In this satellite image, vessels are seen at the site of the oil spill June 15, 2010 in the Gulf of Mexio.

The owner of the drilling rig that exploded at the outset of the 2010 Gulf of Mexico oil spill catastrophe started its defense Monday at a trial designed to determine the disaster's causes and assign fault to the companies involved.

Transocean called its first witness, well control expert Calvin Barnhill, on the 13th day of the trial. Transocean president and CEO Steven Newman was scheduled to testify Tuesday.