This concept be seen as a variant on the SparBuoy concept. The motivating philospohy is to take what works: a boat; and then make it cheaper, more durable and stable, at the expense of mobility.
I have a boatload of notes on this thing, but nothing particularly readable yet, but im very excited about the results so far.
• Effective real estate (exl hull space): 2000-3000 m2 • Estimated cost (inc: materials, construction; exl: interior, utilities): 2M$ • Water displacement: 6000 cube, 6000MT • Concrete hull: 750 cube, 1800MT • Ballast: 1000 cube, 3000MT • Actual building: 1200MT (150MT steel skeleton, 250MT concrete panels, lots of MT-budget to spare)
These are not at all optimistic estimates: any estimate is taken from the worst data i could find, then adding another factor two in the discouraging direction. I want to emphasize that everything i propose here is technically and financially very down to earth, nothing fancy.
Basically, its a Concrete shell (0.5m wall thickness near the bottom, 20m radius, 20m depth), with a steel deck on top, and ballast at the bottom. No windows, nothing fancy: stable flotation under all circumstances. This combination provides the foundation for an otherwise rather unexciting and hence affordable condo. Only the first ten meters or so are 'engineering driven' (crest-through 20m), that is, they should expect waves to hit them, but that can be taken care of. Above the waves, we can add on lots of square meters, and justify the cost of this thing. The idea is not for this condo to be able to take anything the sea might throw at it, the idea is, if something goes wrong above the waterline, your carpet is ruined, perhaps your entire apartment needs to be rebuilt, but during a storm that bad, youll be safe inside the hull. Once every ten lifetimes or so.
It might be hard to believe, but its center of gravity is about a meter below its center of flotation. That guarantees it will not turn upside down after some critical angle, like boats do. Its weakness is that im pretty sure its going to roll quite badly in heavy weather, and there isnt much that can be done about this by varying any of the parameters involved (keeping cost in mind).
One way to tackle this is to arrange such a design in clusters, connected such as to be able to freely move up and down, but being linked in terms of rolling motion. They could still tip over as a whole, but by clevely choosing the spacing between them, and match them out-of-phase with the wavelength of the wavespectrum that does the most rocking, this problem can be eliminated more or less completely.
Ive since come to realize that vertical stability (keeping up up and down down) is the expensive part of any seastead design. If you have a hypothetical rigid vertical rod sticking out of the water (even if it can support zero vertical load), building a seastead next to it becomes an order of magnitude simpler.
Toying with that realization, and what i call 'distributed stability', yield some even more pleasant simplifications, but they are still very much a WIP: im not done with the math yet, so ill refrain from any grandiose claims.
Further work on this is what is keeping me from turning all my notes, motivations and calculations into something readable to put here, but i will do so in due time. And as you can tell, im a wiki-noob. Feel free to rearrange.