Design and study of gas-tight premium threads for tubing and casing

Two premium threaded connections with buttress threads and torque shoulders are established in this paper. One premium threaded connection uses the taper difference fit model and the other uses the arc and straight line fit model for the sealing surfaces of the pipe body and the collar body. 3D models are established to consider the frictional shear stress during the make-up process. The finite element analyses of the two connection models are performed to obtain the stress distribution and deformation-induced space variation on the connection models. In addition, the characteristics of the designed structures are analyzed and the tubing and casing make-up curves are deduced based on these characteristics. Finally, an experimental test with the arc and straight line fit model is carried out. As a result, the arc and straight line fit model shows good gas seal performance in the make-up and break-out operation tests. This study is expected to provide a theoretical guide to practical operations in China, thereby reducing the damage probability of tubing and casing during make-up.