>>2216360I don't think the underwater part needs to be a strong structure as it will be filled with water mainly.
Bouyancy is calculated by Archimedes flotation principle so a floating object displaces its own weight of fluid, so the spars bouy can be thought of as a very tall ship. the water column would be the ship's hull, the air part of the column + platform weight would be equivalent to the interior of a ship (inside air = much lighter than water + cargo etc). You basically just need to calculate how much water is displaced by the air part of the column and that needs to weigh more than the platform.
Another idea i saw was a self righting boat
https://www.youtube.com/watch?v=iJCQKuMq3vY>>2216390I just read your post, we are thinking the same thing, that would make it alot cheaper to make. I was even thinking does the lower section have to be solid at all, it only has to provide as a drag against the water, if that is true that opens up different possibilities, ie collapsible sheeting or even netting???
if you haven't seen ships stability gyros you should check them out.
https://www.youtube.com/watch?v=Gaffw84Ijx4https://www.youtube.com/watch?v=-CpSL35Hxgsthe is also pumping around fluids to counteract roll
https://www.youtube.com/watch?v=A2QfV11XYD8 