* Pressure change on Feb. 25 signalled trouble, govt told
* Troubled well had been plugged year earlier
* Company to drill two relief wells
* Experts from BP's Macondo disaster called in
By Karolin Schaps and Muriel Boselli
LONDON/PARIS, March 30 (Reuters) - Total repeatedly assured workers on its North Sea Elgin platform that a leak was impossible until just hours before evacuating them as potentially explosive gas spread, a senior union official said on Friday.
French energy company Total, the platform's operator, said it had detected the first signs of trouble a month before Sunday's leak erupted as pressure rose in a well that had been capped a year earlier.
Safety concerns among the 238 workers aboard were then raised but repeatedly ignored, union representative Jake Molloy told Reuters.
"The workforce were repeatedly told that a failure in Annulus C (the pipe casing) could not happen and even if it did, a design feature would prevent a gas leak," Molloy, head of the RMT union's offshore oil and gas workers section, said.
"Several discussions between workers and Total technical authorities happened throughout the preceding weeks, up to and including a few hours before the event," he said.
A Total spokeswoman declined to make immediate comment.
The leak has spewed an estimated 200,000 cubic metres of natural gas into the air daily since Sunday, enveloping the Elgin platform in a potentially explosive gas cloud.
All workers were evacuated and a two-mile exclusion zone has been set up for safety reasons with fire-fighting ships on standby.
A team of international experts is advising Total on how to plug the leak, including U.S. firefighting and engineering firm Wild Well Control which helped tackle BP's Gulf of Mexico oil spill in 2010 and Kuwait's raging oil fires in 1991.
Total on Friday said it would move ahead with drilling two relief wells as soon as possible, a process that could take six months given that the well involved bores into a gas pocket beneath 4 kilometres of rock.
Drilling could begin in 7-10 days, Total UK Managing Director Philippe Guys told a news conference in Aberdeen.
"On February 25 we started observing irregular pressure on the plug on the G4 well on the Elgin field," Guys said, noting it had been plugged a year earlier.
(Then) on March 25 we observed a sudden pressure increase followed by an escape of mud and gas."
Britain's Health and Safety Executive confirmed it was informed by Total on Feb. 26 about a pressure fluctuation, a spokesman said.
"Total were to monitor the pressures and bleed down pressures as required to keep them within tightly defined limits," he said.
Total earlier said it hoped the leak would run itself dry as reservoir pressure dropped, removing the need for relief drilling that could take months and cost up to $3 billion.
FLARE STILL BURNING
Total must also decide what to do about extinguishing a flare only 100 metres away from where the gas is leaking which was lit as part of the evacuation of the platform to relieve pressure in the troubled well.
Water could be dropped from a helicopter or nitrogen sprayed to starve the flame of oxygen, the UK energy department said.
"Total has assured the government the platform is designed so the flare is located in a position where the prevailing wind blows the gas release away from it," the department said.
Total has dismissed the risk of a blast at the platform while one engineering consultant has warned Elgin could become "an explosion waiting to happen".
On Friday the UK government said Total's response to the incident had been very effective, noting the platform was evacuated in three and a half hours.
SAFETY LESSONS
UK Energy Minister Charles Hendry said the potential permanent shutdown of the rig would be decided by the operator and regulators though seeking safety lessons would be important.
Union officials have called the frequency of safety lapses in the British sector of the North Sea's energy business intolerable.
The North Sea witnessed the world's deadliest offshore oil disaster 24 years ago when 167 people were killed in the Piper Alpha platform fire.
Royal Dutch Shell shut its Shearwater platform near the Elgin rig as a "precautionary measure" on Tuesday.
The Elgin platform sits in waters less than 100 metres deep and 240 km (150 miles) off the east coast of Scotland.
The company and UK authorities have said they expect "minimal" environmental impact from the gas leak and escaped light oil on the water.
Total shares have fallen around 7 percent since the incident, wiping off around 7 billion euros ($10.62 billion) off the company's market value.