Fatigue damage analysis method of offshore wind turbine foundation

The systems of offshore wind turbines are established in a complex marine environment, and are subjected to all kinds of ocean loadings such as waves, currents and wind for a long time. It is found that offshore wind turbine foundations are seldom damaged due to the problem of strength and stability under normal operation conditions. Most of the damages are caused by the fatigue damage under long-term loads. Aimed to this problem, the monopile foundation of offshore wind turbine is analyzed as an example in this study. The fatigue damage is analyzed systematically using the method of time-domain, combining the rain-flow counting method, S–N curve with linear and nonlinear fatigue damage models. In addition, a modified nonlinear fatigue damage model is proposed based on the Corten-Dolan and Manson-Halford nonlinear fatigue damage models, which can consider the modification of load interactions and the effect of loading sequence. In this way, the short-term fatigue damage value of offshore wind turbine foundation can be obtained. On the basis, the long-term probability distribution function of wind speed is obtained according to the actual monitoring results of wind load, and the fatigue damage value of the offshore wind turbine foundation in the design life can be obtained. Finally, a complete fatigue damage analysis method of offshore wind turbines is proposed, which is applicable to both short-term fatigue damage and long-term fatigue damage.