Moisture Content and TML (Transportable Moisture Limit) Determination: Test Methods, Standards and C

Transportable Moisture Limit (TML) determination and moisture content testing are critical safety requirements for bulk mineral cargoes classified as Group A under the IMO IMSBC Code. This article explains the technical principles of TML and moisture content determination, the three internationally recognized test methods, applicable ISO standards, and how shippers, terminals and carriers use the results to prevent cargo liquefaction and vessel capsizing at sea.

Technical Overview

Liquefaction is one of the most serious hazards in the maritime transport of solid bulk cargoes. Fine-grained mineral cargoes such as iron ore fines, nickel ore, bauxite and metal concentrates contain moisture within their pore spaces. When a ship rolls and pitches in a seaway, cyclic loading can cause pore water pressure to build up, dramatically reducing the shear strength of the cargo until it behaves like a viscous slurry. This phenomenon, known as liquefaction, can cause cargo to shift suddenly, leading to loss of stability and, in documented cases, the total loss of vessels and crews.

Two measured parameters control this risk. The moisture content (MC) is the percentage of water in the cargo, calculated as the mass of water lost on drying divided by the mass of the wet material. The Transportable Moisture Limit (TML) is the maximum moisture content at which a cargo may be safely shipped without liquefying under dynamic ship motion. The TML is defined as 90% of the Flow Moisture Point (FMP), which is the moisture content at which the material just begins to exhibit plastic flow under standardized test conditions.

Under the International Maritime Solid Bulk Cargoes (IMSBC) Code, developed by the International Maritime Organization (IMO), cargoes that may liquefy are classified as Group A. Before shipment, the shipper must provide a TML certificate and a signed moisture content declaration confirming that the actual moisture content is below the TML. These determinations are performed by accredited third-party laboratories using the standardized procedures set out in Section 4 and Appendix 2 of the IMSBC Code, supported by international standards such as ISO 3087, ISO 3082, ISO 12742 and ISO 12743.

Testing Samples

Iron Ore Fines and Concentrates

Iron ore fines are one of the highest-volume Group A cargoes and are particularly susceptible to liquefaction due to their high proportion of fine particles. Sampling is carried out in accordance with ISO 3082, and moisture content is determined per ISO 3087. Representative sampling across the entire stockpile or consignment is essential, as moisture distribution is rarely uniform, especially after rainfall or stockpile drainage.

Nickel Ore and Laterites

Nickel ore is typically clay-rich with high natural moisture and pronounced variability between mining faces and stockpiles. Because it is often loaded in tropical regions with heavy rainfall, close-moisture monitoring and careful sample sealing are required to ensure the laboratory result genuinely represents the cargo as loaded.

Mineral Concentrates

Copper, lead, zinc, tin and other mineral concentrates are fine-grained, high-density products that frequently fall under Group A. Sampling and TML determination for concentrates follow ISO 12743 and ISO 12742 respectively, and the flow table method is generally well suited to their particle size range.

Bauxite

Following a series of liquefaction incidents involving bauxite shipments, the IMSBC Code was amended to introduce stricter Group A requirements for certain bauxite types. Dedicated and modified Proctor/Fagerberg-based procedures have been adopted for these cargoes, and shippers should verify the applicable test procedure against the current individual schedule.

Other Group A Cargoes

Coal fines and certain coals, sinter feed, ilmenite, fluorspar fines, mineral sands, filter cakes, slurries and other fine-grained mineral products may also be classified as Group A. All samples must be transported in sealed, airtight containers to prevent moisture loss, accompanied by full chain-of-custody documentation, and delivered to the laboratory without delay.

Testing Items

Typical testing programs for moisture content and TML determination include the following items:

• Total (as-received) moisture content of the cargo lot
• Flow Moisture Point (FMP) determination
• Transportable Moisture Limit (TML) calculation, equal to 90% of the FMP
• Particle size distribution and sieve analysis, used to select the appropriate TML test method
• Documentation support for the shipper's declaration and TML certificate required by the IMSBC Code
• Comparative moisture checks at load port, including re-testing after significant rainfall

Testing Methods

Sample Preparation and Method Selection

On receipt, samples are homogenized, split into representative test portions and subjected to sieve analysis. The particle size distribution determines which TML procedure applies: the flow table test is generally suitable for fine materials with a maximum grain size of approximately 7 mm, with provisions in the IMSBC Code for materials containing coarser fractions; the penetration test is applied to fine cohesive materials, with a variant covering coarser materials and a variant developed for certain coals; and the Proctor/Fagerberg test is used for materials with a maximum particle size of approximately 5 mm, including modified variants adopted for bauxite.

Moisture Content Determination

Moisture content is determined by oven drying at 105 ± 5 °C until constant mass is reached, following ISO 3087 for iron ores, ISO 589 for hard coal and equivalent national procedures. The moisture content is calculated as the ratio of the mass of water removed to the mass of the original wet specimen, expressed as a percentage. Duplicate determinations are performed and must agree within specified tolerances; rapid moisture analyzers may be used for screening purposes, but oven drying remains the reference method for certification.

Flow Table Test

The flow table test is the most widely applied TML method for mineral concentrates and fine ores. A test portion is mixed with water and compacted into a cylindrical mold on a standardized, cam-operated flow table. The mold is removed and the table is raised and dropped 25 times from a height of 12.5 mm. After each run, the spread of the specimen and the appearance of a wet ring or plastic flow at the base are observed and recorded. The test is repeated at successive moisture increments bracketing the critical point, and the results are plotted as flow versus moisture content. The Flow Moisture Point is identified at the onset of plastic flow, and the TML is reported as 90% of the FMP.

Penetration Test

The penetration test uses Proctor-type compaction equipment, with the specimen compacted in layers using a standard hammer in a compaction mold. A penetration bit is applied to the compacted surface and the penetration depth is recorded with a dial gauge at each moisture level. As moisture content increases toward the liquefaction threshold, penetration increases sharply; the FMP is determined from the penetration–moisture curve, and the TML is again taken as 90% of the FMP. This method is preferred for very fine, cohesive materials and for certain coal cargoes.

Proctor/Fagerberg Test

The Proctor/Fagerberg method evaluates TML from compaction behavior. A series of specimens is compacted at progressively decreasing moisture contents, starting from a near-saturated condition, and the wet density of each specimen is measured and converted to dry density. The wet and dry density curves are constructed graphically, and the TML is derived from the geometric relationship between them. This approach suits coarser, less cohesive materials and forms the basis of the modified procedures introduced for bauxite in recent IMSBC Code amendments.

Calculation and Reporting

The final report states the test method used, the measured moisture content, the Flow Moisture Point, the TML (rounded conservatively), the date and location of sampling, and the accreditation status of the laboratory. Under the IMSBC Code, TML certificates are typically valid for up to six months, provided the material composition does not change, while moisture content sampling is normally required within approximately seven days of the commencement of loading. Re-testing is mandatory where rain, snow or other conditions may have altered the moisture content between sampling and shipment.

Applications

Moisture content and TML determination serve a wide range of practical purposes across the bulk shipping supply chain:

• Shipper compliance with the cargo information, TML certificate and moisture declaration requirements of the IMSBC Code
• Load port acceptance procedures and terminal quality control prior to and during loading operations
• Port State control, flag State and charterer inspections, including the master's right to request verification testing
• Mine and beneficiation plant process control, including stockpile management, filtration and dewatering optimization
• Pre-shipment surveys and cargo superintendence for traders, insurers and P&I interests
• Loss prevention and post-incident investigation following suspected liquefaction events
• Classification of new or modified bulk cargoes and justification of Group A assignments

FAQ

What is the difference between moisture content and TML?

Moisture content is the actual water content of the cargo as shipped, while the TML is the maximum moisture level at which the cargo can be safely transported without liquefying. Safe carriage requires the declared moisture content to be below the TML, with a practical margin to account for moisture migration and heterogeneity.

Why is the TML set at 90% of the Flow Moisture Point?

The 10% reduction provides a safety margin that compensates for natural moisture variability within a stockpile, moisture migration during the voyage, sampling uncertainty and test variability. It is a mandatory convention under the IMSBC Code.

Which TML test method applies to my cargo?

Method selection depends primarily on particle size distribution and material behavior, following the decision criteria in Appendix 2 of the IMSBC Code. The flow table, penetration and Proctor/Fagerberg methods each cover specific grain size ranges, and an accredited third-party laboratory can confirm the correct procedure based on sieve analysis.

What happens if the moisture content exceeds the TML?

The cargo should not be accepted for carriage in the normal way. Options include further dewatering and drainage followed by re-testing, or carriage only under the special provisions of the IMSBC Code for specially constructed or fitted ships. Loading non-compliant Group A cargo exposes all parties to severe safety and legal consequences.

How long are TML and moisture content certificates valid?

TML results are generally accepted for up to six months, provided the cargo source and processing remain unchanged, while moisture content determinations are required shortly before loading, typically within seven days. Adverse weather between sampling and shipment triggers additional verification testing.

How should samples be taken and preserved?

Samples must be collected as representative increments across the full consignment using systematic sampling plans, then sealed immediately in airtight containers to prevent moisture loss and delivered promptly to the laboratory with full documentation.