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OilandGasEngineer

Specificationrequirementsforsteelundersourservice21stFebruary2013

Formorethan50yearsdocumentshavebeenpresentedaboutthesourservicedamageofsteel.Inspiteofthatlongperiodoftimeoil,gasandengineeringcompaniesaswellasmanyorganisationslikeNACEorEFCarestillworkingonimprovedsourserviceregulationsinordertoavoidsevereanddangerousdamageoftheequipment.Generalrulesorrecommendedpracticestreatmainlythepreventionofsourservicerelatedhydrogeninducedcracking(HIC).

Thisarticlesummarisessuchrequirementsinthefieldofpressurevesselapplicationsespeciallyfocussingonspecificationrequirementsfortheselectionandtreatmentofthesteelplates.ItwillalsobeshowninwhichwayasteelproduceruseshisknowhowandhislargeexperiencetoassureahomogeneousHICresistanceofthepressurevesselplates.Thisisonlypossiblebyapplyingaspecialproductionroute.Sometimessteelplatespecificationsaremadeinordertoprescribethemanufacturingprocessandthematerialpropertiesaimingtoreceiveahighqualityproductwithexcellentinservicebehaviour.Howeverthishastobedoneonlywiththenecessaryknowledgeabouttheinfluenceofthespecificsteelmakingparametersonthesourservicepropertiesinordertoreachtheaimedresistanceandtoavoidnonacceptableorcontradictoryspecificationitems.

Althoughtheproblemoffailureofpressurevesselsoperatedinsourserviceshasbeenfacedforseveraldecades,researchisnowadaysstillgoingontoenableenduserstooperatetheirinstallationssafely.Thephenomenonofhydrogeninducedcracking(HIC)seemstobeunderstoodandtheexistingstandardsas,forexample,NACETM0284(1),EFC16(2),whichenabletheoperatorstospecifytheappropriatesteelplatesforsourserviceenvironmentarewidelyused.Astheseprescriptionsareadditionallymixedwithfurtherrequirementsegfromoilandgascompaniesthevarietyofsourservicespecificationsisverylarge.Itisachallengeforthesteelmanufacturertocoverthesespecificationsbyanappropriateproductionandmarketingconceptforthesespecialsteels.ItisimportanttoestablishaneconomicplateproductionrouteprovidingplateswithwarrantedhomogeneousHICresistance.

Corrosionmechanisms

Thefirststeptohydrogeninducedcorrosiondamagesistheabsorptionofhydrogenbythesteel.BasicconditionthereforeisacorrosionreactioninawetandsourenvironmentunderpresenceofH2Saspromoter.Thecorrosionreactioncreatesasourceofatomichydrogenmovingfreeinthesouratmosphere.Aftertheatomichydrogenisabsorbedinthesteelitwillmigratetoareassuchasimperfections

orinclusionswhereitrecombinestoH2.Thismayleadtoenormouspressurescausingcracks.TheprincipleofthedamagereactionisshownschematicallyinFig.1.AmongthevarioustypesofdamagemechanismswhichcanbefoundinthepresenceofH2SthemostlyunderstoodcaseisHIC.Cracksofthistypearemoreorlessparalleltotheplatesurfacewithsomesteps.Mostlytheyarelocatedinmidthicknesspositionoftheplate.HydrogenBlisteringisaspecialcaseofHICduetothevicinityofthematerialsurface.

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HICtestingprocedure

TestspecimenscutfromtheplateinaccordancewiththetestmethodareexposedinaH2SsaturatedsolutionwithdefinedpHlevel.Afterexposureoftypically96hoursinthesolutioneachspecimenisequidistantlycutintothreecrosssections.ThesesectionsareinvestigatedafterpolishingfortheamountofcracksasshowninFig.2.

ThemostcommonspecificationforevaluationofHICsusceptibilityisNACETM02842003:EvaluationofPipelineandPressureVesselSteelsforResistancetoHydrogenInducedCrackingOrigin:1984forevaluationandcomparisonoftestresultsTestsolution:pH3(solutionA)orpH5(solutionB)bothsaturatedwithH2STestspecimens:positiondimensionsandpreparationdefinedDuration:96hEvaluation:equidistantcuttingmetallographicevaluationnoacceptancecriteriadefinedthesearetobeagreedbetweenpurchaserandsupplier.

Specialproductionroute

AtDillingerHtteallHICresistantplatesbothforpressurevesselsandforlinepipeareproducedfollowingaspecialroutewhichensuresgoodmaterialpropertiesinregardtosourserviceoperation.TheproductionrouteisappliedtoobtainahomogenousHICresistanceleveloverthewholeplateandconsistsofspecialtechnicaltreatmentsandimprovedqualityassurancemeasures(3).

ThevariousdifferentprocessesappliedinthesteelplantareshowninFig.3.Afterapredesulphurisingthedecarburisationdenitrogenisationanddephosphorisationintheconvertertakeplace.Afterwardstheanalysisadjustmentisperformedintheladle.Inafirststeptogetherwithadjustmentoftemperatureoftheheatforlatercastingaroughadjustmentofthechemicalanalysisiscarriedout.

Subsequentlythefineadjustmentofthechemicalanalysisandtheremovalofnitrogenhydrogenandsulphurarecarriedoutundervacuum.Oneofthemaintargetsincastingbesidestoobtaintheinitialgeometryforrollingistoavoidsegregationsinthesteel.IncontinuouscastingverticalcastertypesshowbestresultsinregardtohomogeneouslowleveldistributionofnonmetallicinclusionsandsegregationswhichisessentialforHICresistantsteels.ThereasonisshowninFig.4.Inthecurvedcastertypetheinclusionsgettrapedatthesoliduslineinroughlyquarterthicknessoftheslab.Inverticalcastingthecaststrandwillonlybebentaftercompletesolidificationthisallowsnonmetallicinclusionsmostlyoxidesintheliquidsteeltofloattothesurface.Thecentrelinesegregationandshrinkagecavitiescanbeminimisedbysoftreductionandexactalignmentoftherollsofthecaster.IngotcastingisalsousedtoproduceHICresistantplates.

Platemakingprocess

Aftercastingtheslabswillbepreparedforthefollowingrollingprocessasegnotthewholelengthofthecaststrandcanbeusedforplateproduction.Reasonsthereforemaybeegsocalledcastingincidentsinfluencingthehomogeneityoftheslab.Thiscanoccurifoneofthelargenumberofproductionparametersisoutofthespeciallyadjustedtargetvalue.Qualityassurancemeasureshavetobetakenatthebeginningandtheendoftheincidentriskrange.

Afterbeingreheatedinthepusherfurnacestoalevelof1100to1250Ctheslabsarerolledtoplates.Theapplicationofthehighshapefactorrollingensuresacompletedeformationeveninthecoreofthefinalplatebyusingveryhighformingforcesandcreatesahomogenousmicrostructure.Afterhydrogeneffusionoftheplatethefinalstepintheplatemillisthenormalising.HoweverthefullHICresistanceoftheplatecanonlybeobtainedafterastressrelievingtreatment.Asnormallytheplatesundergoastressrelievingafterweldingatcustomer'sworkshopthisisusuallynotdoneinthemill.

TheeffectofthespecialproductionrouteontheproductpropertiesisshowninFig.5.InthispicturetheHICvaluesobtainedintheHICtestaccordingtoNACETM0284betweenintentionallyproducedHICresistantplatesandplatesspecifiedwithsome(eglowsulphur...)elementsofthespecialproductionroutearecompared.ThesesocalledPseudoHICplatesshowaprobabilityofabout37percentforobtainingvaluesoflessthan2percentCLRwhereasplatesproducedfollowingtheentirespecialproductionroutearelikelytoobtainCLRvaluesbelow2percentinapproximately88percentofthecases.ThismeansthatthespecialrouteprovidesnotonlyclearlybetterresultsinregardtotheHICpropertiesbutalsothedeliverytimesaresaferastheriskinfailingtheHICtestisverysmall.

ThemarketinHICresistantplatesiscomplexasalmosteveryoilandgascompanyaswellasmajorengineeringcompaniesissuetheirownspecificationforHICresistantplates.Oftenthesespecificationsincluderestrictionsforseveralproductionparameters.SpecifyingproductionparameterslikesulphurphosphoruscalciumtosulphurratioZ35propertiesandsooninducesdifficultiesasthislimitsthepossibilityofspecialsteeldesign.

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TheresultofthispracticeisoftenenoughthatasteelwithalltherestrictionsandhomogenousHICresistancecannotbeproduced.Insteadofinterferingsteelproductionbyadditionalprescriptionsitwouldbemorehelpfultogivefreedomforsteeldesigntothemanufacturerandspecifyotheritems.IfHICresistantmaterialisneededtheresistancelevelshallbespecifiedproperlytogetherwithamanufacturerswarrantyforhomogenousmaterialpropertiestoavoidPseudoHICmaterialtobeoffered.TheuseofPseudoHICplatescreatesanadditionalriskinoperatinginstallationsasshowninFig.5.

ToofferbestservicetoendusersDillingerHtteisalwaysinterestedtobeincontacttospecificationwritersinordertogivesupportfrommanufacturer'ssideandtodiscussspecifications.AdiscussionwithenduserssharingtheirexperiencesusingHICresistantplateswouldalsobeveryfruitfulandnecessaryforthefurtherdevelopmentofthesesteeltypes.WhoeverwantstodiscussthisshouldfeelfreetocontactDillingerHtte.

Apartfromspecificationsthemanufacturersfaceotherchallenges.VeryoftentherequestedquantitiesofHICresistantmaterialareverysmallandgodowntoasfewas20tperorder.Ifyoutakeintoconsiderationthateconomicsteelproductionisboundtolargecastingsequencesitisclearthattherehavetobemeasurestoenablehighqualityproductionatreasonablecost.Toalloweconomicalproductionacertainstandardinproductionwasdefined.Fig.6showssuchastandardisationforplatematerialforsourservice.

TheCLRCTRandCSRvaluesshowninFig.6canbeofferedasaverageofallthesectionsexamined.Howeverseveralspecificationsrequirethesevaluesasanaverageofthreesectionsorevenassinglevaluesoutoftheinvestigationofonesection.Inthesecasesthetechnicalfeasibilitydecreasesasproductionriskincreases.

RecentinvestigationsshowedthatthestandardisedofferschemedisplayedinFig.6canbeextended.Theprogressconcernsboththerollingofthickerplatesandimprovedcapabilitiesinregardtoinvestigationsofsinglecrosssectionsorcrosssectionsfromonetestbar.Fig.7showstheseextendedpossibilitiesofDillingerHtteGTS.InadditiontothepublishedvaluessofarDillingerHttenowoffersDICREST5materialuptoaplatethicknessof110mmandisalsoabletoofferegCLRvaluesof10percentasanaverageofthreesectionsexaminedandCLR15percentforsinglesectionexamination.AnyotheradditionalrequirementissubjecttoagreementbetweenDillingerHtteandthepurchaser.

AnotherchallengeamanufacturerfacesforHICresistantplatesistheurgentneedformaterialincaseofinstallationbreakdown.Inthiscasetheuserimmediatelyneedsplatesfulfillingall(most)requirementsoftheinitialspecification.Theshortdeliveriesandsmallorderbatchescannormallynotbemetbyamanufacturer.FortheseurgentcasesDillingerHttetogetherwithAncoferWaldramSteelplatesintheNetherlandsandDillingerMiddleEastintheUnitedArabEmiratesoffersHICresistantplatesfromstockwithastrongspecificationcoveringalotofthemostcommonrequirements.ThismakesitpossibletogetHICresistantplatesevenwithshortdeliveriesincaseofemergency

Conclusion

H2Srelatedcrackingisrecognisedasadangerinthepetroleumindustryandtheimportanceoftheissueisgrowingasmoreandmoreequipmentoperatesundercriticalsourserviceconditions.

ToobtainplateswithhomogenousHICresistanceoverthewholeproductaspecialproductionrouteisnecessary.Aplatemanufacturercanbefacedwithproblemsduetospecificationsprescribingelementsoftheproductionroutebylimitingcertainparameterssuchascalciumtosulphurratioorphosphoruscontent.

Themorecustomersspecificationstakeinfluenceonthesteeldesignthemorethiscancreatedifficultiestoapplyanadaptedproductionroute.Thiscanleadalsotounnecessarycostincrease.ThebetterwaytospecifyHICresistantplatesistodefinetheacceptancecriteriaandthetestmethodandtospecifyamanufacturerscommitmenttoachievethisminimumspecifiedtestcriterialevelhomogeneouslyoverthewholeplate.ThemethodtocalculatetheHICacceptancecriteriameanshowtocalculatetheaveragevaluesstronglyinfluencestheacceptancelevelforHICrequirementsastheproductionriskchangesadequately.

DillingerHtteoffersstandardisedHICresistantDICRESTplatescoveringalargevarietyofinternationalsourservicespecifications.Itisalsopossibletoproducesuchplateswithatailormadedesignaccordingtoindividualcustomerneeds.

Enter57oratwww.engineerlive.com/iog

AnjaDemmerathGnterLuxenburgerMartinSteffenAndreasThiemearewithDillingerHtteGTSDillingenSaarGermany.www.dillinger.de(http://www.dillinger.de)

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