Dennis Riley, 52, Robert Broome, 48, Andrew Jenkins, 33, and Julie Jones, 54 died after a storage tank exploded at the site in 2011. A fifth worker, Andrew Philips, also sustained major injuries.
Swansea Crown Court heard how on 2 June 2011, the five workers were emptying a tank in the Amine Recovery Unit using a vacuum tanker when the incident took place shortly after 6pm. B&A Contracts Ltd, a long-term contractor at the refinery, was carrying out the work, with support from another contractor, Hertel.
A fireball resulting from the explosion severed a five-tonne tank roof, which shot 55 metres out, hitting a butane storage sphere. The roof narrowly missed a pipe track where flammable materials were carried.
Ignition of a highly flammable atmosphere within the tank, during what should have been a routine emptying operation in preparation for further cleaning and maintenance, was the likely cause of the explosion, said HSE.
Its investigation also found ‘long-standing failures’ within the refinery safety management systems. As a result, the risks posed by flammable atmospheres inside the Amine Recovery Unit were not understood or controlled.
Valero Energy UK Ltd of Wood Street, London, which bought the refinery off Chevron Limited, was fined and ordered to pay costs of £1 million for breaches of sections 2(1) and 3(1) of the Health and Safety at Work Act 1974.
B&A Contracts Ltd of Hubberston Road, Pembrokeshire was also fined £120,000 and ordered to pay costs of £40,000.
Speaking after the final hearing on 6 June, investigating 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.
He added: "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.”