(197d) Modeling Underwater Penetration of Lng Carrier

An underwater breach of a tank in a double-hulled LNG carrier poses several challenging questions to be addressed by an adequate model. Initially, sea water rushes in as LNG flows out, both mixing in the available double-hull space. The vaporizing LNG could possible experience a rapid phase transition (RPT) during this mixing. After this hull space is flooded, the pressure balance favors continued LNG outflow until the level in the breached tank is lowered sufficiently. The vacuum breaker valve capacity may not be adequate to avoid head-space pressure decrease even accounting for LNG flash evaporation in the tank. At a point the pressure balance across the breach will allow inflow of water into the tank. As this incoming water freezes, the released heat of fusion vaporizes some LNG inside the tank. This added evaporated vapor acts to raise the pressure in the tank and thereby oppose further water inflow. An oscillating flow pattern is likely from pressure considerations. In addition, freezing water can build up near the breach and constrict LNG outflow and water inflow.

This paper describes the construction of a model that accounts for the following phenomena: 1) LNG tank head pressure decreasing by the piston effect of outflow; 2) Internal evaporation and air inflow from a vacuum breaker; 3) LNG and sea water filling the hull space to flooding; 4) LNG evaporation in the hull space and outflow through the outer breach; 5) LNG outflow and water inflow by pressure balance consideration at the inner breach; 6) the effect of water inflow evaporating LNG and the resulting pressure increase inside the tank. The model does not allow for ice build up at the inner breach constricting flow. Predictions show tank pressure fluctuations, LNG outflow rates, and water inflow rates over time.