Development of Corrosion Wastage Assessment Methodology for Water Ballast Tanks: An Aging Bulk Carrier

Abstract

Various environmental factors, operating conditions, transport routes, the type of transported cargo, maintenance practices, and other external and internal factors significantly influence the development of corrosion. Cargo holds and water ballast tanks are particularly susceptible to corrosion damage. This study investigates the extent to which steel thickness reduction due to corrosion contributes to the degradation of steel structures and whether this reduction aligns with the adopted Common Structural Rules (CSR). The analysis is based on an aging bulk carrier and three types of ballast tanks within the cargo hold area: top-side tanks, hopper-side tanks, and double-bottom tanks. Thickness measurements were conducted on nine specific transverse structural locations, and a corrosion wastage assessment methodology was developed based on a nonlinear stochastic model. The corrosion growth rate was modeled using a probabilistic approach where the corrosion rate parameter d0 follows a Weibull distribution. The model also incorporates 95% confidence intervals to reflect uncertainty and assess early risk exceedance relative to CSR corrosion margins. The results revealed significant differences in corrosion behavior among ballast tank areas and identified critical zones where corrosion thresholds are reached earlier than expected. The proposed methodology demonstrates its applicability in assessing structural degradation patterns and validating CSR-based corrosion allowances.