The companies involved in the worst offshore oil spill in U.S. history are trying to prevent government investigations blaming them for the disaster from being used against them by the people and businesses who are suing them. Billions of dollars are potentially at stake in a trial scheduled for February to determine whether rig owner Transocean can limit what it pays those making claims under maritime law and to assign percentages of fault to Transocean and other companies involved...BP, Transocean and cement contractor Halliburton filed motions...in federal court in New Orleans seeking to keep certain government oil spill reports out of the civil case. BP also wants a judge to bar plaintiffs' lawyers from using past criminal, civil and regulatory proceedings against the British firm in the civil case. The filings deal with the two most comprehensive federal investigations of the disaster...There was no immediate ruling.