The investigation also found there had been longstanding failures within the refinery safety management systems and as a result the risks posed by flammable atmospheres within the Amine Recovery Unit were not understood or controlled.
At the time of the incident the refinery was operated by Chevron Limited, but ownership changed in August 2011 when the sale to Valero was completed.
Valero Energy UK Ltd of Wood Street, London, pleaded guilty to breaching Sections 2(1) and 3(1) of the Health and Safety at Work etc Act 1974. The company have been fined £5 million and ordered to pay costs of £1 million.
B & A Contracts Ltd of Hubberston Road, Pembrokeshire pleaded guilty to breaching Sections 2(1) and 3(1) of the Health and Safety at Work etc Act 1974.They have been fined £120,000 and ordered to pay costs of £40,000.
Speaking after the hearing, HSE inspector Andrew Knowles said: “This incident, which had devastating consequences for all of those involved, was entirely preventable. Many opportunities to take action to control risk were missed, that would have prevented the incident from occurring. It is important to realise that the incident could have had even more serious consequences had the butane sphere or pipe track been damaged by the flying tank roof.
Detective Superintendent Anthony Griffiths said: “Officers from Dyfed-Powys Police worked closely with the Health and Safety Executive to support them in the very complex investigation to establish the cause of this tragic incident. We hope that the lessons learned ensure that a tragedy of this nature doesn’t happen again. Our thoughts remain with all the families involved.”