Lightest GRP structures

Discussion in 'Class Societies' started by Morgig, Apr 10, 2013.

  1. Morgig
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    Morgig Junior Member

    Yes this is one way to approach this but I've only really found to work with an exact copy of the previous boats running at the same speed and displacement. (we have used this argument when rules have changed during the build of a run of boat, such as with the LRS SSC composite rules for UD tapes recently)

    This also requires the surveyor has to accept the assumption that wave height/operating conditions are the same, which is not always possible if you have the same design for different client operating out of different ports.

    When it comes down to it I always find it easier to design the structure to class rules when you have an intelligent surveyor at the other end that you can discuss the validity of the class ‘model’ to your boat.

    This is not the case when you are just using their rules as a standard and have to justify why discrete areas of the structure look to be failing class rules to a non technical end user or yard.

    I assume here you are using their simple formulas for composite plate thicknesses etc and assuming a typical E value for your laminate, rather than calculating individual laminate stresses as per LRS?
     
  2. Ad Hoc
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    Ad Hoc Naval Architect

    That is not the job/role of the surveyor. The case must be made to the plan approval office. The surveyor only carries out instructions from plan approval and checks the quality and adherence to approved plans.

    No, I always get coupons made. Then use those values.
     
  3. TANSL
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    TANSL Senior Member

    I have seen calculations for GRP justification of scantlings by Det Norske Veritas, "Rules for Construction of Vessels less than 15 meters", which with a few multiplications gave the thickness of the various panels. In the calculations that I have seen was not made any study of the stresses borne by each of the layers. The calculation was very fast but, I fear, that no rigorous entity will accept them, if not done this study of the layers. We all know that, when using heterogeneous materials such analysis is compulsory. In my olpinión, the Lloyd's SSC allows you to perform these calculations with enough agility, and allows easy changes in layers distribution, if necessary.
     
  4. Alik
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    Alik Senior Member

    This is simplified method used in Nordic Boat Standard. In HSC rules, DNV rules assumes more complicated analysis, actually there are 3 levels of analysis used in DNV HSC.
     
  5. TANSL
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    TANSL Senior Member

    I see Alik. The comment that comes to mind is that for HSC vessels requirements are very strong, unusual, while a fishing boat or any other work boat, even a recreational boat, although not subject to such requests, should not be calculated by simplified methods. Also for them is compulsory in-depth analysis of its structure (when they are built with non-homogeneous materials, such as GRP).
    In such case it is normal to meet the minimum thickness, but the layer analysis can demonstrate that the distribution is not acceptable. Not to mention the reinforcements.
     
  6. Morgig
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    Morgig Junior Member

    I don't think these DNV rules are still in force as I can't find them on the DNV web site.

    However something like these rules would be useful for smaller vessels built of conventional GRP were there is a limited design budget.

    Obviously for more demanding applications (larger/faster/different materials etc) a more detailed analysis would be required.
     
  7. Alik
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    Alik Senior Member

    They are in force; You need to search for Nordic Boat Standard. It is not on DNV website, but if You read preamble of the rules You will understand why.
     
  8. Morgig
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    Morgig Junior Member

    Although I'd normally agree with this statement in practice we find depends on the offices and people you are dealing with.

    This is an interesting approach I assume you base the layup of the coupon on previous experience and then test these against rule requirement.

    We have only tended to get coupons tested for class when we are using more advanced materials with properties not found in the class rules. Obviously class rule properties are generic and a test is always better but it comes with some cost in terms of time and money.
     
  9. Ad Hoc
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    Ad Hoc Naval Architect

    It shouldn't do. All cases I have argued, LR, DNV have all been direct with plan approval HQ. Although with ABS, they tried to allow the local plan approval office whom was also the surveyor. But this didn't lead to meaningful results, so I eventually went to head office and got my result.

    Kind of yes. It does depend upon the Class being used. Since with DNV it is very simple and straight forward. LR is a bit more complex. Knowing previous values/layup and their mechanical properties, allows much more flexibility in what I want/need.

    Not as expensive nor as time consuming as you think. And when you consider the "savings", since you will have all the hard data to hand, for me, its a no brainer. Coupon tests always done first, keeps Class off your back too :D
     
  10. meren
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    meren Junior Member

  11. rxcomposite
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    rxcomposite Senior Member

    The problem in comparison is that LR seems to be too conservative in its material property derivation. It offers its own formula for hand calculations and even suggest the Rule of Mixture (ROM) formula Tult=Vf Tf + Vr Tr if no coupon test data is available. The result is way off in contrast to its own formula.

    DNV does not offer formulae and designers use the standard macromechanics theory in preliminary design studies. ROM is the workhorse of the composite designers and even with correction factors inserted such as Tult=Vf Tf FD + Vr Tr, the values are still high compared to LR.

    Is LR being too conservative in its calculations? Or have they taken a really different route? The formulae in macromechanics theory appears to be consistent with other composites book, even Eric Greens "Marine Composites" and M. Hollmann "Composite Aircraft Design", but LR has a different way of deriving properties.
     
  12. Alik
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    Alik Senior Member

    Actually, I compared the results from LR and ISO formulas and they are quite the same. Some formulas are exactly the same!
     
  13. rxcomposite
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    rxcomposite Senior Member

    Alik, I dont get the same result. Have you tried the ROM of LR and compared it with its own formula for CSM, WR, and Uni? This is LR vs LR.
     
  14. Alik
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    Alik Senior Member

    No, I have not. Pls post we can have a look, if possible.
     

  15. rxcomposite
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    rxcomposite Senior Member

    Here attached. Still a mess but its functional.

    Do you have any links or reference to ISO standards formula so I can add another column?
     

    Attached Files:

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