Royal Dutch Shell PLC said Thursday an operation is under way to stop the leak at its Gannet Alpha platform following the worst North Sea oil spill in more than a decade.
About 1,300 barrels of oil has spewed into the sea since a pipeline at Shell's Gannet Alpha platform was found to be leaking Aug. 12, the company said, though it claims that after shutting the well, only one barrel a day is leaking from the installation to control the build up of pressure in the pipeline.
The pipeline at the Gannet Alpha platform, around 110 miles (180 kilometers) east of Aberdeen, Scotland, still contains up to 660 tons, or 4,620 barrels, of crude oil and needs to be stabilized before it can be shut down.
In the first phase of the operation to shut down the leak, engineers have lowered five giant concrete "blankets" on to a stretch of pipeline to place it back on the seabed after it lifted 4 feet (1.2 meters) off the sea floor.
Work is ongoing to lay concrete blankets to secure the pipeline, Shell spokesman Steve Harris said.
"First, we have to make a risk assessment ... to make sure that we can find a safe way to shut it off," he told the BBC. "We expect the result of the risk assessment very quickly and then divers will go down 300 feet (90 meters) and turn off the remaining amount of oil that is leaking into the sea."
Harris added "work is going on so we can move to the second phase." No time-scale for a full shutdown was given.
Shell's director of exploration and production activities in Europe, Glen Cayley, asserted Thursday the company is "making good progress," but has not yet stopped the remaining leak. The leak is on a relief valve on the pipeline. The pipeline is a "flow line" that carries oil away from the well on the ocean floor.
Cayley said after inspections with remote-controlled submarines and divers, the company expects to "finalize our assessment of how best to close the relief valve safely" later Thursday.
The leak created a slick of about 30,000 acres _ or 27,000 football fields _ at its greatest extent, and the oil company has been criticized by environmental groups for a lack of transparency about the extent of the pollution.
Shell said Thursday the sheen of oil has receded to just 60 acres and will dissipate entirely without reaching shore.
Scottish Environment Secretary Richard Lochhead said he has asked the British government and the company to be more forthcoming with information. On Wednesday, he said damage from the spill to fisheries is expected to be limited, and a single oil-smeared bird has been found so far.
The slick has remained at sea and there have been no repetition of scenes following the Deepwater Horizon disaster in the Gulf of Mexico, where sea life and birds were caught in thick red sludge.
Still, the Royal Society for the Protection of Birds urged Shell to involve the Scottish government and environmental groups in the British government's investigation of the spill, warning that puffins, razorbills and guillemots on the open seas were at risk.
The organization's Scottish director, Stuart Housden, said the agency wants details of when Shell's infrastructure in the area was laid and how well it has been maintained.
"The inquiry must be broad enough to cover the preparedness and investment needed by government and industry to prevent future occurrences," he said in a statement Thursday.
Richard Dixon, director of the World Wildlife Fund Scotland, said the oil is toxic if ingested and could harm marine life and mammals.
"It is hard to understand why they took so long to be open about what is happening given what happened to BP in the Gulf of Mexico," he said. "The public has a right to know what is going on and need confidence in the organization instead of this culture of secrecy."
Dixon said birds and sea life would be damaged by the toxic slick and predicted heavy oil has come to rest on the seabed.
Harris said contingency plans were in place in case of further spillage including planes loaded with detergent ready to tackle a bigger slick.
Kim Blomley, Shell's spokesman in London, said the company has "tried to be as open as is reasonable and possible, given the situation." He pointed to daily media updates and the company's Twitter feed.
The last major spill in the North Sea occurred in 1993, when the oil tanker MV Braer ran aground in the Shetland Islands, spilling around 620,000 barrels of crude into the sea.
Toby Sterling reported from Amsterdam.