During power generation, the rotor of wind turbine needs to be aligned with the incoming wind for optimal energy production. On state-of-the-art upwind wind turbines, this step is usually achieved through the yaw control system in the nacelle. This mechanism, however, adds complexity and provides an additional source for errors for offshore turbines since the offshore condition is humid and salty. The salt-laden air may corrode the steel structures, such as the yaw bearing, increasing the cost and difficulty of maintaining. A new design concept of a semi-submersible floating offshore downwind wind turbine (FOWT) moored by a single-point mooring (SPM) system is proposed to address this issue. Due to this improvement, the operation and maintenance cost will be reduced. Based on a 5MW offshore wind turbine, a 3-column semi-submersible floating foundation with a SPM system is designed to support the wind turbine. This system allows the floating foundation to rotate freely for wind alignment. Through weathervane, the yaw control system will be simplified and the power generation efficiency will be improved. In this study, a coupled system model is developed to simulate the dynamic response of the FOWT system and check its feasibility. The result shows that the combination of the single point mooring and the downwind type wind turbine is effective in terms of weathervane. Under the design condition, the dynamic response of the system is relatively small.
With the rapid development of global economy, energy supply has become a strategic problem. As a kind of renewable energy, wind energy has attracted increasing attention because it is clean and inexhaustible. Vast wind resources exist in the offshore environment with stronger and steadier wind speeds, allowing for significantly higher capacity than those over land (Chan et.al, 2015). According to the report (Olivier et.al, 2016) from the International Renewable Energy Agency (IRENA), by the end of 2015 the global cumulative installed wind capacity has reached 431,498 MW. The offshore cumulative installed wind capacity is 12,161MW while the cumulative installed wind capacity in Asia is 1,078 MW (Whiteman et.al, 2016). China has been exploiting wind energy positively in recent years, and it is planned that the offshore cumulative installed wind capacity will reach 30GW by the end of 2020 (Zhang et.al, 2015).