[ruzbehxyz]

The term ‘Dry-docking’ refers to when a vessel is taken into the dry-dock of a shipyard so that the underwater areas of the hull can be inspected, cleaned, painted and repaired if required.



Dry-docking is a more broad term, where the entire period of repair, inspection is termed as “Dry-docking”. In the actual world, ships may be at the Dock for only 4-5 days for the mandatory hull cleaning and painting. The other time (before and after) is spent at the lay berth where other docking related surveys and maintenance is carried out, which may not be possible to do when the vessel is in service. So the Dry-docking is a also a major maintenance period for any vessel. All ship owners / managers prefer to reduce the docking time, as costs involved are very high and maintenance sometimes goes into millions of dollars.

https://youtube.com/watch?v=OlmBYRoU...jmR8I6Dfg_GZr7

Regulatory requirement:
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 inspection of the hull in a dry dock twice within a five-year period. This survey is often referred to as “Docking survey”. Docking includes mandatory inspection and maintenance of the hull, propeller, rudder, etc. and other parts which are always immersed in water and are generally inaccessible by ship’s crew when the ship is in service.
Before the age of 15 years, the actual docking can take place every 5 years +- 6 months, with the intermediate one being relaxed to an IWS (In-water survey), where specialist divers inspect the hull and clean if required, to the satisfaction of the vessel’s Classification society.


Phases of Dry-Docking:

Phase 1: Preparation.
This involves thorough planning. In this stage, every aspect of the dry-docking procedure is carefully planned. Dry dock specs are prepared by the vessel and sent across to the vessel manager. The vessel managers discusses the plan with owners and certain jobs are added / removed from the final list. The list of jobs are provided to the shipyard and quotations obtained. Lot of to and fro emails between the office and shipyard. Finally the Shipyard is booked. Some non-mandatory jobs may be deleted, depending on the budget of the ship owner.


Phase 2: Arrival at the ship-yard.
The vessel arrives at the shipyard empty (only with Ballast water) and is safety docked at a lay berth / anchorage. Jobs are identified by the Shipyard and discussed with the crew. Daily meetings ensure this.


Phase 3: Docking of the vessel.
This is a critical phase and I will not get into the technical details. Before docking, the stability of the vessel is worked out by the Master & Chief officer, and exchanged with the Dock Master. Once agreed, the Dock Master then sets the wooden / concrete blocks for the vessel at the Dock, as per the Docking plan of the vessel which was provides earlier. The vessel needs to have minimum Ballast on board with a desirable trim by the stern. Usually at this stage, the ships engines are not operational and all movements are done by Tug boats. On the day, the vessel is towed into the Dry Dock and positioned by using various tugs. Once in line, the gates are closed and water is pumped out. Gradually the vessel sits on the blocks. In between, Divers go down to check the positioning of the vessel with respect to the blocks placed before. Once the vessel is fully on the blocks, the bottom plugs are opened by the shipyard and the balance Ballast water is left to drain by gravity. If possible, the vessel's ballast pumps are also used to assist in quickly emtying the Ballast tanks. At a Dry-Dock, most of the vessel's machinery is in-operational and shore power is provided by the shipyard.

https://youtube.com/watch?v=ubnhD0uc...19n41WwZyymRpF

Phase 4: Maintenance and Repair.
Though the maintenance and repair happen at all stages and on a daily basis, here we are referring to the maintenance for the Hull which cannot happen when the ship is afloat. The Hull is inspected by the Class inspector and others. The maintenance plan is discussed and the Hull maintenance starts, which ends with painting of the Hull with “Anti-Fouling” Paint. Various other openings, Rudder, Propeller etc. are also inspected and maintenance carried out.


Phase 5: Undocking.
This is another critical part like dry-docking. Watertight integrity of the plugs removed before and of various openings is checked using a vacuum test. Just one day before undocking, the Chief officer takes in the same amount of Ballast water, just like how it was before. Since the vessel’s pumps are usually in-operational, the water is provided by the shipyard using fire hoses. On the day, the shipyard pumps water in the dock and slowly the vessel starts floating. Once afloat, the dock gate is opened and the vessel is towed to a lay berth, to complete the balance maintenance jobs.

https://youtube.com/watch?v=EOHtotlQ...1X-reQSax8wZhU

Phase 6: Testing and Trials, and Departure from shipyard.
During this stage, various machinery which have been opened up for maintenance are to be tried out satisfactorily, some in presence of a Class surveyor. If major jobs are done on the Main Engine, then sea trials may also be done. Once all machinery are tried out and all certification completed, the vessel is allowed to sail out from the shipyard, thus completing it's docking period and going back into trading service.



I have just provided a brief introduction on docking of ships. All technical details are excluded, as the article would then become boring. Now let the photos and videos do the talking.





































Attachment 2647566






Note: All photgraphs and videos are taken by the Author. No permission, either express or implied, is granted for the electronic transmission, storage, retrieval, transfering or printing of the photographs.

[that_sedate_guy]

Originally Posted by ruzbehxyz

Once the vessel is fully on the blocks, the bottom plugs are opened by the shipyard and the balance Ballast water is left to drain by gravity.

If my understanding serves any purpose, the vessel initially at sea level enters the dock at the same level, right? In that case, where does the dock water empty to? Or is there some glaring detail I've missed?
Really interesting read, btw. Thanks!

[ruzbehxyz]

Originally Posted by that_sedate_guy

If my understanding serves any purpose, the vessel initially at sea level enters the dock at the same level, right? In that case, where does the dock water empty to? Or is there some glaring detail I've missed?
Really interesting read, btw. Thanks!

The dock water is pumped out using high capacity pumps and gravity. During the process, as the water level goes down, the vessel starts moving down and gradually sits on the blocks, aft / stern area first. Eventually all water is pumped out, to access the dock for everyone for maintenance and inspection purpose.

This video would explain further:

https://youtu.be/wJVvkMmur0g?si=AcM-swnWoXZkKrTv

[arijitkanrar]

Thanks for the detailed description.
Here is something I've always wondered, how is the ship stabilized when in dry-dock.
In the pics you've shown a flat bottomed hull which I guess are easier to stabilize?
What about other ships which are not round bottomed?
Are there points on the ship where supports can be attached? Is there weight on the keel of the ship when dry-docked?
What about situations where major refits and overhauls are carried out? For e.g. replacing major machinery where there is a possibility of the CoG shifting considerably. Are additional supports used?

[sagarpadaki]

Thanks a lot for the details of Dry docking. I found the whole process fascinating!

I see that the hull is painted red oxide color. Is this a standard color? Or can the color be chosen? Also, how are the areas of the hull that are hidden/not accessible due to the areas resting on the concrete blocks painted?

[ruzbehxyz]

Originally Posted by arijitkanrar

Thanks for the detailed description.
What about other ships which are not round bottomed?

Most merchant ships are flat bottom. There are two photos above, which are taken from below the hull, showing the flat bottom. This in fact helps the ship to dock.

Is there weight on the keel of the ship when dry-docked?

The Hull is designed to take the entire weight when outside the water. In fact, ships are built on blocks or in a shipyard part by part and then floated.

What about situations where major refits and overhauls are carried out? For e.g. replacing major machinery where there is a possibility of the CoG shifting considerably. Are additional supports used?

This comes under a subject called “Stability” and very complex to explain here. You can read about Ship Stability online.

[srini1785]

Fascinating.

A few noob questions.

1. What is the turnaround time between dry docking and back to the sea? how is it charged?

2. How do you test the integrity of the steel (or the material used) in the bottom part (Hull is it called?) ultrasound testing or something?

I suppose that there are limitations on the size of the ships which can come for dry docking.

[that_sedate_guy]

Originally Posted by srini1785

2. How do you test the integrity of the steel (or the material used) in the bottom part (Hull is it called?) ultrasound testing or something?

When I visited the Cochin shipyard for a training session, we were given a briefing on the weld shop and its processes followed by the NDT (Non Destructive Testing) lab.
NDT methods are used simply as a verification of an already tested and prototyped design. X-ray probes are also used in addition to US (ultrasound) probes.
Not sure about the recent improvements to the processes and the standards now though.
The plate steel used in fabricating the decks can sometimes be 100mm thick. That's four inches! One would be surprised to know that a lot of risk mitigation in such large scale industries is done at an Engineering level itself. The weld strength and rivet load calculations are taken very seriously. Once the calculations are approved, they are applied to proto-welds on mock-up plates which are subjected to tensile, compressive and impact loads. This is actually considered R&D as opposed to testing because the inherent load capacity of the weld structure is tested for each different bevel angle, surface finish and so many more variables. After all these steps are passed, it is then that the actual welds are done. Rivers undergo similar stringent processes.

[Gannu_1]

Originally Posted by arijitkanrar

In the pics you've shown a flat bottomed hull which I guess are easier to stabilize?

Yes but before the ship arrives at the dry dock or the shiplift in our case (L&T Shipyard at Kattupalli, Chennai), ship owners send the docking plan which gives the information to the yard for docking the ship safely. Information includes position of keel blocks, location of underwater appendages and projections like a bulbous bow, sonar dome, fin stabiliser, rudder, etc.

Originally Posted by arijitkanrar

What about other ships which are not round bottomed?

You’ll need customised trestles to support them. Like these:





Most yards which do repairs and refits of high-speed crafts would have them.

Originally Posted by arijitkanrar

Are there points on the ship where supports can be attached?

Usually the dock blocks at the keel.

Originally Posted by arijitkanrar

Is there weight on the keel of the ship when dry-docked?

The entire hull weight is transferred to the dock blocks. In addition to the docking plan, the weight distribution curve also provides the length-wise weight distribution, bending moments and CG. Something like this:

Originally Posted by arijitkanrar

What about situations where major refits and overhauls are carried out? For e.g. replacing major machinery where there is a possibility of the CoG shifting considerably. Are additional supports used?

Not required. Cut the hull on the decks or the side-shell depending on the easiest route, remove the old equipment, make changes to the hull for seating the new equipment, clear the survey, lower the new equipment and install it.

Originally Posted by sagarpadaki

I see that the hull is painted red oxide color. Is this a standard color? Or can the color be chosen?

The reddish-brown color is due to the copper content in the anti-fouling paint applied in the underwater hull. The color can’t be chosen.

Originally Posted by sagarpadaki

Also, how are the areas of the hull that are hidden/not accessible due to the areas resting on the concrete blocks painted?

During painting, they move the ship by one block to the forward or aft.

[shipnil]

Originally Posted by that_sedate_guy

If my understanding serves any purpose, the vessel initially at sea level enters the dock at the same level, right? In that case, where does the dock water empty to? Or is there some glaring detail I've missed?
Really interesting read, btw. Thanks!

I might be wrong, but you wanted to know how the water can be pumped out when the level is same. Well, the dock is like big tank, with gates on one side that open to the sea. Once the ship comes in, the gates are completely shut and then the water from the 'closed tank' is pumped back into the sea.

[adi.mariner]

Apart from the dry dock mentioned above, there is also a ship lift system in which the entire ship is lifted out of the water with winches and then moved about using rails. This is convenient for smaller ships and multiple ships can be dry docked at the same time.

[Mr.U]

Originally Posted by Gannu_1

During painting, they move the ship by one block to the forward or aft.

This not the case always. It depends on the equipment the yard has and also on how big the ship is. Most of the time these areas are not touched. The paint on the bottom hull doesn’t have much wear and tear as ideally it does not come in contact with anything other than water through out its life.

Each time the ship is placed in dry dock, they manoeuvre the ship in such a manner as to place the previously unpainted areas to be between the support piers. That way they can be cleaned and repainted. The next time in dry dock, they position it so that the other areas can be repainted.

The ships have a three position rotation schedule. If the hull was resting on block position 2 the last time it was painted, it would be located on block position 3 this time, allowing the old paint in position 2 to be removed and replaced. The next time it was drydocked, the ship would be located on block position 1, and so on.

However if these areas need to be painted, repaired or serviced then a method called fleeting is used. In this method the dock is flooded and the ship is moved a little to access the covered spots. The shipyard has the expertise to measure exactly where the blocks will touch.

[RJK]

Originally Posted by adi.mariner

Apart from the dry dock mentioned above, there is also a ship lift system in which the entire ship is lifted out of the water with winches and then moved about using rails. This is convenient for smaller ships and multiple ships can be dry docked at the same time.

This looks like chowgule’s yard at Lavgan
I spent 3 months there last year refurbishing one of our oil rigs, must say good for ships and supply vessels but terrible yard for rigs

[V.Narayan]

Originally Posted by ruzbehxyz

The term ‘Dry-docking’ refers to when a vessel is taken into the dry-dock of a shipyard so that the underwater areas of the hull can be inspected, cleaned, painted and repaired if required.

Thank you Captain for sharing this and enriching our naval threads multi-fold. Dry docking is an art and science rolled in one. Watching it is extra ordinary. To ensure that the vessel all along its length and beam will settle exactly correctly on those blocks underwater is mind blowing.

[Jeroen]

Thanks for sharing! It brings back many memories from my own merchant navy days!

Choosing which drydock to use was also a bit of a thing. It needed to be near the route of the vessel, so you would not loose too much time. Workmanship and turn around times were important parameters too.

We often used Japanese dry docks in the 70-80s for vessel in that part of the world. They would send a team ahead of the drydocking to come aboard and check each job in detail. Their preparation was meticulous.

When I worked in the offshore I found myself working the Northsea of course. Many of the AHT operators would never dry dock their vessels in any of the UK ports. The risk of a strike and the vessel getting stuck was just to great. So we sailed to the Netherlands. The additional sailing time was well worth the money!

Jeroen