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):

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 (1031–1095) wrote in his Dream Pool Essays:

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). This drydock currently holds the worlds oldest commissioned warship HMS Victory.

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, Northern Ireland, 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. The largest graving dock of the Mediterranean as of 2009 is at the Hellenic Shipyards S.A. (HSY S.A., Athens, Greece). The by far largest roofed dry dock is at the German Meyer Werft Shipyard in Papenburg, Germany, it is 504m long, 125m wide and stands 75m tall.

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 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 anti-submarine warfare warships have protruding sonar domes, requiring that the hull of the ship be supported several metres 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, fortified drydocks were used by the Germans to protect their submarines from Allied air raids (see Submarine pen); however, their effectiveness in that role diminished towards the end of the war as bombs became available that could penetrate them. Today, covered drydocks are usually used only when servicing or repairing a fleet ballistic missile submarine. Another advantage of covered drydocks is that work can take place 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.

A typical floating drydock involves multiple rectangular sections. These sections can be combined to handle ships of various lengths, and the sections themselves can come in different dimensions. Each section contains its own equipment for emptying the ballast and to provide the required services, and the addition of a bow section can facilitate the towing of the drydock once assembled. For smaller boats, one-piece floating drydocks can be constructed, potentially coming with their own bow and steering mechanism.
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.

The "Hughes Mining Barge", or HMB-1, is a covered, floating drydock that is also submersible to support the secret transfer of a mechanical lifting device underneath the Glomar Explorer ship, as well as the development of the Sea Shadow stealth ship.

File:DALPHIN I.jpg|The towboat, DOLPHIN I, in a floating drydock on the Mississippi River in Algiers, New Orleans, Louisiana.
File:Blohm+Voss Dock10 Hafen Hamburg 2.jpg|Blohm + Voss Dock 10, at the Port of Hamburg.
File:Floating dock 2008 G1.jpg|Floating drydock located in Sevastopol.
File:Floating drydock fo a small boat.jpg|A floating drydock (or boat lift) in a private marina, used to keep small boats out of the water while not in use.

Alternative drydock systems

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.

They may also be used for the prefabrication of the elements of an immersed tube tunnel, before they are floated into position.

See also

• Space dock

• Semi-submersible

• St. Nazaire Raid — an attack on a drydock during WW2.