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Abstract--Based on the 5 MV wind turbine of a certain renewable energy institute in America, the model of a floating offshore wind turbine spar platform mooring system has been established by Orcaflex. By calculating the load on the wind turbine, the hydrodynamic analysis of the wind turbine mooring system was researched and the mooring tension of the mooring system was analyzed in different load conditions. With the change of different fairlead position and different layouts of the fairleads, the optimization design of the mooring system has been given. Keywords-wind turbine; spar platform; hydrodynamic analysis; mooring tension I INTRODUCTIONA satisfying mooring system for the offshore wind turbines is very complex. The almost constant wind, current and wave drift force would cause the natural position of the system to move and produce the impact load at the equilibrium position which would make the mooring chain appear tension-relaxation alternating motion. It would cause the mutation of the cable effective tension.At present, the floating offshore wind turbine spar is only applied to the shallow water depth of 30m.[1-2] In this paper, based on the time domain coupled dynamic analysis method, the applicable water depth has reached up to 70m. II MATHEMATICAL FORMULATION A. The Description of the Wind SpeedThe change of the mean wind speed along the height generally conforms to the exponential law or logarithm law. Marine structures always uses American Petroleum Institute (API) spectrum. So Orcaflex modelling process also chose American Petroleum Institute (API) wind spectrum.Where z is the height above sea level, h is the reference height above sea level, generally taken to be 10 m, ( ) u z is the mean wind speed at the height of z, ( ) u h is the mean wind speed at the reference height. And n is the wind profile index. B. Theory of the Motion of the Wind Turbine BladesThe wind turbine blades are subjected to A is the blade area for the vertical wind direction , V is the wind speed of the blade center, d is the wind blade width, a ρ refers to the density of the air, here taken as 1.3kg/m 3 . C. The Load Calculation of the Tower and the Selection of the Wave TheoryThe tower can be viewed as a slender rod. In Orcaflex, it is calculated by the Morison formula. The acceleration of air in the calculation of wind load is small, so the inertia force term can be ignored. Dean pointed out that in a variety of water depth, the Airy wave theory gives good results.[3] So we used the Airy wave theory in the modelling process . D. Calculation of Current Load and Wave LoadThe performance of the mooring line is equivalent to a nonlinear spring. The line is divided into a series of line segments which are then modelled by straight massless model segments with a node at each end. The model segments only model the axial and torsional properties of the line. The other properties are all lumped to the nodes. [4][5][6][7][8]