DRY DOCKINGS OF SHIP: Short Question & Answer

Dry Docking:  Methods of dry docking of ships.  Inspection and routine overhauling of underwater fittings and hull.  Measurement of clearances and drops.  Removal and fitting of propellers (with and without Key).

Part – A

                      (Short Questions)

1. What is a Dry Dock?

Ans;- The 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 a 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 the hull and other areas of the ship which have been exposed to seawater for a long time are available for carrying out maintenance and repair works.

2.  Whatis the Solas requirement for vessels to Dry Dock?

Ans;- As per the SOLAS requirement, all Merchant's vessels require a complete inspection of the 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 that are immersed in water and are normally inaccessible by ship staff at normal sailing period.

For Passenger's vessel, the inspection of the bottom of the ship is to be done annually. Two of such inspections in a period of five years must be carried in dry dock and the maximum interval between these inspections is 3 years.

How many types of Dry Docks are there?

There are mainly two types of dry dock procedure:

1) Graving dock:

This type is normally constructed on land near the coastal water with a rectangular solid concrete construction with blocks, walls, and gates. The vessel is shifted inside the dock and rested in the blocks. After the ship is in the required position, the gate is closed and water is removed.

2) Floating dock:

A Floating Dock is a kind of a pontoon with sponsons on both sides. The pontoon is divided into the number of tanks. When the ship has to be docked, the tanks are filled with water due to which the dock submerges into the water. The ship is then navigated into the dock and the tanks are emptied so that the dock rises above the water.

The pumping of water is done with the help of ballast pumps which are located in sponsons. The sponsons are also equipped with cranes and bollards. Both the sponsons are connected with a bridge. Apart from the ballast pumps, the water is also filled manually with the help of manual valves.

To prevent any movement of the ship, keel blocks are provided. These keel blocks are placed on the tanktops. For increased stability side blocks are also provided. The side blocks are provided in such a way that the forces they exert don't damage the hull. The special reinforcement material is provided to prevent this.

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.

3). What are the 3 requirements for a Dry Dock?

Ans;- 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 center 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 centerline of the 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 the hull and the keel of the vessel.