Energy giant Total was trying Wednesday to contain a gas leak that forced the evacuation of a well off the coast of Scotland.
A union representing workers on the rig warned there was an "urgent need" to stop the leak, which began Sunday.
"If the gas cloud somehow finds an ignition source, we could be looking at complete destruction," said Jake Molloy, an official with the National Union of Rail, Maritime and Transport Workers.
"This is an unprecedented situation and we really are in the realms of the unknown," Molloy said of the leak from the Elgin platform in the North Sea, about 150 miles (240 km) east of the Scottish city of Aberdeen.
But oceanographer Simon Boxall said the risk of explosion may not be as high as initially feared.
"Initially the risk of explosion was seen as being very high. There were reports coming through of a large gas cloud enveloping the whole rig," he told CNN.
But the fact that there has been no blast when the flare on the gas rig is still burning "obviously contradicts the idea that there was a very high risk of explosion," he said.
"Perhaps the quantity of gas is not that great as first thought," he said.
For the moment, the wind seems to be blowing the gas cloud away from the flare on the Elgin rig.
Nearly 240 workers were taken off the rig as the problem developed Sunday, Total said in a statement.
"Investigations are ongoing to analyze the causes and to determine the remediation of the gas leak," the company said.
Total confirmed there was a "sheen on the water in the vicinity of the platform," but said there was no indication of environmental damage. There have been no injuries, the company said.
Shell partly evacuated two of its nearby platforms as a "purely precautionary" measure, it said.
The Shearwater crew was reduced from 90 to 38, and the Hans Deul crew is in the process of being reduced from 106 to 38. Drilling was suspended on the Hans Deul rig, Shell UK said.
The Elgin leak has echoes of the BP Deepwater Horizon spill in the Gulf of Mexico, but there are significant differences. The Elgin is in significantly shallower water, which could make problems easier to fix -- but it is leaking gas, rather than oil. Gas catches fire much more easily.
"This is nothing on the scale of the Gulf spill two years ago," Boxall said. "This is a relatively light spill. The gas itself is dispersing quite rapidly."
But there is one parallel, he pointed out.
"Ironically it sounds as if, just like with the Deepwater Horizon, they were closing off a well and somewhere along the line something went wrong," he said.
"The hope is that there is so little gas pressure in there that it will just blow itself out," he said of the Elgin spill. Failing that, Total may have to dig a bypass well to close off the leak from another angle, as BP did with the Gulf spill, he said.
Any effect on the environment is likely to be strictly local, he predicted.
Total's share price fell 7% Tuesday on news of the leak, but market analyst firm Jeffries International said Wednesday that it thought the market had overreacted.
The North Sea was the scene of the world's worst offshore rig disaster, the Piper Alpha explosion, which killed 167 people in 1988.