Two tax consultants from the Norwegian branch of auditor Ernst & Young are accused of gross negligence having aided and abetted the company Transocean in providing incorrect or incomplete information to the tax authorities in the period from 2000 to 2002.
E&Y has maintained that the employees have followed the rules and regulations to the books and have done nothing wrong.
Houston-based Transocean's own estimates indicate that it may have to fork out $560 million plus interest in a worst-case scenario.
Two days ago, Norway's financial crimes unit had filed charges of tax evasion against rig giant Transocean after an exhaustive six-year investigation.
However, the drilling contractor Transocean had announced that the company would continue to co-operate with authorities following a court ruling on documents seized from lawyers during a tax investigation of its former Norwegian unit.