Summary. A new method of connection makeup was developed to provide a leak-free seal up to rated working pressure and tension for API 8-round casing and tubing threads. For many years, industry has used various torque-only and torque-turn techniques to make up these threads. Current industry needs and regulatory test requirements are raising the pressure-sealing needs and tensile-load requirements for drilling and production tubulars. This new method, torque-position, accounts for variances in thread tolerances, coupling coating or plating, and thread compound. Advanced finite-element computer modeling helps to establish key makeup parameters. This paper discusses the problems with current makeup methods and evaluates the new method, including its field application for casing and tubing. Introduction The API 8-round thread is one of the most widely used casing and tubing threads in the oil and gas industry. Its ease of threading and low production costs make it readily available for many applications. Because of potential leak problems, however, it is often limited to working-pressure applications below 5,000 psi [34 MPa], especially where a gas-tight connection is required. Even at pressures below 5,000 psi [34 MPa], leaks can occur during various pressures below 5,000 psi [34 MPa], leaks can occur during various production modes if proper inspection, handling, and makeup production modes if proper inspection, handling, and makeup procedures are not observed. procedures are not observed. Field Leeks Torque-Only Method. Connection leaks were encountered when heavy-wall, 7-in. [ 178-mm] -OD, 4 1 -lbm/ft [61 -kg/m], Grade P-110, 8-round production casing was run in the bottom section of P-110, 8-round production casing was run in the bottom section of a casing string. The plain-end casing was used, the threads were gauged to API specifications, couplings were bucked on to specified torque values, and the casing was run m two wells. Both wells developed leaks during completion and subsequent production. To solve the problems, 5-in. [127-mm) -OD liners were cemented across the 7-in. [178-mml -OD, heavy-wall casing to cover multiple leaks. An analysis of the casing-thread inspection and gauging did not reveal any problems. An analysis of the couplings used in both strings, however, revealed that some were phosphate-coated and some were tin-plated. Even though the specified torque values had been used for each of coating during makeup, subsequent testing revealed that much higher torque than specified was required for the phosphate-coated couplings to provide a leak-free connection. A review of the torque makeup tapes provided by the casing-tong service company showed very high torque values where the phosphate-coated couplings had been run in the casing string and phosphate-coated couplings had been run in the casing string and that the connection leaks occurred in those intervals. Two recent tubing leaks in offshore wells were attributed to improper thread engagement when Grade N-80 pup joints were madup into a Grade P-110 packer material and into a Grade P-110 gas-lift mandrel. Even though both had been pressure tested for a few seconds while being run in the hole, both leaked during later production and stimulation. Analyses of both leaks showed that the N-80 pin had not been made up far enough into the P-110 box material. The connections had leaked, the threads eventually washed out, and the connections parted. Both wans required workovers to replace the assemblies. In both cases, torque values higher than those normally required for Grade N-80 were needed to obtain adequate thread engagement to prevent leaks. Torque-Turn Method. Several connection leaks were also experienced in 7%-in. (194-mm) -OD, 39-lbm/ft [58-kg/m], P-110, long-thread-and-coupling (LTC) casing with tin-plated couplings. Thes leaks occurred during pressure testing and shortly after the casing was pressure tested to 4,500 psi [31 MPa] with drilling mud inside the casing. Remedial work to stop the leaks was time-consuming and costly. The first well to experience this problem had threaded and coupled pipe from the supplier, with the field-end makeup being torque-turned while the casing was run. About 15% of the millend connections made up further as the field end was made up by torque-turn. This indicated that the mill end was undertorqued; a check of mill-end makeup procedures could not determine the makeup torque value used. Also, the thread compound used at the mill could not be positively identified for analysis. If casing is bought threaded and coupled, the made-up pin and coupling cannot be thread gauged or even visually inspected. Also, the threading tolerances of each pin end and opposite coupling ends are not the same because of the threading sequence. Therefore, thread gauging the pin and open end of the coupling will not indicate the thread tolerances of the threaded and coupled connection. This condition allows problems to go undetected with respect to thread lead, height, taper, pitch diameter, and ovality. Discussion. These leak problems can be related to improper makeup resulting from use of either the torque-only or the torque-turn makeup method. The torque-only method, commonly used for millend makeup of casing and tubing, uses API torque values and a check of the coupling face with respect to the last thread scratch on the pin. This torque value is sometimes adjusted to obtain rec-ommended engagement (within two thread turn) on several joint(usually 10 to 20 joints for an order of casing or tubing) and to use an average torque value for the remainder of the order without a check of the actual makeup position. With torque-turn, a method used successfully for many years, the smoother finish of current machining techniques and various thread tolerances caused problems with the turn count. The turn count starts once a reference torque is reached during makeup. This reference torque varies from 50 to 300 lbf-ft [68 to 407 N - m] for various tubing sizes and material grades to 10% of the minimum makeup torque values for casing. Because of the low reference torque values, the turn counter may start recording turns at different positions of pin and coupling engagement. While this method has positions of pin and coupling engagement. While this method has served industry well, the changes in the thread-surface friction coefficient, thread manufacturing methods, and thread compound differences and the variations in the coating or plating friction coeffi-cient have given erratic results at times. Also, as mentione earlier, if the mill end of threaded and coupled pipe has not been torque-turned, the mill end of the coupling can turn during field makeup. This can then introduce a potential leak path when the thread compound is disturbed, especially if the pipe has been on the rack for a long time. Problems Affecting Makeup Problems Affecting Makeup The API 8-round tubing and casing a is a very forgiving when made up with appropriate torque values and with the proper coating or plating on the couplings. If excessive torque values are used and if the coating or plating (zinc, phosphate, or tin) is not properly applied, however, galling and leaks can be a problem. properly applied, however, galling and leaks can be a problem. SPEDE September 1990 P. 233
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