The Fatigue Assessment Of Support Structures For The Offshore Wind Turbine In The Ice Region

It is beneficial to the offshore wind power because of broad sea,less obstacles and abundant wind resources.The offshore wind turbine is prone to fatigue damage that is subjected to the wind,wave and current load in the long term.The offshore wind turbine is exposed to the ice load in the high latitude areas in winter,such as Bohai and the Baltic Sea.In this paper,the fatigue assessment of the offshore wind turbine support structures in Bohai is established in time domain and frequency domain.The main research contents are as follows:(1)The ice and wind data of Liaodong Bay in Bohai Sea is sorted out and analyzed,the environment parameters in the ice region are presented.The sea ice parameters mainly include velocity,thickness,drift direction and compression strength.The wind parameters mainly include velocity.The joint probability density function of wind and ice can be calculated and the fatigue conditions are established provided that wind velocity and ice velocity are mutual independent,the direction of wind and ice are the same and the ice parameters are mutual independent.(2)Taking monopile offshore wind turbine for research objects,the fatigue analysis of offshore wind turbine support structure is performed in time domain under combined wind and ice load.The wind load and ice load of three kinds of ice-force model,intermittent crushing,frequency lock-in crushing and continuous crushing,are calculated by FAST software.Fatigue induced by the wind load and ice load are computed separately and fatigue of support structure subjected to combination wind load with ice load is calculated.The contribution of different ice failure modes to the fatigue damage of the support structure of the offshore wind turbine is studied.(3)The continuous crushing ice load can be described by the ice force spectrum and wind load can be represented by the wind spectrum.Therefore,the fatigue damage combination and spectral analysis method can be employed to evaluate fatigue.In the time domain,on the basis of the full time domain calculation results,the combination methods of fatigue damage that is Tempel method,DNV method,Huang-Moan method and Ma-Han method is compared and analyzed.In the frequency domain,based on the linearization theory of maximum principal stress under the condition of multi-load input in the linearsystem,a spectral analysis method for offshore wind turbine support structures under the combined action of wind and ice loads is established.Taking the monopile and tripod offshore wind turbines as the study objects,the applicability and accuracy of time-domain fatigue damage combination method and spectral analysis method for different forms of offshore wind turbine support structures are verified.