Dynamic analysis of a floating hybrid spar TLP concept for wind monitoring applications in deep sea

Abstract

This paper presents an investigation to assess the motions experienced by a floating hybrid spar-tension leg platform (TLP) structure when supporting a wind monitoring lattice tower in deep water conditions of the Central Mediterranean through a numerical study using the software package ANSYS® AQWA™. The structure supporting the tower involves a four-legged system with a single cylindrical spar used to provide the required buoyancy and retain the four tethers under tension. The four horizontal spokes linking the spar to the tethers are located above water level to facilitate the installation of a LiDAR System and guy wires for supporting the wind monitoring tower. A parametric analysis was carried out for different geometrical and met ocean conditions, and the simulations were restricted to regular (single frequency) wave and constant wind speed conditions only. The Morison formulation was used to resolve the hydrodynamic loads in a time domain. The study shows how the natural periodic times of the floating wind-monitoring mast structure decrease with increasing buoyancy-to-weight ratios. From the time-series simulations on the floating structure it was evident that slender spars experience smaller displacements. This is a favourable result as it means that it would have a lower impact on the wind measurements taken by the cup-type anemometers installed on the tower. Finally, a sensitivity analysis was carried out to examine the variations of surge motion predictions resulting from deviations in the hydrodynamic coefficients. It was observed that the platform surge motion is more sensitive to deviations in the added mass coefficient than the drag coefficient.