Abstract

Thread analysis is a critical structural analysis for downhole applications in the oil and gas industry, where strength and sealing performance of the thread connection is evaluated under the design load envelope. The first step of thread analysis is to reasonably model the initial make-up-torque to account for the pre-stressing effects caused by the torque-induced axial force at the bearing surfaces. This modelling is typically done by calculating and introducing an interference fit in 2D axisymmetric simulations. The well-known power-screw equation, which estimates the torque-induced axial forces on the thread bearing surface, is commonly used to find the axial force at the bearing surface. The initial interference is then determined by calculating the axial deformation of the threaded connection. However, interference calculations become challenging if the thread design and geometry are not conventional, such as threads with two nose-to-shoulder bearing surfaces (double-shoulder), where the power-screw equation is no longer applicable. The goal of this paper is to establish a general method to numerically calculate the interference fit applicable to both single-shoulder and double-shoulder threads using 2D axisymmetric models. The fundamental difference between make-up-torque mechanisms of single-shoulder and double-shoulder threads is highlighted and the application of power-screw equation is extended using classical mechanics to establish the torque-axial force relationship for double-shoulder threads through a new equation. Full 3D simulations followed by 2D axisymmetric finite element analyses are conducted to better understand the mechanics of the problem and verify the torque-axial force relationship. The analysis results confirm the substantial difference in mechanics of the single-shoulder and double-shoulder threads and the way torque-induced axial force is distributed. Finite element analyses reveal the high fidelity of the proposed methodology, where the results of 3D and 2D simulations as well as the predictions of the proposed equation come into good agreement. The results of this study can substantially improve the make-up torque simulation method using 2D axisymmetric models.