Operator Total says irregular pressure was detected a month before gas leak at the Elgin platform off Scotland’s coast.

Signs of trouble aboard a North Sea drilling platform where a natural gas leak in a plugged well has triggered fears of a massive explosion, operator Total has said.

The leak began on Sunday and remains unchanged at the Elgin platform off Scotland’s east coast, where all 238 workers were evacuated and a two-mile exclusion zone has been set up for safety reasons.

“On February 25 we started observing irregular pressure on the plug on the G4 well on the Elgin field,” Total’s UK Managing Director Philippe Guys told a news conference in Aberdeen on Friday.

He said the company responded within days to regulate the pressure in the well, which had been plugged in February 2011.

Even so, the company had repeatedly assured workers that a leak was impossible until just hours before evacuating them, union representative Jake Molloy told the Reuters news agency on Friday.

Safety concerns among workers aboard the platform were raised but repeatedly ignored, Molloy said.

“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.

“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.

Research vessel

Meanwhile, environmental rights group Greenpeace said it would send a research vessel to the Elgin platform where Total is preparing to sink two relief wells to stop the gas leak along with a plugging operation.

“Because of Total’s insufficient information policy the independent ecological organisation has decided to evaluate the situation on the spot,” the group said in a statement on Friday.

“It will provide videos and photos,” it added.

Greenpeace said experts on board the ship would take samples to measure air, water and soil pollution.

They would also try to find the gas leak using an infrared camera to check on the information from Total.

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.

Fire-fighting ships are on standby at the site and a team of international experts is advising Total on how to plug the leak.

Risk of blast

The company 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 dismissed the risk of a blast at the platform while one engineering consultant has warned Elgin could become

“an explosion waiting to happen”.

The French energy giant has seen around $10bn wiped off its stock value since crew on the rig were evacuated on Sunday when the leak was spotted.

The company and UK authorities have said they expect “minimal” environmental impact from the gas leak.

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.