A Diamond Offshore Drilling Inc. subsidiary signed a contract with Keppel AmFELS shipyard in Brownsville, Tex., to build a moored semisubmersible drilling rig using an existing hull.

Total cost is expected to be $300 million. Delivery is scheduled for third-quarter 2013.

Larry Dickerson, Diamond Offshore president and chief executive officer, said the Ocean Onyx will be built from a Diamond Offshore cold-stacked unit, which operated as the Ocean Voyager.

Dickerson expects the Ocean Onyx can be placed into service in half the time and at half the cost of a newbuild semi.

The Ocean Onyx will be designed to operate in 6,000 ft of water. It will have a variable deck load of 5,000 long tons, a five-ram blowout preventer, and housing for 140 people.