What Shell divers did to stem the leak

THE team of specialist divers brought in by Shell to close off the leaking valve, more than 300ft below the surface, took just two hours to complete the operation.

The divers closed off the relief valve which was still leaking a small amount of oil.

They then closed a second valve, which was not leaking, to maintain the pressure balance in the 24in main pipeline, where some 660 tonnes of oil could still be trapped.

Shell began the operation by laying concrete weights, known as "rock mattresses", on Thursday. These were used to pin the pipeline to the seabed.

This enabled valves at the end of the affected length to be closed safely.

The "rock mattresses" are formed using high- strength concrete pieces linked together with a network of high-strength synthetic ropes.

This forms a continuous moveable barrier, which is highly flexible because of its segmented structure.

It is understood that at least five such "rock mattresses" were positioned over the pipe to secure the flow line.

They will ensure it does not move or create further problems.