Importance of the Pin-End Effect on Leak Assessment in Threaded Connections

Abstract

Because pin and box are offset, applied axial tension (or compression) creates axial shear across engaged threads that opens a connection and reduces leak resistance. Less understood is the pin-end effect which does the same thing. Internal pressure acting on the pin nose pushes the pin out of the box, creating an additive axial force that contributes to the thread shear and decreases leak resistance. This paper addresses these axial loads for threads with non-symmetric flank angles. Multiple new concepts are presented. First, the thread shear equation, which depends on the pin-end effect, is developed for Buttress type threads in terms of the two flank angles. Second, the leak ratings for specific Buttress connections are presented in terms of a new leak criterion with dependence on backup pressure and axial force. Third, the leak dependence of Buttress type threads on backup pressure is shown to be significant and suggests that qualification testing of any connection for leak, without backup pressure, is not adequate.