Oil tankers carry different types of oil cargo in their cargo tanks and it often happens that after discharging the oil cargo in some port, the ship sails without any cargo to some other destination. In order to do so, it has to take ballast from the sea to get better draught and stability.
For this reason, ballast water is taken into cargo tanks wherein generally oil cargo would have been carried. It is to note that the ballast water carried in cargo tanks has to be discharged out at sea before the next cargo loading. Therefore, Oil Discharge monitoring and control system (ODMCS) is used to prevent the pollution of ocean by oil due to the discharge from ballast and bilge spaces.
As per MARPOL 73/78 Annex I, all the oil tankers of 150 GT and above must have an approved Oil Discharge Monitoring System. The system must have provision to work in manual operating mode if the auto system is not working.
An ODMCS consists essentially of four systems:
- An Oil content meter: The oil content meter is used to analyze the content of oil in the water that is to be discharged overboard. This oil is expressed in parts per million (PPM).
- A flow meter: The flow rate of the oily water to be discharged is measured at the discharge pipe.
- A computing unit: A computing unit calculates the oil discharge in litres/nautical miles and the total quantity, along with date and time identification.
- An overboard valve control system: The auto control valve is installed at the overboard so that it must close and stop the discharge when permissible limit has been reached.
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