Knowledge / Maritime cargo

Bulk-cargo liquefaction: moisture, sampling and acceptance limits

Cargo identity, representative sampling, moisture control and acceptance decisions for liquefaction and dynamic separation.

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A dry-looking surface does not prove that a bulk cargo will remain stable at sea. In some fine-grained materials, water, particle structure and repeated motion combine to change strength or redistribute the cargo. The consequence can extend well beyond commercial damage: cargo movement can seriously impair ship stability. A defensible acceptance decision depends on correct cargo identification and current evidence that represents the material actually being delivered. This article explains the mechanism and evidence chain; it is not a laboratory procedure or an instruction to accept a particular shipment.

Identify the cargo and the applicable edition

The IMSBC Code provides the principal framework for solid bulk cargoes; grain is addressed separately. A commercial product name may differ from the Bulk Cargo Shipping Name. Changes in particle distribution or production can invalidate an assumption that a previous shipment's classification still applies. Group A concerns physical instability hazards; Group B concerns chemical hazards, and a cargo can fall into both groups.

Edition control matters. As at 6 October 2026, voluntary early application of amendment 08-25 has begun, with mandatory entry into force set for 1 January 2027. IMO's MSC 110 summary distinguishes these dates. The latest published text should not automatically be described as mandatory everywhere without checking flag-State and port implementation.

Amendment 07-23, forming the 2023 text, entered into force on 1 January 2025; the mandatory baseline and voluntary early application must be distinguished in a 2026 assessment. MSC.539(107).

Why particle contact matters

In dry or sufficiently draining granular material, resistance to shear depends substantially on contacts between particles. Under repeated loading, the particle skeleton may tend to become more compact. If water cannot escape as quickly as the pore volume decreases, pore-water pressure can rise. Effective stress carried by the particle contacts falls, reducing resistance to shear. This basic mechanics explains how apparently solid material can develop highly mobile behaviour.

The process can develop cumulatively before one conspicuous large wave arrives. Material that looks stiff or cohesive in a grab at the terminal is not thereby characterized for prolonged vibration and motion. Particle sizes, their distribution, permeability, mineral properties and the location of moisture all matter. A limit established for another material cannot be transferred merely because the cargo has a similar appearance or trade description.

Dynamic separation is a distinct mechanism

Dynamic separation can produce an upper slurry layer of water and fines, creating free-surface effects and uneven cargo distribution. AMSA's official explanation discusses this mechanism, particularly for bauxite fines, and distinguishes it from conventional liquefaction. The same mechanism or laboratory method must not be assumed for every wet bulk material.

Operationally, the hazard need not wait until the entire cargo visibly flows like water. Local wet regions, liquid at the surface or unusual vessel motions can point to changing conditions. A list of symptoms cannot establish a remote diagnosis, however. Observations should be compared with the cargo information and escalated through the ship's safety-management arrangements. Waiting to establish a precise label for the mechanism should not delay protective decisions.

A TML value is not a cargo passport

The transportable moisture limit, or TML, is established by an appropriate test method. Group A cargo on an ordinary vessel must have actual moisture below its TML; special-ship exceptions require separate approval. A dry result in the auxiliary can test does not establish compliance. These boundaries are addressed in sections 7 and 8 of the 2023 IMSBC text.

TML and current moisture answer different questions. One describes a tested transportability boundary; the other describes the condition of the shipment. A laboratory's precise moisture measurement cannot repair a sample taken from the wrong stockpile. A correctly located sample can also mislead if it loses moisture before testing. Reliability therefore starts with representativeness and preservation, not with the number of decimal places on the certificate.

Connect the sample to the shipment

A representative sampling plan accounts for different stockpile regions, depths and production periods. Taking material only from an accessible, sun-dried surface may overlook wetter material beneath it. Sampling location, time, lot boundaries, container integrity and transfer to the laboratory need traceability. Rain protection and drainage arrangements must work in the field, rather than existing only in a written procedure.

New material added after sampling, mixed production lots or substantial rainfall can weaken the relevance of previous evidence. Looking only at the certificate date is then inadequate. The issue is whether the documented material has changed physically. Separating accepted stock from material awaiting assessment is a practical control against a loader collecting from the wrong pile.

Sampling uncertainty and test uncertainty should also be distinguished. Repeating a laboratory measurement can improve understanding of the material in the sample jar, while leaving an unrepresentative sampling strategy untouched. An investigation should ask where disagreement entered the chain before commissioning more repetitions of the same downstream test.

A numerical example with explicit uncertainty

Assume a hypothetical lot has a laboratory-established TML of 9.0 percent and measured moisture of 8.4 percent. The numerical difference is 0.6 percentage points. It is not a probability of safe behaviour or a prediction of how many days the cargo can withstand at sea. This example assumes correct classification, representative sampling, an appropriate method and no subsequent change in the lot. It illustrates comparison of two quantities only.

If the stated measurement uncertainty is ±0.2 percentage points, the reported interval is approximately 8.2–8.6 percent. That interval does not necessarily include sampling error or every spatial variation in the cargo. If three truckloads give 8.0, 8.4 and 9.2 percent, casually averaging them does not eliminate the wetter material. Lot definition and conformity assessment must follow the competent sampling and control procedure.

After rain, presenting the original 8.4 percent certificate again is not evidence of the new condition. Assessment must examine what was exposed, how protected and exposed material were separated, and whether renewed sampling is required. The objective is current evidence describing the actual shipment, rather than a number that conveniently permits the vessel to depart.

Test-method and timing boundaries

TML is not obtained by one universal test or multiplier for every material. Suitability of a flow-table, penetration or compaction-based method depends on cargo characteristics. A report containing only the TML number is incomplete as an explanation: the method and sample must be appropriate to the cargo. Taking a ratio used in one method and presenting it as a general rule for all Group A materials conceals important differences.

Sample preparation is part of the result. Removing coarse particles, drying, mixing or dividing the sample must follow the defined method. Preparation that changes the behaviour of the material without being part of that method can introduce bias. A laboratory's competence and report traceability therefore matter alongside its name. This educational discussion does not provide a laboratory recipe for conducting the tests.

IMSBC 4.5 requires TML testing within six months before loading, repeated if characteristics change. Moisture sampling and testing should be as close as practicable to loading commencement, never more than seven days beforehand. Significant rain or snow requires evidence that moisture remains below TML.

Those intervals are not automatic guarantees. Yesterday's sample is not current evidence if it represents the wrong lot; a recent TML can also lose relevance after a production change. Timing and change control must work together. Information exchange between shipper, terminal and vessel therefore concerns preservation of the physical condition represented by the certificate, as well as transmission of the certificate itself.

Acceptance and voyage boundaries

Where cargo information, tests and observed condition conflict, acceptance should pause for competent resolution. Loading questionable material first and correcting the paperwork later transfers the hazard from shore to ship. Rain, changed material or free water observed during loading should be recorded and managed through the agreed stop process. A general educational article cannot accept cargo on behalf of the master by comparing two numbers.

Unexpected list or motion at sea requires ship-specific assessment; improvised ballast changes can worsen the situation. Action depends on approved vessel information, the master's judgement and appropriate expert support. Uncontrolled entry into a hold to inspect the cargo is unacceptable because atmospheric and physical hazards require their own controls. The strongest defence is to maintain cargo identity, sampling, testing and change control as one continuous chain before the problem develops offshore.

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