Statistical properties of tanker net section modulus

Section modulus is affected by several uncertainties due to structural product manufacturing tolerances, fabrication imperfections and corrosion depths. As a matter of course, the section modulus is a random variable. With reference to corrosion effects, the actual Rules adopt the so-called net scantling approach, for the strength checks of platings, ordinary stiffeners and primary supporting members: given corrosion additions should be added to the net scantlings to obtain relevant gross thicknesses. On the other hand, for the assessment of the hull girder section modulus the gross scantling approach is adopted or the sectional properties have to be calculated by deducting half the corrosion additions from the gross thicknesses of the members contributing to the longitudinal strength. In this paper, the significance of this design procedure is examined in light of the statistical properties of section modulus, evaluated according to a purposely developed procedure and the corrosion wastage model, and corresponding statistical data, proposed by Paik et al, 2003, for oil tanker structures. In particular, the calculations have been performed for a Floating Storage Offloading structure, for which relevant geometrical data were available as well as a significant set of gauged thicknesses. Obtained results are fully discussed.