The performance of the ship hull deteriorates randomly over time under corrosion attacks. To ensure the safe operation of a ship, dry-docking inspections are carried out on a regular basis to inspect, recoat, and renew structural members. The convention “at least two inspections every five years”, implemented in the shipping industry, is empirically determined without much numerical evidence. Considering the expensive cost of drydocking inspections, it is crucial to study the optimal inspection interval in the presence of uncertainty. This paper proposes a risk-based maintenance decision-making framework for ships to address the optimal drydocking inspection. The minimum expected cost rule is used to explore the economically optimal inspection interval. Monte Carlo simulations are employed to obtain the probability distribution of the life-cycle cost. The costs considered include the cost of dry-docking and member renewal as well as monetary consequences of hull failure. A ship hull is utilized to illustrate the application of the proposed framework.

Introduction

The loss of a ship has significant implications in terms of environmental impact, economic loss, and crew casualties. Metal loss corrosion is a primary concern for the safety of aging ships [1,2]. By decreasing the modulus of the hull cross-section, growing corrosion reduces the ultimate strength against external loadings. To maintain the satisfactory integrity of the hull, ships are dry-docked for inspection at a minimum twice in a 5-year period [3]. During the inspection, the thickness of the structural members of the hull is measured using non-destructive tools (e.g., ultrasound inspection), structural renewal is performed for the critical members and new protective coatings are applied to the hull surface to protect against corrosion [4]. Dry-docking is an expensive process and accounts for the largest maintenance cost throughout the ship service life. The cost of one dry-docking can be as high as $0.2 M to $0.7 M [4]. Dry-docking can also adversely affect the flexibility of operational schedules by taking a ship out of service. It is of paramount importance to explore a cost-effective inspection schedule that allows the ship to stay longer in water while guaranteeing an acceptable level of safety. Determining the optimal inspection is not a simple task. Various uncertainties are associated with the performance of the hull. Specifically, the bending moment induced by still water and sea waves experienced during one voyage is uncertain; the spatial variability of geometric and material properties associated with different structural members, the uncertainty associated with the prediction of bendingresistant capacity, and the stochastic corrosion growth result in the uncertain structural performance of the hull.