The blaze was confined to a specific set of equipment and no environmental damage was caused, the company said.
A special investigative committee has been formed to determine the cause of the fire, Petrobras said, adding that the appropriate government officials had been informed of the accident.
All safety procedures were followed and the systems at the plant functioned as designed, the energy company said.
This is the fourth accident at a Petrobras plant since November.
Labor unions have accused the company of threatening safety procedures and systems due to higher production, which surged from 85% of capacity to 98% of capacity as a result of growing demand for petroleum and petroleum derivatives.