Dynamic Characteristics And Vibration Control Of Floating Wind Turbine With Double Cable Mooring System

With the increasing demand for energy and the exhaustion of traditional non-renewable energy,wind energy as one of the most valuable renewable energies has been widely concerned by all over the world.Offshore wind power has many advantages over onshore wind power,and floating wind turbines have magnificent prospects as a new direction of offshore wind power development.Floating wind turbines are usually located in the harsh environment of deep sea and are easily affected by extreme loads.In order to ensure the safe and reliable operation of floating wind turbine,this thesis proposed a new double-cable mooring system for semi-submersible wind turbines,and evaluated its effectiveness in reducing the dynamic response of wind turbine platform and the dynamic performance of the mooring system.At the same time,the tuned mass damper(TMD)was installed in the cabin of the wind turbine to further reduce the dynamic response of the floating wind turbine and improve its stability.The main work and conclusions of this thesis are as follows:(1)The research on mooring system and vibration reduction control of floating wind turbine at home and abroad was discussed,the common floating wind turbine platform at present were revisited,the wind load,wave load and current load on floating wind turbines were systematically revisited,and the basic theories and calculation formulas of floating wind turbines were reviewed.(2)Modeling of conventional 5MW OC4-Deep Cwind semi-submersible wind turbine with single-cable mooring system was carried out through professional hydrodynamic analysis software Orca Flex,which was divided into four parts: upper generator set,middle tower,lower semi-submersible platform and mooring system.The corresponding frequencies and mode shapes are obtained by modal analysis.The dynamic responses under normal operating condition and storm self-existing condition were calculated respectively,the response characteristics in time and frequency domain were obtained.According to the dynamic response data,the conventional semi-submersible platform with single-cable mooring system has a large response in the three directions of surge,heave and pitch.(3)A new type of double-cable mooring system is proposed to overcome the shortcoming of single-cable mooring system which can not take into account both vibration reduction performance and fatigue resistance performance.The new double-cable mooring system consists of main cable,auxiliary cable,anchor device and vertical boom connecting the main cable and auxiliary cable.The novel double-cable mooring system was modeled in Orca Flex,and the advantages of the double-cable mooring system compared with the original single-cable mooring system for vibration control of the floating wind turbine platform were analyzed under normal operating condition and storm self-existing condition.The results show that compared with the original single-cable mooring system,the vibration response of the floating wind turbine with double-cable mooring system is significantly reduced,but the influence of variation of cable pretension and the position of cable guide hole on the vibration response of the floating wind turbine is not obvious.(4)According to the SDOF-TMD optimization theory,related formulas of SDOF-TMD for vibration control were derived,and TMD stiffness parameter and damping parameter were optimized.Considering cabin space constraint,TMD parameters need to be optimized under stroke constraint.The dynamic simulation analysis of semi-submersible wind turbine with double-cable mooring system under certain sea conditions were carried out,in which TMD with and without stroke limit and without TMD were compared for their vibration reduction effects.The equivalent damping ratio of TMD with stroke limit and TMD without stroke limit and TMD without double cable mooring semi-submersible wind turbine in floating wind turbine was compared and analyzed.The results show that the addition of TMD has obvious vibration reduction effect on the floating wind turbine,and the vibration reduction effect of TMD without travel limit is more obvious than that of TMD with stroke limit.However,considering the actual situation,it is necessary to impose stroke limit on TMD.