The difference between liquefaction and dynamic separation explained
NorthStandard P&I Club has published a guide on Bulk Cargo Liquefaction and Dynamic Separation, explaining more about the differences between the two and presenting best practices to mitigate the phenomenen.
WHAT IS DYNAMIC SEPARATION?
Dynamic separation occurs during a voyage where the moisture in the cargo rises to the surface of a stow as the cargo consolidates due to vibration and vessel motion. This is caused by a mechanism similar to liquefaction, but the cargo contains enough fine particles such that the void spaces between particles are very small and impede the moisture flowing through the cargo. As the water becomes pressurized and begins to flow, fine particles are carried with it and create a slurry.
Unlike liquefaction where a portion of the cargo in its entirety flows like a liquid, dynamic separation generates a slurry of water and fine particles which is driven away from the consolidating portion of the stow to areas of lower pressure and collects on the top of the denser (consolidated) portion of the stow. In response to vessel motions, this slurry has a free surface effect causing incremental shifting of the centre of gravity in the hold, leading to a reduction of the vessel’s metacentric height (GM), negatively affecting the ship’s stability.
DIFFERENCES BETWEEN LIQUEFACTION AND DYNAMIC SEPARATION Liquefaction Definition Cargo state Cause Mechanism Movement Effect on ship stability
Cargo behaves like a liquid when the moisture content exceeds the transportable moisture limit (TML).
Entire cargo or significant portions flow as a liquid.
Excess moisture causes the cargo to lose its solid structure and behave like a liquid.
Moisture saturates the cargo, reducing friction between particles, causing the cargo to flow.
Cargo can flow freely, potentially shifting within the hold and causing instability.
Can cause significant shifts in the center of gravity, leading to dangerous angles of list and potential capsizing.
Detection Prevention
Examples of affected cargoes
Requires monitoring of moisture content before and during loading.
Ensure moisture content is below the TML before loading.
Iron ore fines, nickel ore, bauxite. Dynamic Separation
The moisture in the cargo rises to the surface during the voyage, creating a slurry of water and fine particles.
Slurry of water and fine particles forms on top of the consolidated cargo.
Vibration and vessel motion cause moisture to migrate to the surface, carrying fine particles with it.
Consolidation due to vibration and motion increases pressure, pushing water and fine particles to the surface.
Slurry forms a free surface on top of the cargo, leading to shifting of the center of gravity.
Creates a free surface effect that incrementally shifts the center of gravity, reducing metacentric height (GM) and negatively affecting stability.
Can be harder to detect as it develops during the voyage.
Requires proper sampling and testing, and awareness of the cargo's condition throughout the voyage.
Similar to liquefaction-prone cargoes but more influenced by the presence of fine particles and vessel motion.
96 | ISSUE 109 | SEP 2024 | THE REPORT
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