Study On Coupled Analysis Methods Of The Floating Offshore Wind Turbines System

With the increasingly serious condition of energy deficiency and environmental pollution,wind energy,as a renewable energy,is attracting more and more attention.Compared to the land and near shore area,deep sea area has more superiority,for the advantages of land saving and higher wind speed.Some kinds of floating wind turbine concepts have been proposed now.Most times,the turbine wind force is been simplified.Little research about floating offshore wind turbines(FOWT)has been published,considering aerodynamics,control system,drivetrain and generator,platform and its mooring system.In the present study,coupled analysis methods are studied for the hydrodynamic responses of a spar type FOWT.Firstly,this thesis summarizes the previous domestic and overseas research on FOWT system,reviews the theories to calculate wave,wind and current loads.A numerical code,known as COUPLE,is been used to calculate the response of the OC3-Hywind Spar.The RAOs and the mooring load-displacement curves of the support platform are obtained.The FAST code is used to establish a complete aerodynamics model and wind turbine mechanical model for the NREL 5MW offshore wind turbine.After that,the COUPLE code is been modified for the uncoupled and limited coupled analysis of a FOWT.Finally,these coupling methods(uncoupled,limited coupled and coupled)are used to get the motions of the FOWT and the top tension of the mooring lines.The results of the uncoupled and coupled methods indicate that the surge motion of the FOWT is magnified and the pitch motion is reduced when using the uncoupled method.The pitch position of the turbine is the reason of this phenomenon.The results of the limited coupled and coupled methods indicate that,the motion between the platform and the wind turbine can significantly increase the pitch of the FOWT.The heave predicted using the limited couple analysis is much greater than that using the coupled analysis.It is because the heave is resonated by the pitch motion in the use of limited coupled analysis since their natural frequency is very close.The results of this study may provide some guidance to the design of FOWT and the related theoretical research in the future.