TML sampling and testing determines the Transportable Moisture Limit of solid bulk cargoes that may liquefy during sea transport, as required by the IMO International Maritime Solid Bulk Cargoes (IMSBC) Code. Certified TML results, compared against the actual moisture content of the cargo, are the primary safeguard against liquefaction-related vessel casualties in the dry bulk trade. This technical guide explains the sampling procedures, applicable test methods, cargo types, and compliance requirements involved in TML testing.
Technical Overview
Transportable Moisture Limit (TML) sampling and testing is the mandatory analytical process used to determine whether a solid bulk cargo can be shipped safely without the risk of liquefaction. The TML is defined as the maximum moisture content at which a cargo remains safe for transport in a ship's hold under the vibration, rolling, and slamming forces encountered at sea. Under the International Maritime Solid Bulk Cargoes (IMSBC) Code, which became mandatory through the SOLAS Convention in 2011, every cargo classified as Group A, meaning a cargo that may liquefy, must be certified with both a TML value and an actual moisture content before loading commences.
Liquefaction occurs when cyclic stresses generated by vessel motion cause pore water pressure to build up within a saturated, fine-grained mass. The cargo loses shear strength, begins to behave like a dense fluid, and can shift rapidly to one side of a hold, producing sudden list, loss of stability, and in severe cases capsize. Multiple bulk carriers and their crews have been lost to liquefaction of nickel ore, iron ore fines, and bauxite cargoes over the past two decades, which makes TML certification one of the most safety-critical procedures in the maritime industry.
- Transportable Moisture Limit (TML): the maximum moisture content permitted for safe carriage of a Group A cargo.
- Flow Moisture Point (FMP): the experimentally determined moisture content at which the material first develops flow characteristics under standardized dynamic loading.
- Moisture Content (MC): the actual water content of the cargo at the time of shipment, which must be below the TML.
- Group A cargo: a cargo that may liquefy if shipped at a moisture content in excess of its TML.
For the flow table and penetration test methods, the TML is calculated as 90 percent of the Flow Moisture Point, providing a safety margin between certified behavior and the onset of flow. The shipper must declare the TML and moisture content on the cargo information document, and the master has the right to refuse cargo that arrives without valid certification or that shows visible signs of high moisture.
Testing Samples
TML testing applies primarily to fine-grained, moisture-sensitive Group A bulk cargoes. Because liquefaction potential depends on particle size distribution, mineralogy, and moisture retention, samples must be collected in a manner that faithfully represents the entire parcel to be loaded. Typical sampling points include mine stockpiles, conveyor belt transfer points, blending yards, barges, truck loads, and ship holds, with increments taken systematically across the full depth and cross-section of the material.
Iron Ore Fines
Fine iron ore products, including sinter feed and fines from beneficiation plants, are among the most widely tested Group A cargoes. Export flows from Australia, Brazil, India, and West Africa are sampled at mine sites and loadout terminals in accordance with ISO 3082, which defines increment mass, number, and precision requirements for iron ore lots. Particular attention is given to cargo exposed to rainfall between sampling and loading, which requires fresh moisture content determinations.
Nickel Ore
Laterite nickel ore shipped from Indonesia and the Philippines is historically the highest-risk liquefaction cargo, combining fine particle size, high natural moisture, and strong moisture heterogeneity within a single parcel. Sampling protocols require layered extraction from stockpiles and holds, with increments taken from different horizons so that the wettest material in the parcel is captured rather than averaged away.
Bauxite
Following the loss of vessels carrying bauxite, notably in 2015, the IMSBC Code introduced a revised bauxite schedule and a dedicated Modified Proctor/Fagerberg test procedure. Bauxite from major exporters such as Guinea is sampled to assess both the tri-hydrate gibbsite content and the saturation behavior that governs its transportable moisture limit.
Mineral Concentrates
Flotation concentrates of copper, lead, zinc, tin, and nickel are produced as fine, relatively cohesive filter cakes whose delivered moisture often sits close to the flow point. These materials are sampled during dewatering, bagging or stockpiling, and at the load port in accordance with ISO 12743, and they are routinely certified using the Proctor/Fagerberg or penetration methods.
Other Group A Cargoes
Additional materials subject to TML testing include mill scale, sinter feed, mineral sands such as ilmenite, quarry fines, coal slurries and filter cakes, and certain industrial residues. When a new or unlisted cargo is suspected of liquefaction potential, shippers should arrange TML characterization and consult the competent authority, because classification of a cargo as Group A triggers the full certification requirement.
Testing Items
A complete TML survey consists of a coordinated set of determinations. The core test program includes the following items, each reported with sampling date, location, lot identification, and applicable method reference.
- Transportable Moisture Limit (TML): the certified limit used for the shipper's declaration.
- Flow Moisture Point (FMP): the directly measured flow threshold from which the TML is derived.
- Moisture Content (MC): the actual water content of the cargo at the time of sampling, determined by oven drying.
- Particle size distribution: sieve analysis confirming the grading applicable to the selected test method.
- Bulk density and dry density: parameters required for saturation calculations in the Proctor/Fagerberg methods.
- Supplementary geotechnical indicators, such as plasticity characterization, where required for cargo assessment or dispute resolution.
The decision logic of the certification is simple: the parcel is acceptable only when the declared moisture content is lower than the TML, and any cargo that has been wetted after testing, blended with different material, or stored for an extended period must be resampled and retested before loading.
Testing Methods
Sampling and Sample Handling
Representative sampling follows recognized standards such as ISO 3082 for iron ore, ISO 12743 for mineral concentrates, and the corresponding coal sampling standards for carbonaceous cargoes. Primary increments of specified minimum mass are collected at set intervals from moving streams, belts, or systematic grids, then combined into gross samples and reduced by riffle splitting. Because moisture loss or gain between sampling and analysis invalidates the result, test samples are sealed immediately in airtight containers, labeled, and transported under documented chain of custody to the laboratory.
Sample Preparation
In the laboratory, the gross sample is homogenized and split into test portions. For FMP determination, a series of portions is prepared at progressively higher moisture contents by controlled drying or moisture addition, so that the flow point can be bracketed between a non-flowing and a flowing specimen. The grading of the material is preserved, and oversize particles are handled strictly in accordance with the provisions of the selected test method.
Flow Table Test (FTT)
The flow table test is applicable to fine, relatively cohesive materials with a maximum particle size of approximately 7 mm. A truncated conical specimen is molded on a standardized flow table and subjected to a defined sequence of table drops. Plastic flow is judged to have occurred when the specimen deforms beyond the geometric criteria of the method after the specified number of drops. The FMP is bracketed from replicate specimens, and the TML is calculated as 90 percent of the FMP.
Penetration Test (PT)
The penetration test is suitable for relatively cohesive materials with particle sizes up to approximately 25 mm. Specimens are compacted into a standard Proctor-type mold at a series of moisture contents, and a calibrated penetration bit is applied under a standardized load after dynamic conditioning of the sample. The flow state is indicated when the measured penetration of the bit exceeds the defined threshold, allowing the FMP to be bracketed and the TML to be established at 90 percent of the flow point.
Proctor/Fagerberg Test (PF)
The Proctor/Fagerberg method is designed for fine, relatively cohesive materials such as mineral concentrates. Specimens are compacted at several compaction energies and moisture contents, and the results are expressed as the relationship between dry density and degree of saturation. The TML is determined graphically from this compaction-saturation relationship at the defined saturation criterion, rather than from a direct observation of flow.
Modified Proctor/Fagerberg Test for Bauxite
A modified version of the Proctor/Fagerberg procedure was developed specifically for bauxite, whose coarse, porous grading behaves differently from typical concentrates. The modified apparatus and analysis account for the unique compaction and saturation response of bauxite and are mandatory for TML certification of bauxite cargoes under the current IMSBC Code provisions.
Moisture Content Determination
The actual moisture content is determined by drying test portions in a ventilated oven at 105 degrees Celsius, plus or minus 5 degrees, to constant mass, in accordance with methods such as ASTM D2216, ISO 3087, and the relevant coal moisture standards. Rapid indicative techniques, including microwave ovens and carbide moisture meters, are useful for screening and stockpile monitoring but are not acceptable substitutes for oven-dried certified results.
Quality Assurance
Certified TML testing should be performed by laboratories operating under ISO/IEC 17025 accreditation or by a competent person recognized by the flag or port state administration. Duplicate determinations, retained reference samples, documented equipment calibration, and full traceability from increment to certificate are essential elements of a legally defensible result.
Applications
TML sampling and testing supports safety and compliance across the entire dry bulk supply chain. Typical applications include:
- Pre-shipment certification: production of the TML certificate and moisture content declaration required by the IMSBC Code before loading Group A cargoes.
- Load port supervision: verification by independent surveyors that cargo delivered alongside matches the certified material and remains below the TML.
- Port state and terminal control: acceptance checks performed by port authorities and terminal operators before a cargo is admitted to the berth.
- Marine insurance and P&I surveys: pre-loading condition surveys and moisture verification arranged by clubs and charterers to manage liquefaction exposure.
- Mine and stockpile management: routine moisture monitoring, drainage, blending, and stockpile rotation decisions that keep cargo within certified limits.
- Incident investigation and claims: post-incident retesting of retained samples to establish liability after list, shift, or cargo damage events.
- New cargo assessment: laboratory characterization supporting the classification of unlisted materials under the IMSBC Code.
In every application, the value of the test depends on the integrity of the sample. A precise laboratory result from an unrepresentative sample provides false assurance, which is why sampling competence is treated as seriously as analytical competence throughout the industry.
FAQ
What does TML stand for?
TML stands for Transportable Moisture Limit, the maximum moisture content at which a Group A solid bulk cargo can be safely transported by sea without liquefying under the dynamic forces acting on the ship.
How is the TML different from the moisture content?
The TML is a certified limit derived from laboratory testing, usually 90 percent of the Flow Moisture Point. The moisture content is the actual water present in the cargo at the time of shipment. Carriage is permissible only when the actual moisture content is below the TML.
Which TML test method applies to which cargo?
Method selection depends on particle size and cohesion. The flow table test suits fine materials up to about 7 mm, the penetration test suits cohesive materials up to about 25 mm, the Proctor/Fagerberg test is used for fine concentrates, and the Modified Proctor/Fagerberg test is mandatory for bauxite.
How long are TML and moisture certificates valid?
Moisture content must be determined as close as practicable to loading, and many port states and insurers require the declaration to be issued within approximately seven days of the commencement of loading. TML results remain valid as long as the material and its characteristics are unchanged, but retesting is required after rainfall exposure, blending, or long storage.
What happens if the cargo moisture exceeds the TML?
The cargo is considered unsafe and must not be loaded. Remedial options include draining, drying, blending with drier material followed by retesting, or withholding the parcel entirely. Loading a cargo above its TML exposes the shipper, terminal, and carrier to serious safety and legal consequences.
Can a grab test or can test replace laboratory TML testing?
No. Simple field indicators such as shake tests and can tests may reveal gross moisture problems, but they are qualitative only. The IMSBC Code requires certified laboratory determination of the TML and moisture content by an approved method for all Group A cargoes.
