Abstract
Loading and contact stress distribution on the thread teeth in tubing and casing premium threaded connections are of great importance for design optimization, pretightening force control, and thread failure prevention. This paper proposes an analytical method based on the elastic mechanics. This is quite different from other papers, which mainly rely on finite element analysis. The differential equation of load distribution on the thread teeth was established according to equal pitch of the engaged thread after deformation and solved by finite difference method. Furthermore, the relation between load acting on each engaged thread and mean contact stress on its load flank is set up based on the geometric description of thread surface. By comparison, this new analytical method with the finite element analysis for a modified API 177.8 mm premium threaded connection is approved. Comparison of the contact stress on the last engaged thread between analytical model and FEM shows that the accuracy of analytical model will decline with the increase of pretightening force after the material enters into plastic deformation. However, the analytical method can meet the needs of engineering to some extent because its relative error is about 6.2%~18.1% for the in-service level of pretightening force.
Highlights
Premium threaded connections have been widely used for maintaining structural and sealing integrity of tubing and casing strings in HPHT wells
Suppose the pretightening force Ft from sealing surface and shoulder contact is 1000 KN; the axial load acting on each engaged thread FiP and the mean contact stress on load flank pi can be calculated according to the analytical model proposed in this paper; the results are shown in Figures 6 and 7, respectively
One can find that both curves are identical in trend and that the minimum and maximum values obtained by the proposed analytical method are close to those of finite element method (FEM)
Summary
Premium threaded connections have been widely used for maintaining structural and sealing integrity of tubing and casing strings in HPHT wells. The vast majority of the analytical models mentioned above are established from screw threaded connection or cylindrical pipe threaded connection and are not applicable to premium connection: its tapered thread connection type contains special metal-to metal seal, interior rotary torque shoulder, and the specialized tooth form of buttress. And Macke [19] developed a testing apparatus to measure the axial displacement of the threads and compare their experimental data by modifying Sopwith equations They found out that the frictional coefficient between the lubricated bolt and nut has little effect on the thread load distribution. Based on elastic mechanics theory and some simplifications, this paper proposed an analytical method to calculate the load distribution and contact stress on each engaged thread tooth for premium threaded connections. The 2D FEM validated the analytical model, and the comparison of the contact stress on the last engaged thread between two methods investigated the accuracy of the analytical model and its practicality in engineering
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
