BP began conducting the first pressure and diagnostic tests on a five-story blowout preventer (BOP) atop its Macando well bore on May 25 in preparation for a “top kill” attempt to stop the oil that has been gushing from the well and into the Gulf of Mexico since the April 20 explosion.

In the “top kill” operation, a vessel will pump drilling mud at a rate of 40 barrels to 50 barrels a minute into two 3-in. choke-and-kill lines at the bottom of the BOP. If the mud stops the oil from coming up the well, concrete then would be pumped in to seal the well. But if the oil keeps coming up through the BOP, a “junk shot” of high-pressure debris would be tried next in an attempt to plug leaks in the BOP.