Elgin platform gas leak: Work 'could take six months' says Total
Oil company Total has revealed it could take six months to drill a relief well to stop the gas leak on its Elgin platform in the North Sea.
The company is looking at several options to stem the flow of gas following Sunday's incident.
Jake Molloy, of the RMT union, said the potential remained for "catastrophic devastation".
Exclusion zones have been put in place around the platform. Shell also announced a shutdown of a platform.
The Scottish government said ministers were being kept "fully informed of developments".
A cloud of gas was reported to be surrounding the platform, which is located 150 miles (240km) off Aberdeen.
Coastguards said shipping was being ordered to keep at least two miles away and there was a three-mile exclusion zone for aircraft.
Shell has moved 120 non-essential staff from the Shearwater platform and Hans Deul drilling rig, about four miles from the Elgin, because of the drifting gas.
The oil giant said the move was a "precautionary measure".
Shell later said: "Further to the precautionary safety measures we took yesterday following Total's gas leak at Elgin, we have now brought forward plans to carry out maintenance at Shearwater.
"This will take place from today, starting four days ahead of schedule. We are therefore shutting down production in a controlled manner."
Total, which operates the Elgin platform, said the situation was stable but it had not yet been able to identify the source of the leak.
A sheen of between two and 23 tonnes of gas condensate, measuring six nautical miles in length, has been reported on the water nearby, and Total has activated its Oil Pollution Emergency Plan.
Jake Molloy, of the RMT union which represents offshore workers, was asked if the incident was the most serious in the North Sea since the 1988 Piper Alpha oil platform tragedy which saw 167 men die.
He told BBC Scotland: "Fortunately we have dealt with the human side of it, but the potential exists for catastrophic devastation.
"If it somehow finds an ignition source we could be looking at complete destruction."
Wullie Wallce, of the Unite union, called for a full evacuation and power down of all oil platforms within a five-mile radius of the incident.
He said: "The risk may be low but our concern is that if the drifting gas was to hit any of the neighbouring installations the results could be catastrophic.
"We would call on the oil and gas industry and the Health and Safety Executive not to take any chances here."
David Hainsworth, health, safety and environment manager for Total E&P UK, told BBC Scotland's Good Morning Scotland programmme the situation was "stable" and there had been no major change overnight.
He said: "We've been monitoring overnight. There seems to be no evolution in the rate of gas release.
"The area was over-flown twice yesterday by spotter planes to look at the sheen on the sea - that is about the same, at six nautical miles in length."
Mr Hainsworth said all the power was turned off when the platform was evacuated to minimise the risk of igniting an explosion.
He added: "Clearly there is a risk of ignition and a fire.
"We've taken away all the usual sources of ignition such as electrical power but yes there is a possibility.
"We believe it is low but you never say never."
Mr Hainsworth said they were evaluating the options for dealing with the leak.
Options include drilling a relief well, and another possibility could be to carry out a "dynamic kill" - pumping heavy mud into the well to suppress the flow of gas.
Mr Hainsworth said there was a possibility that the leak could stop of its own accord.
He said: "I would say the best case scenario is that the gas is not very productive from this area and that it dies off over the coming days and weeks."
Dr Simon Boxall, an oceanographer at Southampton University, told BBC Scotland that this was not a deepwater drilling rig and platform but it was unusual in that they were drilling down 5km (3.1 miles) into the sea bed.
He said: "It is a very deep well. The gas they are bringing up is what we call sour gas.
"That gas has a high proportion of hydrogen sulphide and carbon dioxide and that makes it very flammable and quite poisonous.
"So the big problem they have got is dealing with a very combustible gas - unlike Deepwater Horizon where we were dealing with crude oil which ironically is very difficult to light sometimes."
All 238 workers were removed from the Elgin installation and the nearby Rowan Viking drilling rig by helicopter on Sunday.
The UK Department of Energy and Climate Change (DECC) has been monitoring the situation and said there was no indication of a risk of significant pollution to the environment.
RSPB Scotland called for "complete transparency" from Total.
Stuart Housden, director of RSPB Scotland, said: "We hope that, second to minimising risks to people, environmental considerations will be foremost in the mind of Total when considering their response to this situation.
"We urgently need to know exactly what environmental impacts the leaking substances could have."