Stress Analysis

12/19/2014 StressAnalysisofundergroundPiping|WhatisPiping

http://www.whatispiping.com/undergroundpiping 1/8

Undergroundorburiedpipingareallpipingwhichrunsbelowgrade.Ineveryprocessindustrytherewillbefewlines(Sewerordrainagesystem,SanitaryandStormWaterlines,Firewaterordrinkingwaterlinesetc),partofwhichnormallyrunsunderground.Howeverthetermburiedpipingorundergroundpiping,intruesense,appearsforpipelineindustryasmilesoflongpiperuncarryingfluidswillbethere.

Analyzinganundergroundpipelineisquitedifferentfromanalyzingplantpiping.Specialproblemsareinvolvedbecauseoftheuniquecharacteristicsofapipeline,coderequirementsandtechniquesrequiredinanalysis.Elementsofanalysisincludepipemovements,anchorageforce,soilfriction,lateralsoilforceandsoilpipeinteraction.

Toappreciatepipecoderequirementsandvisualizeproblemsinvolvedinpipelinestressanalysis,itisnecessarytofirstdistinguishapipelinefromplantpiping.Uniquecharacteristicsofapipelineinclude:

Highallowablestress:Apipelinehasarathersimpleshape.Itiscircularandveryoftenrunsseveralmilesbeforemakingaturn.Therefore,thestressescalculatedareallbasedonsimplestaticequilibriumformulaswhichareveryreliable.Sincestressesproducedarepredictable,allowablestressusedisconsiderablehigherthanthatusedinplantpiping.

Highyieldstrengthpipe:Toraisetheallowable,thefirstobstacleisyieldstrength.Althoughapipelineoperatingbeyondyieldstrengthmaynotcreatestructuralintegrityproblems,itmaycauseundesirableexcessive

What is PipingAnattempttoexplainprocesspipingengineeringbasicsindetailtohelpalltheprocesspipingengineeringprofessionalsaroundtheworld.

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13thOctober2014 want2learn CaesarII 1Comment

Basics for Stress Analysis of UndergroundPiping using Caesar II

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deformationandpossibilityofstrainfollowup.Therefore,hightestlinewithaveryhighyieldtoultimatestrengthratioisnormallyusedinpipelineconstruction.Yieldstrengthinsomepipecanbeashighas80percentofultimatestrength.Allallowablestressesarebasedonlyonyieldstrength.

Highpressureelongation:Movementofpipelineisnormallyduetoexpansionofaverylonglineatlowtemperaturedifference.Pressureelongation,negligibleinplantpiping,contributesmuchofthetotalmovementandmustbeincludedintheanalysis.

Soilpipeinteraction:Themainportionofapipelineisburiedunderground.Anypipemovementhastoovercomesoilforce,whichcanbedividedintotwocategories:Frictionforcecreatedfromslidingandpressureforceresultingfrompushing.Themajortaskofpipelineanalysisistoinvestigatesoilpipeinteractionwhichhasneverbeenasubjectinplantpipinganalysis.

Normallytheselinesdoesnothavehighdesigntemperatures(oftheorderof60to80degreecentigrade)andonlythermalstresscheckingissufficientforundergroundpart.CommonmaterialsusedforundergroundpipingareCarbonSteel,Ductileiron,castIron,StainlessSteelandFRP/GRP.

InthisarticleIwilltrytoexplainthestepsfollowedwhileanalysingsuchsystemsusingCaesarII.Howeverthisarticledoesnotcoverthebasictheoryforanalysis.

InputsRequiredforAnalysis:

BeforeproceedingforanalysisofburiedpipingusingCaesarIIcollectthefollowinginformationfromrelateddepartment1.IsometricdrawingsorGAdrawingsofthepipelinefromPipinglayoutDepartment.2.Lineparameters(Temperature,Pressure,Material,FluidDensity,etc)fromprocessDepartment.3.SoilPropertiesfromCivilDepartment.

CaesarIIforUndergroundPipingAnalysis:

TheCAESARIIundergroundpipemodelerisdesignedtosimplifyuserinputofburiedpipedata.ToachievethisobjectivetheModelerperformsthefollowingfunctionsforanalyst:

Allowsthedirectinputofsoilproperties.TheModelercontainstheequationsforburiedpipestiffnessesthatareoutlinedlaterinthisreport.Theseequationsareusedtocalculatefirstthestiffnessesonaperlengthofpipebasis,andthengeneratetherestraintsthatsimulatethediscreteburiedpiperestraint.

Breaksdownstraightandcurvedlengthsofpipetolocatesoilrestraints.CAESARIIusesazoneconcepttobreakdownstraightandcurvedsections.Wheretransversebearingisaconcern(nearbends,tees,andentry/exitpoints),soilrestraintsarelocatedincloseproximityandwhereaxialloaddominates,soilrestraintsarespacedfarapart.

Allowsthedirectinputofuserdefinedsoilstiffnessesonaperlengthofpipebasis.Inputparametersincludeaxial,transverse,upward,anddownwardstiffnesses,aswellasultimateloads.Userscanspecifyuserdefinedstiffnessesseparately,orinconjunctionwithCAESARIIsautomaticallygeneratedsoilstiffnesses.

ModelingstepsfollowedinCaesarII:

Themodelingofburiedpipingisveryeasyifyouhaveallthedataatyourhand.Thefollowingstepsarefollowedformodeling:

Fromtheisometricmodelthelineinthesamewayasyoufollowincaseofabovegroundpipemodeli.e,enterlinepropertiesinCaesarSpreadsheet,enterlengthsbybreakingthelineintoseveralnodesorselectanexistingjobforconvertingitintoanundergroundmodel.



AnalystcanstarttheBuriedPipeModelerbyselectinganexistingjobandthenchoosingInputUndergroundfromtheCAESARIIMainMenu.TheModelerisdesignedtoreadastandardCAESARIIinputdatafilethatdescribesthebasiclayoutofthepipingsystemasifitwasnotburied.FromthisbasicinputCAESARIIcreatesasecondinputdatafilethatcontainstheburiedpipemodel.Thissecondinputfiletypicallycontainsamuchlargernumberofelementsandrestraintsthanthefirstjob.Thefirstjobthatservesasthepatternistermedtheoriginaljob.Thesecondfilethatcontainstheelementmeshrefinementandtheburiedpiperestraintsistermedtheburiedjob.CAESARIInamestheburiedjobbyappendingaBtothenameoftheoriginaljob.

WhentheBuriedPipeModelerisinitiallystartedup,thefollowingscreenappears:

Fig.1:SampleCaesarIISpreadsheetforBuriedPiping

Thisspreadsheetisusedtoentertheburiedelementdescriptionsforthejob.Theburiedelementdescriptionspreadsheetservesseveralfunctions:

Allowsanalysttodefinewhichpartofthepipingsystemisburied.

Allowsanalysttodefinemeshspacingatspecificelementends.

Allowstheinputofuserdefinedsoilstiffnesses.

From/Tonode:

AnyelementofpipeinCAESARIIcanbedefinebytwoelementsfirstisstartpointandsecondisendpoint.Inburiedpipemodel,beforeconversiontheFrom/Tonodesremainssameasunburiedmodel.

Soilmodelno.:Thiscolumnisusedtodefinewhichoftheelementsinthemodelareburied.Anonzeroentryinthiscolumnimpliesthattheassociatedelementisburied.A1inthiscolumnimpliesthattheanalystwishestoenteruserdefinedstiffnesses,onaperlengthofpipebasis,atthispointinthemodel.Thesestiffnessesmustfollowincolumnnumbers6through13.Anynumbergreaterthan1inthesoilmodelno.columnpointstoaCAESARIIsoilrestraintmodelgeneratedusingtheequationsoutlinedlaterunderSoilModelsfromanalystenteredsoildata.

From/Tomeshtype:AcriticalpartofthemodelingofanundergroundpipingsystemistheproperdefinitionofZone1bearingregions.Theseregionsprimarilyoccur:Oneithersideofachangeindirection



ForallpipesframingintoanintersectionAtpointswherethepipeentersorleavesthesoilCAESARIIautomaticallyputsaZone1meshgradientateachsideofthepipeframingintoanelbow.NoteitistheanalystsresponsibilitytotellCAESARIIwheretheotherZone1areasarelocatedinthepipingsystem.

Userdefinedstiffness&ultimateload:Thereare13columnsinthespreadsheet.Column6to13carrytheuserdefinedsoilstiffnessesandultimateloadsifanalystdefinessoilmodel1.Analysthastoenterlateral,axial,upward,downwardstiffnesses&loads.

Procedure:

1. Selecttheoriginaljobandentertheburiedpipemodeler.Theoriginaljobmustalreadyexist,andwillserveasthebasisforthenewburiedpipemodel.Theoriginalmodelshouldonlycontainthebasicgeometryofthepipingsystemtobeburied.Themodelerwillremoveanyexistingrestraints(intheburiedportion).Addanyundergroundrestraintstotheburiedmodel.RenametheburiedjobifCAESARIIdefaultnameisnotappropriate.

2. EnterthesoildatausingSoilModels.

3. Describethesectionsofthepipingsystemthatareburied,anddefineanyrequiredfinemeshareasusingtheburiedelementdataspreadsheet.

4. ConverttheoriginalmodelintotheburiedmodelbytheactivationofoptionConvertInput.Thisstepproducesadetaileddescriptionoftheconversion.

5. ExittheBuriedPipeModelerandreturntotheCAESARIIMainMenu.Fromheretheanalystmayperformtheanalysisoftheburiedpipejob.

Thestepstocreatesoilmodelandfewotherimportantconsiderationswillbepublishedinfuturepostsonundergroundpipinganalysis.

RelatedPosts:

1. UndergroundPipingStressAnalysisProcedureusingCaesarII

2. StressAnalysisofVerticalReboilerPipingusingCaesarII:Part3of3

3. StepbyStepMethodsforWRC107andWRC297CheckinginCaesarII

4. StressAnalysisofColumnpipingsystemusingCaesarII

5. StressAnalysisofPumpPiping(Centrifugal)SystemusingCaesarII

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StarSaturday20September2014,1:13am

Ireadalotofinterestingarticleshere.Probablyyouspendalotoftimewriting,

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