HFO: the fuel that refuses to behave

Heavy fuel oil was never meant to be simple. It is the leftover of the refining process, the fraction nobody else wants, blended down into something a ship’s engine can burn without falling apart.

That has always made it a compromise fuel. What has changed in recent years is how many different compromises now sit inside a single bunker delivery, and how little margin is left once a vessel starts using it.

Those who have spent years sailing on ships that use HFO have one or two horror stories to tell about fuel-related disasters. The shift to very low sulphur fuel oil was supposed to close one long-running chapter of trouble. Sulphur compliance has, broadly, been achieved. But solving one problem in a blended fuel usually creates several more, because the components used to meet a sulphur target are not chosen for compatibility with one another. They are chosen because they are available, and because they are cheap enough to make the blend work commercially. The result is a fuel that can pass every parameter on a certificate of quality and still behave unpredictably once it is sitting in a ship’s tank for two or three weeks.

This is the part of the current HFO picture that deserves far more attention than it gets. Off-specification results still happen, and sulphur, water and sediment remain the recurring offenders, but outright non-compliance is not really the industry’s biggest problem anymore. The bigger problem is fuel that is technically on-spec and still causes chaos. Purifiers overloaded with sludge. Filters blocking faster than expected. Injectors wearing through cat fines that should never have made it past treatment. None of that necessarily shows up as a failed test result. It shows up three weeks into a voyage, usually at the worst possible moment.

Total sediment potential is a useful lens on this, because it captures instability rather than a single contaminant. A fuel can sit comfortably within its viscosity, density and sulphur limits and still be a poor blend, chemically unstable, prone to dropping sludge once storage conditions, heating cycles or mixing with other bunkers on board start to work on it. Engineers have always known that two fuels which look identical on paper can perform very differently at sea. What has changed is the frequency with which that gap now matters, because blending has become more aggressive and the range of components going into a modern residual fuel has widened considerably.

Some of that widening is driven by decarbonisation pressure working in an odd direction. Refiners chasing sulphur and, increasingly, carbon targets are reaching for blend components that would have been unusual a decade ago. Shale-derived oils are one example that has surfaced repeatedly this year, turning up in blends supplied out of major Asian bunkering hubs at concentrations high enough to raise real questions about stability, even where the finished fuel technically satisfied ISO 8217. None of this means every cargo containing an unconventional component is dangerous. It means the safety margin that operators used to take for granted, the assumption that an on-spec fuel is a safe fuel, can no longer be relied upon without question.

Cat fines still a problem

Cat fines remain the most mechanically punishing of the recurring issues, and they have not gone away simply because attention has shifted towards carbon and compliance. Aluminium and silicon abrasion continues to wreck injection equipment and cylinder liners in engine rooms that do not treat fuel aggressively enough before combustion. The uncomfortable truth is that purifier settings calibrated for an earlier generation of heavy fuel are not always adequate for today’s blends, and crews are still catching this the hard way, through wear rates rather than through warnings on a bunker delivery note.

Water content sits alongside sediment as the other quiet commercial drain on the industry. Most cases are manageable onboard; a percentage or two of water is not, by itself, catastrophic for a well-run separator, but it is never free. It costs fuel treatment time, it costs separator throughput, and at scale across a fleet it costs real money in fuel that was paid for as oil and delivered partly as water. Shipowners rarely make headlines over this kind of loss. They absorb it quietly, voyage after voyage, and it never quite goes away.

Biofuel blends

None of this is happening in a vacuum. It is landing on an industry that is simultaneously trying to manage the early stages of a fuel transition, bringing biofuel blends, principally FAME-based products, into the same bunkering infrastructure that has always handled straight residual fuel. The good news, such as it is, is that biofuel blending itself has not so far introduced a distinct new category of failure.

Most of the trouble still traces back to the conventional fuel component rather than the bio-derived fraction. But blending complexity of any kind adds variables, and variables are exactly what a stability-sensitive fuel does not need. Cold flow behaviour, storage duration, and compatibility with whatever is already in a ship’s tanks all become more sensitive to get wrong as blend recipes multiply.

The commercial and operational consequences of all this are not abstract. A vessel that de-bunkers a non-compliant cargo loses days and incurs costs that rarely fall cleanly on the party responsible for the problem. A vessel that keeps a marginal fuel and suffers filter or injector damage mid-voyage faces an unplanned repair bill, lost time, and in the worst cases a genuine safety event, particularly where flash point has drifted below the regulatory minimum. Insurers, class societies and flag administrations all end up drawn into disputes that a simple pass or fail test result was supposed to prevent. The paperwork says compliant. The engine room tells a different story.

None of this suggests the industry is powerless, though. Targeted fuel additives, properly formulated for the fuel chemistry in question rather than sold as a universal cure, can do a good deal to narrow the gap between a marginal blend and a stable one, improving combustion efficiency, keeping catalytic fines and sediment in suspension long enough for onboard separation to catch them, and reducing the sludge formation that drives filter blockage. Equally important, and too often overlooked, is proper homogenisation of fuel in storage and service tanks. A blend that has been left to stratify, with heavier and lighter fractions settling unevenly over a voyage, will behave far worse in practice than the same fuel kept properly mixed and circulated. Neither additive treatment nor homogenisation is a substitute for sound procurement and rigorous testing, but used well and applied consistently, together they can materially reduce the operational risk sitting inside an otherwise compliant cargo.

A shift in mindset

For operators, the practical answer has to be a shift in mindset rather than simply more testing. Routine specification analysis against ISO 8217 remains essential; nobody sensible is arguing for less of it, but it was never designed to catch the kind of conditional, in-service risk that a marginal, complex blend can carry. Supplier due diligence matters more than it used to.

So does a willingness to commission additional analysis, stability and compatibility testing in particular, on cargoes that look unusual even when the standard certificate comes back clean. Purifier performance needs checking before and after treatment, not assumed. And crews need to be told, plainly, that an on-spec bunker note is not the same thing as a guarantee of trouble-free running.

There is also a wider industry conversation that has not caught up with where the fuel supply chain actually is. Much of the public debate around marine fuel still frames the challenge as one of compliance, sulphur limits, emission control areas, decarbonisation targets, all of which matter, but all of which sit on top of a more basic question that gets far less airtime: is the fuel itself fit for purpose in service, not just fit for purpose on a test bench? That question is becoming harder to answer with confidence, precisely because the blends being supplied are more varied, more commercially engineered, and less predictable than the residual fuels the industry built its testing regimes around.

Geography and infrastructure still play their part in all of this, and it is worth being honest about where the risk concentrates. The major bunkering hubs, the ports handling the greatest volume and the greatest diversity of blend components, tend to show the widest spread of results, from entirely unremarkable fuel through to the more troubling outliers.

That is not really a surprise. High-volume blending operations, working with variable feedstock and tight commercial margins, are always going to produce more variation than smaller, simpler supply chains. It does mean, though, that a vessel’s exposure to risk is shaped as much by where it bunkers as by anything written on the delivery note, and owners planning voyages around the busiest hubs need to factor that into their fuel management, not treat it as background noise.

There is also a question of transparency that the industry has been slow to resolve. Operators are entitled to know, in reasonably specific terms, what has gone into the fuel they are burning, not simply that it meets a set of aggregate limits. That matters for engine room decision-making; it matters for emissions reporting as blended and biofuel products become more common, and it matters commercially when a dispute arises over damage or delay.

At present, the information supplied with a bunker delivery rarely goes much beyond the standard test certificate, and that certificate, however accurately completed, was never designed to describe blend composition or in-service behaviour. Closing that gap will take cooperation between suppliers, testing houses and owners that, frankly, does not yet exist in most parts of the market.

None of the above should trigger panic stations. Most fuel delivered to most ships most of the time performs adequately, and the outright horror stories remain a minority rather than the norm. But the margin for error has narrowed, and the industry’s tools for spotting trouble before it happens have not kept pace with the sophistication of modern blending.

Testing regimes built around a pass or fail reading of a fixed set of parameters were designed for a simpler fuel market than the one now supplying the world’s shipping. Heavy fuel oil was always a compromise. The compromise is simply getting more complicated, and it is the ships burning it and the crews running them who are left managing the consequences.