First estimate (hand calc.) for force on slender structures

Discussion in 'Hydrodynamics and Aerodynamics' started by Murdock86, Jun 21, 2013.

  1. Murdock86
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    Murdock86 Junior Member

    There is a concept for a ballast system that we are investigating.
    I did some spreadsheet/hand calculations earlier this week using Morison equations. Need some experienced advice on a couple of points there; please see pdf attached.

    This is just an initial estimate and the idea is to get some feel of the design forces, and then we'd go back to the drawing board :cool: Thanks in advance!

    Attached Files:

  2. Ad Hoc
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    Ad Hoc Naval Architect



    1) is the weight of the frame system a small percentage of the displacement of the barge it is attached to?

    In reply to your questions

    1) The drag you are calculating (from the Morison Eqn) have you also included the frictional drag, from its WSA, or are you just using the XSA to the flow (i.e. its shape/form - as per the Morison Eqn)?

    And leading on from this, the forces you are calculating is this when the barge is stationary or when the barge is moving?

    Since if the values you have calculated are when the barge is stationary, then yes, you must add the wave velocity (as this is the drag on the system), but if the barge is under way, this velocity is then your barges forward speed (and this is causing drag on the barge).
  3. Petros
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    Petros Senior Member


    I think you are trying to develop forces from a conceptual understand of fluid motion. the problem with this is nature knows nothing about your assumptions, and does what it does and will often subject your designs to unforeseen forces way beyond what you thought were reasonable loads. when forces are predictable, this is not a bad approach, but when dealing what nature might through at you, you have to go with the experience compiled by a rating agency. Yes, you need to consider the wave loads, but do it that way industry does it.

    I suggest you investigate the loads recommended by the various rating rules, you can get German Lloyds on the internet for free. It groups the different boats by sized, usage, and types of waters they are designed to be used on (inland, coastal, ocean, etc). And from there the tables will give you all the forces on the hull you need to accommodate. these are based on many years, even centuries, of observing what failed and what survives. They are by nature very conservative, but it is the best way we know how to accommodate the "unforeseen circumstances" that regular come up with ships at sea, as far as hull loads are concerned.

    Unless you are designing a temporary structure that will be used in controlled and predictable conditions, use the tried and true approach to accommodating hull loads from one of the class rating organizations.

    good luck.
  4. Ad Hoc
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    Ad Hoc Naval Architect

    All he is trying to do is estimate the loads on a frame structure that is immersed from the side of a barge. That is all. At first glance it appears the forces are low – because it is an “open cell”. But it is not, which he is finding out. We did a similar analysis for a client using a similar framing system several years ago, but for a totally different application. They too didn't appreciate the loads that are imposed by a "simple framing" system.

  5. jehardiman
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    jehardiman Senior Member

    Yes, he will way underestimate the forces if he used the method he proposed on a close structure with no inertial forces.

    So for 1) This is why when the added wave load looked too high. you must also add the wave orbital forces onto any forward velocity of the barges as wave celerity of the subject wave periods are well above barge speed. Also, it those are the actual proportions, the they are not slender body even though the elements may be, they are a close structure more like a grillage...added mass will be huge so you might as well make it a closed freeflood to reduce flutter.

    And 2) No, inertia forces will dominate for your given wave (and will up until ~12 sec period then orbital acceleration loses out to orbital velocity) and you need to look at the full wave cycle because the maximum load will not be when the wave is on the quarter periods, but on the 1/8th periods (i.e. 1/8t, 3/8t, 5/8t, 7/8t).
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