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WELLBORE TUBULARS Premium-Connection Design High resistance to fatigue loading together with high torque and compression capacities is necessary for connections used for drilling with casing. On the other hand, cost-effective solutions must be found to provide connections for shallow wells for which sophisticated premium connections are not economical. The full-length paper describes the development and evaluation of premium and semipremium connections for tubing/casing. Introduction Drilling
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  74 JPT ã MAY 2006 High resistance to fatigue loading together  with high torque and compression capaci-ties is necessary for connections used for drilling with casing. On the other hand,cost-effective solutions must be found toprovide connections for shallow wells for  which sophisticated premium connectionsare not economical. The full-length paper describes the development and evaluationof premium and semipremium connectionsfor tubing/casing. Introduction Drilling with casing srcinally was usedfor nondemanding vertical wells where, inalmost all cases, American Petroleum Inst.(API) connections (sometimes with modi-fications to improve torque capacity) wereused. As drilling with casing was usedin more demanding and critical applica-tions such as horizontal, deviated, and off-shore wells where most wells are drilleddirectionally, new connection types wereneeded. Requirements for these connec-tions include reliability, sealability, highovertorque capacity, and ultrahigh fatigueresistance because the stresses produced bybending and vibrations while rotating canreduce connection life. Considerations Use of oil-country-tubular-goods (OCTG)connections for drilling began as an attemptto reduce costs. Standard OCTG casingconnections are designed to withstand stat-ic loads of tension and compression, inter-nal and external pressure, and bending.Torque requirements for these connectionsare low. However, for drilling with casing,some critical issues that become importantfor drilling directional and deviated wellsinclude the following.ã Wear protection.ã High torque and compression resis-tance.ã High fatigue resistance.ã Cost effectiveness. Wear protection is provided by meansof centralizers or other hardened surfaceslocated close to the pipe end to avoiddamage to the connection. Higher torqueand compression requirements are met bydesigns that include robust torque shoul-ders, thread interferences, and reduced gapsbetween mating threads. Fatigue resistanceis a very important issue for drilling withcasing and perhaps the most difficult chal-lenge to address. Fatigue Response of OCTG Connections.  Fatigue is a complex process that affectsmechanical components subjected to cyclicloads. Fatigue failure is not a sudden crack,instead it involves different failure phasesthat occur until component failure becomesevident. This failure occurs at stress levelsmuch less than the elastic limit of the mate-rial. When subjected to cyclic loading, ordi-nary OCTG connections are weaker thandrillpipe connectors that are designed towork with alternating stresses. The fatigue-failure location of threaded connections isin the area with the highest stress-concen-tration factor (SCF). Location of the SCFcan be estimated with finite-element analy-sis (FEA), which also can yield a numericalvalue of the SCF. For standard premiumOCTG connections with loads expected indrilling with casing applications, the SCFnormally is greater than 2. However, froma fatigue perspective, it is desirable that theSCF be as low as possible. Threaded-con-nection response to fatigue loads can beplotted as a stress vs. number-of-cycles-to-failure curve.The alternating stress for a drilling withcasing operation is produced by the stringdogleg while the string is rotated. The meanstress is produced by the axial loads appliedon the string. Consequently, for a longerlife, a balance between the axial load (meanstress) and the maximum dogleg that can beapplied while rotating must be achieved. This article, written by Assistant TechnologyEditor Karen Bybee, contains highlights of  paper OTC 17221, “Premium & Semi-PremiumConnections Design Optimization for VariedDrilling-With-Casing Applications,” by N.J. Santi and G.E. Carcagno,  SPE, Tenaris, and R. Toscano, Centro Investigation Industria l  , prepared for the 2005 Offshore TechnologyConference, Houston, 2–5 May.Copyright 2005 Offshore TechnologyConference. Reproduced by permission. Premium-Connection Design WELLBORE TUBULARS Fig. 1—Semipremium tubing and cas-ing connection. The full-length paper is available for purchase at the OTC Library:www.otcnet.org. The paper has not been peer reviewed. Fig. 2—Premium connection.  JPT ã MAY 2006 75 Connection Development A development program was launched toprovide alternative connections for drill-ing with casing. Different options fromsemipremium to premium fit-for-purposeconnections with enhanced fatigue life wereevaluated and tested. For the semipremiumconcept, a threaded and coupled (T&C)design was chosen because T&C designshad been used for similar applications.Main features of this design include a tight-er tolerance range than API connections, arobust torque shoulder that allows stablemakeup and extra torque capacity withcontrolled stresses along the threaded area,and a robust thread design (Fig. 1). For the premium connection, a toughcasing design on an upset joint based ona proven design for tubing drilling andworkstring applications was chosen. Tworobust shoulders were included that allowsufficient prestress levels while having anovertorque capability that ensures no dam-age on the connection as a result of suchprestress (Fig.2). Different design alternatives were evalu-ated by use of FEA, full-scale fatigue tests,and make-and-break tests. In addition,magnetic-particle inspections to check forthe presence of cracks were performedon specimens that did not fail duringfatigue tests. FEA FEA was performed for a premium con-nection and a standard T&C connectionto evaluate fatigue response. Connectionswere modeled in extreme configurations of thread and seal, and two stress levels weresimulated with a mean stress of 30% of the minimum API yield strength. The FEAcode used incorporates an element that iseffective and reliable for OCTG-connectionanalysis. Small displacements and strainswere considered in the model. An elasto-plastic material model, von Mises yield cri-terion, and a bilinear model with isotropichardening were used. Nonlinear contactconditions were handled with a Lagrangemultiplier contact algorithm.In all cases, the makeup-torque value wasincreased to a higher value than that usedfor running casing to ensure good prestressin the connection to ensure that both shoul-ders were closed during the loading cycle.Control points were in the most sensi-tive connection areas. The SCF map andthe graphic of results show that the high-est stresses for different alternating loadsare produced at either the first or sec-ond pin or box threads, depending on theapplied stress. This is different from whatis observed normally on T&C connectionswhere the highest SCFs are located withinthe last imperfect threads of the pin.The SCFs on the premium connector donot exceed 2 and in some cases are closer to1, which means that the connection meetsthe design target of an SCF less than 2. Fatigue-Resistance Tests The premium and semipremium connec-tions that underwent the full-scale testingprogram were manufactured from 7-in.,32-lbm/ft L80 material. The samples weremanufactured according to standard manu-facturing procedures; however, the integralupset joints were manufactured on cou-pling stocks with the upset ends turned toreduce any possible imperfections.The full-scale tests were performed onthree samples of the T&C and three sam-ples of the premium connection by use of aresonant fatigue rig. The tests were plannedto stop when the connections leaked. Bothconnections were tested at the stress levelsrecommended for this application, but themean stress levels were slightly different tosubject the upset end to a worst condition.Run-out limit was set at 12 million cycles,in the event that failure did not occur ear-lier. Stress levels were chosen to simulatebending conditions of 19, 14.3, 9.5, and25.6°/100 ft with a 236.5-kips mean tensionload for the premium connection and 220kips for the T&C connection. Fatigue-Test Results. The T&C connec-tions behaved very well, achieving satisfac-tory results and providing an experimentalSCF of 1.3 with a 95% survival probability.According to the results, this connectioncould drill at 9.5°/100 ft for more than 80days with a 220-kips tension load.All the failures in the premium connec-tions occurred on the pipe body on the transi-tion area away from the connection. The firstconnection tested achieved 868,000 cycles,and failure occurred at the tong marks. Thisconnection was replaced by a spare specimenthat had the tong marks ground out. Thisspare specimen achieved more than 3 millioncycles and failed in the transition area. Thismeans that the 7-in. premium connection candrill 20 days at 100 rev/min, 19°/100 ft, and atensile load of 236.5 kips. The experimentalSCF for the upset connections was 0.9 with a95% survival probability. Magnetic-Particle Inspection. After thefatigue tests, the connections were brokenout and inspected with magnetic particles.The inspection showed that for the upsetconnections, cracks developed at the transi-tion area or at the tong marks for the firstupset connection (Fig. 3) and at the transi-tion for the others. No cracks were foundon threads and seals. Visual inspection con-firmed that no damage appeared on threadsand seals despite high makeup torque. Nocracks were found in the threaded area onthe T&C semipremium connection. Discussion Results showed that the connections satis-factorily achieved the requirements estab-lished at the beginning of the developmentprogram. For the semipremium connection,the purpose was to obtain better behaviorthan API connections currently used fordrilling-with-casing applications. For pre-mium connections, it was demonstratedthat the design satisfies the objectives inall cases, with the exception of the failurecaused by tong marks. The connectiontested at 14.5 ksi and stopped at 13 millioncycles achieved more than 770,000 cycleswhen retested at 36.2 ksi. Cumulative dam-age on the pipe supports a fatigue-lifeestimate of 50 million cycles at 14.5 ksi(9.5°/100 ft) with no damage on the con-nection. In addition, the T&C connectionsalso were overtorqued to 160% of the nor-mal value, while the upset connections wereovertorqued to as much as 290% optimumtorque, which demonstrates the overtorquecapacity of these connections. Conclusions 1. The premium connection demonstrat-ed an ultrahigh fatigue resistance withoutshowing any failure on the connection.2. The bending angles at which the con-nections were tested make them suitablefor deviated wells, in particular the upsetconnections that also allow very high angleswith a good life expectancy.3. The semipremium connection presentsa very good balance between performanceand cost that makes it appropriate forless-demanding wells with no sealabilityrequirements. JPT Fig. 3—Magnetic-particle inspection of tong marks.
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