Prediction Of Motion Responses For The Floating Offshore Wind Turbine Under Extreme Conditions

The "30·60" carbon peak and carbon neutrality commitment stimulate the leapfrog development of renewable energy in China.With a clear carbon-neutral target,floating offshore wind power has become the workhouse of clean energy.With that said,this research aims to address the encountered difficulties for offshore energy extractors,especially for the semisubmersible floating offshore wind turbine.The numerical methods are employed to investigate its fully coupled effect from aerodynamic,hydrodynamic and the mooring system under the combined action of wind and waves.Besides,the accurate prediction method and its application for the motion responses of the floating wind turbine under extreme conditions are also studied.First,a numerical wave flume based on the waving force method is developed.The wavemaking accuracy is verified,which lays a solid foundation for accurately solving the hydrodynamic load of waves acting on a floating semi-submersible platform.Specifically,the waving force method can significantly reduce the computation cost while improving the efficiency during the numerical simulation of small regular and irregular wave.However,the selection of grid and time-step largely affect the accuracy of wave-making.To deal with the attenuation of wave height caused by numerical dissipation,the method of wave height correction is used to secondly simulate regular and irregular waves,thus improving the precision of wave-making to a certain extent.Second,taking OC4 Deep Cwind semi-submersible platform and NREL 5MW baseline wind turbine as the research objects,a series of models including the rotor's aerodynamics,the semi-submersible platform's hydrodynamics,the mooring chain performance are built and coupled together to research fully coupled effect on the floating offshore wind turbines.Before coupled simulations,each part was verified separately.Then,the fully coupled model is employed to conduct the research on the motion response of floating wind turbines both under working and extreme sea conditions.The results both on heave and pitch obtained from fully coupled model remains consistent with that from the FAST(Fatigue,Aerodynamics,Structures,and Turbulence),a program for the calculation of horizontal axis wind turbine developed by NREL.Finally,the semi-submersible platforms with two different types of damping plate are introduced,including the independent damping plate platform,Wind Float,and the integrated damping plate platform,Hexa Semi.Moreover,the methods of static water free attenuation,motion responses prediction of floating wind turbine under extreme sea conditions,and time and frequency domain analysis were applied to study the influence from damping plates on the hydrodynamic performance.It is found that the integrated damping platform,Hexa Semi,has better hydrodynamic performance because the larger vortex circumference contributes to the rapid release of energy to the surrounding fluid during its movement.In addition,the new integrated platform,can also be used as a reference for engineering application,especially for the future development of floating offshore wind power.