A SERIES of systemic failures led to a fatal explosion at the Chevron oil refinery in Pembroke, Swansea crown court heard yesterday (Tuesday).

The storage tank explosion at the-then Chevron plant on June 2, 2011, claimed the lives of four workers: Dennis Riley, aged 52; Robert Broome, aged 48; Julie Jones, aged 54, and 33-year-old Andrew Jenkins died in the accident. A fifth worker Andrew Phillips suffered life-changing injuries.

The powerful blast blew the roof of the storage tank more an 150ft feet across the site sending it crashing into a bupane gas tank, which fortunately remained intact.

A sentencing hearing at Swansea Crown Court heard that the plant had become "fundamentally unsafe" due to a series of errors and failings that contributed to the fatal incident.

Oil company Valero Energy UK, which now owns the Pembroke refinery, and local firm B&A Contracts both pleaded guilty to breaches of health and safety rules when they appeared before Haverfordwest magistrates court on October 11 of last year.

The court heard that on June 2, 2011, workers were emptying a tank at the part of the refinery where substances used to clean crude oil were processed and recycled.

Andrew Langdon QC, prosecuting, said B&A Contracts had the job of pumping sludge out from the bottom of tank 302 to prepare it for cleaning and maintenance.

The work was carried out by lowering a hose from the roof into the tank so that the contents could be pumped into a bowser.

The court heard the hose should have been earthed but was not.

Ten minutes into the operation the contents of the tank suddenly ignited, causing a massive explosion and fire ball.

Mr Langdon said there were two possible sources of ignition; static electricity generated by the hose, or substances in the tank that spontaneously combusted on contact with air.

The prosecution case was that the "overwhelmingly likely" cause was an electrostatic spark. However, it was not possible to determine, to the legal burden of proof, which of the two possible methods had caused the blast.

He added that little or no consideration seemed to have been given to the possibility of the atmosphere in the tank being contaminated with highly flammable gasses such as methane, ethane and hexane - a mistake that was to prove fatal.

The risk of hydrocarbon vapours in the storage vessel "should have been foreseen, and acted upon", he said.

Mr Langdon described five areas of "overlapping systemic failures" which were factors in the explosion.

These included inadequate risk assessments carried out by Chevron and inadequacies in gathering and recording safety data and information.

There were weaknesses in the way on-site permits for carrying out jobs were issued and problems with the way contractors were managed and monitored.

There were also lack of proper working practices or procedures for the task being undertaken, and such procedures as were in place were not followed.

"It is unusual to find so many parallel systemic failures, each of which had a direct causation," he said.

"These are manifest failings which paint a picture of a workplace which had become, over time, fundamentally unsafe."