Assessment of Fatigue Damage Initiation in Oil and Gas Steel Pipes

Steel pipe structures used in oil and gas industry, such as drill pipes, rigid risers and pipelines, undergo the action of cyclic loadings that can cause their failure by fatigue. These structures are made of high strength steels, such as API 5L grades X for instance. A consistent evaluation of the fatigue behavior should fundamentally be based on a local approach, in the dislocation scale, and take into account the micromechanisms of fatigue damage initiation, including microdeformations and microstructural changes, which precede the macrocrack propagation leading to final failure. In this work, the microstructural mechanisms of fatigue damage initiation in API 5L X60 grade steel pipes are investigated. Material properties of API 5L X60 steel are estimated according to chemical composition analyses, microscopic analyses, uniaxial tensile tests and Vickers micro-hardness tests. Samples are submitted to fatigue tests with reversed stress bending loadings. Microdeformations and residual stresses are measured with the aid of the X-ray diffraction method in real-time during fatigue tests. A numerical model is developed to reproduce the fatigue test loadings. The aim of the work is to provide ground for the development of a microstructural criterion for fatigue damage initiation in API 5L X60 grade steel pipes from the obtained experimental results. This criterion could allow a good prediction of the residual life of steel pipes previously submitted to fatigue loadings, before macroscopic cracking, and help to increase the reliability of oil and gas pipes.