Design, modelling and analysis of a combined semi-Submersible floating wind turbine and wave energy point-Absorber

Floating platforms for offshore wind tend to be very expensive and different solutions have been proposed to increase their cost-effectiveness. One of them involves the combination of offshore wind generation with other forms of ocean renewable energy, as is the subject of the FP7 project Marina Platform. In particular, wave energy from the sea has been investigated since the '70s and although a few technologies have already reached a pre-commercial stage, their actual economic feasibility can still be questioned so that the possibility of sharing cables, moorings and even the structure with offshore wind turbine is very interesting also from the point of view of wave energy developers. This paper presents the design, modeling and analysis of a combined concept composed of a semi-submersible platform hosting a single 5 MW wind turbine and a heaving pointabsorber consisting of a floating cylinder placed at the geometric center of the platform. A preliminary design of the concept is carried out by a frequency-domain analysis focused on the definition of the most suitable geometry with the objective of a limited dynamic response of the platform and satisfactory wave power absorption at the same time. It is shown how the requirement of maintaining reduced amplitude on the platform motions imposes the adoption of relatively slender cylinders as pointabsorbers. After a conventional catenary mooring arrangement is assumed and its basic line parameters determined by applying a quasistatic approach, a global coupled time-domain model is built based on the Cummins equation and the use of panel codes (e.g. WAMIT, AQWA) for the computation of the hydrodynamic coefficients. Moorings are modeled as individual catenary lines whereas the dynamics of the wind turbine are modeled by introducing thrust and power curves as function of the motions of the platform, after previous determination with the Blade Element Momentum theory. The analysis is carried out over a set of operational sea states for different locations around Europe. Through the analysis of power performance, platform and point-absorber motions and mooring tensions, it is shown how the introduction of a Wave Energy Converter (WEC) can occasionally have a positive effect on the whole response of the platform though the significance of its energy contribution is relatively small and additional synergies have to be sought to justify its adoption.