Enhancing pitch stability in floating wind platforms: the hydrodynamic effects of thin plates

Abstract

The Floating Offshore Wind Turbines (FOWT) technology is still at an early stage looking for ways to make it competitive. In the case of semi-submersible FOWT the large structure has been pointed as an option to tackle the final energy price and therefore, smaller/cheaper platforms are desired. However, this would have implications on the platform hydrodynamic performance. This thesis discusses the use of thin disks as pitch motion suppression surfaces for FOWT platforms, employing three methodologies; potential flow, Computational Fluid Dynamics (CFD) and Experiments. With NEMOH a boundary element method code, the added mass and excitation forces/moments were calculated for disks at different angles and platform geometries, identifying the main parameters for exploration. Two CFD campaigns were carried out to characterize relevant parameters of a thin disk at a range of disk-platform relative angles on rotational motion. A novel experimental campaign was conducted for validation, benchmarking and exploring purposes. This campaign characterized a thin disk under a large range of Keulegan-Carpenter (KC) and Reynolds (Re) numbers, by using the forced oscillation motion technique for rotational motion around a fixed point in space, representing a platform centre of rotation.

By using potential flow and CFD calculations, it was observed that the main parameter to define the performance of a disk as a pitch motion reduction surface on a semi-submersible platform is the (here defined) disk-platform relative γ angle. When this angle reaches 0◦ the hydrodynamic forces are maximized, and lower accelerations are observed. On the experimental campaign, a small dependency on Re and frequency was observed for both the inertial and damping hydrodynamic forces, agreeing with the reviewed literature of thin disks under heave motion. The added mass trend for high KC values was corrected with the experimental results, agreeing with the second CFD campaign.

The obtained results were used to develop empirical formulae that can have multiple applications in the early design stages when disks are applied for pitch motion reduction. This includes the ability to obtain approximated added mass and damping coefficients, taking into consideration the angle of the disk, the platform design and the KC condition. To study the applicability of the numerical, experimental and empirical results on larger scales, four different disk scales were tested using CFD, obtaining good agreement among the disks on the quadratic damping coefficient.

In conclusion, the pitch motion reduction surfaces approach, applied on thin disks, for semi-submersible platforms was proved to be promising. When the Windfloat heave plates added mass was compared against the proposed approach, a 25% and 78% of increment is observed for added mass and linear damping respectively.