Dynamic Response And Fatigue Analysis Of Floating Wind Turbine Foundation Based On Global Coupling

Compared with landbase wind turbines,the three pontoons semi-submersible wind turbine is not affected by the topography and geomorphology.The wind energy in the deep sea is richer than that in the offshore areas,so it has a large development space.Therefore,the floating wind turbine is suitable for deep sea and is more welcomed.Floating wind turbines would be affected by the coupling of wind and wave loads during service period,and the ocean environment changes rapidly.Hence,the research of influences of the different combinations of wind and wave on the sensitivity of floating wind turbines has great reference value for practical applications,and it is also necessary to study the fatigue damage of the floating foundations.Floating wind turbine is a multibody which is composed by many parts,such as mooring lines,pontoons,tower,blades and gear box.There is interaction between each components,the wind and the wave loads would influence the motion and the force of the wind trubine;the movements of the floating wind turbine could also have influence on the wind load.So the wind turbine could be considered as a full-coupled system which should be analysised with full-coupled theory.In this paper,OC4-DeepCwind the three pontoons semi-submersible wind turbine is taken as the research object.Firstly,the natural characteristics are verified,and the forecasting method is verified based on the experimental data and the forecast results of the FAST,and the influence of wind-wave coupling effect on its hydrodynamic response is studied.Then,the relevant parameters of wind and wave load that are applied to the three pontoons semi-submersible wind turbine have been changed,and the sensitivity of the three pontoons semi-submersible wind turbine to the environmental load is studied.Finally,the fatigue damage analysis of the braces of the supporting structure of the three pontoons semi-submersible wind turbine is carried out according to the S-N curve and the fatigue cumulative damage theory.(1)Based on the numerical simulation method,multi-body coupled hydrodynamic model of the three pontoons semi-submersible wind turbine is established,and the natural characteristics and prediction methods are verified to ensure its feasibility.The coupling effect of wind and wave is analyzed by comparing the simulation results of the joint effect of wind and wave loads with wind and wave loading effect separately.The differences between effects of joint loads of wind and wave and wave loads alone on thrust of the three pontoons semi-submersible wind turbine is not obvious,but the axial force of the mooring lines is obviously affected by the wave load;when the wave load is not big,the coupling effect of wind and wave is more effective to the force of the mooring lines and the motion of the floating foundation of the three pontoons semi-submersible wind turbine compared with the effect of wave alone.(2)The different combinations of wind and wave loads have been considered to find out the sensitivity of the coupled three pontoons semi-submersible wind turbine.The calculation results show that the reduction of the pontoon spacing would increase the bending moment between the braces but reduce the axial force of the mooring lines and increase the stability of the floating foundation of the three pontoons semisubmersible wind turbine.The combinations of different wind and wave loads have different effects on motion of the floating foundation of the three pontoons semisubmersible wind turbine and the axial force of mooring lines.(3)The fatigue hot spot of has been determined and the fatigue damage of the braces of the three pontoons semi-submersible wind turbine has been calculated according to the S-N curve and the linear fatigue cumulative damage theory.Analysis of the fatigue damage of the floating foundation of the three pontoons semisubmersible wind turbine is carried out through changing the structure of braces.The results show that the brace with the same direction of the wave and wind has the biggest fatigue damage.The fatigue damage of the cross braces can be effectively reduced with the changes of the wall thickness and diameter of the cross brace within a certain small range.