Bristow helicopter pilots are set to take strike action over five week in a dispute over pay.
British Airline Pilots Association (BALPA) said its members are "forced to strike" by management which "belligerently ignores" calls for a fair a reasonable pay offer.
Pilots and tech crew were on strike on Tuesday and will continue to take action today and tomorrow.
'Bristow has shown contempt for its employees'
Amy Leversidge, BALPA General Secretary, said. "Time and again Bristow management has shown the contempt it has for employees by failing to present an offer that is anywhere near acceptable to our members."
She added: "These loyal pilots and technical crew operate in some of the most treacherous and challenging conditions transporting workers to the oil and gas rigs in the North Sea and providing vital search and rescue services.
"There are tens of thousands of workers offshore at any time working in the oil and gas industry and every single one is able to get to work and get home again because of helicopter pilots."
Bristow said it's "disappointed" and has urged the union back to negotiating table.
"We are disappointed that Balpa is leading their members out on strike action despite receiving an offer which provides significant enhancements to pay and conditions," a statement said.
"We are calling on Balpa to continue working with us towards a realistic and sustainable solution."
The group has confirmed contingency plans are in place.
The Maritime and Coastguard Agency said: "HM Coastguard has developed contingency plans to ensure search and rescue response across the UK continues.
"It would be inappropriate to comment on the industrial dispute between Bristow and Balpa while discussions remain ongoing."