The offshore oil industry has taken a major option for FPSO units adapted for the complete range of water depths and environmental conditions. Solutions based on new purpose built designs or conversions of existing tankers are available depending on the strategy of oil companies. The cost related to the FPSO hull may be less important compared to the costs related to topside and sub-sea equipments. However, structural issues experienced on both purpose built FPSOs or converted oil tankers draw the attention to the importance of the strength and mainly fatigue verification of the hull in the design / conversion stage and show the vulnerable steps during tight schedules. Repair / reinforcement activities are much more expensive when performed offshore compared to shipyard. The fatigue phenomenon of the hull structure is strongly dependent on the detail design and on the protection against the corrosion inside the ballast and cargo tanks. Codes based on simplified approaches are developed to scan potential problems on connections of longitudinal stiffeners throughout the unit length using nominal stress methodology and stress concentration factors. More extensive direct 3D finite element analyses are necessary in order to prevent failures which cannot be treated within the simplified approaches. This document presents the methodology of analysis, lessons learned and examples coming from the performance of existing FPSOs (purpose built or conversions) and the implications of the hull structural design of the FPSO's on the final maintenance costs

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