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 work or modernization . An example of such an application is the lengthening of ships . 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 . The cutting of the hull is done inside a cofferdam attached directly to the hull of the ship ; the cofferdam 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 , the use of a drydock might be even more expensive . '' See also caisson . '' 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 , since methane hydrates froze in the upper levels
Deepwater Horizon oil spill
Deepwater Horizon oil spill
Cofferdam