Development of simplified wear estimation method considering rolling motion between mooring chain links for floating structures

For the development of floating offshore wind turbines (FOWTs) in Japan, reliability assessment and cost reduction by efficient maintenance on mooring chains are part of the key issues because sites deeper than 60 m require mooring systems for station keeping. In this paper, attempts have been made to simplify the wear estimation method which was previously proposed by using a 3-D rigid-body link model for improvement of accuracy. These investigations are performed about a spar-type floating structure moored with three catenary lines at Goto, Nagasaki prefecture, Japan. In the previous study, it was confirmed that the estimation method using the 3-D link model can calculate the interlink wear amount closer to the chain diameter measurements and more reasonable than the method using the conventional mass-spring model by removing the rolling distance from the displacement of contact points. However, the estimation method has technical difficulty in terms of calculation of contact properties and analysis costs for the mooring chain design. For this reason, the wear estimation method is simplified and verified by considering the rolling motion between mooring chain links modeled by the widely employed mass-spring model based on the method proposed by Mooring Integrity JIP. Firstly, the wear due to the rolling is reproduced by a three-dimensional FE analysis to investigate the actual wear phenomenon including the effect of the proof load test as well as the idealized configuration. Secondly, the application method is proposed against the response of the mooring chain subjected to irregular waves Finally, a comparison with the measurement is conducted to verify the proposed wear estimation method.