An underwater breach of a tank in a double-bulled LNG carrier poses several challenging questions to be addressed by an adequate model. This article describes a model that accounts for the following phenomena: (1) LNG tank vapor space pressure decreasing by the piston effect of outflow (the effect a decreasing water level pulling down the pressure); (2) LNG evaporation in the bull space and vapor outflow through the outer breach: (3) LNG outflow and gas inflow to the LNG tank at the inner breach; (4) Ice formation by LNG evaporative cooling. The model does not allow for localized ice build up at the inner breach constricting flow. Predictions show tank pressure, LNG outflow rates, water inflow rates over time, the duration of the event, and total LNG losses. Losses are shown to be considerably less with double-bull than with single bull construction. (C) 2008 American Institute of Chemical Engineers Engineers Process Saf Prog 27: 336-344, 2008