On the relative importance of loads acting on a floating verticalaxis wind turbine system when evaluating the global system response

Interest in offshore floating wind turbines has been growing over the last decade. While a number of studies have been conducted to model the dynamics of offshore floating HAWT systems (e.g. OC3-Phase IV, OC4-Phase II), relatively few studies have been conducted on floating VAWT systems, despite their potential advantages.

Due to the substantial differences between HAWT and VAWT systems, analysis procedures employed for a floating HAWT analyses cannot be extended to use for floating VAWT systems. Here, the main aim is to provide a systematic analysis and comparison of the forces acting on a reference offshore floating VAWT, considering a turbulent wind field and stochastically generated waves, to assess the more critical loads and distinguish them from those with negligible effect, when estimating the global system response.

The floating VAWT system considered is comprised of a 5MW rotor supported by the OC4-Phase II semisubmersible. Using the coupled model of dynamics for VAWT "FloVAWT", the global response of the system is estimated for a set of load cases, allowing the assessment of the contributions of individual force components.

In particular, the simulations allow us to assess the impact of the VAWT aerodynamic forces, the platform hydrodynamic forces, and the mooring forces. The results help evaluate the relative importance of hydrodynamic with respect to aerodynamic forces, depending on the loading condition. A deeper insight into the aerodynamic forces is provided, which shows the impact of a) the roll/pitch inclination and b) the roll/pitch motion velocities on the rotor aerodynamic response and, eventually, on the global response of the platform.