Workers at the giant Shetland Gas Plant are set to launch a series of strikes later this month in a dispute over wages and holiday pay.
The Unite union said more than 80 staff employed by Aker Solutions at the Total-operated site would stage strikes every Monday and Wednesday from 20 November.
It claimed the terminal could be forced to shut down as a result of the action.
Total said it would be able to continue to operate the plant safely.
The plant processes about 10% of the UK's gas supply.
Unite said 92% of its members voted for strike action, after long-standing talks between the union and Aker failed to make progress.
The union said it had tabled a 4% wage claim but had only been offered 1%. It also claimed Aker owed every worker several years of back-dated holiday pay.
A spokesman for Aker said the company was disappointed with the planned action, but added that it remained "committed to finding a positive resolution to the situation."
'Out of patience'
Unite Scotland regional officer John Boland said: "Aker Solutions' management have consistently refused to conduct meaningful negotiations with Unite over wages and holiday pay.
"After nine months of this, the patience of the workers at the plant has run out - and no wonder."
He added: "Anyone looking at the facts would conclude that it is the company who are being intransigent and irresponsible.
"It's time the company lived up to its billing and brought some real Aker solutions to the negotiating table."
A Total spokesman said: "Safety is a core value for Total and we will not allow it to be compromised because of this strike.
"Despite this strike action, Total will be able to continue to operate the Shetland Gas Plant safely.
"Total have identified measures that will ensure safe operations during the strike."