Oil workers responding to what was assumed to be a false gas alarm on a North Sea installation found themselves trapped by a “highly flammable” hydrocarbon release capable of killing everyone onboard.

The ten crew members were left stranded on board the normally unmanned platform unable to seek shelter in the safety refuges due to the cloud of un-ignited gas “compromising” escape routes off the platform.

They had been sent to respond to a gas alarm on the Davy installationin the Southern North Sea which is said to have been going off for four days prior to their arrival.

After being dropped off by a helicopter, which then departed, the found themselves in serious danger and unable to reach suitable safety gear.

A power outage then left them only able to communicate using a hand held radio, while the installation manager was left unable to view fire detection systems on the production deck.

As well as exposing them to “serious” risk of asphyxiation there was also the possibility of a “fire or explosion”, according to safety watchdogs.

Helicopters are normally warned to stay away from potential hydrocarbon releases as engines can trigger fires and explosions.

Unions have branded the situation a disgrace.

RMT union regional organiser Jake Molloy said: “It’s staggering that the guys involved could find themselves in that situation.

“Not only did they get put into a hazardous situation, they flew into it. The potential for a a gas cloud ignition that would have killed everyone on board the helicopter, including the flight crew, is significant.

“They then found themselves without any means of escape because the safe refuge, the escape routes and access to survival equipment was compromised because of the gas cloud.

“The most galling and astonishing part of it is from what I’m hearing from the guys involved is that the alarm had been ringing for four days.

“It had been acknowledge but on the basis that they assumed it was a false alarm.”

The crew had to wait in a unprotected area on the tiny platform, which is only the diameter of a helicopter landing pad until the gas cloud dissipated and a helicopter could return to rescue them.

Due to the layout of the platform and the position of the gas cloud they were separated from emergency lifesaving gear, such as life jackets and escape equipment throughout their ordeal.

The Health and Safety Executive released a scathing investigation report and prohibition notice into the incident yesterday.

The inspector who ran the probe said: “The location of the gas release – and its associated gas cloud – compromised the designated escape route for all personnel to the temporary refuge and the primary muster point on the installation’s cellar deck, leaving them no option but to muster in an unprotected area below the helideck.

“The flammable gas release, and the potential for its ignition, meant that the preferred means of evacuation from the installation – by helicopter from the helideck – was too dangerous to be used.

“The same compromised route to the cellar deck prevented access by the crew to the installation’s designated means of escape to sea – the Selantic Chute and its associated life-raft – and to any of the crew’s personal protective equipment – personal descent devices, life jackets and abandonment suits, – required to secure a good prospect of survival having escaped to sea.”

The Offshore Installation Manager (OIM) started the emergency shutdown of the platform when the leak was noticed.

But a loss of electrical power to a fire and gas systems panel meant he lost any oversight of the status of the plant and the associated gas, heat and smoke detection elements on the production deck.

The same power loss also stopped the platform from using radios, telephone and microwave links to talk with the neighboring control room.

The HSE inspector added: “This prevented the clear and accurate communication with external agencies necessary for the OIM to coordinate an effective emergency response.

“These finding lead me to the opinion that the arrangements and equipment provided for emergency response on the installation as described in the installations emergency response plan did not provide sufficient protection for personnel on the installation during a foreseeable major incident.”

Platform operator Perenco has confirmed an investigation is ongoing.

A spokesman for the firm said: “Perenco confirms that there was a gas release on the Davy Platform.

“The correct response procedures were followed, which included an emergency shutdown of the platform and notifying The Health & Safety Executive.

“There were no injuries to personnel or damage to equipment.

“Throughout the incident, the crew was supported by a safety standby vessel.

“An investigation into the cause of the release was subsequently launched and is ongoing.”