What makes this such a difficult leak to stop is the mile of depth to the bottom and something around 8,000 to 9,000 psi (pounds per square inch) at the leak. “Leak” is not the right word because it implies a dribbling while the discharge is a blasting force under incredible pressure from deep inside the core of the Earth.
At a depth of a mile the pressure of the sea water is already 2400 psi so the pressure of the oil squirting out has to be tremendous. I think the possibility of their plan to cut off the damaged oil riser and place a second BOP is wishful thinking.