Experimental and Numerical Study for Weathervane and Stability Performance of Single-Point-Moored FOWTs Under Wind-Current Coexisting Field
11th International Workshop on Ship and Marine Hydrodynamics (IWSH2019), Paper 9
Contribution to Conference
Recently, Floating Offshore Wind Turbines (FOWTs) are attracting a great deal of attention as offshore renewable energy source in terms of its energy efficiency and potential. Wind turbines normally weathervane using the yaw control system. However, the yaw control system is found to be one of the major reasons for the failure of wind turbines. Especially, in the case of Floating Offshore wind turbines, when the yaw control system breaks down, it becomes all the more difficult to repair it on the sea because of the high altitude and waves. Also if the failure is severe, the wind turbine need to be towed back to docks increasing maintenance costs. As one of the solution of this problem, Single Point Mooring (SPM) system is proposed. FOWTs moored to SPM system can rotate around the mooring point and follow the wind direction without yaw control system. In this study, the weathervane and stability performance of FOWTs moored to SPM system in the wind and current coexisting field is considered through a series of experiments and numerically simulating time-series history. First, we designed and manufactured 1/200 scale models of two types FOWTs, a semi-submersible and spar, based on Froude’s scaling law. The scaled models are experimented in a circulating water tank at Osaka Prefecture University, Osaka, Japan. It was found out that the weathervane performance of both the FOWTs types was acceptable in rated wind and slow speed current condition. However, in the rated wind and high speed current condition, the weathervane performance especially of the semi-submersible type was not acceptable and instability oscillation of spar type was observed. In addition, a numerical simulation program was developed with an assumption of modeling the SPM-FOWTs as a double pendulum. In this time history program, the effect of the bearing on the mooring buoy is considered for clear understanding. The result of the program is discussed and compared to the results of the experiments.
Hydrodynamics of renewable marine energy and ocean resources