Tankers accumulate oily water in slop tanks from cargo tank washing operations and ballasting. This oily water mixture must be monitored before it can be discharged overboard. Under MARPOL Annex I and MEPC.108(49), crude oil, product and chemical tankers are required to use type-approved Oil Discharge Monitoring Equipment (ODME) to monitor, record and control slop tank and ballast discharge. Rivertrace’s SMART ODME provides compliant discharge monitoring and integrated data logging to meet these requirements.
| Key thing to remember
ODME is not the same as a standard 15ppm bilge alarm. ODME is a separate, more complex monitoring system designed specifically for tanker slop and ballast discharge operations, measuring discharge rate in litres per nautical mile and cargo fraction rather than oil concentration in ppm. The applicable standard is MEPC.108(49) — not MEPC.107(49). Installing a standard bilge alarm on a slop discharge system does not satisfy the MEPC.108(49) requirement. |
What Is a Slop Tank and What Does It Contain?
A slop tank is a dedicated tank on a tanker used to collect the oily water and cargo residues generated during cargo tank washing and ballasting operations. When a tanker discharges its cargo, the cargo tanks are washed with water to remove cargo residues. This wash water — mixed with oil, emulsifiers and cleaning agents — is directed to the slop tank for retention.
Slop tanks also receive ballast water from cargo tanks where seawater has been used to ballast the vessel after cargo discharge. This ballast water picks up oil residues from the tank walls and bottom, creating an oily water mixture in the slop tank.
The contents of a slop tank typically include:
- Residual crude oil or product cargo entrained in the wash water
- Emulsified oil and water from tank washing operations
- Chemical cleaning agents and demulsifiers used during tank washing
- Ballast water contaminated with cargo residues
- Sludge and sediment from the cargo tank floor
This mixture cannot be discharged directly overboard — it must be monitored by type-approved ODME and the discharge must meet the limits set by MEPC.108(49) before any discharge can proceed.
What Regulations Apply to Slop Tank Discharge?
Slop tank discharge from crude oil, product and chemical tankers is governed by MARPOL Annex I and IMO resolution MEPC.108(49) — the Revised Guidelines and Specifications for Oil Discharge Monitoring and Control Systems for Oil Tankers — as amended by MEPC.240(65).
The key discharge conditions under MEPC.108(49) are:
- The instantaneous rate of discharge must not exceed 30 litres per nautical mile
- The total quantity of oil discharged must not exceed 1/30,000 of the total quantity of the previous cargo
- The vessel must be more than 50 nautical miles from the nearest land
- The vessel must be proceeding en route
- The discharge monitoring and control system must be in operation
- All discharge operations must be recorded in the Oil Record Book Part II
Unlike the MARPOL bilge water standard — which sets an absolute oil concentration limit of 15ppm — the tanker slop discharge standard is expressed as a rate of discharge in litres per nautical mile and as a fraction of the previous cargo. This requires a fundamentally different monitoring approach from a standard oil content monitor.
Which Vessels Are Required to Fit ODME?
The ODME requirement under MEPC.108(49) applies to a specific category of vessels involved in tanker cargo operations. The table below sets out which vessel types are required to fit ODME and under what conditions.
| Vessel type |
When ODME is required |
| Crude oil tankers |
All crude oil tankers of 150 GT and above built after 2005; ODME required for slop and ballast discharge under MEPC.108(49) |
| Product and chemical tankers |
Required where tank washing and ballast discharge operations take place at sea |
| Combination carriers |
OBO and ore/oil vessels that carry oil cargo — ODME requirements apply to tanker-related discharge operations |
| FPSOs converted from tankers |
Where the FPSO retains tanker-derived slop tank, tank washing or ballast discharge systems — ODME requirements apply to those systems |
For marine vessel operators unsure whether their vessel requires ODME under MEPC.108(49), Rivertrace can advise on the applicable regulatory requirements for specific vessel types and discharge applications.
How Is ODME Different From a Standard 15ppm Bilge Alarm?
ODME and a standard 15ppm bilge alarm are both oil monitoring instruments but they are designed for completely different discharge applications, regulated by different MEPC resolutions and measure discharge in fundamentally different ways.
Standard 15ppm bilge alarm — MEPC.107(49)
A standard bilge alarm measures oil concentration in parts per million in oily water separator discharge from a vessel machinery space. It operates under a simple threshold — if oil concentration exceeds 15ppm, discharge stops. It is designed for relatively low-flow, stable composition bilge water streams.
ODME — MEPC.108(49)
ODME measures and records oil discharge as an instantaneous rate in litres per nautical mile, combined with calculation of the total cargo fraction discharged. It integrates with the vessel’s pumping systems and valve controls to automatically stop discharge if either the rate limit or the total cargo fraction limit is exceeded. It also provides an integrated electronic discharge record for the Oil Record Book Part II.
ODME is a significantly more complex system than a bilge alarm. It must be integrated with vessel speed, position and pumping data to calculate the instantaneous discharge rate and cumulative cargo fraction. The measuring cell must be replaced every five years to maintain type approval accuracy.
| A bilge alarm does not satisfy the ODME requirement
Fitting a standard MEPC.107(49)-approved bilge alarm to a tanker slop or ballast discharge system does not comply with the MEPC.108(49) ODME requirement. The two systems operate under different resolutions, measure discharge in different ways and produce different discharge records. A PSC inspection that finds a bilge alarm installed where ODME is required will result in a deficiency notice and potentially a port detention. |
What Does the Rivertrace SMART ODME Do?
The SMART ODME is a type-approved oil discharge monitoring and control system for crude oil, product and chemical tankers, meeting the requirements of MEPC.108(49) and MEPC.240(65). It continuously monitors, records and controls the discharge of slop water and ballast from cargo tanks.
Key capabilities of the SMART ODME include:
- Continuous measurement of oil-in-water concentration in the discharge stream
- Instantaneous discharge rate calculation in litres per nautical mile using vessel speed and flow rate data
- Automatic discharge control — stops discharge if the instantaneous rate exceeds 30 litres per nautical mile or if the total cargo fraction limit is being approached
- Vessel position and speed data integration for accurate discharge rate calculation and record-keeping
- Integrated data logging and Oil Record Book Part II output — providing the electronic discharge record required by MEPC.108(49)
- Remote access to discharge records via the Rivertrace digital compliance portal
- Pneumatic and electric pump options to suit different vessel configurations
Why Is ODME Servicing and Calibration Essential for Compliance?
An ODME system that is not correctly calibrated and serviced cannot produce accurate discharge records. Port state control inspections, SIRE audits and RightShip inspections all examine the calibration status and service history of ODME systems. Non-compliance findings related to ODME are among the most serious compliance risks for tanker operators.
Annual calibration
ODME systems must have their measuring cell accuracy verified at regular intervals. An uncalibrated or out-of-certificate ODME system may produce inaccurate discharge records — leaving operators unknowingly non-compliant with MEPC.108(49) even while the system appears to be functioning.
Five-year measuring cell replacement
The SMART ODME measuring cell is a precision optical device that must be replaced every five years to remain accurate and within type approval specification. This requirement is set out in MEPC.108(49). All routine maintenance of the ODME — including cell replacement — must be recorded in the vessel’s maintenance records.
Documentation and record-keeping
The calibration certificate, maintenance records and service history must be held onboard and available for inspection. A port state control inspector who finds that calibration records are missing or that the measuring cell has not been replaced within the required interval will treat this as a compliance failure regardless of whether the system is otherwise functioning correctly.
How Does the Rivertrace Fleet Compliance Assurance Service Support Tanker Operators?
Rivertrace offers a Fleet Compliance Assurance Service designed to support tanker operators in maintaining long-term ODME compliance across their fleet. The service goes beyond equipment supply to provide a structured compliance programme covering calibration, servicing, cell replacement and documentation.
The Fleet Compliance Assurance Service provides:
- Fixed-price service and spares agreements — supporting cost predictability and compliance planning across multiple vessels
- Calibrated measuring cell exchange and certified test data — ensuring replacement cells meet type approval accuracy requirements
- Digital certificate storage and access — calibration certificates and service records available online for inspection preparation
- Fleet-wide planning and scheduling — proactive scheduling of calibration and cell replacement before deadlines are reached
- Trained global engineer network — enabling fast turnaround on service visits and cell replacements in major port locations
| “With Rivertrace, we don’t just deliver ODME monitors — we create peace of mind between inspections.”
Martin Saunders, Managing Director, Rivertrace |
What Are the Compliance Risks of an Unserviced ODME System?
The table below summarises the main compliance risks associated with failing to maintain an ODME system correctly, and the consequences of each.
| Compliance risk |
Potential consequence |
| Outdated measuring cell |
Inaccurate readings and discharge violations; measuring cell must be replaced every 5 years under MEPC.108(49) |
| Missing calibration certificate |
Vessel detention or off-hire during PSC inspection if calibration records cannot be produced |
| Unserviced ODME system |
Inaccurate monitoring, reputational damage and financial penalties from non-compliant discharge |
| Incomplete Oil Record Book entries |
Enforcement action and port state control findings; operators cannot demonstrate compliant discharge |
The Rivertrace SMART ODME, when correctly serviced and calibrated, provides a verifiable and automated audit trail — giving operators confidence during inspections and demonstrable proof of compliance with MEPC.108(49) discharge requirements.
Who Is Responsible for ODME Compliance on a Tanker Fleet?
ODME compliance touches several roles within a tanker operating company:
Ship operations and engineering teams
Responsible for ensuring the ODME system is functioning correctly, that discharge operations are carried out within the permitted limits, and that Oil Record Book Part II entries are completed accurately for every discharge event.
Technical superintendents and fleet managers
Responsible for scheduling ODME servicing, cell replacement and calibration across the fleet — and for ensuring that service records and calibration certificates are current and available onboard for inspection at all times.
Procurement teams
Responsible for maintaining ODME spare part availability and for managing service contracts that ensure calibration and cell replacement obligations are met on schedule across multiple vessels.
Frequently Asked Questions
What is a slop tank on a tanker?
A slop tank is a dedicated tank used to collect oily wash water and cargo residues from cargo tank washing and ballasting operations. After cargo is discharged, the cargo tanks are washed with water to remove cargo residues. This wash water and contaminated ballast water is directed to the slop tank before treatment and monitored discharge under MEPC.108(49).
What is the difference between ODME and a standard bilge alarm?
A standard bilge alarm is designed for oily water separator discharge from a vessel machinery space under MEPC.107(49), measuring oil concentration against a 15ppm limit. ODME is designed for tanker slop and ballast discharge under MEPC.108(49), measuring discharge rate in litres per nautical mile and total cargo fraction. They are completely different systems for different discharge streams and cannot be substituted for one another.
How often does the SMART ODME measuring cell need to be replaced?
The ODME measuring cell must be replaced every five years to maintain type approval accuracy under MEPC.108(49). The replacement must be recorded in the vessel’s maintenance records. An out-of-date measuring cell is a compliance finding during port state control inspection or SIRE audit, regardless of whether the system otherwise appears to be functioning.
What does the Oil Record Book Part II record for ODME discharge operations?
The Oil Record Book Part II records all tanker slop and ballast discharge operations, including the date and position of discharge, the quantity discharged, the ODME readings during discharge, and any alarm activations or automatic discharge stops. The record must be available for inspection by port state control authorities and must be maintained accurately for every discharge event.
Does the ODME requirement apply to FPSOs converted from tankers?
Yes. Where an FPSO has been converted from a crude oil, product or chemical tanker and retains tanker-derived slop tank, tank washing or ballast discharge systems, the MEPC.108(49) ODME requirement applies to those specific discharge streams. The topsides produced water discharge from the FPSO’s processing system is subject to separate field permit requirements and requires a different type of monitoring equipment.
