Britain's new multi-million-pound wellhead capping device to prevent oil leaks may not be strong enough to tackle blowouts on wells in the deepest waters around the British Isles, it is claimed.
The cap was commissioned in the wake of BP's Gulf of Mexico oil spill to calm fears about drilling, especially waters west of the Shetlands.
Designed by the UK Oil Spill Prevention and Response Advisory Group (OSPRAG), the cap would have a good chance of working on the vast majority of the 11,000 wells on the UK continental shelf.
It is capable of capping a well flow of up to 75,000 barrels a day and in water depths of up to 1,670m (5,500 feet).
But the environmental group Greenpeace says there are wells where the cap is unlikely to be effective in the event of a major blow-out.
The deepest wells are said to be the ones at greatest risk and the deepest North Sea well was drilled in 2001 in more than 1,800m of water. And ExxonMobil and Providence Resources are about to start drilling around 10 exploration and development wells at their Dunquin oil and gas field at depths of around 1,700m.
In evidence to the UK energy committee, French oil giant Total acknowledges: "[North Sea] wells have been drilled in water depths up to 1,800m." Oil and Gas UK, the industry body, submitted a memorandum saying: "The North Sea with a water depth range of approximately 100ft to 700ft, was considered to be deep water 40 years ago.
"Depths west of Shetland vary from approximately 500ft to 6,000ft (1,800m) plus."
Charlie Kronick, Greenpeace's senior climate adviser, said there were "very serious questions" over the capability of the cap in extreme deep water. "Pretty much all the technology brought to bear on the BP well was for shallow waters and [the UK] cap is just a slightly upgraded version.
"It raises very serious questions about whether oil companies can ever respond to an accident in very deep water.
"In the west of Shetland, companies are going to be at the absolute edge of technology – in exactly the same way as BP in the Gulf of Mexico."
Greenpeace is challenging the safety of deep water drilling near the Shetland Islands, where BP, Total and Chevron operate, in a landmark court case brought against the UK.
Brian Kinkead, leader of OSPRAG's Technical Review Group, said: "The capping device has been developed to be deployed in the widest possible range of spill scenarios,
"Its limit of a depth of 1,670m meets the requirements of current forecasted drilling operations. However, we believe that the capping device could ultimately be rated for even deeper waters and we plan to carry out this evaluation at a later date."