DRYDOCKING 101

TYPES OF DRY DOCKS AND DRYDOCKING METHODS

There are four major types of dry docks. Each major type of dry dock has several types or versions. Additionally, lifts, cranes, or air bags are other methods of drydocking vessels.

Basin or Graving Dry Docks

Basin or graving docks are a permanent hole in the ground which are filled and drained with water to dry dock ships. Due to their construction they do not pose the same strength and stability problems faced in other types of dry docks. It is extremely important that the Dock Master investigate the point at which a vessel with substantial trim will become unstable in a basin, since it is not possible to accommodate trim except to some degree by shoring or blocking on an incline.

Railway Dry Docks

The railway dock has a built-up cradle to bring the dock and vessel up to yard grade, and a decked-over cradle to be used as a working platform, and docking platforms along the sides to facilitate warping in vessels, handling lines, and hauling sliding bilge blocks. A railway Dry Dock often has a curved track so that it can accept a vessel floating with a trim but dock it at the top of the track with its keel parallel to the ground.

The principle of hauling the vessel up an inclined track results in the need for pull equal to only about 10% of the live load. It also, more importantly, means that the cradle is continuously and safely supported during the whole docking operation, and any failure of equipment is unlikely to cause severe damage to the ship.

Vertical Lift Dry Docks

The vertical lift dry dock is the extreme case of making a railway’s incline steeper and steeper to the limit. All of its support must be transmitted through the cables, chains, or hydraulic components to the edges of the lift platform carrying the ship. Also, it occupies no space out into the water as does a railway.

A lift consists of a series of hoists or jacks on each side of the platform, with the lifting machinery mounted on a pier structure on each side. The motion of the individual hoists must be synchronized. Because of load concentrations of the vessels, certain hosts carry more than others, to the extent that the lift capacity has to be greater than what the heaviest vessel might otherwise require by a ratio as high as 1½ to 1.

The vertical lift solution is economical for existing deep-water berths where dredging and fill requirements are minimal. The lift is not able to take a trim, a disadvantage when some vessels need to be dry docked.

Floating Dry Docks

The floating dry dock’s major difference from the other types of dry docks is that the forces between the blocks and the ship are a function of buoyancy as well as load. Since this kind of support requires the ship to redistribute its weight the same way it does when afloat, the ship is supported in close to its natural condition.

Floating dry docks consist of a buoyancy chamber, or a series of integrated buoyancy chambers, that can be flooded in a highly controlled fashion, thereby submerging the dock to a preselected

draft over a series of blocks which form a cradle of blocks within the dock basin. The blocks form a cradle that matches the shape of the underwater body of the ship to be drydocked. When a ship is carefully positioned over the blocks, the buoyancy chamber(s) is dewatered, restoring buoyancy and thereby lifting the ship clear of the surrounding water.

A particular advantage of floating dry docks as compared to graving docks or marine railways is the ability to adjust, by ballasting, the floating dock to match the trim or the list of the ship to be docked.

The mobility of floating dry docks is another plus-factor as an investment, since they can readily be moved – making for flexibility in developing yards and for salability.     (…more)

Lifts and Cranes