understanding damage stability

Have you been breaking your head to understand damage stability?

Not only this topic is difficult to understand, it is boring too. I know and I understand.

But boring topic cannot be the explanation of not knowing what we ought to know in the real world, on the high seas.

In 2010, Paris MOU and Black Sea MOU carried out a concentrated inspection campaign on tanker damage stability.

At the end of this campaign, Pat Dolby, co-ordinator of this CIC commented,

The most significant finding from the campaign was that 16.2% of tankers that were inspected, the master could not demonstrate that the ship was complying with damage stability. 

16.2%. That is a huge number that definitely shows that there is a void in understanding this topic.

So today, let us discuss and demystify this topic of damage stability.

Why damage stability?

Do you remember that scene from the movie “Titanic”, where after the ship hit the iceberg, the naval architect lays down the ship’s plans in front of the captain?

While he was trying to brief the captain about the situation, the captain just had this one question.

How much time do we have?

titanic sinking damage stability

And he said, an hour, two at the most.

How did he arrive at this number and how was he sure that an unsinkable ship is going to sink?

I will answer that question later but that is what damage stability is all about. All we are interested in is knowing that after an incident if the ship will remain afloat or sink?

All the rules about damage stability are trying to keep the ships safe even after one or more compartments are breached.

Consider this.

You have two ships both identical in every respect but the first one has only one tank (or cargo hold) and the second one has two tanks (or cargo holds).

Which one do you think is safer?  Easy answer, right?

The one with the two cargo tanks.

This is because if one compartment is flooded, the first ship will have 100% of the cargo space flooded. The second ship would still have 50% of the cargo space intact.

What I am trying to prove here is that more the subdivisions of the ship, safer it would be.

But the ship owners cannot divide the cargo spaces in 20 or 50 compartments. This would mean the use of more steel, more money to build the ship and lesser cargo space to use.

Shipowner cannot build a ship with just one compartment either. That is too unsafe.

So how many minimum subdivisions a ship must have?

All in all, there are three approaches to building a ship that can withstand damage to its compartments.

All the three approaches for damage stability are just aiming to find that answer. So let us discuss each of these approaches of damage stability.

1. Floodable length and factor of subdivision

This is an old approach but it is still important to discuss this because this approach lays the foundation to understand damage stability.

In this approach, the number of subdivisions required is calculated by knowing the floodable length along the ship.

Floodable length is the length of the compartment which if flooded will cause the ship to sink up to the margin line.

Let us understand this by building a ship.

We have a ship and we need to put subdivisions (bulkheads) to it to divide the ship into compartments.

We create one compartment in the midship by placing two bulkheads (let us mark this bulkhead as A & B).

Floodable length

The length of this compartment (Length AB) need to such that if this compartment is flooded, the ship will sink to a point where margin line is just submerged.

This is the floodable length at this point.

Now we want to place another bulkhead aft of midship. Again this bulkhead needs to be at a location (C) such that if compartment AC is flooded, the ship will sink to a point where margin line is just submerged.


And with this same approach, we can decide the location of other bulkheads along the ship’s length.

Floodable length 3

When calculating the floodable length, One thing that we need to keep in mind is that we need to flood the compartment to the full width of the ship even when we have or plan to have a centerline bulkhead.

Floodable length Curve

Our ship is ready now with all the compartments it needs. This ship would not sink if any one compartment is breached and flooded.

But if you would have noticed I have drawn larger compartment in the midship area. This means that I have shown large floodable length closer to the midship area.

This is because if the midship compartment is flooded, the ship will sink bodily (with least trim).

But as we move away from the midship, the flooded compartment will trim the vessel. This would make a smaller compartment to sink the ship up to the margin line.

margin line submerged floodable length

So the bottom line is that the floodable length changes along the length of the ship.

Floodable length curve represents the maximum floodable length of the ship along the ship’s length. This curve is obtained by vertically plotting the floodable length along the ship’s length.

floodable length curve plotted

Checking the damage stability compliance: Floodable curve method

So far I have given the basic idea of what floodable length is and how floodable length curve is obtained.

Ships, that are required to comply with this method of damage stability would be provided with the floodable length curve.

The damage stability rules for the ships would be something like…

The ship should be able to survive the breach (flooding) of any one (two or three) compartment.

To check if the ship would comply with this damage stability requirement, the floodable length curve is superimposed on the ship’s plan.

Then one compartment by one, the damage stability compliance is checked. The length of the assumed damaged compartment is plotted vertically at the center of the compartment.

If this length is below the floodable length curve, this compartment complies with the damage stability requirements of one compartment standard.

Floodable Length Curve compliance

Same is done with other compartments.

floodable length curve damage stability compliance

As we can see, all the length triangles are within the floodable length curve of the ship. This means that this ship complies with one compartment standards of the damage stability.

Now let us check the damage stability compliance for two compartment standard. In this case, we will assume the flooding of two compartments and compare the length triangle with the floodable length curve of the ship.

Damage stability compliance with Two compartment standard

Again same is done assuming flooding of any two adjacent compartments.

floodable length curve with two compartment

Clearly, this ship does not comply with damage stability requirements of two compartment standards.

If we need to comply with two compartment standards, this ship needs to have more compartments, the length of which need to be such that even when two compartments are flooded it will be below the floodable length curve.

Maybe the below subdivision of the ship will be able to satisfy the damage stability requirement for two compartment standard.

Floodable length curve compartments

Let us check the damage stability compliance to two compartment standard.

damage stability compliance two compartment standard

As you can see, this ship is a two compartment standard ship now.

We can go on in a similar way if we want to build a three compartment or four compartment ship.

Remember, Titanic was a four compartment ship and so was called the unsinkable ship.

Finally, if you are still unsure of this concept, watch this video.


2. Damage stability: Probabilistic damage assessment

Damage stability calculations by probabilistic damage assessment is required by SOLAS Chapter II-1, part B. This is required for cargo ships 80 m in length and upwards and to all passenger ships regardless of length.

This approach uses the concept of probability to ensure that ships can survive damage to its compartment(s).

There are two probability factors that are used in this approach.

  • Probability that a particular compartment(s) will damage in an incident (factor “p”)
  • the probability that ship will survive if that compartment(s) is flooded (Factor “s”)

Used as the requirement for the cargo ships and passenger ships.

Multiplying these two factors (p x s) will give the probability of surviving that damage case.

Let us again take our 8 compartment ship and calculate the probability of surviving damage to one compartment.

probability of one compartment flooding

Now we need to calculate the probability of surviving two compartment damage.

probability of surviving two compartments

While it may seem repetitive but let us also calculate the probability of surviving three compartment damage.

probability of surviving three compartments

The value of S in all these will either be 0 or 1. This is because when we have considered a damage, the ship will either survive (probability 1) or not survive (probability 0).

So if this ship is three compartment ship, there is no need to consider the probability of survival for four and more compartments because it will be zero.

But there is still one thing to consider. At what drafts we need to consider all these damages?

SOLAS requires that these should be considered at three drafts.

  • Deepest subdivision draught (ds): Which corresponds to the Summer Load Line draught of the ship.
  • Light service draught (dl): Service draught corresponding to the lightest anticipated loading and associated tankage, including, however, such ballast as may be necessary for stability and/or immersion.
  • Partial subdivision draught (dp):  light service draught plus 60% of the difference between the light service draught and the deepest subdivision draught.

So, for example, all these three tables I made above need to be made for these three initial (before damage) drafts of the ship.

So for deepest subdivision draft we will have

Probability of survival at draft ds

For Light service draught (dl),

Probability of survival at light draught dl

And finally, for partial subdivision draft, we will have

Probability of survival at partial subdivision draught dp

3. Damage stability compliance: Probabilistic method

Finally the bottom line. How would a ship comply with the damage stability requirements?

As per SOLAS Chapter II-1, part B-1, Regulation 6, the ship complies with damage stability when

Attained Subdivision Index > Required subdivision index 

Attained Subdivision Index

As per SOLAS, attained subdivision index is calculated by the formula

Attained subdivision index

Required Subdivision Index

SOLAS chapter II-1, Reg 7 gives the formula to calculate the required subdivision index for a ship.

These formulas are different for different type and size of the ship.

Required Subdivision Index


This would be the minimum required value of subdivision index.

If the actual value of subdivision index (Attained value) is less than the required, the subdivisions need to be re-arranged or increased to have attained subdivision index to be more than required subdivision index.

Damage stability by Deterministic damage assessment

Damage stability calculations by this method is required for all types of tankers.

Unlike probabilistic method that uses the concept of probability, the deterministic method defines the variables in quantifiable terms.

In this method,

  • the damaged area is defined (damage assumption); and
  • The minimum required value of the stability factors is defined (Survival requirements)

In all the cases of damage assumptions, the vessel should have the stability factors value more than the survival requirements.

Let us take the example of IBC code that sets the rules for the chemical tankers.

Damage assumptions as per IBC code are

1. Extent of damage

This defines the extent the hull of the chemical tanker needs to be assumed damaged.

Damage assumption (Extent of damage) IBC code


2. Flooding assumption

This defines the flooding assumptions that need to be considered after the assumed damage to the hull of the chemical tanker.

flooding assumptions IBC code


3. Standard of damage

The dimensions of assumed damage are considered in the “extent of damage” section. Standard of damage defines the assumed location of the damage along the ship’s length.


Survival Requirements as per IBC code

We have considered all the damage assumptions required as per IBC code.

In all the possible cases as per the damage assumptions, the ship should survive.

But in the deterministic approach, survival does not just mean that ship should not sink. The deterministic approach gives the minimum stability criteria values that the ship must have with assumed damage as defined.

As per IBC code, these survival requirements are for two phases of flooding.

  • In any stage of flooding
  • At final equilibrium after flooding

In any stage of flooding

survival requirement IBC Code damage stability 1

At final equilibrium after flooding


Damage stability compliance: Deterministic Approach

With probabilistic approach and floodable length curve, the damage stability compliance is dealt with at the stage of construction of the ship.

But ensuring compliance with the deterministic approach is different.

In the real world, there can be endless combinations of loading conditions of a ship. In each of these loading conditions, we need to apply the damage assumptions.

We then need to check if the survival requirements as defined by the IMO in various conventions are satisfied.

Off course, all these cases cannot be documented and checked during construction stages.

Instead, the damage stability criteria is checked for most probable loading conditions.

But during normal ship operations and before loading, the chief officer need to check and confirm that damage stability criterions are met.

How to check if the proposed stowage plan satisfies the damage stability requirements?

Well, there are few methods to check this but I will have look ahead approach here. As per the new requirements, the loadicators fitted on tankers need to have damage stability calculation capabilities.

So before a stowage plan is finalized, we need to check from the loadicator if this stowage satisfies the damage stability requirements.

If not, the chief officer needs to make required amendments to the stowage until the damage stability requirements are met.


If we are not checking the damage stability of the ship, not only we are risking the environment but we are risking our lives too.

It is so important that ships are able to survive any damage sustained during the adventures it carries on the high seas.

The first step toward complying with the damage stability is to understand what it is and what is required of us.

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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.

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Jul 9, 2017

Wonderfully explained sir, I wish many more various topics from you as you always do the explanations in very different ways,example based, thanking you

Rajeev Jassal
Rajeev Jassal
Jul 19, 2017

Glad you liked it Indranil...

Avinash Nayak
Avinash Nayak
Jul 19, 2017

Great explanation Sir...It cleared the concept regarding damage stability and how to calculate. Kindly advise, how we will know whether the ship is constructed as per probablistic method or deterministic method?

Rajeev Jassal
Rajeev Jassal
Jul 19, 2017

Ships are built as per the requirements of SOLAS chapter II. The damage stability requirements in SOLAS chapter II are based on probabilistic approach. However, the oil tankers, chemical tanker, and gas tankers are required to be built as per the requirements of MARPOL Annex I, IBC Code and IGC code respectively. These damage stability requirements are based upon the deterministic approach.

Jul 22, 2017

Appreciate your effort sir, kindly advise if cross flooding arrangements is only for passenger vessel or for cargo vessel also, and when is it required, which tanks should have this arrangements, TIA.

Rajeev Jassal
Rajeev Jassal
Jul 29, 2017

Cross flooding arrangements are applicable for passenger ships though it is not compulsory. Passenger ships just need to comply with the damage stability requirements as I have discussed in this post. Having cross flooding arrangements increases the chances that passenger ship would comply with the damage stability requirements. For the areas for cross flooding, it is the decision of the ship owner and naval architect designing the ship. For example, considering no cross flooding arrangements the passenger ship might be failing one damage case (Say damage to No 4 compartment) for damage stability requirements. By having cross flooding arrangements for No 4 compartment may cause the ship to comply with the damage stability requirements for this damage case. Or shipowner may choose to have the cross flooding arrangements for the entire length of the ship. If the ship complies with the damage stability requirements without cross flooding arrangements, they may choose to not fit the cross flooding arrangements. The only requirement is to comply with damage stability, with or without cross flooding arrangements does not matter.

Anant Batra
Anant Batra
Jul 29, 2017

Thanks for clarifying that sir.

Harsh vardhan Anand
Harsh vardhan Anand
Jul 29, 2017

I am going for my mates written exam next week. It is a very nice explanation. Found it very helpful. Is there any other platform where i could contact you to clear my doubts in future ?

Rajeev Jassal
Rajeev Jassal
Jul 29, 2017

Glad you found it useful Harsh... If you have any question you can post it on our SEAQA section, this way fellow seafarers can also benefit from your questions and the answers to those questions. Alternatively, you can send me an email at support@myseatime.com. On average I get 10-15 emails a day and sometimes there isn't enough time in a day to respond to all of them but I try my best.

Aug 5, 2017

If loadicator is not working how I will ascertain that on departure the tanker vessel is complying with damage stability requirements

Rajeev Jassal
Rajeev Jassal
Aug 5, 2017

We need to check the KG curve in the damage stability booklet. The curve plotted with KG on one axis and draft on the other axis. We need to take our draft and see the maximum KG allowed from the curve. Ship's actual KG should be less than this value. On many ships (specially the old ships) this KG curve may not be there. Owners need to contact Yard and ship's class for these curves.

Anurag Choudhary
Anurag Choudhary
Aug 5, 2017

Thanks a lot sir!!

Aug 5, 2017

Thanks so much with your explanation ...I study naval architecture ...I have a question about how to calculate the floodable lentgh in theory form ...I know a method of iteration but I don't undertand how to start ..I need to asum the draf, the trim ...if you know a reference that explain that I would apreciate it!!!

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