New HDPE pontoon under old steel Pontoon

Discussion in 'Boat Design' started by gdzle, Apr 8, 2020.

  1. gdzle
    Joined: Apr 2020
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    Location: Australia

    gdzle New Member

    Hey All,

    I'm not sure what thread this belongs in, but essentially I have an existing coffin shape fully welded steel plate pontoon in a mining environment. The thing is overloaded on one side because of some piping and we're trying to add overall buoyancy to the system, but particularly on the overloaded side. There's a walkway on the pontoon and occasionally when the pipes get bogged, the pontoon can sink enough that the walkway ends up being partially submerged which we don't want. The existing pontoon is about 4.0m x 4.0m plan area.

    We've come up with this solution attached in photo, essentially some HDPE modular floats bolted to a fully welded stainless steel frame at the bottom (150 Channels). The floats have spin weld fittings to allow them to filled with water to sink then re-float once in position. Typically I've seen the frame directly seated on the top side floats, which obviously provides better buoyancy. However, long story short the mine site guys we're working with are strongly against this conventional arrangement with frame on top. So we've tried to flip the typical system upside down and have the frame sit under the existing steel pontoon and the new pontoons sitting either side of the existing one. This way they can just sink the new system enough to be able to move the frame beneath the existing pontoon (Existing pontoon is pink in the photo). The HDPE floats then sit on top of the stainless steel frame, and are connected by threaded rods to just an fully welded equal angle top frame. The frame will be linked by chains to the existing pontoon that already has lugs welded to it. I've asked fabricators if they've done this before, and most of them said the frame is always at the top. I've sized the floats based on the loads on the existing pontoon.


    So i'm just wanting to know if anyone has seen an arrangement like this before and if they can offer any advice? I don't think the frame needs any lateral bracing since there's no real lateral loads, and its all fully welded, but I'd like to hear if people have a different opinion on this. I'm just mainly concerned the about the frame because its quite wide.
     

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  2. Barry
    Joined: Mar 2002
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    Barry Senior Member

    The bottom cross members that go under the existing buoyancy chamber will not be carrying much if any weight of this chamber. The new "wings" will be pushing up the ends of the cantilevered cross members as the buoyant forces
    will causes these members to deflect or bend upwards. When the new wings are helping to carry the load, the beams deflect, the main vertical load will be acting on the edge of the existing chamber making the cross frame redundant.

    There are quite a few parameters that we don't know but if the existing chamber has a significant wall thickness and the lugs that you refer to are welded both at the bottom and top of the side,
    ( and perhaps if the original chamber can support the forces that the wings will provide) I would consider, (after the rest of the information is made available) to go with two independant wings and pin them to the lugs.


    Certainly you can put lugs or something to resist this deflection, but the lower new crossbeams under the existing chamber will not be carrying much load and this structure is not required

    Instead, if you can weld lugs to the existing chamber at the bottom and top of the side of the existing chamber, you would be able to just build two new chambers and bolt or pin it to the side of the existing chamber
     
  3. fallguy
    Joined: Dec 2016
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    Location: usa

    fallguy Senior Member

    I have little to offer, but a thought provoking question. Why would you want to ballast the floats? I think in simple terms, generally, which may be entirely useless, but a reserve buoyancy is usually just there when needed. By designing the shape correctly, the reserve buoyancy would increase for every added unit of submersion.

    Take my words with a grain of salt please, perhaps the buoyancy you need is too great for the idea, but a square shape does not increase buoyancy for every unit of submersion.

    perhaps you would prefer the static method, and you don't mind constantly adjusting the ballasting..
     
  4. Blueknarr
    Joined: Aug 2017
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    Location: Colorado

    Blueknarr Senior Member


    The new floats will not remain flooded. To install the new floats they must be in alignment with the old ones. Either temporarily partially sink the new or lift the barge with a crane. Pumping water is a lot easier and cheaper than hiring a crane.
     
  5. fallguy
    Joined: Dec 2016
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    Location: usa

    fallguy Senior Member

    ahh, just to facilitate the build, underwater welding too boot! Might be easier to use the tides..ez for me to say; harder to implement

    thanks
     
  6. Blueknarr
    Joined: Aug 2017
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    Location: Colorado

    Blueknarr Senior Member

    I thought a bit more after my first post.

    It might be handy to be able to partially flood the new floats to help keep the barge in trim with the unbalanced loads.

    Keep some of the lower cross members. They will prevent the new wing-toons from angling up on the outside.
     
  7. gdzle
    Joined: Apr 2020
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    Location: Australia

    gdzle New Member


    Yeah that's right, we did some calcs and found that the front pontoons (ie. walkway end) will be sitting higher than the back ones due to the piping loads. Plan is to fill those front ones up just enough to try balance the system a bit more. We don't want the existing pontoon to sit too high because it's connected to a floatline and the main floating mining plant, so it needs to sit level for the walkways, but provide just enough buoyancy to keep the walkway at 75mm freeboard.
     
  8. gdzle
    Joined: Apr 2020
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    Location: Australia

    gdzle New Member

    Thanks Barry, the wall thickness of the existing steel pontoon is 6mm. plate, but its adequately stiffened with 6FL internally so deflection of the platework shouldn't be an issue. Additionally I spoke to some fabricators who said that the HDPE pontoons shouldn't be deflecting much (I don't have much experience with HDPE design). My main concern was the new frame on the underside, I know it's fully welded and like you said it's predominantly taking vertical loads. I am hoping that with the front HDPE pontoons they can be somewhat partially filled with water to counteract the eccentric loading we're seeing in those pictures. Hopefully if the vertical load distribution is more balanced and the frame is rigid enough by fully welded connections and bolted connections to the HDPE floats, then stress in the frame will be better distributed.
     

  9. ondarvr
    Joined: Dec 2005
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    Location: Monroe WA

    ondarvr Senior Member

    I've built frames and used plastic drums as flotation.

    I ran air lines to each drum and adjusted the volume in each drum to balance the load. It worked well for what we needed to do.
     
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