A maritime authority in London said Monday it opened an investigation into Transocean after a rig became loose from a tow and struck ground.

The British Marine Accident Investigation Branch said in a statement it "commenced an investigation into the grounding of a semi-submersible rig, Transocean Winner, on the west coast of Lewis after it became detached from the tug boat Alp Forward."

The rig was last contracted to oil company Marathon for operations in the Norwegian waters of North Sea at a day rate of $498,000. The contract award was announced in late 2013.

There were no published statements from Transocean on the grounding. A spokesperson for the company told the Scottish Daily Record the rig lost its tow during severe weather off the western coast of the country.

"Transocean is working closely with authorities to resolve the situation as quickly as possible," the representative was quoted as saying.

No personnel were on board at the time of the grounding and there was no word of a release of any material on the rig.

The 2012 grounding of the drillship Kulluk was one of the black marks hanging over Royal Dutch Shell's $5 billion Arctic oil exploration plans for the Alaska waters in the Chukchi and Beaufort seas. The U.S. Coast Guard blamed harsh winter conditions and the company's efforts to escape Alaskan tax laws for the incident.

A cascading series of failures at a well below Transocean's Deepwater Horizon rig, leased to BP, left 11 people dead and triggered the worst offshore accidental oil spill in the history of the industry in 2010.