The dry dock is a structured area wherein construction, repairs and maintenance of merchant vessels and boats are carried out. The unique construction allows the water to be filled up in an area, also known as a lock so that vessels can be manoeuvred in and out of the area. Once the vessel enters the dry dock, the gates are closed and the seawater is drained out so that hull and other areas of the ship which have been exposed to seawater for a long time are available for carrying out maintenance and repair works.
As per SOLAS requirements, all Merchant vessels require a complete inspection of the hull in a dry dock twice within 5 year period and the intermediate survey must not be more than 36 months. This includes maintenance of hull, propeller, rudder etc. and other parts which are immersed in water and are normally inaccessible by ship staff at a normal sailing period.
For passenger vessel the inspection of the ship’s bottom is to be done annually. Two of such inspections in a period of five years must be carried in dry dock and the maximum interval between these inspections should be 3 years.
Dry Dock Operations Guides for Marine Engineers & Deck Officers
Types of Dry Dock
There are different types of drydocks that are used for repairing and cleaning the ship’s bottom part:
- Graving dock
- Floating dock
- Marine Rail Dock
- Marine mobile lifts
Among these, the marine mobile lifts and ship lifts are mainly used for small vessels such as recreational yachts, tugs pilot boats etc. Nowadays, there are mainly two types of dry dock procedures from the above lists which are used for seagoing vessels:
1) Graving dock:
This type of dry dock is normally constructed on land near the coastal water with a rectangular solid concrete construction with blocks, walls, and gates. The vessel is shifted inside the dock and rested in the blocks. After the ship is in the required position, the gate is closed and water is removed.
Related Read: Understanding Ship Stability During Dry Dock
In the earlier days, the graving docks were constructed using stones and timber. Now, steel and concrete structure is used to make the enclosure and heavy steel gate is used to seal the dock to stop the ingress of water once the ship is standing on the blocks. The gates can be in two parts with each hinged to the sides and can be hydraulically operated or can be of one solid steel structure supported on roller over the track, which can be retracted inside the dock walls when opening the gate.
Advantages of Graving Dry Dock:
- It can accommodate bigger size vessels when compared to other dry docking systems
- It is cheaper for dry-docking a similar sized vessel as compared to other types
- The graving dry dock can be used to perform retrofitting, modification etc. which is difficult to achieve in other types
- The supply of spares, machinery, services to graving dock is very much accessible due to its location-based near the land
- New advanced graving docks have welding, hot-work and other workshop located inside the dock in an elevated surface (above the water surface when the dock is filled) giving quick access and workflow in the dock
- Retractable ramps in new types of graving docks make it easy to supply spare, machinery and saves a lot of time and manpower to transfer them inside the dock
- A bigger graving dock can be used to repair more than two ships at a time and some modern graving docks have two gates at both ends, making it easier to repair and re-float the ships independently
Disadvantages of Graving dock:
- When re-flooding the dock, all the machinery and equipment needs to be taken out from the dock, which takes time
- The maintenance cost of the graving dock increases as per the age of the dock and becomes very high
- Any problem with the dock gate will make the whole dock non-operational
- The docking and undocking process in the graving dock takes time as compare to other types
- If the dock holds multiple ships for repair, the complete operation needs to be stopped if any one of the vessels needs to be taken out of the dry dock as it will require filling of water for refloating
2) Floating dock:
A floating dock is in the form of “U” structure which is mainly used in salvage, to carry ships that have met with an accident and is damaged to an extent that has made them unable to sail further to a coastal dock. However, now many regular sea-going, small and mid-size vessels are also drydocking it in a floating dock. Several “U” type floating docks can be joined to carry a large vessel.
The ship is brought near the channel where the floating dock will partly submerge itself and the ship slides inside the dock. Once the ship is in the position, the floating dock is then de-ballasted to drain the water from its hollow floors and walls to support the vessel on the blocks arranged on the floor of the floating dock.
A valve is provided which can be opened to fill up the chambers with water and which will make the dock immersed in water so that the ship can sail out. The water is pumped out of the chamber which will allow the dock to rise, exposing the underwater area of the ship for maintenance or carrying the ship to repair the facility.
Related Read: What is Ballasting and De-ballasting?
The floating dry dock is usually built using steel framing which is similar to that of seagoing vessels with ballast tanks provided on sides and at the bottom to raise and lower the dock. The floating dry docks are commonly operated in sheltered harbours as there is no wave or natural tidal activities present there.
Advantages of Floating dock:
- They can be propelled to the location of a salvage vessel near the harbour
- They are cheaper to maintain as compare to graving docks and can get a higher resalable return
- They can be installed near or away from the shore inside the harbour, making them a portable and space-saving structure without taking space of the shore facility
- The complete floating dock can be aft or forward trim by ballasting the dock, which further assists the ship or the damaged vessel which cannot be given a trim
- Additional mooring equipment is needed for the floating dock to make it stable
- The floating dry dock can be altered and increases in size in all dimensions by extensive retrofitting/ rebuilding
- They can also be split into two different floating docks independent of each other
Disadvantages of Floating Dock:
- The supply of store, equipment, and manpower is usually done from one access point gangway which makes the operation slow
- The maintenance cost of the floating dry dock is similar to that of a ship as the hull of the floating dock is submerged in the salt water
- The floating dry dock operation will effect if there are tides or during windy weather
- When re-flooding the dock, all the machinery and equipment needs are to be taken out from the dock which takes time
The blocks inside the dry dock play a very critical role in taking up the ship’s load and distributing it among the number of blocks placed below the keel plates. Every ship is provided with a docking manual which will provide guide block plan approved by Ship Classification Society.
Ship master/ chief officer and the dry dock master must understand the blocking plan for the particular vessel and how the ship will behave under load when resting on the blocks, which include various factors that can affect the amount of load taken by a block.
The blocks are usually constructed from one material as the stiffness of all the blocks will be similar. If the blocks are constructed from different materials, the force exerted on the blocks with smaller elasticity module will squeeze it more as compared to the block which is stiffer. This can lead to damage to the block or the ship’s hull as the distribution of the force will be uneven.
The most common material used for the blocks are:
- Concrete with steel
- Timber blocks
- Timber on top and concrete at the bottom
In timber block, which is added into a keel track built of blocks with concrete or steel bases, the timber block will take a much smaller load relative to the others.
Apart from the keel blocks, side blocks are also arranged to support the ship. They are less stiffer than the keel blocks as a stiffer side block will overload the vessel and may damage the structure. The height of the slide blocks is usually similar or more than that of the keel blocks.
Important factors which affect the loading on the blocks are:
- The initial height of the block: The height of the block is a very critical factor as the load on the individual block will be determined by the load shared by all the other blocks. If the placement and height of the blocks are according to the ship’s hull, the load distribution will be even
- The contact area of the block: The contact area of the block with the ship’s hull determines the load distributed on the block. If the contact area of the block is smaller in size, this means the load exerted on this block will be lesser than that with a larger contact area.
- The material of the block: As stated earlier, different material will react differently to the load exerted by the ship. Hence, the material of the block used for the keel block and for the side block plays an important role in aligning the ship in the dry dock.
- Placement of the block: The blocs in the dry dock needs to be arranged as per the docking plan of the ship as there are many types of equipment and parts which can get damaged if the block position is not altered. For e.g. for echo sounder, anodes etc., the blocks need to be removed so that these equipment tools can have the space to settle once the ship rests on the blocks.
Choosing a Type of Dry Dock:
The criteria to select a type of dry dock depends on the following factors:
- The size of the vessel: The graving dock is used to accommodate larger size as compared to any other type. If the ship owner/ manager has to dry dock a large oil tanker, they will go for graving dock. If the vessel is about 10000 tonnes, Marine railway type can be used. If a boat or small yacht of up to 250 tonnes needs a repair in the dock, a mobile marine lift can be used
- The condition of the vessel: If the vessel propulsion plant is not working or there are damages which can make a vessel immobilized, floating docks are commonly used in such condition
- Types of repair: The choice of the type of dry dock also depend on the types of repair the ship wants to undergo. For the normal scheduled dry dock, a floating dock can be selected, however, if there are major retrofitting or massive parts/ machine fitting is required, the graving dock is selected as they are usually located near to the shipyard and it is easy to move the material from land to dock as compare to the floating dock
- Schedule of the vessel: The location and the type of the dry dock will be decided as per the current schedule of the vessel and how easy it is for the ship to reach the dock after unloading all the cargo to the last port of call
- Budget: The most important factor to select the type of dry dock is the budget allotted to the ship
Related Read: How Cost Estimation is done for Ship’s Dry Dock?
Three Requirements for Dry Dock
Stability is the most important requirement for getting a ship safely into a dry dock. The three important parameters which must be ensured before entering the dry dock are:
1) Adequate Initial G.M:
When the ship touches the blocks, there is a reaction at the point of contact which raises the center of gravity “G” and reduces the metacentric height “G.M” so that adequate initial metacentric height is required to compensate the same.
2) Vessel to be Upright:
While entering the dock the vessel needs to be upright, which means that there should be no port or starboard list when the ship touches the blocks. If the point of contact of the ship and keel blocks is outside the centre line of a vessel, it may force the vessel to tip over.
3) Small or Moderate Trim Aft:
A moderate trim aft is usually kept when making the ship’s keel sit on the keel block. As the water level in the dock lowers, the slight trim allows the accending of stern and bow in tandem rather than simultaneously which will reduce the load and pressure on a hull and the keel of a vessel.
Dry dock procedure
Once the type of dry dock is selected by the ship manager, the next process is to prepare the ship to enter the dry dock and later place the ship’s keel carefully on the blocks provided in the dry dock floor. Some important points to note are:
- Make sure the ship is prepared to enter the dry dock by having least ballast and no cargo carried on board
- If the floating dock is used, the movement of the ship will depend on the requirement of the docking master and the condition of the vessel. If the vessel is immobilized due to an accident, the floating dock will move towards the vessel and if the vessel is functioning properly, it may propel inside the dock
- Once the ship enters the dry dock, it is moored to the dock
- The docking master will inform the ship’s crew beforehand about the trim requirement which they need to maintain
- Before emptying the dock, the Ship and dock master needs to ensure equipment like echo sounder or log sensor does not hit any blocks and are clear from any obstruction. For this, divers are sent to check that all such equipment is clear of the blocks
- The dock master will give the order to pump the water out from the dock and slowly the ship will sit on the keel blocks
- The docking master and the ship’s crew must ensure the point of contact of the ship and keel blocks does not lie outside the center line of a vessel as it may lead to tumbling of the ship
- Once the ship sits on the dock, the cleaning and repairing process starts
Duties of crew members
While dry docking the ship, the duty of the ship’s crew will be:
- To keep the ship at minimum ballast condition
- To keep the aft trim as requested by the dockmaster
- To moore the vessel by assisting dock crew once the ship comes inside the dry dock
- To keep a check on the stability of the vessel while the water is lowered in the dock
- Once the ship is sitting on the blocks, ship’s crew will pump out the ballast
- To prepare the ship to connect to the shore power
- Once the dry dock water is pumped out, de-ballast tanks using gravity
- Remove the drain plugs from the bottom of the ship for various tanks
Drydocking is one operation which allows the ship’s crew to learn those areas of the ship which cannot be explored when the ship is sailing. It also helps the ship manager to assess the condition of the ship’s hull and those machineries which are not accessible when the ship is in water. Bringing the ship in the dry dock is teamwork between ship’s crew and dry dock personal and proper communication is the key for the safe laying of the ship’s keel on the keel blocks.
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