| Clean and renewable wind energy has been utilized for thousands of years,and continuous recent efforts have made it the world's fastest growing and most promising renewable energy source.Turbine installation has spread from the land into shallow coastal seas,and offshore deep water will be next.Floating wind turbines are most suitable for this application,and they are being studied and developed with keen interest in many countries.Floating offshore supports for wind turbines can be divided into three main types according to the physical principles used to achieve static stability: spar-type,semisubmersible-type,and TLP-type.Among these,the TLP-type shows excellent comprehensive performance and low impact on the performance of the wind turbine;it thus has great potential for broad application.Compared with the traditional fixed wind turbines installed on land or in shallow water,floating turbines have the problems of nonlinear hydrodynamics and large movement in addition to the problems of unsteady aerodynamics and aerodynamic elasticity,which make the turbine and its support a very complex aerodynamic–hydrodynamic–servo-elastic multi-body system.Extensive and detailed research conducted by international research institutions has resulted in a variety of proposals for TLP-type floating wind turbines.However,research on TLP-type floating wind turbine in China remains in the early stage,with the focus on the existing concepts and its dynamic characteristics.To play a leading role in deep-water offshore wind energy development requires urgent conceptual design studies of TLP-type floating wind turbines,including theoretical calculations and model testing,as a sound technical base for future application.For the reasons given above,this paper mainly focuses on four issues: one is the conceptual design of a new multi-column TLP-type floating wind turbine;the other is the research on the model test method of the TLP-type floating wind turbine.The third is the research on the establishment and the coupled dynamic response of the TLP-type floating wind turbine.The fourth is the research on the ultimate load and the fatigue load calculation of the key components of the floating wind turbine.The conclusions are summarized as follows:Following a comprehensive and systematic investigation of previous works regarding TLP-type floating wind turbine systems,this paper summarizes the conceptual design methodology,the design basis and and the platform innovation principles.A novel multicolumn TLP-type floating wind turbine concept(SAFOWind)is then proposed,for which theoretical analyses,numerical simulations,and model tests are reported.SAFOWind incorporates the advantages of second-generation TLPs,and has a small waterline area,deep draft,multiple columns,and an external mooring system.Initial conceptual design determined the system's general arrangement and main dimensions.The stiffness matrix of the TLP mooring system was deviated,and the influence of relevant design parameters on the system's quasi-static performance and natural frequency was analyzed.The optimal displacement and general design parameters were determined by optimizing the pre-tension coefficient.The results of model tests show that the SAFOWind concept has excellent motion performance in waves and thus meets the design requirements.The specially designed tempory buoyancy module allows for integral assembly in the dock and transport to the installation site by wet tow,resulting in significant savings on marine operation times and lower the installation costs.Additional cost analysis for a floating offshore wind farm based on the 5-MW SAFOWind concept estimated the main economic indicators such as cost per kilowatt capacity and cost per kilowatt-hour electricity generation.Comprehensive wave-basin model testing was performed to verify the SAFOWind concept and obtain its dynamic motion responses and global performance under combined wind,wave,and current loads.A proper scaling law was established,and a model wind turbine was built based on the optimal design of the model blades.The effect of Reynolds number on the model wind turbine's aerodynamic performance was analyzed.To improve the precision of testing,an elastically similar model of the turbine tower was designed that perfectly modeled the flexibility of the tower structure.The BEM-3D method was constructed by combining the three-dimensional airfoil aerodynamic coefficients with the BEM theory.The model test results show that the proposed BEM-3D method significantly improves the calculation accuracy compared with the traditional BEM-2D method.The test plan consisted of still water tests,white noise wave tests,irregular wave tests,and combined wind,wave,and current tests.The technical feasibility of the system was assessed in terms of the natural periods of motion,damping,motion response amplitude operators(RAOs),and tendon tensions.A fully coupled dynamic model including a turbine,control system,tower,support platform,and TLP mooring system was established based on the theoretical analysis.By comparing the results of the model test with the numerical results,the fully coupled dynamic model is verified and improved.Coupled dynamic responses under wind,wave,and current loads were obtained by numerical simulation in the time domain.The numerical model considered the second-order wave force transfer function(QTF),and thus investigated the influence of second-order wave force.The effects of wave approach angle and wind–wave load on the system's dynamic performance were also studied.The results show that:(1)The tower flexibility has a great influence on the pitch motion response of the TLP-type floating wind turbine,which can significantly change the natural frequency of the pitch motion.(2)The second-order wave force effects high-frequency response of the pitch motion greatly,which also has an important impact on the nacelle surge acceleration,tower load and tension fluctuations.(3)By incorparating the first and second-order wave load,the numerical results of tether tension agree well with the experimental results.(4)The variations of the surge and pitch motion amplitude under the combined load cases are lower than the wave load only case,namely the existence of the wind and current loads have a stabilized effect to the motions due to the added damping.(5)The generator power output is similar to that of the fixed wind turbine installed on land,which shows the SAFOWind concept has small dynamic motion responses and thus has little effect on the power output.Extreme and fatigue load analysis was performed and local extremes were extracted using methods of integrated time separation and peak-over-threshold.A sufficient number of local extremes were extracted,while simultaneously ensuring their independence.A fatigue load calculation process was proposed and used to predict the extreme and fatigue loads of the system.The effects of simulation duration,wind turbulence intensity,highorder wave load,and wind–wave loads on structural fatigue damage was examined.Structural load was also compared between the SAFOWind system and an NREL offshore 5-MW baseline wind turbine installed on land.The results show that:(1)The fatigue damage equivalent load of the tower bottom bending moment will increase about 10% by raising the level of wind turbulence intensity.(2)The second-order wave load has a significant impact on the fatigue damage.Considering the second-order wave load,the structural fatigue damage has increased by 85% and 25% respectively under the action of wave load alone case and the combination of wind and wave load case.(3)The fatigue damage caused by the combined wind and wave load is far greater than the wind load and wave load acting alone and the result of superposition.In summary,this paper introduces the concept of a novel multi-column TLP floating wind turbine,in a project supported by the National Basic Research Program(973 Program)and having independent intellectual property rights.Numerical and experimental studies were performed to investigate the coupled dynamics of the system,and extreme and fatigue load analysis were conducted to evaluate its structural safety.The outcomes of this study are expected to provide a theoretical basis and technical support for future works regarding the design and analysis of floating offshore wind turbines. |
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