Repsol platforms affected include the Arbroath, Auk, Bleo Holm, Claymore, Clyde, Fulmar Alpha, Piper Bravo, Saltire, Tartan Alpha, and the Flotta oil terminal, Unite added.

"The dispute centres on the removal of a 10% equal time payment, years of below inflationary pay increases as well as issues around payments for Offshore Energies UK (OEUK) medicals, mileage and stand in duties," the union said in a statement.

In a separate dispute, Petrofac workers at BP will take strike action from Dec. 29-31 at installations including Andrew, Clair, Clair Ridge, ETAP, and Glen Lyon floating production, storage and offloading (FPSO) facility.

Clair Ridge can produce around 120,000 barrels per day (bpd) and the Glen Lyon FPSO vessel can process up to 130,000 bpd, according to BP's website.

The Petrofac BP dispute involves 76 union members and centres on their work rotation, Unite said in a statement.

The action at Repsol installations follows workers rejecting the latest pay offer from Petrofac by 79%, on an 89% turnout.

A further 48-hour strike will impact the Montrose platform from Dec. 29-31, Unite added.

(Reporting by Arundhati Sarkar and Swati Verma in Bengaluru; Editing by Alexander Smith)