Here's a particularly interesting "enclosed/confined space entry" incident that sort of wasn't, from the Step Change In Safety Website. It highlights an often over-looked hazard - lighter than air gases coming out of a tank:
"While assisting with pre-entry checks prior to entering a tank to carry out quarterly maintenance checks. (The) I(njured) P(erson) lost consciousness and collapsed on deck.
The IP lifted the manhole cover to gain access to the tank for the purpose of air sampling as part of their procedure prior to entering the tank to carry out quarterly maintenance. the IP was working next to his supervisor who began to lower gas sampling equipment into the tank. Within 1 minute of the tank entry cover being opened, while the sampling equipment was approximately 3 metres into the 6 metre deep tank, the gas sensor alarm activated and the IP, who was still outside the tank, lost consciousness. The IP subsequently recovered fully with no apparent residual effects
Subsequent gas sampling during the investigation into the incident recorded unexpectedly high levels of hydrogen within the tank, the presence of which is probably due to the electrolytic reaction between the sacrificial anodes in the tank and the steel within the ballast tank.
This alert has been issued to heighten awareness of the potential for this unusual situation to arise. While one would normally expect heavier than air gasses or lack of oxygen due to scavenging in these spaces One must always be aware of the potential presence of other gases at all times. Due to the unusual nature of the gases involved it is recommended that risk assessment of the proposed task should always include the potential for this lighter than air gas with suitable risk reduction being put in place. Further information and recommendations will be posted when investigations into this incident have been completed