A ship operates using mainly four main kinds of liquids: fuel or diesel oil, lube oil, fresh water, and seawater. These liquids are stored at various places on the ship in their dedicated tanks. Apart from these, there are other liquids such as oily bilge and sludge are also generated on board from the working of machinery.

Thus, several numbers of tanks are provided onboard ships to store different kinds of fluid which are used or generated for normal operation of the marine engine and auxiliary machinery.

Since functions and properties of all liquids mentioned above are different, they are stored separately and care must be taken not to mix any two different fluids even when there is a leakage or failure in the boundaries of the tank separating them by using Cofferdam or void space.

It’s confusing when using the terminology such as Void and Cofferdam on ship as usual because of the function and definition is quite not clearly.

Cofferdam

To avoid the above-mentioned situation is a void or empty compartment is provided between the tanks to prevent two different liquids from mixing with each other. This space is known as a cofferdam. Space avoids intermixing of two different liquid when there is a leak from the boundary separating the two liquid.

• The cofferdam is provided with manholes for entry and inspection
• It is also provided with sounding pipe to check leakages 
• It is always maintained dry to detect an early leak.

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Where is Cofferdam Provided?

Cofferdam dam are there on all the ships. They are provided:

• Between fuel oil tanks, and lube oil and fresh water tanks in the engine room.
• All around main engine lube oil drain or sump tank to separate it from other double bottom tank
• Between other different grade liquid tanks like diesel oil and fuel oil service tanks.
• Cofferdam is also provided to give access to the holding down bolts for main engine
• Between cargo space and machinery space.
• Pump room also may be a part of cofferdam.

Cofferdam is a type of enclosed space and all enclosed space entry precautions must be taken before making an entry in the same.

How about Void space:

“Void space” is an enclosed space in the cargo area external to a cargo containment system, other than a hold space, ballast space, fuel oil tank, cargo pump or compressor room, or any space in normal use by personnel ~  void space will not be used in most case.

Openings to void spaces adjacent to cargo tanks should be so designed and fitted as to prevent the entry of water, sulphur or cargo vapor.

Hence, from above reference, the definition of these two space can be more easy to understand.