Structural assessment and platform actuation of a novel semi-submersible floating offshore wind turbine substructure

Abstract

Floating offshore wind turbines (FOWTs) represent a valuable resource as governments plan how to transition from fossil fuels to renewables. However, there are both technical and economic challenges to overcome before floating offshore wind becomes a commercially viable option. Due to high deployment, capital, and development costs (due to the relative novelty of the technology), floating offshore wind currently has a much higher energy cost than other renewable options such as fixed bottom offshore or land-based wind turbines. The design of FOWTs also presents many engineering hurdles, such as maintaining platform stability without excessive system mass and cost, keeping the Levelized Cost of Energy (LCOE) competitive with other energy production methods, and ensuring the structure can withstand environmental loading from waves, wind, and turbine actions. SpiderFLOAT is a novel semi-submersible floating offshore wind substructure designed by the USFLOWT team under the ARPA-E ATLANTIS program. SpiderFLOAT is a flexible substructure consisting of a central column with three legs attached by moment-free connections and supported by tensioned stay-cables. Buoyancy cans are attached at the ends of the legs. This structure is designed to shed loading via structural compliance, reducing the structural requirements to sustain wind and wave loading in an offshore environment. This keeps system mass, and by extension material costs, lower than other FOWT substructure designs. This thesis investigates a few SpiderFLOAT substructure alterations to the design to mitigate loading and improve system stability while keeping system mass low. Two types of design alterations were considered: alterations to the structural geometry and the introduction of substructure actuation mechanisms. Structural alterations included system draft, buoyancy can size, stay cable attachment points, and the number of legs. Actuation mechanisms included the tension of the stay cables, the ballast level in the buoyancy cans, and adjustments in mooring-line length and thus tension.