Dry dock

A dry dock is a specialized facility used for the construction, maintenance, and repair of watercraft.

Because ships spend much of their time submerged, their hulls are subject to constant wear. They can accumulate rust or become covered in marine life like barnacles. Dry docks provide a way to remove a vessel from the water entirely. This allows engineers to inspect and service the parts of the ship that are usually hidden beneath the waves.

One of the most common methods is the graving dock, which is a traditional basin made of concrete or earthen berms.

To use this dock, workers open large gates to allow a vessel to float inside the basin. Once the ship is positioned, the gates are closed tightly. Large pumps then remove the water from the basin. As the water level drops, the ship settles onto specific support blocks on the floor. These blocks, known as keel and bilge blocks, must be placed according to a precise docking plan. It is vital that these blocks align with the ship's structural members to prevent damage from the vessel's immense weight.

A floating dry dock operates through a different mechanical process using buoyancy.

This structure is a U-shaped pontoon containing floodable buoyancy chambers. To receive a ship, workers open valves to fill these chambers with water. This causes the dock to sink lower until its deck is submerged. The ship then moves into the center of the U-shape. When the water is pumped out of the chambers, the dock rises and lifts the ship out of the sea. Large floating docks often consist of multiple rectangular sections that can be combined to accommodate various ship lengths.

There are several other ways to move ships for repair. Marine railways can handle larger ships weighing up to 3,000 tons. Shiplifts are used for both new construction and repairs for vessels between 800 and 25,000 tons. Smaller boats might use a slipway or a patent slip. These diverse systems ensure that everything from a small boat to a massive warship can be serviced effectively.

The history of dry docking shows how maritime technology has evolved over centuries. In the 10th century, China was already utilizing dry docks.

Before modern docks existed in Europe, sailors practiced careening. This involved floating a ship over a hard sand beach at high tide and letting it rest on its side as the tide receded. The oldest surviving purpose-built dry dock in Europe was commissioned by Henry VII of England in 1495 at HMNB Portsmouth. This early dock was a timber-lined excavation. Workers had to dig away a bank of rock and clay by hand to let ships in, a task that typically took 29 days of constant work.

Modern dry docks have reached incredible scales. The largest roofed dry dock is located at the Meyer Werft Shipyard in Germany. It measures 504 meters long, 125 meters wide, and stands 75 meters tall. In the United States, Dry Dock 12 at Newport News Shipbuilding is the largest. The largest floating dock in North America is named The Vigorous, located in Portland, Oregon. Some docks are even built with roofs to protect vessels from spy satellites or to allow work to continue during bad weather. During World War II, German engineers used fortified dry docks to protect submarines from air raids.

Dry docks are also essential for massive civil engineering projects. They are used to build large components for bridges, dams, and tunnels. For example, the Oosterscheldekering dam in the Netherlands used a dry dock to construct concrete pillars. Each of these pillars weighed 18,000 tonnes. These pillars were built in the dock and then towed to their final location on the seabed. Similarly, dry docks were used to prefabricate elements for the Silver Line immersed tube tunnel in Boston. This shows that the technology used to fix ships is also vital for building the infrastructure of our world.