Abstract: A tanker ship has upper and lower tanks for carrying cargo. The upper and lower tanks are separated from each other by an intermediate deck. The lower tank is filled with cargo to the intermediate deck such that the cargo is slightly pressurized and there is no ullage space. As the tanker sails, the cargo in the lower tank is likely to change pressure due to temperature changes. These pressure changes are detected by a pipe extending from the lower tank through the intermediate deck up to an ullage space in the corresponding upper tank. A siphon is used to transfer cargo between the upper and lower tanks to equalize the pressures. In the event that the lower tank is breached, sensors are used to detect the resulting drop in pressure from the lower tank cargo. The sensor system provides a warning to the crew, closes the siphon to prevent cargo transfer from the upper tank to the lower tank and opens an auxiliary pipe located in the upper portion of the lower tank.