Upper Engine Room - Main Deck Level. Initial view: Gray tank is the DC (direct contact) heater which heats and deaerated feed-water on it's way to the boilers. Panning to the right, The white tank is the main engine lube oil gravity tank. Oil is pumped up to and gravity fed from this tank to the main engine bearings. In case of power failure at full speed, this tank provides approximately 5 minutes of oil pressure by gravity to the main engine so the shaft can be stopped before damage occurs.