Dynamics of a Y-shaped semi-submersible floating wind turbine: a comparison of concrete and steel support structures

Concrete semi-submersible floating wind turbines (FWTs) have generated increasing interest in the offshore wind energy industry due to their superiority over steel structures in aspects of construction and maintenance. However, the influence of construction materials on the dynamics of FWTs was scarcely studied before. The dynamics of two FWTs, mounted on a concrete and a steel Y-shaped semi-submersible platform respectively, are investigated by means of 1:60 scale combined wind and wave model tests and coupled multi-body simulations. The results indicate that the steel structure with a lower centre of gravity exhibits advantages in platform pitch motion alleviation. The steel structure is also subjected to lower pitch-induced tower base loads and nacelle acceleration than its concrete counterpart. However, an opposite trend is found in the wave-frequency responses, leading to an insignificant difference of tower base loads and nacelle acceleration between the two structures. ABBREVIATIONS: FWTs: Floating wind turbines; CoG: Centre of gravity; CoB: Centre of buoyancy; LCP3: Lightweight prestressed concrete platform; HSP3: High-strength steel platform; WTWF: Wind tunnel and wave flume joint laboratory; ROC: Rated operational condition; JONSWAP; Joint North Sea Wave Observation Projection; RAO: Response amplitude operator; BEM: Blade element momentum theory; DOF: Degree of freedom; GDW: Generalised dynamic wake theory; PSD: Power spectral density; QTF: Quadratic transfer function.