Study On Dynamic Green Function Of Seawater-TI Saturated Poroelastic Seabed And Seismic Response Of Offshore Wind Turbine

Dynamic Green’s functions are the basic solutions to the dynamic response of soil and the dynamic interaction of soil-structure under dynamic loads such as earthquake,wave and traffic loads,as well as the foundation of boundary element method.In recent years,with the booming offshore wind power,offshore oil and combustible ice exploitation and other marine engineering,the dynamic Green’s functions in marine environment have significant theoretical value to solve the dynamic interaction of soil-marine structure.Considering the anisotropic physical and mechanical properties of seabed soil in horizontal and vertical directions,it is more appropriate to regard the seabed as transversely isotropic saturated media.In addition,most of the offshore wind farms are located in the circum-Pacific seismic belt,and vulnerable to the strong seismic pulse caused by faults.In order to ensure the safety of offshore wind turbine during the service,it is necessary to investigate the time domain seismic response of pile-supported wind turbine considering the seawater-seabed-structure coupling effect.Given the above background,the dynamic Green’s functions and the point source pulse in the marine environment are studied analytically by virtue of a seawater-seabed coupling model.Then,the seismic response characteristics of pile-supported wind turbine in transversely isotropic seabed soil are numerically analyzed by using finite difference software(FLAC^3D).The main research contents are as follows:（1）Based on Biot’s motion theory and compressible fluid governing equation,the seabed soil is regarded as a transversely isotropic saturated porous medium with finite thickness,and then the theoretical model of vertical coupled vibration of seawater-seabed soil is established in cylindrical coordinate system;the motion equation of soil skeleton displacement-pore pressure form（u-p）is derived through direct decoupling;finally,the steady dynamic responses of the seawater-seabed coupled model under the action of surface disk load,ring load and point load are solved by using Hankel integral transformation,taking into account the boundary conditions of the free surface of seawater,bedrock surface and the continuity condition of seabed surface.The influences of some related factors on the dynamic Green’s functions,such as on the degree of anisotropy,frequency,depth of bedrock,depth of seawater,permeability coefficient and porosity of seabed soil,are discussed finally.（2）In the light of three kinds of seawater:finite depth seawater,half space seawater and full space seawater,the motion equation of seawater excited by point sound source is solved by using Fourier transform and Hankel integral transform,and then the hydrodynamic pressure and vertical displacement of seawater are obtained in the transform domain under the action of Ricker wavelet pulse.A singular integral is solved with the help of residue theorem during the inverse transformation,and then the time-domain dynamic response of seawater subjected to the pulse of a point sound source is studied.In addition,the solutions are further derived considering the damping effect of underwater flexible medium on the absorption of incident wave.The dynamic response of seawater and its acoustic wave propagation characteristics with different seawater depth,sound source location and bottom boundary damping are investigated.（3）The seismic responses of offshore wind turbine supported by inclined pile group with high pile cap in transversely isotropic saturated seabed soil are numerically investigated by using FLAC^3D.The pile group foundation is simulated by pile element,and the blade,tower,engine room and hub are simulated by thin-walled cylindrical,rectangular and hemispherical shell elements with variable cross-section,respectively The additional mass method is used to calculate the hydrodynamic pressure caused by earthquake The measured acceleration time history of Kobe seismic wave is applied to the bottom of seabed soil along the horizontal direction after baseline correction,and the free field boundary is set to reduce the wave reflection interference.The effects of anisotropy degree of seabed soil,inclination angle of inclined pile,pile diameter and near fault impulse effect on the seismic response of offshore wind turbine are studied.