Moisture migration and the wet base

Moisture migration is defined at IMSBC Code section 1.7 as the movement of moisture contained in a cargo by settling and consolidation of the cargo due to vibration and the ship’s motion, in which water is progressively displaced and may result in some portions or all of the cargo developing a flow state. It is the mechanism behind the wet base, the case that makes a cargo dangerous even where the shipper’s certificate is accurate.

Section 7.2.3 states the position directly: a cargo shift may occur when the moisture content exceeds the transportable moisture limit, and some cargoes susceptible to moisture migration may develop a dangerous wet base even if the average moisture content is less than that limit. The average is a bulk figure; the failure is local. Water displaced downward through a compacting mass accumulates at the tank top, where it can mobilize a layer of fines under continued rolling.

Section 4.2.2.11 is the corresponding declaration item, requiring the shipper to state the likelihood of formation of a wet base with reference to section 7.2.3. The full article will cover the physics, the cargoes most susceptible, the monitoring available on passage and why the declaration item is so often returned blank.