Study On Dynamic Response Of Wind Turbine With Pile Foundation Considering Non-Gaussian Characteristics Of Wind Load

Pile Foundation is an important part of offshore wind turbine.Monopile is the most widely used foundation form at present.The offshore wind turbine will be subjected to environmental loads such as wind,wave and current during its service,in which wind load is the main environmental load.The wind field is generally assumed to be Gaussian distribution in the codes and researches at home and abroad.However,it has been proved that the wind load has significant non-Gaussian characteristics.Therefore,it is necessary to use the limited sample data to construct a reasonable mathematical model to simulate non-Gaussian wind field.Furthermore,it is transformed into the corresponding non-Gaussian wind load acting on the offshore wind turbine.The dynamic response,extreme value prediction and fatigue damage assessment of monopile under non-Gaussian and Gaussian wind loads would be compared.The main work and achievements of this paper are as follows:Firstly,based on the open-source data of NERL-5MW offshore wind turbine,the software Turbsim is used to simulate the random wind field,and FAST is used to transform the wind load.Then,the full-structure finite element model of 5MW wind turbine is established by ABAQUS software,and the validity of the numerical model is verified.Secondly,in this paper,the three-dimensional dynamic response of monopile in nonGaussian wind field and Gaussian wind field is simulated,and the results are compared.The results show that the amplitude and the degree of oscillation of the stress time history curve in the middle and high wind speed section are greatly influenced by the non-Gaussian characteristics of wind load.In statistics,the mean and variance of stress time history are hardly affected by non-gaussian characteristics.However,with the increase of the wind field kurtosis,the dispersion degree of stress time-history in the middle-high wind speed section increases.The skewness and kurtosis of stress time history are positively correlated with skewness and kurtosis of wind field.Thirdly,the average conditional exceedance rate method(ACER)is used to calculate the short-term extreme stress distribution at the mudline of monopile.The influence of kurtosis and skewness on short-term extreme value is compared.It is found that the short-term extreme value of non-Gaussian wind field is larger than that of Gaussian wind field in low wind speed section and high wind speed section,and the maximum difference ratio is 18%.In the middle wind speed section,as the kurtosis increases,the corresponding short-term extremes of the nonGaussian and Gaussian wind fields alternate in the lead.When the skewness is changed,the short-term extreme value of the corresponding non-Gaussian wind field is smaller than that of the Gaussian wind field.On the whole,kurtosis affects the short-term extreme value more than skewness.Fourthly,on the basis of the short-term extreme value,the long-term extreme value of a monopile during operation is obtained by combining the probability density function of wind velocity distribution.Furthermore,the long-term extreme values of 10-year and 50-year return under Gauss and non-Gaussian wind loads are compared and analyzed.The results show that the long-term extreme value under non-Gaussian wind field is 14.2% larger than that under Gaussian wind field.When the non-gaussian characteristic of wind load is taken into account,the safety margin of the fan foundation structure will be reduced from 39% to 30%.Kurtosis has great influence on the long-term extreme value,but skewness has little influence on the long-term extreme value.Fifthly,the rain-flow counting method is used to deal with the stress time-history at the mudline,and the equivalent stress amplitude is calculated.Then,the fatigue damage value of monopile is calculated by using stress-fatigue life curve and Miner’s linear fatigue damage theory.when non-Gaussian wind load is considered,the equivalent stress amplitude and fatigue damage rate of monopile under most wind speed series increase,but the number of cycles decrease.The influence of non-Gaussian wind field(0,7)is significantly greater than that of Gauss wind field.