Enhancing Trapezoidal Threads Using a Parametric Numerical Approach

A parametric program designed for modeling tapered, trapezoidal threaded connections is used in combination with Abaqus to investigate the behavior of couplings subjected to static load combinations containing make-up, axial tension and internal pressure. Three criteria are defined and used to quantify the performance of the connection: load distribution, helical gap size between the threads and the amount of global plasticity. From these parameters, the load distribution provides valuable information about the effectiveness of the load bearing characteristics of the thread and can be used to detect possible overstressing of the connection. The helical gap size is used to estimate its tendency to provide a leak tight thread seal, while the global plasticity reflects the total amount of plastic deformation within the connection. During the investigation, the effects of taper angle, load flank angle, wall thickness, size of threads and initial thread clearance are considered. The presented modeling approach consists of three parts. First, the optimal make-up position for a trapezoidal threaded 4.5 inch TN80 connection is calculated using the plasticity criterion. Next, the results for the three performance parameters as a function of both axial tension and internal pressure are discussed. Finally, after investigating the various isolated effects induced by geometric changes, a newly defined, enhanced threaded geometry is suggested and compared with the standardized API-buttress thread.