Unions representing Brazilian oil workers held a 24-hour job action Thursday on offshore-drilling platforms owned by state-run oil giant Petroleo Brasileiro SA (PBR, PETR4.BR).  

In a statement, the National Oil Workers Federation said workers on 33 of the 39 offshore platforms in the huge
Campos Basin near Rio de Janeiro were taking part in the job action. Workers on the platforms will perform only essential services throughout Thursday. Normal activities will resume Friday, the federation said.

Workers are protesting changes in calculation and payment of overtime. Thursday's job action was designed as a "warning" to Petrobras, the federation said. No further job actions are currently scheduled.

In its own statement, Petrobras said it was "taking all steps necessary to assure normal operations and no interruption in supplies of [fuel and other] products to the public." Through a spokeswoman, the company declined to comment on the specific issue of overtime pay.