What is meant by Dry Docking ?

Dry docking of a vessel refers to taking the vessel into a dry dock and placing her on a series of blocks (known as ‘keel blocks’ as they are placed under the keel of the vessel) arranged on the bottom of the dry-dock. The keel blocks are fabricated out of steel and have wooden tops so that the ship’s hull plating will not come into hard contact with steel, which may result in damage to the plate. Also the wooden tops are shaped as wedges so that the height of the block can be adjusted by pushing two wedges placed with their tapers in the opposite direction, into each other or pulling them apart.

What are Keel Blocks ?

This facility of adjusting the height is necessary because the ship’s bottom plate may not be exactly in a straight line (even a few millimeters of difference can result in overloading the hull structure and deforming the same and all classification rules allow certain tolerance in the straightness of the keel) and blocks of the same height can result in some portion of the hull not touching the blocks below that. This will in turn load the adjacent blocks which are touching the hull.

What is Docking Plan ?

The number of keel blocks, their spacing and exact location are decided taking into consideration the ship’s hull structure viz. the frame spacing, location of additional brackets (known as docking brackets) and stiffeners provided specially for the purpose of strengthening the hull at the locations where the hull is resting on the blocks and also distributing the weight evenly on the blocks. The shipyard which builds the vessel prepares a drawing called ‘docking plan’ which gives the exact locations where the blocks should support the hull when the vessel is dry-docked. The repair yard undertaking the repairs/docking will need this drawing in advance so that they can arrange the blocks accordingly before flooding the dock prior to docking of the vessel.

What are the Ballast Requirements for Dry Docking ?

The docking plan also specifies the number of blocks and the minimum load carrying capacity of the blocks. As the entire weight of the ship in the docking condition has to be borne by the keel blocks, it is also necessary to limit the weight of the ship while she is being docked. Thus the extra weight of ballast, a minimum of which is required to be on board to provide stability when the vessel is afloat/sailing, has to be reduced to the maximum just before the vessel enters the dry-dock. Since the weight of ballast required for the vessel to stay upright in afloat condition may be more than the weight acceptable by the keel block, it may be necessary to take the vessel into the dock with that much ballast and then de-ballast the vessel when she is inside the dry-dock but is still afloat. In such a case, the vessel will be taken into the dock and then de-ballasted while the dock is being emptied after closing the dock gate.

Dock Sill Height

There is one more reason for reducing the docking draft of the vessel and that is the ‘sill height’ of the dry dock. Although the depth of water in the channel may be sufficient to accept vessels of a certain draft, the sill height at the entrance of the dock may be insufficient to allow this draft. Due to this reason, the vessel is docked when the depth of water near the dock entrance is maximum, which condition exists during the high tide only. Thus docking and un-docking dates have to be planned after taking into consideration the tidal variations at the place where the dry-dock is situated.

Dry Docking Interval

Every vessel needs to be taken into a dry-dock once in a while in order to examine the condition of the under-water hull as well as some under-water fittings. All Classification Societies have rules covering this requirement. The docking intervals are decided after taking into consideration many facts like the facilities provided on board for ‘condition monitoring’ as well as for inspecting the hull in the afloat condition and ascertaining the state of the hull and other items under the water line. These intervals may later be changed depending upon the age of the vessel, the condition of the hull and under-water fittings during a survey etc.

What are Important Classification Society Rules on Dry Docking ?

A minimum of two Docking Surveys are to be held in each five-year Special Survey period and the maximum interval between successive docking. Surveys is not to exceed three years. One of the two Docking Surveys required in each five year period is to coincide with the Special Survey. Consideration may be given in exceptional circumstances to an extension of this interval not exceeding three months beyond the due date.
The class may accept an In-water Survey (IWS) in lieu of the intermediate docking between Special Surveys. Special Surveys are different from routine surveys which may be carried out annually. The scope of inspection is more during the Special Surveys and the timing of the Special Surveys also is covered by the class rules.

Special Surveys – Class Rule Requirements

All ships to be subjected to Special Surveys in accordance with the requirements. These Surveys become due at five-yearly intervals, the first one five years from the date of build or date of Special Survey for Classification as recorded in the Register Book, and thereafter five years from the date recorded for the previous Special Survey. Consideration may be given to any exceptional circumstances justifying an extension of the hull classification to a maximum of three months beyond the fifth year. If an extension is agreed the next period of hull classification will start from the due date of the Special Survey before the extension was granted. In this context ‘exceptional circumstances’ means unavailability of dry-docking facilities, repair facilities, essential materials, equipment or spare parts or delays incurred by action taken to avoid severe weather conditions.

Docking Surveys – Class Rule Requirements

At Docking Surveys or In-water Surveys the Surveyor is to examine the ship and machinery, so far as necessary and practicable, in order to be satisfied as to the general condition.
For oil tankers (including ore/oil and ore/bulk/oil ships), chemical tankers and bulk carriers over 15 years of age the intermediate docking between Special Surveys is to be held in dry-dock. Further, this survey is to be held as part of the Intermediate Survey.
Where a ship is in dry-dock or on a slipway it is to be placed on blocks of sufficient height, and proper staging is to be erected as may be necessary, for the examination of the shell including bottom and bow plating, keel, stern, stern frame and rudder. The rudder is to be lifted for examination of the pintles if considered necessary by the Surveyor.