Effect of wind, wave and current on movement characteristics of array of floating wind turbine Spar platform

Abstract: The demand for renewable energy has increased steadily in recent years in response to the shortage of traditional energy as well as the environment pollution. Among the available renewable energy resources, wind energy is considered to be one of the most promising forms of energy. Compared with the onshore wind energy, the offshore wind energy is optimized due to its high speed, small shear force and saving of resources. It is the definite tendency for the future wind farms to be offshore, from shallow to deep sea, and change to floating type. Pursuant to the limitation of technology and budget, the current offshore wind turbines mostly adopt the fixed base and the location is normally within 30 m shallow sea area. The economic analysis shows that for the better wind resources in the deep sea area with more than 60 m depth, the floating type platform should be adopted. Obviously, the stability of platform structure is the fundamental guarantee of the safe operation for the offshore floating wind turbine. In order to investigate the dynamic response of wind turbine platform matrix, this paper proposes a multi-platform array scheme of share mooring system with 3×3 square array; the models are established based on the OC3-Hywind Spar Buoy platform with NREL 5MW wind turbine, and with the radiation and diffraction theory and the finite element method, the comparison of dynamic response characteristicsis carried out between the single platform and the 3×3 square array platform. The dynamic response of floating platform with no wind operation is resolved. The velocity fluctuation of the floating platform motion is converted into the velocity fluctuation of the wind flow at the rotor, which is then superposed with the inflow wind speed to be taken as the relative wind speed. The blade element momentum theory and the variations are used to resolve the wind loading. The first order wave force on the floating platform is resolved with the radiation and diffraction theory. The near field method is used to resolve the second order average drift force. The current load is simplified just as the drag force. The simulation is carried out by the commercial FEM software ANSYS AQWA. The results are as follows: the surge, heave and pitch response of Spar platform are all concentrated in low frequency waves, whose frequency is lower than 0.5 rad/s; the platforms located on the 4 vertexesin the 3×3 square array have higher swaying response with the response amplitude between -0.2 and 0.2 m, and the swaying response of other platforms can be neglected. With the intensification of environmental load, single platform's surge response increases sharply with the response amplitude increasing from -0.25-0.25 m to -1.5-1.5 m, but multiple platforms' surge response does not change significantly. At the same time, the multi-platform pitching response is significantly lower than the single platform. The results verify the effectiveness of the scheme in this paper and therefore provide theoretical reference for the construction of the future wind farm on the sea.