Right now I'm trying to design a floatation device for putting 1000m under the sea (~1500 psi external pressure). The nicest looking and most convenient shape would be a torus but I'm having trouble estimating stress on the thing. I'm looking for around 20lb of buoyancy or more and dimension are roughly: OD: 16" ID: 5" Cross section d~5" At the company I work for they've done testing before with plastic floats of this kind with not so good results. As soon as they deform a little it becomes a positive feedback type situation creating a greater stress concentration and it crushed at relatively low pressure. I guess this means it's a hoop stress type buckling situation but I'm a little fuzzy on how to approach the calculations.