What to check on a ODME System.
An oil discharge monitoring and control system is a measurement and interlock system, not a treatment plant - it does not remove oil, it decides in real time whether the water reaching the overboard valve is clean enough to discharge and stops the discharge automatically if it is not. It is fitted specifically to oil tankers, sitting between the cargo tank stripping or slop tank system and the overboard discharge, and its record is what a Port State Control officer checks first when reviewing a tanker's compliance with MARPOL Annex…
What sets an ODME apart
An oil discharge monitoring and control system is a measurement and interlock system, not a treatment plant - it does not remove oil, it decides in real time whether the water reaching the overboard valve is clean enough to discharge and stops the discharge automatically if it is not. It is fitted specifically to oil tankers, sitting between the cargo tank stripping or slop tank system and the overboard discharge, and its record is what a Port State Control officer checks first when reviewing a tanker's compliance with MARPOL Annex I discharge limits.
Main components
Oil content meter
An optical or fluorescence-based sensor that continuously analyses a sample stream drawn from the discharge line and reports oil content in parts per million.
Sample and flow system
Pumps and piping that draw a representative sample from the main discharge line to the sensor without losing accuracy through settling or dilution in the sample line itself.
Control and interlock unit
Compares the measured oil content and discharge rate against the calculated MARPOL limit for the voyage and automatically closes the overboard valve if either is exceeded.
Data recorder
Logs oil content, flow rate, ship's position and time continuously, producing the record used to demonstrate compliance and, where fitted, feeding the oil record book.
Overboard discharge valve
A fail-safe valve, normally closed, that only opens under the ODME's control and shuts immediately on any alarm or system fault.
Selection and sizing
| Parameter | Relevance |
|---|---|
| Sensor technology | Optical/fluorescence types differ in sensitivity to crude versus refined product residues |
| Discharge rate range | Must cover the pump capacity fitted for cargo tank stripping and slop discharge |
| Position input | Needs a reliable GNSS feed, since discharge is only permitted at the required distance from land |
| Type approval scope | Must match the tanker size category and trading pattern under MARPOL Annex I |
Regulations and class
MARPOL Annex I, Regulation 31 requires oil tankers of 150 gross tonnes and above to carry an approved ODME as a condition of discharging any oily water overboard rather than retaining it for shore disposal. The system must be type-approved to the IMO performance standard in resolution MEPC.108(49), which sets the required accuracy, alarm and automatic stopping functions. Class and flag state surveys check the calibration record, the automatic stopping function under test conditions, and that the recorder has not been tampered with, since falsified ODME records have been a recurring focus of Port State Control detentions.
Typical faults
- Sensor fouling - wax or sediment coats the optical cell, drifting the reading low and risking a discharge that is actually over the limit.
- Sample line blockage - a partially blocked sample line starves the sensor of a representative flow, producing unstable or clearly wrong readings.
- Position feed loss - if the GNSS input drops out, the system should default to the most restrictive discharge condition, but a poorly integrated unit can instead simply stop logging position.
- Calibration drift - infrequent calibration against a known reference lets the reading drift gradually enough that it is not obvious from day-to-day operation.
- Bypass tampering - a documented cause of major fines and detentions, where the interlock is defeated to discharge without meeting the limit.
What to look for in a supplier
- Type approval certificate current for MEPC.108(49) and matching the ship's tanker category
- Calibration service availability in the ports the ship actually trades to, not just at the builder's yard
- Tamper-evident recorder design and a clear audit trail, given how closely this record is checked in Port State Control
- Sensor cleaning and sample line maintenance documented in the manufacturer's schedule, not left to generic guidance
Treat the ODME record with the same discipline as the oil record book - the two are cross-checked by inspectors, and an unexplained gap or mismatch between them draws attention faster than almost anything else on a tanker inspection.