CHICAGO (AFP) ― British oil giant BP must cover some of the eventual claims against rig owner Transocean arising from the Gulf of Mexico oil spill, a U.S. judge ruled Thursday, with any bill set to run into billions of dollars.

The mixed ruling found BP must indemnify the Swiss drilling rig operator from “compensatory damages” related to the 2010 spill even if the pollution and its impacts are the result of Transocean’s “gross negligence.”

But BP does not need to cover any punitive damages or civil penalties that may be assessed under the U.S. Clean Water Act, nor is it responsible for covering the costs of Transocean’s legal fees, Judge Carl Barbier ruled.

An explosion on the BP-leased Deepwater Horizon rig killed 11 people on April 20, 2010. The rig then sank and the Macondo well gushed oil into the ocean for 87 days, blackening the southern U.S. shoreline and crippling the local tourism and fishing sectors.