Fully Coupled Analysis And Vibration Control Of Bottom Fixed Offshore Wind Turbine Under Earthquakes

The offshore wind energy in China will develop prosperously based on the planning of the 13th five year of Chinese government and the demanding of society developing.The offshore wind farms will confront with complex environmental conditions by in comparison with the onshore wind farms,such as wave,wind,current and sea ice loads.Besides,the earthquakes should be considered in the designing of offshore wind farms in the specific regions of China,such as Bohai Bay and East China Sea.While the definitions of seismic load combinations for offshore wind turbines are insufficient in the released design standards of offshore wine turbines,such as the IEC standard only point that the states of offshore wind turbine should be considered in the seismic analysis,but the detailed control strategies are not given.Definitions of seismic load conditions in domestic standards are mainly refer to the standards of onshore wind turbines or the standards of seismic design of building.The following topics are performed in order to instruct the planning and designing of offshore wind farms properly:(1)Dynamic model test of bottom fixed offshore wind turbine(OWT)Dynamic model test of a bottom fixed OWT is performed in order to prove the influence of the environmental loads on the structural responses under earthquakes.Elastic similarity and Froude number similarity are applied in the design of the test model.Morover,the Newton similarity is used to scale the thrust loads of the prototype.Dynmaic test model is established based on the hydro-elastic similarity,which is derived according to the elastic and Froude number similarity,scaled thrust loads are determined by using the Newton similarity,and the equivalent rotor disk is used to model the rotor blades and the hub.Basic parameters of the fan of the simplified wind generation systems are determined based on the scaled thrust loads and geometries of the equivalent rotor disk.Development of the simplified wind generation system,the system is mainly consist of wind generation part,support structure and numerical simulation procedures.Dynamic model test of OWT under seismic,environmental loads and combined seismic load combinations are performed by using the simplified wind generation system and joint wind,wave and earthquake simulation system.Dynamic characteristics of OWT under seismic excitaitons and the influence of environmental loads are analyzed.(2)Numerical analysis of the bottom fixed offshore wind turbineInitially,the governing equation of motion of the OWT is derived based on the blade momentum theory,Morison formula and fundamental theories of structural dynamics.A seismic module is compiled and added to the coupled analysis model of OWT under wind and wave load with the intention of establish fully coupled analysis model in the seismic analysis of offshore wind turbine.Dyanmic characteristics of structural responses under different seismic load conditions are analyzed,also the influence of mechanical control strategies are discussed,such as variable speed control,blade feather or high speed shaft break.Governing equation of mition of OWT under seismic,aerodynamic and hydrodynamic load is derived based on blade momentum theory,Morison formula and fundamental theories of structural dynamics.The seismic module is compiled with the intention of establish fully coupled analysis model of OWT under seismic,wind and wave load.Newly compiled seismic module is validated based on the seismic analysis of an onshore wind turbine.Structural responses under earthquakes are analyzed based on the fully coupled analysis model,the dynamic charactersitics of OWT under seismic loads are proved.Further,fully coupled analysis of OWT under seismic,wind and wave load are performed by using the coupled analysis modle,the influence of mechanical control strategies and environmental loads are discussed in detail.(3)Vibration control of bottom fixed offshore wind turbineDfferent combinations of basic parapeters of TMD are determined based on the dynamic characteristics of OWT under seismic excitations.The applicability of TMD in the vibration control of OWT under earthquakes are proved,then the most effective design rules of TMD for OWT are suggested.Furthermore,the MTMD are proposed based on the optimal design rulses of TMD with intention of achieve more effective control effects of OWT.Reduction of the bending moment is determined as the assessment index to select the most effective MTMD for OWT under different load conditions.The TMD module is recompiled based on the governing motion of equation of MTMD in order to establishe the numerical modle of MTMD,which be can applied to the nacelle,tower and flange of OWT simultaneously.Different combinations of parameters of TMD are selected based on the dynamic characteristics of OWT under earthquakes,and the most effective parameters of TMD are determined by in comparion with the structural responses under different TMDs.Then the parameters of MTMD are selected based on the optimal parameters of TMD and structural responses under combined siemsic load conditions.Reduction of bending moment under different load conditions is selected as the assessment index for the determination of the most effective design parameters of MTMD,and the general design rules of MTMD are suggested based on the optimal MTMDs under different load conditions.