Maxsurf Modeler Tanks

Discussion in 'Stability' started by Mitch1990, May 14, 2020.

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  1. Mitch1990
    Joined: Feb 2020
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    Mitch1990 Junior Member

    Hi,

    I am very new to Maxsurf and am terrible at it, I am at the point where I understand that surfaces create decks and bulkheads. One of my design parameters is to include a 250m3 fuel tank in the double bottom. I used the ABS rules to determine a double bottom depth of 0.9m (ish). Is there a way to use the volume of the tank to govern the location of the FWD bulkhead of said tank? At the moment I am manually iterating to it.

    Also, where do I find the minimum number of water tight bulkheads in ABS? In DNVGL there is a table with lengths that specify bulkhead numbers. This does not appear to exist in ABS in Part 3 and it directs me to the SOLAS convention which is not the easiest thing I have ever read and well beyond me at this stage.
     
  2. TANSL
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    TANSL Senior Member

    I think you should proceed in reverse. Deduct, according to the service of the tank, the volume that you need and establish its situation so that its CoG is, more or less, in the position that interests you. Then you can make 4 or 5 sections of that tank, integrate them and check that you get the desired volume.
    Regarding the number of watertight bulkheads, place one at the fore peak, another at the after peak, and do a study of floodable lengths (Maxsurf does it quickly) to determine the maximum length that each compartment of your boat can have. That gives you the minimum number of watertight transverse bulkheads your boat should have.
     
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  3. Mitch1990
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    Mitch1990 Junior Member

    The floodable length analysis is ABS compliant. I know there was a move towards damage simulation for damage stability analysis and survivability moving away from strict directions.
     
  4. TANSL
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    TANSL Senior Member

    Stability after damage would be the second and most complicated step, but to know the number of watertight bulkheads, floodables lengths analysis are sufficient. Although, of course, there may be circumstances that lead to an increase in the minimum number of bulkheads.
     
  5. Mitch1990
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    Mitch1990 Junior Member

    Thank you for you help. Our lecturer is very much wanting us to use the subdivision requirements provided in the SOLAS convention, so I will do as I am told. I have started trying to understand how the limiting KG curve works in Maxsurf and it appears to be independent of the load state which roughly makes sense as it is use to govern loading conditions. I am using the cargoship_3surface model which is a sample with maxsurf for this and it has a DWL of 5.25m, LBP is 100m and Breadth is 20m. Why then do I get a negative GM at just over 1000 tonnes? I haven't added any data to loading condition. I am new to both maxsurf and naval architecture
     
  6. TANSL
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    TANSL Senior Member

    I can't exactly answer your question about Negative GM Value. What I can tell you is that, I don't know how it does it or why, Maxsurf defines a KG value when, for example, it calculates the hydrositatic values. I imagine it assumes that the KG is at a certain percentage of the depth of the used surface model. And, until you define a specific load condition, Maxsurf is using the KG that he has invented.
     
  7. Mitch1990
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    Mitch1990 Junior Member

    When I run the upright hydrostatic analysis with a KG set by me, it shows GM approaching zero and going negative around 4000 tons which makes more sense. I am obviously missing some fundamental concept in transverse stability. I will keep reading
     
  8. TANSL
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    TANSL Senior Member

    Try with a KG value of, aprox. 90% of the hull depth. This is a "normal value" for the light ship. Then add all other weights.
    Coulod I see your model in Maxsurf?
     
  9. Mitch1990
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    Mitch1990 Junior Member

    It is just the sample ship provided with the maxsurf. We have been told too add two tanks and the required water tight bulkheads to it which I have done in a separate model which is titled modified.
     

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  10. TANSL
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    TANSL Senior Member

    Sorry but my Maxsurf doesn't allow me to handle so many surfaces.
    I don't know if you have correctly defined the Zero point and the Frame of reference. If you haven't, you should do it before doing anything else.
    You should define some loading condition, at least the light ship.
    If the tanks rest on the hull, all you have to do is give Maxsurf the data of the extreme points of each tank. You don't need to define watertight bulkheads as such, just give the limits of each tank.
    I do not understand what the definition of tanks has to do with the calculation of the KG limits. Can you explain to me what you really need to do, what is the final calculation you want to carry out?
     
  11. Mitch1990
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    Mitch1990 Junior Member

    I need to generate a limiting KG curve and based on the upright hydrostatic data the output of the limiting KG seams incorrect to me. I have my zero point set at the baseline for vertical and the FP for longitudinal so all the values are negative. When I run the limiting KG analysis, it shows the CG solid to be above the top of the model???
    Annotation 2020-05-15 150312.png Annotation 2020-05-15 150409.png Annotation 2020-05-15 151137.png
     
  12. Mitch1990
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    Mitch1990 Junior Member

    Turns out that I have not defined the criteria correctly for Severe wind and rolling. My bad, but thank you for your help
     
  13. TANSL
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    TANSL Senior Member

    Sorry but I don't understand where you want to go or how. You start by talking about tanks, watertight bulkheads, negative GM, KG above the model, "solid" CG (???) and you end up realizing that you have not defined the stability criteria. How, then, did you want to calculate the KG limit curve? It is absurd.
    By the way, if it is a matter of increasing the KG value until some stability criterion is no longer met, why is the KG not going to be above the model (as you say)?
    And another issue, it has a very poorly defined margin line. Thus it is not possible for Maxsurf to make any correct calculation.
     
  14. Mitch1990
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    Mitch1990 Junior Member

    Sorry about that, I had selected a criteria based on what I was told to do but didn't full understand the criteria so when I ran the limiting KG analysis it failed to converge. The criteria I have failing on severe wind which appears to need specific definition. I have sorted my KG issues, thank you for you help and sorry for wasting your time. I am now back to trying to calculate the number of watertight bulkheads based on IMO stability regulations which is very convoluted. Has anyone got an example on how to do this? It seams a little odd that some class societies give clear definition based on vessel length if you have not conducted damage control probability analysis and then for ABS to say conduct in accordance with IMO stability direction.

    I wouldn't even know where to start on severe wind compliance

    Sorry again.
     

  15. TANSL
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    TANSL Senior Member

    You don't need to apologize, simply, to get better help, you should better focus your questions. Here most of us like to help.
    IMO has publications on the stability criteria applicable to each type of ship, stability of the intact ship and the ship after damage. There you will find the criteria, some explanations that are not always easy to understand, and some formulas, such as those of waves and intense wind (Chapter 3.2.2).
     
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