2.3 BALLASTING PRACTICES

The majority of vessels engaged in international and coastal commercial trading undertake ballasting operations as part of their regular routines. Ballasting operations are usually performed in sheltered port waters where adverse weather conditions have minimal impact on ship stability. They can also take place at sea.

2.3.1 Ballast water distribution

Ships' masters arrange the placement of ballast in cargo holds and ballast tanks to achieve a uniform distribution of weight throughout a vessel's structure. This prevents unsafe stresses and bending moments - hogging, sagging and shearing - in the hull (Figure 2). It is important that ballast is distributed correctly throughout a ship to ensure that safe bending moments and shear stresses in the hull are not exceeded, resulting in structural failure of the vessel. The ratio of cargo to ballast, and the arrangement or placement of cargo and ballast onboard a ship, are therefore fundamental to a ship's safety and structural integrity.

Modern ships are equipped with loading computers which allow the masters to calculate ship displacement, trim, stability, draft, and longitudinal strength data for any quantities and distribution of cargo, fuel, fresh water and sea ballast⁹ This means that quantities and correct placement of ballast can be calculated quickly and accurately.

FIGURE 2
Stresses exerted on ships' hulls

Hogging

Sagging

Shearing

Source: B. Hayden after Green, in Ballast Water. A Marine Cocktail, Royal Society of New Zealand, Miscellaneous Series 30, 1995, 7.

Ballast tanks are usually kept either completely full or empty in an effort to minimise the free surface effect caused by partially filled (slack) tanks. Slack tanks can compromise vessel stability and ballast tanks (or cargo holds) may
be physically damaged by the force of large volumes of water striking the tank boundaries.

Ballast intake and discharge procedures are carried out in accordance with the design characteristics and operating requirements of an individual ship or class of ships.

2.3.2 Volumes of ballast water

Variables such as vessel type and design, cargo load, salinity of water, weather conditions, amount of fuel and fresh water on-board, sea state and the Master's discretion will influence the quantity of ballast carried by a vessel at a given time.

Typically some 30-40% of ship dwt (deadweight tonnage) can be carried as ballast water, although ballast capacities on a large bulk carrier may be as much as 60% of deadweight tonnage¹⁰ More ballast is usually carried in inclement weather - referred to as "heavy ballast condition" - than in calm conditions.

For ships visiting Victoria, average ballast capacities as a percentage of dwt for different vessel types are shown in Table 3.

TABLE 3
Average ballast capacity as a percentage of dead weight tonnage¹¹

Type of Vessel	Ballast as % of dwt
Roll On/Roll Off	38 %
Bulk Carrier	41%
Tanker	26%
Container	30%
General Cargo	35%

Source: Walters, S. (1996), Ballast Water, Hull Fouling and Exotic Marine Organism Introductions via Ships - A Victorian Study, Publication 494, May 1996, Environment Protection Authority (Victoria).