Quantitative wear estimation for mooring chain of floating structures and its validation

For reducing maintenance cost of floating offshore wind turbine structures, it is necessary to establish quantitative wear estimation method for the mooring chains. In this paper, attempts have been made to improve the accuracy and applicability of the estimation method in the following steps. By using the wear analysis method between the links of mooring chain that Gotoh et al. investigated with the finite element analysis software (MSC. Marc), the expression of the wear amount was obtained, which was a proportional functional form associated with sliding angle and tension. The relative sliding angle and tension between links were analyzed by using a multibody dynamics software (MSC. Adams). A spar-type floater moored with three catenary mooring lines at Goto, Nagasaki prefecture, Japan was analyzed. Here, the floating body was modelled as a rigid body and mooring chains were modelled by mass-spring (lumped mass) model. From these results, the wear amounts calculated by using the estimation formula and relative sliding angle and tension between links were compared with the measured wear amounts for mooring chain of the floater which was deployed for about one-year at Goto. The cases with only waves and those with wind and waves were analyzed. From the comparison between the simulation results and the measured ones, it was found that the proposed method can fairly predict the wear amount of mooring chains. However, it was also found that the proposed method has a tendency to overestimate the measured results. These reasons were discussed in the paper.