System reliability of corroded ship hull girders

Abstract

Reliability assessment is an indispensable task in risk-based corroding ship maintenance planning. For simplicity and computational efficiency it is commonly assumed that the reliability of a ship is computed as a component-based reliability problem defined by the plastic collapse of the one-bay section at mid-ship. However, this assumption may underestimate the likelihood of ship failure since there are many bays of sections subjected to very high vertical bending moments near the mid-ship and the failure of any section can lead to the loss of the entire ship. The failures of the individual one-bay sections are spatially dependent due to the spatial variability of variables, such as corrosion and yield strength, among others. Therefore, system-based reliability is more appropriate for describing the safety level of ships under uncertainty. This paper utilises an improved first-order reliability method (FORM) recently developed to assess ship system reliability. The methodology developed is demonstrated on a double hull girder tanker subjected to corrosion. The effect of the spatial dependence among the variables associated with different sections on the computed failure probability of the ship is studied by parametric analysis. The results indicate that the dependence of the model error for predicting the bending capacity associated with different sections impacts the failure probability significantly, and the underestimation of this probability can be non-trivial if only a single one-bay section is considered.