Drydock

Drydock

[drahy-dok]

A drydock 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. Drydocks are used for the construction, maintenance, and repair of ships, boats, and other watercraft.

History

Ptolemaic Egypt

According to the ancient Greek author Athenaeus of Naucratis, the drydock was invented in Ptolemaic Egypt, some time after the death of Ptolemy IV Philopator (reigned 221-204 BC):
But after that (the reign of Ptolemy IV Philopator) a Phoenician devised a new method of launching it (a ship), having dug a trench under it, equal to the ship itself in length, which he dug close to the harbour. And in the trench he built props of solid stone five cubits deep, and across them he laid beams crosswise, running the whole width of the trench, at four cubits' distance from one another; and then making a channel from the sea he filled all the space which he had excavated with water, out of which he easily brought the ship by the aid of whatever men happened to be at hand; then closing the entrance which had been originally made, he drained the water off again by means of engines (organois); and when this had been done the vessel rested securely on the before-mentioned cross-beams.

Since Athenaeus recorded the event 400 years later (around 200 AD), there is sufficient reason to believe that drydocks had been known throughout classical antiquity.

China

Drydocks appeared in China by 1070 A.D. In 1088, Song Dynasty scientist and statesman Shen Kuo (10311095) wrote in his Dream Pool Essays:

At the beginning of the dynasty (c. +965) the two Che provinces (now Chekiang and southern Chiangsu) presented (to the throne) two dragon ships each more than 200 ft. in length. The upper works included several decks with palatial cabins and saloons, containing thrones and couches all ready for imperial tours of inspection. After many years, their hulls decayed and needed repairs, but the work was impossible as long as they were afloat. So in the Hsi-Ning reign period (+1068 to +1077) a palace official Huang Huai-Hsin suggested a plan. A large basin was excavated at the north end of the Chin-ming Lake capable of containing the dragon ships, and in it heavy crosswise beams were laid down upon a foundation of pillars. Then (a breach was made) so that the basin quickly filled with water, after which the ships were towed in above the beams. The (breach now being closed) the water was pumped out by wheels so that the ships rested quite in the air. When the repairs were complete, the water was let in again, so that the ships were afloat once more (and could leave the dock). Finally the beams and pillars were taken away, and the whole basin covered over with a great roof so as to form a hangar in which the ships could be protected from the elements and avoid the damage caused by undue exposure. (Wade-Giles spelling.)

Renaissance Europe

In Europe, the first drydock, and the oldest surviving drydock, was commissioned by Henry VII of England at HMNB Portsmouth in 1495 (see The Tudors and the Royal Navy).

Possibly the earliest description of a floating dock comes from a small Italian book printed in Venice in 1560, called Descrittione dell'artifitiosa machina. In the booklet, an unknown author asks for the privilege of using a new method for the salvaging of a grounded ship and then proceeds to describe and illustrate his approach. The included woodcut shows a ship flanked by two large floating trestles, forming a roof above the vessel. The ship is pulled in an upright position by a number of ropes hanging down from the roof.

Modern times

Harland and Wolff Heavy Industries in Belfast, is currently the largest in the world and is close to the slipway on which the ill-fated was constructed. The massive cranes are named after the Biblical figures Samson and Goliath. Goliath stands 96m tall, while Samson is taller at 106m.

Northrop Grumman Newport News Shipbuilding's Dry Dock 12 is the largest drydock in the USA. The Saint-Nazaire's Chantiers de l'Atlantique owns one of the biggest in the world : 3937 feet x 196.850 feet.

Types

Graving

The classic form of drydock, properly known as graving 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. A distinction needs to be made between drydocks that are intended for ship construction sites, and those that are designed for the maintenance and repair of ships - especially for the repair of battle damage. More routine use of drydocks is for the cleaning (removal of barnacles and rust) and re-painting of ship's hulls.

Some fine-tuning of the ship's position can be done by divers while there is still some water left to maneuver 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 anti-submarine warfare 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 re-enter the dry dock and the ship is carefully refloated.

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.

Drydocks used for building Navy vessels may occasionally be built with a roof. This is done to prevent spy satellites from taking pictures of the drydock and any ships or submarines that may be in it. During World War II covered drydocks 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 drydocks are usually used only when servicing or repairing a fleet ballistic missile submarine. Another advantage of covered drydocks is that one can work independently of the weather. This can save time in bad weather.

Floating

A floating drydock is a type of pontoon for dry docking ships, possessing floodable buoyancy chambers and a "U" shaped cross-section. The walls are used to give the drydock stability when the floor is below the water level. When valves are opened the chambers are filled 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 drydock rises and the deck is cleared of water, allowing work to proceed on the ship's hull.

Shipyards operate floating drydocks, among other means of hauling or docking vessels. The advantage of floating drydocks is that they can be moved all over the world and also can be sold second-hand. During World War II, the U.S. Navy used such (floating) drydocks 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 uses for ships

Apart from graving docks and floating drydocks, ships can also be drydocked and launched by:

  • Mobile boatlift (also called Travelift, for vessels up to 1000 metric tons)
  • Slipway, patent slip — For repair of smaller boats and the newbuilding launch of larger vessels
  • Marine railway — For repair of larger ships up to about 3000 tons ship weight
  • Shiplift — For repair as well as for newbuilding. From 800 to 25000 ton shipweight

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 65 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

References

External links

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