This makes the stack considerably larger than its counterpart on Statoil’s Norne production ship further north, and allows it to handle three times the volume of gas under significantly higher pressure.

Not only is the stack used to transmit wellstreams from the turret to the process facilities while the ship rotates, but it also conveys gas for injection back into the  reservoir to provide pressure support.

The latter was one of the most difficult challenges in the whole Åsgard development, requiring a 12-inch injection unit in the stack suitable for an operational pressure of 475 bar.

Able to handle 23 million standard cubic metres of gas and 200,000 barrels of oil daily at this pressure, the swivel stack is the largest ever built in capacity terms and the most complex.

It is equipped with a special internal inspection and cleaning system to protect the seals, allowing the stack to cope with quartz-like particles (propants) carried in the wellstreams.

Propants are used to keep artificial fractures open in the reservoir rock.

This fracturing process has been applied to stimulate the flow of oil from Smørbukk, which is one of the most difficult reservoirs on the Norwegian continental shelf.