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Dry Docking of Ships – Understanding Stability And Docking Plan
By
Soumya Chakraborty
April 10, 2021
Dry Dock, Naval Architecture
The process of docking and undocking of ships might not seem like an important operation. However, it’s a
process that is carried out more than once, not only by shipbuilding yards during the construction of a ship, but
also as regular part of the ship’s lifetime. The understanding of the process of docking is specialised, and hence,
not many naval architects or engineers are thorough with the inner details of docking. Professionals who are
specialised in carrying out docking and undocking of ships are commonly known as Dock Masters.

But why is it important for ship designers to know about the docking processes? It is because, the process of
docking is done by the aid of a series of drawings and plans, which are prepared by designers, based on certain
calculations. Hence, knowing docking processes, docking calculations, and understanding how to read docking
plans is an important skill for designers working in shipyards.

From time to time, it becomes important to carry out repairs in the underwater portion of the hull. Such repairs
may include renewal of the sacrificial anodes, refit of the propellers, overhauling of the propulsion shafts, repair
of rudders, underwater hull blasting to remove fouling, etc. In order to carry out these repairs, the underwater
portion of the hull needs to be made accessible, which is the purpose served by a dry dock. It has also become a
common practice in large shipyards to build their ships on dry docks, and float it out when ready for trials. For
such procedures, the docking plans need to be prepared taking into consideration the increase in weight of the
ship structure along the building time. Once the ship has been built, the dry dock is flooded and the ship is
undocked. The calculations for undocking also play a major role in the process because it is during undocking that
the the ship is at a risk of capsizing.

Stability during Docking:

When the ship enters a dry dock, it must have a positive metacentric height; and is usually trimmed by stern. The
floor of the dry dock is lined with keel blocks, which are so arranged such that they can bear the weight of the
ship. When the ship enters the dry dock, her centerline is first brought in line with the centerline of the keel
blocks by using a combination of plum lines and Leica theodolite.

The dock gates are then closed and the water is pumped out of the dock in stages. Since the ship has a trim by
stern, the stern of the ship will first sit on the keel blocks. The rate of pumping out water is reduced as the stern is
almost about to touch the keel blocks. The reason is, it is from this stage of the docking procedure when the
stability of the ship starts getting critical. The interval of time from when the stern takes the blocks to the moment
when the entire ship’s weight is borne by the blocks is called Critical Period. We will understand the details a little
later.

When the stern of the ship takes the blocks, it is fixed to the shores (sides of the dock). This is carried out from aft
to forward so that by the time the entire ship takes the blocks, it is fixed to the shores. When the ship is
completely borne by the blocks, water is pumped out quickly from the dock.

So what happens during the critical period and why?

When the ship’s stern just touches the keel blocks, part of the ship’s weight is being borne by the keel blocks. The
contact between the stern and the keel block creates a normal reaction or upthrust. The magnitude of this
upward normal reaction increases as the water level in the dry dock reduces. It is this upthrust that creates a
virtual reduction in the metacentric height of the ship. Hence it is very crucial to maintain sufficient positive
metacentric height before docking, lacking which, the ship may heel over to either side, or even slip off the keel
blocks and capsize.
Related Read: Ship Stability – What Makes a Ship Unstable?
The purpose, hence, is to calculate the metacentric height of the ship at different stages of the docking process,
and ensure that it does not fall below the safe limit. Follow the figure underneath, which shows a ship that has
just touched the keel block by its stern. The location of the center of floatation (F) is known from the hydrostatic
curves at the given displacement. Since the location of the stern is a known point, its distance from the center of
floatation (l) can be calculated instantly.

ship during dry docking

Figure 1: Longitudinal view (elevation) of a ship during dry docking.
The moment to change trim by 1 cm (MCTc) is a hydrostatic parameter that is obtained from the hydrostatic
curves. So, for a known value of trim, the following equation is obtained:

CG critical period
Figure 2: Transverse view for critical period during dry docking.

The above figure shows the transverse view of a ship in the critical period, which has been inclined by an external
force to an angle theta (Ɵ). The weight of the ship (W) acts vertically down through the center of gravity (G). The
upward reaction force (P) acts vertically upwards through the keel of the ship. This is a normal reaction force, and
is equal to the portion of the weight of the ship being borne by the keel blocks. For equilibrium, the remaining
portion of the weight of the ship (W-P) will be supported by the buoyancy, which will act through the initial
metacentric height of the ship (M).

Related Read: Intact Stability Criteria & Inclining Experiment

What we have now, are three vertical parallel forces acting on the ship:

Weight (W) acting downward.

Keel block upthrust (P) acting upward.
Buoyancy (W-P) acting upward.
The upthrust force (P) can be considered to have an effect similar to that of removal of a weight from the ship.
This has the virtual effect of rising the center of gravity of the ship from the point ‘G’ to ‘G1’. The metacentric
height therefore reduces from GM to G1M, as shown in the diagram below:

CG during dry docking

Figure 3: Virtual rise in CG during dry docking.
The virtual reduction in metacentric height at any stage of the docking process can be calculated by the following
expression:

loss of metacentric height

This calculation must be carried out for the condition when the ship has just touched the keel blocks throughout
its length. It is at this point that the keel block upthrust is maximum, and the risk of tipping over or slipping from
keel blocks is most likely if the metacentric height is too low or negative.

Docking Plans:
A docking plan is a document that is prepared for every ship during its preliminary design phase. All the
information required to bring a ship to a dry dock are included in its docking plan. While most of the information
is condensed into drawings, one must also refer to the textual references and notes provided, because they also
inform us about the type of dry dock that is being used, and technical specifications of dry dock that should be
met before the ship is docked.

The contents of a typical docking plan are discussed below:

Table of Hydrostatics and Hydrostatic Curves:

The first few hydrostatics that must be checked before a ship enters a dry dock are:

Forward draft.
Aft draft.
Longitudinal Center of Buoyancy.
Moment to Change Trim by 1cm.
Center of Gravity.
Longitudinal Center of Floatation.
Transverse metacenter.
Since it is not preferred to provide the information in an easily obtainable manner, a tabular form of hydrostatic
data is more preferable than the curves. In case of intermediate values, interpolation methods are used, and then
tallied with the curves.

Docking Drawing – Elevation View:

The elevation view of the docking drawing gives the following information:

Location of the Longitudinal Reference Point (LRP), i.e. the point from which all the longitudinal dimensions are
measured.
Location of Aft Perpendicular and Forward Perpendicular.
Location of the end of skeg.
Frame spacing.
Longitudinal clearance required for removal of shaft.
Longitudinal clearance required for removal of rudder.
Location of draft marks along the ship.
Location of the first and last keel blocks.
Docking Drawing – Plan View:
The plan view would show the location of the keel blocks along the centerline of the ship. Most large ships with
wide beam are also placed on a series of side blocks, and the layout of the same is laid out in the plan view. The
position of every hull opening and hull protrusions (both, above and below the waterline) are also marked in this
drawing, in order to make sure they do not interfere with the dock blocks.

Cross Section at Propellers:

Sufficient clearance should be ensured between the propeller tip and the dock floor. There should also be
adequate longitudinal and transverse clearances for enabling removal of the propeller. It is due to this reason, the
cross sectional view of the propeller (usually looking forward) is provided in the dock plans. The centerline of the
ship, centerline of the propeller disc, distance between the ship’s centerline and the propeller centerline, and keel
line of the amidship is also shown in the same diagram, as illustrated below.

propeller clearances
Figure 4: Propeller clearances in a docking plan.
Keel Profile:
The keel profile shows the elevation of the keel line along the ship’s length. This profile is used to determine the
height of the keel blocks at each longitudinal position, taking into consideration the load distribution curve of the
ship.
Bilge Keel Clearances:
The bilge keel extends as an appendage from the sides of the hull. The distance from the bilge keel to the ship’s
centerline must be specified in the docking plan. The height of the bilge keel from the ship’s keel is used to ensure
there is sufficient vertical clearance available during docking and undocking.

Related Read : Understanding Intact & Damage Stability Of Ship

Load Distribution and Block Pressure:

Since the keel blocks bear the weight of the ship, the load distribution curve for the keel blocks is derived from
the weight curve of the ship, which is comprised of a combination of distributed and concentrated weights. The
weight of the hull girder and superstructure are distributed along the ship’s length. But weights like that of
machinery, equipment, transverse bulkheads, fuel oil, fresh water are considered as concentrated weights.

Weight Curve of a ship

Figure 7: Weight Curve of a ship.

The figure above shows the nature of the weight curve for a ship (shown in blue). In case of dry docking, the
buoyancy curve becomes non-existent. However, there is an important change that is made in the weight curve. If
you notice carefully, when a ship is afloat, it is supported by buoyancy throughout its length. But a ship on a dry
dock is not supported by keel blocks throughout its length. This is because of the shape of the ship’s hull. Notice
in the figure below, that a significant length of the ship forward and aft are not supported by the keel blocks
directly, due to the overhangs at the bow and stern.

Keel bearing length

Figure 8: Keel bearing length and Overhang.
Though the weight of the overhang regions is not directly exerted on the keel blocks, the total weight of the ship
is supported by the total area of the keel blocks. To compensate for this effect, the weight curve of the ship is
accordingly changed so as to transfer the weights of the overhang regions onto the region supported by the
blocks (also called keel bearing length).

The maximum pressure that can be exerted on the blocks is a function of the material used for the blocks. This
value being a constant, the minimum block area required for each block is calculated. If you observe the nature of
the weight curve, it is usually high at the mid ship region and decreases at the forward and aft. It is due to this
reason that the weight bearing area of the keel blocks increase as we move towards the mid ship. The pressure
exerted on the keel blocks is called the block pressure, and the Average Block Pressure is the total weight of the
ship divided by the total bearing area.

Now, docking plans are created for every ship taking into consideration the dry dock where it is likely to be dry
docked during most of its major repair and refits. But there may arise situations where a vessel is to be dry
docked at a different dock, where the docking plan is to be modified to suit the dry dock. There are a number of
conditions that must be met in order to prevent any structural failure in such cases:

The number of blocks may be different from the original docking plan, but the total bearing area must be
sufficient to maintain the block pressure below the material safe limits. This ensures that the hull does not have
excessive loads that could cause damage to itself.

The floor of the dock has its own strength limits, which depends on the material of the dock floor. This makes the
load on the dock floor per unit length a limiting parameter too. Now, if any block is removed or repositioned from
the original plan, the load on the dock floor per unit length must be recalculated and checked for the given factor
of safety.
The dock floor is strengthened underneath by transverse frames that run along the breadth of the dock. When
keel blocks or side blocks are repositioned from the original dock plan, it should be ensured that the new position
of the block is such that it comes under a strength bearing member of the ship (bulkhead, longitudinal girder, etc.)
and also sits on top of a dock floor transverse. This is to ensure that there is a proper stress flow from the ship’s
hull to the keel blocks, and finally to the dock floor.
It must be checked that the new positions of the keel blocks do not interfere with any underwater opening or
protrusion. The heights of the new blocks should be calculated by interpolating the values of the two closest
values from the original docking plan.
side blocks
Figure 9: Positioning of side blocks on strength bearing members.

It is evident from the article, that the calculations and analyses that go into creating docking plans and executing
the process require equal attention from aspects of stability, as well as the strength of the ship. Most of the
failures in dry docking or undocking of ships have been due to

Improper evaluation of loading of the ship.

Improper loading of the ship during docking (concentrated weights like fuel oil, lube oil, fresh water, ballast
water, etc. should not be stored in overhang regions during dry docking).
Improper stability assessment of the ship during the critical period.
While these are the primary reasons, there are a series of many which can only be discussed in detail in courses
that are designed specifically for dock masters. Today, dock masters use software to pre-determine the docking
plans of ships, and the results are then tallied with the existing docking plan. Even then, this procedure remains
one of the most crucial ones in the ship-building and ship repair industries, owing to its demand for extremely low
margin for error.

Further Reading:

Ship Stability : Damage Stability Of Ships

Ship Stability: Understanding Curves of Static Stability
What is Extended Dry-Docking of Ships?

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Do you have info to share with us ? Suggest a correction

About Author
Soumya is pursuing Naval Architecture and Ocean Engineering at IMU, Visakhapatnam, India. Passionate about
marine design, he believes in the importance of sharing maritime technical knowhow among industry personnel
and students. He is also the Co-Founder and Editor-in-Chief of Learn Ship Design- A Student Initiative.

Read More Articles By This Author >

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Post Tags:
#dry dock
BE THE FIRST TO COMMENT
20 Comments
AHMAD says:
February 11, 2017 at 1:22 am
ITS VERY FANTASTIC

kailash dahiya says:

September 26, 2017 at 8:53 am
it was very easy to understand….thx for such a great blog

Anish says:
October 2, 2017 at 9:38 pm
Glad it is useful to fellow seafarers.

Ashok Chaudhary says:

December 31, 2017 at 2:11 pm
1-why ship enters in dock with trim by stern .
2- how to paint the area between keel blocks and keel.

Anish says:
January 18, 2018 at 12:51 pm
@Ashok: Use our forums to ask your exam related queries: https://forums.marineinsight.com

Jason says:
May 17, 2018 at 3:06 pm
It is a great blog ,THKS!

Is there any rule or instruction for ship docking and undocking?

Anish says:
May 19, 2018 at 8:38 am
@Jason; Thank you for your kind words.

The docking/ undocking procedure will vary on size and type of the ship. There may be local regulations / yard
requirements which you need to follow. There are no broader rules for docking /undocking.

Jason says:
May 25, 2018 at 2:11 pm
@ Anish; thanks for your reply, is there any local regulations / yard requirements for ship docking/undocking
available in your hand? will you pls sent to me if you can do me a favor?
email box: wangxiaohua@cmhk.com

Srini says:
April 27, 2019 at 10:42 am
Amazing article and was very interesting to read. Engineering is an amazing field. Thabks for the article and the
level of detail.

Anish says:
April 30, 2019 at 1:31 pm
@Srini: Thank you. Glad you enjoyed reading it.

Baghyarajan Muralikrishnan says:

May 3, 2019 at 12:44 am
Hi Anish, Informative and In simple words. Well Done. Thanks.

Gbadamosi Idris says:

June 12, 2019 at 5:10 am
Thanks so much for the rightful information on dry docking,very explanatry.

Anish says:
June 12, 2019 at 3:50 pm
@Gbadamosi: Glad the information is helpful. ????????

Sanjiv Yadav says:

June 20, 2019 at 9:23 pm
I just wanted to know how block area is painted which supports the vessel?
Also what is Drydock Version 1 & 2?

A. Trivedi says:
July 30, 2019 at 11:54 pm
Hi Anish, well explained. Pls show the reverse act during un-docking when ship begins to float & recreation of COB
upward

Anish says:
August 8, 2019 at 1:16 pm
@A.Trivedi: Well noted your suggestion. I will definitely pass the topic to my editorial team. Thank you. ????????

Heri K. Mwaruka says:

June 14, 2020 at 11:16 pm
This has covered almost everything one will need to know. Thanks for the article.
Will you please prepare another article on UN DOCKING OF A SHIP?
All the best

Anish says:
June 29, 2020 at 10:54 am
@Heri:
Thank you for the suggestion –
Sure. Meanwhile, please check this article – https://www.marineinsight.com/guidelines/checks-to-perform-on-
ship-before-coming-out-of-the-dry-dock/

umairrao says:
February 6, 2021 at 4:57 am
Hi. Very nicely brief one more question is that if tanker Is in loaded and had to proceed dry dock with cargo
onboard what precautions we have to take kindly inform

Subhash Gupta, says:

March 26, 2021 at 5:17 pm
Hi
How is the weight coming on each keel block determined. Is it measured using load cells or by other means?

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