May 21, 2008 at 12:00 pm #493
Perhaps we’re getting ahead of ourselves on designs. The first step in a design process is to identify key requirements:
- Minimize vertical travel from wave action
- Horizontal stability
- Useful internal volume (what constitutes adequate will depend on further discussion, but this is the starting point)
- Useful surface area (see point 3)
- Lifespan of decades, or longer. Call it an absolute minimum of 50 years’ durability
- Withstand extremes of weather conditions
- Accessible to some form of support vessel (commercial ships, private vessels, submarines, Naval vessels)
- Able to be largely self-sufficient in terms of producing necessities of life (food, water, shelter)
- Graceful failure mode
- Systems for:
- temperature maintenance
- fresh water
- basic to advanced medical
- Electrical generation
- communications (internal, external)
- emergency evacuation
- fire suppression
- food processing
- basic repairs (machine shop)
Basic considerations to achieve these goals:
- OK, so the purpose of having the majority of the buoyancy below the wave line is so that wave movement minimizes displacement changes (by being higher or lower on the hull at any given point) thus minimizing the vertical travel of the entire structure. Ideally, there would be no displacement at the wave line, but a spar design almost certainly requires some sort of internal access to displacement lower in the hull, which means a hollow tube. Other ideas I’ve had include baffles, much like a fuel or other type of mobile tank for carrying liquids has baffles to prevent sloshing and changes the center of gravity rapidly, perhaps horizontal extrusions from the hull (extended rings) well below the wave line could greatly incease the drag from vertical movement, thus reducing the Spar’s tendency to bob in the waves.
- on a spar design horizontal stability is mostly achieved by ballast
- Some displacement may be left empty to adjust ballast, but some of it needs to house people and systems.
- Surface area has pros and cons that impact on other design issues, like stabililty and durability
- both materials and engineering impact here
- Largely an engineering function, but also geographical placement
- Design issue
- Very complex- design, geography, systems and expertise
- lots of subdisciplines- this will largely depend on people who can fix things. This is where the human capital comes into play. The most robust design cannot guarantee idiot-proof systems that run without maintenance for decades.
On this last note: if you wish to become a Seasteader, the most important requirement is that you are going to have to build yourself into a Seasteader before you can hope to build or even live on a Seastead. If you’re not useful in multiple disciplines, there is probably no room for you aboard.May 21, 2008 at 2:28 pm #2175
- “Minimize vertical travel from wave action”
- This is achieved by either of two ways: minimizing the section at the waterline (spar approach does that) so that waves have little effect on total buoyancy, or maximizing it so that the waves’ peaks are counterbalanced by the waves’ valleys, and vice-versa, for a sufficient range of wave periods. This means that the call for vertical stability triggers a pull in either direction of design: vertically elongated or broadly spread. This has other implications: a non-boxy shape, elongated in either direction, will not work as well mechanically if it is built out of concrete because such shapes favor tensile and shear stresses over compression.
- “Horizontal stability”
- This is achieved by either pulling the center of mass well below the center of buoyancy (by adding lots of mass as ballast) or by designing the shape to have its buoyancy spread more horizontally than vertically. This gives us another pull in opposite directions, towards elongated or spread shapes. There is another aspect to horizontal stability, which is to have the center of mass closer to where the horizontal efforts will apply (the waterline or the mooring point, in our case), so as to minimize the tilting torques. This consideration favors the spread approach over the ballast one.
- Useful internal volume [and] Useful surface area
- That’s mostly a question of pure scale, although I would say that we can make better use of an horizontally-spread living space than a vertical one.
- Lifespan of decades, or longer.
- Here, the ferrocement wins hands down. Submerged steel requires periodic drydock inspection, and possibly bioufouling, emerged steel requires periodic inspection too for corrosion. Aluminium is too pricey. With a lifespan in the decades, it might be good to consider also the option of growing natural coral reefs on the bottom of seasteads. One could maybe even figure out some way to have a continually growing seastead, with one end covering itself with coral and being progressively submerged deeper while the structure is extended periodically at the other end (with additional flotation to compensate the weight of the reef). That would be kinda like the subduction of tectonic sheets.
- Withstand extremes of weather conditions
- This aspect puts a minimum scale filter on designs: they must be large enough to survive the known conditions. We have most of the numbers already (100′ tall waves, 250 mph winds, 4°C saltwater and 50°C under the sun), and they mean there’s a necessary minimum cost to entry, too. The first sea-going prototype will be BIG, but we already knew that.
The rest of the points concern the infrastructure rather than structure. I think I got an idea for an incremental design that follows the spread approach. I’ll try to have a virtual model built soon, and the “subduction” idea illustrated, too.May 21, 2008 at 3:14 pm #2176
Brent Spar (steel) was in the water from 1971 until 1998, was it ever hauled out? 27 years- tthat’s not the timeline we’re looking for, but it would be interesting to see what the integrity of the structure was like at the end, and what the maintenance cycle was.
May 21, 2008 at 3:30 pm #2179May 21, 2008 at 3:59 pm #2182
- Also, on that idea of gradually growing the seastead as one end sinks- perhaps that could be part of a decades-long attempt to create an artificial island. The seastead stay essentially in one place where the seabed is fairly shallow, and over several decades of living, commerce, etc., a breakwater is built up, a large base of artificial reef is built up and coral cultivated upon it until eventually, you can walk and grow things on it.
- Finally, the seastead is grounded or scuttled in place, becoming a permanent part of the landmass.
Correct me if I’m wrong, but wouldn’t ferrocement require periodic inspections as well? Flexing and temperature changes could cause cracks that then allow the rebar inside to rust and decay, could eventually start leaks- and how do you repair that in the open ocean? Steel can have patches welded on even underwater.
May 21, 2008 at 4:24 pm #2183
- Is there a source on your idea that ferrocement is the more durable material, given the type of structure we’re talking about?
I might be wrong but carbon fiber composites are a serious possibility. I know that this might not be such a realistic proposal at the time but would it not be possible to eventually construct seasteads out of carbon fiber reinforced composite materials. they dont rust, they are strong, light, durable, they will not degrade because of the ocean, the only downside i can think of right now is the expense
May 21, 2008 at 4:42 pm #2185
- to point out the obvious, any structure that is constantly in such an extreme enviroment like the ocean, needs to be constantly checked for any weaknesses in the existing materials. There is not a single material that i have ever heard about that will not eventually weaken if it is exposed to the ocean for extended periods of time.
Carbon fibre is probably a very good choice for structures on the platform, provided the expense can be brought down to a reasonable level. Particularly covered structures like geodesic domes with panels of carbon fiber.May 21, 2008 at 5:14 pm #2188
This reply is probably going to be too short.
The major design constraint is economics. Everything else is traded off vs. cost. Next on the list is safety. There is no such thing as absolute safety. It is safety relative to other designs — oil platforms, cruise ships, sail boats, etc. We have been leaning towards buoyancy redundancy — if the spar buoyancy fails, the safety hull takes over. Next on the list is comfort. Again, comfort is relative to other alternatives, oil platforms, cruise ships, sail boats, etc. A better way to state the issue is that we want as much comfort as is economically possible. For example, if people wind up getting seasick during some really bad weather, but are fine otherwise, that might be a reasonable trade-off. Structure longevity is next on the list. No structure lasts for ever. Years of longevity are are required, but decades may not be economically feasible. 25 years is a typical number that is discussed in the off-shore industry. Complete self-sufficiency is a “nice to have”, but not terribly realistic. A goal of mostly energy and water self-sufficient is a first goal. Food self-sufficiency is going to be really tough.
May 21, 2008 at 5:43 pm #2189
- Economically feasible!
- Relatively safe
- Comfortable (within reason)
- Durable (within reason)
- Partially self-sufficient (within reason)
Quite Right. I hadn’t really prioritized, just started off listing some requirements- got too granular too quick, but that’s what this kind of collaboration is for.May 21, 2008 at 6:27 pm #2193
Carbon fiber reinforced concrete? Or cheaper, but almost as strong: Basalt Fiber.May 21, 2008 at 9:13 pm #2199
Naval experiments , http://www.stormingmedia.us/38/3859/A385963.html , showed plain carbon steel fiber to have the best durability in a marine environment, followed by alkali resistant glass fiber. The basalt fiber would need to be alkali resistant to use in concrete. I favor a chopped steel – foamed flexible concrete like Ductal, http://www.ductal.com/ , or University of Michigan’s composite, http://www.umich.edu/news/index.html?Releases/2005/May05/r050405 (don’t know if concrete in link is foamed nor what material fibers) over labor intensive steel rebar which is also prone to corrosion and spalling as the corrosion propagates down the length of the rebar. The foam/fiber composite can be cast in a neutrally bouyant mold system in a protected harbor with sections joined and then pumped out to elevate above the water. Info and experiments with plain foam concrete are at http://pelagic.wavyhill.xsmail.com/ .
The design can be optimized with integrated arches and material thickness to support itself and minimize moment and sheer loads. Using a flexible fiber reinforced material prevents cracking by allowing elastic deformation under varying loads. Foaming the material gives it inherent floatation.May 21, 2008 at 9:18 pm #2200
Sounds expensiveMay 22, 2008 at 2:50 pm #2258
WRT the superior durability of ferrocement, I’m only repeating the online book’s prejudices. I’ll have to research that a bit deeper.May 23, 2008 at 12:44 pm #2253
With a design lifespan of decades, if not longer, I think we need to add maintainability to the list of design requirements. The seasteaders will have to maintain their home in situ, so designing for this necessity is a must. This consideration affects both the configuration of the structure as well as the choice of materials. Generally the cost of maintenance over the lifecycle of a system is far higher than the initial costs of construction. So in examining the economics of seasteading we need to consider the life cycle costs, including the costs of maintenance, not just the construction costs.
“He who does not risk, cannot win.” — John Paul JonesMay 27, 2008 at 2:40 pm #2483
SPAR IS TOO PRICELY! economics first!
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