Ballast Water Defined
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What is ballast?
Why do ships carry ballast water?
- Why is ballast water an issue?
- Discharges in Canadian waters - Statistics
Ballast is defined as any solid or liquid that is brought on board a vessel to increase the draft, change the trim, regulate the stability or to maintain stress loads within acceptable limits. Prior to the 1880s, ships used solid ballast materials such as rocks and sand, which had to be manually shoveled into cargo holds, and similarly discharged when cargo was to be loaded on board. If not properly secured, solid ballast was prone to shifting in heavy seas causing instability.
With the introduction of steel-hulled vessels and pumping technology, water became the ballast of choice. Water can be easily pumped in and out of ballast tanks, requires little manpower, and as long as tanks are kept full, poses little to no stability problems.
Diagram: Courtesy of GloBallast
Ships are designed and built to move through water carrying cargo, such as oil, grains, containers, machinery and people. If the ship is travelling without cargo, or has discharged some cargo in one port and is on route to its next port of call, ballast may be taken on board to achieve the required safe operating conditions. This includes keeping the ship deep enough in the water to ensure efficient propeller and rudder operation and to avoid the bow emerging from the water, especially in heavy seas.
Water has a good weight-to-volume ratio and is carried in separate tanks used just for ballast, or in empty cargo tanks. When a vessel is departing a port, water and any sediment that may be stirred up, is pumped into the ballast tanks and released again when it takes on cargo at the next port. To prevent large foreign objects from entering the ship with the ballast water, ships' sea chests are covered with grates.
Safety, weather conditions, a ship's load, and the route taken are the primary factors that determine how much ballast water is taken on board a vessel. More ballast is necessary for ships to sit lower in the water during stormy weather, or to allow for passage under a bridge. Ballast water is also used to balance the ship as it uses up fuel during a long voyage, or during loading and unloading operations.
Ocean going vessels can be classified as general cargo ships, bulk carriers, tankers, chemical tankers, container ships, ore carriers, passenger ships Ro/Ro carriers, ferries and tug/barge combinations. The number and size of ballast tanks vary according to the vessel type and design, and are positioned in order to minimize hull stresses. Double bottom ballast tanks that run the length of the vessel, and are integrated with the hull girder, provide stability. Most ships are equipped with a range of ballast capabilities and capacities, but generally it is 25 to 30 per cent of their dead weight tonnage.
There are thousands of aquatic species that may be carried in ships' ballast water, including bacteria and other microbes, micro-algae, and various life stages of aquatic plant and animal species. Ships travelling in Canadian waters carry thousands of tonnes of ballast water annually, making Canada vulnerable to the introduction of alien species from the ballast water discharged.
Species are considered alien if they are not native to a given ecosystem. They are also referred to as exotic, non-native, or non-indigenous species. Alien species are considered to be invasive when their introduction causes, or is likely to cause, harm to the environment, the economy, or human health.
The introduction and spread of alien invasive species is a serious problem that has ecological, economic, health and environmental impacts, including loss of native biological diversity. The impacts depend on the origin of the organisms and the location of the point of discharge.
For recent statistics on ballast water discharges in Canadian waters, please select this link.
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