Study on the Dynamic Behaviours of an Articulated Offshore Wind Turbine under the Severe Sea State

In this paper, an articulated offshore wind turbine (AOWT), which consists of ballast tank, buoyancy tank and middle column, is proposed to solve the challenges that both fixed and floating offshore wind turbine couldn’t meet in the the medium-water-depth areas. Through establishing high-dimensional three-objective optimization mathematical model, the main foundation dimensions are determined by non-dominated sorting genetic algorithm Ⅲ (NSGA- Ⅲ ) and corresponding optimization algorithm code is programmed in the MATLAB development environment. In order to to verify the robustness of the AOWT, different severe sea states, including extreme wind speed and extreme stochastic wave scenarios are chosen. For survival sea state, as the turbine is in the parked condition, the aerodynamic load on rotating blades is ignored and the wind pressure loads are calculated based on the empirical formula. The first and second order wave loads are all considered, corresponding hydrodynamic coefficients are simulated by the 3-D potential flow theory. The simulations are conducted in the time domain and dynamic responses are numerically simulated by our in-house aero-hydro coupled code. In previous works, we have investigated AOWT dynamic behaviours under the operational scenarios. It is found that AOWT shows robust performance under the rated sea state. However, as a permanent moored offshore wind turbine, the AOWT will suffer different kinds of severe sea state during its lifetime. Towards this end, we performed a series of simulations to study the AOWT dynamic response under the severe sea status and the results of wave forces, tensions on the articulated joint and foundation motion in the pitch are summarized and analysed. Through the simulation results, we can have a clear understanding of the structure response. The safety and stability of AOWT under different severe sea states are thoroughly investigated, which reflect the validity of physical design to a certain extent. Furthermore, it also points out that the study of dynamic response under severe wave scenarios is necessary for structural design.