The Brent Spar may eventually be cleaned out and sunk in shallow North Sea waters, turning it into a gigantic artificial reef, Shell said yesterday.
But the company said it did not want to pursue its original plan of sinking its oil storage buoy in deep water with residual contaminants still in its tanks.
Shell UK was setting out its new strategy for deciding how to dispose of the Spar, a 14,500 tonne, 450ft-tall redundant oil tank which spent 19 years in the North Sea.
In June the company's plans for deep-sea disposal were dropped at the last minute after the successful protest campaign by the environmental group Greenpeace. The Spar is now in a Norwegian fjord.
Today, Shell will formally invite engineering and construction companies to express their interest in disposing of the structure. It has already received more than 200 offers.
Shell will select up to 30 companies to prepare more detailed proposals, from which a shortlist of six will be invited to draw up plans in depth. Then Shell will choose one to submit to the Government for the necessary permission.
Shell will choose what it regards as the best practicable environmental option (BPEO) - the one offering the best combination of minimising threats to workers' safety, damage to the environment and cost while maximising public acceptability.
Previously it argued that the BPEO for the Brent Spar was deep-sea dumping. The Government fully accepted this and defended it, which is why ministers were so angry when Shell backed down under pressure from Greenpeace. They insist Shell will have to draw up a highly persuasive case to gain permission for any other option.
Asked if deep-sea dumping could emerge as the BPEO once again, senior Shell UK executives refused to rule it out entirely. Heinz Rothermund, a managing director of Shell UK, said: "We should not be speculating now" on what would emerge after more than a year of discussions and planning, during which environmental groups would be consulted and reports published. But "good, convincing" alternatives were expected to emerge, even if they were more costly.
Shell's Brent Spar project manager, Eric Faulds, said 95 redundant rigs had been dumped in shallow waters off the United States' coast to form reefs, boosting marine life. If this option was ever chosen for the Brent Spar it would first have some 100 tonnes of oily silt removed from its tanks.
Greenpeace UK's campaigning director, Sarah Burton, said yesterday: "We have to give Shell the benefit of the doubt." But the group would strongly oppose turning the Brent Spar into an undersea reef.