* Elgin may become "an explosion waiting to happen" - consultant
* Flare left burning to prevent pressure build-up - UK govt
* Total shares under pressure
By Sybille de La Hamaide and Karolin Schaps
PARIS/LONDON, March 28 (Reuters) - Explosive natural gas is leaking from Total's Elgin North Sea platform less than 100 metres from a flare which workers left burning as they evacuated the rig, the French energy company said on Wednesday.
Total dismissed the risk of a blast at the platform, 240 km (150 miles) off the east coast of Scotland, and the British government said the flame had to remain burning to prevent excess gas pressure from building up.
But one energy industry consultant said Elgin could become "an explosion waiting to happen" if the oil major did not rapidly stop the leak which is above the water at the wellhead.
Total's share price has dropped about 7 percent in the past two days, although some analysts said the leak did not appear to be as serious as the oil leak that caused BP's Deepwater Horizon disaster in 2010, the world's worst marine oil spill.
A spokesman for Total UK said the flare was on a separate platform from the leak, albeit only a short distance away.
"The flare is still burning but is not posing a risk. The leak is on the wellhead platform and the flare is on the Processing, Utilities and Quarters platform. There is a gap of 90 metres (300 feet) between the two," he said in the Scottish city of Aberdeen.
Total said it expected the flare to exhaust itself and burn out. The company was looking at ways to extinguish the flare if this did not happen.
David Hainsworth, a health, safety and environment manager at Total, told the BBC the priority had been the safety of the 238 staff of the platform when it was evacuated on Sunday.
Memories are still raw in the North Sea industry of the Piper Alpha platform fire 24 years ago, when 167 people were killed in the world's deadliest offshore oil disaster.
Hainsworth said the flare was still alight, adding that "we don't believe it has been reduced in size". He could not say how long it would take to extinguish the flame, and whether that would be "an hour, or 24 hours or two days" - or even longer.
The British government said the flame was still alight as part of the safety system triggered during the evacuation to burn off excess gas.
"At the moment wind is taking the gas cloud away from the flame and weather conditions are forecast to remain stable for the next few days," said a spokesman for the Department of Energy and Climate Change. "We hope that the pressure will be such that the flame will naturally go out by itself, but Total are not taking that for granted."
Industry consultant John Shanks said the stakes were high for the offshore industry. "The news this morning that the flare is still burning on the platform is thus unwelcome," said Shanks, who works at RiserTec, a specialist engineering consultancy based in Aberdeen.
"Under normal conditions, the deeper the leak, the more difficult remedial work will be. However, if gas continues to leak at a steady or increased rate over a sustained period of time, the platform could become an explosion waiting to happen."
A spokesman for Total in Paris said a solution to plugging the leak was still being evaluated and it was "a question of days". "We have not precisely identified the cause of the incident," he said.
The British energy ministry said Total was considering two options to plug the leak: drilling a relief well that would allow the flow of gas to be shut off, or blocking the well with heavy mud.
Total warned on Tuesday that it could take six months to halt the flow of gas in an accident that has thrown a spotlight on the safety record of energy production in the British sector of the North Sea.
Hainsworth said then that some weeks ago Total engineers had decided to pump mud into redundant piping on a gas reservoir which had been plugged about a year ago. This appeared to result in the escape of gas from the outer casing of the well.
The Piper Alpha disaster led to a major review of offshore safety rules, underpinning the Britain's current regime. Industry body Oil and Gas UK said hydrocarbon leaks from offshore rigs have fallen 70 percent over the past 15 years, though figures suggest they remain high compared with Norway.
The UK sector recorded 155 cases of hydrocarbon releases in 2010-2011, against only eight in 2010 alone in the Norwegian sector.
"Obviously the UK has more rigs in our sector of the North Sea compared with Norway, but like for like we're still seeing many more incidents," said one energy union official who asked not to be identified. "This is the type of thing we're seeing more and more, and as a union we're getting sick of it."
Six months ago Total showed Elgin, one of the newer North Sea platforms which is due to keep producing for another 25 years, to a group of journalists, including a Reuters reporter.
On top of its triangular structure on huge yellow-painted ladder legs was a labyrinth of stacks of staff quarters next to an enormous on-board gas treatment plant, powered by engines with exhaust pipes as wide as two cars. The platform boasts two gymnasiums and a sea view cafeteria.
Credit ratings agency Fitch said current reports of the four-day leak suggest the unfolding incident was not as serious as the explosion at the Deepwater Horizon platform which resulted in oil pouring into the Gulf of Mexico.
"The Elgin leak is a surface gas leak rather than an underwater oil leak, making its potential for environmental damage far lower than in the Deepwater Horizon case," Fitch said in a statement.
Fitch said accidents like this were unpredictable and difficult to resolve but it considered the potential was low for this leak to escalate to a crisis on the scale of Deepwater Horizon.
The explosion at the Deepwater Horizon rig killed 11 workers and ruptured BP's Macondo well, unleashing millions of barrels of oil into the Gulf of Mexico. BP struck a deal estimated at $7.8 billion with businesses and individuals suing over the spill.
Analysts' estimates for the costs to Total of the Elgin leak ranged from $150 million to $2.7 billion, depending on how long the company takes to bring it under control.
In the worst-case scenario of an explosion on the platform, however, costs could soar to at least $10 billion, not including possible environmental fines, they said.
FRONTIERS OF TECHNOLOGY
The Elgin well, which pumps about three percent of Britain's gas output from six km (nearly four miles) below the seabed, pushes the frontiers of technology.
With British energy exploration moving from the North Sea into the Atlantic, environmental group Greenpeace said the Elgin incident highlighted the dangers of operating in ever more hostile environments further afield.
"A spill in the Arctic would be much harder to contain and close to impossible to clean up. Yet that is where the oil and gas companies now want to go in their search for more lethal riches," John Sauven, Greenpeace executive director, said in a statement.
Simon Boxall, a marine expert and oceanographer at the National Oceanography Centre, Southampton, said that what seemed like a very serious incident 24 hours ago now appeared to be settling down, but risks remained.
"There is still a risk and they do need to stop the leak, because a chronic leak would build up hydrogen sulphide in the water. It's a problem that needs solving and it's not doing the environment any good, but it's not a major incident at this stage," he said.
Total said a preliminary assessment suggested there had been no significant impact on the environment. A surveillance flight had spotted a gas cloud heading east and a sheen on the sea surface. Seawater samples had been collected for testing.
Fire fighting vessels were at the field as a precaution and a robot vessel was on standby, it said in a statement.
The loss of oil and gas output from Elgin - as well as the prospect of a big repair bill - helped drive Total's share price down six percent on the Paris bourse on Tuesday and 1.4 percent on Wednesday.