Optimization of Semi-Submersible Hull Design for Floating Offshore Wind Turbines

Abstract As wind energy developers start venturing into deeper waters with depths greater than 65 meters, the floating offshore wind turbines (FOWTs) are becoming the preferred solutions there over bottom-fixed structures. This paper summarizes the design of a semi-submersible platform for hosting a 15MW turbine. While semi-submersible origins from the oil & gas industry, the FOWT presents a different set of requisites and conditions for designing it. Recent existing and planned projects can be characterized by excessive use of steel and dimensions, compared to bottom-fixed structures. This paper aims to optimize the hull structure of a semi-submersible platform to be as cost-effective as possible while fulfilling the following design considerations: strength, vessel stability in still water and dynamic conditions, constructability, and operability. Meanwhile, the design is made to satisfy the rule requirements of major classification societies. Through a literature survey, data on existing semi-submersible projects is gathered and analyzed. The ratios of vessel displacement to hull steel weight relations are presented to show the trend. A high-level overview of class rules is given. An example platform, TaidaFloat, designed to carry a 15MW turbine is introduced. Its application is targeting a water depth from 65 to 100 meters with a deployment in Taiwan Strait in mind. The platform is designed against some limits and constraints, e.g. ensuring that it meets stability criteria. Its internal structural arrangement is preliminarily developed, and the steel weight is compared to recent projects. Features of the design are introduced, and their advantages are summarized.