What Are The Types Of Dry Docks?

What Are The Types Of Dry Docks - Merchant Navy Info - Blog

A dry dock is a structured area where commercial ships and boats are built, repaired, and maintained. This unique design or arrangement allows water to be refilled in an area. Also known as a lock, allowing ships to enter and exit the area. When a ship enters a dry dock. The gates are closed, and the seawater is drained, exposing the hull and other parts that have been exposed to seawater for extended periods of time for maintenance and repair work.

Dry Dock Survey

According to SOLAS requirements, every commercial vessel must undergo a full hull survey in dry dock. Twice within five years and an interim survey within 36 months. This includes maintenance of the hull, propeller, rudder, etc. Other parts are underwater and normally inaccessible to personnel while the vessel is underway. The bottom of a passenger ship must be inspected annually. Such tests must be carried out in dry dock twice in 5 years. With an interval of up to 3 years.

Types of Dry Docks 

Different types of dry docks are used for ship repair and cleaning.

 The main ones are: 

1 Graving dock 

2 Floating dock 

3 Marine rail dock 

4 Ship lift 

5 Mobile marine lift 

Among these, mobile marine lifts and ship lifts are mainly used for small vessels such as pleasure yachts and tugboats. Masu. Currently, there are two main types of graving dock methods used for ocean-going vessels, as shown in the list above.

1) Graving Dry

Dock This type of dry dock is usually constructed on land near the coast.

It is rectangular and made of solid concrete with blocks, walls and gates. The ship is pushed into a dry dock and placed on blocks. Once the vessel is in the desired position, the gates are closed, and the water is removed. In the past, grave graving docks were built of stone and wood. Steel and concrete fencing are now in use, and heavy steel gates seal off the dock to prevent water from entering once the ship reaches the block. The gate consists of two parts, each hinged on the side and operated hydraulically, and a sturdy part that rests on rollers on a track and retracts into the graving dock wall when the gate is opened. May consist of steel construction.

Advantages of Graving Dry Dock 

  1. Can accommodate larger vessels compared to other dry dock systems.
  2. It is cheaper to dry dock ships of similar size than other types of ships.
  3. Graving Dry Dock allows for modifications and alterations that are difficult to perform with other types.
  4. Due to its close location to land, the cemetery dock has easy access to supplies of spare parts, machinery and services.
  5. New advanced grab dock allows for in-dock welding, hot work, and other workshops to be carried out on elevated ground (or above water if the dock is full), allowing quick access and maneuvering to the dock. It will be.
  6. The new grab dock’s retractable ramp facilitates the deployment of replacement machines, significantly saving time and manpower for moving these machines around the dock.

Grave docks larger than Seven can repair two or more ships at the same time, and some modern grave docks have two gates at each end, allowing ships to be repaired and resurfaced individually.

Disadvantages of Graving Dry Dock

  1. If the dry dock floods again, all machinery and equipment must be removed from the dock, which is time-consuming.
  2. The cost of maintaining a graveyard dock increases as the dock ages and becomes very expensive.
  3. A problem with the dock door will render the entire dock inoperable.
  4. The Grabdock docking and undocking process takes longer than other types.
  5. If there is more than one ship in the dock for repairs, and one of them needs to be removed from the dry dock, the entire process will stop as water needs to be added to resurface it. Is needed.

2) Floating Docks 

Floating docks are shaped like a “U” structure and are primarily used for salvage to transport vessels that have been in an accident and are too damaged to proceed to a coastal dock. However, many regular ocean-going vessels and small and medium-sized vessels now moor on floating docks. To transport large ships, multiple “U” shaped floating docks can be connected. The ship is brought near a canal where the floating dock partially sinks, and the ship slides into the dock. 

Once the ship is in position, remove the ballast from the floating dock and drain  water from the hollow floor and walls that support the ship with blocks placed on the floor of the dock. It is equipped with a valve that can be opened to fill the chamber with water, submerge the dock and launch the ship. Water is pumped out of the chamber, and the dry dock is raised to expose the ship’s underwater areas for maintenance and ship repairs. Floating docks are typically constructed of steel frames similar to ocean-going ships, with ballast tanks on the sides and bottom for raising and lowering the dock. Floating dry docks typically operate in sheltered harbours free of wave or natural tidal activity.

Advantages of Floating Dry Docks 

  1. Can be driven to  salvage vessel locations near the port 
  2. Lower maintenance costs and higher resale income compared to grave docks  
  3. It can be installed near or at a distance from shore in the port. This allows for a highly portable and space-saving structure that does not take up space on land facilities.
  4. The completed floating dry dock can be trimmed aft or forward by ballasting the dock, which can further help vessels that cannot be trimmed or are damaged.
  5. Additional anchorage equipment is required to stabilize floating dry docks.
  6. The floating dry dock can be modified and expanded to any dimension through extensive renovation/reconstruction 
  7. It is also possible to split it into two different independent floating docks  of the floating dock


  1. The supply of warehouses, equipment and labour is usually done in the following ways.
  2. There is a corridor of access points, which slows down the operation.
  3. The maintenance cost of a floating dock is similar to that of a ship because the hull is submerged in salt water.
  4. Floating dry dock operations are affected by tidal and windy weather conditions.
  5. If the dock were to flood again, all necessary machinery and equipment would have to be removed, which would take time.

Dry dock block 

The blocks in the dry dock play an important role in receiving the ship’s cargo and distributing it to the blocks placed below the keel plate. Each ship has a docking manual that includes a classification society-approved command block plan. The Master/First Officer and the Dry Dock Master should plan their respective vessels’ block plans and underload when resting on the block, including the various factors that may affect the load that the block can absorb. You need to understand how the ship works. blocks are usually made of one material so that the stiffness is comparable. If the blocks are made of different materials, a block with a lower modulus of elasticity will be compressed more by an applied force than a stiffer block. The uneven force distribution can cause damage to the blocks and hull. The most commonly used materials  for  blocks are: 

  1. Steel and concrete 
  2. Wooden blocks 
  3. Wood on top and concrete on the bottom 

When adding wood blocks to a keel rail made of concrete or concrete blocks, since it is steel-based, the load on the wood blocks is much lower than that of other blocks. In addition to the keel block, there are also side blocks that support the ship. They are less rigid than the keel blocks, as stiff side blocks can overload the ship and damage the structure. The height of the sliding block is usually the same as or greater than the height of the keel block.

The important factors that influence the loading of dry dock blocks are:

The initial height of the block

The height of the block is an important factor because the load on an individual block is determined by the load shared by all blocks. Other blocks, too. If the placement and height of the blocks correspond to the hull, the load distribution will be uniform.

Block contact area

The contact area between the block and the hull determines the load distributed to the block. If the contact area of ​​a block is small, it means that the load on that block is smaller than if the contact area is large.

Block material

As already mentioned, different materials react differently to the loads exerted by the ship. Therefore, the material of the blocks used for the keel block and side blocks plays an important role in the orientation of the ship in the dry dock.

Block placement

Blocks in the dry dock must be placed according to the ship’s dock plan. Many types of equipment and parts can be damaged if the block is not repositioned. For example,  blocks such as depth sounders and anodes must be removed to make room for this equipment after the ship is berthed above them.

Dry Dock Type Selection 

The criteria for selecting the dry dock type for your vessel is determined by the following factors:

Ship Size

Grave Dock accommodates larger sizes than any other type. When ship owners/managers need to dry dock  large oil tankers, they  go to graveyard docks. The ship-rail type can be used when the ship weighs approximately 10,000 tons. If your boat or small yacht weighing up to 250 tons needs to be repaired at the dock, a mobile vessel lift can be used.

Ship status

Floating docks are often used when a vessel’s propulsion system fails or is damaged, and the vessel may become immobile.

Type of repair

The choice of dry dock type also depends on the type of repair the ship wishes to undergo. You can choose a floating dock instead of a regular dry dock. However, if a major refurbishment or assembly of large parts/machines is required, a grab dock is usually located near the shipyard and is easier to transport materials from shore to the dock than a floating dock. Selected.

Vessel Schedule

Dry dock location and type are determined based on the vessel’s current schedule and how easy it is for the vessel to arrive at the dock after all cargo has been discharged at the last port. It will be done.


The most important factor in choosing the type of dry dock is the budget allocated to the vessel.

Dry-docking Requirements  

Stability is the most important requirement for safely drydocking a vessel. Three important parameters that must be checked before entering  dry dock are: 

1) Proper initial GM

When a ship contacts a block, the reaction at the point of contact increases the center of gravity, “G,” and decreases the center of gravity. Metacentric Height “G.M.” Therefore, sufficient initial metacentric height is required to compensate.

2) The ship must be upright

The ship must be upright when entering the dock. There should be no port or starboard side when the vessel touches the block. If the point of contact between the ship and the keel block is outside the ship’s centerline, the ship may capsize.

3) Small or medium aft trim

When a ship’s keel is on her block, a medium aft trim  is usually maintained. As the water level at the dock drops, a slight trim allows the stern and bow to be raised at the same time instead of at the same time, reducing stress and pressure on the hull and keel.

Dry Docking Procedures 

Once the ship manager has selected the dry dock type, the next step is to prepare the ship for dry docking and place the ship’s keel in a designated block on the dry dock floor. Place it carefully.

 Important points to note are: 

  1. Ensure the vessel is ready for dry docking with as little ballast and cargo on board as possible.
  2. When using a floating dock, the movement of the vessel depends on the instructions of the dockmaster, the requirements, and the condition of the vessel. If a ship becomes stuck due to an accident, the floating dock will move towards it, and if the ship is in good working order, it can proceed within the dock.
  3. When a ship arrives at a dry dock, it is moored to the dock.
  4. At berth, the master notifies the ship’s crew in advance of the trim requirements that must be met.
  5. Before emptying a dock, vessels and dock masters should ensure that equipment such as depth sounders and log sensors do not hit blocks and are clear of obstructions. To do this, a diver is sent to ensure that all equipment is properly equipped with a block.
  6. The harbormaster orders water to be pumped out of the dock, and the ship slowly sits on the keel block.
  7. Captains and crew at anchor should ensure that the point of contact between the vessel and the keel block is not outside the vessel’s centreline, as this could cause the vessel to capsize.

Duties of ship’s crew while in dry dock.

While dry docking a ship,  the ship’s crew (under the direction of the chief engineer and first officer) have the following duties: 

  1. Maintain the ship in a minimum ballast condition.
  2. Maintain stern trim as required by the port Captain 
  3. Provides support to the dock crew as soon as the ship enters dry dock and ensures that the ship is moored 
  4. Controls the stability of the ship while  water is being lowered to the dock 
  5. Once the ship is in the block, the ship’s crew pumps out the ballast 
  6. Prepares the ship for connection to  shore power 
  7. Once the dry dock is drained, gravity is used to remove the ballast from the tank.
  8. Remove the various tank drain plugs from the bottom of the vessel.

Dry docking is a process that allows a ship’s crew to learn about areas that cannot be explored while underway. It also helps ship managers assess the condition of hulls and machinery that is not accessible when the vessel is underwater. Moving a ship into a dry dock is a team effort between the ship’s crew and dry dock personnel. Proper communication is key to safely placing the ship’s keel on her keel block.

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