BP prepared on Tuesday to try sealing off its runaway well with a new cap that it says could for the first time in 12 weeks finally arrest the flow of oil spewing from the floor of the Gulf of Mexico.

The British energy giant has suffered numerous setbacks in its struggle to control the 85-day-old gusher that stands as the worst offshore oil spill in U.S. history. And BP cautioned that tests of its latest containment system were not sure to succeed.