Design characteristics that improve the fatigue life of threaded pipe connections

Threaded pipe connections are commonly used to connect risers, tendons, drill pipes and wellcasing strings. In these applications fatigue resistance plays an important role. A large variety of patenteddesign features exist, all claiming to improve the connection’s fatigue life. However, patent documents onlycontain claims and numerical or experimental data about these connection’s performance is generally notpublished. This makes it hard to make a quantitative comparison between different designs. In this study anoverview is given of fatigue resistant threaded connections. Two major methods to improve the fatigue lifeof a connection were identified. First of all, local stress concentrations can be reduced by optimizing thegeometry of the threads. Second the global shape of the connection can be optimized to obtain a moreuniform load distribution.Using a parametric finite element model, different designs were compared. The connections were modelledby a 2D axisymmetric geometry with non-linear material properties and elaborate contact conditions.Selected designs have been subjected to experimental tests in a four-point bending fatigue setup. Theexperimental tests serve as a validation for the results of the numerical simulations. It was found that themultiaxial stress distribution at the thread roots is the defining factor for the fatigue life of the connection.Nevertheless, these stresses can be changed by the global geometry of the connection. It can beconcluded that the fatigue life of threaded connections is determined by a combination of global and localaspects which should both be analysed for fatigue life calculations.