Simulating an ultimate bending moment of ageing hulls: a bulk carrier case study

Abstract

Ship structural assessments generally consider service loadings anticipated in design phase. Bending moments corresponding to these conditions are then evaluated based on yielding and buckling criteria derived from the allowable stress threshold and linear-elastic behavior of the material. Still, ships might experience extreme and once-in-a-lifetime events such as overloading, extreme bow waves, or grounding. When tied with ageing process resulting from corrosion, these events can accelerate structural deterioration, leading to the catastrophic hull failures, such as “brake-in-two” or loss of the ship. Unfortunately, numerous hull collapses have been recorded in recent years. The collapse of the hull girders (i.e., ultimate strength) of large deep-sea ships is primarily determined by the maximum or ultimate bending moment the hull can withstand. These are highly non-linear problems in terms material behavior and geometry. Therefore, the industry and classification societies have recognized an issue and currently are in the process of developing sophisticated ultimate strength assessment procedures in their technical standards. Most notable ones are delivered for bulk carriers and oil tankers in Common Structural Rules (IACS CSR). The work presented here is a simulation of the ultimate strength performed on a case study of an intact and corrosion-induced ageing bulk carrier, by using two numerical methods: progressive-collapse analysis defined by IACS and nonlinear finite element method. The nature of the hull girder collapse is presented along with the extent of corrosion impact of the reduction of ultimate strength.