刘成举论文翻译英文版.docx
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刘成举论文翻译英文版.docx
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刘成举论文翻译英文版
RoleofMicrostructureinSuckerRodStringFailuresinOilWellProduction
SofianeBenhaddadandGlenLee
(Submitted12December2000;inrevisedform20February2001)
Sucker-rodpumpsareoperatinginveryaggressiveenvironmentsinoilwellproduction.Thecombinedeffectofacorrosiveenvironmentandsignificantmechanicalloadscontributetofrequentcasesoffailureoftherodstringduringoperation.Standardsandrecommendationshavebeendevelopedtocontrolandavoidthosefailures.Thisstudypresentsvariousfailurecasesofsuckerrodsindifferentapplications.Theheattreatmentofthesteelmaterialandtheresultingmicrostructureareanimportantfactorinthebehaviorofthesuckerrod.Aspheroidizedmicrostructurepresentsaweakerresistancetocorrosionaffectingtherodlife.Non-metallicinclusionsareapittingpreferentialsiteleadingtofatiguecrackinitiation.Heterogenousmicrostructureasbandedmartensiteandferrite/pearlitedecreasestheductilityofthematerialaffectingthefatiguepropagationresistance.
Keywords:
corrosion-fatigue,inclusions,pitting,steelmicrostructure,suckerrod
1.Introduction
TheoilproductionwellsinwesternCanadagenerallyoperateinaggressiveenvironments,includingcorrosiveelements,highvelocityofthefluids,andconsiderabledepthforcompletion.Determinationandanalysisofthenecessarydataarerequiredforselectionofmaterialsforwellcomponents.Materialsforthetubing,casing,andsuckerrodstring,aswellasasuitablecoatingorchemicalprotectionprogramareselectedappropriatelyforeachoperatingwell.Insuchanaggressiveenvironment,themostvulnerabledownholecomponentisthesuckerrodstringduetoitsfunction.
Specificationsandstandards[1-4]havebeenwrittenfortheimprovementoftheperformanceandlifeexpectationofsuckerrods.Therecommendationsofthesecodesandstandardsaredirectedtowardsabetterchoiceofmaterialforgoodcorrosionresistanceandabetterdesignoftherodstringforanevendistributionofstresses.Theresultingrecommendationsarerestrictiveandmaypaysignificantattentiontoeconomicfactors.Therefore,manyoperatorscontinuetousecarbonoralloysteelcomponentswithanappropriatechemicalinhibitionprogram.Carbonsteelispreferredoverothermaterialsduetoitsmachinability,highstrength,andlowcost.
Theuseofcarbonsteelrodstringsinaggressiveenvironments,withoutproperlyconsideringthecomplexinteractionsbetweenthematerialanditsserviceenvironment,leadstofailures.Inthiscontext,numerousrodstringfailureshavebeenstudiedthroughmetallurgicalinvestigationthatincludedmacro-observation,chemistry,fractography,microscopy,andhardnesstesting.Thispapersummarizesseveralofthestudiedcasesanddividesthecasesaccordingtothemainrootcauseoffailure:
mechanicallyinducedfailureandenvironmentallyinducedfailure.
2.MechanicallyInducedFailures
Allsuckerrodfailurecaseswererelatedtothemostcommonlyusedmaterials:
carbonoralloysteel.Theserodstringswereoperatingunderoneofthreeconditions:
1)inconjunctionwithachemicalcorrosioninhibitionprogram;2)inconjunctionwithaprotectivecoating;and3)withoutanyprotectionfromtheserviceenvironment.Casesaresummarizedalongwiththedetailsofthefailureanalysis.
2.1RoleoftheMicrostructure
2.1.1Observation.Threesuckerrodsbrokeatthepinend(Fig.1)andproducedthesamemicroscopicrupturewithidenticalfeatures.Therodswerefabricatedfromcarbonoralloysteel.Theobservationofthesurfaceoftherupturerevealedthreedistinctzones(Fig.2).Thefirstzone,correspondingtothecrackinitiation,revealedpitsandcorrosion.Thesecondzonewasflatandsmooth,indicatingamacroscopicallybrittlefracture.Thesecondzonesurfacewascoveredwithbeachmarks,suggestingfatiguecrackpropagation.Thethirdandlastzonewasashearlip.Thethirdzonesurfacewasroughandcoveredwithdimples(Fig.3),showingthatthefinalruptureprocesswasductileoverload.Allthesefeaturesindicatedthatcorrosion-fatiguewasthecauseoffailure.Thiswasconfirmedwiththepresenceoffatiguestriationsonthesurfaceofthesecondzone(Fig.4).Thepittingofthesurfaceoftherodintroducedastressconcentrationatthe“pittips”andledtotheinitiationandpropagationoffatiguecracksduetocyclicloadsresultingfromtheoperationofthepump.Theaggressivedown-holeenvironmentmayhaveacceleratedthecrackgrowth.
2.1.2Microscopy.Microscopyobservationswerecompletedonsectionsremovedfromtherodthroughthesurfaceoftherupture.Themicrostructurewasferriteandpearlitewithnumerousinclusionsdispersedthroughoutthestructure(Fig.5).TheinclusionswereanalyzedasMnSparticles.Theinclusionswereelongatedparalleltotherodlengthandcontainednumerousmicro-cracks.Additionally,theinclusionsprovidedpreferentialsitesforpittinginthematerial(Fig.6).
Severalstudieshavehighlightedthedetrimentaleffectofinclusionsinthesteel[5-8]andshownthatMnSparticlescreateamicro-galvaniccouplewiththesteel:
MnSbeingtheanodeandthesteelthecathode.OurobservationswereconsistentwithpreviousworksthatshowedthatthepresenceofMnSinclusionsatthesurfaceleadstopitformationinacidenvironments.
Thepitsformedatthesurfaceoftherodprovidedlocalstressconcentrationthatalteredtheresistancetofatigueinitiation.Thepits,coupledwiththecyclescreatedintheoperationofthepump,haveservedtoinitiatefatigue.Thecrackisthenpropagatedthroughthethicknessoftherod.
Inoneofthefailurecasesstudied,elongatedbandsinthemicrostructurewereobserved(Fig.7).Thesebandscorrespondedtotherupturefeaturesofthefracturesurface.Microhardnesstestingofthebandsandthebulkofthestructureshowedasignificantvariance,withthehardnessofthebulkbeing293HVandthebandsignificantlyharderat411HV.Thehighhardnessofthebandsisindicativeofabrittlematerial.Itcorrespondstothelocationofinclusionsandmicro-cracksinthematerial.Additionally,thebandareasshowsegregationofchrome,whichleadstothehigherhardness.Thebandingresultedfrompartialaustenitizing,whichcausedtheoriginalbandedferriteandpearlitemicrostructuretotransformtobandsofveryhardmartensiteandbandsofferrite/pearlite.ThepresenceofthehardmartensitebandscombinedwithMnSinclusionstopromotetheinitiationoffatiguecracksandleadtoadecreasedresistancetocrackpropagation.
Ahighnumberofinclusionsinthesteelisextremelydetrimentalandtheuseofhigherqualitysteelispreferred.Changesinthefabricationprocessfortherods,includingimprovedqualitycontrol,resultedinhomogeneousmicrostructurewithanevenchemistryandhardnessanddecreasedthetendencyforfailure.
Fig.1Suckerrodbrokenatthepinend
Fig.2Surfaceoftheruptureofthesuckerrod
Fig.3Surfaceofthelaststageoftherupture
Fig.4Fatiguestriationsonthesurfaceoftherupture
Fig.5Inclusionsinthemicrostructure
Fig.6Pitsatinitiationstageoftherupture
Fig.7Bandsinthemicrostructure
2.2CoatingandDesign
Twoofthemajorfactorscontributingtothefailureofsuckerrodsare:
1.Corrosionatthesurfaceoftherodsand,
2.Stressconcentrationsinducedbymechanicaldesign.Theeliminationofthesefactorsisachievedby:
a)Protectionofthesurfacewiththeapplicationofeitheracorrosionresistantcoatingorchemicalinhibitionoftheoilfieldenvironment,and/or
b)Betterdesignofthesuckerrodtoremovestressconcentrations.
2.2.1Observation.Inthefollowingcase,althoughdesignimprovementsweremade,therodfailedinservice.Asocketwastightenedaroundthepintoreducethestressconcentrationsappliedonthepinendoftherod.However,therodbrokerightattheedgeofthesocketafterafewmonthsofoperationinsourservice(Fig.8).Thesurfaceoftherupturewasexaminedafteracidcleaning.Thefracturesurfacecontainedthesamefeaturesaspreviouslydiscussed:
threedistinctzonescorrespondingtotheinitiation,propagation,andfinalfractureportionofthefailure(Fig.9).Cracksandcorrosiondepositswereobservedontheinitiationzone.Striationsandhammeringmarkswereobservedinthepropagationstagezone.Dimplescoveredthefinalrupturezone.Thesuckerrodfailedbycorrosion-fatiguemode.Thecorrosionparticlesweredeterminedtobeironsulfide(x-raydiffraction)indicatingtheaggressivenatureoftheserviceenvironment.
2.2.2Microscopy.Atransversecrosssectionthroughthediameteroftherodshowedthatcrackinitiationwasfollowedbythepropagationofanearlystraightcrack.Smallcurveswereobservedasthecrackpassedthroughthecoatingduetothepresenceofvoidsinthecoatinglayer(Fig.10).Micro-crackswereobservednearthesurfaceandalargesecondarycrackwasobservedattheendduetothehighdeformation(Fig.11).Therodmaterialwasoftype8610andspraycoatedwithaNi-Cr-Beutecticalloy.Sincethematerialwasoperatinginsourservice,itshouldconformtotherequirementsofNACEStandardMR0176[1].ThecompositionofthecoatingwasslightlydifferentthantheminimumNACErecommendation.[1]Bothchromeandboroncontentswerejustovertherecommendedmaximum.Thehardnessofthecoatingwas63HRC,whiletheminimumrequired[1]is55HRC,andthecoatingthicknesswas0.3mmwhiletheminimumrequired[1]is0.25mm.
Theseresultsleadtotheconclusionthatthesprayedmetalcoatingconformedtothereferencerequirements.Themaincauseofthefailurewasthestressconcentrationattheendofthesocketwheretherodfailed.T
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