DNV Offshore Structure.pdf

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OFFSHORE STANDARD

DET NORSKE VERITAS

DNV-OS-C201

STRUCTURAL DESIGN OF OFFSHORE

UNITS (WSD METHOD)APRIL 2002



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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 Operation

Amendments and corrections

First issue (March 2001) had several missing items, which have now been inserted:

— Sec.11: Sea pressures— Sec.12: Permanent loads, variable functional loads, environmental loads, windloads, waves, current, accidental loads, fatigue

loads, collision, dropped object, fire, explosion, unintended flooding, preload, capacity, overturning stability, air gap, redun-dancy, brace arrangement, structural detailing.

Sec.4 Table D3: Material selection table has been amended to include design temperature of 10°C and possibility for increased

use of NV A grade.Printing errors have been corrected.


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Offshore Standard DNV-OS-C201, April 2002

Contents –

CONTENTS

Sec. 1 Introduction........................................................... 7

A. General....................................................................................7A 100 Introduction.......................................................................7

A 200 Objectives .........................................................................7A 300 Scope and application .......................................................7A 400 Other than DNV codes......................................................7

B. Normative References ............................................................7B 100 General..............................................................................7B 200 Offshore standards............................................................7

C. Informative References...........................................................7C 100 General..............................................................................7

D. Definitions ..............................................................................8D 100 Verbal forms .....................................................................8D 200 Terms ................................................................................8

E. Abbreviations and Symbols....................................................9E 100 Abbreviations....................................................................9

E 200 Symbols ..........................................................................10

Sec. 2 Design Principles................................................. 12

A. Introduction ..........................................................................12A 100 General............................................................................12A 200 Aim of the design............................................................12

B. General Design Considerations ............................................12B 100 General............................................................................12

C. Design Conditions ................................................................12C 100 Basic conditions..............................................................12

D. Loading Conditions ..............................................................12D 100 General............................................................................12D 200 Load................................................................................13

E. Design by the WSD Method.................................................13E 100 Usage factors and permissible stresses...........................13E 200 Basic usage factors.......................................................... 13

F. Design Assisted by Testing ..................................................13F 100 General............................................................................13F 200 Full-scale testing and observation of performance of

existing structures ........................................................... 13

G. Probability Based Design .....................................................13G 100 Definition........................................................................13G 200 General............................................................................13

Sec. 3 Loads and Load Effects...................................... 14

A. Introduction ..........................................................................14A 100 General............................................................................14

B. Basis for Selection of Loads.................................................14B 100 General............................................................................14

C. Permanent Loads ..................................................................14C 100 General............................................................................14

D. Variable Functional Loads....................................................14D 100 General............................................................................14D 200 Variable functional loads on deck areas .........................15D 300 Tank pressures ................................................................15D 400 Miscellaneous loads........................................................ 16

E. Environmental Loads............................................................16E 100 General............................................................................16

E 200 Environmental loads for mobile units.............................16E 300 Environmental loads for site specific units.....................16E 400 Determination of hydrodynamic loads ...........................16E 500 Wave loads...................................................................... 16E 600 Wave induced inertia forces ...........................................16E 700 Wind loads ...................................................................... 16

E 800 Earthquake......................................................................17E 900 Vortex induced oscillations ............................................17E 1000 Current ............................................................................17E 1100 Tidal effects....................................................................17

E 1200 Marine growth ................................................................17E 1300 Snow and ice accumulation ............................................17E 1400 Direct ice load.................................................................17E 1500 Water level, settlements and erosion..............................17E 1600 Appurtenances and equipment........................................17

F. Combination of Environmental Loads..................................18F 100 General............................................................................18

G. Accidental Loads ..................................................................18G 100 General............................................................................18

H. Deformation Loads...............................................................18H 100 General............................................................................18H 200 Temperature loads ..........................................................18H 300 Settlements and subsidence of sea bed...........................18

I. Load Effect Analysis ............................................................18I 100 General............................................................................18I 200 Global motion analysis ...................................................19I 300 Load effects in structures and soil or foundation............19

Sec. 4 Selection of Material and InspectionPrinciples ............................................................. 20

A. General..................................................................................20A 100 ........................................................................................20

B. Design Temperatures............................................................20B 100 General............................................................................20B 200 Floating units ..................................................................20B 300 Bottom fixed units ..........................................................20

C. Structural Category...............................................................20C 100 General............................................................................20C 200 Selection of structural category ......................................20C 300 Inspection of welds .........................................................20

D. Structural Steel .....................................................................21D 100 General............................................................................21D 200 Material designations......................................................21D 300 Selection of structural steel.............................................22

Sec. 5 Structural Strength ............................................ 23

A. General..................................................................................23A 100 General............................................................................23A 200 Structural analysis...........................................................23A 300 Ductility ..........................................................................23A 400 Yield check.....................................................................23

A 500 Buckling check ...............................................................23

B. Flat Plated Structures and Stiffened Panels..........................23B 100 Yield check .....................................................................23B 200 Buckling check ...............................................................23B 300 Capacity checks according to other codes......................24

C. Shell Structures.....................................................................24C 100 General............................................................................24

D. Tubular Members, Tubular Joints and Conical Transitions .24D 100 General............................................................................24

E. Non-Tubular Beams, Columns and Frames..........................24E 100 General............................................................................24

Sec. 6 Section Scantlings............................................... 25

A. General..................................................................................25A 100 Scope...............................................................................25

B. Strength of Plating and Stiffeners.........................................25B 100 Scope...............................................................................25




– Contents

B 200 Minimum thickness.........................................................25B 300 Bending of plating...........................................................25B 400 Stiffeners .........................................................................25

C. Bending and Shear in Girders...............................................26C 100 General ............................................................................26C 200 Minimum thickness.........................................................26C 300 Bending and shear...........................................................26C 400 Effective flange ...............................................................26

C 500 Effective web ..................................................................26C 600 Strength requirements for simple girders........................26C 700 Complex girder systems..................................................27

Sec. 7 Fatigue................................................................. 28

A. General..................................................................................28A 100 General ............................................................................28A 200 Design fatigue factors .....................................................28A 300 Methods for fatigue analysis ...........................................28A 400 Simplified fatigue analysis..............................................28A 500 Stochastic fatigue analysis ..............................................29

Sec. 8 Accidental Conditions ........................................ 30

A. General..................................................................................30

A 100 General ............................................................................30

Sec. 9 Weld Connections............................................... 31

A. General..................................................................................31A 100 Scope...............................................................................31

B. Types of Welded Steel Joints ...............................................31B 100 Butt joints........................................................................31B 200 Tee or cross joints ...........................................................31B 300 Slot welds........................................................................32B 400 Lap joint ..........................................................................32

C. Weld Size.............................................................................. 32C 100 General ............................................................................32C 200 Ordinary fillet welds .......................................................32C 300 Partly penetration welds and fillet welds in cross

connections subject to high stresses................................33C 400 Connections of stiffeners to girders and

bulkheads, etc..................................................................33C 500 End connections of girders..............................................34C 600 Direct calculation of weld connections...........................34

Sec. 10 Corrosion Protection.......................................... 35

A. General..................................................................................35A 100 Scope...............................................................................35

B. Acceptable Corrosion Protection..........................................35B 100 Atmospheric zone ...........................................................35B 200 Splash zone .....................................................................35B 300 Submerged zone..............................................................35B 400 Internal zone....................................................................35

B 500 Corrosion additions.........................................................35C. Cathodic Protection ..............................................................36C 100 General ............................................................................36C 200 Protection by sacrificial anodes ......................................36C 300 Protection by impressed current......................................36C 400 Cathodic protection monitoring system..........................36C 500 Testing of effectiveness of corrosion protection

system.............................................................................36

D. Coating..................................................................................37D 100 Specification....................................................................37D 200 Coating application .........................................................37

Sec. 11 Special Considerations for Column StabilisedUnits ..................................................................... 38

A. General..................................................................................38A 100 Scope...............................................................................38

B. Structural Categorisation ......................................................38B 100 General ............................................................................38B 200 Structural categorisation .................................................38

C. Material Selection.................................................................38C 100 General............................................................................38C 200 Design temperatures........................................................38

D. Inspection Categories............................................................39D 100 General............................................................................39D 200 Categorisation and inspection level for typical

column stabilised unit details..........................................39

E. Design and Loading Conditions ...........................................41E 100 General............................................................................41E 200 Load point .......................................................................41E 300 Permanent loads ..............................................................41E 400 Variable functional loads ................................................41E 500 Lifeboat platforms...........................................................41E 600 Tank loads.......................................................................42E 700 Environmental loads, general..........................................42E 800 Sea pressures...................................................................42E 900 Wind loads ......................................................................43E 1000 Heavy components..........................................................43E 1100 Deformation loads...........................................................43E 1200 Accidental loads..............................................................43E 1300 Fatigue loads ...................................................................43E 1400 Combination of loads......................................................43

F. Structural Strength ................................................................43F 100 General............................................................................43F 200 Global capacity ...............................................................43F 300 Transit condition .............................................................43F 400 Method of analysis..........................................................44F 500 Air gap.............................................................................44

G. Fatigue ..................................................................................44G 100 General............................................................................44G 200 Fatigue analysis...............................................................45

H. Resistance Against Collision, Dropped Objects, Fire andExplosion ..............................................................................45

H 100 General............................................................................45H 200 Collision..........................................................................45H 300 Dropped objects ..............................................................45H 400 Fire ..................................................................................45

H 500 Explosion ........................................................................45H 600 Heeled condition .............................................................45

I. Redundancy ..........................................................................46I 100 General............................................................................46I 200 Brace arrangements.........................................................46

J. Structure in Way of a Fixed Mooring System......................46J 100 Structural strength...........................................................46

K. Structural Details ..................................................................46K 100 General............................................................................46

Sec. 12 Special Considerations for Self ElevatingUnits ..................................................................... 47

A. General..................................................................................47A 100 Scope and application .....................................................47

B. Structural Categorisation and Material Selection .................47B 100 Structural categorisation .................................................47B 200 Material selection............................................................47B 300 Design temperature.........................................................47B 400 Selection of structural steel.............................................48B 500 Inspection categories.......................................................48

C. Design and Loading Conditions ...........................................48C 100 General............................................................................48C 200 Transit .............................................................................48C 300 Installation and retrieval..................................................49C 400 Operation and survival....................................................49

D. Environmental Conditions....................................................49

D 100 General............................................................................49D 200 Wind................................................................................49D 300 Waves..............................................................................50D 400 Current ............................................................................50D 500 Temperature ....................................................................50D 600 Snow and ice...................................................................50




Contents –

E. Method of Analysis ..............................................................50E 100 General............................................................................50E 200 Global structural analysis................................................51E 300 Local structural analysis ................................................. 51E 400 Fatigue analysis...............................................................51

F. Design Loads........................................................................51F 100 General............................................................................51F 200 Permanent loads..............................................................51

F 300 Variable functional loads................................................52F 400 Environmental loads, general ......................................... 52F 500 Wind loads...................................................................... 52F 600 Waves..............................................................................52F 700 Current............................................................................52F 800 Wave and current............................................................52F 900 Sea pressures during transit ............................................53F 1000 Heavy components during transit ...................................53F 1100 Accidental loads..............................................................54F 1200 Fatigue loads ...................................................................54F 1300 Combination of loads...................................................... 54

G. Structural Strength................................................................54G 100 General............................................................................54G 200 Global capacity ............................................................... 54G 300 Footing strength..............................................................54

G 400 Leg strength .................................................................... 54G 500 Jackhouse support strength .............................................55G 600 Hull strength ...................................................................55

H. Fatigue Strength.................................................................... 55H 100 General............................................................................55H 200 Fatigue analysis...............................................................55H 300 Worldwide operation ......................................................55H 400 Restricted operation ........................................................55

I. Accidental Conditions ..........................................................55I 100 General............................................................................55I 200 Collisions ........................................................................55I 300 Dropped objects.............................................................. 56I 400 Fires ................................................................................56I 500 Explosions....................................................................... 56I 600 Unintended flooding .......................................................56

J. Miscellaneous requirements .................................................56J 100 General............................................................................56J 200 Pre-load capasity.............................................................56J 300 Overturning stability.......................................................57J 400 Air gap ............................................................................ 57J 500 Structural detailing..........................................................57

Sec. 13 Special Considerations for TensionLeg Platforms (TLP) ........................................... 59

A. General..................................................................................59A 100 Scope and application .....................................................59A 200 Description of tendon system .........................................59

B. Structural Categorisation, Material Selection and Extent of

NDT......................................................................................60B 100 General............................................................................60B 200 Structural categorisation ................................................. 60B 300 Material selection............................................................60B 400 Design temperatures .......................................................60B 500 Inspection categories.......................................................61

C. Design Criteria...................................................................... 61C 100 General............................................................................61C 200 Design conditions ...........................................................61C 300 Fabrication ......................................................................62C 400 Mating.............................................................................62C 500 Sea transportation ...........................................................62C 600 Installation ......................................................................62C 700 Decommissioning ...........................................................62C 800 Design principles, tendons.............................................. 62

D. Design Loads........................................................................63D 100 General............................................................................63D 200 Load categories............................................................... 63

E. Global Performance..............................................................63E 100 General............................................................................63

E 200 Frequency domain analysis.............................................63E 300 High frequency analyses.................................................64E 400 Wave frequency analyses ...............................................64E 500 Low frequency analyses .................................................64E 600 Time domain analyses ....................................................64E 700 Model testing ..................................................................65E 800 Load effects in the tendons .............................................65

F. Structural Strength ................................................................65

F 100 General............................................................................65F 200 Hull.................................................................................65F 300 Structural analysis...........................................................65F 400 Structural design.............................................................66F 500 Deck................................................................................66F 600 Extreme tendon tensions.................................................66F 700 Structural design of tendons ...........................................66F 800 Foundations.....................................................................66

G. Fatigue ..................................................................................66G 100 General............................................................................66G 200 Hull and deck..................................................................67G 300 Tendons...........................................................................67G 400 Foundation ......................................................................67

H. Accidental Condition............................................................67

H 100 Hull.................................................................................67H 200 Hull and deck..................................................................67H 300 Tendons...........................................................................67H 400 Foundations..................................................................... 67

Sec. 14 Special Considerations for Deep DraughtFloaters (DDF)..................................................... 68

A. General..................................................................................68A 100 Introduction.....................................................................68A 200 Scope and application .....................................................68

B. Non-Operational Phases .......................................................68B 100 General............................................................................68B 200 Fabrication ......................................................................68B 300 Mating.............................................................................68B 400 Sea transportation ...........................................................68

B 500 Installation ......................................................................68B 600 Decommissioning...........................................................68

C. Structural Categorisation, Selection of Material andExtent of Inspection..............................................................68

C 100 General............................................................................68C 200 Material selection............................................................69C 300 Design temperatures .......................................................69C 400 Inspection categories ......................................................69C 500 Guidance to minimum requirements ..............................69

D. Design Loads ........................................................................70D 100 Permanent loads..............................................................70D 200 Variable functional loads................................................70D 300 Environmental loads .......................................................70D 400 Determination of loads ...................................................70

D 500 Hydrodynamic loads.......................................................70D 600 Combination of environmental loads..............................70

E. Load Effect Analysis in Operational Phase..........................70E 100 General............................................................................70E 200 Global bending effects....................................................71

F. Load Effect Analysis in Non-Operational Phases ................71F 100 General............................................................................71F 200 Transportation.................................................................71F 300 Launching .......................................................................71F 400 Upending.........................................................................71F 500 Deck mating....................................................................71

G. Structural Strength ................................................................71G 100 Operation phase for hull .................................................71G 200 Non-operational phases for hull......................................72

G 300 Operation phase for deck or topside ...............................72G 400 Non-operational phases for deck or topside ...................72

H. Fatigue ..................................................................................72H 100 General............................................................................72H 200 Operation phase for hull .................................................72




– Contents

H 300 Non-operational phases for hull......................................73H 400 Splash zone .....................................................................73H 500 Operation phase for deck or topside ...............................73H 600 Non-operational phases for deck or topside....................73

I. Accidental Condition............................................................73I 100 General ............................................................................73I 200 Fire ..................................................................................73I 300 Explosion ........................................................................73

I 400 Collision..........................................................................73I 500 Dropped objects ..............................................................73I 600 Unintended flooding .......................................................74I 700 Abnormal wave events....................................................74

App. A Cross Sectional Types ........................................ 75

A. Cross Sectional Types ..........................................................75A 100 General ............................................................................75A 200 Cross section requirements for plastic analysis ..............75A 300 Cross section requirements when elastic global

analysis is used................................................................75

App. B Methods and Models for Design of ColumnStabilised Units ................................................... 77

A. Methods and Models.............................................................77A 100 General............................................................................77

A 200 World wide operation......................................................77

A 300 Benign waters or restricted areas....................................77

App. C Permanently Installed Units.............................. 78

A. Introduction...........................................................................78A 100 Application......................................................................78

B. Inspection and Maintenance .................................................78B 100 Facilities for inspection on location................................78

C. Fatigue ..................................................................................78C 100 Design fatigue factors .....................................................78

C 200 Splash zone for floating units..........................................78




Sec.1 –

SECTION 1INTRODUCTION

A. General

A 100 Introduction101 This document is the DNV offshore standard for struc-tures based on the principle of Working Stress Design (WSD).

A 200 Objectives

201 The standard specifies general principles and guidelinesfor the structural design of offshore structures.

202 The objective for this standard is, in combination withreferred standards, recommended practices, guidelines etc., togive a minimum and a uniform level of safety for all structuresand structural components.

A 300 Scope and application

301 The standard is in principle applicable to all types of off-

shore structures of metallic materials. However, this standardis specially meant for the type of units:

— column stabilised— self elevating— tension leg platform— deep draught floaters.

302 For other materials than steel, the general design princi-ples given in this standard may be used together with relevantmaterial standards, codes or specifications.

303 The standard is applicable to the design of structures in-cluding substructures, topside structures and hulls.

A 400 Other than DNV codes

401 In case of conflict between requirements of this standardand a reference document other than DNV documents, the re-quirements of this standard shall prevail.

402 Where reference is made to codes other than DNV doc-uments, the valid revision shall be taken as the revision, whichwas current at the date of issue of this standard, unless other-wise specified.

403 When code checks are performed according to otherthan DNV codes, the usage factors as given in the respectivecode shall be used.

B. Normative ReferencesB 100 General

101 The standards given in Table B1 and Table B2 includeprovisions, which through reference in this text constitute pro-visions for this standard.

B 200 Offshore standards

201 The offshore standards given in Table B2 are referred toin this standard.

C. Informative References

C 100 General101 The documents listed in Table C1 and Table C2 includeacceptable methods for fulfilling the requirements in the stand-ard and may be used as a source of supplementary information.

Other recognised codes or standards may be applied providedit is shown that they meet or exceed the level of safety of theactual DNV standard.

102 The publications given in Table C2 are referred to in thisstandard.

Table B2 DNV Offshore Standards

Reference Title

DNV-OS-A101 Safety Principles and Arrangement

DNV-OS-B101 Metallic MaterialsDNV-OS-C301 Stability and Watertight Integrity

DNV-OS-C401 Fabrication and Testing of Offshore Structures

DNV-OS-E301 Position Mooring

DNV-OS-E401 Helicopter Decks

Table C1 DNV Recommended Practices, Classification Notes

and other references

Reference Title

DNV-RP-C103 Column Stabilised Units

DNV-RP-C202 Buckling Strength of Shells

DNV-RP-C203 Fatigue Strength Analysis of Offshore SteelStructures

DNV Classifica-tion Note 30.1

Buckling Strength Analysis


Foundations


Environmental Conditions and EnvironmentalLoads


Structural Reliability Analysis of Marine Struc-tures


Strength Analysis of Main Structures of Self el-evating Units

DNV-OS-C101 Design of Offshore Steel Structures, General(LRFD method)

DNV-OS-C103 Structural Design of Column Stabilised Units(LRFD method)

DNV-OS-C104 Structural Design of Self Elevating Units (LR-FD method)

DNV-OS-C105 Structural Design of Tension Leg Platforms(LRFD method)

DNV-OS-C106 Structural Design of Deep Draught FloatingUnits

DNV-OS-F201 Dynamic Risers

Table C2 Other references

Reference Title

AISC-ASD Manual of Steel Construction ASD

API RP 2A - WSD

with supplement 1

Planning, Designing and Constructing Fixed

Offshore Platforms - Working Stress DesignAPI RP 2T Planning, Designing and Constructing Tension

Leg Platforms

BS 7910 Guide on methods for assessing the acceptabili-ty of flaws in fusion welded structures

ISO 13819-1 Petroleum and natural gas industries – Offshorestructures – Part 1: General requirements

NACE TPC Publication No. 3. The role of bacteria in corro-sion of oil field equipment

SNAME 5-5A Site Specific Assessment of Mobile Jack-UpUnits




– Sec.1

D. Definitions

D 100 Verbal forms

101 Shall: Indicates a mandatory requirement to be followedfor fulfilment or compliance with the present standard. Devia-tions are not permitted unless formally and rigorously justified,and accepted by all relevant contracting parties.

102 Should: Indicates a recommendation that a certaincourse of action is preferred or particularly suitable. Alterna-tive courses of action are allowable under the standard whereagreed between contracting parties but shall be justified anddocumented.

103 May: Indicates a permission, or an option, which is per-mitted as part of conformance with the standard.

104 Can: Can-requirements are conditional and indicate apossibility to the user of the standard.

D 200 Terms

201 Accidental condition: When the unit is subjected to ac-cidental loads such as collision, dropped objects, fire explo-sion, etc.

202 Atmospheric zone: The external region exposed to at-mospheric conditions.

203 Cathodic protection: A technique to prevent corrosionof a steel surface by making the surface to be the cathode of anelectrochemical cell.

204 Characteristic load: The reference value of a load to beused in the determination of load effects. The characteristicload is normally based upon a defined fractile in the upper endof the distribution function for load.

205 Classic spar: Shell type hull structure.

206 Classification note: The classification notes cover prov-en technology and solutions which is found to represent goodpractice by DNV, and which represent one alternative for sat-isfying the requirements given in the DNV rules or other codesand standards cited by DNV. The classification notes will inthe same manner be applicable for fulfilling the requirementsin the DNV offshore standards.

207 Coating: Metallic, inorganic or organic material appliedto steel surfaces for prevention of corrosion.

208 Column stabilised unit: A floating unit that can be relo-cated. A column stabilised unit normally consists of a deck structure with a number of widely spaced, large diameter, sup-porting columns that are attached to submerged pontoons.

209 Corrosion addition: Extra steel thickness that is allowedrusted away during design lifetime.

210 Damaged condition: The unit capability to withstand

loads after damage caused by accidental loads.

211 Deep draught floater (DDF): Is a unit categorised witha relative large draught. This large draught is mainly intro-duced to obtain sufficiently high eigenperiod in heave and re-duced wave excitation in heave such that resonant responses inheave can be omitted or minimised.

212 Design temperature: Normally used for the lowest meandaily temperature to which the structure may be exposed toduring installation and operation.

213 Driving voltage: The difference between closed circuitanode potential and the protection potential.

214 Dynamic upending: A process where seawater is filledor flooded into the bottom section of a horizontally floating

DDF hull and creating a trim condition and subsequent waterfilling of hull or moonpool and dynamic upending to bring thehull in vertical position.

215 Expected loads and response history: Expected load andresponse history for a specified time period, taking into ac-

count the number of load cycles and the resulting load levelsand response for each cycle.

216 Expected value: The most probable value of a load dur-ing a specified time period.

217 Fail to safe: A failure shall not lead to new failure,which may lead to total loss of the structure.

218 Fatigue: Degradation of the material caused by cyclicloading.

219 Fatigue critical: Structure with fatigue life less thanthree times the design fatigue life.

220 Guidance note: Information entered in the standard inorder to increase the understanding of the requirements.

221 Hard tank area: Usually upper part of the hull providingsufficient buoyancy for a DDF unit.

222 High frequency (HF) responses: Defined as rigid bodymotions at, or near heave, roll and pitch eigenperiods due tonon-linear wave effects.

223 Hindcasting: A method using registered meteorologicaldata to reproduce environmental parameters. Mostly used for

reproducing wave parameters.224 Inspection: Activities such as measuring, examination,testing, gauging one or more characteristics of an object orservice and comparing the results with specified requirementsfor determine conformity.

225 Installation: A temporary condition where the unit is un-der construction such as mating or in preparation for operation-al phase such as upending of DDFs, lowering the legs andelevating the self elevating units or tether pretension for TLPs.

226 Load effect: Effect of a single design load or combina-tion of loads on the equipment or system, such as stress, strain,deformation, displacement, motion, etc.

227 Lowest daily mean temperature: The lowest value on the

annual mean daily temperature curve for the area in question.For seasonally restricted service the lowest value within thetime of operation applies.

228 Low frequency (LF) responses: Defined as TLP rigidbody non-linear motions at, or near surge, sway and yaw eigen-periods.

229 Lowest waterline: Typical light ballast waterline forships, transit waterline or inspection waterline for other typesof units. Extreme inspection waterline is not considered.

230 Material strength: The nominal value of materialstrength to be used in the determination of the design resist-ance. The material strength is normally based upon a 5% frac-tile in the lower end of the distribution function for materialstrength.

231 Mean: Statistical mean over observation period.

232 Non-destructive testing (NDT): Structural tests and in-spection of welds with radiography, ultrasonic or magneticpowder methods.

233 Offshore standard: The DNV offshore standards aredocuments which presents the principles and technical require-ments for design of offshore structures. The standards are of-fered as DNV’s interpretation of engineering practice forgeneral use by the offshore industry for achieving safe struc-tures.

234 Operating conditions: Conditions wherein a unit is onlocation for purposes of production, drilling or other opera-tions, and combined environmental and operational loadings

are within the appropriate design limits established for suchoperations.

235 P-delta effect: Global bending or shear effects in DDFunits due to relatively high roll or pitch angles in harsh envi-ronment.




Sec.1 –

236 Potential: The voltage between a submerged metal sur-face and a reference electrode.

237 Recommended Practice (RP): The recommended prac-tice publications cover proven technology and solutions whichhave been found by DNV to represent good practice, andwhich represent one alternative for satisfying the requirementsgiven in the DNV offshore standards or other codes and stand-ards cited by DNV.

238 Redundancy: The ability of a component or system tomaintain or restore its function when a failure of a member orconnection has occurred. Redundancy can be achieved for in-stance by strengthening or introducing alternative load paths.

239 Reference electrode: Electrode with stable open-circuitpotential used as reference for potential measurements.

240 Reliability: The ability of a component or a system toperform its required function without failure during a specifiedtime interval.

241 Representative value: The value assigned to each loadfor a design situation.

242 Resistance: The reference value of structural strength to

be used in the determination of the design strength. The resist-ance is normally based upon a 5% fractile in the lower end of the distribution function for resistance.

243 Ringing: Defined as the non-linear high frequency reso-nant response induced by transient loads from high, steepwaves.

244 Riser frame: Framed steel structures installed at differ-ent vertical elevations along the hull or moonpool in order toseparate the different risers.

245 Risk: The qualitative or quantitative likelihood of an ac-cidental or unplanned event occurring considered in conjunc-tion with the potential consequences of such a failure. Inquantitative terms, risk is the quantified probability of a de-fined failure mode times its quantified consequence.

246 Self elevating unit: Jack-up. A mobile unit that can be re-located floating on the hull and that is bottom founded in its op-eration mode. The unit reaches its operation mode by loweringthe legs to the sea floor and then jacking the hull to the requiredelevation.

247 Shakedown: A linear elastic structural behaviour is es-tablished after yielding of the material has occurred.

248 Slamming: Impact load on an approximately horizontalmember from a rising water surface as a wave passes. The di-rection of the impact load is mainly vertical.

249 Specified minimum yield strength (SMYS): The mini-mum yield strength prescribed by the specification or standardunder which the material is purchased.

250 Specified value: Minimum or maximum value duringthe period considered. This value may take into account oper-ational requirements, limitations and measures taken such thatthe required safety level is obtained.

251 Splash zone: The external region of the unit that is mostfrequently exposed to wave action.

252 Springing: The high frequency non-linear resonant re-sponse induced by cyclic (steady state) loads in low to moder-ate sea states.

253 Strake: Usually helical devices (strake) welded to outerhull with the purpose of reducing the cross-flow motion (VIVinduced) of DDF hull due to current (mainly). Also the termsuppression device may be used to describe the strake.

254 Submerged zone: The part of the installation, which isbelow the splash zone, including buried parts.

255 Survival condition: A condition during which a unit maybe subjected to the most severe environmental loadings forwhich the unit is designed. Drilling, production or similar op-

erations may have been discontinued due to the severity of theenvironmental loadings.

256 Target safety level: A nominal acceptable probability of structural failure.

257 Temporary conditions: A not operational condition thatmay be a design condition, e.g. mating, transit or installationphases.

258 Tensile strength: Minimum stress level where strainhardening is at maximum or at rupture.

259 Tension leg platform (TLP): A buoyant unit connectedto a fixed foundation by pre-tensioned tendons. The tendonsare normally parallel, near vertical elements, acting in tension,which usually restrain the motions of the TLP in heave, rolland pitch. The platform is usually compliant in surge, swayand yaw.

260 Transit conditions: All unit field movements or move-ments from one geographical location to another.

261 Truss spar: Truss structure for the hull part below hardtank area.

262 Unit: A general term for a column stabilised or self ele-vating offshore installation. The term installation includes allunits and is most commonly used in these offshore standards.

263 Usage factor: The ratio between permissible and thecharacteristic strength of the structural member.

264 Verification: Examination to confirm that an activity, aproduct or a service is in accordance with specified require-ments.

265 Wave frequency (WF) responses: Linear rigid body mo-tions at the dominating wave periods.

266 Ultimate strength: Corresponding to the maximum loadcarrying resistance.

E. Abbreviations and Symbols

E 100 Abbreviations

101 The abbreviations given in Table E1 are used in thisstandard.

Table E1 Abbreviations

Abbreviation In full

AISC American Institute of Steel Construction

API American Petroleum Institute

ASD allowable stress design

BS British Standard (issued by British Standard Insti-

tution)

CTOD crack tip opening displacement

DDF deep draught floaters

DFF design fatigue factor

DNV Det Norske Veritas

DP dynamic positioning

EHS extra high strength

FE finite elements

HAT highest astronomical tide

HF high frequency

HISC hydrogen induced stress cracking

HRTLP heave resisted TLP

HS high strengthIC inspection category

IIP in service inspection program

ISO International Organisation for Standardisation

LAT lowest astronomic tide




– Sec.1

E 200 Symbols

201 The following units are used in this standard:

202 The following Latin characters are used in this standard:

LF low frequency

LRFD load and resistance factor design

MPI magnetic particle inspection

MSL mean stillwater line

NACE National Association of Corrosion Engineers

NDT non destructive testing

NS normal strength

QTF quadratic transfer function

RAO response amplitude operator

RP recommended practice

SCF stress concentration factor

SMYS specified minimum yield stress

SNAME Society of Naval Architects and Marine Engi-neers

TLP tension leg platform

TLWP tension leg wellhead platform

VIV vortex induced vibrations

WF wave frequencyWSD working stress design

g gram

k kilo

m meter

cm centimetre

mm millimetre

t tonne

N Newton

s second.

a sectional area of weld

the intercept of the design S-N curve with the log Naxis

a0 total connection area at supports of stiffeners

ah horizontal acceleration

av vertical acceleration

b breadth of plate flange

be effective flange width

c flange breadthd web height

dp diameter of pipe

f distributed load factor for primary design

f r strength ratio

f u lowest ultimate tensile strength

f w strength ratio

f y yield stress

g0 acceleration due to gravity

h the shape parameter of the Weibull stress range dis-tribution

hD dynamic pressure head due to flow through pipes

hop1 vertical distance from the load point to the top of airpipe

hop2 vertical distance from the load point to the positionof maximum filling height

Table E1 Abbreviations (Continued)

Abbreviation In full

a

hpc vertical distance from the load point to the positionof maximum filling height

hs vertical distance from the load point to the top of thetank

k roughness height

k a factor for aspect ratio of plate field

k m bending moment factork r correction factor for curvature perpendicular to thestiffeners

k pp factor dependent on support condition for plate

k ps factor dependent on support condition for stiffener

k τ shear force factor

l stiffener span

l0 distance between points of zero bending moments

m the inverse slope of the S-N curve

ni the number of stress variations in i years

n0 total number of stress variations during the lifetimeof the structure

p lateral pressureps sea pressure

pe sea pressure

p0 valve opening pressure

q distributed load

qc contact pressure

r root face

s stiffener spacing

t thickness

t0 net thickness abutting plate

tf thickness of flange

tk corrosion addition

tm factor used in formulas for minimum plate thickness

tp thickness of pipe

tw web thickness

tW throat thickness of weld

xD load effect with a return period of D-year

zb vertical distance

A area

AW web area

C buckling coefficient

Ce effective plate flange factor

CD hydrodynamic coefficient, drag

CM hydrodynamic coefficient, added massD number of years

Dm diameter of member

DB depth of barge

E modulus of elasticity, 2.1 105 N/mm2

FV maximum axial force

Fx( x) long-term peak distribution

Hs significant wave height

KC Keulegan-Carpenter number

L length

Li variable used in determining splash zone

M bending moment

Mc mass of component

Me eccentricity moment

Mp plastic moment resistance




Sec.1 –

203 The following Greek characters are used in this stand-ard:

α length ratio β coefficient depending on type of structure and

reduced slenderness β w correlation factorε relative strain

Γ ( ) the complete gamma functionγ c contingency factorη 0 basic usage factorη p maximum permissible usage factorϕ angle between the stiffener web plane and the

plane perpendicular to the platingλ reduced slenderness parameterθ rotation ρ densityσ stressσ e elastic buckling stressσ fw yield stress of weld depositsσ j equivalent stress for global in-plane membrane

stress∆σ ampl_n0 extreme stress amplitude

∆σ ni extreme stress range∆σ n0 extreme stress rangeσ p permissible stressσ p1 permissible bending stressσ p2 permissible bending stressσ ⊥ normal stress perpendicular to an axisτ shear stressτ p permissible shear stressτ ⊥ shear stress perpendicular to an axisτ || shear stress parallel to an axisψ stress ratio.

My elastic moment resistance

N number of stress cycles to failure

ND total number of load effect maxima during D years

Np number of supported stiffeners on the girder span

Ns number of stiffeners between considered sectionand nearest support

P loadPE Euler buckling load

PH horizontal force

Pp average point load

PV vertical force

R radius of curvature

S stress range

Sg girder span

SZL lower limit of the splash zone

SZU upper limit of the splash zone

T wave period

TE

extreme operational draught

TTH heavy transit draught

TZ average zero-upcrossing period

Ui variable used in determining splash zone

Um maximum orbital particle velocity

Z steel grade with proved through thickness properties

Zs section modulus for stiffener section

Zg section modulus for simple girder section.




– Sec.2

SECTION 2DESIGN PRINCIPLES

A. Introduction

A 100 General101 This section describes design principles and designmethods including:

— working stress design method— design assisted by testing— probability based design.

102 General design considerations regardless of designmethod are also given in B.

103 This standard is based on the working stress design(WSD) method also known as the allowable stress method.

104 Direct reliability analysis methods are mainly consid-ered as applicable to special case design problems, to calibrate

the usage factors to be used in the WSD method and for condi-tions where limited experience exists.

105 As an alternative or as a supplement to analytical meth-ods, determination of load effects or resistance may in somecases be based either on testing or on observation of structuralperformance of models or full-scale structures.

A 200 Aim of the design

201 Structures and structural elements shall be designed to:

— sustain loads liable to occur during all temporary, operat-ing and damaged conditions if required

— maintain acceptable safety for personnel and environment— have adequate durability against deterioration during the

design life of the structure.

B. General Design Considerations

B 100 General

101 The design of a structural system, its components anddetails should, as far as possible, account for the followingprinciples:

— resistance against relevant mechanical, physical andchemical deterioration is achieved

— fabrication and construction comply with relevant, recog-nised techniques and practice

— inspection, maintenance and repair are possible.

102 Structures and elements thereof, shall possess ductile re-sistance unless the specified purpose requires otherwise.

103 The overall structural safety shall be evaluated on thebasis of preventive measures against structural failure put intodesign, fabrication and in-service inspection as well as theunit’s residual strength against total collapse in the case of structural failure of vital elements.

104 Structural connections are, in general, to be designedwith the aim to minimise stress concentrations and reducecomplex stress flow patterns.

105 Fatigue life improvements with methods such as grind-ing or hammer peening of welds should not provide a measur-

able increase in the fatigue life at the design stage. The fatiguelife should instead be extended by means of modification of structural details. Fatigue life improvements based on meanstress level should not be applied.

106 Transmission of high tensile stresses through the thick-

ness of plates shall be avoided as far as possible. In caseswhere transmission of high tensile stresses through thickness

occur, structural material with proven through thickness prop-erties shall be considered used.

107 Structural elements may be fabricated according to therequirements given in DNV-OS-C401.

C. Design Conditions

C 100 Basic conditions

101 Different modes of operation or phases during the life of structure may be governing for the design. The following de-sign conditions given in Table C1 shall normally be consid-ered.

102 Relevant load cases shall be established for the variousdesign conditions based on the most unfavourable combina-tions of functional loads, environmental loads and/or acciden-

tal loads, see Sec.3.103 Limiting environmental and operational conditions (de-sign data) for the different design conditions shall be specified.The limiting conditions shall be stated in the operation manual.

D. Loading Conditions

D 100 General

101 Each structural member shall be designed for the mostunfavourable of the loading conditions given in Table D1.

102 For each of the loading conditions in Table D1 and foreach structural element, the most unfavourable combinations,

position, and direction of the forces have to be used in the anal-ysis.

103 All directions of wind, waves and current relative to theunit are normally to be assumed equally probable.

104 If, however, statistics show clearly that wind, waves and

Table C1 Design conditions

I n s t a l l a t i o n

O p e r a t i n g

S u r v i v a l

T r a n s i t

A c c i d e n t a l

D a m a g e d

Column stabilised unit x x x x x x

Self elevating unit x x x x x x

Tension leg platforms x x x x

Deep draught floaters x x x x

Table D1 Loading conditions

Case Description

a) functional loads

b) maximum environmental loads and associated functionalloads

c) accidental loads and associated functional loads

d) annual most probable value of environmental loads and asso-ciated functional loads after credible failures, or after acci-dental events

e) annual most probable value of environmental loads and asso-ciated functional loads in a heeled condition corresponding toaccidental flooding




Sec.2 –

current of the prescribed probability are different for differentdirections, this may be taken into account in the analysis. It isassumed that orientation of the unit will be under completecontrol of the operator.

D 200 Load

201 The representative values for load component in the dif-

ferent design conditions shall be based on Sec.3.202 For the design conditions, installation and transit, theloads may be based on specified values, which shall be select-ed dependent on the measurers taken to achieve the requiredsafety level. The value may be specified with due attention tothe actual location, season of the year, weather forecast andconsequences of failure.

E. Design by the WSD Method

E 100 Usage factors and permissible stresses

101The permissible usage factor, η p, is defined as the ratiobetween permissible stress and a stress representing the char-

acteristic strength or capability of the structural member.

102 The permissible usage factors and stresses are a functionof:

— loading condition

— failure mode

— importance of strength member.

103 Stresses shall be calculated using net scantlings, i.e. withany corrosion addition deducted.

104 If the residual strength of the unit after collapse of a vitalstructural member does not satisfy the accidental damage cri-teria, the usage factors in Table E1 for the pertinent vital struc-tural members shall be multiplied by a factor 0.9.

E 200 Basic usage factors

201 The basic usage factor, η 0, is given in Table E1.

202 The target component safety level is achieved by usingusage factors, which take into account the variation in load andresistance and the reduced probabilities that various loads willact simultaneously.

203 The basic usage factors account for:

— possible unfavourable deviations of the loads

— the reduced probability that various loads acting togetherwill act simultaneously

— uncertainties in the model and analysis used for determi-nation of load effects

— possible unfavourable deviations in the resistance of mate-rials

— possible reduced resistance of the materials in the struc-ture, as a whole, as compared with the values deducedfrom test specimens.

F. Design Assisted by Testing

F 100 General

101 Design by testing or observation of performance is ingeneral to be supported by analytical design methods.

102 Load effects, structural resistance and resistance againstmaterial degradation may be established by means of testing orobservation of the actual performance of full-scale structures.

F 200 Full-scale testing and observation of performanceof existing structures

201 Full-scale tests or monitoring on existing structures maybe used to give information on response and load effects to beutilised in calibration and updating of the safety level of thestructure.

G. Probability Based Design

G 100 Definition

101 Reliability, or structural safety, is defined as the proba-bility that failure will not occur or that a specified criterion willnot be exceeded.

G 200 General

201 This section gives requirements for structural reliabilityanalysis undertaken in order to document compliance with theoffshore standards.

202 Acceptable procedures for reliability analyses are docu-mented in the Classification Note 30.6.

203 Reliability analyses shall be based on level 3 reliabilitymethods. These methods utilise probability of failure as a

measure and require knowledge of the distribution of all basicvariables.

204 In this standard, level 3 reliability methods are mainlyconsidered applicable to:

— calibration of level 1 method to account for improvedknowledge. (Level 1 methods are deterministic analysismethods that use only one characteristic value to describeeach uncertain variable)

— special case design problems

— novel designs where limited (or no) experience exists.

205 Reliability analysis may be updated by utilisation of newinformation. Where such updating indicates that the assump-

tions upon which the original analysis was based are not valid,and, the result of such non-validation is deemed to be essentialto safety, the subject approval may be revoked.

206 Target reliabilities shall be commensurate with the con-sequence of failure. The method of establishing such target re-liabilities, and the values of the target reliabilities themselvesshall be specially considered in each case. To the extent possi-ble, the minimum target reliabilities shall be based on estab-lished cases that are known to have adequate safety.

207 Where well established cases do not exist, for example,in the case of novel and unique design solution, the minimumtarget reliability values shall be based upon one (or a combina-tion) of the following considerations:

— transferable target reliabilities similar as for existing de-sign solutions

— internationally recognised codes and standards.

See also Classification Note 30.6.

Table E1 Basic usage factors η 0

Loading conditions

a) b) c) d) e)

η 0 0.60 1) 0.80 1) 0.80 1.00 1.00 2)

1) The usage factorη 0 for environmental loads may be increased to 0.69in load condition a) and 0.92 in load condition b) if the structure is un-manned during extreme environmental conditions.

2) If a basic usage factor of 0.75 is applied, environmental loads may bedisregarded.




– Sec.3

SECTION 3LOADS AND LOAD EFFECTS

A. Introduction

A 100 General101 The requirements in this section define and specify loadcomponents and load combinations to be considered in theoverall strength analysis as well as design pressures applicablein formulae for local design.

102 Impact pressure caused by the sea (e.g. slamming or bowimpact) or by liquid cargoes in partly filled tanks (sloshing) arenot covered by this section. Design values are given in the sec-tions for special considerations for each type of unit.

103 For loads from mooring system, see DNV-OS-E301.

B. Basis for Selection of Loads

B 100 General101 Unless specific exceptions apply, as documented withinthis standard, the loads documented in Table B1 and Table B2shall apply in the temporary and operational design conditions,respectively.

102 Where environmental and accidental loads may act si-multaneously, the representative values may be determinedbased on their joint probability distribution.

C. Permanent Loads

C 100 General

101 Permanent loads are loads that will not vary in magni-tude, position or direction during the period considered. Exam-ples are:

— mass of structure— mass of permanent ballast and equipment— external and internal hydrostatic pressure of a permanent

nature— reaction to the above e.g. articulated tower base reload.

102 The representative value of a permanent load is definedas the expected value based on accurate data of the unit, massof the material and the volume in question.

D. Variable Functional Loads

D 100 General

101 Variable functional loads are loads which may vary inmagnitude, position and direction during the period under con-sideration, and which are related to operations and normal useof the installation.

102 Examples are:

— personnel

— stored materials, equipment, gas, fluids and fluid pressure— crane operational loads— loads from fendering— loads associated with installation operations— loads associated with drilling operations

Table B1 Basis for selection of representative loads for temporary design conditions, e.g. installation and transit design conditions

Load category

Operation design conditions

Strength(loading condition a and b)

Fatigue

Accidental

Intact structure(loading condition c)

Damaged structure(loading condition d and e)

Permanent Expected value

Variable Specified value

Environmental Specified value 1) Expected load history Specified value Specified value

Accidental Specified value

Deformation Expected extreme value

1) Not applicable for loading condition a

For definitions, see Sec.1

See the DNV Rules for Planning and Execution of Marine Operations.

Table B2 Basis for selection of representative loads for in-place design conditions, e.g. operating and survival

Load category

Operation design conditions

Strength(loading condition a and b)

Fatigue

Accidental

Intact structure(loading condition c)

Damaged structure(loading condition d and e)

Permanent Expected value

Variable Specified value

Environmental

Annual probability 1) beingexceeded = 10–2 for the load

effect (100 year return period)2)

Expected load history Not applicableLoad with return period not

less than one year.

AccidentalSpecified value, see also

DNV-OS-A101

Deformation Expected extreme value

1) The probability of exceedance applies.

2) Not applicable for loading condition a




Sec.3 –

— loads from variable ballast and equipment— variable cargo inventory for storage vessels— helicopters— lifeboats.

103 The variable functional load is the maximum (or mini-mum) specified value, which produces the most unfavourableload effects in the structure under consideration.

104 The specified value shall be determined on the basis of

relevant specifications. An expected load history shall be usedin fatigue design.

D 200 Variable functional loads on deck areas

201 Variable functional loads on deck areas of the topsidestructure shall be based on Table D1 unless specified otherwisein the design basis or design brief. The intensity of the distrib-uted loads depends on local and global aspects as shown in Ta-

ble D1. The following notations are used:

D 300 Tank pressures

301 The hydrostatic pressures given in the 302 to 311 arenormative requirements. Other requirements for the hydrostat-ic pressure in tanks may be given in the sections for specialconsiderations for each type of unit.

302 The structure shall be designed to resist the maximumhydrostatic pressure of the heaviest filling in tanks that may oc-cur during fabrication, installation and operation.

303 Hydrostatic pressures in tanks should normally be basedon a minimum density equal to that of seawater, ρ = 1025 kg/ m3. Tanks for higher density fluids (e.g. mud) shall be de-signed on basis of special consideration. The density, upon

which the scantlings of individual tanks are based, shall be giv-en in the operating manual.

304 Pressure loads that may occur during emptying of wateror oil filled structural parts for condition monitoring; mainte-nance or repair shall be evaluated.

305 Hydrostatic pressure heads shall be based on tank fillingarrangement by for example pumping, gravitational effect, ac-celerations as well as venting arrangements.

306 Pumping pressures can be limited by installing appropri-ate alarms and auto-pump cut-off system (e.g. high level andhigh-high level with automatic stop of the pumps). In such asituation the pressure head can be taken to be the cut-off pres-sure head hpc.

307 Dynamic pressure heads resulting from filling throughpipes by pumping shall be included in the design pressurehead.

308 If not given in Sec.11 to Sec.14, the maximum internalpressure in tanks shall be taken as the largest of pressure p1 and

p2 given below:

309 Systems installed to limit the pressure to hpc can be tak-en into account, see for example Sec.11.

310 In a situation where design pressure head might be ex-

ceeded, should be considered as an accidental condition.311 The tank pressures given in this section refer to staticpressures only. When hydrostatic pressure is combined withhydrodynamic pressure caused by the motion of the unit, thepressure p1 shall not be combined with the dynamic pressure

Table D1 Variable functional loads on deck areas

Local design * Primary design * Global design *

Area Distributed load, q(kN/m2)

Point load, P(kN)

Apply factor to distributedload

Apply factor to primarydesign load

Storage areas q 3) 1.5 q 3) 1.0 1.0

Lay down areas q3) 1.5 q 3) f 4) f 4)

Lifeboat platforms 9.0 9.0 1.0 may be ignored

Area between equipment 5.0 5.0 f 4) may be ignored

Walkways, staircases andplatforms

4.0 4.0 f 4) may be ignored

Walkways and staircases

for inspection only

3.0 3.0 f 4) may be ignored

Areas not exposed to otherfunctional loads

2.5 2.5 1.0 -

Notes:

1) Wheel loads to be added to distributed loads where relevant. (Wheel loads can normally be considered acting on an area of 300 x 300 mm.)

2) Point loads to be applied on an area 100 x 100 mm, and at the most severe position, but not added to wheel loads or distributed loads.

3) The loads shall be evaluated for each case. Lay down areas should not be designed for less than 15 kN/m2.

4) , where A is the loaded area in m2.

5) Global load cases should be established based upon “worst case”, representative variable load combinations, complying with the limiting global criteriato the structure. For buoyant structures these criteria are established by requirements for the floating position in still water, and intact and damage stabilityrequirements, as documented in the operational manual, considering variable load on the deck and in tanks.

* Local design: e.g. design of plates, stiffeners, beams and bracketsPrimary design: e.g. design of girders and columnsGlobal design: e.g. design of deck main structure and substructure

f min 1.0; (0.5 3 A ) ⁄ +{ }=

ρ = density of liquid (kg/m3)

g0 = 9.81 m/s2

hpc = vertical distance (m) from the load point to theposition of maximum filling height. If no con-trol devices are used, the pressure heightshould be considered to the top of the air pipe.For tanks adjacent to the sea that are situatedbelow the extreme operational draught, hpcshall normally not be taken less than the ex-treme operational draught

hD = dynamic pressure head due to flow throughpipes

hs = vertical distance (m) from the load point to thetop of the tank

p0 = 25 kN/m2 in general. Valve opening pressurewhen exceeding the general value.

p1 ρ g0 hpc hD+( ) kN m2

⁄ ( )=

p2 ρ g0 hs p0+ kN m2

⁄ ( )=




– Sec.3

(hD) due to flow resistance in the pipe.

D 400 Miscellaneous loads

401 Railing shall be designed for 1.5 kN/m, acting horizon-tally on the top of the railing.

E. Environmental Loads

E 100 General

101 Environmental loads are loads which may vary in mag-nitude, position and direction during the period under consid-eration, and which are related to operations and normal use of the installation. Examples are:

— hydrodynamic loads induced by waves and current— inertia forces— wind— earthquake— tidal effects— marine growth

— snow and ice.102 Practical information regarding environmental loadsand conditions are given in Classification Note 30.5.

E 200 Environmental loads for mobile units

201 The design of mobile offshore units shall be based on themost severe environmental loads that the structure may expe-rience during its design life. The applied environmental condi-tions shall be stated in the design basis or design brief. Unlessotherwise stated in the design brief, the North Atlantic scatterdiagram should be used for strength and fatigue for unrestrict-ed world wide operation.

E 300 Environmental loads for site specific units

301 The parameters describing the environmental conditionsshall be based on observations from or in the vicinity of the rel-evant location and on general knowledge about the environ-mental conditions in the area. Data for the joint occurrence of for example wave, wind and current conditions should be ap-plied.

302 According to this standard, the environmental loadsshall be determined with stipulated probabilities of exceed-ance. The statistical analysis of measured data or simulateddata should make use of different statistical methods to evalu-ate the sensitivity of the result. The validation of distributionswith respect to data should be tested by means of recognisedmethods.

303 The analysis of the data shall be based on the longest

possible time period for the relevant area. In the case of shorttime series the statistical uncertainty shall be accounted forwhen determining design values. Hindcasting may be used toextend measured time series, or to interpolate to places wheremeasured data have not been collected. If hindcasting is used,the model shall be calibrated against measured data, to ensurethat the hindcast results comply with available measured data.

E 400 Determination of hydrodynamic loads

401 Hydrodynamic loads shall be determined by analysis.When theoretical predictions are subjected to significant un-certainties, theoretical calculations shall be supported by mod-el tests or full scale measurements of existing structures or bya combination of such tests and full scale measurements.

402 Hydrodynamic model tests should be carried out to:— confirm that no important hydrodynamic feature has been

overlooked by varying the wave parameters (for new typesof installations, environmental conditions, adjacent struc-ture, etc.)

— support theoretical calculations when available analyticalmethods are susceptible to large uncertainties

— verify theoretical methods on a general basis.

403 Models shall be sufficient to represent the actual instal-lation. The test set-up and registration system shall provide abasis for reliable, repeatable interpretation.

404 Full-scale measurements may be used to update the re-sponse prediction of the relevant structure and to validate theresponse analysis for future analysis. Such tests may especiallybe applied to reduce uncertainties associated with loads andload effects, which are difficult to simulate in model scale.

405 In full-scale measurements it is important to ensure suf-ficient instrumentation and logging of environmental condi-tions and responses to ensure reliable interpretation.

406 Wind tunnel tests should be carried out when:

— wind loads are significant for overall stability, offset, mo-tions or structural response

— there is a danger of dynamic instability.

407 Wind tunnel test may support or replace theoretical cal-

culations when available theoretical methods are susceptible tolarge uncertainties (e.g. due to new type of installations or adjacent installation influence the relevant installation).

408 Theoretical models for calculation of loads from ice-bergs or drift ice should be checked against model tests or full-scale measurements.

409 Proof tests of the structure may be necessary to confirmassumptions made in the design.

E 500 Wave loads

501 Wave theory or kinematics shall be selected accordingto recognised methods with due consideration of actual waterdepth and description of wave kinematics at the surface and thewater column below.

502 Linearised wave theories (e.g. Airy) may be used whenappropriate. In such circumstances the influence of finite am-plitude waves shall be taken into consideration.

503 Wave loads can be determined according to Classifica-tion Note 30.5.

504 For large volume structures where the wave kinematicsis disturbed by the presence of the structure, typical radiationand diffraction analyses shall be performed to determine thewave loads (excitation forces or pressures).

505 For slender structures (typically bracings, tendons, ris-ers) where the Morison equation is applicable, the wave loadscan be estimated by careful selection of drag and inertia coef-ficients (see Classification Note 30.5).

E 600 Wave induced inertia forces

601 The load effect from inertia forces shall be taken into ac-count in the design. Examples where inertia forces can be of significance is:

— heavy objects— tank pressures— flare towers— drilling towers— crane pedestals.

E 700 Wind loads

701 The wind velocity at the location of the installation shall

be established on the basis of previous measurements at the ac-tual and adjacent locations, hindcast predictions as well as the-oretical models and other meteorological information. If thewind velocity is of significant importance to the design and ex-isting wind data are scarce and uncertain, wind velocity meas-urements should be carried out at the location in question.




Sec.3 –

702 Values of the wind velocity should be determined withdue account of the inherent uncertainties.

Guidance note:

Wind loads may be determined in accordance with ClassificationNote 30.5.

---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---

703 The pressure acting on vertical external bulkheads ex-posed to wind is in general not to be taken less than 2.5 kN/m2

unless otherwise documented.

E 800 Earthquake

801 Relevant earthquake effects shall be considered for bot-tom fixed structures.

802 Earthquake excitation design loads and load historiesmay be described either in terms of response spectra or interms of time histories. For the response spectrum method allmodes of vibration which contribute significantly to the re-sponse shall be included. Correlation effects shall be account-ed for when combining the modal response maximum.

803 When performing time-history earthquake analysis, theresponse of the structure/foundation system shall be analysedfor a representative set of time histories. Such time historiesshall be selected and scaled to provide a best fit of the earth-quake motion in the frequency range where the main dynamicresponse is expected.

804 The dynamic characteristics of the structure and itsfoundation should be determined using a three-dimensionalanalytical model. A two-dimensional or asymmetric modelmay be used for the soil and structure interaction analysis pro-vided compatibility with the three-dimensional structural mod-el is ensured.

805 Where characteristic ground motions, soil characteris-tics, damping and other modelling parameters are subject togreat uncertainties, a parameter sensitivity study should be car-ried out.

806 Consideration shall be given to the possibility that earth-quakes in the local region may cause other effects such as sub-sea earth slides, critical pore pressure built-up in the soil ormajor soil deformations affecting foundation slabs, piles orskirts.

E 900 Vortex induced oscillations

901 Consideration of loads from vortex shedding on individ-ual elements due to wind, current and waves may be based onClassification Note 30.5. Vortex induced vibrations of framesshall also be considered. The material and structural damping

of individual elements in welded steel structures shall not beset higher than 0.15% of critical damping.

E 1000 Current

1001 Current design velocities shall be based upon appropri-ate consideration of velocity and height profiles and direction-ality.

Guidance note:

Further details regarding current loads are given in ClassificationNote 30.5.


E 1100 Tidal effects

1101 For floating structures constrained by tendon mooringsystems, tidal effects can significantly influence the structure’sbuoyancy and the mean loads in the mooring components.Therefore the choice of tide conditions for static equilibriumanalysis is important. Tidal effects should be considered in

evaluating the various responses of interest. Higher mean wa-ter levels tend to increase maximum mooring tensions, hydro-static loads, and current loads on the hull, while tending todecrease under deck wave clearances.

1102 These effects of tide may be taken into account by per-forming a static balance at the various appropriate tide levelsto provide a starting point for further analysis, or by making al-

lowances for the appropriate tide level in calculating extremeresponses.

Guidance note:

For example, the effects of the highest tide level consistent withthe probability of simultaneous occurrence of other extreme en-vironmental conditions should be taken into account in estimat-ing maximum tendon tensions for a TLP.


E 1200 Marine growth

1201 Marine growth is a common designation for a surfacecoating on marine structures, caused by plants, animals andbacteria. In addition to the direct increase in structure weight,

marine growth may cause an increase in hydrodynamic dragand added mass due to the effective increase in member dimen-sions, and may alter the roughness characteristics of the sur-face.

E 1300 Snow and ice accumulation

1301 Ice accretion from sea spray, snow, rain and air humid-ity shall be considered, where relevant.

1302 Snow and ice loads may be reduced or neglected if snow and ice removal procedure is established.

1303 Possible increases of cross-sectional area and changesin surface roughness caused by icing shall be considered,where relevant, when determining wind and hydrodynamicloads.

1304 For buoyant structures the possibility of uneven distri-bution of snow and ice accretion shall be considered.

E 1400 Direct ice load

1401 Where impact with sea ice or icebergs may occur, thecontact loads shall be determined according to relevant, recog-nised theoretical models, model tests or full-scale measure-ments.

1402 When determining the magnitude and direction of theloads, the following factors shall be considered:

— geometry and nature of the ice

— mechanical properties of the ice

— velocity and direction of the ice— geometry and size of the ice and structure contact area

— ice failure mode as a function of the structure geometry— environmental forces available to drive the ice— inertia effects for both ice and structure.

E 1500 Water level, settlements and erosion

1501 When determining water level in the calculation of loads, the tidal water and storm surge shall be taken into ac-count. Calculation methods that take into account the effectsthat the structure and adjacent structures have on the water lev-el shall be used.

1502 Uncertainty of measurements and possible erosion

shall be considered.

E 1600 Appurtenances and equipment

1601 Hydrodynamic loads on appurtenances (anodes, fend-ers, strakes etc,) shall be taken into account, when relevant.




– Sec.3

F. Combination of Environmental Loads

F 100 General

101 Individual environmental loads are commonly definedby an annual probability of exceedance level, e.g. 10-2 or 10-1.The long-term variability of multiple loads is described by ascatter diagram or joint density function including informationabout direction. Contour curves can then be derived whichgive combination of environmental parameters, which approx-imately describe the various loads corresponding to the givenprobability of exceedance.

102 Alternatively, the probability of exceedance can be re-

ferred to the load effects. This is particularly relevant when di-

rection of the load is an important parameter.

103 The load intensities for various types of loads can be

combined according to the probabilities of exceedance as giv-

en in Table F1.

104 In a short-term period with a combination of waves andfluctuating wind, the individual variations of the two load

processes can be assumed uncorrelated.

G. Accidental Loads

G 100 General

101 Accidental loads are loads related to abnormal opera-tions or technical failure. Examples of accidental loads areloads caused by:

— dropped objects— collision impact— explosions— fire— change of intended pressure difference— accidental impact from vessel, helicopter or other objects— unintended change in ballast distribution— failure of a ballast pipe or unintended flooding of a hull

compartment— failure of mooring lines— loss of dynamic positioning (DP) system causing loss of

heading.

102 Relevant accidental loads should be determined on thebasis of an assessment and relevant experiences. With respectto planning, implementation, use and updating of such assess-ment and generic accidental loads, see DNV-OS-A101.

103 For temporary design conditions, the representative val-ue may be a specified value dependent on practical require-ments. The level of safety related to the temporary design

conditions shall not be inferior to the safety level required forthe operating design conditions.

H. Deformation Loads

H 100 General

101 Deformation loads are loads caused by inflicted defor-mations such as:

— temperature loads— built-in deformations— settlement of foundations— the tether pre-tension on a tension leg platform (TLP).

H 200 Temperature loads

201 Structures shall be designed for the most extreme tem-perature differences they may be exposed to. This applies, butnot limited, to:

— storage tanks— structural parts that are exposed to radiation from the top

of a flare boom. For flare born radiation a one hour meanwind with a return period of one year may be used to cal-culate the spatial flame extent and the air cooling in the as-sessment of heat radiation from the flare boom

— structural parts that are in contact with pipelines, risers orprocess equipment.

202 The ambient sea or air temperature is calculated as anextreme value with an annual probability of exceedance equalto 10-2 (100 years).

H 300 Settlements and subsidence of sea bed301 Settlement of the foundations into the sea bed shall beconsidered.

302 The possibility of, and the consequences of, subsidenceof the seabed as a result of changes in the subsoil and in theproduction reservoir during the service life of the installation,shall be considered.

303 Reservoir settlements and subsequent subsidence of theseabed should be calculated as a conservatively estimatedmean value.

I. Load Effect Analysis

I 100 General

101 Load effects, in terms of motions, displacements, or in-ternal forces and stresses of the structure, should be deter-mined considering:

— the spatial and temporal nature including:

— possible non-linearities of the load— dynamic character of the response

— the relevant conditions for design check — the desired accuracy in the relevant design phase.

102 Permanent, functional, deformation, and fire loads can

generally be treated by static methods of analysis. Environ-mental (wave and earthquake) loads and certain accidentalloads (impacts, explosions) may require dynamic analysis. In-ertia and damping forces are important when the periods of steady-state loads are close to natural periods or when transientloads occur.

Table F1 Possible combinations of environmental loads to represent combinations with 10-2 annual probability of exceedance for

loading condition b and loads with return period not less than one year for loading condition d and e

Condition Wind Waves Current Ice Sea level

Strength(loading condition b)

10-2 10-2 10-1 10-2

10-1 10-1 10-2 10-2

10-1 10-1 10-1 10-2 mean water level

Accidental(loading condition d and e)

return period not lessthan one year







Sec.3 –

103 In general, three frequency bands need to be consideredfor offshore structures:

104 A global wave motion analysis is required for structureswith at least one free mode. For fully restrained structures a

static or dynamic wave-structure-foundation analysis is re-quired.

105 Uncertainties in the analysis model are expected to betaken care of by the basic usage factors. If uncertainties areparticularly high, conservative assumptions shall be made.

106 If analytical models are particularly uncertain, the sensi-tivity of the models and the parameters utilised in the modelsshall be examined. If geometric deviations or imperfectionshave a significant effect on load effects, conservative geomet-ric parameters shall be used in the calculation.

107 In the final design stage theoretical methods for predic-tion of important responses of any novel system should nor-mally be verified by appropriate model tests. (See Sec.2 E200).

108 Earthquake loads need only be considered for restrainedmodes of behaviour. See sections with special considerationsfor each type of unit for requirements related to the differentobjects.

I 200 Global motion analysis

201 The purpose of a motion analysis is to determine dis-placements, accelerations, velocities and hydrodynamic pres-sures relevant for the loading on the hull and superstructure, aswell as relative motions (in free modes) needed to assess airgap and green water requirements. Excitation by waves, cur-rent and wind should be considered.

I 300 Load effects in structures and soil or foundation

301 Displacements, forces or stresses in the structure andfoundation, shall be determined for relevant combinations of loads by means of recognised methods, which take adequateaccount of the variation of loads in time and space, the motionsof the structure and the design condition which shall be veri-

fied. Characteristic values of the load effects shall be deter-mined.

302 Non-linear and dynamic effects associated with loadsand structural response, shall be accounted for when relevant.

303 The stochastic nature of environmental loads should beadequately accounted for.

304 Description of the different types of analyses are cov-ered in the sections for special considerations for each type of unit and recommended practices.

High frequency(HF)

Rigid body natural periods below dominat-ing wave periods (typically ringing andspringing responses in TLP’s).

Wave frequency(WF)

Area with wave periods in the range4 to 25 s typically. Applicable to all off-shore structures located in the wave activezone.

Low frequency(LF)

This frequency band relates to slowly var-ying responses with natural periods abovedominating wave energy (typically slowlyvarying surge and sway motions for col-umn stabilised units as well as slowly var-ying roll and pitch motions for deepdraught floaters).




– Sec.4

SECTION 4SELECTION OF MATERIAL AND INSPECTION PRINCIPLES

A. General

A 100101 This section describes the selection of steel materialsand inspection principles to be applied in design and construc-tion of offshore steel structures.

B. Design Temperatures

B 100 General

101 The design temperature is a reference temperature usedas a criterion for the selection of steel grades. The design tem-perature shall be based on lowest daily mean temperature.

102 In all cases where the service temperature is reduced by

localised cryogenic storage or other cooling conditions, suchfactors shall be taken into account in establishing the minimumdesign temperatures.

B 200 Floating units

201 The design temperature for floating units shall not ex-ceed the lowest service temperature of the steel as defined forvarious structural parts.

202 External structures above the lowest waterline shall bedesigned with service temperatures equal to the lowest dailymean temperature for the area(s) where the unit is to operate.

203 Further details regarding design temperature for differ-ent structural elements are given in the object standards.

204 External structures below the lowest waterline need notbe designed for service temperatures lower than 0°C. A higherservice temperature may be accepted if adequate supportingdata can be presented relative to the lowest average tempera-ture applicable to the relevant actual water depths.

205 Internal structures in way of permanently heated roomsneed not be designed for service temperatures lower than 0°C.

B 300 Bottom fixed units

301 For fixed units, materials in structures above the lowestastronomical tide (LAT) shall be designed for service temper-atures down to the lowest daily mean temperature.

302 Materials in structures below the lowest astronomicaltide (LAT) need not be designed for service temperatures low-

er than of 0°C. A higher service temperature may be acceptedif adequate supporting data can be presented relative to thelowest daily mean temperature applicable for the relevant wa-ter depths.

C. Structural Category

C 100 General

101 The purpose of the structural categorisation is to assureadequate material and suitable inspection to avoid brittle frac-ture. The purpose of inspection is also to remove defects thatmay grow into fatigue cracks during service life.

Guidance note:

Conditions that may result in brittle fracture are sought avoided.Brittle fracture may occur under a combination of:

- presence of sharp defects such as cracks- high tensile stress in direction normal to planar defect(s)- material with low fracture toughness.

Sharp cracks resulting from fabrication may be found by inspec-tion and repaired. Fatigue cracks may also be discovered during

service life by inspection.High stresses in a component may occur due to welding. A com-plex connection is likely to provide more restraint and larger re-sidual stress than a simple one. This residual stress may be partlyremoved by post weld heat treatment if necessary. Also a com-plex connection shows a more three-dimensional stress state dueto external loading than simple connections. This stress state mayprovide basis for a cleavage fracture.

The fracture toughness is dependent on temperature and materialthickness. These parameters are accounted for separately in se-lection of material. The resulting fracture toughness in the weldand the heat affected zone is also dependent on the fabricationmethod.

Thus, to avoid brittle fracture, first a material with a suitable frac-ture toughness for the actual design temperature and thickness isselected. Then a proper fabrication method is used. In special

cases post weld heat treatment may be performed to reduce crack driving stresses, see also DNV-OS-C401. A suitable amount of inspection is carried out to remove planar defects larger than ac-ceptable. In this standard selection of material with appropriatefracture toughness and avoidance of unacceptable defects areachieved by linking different types of connections to differentstructural categories and inspection categories.


C 200 Selection of structural category

201 Components are classified into structural categories ac-cording to the following criteria:

— significance of component in terms of consequence of fail-

ure— stress condition at the considered detail that together withpossible weld defects or fatigue cracks may provoke brittlefracture.

Guidance note:

The consequence of failure may be quantified in terms of residualstrength of the structure when considering failure of the actualcomponent.


202 Structural category for selection of materials shall be de-termined according to principles given in Table C1.

C 300 Inspection of welds

301 Requirements for type and extent of inspection are givenin DNV-OS-C401 dependent on assigned inspection category

Table C1 Structural categories for selection of materials 1)

Structuralcategory

Principles for determination of structural category

Special Structural parts where failure will have substantialconsequences and are subject to a stress conditionthat may increase the probability of a brittle frac-ture.2)

Primary Structural parts where failure will have substantialconsequences.

Secondary Structural parts where failure will be without signif-icant consequence.

1) Examples of determination of structural categories are given in the var-ious object standards.

2) In complex joints a triaxial or biaxial stress pattern will be present. Thismay give conditions for brittle fracture where tensile stresses are

present in addition to presence of defects and material with low fracturetoughness.




Sec.4 –

for the welds. The requirements are based on the considerationof fatigue damage and assessment of general fabrication qual-ity.

302 The inspection category is by default related to the struc-tural category according to Table C2.

303 The weld connection between two components shall beassigned an inspection category according to the highest of the joined components. For stiffened plates, the weld connectionbetween stiffener and stringer and girder web to the plate maybe inspected according to inspection category III.

304 If the fabrication quality is assessed by testing, or wellknown quality from previous experience, the extent of inspec-tion required for elements within structural category primarymay be reduced, but not less than for inspection category III.

305 Fatigue critical details within structural category prima-ry and secondary shall be inspected according to requirementsin category I.

306 Welds in fatigue critical areas not accessible for inspec-tion and repair during operation shall be inspected according torequirements in category I during construction.

D. Structural Steel

D 100 General

101 Where the subsequent requirements for steel grades aredependent on plate thickness, these are based on the nominal

thickness as built.102 The requirements in this subsection deal with the selec-tion of various structural steel grades in compliance with therequirements given in DNV-OS-B101. Where other, agreedcodes or standards have been utilised in the specification of steels, the application of such steel grades within the structureshall be specially considered.

103 The steel grades selected for structural components shallbe related to calculated stresses and requirements to toughnessproperties. Requirements for toughness properties are in gen-eral based on the Charpy V-notch test and are dependent on de-sign temperature, structural category and thickness of thecomponent in question.

104 The material toughness may also be evaluated by frac-

ture mechanics testing in special cases.

105 In structural cross-joints where high tensile stresses areacting perpendicular to the plane of the plate, the plate materialshall be tested to prove the ability to resist lamellar tearing, Z-quality, see 203.

106 Requirements for forging and castings are given inDNV-OS-B101.

D 200 Material designations

201 Structural steel of various strength groups will be re-ferred to as given in Table D1.

202 Each strength group consists of two parallel series of steel grades:

— steels of normal weldability— steels of improved weldability.

The two series are intended for the same applications. Howev-er, the improved weldability grades have in addition to leanerchemistry and better weldability, extra margins to account forreduced toughness after welding. These grades are also limitedto a specified minimum yield stress of 500 N/mm2.

203 Within each strength group different grades, dependingupon the required impact toughness properties, are defined.The grades are referred to as A, B, D, E, F or AW, BW, DW,EW for improved weldability grades as shown in Table D2.

Additional symbol:

Z = steel grade of proven through-thickness properties.This symbol is omitted for steels of improved welda-bility although improved through-thickness properties

are required.

Table C2 Inspection categories

Inspection category Structural categoryI Special

II Primary

III Secondary

Table D1 Material designations

Designation Strength groupSpecified minimum yield stress

f y (N/mm2)1)

NVNormal strengthsteel (NS)

235

NV-27

High strengthsteel (HS)

265

NV-32 315

NV-36 355

NV-40 390

NV-420

Extra highstrength steel(EHS)

420

NV-460 460

NV-500 500

NV-550 550

NV-620 620NV-690 690

1) For steels of improved weldability the required specified minimumyield stress is reduced for increasing material thickness, see DNV-OS-B101.




– Sec.4

D 300 Selection of structural steel

301 The grade of steel to be used shall in general be relatedto the design temperature and thickness for the applicablestructural category as shown in Table D3.

302 Selection of a better steel grade than minimum requiredin design shall not lead to more stringent requirements in fab-rication.

303 Grade of steel to be used for thickness less than 10 mmand/or design temperature above 0°C will be specially consid-ered in each case.

304 Welded steel plates and sections of thickness exceeding

the upper limits for the actual steel grade as given in Table D3shall be evaluated in each individual case with respect to thefitness for purpose of the weldments. The evaluation should bebased on fracture mechanics testing and analysis, e.g. in ac-cordance with BS 7910.

305 For regions subjected to compressive and/or low tensilestresses, consideration may be given to the use of lower steelgrades than stated in Table D3.

306 The use of steels with specified minimum yield stressgreater than 550 N/mm2 (NV550) shall be subject to specialconsideration for applications where anaerobic environmentalconditions such as stagnant water, organically active mud(bacteria) and hydrogen sulphide may predominate.

307 Predominantly anaerobic conditions can for this purpose

be characterised by a concentration of sulphate reducing bac-teria, SRB, in the order of magnitude >103 SRB/ml (methodaccording to NACE TPC Publication No.3).

308 The steels' susceptibility to hydrogen induced stresscracking (HISC) shall be specially considered when used forcritical applications (such as jack-up legs and spud cans). Seealso Sec.10.

Table D2 Applicable steel grades

Strength group

GradeTest temperature

(ºC) Normalweldability

Improvedweldability

NS A - Not tested

B 1) BW 0

D DW -20

E EW -40

HS A AW 0

D DW -20

E EW -40

F - -60

EHS A - 0

D DW -20

E EW -40

F - -60

1) Charpy V-notch tests are required for thickness above 25 mm but is subject to agreement between the contracting parties for thickness of 25mm or less.

Table D3 Thickness limitations (mm) of structural steels for

different structural categories and design temperatures (ºC)

Structuralcategory

Grade ≥10 0 -10 -20

Secondary

AB/BWD/DWE/EW

AH/AHW

DH/DHWEH/EHW

FHAEH

DEH/DEHWEEH/EEHW

FEH

306015015050

10015015060150150150

306015015050

10015015060150150150

255010015040

8015015050100150150

204080

15030

601501504080

150150

Primary

AB/BWD/DWE/EW

AH/AHWDH/DHWEH/EHW

FHAEH

DEH/DEHWEEH/EEHWFEH

304060150255010015030

60150150

203060150255010015030

60150150

10255010020408015025

50100150

N.A.204080153060

15020

4080150

Special

D/DWE/EW

AH/AHWDH/DHWEH/EHW

FHAEH

DEH/DEHWEEH/EEHW

FEH

3560102550100153060150

3060102550100153060150

2550

N.A.204080102550100

2040

N.A.153060

N.A.204080

N.A. = no application




Sec.5 –

SECTION 5STRUCTURAL STRENGTH

A. General

A 100 General101 This chapter gives provisions for checking of ultimatestrength for typical structural elements used in offshore steelstructures.

102 The ultimate strength capacity (yield and buckling) of structural elements shall be assessed using a rational, justifia-ble, engineering approach.

103 The structural capacity of all structural components shallbe performed. The capacity check shall consider both exces-sive yielding and buckling.

104 Simplified assumptions regarding stress distributionsmay be used provided that the assumptions are made in accord-ance with generally accepted practice, or in accordance with

sufficiently comprehensive experience or tests.105 The corrosion addition as given in Sec.10 B500 shall notbe accounted for in the determination of the resistance.

A 200 Structural analysis

201 The structural analysis may be carried out as linear elas-tic, simplified rigid-plastic, or elastic-plastic analyses. Bothfirst order or second order analyses may be applied. In all cas-es, the structural detailing with respect to strength and ductilityrequirement shall conform to the assumption made for theanalysis.

202 When plastic or elastic-plastic analyses are used forstructures exposed to cyclic loading (e.g. wave loads), checksshall be carried out to verify that the structure will shake down

without excessive plastic deformations or fracture due to re-peated yielding. The cyclic load history needs to be defined insuch a way that the structural reliability in case of cyclic load-ing (e.g. storm loading) is not less than the structural reliabilityfor ultimate strength for non-cyclic loads.

203 In case of linear analysis combined with the resistanceformulations set down in this standard, shakedown can be as-sumed without further checks.

204 If plastic or elastic-plastic structural analyses are usedfor determining the sectional stress resultants, limitations tothe width thickness ratios apply. Relevant width thickness ra-tios are found in the relevant codes used for capacity checks.

205 When plastic analysis and/or plastic capacity checks are

used (cross section Type I and II, according to Appendix A),the members shall be capable of forming plastic hinges withsufficient rotation capacity to enable the required redistribu-tion of bending moments to develop. It shall also be checkedthat the load pattern will not be changed due to the deforma-tions.

206 Cross sections of beams are divided into different typesdependent of their ability to develop plastic hinges. A methodfor determination of cross sectional types is found in AppendixA.

A 300 Ductility

301 It is a fundamental requirement that all failure modes aresufficiently ductile such that the structural behaviour will be in

accordance with the anticipated model used for determinationof the responses. In general all design procedures, regardless of analysis method, will not capture the true structural behaviour.Ductile failure modes will allow the structure to redistributeforces in accordance with the presupposed static model. Brittlefailure modes shall therefore be avoided or shall be verified to

have excess resistance compared to ductile modes, and in thisway protect the structure from brittle failure.

302 The following sources for brittle structural behaviourmay need to be considered for a steel structure:

— unstable fracture caused by a combination of the followingfactors: brittle material, low temperature in the steel, a de-sign resulting in high local stresses and the possibilities forweld defects

— structural details where ultimate resistance is reached withplastic deformations only in limited areas, making the glo-bal behaviour brittle

— shell buckling— buckling where interaction between local and global buck-

ling modes occurs.

A 400 Yield check

401 Structural members for which excessive yielding is apossible mode of failure, shall be investigated for yielding.

402 Local peak stresses from linear elastic analysis in areaswith pronounced geometrical changes, may exceed the yieldstress provided that the adjacent structural parts has capacityfor the redistributed stresses.

403 Yield checks may be performed based on net sectionalproperties. For large volume hull structures gross scantlingsmay be applied.

404 For yield check of welded connections, see Sec.9.

A 500 Buckling check

501 Requirements for the elements of the cross section not

fulfilling requirements for cross section type III need to bechecked for local buckling.

502 Buckling analysis shall be based on the characteristicbuckling resistance for the most unfavourable buckling mode.

503 The characteristic buckling strength shall be based onthe 5th percentile of test results.

504 Initial imperfections and residual stresses in structuralmembers shall be accounted for.

505 It shall be ensured that there is conformity between theinitial imperfections in the buckling resistance formulas andthe tolerances in the applied fabrication standard.

Guidance note:

If buckling resistance is calculated in accordance with Classifi-cation Note 30.1 or DNV-RP-C202, the maximum imperfectionsas given in Classification Note 30.1 shall not be exceeded.


B. Flat Plated Structures and Stiffened Panels

B 100 Yield check

101 Yield check of plating and stiffeners may be performedas given in Sec.6.

102 Yield check of girders may be performed as given inSec.6.

B 200 Buckling check

201 The buckling stability of plated structures may bechecked according to Classification Note 30.1.

202 In case the stiffened panel is buckling checked as a stiff-




– Sec.5

ener with effective plate width, the plates between the stiffen-ers need not to be checked separately.

203 In case an unstiffened flat plate panel is bucklingchecked according to Classification Note 30.1, the maximumpermissible usage factor (η p ) may be 10% higher than basicusage factor η 0 (see C103).

B 300 Capacity checks according to other codes

301 Stiffeners and girders may be designed according to pro-visions for beams in recognised standards such as AISC-ASD.

Guidance note:

The principles and effects of cross section types are included inthe AISC-ASD standard.


C. Shell Structures

C 100 General

101 The buckling stability of shell structures may be

checked according to Classification Note 30.1.

102 For interaction between shell buckling and columnbuckling, Classification Note 30.1 may be used.

103 If Classification Note 30.1 is applied, the maximum per-missible usage factor η p for shells is given by:

D. Tubular Members, Tubular Joints and Coni-

cal TransitionsD 100 General

101 Tubular members without external pressure may bechecked according to Classification Note 30.1. Tubular mem-bers with external pressure and with compact cross sectionsmay be checked according to Classification Note 30.1.

Guidance note:

Compact tubular cross section is in this context defined as whenthe diameter (D) to thickness (t) ratio satisfy the following crite-ria:


102 Tubular members with external pressure, tubular jointsand conical transitions may be checked according to API RP2A.

E. Non-Tubular Beams, Columns and Frames

E 100 General

101 The design of members shall take into account the pos-sible limits on the resistance of the cross section due to localbuckling.

102 Buckling checks may be performed according to Classi-fication Note 30.1.

103 Capacity check may be performed according to recog-nised standards such as AISC-ASD.

β = coefficient depending on type of structure and re-duced slenderness, see Table C1

η 0 = basic usage factor, see Sec.2 Table E1.

Table C1 The coefficient β for shell buckling

Type of structure λ ≤ 0.5 0.5 < λ < 1.0 λ ≥ 1.0

Unstiffened flat platepanels

1.1 1.1 1.1

Girder, beams stiffen-ers on shells

1.0 1.0 1.0

Shells of single curva-ture (cylindricalshells, conical shells)

1.0 1.2 - 0.4 λ 0.8

Note that the slenderness is based on the buckling mode under consideration

λ = reduced slenderness parameter

ηp βη0=

f y

σe-----

f y = specified minimum yield stress

σ e elastic buckling stress for the buckling mode un-der consideration.

where E = modulus of elasticity and f y = minimum yieldstress.

D

t---- 0.5

E

f y----≤




Sec.6 –

SECTION 6SECTION SCANTLINGS

A. General

A 100 Scope101 The requirements in this section are applicable for:

— plate thicknesses and local strength of panels— simple girders.

Procedures for the calculations of complex girder systems areindicated.

B. Strength of Plating and Stiffeners

B 100 Scope

101 The requirements in this section will normally give min-imum scantlings for plate and stiffened panels with respect toyield. Dimensions and further references with respect to buck-ling capacity are given in Sec.5.

B 200 Minimum thickness

201 The thickness t of structures should not to be less than:

B 300 Bending of plating

301 The thickness t of plating subjected to lateral pressureshall not be less than:

B 400 Stiffeners

401 The section modulus Zs for longitudinals, beams, framesand other stiffeners subjected to lateral pressure shall not beless than:

402 For watertight bulkhead and deck or flat structures ex-posed to sea pressure (compartment flooded), see Sec.2 D100loading condition e), 401 applies, taking:

403 The requirement in 401 shall be regarded as the require-ment about an axis parallel to the plating. As an approximationthe requirement to standard section modulus for stiffeners at anoblique angle with the plating may be obtained if the sectionmodulus is multiplied by the factor:

404 Stiffeners with sniped ends may be accepted where dy-namic stresses are small and vibrations are considered to be of

small importance, provided that the plate thickness t supportedby the stiffener is not less than:

In such cases the section modulus of the stiffener calculated asindicated in 401 is normally to be based on the following pa-rameter values:

The stiffeners should normally be snipped to an angle of max-

imum 30°.

tm = 7.0 for primary structural elements

= 5.0 for secondary structural elements

f y = minimum yield stress in N/mm

2

, defined in Sec.4Table D1.

k a = correction factor for aspect ratio of plate field

= (1.1 − 0.25 s/l)2

maximum 1.0 for s/ l = 0.4

minimum 0.72 for s/ l = 1.0k r = correction factor for curvature perpendicular to

the stiffeners.

(1− 0.5 s/R)

R = radius of curvature in m

s = stiffener spacing in m, measured along the plat-ing

p = lateral pressure in kN/m2 as given in Sec.3 D

σ p1 = permissible bending stress

= 1.3 (σ p−σ j), but less than σ p = η 0 f yσ j = equivalent stress for global in-plane membrane

stress

η 0 = basic usage factor, see Sec.2 Table E1

t 15.3tm

f y

--------=

t 15.8k ak rs p

σpl k pp

--------------------103 (mm)=

σ j σx 2

σy 2

2σx– σy 3τ2

++=

f y = minimum yield strength, see Sec.4 Table D1

k pp = fixation parameter for plate

= 1.0 for clamped edges

= 0.5 for simply supported edges.

l = stiffener span in m

k m

= bending moment factor, see Table C1

σ p2 = permissible bending stress dependent on thetype of loading condition, see Sec.2 D100

= 0.6 f y – σ j (N/mm2) for loading condition a)

= 0.8 f y – σ j (N/mm2) for loading condition b)

k ps = fixation parameter for stiffeners

= 1.0 if at least one end is clamped

= 0.9 if both ends are simply supported.

ϕ = angle between the stiffener web plane and theplane perpendicular to the plating.

k m = 8

k ps = 0.9

Zs

l2s p

k mσp2k ps

------------------------ 106 (mm

3), minimum 15000 mm

3=

σp2 f y σ j (N mm2

) ⁄ –=

1

ϕcos------------

t 19l 0.5 s–( ) s p

f y-------------------------------- (mm)=




– Sec.6

Guidance note:

For typical sniped end details as described above, a stress range

lower than 30 MPa can be considered as small dynamic stress.


C. Bending and Shear in Girders

C 100 General

101 The requirements in this section give minimum scant-

lings to simple girders with respect to yield. Further proce-

dures for the calculations of complex girder systems are

indicated.

102 Dimensions and further references with respect to buck-

ling capacity are given in Sec.5.

C 200 Minimum thickness

201 The thickness of web and flange plating shall not be less

than given B201.

C 300 Bending and shear

301 The requirements for section modulus and web area giv-

en in 602 and 603 apply to simple girders supporting stiffeners

or other girders exposed to linearly distributed lateral pressure.

It is assumed that the girder satisfies the basic assumptions of

simple beam theory and that the supported members are ap-

proximately evenly spaced and similarly supported at both

ends. Other loads should be considered in each case based on

the same beam-theory.

302 When boundary conditions for individual girders are not

predictable due to dependence of adjacent structures, direct

calculations according to the procedures given in F will be re-

quired.

303 The section modulus and web area of the girder shall be

taken in accordance with particulars as given in 600 or 700.

Structural modelling in connection with direct stress analysis

shall be based on the same particulars when applicable.

C 400 Effective flange

401 The effective plate flange area is defined as the cross-

sectional area of plating within the effective flange width. The

cross section area of continuous stiffeners within the effective

flange may be included. The effective flange width be is deter-

mined by:

Figure 1Graphs for the effective flange parameter C

C 500 Effective web

501 Holes in girders will generally be accepted provided theshear stress level is acceptable and the buckling capacity andfatigue life is documented to be sufficient.

C 600 Strength requirements for simple girders

601 Simple girders subjected to lateral pressure and whichare not taking part in the overall strength of the unit, are tocomply with the following:

— section modulus according to 602— web area according to 603.

602 Section modulus Zg:

603 Web area AW:

Ce = as given in Fig.1 for various numbers of evenlyspaced point loads (Np) on the span

b = breadth of plate flange in mm

= may be determined as:

= (l1 + l2)/2 (mm)

l1 , l2 = span of supported stiffener on both side of thegirder respectively

l0 = distance between points of zero bending mo-ments

= Sg for simply supported girders

= 0.6 Sg for girders fixed at both ends

Sg = girder span as if simply supported, see 602.

be Ceb mm( )=

Sg = girder span in m. The web height of in-planegirders may be deducted. When bracket(s) arefitted at the end(s), the girder span Sg may be re-duced by two thirds of the bracket arm length(s),provided the girder end(s) may be assumedclamped and provided the section modulus at thebracketed end(s) is satisfactory.

b = breadth of load area in m (plate flange), b may bedetermined as:

= 0.5 (l1 + l2) , l1 and l2 are the spans of the sup-ported stiffeners

k m = bending moment factork m-values in accordance with see Table C1 maybe applied

σ p2 = bending stress

= f y − σ jσ j = equivalent stress for global in-plane membrane

stress.

k τ = shear force factor. k τ -values in accordance withTable C1 may be applied

Ns = number of stiffeners between considered sectionand nearest support. The Ns-value shall in no casebe taken greater than (Np + 1)/4

Zg

Sg 2 b p

k mσp2-------------------- 10

6

mm3

( )=

AW

k τSgbp Ns– Pp

τP

------------------------------------- 103 (mm

2)=




Sec.6 –

604 For watertight bulkhead and deck or flat structures ex-

posed to sea pressure (compartment flooded) loading conditione), 602 and 603 apply, taking:

605 The k m- and k τ -values in 602 and 603 may be calculatedaccording to general beam theory. In Table C1, k m- and k τ -values are given for some defined load and boundary condi-tions. Note that the smallest k m-value shall be applied to sim-ple girders. For girders where brackets are fitted or the flangearea has been partly increased due to large bending moment, alarger k m-value may be used outside the strengthened region.

C 700 Complex girder systems

701 For girders that are parts of a complex 2- or 3-dimen-sional structural system, a complete structural analysis mayhave to be carried out to demonstrate that the stresses are ac-ceptable.

702 Calculation methods or computer programs applied areto take into account the effects of bending, shear, axial and tor-

sional deformations.703 The calculations are to reflect the structural response of the 2- or 3-dimensional structure considered, with due atten-tion to boundary conditions.

704 For systems consisting of slender girders, calculationsbased on beam theory (frame work analysis) may be applied,with due attention to:

— shear area variation— moment of inertia variation— effective flange— lateral buckling of girder flanges.

705 The most unfavourable of the loading conditions givenin Sec.2 D100 shall be applied.

706 For girders taking part in the overall strength of the unit,stresses due to the design pressures given in Sec.3 shall becombined with relevant overall stresses.

Np = number of supported stiffeners on the girder span

Pp = average “point load” (kN) from stiffeners be-tween considered section and nearest support

τ p = 0.3 f y (N/mm2) for loading condition a)

= 0.4 f y (N/mm2) for loading condition b).

σ p2 = 0.91 f y (N/mm2) in 602

τ p = 0.5 f y (N/mm2) in 603.

Table C1 Values of km and kt

Load and boundary conditions Bending moment and shear force factors

Positions 1k m1k τ 1

2k m2

-

3k m3k τ 3

1Support

2Field

3Support

120.5

24 120.5

-0.38

14.2 80.63

-0.5

8 -0.5

150.3

23.3 100.7

-0.2

16.8 7.50.8

-0.33

7.8 -0.67




– Sec.7

SECTION 7FATIGUE

A. General

A 100 General

101 In this standard, requirements are given in relation to fa-tigue analyses based on fatigue tests and fracture mechanics.See DNV-RP-C203 for practical details with respect to fatiguedesign.

102 The aim of fatigue design is to ensure that the structurehas an adequate fatigue life. Calculated fatigue lives can alsoform the basis for efficient inspection programmes during fab-rication and the operational life of the structure.

103 The resistance against fatigue is normally given as S-Ncurves, i.e. stress range (S) versus number of cycles to failure(N) based on fatigue tests. Fatigue failure is normally definedas when the crack has grown through the thickness.

104 The S-N curves shall in general be based on a 97.6%probability of survival.

105 The design fatigue life for the structure componentsshould be based on the structure service life specified. If a serv-ice life is not specified, 20 years should be used.

106 To ensure that the structure will fulfil the intended func-tion, a fatigue assessment shall be carried out for each individ-ual member, which is subjected to fatigue loading. Whereappropriate, the fatigue assessment shall be supported by a de-tailed fatigue analysis. It shall be noted that any element ormember of the structure, every welded joint and attachment orother form of stress concentration is potentially a source of fa-tigue cracking and should be individually considered.

A 200 Design fatigue factors

201 Design fatigue factors (DFF) shall be applied to increasethe probability for avoiding fatigue failures.

202 The DFFs are dependent on the significance of the struc-tural components with respect to structural integrity and avail-ability for inspection and repair.

203 DFFs shall be applied to the design fatigue life. The cal-culated fatigue life shall be longer than the design fatigue lifetimes the DFF.

204 The design requirement can alternatively be expressedas the cumulative damage ratio for the number of load cyclesof the defined design fatigue life multiplied with the DFF shall

be less or equal to 1.0.

205 The design fatigue factors in Table A1 are valid for unitswith low consequence of failure and where it can be demon-strated that the structure satisfies the requirement to damagedcondition according to the accidental design condition withfailure in the actual joint as the defined damage.

Guidance note:

For units inspected during operation according to DNV require-

ments, the DFF for outer shell should be taken as 1. For units in-spected afloat at a sheltered location, the DFF for areas above 1m above lowest inspection waterline should be taken as 1, andbelow this line the DFF is 2 for the outer shell. Splash zone is de-fined as non-accessible area.

Where the likely crack propagation develops from a locationwhich is accessible for inspection and repair to a structural ele-ment having no access, such location is itself to be deemed tohave the same categorisation as the most demanding categorywhen considering the most likely crack path. For example, a welddetail on the inside (dry space) of a submerged shell plate shallbe allocated the same DFF as that relevant for a similar weld lo-cated externally on the plate.


206 The design fatigue factors shall be based on special con-siderations where fatigue failure will entail substantial conse-quences such as:

— danger of loss of human life, i.e. not compliance with theaccidental criteria

— significant pollution— major economical consequences.

Guidance note:

Evaluation of likely crack propagation paths (including directionand growth rate related to the inspection interval), may indicatethe use of a different DFF than that which would be selectedwhen the detail is considered in isolation. For example where thelikely crack propagation indicates that a fatigue failure starting ina non critical area grows such that there might be a substantial

consequence of failure, such fatigue sensitive location is itself tobe deemed to have a substantial consequence of failure.


207 Welds in joints below 150 m water depth should be as-sumed inaccessible for in-service inspection.

208 Sec.11 to Sec.14 define the design fatigue factor to beapplied for typical structural details.

A 300 Methods for fatigue analysis

301 The fatigue analysis should be based on S-N data, deter-mined by fatigue testing of the considered welded detail, andthe linear damage hypothesis. When appropriate, the fatigueanalysis may alternatively be based on fracture mechanics.

302 In fatigue critical areas where the fatigue life estimatebased on simplified methods is short, a more accurate investi-gation or a fracture mechanics analysis shall be performed.

303 For calculations based on fracture mechanics, it shouldbe documented that the in-service inspections accommodate asufficient time interval between time of crack detection and thetime of unstable fracture. See DNV-RP-C203 for more details.

304 All significant stress ranges, which contribute to fatiguedamage in the structure, should be considered. The long termdistribution of stress ranges may be found by deterministic orspectral analysis. Dynamic effects shall be duly accounted forwhen establishing the stress history.

A 400 Simplified fatigue analysis

401 Simplified fatigue analysis may be undertaken in orderto establish the general acceptability of fatigue resistance, or asa screening process to identify the most critical details to beconsidered in a stochastic fatigue analysis, see 500.

402 Simplified fatigue analyses should be undertaken utilis-

Table A1 Design fatigue factors (DFF)

DFF Structural element

1 Internal structure, accessible and not welded directlyto the submerged part

1 External structure, accessible for regular inspectionand repair in dry and clean conditions

2 Internal structure, accessible and welded directly tothe submerged part

2 External structure not accessible for inspection and re-pair in dry and clean conditions

3 Non-accessible areas, areas not planned to be accessi-ble for inspection and repair during operation




Sec.7 –

ing appropriate conservative design parameters. A two-param-eter, Weibull distribution (see DNV-RP-C203 2.14.) may beutilised to describe the long-term stress range distribution:

403 When the simplified fatigue evaluation is based on dy-namic stress from the global analysis, the stresses should bescaled to the return period of the minimum fatigue life of the

unit. In such cases, scaling may be undertaken utilising the ap-

propriate factor found from the following:

A 500 Stochastic fatigue analysis

501 Stochastic fatigue analyses shall be based upon recog-nised procedures and principles utilising relevant site specificdata or North Atlantic environmental data.

502 Simplified fatigue analyses should be used as a “screen-ing” process to identify locations for which a detailed, stochas-tic fatigue analysis should be undertaken.

503 Fatigue analyses shall include consideration of the direc-tional probability of the environmental data. Providing that itcan be satisfactorily checked, scatter diagram data may be con-

sidered as being directionally specific. Scatter diagram forworld wide operations (North Atlantic scatter diagram) is giv-en in Classification Note 30.5. Relevant wave spectra and en-ergy spreading shall be utilised as relevant.

504 Structural response shall be determined based uponanalyses of an adequate number of wave directions. Transferfunctions should be established based upon consideration of asufficient number of periods, such that the number, and valuesof the periods analysed:

— adequately cover the wave data— satisfactorily describe transfer functions at, and around,

the wave “cancellation” and “amplifying” periods (Con-sideration should be given to take account that such “can-cellation” and “amplifying” periods may be different fordifferent elements within the structure)

— satisfactorily describe transfer functions at, and around,the relevant excitation periods of the structure.

505 Stochastic fatigue analyses utilising simplified structur-al model representations of the unit (e.g. a space frame model)may form basis for identifying locations for which a stochasticfatigue analysis, utilising a detailed model of the structure,should be undertaken (e.g. at critical intersections).

n0 = total number of stress variations during thelifetime of the structure

= extreme stress range that is exceeded onceout of n0 stress variations. The extreme stressamplitude:

is thus given by

γ c = contingency factor

= 1.1, if not otherwise stated in the sections 11-14 for each object

h = the shape parameter of the Weibull stressrange distribution

= the intercept of the design S-N curve with thelog N axis (see DNV-RP-C203 2.3)

= is the complete gamma function (see DNV-RP-C203 2.14)

m = the inverse slope of the S-N curve (see DNV-RP-C203 2.14)

DFF = design fatigue factor.

∆σn0

1

γ c----

n0( )ln( )

1

h---

DFF( )

1

m----

-------------------------a

n0Γ 1

m

h----+

-----------------------------

1

m----

=

∆σn0

∆σampl_n0

∆σn0

/ 2

a

Γ 1m

h----+

ni = the number of stress variations in i years appro-

priate to the global analysis= the extreme stress range that is exceeded once

out of ni stress variations.

∆σn0∆σni

n0log

n ilog---------------

1

h---

=

∆σni




– Sec.8

SECTION 8ACCIDENTAL CONDITIONS

A. General

A 100 General101 In principle accidental condition shall be assessed for allunits. Safety assessment is carried out according to the princi-ples given in DNV-OS-A101.

102 Structures shall be checked in accidental condition intwo steps:

a) Resistance of the structure against design accidental loads.Structural capacity can be calculated according to themethods given in Sec.5.

b) Post accident resistance of the structure against environ-mental loads. Should only be checked when the resistanceis reduced by structural damage caused by the design acci-dental loads.

103 The overall objective of design against accidental loadsis to achieve a system where the main safety functions are notimpaired by the design accidental loads.

104 The design against accidental loads may be done by di-rect calculation of the effects imposed by the loads on thestructure, or indirectly, by design of the structure as tolerableto accidents. Examples of the latter are compartmentation of

floating units which provides sufficient integrity to survivecertain collision scenarios without further calculations.

105 The inherent uncertainty of the frequency and magni-tude of the accidental loads, as well as the approximate natureof the methods for determination of accidental load effects,shall be recognised. It is therefore essential to apply sound en-gineering judgement and pragmatic evaluations in the design.

106 If non-linear, dynamic finite element analysis is appliedfor design, it shall be verified that all local failure modes (e.g.strain rate, local buckling, joint overloading, and joint fracture)are accounted for implicitly by the modelling adopted, or elsesubjected to explicit evaluation.

Typical accidental loads are:

— impact from ship collisions

— impact from dropped objects— fires— explosions

— abnormal environmental conditions— accidental flooding.

The different types of accidental loads require different meth-ods and analyses to assess the structural resistance.




Sec.9 –

SECTION 9WELD CONNECTIONS

A. General

A 100 Scope101 The requirements in this section are related to types andsize of welds.

B. Types of Welded Steel Joints

B 100 Butt joints

101 All types of butt joints should be welded from bothsides. Before welding is carried out from the second side, un-sound weld metal shall be removed at the root by a suitablemethod.

B 200 Tee or cross joints

201 The connection of a plate abutting on another plate maybe made as indicated in Fig.1.

202 The throat thickness of the weld is always to be meas-ured as the normal to the weld surface, as indicated in Fig.1 d.

203 The type of connection is normally to be adopted as fol-lows:

a) Full penetration weld

Important cross connections in structures exposed to highstress, especially dynamic, e.g. for special areas and fa-tigue utilised primary structure. All external welds in wayof opening to open sea, e.g. pipes, sea chests or tee-jointsas applicable.

b) Partly penetration weld Connections where the static stress level is high. Accepta-ble also for dynamically stressed connections, providedthe equivalent stress is acceptable, see C300.

c) Fillet weld

Connections where:

— stresses in the weld are mainly shear— direct stresses are moderate and mainly static— dynamic stresses in the abutting plate are small.

Figure 1Tee or cross joints

204 Double continuous welds are required in the followingconnections, irrespective of the stress level:

— oiltight and watertight connections

— connections at supports and ends of girders, stiffeners and

pillars— connections in foundations and supporting structures for

machinery

— connections in rudders, except where access difficultiesnecessitate slot welds.

205 Intermittent fillet welds may be used in the connectionof girder and stiffener webs to plate and girder flange plate, re-spectively, where the connection is moderately stressed. Withreference to Fig.2, the various types of intermittent welds areas follows:

— chain weld

— staggered weld

— scallop weld (closed).206 Where intermittent welds are accepted, scallop weldsshall be used in tanks for water ballast or fresh water. Chainand staggered welds may be used in dry spaces and tanks ar-ranged for fuel oil only.




– Sec.9

Figure 2Intermittent welds

B 300 Slot welds

301 Slot weld, see Fig.3, may be used for connection of plat-ing to internal webs, where access for welding is not practica-ble, e.g. rudders. The length of slots and distance between slotswill be considered in view of the required size of welding.

Figure 3Slot welds

B 400 Lap joint

401 Lap joint as indicated in Fig.4 may be used in end con-nections of stiffeners. Lap joints should be avoided in connec-tions with dynamic stresses.

Figure 4

Lap joint

C. Weld Size

C 100 General

101 The sizes of weld connections shall be as given in 200 to

500.

If the yield stress of the weld deposit is higher than that of the

base metal, the size of ordinary fillet weld connections may be

reduced as indicated in 102.

The yield stress of the weld deposit shall in no case be less than

given in DNV-OS-C401.

102 Welding consumables used for welding of normal steeland some high strength steels are assumed to give weld depos-

its with yield stress σ fw as indicated in Table C1. If welding

consumables with deposits of lower yield stress than specified

in Table C1 are used, the applied yield strength shall be clearly

informed on drawings and in design reports.

103 The size of some weld connections may be reduced:

a) Corresponding to the strength of the weld metal, f w:

b) Corresponding to the strength ratio value f r, base metal toweld metal:

Ordinary values for f w and f r for normal strength and high

strength steels are given in Table C1.

104 When deep penetrating welding processes are applied,

the required throat thicknesses may be reduced by 15% provid-ed sufficient weld penetration is demonstrated.

C 200 Ordinary fillet welds

201 Where the connection of girder and stiffener webs and

plate panel or girder flange plate, respectively, are mainly

shear stressed, fillet welds as specified in the following will

normally be adopted.

202 Unless otherwise calculated, the throat thickness of dou-

f y = yield stress of base material, abutting plate (N/ mm2)

σ fw = yield stress of weld deposit (N/mm2)

f w

σfw

235---------

0.75

or=

f r

f y

σfw

---------

0.75

minimum 0.75=

Table C1 Strength ratios, f w and f r.

Base metal Weld deposit Strength ratios

Strength group Designation Yield stressσ fw

(N/mm2)

Weld metal Base metal/weld metal

Normal strength steels NV NS 355 1.34 0.75

High strength steels NV 27NV 32NV 36

NV 40

375375375

390

1.40

1.44

0.750.880.96

1.00

f w

σfw

240---------

0.75

= f r

f y

σfw

---------

0.75

=




Sec.9 –

ble continuous fillet welds tW should not be less than:

203 The throat thickness of intermittent welds may be as re-quired in 202 for double continuous welds provided the weldedlength is not less than:

— 50% of total length for connections in tanks— 35% of total length for connections elsewhere.

Double continuous welds shall be adopted at stiffener endswhen necessary due to bracketed end connections.

204 For intermittent welds, the throat thickness tW is not toexceed:

Chain welds and scallop welds

Staggered welds

If the calculated throat thickness exceeds that given above, theconsidered weld length shall be increased correspondingly.

C 300 Partly penetration welds and fillet welds in crossconnections subject to high stresses

301 In structural parts where dynamic stresses or high statictensile stresses act through an intermediate plate, see Fig.1,

penetration welds or increased fillet welds shall be used.302 When the abutting plate carries dynamic stresses, theconnection shall fulfil the requirements with respect to fatigue,see Sec.7.

303 When the abutting plate carries tensile stresses higherthan 100 N/mm2, the throat thickness tW of a double continu-ous weld shall not be less than:

C 400 Connections of stiffeners to girders and bulk-heads, etc.

401 Stiffeners may be connected to the web plate of girdersin the following ways:

— welded directly to the web plate on one or both sides of thestiffener

— connected by single- or double-sided lugs— with stiffener or bracket welded on top of frame

— a combination of the connections listed above.In locations with great shear stresses in the web plate, a double-sided connection or stiffening of the unconnected web plateedge is normally required. A double-sided connection may betaken into account when calculating the effective web area.

402 Various standard types of connections are shown inFig.5.

Other types of connection will be considered in each case.

Figure 5Connections of stiffeners to girders

403 Connection lugs shall normally have a thickness not lessthan 75% of the web plate thickness.

404 The total connection area a0 (parent material) at sup-ports of stiffeners is normally not to be less than:

t0 = net thickness (mm) of abutting plate. Within60% of the middle of span for stiffeners and forgirders, t0 need normally not be taken greater

than 11 mm, however, shall in no case be lessthan 0.5 times the net thickness of the web.

σ = calculated maximum tensile stress in abutting plate(N/mm2)

r = root face (mm), see Fig.1 d

t0 = net thickness (mm) of abutting plate.

tw 0.43 f rt0 mm( ), minimum 3 mm=

tw 0.6 f rt0 mm( )=

tw 0.75 f rt0 mm( )=

tw1.36

f w---------- 0.2

σ

270--------- 0.25–

r

t0

----+ t0 mm( )

minimum 3 mm

=

c = detail shape factor as given in Table C1

σ p = permissible stress (N/mm2)

= η 0 f yη 0 = allowable usage factor, see Sec.2

f y = minimum yield strength, see Sec.4

l = span of stiffener (m)

s = spacing between stiffeners (m)

p = lateral pressure (kN/m2).

Table C2 Detail shape factor c

Type of connec-tion (see Fig.5)

I

Web to web con-nection only

II Stiffener or bracket on top of

stiffener

Single-sided Double-sided

abc

1.000.900.80

1.251.151.00

1.000.900.80

a0 3c

σp

------103

l 0.5s–( ) s p mm2

( )=




– Sec.9

405 Weld area a shall not be less than:

406 The weld connection between stiffener end and bracket

is principally to be designed such that the shear stresses of the

connection corresponds to the permissible stress.

407 The weld area of brackets to stiffeners which are carry-

ing longitudinal stresses or which are taking part in the

strength of heavy girders etc., shall not be less than the section-

al area of the longitudinal.

408 Brackets shall be connected to bulkhead by a double

continuous weld, for heavily stressed connections by a partly

or full penetration weld.

C 500 End connections of girders

501 The weld connection area of bracket to adjoining girdersor other structural parts shall be based on the calculated normal

and shear stresses. Double continuous welding shall be used.

Where large tensile stresses are expected, welding according to

300 shall be applied.

502 The end connections of simple girders shall satisfy the

requirements for section modulus given for the girder in ques-

tion.

Where the design shear stresses in web plate exceed 75 N/

mm2, double continuous boundary fillet welds shall have

throat thickness tW not less than:

C 600 Direct calculation of weld connections

601 The distribution of forces in a welded connection may be

calculated on the assumption of either elastic or plastic behav-

iour.

602 Residual stresses and stresses not participating in the

transfer of load need not be included when checking the resist-

ance of a weld. This applies specifically to the normal stress

parallel to the axis of a weld.

603 Welded connections shall be designed to have adequate

deformation capacity.

604 In joints where plastic hinges may form, the welds shall

be designed to provide at least the same resistance as the weak-

est of the connected parts.

605 In other joints where deformation capacity for joint rota-

tion is required due to the possibility of excessive straining, the

welds require sufficient strength not to rupture before general

yielding in the adjacent parent material.

Guidance note:

In general this will be satisfied if the design resistance of the weld

is not less than 80% of the design resistance of the weakest of the

connected parts.


606 The resistance of fillet welds is adequate if, at everypoint in its length, the resultant of all the forces per unit lengthtransmitted by the weld does not exceed its resistance.

607 The resistance of the fillet weld will be sufficient if boththe following conditions are satisfied:

Figure 6Explanation of stresses on the throat section of a fillet weld

a0 = connection area (mm2) as given in 404.

τ = calculated shear stress (N/mm2)

t0 = net thickness (mm) of web plate

a f ra0 mm2

( )=

tw

τ

174f w---------------f

rt0 mm( )=

σ ⊥ = normal stress perpendicular to the throat

τ ⊥ = shear stress (in plane of the throat) perpendicu-lar to the axis of the weld

τ || = shear stress (in plane of the throat) parallel to theaxis of the weld, see Table C3

f u = nominal lowest ultimate tensile strength of theweaker part joined

β w = appropriate correlation factor, see Table C3

η 0 = basic usage factor. ref Sec. 2 E

Table C3 The correlation factor β w

Steel grade Lowest ultimate tensile

strength f u

Correlation factor β w

NV NS 400 0.83

NV 27 400 0.83

NV 32 440 0.86

NV 36 490 0.89

NV 40 510 0.9

NV 420 530 1.0

NV 460 570 1.0

σ⊥

23 τ

||2

τ⊥

2+

+

f u

βw

-------η0

and σ⊥

f uη0≤

≤




Sec.10 –

SECTION 10CORROSION PROTECTION

A. General

A 100 Scope101 In this section the requirements regarding corrosion pro-tection arrangement and equipment are given.

B. Acceptable Corrosion Protection

B 100 Atmospheric zone

101 Steel surfaces in the atmospheric zone shall be protectedby coating.

B 200 Splash zone

201 Steel surfaces in the splash zone shall be protected by

coating.202 The splash zone is that part of an installation, which isintermittently exposed to air and immersed in the sea. The zonehas special requirements to fatigue for bottom fixed units andfloating units that have constant draught.

Guidance note:

Constant draught means that the unit is not designed for changingthe draught for inspection and repair for the splash zone and othersubmerged areas.


203 For floating units with constant draught, the extent of thesplash zone shall extend 5 m above and 4 m below this draught.

204 For bottom fixed structures, such as jackets and TLPs,the definitions given in 205 to 207 apply.

205 The wave height to be used to determine the upper andlower limits of the splash zone shall be taken as 1/3 of the waveheight that has an annual probability of being exceeded of 10-2.

206 The upper limit of the splash zone (SZU) shall be calcu-lated by:

where:

The variables (Ui) shall be applied, as relevant, to the structurein question, with a sign leading to the largest or larger value of SZU.

207 The lower limit of the splash zone (SZL) shall be calcu-lated by:

where:

The variables (Li) shall be applied, as relevant, to the structurein question, with a sign leading to the smallest or smaller value

of SZL.

B 300 Submerged zone

301 Steel surfaces in the submerged zone, including splashzone areas below normal operating draught, shall be cathodi-cally protected, preferably in combination with coating. Forcoated submerged steel the cathodic protection current densitycan be reduced.

B 400 Internal zone

401 Tanks which are exposed to sea water or other corrosiveliquids, typically water ballast tanks, shall be protected bycoating. Sacrificial anodes shall be used in combination withcoating where relevant, for example water ballast tanks that

will stay empty less than approximately 50% of the time.

402 Tanks which are empty or only partly filled with sea wa-ter shall be protected either by coating, corrosion addition or acombination of these methods. De-humidifying equipment canbe used for corrosion prevention in spaces designed to be dry.

403 Fresh water tanks shall be coated. Health authorities re-quirements for certification of the coating with respect to tox-icity, taste and smell shall be complied with.

404 Areas with high fatigue utilisation shall be protected bycathodic protection or coating unless the effect of unprotectedsteel has been accounted for in the fatigue evaluation. Regard-ing the use of aluminium coating, see D102. To facilitate in-service inspections in ballast tanks and in areas where crack

detection is important, light coloured, hard coatings for exam-ple on epoxy or similar basis shall be used.

405 Magnesium anodes and impressed current systems shallnot be used in tanks.

406 Corrosion protection of closed spaces impossible to in-spect after final welding is subject to special consideration.

407 Internal surfaces of structural members that may not staydry or will not be sealed off from the atmosphere, shall be pro-tected by coating. For internal surfaces of compartments thatwill remain sealed off and dry for the design life of the struc-ture, coating is not required.

408 In ballast tanks which may become gas hazardous areas

due to being located adjacent to for example fuel tanks or oilstorage tanks for liquids with flash point less than 60°C, alu-minium anodes shall be so located that the kinetic energy de-veloped in event of their loosening and falling down is lessthan 275 J. Fillet welds for attachment of anodes shall be con-tinuous and of adequate cross section. Attachment by clampsfixed by set-screws shall not be applied in potentially gas haz-ardous areas. Attachments by properly secured through-boltsmay, however, be applied.

409 Tanks in which anodes are installed shall have sufficientholes for the circulation of air to prevent gas from accumulat-ing in pockets.

B 500 Corrosion additions

501 Unprotected steel (plates, stiffeners and girders) in tanksshall be given a corrosion addition tk as follows:

— one side unprotected: tk = 1.0 mm

— two sides unprotected: tk = 2.0 mm.

U1 = 60 % of the wave height defined in 205

U2 = highest astronomical tide level (HAT)

U3 = foundation settlement, if applicable

U4 = range of operation draught, if applicable

U5 = motion of the structure, if applicable.

L1 = 40% of the wave height defined in 205

L2 = lowest astronomical tide level (LAT)

L3 = range of operating draught, if applicable

L4 = motions of the structure, if applicable.

SZU U1 U2 U3 U4 U5+ + + +=

SZL L1 L2 L3 L4 L5+ + + +=




– Sec.10

Guidance note:

Corrosion addition should be used in the splash zone if an ordi-nary paint coating only is applied to the structure. If a thermallysprayed aluminium coating, glass flake reinforced polyester orepoxy coating, or similar thick film coating is used, corrosion ad-dition should not be needed.


502 When a corrosion addition has been added to the scant-lings, this shall be clearly noted in design documentation andstructural drawings.

C. Cathodic Protection

C 100 General

101 The cathodic protection system shall deliver sufficientprotective current to maintain the potential at all steel surfacesof the structure in the submerged zone between − 0.80 V and −1.10 V versus the Ag/AgCl/sea water reference electrodethroughout the design life of the installation.

102 These potentials apply to normal sea water (salinity 32to 38 g/l) and saline mud.

103 If potential measurements are carried out in brackishwater, either a Ag/AgCl reference electrode with closed elec-trolyte compartment or permeable membrane (not dependenton chloride concentration) or of other type (Cu/CuSO4, Zn orsaturated calomel electrode SCE) shall be used.

Guidance note:

The following relationships should be valid between potentials(volts V) measured with Ag/AgCl and other reference cells:


104 The current density needed to achieve the above poten-tials shall be selected on the basis of the worst environmentalconditions that the unit can be expected to be exposed to withrespect to corrosivity.

105 If the unit is operating in an environment with worseconditions than originally intended, additional protection maybe required.

106 The reduction in current density for coated surfaces

compared with bare steel, will be dependent on the quality of the coating system.

107 Cathodic protection systems for steels with specifiedminimum yield stress > 550 N/mm2 are subject to special con-sideration for applications where hydrogen induced stresscracking (HISC) may be anticipated. See 108 and 109 andSec.4 D300.

108 Qualification testing shall be carried out for critical ap-plications such as legs and spud cans. Test conditions: Anaer-obic, hydrogen sulphide containing environment and cathodicprotection potentials of − 1.1 V Ag/AgCl, or more negative po-tentials. The test procedure, slow strain rate method or similar,should be agreed.

109If not documented by testing that cathodic protection to− 1.1 V Ag/AgCl is harmless (for steel with specified mini-

mum yield stress higher than 550 N/mm2), the cathodic protec-tion potential shall be limited by using special anodes of controlled voltage type (with diodes or similar), or other meth-od.

C 200 Protection by sacrificial anodes

201 A cathodic protection system by sacrificial anodes shallbe designed to maintain the required potential during the peri-od between complete re-installation of the anodes. This periodshall not be less than 5 years.

202 The anodes shall be located so as to give a uniform cur-rent distribution to the steel structure.

Guidance note:Installation of a permanent cathodic protection monitoring sys-tem based on potential readings from fixed reference electrodesmay be advantageous.


203 The anode core shall be designed to support the anodeand to maintain the anode shape during the later stages of theanode life.

C 300 Protection by impressed current

301 The impressed current anodes shall be located andshielded in such way as to give a protective current distributionbeing as uniform as possible.

302 Due to the risks of detrimental overprotection, im-pressed current anodes shall not be located close to areas withhigh stresses.

303 For complicated structures such as frameworks of pipes,the impressed current system should be designed to provide1.25 to 1.5 times the calculated current demand, in order tocompensate for inefficient current distribution.

304 Installation of a permanent control and monitoring sys-tem is required in order to provide adequate cathodic protec-tion and avoid over-protection.

305 The impressed current system should be arranged so thatthe risk of damages to anodes, cables and reference electrodesis minimised.

C 400 Cathodic protection monitoring system

401 A monitoring system for impressed current cathodicprotection systems shall be provided.

402 The monitoring system shall be based on potential read-ings from fixed reference electrodes and shall be suitable formeasuring and recording the level of protection of representa-tive parts of the submerged structure. Locations of referenceelectrodes should be selected with special attention to areaswhere under- or overprotection may be expected.

403 It shall be measured that the monitoring system is func-tioning satisfactorily. If satisfactory functioning can not beproved, potential measurements by divers or submersibles maybe required.

404 For steels with specified minimum yield stress higherthan 550 N/mm2 in anaerobic environment, cathodic protec-tion potentials shall be monitored to ensure compliance withthe target range indicated in the Guidance note below. In casethe target range is exceeded, inspection with respect to possi-ble HISC shall be carried out.

Guidance note:

The target potential range for steels susceptible to HISC in anaer-obic, sulphide containing environment is − 770 to − 30 mV ver-sus the Ag/AgCl reference electrode.


C 500 Testing of effectiveness of corrosion protectionsystem

501 After the cathodic protection system is put into opera-tion, an initial survey shall be performed to establish that allsubmerged areas are adequately protected. During this surveythe structure shall be in normal operating condition. This initialsurvey shall be carried out within:

Ag/AgCl/sea water Zn Cu/CuSO4 SCE

− 0.80 + 0.25 − 0.85 − 0.79

− 0.90 + 0.15 − 0.95 − 0.89− 1.10 − 0.05 − 1.15 − 1.09




Sec.10 –

— 6 months after delivery for sacrificial anode systems— 3 months after delivery for impressed current systems.

Guidance note:

Lowering of reference electrode in a line is usually sufficient. Po-tential readings utilising divers or submersible may be requiredin special cases.


D. Coating

D 100 Specification

101 A coating specification shall include description of:

— steel surface treatment for coating application, includingshop-primer

— control of temperatures and climatic conditions duringblast cleaning and coating application

— coating systems, including coating types, number of coatsand film thicknesses

— coating allocation schedule (which coatings where) quali-ty control or inspection requirements.

102 The use of aluminium coating is generally not recom-mended in tanks for liquids with flash point below 60°C, in adjacent ballast tanks, in cofferdams, in pump rooms or on decksabove the mentioned spaces nor in any other area where gasmay accumulate. Organic coatings, for example on epoxy ba-sis, containing up to 10% aluminium by weight in the dry filmare, however, acceptable in the mentioned areas.

D 200 Coating application

201 Regarding coating application, see DNV-OS-C401.




– Sec.11

SECTION 11SPECIAL CONSIDERATIONS FOR COLUMN STABILISED UNITS

A. General

A 100 Scope101 The requirements and guidance documented in this Sec-tion are generally applicable to all configurations of columnstabilised units, including those with:

— ring (continuous) pontoons— twin pontoons.

102 A column stabilised unit is a floating unit that can be re-located. A column stabilised unit normally consists of a deck structure with a number of widely spaced, large diameter, sup-porting columns that are attached to submerged pontoons.

103 Column stabilised units may be kept on station by eithera passive mooring system (e.g. anchor lines), or an active

mooring system (e.g. thrusters), or a combination of thesemethods.

104 A column stabilised unit may be designed to function ina number of modes, e.g. transit, operational and survival. Lim-iting design criteria for modes of operation shall be establishedand documented. Such limiting design criteria shall includerelevant consideration of the following items:

— intact condition - structural strength— damaged condition - structural strength— air gap— compartmentation and stability.

B. Structural Categorisation

B 100 General

101 The structural application categories are determinedbased on the structural significance, consequences of failureand the complexity of the joints and shall be selected accordingto the principles as given in Sec.4.

102 The steel grades selected for structural components areto be related to weldability and requirements for toughnessproperties and are to be in compliance with the requirementsgiven in the DNV-OS-B101.

B 200 Structural categorisation

201 Application categories for structural components are de-fined in Sec.4. Structural members of column stabilised unitsare normally found in the following groups:

Special category

a) Portions of deck plating, heavy flanges, and bulkheadswithin the upper hull or platform which form «Box» or «I»type supporting structure which receive major concentrat-ed loads.

b) External shell structure in way of high stressed intersec-tions of vertical columns, decks and lower hulls.

c) Major intersections of bracing members.

d) «Through» material used at connections of vertical col-umns, upper platform decks and upper or lower hulls

which are designed to provide proper alignment and ade-quate load transfer.

e) External brackets, portions of bulkheads, and frameswhich are designed to receive concentrated loads at inter-sections of major structural members.

f) Highly stressed elements of anchor line fairleads, cranepedestals etc. and their supporting structure.

Fig.1 to Fig.4 show examples of structural application catego-ry.

Primary category

a) Deck plating, heavy flanges, and bulkheads within the up-per hull or platform, which form «Box» or «I». type sup-porting structure which do not receive major concentratedloads.

b) External shell structure of vertical columns, lower and up-per hulls, and diagonal and horizontal braces.

c) Bulkheads, decks, stiffeners and girders, which provide lo-cal reinforcement or continuity of structure in way of in-tersections, except areas where the structure is consideredfor special application.

d) Main support structure of heavy substructures and equip-ment, e.g. anchor line fairleads, cranes, drill floor sub-structure, life boat platform, thruster foundation andhelicopter deck.

Secondary category

a) Upper platform decks, or decks of upper hulls except areaswhere the structure is considered primary or special appli-cation.

b) Bulkheads, stiffeners, flats or decks and girders in verticalcolumns, decks, lower hulls, diagonal and horizontal brac-

ing, which are not considered as primary or special appli-cation.

c) Deckhouses.

d) Other structures not categorised as special or primary.

C. Material Selection

C 100 General

101 Material specifications shall be established for all struc-tural materials. Such materials shall be suitable for their in-tended purpose and have adequate properties in all relevantdesign conditions. Material selection shall be undertaken in ac-

cordance with the principles given in Sec.4.102 When considering criteria appropriate to material gradeselection, adequate consideration shall be given to all relevantphases in the life cycle of the unit. In this connection there maybe conditions and criteria, other than those from the in-service,operational phase, that provide the design requirements in re-spect to the selection of material. (Such criteria may, for exam-ple, be design temperature and/or stress levels during marineoperations.)

103 In structural cross-joints essential for the overall struc-tural integrity where high tensile stresses are acting perpendic-ular to the plane of the plate, the plate material shall be testedto prove the ability to resist lamellar tearing. (Z-quality).

104 Material designations are defined in Sec.4.

C 200 Design temperatures

201 Design temperature is a reference temperature used as acriterion for the selection of steel grades. The design tempera-ture shall be based on Lowest mean daily temperature.




Sec.11 –

202 External structures above the light transit waterline shallbe designed for service temperatures down to the lowest meandaily temperature for the area(s) where the unit is to operate.However, for column stabilised units of conventional type, thepontoon deck need normally not be designed for service tem-peratures lower than 0°C.

203 External structures below the light transit waterline need

not be designed for service temperatures lower than 0°C.204 Internal structures of columns, pontoons and decks areassumed to have the same service temperature as the adjacentexternal structure if not otherwise documented.

205 Internal structures in way of permanently heated roomsneed not to be designed for service temperatures lower than0ºC.

206 For operation in areas where undercooled water may oc-cur, such condition should be considered when selecting mate-rials.

D. Inspection CategoriesD 100 General

101 Welding, and the extent of non-destructive examinationduring fabrication, shall be in accordance with the require-ments stipulated for the appropriate inspection category as de-fined in Sec.4.

102 Inspection categories determined in accordance withSec.4 provide requirements for the minimum extent of re-quired inspection. When considering the economic conse-quence that repair may entail, for example, in way of complexconnections with limited or difficult access, it may be consid-ered prudent engineering practice to require more demandingrequirements for inspection than the required minimum.

103 When determining the extent of inspection, and the loca-tions of required NDT, in additional to evaluating design pa-rameters (for example fatigue utilisation) consideration should

be given to relevant fabrication parameters including;

— location of block (section) joints

— manual versus automatic welding

— start and stop of weld etc.

D 200 Categorisation and inspection level for typicalcolumn stabilised unit details

201 Fig.1 to Fig.4 illustrate minimum requirements for struc-tural categorisation, and inspection for typical column stabi-lised unit configurations.

202 In way of the pontoon and column connection as indicat-ed in Fig.1 and Fig.2, the pontoon deck plate should be the con-tinuous material. These plate fields should be material withthrough-thickness properties (Z-quality material).

203 Shaded areas indicated in the figures are intended to bethree-dimensional in extent. This implies that, in way of theselocations, the shaded area logic is not only to apply to the outersurface of the connection but is also to extend into the struc-ture. However, stiffeners and stiffener brackets within this areashould be of primary category and the bracket toe locations onthe stiffeners should be designated with mandatory magneticparticle inspection (MPI).

204 The inspection categories for general pontoon, plate buttwelds and girder welds to the pontoon shell are determinedbased upon, amongst others: accessibility and fatigue utilisa-tion.

205 Major bracket toes should be designated as locationswith a mandatory requirement to MPI. In way of the brace con-nections as indicated in Fig.3 the brace and brace bracket platefields should be the continuous material. These plate fieldsshould be material with through-thickness properties (Z-quali-ty material).

206 In way of the column and upper hull connection as indi-cated in Fig.4 the upper hull deck plate fields will normally bethe continuous material. These plate fields should be materialwith through-thickness properties (Z-quality material).

Figure 1Pontoon and column connection, twin pontoon design

Primary Area: ICI 1)

Column

Pontoon(Z-Quality

Around the Column Diam.

Primary Area: ICI 1)

Column

Pontoon(Z-Quality

Around the Column Diam.

1 m

1 m

Pontoon Top

Radius 1 m

This is normally fatigue critical, and hence the inspection category is

increased from II to I, see Section 4 C204.

1)




– Sec.11

Figure 2Column and ring pontoon connection, ring pontoon design

Figure 3Brace connection




Sec.11 –

Figure 4Connection column and upper hull

E. Design and Loading Conditions

E 100 General

101 The general definition of design and loading conditionsis given in Sec.2 A100 whilst the loading conditions that shallbe considered within each design condition are defined in 102.

102 Relevant combinations of design and loading conditionsare given in Table E1.

E 200 Load point

201 The load point for which the pressure for a plate fieldshall be calculated, is defined as midpoint of a horizontallystiffened plate field, and half of the stiffener spacing above thelower support of vertically stiffened plate field, or at loweredge of plate when the thickness is changed within the platefield.

202 The load point for which the pressure for a stiffener shallbe calculated, is defined as midpoint of the span. When thepressure is not varied linearly over the span, the pressure shallbe taken as the greater of the pressure at the midpoint, and theaverage of the pressures calculated at each end of the stiffener.

203 The load point for which the pressure for a girder shallbe calculated, is defined as midpoint of the load area.

E 300 Permanent loads

301 Permanent loads are loads that will not vary in magni-tude, position, or direction during the period considered, and

include:

— lightweight of the unit, including mass of permanently in-

stalled modules and equipment, such as accommodation,helicopter deck, drilling and production equipment— hydrostatic pressures resulting from buoyancy— pre-tension in respect to mooring, drilling and production

systems (e.g. mooring lines, risers etc.) see DNV-OS-E301.

E 400 Variable functional loads

401 Variable functional loads are loads that may vary inmagnitude, position and direction during the period under con-sideration.

402 Except where analytical procedures or design specifica-tions otherwise require, the value of the variable loads utilisedin structural design shall be taken as either the lower or upper

value, whichever gives the more unfavourable effect. Variableloads on deck areas for local design are given in Sec.3 D200.

403 Variations in operational mass distributions (includingvariations in tank load conditions in pontoons) shall be ade-quately accounted for in the structural design.

404 Design criteria resulting from operational requirementsshall be fully considered. Examples of such operations may be:

— drilling, production, workover, and combinations thereof — consumable re-supply procedures— maintenance procedures— possible mass re-distributions in extreme conditions.

405 Dynamic loads resulting from flow through air pipesduring filling operations shall be adequately considered in thedesign of tank structures.

E 500 Lifeboat platforms

501 Lifeboat platforms shall be checked for the strength andaccidental design conditions if relevant. A dynamic factor of

Table E1 Relevant design and loading conditions

Design condi-tions

Loading conditions

a) b) c) d) e)

Installation x x

Operation x x x x x

Survival x x x

Transit x x x




– Sec.11

0.2 g0 due to retardation of the lifeboats when lowered shall beincluded in both strength and accidental design conditions.

E 600 Tank loads

601 A minimum density ( ρ ) of 1.025 t/m3 should be consid-ered in the determination of the required scantlings of tank structures.

602 The extent to which it is possible to fill sounding, vent-ing or loading pipe arrangements shall be fully accounted forin determination of the maximum pressure to which a tank maybe subjected to.

603 Dynamic pressure heads resulting from filling of suchpipes shall be included in the pressure head where such loadcomponents are applicable.

604 The internal pressure in full tanks shall be taken as:

a) For tanks with the maximum filling height being to the topof the air pipe, the largest of case 1 and 3 to be applied.

b) For tanks with the maximum filling height being less thanto the top of the air pipe, the largest of case 2 and 3 to beapplied.

Case 1: For tanks with maximum filling height to the top of theair pipe

Case 2: For tanks with maximum filling height less than to thetop of the air pipe

Case 3: For all tank types

Guidance note:

The valve opening pressure can be reduced if the actual pressure

is documented.


605 For external plate field boundaries, it is allowed to con-sider the external pressure up to the lowest waterline occurring

in the environmental extreme condition (including relative mo-tion of the unit).

Guidance note:

For preliminary design calculations, av may be taken as 0.3 g0and external pressure for external plate field boundaries may betaken up to half the pontoon height.


606 In cases where the maximum filling height is less thanthe height to the top of the air pipe, it shall be ensured that thetank will not be over-pressured during operation and tank test-ing conditions.

E 700 Environmental loads, general

701 General considerations for environmental loads are giv-en in Sec.3 E and Sec.3 F.

702 Combination of environmental loads is stated in Sec.3Table F1.

703 Typical environmental loads to be considered in thestructural design of a column stabilised unit are:

— wave loads (including variable pressure, inertia, wave“run-up”, and slamming loads)— wind loads— current loads— snow and ice loads.

704 The following responses due to environmental loadsshall be considered in the structural design of a column stabi-lised unit:

— dynamic stresses for all design conditions— rigid body motion (e.g. in respect to air gap and maximum

angles of inclination)— sloshing— slamming induced vibrations

— vortex induced vibrations (e.g. resulting from wind loadson structural elements in a flare tower)— environmental loads from mooring and riser system.

705 For column stabilised units with traditional catenarymooring systems, earthquake loads should be ignored.

706 Further considerations with respect to environmentalloads are given in Classification Note 30.5.

E 800 Sea pressures

801 For load conditions where environmental load effectsare to be considered the pressures resulting from sea loadingare to include consideration of the relative motion of the unit.

802 The sea pressure acting on pontoons and columns of col-

umn-stabilised platforms in operating conditions shall be takenas:

where

and

av = maximum vertical acceleration, (m/s2), beingthe coupled motion response applicable to thetank in question

hop1 = vertical distance (m) from the load point to thetop of air pipe

hD1 = pressure head due to flow through air pipes, seealso Sec.3 D311.

g0 = 9.81 m/s2, acceleration due to gravity.

hop2 = vertical distance (m) from the load point to theposition of maximum filling height. For tanksadjacent to the sea that are situated below the ex-treme operational draught (TE), hop2 should notbe taken as being less than TE.

hs = vertical distance (m) from the load point to thetop of the tank

p0 = 25 kN/m2 in general

= valve opening pressure when exceeding the gen-eral value.

p1 ρ hop l g0 av+( ) hD1 g0+[ ] kN m2

⁄ ( )=

p2 ρ hop 2 g0 av+( ) hD1 g0+[ ] kN m2

⁄ ( )=

p3 ρ g0hs 1av

g0

-----+ p0 kN m

2 ⁄ ( )+=

TE = extreme operational draught (m) measured verti-cally from the moulded baseline to the assignedload waterline

p ps p+ e =

ps 10 Cw TE zb–( ) kN m2

⁄ ( ) 0 ≥=

pe 10 Cw DD zb–( ) kN m2

⁄ ( ) for zb TE≥=

pe

10 Cw

DD

TE

–( ) kN m2

⁄ ( ) for zb

TE

<=




Sec.11 –

803 The Smith effect (Cw = 0.9) shall only be applied forloading conditions including extreme wave conditions.

E 900 Wind loads

901 The pressure acting on vertical external bulkheads ex-posed to wind shall in general not be taken less than 2.5 kN/m2for local design.

902 Further details regarding wind loads are given in Classi-fication Note 30.5.

E 1000 Heavy components

1001 The forces acting on supporting structures and lashingsystems for rigid units of cargo, equipment or other structuralcomponents should be taken as:

For components exposed to wind, a horizontal force due to thegust wind shall be added to PH.

E 1100 Deformation loads

1101 Deformation loads are loads caused by inflicted defor-mations, such as:

— temperature loads— built-in deformations.

Further details and description of deformation loads are given

in Sec.3 H.

E 1200 Accidental loads

1201 The following accidental conditions shall be consid-ered in respect to the structural design of a column stabilisedunit:

— collision— dropped objects, e.g. from crane handling— fire— explosion— unintended flooding.

1202 Requirements and guidance on accidental loads aregiven in Sec.3 and generic loads are given in DNV-OS-A101.

E 1300 Fatigue loads

1301 Repetitive loads, which may lead to significant fatiguedamage, shall be evaluated. The following listed sources of fa-tigue loads shall, where relevant, be considered:

— waves (including those loads caused by slamming and var-iable (dynamic) pressures)

— wind (especially when vortex induced vibrations may oc-cur)

— currents (especially when vortex induced vibrations mayoccur)

— mechanical loading and unloading (e.g. crane loads).

The effects of both local and global dynamic response shall beproperly accounted for when determining response distribu-tions related to fatigue loads.

1302 Further considerations in respect to fatigue loads aregiven in DNV-RP-C203 and Classification Note 30.5.

E 1400 Combination of loads

1401 Structural strength shall be evaluated considering allrelevant, realistic load conditions and combinations. Scant-lings shall be determined on the basis of criteria that combine,in a rational manner, the effects of relevant global and local re-sponses for each individual structural element.

1402 A sufficient number of load conditions shall be evalu-ated to ensure that the characteristic largest (or smallest) re-

sponse, for the appropriate return period, has been established.

F. Structural Strength

F 100 General

101 Both global and local capacity shall be checked with re-spect to strength. The global and local stresses shall be com-bined in an appropriate manner.

102 Analytical models shall adequately describe the relevantproperties of loads, stiffness, displacement, response, satisfac-tory account for the local system, effects of time dependency,damping, and inertia.

103 The loads shall be combined in the most unfavourableway, provided that the combination is physically feasible andpermitted according to the load specifications.

F 200 Global capacity

201 Gross scantlings may be utilised in the calculation of hull structural strength, provided a corrosion protection systemin accordance with Sec.10 is maintained.

202 Strength capacity check shall be performed for all struc-tural members contributing to the global and local strength of the column stabilised unit. The structures to be checked in-cludes, but are not limited to, the following:

— outer skin of pontoons

— longitudinal and transverse bulkheads, girders and decksin pontoons

— connections between pontoon, columns and bracings— bracings— outer skin of columns— decks, stringers and bulkheads in columns— main bearing bulkheads, frameworks and decks in the

deck structure— connection between bracings and the deck structure— connection between columns and the deck structure— girders in the deck structure.

203 Redistribution of loads is allowed if some panels showto be over-utilised provided the total capacity is satisfactoryand all the other relevant design conditions are fulfilled.

F 300 Transit condition

301 The structure shall be analysed for zero forward speed.For units in transit with high speed, also maximum speed shallbe considered in the load and strength calculations.

Cw = reduction factor due to wave particle motion(Smith effect)

= 0.9 unless otherwise documented

DD = vertical distance in m from the moulded baselineto the underside of the deck structure(the largest relative distance from moulded base-line to the wave crest may replace DD if this is

proved smaller)zb = vertical distance in m from the moulded baseline

to the load point

ps = static sea pressure

pe = dynamic sea pressure.

av = vertical acceleration (m/s2)

ah = horizontal acceleration (m/s2)

Mc = mass of component (t)

PV = vertical force

PH = horizontal force.

PV g0 av±( )Mc kN( )

PH ahMc kN( )=

=




– Sec.11

Guidance note:

Roll and pitch motion at resonance should be somewhat smallerthan calculated by a linear wave theory due to flow of water ontop of the pontoons. This effect may be accounted for providedrational analysis or tests prove its magnitude.


302 Slamming on bracings shall be considered as a possible

limiting criterion for operation in transit. The effect of forwardspeed shall be accounted for in the slamming calculations.

F 400 Method of analysis

401 The analysis shall be performed to evaluate the structur-al capacity due to global and local effects.

402 Model testing shall be performed when significant non-linear effects cannot be adequately determined by direct calcu-lations. In such cases, time domain analysis may also be con-sidered as being necessary. Model tests shall also be performedfor new types of column stabilised units.

403 Where non-linear effects may be considered insignifi-cant, or where such loads may be satisfactorily accounted forin a linear analysis, a frequency domain analysis can be under-

taken. Transfer functions for structural response shall be estab-lished by analysis of an adequate number of wave directions,with an appropriate radial spacing. A sufficient number of pe-riods shall be analysed to:

— adequately cover the site specific wave conditions— satisfactorily describe transfer functions at, and around,

the wave “cancellation” and “amplifying” periods— satisfactorily describe transfer functions at, and around,

the heave resonance period of the unit.

404 Global, wave-frequency, structural responses shall beestablished by an appropriate methodology, for example:

— a regular wave analysis

— a “design wave” analysis— a stochastic analysis.

405 A global structural model shall represent the global stiff-ness and should be represented by a large volume, thin-walledthree dimensional finite element model. A thin-walled modelshould be modelled with shell or membrane elements some-times in combination with beam elements. The structural con-nections in the model shall be modelled with adequate stiffnessin order to represent the actual stiffness in such a way that theresulting responses are appropriate to the model being ana-lysed. The global model usually comprises:

— pontoon shell, longitudinal and transverse bulkheads— column shell, decks, bulkheads and trunk walls

— main bulkheads, frameworks and decks for the deck struc-ture (“secondary” decks which are not taking part in theglobal structural capacity should not be modelled)

— bracing and transverse beams.

406 The global analyses are used to analyse the structurethrough several stages, such as:

— built-in stresses due to fabrication or mating— environmental loads— different ballast conditions including operating and sur-

vival— transit.

407 Wave loads should be analysed by use of sink sourcemodel in combination with a Morison model when relevant.

For certain designs a Morison model may be relevant. Detailsrelated to normal practice for selection of models and methodsare given in Appendix B.

408 When utilising stochastic analysis for world wide oper-ation the analyses shall be undertaken utilising North Atlantic

scatter diagram given in Classification Note 30.5.

409 For restricted operation the analyses shall be undertakenutilising relevant site specific environmental data for the ar-ea(s) the unit will be operated. The restrictions shall be de-scribed in the operation manual for the unit.

F 500 Air gap

501 Positive air gap should in general be ensured for waveswith a 10 -2 annual probability of exceedance. However, localwave impact is acceptable if it can be demonstrated that suchloads are adequately accounted for in the design and that safetyto personnel is not significantly impaired.

502 Analysis undertaken to check air gap should be calibrat-ed against relevant model test results when available. Suchanalysis should take into account:

— wave and structure interaction effects— wave asymmetry effects— global rigid body motions (including dynamic effects)— effects of interacting systems (e.g. mooring and riser sys-

tems)— maximum and minimum draughts.

503 Column “run-up” load effects shall be accounted for inthe design of the structural arrangement in the way of the col-umn and bottom plate of the deck connection. These “run-up”loads shall be treated as environmental load component, how-ever, they should not be considered as occurring simultaneous-ly with other environmental loads.

504 Evaluation of sufficient air gap shall include considera-tion of all affected structural items including lifeboat plat-forms, riser balconies, overhanging deck modules etc.

G. Fatigue

G 100 General

101 Units intended to follow normal inspection requirementsaccording to class requirements, i.e. inspection every fiveyears in sheltered waters or dry dock, may apply a design fa-tigue factor (DFF) of 1.0.

102 Units intended to stay on location for prolonged surveyperiod, i.e. without planned sheltered water inspection, shallcomply with the requirements given in Appendix C.

103 Local effects, for example due to:

— slamming— sloshing— vortex shedding— dynamic pressures— mooring and riser systems

shall be included in the fatigue damage assessment when rele-vant.

104 In the assessment of fatigue resistance, relevant consid-eration shall be given to the effects of stress concentrations in-cluding those occurring as a result of:

— fabrication tolerances (including due regard to tolerancesin way of connections involved in mating sequences orsection joints)

— cut-outs— details at connections of structural sections (e.g. cut-outs

to facilitate construction welding)

— attachments.105 Local detailed FE-analysis of critical connections (e.g.pontoon and pontoon, pontoon and column, column and deck and brace connections) should be undertaken in order to iden-tify local stress distributions, appropriate SCFs, and/or extrap-




Sec.11 –

olated stresses to be utilised in the fatigue evaluation. Dynamicstress variations through the plate thickness shall be checkedand considered in such evaluations, see DNV-RP-C203, forfurther details.

106 For well known details the local FE-analysis may beomitted, provided relevant information regarding SCF areavailable.

107 Principal stresses (see DNV-RP-C203 2.2) should be ap-plied in the evaluation of fatigue responses.

G 200 Fatigue analysis

201 The basis for determining the acceptability of fatigue re-sistance, with respect to wave loads, shall be in accordancewith the requirements given in Appendix B. The required mod-els and methods are dependent on type of operation, environ-ment and design type of the unit.

202 For world wide operation the analyses shall be undertak-en utilising environmental data (e.g. scatter diagram, spec-trum) given in Classification Note 30.5. The North Atlanticscatter diagram shall be utilised.

203 The analyses shall be undertaken utilising relevant site

specific environmental data for the area(s) the unit will be op-erated. The restrictions shall be described in the OperationManual for the unit.

204 In simplified fatigue analysis based on a two parameterWeibull distribution as described in Sec.7, a Weibull shape pa-rameter h = 1.1 should be used for a two pontoon column sta-bilised unit.

A Pierson-Moskowitch spectrum and a cos4 spreading func-tion should be used in the evaluation of column stabilisedunits.

H. Resistance Against Collision, Dropped

Objects, Fire and Explosion

H 100 General

101 The general basis for estimating the effect of crediblecollision and dropped object is given in Sec.3 G.

102 The credible collision against a column of column stabi-lised units will normally only cause local damage of the col-umn, i.e. loading condition c) and d) need not be checked.However, in cases when the columns are especially slender,the global strength of the unit at the moment of collision andthe residual strength after collision shall be checked accordingto Sec.5.

103 The credible collision or dropped object against a brac-ing shall be assumed to cause complete failure of the bracing,which then shall be assumed non-effective for check of the re-sidual strength of the unit after collision, i.e. loading conditiond).

104 For especially strong bracings, the damage may be lim-ited to local denting. The residual strength of the bracing maybe included for check of the unit after the accident.

105 The structural arrangement of the upper hull shall beconsidered with regard to the structural integrity of the unit af-ter the failure of relevant parts of any primary structural ele-ment essential for the overall integrity caused by fire orexplosion. Where considered necessary, a structural analysismay be required with strength criteria as loading condition d).

H 200 Collision

201 A collision between a supply vessel and a column of col-umn-stabilised units shall be considered for all elements of theunit that may be exposed to sideway, bow or stern collision.The vertical extent of the collision zone shall be based on thedepth and draught of the supply vessel and the relative motion

between the supply vessels and the unit.

202 A collision will normally only cause local damage of thecolumn. However, for unit with slender columns, the globalstrength of the unit shall be checked.

203 A collision against a bracing will normally cause com-plete failure of the bracing and its connections (e.g. K-joints).These parts shall be assumed non-effective for check of the re-

sidual strength of the unit after collision.

H 300 Dropped objects

301 Critical areas for dropped objects shall be determined onthe basis of the actual movement of potential dropped objectsrelative to the structure of the unit itself. Where a dropped object is a relevant accidental event, the impact energy shall beestablished and the structural consequences of the impact as-sessed.

302 A dropped object on a bracing will normally cause com-plete failure of the bracing or its connections (e.g. K-joints).These parts are assumed to be non-effective for the check of the residual strength of the unit after dropped object impact.

303 Critical areas for dropped objects shall be determined on

the basis of the actual movement of loads assuming a drop di-rection within an angle with the vertical direction:

— 10° in air— 15° in water.— 5° in air for bottom supported units (TLP).

Dropped objects shall be considered for vital structural ele-ments of the unit within the areas given above.

H 400 Fire

401 The main load bearing structure that is subjected to a fireshall not lose the structural capacity. The following fire scenar-ios shall be considered:

— fire inside the unit— fire on the sea surface.

402 Further requirements concerning accidental conditionevents involving fire is given in DNV-OS-A101.

403 Assessment of fire may be omitted provided assump-tions made in DNV-OS-D301 are met.

H 500 Explosion

501 In respect to design, considering loads resulting from ex-plosions, one or a combination of the following design philos-ophies are relevant:

a) Hazardous locations are located in unconfined (open) lo-cations and that sufficient shielding mechanisms (e.g.blast walls) are installed.

b) Locate hazardous areas in partially confined locations anddesign utilising the resulting, relatively small overpres-sures.

c) Locate hazardous areas in enclosed locations and installpressure relief mechanisms (e.g. blast panels) and designfor the resulting overpressure.

502 As far as practicable, structural design accounting forlarge plate field rupture resulting from explosion loads shouldbe avoided due to the uncertainties of the loads and the conse-quences of the rupture itself.

H 600 Heeled condition

601 Heeling of the unit after damage flooding as described inDNV-OS-C301 shall be accounted for in the assessment of structural strength. Maximum static allowable heel after acci-dental flooding is 17° including wind. Structures that are wetwhen the static equilibrium angle is achieved, shall be checked




– Sec.11

for external water pressure.

Guidance note:

The heeled condition corresponding to accidental flooding intransit conditions will normally not be governing for the design.


602 The unit shall be designed for environmental conditioncorresponding to one year return period after damage. SeeSec.2 Table E1 note 3.

603 Local exceedance of the structural resistance is accepta-ble provided redistribution of forces due to yielding, bucklingand fracture is accounted for.

604 Wave pressure, slamming forces and green sea shall beaccounted for in all relevant areas. Local damage may be ac-cepted provided progressive structural collapse and damage of vital equipment is avoided.

605 Position of air-intakes and openings to areas with vitalequipment which need to be available during an emergency sit-uation, e.g. emergency generators, shall be considered takinginto account the wave elevation in a one year storm.

I. Redundancy

I 100 General

101 Structural robustness shall, when considered necessary,be demonstrated by appropriate analysis. Slender, main loadbearing structural elements shall normally be demonstrated tobe redundant in the accidental design condition.

I 200 Brace arrangements

201 For bracing systems the following considerations shall

apply:

a) Brace structural arrangements shall be investigated for rel-evant combinations of global and local loads.

b) Structural redundancy of slender bracing systems (seeI100) shall normally include brace node redundancy (i.e.all bracings entering the node), in addition to individualbrace element redundancy.

c) Brace end connections (e.g. brace and column connec-tions) shall normally be designed such that the brace ele-ment itself will fail before the end connection.

d) Underwater braces shall be watertight and have a leakagedetection system.

e) When relevant (e.g. in the self-floating, transit condition)the effect of slamming on braces shall be considered.

J. Structure in Way of a Fixed Mooring System

J 100 Structural strength

101 Local structure in way of fairleads, winches, etc. form-ing part of the position mooring system is, as a minimum, to becapable of withstanding forces equivalent to 1.25 times thebreaking strength of any individual mooring line. The strengthevaluation should be undertaken utilising the most unfavoura-ble operational direction of the anchor line. In the evaluation of the most unfavourable direction, account shall be taken of rel-ative angular motion of the unit in addition to possible line leaddirections. The allowable usage factor may be increased to 1.0in this case.

K. Structural Details

K 100 General

101 In the design phase particular attention should be givento structural details, and requirements for reinforcement in ar-eas that may be subjected to high local stresses, for example:

— critical connections— locations that may be subjected to wave impact (including

wave run-up effects along the columns)

— locations in way of mooring arrangements— locations that may be subjected to damage.

102 In way of critical connections, structural continuityshould be maintained through joints with the axial stiffeningmembers and shear web plates being made continuous. Partic-ular attention should be given to weld detailing and geometricform at the point of the intersections of the continuous platefields with the intersecting structure.




Sec.12 –

SECTION 12SPECIAL CONSIDERATIONS FOR SELF ELEVATING UNITS

A. General

A 100 Scope and application101 This standard applies to all types of steel self elevatingunits or jack-ups.

102 A self elevating unit or jack-up may be designed to func-tion in a number of modes, e.g. transit, operational and surviv-al. Limiting design criteria for going from one mode to anothershall be clearly established and documented. Such limiting de-sign criteria shall include relevant consideration of the follow-ing items:

— intact condition, structural strength— damaged condition, structural strength— fatigue strength— accidental damage

— air gap— overturning stability— compartmentation and floating stability.

103 For novel designs, or unproved applications of designswhere limited or no direct experience exists, relevant analysesand model testing, shall be performed which clearly demon-strate that an acceptable level of safety is obtained.

B. Structural Categorisation and Material Selec-tion

B 100 Structural categorisation101 Application categories for structural components are de-fined in Sec.4. Structural members of self elevating units arenormally found in the following groups:

Special category

a) Vertical columns in way of intersection with the mat struc-ture.

b) Highly stressed elements of bottom of leg, including legand spudcan or mat connection.

c) Intersections of lattice type leg structure, which incorpo-rates novel construction, including the use of steel cast-ings.

d) Highly stressed elements of guide structures, jacking andlocking system(s). Jack-house and support structure.

e) Highly stressed elements of crane pedestals, etc. and theirsupporting structure.

Primary category

a) Combination of bulkhead, deck, side and bottom platingwithin the hull which form «Box» or «I» type main sup-porting structure.

b) All components of lattice type legs and external plating of cylindrical legs.

c) Jack-house supporting structure and bottom footing struc-ture, which receives initial transfer of load from legs.

d) Internal bulkheads, shell and deck of bottom mat support-ing structure which are designed to distribute major loads,either uniform or concentrated, into the mat structure.

e) Main support structure of heavy substructures and equip-ment, e.g. cranes, drill floor substructure, life boat plat-

form and helicopter deck.

Secondary category

a) Deck, side and bottom plating of hull except areas wherethe structure is considered primary or special application.

b) Bulkheads, stiffeners, decks and girders in hull that are notconsidered as primary or special application.

c) Internal bulkheads and girders in cylindrical legs.

d) Internal bulkheads, stiffeners and girders of bottom matsupporting structure except where the structure is consid-ered primary or special application.

B 200 Material selection

201 Material specifications shall be established for all struc-tural materials. Such materials shall be suitable for their in-tended purpose and have adequate properties in all relevantdesign conditions. Material selection shall be undertaken in ac-cordance with the principles given in Sec.4.

202 When considering criteria appropriate to material gradeselection, adequate consideration shall be given to all relevantphases in the life cycle of the unit. In this connection there maybe conditions and criteria, other than those from the in-service,operational phase, that provide the design requirements in re-spect to the selection of material. (Such criteria may, for exam-ple, be design temperature and/or stress levels during marineoperations.)

203 In ‘special areas’ structural cross-joints where high ten-sile stresses are acting perpendicular to the plane of the plate,the plate material shall be tested to prove the ability to resist la-mellar tearing. (Z-quality).

B 300 Design temperature

301 The design temperature is not to exceed the lowest serv-ice temperature of the steel as defined below for the variousstructural parts.

302 External structures above the lowest astronomical tide(LAT) for the unit in operation shall be designed for servicetemperatures down to the lowest, average, daily, atmospherictemperature for the draft(s) and area(s) where the unit is to op-erate.

303 External structures above the light transit waterline dur-ing transportation shall be designed for service temperaturesdown to the lowest daily mean temperature for the area(s)where the unit shall be transported.

Guidance note:

If data giving the lowest daily mean temperature are not availa-ble, other criteria may be accepted after special consideration.


304 External structures below the light transit waterline dur-ing transportation and below the lowest astronomical tide(LAT) during operation need not to be designed for servicetemperatures lower than 0°C

305 Internal structures of maths, spud cans, legs and hull are

assumed to have the same service temperature as the adjacentexternal structure if not otherwise documented.

306 Internal structures in way of permanently heated roomsneed not to be designed for service temperatures lower than0°C.




– Sec.12

B 400 Selection of structural steel

401 The grade of steel to be used is in general to be relatedto the design temperature and thickness as shown in Sec.4 forthe various application categories.

402 A lower service temperature than given in the Sec.4 Ta-ble D3 for the relevant steel grade may be considered when astress relieving heat treatment is carried out after welding.

403 For regions subjected to compressive and/or low tensilestresses, consideration will be given to the use of lower steelgrades than stated in the Sec.4 Table D3.

404 The toughness requirements for steel plates, sectionsand weldments exceeding the thickness limits in Sec.4 TableD3 shall be evaluated in each separate case.

405 Grade of steel to be used for thicknesses less than 10 mmand/or design temperature above 0°C should be specially con-sidered in each case.

406 Use of steels in anaerobic conditions or steels suscepti-ble to hydrogen induced stress cracking (HISC) should be es-pecially considered as specified in Sec.4.

B 500 Inspection categories501 Welding, and the extent of non-destructive examinationduring fabrication, shall be in accordance with the require-ments stipulated for the appropriate inspection category as de-fined in Sec.4.

502 Inspection categories determined in accordance withSec.4 provide requirements for the minimum extent of re-quired inspection. When considering the economic conse-quence that repair may entail, for example, in way of complexconnections with limited or difficult access, it may be consid-ered prudent engineering practice to require more demandingrequirements for inspection than the required minimum.

503 When determining the extent of inspection, and the loca-tions of required NDT, in additional to evaluating design pa-

rameters (for example fatigue utilisation) consideration shouldbe given to relevant fabrication parameters including; locationof block (section) joints, manual versus automatic welding,start and stop of weld etc.

C. Design and Loading Conditions

C 100 General

101 The general definition of design conditions is given inSec.2 A100 whilst the loading conditions within each designcondition are defined in 102.

102 The structure shall be designed to resist relevant loads

associated with conditions that may occur during all stages of the life-cycle of the unit. The conditions that should be consid-ered are:

— transit condition(s)— installation condition— operating condition(s)— survival condition— retrieval condition.

103 Relevant load cases shall be established for the variousdesign conditions based on the most unfavourable combina-tions of functional loads, environmental loads and/or acciden-tal loads. Analysis shall include built in stresses due toassembly of the structure during fabrication.

104 Limiting environmental and operating conditions (de-sign data) for the different design conditions shall be specifiedby the builder.

105 If it is intended to dry dock the unit the footing structure(i.e. mat or spudcans) shall be suitably strengthened to with-

stand such actions.

106 The relevant design and loading conditions for self ele-vating units are shown in Table C1.

C 200 Transit

201 A detailed transportation assessment shall be undertakenwhich includes determination of the limiting environmentalcriteria, evaluation of intact and damage stability characteris-tics, motion response of the global system and the resulting, in-duced loads. The occurrence of slamming loads on thestructure and the effects of fatigue during transport phasesshall be evaluated when relevant.

Guidance note:

For guidance on global analysis for the transit condition see Clas-sification Note 31.5, 5.3 and for environmental loading see Clas-sification Note 30.5.


202 The structure shall be analysed for zero forward speed inthe transit analysis.

203 The legs shall be designed for the static and inertia forc-es resulting from the motions in the most severe environmentaltransit conditions, combined with wind forces resulting fromthe maximum wind velocity.

204 The leg positions for both field moves and ocean moves

shall be assessed when considering structural strength for tran-sit condition.

205 In lieu of a more accurate analysis, for the ocean transitcondition the legs shall be designed for the following forcesconsidered to act simultaneously:

— 120% of the acceleration forces caused by the roll andpitch of the platform

— 120% of the static forces at the maximum amplitude of rollor pitch

— wind forces from a 45 m/s wind velocity.

Note, that the effect of heave, surge and sway are implicitly ac-counted for by use of the 20% upscaling of the motions.

206 For the field transit position the legs may be designed forthe acceleration forces caused by a 6° single amplitude roll orpitch at the natural period of the unit plus 120% of the staticforces at a 6° inclination of the legs unless otherwise verifiedby model tests or calculations.

207 Dynamic amplification of the acceleration forces on thelegs shall be accounted for if the natural periods of the legs aresuch that significant amplification may occur.

208 If considered relevant, the effect of vortex shedding in-duced vibrations of the legs due to wind shall be taken into ac-count.

Guidance note:

For guidance relating to vortex induced oscillations see Classifi-cation Note 30.5, 7.


209 The hull shall be designed for global mass and sea pres-sure loads, local loads and leg loads during transit.

210 Satisfactory compartmentation and stability during all

Table C1 Relevant design and loading conditions

Design conditions Loading conditions

a) b) c) d) e)

Installation XOperation X X X X

Retrieval X

Survival X X

Transit X X X




Sec.12 –

floating operations shall be ensured, see DNV-OS-C301.

211 Unless satisfactory documentation exists demonstratingthat shimming is not necessary, relevant leg interfaces (e.g. legand upper guide) shall be shimmed in the transit condition.

212 All aspects of transportation, including planning andprocedures, preparations, seafastenings and marine operationsshould comply with the requirements of the warranty authori-

ty.

C 300 Installation and retrieval

301 Relevant static and dynamic loads during installationshall be accounted for in the design, including consideration of the maximum environmental conditions expected for the oper-ations and leg impact on the seabed.

Guidance note:

Guidance relating to simplified analytical methodology for bot-tom impact on the legs is given in Classification Note 31.5, 5.8.


302 The capacity of the unit during pre-loading must be as-sessed. The purpose of pre-loading is to develop adequate

foundation capacity to resist the extreme vertical and horizon-tal loadings. The unit should be capable of pre-loading to ex-ceed the maximum vertical soil loadings associated with theworst storm loading.

Guidance note:

Guidance relating to pre-loading is given in Classification Note30.4, 1 and 8.


303 The hull structure must be analysed to ensure it can with-stand the maximum pre-loading condition.

304 The structural strength of the hull, legs and footings dur-ing installation and retrieval shall comply with the strengthcondition given in Sec.5 of this standard.

C 400 Operation and survival

401 The operation and survival conditions cover the unit inthe hull elevated mode.

402 A detailed assessment shall be undertaken which in-cludes determination of the limiting soils, environmental andweight criteria and the resulting, induced loads.

403 Dynamic structural deflection and stresses due to waveloading shall be accounted for if the natural periods of the unitare such that significant dynamic amplification may occur.

404 Non-linear amplification (large displacement effects) of the overall deflections due to second order bending effects of the legs shall be accounted for whenever significant.

405 Critical aspects to be considered in the elevated condi-tion are structural strength, overturning stability and air gap.

406 The structural strength of the hull, legs and footings dur-ing operation and survival shall comply with this section andSec.5. The strength assessment should be carried out for themost limiting conditions with the maximum storm conditionand maximum operating condition examined as a minimum.

Guidance note:

The hull will typically comprise the following elements:

- decks- longitudinal bulkheads- transverse frames- longitudinal girders and stringers- stringers and web frames on the transverse bulkheads

- jackhouses.


407 The strength of the hull shall be assessed based on thecharacteristic load conditions that result in maximum longitu-

dinal tension and compression stresses (for yield and bucklingassessment) in deck and bottom plating.

408 The effect of large openings in the hull (e.g. drill slot)which affect the distribution of global stresses should be deter-mined by a finite element model accounting for three-dimen-sional effects.

D. Environmental Conditions

D 100 General

101 All environmental phenomena, which may contribute tostructural damage, shall be considered. Such phenomena arewind, waves, currents, ice, earthquake, soil conditions, temper-ature, fouling, corrosion, etc.

102 The specified environmental design data used for calcu-lating loads for intact structure are to correspond with the mostprobable largest values for a return period of 100 years.

103 For damaged structure calculations a return period of one year shall be used.

104 The environmental design data may be given as maxi-mum wave heights with corresponding periods and wind- andcurrent velocities and design temperatures or as acceptable ge-ographical areas for operation. In the latter case the builder isto specify the operational areas and submit documentationshowing that the environmental data for these areas are withinthe environmental design data.

105 The statistical data used as a basis for design must covera sufficiently long period of time.

Guidance note:

In many cases environmental data can be supplied by DNV. Seealso Classification Note 30.5.


D 200 Wind

201 Wind velocity statistics shall be used as a basis for a de-scription of wind conditions, if such data are available. Threekinds of wind velocities shall be considered as given in 202 to204.

202 Sustained wind velocity is defined as the average windvelocity during a time interval (sampling time) of one minute.The most probable highest sustained wind velocity in a periodof N years will be referred to as the «N years sustained wind».This is equivalent to a wind velocity with a recurrence periodof N years.

203 Gust wind velocity is defined as the average wind veloc-ity during a time interval of 3 s. The «N years gust wind veloc-ity» is the most probable highest gust velocity in a period of Nyears.

204 One hour wind velocity is defined as the average windvelocity during a time interval of one hour.

205 Characteristic wind design velocities shall be basedupon appropriate considerations of velocity and height profilesfor the relevant averaging time.

Guidance note:

Practical information in respect to wind conditions, including ve-locity and height profiles, is documented in Classification Note30.5.


206 When wind tunnel data obtained from reliable and ade-quate tests on a representative model of the platform are avail-able, these data will be considered for the determination of pressures and resulting forces.




– Sec.12

D 300 Waves

301 Wave conditions which shall be considered for designpurposes, may be described either by deterministic (regular)design wave methods or by stochastic (irregular sea state)methods applying wave energy spectra.

302 Short term irregular sea states are described by means of wave energy spectra, which are characterised by significant

wave height (HS), and average zero-upcrossing period (TZ).Analytical spectrum expressions are to reflect the width andshape of typical spectra for the considered height.

The shortcrestedness of waves in a seaway, i.e. the directionaldispersion of wave energy, may be taken into account. Theprincipal direction of wave encounter is defined as the direc-tion of maximum wave energy density.

Guidance note:

For open sea locations the Pierson-Moskowitz (P-M) type of spectrum may be applied. For shallow water, or locations with anarrow “fetch”, a more narrow spectrum should be considered(e.g. Jonswap spectrum).

Practical information in respect to wave conditions is document-ed in Classification Note 30.5, 3 and Classification Note 31.5,

3.2.


303 The long term behaviour of the sea is described bymeans of a family of wave spectra, the probability of occur-rence for each spectrum being taken into account.

304 For this purpose one needs the joint probability densityfunction for HS and TZ, which can be obtained from wave sta-tistics. A description of the long term sea states based on theuse of hindcastings can also be accepted. Wave statistics for in-dividual principal directions of wave encounter should beused, otherwise conservative assumptions shall be introduced.

Extreme wave heights are expressed in terms of wave heightshaving a low probability of occurrence.

The «N year wave height» is the most probable largest individ-ual wave height during N years. This is equivalent to a waveheight with a return period of N years.

305 In deterministic design procedures, based on simple reg-ular wave considerations, the wave shall be described by thefollowing parameters:

— wave period— wave height— wave direction— still water depth.

The choice of an appropriate design wave formulation has tobe based on particular considerations for the problem in ques-

tion. Shallow water effects shall be accounted for.306 The design waves shall be those, which produce themost unfavourable loads on the considered structure, takinginto account the shape and size of structure, etc.

The wave period shall be specified in each case of application.It may be necessary to investigate a representative number of wave periods, in order to ensure a sufficiently accurate deter-mination of the maximum loads.

D 400 Current

401 Adequate current velocity data shall be selected from thestatistics available. Different components of current shall beconsidered, such as tidal current and wind generated current.

402 The variation of current velocity over the water depthshall be considered when this is relevant.

D 500 Temperature

501 The design temperature shall be specified as necessaryfor the areas where the unit is to operate or be transported, see

Sec.2 C200.

D 600 Snow and ice

601 Snow and ice shall be considered as necessary for the ar-eas where the unit is to operate or be transported.

E. Method of Analysis

E 100 General

101 Structural analysis shall be performed to evaluate thestructural strength due to global and local effects.

102 The following responses shall be considered in the struc-tural design whenever significant:

— dynamic stresses for all design conditions— non-linear wave loading effects, (e.g. effect of drag and fi-

nite wave elevation)— non-linear amplification due to second order bending ef-

fects of the legs (P-delta effect)— slamming induced vibrations

— vortex induced vibrations (e.g. resulting from wind loadson structural elements in a flare tower)

— wear resulting from environmental loads at riser systeminterfaces with hull structures.

103 Non-linear amplification of the overall deflections dueto second order bending effects of the legs shall be accountedfor whenever significant. The non-linear bending responsemay be calculated by multiplying the linear leg response by anamplification factor as follows:

104 In the unit elevated mode the global structural behaviourmay be calculated by deterministic quasi-static analysis, di-rectly considering non-linear wave and leg bending effects.The effect of dynamics should be represented by an inertiaforce contribution at the level of the hull centre of gravity or bya dynamic amplification factor, as specified in ClassificationNote 31.5.

105 In case of significant uncertainties related to the non-lin-ear, dynamic behaviour, stochastic time domain analysis maybe performed. The selection of critical seastate for the analysisshould be properly considered.

Guidance note:For shallow waters the significant wave height should be correct-ed as shown in Classification Note 30.5, 3.2.11.

The irregular wave simulation may be performed as presented inClassification Note 30.5, 3.2.12.


106 Where non-linear loads may be considered as being in-significant, or where such loads may be satisfactorily account-ed for in a linearised analysis, a frequency domain analysis canbe undertaken. Transfer functions for structural response shallbe established by analysis of an adequate number of wave di-rections, with an appropriate radial spacing. A sufficientnumber of periods shall be analysed to:

— adequately cover the site specific wave conditions— to satisfactorily describe transfer functions at, and around,

the wave ‘cancellation’ and ‘amplifying’ periods— to satisfactorily describe transfer functions at, and around,

the resonance period of the unit.

P = axial load on one leg

PE = Euler buckling load for one leg.

α1

1 P PE ⁄ –----------------------=




Sec.12 –

107 As an alternative to time domain analysis model testingmay be performed when non-linear effects cannot be adequate-ly determined by direct calculations. Model tests should alsobe performed for new types of self elevating units or jack-ups.

108 For independent leg units, the static inclination of thelegs shall be accounted for. The inclination is defined as thestatic angle between the leg and a vertical line and is due to

fabrication tolerances, fixation system and hull inclination, asspecified in Classification Note 31.5.

109 The seabed conditions, and therefore the leg and soil in-teraction, need to be considered as it can effect the following

— leg bending moment distribution— overall structure stiffness and therefore the natural period

of the unit— load distribution on the spudcans.

110 The leg and soil interaction shall be varied as necessarybetween an upper and lower bound to provide the worst re-sponse at the bottom leg and footing area and at the jackhouselevel.

Guidance note:As the leg and soil interaction is difficult to predict, it is accept-able and conservative to assume pinned and fixed conditions asthe upper and lower bounds.

For further guidance see Classification Note 30.4 Sec.8 and Clas-sification Note 31.5, Sec.3.6 and Sec.5.5.


111 The leg and hull connection can be designed by any of or combination of the following methods:

— a fixation system, i.e. rack chock — a fixed jacking system, i.e. pinions rigidly mounted to the

jackhouse

— a floating jacking system, i.e. pinions mounted to the jack-house by means of flexible shock pads

— a guiding system by upper and lower guides.

The characteristics and behaviour of the actual leg and hullconnection system need to be properly represented in the ap-propriate global and local analyses.

Guidance note:

Practical information in respect to modelling leg and hull inter-action is documented in Classification Note 31.5 Sec.5.4 orSNAME 5-5A, Section 5.6.


E 200 Global structural analysis

201 A global structural model shall represent the global stiff-ness and behaviour of the platform. The global model shouldusually represent the following,:

— footing main plating and stiffeners— leg truss or shell and stiffeners— jackhouse— main bulkheads, frameworks and decks for the deck struc-

ture ("secondary" decks which are not taking part in theglobal structural capacity should not be modelled)

— mass model.

202 Depending on the purpose of the analysis and possible

combination with further local analysis the different level of idealisation and detailing may be applied for a global structure.The hull may either be represented by a detailed plate and shellmodel or a model using grillage beams. The legs may be mod-elled by detailed structural models or equivalent beams, or acombination of such.

Guidance note:

For further guidance regarding modelling procedures see Classi-fication Note 31.5 or SNAME 5-5A.


E 300 Local structural analysis

301 An adequate number of local structural models should

be created in order to evaluate response of the structure to var-iations in local loads. The model(s) should be sufficiently de-tailed such that resulting responses are obtained to the requireddegree of accuracy. A number of local models may be requiredin order to fully evaluate local response at all relevant sections.The following local models should be analysed in the evalua-tion of strength:

— footing, mat or spudcan— stiffened plates subjected to tank pressures or deck area

loads— leg and hull connection system including jackhouse sup-

port structure— support structure for heavy equipment such as drill floor

and pipe racks

— riser hang off structure— crane pedestal support structure— helicopter deck support structure.

302 A detailed FE model should be applied to calculate thetransfer of leg axial forces, bending moments and shears be-tween the upper and lower guide structures and the jackingand/or fixation system. The systems and interactions should beproperly modelled in terms of stiffness, orientation and clear-ances. The analysis model should also include a detailed modelof the leg in the hull interface area, the guides, fixation and/or jacking system, together with the main jackhouse structure.

Guidance note:

The detailed leg model should normally extend 4 bays below andabove the lower and upper guides, respectively


Guidance note:

For further guidance regarding modelling procedures see Classi-fication Note 31.5 or SNAME 5-5A.


E 400 Fatigue analysis

401 The fatigue life shall be calculated considering the com-bined effects of global and local structural response. The ex-pected dynamic load history shall be specified in the designbrief as basis for the calculations.

402 Stress concentration factors for fatigue sensitive struc-tural details that cannot be obtained from standard tables, shallbe determined by a finite element analysis.

F. Design Loads

F 100 General

101 The requirements in this section define and specify loadcomponents and load combinations to be considered in theoverall strength analysis for self elevating units as well as localscantlings.

102 Other considerations regarding design loads are given inSec.3.

F 200 Permanent loads

201 Permanent loads are loads that will not vary in magni-tude, position, or direction during the period considered and in-clude:




– Sec.12

— 'lightweight' of the unit, including mass of permanently in-stalled modules and equipment, such as accommodation,helicopter deck, drilling and production equipment

— permanent ballast— hydrostatic pressures resulting from buoyancy— pretension in respect to drilling and production systems

(e.g. risers, etc.).

F 300 Variable functional loads

301 Variable functional loads are loads that may vary inmagnitude, position and direction during the period under con-sideration.

302 Except where analytical procedures or design specifica-tions otherwise require, the value of the variable loads utilisedin structural design should be taken as either the lower or upperdesign value, whichever gives the more unfavourable effect.Variable loads on deck areas for local design are stated inSec.3.

303 Variations in operational mass distributions (includingvariations in tank load conditions) shall be adequately account-ed for in the structural design.

304 Design criteria resulting from operational requirementsshould be fully considered. Examples of such operations maybe:

— drilling, production, workover, and combinations thereof — consumable re-supply procedures— maintenance procedures— possible mass re-distributions in extreme conditions.

305 Dynamic loads resulting from flow through air pipesduring filling operations shall be adequately considered in thedesign of tank structures.

306 Lifeboat platforms shall be checked for the loading con-ditions in Table D1, Sec.3 D as relevant. A dynamic factor of

0.2 g0 due to retardation of the lifeboats when lowered shall beincluded.

F 400 Environmental loads, general

401 General considerations for environmental loads are giv-en in Sec.3 E, F, G, H and I.

402 Combination of environmental loads is stated in Sec.3 F.

Guidance note:

Further considerations with respect to environmental loads aregiven in Classification Note 30.5.


F 500 Wind loads

501 In conjunction with maximum wave forces the sustainedwind velocity, i.e. the 1 minute average velocity, shall be used.If gust wind alone is more unfavourable than sustained wind inconjunction with wave forces, the gust wind velocity shall beused. For local load calculations gust wind velocity shall beused.

502 For structures being sensitive to dynamic loads, for in-stance tall structures having long natural period of vibration,the stresses due to the gust wind pressure considered as staticshall be multiplied by an appropriate dynamic amplificationfactor.

503 The possibility of vibrations due to instability in the flowpattern induced by the structure itself should also be consid-

ered.

F 600 Waves

601 The basic wave load parameters and response calcula-tion methods in this standard shall be used together with a

wave load analysis with the most unfavourable combinationsof height, period and direction of the waves.

602 The liquid particle velocity and acceleration in regularwaves shall be calculated according to recognised wave theo-ries, taking into account the significance of shallow water andsurface elevation.

Linearised wave theories may be used when appropriate. In

such cases appropriate account shall be taken of the extrapola-tion of wave kinematics to the free surface.

603 The wave design data shall represent the maximumwave heights specified for the unit, as well as the maximumwave steepness.

The wave lengths shall be selected as the most critical ones forthe response of the structure or structural part to be investigat-ed.

Guidance note:

Practical information in respect to wave conditions, includingwave steepness criteria and wave "stretching", is documented inClassification Note 30.5, Sec.3.


604 For a deterministic wave analysis using an appropriatenon-linear wave theory for the water depth, i.e. Stokes’ 5th orDean’s Stream Function, the fluid velocity and acceleration of the maximum long-crested 100 year wave may be multipliedby a kinematics reduction factor of 0.86. The scaling of the ve-locity shall be used only in connection with hydrodynamic co-efficients as defined for mobile units in 803, i.e. CD = 1.0 forsubmerged, cleaned jack-up members.

Guidance note:

The kinematics reduction factor is introduced to account for theconservatism of deterministic or regular wave kinematics tradi-tionally accomplished by adjusting the hydrodynamic properties.


F 700 Current

701 Characteristic current design velocities shall be basedupon appropriate consideration of velocity and height profiles.The variation in current profile with variation in water depth,due to wave action shall be appropriately accounted for.

Guidance note:

Practical information in respect to current conditions, includingcurrent stretching in the passage of a wave, is documented inClassification Note 30.5 Sec.4.


F 800 Wave and current

801 Wave and current loads should be calculated usingMorison’s equation.

Guidance note:

For information regarding use of Morison’s equation see Classi-fication Note 30.5, 6.


802 Vector addition of the wave and current induced particlevelocities shall normally be used for calculation of the com-bined wave and current drag force. If available, computationsof the total particle velocities and acceleration based on more

exact theories of wave and current interaction will be pre-ferred.

803 Hydrodynamic coefficients for circular cylinder in oscil-latory flow with in-service marine roughness, and for high val-ues of the Keulegan-Carpenter number, i.e. KC > 37, may be




Sec.12 –

taken as given in Table F1.

The Keulegan-Carpenter number is defined by:

804 The roughness for a “mobile unit” applies when marinegrowth roughness is removed between submersion of mem-bers.

805 The smooth values will apply above MWL + 2 m and therough values below MWL + 2 m, where MWL is the mean stillwater level, as defined in Classification Note 30.5, Figure 4-2.

806 The drag coefficient CD dependence on roughness maybe interpolated as:

807 The values in 806 apply for both stochastic and deter-ministic wave analysis when the guidance given in 604 is fol-lowed.

808 Tentative values of the drag coefficient as a function of

Kulegan-Carpenter number for smooth and marine growthcovered circular cylinders for supercritical Reynolds numbersare expressed as:

809 The formula in 808 is valid for free flow field withoutany influence of a fixed boundary. For KC < 10 the formula isexpected to be conservative.

810 Assumptions regarding allowable marine growth shallbe stated in the basis of design.

811 For non-tubular members the hydrodynamic coeffi-cients should reflect the actual shape of the cross sections andmember orientation relative to the wave direction.

Guidance note:

Hydrodynamic coefficients relevant to typical self elevating unitor jack-up chord designs are stated in Classification Note 30.5, 5and Classification Note 31.5, 4.5. See also SNAME 5-5A.

Equivalent single beam stiffness parameters for lattice-type legsmay be obtained from Classification Note 31.5, 5.6.


F 900 Sea pressures during transit

901 Unless otherwise documented, the sea pressure p actingon the bottom, side and weather deck of a self elevating unit intransit condition is to be taken as:

and for weather decks:

or for sides and bottom:

902 In cases where pressure difference on bulkhead sides isinvestigated, i.e. transit condition, the pressures shall be com-bined in such a way that the largest pressure difference is usedfor design.

F 1000 Heavy components during transit

1001 The forces acting on supporting structures and lashingsystems for rigid units of cargo, equipment or other structuralcomponents should be taken as:

For units exposed to wind, a horizontal force due to the designgust wind shall be added to PH.

Guidance note:

For self elevating units or jack-ups in transit condition, ah and avneed not be taken larger than 0.5 g0 (m/s2).


Table F1 Hydrodynamic coefficients C

Surface condition Drag coefficient

C D (k/Dm)

Inertia coeffi-cient

C M (k/Dm)

Multiyear roughnessk/Dm > 1/100

1.05 1.8

Mobile unit (cleaned)k/Dm < 1/100

1.0 1.8

Smooth memberk/Dm < 1/10000

0.65 2.0

k = the roughness height

Dm = the member diameter

Um = the maximum orbital particle velocity

T = the wave period.

Kc

UmT

Dm

------------=

CD

CD

k D ⁄ m

( )

0.65 k D 1 10000 ⁄ < ⁄ ;

0.65 2.36+0.34 log10

k Dm

⁄ ( )( );1 10000 k Dm

1 250 ⁄ < ⁄ < ⁄

1.0 1 250 k D

m

1 100 ⁄ < ⁄ < ⁄ ;

1.05 1 100 k < ⁄ Dm

1< ⁄ 25 ⁄ ;

==

CD

CD

k D ⁄ m

( )

0.65 k D 1 10000 ⁄ < ⁄ ;

0.65 2.36+0.34 log10

k Dm

⁄ ( )( ) ;1 10000 k Dm

1 250 ⁄ < ⁄ < ⁄

1.0 1 250 k Dm

1 100 ⁄ < ⁄ < ⁄ ;

1.05 1 100 k < ⁄ Dm

1< ⁄ 25 ⁄ ;

==

CD

CD

k Dm

⁄ ( )

1.45 for Kc 10<

2

Kc

5–( )1 5 ⁄

------------------------------- for 10 Kc

37< <

1.0 for 37 Kc

<

=

ps = static sea pressure

pe = dynamic sea pressure

TTH = heavy transit draught (m) measured verticallyfrom the moulded baseline to the uppermosttransit waterline

DB

= depth of barge (m)

L = greater of length or breadth (m)

zb = vertical distance (m) measured vertically fromthe moulded baseline to the load point.

av = vertical acceleration (m/s2)

ah = horizontal acceleration (m/s2)

Mc = mass of component (t)

PV = vertical force

PH = horizontal force.

p ps pe±=

ps 10 TTH zb–( ) kN m2

⁄ ( )=

pe 10 0.75 DB 0.07 L zb–+( ) kN m2

⁄ ( )=

pe 6.0 kN m2

⁄ ≥

pe 10 TTH 0.07 L zb–+( ) kN m2

⁄ ( )=

PV g0 av±( )Mc (kN)=

PH ahMc (kN)=




– Sec.12

F 1100 Accidental loads

1101 The following accidental conditions shall be consid-ered in respect to the structural design of a self elevating unit:

— collision— dropped objects (e.g. from crane handling)— fire— explosion— unintended flooding during transit.

1102 Additional considerations in respect to accidental loadsare given in DNV-OS-A101.

F 1200 Fatigue loads

1201 Repetitive loads, which may lead to possible signifi-cant fatigue damage, shall be evaluated. The following listedsources of fatigue loads shall, where relevant, be considered:

— waves (including those loads caused by slamming and var-iable (dynamic) pressures)

— wind (especially when vortex induced vibrations may oc-cur)

— currents (especially when vortex induced vibrations mayoccur)— mechanical vibration (e.g. caused by operation of machin-

ery)— mechanical loading and unloading (e.g. crane loads).

The effects of both local and global dynamic response shall beproperly accounted for when determining response distribu-tions related to fatigue loads.

1202 Further considerations in respect to fatigue loads aregiven in DNV-RP-C203.

F 1300 Combination of loads

1301 Load combinations for the design conditions are ingeneral given in Sec.3.

1302 Structural strength shall be evaluated considering allrelevant, realistic load conditions and combinations. Scant-lings shall be determined on the basis of criteria that combine,in a rational manner, the effects of relevant global and local re-sponses for each individual structural element.

1303 A sufficient number of load conditions shall be evalu-ated to ensure that the characteristic largest (or smallest) re-sponse, for the appropriate return period, has been established.

Guidance note:

For example, maximum global, characteristic responses for a self elevating unit or jack-up may occur in environmental conditionsthat are not associated with the characteristic, largest, waveheight. In such cases, wave period and associated wave steepnessparameters are more likely to be governing factors in the deter-mination of maximum and minimum responses.


G. Structural Strength

G 100 General

101 Both global and local capacity shall be checked with re-spect to strength. The global and local stresses shall be com-bined in an appropriate manner.

102 Analytical models shall adequately describe the relevantproperties of loads, stiffness, displacement, satisfactory ac-

count for the local system, effects of time dependency, damp-ing, and inertia.

103 The loads shall be combined in the most unfavourableway, provided that the combination is physically feasible andpermitted according to the load specifications.

104 The usage factors in Sec.2, Table E1 shall be used forself elevating units.

G 200 Global capacity

201 Gross scantlings may be utilised in the calculation of hull structural strength, provided a corrosion protection systemin accordance with Sec.10 is maintained.

202 The strength capacity shall be checked for all structuralmembers contributing to the global and local strength of theself elevating unit or jack-up. The structure to be checked is allplates and continuous stiffeners included in the followingstructures:

— main load bearing plating in mat and spudcan type foot-ings

— all leg members in truss type legs— outer plating in column type legs— jackhouse supporting structure— main bearing bulkheads, frameworks and decks in the hull

structure— girders in the hull structure.

203 Redistribution of stresses is allowed if some panels areshown to be over-utilised provided the total capacity is satis-factory and all the other relevant design conditions are ful-filled.

204 Design principles for strength analysis are given in Sec.5and Classification Note 31.5.

205 Initial imperfections in structural members shall be ac-counted for. For lattice leg structure this will include imperfec-tions for single beam elements as well as for complete legassembly.

G 300 Footing strength

301 In the operating condition account shall be taken of theforces transferred from the legs and the seabed reaction, the in-

ternal structure shall be designed to facilitate proper diffusionof these forces.

302 High stress concentrations at the connection between legand mat/spudcan shall be avoided as far as possible.

303 The effect of an uneven distribution of critical contactstresses over the foundation area shall be examined taking intoaccount a maximum eccentricity moment from the soil result-ed from 304, uneven seabed conditions and scouring.

304 For separate type spudcans the maximum eccentricitymoment Me should not be taken less than:

The corresponding critical contact pressure qc should not be

taken less than:

For other types of bottom support, e.g. mats special considera-tions should be made.

G 400 Leg strength

401 The boundary conditions for the legs at the seabed shallbe varied within realistic upper and lower limits when thescantlings of the legs are determined. The variation in bound-ary conditions is to take into account uncertainties in the esti-mation of soil properties, non-linear soil-structure interaction,

FV = maximum axial force in the leg accounting forfunctional loads and environmental overturningloads

R = equivalent radius of spudcan contact area.

Me 0.5 FVR=

qc

FV

R2

-------=




Sec.12 –

effects due to repeated loadings, possible scouring, etc.

402 When determining the forces and moments in the legs,different positions of the hull supports along the legs shall beconsidered.

403 Due attention shall be paid to the shear force in the legbetween supporting points in the hull structure, and the posi-tion and duration of load transfer between the leg and hull.

404 Lattice-type legs shall be checked against overall buck-ling, buckling of single elements and punching strength of thenodes, see Sec.5.

G 500 Jackhouse support strength

501 Special attention shall be paid to the means for the legsupport, the jackhouses, the support of the jackhouse to themain hull, and the main load transfer girders between the jack-houses.

G 600 Hull strength

601 Scantlings of the hull shall be checked for the transitconditions with external hydrostatic pressure and inertia forceson the legs as well as for the pre-loading and elevated condi-tions, see Sec.5.

.

H. Fatigue Strength

H 100 General

101 General requirements are given in Sec.7, and guidanceconcerning fatigue life are given in DNV-RP-C203 and Clas-sification Note 31.5, 7.

102 Units intended to follow normal inspection requirementsaccording to class requirements, i.e. dry dock or sheltered wa-ter inspection every 5 years, may apply a design fatigue factor(DFF) of 1.0. For classification, see Sec.7 A200.

103 Units intended to stay on location for prolonged surveyperiod, i.e. without planned sheltered water inspection, shallcomply with the requirements given in Appendix C.

104 Assumptions related to the resistance parameters adopt-ed in the fatigue design, e.g. with respect to corrosion protec-tion, shall be consistent with the in-service structure (seeDNV-RP-C203).

H 200 Fatigue analysis

201 The required models and methods for fatigue analysisfor self elevating units or jack-ups are dependent on type of op-eration, environment and design type of the unit. For units op-erating at deeper waters where the first natural periods are in arange with significant wave energy, e.g. for natural periodshigher than 3 s, the dynamic structural response need to be con-sidered in the fatigue analysis.

H 300 Worldwide operation

301 For world wide operation the analyses shall be undertak-en utilising environmental data (e.g. scatter diagram, spec-trum) given in Classification Note 30.5. The North Atlanticscatter diagram shall be utilised.

H 400 Restricted operation401 The analyses shall be undertaken utilising relevant sitespecific environmental data for the area(s) in which the unitwill be operated. The restrictions shall be described in the Op-eration Manual for the unit.

I. Accidental Conditions

I 100 General

101 Satisfactory protection against accidental damage is tobe obtained by the following two means:

— low damage probability— acceptable damage consequences.

102 The capability of the structure to redistribute loadsshould be considered when designing the structure. The struc-tural integrity is to be intact and should be analysed for the fol-lowing damaged conditions:

— removal of one node, for lattice type legs— fracture of primary girder in the upper hull.

After damage requiring immediate repair, the unit is to resistfunctional and environmental loads corresponding to a returnperiod of one year.

103 Analysis as stated is to satisfy relevant strength criteriagiven in this standard. The damage consequences of other ac-cidental events shall be specially considered in each case ap-

plying an equivalent standard of safety.

Guidance note:

Energy absorption by impact types of accidental events requiresthe structure to behave in a ductile manner. Measures to obtainadequate ductility are:

- make the strength of connections of primary members in ex-cess of that of the member

- provide redundancy in the structure, so that alternate load re-distribution paths may be developed

- avoid dependence on energy absorption in slender memberswith a non-ductile post buckling behaviour

- avoid pronounced weak sections and abrupt change instrength or stiffness

- use non-brittle materials.


104 The loads and consequential damage due to accidentalevents such as:

— collision— dropped objects (e.g. from crane handling)— fire— explosion— unintended flooding during transit

are not to cause loss of floatability or capsizing during transit,on-bottom instability in operation or survival conditions, pol-lution or loss of human life. Requirements for compartmenta-tion and stability are given in DNV-OS-C301.

105 Generic design accidental loads are given in DNV-OS-A101. An analysis, proving that the minimum design loads canbe applied after damage, shall be performed.

I 200 Collisions

201 Collision by a supply vessel against a leg of a self-ele-vating unit or jack-up is to be considered for all elements thatmay be hit either by sideways, bow or stern collision. The ver-tical extent of the collision zone is to be based on the depth anddraught of visiting supply vessels.

Guidance note:

Simplified procedures for calculation of vessel impact on self-el-evating unit or jack-up legs may be found in Classification Note31.5.


202 A collision will normally only cause local damage of theleg, however, the global strength of the unit shall also bechecked. With lattice type legs the damaged chord or bracingand connections are assumed to be non-effective for check of




– Sec.12

residual strength of the unit after collision.

203 Assessment of dynamic effects and non-linear structuralresponse (geometrical and material) shall be performed as partof the impact evaluation.

I 300 Dropped objects

301 Critical areas for dropped objects shall be determined on

the basis of the actual movement of potential dropped objects(e.g. crane or other lifting operation mass) relative to the struc-ture of the unit itself. Where a dropped object is a relevant ac-cidental event, the impact energy shall be established and thestructural consequences of the impact assessed.

302 A dropped object against a chord or bracing will normal-ly cause complete failure of the element or its connections.These parts are assumed to be non-effective for the check of the residual strength of the unit after dropped object impact.

303 Critical areas for dropped objects are to be determinedon the basis of the actual movement of loads assuming a min-imum drop direction within an angle with the vertical direc-tion:

— 5 degrees in air— 15 degrees in water.

Dropped objects are to be considered for vital structural ele-ments of the unit within the areas given above.

I 400 Fires

401 The structure that is subjected to a fire shall have suffi-cient structural capacity before evacuation has occurred. Thefollowing fire scenarios shall be considered:

— jet fires— fire inside or on the hull— fire on the sea surface.

402 Further requirements concerning accidental limit stateevents involving fire is given in DNV-OS-A101.

403 Assessment of fire may be omitted provided assump-tions made in DNV-OS-D301 are met.

I 500 Explosions

501 In respect to design, one or more of the following maindesign philosophies will be relevant:

— ensure that hazardous locations are located in unconfined(open) locations and that sufficient shielding mechanisms(e.g. blast walls) are installed

— locate hazardous areas in partially confined locations anddesign utilising the resulting, relatively small overpres-sures

— locate hazardous areas in enclosed locations and installpressure relief mechanisms (e.g. blast panels) and designfor the resulting overpressure.

502 As far as practicable, structural design accounting forlarge plate field rupture resulting from explosion actionsshould be avoided due to the uncertainties of the actions andthe consequences of the rupture itself.

503 Structural support of blast walls and the transmission of the blast action into main structural members shall be evaluat-ed when relevant. Effectiveness of connections and the possi-ble outcome from blast, such as flying debris, shall beconsidered.

I 600 Unintended flooding

601 Heeling of the unit, during transit condition, after dam-age flooding as described in DNV-OS-C301 shall be account-ed for in the structural strength. Maximum static allowableheel after accidental flooding is 17 degrees including the effectof wind. Structures that are wet when the static equilibrium an-

gle is achieved shall be checked for external water pressure.

602 The unit shall be designed for environmental conditioncorresponding to 1 year return period after damage flooding.

603 Local exceedance of the permissible load level is accept-able provided redistribution of forces due to yielding, bucklingand fracture is accounted for.

604 Wave pressure, slamming forces and green sea shall beaccounted for in all relevant areas. Local damage may be ac-cepted provided progressive structural collapse and damage of vital equipment is avoided.

605 Position of air-intakes and openings to areas with vitalequipment which need to be available during an emergency sit-uation e.g. emergency generators, shall be considered takinginto account the wave elevation in a 1 year storm.

J. Miscellaneous requirements

J 100 General

101 Some special items need to be considered in relation to

robust design and safe operation of self-elevating units or jack-ups. Further details may be found in Classification Note 30.5.

J 200 Pre-load capasity

201 Impact forces occurring during installation and retrievalconditions are to be satisfactorily accounted in the design. Ananalytical method is described in Classification Note 31.5Sec.5.8.

202 Units with separate footings which are designed for apinned leg-bottom connection are to have a capability to pre-load the legs up to at least 100% of the maximum design axialloads in the legs accounting for functional loads and environ-mental overturning loads.

For units that shall operate in soil conditions where exceedanceof the soil capacity will result in large penetrations, a pre-loadhigher than the maximum survival load case axial load will berequired. Examples of such soils are generally soft clays, orconditions where hard soils are underlain by softer soils andthere is a risk of a punch-through failure.

A recommended approach for determination of required pre-load is given in Classification Note 30.4.

203 Units with separate footings where the design is basedon a specified moment restraint of the legs at the sea bottom areto have a capability to pre-load the legs up to a level whichshall account for the maximum design axial loads in the legsdue to functional loads and environmental overturning loadsplus the specified moment restraint at the bottom.

In lieu of a detailed soil/structure interaction analysis the re-quired pre-load may in this case be determined by the follow-ing factor:

For cohesive soils, e.g. clay:

For cohesionless soils, e.g. sand:

FVP = minimum required pre-load on one leg

FVP

FV

----------1

12 A

πR2

-----------MU

FV

----------–

--------------------------------=

FVP

FV

----------1

1 2 A

πR2

-----------MU FV

----------–

--------------------------------

2

=




Sec.12 –

204 For cohesionless soils, the above requirement to pre-load capacity may be departed from in case where a jetting sys-tem is installed which will provide penetration to full soil con-tact of the total spud-can area.

205 The potential of scour at each location should be evalu-ated. If scour takes place, the beneficial effect of pre-loadingrelated to moment restraint capacity may be destroyed. At lo-cations with scour potential, scour protection should normallybe provided in order to rely on a permanent moment restraint.

J 300 Overturning stability

301 The safety against overturning is determined by theequation:

302 The stabilising moment due to functional loads shouldbe calculated with respect to the assumed axis of rotation.

For self-elevating units or jack-ups with separate footings theaxis of rotation may, in lieu of a detailed soil-structure interact-ing analysis, be assumed to be a horizontal axis intersecting theaxis of two of the legs. It may further be assumed that the ver-tical position of the axis of rotation is located at a distanceabove the spudcan tip equivalent to the lesser of:

— half the maximum predicted penetration or— half the height of the spudcan.

For self-elevating units or jack-ups with mat support, the loca-tion of the axis of rotation may have to be specially considered.

303 The overturning moment due to wind, waves and currentshould be calculated with respect to the axis of rotation definedin 102.

The overturning stability is to be calculated for the most unfa-vourable direction and combination of environmental andfunctional loads according to the load plan for the unit. The dy-namic amplification of the combined wave and current load ef-fect should be taken into account.

304 The lower ends of separate legs are to be prevented fromsideway slipping by ensuring sufficient horizontal leg and soil

support.

J 400 Air gap

401 Clearance between the hull structure and the wave crestis normally to be ensured for the operating position.

402 The requirement to the length of the leg is that the dis-tance between the lower part of the deck structure in the oper-ating position and the crest of the maximum design wave,including astronomical and storm tides, is not to be less than10% of the combined storm tide, astronomical tide and heightof the design wave above the mean low water level, or 1.2 m,whichever is smaller. Expected subsidence of the structure isto be taken into account.

403 Crest elevation above still water level is given in Fig.1.

404 The maximum design wave elevation applied for calcu-lation of air gap shall not include the kinematic reduction fac-tor that may be applied for wave force calculations as given inF604.

405 A smaller distance may be accepted if wave impact forc-es on the deck structure are taken into account in the strengthand overturning analysis.

406 Clearance between the structure and wave is to be en-sured in floating condition for appendices such as helicopterdeck, etc.

J 500 Structural detailing

501 In the design phase particular attention should be givento structural detailing, and requirements for reinforcement inareas that may be subjected to high local stresses, for example:

— critical connections (see Sec.2 B)

— locations that may be subjected to wave impact

— locations that may be subjected to accidental or operation-al damage.

502 In way of critical connections, continuity of strength isnormally to be maintained through joints with the axial stiffen-ing members and shear web plates being made continuous.Particular attention should be given to weld detailing and geo-metric form at the point of the intersections of the continuousplate fields with the intersecting structure.

FV = maximum axial force in the leg accounting forfunctional loads and environmental overturn-ing loads

MU = minimum moment restraint of the leg at theseabed

A = area of spud-can in contact with soil

R = equivalent radius of spud can contact area.

MO = overturning moment, i.e. caused by environ-mental loads

MS = stabilising moment, i.e. caused by functionalloads

γ s = safety coefficient against overturning

= 1.1.

γ sMs

MO

---------≤




– Sec.12

Figure 1Crest elevation




Sec.13 –

SECTION 13SPECIAL CONSIDERATIONS FOR TENSION LEG PLATFORMS (TLP)

A. General

A 100 Scope and application101 This standard provides requirements and guidance to thestructural design of TLPs, fabricated in steel, in accordancewith the provisions of this standard. The requirements andguidance documented in this standard are generally applicableto all configurations of tension leg platforms.

102 A tension leg platform (TLP) is defined as a buoyantunit connected to a fixed foundation by pre-tensioned tendons.The tendons are normally parallel, near vertical elements, act-ing in tension, which usually restrain the motions of the TLPin heave, roll and pitch. The platform is usually compliant insurge, sway and yaw. Fig.1 shows an example of a TLP con-figuration.

Figure 1Example of TLP configuration

103 A TLP is usually applied for drilling, production and ex-port of hydrocarbons. Storage may also be a TLP mission.

104 A TLP may be designed to function in different modes,

typically operation and survival. Also horizontal movement(e.g. by use of catenary or taut mooring) of TLP above wells

may be relevant. Limiting design criteria when going from onemode of operation to another shall be established.

105 The TLP unit should also be designed for transit reloca-tion, if relevant.

106 For novel designs, or unproved applications of designswhere limited, or no direct experience exists, relevant analysesand model testing shall be performed which clearly demon-strate that an acceptable level of safety can be obtained, i.e.safety level is not inferior to that obtained when applying thisstandard to traditional designs.

107 Requirements concerning riser systems are given inDNV-OS-F201.

108 In case of application of a catenary or taut mooring sys-tem in combination with tendons, see DNV-OS-E301.

109 Requirements related to floating stability (intact anddamaged) are given in DNV-OS-C301.

A 200 Description of tendon system

201 Individual tendons are considered within this standard asbeing composed of three major parts:

— interface at the platform— interface at the foundation (seafloor)— link between platform and foundation.

202 Tendon components at the platform interface shall ade-quately perform the following main functions:

— apply, monitor and adjust a prescribed level of tension tothe tendon— connect the tensioned tendon to the platform— transfer side loads and absorb bending moments or rota-

tions of the tendon.

203 Tendon components providing the link between the plat-form and the foundation consist of tendon elements (tubulars,solid rods etc.), termination at the platform interface and at thefoundation interface, and intermediate connections of cou-plings along the length as required. The intermediate connec-tions may take the form of mechanical couplings (threads,clamps, bolted flanges etc.), welded joints or other types of connections. Fig.2 shows an example of a TLP tendon system.




– Sec.13

Figure 2Example of TLP tendon system

204 Tendon components at the foundation interface shall ad-equately perform the following main functions:

a) Provide the structural connection between the tendon andthe foundation.

b) Transfer side loads and absorb bending moments, or rota-tions of the tendon.

205 The tendon design may incorporate specialised compo-nents, such as:

— corrosion-protection system components— buoyancy devices— sensors and other types of instrumentation for monitoring

the performance and condition of the tendons— auxiliary lines, umbilicals etc. for tendon service require-

ments and/or for functions not related to the tendons— provisions for tendons to be used as guidance structure for

running other tendons or various types of equipment— elastomeric elements.

B. Structural Categorisation, Material Selectionand Extent of NDT

B 100 General

101 Selection of materials and inspection principles shall bebased on a systematic categorisation of the structure accordingto the structural significance and the complexity of the joints

and connections as given in Sec.4.102 In addition to in-service operational phases, considera-tion shall be given to structural members and details utilisedfor temporary conditions, e.g. fabrication, lifting arrange-ments, towing and installation arrangements, etc.

103 For TLP structures, which are similar to column stabi-lised units, the structural categorisation and extent of inspec-tion for the structural components should follow therequirements as given in Sec.11. For TLPs, which are similarto deep draught floaters, the structural categorisation and ex-tent of inspection for the structural components should followthe requirements as given in Sec.14.

B 200 Structural categorisation

201 Application categories for structural components are de-fined in Sec.4. Structural members of foundations, tendons andtheir connections should be found in the following groups:

Special category

a) Tendon interfaces with the foundation and the TLP hull.

b) Complex tendon and tendon connections.

Primary category

a) Simple tendon and tendon connections.

b) Interface arrangements outside locations of complex con-nections including general stiffened plate fields (e.g. athull interface).

Secondary category

a) Normally no locations are relevant for tendons or tendoninterfaces.

B 300 Material selection

301 Material specifications shall be established for all struc-tural materials. Such materials shall be suitable for their in-tended purpose and have adequate properties in all relevantdesign conditions. Material selection shall be undertaken in ac-cordance with the principles given in Sec.4.

302 Examples of considerations with respect to structuralcategorisation of tendons and tendon interfaces are given in theFig.3 and Fig.4. These examples provide minimum require-

ments.303 The structural categorisation given in 200 assumes thatthe tendon system is demonstrated to have residual strength,and that the TLP structural system satisfies the requirements of the accidental damaged condition with failure of the tendon (ora connection in the tendon) as the defined damage. If this is notthe case, then structural category special shall be used for thetendon system and its connections.


B 400 Design temperatures

401 For TLPs, materials in structures above the lowest astro-nomical tide (LAT) shall be designed for service temperaturesdown to the lowest average daily atmospheric temperature for

the area(s) where the unit is to operate.402 Materials in structures below the LAT should be de-signed for service temperatures of 0°C. A higher service tem-perature may be used if adequate supporting data showsrelative to the lowest average temperature applicable to the rel-




Sec.13 –

evant actual water depths.

B 500 Inspection categories

501 Welding and the extent of non-destructive examinationduring fabrication, shall be in accordance with the require-ments given for the appropriate inspection category as definedin Sec.4.

502 Inspection categories provide requirements for the min-imum extent of required inspection. When considering the eco-nomic consequence that repair during in-service operation mayentail, for example, through complex connections with limitedor difficult access, it may be considered prudent engineeringpractice to require more demanding requirements for inspec-tion than the required minimum.

503 When determining the extent of inspection and the loca-tions of required NDT, in additional to evaluating design pa-rameters (for example fatigue utilisation), considerationshould be given to relevant fabrication parameters including:

— location of block (section) joints— manual versus automatic welding— start and stop of weld, etc.

The Fig.3 and Fig.4 show examples of structural categorisationand inspection category.

Figure 3Principles of the extent of special structure at tendon foundation

* Special if damaged condition is not fulfilled, see 303.

Figure 4Example of tendon connection

C. Design Criteria

C 100 General

101 The following basic design criteria shall be compliedwith for the TLP design:

a) The TLP shall be able to sustain all loads liable to occur

during all relevant temporary and operating design condi-tions for all applicable design conditions.

b) Direct wave loads on the deck structure should not occurin the operating condition. Direct wave loads on the deck structure may be accepted as an accidental condition pro-vided that such loads are adequately included in the de-sign.

c) Momentary (part of a high frequency cycle) loss of tendontension may be accepted provided it can be documentedthat there will be no detrimental effects on tendon systemand supporting (foundation and hull) structures.

102 Operating tolerances shall be specified and shall beachievable in practice. The most unfavourable operating toler-

ances should be included in the design. Active operation shallnot be dependent on high reliability of operating personnel inan emergency situation.

Guidance note:

Active operation of the following may be considered in an emer-gency situation, as applicable:

- ballast distribution- weight distribution- tendon tension- riser tension.


C 200 Design conditions

201 The structure shall be designed to resist relevant loadsassociated with conditions that may occur during all stages of the lifecycle of the unit. Such stages may include:

— fabrication— site moves




– Sec.13

— mating— sea transportation— installation— operation— decommissioning.

202 Structural design covering marine operation and con-struction sequences shall be undertaken in accordance withthis standard.

203 Marine operations may be undertaken in accordancewith the requirements stated in the DNV Rules for Planningand Execution of Marine Operations. All marine operationsshall, as far as practicable, be based upon well proven princi-ples, techniques, systems and equipment and shall be under-taken by qualified, competent personnel possessing relevantexperience.

204 Structural responses resulting from one temporary phasecondition (e.g. a fabrication or transportation operation) thatmay effect design criteria in another phase shall be clearly doc-umented and considered in all relevant design workings.

C 300 Fabrication301 The planning of fabrication sequences and the methodsof fabrication shall be performed. Loads occurring in fabrica-tion phases shall be assessed and, when necessary, the struc-ture and the structural support arrangement shall be evaluatedfor structural adequacy.

302 Major lifting operations shall be evaluated to ensure thatdeformations are within acceptable levels, and that relevantstrength criteria are satisfied.

C 400 Mating

401 All relevant load effects incurred during mating opera-tions shall be considered in the design process. Particular at-tention should be given to hydrostatic loads imposed during

mating sequences.

C 500 Sea transportation

501 A detailed transportation assessment shall be undertakenwhich includes determination of the limiting environmentalcriteria, evaluation of intact and damage stability characteris-tics, motion response of the global system and the resulting, in-duced load effects. The occurrence of slamming loads on thestructure and the effects of fatigue during transport phasesshall be evaluated when relevant.

502 In case of transportation (surface and subsurface) of ten-dons; this operation shall be carefully planned and analysed.Special attention shall be given to attachment or securing of buoyancy modules. Model testing shall be considered.

503 Satisfactory compartmentation and stability during allfloating operations shall be ensured.

504 All aspects of the transportation, including planning andprocedures, preparations, seafastenings and marine operationsshould comply with the requirements of the warranty authori-ty.

C 600 Installation

601 Installation procedures of foundations (e.g. piles, suc-tion anchor or gravity based structures) shall consider relevantstatic and dynamic loads, including consideration of the maxi-mum environmental conditions expected for the operations.

602 For novel installation activities (foundations and ten-dons), relevant model testing should be considered.

603 Tendon stand-off (pending TLP installation) phasesshall be considered with respect to loads and responses.

604 The loads induced by the marine spread mooring in-

volved in the operations, and the forces exerted on the struc-tures utilised in positioning the unit, such as fairleads and padeyes, shall be considered for local strength checks.

C 700 Decommissioning

701 Abandonment of the unit shall be planned for in the de-sign stage.

C 800 Design principles, tendons

801 Essential components of the tendon system shall be de-signed on the principle that, as far as practicable, they shall becapable of being inspected, maintained, repaired and/or re-placed.

802 Tendon mechanical components shall, as far as practica-ble, be designed “fail to safe”. Consideration shall be given inthe design to possible early detection of failure for essentialcomponents, which cannot be designed according to this prin-ciple.

803 Certain vital tendon components may, due to their spe-cialised and unproven function, require extensive engineeringand prototype testing to determine:

— confirmation of anticipated design performance— fatigue characteristics— fracture characteristics— corrosion characteristics— mechanical characteristics.

804 The tendon system and the securing or supporting ar-rangements shall be designed in such a manner that a possiblefailure of one tendon is not to cause progressive tendon failureor excessive damage to the securing or supporting arrangementat the platform or at the foundation.

805 A fracture control strategy should be adopted to ensureconsistency of design, fabrication and in service monitoring

assumptions. The objective of such a strategy is to ensure thatthe largest undetected flaw from fabrication of the tendons willnot grow to a size that could induce failure within the designlife of the tendon, or within the planned in-service inspectioninterval, within a reasonable level of reliability. Elements of this strategy include:

— design fatigue life— fracture toughness— reliability of inspection during fabrication— in-service inspection intervals and methods.

806 Fracture mechanics should be used to define allowableflaw sizes, estimate crack growth rates and thus help define in-

spection intervals and monitoring strategies.807 All materials liable to corrode shall be protected againstcorrosion. Special attention should be given to:

— local complex geometries— areas that are difficult to inspect and/or repair— consequences of corrosion damage— possibilities for electrolytic corrosion.

808 All sliding surfaces shall be designed with sufficient ad-ditional thickness against wear. Special attention should begiven to the following:

— cross-load bearings— seals— ball joints.

809 Satisfactory considerations shall be given to settlementor subsidence, which may be a significant factor in determin-ing tendon-tension adjustment requirements.




Sec.13 –

D. Design Loads

D 100 General

101 Design loads are, in general, defined in Sec.3. Guidanceconcerning load categories relevant for TLP designs are givenin 200.

D 200 Load categories

201 All relevant loads that may influence the safety of thestructure or its parts from commencement of fabrication to per-manent decommissioning should be considered in design. Thedifferent loads are defined in Sec.3.

202 For the deck and hull of the TLP, the loads are similar tothose described in Sec.11 for TLPs similar to column stabilisedunits. TLPs similar to deep draught floaters shall be designedwith loads as given in Sec.14. Loads are described in 101 and201 with the exception of the tendon loads (inclusive potentialringing and springing effects).

203 In relation to determination of environmental conditionsand loads, see Classification Note 30.5.

204 The wave loads on the tendons can be described as rec-ommended in Sec.3 (and Classification Note 30.5) for slenderstructures with significant motions.

205 The disturbance of wave kinematics from hull (columnsand pontoons) in relation to the riser system and tendons shallbe accounted for if it is of importance.

206 The earthquake loads at the foundation of the tendonsare described in Sec.3 and DNV-OS-C101 Sec.11.

207 The following loads should be considered:

— permanent loads— variable functional loads— environmental loads— deformation loads

— accidental loads.

208 For preliminary design stages it is recommended that"contingency factors" are applied in relation to permanentloads to reflect uncertainties in load estimates and centres of gravity.

209 "Contingency factors" should also be considered for ear-ly design stages in relation to variable functional loads, espe-cially for minimum facilities TLPs (e.g. TLWP and MiniTLP).

210 The environmental loads are summarised as:

— wind loads

— mean (sustained) wind— dynamic (gust) wind.

— wave and current loads

— loads on slender members— loads induced by TLP motions— slamming and shock pressure— wave diffraction and radiation— mean drift forces— higher order non-linear wave loads (slowly varying,

ringing and springing)— wave enhancement— vortex shedding effects.

— marine growth— snow and ice accumulation— direct ice loads (icebergs and ice flows)— earthquake— tidal and storm surge effects.

E. Global Performance

E 100 General

101 The selected methods of response analysis are depend-ent on the design conditions, dynamic characteristics, non-lin-earities in loads and response and the required accuracy in theactual design phase.

Guidance note:For a detailed discussion of the different applicable methods forglobal analysis of tension leg platforms, see API RP 2T.


102 The selected methods of analysis and models employedin the analysis shall include relevant non-linearities and mo-tion-coupling effects. The approximations, simplificationsand/or assumptions made in the analysis shall be justified, andtheir possible effects shall be quantified for example by meansof simplified parametric studies.

103 During the design process, the methods for analytical ornumerical prediction of important system responses shall beverified (calibrated) by appropriate model tests.

104 Model tests may also be used to determine specific re-sponses for which numerical or analytical procedures are notyet developed and recognised.

105 Motion components shall be determined, by relevantanalysis techniques, for those applicable design conditionsspecified in Sec.2. The basic assumptions and limitations asso-ciated with the different methods of analysis of global per-formance shall be duly considered prior to the selection of themethods.

106 The TLP should be analysed by methods as applicable tocolumn stabilised units or deep draught floaters when the unitis free floating, respectively. See Sec.11 or Sec.14.

107 The method of platform motion analysis as outlined inthis standard is one approximate method, which may be ap-plied. The designer is encouraged also to consider and applyother methods in order to discover the effects of possible inac-curacies etc. in the different methods.

E 200 Frequency domain analysis

201 Frequency domain HF, WF and LF analyses techniquesmay be applied for a TLP. Regarding load effects due to meanwind, current and mean wave drift, see Sec.3.

202 For typical TLP geometries and tendon arrangements,the analysis of the total dynamic load effects may be carriedout as:

— a high frequency (HF) analysis of springing— a wave frequency (WF) analysis in all six degrees of free-

dom— a low frequency (LF) analysis in surge, sway and yaw.

203 The following assumptions are inherent in adopting suchan independent analysis approach:

— the natural frequencies in heave, roll and pitch are includ-ed in the wave frequency analysis

— the natural frequencies in surge, sway and yaw are includ-ed in the low frequency analysis

— the high and low natural frequencies are sufficient separateto allow independent dynamic analysis to be carried out

— the low frequency excitation forces have negligible effecton the wave frequency motions

— the low frequency excitation forces have a negligible dy-

namic effect in heave, roll and pitch— tendon lateral dynamics are unimportant for platformsurge and sway motions.

204 Typical parameters to be considered for global perform-ance analyses are different TLP draughts, wave conditions and




– Sec.13

headings, tidal effects, storm surges, set down, foundation set-tlement (s), subsidence, mis-positioning, tolerances, tendonflooding, tendon removal and hull compartment(s) flooding.Possible variations in vertical centre of gravity shall also be an-alysed (especially if ringing responses are important). Thismay be relevant in case of:

— changes in topside weights (e.g. future modules)

— tendon system changes (altered utilisation)— changes in ballast weights and distributions.

E 300 High frequency analyses

301 Frequency domain springing analyses shall be per-formed to evaluate tendon and TLP susceptibility to springingresponses.

302 Recognised analytical methods exist for determinationof springing responses in tendons. These methods include cal-culation of quadratic transfer functions (QTFs) for axial ten-don (due to sum frequency loads on the hull) stresses which isthe basis for determination of tendon fatigue due to springing.

303 Damping level applied in the springing response analy-ses shall be duly considered and documented.

E 400 Wave frequency analyses

401 A wave frequency dynamic analysis may be carried outby using linear wave theory in order to determine first-orderplatform motions and tendon response.

402 First order wave load analyses shall also serve as basisfor structural response analyses. Finite wave load effects shallbe evaluated and taken into account. This may for example, beperformed by use of beam models and application of Morisonload formulation and finite amplitude waves.

403 In linear theory, the response in regular waves (transferfunctions) is combined with a wave spectrum to predict the re-sponse in irregular seas.

404 The effect of low-frequency set-down variations on theWF analysis shall be investigated by analysing at least two rep-resentative mean offset positions determined from the low fre-quency analysis.

405 Set-down or offset induced heave motion may be includ-ed in the wave frequency RAOs.

406 A sufficient number of wave approach headings shall beselected for analyses (e.g. with basis in global configuration,number of columns, riser configuration etc.).

407 In determination of yaw induced fatigue responses (e.g.tendon and flex element design) due account must be given towave spreading when calculating the long term responses.

E 500 Low frequency analyses

501 A low frequency dynamic analysis could be performedto determine the slow drift effects at early design stages due tofluctuating wind and second order wave loads.

502 Appropriate methods of analysis shall be used with se-lection of realistic damping levels. Damping coefficients forlow frequency motion analyses are important as the low fre-quency motion may be dominated by resonant responses.

E 600 Time domain analyses

601 For global motion response analyses, a time domain ap-proach will be beneficial. In this type of analyses it is possibleto include all environmental load effects and typical non-lineareffects such as:

— hull drag forces (including relative velocities)— finite wave amplitude effects— non-linear restoring (tendons, risers).

602 Highly non-linear effects such as ringing may also re-

quire a time domain analysis approach. Analytical methods ex-ist for estimation of ringing responses. These methods can beused for the early design stage, but shall be correlated againstmodel tests for the final design. Ringing and springing re-sponses of hull and deck may however be analysed within thefrequency domain with basis in model test results, or equiva-lent analytical results.

603 For deep waters, a fully coupled time domain analysis of tendons, risers and platform may be required. This may for ex-ample, be relevant if:

— model basin scale will not be suitable to produce reliabledesign results or information

— consistent global damping levels (e.g. in surge, sway andyaw) due to the presence of slender structures (risers, ten-dons) are needed

— it is desirable to perform the slender structure responseanalyses with basis in coupled motion analyses.

604 A relevant wave spectrum shall be used to generate ran-dom time series when simulating irregular wave elevations andkinematics.

605 The simulation length shall be long enough to obtainsufficient number of LF maxima (surge, sway, and yaw).

606 Statistical convergence shall be checked by performingsensitivity analyses where parameters as input seed, simulationlength, time step, solution technique etc. are varied.

607 Determination of extreme responses from time domainanalyses shall be performed according to recognised princi-ples.

608 Depending on selected TLP installation method, timedomain analyses will probably be required to simulate the sit-uation when the TLP is transferred from a free floating modeto the vertical restrained mode. Model testing shall also be con-sidered in this context.

Guidance note:

Combined loading

Common practice to determine extreme responses has been toexpose the dynamic system to multiple stationary design envi-ronmental conditions. Each design condition is then described interms of a limited number of environmental parameters (e.g. Hs,Tp) and a given seastate duration (3 to 6 hours). Different combi-nations of wind, wave and current with nearly the same return pe-riod for the combined environmental condition are typicallyapplied.

The main problem related to design criteria based on environ-mental statistics is that the return period for the characteristicload effect is unknown for non-linear dynamic systems. This willin general lead to an inconsistent safety level for different designconcepts and failure modes.

A more consistent approach is to apply design based on responsestatistics. Consistent assessment of the D-year load effect will re-quire a probabilistic response description due to the long-termenvironmental loads on the system. The load effect with a returnperiod of D-year, denoted xD, can formally be found from thelong-term load effect distribution as:

The main challenge related to this approach is to establish thelong-term load effect distribution due to the non-linear behav-iour. Design based on response statistics is in general the recom-mended procedure and should be considered wheneverpracticable for consistent assessment of characteristic load ef-fects.

ND = total number of load effect maxima during Dyears

Fx( x) = long-term peak distribution of the (generalised)load effect

Fx xD( ) 11

ND

--------–=




Sec.13 –

Further details may be found in Appendices to DNV-OS-F201.


E 700 Model testing

701 Model testing will usually be required for final check of TLP designs. The main reason for model testing is to check

that analytical results correlate with model tests.702 The most important parameters to evaluate are:

— air gap— first order motions— total offset— set-down— WF motions versus LF motions— tendon responses (maximum, minimum)— accelerations— ringing— springing.

703 The model scale applied in testing shall be appropriatesuch that reliable results can be expected. A sufficient numberof seastates needs to be calibrated covering the relevant designconditions.

704 Wave headings and other variable parameters (waterlevels, vertical centre of gravity, etc.) need to be varied andtested as required.

705 If HF responses (ringing and springing) shows to begoverning for tendon extreme and fatigue design respectively,the amount of testing may have to be increased to obtain con-fidence in results.

E 800 Load effects in the tendons

801 Load effects in the tendons comprise mean and dynamiccomponents.

802 The steady-state loads may be determined from the equi-librium condition of the platform, tendon and risers.

803 Tendon load effects arise from platform motions, anyground motions and direct hydrodynamic loads on the tendon.

804 Dynamic analysis of tendon responses shall take into ac-count the possibility of platform heave, roll and pitch excita-tion (springing and ringing effects).

805 Linearised dynamic analysis does not include some of the secondary wave effects, and may not model accurately ex-treme wave responses. A check of linear-analysis results usingnon-linear methods may be necessary. Model testing may alsobe used to confirm analytical results. Care shall be exercised ininterpreting model-test results for resonant responses, particu-larly for loads due to platform heave, roll and pitch, sincedamping may not be accurately modelled.

806 Lift and overturning moment generated on the TLP bywind loads shall be included in the tendon response calcula-tions.

807 Susceptibility to vortex induced vibrations shall be eval-uated in operational and non-operational phases.

808 Interference (tendon and riser, tendon and tendon, ten-don and hull, tendon and foundation) shall be evaluated fornon-operational as well as the operational phase.

F. Structural Strength

F 100 General

101 General considerations in respect to methods of analysisand capacity checks of structural elements are given in Sec.5.

102 The TLP hull shall be designed for the loading condi-

tions that will produce the most severe load effects on thestructure. A dynamic analysis shall be performed to derivecharacteristic largest stresses in the structure.

103 Analytical models shall adequately describe the relevantproperties of loads, stiffness and displacement, and shall ac-count for the local and system effects of, time dependency,damping and inertia.

F 200 Hull

201 The following analysis procedure to obtain characteris-tic platform-hull response shall be applied:

Analysis of the initial mean position

In this analysis, all vertical loads are applied (masses, liveloads, buoyancy etc.) and equilibrium is achieved taking intoaccount pretension in tendons and risers.

Mean offset

In this analysis the lateral mean wind, mean wave-drift andcurrent loads are applied to the TLP resulting in a static offsetposition with a given set-down.

Design wave analysis

To satisfy the need for simultaneity of the responses, a designwave approach may be used for maximum stress analysis.

The merits of the stochastic approach are retained by using theextreme stochastic values of some characteristic parameters inthe selection of the design wave and is applied to the platformin its offset position. The results are superimposed on thesteady-state solution to obtain maximum stresses.

Spectral analysis

Assuming the same offset position as described under meanoffset and with a relevant storm spectrum, an analysis is carriedout using ‘n’ wave frequencies from ‘m’ directions. Tradition-al spectral analysis methods should be used to compute the rel-

evant response spectra and their statistics.202 For a TLP hull, the following characteristic global sec-tional loads due to wave forces shall be considered as a mini-mum, see also Sec.11:

— split forces (transverse, longitudinal or oblique sea for oddcolumned TLPs)

— torsional moment about a transverse and longitudinal, hor-izontal axis (in diagonal or near-diagonal)

— longitudinal opposed forces between parallel pontoons (indiagonal or near-diagonal seas)

— longitudinal, transverse and vertical accelerations of deck masses.

203 It is recommended that a full stochastic wave load anal-

ysis is used as basis for the final design.204 Local load effects (e.g. maximum direct environmentalload on an individual member, wave slamming loads, externalhydrostatic pressure, ballast distribution, internal tank pres-sures etc.) shall be considered. Additional loads from for ex-ample, high-frequency ringing accelerations shall be taken intoaccount.

F 300 Structural analysis

301 For global structural analysis, a complete three-dimen-sional structural model of the TLP is required. See Sec.5 andAppendix B.

302 Additional detailed finite-element analyses may be re-quired for complex joints and other complicated structural

parts to determine the local stress distribution more accuratelyand/or to verify the results of a space-frame analysis, see alsoSec.11.

303 Local environmental load effects, such as wave slam-ming and possible wave- or wind-induced vortex shedding,




– Sec.13

shall be considered as appropriate.

F 400 Structural design

401 Special attention shall be given to the structural designof the tendon supporting structures to ensure a smooth transferand redistribution of the tendon concentrated loads through thehull structure without causing undue stress concentrations.

402 The internal structure in columns in way of bracingsshould be designed stronger than the axial strength of the brac-ing itself.

403 Special consideration shall be given to the pontoonstrength in way of intersections with columns, accounting forpossible reduction in strength due to cut-outs and stress con-centrations.

F 500 Deck

501 Structural analysis and design of deck structure shall fol-low the principles as outlined in Sec.11, additional load effects(e.g. global accelerations) from high-frequency ringing andspringing shall be taken into account when relevant.

502 In the operating condition, positive air gap should be en-

sured. However, wave impact may be permitted to occur onany part of the structure provided that it can be demonstratedthat such loads are adequately accounted for in the design andthat safety to personnel is not significantly impaired.

503 Analysis undertaken to document air gap should be cal-ibrated against relevant model test results. Such analysis shallinclude relevant account of:

— wave and structure interaction effects— wave asymmetry effects— global rigid body motions (including dynamic effects)— effects of interacting systems (e.g. riser systems)— maximum and minimum draughts (set-down, tidal surge,

subsidence, settlement effects).

504 Column ‘run-up’ load effects shall be accounted for inthe design of the structural arrangement in way of the columnand deck box connection. These 'run-up' loads should be treat-ed as an environmental load component, however, they neednot be considered as occurring simultaneously with other envi-ronmental responses.

505 Evaluation of air gap adequacy shall include considera-tion of all influenced structural items including lifeboat plat-forms, riser balconies, overhanging deck modules etc.

F 600 Extreme tendon tensions

601 As a minimum the following tension components shallbe taken into account:

— pretension (static tension at MSL)— tide (tidal effects)— storm surge (positive and negative values)— tendon weight (submerged weight)— overturning (due to current, mean wind or drift load)— set-down (due to current, mean wind or drift load)— WF tension (wave frequency component)— LF tension (wind gust and slowly varying drift)— ringing (HF response).

602 Additional components to be considered are:

— margins for fabrication, installation and tension readingtolerances

— operational requirements (e.g. operational flexibility of

ballasting operations)— allowance for foundation mis-positioning— field subsidence— foundation settlement and uplift.

603 Bending stresses along the tendon shall be analysed and

taken into account in the design. For the constraint mode thebending stresses in the tendon will usually be low. In case of surface, or subsurface, tow (non-operational phase) the bend-ing stresses shall be carefully analysed and taken into accountin the design.

604 For nearly buoyant tendons the combination of environ-mental loads (axial and bending) and high hydrostatic water

pressure may be a governing combination (buckling).605 Limiting combinations (envelopes) of tendon tensionand rotations (flex elements) need to be established.

606 For specific tendon components such as couplings, flexelements, top and bottom connections etc. the stress distribu-tion shall be determined by appropriate finite-element analy-sis.

607 If temporary (part of a high frequency cycle) tendon ten-sion loss is permitted, tendon dynamic analyses shall be con-ducted to evaluate its effect on the complete tendon system andsupporting structures. Alternatively, model tests may be per-formed. The reasoning behind this is that loss of tension couldresult in detrimental effects from tendon buckling and/or dam-age to flex elements.

F 700 Structural design of tendons

701 The structural design of tendons shall be carried out ac-cording to this standard with the additional considerations giv-en in this subsection.

702 Buckling checks of tendon body may be performed ac-cording to API RP 2T.

703 When deriving maximum stresses in the tendons rele-vant stress components shall be superimposed on the stressesdue to maximum tendon tension, minimum tendon tension ormaximum tendon angle, as relevant.

704 Such additional stress components may be:

— tendon-bending stresses due to lateral loads and motionsof the tendon— tendon-bending stresses due to flex-element rotational

stiffness— thermal stresses in the tendon due to temperature differ-

ences over the cross sections— hoop stresses due to hydrostatic pressure.

F 800 Foundations

801 Foundation design may be carried out according toDNV-OS-C101 Sec.11.

802 Relevant combinations of tendon tensions and angles of load components shall be analysed for the foundation design.

803 For gravity foundations the pretension shall be compen-

sated by submerged weight of the foundation, whereas the var-ying loads may be resisted by for example suction and friction.

G. Fatigue

G 100 General

101 Structural parts where fatigue may be a critical mode of failure shall be investigated with respect to fatigue. All signif-icant loads contributing to fatigue damage (non-operationaland operational) shall be taken into account. For a TLP, the ef-fects of springing and ringing resonant responses shall be con-sidered for fatigue.

102 Fatigue design may be carried out by methods based onfatigue tests and cumulative damage analysis, methods basedon fracture mechanics, or a combination of these.

103 General requirements and guidance to fatigue design aregiven in Sec.7 and DNV-RP-C203.




Sec.13 –

104 Careful design of details as well as stringent quality re-quirements for fabrication is essential in achieving acceptablefatigue strength. It shall be ensured that the design assumptionsmade concerning these parameters are achievable in practice.

105 The results of fatigue analyses shall be fully consideredwhen the in-service inspection plans are developed for the plat-form.

G 200 Hull and deck

201 Fatigue design of hull or deck structure shall be per-formed in accordance with principles given in Sec.11 orSec.14, as appropriate.

G 300 Tendons

301 All parts of the tendon system shall be evaluated for fa-tigue.

302 First order wave loads (direct or indirect) will usually begoverning, however also fatigue due to springing shall be care-fully considered and taken into account. HF and WF tendon re-sponses shall be combined realistically.

303 In case of wet transportation (surface or subsurface) to

field, these fatigue contributions shall be accounted for in de-sign.

304 Vortex induced vibrations shall be considered and takeninto account. This applies to operation and non-operational(e.g. tendon stand-off) phases.

305 Series effects (welds, couplings) shall be evaluated.

306 When fracture-mechanics methods are employed, realis-tic estimates of strains combined with maximum defect sizeslikely to be missed with the applicable NDT methods shall beused.

G 400 Foundation

401 Tendon responses (tension and angle) will be the maincontributors to fatigue design of foundations. Local stressesshall be determined by use of finite element analyses.

H. Accidental Condition

H 100 Hull

101 Requirements concerning accidental events are given inSec.7 and Sec.11.

102 Units shall be designed to be damage tolerant, i.e. cred-ible accidental damage, or events, should not cause loss of glo-bal structural integrity. The capability of the structure toredistribute loads should be considered when designing thestructure.

103 In the design phase, attention shall be given to layoutand arrangements of facilities and equipment in order to mini-mise the adverse effects of accidental events.

104 Satisfactory protection against accidental damage maybe obtained by a combination of the following principles:

— reduction of the probability of damage to an acceptablelevel

— reduction of the consequences of damage to an acceptablelevel.

105 Structural design in respect to the accidental conditionshall involve a two-stage procedure considering:

— resistance of the structure to a relevant accidental event— capacity of the structure after an accidental event.

106 Global structural integrity shall be maintained both dur-ing and after an accidental event. Loads occurring at the timeof a design accidental event and thereafter shall not cause com-plete structural collapse.

107 Requirements for compartmentation and stability in thedamage condition are given in DNV-OS-C301. When the deck structure becomes buoyant in satisfying requirements for dam-age stability, consideration shall be given to the structural re-sponse resulting from such loads.

H 200 Hull and deck

201 The most relevant accidental events for hull and deck designs are:

— dropped objects— fire— explosion— collision— unintended flooding— abnormal wave events.

202 Compartmentation is a key issue for TLP’s due to thefine balance between weight, buoyancy and pretensions. SeeDNV-OS-C301.

H 300 Tendons

301 The most relevant accidental events for the tendons are:

— missing tendon— tendon flooding— dropped objects— flooding of hull compartment(s).

302 Missing (e.g. due to change-out, or inspection) tendonrequires analysis with environmental loads with 10-2 annualprobability of exceedance to satisfy the accidental criteria. Thesame applies to tendon flooding, if relevant.

303 For accidental events leading to tendon failure, the pos-sible detrimental effect of the release of the elastic energystored in the tendon may have on the surrounding structureshall be considered.

304 Dropped objects may cause damage to the tendons and

in particular the top and bottom connectors may be exposed.Shielding may be required installed.

305 Flooding of hull compartments and the effects on designshall be analysed thoroughly.

H 400 Foundations

401 Accidental events to be considered for the foundationsshall as a minimum be those listed for tendons.




– Sec.14

SECTION 14SPECIAL CONSIDERATIONS FOR DEEP DRAUGHT FLOATERS (DDF)

A. General

A 100 Introduction101 A deep draught floater (DDF) is categorised with a rela-tive large draught. This large draught is mainly introduced toobtain sufficiently high eigenperiod in heave and reducedwave excitation in heave such that resonant responses in heavecan be omitted or minimised.

102 A DDF can have multi vertical columns, single columnwithout, or with (e.g. classic and truss spar) moonpool.

103 The unit is usually kept in position by a passive mooringsystem. The mooring system may also be activated in case of horizontal movements above wells (drilling riser placed verti-cally above well).

104 The deck or topside solution may be modular, or inte-

grated type.

A 200 Scope and application

201 The DDF unit may be applied for drilling, production,export and storage.

202 A DDF unit may be designed to function in differentmodes, typically operational (inclusive horizontal movementabove wells) and survival. Limiting design criteria when goingfrom one mode of operation to another shall be established.

203 The DDF unit should also be designed for transit reloca-tion, if relevant.

204 For novel designs, or unproved applications of designswhere limited, or no direct experience exists, relevant analyses

and model testing shall be performed which clearly demon-strate that an acceptable level of safety can be obtained, i.e.safety level is not inferior to that obtained when applying thisstandard to traditional designs.

205 Requirements concerning mooring and riser systems arenot considered in this standard. See DNV-OS-E301 and DNV-OS-F201.

206 Requirements related to floating stability is given inDNV-OS-C301.

B. Non-Operational Phases

B 100 General

101 In general the unit shall be designed to resist relevantloads associated with conditions that may occur during allphases of the life cycle of the unit. Such phases may include:

— fabrication— load-out, load-on— sea transportation (wet or dry)— assembly of hull main sections— installation (dynamic upending, launching, deck mating,

jacking)— relocation (drilling mode, new site)— decommissioning.

102 Structural design covering marine operations and con-

struction sequences shall be undertaken in accordance withthis standard.

103 Marine operations may be undertaken in accordancewith the requirements stated in the DNV Rules for Planningand Execution of Marine Operations.

104 All marine operations shall, as far as practicable, bebased upon well proven principles, techniques, systems and

equipment and shall be undertaken by qualified, competentpersonnel possessing relevant experience.

105 Structural responses resulting from one temporary phasecondition (e.g. construction or assembly, or transportation)that may effect design criteria in another phase shall be clearlydocumented and considered in all relevant design workings.

B 200 Fabrication

201 The planning of fabrication sequences and the methodsof fabrication shall be performed. Loads occurring in fabrica-tion phases shall be assessed and, when necessary, the struc-ture and the structural support arrangement shall be evaluatedfor structural adequacy.

202 Major lifting operations shall be evaluated to ensure that

deformations are within acceptable levels, and that relevantstrength criteria are satisfied.

B 300 Mating

301 All relevant load effects incurred during mating opera-tions shall be considered in the design process. Particular at-tention should be given to hydrostatic loads imposed duringmating sequences.

B 400 Sea transportation

401 A detailed transportation assessment shall be undertakenwhich includes determination of the limiting environmentalcriteria, evaluation of intact and damage stability characteris-tics, motion response of the global system and the resulting, in-

duced load effects. The occurrence of slamming loads on thestructure and the effects of fatigue during transport phasesshall be evaluated when relevant.

402 Satisfactory compartmentation and stability during allfloating operations shall be ensured.

403 All aspects of the transportation, including planning andprocedures, preparations, seafastenings and marine operationsshould comply with the requirements of the warranty authori-ty.

B 500 Installation

501 Installation procedures of foundations (e.g. piles, suc-tion anchor or gravity based structures) shall consider relevantstatic and dynamic loads, including consideration of the maxi-

mum environmental conditions expected for the operations.502 For novel installation activities, relevant model testingshould be considered.

503 The loads induced by the marine spread mooring in-volved in the operations, and the forces exerted on the struc-tures utilised in positioning the unit, such as fairleads and padeyes, shall be considered for local strength checks.

B 600 Decommissioning

601 Abandonment of the unit shall be planned for in the de-sign stage.

C. Structural Categorisation, Selection of Mate-rial and Extent of Inspection

C 100 General

101 Application categories for structural components are de-




Sec.14 –

fined in Sec.4. For novel designs of DDF, the structural cate-gorisation shall be based on the definition in Sec.4.

102 Structural members of a DDF of caisson type are nor-mally found in the following group:

Special category

a) Portions of deck plating, heavy flanges, and bulkheads

within the structure, which receive major concentratedloads.

b) External shell structure in way of highly stressed connec-tions to the deck structure.

c) Major intersections of bracing members.

d) External brackets, portions of bulkheads, and frameswhich are designed to receive concentrated loads at inter-sections of major structural members.

e) Highly stressed elements of anchor line fairleads, cranepedestals, flare boom etc. and their supporting structure.

Primary category

a) Deck plating, heavy flanges, transverse frames, stringers,and bulkhead structure, which do not receive major con-centrated loads.

b) Moonpool shell.

c) External shell and diagonal and horizontal braces.

d) Bulkheads, decks, stiffeners and girders which provide lo-cal reinforcement or continuity of structure in way of in-tersections, except areas where the structure is consideredspecial application.

e) Main support structure of heavy substructures and equip-ment, e.g. anchor line fairleads, cranes, drill floor sub-structure, lifeboat platform, thruster well and helicopterdeck.

Secondary category

a) Upper platform decks, or decks of upper hulls except areaswhere the structure is considered primary or special appli-cation.

b) Bulkheads, stiffeners, flats or decks and girders, diagonaland horizontal beam columns, which are not considered asprimary or special application.

c) Non-watertight bulkheads internal outfitting structure ingeneral, and other non-load bearing components.

d) Certain large diameter vertical columns with low length todiameter ratios, except at intersections.

C 200 Material selection

201 Material specifications shall be established for all struc-tural materials utilised in a DDF unit. Such materials shall besuitable for their intended purpose and have adequate proper-ties in all relevant design conditions. Material selection shallbe undertaken in accordance with the principles given in Sec.4.

202 When considering criteria appropriate to material gradeselection, adequate consideration shall be given to all relevantphases in the life cycle of the unit. In this connection there maybe conditions and criteria, other than those from the in-service,operational phase, that provide the design requirements in re-spect to the selection of material. (Such criteria may, for exam-ple, be design temperature and/or stress levels during marineoperations.)

203 In structural cross-joints essential for the overall struc-tural integrity where high tensile stresses are acting normal tothe plane of the plate, the plate material shall be tested to provethe ability to resist lamellar tearing (Z-quality).


C 300 Design temperatures

301 External structures above the inspection waterline shallbe designed for service temperatures down to the lowest dailymean temperature for the area(s) where the unit is to operate.

302 External structures below the inspection waterline neednormally not be designed for service temperatures lower than0°C.

303 Internal structures are assumed to have the same servicetemperature as the adjacent external structure if not otherwisedocumented.

304 Internal structures in way of permanently heated roomsneed normally not be designed for service temperatures lowerthan 0°C.

C 400 Inspection categories

401 Welding, and the extent of non-destructive examinationduring fabrication, shall be in accordance with the require-ments stipulated for the structural categorisation as defined inSec.4.

402 Inspection categories determined in accordance withSec.4 provide requirements for the minimum extent of re-quired inspection. When considering the economic conse-quence that repair during in-service operation may entail, forexample, through complex connections with limited or diffi-cult access, it may be considered prudent engineering practiceto require more demanding requirements for inspection thanthe required minimum.

403 When determining the extent of inspection and the loca-tions of required NDT, in addition to evaluating design param-eters (for example fatigue utilisation), consideration should begiven to relevant fabrication parameters including:

— location of block (section) joints— manual versus automatic welding— start and stop of weld etc.

C 500 Guidance to minimum requirements

501 The Fig.1 illustrates minimum requirements for selec-tion of the structural category for one example of structuralconfigurations of a DDF unit. The indicated structural catego-risation should be regarded as guidance of how to apply therecommendations in Sec.4.




– Sec.14

Figure 1Example of typical structural categorisation in the hard tank area

D. Design Loads

D 100 Permanent loads

101 The type and use of permanent ballast (e.g. within softtank of DDF units) for stability reasons must be carefully eval-uated with respect to long term effects related to corrosion,wash out etc.

D 200 Variable functional loads

201 All relevant combinations of filling of hard tanks for theoperation phase shall be taken into account in design.

202 Hydrostatic or hydrodynamic differential pressures act-ing on the hull or buoyancy tanks during launch and upendingsequences shall be analysed or determined and taken into ac-count in design of the hull.

203 All relevant combinations of differential pressures due

to filling of ballast tanks, produced fluids, compressed air etc.shall be taken into account in design.

D 300 Environmental loads

301 If sufficient environmental data is available, environ-mental joint probability models may be developed and appliedin the design of DDF units. This is especially important in ar-eas with for example high loop current and frequently occur-ring hurricanes.

302 Due to the geometry (deep draft and large volume) of DDF units the current loadings may be of high importance fordesign of mooring and riser systems and in relation to VIV(hull, risers) hence attention must be put on the description of magnitude and distribution of current with depth.

D 400 Determination of loads

401 Calculation of hydrodynamic loads may be carried outaccording to Classification Note 30.5.

402 Hydrodynamic model tests should be carried out to:

— confirm that no important hydrodynamic feature has beenoverlooked (for new type of units, environmental condi-tions, adjacent structures, Mathieu instability etc.)

— support theoretical calculations when available analyticalmethods are susceptible to large uncertainties (e.g. in eval-uating the need of VIV suppression devices (typicallystrakes on DDF hull))

— verify theoretical methods and models on a general basis.

403 Wind tunnel tests should be performed when:

— wind loads are significant for overall stability, motions orstructural response

— there is a danger of dynamic instability.

404 Models applied in model tests shall be sufficient (rea-sonable scale and controllable scaling effects) to represent theactual unit. The test set-up and registration system shall pro-vide a sound basis for reliable, repeatable interpretations.

405 A correlation report (tests and calculations) shall be pre-pared for validation purposes (design documentation).

D 500 Hydrodynamic loads

501 Resonant excitation (e.g. internal moonpool resonance,sloshing and roll and pitch resonance) shall be carefully eval-uated. Wave on deck via moonpool has to be considered forDDF concepts with relatively short distances between moon-pool and the outer wave active zone.

502 If hydrodynamic analyses of a DDF are performed withthe moonpool 'sealed' at the keel level it must be validated thatthe results are equivalent to 'open' DDF hydrodynamic analy-ses. Special focus should be placed on the heave motion pre-diction (important for riser system) by using consistent addedmass, total damping and excitation forces such that the eigen-period and response in heave can be determined correctly.

503 In case of a truss DDF with damping and added mass

plates and where it is possible that resonant, or near resonantheave motion may occur, the theoretical predictions should bevalidated against model test results.

504 If VIV suppression devices (e.g. spiral strakes) are at-tached to the hull, the increased loads (drag, inertia) must betaken into account. This applies to the operational as well asnon-operational phases.

505 Simulation of loads and responses on risers in the moon-pool area shall be carried out according to a recognised code.

Guidance note:

DNV-OS-F201 may be applied for this purpose.


D 600 Combination of environmental loads601 In areas with high current (e.g. loop current, or high sub-surface current) special attention must be given to the joint oc-currence of wind, waves and current. Joint probability models(loads and load effects) are recommended.

If not more accurate are available, the combination of environ-mental loads may be taken according to Sec.3 Table F1.

E. Load Effect Analysis in Operational Phase

E 100 General

101 Global, dynamic motion response analysis taking into

account loads from wind (static and gust), waves (wave fre-quency and low frequency) and current shall be performed.Time domain analysis is the preferred option.

102 Coupled analyses may be performed for DDF units inorder to determine the coupling effects due to the presence of




Sec.14 –

mooring and risers. These coupled analyses will mainly pro-vide viscous damping estimates for slowly varying motions(all six degrees of freedom). When utilising viscous dampingestimates from coupled analyses the actual riser installationprogram must be taken into consideration.

103 Depending on actual water depth, dimensions and ge-ometry and mooring system, DDF units will typically experi-ence the following eigenmodes or eigenperiods:

— surge or sway; 120 to 200 s— heave; 20 to 35 s— roll or pitch; 50 to 90 s

The simulation length for determination of the different loadeffects must be sufficient such that reliable extreme responsestatistics can be obtained.

Guidance note:

Combined loading

Common practice to determine extreme responses has been toexpose the dynamic system to multiple stationary design envi-ronmental conditions. Each design condition is then described interms of a limited number of environmental parameters (e.g. Hs,Tp) and a given seastate duration (3 to 6 hours). Different combi-

nations of wind, wave and current with nearly the same return pe-riod for the combined environmental condition are typicallyapplied.

The main problem related to design criteria based on environ-mental statistics is that the return period for the characteristicload effect is unknown for non-linear dynamic systems. This willin general lead to an inconsistent safety level for different designconcepts and failure modes.

A more consistent approach is to apply design based on responsestatistics. Consistent assessment of the D-year load effect will re-quire a probabilistic response description due to the long-termenvironmental loads on the system. The load effect with a returnperiod of D-year, denoted xD, can formally be found from thelong-term load effect distribution as:

The main challenge related to this approach is to establish thelong-term load effect distribution due to the non-linear behav-iour. Design based on response statistics is in general the recom-mended procedure and should be considered wheneverpracticable for consistent assessment of characteristic load ef-fects.

Further details may be found in Appendices to DNV-OS-F201.


E 200 Global bending effects

201 Global bending and shear forces along the length of thestructure due to environmental load effects shall be deter-mined. This applies to first order wave effects, as well as P-del-ta effects due to platform heel or tilt.

202 Global bending and shear forces in the hull will be influ-enced by the non-linear restoring effect from the mooring sys-tem. This additional load effect shall be analysed and takeninto account in design of the hull structure.

F. Load Effect Analysis in Non-OperationalPhases

F 100 General

101 All temporary phases shall be carefully evaluated and

sufficient level and amount of analyses shall be performed ac-cording to this standard. Further details regarding non-opera-tional conditions may be found in the DNV Rules for Planningand Execution of Marine Operations.

F 200 Transportation

201 In case of wet tow in harsh environment (e.g. overseas),model tests shall be performed as a supplement to motion re-sponse analyses. Non-linear effects (e.g. slamming, globalbending or shear, green seas) shall be taken into account.

202 Motion response analyses shall be performed for drytransports on for example heavy lift vessel, or barge. Specialattention to:

— roll motions (roll angles, accelerations, viscous roll damp-ing)

— slamming pressures and structural responses— global strength (vessel, DDF unit)— strength of sea-fastening— stability, overhang.

F 300 Launching

301 Launching may be an alternative way of installation orupending a DDF (e.g. truss spar). Model testing of the launchprocess may be required if there is limited or no experiencewith such operations for similar concepts.

F 400 Upending

401 Pre-upending phases shall be analysed with respect toglobal bending moments and shear forces in the hull. In case of wave load effects in this pre-upending phase may be relevant,this shall be analysed and taken into account.

402 In case of dynamic upending, analyses shall be per-formed in order to determine global and local load effects inthe DDF unit with its appurtenances.

403 Hydrostatic or hydrodynamic differential (outside and

inside) pressures during dynamic upending shall be deter-mined and further used in design of the hull structure.

404 Model testing of the dynamic upending may be avoidedif the applied simulation software has been validated againstsimilar or relevant operations and showing good correlation.

405 In case of lift assisted upending offshore, the limitingenvironmental criteria must be carefully selected. Dynamicanalyses of the system (lift vessel, lifting gear, DDF unit) willbe required in order to determine responses in lifting gear andDDF unit.

F 500 Deck mating

501 Offshore installation of deck structure and modules willrequire refined analyses in order to determine the governing re-sponses. This applies to lifting operations as well as float-overoperations with barge. Important factors are limiting environ-mental criteria, impact responses and floating stability require-ments.

502 Floating concepts (such as jack-ups) utilising jacking of legs to desired draft and subsequent deballasting to obtain suf-ficient air-gap, shall be carefully evaluated or analysed with re-spect to limiting environmental criteria.

G. Structural Strength

G 100 Operation phase for hull

101 For global structural analysis, a complete three-dimen-sional structural model of the unit is required. This may be acomplete shell type model, or a combined shell and space-frame model.

102 Additional detailed finite-element analyses may be re-

Fx( x) = long-term peak distribution of the (generalised)load effect

ND = total number of load effect maxima during Dyears

Fx xD( ) 1

1

ND--------–=




– Sec.14

quired for complex joints and other complicated structuralparts (e.g. fairlead area, hard tank area, column and brace con-nections, strake terminations and interactions, deck and hullconnections, riser frame and hull connections, curved flanges)to determine the local stress distribution more accurately.

Guidance note:

In order to be able to assess the effect of all possible tank filling

configurations, a local FEM-model covering the hard tank areamay be utilised. Only those tanks used in the normal operation of the platform shall as a minimum be modelled. The stresses froma local FEM-model should be superimposed to global stresses.


103 The additional global bending and shear due to P-deltaand mooring restoring effects shall be combined with first or-der wave effects in a consistent way.

104 The same applies to combining the loads from the riserson riser frames in the moonpool and transfer into the hull struc-ture. Horizontal forces as well as vertical (friction from risersystem) shall be taken into account.

105 If VIV suppression devices (e.g. strakes) are installed,both local (direct wave and current loads) and global bendingeffects should be considered in design of the suppression de-vices.

G 200 Non-operational phases for hull

201 Finite element analyses will be required performed foroverseas wet tow and dry tow in harsh environment.

202 For dry tow this implies that the complete structural sys-tem (hull sections, sea-fastening, transport vessel) shall bemodelled such that reliable stress-distributions can be ob-tained.

203 For wet tow in harsh environment special emphasis mustbe put on the simulation or modelling of the hydrodynamicwave pressures or accelerations acting on the wet hull struc-ture. Further the non-linear hogging and sagging bending orshear effects due to the shape of the hull should be properlysimulated or accounted for in the design.

204 The level or amount of finite element analyses for theupending process needs to be evaluated. As a minimum, thefollowing considerations shall be made:

a) Global bending moments and shear forces to be compared(location and level) for the operational phase and pre-up-ending or dynamic upending.

b) Possibilities for local and global buckling (e.g. skirt areafor a classic spar) due to global load effects and lateral dif-ferential pressures needs to be assessed or analysed.

G 300 Operation phase for deck or topside

301 Structural analysis of deck structure shall, in general,follow the same principles as outlined for the hull.

302 Horizontal accelerations at deck level due to wave load-ing will be high for some DDF units in harsh environment. De-tailed FEM analyses of the deck and hull connections shall beperformed in such instances.

G 400 Non-operational phases for deck or topside

401 Typical non-operational phases as fabrication, transpor-tation and installation of deck and topside modules shall be as-sessed and analysed to a sufficient level such that the actualstress level can be determined and further used in the designchecks.

H. Fatigue

H 100 General

101 Criteria related to DFFs are given in Sec.6.

102 DNV-RP-C203 presents recommendations in relation tofatigue analyses based on fatigue tests and fracture mechanics.

H 200 Operation phase for hull

201 First order wave actions will usually be the dominatingfatigue component for the hull in harsh environment. The longterm distribution of wave induced stress fluctuations need to bedetermined with basis in the same type of load effect and finiteelement analyses as for strength analysis.

Guidance note:

Early phase evaluation or analysis of fatigue may incorporatemodelling the hull as a beam with associated mass distributionand simulation of wave actions according to Morison formula-tion, or preferably, performing a radiation or diffraction analysis.

Final documentation related to first order wave induced fatiguedamage should incorporate a stochastic approach. This impliesestablishing stress transfer functions, which are combined with

relevant wave spectra (scatter diagram) in order to obtain long-term distribution of stresses. The stress transfer functions shouldbe obtained from FEM analyses with appropriate simulation of wave loads (radiation or diffraction analysis). The P-delta effectdue to platform roll and pitch must be taken into account.


202 As for strength assessments, the P-delta effect due toplatform roll or pitch shall be taken into account. This impliesthat both first order and second order, slowly varying roll orpitch motions need to be considered and taken into account if contributing to fatigue damage in the hull.

203 For special fatigue sensitive areas, local stress concen-trations shall be determined by detailed finite element analy-

ses.204 Typical fatigue sensitive areas for DDF units will be:

— hull and deck connections— collision ring area— hull and deck and stiffener connections at location of peak

wave induced global bending moments— fairlead area— hard tank area— column and brace connections— strake and hull connections and strake terminations— riser frame and hull connections— hard tank and truss spar connections— tubular joints.

205 Fatigue analyses shall be performed to check that thehull strakes have sufficient fatigue lives. Relative motions be-tween the hull and disturbed wave kinematics around strakesmust be properly taken into account. Hydrodynamic pressuresfrom a radiation and diffraction analysis in combination with aMorison formulation (inertia and drag) will be sufficient to de-scribe the environmental loads on the strakes.

206 Vortex induced vibration (VIV) load effects from moor-ing system (global hull cross-flow motions) into the fairlead orhull areas shall be outlined and taken into account if signifi-cant. The same applies to VIV load effects from riser systeminto the riser frame or hull areas.

207Allowance for wear and tear shall be taken into accountin areas exposed to e.g. friction and abrasion. For a DDF unit

this will typically be interfaces between hull and risers (keellevel, intermediate riser-frames, deck level). These relativemotions are caused by movements of the unit and risers andsubsequent pull-out and push-up of the risers in the moonpool.




Sec.14 –

H 300 Non-operational phases for hull

301 Wet, overseas transports in harsh environment will re-quire quite detailed analyses to determine the fatigue damageduring this temporary phase. Both global and local wave loadeffects shall be taken into account. Some level of monitoringof weather and load effects during towage will be requiredsuch that it is possible to recalculate the actual fatigue contri-bution during wet tow.

302 Dry, overseas transports will usually be less exposed tofatigue damage. It is however, required almost the same levelof FE analyses as for wet tow in order to determine the stressfluctuations in hull, sea-fastenings and transport.

H 400 Splash zone

401 The definition of ‘splash zone’ as given Sec.10 B200, re-lates to a highest and lowest tidal reference. For DDF units, forthe evaluation of fatigue, reference to the tidal datum should besubstituted by reference to the draught that is intended to beutilised when condition monitoring shall be undertaken. Therequirement that the extent of the splash zone is to extend 5 mabove and 4 m below this draught may then be applied.

Guidance note:If significant adjustment in draught is possible in order to providefor satisfactory accessibility in respect to inspection, mainte-nance and repair, a sufficient margin in respect to the minimuminspection draught should be considered when deciding upon theappropriate design fatigue factors. As a minimum this marginshall be at least 1 m, however it is recommended that a larger val-ue is considered especially in the early design stages where suf-ficient reserve should be allowed for to account for designchanges (mass and centre of mass of the unit). Considerationshould further be given to operational requirements that may lim-it the possibility for ballasting and deballasting operations.

When considering utilisation of remotely operated vehicle(ROV) inspection, consideration should be given to the limita-tions imposed on such inspection by the action of water particlemotion (e.g. waves). The practicality of such a consideration may

be that effective underwater inspection by ROV, in normal seaconditions, may not be achievable unless the inspection depth isat least 10 m below the sea surface.


H 500 Operation phase for deck or topside

501 Wave induced horizontal accelerations and P-delta ef-fects will usually be governing for fatigue design of deck struc-ture and topside modules and shall be duly taken into account.

502 A stochastic approach is the preferred option for deter-mination of final fatigue damage for the deck or topside. SeeGuidance Note to 201 for the hull.

503 Deck and hull connections, joints in deck structure,

module supports etc. will typically be fatigue sensitive areas.The amount or level of detailed FE analyses for these jointsneed to be considered. For the deck and hull connection somelevel or amount of detailed FE analyses shall be performed, atleast for units located in harsh environment.

H 600 Non-operational phases for deck or topside

601 Fatigue damage of deck structure and topside modulesshall be documented if the stress fluctuations in the differentphases are significant.

I. Accidental Condition

I 100 General101 The objective of this subsection is to provide supple-mental guidance related to design for accidental condition asoutlined in Sec.7.

102 Units shall be designed to be damage tolerant, i.e. cred-

ible accidental damage, or events, should not cause loss of glo-bal structural integrity. The capability of the structure toredistribute loads should be considered when designing thestructure.

I 200 Fire

201 Deck area will be limited for some DDF concepts. Po-tential fire scenarios shall therefore be carefully consideredand taken into account in design and layout planning.

I 300 Explosion

301 As for fire, the limiting deck space and protected moon-pool area (potential gas or oil leakage) for some DDF units re-quire that explosions are carefully considered in the designprocess.

302 In respect to design considering loads resulting from ex-plosions one, or a combination of the following main designphilosophies are relevant:

a) Ensure that the probability of explosion is reduced to a lev-el where it is not required to be considered as a relevant de-sign loadcase.

b) Ensure that hazardous areas are located in unconfined(open) locations and that sufficient shielding mechanisms(e.g. blast walls) are installed.

c) Locate hazardous areas in partially confined locations anddesign utilising the resulting, relatively small overpres-sures.

d) Locate hazardous areas in enclosed locations and installpressure relief mechanisms (e.g. blast panels) and designfor the resulting overpressure.

303 As far as practicable, structural design accounting forlarge plate field rupture resulting from explosion loads shouldnormally be avoided due to the uncertainties of the loads andthe consequence of the rupture itself.

Structural support of blast walls, and the transmission of theblast load into main structural members shall be evaluatedwhen relevant. Effectiveness of connections and the possibleoutcome from blast, such as flying debris, shall be considered.

I 400 Collision

401 Safety assessments shall be the basis for determinationof type and size of colliding vessel and impact speed.

402 Collision impact shall be considered for all elements of the unit, which may be impacted by sideways, bow or sterncollision. The vertical extent of the collision zone shall bebased on the depth and draught of attending vessels and the rel-ative motion between the attending vessels and the unit.

403 Resistance to unit collisions may be accounted for by in-direct means, such as, using redundant framing configurations,collision ring in splash zone and materials with sufficienttoughness in affected areas.

I 500 Dropped objects

501 Critical areas for dropped objects shall be determined onthe basis of the actual movement of potential dropped objects(e.g. crane actions) relative to the structure of the unit itself.Where a dropped object is a relevant accidental event, the im-pact energy shall be established and the structural consequenc-es of the impact assessed.

502 Generally, dropped object assessment will involve thefollowing considerations:

a) Assessment of the risk and consequences of dropped objects impacting topside, wellhead, and riser system inmoonpool and safety systems and equipment. The assess-ment shall identify the necessity of any local structural re-inforcement or protections to such arrangements.




– Sec.14

b) Assessments of the risk and consequences of dropped objects impacting externally on the hull structure (shell, orbracings) and hull attachments such as strakes, fairleadsand pipes. The structural consequences are normally fullyaccounted for by the requirements for watertight compart-mentation and damage stability and the requirement forstructural redundancy of slender structural members.

I 600 Unintended flooding601 A procedure describing actions to be taken after relevantunintended flooding shall be prepared. Unintended filling of hard tanks, collision ring and bracings for a DDF will be themost relevant scenarios for the operation phase.

602 It must be ensured that counter-filling of tanks and unituprighting can be performed safely and without delays.

603 Structural aspects related to the tilted condition andcounter-flooding (if relevant) shall be investigated. This ap-plies to the complete unit including risers and mooring system.

604 If the unit can not be brought back to the design draughtand verticality by counter-ballasting and redistribution of bal-last water, this must be taken into account in design of the unit.

I 700 Abnormal wave events

701 Abnormal wave effects are partly related to air-gap andwave exposure to deck or topside structures. Consequencesfrom such wave impacts shall be evaluated and taken into ac-count in design of the relevant structural parts.

702 In areas with hurricanes, special considerations have tobe made with respect to selection of relevant sea states to beapplied in design of the unit.




App.A –

APPENDIX ACROSS SECTIONAL TYPES

A. Cross Sectional Types

A 100 General101 Cross sections of beams are divided into different typesdependent of their ability to develop plastic hinges as given inTable A1.

Figure 1Relation between moment M and plastic moment resistance Mp,and rotation θ for cross sectional types. My is elastic moment re-sistance

102 The categorisation of cross sections depends on the pro-portions of each of its compression elements, see Table A3.

103 Compression elements include every element of a crosssection which is either totally or partially in compression, due

to axial force or bending moment, under the load combinationconsidered.

104 The various compression elements in a cross sectionsuch as web or flange, can be in different classes.

105 The selection of cross sectional type is normally quotedby the highest or less favourable type of its compression ele-ments.

A 200 Cross section requirements for plastic analysis

201 At plastic hinge locations, the cross section of the mem-ber which contains the plastic hinge shall have an axis of sym-metry in the plane of loading.

202 At plastic hinge locations, the cross section of the mem-ber which contains the plastic hinge shall have a rotation ca-pacity not less than the required rotation at that plastic hingelocation.

A 300 Cross section requirements when elastic globalanalysis is used

301 When elastic global analysis is used, the role of crosssection classification is to identify the extent to which the re-sistance of a cross section is limited by its local buckling resist-ance.

302 When all the compression elements of a cross section aretype III, its resistance may be based on an elastic distributionof stresses across the cross section, limited to the yield strength

at the extreme fibres.

Table A1 Cross sectional types

I Cross sections that can form a plastic hinge with the rotationcapacity required for plastic analysis

II Cross sections that can develop their plastic moment resist-ance, but have limited rotation capacity

III Cross sections where the calculated stress in the extremecompression fibre of the steel member can reach its yieldstrength, but local buckling is liable to prevent developmentof the plastic moment resistance

IV Cross sections where it is necessary to make explicit allow-ances for the effects of local buckling when determiningtheir moment resistance or compression resistance

Table A2 Coefficient related to relative strain

NV Steel grade 1) ε 2)

NV-NS 1

NV-27 0.94

NV-32 0.86

NV-36 0.81

NV-40 0.78

NV-420 0.75

NV-460 0.72

NV-500 0.69

NV-550 0.65

NV-620 0.62

NV-690 0.58

1) The table is not valid for steel with improved weldability. SeeSec.4, Table D1, footnote 1).

2)

ε235

f y

--------- where f y is yield strength=




– App.A

Table A3 Maximum width to thickness ratios for compression elements

Cross section part Type I Type II Type III

d = h - 3 tw3)

d / tw ≤ 33 ε d / tw ≤ 38 ε d / tw≤ 42 ε

d / tw ≤ 72 ε 2) d / tw ≤ 83 ε d / tw ≤ 124 ε

when α > 0.5:

when α ≤ 0.5:

when α > 0.5:

when α ≤ 0.5:

when ψ > -1:

when ψ ≤ -1:

d / tp ≤ 50 ε 2 d / tp ≤ 70 ε 2 d / tp ≤ 90 ε 2

1) Compression negative

2) ε is defined in Table A23) Valid for rectangular hollow sections (RHS) where h is the height of the profile

4) C is the buckling coefficient. See e.g. Classification Note 30.1, Table 3.2, No. 4 and 7 or Eurocode 3 Table 5.3.3 (denoted k σ )

5) Valid for axial and bending, not external pressure.

d tw ⁄ 396ε

13α 1–----------------≤

d tw ⁄ 36εα

--------≤

d tw ⁄ 456ε

13α 1–-------------------≤

d tw ⁄ 41.5ε

α--------------≤

d tw ⁄ 126ε

2 ψ +-------------≤

d tw ⁄ 62ε 1 ψ –( ) ψ ≤

Rolled: c tf ⁄ 10ε α ⁄ ≤

Welded: c tf ⁄ 9ε α ⁄ ≤

Rolled: c tf ⁄ ( ) 11ε≤

Welded: c tf ⁄ ( ) 10ε≤

Rolled: c tf ⁄ 15ε≤

Welded: c tf ⁄ ( ) 14ε≤

Rolled: c tf ⁄ 10ε α ⁄ ≤


Rolled: c tf ⁄ ( ) 10ε α ⁄ ≤


Rolled: c tf ⁄ ( ) 23ε C 4 )

≤

Welded: c tf ⁄ 21ε C≤

Rolled: c tf ⁄ ( )10ε

α α------------≤

Welded: c tf ⁄ 9ε

α α------------≤

Rolled: c tf ⁄ ( )11ε

α α------------≤

Welded: c tf ⁄ ( )10ε

α α------------≤

Rolled: c tf ⁄ ( ) 23ε C≤

Welded: c tf ⁄ 21ε C≤




App.B –

APPENDIX BMETHODS AND MODELS FOR DESIGN OF COLUMN STABILISED UNITS

A. Methods and Models

A 100 General101 The guidance given in this appendix is normal practicefor methods and models utilised in design of typical columnstabilised units i.e. ring-pontoon design and two-pontoon de-sign.

102 Table A1 gives guidance on methods and models nor-mally applied in the design of typical column stabilised units.For new designs deviating from well-known designs, e.g. bythe slenderness of the structure and the arrangement of the loadbearing elements, etc., the relevance of the methods and mod-els should be considered.

A 200 World wide operation

201 Design for world wide operation shall be based on the

environmental criteria, e.g. North Atlantic scatter diagram giv-en in Classification Note 30.5.

202 The simplified fatigue method described in Sec.5 maybe utilised with a Weibull parameter of 1.1 in combination

with a contingency factor of 1.1. For units intended to operatefor a longer period, see definition “Y” below, the simplified fa-tigue method should be verified by a stochastic fatigue analysisof the most critical details.

A 300 Benign waters or restricted areas

301 Design for restricted areas or benign waters shall bebased on site specific environmental data for the area(s) theunit shall operate.

302 The simplified fatigue method described in Sec.7 maybe utilised with a Weibull parameter calculated based on sitespecific criteria.

303 When a simplified fatigue method is utilised, a contin-gency factor of 1.1 shall be applied to the response amplitude.

Table B1 Methods and models which should be used for design of typical column stabilised units

Two-pontoon semisubmersible Ring-pontoon semisubmersible

Hydrodynamicmodel, Morison

Global structuralstrength model

Fatigue method Hydrodynamicmodel, Morison

Global structuralstrength model

Fatigue method

Harsh environment orWorldwide

X 1 4 6 1 5 7

Y 1 4 7 1 5 7

Benign waters or re-stricted areas

X 2 3 6 1 5 7

Y 1 4 6 1 5 7

Definitions

X-unit following normal class survey intervals (survey in sheltered waters or drydock every 4 to 5 years).Y-unit located for a longer period on location – surveys carried out in-water at location.

Hydrodynamic models

1) Hybrid model - Sink-source and/or Morison (when relevant, for calculation of drag forces).

2) Morison model with contingency factor 1.3 for strength and 1.1 for fatigue.

Global structural models

3) Beam model.

4) Combined beam and shell model. The extent of the beam and shell models may vary depending on the design. For typical beam structures a beam modelalone may be acceptable.

5) Complete shell model.

Fatigue method

6) Simplified fatigue analysis. Contingency factor of 1.1 shall be applied, as given in Sec.7 A402.

7) Stochastic fatigue analysis, based on a screening process with simplified approach to identify critical details.

Harsh environment or Worldwide

— Units (X) designed for operation based on world wide requirements given in Classification Note 30.5.

— Units (Y) designed for operation based on site specific requirements.

Benign waters or restricted areas

— Units (X) designed for operation based on site specific criteria for benign waters or restricted areas.

— Units (Y) designed for operation based on site specific criteria for benign waters or restricted areas.




– App.C

APPENDIX CPERMANENTLY INSTALLED UNITS

A. Introduction

A 100 Application101 The requirements and guidance given in this Appendixare supplementary requirements for units that are intended tostay on location for prolonged periods, normally more than 5years.

102 Permanently located units shall be designed for site spe-cific environmental criteria for the area(s) the unit will be lo-cated.

B. Inspection and Maintenance

B 100 Facilities for inspection on location

101 Inspections may be carried out on location based on ap-proved procedures outlined in a maintenance system and in-spection arrangement, without interrupting the function of theunit. The following matters should be taken into considerationto be able to carry out condition monitoring on location:

— arrangement for underwater inspection of hull, propellers,thrusters and openings affecting the unit’s seaworthiness

— means of blanking of all openings— marking of the underwater hull— use of corrosion resistant materials for propeller— accessibility of all tanks and spaces for inspection— corrosion protection— maintenance and inspection of thrusters— ability to gas free and ventilate tanks— provisions to ensure that all tank inlets are secured during— inspection— testing facilities of all important machinery.

C. Fatigue

C 100 Design fatigue factors

101 Design Fatigue Factors (DFF) are introduced as fatiguesafety factors. DFF shall be applied to structural elements ac-cording to the principles in Sec.7. See also Fig.1.

Figure 1Example illustrating considerations relevant for selection of DFF in a typical section

102 Fatigue safety factors applied to the unit will be depend-ent on the accessibility for inspection and repair with specialconsiderations in the splash zone, see 200.

C 200 Splash zone for floating units

201 For fatigue evaluation of floating units, reference to thed h h i i d d b ili d d i di i i

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