The Seasteading Institute

towers are essentially large diameter concrete pipes standing vertically on a concrete barge with a platform built across the top. The barge was then towed out and sunk with water filling the pipes up to its level. I have no idea how thick they are but since they are not under pressure I would guess about 6 to 8 inches thick…

The record of 303m is held by the Troll A offshore platform. It’ll set you back some 26 billion English Pounds.

http://en.wikipedia.org/wiki/Troll_A_platform

http://en.wikipedia.org/wiki/Condeep

There might be cheaper ways to build from the ocean floor though, if you don’t need to use the space inside the towers, but to me this way of seasteading seems both expensive and inflexible. Perhaps it could be worth trying if you found a very shallow seamount in international waters.

Looks like a version of Clubstead. [See: the front page of the website.] The only major difference (besides function/purpose) is in the added cost of attaching it to the sea floor instead of allowing it to float, along with all the benefits and drawbacks of being completely stationary.

So, to breifly answer your question before the big debate reignights on the subject: Added cost of materials to fasten it to the sea floor is one negative. Another reason that it hasn’t been discussed here recently is that many people want a somewhat mobile structure, and all the benefits that come with it. It’s been considered before but the current focus seems to be on floating structures.