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Alternatives to spar designs
Fri, 05/16/2008 - 23:03 — thebastidge
What about wide cones?
- One of the things that bothers me about spar designs is the it is dependent upon an otherwise-non-functional ballast to maintain orientation.
- I understand that nearly every water-borne platform of any serious size ever designed has made use of ballast. I don't have an issue with that.
- But rather than creating further stresses and mechanical challenges to righting our habitat, why not try to make it as stable as possible by its very shape?
- A traditional boat hull, while it might not ride perfectly without ballast, is reasonably good at keeping itself upright.
- The sharper the "V" of the hull, the more it needs ballast to keep it from rolling sideways, but only because it is bilaterally symmetrical.
- A cone would be radially symmetrical and wouldn't have any reason to tip.
- A good portion of the buoyancy/displacement would still be below the wave line with a habitat of any size, giving it the desired vertical stability
- The surface area subject to wind/ sail area/ shearing force could be a fairly oblique surface
- A wide/large enough cone shape might possibly be designed with a lagoon/open water shaft in the middle- potentially useful? I don't know.
- Safer because if you lose the ballast either accidentally or because some contingency makes it necessary, you still don't capsize
- Conversely- more cross section is presented to wave action, which may add to rolling motions
- Then you also get to maximize your solar area by making the top a matching cone or tratrahedral shape, rather than perching a platform at the top of a pole.
- You also don't have to make it as long to achieve the same internal volume (and therefore displacement for buoyancy),
- subjecting it to less shearing force
- Giving it a shallower draft- aids in close-in mobility and in finding gentle harbors
- obviating the need to manufacture it on its side and then tilt it vertical in a risky and technically difficult operation
You still have the option to attach a good heavy ballast to the bottom section, either in the interior of the cone, or as I mentioned before, in concentric rings of "troughs" around the outside
it will look like an childs "top"
the ballast being the structure itself. densest in the center with a shaft that serves as a light collection platform towering above the structure. I'm not an engineer, just thinking aloud...
a Mound !
That's basically a mound, isn't it ? My reflexions on the spar design have led me to believe it's our best bet for low-cost early designs: simply pile up lots of floating material until you got a big enough mound floating in one piece.
A note about ballast: you don't need ballast for stability at all. For example anything with three feet is isostatic, add just another to make sure you won't capsize if you lose one foot by accident. Another way to achieve stability, is to design your structure to be at its lowest vertical mass distribution (= make its mass more horizontally spread than vertically).
Well, in its most extreme
Well, in its most extreme example, bend a thin piece of aluminum (A coke can should work) into a "V" and seal each end, and then try to float it.
It's also a fairly simple shape for manufacturing purposes.
An upside-down cone. Sort of
An upside-down cone. Sort of like a spar buoy, but flotation accelerates (or some such) with added weight and more naturally structurally sound than a big cantilevered platform on a stick. Or, like someone suggested (regarding the shape of the counterweight though) you could make the shape exponential for increased distance and hence leverage from the counterweight.
http://img404.imageshack.us/my.php?image=sparbuoyvariantza1.png
Curves are tough
Complex shapes require more expertise, time, and expense to manufacture than simpler shapes like cylinders or cones. A funnel or "trumpet" shape is much more difficult to create.
Concave? It´s still a
Concave? It´s still a cylindrical shape at any given depth. Are you sure about this? I agree that in a 2-D drawing it is concave, but I´m not so sure a 3D shape would have the same structural drawback. I could be wrong though... I guess the pressure works perpendicular to the surface...
On further thought you may
On further thought you may be right. Depth pressure should crush laterlally, which would be a cylinder at any given point.
An engineer would have to
An engineer would have to settle this. I couldn´t tell one way or the other. I agree it would be harder to build, with concrete at least. Perhaps not so much if you used welded sheet steel. Or, on second thought that might still be lot more work than a cylinder.
Concrete bubbles floating foam
I've started working on a simple kind of construction, where you first manufacture hollow concrete spheres in various diameters (4 meters, 3 meters, 2 meters and 1 meter), then pile up the larger ones in the water and keep them together with a net, then add smaller ones (one diameter kind at a time) and finally pour some more concrete on top of this "floating foam" (maybe the spheres could have been initially dropped onto a floating plastic film, so it does not leak through) and let it cure to get a rather large floating platform to build on. Extending the island would be a matter of pouring more spheres and attaching them to the structure (ports and trusses and pretty much any kind of support could be integrated at the last stage). I'll try to get pictures up, and you can have a direct look at the 3D model in Second Life, in region Everlite.
It's a bit like Spiralisland, but with foam-core concrete spheres replacing the plastic bottles. That should make it much more durable and storm resistant. Buoyancy can be added simply by sinking a few more spheres below to adjust the tilt or rise the structure a bit.
More alternatives
Apparently, the weight of the upper emerged structure can be used as a way to constrain the underlying buoyancy structure in compression, for relieving bending and shear stresses caused by the waves. There are not a great number of structures that allow this, so I think any ferrocement-built seastead will have to resort to one of them.
The truncated pyramid falls
The truncated pyramid falls more in the 'Akido' method of handling waves mentioned in the Seasteading book, but the pyramid needs gentler slopes and a depth of 50' to a height of 50', overall height 100' for rogue waves.
Aikido indeed
As an enthusiastic aikidoka, this is the kind of solution that fits me :) Why the gentler slopes ?
gentler slopes
The waves hit at a more oblique angle, which allows them to move upward along the slopes or downward along the slopes rather than smacking with full force into a flat wall. Resisting a force takes more inertia and structural strength than diverting that force. The further it is diverted from it's original path, the more inertia (mass) and structural strength needed.
So the gentler the slope, the less we have to divert the force of the wave. Unless we just let it wash completely over the top, which could be bad too.
Another try
What about this kind of structure: pyramid villa (cut view)
no go
Even islands can't take big waves washing completely over them. It takes years to recover from that. Look at the Tsunamis and storms in Asia the last few years. Closing in on millions dead. One advantage to the opean ocean is being able to ride out such big waves rather than resisting them. The whole point of minimizing surface presented to wave action is to damp reactions to waves, not meet them head on, and not to let them swamp us (and wash away everything we have worked for and depend on for survival).
Tsunamis not a threat in the open sea
The slopes on the pyramid brake the waves down: they start to shoal immediately and collapse before they reach the summit. Also, the kind of waves that ruin islands and kill millions, tsunamis, are completely invisible to this sort of structure floating in the sea, they become dangerous only when the water becomes shallow at the coastline: that's when they shoal from a 100 km wavelength and <1 meter height to a >25 meters height and to a comparatively reduced wavelength, by braking on the sea floor.
Rogue waves
http://en.wikipedia.org/wiki/Rogue_wave_(oceanography)
Braking and breaking
I'm aware of freak waves. They're not very destructive to a "beached" design, for the same reason that you don't see news that freak waves damaged shore equipment despite their occuring all the time everywhere: they get braked down by the slope (while tsunamis pass beneath the structure with minimal interaction as their displacement is spread on hundreds of meters of depth). They do impose severe constraints on any ocean-going design, though, and in the case of a truncated pyramid design the important characteristic is not so much the height than the wavelength.
Another try
(Duplicated comment)
It seems to me that any
It seems to me that any structure with an cross section area this large at the water line would be very much affected by waves. Unless you built it huge that is. This close to the water you´d run the risk of large waves washing over the whole habitat as well.
Mech engineer needed
As long as the structure is tall enough, and can rest on just half of its wet surface and not break, it should be OK.
Steel pylons
Or another solution might be to put a gap between the habitat and thewide floating block, for bigger waves to wash over and leave without damage: example
Bobbing in the waves. . .
Jesrad - I think that vtoldude is pointing out that these structures would probably bob in the waves, which is something the spar design sought to minimize.
Your point, if I understand correctly is that this really isn't an issue as long as they're not going to overturn, even in the worst of weather. I think you're at least on to something there.
Once again, by the way, these are really cool renderings.
Rolling period
This structure does not bob in waves if its roll period is significantly higher than the average period of waves (around 7 seconds). And, considering the size (64 meters along the side on the rendered one) and thickness of walls (0.5 meter expected) the roll period on this thing will be measured in minutes. It won't even resonate in large waves at all. And it would take a killer asteroid strike to generate a wave tall enough to overturn it. I asked my brother for his opinion, and he says the truncated pyramid design is mechanically sound. Now the only problem left is what to put on the "beaches" that won't easily be torn off...
I see
You make a convincing argument. While some motion would be unavoidable, I suppose, you're probaby right - it would be nothing like vigorous "bobbing."
What if the rolling period
Resonance
Yes, if there is a continuous stream of waves with a period the exact multiple of the rolling period for the structure, it will resonate and start rocking. But exact period multiples are very unlikely (not enough to discount it, of course), and their effect is also simultaneously negated by any other different frequencies present, which is the case in the open sea: there are always smaller, lower period waves coexisting with bigger, higher period waves. And the rocking would be damped by the drag from the water, which is much higher than in air, so it could not exceed a given degree of roll - no overturning, no structure failure, but a severe case of sea-sickness. In any case, changing the direction of the pyramid so that its corner, instead of its side, faces the waves directly, would change its rolling period (it's not the same period value along the sides and across the diagonal) and that would stop the resonating. And if you're still worried, just put ballasts on the sides instead of rigid foam cores, and either use active ballasting to oppose the rocking, or static ballasting to change the roll period.
Pictures
I love your pictures. Please lets get them posted on the site somewhere. I think you have already gotten some of them posted.
Is there any way I could get you to draw some pictures for me? I have a backlog of images that need to get rendered to convey the basic ideas that I have.
Sure !
Let me know what sort of pictures you'd need, I'll happily have try at it.
Contact me off list
Please send me an E-mail off-list. Wayne at Seasteading Dot Org should do the trick.
Big Waves
Big waves at sea contain a massive amount of energy. The spar design is all about trying not to get killed by big waves.
All designs don't use ballast-
TLP's (tension leg platforms) don't. They use the buoyancy of the structure against tight mooring lines.
http://www.atlantia.com/semisub/images/semisub_FPU.jpg
In this picture all the lines you see going to the bottom are risers (vertical pipelines). You can see the anchorlines (under tension) leading off at about a 30deg angle.
Here's a better pic- http://www.atlantia.com/semisub/images/semisub_upclose.jpg
It's simple, very stable and resistant to failure. For instance a flooding of one pontoon results in a minor list that direction. The buoyancy of the structure and the opposing tensioned anchor lines preserve stability while you deal with the problem.
It's simple to build and deploy. How I'd do it is build the base and the legs up to about 40' and launch it. Move it to a spot about 30' deep and ballast (flood it) down to the bottom. Then working from a barge, continue construction. The legs would be cylindrical, steel (or frp rebar) reinforced poured aggregate concrete. The deck pre stressed concrete like a parking garage. I think a truncated pyramid superstructure would work well.
Deployment is simple. Pump out the ballast water, tow it to location, ballast down (pump water in), connect the tension legs (anchor lines), and pump the water out tensioning the legs.
The anchors would be suction piles or "SEA" (suction embedded anchors). See www.sptoffshore.com go to products, SEA anchors.
Simple and stable, probably the most square footage per dollar possible. It could be constructed by the owners without a shipyard. Shipyards don't build ships, people build them and the shipyard is a middle man so to speak. A middle man trying to cover overhead between projects by adding it to your ship. pet peeve there.
A smaller design could be steel, but again it could be self constructed after launching the base. Or the base could be self constructed in cofferdam or drydock.
Here's an interim solution for construction housing and support. It's an ex Navy YRST II dive support barge that accomodates 100. I've inspected it personally. Cheap at twice the price and much nicer than the pics would indicate.
http://www.freewebs.com/riversummers/yrst2forsale.htm
How do you move it? Will
How do you move it? Will moving disrupt life on the platform?
how do you move it?
Towboats. The tension in the legs or anchor chains (called legs cause they are rigid under tension) is produced by ballasting down (flooding the base and legs), connecting the anchor chains, and pumping the water out thus tensioning the anchors against the large amount of buoyancy. In the flooded or ballasted down condition, the vessel is stable, like a semi-submersible. It can be towed and even can be moved to shallow water and set on the bottom.
Moving is releasing the anchors and towing. The suction piles are just big pipes with a top welded on. They are so cheap they can be abandoned (or left to return to later). The chains (legs) can have buoyancy built into them (attached) and can be towed rather than recovered onboard.
Externally Ballasted Spar is like TLP
Somewhere in one of the forums I talk about the mooring issue in greater detail. Suffice to say, that an externally ballasted Spar is kind of like a TLP, but without the mooring lines.
DP & Energy.
I'm getting the idea that the plan is for a DP spar. I've only run smaller DP vessels. I've been on larger ones but not for a long time.
DP has to be sized for stationkeeping in maximum conditions. I would think that would equivocate to 4 or 5 knot motive ability. That sounds very expensive in terms capital cost, maintenance, and energy, especially for a vessel that only needs to move occasionally and has such a high waterplane area.
Deepwater (I'll call that 300-1500 meters) mooring technology has progressed much in the last 5-10 years. There are several applications that are rapidly developing that will advance it to the status of everyday use in the next year or two. I haven't done any hard research, but I'd bet a case of Corona that the cost difference between DP for such a vessel and mooring (even with moving several times a year) is an order of magnitude.
I would think the operations will require a utility/supply vessel. The smaller ones like the 110' to 130' class are cheap to buy and operate. You can set several suction piles a day in 900-1000 msw with a 130' utility vessel. You could also set moorings for the undersea current and floating offshore wind generator industries which are about to explode. Walt Musial of the NREL in Boulder, who's been in the wind industry for several decades, says that deepwater (in this case 20-60 msw) wind power has the potential to supply all of our power requirements in the US. (Excepting the transmission obstacles, but then the majority of our power requirements are concentrated on the coasts anyway). What a business to be in! Here's a patent on setting them with an ROV. I think I've got a better method that provides more precise placement. 9It may infringe though).
Design elaboration- I was thinking that a 5 sided platform would be practical. Five corner pontoons and a center one. A good sized seastead could maybe support a central 2 or 3MW unit and five 500kw-750kw units (mostly restricted by spacing requirements). That's a lot of energy. Enought to sell. And then there's the carbon trade credits that can also be sold or owned by a sponsor or VC. Lots of big companies will need them very soon.
Thebastidge said in another thread- <<You need to find a reason for being on a seastead that doesn't exclusively revolve around avoiding the laws of where you're at.>> Commerce would seem to be the most likely. Renewable energy is the "oil" of the future, short and long term.
Don't want to be tied to a subsea cable? Okay, make and ship H2. And it won't be H@ genreated from natural gas, it will be CER genereating H2. Not to mention how good everyone would feel about saving the polar bears. Or the postive PR benefits of of being in the planet-saving renewable energy business.