Strike action is taking place on a number of North Sea oil and gas installations in a row over pay.
The Unite union said nearly 150 members employed by Petrofac on Repsol-Sinopec operated assets were involved.
The action involves a continuous overtime ban and a 48-hour stoppage on Wednesday and Thursday. Another two day strike is due to begin on 30 November.
Unite said the action, involving roles such as technicians, would cause "significant disruption".
A Petrofac spokeswoman said: "We continue to work closely with our teams and our client to ensure there is no increased risk to safety during periods of industrial action.
"Through regular reviews of the remuneration of our offshore workforce, we ensure fair compensation aligned with the market.
"Our latest review resulted in enhancements, including: a salary increase and commitment to an additional increase in January 2023, an equal time allowance and increased additional and training day payments."
Meanwhile, Unite also announced members had accepted an improved pay offer from the Catering Offshore Trade Association (Cota).
The union said the deal covered 3,000 workers in offshore catering.