Experimental analysis and modelling of the fatigue behaviour of threaded pipe connections

Threaded pipe connections are commonly used in the oil and gas industry in particular to connect casing strings, drillpipe strings or production and transportation risers and pipelines. In order to maintain a sealed and secure connection while in service, the connections are generally preloaded. To introduce this preload, the connections commonly have tapered threads and they are assembled by applying a certain make-up torque. The combination of the make-up torque and external loads results in a multiaxial stress distribution over the coupling, where the coupling’s threads act as stress raisers. When used in offshore environments, waves and currents cause dynamic loads acting on the pipelines. The pipe connections remain the weakest points because fatigue cracks can initiate in the connection’s threads. A wide variety of patented threaded pipe connection designs exists, all claiming to improve a connection’s fatigue life. However, experimental data for such designs, available in literature, is limited. Published studies generally comprise experiments on a single connection type. In addition, as no uniformity in testing setup, loading conditions and damage detection techniques exists, results from different studies can not be used for a detailed fatigue analysis to compare the designs. Furthermore, current design curves in codes and standards lead to overly conservative or inaccurate results. The aim of this work is to provide a better understanding of the fatigue mechanisms of threaded pipe connections and to study the effect of different design features on a connection’s fatigue life. Final goal is to formulate guidelines for new fatigue resistant connection designs. In this study, the API Line Pipe connection is used as a reference. Several modifications and design features are applied to this connection type. To simulate the influence of these modifications, a parametric 2D axisymmetric finite element model, called ThreadGen, is developed. The results of this model are compared with a 3D finite element simulation to prove its validity for both make-up and bending. In addition the results of the 2D axisymmetric simulations are validated by static strain gauge measurements during a make-up test, an axial tension test, a bending test and an internal pressure test. The validated model is then used to evaluate the influence of connection properties and design features on the threaded connection’s behaviour. Test rigs are developed to perform fatigue experiments on three scales: the small scale experiments on 1” (33.4 mm outer diameter) connections are performed in four-point bending; the medium scale tests on 4.5” (114.3 mm) connections are carried out under axial tension and in four-point bending; for full scale testing of specimens in the range of 150 mm to 500 mm in diameter a resonant bending fatigue setup is developed. A significant part of this work is dedicated to this last setup. Two plain pipes without a threaded connection are tested in the setup to characterize its dynamic behaviour and to measure the response of a pipe subjected to a certain excitation frequency. The testing of full scale threaded pipe connections in this setup is part of future research. The majority of the performed fatigue tests comprises small scale experiments. Several modified configurations are tested. For each configuration an S-N curve is constructed, so that the effect of a certain configuration on the connection’s fatigue life can be quantified. It is observed that a local modification of the threaded connection’s geometry as well as the connection’s contact conditions can have an important influence on the fatigue life of the connection. During the medium scale four-point bending tests, advanced measuring techniques are used to monitor the fatigue crack growth during the test. Although these are existing techniques, they are applied to crack sizing in threaded connections for the first time. A dynamic 3D optical displacement measurement system is used to detect changes in the bending deflection shape of the pipe as a result of a propagating crack. Besides, a local crack opening measurement is carried out and modal analysis techniques are applied to monitor changes of the pipe’s eigenfrequencies. To visualize the crack fronts at different moments during the tests, a beach marking technique is used so that the exact crack shapes can be seen during post-mortem analysis. It is shown that a crack initiates at the root of the last engaged thread of the male part of the connection, and propagates gradually over a large segment of the circumference, forming a long shallow crack. When the crack penetrates the pipe wall, it rapidly increases in size along two crack fronts. The observed crack shapes do not have a semi-elliptical shape as commonly used in fracture mechanics. A fatigue crack growth analysis that considers the crack as an annular flaw, is effective in describing the crack growth behaviour. The results from the finite element simulations and the experimentally obtained S-N curves are combined in a multiaxial damage evolution law. Using this fatigue analysis, the observed trends in the fatigue lives of the configurations are explained. The general concept of using a connection’s thread load distribution as a measure for its fatigue life is proven to be inaccurate. The main reason for this is that the load distribution is related to the axial stresses over the connection. However, the fatigue life of a threaded connection is determined by the local multiaxial stress distribution and strain range around the root of the last engaged thread. These local conditions are not only the result of the load distribution, but they are also affected by the hoop stress introduced during make-up, which can additionally be affected by a changed connection stiffness. In addition, the multiaxial damage evolution law is used to analyse the influence of several features on a connection’s fatigue life. Not for all patented modifications an increased fatigue life is predicted when applied to the API Line Pipe connection. It is finally stated that to optimize a fatigue resistant connection, it is appropriate to combine several design features. The thread shape can be optimized to obtain a low stress concentration factor and reduce the local strains at the thread root. The connection global geometry and make-up conditions can be optimized to improve the load distribution over the threads and reduce local stresses and strains at the threads.