Post-Katrina performance of a turret-moored floating storage and offloading unit in the Gulf of Mexico

Abstract

Conventional design of ships and offshore platforms relies on performing a short-term response analysis for metocean conditions with a 100-year return period. This approach is efficient but not necessarily conservative when compared with a comprehensive long-term response analysis that considers platform responses for storms encountered over the lifetime of the platform. Turret-moored floating production, storage and offloading systems are sensitive to non-collinear wind, waves, and current conditions. During hurricanes in the Gulf of Mexico such conditions frequently occur and, depending on the resulting weathervaning characteristics of a turret-moored tanker, they may have a strong impact on vessel motions, mooring line loads, and riser performance. In recent years some very strong hurricanes have occurred in the Gulf of Mexico, e.g. Katrina and Rita in 2005. When including these recent storm data in the long-term response analysis they have a marked effect on the extrapolated 100-year long-term responses. The post-Katrina long-term responses of a generic turret-moored floating storage and offloading unit are evaluated and compared against pre-Katrina analysis results. The results of the analysis are used to stipulate response-based design criteria which are simple short-term design sea states that can reproduce a given long-term response (e.g. roll).