Control and Monitoring Oily water discharge

Oil Discharge monitoring and control system

Oil Discharge monitoring and control system

Oil tankers, for their ballast voyages carry ballast water into cargo tanks wherein generally oil cargo would have been carried. This ballast water 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.

Over view

A computer unit.

Intended to be installed in the cargo control room or in an equivalent non-hazardous area. The computer unit controls and receives data from the other ODME components. This information is treated for computing and control purposes and is stored into a flash memory, which could be displayed or printed out an external computer or printer, according to the requirements of the IMO resolutions.

A Zener barrier unit

Intended to be installed in the engine room or equivalent safe area. The Zener barrier module converts electrical control signals from the computer unit to pneumatic or electrical supply signals for operation of the components that are a part of the ODME 2005 equipment. The Zener barrier module contains electrical power supply and zener barriers for the analysing unit and for up to two 4-20mA loop powered flow meters.

An Analysing unit

Intended to be installed in the hazardous area. The analysing cabinet contains the oil content measuring unit, a sample feed pump of shear type, a fresh water valve for cleaning purposes and a pressure transmitter that monitors the sample flow through the measuring cell. This compact design makes it possible to install the unit close to the sampling point, which make the sampling pipes distance between the sampling probes and the analysing cabinet as short as possible.

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 liters/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 analyzed 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

• The vessel must be en-route 
• The vessel should not be in special areas. 
• The tanker must be 50 nautical miles away from land.
• The instantaneous rate of discharge of oil content does not exceed 30   liters per nautical mile.
• 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.

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

• Discharge rate of the pump which is discharging the oily water mixture overboard.
• The location of the ship in latitude and longitude.
• Date and time of the discharge.
• The total quantity that has been discharge overboard.
• 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.