Ultimate and accidental limit state design for mooring systems of floating offshore wind turbines

The paper deals with the design of mooring systems for tri-floater offshore wind turbines. Both ultimate (ULS) and accidental (ALS) limit states are examined; correspondingly 50 years and 1 year return period environmental loads are associated with. Two turbine conditions, namely power production and parked wind turbine, are analyzed; to the first condition the ULS is applied, to the second one both ULS and ALS are considered. The platform static demand is assessed in terms of static forces due to turbine thrust, wind and current drag forces and wave steady drift forces. The platform dynamic offset is determined considering both wave-frequency linear and low-frequency second order motions. Mooring patterns with 6, 9 and 12 l9nes are considered; both chain cables and steel wire ropes are assumed for the mooring lines. Water depth incidence is examined in the range between 50 and 300 m. In each case the mooring system is dimensioned in such a way that the mooring system weight is deternined. The Dutch tri-floater design is assumed as reference structure and three candidate sites in Southern Mediterranean Sea are considered. It is found that both platform maximum admissible offset and line pattern significantly influence the overall weight of the mooring system.