Equipment Guide · ODME — Oil Discharge Monitoring
After tank washing, the slop tank holds oil floating on mostly-clean water — and MARPOL lets a tanker discharge that settled water at sea through exactly one guarded gate: the Oil Discharge Monitoring Equipment. It reads oil content, flow and ship's speed, computes litres of oil per mile of track, totals every litre against the 1/30,000 allowance, and closes the overboard valve itself the instant any limit is crossed. Tap a step or press "Play" to follow the water from the slop tank to the sea — then run a decant yourself in the simulator and see the interlock act.
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Take a tanker that carried 90,000 m³ of crude. The total quantity of oil it may discharge with the tank-washing water from that cargo is 1/30,000 of it — just 3 m³, or 3,000 litres, for the entire operation. The ODME's totalizer counts every litre toward that ceiling and closes the overboard valve for good when it is reached. This is why the instantaneous 30 L/nm limit alone is not the real constraint: at 12 knots and 300 m³/h you would need roughly 1,200 ppm to trip it, but water that dirty devours the 3,000-litre budget in a few hours. The cleaner the water you send over — long settling, gentle pumping — the further the budget stretches, and the more slop space you win back for the next cargo.
Set the ship's speed, the oil content the meter is reading and the pump's flow — the ODME computes litres per nautical mile live. Stay under 30 L/nm and inside the 3,000-litre totalizer budget and the water keeps going over the side. Cross a limit — or stop the ship — and the overboard valve closes itself and the flow swings back to the slop tank. Reopen with the valve's + once you have fixed the numbers.
Rate gauge marks — caution above 24 · interlock at 30 L/nm. Totalizer ceiling 3,000 L = 1/30,000 of this 90,000 m³ cargo. Sim time runs fast: each second ≈ 5 minutes of pumping.
The rate is a balance of three numbers: concentration (ppm) times flow spread along the ship's track (speed). The ODME never negotiates — over 30 L/nm, ship not making way, or budget spent, and it drives the overboard valve shut itself, swinging the discharge back to the slop tank. Every event lands in its memory: the log line below is what a Port State inspector will read later.
Press Start to run the decant — then push the oil content or the flow, or stop the ship, and watch the ODME close the overboard valve itself.
Working principle — explained in our own plain words with our own schematic from general marine knowledge (not copied from any OEM manual). Conceptual; varies by ship/model. Reviewed and verified by a Master Mariner.