The operators of one of the largest oil refineries in Europe have been fined £400,000 today (Monday) for breaching health and safety at work regulations.

Valero Energy Uk Ltd most also pay £60,614 in prosecution costs as well as their own defence costs.

The company admitted breaching Section 2 of the Health and Safety at Work Act by failing to ensure a safe working environment.

Swansea crown court heard how David Thomas, now aged 55, fell from a walkway linked to an oil tanker being unloaded at Berth 6 at the Rhoscrowther refinery on March 5, 2012.

The access tower had suddenly descended and his legs became tangled in loose cables.

Rupert Lowe, prosecuting, said he ended up dangling over the side after a plunge of more than 10 feet.

But he managed to use his experience as a rock climber to remain calm and to grab hold of a cross beam to take the weight from his legs.

Mr Thomas was rescued by crew members from the tanker and Valero workers and taken to Withybush hospital, Haverfordwest, before being flown by air ambulance to Swansea’s Morriston hospital for specialised treatment to his injured legs.

Mr Lowe said he suffered fractures to both his legs and his left knee was dislocated. He spent 17 days in hospital and may one day need a replacement knee.

The injuries had proved to be permanent, added Mr Lowe, and Mr Thomas had now developed arthritis and had been unable to return to work.

The cause of the accident, said Mr Lowe, was the poor design of the access walkway, where there had been an accident in 2010 when, again, it descended without warning.

There were many mechanical difficulties, he added, and one was the tendency of the control cables to operate but not for the walkway to actually lower until someone stepped onto it.

A maintenance company employed by Valero had reported various issues over the years, and at one stage warned of “a potentially fatal accident waiting to happen.”

Mr Lowe said the prosecution accepted that Valero had responded to some of the issues raised by the maintenance company, but not adequately.

A proper risk assessment had not been carried out, there had been insufficient training and instructions, previous incidents had not been investigated properly and some recommendations had not been acted upon.

Mark Watson, representing Valero, said the company had an excellent safety record and had inherited the walkway from Chevron, the previous owners of the refinery.

In hindsight, he said, it was the very design of the equipment that had led to the problems.

The walkway had since been mothballed and another system was under design.

Judge Peter Heywood said the breaches were serious, as were the injuries suffered by Mr Thomas.

”Clearly, this piece of equipment had problems from its inception. There were significant design problems,” he added.

But Valero had failed to “focus” properly after a series of incidents and the “strident” warning from the maintenance firm had proved to be true.

Mr Watson said the fine and costs order would be paid in full within 28 days.