Structural Strength Analysis For Blades And Fatigue Analysis For Tower Of Floating Offshore Wind Turbine Under Wind And Wave Loads

In order to lower the reliance on the traditional fossil energy and solve the increasingly serious environmental pollution,the clean and renewable energy has become a research hotspot.As one of renewable energy resources,wind energy is the most promising to be reliable and practical.However,due to the stringent requirements for appropriate location and the visual and acoustic impact to residents,the traditional fixed wind turbines are hard to develop further.In recent years,more researchers have turned their attention to the offshore wind turbines.With the depth of water and size of wind turbine increasing,the support structures of offshore wind turbines has gradually turned to floating support structures from fixed support structures.For the floating offshore wind turbines,the floating platform has to sustain the six degree of freedom(DOF)motion due to the integrated wind and wave loads,which makes the loads applying on the wind turbine much more complicated.Therefore,for the sake of the normal operation of floating offshore wind turbines,it is of great importance to conduct the structural strength analysis and fatigue analysis for the critical structural components.This paper addresses a study on the using of numerical analysis method to conduct structural strength analysis for blades and fatigue analysis for tower of floating offshore wind turbine under stochastic wind and wave loads.The major content of this paper includes:1.Considering the effect of floating platform on the blade,the formula of velocity and acceleration of every blade element are proposed.Based on the formula,the aerodynamic and inertial loads are calculated.Meanwhile,the detailed procedure of using the finite element method to conduct the structural strength analysis for blade of floating offshore wind turbines is proposed.2.The blade of a spar-type platform supported 10 MW wind turbine proposed by DTU is chosen as the research object.Under different load cases,the distribution of aerodynamic loads along the blade,the bending moments at blade root,the displacements at blade tip and Von Mises effective stress of blade are analyzed.3.In order to obtain the time series of axial stress at tower base,the simplified model of tower base is proposed.The Rainflow Counting method is applied to deal with the time series of axial stress to get the number of cycles corresponding to different mean axial stresses and ranges of axial stress.The effect of mean axial stress on the cumulative fatigue damage is corrected by Goodman correction.Based on the joint probability of wind and wave,using the Miner liner cumulative damage theory,the short-term cumulative fatigue damages at the different positions of tower base are calculated.On the basis of load cases which are included in the long-term fatigue analysis,the long-term cumulative fatigue damages and fatigue damage at the different positions of tower base are obtained.4.The tower base of the NREL 5MW wind turbine supported by OC3-Hywind Spar is chosen as the research object for fatigue investigation.Under different load cases,the axial stresses,the short-term cumulative fatigue damages,the long-term cumulative fatigue damages and fatigue life at the different positions of tower base are analyzed.Meanwhile,the effects of different simulation length and misaligned wind and wave on the fatigue damage are also analyzed.The cumulative fatigue damages at the different positions of tower base under wind-only and wave-only condition are compared.Results and conclusions in this paper may benefit the structural design and material selection of floating offshore wind turbines.