Chevron Corp and Transocean Ltd have agreed to change offshore safety and operating procedures as part of a lawsuit seeking nearly $20 billion in damages for a November 2011 oil spill, Brazilian public prosecutors told Reuters on Friday.

The agreement, known as a "change-of-conduct accord," was drafted at the companies' request with federal prosecutors who are handling two civil lawsuits against Chevron and Transocean, the prosecutors' press office in Rio de Janeiro said.

The lawsuits are the largest-ever environmental prosecutions in Brazil. The press office said it did not know what the companies had agreed to do in the accord.

Details of the agreement will be presented to a public hearing on December 14 in Rio de Janeiro by federal prosecutor Gisele Porto, the press office said. After the hearings and possible changes, the agreement is expected to be presented to the federal judge in Rio who is hearing the lawsuits.

Porto is lead prosecutor on the civil lawsuits against Chevron, the No. 2 U.S. oil company, and Transocean, the world's largest offshore oil drilling rig operator, for the 3,600-barrel spill in the Frade field northeast of Rio de Janeiro. No one was hurt in the accident.

By comparison, more than 5 million barrels of oil spilled in the 2010 Deepwater Horizon disaster in BP Plc's Macondo field in the Gulf of Mexico.

Chevron officials were not immediately available for comment. Transocean's press officer in Houston declined to comment. Previously, both companies said they committed no crime and acted correctly during and after the spill.

(Reporting by Jeb Blount; editing by John Wallace)