There are 50 more Brent Spars waiting to happen

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The single most powerful argument Greenpeace deploys against the dumping of the Brent Spar is that it will set a grim precedent.

With some 400 oil and gas installations in the North Sea, the prospect raised by environmentalists and politicians is of hundreds being sunk all over the place as the hydrocarbons beneath the seabed run out. It won't be quite like that. But the oil companies working in British waters agree that, as they become redundant over the next few decades, about 50 of the 200 platforms in the UK sector of the continental shelf will, in part, be left out at sea to rust away on the bottom.

Their topsides - the complex mass of living quarters, machinery, cranes, helideck and oil and gas processing hardware - will be taken back to land. Then, to comply with international maritime law, the support structure of legs and struts which hold the topsides above the waves will have to be chopped off at a depth which allows 55 metres (about 180 feet) between the sea surface and the highest point of the platform's sunken remains.

That leaves the owners with the problem of what to do with this chopped- off, upper portion of the support structure, some 200 feet tall and weighing several thousand tonnes.

It could be taken by barge back to land for scrapping, but the industry is expected to argue that in several, perhaps most cases, the best overall option is to leave it next to the lower portion on the seabed. It will either be dropped there, using large cranes on a barge, or it will be toppled over, probably by using a sequence of explosive charges to cut through the legs.

This is a form of dumping at sea, albeit only partial. But it is quite different from what is happening with the Brent Spar facility. This is being towed hundreds of miles out of the North Sea into the Atlantic, then sunk - almost in its entirety - to a depth of 6,000 feet.

The Brent Spar, a huge floating oil storage tank, is a very different structure from the platforms which are used to drill for oil and gas. That is why Shell and the Government repeatedly claim that its disposal in deep water does not set a precedent for the rest of the North Sea.

But why won't the oil companies remove all of their platforms as they become redundant, and leave the North Sea as they found it a quarter of a century ago?

The UK Offshore Operators Association, which represents most of them, says operators will remove entire platforms in the southern North Sea, where the water is less than 75 metres deep. These structures, about 150 of them, will be taken back on land.

The issue of partial dumping only arises for the 53 platforms in the wilder, deeper water of the UK sector of the central and northern North Sea. The main reasons why the entire structures will not be taken back to land are the hazards to human life and the enormous cost involved in doing this.

The owners have to persuade the Government that whatever option they go for is the combination of reducing threats to human safety, threats to the environment and minimising costs. They will probably be able to justify leaving a large part of their platforms on the seabed to ministers, but the international furore over the Brent Spar must now be making them wonder how they can convince the public.

The oil industry and the Government have been debating and discussing the decommissioning of old oil installations for years, but the wider public and the media, outside Scotland, have not been engaged.

The decision to allow deep-sea dumping of the Brent Spar followed extensive discussions between Whitehall departments and consultation with fishermen and Government wildlife organisations.

It took owners Shell three years of work to build up its case. Yet the first Government announcement about the Brent Spar was only in February this year, when Energy Minister Tim Eggar issued a press release saying the Government was prepared to accept its disposal at sea.

Ministers and oil company executives are now wondering if a more open debate and more consultation are needed in the run-up to North Sea disposal decisions.

Nicholas Schoon