Valero Energy Uk Ltd has been fined £5m for an explosion at the Pembroke oil refinery that killed four workers.

The company must also pay £1m towards prosecution costs.

Swansea crown court heard that Valero bought the refinery from Chevron two months after the explosion on June 2, 2011, and that Chevron would pay both the fine and costs.

B and A Contracts Ltd, who were carrying out work on Tank 302 when the explosion happened, were fined £120,000 and must pay £40,000 in prosecution costs.

Western Telegraph:

(Above, the victims of the Chevron Pembroke Refinery explosion.)

Valero were given 28 days to pay in full. B and A will pay £40,000 a year for four years.

Both firms admitted two breaches each of the Health and Safety of Work Act by failing to ensure the safety of employees and others.

The judge, Mr Justice Lewis, said the fines had to reflect the seriousness of the offending and the consequences but also the companies' ability to pay.

The court heard how a massive explosion ripped through Tank 302 at 6.18pm on June 2, 2011, killing Andrew Jenkins, Julie Jones, Robert Broome and Denis Riley. Andrew Phillips survived but suffered terrible burns.

Western Telegraph:

In a lengthy judgement handed down to a packed court, Mr Justice Lewis said the most likely cause was the use of a piece of machinery owned and operated by B and A which had not been earthed and could have triggered a spark.

But the offending was also caused by "systemic failures" by Chevron which could be traced back to 1998. B and A, said the judge, carried out a risk assessment and then failed to comply with it.

The explosion happened as B and A workers carried out maintenance work on Tank 302, part of the Amine Recovery Unit. The process, known as scrubbing, involved mixing amine with oil products under pressure, the aim being to remove hydrogen sulphide.

During May, 2011, it was decided to empty Tank 302 for cleaning and maintenance. The operations adviser decided to postpone the work as he would be away. He contacted the integrity co-ordinator to ask for the postponement but in a misunderstanding the co-ordinator thought he was talking about a different tank and the work went ahead.

The residue of liquid was to be siphoned off but there was a loss of power and the worker involved did not realise the operation had not been completed successfully.

"The operation proceeded on the basis that the tank was empty. It was not," said the judge.

On May 31 a gas reading taken inside the tank showed the level of inflammable gases were high at 66 per cent.

"It is not possible for the court to decide whether the results of the gas test were not properly communicated or, if they were, their significance was not understood," said the judge.

Either way, "Chevron accepted that it failed to heed and react properly to the results that confirmed the presence of a potentially explosive atmosphere."

The blast lifted off the five tonne roof and threw it 55 metres away.

Andrew Jenkins was on top of the tank and Julie Jones was standing on a platform between Tanks 302 and 303.

Andrew Phillips and Robert Broome were on the ground and Denis Riley was sitting on a kerb making notes.

"The flammable atmosphere in the tank ignited," said the judge.

"Chevron's culpability is high. There were serious and in some cases systemic failures."

Mr Justice Lewis said B and A was also responsible for serious failings. Its own health and safety manager was not aware of the relevant safety standards and the company did not adhere to its own risk assessment and method statement.

In a statement following the case, Chevron said: "Chevron fully accepts its responsibility for the tragic events of June 2, 2011. We always seek top operate safely but we reognise that we did not live up to our own standards on this occasion. 

"We carried out an extensive review following the incident and implemented chanegs to avoid a recurrence.

"Chevron continues to remember those individuals, families and colleagues affected by the incident."

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 of Dyfed-Powys Police, 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.”

An HSE-released video of the explosion may be seen here.