Iron ore loading limits

As bulk carriers have increased in size and loading rates have been speeded up, small deviations from loading plans can cause structural instability for an iron ore carrier.

The aim when loading a large bulk carrier is to maintain girder strength which is achieved by evenly distributing the cargo. If the weight of the cargo is not smoothly distributed over the holds, the hull will experience large bending and shearing loads which can cause high stresses in the structure.

A ship might, for example, be loaded using an alternate loading pattern. For a capesize this could mean holds 1, 3, 5, 7 and 9 are loaded with iron ore and holds 2, 4, 6 and 8 would be left empty.

Although this configuration generates high shear forces on the vessel this is not a danger if the ship has been designed to accommodate such an arrangement. However, Gijsbert de Jong, product manager for dry bulk carriers at classification society Bureau Veritas, says the main issue is how to arrive at this condition in a safe and controlled manner.

“In order to do so we need to ensure that at any time during the cargo loading process the hull girder loads do not exceed the strength capacity of the ship structure,” he said. “That means that the sequence and speed of loading iron ore and discharging of ballast water needs to be carefully prepared and agreed between the loading terminal and the master.”

Key points are the number of pours used to fill the cargo holds, the cargo loading rate and ballast water discharging rate.

“For obvious reasons of efficiency the loading terminal would prefer the highest possible cargo loading rate using a single pour to fill each hold,” he adds. “The last point may prove to be an issue, as not all bulk carriers have been designed for single pour loading.”

He says that while in principle high loading rates of up to 16,000 tonne per hour do not necessarily cause problems, if the process is not well controlled and there is a problem with the synchronisation of loading and de-ballasting, causing a deviation from the agreed loading plan, a dangerous situation can occur. “If the hull girder is overloaded, large scale structural damage can result and – in the worst case scenario – structural collapse,” he says.