Beijing ? China’s third-biggest oil producer launched an $18.5 billion bid for U.S. oil company Unocal Corp., setting up a possible contest with rival bidder Chevron Corp.

The bid by state-owned CNOOC Ltd. on Thursday was the biggest yet in a multibillion-dollar wave of foreign takeover attempts by Chinese companies trying to secure a place in the top ranks of global competitors.

Unocal, based in El Segundo, Calif., already had agreed to be acquired by Chevron for $16.6 billion, but said it would evaluate the CNOOC offer. Unocal said its board’s earlier recommendation to shareholders to accept the Chevron offer remained in place.

CNOOC’s chairman and CEO, Fu Chengyu, called his company’s bid friendly and said it would be superior for Unocal shareholders.

“The deal is fully financed, subject to customary closing conditions, and priced in line with market values for comparable businesses,” Fu said.