The paper presents the conversion of an oil tanker into FPSO to be operated in Gulf of Mexico (GoM) considering the strength and fatigue analyses. First, hydrodynamic analysis is performed for a typical full load condition and a typical ballast condition in order to determine hydrodynamic loads for ultimate strength and fatigue for operating as an FPSO in GoM. The section scantlings are then generated to assess the hull girder ultimate strength based on still water and site-specific wave loads. Fatigue analysis is carried out to estimate of used and remaining fatigue life, which is based on DNVGL Classification Note 30.7: 'Fatigue Assessment of Ship Structures'. The analysis comprises a check of the longitudinal material amidships, and is based on the approved drawings applying 'as measured' scantlings taken from UTM measurements. The actual trade pattern of the vessel, and other assumptions, has a large effect on the fatigue life.