Wave-induced collision loads and moments between a spar-buoy floating wind turbine and an installation vessel

Abstract

Spar-buoy floating wind turbines (FWTs) have been deployed at full-scale off the coast of Scotland. However, their deep draught restricts their wider-suitability for assembly and installation at shallow water ports. Here a barge-type installation vessel is investigated experimentally for supporting a FWT such that a draught reduction of up to 30% can be achieved. Tests in irregular waves are conducted to understand the design loads required of a mechanical linkage between the vessel and FWT. Overturning moments increase by over double for a doubling in significant wave height, Hs, and peak overturning moment occurs at the pitch eigenfrequency of the combined vessel and FWT. For Hs=1.5 m, these loads can be accommodated for with a steel truss-frame structure. Collisions between vessel and FWT are also tested for regular head waves and show that, whilst a number of collision forces exceed those of the relevant design standards, these could be reduced to within the existing design limits by use of a fender.