* Reports said stain found in Petrobras offshore field
* ANP says navy overflights turn up no sign of oil stain
* Petrobras says it "confirmed normality of operations"
* Petrobras reported problem in oil field Wednesday -Ibama
BRASILIA, May 17 (Reuters) - Brazil's state-owned oil company Petrobras, its navy, and oil industry regulators rushed to investigate multiple reports of an oil leak from an offshore field on Thursday, but said they found no signs of oil in the water.
Petrobras itself had raised the alarm on Wednesday, reporting an oil problem near the giant P-57 oil production ship in the Jubarte field, Brazil's environmental protection agency Ibama said.
The navy said it received reports on the spill as early as Wednesday, and the Folha de Sao Paulo daily newspaper said oil workers reported a 1 km (0.6 mile) long oil stain near the P-57 when they returned from their shifts at sea.
Boats and aircraft were deployed to investigate, but found nothing.
"After news of a supposed stain near the P-57, there resulted this afternoon a verification on location that confirmed the normality of operations in the region," Petrobras said late on Thursday.
It added that "all its control systems on production and drilling units off the coast of Espirito Santo state are operating within the parameters of normality."
A leader of the union that represents workers on the P-57 told Reuters a small spill could have dispersed before a formal inspection took place.
"It's almost impossible to imagine that Petrobras would have informed Ibama of something if they didn't at least see oil in the water," said Valnisio Hoffman, head of administration at Sindipetro Espirito Santo union. "You don't report something that's not there."
A Petrobras spokeswoman declined to confirm or deny whether Petrobras informed Ibama of a spill on Wednesday.
Hoffman, who has worked on the P-57 and is in regular contact with its principal officers, said it was unlikely that a leak came from the huge production ship, known as an FPSO. Oil or petroleum-based drilling fluid may have leaked from nearby drilling platforms or from other FPSOs, he said.
There are two additional Petrobras production ships and a drilling rig on lease to Petrobras from Switzerland-based Noble Corp. within six to 10 km of the P-57.
Oil-fueled service and cargo ships regularly dock with the vessels or pass through the field, 85 km from shore. Jubarte sits astride one the busiest marine areas on Brazil's coast.
Jubarte produced 196,000 barrels of oil and natural gas equivalent per day (boepd) in February, or about 8 percent of Brazil's total output of 2.63 million boepd, according to Brazil's oil regulator, the ANP.
To tap its growing reserves, Petrobras plans to spend about $225 billion over five years to more than double output to about 6 million boepd in 2020. The vast majority of that oil will come from offshore fields such as Jubarte near Rio de Janeiro and Sao Paulo.
Recent spills, though, have cast a spotlight on Brazil's ambitious plans and its capacity to develop its giant, but technically challenging deepwater fields.
Discoveries over the past five years rank among the largest anywhere in the last three decades and could allow Brazil to leapfrog the United States as the world's third-biggest oil producer. They are also in some of the deepest seas ever considered for oil development.
A spill in the Frade field south of Jubarte in November led to civil lawsuits seeking about $20 billion in damages and criminal charges against Chevron, which operates the field, as well as Transocean, its drilling contractor, and 17 of the two companies' employees.
Chevron and its partners in the field decided to shut down output in Frade after additional, unexplained leaks were found in field waters in March. Frade produced 64,000 barrels a day of oil in February, the ANP said.
Chevron and Transocean deny any wrongdoing. Chevron owns 52 percent of Frade and Petrobras owns 30 percent. The rest is owned by a Japanese group led by Inpex and Sojitz Corp .
Petrobras said on Thursday it is unlikely Frade will be shut permanently, and that a reopening is likely after resolving the causes of the November spill.
Petrobras preferred shares, the company's most-traded class of stock, fell 4.46 percent to 18.43 reais in Sao Paulo, its lowest since October. The benchmark Bovespa index of the most-traded stocks on the Sao Paulo stock exchange fell 3.31 percent.