Comparative study among non-redundant and redundant stationkeeping systems for floating offshore wind turbines on intermediate water depth

Abstract The paper focuses on the design and selection of mooring systems for Floating Offshore Wind Turbines, deployed on intermediate water depths, and provides a comparative analysis among non-redundant and redundant layouts, consisting of 3 and 9 lines, respectively. After developing the non-linear time-domain hydrodynamic model, the Ultimate, Accidental and Fatigue Limit State Design Conditions for mooring systems are briefly discussed. Subsequently, an improved optimization procedure, devoted to detect the minimum weight configuration of the mooring system, constrained to the fulfilment of several restraint criteria, is outlined and applied to a reference case study. The OC4-DeepCwind semisubmersible tri-floater platform, equipped with the 5 MW NREL wind turbine, is assumed to be deployed in the Northern North Sea. The optimization procedure is embodied to preliminarily detect a set of candidate configurations, satisfying the Ultimate and Accidental Limit State design conditions, that are subsequently checked to assess the expected fatigue life. The preliminarily design of anchor foundations is also performed. Finally, a cost analysis is carried out to investigate the impact of the line number on the total installation costs of the stationkeeping system. Some considerations, concerning the impact of line number on the expected cost due to maintenance operations, are also provided.