Consultation regarding ISO 12215-5

Discussion in 'Class Societies' started by TANSL, Nov 25, 2020.

  1. Ad Hoc
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    Ad Hoc Naval Architect

    In a round about way - yes.

    It all started with the paper by Heller/Jasper On the Structural Design of Planing Craft in 1961...where they attempted to rationalise a methodological procedure for the design of such vessels.
    This was a ground breaking paper at the time. Then there was the Structural Design of Aluminium Crew Boats by Spencer in 1975 and then Considerations on the Structural Design of High Speed Performance Marine vehicles in 1977 by Allen and Jones. Another seminal paper. They introduced further refinements to the Ar ratio.

    These papers introduced the aspect area, or panel area ratio... the simple length x breadth of a panel subjected to a bottom slamming pressure.
    This area ratio has many different variations, but all subtly similar.

    DNV adopted this in their 1985 rules, the simple orange booklet Classification of High Speed Light Craft- for those that remember the DSC Code as it was termed- that was only about 10-15 pages thick!
    These ruels adopted the simplified method of area ratio = 0.70.Displacement/draft with some caveats.

    Then when DNV Golden rules came out in 1991, the big thick ones (now DNV-GL) further R&D was done by DNV (and others like LR), noted in their papers, including Response of Fast Craft Hull Structures to Slamming Loads, by Hayman/Huag/Valsgaard in 1991 in some way to explain their new rules.
    Thus the "simplified" area ratio first adopted back in the 1970s has evolved into further limitations and variations/caveats, depending upon whether it is a panel or stiffener.

    So it is a kind of empirical based ratio, with a mix of old work and new work and simple regression based analysis of R&D for sensitively analysis of panels sizes etc to arrive at a set of parameters that "best fit" the research to formulate a set of rules. And also noted in my excel spreadsheet of DNV rules:

    upload_2020-11-27_10-6-57.png

    But the bottom line is, it is not the Design Pressure that is really the focus, it is the final structural arrangement.
     
    Last edited: Nov 26, 2020
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  2. rxcomposite
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    rxcomposite Senior Member

    Tansl- You are analyzing a plate and stiffener (girder, transverse, longitudinals, ect). The pressure derived for a plate gets multiplied by a factor (usually o.5) so it is less. There are many factors used for different members, the same for the deflection criteria.

    What you are showing is a software derived calculation but what is inside is derived from a rule. Typical LR shows how the panel pressure formula gets multiplied by a factor for a stiffening member + plate. LR rule is downloadable from the net so it it is safe to post here. Cant find it in ISO 12215-5
     

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  3. DCockey
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    DCockey Senior Member

    I've heard several times that it is a mistake to take the "design pressure" from a particular design rule and attempt to use it another design rule, or try to use it directly in a "first principles" desi
     
  4. rxcomposite
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    rxcomposite Senior Member

    Attached are snippets from a software. Note the differing pressures of members but belonging to the same group, ie bottom.
     

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  5. Ad Hoc
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    Ad Hoc Naval Architect

    Correct.
    Since each rule uses it own fudge factors and methodology to arrive at a set of prescriptive rules for design.
    Case in point...a comparison between DNV and ABS, using the same ratios for design pressure:

    upload_2020-11-27_12-7-46.png

    It is not about the "absolute" value calculated. It is about the final design solution...how you get there is different depending upon which set of prescriptive rules one uses.
     
  6. rxcomposite
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    rxcomposite Senior Member

    None in my rulebook. The longitudinals + attached plate (effective width) uses the same derived pressure for the plate for calculations. It is "rationalized" into a wide flange beam with a very wide bottom flange.
     
  7. rxcomposite
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    rxcomposite Senior Member

    Like this. It uses the designed plate pressure.
     

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  8. baeckmo
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    baeckmo Hydrodynamics

    .....which demonstrates Ad Hoc's statement about the differences between different rules. I think it would be correct to say that the "design pressure" must not be understood as equal to the physical, hydraulic pressure acting on the structure. Instead, it is a corrected value, reflecting the influence from local pressure peaks, allowable deflections, empirical compensations for fatigue aso aso, all as interpreted by the specific rule writers. The more calculating "refinements" introduced into a set of rules, the further away from the physical pressure you get with this approach.

    I must confess that I too find it a bit frustrating that corrections are applied to the physical phenomenon (pressure) that actually presents the load, instead of introducing all corrections into the structural components calculation, but it seems to follow the old tradition of empirical "safety factors". There you actually introduce a load increase to be safe when the calculation method is unsafe. The more "holes" you find in the calc method, the more detail corrections, much the same as tax regulations; "how much have you earned, how much have you left, send all of it in"........
     
  9. TANSL
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    TANSL Senior Member

    That explains things somewhat, or tries to do so. It is what I am looking for, not what each regulation does, but why , together with a technical justification or a justification derived from actual practice.
    Thank you all for your attempts at explanation. It has been very interesting and very enlightening. Thank you.
     
  10. rxcomposite
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    rxcomposite Senior Member

    And that is how the accountant qualified to work in the IRS.:D
     
  11. TANSL
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    TANSL Senior Member

    What is the theoretical basis that supports the concept of "design area", what is it intended to take into account with this concept?
     
  12. Ad Hoc
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    Ad Hoc Naval Architect

    Can you define what you mean by:

    Thanks
     
  13. TANSL
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    TANSL Senior Member

    I am not the one to give a definition of this type so I will abide by what the ISO 12215-5 standard says.
    Design Area.gif
    Once this is known, could you now, please, answer my question?. Thanks
     
  14. Ad Hoc
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    Ad Hoc Naval Architect

    That is the answer then :).,. the words you are using are from ISO.

    Design Area - is just a two words to describe the area under investigation, for structural design.
    Since when a vessel is in a sea and is subjected to wave impact laods, there is slamming. So, how do you define the location and extent of the slamming to design the structure to withstand said impact/slamming loads?.... the whole boat the entire breadth of the boat?... obviously not!

    Thus the term, for ISO, is Design Area.

    It is merely a reference to what you as the designer, has selected to be your limits for the impact area that is supported by structure. It is an area that will be subjected to said slamming load and is supported by structure. The structure then defines the size of this "area" and this is defined simply as the length of the panel and the breadth of the panel - to provide an 'area'... or commonly referenced as the frame spacing and the stiffer spacing.
    There is no magic to it, it is just words...

    As noted in my post above, everyone uses subtly different words and methods to describe the same thing as noted in ISO, viz:

    Heller/Jasper used the term G or half girth..as a reference plane.
    Spencer used the term - impact reference area
    Allen & Jones used the same - impact reference area
    DNV uses Areff - reference area from impact loads
    BV uses Sr - reference area.

    In the case of Spence, Allen/Jones et al, they use Ar = 25 x Displacement/draft
    DNV-GL uses reference area Areff = 0.7 x Displacement/T
    BV uses Sr = 0.7 x Displacement/T

    DNV-GL also add additional factors Kred, reduction factors for the design load area

    ISO uses "Design Area" , not by the ref in 7.5 and 9.1.1/2 and 9.2.1/2.
    The definitions used in ISO 9.1.1/2 and 9.2.1/2 are very similar to the reduction/fudge factors for Kred used in DNV-GL rules.

    That's it..it is just words to describe how the impact/slamming load is applied to a vessel in terms of 2 geometric dimensions....the length and breadth of a panel that is supported by structure that is subjected to a slamming load.
     

  15. TANSL
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    TANSL Senior Member

    Yes, Ad Hoc, thanks for your explanations, all that is obvious, what everyone can know by reading the Regulations, but why is it said that a value greater than a certain number will not be taken? What does it mean this limitation and what effect does it intend to achieve, or does it intend to avoid?
    Thank you once again for your time.
     
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