Presentations

Abstract

This study investigates the techno-economic benefits of using steel wire ropes (SWR) or synthetic ropes such as polyester ropes (PR) and high-modulus polyethylene (HMPE), whichever is most suitable, for the station keeping of floating offshore wind turbines (FOWT). Three floaters are examined under harsh (OA-class) and moderate (OB-class) environmental conditions, supporting the DTU 10MW reference turbine [1]: an up-scaled OC4 semi-submersible [2], an up-scaled OC3 spar buoy [3] (both originally designed for 5MW), and a tension-leg platform (TLP) [4]. The semi-submersible and TLP were evaluated at 200 m water depth, and the spar buoy at 300 m. For the semi-submersible and spar-buoy configurations, three mooring system variants were designed: a conventional chain-only catenary system, and two hybrid catenary designs combining chain with either steel wire or polyester ropes. For the TLP, steel wire ropes were compared against HMPE alternatives. All mooring systems feature redundant layouts (3×2 for semi-submersible and spar-buoy, 3×3 for TLP) with drag-embedded anchors. The comparative assessment focused on key performance indicators (KPIs) including mooring lines material cost and dynamic tension, platform motions, and wind turbine loading. Results confirm that all examined mooring solutions are technically feasible. Hybrid catenary systems and TLPs incorporating SWR or synthetic ropes proved effective for station-keeping. Chains predominantly failed due to fatigue, whereas ropes were governed by ultimate strength. The effectiveness of rope-based solutions was found to depend on multiple interdependent parameters, including allowable offset, pretension requirements, and installation site. Economically, hybrid systems showed significant promise, particularly in terms of material cost reduction. Rope properties (e.g., lower weight) may further reduce installation, transportation and maintenance costs, though this warrants further investigation. Overall, rope-based mooring systems offer a strong balance between dynamic performance and cost efficiency for FOWTs, provided that site-specific and floater-dependent constraints are properly addressed.