A Small-Footprint Passive Constant-Tension Mooring Concept for Floating Offshore Wind Turbines: Theoretical Analysis and Experimental Investigation

Conventional catenary mooring systems for floating offshore wind turbines (FOWTs) face significant challenges related to high installed cost and large seabed footprint. To address these challenges, this paper proposes a novel length-varying tension-fixed (LVTF) mooring concept characterized by a compact seabed footprint. Theoretical formulations are developed, and parametric study results are presented on the fundamental station-keeping characteristics of a single-anchor LVTF mooring concept for a 15 MW semi-submersible FOWT. Preliminary design of a multi-anchor passive LVTF mooring system in 100 m water depth with a mooring anchor radius of 70 m is performed for a 16 MW multi-column Spar-type FOWT for potential deployment in the South China Sea. A wave basin model test campaign is conducted to investigate the station-keeping performance of the passive LVTF mooring system for the Spar-type FOWT under extreme typhoon conditions. The model test results, including platform global motions and mooring line tensions, are compared with those of the conventional catenary mooring design. It is found that the small-footprint passive LVTF mooring system can provide adequate station-keeping performance for the 16 MW Spar-type FOWT with peak mooring line tension significantly lower than that of the equivalent catenary system. A preliminary cost analysis indicates that an installed-cost reduction of up to 65% may potentially be achieved in comparison with the conventional catenary mooring design.