Ballast tank hydrogen
- Safety Flash
- Published on 23 July 2008
- Generated on 1 March 2026
- IMCA SF 12/08
- 2 minute read
A Member has reported a serious confined space incident in which a crew member was injured.
What happened?
The incident occurred during quarterly planned maintenance of the leakage detection system in the base of one of the legs of a semi-submersible accommodation unit alongside fixed production platform.
A crew member lifted the manhole cover to gain access to the tank to undertake planned maintenance. The crew member was working next to his supervisor who began to lower gas sampling equipment into the tank as part of normal pre-entry checks. Within a minute of the manhole cover being lifted, the gas sampling equipment (which was 3m down into the 6m height of the tank) gave an alarm, and the crew member lost consciousness.
Subsequent gas sampling during the investigation was undertaken and recorded unexpectedly high levels of hydrogen. The presence of hydrogen can be explained by the electrolytic reaction between the sacrificial anodes and the steel within the ballast tank below the tank being worked upon.
The crew member who lost consciousness recovered fully with no residual ill health effects.
The company involved made the following recommendations:
- Vent ballast tanks regularly in order to prevent hydrogen build-up.
- Ensure appropriate steps are taken to purge gases from ballast tanks prior to tank opening.
- Using appropriate equipment, conduct tests for the presence of hydrogen before tank entry.
- Remain mindful of the potential for build-up of hydrogen in ballast tanks where sacrificial anodes are used.
- Review gas sampling procedure.
IMCA Safety Flashes summarise key safety matters and incidents, allowing lessons to be more easily learnt for the benefit of the entire offshore industry.
The effectiveness of the IMCA Safety Flash system depends on the industry sharing information and so avoiding repeat incidents. Incidents are classified according to IOGP's Life Saving Rules.
All information is anonymised or sanitised, as appropriate, and warnings for graphic content included where possible.
IMCA makes every effort to ensure both the accuracy and reliability of the information shared, but is not be liable for any guidance and/or recommendation and/or statement herein contained.
The information contained in this document does not fulfil or replace any individual's or Member's legal, regulatory or other duties or obligations in respect of their operations. Individuals and Members remain solely responsible for the safe, lawful and proper conduct of their operations.
Share your safety incidents with IMCA online. Sign-up to receive Safety Flashes straight to your email.