Mooring System Research And Coupled Analysis Of A Floating Platform For Offshore Wind Turbines

Due to the depletion of traditional fossil energy, wind energy is widely attracting more and more attention as a kind of clean and renewable energy. A t the same time, with the rise of the offshore wind power industry, research on the floating offshore wind turbine(FOWT) has gained much concern recently. The floating offshore wind turbines usually operate in limited sea area connected with the mooring sy stem which is moored on the seabed. A safe and reliable mooring system can secure the dynamic stability of floating offshore wind turbines against the wind, wave and current load s. When the other requirements are the same, t he power-generating efficiency of wind turbine has closely relationship with the area of the rotor which is calculated through the 6 DOFs movement of the platform. In a word, describing the motion characteristics of wind turbines well and truly can support the evaluation of the wind turb ine powergenerating efficiency.The semi-submersible floating offshore wind turbine gained much attention for its well stability and widely depth adaptability. Two traditional semi-submersible floating offshore wind turbines made up of different materials are selected as the object, and the research mainly includes the several following aspects:(1)The preliminary design method of the mooring system of the offshore floating structures is summarized, which provided the guidance for designing the mooring system of the 5MW steel semi-submersible floating offshore wind turbine. The detailed design based on dynamic analysis is completed by time-domain calculating software Orcaflex, and the results of the analysis agree well with the criterion which indicates that the preliminary design method is simple and applicable and provides valuable reference for mooring system design of steel FOWT in future.(2)The effects of mooring line parameters(fairlead location, pretension and angle between lines) on the power-generating efficiency of wind turbine, dynamic response of the system and effective tension of mooring chains are analyzed by using the dynamic analysis method, and the variations of pitch movement, offset and effective tension with corresponding parameters are gained based on the results of parametric analysis which provides similarly valuable reference for mooring system design of FOWT.(3)The fully coupled dynamic analysis of a FOWT system in the time domain includes aeroloading, tower/blade elasticity, blad e-rotor dynamics and control, mooring dynamics and platform motions. A reinforced concrete semi-submersible FOWT is selected as the object, and in order to perform the fully coupled dynamic analysis by solving the dynamic equation in FAST, the hydrodynamic data calculated by WAMIT is imported into Orca Flex to calculate the loads. With the purpose of calculating the dynamic response and evaluating the power-generating efficiency of wind turbine, the real pitch of the rotor can be obtained through the transformation of coordinate. The results show that different degrees of motion need to be specifically controlled in different sea states and the movement of platform and mooring line response also differed when subjected to environmental loads.