Trans RINA, Vol 161, Part A4, Intl J Maritime Eng, Oct-Dec 2019
equation 2. The ship used is a bulk carrier similar to the Bulk Jupiter, loaded in the same way.
Figure 5. Liquefaction versus dynamic separation (Australian Governement, 2018) (Australian Maritime Safety Authority, 2017)
The liquefaction part shows that the complete cargo quantity starts to behave as a high-density liquid, moving in synchrony with the rolling movement of the ship and compromising the stability due to the free surface effect.
If, however, the cargo is composed of different grain sizes (more than 30% of fine particles less than 1 mm and more than 40% of particles less than 2.5mm (both)) (IMO, 2017), the smaller sizes will migrate during loading and carriage to the bottom and impede normal drainage of the cargo. The increased moisture content in the lower parts will reduce the angle of repose.
At the surface of the cargo a slurry (green in the top part of Figure 5) will start to form. Free water will be pushed upwards due to moisture migration, particle rearrangement and compactation (Rose, 2014) and mix with cargo particles. This slurry will behave as a high viscosity mixture such as wet concrete. The bulk of the cargo underneath the slurry layer remains undisturbed.
As a consequence of the rolling and pitching movements of the ship the slurry accumulates on the low side and the free slurry surface causes vessel to develop list or heel. This list will gradually increase as the slurry increases in mass and density as more particles are entrained in the liquid and are deposited to the lower end of the hold.
The sloshing of the slurry will erode the solid cargo beneath, further increasing the list. When the list becomes bigger than the angle of repose (AOR), the lower part of the cargo will suddenly shift to one side and eventually cause the ship to capsize.
The scenario above fully corresponds with the atypical ship motion observed by the crew (ClassNK, 2018). This wobbling is caused by the movement of a free surface slurry over the top of the cargo which is out of phase with the roll period of the ship.
To gain further insight into the stability behavior of a ship loaded with bauxite, we set up a simulation, using
Figure 6. The virtual rise and horizontal shift of G and the consequently decrease of the righting arm GZ, as a result of cargo shift. GZ in m & θ in degrees
Figure 6 shows the red original GZ-curve of our model ship with an initial GM of 7.04m. Given the remarks formulated by Menkiti & Evans, the free surface effect is negligible. Due to a sliding of the bauxite cargo in all five of the cargo holds, the GZ value is reduced to the blue curve, which shows the simulated GZ value after a cargo shift of in total 4400 tons. This correction is calculated on the dimensions of the holds (approx. 28x24m) of our model ship, with a cargo density of 1.5 ton/m³ and a displacement of 59718 t.
A permanent list of 10° (intersection with the horizontal axis) developed. This is an unsafe situation because after the next wave this permanent list will increase. For ships with a sufficient stability, this build up in list will gradually decrease since the righting lever GZ increases with angle of heel θ. The shifting of the cargo will not necessarily cause capsizing. On the contrary, when the stability of the ship is insufficient, the ship will heel further than the maximum GZ value, the heeling moment caused by the shift of cargo will become greater than the righting moment and in a short period of time the ship will founder.
The dynamic stability (work done by the righting moment) of the ship is represented by the area under the GZ curves. In heeled condition (blue curve), the dynamic stability is drastically decreased. In bad weather, where rolling angles are already significant, this reduced dynamical stability will result in even larger angles causing further sliding of the cargo. Hence, whenever confronted with liquefaction/sliding on board, it is of the utmost importance to avoid regions with bad weather.
4. CONCLUSION: HOW TO AVOID?
The captain and crew can avoid this awkward situation by careful monitoring of the cargo when loading. If in doubt, a simple “can test” can give an indication of the real humidity content of the cargo. This so called ‘can test’ is exactly what
©2019: The Royal Institution of Naval Architects
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