Over 600 tonnes of oil at risk in North Sea spill

A total of 660 tonnes of toxic oil remains trapped inside the leaking system - and there are fears of a sudden and catastrophic release if plans to use divers to close a leaking valve go wrong.

Shell has estimated that 216 tonnes have already escaped into the North Sea.

Hercules transport planes with dispersants were put on stand-by yesterday to deal with any emergency and specialist vessels with booms and other containment equipment were heading for the scene, 112 miles off the Aberdeen coast, as a small amount of oil continued to spill from the 20-year-old pipeline.

Hugh Shaw, the Scottish Secretary's Representative for Maritime Salvage and Intervention, said he had asked for containment plans to be put together to deal with a worst-case scenario.

"I want to make sure that everything that can be done is being done in the unlikely event that we do lose any other oil from this main line," he said.

"I think everything I am asking Shell to do at the moment is being done. It is very much a team effort." Shell revealed yesterday that it planned to use divers to manually close the leaking valve, believed to be the only remaining source of the oil spill.

But Mr Shaw, whose duty is to ensure that any plans to deal with the spill are in the best interests of the UK, said he had still to give the green light to the operation as technical discussions were continuing about the risk to the main 2ft pipeline.

There was a risk the operation could lead to further stress being placed on the system, leading to an uncontrolled release of the remaining trapped oil.

Mr Shaw agreed Scotland potentially faced its biggest environmental threat since the 1993 Braer disaster, with the risk of a slick spreading into the Norwegian sector of the North Sea.

He said: "I would rather live with a small quarter tonne or half a tonne on the surface than be faced with the magnitude of another 600 tonnes possibly coming up in an uncontrolled manner. I am simply looking for assurances on that side. As soon as that is given – if we agree this is the best course of action – then obviously the green light will be given for that part of the operation."

He added: "Although it is my desire that we stem this flow as quickly as possibly I need to make sure that the correct safeguards are in place."

He revealed that he has also asked Shell to come up with a "containment device" to trap the oil leaking into the North Sea. In the meantime Shell, with his backing, has already started to deploy a fleet of specialist vessels and aircraft to the scene.

"It is obviously my hope that we don't have to use such equipment but I feel a lot happier that Shell have taken these steps and at least we have the equipment on scene, should it be required," he said.

The oil spill was already being regarded as "significant", and Mr Shaw warned: "Obviously the potential is there for another significant loss. That is why we are taking this is in a slow, methodical manner. We don't wish to take any short cuts that could give us greater problems."

Glen Cayley, the technical director of Shell's exploration and production activities in Europe, said much of the marine growth around the relief valve where the leak is continuing had been removed and that Shell was "assessing the risk" of using divers to manually close the valve.

Shell estimated that a total of 216 tonnes had leaked into the North Sea since Wednesday last week and that a maximum of another 660 tonnes remained trapped in the 8in flow line where the leak started and the main 2ft diameter pipeline into which it feeds. Mr Cayley said: "We don't know precisely what's in the 24in line."

The use divers was not a "trivial exercise", he said, adding: "We would hope to start in the next 24 hours. We will likely close the valve, if the valve is closable, in daylight so we can observe any additional oil to the surface. We are looking at additional mitigation plans should that valve fail which would involve containment.

"We are making good progress in stopping the leakage from the flow line to the Gannet platform. The flow rate currently stands at less than one barrel a day.

"Today we are also continuing works to secure the flow line. To ensure the stability of the flow line and decrease the leak we depressurised the line. A related effect of that crucial step to secure the situation is that we are seeing some buoyancy in the line and will address this today through use of rock mattresses – a known industry procedure to secure pipelines."

Echoing an apology issued by Shell earlier this week, Mr Cayley said: "I must stress again how much we regret this incident, that the situation is under control and we are working towards a swift solution. However I cannot stress enough the need to undertake detailed risk assessments and ensure any work considered is undertaken safely."

Jim McKie, the chairman of the Environmental Standing Advisory Group advising Mr Shaw, said he believed the impact of the spill on wildlife and the environment had been "minimal".

He said: "Because of the low number of sea birds in the area and the small amount of fishing effort in the area we would regard currently the environmental risk to be low."

Scottish environment secretary Richard Lochhead said yesterday he been in further contact with the energy giant and had underlined the need for better communication and greater openness and transparency.

Mr Lochhead said: "We have made clear the Scottish Government's primary role is to assess and advise on the impact this spill may have on the marine environment."

He added: "The current information we have is that only one oiled bird has been spotted – there is no evidence of any significant impact. We do not expect fisheries to be impacted, but again monitoring will be undertaken. Marine Scotland's fisheries research vessel Scotia has been diverted to the area and tasked to take samples from the incident area for analysis."

Shell announced yesterday that production on Gannet will be shut down for 30 days from today in line with a planned summer shutdown.