Alvin operations are carried out in an environment imagined to be extremely hostile and the degree of risk to the vehicle’s passengers can be easily exaggerated. This section is intended to address and concerns that a first time user may have.
Perhaps the most misunderstood hazard is that of the extreme pressure at our maximum operating depth of 6,500 meters (21,325 feet). At no time are the pilot and observers exposed to anything other than normal sea-level pressure. The hull of the submersible is built to withstand an external pressure 25-percent greater than that experienced at maximum operating depth. Thus its integrity is assured even in the unlikely event that the normal operating depth is exceeded.
In order to ensure that no water leaks into the cabin, all openings for wires, viewports and the entrance hatch are designed in such a way that they provide a tighter seal the deeper the submersible dives. These devices are thoroughly checked prior to each dive.
While the submersible weighs 20 tons in air, it is so configured that when submerged in salt water it is exactly neutral, requiring only a slight force to move it up or down. It is therefore possible, even if all normal power is lost, to ascend to the surface by dropping a small amount of weight. In order to guarantee that this can always be done, four emergency batteries are provided within the cabin, any of which is capable of operating all of the weight releases. In primary operation, six releasable weights are carried at the start of the dive; two of these are dropped when the submersible reaches bottom to effect neutral buoyancy. Releasing two additional weights will make the submersible buoyant enough to return to the surface with no further action. The remaining weights are available as needed. If for any reason additional buoyancy is needed, both of the large battery tanks may be released. The batteries, each of which weighs 1,450 pounds in water, are mounted on separate release devices which may be activated by any of the emergency batteries. Both manipulators and the sample basket are releasable which would reduce weight, but more importantly, may eliminate the cause of the emergency if resulting from entanglement. The surface support ship is always in the immediate vicinity and in continuous communication with the submersible’s occupants.
Each year the safety equipment and the other systems receive a thorough evaluation by the technicians and engineers of the Alvin Group. Their work is reviewed and checked by a U.S. Navy certification team and any discrepancies are addressed before authorization to operate is granted. Prior to each dive, all systems and components are functionally operated by the crew and the pilot. Any safety-related failure or malfunction discovered during these checks must be corrected and the equipment rechecked before a dive can begin.
Nearly sixty years of operation, encompassing more than 5,100 dives, attest to the effectiveness of the rigorous program which has assured, and will continue to assure, the safety of Alvin operations.