Assesing for Racking of a pontoon boat in waves with a moon pool.

Discussion in 'Boat Design' started by iang, Dec 16, 2015.

  1. iang
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    iang Junior Member

    Hello,

    I am currently designing a pontoon boat which will be used to deploy a tidal turbine. Here's a picture of what I'm thinking right now.

    [​IMG]

    The question I have is how to best go about designing the frame to withstand racking due to wave loading? I know pontoon boats get designed all the time and can handle racking, but most pontoon boats don't have a huge moon pool cut out of them like mine will.

    Right now what I have been doing to design the structural frame is taking the buoyancy force of one pontoon an using that as a cantilevered force on the frame assuming the other side of the frame is clamped. I'm thinking this is a pretty conservative calculation compared to what will actually happen. I'm looking to make the frame lighter and cheaper than what I currently have drawn in the picture above.

    A few more details:
    Those pontoons are 45' long 42" in diameter. The platform will be moored to that bridge pier in about 50' of water. Typical waves in this area are about 3' and often times are due to shipping traffic and recreational boats. Maximum current at this site is 4.1kt.

    Any help would be great. Thanks!
     
  2. gonzo
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    gonzo Senior Member

    If you design hinged attachments, there will be no racking. Also, the pontoon will always float flat on the water.
     
  3. TANSL
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    TANSL Senior Member

    But how can you say that?. Of course it wil be racking. You could use, I guess, similar solution to those of large hatches on the deck of a ship.
     
  4. iang
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    iang Junior Member

    TANSL can you show me an example of what you mean by "large deck hatches on the deck of a ship"?
     
  5. TANSL
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    TANSL Senior Member

    Sorry my friend, I have no photographs or images on that subject. Imagine how openings are on deck, hatches, of a bulk-carrier or a cargo ship. See if you can, the rules of any Classification Society to see how to attach beams and deck girders and how to reinforce the corners of the openings.
     
  6. Ad Hoc
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    Ad Hoc Naval Architect

    It is not worth trying to formulate your own formula from first principles.

    Just use any Classification societies rules. I would suggest using DNV as it is much quicker/easier to arrive at an answer.

    All you need is:
    1) Displacement
    2) Distance between CL to CL of each hull
    3) Lwl

    Then assume an acg of 1.0, which is the default.

    Simple :)
     
  7. Ad Hoc
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    Ad Hoc Naval Architect

    A hinged joint shall mean no bending moment about the joint, but there shall still be the reaction loads as each hull moves independently from each other.

    No you couldn't.
    Those large hatches do not have sufficient resistance to torsional loads that are required for such a multihull arrangement. There would be far too much displacement.
     
  8. gonzo
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    gonzo Senior Member

    Maybe I'm confused as to what the OP is calling racking. Is this on the horizontal plane of the deck or in the sliding supports like I interpreted it?
     
  9. fredrosse
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    fredrosse USACE Steam

    Place a boom at each end of the two cylindrical hulls (four booms total), with the four booms joining at the center of the structure, about 20 feet +/- above the deck. This structure will resist racking very well, and could also serve as a lifting point for the water turbine. With this structural arrangement you can treat each boom as a structiral element of a truss, neglecting bending moments in the booms (tension or compression only).

    "Right now what I have been doing to design the structural frame is taking the buoyancy force of one pontoon an using that as a cantilevered force on the frame assuming the other side of the frame is clamped. I'm thinking this is a pretty conservative calculation compared to what will actually happen. I'm looking to make the frame lighter and cheaper than what I currently have drawn in the picture above."....................Yes, that is too conservative an analysis. With the structure described above, consider only two horizontal beams, one at each end of the pontoons, traversing from pontoon to pontoon. The pontoons themselves form the remaining elements of the truss structure.
     

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  10. Ad Hoc
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    Ad Hoc Naval Architect

    Gonzo

    He means, or should do, this:

    Cat Beams.jpg

    which results in this type of analysis if one cannot do by hand:

    Box beam raft structure - FEM.jpg

    with the classical "S" shape between the two hulls:

    Box beam raft structure - S shape.jpg

    As can be seen..a flat plate, such as a hatch is insufficient for torsional shear flow.
     
  11. TANSL
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    TANSL Senior Member

    Has you ever heard about the torsion problems experienced by some general cargo ships or container ships or dredgers?. I do, and I worked on that topic.
    I did not say that the solutions are the same, but may be similar.
    I hope, for my sake and for the OP, you give us a lecture, as you get used, showing how wrong I am. Thanks.
    No, it can't be seen. I do not know what you intended to demonstrate with these beautiful figures.
    fredrosse, without much figure, seems to be much closer to a solution. I think the OP would prefer this to a demonstration of our great wisdom.
     
  12. Ad Hoc
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    Ad Hoc Naval Architect

    Ok..since your frame of reference is a container ship, let’s look at a container ship then.

    Take the examples below, of ship A B and C. Ship A being a standard conventional ship with a bottom sides and a deck only. However it is a “closed cell” for shear flow. Ship B, a typical container ship type arrangement, with wing tanks, closed cell type. And ship C, a typical working boat with no deck across at all, an open cell as such.

    Equating all to a torsional modulus using standard Bredt-Batho Theory for cells and torsional shear flow we can demonstrate the following:

    Torsional Resistance-1.jpg Torsional Resistance-2.jpg

    And the final results are shown using the first ship A, a closed cell, as the comparison, thus:

    Torsional Resistance-3.jpg

    So we can see that Ship B, which does not have a full deck but wing tanks has an angle of twist 23 times that of a closed cell, and the shear stress is 5 times that of the closed cell.

    If we now look at the open deck ship C, it has an angle of twist of 15,424 times that of the closed cell and a shear stress of 257 times that of the closed cell.

    Thus, this is why container ships try to emulate that torsional stiffness of a closed cell vessel like Ship A as much as possible by using wing tanks, as it can be clearly seen to be superior to that of an open cell like Ship C. But the raison d'etre of a container ship is to have as many containers in the holds as possible. Ergo the largest volume possible..an arrangement that is Ship C, but clearly it requires significantly more structure to withstand the same torsional loads that are easily accommodated by a closed cell Ship A. Ergo Ship B is the compromise.

    This is very basic structural analysis of torsional shapes.

    It very easy to demonstrate that a box or a closed cell is infinitely superior to resist torsional loads by the shear flow paths of a closed cell using standard theory. Thus a flat plate - like a hatch - is extremely poor in resisting torsional loads whereas a closed cell, like a box, is extremely good.
     
  13. fredrosse
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    fredrosse USACE Steam

    A simple picture of what I would consider maximum racking load case for basic quasi-static loads in each of the boom and cross member elements. The forces in these elements have their load directions defined by assuming no moments at their endpoints, beyond that, fairly simple geometry/math to get the tension & compression loads in these members. Beyond simple tensile and compressive stress in the truss model, of course buckling criteria must be examined in the compressed members.

    For the racking case the two vectors add up to the total displacement, and act at about 75%/25% of each pontoon's length.

    Additional evaluation of dynamic loads tending to distort the deck from a rectangle into a parrallagram, as well as adequate moment resistance of the pontoon connecting cross members to support the pontoons with transverse dynamic loads is required. These stress considerations will probably not cause any significant hardship to the design.

    Once you have the stress model setup (I would use EXCEL) you can fool around with the placement of the four booms to the deck area, as well as the overall structure height, to arrive at the best arrangement here.

    I am sure there are additional criteria here, just wanted to outline the "racking" problem. Any comments from our NA friends is welcomed.
     

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  14. TANSL
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    TANSL Senior Member

    No, probably my difficulty in explaining make you unable to understand me. Or perhaps there are other reasons which I'd rather not imagine. My "frame of reference" is not a container ship. I never said that. What I am saying is that the problems of torque, serious, occur in other types of boats, monohulls. For this I have spoken of similar solutions to those taken in bulk-carriers, container ships and dredgers. Have I explained now correctly ?. If not, and you still do not understand I beg you not try to interpret what you would like but you ask me. So we avoid that you say that I say what I do not say.
    But if you want to be constructive, rather than highlight my mistakes, why not give a solution to the problem at hand ?. I do not have it, I would have already gaven it, if any.
     

  15. Ad Hoc
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    Ad Hoc Naval Architect

    Examples given above is of various type of monohull, including a container ship, but applicable to any vessel that is of similar arrangement.

    A monohull is a monohull. A container ship is a container ship etc...that is all very clear.

    Thus either you understand the theory of shear flow with regards to torsion, or you do not. It is not a difficult concept to grasp for anyone that has knowledge of structural analysis and design.

    Thus whether a monohull...of what ever type..or a multihull, the issue remains.

    And I think this goes to the heart of the matter. Comprehension.

    An answer has been given, but either you have not read it, or it is beyond your comprehension of either English language or technical ability.
     
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