Shell plate thickness limitation

Discussion in 'Boat Design' started by saeedfa, Aug 10, 2020 at 1:00 PM.

  1. saeedfa
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    saeedfa Junior Member

    hi
    i start to structural assessment of a boat that is Ice service class, so below water line the shell plate thickness is 12 mm, and above water line the shell plate thickness is 4 mm.
    my question is, is there any limitation or requirement that say about changes in plate thickness? for example it say maximum change in plate thickness must be 2:1, so i must use 6 mm plate beside the 12 mm plate, or any thing like that.
    i use DNVGL rule, so if there is any reference in DNVGL it is better for me.

    thank you
     
  2. bajansailor
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    bajansailor Marine Surveyor

    What type / size of vessel are you designing Saeed?
    A plate thickness of 4 mm above the waterline sounds very light, even for a relatively small vessel.
    And especially so in comparison to 12 mm thickness of the hull bottom.
     
  3. saeedfa
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    saeedfa Junior Member

    a vessel with ice class, length about 30 m, and 9 m breadth. shell plate at bottom and side below water line is 12 mm, but calculation said that for shell plate above waterline a 4 mm plate thickness is good. (according to DNVGL - HSLC rules)
    but probably i use 6 mm plate instead of 4 mm for shell above water line.
    i want to know is there any requirement for the thickness of two different plate that connect together? for example it say if two different plate connect together, the ratio of thickness of the thicker plate to thinner plate must not more than 2, some thing like this ..
     
  4. TANSL
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    TANSL Senior Member

    As far as I know the limitation that you think does not exist but there must be a transition zone between both thicknesses. The length of that transition is regulated by the regulations, although I do not remember it at this time. You must find this information in the DNV GL regulations.
     

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  5. SamSam
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    SamSam Senior Member

    It might have to do with relative flexibility. A work around might be to go from 12mm to a transition band of 8 or 6mm plate and the rest would be 4mm, although it might not be worth the effort.
     
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  6. Ad Hoc
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    Ad Hoc Naval Architect

    When considering butt welds, and when referring to Class rules like DNV-GL... the rule is if the difference in thickness between the 2 butting plates exceeds 2mm, then there must be a taper.
    DNV-GL requires a minimum of 3:1.

    As a general rule, we use 4:1 as a taper.

    But as noted by SamSam. Regardless what the rule may or may not say.. Class rules must always be considered as a minimum.
    But now, you're getting into fit-for-purpose.

    Going from a 12mm to a 4mm is too much of a transition. There are many reasons why, but just from a pure production point of view, a thick plate butting into a thin plate is not good practice, owing to the large difference required in heat input for a 12mm and a 4mm plate. Thus a transition from the structural point of view... smooth change in relative local stiffness, and also a production point of view is used. So a plate of either 8mm or 6mm would suffice.
     
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  7. saeedfa
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    saeedfa Junior Member

    Thank you guys
    I take my answer from your comments
    Thank for your helps
     
  8. Barry
    Joined: Mar 2002
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    Barry Senior Member


    Would you put the butt weld on a stringer/frame or mid stringer?
    ie limit flex at the weld
     

  9. Ad Hoc
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    Ad Hoc Naval Architect

    As a general rule, we place frames around 200mm from the butt/seam.
    For stringers around 50mm from the butt.
     
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