Fatigue Life Prediction of Mooring Chains Subjected to Tension and Out of Plane Bending

The present paper describes the behaviour of chain segments that are subjected to pretension and a rotation angle at the segment end. The behaviour such segments has been investigated both experimentally and by finite element modelling. The purpose is to carry out fatigue life predictions. A full scale test with a studless chain segment with a diameter of 125 mm has been carried out to shed more light on the behaviour. The test corroborated the assumption that the chain segment behaves semi-rigidly under the given conditions due to locking of the inter-link hinge mechanism. The influence of various important parameters on the bending effect was studied. The chain segment has a significant flexural stiffness and an intra-link bending curvature for links that are in a flat position with respect to the inflicted end rotation. The associated Out of Plane Bending (OPB) stresses are significant for critical links close to the end hang-off. This effect must be taken into account when carrying out fatigue life predictions. There is still no guidance given in rules and regulations for this case. In the present work it is suggested that a hot spot method is applied for fatigue life predictions. Under a combined loading mode, defined by tension and OPB, the maximum principal stress range in the link bend area must be determined by refined finite element analysis. Subsequently, the fatigue life can be predicted by using an appropriate hot spot S-N curve. The paper points out how a good fatigue design of the hang-off area can reduce the effect of OPB. A Buoy Turret Loading (BTL) is used as a case study to demonstrate both the design proposal and the fatigue life prediction methodology.