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 tank 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 – normally, on Chemical Tanker all the Oil-water will be stored in SLOP tank.

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.

Main Parts of ODME

An ODMCS consists essentially of four systems:

1. 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).

2. A flow meter: The flow rate of the oily water to be discharged is measured at the discharge pipe.

3. A computing unit: A computing unit calculates the oil discharge in litres/nautical miles and the total quantity, along with date and time identification.

4. 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.

Working

The oily mixture is pumped out to the sea through ODMCS by a pump. A sampler probe and a flow meter sensor is connected at the discharge pipe, before the overboard valve, to sense the oil content and the flow of mixture.

The data provided by the two sensors are fed in a control unit wherein it is analysed and the discharge valve is controlled by the same.

If the control unit senses a rise in the ppm and flow comparing to the permissible value, it will shut the overboard valve and open the re-circulation valve which is connected to slop tank of the ship.

Regulatory requirements for oil mixture discharge from cargo space

• Tanker vessel must be enroute (the ship is under way at sea on a course or courses, including deviation from the shortest direct route)
• The vessel should not be in special areas – discharge prohibited .
• The tanker must be 50 nautical miles away from land.
• The instantaneous rate of discharge of oil content does not exceed 30 litres per nautical mile (1.852 km).

In mean of Instantaneous rate of discharge – IRD ( oil content discharge rate )

• The total quantity of discharge must not exceed 1/30000 of the total quantity of the residue formed cargo.

• The tanker must have operational and approved ODMCS.

From the machinery space bilges of all ships, except from those of tankers where the discharge is mixed with oil cargo residue:

• The ship is not within a Special Area;
• The ship is more than 12 nautical miles from the nearest land;
• The ship is en route;
• The oil content of the effluent is less than 15 parts per million. And;
• The ship has in operation an oil discharge monitoring and control system, oily-water separating equipment, oil filtering system or other installation required by this Marpol Annex I

As per the regulation, the following inputs must be recorded by the system:

1. Discharge rate of the pump which is discharging the oily water mixture overboard.
2. The location of the ship in latitude and longitude.  ( GPS )
3. Date and time of the discharge.
4. The total quantity that has been discharge overboard.
5. Oil content of the discharged mixture in PPM.

All the records of ODMCS must be stored on board ships for not less than 3 years.