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

The paper deals with the catenary mooring system design for tri-floater floating offshore wind turbines. Both ultimate (ULS) and accidental (ALS) limit states are examined, under 50 and 1 year return period environmental loads. Both power production and parked wind turbine conditions are analysed; for the former the ULS is applied, for the latter both ULS and ALS are considered. The platform static demand is assessed in terms of turbine thrust, wind, current and wave steady drift forces. The dynamic offset is determined considering both wave and low-frequency motions. Mooring patterns with 6, 9 and 12 chain cable and steel wire rope lines are considered. Water depth incidence is examined in the range between 50 and 300 m and the mooring system is dimensioned so that the relevant weight is determined. The Dutch tri-floater is assumed as reference structure and three candidate sites in the Southern Mediterranean Sea are considered. It is found that platform admissible offset and line pattern significantly influence the mooring system weight; obtained results show that 9 and 12 line configurations are the necessary choice and the mooring line weight is independent of water depth between 100 and 200 m, while increases out of this range.