Barge Stability

Discussion in 'Boat Design' started by maritimeman, Mar 2, 2023.

  1. maritimeman
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    maritimeman Junior Member

    Hey all,

    I'm new to boats but I am curious to better understand barge stability for a project I'm working on.

    What makes a barge more stable? More specifically, I'm looking at standard flat-top ABS barges versus semi-submersible barges, and trying to understand which is better for stability.

    I haven't been able to find too much information online, but is it accurate to say that the lower your center of gravity (like a semi-submersible barge that can lower it's Cg using ballast tanks underneath) the more stable it is? Or perhaps is it more a function of how submerged the vessel is? Which is to say, is a highly loaded ABS barge fairly stable? Or more broadly, is a highly loaded ABS barge more stable than an unloaded one?

    Thanks in advance - if I'm omitting any key information please ask and I'll answer, just might not be aware of it.
     
  2. TANSL
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    TANSL Senior Member

    The stability of a ship cannot be quantified, it is not defined with a number. There are a series of stability criteria, specific to each type of ship, and if a ship meets them, that ship "is stable." However, if you choose only one of these criteria, you will be able to compare which boat, among those of a certain sample, meets it more easily.
    In general, the stability criteria, and above all the initial stability, for a given displacement, are met more easily the lower the CoG is.
     
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  3. jehardiman
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    jehardiman Senior Member

    First of all, welcome to the forum Maritime Man...

    This is so win!

    As stated there are too many types of "stability", each predicated on a different operational scenario, that Naval Architects adjust each vessel to met the necessary stability requirements for that vessels Statement of Requirements (SoR).

    That stated, there are two different types of "stability" that need to be conformed too. The first is actual physical "stability", where the vessel can perform its necessary functions in the SoR environment. The second type of "stability" is the operator perceived physical movement, which leads to operational confidence. Failure of one or both of these different types of "stability" can lead to a catastrophic event.

    EDIT: I don't think going into "Spar Buoy" stability vs "Pie Tin" stability is going to help at this time. Perhaps Maritimeman can say what he is looking at this barge to accomplish...
     
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  4. maritimeman
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    maritimeman Junior Member

    Understood, thanks guys - perhaps I should sharpen up my language.

    @jehardiman, noted on your last edit - I wish I could be more transparent here but I'm actually bound under NDA so I can't disclose specifically! But I'm not well versed in these matters yet eager to learn, so I'm hoping you'll still be open to helping me understand, my opaqueness notwithstanding.

    Suffice it to say that the "stability" I'm interested in is the latter - operator perceived physical movement. Or, perhaps to put it to a tighter, less relative judgement - I'm going for recreating as much of a "land-like" surface in the ocean as possible. Perhaps most precisely - minimal change of angle in any degree of freedom (pitch, yaw, and roll). Roll and pitch in this case are the most concerning, though yaw matters.

    To me, Yaw seems most straightforward (and please do correct me if I'm mistaken here) in that, if properly moored (perhaps at all four corners) your yaw should be pretty minimal, as you're fairly well constrained from rotating. Pitch and roll, on the other hand, would be tougher since that's predicated largely on the "motion of the ocean." This is what I'm interested in and refer to as "stability" - and where I posited that perhaps a lower center of gravity (such as under-barge ballast tanks) improves the response of the barge to waves - or rather, minimizes resulting pitch and roll.

    I've never been on an oil rig/platform before, but I always assumed they felt pretty "stationary." As I understand it, a lot of these float, so there must be something intentional (combination of platform design, mooring, etc) that makes these feel like relatively solid ground. I want to see what principles guide this, and how they can be applied on a smaller scale (such as an ABS barge or semi-submersible).

    P.S. Thanks for the welcome, jehardiman!
     
  5. jehardiman
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    jehardiman Senior Member

    Oh, where to start....Oil Rig/Platform....Is the rig intended to be a shoal water Jack-up, a mid water moored, a deep water TLP or a benthic DP semi-sub? As you can see the SoR is so tightly rolled into this as to be the main driving factor behind the term "stationary". Even connected to the wellhead, a "ridgid" drill string can have significant movement depending on the scenario.
     
  6. maritimeman
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    maritimeman Junior Member

    I think a rig is more of an example to illustrate what I'm going for - I wouldn't be using anything similar to a platform.

    In my case it's an ABS barge I'm trying to make "stay level"
     
  7. jehardiman
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    jehardiman Senior Member

    So like a Crane Barge....Crane barges are limited in pitch and roll to prevent putting dynamic loads on the boom. In most case, operational loads in the US are limited by OSHA, rather than ABS survival loads. See OSHA 1926.1437 - Floating cranes/derricks and land cranes/derricks on barges.
     
  8. Ad Hoc
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    Ad Hoc Naval Architect

    More stable than what???

    As noted above by both T ans JEH, there are 2 'basics' to this, the statical stability..and how does it compare with "codes" and the physical stability of how it "feels".

    That is something you need to quantify, as it can be all relative.

    What you are now starting to delve into is the pure motion response to a perturbation.

    This governed by many factors but one can "boil it down" to the waterplane area and its relationship with the restoring forces of the vessel.

    Take heave, the natural period of heave is proportional to the waterplane area (WPA), and inversely proportional to the displacement.
    So in this acse one requires a low, or small WPA compared to the displacement.

    Similarly natural period of roll, is proportional to the mass moment of inertia and inversely proportional to the metacentric height.
    In this case a high mass inertia coupled with a low GM yields a long natural period of roll. The Queen Mary was famous for having a long "lazy" period of roll, circa 30seconds....she had a very large displacement, but a low GM.

    Similarly with Pitch but in the Long.t axis.

    The vessel's ability to restore itself, the WPA and GM....are attributes of the hull shape.
    Thus it is not necessarily a low KG, but where the KG is in relation to the hull's attributes when heeled.
     
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  9. rberrey
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    rberrey Senior Member

    Our big ringer barge was built to purpose , we had tanks forward in the rake we could drain in order to tow the barge up if needed , sponsons that could be added for more flotation , spuds to keep it in place . Ocean waves might have affected it,s stability , but a 4' wave , short chop had no affect on the stability moving or stationary , or the ability of the crane and barge to do what it was designed for . A 90' barge with a crane on it is another matter , it was not designed for the sole purpose of having a crane moving forward and back , swinging side to side with weight being concentrated in a given area , it affects the stability . That is why you will see a crane or track hoe ect chained down on a barge , even if it can walk forward it is chained down .
     
  10. portacruise
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    portacruise Senior Member

    Would some kind of SWATH design help?
     
  11. mitchgrunes
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    mitchgrunes Senior Member

    First off, be aware that I don't have engineering level knowledge. But:

    I notice there are catamaran-like barges. I guess the idea is that you are somewhat stable in somewhat bigger waves, if you have sufficient distance between the hulls, because the maximum tilt angle for a given wave height and length is smaller if the hulls are far apart.

    Maximum wave length and height seem like pretty important parameters to know before you estimate stability. In particular, any boat that is designed to be stable in small waves will be extremely unstable in waves comparable to the width and height of the boat, especially if the waves break over the boat. I've tried to convince people this is true for kayaks - that "stable" kayaks aren't always better in big waves, especially if the waves break over the sides, though that is a very different domain from barges. Boats stabilized by buoyancy tend to stay "flat" to the surface of the water - but when that surface tilts, you tend to tilt with it, and when the surface rolls over on itself, and is comparable to or larger than the size of the boat, you tend to roll over with it.

    But if you have a big enough barge, that might not normally be a factor, because your barge is hopefully much wider and longer than the wave length and height. Though open water waves have occasionally been recorded over 30 meters high; I guess you need a very big (and very solidly built) barge to be stay stable in those conditions.

    People have tried to stabilize ships with really big momentum wheels, and controllable fins. Does anyone know if that is practical on a big barge, or only something smaller?

    Of course, many oil rigs take a completely different approach to stabilization. They are anchored into the bottom, and the platform is supposed to be high enough above the surface that waves (and wind) can't interact (much) with them. That's completely different from buoyancy-stabilized boats. But they aren't cheap. And even some of them have run into trouble in severe storms.
     
  12. gonzo
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    gonzo Senior Member

    Boats are not stabilized by buoyancy. They are stabilized by the couple created by the horizontal distance between the center of buoyancy and the center of mass.
    I think you are oversimplifying the way monohulls and multihulls behave by a too broad generalization. A very large breaking wave may knock down a monohull but capsize a multihull. A barge may have waves breaking against and over the side. This means that the barge is not rolling too much but staying horizontal, which is an advantage. There is no overall best, just best for a particular application and operating conditions. I'm sure you have read that time and time over in this forum.
     
    Last edited: Mar 25, 2023
  13. TANSL
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    TANSL Senior Member

    I don't know if you understand what stability is (I think not) because, when a ship is in equilibrium, the horizontal distance between the center of mass and the center of buoyancy is zero.
     
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  14. BlueBell
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    BlueBell . . . _ _ _ . . . _ _ _

    The crane operator.
    (If they're good)
     

  15. TANSL
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    TANSL Senior Member

    This statement is totally confusing, it means nothing.
     
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