Model tests on the dynamic characteristics of semi-submersible wind turbines with central and eccentric arrangements

[Objective] Harnessing high-quality deep-sea wind energy, semi-submersible wind turbines have emerged as a prevalent structural solution in the offshore wind energy industry. In particular, the Y-shaped semi-submersible platform, featuring a centrally arranged wind turbine, and the Δ-shaped semi-submersible platform, with an eccentrically arranged wind turbine, are two dominant configurations for deep-sea applications. However, the systematic quantification of their dynamic characteristic disparities remains lacking, which can be attributed to various factors, such as incomplete numerical methodologies, variations in turbine power capacities, divergent design standards, and construction techniques among existing prototypes, as well as potential technical and commercial confidentiality constraints. [Methods] To facilitate an equitable comparison, the optimized design of centrally and eccentrically arranged semi-submersible platforms and mooring systems suitable for the DTU 10-MW wind turbine is obtained, with minimizing the system costs and cumulative fatigue damage as the objectives and the main dimensions of the platform and mooring as the key variables. Subsequently, semi-submersible wind turbine test models with a scale ratio of 1∶70 were designed and established based on the similarity criterion. Dynamic characteristic testing was conducted using scaled model tests under combined wind and wave conditions, focusing on the rated operation and extreme survival mode of the wind turbines. A comparative analysis was conducted to assess the effects of wind-only, wave-only, and combined wind-wave conditions on dynamic responses of the two semi-submersible wind turbines. Through statistical analysis of the time and frequency domains of key dynamic performance indicators, such as platform motion, nacelle acceleration, tower base bending moment, and mooring fairlead tension, the effect of different loads, the coupling characteristics among various dynamic responses, and the excitation mechanisms were investigated. [Results] The results indicate that the pitch natural periods of the two semi-submersible wind turbines are similar. The larger vertical static water stiffness of the Y-shaped semi-submersible wind turbine results in a shorter heave natural period. The Δ-shaped semi-submersible platform's four-line mooring system demonstrates greater structural stiffness over the Y-shaped semi-submersible platform's three-line system, resulting in a reduced surge natural period. The eccentric arrangement of the Δ-shaped semi-submersible wind turbine is prone to the coupling effects of the heave and pitch; the pitch amplifies the vertical motion of the wind turbine. The difference in mooring stiffness caused by different mooring schemes leads to a significantly smaller surge response and marginally smaller pitch response in the Δ-shaped semi-submersible wind turbine compared with that in the Y-shaped one. Nacelle accelerations and tower base bending moments are more pronounced in the Y-shaped semi-submersible wind turbine under long-period extreme waves, whereas the Δ-shaped semisubmersible wind turbine exhibits higher responses under short-period operational waves. Nonlinear mooring system behavior driven by load-induced equilibrium shifts causes upstream lines to enter a tensioned, nonlinear stiffness regime under combined wind-wave loading, exacerbating fairlead tension fluctuations and spectral peak magnitudes. [Conclusions] This study highlights the necessity of accounting for the dynamic characteristic differences between Y-shaped and Δ-shaped semi-submersible wind turbines for various sea states and limit states during engineering design. Furthermore, by integrating the dynamic characteristics observed in typical working conditions with full-lifecycle sea condition data, this research provides a quantitative framework for the selection and design optimization of deep-sea floating wind turbine platforms.