Deepwater Horizon – Part 2 – Mitigation Efforts
The Deepwater Horizon sank on April 22nd with over 700,000 gallons of diesel fuel on board.
For more information on how Deepwater Horizon got to this point, read Deepwater Horizon – Part 1
If the 700,000 gallons of stored diesel fuel on board Deepwater Horizon wasn’t enough, British Petroleum (BP), the U.S. Coast Guard and other responders used cameras mounted on remotely operated vehicles (ROV) and found two leaks from the well riser pipe near the sea bottom.
The locations of the leaks on the riser pipe were directly above the 50-foot tall blowout preventer (BOP, see photo below), and at the end of the riser pipe.
Photo adapted from www.nola.com
Deepwater Horizon Mitigation Efforts
On May 7th, BP determined the BOP broke and would not stop the estimated flow of 5,000 barrels per day from the well.
The U.S. government had failed to require oil companies to develop blowout containment technology for under the seafloor (National Commission, 2011, p.135), so BP had to stop the flow of oil from the BOP and riser pipe above it.
BP considered as early as April 25th, five days after the rig explosion, that a “relief well” might ultimately be needed to “kill” the well and stop the flow of oil from it.
On May 8th, BP placed a 40-foot tall, 98-ton dome over the top of the BOP to collect oil rising to the ocean surface. Think of the dome as a thimble with a cord tied to the top.
But, expanding methane dissolved in the oil cooled to the point of creating water-methane hydrates (a form of slushy ice), which clogged the riser pipe.
During the effort, engineers realized that they underestimated the flow rate of the oil which actually ranged from 12,000 to 25,000 barrels per day.
Later studies showed that even the 25,000 barrels per day estimate proved low.
On May 12th, President Obama ordered Noble Prize winner and Secretary of Energy, Steven Chu to form a scientific team to come up with a plan.
The plan was called, “top kill”.
Beginning on the afternoon of May 26th, BP pumped 100,000 gallons of dense drilling mud into the Macondo well through the top of the BOP and injected cement through the side of the BOP.
After three attempts, operation top kill was stopped out of fears that the well casing immediately beneath the seafloor would rupture and create a new spill along the sides of the casing.
On May 29th, BP proposed a new plan, “top hat”, to cut off the kinked riser pipe from the top of the BOP and install a new pipe from the top of the BOP to a surface ship named, Discoverer Enterprise that would capture the oil.
This engineering plan worked and on June 16th and Discoverer Enterprise began collecting 15,000 barrels of oil per day. With top hat in place, the flow of oil decreased significantly; however, an unknown amount of oil continued to leak out beyond that captured.
On July 12th, BP received approval for operation “top stack”.
This plan involved removing “top hat” and replacing it with a new modified BOP called “top stack”.
After the installing the new BOP, academic advisors, BP engineers, and Secretary Chu’s team slowly began to shut the well in by closing the valves on the top stack and increasing the pressure inside the well casing to over 6,000 psi.
At these pressures, the casing could collapse and cause a catastrophic failure of the entire well. But on July 15th, 87 days after the explosion, all flow from the Macondo wells was stopped.
On August 2nd, BP began operation “static kill”, whereby they pumped dense mud through the top of the new BOP to push oil and gas far enough back into the reservoir to allow drillers to cement the well closed.
On August 8th, the new cement job had passed all tests – the Macondo well was sealed off and no longer leaking.
Finally, the relief well BP began during the first week of May reached the Macondo well on September 19th, 152 days after the explosion. Through this well, BP injected cement into the bottom of the failed well effectively killing the well, and the spill was over.
Ultimately, it took thousands of scientists working together to come up with solutions that ended the Deepwater Horizon mitigation.
The solution was as complex as the original failure. The well could not be shut-in with the flip of a switch; a cascade of imagination, precise calculations, and human dare came up with the solution.
In the end, Macondo proved that solutions to complex problems could be equally complicated.
You can read the third part of this Deepwater Horizon Series by clicking the following link: Deepwater Horizon – Part 3 – Aftermath
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