effect of double bottom on floodable length calculation?

Discussion in 'Boat Design' started by sirhamid, Dec 3, 2013.

  1. sirhamid
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    sirhamid Junior Member

    hi all:cool:
    what is the effect of double bottom on calculation of flood-able length of a vessel?:idea::confused:
     
  2. TANSL
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    TANSL Senior Member

    Speaking from memory for a rapid response, I seem to remember that if the height of the double bottom is less than 1.5 m (or a ratio of maximum width), has no effect on stability after damage. It all depends on the height of damage that has to be considered in accordance with the regulations.
    Where the double bottom has a height greater than said minimum height, can be considered as the compartment that suffers damage while those above it do not reach their failure through the bottom. Maybe they can damage through the side.
     
  3. sirhamid
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    sirhamid Junior Member


    guess the accident on side at water line.
    please calculate without regulation used.:!:
     
  4. TANSL
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    TANSL Senior Member

    In that case, the double bottom is not flooded and therefore the volume of the flooded compartment is smaller, which is very good.
    Beware of the side damage, or at the bottom, does not affect the two compartments. The regulations call for both dameges to be studied, bottom and side.
     
  5. Ad Hoc
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    Ad Hoc Naval Architect

    The reason for using a double bottom is to improve the damage stability of a vessel. So, whatever statutory requirements you must comply with in those set of rules shall be a set of criteria. Part of the criteria, shall be the extent of the damage, vertically, athwartships and longitudinally. If you place the double bottom outside of this "zone" of damage, the double bottom greatly improves you final stability post damage.
     
  6. jehardiman
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    jehardiman Senior Member

    Be very careful here, because an unflooded double bottom will usually not improve damage stability! This is because the effect of an unflooded double bottom will raise the KG faster than the KB with the same loss of waterplane inertia resulting in a lower damaged GM (assuming standard arrangements where the double bottom volume is small compared to the compartment).

    Under the new SOLAS probablistic damage rules, damage is assumed to extend from the keel to 12.5m above the waterline and to 1/2 the beam, rendering double bottoms moot for collision. Double bottoms are to improve hull girder strength and limit grounding/bilging damage, not flood control for side collision.

    http://www.imo.org/blast/blastDataHelper.asp?data_id=24855&filename=281(85).pdf
     
    Last edited: Dec 3, 2013
  7. DCockey
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    DCockey Senior Member

    My impression is a primary political/popular (in contrast to technical) reason for mandating double bottoms on tankers is the idea that a double bottom may prevent oil spills in the even of a grounding or allision with a rock.
     
  8. Ad Hoc
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    Ad Hoc Naval Architect

    You must be referring to significantly larger vessels that I design. I would have to dig out old notes from when I was a student (and had hair) to investigate that further, but I defer to your judgement on that, as its been over 25 years since I did big ship stuff.

    On all the HSC vessels I design a DB assists in the raking damage, which is where the hull is damaged for 55% of its length from the FP and/or 35% of its length anywhere. Without a DB almost all vessels fail the damage stability requirements. Monohulls struggle and result in much larger than necessary, or very poor motions. Since to counter such damage, the beam needs to be wider and an increase in freeboard to prevent the deck line/opening of progressive flooding from being immersed. It does generally raise the VCG from a non-DB version simply to pass the onerous damage stability that being the raking damage rules, i.e using DBs outside of the zone of damage.
     
    Last edited: Dec 4, 2013
  9. TANSL
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    TANSL Senior Member

    Remember that is not the immersion of the deck but the margin line.
    I think if the double bottom has a vertical dimension less than the height of the damage, it will have no influence on the stability after damage.
     
  10. jehardiman
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    jehardiman Senior Member

    Actually, they changed the "margin line" to the "flooding deck", i.e. where the ship downfloods, read the IMO resolution I linked. IMO really needs to get a full copy of SOLAS on the net, but I think there is money to be made selling copies.
     
  11. TANSL
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    TANSL Senior Member

    Sorry, I'm not up to date. Is gone the margin line?
    There may be missing in the probabilistic method. In the traditional method, I think not ????
     
  12. sirhamid
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    sirhamid Junior Member

    Dears
    Your Answers Change My Question Way!!!!!
     
  13. TANSL
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    TANSL Senior Member

    The method of floodable length:
    - Not considered asymmetric flooding.
    - Only consider the effect of transverse bulkheads that extend to the bulkhead deck.
    -Not considered neither longitudinal nor transverse stability.
    Does this answer your question?. If not, I recommend you read the SOLAS as far as damage stability concerns ..
     
  14. sirhamid
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    sirhamid Junior Member

    paradox


    dear TANSL
    if the bulkhead or any suggestion about place not effect on F.L but why we can consider the permeability factor for place and changed the flood-able length?
     

  15. TANSL
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    TANSL Senior Member

    You can outline the floodable length curves for various permeabilities, 85 and 96 for example. Subsequently, for each real compartment, once assigned its permeability, you can deduct from these curves what is its maximum allowable length.
     
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