Near miss: onboard O₂ bottle leaked into diving bell

What happened?

The entire contents of the onboard O₂ bottle leaked into the diving bell during a bell run at -140m, causing a much higher than normal PPO₂ reading. (2.2 PPO₂). The incident occurred when the bellman left the Oxygen add valves cracked open during bell pre-dive checks. The contents of the single 50 litre bottle drained into the bell and the bell run was aborted, due to the lack of O₂ which might have been required in an emergency.

What went wrong?

• The Bellman got distracted during bell pre-dive checks and both needle valves were left cracked open;
• There was a long delay before the high O₂ readings reached the topside analysers in dive control due to the 300m bell umbilical;
• The portable O₂ analyser in the bell was not believed because it had been unreliable in the past;
• The bell onboard O₂ pressure gauge was not easily read from inside the bell.

What were the causes

• Insufficient mechanical safeguards were in place to prevent this from happening. In the past, many saturation systems had an O₂ buffer tank installed in the bell. There was a three-way valve attached to it that would only allow the tank to be either filled or drained, but only one operation at a time could be performed. These tanks are not always present now;
• The time it took for bell atmosphere to reach the topside analyser in dive control was unusually long;
• The high reading on the bell portable analyser was not believed as it had not been reliable in the past;
• Human error was also a factor as the bellman got distracted during bell checks and forgot to close the two supply valves.

What lessons were learned? What actions were taken?

• If more than one O₂ bottle had been online, the outcome could have been worse;
• The bell run was aborted, and the bell atmosphere was flushed through;
• The bell divers UPDT was calculated and they were found to be within acceptable limits;
• The divers were stood down and monitored;
• The O2 plumbing was leak checked;
• Additional checks were added to ensure that all of the O₂ valves were secure after use;
• An additional O₂ analyser was added in the bell;
• Buffer tanks were sourced for any system that did not have one already installed;
• Portable O₂ analysers with alarms, suitable for use in the bell were sourced.

Members may wish to refer to the following incidents:

• High potential near-miss: Poor Oâ‚‚ content in supplied air – diver temporarily lost consciousness
• Use of pre-mixed nitrox gases
• Failure to follow gas quad procedure