This paper is to investigate and compare hydrodynamics of four different floating platforms for wind turbines, including the natural frequencies, RAOs and standard deviations of motions subjected to the possible sea states of East China Sea at five different water depths (30-200 m) based on potential wave theory. The results show that the semisubmersible platform has smaller natural frequencies and standard deviations than the barges in heave and pitch and that the water depth has significant effects on the floating platforms when it is less than 62.5m in the tested cases. The effects of a heave plate with different diameters on heave RAOs is also studied by using the dipole method with viscosity estimated by an empirical formula, and find that the heave RAOs may not be reduced by the heave plate and the increase of its diameter.
Compared with onshore wind resource, offshore wind energy has much more potential and becomes increasingly popular. Currently, most of the offshore wind turbines are installed on the fixed support platforms, which are based on the mature technology of the onshore wind turbines. However, the technology for fixed offshore platforms may not be economical when water is deeper than 50m (Robertson and Jonkman, 2011). In contrast, the floating offshore wind turbines (FOWT) will be economically better in the deeper sea areas. Previous studies by Withee (2004) and Lee (2005) have revealed some encouraging results for some floating wind turbine systems.
Different from oil & gas offshore floating structures, the floating structure of offshore wind turbine must withstand the vast overturning moment from the wind turbine thrust load. In terms of how FOWT achieve its hydrostatic stability in roll and pitch, there are three primary categories. The tension leg platform (TLP) obtains restoring moments mainly by the integration of the mooring system and surplus buoyancy in the platform, spar buoy primarily depends a deep draft combined with ballast, and semi-submersibles rely on the large water plane area. Jonkman and Matha (2011) analyzed the dynamic response of three primary floating platforms mentioned above and compared with the onshore wind turbine. The increased loads on floating wind turbine were found in their work in contrast to land-based wind turbine. The characteristics of dynamic response for the TLP, Spar and semisubmersible platforms with vertical axis rotor were investigated by Cheng (2015).