See L. White and E. A. Prentis, Cofferdams (2d ed. 1956).
Watertight enclosure from which water is pumped to expose the bed of a body of water in order to permit the construction of a pier or other hydraulic work. Cofferdams are made by driving metal sheetpiling (a series of thin, interlocking panels) into the bed to form a watertight fence. Roman engineers used cofferdams to found the piers of their arch bridges and aqueducts. Seealso caisson.
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The cofferdam is also used on occasion in the shipbuilding and ship repair industry, when it is not practical to put a ship in drydock for repair or alteration. An example of such an application is certain ship lengthening operations. In some cases a ship is actually cut in two while still in the water, and a new section of ship is floated in to lengthen the ship. Torch cutting of the hull is done inside a cofferdam attached directly to the hull of the ship, and is then detached before the hull sections are floated apart. The cofferdam is later replaced while the hull sections are welded together again. As expensive as this may be to accomplish, use of a drydock may be even more expensive. See also caisson.
The division between the tanks and the hull of a double-hulled vessel is not normally called a cofferdam, although it carries this function.