CathodicProtectionDesigninDeepWater:BeSafe,NotSorry!byJimBritton(1999)AbstractOffshore structures in deepwater are now quite common.However, a deepwater developmentprojectstillrequiressignificantcapitalinvestmentonthepartoftheoperator.Corrosionfailureisnotacceptable.Thispaperpresentsacommon senseapproach tocathodicprotectiondesignondeepwaterprojects.SomepracticaltipsforavoidingpitfallsarepresentedaswellasananalysisoftheroleofROV'sinmaintenanceandmonitoringoftheseassets.IntroductionTheworldwidedevelopmentofdeepwateroilandgasprospectshasresulted intheemergenceofmanyalternativeschemesandstructuresdesignedtoproduceoilandgasmorecosteffectively.Earlydevelopmentsused large fixed jacketstructuressuchastheShellCognacandBullwinkleplatforms,whicharelocatedinwaterdepthsof1000feetand1350feetrespectivelyintheGulfofMexico.Asdrillingmovedtoevengreaterdepths,thesefixedstructureswerenolongercosteffective,andnewstructuredesignsemerged:TensionLegPlatforms(TLP's),FloatingProductionSystems(FPS's),SPARdesigns.Allofthesestructureshaveprocessfacilitieslocatedonthesurface,abovesubseawellheads.Thecompletionofremotesubseawellsconnectedbyflow linesandcontrolumbilicalstoasurfaceproductionfacilityisnowthemostcommondesign,beingaverycosteffectivemethodfordevelopingsmallerreservoirs.Theseremotewellsareoftenseveralmilesfromtheproductionfacility,anditisnotuncommonforseveralsubseawellstobeconnectedtoasinglesubseamanifoldstructure,whichisinturnconnectedbypipelinetoaproductionorstoragefacilityonthesurface.Cathodicprotection(CP)forthesewells,manifolds,flowlines,jumpersandumbilicalsisthemainfocusofthispaper.EnvironmentalConsiderationsUnderstandingDeepWater Therearethreemajorfactors impactingcathodicprotectiondesign indeepwater.ItisimportanttounderstandthesefactorsandhowtheyworktogethertodriveCPdesignsinthisenvironment:1.WaterTemperature Asdepth increases,water temperaturedecreases,and temperaturehasadirect affect onwater resistivity. By affecting the solubility of nutrient salts, lower temperatureschangethecompositionandmorphologyofcalcareousdepositsformedatthecathode.2.WaterResistivityTheincreaseofwaterresistivityraisestheanodetoseawaterresistance,andthisincrease in resistancedecreasescurrentoutput from fixedanodes.Water resistivity is thereforeamajor factor insizinganodes (rather thanusingstocksizes) tomeet thedesiredweigh tocurrentratios.

3. Calcareous Deposits These deposits are formed on the cathode surface as a result ofelectrochemical reactions associated with cathodic protection. This phenomenon is the majorcontributing factor towhy cathodicprotection systemswork in seawater.The calcareousdeposit,actingasabarriercoating,dramaticallyreducesthecurrentdensityrequiredforcathodicprotectiontooccur.Calcareousdeposits formmuch slower incoldwater,and ingeneralare lessdense thandepositsformedinwarmerwaters.Lessdensedepositsrequireahighercurrentdensitytomaintainrequiredcathodicpolarization.Insummary,designingdeepwatercathodicprotectionsystemsrequires the followingbasicdesigncriteriamodification:1.Usehigher initial (polarization)andmaintenance currentdensity values thanwouldbeused inshallowwarmwater.2.Usehigherseawaterresistivityvalueswhencomputinganoderesistance/currentoutput.3.Usecoatingstoreducecathodicprotectioncurrentrequirements.CathodicProtectionDesign(basicconsiderations)DesignConservatively If"conservative" isdefinedas"beingwithinsensible limits," thenwemustdesigndeepwatercathodicprotectionsystemsveryconservatively.Thecostassociatedwithdeepwateroilandgasprojectsismeasuredintens,orevenhundredsofmillionsofdollars.Costsassociatedwithinsiturepairsquicklyescalatewhenthereisaproblem.Forthisreason,corrosionfailuresarenotanoption.Cathodicprotectionsystemsforthesepiecesofequipmenthaverelativecostsinthetensofthousandsofdollars.Cathodicprotectionisrelativelycheap;useitwisely.Trytogetitrightthefirsttime.UseCoatingsAspreviouslystated,cathodicprotectiondesignincoldwaterrequiresmorecathodicprotectioncurrentperunitarea,andincoldwaterastandardanodegeometrywillmakelesscurrentavailable. ItquicklybecomesobviousthatcoveringupsomesurfaceareawithcoatingsmakesaCPdesignmuchmoremanageable.Coatingsalsoprovideadditionalbenefitssuchas increased insituvisibility,andcorrosionprotectionduringonshorefabricationandstorage.CoatingEfficiencyWhatistheappropriatecoatingbreakdownfactortouse?Thisquestionhasbeendebatedamong corrosionengineers fordecades.Publishedguidelines vary in their adviceon thissubject.Oneprominentcodeconsiderssomecoatingscompletelyworthlessafter20years[1].Thesameguidelinelaterprovidesthisstatement:"Operator'sexperienceofaspecificpaintcoatingsystemmayjustifytheuseoflessconservativecoatingbreakdownfactorsthanspecifiedinthisdocument."Clearlyconservativecommonsensemustrule.Forpipelines,itiscommonpracticetoassignacoatingbreakdownfactorof5%overthedesign life.Forcoatingsonseawater immersedsurfacesnearthebottom,wewouldrecommendincreasingthisnumbersto1015%initialdamageand2025%overthedesignlife.Thiswillrequiremoreanodes,butremember:besafe,notsorry.

QualityControlItiscriticalthatsacrificialanodesworkasdesigned.Anodeswhichfailtoactivate,orwhichperformwithsignificantlyreducedgalvanicefficiencycouldmeanthatananoderetrofitwillberequiredprematurely.Deepwateranoderetrofitprojectsarenotimpossible,buttheyareusuallyveryexpensive. Anode performance can be guaranteed if the following quality control guidelines areobserved: 1.Write a clearly defined specification with special emphasis on electrochemical potential andefficiencytesting.Anyseriousdeficienciesinchemicalcompositionwillbeexposedduringthesetests.AhelpfuldocumenthasrecentlybeenrevisedbyaNACET7Ltechnicalcommittee[2].2.Useananodealloywithaproventrackrecord.TheAlInZnSianodealloysarepreferablebecausetheirbehavior inmudenvironments ismorepredictable,andbecausethereare lessenvironmentalconcernsthanwithMercury(Hg)orTin(Sn)activatedmaterials.Aluminum(AlInZnSi)anodesaremoreefficientandlighterthanzincanodesandhavethehighestavailableopencircuitpotential.3. Ensure that the anode specifications require that the testing be performed at the minimumanticipatedservicetemperature.4. Specify an anode chemistry that will work in cold water and cold saline mud. A suggestedmodificationofanambienttemperaturechemicalcompositionversusoneintendedforcoldwaterispresentedinTable1.Thiswillresultinfewerrejectsatthetestingphaseandwillreducetheriskofanodesnotactivatingwheninstalled.5.ExpectwhatyouInspect.Usequalifiedthirdpartyinspectorstoassureanodequality.Table1AnodeChemistryModificationforColdWaterService

Element TypicalComposition ColdWaterCompositionIron(Fe) 0.10%max 0.07%maxZinc(Zn) 2.87.0% 4.755.25%Copper(Cu) 0.006%max 0.005%maxSilicon(Si) 0.20%max 0.10%maxIndium(In) 0.010.03% 0.015%0.025%Cadmium(Cd) NotSpecified 0.002%maxOthers(each) 0.02%max 0.02%maxAluminum(Al) Remainder Remainder

SpecialConsiderationsforSubseaProductionEquipmentThemechanicalcomplexityofasubseawellheadassemblyorasubseaproductionmanifoldcausesanumber of unusual problems for a cathodic protection designer. The wide array of specializedmaterialsusedcanresultinunexpectedcompatibilityissues.Itisimportanttobeawareofpitfallsandtodesignthecathodicprotectionsystemsaroundthem.Someproblemsspecifictosubseaproductionequipmentareaddressedinthefollowingsections.A.ElectricalContinuity Manyof thesubseastructurescurrentlybeing installedhavehundredsofindividualcomponentsandmanymovingparts.Atypicalsubseawellheadtreeisshownbelow(Figure1).Theanodesforthesedevicesaretypicallyweldedtothesupportframe(s).Ifallthepiecesoftheassembly are not electrically connectedwith a sufficiently low resistance to the frame, then anunbondedcomponentisfreetocorrodeatwhateverrateappliestothatmaterialinseawater.Somecausesofdiscontinuityare:1. Coated components joined by bolting. This often results in the fastener and one of the twocomponentsbeingoutofcircuitwiththecathodicprotectionsystem.Thiscausesspecialproblemsifthefastenerismadefromamaterialsubjecttocrevicecorrosion.2.Fluorocarbon(Xylan)coatedfastenersthatareusedbecauseoftheirpredictabletorquepropertiesand their limitedatmosphericcorrosionprotectionareoften left isolatedbecauseof thedielectricpropertiesofthecoating.Damagetotheheadsofboltsandnutsfromwrenchesleavesthefastenerstocorrodefreely.3.Necessaryarticulatedjointsoftenresultsindiscontinuitiesacrossthearticulationmechanism.4.Mechanicalconnectorsandstabbingpoststhatmayappeartoguaranteeelectricalcontactcanandhaveleftentiresectionsofsubseaassembliesdiscontinuous.

FIGURE1.TypicalSubseaWellheadAssemblyFigure2.CathodicProtectionTestPointonUpperLeftCorerofPanel

Figure3.TypicalElectricalDiscontinuityAcrossLinkageMechanism

B.ContinuityTestingMostsubseadevelopmentprojectsincludeaphasecalledSystemIntegrationTesting(SIT).Allthevariouscomponentsofthesystemarestackeduporconnectedondry landtosimulatethecompletedoffshoreinstallation.Thisisaperfectopportunitytoverifyelectricalcontinuitythroughouttheassembly.Continuityisverifiedfromacommontestpointonthecomponentwheretheanodesareattachedtoallothercomponentsthatcomprisetheassembly.Astandardmultimetercanbeusedtochecktheresistance,werecommendthatavalueof0.3orlessbeverified(excludingtest lead resistance). The same test points (Figure 2) are used as inspection points for life cyclemonitoringofthesystem.Discontinuities(andtherewillusuallybesome)canbefixedonshorequiteeasily.AnexampleofatypicaldiscontinuityfoundduringaninspectionisshowninFigure3.FixingContinuityProblemsSomewaystoaddresscontinuityproblemsare:1.Tackweldacrossboltedjoints.2.Usestainlesssteelwirejumpersbetweencomponents.3.Usestar(serrated)washersunderboltbeads.4.Removecoatingsunderboltbeads.5.Useconductivefastenercoatings.6.Locateanodesonmorecomponents.C.MaterialIncompatibilitiesThewidearrayofmaterialsandcorrosionresistantalloysusedontheseprojectscancauseproblems.Itis importanttoreviewallmaterialsthatwillbeexposedtocathodicprotectionfortheirsusceptibilitytohydrogendamageattheexpectedpotential.Materialsthatshouldbecloselycheckedinclude:somealloysoftitanium,somestainlesssteels(400seriesforexample)and174PH. Sometimes it isdifficult toprevent components from receiving cathodicprotectioneventhoughthematerialselectionengineermaynothaveintendedit.

beadequatelyprotected.Careshouldbetakentoensurethattheseareasareprovidedwithanodesinsideandoutsidethecompartment.Otherareaspotentiallysubjecttoshieldingincludecomponentswithmechanicallyattacheddielectricbuoyancymodules(syntacticfoam),thesecanbehavelikeultrathickdisbondedcoatings.Controlpodsoftenhavecontroltubingmanifoldsunderaprotectivesteelcover.Theseareasmustreceivespecialattentionfromthecathodicprotectiondesigner.E.MudBuriedSteelThemudburiedsteel(pilings,wellcasings,mudmatsanchorsetc.)hasasurfaceareawhichisoftenanorderofmagnitudegreaterthanthesteelsurfaceinthewaterzone.Inadditiontothe largearea,thesurfacesareusuallynotcoated.Eventhoughtheseareasrequiresignificantlylower current densities, the large area can amount to a disproportionately high anode weightrequirement,andavailableanodeattachmentrealestate(areasatwhichtopossiblyconnectanodes)isusuallyatapremium.Besurethattheseareasareaccountedforconservativelyandconsiderthefollowingpointsanddesignoptionswhenaccountingforthemudburiedarea.1.Usecurrentattenuationmodelstocalculatehowfaritwouldbepossibleforananodeto"throw"current.Don'taccountforsteelareasbelowthis,becausethecorrosionratewillusuallybeminimal.Ourdesignersusuallyonlyaccount for the first200 feetofpilingsandwell conductorsbelow theseabed,usingamaintenancecurrentdensityof20mA/m2(2mA/ft2).2.Considertheuseofsealedthermalsprayedaluminumcoatings(TSA)toreducecurrentrequirement.Inthiscase,wesuggestusingthesamecurrentdensity,stillonthefirst200feet,butapplyacoatingfactorof15%bare.3.InstallattachmentsonthetopofpilingswhichsimplifyROVattachmentofcables.Withthisprovisioninplace,itisrelativelysimpletoinstallsomeseabedanodesledsataremotelocationtoprovidethecurrenttotheseareasandtopreventdrainfromtheclosefittedanodesonthesubseaequipment.Thisprovisionisrecommendedonallofthesetypesofprojects.4.Don'tforgetthatpilingshaveaninsidetoo.Thetopsectionofpilingsnormallyextendsomedistanceabovethepileguide;theyareusuallybaresteelandthusrepresentasignificantpercentageoftheseawaterimmersedbaresteelarea.Don'tforgetthis!Cathodicprotectioncurrentwillbelosttotheinsideaswellastheoutsidesurfacesofthesepilings.Coatingtheuppersectionsofthesepilings isstronglyrecommended.SealedTSAcoatingswillworkverywellhere,andtheycanbeappliedtobothsurfaces.F. ROV Compatibility At extreme depths >1000 feet, allwork on the facilitieswill have to beperformedbyaRemotelyOperatedVehicle(ROV).Thesemachines,whilecontinuallyimproving,arestillmachineswithlimiteddexterityandatether/umbilical.TokeepthecathodicprotectiondesignROVfriendly,followthesesimplesteps:1.Placeanodes inareaswheretheywillnot interferewithROVoperations,andavoiddesignsthatintroducepotentialsnagpointsfortheROVumbilical/tether.

2.WhenplacingROVtestpoints,includeagrabrailforthevehicletoholdwhilemonitoringthepoint.ThiswillreducewearandtearontheROV'smanipulatorsandonthemonitoringequipment.Figure2showsatypicalCPtestpointonatree(notethegrabrail).AnROVstabtypecurrentdensitysensorisshowninFigure4(again,notethegrabrail).

Figure4.ROVStabonaCurrentDensitySensorGrabRailonLeft3.Forpotentialmonitoring,provideclearlymarked,baresteelteststab locations.Thehighqualitycoatingsusedonmanyofthesefacilitiesdonotallowstabbingprobestoeasilypenetrate.Attemptingtoestablishacleantipcontactthroughahighqualitymarineepoxypaintcoulddamagethesubseaequipment.ItalsoputstremendousstrainontheROVhydraulicsystems.4.Whenspecifyingcathodicprotectionprobesorcurrentprobes,selectamountthatwilleasilyfittheROVmanipulatorandwhichwill requireminimum interface to theROVelectronicand fiberopticsystems. Figure 5 shows a recently developed self contained deepwater CP probe designed foroperationatupto10,000feet,whilerequiringnoelectricalinterfacetotheROV.AstandardtipcontactCPprobe,whichrequiressurfaceinterfacing,isalsoshown(Figure6).Figure5.10,000FeetRatedSelfContainedCPProbe(updatedimage)

Figure6.StandardDualElementROVCPProbe(updatedimage)

SpecialconsiderationsfordeepwaterpipelinesQ:Whatisspecialaboutadeepwaterpipeline?A:Basicdesigncriteria fordeepwaterpipelinesand flowlinesarenotmuchdifferent fromshallowwatersystems,buttherearemorerisksassociatedwithapipelinelocatedindeepwater.Thefollowingsuggestionsareofferedtominimizetheriskofproblemswiththesecathodicprotectionsystems.Someofthesesuggestionswouldincidentallyimproveshallowwatersystemreliabilityalso.A.AnodeTapersMostdeepwaterpipelinesarenotstabilizedwithconcreteweightcoatings,thusthebraceletanodescommonlyusedareproudofthepipeline.Asaresultanodessometimesdetachduringthe layprocessduetotheanodesnaggingonthepipe layequipment[3][4].Theuseofcastpolyurethanetaperstoanchortheanodeandprovideasmoothdiametertransitionhasproventobeaveryeffectiveremedytothisproblem.Placingbothanodehalvesaparticularwaycanalsoreducetheriskofsnagging.Onewanttobesurethatalloftheanodeswillarriveontheseabedatthesixo'clockposition.Thisprocessisnotasfavorableasthetapersbutmaybetheonlyworkablesolutionifanodeshavetobeattachedonthelaybarge[5].B.PipeLay Inspection Thebestopportunity to identifyand repairproblems isalwaysduring theinstallation.Technologyisavailablewhichwillverifythatanodesareattachedtothepipeandthatthecoating isnotbadlydamagedbeforethepipehitstheseabed[5]. Inaddition,therequiredpostlayinspectionperformedbyalloperatorstocheckthelocationofthelineisalsoagreatopportunitytoverifythepipeline'scorrosioncontrolsystemperformance.SpecifythatacathodicprotectionprobebeattachedtotheROVduringthepostlay(asbuilt)inspectionrun.CathodicProtectionTestingandMonitoringwithROV'sIntroductionWithouttheROVtherewouldbenooilandgasdevelopmentinwaterdepthsgreaterthan1500feet.Theseunderwaterrobotsarecomplexmachineswithoutbrains.Wehavetomakeitsimpleforthemtodotheworkofcorrosiontechnicians[6].Withsubseadevelopmentswedonothavetheluxuryofrunningcablestothesurface.Therefore,wemustusetheROV.EquipmentmadeespeciallyforROV'sisnowavailabletoperformthefollowingtasks:ElectricalPotentialSurveysAwiderangeofequipmentisavailableforpointandcontinuouspotentialsurveysonsubseaequipmentandtheassociatedpipelines.

CurrentDensityandAnodeCurrentMeasurement Historically thishasbeenachievedusing theelectric fieldgradientmethod [7].Anewlydeveloped fixedmonitoring instrumentallowsacurrentdensitymonitorormonitoredanode tobedeployed formeasurementwithanROV (Figure4).Bystabbingaselfcontainedreadout into the fixedcontacts thevoltagedropshuntcanbeaccuratelyrecorded.ConclusionsWhendesigningaCPsystemindeepwater,besafe,notsorry!Paycloseattentiontothestructure,itscontinuity,what it'smade of, its geometry, and how it will be protected over its life cycle. Beconservativewithcoatingandcathodicprotectiondesigns,butmakeretrofitprovisionsbyprovidingsimpleROVattachmentpoints.Ensurethatonlyqualitymaterialsareused inthecorrosioncontrolsystem.Andlastbutnotleast,hireacorrosionengineertoreviewallthedesignspecifications,coatingsvs.materialsvs.cathodicprotection.References[1]DelNorskeVeritas(DnV)RecommendedPracticeRP8401"CathodicProtectionDesign"1993[2]NACEInternationalTMOI9098"ImpressedCurrentTestMethodforLaboratoryTestingofAluminumAnodes"[3]CORROSION'97Paper470R.H.Winters,A.Holk"CathodicProtectionRetrofitofanOffshorePipeline"[4]CORROSION'93Paper527LJ.RipponetaI."ShortingPipelineandJacketCathodicProtectionSystems"[5]OffshoreMagazineApril1996J.Britton"ProtectingPipelineCorrosionControlSystemsDuringPipeLay"[6]UnderwaterMagazineFall98P.47J.Britton"AnROYinterfaceforCorrosionMonitoringEquipment"[7]CORROSION'92Paper422J.Britton"ContinuousSurveysofCathodicProtectionSystemPerformanceonBuriedPipelinesintheGulfofMexico"