U.S. Navy Ship Salvage Manual Volume 6 - Oil Spill Response

S0300-A6-MAN-060Damaged deep tanks on ships will not necessarily spill all their contents. If a deep tank containingoil less dense than the surrounding water is holed below the waterline, oil flows out until the oilwithin the tank reaches a level where the sea pressure and oil pressure balance as shown in Figure3-6. The depth of oil above the hole is deeper than the depth of water outside the tank because ofthe oil’s lower density. Some additional oil will flow out as the relative heights of the oil-waterinterface and the upper edge of the hole fluctuate because of passing waves or vessel rolling,establishing a shallow water bottom beneath the oil. For ships stranded in tidal areas, additionaloil will flow out as the water level falls below that at the time of the casualty. Little oil will bespilled on subsequent tide cycles, unless later low tides are lower than the previously experiencedlowest water level. It was estimated that in the EXXON VALDEZ casualty, approximately half theoil spilled flowed out of the damaged tanks in the first 20 minutes after the grounding, while mostof the remaining oil spillage occurred over the next 24 hours as the tide rose and fell.It is not always necessary to empty a damaged tank completely to stop oil leakage. Deepening orestablishing water bottoms provides a buffer that can prevent further discharge of liquids lighterthan water. As a practical matter, a water bottom has been established when the cargo pumpsFigure 3-6. Oil Outflow and Formation of Water Bottom.3-14