Not sure if this has been discussed before.
http://www.eurekalert.org/pub_releases/2010-07/aiop-acw072710.php
Why not figure out how to use what is abundant and free and already located at the seastead location? Water. If you can make it solid by turning it into high temperature ice, or by other means, then you have a spar bouy setup in place.
If barium fluoride, and the condensing process, turns out to be cheaper than concrete construction and shipping, then I think we have a winner.
I doubt that will be the case for the next hundred years, however, but I’d be delighted if/when I’m proven wrong here.
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There is no fate but what we make for ourselves. Each to his fate.