Offshore workers in the North Sea plan to strike for five days over the next few months amid a dispute over planned changes to shift patterns.
Unite members on the Elgin-Franklin, North Alwyn and Dunbar platforms were initially scheduled to strike on March 11 and 27 and April 12.
Late on Monday night, the union confirms a further two 24-hour stoppages would be pencilled in for March 19 and April 2.
Aker and Petrofac previously proposed a change to the on-off shift rotation from two weeks on, three weeks off to a three-three and three-four basis.
A reduction to the existing terms and conditions from parent company Total is also part of the grievances.
John Boland, Unite regional industrial officer, said: “The actions by Aker and Petrofac at the behest of Total are brutal and scandalous.
“Instead of responding to the legitimate concerns our members have over the imposition of the hated 3:3 or 3:4 rotas in a responsible and constructive manner, the companies have responded by serving redundancy notices.
“This disgraceful behaviour will not go unchallenged and for this reason we have had no option but to increase the scheduled days of industrial action.
“Unite will robustly defend the interests of our members and will confront any intimidation by Aker and Petrofac blow by blow until they see sense.”
Members on Shetland Gas Plant are considering further industrial action to the five dates previously announced: March 13 and 27, April 10 and 24, and May 8.
Elgin-Franklin platform members voted 88.2% in favour for industrial action, 95.1% on the Shetland Gas Plant and 93.9% on the North Alwyn and Dunbar platforms.
John Clark, Unite regional industrial officer, said: “The severe actions taken by Aker at the Shetland Gas Plant shows complete disregard for Unite’s members whose loyalty has been rewarded with attempts to impose the dreaded 3:3/3:4 rota system.
“No constructive dialogue has been afforded through the period of negotiation with a take it or leave it attitude being stubbornly emphasised by Aker’s representatives.
“This draconian behaviour will be fiercely challenged and Unite will continue to defend the interests of all its members in the forthcoming dispute.”