What is the difference between dynamic separation and liquefaction?

Overview

A quick and simple illustration on the different phenemena : dynamic separation process and liquefaction.

The Australian Maritime Safety Authority (AMSA) has provided a good illustration of what is dynamic separation and liquefaction. This could assist members to understand better these different phenemona that are moisture-related instabilities which will affect the ship's stability when transporting bauxite cargoes in bulk.

What is dynamic separation?

Dynamic separation occurs during a voyage where the moisture in the cargo migrates to the surface of a hold as the cargo compacts from underneath due to vibration and vessel movement. This occurs when the cargo contains enough small particles that stop the moisture draining from the cargo.

As a slurry of water and fine particles collects on the top of the cargo it creates a free surface effect in the hold. Over time, the sloshing motion of the slurry can deposit loose cargo unevenly in the hold. The research also shows that different wave periods of the slurry in different holds results in the ship having a peculiar motion rather than a regular roll period in a seaway.

Modelling shows that over time, cargo collects on one side of the holds. As this occurs in adjacent holds, the ship’s weight distribution and stability become affected. The result in more extreme circumstances is the ship progressively listing to one side. As this list increases, so does the action of the free surface water, potentially undermining the surface of the cargo, resulting in a shift and capsize.

 

For more information on liquefaction, please refer to sections 7 and 8 of the International Maritime Solid Bulk Cargoes Code.

Source: AMSA

Ai Cheng Foo-Nielsen
in Copenhagen, DK

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