Closing the valve is a key step. It was a careful and complex operation conducted by skilled divers, with support from our technical teams onshore. But we will be watching the line closely over the next 24 hours and beyond. Our next task is to remove the residual oil from inside the depressurized flowline, and that will take time.

—Glen Cayley, Technical Director of Shell’s exploration and production activities in Europe, based in Aberdeen

Diver laying concrete mats. Source: Shell. Click to enlarge.

Several days ago, to ensure the stability of the flowline and decrease the leak, Shell depressurized the line. A related effect of that step was some buoyancy in the line, which Shell addressed through the use of “rock mattresses”; to secure the flowline to the seabed, 24 concrete mats have now been laid. More will be put down in days to come, Shell said.

The 8-inch (20 cm) flowline in which the leak occurred sits within a 24-inch (61 cm) pipeline that carries other essential lines: a line through which gas is pumped to pressurize the well; and another line containing support systems such as power and hydraulics.

Cross section Of 24” casing/carrier line. Click to enlarge.

Three vessels are on site with dispersants and specialized oil spill response equipment if needed.

Shell said that around 218 tonnes of oil have entered the North Sea from the flowline since the problem was first detected on 10 August. The Marine Coastguard’s latest estimate is that the sheen currently covers an area of 6.7 square km and is 3.62 tonnes by volume.

Shell says it has set up an investigation team to establish the cause of the leak and will also co-operate with government authorities as they conduct their own investigations, including supplying them with pipeline integrity reports and other information.