dry-docking-a-ship

If you have the experience of dry docking of a ship, you would agree that dry docking is a great experience.

I personally love to be on a ship due for dry dock. After all, you get to see things which you don’t see during routine operation of the ship.

I was lucky enough to get a chance to be in dry dock in each rank I have served on. I was even lucky to experience the double-hull conversion of a tanker during dry dock.

But if you have not been to a dry dock, there would be one thing that might come to your mind on hearing the word Dry Dock. And that is dry dock calculation that we read in ship stability, probably during our Mate’s exams.

That’s purely theory part. And I believe theory without practical experience is just a theory.

So here I am going to write about the practical aspect of taking a ship to dry dock.

But before I proceed, in layman’s terms I will summarize what we had read about dry docking in ship stability.

We read and understood these things

  • The time from “when Stern touches the blocks” to when “full ship is on the blocks” is the critical period.
  • During the critical period, the vessel’s GM reduces. This is because vessel’s ‘Gravitational center G moves upwards when Stern touches the blocks.
  • It is required and a good practice to have the least trim while docking so that the critical time is minimal.

Preparing for Dry Dock

Well, I am not going to the company specific parts of dry docking, like preparing repair specifications.

I will specifically be talking about taking the ship into the dock and making it sit on the blocks.

And then, of course, bringing the ship out of dry dock.

Days before the planned dry dock, Dock master will make the first contact with the ship. He can do so either directly or through the company representative such as superintendent.

The dock master has a huge responsibility of calculating the stresses on the dock as well as the ship’s structure. Any miscalculation can lead to serious accidents resulting in huge damages. These damages can be to the ship as well as dock itself.

The dock master is trained for block arrangement and stability during dry docking. For all these calculations, dock master needs certain information from the ship. Among other things, he will ask for

  • a copy of Vessel’s Docking plan
  • Arrival Stability conditionPre-docking condition
  • Pre-docking condition

Some of the content of his email might look something like this

First email from dock master before dry docking

From the docking plan, dock master wants to know

1) Hull structure so that he can arrange the blocks to support the ship’s hull.

2) locations of transducers for log and echo sounders so that these do not come beneath the blocks.

3) Location of sea chests and drain plugs for the same reason.

Based on the docking plan provided by the ship, dock master prepares his own docking plan for the ship. Below are some of the sections of actual docking plan prepared by dock master for s ship arriving for dry dock.

Plan view of docking plan made by dock master

As you can see, dock master has planned which blocks he needs to remove and where he needs to put blocks. He also has specifically marked the location of echo sounder and speed log.

Have a look at the more closer view below, which is again from same docking plan prepared by dock master.

Bow Longitudinal View Docking plan

I think the above image make it more clear about what dock master is trying to achieve from the docking plan. If you want to see the stern view too. Here is it.

Stern Lonitudinal view Docking plan

And this is no theory. This is an actual docking plan prepared by dock master for a ship arriving at dry dock.

I know I am kind repeating myself but I can’t say it enough. That is because I get excited to see real thing than just theory.

Stability condition and weight distribution

Apart from docking plan, dock master would ask arrival weight distribution of the ship.

There can be up to 4 stages for which stability calculations are required. These stages are

  • Arrival Dry docking port
  • Pre-docking condition
  • Ship sitting on the blocks but dock not yet empty (also called wet condition by dock master)
  • Ship on the blocks and dock empty (called Dry condition by docking master

Let’s discuss each of this condition

Arrival Dry docking port

On arrival dry docking port, you need to have least possible ballast. By least possible I mean, propeller should be immersed. And also you should be complying with all stability requirements.

Pre-docking stability condition

So we know that we cannot arrive with zero ballast as our propeller need to be immersed and the ship needs to be stable.

But what is the logic behind having other three conditions? Why can’t we just remove all the ballast and go inside the dock?

Let’s understand the logic behind these conditions

Docking with zero ballast is the ideal condition. But most of the times this would not be possible. That is because docking master would limit you for the maximum trim that you can have. In zero ballast condition, your trim may be more than 2 meters.

Dock master would want you to reduce the trim to around 0.5 meters. This depends on the dock on how much trim you can have before docking.

We have already discussed the reason for the need of least trim while docking. This is to have the least critical period. More trim we have more will be the time required to bring the vessel from stern on the block to full ship on the block. And this is the critical period with least GM value. We do not want to have the ship in the critical period for longer time.

Most of the ships will have considerable stern trim in light weight condition. So most ships will need to have some ballast forward in Pre-docking condition.

The amount of ballast would depend on how much trim dock master has advised you to have.

Stability condition while ship on blocks but dock not empty

When the ship is on the blocks, you have already passed the critical period. Dock master will tell you to start deballasting. The only concern dock master will have is the ship should not refloat.

The ship can refloat if dock deballasting cannot compensate for the decrease in the draft because of deballasting.

The condition is monitored by the dock master and he would tell you on how much ballast you can remove in this condition.

But the question is why the dock master need the vessel to remove the ballast concurrently when he empties the dock?

This is because dock water does not want to have more weight on the blocks. When the ship is sitting on the blocks but has water inside the dock, there is a certain amount of buoyancy ship has. This buoyancy acts like upthrust which reduces the effective weight acting on the blocks.

This condition will be discussed by the dock master and he will advise when and how much ballast you can remove.

As I said earlier, dock master bases his calculations on not to allow the ship to refloat.

Ship on the blocks and dock empty

When the ship is on the blocks and there is no danger of ship re-floating, dock master will tell to take out all ballast.

Dock masters sometimes call this condition as the Dry condition.

A Complete guide of taking a ship to dry dock

Procedures for taking the ship to dry dock

Now that we know about the stability part, let’s look into each stage of taking the ship to the dry dock.

Arrival to dry docking port

As I mentioned, you would arrive with least ballast. That would be arrival dry docking port condition. Even though the ship will be complying with draft and stability requirements, but the ship will be light. Lighter than usual ballast condition. So before you arrive at this condition, it is important to scan the weather reports. You would not want to arrive in light condition if the weather prediction is rough.

Most of the time, the ship is taken to the lay-up berth before going into the dry dock. If not, vessel needs to be at anchor for deballasting to arrive at the Pre-docking condition.

While at anchor, dock safety inspector will board the vessel. He will do the gas check of all the compartments to make sure that vessel is gas free. He will then issue a gas free certificate. He will also give safety booklet of the dock which will have all the safety regulations of the dock.

Docking of the vessel

The vessel will dock when it has achieved the pre-docking condition. In this condition, vessel will have least ballast to achieve the required trim.

Before docking, dock master will board the vessel. He will discuss the docking procedures with master and chief officer. He will give the mooring arrangement plan while docking. He will specify the Panama leads from where the moorings will be passed.

Apart from this, he will also discuss the ballast condition at each stage.

For shifting to the dock, the pilot will board the vessel. As the ship’s engine will not be available, the ship will have a number of tugs to move the ship to dry dock. The number of tugs would depend on how big the ship is and how powerful the tugs are. In any case, all ships can expect 5 tugs or more.

Out of these 5 tugs usually, 2 will only be assisting for pushing. Different docks can have different arrangements for making fast the tugs. It will all depend upon the location, tidal current, and local factors.

One of such arrangement can be two tugs made fast forward, one made fast aft and two tugs standby. The one tug made fast aft will have one line on each side of the poop deck to have better control in handling the ship.

Depending upon the dock, the ship will either enter stern in or bow in.

The pilot will bring the ship parallel to the dock. When the stern (or bow whichever is entering first) is close to the dock knuckle, docking master will take over from the pilot. Docking Master is different from dock master. Docking Master may not board the vessel and will be giving instructions to the tugs from the dock itself.

When the ship is inside the dock, ship’s crew need to pass the mooring lines as per the agreed mooring arrangement. Usually forward and aft will have two lines on each side. Out of two lines on each side, one on each side can be shore line. But this can be different and mooring arrangements will be advised by the dock master.

When the ship is made fast with the moorings, docking master will sign off and dock master will take over.

Vessel on the blocks

Before dock master starts to remove dock water, a diver will make an underwater inspection. The diver will ensure that echo sounder and log sensors are clear and not sitting under the blocks. He will also ensure physically that vessel’s centerline is in line with the blocks. It is a good practice to switch off the echo sounder and speed log now.

After the diver has made his inspection, dock master will start pumping out dock water.

Dock master will let the vessel know when Stern has touched the blocks and when the ship is on the blocks.

After the ship is on the blocks, dock master will tell to start pumping out ballast to arrive at the wet condition.

As the dock water is being pumped out, at one point the water will go down from the generator cooling water sea chest.

After this point ship will get power supply from shore.

A shore electrician will board the ship (Through basket and shore crane) and make arrangement to connect the shore power. Ship’s Electrician should coordinate with him to have the shore power connected.

You should check if shore power will be enough for running the ballast pump and mooring winches. If not, this should be discussed with dock master in advance. He will then ensure that water level does not go below sea chest until you have pumped out required ballast.

Once on shore power, dock master would continue to dry the dock. He will tell you to take out all ballast accordingly. You may do so with gravity as same might be more effective.

Once the dock is dry and ship sitting on the blocks, you can line up to deballast all ballast tanks by gravity. This is to let all the water drain whatever is left in the ballast tanks.

So now you have already brought the ship to the dry dock. It is a wonderful view to see the ship out of the water. You should not wait to go down in the dock and have a look at her.

Removing the Drain plugs

The Ship repair manager will now request the chief officer to witness the removal of the bottom plugs.

As you know, each tank which forms part of the hull has a bottom plug to drain the water in dry dock.

Removing bottom plugs ensure that the tanks are empty and dry. As bottom plug of each tank is removed, it is important to label it. This will ensure that bottom plugs are not interchanged while fitting back.

Though plugs of all the ballast tanks are of the same size, it is best practice to fit back plugs which belong to each tank.

If you wish to experience how we remove the bottom plugs, watch this video.

Departure from Dry Dock

After few hectic days in dry dock, it would be time to leave dry dock. We need to be equally attentive in leaving the dry dock as we were while coming into the dry dock.

Before dock master floods the dry dock, all the underwater things need to be in order. This includes sea chests, ICCP system, echo sounder sensors, log sensors and drain plugs.

Echo sounder, log and drain plugs are tested for air and water tightness. Testing involves first putting soap solution around the drain plug. Then we create the vacuum around drain plug and look for any bubbles.

If you have never experienced this testing before, here is a video of testing of the drain plug.

After all these integrity tests are complete, it is time to leave the dry dock.

The best approach of leaving the dry dock is to follow exactly how the ship came into the dry dock.

It would involve

  1. Filling the ballast to bring the ship to wet condition.
  2. passing the lines as was in the arrival condition
  3. Flooding the dock
  4. Filling the ballast to pre-dry dock condition
  5. Flooding the dock up to the level where the ship is fully afloat.
  6. Disconnecting the shore supply and taking ship’s generators on load
  7. Taking the ship out of the dock with the help of tugs.

Finally to summarize the process of bringing a ship to the dry dock,  here is an Infographic

Infographic: Taking a ship to dry dock

Conclusion

Dry docking is a great experience for those who do not want to stop learning. The special thing about dry docking is that there are plenty things that a seafarer can learn each time he attends a dry dock.

While it might seem to be a difficult process but if we view the whole process logically, it would seem a routine.

If you are going to a dry dock, Enjoy the learning process. And let me know in the comments below if there was anything that should form a part of this guide.

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Capt Rajeev Jassal

About Capt Rajeev Jassal

Capt. Rajeev Jassal has sailed for over 19 years mainly on crude oil, product and chemical tankers. He holds MBA in shipping & Logistics degree from London. He has done extensive research on quantitatively measuring Safety culture onboard and safety climate ashore which he believes is the most important element for safer shipping.

49 Comments

Jayraj
Jayraj
Apr 11, 2016

A wonderful explanation Sir , though I haven't got a chance to be at drydock , but I could Imagine the process .. Thanks.

Rajeev Jassal
Rajeev Jassal
Apr 22, 2016

Glad that you liked it Jayraj

Anooj pradhan
Anooj pradhan
Sep 11, 2016

Excellent explanation sir

Rajeev Jassal
Rajeev Jassal
Nov 26, 2016

Glad you liked it Anooj..

rag
rag
May 10, 2016

Loved your web blog and its excellent.

Rajeev Jassal
Rajeev Jassal
May 10, 2016

Glad that you liked it Rag..

Lokesh
Lokesh
Jul 5, 2016

Very informative and useful.

Rajeev Jassal
Rajeev Jassal
Jul 5, 2016

Glad you found it useful Lokesh..

John
John
Jul 15, 2016

Although the principal of testing a bottom plug is the same, the video above is actually a vacuum test to ensure that a transducer head (most likely an Echo Sounder) is properly sealed.

Rajeev Jassal
Rajeev Jassal
Jul 16, 2016

You are absolutely right John.

Sanjay Waghmode
Sanjay Waghmode
Jul 17, 2016

Awesome sir, i found it very much useful during drydock of NCC ABHA. It was my first drydock.

Rajeev Jassal
Rajeev Jassal
Jul 18, 2016

Glad you found it useful Sanjay. I hope the dry dock of ABHA went all well...

Amin
Amin
Sep 5, 2016

Excellent explanation sir.

Rajeev Jassal
Rajeev Jassal
Sep 7, 2016

Glad you liked it Amin..

ankit vyas
ankit vyas
Sep 6, 2016

sir pl post article about legal requirement during dry dock for chief mate candidates this blog is really helpful for orals

Rajeev Jassal
Rajeev Jassal
Sep 7, 2016

I will include that in one of the future blogs Ankit..

VS Ramamurthy
VS Ramamurthy
Sep 11, 2016

Dear Rajeev Sir: I was surfing through for Keel Blocks and types of keel blocks for dry docking, and stepped over your blog. Its amazing to know the facts, and its the same even with us in dockyard. I have been lucky to see a ship docking and undocking. Can you share some of details on keel blocks, it would be a great help to me. Regards, Ramamurthy, Visakhapatnam, India

Rajeev Jassal
Rajeev Jassal
Oct 16, 2016

Glad you found it useful Ramamurthy. I have one post on ship structures. Hope that could be of some help.

abhimanyu
abhimanyu
Sep 11, 2016

very informative and great article for orals prep !

Rajeev Jassal
Rajeev Jassal
Sep 15, 2016

Glad you liked it Abhimanyu..

muangmon.j
muangmon.j
Sep 15, 2016

that is good share for one

Rajeev Jassal
Rajeev Jassal
Sep 15, 2016

Thanks..

jaison
jaison
Sep 19, 2016

very informative article sir..helped a lot

Rajeev Jassal
Rajeev Jassal
Oct 9, 2016

Glad you liked it Jaison,,

Mohammed Alhassan
Mohammed Alhassan
Oct 2, 2016

A very good and concise explanation especially for crew going to dock for the first time.

Rajeev Jassal
Rajeev Jassal
Oct 9, 2016

Thanks Mohammed..

Swapnil Ogale
Swapnil Ogale
Oct 13, 2016

Thank You very much sir for this wonderful article. Immensely helped me while answering my mates orals. Had never been to a drydock but could visualize perfectly.

Rajeev Jassal
Rajeev Jassal
Oct 16, 2016

Glad that it could be some help to you Swapnil..

Abhishek Kumar
Abhishek Kumar
Oct 18, 2016

Thanks a ton for the article sir.Can you tell me the name of the instrument which does vacuum test of bottom plug?

Rajeev Jassal
Rajeev Jassal
Oct 28, 2016

There is no special name for this. It is simply called "Leak Test Vacuum box".

Prashant kr. Rai
Prashant kr. Rai
Oct 20, 2016

Sir can u pls tell me how come dock master will know that ship bottom had touched the blocks,What are the methods?

Rajeev Jassal
Rajeev Jassal
Oct 28, 2016

He knows the height of the blocks and when the draft of the ship is equal to this height, he would know that ship has touched the blocks. In case of floating dock, he will notice the additional weight on the online loadicator of the dock.

Satish
Satish
Oct 28, 2016

Sir, The way you explained is very simple and one can parallely imagine....just got the feeling that I was physically witnessing the whole scenario.....felt good reading....Thank u

Rajeev Jassal
Rajeev Jassal
Oct 28, 2016

Thanks Satish. Glad you found it interesting..

Soumen Guha Roy
Soumen Guha Roy
Nov 1, 2016

Sir,this an awesome post as the others from you,very helpful for orals.Can you post something simple regarding stability,it would be quite helpful.Thanks.

Rajeev Jassal
Rajeev Jassal
Nov 2, 2016

Glad you liked it Soumen. I will write on stability soon..

mohsen salmanpour
mohsen salmanpour
Nov 4, 2016

Hi. excellent explanation. i have a question about dock master responsibility for any accident happened for the ship , dock crew and dry dock itself during docking & un-docking the vessels specially if during this procedure fatal accident causing human life to be lost. thanks a lot

Rajeev
Rajeev
Nov 11, 2016

It is not the dock master alone that would be responsible but the dry dock company. In case of fatal accidents, the ship owner might appoint a maritime lawyer and make monetary claims for the family of the seafarer.

Neeraj
Neeraj
Nov 26, 2016

Excellent blog sir, very informative, very practical , beautifully explained. I had few doubts about it, got cleared. Thanks a lot sir

Rajeev Jassal
Rajeev Jassal
Nov 26, 2016

Glad you liked it Neeraj..

jourlan eballar
jourlan eballar
Dec 19, 2016

Sir when to do the de-ballasting once the Ships is already inside

Rajeev Jassal
Rajeev Jassal
Dec 29, 2016

Dock master will advise when to start the deballasting. It is started when ship is aligned to the blocks. Dock master will advise to keep the minimum draft to keep the A/E running. When the shore supply is connected he will advise to keep on deballasting.

dani
dani
Jan 4, 2017

Good day sir. Maybe i'm off the point a bit but what usually happens with staff members during a dry dock period of 2 to 3 weeks? For example on a cruse ship with hundreds of employees from all over the world.

Rajeev Jassal
Rajeev Jassal
Jun 23, 2017

The required staff stays onboard. I do not know about cruise ships but I guess the catering staff and all other staffs required for passengers are sent on leave and staff responsible for the operations of the ship remains on board.

Dhiren Chhaya
Dhiren Chhaya
Feb 11, 2017

Excellent blog....Sir. Accidentally stumbled upon this blog and though not connected with Marine Engg. ; enjoyed reading.. Am basically a Mech. Engr.... enhanced knowledge.

Rajeev Jassal
Rajeev Jassal
Jun 23, 2017

Thanks, Dhiren...

Adriana Angulo
Adriana Angulo
Jun 21, 2017

I'm glad to find this blog, I loved your complete explanation, it made it all clear for me . Thank you.

Rajeev Jassal
Rajeev Jassal
Jun 23, 2017

Glad you liked it, Adriana...

SHIVEN
SHIVEN
Jul 16, 2017

m making presentation on dry dock u made my job a piece of cake with this data .

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