Ocean Rig, a DryShips subsidiary, has exercised options to build two ultra-deepwater drillships with a South Korean shipyard.

The two vessels are expected to be delivered by July and October 2013, and will be capable of drilling to a depth of 12,000ft.

The fresh order follows news that DryShips secured options with Samsung Heavy Industries in November 2010 to build four drillships, each estimated to cost $600m.

DryShips owns six ultradeepwater drilling units, comprising two semisubmersible rigs and four drillships, two of which will be delivered to the company in 2011, reports Reuters.