Cargo liquefaction poses significant risks to maritime safety, particularly for vessels carrying solid bulk cargoes. In June 2023, the IMO’s Maritime Safety Committee adopted Resolution MSC.539 (107), introducing the latest amendments (07-23) to the International Maritime Solid Bulk Cargoes (IMSBC) Code. These amendments, which will enter into force on January 1, 2025, provide critical guidelines aimed at preventing cargo liquefaction during sea voyages.
Meaning of cargoes which may liquefy
Cargoes which may liquefy are those that contain a certain proportion of fine particles and a certain amount of moisture. They may liquefy if shipped with a moisture content in excess of their Transportable Moisture Limit.
Definition of Cargo Liquefaction
Liquefaction occurs when bulk cargoes that initially appear dry, such as mineral concentrates, contain enough moisture that they behave like a liquid when subjected to the vibration and movement of a ship at sea. This condition often arises when the spaces between particles in the cargo compact, increasing water pressure and reducing the friction between particles. If not managed, this phenomenon can lead to a dangerous shift in cargo, potentially resulting in the vessel “capsizing”.
Understanding Group A Cargoes
Group A consists of cargoes which possess a hazard due to moisture that may result in liquefaction or dynamic separation if shipped at a moisture content in excess of their transportable moisture limit.
Definition of Transportable Moisture Limit
The Transportable Moisture Limit (TML) of a Group A cargo means the maximum moisture content of the cargo which is considered safe for carriage in ships not complying with the special provisions of 7.3.2. It is determined by the test procedures approved by a competent authority, such as those specified in paragraph 1 of appendix 2 of the IMBSC Code.
Key Conditions Leading to Liquefaction
Cargoes prone to liquefaction are categorized as Group A cargoes. These cargoes contain a mixture of small particles and moisture, which can become fluid under specific conditions. The IMSBC Code highlights several key factors that lead to liquefaction:
- The compaction of the cargo due to the ship’s motion and the reduction of the volume of the spaces between the particles.
- The reduction in space between cargo particles and increase in water pressure in the space.
- Increased water pressure between particles, leading to reduced friction between the cargo particles and a reduction in the shear strength of the cargo.
- Cargoes may also undergo spontaneous heating, further compounding the risks.
Certain cargoes, despite appearing dry on the surface, may undergo moisture migration, developing a dangerous wet base that could cause the cargo to shift unexpectedly, increasing the risk of capsizing.
In the resulting viscous fluid state, cargo may flow to one side of the ship with a roll but not completely return with a roll the other way. Consequently, the ship may progressively reach a dangerous heel and capsize quite suddenly.
Guidelines for Safe Transport
The IMSBC Code stipulates that Group A cargoes should only be loaded if their moisture content is below the TML. This measure ensures the cargo remains stable during the voyage. Additionally, cargoes with moisture content exceeding the TML may still be carried on ships that are specially constructed or fitted to confine cargo shifts. These ships must have structural boundaries or portable divisions designed to limit cargo movement to a safe level.
The Code also specifies several provisions to further mitigate the risks:
- Cargo spaces must be kept dry, with adequate precautions taken to prevent the ingress of water during the voyage.
- Cargoes that contain liquids (other than packaged goods) should not be stowed in the same cargo space above or adjacent to these solid bulk cargoes.
- Masters must avoid using water to cool cargoes at sea, as this could inadvertently bring the moisture content of the cargo to dangerous levels. In extreme cases, water spray may be applied with caution.
Specially Constructed or Fitted Cargo Ships
The Code provides guidelines for ships specially constructed to handle cargoes prone to liquefaction:
- Specially constructed ships have permanent boundaries designed to confine any shift in cargo to an acceptable limit.
- Specially fitted ships use portable divisions, carefully designed to withstand the immense forces of cargo flow. These divisions must meet strict design and positioning standards to minimize the risk of heeling or capsizing due to cargo movement.
For approval, such ships must submit relevant structural drawings and stability calculations to the appropriate authorities, ensuring that the vessel is equipped to handle the unique risks associated with liquefiable cargoes.
Conclusion
The amendments to the IMSBC Code underscore the critical importance of managing the risks associated with cargo liquefaction. Adhering to these guidelines is essential for ship operators and masters to ensure the safety of their vessels and crews to prevent cathastropic losses and claims.
For comprehensive guidance, specialized risk assessment, and legal advice on navigating the IMSBC Code amendments, please contact Interwits.
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