The Ormen Lange gas field, which feeds the Langeled pipeline under the North Sea, covers 20 percent of Britain's demand for natural gas.
The upgrade will increase the output capacity to 70 million cubic metres (mcm) per day, from the current output level of about 50 mcm per day, Shell executives told Reuters during a visit of the plant, situated on Norway's west coast.
Separately, Shell said it would raise the capacity of its Nyhamna gas processing plant to 84 mcm per day from 70 mcm per day, to accommodate the future output from the Aasta Hansteen field, which is under development by Statoil and is expected to start production in the fourth quarter of 2018.
Shell's partners at Ormen Lange include Statoil, ExxonMobil, DONG Energy and Norway's state-owned Petoro.