Tanker Conversion into FPSO vessel: Part 1 – Longitudinal Strength Assessment of the Candidate Tanker

Longitudinal strength assessment is a regulatory requirement that precedes Tanker conversion to FPSO vessel in order to verify capacity and reveal the extent of renewal necessary for structural members of Tanker for the new service. This paper therefore presents Part 1, Longitudinal strength assessment of a Tanker “MV Energy concentration” for the actual conversion project that will be presented in Part 11. Use is made of elastic-plastic principles together with IACS (International Association of Classification Societies) Common Structural Rules and DnV design Codes for the prescribed analyses. Results reveal that the candidate Tanker requires structural modification in a few but critical structural elements for the new service. The section modulus for deck Longitudinals determined as 332543.296cm3 fell short by about 94% relative to the Rule required minimum value requiring serious intervention as do the deck girders with a Section modulus of 559421.4579 cm3 and capacity inadequacy of 99% relative to Rule required minimum. The deck plate thickness requirement also fell short by a minimum of about 30% requiring intervention. Conclusively, even with the confirmed buckling capacity adequacies of the longitudinal bulkheads and side-shells and section moduli at the deck and keel, some level of renewal is still necessary considering the effect of the severe site-specific environmental forces when being converted into FPSO and constrained to maintain position without dry-docking. Besides this, a very conservative analysis has been conducted as corrosion wastages were not considered and so, renewal is recommended as actual values will be lower than reported