Analysis of Mooring Lines for Wave Energy Converters: A Comparison of De-Coupled and Coupled Simulation Procedures

Mooring systems for wave energy converters (WEC) have to be designed to survive the cyclic loads and motions they are subjected to as a result of the wave load-WEC interaction and the motions of the WEC in the random elevation of the sea surface. The current study compares four simulation procedures for the analysis of fatigue of WEC moorings. The objective is to recommend the type of simulation procedure that can be used to make reliable fatigue design of WEC mooring systems at a reasonable computational effort. A cylindrical floating WEC with four spread mooring lines is chosen for case study. The mooring dynamics of the WEC is simulated using coupled and de-coupled approaches in the time domain. In total, four types of simulation procedures are compared using the commercial simulation software DNV DeepC and an in-house code MOODY. A parameter sensitivity analysis of environmental conditions, model and numerical parameters is presented. The results are compared with respect to fatigue damage calculated using a stress-based approach and the rainflow counting method. It is found that a de-coupled approach, using DNV DeepC to simulate the buoy’s motions and cable response, is recommended since (i) it gives reliable results in terms of motion and stress responses of the buoy, mooring lines and accumulated fatigue damage, (ii) it requires the least model preparation by the engineer and the computation time is reasonable.