Before we come to ODMCS tests, it is better to first do a recap on the requirements of ODMCS. ODMCS (Oil discharge monitoring control system), sometimes also called ODME (Oil discharge monitoring equipment) is an equipment required under Marpol Annex 1 and is required to monitor discharge of oily mixture from cargo tanks of oil tankers.

ODME oil discharge

The Marpol requires following for discharging oily Mixture

  1. The tanker should not be in special area
  2. The tanker should be Enroute
  3. The tanker must be more than 50 Nautical miles from the nearest land
  4. The instantaneous rate of discharge should not exceed 30 litres per Nautical mile
  5. The total quantity of oil discharged should not exceed 1/30000 of the total quantity of cargo of which the residues formed a part.
  6. The tanker must have an approved ODMCS and it should be operational

ODMCS is installed to ensure compliance with the requirements no 4 and 5 and so the testing of ODMCS should be based upon to find if

  1. The ODMCS would actually stop if “instantaneous rate of discharge” exceeds 30 litres per nautical mile.
  2. The ODMCS should stop when total oil discharged has reached the allowed limits as per point 5 above.

It is general practice among many seafarers to test the ODMCS for only IRD, but as I have pointed out, the regulation require the ODMCS to stop when total oil discharged limit exceeds and thus this is an equally important test too.

Test for Instantaneous rate of Discharge

The formula for IRD is

IRD (in litres/Nautical mile)=  Oil content(ppm) x Flow rate (m3/hr) / Speed (Knots) x 1000

For ODMCS discharge valve to open, the IRD should be less than 30 L/NM and we just play with oil content, flow rate and speed (enter these values manually) to get the IRD to say 29.8 L/NM and discharge valve should open. We then increase the oil content a bit to make the IRD 30.1 L/NM to see if the discharge valve closes. Pretty simple. However this is just the simulation test which just tells us that the computing unit of ODMCS is working alright.

It does not tell us if in actual operation the oil content meter and flow meter will give correct reading of oil content and sample flow respectively. For this reason, the oil majors require the annual calibration of oil content meter and flow meter by an external competent company. Master and ship staff should ensure that such annual test has been carried out and certificate is available onboard.

It is also important in actual operation of ODMCS that sample is drawn from the discharge line and fed to oil content meter so that it can measure the oil content in the oily water. This sample is drawn and circulated through oil content meter by Sample pump. This makes the sample pump extremely important for the functioning of ODMCS and hence sample pump should be checked monthly for its free movement by rotating with hand

Test for Total quantity Discharged 

First of all let us understand the rationale behind this requirement. The first requirement was that while the ship is moving, you can discharge 30 litres of oil contained in the oily water provided this 30 litre is spread over one nautical mile. If the second requirement is not there, one could keep on diluting the oily mixture to make sure IRD is always less than 30 L/NM and pump out entire tank content. This requirement limits the total oil that a ship can discharge irrespective of complying with the first requirement.

So if the vessel was carrying 45000 m3 of oil cargo and oily mixture are formed from washing or ballasting these tanks, the total oil that can be discharge through ODMCS is 45000/30000 which is 1.5 m3 or 1500 Ltrs. This figure need to be entered in the ODMCS  before actual run of ODMCS.

Now how to test if after achieving this limit the ODMCS will shut the discharge valve and open re-circulation valve. Test is again pretty simple, set a lower value and run the ODMCS in simulation mode with IRD say around 29 L/NM. You would see the Total oil discharged reading go up. When you reach the set value of total oil discharged, the ODMCS should shut the discharge valve.

Even though there may be some other small tests like power failure test etc. Checking and testing the ODMCS and associated components as mentioned above would most likely ensure that your ODMCS would be ready when you require it.

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Capt Rajeev Jassal

About Capt Rajeev Jassal

Capt. Rajeev Jassal has sailed for over 19 years mainly on crude oil, product and chemical tankers. He holds MBA in shipping & Logistics degree from London. He has done extensive research on quantitatively measuring Safety culture onboard and safety climate ashore which he believes is the most important element for safer shipping.

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Arji Shanmukha Sai
Arji Shanmukha Sai
Apr 16, 2016

Sir, does ODMCS have 2 modes of operation. One for decanting, I.e, ird not more than 30 ltr per nm and 15 ppm for clean ballast? Because in the definition of clean ballast they have mentioned about the 15 ppm requirement. Awaiting for your valuable reply.

Rajeev Jassal
Rajeev Jassal
Apr 22, 2016

Arji, If ship is fitted with clean ballast tanks, 15ppm device is required. Now it is upto owners how they wish to satisfy this requirement. There are very few ODMCS with two mode of operations. Owners can either fit these or they can have 15ppm unit installed for clean water ballast tanks and ODMCS for cargo tanks. But you would hardly see ships with clean ballast tanks nowdays even when MARPOL may allows certain size of the ship to have these. This is because nobody want to do business with these ships because of environmental concerns.

Dec 31, 2018

15ppm mode is to be used when discharging clean ballast or any heavy weather ballast carried in washed cargo tanks.

Apr 12, 2017

I was in searching of such a device that can tackle all the oil and gas calculations. Then one of my friend suggested me Petroleum Calculator , where i got calculator that is very helpful in my oil and gas company.

karishma jain
karishma jain
Mar 28, 2018

how to test in front of PSCO hat 15 ppm monitor is working? thank u sir

Mar 7, 2019

that is very same as 30l per nm,first to set value 14ppm,then 16ppm,to wacth two valves.

Marcelo Recio
Marcelo Recio
Jul 2, 2019

Sir. I found your explanations very interesting and simple to understand as usual. Now , I would highly appreciate if you can help me with this question: A few days ago, I performed a FSI on a tanker. The Master exhibited an ODMC calibration certificate issued more than one year ago, but with no period of validity. With regard to the IOPP certificate, it had recently passed the annual survey by Class with no observations. Res MEPC 108 refers to annual checks and requires a certificate of calibration to be available to PSC and other inspectors but doesn´t state clearly if annual check is just a check by Class or involves also the equipment´s calibration. So; Should I have regarded that certifcate as invalid (overdue) or it may be regarded as still in compliance? Thank you very much in advance. Kind regards.

Rajeev Jassal
Rajeev Jassal
Jul 3, 2019

Annual Calibration of ODMCS is not a statutory requirement. Most of the tankers have now started to perform annual shore calibration of ODMCS because of the requirement by "Exxon Mobil oil major" but it is still not required by other oil majors like Shell and BP. So with regard to FSI, it is OK if they did not have it.

Marcelo Recio
Marcelo Recio
Jul 4, 2019

Thank you very much Captain for your valuable information. Kind regards.

Aug 17, 2019

Dear sir, How often ODME is tested?

Omkar Samant
Omkar Samant
Sep 8, 2020

Good Day Sir, I work on an oil tanker and found your article extremely helpful, I have always referred my Juniors to read this article for better understanding ODMCS and its testing. A question that comes to mind is how does the Oil Content Monitor actually function in measuring the PPM of the oily water passing into the discharge line. Also just as we test the IRD computing unit by playing with the PPM setting, can we also do that by playing with the flow setting in m3/hr or Speed setting in kts? Your advice will be much appreciated.

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