A Ballast tank is a space within a ship, boat or floating platform that holds any solid or liquid with the purpose of regulating Draft, trim, stability or stress loads of the vessel or structure.[1]
History
Historically, ballasting systems for vessels involved manually loading solid materials on board to achieve the desired Draft of the vessel, whether that be just deep enough in the water to achieve increased propeller immersion, or to have the vessel sit very deep in the water in order to have maximum stability when moving without any cargo or passengers on board. This was done because ships would some times have to make voyages without any cargo when going to ports to pick up products destined for other ports but no cargo was going to that port of call, in which case the ship would carry ballast to keep stable and drafting in the water properly. Up until the mid nineteenth century solid ballast was the only method for altering the way a ship was situated in the water, this was done by adding or removing material from various holds on the ship by laborers in ports. Solid ballast consisted of a wide variety of materials selected for their weight, in-expense, and ease of procurement in a given port, they included; earth, stone, gravel, sand, pig iron, and sometimes even ordnance and rubble were used on military vessels.[2]
Ships
The primary factor that allowed ships to transition from solid ballast to liquid ballast systems was the shift in ship building in the nineteenth century from wooden ships to iron steamships. The first steamships that incorporated water ballast tanks were colliers in Europe during the mid nineteenth century because it allowed for significant decreased costs in making the round trips from Tyne to London and back. The coal was mined in Tyne and needed in London, so the leg of the voyage back to Tyne was one without cargo, which means the cargo ships had to carry ballast[3]. By using sea water to ballast the ship instead of buying, loading and then unloading the ballast in Tyne, ship operators cut time spent in port and the costs associated with using solid ballast. Today ships implement a complex system of ballast tanks, pipes and pumps to move sea water through out the ballasting system to various tanks as well as bring more water on board or discharging it over board[4].
Tank Coatings
Due to the corrosive nature of salt water stored in the metal tanks on board vessels as ballast it has been a constant challenge for ship operators to prevent the rusting and surface pitting caused by the sea water being in contact with the steel walls of the tanks for sometimes weeks on end. It is now common practice to coat the insides of tanks with anti-corrosion or corrosion resistant coatings ranging from epoxy based paints applied in thick, 2-3 mm coats, to anti corrosion coatings of inorganic zinc compounds.[5] The inorganic zinc coatings effectively line the tank with a sacrificial anode which is consumed by the salt water via galvanic corrosion.
Submarines
In submarines ballast tanks are used to allow the vessel to submerge, water being taken in to alter the vessel's buoyancy and allow the submarine to dive. When the submarine surfaces, water is blown out from the tanks using compressed air, and the vessel becomes positively buoyant again, allowing it to rise to the surface. 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. Up until modern submarine designs became widely used submarines also commonly incorporated a superstructure around the watertight cylindrical part of the vessel and water was allowed to enter the spaces between the two in order to prevent undue stresses on the superstructure when fully submerged. [6]
Environmental concerns
Ballast water taken in to the ballast tanks of a vessel from one body of water and discharged into another body of water has the potential to introduce invasive species of aquatic life. This hazard is highly dependent upon the port of ballasting and discharge, volume of water, duration of storage of the water, and if any ballast treatment systems are implemented to mitigate survival of invasive species in the tanks[7]. The discharging of water from ballast tanks, which were filed in foreign ports has been responsible for the introduction of species that cause environmental, ecological and economic damage[8]. Some examples include; zebra mussels in the Great Lakes of Canada and the United States, Asian Kelp in New Zealand and the United States, Mitten Crabs in North American and Europe, and North Pacific Sea-stars in Austraila.