A diver emerges from the diving bell. (Photos: Alexis Rosenfeld).
On June 28, 1996, four Comex divers left their saturation chambers following 12 days of confinement. In line with the objectives of Hydra 12, the latest in a series of experimental dive programs by Comex, they had achieved eight working dives at a depth of 210 meters over a five-day period.
The exercise was performed offshore Cannes, southern France at a spot sheltered from the prevailing east and west winds. Four of the dives were performed breathing the standard helium mixture heliox, with the rest managed by breathing the newer hydreliox mix of hydrogen, oxygen and helium. Comex has been pushing hydreliox for some time as a better medium for saturation diving in moderate water depths.
On a repetitive basis, the divers connected various pipe sections on a specially constructed sea-floor working platform: the aim was to compare their performance when using the two breathing mixtures alternately. Each dive lasted from two to six hours, depending on the tasks involved. These comprised:
- vertical installation of riser bottom section.
- alignment of the riser with the straight pipeline section and adjustment of the inter-flange gap.
- installation of spool-piece and fitting of threaded rods.
- installation of Hydratight torquing system.
- disassembly of the assembled parts and reconfiguration in the initial state for the next dive.
According to Comex, divers breathing the hydrogen mixture demonstrated a higher efficiency with regard to task analysis and also a greater working capacity when the manipulations demanded a sustained effort. A spokesperson commented: "The ergonomic effectiveness of hydrogen was thus clearly demonstrated in actual open sea working conditions. It is now proven that hydrogen is the best component in breathing mixtures for mid-deep or very-deep dives.
"The helium to hydrogen and hydrogen to helium switches were also perfectly mastered at both physiological and technical levels.
"Following the hydreliox dives, the Doppler ultra-sound tests showed no circulating bubbles in the divers' cardiovascular system. This proves beyond doubt a perfect hydrogen dosage within the breathing mixture. "In addition, the `surface loop' closed circuit breathing system worked perfectly giving evidence of its total reliability and demonstrating how easy it could be to add a `hydrogen out' deck container on a helium-diving support system."
Comex maintains there is a long-term role for divers, despite the advances in diverless technology below 200 meters. Four years ago the hydrogen mix was used successfully on Hydra-10, which involved a simulated 701 meter depth dive at the company's hyperbaric research center.
As back-up for these latest trials, the vessel spread included the underwater operations support barge Nyroca and the INPA (National Professional Diving Institute) diving support barge. Nyroca, moored to four buoys, supported the submerged working platform which was suspended by a cable to the 210 meter test depth.
The divers breathed standard helium mixtures within INPA's saturation chambers, switching to hydreliox for their seabed tasks. The hydrogen-adapted gas reclaim system loop was linked to the diving assembly via the bell umbilical. Oceanographic research ship Minibex monitored the divers in situ using its ROV Super Achille. Outside observers watched the tests from the transparent hull of the submarine Remora 2000.
Comex claims the helium-in/hydrogen-out technique could be adapted to most saturation systems on the large DSVs operating in the North Sea, Brazil, and Gulf of Mexico.
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