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Summary The doping of thread-TCG (TCG) poses and environmental pollution threat, but it is essential to the performance of tubular connections. This paper deals with an innovative tubular-connection coating without TCG, and describes the galling resistance and connection performance under field conditions. To develop the new coating technique without TCG, pin-on-disk-type tribological tests were conducted. For the connections with the new coating, performance was evaluated with power tongs and full-sized testers. As a result of the testing, the make-and-break (M&B) of the connection without any liquid lubricant was accomplished by means of the new coating. The connection can maintain the same performance as the conventional connection with TCG. This new connection coating can solve the problems induced by TCG and simplify the running works of casing and tubing pipes.
Introduction Oil Country Tubular connections used for oil and gas wells have been constructed with TCG for several decades. The galling resistance of the connection during makeup is directly affected by the TCG. Recently, the environmental impact of the TCG has become an issue in the oil and gas industry. One of the problems addressed is environmental pollution caused by heavy metal particles contained in TCG. The response has been the development of commercialized, environmentally safe TCGs. Other problems include accumulation at the well bottom, contamination of the reservoir, and contamination of production fluids. The two primary reasons for the use of TG are to affect a thread seal and to provide lubrication of the sliding surfaces during assembly of the connection.
The objective of this study was to develop a new tubular connection coating that can display the same performance as the conventional connections without the use of TCG. The key technology is the new self-lubricating surface treatment, thus eliminating the need for the TCG. The primary contribution of this technology is the elimination of the previously mentioned problems attributable to the TCG. Secondary contributions are reduced labor while running the pipe, simplified makeup operations, and cleaner and safer working environments. This will produce a more efficient pipe-running operation.
With a rubbing-wear test device, the galling resistance of the conventional metal plating, the dispersion plating, and the resin coating was evaluated. Then, the galling resistance of a 7-in. outside diameter (OD) premium connection was evaluated without the use of TCGs. Additionally, the durability of the resin coating was validated at temperatures of -10 and 170°C. Connection performance, gas-sealing ability, and sulfide stress-cracking (SSC) resistance were evaluated with a special full-sized load frame.
The new coating, consisting of phosphate coating and modified polymide resin containing MoS2, indicated the best galling resistance. No galling was observed in 10 M&B assembly tests. TCG was not applied. The galling resistance was demonstrated in this durability test. The connections showed good sealing ability without TCG, and the SSC resistance was not reduced.
Design Concept of the Dope-Free Connection The dope-free connection is defined as tubular connection makeup without any liquid lubricant (e.g., TCG) with the same performance as a conventional connection.
The design concept of the dope-free connection is to provide self-lubrication by means of the surface treatment. The main functions of the TCG are to improve the galling and leak resistances of the connection. In general, the galling resistance is necessary for the reuse of the connection in the fields. Here, galling is regarded as the surface damage that occurs in the severe stage of adhesive wear between the interacting sliding surfaces under high contact of bearing pressure. The galling resistance of the conventional connection is a function of the TCG and surface treatment. Regarding the sealing ability, TCG is indispensable for thread-seal connections such as American Petroleum Inst. buttress and round thread. The TCG may also be effective for improving the sealing for the premium connection, though it is not necessary to use TCG. Accordingly, the research was limited to premium connection with a metal-to-metal seal. The targets were galling resistance without a liquid lubricant and leak resistance without a sealant. The SSC resistance and the durability of the surface treatment were also investigated. From the SSC tests, the potential of SSC induced by MoS2 was considered. The durability tests were conducted to evaluate the performance deterioration of the resin coating with time-dependant swelling under free-state and makeup conditions.
Method for Evaluation of Connection Performance Three types of test were performed to develop the dope-free connection. The coupon test conducted by use of a galling-test machine to select the optimum surface-treatment candidate for gall resistance. The field simulation test was run to validate the performance properties of the connection with the new coating. The numerical simulation test estimated the torque developed and the stress state of the connection.
Coupon Test of Surface Treatment.
Fig. 1 illustrates the pin-on-disk-type galling-test machine. The test simulates the sliding and contact conditions of a tubular-connection, metal-to-metal seal. The test machine, however, was not used to predict the galling resistance of the connection, but, rather, to rank the surface treatments. The pin has a polished spherical surface with a radius of 24 mm. The disk has a flat surface of 250 mm diameter, finished by turning. The maximum surface roughness is about 0.007 mm, as large as that of the seal surface for a conventional premium connection. Both materials are heat-treated 1% Cr/0.2% Mo steel, and the yield stresses are 800 MPa. Surface treatment was applied only to the pin.
Table 1 summarizes the surface treatments, which are classified into three groups. The metal-plating group includes the popular treatments used for conventional connections. The dispersion-plating group is one of metal plating in which the solid lubricant is dispersed into to the metal matrix. The resin-coating group is characterized by the solid lubricant bound by the resin. Here, the super engineering plastics represented by modified polymide resin and epoxy resin were used.
Coupon Test of Surface Treatment.
Fig. 1 illustrates the pin-on-disk-type galling-test machine. The test simulates the sliding and contact conditions of a tubular-connection, metal-to-metal seal. The test machine, however, was not used to predict the galling resistance of the connection, but, rather, to rank the surface treatments. The pin has a polished spherical surface with a radius of 24 mm. The disk has a flat surface of 250 mm diameter, finished by turning. The maximum surface roughness is about 0.007 mm, as large as that of the seal surface for a conventional premium connection. Both materials are heat-treated 1% Cr/0.2% Mo steel, and the yield stresses are 800 MPa. Surface treatment was applied only to the pin.
Table 1 summarizes the surface treatments, which are classified into three groups. The metal-plating group includes the popular treatments used for conventional connections. The dispersion-plating group is one of metal plating in which the solid lubricant is dispersed into to the metal matrix. The resin-coating group is characterized by the solid lubricant bound by the resin. Here, the super engineering plastics represented by modified polymide resin and epoxy resin were used.