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Shell oil leak scrutinised

Shell was this week facing continued scrutiny of its handling of the Gannet North Sea oil spill, after an operation to close two relief valves stemmed the flow of oil leaking from the Gannet F subsea well.

“A lack of transparency”

Shell said the leak had stopped after divers closed the valves on Friday, but Scottish environment secretary Richard Lochhead criticised the company for its lack of transparency about the incident.

“With so much [oil and gas] activity and infrastructure in our seas there is always a risk of incidents,” said Lochhead.

“Not only should every effort be taken to minimise these risks, but when incidents do happen openness and transparency must be the guiding principles.”

He was speaking ahead of the government investigation of the accident.

The Department for Energy and Climate Change and the Health and Safety Executive was investigating the causes of the incident, and Shell has launched its own internal investigation.

The leak is in a 4.1km long, 610mm diameter carrier line which contains a pressurised 200mm diameter flowline, a smaller gas line and a utilities line (see drawing).

Screen_shot_2011_08_23_at_12.23.46

Affected pipeline on sea bed

Pressurised gas is pumped through the gas line from the Gannet A platform to the Gannet F well located 185km off Aberdeen, where it forces oil out of the well and back through the flowline towards the platform. The utilities line carries substances including chemical injections for the wellhead, and hydraulic fluid.

At the end of the carrier line is a towhead with relief valves. The flowline, gas line and utilities line enter the towhead bundled within the carrier line and exit the towhead as three separate lines, so that they can run the 70m to the well head in the specific direction needed to reach their connection point.

On 11 August, Shell first attempted to stop oil escaping from the flowline despite not knowing the exact location of the leak.

It closed off the wellhead, depressurising the flowline and stopping the flow of gas into the gas line, hoping that these measures would prevent oil leaking into the sea. But in the following days it became clear that oil was still escaping, and this time a relief valve on the towhead was identified as the escape point.

On 17 August Shell began operations to close the leaking valve − and a second valve as a precaution − by sending divers and remotely operated vehicles (ROVs) to “remove panels and gratings to assess the ability to close the valve safely”. 

Oil now naturally dispersing

The two valves were successfully closed two days later. As NCE went to press, Shell was monitoring the results of this operation, and was reporting that oil flow had ceased.

Depressurising the flowline on 11 August had the unwanted effect of making the carrier line buoyant, so concrete mats weighing 5t each were placed on top of the carrier line to anchor it to the sea bed.

Forty five mats were in place as NCE went to press. Shell is now considering how to secure and remove around 600t of residual oil thought to still be in the depressurised flowline. More than 218t of oil has spilled since the leak was first detected, but government aircraft surveillance has reported that the slick in the North Sea is dispersing naturally.

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