Elastoplastic Analysis of the Residual Stress in Chain Links

Abstract

Mooring lines of offshore oil exploitation platforms consist of long lengths of steel chain links, wire ropes and other accessories. Usually, these lines are designed for an operational life of about 20 years and periodic inspections are mandatory for monitoring the structural integrity of these components. The failure of a single element in a mooring line can cause incalculable environmental damage and severe economic losses. The ocean adverse environment loading produced by the combination of the wind, waves and currents leads to a complex alternate loading that can promote fatigue and crack propagation. Residual stress plays a preponderant part in the structural integrity of a mechanical component subjected to such loading. Offshore mooring line components as chain links enter in operation with a residual stress field created by the proof test dictated by offshore standards. However, the traditional design of such mechanical components does not consider the presence of residual stress. This study concerns about predict the residual stress field present in stud and studless chain links prior to operation to compare the fatigue life predicted by the traditional design methodology with the one predicted considering the residual stresses states present before operation. Numeric simulations with an elastoplastic finite element model are used to estimate the residual stress along the chain link that are present after the proof test and before operation. The results indicate that the presence of residual stresses modify significantly the fatigue life of the component.