Dynamics Analysis And Design Optimization Of Spar-type Floating Offshore Wind Turbine

The floating offshore wind turbine(FOWT)system shows promising potential for application in harvesting wind power in deep-water area.The dynamic responses of FOWT have great influence on the failures of key positions and the power output of wind turbine.The coupling effects of the FOWT system include those among the subassemblies,those among environmental loads as well as those between loads and motions.Due to these coupling effects,the dynamic behavior of FOWT becomes a complicated problem.This thesis focuses on the dynamics and optimization of the spar-type FOWT,including the dynamics modelling,the analysis of dynamic responses under external loads and the design optimization of the system.The main contents and conclusions of this thesis are summarized as follows.(1)The complete nonlinear time-domain equation of motions(EOMs)of thespar-type FOWT system are formulated based on the theorem of momentof momentum and the theorem of motion of the center of mass.Theformulas of external loads are also derived.The capability of in-housecode is verified by comparing its simulation results with those predictedby FAST and experimental data.(2)The influence of environmental loads(i.e.,loads induced by wind,waveand current),rotor rotation and nacelle yaw on the dynamic responses ofFOWT are investigated.The mean positions and variations of themotions are determined by rotor thrust and wave forces,respectively.Theloads induced by current increase the mean values of surge and pitchwhile it has little influence on the variations of these motions.Thefrequency components near the natural frequencies of surge and pitch arereduced by the aerodynamic damping.This damping is enhanced by theincrease of wind velocity.Due to the layout of mooring system,the peakvalues and standard deviation of the tension of downstream mooring lineare larger than those of upstream mooring line.The gyroscopic momentgenerated by the rotor rotation is the main contributor to the yaw motionof the substructure.(3)A novel spar-buoy model with a relatively shallower draft is designedand its hydrodynamic characteristics are also investigated.The designparameters of the mooring system are selected based on an optimizationprocedure to obtain the optimum solution.The effects of the heave plateon suppressing the dynamic responses are also discussed.It is found thatdiffraction effects are negligible in moderate to severe sea states,radiation damping in most modes of motion is small,and flow separationwill occur in severe sea states along the upper regions of the spar.Themean position of pitch only increases with the depth of fairlead.Thetension of mooring line is only influenced by the depth of fairlead andthe radius of anchor.When the depth of fairlead gets close to the locationof the center of mass of the whole system in static position,the naturalperiod of pitch always reaches its maximum value.The heave plate cansuppress heave motion effectively while it has little influence on othermodes of motion.This thesis can serve as a reference work for the design,fabrication and deployment of spar-type FOWT system.