In a recent safety flash, IMCA analyses an incident which regarded a bailout manifold failure, and provides lessons learned.
The incident
A diving bell bailout manifold failed under pressure. No divers were in the bell at the time. The bell was being prepared for diving operations whilst the DSV was waiting on tides and bell checks successfully completed. The bell man had left the bell and was back in the chamber, and the bell trunk had been isolated from the system at 31msw, with the system itself at 32msw.
A little over an hour after the bell checks, the dive supervisor heard a sudden rush of gas over the comms. A check of the bell internal camera showed visible misting (as from a gas leak). Initial investigation lead to the discovery of four pieces of a failed nut from the bailout manifold assembly.
Diving operations were immediately suspended. The decision was taken to surface both bells and remove all bailouts for inspection.
Probable cause
Forensic testing of the nuts from the manifold assembly supported the original root cause analysis which suggested that lateral force had likely been introduced due to inappropriate securing mechanisms.
This led to stress corrosion cracking in the nut and allowed material ingress which caused internal corrosion over time.
Actions taken
- Replaced these particular bailout manifold assemblies with a more robust version currently in use elsewhere in the fleet;
- Ensure familiarity with planned maintenance tasks relating to bailout alignment, integrity and boots;
- Circulate to industry: the forensic analysis report indicates the nuts used are manufactured from copper alloy CW614N which is susceptible to stress corrosion cracking and dezincification. It is known that these are widely used on manifold assemblies in the industry.