| Offshore wind turbines are often in complex environmental loads caused by earthquakes,stormy waves and so on.The slender structure of offshore wind turbine makes it sensitive to mass,stiffness distribution and vibration frequency.The dynamic research of offshore wind turbine under earthquake is the basis of its seismic design,and it is of great value to the project.Rayleigh wave is a kind of interface wave propagating in composite mode in layered medium.Due to the different elastic properties of each layer,the established characteristic equation includes some periodic functions and sometimes plural equations,which increases the difficulty of solving.In this paper,the calculation model of pile supported offshore wind turbine in the lower seabed during the passage of Rayleigh wave is established in order to analyze the influence of Rayleigh wave on the dynamic response of offshore wind turbine in layered foundation.The main research contents are as follows:(1)The seabed soil is simulated as single-phase elastic medium.The free field is discretized and transformed into two-dimensional bar elements by finite element method.The fluid layer and the soil layer are regarded as sliding connections;The stiffness matrix and mass matrix of fluid-layered medium are established based on the wave equation of elastic medium and the hydrodynamic equation.And couping the analytical solution of half-space soil to the global stiffness matrix.The characteristic equation is solved by MATLAB to obtain the free field displacement under the action of Rayleigh wave.(2)The seabed foundation is simulated as a fluid layered soil half space model.The fluid layer is regarded as ideal fluid and let the hydrodynamic pressure on the surface be zero;The wind turbine structure is divided into three sections: tower,immersed pile and embedded pile.The tower is regarded as a hollow pile with variable cross-section,and the engine room and blade on the top of the tower are simplified as particles providing horizontal force and moment of inertia.The immersion section is divided into upper water section and underwater section.According to the extended Morison equation,the influence of hydrodynamic force on the structure is considered;Using the idea of plane strain model,the influence of vertical strain in free field is ignored.The horizontal dynamic impedance of free field is solved by wave equation in cylindrical coordinate system and substituted into the vibration differential equation of embedded pile.According to Pasternak model and consider the existence of soil shear layer;The analytical solution of the vibration equation of tower and immersed pile is derived,and the expression of horizontal displacement of embedded pile is obtained by finite difference method.The horizontal displacement and internal force of the offshore wind turbine model are solved according to the structural boundary conditions and continuity conditions.The analysis shows that the fluid layer has a certain embedding effect on the structure and dissipation effect on Rayleigh wave energy;With the increase of pile diameter and wall thickness,the structural displacement decreases;The pile diameter has a great influence on the resonance frequency of single pile wind turbine.(3)Based on the single pile offshore wind turbine model,the single pile foundation is changed into pile group support.It is assumed that the tower,bearing platform and pile foundation are rigidly connected,and there is no relative displacement at the tower bottom,bearing platform and pile top.Ignore the vertical displacement and rotation effect of bearing platform;The expression of displacement attenuation function is obtained from the displacement function of free field in cylindrical coordinate system.When analyzing and calculating any pile in the pile group,in addition to its own vibration,the vibration radiation of other piles as active piles in the free field is considered at the same time;The attenuation function is substituted into the differential equation of the buried section of pile group,The equation of embedded ppile considering attenuation function is solved jointly with the vibration equation of other parts.The effects of pile spacing,pile angle and other parameters on the horizontal dynamic response of offshore wind turbine supported by pile groups are studied.It is concluded that the attenuation function and pile pile interaction factor are obviously affected by frequency,pile spacing and inter pile angle;Compared with single pile,the pile foundation parameters such as concentrated mass at the top of the tower,pile diameter and wall thickness have less influence on the resonant frequency of wind turbine supported by pile group foundation. |
