The terms oil discharge monitoring system and oil content monitor are frequently used interchangeably — but they refer to fundamentally different instruments, governed by different MARPOL regulations, measuring discharge in different ways and required on different vessel types. Understanding the distinction is important for anyone involved in purchasing, specifying or operating discharge monitoring equipment on tankers or other vessels. For a detailed product comparison focused specifically on FPSO applications, see ODME vs. Standard Oil Content Monitor: Which Does an FPSO Need?.
| Key thing to remember An oil discharge monitoring system — commonly abbreviated as ODME or ODMS — is an integrated control system that monitors, records and controls tanker slop and ballast discharge under MEPC.108(49). It is not simply a measuring instrument. It combines oil concentration measurement with flow metering, vessel speed and position data, pump control integration and electronic discharge recording. An oil content monitor measures oil concentration only — it is a different instrument for a different application. |
What Is an Oil Discharge Monitoring System?
An oil discharge monitoring system — Oil Discharge Monitoring Equipment (ODME) under MARPOL terminology — is an integrated control and recording system fitted to crude oil, product and chemical tankers to monitor, record and automatically control the discharge of oily water from cargo tanks, tank washings and ballast systems.
Unlike a standard oil content monitor, which measures oil concentration in parts per million and stops discharge if the concentration exceeds 15ppm, an ODME measures discharge in terms of rate and quantity rather than concentration. The system calculates the instantaneous rate of oil discharge in litres per nautical mile — using a combination of oil concentration measurement, flow metering and vessel speed data — and stops discharge automatically if this rate exceeds the permitted limit.
ODME also tracks the total quantity of oil discharged as a fraction of the previous cargo — a cumulative record that must not exceed 1/30,000 of the cargo quantity. Both the instantaneous rate limit and the cumulative cargo fraction limit must be maintained simultaneously throughout any discharge operation.
What Are the Components of an Oil Discharge Monitoring System?
An ODME is a multi-component system rather than a single instrument. The table below sets out the main components and what each one does.
| System component | What it does |
| Oil content measuring unit | The analyser that measures the oil concentration of the discharge stream — analogous to the measurement cell in a standard OCM, but part of a larger integrated system |
| Flow meter | Measures the volumetric flow rate of the discharge stream in cubic metres per hour — required to calculate instantaneous discharge rate in litres per nautical mile |
| Vessel speed input | Receives vessel speed data — typically from GPS or the vessel navigation system — to complete the litres per nautical mile calculation |
| Vessel position input | Receives GPS position data to record the vessel location during each discharge event — required for the Electronic Discharge Record |
| Pump control interface | Connects to the cargo pump and discharge valve controls to automatically stop discharge if the rate limit or cargo fraction limit is exceeded |
| Processing and calculation unit | Combines oil concentration, flow rate and vessel speed data to calculate the instantaneous discharge rate in litres per nautical mile and cumulative cargo fraction discharged |
| Electronic Oil Discharge Monitoring Record | The data logging system that records all discharge operations, alarm events and vessel position with timestamps — satisfying the MEPC.108(49) record-keeping requirement |
| Display and alarm panel | Provides real-time readout of oil concentration, discharge rate, cumulative cargo fraction and alarm status to the officer of the watch |
The SMART ODME from Rivertrace integrates all of these components into a single system meeting MEPC.108(49) and MEPC.240(65) requirements, with pneumatic and electric pump interface options to suit different tanker configurations.
How Is an ODME Different From a Standard Oil Content Monitor?
The table below sets out the key differences between an oil discharge monitoring system and a standard oil content monitor across all the dimensions that matter for specification, procurement and compliance.
| Oil discharge monitoring system (ODME) | Oil content monitor (OCM) | |
| Full name | Oil Discharge Monitoring Equipment (ODME) | Oil Content Monitor (OCM) / 15ppm bilge alarm |
| Applicable regulation | MARPOL Annex I — MEPC.108(49) and MEPC.240(65) | MARPOL Annex I — MEPC.107(49) |
| Discharge stream | Tanker cargo tank slop water, tank washings, ballast water | Oily water separator discharge from vessel machinery space |
| What it measures | Instantaneous discharge rate in litres per nautical mile; total oil discharged as a fraction of previous cargo | Oil concentration in the discharge in parts per million |
| Discharge limit | 30 litres per nautical mile; total not to exceed 1/30,000 of previous cargo | 15ppm oil concentration limit |
| System integration | Integrates with vessel speed, position and pump controls to calculate discharge rate and control discharge | Integrates with OWS discharge valve to stop discharge above 15ppm |
| Data logged | Position, speed, discharge rate, cumulative cargo fraction, alarm events — Electronic Oil Discharge Monitoring Record | Oil concentration, discharge on/off, alarm events — Oil Record Book Part I |
| Measuring cell replacement | Every 5 years under MEPC.108(49) | Calibration verification at IOPP survey intervals |
| Vessel type | Crude oil, product and chemical tankers; FPSOs with tanker-derived discharge systems | All ships of 400 GT and above |
| Rivertrace product | SMART ODME | SMART BILGE+ |
The most fundamental difference is what each system measures and why. A standard oil content monitor answers the question: is the oil concentration in this discharge below 15ppm? An ODME answers a different, more complex question: is the rate of oil discharge in litres per nautical mile within the permitted limit, and is the total cumulative cargo fraction within the 1/30,000 limit? These are completely different compliance questions requiring completely different measurement approaches.
How Does an ODME Calculate the Oil Discharge Rate?
The oil discharge rate in litres per nautical mile is not a direct measurement — it is a calculated value derived from three inputs that the ODME system collects simultaneously:
- Oil concentration — the oil content of the discharge stream, measured in ppm by the oil content measuring unit in the ODME
- Flow rate — the volumetric flow rate of the discharge stream, measured in cubic metres per hour by the flow meter
- Vessel speed — the vessel’s speed over the ground in knots, received from the GPS or navigation system
The calculation works as follows: the instantaneous oil discharge rate in litres per hour is calculated by multiplying the oil concentration in ppm by the volumetric flow rate. This hourly rate is then divided by the vessel speed in nautical miles per hour to produce the instantaneous discharge rate in litres per nautical mile.
If this calculated rate exceeds 30 litres per nautical mile at any point during discharge, the ODME automatically closes the discharge valve and stops the discharge. The system also accumulates the total oil discharged as a running total of cargo fraction, stopping discharge if the 1/30,000 limit is approached.
What Does the Electronic Oil Discharge Monitoring Record Contain?
One of the most important functions of an ODME is the Electronic Oil Discharge Monitoring Record — the continuous data log that forms the compliance record for all tanker slop and ballast discharge operations. This record must be available for inspection by port state control authorities at all times.
The Electronic Oil Discharge Monitoring Record captures:
- Date and time of each discharge event — start and stop
- Vessel position (latitude and longitude from GPS) at the time of discharge
- Vessel speed during the discharge
- Oil concentration in the discharge stream throughout the event
- Calculated instantaneous discharge rate in litres per nautical mile
- Cumulative total cargo fraction discharged
- Any alarm activations and the corresponding response
- Any automatic discharge stops triggered by the system
This electronic record replaces the manual Oil Record Book Part II entries that were required before electronic ODME recording was accepted. The ODME record provides a more complete and tamper-resistant record of discharge operations than manual recording — which is one of the reasons MEPC.108(49) mandates electronic recording as part of the ODME system requirement.
Why Does an ODME Require Vessel Speed and Position Data?
Vessel speed and position data are not optional inputs to the ODME — they are essential for both the discharge rate calculation and the compliance record.
Speed data for discharge rate calculation
The discharge limit of 30 litres per nautical mile is expressed as a rate relative to the distance the vessel travels — not as a rate relative to time. A vessel travelling at 10 knots covers one nautical mile every 6 minutes. The same discharge flow rate that produces 30 litres per nautical mile at 10 knots would produce 60 litres per nautical mile at 5 knots. The vessel speed must therefore be continuously factored into the discharge rate calculation — and the discharge must be automatically controlled to stay within the 30 litres per nautical mile limit at whatever speed the vessel is travelling.
Position data for the compliance record
The MARPOL requirements for tanker discharge specify that discharge can only take place more than 50 nautical miles from the nearest land. The ODME records vessel position from GPS throughout the discharge operation, providing a continuous record that confirms the vessel was beyond the 50 nautical mile limit during the discharge. Without GPS position recording, the operator cannot demonstrate this compliance condition was met during a specific discharge event.
Which Vessels Are Required to Fit an ODME?
ODME is required under MEPC.108(49) on crude oil tankers of 150 gross tonnage and above, and on product and chemical tankers of any size certified to carry oil in bulk. The requirement applies to all such vessels on international voyages.
Vessels that may also require ODME in specific circumstances include:
- Combination carriers (OBO and ore/oil vessels) that carry oil cargo — ODME requirements apply to the tanker-related discharge operations on these vessels
- FPSOs converted from crude oil, product or chemical tankers — where the FPSO retains tanker-derived slop tank, tank washing or ballast discharge systems. See the slop tank discharge monitoring application page for more detail on FPSO-specific ODME requirements
What Are the Calibration and Maintenance Requirements for an ODME?
ODME systems have specific calibration and maintenance requirements under MEPC.108(49) that are more demanding than those for a standard oil content monitor:
Measuring cell replacement
The oil content measuring unit within the ODME must have its measuring cell replaced every five years. This is a mandatory requirement under MEPC.108(49) — an expired measuring cell is a compliance finding during port state control inspection or SIRE audit, regardless of whether the system otherwise appears to be functioning correctly.
Annual calibration check
The accuracy of the ODME oil measuring unit must be verified annually. The calibration certificate must be available onboard for inspection.
Flow meter verification
The flow meter accuracy must be verified periodically to confirm that the discharge rate calculation remains accurate. A flow meter that under-reads will cause the system to calculate a lower discharge rate than is actually occurring — potentially allowing discharge above the permitted limit to proceed without triggering an automatic stop.
Position and speed data verification
The GPS position and vessel speed inputs to the ODME must be verified to confirm that accurate data is being received and that the discharge rate calculation is based on correct vessel speed information.
| An expired measuring cell is an immediate compliance finding Port state control inspectors and SIRE auditors check the measuring cell replacement date as a standard ODME inspection item. A measuring cell that has not been replaced within the five-year requirement specified in MEPC.108(49) is treated as a compliance deficiency regardless of whether the ODME is otherwise functioning correctly. Measuring cell replacement must be scheduled proactively before the five-year deadline — not reactively when an inspector raises a finding. |
What Is the Difference Between ODME and ODMS?
ODME (Oil Discharge Monitoring Equipment) and ODMS (Oil Discharge Monitoring System) are used interchangeably in industry to refer to the same type of equipment — the integrated system fitted to tankers for slop and ballast discharge monitoring under MEPC.108(49). The IMO MARPOL text uses ODME; some manufacturers and operators use ODMS. Both refer to the same integrated monitoring and control system.
The Rivertrace product for this application is the SMART BILGE+ for standard bilge water OCM applications under MEPC.107(49), and the SMART ODME for tanker slop and ballast discharge monitoring under MEPC.108(49). The two products are completely different — the SMART BILGE+ measures oil concentration in ppm for the 15ppm bilge water limit, while the SMART ODME is an integrated system that calculates discharge rate in litres per nautical mile and monitors cumulative cargo fraction.
Frequently Asked Questions
What is an oil discharge monitoring system?
An oil discharge monitoring system — ODME under MARPOL terminology — is an integrated system fitted to crude oil, product and chemical tankers to monitor, record and control the discharge of oily water from cargo tanks, tank washings and ballast systems under MEPC.108(49). It combines oil concentration measurement with flow metering, vessel speed and position data, automatic pump control and electronic discharge recording. It is fundamentally different from a standard oil content monitor, which simply measures oil concentration in ppm.
What does an ODME measure?
An ODME measures the instantaneous rate of oil discharge in litres per nautical mile — calculated from oil concentration, flow rate and vessel speed — and tracks the cumulative total of oil discharged as a fraction of the previous cargo. It automatically stops discharge if the instantaneous rate exceeds 30 litres per nautical mile or if the cumulative 1/30,000 cargo fraction limit is being approached. It also records vessel position, speed, oil concentration and discharge rate throughout each discharge event.
What is the difference between ODME and a 15ppm bilge alarm?
A 15ppm bilge alarm is a type-approved oil content monitor for oily water separator discharge from a vessel machinery space under MEPC.107(49). It measures oil concentration in ppm and stops discharge above 15ppm. ODME is a completely different integrated system for tanker slop and ballast discharge under MEPC.108(49). It measures discharge rate in litres per nautical mile rather than oil concentration in ppm, integrates with vessel speed and position data, connects to pump controls and produces an Electronic Oil Discharge Monitoring Record rather than an Oil Record Book Part I entry.
Why does an ODME need vessel speed data?
The tanker slop discharge limit of 30 litres per nautical mile is expressed relative to the distance the vessel travels, not relative to time. The same volumetric discharge flow rate produces a higher litres per nautical mile rate when the vessel is moving slowly. Vessel speed must therefore be continuously factored into the discharge rate calculation and the discharge automatically controlled to stay within the 30 litres per nautical mile limit at whatever speed the vessel is travelling.
How often does the ODME measuring cell need to be replaced?
The ODME measuring cell must be replaced every five years under MEPC.108(49). This is a mandatory requirement and an expired measuring cell is a compliance finding during port state control inspection or SIRE audit regardless of whether the ODME is otherwise functioning. Measuring cell replacement must be scheduled proactively before the five-year deadline.