Vessels and Ballast Water - Global Maritime Transport and Ballast Water Management: Issues and Solutions - page 13

The Importance of Ballast for Vessels

Commercial vessels are built for the transport of various commodities or people by
the sea or inland waterways. When a vessel is not fully laden, additional weight is
required to compensate for the increased buoyancy that can result in:

- the lack of propeller immersion,

- inadequate transversal inclination, i.e., heeling,

- inadequate longitudinal inclination, i.e., trim,

- static and dynamic stresses on the vessel's hull including shear and torsion

forces, bending moments and slamming, and

- static and dynamic transversal and longitudinal instability,

in order to provide for the vessel's seaworthiness. This implies that not only com-
mercial vessels, but also some other vessels (e.g., navy vessels, bigger pleasure
boats) use ballast water to provide for adequate seaworthiness (David 2007 ).
The material used for adding weight to the vessel is referred to as ballast.
Historically, ballast material was solid (e.g., sand, rocks, cobble, iron). After the
introduction of iron, replacing wood, as basic vessel building material in the middle
of the nineteenth century, the doors were opened to new technologies. Loading of
water (i.e., ballast water) in cargo holds or tanks (i.e., ballast water tanks) was
shown to be easier and more effi cient, and hence was adopted as a new practice of
increasing importance.
A vessel deemed to be “not fully laden” is a situation when she is not at her maxi-
mum allowed draught; i.e., when her carrying capacity in terms of weight, i.e.,
deadweight (DWT), is not fully exploited. This is typically a dynamic situation dur-
ing cargo operations in a port; i.e., a vessel will experience changes in loading as it
loads and/or unloads cargo. This condition may also result from either the lack of
cargo available for transport, or occurs when cargo is light and the total volume of a
vessel's cargo spaces becomes a limiting factor (David 2007 ). However, even when
a vessel is fully loaded it can require ballast water operations due to a non-equal
distribution of weights on the vessel; i.e., loading of non-homogeneous cargoes,
e.g., general cargoes, very heavy cargoes or heavy containers on top of light
containers.
Other dynamic factors may also require ballast water operations, such as weather
and sea conditions on the route, the approach to shallow waters, and the consump-
tion of fuel and diesel oil during the voyage. According to expected weather condi-
tions, a vessel would sail in a heavy ballast condition, i.e., maximum ballast loaded,
when expecting bad weather, or a light ballast condition, i.e., partial ballast loaded,
when it is ensured that the weather conditions and rough seas will not impair the
vessel's stability, e.g., when approaching a port or inland waterways. Vessels would
go from heavy ballast to light ballast conditions when safe and weather as well as
sea conditions are favourable to consume less fuel, and when in save haven close to
a port or at the ports anchorage, to get ready for loading cargo. When approaching
shallow waters a vessel may also need to discharge some ballast water to provide for
less draught, or when she needs to sail below a bridge she may need to add ballast