| Compared with offshore fixed base wind turbines,offshore floating wind turbines are typical equipment for deep-sea wind power development.Under the action of wind,wave and current multi-field coupling,the operating environment is harsh,and there are significant unsteady three-dimensional flow characteristics,which are not only depends on the rotor operating conditions,but also related to the amplitude and frequency of the platform motion.The traditional BEM method has theoretical limitations and cannot fully describe this kind of three-dimensional characteristic.So it is not suitable for the aerodynamic analysis of floating wind turbines.However,the free wake vortex method could reduces the model's dependence on the empirical relationship,and calculate the three-dimensional flow velocity and induced velocity of the floating wind turbine more accurately under the six-degree-of-freedom movement of the platform,so as to evaluate load more precisely.In view of this,an accurate structural dynamics simulation model was established with the Technical university of Denmark(DTU)10 MW reference wind turbine.Based on the free wake vortex method,the 10 MW floating wind turbine with a wide range of blade tip speed ratio was studied.The load characteristics of floating wind turbine rotors under six degrees of freedom movement were analyzed.The output power characteristics of the turbines were analyzed with the platform surge and pitch motion,and reduce turbine failure rate based on reliability growth.The finite element model of wind turbine blade was established,and the structure layer of the blade was optimized based on the load corresponding to the maximum deflection of the floating wind turbine,so as to improve the blade reliability.Around the above contents,under the funding of national key research and development program subject(2018YFB1501304)"Optimization design,batch manufacturing process and testing technology of key components of transmission chain" for large offshore wind turbine,main contributions were made as follows:(1)Analysis of the limitation of blade element momentum and verification of free wake vortex method.The calculation and iterative process of the blade element momentum and the free wake vortex method were briefly described and derived.The limitations of blade element momentum theory were discussed through the reduced frequency and the validity of the momentum balance hypothesis.The aerodynamic instability of the wind turbine blade starts from the inner side of the blade.With the disturbance frequency increasing,the aerodynamic instability spreads to the-outside of the blades along the spanwise.The stronger the disturbance frequency,the more aerodynamically unstable.Momentum balance failure is most likely to occur on the outer side of the blade.With low wind speeds,the proportion of momentum balance failure is greater.Based on the free wake vortex method,the aerodynamic performance of the NREL VI phase wind turbine was analyzed.The accuracy of the free wake vortex method was verified by UAE VI phase axial and yaw experiments.The free wake vortex model can accurately calculate the relative velocity and induced velocity,and then accurately evaluate the load.(2)Study on the load characteristics of the floating turbine blades under six degrees of freedom movement of the platform.A parametric model was established with the DTU 10 MW reference wind turbine as the research object.The accuracy of the parametric model was ensured by adjusting vortex core parameters and wake discrete parameters.Simulation analysis of DTU 1 0 MW wind turbine was carried out based on free wake vortex method.Compared with the axial condition,the three-dimensional flow characteristics under the condition of 30° yaw are more significant.There is a coupling effect between the platform's six degree of freedom motion and the rotor's rotating motion.This coupling effect will affect the thrust of the rotor,enhance the fluctuation of the rotor's thrust,and increase the fatigue load and ultimate load of the blade.Compared with the DTU 10 MW fixed basis wind turbine,the standard deviation of the floating wind turbine's rotor thrust is 2.291 times more than that of onshore wind turbines,the maximum rotor thrust of the floating wind turbine is 1.635 times,the standard deviation of blade root out-of-plane bending moment of the floating wind turbine is 1.389 times,the maximum blade root out-of-plane bending moment of the floating wind turbine is 1.344 times,the maximum blade root out-of-plane moment short-term damage equivalent load of the floating wind turbine is 1.116 times.The rotor thrust,blade root bending moment and blade tip deflection of floating wind turbine are mainly caused by wind load,while platform's six degree of freedom motion affects the fluctuation of rotor thrust,blade root bending moment and blade tip deflection.(3)Analysis of influence of platform surge and pitch motion on the output power of floating wind turbine.Under the effect of platform surge and pitch motion,the synthetic flow field power of DTU 10 MW floating wind turbines should be described by the combined velocity Ussum instead of the incoming wind speed U?.The amplitude and frequency of the platform's surge and pitch motion have similar effects on the instantaneous output power Pi and instantaneous thrust Ti of DTU 10 MW floating wind turbine,as well as on the output power and rotor thrust hysteresis,with a phase lag of 14.4°.The larger the amplitude and frequency of platform surging and pitching motion are,the greater the instantaneous output power Pi and instantaneous thrust Ti will be.For different movement amplitudes of the platform,the maximum increase of average power coefficient CP0 under S3 condition was 29.89%.For different movement frequencies of the platform,the maximum increase of average power coefficient CP0 under the condition of the frequency being 0.2 Hz was 14.94%.With the amplitude and frequency of the platform's surge and pitch motion increasing,the probability of DTU 10 MW floating wind turbine operating in abnormal working conditions will be increased.Although the coupled motion of the platform can improve the average power output,the power fluctuation will be increased.which will cause greater fatigue damage to the turbine components.Taking the reduced frequency k1 and k2 as the dependent variables of the power characteristics,the power characteristic curve function CPa(?,?up,k1,k2)of the floating wind turbine was derived.The function varies with the tip ratio ?,blade pitch angle ?lip and simplified frequency k1 and k2.(4)Study on blade layers optimization of offshore floating wind turbine based on particle swarm optimization algorithm.Through the particle swarm optimization,with the objective of minimizing the weight of wind turbine blade,based on the load corresponding to the maximum tip deflection of DTU 10 MW floating wind turbine,under the constraints of meeting the maximum stress,bending,deflection,natural frequency of the blade,and Tsai-Wu failure criteria,adjusting the blade layer thickness and the chord width of the main beam,the blade structural layer of the DTU 10 MW wind turbine was optimized.The weight of optimized blade compared to DTU 10 MW blade decreased by 12.376%.BALSA material was added to the optimized blade main beam,and the chord position distribution of the main beam was adjusted from 0.275c to around 0.35c.Among the first ten modes,from the first to fifth modes vibration modes were basically the same.From the 6th to 10th mode shapes,the 10th order mode has the largest mode difference of 7.29%.According to the Tsai-Wu failure criterion,the strength index values of the blade's main beam,shear web,and skin were all less than the critical value 1.Among them,maximum Tsai-Wu intensity factor of the main beam,shear web and skin were 0.542,0.661 and 0.621 respectively,it can be seen that the blade optimization results are reliable.Based on the free wake vortex method,the load characteristics of offshore floating wind turbines were studied in depth,which can be used to guide the design of safe and efficient floating wind turbines.Optimization of blade structure layup based on particle swarm optimization algorithm can effectively reduce the weight of the blade,reduce the failure rate and improve the reliability of the turbine. |
PDF Full Text Request