Review of the Performance of High Strength Steels Used Offshore

8/20/2019 Review of the Performance of High Strength Steels Used Offshore

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HSEHealth & Safety

Executive

Review of the performance of highstrength steels used offshore

Prepared by Cranfield Universityfor the Health and Safety Executive 2003

RESEARCH REPORT 105


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HSEHealth & Safety

Executive

Review of the performance of highstrength steels used offshore

Professor J Billingham, Professor J V Sharp,Dr J Spurrier and Dr P J Kilgallon

School of Industrial and Manufacturing ScienceCranfield University

CranfieldBedfordshire

MK43 0AL

High strength steels (yield strength >500MPa to typically 700MPa) are increasingly being used in

offshore structural applications including production jack-ups with demanding requirements. They offer

a number of advantages over conventional steels, particularly where weight is important. This review

considers the types of steel used offshore, their mechanical properties, their weldability and their

suitability for safe usage offshore in terms of fracture, fatigue, static strength, cathodic protection and

hydrogen embrittlement performance. In addition, this review addresses the performance of high

strength steels at high temperatures and at high strain rates. It outlines the difficulties in working with

the very limited published codes and standards and discusses performance in the field. Current design

restrictions such as limits on yield ratios, susceptibility to hydrogen cracking including the influence of

SRBs, and the management of the behaviour of such steels in seawater under cathodic protection

conditions are discussed. Recommendations are made to encourage the wider use of high strength

steels in the future and areas where further study is required are identified.

This report and the work it describes were funded by the Health and Safety Executive (HSE). Itscontents, including any opinions and/or conclusions expressed, are those of the authors alone and do

not necessarily reflect HSE policy.

HSE BOOKS


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© Crown copyright 2003

First published 2003

ISBN 0 7176 2205 3

All rights reserved. No part of this publication may bereproduced, stored in a retrieval system, or transmitted inany form or by any means (electronic, mechanical,photocopying, recording or otherwise) without the priorwritten permission of the copyright owner.

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

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CONTENTS Page

NOMENCLATURE.................................................................................... vii

SUMMARY................................................................................................ ix

1.INTRODUCTION.................................................................................. 1

2. THEUSEOFHIGHSTRENGTHSTEELSOFFSHORE....................... 3

3. MECHANICALPROPERTIESOFHIGHSTRENGTHSTEELS........... 6

3.1SteelTypeandProcessRoute.................................................... 63.2 MetallurgicalandCompositionalConsiderations..................... 63.3YieldRatioConsiderations......................................................... 7

4. CODESANDSTANDARDS................................................................. 234.1 AllProperties............................................................................... 234.2Fatigue.......................................................................................... 244.3FractureToughness..................................................................... 244.4HydrogenCracking...................................................................... 244.5DefectAcceptanceCriteria.......................................................... 25

4.6CorrosionProtection................................................................... 25

4.7StaticStrengthofTubularJoints................................................ 254.8 ImpactProperties......................................................................... 264.9HighTemperatureProperties...................................................... 26

5. FABRICATIONANDWELDING.......................................................... 29

6. TOUGHNESS....................................................................................... 346.1 DuctiletoBrittleTransition........................................................ 346.2 CharpyV-NotchValuesandHighStrengthSteel..................... 356.3 FractureMechanicsValues....................................................... 366.4 FractureToughnessofHighStrengthSteels............................ 376.5 FlawAssessmentConsiderationsforHighStrengthSteels.... 396.6 SummaryofToughnessConsiderations................................... 40

7. FATIGUEINHIGHSTRENGTHSTEELS........................................... 467.1 Introduction................................................................................. 467.2 FatigueCrackPropagation........................................................ 46

7.2.1 EffectofSteelStrengthonFatigueCrackGrowthRate 467.2.2 ParentMaterials............................................................... 477.2.3 HAZ................................................................................... 47

7.2.4 WeldMetals...................................................................... 477.2.5 FatigueThresholds.......................................................... 47

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7.3 EffectofSRBandSulphides...................................................... 477.4 SNData........................................................................................ 487.5 FatigueImprovementTechniques............................................. 487.6 Summary..................................................................................... 49

8. CATHODICPROTECTION................................................................... 648.1 Introduction................................................................................. 648.2 ProtectionCriteria....................................................................... 648.3 AvoidingOverprotectionProblems........................................... 658.4 Summary..................................................................................... 66

9. HYDROGENEMBRITTLEMENT.......................................................... 709.1 Introduction................................................................................. 709.2 SourcesofHydrogeninSteelsExposedtotheMarine

Environment................................................................................ 709.3 RecentLiteratureReview........................................................... 71

9.4 EffectofWelding......................................................................... 729.5 HETesting................................................................................... 729.6 HydrogenEmbrittlementTestResults...................................... 729.7 Summary..................................................................................... 74

10.HIGHTEMPERATUREPROPERTIES................................................ 8110.1 Summary................................................................................... 82

11.HIGHSTRAINRATES........................................................................ 8411.1 Summary................................................................................... 85

12.FIELDPERFORMANCEOFFSHORE................................................. 8812.1 Introduction............................................................................... 8812.2 ProductionJack-ups................................................................. 88

12.2.1 BPHarding..................................................................... 8812.2.2 Siri.................................................................................. 8812.2.3HangTuahACEPlatform.............................................. 8912.2.4 Elgin-Franklin................................................................ 89

12.3 DrillingJack-ups....................................................................... 8912.4 TensionLegPlatforms............................................................. 91

12.5 Summary................................................................................... 91

13.INSPECTIONANDREPAIR............................................................... 9413.1 Summary.................................................................................. 94

14. DESIGNRESTRICTIONS................................................................... 9614.1 BucklingofMembers............................................................... 9614.2 StaticCapacityofTubularJoints............................................ 9614.3 DraftISOStandardRecommendationsforHigh

StrengthSteels......................................................................... 96

15. SUMMARYANDCONCLUSIONS..................................................... 99

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APPENDIX1–OTHERSTRUCTURALAPPLICATIONSOFHIGHSTRENGTHSTEELS–BOLTS&

THREADEDFASTENERS................................................ 102

APPENDIX3............................................................................................. 104

APPENDIX6–FRACTURETOUGHNESSCONCEPTS.......................... 105A6.1 DuctiletoBrittleTransition:AnIntroductiontoFracture.... 105A6.2 CharpyV-NotchValues:Background................................... 105A6.3 CharpyVNotchValuesforHighStengthSteel.................... 107A6.4 FractureMechanicsTests:Background............................... 108

A6.4.1 BrittleMaterials........................................................ 108A6.4.2 DuctileMaterials....................................................... 109

A6.5 FractureMechanicsValuesforHighStrengthSteels.......... 110A6.6FlawAssessmentConsiderationsforHighStrengthSteels 111

APPENDIX9 ............................................................................................ 117A9.1 SlowStrainRateTesting......................................................... 117A9.2 FractureMechanicsTesting................................................... 117

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NOMENCLATURE

a Cracklengthparameter

B ThicknessoffracturespecimenCEorCE Carbonequivalent

CP Cathodicprotection

CR Controlledrolled

CTOD Cracktipopeningdisplacement

Cv Charpyimpactenergy

� Cracktipopeningdisplacementvalue

da/dN Crackgrowthrate

� Stressintensityfactorrange

� Young’smodulus

�� Embrittlementindex

FCAW Fluxcoredarcwelding

FCGR Fatiguecrackgrowthrate

FMD Floodedmemberdetection

HAC Hydrogenassistedcracking

HAZ Heataffectedzone

HE Hydrogenembrittlement

HIC Hydrogeninducedcracking

HSLA Highstrengthlowalloy

HSS Highstrengthsteels

HV Vickershardness

ICCP Impressedcurrentcathodicprotection

ISO InternationalStandardsOrganisation

J JoulesKapp Appliedstressintensityfactor

Kc Apparenttoughness

K1A Arresttoughness

K1c Materialtoughness

K1D Stressintensityfactortokeepcrackin

motion

KISCC Fracturetoughnessunderconditionsof

stresscorrosioncracking

Kth ThresholdvalueofK

LBZ Localbrittlezones

MAC Martensite-austenitecontentMMA Manualmetalarc

MPa MegaPascals

Q&T Quenchandtempered

�y Yieldstress

Rcurve Resistancecurve(energyperunitareaof

crackextension)

Re Specifiedmin.yieldstrength(infracture

toughnessequations)

SACP Sacrificalanodecathodicprotection

SAW Submergedarcwelding

SMYS Specifiedminimumyieldstrength

S-N Stressversusno.ofcyclesinfatigue

SRB Sulphatereducingbacteria

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SSCC Sulphidestresscorrosioncracking

SSRT Slowstrainratetesting

TLP Tensionlegplatform

TMCP Thermomechanicallycontrolled

processing

TMCR ThermomechanicallycontrolledrollingTT Ductiletobrittletransitiontemperature

UKCS UKContinentalshelf

UTS Ultimatetensilestrength

� Poisson’sratio

YR Yieldratio(�y /UTS)

YS Yieldstrength

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SUMMARY

High strength steels (yield strength >500MPa to typically 700MPa) are increasingly being

used in offshore structural applications including production jack-ups with demanding

requirements. Theyofferanumberofadvantagesoverconventionalsteels,particularlywhere

weightisimportant.Thisreviewconsidersthetypesofsteelusedoffshore,theirmechanicalproperties,theirweldabilityandtheirsuitabilityforsafeusageoffshoreintermsoffracture,

fatigue, static strength, cathodic protection and hydrogen embrittlement performance. In

addition, thisreviewaddresses theperformanceofhigh strengthsteels athightemperatures

andathighstrainrates.Itoutlinesthedifficultiesinworkingwiththeverylimitedpublished

codesandstandardsanddiscussesperformancein thefield.Currentdesignrestrictionssuch

aslimitsonyieldratios,susceptibilitytohydrogencrackingincludingtheinfluenceofSRBs,

and themanagement of the behaviour of such steels in seawater undercathodicprotection

conditions are discussed. Recommendations are made to encourage the wider use of high

strengthsteelsinthefutureandareaswherefurtherstudyisrequiredareidentified.

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1. INTRODUCTION

Fixedoffshorestructures areconventionallyconstructed frommediumgradestructural steels,with

yieldstrengthstypicallyintherangeof350MPa.Thesesteelsarewelldocumentedandcoveredby

existingcodesandstandards.However,inrecentyearstherehasbeenanincreasinginterestintheuse

of higher strength steels for these installations, recognising the benefits from an increase in thestrengthtoweightratioandtheassociatedsavingsinthecostofmaterials.Asaresult,significant

partsofseveralplatforms(jacketandtopsides)havebeenconstructedfrom400–450MPasteeland

installedintheNorthSea. However,todate,fatiguesensitivecomponents(e.g.tubularjoints)have

generallybeenfabricatedfrommediumstrengthsteelbecauseofthebetterknowledgeonthesesteels

regardingfatigueperformanceandthelackofincreasedperformanceofhighstrengthsteelsinthis

area.

Theprincipalapplicationofveryhighstrengthsteelsoffshorehasbeeninthefabricationofjack-ups.

Steelswithnominalyieldstrengthsintherange500–800MPaarenormallyusedinfabricationof

legs,rackandpinionsandspudcans.Theseunits,usedprimarilyfordrilling,havemanyyearsof

satisfactoryexperienceinuse,operatinginavarietyofwaterdepths,butarenormallybroughtinto

drydockforinspectionat5yearintervals,whereanydamageorcrackingcanbefoundandrepaired.Inrecentyearstherehasbeenincreasinginterestintheuseofjack-upsforproduction,whereperiodic

drydockinspectionisnotpossible.Twojack-upsforproduction,utilisinghighstrengthsteels,are

nowinstalled(Hardingin1996,Siriin1998)andathird(Elgin-Franklin)isduetobeinstalledshortly

on the UKCS. High strength steels have also been used in tethering attachments for floating

structures in TLPs (tension leg platforms) and for mooring lines with semi-submersible module

offshoredrillingunits(MODUs).

A considerable amount of research has been undertaken on high strength steels in recent years

providingnewdatatosupportoffshoreapplications.However,overallthereislimitedinformationof

the long-term use of high strength steels in seawater, particularly under the severe environment

conditionstowhichstructuresontheUKCSaresubjected.Particularconcernswiththeuseofhigher

strengthsteelsarethegreatersusceptibilitytohydrogencrackingwhichcanbeenhancedwhenSRBs

arepresent,theirfatigueandfractureperformance,and,foroffshoreapplications,theirperformanceat

highertemperaturesasaresultoffire.

Most codes and standards relate to medium strength steels and in most cases the use of design

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REFERENCES

1.01 Billingham J, Healy J, and Spurrier J, ‘Currentandpotentialuseofhighstrengthsteelsin

offshorestructures’,Publication95/102MTD,Sept.1995,51pages,ISBN1-870553-24-1

1.02 Billingham J, Healy J, and Bolt H, ‘Highstrengthsteels–thesignificanceofyieldratioandworkhardeningforstructuralperformance’,MarineResearchReview9,36pages,published

MTD1997,ISBN1-870553-27-6

1.03 Billingham J, Sharp J V, Spurrier J, and Stacey A, ‘Theuseofhighstrengthstructuralsteels

inoffshoreengineering’,Intnl.SymposiumonSafetyinApplicationofHighStrengthSteel,

Trondheim,July1997

1.04 Sharp J V, Billingham J, and Stacey A, ‘Performanceofhighstrengthsteelsusedinjack-ups

inseawater’MarineStructures,Vol.12,1999,349-369,ISSN0951-8339,Elsevier1999

1.05 Healy J, and Billingham J, ‘Areviewofthecorrosionfatiguebehaviourofstructuralsteelsin

thestrengthrange350-900MPa,andassociatedhighstrengthweldments’,OffshoreTechnologyReport,OTH532,Publ.Health&SafetyExecutive,1997,ISBN0-71762409-9

1.06 Robinson M J, and Kilgallon P J, ‘Reviewoftheeffectsofsulphatereducingbacteriainthe

marineenvironmentonthecorrosionfatigueandhydrogenembrittlementofhighstrength

steels’,OffshoreTechnologyReport,OTH55598,HSEBooks(1999),Sudbury,Suffolk

1.07 Robinson M J, and Kilgallon P J, ‘Reviewoftheeffectsofmicrostructureonthehydrogen

embrittlementofhighstrengthoffshoresteels’,OffshoreTechnologyReport(inpress–no

numberyetissued)HSEBooks(1999),Sudbury,Suffolk

1.08 Stacey A, Sharp J V and King R N, ‘HighStrengthSteelsusedinOffshoreInstallations’,

Proceedings15thInternationalConferenceOffshoreMechanicsandArcticEngineering,Florence,Italy,June1996,VolIII

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2. THEUSEOFHIGHSTRENGTHSTEELSOFFSHORE

Traditionally, offshore structures have been fabricated with moderate strength steels with yield

strengthsupto350MPa[2.01],mainlyproducedbythenormalisingroute.However,therehasbeena

significant growth over the past twenty years in the use of high strength steels in the offshoreengineeringindustry,primarilydrivenbyadesiretosaveweightandcost[2.02].Table2.1showsthe

main application areas involved which vary with the strength of steel used. Such steels are also

generallyproducedbyalternativeprocessingroutessuchasthermomechanicalcontrolledprocessing

(TMCP),andquenchingandtempering(Q&T).

Theprincipaladvantageofusingthesestructuralmaterialsistheirincreasedstrengthtoweightratio

andtheattendantsavingsinmaterialscostsandconstructionschedules[2.03]duetoreducedamounts

ofwelding. The most importantincreases have occurred in the topside areasof jacket structures

where the weight saving has not onlyproducedoverall savings inmaterials usedbut has allowed

cranebargeinstallationofmorecompletetopsideprocessingandaccommodationunits[2.04]with

significantrelatedcostsavings.Asurveyundertakenin1995[2.05]indicatedthattheproportionof

highstrengthsteel (definedas>350MPayieldstrength)usedin offshorestructures increasedfromlessthan10%toover40%overlittlelessthanadecade.

Morerecentapplications,especiallywithsmaller,lighterstructures,involvetheuseofsuchsteelsin

thejacketmembersthemselvesalthoughtherearestillusuallyrestrictionsto theiruseinnodalareas

becauseofconcerns related tofatigueperformance. Itis likely that theuse ofsuchmaterialswill

continue to increase as the steels become more widely available andconstruction yardsget more

experiencedinfabricationprocedures.Todate,moststeelshavebeenrestrictedto450gradesbut

researchanddevelopmentprogrammes[2.06]haveindicatedthatsteelgradesupto550MPacanbe

produced which are readily weldable and possess excellent fracture toughness. Such steels will

increasinglybeutilisedastheybecomemorewidelyavailable.

Higherstrengthsteels(>550MPaandoftenupto700MPa)areusuallyproducedbythequenchingandtemperingrouteandhavetraditionallybeenusedoffshoreinmobilejack-updrillingrigswhichdonot

stay permanently on station and are periodically dry docked, allowing inspection and repair

programmestobeimplemented[2.07].Theprincipalapplicationofhighstrengthsteelsinjack-upsis

inthefabricationofthelegsbecauseoftherequirementtominimiseweightduringthetransportation

stage. Ingeneral,each lattice leg iscomposedofthreeorfour longitudinalchordmemberswhich

maycontainarackplateforelevatingthehullandaseriesofhorizontalanddiagonaltubularbraces

whichconnectthechordstoformatruss.Supplementarybraces(spanbreakers)arefrequentlyused

between main brace mid-points to increase the buckling resistance. The rack plate is very thick,

varyingtypicallybetween150and250mm.Thechordshellcansareusuallyfabricatedfromplate

with a wall thickness between 35 and 80mm and with a diameter in the range 800 to1200mm.

Weldabilityandgoodtoughnessandductilityareimportantmaterialconsiderationsinthisapplication

and thesteel makerprovides thisby carefulcontrolofalloy composition andbyprocessing[2.01;

2.04].

Steelsofsimilarstrengthlevelshaveonlycomparativelyrecentlybeenusedinproductionjack-ups

permanentlyonstationinNorthSeaprojectsintheHardingandSirifields,andintheElginjacket

which was installed in 2001. In such installations, fatigue, corrosion fatigue and hydrogen

embrittlementbecomemajordesignconsiderationsandthesteelsusedhavetobecarefullyreassessed.

TheFrenchTPG500designisagoodexampleofthistypeofstructure.Itcanbebuiltonshoreasone

completeunitandfloatedouttosite.Onceonstationthelegscanbeloweredtotheseabedandthe

deckjackedupforoperation,thusreducingtheneedforheavyliftoperationsduringinstallationand

producingsignificantcostsavings.Asecondbenefitinthisdesignisthatitisareusableproduction

facility,sinceitcanberefloated,removedfromonesitetoanother,andcommenceoperationsinthenew field. Toprovide the required fatigue life the legsof thestructure have incorporated forged

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nodes(fabricatedbyCreusot-Loire),thusreducingthestressconcentrationsnormallyseeninwelded

nodes. TheHardingplatformisin110mwaterdepthandthelowerpartofthestructurecomprisesa

concrete base to remove any potential problems of hydrogen embrittlement related to sulphate

reducingbacteriainthemudzone. Ajack-upwasalsoinstalledasa permanentinstallationin the

Danishsector(Sirifield,60mdepth)in1998.ThemajorityofthelegsectionsontheSiriplatformare

madefromthicksection(65-110mm)690gradehighstrengthsteel.TheElginstructureusesarange

of high strength steels including 500MPa steel in structural members, chords and bracings and

700MPasteelintheracks.Ituseslowerstrengthsteels(350MPa)inthelowerpartofthestructure

whicharepiledintotheseabedtoavoidpotentialhydrogenembrittlementproblems.

Otherapplicationsforhighstrengthsteelsarefoundinmooringattachmentsforfloatingstructures

such astension leg platforms (TLPs). These structures are fixedbyvertical tension legs topiled

foundation templates on the seabed. One ofthe firstsuchdesigns for theHutton fieldin theUK

sectorused16tensionlegs(4ateachcorner).Eachlegisathickwalledsteeltubular,manufactured

fromalowalloysteel(3.5%Ni,Cr,Mo,V)withaminimumyieldstrengthof795MPa[2.08].The

individual components of each leg are forged and connected by screwed couplings. The steel

composition was selected to provide the highest possible strength, commensurate with adequate

fracture toughness. Resistance to both stress corrosion cracking and corrosion fatigue were alsoimportant.Thechoiceofscrewedconnectorswasbasedonthefactthattherewereinsufficientdata

available on the corrosion fatigue performance of welded tubulars to guarantee safe performance

undertheenvisageddesignlifeofthestructures.Alargetestprogrammewasundertakentojustify

thechoice ofmaterial. The platformhas now been inoperation for almost 20yearswithoutany

significantproblemswiththetethers.

SincethenTLPtypeplatformshavebeeninstalledinseveralotherfields,bothinNorwayandinthe

GulfofMexico.IntheHeidronfield,forexample,aweldedTMCP(thermo-mechanicallyprocessed)

microalloyedX70pipelinesteelwasusedforthetethers.Thestructureisin270mwaterdepthand

comprisedtubulartensionlegelementsthatwere1mdiameterand38mmthick.Thesteelhadayield

strengthof500MPaandimpacttoughnessof60Jat-40ºC.Otherstructureshavebeenusedinmuch

deeperwaters,mainlyintheUS(AugerTLPin872m,installedin1994;MarsTLPin896mofwaterin1996),whereconventionalfixedplatformsareuneconomic. Theseprojectsinvolved76mmthick

415MPacomponentsofweldableTMCPsteel.PlansareinhandforevendeeperwaterTLPunits,but

itisnowrecognisedthattheavailabilityofsuitablehighstrengthmaterialsforthetethersislimiting

furtherdevelopment.

Otherfloatingstructuressuchassemi-submersibles,usedweldedhigherstrengthsteelanchorchains

or wire ropes as their mooring attachments. Such steel chain and wire rope components are

consideredoutsidethescopeofthepresentreview.

Other, more specialised, usage areas include flanges and repair clamps where threaded fasteners

providethemainloadtransfermechanism. Suchboltsandthreadedfastenersarediscussedinmore

detailinAppendixI.

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Table2.1Highstrengthsteelsusedoffshore

Strength Process Route Application Area

MPa (grade)

350(X52) Normalised Structures

TMCP Structures&Pipelines

450(X65) Q&T Structures

TMCP Pipelines

550(X80) Q&T Structures&Moorings

TMCP Pipelines

650 Q&T Jack-ups&Moorings



REFERENCES

2.01 Billingham J, ‘Steel–aversatileadvancedmaterialinmarineenvironments’,Ironmakingand

Steelmaking1994,Vol.21,No.6,p422

2.02 Billingham J, Healy J, and Spurrier J, ‘Currentandpotentialuseofhighstrengthsteelsin

offshorestructures’,MTDPublication95/102(1995),ISBN1-870-533-24-1

2.03 Rodgers K J, and Lockhead J C, ‘Theweldingofgrade450offshorestructuralsteels’,Proc

Conf.onWeldingandWeldPropertiesintheOffshoreIndustry,London,April1992

2.04 Webster S E, ‘Structural materials for offshore structures – past, present and future’,

Proc.Conf.onSafeDesignandFabricationofOffshoreStructures,IBC,London,Sept.1993

2.05 Healy J, and Billingham J, ‘High strength steels – a viable option for offshore designs’,

EuroforumConference–LatestInnovationsinOffshorePlatformDesignandConstruction,

London1996

2.06 ‘The influence of welding on materials performance of high strength steels offshore’,

Managed Programmes of University Research, Marine Technology Centre, Cranfield

University,1985-1994

2.07 Sharp J V, Billingham J, and Stacey A, ‘Performanceofhighstrengthsteelsusedinjack-ups'’

JournalofmarineStructures,12(1999),349

2.08 Salama M N, and Tetlow J H, Proc.ofOffshoreTechnologyConference,Houston,Texas,

1983,Paper449.

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3. MECHANICALPROPERTIESOFHIGHSTRENGTHSTEELS

3.1 STEELTYPEANDPROCESSROUTEIn general, thestrength ofa steel iscontrolled by its microstructurewhichvariesaccording to its

chemical composition, its thermal history and the deformation processes it undergoes during its

productionschedule. Inaddition,structuralsteelforoffshoreapplicationsmustbe readilyweldablesincethisisthetraditionalfabricationrouteforoffshorestructures.Structuralsteelsforoffshoremust

thereforebeavailableinmoderatetothicksections(30–100mm)andmustexhibitgoodtoughnessto

avoid the possibilityof brittle failure, in addition to showing good weldabilityandhigh strength.

Suchoverallrequirementsareoftendifficulttoachievebecauseanincreaseinoneoftheseproperties

oftenleadstoadecreaseintheothers.

Table3.1belowshowsthestrengthrangesandprocessroutesforhighstrengthsteelsusedinavariety

ofoffshoreengineeringapplications.

Most conventional structures use only moderate strength steel produced by the normalised or

thermomechanically processed routes (TMCP) but at higher strength levels there are processing

thicknessrestrictionstoTMCPsteelsandnormalisingcannotproducethestrengthlevelsrequiredinthenecessary section thicknesses. Quenchingand tempering is therefore the standardproduction

routeforveryhighstrengthstructuralsteel.Thelimitationsthatapplytothedifferentprocessroutes

inrespectofstrengthorthicknessrangesareshowninTable3.2.

3.2 METALLURGICALANDCOMPOSITIONALCONSIDERATIONSTheoffshorepipeline industry,for many years, hasusedhigh strengthsteels andtoday commonly

uses X70 steel grade ( ! 450MPa) with excellent toughness and weldability properties [3.01].Significant benefits in suchdevelopments have comefrom understanding the complex chemistries

developedforthesteelplustheuseofextensivethermomechanicalprocessing,primarilytoproduce

fine grained microstructures, including controlled rolling, thermomechanical controlled processing

and accelerated cooling. Many of the principles involved in such developments, particularly the

complexinteractionsbetweenstrength,toughnessandweldabilityasinfluencedbysteelchemistry,

heat treatments and thermal processing [3.02] have been carried over into higher strength steel

development.

Many of these well understood metallurgical principles can be utilised to satisfy the overall

mechanicalpropertyrequirementsforhighstrengthstructuralsteels,namely:

" reducedcarboncontenttoimproveweldabilityandtoughness;" decreasedgrainsize(ferriteand/orbainite)togiveincreasedstrengthandincreasedtoughness.

This is usually achieved by microalloying with Nb, V or Al and by some form of

thermomechanicalprocessing;

" decreased impuritycontent (S, P,O)to increase toughness inparticular and through thicknesshomogeneity,i.e.theuseofcleansteeltechnology;

" increased alloying with Ni, Cr, Mo and Cu to give solid solution and transformationstrengthening,especiallyatthehigherstrengthlevels.

Relativelysmallchangesincompositionand/orvariationsinprocessingroutecansignificantlyaffect

theresultingmechanicalpropertiesasshowninTable3.3.Thistableshows‘old’and‘new’versions

ofsteelswithin3standardsteelgrades,i.e.355,450and690MPayieldstrength. Althoughallofthe

steelswithin a particular grade satisfy thegrade requirements (primarilywith respect to specified

minimum yield strength) it can be seen that the ‘newer’ versions show much improved overall

properties by combining the required yield strength with improved toughness (improved Charpy

impactperformance{Cv}),andimprovedweldability(lowercarbonequivalentvalues[CE1}). This

Carbon equivalent CE is defined as CE=C+Mn/6+(Cr+Mo+V)/5+(Ni+Cu)/15 – See page 30 for more details.

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has been achieved primarily by controlling the microstructure through changes in chemistry and

thermalprocessing.

Manyengineersanddesignersdonotappreciatethatthemechanicalpropertiesofaparticularsteel

can vary significantly within a specified steel grade (i.e. steel with a specified minimum yield

strength).Figure3.1showsthevariationinmechanicalpropertiesforthreeoffshoresteelgradeswith

minimumyieldstrengthsof355,420and450MPa[3.03].Forthe450MPasteel,forexample,itcan

beseen that the yieldstrengthcanvaryby100MPa from440 to540MPa (+20%ondesign yield

value),withameanvalueatapproximately500MPa.Suchvariationsareproducedbyvariationsin

steelcompositionandprocessingwhichaffectallthemechanicalpropertiesasshowninFigure3.2for

a450MPasteel.Suchvariationscanhaveseriousimplicationsforthedegreeofweldovermatching

or undermatching that occurs in the final structure. The range of properties achievable within a

particulargradecanalsovarysignificantlywithprocessrouteasshowninFigure3.3whichillustrates

themuchwidervariation obtainedfrom theTMCProutethan fromeither the normalised routeat

lowerstrengthlevelsorthequenchedandtemperedrouteathigherstrengthlevels[3.04].Variations

can also occur with plate thickness and with steel manufacturer [3.05]. It is important that this

potentialvariationinyieldstrengthisrecognisedatthedesignstage.

Typical compositions and properties for high strength steels produced by the normalised,

thermomechanically controlledprocessedandquenchingand tempered routes are shown inTables

3.4,3.5and3.6respectively. Othertypicalcompositionandmechanicalpropertiesaregivenin the

draftDNVOffshoreStandardOS-B101MetallicMaterials,(May2000).Ingeneralforsuchsteelsthe

strengthincreasesasthehardenabilityandthecompositionrelatedcarbonequivalentvaluesincrease

(seeFigure3.4).foreachprocessroute,buttheparticularprocessrouteselectedgenerallyhasamore

significantinfluenceonyieldstrength.Bymakinguseofstrengthimprovementsassociatedwiththe

processingroute,itispossibletoproducesteelsofthesamestrengthlevelatleanerchemistriesand

hencelowercarbonequivalentvalues,whichshowimprovedweldability.Byusingcarefulcontrolof

compositionandprocessing,steelscanthereforeusuallybeproducedwithexcellentcombinationsof

strengthandtoughnesscombinedwithexcellentweldability.Ingeneral,asthestrengthincreasesthe

weldabilityinparticulardecreasesandmorecontrolovertheweldingproceduressuchasincreasedlevelsofpreheatingareusuallyrequired.Moreover,ingeneral,thetoughnessofveryhighstrength

steels(690MPa)isinferiortothatofsteelswithlow(350MPa)orintermediate(450-550MPa)levels

ofstrength.

3.3 YIELDRATIOCONSIDERATIONSThestressstrainbehaviourofhighstrengthsteelsdifferssomewhatfromthatoflowerstrengthsteels

in that they generally show reduced capacity for strain hardening after yielding and reduced

elongationasshowninFigure3.5.Thisisbecausethesteelstrengtheningmechanismsusedinhigh

strength steeldevelopmenthave been selected specifically to increase the yieldstrength and have

muchlessinfluenceinsubsequentstrainhardeningbehaviour.Onemeasuretoillustratethisdifferent

y)toultimatetensilestrength(UTS),andwhichgenerallyincreasesasthestrengthofthesteelincreasesasshowninFigure

3.6forarangeofoffshoresteelgrades[3.06].YRisnot,however,auniquemeasureofhowthesteel

behavesbecausesteelswithverydifferentstressstraincurvescanhavethesamevalueofYR[3.06].

TherearerestrictionsinstructuraldesigncodestoreflectthischangedbehavioursuchthatYRfor

materialtobeusedforstructuralmembersisnotallowedtohaveavaluegreaterthan0.85indesign

equationstoensurethatthereisadequateductilityinthemembertodevelopplasticfailurebehaviour

as a defenceagainst brittle fracture.Design aspects related toYRaregiven in section 13of this

report.Examinationofadifferentdatabase[3.07]showninFigure3.7showsthatgenerallysteels

withyieldstrengthsupto500MPacansatisfythisgeneralrequirementsbutthatveryhighstrength

steelsdonot.

Theyieldratioisnotdirectlyrelatedtothecapabilityofagivensteeltowithstandplasticstrainafter

yieldandbeforefracture. Inolderhighstrengthsteels,elongationgenerallydecreasesasyieldratio

7


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increases,butmoderncleansteelswithlowcarboncontentandlowlevelsofimpurityhavesignificant

elongation even at the highest strength (690 grade) and yield value ratio (0.95), giving more

confidenceastotheirdeformationcapability[3.05].Analternativemeasureisthegeneralelongation

whichisusuallysubstantialinmodernsteelwithhighyieldratios.

Currentdesignequationsarebasedontestdatafrommediumstrengthsteels,wheresomedegreeof

strainhardening ispresent. Lack ofstrainhardening can lead toprematurecracking, whichcould

havesignificant implicationsfortubular joints inservice. As a result,design codeshaveplaceda

limitontheyieldratio(typically0.7).ExaminationofFigure3.7emphasisestherangeofvaluesthat

can occur within particular strength grades, largely due to the different methods of production,

differences in steel chemistry, and differences in section thickness that occur (see section 3.2).

Indeed,fromthisdiagramitcanbeseenthat350MPasteelsgenerallyshowayieldratiorangingfrom

0.6 to0.8, that 450MPa steels havevalues rangingfrom0.7 to0.87,whereas690MPasteelshave

valuesrangingfrom0.9to0.95.Elongationgenerallydecreasesinlinewithincreasingyieldratio;

thereforefor350–450steels,elongationsaregenerallyoftheorderof20–35%,whereasfor690

steels,valuesof14–18%aremoretypical[3.06].

ExaminationofFigure3.7showsthatmanysteels,evenatstrengthlevelsupto400MPahaveyieldratios above 0.7, which could include many steels purchased at grade 355 level. It is therefore

possiblethatsomeearlierstructuresmighthavenodaljointswhichdonotsatisfythecurrentdesign

codesalthoughtheyhaveperformedperfectlysatisfactorilyinservice.

Therehas for some time been a feeling that the code restrictionsfor nodalconnections are rather

conservativeinrespectofhighstrengthsteelsbecauseintuitivelyitwouldbeexpectedthatjointload

capacity would increase in line with yield strength, whereas the code restrictions impose severe

limitations.Forexample,onincreasingtheyieldstrengthfrom355to532MPa(a50%increase)the

designerisonlyallowedtoincreasetheallowabledesignstressby23%whentheyieldratiois0.85

(YR = 0.85,design stress = 0.7UTS = 0.7 x626 =438MPa). Otherexamplesof the restrictions

imposedbythecodearegiveninAppendix3.InitialfiniteelementstudiesonXjointdeformation

behaviourbyBOMELandCranfield[3.06]indicatedthatjointswithhighYRhadsignificantlyhigher joint capacities than joints with low ratios. For example, a joint with a YR of 0.93 (490MPa$y,525MPa UTS) showed a 28% increase in joint capacity compared to a lower strength steel$y =350MPawithaYRof0.66and the samevalueofUTS(525MPaUTS). Existing structuralcodes

wouldhaverestrictedthecapacityofboththesejointstothesamevalue(YR=0.66$D=$y=350,YR0.93,$D=2/3xUTS=2/3x525=350).Despitetheaboveenhancedin-jointcapacity,thecapacity does not increase linearly with yield stress as indicated by the design equations (static

strengthforDTjoints):

%P- F MIN*,

,17.0 )

' T2 5.2( & Q)14 %y+ YR(

where P = staticdesign strength, Fy = yield strength, YR = yield ratio, T = wall thickness, % =

diameterratiod/D,andQ% isageometricfactor,definedasQ% =0.3/ %(1-0.833%)for% >0.6andQ%=1for% 600MPa)[3.10]showedthattherecommendedrestrictionof0.7UTSwasjustified. However,the

analyseswerecarriedout usingthe HSE GuidanceNotesequations whereultimatestrength isthefailurecriterion.Thepointwasalsomadethatthedatawereverylimitedandthattherangeofjoint

8


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typestestedwaslimited(1T,8K/YT,2DT).Inaddition,thegeometryrangewaslimited(lowest

gammawas14.8,highestbeta0.43). Therangeofyieldratioswas0.88to0.94.

A later study for the European Commission [3.11] which involved some relatively large scale

experimental tests by BOMEL, TNO and Delft Universities largely confirmed these earlier

experiments. This programme included a significant finite element study, a comprehensive re

examinationofthetestdatabaseusedinsettingupthestructuralcodeformulationsandanexpanded

experimentaltestprogrammeinvolving2seriesoftests(compressionandtension)onDTjointsmade

from 350, 450 and 690MPa steels. The finite element analyses successfully reflected the

experimentalDTtestjointdata.Theyalsoisolatedandquantifiedaccuratelytheeffectofgeometric

imperfectionsinthe testjoint.Theearlierindicationsthatthedesignstressfortubularjointsshould

beraisedfromtheYR=0.7valueintheguidancewereconfirmed.Theauthorsconcludedthatthe

conservatismutilisedinhighstrengthsteeltubularsteeldesigncouldbereducedbychangingtheYR

=0.7limitinthedesignequationto0.8forbothcompressionandtensionjoints.Thiswouldthen

enablemorewidespreadusetobemadeofhighstrengthsteelintubularjointdesign.Theauthorsalso

recommendedthatmoretestsontubularjoints,especiallyatthehighergradesofsteelandespecially

intension,wererequired.

A recentlypublishedpaper [3.12] from ThyssenKrupp StahlAGanalysed transition temperatures

from Charpy V-tests and the fracture mechanics transition temperature for 690MPa steels against

yieldratioandfoundnocorrelation,butitwasrecognisedthattheseresultswerefromsmallscale

tests.Anotheranalysisofthemaximumnetstressversustesttemperatureofthewideplatetestsfor

690MPasteels,withyieldratiosrangingfrom0.87to0.93showedthatthehighestloadswereinthe

steelswiththehighestyieldratio.Theauthorsconcludedthatyieldratioisnotagoodmeasureof

componentsafety,andthatotherfactorsshouldbetakenintoaccount. Thispaper[3.12]alsolisted

limitationsonyieldratioinvariousdesigncodesandmaterialsstandards(bothonshoreandoffshore)

rangingfrom0.7to0.93forvariouscomponents.

Since1996, severalPanelshavebeenmeeting todraft anew ISOstandardforoffshorestructures.

ThisincludesaPaneldraftingasectiononthestaticstrengthoftubularjoints.ThePanelhasreexaminedthetestdataonjointstrengthanddevelopedsomeimproveddesignequations.However,

becauseofthelackofdataonhigherstrengthsteeljoints,thePanelconcludedthattheseequations

shouldbelimitedtosteelswithyieldstrengthslessthan500MPa(seesection4).However,evenfor

thesesteels,itwasconsiderednecessarytoimposealimitoftheyieldratio(sincesomelowerstrength

steelshaveyieldratiosgreater than0.7). ThePanelconcluded thatonthebasisof test results for

lowerstrengthsteels,thelimitingyieldratioshouldbe0.8.

For higher strengthsteels (yieldstrengthgreater than 500MPa) the Panelconcluded that use ofa

limitingvalue of yield ratio of0.8 may be adequate to permit theultimate compression capacity

equationstobeusedforjointswithstrengthsintherange500-800MPaprovidedadequateductility

canbedemonstratedintheHAZandparentmaterial.Itisunclearhowthisdemonstrationofadequate

ductility canbeprovidedeither interms ofmechanicalpropertydataof the steels concerned orinidentifiedtestprocedures.

Laterexaminationofsomeoftheavailablestaticstrengthdata[3.13]hasconcludedthatalthoughthe

factorforcompressionloadingcouldberelaxedto0.8,thefactorfortensionloadingshould,indeed,

beloweredto0.5basedonthedesigncapacitybeingrelatedtofirstcrackingratherthantoultimate

strengthasin,forexample,theAPIRP2Acode.Failuremodesinthecompressiontestsinvolvedan

indentationofthechordofabout30%ofthediameter.Cracksappearedinthetensionspecimensat

loadsofaround50%ofthemaximumloadreachedinthetests.However,itshouldberecognisedthat

theserecommendationsarebasedonverylimitedtestdata.

Overall, the design ofhigh strengthwelded joints for static strength isunclear basedon thevery

limitedexistingdata.Whenfailureisdefinedastheonsetofcracking(undertensionloading,e.g.as

inAPIRP2A)itwouldappearthattheexistingdesignequationsareunconservativeforhighstrength

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steeljoints,evenwithayieldratioof0.7.Forcompressionloadingalimitingyieldratioof0.8would

appear appropriate, provided there isadequateductility present inboth the HAZandparent plate.

Furtherdataarerequiredtoresolvetheseuncertainties.

Analternativeapproachtotheconcernsregardingtheinfluenceofhighyieldratioanddeformation

capacityofhighstrengthsteeltubularjointsistoredesignthesteelanditsproductionroutetodevelop

highstrengthsteelswithloweryieldratios. Japanesestudies[3.14]aimedatdevelopingsteelsfor

earthquake resistant structures have utilised accelerated cooling and intercritical quenching

procedures toproduceamicrostructureof ferrite dispersed inabainiticmatrixwhichcan produce

highstrengthsteel500MPainthicksectionswithayieldratioof0.7andCEvaluesof0.4which

would indicate reasonable weldability. 700MPa steels with YR of only 0.83 have also been

developedbutthesearelessweldable(CE0.52)[3.15].Theweldabilityofsuchsteelisinferiorto

modernHSLAsteelsusedoffshorebutcouldalmostcertainlybeimprovedwithfurtherdevelopment.

Castingscanofferadvantagesoverweldedstructuralfabricationsbecausethejointintersectionscan

be easily contoured to reducestress concentrationeffects,at say nodal joints for example, with a

correspondingincreaseinfatiguelife[3.16].Inconventionalweldednodaljointsthefatiguelifeis

decreasedbecauseofthemicrocrackingthatexistsintheweldtoewhichiseliminatedincastjointswithacorrespondingsignificantincreaseinfatiguelifeasshowninFigure3.8.Highstrengthsteel

castingsareavailablewhichhaveattractivecombinationsofstrengthandtoughnessproperties[3.17;

3.18]. Steelswithyieldstrengthsupto690MPaareavailable.Theycannotderivetheirstrengthfrom

processingsousuallyhaveadditionofnickelandchromiumtosuppresstransformationtemperatures

andproducelowcarbonmartensiticorbainiticstructures.Theexcellentthroughthicknessproperties

of castings have also opened up new markets in lifting attachments, spreader bars, and pad eyes

[3.16].

REFERENCES

3.01 ‘SteelsforLinepipeandPipelinefittings’,Proc.Intnl.Conf.London1981,publMetalsSociety

3.02 Billingham J, ‘Steel–AVersatileAdvancedmaterialsinmarineEnvironments’,Ironmaking

andSteelmaking,Vol.21,No.6,452,1994

3.03 Billingham J, Healy J, and Spurrier J, ‘CurrentandPotentialUseofHighStrengthSteelsin

OffshoreStructures,MTDpublication95/102,published1996,ISBN1-870553-24-1

3.04 Denys R, ConfEvaluationofMaterialsinSevereEnvironments,ISIJapan,Vol.2,1989.

3.05 Healy J, and Billingham J, MetallurgicalConsiderationsoftheHighYieldtoUltimateRatio

in High Strength Steels for Use in Offshore Engineering’, 14th

Intnl.Conf. OMAE 1995,

Vol.III,365

3.06 Billingham J, Healy J, and Bolt H, ‘HighStrengthSteel–theSignificanceofYieldRatioand

WorkHardeningforStructuralPerformance’,MarineResearchReview9,publMTD1997,

ISBN1-870553-27-6

3.07 Willcock R T S, ‘Yield:TensileRatioandSafetyofHighStrengthSteels’,HSEReport,Mat

R108,1992

3.08 Healy B E, and Zettlemoyer N, ‘Inplanebendingstrengthofcirculartubularjoint’,Proc.5th

Intnl.SymposiumonTubularStructures,ed.MGCoutieandGDavies,EandFNSpan,1993

10


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3.09 Wilmhurst S R and Lee M M K, Non-linear FEM Studyof Ultimate Strength of Tubular

MultiplanerDoubleKJoint’,Proc12thOMAE,Glasgow1993.

3.10 Lalani et al, ESCSReport7210MC/6021996

3.11 StaticStrengthofHighStrengthSteelTubularJoints,ECSC7210MC/6021996,reportedin

Wicks P J, and Stacey A, ‘StaticStrengthofHighStrengthSteelTubularJoints’,Proceedings

ETCE/OMAE2000Conference,Paper2081,February2000,Publ,ASME

3.12 Kaiser H J, Kern A, Niessen T, and Schriever, ‘ModernHighStrengthSteelswithMinimum

YieldStrength upto 690MPa and HighComponent Safety’,Proc. 11thIntnl.Offshoreand

PolarEngineeringConference,Norway2001,ISBN1-880653-55-9.

3.13 PrivateCommunicationwithNWNicholls

3.14 Shikani N, Kurihora M, Tagawa H, Salkui S, and Watanabe I, ‘Developmentofhighstrength

steelwithlowyieldratioforlargescalesteelstructures’,Proc.ofMicroalloying88,Chicago

1988,p481

8

3.15 Toyoda M, ‘StrainHardenabilityofHighStrengthSteelsandMatchingPropertiesinWelds’,th

Intnl.OMAEConf.1989

3.16 Marston G J, ‘NovelApplicationofStructuralSteelCastingsintheOffshoreIndustry’The

SafeDesignandFabricationofOffshoreStructures,IBCConf.,London,1993

3.17 Richardson R C, ‘HigherStrengthCastSteelforOffshoreStructures’,WorldExpro161

3.18 Cowling M J , ‘Fatigue Performance of Cast Steel Intersection for Offshore Structures, in

FatigueCrackGrowthinOffshoreStructures,ed.WDDover,SDharmavason,FPBrennan

andKJMarsch,EMAS1995.

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Table3.1Strengthrangesandprocessroutesforhighstrengthsteelsusedinoffshoreengineering

Type Strength levels used Process

of structure (MPa) Route

Jacketstructuresandtopsides 350–500 NormalisedQ&T

TMCP

Pipelines 350–550 TMCP

(X52)(X80)

Jack-ups/Moorings 500–850 Q&T

Table3.2Steelprocessingroutesforproductionofhighstrengthstructuralsteels

Normalised Usually


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Table3.3EffectofchangesinprocessingandalloyingmethodologyonmechanicalpropertiesofGrade35

Steeldesign Process

Chemical Composition

B C Mn Si Ni Cr Mo Cu S P Al

Grade 355BS4340

50D

NormalisedOLD

- 0.20 1.35 0.30 - - 0.016 0.015 0.02

BS7191

355EMZ

Normalised

NEW

- 0.11 1.50 0.40 0.15 0.15 0.005 0.015 0.03

BS4360

50D

TMCP - 0.07 1.49 0.21 0.38 0.02 0.002 0.008 0.02

Grade450Q1N

Q&T

OLD

- 0.18 0.4 0.30 3.0 1-1.8 0.015 0.005 0.02

BS7191

450EMZ

Q&T

NEW

- 0.11 1.49 0.3 0.52 0.11 0.001 0.010 0.03

Dillinger

450TMCP

TMCP - 0.09 1.50 0.3 - - 0.001 0.007 0.03

Grade 690Q2N

Q&T

OLD

- 0.11 0.42 0.23 3.40 1.48 0.46 0.03 0.001 0.012 0.026

OX812 Q&TNEW

- 0.11 0.89 0.26 1.18 0.46 0.38 0.15 0.003 0.008 0.07

SE702 Q&T

NEW

0.0027 0.125 1.05 0.25 1.4 0.5 0.45 0.20


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Table3.4TypicalcompositionandmechanicalpropertiesofnormalisedsteelsproducedinEurope–yieldstre

Typical composition (by weight %)

Thickness CE IIW Ty(mm) C Mn Si S P Nb V Al Cu Ni Cr Mo

25 0.20 1.35 0.42 0.016 0.015 0.028 - 0.022 - - - - 0.43 360MP

20 0.22 1.0- 0.55 0.030 0.035 - - - 0.3 0.5- 0.2 0.1 0.52 420MP1.6 max 0.7

20 0.22 1.6

- 0.60 0.49 0.02 0.37 460MPa/2

Table3.6Typicalcompositionandmechanicalpropertiesofquenchedandtemperedsteels–yieldstrengthra

Typical composition (by weight %)

Thickness CE IIW(mm) C Mn Si S P Nb V Al Ti Cu Ni Cr Mo B

6–140 0.18 0.1- 0.15- 0.075 0.015 -


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Figure3.1Variationinyieldstrengthfor355,420and450gradesteels

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Figure3.2Showingtypicalvariationinmechanicalpropertiesforagrade450steel(35- y=430MPa,m

20%,samplesizeN=94)

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Figure3.3Variationinmechanicalpropertieswithprocessrouteforsteelgrades350and420afterDenys

[3.04]

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Figure3.4Effectofcarbonequivalentvalueandsteelprocessingrouteonplatestrength

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Figure3.5Typicalstress-straincurveforGrades355,450and690steel

Truestressstrainlinesfordifferentsteelgrades

Load-deflectionlinesfordifferentsteelgrades

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Figure3.6Offshoregradesteels,nominalthickness50mm

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Figure3.7Yieldratioof200castandwroughtironhighstrengthsteels

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Figure3.8Showingcomparisonofweldedandcaststeelproperties

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4.CODESANDSTANDARDS

Detailedcodesandstandardsnowexistformediumstrengthstructuralsteels,coveringmostofthe

aspects relevant to offshore design. However, for higher strength steels (YS 500-600MPa) the

availablecodesandstandardsarelimitedandforevenhigherstrengthsteels(>600MPa),almostnon

existent. This section reviews theavailable codes and standards, both published and thosebeingdevelopedatpresentforoffshoreuse.Intermsofoffshorehazards,table4.1showsthecurrentstatus

ofcodesandstandards.Thecurrentstatusofcodesandstandardswillnowbereviewedintermsof

materialsproperties.

4.1 ALLPROPERTIESHSE/D.EnergyOffshoreGuidancehasbeendevelopedovermanyyears,withthefirsteditionbeing

published in1974.Thefourth edition was published in1990, [4.05] with significant amendments

being added up to 1995 [4.07]. It is particularly strong in materials properties, performance of

structuralcomponentsetc,andhasseveralsectionsdevotedtohighstrengthsteels. Inparticularthe

controlofhydrogen assistedcracking isaddressed in somedetail, morethan inany otherexisting

codeorstandard.HoweverasaresultoftheissueoftheDCRRegulations[4.08]in1996ithasbeen

withdrawn,althoughitisstillavailableasadocumentforconsultation.

Two ASTM standards [4.01,4.02] cover some aspects of the requirements ofhigh strength steels,

although these are limited in their applicability. A808 is concerned with high strength, low ally

carbon,manganesesteels of structuralquality,whilst A514providesa specification forhigh yield

strengthQ&Talloysteels,intendedprimarilyforuseinweldedbridgesandotherstructures.

TherecentlypublishedNORSOKstandardontheDesignofSteelstructures[4.09]includesfivesteel

qualitylevels(DC1-DC5).HoweverallofthesegradesarelimitedtosteelswithYSequaltoorless

than500MPa.Forhigherstrengthsteelsitisstatedthatthefeasibilityofsuchaselectionofsteelshall

beassessedineachcase.

TheInternationalAssociation ofClassificationSocieties(IACS)providesa setof requirements forhighstrengthquenchedandtemperedsteels[4.11],forsteelswithYSintherange420–690MPa,

dividedintosixgroups.Therequirementsincludemethodofmanufacture,mechanicalproperties,and

inspectionduringmanufacture.

TheDnVoffshorestandard[4.04]groupssteelsintothreemaingrades,thehighestofwhich(extra

highstrength)coversmaterialswithyieldstrengthsfrom420-690MPa.Thesegradesarelinkedto

impact toughness properties according to weldability requirements, but the improved weldability

gradeislimitedtoamaximumYSof500MPa.AstatementisalsomadethatsteelswithYS>550MPa

shall be subject to special considerations for applications where anaerobic conditions may

predominate.

SimilarlyinthedraftISOStandardforfixedstructures(19902)[4.10]steelsareclassifiedintofive

groups,withGradeVcoveringsteelswithYSupto500MPa.Itisalsostatedthatfurthergroupsmay

beaddedwhendatabecomesavailable.Thedraftstandardincludesanimportantstatementonhigher

strengthsteels,whichis:

'Althoughsteelswithyieldstrengthsinexcessof500MPa(73ksi)arecurrentlyavailable,no

agreedstandardexistsforoffshorefixedplatformstructuraluse.Thesearenotrecognisedasoffshore

fixedplatformstructuralgradesandusersshouldtakecaretoensurethatductility,fracturetoughness

and weldability will be adequate for the intended application. Attention is drawn to the need to

considerfatigueandcorrosionconditions,includingthetendencyforhigherstrengthsteelstobemore

susceptibletohydrogenembrittlementandcertaintypesofstresscorrosion.Particularcareshouldbe

exercisedwherehighstrengthisdevelopedasaresultofalloyadditions'.

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AseparateISOTechnicalgroupisdevelopingastandardforjack-ups,whichisexpectedtoinclude

guidanceonhigherstrengthsteels,appropriatetojack-ups,buthasyettobedrafted.

4.2 FATIGUENewGuidancewaspublishedbyHSEin1995[4.07].ThisincludedamodifiedsetofS-Ncurves,but

thesewererestrictedtosteelswithyieldstrengthequaltoor lessthan500MPa,asitwasconcluded

thatthetestdataavailablewereinsufficientforhigherstrengthsteels.Thiswasparticularlytruefor

fatigueinseawaterundercathodicprotectionandfreecorrosionconditions,wherethedataavailable

onhighstrengthsteeljointswereextremelylimited.TheHSEGuidancerecommendedthatforhigher

strengthsteels,datafromanapprovedtestprogrammeareusedtodetermineappropriateS-Ncurves,

orfracturemechanicsconstants.Following incorporationof theDCRRegulationsoffshorein1996

theHSEGuidancehasbeenwithdrawn.

DnV Rules [4.05] include S-N curves and fracturemechanics constants for steels with YS up to

500MPa.

TheNORSOKstandard[4.09]providesrecommendedS-Ncurvesforsteels,bothinairandseawater.

AsnotedearliertheseapplytosteelswithsteelqualitylevelsfromItoV,themaximumyieldstrengthbeing500MPa.Forsteelsofhigherstrengthitisstatedthatthefeasibilityofsuchaselectionshallbe

assessedineachcase.

The draftISOstandard [4.10] states that the limitedamount oftestdataforplatejoints withyield

strengthsupto540MPaandtubularjointsmanufacturedfromhighstrengthsteelwithyieldstrengths

up to700MPasuggeststhat fatigueperformance in seawaterunder CPandunder freecorrosionis

similartothatformediumstrengthsteels,buttestdatashouldbeusedtodetermineappropriateS-N

curves. Inaddition,thedraftstandardindicatesthatforevenhigherstrengthsteels(700–800MPa)

the effect of seawater on the fatigue performance of these materials is considered to be more

detrimentalthanformediumstrengthsteelsbecauseoftheirgreatersusceptibilitytocrackingfrom

hydrogenembrittlement. Inparticular,it isnotedthat several studieshaveshown thatexcessively

negativeCPprotectionpotentialscanbeacauseofcrackingduetothegenerationofhydrogenwhichenhancescrackgrowthrates.Itisstatedinconclusionthatitisimportantthatthefatigueperformance

ofhighstrengthsteelsisunderstoodandthatappropriatelevelsofCPareapplied.

4.3 FRACTURETOUGHNESSMost codes and standards recommend the need to avoid brittle fracture. Good specifications are

publishedformediumstrengthsteelsbutgenerallythereisverylimitedguidanceforhigherstrength

steels.OverallavoidanceofbrittlefractureisbasedonrecommendingaminimumvalueofCharpy

energyvaluesaccordingtoyieldstrength.OnthisbasistheInternationalAssociationofClassification

Societies(IACS)[4.11]hasrecommendedforhighstrengthQ&Tsteelsthattheaverageenergyfrom

acharpyVnotchtestshouldbeRe/10forthelongitudinaldirection,and2/3ofthisforthetransverse

direction,i.e.for690MPasteels(Fgrade)Charpyenergyvaluesof69Jand46Jatatesttemperature

of -60oC with minimum individual values of 70% of the minimum average, i.e. 48J and 32Jrespectively[4.11].Possiblelimitationsonthisrequirementareconsideredinsection6.

4.4 HYDROGENCRACKINGAsaresultofthedetectionofcrackinginjack-upsinthelate1980sasignificantresearchprogramme

was undertaken on high strength steels which led to new guidance being developed to minimise

crackinginpractice.

HSEpublishedanamendmenttoitsGuidance[4.07]witharecommendationthattheCPlevelshould

belimitedtoanegativevoltagenolowerthan-850mV(Ag/AgCl).Toachievethisspecialmeasures

wererecommended,suchasvoltagelimitingdiodestokeeppotentialswithintherecommendedlimits.

Inadditionsteelsproposedforuseoffshoreinconditionswherethereisavulnerabilitytohydrogencracking should be assessed using, for example, slow strain rate testing. High strength steels

(YS>650MPa) should beexamined for the possibilityofhydrogendamage in service,both in the

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parentmaterialandintheweldments.TheHSEGuidancewassupportedbyapublishedOTHreport

[4.13] which provided data on the performance of several steels and on the recommended test

methods.

The DnV Offshore standard [4.04] also provides guidance on the use of high strength steels in

seawaterwithCP. In this case the recommended rangefor steels susceptible tohydrogen induced

crackingis-770mVto-830mVforsteelswithYSlargerthan550MPa,whichissimilartotheHSE

recommendations.

4.5 DEFECTACCEPTANCECRITERIABS7910publishedin1998[4.06]containsdataforcalculatingcrackgrowthunderstaticandcyclic

loadingconditions.Recommendedvaluesoftheconstants(A,m)aregivenforsteelswithyield

strengthsupto600MPa,thusenablingthefatiguecrackgrowthrateacceptanceofdefectsinhigher

strengthsteelsfoundduringinspectionorassumedduringdesigntobequantified.Forhigherstrength

steels(>600MPa)itisrecommendedthattestdataarerequired.

4.6 CORROSIONPROTECTION

It is stated in the NORSOK standard [4.14] that for high strength steels (YS>700MPa) a specialevaluationisrequiredwithrespecttohydrogenimpact.(SeeEN10002,metallicmaterials.Tensile

testing.Part1methodoftest).

TheDnVcode[4.04]providesbothgeneralrequirementsforcathodicprotectionaswellasspecific

needsforhighstrengthsteels.Steelswithspecifiedminimumyieldstrengths>550MPaaresubjectto

special considerations for applications where hydrogen induced stress cracking (HISC) may be

anticipated,wherequalificationtestingshouldbecarriedoutforcriticalapplicationssuchaslegsand

spudcans.Intheabsenceof suchtestingtodemonstratethathighnegativeCPlevelsarenotharmful

itisstatedthattheCPlevelshouldbelimitedbytheuseofspecialanodesorcontrolledvoltagetype

(e.g. with diodes) or by other methods. CP potentials levels should also be monitored to ensure

compliance with the target range, which is set to be within the limits of -770mV to -830mV

(Ag/AgCl).InthecaseofobservedexceedanceofthisrangeitisrecommendedthatinspectionforHISCshouldbecarriedout,

Section19ofthedraftISOstandard[4.10]isconcernedwithcorrosioncontrol,andincludesasection

oncathodicprotection.Thisstatesthatbecauseoftherisksofhydrogeninducedstresscrackingsteels

with minimum yield strengths in excess of 720MPa should not be used for critical cathodically

protectedcomponentswithoutspecialconsiderations.Inaddition,itisstatedthatanyweldingorother

fabrication affecting ductility or tensile properties, should be carried out according to a qualified

procedure,whichlimits hardness toHV350. Itis expectedthat thiswill restrict the useofwelded

structuralsteelstoapproximately550MPamaximumspecifiedminimumyieldstrength.

For medium strength steels the recommendedpotential range is -0.8 to -1.1volts (Ag/AgCl). For

somehigherstrengthsteelsthenegativeendofthisrangeisexpectedtobedetrimental,intermsofhydrogencrackingetc.

4.7 STATICSTRENGTHOFTUBULARJOINTSCurrentoffshoredesigncodesprovideequationsfordeterminingthestaticstrengthofvariousclasses

oftubularjoints.Thestrengthisgenerallyproportionaltoyieldstrength,butdataindicatethatthis

proportionalityislimitedtolowerstrengthsteels.Asaresultthebasicequationsarelimitedtosteels

with YS


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For higher strengthsteels thedraft ISOstandard recommends that the basicultimate compression

capacityequationsmaybeused,togetherwitharatiooftheyieldtoultimatestrengthlimitedto0.8,

providedadequateductilitycanbedemonstratedinboththeHAZandparentmaterial(however,the

criteriafordemonstratingthisarenotprovided). ThedraftISOstandardhighlightsthatthe limiton

yieldratioforthetensioncapacityofjointsbasedonfirstcrackingmayneedfurtherinvestigation(see

section3).

The static strength of cracked high strength steel joints is of interest, particularly for the use of

floodedmemberdetection.Somerecentlypublisheddataforhighstrengthsteels(SE702)[4.16]have

beenmadeavailablefromaseriesofninestatictestsperformedonlargepre-crackedweldedtubular

joints(sixTjoints,threeYjoints). Thesewereloadedtofailureinaxialandout-of-planebending.

Allspecimenshadaleastonethroughthicknesscrack.Theresultswereanalysedintermsofboth

lossofstaticcapacityduetothecrackingandbyfailureassessmentdiagrams(FAD).Thereduction

instaticstrengthcomparedtocrackedmediumstrengthsteelswasabout5%greater,possiblydueto

differences incrackpath (the cracks in theSE702steel stayed closer tothe weldwhengrowing).

UsingtheFADapproach,somediscrepancieswerefoundforaTjointwithtwocracks,givinglow

values.Thiswasconsideredpossiblyduetotheinadequacyofthemultiplecrackcorrectionused.

TheFADapproachalsodemonstratedtheimportanceofthefracturetoughnessvalueused.FortheSE702 steel it was necessary to use the KQ (nominal) toughness value rather than the maximum

toughnessvalue,Kmax,forwhichmanyoftheresultswereunconservative.

4.8 IMPACTPROPERTIESLowspeedimpactscanarisefrombothshipimpactanddroppedobjects.Inthesecasestheabilityof

thehighstrengthsteeltoabsorbtheappropriateenergyisoneofthemainperformancerequirements.

Currentcodesand standards specify the levelofimpact energy tobeabsorbed during shipimpact

(typically4MJ)butthisisnotrelatedtomaterialspropertiesoryieldstrength,evenformediumgrade

steels.

High speed impacts can be the result ofexplosions and theenergies ofprojectiles can vary fromseveral kilojoules to several hundred kilojoules. There is no published guidance on the required

materialpropertiesandthepossibleeffectofyieldstrength.

4.9 HIGHTEMPERATUREPROPERTIESHigh strengthsteelcomponents offshorecanbe subjected tohigh temperatures asa resultof fire,

eitherontheseaorfromajetfire.Unprotectedsteelscanexperiencetemperaturesupto1200oCina

short space of time. Guidance is normally associated with either limiting temperatures (e.g. by

passive protection requirements) or bydesign based on data for lower strength steels at elevated

temperatures[4.16].SomenewdatahaverecentlybeenobtainedforsteelswithYS~450MPa[4.17]

(see section 10). For even higher strength steels the data on high temperature performance are

extremelylimited.

REFERENCES

4.01 ASTMstandardA514,’Standardspecificationforhighyieldstrength,Q&Talloysteelplatesuitableforwelding’,2000,ASTM

4.02 ASTMstandardA808,’Standardspecificationforhighstrength,lowalloycarbon,

manganese,columbium,vanadiumsteelofstructuralqualitywithimprovednotchtoughness’,

2000,ASTM

4.03 AWSStructuralWeldingCode,Steel,D1.1-96

4.04 DNVOffshoreStandard,DesignofOffshoreSteelStructures,General(LRFD),2000

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4.05 Dept.ofEnergy,’OffshoreInstallations-GuidanceonDesign,Constructionandcertification’,

HMSO,London,1990

4.06 BritishStandard’Guidanceonmethodsforassessingtheacceptabilityofflawsinwelded

structures,BS7910,1991

4.07 HSE,’OffshoreInstallations-GuidanceonDesign,ConstructionandCertification’,HSE

Books,1995,amendmentno.5

4.08 HSE, ’OffshoreInstallations&Wells(Design&Constructionetc)Regulations’,HSEBooks,

1996

4.09 NORSOK,’DesignofSteelStructures’N-004,1998

4.10 ISO’Petroleum&NaturalGasIndustries,’FixedSteelOffshoreStructures,ISOCD19902,

tobepublished

4.11 InternationalAssociationofClassificationSocieties(IACS),‘Unifiedrequirements,Section

W16,HighStrengthQ&TsteelsforWeldedStructures’,IACS,London,1994.

4.12 DnVOffshorestandard,Metallicmaterials,OS-B101,2000

4.13 Abernethy, K, Fowler, C M, Jacob, R, Davey V.S.,'Hydrogencrackingoflegsandspudcans

onjack-updrillingrigs-asummaryofresultsofaninvestigation',HSEReportOTH91351,

HSEBooks

4.14 NORSOK,'CathodicProtection',M503,1997

4.15 API,RecommendedPracticeforplanning,designingandconstructingfixedoffshoreplatforms,APIRP2A20

thedition,API,Washington,USA

4.16 Talei-Faz B, Dover W D, Brennan F P,‘Staticstrengthofcrackedhighstrengthsteeltubular

joints’,UCLFinalreportforHSE,2001

4.17 Steel Construction Institute, 'Experimental data relating to the performance of steel

components at high temperatures', OffshoreTechnology report, OTI 92 602, HSE Books,

1992

4.18 SteelConstructionInstitute,‘Elevatedtemperatureandhighstrainratepropertiesofoffshore

steels’,OffshoreTechnologyreportOTO0202001,HSEBooks.

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Table4.1

Offshore MaterialsPerformance Codes&Standardsfor Comments

Hazard

Structural

Failure

Requirement

Materialsspecifications

HSS

ASTMstandards,

A514,A808[4.01,4.02],

DnVStandard[4.04]

DnVcode[4.04]covers

steelgradesupto

690MPa

Weldingspecifications AWSCode[4.03] Coverssteelgradesupto

690MPa

Fatigueofwelded

joints,members

Limited Mostcodesprovidea

limitof500MPaon

yieldstrength(YS)for

applicability.Specific

testsareproposedfor

highstrengthsteels(HSS)todevelop

datatosupport

application

Fracturetoughnessof

steels

IACSrecommendations

[4.11]

MinimumCharpyvalues

ofYS/10

Hydrogen

Embrittlement HSEGuidance[4.05,

4.07],DnVRules[4.04]

Controlofhydrogen

assistedcrackingisbest

describedinHSE

GuidanceNotes[4.04]

Staticstrengthoftubular

joints

Factortobeappliedfor

higherstrengthsteelsin

severalstandards,

includingdraftISO

Modifiedfactor,based

onyieldratio(under

discussionindraftISO

standard)

Defectacceptance

criteria

BS7910[4.06] DataavailableforHSS

(yieldstrengthupto

600MPa)

Corrosionprotection Limited,HSEGuidance,

DnVcode[4.04]

Mostdataprovidedfor

mediumstrengthsteels

Inspection&repair Verylimited -

Boat

Impact

Impactperformance,

largestraincapacity

None LackofdataforHSS

Fire(onthe

sea,jetfire

etc.)

Hightemperature

performance

Verylimited LackofdataforHSS

Blast Highstrainrate

performance

Verylimited LackofdataforHSS

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5.FABRICATIONANDWELDING

Mosthighstrengthsteelapplicationsoffshoreinvolveweldedfabrication.Formingandweldingcosts

arethemostsignificantiteminthecostofajacketstructure,comprisingupto57%oftotalcostsin

onereportedanalysis[5.01].Improvementsinthisareaarelikelytocomefromweldingsincecold

forming of plates is generally considered to be an efficient and economical process. Weldingprocesseswhichgivegreaterproductivityand/orincorporateareductionoreliminationofpre-and

post-welding heating could provide major cost savings, and significant progress has been made.

However,asthestrengthofthesteelincreasesthepre-heatingrequirementbecomesgreaterassuch

steelsare usually more highly alloyed. If plate thickness is less than40mm, stress relieving heat

treatmentisnotrequiredforgradeswithyieldstrengthsupto450MPa.Thiscanleadtosignificant

timeandcostsavingsinthefabricationprocedures.Otherareastoconsiderarethedevelopmentof

weldingprocesseswithimprovedwelddepositionrates.

Reportsfromfabricators[5.02;5.03;5.04]indicatethatatleastupto450MPastrengthlevels,welding

isnomoreexpensiveordifficultforawellorganisedyardthanweldingthenormal355grade.With

thehigheststrengthgrades,however,moreprecautionshavetobetaken.

Weldabilityofsteel isa termthatisusedto indicatetheeasewithwhichsoundweldmentscanbe

produced using normal welding procedures. The weldment comprises both the weld and the

associatedheataffectedzone.Weldingdefectssuchasporesandcrackscanbeproducedaswellas

undesirablemicrostructures in the weld andits associated heat affected zonewhichcan lower the

resultingmechanicalpropertiesofthejoint.

Variations in the welding process, such as steel dimensions, weld geometry, heat input and steel

compositionallinfluence theresultingmicrostructure.Nomogramsinvolvingthermalseverity- joint

thickness (mm),heat input of the weld (kJ/mm) and weld preheat required (ºC) are oftenused to

indicate the necessary welding procedure to be followed to produce a sound crack-free joint in

relationtotheparticularcompositionofthesteelusedwhichisusuallyrelatedtocarbonequivalent

value.Ingeneralasteelwithlowercarbonequivalentvaluehasimprovedweldabilitycomparedtoahighercarbonequivalentsteel.Thetwomostcommonlyspecifiedcarbonequivalentequationsare

thatrecommendedbytheInternationalInstituteofWeldingwhichcoversawiderangeofsteels:

Mn Cr& Mo& V Ni& CuCE- CE IIW- C& & &

6 5 15

andtheItoandBessyoequivalentwhichisoftenpreferredformodernlowcarbonsteels:

Si Mn& Cu& Cr Ni Mo VCE- PCM - C& & & & & & B5

30 20 60 15 10

ThislatterequationistheoneusedforhighstrengthsteelsinthedraftDNVMetallicMaterialsOS-

B101 Standard (May 2000). In this guidance document, steels with improved weldability have

reduced carbon contents and limitations on the levels of chromium, nickel and molybdenum

comparedtosteelsofnormalweldability,i.e.theymusthavereducedCEvalues.

An alternative approachmore commonlyused inotherpartsofthe world is the Gravillediagram

showninFigure5.1whichseparatesthesteelsintothreezonesratedbytheireaseofweldability–

zoneIeasilyweldable,zoneIIweldablewithcare,andzoneIIIdifficulttoweld,Fromthisdiagram

itcanbeseenthatweldabilitydecreasesasthecarbonequivalentvalueincreasesbutthediagramalso

emphasises theextremelyimportanteffect ofcarboncontent onweldability. Reducing thecarbon

contentofasteelisthemosteffectivewaytoimproveitsweldability.

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Astheparentstrengthincreases,greaterprecautionsareneededtoensurethatweldingproceduresare

satisfactory. The strength increases in the weld are normally produced by alloying since

strengtheningproceduressuchasthermomechanicalprocessingcannotbeutilisedintheweldmetal.

Thewelds therefore become more hardenable andprecautions are required toprevent weld metal

hydrogencracking. Theweldability ofmodern steels hasbeengreatly improvedbytheirextreme

cleanliness, and by their low carbon content and low carbon equivalent values. Low hydrogen

consumablesare important inreducingthepossibilityofhydrogencrackingandcanalsoleadto a

reductioninthepre-heatingrequirements.Nomajorproblemshavebeenreportedinweldingsteels

upto500MPayieldstrength[5.04]inmoderatesectionsizes.Athighstrengthlevels,preheatingis

requiredand steelmakersaredevotingconsiderable attention to improving the weldabilityof such

steelstotrytoreducefabricationcosts.For690gradesteels,forexample,preheattemperaturesof

125ºCarerecommended,andelectrodesandfluxeswithverylowhydrogencontentmustbeusedin

ordertopreventhydrogencracking[5.05].

TheformationofhardorbrittlephasesintheweldHAZor,indeed,inthewelditselfduringmulti

passwelding,canaffectthetoughnessoftheweldanditsabilitytowithstandexposuretohydrogen.

Important factors are the grain size in the grain coarsened HAZ near to the fusion line and the

microstructural changes that occur in the weld metal during subsequent weld depositions duringmulti-pass welding. In general, the Charpy toughnessof the coarse grained HAZ decreaseswith

increasingheatinputandincreasingimpuritycontent. Becauseof thisthereareoftenrestrictionsin

the upper levels of heat input that can be used (e.g. 3.5kJ/mm for submerged arc welding) but

productivity is not greatly compromised because of the generally thinner sections and smaller

volumesofdeposited weld metalutilised. Steel that couldbewelded satisfactorilyathigherheat

inputlevelswouldoffereconomicadvantages[5.06]andrecentwork[5.07;5.08]showedthatcertain

steels and weld consumables did satisfy these requirements and could offer further economic

advantages.

Published literature indicates that there are weld consumables which can produce the necessary

materialpropertiesrequiredinservice,evenforthe690gradesteels[5.09].However,atthehighest

strength levels envisagedthereis muchlessexperienceandavailability ofweldconsumableswithsuitable properties, particularly in respect of toughness. In addition, significant pre-heating and

interpass control are necessary in order to avoid hydrogen cracking problems. Weld metal

microstructuresaredeterminedprimarily by thechemicalcomposition, theamountofnon-metallic

inclusionspresentinthemicrostructurewhichaffectphasenucleation,andbythecoolingrate.Alloy

design aims to maximise the amount of acicular ferrite present and to minimise the effects of

undesirable microstructures such as coarse grain size, grain boundary ferrite and coarse

martensite/austenite/carbideconstituents(MAC).

The welding consumables employ sophisticated alloying techniques, incorporating the optimum

balance ofdeoxidisingelements (aluminium,siliconand manganese) toproduce ahigh densityof

small non-metallicinclusionswhichareknown to act as intragranularnucleationsitesforacicular

ferrite. Thecarbon content is generally kept low to aid weldability, so the increased strength isachievedthroughadditionsofmolybdenuminSAWwiresandtitanium-boroninFCAWwires,and

theimpacttoughnessisimprovedwithnickeladditions.IntheCranfieldstudy[5.08]consumablesup

to550MPayieldstrengthshowedadequatetoughnessthroughouttheweld,withuppershelfvalues

>150Jandthe50Jimpacttransitiontemperaturesbelow-60°C.Allweldshadlowhardnessvalues

and showed no indication of hydrogen cracking. Acicular ferrite was the major microstructural

featureoftheweldsandmicrostructuresgenerallycoarsenedasheatinputincreased.

ForbothSAWandFCAW,690MPaconsumablesshowedmixedmicrostructurescontainingacicular

ferrite,martensiteandpolygonalferrite.Impactpropertieswereinferiortothe550MPaweldswith

uppershelfvaluesrangingfrom80-100Jand50Jimpacttransitiontemperaturesbetween–50and-

80°C.Itwasconcludedinthisstudythatmoredevelopmentworkisneededbeforetheseconsumables

canbespecifiedgenerallyforoffshoreapplication.

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Inmostweldedstructuresitisconsidereddesirabletoovermatchtheyieldstrengthoftheparentplate

andtherelatedHAZ.Thisisbecauseiftheweldsundermatchthenanyenforceddeformationwillbe

concentratedintherelativelysmallweldmetalvolumesleadingtohighstraininthesezones.Ifsuch

zoneshavereducedtoughnessvalues,whichisgenerallythecaseatthehigheststrengthlevels,then

there is an increased likelihood of failure. Weld metal specifications often call for 20 to 30%

overmatch which is easily achievable in 450 and 550 grades with satisfactory weld property

performance.Theproblemarisesinproducingthenecessarycombinationofpropertiesintheweld

metal requiredat the highest strength levels, i.e. 690 grade. In otherapplications such asstorage

tanks,undermatching weldshavebeenused inhigh strengthsteel structures which have generally

performed satisfactorily because the weld metals have good toughness. The normal statistical

variationsinyieldstrengththatoccurinsteelplate(discussedinSection3),alsooccurinweldmetals.

Thisposesanadditionalproblembecausethereisadistinctpossibilitythat,unlesssignificantlevels

ofovermatcharespecified,thelowerstrengthweldmetalswillundermatchthehigheststrengthparent

materialinparticularprojectfabricationprogrammes,leadingtocertainjointsbeingundermatched.

The importance of this effect is provoking considerable interest at the moment, particularly with

regardtothehighergradesteels.

Ithasbeenreportedthattheresidualstressesinrestrainedhighstrengthsteeljointsarelessthanthoseencounteredinlowerstrengthsteelswhichcanhaveanimportantinfluenceonsubsequentfatigueand

fracturebehaviour.Bennettetal[5.05]claimedthattheresidualstressesina40mmthickrestrained

jointwereonlyapproximatelyonehalfofthoseobtainedwithmildsteelandwerelargelyindependent

ofheatinput. Theexplanationofthiseffectwasthoughttobepartialcounterbalancingofthethermal

contraction stressesby themartensitic/bainitic transformationwhich occurredduring cooling. The

potential benefit of this different behaviour seems not to have been utilised significantly to date.

Furtherworkisneededtoprovideconfidenceinthisapproach.

ArecentreportdetailstheweldingpracticesandproceduresusedforweldingtheElginjacket[5.09].

Highstrengthsteels,Superelso500andSuperelso600,wereusedontheprojectinlegchords.These

materialshavespecifiedminimumyieldstrengthsof450MPaand550MPsandyieldratiosof0.78

and 0.80. Significantuse was made ofMMA welding during thefabricationbecause of its fullypositionalweldingonsitecapability.Oerlikonelectrodes,Tenacito70,wereusedwhichgaveyield

strengthsbetween490and550MPaandexcellentCharpyV-notchtoughnessvaluesof130Jat-40ºC.

Thewelding of thechord to theprefabricated 160mmthick rack sections was carried out with a

minimumpreheating temperature of150ºC followedbya dehydrogenationtreatmentof2 hours at

200ºC. Verylowrepairrates(


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5.07 HighStrengthSteelsinOffshoreEngineering,MTDPublication95/100,ISBN1-870553-21-

7,1995

5.08 Billingham J, Blackman S, and Norrish J, ‘Furtherassessmentofhighstrengthweldmetals

foruseinoffshoreengineeringapplications’,CranfieldFinalReport,January1998

5.09 Bews R, ‘MMAWeldingGasfromStrengthtoStrength’WeldingandMaterialsFabrication,

July/August2000.

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Figure5.1CriteriaofSteelWeldability–CrackingSusceptibility

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6. TOUGHNESS

Toughness may belooselydescribed asa measureofthe resistance tofailurein thepresenceofa

crack,notchor similar stress concentrator. High toughness therefore isgenerally recognised asa

desirablepropertyforoffshoresteels.

Ahightoughnessmaterialisonewhereaconsiderableamountofplasticdeformationisrequiredat

thecracktipbeforethecrackcanbemadetoadvance.Conversely,iftheapplicationofstresscauses

theelasticfailureofatomicbondsatthecracktip,relativelylittleenergyofdeformationisinvolved,

andtheresultisabrittlefracture.

Theword‘toughness’isusedfortwoquiteseparatequantities. Theyaremorecorrectlydescribedas

‘ImpactToughness’and‘FractureToughness’.

Impacttoughnessisanenergymeasurement(Joules,orft-lbs)andcommonlyrelatestotheCharpyV

notchtest.Fracturetoughnessisacalculatedvalueforthecriticalstressintensityfactor(N.m-3/2

or

MPa -tip-opening-displacement(CTOD)testsor

J-integraltests.

Relationshipsbetweenthesequantitiesareempirical.Therelationshipshavebeenwellvalidatedover

manyyearsforstructuralsteelsinmoderatesectionthickness.Thishaspermittedthemorereadily

availableCharpyimpactdatatobeusedasanindicatortotheadequacyofthefracturetoughness.

Wherethesamerelationshipsbetweenimpacttestingandfracturetoughnessareextendedtothick

sectionedmaterialandtohighstrengthsteel,specificationsshouldbeviewedwithcautionuntilithas

beendemonstrated thatadequatefactorsof reserveare incorporated forthenewconditions.Direct

testing for fracture toughness may be preferable, particularly since it can reduce some of the

uncertaintiesrelatedtotheeffectofmaterialthickness.

Inferriticsteels,thefracturetoughnessisaffectedbytemperature,bystrainrateandbygeometry.

Thelatter influence isalsoknownas ‘the stress state’, ‘thedegreeof triaxiality’ or‘the thickness

effect’.Theapparentchangesintoughnessthatresultfromthegeometryarenotquantifiedatallby

theCharpytest,whichalwaysusesastandardsmall(10mmthick)specimen.Despitethis,Charpy

resultsarewidelyusedinmaterialsselectionandincurrentcodesandstandards.

6.1 DUCTILETOBRITTLETRANSITION

Boththeimpacttoughnessandthefracturetoughnessofferriticsteelarecharacterisedbyaductile-to-

brittletransitionasthetemperatureisreduced.Thiscorrespondswithachangeinthemechanismof

crackmovement,fromplasticbluntingandplastictearing(ductilecontrol)at thehighertemperaturetocleavage(brittlefracture)atthelowertemperature.Thetransitionoccursoverarelativelynarrow

range of temperature, typically 30ºC, but often involves considerable experimental scatter. As a

result,thereareseveraldifferentdefinitionsofthetransitiontemperaturefromthesamedata.Low

transitiontemperaturesandhigh‘upper-shelf’valuesoftoughnessareseenasbeneficial.

Thetransition temperature (TT) isnot an invariant propertyof a givenmaterial,even for a fixed

composition,grainsizeetc. TheTTvarieswiththestateofstress(whichmeansthatitdependsonthe

sizeandgeometrythathasbeenused)andrateofloading.Increasingthethicknessofthespecimen,

orincreasingtherateofloading,produceanincreaseintheTT.

Without these complications, it would be relatively simple to avoid brittle fracture - the basic

requirementwouldbetoselectmaterialforwhichtheTTwasbelowtheservicetemperaturerange.Itwouldstill benecessary to take into account that fabrication processes such asweldingaffectthe

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materialanditsTT,bothasaresultofchangesinthematerialfromthethermalcyclingandfromthe

introductionofflaws(thathavetheeffectofincreasingtheTT).Itwouldalsobenecessarytoensure

thattheresultsrelatedtothecorrectenvironmentalexposure.

Unfortunately, the commence

Documents

Review of the Performance of High Strength Steels Used Offshore