| Recently,the offshore wind power is extending towards the deeper water.However,when the water depth is over 50m,as the fixed foundation for offshore wind turbines are no longer practical because of its excessively high cost.Therefore,the floating offshore wind turbines(FOWTs)become a major method to exploit the wind energy in deep waters.The semi-submersible floating wind turbines,which has the advantages of suiting to wide range of water depth,installed at the harbor and easy to tow,high hydrodynamic performances,has become one of the most popular FOWTs.The major task of this paper is to investigate the problems which one may deal with when processing the loads analysis and platform strength analysis of the semi-submersible wind turbines.The main content includes:(1)In order to study effects of the different simulation lengths on the loads of wind turbines,a series of typical operational sea states were selected and a total of 1860 time domain simulations of different simulation lengths,i.e.3h,1h,20min and 10min,were implemented.The results of various loads,e.g.the tower base moment and blade root moment,were drawn from the simulations.The results show that the blade loads are insensitive to the length of the simulation while the tower base loads increases with the length of the simulation positively.However,if the total lengths of the simulations are the same,the tower base loads remain equal regardless of the change of the length of a single simulation.Due to the impacts of the platform yaw motion,the yaw bearing moments of the nacelle are underestimated by the 10-min simulations.In summary,it is recommended that when conucting the loads analysis of the semi-submersible wind turbines,a simulation length of 20 min should be used and a proper load factor should be multiplied by the tower base loads.(2)With the purpose of studying the impacts of wind-wave misalignment on the catenary moorings,the coupling models of a semi-submersible and a spar type FOWT were built and the time domain simulations were conducted under different typical sea states.The platform motions and mooring line tensions were obtained and analyzed.The results show that the aligned wind-wave generates the greatest mooring tensions and the highest platform motions in all degrees of freedom(except for the yaw motion).However,under the moderate sea states,when the wind turbine is parked,misaligned wind and wave may cause larger mooring fatigue damage.Therefore,it is suggested that when assessing the effect of the moderate sea state on the fatigue damage of mooring lines,at least two intercross angles,e.g.the 0° and the 90°should be considered.When assessing the fatigue life of the mooring lines,the wind-wave misalignment should be considered according to the wind wave direction scatter diagram.(3)The random design wave method was proposed to determine the worst wave load conditions in the progress of the strength analysis of the semi-submersible FOWTs,in which a series of response loads were selected according to the structure characteristics of the triangle FOWT platform.As a case study,the global strength analysis of the OC4-DeepCwind platform was conducted.It is shown that for the semi-submersible platform,whose column are connected by the slender beams,the stress level is extremely high around the beam connections.This should be highly appreciated when doing the strength analysis.Besides,the heave property of the platform is vital to the strength of the platform,since if the heave period of the platform is in the rang of the typical wave period interval(2～20s),the platform stresses will be very sensitive to the wave loads effect,which poses great threat to the platform safety. |
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