Bottom-supported tension leg towers with inclined tethers for offshore wind turbines

Abstract

Offshore wind with high energy density and low turbulence has attracted the global scientific community towards the development of the Offshore Wind Turbines (OWTs). Today, most of the shallow water sites are already occupied, and the industry is looking to install turbines at the deep water sites (water depth > 50m). However, an OWT support structure designed with the conventional concepts becomes massive and expensive in deeper waters. In contrast, floating OWTs are more expansive than the bottom supported ones, and considered suitable for a water depth of 60 m or more. A novel support structure concept termed a 'bottom supported tension leg tower' (BSTLT) has been proposed recently and the possibility of using bottom supported structures in deeper waters is shown. In this paper, an extension of the BSTLT concept has been achieved using inclined tethers with a monopile. The preliminary technical feasibility including the sensitivity of the natural frequencies and dynamic analyses considering the in-place operating and the extreme design conditions has been studied for 50 m water depth. The results show that the proposed concept can be utilized to make the monopile based support structures more costefficient for the water depth stated above.