Time-varying ultimate strength of aging tanker deck plate considering corrosion effect

This paper presents a semi-probabilistic approach to assess the time-varying ultimate strength of aging tanker's deck plate considering corrosion wastage. The procedure includes (1) defining the limit state function of deck plate failure, (2) determining the time-varying probability density function of corrosion wastage and values for selected severity levels of corrosion, (3) calculating the time-varying ultimate strength of deck plates corresponding to the selected levels of corrosion wastage, (4) determining the time-varying maximum nominal stress, (5) calculating when the deck plate reaches the ultimate strength limit state, and (6) determining the inspection intervals based on the risk of ultimate strength failure. A nonlinear corrosion model was proposed for deriving the time-varying probability density function of deck plates' corrosion wastage. The probability density function was determined based on a statistical analysis of the American Bureau of Shipping (ABS) corrosion wastage database [Wang G, Spencer J, Sun HH. Assessment of Corrosion Risks to Aging Oil Tankers In: 22nd International Conference on Offshore Mechanics and Arctic Engineering, Cancun, Mexico, 8–13 June 2002]. To simplify the calculations of ultimate strength, a semi-probabilistic model was adopted. Three levels of corrosion severity were considered, and the corresponding values of corrosion wastage were used instead of the probability density function. The increasing nominal stresses as ships age were also taken into account considering the loss of global hull girder section modulus. The loads of still-water-induced and wave-induced bending moments were treated as deterministic values. The ultimate strength calculation was based on the latest International Association of Classification Societies (IACS) – Common Structure Rule (CSR) formula [Common Structural Rules for Double Hull Oil Tankers, http://www.iacs.org.uk/; 2007]. Deck failure was defined as deck plate's ultimate strength becoming lower than the maximum nominal stress. An inspection should be conducted before such failure takes place. A total of nine sample tankers, designed in 1970s, 1980s and 1990s were selected for demonstrating this approach. Time to deck plate's failure by ultimate strength varies in a wide range depending on the initial designs and corrosion severity at both local plate and global hull girder levels.