74 JPT • 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 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.

IntroductionDrilling with casing originally was used for nondemanding vertical wells where, in almost all cases, American Petroleum Inst. (API) connections (sometimes with modi-fications to improve torque capacity) were used. As drilling with casing was used in more demanding and critical applica-tions such as horizontal, deviated, and off-shore wells where most wells are drilled directionally, new connection types were needed. Requirements for these connec-tions include reliability, sealability, high overtorque capacity, and ultrahigh fatigue resistance because the stresses produced by bending and vibrations while rotating can reduce connection life.

ConsiderationsUse of oil-country-tubular-goods (OCTG) connections for drilling began as an attempt to reduce costs. Standard OCTG casing connections are designed to withstand stat-

ic loads of tension and compression, inter-nal and external pressure, and bending. Torque requirements for these connections are low. However, for drilling with casing, some critical issues that become important for drilling directional and deviated wells include the following.

• Wear protection. • High torque and compression resis-

tance. • High fatigue resistance. • Cost effectiveness.Wear protection is provided by means

of centralizers or other hardened surfaces located close to the pipe end to avoid damage to the connection. Higher torque and compression requirements are met by designs that include robust torque shoul-ders, thread interferences, and reduced gaps between mating threads. Fatigue resistance is a very important issue for drilling with casing and perhaps the most difficult chal-lenge to address.

Fatigue Response of OCTG Connections. Fatigue is a complex process that affects mechanical components subjected to cyclic loads. Fatigue failure is not a sudden crack, instead it involves different failure phases

that occur until component failure becomes evident. This failure occurs at stress levels much less than the elastic limit of the mate-rial. When subjected to cyclic loading, ordi-nary OCTG connections are weaker than drillpipe connectors that are designed to work with alternating stresses. The fatigue-failure location of threaded connections is in the area with the highest stress-concen-tration factor (SCF). Location of the SCF can be estimated with finite-element analy-sis (FEA), which also can yield a numerical value of the SCF. For standard premium OCTG connections with loads expected in drilling with casing applications, the SCF normally is greater than 2. However, from a fatigue perspective, it is desirable that the SCF be as low as possible. Threaded-con-nection response to fatigue loads can be plotted as a stress vs. number-of-cycles-to-failure curve.

The alternating stress for a drilling with casing operation is produced by the string dogleg while the string is rotated. The mean stress is produced by the axial loads applied on the string. Consequently, for a longer life, a balance between the axial load (mean stress) and the maximum dogleg that can be applied while rotating must be achieved.

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper OTC 17221, “Premium & Semi-Premium Connections Design Optimization for Varied Drilling-With-Casing Applications,” by N.J. Santi and G.E. Carcagno, SPE, Tenaris, and R. Toscano, Centro Investigation Industrial, prepared for the 2005 Offshore Technology Conference, Houston, 2–5 May.

Copyright 2005 Offshore Technology Conference. 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 • MAY 2006 75

Connection DevelopmentA development program was launched to provide alternative connections for drill-ing with casing. Different options from semipremium to premium fit-for-purpose connections with enhanced fatigue life were evaluated and tested. For the semipremium concept, a threaded and coupled (T&C) design was chosen because T&C designs had been used for similar applications. Main features of this design include a tight-er tolerance range than API connections, a robust torque shoulder that allows stable makeup and extra torque capacity with controlled stresses along the threaded area, and a robust thread design (Fig. 1).

For the premium connection, a tough casing design on an upset joint based on a proven design for tubing drilling and workstring applications was chosen. Two robust shoulders were included that allow sufficient prestress levels while having an overtorque capability that ensures no dam-age on the connection as a result of such prestress (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 for the presence of cracks were performed on specimens that did not fail during fatigue tests.

FEAFEA was performed for a premium con-nection and a standard T&C connection to evaluate fatigue response. Connections were modeled in extreme configurations of thread and seal, and two stress levels were simulated with a mean stress of 30% of the minimum API yield strength. The FEA code used incorporates an element that is effective and reliable for OCTG-connection analysis. Small displacements and strains were considered in the model. An elasto-plastic material model, von Mises yield cri-terion, and a bilinear model with isotropic hardening were used. Nonlinear contact

conditions were handled with a Lagrange multiplier contact algorithm.

In all cases, the makeup-torque value was increased to a higher value than that used for running casing to ensure good prestress in 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 and the graphic of results show that the high-est stresses for different alternating loads are produced at either the first or sec-ond pin or box threads, depending on the applied stress. This is different from what is observed normally on T&C connections where the highest SCFs are located within the last imperfect threads of the pin.

The SCFs on the premium connector do not exceed 2 and in some cases are closer to 1, which means that the connection meets the design target of an SCF less than 2.

Fatigue-Resistance TestsThe premium and semipremium connec-tions that underwent the full-scale testing program were manufactured from 7-in., 32-lbm/ft L80 material. The samples were manufactured according to standard manu-facturing procedures; however, the integral upset joints were manufactured on cou-pling stocks with the upset ends turned to reduce any possible imperfections.

The full-scale tests were performed on three samples of the T&C and three sam-ples of the premium connection by use of a resonant fatigue rig. The tests were planned to stop when the connections leaked. Both connections were tested at the stress levels recommended for this application, but the mean stress levels were slightly different to subject 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 simulate bending conditions of 19, 14.3, 9.5, and 25.6°/100 ft with a 236.5-kips mean tension load for the premium connection and 220 kips for the T&C connection.

Fatigue-Test Results. The T&C connec-tions behaved very well, achieving satisfac-tory results and providing an experimental SCF of 1.3 with a 95% survival probability. According to the results, this connection could drill at 9.5°/100 ft for more than 80 days 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 first connection tested achieved 868,000 cycles, and failure occurred at the tong marks. This connection was replaced by a spare specimen

that had the tong marks ground out. This spare specimen achieved more than 3 million cycles and failed in the transition area. This means that the 7-in. premium connection can drill 20 days at 100 rev/min, 19°/100 ft, and a tensile load of 236.5 kips. The experimental SCF for the upset connections was 0.9 with a 95% survival probability.

Magnetic-Particle Inspection. After the fatigue tests, the connections were broken out and inspected with magnetic particles. The inspection showed that for the upset connections, cracks developed at the transi-tion area or at the tong marks for the first upset connection (Fig. 3) and at the transi-tion for the others. No cracks were found on threads and seals. Visual inspection con-firmed that no damage appeared on threads and seals despite high makeup torque. No cracks were found in the threaded area on the T&C semipremium connection.

DiscussionResults showed that the connections satis-factorily achieved the requirements estab-lished at the beginning of the development program. For the semipremium connection, the purpose was to obtain better behavior than API connections currently used for drilling-with-casing applications. For pre-mium connections, it was demonstrated that the design satisfies the objectives in all cases, with the exception of the failure caused by tong marks. The connection tested at 14.5 ksi and stopped at 13 million cycles achieved more than 770,000 cycles when retested at 36.2 ksi. Cumulative dam-age on the pipe supports a fatigue-life estimate of 50 million cycles at 14.5 ksi (9.5°/100 ft) with no damage on the con-nection. In addition, the T&C connections also were overtorqued to 160% of the nor-mal value, while the upset connections were overtorqued to as much as 290% optimum torque, which demonstrates the overtorque capacity of these connections.

Conclusions1. The premium connection demonstrat-

ed an ultrahigh fatigue resistance without showing any failure on the connection.

2. The bending angles at which the con-nections were tested make them suitable for deviated wells, in particular the upset connections that also allow very high angles with a good life expectancy.

3. The semipremium connection presents a very good balance between performance and cost that makes it appropriate for less-demanding wells with no sealability requirements. JPT

Fig. 3—Magnetic-particle inspection of tong marks.