Study Of Dynamic Characteristics On Floating Offshore Wind Turbine Platform

Offshore wind energy has the advantages of high wind speed, low turbulence intensity, steady dominant wind direction. Exploiting offshore wind energy won't occupy increasingly tense land space while supplying clean and efficient power, which is paticularily important for densely populated east coast in China where power demands are high. But offshore wind energy is even abundant in waters more than 10 km away from coast with a depth above 30m, where bottom-fixed offshore wind turbines are no longer applicable. In the purpose of improving the industrial and economical efficiency in deep water wind energy exploitation, the technology that installing wind turbine on floating platform to form a floating offshore wind turbine (FOWT) is put forward. FOWT is subjected to various environment loads such as wind, wave and current loads, and its response is largely influenced by coupling and non-linear dynamic effects, whick make it necessary to study the dynamic charastic of FOWT. In this thesis, issues in the following aspect are addressed:(1) The influence of aerodynamic damping on frequency domain response of floating wind turbine platform is anlyzed. The 5MW wind turbine and the spar type floating platform model by the National Renewable Energy Labotary (NREL) is selected as an example. The aerodynamic damping matrix is established according to aerodynamic damping calculation method. Hydrodynamic coefficients of the floating platform are computed based on the three-dimensional potential flow theory, and mooring system stiffness is taken into account as linear spring. Equations of motion of the floating platform with or without considering the influence of aerodynamic damping are established and solved in frequency domain. The influence of aerodynamic damping on floating platform's motion is analyzed in frequency domain through response amplitude operaters (RAOs) and the response spectrum derived from RAOs and JONSWAP wave spectrum.(2) A time domain coupling analysis is conducted for a semi-submersible FOWT. The coupling time domain analysis model was built using Orcaflex and FAST software based on retardation function method, and basic procedure and fundmental coupling theory for couping analysis was elaborated briefly. Couping effects between aerodynamic load and tower top displacement and floating platform pitch motion, and between mooring line tension and floating platform surge motion, are analyzed. Transient dynamic responses in mooring line tension, aerodynamic load and tower base bending moment caused by wind turbine start-up are then investigated. Frequency characteristic and sensitivity to working condition and loading direction of floating platform motion response are finally studied.(3) The influence of aerodynamic loads on fatigue performance of floating platform mooring lines is investigated. The NREL 5MW offshore wind turbine and a semi-submercible floating platform is selected as an example, a corresponding mooring system is then designed. Based on stochastic environment loads calculation theory, the coupling model for the FOWT is built using FAST and Orcaflex analysis software, and five coupling analytical cases are included according to different models. Considering the long-term distribution of ocean environment conditions, the fatigue performance of mooring lines is then investigated using Miner's cumulative damage theory, rain flow counting method and mooring line T-N curve. The influence of aerodynamic loads is eventually analyzed by comparisons of mooring lines fatigue damage in each analytical case.