The Spar-type FOWT, which is a kind of the stable offshore wind generator, has been widely adopted and investigated in recent years. As a permanent mooring structure, it faces the issue on mooring line fracture. In the present work, the simulations are conducted in time domain to investigate its transient response in scenarios with fractured mooring lines. Towards this end, our in-house code SFND, which is a coupled aero-hydro-elastic numerical model is adopted to perform the simulations. The methodology includes a blade-element-momentum model for aerodynamics, a nonlinear model for hydrodynamics, a nonlinear restoring model of SPAR buoy, and a fully nonlinear dynamic algorithm for intact and fractured mooring lines. The simulations are conducted under both stochastic and freak wave scenarios. The motions of platform, the tensions in the mooring lines and the power generation performance are documented in different cases. According to the results, the large drift motion is observed and the transient response is discussed.
During the recent decades, the wind energy has attracted more and more attention because of its advantages and features, such as no pollution, no carbon emission, and so on. However, with the issues on the land limitation and the noise, the installation of the onshore wind turbines nearly reaches the bottleneck. Therefore, the wind turbines are designed to be supported by the offshore foundations, in order to catch the offshore wind energy, which is less turbulence and more strength than the onshore one. Generally, the fixed foundations, including pile, gravity, jacket, etc., are widely adopted. Nevertheless, according to previous research, the costs and difficulties of the installation and maintenance increase exponentially when the water depth exceeds 50m (Leimeister et.al, 2020). To overcome this situation, the floating offshore wind turbines (FOWTs) are proposed.
The conceptual designs of the floating foundation are basically based on the experiences from the oil and gas industry (Hsu, 2017). Hereby, the different types of the floating foundations can be majorly divided into three types, which contains the Spar type, the Semi-submersible type and the Tension Leg Platform (TLP) type. Among these innovative designs, the Spar buoy shows both well hydrodynamic performance and robustness according to numerical simulations and wave basin tests (Yang et.al, 2020, Salehyar et.al, 2017, Li et.al, 2018a, Duan et.al, 2016). Even more, the first floating wind farm, Hywind Scotland, also adopted five Spar-type FOWT and successfully generate power more than two years.