| With the increasing consumption of global energy and shortage of fossil fuels,renewable energy exploration and development has been an important development strategy around the word.As one of the most promising green renewable energy sources,the development and utilization of wind power has been moving from onshore to offshore areas,but the environmental conditions are more complex and severe in offshore areas.Therefore,the design and analysis of aerodynamic performance,hydrodynamic characteristics and structural properties plays an important role in maintaining the stability of floating offshore wind turbines(FOWTs).In this dissertation,a Spar-supported 5MW FOWT was studied to research its hydrodynamic performance,and further discussed the related strategies to improve its kinematic performance and verified the effectiveness with numerical simulation.The main work is listed as follows:1.Frequency-domain characteristics are gained through modeling and calculation of a Spar-supported FOWT based on the three-dimensional potential flow theory and numerical simulation.The frequency-domain response were analyzed including amplitude operator,added mass,radiation damping,first-order wave exciting force and second-order drifting force.The frequency-domain results show that surge,heave and pitch RAO tend to arrive at high level when the wave frequency is relevantly low,which means the natural frequency in the three main degrees of freedom(DOFs)is low(less than 0.4 rad/s).The second-order drifting force is one order smaller than the first-order exciting force.Water depth doesn't make a difference in the natural frequency in surge,heave and pitch motions,but the peak RAOs and added mass responding to the natural frequency have the tendency to decrease when the water depth increases.2.Describes the coupling time-domain analysis method considering environmental load,mooring system and the interaction of the components of the FOWT system and the theory of calculating mooring force.And further obtain the time experience of motion response,cable tension,mooring force and drifting force,together with the spectrum density curve of related parameters.Time-domain results and power spectral density curves show that surge potion mainly behaves low-frequency(about 0.04 rad/s)response due to the drifting force,while heave and pitch tend to be more affected by wave-frequency response.The cable section tension of three mooring lines are the same order of magnitude,but the mooring line distributed in the same direction of load direction has lager section tension than the other two mooring lines.Mooring force in the heave DOF is one order larger than surge,which means the mooring force mainly affects and restrains vertical motion.Second order drifting force cannot be ignored for it's the cause of long-period horizontal motion although it's one order smaller than first order wave exciting force.3.Brief review of the internal resonance and vortex-induced vibration(VIV)problem of Spar platform in the current.Referring to the research experience of offshore oil platforms,similar method are introduced into the Spar-supported FOWTs about the attachment of helical strake and heave damping plate.Numerical simulations are carried out to verify the effectiveness in improving the kinematic performance of the strategies and compare the hydrodynamic parameters to the results obtained from the formal calculation.The simulation results indicate that the heave natural frequency has been increased with the attachment of helical strake and heave plates,which means the natural period has been decreased to avoid resonance with incident wave.Also the peak pitch RAO got decreased obviously.In addition,added mass and radiation damping get obviously increased so that heave motion can be restrained to some extent.To synthesize frequency-domain and time-domain analysis,the attachment of helical strake and heave plates can both succeed to optimize the hydrodynamic response and kinematic performance of Spar-supported offshore wind turbines. |
PDF Full Text Request