A liquid containment system for an ocean-going vessel. The liquid containment system includes a tank having converging upper walls. The converging upper walls of the tank contain a substantial portion of the liquid therebetween and reduce the free surface area associated with the liquid. The upper converging walls of the tank extend above the horizontal deck of the vessel, but still allow sufficient deck space for supporting various required equipment. In one embodiment, the liquid containment system is a prismatic membrane tank designed to receive and hold liquefied natural gas (LNG).