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DRY DOCKING-1 (Basic Drydocking)

Q. As a Chief Officer, how will you prepare your ship for dry docking?
Ans. My preparation for dry docking will be under the following heads:

1. Stability including trim, GM, FSC, etc. GMvirtual for critical instant is calculated allowing for maximum free surface effect for the respective tanks where relevant.

2. List of repairs to be done in the dry dock is made ready.

3. Preparations are commenced for surveys and inspections to be done, in the dry dock.

4. Responsibilities of duty officers and crew in the dry dock are laid down and explained.

5. Sailing time to anchorage and anchorage period available before dry docking is estimated. The work which must be done during this period is listed and planned.

6. Ship-specific work, e.g. tank cleaning and sludge management in case of a tanker, is planned out.     

7. A careful study of previous dry docking including of the docking plan is made .

8. Coordination of senior officers with Master; communication with office & drydock; etc is planned out.

9. Taking note of facilities available at dry dock; reviews about drydock; weather prevailing at drydock; etc are discussed.

Q. What precautions must be taken in respect of ship’s stability?
Ans. The GM must be as high as possible with minimum free surface. Trim should be within the range provided by the Dock Master. All the movable weights must be secured. Liquids from high tanks can be shifted to lower tanks. As far as possible, the tanks must be empty or full. The soundings must be accurately taken. Also, it should be confirmed that no tank is leaking.

Ideally, the weight distribution while coming out of dry dock should be same as at the time of coming in. Hence, the tanks to be with ballast must be chosen in such a way that for the inspection purpose, it is not required to empty them in dry dock. Many a times, a tank is emptied in a dry dock and then filled up by a hose. This is time consuming and therefore expensive. With the harmonization of surveys, it may be little difficult to allow many DBs empty / full (as per our liking). The draining and refilling of tanks however, must be kept to a minimum. A guidance may be taken from loading and stability manual and records from the past dry docking.

Q. Why is a ship normally, dry docked by stern?
Ans. The stern is strengthened for this purpose. Stern frame is made up of solid cast steel, which can withstand high compressive stresses. A ship in ballast condition is normally down by stern hence with minimum adding of ballast in DBs, the dry-dock condition may be achieved.

A ship enters a conventional drydock, head first. An outward or seaward declivity is arranged for water to drain off from the drydock. This means, the ship must have trim larger than the declivity of blocks.

Q. What are the other advantages of dry-docking with stern trim?
Ans. It is very important that the first block that is taken by the ship is positioned perfectly with respect to the ship. The rest of the blocks will automatically be taken at desired appropriate locations. The other blocks are placed taking into consideration the docking plan showing the last dry-dock’s positions of blocks and the various features on hull such as drain plugs, transducers, etc.

Q. What information is generally provided to the dock master?
Ans. Drydock office is generally provided with the following information, in addition to the answers to the questionnaire provided by the yard :

Q. What is docking plan? What does it show?
Ans. Docking plan is a schematic plan, whose purpose is to assist the crew of drydock arrange blocks beforehand. It shows the location of any feature that may be present on the ship’s hull, especially below the bilge keel line. The locations of rudder, propeller, bilge keel, brackets / sockets used for cathodic protection, drain plugs, transducers, any device for measuring speed or fitted for anti-rolling purpose, etc. are indicated.

Q. What decides min & max trim?
Ans. The minimum trim is decided by the requirement from the Dock Master and the maximum trim is decided by the minimum virtual GM, the ship must have at the critical moment.

Q. In what way is a shell expansion plan different from docking plan?
Ans. Shell expansion plan is a proportional area plan showing the various plates, frames and strakes on hull. This plan in a way depicts a three dimensional shell on the two dimensional plane. But since the proportion is maintained a rectangular plate will appear rectangular and a stealer plate will appear similar to its actual shape. The purpose is to provide address to each and every point on shell. Thus, a dent can be described as extending between particular frames and on a particular strake with the dimensions and depth of the dent. The other important data includes scantling of hull plate, location of water tight bulk heads, etc.

Q. What is critical period during dry-docking in a graving dry-dock?
Ans. The critical period in a graving dry-dock starts from the time that the stern takes the block and lasts till the blocks are taken all over, during the flooding of the dry-dock.

Q. Why is this period called critical?
Ans. During this period the ship is neither free floating with original stability nor sitting all over on blocks, with additional securing. The ship throughout is pivoted with decreasing GM, draft, WPA, etc. Any upsetting moment can be dangerous in this period. Moreover, the growing up-thrust continues to work from the same block and in the same part of the ship’s hull.   

Q. How is a vessel aligned in a dry-dock over the blocks?
Ans. To ensure that the ship stays in the middle, a wire through the dock, with a little flag, or a wooden plate, which shows the middle of the line of blocks is rigged up. When the hull almost meets that wire or line, the ship is stopped and the stern is also aligned. Some fine-tuning of the ship’s position can be done by divers while there is still some water left to maneuver about. It is extremely important that supporting blocks conform to the structural members so that the ship is not damaged when its weight is supported by the blocks. Blocks are pre-arranged with the help of docking plan.

Q. Why are some of the graving drydocks, stair-shaped on sides?
Ans. This shape helps in two ways; firstly, it helps placing wooden poles in some drydocks to additionally support the vessel in drydock; secondly, it reduces the amount of water to be pumped out. When the ship takes the blocks, all over, the wooden poles are secured, with ropes maintaining the poles to the hull sides, at good height and with wedges against the concrete walls. This is why; these drydocks are ‘staircase’ shaped. The timber poles sit on a step or at least supported well this way. The wedges are driven in between the beams and the dock’s wall.

Q. What precaution regarding bottom plugs, must be taken?               
Ans. While removing bottom plugs, one must ensure that the tanks are empty and dry. As bottom plug of each tank is removed, it is important to label it, tank-wise. This will ensure that bottom plugs are not interchanged, while fitting back.

Q. What is ‘bouncing the hull’, in dry-docking?
Ans. Bounce the hull, is for redoing the anti-fouling. Once the initial coat is applied, the ship is temporarily floated, just long enough to reposition so that surfaces covered by the blocks are now free to be worked on.

Q. How often is drydocking necessary?
Ans. A docking survey should be carried out twice within a 5 year period. The intermediate survey must be completed within 3 years. One of the two docking surveys within the 5 years period should coincide with a special survey. An in water survey’ may be accepted in lieu of the intermediate survey.

Q.  What special precautions must be taken by the Master, upon knowing that his ship is due a dry-dock?
Ans. Master’s active involvement in the ship’s business & profit earning is extremely important. He can guide the company regarding the geographical & climate-wise suitability of dry-dock. He can also guide the company with his experience and also with the experience of officers about a particular dry-dock. Preparation for dry dock begins after the ship sails from the previous port. The Master must, at the earliest call up a meeting of the senior officers & discuss with them the various aspects such as; preparation of specification sheets describing individual jobs to be submitted to various dry docks for pricing. The jobs are priced individually and as a whole. This allows the ship managers to streamline the jobs to provide maximum value for money. The Chief Officer & Chief Engineer can be asked to dig out the details and jobs done in the last dry-docking, study various plans, etc. The works to be done at anchorage & in dry-dock must be planned. General as well as ship’s type specific works to be done must be planned out very carefully and the officers must be instructed accordingly.

Q.  What works are carried out in a drydock?
Ans. One of the most important tasks and responsibilities of the ship’s staff, being the safety and fire fighting responsibilities of the vessel, are handed over to the dry dock safety department, for the duration of the dry and wet dock period. All hot works, tank entries or jobs, requiring special safety measures carried out by ship’s crew, must be first agreed with the dry dock safety department. A daily meeting is held to discuss forthcoming jobs and any special requirement. This also allows the vessel’s staff and company representatives to monitor the progress of the jobs in dry-dock.

Where a ship is in dry dock or on a slipway, she 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 hull, including bottom and bow plating, keel, stern, stern-frame and rudder.

Q. Why the shore power is required in a dry dock?
Ans. In dry dock, the auxiliary engine-generators, which produce the electrical power, cannot function in want of supply of cooling water. The auxiliary engine therefore, has to be switched off before the water is pumped out. The chief engineer must ensure the relevant precautions in respect of above.

Q. What precautions are taken prior to re-flooding a dry-dock?
Ans. A check list is referred, to ensure whether all the jobs are accomplished. Plugs, valves, items removed or renewed are carefully checked by relevant person in-charge. Dry-dock bottom is carefully inspected by a search team. All tanks are sounded and compared with the soundings when the ship had entered the dry-dock. Each of the platings that had to be replaced is also checked for water-tightness. Sometimes, a dockworker may be relaxing in a remote area next to a block. After a due safety and seamanship oriented check, the dock is flooded. The ship must not show any development of list. The ‘getting out’ operations are similar to the ‘getting in’ operations, though, of course in a reverse order.

The rudder plug and vent are checked. The Impressed Current Cathodic Protection System (ICCP) is checked. All sea inlets and sea chests gratings are checked. Echo sounder and logs are checked. It is ensured that all the items sent ashore are returned after due attendance. A consolidated report is then reviewed by the Master. Water is filled till sea-chests are well under water. Generators are then started, all the sea valves which were overhauled are checked for tightness.

Q. How is the GM at critical instant calculated?
Ans. KG of ship, after accounting for the weights, liquids, etc. is calculated. KM for the draft at critical moment is read from the tables. KMCRITICAL – KG = GMS. Free surface correction is allowed using the displacement for the draft at critical moment. The upthrust at critical moment ‘P’ is calculated using the formula PCR × LCF= trc x 100 MCTC (trc = Change of trim required for vessel to sit all over), virtual loss of GM = FSC and virtual loss of GM due to dry-docking is subtracted from GMS. FSC is seen for virtual displacement at relevant stage. Virtual loss due dry-docking is given by:
or

Q.  What best can be done if the GM at critical instant is very low?
Ans: Following can be done to improve the GM at critical instant:

1. Reducing KG by lowering the weights or filling up a DB symmetrical about centre line.

2. Reducing the free surface correction as much as possible. 3. Reducing the trim as much as possible because the virtual loss of GM due to dry-docking is directly proportional to the trim change.

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