Damage Stability

[stalin]

Hi Everybody,

I am doing damage stability calculations as per MARPOL, Reg 28, for a double hull oil tanker. MARPOL says, for any operating draught for full or partial loaded conditions, specified damages to be applied.

Can any one please clarify whether only partial loading condition (100% cargo and 10% consumables) is enough or both full and partial loading conditions are required ?

Thank you in advance.

[Guillermo]

Unified Interpretation 46 - Operating draught
With regard to the term “any operating draught reflecting actual partial or full load conditions”, the information required should enable the damage stability to be assessed under conditions the same as or similar to those under which the ship is expected to operate.

Previous Unified Interpretation 46 - Operating draught (applies to vessels constructed between 1979 and 1984)
With regard to the term “any operating draught reflecting actual partial or full load conditions”, the information required should enable the damage stability to be assessed under conditions the same as or similar to those under which the ship is expected to operate. For this purpose, until 2 October 1984 , in addition to the calculation for the full load condition, a limited number of partial load conditions should be investigated. After 2 October 1984, more detailed information covering partial operational load conditions would be required depending upon more varied operational demands.”

[stalin]

Thank you Guillermo.

Somewhere in the rule interpretations of MARPOL, I read, damage is directly applied to all the loading conditions in the approved Intact stability manual.

However, now with the fully loaded departure initial condition, some lesser extent damages are not meeting the required damage stability criteria.
Due to the higher angle of heel, range of stability is poor (below 20 deg) beyond the position of equilibrium. Down flooding angle is only 35 deg, which is the reason for poor range.

With this scenario, initial loading condition cannot be changed as it was approved already and the position of downflooding point cannot be altered since it is Engine room air inlet.

Can you please advise what could be corrective measure to improve range of stability? Please help.

[Guillermo]

What do you mean by "range of stability is poor (below 20 deg) beyond the position of equilibrium". May you please post the GZ curve?

I'm surprised about that small downflooding angle. May you post also the general drawings of the vessel and the section where the air intake is?

It's difficult to help without knowing things in deep and performing a proper analysis.

[stalin]

Thank you for your prompt reply Mr.Guillermo.

The vessel is a single hull oil tanker converted to double hull 2 years ago. Downflooding angle has been taken from the existing stability booklet before conversion. The maximum draft shown in that downflooding angle curve is 10.6m, but now the maximum draft has been increased to 11.4, with which the angle is 36.85 deg. The GZ curve and the required drawings are attached for your quick reference.

Kindly go through once and suggest.

[Guillermo]

Well, scantling draft (10.36 m) shall not be surpassed, so load condition has to be altered to match that requisite, I'm afraid.

I'm surprised to see the downflooding angle has been geometrically found. What should be done, in my opinion, is to fix the downflooding point coordinates and then run the stability program under free trim and constant displacement conditions, to solve the real downflooding angle.

Cheers.

[stalin]

Maximum draft has been raised to 11.4m and it was approved by Statutory authority. Fully loaded condition has been approved at 11.4m meeting all strength requirements.

Regarding downflooding angle, I calculated the same way as you said, in the stability program by defining the unprotected opening with coordinates. But we need to show geometrically the downflooding angle from LWL and graph at various drafts in the stability booklet.

[Guillermo]

OK then.

How did you get the tangent point at the maximum GZ, to find the correspondent angle?
Visually it seems to rather be +/- 33 deg (or even 34 deg) instead of +/- 29 deg.
Down flooding angle of 35+ deg is still bigger than maximum GZ angle, even it being 33 or 34 deg, so no problem on that side.

But this is not important here.

Rule says:
.3. The stability in the final stage of flooding shall be investigated and may be regarded as sufficient if the righting lever curve has at least a range of 20º beyond the position of equilibrium in association with a maximum residual righting lever of at least 0.1 m within the 20º range; the area under the curve within this range shall not be less than 0.0175 m•rad. Unprotected openings shall not be immersed within this range unless the space concerned is assumed to be flooded. Within this range, the immersion of any of the openings listed in subparagraph 3.1 of this paragraph and other openings capable of being closed weathertight may be permitted.

(Bolded is mine)
It seems there is no problem to comply. You have a range in excess of 20 deg complying with the 0.1 m requisite. Why are you limiting the range at maximum GZ or whatever?
See attached image.

Cheers.

[stalin]

Thank you for the prompt reply sir.

If you look at the diagram, Unprotected opening ER immerses at 29 deg, and here GZ curve is limited(not max GZ). Since the initial steady heeling angle is 11.8 deg, range is calculated from that point to 29 deg, which comes to 17.3 deg.

Please clarify how did you take 36 deg as maximum limit to range.

[Guillermo]

Your own words, post #5: "....now the maximum draft has been increased to 11.4, with which the angle is 36.85 deg"

[stalin]

When the ship heels to 36 deg in the intact condition with 11.4m draft, downflooding angle immerses.

But with the damage condition, which I posted last time, ER immerses at 29 deg itself.

[Guillermo]

Sorry I did not understand properly.
What about rising/centering the air intake? Is it possible?


Remember: "...a range of 20º beyond the position of equilibrium in association with a maximum residual righting lever of at least 0.1 m within the 20º range"

[stalin]

No sir, it is not possible.

Can we take exemption from Statutory Authority regarding this? Do they allow non compliance of criteria for certain damage cases?

[Guillermo]

Probably not, except perhaps if the vessel is only intended to navigate within the flag waters. Not possible for international voyages, up to my knowledge.

Is it possible to further sub-divide the compartments in conflict?