Trans RINA, Vol 161, Part A4, Intl J Maritime Eng, Oct-Dec 2019


Previous tests by Menkiti & Evans indicated that Handymax vessels induce the largest forces in the cargo as they roll and pitch more when compared to other bulk carrier types.


To evaluate the risk of liquefaction, the load induced by the cyclic motions of the ship such as rolling, pitching and slamming is expressed in terms of induced cyclic shear stress (colored full lines on the graph) and this is compared with the liquefaction resistance (Resistance Cyclic Stress Ratio) of the bauxite cargo (blue diamonds). For all the test conducted the Induced Cyclic resistance Ratio is smaller than the Resistance Cyclic Stress Ratio and liquefaction will never occur.


Evidence from real world shipments of bauxites shows that instabilities due to moisture cannot be explained by liquefaction phenomena, but can be under a “dynamic separation” mechanism of instability.


An in-depth study was carried out by a group of experts called Global Bauxite Working Group or GBWG. This study showed that certain types of bauxite cargoes with a large proportion of smaller particles could be subject to a newly-identified phenomenon, known as “dynamic separation”, when there is excess moisture in the cargo.


In response to this study, the IMO published (on September 20, 2017) its circular CCC.1/Circ.2/Rev.1, stating that certain bauxite cargoes should be treated as Group A cargoes (cargoes which may liquefy).


This new group of bauxite cargo will be known under the name of “Bauxite Fines”. In the same circular a new test procedure for determining the TML for bauxite and amendments to the individual schedule for Bauxite of Group C were established. IMSBC will be adapted and the date for entry into force of these draft amendments to the IMBSC Code is expected to be 1 January 2021. Ships carrying bauxite (or other mineral ores) have an important initial static stability. An excessive GM value results in shorter rolling periods and high accelerations which may trigger liquefaction (DNV-GL, 2015). The moisture is likely to migrate downwards, resulting in an increasing moisture level towards the bottom of the hold. During the voyage, the vibrations and movements of the ship will also trigger the migration of fine particles to the bottom of the hold, leading at the same time to a compaction of the cargo and an increase of the cohesive forces. This compaction will hamper the drainage of the humidity by the ship’s bilge system. (Australian Governement, 2018)


This stratification will result is an unstable lower part of the cargo. The increased portion of fines and the high humidity will increase the risk of sliding of the cargo.


The behavior of this lower part is governed by the angle of repose (AOR). The angle (see Figure 4) of repose is the steepest slope of the unconfined material, measured from


Figure 4. Angle of repose


The AOR is a characteristic related to interparticular friction or resistance to movement between particles.


However, the AOR is a complex function of many parameters. Particle size and shape, granulation, density, pressure, surface area, cohesion, moisture content and the material's coefficient of friction all play a role when determining its value. DNV-GL points out that the same cargo might be unstable when the humidity is high or low but is stable at average moisture levels (DNV-GL, 2015). Literature study shows that the AOR of bauxite varies between 20 and 44° (RKM, 2016). The Bulk Jupiter, at the time of the accident, was sailing the leftover of a tropical storm.Wave heights of 6 meters and wind speeds up to 35 knots were reported. The chief cook, the only survivor, mentioned heavy rolling and pitching.


The granulation and humidity of each shipment of bauxite is different and unknown. The Australian Maritime Safety Authority (Australian Governement, 2018), Miller et al. (2015) and Lie et al. (2017) suggest that during handling, loading and the carriage of the bauxite ore a vertical and horizontal stratification occurs in function of the lump size. As such, the angle of repose is not a uniform value but will vary throughout the cargo in function of the height, time after loading and the movements of the ship at sea. Prediction of the behavior of each layer becomes difficult if not impossible.


3.3


DYNAMIC SEPARATION AND LIQUEFACTION COMPARED


Both “Liquefaction” and “Dynamic Separation” are thus caused by excess humidity in the cargo, beyond the TML. However, the further physical manifestation differs markedly (Figure 5).


A-422 ©2019: The Royal Institution of Naval Architects


the horizontal plane on which the material can be heaped without collapsing.


If the stratification occurs parallel to the hold bottom, this lower part of the cargo will shift to one side when the rolling angle of the ship to port or starboard is greater than the angle of repose as shown in the upper part of Figure 5 (dynamic separation).When this happens, a permanent list is created and this may eventually lead to the capsizing of the ship.


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