The mooring break is an important issue for floating offshore wind turbine (FOWT). A semi-submersible FOWT is used to study the transient response due to the mooring break under typhoon conditions in South China Sea. The 5MW class wind turbine, with 6 catenary lines mooring system, is used in present analysis. The focus of this paper is to investigate the dynamic response, mainly including the platform motion and the tension of catenary mooring lines with mooring break. The results provide some valuable proposal for mooring design of FOWT.
Most of countries around the world are facing an energy transition due to the depletion of fossil energy. Compared with the traditional three fossil energy sources (coal, oil, natural gas), wind power generation converts mechanical energy in the air into electrical energy without generating pollutant emissions. Wind energy, special the offshore wind, has received wide attention as a green renewable energy source in the past decade. As the positioning structure of the FOWT, the mooring system plays a crucial role in the safe and stable operation of the FOWT. Therefore, it is significant to understand the impact of the mooring system on the response of the FOWT system.
For the mooring system dynamics of the FOWT, namely the cable dynamics module, the main theoretical basis is: quasi-static method, lump mass and rod theory. In the research of the FOWT mooring system, Hall et al. ignored the torsion and bending stiffness of the mooring line. Based on the validation results against the experimental data, they concluded that the lumped mass method is superior to the quasi-static method in the coupled dynamic analysis (Hall and Goupee, 2015). Benassai et al. numerically analyzed the effects of catenary mooring system and vertical tension line mooring system on the dynamic response of FOWT (Benassai et al., 2014). Drag force induced by the current cannot be considered in the classical catenary mooring equation, while finite element method based analysis is computationally intensive. In this regard, Park et al. proposed using a discrete catenary equation to simulate mooring by dividing the mooring lines into three parts, the suspended part, the grounding part and the lying bottom part, and respectively solved the equations of different units(Park et al., 2017). Qiao et al. simulated the FOWT mooring system into a damping mechanism that absorbs the kinetic energy of the platform, thereby establishing a numerical model to evaluate the damping characteristics of the FOWT mooring system(Qiao and Ou, 2014). Based on the excitation cylinder model, Cheng Zhengshun et al. coupled the developed aerodynamic code with SIMO-RIFLEX and performed a fully coupled dynamic analysis of the vertical axis wind turbine numerical model. It was found that it is feasible to use SIMO-RIFLEX-AC for computation of FOWT full system dynamic response (Cheng et al., 2017). In addition, Motohiko Murai et al. studied the dynamic response and mooring tension changes of cable-breaking platforms where mooring and seabed soil friction were destroyed(Murai and Suyama, 2015). According to the mooring design standard provided by DNV, Campanile et al. analyzed the frequency domain response of semi-submersible FOWTs in different working conditions for different depth ranging from 50m to 350m deep. Through budget analysis, it was concluded that the mooring system with 6 mooring lines has better economy while meeting the positioning requirements(Campanile et al., 2018). For mooring break, Yan Li studied the transient response of a SPAR FOWT when some mooring line fails and the pitch angle turn to 90° (Li et al., 2018). Y.H. Bae has checked the response of the three-mooring-line system of a Semisubmersible FOWT when a line is broken and finally consider turbine shut-down scenario(Bae et al., 2017).
