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Marine wildlife could benefit if some de-commissioned oil rigs were left on the sea bed, a survey says.

This challenges the conventional wisdom that the sea bed should be restored to its pristine state when a rig's life ends.

The paper says over the 30-year lifetime of an oil rig, creatures have often colonised the structure to form a reef.

It says this artificial habitat can be more valuable than the original seabed.

It can also protect sea creatures from fishing.

The paper from the University of Technology in Sydney, Australia, is based on a survey of 40 experts from academia, government and consultancies.

Their focus was on the North Sea - but the authors say the principles are applicable anywhere.

More than 90% of the experts surveyed said governments should abandon the principle that oil rigs should always be removed.

Instead, there should be a more flexible, case-by-case approach to de-commissioning.

It warns that the process of removing the rigs can be damaging to the environment in its own right.

What is the case for and against?

The paper drew a sceptical response from Greenpeace. Their spokesman Doug Parr told BBC News: "If companies want to propose artificial reefs for ecological benefit there are international legal processes for doing so which will need to be justified to the full range of scientific opinion.

"The North Sea is not a natural environment for hard structures and leaving rigs there is a distortion of the ecosystem - a raft of plastic bottles accumulates marine life, but no-one is arguing we should create more.

"We should be wary of proposals that look like a convenient way of oil companies avoiding their responsibility to clean up after themselves."

But David Johns, head of the Plankton Recorder Survey at the Marine Biological Association and co-author of the paper, said it was time for a re-think.

He said multiple factors had to be taken into account when de-commissioning. He said: "There needs to be a case-by-case evaluation.

"These sites can become refuges for many under-threat organisms. And the physical presence of a sub-sea structure limits fishing effort in the local area."

Will a change in policy save cash?

If the findings are adopted by governments it would offer a financial boon to the oil industry - and also in future to the wind industry, which will undergo a similar de-commissioning issues as turbines age.

The paper says under the current regulations, based on OSPAR Decision 98/3, more than 1300 offshore oil and gas installations will have to be brought to shore over the coming decades.

Costs are estimated at €50-100bn - of which Europe's governments will have to pay more than 60%, due to tax redemptions and co-ownership. In the future, some 10,000-20,000 existing and future wind turbines will have to be removed in a similar way.

What species will benefit?

One of the partners in the new report was North Sea Futures, run by an environment consultant, Anna-Mette Jorgensen.

She told BBC News: "Current policy is very unlikely to be good for the environment or the taxpayer. The ecosystem has adapted to these structures. There are lots of fish round them - and they are one of very few areas protected from fisheries."

She said the endangered lophelia cold water coral in the North Sea appeared to be using the rigs to colonise new areas.

Have we done this before?

The rig removal debate has been lively in the US Gulf of Mexico, where officials instigated a Rigs-to-Reef programme.

This involves cutting off the rig from its foundations and towing it to where it will make a useful reef, or slicing off the top of the rig and either placing it on to the sea floor or towing it to another site.

The issue is sensitive in the UK, where there was a huge battle in the 1995 over a North Sea oil loading buoy called Brent Spar.

Shell wanted to dispose it in the deep ocean, but Greenpeace won a public battle to get the structure disposed of on land, arguing that firms should face the same rules as individuals when it comes to dumping in the sea. This led directly to the imposition of the rules that still apply today

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