A Reduced Draft Spar Concept for Large Offshore Wind Turbines

Abstract

This paper describes a new floater type: the Reduced Draft Spar (RDS). The RDS is in essence a spar, and so stability in operation is achieved by having the center of gravity below the center of buoyancy. Spars thus need a relevant draft and some ballast at their bottom. The RDS instead, and compared to classic spars, increases the mass below the center of buoyancy to substantially reduce the draft. This counter-intuitive approach considerably increases the overall mass of the solution. But fortunately this additional mass can be provided by cost-effective solid ballast. In the same way gravity-based structures weigh much more than jackets or monopiles yet they can still be economically feasible, the RDS is considerably heavier than classic spars. Thereby, the RDS can have the benefits of reduced draft solutions like semis while keeping the inherent simplicity of spars. The RDS concept replaces the main cylinder of classic spars by a shorter one, which is in turn held by a large caisson at the bottom of the floater. This allows the assembly of the Wind Turbine (WT) onshore and gives to the RDS enough floating stability to perform the Transport and Installation (T&I) marine operations with a significant reduction of auxiliary means. The proposed floater is made of concrete. It supports an 8 MW turbine in a generic North Sea offshore location. Besides and like in some semis, the unit is fitted with an Active Ballast System (ABS) used to compensate the environmental mean loads (mainly the WT mean thrust). In the paper, a parametric design process is used to obtain the platform main dimensions. The intact stability, both in operation and during all marine operations phases, is checked taking into consideration reasonable design margins. The dynamic response of the RDS to extreme wind, waves and currents is also analyzed. A state-of-the-art seakeeping program coupled with a simplified aerodynamic load model accounts for the effect produced by the wind dynamics on the unit response. The performance of the platform in operation is similar to that of classic spars. Therefore, the paper focuses on the study of the survival conditions. Since the platform cross section is high, survival current loads become differentiating. The dynamic loads at the mooring lines are thus analyzed to assess their feasibility in severe storm environmental conditions, which rule over the mooring design.