The International Maritime Solid Bulk Cargoes Code (IMSBC Code) categorises solid bulk cargoes into three groups; Group A, Group B and Group C. A Group A solid bulk cargo are those which may liquefy if transported with a gross water content by weight exceeding the Transportable Moisture Limit (TML).
Section 7.2 of the 2013 Edition of the International Maritime Solid Bulk Cargoes Code (IMSBC Code) states the following regarding Group A solid bulk cargoes:
Note: Section 7.2 of the International Maritime Solid Bulk Cargoes Code (IMSBC Code) has been modified in the 2014 Edition.
Group A cargoes contain a certain proportion of small particles and a certain amount of moisture. Group A cargoes may liquefy during a voyage even when they are cohesive and trimmed level. Liquefaction can result in cargo shift. This phenomenon may be described as follows:
- the volume of the spaces between the particles reduces as the cargo is compacted owing to the ship motion, etc.;
- the reduction in space between cargo particles causes an increase in water pressure in the space; and
- the increase in water pressure reduces the friction between cargo particles, resulting in a reduction in the shear strength of the cargo.
Liquefaction does not occur when the cargo consists of large particles or lumps and water passes through the spaces between the particles and there is no increase in the water pressure.
A cargo shift caused by liquefaction may occur when the moisture content exceeds the TML. Some cargoes are susceptible to moisture migration and may develop a dangerous wet base even if the average moisture content is less than the TML. Although the cargo surface may appear dry, undetected liquefaction may take place, resulting in shifting of the cargo. Cargoes with high moisture content are prone to sliding, particularly when the cargo is shallow and subject to large heel angles.
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.
- ↑ International Maritime Organisation, International Maritime Solid Bulk Cargoes Code, 2013 Edition, p.34, London: International Maritime Organization.
- ↑ International Maritime Organisation, International Maritime Solid Bulk Cargoes Code, 2014 Edition, London: International Maritime Organization.