Offshore drilling contractor Ocean Rig UDW Inc. filed for bankruptcy protection in the U.S. to block distressed debt investors from interfering with a debt restructuring that will slash $3.7 billion in debt from its books. The Cayman Islands holding company filed Monday night for protection under chapter 15, the section of the U.S. code dealing with cross-border insolvencies. Ocean Rig is one of a number of offshore rig owners and…