Probabilistic Assessment of the Risk of Cargo Shifting Onboard Ships in Waves

Doktorsavhandling, 2003

This thesis is the result of work within the research project "Probabilistic Models for Ship Safety in Rough Seas". One major danger in sea transportation is the shifting of cargo. The aim of the project has been to develop a methodology for assessing the risk of cargo shifting as well as performing some case studies. The idea has been that a more probabilistic approach to the problem of cargo shifting is needed. In the methodology the interaction of wave, ship and cargo is modelled and combined with statistical methods in order to estimate the risk of cargo shifting. The main aim has been to assess the long-term risk of cargo shifting for a vessel on a specific route, but also short-term risk has been addressed. Linear waves and ship motion transfer functions are used to calculate the ship motion induced forces acting on a cargo unit. A basic model of a container has been used for determining shifting criteria. Comparing time series of the ship motions induced forces with the shifting criteria gives the probability of shifting for a cargo unit. During its lifetime a ship will encounter different conditions, in which the probability of cargo shifting will differ. Defining such conditions as unique combinations of wave, ship and cargo parameters, the probability of occurrence of each condition can be estimated from wave statistics and knowledge of the operating profile of the vessel. In each condition the probability of at least one cargo shift can be estimated, and in combination with the frequency of occurrence the total risk of cargo shifting can be estimated. Computational examples have shown that the influence of wave, ship and cargo parameters can be assessed, especially with short-term risk.