The Sleipner platform in the North Sea, a mammoth steel and cement structure, has successfully buried millions of tonnes of CO2 under the seabed for the past 12 years in a pioneering project.

Using a simple metallic tube measuring 50 centimetres (20 inches) in diameter, the platform operator, Norwegian oil and gas group StatoilHydro, has injected some 10 million tonnes of CO2 into a deep saline aquifer one kilometre (0.6 miles) under the sea.

"We bury every year the same amount of CO2 as emitted by 300,000 to 400,000 cars," said Helge Smaamo, the manager of the Sleipner rig, a structure so large that the 240 employees ride three-wheeled scooters to get around.

The project is far from a philanthropic initiative to save the climate: StatoilHydro decided to test the carbon capture and storage (CCS) idea 250 kilometres (155 miles) off the Norwegian coast for purely financial reasons.

The natural gas extracted by Sleipner has a carbon dioxide content of nine percent, almost four times the commercial quality target of 2.5 percent, requiring the company to reduce the level by filtering it with amines on a platform adjacent to the main structure.

Since it was already being filtered, the question was then whether to release the CO2 into the atmosphere or to capture it.

A carbon tax imposed as of 1991 on Norway's offshore sector led the group to opt for the second solution, despite an initial cost of 100 million dollars to drill a well and install a compressor, and annual operating costs of five million dollars.

"We save money by injecting (CO2) gas rather than releasing it," said Olav Kaarstad, a special advisor at StatoilHydro.

There are no figures available on how much StatoilHydro has saved, but with the carbon tax at its current level of 66 dollars per tonne, StatoilHydro would have to pay 66 million dollars a year to release one million tonnes of CO2 into the atmosphere.

The Sleipnir platform, the eight-legged steed that belonged to the Norse god of war Odin, has become a textbook case, with visitors flocking to the site by helicopter to study the project.

Far below the waves of the raging North Sea, the seabed of watertight calcium rock called mudstone has yet to leak any CO2, according to independent studies.

But all this does not make Sleipner a "green platform."

The enormous gas and diesel powered generator that provides electricity and compresses the gas, and the flare that burns off the impurities, together release a total of 900,000 tonnes of CO2 per year -- as much as the volume of gas buried under the seabed each year.

The CCS technology could one day be expanded to other industries.

"The main markets for carbon capture storage are the large stationary sources of CO2 such as coal-power plants, natural gas refining, fertilisers and petrochemical plants, and iron, steel and cement plants," Kaarstad said.

The idea of CCS is however a hotly debated idea, even among environmentalists.

Greenpeace, which published a report in early May entitled "False hope. Why carbon capture and storage won't save the climate," is spearheading the opposition.

Its list of complaints is long.

It says an efficient and affordable version of the technology will not be ready in time to contribute to the global CO2 emissions reductions the UN-based International Panel on Climate Change says must start by 2015 in order to limit global warming to a two-degree increase.

The method also consumes a lot of energy, it is expensive and there is always the risk of leaks, it argues.

Greenpeace considers the research on carbon capture and storage a waste of limited scientific and financial resources and says it would prefer to see more focus on energy efficiency and renewable energies.

"The real solutions to stopping dangerous climate change lie in renewable energy and energy efficiency that can start protecting the climate today," it wrote in its report.

"Technically accessible renewable energy sources -- such as wind, wave and solar -- are capable of providing six times more energy than the world currently consumes -- forever," it said.

But other environmentalists are more positive.

"We estimate that carbon capture and storage represents 30 percent of the tools available to reduce emissions, or 50 percent in rich countries," Frederic Hauge, the head of the Norwegian environmental group Bellona, told AFP.

"Those who criticise CCS don't take the fight against global warming seriously," he added.

"We are so dependent on fossil fuels that there is no other solution to tide us over until renewable energies are more widespread. As it stands today, gas- and coal-fired plants release 100 percent of their CO2, so anything that can bring this number down is good," he said.

According to experts, the future of the method depends on its cost.

Carbon capture and storage currently costs around 60 euros per avoided tonne of CO2, but that cost would have to be at least halved to make it a viable alternative to industries, which can currently buy CO2 emissions rights for around 25 euros per tonne.

"We need to get the cost down for the technology and to have a higher CO2 price, and, not only that, to have more certainty about how long we will have a high carbon price," Kaarstad said.

"Then things will sort themselves out by themselves," he said.

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