Modelling linearly excited resonant heave response of a semi-submersible floating wind turbine

Accurate prediction of heave motion is essential for evaluating mooring loads and dynamic cable fatigue, which are key considerations for the safe and efficient design of floating wind turbines. For semi-submersible platforms equipped with heave plates, this task remains challenging because viscous effects influence both damping and added mass, leading to shifts in natural frequency that are not captured by potential-flow theory. This study develops a numerical model that combines a potential flow solver with Morison-type viscous drag and incorporates empirical corrections to account for viscosity-induced variation in added mass. The model successfully reproduces experimentally determined heave responses in both frequency and time domains, demonstrating that inclusion of added mass variation is essential for reliable prediction — an aspect that is seldom addressed in existing dynamic models. Further, the results show that employing undisturbed water particle velocities in the Morison formulation provides a practical and satisfactory representation of viscous drag, avoiding the additional complexity associated with using the disturbed wave field. These findings highlight the effectiveness of the proposed approach and its applicability to the dynamic analysis and design of floating wind turbines with heave plates.