Numerical and experimental study of the fatigue of threaded pipe couplings

Abstract

Threaded pipe couplings are used in the oil and gas industry as an alternative for welding and in applications where pipes should be frequently coupled and uncoupled as is the case for drill pipes. To maintain a sealed and secure connection while being subjected to external variable loads, they are commonly preloaded by using threaded connections with a conical shape. In this study a series of finite element analyses of conical threaded API Line Pipe couplings is carried out to quantify the influence of its contact parameters (coefficient of friction, preload and taper angle). In the assembled pipes, small sliding of the thread contact surfaces is still possible due to elastic deformation under external loads. This means, the contact zones of the threads can change under load, which complicates the analysis of the couplings. It was found that the coefficient of friction between the threads had a significant influence on this sliding. The shape of the threads inherently causes local stress concentrations, which can in turn initiate fatigue cracks. To validate the results of the finite element simulations, both static and dynamic tests were carried out on an API Line Pipe coupling. The preloading of the connection was carried out on a torque machine and an experimental fatigue test was carried out on a four-point bending test setup. Strains measured by strain gauges on the connections are in good agreement with the strains predicted by the finite element simulations. Appearing fatigue cracks emanated from the root of the last engaged thread of the male part of the connection, which is the region with the highest stress concentration factor in the numerical model.