DETERMINISTICPIPELINEINTEGRITYASSESSMENTTOOPTIMIZECORROSIONCONTROLANDREDUCECOSTM.J.J.SimonThomas,L.H.PragerandC.V.M.VoermansShellGlobalSolutionsInternationalB.V.Badhuisweg31031CMAmsterdamTheNetherlandsB.P.MiglinandB.F.M.PotsShellGlobalSolutions(US)3333Highway6SouthHouston,Texas77082l.J.Rippon,ShellGlobalSolutionsInternationalB.V.Volmerlaan82288GDRijswijkTheNetherlandsABSTRACTAmethodologywasdevelopedtoimprovetheintegritymanagementofpipelines,takingaccountofinternalcorrosion,externalcorrosionandthirdpartydamage.Thesystemcanbeusedtodefinethecorrosioncontrolandmonitoringneedsofnewandexistinglines.Italsoservestodeterminetheintegrity,optimizeinspectionandmanagerepairandrehabilitationofexistinglines.Theapproachtotheevaluationofthedegradationrateandfitnessforpurposeofthepipelineisdeterministic.Themethodologyconsiderstherelativecontributionofallrelevantfactors,suchasCO2,H2S,02,organicacidsandbacteriaoninternalcorrosion,soilcorrosivity,coatingconditionandcathodicprotectiononexternalcorrosionandoperatingenvironmentonthirdpartydamage.Theinspectionplanningfollowsarisk-basedapproach.Thesystemhasbeenappliedtoalargenumberofpipelinesallovertheworldandshowntoaddvaluetothebusinessofoperators.1INTRODUCTIONPipelineintegritymanagementhistoricallyhasbeenbasedprimarilyonthecompositionoftheproducedfluids(internalprotection)andthesoilconditions(externalprotection).Thisapproachhasresultedinexcellentdesignsfromthepointofviewofthepipelineprojectengineer.Forexample,CO2corrosionisusuallyaccountedforinmuchdetailandtheuseofalternativematerialsorcorrosioninhibitionisselectedonthebasisofaccuratemodelstocopewithinternalcorrosion.Inasimilarmanner,suitablecoatingsandcathodicprotectionsystemsaredesignedtopreventexternalcorrosion.Oncethepipelineisinservice,itoftenturnsoutthatotherfactors,usuallyofanoperationalnature,poseaseriousthreattotheintegrity.Inmanycasesoxygeningressandthepresenceofbacteria,orthereliabilityoftheinhibitorpumparedeterminingtheinternalcorrosionrateandhencetherelatedoperatingexpenditures.Thecorrectoperationofcathodicprotectionsystemsandtimelycoatingrepairstendtodeterminethecostsofexternalcorrosion.Anothermajorclassofthreatsisthatofthirdpartydamage,whichcanvaryfromaccidentaldamageduetodiggingortrawlerboards,tosabotageandtheftofproduct.Historicaldataonpipelinefailuresareusedtodeterminethemainexposurestopipelineintegrity,henceenablingmeasurestobetakenonfailureprevention.Itshouldberealizedthatthefailurefrequenciesarehighlydependentonthelocalcircumstances.E.g.,Table1showsthatthemaincausesoffailureinEuropeandtheUSareexternaldamage,mechanicaldamage(manufacturingandconstructiondefects)andcorrosion1.Itisobviousthataneffectiveintegritymanagementprogramhastotakeallpotentialthreatsintoaccount.Monitoringandinspectionprogramsshouldbedesignedsuchthattheydetectandpreferablynegatethesethreatsinatimelymanner.Theprogramshouldbeusedtore-directpipelinemaintenanceonthebasisofthemonitoringandinspectionresults.Itshouldalsoproviderelevantinformationtodirectrepairandrehabilitationofthepipeline.Aprocesswasdeveloped,whichformsthebasisforasoftwarepackagenamedPIPERBA(1).IthasbeenadoptedbymostmajorShellpipelineoperatorsaswellasbysomethirdparties.Thewholepackagehasbeendescribedelsewhere2.Thispaperconcentratesondescribingtheintegrityassessmentpartofthisprogram.DETERMINISTICINTEGRITYASSESSMENTThebasicintegritymanagementprocessisdepictedinFigure1.Themostimportantinputsandsupporttoolsareshownaswell.Themainstepsoftheintegritymanagementprocesswillbediscussedfortheprincipalthreatsconsideredinthecurrentversionoftheprogram,internalcorrosion,externalcorrosionandthird-partydamage.o)ProprietarycomputerprogramPIPE-RBA=PipelineRisk-BasedAssessment2InternalcorrosionThemostcommoninternalcorrosionmechanismsinpipelinestransportingproducedfluidsareCO2corrosion,H2Scorrosion,organicacidcorrosion,oxygencorrosionandbacterialcorrosion.In-housemodelsexisttocalculatetheexpectedcorrosionratesifnoprotectivemeasuresaretaken.TheearlierCO2corrosionmodelhasbeendiscussedelsewhereindetail3,4.ThelatestversionoftheresultingcomputerprogramHYDROCOR2)alsoestimatesthecorrosionduetothepresenceoforganicacids,oxygen,bacteriaandhydrogensulfide5.Aspecialversionwasdevelopedtosupportthepipelineintegritymanagementprogramdescribedinthispaper.Figure2showstheinputandoutputscreen.Forconvenience,theinputfieldsarepre-selectedonthebasisofthetypeofservicethelineisoperatingin.Thiswaytheuseronlyhastogatherandenterrelevantdata.Figure2showsthattheprogramalsotakeskeyoperationalexperienceintoaccounttoadjusttheestimatesofthecontributingcorrosionrates.Thisincludessuchfactorsasthepigginghistorytomoderatethechancesofunderdepositcorrosionandtheavailabilityofnutrientsforbacterialgrowth.Finally,itconsidersthechemicalsavailability,i.e.thecomplianceofchemicalsinjectionprogramswiththeagreedstrategy.Thisavailabilityofcorrosioncontrolisdeemedtobeextremelyimportantandcanbeexpressedasfollows,inthiscaseforinhibition:CR=FinhXCRinh+(lFinh)XCRuninh(1)whereFinhstandsfortheinhibitoravailability,CRinhistheinhibitedcorrosionrateandCRuninhtheuninhibitedrate.Ifthesystemreliesonbiocideinjectiontocontrolbacterialcorrosion,thesubscriptsinhibitedanduninhibitedshouldbereplacedaccordingly.Asimilarapproachisappliedtocalculatetheconsequenceoftheavailabilityofotheroperationalsystems,e.g.dewpointcontrol.Thelaststepistocombinethecalculatedcorrosionratewithcorrosionmonitoringdata,inspectiondataandoperationalexperience.Thisaspectwillbediscussedinmoredetailbelow.ThusthestepsthatwereshowninageneralformatinFigure1,canbeshownforinternalcorrosionasinFigure3.Itisnotedthatinthisterminologycorrosionmonitoringincludescorrosionratemeasurements,residuals,trackingofoperationalparameterssuchascomplianceofthechemicalinjectionsystemwiththeagreedgoals,etc.Externalcorrosion-onshorelines(buriedorexposed)Theexternalcorrosionthreatsthataretakenintoconsiderationareatmosphericcorrosion,corrosionofburiedsteel,corrosionduetointerferenceofcathodicallyprotectedsystemsandfailuresofisolationjoints.Protectionagainstexternalcorrosionisalwaysbasedonanexternalcoatingandcathodicprotectiontopreventcorrosionatdamagedareasofthecoating.Thepipelineisanalysedtodeterminethechancesthatthesethreatsresultincorrosion.Thefirststepistodeterminetheunprotectedcorrosionratebasedonthesoilcorrosivityalongthepipelinefrommeasuredresistivitydataandsoiltype.Thisdeterminestheprincipalexternalcorrosion(2)ProprietarycomputerprogramtocalculateCO2corrosion,estimatecorrosionduetobacteria,oxygenandorganicacids.3threatandformsthebasisforthedesignofthecorrosioncontrolsystemsinplace.TypicalcorrosionratesofburiedsteelaregiveninTable2.Nextthequalityofthecathodicprotectionisassessed,basedontestpostandcloseintervalpotentialsurveydata.Thelatterarealsousedtoassessprotectionagainstinterferencerisksandtheoccurrenceofisolationjointfailures.CRextemal,CP=FxCRprotected-I-(l-F)xCRunprotected(2)whereCRarethecorrosionratesandFistheavailabilityofthecathodicprotectionsystem.ThefactorsareshowninTable3.Thisresultsinanestimatedcorrosionrateofexposedsteeltakingintoconsiderationtheperformanceofthecathodicprotectionsystemforacertaintimeperiod.Thetypesofinterferencethataretakenintoaccountarecathodicprotectionofotherlines,eithercrossingorparalleltothelineunderconsideration,isolationjointsandflanges,DCandACtraction,andpowerlines.Thecoatingeffectivenessisassessedfromcurrentdensitymonitoringdataandoverthelinepotentialsurveysasdiscussedbelow.TheexternalcorrosionassessmentprocessisshowninFigure4.ExternalCorrosion-offshoresubsealinesForsubsealinestheexternalcorrosionthreatsthataretakenintoconsiderationarecorrosioninseawater,corrosionduetointerferenceofcathodicallyprotectedsystemsandfailuresofisolationjoints.(Thelattertwoaremuchlessimportantthanforonshorelinesbecausealmostalltheoffshorecathodicprotectionsystemsusesacrificialanodes).Protectionagainstexternalcorrosionisalwaysbasedonanexternalcoatingandcathodicprotectiontopreventcorrosionatdamagedareasofthecoating.Thepipelineisanalysedtodeterminethechancesthatthesethreatsresultincorrosion.Forimpressedcurrentcathodicprotectionsystems,asimilarapproachtodeterminingtheextemalcorrosionrateisappliedasfortheonshorecase.Forsacrificalcathodicprotectionsystems,theexternalcorrosionratewillbezero