Dry docking in merchant navy is the art of putting the vessel in such conditions where the dock is dry. The vessel has to be physically removed from the water and put in such a condition that it is not in contact with the water. As a result, each and every part of the ship is completely visible and is accessible which can be cleaned, surveyed, measured and for which the repairs could be carried out in such a way that the vessel is tested, certified and surveyed in a safe environment without any danger to either the vessel or to the personnel onboard.
CLASSIFICATION OF DOCK:
1. DRY DOCK: Dry docks are docks that are used to maintain ships. A dry dock is a small basin that can be filled to allow a load to be floated in and then emptied to allow the load to land on a dry platform. It consists of a long chamber with sidewalls, a semicircular end wall, and a floor. The open end of the chamber is equipped with a gate that serves as the dock’s entrance. Ships, vessels, or other watercraft are constructed, maintained, and repaired on dry dock.
2. WET DOCK: These are also called harbor docks. Wet docks are necessary for the berthing of vessels in order to facilitate the loading and unloading of passengers and cargo.
DRY DOCKING SURVEY
Twice during a five-year span ,all merchant vessels must have a complete hull survey in a dry dock and the intermediate survey must not take more than 36 months, according to SOLAS. Dry docking deals with the hull, propeller, rudder, and other underwater parts of the ship that are normally inaccessible to crew members when the ship is sailing. Dry docking is expected for every vessel at least once every five years. A typical dry docking project takes 10-14 days to complete. If the dry dock is not properly maintained, costs and time can escalate, and critical flaws can go undetected and uncorrected. Dry docking is used to expose underwater components for inspection and repair. The ship’s underwater section must be inspected on a regular basis for damage caused by the sea water. The dry docking process is one of the most cost-effective approaches for ship repair and maintenance.
PLANS OF VESSEL REQUIRED BY THE DRY DOCK
1. SHELL EXPANSION PLAN– A plan depicting the seams and butts, thickness, and welding of all plates that make up the shell plating, framing, and other components.
2. DRY DOCK PLAN– It contains all crucial details about the ship’s layout and design, structural stability points, and the precise position of all underwater parts, such as propellers, shafts, seawater inlets and outlets, and other appendages.
3. CAPACITY PLAN– A plan showing availability of the freight, fuel, freshwater, water ballast, and other spaces, as well as cubic or weight volume lists and a scale showing deadweight capacities at various draughts and displacements.
4. LSA/FFA PLANS– It provides international requirements for life-saving appliances.
5. GENERAL ARRANGEMENT PLANS– General arrangement or GA plans depicts the division and arrangement of the ship. The overall layout of a vessel can be described as the allocation of volumes for all critical operations, all of which are properly coordinated for position and access.
DRY DOCKING- PROCEDURE
The technique for dry docking a ship is unquestionably not unrestricted. It requires a very long time of preparation, thinking about careful insights about the boat, and setting up the dock appropriately. Indeed, even the smallest mistake in estimations can prompt extreme outcomes.
The different phases of dry docking are as follows:
- PRE-DOCKING PREPARATION: Dry docking a ship requires meticulous planning. The Dockmaster will prepare a docking plan few days before a ship is to be dry docked, taking into account all the minute specifications of the ship’s construction, such as the hull layout, drain plug positions, and echo sounders under the ship, to ensure that they are not damaged during docking.
- PREPARING A DOCKING PLAN: The method of dry docking a ship must be carefully prepared in order to be carried out smoothly. To assess the ship’s height, weight, and structure, the Dockmasters use a set of drawings, precise measurements, and calculations. The docking plan outlines how to dock the ship successfully as well as how to undock it quickly. It also lowers the chances of a ship capsizing while undocking.
- UNDERSTANDING THE STABILITY CONDITIONS: While docking, the ship must maintain its stability. To properly position the container, it is important to decide how to distribute the weight of the container. Since the keel valve covers almost the entire weight of the ship, a slight error in the stability measurement can lead to an accident. It is necessary to thoroughly check the stability of the ships before actually operating it.
- ARRIVAL: The ship must fulfill all of the stability criteria when it arrives at the dry docking terminal. The ship’s propeller must not be submerged, and the ballast must be kept to a minimum. It is better to avoid dry docking if there is a chance of bad weather. On arrival, the Cleaners and the Dockmaster board the ship.
- ACTUAL DOCKING: Since the ship’s engine is not usable, it must be towed into the dock after it has accomplished the pre-docking conditions. The ship is moored and secured until it is within the docks. Workers then pick up and begin cleaning procedures such as pumping out dock water, making ballast pumps, and removing drain plugs.
TYPES OF DRY DOCK
Following are the 5 types of dry docks which are used for cleaning and maintenance of the ship:
1 GRAVING DOCK: It is excavated from the land and close to the sea by means of floating caisson gates. Sil is the term for the bottom edge of the dock beneath the gate. The sides of the dock are terraced with concrete steps into which shores are heeled. Along the centerline of the dock, huge blocks of timber are built into keel blocks.
These blocks are tied together to prevent them from toppling or being tripped as the vessel is sewed or when re-floated. When the water levels on both sides are equal, the dock is filled with river water and the gate is opened.
2. FLOATING DOCK: It ensures that the vessel’s centre of gravity and dock’s centre of buoyancy is in one line. It is a huge, flat, subdivided steel tank like a ship’s double bottom, on the upper centerline of which is a line of keel blocks. The floating dock forms the bottom of the dock which has no ends but high narrow sides which are also tanks. Both in the double bottom and sides are flooded over the dock centre line and the water is pumped dry until the ship is clear of the water.
To keep the dock properly trimmed, the ship’s centre of gravity must be vertically above the centre of the dock’s buoyancy. The two eventually form one floating craft; the Dockmaster will require details regarding the ship’s stability, trim, and loading condition and may require any of these to be changed.
3. MARINE RAIL DOCK OR SLIP WAY: An inclined railway running from the shore deep into the water is known as a marine rail dock. This railway track is used to pull a ship that needs repair out of the water. Lift capacities range from 100 to 6000 tonnes.
The main components of the marine dock are as follows:
- Cradle: The cradle which is on a roller, is lowered into the water along an inclined track until there is enough water to cover the cradle.
- Track: The track slope should be chosen to match the natural slope of the site in order to reduce dredging while still providing the necessary draughts over the blocks for docking the vessel
4. SHIPLIFT: It consists of a concrete platform that is raised and lowered vertically by a series of hoists in synchrony. The platform is submerged first, followed by the ship that was floating above the support, and eventually, the platform, support, and ship are raised and levelled.
5. MARINE MOBILE LIFT: Marine mobile liFT is also used for small boats like recreational yachts, tugboats, and pilot boats. It is a kind of device on a ship to dry dock or launch repaired equipment. It usually needs to be customized to achieve the handling of the ship.
WHY IT IS NECESSARY TO HAVE A DRY DOCK
For better results and efficient operation, dry docking is needed to inspect and correct mechanical defects in the vessel in its initial dry state. Sweep, scrub, and ready the ship by descaling all rusted areas of the hull and then paint the ship’s hull to restore its original speed and fuel consumption.
A surveyor inspects a ship in clear water with the hull visible on the outer surface until it is taken for repairs. The surveyor looks for flaws and points them out, which are then fixed using dry docking.
The purpose of dry docking is to ensure that ships are operational and their class license is not revoked. To ensure seaworthiness, structural machinery and various components are inspected and maintained. Drydocking is often necessary if a ship’s underwater structure has been damaged by grounding, collision, or any other damage that will impact the water’s integrity of the ship’s hull.