Dry docking means place a ship in a dry dock. Dry docking is the process and Dry-dock is the place where it is done. It’s done in China first time and In India Bombay Port Trust is giving dry docking facility. It can be done by various methods according to size, type and requirement of ships.
Methods of Dry-docking (It’s also called as Types of Dry-dock)
- Graving Dock
- Floating Dock
- Ship Lift
- Slip Way
- Marine Railway
In all dry-docks, Keel blocks and Bilge blocks are arranged for landing of ship’s hull in a proper manner. Number of blocks, their positions and spacing are decided by the hull structure of the ship. For this Docking Plan is carried out.
Keel Block – It’s a docking block used to support ship’s keel.
Bilge Block – It supports the bilge of the ship at the turn of the bilge while in a dry-dock.
It’s a land-based dry dock and it simplifies the logistics.
It’s a narrow basin type, usually made up of concrete and closed by gates either with the lock gate or with Caisson (watertight door).
Inside the basin, the ship will be floating with its longitudinal axis with the basin axis. Then water is pumped out slowly.
Keel blocks as well as bilge block are placed on the bottom according to the docking plane of the ship.
It has size limitation.
It’s like a buoyant structure also known as pontoon (floating type chamber).
It has number of floodable buoyancy chamber.
It looks like ‘U’ i.e U cross-section shape.
Wall of the floating dock is mainly used for stability to keep it upright.
When the ship has to be docked, the tanks (chambers) are filled with water. Due to the weight of the water dock submerges into the water. Then ship is taken into the dock and tanks/chambers are emptied. Due to release of water, dock floats at the water. Ballast pump is used in pumping of water.
It’s an alternative for slipway, floating and gravity dock.
It consists of a structural platform that is lifted or lowered exactly vertically, by a number of hoists.
Ship is mechanically lifted and placed at desired place.
This type of dock is also called mechanical lift dock.
Slip Way : (Boat ramp)
It is dedicated for very small boat like craft, yacht etc.
A slip way or rail or ram is made up of steel placed on the inclined surface.
It is an inclined plane made up of steel.
It has cradle on it to which a ship first will float and after ballasting the ship will sit on the cradle.
By the means of mechanical arrangement the cradle is hauled up to the shore for repairing.
Note : With a floating dock, a ship can exceed its length with no problem. But it would be a big problem with a land based (graving dock) dock.
Purpose of Dry-docking
- Docking survey
- For internal machinery survey item
- Inspection of stern frame along with rudder, rudder jumping clearance, rudder internal inspection and painting.
- Propeller check along with its disc surface curvature and propeller drop.
- Inspection of anchor
- Inspection of chain cables
- Inspection and replacement of sacrificial anode
- Sea chest and valves
Critical Period: The period of time from the critical moment when the keel of the vessel first touches the line of blocks and achievement of stability on the blocks.
According to Classification Societies, Docking survey is done twice in 5 year. An intermediate survey is done within 3 years. A special survey should coincide with 2 specified survey within 5 years. In-water survey is also accepted in place of intermediate survey.
Survey Check List : It is the list of different items which have to be inspected or checked or repaired.
Preparation of Dry-docking : (Planning)
- Before dry docking, all information regarding dry docking from the head office and from assigned dockyard is to be collected. The ship staff has to prepare all repair list and due survey item of machinery. To prepare all necessary spare stores, drawing (ship’s drawing), manuals, certificates, special tools and measuring instrument. And also, Label all sea valves and cocks, and mark with tags and colour coding.
- To keep emergency fire pump, emergency generator, air compressor, air bottle and portable fire extinguishers in good working order.
- Locked fixed fire fighting installation room as per dockyard rule.
- Shutdown of the auxiliary boiler, Oily water separator, sewage treatment plant, bilge water overboard valve in clean and closed condition.
- Fill up settling and service tank of diesel oil and fuel oil (hfo).
- Clean bilge holding tank, sludge tank, waste oil tank and give all this tank content to shore reception facility.
- Prepare receiving of store supply items, ISC flange, standard discharge coupling and arrangement for AC.
- Follow dry dock fire safety regulation.
- Adjust the require draft and trim of the vessel with deck officer.
- Take all soundings
- Main engine crankshaft deflection and its record.
During Dry-docking : (Entering & Docking)
- Always check : During Entering-(Metacentric height, Upright conditions and Trim by Aft)
- To discuss with the attending superintendent and dockyard manager about the repair job, scantling work and duration of docking period.
- Take & record all the clearance, deflection and wear down.
- Besides ship’s staff repair job always assist attending surveyor and record all survey item.
- Complete all under water job, check all repair job by shore staffs and ship staffs.
- Make daily work progress report.
After Dry-docking (Undocking)
- During the first stage of flooding of dry dock, check water tightness of ship’s hull and Engine room.
- Check all repair jobs
- Check all measurement data and collect from dockyard and compare it with earlier data.
- When sea water level cover up upto high sea chest level, open all the sea-suction valve, purge air from sea chest, cooling sea water pump, all filters casing and then run all the pump sea to sea till discharge pressure to your satisfaction.
- Start the generator and run it to your satisfaction and then cut off shore supply.
- Ensure all special tools, store items etc.
- Work done certificate by the shipyard are to be checked, stamp, sign and send.
- Dry docking report are to be generated for the head of office and copy to be retained on board.
- Now, finally with the permission of dry dock, you are ready to cast off the dock.