Floating offshore wind turbines (FOWT) are designed for harnessing offshore wind power from deep ocean water locations. The dynamic interactions between the various components of a floating wind turbine are very important in the design of such structures. The spartype floaters for floating offshore wind turbines have received wide popularity in recent times. In the case of spar-type offshore oil platforms, heave plates are often used in order to limit the heave response of the structure, especially for long waves. The effect of heave plates for such structures are two folds; they increase the added mass and the damping of the structure. However, the feasibility of using heave plates for FOWT structures has not been intensively studied so far. For the present study, the OC3-Hywind spar designed with a 5MW National Renewable Energy Laboratory (NREL) wind turbine is used. A coupled aero-turbine-floater-mooring analysis of the FOWT structure is carried out using the HydroD-HAWC2 model in order to study the effect of heave plates on the hydrodynamic motion. A heave plate in the form of a circular disk attached to the keel of the spar is studied for different diameters of the disks. The Effective response amplitude operators (RAO) of the FOWT are obtained for different disk configurations. Based on the results, recommendations are made on the use of heave plates for FOWT structures. Further, the hydrodynamic response analysis of the wind turbine with heave plate is performed for an operating environmental condition.
The total potential of offshore wind energy in Europe is expected to reach 7.7 GW by the end of 2018 (Wind Europe, 2017). It was observed that there is a gradual increase in the average water depth of the offshore wind farms installed in the recent times and the tendency shows it will further move offshore. Even though the fixed installations rule the offshore wind industry, their feasibility is limited to certain water depths. According to Wind Europe (2017), 80% of the offshore wind resource is located in waters 60m and deeper in European seas, where the fixed offshore wind turbines are not economically attractive. In order to explore this enormous potential of offshore wind resource, it is expected that there will be more demand for floating offshore wind turbines by 2030. Among floating wind turbines, the spar-buoy concept Hywind by Equinor (Jonkman, 2010a) has got wide recognition. Equinor has already commercialised the concept by building the world's first floating wind farm in Peterhead, Scotland.