Resrarch Of Coupling Mechanism Of Structural Dynamic Responses Of Offshore Wind Turbines With Pile-soil Interactions

China is a big resource consumption country.With the gradual reduction of coal,oil and other resources,the demand for renewable energy is becoming more and more urgent.As global warming intensifies,the Chinese government solemnly pledged 2030 carbon peak and2060 carbon neutralisation targets.Meanwhile,the global consensus reached on developing and utilizing renewable energies.As an efficient and clean renewable resource,the offshore wind energy became an important part of renewable energy system in China and received strong support from the government in recent years.The constructured coastal and offshore wind farms in China mainly adopted fixed bottom substructures,such as gravity,monopile,jacket and et al.The offshore wind turbines(OWTs)need to suffer complex environmental loads such as winds,waves,currents,earthquakes and sea ice during the operation period,owing to the the complex marine environment conditions in the offshore regions in China.Presently,the semi-integrated analysis method is used to analyze the dynamic characteristics and evalueate the structural safety of OWTs under stochastic environmental loads.The main disadvantage of the semi-integrated analysis method is the separated modeling of rotor-nacelle assembly(RNA)and support system,which ignores the coupling effect between RNA and the support system,and the influence of aerodynamic damping on the responses of support structure under waves,currents,sea ice and the other loads.In order to compensate the above deficiencies,the researchers proposed an integrated aero-hydro-elastic analysis method for OWTs under winds and waves.However,the proposed coupled analysis method can not simulate the combined wind,wave and seismic loads cases,and study the influence of pile-soil interactions(PSI)on the coupling mechanism of seismic responses of OWTs.In order to mitigate the defects of the presented fully coupled analysis method,the linearization method of pile foundation is recommended according to the nonlinear pile-soil interaction models and the pile foundation linearization theories,which can be applied to the fully coupled analysis of OWTs.Based on this method,combined with the fully coupling analysis theories and structural dynamics theories,the coupled analysis method for OWTs with pile-soil interactions under the wind,wave and earthquake loads is proposed.Then,the coupled aero-hydro-elastic support system-elastic foundation analysis model is established in the updated time domain coupled simulation tool FAST v8.The influence of equivalent models of pile-soil interactions on the dynamaic characteristics and coupling mechanism of OWTs are investigated using the developted coupled analysis model.Accroding to the performed inveatigations,the research progress can be summarized as:(1)Based on the nonlinear p-y,t-z and q-z curves and linear Vinkel foundation model,the linearization method for OWT pile foundation is derived,which is coupled with the degrees of freedom(DOFs)of pile head node and can be applied in the coupled analysis of OWTs.(2)The fully coupled analysis method for OWTs with pile-soil interactions under wind,wave andseismic loads is established,based on the proposed pile foundation linearization method,coupling analysis theores and structural dynamics theories.Sequentially,the fully coupled RNA-tower-substructure-pile foundation analysis model is established in the updated simulation tool FAST v8.(3)In the study,the PSI is equivalent as fixed pile head nodes(apprently fixed boundary,AF),the equivalent pile length with six times the pile diameter at pile head s(equivalent pile boundary,EP)and coupled springs at pile heads(coupled spring boundary,CS).Then,the variation of dynamic characteristics of OWT with different PSI equivalent models is studied based on the established fully coupled analysis model.(4)Further,the defects of equivalent AF and EP boundary conditions in the coupledanalysis of OWT are proved,acceoding to the coupled structural responses of monopile and jacket OWTs in the time and frequency domains under combined wind,wave and seismic loads.Meanwhile,it is verified that the coupled RNA-tower-substructure-CS at pile head analysis model should be established in the analysis,in order to ensure the simulation accuracy of coupled structural responses of OWTs under complex environmental loads.