A cofferdam (also called coffer) is an enclosure within a water environment constructed to allow water to be pumped out to create a dry work environment. Commonly used for oil rig construction and repair, bridge and dam work, the cofferdam is usually a welded steel structure that is temporary and is typically dismantled after work is completed. Its components consist of sheet piles, wales, and cross braces.
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.
Cofferdams have also been used to recover ships sunk in shallow waters, such as the USS Maine, or Robert de LaSalle's La Belle.
A 100 ton open caisson that was lowered more than a mile to the sea floor in attempts to stop the flow of oil in the Deepwater Horizon oil spill has been called a cofferdam. It did not work as methane froze in the upper levels preventing the containment.
The division between the tanks and the hull of a double-hulled vessel is not normally called a cofferdam, although it serves this function.
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