Viscosity and nonlinear resonant heave response of a semi–submersible floating wind energy platform

Floating offshore wind energy represents a promising frontier in marine renewable energy, enabling deployment in deeper waters. Among the various solutions for floating wind energy, semi-submersible platforms have emerged as the most viable option. The success of this technology depends on their hydrodynamic performance. Current engineering practice for design and operability assessment has primarily focused on short wave excitation, with less attention to long-period waves. Swells carry energy near the natural periods of these structures and are more likely to induce resonant responses in heave, in which scenario the prediction is challenging and less understood. This may lead to over-conservative designs and thus unnecessarily high cost. To address this gap, this study examines resonant responses driven by long-period waves through linear processes. In resonance, viscous effects play a critical role, e.g., in determining response amplitudes. However, estimating viscous effects is challenging as a result of their underlying complex physics and nonlinearity. To better understand the viscous effects and their impact on floating wind turbines, a series of scaled model tests was analysed for a 10-MW floating wind energy platform. To facilitate the interpretation of the experimental results, inviscid flow calculations were also performed. The results indicate that viscous damping is positively correlated with the Keulegan–Carpenter ( KC ) number that characterizes the relative velocity between the floating system and the surrounding water particles. A striking observation is that viscosity can significantly alter the added mass, which is key to the estimation of the natural frequency of the floating system.