| MGO | MDO | |
|---|---|---|
| ISO 8217 grade | DMA | DMB / DMZ |
| Composition | Pure distillate | Distillate + residual blend |
| Max viscosity | 6.0 cSt at 40°C | 11.0 cSt at 40°C |
| ECA compliant? | Yes (DMA) | DMZ only (≤0.10% S) |
| Heating required | No | No |
| Typical use | ECAs, small vessels, modern eco-ships | Ocean passages, older engines |
MGO and MDO are the two distillate fuel grades that ships use when heavy fuel oil is either unsuitable or prohibited. The difference between them — in viscosity, sulphur content, ISO grade, and cost — determines which fuel is loaded, when it must be used, and what the chief engineer must document to satisfy MARPOL Annex VI.
This article covers fuel properties and ISO 8217 grades, the sulphur compliance regime under MARPOL, what a bunker delivery note must contain, VLSFO compatibility risks, and what port state control examines when it boards.
What are MGO and MDO, and how do they differ from HFO?
Marine gas oil (MGO) and marine diesel oil (MDO) are distillate fuels produced from the lighter fractions of crude oil refining. Heavy fuel oil (HFO) is a residual product — what remains after the lighter fractions are removed. The fundamental difference is refining depth: distillates are cleaner, lower in sulphur, lower in viscosity, and require no heating to pump or inject.
MGO consists exclusively of distillates — no residual blending. MDO is a blend of distillates with a small proportion of residual fuel, which raises its viscosity and density slightly above MGO but keeps it pumpable at ambient temperature. HFO requires heating to 120–130°C before it can be pumped and injected — a fuel treatment system that small vessels and many modern eco-ships avoid entirely.
The practical consequence for shipboard operations is that MGO and MDO can be used in any diesel engine without preheating, purification is simpler, and the fuel is compatible with the engine’s standard injection timing. How a fuel oil purifier works differs between distillate and residual fuels — purifier settings, temperature, and throughput rate must be adjusted when changing fuel grade.
What are the ISO 8217 grades for MGO and MDO?
ISO 8217:2017 is the international specification standard for marine fuels. It classifies distillate marine fuels under the ‘DM’ series and residual fuels under the ‘RM’ series. MGO corresponds to grade DMA — the cleanest distillate grade. MDO corresponds to grades DMB and DMZ, which permit a small residual content and slightly higher viscosity and density than DMA.
| Property | MGO (DMA) | MDO (DMB/DMZ) | VLSFO (RMG/RMK) |
| ISO 8217 grade | DMA | DMB / DMZ | RMG 380 / RMK 700 |
| Density at 15°C | Max 890 kg/m³ | Max 900 kg/m³ | Max 991 kg/m³ |
| Viscosity | Max 6.0 cSt at 40°C | Max 11.0 cSt at 40°C | 380–700 cSt at 50°C |
| Flash point | Min 60°C | Min 60°C | Min 60°C |
| Sulphur limit (global) | Max 0.50% | Max 0.50% | Max 0.50% |
| Sulphur limit (ECA) | Max 0.10% ✓ | Max 0.10% (if compliant grade) | Not ECA-compliant |
| Heating required | No | No | Yes (120–130°C) |
| Cetane index | Min 40 | Min 35 | Not specified |
The DMZ grade introduced in ISO 8217:2017 bridges DMA and DMB — it permits higher viscosity than DMA but requires zero sulphur above 0.10%, making it explicitly ECA-compliant. This grade was created to address VLSFO compatibility issues by providing a defined blended distillate option within the ISO framework.
What does the ISO 8217 flash point requirement mean operationally?
SOLAS Chapter II-2 Regulation 4 requires all fuel oil used on board to have a flash point of not less than 60°C. ISO 8217:2017 mirrors this requirement for all DM and RM grades. A bunker delivery note that shows a flash point below 60°C is grounds for rejecting the fuel — the chief officer or chief engineer must note the discrepancy on the BDN before the delivery hose is disconnected.
Flash point is not viscosity — a low flash point fuel flows easily but ignites at dangerously low temperatures. The 60°C minimum provides a safety margin against ignition in the fuel handling system. Fuels with flash points between 60°C and 70°C require careful handling near hot surfaces in the engine room.
What sulphur limits does MARPOL Annex VI impose on MGO and MDO?
MARPOL Annex VI Regulation 14 sets global and ECA sulphur limits for fuel oil used on ships. The global sulphur cap has been 0.50% m/m since 1 January 2020. Inside Emission Control Areas (ECAs), the limit is 0.10% m/m. Vessels must use fuel that meets the applicable limit for their current geographic position — not their flag state or port of registry.
MGO (DMA grade) typically has sulphur content well below 0.10% and satisfies both the global cap and the ECA limit without modification. Standard MDO (DMB grade) may contain up to 0.50% sulphur — it meets the global cap but does not meet the ECA 0.10% limit unless specifically supplied as a low-sulphur grade. The BDN must state the actual sulphur content of the fuel delivered.
What are the Emission Control Areas where the 0.10% limit applies?
- Baltic Sea ECA — established under MARPOL Annex VI, effective 2015
- North Sea ECA — established under MARPOL Annex VI, effective 2007
- North American ECA — covers waters within 200nm of US and Canadian coasts, effective 2012
- US Caribbean Sea ECA — covers waters around Puerto Rico and US Virgin Islands, effective 2014
Vessels that cannot or will not use 0.10% sulphur fuel in ECAs may fit an exhaust gas cleaning system (EGCS) instead — a technology that removes sulphur compounds from the exhaust after combustion. Ship scrubbers are the approved MARPOL Annex VI equivalent arrangement under Regulation 4, but their use in certain ports is now restricted by local regulations that prohibit open-loop scrubber discharge.
What does MARPOL Annex VI Regulation 14.6 require when changing fuel before entering an ECA?
MARPOL Annex VI Regulation 14.6 requires ships to carry a written procedure for fuel oil changeover before entering an ECA. The procedure must show the volume of low-sulphur fuel in each tank and the date, time, and position where the changeover is completed. This information must be recorded in the ship’s log book.
The changeover must be completed before the vessel crosses the ECA boundary — not after. A vessel still burning 0.50% fuel inside an ECA boundary is in violation regardless of whether a changeover is in progress. PSC officers specifically check the log book entries and the BDN date against the vessel’s track to verify compliant changeover timing.
What must a bunker delivery note contain under MARPOL?
MARPOL Annex VI Regulation 18.5 requires a bunker delivery note (BDN) to be provided for each fuel oil delivery. The BDN must be retained on board for at least three years after delivery. It is a primary document of compliance — PSC officers examine it at every port call when an Annex VI inspection is conducted.
What information is mandatory on a MARPOL-compliant BDN?
- Name and IMO number of the receiving ship
- Port of bunkering
- Date of commencement of delivery
- Name, address, and telephone number of the fuel oil supplier
- Product name(s)
- Quantity delivered in metric tonnes
- Density at 15°C in kg/m³
- Sulphur content as a percentage m/m
- Declaration that the fuel oil supplied conforms to MARPOL Annex VI Regulation 14.1 or 14.4
The chief engineer or duty officer must verify the BDN against the supplier’s certificate of quality before signing. A BDN signed without verification creates a compliance risk — if the fuel is subsequently found off-specification, a signed BDN without notation makes it harder to demonstrate the ship was not aware of the deficiency.
What is the MARPOL representative sample and how long must it be retained?
MARPOL Annex VI Regulation 18.8 requires a representative sample of each fuel oil delivery to be sealed and retained on board for 12 months after the date of delivery. The sample must be taken using a drip sampling method during the delivery — not from the bunker tank after delivery is complete. Bunkering operations require the bunker surveyor or duty engineer to supervise sample collection and witness the sealing and labelling. A correctly retained MARPOL sample is the ship’s primary defence in an off-specification fuel dispute.
The sample label must show the vessel name, bunker port, date, grade, and the name of the witness. If a dispute arises over fuel quality — engine damage, filter clogging, or PSC non-compliance allegation — the MARPOL sample is sent to an independent laboratory. A missing or improperly labelled sample significantly weakens the ship’s position in any subsequent claim.
What are the compatibility risks of VLSFO with MDO and MGO?
Very Low Sulphur Fuel Oil (VLSFO) — supplied as ISO 8217 grades RMG 380 or RMK 700 with sulphur content at or below 0.50% — is the residual fuel grade that most deep-sea vessels adopted after the 2020 global sulphur cap. It is not a distillate fuel. Mixing VLSFO with MDO or MGO in tanks or fuel lines creates compatibility risks that distillate-to-distillate mixing does not.
VLSFO produced from different crude sources or by different refinery processes has highly variable chemical composition. When a vessel changes from VLSFO to MDO — or back — residual VLSFO in the system mixes with the incoming distillate. The resulting blend may be unstable: asphaltenes in the VLSFO can precipitate out of solution when the blend’s solubility balance is disturbed, forming sludge that blocks filters, purifiers, and injection equipment.
Cat fines — catalytic cracking residues carried over from the refinery into VLSFO — cause severe abrasive wear on fuel injection equipment and piston rings. MDO and MGO do not carry cat fines.
A vessel switching from VLSFO to MDO for ECA entry carries residual VLSFO cat fines in the system until the distillate has flushed them through — the fuel purifier must run at reduced throughput and the filter differential pressure monitored closely during the changeover period. Contaminated fuel displaced during changeover may be directed to slop tanks, where it is segregated until it can be processed through the purifier or discharged ashore.
What is the correct procedure for changing from VLSFO to MDO before an ECA?
- Confirm MDO tank quantity — sufficient for ECA transit plus reserve
- Calculate changeover position — allow sufficient time at reduced speed to complete before the ECA boundary
- Switch settling tank supply to MDO — monitor transfer pump operation
- Run purifier at reduced throughput — increase efficiency at the lower viscosity of distillate
- Monitor service tank density — confirm VLSFO is displaced before ECA entry
- Monitor fuel filter differential pressure — increased dp indicates cat fine or sludge carry-over
- Record changeover in log book — date, time, position, tank quantities, sulphur content of fuel in use
- Retain the BDN and MARPOL sample for both fuel grades on board
What does a bunker survey verify about MDO and MGO quality?
A bunker survey verifies the quantity, quality, and MARPOL compliance of fuel delivered to the ship. The bunker surveyor witnesses the entire delivery — from tank sounding before and after to sample collection and BDN signing. The quantity dispute between the ship’s figure and the supplier’s flow-meter figure is the most common bunker survey finding; quality disputes are less frequent but more commercially significant.
The bunker surveyor’s role includes checking the BDN for MARPOL-required information, witnessing the MARPOL representative sample collection and sealing, and noting any discrepancies between the declared specification and the supplier’s certificate of analysis. Off-specification fuel — sulphur above declared, flash point below 60°C, viscosity outside the ISO 8217 limit — is noted in the survey report and forms the basis of a supplier claim.
Independent fuel testing services — including Lloyd’s Register FOBAS, Veritas Petroleum Services (VPS), and DNV Petroleum Services — analyse samples against ISO 8217 parameters and provide a certificate of analysis. Shipping companies routinely send every bunker sample for independent analysis. An off-spec finding triggers a formal claim against the supplier, and the MARPOL sample is retained until the claim is resolved.
What does port state control examine regarding fuel oil compliance?
Port state control officers conducting an MARPOL Annex VI examination check four documents: the bunker delivery notes for fuels on board, the fuel oil record book (FORB), the MARPOL representative samples, and the written ECA fuel changeover procedure. For tanker operators, the same fuel documentation is reviewed during vetting inspections — SIRE and CDI auditors treat fuel sulphur compliance and BDN retention as standard checklist items.
Deficiencies in any of these — missing BDN, incomplete FORB entries, absent samples, or no changeover procedure — result in a deficiency notation and, for serious non-compliance, detention.
The fuel oil record book must record every fuel oil transfer, bunkering operation, and ECA changeover. An FORB that shows a gap in entries during a period when the vessel transited an ECA triggers a detailed PSC examination. Port state control officers cross-reference the FORB entries against the AIS track — a vessel whose FORB shows no ECA changeover but whose AIS track shows it entered the Baltic Sea is facing a serious non-compliance allegation.
The in-use sample — taken from the fuel supply line to the main engine while the vessel is in port — must have sulphur content matching the fuel declared in the FORB for the port. A mismatch between the in-use sample test result and the declared fuel grade is treated as direct evidence of non-compliance. The consequences range from a deficiency requiring documentary correction to a case referral to the flag state for potential prosecution.
Frequently Asked Questions
What is the difference between MGO and MDO?
MGO (marine gas oil, ISO 8217 grade DMA) is a pure distillate with maximum viscosity 6.0 cSt at 40°C and maximum sulphur 0.10% when supplied as ECA-grade. MDO (marine diesel oil, ISO 8217 grade DMB or DMZ) permits a small residual content, has a higher maximum viscosity of 11.0 cSt at 40°C, and may contain up to 0.50% sulphur in standard grade. Both have a minimum flash point of 60°C under SOLAS and ISO 8217.
Can MDO be used inside an Emission Control Area?
Standard DMB-grade MDO with sulphur content above 0.10% cannot be used inside an ECA. DMZ-grade MDO — specifically supplied with sulphur at or below 0.10% — is ECA-compliant. The BDN must state the actual sulphur content of the delivered fuel. The vessel must verify the sulphur figure on the BDN before accepting delivery if ECA operation is planned.
How long must a bunker delivery note be retained on board?
MARPOL Annex VI Regulation 18.5 requires bunker delivery notes to be retained on board for not less than three years after the date of delivery. The MARPOL representative sample must be retained for 12 months. Both must be available for inspection by PSC officers at any port call during the retention period.
What is the fuel oil record book and who must sign it?
The fuel oil record book (FORB) records all bunkering, internal fuel transfers, and ECA fuel changeovers. It is required under MARPOL Annex VI Regulation 17 for vessels of 400 GT and above. Each completed operation must be signed by the officer responsible and the master must countersign each page. The FORB must be retained on board for three years after the last entry.
What causes lacquering when burning MGO?
Lacquering occurs when MGO combustion produces dry, hard deposits that adhere to the cylinder liner surface and disrupt the oil film. MGO has lower lubricity than MDO or HFO — it contains fewer lubricating components. Some operators add lubricity improver additives to MGO to reduce liner wear. The risk is highest when switching from HFO or VLSFO to MGO on engines not designed to handle low-lubricity fuels.
Is VLSFO the same as MGO or MDO?
No. VLSFO (Very Low Sulphur Fuel Oil) is a residual blended fuel — ISO 8217 grades RMG or RMK — with sulphur at or below 0.50% to meet the 2020 global cap. It is not a distillate. It requires heating, purification, and cannot be used in ECAs. MGO and MDO are distillates that require no heating and MGO/DMZ grades are ECA-compliant. VLSFO and distillates are also potentially incompatible when mixed.
The long-term trajectory of marine fuel is away from both HFO and VLSFO toward LNG, methanol, and ammonia as zero-carbon or low-carbon alternatives. Dual-fuel engine technology bridges the transition — vessels ordered today are frequently built with dual-fuel capability that allows operation on LNG, methanol, or MGO from the same engine platform, protecting the asset against future fuel regulation changes.
What happens if a ship is found using non-compliant fuel in an ECA?
Non-compliant fuel use in an ECA is a MARPOL Annex VI violation reportable to the flag state Administration. PSC officers may detain the vessel pending fuel change, issue a deficiency report, and refer the case to the flag state for enforcement action. Fines vary by flag state and port state — in the United States, MARPOL violations can result in significant financial penalties and in some cases criminal prosecution of the responsible officer. Port state control procedures vary by MoU region but all treat fuel sulphur non-compliance as a high-priority deficiency category.
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