Ice-induced vibrations under continuous brittle crushing for an offshore wind turbine
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Offshore wind structures located in ice infested waters are subjected to actions from moving ice and forces are generated when a drifting ice crushes against the structure. The main purpose of this Master s thesis is to implement numerical ice loading models on an offshore wind turbine monopile structure, analyze ice induced vibrations and assess the importance of ice loads on offshore wind turbines.A model of offshore wind turbine monopile structure for time domain simulation will be established on FEDEM and ice loading spectral model will be implemented first. Ice force spectrum will be calculated from empirical formulas and ice loading time series will be generated and applied on structure model directly.In order to assess the performance of the spectral model, another ice-structure interaction model, Määttänen-Blenkarn model, is also applied on the offshore wind turbine monopile structure. The time domain simulation results from two models are analyzed and compared. The feasibility of both models for different ice speeds are investigated.The complex ice properties define the difficulty in ice load calculation. The relative speed between structure and ice will influence ice crushing strength. The existing spectral model is only feasible for continuous brittle crushing regime where structure response have little influence. But Määttänen-Blenkarn model considers structure response through stress rate dependent ice crushing strength and is also applicable for low to intermediate ice speeds. Simulation results also illustrate that different ice crushing modes of different ice speeds can be obtained from Määttänen-Blenkarn model and the results of spectral model for different ice speeds have similar characteristics. To better evaluate the impact of ice loads on offshore wind turbine structure, the coupling model of wind and ice loads will be applied in time domain simulation to investigate the joint effect of wind and ice loading. Fatigue damage assessment is also performed to see the structure damage induced by ice loads. The results illustrate that based on current model and data, ice loads are larger than wind loads and the fatigue damage caused by ice loads should be considered.