Page 33 - Ship Construction.DJ Eyres 6Ed

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22 Ship Construction
Bulk Carriers

A wide range of bulk commodities are carried in bulk carriers including
coal, grain, ore, cement, alumina, bauxite, and mineral sand plus shipments
of products such as packaged steel and timber.
The large bulk carrier originated as an ore carrier on the Great Lakes at
the beginning of the 20th century. For the period of the Second World War
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dedicated bulk carriers were only built spasmodically for ocean trading,
since a large amount of these cargoes could be carried by general cargo
tramps with the advantage of their being able to take return cargoes.
งานห้องสมุด ศูนย์ฝกพาณิชย์นาวี
A series of turret-deck steamers were built for ore carrying purposes
between 1904 and 1910 and a section through such a vessel is illustrated in
Figure 3.4(a). Since 1945 a substantial number of ocean-going ore carriers
have been built of uniform design. This form of ore carrier with a double
bottom and side ballast tanks first appeared in 1917, only at that time the
side tanks did not extend to the full hold depth. To overcome the disadvan-
tage that the ore carrier was only usefully employed on one leg of the voyage
the oil/ore carrier also evolved at that time. The latter ship type carried oil in
the wing tanks as shown in Figure 3.4(c), and had a passageway for crew
protection in order to obtain the deeper draft permitted tankers.
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The common general bulk carrier that predominated in the latter half of
the twentieth century took the form shown in Figure 3.4(d) with double
bottom, hopper sides and deck wing tanks. These latter tanks have been used
for the carriage of light grain cargoes as well as water ballast. Specific
variations of this type have been built, Figure 3.4(e) shows a ‘universal bulk
carrier’ patented by the McGregor International Organization that offered
a very flexible range of cargo stowage solutions. Another type shown in
Figure 3.4(f) had alternate holds of short length. On single voyages the vessel
could carry high density cargoes only in the short holds to give an accept-
able cargo distribution. Such stowage is not uncommon on general bulk
carriers with uniform hold lengths where alternate hold loading or block
hold loading may be utilized to stow high density cargoes. With such
loading arrangements high shear forces occur at the ends of the holds
requiring additional strengthening of the side shell in way of the bulkheads.
A general arrangement of a typical bulk carrier shows a clear deck with
machinery aft. Large hatches with steel covers are designed to facilitate
rapid 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 size of this type of ship has also steadily
increased and bulk carriers have reached 250 000 tonnes deadweight.
Ships of the general bulk carrier form experienced a relatively high casu-
alty rate during the late 1980’s and early 1990’s (between 1980 and 2000
some 170 bulk carriers were totally lost) giving rise to concern as to their
design and construction. Throughout the late1990’s bulk carrier safety