A Multistate Approach to Reliability Analyses of a Ship Hull Structure

Structural damage in the form of corrosion, fatigue, damage, cracks and fouling can significantly reduce the structural integrity of ships in navigation and decrease the navigation safety. Therefore, numerous studies aim to improve international rules and regulations, and ensure proper maintenance of ships and timely inspections. Classification societies, flag states and port states strive to conduct appropriate inspections of ships with the aim of preventive detection of defects. Through the application of International Safety Management system, companies strive to improve maintenance systems, monitor the condition of ships and conduct risk assessments to reduce potential accidents and negative consequences for people, material goods and the environment. By means of advanced Structural Health Monitoring, the observation and analysis of the physical, chemical or electrical characteristics of components or systems are conducted over time. Such examinations tend to identify the variations that lead to degradation of the current or future performance of the inspected systems and their components. The condition of hull structures is monitored by mandatory requirements which are prescribed by classification societies e.g. the number of thickness measurements of hull structure areas and elements. The measurements create extensive databases that are further used to monitor the condition of existing ships and predict their future conditions. This study relies on a database of 25 bulk carriers aged between 5 and 25 years. The measurements were performed during regular special surveys on the total of 110 fuel tanks located in double bottom area of ships, which provided 3070 measured data in total. The values of the thickness diminution of steel plates due to corrosion were obtained through the measurements of the thickness of the steel plates by means of ultrasound thickness gauging equipment, in accordance with the rules of classification societies. Based on those rules and allowable substance and extensive corrosion, the paper considers the excessive corrosion values (above 20%) that were identified as failures and required the replacement of corroded surfaces. The multistate approach to the reliability analysis of the steel plates of inner bottom plating and the improvement of reliability after critical conditions showed that the usability of the analyzed ships significantly dropped after 15 years of exploitation.