Investigation on possible layouts of a catamaran floating breakwater behind a wave energy converter

This experimental study is part of our R&D study of a multipurpose structure formed by catamaran floating breakwater (FB) placed behind a new type of wave energy converter, named ShoWED. The structure achieves the double aim of generating electrical energy and of protecting marinas. The specific objective of this work is to compare innovative FB layouts in terms of wave reflection, transmission and forces. Physical model tests (in scale 1:20) on the FB were carried out in the absence of the Wave Energy Converter (WEC). In particular, two geometries of catamaran type structures are investigated: one with a spacing between the pontoons equal to the sum of the two hulls, the other with double spacing. It was found that the transmission coefficient of the catamaran structure is largely reduced compared to the traditional ones, assuming same total mass. In fact, the dissipative mechanism plays an essential role. The larger catamaran does not perform significantly better than the narrower one. The performance of the converter is studied in scale 1:1. A peculiarity of these latter tests is that the real Power Take Off (PTO) was tested, although under smaller waves, allowing to measure the produced electrical energy as a function of the real external electrical impedance. It was found that a modification of the electrical impedance of the PTO affects the hydrodynamic response of the WEC floating arm: this poses a warning on the interpretation of the existing experimental investigations that model the PTO systems through mere resistive devices.