Monolithic concrete off-shore floating sturcture for wind turbines

Abstract

A new concept of a SPAR floating platform is being developed in the KIC-Innoenergy project AFOSP (Alternative Floating Platform Designs for Offshore Wind Turbines using Low Cost Materials). Members of the consortium are Gas Natural Fenosa, University of Stuttgart and Universitat Politècnica de Catalunya. The main differentiating aspects with respect to other SPAR prototypes are the monolithic nature of the whole structure, including both, the platform and the tower, the use of post-tensioned concrete as main material and the installation process. A comparison between similar steel and concrete designs demonstrates that the material cost for the concrete structure is around one third of the steel one. Considering that the concrete Oil & Gas platforms are virtually free of maintenance and have an extended lifetime, the real cost is less than 1/3 of the steel one, while the offshore tasks and the moorings systems have similar costs for both alternatives. Following a hydrostatic pre-design of the structure, coupled aero-servo-hydro-elastic analyses for a 5MW wind turbine have been performed by using an in-house FOWT model with a reduced number of degrees of freedom (DOF) and simplified aero- and hydrodynamics. With the obtained loads, the most relevant structural members have been checked, including a simplified fatigue limit state analysis, obtaining a lifetime over 50 years. Finally, a comprehensive cost analysis has been performed to obtain the Levelized Cost Of Energy (LCOE) for the developed platform.