Transportable Moisture Limit (TML) testing is a mandatory safety assessment for solid bulk cargoes that may liquefy during sea carriage, as required by the International Maritime Solid Bulk Cargoes (IMSBC) Code. This article explains the technical principles, accepted laboratory test methods, applicable cargo types, sampling procedures, and compliance requirements of TML testing to help shippers, ship operators, and terminal personnel prevent liquefaction-related maritime casualties.
Technical Overview
The Transportable Moisture Limit (TML) is defined by the International Maritime Solid Bulk Cargoes (IMSBC) Code, adopted under the SOLAS Convention, as the maximum moisture content of a solid bulk cargo that is considered safe for carriage in the cargo holds of a ship. The TML is calculated as 90 percent of the Flow Moisture Point (FMP), which is the experimentally determined moisture content at which a saturated cargo develops a flow state under the influence of cyclic loading, vibration, or ship motion. Because laboratory and natural conditions cannot be perfectly replicated, the 10 percent safety margin is applied to account for measurement uncertainty, moisture migration within the hold, and cargo heterogeneity.
Cargoes that contain a significant proportion of fine particles and are prone to liquefaction are classified as Group A cargoes under the IMSBC Code. When such cargo is loaded with excessive moisture, the pore water pressure generated by ship vibration and rolling can cause the cargo to lose grain-to-grain contact and behave like a viscous fluid. This liquefaction can trigger a sudden cargo shift, drastic loss of stability, and capsize, sometimes within minutes, which is why nickel ore, iron ore fines, and similar materials have caused numerous vessel losses and fatalities. Accurate TML determination and strict moisture control are therefore fundamental elements of maritime safety regulation.
Under the IMSBC Code, the shipper is legally responsible for providing a signed cargo information declaration that includes the TML, the actual moisture content, the cargo group classification, and other transport-relevant properties before loading commences. Cargo with a moisture content exceeding its TML must be rejected for shipment, and ports, masters, and competent authorities are entitled to verify the declared values through independent testing at accredited third-party laboratories. TML testing thus serves as the core technical control point in the safe ocean transportation of liquefiable solid bulk cargoes.
Testing Samples
Iron Ore Fines
Iron ore fines are one of the most frequently tested Group A cargoes. Due to their high density and fine particle size distribution, they are highly susceptible to moisture migration and liquefaction. Testing typically follows the flow table method or the modified penetration methods recognized in recent IMSBC Code amendments, with sampling conducted in accordance with ISO 3082.
Nickel Ore
Nickel ore, particularly lateritic nickel ore with high clay and moisture content, is historically associated with severe liquefaction casualties. Because of its natural stickiness and variable moisture retention, laboratory testing must carefully bracket the flow moisture point using multiple moisture-conditioned test portions prepared from a fully homogenized gross sample.
Mineral Concentrates
Concentrates of copper, lead, zinc, and nickel produced by froth flotation retain substantial process moisture and are classic Group A materials. Their TML is commonly determined by the flow table test, with sampling and moisture determination performed in accordance with ISO 12743 and ISO 12742.
Bauxite
Bauxite has been reclassified into several Group A and Group A and B schedules after liquefaction incidents. Standard and tri-hydrate bauxite fines require TML certification, and attention must be paid to the proportion of fine particles produced during handling and moisture adjustment at the stockyard.
Coal and Other Fine-Grained Bulk Cargoes
Coal fines, sand, mineral sands such as ilmenite, fluorspar, manganese ore fines, iron and steel making residues, and certain processed mineral products may also fall under Group A or dual Group A and B classification. The applicable test method depends on particle size, plasticity, and the schedule requirements of the IMSBC Code.
Testing Items
A complete TML examination program for a solid bulk cargo generally includes the following test items:
- Transportable Moisture Limit (TML) determination for the cargo as presented for shipment
- Flow Moisture Point (FMP) determination by an approved laboratory procedure
- Actual moisture content (MC) determination of the cargo lot at the time of loading
- Particle size distribution analysis, including the percentage of particles finer than 1 mm and fines content relevant to liquefaction behavior
- Bulk density and stowage factor measurement for cargo planning purposes
- Cargo group classification assessment (Group A, Group B, or Group C) based on physical properties and hazard characteristics
Testing Methods
Flow Table Test
The flow table test is the classical reference method described in the IMSBC Code and standardized in ISO 12742. A test specimen is compacted in a molded ring mounted on a mechanically operated flow table and subjected to a standardized dropping sequence, typically 25 drops repeated over two cycles. Flow moisture is identified when the specimen surface exhibits visible plastic deformation, such as the emergence of a wet glossy ring or a distinct change in the molded crest shape. The test is repeated on specimens with incrementally adjusted moisture contents until the FMP is bracketed between a flow and a non-flow condition, after which the TML is calculated as 90 percent of the interpolated FMP. The method is best suited to fine-grained materials with a high proportion passing the 1 mm sieve.
Penetration Test
The penetration test, also known as the Proctor/Fagerberg method, uses a cylindrical compaction mold and a standard compaction hammer to prepare specimens at increasing moisture contents. A penetration bit is applied under a constant load, and the moisture content at which the penetration depth increase exceeds the specified criterion indicates the flow state. Modified versions of this method have been developed and recognized by the IMO for iron ore fines and other coarse or high-density cargoes where the flow table test is not applicable. The test is particularly useful for materials containing particles up to several millimeters in size.
Paste Test
The paste test is a supplementary qualitative procedure in which a ball of cargo paste is placed in a dish and struck against a hard surface in a standardized manner. The appearance of free moisture or a wet sheen on the paste surface indicates the material may be near or above its flow moisture point. While the paste test cannot by itself determine a certified TML value, it provides a rapid screening tool for cargo inspectors and loading supervisors at the stockpile or shipside.
Laboratory Moisture Content Determination
Moisture content is determined by weighing a representative test portion before and after drying to constant mass in a ventilated oven at 105 degrees Celsius plus or minus 5 degrees Celsius, following standards such as ISO 3087 for iron ores, ISO 9599 for sulfide concentrates, and ASTM D2216 for general soils and aggregates. The moisture content declaration accompanying the cargo must normally be based on samples taken not more than seven days before loading commences, with re-testing required if significant rain exposure occurs before or during loading.
Sampling and Sample Preparation
Reliable TML results depend entirely on representative sampling. Increment samples are collected from conveyor belts, stockpiles, or ship holds using systematic sampling plans that account for cargo heterogeneity, and are combined and reduced by quartering or riffling to produce test samples. Samples intended for moisture determination must be sealed immediately in airtight moisture-tight containers with minimal headspace, and container walls must be inspected for condensation before analysis. Accredited third-party laboratories follow documented chain-of-custody procedures to ensure the analytical results are legally defensible.
Applications
TML testing is applied throughout the solid bulk cargo supply chain. Mining companies and trading houses commission TML determination and moisture monitoring before nominating cargo for shipment, ensuring that stockpiles are drained, conditioned, or blended to remain safely below the TML. Port terminals and loading facilities use TML certificates and routine moisture checks as gate criteria for accepting cargo onto the berth, particularly during and after heavy rainfall.
Ship masters and marine superintendents rely on validated TML documentation to verify that the cargo information declaration complies with the IMSBC Code before authorizing loading, and may order independent laboratory checks when documentation appears inconsistent with the observed cargo condition. Marine insurers, hull and machinery clubs, and legal counsel engaged in casualty investigation or cargo claims also depend on TML and FMP test data to establish liability following liquefaction incidents.
- Pre-shipment certification of Group A cargoes for cargo information declarations
- Port state control and flag state verification of declared moisture values
- Rain-affected cargo re-testing before and during loading operations
- Cargo blending and moisture conditioning studies at mine sites
- Technical evidence for liquefaction casualty investigation and dispute resolution
FAQ
What is the difference between TML, FMP, and moisture content?
The Flow Moisture Point (FMP) is the laboratory-determined moisture content at which a cargo begins to flow under vibration or cyclic loading. The TML is the maximum permissible moisture content for shipment and is fixed at 90 percent of the FMP to provide a safety margin. The moisture content is the actual water content of the cargo presented for loading, and it must be below the TML for the cargo to be accepted.
How often must TML testing be performed?
Industry practice and most flag state guidance require TML testing not more than six months before loading, provided the cargo source, mineralogy, and handling process remain unchanged. The declared moisture content, however, must be determined from samples taken much closer to loading, normally within seven days of the start of cargo operations, and after any significant rainfall the cargo must be re-tested.
What happens if the moisture content exceeds the TML?
Cargo with moisture content above its TML is prohibited from loading under the IMSBC Code. The shipper must drain, dry, or blend the cargo to reduce moisture, and fresh samples must be tested to confirm compliance. Loading wet cargo exposes all parties to criminal and civil liability, and the ship master has the authority and obligation to stop loading operations.
Which cargoes require TML certification?
All cargoes listed in the IMSBC Code as Group A, or dual Group A and B, require TML certification and moisture declaration. Typical examples include iron ore fines, nickel ore, various mineral concentrates, certain bauxite schedules, coal fines, mineral sands, and any cargo not listed in the Code that contains fines and may liquefy, which must be assessed by the competent authority of the loading port.
How should samples be collected and preserved for TML testing?
Samples must be collected systematically from moving streams or representative points across the stockpile to reflect the entire cargo lot, and each sample for moisture determination must be sealed immediately in airtight containers to prevent moisture loss or gain. Accurate labeling, tamper-evident sealing, and documented chain of custody from sampling to laboratory analysis are essential for the certificate to be accepted by carriers and port authorities.
Can a stockpile TML result be reused for every shipment?
No. A TML certificate applies to a specific cargo with defined characteristics. Changes in ore body, beneficiation process, particle size distribution, or blending proportions can shift the flow moisture point significantly, so new testing is required whenever the material characteristics change, even if the previous certificate is still within its validity period.