Dry dock
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U.S. Navy submarine USS Greeneville in dry dock following collision with a fishing boat. |
A
dry dock is a narrow basin or vessel that can be flooded to allow a load to be floated in, then drained to allow that load to come to rest on a dry platform.
Graving dry dock
A graving dry dock is a narrow basin, usually made of earthen berms and concrete, closed by gates or by a caisson, in which a vessel may be floated and the water pumped out, leaving the vessel supported on blocks. The keel blocks as well as the bilge block are placed on the floor of the dock in accordance with the "docking plan" of the ship.
Some fine-tuning of the ship's position is done by
scuba divers while there is still some water left to manoeuvre it about. Each ship has a diagram of the shape of its bottom. It is extremely important that supporting blocks conform to this shape so that the ship is not damaged when its weight is supported by the blocks. Some
ASW warships have protruding
sonar domes, requiring that the hull of the ship be supported several meters from the bottom of the drydock.
Once the remainder of the water is pumped out, people can walk around in the dry dock, and the ship can be freely inspected or serviced. When work on the ship is finished, water is allowed to reenter the dry dock and the ship is carefully refloated.
A graving dock is a multi-purpose dock. It can be used for new building, for repair, and for maintenance. The word "graving" was more often used to denote the cleaning of a ship's bottom.
Modern graving docks are box-shaped, to accommodate the newer, boxier ship designs, whereas old drydocks are often shaped like the ships that are planned to be docked there. This shaping was advantageous because such a dock was easier to build, it was easier to side (support) the ships, and less water had to be pumped away.
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U.S. Navy ballistic missile submarine USS Michigan inside a flooded drydock |
Dry docks that are used for building Navy vessels may occasionally be built with a roof. This is done to prevent
spy satellites from taking pictures of the dry dock and any ships or submarines that may be in it. During
World War II covered dry docks were frequently used by submarine fleets to protect them from enemy air raids, however their effectiveness in that role diminished after the Second World War. Today covered dry docks are usually used only when servicing or repairing a fleet ballistic missile submarine. Another advantage of covered dry docks is that one can work independent of the weather. This can save considerable time.
The
Chinese invented dry docks no later than
1070.
The first dry dock in Europe, and the oldest surviving dry dock, was commissioned by
Henry VII of England at
HMNB Portsmouth in
1495 (see
The Tudors and the Royal Navy).
The largest dry dock in the world is in
Belfast,
Northern Ireland measuring 556m by 93m, with a capacity of 1.2million tonnes.
Floating dry dock
A floating dry dock is a
salvage platform which has floodable
buoyancy chambers in a "U" shaped cross-section that can support a ship. When
valves are opened to fill the chambers with water, the dry dock floats lower in the water, allowing a ship to be moved into position inside. When the water is pumped out of the chambers, the dry dock rises and the deck is cleared of water, allowing work to proceed on the ship's hull.
Floating dry docks are often operated by shipyards, but can also be moved around the world as needed — some split into sections for easier towing. During
World War II, the
U.S. Navy used such dry docks extensively to provide maintenance in remote locations. One of these, the 850-foot
AFDB-3, saw action in
Guam, was mothballed near
Norfolk, Virginia, and was eventually towed to
Portland, Maine, to become part of
Bath Iron Works' repair facilities.
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Photos of USS Samuel B. Roberts on blocks in AFDB-3 in 1988Other ways of drydocking ships
Apart from graving docks and floating drydocks, ships can also be drydocked and launched by:
* Mobile boatlifts (also called Travelifts, up to about 250 tons weight)
*
Slipways,
Patent slip For repair of smaller boats and the newbuilding launch of larger vessels
*
Marine Railways for repair of larger ships up to about 3000 tons ship weight
*
Shiplifts for repair as well as for newbuilding. From 800 to 25000 ton shipweight
Some drydocks are used during the construction of bridges, dams, and other large objects. For example, the drydock on the artificial island of
Neeltje-Jans was used for the construction of the
Oosterscheldekering, a large dam in the
Netherlands that consists of 62 concrete pillars weighing 18,000 tonnes each. The pillars were constructed in a drydock and towed to their final place on the seabed.
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Floating drydock in Singapore |
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Dry Docks in Science Fiction*
Semi-submersible*
St. Nazaire Raid - an attack on a dry dock during
WW2.
