The HSE (Health and Safety Executive) has issued an improvement notice following the incident which happened in October last year.
It comes a day after the company said it would be reducing its headcount by 600 staff from its North Sea operations.
BP said the incident had occured after the platform was shut down to enable maintenance work to take place.
The notice from the HSE to BP, said: “You failed to take appropriate measures to prevent the uncontrolled release of flammable gas from the Mungo riser lift gas line on level 1 of the ETAP PDR platform in the early hours of 1st October 2015.
“The release occurred when you pressurised the riser lift gas line to 60 bar pressure.
“You failed to take appropriate measures to control maintenance activities in that you had previously checked this line section by line walking on the 11th September 2015, following your Turn Around activities, but had since then conducted pressure testing which involved isolations and related modifications to valve positions in the early hours of the 26th September.
“You failed to adequately record and monitor those isolations and modifications and you fail.”
The company has been told it has until February 29th to compley with the notice.
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