Virtually all liquids transported by sea - such as crude oil and finished petroleum products - are carried in bulk, and tankers now form by far the greatest percentage of the world fleet of merchant ships. The second biggest group consists of solid bulk cargo carriers. The goods carried in this way include coal, grains, ores, concentrates, fertilizers and animal feeds. From the shippers' point of view bulk carriage has numerous advantages. A bulk cargo can be loaded and unloaded far more quickly than one that is unitized, thereby leading to great savings in time and money. However, there are a number of dangers in the carriage of bulk cargoes which include the following:
1. Improper weight distribution resulting in structural damage. This can be caused by putting too much weight on the inner bottom of the ship or by wrongly distributing the cargo between holds, leading to excessive stresses on the ship's structure.
2. Improper stability and cargo shift. Another result of improper loading can be excessive stability. This leads to the normal rolls of the ship becoming shorter but much more violent. Apart from being extremely uncomfortable for those on board, this can in turn lead to damage to the ship's structure.
Stability of the ship can also be affected by the cargo shifting during the course of the voyage. This can occur because the cargo is inadequately trimmed (levelled-off) or improperly distributed. In some cases cargoes can liquify as a result of vibration and the motion of the ship and then slide or flow to one side of the cargo hold. This usually happens when the cargo consists of finely-grained material, such as fine coal and ore concentrates, which are damp when loaded. However it occurs, a shift of cargo can lead to the ship listing and ultimately capsizing.
3. Spontaneous heating. Some cargoes carried in bulk have a tendency to heat spontaneously during the course of a voyage. The result can be a fire or explosion.
4. Chemical hazards. These hazards include the emission of toxic or explosive gases, oxygen depletion, spontaneous combustion or severe corrosion.
Measures to counter these problems have been adopted at both a national and international level.