Transocean's highest-ranking drilling employee on the oil rig that exploded and sank in the Gulf of Mexico has refused to testify in civil lawsuits over the disaster. Jimmy Harrell, the rig's offshore manager, was in charge of drilling activities on the Deepwater Horizon, which exploded on April 20 last year while drilling a BP well off the Louisiana coast. Court records show Mr Harrell told federal investigators last year that he was in command of the rig before the explosion that killed 11 workers and caused the worst offshore oil spill in the United States.