This paper proposes three kinds of SWA (Small Water-plane Area) trifloater foundations, aiming to study this new-type floating foundation for offshore wind turbines with excellent hydrodynamic response performance. The overall hydrodynamic model including foundationtower-wind turbine was established by using the SESAM software. Based on three-dimensional potential flow theory and Morrison formula, combined with the environmental loads of wind, wave and current in South China Sea, the hydrodynamic responses of these three SWA tri-floater foundations and constant section tri-floater foundation are calculated and compared in both frequency and time domain. The results show that the wave excitation loads acting on the foundation could be decreased effectively by reducing the water-plane area of columns. In addition, the motion responses of foundation are sensitive to the stiffness and water-plane area. The SWA scheme is possible to be adopted for FOWT (Floating Offshore Wind Turbine) by designing the foundation appropriately.
With the development trend of offshore wind turbine from shallow water to deep water, the floating foundations are becoming increasingly popular compared with the traditional fixed foundations. Generally, the floating foundation can be roughly divided into three categories: Spar type, TLP (Tension Leg Platform) type and Semi-submersible (Zhang C, 2017).
FOWT is a combination system. In general, it could be divided into two main parts: the wind turbine subassembly and floating foundation. The upper tower suffers from wind loads and the blades suffer from aerodynamic loads. The foundation suffers from wave and current loads simultaneously. The interaction of various loads due to wind, wave and current brings great challenges for the design of floating structure. The improvement of stability and reduction the response amplitude of floating structure when subject to environment loads is a reason of concern. Meng, He, Zhou, Zhao and Liu (2018) proposed a fully coupled aero-hydro-servo-elastic time domain model by FAST software to investigate the dynamic response characteristics of a 6MW spar-type FOWT under the multi interaction of wind & wave loads. The results show that the mean value of dynamic response was mainly dependent on the wind-induced action. Zhang and Kim (2018) presented a fully coupled numerical simulation method to analyze the dynamic response of a semi-submersible floating wind turbine under the combined wind-wave excitation environment loads. Ye and Ji (2019) studied on the dynamic response of a 5MW spar-type offshore wind turbine by taking into account the interaction of current, wave and wind. The effects of hydrodynamic & aerodynamic loads on dynamic response of floater is calculated by time history and its FFT (Fast Fourier Transformation) spectrum results. Bahramiasl, Abbaspour and Karimirad (2018) carried out a study on influence of floater responses by the gyroscopic effect of rotating rotor and wind heading angle. It was found that the peak of spectrums could be shifted to a higher frequency by increasing the rotor rotation velocity. In addition, the heave and pitch response amplitudes in time domain as well as heave, sway and surge motion in frequency domain could be changed with the changing of heading angle of wind. Huijs, Vlasveld, Gormand, Savenije, Caboni, LeBlanc, Ferreira, Lindenburg, Gueydon, Otto and Paillard (2018) integrated design a semi-submersible floating VAWT (Vertical Axis Wind Turbine) with active blade pitch. The result illustrated that the weight of floater can be reduced by 20% under the same rated power. Chen, Hu, Duan (2018) compared the dynamical characteristics of a 5MW spar-buoy with semi-submersible floating wind turbine by the model testing results. It was found that the sparbuoy floating wind turbine is more sensitive to wind loading due to the smaller water plane, whereas the semi-submersible floating wind turbine is more sensitive to wave loading, especially for the second order difference frequency wave loading. Liapis, Lu, Li and Peng (2015) used the model testing and CFD simulation methods to investigate a design of a semi-submersible utilizing heave plates. They concluded that heave motion can be significantly reduced by adopting heave plates. Jang, Park, Kim, Kim and Hong (2019) conducted a numerical and experimental study on a semi-submersible multi-unit offshore floater with four wind turbines. The study indicates that the heave and pitch motions of floaters had been reduced effectively due to the effect of heave plates, and their natural frequencies were also changed.