A fuel can meet the sulphur limit, arrive with correct paperwork, and still wreck an engine. Since the 0.50% cap reshaped the bunker pool, the operational risk moved from compliance to compatibility, and it is managed before the hose is connected rather than after.
Compliant Does Not Mean Interchangeable
The sulphur cap solved an emissions problem and created an operational one. Before it, the residual fuel pool was relatively uniform in character. Afterwards, refiners met the limit through blending, and the result is a family of very low sulphur products whose composition varies with refinery configuration, feedstock, and region. Two stems can both certify at under 0.50% sulphur, both satisfy the specification, and behave completely differently in a fuel system.
That is the practical shift operators have had to absorb. The compliance question, is this fuel legal, is now the easy part and is answered on the bunker delivery note. The harder question, will this fuel work in this ship alongside what is already in the tanks, is answered by testing, planning, and segregation, and it is the one that damages machinery when it is skipped.
Two fuels can each meet the specification perfectly and still be ruinous when mixed. Compliance is a property of a fuel. Compatibility is a property of a pair.
Availability Is Not Uniform
MARPOL Annex VI obliges parties to take reasonable steps to promote the availability of compliant fuel oil and to notify the Organization where it is unavailable, and it provides for a fuel oil non-availability report where a ship genuinely cannot obtain compliant fuel despite best efforts. That mechanism exists precisely because availability is uneven.
For an operator planning a voyage, the useful consequence is that grade availability, not just price, belongs in the bunkering decision. Major hubs offer a wide choice of grades, competing suppliers, and reliable barge capacity. Smaller ports may offer one grade from one supplier with limited flexibility on timing. Where a vessel’s trading pattern includes such ports, planning the stem at the previous hub is frequently the better answer than arriving with empty tanks and taking what is offered.
The non-availability route is genuinely a last resort rather than a planning tool. It requires evidence of the efforts made to obtain compliant fuel, and it is not a substitute for having planned the bunkering properly. An operator who reaches for it routinely is describing a scheduling failure rather than a market one.
Where Quality Actually Varies
Beyond sulphur, several parameters differ meaningfully between compliant fuels, and each has a distinct consequence aboard.
Stability and compatibility: Whether the fuel holds its asphaltenes in suspension, and whether it will do so when commingled with a different blend. Instability produces sludge that blocks filters and separators.
Catalytic fines: Hard aluminium and silicon particles carried over from refining. They are abrasive, and inadequate settling and purification lets them reach fuel pumps and liners, where the wear is rapid and expensive.
Viscosity and cold flow: Blended low sulphur fuels vary widely, and pour point and wax behaviour matter for heating and handling in a way heavy fuel practice did not always require.
Ignition and combustion quality: Variation affecting how the fuel burns, with consequences for engine performance that may not appear immediately.
Flash point: A safety parameter with a hard minimum, and an off-spec result here is a serious matter rather than a quality inconvenience.
Compatibility deserves the most attention because it is the parameter that depends on two fuels rather than one. A stem can be entirely within specification and still be incompatible with the fuel already aboard, and the resulting sludge appears in the separators and filters within days. This is why segregation is the default discipline: keeping stems in separate tanks where possible, and where commingling is unavoidable, testing compatibility before mixing rather than discovering it afterwards.
The Sampling Regime Is the Protection
Everything an operator can later prove about a fuel rests on the samples taken at delivery. MARPOL Annex VI requires the bunker delivery note to be accompanied by a representative sample of the fuel oil delivered, sealed and signed, retained aboard for the required period. That MARPOL sample is the reference point in any subsequent dispute, and its handling is not a formality.
Practically, this means the sample must be drawn continuously throughout the delivery at the receiving ship’s manifold, so it genuinely represents what came aboard rather than the first or last portion. It must be witnessed and sealed with seal numbers recorded on the delivery note, and additional samples are typically drawn for the ship’s own testing and for the supplier. Where the sampling point or the method is not as required, the operator’s ability to prove anything later is weakened at exactly the moment it matters.
The sample is the only evidence that survives the voyage. Everything about a fuel dispute, months later, turns on whether it was drawn properly and sealed in front of the right people.
The Pre-Bunkering Sequence
Reduced to what a vessel actually does, protection against a bad stem is a short sequence carried out in order, before and during delivery.
The last step is the one most often compressed by schedule pressure, and it is the one that prevents the expensive outcome. Laboratory turnaround on a bunker sample is short relative to the cost of running an off-specification or incompatible fuel through main engine systems, and a vessel that can burn its previous stem while the new one is tested has bought itself the option to reject.
What This Costs, and What It Saves
None of this is elaborate. It is a specification written properly, a tank plan decided in advance, samples drawn to the required method, and a testing habit that treats analysis as routine rather than exceptional. Set against it is the alternative: filters and separators blocked by incompatible blends, abrasive wear from cat fines that shortens component life, an engine problem at sea, or a dispute in which the operator cannot prove what was delivered because the sampling was not done properly.
The sulphur cap made fuel compliance a documentary matter, which is straightforward. What it did not do is make fuel uniform. Availability varies by port, quality varies by blend, and compatibility varies by combination, and all three are managed before a hose is connected rather than after a separator starts filling with sludge.
Frequently Asked Questions
If a fuel meets the sulphur limit, is it safe to use?
Not necessarily. Meeting the 0.50% limit answers the compliance question but says nothing about stability, compatibility with fuel already aboard, catalytic fines content, viscosity behaviour, or combustion quality. Because compliant low sulphur fuels are blended products that vary by refinery, feedstock and region, two fully compliant stems can behave very differently and can be incompatible with one another.
What is the MARPOL sample and why does it matter?
MARPOL Annex VI requires the bunker delivery note to be accompanied by a representative sample of the fuel delivered, sealed, signed, and retained aboard for the required period. It is the reference sample in any later dispute about what was actually supplied. To be meaningful it must be drawn continuously throughout delivery at the receiving ship’s manifold, witnessed, and sealed with the seal numbers recorded on the delivery note.
Why is compatibility treated separately from quality?
Because compatibility is a property of two fuels together rather than one fuel alone. A stem can be fully within specification and still be incompatible with the fuel already in the tank, causing asphaltenes to drop out as sludge that blocks filters and separators. This is why segregating stems in separate tanks is the default practice, and why compatibility should be tested before commingling rather than assumed.
What is a fuel oil non-availability report?
It is the mechanism under MARPOL Annex VI for a ship that cannot obtain compliant fuel oil despite best efforts, supported by evidence of the attempts made. It is a last resort rather than a planning tool. Because availability of specific grades varies considerably between major hubs and smaller ports, the better approach is to plan the stem where supply is reliable rather than arrive somewhere with empty tanks and limited options.
Sources: IMO MARPOL Annex VI, Regulation 18, Fuel oil availability and quality (bunker delivery note, representative sample, and fuel oil non-availability reporting) · IMO MARPOL Annex VI, Regulation 14, Sulphur oxides and particulate matter · ISO 8217, Petroleum products, Fuels (class F), Specifications of marine fuels · IMO MSC-MEPC.2/Circ.18 (11 July 2024), Guidelines for the sampling of fuel oil for determination of compliance, which revoked Resolution MEPC.182(59) and extended use of the MARPOL delivered sample to the SOLAS II-2/4.2.1 flashpoint check · IMO Resolution MEPC.320(74), Guidelines for consistent implementation of the 0.50% sulphur limit
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