Review of single and double hull tanker designs has been conducted to identify the relevant aspects of the hull structure in the context of conversion into future FPSO systems. Typical designs have been selected and structural analyzes procedures for verification of the hull girder strength and the primary and secondary structures have been reviewed regarding yielding, buckling and fatigue strength. The typical defects and the main degradation modes based on the return of experience with tankers are also identified and a set of hull design parameters are described in order to evaluate the hulls candidates to conversion.

The paper provides guidelines to assist the selection of trading tankers for conversion into FPSO. Based on a set of hull design parameters, different hull configurations are evaluated and the respective advantages and disadvantages assessed in order to identify the best candidates for conversion.

Reserve of strength and corrosion margins have to be analyzed for each hull and compared with the FPSO project requirements, including field design life, environmental conditions and corrosion rates. Lists of typical defects and hazards based on the return of experience with tankers are also discussed. The paper also discuss the different designs based on the several parameters analyzed in order to provide guidelines to assess the hull structure condition, estimate the repair work at conversion and the inspection effort along the FPSO life.

FPSO conversion projects based on existing trading tankers are still alternatives to new constructions for developments in areas like West Africa, Brazil and South East Asia. Nevertheless there are few tankers built before 1985 still available for conversion. Consequently single hull tankers built after mid 80s and double hulls built after early 90s became natural candidates to conversion into FPSO. Nevertheless, the decision making process regarding the selection of such hulls faces the higher cost of refurbishment of single hulls during the conversion and the high cost of purchase of more recent double hulls.


  • FPSOFloating Production, Storage and Offloading system

  • FSOFloating, Storage and Offloading system

  • HTSHigh Tensile Steel

  • IMRInspection, Maintenance and Repair

  • MARPOL International Convention for the Prevention of Pollution from Ships

  • MIC Microbial Induced Corrosion

  • SBT Segregated Ballast Tank

  • SWBM Still Water Bending Moment

  • SWSF Still Water Shear Force

  • VLCC Very Large Crude Oil Carrier

  • VWBM Vertical Wave Bending Moment

  • VWSF Vertical Wave Shear Force


Although we see a shift towards new-build FPSO's, in particular for developments in harsh environment conditions, conversion seems to remain the basis for several projects in areas where benign environmental conditions are predominant, such as West Africa, Southeast Asia and Brazil.

Therefore, the possibility of fast track schedules to have an early first oil date is also a important parameter in the decision process for selection of the floating unit type of hull: converted or new-build.

Data concerning VLCC tankers built between 1973 and 2004 have been reviewed in order to identify the main types of oil tanker still operating and that may be available for FPSO conversion projects.

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