Abstract
The effect of out-of-plane bending (OPB) on the fatigue life of moorings has been the focus of extensive study over the last 15 years. Classification societies now require it to be considered in mooring chain fatigue life calculations.
A fatigue analysis of the mooring for a North Sea FPSO was required to support life extension. This paper describes the development of a procedure for the calculation of the fatigue life of the mooring chain with a thorough treatment of fatigue stresses including out-of-plane bending and interaction with the fairlead chainwheel. To include these effects, an integrated method has been developed.
A nonlinear time-domain simulation of the vessel and mooring system is used to calculate time histories of tension, and declination and azimuth angles relative to the vessel for multiple seastates representing the wave-induced fatigue environment. A detailed finite element model of the vessel’s top chain and chainwheel is used to generate transfer functions between chain tension, azimuth and declination angles relative to the vessel, and Dang Van stress.
The mooring time history output and the stress transfer functions are combined to produce time histories of Dang Van stress at nodes across the chain link surface for each seastate. The resulting nodal stress time histories are rainflow counted. Damage is then calculated and summed across all seastates using Miner’s rule.
The results for the integrated method show good agreement with that for tension-tension using chain S-N curves if OPB is omitted, providing validation of the approach. Including the effect of OPB, the increase in annual chain fatigue damage is equivalent to the application of a stress concentration factor of 1.5. The complex history of the particular vessel, including the effect of two deployments and a chain refresh strategy, results in a slight increase in calculated life of the critical link compared to tension-tension fatigue alone when all effects are included.
