Cofferdam
A Cofferdam is a narrow, normally empty and sealed space between two adjacent tanks or between a tank and another compartment, used to keep their contents apart. It is bounded by watertight or oiltight bulkheads and is kept dry and gas-free, so that a leak from one side cannot pass directly into the other.
Cofferdams are required where incompatible or hazardous liquids must be separated — for example between fuel oil and fresh water or cargo tanks, or around the machinery of a tanker — and they give a space to detect and contain leakage. They are entered only as enclosed spaces, under permit, after gas-freeing.
A cofferdam is a simple but vital safety separation: it stops fuel or cargo leaking into fresh water, machinery or accommodation, and gives an inspectable space where a leak shows up before it becomes a contamination or fire risk. It is a standard feature of tanker and machinery-space arrangements.
Why must a cofferdam be gas-freed before entry?
Because it is a sealed, unventilated space adjoining tanks, a cofferdam can accumulate a hazardous or oxygen-deficient atmosphere from leakage; it is treated as an enclosed space and only entered under permit after testing and gas-freeing.