely to form such aconfined space. The first consideration is to avoid entry when there is a reasonably foreseeablerisk of serious injury from any hazardous substance or condition. Based on a risk assessment,

measures can then be adopted to reduce the risk to an acceptable level.

The Noise at Work Regulations 1989 require an assessment of the exposure of employees tonoise to be carried out when the first action level of 85 dB (A) or the peak action level of 200

pascals is exceeded. Hearing protection is only acceptable after all reasonably practicalmeasures have been taken to reduce exposure at source.

The Supply of Machinery (Safety) Regulations 1992, which implement the Machinery Directive89/392/EEC, places duties on the responsible person who supplies relevant machinery and/or relevant safety components in the UK market. Relevant machinery and safety components aredefined within the Regulations. The Regulations require machinery etc to satisfy the essentialhealth and safety requirements (EHSRs), and to undergo the appropriate conformity assessment

procedures to demonstrate that the equipment has met the EHSRs and is safe.

The Gas Act 1995 authorises the Public Gas Transporter to require the fitting of supply protection devices to protect against excess reverse pressures, low inlet pressures, large rates of change of flow and undue pressure/flow perturbations.

The Dangerous Substances and Explosive Atmospheres Regulations DSEAR 2002, together with the associated Approved Codes of Practice, 17,18219 implement Directive 1999/92/EC(the ATEX "Workplace" Directive) and are concerned with area classification and the selectionand use of equipment for use in hazardous areas.

The Gas Safety (Installation and Use) Regulations 1998 mainly apply to domestic premises.However, regulation 38 covering the use of antifluctuators and valves applies to all gas users.



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This reaffirms the need to notify the Public Gas Transporter and carry out any requirements set by the Transporter in order to protect other consumers from danger. This is not relevantoffshore.

The Electricity at Work Regulations 1989 set out the safety requirements for electricalinstallations and the safety of people working with or near such systems. They impose duties primarily on the occupier of the premises but also in certain cases on employees working on thesystem, including contractors.

The Equipment and Protective Systems Intended for Use in Potentially Explosive AtmospheresRegulations 1996, which implement Directive 94/9/EC (the ATEX `Equipment' Directive), areconcerned with the supply of equipment and protective systems for use in potentially explosiveatmospheres.

The Pressure Systems Safety Regulations 2000 set requirements for pressure systems containinga relevant fluid. A relevant fluid is defined as steam, at any pressure, a gas or a liquid which

would have a vapour pressure greater than 0.5 bar above atmospheric. Gases dissolved under pressure are also considered relevant fluids. The Regulations impose requirements on designers,manufacturers, suppliers, owners and users of pressure systems, together with employers of

people who modify or repair such systems. The intention of the Regulations is to prevent therisk of serious injury from stored energy as a result of a failure of the pressure system or part of it. The design requirements of the Pressure Systems Safety Regulations (regulations 4 and 5(1)and (4)) are specifically disapplied for equipment designed and supplied in accordance with thePressure Equipment Regulations 1999.

The Pressure Equipment Regulations 1999, which implement the Pressure Equipment Directive97/23/EC, put duties on the responsible person who places pressure equipment on the UK market or puts such equipment into service in the UK. The Regulations apply to the design,

manufacture and conformity assessment of pressure equipment and assemblies of pressureequipment with a maximum allowable pressure greater than 0.5 bar. The Regulations requireequipment (as defined) to satisfy the essential safety requirements and to undergo theappropriate conformity assessment procedures to demonstrate that the equipment has met theessential safety requirements and is safe. The conformity assessment procedures are based onthe level of hazard, which is determined by classifying the equipment according to criteria laiddown in the Regulations.

The Dangerous Substances and Explosive Atmosphere Regulations 2002 together with theassociated Approved Codes of Practice 17,18,19 implement Directive 1999/92/EC (the ATEX“Workplace” Directive) and are concerned with area classification and the use of equipment for use in hazardous areas.



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10 HAZARDS AND FAILURE MODES

10.1 WHAT CAN GO WRONG

Gas turbines are complex high speed components, with tight dimensional tolerances, operatingat very high temperatures. As such, components are subject to a variety of degradationmechanisms in service. These are dominated by creep, fatigue, erosion and oxidation withimpact damage an issue if components fail or following maintenance. Creep may eventuallylead to failure but is of most concern because of the dimensional changes it produces incomponents subject to load and temperature. A major part of maintenance is checking of dimensions and tolerances. Fatigue is or particular concern at areas of stress concentration suchas the turbine blade roots.

Gas turbines are very reliable if run at a steady state, indeed if all the main factors such as fuelflow, and air flow are constant. Usually problems are associated with a change in one of these

external inputs.

A mechanical failure of the turbine may cause substantial mechanical damage within theacoustic enclosure, but is less likely to cause major injury / damage outside unless blades or other missiles are thrown. The greater risk is the uncontrolled release of fuel (gas or liquid); thismay or may not be associated with a mechanical failure. There are well-understood risks tomaintenance personnel during overhaul work; the greater safety risk is that a major failure isinitiated by inadequate or incomplete maintenance work during subsequent operation.

Any mechanical failure of the turbine, or an explosion within the acoustic enclosure, coulddisrupt fuel pipework, with the potential for a significant release. Missiles, in the form of ejected compressor blades or other high-speed components, may be thrown in a mainly radial

direction, with the potential to damage people or critical systems at some distance from theturbine.

10.2 FAILURE MECHANISMS AND ANALYSIS

Turbine components must withstand high temperatures, high speeds of operation and fluctuatingstresses. The major degradation mechanisms that arise from this combination are:

x Creep

x Thermo-mechanical fatigue

x High cycle fatigue

x Embrittlement

x Corrosion, environmental attack x Erosion

x Oxidation

x Foreign object damage

Gas turbine components operate to high dimensional tolerances so any mechanism causingchanges in dimensions or shape is relevant to performance, not only mechanisms causingcracking, overload or failure.

Creep

Creep refers to progressive deformation under load experienced increasingly at higher temperatures. Components affected by creep include:



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Blade shrouds, turbine blades (producing twisting) turbine vanes and combustion hardware.Creep resistance is a major design consideration.

Thermo-mechanical fatigue

This refers to fatigue damage arising from the thermo-mechanical loads experienced duringstart-up, operation and shut down of gas turbines. This may lead to crack initiation and growthleading eventually to failure. Components affected include: turbine blades, vanes, combustionhardware. Characteristic features are wedge-shaped cracks with oxidation.

High-cycle fatigue

High cycle fatigue refers to fatigue damage arising from the rapid stress cycling experienced byturbine components during normal operation. Components affected include: turbine blades,vanes, discs, compressor blades and vanes. Cracking is most likely at areas of stressconcentration such as the blade fir tree roots, pitting or surface damage.

Metallurgical embrittlement

This most commonly occurs in turbine components due to formation of brittle phases such as V,

P and Laves phase on high temperature ageing. Such phases which are topologically similar tothe base material reduce toughness. Temper embrittlement by P, Sn or Sb is also possible if

solute levels are high, for components operated in the embrittlement range (400-650 qC) or if incorrectly heat treated during manufacture.

Environmental attack

Environmental attack is possible for all components. The environment in a gas turbine arising

from combustion processes is very corrosive. Erosion will occur due to the high velocity of air movement within the turbine. All components are susceptible to progressive oxidation,

particularly the combustion system which sees the highest temperatures.

Foreign body damage

Damage from foreign bodies is not uncommon and could affect any rotating or stationarycomponent in the gas stream. The operating speeds of gas turbines are such that any foreign

body passing through the flow stream is likely to cause significant damage. Causes are debrisleft in during maintenance or from failure of individual components. Domestic object damage

(DOD) arising from articles left on in maintenance such as bolts, spanners etc. is usually easilyidentifiable.

Manufacture or repair

Casting and weld defects such as hot tears can be introduced on manufacture or during repair.

Failure analysis

Failure analysis follows similar principles to other rotating and static components to identify theorigin and cause of failure. This may be supported by metallurgical assessment, microscopy,fracture mechanics analysis (FMA), stress analysis, operational analysis and fuel, air and water analysis.



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Table 3 Causes of degradation mechanisms, hot gas components

Duty Degradation mechanism

Continuous duty application RuptureCreep deflectionHigh cycle fatigueCorrosionOxidationErosionRubs/ wear Foreign object damage

Cyclic duty application Thermal mechanical fatigueHigh cycle fatigueRubs/wear Foreign object damage

Materials

Gas turbine components are prone to materials degradation12. They spin fast, are very hot insome parts, operate in a corrosive, oxidising and erosive environment and exhibit hot and coolthermal cycling. Mechanisms include diffusion, ageing, formation of grain-boundary phases andchanges in microstructure.

In gas turbines nimonic alloys are mainly used for the blades, Co-Ni based alloys for compressor components, Hastelloys (Fe-based) in heater areas. Steel is no longer used muchand increasingly taken out of turbines. This is in contrast to steam turbines which mainly use

stainless steel. Gas turbines have much tighter tolerances. Ceramics have been used for turbine blades in some Rolls Royce designs.

Co and Ni superalloys are widely used because of strength combined with good oxidation andenvironment resistance. Superalloys typically have 10-12 alloying elements present for specificreasons.

x Ni or CO as matrix

x Re, W, Mo, Nb and Ta for creep hardness. These elements have a higher diameter than the Ni or Co matrix giving solid solution strengthening.

x Precipitate hardening is provided by elements like Al. For example J' Ni3Al is amain precipitate. The matrix is austenitic.

x Higher strength alloys are cast (In738, ReneCo, GTD11), lower strength alloysare forged

x Precipitate hardening is provided by carbides at the grain boundaries. Thealloys are typically give a solution heat treatment followed by one or moreageing cycles.

x The grain size is controlled. Fine grains give a lower creep strength, but better fatigue resistance. Discs are fine-grained and forged. Blades and rotors arecourse grained and cast.

x Oxidation resistance comes from formation of a thin protective oxide layer suchas Al2O3, NiAl2O4 or Cr 2O3.

x Thermal cycling, for example stressing on start-up or shut-down can cause

spalling of the oxide layers.



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x Cr 203 is better than Al203 for hot corrosion resistance. Salt or oxide deposit onthe surface is accelerated at high temperature.

x Cost is about $10,000 per directionally solidified turbine blade

Fine-grained components are typically produced by powder metallurgy or forging. Controlledorientation by: directional solidification (DS), single crystals (SX) methods. These result in nograin boundaries and optimum orientation control.

Table 4 Function of alloying additions in IN738 Nimonic turbine blade material

Alloy Element Purpose

Cr Oxidation and hot-corrosion resistance

Mo, W Solid solution strengthening

Al, Ti Precipitates

Co, Ni Base material

Ni Hardening

Cb Precipitation J'

Ta Beneficial in oxidation

Air Compressors

There is a much greater risk of air compressor failures in large GTs12

. Factors that impact onfailure in new designs include: 3D aerofoils, controlled diffusion profile, reduced aerofoil count,stages unique with longer chords, smaller clearances, higher pressure ratios, thinner leadingedges, wet operation. Safety margins are calculated by finite element methods FEM. Newdesigns have resulted in higher cost (typically 10x blade cost). There may be different or additional stresses and more risk to the blades which in newer designs generally operate withsmaller operating margins

Combustors

The combustion chamber and transition zone must encounter very high temperatures, increased pressure and buffeting from the air flow can occur, particularly for protruding sections.Importantly the high pressure (HP) air flow goes directly from the combustor to the power turbine (PT). The power turbine is the most expensive part of the gas turbine and costliest torepair. For this reason it is very important to maintain the integrity of the combustor andtransition zone. Any cracks and debris cannot be tolerated because of the potential for knock-ondamage to the power turbine. Inspection of these regions forms an important part of maintenance and acceptance criteria are necessarily tight.

Combustion systems are more complex than previously with multiple (often 9 or more)combustor nozzles compared to the single combustor used in early GTs. Flashback is a major

problem in the turbine aggravated by any liquids. New factors including very low emission on

gas, premix, multiple injection points, staged operation with complex controls. Combustor



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design and cooling designs are increasingly complex. It is not possible to guarantee there will be no liquid. These factors have reduced life of Thermal Barrier Coatings (TBCs) used oncombustors and transition systems, and arguably cost and risk have also increased.

Turbines

Turbine components are subject to creep, fatigue, corrosion, erosion and oxidation. These mayaffect clearances or initiate cracking. Most critical are areas of stress concentration such as thefir tree roots used for blade attachment. Entrance temps in the power turbine are typically 1300- 2600 ˚C and ensuring uniform cooling is an issue (cooling air typically at 1100 ˚C). Thermal

barrier coatings TBCs are used on the inside and outside of blades. Designers are now lookingat steam cooling for large turbines, but this requires an auxiliary boiler on start up. On largeturbines there may be 100 or more blades per rotor each costing around $20,000. In singlecrystal blades repair is difficult. Aerofoils are ultra-high cost and the margin to avoid melting isdifficult.

10.3 PM84 ADVICE ON MECHANICAL FAILURES

Mechanical failures in gas turbine plant are covered in Paragraphs 60 to 68 of PM84. This notesthe following.

The frequency of mechanical failures on GT plant is low. However, if it occurs in close proximity to other plant the consequences can be severe. This is a particular concern on offshoreinstallations where other highpressure pipework/plant containing flammable materials can bedamaged.

On some thin-walled machines, blade failure can result in blades being ejected at high speedthrough the rotor housing. The casings of some machines are protected to withstand such

failures. The need for additional protection should be considered as part of the risk assessment.Failures can occur for a variety of reasons such as overload, deterioration or damage incurred inuse. To further reduce the risk of turbine failure, appropriate measures should be implementedto monitor blade condition for erosion, corrosion and damage. Air inlets may be screened to

prevent the entry of foreign bodies into the turbine intakes. In such cases precautions should betaken to avoid hazards from ice formation where icing conditions can occur.

Turbines and their housings are precision components which run at high temperature. Avibration footprint at first run up/run down and steady state can provide a valuable reference

point. To avoid damage, procedures need to be followed when starting and stopping the GT.These procedures are intended to mitigate the rate of expansion or contraction of the blades andhousing. If they are not followed, the rates of expansion can differ and damage can occur.

While running, the rotational speed of GTs should be controlled within safe limits to prevent blades from being overloaded and damaged due to centrifugal force. Any safety features provided for this purpose, such as overspeed protection, need to be maintained in good workingorder and tested both off-line and on-line. For instance, overspeed testing can be achieved bycausing a trip during recommissioning from an outage. On some machines a trip condition can

be simulated by control software, while other machines can only achieve this by actuallyoverspeeding the machine, when careful consideration needs to be given to any increased risksfrom carrying out such a test.

Blades erode and shaft bearings may wear in use and this can upset the balance of the GT. If theerosion and/or wear are allowed to progress beyond safe limits then mechanical failure canoccur due to the lack of balance. GTs should be inspected and maintained at set intervals to



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protect against damage or wear which may lead to safety-critical failure. Periodic preventivemaintenance should be carried out to set schemes to determine the levels of deterioration or damage on blades and shaft bearings. When formulating the inspection scheme or carrying outmaintenance, the manufacturer's recommendations on inspection intervals or replacement

criteria for parts should be taken into account.

Condition monitoring may be used to assess the condition of blades and bearings withoutresorting to costly strip-downs. However, periodic inspection still needs to be carried out atappropriate intervals and care needs to be taken over the correct interpretation of data obtained.This often means that condition-monitoring data needs to be collected over a period of time andcompared with the results of periodic inspections before users can have sufficient confidence inits interpretation. Also, a history of data is often needed to measure vibration trends, which canindicate when blades or bearings need to be replaced. Only those who are competent to make

judgments on its significance should undertake the interpretation of such data.

Airborne contaminants can enter via the air inlet and become deposited on the compressor

blades. Compressor blade cleaning, as well as maintaining the efficiency of the GT, may alsolessen any possible risk of blade failure. Air inlet filters should be cleaned and maintainedregularly.

Gearboxes should be maintained, taking into account the manufacturer's instructions. Thecorrect grade of oil should be used and replaced at the correct intervals specified by themanufacturer.

Major gearbox failures have been known to cause injury as the larger units transmit significantshaft power. Vibration monitoring and oil debris analysis can give early warning of damage.Continued contact with the supplier and participation in user networks can supply usefulinformation on problems encountered in use, what to look for during inspection and the

necessary frequency of inspection intervals.

10.4 ANECDOTAL INFORMATION

Cracks have been found even in younger turbines (5-10 years) in rotors and compressors. Thesecan be managed by blending out without needing to condemn the component. Given thereliability of modern turbines there is a temptation to save on maintenance costs by not lookingfor cracks in newer plant. Changes in duty cycle can result in unanticipated damage. Modes may

be unusual.

Offshore operators have reported incidences of cracking in newer gas turbines such as cracking

of the turbine casings in aeroderivative engines. Maintenance is usually subcontracted out tothe supplier or turbine specialist and a lot of reliance is placed on their expertise. In one examplethe operator had been told that one of the turbine blades contained a defect but this was withinacceptable limits for operation. No specific information was supplied on the defect. There wassome concern by the operator that rejection criteria may be less stringent than commonlyapplied in offshore operations.



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10.5 ACCIDENT, INCIDENT AND DANGEROUS OCCURRENCE DATA

Data extracted

A review of HSE accident and RIDDOR databases was made in June 2004 to identify incidentsthat related to turbines or driven equipment. Accident data was extracted from the ORIONdatabase using the keywords: Turbines, Generator, Generation and Compressor. The data werereviewed and incidents not directly relating to the criteria, such as ‘walking past the compressor

when IP slipped’ were excluded. Those suggesting, for example, issues of work on compressorsin confined space were retained. This generated three separate databases:

x Dangerous occurrences were extracted from ORION using the same search criteriamentioned above but excluding any incidents which are captured instead inhydrocarbon release (HCR) data. Again the incident reports were reviewed for relevance and those considered inappropriate removed, e.g. ‘scaffolding board fell onto

roof of compressor house’. (do.xls)

x Hydrocarbon Release (HCR) data was extracted using the search criteria: Systems - Gascompression, Utilities -gas and power generated turbines and fuel gas, oil - diesel and

power generated turbines, Equipment – Turbines, Compressors. Note this includesdifferent types of compressors e.g. air compressors and re-compressors, though noincidents were found for the latter ( HCR data.xls)

x Data on accidents was also extracted using the same search criteria as for dangerousoccurrences (accidents.xls)

Incidents classifying equipment type as pumps were available but not included. The data

extracted covered the 14 years from 1991 to 2004

Analysis of Data

A total of 278 dangerous occurrences meeting these criteria were reported in the Period fromJanuary 1991 to March 2004. These were classified in terms of the type of equipmentmentioned and in terms of turbines and driven equipment. The total number of occurrences inthis period is not known. The dangerous occurrences (DO) database classifies the consequencesof incidents, with terms including: ‘explosion, fire offshore, fail vessel , and fail offshore’ . Theterm ‘ fail fairground’ is used where data has not correctly copied across. It is not possible to be

precise from the DO data as it is not always mentioned if compressors were turbine driven andthe term generators is sometimes used, as it is in safety cases, for turbines. The causes of the

occurrences relating to turbines were also classified and reviewed.

The main cause of dangerous occurrence was leakage of gas, fuel or oil which subsequentlyignited. There were a number of cases of internal explosion due to excess fuel ingress into thecombustor on start up, problems with bearing seizure, in one case leading to shaft failure. Theexhaust lagging was prone to ignition following leaks and loss of lagging had occurred in onecase due to severe storm conditions. Taking only incidents where turbine was specificallymentioned, approximately 45% of classified incidents were associated with the turbine (SeeFigure 40 below). It is likely that a proportion of the generators are in fact turbines and thatsome of the compressors were turbine driven. The fires were extinguished in almost all cases

by the fire and safety system. Note that Halon fire extinguishing systems have been withdrawnoffshore for environmental reasons.



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Figure 37 below shows the number of dangerous occurrences reported by year for thesecomponents from 1991 to 2004. The number of reported incidents has fallen in recent yearscompared to 10-15 years ago. The highest number was recorded in 1992, falling progressivelyuntil 2000. The year 2000 recorded a rise followed by a progressive decrease in the following

years. The rise in 1992 coincides with change in HSE reporting procedures for dangerousoccurrences. It is believed this reflects better reporting criteria rather than an actual increase inthe number of incidents. A similar change is considered to be the reason for the rise in 2000.The subsequent decreases indicate improved reliability of gas turbines and indicate the value of monitoring dangerous occurrences in reducing risk.

0

10

20

30

40

50

60

Total

1991

1992

1993

1994

1995

1996

1997

1998

1999

2000

2001

2002

2003

2004

Count of Incident Id

Year

Figure 37 Number of dangerous occurrences associated with turbines, generators and

compressors offshore by year. Analysis of data from ORION database 1991 to 2004

The ORION database normally categorises each incident in terms of the type of incident, for example fire offshore or release offshore. Figure 38 summarises the consequences for theincidents where a consequence was given. This comprised less than half of the 278 incidentsidentified. For 170 of the incidents the type was not described and a default description of

failed fairground was given.

In order to better understand the actual incidences occurring on offshore gas turbines wereviewed the comments in the database in those cases where no consequence had been given.The type of incident in most cases could be inferred directly from the comments. Figure 39

below gives an analysis of all the incidents by consequences including those inferred. The mostcommon incidents were fire or smoke, with release of gas next common.

Common causes of fire and smoke involved leakage of lubricating oil or hydrocarbon onto theexhaust lagging or hot parts of the combustion system, followed by smoke or ignition. Fuelleakages could also lead to ignition. Gas releases were commonly associated with failure of seals. Many parts of the turbine casing are hot so any leakage of gas, oil, fuel or hydrocarbon

poses a risk of ignition. Ignition can be a problem in newer low emission turbines. If the fuelfails to ignite then there is a risk of subsequent ignition or explosion when the air/fuel mixturereaches the hot exhaust system. There were relatively few incidents (around 20) associated withthe electrical systems.



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Figure 38 Analysis of dangerous occurrences by reported consequence associatedwith turbines, generators and compressors offshore by year. Analysis of data from

ORION database 1991 to 2004.

Figure 39 Analysis of dangerous occurrences offshore by reported or inferredconsequence associated with turbines, generators and compressors. Consequenceinferred from comment in ORION database where not specifically given. Analysis of

data from ORION database 1991 to 2004.



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A different analysis of the same data by equipment type is given in Figure 40 below. Thisshows the proportion of failures associated with each equipment type. Turbines do feature

prominently accounting for approximately 45% of these reported incidents. Detailedinterpretation is difficult as we are dealing with only a small subset of the overall DO data

extracted with specific criteria.

Figure 40 Proportion of dangerous occurrences associated with turbines,generators, compressors and driven equipment.

To understand better the causes of incidents an analysis was made by turbine system, taking

only the reported incidents where a gas turbine was specifically mentioned. This analysis isshown in Figure 41 below. This illustrates clearly that the exhaust system, particularly thelagging is a common location for fire. The gas generator (GG), in particular the hot combustion

parts, and the fuel system also feature significantly. The next most significant systems in termsof dangerous occurrences are the Power Turbine (PT) and the lube oil system.

Figure 41 Proportion of dangerous occurrences associated with turbines,generators, compressors and driven equipment broken down by system



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10.6 IMIA INDUSTRIAL GAS TURBINE MEMBERS FAILURE STATISTICS

The International Association of Industrial insurers IMIA carried out a survey in 1993 of thecauses of failure of industrial gas turbines37. This covered turbines from <10MW to very large

turbines (>100MW) in the period 1984 to 1992. The information received detailedapproximately 60 failure instances totalling some £44M in terms of claims.

The arising data is summarised in Figure 42 and Figure 43. It was found that failure was mostlikely to occur in the first 3 years of operation. Design faults and maintenance induced faults(MIFS) such as leaving a rag in the machine accounted for approximately half the number of failures. Looking in terms of cost (Figure 43) design faults account for approximately 70% of failures.

Information from individual insurance companies in the IMIA report 37 reveals someinteresting information: one failure database covering the period 1986 to 1991 concluded thatgas turbines were more reliable than most plant coming number 25 in their rankings.

Percentage of total loss frequency was 0.25% compared with 22.12% for boilers which cametop. However, when the losses were ranked by cost gas turbines came out on top with 22% of total payout.

Faulty Design

70%

Lack of Maintenance

14%

Other Causes

16% Faulty Design

Lack of Maintenance

Other Causes

Figure 42 Gas turbine failures by failure category (numbers of failures). Data from IMIA

Reference37

. www.imia.com

The average cost of failure in the time period of the survey (1984 to 1992) varied fromapproximately £220k for turbines <10MW, £400k for 11-49MW turbines to £500 for 50-99MWturbines. For gas turbines >100MW the average failure cost was very much greater at £5M.



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Maintenance Induced

Faults

15%

Design Faults

35%

Faulty Manufacture

11%

Faulty Installation

9%

Unknown

5%

Miscellaneous

7%

Misuse7%

Lack of Maintenance

11%Maintenance Induced Faults

Design Faults

Faulty ManufactureFaulty Installation

Unknown

Miscellaneous

Misuse

Lack of Maintenance

Figure 43 Gas turbine failures by failure category (costs of failures). Data from IMIAReference 37. www.imia.com

10.7 RELIABILITY DATA FOR GAS TURBINES

The Norwegian OREDA database provides useful information on reliability of mechanicalequipment offshore including gas turbines and turbine packages. OREDA was last updated in

2002 with previous issues in 1997, 1992 and 1984. The reliability data is grouped by years between the different issues. This is so that the statistics for current more modern equipmentare not biased by poorer statistics for early developmental models.

The 1984 OREDA data for gas turbine driven compressor packages, showed that 85% of failures were associated with the turbine drive unit. the critical failure rate per 10

6hours varied

between 460 and 1700 and for all failure modes from 3300 to 4800. OREDA is very useful ingiving comparative reliability data, although It should be noted that the populations of givencomponents studied in OREDA are often quite low.

OREDA also gives information on typical testing and maintenance strategy.

10.8 SUMMARY TABLES BY SYSTEM AND COMPONENT

A summary of the main failure modes and associated hazards is given below inTable 5

.



7 1

T a b l e 5 S u m m a r y o f m a i n f a i l u r e m o d e s a

n d a s s o c i a t e d h a z a r d s b y s y s t

e m a n d c o m p o n e n t .

I D

S y s t e m

C o m p o n

e n t

F a i l u r e M o d e

H a z a r d s

A I

A i r I n t a k e

A i r i n l e t

C r a c k i n g , f r a c t u r e - i m

p a c t d a m a g e d u e t o i n g e s t e d

d e b r i s o r d u r i n g m a i n t e n a n c e

K n

o c k - o n d a m a g e t o t u r b i n e . V a r i a b l e a i r f l o w t o

t u r b i n e

A C

A i r C o m p r e s s o r

R o t o r A

s s e m b l y

S e e i n d i v i d u a l c o m p o n e n t s b e l o w

L P c o m p r e s s o r

( 1 ) S t a t o r o r r o t o r b l a d e d a m a g e d u e t o f o r e i g n o b j e c t

i n g e s t i o n ( 2 ) D i s c f a t i g u e f a i l u r e ( v e r y r a r e ) ( 3 ) F a i l u r e

d u e t o b u i l d e r r o r o r ( 4 ) F a i l u r e f o l l o w i n g s u r g e .

P r o j e c t i l e d a m a g e t o p e r s o n n e l . D e b r i s i n a i r f l o w .

N o

n - u n i f o r m a i r f l o w . K n o c k - o n d a m a g e t o

c o m p r e s s o r a n d t u r b i n e

H P c o m p r e s s o r

A s f o r L P C o m p r e s s o r , p l u s ( 5 ) B l e e d a i r v a l v e f a i l u r e ( w h e r e f i t t e d ) , c a u s i n g d e b r i s t o f a l l i n t o a i u r f l o w )

R o t o r D

i s c s

P r i m a r y f a i l u r e v e r y r a r e , f a t i g u e m o s t l i k e l y .

R o t o r b l a d e s

F a i l u r e d u e t o i m p a c t o f f o r e i g n o b j e c t s m o s t c o m m o n :

o v e r t e m p e r a t u r e i n t u r b i n e b l a d e s , e s p e c i a l l y H P S t a g e

1 b l a d e s , a l s o c o m m o n .

P r o j e c t i l e r i s k t o p e r s o n n e l . K n o c k o n d

a m a g e .

A c

c e l e r a t e d c r e e p d a m a g e a n d o x i d a t i o

n i f

o v

e r t e m p e r a t u r e .

S t a t o r s

F o r c o m p r e s s o r s , m o s

t l i k e l y f a i l u r e m o d e d u e t o

i m p a c t f r o m i n g e s t e d a r t i c l e s ; F o r t u r b i n e s , ( 1 )

i m p a c t

d a m a g e d u e t o u p s t r e a

m m e c h a n i c a l f a i l u r e ( 2 ) e r o s i o n

d u e t o t e m p e r a t u r e e f f

e c t s o r ( 3 ) f a i l u r e d u e t o

o v e r t e m p e r a t u r e m o s t

l i k e l y f a i l u r e c a u s e s .

K n

o c k o n d a m a g e . V i b r a t i o n o r p o o r p e

r f o r a m n c e i f

a i r

f l o w r e s t r i c t e d .



7 2

I D

S y s t e m

C o m p o n

e n t


H a z a r d s

A C

C o m p r e s s o r

( c o n t . )

B e a r i n g s

B e a r i n g s , i f

t r e a t e d c o

r r e c t l y , s

h o u l d n o t f a i l , b u t

b e a r i n g f a i l u r e i s p e r c e i v e d a s c o m m o n .

T a k i n g

i n s u f f i c i e n t c a r e d u r i n

g i n s t a l l a t i o n , o r i n c o r r e c t

t r a n s p o r t a t i o n o f a n u n i n s t a l l e d e n g i n e n e w l y o u t o f

o v e r h a u l o r r e p a i r , c a n c a u s e d a m a g e l e a d i n g t o

i n f a n t i l e f a i l u r e .

O v e r t e m p e r a t u r e , i n s u

f f i c i e n t o r i n c o r r e c t l u b r i c a t i o n

a l s o a r e c o m m o n c a u s

e s o f b e a r i n g d i s t r e s s l e a d i n g t o

f a i l u r e .

G a s T u r b i n e s

r e l y o n t h e i r b e a r i n g s t o f u n c t i o n

c o r r e c t l y ; a b e a r i n g f a

i l u r e i n v a r i a b l y w i l l l e a d t o v e r y

e x t e n s i v e a n d e x p e n s i v e d a m a g e t o r o t o r s , b l a

d e s a n d

c a s i n g s .

M e c h a n i c a l f a i l u r e . P r o j e c t i l e r i s k t o p e r s o n n e l .

E x

t e n s i v e k n o c k - o n d a m a g e t o o t h e r c o m p o n e n t s

S e a l s

S e a l s r e l y a b s o l u t e l y o n c o r r e c t h a n d l i n g p r o c e d u r e s

d u r i n g i n s t a l l a t i o n , a n d s o m e a l s o r e l y o n c o r r e c t

l u b r i c a t i o n a n d r o t o r b

a l a n c e ( i e , l a c k o f v i b r a t i o n )

w h i l s t r u n n i n g . A d a m

a g e d s e a l m u s t n o t b e f i t t e d ; s e a l

f a i l u r e s o o n a f t e r g a s t u r b i n e r e p a i r o r o v e r h a u l a c t i v i t y

u s u a l l y ( b u t n o t a l w a y

s ) i s d u e t o i n c o r r e c t m a i n t e n a n c e

p r o c e d u r e ( s ) .

G a

s l e a k a g e w i t h f i r e o r e x p l o s i o n r i s k .

P o o r

p e r f o r m a n c e .

A c c e s s o

r y d r i v e

W e a r a n d c o r r o s i o n o r c r a c k i n g o f g e a r w h e e l s ,

C o r r o s i o n o r c r a c k i n g

o f s h a f t s

M e c h a n i c a l m a l f u n c t i o n . R e d u c e d t i m i n

g a n d

e f f i c i e n c y .



7 3

I D

S y s t e m

C o m p o n

e n t


H a z a r d s

A C

C o m p r e s s o r

( c o n t . )

C a s i n g

F a i l u r e u n u s u a l u n l e s s d u r i n g c a t a s t r o p h i c f a i l u r e o f

i n t e r n a l e n g i n e c o m p o

n e n t s ( e g , b l a d e ( s ) o r a d i s c )

w h i l s t r u n n i n g ; h o w e v

e r , v

i b r a t i o n c a n c a u s e c r a c k i n g

d u e t o f a t i g u e ; i m p a c t

f r o m e x t e r n a l s o u r c e s - t o o l s

d u r i n g m a i n t e n a n c e o r c a r e l e s s n e s s w h i l s t a c c e s s i n g t h e

e x t e r n a l s u r f a c e s d u r i n g m a i n t e n a n c e , f o r i n s t a n c e - c a n

c a u s e d a m a g e w h i c h c

a n d e t e r i o r a t e a n d c a u s e c a s i n g

f a i l u r e a t a l a t e r d a t e .

P r o j e c t i l e d a m a g e t o p e r s o n n e l o r a d j a c e n t p i p e w o r k .

G a

s l e a k w i t h r i s k o f f i r e o r e x p l o s i o n

A C

C o m p r e s s o r

( c o n t . )

C o w l s

A s f o r c a s i n g s ; m o s t l i k e l y s o u r c e i s d a m a g e c a u s e d

d u r i n g m a i n t e n a n c e , l e a d i n g t o f a i l u r e a t a l a t e r d a t e .

P r o j e c t i l e d a m a g e t o p e r s o n n e l o r a d j a c e n t p i p e w o r k .

G a

s l e a k w i t h r i s k o f f i r e o r e x p l o s i o n

I n l e t g u i d e v a n e s

I m p a c t d a m a g e d u e t o

f o r e i g n o b j e c t s , o r s u r g e .

P o

o r t u r b i n e o p e r a t i o n . R i s k o f k n o c k - o n p r o j e c t i l e

d a m a g e

S h a f t s

V e r y r a r e ; f a t i g u e m o s t l i k e l y

C o u p l i n g s

V e r y r a r e , f a

t i g u e m o s t l i k e l y

G G

G a s G e n e r a t o r

C o m b u s

t i o n c h a m b e r s F a t i g u e d u e t o w e a k e n

i n g a f t e r f o r m a t i o n o f c r a c k s i n

c o m b u s t i o n c h a m b e r l i n e r s ; s e c o n d a r y f a i l u r e

f o l l o w i n g s u r g e , o r f o r e i g n o b j e c t i n g e s t i o n , o r

c o m p r e s s o r b l a d e f a i l u r e , e t c .

H i

g h r i s k o f k n o c k - o n d a m a g e t o p o w e r t u r b i n e

F u e l n o z z l e s

F a i l u r e d u e t o v i b r a t i o

n e f f e c t s ; a l s o f a i l u r e d u e t o

i n c o r r e c t o r i m p r o p e r l y f i l t e r e d f u e l o r f u e l a d d i t i v e s .

H i

g h r i s k o f k n o c k - o n d a m a g e t o p o w e r t u r b i n e .

I n c o r r e c t f u e l / a i r m i x i n g w i t h r i s k o f e x

p l o s i o n .

H i

g h e r e m i s s i o n s .



7 4

I D

S y s t e m

C o m p o n

e n t


H a z a r d s

G G


( c o n t )

S w i r l v a

n e s

I m p a c t d a m a g e o r e r o s i o n .

N o

n - u n i f o r m f u e l m i x i n g a n d i g n i t i o n

C o a t i n g s

N o r m a l l y c o a t i n g s , e s p e c i a l l y t h o s e o n t h e h o t t e s t

c o m p o n e n t s o r p a r t s o

f g a s t u r b i n e s , f a

i l d u e t o

o v e r t e m p e r a t u r e e f f e c

t s .

A c

c e l e r a t e d d e g r a d a t i o n l e a d i n g t o f a i l u

r e o f

c o m p o n e n t b y c r e e p , o x i d a t i o n o r e r o s i o n .

L i n e r s

I m p a c t o r o v e r t e m p e r a t u r e .

G a

s l e a k a g e .

H i g h r i s k o f d a m a g e t o P o w e r

T u

r b i n e ( P T ) d o w n s t r e a m

T r a n s i t i o n p i e c e

I m p a c t o r o v e r t e m p e r a t u r e .

G a

s l e a k a g e .

H i g h r i s k o f d a m a g e t o P o

w e r T u r b i n e

( P T ) d o w n s t r e a m

B u c k e t

I m p a c t , i n c o r r e c t l y a d

j u s t e d f u e l f l o w ( o v e r f u e l l i n g ) o r

v i b r a t i o n .

G a

s l e a k a g e .

H i g h r i s k o f d a m a g e t o P o

w e r T u r b i n e

( P T ) d o w n s t r e a m

P T

P o w e r T u r b i n e

C a s i n g

C a r e l e s s n e s s d u r i n g m

a i n t e n a n c e - i m p a c t o f s o m e s o r t ,

o r i n t e r n a l e n g i n e c o m

p o n e n t f a i l u r e c a u s i n g t h e c a s i n g

t o r u p t u r e , o r p r o l o n g e d e x p o s u r e t o e x c e s s i v e

v i b r a t i o n .

G a

s l e a k a g e . R

i s k o f f i r e o r e x p l o s i o n .

S h a f t s

U n u s u a l a s a p r i m a r y c a u s e , b u t f a t i g u e m o s t l i k e l y .

H i g h e n e r g y m e c h a n i c a l f a i l u r e

H P T u r b

i n e

O v e r t e m p e r a t u r e ; i m p

a c t d a m a g e t o b l a d e s ; b e a r i n g

f a i l u r e .

P r o j e c t i l e r i s k t o p e r s o n n e l a n d a d j a c e n t p i p e w o r k .

K n

o c k - o n f a i l u r e o f o t h e r P T c o m p o n e n

t s .

A c

c e l e r a t e d d e g r a d a t i o n .



7 5

I D

S y s t e m

C o m p o n

e n t


H a z a r d s

P T


( c o n t . )

L P T u r b

i n e

A s a b o v e .


A c

c e l e r a t e d d e g r a d a t i o n . K n o c k - o n f a i l u r e o f o t h e r

P T

c o m p o n e n t s

D i s c s

U n u s u a l a s a p r i m a r y c a u s e , b u t f a t i g u e m o s t l i k e l y .


A c

c e l e r a t e d d e g r a d a t i o n . K n o c k - o n f a i l u

r e o f o t h e r P T

c o m p o n e n t s

B l a d e s

O v e r t e m p e r a t u r e ; i m p

a c t d a m a g e ; b e a r i n g f a i l u r e .


A c

c e l e r a t e d d e g r a d a t i o n . K n o c k - o n f a i l u r e o f o t h e r

P T

c o m p o n e n t s

N o z z l e g u i d e v a n e s

O v e r t e m p e r a t u r e , i m p

a c t d a m a g e .

R e

d u c e d t u r b i n e p e r f o r m a n c e .

B e a r i n g s

L u b r i c a t i o n i s s u e s - l a

c k o f , i n c o r r e c t t y p e , o r

o v e r t e m p e r a t u r e c a u s e d b y s a m e . S e e n o t e s f o r

C o m p r e s s o r B e a r i n g s , w

h i c h a l s o a p p l y h e r e .

M e c h a n i c a l f a i l u r e . P r o j e c t i l e r i s k t o p e r s o n n e l .

E x

t e n s i v e k n o c k - o n d a m a g e t o o t h e r c o m p o n e n t s

B e a r i n g

s e a l s

S e e a l s o c o m m e n t s f o r C o m p r e s s o r s e a l s a b o v e .

P T

s e a l s a l s o c a n s u f f e r f r o m e x t e n d e d e x p o s u r e t o h i g h

t e m p e r a t u r e s o r h i g h l

e v e l s o f v i b r a t i o n .

L o

s s o f l u b r i c a t i o n l e a d i n g t o b e a r i n g s e i z u r e a n d

f a i l u r e . G a s l e a k a g e w i t h f i r e o r e x p l o s i o n r i s k . P o o r


A i r a n d

g a s s e a l s

T h e s e r e l y o n p r e s s u r e d i f f e r e n t i a l s : i f t h e d i f f e r e n c e

d o e s n o t e x i s t , t h e n t h e y f a i l .

G a

s l e a k a g e w i t h f i r e o r e x p l o s i o n r i s k .

P o o r


S u p p o r t

r i n g s

C o r r o s i o n , e r o s i o n , c r a c k i n g

L o

s s o f f u n c t i o n

E X

E x h a u s t

V i b r a t i o n , i m p a c t d a m

a g e o r d a m a g e d u e t o o v e r -

t e m p e r a t u r e a r e m o s t c o m m o n c a u s e s o f e x h a u s t

f a i l u r e .

L e

a k a g e o f h o t e x h a u s t g a s e s . R i s k o f p

e r s o n n e l

i n j u r y . I g n i t i o n o f f u e l o r l u b r i c a n t l e a k e d i n t o

i n s u l a t i o n .



7 6

I D

S y s t e m

C o m p o n

e n t


H a z a r d s

F S

F u e l s y s t e m

F u e l t a n

k

C o r r o s i o n , i m p a c t d a m

a g e t o t h e e x t e r i o r .

F u

e l l e a k . R i s k o f f i r e o r e x p l o s i o n .

F u e l p u m p

F a t i g u e f a i l u r e o f i n t e r n a l c o m p o n e n t s ; e l e c t r i c a l

p r o b l e m s o r , s

i m p l y , o l d a g e .

L o

s s o f t u r b i n e i g n i t i o n . P o t e n t i a l r i s k o f e x p l o s i o n .

P i p i n g

O v e r p r e s s u r e ; f o r s o l i d p i p e s , i n a p p r o p r i a t e u s e ( e g , a s

h a n d h o l d ) w h i l s t f i t t e d - f l e x i b l e p i p e s s u f f e r n t h e

s a m e w a y .

O c c a s i o n a

l l y , b u

t u n u s u a l l y , c o r r o s i o n f o r

s o l i d p i p e s ; c h a f i n g b y s e c u r i n g c l i p s ; a c c i d e n t a l

d a m a g e t o b r a i d i n g f o

r f l e x i b l e p i p e s .

F u

e l , w a t e r o r l u b r i c a n t l e a k . R i s k o f f i r

e o r e x p l o s i o n

F u e l i n j e c t i o n s y s t e m

C o n t a m i n a t i o n b y f o r e i g n o b j e c t s - u s u a l l y d i r t

i n t r o d u c e d i n t h e f u e l

- o r f a i l u r e o f i n t e r n a l

c o m p o n e n t s - s h a f t s o r s e a l s .

O c c a s i o n a l l y , f a

i l u r e o f

p o w e r s u p p l y o r i n c o r

r e c t s c h e d u l i n g d u r i n g s e t u p a t

m a n u f a c t u r e c a n c a u s e p r o b l e m s .

R e

s t r i c t e d f u e l f l o w , n o n - u n i f o r m c o m b

u s t i o n . P o o r

t u r b i n e o p e r a t i o n . R i s k o f d a m a g e t o c o m b u s t i o n

s y s t e m

S T

S t a r t e r

F a i l u r e t o s t a r t . F u e l b u i l d u p i n c o m b u s t i o n c h a m b e r F a

i l u r e t o s t a r t . R i s k o f e x p l o s i o n i f f u e l b u i l d - u p .

L O

L u b e O i l S y s t e m

P u m p s

F a i l u r e o f l u b r i c a n t f l o w

S e i z i n g o f c o m p o n e n t s , o v e r h e a t i n g , f a i l u r e

T a n k s

L e a k a g e , p r e s s u r e b u i l d u p

L u

b r i c a n t l e a k . R i s k o f f i r e , e x p l o s i o n .

S e a l s

L e a k a g e

L u

b r i c a n t l e a k . R i s k o f f i r e , e x p l o s i o n .

A G

A u x i l i a r y G e a r b o x

A l l

M e c h a n i c a l f a i l u r e . L o s s o f f u n c t i o n .

L o

s s o f d r i v e . M e c h a n i c a l f a i l u r e .

C L

C o o l i n g s y s t e m

O v e r h e a t i n g o f b l a d e s

a n d o t h e r c o m p o n e n t s .

A c c e l e r a t e d c r e e p , o x i d a t i o n a n d e r o s i o n d a m a g e

R i s k o f c o m p o n e n t d i s t o r t i o n , O x i d a t i o n o r f a i l u r e .

S e

i z u r e . M e c h a n i c a l d a m a g e t o c o m p o n

e n t s . R

i s k o f

p r o j e c t i l e d a m a g e



7 7

I D

S y s t e m

C o m p o n

e n t


H a z a r d s

C S

C o n t r o l s y s t e m ( o n S k i d ) O p e r a t i o n a l c o n t r o l s

I n d i c a t i o n s f a i l

O p

e r a t o r u n a w a r e o f h a z a r d o u s c o n d i t i o

n s

O p e r a t o r c o n t r o l s f a i l

O p

e r a t o r u n a b l e t o e x e r c i s e s a f e c o n t r o l

S e n s o r f a i l u r e

A u

t o m a t i c o p e r a t i o n t o w i t h i n u n s t a b l e o r h a z a r d o u s

r e g i o n

S p u r i o u s o u t p u t

O p

e r a t i o n t o w i t h i n u n s t a b l e o r h a z a r d o

u s r e g i o n

S e n s o r f

a i l u r e

E S D c o n d i t i o n n o t d e t e c t e d

U n

r e v e a l e d f a i l u r e i n r e d u n d a n t E S D c o

n f i g u r a t i o n -

d e g r a d a t i o n o f s a f e t y m a r g i n s

E S D s y s t e m f a i l u r e

E S

D c o n d i t i o n n o t d e t e c t e d - p l a n t o p e r a t i o n b e y o n d

s a f e t y l i m i t s

F a i l t o o p e r a t e o n d e m

a n d

H a

z a r d o u s p l a n t c o n d i t i o n u n m i t i g a t e d

E S

E l e c t r i c a l S y s t e m s

A l l

I n c o r r e c t s e l e c t i o n f o r

h a z a r d o u s a r e a

I g n i t i o n o f h a z a r d o u s a r e a f l a m m a b l e s u

b s t a n c e f r o m

i n t e r n a l s p a r k s o r h o t s u r f a c e s

I n a d e q u a t e m a i n t e n a n c e

D e

g r a d a t i o n o f h a z a r d o u s a r e a p r o t e c t i o

n p r i n c i p l e s

D e

g r a d a t i o n o f s a f e g u a r d s a g a i n s t e l e c t r o c u t i o n d u e

t o

d i r e c t o r i n d i r e c t c o n t a c t w i t h h a z a r d o u s e l e c t r i c a l

c h a r g e



7 8

I D

S y s t e m

C o m p o n

e n t


H a z a r d s

E S


( c o n t )

A l l

I n a d e q u a t e m a i n t e n a n

c e

H a

z a r d o u s r e l e a s e o f e l e c t r i c a l e n e r g y l e a d i n g t o f i r e

o r

e x p l o s i o n

I n a d e q u a t e l a b e l l i n g

I n a d e q u a t e i s o l a t i o n f o r w o r k i n g o n e l e c t r i c a l

e q u i p m e n t

C a b l e s

I n a d e q u a t e s u p p o r t s

M e c h a n i c a l d a m a g e


I n s u l a t i o n d a m a g e

S h e a t h d a m a g e



E l e c t r i c a l f a u l t


C i r c u i t t r i p / f u s e - l o s s o f f u n c t i o n

S p

a r k s - i g n i t i o n o f h a z a r d o u s a r e a f l a m

m a b l e

s u b s t a n c e

C o n d u i t

s y s t e m s


C a

b l e d a m a g e

I n g r e s s o f h a z a r d o u s a r e a f l a m m a b l e s u b s t a n c e -

v i o l a t i o n o f h a z a r d o u s a r e a p r o t e c t i o n p

r i n c i p l e

E n c l o s u r e




r i n c i p l e


D a

m a g e t o d o o r s e a l s - v i o l a t i o n o f h a z a r d o u s a r e a

p r o t e c t i o n p r i n c i p l e



7 9

I D

S y s t e m

C o m p o n

e n t


H a z a r d s

E S


( c o n t )

E n c l o s u r e

C o n d u i t e n t r y p o i n t s n

o t b l a n k e d



r i n c i p l e

I n a d e q u a t e c a b l e g l a n d s



r i n c i p l e

C o v e r s l e f t o f f

E x

p o s u r e o f p e r s o n s t o e l e c t r o c u t i o n b y

d i r e c t c o n t a c t

w i

t h h a z a r d o u s e l e c t r i c a l c h a r g e

I n a d e q u a t e c o v e r o r d o o r s e a l s

V i o l a t i o n o f h a z a r d o u s a r e a p r o t e c t i o n p

r i n c i p l e ( e . g .

b o

l t s l e f t o u t o f f l a m e p r o o f e n c l o s u r e d

o o r )

I n a d e q u a t e s h r o u d i n g

o f l i v e p a r t s

E x

p o s u r e o f p e r s o n s t o e l e c t r o c u t i o n b y

d i r e c t c o n t a c t

w i

t h h a z a r d o u s e l e c t r i c a l c h a r g e

S w i t c h g

e a r


S u

p p l y t r i p / f u s e - l o s s o f f u n c t i o n

S p

a r k s - i g n i t i o n o f h a z a r d o u s a r e a f l a m

m a b l e

s u b s t a n c e

I s o l a t o r s

M e c h a n i s m m a l f u n c t i o n

U n

a b l e t o s w i t c h o r i s o l a t e l o a d s . H e a t i n g o r a r c i n g -


r i n c i p l e

C i r c u i t b r e a k e r s

M e c h a n i s m m a l f u n c t i o n

C a

t a s t r o p h i c f a i l u r e o n f a u l t i n t e r r u p t i o n - e x p l o s i o n

M e c h a n i c a l D r i v e

G e a r b o x

M e c h a n i c a l f a i l u r e S e e R R 0 7 6

P r o j e c t i l e r i s k t o p e r s o n n e l . L o s s o f d r i v

e t o d r i v e n

e q u i p m e n t . R i s k o f s u r g e o r d a m a g e t o d r i v e n

e q u i p m e n t .

M a i n d r i v e c o u p l i n g

M e c h a n i c a l f a i l u r e S e e R R 0 7 6

P r o j e c t i l e r i s k t o p e r s o n n e l . L o s s o f d r i v

e t o d r i v e n

e q u i p m e n t . R i s k o f s u r g e o r d a m a g e t o d r i v e n

e q u i p m e n t .



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10.9 OTHER HAZARDS

There are a number of hazards for gas turbines that are not associated with a particular systemor component. Many of these are generic issues offshore and not specific to gas turbines.These include:

x Access to enclosures

x Risk of fire or explosion from gas or fuel leakage

x Risk of projectile damage

x Risk of fire from lubricant leakage

x Risk of injury from touching hot components, particularly exhaust andcombustion systems

x Build up of harmful gases or explosive mixtures in enclosures



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11 MAINTENANCE AND INSPECTION

Gas turbines have to withstand harsh conditions including high flow, temperature and pressure.All materials are subject to degradation by mechanisms including fatigue, creep, erosion andoxidation.

Some failure modes can be catastrophic with a risk of projectile damage to nearby personnel, pipework or systems. Such damage is often extensive. There is documentary evidence of projectile parts following turbine failures cutting the turbines in half. The packaging and theenclosures seeks to contain any possible failure.

It is very important to comply with manufacturers inspection guidance. Inspection intervals aretypically based on elapsed time or number of starts or incursions, if the latter can be monitored.The control system may monitor the number of starts or incursions using a cycle counter or justthe number of starts. A sequence Start > Operation > Stop up and down would count as one

cycle. Incursions may lead to shut down of turbine. Modern control systems include software tomonitor incursions and operation.

11.1 OVERVIEW

Maintenance costs and availability of plant are two of the most important concerns to equipmentowners. For maintenance programmes to be fully effective, equipment owners have developed ageneral understanding of the relationship between their operating plans and priorities for the

plant, the skill level of operating and maintenance personnel, and the manufacturer'srecommendations regarding the number and types of inspections, spare parts planning, andother major factors affecting component life and proper operation of the equipment.

The primary factors, which affect the maintenance planning process, are shown below in Figure44.

Figure 44 key factors affecting maintenance planning. Courtesy GE.



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Parts unique to the gas turbine requiring the most careful attention are those associated with thecombustion process together with those exposed to high temperatures from the hot gasesdischarged from the combustion system. They are called the hot-gas-path parts and include:

x Combustion liners,x End caps,

x Fuel nozzle assemblies,

x Crossfire tubes,

x Transition pieces,

x Turbine nozzles,

x Turbine stationary shrouds

x Turbine buckets.

The basic design philosophy and recommended maintenance for heavy-duty gas turbines is toensure maximum periods of operation between overhauls and inspection, and perform in-place,on-site inspection and maintenance using local trade skills to disassemble, inspect and re-assemble.

In addition to maintenance of the basic gas turbine, the control devices, fuel meteringequipment, gas turbine auxiliaries, load package, and other station auxiliaries also require

periodic servicing. Analysis of scheduled outages and forced outages show that the primarymaintenance effort is attributed to five basic systems:

x Controls and accessories,

x Combustion,

x Turbine,

x Generator

x Balance of plant.

The unavailability of controls and accessories is generally composed of short-duration outages,whereas conversely the other four systems are composed of fewer, but usually longer durationoutages.

11.2 INSPECTION & REPAIR

Refurbishment of Gas Turbine Components

Overhaul and refurbishment of gas turbine components is usually carried out in specialistworkshops. This will typically follow the following sequence:

x Receipt

x Evaluation of condition

x Disassembly

x Cleaning and stripping



83

x Dimensional checking

x Define workscope

x Heat treatment

x Welding, brazing, blending

x Coating

x Final inspection

x Verification

Verification is needed to provide own assessment that all necessary has been done. Evaluationof incoming condition is of crucial importance. Good workshops usually have an in-houserepair shop. Serial numbers are usually on the edges of components or cast on.

Evaluation of damage

Damage can occur in shipping and handling. On receipt it should be confirmed thatcomponents are in the expected condition; determine the cleaning and stripping required andthen define the inspection procedure. It is very important that this evaluation of components isdone up-front.

Figure 45 Changeout of RB211 Coberra gas generator. Courtesy Rolls Royce.



84

Disassembly

Disassembly can include rotating parts and parts subject to oxidation or heat damage. Theseinclude the turbine discs, support rings, core plugs, cowl wraps and blades. It is often found that

vanes need re-welding if previously repaired to correct poor penetration. Components arestripped and cleaned chemically or thermo-mechanically; usually with Aluminium Oxide and NOT sand. Good repair shops will heat tint rotating parts, turbine blades and buckets at ~1100F(590 ºC) to show up areas of oxidation damage.

Dimensional checking

Gas turbine components operate to high tolerances. Dimensional checking is crucial early inmaintenance to ensure correct fit. This is done by physical measurements, ultrasonics and byuse of a computer measurement machine (CMM). Fixtures are used that simulate actual fitting.

Non-destructive testing (NDT)

NDT methods applied in maintenance include visual, penetrant, magnetic particle, ultrasonicand X-radiography. Penetrant is used for side-wall inspections. Blades out MPI is normal for turbine blades. In-situ inspection is possible using specialised ultrasonic methods and MPI of

blade end faces.

MPI and penetrant methods are used to look for cracking of casings, cowls and components inthe combustion system and hot gas path. Transient thermography with thermal signalreconstruction (TSR) signal processing has recently been applied for inspection of compressor and turbine blades, transition pieces and vane inspection. This method can show up loss of wallthickness (Figure 46 and Figure 47).

Figure 46 Thermography images of turbine blade component and vane showing wallthinning, internal air channels, and misaligned or missed channels: (a) conventional

thermography or turbine blade (b) thermography of turbine blade with TSR processing,(c) thermography of vane with TSR processing. . Images Courtesy Thermal Wave Inc.

www.thermalwave.com



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Metallurgical Examination

Metallurgical examination is important, particularly for hot section equipment. It is very

important that the repair shop has a metallurgist in house.

Defining of workscope

This combines evaluation, customer requirements, repair facility, inspection and standards.Quality assurance (QA) is mandatory. Good practice includes use of an in-house repair shop,having a suitably experienced metallurgist on site, and taking time in gas turbine repair.

Processes

Heat treatment is an important part of repairs for Ni-based superalloys. This needs to beundertaken in a controlled atmosphere, under vacuum or hot isostatic pressure (HIP).

Temperature is controlled using 3 thermocouples; one new, one slightly used and one older,which are replaced in a cycle. These need to be physically on the part.

Nozzle and Vanes

Most repairs to nozzles and vanes are done with TIG welding. GSAW (stick), GMAW (MIG),GTAW (TIG) and micro-plasma welding. It is detrimental if too much weld is left on as it needsto be ground off leaving more scope for defects.

Figure 47 Thermography images of turbine vane using TSR processing showingvariations in local wall thickness. Courtesy Thermal Wave Inc. www.thermalwave.com

Buckets and Blades

Repair of these components involves brazing processes, blending, coating and final inspection.Dirt and oxide is removed. By brazing and diffusing at high temperature it is possible to buildup a thin wall of material to restore dimensions and wall thickness. But, this will not restore loststrength. The component is re-profiled by blending and recoated. Thermal barrier coating isused on aero foils.



86

Final inspection is made to check metallurgical experience, dimensions, appearance, ensurefunctionality. This is best done by someone who understands the tolerances. Water is used tocheck there is no blockage of cooling holes.

Quality records

These include Heat treatment checks, other process records, direct material and documentation.

11.3 MAINTENANCE GUIDANCE

The inspection and repair requirements outlined in Maintenance and Instructions Manuals provided to owners establishes a pattern of inspections. In addition, supplementary informationis provided through a system of Technical Information Letters. This updating of information,contained in the maintenance and instructions manual, assures optimum installation, operationand maintenance of the turbine.

Many of the Technical Information Letters contain advisory technical recommendations toresolve issues and improve the operation, maintenance, safety, reliability or availability of theturbine. The recommendations contained in Technical Information Letters should be reviewedand factored into the overall maintenance planning program.

For a maintenance program to be effective, from both a cost and turbine availability standpoint,owners must develop a general understanding of the relationship between their operating plansand priorities for the plant and the manufacturer's recommendations regarding the number andtypes of inspections, spare parts planning, and other major factors affecting the life and proper operation of the equipment.

The heavy-duty gas turbine is designed to withstand severe duty and to be maintained on-site,with off-site repair required only on certain combustion components, hot-gas-path parts androtor assemblies needing specialized shop service. The following features are designed intoheavy-duty gas turbines to facilitate on-site maintenance:

x Casings, shells and frames are generally split on the machine horizontal centreline.Upper halves may be lifted individually for access to internal parts. With upper-half compressor casings removed, all stator vanes can be slid circumferentially out of thecasings for inspection or replacement without rotor removal. On most designs, thevariable inlet guide vanes (VIGVs) can be removed radially with upper half of inletcasing removed. With the upper-half of the turbine shell lifted, each half of the firststage nozzle assembly can be removed for inspection, repair or replacement without

rotor removal. On some units, upper-half, later-stage nozzle assemblies are lifted withthe turbine shell, also allowing inspection and/or removal of the turbine buckets.Turbine buckets are generally moment weighed and computer charted in sets for rotor spool assembly so that they may be replaced without the need to remove or rebalancethe rotor assembly.

x Bearing housings and liners are generally split on the horizontal centreline so that theymay be inspected and replaced, when necessary. The lower half of the bearing liner can

be removed without removing the rotor. Seals and shaft packings are usually separatefrom the main bearing housings and casing structures and may be readily removed andreplaced. On most designs, fuel nozzles, combustion liners and flow sleeves can be

removed for inspection, maintenance or replacement without lifting any casings. Ingeneral, all major accessories, including filters and coolers, are separate assemblies that



87

are readily accessible for inspection or maintenance. They may also be individuallyreplaced as necessary.

Inspection aids can be built into heavy-duty gas turbines to assist with inspection procedures.

These provide for visual inspection and clearance measurement of some of the critical internalturbine gas-path components without removal of the gas turbine outer casings and shells. These procedures include gas-path borescope inspection and turbine nozzle axial clearancemeasurements. An effective borescope inspection program can result in removing casings andshells from a turbine unit only when it is necessary to repair or replace parts. Boroscope accesslocations for a gas turbine are shown below in Figure 48.

Figure 48 Typical gas turbine boroscope access locations. Courtesy GE.

There are many factors that can influence equipment life and these must be understood andaccounted for in the owner's maintenance planning. Starting cycle, power setting, type of fuelused and level of steam or water injection are key factors in determining the maintenanceinterval requirements as these factors directly influence the life of critical gas turbine parts. Inthe approach of one of the major equipment suppliers (GE) to maintenance planning, a gas fuelunit operating continuous duty, with no water or steam injection, is established as the baselinecondition, which sets the maximum recommended maintenance intervals. For operation thatdiffers from the baseline, maintenance factors are established that determine the increased levelof maintenance that is required. For example, a maintenance factor of two would indicate amaintenance interval that is half of the baseline interval.

Gas turbines are affected in different ways for different service-duties. Thermo-mechanical

fatigue is the dominant limiter of life for peaking machines, while creep, oxidation, and



88

corrosion are the dominant limiters of life for continuous duty machines. Interactions of thesemechanisms are considered in the design criteria, but to a great extent are second order effects.For that reason, maintenance requirements are based on independent counts of starts and hours.Whichever criterion limit is first reached determines the maintenance interval.

An alternative approach, converts each start cycle to an equivalent number of operating hours(EOH) with inspection intervals based on the equivalent hours count. This logic can create theimpression of longer intervals; while in reality more frequent maintenance inspections arerequired. Different approaches to setting maintenance time are summarised below in Figure 49.

Figure 49 Gas turbine maintenance requirements. Courtesy GE.

Fuels

Fuels burned in gas turbines range from clean natural gas to residual oils. Heavier hydrocarbonfuels have a maintenance factor ranging from three to four for residual fuel and two to three for crude oil fuels. These fuels generally release a higher amount of radiant thermal energy, whichresults in a subsequent reduction in combustion hardware life, and frequently contain corrosive

elements such as sodium, potassium, vanadium and lead that can lead to accelerated hotcorrosion of turbine nozzles and buckets.

Some elements in these fuels can cause deposits either directly or through compounds formedwith inhibitors that are used to prevent corrosion. These deposits impact performance and canlead to a need for more frequent maintenance. Distillates, as refined, do not generally containhigh levels of these corrosive elements, but harmful contaminants can be present in these fuelswhen delivered to the site. Two common ways of contaminating number two distillate fuel oilare: salt water ballast mixing with the cargo during sea transport, and contamination of thedistillate fuel when transported to site in tankers, tank trucks or pipelines that were previouslyused to transport contaminated fuel, chemicals or leaded gasoline. Natural gas fuels aregenerally considered to be the optimum fuel with regard to turbine maintenance.



89

Figure 50 Hot-gas-path maintenance intervals. Courtesy GE

Table 6 Maintenance Factors – hot-gas-path

Hot gas path inspectiona 24,000 hours or 1200 starts

Major inspectionb 48,000 hours or 2400 starts

Factors impacting maintenance

Hours factors

x Fuel Gas 1Distillate 1.5Crude 2 to 3Residual 3 to 4

x Peak load

x Water/steam injection Dry control 1 (GTD-222)

Wet control 1.9 (5% H2O GTD-222)Starts Factors

x Trip from full load 8

x Fast Load 2

x Emergency start 20

a,b Criterion is hours or starts – whichever occurs first

The importance of proper fuel quality has been amplified with Dry Low NOx (DLN)combustion systems. Proper adherence to equipment manufacturer’s fuel specifications isrequired to allow proper combustion system operation, and to maintain applicable warranties.

Liquid hydrocarbon carryover can expose the hot-gas-path hardware to severe over temperature



90

conditions and can result in significant reductions in hot-gas-path parts lives or repair intervals.Owners can control this potential issue by using effective gas scrubber systems and bysuperheating the gaseous fuel prior to use to provide a nominal 50°F (28°C) of superheat at theturbine gas control valve connection. The prevention of hot corrosion of the turbine buckets and

nozzles is mainly under the control of the owner. Undetected and untreated, a single shipment of contaminated fuel can cause substantial damage to the gas turbine hot gas path components.

Potentially high maintenance costs and loss of availability can be minimised or eliminated by:

x Placing a proper fuel specification on the fuel supplier. For liquid fuels, each shipmentshould include a report that identifies specific gravity, flash point, viscosity, sulphur content, pour point and ash content of the fuel.

x Providing a regular fuel quality sampling and analysis program. As part of this program,an online water in fuel oil monitor is recommended, as is a portable fuel analyser that,as a minimum, reads vanadium, lead, sodium, potassium, calcium and magnesium.

Water (or steam) Injection

Water (or steam) injection for emissions control or power augmentation can impact on the livesof turbine parts and maintenance intervals. This relates to the effect of the added water on thehot-gas transport properties. Higher gas conductivity, in particular, increases the heat transfer tothe buckets and nozzles and can lead to higher metal temperature and reduced parts lifetime.The impact on part life from steam or water injection is related to the way the turbine iscontrolled. The control system on most base load applications reduces firing temperature aswater or steam is injected.

Cyclic Effects

For the starts-based maintenance criteria (as opposed to the hours-based maintenance criteriadescribed earlier), operating factors associated with the cyclic effects produced during start-up,operation and shutdown of the turbine must be considered. Operating conditions other than thestandard start-up and shutdown sequence can potentially reduce the cyclic life of the hot gas

path components and rotors, and, if present, will require more frequent maintenance and partsrefurbishment and/or replacement. A typical gas turbine start-stop cycle is illustrated in Figure

51.

Thermal mechanical fatigue testing has found that the number of cycles that a part canwithstand before cracking occurs is strongly influenced by the total strain range and the

maximum metal temperature experienced. Any operating condition that significantly increasesthe strain range and/or the maximum metal temperature over the normal cycle conditions willact to reduce the fatigue life and increase the starts-based maintenance factor.

Rotor

In addition to the hot gas path components, the rotor structure maintenance and refurbishmentrequirements are affected by the cyclic effects associated with start-up, operation and shutdown.Maintenance factors specific to an application's operating profile and rotor design must bedetermined and incorporated into the operators maintenance planning. Disassembly andinspection of all rotor components is required when the accumulated rotor starts reach theinspection limit.



91

Figure 51 Turbine start-stop cycle – firing temperature changes

Combustion System

A typical combustion system contains transition pieces, combustion liners, flow sleeves, head-end assemblies containing fuel nozzles and cartridges, end caps and end covers, and assortedother hardware including cross-fire tubes, spark plugs and flame detectors. In addition, there can

be various fuel and air delivery components such as purge or check valves and flex hoses.

GE, for example, provides several types of combustion systems including standard combustors,Multi-Nozzle Quiet Combustors (MNQC), IGCC combustors and Dry Low NOx (DLN)combustors. Each of these combustion systems have unique operating characteristics and modesof operation with differing responses to operational variables affecting maintenance andrefurbishment requirements. The maintenance and refurbishment requirements of combustion

parts are impacted by many of the same factors as hot gas path parts including start cycle, trips,fuel type and quality, firing temperature and use of steam or water injection for either emissionscontrol or power augmentation.

Combustion maintenance is performed, if required, following each combustion inspection (or

repair) interval . It is expected and recommended that intervals be modified based on specific

experience. Replacement intervals are usually defined by a recommended number of combustion (or repair) intervals and are usually combustion component specific. In general, thereplacement interval as a function of the number of combustion inspection intervals is reduced if the combustion inspection interval is extended. For example, a component having an 8,000 hour combustion inspection (CI) interval and a 6(CI) or 48,000 hour replacement interval would havea replacement interval of 4(CI) if the inspection interval was increased to 12,000 hours tomaintain a 48,000 hour replacement interval.

Off Frequency Operation

Heavy-duty single shaft gas turbines are generally designed to operate over a 95% to 105%speed range. However, operation at other than rated speed has the potential to impact

maintenance requirements. Depending on the industry code requirements, the specifics of the



92

turbine design and the turbine control philosophy employed, operating conditions can result thatwill accelerate life consumption of hot gas path components. Where this is true, the maintenancefactor associated with this operation must be understood and these speed events analysed andrecorded so as to include in the maintenance plan for this gas turbine installation. Generator

drive turbines operating in a power system grid are sometimes required to meet operationalrequirements that are aimed at maintaining grid stability under conditions of sudden load or capacity changes. Most codes require turbines to remain on line in the event of a frequencydisturbance. For under-frequency operation, the turbine output decrease that will normally occur with a speed decrease is allowed and the net impact on the turbine as measured by amaintenance factor is minimal. In some grid systems, there are more stringent codes that requireremaining on line while maintaining load on a defined schedule of load versus grid frequency.

Air Quality

Maintenance and operating costs are also influenced by the quality of the air that the turbineconsumes. In addition to the deleterious effects of airborne contaminants on hot-gas-path

components, contaminants such as dust, salt and oil can also cause compressor blade erosion,corrosion and fouling. Twenty-micron particles entering the compressor can cause significant

blade erosion. Fouling can be caused by sub micron dirt particles entering the compressor aswell as from ingestion of oil vapours, smoke, sea salt and industrial vapours. Corrosion of compressor blading causes pitting of the blade surface, which, in addition to increasing thesurface roughness, also serves as potential sites for fatigue crack initiation. These surfaceroughness and blade contour changes will decrease compressor airflow and efficiency, which inturn reduces the gas turbine output and overall thermal efficiency.

Inlet Fogging

One of the ways some users increase turbine output is through the use of inlet foggers. Foggers

inject a large amount of moisture in the inlet ducting, exposing the forward stages of thecompressor to a continuously moist environment. Operation of a compressor in such anenvironment may lead to long-term degradation of the compressor due to fouling, material

property degradation, corrosion and erosion. Experience has shown that depending on thequality of water used, the inlet silencer and ducting material, and the condition of the inletsilencer, fouling of the compressor can be severe with inlet foggers.

As an example, for turbines with Type 403 stainless steel compressor blades, the presence of moisture will reduce blade fatigue strength by as much as 30% as well as subject the blades tocorrosion. Further reductions in fatigue strength will result if the environment is acidic and if

pitting is present on the blade. Pitting is corrosion-induced and blades with pitting can seematerial strength reduced to 40% of its virgin value. The presence of moisture also increases thecrack propagation rate in a blade if a flaw is present. Water droplets, in excess of 25 microns indiameter, will cause leading edge erosion on the first few stages of the compressor. Thiserosion, if sufficiently developed, may lead to blade failure. Additionally, the roughened leadingedge surface lowers the compressor efficiency and unit performance.



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Maintenance Inspections

Maintenance inspection types may be broadly classified as:

x Standby,x Running

x Disassembly inspections

The standby inspection is performed during off-peak periods when the unit is not operating andincludes routine servicing of accessory systems and device calibration. The running inspectionis performed by observing key operating parameters while the turbine is running. Thedisassembly inspection requires opening the turbine for inspection of internal components and is

performed in varying degrees. Disassembly inspections progress from the combustioninspection to the hot-gas-path inspection to the major inspection as shown in the figure below.

Standby Inspections

Standby inspections are performed on all gas turbines but are applicable particularly to gasturbines used in peaking and intermittent-duty service where starting reliability is of primaryconcern. This inspection includes routinely servicing the battery system, changing filters,checking oil and water levels, cleaning relays and checking device calibrations. Servicing can

be performed in off-peak periods without interrupting the availability of the turbine. A periodicstart-up test run is an essential part of the standby inspection.

The turbine suppliers Maintenance and Instructions Manual , as well as the Service Manual

Instruction Books, contain information and drawings necessary to perform these periodicchecks. Among the most useful drawings in the Service Manual Instruction Books for standbymaintenance are the control specifications, piping schematic and electrical configuration. Thesedrawings provide the calibrations, operating limits, operating characteristics and sequencing of

all control devices. This information should be used regularly by operating and maintenance



94

personnel. Careful adherence to minor standby inspection maintenance can have a significanteffect on reducing overall maintenance costs and maintaining high turbine reliability. It isessential that a good record be kept of all inspections made and of the maintenance work

performed in order to ensure establishing a sound maintenance program.

Running Inspections

Running inspections consist of the general and continued observations made while a unit isoperating. This starts by establishing baseline operating data during initial start-up of a new unitand after any major disassembly work. This baseline then serves as a reference from whichsubsequent unit deterioration can be measured. Data should be taken to establish normalequipment start-up parameters as well as key steady state operating parameters. Steady state isdefined as conditions at which no more than a 5°F/3°C change in wheel space temperatureoccurs over a 15-minute time period. Data must be taken at regular intervals and should berecorded to permit an evaluation of the turbine performance and maintenance requirements as afunction of operating time.

This operating inspection data, includes: load versus exhaust temperature, vibration, fuel flowand pressure, bearing metal temperature, lube oil pressure, exhaust gas temperatures, exhausttemperature spread variation and start-up time. This list is only a minimum and other parametersshould be used as necessary. A graph of these parameters will help provide a basis for judgingthe conditions of the system. Deviations from the norm help pinpoint impending trouble,changes in calibration or damaged components.

11.4 DISASSEMBLY INSPECTIONS

Combustion Inspection

The combustion inspection is a relatively short disassembly shutdown inspection of fuelnozzles, liners, transition pieces, crossfire tubes and retainers, spark plug assemblies, flamedetectors and combustor flow sleeves. This inspection concentrates on the combustion liners,

transition pieces, fuel nozzles and end caps which are recognized as being the first to requirereplacement and repair in a good maintenance program. Proper inspection, maintenance andrepair of these items will contribute to a longer life of the downstream parts, such as turbinenozzles and buckets.

Hot-Gas-Path Inspection

The purpose of a hot-gas-path inspection is to examine those parts exposed to high temperatures

from the hot gases discharged from the combustion process. The hot-gas-path inspectionincludes the full scope of the combustion inspection and, in addition, a detailed inspection of theturbine nozzles, stationary stator shrouds and turbine buckets. To perform this inspection, thetop half of the turbine shell must be removed. Prior to shell removal, proper machine centrelinesupport using mechanical jacks is necessary to assure proper alignment of rotor to stator, obtainaccurate half-shell clearances and prevent twisting of the stator casings.

The first-stage turbine nozzle assembly is exposed to the direct hot-gas discharge from thecombustion process and is subjected to the highest gas temperatures in the turbine section. Suchconditions frequently cause nozzle cracking and oxidation and, in fact, this is expected. Thesecond- and third-stage nozzles are exposed to high gas bending loads, which, in combinationwith the operating temperatures, can lead to downstream deflection and closure of critical axialclearances. To a degree, nozzle distress can be tolerated and criteria have been established for



95

determining when repair is required. These limits are contained in the Maintenance andInstruction Books previously described. As a general rule, first stage nozzles will require repair at the hot-gas path inspection. The second- and third-stage nozzles may require refurbishment tore-establish the proper axial clearances. Normally, turbine nozzles can be repaired several times

to extend life and it is generally repair cost versus replacement cost that dictates the replacementdecision.

Coatings play a critical role in protecting the combustion buckets operating at high metaltemperatures to ensure that the full capability of the high strength superalloy is maintained andthat the bucket rupture life meets design expectations. This is particularly true of cooled bucketdesigns that operate above 1985°F (1085°C) firing temperature. Significant exposure of the basemetal to the environment will accelerate the creep rate and can lead to premature replacementthrough a combination of increased temperature and stress and a reduction in material strength.This degradation process is driven by oxidation of the unprotected base alloy. In the past, onearly generation uncooled designs, surface degradation due to corrosion or oxidation wasconsidered to be a performance issue and not a factor in bucket life. This is no longer the case at

the higher firing temperatures of current generation designs. These factors are illustrated inFigure 52.

Given the importance of coatings, it must be recognized that even the best coatings availablewill have a finite life and the condition of the coating will play a major role in determining

bucket replacement life. Refurbishment through stripping and recoating is an option for extending bucket life, but if recoating is selected, it should be done before the coating has

breached to expose base metal.

Figure 52 Stage 1 bucket oxidation and bucket life. Courtesy GE



96

11.5 MAJOR INSPECTION

The purpose of the major inspection is to examine all of the internal rotating and stationary

components from the inlet of the machine through the exhaust section of the machine. A major inspection should be scheduled in accordance with the recommendations in the owner'sMaintenance and Instructions Manual or as modified by the results of previous borescope andhot-gas-path inspection. The work scope involves inspection of all of the major flange-to-flangecomponents of the gas turbine which are subject to deterioration during normal turbineoperation. This inspection includes previous elements of the combustion and hot-gas-pathinspections, in addition to laying open the complete flange-to-flange gas turbine to thehorizontal joints, as shown in Figure ##, with inspections being performed on individual items.

Prior to removing casings, shells and frames, the unit must be properly supported. Proper centreline support using mechanical jacks and jacking sequence procedures are necessary toassure proper alignment of rotor to stator, obtain accurate half shell clearances and to prevent

twisting of the casings while on the half shell.

Typical major inspection requirements for all machines are:

x All radial and axial clearances are checked against their original values (opening andclosing).

x Casings, shells and frames/ diffusers are inspected for cracks and erosion.

x Compressor inlet and compressor flowpath are inspected for fouling, erosion, corrosionand leakage. The IGVs are inspected, looking for corrosion, bushing wear and vanecracking.

x Rotor and stator compressor blades are checked for tip clearance, rubs, impact damage,corrosion pitting, bowing and cracking.

x Turbine stationary shrouds are checked for clearance, erosion, rubbing, cracking, and build-up.

x Seals and hook fits of turbine nozzles and diaphragms are inspected for rubs, erosion,fretting or thermal deterioration.

x Turbine buckets are removed and a non-destructive check of buckets and wheeldovetails is performed (first stage bucket protective coating should be evaluated for remaining coating life). Buckets that were not recoated at the hot-gas-path inspectionshould be replaced.

x Rotor inspections recommended in the maintenance and inspection manual or byTechnical Information Letters should be performed.

x Bearing liners and seals are inspected for clearance and wear.

x Inlet systems are inspected for corrosion, cracked silencers and loose parts.

x Exhaust systems are inspected for cracks, broken silencer panels or insulation panels.

x Check alignment - gas turbine to generator/gas turbine to accessory gear.



97

11.6 TURBINE BORE INSPECTIONS

Inspection of the bore and inside of rotors in gas and steam turbines has been difficult due to the poor access and illumination. Commercial ultrasonic systems are now available that deploy

arrays of ultrasonic probes inside the bore to look for signs of cracking. A new commercialsystem is shown below in Figure 53.

Figure 53 Ultrasonic turbine bore inspection system. Deploys arrays of ultrasonicprobes. Courtesy Phoenix

11.7 CLEANING

Gas turbine operation produces deposits and fouling which can affect smooth operation. It isnormal practice to clean the turbine at regular intervals. This is commonly done by the injectionof water droplets. It is very individual how this is done, different methods may be required for on-line washing for 2-stage and one-stage turbines. Cleaning is characterised by flow rate and

pressure. To assist cleaning and optimise the process models have been developed for washingsystems17. Such models may typically plot air flow rate versus power output. Smaller dropletsare desirable to avoid erosion. Injection may be into crossflow or parallel. Higher momentum(size, velocity) gives better air flow penetration.

Washing frequency depends on the installation profile. Economic analysis is commonly used to balance cleaning with operational requirements. For cleaning and other monitoring andmaintenance the following definitions are used:

x off-line not firing fuel,

x on-line firing fuel.

At high pressures small droplets are preferred, at low pressures 100-200Pm particles are typical.In off-line cleaning the gas turbine is run at crank speed for cleaning; on-line the GT is run atup-speed. There is a risk of running in flutter mode if too much water.



98

Key points to note for gas turbine washing include:

x On-line washing may not restore full power since dirt may be moved down thecompressor section settling at the high pressure sections.

x On-line washing needs to be supplemented with off-line washing to restore near-full power conditions. To be effective, on-line washing needs to be carried out frequently(once every 72 hours is typical)

x After each detergent on-line wash, a rinse wash should be applied to remove residuefrom the injection nozzles.

x Effectiveness of washing techniques depends on the type of fouling experienced, theselected washing liquid and the location of the injection nozzles.

x Solvent-based detergents are the most effective cleaning detergents. Water-baseddetergents are less effective.

x Logging of performance records before and after washing are crucial to the washingoperation.

x Demineralised water with purity in accordance with Manufacturer's recommendations is best used for washing. The critical issue is corrosion of the hot gas path due toimpurities.

x Selection of washing detergent needs to be based on the lowest possible ash content tominimise hot gas path corrosion.

x For off-line washing, waste water handling shall be considered.

The cleaning of gas turbines has been modelled at Cranfield University and reported at

Turbo200417

. Particles deposit during operation; reduce inlet size and affect blade performance.To clean it is normal to inject water droplets upstream of the compressor. There are a variety of intake ducts on a given GT and a variety of operating conditions. Experiments are costly anddifficult; therefore it is preferable to do numerical analysis. Filter loss is included as acorrection, not explicitly included in the model domain. Outlet power and inlet mass flowdepend on installation, altitude (m), high ambient temperatures.

The Cranfield study modelled two scenarios: design point (DP) and a High Desert extremeheavy duty installation (HD). At 870m in the HD environment power is down 23% and inletmass flow significantly reduced.

Droplet trajectories were modelled for a for 40˚ solid cone pattern. Flow was disrupted by the

bearing support struts. In gas turbine cleaning a complete wetting of first blade row from hub totip is essential. The spray centre line at the IGV was modelled. Better penetration was observedas the velocity goes up. In HD conditions lower flow occured, better penetration above shaftcone, adverse below. Shading occurred from the support struts. Jets impinged on the casing andhit the support strut. The effect of particle size 300um and 50-500um was modelled.

It was concluded that operating condition has an effect on spray injection, Droplet trajectoriesmodelled based on momentum balance showed droplet diameter, injection velocity andinjection angle to be key factors. Small GTs have lower mass flow but similar operatingvelocities to large GTs. The injection angle provides a simple method of compensation. It is acommon configuration to have vertical inlet ducts; in this situation the shaft cone is an obstacle.



99

11.8 SUMMARY BY SYSTEM AND COMPONENT

A summary of inspection practice by system and component is given below in Table 7. It should be noted this is a general summary and actual inspection practice will vary between

manufacturer and turbine type; industrial or aero-derivative. Manufacturers may choose toconcentrate on specific areas dependent on the service experience with specific models and pastinspection and service experience on a given installation. There are areas such as thecombustion, hot gas path, exhaust and fuel systems that are common locations for degradationin service and reported incidents (see Section 10.5).



1 0 0

T a b l e

7 S u m m a r y o f i n s p e c t i o n p r a c t i c e f o r g a s t u r b i n e s y s t e m s a n d c o m p o n e n t s

I D

I n s p e c t i o n

P u r p o s e o f I n s p e c t i o n

S y s t e m

C o m p o n e n t s

I n s p

e c t i o n

S I

S t a n d b y I n s p e c t i o n s

S t a n d b y i n s p

e c t i o n s a r e

p e r f o r m e d o n

a l l g a s t u r b i n e s b u t

a r e a p p l i c a b l e p a r t i c u l a r l y t o g a s

t u r b i n e s u s e d

i n p e a k i n g a n d

i n t e r m i t t e n t - d

u t y s e r v i c e w h e r e

s t a r t i n g r e l i a b

i l i t y i s o f p r i m a r y

c o n c e r n .

S t a r t - u p , l u b r i c a t i o n ,

c o o l i n g , c l e a n i n g a n d

f u e l s y s t e m s

V a r i o u s

T h i s

i n s p e c t i o n i n c l u d e s r o u t i n e l y s e r v i c i n g t h e b a t t e r y

s y s t e m , c h a n g i n g f i l t e r s , c h e c k i n g o i l a n d

w a t e r l e v e l s ,

c l e a

n i n g r e l a y s a n d c h e c k i n g d e v i c e c a l i b r a t i o n s . S e r v i c i n g

c a n

b e p e r f o r m e d i n o f f - p e a k p e r i o d s w i t h

o u t i n t e r r u p t i n g

t h e a v a i l a b i l i t y o f t h e t u r b i n e . A p e r i o d i c s t a r t - u p t e s t r u n i s

a n e

s s e n t i a l p a r t o f t h e s t a n d b y i n s p e c t i o n

.

R I

R u n

n i n g I n s p e c t i o n s

R u n n i n g i n s p

e c t i o n s c o n s i s t o f

g e n e r a l a n d c o n t i n u e d

o b s e r v a t i o n s

m a d e w h i l e a u n i t i s

o p e r a t i n g .

E s t a b l i s h i n g b a s e l i n e o p e r a t i n g

d a t a d u r i n g i n i t i a l s t a r t - u p o f a n e w

u n i t a n d a f t e r a n y m a j o r

d i s a s s e m b l y

w o r k . T h i s b a s e l i n e

s e r v e s a s a r

e f e r e n c e f r o m w h i c h

s u b s e q u e n t u

n i t d e t e r i o r a t i o n c a n

b e m e a s u r e d

.

G a s T u r b i n e

O n - l i n e s e n s o r s

O p e

r a t i n g i n s p e c t i o n d a t a t o e s t a b l i s h n o r

m a l e q u i p m e n t

s t a r t - u p p a r a m e t e r s a s w e l l a s k e y s t e a d y

s t a t e o p e r a t i n g

p a r a

m e t e r s . S t e a d y s t a t e i s d e f i n e d a s c o

n d i t i o n s a t w h i c h

n o m

o r e t h a n a 5 ° F / 3 ° C c h a n g e i n w h e e l s p a c e

t e m p e r a t u r e o c c u r s o v e r a 1 5 - m i n u t e t i m e

p e r i o d .

D a t a

m u s t b e t a k e n a t r e g u l a r i n t e r v a l s a n

d s h o u l d b e

r e c o

r d e d t o p e r m i t a n e v a l u a t i o n o f t h e t u r b i n e p e r f o r m a n c e

a n d

m a i n t e n a n c e r e q u i r e m e n t s a s a f u n c t

i o n o f o p e r a t i n g

t i m e

.

T h i s

o p e r a t i n g i n s p e c t i o n d a t a i n c l u d e s : l o

a d v e r s u s

e x h a u s t t e m p e r a t u r e , v i b r a t i o n , f u e l f l o w a

n d p r e s s u r e ,

b e a r i n g m e t a l t e m p e r a t u r e , l u b e o i l p r e s s u r e , e x h a u s t g a s

t e m p e r a t u r e s , e x h a u s t t e m p e r a t u r e s p r e a

d v a r i a t i o n a n d

s t a r t - u p t i m e .

T h i s

l i s t i s o n l y a m i n i m u m a n d o t h e r p a r a

m e t e r s s h o u l d b e

u s e d a s n e c e s s a r y . A g r a p h o f t h e s e p a r a

m e t e r s w i l l h e l p

p r o v

i d e a b a s i s f o r j u d g i n g t h e c o n d i t i o n s

o f t h e s y s t e m .

D e v i a t i o n s f r o m t h e n o r m h e l p p i n p o i n t i m

p e n d i n g t r o u b l e ,

c h a n g e s i n c a l i b r a t i o n o r d a m a g e d c o m p o

n e n t s .



1 0 1

I D

I n s p e c t i o n


S y s t e m

C o m p o n e n t s

I n s p

e c t i o n

D C I

D i s a

s s e m b l y - C o m b u s t i o n

I n s p

e c t i o n

R e l a t i v e l y s h o r t d i s a s s e m b l y

s h u t d o w n i n s

p e c t i o n .

C o n c e n t r a t e s

o n t h e c o m b u s t i o n

l i n e r s , t r a n s i t i o n p i e c e s , f u e l

n o z z l e s a n d e n d c a p s w h i c h a r e

r e c o g n i z e d a

s b e i n g t h e f i r s t t o

r e q u i r e r e p l a c e m e n t a n d r e p a i r i n

a g o o d m a i n t e n a n c e p r o g r a m .

C o m b u s t i o n

L i n e r s , t r a n s i t i o n

p i e c e s , f u e l n o z z l e s ,

e n d - c a p s

R e l a

t i v e l y s h o r t d i s a s s e m b l y s h u t d o w n i n s p e c t i o n o f f u e l

n o z z l e s , l i n e r s , t r a n s i t i o n p i e c e s , c r o s s f i r e

t u b e s a n d

r e t a i n e r s , s p a r k p l u g a s s e m b l i e s , f l a m e d e t e c t o r s a n d

c o m

b u s t o r f l o w s l e e v e s .

T h i s

i n s p e c t i o n c o n c e n t r a t e s o n t h e c o m b

u s t i o n l i n e r s ,

t r a n s i t i o n p i e c e s , f u e l n o z z l e s a n d e n d c a p s w h i c h a r e

r e c o

g n i z e d a s b e i n g t h e f i r s t t o r e q u i r e r e p l a c e m e n t a n d

r e p a

i r i n a g o o d m a i n t e n a n c e p r o g r a m . P r o p e r i n s p e c t i o n ,

m a i n t e n a n c e a n d r e p a i r o f t h e s e i t e m s w i l l c o n t r i b u t e t o a

l o n g

e r l i f e o f t h e d o w n s t r e a m p a r t s , s u c h a s t u r b i n e n o z z l e s

a n d

b u c k e t s .

H C P

D i s a

s s e m b l y - H o t g a s p a t h

i n s p

e c t i o n

T h e p u r p o s e

o f a h o t - g a s - p a t h

i n s p e c t i o n i s

t o e x a m i n e t h o s e

p a r t s e x p o s e

d t o h i g h

t e m p e r a t u r e s

f r o m t h e h o t g a s e s

d i s c h a r g e d f r

o m t h e c o m b u s t i o n

p r o c e s s .

H o t g a s p a t h

A s c o m b u s t i o n

i n s p e c t i o n . A l s o

t u r b i n e n o z z l e s ,

s t a t i o n a r y s t a t o r

s h r o u d s a n d t u r b i n e

b u c k e t s

T h e

h o t - g a s - p a t h i n s p e c t i o n i n c l u d e s t h e f u l l s c o p e o f t h e

c o m

b u s t i o n i n s p e c t i o n a n d , i n a d d i t i o n , a d e t a i l e d

i n s p

e c t i o n o f t h e t u r b i n e n o z z l e s , s t a t i o n a r y s t a t o r s h r o u d s

a n d

t u r b i n e b u c k e t s .

T o p

e r f o r m t h i s i n s p e c t i o n , t h e t o p h a l f o f

t h e t u r b i n e s h e l l

m u s

t b e r e m o v e d .

M I

M a j o r I n s p e c t i o n

T h e p u r p o s e

o f t h e m a j o r

i n s p e c t i o n i s

t o e x a m i n e a l l o f t h e

i n t e r n a l r o t a t i n g a n d s t a t i o n a r y

c o m p o n e n t s

f r o m t h e i n l e t o f t h e

m a c h i n e t h r o

u g h t h e e x h a u s t

s e c t i o n o f t h e

m a c h i n e .

A l l s i g n i f i c a n t

c o m p o n e n t s

V a r i o u s

T h e

w o r k s c o p e i n v o l v e s i n s p e c t i o n o f a l l

o f t h e m a j o r

f l a n g e - t o - f l a n g e c o m p o n e n t s o f t h e g a s t u

r b i n e w h i c h a r e

s u b j e c t t o d e t e r i o r a t i o n d u r i n g n o r m a l t u r b

i n e o p e r a t i o n .

T h i s

i n s p e c t i o n i n c l u d e s p r e v i o u s e l e m e n t s o f t h e

c o m

b u s t i o n a n d h o t - g a s - p a t h i n s p e c t i o n s . T h e c o m p l e t e

f l a n g e - t o - f l a n g e g a s t u r b i n e i s l a y e d o p e n

t o t h e h o r i z o n t a l

j o i n t

s t o a l l o w i n s p e c t i o n s t o b e p e r f o r m e d

o n i n d i v i d u a l

i t e m

s .

A m a j o r i n s p e c t i o n s h o u l d b e

s c h e d u l e d i n

a c c o r d a n c e w i t h t h e

r e c o m m e n d a

t i o n s i n t h e o w n e r ' s

M a i n t e n a n c e

a n d I n s t r u c t i o n s

M a n u a l o r a s

m o d i f i e d b y t h e

r e s u l t s o f p r e

v i o u s b o r e s c o p e a n d

h o t - g a s - p a t h

i n s p e c t i o n .

A i r I n t a k e

A i r i n l e t

I n s p

e c t i o n f o r c o r r o s i o n , c r a c k e d s i l e n c e r s a n d l o o s e p a r t s



1 0 2

I D

I n s p e c t i o n


S y s t e m

C o m p o n e n t s

I n s p

e c t i o n

M I


S e e a b o v e

C o m p r e s s o r

I n l e t a n d f l o w p a t h

C o m

p r e s s o r i n l e t a n d c o m p r e s s o r f l o w p a t h a r e i n s p e c t e d

f o r f o u l i n g , e r o s i o n , c o r r o s i o n a n d l e a k a g e

.

R o t o r A s s e m b l y

R o t o

r a n d s t a t o r b l a d e s a r e c h e c k e d f o r t i p c l e a r a n c e , r u b s ,

i m p a c t d a m a g e , c o r r o s i o n p i t t i n g , b o w i n g

a n d c r a c k i n g . A l l

r a d i a l a n d a x i a l c l e a r a n c e s a r e c h e c k e d a

g a i n s t t h e i r

o r i g i n a l v a l u e s ( o p e n i n g a n d c l o s i n g ) .

G e n e r a t o r , A c c e s s o r y

D r i v e s , D r i v e n

e q u i p m e n t

C h e

c k a l i g n m e n t - g a s t u r b i n e t o g e n e r a t o

r / g a s t u r b i n e t o

a c c e

s s o r y g e a r . D r i v e n e q u i p m e n t w i l l b e

s u b j e c t t o i t ' s o w n

m a i n t e n a n c e r e q u i r e m e n t s

C a s i n g , C o w l s a n d

f r a m e s / d i f f u s e r s

C a s i n g s , s h e l l s a n d f r a m e s / d i f f u s e r s a r e i n s p e c t e d f o r

c r a c

k s a n d e r o s i o n .

I n l e t g u i d e v a n e s

T h e

i n l e t g u i d e v a n e s ( I G V s ) a r e i n s p e c t e

d , l o o k i n g f o r

c o r r o s i o n , b u s h i n g w e a r a n d v a n e c r a c k i n

g .

B e a r i n g s a n d S e a l s

B e a r i n g l i n e r s a n d s e a l s a r e i n s p e c t e d f o r

c l e a r a n c e a n d

w e a

r . B e a r i n g h o u s i n g s w i l l b e e x a m i n e d

f o r c o r r o s i o n ,

w e a

r a n d c r a c k i n g

G a s G e n e r a t o r ( G G )

C o m b u s t i o n s y s t e m

C o m

b u s t i o n a n d h o t g a s p a t h i n s p e c t i o n a

s a b o v e . C h e c k

f u e l

n o z z l e s f o r e r o s i o n a n d b l o c k a g e . R e m o t e v i s u a l

i n s p

e c t i o n i n t e r n a l l y b y b o r o s c o p e .

F u e l s y s t e m

I n s p

e c t f o r l e a k s , i n t e g r i t y o f p i p e s y t e m s ,

b l o c k a g e o f

f i l t e r

s , c o n d i t i o n a n d b l o c k a g e o f f u e l n o z z

l e s .

B u c k e t

T u r b

i n e b u c k e t s a r e r e m o v e d a n d a n o n - d

e s t r u c t i v e c h e c k

o f b u c k e t s a n d w h e e l d o v e t a i l s i s p e r f o r m e d ( f i r s t s t a g e

b u c k e t p r o t e c t i v e c o a t i n g s h o u l d b e e v a l u a t e d f o r r e m a i n i n g

c o a t i n g l i f e ) .

B u c k e t s t h a t w e r e n o t r e c o a t e d a t t h e h o t - g a s - p a t h

i n s p

e c t i o n s h o u l d b e r e p l a c e d .

T r a n s i t i o n P i e c e

V i s u

a l e x a m i n a t i o n f o r o x i d a t i o n , c r a c k i n g , e r o s i o n a n d l o s s

o f w

a l l t h i c k n e s s . I n t e r n a l v i s u a l i n s p e c t i o n

u s i n g

b o r o

s c o p e s .

N o n

- i n t r u s i v e N D E e x a m i n a t i o n b y t h e r m

o g r a p h y a n d

o t h e

r m e t h o d s . P e n e t r a n t a n d M P I i n s p e c t i o n s i n m a j o r

o v e r h a u l s .

T u r b i n e N o z z l e s

S e a l s a n d h o o k f i t s o f t u r b i n e n o z z l e s a n d

d i a p h r a g m s a r e

i n s p

e c t e d f o r r u b s , e r o s i o n , f r e t t i n g o r t h e r m a l d e t e r i o r a t i o n .



1 0 3

I D

I n s p e c t i o n


S y s t e m

C o m p o n e n t s

I n s p

e c t i o n


( P T )

C a s i n g

C a s i n g s , s h e l l s a n d f r a m e s / d i f f u s e r s a r e i n s p e c t e d f o r

c r a c

k s a n d e r o s i o n .

S h a f t s

N D E

a n d v i s u a l e x a m i n a t i o n f o r c r a c k i n g .

N D E m e t h o d s

m a y

i n c l u d e s p e c i a l i s e d u l t r a s o n i c s ( U T ) M P I , a n d

p e n e t r a n t m e t h o d s

M I


S e e a b o v e


( P T )

( C o n t i n u e d )

R o t o r s

R o t o

r i n s p e c t i o n s r e c o m m e n d e d i n t h e m a i n t e n a n c e a n d

i n s p

e c t i o n m a n u a l o r b y T e c h n i c a l I n f o r m a t i o n L e t t e r s

s h o u l d b e p e r f o r m e d . T h i s m a y i n c l u d e d i s m a n t l i n g , v i s u a l

e x a m i n a t i o n , d i m e n s i o n a l c h e c k i n g , a s s e s s m a n t o f c o a t i n g

c o n d i t i o n s a n d N D E e x a m i n a t i o n b y m a g n

e t i c p a r t i c l e

i n s p

e c t i o n ( M P I ) , r a d i o g r a p h y , t h e r m o g r a p h y , u l t r a s o n i c s

a n d

o t h e r m e t h o d s f o r c r a c k i n g e r o s i o n o r c o r r o s i o n

d a m

a g e , l o s s o f w a l l t h i c k n e s s a n d b l o c k i n g o f c o o l i n g

h o l e

s .

N o z z l e g u i d e v a n e s

a n d s h r o u d s

T u r b

i n e s t a t i o n a r y s h r o u d s a r e c h e c k e d f o

r c l e a r a n c e ,

e r o s

i o n , r u b b i n g , c r a c k i n g , a n d b u i l d - u p .

B e a r i n g s a n d S e a l s

B e a r i n g l i n e r s a n d s e a l s a r e i n s p e c t e d f o r

c l e a r a n c e a n d

w e a

r . B e a r i n g h o u s i n g s w i l l b e e x a m i n e d

f o r c o r r o s i o n ,

w e a

r a n d c r a c k i n g

E x h a u s t

E x h a u s t b a f f l e s ,

s i l e n c e r , i n s u l a t i o n

E x h a u s t s y s t e m s a r e i n s p e c t e d f o r c r a c k s

, b r o k e n s i l e n c e r

p a n e l s o r i n s u l a t i o n p a n e l s .

G e n e r a t o r , A c c

e s s o r y

D r i v e s , D r i v e n

e q u i p m e n t

V a r i o u s

C h e

c k a l i g n m e n t - g a s t u r b i n e t o g e n e r a t o

r / g a s t u r b i n e t o

a c c e

s s o r y g e a r . D r i v e n e q u i p m e n t w i l l b e

s u b j e c t t o i t ' s o w n

m a i n t e n a n c e r e q u i r e m e n t s

C o n t r o l s y s t e m

( o n

S k i d )

O p e r a t i o n a l c o n t r o l s

C h e

c k f u n c t i o n a l i t y a n d s e n s o r c o n d i t i o n .

C h e c k i n t e g r i t y o f

e l e c

t r i c a l s y s t e m s . N o t e s p e c i f i c g u i d a n c e

i n P M 8 4 .

E l e c t r i c a l s y s t e

m s

V a r i o u s

M a i n t e n a n c e i n l i n e w i t h B S E N 6 0 0 7 9 - 1 7

a n d o t h e r I E C

r e g u

l a t i o n s c o n c e r n i n g e l e c t r i c a l e q u i p m e

n t i n h a z a r d o u s

e n v i r o n m e n t s . N o t e s p e c i f i c g u i d a n c e i n P M 8 4

C l e a n i n g S y s t e

m s

V a r i o u s

G a s

t u r b i n e s a r e s u b j e c t t o p e r i o d i c i n - s i t u

c l e a n i n g b y

i n j e c

t i o n o f w a t e r d r o p l e t s t a n d o t h e r m e t h o d s t o c l e a r t h e

f l o w

p a t h a n d r e m o v e a n y a c c u m u l a t e d f o

u l i n g o r d e b r i s .



105

12 OPERATIONAL ISSUES

12.1 HAZARDS

Hazards associated with operation of GTS are covered in PM84 Paragraphs 6 to 11. HSEGuidance Note PM84 is reproduced in full in Appendix 3. The fuel supply to a GT has to be athigh pressure. Typically, industrial units require natural gas up to 30 barg and some machinesrequire fuel up to 50 barg. The pipework supplying the fuel to the turbine combustion chambersis often highly complex since the fuel is supplied to one or more annular distribution manifoldsconnected to numerous individual burners. A combination of flanges, flexible pipes, valves and

bellows may be used, each being a potential leak site. Leaks are therefore foreseeable. Leaksmay be ignited immediately producing a flame, or may lead to the accumulation of a flammablefuel air mixture. The delayed ignition of such a mixture within a confined space, such as anacoustic enclosure, can lead to an explosion with potential for injury and major plant damage. Aleak of liquid at high pressure can produce a mist, which is flammable at a temperature below

the flashpoint of the liquid, so that leaks of liquid fuels, lubricating oils and hydraulic fluidsmay also result in fires or explosions.

The burning of fuel in the GT may produce high surface temperatures capable of igniting a leak.In the case of aero-engines the casings may glow dull red due to the heat produced. On larger

plant, hot surfaces in excess of 520°C have been found during normal operation. In certaincircumstances such temperatures are sufficient to ignite leaks of mist or vapor from liquid fuels,lubricating or hydraulic oils, as well as gaseous fuels.

GTs are a significant noise source capable of causing noise-induced hearing loss as well as producing environmentally unacceptable noise. For these reasons they are often installed withinan acoustic enclosure.

Other explosion hazards may be present within the GT. An excess of flammable fuel/air mixturemay accumulate within the turbine inlet or exhaust system, which can be ignited (especially atstart-up).

Due to the high operating speeds mechanical failure can occur, in particular with turbine andcompressor blades and discs. Such failures can lead to a loss of containment, risk of injury or damage from projectiles, mechanical damage, and fire and explosion risks from plantdisruption.

Electric shock and electromagnetic field hazards may also exist on generators and turbineauxiliary systems.

12.2 START-UP AND SHUT-DOWN

Explosions within fired plant at start-up, due to the ignition of accumulated fuel, are a well-recognized hazard, and measures should be adopted to control this hazard. Such measuresidentified in PM84 should include adequate gas path purging (at least three volume changes)

before startup, a high standard of isolation to prevent leakage during shutdown and a controlledduration for attempted ignition based on flame or combustion detection. Arrangements should

be provided to drain any accumulation of liquid fuel from the GT casing. These precautions arenormally inherent within the GT control package provided by the manufacturer. Care shouldalso be taken with the design of drain lines, to minimise risks when changing from a liquid fuelto gas, by preventing gas from entering sump tanks. Consideration should also be given to

fitting gas detectors in such tanks.



106

12.3 SURGE PREVENTION

Surge is a backflow in pressure giving a momentary change in the direction of airflow. This is

different to overspeed . This flow reversal is accompanied by high fluctuating load on thecompressor bearings Surge must be avoided at all costs as it can cause damage to the turbine,combustion chamber or back-end of the compressor; damage may be severe. There are a varietyof causes. Causes could include blockage of air supply, blockage of fuel or other transientchanges. In some circumstances it is possible to get a locked-in surge with pressure waves

bouncing back and forth.

Normally the gas turbine may carry on with little affect. In other circumstances surge can causesevere damage, depending how deep the extent of the pressure variation. If surge conditions aremet, there is little an operator can do physically to stop or avoid a surge. Surge is best avoided

by keeping operation within strictly controlled boundaries which have been previously definedand modelled by the turbine supplier. Some protection is afforded by surge-protection systems

and recycle valves which open to control pressure differentials if pressure variations potentiallyleading to surge are monitored. Because of the potential consequences it is important to beassured of the operator's competence in this area.

Surge is relevant to the air compressor in the turbine and to driven compressors. In the contextof gas turbines surge is possible in the Compressor (GC) of the GT. Where the GT drives a gascompressor, surge must also be avoided in the driven equipment. Surge has the potential tocause significant damage to a GT or compressor. If the discharge volume of trapped gas goes

past the stability limit a lot of load can be transferred to the thrust bearings. The GT usuallysurvives but the high stress conditions can lead to overheating of the machine.

Surge is avoided primarily by careful control of operating conditions so that the GT stays within

stability limits ( Figure 54). This is an important part of gas turbine design. Turbine supplierswill run simulation models to ensure that the conditions that could give rise to surge in a givendesign are well understood. A gas turbine will include a recycle loop with control valves

between the power turbine (PT) and gas compressor (GC) for surge prevention. Other ways of minimising the likelihood of surge include: active and passive methods to increase the stability,simulation to improve the accuracy of determining the stability (surge) limit, and simulation to

better understand the interaction between the compressor, the anti-surge devices (controlsystem, valves) and the station piping layout (coolers, scrubbers, check valves). A detailed studyof surge avoidance in centrifugal compressors driven by two-shaft gas turbines was given atIGTI2004 by Kurz and White

7The possible operating points of a centrifugal gas compressor

are limited by maximum and minimum operating speed, maximum available power, choke flow,and stability (surge) limit

Surge is most likely during rapid or emergency shutdown. When the power supply is cut, therotating system slows down in inertia, gas is trapped with less head than normal operation.Pressure is reduced, the ESD works against the emergency shutdown. Whether surge occurswill depend on a number of factors including mass and energy balance, momentum, valvecharacteristics, compressor characteristics and system inertia. Shutdown is rapid; as a rule of thumb ~30% of speed is lost in the first second.



107

Figure 54 Limits of stable airflow

Each stage of a multi-stage air compressor possesses certain airflow characteristics that aredissimilar from those of its neighbour; thus to design a workable and efficient compressor, thecharacteristics of each stage must be carefully matched. This is a relatively simple process toimplement for one set of conditions (design mass flow, pressure ratio and rotational speed), butis much more difficult when reasonable matching is to be retained with the compressor operating over a wide range of conditions such as a gas turbine encounters.

If the engine demands a pressure rise from the compressor which is higher than the blading cansustain, surge occurs. In this case there is an instantaneous breakdown of flow through themachine and the high pressure air in the combustion system is expelled forward through thecompressor with a loud 'bang' and a resultant loss of engine thrust. Compressors are designedwith adequate margin to ensure that this area of instability is avoided

Models can give an understanding of the conditions leading to surge and aid prevention7. Recentexperience of using models by Statoil in the Troll field [Bjorge IGTI 2004] showed that modelscan give insight to the factors causing surge and what protects the system, for example highinertia, slow power decay and power-loss delay. Modern control systems with real timemonitoring of exhaust temperature and feedback can adjust performance of all parts of theturbine (air compression, fuel input etc.) to prevent surge

A key factor in surge prevention is the downstream volume. Proper sizing of the system and pressure side volume is essential. The volume downside of the check valve should be reduced asmuch as possible. The size of the downstream volume is very important to get stability in thecompression system. The pressure coupling between the compressor and gas turbine is also an

important factor in determining the size of discharge volume.

Simplified models are available from suppliers to look at surge issues. These have beenvalidated against test data. The turbine supplier will normally determine what ESD valve touse. In operation stepping to idle is preferred to ESD. Supplier experience is that ~90% of ESDsare preventable. Surge in driven compressors may also impact on gas turbine integrity. Surgeavoidance in compressors is covered in Reference 7

12.4 RECYCLE FACILITY

The usual method for surge avoidance, anti-surge control , consists of operation and control of arecycle loop. This can be activated by a fast acting valve, the anti-surge valve, when the control

system detects that the compressor is approaching its surge limit. Typical control systems use



108

suction and discharge pressure and temperature, together with the inlet flow into the compressor as input to calculate the relative distance, the surge margin, of the present operating point to the

predicted or measured surge line of the air compressor or driven compressor.

If the surge margin reaches a preset value (often 10%), the anti-surge valve starts to open,thereby reducing the pressure ratio of the compressor and increasing the flow through thecompressor. The situation is complicated by the fact that the surge valve also has to be capableof precisely controlling low. Additionally, some manufacturers place limits on how far intochoke (or overload) they allow their compressors to operate. The surge prevention system maycause associated noise and vibration problems.

12.5 CONTROL SYSTEMS

Control systems have been covered in detail in Section 0 and are also covered in PM84. Gasturbines are complex machines and synchronisation and controlled operation of the different

systems is essential to ensure smooth operation and avoid surge or instability. It is importantthat operators are fully familiar with the operation of the control system and any warningindicators that may be indicative of a deviation from normal operating conditions. Gas turbinesare tolerant and reliable in normal operation provided fuel flow and input of air remain uniform.Control is often undertaken through monitoring of exhaust temperature. In turbine packages,the turbine control must respond to the operating requirements for the driven equipment such asalternators, pumps or compressors.

A major recent North Sea incident occurred where a maintenance engineer had shut off part of the control system during routine maintenance. The logic controlling the purging system for thecombustion chamber had been bypassed. There was a flame-out problem on restart. Asignificant explosion occurred due to build up of fuel within the chamber damaging the power

turbine and exhaust and taking out the waste heat recovery systems. HSE inspectorsinvestigating were surprised that no controls were in place to prevent the maintenance engineer switching off this safety control system.

Maintenance of gas turbines is usually subcontracted with the maintenance companies followingtheir own procedures. Gas turbines operate within clearly defined margins. Normal practice

before carrying out a new procedure or bypassing control systems in this way would be tocontact the turbine manufacturer who would simulate on their computer models and assure the

planned intervention would be OK

The key issue is that before overriding or modifying any part of the control system, the

maintenance engineer should check with the manufacturer that this is safe. Such changes need

to be undertaken by personnel with appropriate training and authorisation. Shortcuts are to beavoided.

12.6 VIBRATION MONITORING

Vibration monitoring is used primarily to monitor conditions of bearings, blade tip rub, bladeintegrity. Any imbalance in these causes vibration. Vibration monitoring gives an earlywarning of any issues before they have time to cause major damage.

If the operating conditions imposed upon the compressor blade departs too far from the designintention, breakdown of airflow and/or aerodynamically induced vibration will occur. These

phenomena may take one of two forms; the blades may stall because the angle of incidence of



109

the air relative to the blade is too high (positive incidence stall) or too low (negative incidencestall). The former is a front stage problem at low speeds and the latter usually affects the rear stages at high speed, either can lead to blade vibration which can induce rapid destruction.

12.7 FIRE DETECTION REQUIREMENTS

Fire and gas detection is essential in and around the acoustic enclosures. Advice on gasdetection can be found in PM84. At least one gas detector should always be installed if the GThas a gaseous fuel supply. The best location for gas detection is in the ventilation outlet becausea leak will always reach it. The detector should be located sufficiently downstream to ensureadequate mixing within the outlet duct. Additional detectors can also be used within theenclosure to increase the probability of detecting small leaks. As well as considering the bestlocation for such additional detectors, care needs to be taken that they are not exposed totemperatures above their operating range. Some large units have successfully used pipedsampling systems to monitor for gas from potential leaks. The sampling regime of these systemsmeans they are slow to respond but may be valuable as an additional source of warning of small

leaks. In the case of a turbine hall, CFD modelling work suggests it is useful to model likelyfuel dispersions around the GTs to identify the best location for gas detectors30. Theeffectiveness of such detectors may also be improved by providing baffles, which will direct

predicted flows towards them.

The settings for gas detectors placed around a GT should be dictated by their purpose. Gasdetectors in the ventilation outlet from the enclosure should be set to alarm at the lowestreasonably practicable level, preferably below 5% of the lower explosive limit (LEL) but notexceeding 10%. Ventilation inlets should be located in a safe area, but if there is a possibility of a flammable mixture being drawn into the enclosure via the air inlets, then further fast-actinggas detectors will be required. In the event of a gas alarm safe plant rundown should beinitiated. During this period, the ventilation should run at its maximum rate. The increase in

ventilation may reduce the gas concentration, but this should not cancel the alarm or delay therundown. It should only be possible to cancel alarms manually and preferably only after the

plant has shut down. High-level trips should also be set as low as reasonably practicable, but nohigher than 25% of the LEL and should initiate automatic GT trip with gas supply valves beingfully closed. Intermediate detector settings, between the alarm and trip settings, may be valuableas a means of initiating automatic controlled shutdown of larger turbines. Very sensitivedetectors may be valuable as a means of early warning of a gas leak, which may enable safeaccess to investigate the leak source.

Gas detectors should be selected in accordance with BS EN 50073 and installed and calibratedregularly in accordance with manufacturers' recommendations. In-situ calibration facilities arerecommended if plant is expected to run continuously for long periods. The use of additional

detectors or recalibration may be required for different fuels. However, recalibration must bestrictly controlled to prevent the incorrect setting of detectors. Where spurious trips must beminimised, such as at larger plant or critical supply installations, a voting system based on anumber of detectors in the ventilation outlet may be used. For example, activation of any oneout of three detectors would initiate an alarm. However, any two out of three detectors above thetrip level would be required to automatically shut down the fuel supply. Displays of gas levels,recording and trending facilities can also add to reliability and aid the diagnosis of faults.

12.8 PRECAUTIONS AGAINST FIRE

Guidance on precautions against fire is given in PM84 paragraphs14-22. Minimizing the risk of

fuel and oil leakage and controlling the presence of sources of ignition will reduce the risk of



110

fire. The presence of exposed hot surfaces during normal operation precludes complete controlover sources of ignition.

The fuel supply should be interlocked in a fail-safe manner with the fire and gas detection

systems. It should also be possible to manually isolate the fuel supply from a safe positionoutside any enclosure around a GT.

Many oil fires, in particular oil-soaked insulation fires, have occurred. Insulation materials inareas susceptible to oil leaks or likely to be exposed to such fluids during general maintenancecan include a protective film or metal skin. This should be carefully installed to avoid

puncturing, and seams should be taped or folded in such a way as not to collect fluids. Further protection of high risk pipes can be achieved by the use of double-walled pipe systems tocontain any leak. To minimise risk, lubrication and hydraulic oil systems should be designedand constructed to recognised engineering standards.

Once a GT is in service, a regular scheme of inspection for leaks of both fuel and oil should be

developed and implemented. This should be carried out in accordance with a safe system of work to minimise the risk to those carrying out the inspection. Guidance on access to enclosuresis given in paragraphs 54-57. Such an inspection scheme should be regularly reviewed andmodified according to user experience. Results of inspections should be recorded. While visualinspections can help identify liquid leaks they will not detect gas fuel leaks.

A fixed fire protection system should be installed to mitigate the consequences of a fire on theGT. This should be to an appropriate standard, such as NFPA 750, BS ISO 14520PM or BS5306 and, as a minimum, designed to be capable of at least suppressing a fire on the GT or within the GT enclosure. The design and installation of fixed fire protection systems is aspecialist field and it is recommended that companies experienced in fire protection engineeringare consulted.

In considering the design of a fire protection system, careful attention also needs to be given toits interactions with other parts of the installation and personnel. These may include:

a) The ventilation system;

b) The isolation of the fuel supply to reduce fire loading and the risk of explosion once thefire has been extinguished;

c) The isolation of the electrical supply;

d) The choice of extinguishant to minimise the risk of electrocution or asphyxiation;

e) The environment in which the GT is installed; and

f) The means of access to the enclosure and the location of emergency shutdown pushbuttons and fuel isolation devices.

g) The openings into the enclosure should be fitted with an automatic closing damper.

The early and reliable detection of fire is critical to the successful performance of the fire protection system. Key to this is the careful choice and siting of fire detectors in the GTenclosure. No single type of fire detector is the best in all situations and typically a combinationof thermal, flame and smoke detectors will be appropriate. The choice should be based on ananalysis of the characteristics of the potential fires that might occur in the GT enclosure andtheir particular causes. Fire detectors should comply with the relevant part of BS EN 54 and

should be installed in accordance with the recommendations of BS 5839 and BS 7273. A



111

manual release facility for the fire protection system should also be provided in accordance withthe recommendations of BS 7273.

Where a fixed fire protection system is installed it should be regularly inspected and properly

maintained in accordance with BS 5839 and BS 7273. The fire protection system should be periodically inspected and serviced by a competent person with the necessary skills andspecialist knowledge of such systems. A suitable record should be kept of the inspection checks,servicing and maintenance work carried out. The user should carry out a daily check that thesystem is operational and other regular checks and tests detailed in the user instructions

provided by the fire protection system installer. The user should ensure that those withresponsibility for carrying out these tasks are adequately trained.

Exposure to extinguishants that are potentially hazardous should be prevented. This may beachieved by selecting a non-hazardous extinguishant, eg water mist. Alternatively, potentiallyhazardous extinguishants, such as gaseous fire extinguishants, can be used under carefullycontrolled conditions to prevent inadvertent exposure to the extinguishant. The control

requirements depend on the particular extinguishant and its maximum concentration in theenclosure. Details and recommendations on this are contained in BS ISO 14520. Extinguishingsystems that may create an asphyxiation or toxic hazard should be isolated before entry into anenclosure. The isolation procedure should comply with BS ISO 14520 and BS 7273. However,systems based on extinguishants such as water mist do not have to be isolated so the risk of inadvertent isolation is eliminated. Inadvertent exposure to extinguishants should be avoided,even with fire protection systems using concentrations at which there are no observed adversetoxicological or physiological effects, in accordance with BS 5839. PM84-5 A suitable alarmshould be incorporated into the fire protection control system to provide sufficient warning to

people within the enclosure to make their escape before discharge of the extinguishant. Wherethere is a potential visibility hazard, the exits from the enclosure should be adequatelyilluminated. Any air exhausts or air inlet.

12.9 RISK ASSESSMENT FOR ROUTINE ACTIVITIES

Risk assessment should be in place for routine activities such as cab entry, water wash, isolationschemes and start-up checks. Guidance on Risk Assessment for GTs is given in PM84

paragraphs 12-13. Risk assessment should be undertaken by competent people at all stages of the design, manufacture, packaging and commissioning of the GT. This should also include theconsequences of foreseeable abnormal operation impacting on nearby plant, for example on anoffshore platform. Manufacturers and suppliers should not only use existing knowledge of hazards associated with GTs but should also maintain contact with the users of such plant togain information on plant failures. The commissioning stage is particularly important as it

necessarily includes the first admission of fuel to the equipment and also because theresponsibility for managing the plant is being progressively transferred to the user.

Before handover the user should carry out a suitable and sufficient risk assessment on theoperation of the GT. This should include the requirements of the Management of Health andSafety at Work Regulations 1999 (see paragraph 80) and of the Dangerous Substances andExplosive Atmospheres Regulations (see PM84 paragraph 91). For larger plants, whichgenerally present a greater risk, a more detailed risk assessment may be required, including theuse of qualitative or quantitative risk analysis techniques. As well as confirming that the safetyfeatures of the plant meet the agreed specification, the risk assessment should also pay particular attention to operational procedures. Third-party design appraisal may be used to demonstratereduced risk by providing verification that relevant design standards have been met. The

adequacy of the training and experience of those involved with the operation, maintenance,



112

inspection and monitoring of the GT plant should also be confirmed. Consideration should begiven to the site conditions in which the equipment is installed, in order to reduce the risk of environmental or third-party impact; for example weather related motion would affect

performance and lifecycle of components and equipment installed on floating platforms. The

risk assessment should be reviewed at appropriate intervals as operational experience develops.

12.10 ACCESS

Safety issues concerned with access to GT enclosures and confined spaces are covered in PM84Paragaphs 54 to 57.The acoustic enclosure around a GT is likely to be a confined space (seePM84 paragraph 82) as there is a foreseeable risk of serious injury due to the leakage andsubsequent ignition of a flammable fuel. Entry for maintenance when the GT has been shutdown should be under the control of a suitable safe system of work, which may include a permitto work. Such a safe system of work should include the manual isolation of the fuel supply andthe testing of the atmosphere within the enclosure to confirm the absence of flammable or toxic

gases.

Strong justification will be required for entry to an enclosure during turbine operation. All other potential options for carrying out the work from outside the enclosure should be considered before allowing entry. Instrumentation with remote indication should be used to avoid routineentry. CCTV and/or viewing windows can be used where practicable to provide visual checkson machinery conditions. On new plant, both manufacturers and users should try to eliminatethe need for entry. If there is no alternative then it should be restricted to a minimum durationand limited to authorised personnel carrying out specific tasks. The risk assessment shouldidentify why such an entry is required, what the inherent hazards are, and the measures to betaken to reduce them. Thermal and noise hazards should also be considered in setting entryduration. A written safe system of work will be required which may include a permit to enter

and to carry out specified work. Appropriate precautions should be taken to prevent the trappingof personnel inside the enclosure under any foreseeable circumstances.

Due to the increased risk while load and fuel changes are taking place, entry should be prohibited at these times. Such changes can occur automatically. However, entry should not be permitted to the enclosure when there is an imminent planned change. Load changes mayincrease the risk of a leak by an increase in fuel pressure when an idling GT is brought on load.The small variations that occur during normal running are not considered to increase risk.Changing from one fuel to another may increase the possibility of a leak occurring due to theincrease in fuel system pressures or use of different pipework. Similarly, entry at start-up andunder any ongoing uncontrolled emergency condition should not be permitted.

For GTs in a turbine hall, close approach to a running machine and access to hazardous areas inthe vicinity of the GT should be kept to the minimum necessary for safe operation in accordancewith risk assessment.

12.11 HAZARD MANAGEMENT IN HOT-SPOTS

Gas turbines operate at extremely high temperatures, sometimes exceeding 2000ºC in thecombustor and gas generator (Figure 1), the hottest parts of the gas turbine. The exhaustmanifold in particularly can achieve high temperatures and is covered with lagging for safetyreasons. Despite the use of air cooling the turbine casing may also be extremely hot.

These high temperatures pose a risk in terms of injury and burns to personnel and fire ignition

following oil, gas or fuel leak. Rigorous safety measures should be in place to avoid injury to



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personnel from contact with hot surfaces. The integrity of lagging should be checked, particularly after storm conditions.

Analysis of the incidents, dangerous occurrences and accidents on UK installations (Section

10.5 ) indicates ignition from oil and fuel leaks to be responsible for a high proportion of thetotal incidents. Good maintenance and preventative measures against leakage or subsequentignition is important

12.12 PRECAUTIONS AGAINST EXPLOSION

Precautions against explosion are covered in guidance PM84 paragraphs 23 to 30. This includesventilation, dilution ventilation and explosion suppression. If an enclosure is provided, then

precautions should also be taken against explosion hazards. These precautions should be basedon risk assessment. The use of certain fuels having low auto-ignition temperatures (AIT) or ignition energies, such as naphtha or hydrogen enriched fuel, requires specialist advice becauseof their particular hazards. The risk assessment should identify the additional risks posed by

such fuels and any measures necessary to reduce the risk to an acceptable level.

Ventilation was initially installed in acoustic enclosures to assist cooling of the GTs.Subsequently it has been shown that it can also be used as a basis of safety, if designed asdilution ventilation. In practice this means that the ventilation should ensure that there are nostagnant or poorly ventilated spaces and that any leak is effectively mixed with air. Re-circulation and re-entrainment should be minimised, further reducing any accumulation of flammable mixture. This may require a large number of air inlet positions to obtain adequatedistribution and, in extreme cases, supplementary fans or air distributors. Dilution ventilation isonly acceptable as a basis of safety when associated with the use of suitable gas detection. SeePM84 paragraphs 43-45.

In most cases a GT cannot directly comply with the regulations made to implement the ATEXDirective (paragraph 88), because of the requirement to exclude hot surfaces from hazardousareas. The European Commission have published guidance on their website29, which confirmsthat the provision of dilution ventilation will, by preventing an explosion, enable GTs operatingin an enclosure to be regarded as ATEX compliant. Conformity assessment of the ventilationdesign, in the UK, will be required to ATEX Equipment-Group II, Category 3 equivalence, andwill therefore be the responsibility of the final supplier.

While dilution ventilation has now been accepted as the preferred basis of safety, explosionrelief and explosion suppression may be used as additional risk reduction measures. However if either of these techniques were to be used as an alternative basis of safety, then appropriate

justification would be required.

Explosion relief is easier and less costly to fit to new plant than to retrofit. It has the advantageof proven reliability as a basis of safety in many process industries. Strengthening of theenclosure can be used to reduce the vent area required. Modification of existing roof panels may

provide sufficient explosion relief. All such relief panels should be restrained and shoulddischarge to a safe place, preferably in the open air, in order to prevent injury to personnel anddamage to adjacent plant. Any ductwork associated with the relief panels should be designed tocontain the expected pressures.

Explosion suppression is a well established technique in other industries. A suitable suppressantis distributed within an enclosure at the onset of an explosion with such speed that the explosionis quenched and the pressure rise is limited to a small acceptable value. It can be linked to a fireextinguishing system and will similarly preclude access to the plant during normal operation



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unless isolated. Ventilation, fuel controls and fire extinguishing systems may need to be linkedto the suppression system to maintain safety following its operation.

Turbines within spacious halls are unlikely to present an explosion hazard, since foreseeableflammable mixtures are not sufficiently enclosed. Such an arrangement has significantadvantages of accessibility for maintenance, although employees in the building are likely toneed protection against exposure to noise. In turbine halls the use of dilution ventilation as a

basis for safety and ATEX compliance is less applicable, and the focus shifts towards gasdetection. However, the ventilation of such large halls should be designed, and checked, toensure that large accumulations of flammable mixture would not arise from foreseeable leaks,and that such leaks can be detected 8 Screens or baffles may assist the detection of leaks byrestricting the spread of fuel/air mixtures. Access to hazardous areas in the vicinity of the GTshould be restricted to mitigate the residual risk, as noted at PM84 paragraph 57.

GT enclosures may, in exceptional circumstances, be installed in a hazardous area. Their installation in zone 1 areas (see definitions of zones in BS EN 60079109) should be avoided. If

installation is contemplated in zone 2 areas, expert specialist advice should be sought. Suchadvice should include consideration of the following precautions:

a) Combustion air and ventilation air should be drawn from a safe area, i.e. un-zoned,taking wind effects into account;

b) Fast-acting gas detectors should be placed in combustion air and ventilation air intakesto provide alarm and trip functions. These detectors should be set to the lowest levelscompatible with a minimum of spurious operations;

c) Engine exhaust should discharge to a safe place outside any zoned areas, taking windeffects into account;

d) Ventilation should be forced, so as to maintain a positive pressure within the enclosure;

e) A pressure detector should be used to interlock the enclosure pressure with the GT fueltrip;

f) Access to the enclosure should be prevented during GT operation and after engineshutdown until hot surfaces have cooled to a safe level. An assessment of the timerequired to achieve adequate cooling will be required;

g) The enclosure should be constructed to minimise air loss to the outside;

h) In general, the enclosure and associated equipment should comply with BS ENstandards for equipment intended for use in hazardous atmospheres; and

i) Depending upon the regulations applicable to the installation site, certification of

conformity and appropriate marking may also be required.

12.13 VENTILATION

Ventilation requirements andeffectiveness are covered in PM84 paragraphs 31 to 41.

If practicable for new plant, ventilation should be designed so that it passes from potentialhydrocarbon leak sources away from surfaces which are at a high temperature, and not towardsthem. However, in doing so care should be taken not to expose other sensitive components, suchas instrumentation and cable trays, to excessive temperatures. Also any modified ventilationflow should not generate component stresses in the GT casing that could lead to failure. Itshould be noted that the appropriate distribution of ventilation air is more important than itsquantity, and that high ventilation rates may inhibit the detection of small leaks.



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Dilution ventilation air movement should be monitored and interlocked to GT start and tripsequences so that the unit cannot start without sufficient ventilation and GT pre-purging. Thegas shut-off valves should not open and any gas-line vent valves should not close until after the

GT purge cycle is complete. Failure of the ventilation system during running should initiate afuel trip, unless the ventilation is automatically restored from an alternate or emergency power supply. This should also supply the air movement detection instruments, gas detectioninstruments and associated engine trip systems. In the case of battery back-up systems acontrolled shutdown should be initiated within the expected safe period of operation of the

batteries. Reliance must not be placed on battery back-up systems to continue normal running.All types of electrical back-up systems involved in safe operation of the plant will requireregular maintenance and testing to ensure their continued availability.

At turbine start-up, thermally induced flows that are present during normal operation may beabsent. The possibility of gas leaks is also likely to be greater at start-up, for example followingmaintenance operations. The effectiveness of the ventilation under normal operating conditions

and at turbine start-up should therefore be confirmed.

In smaller enclosures the effectiveness of the ventilation may be studied with the use of smokecombined with closed circuit television (CCTV). In larger enclosures (above about 50 m3) tracer gas techniques have been used effectively. However, it has been found that in most casesventilation and gas leakage in these larger enclosures are best predicted by modelling withcomputational fluid dynamics (CFD). Currently other available techniques may fail to take fullaccount of the momentum of the leak. An additional benefit is that CFD permits a quantitativeassessment against the criterion noted below. A CFD approach also has the advantage. thatventilation modifications, if shown to be necessary, can be modelled without actual plantchange, or even before the plant is built.

A quantitative criterion against which to assess dilution ventilation efficiency in enclosures has been proposed [10] and shown to be both conservative [PM84-8] and attainable. It is based on the principle of limiting any foreseeable accumulation of flammable mixture, so that its ignitionwould not present a hazard to the strength of the enclosure or to people. The criterion proposesthat the size of the flammable cloud, as defined by the iso-surface at 50% of the lower explosivelimit (LEL), should be no larger than 0.1 % of the net enclosure volume. This criterion has beendeveloped to allow a common basis for assessment of ventilation effectiveness in enclosures. Itis primarily applicable to a CFD-based approach. The results of any research into this fieldshould be taken into account as they become available.

In adopting a CFD approach, the model should be representative of the plant. The geometry of the enclosure, turbine and associated equipment should be adequately resolved by the CFD grid.It may not prove possible to explicitly resolve small obstacles, such as pipework, fittings etc, inwhich case these should be taken into account by adopting a porosity-based approach. Thenumber and location of ventilation inlets and outlets should be correctly represented, as shouldthe flow rates. Consideration should be given to thermal boundary conditions, and the need tosatisfy an overall heat balance for the turbine enclosure system. Where possible, the CFD modelshould be demonstrated as being representative of actual conditions, by comparison of simulated velocity and temperature fields with in-situ measurements.

The effects of buoyancy in a CFD model should be addressed, since thermally induced naturalconvection flows can be significant. While the main fuel, natural gas, is inherently buoyant, ahigh-pressure release will normally cause a substantial amount of mixing, and the resulting gas

cloud may then be at relatively low concentration. In these circumstances the gas cloud could bemore affected by the background ventilation, including any thermally induced flows, or flows



116

induced by the momentum of the release. The modelling of the gas leak in a CFD approach can be undertaken in one of two ways: either the leak source is resolved explicitly by the CFD grid,or the effects of the leak are introduced as sub-grid scale sources of mass, momentum, energy,and turbulence. In practice, it is usually not feasible to resolve the leak directly at its source, due

to its small dimensions. In such cases it is acceptable to use correlations or a simple jet model to provide a larger pseudo-source a small distance downstream from the leak location, which can be resolved by the CFD grid. In general, this approach is more reliable than use of a sub-gridscale source.

The leak rate to be modelled in CFD simulations should be the largest leak that would just passundetected. This can be calculated as that gas release rate which, when fully mixed in theventilating air passing through the enclosure, just initiates the alarm for a detector located in theventilation outlet. Larger leaks than this should be readily detected and appropriate action taken.Smaller leaks could pass undetected, but present no hazard if the ventilation design has beenvalidated.

A CFD approach should aim to demonstrate that the ventilation is effective for a credible `worst case' . The leak rate should be calculated using the above approach, and the leak location andorientation chosen to produce the largest flammable cloud predicted by CFD modelling. Thiscan be best achieved by an approach which identifies poorly ventilated regions, ie re-circulatingor stagnant flow. Identification of poorly ventilated regions can be achieved by analysingsimulations or measurements. Since it is not possible to know, in advance, which combinationof factors will lead to the largest flammable cloud, a small number of alternative leak locationsand orientations should be simulated. These leak scenarios should be investigated separately toavoid interactions, rather than all modelled within a single simulation.

CFD results should be subject to sensitivity analysis regarding areas of modelling uncertainty.In particular, the sensitivity of the flammable cloud volume to the mesh resolution should be

addressed. This can, for example, be achieved by local grid refinement. The numerical schemesthat are used to estimate fluid flow across the boundaries of grid cells can also have a significantinfluence on the accuracy of the results. Simple schemes may result in over-rapid mixing,

purely as a consequence of numerical errors. This effect is commonly referred to as false, or numerical, diffusion. More advanced numerical schemes should ideally be used to avoidexcessive numerical diffusion.

12.14 FUEL SUPPLY SYSTEMS

Fuel supply systems are covered in paragraphs 48 and 49 of PM84. Fuel pipework should bedesigned, constructed, tested and installed to an appropriate recognised standard. Relevantreferences are given in Institution of Gas Engineers and Managers publication UP/9.14

Replacement pipework should be subject to the same standards. Vulnerable pipework should berouted so as to avoid the likely disintegration plane of ejected turbine disks and blades. Fuel

pipework should also be designed with the minimum of non-welded joints compatible withmaintenance requirements. Assembly and maintenance requirements should be considered at thedesign stage.

All fuel pipelines should be assembled, and reassembled following maintenance, under a qualityassurance scheme. They should also be pressure tested, so far as practicable. All flanges andfittings upstream of any final flanges or connections at combustion chambers should be pressureand leak tested after assembly. Final flanges or connections should be tightened under recordedand controlled quality assured conditions, and leak tested so far as practicable. Adequate accessto all such fuel pipework flanges is thus essential. Where it is possible to produce a small



117

backpressure by spinning the gas turbine, techniques such as the use of proprietary leak detection spray or a tracer gas can be used to aid leak detection33

.

12.15 GAS FUEL

Paragraphs 50 and 51 of PM84 give special precautions for gas fuel

A high standard of automatic isolation, based on two safety shut-off valves meeting class A performance standards, should be fitted to the gas supply to prevent gas from passing intodownstream equipment while the GT is stationary. For systems where the fuel thermal energyinput flow rate exceeds 1.2 MW, the valves should be fitted with a system to prove their effective closure, for example by the fitting of proving switches to detect mechanical overtravel,or by sequential pressure proving, which may use an intermediate vent valve. The latter systemhas the advantage that it effectively tests the valves for leakage at each start-up and shutdown.Further guidance on isolation is given in IGE/UP/9.

For applications where gas supplied by a national gas transporter is further compressed by theend user, safety features will be required to prevent the back feed of high-pressure gas into thedistribution system.

Appropriate measures to prevent this situation during upset conditions may be required by thegas transporter. Such measures could include:

a) a plant inlet `emergency shutdown valve' acting on rising pressure in addition to other plant safety requirements; and

b) a 'non-return valve' at the suction side of the gas compressor package to prevent reverseflow.

Further details are given in IGE /UP/6.

12.16 ADDITIONAL EXPLOSION PRECAUTIONS FOR LIQUID FUELS AND OILS

Additional precautions to avoid explosion with liquid fuels and oils are given in Paragraphs 52and 53 of PM84.

Liquid fuel leaks from high-pressure sources can produce a mist, which can be flammable at atemperature below the flashpoint of the liquid. Ignition of such a mist can have explosive effectssimilar to gas explosions. Effective ventilation should be provided but, because ventilation isless effective in diluting and removing liquid droplets, their formation should be avoided as far as possible. Vulnerable joints and fittings should be minimised. Consideration should be given

to the use of welded joints or the use of double containment pipework, as well as to the use of proprietary mist eliminators (spray shields) or encapsulation to protect remaining vulnerable joints and fittings. Mist detection should be considered as a further risk reduction measure if practicable. So far as possible, joints should be positioned so that leaks do not drip or spray ontohot surfaces. In particular, for liquid fuels of very low AIT such as naphtha, segregation of risk areas, explosion relief or explosion suppression should be considered. This is because of theincreased risk of ignition and the uncertainties of CFD modelling of such releases. Further guidance on liquid fuel installations is given in IGE/UP/9.

High pressure leaks of lubricating oils and hydraulic oils may also produce a flammable mistwith risks similar to those noted above for fuels. The properties of any such flammable fluidsshould be obtained from suppliers and taken into account in a risk assessment. Where necessary,

additional precautions as described above should be considered to reduce the risk. Where other



118

risk reduction measures against flammable oil mists do not provide an adequate level of safety,it will be necessary to use fire-resistant or non-flammable fluids.

12.17 EMERGENCY PROCEDURES

Emergency procedures are covered in Paragraph 59 of PM84. Actions to be taken in the eventof fire or gas alarms should be written into emergency plans and regularly reviewed. Guidancefrom suppliers should be sought and applied. Training in emergency procedures should be givento operators. Instructions should be given on when to shut down under controlled conditions or to trip fuel

supplies immediately, when to summon the emergency services, control of the ventilationsystem, access limitation, and emergency communications. Emergency shutdown controlsshould be located within the control room and at other appropriate locations based on a risk assessment.

12.18 AIR AND GAS SEALS

There are many air and gas seals in gas turbines to separate different regions and pressures of air and gas flow and to facilitate cooling of high temperature components. Air may build up in thelubricant oil used for bearing and seals in the gas turbine. This is separated off in separationtank. Air inlet to the tank is controlled to avoid the risk of explosion, with breather valves toavoid pressure build-up. There have been quite a few incidents associated with blockage of

breather valves, leading to pressure release. This can pose a safety hazard particularly if sour gas is present and in enclosed environments.

12.19 CHANGEOVER IN DUEL FUEL SYSTEMS

Many offshore gas turbines are duel fuel, that is they can also operate on diesel as well as produced gas. There have been a number of incidents associated with fuel changeover. It isimportant to ensure that necessary control sequences are carried out. This includes shutting off the fuel system for conventional gas operation and purging the combustion chambers to clear these of existing fuel build up.



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13 RECENT TRENDS

13.1 MICROTURBINE DEVELOPMENT

A recent trend is the development of small microturbines for simple power generation or drivenequipment applications. The positive features of microturbines are large power for small size.The negatives are fuel requirements and running cost. Applications foreseen include use in thehome to cover grid unreliability, refrigeration, and military use for remote vehicles and sensors.There are potential offshore applications include use on remote installations or where a smalllocal power or drive requirement such as pumping exists.

Compact radial and centrifugal designs have been developed by Hitachi http://www.power-hitachi.com and SWRI GE have been developing microturbines in the CHP project

15. Goal is

33-40% electrical efficiency. Applications seen include power refrigeration and heating.

x NOx <7ppmx 10,000h between major overhauls

x Cost $500/kW

In refrigeration an evaporator, condensor and power module are required. Microturbines tend touse integral single-piece component turbines rather than individual turbine blades.

13.2 DRY LOW EMISSIONS (DLE)

Increasingly stringent emission controls have produced a trend to gas turbines giving low NOxand CO emissions (<25vppm). This is achieved using a dry low emissions (DLE) combustionsystems and requires careful control of air and fuel input and other operating parameters. DLE

versions are available now from most major suppliers.

As an example, design innovations to achieve DLE and give significantly lower NOx emissionsin RB211gas generators in Rolls Royce Trent and Coberra 6000 gas turbines included:

x pre-mix, lean burn combustion in original lean burn designs. Successful initially butdeveloped a noise problem.

x Solution to make fueling asymmetric and moving the location of heat release. Similar problems were found in the secondary zone and removed.

x less cooling air and lower flame temperature to give lower Nox emissions and improvedfuel mixing

x new shorter combustors. These gave more uniform and lower NOx emissions.

x new mixing ducts – fuel in, air gradually goes in

x damping technologies to remove noise.

x pressure wave dumping , resonant cavities take out noise. New combustor gives muchlower noise.

x Closed loop emission control

The temperature is critical to the level of emissions. Too low a temperature leads to CO, toohigh a temperature results in higher NOx emissions as illustrated below in Figure 55.



120

NOxCO

2000Temperature (qC)

C O L e v e l

N Ox

L ev el

0

10065

0

Figure 55 Schematic illustrating the effect of temperature on NOx and CO emissions

13.3 STEAM INJECTION FOR EMISSION REDUCTION AND POWER OUTPUT

An alternative way of reducing NOx and CO emissions to below 3ppm has been reported 35

involving premixing steam with the fuel prior to it’s combustion. The steam is intimately mixedwith the fuel in such away as to suppress the size of the flame and promote combustionefficiency. This combustion consumes much of the excess oxygen and thereby inhibits NOx andCO formation. Very low emission levels have been demonstrated in preliminary laboratory andengine testing.

The use of steam injection to increase power output of gas turbines is already established. It has been reported that steam mass flow typically boosts power output by up to 30%. Withoutconsuming more fuel with a 15% reduction in plant heat rate 35..

13.4 WASTE HEAT RECOVERY UNITS

Waste heat recovery units (WHRU) are increasingly used offshore. These convert waste heatgenerated in the exhaust gases of the gas turbine for hot water, heating, process and other services. This is achieved by integrating a WHRU heat exchanger unit within the exhaustsystem of the gas turbine.

13.5 COMBINED CYCLE GAS TURBINES

Combined cycle gas turbines CCGTs combine a gas turbine with a steam turbine used for secondary power generation

14. The heat generated from the gas turbine is used to produce steam

for the steam turbine. CCGTs therefore have greater efficiency than conventional gas turbines.CCCGTs are more commonly found in power stations than offshore installations. Daily cyclingand weekend shutdowns can reduce component life.



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Figure 56 Advanced combined cycle gas turbine system configuration. Courtesy GE36.

The use of combined cycle is usually associated with larger turbines such as the GE Frame 5 15-18MW to very large gas turbines (>100MW) in conventional utilities. Smaller CCGTs areavailable, for example in the 5-50MW range. The additional topside weight and space necessaryto incorporate an additional steam turbine could limit application offshore.

Combined cycle gas turbines are more complex than conventional GTs. This change in regimeand complexity causes:

x Lower life in nozzles and blades (average 25,000h compared with 40-45,000h previously)

x Higher degradation rate, typically 5-7% in first 10,000h

x High thermal efficiency 45-60%

x Lower availability, typically 10% less –10% ~80%)

Sources of downtime have been summarised as:

x <200MW Turbine 53%, Compressors 30%, Rotor, Auxiliary, Combustors 30%

x >200MW Turbine 28%, Compressor 28%



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Figure 57 Cycle diagram for a combined cycle gas turbine (CCGT) showing steamturbine in axial line with the gas turbine. Courtesy GE power 36

In the conventional power industry manufacturers pay penalties ($Ms) on not meeting power and heating rates. This is aggravated by the instability of low NOX combustors. For CGGTs the

use of Long Term Service Agreements (LTSA) are a future trend. LTSA may be necessary toget financing and insurance cover. The driving forces are: gas turbines pushing designenvelopes, limited operational history, limited parts availability, high degradation rate.



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14 OPERATIONAL SUPPORT GUIDANCE

Guidance on operational activities which will have an effect on the safety and reliable operationof rotating equipment is given in HSE research report RR076 including gas turbine packages.This includes delineation of factors which may indicate that the equipment is being wellmaintained or there are deficiencies. Summary tables are included for major packages to beused on site visits in reviewing the installation.

To avoid confusion this guidance note does not propose a separate system for review of gasturbines and the inspectors and readers are referred to RR076 for the details of the review

process. Some indicators from RR076 that are relevant to gas turbines are reproduced below.Additional indicators that may be indicative of good practice in operation and maintenance of gas turbines are given in Section 15.



1 2 4

T a b l e 8 T h e t a b l e g i v e s e x a m p l e s o f o b s e r v a t i o n s w h i c h w o u l d i n d i c a t e n o t b e s t p r a c t i c e i n t h e

o p e r a t i o n / m a i n t e n a n c e o f a g a s t u r b i n e .

S o u r c e H S E R e s e a r c h R e p o r t R R 0 7 6

I T E

M

T O P I C

K E Y O B S E R V A T I O N

I M P A C T

I N F E R E N C E

A C T I O N

E Q U I P M E N T

V i b r a t i o n

V i b r a t i o n m o n i t o r s n o t

f u n c t i o n i n g , o r a l a r m s i n

S i g n i f i c a n t v i b r a t i o n c a n

b e f e l t

M

a c h i n e d a m a g e , d a m a g e

t o

f u e l l i n e s , f u e l r e l e a s e s .

B

l a d e f a i l u r e s .

O p e r a t o r s n o t p a y i n g a t t e n t i o n t o

R e v i e w v i b r a t i o n m o n i t o r i n g

v i b r a t i o n

l e v e l s , n o t a w a r e o f

p o t e n t i a l d a m a g e

R e v i e w v i b r a t i o n m o n i t o r i n g

p o l i c y / p r a c t i c e

F u e l

H i g h f u e l p r e s s u r e s ( i f

v i s i b l e o n l o c a l g a u g e s )

I n c r e a s e d r i s k o f p i p e o r

j o

i n t f a i l u r e s ( p a r t i c u l a r l y

f l e x i b l e )

F u e l n o z

z l e r e s t r i c t i o n s o r e x c e s s

f u e l f l o w

C h e c k f u

e l p r e s s u r e v s .

v e n d o r f u e l f l o w . C

h e c k

o p e r a t i n g l o g s

M A I N T E N A N C E

V e n t i l a t i o n

A c o u s t i c e n c l o s u r e

v e n t i l a t i o n l o u v e r s v e r y

d i r t y f l a m m a b l e m i x t u r e .

G a s o r o b s c u r e d

O

v e r - h e a t i n g , r i s k o f

f l a m m a b l e m i x t u r e

d e t e c t i o n i n e f f e c t i v e

E f f e c t i v e n e s s o f v e n t i l a t i o n i s n o t

c h e c k e d

C a r r y o u

t a i r f l o w t e s t

V e n t i l a t i o n

E n c l o s u r e d o o r s l e f t o p e n .

P a n e l s r e m o v e d

W

r o n g v e n t i l a t i o n p a t t e r n ,

o v e r h e a t i n g , g a s d e t e c t i o n

i n

e f f e c t i v e . N o i s e

e m i s s i o n s

O p e r a t o r s a r e u n a w a r e o f t h e

p o t e n t i a l h a z a r d

R e v i e w o p e r a t i n g /

m a i n t e n a n c e p h i l o s o p h y

O i l l e a k a g e

O i l a b s o r b e n t p a d s a r o u n d

e n c l o s u r e b a s e

F u e l o r l u b r i c a n t l e a k a g e ?

P o o l f i r e s ? F u e l m i s t ?

O p e r a t o r s u n a b l e t o c o n t a i n f u e l

l e a k

I d e n t i f y f l u i d a n d s o u r c e .

P l a n r e m

e d i a l a c t i o n .

O i l l e a k a g e

O i l a b s o r b e n t p a d s a r o u n d

e n c l o s u r e b a s e

S l i p p e r y f l o o r – p e r s o n n e l

i n

j u r y

O p e r a t o r s h a v e n o t c o n s i d e r e d

r i s k o f i n

j u r y

I d e n t i f y f l u i d a n d s o u r c e .

P l a n r e m

e d i a l a c t i o n .

M o n i t o r

c o n d i t i o n o f f l o o r ,

w a r n i n g

s i g n s , b a r r i e r s

O P E

R A T I O N

A c o u s t i c e n c l o s u r e

V i e w i n g w i n d o w s d i r t y /

o b s c u r e d / n o i n t e r n a l

l i g h t i n g

C

a n n o t s e e i n s i d e

e n c l o s u r e t o c h e c k .

U

n n e c e s s a r y e n t r i e s .

I n c r e a s e d p e r s o n n e l r i s k

C h e c k s ,

i f m a d e , r e q u i r e e n t r y t o

e n c l o s u r e

R e v i e w o p e r a t i n g

i n s t r u c t i o n s - p l a n

i m p r o v e m e n t s

A c c e s s

E n c l o s u r e a c c e s s w i t h o u t

p e r m i t

I n c r e a s e d p e r s o n n e l r i s k .

S e a r c h p r o b l e m s i f t h e r e i s

a n i n c i d e n t .

A d d i t i o n

a l h a z a r d o f e n c l o s u r e

e n t r y i s n o t r e c o g n i s e d


i n s t r u c t i o n s & P T W c

o n t r o l



1 2 5

I T E

M

T O P I C

K E Y O B S E R V A T I O N

I M P A C T

I N F E R E N C E

A C T I O N

O P E

R A T I O N

P e r s o n n e l s a f e t y

A c o u s t i c e n c l o s u r e d o o r

c a n b e p a d l o c k e d

P o t e n t i a l t o t r a p p e r s o n n e l

i n

s i d e

T r a p p i n g

r i s k h a s n o t b e e n

r e c o g n i s e d

A s s e s s h

a z a r d , i d e n t i f y i f

a l t e r n a t i v e e s c a p e e x i s t s .

I n s t r u m e n t s

C o n t r o l p a n e l o r l o c a l

i n s t r u m e n t d i s p l a y s

i n a c c e s s i b l e / d i r t y /

d a m a g e d

O

p e r a t o r s d o n o t m a n a g e

e q u i p m e n t , f a u l t s d e v e l o p

u n - c h e c k e d . A l a r m s m i g h t

b e m i s s e d .

O p e r a t o r s d o n o t r o u t i n e l y c h e c k

t h e s e i n s

t r u m e n t s


i n s t r u c t i o n s - a r e i n s t r u m e n t s

n e c e s s a r y o r r e d u n d a n t .

R e i n s t a t e o r r e m o v e .

E x h a u s t

H i g h d i s c h a r g e

t e m p e r a t u r e ( a l a r m s i n o r

s c o r c h e d d u c t i n g )

C

r e e p f a i l u r e o f b l a d e s .

M

i s s i l e s . E x h a u s t d u c t i n g /

f l e x i b l e f a i l u r e

I n t e r n a l p r o b l e m s w i t h t u r b i n e ,

f u e l c o n t r o l p r o b l e m s

R e v i e w r e c e n t o p e r a t i n g

t e m p e r a t u r e a n d c o n d i t i o n

m o n i t o r i n g d a t a



127

15 EXAMPLES OF GOOD AND BEST PRACTICE

In this context good practice is defined as practice or action that would be expected by anyreasonably trained inspector to be done on an installation. Best practice covers procedures andoperation practice that goes beyond this.

HSE guidance document PM84 [1] covers the main safety factors that need to be considered inthe operation of gas turbines. This includes ventilation, access, fire prevention systems, surge

prevention, and electrical and control systems. PM84 notes that the guidance is not obligatoryto operators. Adherence to the advice in PM84, developed in working groups including users,operators, suppliers and HSE is seen as good practice. Conversely not following the guidancemay be indicative or poor practice or require justification.

There are many basic things that would be considered normal and part of good practice. Theseinclude access arrangements, use of monitoring systems, appropriate ventilation, checking for

leaks of gas, fuel or lubricant. These are necessary from a safety perspective and do notconstitute best practice. Specific indicators of lack of good practice for turbine packages takenfrom RR076 are summarised above in Table 8.

Gas turbines are specialist equipment and maintenance is usually managed by the supplier or specialist contractor under a maintenance agreement. Simple adherence to the recommendationsof such contractors is not in itself indicative of good practice. Best practice would be where theoperator takes and active interest in what has been done at maintenance and any failures or degradation found that may impact on future integrity. For example:

x What is the basis for any components which have been found defective beingleft in service and not replaced.

x Are the rejection criteria for defective components in accordance with offshore practice, where tighter definitions may be used than on onshore applications.

x What is the reason for upgrades or chances to design

x Where cracking has been found or failures have occurred; are these knownlimitations with a given turbine model or new. Are these a result of changes indesign, for example to blade profile or casing material.

From operation experience it is good practice to have technicians that know both disciplines:mechanical (propulsion) and electronic (control). It is highly beneficial to know both to

properly diagnose faults. Electromechanical and digital system experience is important. Dualtrade is beneficial

Maintenance manuals are the first port of call in any maintenance and inspection process.Things that an inspector would need to check for include:

x What is the kit

x Who provided it

x What documentation is available

x Amendment status

x Is the manufacturer aware any issues in this particular installation

x Are Manuals available and being used

x Updates

x Right air, fuel

x Are reasonable precautions being observed



128

A lack of familiarity of relevant platform personnel with these factors would be indicative of poor practice.

There are certain fundamentals in safe turbine operation. These include:

x Not blocking air intakes

x Fuel supply protected, inviolate

x No water (unless intended). water injection used in some GTs to enhance performance).

x Don't block exhausts

x If the GT needs lube oil it is stored in the right containers, recorded, right stuff.

x People operating know what to do

A lack of familiarity of relevant platform personnel with these factors would similarly beindicative of poor practice.

An example of best practice is where the dutyholder has procurement and design specification

documents that bring together best practice from their historical operating experience with gasturbines; see procurement example in Appendix 2. Such documents may also suggestamendments to API procurement standards based on the operators experience. Examples of relevant advice from such procurement and design documents reviewed in Appendix 2 include:

x Identify all changes which are not proven in similar machines produced over the last 5years or where less than 100 000 fired hours have been accumulated in all machines.

x Give attention to off-design conditions which may occur during start-up and shutdown procedures associated with the particular applications of the gas turbine.

x Consider spares availability. A spares inventory comprising either a recommendedrange of individual components or a complete gas generator and/or rotors, or acombination of both, will be dictated by the required plant availability. In some cases,holding a complete spare gas generator may be more economical in the longer term thanholding individual components.

x Consider gas and liquid fuel variability on the installation. Aero-derivative gas turbinesrequire premium gas and liquid fuels. If the gas turbine fuel may be a crude oil, residualfuel oil, very lean gas, refinery mix gas or a gas that is subject to changes of WobbeIndex of more than 10%, then industrial gas turbines may be preferable.

x The site conditions of elevation, humidity and ambient temperature should be taken into

consideration together with the type of fuel (gas/liquid) and combustors and the power requirements of the driven equipment in order to arrive at a realistic site-rated power (rating) of the gas turbine.

x Copper and its alloys shall not be used in the presence of hydrogen sulphides, acetylene,ammonia, ammonium chloride or mercury. Materials for components in contact withgas shall conform to NACE MR0175 if the level of H2S exceeds the levels specifiedtherein.

x The location of the combustion air intake shall be carefully selected so as not to shortenthe life of the gas turbine. Satisfactory access shall be provided and no undue hazardshall be created. If flammable gasses are detected in the combustion air inlet, thesafeguarding system shall shut down the gas turbine.



129

x The combustion air intakes should be as close to the gas turbine as possible, tominimise cost and any power reduction due to pressure loss. The intake shall be locatedin a non-hazardous area or a zone 2 area;

x Air intakes should not be located in a zone 0 or a zone 1 area. The intake should not be

placed beneath a roof of any building within which flammable vapours mayaccumulate.

x Process equipment, pipe flanges and open drains should not be placed within 5 metresof the air intake. Careful consideration shall be given to the area classificationsurrounding the gas turbine installation.

x In marginal cases, it should be investigated whether identical fuels have been used byother operators and any specific design requirements determined, especially in relationto trace elements.

x Gas turbine hot parts are particularly sensitive to alkaline metals such as sodium and potassium. Other elements may have additional restrictions due to environmental

emission limits and the general corrosion requirements of downstream systems.

x Fuel condition. The possibility of liquid entrainment or condensate formation in the fuelgas supply should be avoided by system design. The system should be designed to

prevent this occurring under all conditions, in particular the formation of condensates infuel gas lines under idle conditions.

x Gas Turbine Washing. Advice on key points regarding turbine cleaning practice asidentified in Section 11.7.

Whilst the actual advice may vary between dutyholder and installation, the availability of such prior service information and inclusion in Dutyholder specifications is a sign of best practice.



131

16 LIST OF APPLICABLE GUIDANCE AND REGULATIONS

API 613 - Continuous Duty Gear

API 614 - Lube Oil System

API 616 - Gas Turbines

API 617 Centrifugal compressors for petroleum, chemical and gas service industries

API 617 - Compressors

API 670 - Machinery Protection

API 671 - Flexible Couplings

API 677 - Auxiliary Drive Gear

API RP 11 PGT Packaged combustion gas turbines

ASME B133 - Gas Turbines

ASME PTC 22 Gas turbine power plants

ASME PTC-10 Compressor Testing

ASME PTC-22 Gas Turbine Testing

ASTM D 2880 Specification for gas turbine fuel oils

ATEX Directives 94/9/EC Equipment in Hazardous Environments European Union (EU)

BS 5839: Part 1: 2002 Fire detection and alarm systems for buildings. Code of Practice for

system design, installation, commissioning and maintenance PM84-5

BS 7273: Parts 1-3 Code of Practice for the operation of fire protection measures PM84-6

BS 7273: Parts 1-3 Code of Practice for the operation of fire protection measures PM84-6

BS EN 50073: 1999 Guide for selection, installation, use and maintenance of apparatus for

the detection and measurement of combustible gases or oxygen PM84-11

BS EN 54: Parts 1-11 Fire detection and fire alarm systems PM84-4

BS EN 60079-10: 1996 Electrical apparatus for explosive gas atmospheres. Classification

of hazardous areas PM84-9

BS EN 61508: 2002 Parts 1-7 Functional safety of electrical/electronic programmable

electronic safety related systems PM84-12

BS EN60079-17:2003 British and European standard on electrical apparatus for explosivegas atmospheres; Part 17: Inspection and maintenance of electrical installations in

hazardous areas (other than mines)

BS ISO 14520: Parts 1-15, 2000 Gaseous fire extinguishing systems. PM84-2

BS5306-4: 2001 Fire extinguishing installations and equipment on premises - Part 4

Specification for carbon dioxide systems PM84-3

EEMUA 140 Noise procedure specification. British Standard.

EU Emissions Trading Scheme Regulations 2005

HSE L101 Control and mitigation measures. Dangerous Substances and Explosive

Atmospheres Regulations 2002. Approved Code of Practice and guidance L101 HSE Books

1997 ISBN 0 7176 1405 0 PM84-19



132

HSE L134 Design of plant, equipment and workplaces. Dangerous Substances and

Explosive Atmospheres Regulations 2002. Approved Code of Practice and guidance L134

HSE Books 2003 ISBN 0 7176 2199 5 PM84-18

HSE L138 Dangerous Substances and Explosive Atmospheres Regulations 2002. Approved

Code of Practice L138 HSE Books 2003 ISBN 0 7176 2203 7 (available from autumn 2003)PM84-17

IEC 61511: 2003 Functional safety -Safety instrumented systems for the process industry

sector - Part 1: Framework, definitions, system, hardware and software requirements

PM84-13

IGE SR/25 Hazardous area classification of natural gas installations Institution of Gas

Engineers and Managers PM84-20

IGE/UP/6 Application of positive displacement compressors to natural gas systems

Institution of Gas Engineers and Managers PM84-16

ISO 2324 Gas turbines - acceptance tests

L101 Safe work in confined spaces. Confined Spaces Regulations 1997. Approved Code of

Practice, Regulations and guidance Ll01 HSE Books 1997 ISBN 0 7176 1405 0 PM84-22

NACE MR0175 Sulphide stress cracking resistant metallic material for oil field equipment

NFPA 750:2000. 1 Water mist fire protection systems National Fire Protection Association

(NFPA) National Fire Codes 750:2000. 1 Water mist fire protection systems National

Fire Protection Association (NFPA) National Fire Codes 750:2000. PM84-1

ON58 HSE Offshore Division Operations Note 58 Dangerous Substances and Explosive

Atmospheres Regulations 2002 DSEAR - A short guide for the offshore industry Issue DateJan 2003

ON59 HSE Offshore Division operations Note 59 The Equipment and Protective Systems Intended for use in Potentially Explosive Atmospheres Regulations 1996 EPS - A short

guide for the offshore industry Issue Date Jan 2003

ON63 HSE Offshore Division Operations Notice 63 A Guide to the Equipment and

Protective Systems Intended for Use in Potentially Explosive Atmospheres Regulations 1996

Issue Date Dec 2003

PM84 Guidance Note PM84 Control of safety risks at gas turbines used for power

generation

SI 2005 No 925 The Greenhouse Gas Emissions Trading Scheme Regulations 2005, ISBN

0110727150 The Stationary Office Limited, EU Emissions Trading Scheme Regulations

2005 http://www.og.dti.gov.uk/environment/euetsr.htm



133

17 REFERENCES

1. PM84 Control of safety risks at gas turbines used for power generation , UK Health andSafety Executive HSE, ISBN 0-7176-2193-6, second edition 2003

2. RR076 Machinery and rotating equipment guidance notes, HSE Research Report 076http://www.hse.gov.uk/research/rrhtm/rr076.htm

3. Brun K and Kurz R Gas turbines in oil and gas applications ASME IGTI Turbo 2004Conference, Power for land, sea and air, Vienna Austria, 14-17 June 2004

4. Gas Turbine Theory, second edition, Cohen H, Rogers GFC and Saravanamuttoo,Longman Group Limited , ISBN 0 58244926 x cased, 11927 8 Paper, 4th impression 1977

5. The Jet Engine, Rolls Royce plc, ISBN 0 902121 04 9 (1986).

6. Fruchtal MAN Turbo, Modular approach to Gas Turbines paper M16 ASME IGTITurbo 2004 Conference, Power for land, sea and air, Vienna Austria, 14-17 June 2004

7. Kurz R and White R C Surge avoidance in gas compression systems, GT2004-53066,Proceedings ASME IGTI Turbo 2004 Conference, Power for land, sea and air, ViennaAustria, 14-17 June 2004

8. Elliot J GE Test and Instrumentation, Proceedings ASME IGTI Turbo 2004 Conference,Power for land, sea and air, Vienna Austria, 14-17 June 2004

9. Woodward Redundant Network Controls for Industrial Turbines 14.30 GT2004 53946Proceedings ASME IGTI Turbo 2004 Conference, Power for land, sea and air, ViennaAustria, 14-17 June 2004

10. Woodward Controls, development, design and testing of a standard gas turbine control15:00h Proceedings ASME IGTI Turbo 2004 Conference, Power for land, sea and air,

Vienna Austria, 14-17 June 2004

11. Failure analysis of Turbines Session G T56 Tutorial session, ASME IGTI Turbo 2004Conference, Power for land, sea and air, Vienna Austria, 14-17 June 2004

12. Ludwig M Materials in gas turbines IGTI Session T56 Room G 14:00, ASME IGTITurbo 2004 Conference, Power for land, sea and air, Vienna Austria, 14-17 June 2004

13. Garside R Electrical apparatus and hazardous areas 4th Edition, Published HexagonTechnology Limited, Aylesbury, K ISBN 0 9516848 3 3, 2002

14. Combined Cycle Gas turbines, Paper W56, ASME IGTI Turbo 2004 Conference, Power for land, sea and air, Vienna Austria, 14-17 June 2004

15. Microturbines for Power Generation, Turbo2004 Session K Monday 15.30, ASME IGTITurbo 2004 Conference, Power for land, sea and air, Vienna Austria, 14-17 June 2004

16. Brun K A Novel Centrifugal Flow Gas Turbine Design Paper GT2004-53063 ASMEIGTI Turbo 2004 Conference, Power for land, sea and air, Vienna Austria, 14-17 June2004

17. Gas turbine cleaning IGTI Paper TH33 Cranfield University ASME IGTI Turbo 2004Conference, Power for land, sea and air , Vienna Austria, 14-17 June 2004

18. Standards and codes of practice for hazardous areas, Simplex

19. Water mist fire protection systems National Fire Protection Association (NFPA) NationalFire Codes 750:2000. 1 Water mist fire protection systems National Fire Protection

Association (NFPA) National Fire Codes 750:2000. PM84-1



134

20. Application of positive displacement compressors to natural gas systems IGE/UP/6Institution of Gas Engineers and Managers PM84-16

21. Control and mitigation measures. Dangerous Substances and Explosive Atmospheres

Regulations 2002. Approved Code of Practice and guidance L101 HSE Books 1997

ISBN 0 7176 1405 0 PM84-19

22. Design of plant, equipment and workplaces. Dangerous Substances and Explosive

Atmospheres Regulations 2002. Approved Code of Practice and guidance L134 HSEBooks 2003 ISBN 0 7176 2199 5 PM84-18

23. Dangerous Substances and Explosive Atmospheres Regulations 2002. Approved Code of

Practice L138 HSE Books 2003 ISBN 0 7176 2203 7 (available from autumn 2003)PM84-17

24. Hazardous area classification of natural gas installations IGE SR/25 Institution of GasEngineers and Managers PM84-20

25. Safe work in confined spaces. Confined Spaces Regulations 1997. Approved Code of

Practice, Regulations and guidance Ll01 HSE Books 1997 ISBN 0 7176 1405 0 PM84-22

26. Explosive Atmospheres – Classification of Hazardous Areas (Zoning) and selection of

Equipment HSE www.hse.gov.uk/comah/sragtech/techmeasareaclas.htm

27. Garside R Electrical apparatus in hazardous areas, ISBN 0 9516848 3 3, 4th Edition,2002, Pub Hexagon Technology Limited, Aylesbury, UK.

28. Simplex - An introduction and basic guidance for the selection, installation and

utilisation of apparatus in potentially hazardous atmospheres, Allenwest electrical, AEL406 Nov (1993)

29. ATEX Directives and their application to gas turbines European Commissionhttp://europa.eu.int/comm/enterprise/atex/gasturbines.htm PM84-7

30. Santon R C, CJ Lea, Lewis M J, Pritchard D K, Thyer A M and Sinai Y Studies into the

role of ventilation and the consequences of leaks in gas turbine power plant acoustic

enclosures and turbine halls Trans IChemE Vol 78 Part B May 2000 175-183 PM84-8

31. Santon R C Explosion hazards at gas turbine driven power plants ASME 98-GT-215PM84-10

32. Board statement on restrictions on human exposure to static and time varying

electromagnetic fields and radiation Documents of the NRPB1993 4 (5) PM84-21

33. DA Farthing, L Marley and JA Lees Operational safety and post-maintenance gas leak

detection in GE frame 9001FA gas turbines Proc Instn Mech Engnrs Vol 213 Part A 465-474 PM8-15

34. The application of natural gas fuel systems to gas turbines and supplementary and

auxiliary burners IGE/UP/9 Institution of Gas Engineers and Managers (first revision published July 2003) PM84-14

35. De Biasi V Steam-fuel mix limits Nox and CO below 3 ppm without DLN or SCR GasTurbine World, October-November 2004, pp24-28

36. Smith, R et al, Advanced Technology Combined Cycles, GE Power Systems, Report GER 3936A http://www.gepower.com

37. Sinfield K D Industrial gas turbine development and operating experience IMIA 16-59(93)E IMIA Conference, September 1993

http://www.imia.com/documents/development_and_operating_experience.htm



135

38. Gas Turbine World 2005 Performance Specs, Gas Turbine Power Plant Ratings for

Project planning, Engineering Design and Procurement , 2005 GTW Specifications 23rd

Edition, January 2005 Vol 34 No 6



A-1

APPENDICES

Appendix 1 List of UK installations A1

Appendix 2 Typical procurement package technical specification A2

Appendix 3 HSE guidance note PM84 on gas turbines A3

Appendix 4 Gas turbine suppliers and summary for UK installations A4

Appendix 5 Specification of turbines used in UK sector A5

Appendix 6 Key systems and components A6



A1-1

APPENDIX 1 LIST OF UK INSTALLATIONS

The installations in UK waters can change particularly for mobile and floating installations (FPS,FPSO). This list summarises the position at April 2004.

Installation Dutyholder Fixed/

Mobile

Type of

Fixed

Type of

Mobile

Reg

No.

Location

AH001 AMERADA HESS F FP N/A 246 UK ALBA FSU CHEVRON F FSU N/A 425 UK ALBA NORTHERN CHEVRON F F N/A 409 UK ALWYN NORTH TOTAL E&P UK PLC F F N/A 290 UK AMETHYST BP SNS (N) F NUI N/A 372 UK ANASURIA SHELL U.K. (CENTRAL) F FPSO N/A 500 UK ANDREW BP MBU F F N/A 483 UK ANGLIA A GAZ DE FRANCE F F N/A 408 UK ANGLIA B GAZ DE FRANCE F SUBSEA N/A 0 UK ARBROATH PETROFAC

PRODUCTIONSERVICES

F F N/A 354 UK

ARCH ROWAN ROWAN DRILLING (UK)LTD

M N/A JU 188 USA

ARDMORE(ROWAN GORILLAVII)

ROWAN DRILLING (UK)LTD

F F N/A 603 UK

ARMADA BG INTERNATIONAL F F N/A 478 UK AUDREY PWD49/11A

CONOCO PHILLIPS F NUI N/A 334 UK

AUK A SHELL U.K. (CENTRAL) F F N/A 88 UK BAE 6 TOWERSAIR

BAE F F N/A 0 UK

BALMORAL ENI F FP N/A 301 UK BAR 331 SAIPEM M N/A MH 0 NLBAR PROTECTOR SAIPEM M N/A MH 578 UK BARQUE PB 48/13A SHELL U.K. SOUTHERN

OPSF NUI N/A 364 UK

BARQUE PL 48/14P SHELL U.K. SOUTHERNOPS

F F N/A 470 UK

BEATRICE A TALISMAN ENERGY(UK) LIMITED

F F N/A 160 UK

BEATRICE B TALISMAN ENERGY(UK) LIMITED

F F N/A 161 UK

BEATRICE C TALISMAN ENERGY(UK) LIMITED

F F N/A 271 UK

BELLWELL CONOCO PHILLIPS F SUBSEA N/A 0 UK BERYL A MOBIL NORTH SEA

LIMITEDF F N/A 95 UK

BERYL B MOBIL NORTH SEALIMITED

F F N/A 174 UK

BESSEMER PERENCO UK LIMITED F NUI N/A 490 UK BLEO HOLM BLUEWATER

ENGINEERINGF FPSO N/A 520 UK

BORGHOLMDOLPHIN

DOLPHIN DRILLINGCOMPANY

M N/A SS 341 UK

BORGILADOLPHIN


M N/A SS 582 UK

BORGNY DOLPHIN DOLPHIN DRILLINGCOMPANY

M N/A SS 157 UK

BORGSTEN DOLPHIN DRILLING M N/A SS 170 UK



A1-2


Mobile

Type of

Fixed

Type of

Mobile

Reg

No.

Location

DOLPHIN COMPANYBOULTON CONOCO PHILLIPS F NUI N/A 511 UK BRAE A MARATHON OIL (UK)

LIMITEDF F N/A 192 UK

BRAE B MARATHON OIL (UK)LIMITED

F F N/A 332 UK

BRENT A SHELL U.K. NORTHERNOPS

F F N/A 122 UK

BRENT B SHELL U.K. NORTHERNOPS

F F N/A 107 UK

BRENT C SHELL U.K. NORTHERNOPS

F F N/A 137 UK

BRENT D SHELL U.K. NORTHERNOPS

F F N/A 124 UK

BRIGANTINE BG SHELL U.K. SOUTHERNOPS

F NUI N/A 548 UK

BRIGANTINE BR SHELL U.K. SOUTHERN

OPS

F NUI N/A 554 UK

BRITANNIA BOL (BRITANNIAOPERATOR LIMITED)

F F N/A 489 UK

BRUCE BP (DBU) F F N/A 430 UK BUCHAN A TALISMAN ENERGY

(UK) LIMITEDF FP N/A 89 UK

BULFORDDOLPHIN


M N/A SS 304 UK

BUZZARD FIELD ENCANA (U.K) LIMITED F F N/A 6067 UK BYFORD DOLPHIN DOLPHIN DRILLING

COMPANYM N/A SS 171 NOR

CAISTER 44/23ACM (MURDOCH

FIELD)


CAMELOT CA MOBIL NORTH SEA F NUI N/A 362 UK CAMELOT CB MOBIL NORTH SEA F F N/A 435 UK CAPTAIN TEXACO F FPSO N/A 495 UK

CARRACK A SHELL U.K. SOUTHERNOPS

F F N/A 576 UK

CASTORO 10 SAIPEM M N/A MH 900034 UK CASTORO SEI SAIPEM M N/A SS 9188 UK CECIL PROVINE ROWAN DRILLING (UK)

LTDM N/A JU 200 USA

CENTRAL BRAE MARATHON OIL (UK)LIMITED

F F N/A 377 UK

CHARLES ROWAN ROWAN DRILLING (UK)LTD M N/A JU 182 USA

CLAIR BP (DBU) F F N/A 6020 UK CLAYMORE TALISMAN ENERGY

(UK) LIMITEDF F N/A 120 UK

CLEETON P/Q BP SNS (N) F F N/A 319 UK CLYDE TALISMAN ENERGY


CORMORANT A SHELL U.K. NORTHERNOPS

F F N/A 138 UK

CORVETTE A SHELL U.K. SOUTHERNOPS

F NUI N/A 522 UK

CRYSTAL OCEAN BROVIG M N/A MH 542 UNKNOWN

CRYSTAL SEA BROVIG M N/A MH 552 UNKNOWNCSO ALLIANCE TECHNIP OFFSHORE M N/A MH 579 VARIOUSCSO APACHE TECHNIP OFFSHORE M N/A MH 0 VARIOUS



A1-3


Mobile

Type of

Fixed

Type of

Mobile

Reg

No.

Location

CSOCONSTRUCTOR

TECHNIP OFFSHORE M N/A SS 507 VARIOUS

CSO INSTALLER TECHNIP OFFSHORE M N/A SS 0 VARIOUSDAVY PERENCO UK LIMITED F NUI N/A 491 UK

DEEPSEA BERGEN ODFJELL M N/A SS 462 UK DEEPSEA DELTA ODFJELL M N/A SS 580 UK DEEPSEA TRYM ODFJELL M N/A SS 562 UK DOUGLAS (LBA) BHP BILLITON F F N/A 465 UK DSND MAYO SUBSEA 7 (UK) M N/A MH 0 VARIOUSDSND PELICAN SUBSEA 7 (UK) M N/A MH 900054 VARIOUSDUNBAR TOTAL E&P UK PLC F F N/A 447 UK DUNLIN A SHELL U.K. NORTHERN

OPSF F N/A 136 UK

EAST BRAE MARATHON OIL (UK)LIMITED

F F N/A 454 UK

EIDER SHELL U.K. NORTHERNOPS

F F N/A 350 UK

ELGIN FRANKLIN TOTAL E&P UK PLC F F N/A 540 UK ENSCO 100 ENSCO M N/A JU 331 NLENSCO 101 ENSCO M N/A JU 543 NLENSCO 102 ENSCO M N/A JU 568 UK ENSCO 70 ENSCO M N/A JU 363 NLENSCO 71 ENSCO M N/A JU 309 NLENSCO 72 ENSCO M N/A JU 289 UK ENSCO 80 ENSCO M N/A JU 176 UK ENSCO 85 ENSCO M N/A JU 194 UK ENSCO 92 ENSCO M N/A JU 202 UK ERSKINE TEXACO F F N/A 504 UK ETAP BP MBU F F N/A 512 UK F G McCLINTOCK TRANSOCEAN SEDCO

FOREX

M N/A JU 317 WHE

FORTIES A APACHE NORTH SEALIMITED

F F N/A 76 UK

FORTIES B APACHE NORTH SEALIMITED

F F N/A 103 UK

FORTIES C APACHE NORTH SEALIMITED

F F N/A 82 UK

FORTIES D APACHE NORTH SEALIMITED

F F N/A 104 UK

FORTIES E APACHE NORTH SEALIMITED

F F N/A 314 UK

FRIGG CDPI TOTAL E&P NORGE AS F F N/A 108 UK FULMAR A SHELL U.K. (CENTRAL) F F N/A 152 UK

GALAXY I GLOBAL SANTA FEDRILLING

M N/A JU 420 UK

GALAXY II GLOBAL SANTA FEDRILLING

M N/A JU 518 CAN

GALAXY III GLOBAL SANTA FEDRILLING

M N/A JU 538 UK

GALLEON 48/20PN SHELL U.K. SOUTHERNOPS

F F N/A 455 UK

GALLEON PG SHELL U.K. SOUTHERNOPS

F F N/A 477 UK

GANNET A SHELL U.K. (CENTRAL) F F N/A 384 UK GLAS DOWR BLUEWATER

ENGINEERINGF FPSO N/A 503 AFR

GLOBALPRODUCER III

KERR MCGEE F FPSO N/A 555 UK



A1-4


Mobile

Type of

Fixed

Type of

Mobile

Reg

No.

Location

(LEADON)GLOMAR ADRIATIC IV

GLOBAL SANTA FEDRILLING

M N/A JU 524 USA

GLOMAR

ADRIATIC VI

GLOBAL SANTA FE

DRILLING

M N/A JU 257 UK

GLOMAR ADRIATIC VII


M N/A JU 308 USA

GLOMAR ADRIATIC XI


M N/A JU 214 UK

GLOMAR ARCTIC I GLOBAL SANTA FEDRILLING

M N/A SS 256 USA

GLOMAR ARCTICII


M N/A SS 0 UK

GLOMAR ARCTICIII


M N/A SS 281 UK

GLOMAR ARCTICIV


M N/A SS 244 UK

GLOMAR BALTIC I GLOBAL SANTA FEDRILLING

M N/A JU 380 USA

GLOMAR GRANDBANKS


M N/A SS 299 CAN

GLOMAR LABRADOR I


M N/A JU 300 WHE

GLOMAR NORTHSEA


M N/A SS 204 CAN

GOLDENEYE SHELL U.K. (CENTRAL) F F N/A 4022 UK GRYPHON A KERR MCGEE F FPSO N/A 448 UK HAEWENE BRIM BLUEWATER


HAMILTON (LBA) BHP BILLITON F NUI N/A 468 UK

HAMILTON NORTH (LBA)

BHP BILLITON F NUI N/A 467 UK

HARDING FIELD BP MBU F F N/A 476 UK HEATHER ALPHA DNO HEATHER LTD F F N/A 144 UK HENRYGOODRICH

TRANSOCEAN SEDCOFOREX

M N/A SS 333 CAN

HEWETT FIELD PETROFACPRODUCTIONSERVICES

F F N/A 11 UK

HOTON BP SNS (N) F F N/A 560 UK HYDE 48/6 BP SNS (N) F NUI N/A 446 UK INDE 49/18A PERENCO UK LIMITED F NUI N/A 2 UK INDE 49/18B PERENCO UK LIMITED F NUI N/A 59 UK

INDE 49/23A PERENCO UK LIMITED F F N/A 3 UK INDE 49/23C PERENCO UK LIMITED F F N/A 123 UK INDE 49/23D PERENCO UK LIMITED F NUI N/A 368 UK INDE 49/24J SHELL U.K. SOUTHERN

OPSF F N/A 17 UK

INDE 49/24K SHELL U.K. SOUTHERNOPS

F F N/A 18 UK

INDE 49/24L SHELL U.K. SOUTHERNOPS

F F N/A 145 UK

INDE 49/24M SHELL U.K. SOUTHERNOPS

F F N/A 528 UK

INDE 49/24N SHELL U.K. SOUTHERNOPS

F F N/A 529 UK

IOLAIR TRANSOCEAN SEDCOFOREX

M N/A SS 241 NOR



A1-5


Mobile

Type of

Fixed

Type of

Mobile

Reg

No.

Location

IRISH SEAPIONEER

HALLIBURTONMANUFACTURING &SERVICES LTD

M N/A JU 494 UK

J W MCLEAN TRANSOCEAN SEDCO

FOREX

M N/A SS 70 UK

JACK BATES TRANSOCEAN SEDCOFOREX

M N/A SS 508 UK

JADE PHILLIPS NORTHERNOPS

F F N/A 558 UK

JANICE KERR MCGEE F FPSO N/A 523 UK JOHN SHAW TRANSOCEAN SEDCO

FOREXM N/A SS 388 UK

JUDY-JOANNE PHILLIPS NORTHERNOPS

F F N/A 449 UK

JUNO MINERVA BP SNS (N) F F N/A 566 UK KAN TAN IV MAERSK COMPANY

LIMITEDM N/A SS 248 WHE

KETCH A SHELL U.K. SOUTHERNOPS

F F N/A 531 UK

KITTIWAKE SHELL U.K. (CENTRAL) F F N/A 378 UK KOMMANDOR SUBSEA

SUBSEA 7 (UK) M MH MH 0 VARIOUS

LAPS FIELD MOBIL NORTH SEA F F N/A 8 UK LEIV EIRIKSSON OCEAN RIG LIMITED M N/A SS 549 NOR LEMAN 49/26A SHELL U.K. SOUTHERN

OPSF F N/A 12 UK

LEMAN 49/27A PERENCO UK LIMITED F F N/A 4 UK LEMAN 49/27B PERENCO UK LIMITED F F N/A 5 UK LEMAN 49/27C PERENCO UK LIMITED F NUI N/A 6 UK LEMAN 49/27D PERENCO UK LIMITED F NUI N/A 7 UK

LEMAN 49/27E PERENCO UK LIMITED F NUI N/A 22 UK LEMAN 49/27F PERENCO UK LIMITED F NUI N/A 58 UK LEMAN 49/27G PERENCO UK LIMITED F NUI N/A 287 UK LEMAN 49/27H PERENCO UK LIMITED F NUI N/A 253 UK LEMAN 49/27J PERENCO UK LIMITED F NUI N/A 254 UK LEMAN B 49/26B SHELL U.K. SOUTHERN

OPSF F N/A 13 UK

LEMAN BT 49/26B SHELL U.K. SOUTHERNOPS

F NUI N/A 16 UK

LEMAN C 49/26C SHELL U.K. SOUTHERNOPS

F F N/A 14 UK

LEMAN D 49/26D SHELL U.K. SOUTHERNOPS

F F N/A 15 UK

LEMAN E 49/26E SHELL U.K. SOUTHERNOPS

F F N/A 189 UK

LEMAN F 49/26F SHELL U.K. SOUTHERNOPS

F F N/A 284 UK

LEMAN G 49/26G SHELL U.K. SOUTHERNOPS

F F N/A 285 UK

LENNOX (LBA) BHP BILLITON F NUI N/A 466 UK LOGGS CENTRAL CONOCO PHILLIPS F NUI N/A 326 UK LOGGSSATELLITES


LOMOND BP MBU F F N/A 418 UK LORELAY ALLSEAS M N/A MH 0 VARIOUSLYELL KERR MCGEE F SUBSEA N/A 422 UK

MAERSK CURLEW MAERSK COMPANYLIMITED

F FPSO N/A 510 UK



A1-6


Mobile

Type of

Fixed

Type of

Mobile

Reg

No.

Location

MAERSK ENDEAVOUR

MAERSK COMPANYLIMITED

M N/A JU 211 DK

MAERSK ENDURER


M N/A JU 506 UK

MAERSK ENHANCER


M N/A JU 247 UK

MAERSK EXERTER


M N/A JU 539 DK

MAERSK GALLANT


M N/A JU 439 UK

MAERSK GIANT MAERSK COMPANYLIMITED

M N/A JU 292 NOR

MAERSK GUARDIAN


M N/A JU 293 DK

MAERSK INNOVATOR


M N/A JU 581 UK

MAERSK

JUTLANDER

MAERSK COMPANY

LIMITED

M N/A SS 371 NOR

MAGELLAN GLOBAL SANTA FEDRILLING

M N/A JU 438 UK

MAGNUS 211/12 BP (DBU) F F N/A 203 UK MARIANOS TECHNIP OFFSHORE M N/A SS 575 VARIOUSMARKHAM ST1 LASMO NETHERLANDS

BVF F N/A 456 UK

MCP 01 TOTAL E&P UK PLC F F N/A 119 UK MILLER BP MBU F F N/A 369 UK MONARCH GLOBAL SANTA FE

DRILLINGM N/A JU 346 UK

MONITOR GLOBAL SANTA FEDRILLING

M N/A JU 406 UK

MONTROSE PETROFACPRODUCTIONSERVICES

F F N/A 111 UK

MORECAMBE BAY BRITISH GASHYDROCARBONRESOURCES LIMITED

F F N/A 340 UK

MPSV SHELL U.K. SOUTHERNOPS

M N/A JU 5023 UK

MSV REGALIA PROSAFE OFFSHORELTD

M N/A SS 288 NOR

MURCHISON211/19

CNR (CANADIAN NATIONAL RESOURCE)

F F N/A 158 UK

MURDOCH

COMPLEX

CONOCO PHILLIPS F F N/A 565 UK

NAVIS EXPLORER DOLPHIN DRILLINGCOMPANY

M N/A DS 567 UK

NELSON SHELL U.K. (CENTRAL) F F N/A 407 UK NEPTUNE BP SNS (N) F NUI N/A 537 UK NINIAN CENTRAL CNR (CANADIAN

NATIONAL RESOURCE)F F N/A 153 UK

NINIAN NORTHERN


F F N/A 151 UK

NINIANSOUTHERN


F F N/A 141 UK

NOBLE AL WHITE NOBLE DRILLING M N/A SS 252 UK NOBLE GEORGE

SAUVAGEAU

NOBLE DRILLING M N/A JU 458 UK

NOBLE JULIE NOBLE DRILLING M N/A JU 533 NL



A1-7


Mobile

Type of

Fixed

Type of

Mobile

Reg

No.

Location

ROBERTSON NOBLE LYNDABOSSLER


NOBLE PIET VAN

EDE

NOBLE DRILLING M N/A JU 342 UNKNOWN

NOBLE RONALDHOOPE


NOBLE TON VANLANGEVELD

NOBLE DRILLING M N/A SS 414 UK

NORDIC APOLLO UGLAND STENASTORAGE

F FPSO N/A 547 UK

NORPIPE PHILLIPS NORWAY F F N/A 226 UK NORTHCORMORANT

SHELL U.K. NORTHERNOPS

F F N/A 183 UK

NORTH EVEREST BP MBU F F N/A 417 UK NORTH SEAPRODUCER

NORTH SEAPRODUCTION

F FPSO N/A 499 UK

NORTH WESTHUTTON

BP (DBU) F F N/A 187 UK

NORTHERNPRODUCER

PETROFACPRODUCTIONSERVICES

F FPSO N/A 167 UK

OCEAN ALLIANCE DIAMOND OFFSHOREDRILLING

M N/A SS 359 NOR

OCEAN AMERICA DIAMOND OFFSHOREDRILLING

M N/A SS 45 USA

OCEANGUARDIAN

DIAMOND OFFSHOREDRILLING

M N/A SS 282 UK

OCEAN NOMAD DIAMOND OFFSHOREDRILLING

M N/A SS 264 UK

OCEAN PRINCESS DIAMOND OFFSHOREDRILLING

M N/A SS 218 UK

OCEAN RIG 2 OCEAN RIG LIMITED M N/A SS 550 WHEOCEAN VALIANT DIAMOND OFFSHORE

DRILLINGM N/A SS 385 AFR

OCEANVANGUARD

DIAMOND OFFSHOREDRILLING

M N/A SS 452 UK

OCEAN VICTORY DIAMOND OFFSHOREDRILLING

M N/A SS 42 USA

ORELIA TECHNIP OFFSHORE M N/A DSV 266 VARIOUSOSI (LBA) BHP BILLITON F FSU N/A 480 UK PAUL B LOYDJUNIOR


M N/A SS 398 UK

PETROJARLFOINAVEN

PGS PRODUCTION AS F FPSO N/A 486 UK

PETROJARL I PGS PRODUCTION AS F FPSO N/A 352 NOR PETROLIA PETROLIA DRILLING

LTDM N/A SS 242 UK

PICKERILL PERENCO UK LIMITED F F N/A 401 UK PIPER B TALISMAN ENERGY


POLYCONCORD RASMUSSEN A/S M N/A SS 219 UK POLYCONFIDENCE

RASMUSSEN A/S M N/A SS 374 USA

PORT REGENCY RASMUSSEN A/S M N/A SS 215 UNKNOWNPORT RIGMAR PORT RIGMAR AS M N/A JU 492 NOR

PRIDE NORTHATLANTIC

PRIDE NORTH SEA LTD M N/A SS 208 UK



A1-8


Mobile

Type of

Fixed

Type of

Mobile

Reg

No.

Location

PRIDE NORTH SEA PRIDE NORTH SEA LTD M N/A SS 112 UK PUFFIN SHELL U.K. (CENTRAL) F F N/A 6026 UK RAMFORM BANFF PGS PRODUCTION AS F FPSO N/A 525 UK RAVENSPURN

NORTH

BP SNS (N) F F N/A 356 UK

RAVENSPURN NORTH ST2 & ST3

BP SNS (N) F NUI N/A 357 UK

RAVENSPURNSOUTH

BP SNS (N) F NUI N/A 320 UK

ROCKWATER 1 SUBSEA 7 (UK) M MH MH 0 VARIOUSROUGH FIELD CENTRICA STORAGE

LTDF F N/A 79 UK

ROWANCALIFORNIA


M N/A JU 272 USA

ROWAN GORILLAII


M N/A JU 283 USA

ROWAN GORILLA

III

ROWAN DRILLING (UK)

LTD

M N/A JU 559 USA

ROWAN GORILLAIV


M N/A JU 358 USA

ROWAN GORILLAV


M N/A JU 526 CAN

ROWAN GORILLAVI


M N/A JU 544 USA

ROWAN GORILLAVII


M N/A JU 545 UK

ROWAN HALIFAX ROWAN DRILLING (UK)LTD

M N/A JU 237 USA

S7000 SAIPEM M N/A SS 347 VARIOUSSAFE BRITANNIA PROSAFE OFFSHORE

LTD

M N/A SS 217 NOR

SAFE CALEDONIA PROSAFE OFFSHORELTD

M N/A SS 213 NOR

SAFE LANCIA PROSAFE OFFSHORELTD

M N/A SS 255 UK

SAFESCANDINAVIA

PROSAFE OFFSHORELTD

M N/A SS 553 NOR

SALTIRE A TALISMAN ENERGY(UK) LIMITED

F F N/A 405 UK

SANTA FE 135 GLOBAL SANTA FEDRILLING

M N/A SS 245 UK

SANTA FE 140 GLOBAL SANTA FEDRILLING

M N/A SS 250 UK

SANTA FEBRITANNIA


M N/A JU 35 UK

SCARABEO 6 SAIPEM M N/A SS 280 UK SCHIEHALLION BP (DBU) F FPSO N/A 509 UK SCHOONER A SHELL U.K. SOUTHERN

OPSF F N/A 469 UK

SCOTT FIELD ENCANA (U.K) LIMITED F F N/A 434 UK SEAFOX 2 WORKFOX UK LTD M N/A JU 268 NLSEAFOX 3 WORKFOX UK LTD M N/A JU 259 NLSEAFOX 4 WORKFOX UK LTD M N/A JU 482 NLSEAN P 49/25A SHELL U.K. SOUTHERN

OPSF F N/A 279 UK

SEAN RD SHELL U.K. SOUTHERN

OPS

F F N/A 278 UK

SEAWAY STOLT OFFSHORE M N/A MH 0 VARIOUS



A1-9


Mobile

Type of

Fixed

Type of

Mobile

Reg

No.

Location

COMMANDER SEAWAY CONDOR STOLT OFFSHORE M N/A MH 900010 UNKNOWNSEAWAYDISCOVERY

STOLT OFFSHORE M N/A MH 574 VARIOUS

SEAWAY EAGLE STOLT OFFSHORE M N/A MH 0 VARIOUSSEAWAY FALCON STOLT OFFSHORE M N/A MH 573 VARIOUSSEAWAYKINGFISHER

STOLT OFFSHORE M N/A MH 0 VARIOUS

SEAWELL WELL OPERATORS UK LTD

M N/A MH 311 UK

SEDCO 704 TRANSOCEAN SEDCOFOREX

M N/A SS 83 UK


M N/A SS 394 UK


M N/A SS 220 UK

SEDCO 712 TRANSOCEAN SEDCO

FOREX

M N/A SS 276 UK


M N/A SS 258 UK

SEILLEAN TRANSOCEAN SEDCOFOREX

F FPSO N/A 383 WHE

SEMAC I SAIPEM M N/A SS 9002 UK SHEARWATER SHELL U.K. (CENTRAL) F F N/A 541 UK SHELF EXPLORER TRANSOCEAN SEDCO

FOREXM N/A JU 201 EUR

SKANDI NAVICA SUBSEA 7 (UK) M N/A MH 0 VARIOUSSKIFF PS SHELL U.K. SOUTHERN

OPSF NUI N/A 546 UK

SOLE PIT CLIPPER

48/19A

SHELL U.K. SOUTHERN

OPS

F F N/A 365 UK

SOLITAIRE ALLSEAS M N/A MH 0 VARIOUSSOVEREIGNEXPLORER


M N/A SS 261 UK

STANISLAVYUDIN

SEAWAY HEAVY LIFT M N/A MH 428 VARIOUS

STENA DEE STENA DRILLING LTD M N/A SS 318 UK STENA SPEY STENA DRILLING LTD M N/A SS 221 UK TARTAN A TALISMAN ENERGY


TERN A SHELL U.K. NORTHERNOPS

F F N/A 353 UK

THAMES A 49/28 MOBIL NORTH SEA F F N/A 306 UK

THIALF HEEREMA M N/A SS 349 VARIOUSTHISTLE A DNO THISTLE LTD F F N/A 125 UK TIFFANY ENI F F N/A 400 UK TOG MOR ALLSEAS M N/A MH 20 VARIOUSTOISA POLARIS SUBSEA 7 (UK) M N/A MH 0 VARIOUSTRANSOCEANARCTIC


M N/A SS 463 EUR

TRANSOCEANEXPLORER


M N/A SS 118 UK

TRANSOCEANLEADER


M N/A SS 481 UK

TRANSOCEAN NORDIC


M N/A JU 294 UK

TRANSOCEANPROSPECT


M N/A SS 572 UK



A1-10


Mobile

Type of

Fixed

Type of

Mobile

Reg

No.

Location

TRANSOCEANSEARCHER


M N/A SS 475 EUR

TRANSOCEANWILDKAT


M N/A SS 166 EUR

TRENCH SETTER ALLSEAS M N/A MH 21 VARIOUSTRENT 43/24 PERENCO UK LIMITED F NUI N/A 497 UK TRITON AMERADA HESS F FPSO N/A 536 UK TYNE PERENCO UK LIMITED F NUI N/A 498 UK UISGE GORM BLUEWATER


UNITY APACHE NORTH SEALIMITED

F F N/A 427 UK

VIKING 49/17B CONOCO PHILLIPS F F N/A 10 UK VIKINGSATELLITES


WAVENEY BP SNS (N) F F N/A 521 UK WELL SERVICER TECHNIP OFFSHORE M N/A SS 312 VARIOUS

WELLAND 53/4A MOBIL NORTH SEA F NUI N/A 393 UK WEST ALPHA SMEDVIG LTD M N/A SS 450 UNKNOWNWEST NAVION SMEDVIG LTD M N/A MH 557 UNKNOWNWEST SOLE A BP SNS (N) F F N/A 27 UK WEST SOLE B BP SNS (N) F NUI N/A 28 UK WEST SOLE C BP SNS (N) F NUI N/A 29 UK WEST SOLE

NEWSHAMBP SNS (N) F F N/A 0 UK

WINDERMERE RWE / DEA F F N/A 502 UK

Key

F Fixed

M Mobile

FPSO Floating, Production, Storage and Offloading Vessel

FSU Floating Storage Unit

MH Mono Hull

SS Semi- Submersible

JU Jack-Up

DSV Drilling Service Vessel

NUI Normally Unmanned Installation



A2-1

APPENDIX 2 TYPICAL PROCUREMENT PACKAGETECHNICAL SPECIFICATION

1.1 INTRODUCTION

This Appendix is intended to help inspectors be aware of the factors that may be consideredin procurement of a gas turbine for offshore use. The technical procurement specifications

for gas turbines offshore are difficult to obtain for commercial and practical reasons. Suchinformation is very detailed and confidential in nature. More importantly the engineersinvolved in production and maintenance of the installation usually different to those in theoriginal design team.

The design team would normally be brought together specifically for purpose of procurement and then disbanded once the installation is complete. Whilst Information

relevant to operation and safety would be retained, detailed technical information relating to procurement is normally archived and not easily accessible at a later date. Procurement of specific process or equipment packages may be undertaken in-house or sub-contracted out toa packager or design house. For these reasons it did not prove straightforward to accesstechnical procurement information during the project.

Procurement and design of gas turbines for operation in the UK sector is usually based onthe American API design codes. These are well developed and include standard data formsthat provide the basis for procurement. For gas turbine applications in the oil & gas sector,API 616 is the foundation for most purchase specifications. Operators are reluctant to varyfrom standard package specifications because of the additional regulatory approval that may

be required. For similar reasons the turbines used on a given installation for a given function,

such as power generation, are usually likely to be of very similar specification.

Dutyholders have experience over many years in the procurement of gas turbines. API 616is generic and may not in all cases contain sufficient information regarding offshorerequirements. It is normal for the operator to encompass their own best practice and specificinformation into a Design and Engineering Practice. For example:

x Combustion Gas Turbines – Design and Engineering practice on selection, testingand installation

x Combustion Gas Turbines – Amendments and supplements to API 616

These design documents bring together best practice form the basis for the technical

procurement specifications issued by the requisitioning oil company.

In this Section an example is given of the information that might typically be included in atechnical purchase specification. The information included in practice will depend on theoperator, their normal practices and the turbine requirement. Experienced operators mayseek to incorporate good practice and consistency across their installations. The reliance

placed on the turbine supplier for advice in selection of an appropriate gas turbine will vary.Gas turbines are specialised items of equipment and significant advice and interaction withthe gas turbine supplier in meeting the installation requirements is both advisable andnecessary.

1.2 API CODES



A2-2

The codes give some flexibility, for example; API 616 Foreword states: " Equipment

Manufacturers, in particular, are encouraged to suggest alternatives to those specified when

such approaches achieve improved energy effectiveness and reduce total life costs without

sacrifice of safety and reliability."

The following codes mostly affect the packaging:

x API 616 - Gas Turbines

x API 617 - Compressors

x API 614 - Lube Oil System

x API 670 - Machinery Protection

x API 613 - Continuous Duty Gear

x API 677 - Auxiliary Drive Gear

x API 671 - Flexible Couplings

In addition there are codes governing testing and operation:

x ASME PTC-22 Gas Turbine Testingx ASME PTC-10 Compressor Testing

x ASME B133 - Gas Turbines

API 616 and ASME PTC-22 are the only two principal gas turbine specific codes for oil &gas applications. API 670, 614, 613, etc. are more generic codes.

The codes and associated data sheets cover most aspects of the gas turbine package and oftenform the main basis for procurement. The information includes: definitions, for example:

x ISO Rating,

x Normal Operating Point,x Maximum Continuous Speed,

x Trip Speed, etc; mechanical integrity - blade natural frequencies, vibration levels, balancing requirements, alarms and shutdowns;

x Design requirements and features - materials, welding, accessories, controls,instrumentation, inlet/exhaust systems, fuel systems; inspection, testing, and

preparation for shipment; and

x Minimum testing, inspection and certification documentation requirements. API 616does not cover government local codes & regulations

1.3 SUPPLIER PROCUREMENT ADVICE

The main suppliers of gas turbine normally provide standard forms to assist in the selectionof the most appropriate turbine or turbines for a specific application. The informationrequested is very similar to that included in the API 616 forms.

The supplier needs to take account of the installation layout and hence any zoningrequirements and the optimum configuration for the exhaust, the intended fuel compositionand whether larger turbines or several smaller turbines are preferable. This will depend if the turbine(s) are required for power generation or driven equipment. For safety criticalapplications it is necessary to ensure sufficient redundancy is in place.

The operator will normally have standard data sheets available as part of their procurementsytem.



A2-3

1.4 TYPICAL TECHNICAL PROCUREMENT SPECIFICATION

The main technical basis for procurement is the operators design and engineering practices

and these define what is included in the technical procurement specification. Theinformation that may be included in a typical technical procurement specification issummarized below.

Information included

The design specification specifies gives requirements and recommendations for the type,selection, testing and installation of combustion gas turbine for mechanical and generator drives and for hot gas generation. As an example, the following issues may be addressed.

Introduction

x Definitions

x Selection and evaluation

x Range and variety of gas turbinesx Prototype gas turbines

x Complete unit responsibility

Technical information

x Operating requirements

x Spares inventory

x Type selection – aeroderivative or industrial, one or two shaft

x Site environment and fuel considerations

x Power requirements

x Use of standard packages

x Installation – cranes, safe access, lay down areas, mounting, enclosures, auxiliary

equipment

x Noise levels- limits, support information, general requirements

x Oil tank vents

x Materials – specification,temperature, corrosion and environment resistance,

coatings, certification

x Starting drives - gas expansion starters, hydraulic motors, diesel engines

x Foundations, baseplates and mountings


x Inlet system – intake location, new configurations, material, leak prevention, joints

and movement allowancesx Air compressor cleaning

x Exhaust system – Exhaust emission, height, proximity to process equipment, rain

ingress, maintenance access, recirculation

x Combustion air filtration – requirements, anti-icing, shutters

x Fire protection – ventilation dampers, extinguishing systems, enclosure surveillance

x Acoustic enclosures – accessibility, ventilation, area classification

x Fuels and fuel systems – fuel selection, gas fuels and systems, liquid fuels and

systems, dual fuel systems, power augmentation

x Inspection and tests – general, combustion tests, complete unit or string tests

Appendices in the design and engineering practice advise on issues such as definitions of vital, non-essential and non-essential services and how that impacts on selection, gas turbine



A2-4

enclosure ventilation, mounting and foundation requirements, exhaust stack rain-catcher requirements, key points for gas turbine washing systems. Diagrams of typical installationarrangements may be included.

References

Reference is made to the Operators other design standards, covering for example:

x Requisitioning binder containing data sheets

x Metallic materials - selected standards

x Metallic materials - prevention of brittle fracture

x Noise control

x Installation of rotating equipment

x combustion gas turbines (amendments/ supplements to API 616)

x field inspection prior to commissioning of mechanical equipment

x fire-fighting systems

x data/requisition sheet for equipment noise limitationx data/requisition sheet for gas turbines

Oil Industry Standards

Reference is normally included to relevant oil industry standards, for example:

x API RP 11 PGT Packaged combustion gas turbines

x API 617 Centrifugal compressors for petroleum, chemical and gas serviceindustries

x ASME PTC 22 Gas turbine power plants

x ASTM D 2880 Specification for gas turbine fuel oilsx NACE MR0175 Sulphide stress cracking resistant metallic material for oil field

equipment

x EEMUA 140 Noise procedure specification. British Standard.

x ISO 2324 Gas turbines - acceptance tests

1.5 VARIATIONS TO API 616 IN OFFSHORE APPLICATIONS

API 616 is generic and offshore operators may specify variations based on their serviceexperience, to ensure good practice, and to meet the particular requirements of an offshoreenvironment. The items that may be covered in such amendments include:

x Definitions

x Basic design

x Referenced standards

x Pressure casings

x Combustors and fuel nozzles

x Casing connections

x Rotating elements

x Seals

x Dynamics

x Bearings and bearing housings

x Lubricationx Materials



A2-5

x Name plates and rotational arrows

x Accessories - starting and helper driver gears, couplings and guards mounting plates


x Insulation, weatherproofing, fire protection

x Acoustic enclosure

x Piping and appurtenancesx Inlet coolers

x Fuel system treatment

x Inspection and tests

x Preparation for shipment

x Dynamic analysis for use with modified rotor bearing designs or prototype gasturbines



A2-6



A3-1

APPENDIX 3 HSE GUIDANCE NOTE PM84 ON GASTURBINES



A3-2



A4-3

APPENDIX 4 GAS TURBINE SUPPLIERS AND SUMMARYFOR UK INSTALLATIONS

INTRODUCTION

There has been significant consolidation in the gas turbine market in recent years. Themajority of gas turbines in the North Sea are now provided by Rolls Royce, Solar , GeneralElectric (GE) and Siemens-Westinghouse.

SOLAR

Solar based in Houston USA is one the largest suppliers of gas turbines for the offshoremarket accounting for around 11% of those currently on UK installations. Solar is owned byCaterpillar Group.

Website: http://esolar.cat.com

The Solar turbines commonly used offshore in UK installations and Worldwide are asfollows:


Solar Turbines 11% Saturn 20Centaur GSC 40/50Taurus 60/70Mars 90/100

1MW3-4MW5-7MW8- 10MW

Saturn 20Centaur 40/50Taurus 60/70Mars 90/100Titan 130

1MW3-4MW5-7MW8-10MW15MW

ROLLS ROYCE GAS TURBINES 1

Rolls Royce is the market leader in the UK sector accounting for about 21% of the gasturbines, including the 501, RB211, Avon and Coberra brands. These are all aero-derivativesand adaptations of Rolls-Royce’s aero-engines. Coberra gas turbines use the Avon 1535aeroderivative gas generators

Website: http://www.rolls-royce.com/energy/products/oilgas/gasturb.jsp

The main Rolls Royce gas turbines used offshore in the UK and Worldwide include thefollowing:


Rolls-Royce (Avon,Coberra, RB211)

27% 501Avon 1534/1535Coberra 2000/6000RB211Olympus GT SK30

5MW15MW15MW30MW35MW

501AvonRB211Trent

5MW15MW30MW50MW

1 Paul Fletcher (stand-in for Tomas, Mon PM) Turbine Designer



A4-4

SIEMENS-WESTINGHOUSE2

Siemens aquired Alstom’s industrial gas turbine business www.power.alstom.com in 2003 to

fill in their portfolio for intermediate gas turbines up to 15MW. There are a number of newturbine products: Cyclone, GT10C and GTX100 and an alternative fuels programme.

Siemens also took over the Alstom medium industrial gas turbine business with itsheadquarters in Finspong, Sweden, supplying gas turbines from 15MW to 50MW. At thesame time, Siemens acquired the Alstom industrial steam turbine business, supplyingturbines up to 130MW, its main execution centers being in Finspong, (Sweden), Brno(Czech Republic) and Nuremberg (Germany). The combined businesses are now registeredunder the name of Demag Delaval Industrial Turbomachinery, and are fully owned bySiemens.

Website: www.industrial.turbines.siemens.com

The main Siemens turbines used offshore on UK installations and Worldwide include thefollowing.


Siemens-Westinghouse(Alstom, Ruston, EGT)

46% Tornado G8000/8004Alstom RustonTB3000/4500/5000PGT10

8MW14MW

14MW

TyphoonTornadoCyclonePGT10

5MW8MW13MW14MW

Alstom had previously absorbed the turbine businesses of EGT and Ruston.

2 Siemens Frank Carchedi (Simens (UK) compressor aerodynamics Oil & Gas Mon 14.30



A4-5

The different gas turbine offerings arising historically from the Alstom, EGT, Ruston andSiemens businesses have now been configured into a sequence of Siemens SGT models fromthe SGT-100 formerly the Typhoon, the SGT-200 formerly the Tornado, up to the 100MW

plus SGT1000.

The Siemens gas turbine models used offshore are typically in the 5-30MW range.

GENERAL ELECTRIC OIL AND GAS

GE accounts for around 8% of the gas turbines on UK installations including the industrialFrame5 gas turbine and the LM1600, LM2500, LM5000 and LM6000 series of aeroderivative gas turbines.

LM2500 series aero-derivative gas turbines are used in a number of other manufacturersturbine packages.The PGT series of gas turbines combines an aeroderivative gas turbine witha more rugged industrial power turbine.

Website: www.gepower.com

GE turbines used on UK offshore installations and Worldwide include the following:


General Electric Oil &Gas:

12% GE Frame 5GE-1201/1401A-CLM2500+LM5000/6000

5MW10-15MW25MW40-50MW

GE5GE 10LM 1600LM 2500

5MW10MW16MW25MW

Links:

Gas Turbines- Aero-Derivative:http://www.gepower.com/prod_serv/products/aero_turbines/en/index.htmGas Turbines Heavy Dutyhttp://www.gepower.com/prod_serv/products/gas_turbines_cc/en/index.htmTurbine Generatorshttp://www.gepower.com/prod_serv/products/generators/en/index.htmTurbine Control Systems:http://www.gepower.com/prod_serv/products/turbine_ctrl_sys/en/index.htm

OTHER

Gas turbines currently used offshore in UK waters from other independent turbine suppliersinclude the following:

Company % UK Models UK

ABBDresser Rand

Pratt and Witney

12% ABB GT35Dresser KG2, KG3MTU V16Pratt & Witney ST18

1-2MW12MW2MW



A4-6



A5-1

APPENDIX 5 SPECIFICATION OF TURBINES USED IN UKSECTOR

Example ISO rated performance specifications for gas turbines installed offshore in the UK sector, and manufacturers current models for oil and gas use, are given in the followingTable. These are taken from a variety of sources including supplier’s literature and the Gas

Turbine World 2005 Performance Specifications3.

The values are for information and cannot be guaranteed to be correct. The performancespecification will vary as improvements are made to a given gas turbine and vary for simplecycle and combined cycle applications. The specification will also vary depending onwhether the turbine is being used in power generation, to drive a compressor or mechanicaldrive. For current specifications for gas turbines in these applications, images and crosssections for specific turbines and a wider range of turbine specifications, readers are referred

to the Suppliers web sites and the Gas Turbine World performance specifications. Thesesources provide general background and detail on ISO rating as well as describing the basisfor the turbine performance measures.

3 Gas Turbine World 2005 performance Specifications gas Turbine power ratings for Project Planning, Engineering, Design and procurement, gas Turbine wold Vol 34 No. 6 2005 GTW Specifications



A5-2



A 5 - 3

M a n u f a

c t u r e r M o d e l

Y e a r

I S O

B a s e

R a t i n g

I S O B a s e

R a t i n g

H e a t

R a t e

E f f i c i e n

c y P r e s s u r e

R a t i o

F l o w

S p e c i f i c

P o w e r

T u r b i n e

S p e e d

E x h a u s t

T e m p

A p p r o x

W e i g h t

A p p r o x i m a t e

D i m e n s i o n s

( f t ) C o m m e n t s

M W

k W

B t u / k W h

%

( l b / s e c )

k W / l b

r p m

° F

l b

L W

H

1 s t y e a r

u n i t

a v a i l a b l e

6 0 0 0 h p e r

y e a r o r

m o r e

L o w e r H e a t i n g V a

l u e

( L H

V )

A t b a s e

l o a d

P e r l b o f

a i r p e r

s e c o n d

O u t p u t

s h a f t

G E

G E 5

1 9 9 9

6

M W

5 , 5 0 0

1 1 , 1 3 0

3 0 . 7 %

1 4 . 8

4 3 . 1

1 2 8

1 6 , 6 3 0

1 , 0 6 5

1 1 6 , 1 6 0

1 9

8

1 0 I D L E

G E

G E 1 0

2 0 0 0

1 1

M W

1 1 , 2 5 0

1 0 8 8 4

3 1 . 4 %

1 5 . 5

1 0 4 . 7

1 1 , 0 0 0

9 0 0

7 4 9 7 0

3 0

8

2 0

G E

L M 1 6 0 0 P E

1 9 8 9

1 5

M W

1 4 , 8 9 8

1 0 0 9 4

3 3 . 8 %

2 1 . 3

1 0 9 . 7

1 3 6

7 , 9 0 0

8 9 4

1 9 0 0 0 0

5 0 1 2

1 5 W a t e r

G E

L M 2 5 0 0 +

6 S t g

1 9 9 7

3 0

M W

3 0 , 4 6 3

8 , 8 5 4

3 8 . 5 %

2 2 . 6

1 9 1 . 3

1 5 9

3 6 0 0

9 6 0

2 6 0 , 0 0 0

5 7

9

1 0 D L E

G E

L M 2 5 0 0 P E

1 9 8 1

2 2

M W

2 2 , 3 4 6

9 , 6 3 0

3 5 . 4 %

1 8 . 0

1 5 3 . 6

1 4 5

3 0 0 0

1 0 0 1

2 5 0 , 0 0 0

5 7

9

1 0 d r y

G E

L M 6 0 0 0

1 9 9 2

4 3

M W

4 3 , 0 7 6

8 , 2 5 5

4 1 . 3 %

3 0 . 0

2 8 8 . 8

1 4 9

3 , 6 0 0

8 4 0

6 8 , 3 4 2

3 1 1 4

1 4 S i z e w / o G T

e n c l o s u r e

G E

P G T 1 0

1 0

M W

1 0 , 2 2 0

1 0 9 4 0

3 1 . 2 %

1 3 . 8

9 3 . 3

7 , 9 0 0

9 1 0

5 9 5 3 5

2 7

8

1 3

G E

P G T 1 6

1 9 8 9

1 4

M W

1 3 , 7 2 0

9 7 6 4

3 4 . 9 %

2 0 . 2

1 0 4 . 3

7 , 9 0 0

9 1 9

4 1 8 9 5

2 7

8

1 2

G E

P G T 2 0

2 0 0 1

1 7

M W

1 7 , 4 6 4

9 7 0 6

3 5 . 2 %

1 5 . 7

1 3 7 . 7

6 , 5 0 0

8 8 7

8 3 0 1 8

3 0

G E

P G T 2 0

2 0 0 1

1 7

M W

1 7 , 4 6 9

9 , 7 2 1

3 5 . 1 %

1 8 . 0

1 3 7 . 8

1 2 7

6 , 5 0 0

8 8 7

8 3 , 0 0 3

3 0 1 1

1 1 s i z e w / o G T

e n c l o s u r e

G E

P G T 2 5

1 9 8 1

2 2

M W

2 2 , 4 1 7

9 4 0 3

3 6 . 3 %

1 7 . 9

1 5 1 . 9

6 , 5 0 0

9 7 6

8 3 0 1 8

3 0 1 1

1 1

G E

P G T 2 5 +

1 9 9 6

3 0

M W

3 0 , 2 2 6

8 6 1 2

3 9 . 6 %

2 1 . 5

1 8 5 . 9

6 , 1 0 0

9 3 1

6 7 8 0 4

2 1

G E

P G T 5

5

M W

5 , 2 2 0

1 2 7 2 4

2 6 . 8 %

9 . 1

5 4 . 2

1 0 , 2 9 0

9 7 3

6 1 7 4 0

2 8

8

1 0

R o l l s R o

y c e

5 0 1 - K B 5 S

1 9 9 0

4

M W

3 8 9 7

1 1 7 4 7

2 9 . 1 %

1 0 . 3

3 3 . 9 I b

1 1 5

1 4 2 0 0

1 0 4 0

2 5 0 0 0

R a t i n g s a t s e a

l e v e l , 1 5 d e g C

R o l l s R o

y c e

5 0 1 - K B 7 S

1 9 9 2

5

M W

5 2 4 5

1 0 8 4 8

3 1 . 5 %

1 3 . 9

4 6 . 6 I b

1 1 3

1 4 6 0 0

9 2 8

2 5 0 0 0

N o e x t e r n a l

p r e s s u r e l o s s e s

R o l l s R o

y c e

5 0 1 - K H 5

1 9 8 2

6

M W

6 4 4 7

8 5 0 9

4 0 . 1 %

1 2 . 5

4 0 . 6

1 5 9

1 4 6 0 0

9 8 6

2 5 0 0 0

8

C a s e s t e a m

i n j e c t e d 2 . 7 3

k g / s e c

R o l l s R o

y c e

5 0 1 - K C 5

1 9 9 0

4

M W

4 1 0 0

8 4 9 5

2 9 . 6 %

9 . 4

3 4 . 2

1 5 9

1 3 6 0 0

1 0 6 0

2 5 0 0 0

8

5 0 1 - K C 5 G a s

g e n e r a t o r

R o l l s R o

y c e

5 0 1 - K C 7

1 9 9 2

6

M W

5 5 1 8

7 9 0 2

3 1 . 7 %

1 3 . 5

4 6 . 2

1 5 9

1 3 6 0 0

9 6 8

2 6 0 0 0

8

5 0 1 - K C 7 G a s

g e n e r a t o r

R o l l s R o

y c e

A v o n - 1 5 3 5

1 , 9 8 8

1 5

M W

1 5 , 1 8 0

8 , 6 6 0

2 9 . 4 %

8 . 8

1 7 0

8 9

5 5 0 0

8 2 7

5 0 , 0 0 0

A v o n 1 5 3 5 G a s

G e n e r a t o r

R o l l s R o

y c e

A v o n 2 6 4 8

1 , 9 8 8

1 5

M W

1 5 , 1 8 0

8 , 6 6 0

2 9 . 4 %

8 . 8

1 7 0

9 2

5 5 0 0

8 2 7

5 0 , 0 0 0

A v o n 1 5 3 5 G a s

G e n e r a t o r

R o l l s R o

y c e

A v o n 2 6 5 6

1 , 9 8 8

1 6

M W

1 5 , 6 6 0

8 , 4 0 5

3 0 . 3 %

8 . 8

1 7 0

1 5 3

4 9 5 0

8 1 8

5 2 , 0 0 0

A v o n 1 5 3 5 G a s

G e n e r a t o r



A 5 - 4

M a n u f a


Y e a r

I S O

B a s e

R a t i n g

I S O B a s e

R a t i n g

H e a t

R a t e

E f f i c i e n

c y P r e s s u r e

R a t i o

F l o w

S p e c i f i c

P o w e r

T u r b i n e

S p e e d

E x h a u s t

T e m p

A p p r o x

W e i g h t


D i m e n s i o n s


M W

k W

B t u / k W h

%

( l b / s e c )

k W / l b

r p m

° F

l b

L W

H

1 s t y e a r

u n i t

a v a i l a b l e

6 0 0 0 h p e r

y e a r o r

m o r e


l u e

( L H

V )

A t b a s e

l o a d

P e r l b o f

a i r p e r

s e c o n d

O u t p u t

s h a f t

R o l l s R o

y c e

C o b e r r a

2 6 4 8

1 , 9 8 9

1 5

M W

1 5 , 1 8 0

8 , 6 6 0

2 9 . 4 %

8 . 8

1 7 0 . 1

9 2

5 , 5 0 0

1 , 1 4 1

5 0 , 0 0 0

A v o n 1 5 3 5 G a s

G e n e r a t o r

R o l l s R o

y c e

C o b e r r a

2 6 5 6

1 , 9 9 0

1 6

M W

1 5 , 6 6 0

8 , 4 0 5

2 9 . 4 %

8 . 8

1 6 9 . 4

1 5 4

4 , 9 5 0

1 , 1 4 5

5 2 , 0 0 0

A v o n 1 5 3 5 G a s

G e n e r a t o r

R o l l s R o

y c e

C o b e r r a

6 5 5 6

1 9 9 2

2 6

M W

2 6 , 0 2 5

8 , 6 6 0

2 9 . 4 %

2 0 . 1

2 0 3 . 3

1 3 6

4 , 9 5 0

1 , 3 5 0

5 8 , 0 0 0

R B 1 1 - 2 4 G T


R o l l s R o

y c e

C o b e r r a

6 5 6 2

1 9 9 3

2 6

M W

2 5 , 9 3 0

9 4 1 5

2 9 . 4 %

2 0 . 8

2 0 8 . 7

1 3 6

4 , 8 0 0

1 , 4 0 4

5 8 , 0 0 0

R B 1 1 - 2 4 G T


R o l l s R o

y c e

R B 2 1 1 6 5 5 6

1 9 9 2

2 6

M W

2 6 , 0 2 0

7 1 0 0

3 4 . 6 %

2 0 . 1

2 0 3

1 3 6

4 , 9 5 0

9 1 0

5 0 , 0 0 0

2 9 . 8 1 3

1 4 R B 1 1 - 2 4 G T


R o l l s R o

y c e

R B 2 1 1 6 5 6 2

1 9 9 3

2 9

M W

2 8 , 5 0 0

6 7 0 5

3 6 . 7 %

2 0 . 8

2 0 9

1 3 6

4 , 8 0 0

9 1 7

5 0 , 0 0 0

3 0 . 1 1 3

1 4 R B 1 1 - 2 4 G T


R o l l s R o

y c e

R B 2 1 1 - 6 5 6 2

D L E

1 9 9 3

2 8

M W

2 7 5 2 0

6 7 0 5

3 6 . 3 %

2 0 . 8

2 0 2

1 3 6

4 8 0 0

9 3 2

5 8 0 0 0

R B 1 1 - 2 4 G T

G a s G e n e r a t o r -

S t e a m i n j e c t i o n

R o l l s R o

y c e

R B 2 1 1 - 6 7 6 2

D L E

1 9 9 9

3 0

M W

2 9 5 0 0

6 5 6 5

3 7 . 7 %

2 1 . 5

2 1 1

1 4 0

4 8 0 0

9 2 0

5 7 0 0 0

R B 1 1 - 2 4 G T


R o l l s R o

y c e

R B 2 1 1 - 6 7 6 1

D L E

2 0 0 0

3 2

M W

3 2 1 2 0

6 2 9 0

3 9 . 3 %

2 1 . 5

2 0 8

1 5 4

4 8 5 0

9 3 8

5 7 0 0 0

R B 1 1 - 2 4 G T


R o l l s R o

y c e

T r e n t 6 0

I D L E

1 9 9 6

5 2

M W

5 1 6 8 5

8 1 3 8

4 1 . 9 %

3 4

3 4 1

1 5 2

3 6 0 0

8 2 5

2 8 6 5 9 8

R B 1 1 - 2 4 G T


R o l l s R o

y c e

T r e n t 6 0

W L E

2 0 0 1

5 8

M W

5 8 0 0 0

8 3 4 6

4 0 . 9 %

3 6

3 6 5

1 5 9

3 0 0 0

7 9 4

2 8 6 5 9 8

R B 1 1 - 2 4 G T

G a s G e n e r a t o r -

W a t e r i n j e c t e d

S i e m e n s

T o r n a d o

1 9 8 1

7

M W

6 6 4 0

1 0 7 6 0

3 1 . 0 %

1 1 . 3

6 0 . 6

1 1 0

1 1 0 5 3

8 6 0

1 2 1 2 5 3

3 7 1 1

8 A l s t o m - E G T

S i e m e n s

T y p h o o n

5

M W

4 8 5 0

1 0 2 7 3

3 1 . 0 %

1 4

4 3 . 2

1 1 2

1 7 3 8 4

9 5 4

7 4 5 1 5

2 7

8

1 1 A l s t o m - E G T

S i e m e n s

S G T - 1 0 0

1 9 8 9

4

M W

4 3 5 0

1 1 3 7 0

3 0 . 0 %

1 3 . 0

3 9 . 0

1 1 2

1 6 5 0 0

9 8 1

7 8 1 7 5

3 3

8

1 1 T y p h o o n 4 . 3 5

S i e m e n s

S G T - 1 0 0

1 9 8 9

5

M W

4 7 0 0

1 1 3 0 9

3 0 . 2 %

1 4 . 1

4 2 . 0

1 1 2

1 7 3 8 4

9 7 5

7 8 1 7 5

3 3

8

1 1 T y p h o o n 4 . 7 0

S i e m e n s

S G T - 1 0 0

1 9 9 7

5

M W

5 0 5 0

1 1 2 9 4

3 0 . 2 %

1 4 . 3

4 3 . 0

1 1 7

1 7 3 8 4

1 0 1 5

7 8 1 7 5

3 3

8

1 1 T y p h o o n 5 . 0 5

S i e m e n s

S G T - 1 0 0

1 9 9 8

5

M W

5 2 5 0

1 1 2 0 3

3 0 . 5 %

1 4 . 8

4 6 . 0

1 1 4

1 7 3 8 4

9 8 6

7 8 1 7 5

3 3

8

1 1 T y p h o o n 5 . 2 5

S i e m e n s

S G T - 2 0 0

1 9 8 1

7

M W

6 7 5 0

1 0 8 2 4

3 1 . 5 %

1 2 . 3

6 5 . 0

1 0 4

1 1 0 5 3

8 7 1

1 2 4 0 0 0

4 1

8

1 1 T o r n a d o



A 5 - 5

M a n u f a


Y e a r

I S O

B a s e

R a t i n g

I S O B a s e

R a t i n g

H e a t

R a t e

E f f i c i e n

c y P r e s s u r e

R a t i o

F l o w

S p e c i f i c

P o w e r

T u r b i n e

S p e e d

E x h a u s t

T e m p

A p p r o x

W e i g h t


D i m e n s i o n s


M W

k W

B t u / k W h

%

( l b / s e c )

k W / l b

r p m

° F

l b

L W

H

1 s t y e a r

u n i t

a v a i l a b l e

6 0 0 0 h p e r

y e a r o r

m o r e


l u e

( L H

V )

A t b a s e

l o a d

P e r l b o f

a i r p e r

s e c o n d

O u t p u t

s h a f t

S i e m e n s

S G T - 3 0 0

1 9 9 5

8

M W

7 9 0 0

1 0 9 3 7

3 1 . 2 %

1 3 . 8

6 6 . 0

1 2 0

1 4 0 1 0

9 9 9

1 2 6 0 0 0

4 0

8

1 2 T e m p e s t

S i e m e n s

S G T - 4 0 0

1 9 9 7

1 3

M W

1 2 9 0 0

9 8 1 7

3 4 . 8 %

1 6 . 9

8 7 . 0

1 4 8

9 5 0 0

1 0 3 1

1 6 5 0 0 0

6 1

9

1 3 C y c l o n e

S i e m e n s

S G T - 5 0 0

1 9 6 8

1 7

M W

1 7 0 0 0

1 0 6 0 0

3 2 . 2 %

1 2 . 0

2 0 3 . 5

8 4

3 0 0 0 /

3 6 0 0

7 0 7

3 3 1 0 0 0

6 8 1 6

1 3 G T 3 5 C

S i e m e n s

S G T - 6 0 0

1 9 8 1

2 5

M W

2 4 7 7 0

9 9 8 5

3 4 . 2 %

1 4 . 0

1 7 7 . 3

1 4 0

7 7 0 0

1 0 0 9

3 3 5 0 0 0 1 6 8 1 5

1 7 G T 1 0 B

S i e m e n s

S G T - 7 0 0

1 9 9 9

2 9

M W

2 9 0 6 0

9 4 8 0

3 6 . 0 %

1 8 . 0

2 0 1 . 0

1 4 5

6 5 0 0

9 6 4

3 5 3 0 0 0

6 8 1 5 1

8

G T 1 0 C

S i e m e n s

S G T - 8 0 0

1 9 9 8

4 5

M W

4 5 0 0 0

9 2 1 5

3 7 . 0 %

1 9 . 3

2 8 7 . 0

1 5 7

6 6 0 0

1 0 0 1

3 7 9 0 0 0

5 6 1 5

1 3 G T X 1 0 0

S i e m e n s

S G T - 9 0 0

1 9 8 2

5 0

M W

4 9 5 0 0

1 0 4 5 0

3 2 . 7 %

1 5 . 3

3 8 6 . 0

1 2 8

5 4 2 5

9 5 7

2 7 6 0 0 0

5 0 1 2

1 4

S o l a r

S a t u r n 2 0

1 9 8 5

1

M W

1 2 0 0

1 4 0 2 5

2 4 . 3 %

6 . 8

1 4 . 4

8 3

2 2 5 1 6

9 4 0

1 9 8 0 0

2 0

6

7

S o l a r

S a t u r n 2 0

G S

1 9 8 4

1

M W

1 2 1 0

1 4 0 2 5

2 4 . 3 %

6 . 8

1 4 . 4

8 3

2 2 5 1 6

9 4 0

2 2 0 0 0

1 8

6

7

S o l a r

C e n t a u r 4 0

G S

1 9 9 2

4

M W

3 5 1 5

1 2 2 4 0

2 7 . 9 %

9 . 8

4 1 . 0

8 4

1 4 9 5 0

8 2 0

5 2 3 7 0

2 9

8

7

S o l a r

C e n t a u r 5 0

G S

1 9 9 3

5

M W

4 6 0 0

1 2 2 7 0

2 9 . 3 %

1 0 . 6

4 1 . 6

1 0 9

1 4 9 5 0

5 1 0

2 7 0 8 0

1 8

8

9

S o l a r

T a u r u s 6 0

G S

1 9 9 2

6

M W

5 5 0 0

1 1 2 2 0

3 1 . 5 %

1 1 . 5

4 8 . 3

1 1 8

1 4 9 5 1

9 5 0

6 7 1 4 0

2 9

8

7

S o l a r

T a u r u s 6 5

2 0 0 5

6

M W

6 0 0 0

1 0 3 7 5

3 2 . 9 %

1 5 . 0

4 3 . 2

1 3 9

1 4 9 5 0

1 0 1 7

7 2 7 0 0

3 2

8

1 0

S o l a r

T a u r u s 7 0

1 9 9 4

8

M W

7 5 2 0

1 0 1 0 0

3 3 . 8 %

1 6 . 1

5 9 . 4

1 2 7

1 5 2 0 0

9 0 5

1 1 0 9 2 3

3 4

9

1 1

S o l a r

M a r s 9 0 G S

1 9 9 4

9

M W

9 4 5 0

1 0 7 1 0

3 1 . 9 %

1 6 . 1

8 8 . 5

1 0 7

8 7 0

1 4 9 0 0 0

4 8

9

1 2

S o l a r

M a r s 1 0 0 G S

1 9 9 4

1 1

M W

1 0 6 9 5

1 0 5 1 5

3 3 . 0 %

1 7 . 4

9 1 . 7

1 1 6

1 1 1 6 8

9 0 5

1 3 7 7 5 0

4 8 1 0

1 2

S o l a r

T i t a n 1 3 0 G S

1 9 9 8

1 5

M W

1 5 0 0 0

9 6 9 5

3 5 . 2 %

1 5 . 0

1 0 9 . 8

1 3 7

1 1 1 7 0

9 2 5

1 4 7 5 9 9

4 6 1 0

1 1

O t h e r

K G 2 - 3 C

1 9 6 8

1

M W

1 4 9 9

2 1 2 0 2

1 6 . 1 %

3 . 9

2 8 . 2

5 3

1 8 0 0 0

r p m

1 0 5 8

3 6 3 7 5

2 1

7

9

O t h e r

K G 2 - 3 E

1 9 8 9

2

M W

1 8 9 5

2 0 4 2 1

1 6 . 7 %

4 . 7

3 3 . 0

5 7

1 8 8 0 0

r p m

1 0 2 0

3 8 5 8 0

2 1

7

9

O t h e r

D R 6 0 G

1 9 9 0

1 4

M W

1 3 7 7 5

9 7 5 2

3 5 . 0 %

2 1 . 5

1 0 4 . 0

1 3 2 7 0 0

0 r p m

1 0 2 0

2 2 7 0 7 5

4 2 1 2

1 1

O t h e r

D R 6 1

1 9 8 6

2 2

M W

2 2 3 0 2

9 4 2 2

3 6 . 2 %

1 8 . 1

1 5 0 . 0

1 4 9 5 5 0

0 r p m

9 8 6

3 5 2 7 3 5

4 9 1 3

1 8

O t h e r

D R 6 1 G

1 9 7 3

2 4

M W

2 3 8 7 3

9 8 9

3 7 . 6 %

1 8 . 4

1 5 3 . 0

1 5 6 3 6 0

0 r p m

9 9 2

3 0 8 6 4 5

4 8 1 2

1 8



A 5 - 6

M a n u f a


Y e a r

I S O

B a s e

R a t i n g

I S O B a s e

R a t i n g

H e a t

R a t e

E f f i c i e n

c y P r e s s u r e

R a t i o

F l o w

S p e c i f i c

P o w e r

T u r b i n e

S p e e d

E x h a u s t

T e m p

A p p r o x

W e i g h t


D i m e n s i o n s


M W

k W

B t u / k W h

%

( l b / s e c )

k W / l b

r p m

° F

l b

L W

H

1 s t y e a r

u n i t

a v a i l a b l e

6 0 0 0 h p e r

y e a r o r

m o r e


l u e

( L H

V )

A t b a s e

l o a d

P e r l b o f

a i r p e r

s e c o n d

O u t p u t

s h a f t

O t h e r

D R 6 1 G P

( S A C )

1 9 9 8

3 1

M W

3 1 3 8 0

8 6 3 8

3 9 . 5 %

2 2 . 8

1 9 2 . 0

1 6 3 3 6 0

0 r p m

9 5 9

3 5 2 7 3 5

4 8 1 2

1 8

O t h e r

V e c t r a - 4 0 G

1 9 9 8

3 1

M W

3 1 3 8 0

8 4 1 6

4 0 . 6 %

2 2 . 8

1 8 6 . 0

1 6 9 6 2 0

0 r p m

9 5 5

3 5 2 7 3 5

4 8 1 4

1 8

O t h e r

D R 6 3 G

1 9 9 2

4 3

M W

4 2 9 8 4

8 2 4 5

4 1 . 4 %

2 9 . 6

2 8 2 . 0

1 5 2 3 6 0

0 r p m

8 2 4

4 8 5 0 1 0

6 0 1 4

2 3

O t h e r

S T 1 8

1 9 9 5

2

M W

1 9 6 1

1 1 2 3 7

3 . 0 . 4 %

1 4

1 7 . 6

1 1 1

1 8 9 0 0

9 9 0

7 7 2

5

2

3

C o n v e r s i o n F a c t o r s

º C = ( º

F - 3 2 ) / 1 . 8

º F = ( º C x 1 . 8 ) + 3 2

l b

x 0 . 4 5 4 = k g

k g x 2 . 2 0 5 = l b

l b / h p h x 0 . 6 0 8 = k g / k W h

k g / k W h x 1 . 6 4 4 = l b / h p h

k W x 1 . 3 4 1 = h p

h p x 0 . 7 4 6 = k W

B

t u x 1 . 0 5 5 = k J

k J x 0 . 9 4 8 = B t u

h p h x 2 . 6 8 5 = M J

M J x 0 . 3 7 3 = h p h

1 k W h = 8 5 9 . 8 k c a l = 3 4 1 3 B t u

1 h p - h r = o . 7 4 6 k W h = 2 5 4 5 B t u

B

t u / l b x 2 . 3 2 6 = k J / k g

k J / k g x 0 . 4 3 0 = B t u / l b

J x 0 . 2 3 9 = c a l o r i e

c a l o r i e x 0 . 2 3 9 = J

S o u r c e

S u p p l i e r i n f o r m a t i o n a n d 2 0 0 5 G

T W T u r b i n e S p e c i f i c a t i o n s



A6-1

Appendix 6 Key Systems and Components

Introduction

The gas turbine itself contains three main components:

x Compressor,

x Gas generator (GG) including combustor and gas turbine (GT)

x Power turbine (PT),

Other key systems within the package include the fuel system either natural gas or liquid(pumped), the bearing lube oil system including tank and filters, pumps (main, pre/post,

backup), the starter (usually either pneumatic, hydraulic or variable speed ac motor), cooling

systems, controls (on-skid, off-skid), driven equipment and the seal gas system (compressors).

There is other ancillary equipment external to the turbine package. This includes: the enclosureand fire protection, the acoustic housing, the inlet system including air-filter (self-cleaning,

barrier, inertial) and silencer, the exhaust system including silencer and the exhaust stack, a lubeoil cooler (water, air), the motor control center, switchgear, neutral ground resistor and inletfogger/cooler. A basic description of each of the main systems is included here.

Package Mounting

The gas turbine within an Offshore Package is normally centre-line mounted from the baseframe, ensuring internal alignment while permitting thermal expansion of the machine. Themain drive shaft, which will be at the hot or exhaust end for a mechanical drive package,includes a flexible coupling, as will any auxiliary drive shafts. Flexible connections link to theinlet and exhaust ducts. The fuel manifold is wrapped around the middle of the machine, withmultiple combustor fuel feeds. Hot surfaces will be fitted with heat shields or thermal insulationfor operator safety.

For turbine packages all machine elements are mounted to a common baseframe that issufficiently rigid to maintain machine alignment, despite movement of the supporting structureor vessel. The normal 3-point mounting system eliminates the transmission of twisting forces toand from the baseframe. As many as possible of the ancillary systems e.g. lubrication oilsystem, seal gas support system, are built into the main baseframe in order to save space, andthe weight of additional bases. The control panel may be mounted separately or built on to theend of the baseframe. The former permits control panels for separate machines to be grouped

together; the latter is convenient for pre-wiring.

Acoustic enclosure

The gas turbine is normally enclosed in a acoustic enclosure. The enclosure reduces the risk from the noise hazard but introduces hazards of an enclosure possibly containing flammablegas. The Acoustic Enclosure for an Aero-derivative Gas Turbine is normally close fitting, andfitted out with ventilation and Fire & Gas Detection Systems. The internal space can be tightly

packed, making access to internal components quite difficult. Modern practice is to use amodular approach with units simply replaced for maintenance. A problem on one componenthas the potential to affect adjacent components or systems, whether by release of material,vibration or over-heating. It may be necessary to remove a component to gain access to adjacent

components.



A6-2

The gas compressor and drive gearbox (if fitted) will be outside the acoustic enclosure, but stillvery closely packed with service pipework & cable trunking. Good design should permit readyaccess to compressor bearings, instruments and drive couplings.

The air inlet housing will be located separate from the turbine next to the external cladding of the process area.

Ventilation

Ventilation requirements within the turbine enclosure are important to minimise the risk of fireand explosion following any leak of fuel, gas or oil as well as ensure safety during maintenanceintervention and monitoring. Guidance on ventilation requirements for turbines in offshoreinstallations can be found in the main report and in PM84 (Appendix 3) and HSE ResearchReport RR076.

Fire and gas explosion prevention system

Gas turbines operate at very high temperature particularly in the gas generator, combustor andearly stages of the power turbine. Temperatures are very high also in the exhaust and associatedlagging. There are large amounts of pipework external to the turbine for lubrication of bearingsand seals and fuel supply to the combustor. Gas leaks also can occur if seals becomeineffective. The high temperatures ensure that any leak is likely to lead to ignition and fire.Most dangerous occurrences noted in the HSE RIDDOR and ORION databases are of this type.A robust fire prevention system is required. This is usually based on extinguishing the fireusing an appropriate inert but breathable gas. Halon systems were formerly used but has been

phased out offshore. Guidance on fire prevention systems is given in Section PM84 and RR076.

Extinguishing systems

Water deluge systems shall not be fitted on gas turbine installations (a deluge of water on to ahot gas turbine casing will cause extensive damage). Inergen or C02 gaseous fire extinguishingsystems are often used. Inergen is an agent composed of nitrogen, argon and carbon dioxide,which after a release sufficiently reduces the concentration of oxygen to stop a fire but is safeenough for humans to survive and function in a normal matter. The mixture of nitrogen, argonand carbon dioxide stimulates respiration systems so that survival in a low oxygen environmentis possible. Inergen is therefore safer than C02 and may be the preferred choice for newapplications provided appropriate refills are locally available. Release of the agents can beautomatically and/or manually initiated. For offshore applications, fine water mist systems may

be considered if space is at premium. These systems require a large quantity of nozzles and

tubing to be an effective extinguishing system at the source of the fire.

Enclosure surveillance

For remote unattended locations, the use may be considered of closed circuit television (CCTV)to monitor equipment within an acoustic enclosure. Zoom, pan and tilt controls shall be

provided from the control room. With low light image intensification, CCTV is a useful tool for operators to survey remote equipment.

AIR INTAKE

Air intake to the turbine is through large bore ducting. The air is filtered using a self cleaninginertial barrier filter. De-icing systems are also used to optimise the air condition before entry



A6-3

into the air compressor. Turbines are capable of working across a wide range of inlettemperatures and environmental conditions. Control of air condition and temperature is

particularly critical for turbine performance and efficiency in low emission (DLE) turbines. Asilencer is also fitted to the air intake to minimise noise and vibration.

Air Intake Filter

Air feed to the gas turbine is filtered through a series of filtration elements to ensure cleanlinessof combustion air.

COMBUSTION AIR COMPRESSOR

The air compressor is the first major part of the gas turbine. It’s function is to compress the air before combustion and expansion through the turbine. There are two basic types of compressor,one giving axial flow and the other centrifugal flow. Axial compressors are by far the most used

in modern offshore gas turbines giving higher air flow, pressure ratios, fuel effiviancy andthrust. Centrifugal compressors may be found in older or smaller turbines where its simplicityand ruggedness outweigh any other disadvantages. Both types are driven by the engineturbine and are usually coupled direct to the turbine shaft.

The axial flow compressor is a multi-stage unit employing alternate rows of rotating (rotor) blades and stationary (stator) vanes, to accelerate and diffuse the air until the required pressurerise is obtained. A centrifugal flow compressor is a single or two-stage unit employing animpeller to accelerate the air and a diffuser to produce the required pressure rise.

Figure A6.1 Axial compressor and high pressure turbine rotor in PGT5 gas turbine.Courtesy Nuovo Pigneone

Axial Flow Compressor

An axial flow compressor consists of one or more rotor assemblies that carry blades of airfoilsection. These assemblies are mounted between bearings in the casings which incorporate thestator vanes. The compressor is a multi-stage unit as the amount of pressure increase by eachstage is small; a stage consists of a row of rotating blades followed by a row of stator vanes.



A6-4

Design of blades and stator vanes is highly specialized. The casing and rotor are tapered fromthe front (low-pressure) end to rear (high pressure) to maintain constant axial flow velocity.

The construction of the compressor centres around the rotor assembly and casings. The rotor shaft is supported in ball and roller bearings and coupled to the turbine shaft in a manner thatallows for any slight variation of alignment. The cylindrical casing assembly may consist of anumber of cylindrical casings with a bolted axial joint between each stage or the casing may bein two halves with a bolted centre line joint. One or other of these construction methods isrequired in order that the casing can be assembled around the rotor.

Principles

The rotor is turned at high speed by the turbine so that air is continuously induced into thecompressor, which is then accelerated by the rotating blades and swept rearwards onto theadjacent row of stator vanes. The pressure rise results from the energy imparted to the air in therotor which increases the air velocity. The air is then decelerated (diffused) in the followingstator passage and the kinetic energy translated into pressure.

Stator vanes serve to correct the deflection given to the air by the rotor blades and to present theair at the correct angle to the next stage of rotor blades. The last row of stator vanes usually actas air straighteners to remove swirl from the air prior to entry into the combustion system at areasonably uniform axial velocity. Pressure changes are accompanied by a progressive increasein air temperature as the pressure increases.

The pressure change across each stage can be quite small, typically 1:1 and 1:2. The compressor itself can increase pressure by factors of 30:1 or more. The ability to design multi-stage axialcompressors with controlled air velocities and straight through flow minimizes losses andresults in a high efficiency and hence low fuel consumption. This gives it a further advantage

over the centrifugal compressor where these conditions are fundamentally not so easilyachieved. For high pressure ratios variable-angle stator vanes or interstage blades are used toensure uniform flow and compression across the full speed range.

Figure A6-2 RB211 gas generator showing axial compressor with stator vanes at left

hand side, combustion chamber and initial turbine stages. Courtesy Rolls Royce



A6-5

A single-spool compressor consists of one rotor assembly and stators with as many stages asnecessary to achieve the desired pressure ratio and all the airflow from the intake passes throughthe compressor. The multi-spool compressor consists of two or more rotor assemblies, eachdriven by their own turbine at an optimum speed to achieve higher pressure ratios and to give

greater operating flexibility.

Although a twin-spool compressor can be used for a pure jet engine, it is most suitable for the by-pass type of engine where the front or low pressure compressor is designed to handle a larger airflow than the high pressure compressor. Only a percentage of the air from the low pressurecompressor passes into the high pressure compressor; the remainder of the air, the by-pass flow,is ducted around the high pressure compressor.

Components

The construction of the compressor centres around the rotor assembly and casings. The maincomponents of an axial air compressor comprise:

x Rotors

x Blades

x Stator vanes

x Discs

x Casing

x Rotor shaft

x Bearings and seals

Rotors

The rotational speed of an axial compressor is such that a disc is required to support thecentrifugal blade load. Where a number of discs are fitted onto one shaft they may be coupledand secured together by a mechanical fixing. Generally, the discs are assembled and weldedtogether, close to their periphery, thus forming an integral drum.

Figure A6-3 Detail of Rotor on aeroderivative gas turbine. Courtesy Sulzer.



A6-6

Rotor blades

The rotor blades are of airfoil section (Figure A6-4) and usually designed to give a pressuregradient along their length to ensure that the air maintains a reasonably uniform axial velocity.

The higher pressure towards the tip balances out the centrifugal action of the rotor on theairstream. The blade is twisted from root to tip to give the correct angle of incidence at each point, defined by a stagger angle. Air flowing through a compressor creates two boundary

layers of slow to stagnant air on the inner and outer walls. In order to compensate for the slewair in the boundary layer a localized increase in blade camber both at the blade tip and root has

been introduced. The blade extremities appear as if formed by bending over each corner, hencethe term end-bend.

Figure A6-4 Gas turbine compressor and stator parts including variable angle stators..Courtesy Nuovo Pigneone, EGT

Rotor Disc

Individual rotor blades are attached to the rotor disc. A variety of fixing methods may be used.Fixing may be circumferential or axial to suit special requirements of the stage. In general theaim is to design a securing feature that imparts the lightest possible load on the supporting discthus minimizing disc weight. Rotor discs are then stacked on the rotor shaft (Figure A6-5)

Figure A6-5 Installation of rotor blades. Courtesy Sulzer



A6-7

Stator vanes

The stator vanes are of airfoil section and are secured into the compressor casing or into stator

vane retaining rings, which are themselves secured to the casing (Figure A6-6). The vanes areoften assembled in segments in the front stages and may be shrouded at their inner ends tominimize the vibrational effect of flow variations on the longer vanes. The stator vanes arelocked in such a manner that they will not rotate around the casing.

Figure A6-6 Axial compressor half casing and stator blades

Casings

The construction of the compressor centres around the rotor assembly and casings. Thecylindrical casing assembly may consist of a number of cylindrical casings with a bolted axial

joint between each stage or the casing may be in two halves with a bolted centre line joint. Oneor other of these construction methods is required in order that the casing can be assembledaround the rotor.

Rotor Shaft

The rotor shaft is supported in ball and roller bearings and coupled to the turbine shaft in amanner that allows for any slight variation of alignment.

Airflow Control

Where high pressure ratios on a single shaft are required it becomes necessary to introduceairflow control into the compressor design. This may take the form of variable inlet guide vanesfor the first stage plus a number of stages incorporating variable stator vanes for the succeedingstages as the shaft pressure ratio is increased (fig. 3-15). As the compressor speed is reduced

from its design value these static vanes are progressively closed in order to maintain an



A6-8

acceptable air angle value onto the following rotor blades. Additionally interstage bleed may be provided but its use in design is now usually limited to the provision of extra margin while theengine is being accelerated, because use at steady operating conditions is inefficient andwasteful of fuel. Three types of air bleed systems are used: hydraulic, pneumatic and electronic.

Figure A6-7 Typical variable stator vanes. Courtesy Rolls Royce.

For casing designs the need is for a light but rigid construction enabling blade tip clearances to be accurately maintained ensuring the highest possible efficiency. These needs are achieved byusing aluminium at the front of the compression system followed by alloy steel as compressiontemperature increases. Whilst for the final stages of the compression system, where temperaturerequirements possibly exceed the capability of the best steel, nickel based alloys may berequired. The use of titanium in preference to aluminium and steel is now more common;

particularly in military engines where its high rigidity to density ratio can result in significantweight reduction. With the development of new manufacturing methods component costs cannow be maintained at a more acceptable level in spite of high initial material costs.

Stator vanes are normally produced from steel or nickel based alloys, a prime requirement beinga high fatigue strength when notched by ingestion damage. Earlier designs specified aluminium

alloys but because of its inferior ability to withstand damage its use has declined. Titanium may be used for stator vanes in the low pressure area but is unsuitable for the smaller stator vanesfurther rearwards in the compression system because of the higher pressures and temperaturesencountered. Any excessive rub which may occur between rotating and static components as aresult of other mechanical failures, can generate sufficient heat from friction to ignite thetitanium. This in turn can lead to expensive repair costs and a possible hazard.



A6-9

Figure A6-8 A hydraulically operated bleed valve and inlet guide vaneairflow control system.

In the design of rotor discs, drums and blades, centrifugal forces dominate and the requirementis for metal with the highest ratio of strength to density. This results in the lightest possible rotor assembly which in turn reduces the forces on the engine structure enabling a further reduction in

weight to be obtained. For this reason, titanium even with its high initial cost is the preferredmaterial and has replaced the steel alloys that were favoured in earlier designs. As higher temperature titanium alloys are developed and produced they are progressively displacing thenickel alloys for the disc and blades at the rear of the system.

Materials

Materials are chosen to achieve the most cost effective design for the components in question,in practice for aero engine design this need is usually best satisfied by the lightest design thattechnology allows for the given loads and temperatures prevailing.



A6-10

Figure A6-9 Typical types of fan blades.

Centrifugal impeller material requirements are similar to those for the axial compressor rotors.Titanium is thus normally specified though aluminium may still be employed on the largestlowpressure ratio designs where robust sections give adequate ingestion capability and

temperatures are acceptably low.

Balancing

The balancing of a compressor rotor or impeller is an extremely important operation in itsmanufacture. In view of the high rotational speeds and the mass of materials any unbalancewould affect the rotating assembly bearings and engine operation. Balancing on these parts iseffected on a special balancing machine.

Centrifugal Flow Compressor

Centrifugal flow compressors have a single or double-sided impeller and occasionally a two-stage, single sided impeller is used. The impeller is supported in a casing that also contains aring of diffuser vanes. If a double-entry impeller is used, the airflow to the rear side is reversedin direction and a plenum chamber is required. The impeller shaft rotates in ball and roller

bearings and is either common to the turbine shaft or split in the centre. The impellor shaft isconnected by a coupling, which is usually designed for ease of detachment.



A6-11

Figure A6-10 Two stage cylindrical compressor and two stage turbine. PGT2 gasturbine. Courtesy Nuovo Pigneone.

Impellers

The impeller is rotated at high speed by the turbine and air is continuously induced into thecentre of the impeller. Centrifugal action causes it to flow radially outwards along the vanes tothe impeller tip, thus accelerating the air and also causing a rise in pressure to occur. The engineintake duct may contain vanes that provide an initial swirl to the air entering the compressor.The impeller consists of a forged disc with integral, radially disposed vanes on one or both side

forming convergent passages in conjunction with the compressor casing. For ease of manufacture straight radial vanes are usually employed. To ease the air from axial flow in theentry duct on to the rotating impeller, the vanes in the centre of the impeller are curved in thedirection of rotation.

Diffusers

The air, on leaving the impeller, passes into the diffuser section where the passages formdivergent nozzles that convert most of the kinetic energy into pressure. To maximize the airflowand pressure rise through the compressor requires the impeller to be rotated at high speed,therefore impellers are designed to operate at tip speeds of up to 1,600 ft. per sec. To maintain

the efficiency of the compressor, it is necessary to prevent excessive air leakage between theimpeller and the casing; this is achieved by keeping their clearances as small as possible. Thediffuser assembly may be an integral part of the compressor casing or a separately attachedassembly. In each instance it consists of a number of vanes formed tangential to the impeller.The vane passages are divergent to convert the kinetic energy into pressure energy and the inner edges of the vanes are in line with the direction of the resultant airflow from the impeller. Theclearance between the impeller and the diffuser is an important factor



A6-12

GAS GENERATOR (CORE ENGINE )

Combustion System

The combustion chamber (has the difficult task of burning large quantities of fuel, suppliedthrough the fuel spray nozzles, with extensive volumes of air, supplied by the compressor, andreleasing the heat in such a manner that the air is expanded and accelerated to give a smoothstream of uniformly heated gas at all conditions required by the turbine. This task must beaccomplished with the minimum loss in pressure and with the maximum heat release for thelimited space available. The amount of fuel added to the air will depend upon the temperaturerise required. However, the maximum temperature is limited to within the range of 850 to 1700

qC, determined by the temperature limitations for the materials from which the turbine bladesand nozzles are made.

The air has already been heated to between 200 and 550qC the work done during compression,

giving a temperature rise requirement of 650 to 1150 ºC from the combustion process. Since thegas temperature required at the turbine varies with engine thrust, the combustion chamber mustalso be capable of maintaining stable and efficient combustion over a wide range of engineoperating conditions. Efficient combustion has become increasingly important because of theneed to carbon emissions and atmospheric pollution.

Figure A6-11 Avon gas generator with combustion chamber and surrounding fuelnozzles visible to right.

Combustion Process

Air from the engine compressor enters the combustion chamber at a velocity up to 200 m s-1 sec,Because at this velocity the air speed is far too high for combustion, the first thing that thechamber must do is to diffuse it, i.e. decelerate it and raise its static pressure. The speed of

burning fuel at normal mixture ratios is only a few feet per second, any fuel lit even in thediffused air stream, which now has a velocity of about 80 feet per second, would be blownaway. A region of low axial velocity has therefore to be created in the chamber, so that theflame will remain alight throughout the range of engine operating conditions. Designs of combustor and fuel nozzle are shown in Figure A6-12 below.



A6-13

Figure A6-12 Multiple combustor in EGT typhoon gas turbine. Right, combustionchamber liner. Courtesy EGT, Nuovo Pigneone

In normal operation, the overall air/fuel ratio of a combustion chamber can vary between 45:1and 130:1. However, fuel will only burn efficiently at, or close to, a ratio of 15:1, so the fuelmust normally be burned with only part of the air entering the chamber, in what is called a

primary combustion zone. This is achieved by means of a flame tube (combustion liner ).Approximately 20 per cent of the air mass flow is taken in by the snout or entry section.Immediately downstream of the snout are swirl vanes and a perforated flare, through which air

passes into the primary combustion zone. The swirling air induces a flow upstream of the centreof the flame tube and promotes the desired recirculation. The air not picked up by the snoutflows into the annular space between the flame tube and the air casing.

Through the wall of the flame tube body, adjacent to the combustion zone, are a selectednumber of secondary holes through which a further 20 per cent of the main flow of air passesinto the primary zone. The air from the swirl vanes and that from the secondary air holesinteracts and creates a region of low velocity recirculation. This takes the form of a toroidalvortex,similar to a smoke ring, which has the effect of stabilizing and anchoring the flame . Therecirculating gases hasten the burning of freshly. It is arranged that the conical fuel spray fromthe nozzle intersects the recirculation vortex at its centre. This action, together with the generalturbulence in the primary zone, greatly assists in breaking up the fuel and mixing it with the

incoming air .

The temperature of the gases released by combustion is about 1,800 to 2,000 qC which is far too hot for entry to the nozzle guide vanes of the turbine. The air not used for combustion,which amounts to about 60 per cent of the total airflow, is therefore introduced progressivelyinto the flame tube. Approximately a third of this is used to lower the gas temperature in thedilution zone before it enters the turbine and the remainder is used for cooling the walls of theflame tube. This is achieved by a film of cooling air flowing along the inside surface of theflame tube wall, insulating it from the hot combustion gases. A recent development allowscooling air to enter a network of passages within the flame tube wall before exiting to form aninsulating film of air, this can reduce the required wall cooling airflow by up to 50 per cent.Combustion should be completed before the dilution air, enters the flame tube, otherwise the



A6-14

incoming air will cool the flame and incomplete combustion will result. An electric spark froman igniter plug initiates combustion and the flame is then self sustained.

Figure A6-13 Flame stabilizing and general airflow pattern through a combustionchamber. Courtesy Rolls Royce

Fuel Supply

Fuel is supplied to the airstream by one of two distinct methods. The most common is theinjection of a fine atomized spray into the recirculating airstream through spray nozzles. The

second method is based on the pre-vaporization of the fuel before it enters the combustion zone.In the vaporizing method the fuel is sprayed from feed tubes into vaporizing tubes which are

positioned inside the flame tube. These tubes turn the fuel through 180 degrees and, as they areheated by combustion, the fuel vaporizes before passing into the flame tube. The primaryairflow passes down the vaporizing tubes with the fuel and also through holes in the flame tubeentry section which provide fans of air to sweep the flame rearwards.

Types Of Combustion Chamber

The design of a combustion chamber and the method of adding the fuel may vary considerably, but the airflow distribution used to effect and maintain combustion is always very similar to thatdescribed. Dilution air is metered into the flame tube in a manner similar to the atomizer flame

tube. There are three main types of combustion chamber in use for gas turbine engines. Theseare the multiple chamber, the tubo-annular chamber and the annular chamber.

Multiple combustion chamber

This type of combustion chamber is used on centrifugal compressor engines and the earlier types of axial flow compressor engines. It is a direct development of the early type of Whittlecombustion chamber. The major difference is that the Whittle chamber had a reverse flow but,as this created a considerable pressure loss, the straight-through multiple chamber wasdeveloped by Joseph Lucas Limited. The chambers are disposed around the engine andcompressor delivery air is directed by ducts to pass into the individual chambers. Each chamber has an inner flame tube around which there is an air casing. The air passes through the flame

tube snout and also between the tube and the outer casing as already described .The separate



A6-15

flame tubes are all interconnected. This allows each tube to operate at the same pressure andalso allows combustion to propagate around the flame tubes during engine starting.

Tubo-annular combustion chamber

The tubo-annular combustion chamber bridges the evolutionary gap between the multiple andannular types. A number of flame tubes are fitted inside a common air casing. The airflow issimilar to that already described. This arrangement combines the ease of overhaul and testing of the multiple system with the compactness of the annular system.

Annular combustion chamber

The annular combustion chamber is the design most favoured in modern aero-derivative gasturbines, such as those used offshore. This type of combustion chamber consists of a singleflame tube, completely annular in form, which is contained in an inner and outer casing . Theairflow through the flame tube is similar to that already described, the chamber being open at

the front to the compressor and at the rear to the turbine nozzles.The main advantage of theannular chamber is that, for the same power output, the length of the chamber is only 75 per cent of that of a tubo-annular system of the same diameter, resulting in considerable saving of weight and production cost.

Another advantage is the elimination of combustion propagation problems from chamber tochamber. In comparison with a tubo-annular combustion system, the wall area of a comparableannular chamber is much less; consequently the amount of cooling air required to prevent the

burning of the flame tube wall is less, by approximately 15 per cent. This reduction in coolingair raises the combustion efficiency to virtually eliminate unburnt fuel, and oxidizes the carbonmonoxide to non-toxic carbon dioxide, thus reducing air pollution.The introduction of the air spray type fuel spray nozzle to this type of combustion chamber also greatly improves the

preparation of fuel for

Fuel manifold

The fuel manifold supplies fuel to the combustion nozzles via a series of pipes. Fuel flow andignition is controlled by the control system. The start up process for turbines is controlled to

provide adequate air flow through the compressor before fuel is injected. Excessive build up of fuel due to failed starts has in a number of cases lead to internal explosion within thecombustion chamber and damage to the turbine.

Combustion Nozzles

Fuel is injected into the turbine through a series of injection nozzles. Design is such as to injectthe fuel into reverse flow to ensure uniform dispersion and mixing with the air prior to ignition.Ignition is usually by inductive discharge.



A6-16

TRANSITION PIECE

The transition piece leading from the combustion chamber to the power turbine encounterssome of the highest temperatures in the gas turbine. Oxidation, erosion and cracking of the

transition piece are key concerns. There has been significant development of specialised NDEmethods including thermography for inspection and wall thickness wall loss measurement intransition pieces.

Figure A6-14 Transition piece leading into power-turbine. Courtesy Sulzer

POWER TURBINE (PT)

The power turbine has the task of providing the power to drive the compressor and accessoriesand, in the case of driven equipment of providing shaft power for power generation, thecompressor or pump. It does this by extracting energy from the hot gases released from thecombustion system and expanding them to a lower pressure and temperature. High stresses areinvolved in this process, and for efficient operation, the turbine blade tips may rotate at speedsover 1,500 feet per second. The continuous flow of gas to which the turbine is exposed mayhave an entry temperature between 850 and 1,700 deg C and may reach a velocity of over 2,500feet per second in parts of the turbine.

To produce the driving torque, the turbine may consist of several stages each employing onerow of stationary nozzle guide vanes and one row of moving blades. The number of stagesdepends upon the relationship between the power required from the gas flow, the rotationalspeed at which it must be produced and the diameter of turbine permitted.

The number of shafts, and therefore turbines, varies with the type of engine. High compressionratio turbines usually have two shafts, driving high and low pressure compressors. On someturbines, driving torque is derived from a free-power turbine This method allows the turbine torun at its optimum speed because it is mechanically independent of other turbine andcompressor shafts.



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Figure A6-15 Power turbine rotor. Courtesy Rolls Royce

The mean blade speed of a turbine has considerable effect on the maximum efficiency possiblefor a given stage output. For a given output the gas velocities, deflections, and hence losses, arereduced in proportion to the square of higher mean blade speeds. Stress in the turbine discincreases as the square of the speed, therefore to maintain the same stress level at higher speedthe sectional thickness, hence the weight, must be increased disproportionately. For this reason,the final design is a compromise between efficiency and weight. Turbines operating at higher

turbine inlet temperatures are thermally more efficient and have an improved power to weightratio. The design of the nozzle guide vane and turbine blade passages is based on aerodynamicconsiderations The turbine depends for it’s operation on the transfer of energy between thecombustion gases and the turbine. This transfer is never 100 per cent because of thermodynamicand mechanical losses.

Figure A6-16 Turbine blades PGT2 gas turbine. Courtesy Nuovo Pigneone



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Figure A6-17 A typical turbine blade showing twisted contour

The losses which prevent the turbine from being 100 percent efficient are due to a number of reasons the turbine blades. A further 4.5 per cent loss would be incurred by aerodynamic lossesin the nozzle guide vanes, gas leakage over the turbine blade tips and exhaust system losses;these losses are of approximately equal proportions. The total losses result in an overallefficiency of approximately 92 per cent.

The basic components of the turbine are the combustion discharge nozzles, the nozzle guide

vanes, the turbine discs and the turbine blades. The rotating assembly is carried on bearingsmounted in common to the compressor shaft or connected to it by a self-aligning coupling.

Nozzle guide vanes

The nozzle guide vanes are of an aerofoil shape with the passage between adjacent vanesforming a convergent duct. The vanes are located in the turbine casing in a manner that allowsfor expansion. The nozzle guide vanes are usually of hollow form and may be cooled by passing

compressor delivery air through them to reduce the effects. of high thermal stresses and gasloads.



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Figure A6-18 Typical nozzle guide vanes showing their shape and location. CourtesyRolls Royce.

Turbine discs

Turbine discs are usually manufactured from a machined forging with an integral shaft or with aflange onto which the shaft may be bolted. The disc also has, around its perimeter, provision for the attachment of the turbine blades. To limit the effect of heat conduction from the turbine

blades to the disc a flow of cooling air is passed across both sides of each disc.

Turbine blades

The turbine blades are of an aerofoil shape, designed to provide passages between adjacent blades that give a steady acceleration of the flow up to the 'throat', where the area is smallest andthe velocity reaches that required at exit to produce the required degree of reaction.. The actualarea of each blade cross-section is fixed by the permitted stress in the material used and by thesize of any holes which may be required for cooling purposes (Part 9). High efficiency demandsthin trailing edges to the sections, but a compromise has to be made so as to prevent the bladescracking due to the temperature changes during engine operation.

The method of attaching the blades to the turbine disc is of considerable importance, since thestress in the disc around the fixing or in the blade root has an important bearing on the limiting

rim speed. The blades on the early Whittle engine were attached by the de Laval bulb rootfixing, but this design was soon superseded by the 'fir-tree' fixing that is now used in themajority of gas turbine engines. This type of fixing involves very accurate machining to ensurethat the loading is shared by all the serrations. The blade is free in the serrations when theturbine is stationary and is stiffened in the root by centrifugal loading when the turbine isrotating. Various methods of blade attachment are shown in fig. 5-9; however, the B.M.W.hollow blade and the de Laval bulb root types are not now generally used on gas turbineengines.

A gap exists between the blade tips and casing, which varies in size due to the different rates of expansion and contraction. To reduce the loss of efficiency through gas leakage across the bladetips, a shroud is often fitted. This is made up by a small segment at the tip of each blade which

forms a peripheral ring around the blade tips. An abradable lining in the casing may also be



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used to reduce gas leakage. Active Clearance Control (ACC.) is a more effective method of maintaining minimum tip clearance throughout the turbine cycle. Air from the compressor isused to cool the turbine casing and when used with shroudless turbine blades, enables higher temperatures and speeds to be used.

The flow characteristics of the turbine must be very carefully matched with those of thecompressor to obtain the maximum efficiency and performance of the engine. If, for example,the nozzle guide vanes allowed too low a maximum flow, then a back pressure would build upcausing the compressor to surge; too high a flow would cause the compressor to choke. In either condition a loss of efficiency would very rapidly occur.

Among the obstacles in the way of using higher turbine entry temperatures have always beenthe effects of these temperatures on the nozzle guide vanes and turbine blades. The high speedof rotation which imparts tensile stress to the turbine disc and blades is also a limiting factor.

Figure A6-19 PGT turbine rotor showing fir tree root attachment of turbine blades andblade clearances



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Figure A6-20 Various methods of attaching blades to turbine discs. Courtesy RollsRoyce

Nozzle guide vanes

Due to their static condition, the nozzle guide vanes do not endure the same rotational stresses asthe turbine blades. Therefore, heat resistance is the property most required. Nickel alloys areused, although cooling is required to prevent melting. Ceramic coatings can enhance the heatresisting properties and, for the same set of conditions, reduce the amount of cooling air

required, thus improving engine efficiency.

Turbine discs

A turbine disc has to rotate at high speed in a relatively cool environment and is subjected tolarge rotational stresses. The limiting factor which affects the useful disc life is its resistance tofatigue cracking. In the past, turbine discs have been made in ferritic and austenitic steels butnickel based alloys are currently used. Increasing the alloying elements in nickel extend the lifelimits of a disc by increasing fatigue resistance. Alternatively, expensive powder metallurgydiscs, which offer an additional 10% in strength, allow faster rotational speeds to be achieved.

Turbine blades.

The correct choice of blade material is important. The blades, while glowing red-hot, must bestrong enough to carry the centrifugal loads due to rotation at high speed. A small turbine bladeweighing only two ounces may exert a load of over two tons at top speed and it must withstandthe high bending loads applied by the gas to produce the many thousands of turbine horse-

power necessary to drive the compressor. Turbine blades must also be resistant to fatigue andthermal shock, so that they will not fail under the influence of high frequency fluctuations in thegas conditions, and they must also be resistant to corrosion and oxidization. In spite of all thesedemands, the blades must be made in a material that can be accurately formed and machined bycurrent manufacturing methods.

For a particular blade material and an acceptable safe life there is an associated maximum

permissible -turbine entry temperature and a corresponding maximum engine power. It is not



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surprising that metallurgists and designers are constantly searching for better turbine bladematerials and improved methods of blade cooling. Over a period of operational time the turbine

blades slowly grow in length. This phenomenon is known as creep and there is a finite usefullife limit before failure occurs. The early materials used were high temperature steel forgings,

but these were rapidly replaced by cast nickel base alloys which give better creep and fatigue properties.

Close examination of a conventional turbine blade reveals a myriad of crystals that lie in alldirections (equi-axed ). Improved service life can be obtained by aligning the crystals to formcolumns along the blade length, produced by a method known as Directional Solidification. Afurther advance of this technique is to make the blade out of a single crystal. Each methodextends the useful creep life of the blade and in the case of the single crystal blade, theoperating temperature can be substantially increased. A non-metal based turbine blade can

be manufactured from reinforced ceramics.

The balancing of a turbine is an extremely important operation in its assembly. In view of

the high rotational speeds and the mass of materials, any unbalance could seriously affectthe rotating assembly bearings and engine operation. Balancing is effected on a special

balancing machine.

Bearings and seals

The shafts on the air compressor and power turbine have bearings on both ends and associatedseals to allow free movement of the shaft The bearings are typically high integrity thrust and

journal bearings. The shaft rotates at hign velocity and the bearing must also cope with theaggressive environment and temperature fluctuations. There are a range of potential damagemechanisms ranging from wear and erosion of the surface to rolling contact fatigue andcracking. Degradation is also possible in the seals and bearing support structure. A lubrication

system ensures free flow of oil to the bearings and seals to prevent gas leaks. There are a largenumber of other seals within the turbine to control airflow and dispersion of gases.

Figure 21 Bearing design on modern gas turbine. Courtesy Rolls Royce.



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MECHANICAL DRIVE

Output Shaft and Coupling

The output shaft provides direct drive for driven equipment. In most cases this will be to agearbox to give greater flexibility in the drive speeds for the gas turbine and the equipment.

Drive Gearboxes

The inclusion of a drive gearbox within the machine package allows the manufacturer tooptimise operating speeds of the Gas Turbine driver and Centrifugal Compressor separately.The technical disadvantages of additional skid length, equipment complexity, and weight beingoffset with benefits for the design of compressor and turbine. Gas Turbine packages will includean Auxiliary Gearbox, normally integral to the cold end of the machine. This provides thenecessary linkage for turbine starting, and mechanical drives where required for oil or fuel

pumps. There are a limited number of safety issues from inclusion of a gearbox within a

machine package. The most serious are: the potential for accidental or failure engagement of auxiliary drives, used to rotate the compressor at low speed, leading to massive overspeed andusual disintegration of the drive; bursting of the gear wheels (design or manufacturing flaws),fires due to leakage of lubricating oil.

Main Drive Coupling

The use of flexible couplings within a gas turbine machine package is essential to provide thenecessary degrees of freedom to enable the machine elements to be aligned, and compensate for any flexibility inherent in the installation skid. Misalignment of the coupling, even within itstolerance limits, puts increased loads on adjacent shaft bearings. It also reduces the service lifeof the coupling, as flexible elements are subjected to greater strains. Coupling lubrication

(where required) and inspections needs to be proactively maintained as the coupling hassignificant mass and has the potential to become a dangerous missile if it fails. Loss of drive isnot normally a safety-related incident; special design requirements apply if drive continuity iscritical.

Ancillary Gearbox

Mechanical or electrical power is required to run a number of turbine support systems includingcooling, lubrication and fuel injection. Drive is commonly take from the air compressor shaft inthe cold portion of the engine and converted for turbine system drive using a gearbox or smallgenerator. This should be distinguished from the auxiliary gearbox used for mechanical drive of compressors and driven equipment, described separately in the main report.

Drive couplings

For dual-shaft turbine packages mechanical drive is achieved through an auxiliary gearbox withflexible couplings. These are described in more detail under driven equipment in HSE ResearchReport RR076.

EXHAUST SYSTEM

Exhaust air at very high temperatures is injected from the power turbine into the exhaustsystem. This is cooled and dispersed to the flare stack or waste heat recovery unit (WHRU).Approximately 50% of offshore turbine installations currently include waste heat recovery



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units. Because of the high temperatures the exhaust baffles are coated and lagged. Loss of lagging following storm conditions and ignition of the lagging following an oil or fuel leak arecommon sources of accidents. See analysis of dangerous occurrences and incidents for gasturbines in main report.

The high velocity of the air can generate significant noise. Consequentially the exhaust systemwill also include a silencer. Exhaust configuration may be axial, usual for WHRUs, or radialallowing the exhaust air to be passed to the flare at a higher level in the installation.

ANCILLIARY SYSTEMS

The gas turbine is dependent on various ancillary systems for safe operation, operating procedures and control system must ensure that these are operational prior to turbine start, andat all times during operation.

Lubrication System

The supply of oil for lubrication of bearings and couplings, support to sealing systems andhydraulic operation of actuators requires clean oil at appropriate pressures. For package unitsthis can be delivered from a common system feeding all elements within the package. Oil

pumps may be driven by electrical power or by auxiliary mechanical drives from the turbine.Electrical drives are much simpler and make pump location much easier. Where the installationhas reliable electrical supplies this option would be preferred. If the package is required tooperate in stand-alone manner even after a total electrical failure, then shaft drives are required.

Where a common lubrication system is fitted, in particular one which also provides compressor seal oil, there is a real issue of potential cross-contamination of the oil. Liquid fuel or theheavier fractions of hydrocarbon gases can dissolve in oil, reducing its viscosity and increasing

its flammability. The fire hazard associated with this potential problem will be greatly reducedif the oil system operates under a nitrogen atmosphere.

The most serious issues for the supply of oil to a machine package arise from either failure of the supply that can lead to damage of the machines, or from oil spill or leakage resulting in afuel source for potential fires.

Process Coolers

Process coolers, e.g. Intercoolers, will typically be shell and tube heat exchangers built to arecognised code. ASME and BS 5500 are commonly used. Ideally, cooling will be against aclosed fresh water cooling system, to minimise problems of corrosion, fouling and pollution.

Piping Systems

Piping systems are generally constructed to international standards, special standards arerequired for fuel gas where double skinned piping is installed.

Control and Anti Surge Valves

The gas compressor is likely to have discharge control , recycle and anti-surge control valves,the latter two duties may be combined. These valves are not necessarily provided by the

package vendor, but their specification, design, installation and control must be carefullyintegrated into the operation of the package. Any changes in duty or design must be allowed for in the valve design and set-up.



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Condition Monitoring

Condition monitoring on larger turbine packages will be provided as part of the package.

Vendors will offer their own preferred system, or will agree to tailor a system to suit the client'srequirements. It is important to ensure that the system provided suits the proposed method of operating and maintaining the equipment.

Fuel and Ignition System

The fuel system will take Gas and/or Liquid Fuel from the installation at the available pressure,filter the fuel(s) and raise pressure if necessary. The fuel system will control the rate of supplyof fuel(s) and isolate the supply when necessary.

Starter

To start the turbine it is necessary to rotate the turbine and air compressor, prior to injection andignition of fuel in the combustors. The starter is usually either pneumatic, hydraulic or avariable speed ac motor. Start up is a safety critical event and needs careful control sequences.Explosions have occurred with fuel build up after failed starts causing knock on damagethrough to the exhaust system

All-electric Actuators

All-electric actuator have recently been developed to replace hydraulic actuator systems.Hydraulic systems can suffer leaks, cleanliness issues,, complexity, poor efficiency and requirea separate servo system. Conventional all electric actuators have been tried previously but havesome drawbacks. Can’t be used in hazardous areas, EMI interference, need separate controller

in safe area, interconnect harnesses. A recent paper at IGTI 2004 reported on development of anintrinsically safe, explosion-proof actuator that can be used in zoned areas giving improvedcontrol response without overshoot.

Bearing Lube oil System

The supply of oil for lubrication of bearings and couplings, support to sealing systems andhydraulic operation of actuators requires clean oil at appropriate pressures. For package unitsthis can be delivered from a common system feeding all elements within the package.The oil pumps may be driven by electrical power or by auxiliary mechanical drives from theturbine. Electrical drives are much simpler and make pump location much easier. Where theinstallation has reliable electrical supplies this option would be preferred. If the package is

required to operate in stand-alone manner even after a total electrical failure, then shaft drivesare required.

Power requirements for control valves and other instruments must be considered. As the package lubrication system will be very congested, and fairly inaccessible, oil leaks from pumpseals or pipe joints will be difficult to detect and repair.

Where a common lubrication system is fitted, in particular one which also provides compressor seal oil, there is a real issue of potential cross-contamination of the oil. Liquid fuel or theheavier fractions of hydrocarbon gases can dissolve in oil, reducing its viscosity and increasingits flammability. The fire hazard associated with this potential problem is greatly reduced if theoil system operates under a nitrogen atmosphere.



The most serious issues for the supply of oil to a machine package arise from either failure of the supply that can lead to damage of the machines, or from oil spill or leakage resulting in afuel source for potential fires. Technical and safety aspects of lubrication systems are describedin more detail in RR076.

Oil pumps

The turbine will include oil pumps to provide lubrication to the seals and bearings. These may be motor or shaft driven.

Fuel boost pump (diesel)

Most turbines are capable of duel fuel operation. If diesel is used this will require a fuel boost pump.

Cooling system

Turbines generate extremely high temperatures (2000 ºC or more) in the combustion and gasgenerator systems. These temperatures are sufficient to cause melting or severe oxidation or degradation of components. A sophisticated cooling system using cold air passed axially fromthe outside of the air compressor is used to maintain temperatures within reasonable limits in the

power turbine and subsequent components. The transition piece on the combustor has toencounter particularly high temperatures.

Sealing gas System

As well a soil seals associated with the bearings, the turbine includes a sophisticated gas sealingsystem. This uses pressure differences to prevent leakage of turbine gases and air into

inappropriate parts of the system

Package mounting and skid

Mounting arrangements for the turbine package have been discussed earlier in the main report.This is usually based on 3-point mounting of equipment on robust frames. Mountingarrangements are particularly important on floating installations where larger degrees of tilt may

be incurred.

Anciliary Equipment and Systems

Ancilliary equipment includes the lube oil cooler (water, air), motor control center, switchgear,

neutral ground resistor and inlet fogger/cooler.

Published by the Health and Safety Executive

03 /06

Offshore Turbine Applications

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