Cofferdam (construction)

A cofferdam in repair work is a temporary watertight enclosure that walls off a damaged or working section of the hull so the space inside can be dewatered and worked dry, often without dry-docking the whole ship. It can be a steel box welded over a shell opening or a sealed compartment created by closing off adjacent tanks. The term also names the permanent narrow void built between two tank boundaries to separate incompatible fluids, such as cargo oil and machinery spaces, required by SOLAS and MARPOL for some arrangements. In repairs the temporary cofferdam lets divers and welders work afloat at the waterline.