Dangerous Goods TML Testing: Transportable Moisture Limit Analysis for Solid Bulk Cargoes

Dangerous goods TML testing determines the Transportable Moisture Limit of solid bulk cargoes that may liquefy during sea transport, protecting vessels from cargo shift and capsize. Under the International Maritime Solid Bulk Cargoes (IMSBC) Code, shippers of Group A cargoes must declare a certified TML and actual moisture content before loading. Accredited third-party laboratories perform TML testing using internationally recognized methods such as the flow table test, penetration test, and modified Proctor/Fagerberg compaction procedures.

Technical Overview

The Transportable Moisture Limit (TML) is the maximum moisture content of a solid bulk cargo at which it is still considered safe for transportation by sea. The requirement originates from the International Convention for the Safety of Life at Sea (SOLAS) and is enforced through the International Maritime Solid Bulk Cargoes (IMSBC) Code, which classifies cargoes that may liquefy as Group A materials. Typical examples include iron ore fines, nickel ore, bauxite, fine coal, and a wide range of mineral concentrates.

Liquefaction is the core hazard that TML testing addresses. When a cargo with high moisture content is subjected to cyclic vibration from ship motion, pore water pressure can build up inside the cargo mass. Once the moisture exceeds a critical threshold, the material loses shear strength and begins to behave like a dense slurry. This can cause the cargo to shift suddenly to one side of a hold, producing a dangerous list and, in severe cases, total loss of the vessel. A number of bulk carrier casualties involving liquefiable mineral cargoes have directly driven the tightening of TML certification requirements.

In the flow table procedure, the TML is defined as 90% of the Flow Moisture Point (FMP), the moisture content at which the material first exhibits plastic flow behavior under standardized dynamic loading. Other approved methods establish the TML from compaction and saturation relationships or penetration resistance criteria. Whatever the method, the acceptance rule is the same: the actual moisture content of the cargo at loading must be below the certified TML, and shippers must declare both values in the cargo information furnished to the master.

Relevant reference standards include the IMSBC Code test procedures in Appendix 2, ISO 12742 for the flow-table determination of TML of copper, lead, and zinc sulfide concentrates, ISO 3087 for moisture content of iron ores, ISO 589 for moisture of hard coal, ASTM D2216 for laboratory water content, and EN 1097-5 for moisture determination of mineral materials by oven drying.

Testing Samples

Iron Ore Fines and Iron Concentrates

Iron ore fines are among the most frequently tested Group A cargoes. Fine particle size combined with high natural moisture makes them highly susceptible to liquefaction, and major loading ports routinely require valid TML and moisture certificates before stockpiles are released for shipment. Representative samples are typically collected from stockpiles, conveyor belts, or ship holds following ISO 3082 sampling procedures.

Nickel Ore and Lateritic Ores

Nickel laterite ore is notorious for its high clay content and retained moisture, and it has been implicated in multiple fatal capsize incidents. TML testing for nickel ore places particular emphasis on careful sample handling, since drying, homogenization, or storage errors can significantly distort both the moisture content and the measured TML.

Bauxite

Following the recognition that certain bauxite types can liquefy, specific test procedures and amendments to the IMSBC Code have been developed for this cargo. Testing distinguishes between bauxite fractions and fines that remain Group A and newer bauxite schedules that may qualify for different treatment depending on particle size distribution and measured behavior.

Coal and Fine Coal

Coal cargoes present a dual hazard: chemical (spontaneous heating and gas emission) and physical (liquefaction of fine fractions). The penetration test and modified Proctor/Fagerberg procedures for coal are commonly applied, and results are reported together with moisture determination performed according to ISO 589.

Sulfide and Other Mineral Concentrates

Copper, lead, and zinc concentrates, together with materials such as ilmenite, chrome ore, manganese ore, fluorspar, sinter feed, and mill scale, are regularly submitted for TML determination. ISO 12743 provides the sampling framework for sulfide concentrates, and the flow table method described in ISO 12742 is widely applied to these materials.

Testing Items

A complete TML evaluation for a liquefiable dangerous goods cargo typically comprises the following determinations:

• Transportable Moisture Limit (TML) determination by an approved laboratory procedure
• Flow Moisture Point (FMP) identification in flow-table-based testing
• Actual moisture content of the cargo, determined by oven drying at 105 ± 5 °C
• Comparison of declared moisture content against the certified TML with safety margin assessment
• Particle size distribution analysis to confirm the applicability of the selected TML method
• Dry density and degree of saturation relationships in Proctor/Fagerberg compaction testing
• Issue of the TML certificate and moisture content declaration for cargo information documentation

Testing Methods

Flow Table Test

The flow table test is the most widely applied TML method and is suitable for mineral concentrates and other homogeneous fine materials with a maximum particle size of approximately 1 mm, with only a limited tolerance for coarser particles. A series of prepared specimens at increasing moisture contents are placed in a mold on a flow table apparatus conforming to ASTM C230 dimensions and subjected to a standardized number of drops. The Flow Moisture Point is identified as the moisture content at which the specimen first shows plastic deformation or flow, and the TML is calculated as 90% of the FMP. ISO 12742 details the procedure for sulfide concentrates.

Penetration Test

The penetration test, based on penetrometer principles consistent with BS 1377-7, is a correlative method used primarily for coal and similar granular materials. A penetration disk or cone is applied to specimens prepared at graded moisture levels, and the relationship between penetration resistance and moisture is established. The flow condition is identified from the moisture-penetration curve, and the TML is derived with the required safety margin. The method can accommodate somewhat coarser particle sizes than the flow table procedure.

Modified Proctor/Fagerberg Compaction Test

The Proctor/Fagerberg family of tests determines the TML from compaction behavior. Specimens are compacted at progressively higher moisture contents, and the dry density and degree of saturation are calculated for each point. The TML is taken as the moisture content corresponding to the defined saturation criterion on the compaction curve, typically around 70% saturation depending on the variant used. Dedicated variants exist for mineral concentrates, coal, and bauxite, each calibrated for the characteristic behavior of that cargo type.

Vibration Table Test

The vibration table method subjects a molded specimen to controlled vertical vibration at stepped moisture contents. The flow moisture condition is detected when the specimen surface exhibits visible plastic deformation or slumping under dynamic load. This method has historically been applied to fine materials such as fine coal and certain bauxite fractions where other procedures are less suitable.

Moisture Content Determination

Supporting moisture measurements are performed by drying representative sub-samples to constant mass in a ventilated oven at 105 ± 5 °C, following ISO 3087 for iron ores, ISO 589 for coal, ASTM D2216 for general laboratory practice, or EN 1097-5 for mineral aggregates. Moisture checks should be repeated close to and, where required, during loading, since rain exposure on stockpiles can rapidly push moisture above the certified limit.

Applications

TML testing serves multiple stakeholders across the bulk shipping supply chain. Shippers and traders use certified results to prepare the mandatory cargo information declarations required before loading can commence. Ship masters and charterers rely on TML and moisture data for pre-load risk assessment and to reject or conditionally accept cargo that fails the criteria. Producers and mine operators integrate routine TML monitoring into stockpile management and cargo preparation, while insurers, P&I interests, and legal teams commission retrospective testing during casualty and cargo damage investigations.

• Pre-shipment compliance certification for Group A solid bulk cargoes
• Port authority and terminal acceptance checks prior to loading
• Quality control during mining, beneficiation, and stockpiling operations
• Verification testing after heavy rainfall or prolonged stockpile storage
• Technical support for cargo claims, casualty investigation, and expert witness work
• Research supporting classification of new cargoes and proposed amendments to the IMSBC Code

FAQ

What is the TML and why is it required?

The TML is the maximum moisture content at which a liquefiable solid bulk cargo may be safely transported by sea. Under the IMSBC Code, any Group A cargo must be certified with a TML and a declared moisture content before shipment, and the actual moisture must remain below the TML throughout loading and the voyage.

When should TML testing be carried out and how long are certificates valid?

TML determination is performed on representative samples of the intended consignment, and industry practice, together with most port state requirements, accepts TML certificates for a period of up to six months before loading. The actual moisture content must be determined much closer to the commencement of loading, typically within seven days, and should be re-checked if the cargo is exposed to rain or other conditions that could change its moisture state. Shippers should always confirm current validity requirements with the competent authority at the loading port.

How much sample is needed for testing?

A representative sample in the range of approximately 20 to 50 kg is normally sufficient, depending on the material and the selected test method. Samples must be collected and divided according to recognized sampling standards such as ISO 3082 or ISO 12743 and transported in sealed, moisture-tight containers to prevent moisture loss before analysis.

What happens if the cargo moisture exceeds the TML?

Cargo with moisture above the TML must not be loaded, as it presents a serious risk of liquefaction and vessel capsize. Options include drying or dewatering the cargo, natural drainage of stockpiles, natural drying in favorable weather, or re-testing after treatment. Only in exceptional cases involving vessels with specially designed arrangements can such cargo be considered, subject to flag and port state approval.

How is the appropriate test method selected?

Method selection depends on cargo type and particle size distribution. Fine concentrates and similar materials generally suit the flow table test, coal is typically assessed by the penetration test or the modified Proctor/Fagerberg coal variant, and bauxite has dedicated procedures. A preliminary sieve analysis is usually performed to confirm which approved method is applicable to the submitted sample.

How long does TML testing take?

Standard turnaround at accredited third-party laboratories is typically three to seven working days from receipt of samples, depending on the method, the number of moisture points required, and whether repeat determinations are needed. Expedited services can often be arranged when loading schedules are tight, though scheduling testing well before the planned shipment window remains best practice.