Special considerations to calculate joint strength of premium connections

Abstract

Premium connections, which usually own modified buttress threads, special mental-to-metal seals and torque shoulder, have been increasingly adopted in casing and tubing strings to improve their joint strength and sealing performance in high pressure-high temperature (HPHT) gas wells. Because of complicated geometric structure, it is very difficult to establish an analytical joint strength model for premium connections while typical finite element analysis (FEA) does not also reveal the general failure law. In fact, the coupling joint strength of premium connection will decrease because of the make-up torque resulted from shoulder and the torque shoulder never allows to separate under limit axial working loads. Considering the make-up torque and prevention of shoulder separation, this paper proposes methods to calculate the joint strength of premium connections with API buttress thread teeth based on elastic mechanics. An example is also analyzed by modifying a premium connection from an API 177.8 mm P110 grade buttress thread casing. The results show that the joint strength of the premium connection is largely influenced by the ratio value ζ of the torque resulted from the shoulder to the total torque and that larger or smaller ratio values ζ both decrease the joint strength. To enhance the joint strength and tensile connection efficiency in practice, not only should the total make-up torque be controlled but the ratio values ζ should also be appropriately controlled. The proposed methods more comprehensively reflect the failure patterns of premium connections and their great significance for joint strength calculations, makeup torque control and connection parameter design.