Double Bottom Tank
All ocean going ships with the expection of tankers, and most coastal vessels are fitted with double bottom which extends from the fore peak bulkhead almost to the after peak bulkhead. The double bottom consists of the outer shell and an inner shell skin or tank top between 1 m and 1.5 m above the keel. This provides a form of protection in the event of damage to the bottom shell. The tank top, being continuous, increases the longitudinal strength and acts as a platform for cargo and machinery. The double bottom space contain a considerable amount of structure and is therefore useless for cargo. It may however, be used for the carriage of oil fuel, fresh water and water ballast. It is sub-divided longitudinally and transversely into large tanks which allows different liquids to be carried and may be used to correct the heel of a ship or to change the trim. Access to these tanks is arranged in the form of manholes with water tight covers.
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| Double Bottom Tank Duct Keel |
Some ships are fitted with a duct keel which extends from within the engine room length to the forward hold. This arrangement allows pipes to be carried beneath the hold spaces and are thus protected against cargo damage. Access onto the duct is arranged from the engine room, allowing the pipes to be inspected and repaired at anytime. At the same time it is possible to carry oil and water pipes in the duct, preventing contamination which occur if the pipes passed trough tanks. Duct keel are particularly important in insulated ships, allowing access to the pipes without disturbing the insulation. Ducts are not required aft since the pipes may be carried through the shaft tunnel.
The duct keel is formed by two longitudinal griders up to 1.83 m apart. This distance must not be exceeded as the griders must be supported by the keel blocks when docking. The structure on each side of the griders is the normal double bottom arrangement. The keel and the tank top centre strake must be strenghthened either by supporting members in the duct or by increasing the thickness of the plate considerably.
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| Duct Keel |
