BP's first attempt to kill the well completely involved procedures called the "top kill" and "junk shot." A top kill pumps heavy drilling mud into the top of the well at rates and pressures high enough to force escaping oil back down the well and into the reservoir. A junk shot complements a top kill. Material (including pieces of tire rubber and golf balls) is pumped into the bottom of a blowout preventer (BOP). That material ideally gets caught on obstructions within the BOP and impedes the flow of oil and gas. By slowing or stopping the flow, a successful junk shot makes it easier to execute a top kill.
MAY 26 | The Top Kill and Junk Shot
BP and federal engineers modeled different scenarios based on different rates at which oil might be flowing from the well. National laboratories engineers used the then-current flow-rate estimate from the well of 5,000 barrels per day, which would turn out to be dwarfed by revised estimates of up to 60,000 barrels per day. BP engineers reportedly felt that the top kill would not work if the flow rate exceeded around 15,000 barrels per day.
During three separate attempts over three consecutive days in later may, BP pumped mud at rates exceeding 100,000 barrels per day and fired numerous shots of junk into the BOP. During each effort, pressures within the well initially dropped, but then flattened, indicating that the top kill had stopped making progress. After the third unsuccessful attempt, BP and the government agreed to discontinue the strategy. The failure of the top kill marked a turning point for the government science teams, with the government significantly increasing its oversight of the containment effort.