Kaefer workers at the Mossmorran plant are walking out today (Monday, November 27) for one week.
It's over a dispute about the contractor not making a cost of living payment for 2023, an extra £1 an hour worked between January and the end of June which reduced to 75p an hour.
They payment was made after an increase of 2.5% for this year.
Also included is an overtime ban for 12 weeks.
Seventy five percent of of Unite members voted in favour of strike action.
The union says it was left with "no option" but to strike "in response to Kaefer’s failure to make such a payment" and is blaming operator Shell for the stand-off.
Shell reported profits of £7.6bn for the first three months of the year, £3.9bn for the three months to the end of June, and profits of £5.1 billion for the third quarter of 2023.
Profits at the company stood at £32.2bn in 2022, double the previous year's total.
Kaefer Limited recorded profits of £1.86m in the year ending December 31, 2022 as turnover also increased to £259.4m. The remuneration package of the highest paid Kaefer Limited director increased to £440,000 up from £356,000 in 2021.
Sharon Graham, Unite general secretary, said: “Unite members employed by Kaefer at the Mossmorran and St Fergus plants have their union’s complete support as they start strike action. Kaefer and the operator Shell, who funds any deal, have refused to make our members a cost of living payment.
“To fund such a payment would be a tiny drop in Shell’s multi billion pound profit ocean. Unite will back our Kaefer members all the way in their fight for better jobs, pay and conditions.”
Workers include scaffold inspectors and supervisors, riggers and rigging inspectors, forklift drivers, general assistants and mechanical supervisors.
Unite says that without these "essential workers" production at the plant near Cowdenbeath will be "severely impacted".
Bob MacGregor, Unite industrial officer, added: “The Kaefer contractors based at Mossmorran and St Fergus deserve a cost of living payment. It’s a damning indictment on both Kaefer and Shell that our members have been forced into taking strike action to get a payment.
“Shell, who own the plants, is amassing billions in the bank and Kaefer is also profitable. We will not rule out further action unless the company gets back round the negotiating table.”