Renewal Scantling Evaluation for FPSO Conversions Considering Pitting Corrosion Effects

When considering an existing tanker for conversion to an FPSO with an expected service life of 15 or more years, a comprehensive reassessment of the hull structure should be conducted and appropriate corrosion margins for the hull girder and local scantlings should be established. Among all structural deteriorations incurred by trading tankers with an age of 10 years or more, corrosion wastage represents the most dominant one. There are two types of corrosion found to occur most frequently in aging tankers, namely general corrosion and pitting corrosion. In the case of general corrosion, the yielding and buckling strength of plating and structural members can be readily determined by deducting the corrosion wastage from the as-built scantling values. However, the determination of the strength reduction due to pitting corrosion is more difficult and complex, and the use of nominal loss of material may be insufficient. This paper discusses the findings from the authors’ investigations of a number of different pitting corrosion distributions, and proposes recommendations for considering pitting corrosion effects during the establishment of renewal scantlings at the time of conversion.