Ballast tank hydrogen

A member has reported a serious confined space incident in which a crew member was injured. 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.