Mooring design and selection for floating offshore wind turbines on intermediate and deep water depths

The paper focuses on design and selection of mooring systems for Floating Offshore Wind Turbines (FOWTs) for intermediate and deep water depths, based on ultimate, accidental and fatigue limit state design conditions, with reference to power production, parked wind turbine and mooring line fault conditions. Platform static and dynamic offsets are determined by a purposely developed 6DOF programme in the frequency domain, accounting for first and second order motions. The Dutch tri-floater platform, together with the well-known 5 MW NREL wind turbine, is assumed as reference design. Besides, two sites, the one in the Southern North Sea, near Dogger Bank on water depths ranging from 50 up to 80 m, the other one in the Northern North Sea, near the Troll field area on water depths ranging from 200 up to 350 m, are assumed as candidate deployment areas, to investigate the effect of: (i) line number, (ii) platform admissible offset and line scope, (iii) offshore/onshore planned maintenance inspections and (iv) mooring technology on relevant installation and maintenance costs. A preliminary cost analysis is also performed and some recommendations are given to correctly choice the most suitable mooring configuration, as a function of water depth.