Analysis Of Coupled Weak Nonlinear Motion Response Of Floating Offshore Wind Turbine System

With the development of economy,the demand of human for energy is increasing rapidly.To satisfy this demand for energy while protecting the environment,we need to find some clean and renewable energy to replace those fossil fuels.And wind power,as a kind of green energy,is drawing more and more attention.This article mainly discussed the issue of coupled dynamic response of floating wind turbine system under complicated environment of wind-wave-current.The research result of this article has a modest importance on offshore wind energy development.This article firstly summarized the research development of floating wind turbine dynamic response,then it separately introduced some of the widely-used relevant theories and research methods,and developed some relevant Fortran program of some of those research methods.In the second chapter,for the calculation of wind forces,it introduced methods about wind field simulation,blade element momentum theory and some simplified methods.In the third chapter,it introduced calculation methods about hydrodynamic forces and hydrostatic forces,which includes frequency-domain and time-domain method of radiation force,diffraction force calculation method,incident wave force calculation method and restoring force calculation method.Calculation method of nonlinear incident wave force and nonlinear restoring force is the main innovation point of this article.Some Fortran program have been developed and validated about all the methods introduced in the third chapter.In the forth chapter,for the simulation of mooring system,it introduced the static catenary method and lumped-mass method and developed relevant Fortran program to do validation and analyses.In the fifth chapter,it combined those methods above and introduced the coupled weak nonlinear dynamic response analysis method and did researches separately on the floating offshore wind turbine platform and floating offshore wind turbine system.The main point of this article is that it introduced the coupled weak nonlinear dynamic response analysis model.For platform analysis,this platform model obtain nonlinear incident wave forces and nonlinear restoring forces on instantaneous wetted body surface.This platform model applies linear potential flow theory to get radiation forces and diffraction forces.The memory effect part of radiation forces and diffraction forces are converted from frequency-domain hydrodynamic coefficients and diffraction forces impulse response function according to the impulse response theory.The mooring forces of platform are obtained by static catenary equation.For floating wind turbine system analysis,this system model applies the same methods as the platform model in calculating hydrodynamic forces and hydrostatic forces.For mooring system analysis,this system model applies lumped-mass method to get mooring forces.For wind forces calculation,this system model applies blade element momentum theory to get rotor wind forces,applies simplified method of AeroDyn to get tower wind forces.Relevant Fortran programs about this coupled weak nonlinear dynamic response analysis method have been developed and some researches and analyses made.From the platform research part,it can be found that the weak nonlinear time-domain method results in great RAO value,and the average position of motion is not on the static balance position.Analyses show that this weak nonlinear time-domain method can take large amplitude wave nonlinearity into consideration and more applicable to terrible ocean environment.From the system research part,it can be found that,when it starts to simulate from the no-wind static balance position,system response will undergo a relatively large displacement and generally evolve into a stable harmonic oscillation,average position of motion will be at the with-wind dynamic balance position.This coupled weak nonlinear dynamic response analysis model of floating offshore wind turbine can take nonlinear mooring force,nonlinear wind force,nonlinear incident wave force and nonlinear restoring force into consideration.Despite of lacking model test data,there is no comparison between simulation data and test data.But relevant programming modules have been validated to some degree,and simulation data can match with each other,so those research results are trustworthy.The research result of this article has a modest importance on revealing dynamic response characteristics of floating wind turbine system under complicated ocean environment and offshore wind energy development.