The term dry docking refers to a regular process which is used for the maintenance and repair of merchant marine vessels. In much the same way as a car is regularly taken to the garage to be serviced, a ship is dry docked in an area called a ‘lock’.
But what is dry docking exactly and what happens during this process?
The main purpose of dry docking is to clean, inspect and repair the parts of a ship which are typically in contact with the sea and so usually submerged and therefore unseen on a day-to-day basis. Dry docking is used to complete a range of maritime maintenance services, such as:
- Hull cleaning, inspection and repairs.
- Painting the vessel with anti-corrosive and anti-fouling paints.
- Cleaning and surveying tanks.
- Examining important components of the ship, such as locking devices, sea suction valves, tail shaft bearings, and the anchor chain.
Dry docking is carried out regularly, with vessels completing a ‘special survey’ every five years, as well as midway surveys in-between this period. A ship will also be dry docked if it has been involved in a collision, has been grounded, or is being sold. It helps to ensure the vessel is operating:
- Safely
- Reliably
- Efficiently
- In accordance with its Flag State’s requirements.
What are dry docks?
A dry dock is a specially-built structure, situated on the shore (i.e. next to a body of water). Dry docks comprise a rectangular basin with a removable wall/gate on the side of the water, which allows it to be filled with, or emptied of, water. This enables the vessel to be manoeuvred into the dry dock (while it is filled with water) and then rest on a dry platform (when it is drained of water) in order to build, repair, clean and inspect it. Shipyards will usually have numerous different sizes of dry dock so that they can service various vessel types, for example a large dry dock which is needed to inspect container ships.
What are floating dry docks?
As their name suggests, floating dry docks do not need to be connected to the land. While they are still located close to the shore, floating dry docks are based in the water, typically in a sheltered harbour where there is no tidal activity. This makes them particularly useful for emergency repairs out at sea and for salvage operations where a damaged ship cannot sail.
Mainly built of steel, these structures have ballast tanks on either side, and on the bottom, which can be filled/emptied of water in order to lower and raise the dock. There are advantages of using floating dry docks compared to dry docks. These include:
- Better accessibility as they can be moved to where the vessel is situated.
- Space-saving as they can be installed further away from the shore, thereby not taking up valuable space.
- Better flexibility as they can be divided into two independent floating docks, or made bigger with retrofitting.
There are, however, also disadvantages of floating dry docks compared to dry docks, such as:
- Slower operations as there is only one access point for the workforce and for supplying equipment.
- More susceptible to operational disruptions caused by tides and/or windy weather.
- Longer time requirements because all equipment needs to be removed from the dock during re-flooding.
- Maintenance costs for floating docks are similar to those incurred by a ship.
How does a dry dock work?
The dry docking process works as follows:
1. Complete preparatory tasks before starting the dry docking process, such as carrying out risk assessments, making a repair list, gathering spare parts which may be needed, and planning for the vessel’s arrival.
2. Arrange supporting blocks in the dock so that the vessel can be aligned correctly, based on the ship’s structure, load bearing points, and other factors.
3. Flood the dry dock by removing the gate and manoeuvre the vessel into the dry dock, positioning it correctly.
4. Close the gates and pump out the seawater so that the dock is dry and the vessel is resting on the supporting blocks, which are anchored to the floor.
5. Carry out any necessary works, such as inspection, cleaning, maintenance and repairs.
6. Flood the dock by removing the gate and undock the vessel, ready to depart for sea trials (as required).
Why do ships have to do dry docking?
Dry docking is a vital element of international shipping. There are three main reasons why ships have to do dry docking. These are:
- It is a regulatory requirement
The SOLAS regulations, set out by the International Maritime Organisation (IMO), require all merchant ships to undergo an intermediate survey within 36 months and a comprehensive study of the hull in a dry dock twice within a five-year period. For passenger ships, they are also required to complete an annual inspection of the bottom of the ship. Not dry docking the vessel may prevent a safety construction certificate from being issued. - It improves a vessel’s efficiency
Over time, a vessel can experience significant wear and tear. Dry docking allows for any issues (e.g. damage, mechanical flaws, worn components) to be identified and repaired/replaced. This ensures the vessel is well-maintained and therefore operating efficiently and performing optimally. Furthermore, dry docking provides an opportunity to upgrade a vessel and make modifications (e.g. increasing cargo space or improving fuel efficiency) that could improve its performance and save money over the long-term. - It ensures the safety of the crew
A well-maintained vessel is more likely to operate safely. Regular dry docking is good practice by responsible ship owners who prioritise the safety and wellbeing of the crew on-board their vessel.