Fatigue Damage Calculation Of Mooring Chain With Out-of-plane Bending

Under the long-term service,mooring chain links will be affected by the combined effects of friction,corrosion,and wear due to the complex marine environment in the deep water.Therefore,the strength and security of mooring chain are inevitably to be affected by this cumulative damage.From the load type,the chain links will be subjected to stretching,bending and other different types of load.The performance of chain is studied extensively when the tensile load is applied.However,the safety performance of chain when exposed to out-of-plane bending(OPB)load studied less.The chain is often subjected to the out-of-plane bending load due to the movement of the floating platform and may cause damage in the fairlead position.It is important to research the stress distribution and fatigue distribution of chain when the OPB load is applied.In this paper,the stress distributions of the chain under different angles of OPB load are compared.The fatigue life and stress distribution of chain are calculated at different time periods when the cumulative damages are considered.And the difference of the results should be compared when the different angles of OPB load are applied separately.Firstly,the finite element model is established and mechanics analysis research of the Grade R4 stud-less chains of 124 mm diameter with different angles of OPB load have been executed by the software ANSYS WORKBENCH 16.2.And the diverse of results are compared when the different angle loads are applied.The maximum equivalent stress of chain under different angles is almost the same.However,the equivalent stress value has changed on the bending area where the chain is transition from straight area to a crown area(contact area).The stress value on the side away from the bending direction increases obviously with the OPB angle increase.Then the Archard theory and fretting theory of wear are used to calculate the wear of chain.Furthermore,the uniform corrosion rate model of the chain is used to simulate the combined of corrosion and wear in the marine environment.Finally based on the modified S-N curve method,the fatigue life of the chain link(the number of cycles required for the chain link to be failure)is calculated.The results show that the fatigue life decreases with the increase of the working time.At 20 years,the fatigue life of the bending area(away from the bending direction)will be less than the fatigue life of the contact area while the angle is 2 degree.Therefore,it is necessary to consider the effect of OPB load when studying the fatigue and stress properties of long working chain.