Qualification of a Semi-Submersible Floating Foundation for Multi-Megawatt Wind Turbines

Abstract

Abstract Recent trends in the wind industry point to the use of increasingly larger and more powerful machines with rated power ranging from 5 to 10MW exclusively designed for offshore use. Floating foundations offer greater flexibility in term of site selection for wind farms, and if properly designed, may result in comparable availability with equivalent offshore turbines on fixed foundations, while reducing the complexity and risks associated with offshore installation. The WindFloat platform is a semi-submersible platform with three columns fitted with a large horizontal water-entrapment plate at the base. The wind turbine and tower are fitted on one of the columns. The platform is designed to support commercially available multi-megawatt wind turbines with no hardware modification to the turbine. The qualification process followed for the development of a 150MW wind farm offshore Portugal is discussed. Because of economic constraints, optimization of the platform is essential to achieve project financial targets. A rational and comprehensive process was followed to optimize the system while maintaining the robustness required to survive in the offshore environment. The design process is based on a combination of advanced numerical analysis and scale model experimentation. Full-scale experimentation is ongoing. Selected design codes and industry standards are applied. The return period of extreme events is adjusted based on experience acquired by the wind industry. Because of the considerable aerodynamic loads generated by the wind turbine and their effects on platform motion, the ability to solve the combined aerodynamic and hydrodynamic problem is necessary. Additional factors, such as tower dynamics and turbine controls must also be taken into account. Development of a coupled hydro-servo-aero-elastic model constitutes a key element of the qualification process. Rules for the design of Mobile Offshore Drilling Units are followed to verify platform stability in intact condition, and to ensure that compartmentation is sufficient to withstand damaged conditions with any compartment flooded. Time-domain coupled analysis of the platform and its mooring system are performed. The hydrodynamic response is also validated with scale model tests. A four-point mooring configuration is adopted. Reliability considerations are discussed to assess the level of redundancy required in the mooring system, in particular with regards to the one-line damaged condition. The platform structural design is conducted using applicable offshore codes from several classification societies, as well as extensive finite-element analysis. Structural dynamics is critical to platform design due to the dynamic excitation of the tower and the corresponding loading of the substructure primary members.