Prediction of Residual Stresses in Mooring Chains and its Impact on Fatigue Life

This paper reports the results of a study conducted to investigate how residual stresses generated during the manufacturing process and subsequent proof loading may affect the fatigue life of mooring chains. The present paper shows the quantitative predictions of residual stress field obtained from finite element models of the fabrication process, and discusses their effect on the fatigue life of chain links depending on the loading mode. The models combine heat transfer analyses for the prediction of temperature histories during heat treatment (quenching and tempering), and stress analyses accounting for the thermo-mechanical history, including proof loading. The manufacturing conditions assumed for the models correspond to data obtained from a chain manufacturer. The predicted residual stress distribution is then combined with the fatigue stress range in service, due to either tension-tension loading or Out-of-Plane Bending (OPB). The effect of the residual stress distribution on the fatigue damage is discussed, and a sensitivity study on the assumptions used in the residual stress prediction is carried out. This determines for which loading conditions the modeling of the heat treatment stage can be neglected so that modeling of the proof loading step is sufficient for assessing the effect on fatigue life.