Workers on the drilling rig that exploded at the outset of the 2010 Gulf of Mexico oil spill catastrophe were “trying to get it right” as they monitored BP’s well for signs of trouble before the blast, an expert witness for the company that owned the rig testified Monday.

“They just misinterpreted what they were seeing,” well control expert Calvin Barnhill said on the 13th day of a trial over the disaster. “I don’t think anybody out there intentionally misinterpreted this situation.”

Barnhill was Transocean Ltd.’s first witness as the Swiss-based company, which leased the rig to BP PLC, started to present its defense. Transocean president and CEO Steven Newman was scheduled to testify Tuesday.

U.S. District Judge Carl Barbier already has heard testimony by more than a dozen witnesses called by the Justice Department and attorneys for Gulf Coast businesses and residents who claim the spill cost them money. The plaintiffs’ lawyers still expect to call another witness to the stand this week, an employee of cement contractor Halliburton.

Tranoscean’s witnesses could take up the rest of the trial’s fourth week. Halliburton and BP plan to call their own witnesses after Transocean finishes.

The trial is designed to determine the causes of BP’s well blowout and assign percentages of fault to the companies involved in the ill-fated drilling project.

The judge is hearing the case without a jury and — barring a settlement — could decide how much more money BP and its contractors owe for their roles in the catastrophe. BP could be on the hook for nearly $18 billion in penalties under the Clean Water Act if the judge finds that it acted with “gross negligence.”

The explosion on the Deepwater Horizon rig in the Gulf of Mexico on April 20, 2010, killed 11 workers and led to the nation’s worst offshore oil spill.

After a plaintiffs’ expert finished testifying Monday, BP attorney Andy Langan asked Barbier to rule that the plaintiffs haven’t proved the London-based oil giant acted with gross negligence or willful misconduct. Barbier said he wasn’t ready to rule on that request yet.

M-I LLC, BP’s drilling fluids contractor on the Macondo well, asked the judge on Monday to dismiss all of the plaintiffs’ claims against the company. M-I attorneys argued the plaintiffs haven’t presented any evidence that M-I made any decision that led to the blowout. Barbier didn’t immediately rule on M-I’s request, either.

The plaintiffs’ attorneys have accused BP and its contractors of cutting corners and sacrificing safety in a rush to save time and money on a project that was behind schedule and millions of dollars over budget.

Barnhill, a petroleum engineer, testified Monday that Tranocean’s drilling crew was properly trained in accordance with industry standards. He described the Deepwater Horizon as a “state-of-the-art” rig.

“I think the rig had drilled in water depths that very few other rigs have ever drilled,” he said.

Geoff Webster, a marine safety expert whose testimony for the plaintiffs ended Monday, concluded that Transocean failed to adequately train crew members or properly maintain the rig.

Barnhill also testified that BP ultimately was responsible for determining whether the results of a crucial safety test were properly interpreted on the day of the blowout.

Two BP rig supervisors, Robert Kaluza and Donald Vidrine, are charged with manslaughter in the 11 rig workers’ deaths and await a separate trial. An indictment last year accuses Kaluza and Vidrine of botching the safety test and disregarding abnormally high pressure readings that should have been glaring signs of trouble just before the explosion.