Actuation Options for the SpiderFLOAT Floating Offshore Wind Substructure

Platform motion in floating offshore wind turbines can produce undesirable effects on the turbine and substructure, including increased loading on critical components and excessive substructure pitch angles and tower top accelerations. Excessive platform motion can be avoided by increasing the floater size, hydrostatic stiffness, turbine control tuning, or by employing substructure actuation. This paper discusses three actuation strategies to better control the mean pitch angle of the innovative floating offshore wind turbine substructure named SpiderFLOAT: active ballasting of the substructure's buoyancy cans, actuation of the tension/stay cables, and actuation of the mooring lines. These three methods aim to minimize the mean system pitch during operational conditions. These methods must respond actively to the range of wind speeds, wave heights and directions, and misalignment of wind and waves that are present under operational conditions. The SpiderFLOAT actuation reduces loading on components of the substructure. The described FOWT substructure and the effects of these actuation mechanisms were simulated using OrcaFlex, a commercial analysis package that incorporates hydrodynamic and aerodynamic loading and structural dynamics. Initial results indicate that the substructure actuation methods are capable of achieving the desired mean pitch offset thereby bringing the mean pitch to zero and reducing some design driving loads.