Dry dock is a structured area wherein construction, repairs and maintenance of merchant vessels and boats are carried out. The unique construction allows the water to be filled up in that area, also known as lock, so that vessels can be maneuvered in and out of the area. Once the vessel enters the dry dock, the gates are closed and the seawater is drained out so that hull and other areas of the ship which have been exposed to sea water for a long time are available for carrying out maintenance and repair works.
As per SOLAS requirement all Merchant vessels requires a complete inspection of hull in a dry dock twice within 5 year period and the intermediate survey must not be more than 36 months. This includes maintenance of hull, propeller, rudder etc. and other parts which are immersed in water and are normally inaccessible by ship staff at normal sailing period.
For Passenger vessel the inspection of the ship’s bottom is to be done annually. Two of such inspection in a period of five year must be carried in dry dock and the maximum interval between these inspections is 3 years.
Types of Dry Dock Procedures
There are mainly two types of dry dock procedure:
1) Graving dock:
This type is normally constructed on a land near the coastal water with a rectangular solid concrete construction with blocks, walls and gates. Vessel is shifted inside the dock and rested in the blocks. After the ship is in required position, gate is closed and water is removed.
2) Floating dock:
A floating dock is a “U” structure used in salvage, to carry ship, which has met with an accident, from mid sea and which is damaged and unable to sail further to go to a coastal dock. Several “U” type floating docks can be joined to carry a large vessel. A valve is provided which can be opened to fill up the chambers with water and which will make the dock immersed in water so that the ship can sail out. The water is pumped out of the chamber which will allow the dock to rise, exposing the underwater area of the ship for maintenance or carrying the ship to repair facility.
Three Requirements for Dry Dock
Stability is the most important requirement for getting a ship safely into a dry dock. The three important parameters which must be ensured before entering the dry dock are:
1) Adequate Initial G.M:
When the ship touches the blocks, there is a reaction at the point of contact which raises the centre of gravity “G” and reduces the metacentric height “G.M” so that adequate initial metacentric height is required to compensate the same.
2) Vessel to be Upright:
While entering the dock the vessel needs to be upright which means there should be no port or starboard list when the ship touches the blocks, the point of contact will be outside the centre line of vessel, which may force the vessel to tip over.
3) Small or Moderate Trim Aft:
The slight trim allows the accenting of stern and bow in tandem rather than simultaneously as it will reduce the load and pressure on hull and the keel of vessel.
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