The sealing mechanism of tubing and casing premium threaded connections under complex loads

With the increase of ultra-deep and extended reach wells, the premium threaded connections (PTCs) of tubing and casing have to withstand extremely high level of combined loads, and often occurs sealing failure under complex loads. In order to solve this problem, the sealing mechanism of PTCs should be analyzed in depth. The existing analytical method can provide the contact stress formula for PTCs, but the extremely nonlinear mechanics of thread interactions under complex load states cannot be captured due to the stringent assumptions. In this article, a nonlinear 3D elastic-plastic finite element model for 7″casing buttress PTCs with the helix angle is established to analyze the stress distribution characteristics on the sealing surface, shoulder and thread. The results show that the preload has important influence on the sealing characteristics of the PTCs, and the sealing performance of the joint depends on the contact stress on the thread, sealing surface and shoulder. The axial tensile load will increase the sealing performance on the thread and weaken the sealing performance on the shoulder and the sealing surface. When a bending moment is applied on the tool joint, the contact stress increases in the compressed side while decreases in the stretched side, and the contact stress and sealing band width of the sealing surface decline, which indicates that the complex loads greatly affect the sealing performance of the sealing surface. Especially, the sealing surface width and the stress obtained with the three-dimensional finite element model are more reasonable than the analytical method due to the consideration of the helix angle of thread and the elastic-plastic characteristics of the material.