Ballast tank

Cross section of a vessel with a single ballast tank at the bottom.

A ballast tank is a compartment within a boat, ship or other floating structure that holds water, which is used as ballast to provide stability for a vessel. Using water in a tank allows for easier adjustment of weight than stone or iron ballast as was used in older vessels. It also allows for ballast to be pumped out to temporarily reduce the draft of the vessel when required to enter shallower water.

History

The basic concept behind the ballast tank can be seen in many forms of aquatic life, such as the blowfish or argonaut octopus,^[1] and the concept has been invented and reinvented many times by humans to serve a variety of purposes. For example, in 1849 Abraham Lincoln, then an Illinois attorney, patented a ballast-tank system to enable cargo vessels to pass over shoals in North American rivers.

Ships

In order to provide adequate stability to vessels at sea, ballast is used to weigh the ship down and lower its centre of gravity. International agreements under the Safety Of Life At Sea (SOLAS) Convention require cargo vessels and passenger ships to be constructed so as to withstand certain kinds of damage. The criteria specify the separation of compartments within the vessel and also the subdivision of those compartments. The International agreements rely upon the states which have signed the agreement to implement the regulations within their waters and on vessels which are entitled to fly their flag. The ballast is generally seawater which is pumped into tanks known as ballast tanks. Depending on the type of vessel, the tanks can be double bottom (extending across the breadth of the vessel), wing tanks (located on the outboard area from keel to deck) or hopper tanks (occupying the upper corner section between hull and main deck). These ballast tanks are connected to pumps which can pump water in or out. These tanks are filled in order to add weight to the ship once cargo has been discharged, and improve its stability. In some extreme conditions, ballast water may be introduced to dedicated cargo spaces in order to add extra weight during heavy weather or to pass under low bridges.

Submarines

Ballast locations on a submarine.

In submersibles and submarines, ballast tanks are used to control the buoyancy of the vessel.

Some submersibles, such as bathyscaphes, dive and re-surface solely by controlling their buoyancy. They flood ballast tanks to submerge, then to re-surface either drop discardable ballast weights, or used stored compressed air to blow their ballast tanks clear of water, becoming buoyant again.

Submarines are larger, more sophisticated and have powerful underwater propulsion. They must travel horizontal distances submerged, require precise control of depth, yet do not descend so deeply, nor need to diver vertically on station. Their primary means of controlling depth are thus their diving planes, in combination with forward motion. At the surface the ballast tanks are emptied to give positive buoyancy. When diving, the tanks are partially flooded to achieve neutral buoyancy. The planes are then adjusted together to drive the hull downwards, whilst still level. For a steeper dive, the stern planes may be reversed and used to pitch the hull downwards.

Submerging is done by opening the vents in the top of the ballast tanks, as well as opening the valves in the bottom. This allows water to flooding into the tank, and allows the air already present inside the tank to escape through the top vents. as the air escapes from the tank, the vessel's buoyancy decreases, thus causing it to sink. In order for the submarine to surface, the vents in the top of the ballast tanks are shut, and compressed air is allowed into the tanks. The high-pressure air pocket pushes the water out through the bottom valves and increases the vessel's buoyancy, causing it to rise. A submarine may have several types of ballast tank: the main ballast tanks, which are the main tanks used for diving and surfacing, and trimming tanks, which are used to adjust the submarine's attitude (its 'trim') both on the surface and when underwater.

Floating structures

Ballast tanks are also integral to the stability and operation of deepwater offshore oil platforms and floating wind turbines.^[2] The ballast facilitates "hydrodynamic stability by moving the center-of-mass as low as possible, placing [it] beneath the [air-filled] buoyancy tank."^[2]

Wakeboard boats

Most wakeboard-specific inboard-engine boats have multiple integrated ballast tanks that are filled with ballast pumps controlled from the helm with rocker switches. Typically the configuration is based on a three tank system with a tank in the center of the boat and two more in the rear of the boat on either side of the engine compartment. Just like larger ships when adding water ballast to smaller wakeboard boats the hull has a lower center of gravity, and increases the draft of the boat. Most wakeboard boat factory ballast systems can be upgraded with larger capacities by adding soft structured ballast bags.

Environmental concerns

Diagram showing the water pollution of the seas from untreated ballast water discharges

Ballast water taken into a tank from one body of water and discharged in another body of water can introduce invasive species of aquatic life. The taking in of water from ballast tanks has been responsible for the introduction of species that cause environmental and economic damage. For example, zebra mussels in the Great Lakes of Canada and the United States.

Non native macroinvertebrates can find their way into a ballast tank. This can cause problems ecologically and economically. Macro-invertebrates are transported by transoceanic and coastal vessels arriving in ports all over the world. Researchers from Switzerland sampled 67 ballast tanks from 62 different vessels operating along geographic pathways and tested for mid ocean exchange or voyage length that had a high chance of macro-invertebrate relocating to a different part of the world. An assessment was done between the relationship of macro-invertebrate presence, and the amount of sediment in ballast tanks. The Switzerland researchers had discovered a presence of a highly invasive European green crab, mud crab, common periwinkle, soft shell clam, and blue mussel in the ballast tanks of the sampled ships. Although the densities of macro-invertebrate were low, invasion of non-native macro-invertebrates can be worrisome during their mating season. The worst thing that can happen is if a female macro-invertebrate is carrying millions of eggs per animal.^[3]

Migration of living animals and settling particle-attached organisms can lead to an uneven distributions of biota at different locations of the world. When small organisms find their way into a ballast tank, the foreign organism or animal can upset the balance of the local habitat. When a local habitat is changed, it can interfere with the natural habitat and potentially damage the existing animal life. Vessel workers check the ballast tank for living organisms ≥50 μm in discrete segments of the drain, it also represents the level of sedimentary of different rock or soil in the tank. Throughout the sample collection, concentrations of organisms and marine life varied in result in the drain segments, patterns also varied in level of stratification in other trials. To have the best sampling strategy for stratified tanks, is to collect various time-integrated samples spaced evenly throughout each discharge.^[4]

All Trans-Oceanic vessels that enter the Great Lakes are required to manage ballast water and ballast tank residuals with ballast water to clear out and exchange for tank flushing. Management and procedures reduce the density and richness of biota effectively in ballast waters and thus reduce the risk of transporting organisms from other parts of the world to non-native areas. Although most ships do ballast water management not all are able to clear the tanks. In an emergency when residual organisms are not able to be cleaned, vessel workers use sodium chloride brine to treat the ballast tanks. Vessels arriving in the Great Lakes, and North Sea ports, were exposed to high concentrations of sodium chloride until the mortality rate of 100% is reached. Results show that an exposure of 115% of brine is extremely effective treatment resulting in a 99.9 mortality rate of living organisms in ballast tanks regardless of the type of organism. There was a median of 0%. About 0.00–5.33 of organisms are expect to survive treatment of the sodium chloride.^[5]

One of the most common problems among vessel construction and maintenance is the corrosion that takes place in the double hull space ballast tanks have in merchant vessels.^[6] Bio-degradation takes place in ballast tank coatings in marine environments. To avoid biodegradation, paint has been a new idea to stop the corrosion of ballast tank. Ballast tanks can carry more than ballast water, most of the time ballast tanks are filled with other bacteria or organisms. Some of these bacteria can that can be picked up from other parts of the world can cause the ballast tank to get damaged. Bacteria from different regions plus the natural bacteria can cause ballast tanks to break down. The natural bacteria community has an interaction of the natural bio-films with the coating, an aspect which is not covered in standard procedures. Researchers have shown that biological activity indeed significantly affects the coating properties. Micro-cracks and small holes have been found in ballast tanks. Acidic bacteria created holes with 0.2–0.9 μm in length and 4–9 μm in width. The natural community caused cracks of 2–8 μm in depth and 1 μm in length. The EIS technique was used to examine the degradation. The bacterial affected coatings decreased in corrosion resistance. The natural community, has a clear loss in coating resistance over time. Also, coating corrosion resistance declines after 40 days of exposure to the natural community, resulting in blisters in the ballast tank. Bacteria might be linked to certain bio-film patterns affecting various types of coating attacks.^[7][8]

See also

References

Briski, E., Ghabooli, S., Bailey, S., & MacIsaac, H. (2012). Invasion risk posed by macroinvertebrates transported in ships' ballast tanks. Biological Invasions, 14(9), 1843–1850.

Robbins-Wamsley, S., Riley, S., Moser, C., Smith, G., et al. (2013). Stratification of living organisms in ballast tanks: How do organism concentrations vary as ballast water is discharged?.Environmental Science & Technology, 47(9), 4442.

Bradie, J., Velde, G., MacIsaac, H., & Bailey, S. (2010). Brine-induced mortality of non-indigenous invertebrates in residual ballast water.Marine Environmental Research, 70(5), 395–401.

De Baere, K., Verstraelen, H., Rigo, P., Van Passel, S., Lenaerts, S., et al. (2013). Reducing the cost of ballast tank corrosion: An economic modeling approach.Marine Structures, 32, 136–152.

Heyer, A., D'Souza, F., Zhang, X., Ferrari, G., Mol, J., et al. (2014). Biodegradation of ballast tank coating investigated by impedance spectroscopy and microscopy. Biodegradation, 25(1), 67–83.