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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
formulaeislimitedtosteelswithyieldstrengths
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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
750 Q&T Jack-ups&Moorings
850 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.
6
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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
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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
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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
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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