Oil Discharge monitoring systems for tankers – calibration best practice

A summary of Oil Discharge Monitoring Systems used on tankers looking at the history, regulation and best maintenance practices.

by Gillian Lovering

It is less than 40 years since oil tankers were permitted to clean tanks after discharge and dispose of the mix of seawater and oil directly into the oceans. The only limitation that existed until the MARPOL Convention came into effect in 1983 was that such discharges should take place at least 50 nautical miles offshore or 100 nautical miles in certain sensitive areas. This was also a time when tankers were not obliged to have segregated ballast tanks and carried ballast in the cargo tanks.

As crude oil prices jumped in the 1960s and 70s, tanker operators began storing the oil and water mix in slop tanks for sufficient time for the oil and water to separate after which the water was discharged and the remaining crude – which could amount to several hundred tonnes – pumped back into cargo tanks for the next cargo to be loaded on top. Crude oil washing systems where some of the cargo itself is used to dislodge residues became mandatory on tankers above 20,000dwt with the advent of MARPOL.

Although the use of water in cleaning tanks and line stripping may be less than it once was, it is still an essential aspect of tanker operation. In order to prevent pollution by oil, the IMO adopted a requirement for tanker discharges that is more stringent than for non-tank vessels.

There is a requirement for all tankers above 150gt to manage the discharge of water that maybe contaminated by oil from cargo tank and other cleaning operations. The IMO regulations and guidelines are contained in Resolution MEPC.108(49).

Under this resolution, all oil tankers of 150gt and above must have approved Oil Discharge Monitoring Equipment (ODME). A more recent resolution MEPC 240(65) for Bio Fuels, became effective on 1 January 2016.

Unlike the arrangements for bilge water where the discharge of treated water is permitted if the oil content is 15ppm or less, the discharge criteria for other wastewater from tankers requires a more sophisticated computing arrangement. This is because the rate of discharge of oil is limited to 30 litres of oil per nautical mile regardless of ship size and the total quantity of discharge must not exceed 1/30000 of the total quantity of the cargo from which the residue was formed.

The ODME comprises:

  • an oil content meter to analyse the content of oil in the water that is to be discharged overboard,
  • a flow meter to measure the rate of discharge,
  • a computing unit to calculate the oil discharge in litres/nautical miles and the total quantity, along with the date and time identification and
  • a control valve to stop discharge when the permissible limit has been reached.

The technology used to measure the oil content in the water is the same as that used in the 15ppm OCMs of other ship types, but the computing functions and flow rates require additional components and technologies. It is these other components that control the separation and discharge operations. Whereas in the bilge treatment, the operation can continue providing the oil content is not above 15ppm, the ODME must cease the overboard discharge if the 30 litres per nautical mile limit is exceeded (as could happen if the ship is moving slowly) or if the overall limit is reached.

Download the guide to oil in water monitoring & discharge


There are other restrictions on discharge beyond those mentioned above and these require that the vessel must be underway, it should not be in any special area where discharge is prohibited, and it must be 50 nautical miles away from any land. These restrictions should be monitored and recorded utilizing a GPS input to the data and the record of ODME operation must be retained on board for a minimum of three years.

To comply with the requirements of MEPC.108(49), the computing and control device must receive the following information:

  • the oil content of the effluent ppm;
  • the flow rate of discharge m3/hour;
  • ship’s speed in knots;
  • ship’s position – latitude and longitude;
  • date and time (GMT); and
  • status of the overboard discharge control.

In operation it is normal for the slop tank contents to be allowed to settle for at least 36 hours so that the lighter oil will separate naturally and rise to the top of the tanks. The water to be discharged is drawn from the bottom of the tank and discharge will usually continue so long as the oil content is not excessive, and the flow rate and other parameters are in line with regulations.
It is necessary for the permitted quantity to be entered manually before operations can commence. This is a simple matter of entering the figure (obtained by dividing the cargo volume by 30,000) into the control system.

It is possible for this step to result in a potentially illegal discharge if the crew enter the wrong figure. This can be accidental or occasionally deliberate. As the ODME records the quantity entered it is possible for Port State Control and other officials to identify the error from cargo documentation and the Oil Record Book and take appropriate action. Sadly, just as with the bilge water separators on ships, some illegal discharges are made by employing sophisticated bypass arrangements.

The IMO has tried to ensure that the ODME should not be circumvented and lays down in Section 5 of the resolution that it ‘should be designed to ensure that user access is restricted to essential controls. Access beyond these controls should be available for emergency maintenance and temporary repair but must require the breaking of security seals or activation of another device that indicates an entry to the equipment.

The seals should be of a design that only the manufacturer or his agent can replace the seals or reset the system following inspection and permanent repairs to the equipment. It also requires that the accuracy of the ODME – as with the OCM for bilge water – should be verified at IOPP renewal surveys and a calibration certificate attesting to that should be retained on board for inspection purposes.

Since the equipment used for determining oil content is in most cases identical to that for the Bilge Water OCM, it would stand to reason that the same criteria regarding calibration apply.

In recent years it has become commonplace for major charterers to insist on certain quality standards and operations being in place on ships carrying their cargoes. One such charterer (ExxonMobil) requires that the ODME must be regularly tested and be verified as fully operational before each use. It also requires that calibration MUST be performed by the ODME manufacturer or persons authorised by the manufacturer annually or else it must be replaced.

As a leading manufacturer of ODME equipment, Rivertrace offers a flexible bespoke solution to suit operator needs. This includes the following value-added services:

  • Calibration Check Certificates issued by approved representatives
  • Annual functional test and simulation certificates
  • Free to use RT Connected Portal to track monitors and automate calibration reminders
  • Calibrated SMART CELL exchange schemes
  • Fixed annual maintenance charges so no hidden costs

Watch an interview with our Managing Director Mike Coomber to learn more about the benefits of choosing the SMART ODME Oil Discharge Monitoring Equipment for your tanker.

Click here to watch Video

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