Shell has been served with a serious safety warning by UK authorities following a gas leak on its Brent Bravo platform in the northern North Sea.
The gas release – described by Shell as low level - was detected on a pipeline valve on 9 June.
Inspectors from the Health & Safety Executive (HSE) issued the operator with a prohibition notice 10 days later, it emerged on Wednesday.
HSE said Shell had not taken measures "to prevent an uncontrolled release of hydrocarbon gas from the facility's gas export system", including in pieces of equipment called "pig traps".
HSE pointed out an unwanted accumulation of gas in pig traps could lead to difficulties in blowing down the export system – effectively derpessurising it - in an emergency.
A Shell spokesperson said: "The team on the platform took immediate action to depressurise the line and to bring the situation under control. These and subsequent actions have addressed the issues raised in the notice, and the affected system has been returned to normal operational service. The safety of our people remains our primary concern."
Jake Molloy, regional organiser of Aberdeen-based trade union RMT-OILC, said: "This gas leak is unfortunate. It should be pointed out, however, that the inability to be able to blow down in the event of accident is something that introduces a major element of risk."
An HSE spokesman said: "HSE can confirm the prohibition notice issued on Brent Bravo was complied with and the investigation will be concluding shortly."