Session: 09-01-04 Wind Energy: Aero-hydrodynamics 4

Paper Number: 126638

126638 - On the Design, Hydrodynamic Modelling and Response Analysis of a New Concept Fowt Platform 

This paper primarily focuses on the design and hydrodynamic analysis of a new improved FOWT concept with a capacity of 5 MW or above. The proposed concept will have smaller dimensions, hence reduced material and displacement. The platform consists of large volume cylindrical structures at its base and near the waterplane in order to increase the distance between the centre of gravity and buoyancy. This is done to improve the stability performance during assembly, transportation and operation for different environmental conditions. Furthermore, the base cylinders work as containers for ballast and effectively reducing vertical motion. While designing the platform, a simplified, semi-automated new approach is employed to reduce the search for design space exploration to adopt the preliminary dimensions.

For a detailed hydrostatic stability analysis, general hydrostatics (GHS) software is used to verify different loading conditions. During the stability analysis, it was found that the platform could be designed with relatively lower displacement to comply with the static stability. But when subjected to wave loads, it exhibited reduced stability with significant motion amplitudes. Nonetheless, the sizing of the platform is primarily determined by the hydrodynamic heave and pitch natural periods, considering them to be outside the wave period range.

Once the preliminary dimensions are chosen, a radiation-diffraction analysis in the frequency domain was performed using potential flow (PF) solver to estimate hydrodynamic coefficients and transfer functions. Ansys Aqwa and BV Hydrostar were used separately for detailed hydrodynamic modelling and analysis of the platform for intercomparison of the numerical results. In both software, only viscous loads from the slender elements were accounted using the drag term from Morison’s equation.
The tower of the turbine is modelled according the description in the existing literature. Rotor-nacelle assembly is modelled as a point mass and placed on top of the tower. Sea water is used as a ballast and modelled as a solid mass with the equal density as sea water. The resulting platform has a 30% reduction in displacement and save more than a 20% of material weight compared to the NREL DeepC platform.The natural period of the platform in heave is found to be 19.4 s and the same for roll and pitch are 28.2 s. Motion RAOs for the platform in heave, roll and pitch show peak values near natural period with acceptable values in comparison to the NREL DeepC platform. Motions RAOs are highly sensitive to the vertical drag resulted from the heave plates, especially in heave compared to roll and pitch whereas transverse drag (along wave directions) has very little contributions. Further details of the study will be presented in the full paper.

Presenting Author: Md Touhidul Islam The University of Edinburgh

Presenting Author Biography: Name: Md Touhidul Islam

Institution: The University of Edinburgh

Email Address: touhidul.islam@ed.ac.uk

Authors:

Md Touhidul Islam The University of Edinburgh
Vengatesan Venugopal The University of Edinburgh