Assessment of Intact Stability Criteria and Survivability for Semi-Submersible FOWT

Abstract This study evaluates the survivability assessment methodologies for semi-submersible Floating Offshore Wind Turbines (FOWTs) against capsizing and sinking risks. Current industry standards employ conservative stability criteria derived from offshore and marine practices, potentially leading to over-designed structures despite FOWTs’ lower risk profile as unmanned facilities. Through a case study of a three-column semi-submersible FOWT stationed with 9 mooring lines, this research compares two stability assessment approaches: the traditional area-ratio-based method and the dynamic-response-based criterion. Results from comprehensive stability analyses show that while the semi-submersible marginally satisfies the area-ratio-based survivability requirement with a ratio of 1.31 (versus required 1.30), it demonstrates substantially higher resistance to capsizing and sinking under dynamic-response-based assessment, achieving a ratio of 2.99 (versus required 0.10). These findings indicate that current survivability assessment by area-ratio method may be overly conservative in evaluating FOWTs’ resistance to capsizing and sinking. The adoption of dynamic-response-based survivability assessment methodologies could enable more accurate evaluation of semi-submersible FOWTs’ resistance to capsizing and sinking, potentially leading to optimized designs that maintain adequate survivability while reducing structural costs—a critical factor in enhancing the economic viability of floating offshore wind energy development.