A double hull ship design has two hulls with a gap between them, providing protection in the event of leaks and minor accidents. It is standard for passenger ships and oil tankers to reduce environmental and financial risks. The design can be retrofitted or built from scratch, but it makes ships less stable and subject to more corrosion. Laws regarding double hull construction vary by nation.
A double hull is a ship design in which the ship features two full hulls with a gap between them, providing some protection in the event of leaks and minor accidents. In this design, a full outer hull covers the bottom and sides of the ship, protecting an inner hull. This approach to shipbuilding is standard for passenger ships, where there is a higher concern for safety, and oil tankers often adopt the design to reduce environmental and financial risks.
New ships can be designed in this layout and built with a double hull from scratch. One concern is that the center of gravity tends to be higher, which will make the ship less stable. Engineers can compensate for this with other aspects of the design to improve stability. It is also possible to retrofit an existing ship to add a double hull in dry dock. This is a major structural change and requires careful engineering and considerable dry dock time to complete the work.
Ships can store ballast between the two hulls or they can use the space to store fuel. This design does not make a ship invulnerable. Serious collisions, grounding and other major marine accidents can break the double hull. This will cause flooding and the release of the ship’s contents. The design can address minor leaks and small accidents and can keep a ship afloat long enough for an evacuation and measures to dump some of the contents to limit losses.
In addition to making ships less stable, double hulls are also subject to more corrosion than traditional designs. Anti-corrosion measures must be installed, and personnel must monitor the condition of the hull to identify any problems as soon as possible. If significant damage begins to occur, the ship may need port repairs and may require dry docking, depending on the nature of the damage. This can increase maintenance costs, although having a double hull can extend the life of the boat, so the tradeoff may be worth it for the owner.
Laws regarding double hull construction vary. Some nations require it for passenger ships, but not others, while others may have requirements for double hulls on oil tankers and ships carrying hazardous materials. Nations preparing new legislation typically alert the shipping industry to give owners and shippers time to start upgrading or selling ships, if necessary, to prepare for changes in the law.
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