Hmmm...I'm not so sure they are exceptional. The upper section is rather light; no heavy machinery like an engine, no heavy tanks like fuel oil. They'd likely only flip during favorable sea conditions so the accelerations are kept to a minimum. And once the spar bit is submerged some meters the influence of surface waves will be reduced.
Edit: the bending stresses when its being towed horizontally could be quite large too...in long-wavelength swell you'd have either the center more bouyant than the ends or the ends more buoyant than the center. Once you flip it it's supported by bouyancy and filled ballast water.
