Draupner imposed design criteria
It is not about how deep – it is about avoid wave crest impacts, and intermitent liquid air contacts.
build big (half a mile is still small in a draupner impact on the surface).
build for duck mode
Have a bow at least the size of Caledonian Star and Bremen who both did survive Draupners. Such a bow can split a Draupner and avoid its impact on the vulnerable rest of the structure – the Ramform is about that http://concretesubmarine.activeboard.com/t51926036/establishing-a-ramform-floating-base-in-the-high-seas-concre/
Prelude is about that …
go submerged, part submerged, minimize splash zone, to avoid wave crest impacts altogether.
build a wave impact safe bubble ( like a rescue pod) http://concretesubmarine.activeboard.com/t46713498/submerged-living-space-bubble-concept-basics/