Shell's Brent Spar oil storage buoy was under tow last night, heading west towards the deep-sea disposal site where it will be sunk - probably soon after the weekend.
The environmental organisation Greenpeace yesterday said its activists would try to disrupt the sinking operation once the gigantic 14,500-ton structure arrived at the North Feni Ridge, 230 miles west of Orkney.
"It is virtually impossible for us to interfere at the moment because the tow is a very hazardous operation - we do not want to be responsible for anything going wrong," a Greenpeace spokesman said.
Greenpeace has chartered the Altair, a Dutch-registered yacht, to follow Shell's five-vessel flotilla, and its Moby Dick, a converted trawler, is also following.
Activists from the organisation spent several weeks on the Brent Spar after boarding it in April. Shell evicted them last month but they boarded again briefly during last week's North Sea Conference in Denmark, when environment ministers from four European nations condemned the British government for allowing the dumping.
Greenpeace claims that sea-dumping poses an environmental risk, while Shell says the risk is minimal and it is the best option taking into account cost, threats to the environment and human safety.
Shell says disposing of the structure on land would cost pounds 46m. However, a 1992 report commissioned by Shell from the Dutch marine engineering company Smit, which has been leaked to Greenpeace, put the total cost at less than pounds 10m.
The pressure group claims the costs of onshore disposal are grossly exaggerating, but yesterday Eric Faulds, the executive in charge of the Brent Spar's disposal, stood by the pounds 46m estimate.
He said the Smit report "was a very early feasibility study", and large extra costs had since been identified. He said dumping at sea would cost about pounds 10m.
Shell warned Greenpeace in a statement that any further attempt to disrupt the tow "could endanger other installations or shipping". It added: "It could also lead to the grounding of the Spar at an unplanned shallow- water location, which is inherently more environmentally sensitive [than the dumping site]."
The towing operation began late on Sunday night 118 miles north-east of the Shetland Islands, in the Brent oil field where the Spar has been anchored for more than 20 years. When it reaches the North Feni Ridge, two explosive charges will blow holes in its buoyancy tanks.
More than 100 tons of material was removed before towing began, but the Spar still carries 100 tons of an oily sludge, which contains toxic metals and low levels of radioactivity. Shell says the contaminants will only affect sea-bed life within a few hundred yards of the sunken Spar.Reuse content