1000m depth rating possible for spheres, 500m for tubular concrete structures
…the (study) results demonstrated the feasibility of near neutrally buoyant concrete structures, having an overall safety factor of three, at depths to 3000 feet for spheres and 1500 feet for cylinders. Greater depths are possible if concretes having a compressive strenght greater than 10.000 psi are used or if negativly buoyant structures are designed.
H.H Haynes and R.D. Rail october 1986 first published sept.1976