It is demonstrated that tank overflow can occur on board a tanker ship due to the communication of cargo tanks containing dissimilar grades of crude oil. The fundamental principles, which enable the flow from one tank to another and the overflow from exhaust devices on top of the tanks, are reviewed along with the governing equations. The case is analyzed firstly for two communicating tanks in terms of the final equilibrium state and of the time scale of the effect until equilibrium is established or tank overflow is occurred, considering all geometrical and liquid transfer parameters. Measures are proposed to prevent the effect from being initiated and to mitigate the results of the effect once it is occurred. This necessarily leads to the need to extend the study to three communicating tanks. The final equilibrium state for three, and potentially more, tanks is analyzed, which allows a pragmatic approach as to how the situation should be handled on board a tanker facing a tank overflow due to density differences of multiple cargo grades. An extended and generalized Risk Analysis model is proposed to supplement existing models.

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