Anchor loads for shallow water mooring of a 15 MW floating wind turbine—Part II: Synthetic and novel mooring systems

Abstract

Novel and synthetic mooring tethers can enable tuning of a floating wind turbine’s mooring system stiffness behaviour, eliciting specific desirable response and load characteristics and potential cost reductions. This paper presents the results of detailed coupled dynamic simulations considering the IEA 15 MW turbine and VolturnUS-S semi-submersible reference platform deployed in the Celtic Sea. The influence of synthetic and novel mooring tethers on anchor loads and platform dynamics in shallow waters is evaluated. Through comparison to results published in Part I, this study highlights that replacing chain with polyester ropes or novel mooring tethers effectively reduces the peak anchor loads by up to 84% and 75% respectively for the smallest footprints (293 m), and up to 80% and 59% respectively for the largest footprints (838 m). However, this results in significantly increased platform excursions, at times exceeding the 25 m allowable limits. The weight of the compliant novel mooring reduces the excursions whilst maintaining lower loads profiles. The excursions can be further minimised by deploying taut moored systems, requiring the anchor to sustain higher loads including, importantly, vertical loads. This paper informs mooring and anchor design optimisation for large-scale floating wind turbines while also advising future research into taut moorings for floating wind turbines.