Study On Time Domain Analysis Method Of Structural Strength Of Floating Wind Turbine Under Freak Waves

In recent years,countries have begun to pay attention to the utilization of floating wind turbine resources,and semi-submersible floating wind turbines have gradually become a research hotspot.Floating wind turbines are a kind of low-cost equipment that can withstand wind and waves.They can also steadily collect wind energy.Given the vast area of China’s oceans,there is a great potential for developing semi-submersible floating wind turbines.Freak wave is an ocean phenomenon with huge wave height,very fast propagation,strong episodic and concentrated energy,which can easily cause great harm to marine engineering structures.In this paper,the concept of X30,a semi-submersible floating turbine configuration proposed by X1 Wind Energy,Spain,is used as the basis for configuration optimization to improve its motion performance.Due to its slender body and light mass,the X30 floating wind turbine is more susceptible to huge wave impacts under the action of freak waves,leading to structural damage.This paper focuses on the analysis of its structural strength under harsh conditions such as freak waves to provide useful references for the development of deep-sea wind energy in China.In this thesis,the complex sea conditions consisting of wind,waves and currents are considered simultaneously,and Fluid Structure Interaction(FSI)is used,i.e.The time domain fully coupled analysis method of Computational Fluid Dynamics(CFD)is applied to calculate the external loads including wind,waves,currents and anchor chain’s tension in real time update,The external loads,such as anchor chain’s tension,are transferred to the structural strength finite element model,and gravity,inertia forces and other loads are added for direct calculation and analysis to obtain the time domain structural stress results and motion response of the floating wind turbine,and the structural strength safety assessment is performed using the simulation results.In order to evaluate the effect of freak waves on the structure,this paper uses a combined wave focusing model and pushes plate wave making theory to simulate freak waves,which are generated by specifying the position and time in the numerical tank through secondary development.And the structural strength stresses under the sea condition of the distorted wave are obtained by the joint fluid-solid coupling simulation.The safety assessment of the structural strength was carried out using the simulation results.The main studies are as follows:(1)In order to adapt to the layout characteristics of different sea areas,a set of numerical tank based on viscous flow theory is established,and the convergence of the mesh density of the numerical tank is analyzed by comparing it with the theoretical value in order to prefer a reasonable set of grid models.Meanwhile,a set of mooring system applicable to this floating turbine is also designed in the numerical tank.(2)A method is established to analyze the time-domain strength calibration of floating wind turbines.This method is based on CFD and direct calculation of structural finite elements,which can simultaneously consider the action of external loads such as wind,waves,currents and anchor chain’s tension on the structure in real time,obtain the time domain structural stress results and motion response,and evaluate the structural strength based on the simulation results.(3)In order to verify the accuracy of the numerical simulation and analysis method for hydrodynamic and load forecasting,a physical model of the tank of the wind turbine system with a design scaling ratio of 1:50 is used in this paper,and a wave test of the floating wind turbine under regular wave conditions is conducted in the physical tank.The attenuation period of the floating turbine was obtained through the free attenuation test,and the tank test results were compared with the numerical simulation results to verify the accuracy of the physical model.And the accuracy of the numerical simulation method was verified by comparing the test and numerical simulation wave pressure results under classical conditions.(4)In this thesis,the sensitivity effects of a series of parameters such as wave height,period and flow velocity on the structural strength of a floating turbine are investigated using a joint fluid-solid simulation method.The results showed that the structural strength of the turbine is strongly influenced by the wave height,period and current velocity.In addition,the maximum equivalent force area of the turbine is mainly distributed at the boundary of the wave blocking plate and at the intersection of the cross brace and diagonal brace at the front of the turbine.The results of this study provided a theoretical basis for the local strengthening of the floating turbine structure design.(5)The focused wave model and the pushes wave making theory are used to successfully simulate the strong nonlinear distortion waves at the specified location and time of the numerical tank.The time domain structural stress distribution of the floating wind turbine under the action of the freak wave is obtained by the joint fluid-structure coupling simulation method,and the structural strength of the wind turbine is evaluated safely based on the simulation results.It is found that the floating wind turbine is subjected to a larger equivalent structural stress during the whole period of the wave action compared to the strong nonlinearity of the wave.In summary,this thesis proposed a method to analyze the time domain strength calibration of floating wind turbines,and used the time domain analysis method to analyze the strength calibration of floating wind turbines under severe sea conditions,especially under the action of deformation waves.The research results provide useful guidance and reference for the design and engineering application of offshore floating wind turbines,and have engineering practical significance.