Explain Dry Docking

A dry dock lifts a boat or ship out of the water.

Whether a submersible ship, with an official number and sometimes a name, or a trench, sometimes with permanent sides and bottom, a dry dock has a single purpose: support a ship, out of water, while construction, repairs or modifications are made. Dry docks have served shipbuilders in these capacities for two thousand years.

History

The word dock carries a slightly different definition for the seaman than it does for the landsman. For seamen, a dock is the water between piers or alongside a pier or wharf for where ships load, unload or undergo repairs. The dry dock is filled with water, then the water is drained after receiving the ship for repairs or maintenance, making the dock dry.

Over 22 centuries, dry docks evolved. The original dry docks, described by the Greek commentator Athenaeus, were built in Egypt, dry trenches where ships were built or repaired. After the work was finished, the trench was opened to the sea and the resulting flood floated the ships. (Reference 1)

In Europe, a dry dock commissioned in 1495 by England's King Henry VII is serving as the home of HMS "Victory" at Portsmouth Dockyard. "Victory" was the flagship of Vice Admiral Horatio Lord Nelson in the Royal Navy's triumph over the combined French and Spanish fleets off Cape Trafalgar, Spain, in 1805, and is the oldest commissioned warship in the world. (REFERENCe: Personal Experience; photos in Reference 2 are for your editorial edification)

Types

There are two types of dry dock. The first is the graving dock, much like the dry dock used by the early Egyptians, and is a purpose-built trench which is pumped out after the ship is positioned inside the dock facility.

Floating dry docks are submersible ships. They are issued official numbers like any other ship, and many are named. The largest floating dry dock in the world is at the Shanghai International Shipping Center in the city of Yangzhou, China. At 410 meters, or 1,345 feet, or slightly more than a quarter mile in length, the dry dock Mt. Emei Shan is large enough to submerge, capture and lift Very Large Crude Carriers, the largest ships in service in the world.

Filling the Dock

Whether a graving dock or a floating dry dock, the bracing on the floor of the dry dock is first adjusted to hold the keel--the backbone of the ship, which carries its weight--then the dry dock is filled with water. This involves opening valves to allow the graving dock to fill with water behind its watertight floodgates

The floating dry dock fills its "flood compartments" with water, causing the floating dry dock to sink into the water in a controlled manner to a predetermined depth based on the draft of the vessel--how deeply the ship sits in the water. For small vessels, this may only be a matter of 15 feet or 20 feet; for the largest vessels, such as supertankers, it may be up to 80 feet or even 100 feet. Even at those depths, the control cabin and part of the structure of the dry dock will still be above water.

The graving dock opens its floodgates to allow the vessel entry.

Securing the Vessel

Whether a graving dock or floating dry dock, the vessel enters slowly, aligning itself as near the center line of the docking chamber as possible to settle onto the braces beneath the keel when the floating dock rises or the graving dock is drained. Wire mooring lines are passed to the vessel and tightened, to keep the ship from tipping on its keel.

Drying the Dock

After the vessel is settled, the graving dock closes its floodgates and pumps the water out of the dock chamber, creating a dry working space around and beneath the ship.

On the floating dry dock, the dock pumps are started, clearing the water from flood chambers, causing the floating dock to rise and lift the ship from the water. Departure from the dry dock simply reverses the procedure.

References

• "The Deipnosophists, Or, Banquet of the Learned, of Athenæus"; Athenæus, Charles Duke Yonge, 1854
• Johns Military History Tours of Britain: Portsmouth Dockyard, Dock Number 2
• CIC-China Shipping International Shipyard Co., Ltd.: Facilities and Services
• "True North"; VLCC Navigates Yangtze River; Wallem Group; Spring 2009