Nostr

Building materials for seasteads

The high seas are (theoretically) open and free, but come with many construction challenges. Waves and winds should be familiar to all of us. I'm going to talk about construction materials options in the presence of the unholy trinity of biofouling, chloride corrosion, and UV.

Aluminium. Lightweight, inexpensive, easily recycled, handles UV well. Moderate difficulty to work and to repair. No particular resistance to biofouling if left in contact with water, but is not vulnerable to burrowing. Is destroyed very quickly by chloride corrosion. Not a practical option unless you are in a low-chloride environment like the Great Lakes of North America.

Cupronickel (including gunmetal, admiralty brass and similar). Moderately easily worked and recycled. Heavy. Invulnerable to UV. Extreme resistance to biofouling (toxic to invertebrates but not mammals or plants). Decent strength in tension compression and shear. Decent creep resistance. Horribly expensive. Resistant but not immune to chloride attack - no crevice corrosion or pitting, but sheds material very slowly across its entire surface. Combined with its cost this is like watching banknotes blow over the side. Can be practical for piping and similar when too small or inaccessible for inspection and maintainence.

Polymers (plastics). Lightweight, inexpensive. Uniquely vulnerable to UV, this can be managed with coatings, free-radical stabilisers and UV-absorbent fillers such as titanium dioxide. Mostly low strength, lower stiffness and no creep resistance unless reinforced with glass fibre, carbon fibre, or metal fibre. Easily recycled... unless you add fillers or reinforcement: you need both. Good resistance to biofouling. Essentially immune to chloride attack (though metal fittings and reinforcement might be vulnerable). Cheap and practical to build, moderately easy to repair, impossible to recycle.

Steel, coated. Very easily worked and repaired, moderately easily recycled. Cheap. Moderately heavy. Immune to UV. Coating can provide good resistance to biofouling. Excellent strength in tension, compression and shear. Excellent creep resistance. Quite vulnerable to chloride corrosion and related sulphate corrosion, can be managed with constant inspection and maintainance. Best option for working boats / infrastructure that expect to be damaged and repaired almost constantly.

Steel, stainless 316. Easily worked and repaired, moderately easily recycled. Expensive. Moderately heavy. Immune to UV, highly resistant to biofouling, more so if coated. Excellent strength in tension, compression and shear. Excellent creep resistance. Resistant to chloride and sulphate corrosion, doubly so if coated. This is not true of the cheaper 304 stainless and similar. Best all-rounder if you can afford it.

Your thoughts? Other options I may have overlooked?

Luke 2y ago

I've heard that Titanium is still too expensive at our scale, but a particular grade of aluminum is what a lot of oil rig pontoons are made of. Oddly, when I look around online now, industry standards say 316 Steel everywhere I look.

Either way, do you know if the thermal spray coatings they do with molten zinc or zinc/aluminium alloys work on both steel & aluminum? Sounds like those are needed for longevity in both cases.

Reply to this note

Please Login to reply.

Discussion

Low Information Voter 2y ago

Molten zinc spray will melt our aluminium, I expect. Maybe not literally, but enough to make it sag and creep under load. Steel would be fine, but need it sandblasted and perfectly dry for adhesion. So while this may be possible in manufacturing on land, its not maintainable "in the field".

There are a lot of different paints and coatings we could use. I wouldn't attempt to choose just one, but its a mature industry with huge economies of scale and many niches.

Luke 2y ago

I can't think of any spar & framework material that is maintainable in the field except concrete. (Which has pro-reef properties underwater) Maybe the goal should be to use whatever metal framework is cheapest on the inside and cover it up completely in concrete? That way it could be patched in the field, and seastead platforms could last centuries.

I've also had the idea to make each spar completely detachable so that if it had a problem we could simply pull a pin and drop that one, depending on dozens or hundreds of other spars underneath the entire structure to absorb the load while a new one is being made/shipped out.

Low Information Voter 2y ago

With a caisson, anything that is weldable is field-maintainable these days. At least, carbon steel is, I have on good authority.

Concrete I am deeply conflicted about. Wire-reinforced concrete is the traditional material for oversized, under-capitalised "heartbreaker" projects that end marriages but never leave the building yard (except eventually to a landfill) :-p

Concrete has very good compressive strength and creep resistance. But is hopeless in tension and shear. The steel wire mesh upgrades "hopeless" to merely "poor". Until it rusts into powder and bursts the surrounding concrete as well. And it will - high pH concrete doesn't protect against chloride for long. Instead of steel reinforcement you might think to try using glass. But the high pH makes glass hydrolyse into gel. You can use stabilised glass with a high % of non-silica glasses, but there's no real advantage (cost or otherwise) over glass reinforced plastic then.

What concrete CAN do is something unique, and Luke I think you alluded to it with your "reef friendly" comment.

Biorock. Aka Seacrete.

If fully submerged in seawater, and if the reinforcing mesh is electrically conductive (metal or carbon fibre), and if a current is impressed on it (with the mesh as the negatively charged terminal), the concrete "grows" and "self-heals" with minerals from the seawater (mostly Brucite). Hard-shelled marine invertebrates colonise the surface and grow at amazing rates, using the electrical current to fix shell-building minerals more efficiently than they can in nature.

At least, that is what the patents and many articles tell us.

I plan on doing some experiments myself, starting this weekend.

For a permanently submerged module (or entire seastead), concrete could be amazing. I planned to do a proper writeup with links, but for now:

https://en.wikipedia.org/wiki/Biorock