Bulk carrier

A bulk carrier is designed to carry loose/unpacked cargoes such as grains, coal, ore, and cement. Bulk carriers can be roughly divided into three categories:

Handy size bulk carriers are of approximately 30.000 tons deadweight, often with their own cargo handling gear for ore, sand, scrap, china clay, grain and forest products. This category bulk carrier represents 71% of all bulk carriers and also has had the highest rate of growth in the last few years. This is partly due to new regulations that put constraints on the construction of larger vessels. Handy size ships are typically 150-200 m in length, with about five cargo holds and about four cranes.

Panamax bulk carriers are of approximately 80.000 tons deadweight. They seldom have cargo gear. Their cargoes are grain and ore and their dimensions are a beam, of 32,31 m, a length of 294,13 m and a draft of 12,04 m.

Cape size bulk carriers are of approximately 160.000 tons deadweight and over, have no cargo gear and carry coal and ore as cargoes.

Ship structure

The general arrangement of a typical bulk carrier shows a clear deck with hatches and an aft superstructure and machinery. Large hatches with steel covers are designed for quick loading and discharge of the cargo. Since the bulk carrier makes many voyages in ballast a large ballast capacity is provided to give adequate immersion of the propeller. The typical bulk carrier has however experienced a relatively high casualty rate during the late 1980s and early 1990s, giving rise to concern as to their design and construction. Throughout the 1990s bulk carrier safety has been a concern of the IMO and others and this continues. Based on accident experience it was concluded that these were due to structural failure which led, to loss of watertight integrity of the side shell followed by progressive flooding through damaged bulkheads the failure having arisen either from excessive hull bending or from excessive trimming stress. Much of the work has concentrated on the hull structure, operational stresses experienced, and damage to protective coatings arising from cargo discharge procedures, poor maintenance and inadequate inspection of the structural integrity of the ship.

The sequence of loading and discharging the cargo is particularly important. It is relatively easy to cause structural damage by large shear forces between full and empty tanks as well as unacceptable hull bending. Many bulk carriers lead particularly demanding working lives. Cargo grabs, bulldozers and hydraulic hammers may cause physical damage to plating, frames and brackets, and if not investigated promptly and repaired where necessary, the overall structure may be weakened. Also the fact that, in loading ports where the cargo delivery rate is high, the inability to pump out ballast water quickly enough may occasionally result in the hull being over-stressed. Even more the cargo itself gives little or no internal support to the insides of the holds. All these possible damages are worsened when the vessel sails in heavy weather, which imposes maximal stress. Thus, we can conclude that a bulk carrier may easily suffer operational structural damage that could lead to sinking.