It may surprise some readers to know how many different types of cargo vessels are traversing our oceans. The type of ship used to move a particular commodity depends on the nature of that commodity as well as the volume being transported. The following details some of the different types of vessels which may be seen at anchorage off Durban and what they may be carrying:

The bulk carrier is one of the most simply designed vessels. As the name implies, their purpose is to carry homogenous cargoes in dry bulk. Dry bulk cargoes may be grain, sugar, coal, rice or iron ore, to name but a few. While bulkers come in a range of sizes from small coastal ships [Handy size] to the large Capesize vessels capable of carrying well over 200,000 tonnes of cargo, they all commonly have a single deck with several (up to nine) clear holds [cargo space] and large hatches on the deck of the vessel for loading and discharging purposes. They occasionally have their own gear [cranes] onboard or, more commonly, not. In that case cargo is typically loaded by shiploader and removed from them by shore cranes.

The “tweendecker” or “multi-purpose” ship is generally designed to allow it to carry most cargoes. Most multi-purpose vessels are able to carry containers in both the holds and on deck, or alternatively bulk cargoes may be carried in the holds. To allow for variations in the stowage factor [the amount of space a cargo takes up], many of these vessels will have movable bulkheads [internal walls] which can be shifted to make a particular hold smaller or larger depending on the cargo which will be carried in it. Most multi-purpose vessels have their own onboard gear.

Container ships are used regularly in the liner trade and carry mostly manufactured goods packed in containers. Container ships are “fully cellular” which means that the vessel’s holds have vertical metal guides into which the containers slide below deck. These vessels will also load tiers of containers on deck which will be lashed together and secured. The size of container vessels is based on the standard 20 foot container and the size of the ship is expressed in TEU’s [twenty feet equivalent units]. The majority of the container ship fleet is around 8000 TEU’s however there are monstrous vessels, with the largest at present being the MSC OSCAR at 19,224 TEU’s.

Roll-on/ Roll-off [“Ro-Ro”] vessels are designed for cargo with wheels. They range in size from ferries to the massive autoliners which regularly call in Durban. Common to all Ro-Ro vessels is that each has a ramp to allow wheeled cargo to be driven or towed into or out of the holds. These ramps may be straight ramps directly off the bow or the stern, or alternatively they may have side access ramps or quarter access ramps which project at an angle from the vessel.

Vessels which carry liquids in bulk are commonly grouped as “tankers.” Crude Oil tankers are generally the largest and may be classed, in order of size, as “ULCCs” Ultra Large Crude Carriers, or “VLCCs” which are Very Large Crude Carriers or Suezmax and Aframax which are medium sized carriers. Product carriers are usually smaller than crude tankers but again are divided into the standard size and a smaller or “handy-size.” In terms of structure, tankers are divided by lateral and longitudinal bulkheads which give the vessels a series of center tanks alongside wing tanks. Generally the wing tanks are only used for water ballast. Tankers are almost always equipped with their own pumps for discharge. Regulations are now in place that newbuild tankers are to be “double hulled” in an attempt to combat pollution incidents.

The expansion of the petro-chemical industry has also occasioned a specialist tanker. Chemical and parcel tankers are generally quite small in size, but contain specialist tanks which are internally coated with types of epoxy or silicates. The different coatings are compatible with the carriage of different chemicals. This is due to the fact that some petro-chemical cargoes are extremely sensitive to the smallest amount of water or rust which could result in the cargo being contaminated.

Both natural gas and petroleum gas are moved in gas carriers, albeit different types of carriers. Natural gas is exactly that, and may be used in its natural form- such as methane. Petroleum gas however is a by-product of oil refinements. It has many uses, the most familiar of which is butane for camp stoves. Both types of gas are carried in liquid form. The two main types of liquid petroleum gas [“LPG”], butane and propane, may be kept in a liquid state so long as they are carried in highly pressurized environments. Cargo tanks in LPG carriers are usually cylindrical, self-supporting and freestanding as well as being insulated. Natural gas on the other hand cannot be liquefied by pressure alone and must be carried at very low temperatures, for example, Methane must be carried at minus 163 degrees Centigrade. One of the most common carriers for these types of cargo is designed using the “Moss system”- easily identifiable by the spherical tanks which protrude high above the ship’s deck. The tanks are insulated and made from an aluminum alloy.

Most liquid natural gas [“LNG”] carriers have been built specifically for a particular contract, although given the recent swing to alternate fuels from petrol and diesel, we may be seeing more of these vessels soon. LNG is typically delivered either to onshore facilities or to Floating Storage and Regasification Units [“FSRU’s”] which are a type of ship, but are more like a floating factory. The current thinking of the Department of Energy is to have FSRU’s anchored in Richards Bay and Ngqura as part of their gas to power problem.

All of the cargo ships are configured and designed in specific ways to deal with their cargoes, making their type and use recognizable at several miles for experienced ship watchers.

This article first appeared in the Sunday Tribune.