The Australian Workers Union and Maritime Union of Australia filed for protected action to legally strike across the massive LNG project a fortnight ago.
Protected action gives the workers legal immunity to conduct a strike without facing consequences imposed by the employer. Unions claim Monadelphous is paying Prelude maintenance workers below industry standard pay and conditions.
The unions are seeking a pay rise and better job security, citing a casualisation of the offshore workforce as a major issue.
Workers will be implementing rolling stoppages and work bans for more than five hours across the facility, essentially rendering the entire day in lost time.
Workers from Monadelphous are currently undertaking engineering and construction work to bring the Prelude FLNG back online.
Energy News understands Shell is working with Monadelphous to minismise the impact to operations and that the industrial action was not related to safety critical work.
"Shell values our contractor workforce, who make a significant contribution to our onshore and offshore activities in Australia, and, as always, the health, safety and wellbeing of those who work on our sites is of the utmost priority," Shell Australia's spokesperson said.
"We can't speak on behalf of another company on any questions specific to the ongoing EA negotiations between Monadelphous and its employees."
Prelude has been sitting idle since February when electical engineering problems saw the vessel shut down. It has not sent a cargo of LNG since.
Monadelphous was awarded the $200 million maintenance service contract at Prelude in 2015. The contract expires in 2022, though it has an option to extend the agreement.