cofferdam -
A temporary watertight structure that is pumped dry to enclose an area underwater and allow construction work on a ship, bridge, or rig to be carried out; a caisson.
cofferdam -
An empty space that acts as a protective barrier between two floors or bulkheads on a ship.
Wikipedia
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.
