Multi-scale Fatigue Analysis Of Jacket Offshore Wind Turbine

Offshore wind energy resources are rich and stable,offshore wind power has become the focus of promoting the development of China’s new energy industry.Combined with China’s geographical location and geological conditions,jacket offshore wind turbine(OWT),which is suitable for 10 ～ 60 m water depth offshore area and has low geological requirements and great stability,has become the development trend of offshore wind power in China.Due to the complexity of the marine load environment of OWT,the joints of jacket foundation are easy to induce cracks under the combined action of wind and wave loads,resulting in stress concentration and fatigue damage accumulation,which makes fatigue failure become the main reason for the failure of jacket OWT foundation structure.Based on this,this paper takes the 5-MW jacket OWT of NREL(National Renewable Energy Laboratory)in its OC4 research project as the research object,and mainly carries out the research from the following three parts:(1)Fatigue load(wind and wave)of offshore wind turbine: Based on the characteristics of the jacket OWT,such as the special geometric shape of the blade,the higher position of the tower and the lower structure of the foundation than the sea level,the main fatigue load of the jacket OWT is the random environmental load of wind and wave.Among them,the wind load of the blade is calculated by the blade element momentum theory,the wind load of the tower is calculated by the load code and other related formulas,and the wave load of the jacket foundation is calculated based on the irregular linear wave theory combined with Morrison equation.The dynamic response of OWT is affected by the interaction of aerodynamic load and hydrodynamic load.Based on the fast wind turbine simulation software,the time domain coupling calculation of aerodynamic and hydrodynamic load is realized to obtain the wind wave combined fatigue load of jacket OWT.(2)Fatigue damage analysis of offshore wind turbine jacket foundation structure:Generally speaking,there are two kinds of fatigue analysis methods for jacket OWT structure: time domain method and frequency domain method.The time domain method has high calculation accuracy but low efficiency,while the frequency domain method has high calculation efficiency but low accuracy.Therefore,in order to accurately and quickly estimate the fatigue damage of OWT jacket foundation,this paper takes the fatigue damage analysis of tubular joints as an example to study the calculation efficiency of time domain method and the calculation accuracy of frequency domain method.In the time domain method,the influence of sample time distance on calculation efficiency is considered,and the shortest sample time distance is given when the calculation accuracy and calculation efficiency meet the requirements.In the frequency domain method,considering the influence of statistical characteristics such as non-Gaussian characteristics,bandwidth characteristics,average stress and other factors on the calculation accuracy,the short-term frequency fatigue damage calculation formula of jacket OWT structure is given.Finally,the joint probability distribution of wind and wave is determined according to the selected research site of jacket OWT,and the random fatigue load of wind and wave is applied for structural dynamic calculation.The long-term fatigue damage of tubular joints is obtained,and the most dangerous condition of fatigue damage is given.(3)Application of multi-scale method to fatigue damage analysis of offshore wind turbine jacket foundation structure: The fatigue failure or failure of OWT structure often starts from local nodes,which has multi-scale characteristics.Traditional OWT usually uses beam element to establish the whole structure finite element model,ignoring the local node structure.In this paper,the multi-scale model of jacket OWT is established.The whole structure is established by beam element,and the local fatigue dangerous node model is established by solid element.The two scale models are coupled with each other.The multi-scale model can accurately obtain the collaborative response of local nodes at the same time of overall analysis.According to the local stress response of the tubular joints of jacket foundation structure,the multi-scale model fatigue analysis is carried out,and the results are compared with the traditional simplified beam model fatigue analysis results,so as to explore the necessity of applying the multi-scale model to the fatigue analysis of jacket OWT structure.In addition,considering the typical multi axial stress state of tubular joints under the combined action of wind and wave,the multi-scale model multi axial fatigue analysis is carried out,and the results are compared with the multi-scale model uniaxial fatigue analysis results to verify the calculation accuracy of multi-scale model fatigue analysis.