U.S. Navy submarine USS Greeneville
in dry dock following collision with a fishing boat.
The dry dock is a place where ships come to be serviced or repainted. The final stages of production in most ships is also done in the dry dock, also called a "graving dock", particularly when used for the complete construction of a ship.
A dry dock will usually be an area below sea level, contiguous with the sea. Support can be set up capable of holding the ship upright when there is no water. The ship enters the dry dock, usually with the help of tugboats if the ship is large.
Large hydraulic gates close, separating the dry dock, and the ship, from the sea. After the ship is in place, powerful pumps start to remove the water from the dry dock, pumping it back into the harbour. This process can take a long time.
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 will have 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.
Dry docks are usually excavated from dry land on the shore. The largest dry dock in the world is in Belfast, Northern Ireland. The oldest dry dock is in Portsmouth Naval Base (in England) and dates back to 1495, Number 2 dock housing HMS Victory.
A floating drydock is a specialised ship used in salvage which is constructed with floodable buoyancy chambers in a "U" shaped cross section that can support the hull of a ship needing repair. Valves can be opened to allow the chambers to fill with water. When this happens the dry dock floats lower in the water. A ship can then be moved into position inside the "U" of the dry dock. The water is pumped out of the chambers and the dry dock rises in the water. It can then be towed, carrying the ship, to a repair facility.
Floating drydocks were used extensively during the Second World War to establish maintenance facilities in remote locations. Several of these docks could be joined to accommodate a long ship.
Semi-submersible salvage ship
MV Blue Marlin
carrying USS Cole
A semi-submersible salvage ship has a long and low well deck between a forward pilot house and an after machinery space. In superficial appearance this is similar to a dry bulk carrier or some forms of oil tanker. It is designed to be flooded to a point where a smaller ship may be brought into the well deck area, aligned with the salvage ship if possible, otherwise positioned on a diagonal. The salvage ship is then pumped dry and is then used to transport the damaged ship to a shipyard for repair. Such a ship, the MV Blue Marlin was used to transport the U.S. guided missile destroyer USS Cole (DDG 67) from Aden, Yemen to Pascagoula, Mississippi, USA, to after it was damaged in a bombing attack October 12, 2000.