Experimental assessment of mooring loads and response of the M4 wave energy converter in complex conditions: the effect of currents and crossing seas

Abstract

Real ocean conditions can be complex. Wave systems often co-exist with substantial currents and/or other wave systems which can differ significantly in their mean directions. Despite this, much of the testing and de-risking of wave energy converters and their associated mooring systems is conducted in unidirectional, unimodal, wave conditions in the absence of current. Through analysis of a new set of experiments conducted at the FloWave Ocean Energy Research Facility, this paper improves understanding of mooring line loads and response in these complex, yet realistic, scenarios. The multi-float M4 wave energy converter (WEC) is assessed in combined wave-current conditions and in directionally spread bimodal crossing wave conditions. The experiments consist of the 1:40 scale M4 model in a 1-2-2 configuration, with an elastic taut-slack dual-buoy mooring configuration where one of the buoys is submerged (as in [1]). Six degree-of-freedom motions along with mooring line loads on the bed (3) and hawser (1) lines are analysed for (i) long-run irregular unidirectional JONSWAP waves conditions with a mean 0.2 m/s current at different angles relative to the wave direction (0^o, 90^o, 180^o)^, and (ii) long-run irregular directionally spread wave conditions consisting of both unimodal and crossing systems (crossing angles of 45^o, 90^o, 135^o). Motions and loads are analysed and additionally compared to unidirectional irregular conditions in the absence of current demonstrating large differences in loading, device orientations and dynamics. The results question the validity of ignoring real-world complexity for floating platform design.

References

[1] Draycott, S., Stansby, P. and Li, G., 2023, September. Experimental measurements of two elastic taut-slack mooring configurations for the multi-float M4 WEC. In Proceedings of the European Wave and Tidal Energy Conference (Vol. 15).