Transocean Ltd., the owner of the doomed Deepwater Horizon rig, today filed suit against BP PLC, demanding that the oil company defend and indemnify it in ongoing litigation over the 2010 oil spill in the Gulf of Mexico.

The Switzerland-based company filed a motion for summary judgment in the U.S. District Court for the Eastern District of Louisiana in which it said BP, which was the well operator, must meet contractual obligations.

The 1998 contract, which was extended at various times, stated that BP would "defend, release, protect, indemnify and hold harmless" Transocean for "any and all fines, penalties and damages associated with environmental pollution," Transocean said in a statement.

The filing is the latest development in the morass of litigation prompted by the April 2010 explosion and spill, which includes a federal government civil enforcement action against the various companies involved in addition to private lawsuits by Gulf Coast residents and other claims the companies have made against each other.

BP has already filed a lawsuit against Transocean, Halliburton Co., the cement contractor on the well, and Cameron International Corp., which built the blowout preventer that failed to stop the flow of oil (Greenwire, April 21).

Legal experts say BP is most likely bound by the indemnification agreements it signed unless it can show that the other companies were guilty of gross negligence.

In a statement, BP said Transocean is "focusing on its interpretation of a contract provision about indemnification to avoid paying its share of any damages."

Every investigation into the disaster has shown that "Transocean's actions played a significant causal role in the accident," the statement said.