Double hull
From Wikipedia, the free encyclopedia
A double hull is a ship hull design and construction method where the bottom and sides of the ship have two complete layers of watertight hull surface: one outer layer forming the normal hull of the ship, and a second inner hull which is somewhat further into the ship, perhaps a few feet, which forms a redundant barrier to seawater in case the outer hull is damaged and leaks.
The space in between the two hull layers is often used as storage tanks for fuel or ballast water.
Double hulls are a more extensive safety measure than double bottoms, which have two hull layers only in the bottom of the ship and not the sides.
Double hulls are significantly safer than single hulls. In case of grounding or other underwater damage, most of the time the damage is limited to flooding the bottom compartment, and the main occupied areas of the ship remain intact. In case of collision with another ship, most of the time the damage is limited to flooding the side compartment, and the main occupied compartments also remain intact.
For these reasons, double hulls or double bottoms have been required in all passenger ships for decades as part of the Safety Of Life At Sea or SOLAS Convention. Their ability to prevent or reduce oil spills led to their being standardized for other types of ships including Oil tankers by the International Convention for the Prevention of Pollution from Ships or MARPOL Convention.
A double hull also conveniently forms a stiff and strong girder or beam structure with the two hull plating layers as upper and lower plates for a composite beam. This greatly strengthens the hull in secondary hull bending and strength, and to some degree in primary hull bending and strength.
Opposing viewpoints have argued that the double hull is actually more dangerous than a single hull. Most of the collisions that the double hull prevents are so minor that they would typically spill little to no oil on a single hull tanker. In addition, the double hull increases the risk of corrosion on the double hull, causing oil to spill into the ballast; not only defeating the purpose of the double hull, but in addition making it increasingly dangerous for the crew on a ship who must inspect those areas.
In addition to this, there is a much larger potential for explosive accidents hapenning due to the increased element of oil mixing with air during a high-energy grounding, as was the case with the Aegean Sea (oil spill).
Possibly the most disturbing fact about the double hull is that it does not protect against major, high-energy collisions or groundings which is what causes the majority oil pollution, despite the fact this is why the double hull was put into United States Legislation.
After the Exxon Valdez oil spill disaster, when that ship grounded on Bligh Reef outside the port of Valdez in Alaska, the US Government required all new oil tankers built for use between US ports to be equipped with a full double hull. This was somewhat ironic, as the damage to the Exxon Valdez was so extensive that it would have penetrated the now-required double hull structure, and was limited to the ship's existing and properly designed double bottom structure. The new regulations, while enhancing general safety, would not have prevented that spill. [citation needed]
One of the downsides of a double hull is that the stability of a ship can be less then that of a single hull. Because the double hull raise the centre of gravity, the metacentric height will be reduced. Futhermore, a double-hulled tanker doesn't need longitudinal bulkheads for longitudinal strength, as the inner hull already provides this. This results in much wider tanks, significantly increasing the free surface effect.[1]
Many in the Shipping Industry consider the Mid-Deck Tanker to be a much more efficent and safer design.[citation needed]
In case of submarines, the double hull structure is significantly different, consisting of an outer light hull and inner pressure hull.
