Numerical Simulation And Experimental Study On Hydrodynamic Characteristic Of Offshore Wind Turbine System

The use of wind power worldwide is in a period of rapid growth. As wind is steadier andstronger over sea than on land, the wind industry has recently stepped to offshore areas.However, the hydrodynamic performance of OWT (offshore wind turbines) is different fromonshore WT (wind turbine) and offshore O&G (oil and gas) platform because of thecomplicated environment. There is no effective method and tool for OWT aerodynamicperformance, hydrodynamic characteristics and motion response etc.So, based on the background of OWT, the theories of aerodynamics, hydrodynamics andcoupled motion were used, numerical simulation and model test were integrated and appliedto the analysis of aerodynamic and hydrodynamic performance of OWT system, theprediction methods of aerodynamic and hydrodynamic loads and responses for OWT werebuilt. This work has theoretical and practical significance to deeply grasp the characteristicsand laws of the aerodynamic and hydrodynamic of OWT, and to provide theoretical methodsand design techniques for developing reliable and efficient offshore wind power devices.In the research of aerodynamics for OWT, the wind turbulence characteristics undernormal wind condition were described by the NTM (normal turbulence model). And theprogram for aerodynamic of rotor with horizontal axis based on BEMT (blade elements andmomentum theory) with blade attack correction was developed. Aerodynamic characteristics,including lift and drag coefficients of blade, axial wind force and torsion of rotor, wereachieved using this program. The characteristics of turbulent wind speed and aerodynamicperformances of a1.5MW OWT under normal wind condition were analyzed by the program.In the research of hydrodynamics for OWT, the methods for different foundation typeswere proposed and applied to simulate hydrodynamic performances of OWTs with monopileand floating foundation. The characteristics and time histories of wave load on the monopileof a1.5MW OWT were studied based on the method combined with the Stokes nonlinearwave theory and Morison equation. Based on3D potential theory for large volume structuresin frequency domain, the hydrodynamic model was established for a5MW semi-submersibleFOWT(floating offshore wind turbine), hydrodynamic characteristics was analyzed, globalmotion response and nacelle acceleration were predicted, motion response level andprobability of FOWT in three typical short sea states were investigated by statistics method. In the research of system coupled characteristics, based on the coupled motion theory offloating structure and mooring system in time domain, a method to solve rotor-floater-linescoupled motion of FOWT was developed and applied to a5MW semi-submersible FOWTsystem. The responses of motions and mooring line tensions under four representative loadcases were investigated. The coupling effect on motion performance and mooring linetensions were studied. Interaction of mooring force and wind load in low frequency wasespecially considered. Comparing to onshore WT and offshore O&G platform, characteristicsof global response and tensions for FOWT mooring system were understood finally.A model test system of a5MW semi-submersible FOWT mooring system has beendesigned and manufactured as well as a test platform of hydrodynamic characteristics. Thetest scheme and test methods for the hydrodynamic performance were presented and thenmodel experiments were carried out. In the model test, the beneficial effect of heave plates onmotion of FOWT was analyzed, the evident influence of constant wind on motion was studied,the characteristics of motion and tensions for FOWT mooring system under load casescombined wind and wave were achieved. Furthermore, the coupled numerical model ofFOWT mooring system was modified by heave damping measured in the test, so reliabilityand validity of numerical simulation method were enhanced.Conclusions of this thesis could lead to a more comprehensive understanding ofaerodynamic and hydrodynamic characteristics for OWT, obtaining of prediction methodsand test means for aerodynamic and hydrodynamic loads and responses for OWT, greatcontribution to the development and utilization of offshore wind energy in China.