The Grey Area in Cargo Liquefaction

Under the International Maritime Solid Bulk Cargoes (IMSBC) Code, a cargo that presents a liquefaction risk is classified as Group A. The requirement that follows appears straightforward. The cargo must be tested to establish its transportable moisture limit (TML), and its actual moisture content must sit below that figure at the point of loading. A cargo even a fraction of a percentage point above its limit cannot be carried.

Set out that way, the regulation reads as a clean pass or fail. In practice, the position facing Owners, Charterers, and Masters is considerably less certain, and understanding why is central to managing the risk properly.

The first complication lies in the testing itself. Several different methods exist for establishing a TML, and they do not always return the same result for the same material. Each method is better suited to particular cargo characteristics, with grain size being one of the principal factors. One test may be appropriate for finer material, another for a defined mid-range, and a cargo that spans a wide range of grain sizes may not map neatly onto any single method. No definitive flow chart exists to direct a prospective Shipper to the correct choice.

"The rules are, on one hand, very strict," says Sam Clarke, a scientist at Brookes Bell specialising in cargo liquefaction. "On the other, the picture is rarely as simple as pass or fail. You can have a cargo where the limit itself shifts depending on which test you use, and that is a difficult message for Shippers to work with."

The consequence is a set of results that can appear contradictory. A moisture content of 8.8 per cent against a TML of 8.7 per cent indicates a cargo that cannot be carried. A repeat of the test returning a TML of 8.9 per cent would render the same cargo acceptable. For an owner presented with competing figures, the safe course is not always obvious from the paperwork alone.

Underlying this is the nature of liquefaction as a physical process. It is not a statistical certainty that can be read directly from a single measurement. The behaviour is governed by interactions between individual grains and by pore fluid pressure within the cargo, a phenomenon that operates at a scale the naked eye cannot assess. A cargo can appear stable and then fail suddenly under the motion of a vessel at sea.

"You cannot look at a cargo and say with certainty that it is going to liquefy," Clarke explains. "It sits in a grey area, because it comes down to what is happening at the scale of the grains and the water between them. That is why it helps to have someone who can look at the specific cargo and advise on how to test it and what the safety picture actually looks like."

Sampling introduces a further layer of difficulty, and it is frequently where apparently compliant cargoes come undone. A measurement of the material’s moisture content is only as reliable as the sample behind it. Material drawn from the top of a stockpile, where water has drained downward, will not represent the cargo as a whole. A moisture content established several days before loading tells the master little if the cargo has been stored uncovered and rained on in the interim, a common situation for many cargoes loaded in tropical regions. The figures may be technically valid while failing to reflect what is actually going into the hold.

The IMSBC Code anticipates part of this. Moisture content must be determined as close as possible to the time of loading, and a TML test must be conducted at least once every six months, or whenever the nature of the cargo changes materially. The provisions exist for sound reasons, but they are demanding to apply consistently, and the codes themselves are long, detailed, and in places open to interpretation.

The role Brookes Bell plays in these situations is one of interpretation and understanding. For Shippers, we can help decide which TML testing method is appropriate for a given cargo and how material should be sampled to produce a representative result of its moisture content. For Owners, we can help interpret certificates and results presented by Shippers. Where a certificate says one thing and the cargo in the hold says another, the value lies in explaining which evidence should carry weight and why.

That capacity to read a situation correctly matters most when a dispute is at stake. A position grounded in a clear scientific understanding of the cargo, the testing, and the sampling stands up to scrutiny in arbitration or court far better than one resting on paperwork accepted at face value.