Dynamic response and power performance of a combined Spar-type floating wind turbine and coaxial floating wave energy converter

Wind turbines need to be spaced at a distance of the order of 1 km apart to reduce the effect of aerodynamic wakes. To increase the density of the power production in the farm, the deployment of wave energy converters (WECs) in the spaces between FWTs could be considered. However, the cost of energy from WECs is still very large. Therefore, the deployments of the WECs will reduce the economic value of the total project. In the present paper, a combined concept involving a combination of Spar-type FWTs and an axi-symmetric two-body WECs is considered. Compared with segregated deployments of FWTs and WECs, this combined concept would imply reduced capital costs of the total project because it will reduce the number of power cables, mooring line and the structural mass of the WECs. However, the effect of the addition of a Torus (donut-shape heaving buoy) on the FWT's motions as well as the power production should first be investigated. In the present study, coupled (wave- and wind-induced response-mooring) analysis is performed using SIMO/TDHMILL3D in the time domain to study the motion behaviour of the combined concept and to estimate the power production from both FWT and WEC under operational conditions. Mooring tension in the combined concept is also compared with the mooring tension in the Spar-type FWT alone. Hydrodynamic loads are determined using HydroD. The validated simplified method called TDHMILL is implemented to calculate the aerodynamic forces as a function of the relative wind velocity. The analysis is performed for several operational conditions according to metocean data taken in the Statfjord field in the North Sea. Finally, the behaviour of the combined concept under operational conditions is assessed, and it is shown to result in a positive synergy between wind and wave energy generation in terms of both capital investment and power production.