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Dry Docking - Part I

It is mandatory to dry dock a ship on a regular basis in accordance with cargo ship safety construction rules based on the 1974 SOLAS regulations. The rules state that cargo ships have to be drydocked twice in an interval of five years with the interval of five years with the interval between consecutive dry dockings being not less than two years and not more than three years. The classification societies have interpreted this rule by providing a regular interval of thirty months with a window period of + or - six months from the anniversary date so as to suit this requirement.
The classification society's rule for periodical surveys has now been amended to operate in accordance with the harmonization of surveys so the surveys and repairs are carried out without hindrance to the operation of the ships.
Accordingly, in a five-year cycle of a survey, the two dry dockings are programmed as the first drydocking during the intermediate survey layup and second drydocking during the special survey layup for renewal of the class certificate.

The above-stated facts apply to regular and routine drydocking. As per classification rules as well as rules for issue of safety construction certificate. The routine drydocking cannot be substituted by an afloat underwater inspection. Such inspections are recommended in special circumstances especially when the ship has missed the scheduled drydocking for various genuine reasons and this inspection by the classification society is carried out only after obtaining a suitable recommendation from the administration.
The recommendation is limited to issue a voyage permission for the ship to reach the shipyard stemmed.
Drydocking has to be carried out after an accidental grounding or collision involving underwater damage. This drydocking will only involve the requirements for satisfactory completion of the repairs for the underwater damage. If, however, this accident has occurred within close proximity of the scheduled drydocking activity may be suitably advanced which the administration will agree.

The commercial operation of bulk carriers tankers and container vessels follow a pattern of movement or transportation called world wide tramping whereby the ships have to sail in accordance to instructions and directives from charterers and route programs are not fixed. Hence the term tramping. The ships are at least out at sea sailing for about 330 to 340 in a calendar year. The execution of the routine, as well as damage repairs, becomes difficult to be accomplished during port stays which are short on an average about one, two or three days in any port. It, therefore, becomes necessary to carry out all accumulated repairs, accumulated surveys and major hull repairs involving a layup and the scheduled drydocking.
The periodical layup, therefore, becomes a major repair activity involving dry docking and has to be planned well in advance so that repair specifications are properly formulated and the drydocking including layup tender is ready for distribution to all concerned shipyards in the vicinity of the ships sailing schedule.
Plans required by shipyard for Drydocking:
The following plans and drawings are required to be submitted to the shipyard for their use to enable the ship to be drydocked safely.
To be submitted before the ship enters drydock
  1. G.A plan
  2. Docking plan
  3. Capacity plan with DWT and displacement/draft. After the ship has docked.
  4. Shell expansion
  5. Midship section
  6. For structural repairs as ordered- Detailed location plans of are as where repairs and renewals are required.

G.A Plan: The general arrangement plan gives a profile view of the ship along with the plan view st main deck level and D.B tank top level and an end view from forward. This drawing is used for berthing purposes alongside repair berths as well as to get a general appearance of the ship. Hence a reduced size drawing would do. 

Docking Plan: The docking plan is a detailed plan view seen from the bottom. It shows all openings for the main sea suction boxes provided at the machinery room area on the underside. The other opening is the individual drain plug holes normally one plug hole per tank in large ships tanks may be provided with two holes per tank. The plan also shows the recommended lines for block laying with a recommended pitch of the blocks are sufficient to safely support bulkers tankers and container ships.  

It should be understood that this docking plan is to be used only for normal drydocking purposes. In the case of ships being dry-docked after a heavy collision or bottom damage, this docking plan will not be suitable. For this purpose, an amended docking plan is made after the ship's bottom is photographed in afloat condition by the shipyard diver and an amended plan made to suit the purpose. The must provide for alternate locations of support for the damaged area where the original blocks cannot be laid for support.

Shell Expansion: It is a two-dimensional drawing of a three-dimensional drawing of a three dimensional surface of the ship's hull form. It is developed from the ship's line plan with the contour lines erected straight on the baseline representing the ship's length. The contour lines on the lines plan are located at corresponding stations indicated by corresponding frame numbers on the length of the baseline. When the ends of the verticals lines on the baseline are joined by a continuous line, the shell expansion outline is obtained. The representation surface is then properly marked by parallel lines both vertical and horizontal lines so that they correspond exactly to the number of strakes forming one half of the hull surface. The strakes are marked with letters A, B, C etc vertically starting from sheer strake A and ending with keel as strake R eg. The strake number are starting from 1 at the stern end to any ending number at the forward bow. Each strake is therefore indicated by a letter to show its level, a number to show its position. 

This plan is very useful for the following information:
  • It is used for marking the location of hull damage on this plan by identifying the strake number, letter and frame number, so that the exact location of the damaged and also suggested repairs are marked in a localized copy.
  • The shell expansion can be used for finding areas of painting surfaces such as topside, boot topping, and bottom areas by applying Simpson's rules directly. In the shell expansion, the vertical scale used is different from the horizontal scale and a suitable adjustment has to be made when calculating areas. This becomes useful in solving disputes concerning areas of preparation and painting.
  • It gives information on the thickness of the original strakes which is indicated by the number in the circle shown in the strake. The quality of steel used is also shown by letters A, B, D, E and AH, BH, DH, EH.
Capacity Plan: This plan is useful in finding the displacement of the ship for a selected mean draft. The displacement is required to be known to decide on the total number of blocks to be used since each block can support only a maximum given mass.  

The capacity plan also gives information on:
  • Volumes of compartments/ tanks in m3.
  • Location of center of volume of compartment/tank. Hence the capacity plan may be used for making minor stability calculations for moments which are required for trim and heel adjustments.
  • For propeller shaft withdrawal and survey. Assembly drawing of the propeller shaft, propeller, and stern tube will be required.
  • Safe stability condition to be provided during docking.
When docking the ship, the ship is slightly trimmed by the stern so that the stern block touches the stern frame first. This is necessary to steady the ship by locating the stern on the block. The stern side is chosen since the stern frame is very strong and can resist the strong single reaction occurring at the instant the stern block touches the stern frame at the coffin plate or bottom of the stern frame.
Almost all modern ships have aft machinery space and when floating in the light condition they trim very badly by stern. This heavy stern trim has to be corrected by filling the forepeak tank sufficiently to result in a slight trim by the stern. Hence all modern ships with aft machinery have their forepeak tanks filled sufficiently to result in slight stern trim. In some cases, the forward ballast tanks may also be filled up to correct the trim to give slight aft trim.

As a general convention or thumb rule, the stern trim is limited to 0.5 meters for ships of length up to 200 meters and 1 meter for ships above 200 meters whilst docking.
In addition to the trim limitation, the shipyard sets the blocks with a declivity to suit the trim and this ensures that all the blocks come into contact at the same time avoiding any condition for a stern reaction to be formed. With the declivity in place, a slightly higher trim will result in a small stern reaction and can be neglected. In graving docks declivity is arranged. In floating dry docks declivity is accomplished by trimming the dry dock suitably and then correcting it to even keel after the ship has completely seated on all the blocks.
The danger of excessive trim results in a dent occurring in the area adjacent the stern frame bottom and coffin plate. In the case of floating docks, the effect of high stern trim will result in the stern blocks being pushed behind and that is because of trim of the dock itself.

The requirements for safe stability during docking and undocking may, therefore, be given by six golden rule as follows:
  1. The mean draft selected should be such that the critical GM on docking is positive ( The Master should verify with the help of the loadictor that the mean draft recommended by the yard is safe and results in a positive critical GM.)
  2. The ship should always be upright during docking and undocking. This requires monitoring of the clinometers during the docking.
  3. Do not exceed the recommended mean draft of the ship.
  4. Do not exceed the recommended stern trim.
  5. Ensure all bilges and bilge tanks are dry and there is no unaccounted water on board.
  6. Soundings of all tanks containing F.W, Ballast, F.O, D.O, and L.O must be recorded and must be the same for each tank at the time of undocking. If F.W is consumed during the stay in dry-dock, the consumed amount must be replenished into the tank before undocking. A very common occurrence is that the fore-peak tank is normally drained out for purpose of the survey. Hence before undocking, the bottom plugs are refitted and the fore-peak tank filled up to the original sounding prior to undocking.
Shore Current Supply and Safety:
Usually, all ships when dry docked take shore supply current. The advantages are as follows:
  • The cost of shore supply is cheaper than the corresponding costs of diesel oil.
  • The Auxiliary engines not being utilized can be overhauled if their routine overhaul is due. In addition, the accessories such as coolers for freshwater and lube oil can be cleaned thoroughly more conveniently than when the ship is floating.
  • Arrangements have to be made for the supply of cooling water ( sea water) in and out.
All ships are provided with a shore power terminal box with proper sockets for the three main phase connecting cables and the Earth cable. It is also provided with a phase sequence meter to verify the phase sequence is correct. It is the duty of the ship's electrician to ensure that the phase connections are correct and their sequence is also correct. A further important check to be made is to ensure that the earth cable is properly earthed and not left unconnected at its end. The electrician also notes down the meter readings at the time of connection and disconnection for verification of power consumed as recorded on work done certificates.
During the stay in drydock electrical power is used only for lighting fresh water supply, ventilation and cabin heaters in winter domestic fridge and galleys. Only small power motors up to a maximum of 1KW rating are in use. The standard supply frequency inland in most parts of the world is 50Hz. While ships standard supply is 50Hz. For small powered motors working continuously at 50Hz supply will not pose any problem, but higher power motors as used for air conditioning may get heated up and they have to be monitored frequently and stopped if they get overheated. Modern shipyards which have special shore supply generators operating at 60Hz does not pose this problem.
The ship's electrician must monitor the operation of the windlass when the anchors and chains are being lowered into the dock especially if the supply is 50Hz as the hydraulic motors can get heated up.

Fire Saftey:
When a ship is being dry-docked in a shipyard the shipyard takes the responsibility of providing all necessary assistance for prevention and extinction of any fire that may occur. 

The shipyard provides two fire watch-keepers who go around and observe all locations where hot work is proceeding. They can be identified by their red-colored boiler suits with the word FIRE printed on the backside. They are on duty during the working shifts of the shipyard. Water pressure is maintained on the ship's mains through the international shore connection ready for use when needed. They keep a few portable fire extinguisher if needed, which they will recharge if used. The role of the ship staff is to cooperate with the yard in putting out an outbreak of fire. Above all the ship staff should not carry out any hot work on their own at any location on the ship. All listed hot work will be carried out by the shipyard. The ship staff may carry out minor hot work on small items within the ship's workshop.


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