Experimental small hexagonal platform

(Larry G) #63

That, or the spuds are installed on location. Another option to towing is to barge them out, as the Hex bridge caisson from IMFS that I am using as a conceptual reference was actually cast on the deck of a barge. In addition to the EPS foam below the deck for buoyancy, the hex shell itself is also a sealed hull, so there’s lots of reserve buoyancy. Putting the spud legs up in the air for travel is a bad idea, agreed. Puts too much weight too high and poses a capsize risk even for such a stable platform, and lowering them adds drag for a tow situation, agreed. But I don’t think it adds too much drag for a rare operation to be practical.

I am planning to build a scale model using fiber cement board (4’x8’ sheets at Home Depot simulate approximately scaled thickness, 80lbs per sheet)) and just use a wooden frame to screw it together and seal with silicone. I think I can place a polycarbonate (plexiglass) panel in one side and use a ruler for scale with a GoPro for recording wave height. At 1" to 1’ (1/12th) scale a single fiber cement board can provide the hexagon base, and two boards should supply all the walls. I can use 2.5" pipe for spud legs. I will probably just use Pipe Straps on the outside for that rather than constructing a complex internal structure. A couple 4’x8’x2" sheets of foam insulation serve as flotation. 6" would support about 323 lbs, 8 inches about 431 lbs.

I have the Columbia River for my wave tank.

My first impression is that you cannot claim both, gravity based AND buoyant support.

If you jack up (lets say 2’) then you HEX will displace a smaller volume of water than when it was free floating. Therefore, the weight of the volume of water the HEX displaces NOW is LESS than the HEX’s own weight. In consequence, no buoyancy.

Yes, it will be displacing less water than the Hex weighs. But it won’t be displacing zero water, therefore is still a buoyant structure AND a gravity based structure. You may be right that it doesn’t matter from a descriptive or legal point of view.

I should be able to simulate the stability by marking my model, and lifting it/re-fastening a couple inches higher and see how big a wave it wakes to make it move. 2" of foam should provide almost 108 lbs of buoyancy. With 6" of foam in the water, I should be able to stand in the middle of it.

With 6 x 8’ sections of pipe, that would simulate 96 ft of spud leg.

The main benefits I foresee from maintaining partial buoyancy are:

  • scaling the deck to longer spans, potentially not necessary in the small hex described here,
  • a graceful failure mode for the spud legs, and
  • the option to move it at a future point.

(Larry G) #64

Spud Ops:



Another concerning fact is that @ certain sea states your HEX’s topsides will be pounded big time by the waves (lateral force) and also that your HEX’s underwater part of the hull will be subjected to an uplifting force.

Wouldn’t that uproot the studs from the bottom?

Isn’t the fact that those stud barges are operated jacked up high and dry over the wave action bellow the main reason to avoid such uprooting?

(EK) #66

I was wondering a bit about that myself. From what I’ve seen, most jackup rigs and similar designs seem to capitalize on having just the legs in the waterline, lessening the wave forces.

Would the spud legs be able to resist the lifting/imbalance due to waves or is that something that would be more based on the interconnectedness of the other hexes?

(Larry G) #67

Subject to testing with models, of course:

I expect the finished hex shell to be in the range of 250-400 tons.
Internal fittings, systems, and furnishings add more weight
I expect to jack up the platform such that water displaced is <100 tons.
The concrete shell is ~12 of wall above the internal deck, with another deck (house) above that.
The displacement cavity below the internal deck is about the same or slightly more ~12-15’ to the bottom of the skirt.

If even half of the skirt-to-internal deck distance is jacked up, that means there is 18’+ from average waterline to the top of the concrete shell.

That means the platform would have to be submerged by a long-wavelength, high enough amplitude wave to equal another 150+ tons of displacement across the ~50’ width of the shell to begin to float the platform. Taking into account the locale specified (Cay Sal Bank) where average wave height is 2-4 ft, and storm surge is ~6-8 ft I think that it’s do-able.

Yes, I believe that most jack-ups are out of the wave zone to deliberately minimize lateral loading. However, most jackup rigs are designed for super precise placement and angles over a drill apparatus. Stability and lack of movement are critical factors in those designs, somewhat less so in mine. In comparison to 3 or 4 spud legs on typical temporary emplacements, I expect to have 6 or 7 legs (one for the point of each angle and possible on in the center) of similar size and structural strength to the standard 4 legs. So 50+% over-build. Compared to the very tall legs that are in common usage, I plan to limit my depth to something under 10 meters (27 meter spuds on barges are pretty common, and jack-up rigs quite a bit more than that.)

I also expect to have multiple hexes eventually added to the first. The more of them placed, the more stability against lateral loading.

The shell is conceived to have additional space in the displacement cavity for over-spec’ing flotation for initial emplacement (towing) safety factor and emergency buoyancy should the legs fail. Such additional flotation can be reduced once on site so that the chance of inadvertent movement is minimized (in the worst case it would simply float lower in the water, causing damage to systems but maintaining a livable surface area until rescue).


i realy love the mother platform feature, self-replication would be an important characteristic of the seastead.
The hexagon is a good choice for the shape, and i would probably add rectangular and triunghiular shapes also.
Whats the maximum load for this structure?

(Larry G) #69

I included some links to concrete slab engineering earlier in the thread.

I don’t really think of engineering this in terms of maximum loads. I think of engineering it in terms of “adequate for living space using rule of thumb common construction techniques,” the way shipbuilders did it for centuries and the way most boat builders still do today for everything except the cutting edge, and pushing the boundaries on size.

(system) #70

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