Cargo Liquefaction and Influence on Ship Stability

Fine grain solid bulk cargo with “sufficient” moisture content may undergo liquefaction during a voyage, posing a danger to the bulk carrier and the crew on-board due to its potential to shift and adversely affect the vessel’s stability. Between the years 2005 to 2017, it is believed that at least 21 bulk carriers have been lost due to cargo liquefaction. Most of these vessels are of 60,000 deadweight tonnes (DWT) and below, i.e. belonging to the “handysize” class. At the present moment, liquefaction is thought to occur through either conventional liquefaction or dynamic separation. In the former, wet granular cargo particles are rearranged through cyclic loads induced by the ship’s motions, resulting in overall compaction of the cargo and a corresponding increase in pore pressure between the particle grains. Shear resistance of the cargo pile decreases and movement of significant portions of the liquefied cargo material may occur, which in turn poses significant risks for the vessel. In dynamic separation, a pile of wet granular cargo particles undergo progressive transformation through intermediate stages, where the moisture separates from the cargo pile, forming fluid slurry comprising water and entrained particles that would be denser than water perched on top of a drier, compacted particle pile. The slurry will slosh with the vessel motion adversely influencing the stability of the vessel. Compared to conventional liquefaction, the compacted particle pile is drier and less susceptible to shift under vessel movement. In this study, a numerical modelling to assess the impact of the two cargo liquefaction mechanisms on a vessel’s stability is undertaken. The numerical models will be described and the results will be discussed.