A cofferdam is a void space typically found between two watertight bulkheads or decks. This space is employed to keep oil, fuel, water, and any other liquids from leaking from one tank or compartment to another. Cofferdams may also be used to locate permanent ballast, to keep the vessel on an even keel. Newly designed vessels are required to be double hulled to prevent the leakage of product in the event of an accident - this is always referred to as a double hull and not as a cofferdam even though it may serve the same purpose.
Coffer dams are typically a single framespace long, extending transversely the width of the tanks requiring separation. The boundary bulkheads may be watertight, or oil tight. The space is generally enclosed with manholes for access and inspection.