Vessels and Ballast Water - Global Maritime Transport and Ballast Water Management: Issues and Solutions - page 28

types of cargoes, which could be divided into specifi cally heavy, e.g., metal rolls,
steel, iron, ore, carbon, oil; and light cargoes, e.g., grains, timber, paper, vehicles,
containers.
In the case of loading a heavy cargo, a vessel will be most probably immersed to
her maximum draught, i.e., one of the load lines, 5 hence needs to discharge all bal-
last to load as much cargo as possible. This means that the vessel will discharge all
ballast except the quantity unable to be discharged, and the quantity needed for
trimming and heeling where appropriate.
Some vessels in ports usually undertake both cargo loading and discharge opera-
tions, e.g., containers, vehicles, general cargoes. In these cases the ballast water
situation fully depends on the quantity of discharged and loaded cargo. If the quan-
tity of discharged cargo is greater than that loaded, it is supposed that the ship will
not discharge ballast and vice versa.
The quantity of ballast water may also depend on weather conditions. When
expecting to sail through bad weather conditions and heavy seas, vessels would be
in heavy ballast condition to improve the safety of navigation.
Tanker vessels carrying heavy oil or vessels specialized for the carriage of orange
juice, for instance, as a rule return to the port of loading empty and therefore require
larger quantities of ballast water for safe navigation. On the other hand, a general
cargo and container vessel will when in operation always carry some cargo, i.e.,
some will be discharged and some loaded at the next port of call. These vessels can
thus carry ballast water taken up in different ports. The quantity of ballast water
carried, however, primarily depends on the cargo handling operations carried out.
Therefore, if a signifi cantly greater quantity of cargo is discharged than loaded, it
may be assumed that ballast water will be required on board, and vice versa (David
et al. 2012 ).
However, when a vessel loads a light cargo, her maximum DWT capacity will
not be exploited, because the limiting factor becomes the volume available to store
the cargo, and not the cargo weight. Some light cargoes are frequently also loaded
on the deck as well as in cargo holds. Consequently, the vessel has diminished trans-
versal stability and needs to improve it by adding ballast in her double bottom tanks.
A typical example is that of loading timber on deck, and this may also be the case
when heavier containers would be loaded on top of lighter containers or on the
upper deck of a car carrier.
The above described situations and conditions show that the ballast water opera-
tions are related to different vessel types, vessel construction, cargo operations and
weather conditions. However, there are no clear limits among all these factors, but
the decision on ballast water operations is under the discretion of the chief offi cer
and direct control of the captain, who is responsible for the vessels stability and
safety (David et al. 2012 ).
5 i.e., appropriate load line according to the IMO Load Line Convention.