Dynamical analysis and control of a combined wind-wave energy conversion platform

The ever-increasing need for larger energy production and for a reduction of dependency on fossil fuels, in favor of renewable energies, has led to the pursuit of new concepts that can exploit natural resources efficiently. In this scenario, offshore wind-wave hybrid platforms have been recently promoted: sharing facilities, infrastructure, and grid connections, gives them the potential to increase energy production at a lower cost. However, an efficient realisation of these two combined technologies requires two potentially conflicting control objectives: on the one hand, for the wind turbine, a reduced movement of the platform is required, which essentially translates to enhanced stability of the structure, so that its behavior resembles standard onshore wind technologies. On the other hand, to maximize the energy produced, wave energy converters (WECs) require optimal control technology which often leads to large amplitude motion, potentially conflicting with the stability required for the wind turbine. The first aim of this thesis is to gain a better understanding of how the energy-maximizing control problem for WEC systems, which can be analyzed in terms of the so-called principle of impedance-matching, interacts with both conversion systems and to elucidate their synergies. The system considered within this study, consists of a three-column semi-submersible platform with a standard horizontal-axis wind turbine (NREL 5 MW) at the top of a tower, sitting on the rear column, and a rectangular flap-type WEC mounted on the main beam. This device is analysed from a closed-loop perspective. In the first place, the WEC control system, synthesised in terms of a proportional-integral (PI) structure, is designed to maximise the energy produced by the WEC, as a function of the (wave) operating conditions. Subsequently, a stabilising controller, based on the so-called Youla-Kučera parameterisation, has been implemented, and compared with the energy-maximising control solution both in terms of interaction with the platform motion, and the final performance achieved, for the same set of wave conditions.