This post is to highlight about Drydocks, more specifically about Floating Drydock.
To ensure seaworthiness of the ships, most ships are required to be drydocked twice every five years.Some ships may have drydocking postponed or the intermediate drydocking waived by performing an UnderWater Inspection in Lieu of Drydocking (UWILD).
Drydocking a ship is a complex task.The ship weighs a lot and the weight must be spread evenly to the dock.While a ship is floating the pressure holding the ship afloat is spread throughout the entire underwater hull.While in drydock the load is spread among wood blocks.There are also interferences such as transducers and irregularities such as flare at the bow.In order to drydock the ship the dockmaster must be aware of the characteristics of the ship’s bottom and it’s weight distribution.Therefore ships carry a docking plan which specifies where the blocks are to be placed, the angle of the side blocks, and the height of the side blocks.It also lists where transducers, sea chests, tailshafts, and other items are located.
1.Types of Drydocks:
- Graving Dock:A graving dock is a channel cut out of a basin or river in which a caisson is placed in front of the opening and water is pumped out. As the water is pumped out the ship is lowered on to the blocks. This type of dock can handle the largest of vessels.
- Marine Railway:This type of dock has blocks mounted on a skid and the skid is lowered into the water on rails. The ship is positioned and chains or line haul up the skid with the vessel on the blocks. In the past horses were used to power these railways. These railways typically do not have as much lifting power as a graving dock or a floating drydock.
- Floating Drydock:This type of dock is U-Shaped and can be sunk to allow the vessel to enter and be positioned.When the ship is in position the water in the ballast tanks of the dock is pumped out and the dock rises to meet the ship.Then the ship is lifted out of the water by the upward motion of the dock.This type of dock typically can lift heavier ships than the railway but not as heavy as a graving dock.The benefits of this dock are that it can be towed anywhere in the world and that it can be ballasted to lift a damaged ship with an excessive list or trim.
- Travel Lift:This system is for smaller vessels and tugboats.The travel lift incorporates a series of straps which are passed under the boat to lift it out of the water.Then the boat and travel lift can be driven to a place on land where the boat can be set on blocks.This system benefits in that no vessels will tie up your hauling machinery.Work does not have to be performed while the boat is in the lift as with the other docking methods.
Now let us see some of the Floating Drydocks.
2.Sudoimport 80 000 tonnes Floating Drydock:
Basic design,Engineering and Built by: Gotaverken Arendal AB, in 1979.
The 80 000 Floating Drydock was primarily designed and built to handle the large nuclear powered icebreakers operating in the artic region of the former Soviet Union.These vessels require services normally not encountered in the ordinary ship repair and maintenance industry and mainly related to the nuclear power plant. Such services calls for systems and capacities far beyond what is provided in ordinary floating drydocks.The floating drydock is of self-docking type and is totally self supported which includes power generation, accommodation and workshops.All relevant and applicable Russian class and authorities rules and regulations were complied with.
- Length over all:330 m.
- Clear width:67,5 m.
- Overall width:88 m.
- Overall height:30 m.
- Immersion depth abv. keel blocks:15 m.
Floating Drydock Lifting Capacity:15 000 Tonnes
Operator: Dubai Drydocks / United Arab Emirates
Basic design and Engineering: GVA Consultants AB
Builder: Dubai Drydocks
Contract Award: September 1992
Commence Operation: December 1994
The floating drydock is designed according to Lloyd’s Register rules for Floating Docks and characterized as a Class “A” Floating Dock.For a max. ship weighing 15 000 tonnes, the pumping time is 180 minutes from the moment the ship touches the heel blocks until the pontoon deck at the outer side of the wing walls emerges from the water.Over the whole length of the dock structural strength will cope a load intensity of 115 tonnes/m on the keel blocks in the center line.Two 12.5 tonnes lifting dock cranes at 21.75 m radius.
- Length overall:205.5 m
- Length over bottom caisson:190.3 m
- Width between inner walls:36.0 m
- Width outside walls:43.0 m
- Clear inside width for a docked ship between dock runways:34.8 m
- Height of bottom caisson:3.5 m
- Height to upper deck above BL:13.1 m
- Height of keel blocks:1.9 m
- Immersion depth above keel blocks, max 6.6 m.
- Two 3.66 m (12 ft) transverse pipe tunnels are arranged, one forward and one aft.
4.Hongkong United Dockyards:
Floating Drydock Lifting Capacity: 40 000 Tonnes
Operator: Hongkong United Dockyards Ltd., Hongkong
Basic design: GVA Consultants AB
Engineering and Built by: Far East Levingston Shipbuilding Ltd., Singapore
Contract Award: July 1993
Commence Operation: May 1995
The 40 000 tonnes lifting capacity dock is designed to comply with Rules and Regulations of Lloyd’s Register of Shipping, as a “+A Floating Dock” classed dock.The main pumping plant is capable of lifting a vessel of 9.5 m max draught in even keel condition and with a displacement of 40 000 tonnes in 130 minutes (from time when ship touches keel blocks to freeboard of 410 mm at centre line caisson).The dock is designed to withstand loads when being sunk to a depth of 10 m above top of keel blocks.The transverse strength over the entire length of the dock will withstand a load intensity of 280 tonnes per m at the centre line.Two traveling cranes, one capable of lifting 70 tons at 27 m and one 30 tons at 27 m radius.
- Length overall:290 .0 m
- Length over caisson:270.0 m
- Width between inner side walls:46.0 m
- Width outside walls, at safety deck:56.0 m
- Height of caisson in CL:5.0 m
- Depth of dock:18.5 m
- Height of keel blocks:1.4 m
- Draught of water over keel blocks:10.0 m
5. Some more photographs of Floating Drydock.
Below see the Floating Drydock in Action:
To download the video click here.
Video Courtesy: Northrop Grumman
In case of any doubts, feel free to to contact OldSailor.
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