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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:

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