A 24-hour strike held a week ago was the first industrial action of its kind in the North Sea in nearly 30 years.
The action on Thursday and Friday on seven Shell-operated platforms follows fresh talks ending without agreement.
Unite and the RMT unions are representing hundreds of workers involved in the dispute over pay and conditions with oil services company Wood Group.
The Aberdeen-based firm provides maintenance and construction to Shell and signed a three-year extension to its contract earlier this year.
Some workers claim they are facing cuts of up to 30%.
Wood Group - which denies that claim - said it was "hugely disappointed that the action was progressing".
The industrial action affects the Shearwater, Gannet, Nelson, Curlew, Brent Alpha, Brent Bravo and Brent Charlie platforms.
BBC Scotland revealed on Tuesday evening that the fresh talks had failed.
A 48-hour strike by offshore workers is getting under way.