Round Steel Pipe Barge Free board Questions

Discussion in 'Boat Design' started by Nick Gear, Feb 15, 2020.

  1. Nick Gear
    Joined: Feb 2020
    Posts: 3
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    Location: New Zealand

    Nick Gear New Member

    I have the following design:
    upload_2020-2-16_11-51-53.png
    The pipes are 762 mm x 12.0 m long, the total weight of the structure (as shown) is 4.1 tonne, the total pipe volume (displacement) is 16.4 m3.

    My question is relating to calculating free board (or draft) on the radial pipe shape? Im assuming this wont be linear in relation to total buoyancy (i.e.) putting stability to one side, as further weight is applied to the structure, the free board isn't simply weight to total reserve buoyancy ratio?
    Would I be right in assuming that i'd need to be calculating the circular segment surface areas of the 762 mm DIA in say 100mm increments to relate back to the various weights im applying..

    Help!
    upload_2020-2-16_12-1-12.png upload_2020-2-16_11-51-53.png upload_2020-2-16_12-1-12.png
     
  2. bajansailor
    Joined: Oct 2007
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    Location: Barbados

    bajansailor Marine Surveyor

    Welcome to the Forum Nick.
    May I ask what is your intended use for this barge?

    The total pipe volume is 16.4 m3 ; in fresh water the displacement at half the pipe depth will be 8.2 tonnes.

    Your calculated weight of the steel work is 4.1 tonnes. So she will be floating at a draft somewhere between D/4 (190.5 mm) and D/2 (381 mm).

    If you draw a circle full size with a diameter of 762 mm, then you could mark on draft marks at a spacing of (say) 20 or 25 mm between the D/4 and D/2 drafts.
    The waterline will be slightly wider at a depth of D/4 + 25 mm (compared to the width at D/4) - you could safely assume that the cross section between these two drafts is a trapezium.
    And then calculate the increase of displacement for each additional increase in draft.
     
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  3. Barry
    Joined: Mar 2002
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    Barry Senior Member

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  4. Nick Gear
    Joined: Feb 2020
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    Location: New Zealand

    Nick Gear New Member

    This is exactly what I was after, thanks Barry
    Intended use is accommodation platform - not a houseboat, but to house two smaller structures, and small commercial tiki bar.
     
  5. Blueknarr
    Joined: Aug 2017
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    Blueknarr Senior Member

  6. Barry
    Joined: Mar 2002
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    Barry Senior Member

    If you look further into the page,if you know the height of the segment, or partial circle, it works out the area
     

  7. Nick Gear
    Joined: Feb 2020
    Posts: 3
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    Location: New Zealand

    Nick Gear New Member

    Thanks Blueknarr.
    We've got limited payload atop the structure, with a wave Hmax <400mm sitting in a sheltered Fjord within a marina.
    Total displacement of structure would be no more than 6.8 - 7.0t including payload.
     
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