NON-INTRUSIVE INSPECTION DNV Rp-G103

8/8/2019 NON-INTRUSIVE INSPECTION DNV Rp-G103

1/82

RECOMMENDED PRACTICE

DET NORSKE VERITAS

DNV-RP-G103

NON-INTRUSIVE INSPECTION

OCTOBER 2007


2/82

Comments may be sent by e-mail to [email protected] subscription orders or information about subscription terms, please use [email protected] information about DNV services, research and publications can be found at http://www.dnv.com, or can be obtained from DNV, Veritas-veien 1, NO-1322 Hvik, Norway; Tel +47 67 57 99 00, Fax +47 67 57 99 11.

Det Norske Veritas. All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, including pho-tocopying and recording, without the prior written consent of Det Norske Veritas.

Computer Typesetting (FM+SGML) by Det Norske Veritas.Printed in Norway

If any person suffers loss or damage which is proved to have been caused by any negligent act or omission of Det Norske Veritas, then Det Norske Veritas shall pay compensation to such personfor his proved direct loss or damage. However, the compensation shall not exceed an amount equal to ten times the fee charged for the service in question, provided that the maximum compen-sation shall never exceed USD 2 million.In this provision "Det Norske Veritas" shall mean the Foundation Det Norske Veritas as well as all its subsidiaries, directors, officers, employees, agents and any other acting on behalf of DetNorske Veritas.

FOREWORD

DET NORSKE VERITAS (DNV) is an autonomous and independent foundation with the objectives of safeguarding life, prop-erty and the environment, at sea and onshore. DNV undertakes classification, certification, and other verification and consultancyservices relating to quality of ships, offshore units and installations, and onshore industries worldwide, and carries out researchin relation to these functions.

DNV Offshore Codes consist of a three level hierarchy of documents:

Offshore Service Specifications. Provide principles and procedures of DNV classification, certification, verification and con-

sultancy services. Offshore Standards. Provide technical provisions and acceptance criteria for general use by the offshore industry as well asthe technical basis for DNV offshore services.

Recommended Practices. Provide proven technology and sound engineering practice as well as guidance for the higher levelOffshore Service Specifications and Offshore Standards.

DNV Offshore Codes are offered within the following areas:

A) Qualification, Quality and Safety Methodology

B) Materials Technology

C) Structures

D) Systems

E) Special Facilities

F) Pipelines and Risers

G) Asset OperationH) Marine Operations

J) Wind Turbines

Amendments and Corrections

This document is valid until superseded by a new revision. Minor amendments and corrections will be published in a separatedocument normally updated twice per year (April and October).

For a complete listing of the changes, see the Amendments and Corrections document located at:http://webshop.dnv.com/global/, under category Offshore Codes.

The electronic web-versions of the DNV Offshore Codes will be regularly updated to include these amendments and corrections.


3/82DET NORSKE VERITAS

Recommended Practice DNV-RP-G103, October 2007

Introduction Page 3

Executive summary

Pressure vessels and pressure systems are required to undergoperiodic, statutory inspection to ensure continued safe and reli-able operation. Traditionally this has been achieved by meansof an internal visual inspection (IVI), however, there can be

very high costs associated with shutting down a vessel (loss ofproduction), isolating it and preparing it for entry. Indeed,these costs can be much higher than the cost of the inspectionitself. Furthermore, the mechanical disturbances involved inpreparing the vessel for internal inspection and reinstating itmay adversely affect its future performance. Finally, and byno means least, man access may also be hazardous. There can,therefore, be significant advantages if inspections are per-formed from the outside of the vessel without breaking con-tainment i.e. non-invasively. However, there needs to be abalance between achieving these benefits and obtaining theinformation required to ensure continued safe and reliableoperation.

While it may often be the preferred option, non-intrusive

inspection (NII) represents a relatively new approach by com-parison to IVI and many engineers responsible for inspectionplanning have yet to build up experience with and confidencein its application. In addition, there are a wide variety of tech-niques available, each with its own specific capabilities andlimitations.

This has been recognised by industry and has led to the devel-opment of a number of guidance documents aimed at assistingplant operators to plan and justify NII. This recommendedpractice is intended to bring these documents together under asingle cover, and to provide a consistent and logical approachat all stages of the non-intrusive inspection process.

This recommended practice is primarily intended for thosewith responsibilities in the planning, implementation and

acceptance of vessel inspections. It is aimed at the inspectionof welded vessels constructed from metals, and related items,

fittings and connections associated with them, although thedocument is equally applicable to forged/spun metal pressurevessels, which contain no welds, as well as storage tanks andlarge diameter pipelines. Although not intended to be covered

by this document, some aspects of this recommended practicemay also be applicable to vessels manufactured from othermaterials and plant items other than pressurised equipment, inwhich case some of the general principles developed may wellstill apply.

The recommended practice provides guidance for:

i) determining when NII is appropriate in principle

ii) information requirements to plan for NII

iii) defining the requirements for the NII method(s) to beselected

iv) selecting methods that meet the requirements

v) evaluating the results of the inspectionvi) documentation requirements.

Acknowledgements

This recommended practice is based on a number of precedingdocuments and as such the permission of the previous authorsto incorporate their work is hereby gratefully acknowledged.In particular, we would like to thank members of the HOIS col-laborative project for their contributions and comments duringthe production of the recommended practice, Mitsui Babcockfor their permission to incorporate elements of the GSP235report covering aspects of the inspection planning procedure

and summary of inspection methods, and ESR Technology Ltdfor compiling the original text for this recommended practice.




Page 4 Introduction




Contents Page 5

CONTENTS

1. INTRODUCTION .................................................. 7

1.1 Background..............................................................7

1.2 Objectives of Non-Intrusive Inspection ................7

1.3 Scope .........................................................................7

1.4 Overview of the Recommended Practice...............8

1.5 Definitions ..............................................................10

2. INTEGRITY REVIEW ....................................... 10

2.1 General Approach .................................................10

2.2 Equipment Profile .................................................10

2.3 Risk Based Inspection Approaches......................11

2.4 Corrosion Risk Assessment ..................................112.4.1 Corrosion Risk Assessment Types ...................................11

2.5 Structural Integrity Assessment..........................12

2.6 Operational Experience ........................................12

3. THE DECISION GUIDANCE PROCESS......... 12

3.1 Introduction ..........................................................12

3.2 Screening ................................................................12

3.3 High-level decision process ..................................133.3.1 Confidence in ability to predict types and locations of

degradation ....................................................................... 143.3.2 Previous inspection effectiveness .....................................153.3.3 Severity and rate of degradation....................................... 163.3.4 NII recommendation.........................................................16

3.4 NII Decision Record ..............................................16

3.5 Examples ................................................................163.5.1 Heat Exchanger Vessel .................................................... 163.5.2 Gas Receiver Vessel ........................................................ 163.5.3 Separator Vessel ...............................................................173.5.4 Absorber Vessel ...............................................................17

4. INSPECTION PLANNING................................. 18

4.1 Introduction ...........................................................18

4.2 The Planning Team ...............................................20

4.3 Inspection Strategy Type .....................................204.3.1 Type A Inspection.............................................................214.3.2 Type B Inspection............................................................. 21

4.3.3 Type C Inspection............................................................. 214.3.4 Selection of Inspection Type ............................................21

4.4 Definition of Vessel Zones ....................................234.4.1 Identification of Zones .....................................................234.4.2 Example............................................................................ 24

4.5 Definition of Degradation Type............................24

4.6 Inspection Effectiveness........................................25

4.7 Required Inspection Effectiveness .......................26

4.8 Coverage.................................................................27

4.9 Selection of Inspection Method ............................274.9.1 Inspection Capability ........................................................274.9.2 Inspection Method Selection Flowcharts.......................... 274.9.3 Inspection Method Selection Criteria ...............................284.9.4 Statistical Methods............................................................29

4.10 Preparation of Work-pack ..................................43

4.11 Inspection Plan Review.........................................43

5. ON-SITE INSPECTION ACTIVITIES............. 44

5.1 Preparation for Inspection ................................... 445.1.1 Individual Responsibilities ...............................................445.1.2 Preparation Tasks .............................................................44

5.2 Performing the Inspection ................................... 45

5.3 Dealing with Non-conformances.......................... 45

5.4 Reporting of Results .............................................45

5.5 Demobilisation....................................................... 46

6. EVALUATION OF INSPECTION.................... 46

6.1 Introduction...........................................................46

6.2 Items to be checked for conformance..................46

6.3 Inspection Method.................................................466.3.1 Type A inspection.............................................................476.3.2 Inspection Types B and C.................................................47

6.4 Procedure...............................................................496.4.1 Type A inspection.............................................................496.4.2 Type B inspection.............................................................496.4.3 Type C inspection.............................................................49

6.5 Data Quality...........................................................496.5.1 Type A inspection.............................................................496.5.2 Type B inspection.............................................................496.5.3 Type C inspection.............................................................49

6.6 Location.................................................................. 496.6.1 Type A inspection.............................................................496.6.2 Type B inspection.............................................................496.6.3 Type C inspection.............................................................49

6.7 Coverage ................................................................ 496.7.1 Type A inspection.............................................................496.7.2 Type B inspection.............................................................506.7.3 Type C inspection.............................................................50

6.8 Critical Non-conformance.................................... 50

6.9 Reportable Indications and flaws ........................506.9.1 Type A inspection.............................................................506.9.2 Type B inspection.............................................................506.9.3 Type C inspection.............................................................50

6.10 Examples ................................................................ 506.10.1 Type A inspection.............................................................506.10.2 Type B inspection.............................................................516.10.3 Type C inspection.............................................................51

7. INSPECTION INTERVAL................................. 51

7.1 Discussion...............................................................51

7.2 Comparison with IVI ............................................ 51

7.3 Detailed Assessment..............................................54

7.4 Examples ................................................................ 557.4.1 Glycol Contactor...............................................................557.4.2 Glycol Flash Drum ........................................................... 567.4.3 Vent Knock Out Drum .....................................................577.4.4 HP Condensate Separator.................................................58

8. REFERENCES..................................................... 60

APP. A REVIEW OF NON INTRUSIVE NDTMETHODS......................................................................... 61

APP. B STATISTICAL APPROACHES TO NII......... 72




Page 6 Contents




Page 7

1. Introduction

1.1 Background

Pressure vessels and pressure systems are required to undergoperiodic, statutory and other non-destructive testing to ensurecontinued safe and reliable operation. This generally includesa requirement to inspect vessels for possible internal degrada-

tion. This is frequently achieved by means of an internal visualinspection (IVI) coupled with the use of surface flaw detectionmethods such as dye penetrant inspection (DPI) and magneticparticle inspection (MPI), however, there can be very highcosts associated with shutting down a vessel (loss of produc-tion), isolating it and preparing for it for entry. Indeed, thesecosts can be much higher than the cost of the inspection itself.Furthermore, the mechanical disturbances involved in prepar-ing the vessel for internal inspection and reinstating it mayadversely affect its future performance. Finally, and by nomeans least, man access may also be hazardous.

There can, therefore, be significant advantages if inspectionsare performed from the outside of the vessel without breakingcontainment i.e. non-invasively. However, there needs to be abalance between achieving these benefits and obtaining the

information required to ensure continued safe and reliableoperation. The acceptability and benefits of non-intrusiveinspection for a particular vessel will depend on a number offactors including:

vessel geometry materials potential deterioration mechanisms and modes locations and sizes of flaws of concern process historic inspection data confidence in inspection capability inspection costs.

Techniques for non-intrusive inspection (NII) of equipment

are becoming increasingly sophisticated and more widelyavailable. While it may often be the preferred option, NII rep-resents a relatively new approach by comparison to IVI andmany engineers responsible for inspection planning have yet tobuild up experience with and confidence in its application. Forthe purposes of this recommended practice, IVI is consideredto include the use of surface flaw detection methods such asDPI and MPI where appropriate. In addition to a general lackof experience, there are a wide variety of methods available tonon-intrusively inspect equipment, each with its own specificcapabilities and limitations.

This has been recognised by industry and a series of structuredguidance documents have been developed Ref. [4], [5] and [6]to assist plant operators to justify and plan NII. Each of these

documents deals with a specific aspect of the NII process, andthere are some minor inconsistencies in notation and approach.This recommended practice is intended to bring all three doc-uments together under a single cover, and to provide a consist-ent and logical approach at all stages of the non-intrusiveinspection process.

Many of the recommendations in this document are not uniqueto non-intrusive inspection since proper planning and adminis-tration is also important for internal inspection. The documentshould be considered in conjunction with other relevant guide-lines such as those contained in the HSE Best Practice for NDTdocument (Ref. [7]).

1.2 Objectives of Non-Intrusive Inspection

It is essential to be clear about the reasons for performing anon-intrusive inspection. The decision to carry out non-intru-sive inspection will normally depend on a number of differentfactors. A primary advantage is likely to be overall cost reduc-tion, but this may arise not because an internal inspection isreplaced, but, for example, when an unscheduled inspection is

required and it is hoped to avoid shutting down the plant bycarrying out the inspection non-invasively. It is important toclarify the objective of the non-intrusive inspection in advance,as this may have an impact on the approach to the inspection.

Questions that should be addressed may include:

Is the inspection to complement an internal inspection pro-gramme?

Is the inspection intended to replace an entire internalinspection or an internal inspection regime?

Potential benefits of performing a non-intrusive inspectioninclude:

Avoids man access which can be hazardous (possibilitiesof flammable or toxic residues which can be difficult toremove, adequate lighting may be difficult to achieve).

Planning for turnaround / shutdown. Identifying whatremedial work is likely at the next turnaround? Carryingout non-intrusive inspection allows the preliminaryinspection to be made before the plant is shutdown, pro-viding an opportunity for the turnaround to be shortenedby long-lead time planning and preparation (for repair and

maintenance based upon the NDT results) to be made inadvance of the start of the turnaround.

Shortening the turnaround. Shutdown duration may bereduced by carrying out most or all of the inspection work inadvance of the shutdown, allowing the turnaround to berestricted to mechanical work. This also simplifies planning.

Removal of requirement to break containment. No need toisolate, drain and purge the vessel. This may include partialbreak of containment, for example access to water/coolantside of a heat exchanger without breaking hydrocarbon con-tainment. This may lead to shortening of the turnaround.

Minimises disturbances to the vessel which could createnew problems.

It may be possible to avoid the need to shut down the ves-sel operation entirely. Inspections can be made on a differ-

ent cycle from any other maintenance, or the inspectionsmay be made at reduced capacity or temperature, ratherthan having to isolate, drain and purge the vessel.

Allows the inspection to be carried out when a potentialproblem is identified, without interfering with other oper-ations. This might occur when either routine surveillanceor unusual operating conditions suggest that damagemight have occurred.

1.3 Scope

This recommended practice is primarily intended for thosewith responsibilities in the planning, implementation andacceptance of vessel inspections. It is aimed at the inspectionof welded vessels constructed from metals, and related items,

fittings and connections associated with them. The documentis equally applicable to forged/spun metal pressure vessels,which contain no welds, as well as storage tanks and largediameter pipelines.

Although not intended to be covered by this document, someaspects of this recommended practice may also be applicableto vessels manufactured from other materials and plant itemsother than pressurised equipment, in which case some of thegeneral principles developed may well still apply.

The recommended practice provides guidance for:

1) determining when NII is appropriate in principle

2) information requirements to plan for NII

3) defining the requirements for the NII method(s) to beselected

4) selecting methods that meet the requirements

5) evaluating the results of the inspection

6) documentation requirements.




Page 8

The scope is limited by the following constraints.

It is assumed that the date of the next inspection followingNII will be determined in the same way as with any otherinspection, based on sound engineering judgement and anunderstanding of the degradation rates and tolerance of theequipment to degradation. This recommended practicetherefore makes no attempt to influence the timing of the

next inspection, other than where the intended scope of theNII inspection has not been fully met. No consideration is given to the relative cost of different

inspection options and the guidance process considerstechnical issues only.

The recommended practice is principally for application topressure vessels.

The recommended practice does not address legislativerequirements, which may in certain countries preclude theuse of non-intrusive methods. The recommended practiceconsiders only the technical aspects of the non-intrusiveinspection planning process, and the user should confirmthat any inspection plan derived using this document satis-fies national legislative requirements.

The recommended practice does not consider the impact

of external degradation mechanisms, for which inspec-tions are intrinsically non-intrusive and would beaddressed by conventional assessment means.

It is also important to note that the recommended practice doesnot aim to comprehensively cover every aspect of planning aninspection by NII but rather seeks to provide structured guid-ance at key stages of the process. Although the document pro-vides guidance on the general principles of non-intrusiveinspection it is not intended to be prescriptive, and readersshould assess each case point by point against their own crite-ria, using the document as a guideline. The recommendedpractice cannot and is not intended to replace sound engineer-ing and commercial judgement by competent personnel.

It should be recognised that with any scheme of examinationthere is a finite probability of missing flaws or degradationwhich could lead to failure. This applies to both invasive andnon-invasive inspection.

1.4 Overview of the Recommended Practice

Non-intrusive inspections generally require a more sophisti-cated approach than internal inspections. The recommendedpractice developed in this document recommends a systematicassessment of each item of equipment to be inspected usingNII. This is a staged process which sequentially considers:

1) when and where inspection is required2) whether NII is appropriate

3) the inspection plan

4) what inspection methods are appropriate

5) requirements during inspection

6) whether the inspection actually performed is adequate.

A summary of the recommended NII process is shown dia-grammatically in Figure 1-1. The colours used on the flowchart indicate the appropriate section of this recommendedpractice to which the action relates.

It is important to recognise that a transfer to a non-intrusive

inspection strategy is likely to require a step-change in theadministration and execution of the inspection. In particular,inspection methods are likely to be more elaborate when com-pared with internal visual inspection. Therefore the inspectionmust be controlled more rigorously, with the procedures (i.e.equipment, settings and reporting criteria) carefully scrutinisedand monitored at all stages in order to ensure that the inspec-tion objectives are met. As a consequence, it is likely that plan-ning of the inspection will need to be considerably morethorough (this is covered in Sec.4). In addition, the reportingformat must be precisely specified. If the results are notrequested in the correct form at the outset of the inspection orare inadequately reported, it can be difficult to transform thedata to the correct format, and useful information may be over-looked or lost.




Page 9

Figure 1-1Overview of NII Procedure.

PrepareEquipment Profile

Is NII possible?Apply screening criteria

(Figure 3-1)

Alternativeinspection

approach required

Yes

Review InspectionPlans

e.g.Access,practicality

Does plan meetrequirements?

Perform inspection

Select pressurevessel for

assessment

Review inspectionresults

Alternativeinspection possibleNo

Evaluateinspection(Figure 6-1)

Inspection meetsNII requirements?

Reducedinspection interval

(Figures 7-1 / 7-2)

No

Inspectioncomplete

Yes

Inspectionaccording to plan?

Yes

1. Integrity Review(section 2)

2. Decision Process(section 3)

3. Planning Process(section 4)

4. Inspection(section5)

5. Evaluation(section 6)

Return tostart

Reduced intervalacceptable/

possible

Alternativeinspectionrequired

No

Yes

Is NII possible?Apply high level decision

process(Figure 3-2)

IntegrityReview

e.g.RBI, CRA etc.

No No

Prepare work-pack

Yes

No

No

Determine date ofnext inspection

Prepare detailedinspection plan

(Figure 4-1)

Yes

Yes




Page 10

1.5 Definitions

In the context of this Recommended Practice the followingdefinitions apply. In certain cases these definitions are thesame as, or are based on, those which appear in other docu-ments such as those of the British Personnel Certification inNon-Destruction Testing (PCN) scheme.

2. Integrity Review

2.1 General Approach

Internal visual inspection (IVI) remains widely used on themajority of vessels and several accepted procedures exist forthe specification of inspection intervals to ensure safe opera-tion. Traditionally, such intervals have been specified on thebasis of legislative requirements, but increasingly plant ownerstake risk considerations into account in order to maximise thecost benefit of inspection. The decision to apply non-intrusiveinspection methods for a particular item of equipment candepend critically on the type and extent of flaws or degradationmechanisms expected. Non-intrusive inspection methods areoften slower and more expensive to apply than internal visualinspections, so that 100% inspection of a vessel is oftenimpractical. If non-intrusive inspection (NII) is to be used inconjunction with, or as an alternative to IVI, then the inspec-tion requirements should be defined such that the risk levelsare not increased. For any given vessel this effectively meansthat there should not be an increase in the probability of failure(POF) when NII is used. Non-intrusive inspection thereforerequires more careful consideration of the parts of the vessel tobe inspected, the flaws/degradation to be detected and theinspection methods to be applied than is normally the case forinternal (e.g. visual) inspections. Nonetheless, many of thedetailed planning considerations will remain the same for NIIas for IVI hence many of the traditional and more recent riskbased inspection (RBI) planning approaches will remain appli-cable.

This section of the recommended practice is intended to provide

an overview of the various data requirements that are necessary inorder to be able to successfully plan for non-intrusive inspection.

2.2 Equipment Profile

The first stage of the non-intrusive inspection planning process

Area Specified zone where inspection will be carriedout. In many instances this area will contain aspecific feature (e.g. weld) which is of particularinterest.

Capability Capability is used to qualitatively describe anNDT methods ability to detect (POD) and sizeflaws.

Certification Procedure used to demonstrate the qualificationof NDT personnel in a method, level and indus-trial sector, and leading to the issue of a certifi-cate.

Competency Capability to perform a given task on the basis ofeducation, training, qualification and experiencefollowing objective assessment. To achieve theappropriate level of competency might require ateam.

Corrosion riskassessment(CRA)

An assessment of the susceptibility of the struc-ture under investigation to all in-service degrada-tion mechanisms that may affect it. The CRA isnot restricted to simply those degradation mecha-nisms related to corrosion.

Coverage Defines the proportion of the structure or regionthereof under consideration that is actually sub-ject to inspection, i.e.

Criticality A function of the risk associated with theinspected equipment, incorporating likelihood ofdegradation occurring and associated conse-quences.

Defect A defect is here taken to be a flaw which rendersthe equipment unfit for its specified service in itscurrent state.

Degradationmechanism

Those mechanisms by which integrity of the pipeor vessel could potentially be impaired e.g. ero-sion, fatigue, creep, brittle fracture, wall loss etc.

Effectiveness A qualitative measure of the probability ofdetecting flaws, taking coverage into account.

Effectiveness = f (POD, Coverage).

Three effectiveness categories are used, (high,medium and low), these being defined by com-parison to the POD for visual inspection. Highimplies a higher POD, medium implies a broadlysimilar POD and low implies a lower POD.

Feature Specific part of area to be inspected i.e. weld,nozzle etc.

Flaw The physical manifestation of a degradationmechanism, in terms of cracking, pitting or wallloss etc.

Inspection body The organisation which manages the perform-ance of the NDT inspection (e.g. inspection ven-dor)

Inspectionmanager

The plant owners representative with overallresponsibility for the inspection.

InspectionMethod

A specific way of applying a NDT method (e.g.Pulse echo, TOFD, Radiography etc.)

Inspectionsupervisor

The leader of the site inspection team with overallresponsibility for coordinating and supervisingthe inspection.

Area inspectedCoverage

Total area under consideration=

Internal Visual Inspection (IVI)

This is considered as an intrusive close visualexamination of all internally accessible platematerial and, where applicable, conventionalmagnetic particle (MPI) or dye penetrant (DPI)inspection of welds. In the context of this Recom-mended Practice, the term IVI is not intended tocover a less rigorous general visual examination.

Non-Intrusive

Inspection (NII)

This refers to any inspection performed from the

outside of the vessel without having to break con-tainment and/or not requiring vessel entry. It maybe performed on-stream or off-stream.The terms non-invasive and non-intrusiveare often used interchangeably.

Operator/technician

Qualified NDT personnel who execute theinspection.

Probability ofdetection (POD)

Probability of detecting a defined flaw type andsize in the area covered by the inspection method.

Procedure A written description of all essential parametersand precautions to be observed when applying anNDT method to a specific test, following anestablished standard, code or specification.

Qualification Evidence of training, professional knowledge,skill and experience as well as physical fitness to

enable NDT personnel to properly perform NDTtasks, which satisfies the requirements ofEN 473 (Ref. [1]) and ISO 9712 (Ref. [2]), e.g.PCN (Ref. [3]), ASNT / TC1A.

Risk basedinspection (RBI)

Process of planning inspection requirementsthrough a detailed assessment of the relativeprobabilities of failure and their associated conse-quences.

Work-pack A complete package of documents (procedures,drawings, standards etc.) relevant to the inspec-tion outlining scope and details of inspection tobe performed.




Page 11

is to carry out a detailed review of the equipment. The intentionis to ensure that the inspection planners have a comprehensiveunderstanding of the design, operation, current condition andanticipated degradation mechanisms that may affect the equip-ment in service. This background information should be com-piled into an equipment profile, which should form a part of theauditable document trail for the inspection. Background infor-mation that should be included in the equipment profile is as

follows:Identity and design

Vessel unique reference number, general arrangement draw-ings, materials, current design basis (pressure, temperature,corrosion allowance, cycling regime etc.)

Type of vessel and function

e.g. separator, heat exchanger, boiler, storage tank, blowdownvessel, reactor, etc.

Operation and service details

Process fluids and possible contaminants, operating tempera-tures, operating pressures, loading and temperature cycles,transients, excursions outside normal operating envelope,

insulation systems.Detailed drawings

Number and type of welds, longitudinal /circumferential shellwelds, welds on the domed end/dished end, nozzles, man-ways, construction details including saddles, supports and sup-port skirts, flanges, compensating plates, insulation etc.

Modifications and repairs

Has the vessel been modified since its original commission-ing? Have any previous flaws or damage been removed orrepaired (dates)? N.B. it is important to maintain records ofany such modifications or repairs.

Previous inspection results

Details of known/previously reported flaws/areas of degrada-

tion. Inspection methods and coverage.

General experience

Flaws/degradation/failures in other similar vessels (if availa-ble)

Complementary information

Information from corrosion monitors etc. providing evidenceon whether degradation is occurring.

Accessibility

General accessibility, access limitations. There may be scaf-folding requirements stated for each vessel. This sometimesaccompanies the data held on vessels or is noted in drawings orisometrics. Though this is primarily a cost issue, there may be

occasions when there are overriding factors that preclude sat-isfactory external access or the construction of suitable scaf-folding.

Safety Requirements

Details should be recorded of any safety requirements pertain-ing to the equipment. Examples of items to consider are listedbelow, however any other safety related information consid-ered pertinent to the inspection should also be recorded.

general safety procedures site-specific procedures including requirements for local

induction courses job-specific risk assessments local safety review requirements for equipment (at stores/

safety officer) safety testing and certification requirements for equipment

Portable Appliance Testing (PAT) certification etc. requirement for inherently safe equipment restrictions on ionising radiations personal protection equipment.

Possible flaws/degradation

Potential failure modes and effects. Possible flaw/degradationlocations. Possible flaw/degradation morphology/ sizes.Reporting criteria. (See also Sec.2.3 and 2.4 below)

The following sections describe in brief some of the estab-lished techniques used to obtain some of this information.

2.3 Risk Based Inspection ApproachesCurrent practice is increasingly to consider the risk presentedby any particular item of equipment to the plant [8] whenestablishing an inspection schedule. Risk based inspection(RBI) processes consider the probability of a failure of theequipment and the associated consequences in order to deter-mine an overall risk ranking. The frequency and extent ofinspection required are then set on the basis of this risk rank-ing. Effective RBI depends heavily on a good knowledge ofthe degradation mechanisms and their growth rates, the inspec-tion history, operating conditions etc. i.e. much of the knowl-edge that goes into RBI is the same as that required to enablejustification and planning for NII.

2.4 Corrosion Risk Assessment

Different non-intrusive inspection methods have differentcapabilities for detecting and sizing flaws, and therefore theinspection manager requires a more detailed knowledge of thetypes and locations of flaws which may be present in a partic-ular item of equipment than is the case for an internal visualinspection. Hence, one of the key sources of information forthe NII assessment is the Corrosion Risk Assessment (CRA).In practice, most rigorous integrity management systems willalready incorporate a consideration of the type and likelihoodof degradation expected for each vessel, as this is also arequirement for most RBI assessments.

The corrosion risk assessment is a formal review of the degra-dation mechanisms to which a particular plant item may besusceptible, along with a determination of the anticipated deg-radation rates. Typically the corrosion risk assessment willconsider flaws or degradation such as:

general corrosion over the whole area local corrosion pitting erosion general or preferential corrosion of welds (including

grooving) loss of or damage to cladding or lining cracking under cladding or lining cracking in or near welds cracking at or near nozzles or other perforations cracking at or from internal fitting welds, nubs etc.

stress corrosion cracking in parent material hydrogen damage (e.g. blistering, stepwise cracking) damage to seals, flange gasket, flange faces damage to internals (not normally detectable by NII).

In addition, it is important to consider the different and possi-bly unusual flaw morphologies which can occur (e.g. micro-biological influenced corrosion) since these aspects caninfluence the selection and capability of non-intrusive inspec-tion methods.

2.4.1 Corrosion Risk Assessment Types

Corrosion risk assessments can be carried out at very differentlevels within different organisations. Given the importance of

the assessment to the NII planning process, it is thereforeimportant to be clear about what level of assessment has beencarried out. The following CRA Types have been defined inorder to describe the nature and extent of the CRA carried out.These CRA Types are used in the NII decision process later inthis recommended practice (Sec.3).




Page 12

CRA Type 1

A basic assessment considering primarily existing inspectionresults and expected degradation based on experience withother similar vessels. This level of assessment is what wouldexist as a minimum to meet inspection planning requirementsby RBI. It would be carried out and reviewed by competentindividuals from appropriate discipline groups including, for

example, inspection, metallurgy/materials, process and engi-neering.

CRA Type 2

A more detailed assessment providing documented considera-tion of at least the following:

the vessel's condition, based on previous inspectionswhenever these have been carried out

the vessel's metallurgy, modification and repair history

the process fluid composition and operating conditions

corrosion management

changes to any of the above factors that may affect inspec-tion requirements

types of degradation growth rates for each type of degradation identified as of

concern (or at least some estimate related to severity, e.g.a likelihood type value from a criticality assessment)

the locations where each type of degradation is likely to beactive.

The assessment should consider existing inspection results andtheoretical predictions. Theoretical predictions should bebased on process and materials information.

CRA Type 3

A comprehensive assessment including an in-depth theoreticalstudy based on process and materials information. The follow-

ing issues need to be addressed:

types of degradation

growth rates for each type of degradation identified as ofconcern

the locations where each type of degradation is likely to beactive

upset conditions that can lead to accelerated degradation

potential for incorrect identification or omission of degra-dation mechanisms.

A Type 3 assessment is carried out in detail on a vessel by ves-sel basis.

CRA Type 4This meets the requirements of a Type 3 assessment withrespect to the theoretical study but also includes considerationof inspection results from the vessel following at least one in-service inspection. Some interpretation/correlation of the pre-dicted degradation and condition found by inspection shallalso have been carried out.

2.5 Structural Integrity Assessment

It is important to have an understanding of the equipmentsability to resist structural damage following degradation. In itssimplest form, this can be knowledge of the margins againstoperating conditions and corrosion allowance incorporated atthe design stage. In certain circumstances however, a morecomprehensive fitness for service assessment will have beencarried out (for example using API 579 [9) which will provideinformation on, for example, critical crack dimensions. Allsuch information provides the inspection manager with evi-dence as to how tolerant the equipment is to degradation,

which in turn will influence the effectiveness of inspectionrequired in order to provide assurance that failure will notoccur in-service between scheduled inspections.

2.6 Operational Experience

Where the user has extensive experience with the same or sim-ilar vessels in the same or similar service, operational experi-

ence provides a very useful corroboration of the theoreticalassessments described previously. Inspection histories outlin-ing the types and sizes of any flaws found in service (or indeedthe absence of flaws), give a clear indication of what inspec-tion requirements are likely to be necessary. Of course, this isonly true if the inspection carried out is appropriate for the deg-radation anticipated.

3. The decision guidance process

3.1 Introduction

Having carried out the integrity review and obtained the nec-

essary information regarding the equipment condition at thetime of the last inspection, it is necessary to determine whetherthe equipment is intrinsically suitable for non-intrusive inspec-tion, or whether an alternative, intrusive technique such as IVIis necessary. This decision process is the second stage of theNII planning process, shown in Figure 1-1. Through applica-tion of a flow chart, the decision guidance process determineswhether NII should be considered for the inspection of a givenpiece of equipment.

The process can be broken down into two main stages, namelyscreening and the high level decision process. The process isshown diagrammatically in Figure 3-1, and is described inmore detail in the following sections.

3.2 Screening

The purpose of this first stage is to rapidly identify those ves-sels for which NII should not be considered or where therequired information can not be obtained from such an inspec-tion. The screening process is based on the users response toeach of the following questions.

Is the vessel intrinsically suitable for NII?

Before proceeding any further with the NII decision process, itis necessary to confirm that the equipment is intrinsicallysuited to inspection by non-intrusive means; that is that thereare no immediately obvious impediments to NII being under-

taken. These include factors such as where there is no access tothe vessel exterior, extreme surface temperatures, geometryconstraints and restrictions to access, as well as any require-ment for inspection of internal fittings.

Has the vessel previously been inspected and is the history stillrelevant?

Vessels with no previous in-service inspection history or forwhich there is reason that the inspection history may no longerbe relevant (due for example to a change in process conditions)should not normally be considered for NII. There are two pos-sible exceptions to this recommendation, as covered by thenext two screening questions.

Is the vessel designed specifically for NII?Where a vessel is designed specifically for inspection by NII,such inspection should be considered from the outset or evenwhen conditions may have changed (while remaining withinthe design intent).




Page 13

Figure 3-1NII Screening Procedure

Is the vessel similar to others for which service history exists?The intent of this question is to identify if there are other ves-sels whose inspection history may be directly relevant to thevessel under consideration. Hence the answer can only betaken as yes for vessels:

substantially the same in terms of design, geometry, con-struction and conditions of service (i.e. normally empty /full, etc.).

and

for which no factors with potential to cause a difference inthe nature, distribution or rate of degradation can be iden-tified.

Similar vessels shall be taken to mean vessels substantially thesame in function, geometry, design, material and construction.Similar service shall be taken to mean substantially the samein each of chemistry, fractions and phase(s) of the vessel con-tents, process type(s), flow rates and temperatures. Whenclassing vessels as similar, justification must be provided.

Is entry scheduled for other reasons?

When the vessel is to be opened for other reasons, advantageshould be taken of the opportunity to perform an internal visualinspection. This does not mean that NII should not be done.However, if it is intended to do NII in parallel with IVI thenthis can be done without additional justification.

3.3 High-level decision process

The next step is to decide whether sufficient information existsto plan the non-intrusive inspection and what inspection effec-tiveness is required. This requires consideration of how confi-dently potential flaw types and locations can be predicted, theeffectiveness of previous inspections, and the severity and rateof any known or predicted degradation. This is achieved usingthe high-level decision process, which determines whether NIIis appropriate in principle, based largely on the use of the deci-sion tree shown in Figure 3-2. This leads the user to a decisionon whether NII is appropriate in principle based on the catego-ries selected for each of three factors. The factors and criteria

STARTSelect vessel for IVI / NII

Yes

No

Has vessel previouslybeen inspected?

Yes

No

Was vessel designedspecifically for NII ?

No

Yes

Is vessel the same asothers for which service

history exists?

Yes

No

Is entry scheduled for otherreasons?

Apply flow chart for highlevel decision(Figure 3 -2)

Yes

No

Can NII be considered

NII is not appropriate

Prepare NII inspectionplan.

Is operating history still

relevant? No

Yes

Perform IVI NII is not appropriate

Is the vessel intrinsicallysuitable for NII?

No

Yes

NII is not appropriate




Page 14

for categorisation are covered below.

3.3.1 Confidence in ability to predict types and locationsof degradation

The decision on whether NII is appropriate in principle isbased to a large extent on confidence in being able to predictall active degradation mechanisms and hence specify methodscapable of identifying the associated flaws. The ability to pre-dict degradation mechanisms relevant to the vessel under con-

sideration and their locations will depend on a number offactors. Evidence can be considered from two main sources, asdescribed below:

theoretical: The nature of the integrity management systemsemployed

evidential : Evidence available from the same or similar ves-sels in the same or similar service (as defined

previously).

Figure 3-2High Level Decision Guidance Chart

Confidence in ability to

predict type andlocation of degradation.

Previous inspectioneffectiveness

Severity and rate ofdegradation

NII Possible

High

Low

Medium

No

Yes

Yes

High

Low

Medium

Yes

Yes

Yes

High

Low

Medium

Yes

Yes

Yes

High

Low

Medium

High

Low

Medium

No

Yes

No

High

Low

Medium

No

Yes

Yes

High

Low

Medium

Yes

Yes

Yes

High

Low

Medium

High

Low

Medium

No

No

No

High

Low

Medium

No

Yes

No

High

Low

Medium

No

Yes

Yes

High

Low

Medium

High

Low

Medium




Page 15

When using evidential information as the basis for predictingtypes and locations of degradation, it is important that the pre-vious inspection results have been considered in detail at theplanning stage for the current inspection and that a Type 1CRA is in place as a minimum. The credibility of the evidenceis directly linked to the amount of evidence available - i.e.number of vessels and previous inspections considered.

The confidence categories have therefore been defined as fol-lows to consider the above factors.

High

Either of the following factors apply:

1) a thorough assessment of potential degradation mecha-nisms gives confidence that all relevant mechanisms andtheir likely locations have been predicted. This assessmenthas been carried out as part of the integrity managementactivity by which inspection planning is conducted. Thisincludes as a minimum for each vessel, documented con-sideration of:

the vessel's condition, based on previous inspections

the vessel's metallurgy, modification and repair his-tory the process fluid composition and operating condi-

tions corrosion management the degradation types, locations and rates changes to any of the above factors that may affect

inspection requirements.

The integrity management plan ensures consideration ofthe above factors by a team of competent individuals fromappropriate discipline groups including, for example, in-spection, metallurgy/materials, process and engineering.The CRA carried out as part of the integrity managementactivities described above should conform to the require-

ments of Type 4.2) the user has extensive experience with the same or similar

vessels in the same or similar service and inspection histo-ries outlining the types of flaws existing are available. Adetailed review of all previous inspection results has beenperformed. These findings have been considered in pre-dicting the type and locations of flaws that may beexpected in the current inspection. Note that extensiveexperience is taken to mean that data is available coveringat least eight inspections in total and not less than twoinspections for the longest serving single vessel used inmaking the judgement, at least one of which should havebeen a close visual inspection (internal or external depend-ing on the nature of the degradation). In addition, a CRAshall have been carried out, this conforming at least to therequirements of Type 1.

Note that a vessel for which the operating conditions havechanged since the previous inspection can not be classed asHigh, except where the conditions can be shown to be morebenign than previously.

Medium

Either of the following apply:

1) a thorough assessment of potential degradation mecha-nisms is considered likely to predict the majority of rele-vant mechanisms and their locations but cannot fully ruleout the possibility of additional degradation mechanisms

being active. This assessment has been carried out as partof the integrity management activity by which inspectionplanning is conducted. It includes as a minimum for eachvessel, documented consideration of:

the vessel's condition, based on previous inspections

whenever these have been carried out the vessel's metallurgy, modification and repair his-

tory the process fluid composition and operating condi-

tions corrosion management the degradation types, locations and rates changes to any of the above factors that may affect

inspection requirements.

The integrity management plan ensures consideration ofthe above factors by a team of competent individuals fromappropriate discipline groups including, for example, in-spection, metallurgy/materials, process and engineering.The CRA carried out as part of the integrity managementactivities described above should conform to the require-ments of Type 2.

2) experience, including inspection histories, is available forthe same or similar vessels in the same or similar servicecovering not less than four inspections in total and not lessthan a single inspection for the longest serving vessel con-sidered in making the judgement. A detailed review of allprevious inspection results has been performed. Thesefindings have been considered in predicting the type andlocations of flaws that may be expected in the currentinspection. In addition, a CRA shall have been carried out,this conforming at least to the requirements of Type 1.

Low

Justification for inclusion in the High or Medium categories, asdefined above, is not possible.

3.3.2 Previous inspection effectiveness

This is included as a further measure of the confidence in abil-ity to predict all relevant flaw types and is also used in definingthe effectiveness required for methods used in the current

inspection as a means of ensuring the probability of failure ismanaged. The intent is to compare the effectiveness of the lastinspection carried out relative to that of a conventional internalvisual inspection, i.e. consisting of close visual examination ofplate material and, where applicable, MPI or DPI.

The following category definitions apply:

High

The inspection performed has a better probability of detectingflaws of concern than does internal visual inspection.

Medium

The inspection performed offers a probability of detectingflaws of concern broadly similar to that of internal visualinspection.

Low

The inspection performed has lower probability of detectingflaws of concern than does internal visual inspection.

In deciding which category to select, the user must consider thefollowing:

1) the probability of detection of the method(s) used withrespect to flaws of concern. This is given by the effective-ness assigned to each method for a given flaw type.

2) the inspection coverage and how this relates to the loca-tions of flaws of concern.

Clearly, the users knowledge of what flaws are likely to be ofconcern and where these flaws may be located is important inassigning effectiveness. For example, if it is known that flawsonly occur in a particular region of the vessel and this regionwas fully inspected with a highly effective method then a High




Page 16

inspection effectiveness may be justified. Conversely, if theuser has little understanding of what flaws might exist then ahigh coverage inspection but using only a single method(which may be capable of detecting only one flaw type) mayresult in a Low ranking.

The user can consider previous inspections on similar vesselsin similar service (as defined above) in determining the cate-gory to select.

3.3.3 Severity and rate of degradation

In making this judgement, the user will consider the worstaffected zone of the vessels and the following category defini-tions apply.

High

The degradation and rate thereof is such that failure of the ves-sel or rejection based on inspection results can reasonably beexpected within the remaining plant lifetime.

Medium

The degradation and rate thereof is such as to be observable

during the plant lifetime but would not be expected to threatenthe integrity of the vessel during this period.

Low

There is no degradation expected or degradation is superficial.

3.3.4 NII recommendation

Having answered the above questions, in conjunction with theflowchart, the suitability of the equipment for inspection byNII can be read from the chart.

3.4 NII Decision Record

As with all matters related to plant safety, it is essential that anauditable record is kept of all factors considered in making

decisions throughout the above decision guidance process.Documentation shall cover as a minimum the following:

1) statement of any changes occurring in process that mayaffect the nature or rate of degradation

2) inspection reports (if not already included in the Equip-ment Profile)

3) justification for acceptance under screening criteria

4) list of vessels considered to be the same as the one underconsideration and justification that degradation can beexpected to be the same

5) justification of selection of category for:

confidence in ability to predict types and locations ofdegradation

previous inspection effectiveness severity and rate of degradation.

3.5 Examples

A number of examples of application of the decision processare presented in this section.

3.5.1 Heat Exchanger Vessel

A heat exchanger vessel (2-HX-05) has been in service for twoyears and is scheduled for its first inspection since commis-sioning. A decision is to be made whether this can be by NII orwhether an IVI should be performed.

The following information is relevant: material (shell and ends): Carbon steel material (tubes): Stainless steel process fluid (shell side): Wet hydrocarbon gas with some

condensation expected

process fluid (tube side): Water.

The vessel is designed to ASME VIII Div 1 with a 3.2 mm cor-rosion allowance. The vessel forms part of a new processstream (Stream 2). This is similar in operation to Stream 1 (inservice for a period of 10 years) but has been designed forgreater efficiency and higher throughput. The vessel underconsideration performs the function of two vessels (1-HX-05

and 1-HX-06) on Stream 1. Both vessels have been subject toinspection every three years. The first two inspection were byIVI but the most recent was by NII, this being regarded as atleast as effective as the IVI in identifying the main degradationmechanism. The shells on both vessels are observed to sufferfrom mild internal corrosion. The average rate has been deter-mined to be approximately 0.15 mm per year. The initialinspections showed that the corrosion rate does vary with posi-tion in the vessel. This is believed to be related to the gas flowrate and condensate impingement. On 1-HX-06, for example,the first inspection revealed one area with localised loss of wallthickness of 1.5 mm. An investigation carried out at the timesuggested that the problem was occurring in a region of partic-ularly high flow where water droplets were likely to beentrained. This was considered to be sufficient to remove the

protective hydrocarbon rich film that would normally beexpected on the steel and lead to accelerated CO2 corrosion.An additional diverter plate was installed during the shut-down. Subsequent inspections revealed that this had resolvedthe problem and the region was no longer subject to higher cor-rosion than the remainder of the vessel.

The design of 2-HX-05 is different from that of the two vesselsit replaces in requiring a higher flow rate and more complexgas flow path. In addition, the gas exit temperature is lowerthan that for 1-HX-06, hence the rate of water condensation isexpected to be slightly higher. The feedstock for Stream 2 isnominally the same as for that for Stream 1 although it doescome from a different source.

Application of the screening process (Figure 3-1):

vessel has had previous inspection and history is still rele-vant?No

vessel designed specifically for NII?No vessel is the same as others for which service history

exists?

The response here should be No. The two vessels for whichservice history exists are similar in overall function but differ-ent in respect of design and process conditions and hence it ispossible that the type and rate of degradation might be differ-ent. The No response at this point indicates that NII is not rec-ommended for this vessel for the forthcoming inspection.

3.5.2 Gas Receiver Vessel

A gas receiver vessel is due for its first inspection three yearsafter entry to service. It one of ten vessels having the samedesign and function. Gas enters the vessels from a common lineand gas take-off is to a manifold. Five of the vessels have beenin service for ten years and each has had three previous inspec-tions, the last two of which have been by NII. Inspection reportsare available for each inspection performed and these have beenreviewed in planning for the current inspection. The reportsindicate that mild internal corrosion has been found on the inte-rior at the bottom of each vessel. The corrosion is fairly uniformacross this region in each of the vessels, with a maximum lossof wall of 1 mm on one of the vessels (the design incorporatesa corrosion allowance of 5 mm). This corrosion is expected asthe gas is not completely dry and a certain amount of moisture

deposition occurs at the bottom of the vessel. The recent inspec-tions by NII have included extensive wall thickness readings inthe corroded regions and TOFD was undertaken on the vesselwelds since fatigue cracking under the pressure cycling wasidentified as a possible failure mechanism. None of the TOFDtests undertaken have identified any cracking.




Page 17

A decision on whether the inspection can be by NII is requiredas this is the preferred option.


vessel has had previous inspection and history is still rele-vant?No

vessel designed specifically for NII?No

vessel is the same as others for which service historyexists? Yes, since the vessel is of the same design, is in thesame service and there are no factors that would beexpected to lead to differences in degradation.

entry is scheduled for other reasons?No.

Proceeding to apply the high level decision guidance flowchart (Figure 3-2):

confidence in ability to predict types and locations of deg-radation. Given that there are a large number of vessels ofthe same design and function and that many of these ves-sels have long inspection histories, the user can directlyconsider evidence available from these vessels in selectingthe category here. Considering the option 2) (category

High), in item 3.3.1, High is justified here previous inspection effectiveness. The previous inspec-tions by NII on the vessels already in service can be takenas Medium, i.e. broadly equivalent to IVI

severity and rate of degradation. Corrosion occurring atthe same rate as on the vessels in service would not beexpected to threaten the integrity of the vessels during thedesign lifetime, hence Medium is selected here.

Following Figure 3-2 with High, Medium, Medium, onearrives at a recommendation that NII is possible in principle.

3.5.3 Separator Vessel

A High Pressure Separator Vessel is due for an inspection. Thevessel has been in service for eleven years. Its first inspection

was after three years in service. The interval to the next inspec-tion was set at four years and this interval has been retained tothe present inspection. A decision on whether this inspectioncan be by NII is required.

The vessel is constructed of carbon steel that is clad internallywith a stainless steel liner. The vessel exterior is fully insulatedand the insulation is protected by galvanised steel sheet.

The integrity management plan for the plant includes technicalreviews, considering the main threats to integrity, on a vesselby vessel basis. This has identified that corrosion and erosionare the only degradation mechanism of concern here. Externalcorrosion (under the insulation) has been identified as havinga higher probability than internal corrosion/erosion. However,if the carbon steel becomes exposed to the process fluids (afterbreakdown of the stainless steel cladding), corrosion can bevery rapid hence integrity of the cladding is important. Corro-sion is more likely to be in the form of pitting than of a gener-alised nature.

Both of the previous inspections have included visual inspec-tion of the vessel interior. This included close visual examina-tion of all accessible internal surfaces. All areas inspected werereported to be in good condition with no visible degradation.This information has been considered in the most recent tech-nical review undertaken in planning for the forthcominginspection. In order to check for signs of external corrosion,sections of insulation were removed during each of theseinspections. The insulation was seen to be dry in each case andmild localised corrosion (




Page 18


vessel has had previous inspection and history is still rele-vant? Yes (NII)

entry is scheduled for other reasons?No.

Proceed to apply the high level decision guidance flow chart(Figure 3-2):

confidence in ability to predict types and locations of deg-radation. The integrity management approach and opera-tional risk assessments performed allow option 1) to beconsidered in determining the category applicable here.While the operational risk assessment has considered indetail the types of degradation likely, the findings cannotbe taken to be fully validated by the inspections carried outto date. It is conceivable that further, unexpected, degrada-tion mechanisms might not have been identified by theinspection, given the limited extent of the NII carried out.Hence a Medium ranking is assigned here

previous inspection effectiveness. The previous inspec-tions (on both vessels) were by NII. The thickness gaugeinspection at fixed points is judged as having a markedly

lower probability that IVI of detecting the flaw type ofmost concern (pitting). Hence the previous inspectioneffectiveness is taken as Low here

severity and rate of degradation. The thickness gaugemeasurements did not reveal any clear loss of wall thick-ness. However, the operational risk study did suggest cor-rosion can be expected, although not sufficient to exceedthe corrosion allowance through the vessel life. Hence aMedium ranking applies here

following Figure 3-2 with Medium, Low, Medium, oneestablishes that NII is not recommended

based on the above recommendation, a decision was madeto perform IVI on one of the vessels and vessel B wasopened for inspection. This consisted of close visualexamination of all internal surfaces and MPI on all nozzle

welds and approximately 20% of shell weld length. Mostof the internal surfaces showed some minor pitting, with amaximum depth of 0.75 mm. No crack like flaws wereidentified in any of the welds

the zinc oxide in vessel A remains in good condition andit would be preferable to continue to operate on the basisof having performed NII rather than opening the vessel forIVI. The decision guidance process is to be followed indetermining whether this is a justifiable approach.

Application of the decision guidance process:

vessel has had previous inspection and history is still rele-vant? Yes (NII)

entry is scheduled for other reasons?No

Proceed to apply the high level decision guidance flow chart:

confidence in ability to predict types and locations of deg-radation. Again the option 1) definitions are considered indetermining the category applicable. The same considera-tions apply here as for vessel B but now the potential exist-ence of unexpected degradation mechanisms can be ruledout based on the findings of the inspection on vessel B.Hence a High is assigned

previous inspection effectiveness. The inspection on ves-sel B was by IVI and the results are relevant to vessel Ahence Medium applies here

severity and rate of degradation. The thickness gaugemeasurements did not reveal any clear loss of wall thick-ness but the internal visual inspection on vessel B showed

pitting up to 0.75 mm depth. Hence a Medium rankingapplies here.

Following Figure 3-2 with High, Medium, Medium, onearrives at a recommendation for that NII can be performed onthis vessel in principle.

4. Inspection planning

4.1 Introduction

The principal objective of inspection planning is to establish acost effective strategy which provides a satisfactory level ofconfidence in the vessels safe and reliable operation until thenext inspection. Inspection planning is a complex task thatdemands consideration of a broad spectrum of issues, rangingfrom detailed technical assessments of the impact of vesseloperating conditions on degradation through to budget plan-ning and allocation. As well as the technical considerations, itis generally also necessary to satisfy certain legislative require-ments. While this aspect is not covered specifically in this rec-ommended practice, since the requirements are usually countryspecific, it remains an important part of the planning process.

The non-intrusive inspection plan for a vessel defines whichparts of the vessel should be inspected, what inspection meth-ods should be used, and what coverage is required. This sectionof the recommended practice provides guidance on determin-ing the most effective non-intrusive inspection plan for a givenvessel, in terms of establishing an appropriate balance betweenvessel integrity and inspection cost / duration. In particular itaddresses the question of how to establish an appropriate bal-

ance between sensitive but relatively slow (and thereforeexpensive) inspection methods, and rapid (and therefore lessexpensive) but less sensitive screening methods.

Approaches to planning will vary from company to companybut, typically, the plan will define at least the following:

timing for the inspection type of inspection method(s) to be used regions of the vessel to be inspected shortlist of inspection service provider(s) qualifications of inspectors, Reporting requirements requirements from plant operations (e.g. shut down) requirements from plant maintenance (e.g. set up scaffolding) safety requirements (equipment and personnel).

Clearly there are many additional considerations that go intothe overall inspection planning activity. In particular, consid-eration should be given to the following:

whether inspection to be performed on stream or off stream temperature during inspection (high temperature is likely

to be the main concern, though difficulties may arise fromthe build-up of ice on low temperature items).

whether the inspection is on the critical path of an outage? whether the inspection is opportunity driven? requirement for comparison with previous/past inspections cost and time constraints.

The steps in planning and implementing a non-intrusiveinspection are the same, regardless of whether the inspectionrequirement has been determined using a risk based inspection(RBI) or more prescriptive approach. However, there can be ahigh degree of synergy between non-intrusive inspection andRBI since both approaches require similar types of informationto be available.

The approach developed here is intended to be systematic,thereby promoting a consistent approach between operators.Nonetheless, it does not remove the need for input and review bycompetent personnel and the results must be reviewed by com-petent inspection planning personnel to check that the inspectionplan is broadly consistent with their engineering judgment.

The main elements in devising an inspection plan are as follows:

Identification and selection of the planning team.

The team to plan, prepare and execute the inspection must becompetent across a range of disciplines. These must be identi-fied and appropriate personnel selected.

Definition of the inspection strategy.

The planning approach will depend in part on the intended




Page 19

inspection strategy, i.e. whether the inspection is intended toconfirm the absence of degradation, or to establish the depth orextent of known active degradation mechanisms.

Definition of vessel zones.

The vessel can be treated as one or more zones, each represent-

ing a particular combination of geometry, material, likelihoodof degradation etc.

Selection of inspection methods and coverage.

Selection of appropriate inspection method(s) and coverage foreach of the defined zones (some zones may require no inspection).

Figure 4-1Inspection Planning Flowchart

STARTSelect vessel for IVI/ NII

Is a technique available

with efficiency >= therequirement?

Review the riskassociated with

performing inspectionwith lower efficiency

Is the risk acceptable?Alternative Inspection

Required

Yes

Determine InspectionStrategy Type

(Figure 4-2)

IdentifyInspection Zones

Determine anticipateddegradation type .

Determine requiredInspection Effectiveness

(Figure 4-4)

Have all relevant defecttypes been considered?

Have all zones beenconsidered?

Determine efficiency ofcandidate inspection

methods(Flowcharts)

Is the technique able tosize defects?

Review the riskassociated with not

accurately sizing defectsIs the risk acceptable?

Alternative InspectionRequired

Equipment Profile(Section2.2)

Decision GuidanceRecords

(From Section3.4)

Prepare Work -pack

No No

No No

No

No

Yes Yes

YesYes

Yes

NII Evaluation




Page 20

4.2 The Planning Team

Inspection planning is generally conducted by a team consist-ing of people with responsibilities in different areas (e.g. pro-duction, process, metallurgy, inspection, maintenance), theobjective being to ensure that the inspection is effective withinthe constraints imposed by sometimes conflicting internalneeds.

The development of a non-intrusive inspection plan can be sig-nificantly more complicated than is the case for an internal (e.g.visual) inspection. A multi-disciplinary approach is requiredwhich is likely to involve appropriate experience in engineer-ing/materials/ process operations/non-destructive testing.

The basic team required to assess the requirements for a non-intrusive inspection should consist of personnel with compe-tencies in the following areas:

general knowledge of the construction of containment ves-sels and systems, materials and materials processing, fab-rication processes etc.

corrosion or materials technology specific knowledge of the systems to be inspected, opera-

tional history and general knowledge, (knowledge of theworking practices and history of the system, safetyrequirements, and the likely conditions at the time ofinspection)

non-destructive testing

nominated person to coordinate the overall process.

Team members may have more than one of the specified skills;it is not necessary for the team to contain individual specialistsin all of the above.

The most effective team is likely to be the smallest team thathas the requisite skills, but the team should not be reducedexcessively, as items are likely to be overlooked. Personnel tocarry out any of these functions should be competent to assess

their own level of expertise in the selected area. One memberof the team should take responsibility for the overall planningprocess.

4.3 Inspection Strategy Type

The objective of any inspection, at the highest level, is to givea high degree of assurance that any degradation with potentialto threaten integrity is detected before the next inspection.However, how it achieves that ambition will vary according tothe specific details of the item under consideration. For exam-ple, the inspection regime for a vessel where the corrosion riskassessment has shown no likely degradation mechanisms willbe very different from the inspection regime for a vessel wherestress corrosion cracking is predicted to be likely.

In the course of defining the inspection schedule the followingthree, closely linked, aspects must therefore be taken into con-sideration:

a) Degradation method:

nature of degradation, global wall thinning or cracking

location of degradation, preferential attack or morerandom.

b) Potential to threaten integrity (what resistance to degrada-tion is embodied in the design)

corrosion allowance

critical crack depth.

c) Degree of assurance feeds into the inspection perform-ance requirements.

Any inspection program specified must also be able to givesome degree of assurance that unexpected damage mecha-nisms are not occurring which might lead to failure of the com-ponent.

The above clearly rests on knowing what degradation mecha-nism to inspect for once this is determined (through corro-sion risk assessment [CRA] or historical evidence) thequestions regarding assurance and potential threats to integritycan be addressed through defining where to inspect and how toinspect. These assessments must take into consideration the

future operating conditions for the component, and not merelyreflect past conditions. For example, where produced fluids areincreasingly sour, it would be misleading to base the inspec-tion requirements on a CRA which has assumed low sulphurcontent, resulting in a downplaying of the importance ofinspection for mechanisms such as sulphide stress cracking(SSC).

The starting point in assessing any deviation from the specifiedinspection plan is an understanding of how the objectives ofthe inspection might be affected, e.g. does the non-conform-ance significantly compromise the ability of the inspection tomeet the overall objectives. It is therefore important that theobjectives be understood, bearing in mind that these may notbe defined in detail in the inspection plan itself (as this defines

the work scope for the inspection team).

As stated above, different vessels will clearly have differentinspection requirements, not just in the detail of the what,where, how, but also in the basis of the approach. For exam-ple, a vessel for which it has been determined that generalisedcorrosion is the main degradation mechanism of concern butwith a low probability would be treated differently to a vesselin which weld cracking by stress corrosion, such as hydrogeninduced stress corrosion cracking (HISCC) or sulphide stresscorrosion cracking (SSCC), is seen a high likelihood. The dif-ferences on this level can form a useful basis for categorisingthey type of inspection in a way that facilitates subsequentassessment of non-conformances. To this end, three inspectiontypes have been defined, as described in Table 4-1 below.

There is a clear difference in emphasis in each of the inspectiontypes and this provides a useful framework for the establish-ment of the inspection plan, as well, as later in the process,treatment of any non-conformances. The three categories arediscussed in more detail in the following sections, and guid-ance on the assignment of an inspection type to a particularitem is given in Sec. 4.3.4.

Table 4-1 Inspection Type Definitions

Type Definition

ADegradation mechanism NOT expected to occur.Inspection is required to confirm there is no onset of the degradation mechanism.

B

Degradation mechanism expected, with low / medium progression.Location of degradation can be predicted.

Not anticipated to impact on vessel integrity in the medium term (typically at least 2 outage periods).Inspection required to confirm CRA predictions.

C

Degradation expected with medium / high progression.Location of degradation can not be predicted.MAY impact on vessel integrity in the medium term (two-outage timeframe).Inspection required to confirm absence of flaws of critical size.




Page 21

4.3.1 Type A Inspection

Type A inspection applies in situations where there is a lowprobability of degradation based on previous inspection his-tory and / or CRA and if degradation is present it will tend tobe general or there is a high confidence that the most likelyareas for degradation can be identified. It is also intended toprovide a general screening for damage due to degradation

mechanisms that might unknowingly be active. The purpose ofthis type of inspection is therefore primarily to confirm thatthere is no degradation active. If degradation is found then fur-ther steps are required to be taken.

Example of Type A

A vessel constructed in Duplex stainless steel and exposed toproduced water, oil and gas at moderate temperature. The mostlikely mechanisms here would be chloride pitting of the shelland chloride SCC of the welds. The probability would typi-cally be very low however, provided the chloride levels are notexcessive and the temperature is moderate. In this situation thecorrosion assessment would typically indicate that degrada-tion, if it does occur, will tend to be found at the bottom of thevessel (where there is contact with water). Provided exposure

to water is similar, there are no other factors leading to prefer-ential degradation. This means there is no need to do a highcoverage inspection a fairly small coverage can give confir-mation that degradation is not active. Its important however toensure that the areas selected for coverage are likely to be rep-resentative of the worst areas. If this is possible (based on thefindings of the corrosion assessment) then very low coveragemay be acceptable 1). A key inspection performance require-ment here is the ability to detect the presence of degradation,even when it is in its early stages.

1) Note that there is a sound statistical foundation for allowing this type ofapproach. A semi-quantitative Bayesian statistics approach can be usedto show that for a situation in which there is a high level of confidence inlimited degradation, a lesser amount of inspection can still have signifi-cant influence on the "degree of assurance".

4.3.2 Type B Inspection

Type B inspection applies when there is some degradationexpected but it is not expected to be such as to threaten integ-rity in the medium term. Medium term is in this case taken asbeing a period equivalent to at least two inspection intervals.This has been adopted as it allows for any missed flaws to beidentified at the subsequent inspection, without threatening theintegrity of the equipment. This inspection applies at a low/moderate coverage and its purpose is to provide sufficientinformation to allow quantified demonstration of the requireddegree of assurance. If the results of the inspection do notallow this then further action is taken.

Example of Type BA vessel constructed in carbon steel that is exposed to wet gas.Corrosion (pitting or more general) is expected but with a lowcorrosion rate. The corrosion assessment has defined three

zones where corrosion conditions may be different. For thisvessel a moderate coverage using corrosion mapping is appro-priate, with a certain amount of coverage in each of the threezones. The coverage should be sufficient to allow a quantifiedstatistical assessment that can be used to demonstrate a highconfidence in estimating the worst flaw. The accuracy and res-olution of the inspection system need to be considered in thesame context. Note the shift in emphasis compared to Type A

here one is using the information provided by the inspectionto say something about the worst degradation that might existin the vessel (including the regions not inspected) where in theType A inspection the emphasis is on identifying the presence(or confirming the absence) of degradation.

4.3.3 Type C Inspection

Type C inspection applies when there is a reasonably highprobability of degradation being present and/or degradationmay be severe and/or degradation has no preferred locations.This inspection will often apply at moderate/high coverage. Itspurpose is to give a high probability that any flaw with poten-tial to threaten integrity is found directly. In the event that suchflaws are found, then further action is required in ord

NON-INTRUSIVE INSPECTION DNV Rp-G103

Documents

Transcript of NON-INTRUSIVE INSPECTION DNV Rp-G103