Welcome to Task 30 –

OC4 Phase 2 (Phase II): Semisubmersible

Floating offshore wind turbines are a good test for aero-hydro-servo-elastic codes because they incorporate features not present in conventional fixed-bottom offshore support structures:

• They introduce very low frequency modes, which can affect the aerodynamic damping and stability of the system.
• They have significant translational and rotational motions of the support structure, which can couple with the motions of the rotor-nacelle assembly.
• They have a mooring system, a new component that must be considered in the overall analysis.
• They need not have a support structure that is slender and cylindrical (although this is nearly the case in the OC3-Hywind system), such that hydrodynamic radiation and diffraction can become important.

Numerous floating-platform concepts are possible for offshore wind turbines, including spar-buoys, tension leg platforms, and barges, and hybrid concepts of these. In OC3 Phase IV, the spar-buoy concept called “Hywind,” developed by StatoilHydro of Norway, was imitated. This concept was chosen because of its simplicity in design, suitability to modeling, and relevance to future commercial projects. In addition, an actual prototype exists. StatoilHydro graciously supplied detailed platform and mooring system data for a conceptual version of the Hywind platform that was developed to support a 5-MW wind turbine model. The primarily ballast-stablized Hywind turbine established a baseline for floating simulation codes but did not test the validity of these codes under conditions that match intended operating conditions for many industry floating concepts. Buoyancy and mooring-line-stablized systems need to be assessed under the same conditions that the spar was in Phase IV of OC3.

This OC4 Phase 2 will conduct a similar comparison of coupled simulation codes using an alternative floating substructure such as a tri-floater or a tension leg platform (TLP). The tri-floater will be used first because it is hydrodynamically more active and therefore may present a better challenge to the codes. However, if the participants agree, the next floating structure used may be a TLP, which has been considered by several groups in the industry. The decision will be strongly influenced by the availability of design data that can be released for public dissemination to the participants.