Oil firm Total is building the plant to handle the output from the Laggan and Tormore fields, now under development. These lie around 90 miles north-west of Shetland.
Work is scheduled to begin this month on the plant, next to the giant Sullom Voe oil terminal on the main island. First gas is expected in the second quarter 2014.
The Aberdeen-based contractor expects that around 500 new jobs will be created on Shetland next year to support the construction phase.
Maroun Semaan, Petrofac’s group chief operating officer, said: “The contract, which has been secured following an extensive project definition and competitive bidding process, forms part of the strategic growth plan for the offshore engineering & operations business and marks our first EPC contract in the UK.”