Cofferdam

A cofferdam is a narrow empty space deliberately built into a ship's structure to separate two adjacent compartments that must be kept apart. It is essentially a void (a sealed space containing neither cargo nor liquid in normal service) placed between the compartments as a buffer. Its purpose is to ensure that if one of the neighbouring spaces leaks, its contents cannot pass directly into the other, and to provide a gas-tight and safety barrier between substances or areas that should not mix or be in direct contact.

Cofferdams are used in several important places aboard ship. They are commonly fitted between fuel oil tanks and cargo tanks, or between different grades of liquid, so that a leak cannot contaminate the cargo or create a dangerous mixture. They separate the machinery spaces or accommodation from cargo tanks on tankers, keeping flammable cargo vapours away from ignition sources and living spaces. They isolate tanks carrying substances that could react dangerously together, and they protect drinking-water tanks from contamination by adjacent tanks. In each case the empty space provides a margin of safety, and because it can be entered and inspected, it also allows leaks from either side to be detected before they cause harm.

Being enclosed voids that may contain little oxygen or accumulated gas, cofferdams are treated as enclosed spaces, so entry into them for inspection or maintenance requires the full precautions (ventilation, gas testing, an entry permit and a standby person) that apply to any enclosed space. Cofferdams are part of the fundamental way ships are subdivided and arranged to contain leaks, prevent contamination, and keep hazardous substances separated, and they reflect the careful design that goes into making a ship safe. Their presence, though hidden in the structure, is an important element of a vessel's safety and pollution-prevention design.