They are small (2.5″ in inside diameter), but they are strong. We are talking about Resinex floatation collars which can be put into extreme depth and which have already been used by Heeremac and Aker Marine in the Gulf of Mexico where they are normally used down to a depth of 200 metres. 
When they were tested in the NATO laboratory in La Spezia, they stood over 100 bars (1000 mts) of depth preassure.
