Research On The Loads Response Of Support Structures Of Offshore Wind Turbine Under Nonlinear Mixed Sea States

Under mixed sea states(combination of wind waves and swells),the5 MW Monopile,Jacket and Tripod offshore wind turbine developed by the National Renewable Energy Laboratory(NREL)are used as the research object.Under the action of irregular linear waves and nonlinear(secondorder)waves,the short-term loads response of the three support structures at the mudline are calculated by the FAST software.Extreme value theory is applied to predict extreme loads.Damage equivalent load is calculated by the MLife software.The specific research content is as follows:The prospect of offshore wind power and the development trend of offshore wind turbines are reviewed.The three support structures of offshore wind turbine to be studied in this paper are introduced.The research significance of this paper is expounded.In addition,related researches on the three and other offshore wind turbine support structures are introduced.The calculation requirements for the ultimate load and fatigue load of offshore wind turbines by relevant design codes are introduced.Wind conditions are simulated by the Turb Sim software.Based on the Ochi-Hubble six-parameter bimodal wave spectrum and the shallow water Stokes wave theory,sea condition is simulated by the Hydrodyn module of the FAST software.The three support structures are placed at a depth of 20 meters.Under the given environmental conditions,the loads response of the fore-aft shear force and the fore-aft bending moment at the mudline under the action of irregular linear waves and nonlinear(second-order)waves are studied.The exceeding probability distribution of the short-term loads of the Monopile support structure is also analyzed.Based on the short-term loads of the three support structures,the block maximum method is used to extract the load extremes.Based on the load extremes extracted under different numbers of the block,the maximum likelihood estimation method is used to estimate the unknown parameters of the generalized extreme value distribution.The influence of different numbers of the block on the type of generalized extreme value distribution is analyzed.The fitting effect of the empirical distribution of the short-term load and the fitted generalized extreme value distribution is tested using a Q-Q chart,and it is found that the increase of the numbers of the block do not affect the fitting effect.Based on the cumulative distribution function of the generalized extreme value distribution,the 84 th percentile and the99.999 th percentile are selected to predict loads,respectively,and the predicted results at the 99.999 th percentile are used as extreme loads.Rainflow counting method is used to handle short-term loads.Based on the predicted extreme loads and the basic principles of fatigue load calculation,the short-term damage equivalent loads of the three support structures are calculated by MLife software.