Finite Element Residual Stress Analysis Applied to Offshore Studless Chain Links

The increasing expansion of deepwater petroleum activities has resulted in new challenges to the design of mooring systems. The complex mooring systems load history, which consists in a combination of wind, waves and currents, could induce nucleation and propagation of cracks in mooring line components. The failure of a single element in a mooring line of an offshore oil exploitation platform can produce incalculable environment damage as well as human and material losses. Offshore mooring line components like chain links must be submitted to a mandatory proof test, dictated by offshore standards, where loads higher than operational loads are applied to the mechanical component, resulting in high levels of residual stresses. Nevertheless, its presence is not considered in traditional design methodologies. Therefore, it is fundamental to develop new and more precise methodologies for assessing the structural integrity of mooring components. In this article, a comparative study is developed considering different approaches: two bidimensional finite element models, two tridimensional finite element models and an analytic model. These analyses establish the drawbacks and goals of using simpler models in the prediction of studless chain links stress distributions and in their fatigue lives. The four finite element models consider large displacements, plasticity and contact phenomena. Moreover, a simple fatigue life analysis is presented, based on SN curve, considering the effect of residual stresses in studless chain links before operation, that is, with loads caused by the proof test.