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A double hull is a ship design where the ship features two complete hulls with a space between them, providing some protection in the event of leaks and minor accidents. In this design, a complete 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 are increased concerns about safety, and tankers commonly adopt the design to reduce environmental and financial risks.
New ships can be engineered with this design and built with a double hull from the ground up. One concern is that the center of gravity tends to be higher, which will make the ship less stable. Engineers may 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 drydock. This is a major structural change and requires careful engineering and substantial time in drydock to complete the work.
Ships can store ballast between the two hulls or may use the space for fuel storage. This design does not make a ship invulnerable. Serious collisions, groundings, and other major marine accidents can breach the double hull. This will result in flooding and a release of the contents of the ship. The design can address minor leaks and small accidents and may keep a ship afloat long enough for an evacuation and measures to offload some of the contents to limit losses.
In addition to making ships less stable, the double hull is also subject to more corrosion than traditional designs. Anti-corrosion measures must be installed, and personnel need to monitor the condition of the hull to identify any problems as early as possible. If significant damage starts to occur, the ship may need repairs in port and could require drydocking, depending on the nature of the damage. This can add to maintenance costs, although having a double hull can extend the life of the ship, so the trade-off may be worth it to the owner.
Laws concerning 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 usually alert the shipping industry to give owners and carriers time to start retrofitting or selling ships, if necessary, to prepare for the changes in the law.