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OilfieldTechnology
February
2014
Saferfatiguemodellingandimprovedlongevity
Astheenvelopeofcoiledtubingusage isexpanded, fatiguemodelling
becomesmore importanttothepredictionof fatigueperformance,
andriskassessment.Commonly, fatigueconsumption istracked in
1 ‑3m increments.Eachtimethesectionof tube isbentorstraightened
eitheronthereeloroverthegooseneck, the fatiguemodel recordsthe
circulatingpressureanddeductssomepercentageof thetotal lifeof the
sectionbeingtracked.Whenthesectioncontainsabiasweld,acommon
practice is foraspike in fatiguetorepresentthederating factoratthe
biasweldapplied inthesoftware.Thesespikescanbeseen inFigure6.
Byhavinga lengthof tubeapproximately50 ‑100m in lengthof thicker
materialwiththebiasweld inthemiddle, theaccurate locationof the
biasweldbecomes less importantbecauseof theadded factorofsafety
fromthethickermaterial.Thisallows forsomestretchduringcycling,
slippageatthecounter,and fieldcroppingof theendbetween jobs
without fearof inaccuratelytrackingthe fatigueonabiasweldregion.
Figure6showsthedifference in fatigueatthebiasweldsusing
SMARTapercomparedtoastandardbiasweld.Thecomparisonwas
madeusingtheCerberus™suiteofsoftware.
Thebiasweldshavebeen
slightlyoffset from thestandardbias
welds in thisexample tohighlight
thedifference in recessed fatigueat
SMARTaperwelds. The inset shows
adecrease in fatiguedue tousing
slightly thickermaterial andhas
reduced fatiguecompared to the
standardbiasweldsbymore than
15%. Inotherwords, thestring
withstandardweldswouldhave
been retiredmuchsooner than the
SMARTaper string. Furthermore,
the tubeoneither sideof thebias
weldshows lower fatiguedue to
the increasedwall thickness. This
increases thesafetyat thebiasweld
bymaking it stronger, and reduces
theemphasisonaveryaccurate
locationof thebiasweld relative to
themodelduringoperations.
Post‑retirementfatigue
testing
Toconfirm the fieldperformance
of itscoiled tubing technology,
Global Tubing, LLChassectioned
retiredstrings toconfirm the
residualperformancebothat the
biasweldand thebasematerial.
Eight casehistorieshavebeen
completed todateand thecompany
continues tovalidatesomestrings
after they return from retirement.
Oneexampleof largerdiameter
tubing (2.625 in./66.68mmOD)
showedveryconsistent results
of thebiasweldversus thebase
material. All thebiaswelds tested
in thisdissectionwerebetween
80 ‑ 100%of themodel (cumulative
field fatigueplus remaining
laboratory fatigue to failure)and
thebasematerialwasbetween100 ‑ 120%of themodel. Inother
words, thebiasweld fatiguewas reduceddue toadditionalwall
thicknessasshown inFigure6andstill showed residualperformance
in linewith themodel.Had theseweldsbeenmanufactured from
thinnermaterials, thespikeswouldhavebeenhigherandcaused
earlier retirement. Thisaddedcushionhasbeenwell received in the
SouthTexas region, and itsuse isspreading throughout theworldas
anadded tool for customers toutilise insafer stringdesign.
Conclusion
Coiled tubingutilisingSMARTaper technology improvessafety,
reliabilityand reach incoiled tubingoperations. The technology
improvessafetyand reliabilityby increasing thewall thicknessat
biaswelds located in thezonewith thehighestamountof fatigue.
Reliabilityandstring lifeare increased15 ‑ 20%by increasing fatigue
performance. Inaddition, the technologycan improve reach in
unconventionalwellsbyquickly transitioningbetweenoptimalwall
thicknesses tominimisebuckling in thehorizontal sectionand reduce
weight.
Figure6.
Fatigueprofileofacommoncoiledstringdesignbeingused inunconventionalapplications.The
diagramsatthebottomshowthe layoutofthestringdesignsandthevariation inwallthicknesses.Thegreen
vertical lines inthediagramsrepresentbiaswelds.Thedips infatigueonthegreenplot(emphasisedbythe
blackbackgroundofthestandardbiasweldstringfatigue)aredueto increasedwallthicknessaroundthebias
weldusingSMARTapertechnology.The insetemphasisesthedifferencebetweenaSMARTaperbiasweldanda
standardbiasweld.The increase inthickness lowerstheaccumulatedplasticbendingfatigueand improvesthe
jointperformancebyover15% inthisexample.
Figure5.
SMARTaperstriptransitionsbetweentwowallthicknesses.Thetaperedsection is linear,andmuch
shorterthantheoverallstrip length,which isdeterminedbythenominalthicknessofthesteel.For instance,a
transitionbetween0.204 in.and0.175 in.canbemadeveryquickly; inas littleas50mwhereasthemajorityof
thestripconsistsofthethinnerwallthicknesstosaveweightand improvereach.
