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.

BP also unveiled other techniques it might use to stop the oil if the “top kill” method does not work—including capping it with a containment device—and released a list of issues it is focusing on in its internal investigation into the cause of the blast and its consequences.

Also, the Environmental Protection Agency on May 24 ordered BP to reduce its use of dispersants on the oil by up to 75% after the company could not come up with a safer alternative to the chemical it has been using, Corexit. BP has used 830,000 gal of Corexit since the spill.

Kent Wells, BP’s senior vice president, said during a May 25 briefing that a top kill of the well may be attempted as early May 26 and “could take anywhere from a half day to a couple of days, depending on what we learn” from tests which will determine static and pump-in pressure at the well.

The company has been unable to perform pressure tests on the blowout preventer that caps the well because a control pod for the BOP was inoperable. The pod now has been repaired and returned, giving BP the ability to turn on and off BOP valves as well as conduct well-bore injection tests at five entry points in the BOP.

The tests may give BP—and an independent panel assigned to determine the well’s oil-flow rate—an indication of how much oil is gushing from the well. BP still says 5,000 barrels a day are escaping from the well; other expert estimates have been much higher. In any case, on a single day, its riser insertion pipe, placed in one of two leaks to capture some of the gushing stream, retrieved 5,000 barrels. BP maintained that amount may have been unusually high, but the numbers underscore that even five weeks after the explosion, neither BP nor anyone else has a very good handle on how much oil is fouling Gulf Coast waters. To date, the dismal efforts to staunch the flow have given the public no reason for optimism.

Doug Suttles, BP chief operating officer, on May 24 gave the top kill a 60% to 70% chance of working. “It’s not a guaranteed success,” he says.

If the top kill doesn’t work, Wells says BP may try removing the riser piping from the BOP and place a containment device, similar to its previous failed containment dome plan, over the BOP. Wells said that removing the riser pipe would create a flat surface and allow for a better seal around the BOP. Removal of the riser, though, could increase flow from the well by 5% to 15%, he said. BP also may completely replace the BOP.

Further, the company is continuing work on drilling two relief wells that it says can also be used to stop the gusher. On May 24, they were at 10,100 ft deep and 8,600 ft deep, respectively, on their way to their target depth of 18,000 ft.

BP announced the preliminary results of an internal investigation that has determined the accident occurred after a failure of “a number of processes, systems and equipment.” Its investigation is focusing on the cement that seals the reservoir from the well; the casing system around the well bore; pressure tests to confirm the well was sealed; procedures to detect and control oil and gas in the well; the BOP emergency disconnect system; the automatic closure of the BOP after it loses connection with the rig, and BOP features that allow remotely activated vehicles to shut the well.

“The honest truth is that this is a complex accident, caused by an unprecedented combination of failures,” said Tony Hayward, BP’s CEO.