Cargoes that have air in the void spaces between the particles allow the free passage of water and air. This means that cargoes with a low moisture content do not liquefy, as there is ample space in the voids between particles to allow any small amounts of liquid to pass.
Similarly, cargoes consisting of large particles do not liquefy because the space between the particles allows water to pass freely with little increase in pressure. However, with well draining particles of a wet cargo where the moisture drains to the tank top and bilges and is not pumped out, there is a risk of sliding, due to the presence of a liquefied zone at the base of the cargo."
(17) 8th to 10th paragraphs - Clarification
The sentences in the following paragraphs can be incorporated into section 7.1.
"In very fine cargoes, cohesive forces oppose the movement of the particles and the pressure of water in the void spaces does not increase. Liquefaction does not occur in fine particulate cargoes.
For liquefaction to occur, the cargo material must be granular, wet and subject to energy from ship motions. When these conditions are present, the particles of cargo move, compressing the void spaces and increasing the pressure of water in the void. When the pressure reaches a certain level, the friction forces between particles decreases, the shear strength of the material becomes very low and the material flows.
Liquefaction does not occur in large or very fine particle cargo, where there is little energy input (calm seas) and when the cargo is dry."
(18) 11th paragraph
I understand that liquefaction easily take place in mineral concentrates obtained through floating separation due to the effect of the residue of surface active agents. However. I consider that such additional explanation is not necessary for safe carriage of cargoes.
"Cargoes of mineral concentrates are particularly susceptible to liquefaction. In part this is due to the size and shape of the particles, but it is also a result of the presence in the cargo of surfactant residue from the refining process. These residues significantly reduce the internal friction, increasing the likelihood of liquefaction in the right conditions."
(19) 12th and 13th paragraph
The sentences in the following paragraphs can be incorporated into section 7.1. (refer to the comment No.20)
"The nature of the movement of a cargo undergoing liquefaction is such that the ship may suffer a sudden and catastrophic loss of transverse stability, leading in the worst case, to capsize.
It is therefore of extreme importance to mariners carrying these cargoes that they have access to good data on the Transportable Moisture Limit (TML) and the moisture content of the particular cargo."
Re: New paragraph 7.1
(20) Taking above comments (12 to 19) into consideration, I propose new paragraph 7.1 as follows:
"7.1.1 Cargoes may shift in cargo holds resulted from ship motion and the shift of cargo may cause capsize. The phenomena of cargo shifting can be categorized into two types, which are "sliding failure" and "liquefaction". The prevent sliding failure, cargo should be trimmed as described in section 5
7.1.2 Some cargoes categorized in Group A in the entries for individual cargoes of this Code may liquefy during voyage even if the cargo is trimmed level to the boundaries of the cargo space and the cargo has cohesion. Typical liquefaction phenomenon can be illustrated, roughly, as following:
(1) The volume of void in particles of the cargo becomes less due to the microscopic movement of particles resulted from cyclic load, when the friction between particles including cohesion is not enough to confine the movement;
(2) The water flow, due to the decrease of void in particles, through the small void is resisted and the pressure of water in void increases. when the cargo contains a certain proportion of small particles and the void does not contain enough air;
(3) The friction between particles becomes small, resulted from the increase of pressure of water in void, and the shear strength of the cargo becomes very low, and
(4) The cargo may flow or become easy to shift.
