Fatigue of Mooring Chains Connected to Offshore Floating Structures Considering Out of Plane Bending Effects

Reliability against fatigue fracture is an issue of major concern in the design of offshore mooring systems with chain segments. The present paper describes the investigation of the effect of OPB (out of plane bending) and IPB (in plane bending) loading modes on the fatigue performance of chain links in critical positions. The hang-off design at the floater is based on long rods with bearings at the connection points to the floating structure. The purpose of the paper is to shed some light on possible design improvements on the connection design when using the available design guidelines for calculation of combined Tension-Bending fatigue in the mooring line. A challenge when using the existing design guidelines is that they often predict very short fatigue lives for what used to be a conventional hang-off design. A possible method for mitigating this is to improve the connection design so that the interlink angles could be reduced due to the slip in the bearings in the end of the rod. This would lower the bending stress to be occurred in those critical links. The present paper investigates the optimum length connecting rods and friction in the bearing on the fatigue lives of top mooring chains. A case study for 125 mm mooring chain and a pretension of 1200 kN is included. A non-linear beam model established according to the BV guidelines showed good ability to model the behavior of the 125 mm chain observed during the test. It is however a challenge to select the right interlink rotational stiffness and the correct bearing friction coefficient. An optimization analyses demonstrated that a rod length should be minimum 3 meters and the friction coefficient should be down to 0.15.