Like a boat, this thing has to perform at least well enough to keep everybody and their cargo alive and undamaged. Some hazards are capsizing, flooding, sinking, breakup, asphyxiation, violent motion, and falling overboard.
Capsizing
The module is 5 times wider than high. We think it would take a tsunami to overturn something with such a low center of gravity. But we don't know enough to make the call. Needs some input from a marine engineering expert.
Flooding
While capsizing is highly unlikely, and sinking impossible so long as floatation cells remain intact, large waves can certainly break over the deck, which is only 7 feet above the nominal water line. This heavy-weather issue is not fully resolved. There are plenty of solutions. Some possibilities are: batten-the-hatches, a conning-tower entrance, a sea-wall around the perimeter, and some kind of double door 'airlock'. If we allow any windows on the outer wall this could be a risk.
Sinking
This is the big no-no. We can make it impossible to sink, and unlikely for major pieces to sink, in the case of a breakup. We do this by leaving a few of the 25 interior cells closed for emergency floatation. The flooded negative bouyancy is rather small, 38 tons compared to 350 tons unflooded bouyancy, and 3 closed cells should be enough. Need to do the math and account for cargo, etc.
Breakup
This is the part that worries us the most at this point because we don't yet know what the compression, tension, and shear strengths of the two concretes will be, nor exactly what forces they must bear. When we do, we will run the numbers. But they will need to be checked by that marine engineer I keep referring to. The one we don't have yet.
Asphyxiation
How can this happen? The design has to include provisions for ventilation. We have to make sure not to circulate any noxious or suffocating fumes, like carbon monoxide and dioxide, fuel, propane, halon, and chlorine. The ventilation should to include detectors, and incorporate, redundancy, yet not provide additional routes for flooding.
Violent motion
Since these platforms float right on the surface with almost no draft, they will be thrown around more by wave action than a conventional boat. Just how much is another heavy weather issue that needs to be examined by an expert. The answers effect what forces the platform must bear, and the comfort and safety of the inhabitants.
Falling overboard
There is no provision in the design yet for any kind of wall or rail at the perimeter. Something will be required. Also life-rings, throw-bags, and dingies must be available. In the expected configuration, dozens of these hexagonal modules will be coupled to form a continuous, flexible carpet. Man-overboard is not only a hazard on the outermost modules. There is a 20” gap between modules due to the coupling bumpers into which someone could fall.
You have to experiance heavy weather at sea to appreciate the violence and scale. Wind forces on this low-profile structure should not be a problem. Wave forces will have to be carefully considered. The modules are highly bouyant so they will ride over waves rather than suffer impact. One set of hazardous forces are inertial, from tilt, lift and fall. The other kind are shear forces arising from uneven lift, as waves pass beneath it.
For waves with a wavelength at least twice the module diameter, or less than half, these forces will be comparatively small. The worst case is when the wavelength equals the diameter. The module could be periodically suspended with either the center or the ends out of water. The mode of failure would be to break in half.
A second durability issue is erosion or corrosion by waves, sun, freeze/thaw cycles, marine organisms, and salt water itself. We can probably discount waves and sun. There is a lot of concrete in use around and in salt water so i think that will not be a problem either. There is one case to beware of. If any of the steel reinforcement gets exposed to salt water, corrosion will propagate slowly through all the connecting steel. The concrete can be shattered by expanding rust. So we take care not to do that.
Small amounts (eg 5%) of air are often entrained in normal density concrete because it improves resistance to freeze/thaw cycles. The way it is finished prevents air cells from appearing at the surface. Cellular concrete, being as much as half air, has a surface pocked with tiny (.5mm) holes. I would expect it to erode readily from freeze/thaw if exposed. Our modules have a skin of normal concrete several inches thick that will protect the cellular concrete from the elements. Cellular concrete has a reputation for higher freeze/thaw resistance than normal concrete.
We come to marine organisms: algae, barnacles, mussels, seaweed, plankton, krill, coral. So far as we know, none of these things will erode normal concrete on a scale of tens of years. It needs to be settled for certain by someone who knows. Since if we go anywhere it will not be in a hurry, we may not care what grows on or around us. Some of it may turn out to be food.
These modules cost something like $6,000 each to produce. If you think of one as a small house and lot., you might want it to last as long as a house. 150-200 years would be a good target. Such a concrete structure on land would be expected to last one or two hundred years. In the ocean, there may be other limiting factors. We know of none yet.