The operator of the UK’s largest oil refinery has been fined for health and safety breaches after a leak of liquid petroleum gas (LPG) was discovered by a worker cycling home at the end of their shift.

Southampton Magistrates’ Court heard how, on 15 November 2015, there was an uncontrolled release of around 15 tonnes of LPG through a valve near to the main roadway used by LPG road tankers visiting Esso Petroleum Company’s refinery in Fawley, Hampshire.

The leak went undetected for around four hours before being discovered by an employee on his way home. It took a further hour to establish the source of the leak with on-site emergency personnel having to enter the area to reset the valve.

An investigation by the Health and Safety Executive (HSE) found that the leak occurred because LPG was put through the pipe work at too a high a pressure for the valve, there was no process in place to detect the discrepancy in the flow in the pipe and that the company had failed to take all measures necessary to prevent a major incident.

Esso Petroleum Company Limited pleaded guilty to breaching Regulation 5(1) of The Control of Major Accident Hazards Regulations (COMAH) 2015 and was fined £500,000.

Speaking after the hearing, HSE inspector, Jonathan Halewood said: “The measures required to prevent incidents should be proportionate to the risks. Where companies handle large quantities of substances that can cause major incidents, such as LPG, they are required to have layers of protection in place to prevent incidents.

“In this incident a number of those layers either failed or were not in place resulting in a significant leak. Even though there was no fire or injury on this occasion, there was potential for a major incident. The prosecution has been brought to highlight the importance of maintaining the layers of protection and preventing this kind of major leak.”