Risk-Based Inspection and Predictive Maintenance Strategy for Oil Tanker Ballast Tank Transverse Structures

Abstract

Ballast tanks are among the most corrosive environments in oil tankers, with progressive thinning of transverse structures in the midship region posing a significant threat to hull girder integrity. Current maintenance practices are largely prescriptive, focusing on minimum-thickness compliance without explicitly relating measured conditions to the probability and consequences of failure. This often leads to reactive decisions and uniformly allocated inspection resources. This study aim was the structural integrity risk of transverse constructions in oil tanker ballast tanks by integrating the probability of failure with its consequences, and applies the resulting risk map to develop a risk-based inspection (RBI) and predictive maintenance (PdM) strategy. The methodology involves three stages: calibration of a probabilistic power-law corrosion model using Condition Assessment Program (CAP) thickness data from a double-hull product oil tanker; computation of ultimate bending capacity and probability of failure using the incremental-iterative method of the IACS Common Structural Rules, the First-Order Reliability Method, and Monte Carlo simulation; and quantification of each structural element's risk on a likelihood-severity matrix. Results show that risk is concentrated rather than uniformly distributed: deck and inner-shell longitudinals have the highest risk scores (9), followed by deck plating, longitudinal bulkhead, and stringer plates (score 8). The probability of failure at 35 years increases from approximately 10⁻⁵ to 2.9×10⁻³ when corrosion dispersion is considered. As risk is primarily governed by the dispersion of wastage rather than its mean, the proposed risk-based strategy is more efficient than conventional uniform periodic inspection.