Problems Encountered With Ships Ballast Tank Discharging
written by: Willie Scott
• edited by: Lamar Stonecypher
• updated: 11/2/2010
When ships have discharged their cargo, they fill their ballast tanks with seawater sometimes containing aquatic exotic species.
Once they reach the next port they discharge the ballast, along with any alien, non-indigenous species that can be detrimental to the local marine life and ecosystems.
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In the old days (even before this old mariner went to sea as a boy) ships used sand and rocks as ballast, so even in those days of wooden ships, mariners knew the advantages of ship ballasting. Nowadays we have a ballasting system comprising of segregated tanks, supplied with seawater from dedicated ballast pumps. This normally happens as the cargo is being discharged, especially on oil tankers.
There are several reasons for ballasting a ship; the main ones being keeping the propeller and rudder in the water maintaining steering and propulsion operations, maintaining the ship's trim and stability in heavy weather, and minimizing topsides wind drag.
However, ballasting and de-ballasting has led to some serious problems, particularly to environmental marine life at the port of discharge.
This is an article on marine engineering and in particular on the marine environment ecosystems that ships ballasting and deballasting can cause. We begin with an overview of a typical ballasting system.
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Applications of a Typical Ballasting System
Oil tankers, container ships, and car transporting vessels are much larger than conventional cargo ships, having large segregated ballast tanks, so we will examine how the ballast system on a VLCC is operated, including the use of the compulsory double hulls.
The fitting of double hulls to all existing oil tankers and to new builds was enforced by the International Maritime Organization (IMO) after several bad tanker accidents involving major polluting heavy crude oil spills. The double hulls on oil tankers now form the ballast tanks, which is logical, as in a collision or grounding it would be the seawater that would escape from the ruptured hull to the ocean, rather than crude oil.
When discharging their cargo, oil tankers take on ballast simultaneously to ensure no extra stress is put on their large hulls, the chicksan loading/discharge arms, or shoreside connections. Ballasting is also used to maintain the ship's level trim during discharge, to ensure all the oil is pumped from the tanks, and so no pockets of oil are left in the tank extremities.
As we have seen, ballasting a ship in foreign ports can include taking onboard some exotic, aquatic species which will remain in the ballast tanks as stowaways until the ballast water is discharged in another port.
It is here that these aquatic, non-native species are introduced to the local waters worldwide, causing numerous problems as shown below:
Great Lakes of Canada and USA
These are approached from the St Lawrence Seaway through a series of locks.
When I was at sea in the sixties our ship used to run between UK ports discharging in Toronto and Detroit, but even in these days there were many ships from numerous foreign exotic ports.
These undoubtedly introduced Aquatic Invasive Species (AIS), animals and plants such as the zebra mussel which rapidly reproduced, quickly populating the waterways and lakes clogging water pipes, filters, and pumping systems. They were also washed ashore at many recreational sites causing foul odors and sharp-edged shell accumulation; subsequent eradication and clean-up of these species cost in excess of $1 billion.
The introduction of North Pacific Sea Star to Australian ports was from ships discharging the larvae along with ballast water loaded in Japan or Korea. The larvae quickly grow to sea-stars of up to 100cm, a danger to local indigenous aquatic life; they poison prey before devouring them.
Black and Caspian Seas
The introduction of the carnivorous North American Comb Jellyfish also known as the sea walnut was introduced by oil tankers discharging ballast water. Its invasion of these waters had a catastrophic effect on their whole ecosystems, decimating fisheries in the Black and Caspian Seas.
NAS - descriptions of various invasive species in USA waters.
NAP - Invasive species in the Great Lakes, Australia and Black Sea.
Invasive Species in Ballast systems – Eradication Methods
1. The exchange of ballast water in mid-ocean.
This is carried out by discharging the original ballast and refilling with the mid-ocean seawater, before entering the cargo discharge port.
2. Purification by UV or Ozone
The use of UV lamps or ozone on the ballast water to kill the invasive species has been suggested.
3. Mechanical control
This involves the use of fine mesh screens (50 Micron) and a backwash system located in the ballast suction inlets. It is designed to trap any invasive species before they can enter into the ballast system.
4. Biological control
Once the invasive species have been identified, the use of a species which control them in their own habitat can be introduced to manage their extermination in their new environment.
5. De-oxygenation of ballast water
This method entails the injection of nitrogen into the ballast tank water, thus starving it of oxygen. De-oxygenation is deemed to not only to eradicate any invasive species, but also mitigate the corrosion of the internals of the ballast tanks.
6. Use of fresh water as ballast
This would require the ballast tanks to be well washed out before filling with fresh water. The fresh water ballast could then be pumped ashore and used for irrigation in arid countries.
7. Coating of ballast tank internal surfaces
There are numerous anti-corrosion tank coatings available today, and these can incorporate anti-fouling additions such as silicon PDS (polydimethyelsiloxanes) to reduce the formation and reproduction of invasive species on the tank surfaces.
However most of the successful anti-fouling coatings are toxic and would be detrimental to indigenous aquatic species if discharged along with the ballast water.