the rotating lug of the twistlock engages, locking the containers together. In addition, the bottom two layers of the stacked containers are connected to the ship with lashing rods. Initially, it was common practice to stow stacks of containers on deck in such a way that the individual stacks were connected to each other laterally using special stowage equipment. In contrast to this, nowadays each stack stands separately so that it can be stowed or unloaded independently of neighboring stacks. A ship carrying containers on deck thus transports a kind of “forest” of independent towers which, in heavy seas, sway back and forth due to the elasticity of the material in much the same way as the ears of corn in a field.
The lashing rods are arranged crosswise in front of and behind each container stack and absorb lateral movements of the stack. Depending on the intended height of the container stacks on deck and their permitted weight, the 5th layer of the stacks are, for example, secured at the bottom by the rods. To allow these rods to be attached, ships have what are known as lashing bridges running across the ship between the rows of containers. The workers can stand on these to work and fasten the rods.
The lashing systems are calculated individually for each ship by specialist companies and are inspected and approved by the classification societies. The results of these calculations are recorded in a Cargo Securing Manual approved by the flag state and in which all details can be found. The weight of the containers is of crucial importance, because the higher a container is in a stack of containers on deck, the less it may weigh.
You can think of this in terms of the weights on a seesaw. On one side of the seesaw is the securing capacity of the lashing system, consisting of twistlocks, lashing rods and turnbuckles, and on the other side there is the weight of the
containers. With this seesaw, the side on which the container weight is sitting must always be up in the air. In other words, the securing system must provide more securing force in an emergency than the containers and their weight require. If containers weighing 28 tonnes are located in the 8th layer, where only containers weighing 5 tonnes are actually permitted, the securing balance of the “seesaw” is massively impacted and the securing capacity is hanging in the air. Applied to the securing system on board, this means that it would be loaded beyond its calculated limits in heavy seas and would fail. The result is that the container stack would sway back and forth in the swell and would lean against the neighboring container stacks. This would in turn overload their securing systems, which already have their hands full holding their own container stacks, and they would therefore also fail. The obvious consequence is a domino effect, such as the one seen recently on the the MSC Zoe.
However, the securing balance on our seesaw can also be disrupted by the cargo in the container itself. This is because the securing systems on board ships are calculated on the assumption that the dynamic behavior of the cargo is neutral. Static masses are assumed for the containers, and it is assumed that this mass always acts at the same point in the container. If a cargo weighing 30 tonnes is inadequately secured in a container, it will break loose in a rough sea and bang from side to side in the container. This violent sideways motion has the same effect on our securing system as a wrecking ball on a house. If the bad weather lasts long enough and the ship’s movements are violent enough, the securing system will fail and cause a domino effect similar to that described above.
The extent to which a ship rolls in the swell will also depend on its stability. The stability referred to here is the ability of a ship to right itself after an external force is applied, for instance as a result
The Report • June 2019 • Issue 88 | 53
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If a cargo weighing 30 tonnes is inadequately secured in a container, it will break loose in a rough sea and bang from side to side in the container. This violent sideways motion has the same effect on our securing system as a wrecking ball on a house.
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