The Reduced-Draft Spar: A Novel Cost-Effective Concept for Floating Offshore Wind Turbines

This paper describes the conceptual design and estimates the CAPEX breakdown of a novel floater type for offshore wind turbines: the Reduced-Draft Spar (RDS). This floater, which resembles a GBS although it is in essence a spar, has excellent seakeeping at a reduced draft. Aiming at reducing the CAPEX, the RDS design allows the installation of the wind turbine at the manufacturing site. Furthermore, no auxiliary means are required to provide stability during towing and in-place installation (mooring and electric hook-up, and ballasting to the operational draft). It also promotes the use of structural concrete and cost-effective high-density ballast for its construction. In addition, the whole concept benefits from the implementation of an Active Ballast System (ABS) to compensate the mean tilt angle while operating. An extensive model test campaign was carried out in summer 2018 at INTA-CEHIPAR model basin to validate the concept. The tests scope was focused on calibrating a state-of-the-art numerical hydrodynamic model for further stages of development. An 8MW RDS model, with a scale factor of 1:50 and a 3-line spread mooring system, was tested in Transport and Installation (T&I), operational and survival conditions to assess the concept feasibility. The ABS was simulated using pre-calibrated counteracting weights. Stability during T&I was also checked. In addition, the unit CAPEX was estimated and compared to equivalent semi and spar units made of steel. The RDS can operate at intermediate water depths (60 to 80 m, where spars cannot) and deep waters, and also avoids the use of expensive auxiliary means for T&I. The results from the model tests confirm a dynamic behavior of the RDS similar to that of classic spars, which is beneficial for the offshore WT. Regarding the CAPEX, estimations indicate relevant savings compared to classic spars or semis. Although it is a large massive unit, the use of concrete combined with heavy ballast makes the concept feasible. Due to the ABS, the required platform's size is smaller and the fatigue life of the WT components increases. Furthermore, an adequate ABS control system increases the net energy production since the energy consumption is negligible compared to the extra generated power. The use of civil construction manufacturing technologies such as floating docks and assemble of pre-manufactured parts leads to major CAPEX savings. The US coast has a huge offshore wind energy resource at water depths greater than 60m, where the RDS floating concept has a promising future. The concept could be used as well in the Offshore Oil& Gas.