Deep Tank is nothing but a tank with special design and purpose. When a ship makes a voyage in a light condition, it is usually desirable to carry a certain amount of water ballast. If the double bottom tanks alone were used for this purpose, the ship might be unduly stiff, so it has become the practice to arrange one of the lower holds so that it can be filled with water when necessary. This permits a large amount of ballast to be carried, without unduly lowering the centre of gravity of the ship. Such a hold is called Deep Tank.
In some cases, it has been found convenient to utilise deep tanks for the carriage of liquid cargoes, or for fuel oil bunkers.
It is generally constructed in Tanker and Cargo ships.
Its location is planned during constructional stage according to need of the ships. In tankers, it is located aft of the collision bulkhead.
When a deep tank is intended for its original purpose, it is basically the same as any other hold. It must, however, be somewhat modified to enable it to carry water ballast.
A washplate must be fitted at the centre-line, to reduce free surface effect. The hatchway must be specially constructed so as to prevent water from escaping from below.
Frames are to be 15% stronger than normal.
Bulkhead stiffners must be spaced not more than 600 mm apart and must be bracketed at the head and foot.
The deck plating which forms the tank top must be at least 1 mm thicker than that of the boundary bulkheads.
Beams may be normal but size of beam should not less than that of the bulkhead stiffners.
Deep Tanks for Bunker
If the deep tanks are dedicated for bunkering also, then the ship’s side and the boundary bulkheads may have additional stiffening in the form of deep, horizontal girders.
A middle line bulkhead must be fitted if the tank extends for the full breadth of the ship.
Quarterline girders must be fitted at the deckhead.
Deep Tanks for Oil as Cargo
Deep tanks may be used for the carriage of oil cargo, provided that its flash point is not less than 60 degree C. In this case, the tank may be constructed in the same way as one built for oil fuel bunkers except that a centre-line bulkhead need not be fitted unless the tank is liable only to be partly filled.
Cofferdams: It’s a narrow void space between two bulkheads or deep tank or floors. Deep tanks are separated by cofferdams. Fuel oil, Cargo oil and water may not be carried in adjacent tanks, unless these are separated from each other by a cofferdam.
Test: Deep tanks are tested by filling them with water to the maximum head which can come on them in practice, provided that this is not less than 2.44 metres above the tank top.
Deep tanks must have sounding pipe, vent pipe & pumping system.
Deep Tank for ballast water and oil i.e. multi-purpose tank should have hold bilge system.
This tank is also used for storing additional fuel for a long voyage and safe side of fuel consumption.
This is generally smaller than cargo hold but much stronger construction.