The agency was hired by the FTSE 100 company after a competitive pitch. It replaces 15-year incumbent Fifth Ring.
Petrofac is an international provider of facilities solutions to the oil & gas production and processing industries. It has charged GH with handling trade media relations and helping increase the firm's profile with key business press in the energy sector.
GH will work closely with Bell Pottinger which handles the brand's corporate and financial activity.
The GH team will be lead by associate director Rowland Barran, who recently joined from Ogilvy. He will report into Petrofac head of corporate communications Hazel Meldrum.