TML Transportation Safety Testing: Transportable Moisture Limit Analysis for Bulk Cargoes

TML transportation safety testing determines the Transportable Moisture Limit of solid bulk cargoes that may liquefy during sea carriage, as required by the International Maritime Solid Bulk Cargoes (IMSBC) Code. Accurate TML and moisture content certification is one of the most effective safeguards against cargo liquefaction, a phenomenon responsible for numerous bulk carrier losses. This article covers the technical principles, applicable sample types, laboratory test methods, acceptance criteria, and regulatory requirements of TML testing.

Technical Overview

Transportable Moisture Limit (TML) testing is a mandatory safety procedure for solid bulk cargoes classified as Group A under the International Maritime Solid Bulk Cargoes Code (IMSBC Code), which was adopted by the International Maritime Organization (IMO) under the SOLAS Convention and became mandatory in 2011. The TML is defined as the maximum moisture content of a cargo at which it can be loaded, transported, and discharged without risk of liquefaction.

Liquefaction occurs when saturated or nearly saturated fine-grained cargo is subjected to cyclic loading from vessel motion, engine vibration, and wave impact. Under these repeated stresses, pore water pressure rises, interparticle contact is lost, and the material suddenly behaves like a dense slurry. If the liquefied mass flows to one side of a cargo hold, the resulting shift and free surface effect can cause progressive listing and, in severe cases, capsize the vessel within minutes. High-profile casualties involving nickel ore and iron ore fines have made TML certification a central focus of bulk shipping safety.

• Flow Moisture Point (FMP): the moisture content at which the material first develops flow behavior under standardized dynamic test conditions.
• Transportable Moisture Limit (TML): fixed at 90% of the measured FMP, providing a built-in safety margin.
• Moisture Content (MC): the actual water content of the cargo at the time of loading, which must remain below the TML.
• Group A cargoes: materials listed in the IMSBC Code as liable to liquefy if the moisture content exceeds the TML.
• Regulatory basis: SOLAS Chapter VI and the IMSBC Code, Section 4 and Appendix 2, which define testing procedures and documentation requirements.

Testing Samples

Iron Ore Fines

Iron ore fines are among the highest-risk liquefaction cargoes due to their fine particle size, high moisture retention, and large shipment volumes. Testing is typically performed using the modified Proctor/Fagerberg method adopted by the IMO following a series of vessel losses, and certificates are required before loading at all major export terminals.

Mineral Concentrates

Copper, lead, zinc, nickel, tin, and other flotation concentrates are produced by wet beneficiation and often shipped with moisture close to saturation. Their fine, relatively uniform particle size makes them well suited to flow table testing, and international standards such as ISO 12742 are widely applied.

Coal Fines and Coal Slurries

Coal may be classified as both Group A and Group B, presenting liquefaction risk in addition to chemical hazards such as methane emission and self-heating. Proctor/Fagerberg and penetration methods are commonly used for coal cargoes with elevated fines content.

Bauxite

Following liquefaction-related casualties, certain bauxite cargoes were reclassified as Group A. Testing with the modified Proctor/Fagerberg procedure accounts for the relatively coarse particle size and clay-rich matrix of bauxite.

Other Group A Cargoes

Testing is also performed on fluorspar, manganese ore fines, chrome ore, mineral sands (ilmenite, rutile, zircon), sinter feed, direct reduced iron fines, sand, and other fine-grained minerals, as well as unfamiliar cargoes that may exhibit liquefaction behavior and must be assessed before carriage.

Testing Items

• Transportable Moisture Limit (TML) determination.
• Flow Moisture Point (FMP) determination.
• Cargo Moisture Content (MC) at the time of sampling and loading.
• MC/TML ratio evaluation and safety margin calculation.
• Bulk density and stowage factor, including compacted density for hold planning.
• Particle size distribution, used to select the appropriate test method and assess liquefaction susceptibility.
• Angle of repose and transportable behavior assessment for cargoes additionally presenting a shifting hazard.

The acceptance criterion for loading is straightforward: the moisture content of the cargo presented for shipment must not exceed its certified TML. If the MC/TML ratio approaches or exceeds 1.0, the cargo cannot be loaded until the moisture content is reduced and verified through retesting.

Testing Methods

Sampling and Sample Preparation

Representative sampling is critical because liquefaction risk depends on localized moisture pockets as well as average moisture. Samples are collected from stockpiles, conveyor belts, or holds in accordance with IMSBC Code Section 4, then reduced by quartering or riffling. The interval between sampling and loading should be as short as practicable, and moisture must be rechecked after heavy rain or other conditions that could alter the water content.

Flow Table Test

The flow table test is best suited to fine-grained materials with particle sizes generally below 1 mm, such as mineral concentrates. A compacted truncated-cone specimen is placed on a standardized flow table and subjected to a controlled series of drops. Specimens are tested at successively higher moisture contents until plastic flow or visible deformation occurs, bracketing the Flow Moisture Point. The TML is then calculated as 90% of the FMP. This method is harmonized with ISO 12742 and Appendix 2 of the IMSBC Code.

Penetration Test

The penetration test, originally developed in Japan, uses a vibrating table with a cylindrical mold and a weighted penetration bit. As moisture content increases, vibration causes progressively deeper penetration of the bit, and the FMP is identified from the sharp change in penetration behavior. The method is typically applied to coal and comparable materials with particle sizes up to approximately 25 mm, where the flow table is unsuitable.

Proctor/Fagerberg Test

The Proctor/Fagerberg method applies incremental compaction energy to specimens prepared at increasing moisture contents in a standard Proctor-type mold, similar to ASTM D698 practice. The saturated bulk density is plotted against moisture content, and the FMP is derived from the characteristic break in the compaction curve. The original procedure was developed for coal and has been extended to a range of fine mineral products.

Modified Proctor/Fagerberg Method

Modified variants of the Proctor/Fagerberg procedure were adopted by the IMO specifically for iron ore fines and bauxite. These versions adjust compaction energy and saturation methodology to reproduce the in-hold stress and drainage conditions of high-density, relatively coarse materials, providing more reliable TML values for these high-risk cargoes.

Moisture Content Determination

Moisture content is determined by oven drying representative sub-samples at approximately 105 degrees Celsius until constant mass is achieved, with the weight loss expressed as a percentage of the wet sample mass. Results from multiple sub-samples are averaged, and the shipper must declare the moisture content alongside the TML certificate in the cargo documentation set.

Applications

• Pre-loading certification by shippers and mining companies for Group A cargo consignments.
• Terminal and port authority verification before acceptance of cargo for stockpiling or shipment.
• Shipmaster and charterer due diligence to confirm that cargo documentation complies with the IMSBC Code.
• P&I club surveys and independent marine cargo inspections following rain events or moisture disputes.
• Classification society and flag state investigations involving cargo behavior and stability incidents.
• Quality control in mineral processing plants for dewatering, filtration, and stockpile moisture management.
• Trade finance and commodity trading, where compliant TML and MC documentation is a condition of shipment release.

FAQ

What is the difference between TML and FMP?

The Flow Moisture Point is the moisture content at which a cargo begins to exhibit flow behavior under dynamic test conditions, while the Transportable Moisture Limit is set at 90% of the FMP. The 10% reduction creates a deliberate safety margin that accounts for sample variability and moisture redistribution during voyage.

Which cargoes require TML testing?

All cargoes classified as Group A in the IMSBC Code require a TML certificate and a moisture content declaration before loading. This includes iron ore fines, nickel ore, various mineral concentrates, certain coals, some bauxite types, and any unfamiliar cargo suspected of liquefaction potential.

What happens if the moisture content exceeds the TML?

The cargo must not be loaded. Remedial options include natural or mechanical drying, drainage, or holding the material until moisture levels fall below the certified TML and verification testing confirms compliance. Loading wet cargo significantly increases the risk of liquefaction and capsizing, and it exposes the shipper and carrier to serious liability.

How long is a TML certificate valid?

The IMSBC Code does not fix a formal expiry date, but the certificate must genuinely represent the cargo actually presented for shipment. If the material source, beneficiation process, or particle size changes, or if a significant time has elapsed, the TML should be re-established through new testing. Moisture content, unlike TML, must always be measured close to the time of loading.

How much sample is needed and how long does testing take?

Depending on the method, laboratories typically require 10 to 25 kg of representative material, collected with full chain-of-custody documentation. Standard turnaround is three to seven working days, although expedited services are available at major load ports. Testing should be performed by laboratories with proven competence in IMSBC Appendix 2 procedures, ideally holding ISO/IEC 17025 accreditation.

Can testing be done at the port instead of at the mine?

Yes. Many major bulk terminals are served by accredited laboratories capable of performing flow table, penetration, and Proctor/Fagerberg tests. Port-side testing is often used to verify stockpile condition or investigate moisture changes after rainfall, while original certification is usually completed near the point of production.