OrcaFlex predictions for a multi-float hinged WEC with nonlinear mooring systems: Elastic mooring force and dynamic motion

The mooring system can affect the motion and energy output characteristics of a WEC. This study investigated the effects of an SPM mooring system in a M4 6-float WEC. The scaled model of M4 was modelled via both Orcaflex and experimental approaches. Compared with experimental results, the accuracy of the numerical results was found to be sensitive to the simulation methods in OrcaFlex, and the filter method supplied a better agreement with experimental results in this study. Following the numerical method selection, another two mooring cables were introduced to compare different stiffness properties with respect to tension reduction. Compared with the other two mooring cables, cable 3 with negative bending has the lowest peak spectral density of tension in all wave conditions; however, its stiffness needs to be optimised to reduce the peak tension in some large wave conditions. According to this, the stiffness of cable 3 is initially adjusted, and the new cable 3 can reduce up to 30% offset distance and 50% peak tension. The results also found that the relative rotational motion of the M4 is not sensitive to stiffness properties, which means the future PTO selection could be independent of the mooring cable.