An apparatus for rapid venting of the compressed air and deballasting of a buoyant air tank in a positively buoyant riser system in the event of a premature drive off or a riser section parting is shown. The buoyancy tank or housing includes an annularly shaped flange positioned on its upper end and retained by explosive bolts. The inner circumference of the flange is sealed to the riser section supported by the buoyancy housing. A sensing means, activated by a parting of the riser, actuates the explosive bolts to release the annularly shaped flange and flood the buoyancy housing while restraining lines limit movement of the buoyancy housing with respect to the riser section.