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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, into 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. It is extremely important that supporting blocks conform to the structural members 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, 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. 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 1.2 million 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. Other ways of drydocking ships Apart from graving docks and floating drydocks, ships can also be drydocked and launched by: Uses other than for ships 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. See also | ||||||||||
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