The Deepwater Horizon oil spill (also known as the Gulf of Mexico Oil Spill) is a large ongoing oil spill caused by an explosion on the Deepwater Horizon offshore oil platform 40 miles southeast of the Louisiana coast on April 20, 2010. Most of the 126 workers on the platform were safely evacuated, and a search and rescue operation began for 11 missing workers. The Deepwater Horizon sank in about 5,000 feet (1,500 m) of water on April 22, 2010. On April 23 the US Coast Guard suspended the search for missing workers who are all presumed dead.

BP was principal developer of the Macondo Prospect oil field where the accident occurred. The Deepwater Horizon, owned by Transocean Ltd., was under a contract with BP to drill an exploratory well. BP was the lessee and principal developer of the Macondo Prospect oil field in which the rig was operating. At the time of the explosion, BP and Transocean were in the process of closing the well in anticipation of later production. Halliburton had recently completed cementing of casings in the well. The U.S. Government has named BP as the responsible party in the incident and will hold the company accountable for all cleanup costs resulting from the oil spill. BP has accepted responsibility for the oil spill and the cleanup costs but indicated that the accident was not their fault as the rig was run by Transocean personnel. 

The sinking of the platform caused oil to gush out of the riser — the 5,000-foot pipe that connects the well at the ocean floor to the drilling platform on the surface. Attempts to shut down the flow, at first estimated at about 1,000 barrels of oil a day, failed when a safety device called a blowout preventer could not be activated. On April 28, government officials said there were three leaks and the well was spilling over 5,000 barrels of oil a day — over 200,000 gallons — nearly a mile below sea level. The exact spill flow rate is uncertain and is part of an ongoing debate. Some independent estimates made in the initial days of the accident put the spill rate as high as high as 25,000 barrels (1,100,000 US gal) a day.

The largest oil spill in U.S. waters is 1989 wreck of the Exxon Valdez, which released about 250,000 barrels (10.8 million barrels) of crude oil into Prince Williams Sound in Alaska. In 2005, Hurricane Katrina caused a spill of eight million gallons of refined oil products in to the Gulf of Mexico that caused significant damage to ecosystems and wildlife. In 1979-80, the Ixtoc 1 exploratory well operated the PEMEX, the Mexican national oi corporation, experienced a blowout and ultimately released about 3.3 million barrels (140 million gallons) of crude oil into the Bay of Campeche in Mexico. Ixtoc did not cause major onshore damage

The oil slick produced by the Deepwater Horizon oil spill covers a surface area of several thousand square miles with the exact size and location of the slick fluctuating from day to day depending on weather conditions.  A satellite image of the surface slick taken on May 17, 2010 showed a surface slick of about 10,170 square miles (26,341 km²), bigger than the state of Maryland. In the weeks following the accident, scientists discovered enormous oil plumes in the deep waters of the Gulf of Mexico. The discovery suggested that that the leak from the broken undersea well could be worse than estimates that the government and BP have given.

The surface slick threatens the ecosystems and the economy of the entire Gulf Coast region, especially Louisiana, Mississippi and Alabama. The U.S. Fish and Wildlife Service reported that up to 20 National Wildlife Refuges could potentially be affected by the spill. Concerns haver also been raised about the environmental impacts of chemical known as "dispersants that have been used to dissipate the oil slick.  By May 17 the National Marine Fisheries Service had closed 24,241 sq mi (62,784 sq km) of oil affected areas to commercial and recreational fishing out of concern for possible contamination. 

Deepwater Horizon

Deepwater Horizon was a ultra-deepwater, dynamically positioned, column-stabilized, semi-submersible mobile offshore drilling unit (MODU).  The rig was 396 feet (121 m) long and 256 feet (78 m) wide and could operate in waters up to 8,000 feet (2,400 m) deep, to a maximum drill depth of 30,000 feet (9,100 m). Built by Hyundai Heavy Industries in South Korea and completed in 2001, the rig was owned by Transocean Ltd. and leased to BP until September 2013.  At the time of the explosion, the rig was on BP's Mississippi Canyon Block 252, referred to as the Macondo Prospect, in the United States sector of the Gulf of Mexico, about 41 miles (66 km) off the Louisiana coast.  The rig commenced drilling in February 2010 at a water depth of approximately 5,000 feet (1,500 m).  The well was planned to be drilled to 18,000 feet (5,500 m), and was to be plugged and abandoned for subsequent completion as a subsea producer.

Explosion and Fire

The fire aboard the Deepwater Horizon reportedly started at 9:45 p.m. CST on April 20, 2010. Survivors described the incident as a sudden explosion that gave them less than five minutes to escape as the alarm went off. Video of the fire shows billowing flames, taller than a multistory building.  After burning for more than a day, Deepwater Horizon sank on April 22, 2010.

The precise cause of the explosion and fire that led to the oil are spill are under investigation.  The current hypothesis about the chain of event is as follows. Transocean, Ltd., representatives said workers had been performing their standard routines with "no indication of any problems" just prior to the explosion. At the time of the explosion the rig was drilling but was not in production. Production casing was being run and cemented at the time of the accident. Once the cementing was complete, it was due to be tested for integrity and a cement plug set to temporarily abandon the well for later completion as a subsea producer. Halliburton said that it had finished cementing 20 hours before the fire. Interviews with rig workers suggest that a bubble of methane gas escaped from the well and shot up the drill column, expanding quickly as it burst through several seals and barriers before exploding. Transocean chief executive Steven Newman stated: "there was a sudden, catastrophic failure of the cement, the casing or both."

Casualties and rescue efforts

At the time of the exploiton there were 126 people on the Deepwater Horizon platform; of these, 115 individuals were evacuated. Most of the workers evacuated the rig and took diesel-powered fiberglass lifeboats to the M/V Damon B Bankston, a workboat that BP had hired to service the rig; some were then evacuated from the workboat by helicopter to regional trauma centers. The United States Coast Guard launched a rescue operation involving two cutters, four helicopters and a rescue plane. After a three-day search covering 5,300 miles, the Coast Guard called off the search for the 11 missing persons, concluding that the "reasonable expectations of survival" had passed. Officials concluded that the missing workers may have been near the blast and not been able to escape the sudden explosion.

The spill

On Friday, April 23, two remotely operated underwater vehicles (ROVs) began scanning the riser to determine if there were any leaks. The discovery of two leaks was made Saturday morning. BP and the Coast Guard initially reported that about 1,000 barrels of oil were coming from the leaks on the riser.  On April 28, the National Oceanic and Atmospheric Administration estimated the leak was likely 5,000 barrels (210,000 US gallons) per day due in part to the discovery of a third leak.

The magnitiude of the leak is a subject of great debate. At the request of Senator Bill Nelson(Florida) and Barbara Boxer, California), BP released four videos of the leaks. Multiple scientists have reviewed those videos, remotely sensed data, and information on the subsurface plume, and concluded that the leak rate is much higher than what BP and the government report. In Congressional testimony on May 19 Professor Steve Wereley of Purdue University reviewed the independent leak rate assessments and reported a range of 20,000 to 100,00 barrels per day. These estimates are far greater than the rate of 5,000 barrels per day cited by BP and the U.S. government. At the higher leak rates, it would take a few days, at most a week, for the spill to exceed the Exxon Valdez's record.

For the first month after the spill, BP and the government repeatedly rejected estimates higher than 5,000 barrels per day, arguing that there is no way to estimate the flow coming out of the pipe accurately. Instead, they relied on measurements of oil on the sea surface made by the Coast Guard and the National Oceanic and Atmospheric Administration. But on May 20, 2010, BP acknowledged that the spill rate was greater than 5,000 barrels per because its own recovery effort was capturing that amount and more was still leaking out. BP's admission bolstered criticism that the government had been lax in measuring the true extent of the spill and its impacts. On May 20, 2010, Jane Lubchenco, head of the National Oceanic and Atmospheric Administration, said that a government task force is working "around the clock" to determine the actual flow rate.

BP has refused to permit scientists to send equipment to the ocean floor that would establish the leak rate with high accuracy. 

BP was subject to intense pressure and criticism that it was impeding independent scientific inquiry into the leak. Rep. Edward J. Markey (D-Mass.) sent a direct request to BP America’s CEO Lamar McKay to release more video footage. Bowing to this pressure, BP announced on May 19, 2010 that will make a live feed of the oil spill made publicly available on the web--an oil gusher webcam. BP said they would release the feed, which went live on May 20, 2010 at the web site of the web site of the U.S. House of Representatives' Select Committee for Energy Independence and Global Warming.  Heavy traffic caused the web site to crash.

Geographic Extent of the Spill

Extent of Surface Oil

Estimates of the extent of the surface oil slick were derived from data on wind and ocean current forecasts, as well as analysis of aerial photography and satellite imagery from a variety of sources. Using these data, New York Times produced a daily map of (1) the “observed extent” where oil was visible on the surface of the water during aerial surveys of the Gulf, and (2) the “probable extent” of the oil slick as estimated by the National Oceanic and Atmospheric Administration of where oil is mostly likely to go. The extents may vary widely from day to day because of changes in wind patterns and ocean currents. One month after accident, the surface slick covered an area of about 16,000 square miles (41,424 square kilometers), an area about twice the size of the state of New Jersey.

By May 12, 2010, the Louisiana Department of Wildlife and Fisheries (LDWF) had confirmed shorleine impact at the South Pass and Whiskey Island. On May 18, 2020, Louisiana officials confirmed that surface oil had reached and pemntrated the marsh ecosystems at the Head of Passes, the region where the main stem of the Mississippi River branches off into three distinct directions at its mouth in the Gulf of Mexico: Southwest Pass (west), Pass A Loutre (east) and South Pass (center).  By May 20, 2010, the Louisiana Department of Environmental Quality had confirmed shoreline impact on the Chandeleur Islands, Whiskey Island, Raccoon Island, South Pass, East Fourchon/Elmers Island, Grand Isle, Trinity Island, Brush Island, and the Pass a Loutre area.  On that date, NOAA, set the total amount of Louisiana shoreline with oil impact to date at 34.52 miles.

On May 19, NOAA concluded that some portion of the oil had reached the Loop Current in the form of "light to very light sheens". The Loop Current is a warm ocean current in the Gulf of Mexico that flows northward between Cuba and the Yucatán peninsula, moves north into the Gulf of Mexico, loops west and south before exiting to the east through the Florida Straits. Once in the Loop Current, oil could be carried into the Florida Keys and the Atlantic Ocean.

Extent of Oil in the Water Column

On May 12, scientists at the National Institute for Undersea Science and Technology (NIUST) discovered large oil plumes in the deep waters of the Gulf of Mexico, including one as large as 10 miles long, 3 miles wide and 300 feet thick in spots. The plumes were recorded at depths of 1,000–1,400 meters. Initial reports suggested that the plumes are depleting the oxygen dissolved in water column, which could pose a threat to marine life forms.

A month after the accidents, some scientists criticized the government of failing to conduct an adequate scientific analysis of the damage and of allowing BP to obscure the spill’s true scope. They point to a 2003 study by the National Academy Sciences that suggested that the oil in a deepwater blowout could break into fine droplets, forming plumes of oil mixed with water that would not quickly rise to the surface. Critics charge that NOAA should have been better prepared to assess the fate and transport of oil below the surface.

Attempts to Stop the Leak

BP's engineers sought to cut off the leak by activating a towering stack of heavy equipment 5,000 feet below the surface of the gulf known as a blowout preventer (BOP). It is a steel-framed stack of valves, rams, housings, tanks and hydraulic tubing that is designed to seal the well quickly in the event of a rapid increase in pressure. This procedure failed. 

On May 7, 2010, BP maneuvered a 98-ton steel containment dome over the worst of the leaks, and planned to funnel the oil through a pipe to the surface, where it would be collected by a drill ship. This procedure failed when the dome’s opening was clogged with gas hydrates — crystal structures that form when gas and water mix and are found in the low temperature and high pressure at the ocean floor.

The first significant success at reducing the release of oil came on May 17, 2010 when robots inserted a four-inch diameter Riser Insertion Tube Tool (RITT) into the Horizon’s riser (21-inch diameter pipe) between the well and the broken end of the riser on the seafloor in 5,000 feet of water. The RITT was expected to work like a straw, sucking the leaking oil into a tanker waiting on the surface where the oil will be separated and then shipped ashore. On May 20, 2010, BP announced that the RITT was recovering about 5,000 barrels of oil a day, yet more continues to flow out.

The Cleanup

BP assumed responsibility for the initial clean up and mitigation efforts.  According to BP Chief Executive, Tony Hayward, "we are taking full responsibility for the spill and we will clean it up and where people can present legitimate claims for damages we will honor them. On April 28, the US military announced it was joining the cleanup operation.

The U.S. government established a "unified command" structure to coordinate the response to the spill. The stated purpose of the unified command is to link the organizations responding to the incident and to provide a forum for those organizations to make "consensus decisions." The Deepwater Horizon Unified Command include BP, Transocean, and the following federal agencies: Minerals Management Service, NOAA, EPA, Homeland Security, the Coast Guard, the Department of the Interior, the Department of State,  the Department of Defense, the Fish and Wildlife Service, the National Park Service, the U.S. Geological Survey(USGS, the Centers for Disease Control (CDC) and the Occupational Safety and Health Administration (OSHA).

As of May 18, the Unifed Command identified these resources employed to respond to the spill: 

• Total response vessels: 950
• Total boom deployed: more than 1.8 million feet (regular plus sorbent boom)
• Oily water recovered: more than 7.65 million gallon
• Dispersant used: more than 590,000 gallons
• Overall personnel responding: more than 20,000 
• 17 staging areas are in place and ready to protect sensitive shorelines, including: Dauphin Island,  Ala., Orange Beach, Ala., Theodore, Ala., Panama City, Fla., Pensacola, Fla., Port St. Joe, Fla., St.  Marks, Fla., Amelia, La., Cocodrie, La., Grand Isle, La., Shell Beach, La., Slidell, La., St. Mary, La.;  Venice, La., Biloxi, Miss., Pascagoula, Miss., and Pass Christian, Miss.

Controlled burns

On April 28, BP performed the first controlled burn of surface oil, also known as an in situ burn.  Fire booms, U-shaped devices that are towed behind two boats and used to pull oil away from the main spill for safe burning, can be used when seas are below 3 feet and when sufficient amounts of oil can be "corralled." Controlled burns continued to be used at the Deepwater Horizon spill site through mid-May, 2010 when conditions were right. This represents the first on-water in-situ burning at a spill since the 1989 test burn during the Exxon Valdez oil spill, which was the first time a fire-resistant boom was used at a spill. The amount of oil burned at the Deepwater Horizon spill site is unknown.

Chemical dispersants

The EPA and Coast Guard approved the use of dispersants, a group of chemicals designed to be sprayed onto oil slicks to accelerate the process of natural dispersion. The dispersants used in the Deepwater Horizon clean-up are Corexit 9500 and Corexit EC9527A, also known as deodorized kerosene. The Environmental Protection Agency (EPA) has pre-approved both for emergencies that are 3 nautical miles (roughly 5 kilometers ) of the shoreline and in water depths greater than 30 feet (10 meters).

Oil spill dispersants do not actually reduce the total amount of oil entering the environment.¹ Rather, they change the inherent chemical and physical properties of oil, thereby changing the oil’s transport, fate, and potential effects. Small amounts of spilled oil naturally disperse into the water column, through the action of waves and other environmental processes. The objective of dispersant use is to enhance the amount of oil that physically mixes into the water column, reducing the potential that a surface slick will contaminate shoreline habitats or come into contact with birds, marine mammals, or other organisms that exist on the water surface or shoreline.  Conversely, by promoting dispersion of oil into the water column, dispersants increase the potential exposure of water-column (fish) and bottom dwelling biota (clams, oysters) to spilled oil. Dispersant application thus represents a conscious decision to increase the risk to one component of the ecosystem (e.g.,the water column) while reducing the load on another (e.g., coastal wetland). Decisions to use dispersants, therefore, involve trade-offs between decreasing the risk to water surface and shoreline habitats while increasing the potential risk to organisms in the water column and on the seafloor.

A 2005 study by the National Research Council on the ecological effects of dispersants concluded that there is insufficient scientific data to assess the net effect of chemical dispersants are on marine and coastal ecosystems. EPA Administrator Lisa Jackson reiterated this point in testimony before the U.S. Sentate Committe on Environment and Public Works on May 18, 2010, when she stated "...the long term effects of dispersants on aquatic life are unknown..."

During the first weeks of May, BP applied dispersant at the sea floor during EPA-sanctioned tests.  On May 7, 2010, after having deployed approximately 15,354 gallons of subsea dispersants, EPA halted subsea dispersant operations, awaiting additional test results in order to resume. Initial studies by EPA indicated that the subsurface application of approximately 10,000-15,000 gallons of dispersants have the equivalent effect on the oil as the surface application of approximately 50,000 gallons of dispersant. Thus, in principle, the subsurface application of dispersants is more efficient than surface application and could result in less dispersants being released into the environment.

On May 15, 2010, the U.S. Coast Guard and U.S. Environmental Protection Agency authorized BP to use dispersants undersea. Government officials stated that preliminary testing results indicate that subsea use of the dispersant is effective at reducing the amount of oil from reaching the surface – and can do so with the use of less dispersant than is needed when the oil does reach the surface. Some scientists are concerned that this practice may contribute to the formation of the underwater oil plume by shaping the oil into smaller droplets. On May 17, U.S. Rep. Edward J. Markey (D-Mass.) sent a letter to EPA Administrator Lisa Jackson asking EPA to respond to concerns about the potential ecological impacts of dispersants.  

On May 19, 2010, the EPA informed BP that the company had to immediately identify and use less-toxic forms of chemical dispersants, suggesting that federal officials were concerned that the unprecedented use of chemical dispersants could pose a significant threat to the Gulf of Mexico's marine life.

Ecological Concerns

The area affected by the Deepwater Horizon oil spill has some of the world's most productive marine and coastal ecosystems. 

Government response

Immediate response

The Coast Guard responded to the explosion and fire on April 20, 2010, treating the event as a Search and Rescue SAR) operation. The next day, on April 21, 2010, the Coast Guard continued its search for the missing eleven people.

Concurrent with the SAR, efforts began to contain the release of oil. The federal government's response to such an event os governed by the National Contingency Plan (NCP) a blueprint for responding to both oil spills and hazardous substance releases. Pursuant to the NCP, the Administration named Rear Admiral Mary Landry the Federal On-Scene Coordinator. A Regional Response Team was established, including representatives of the Coast Guard, Department of Homeland Security  (DHS), Department of Commerce (DOC)/National Oceanic and Atmospheric Administration  (NOAA), Department of the Interior (DOI) and the Environmental Protection Agency (EPA), as  well as state and local representatives.

On April 22 a National Response Team (NRT), an organization of 16 federal departments and agencies responsible for coordinating emergency preparedness and response to oil and hazardous substance pollution events, convened its first daily meeting with leadership from across the federal government. Participants in the meeting included the White House, U.S. Coast Guard, the Department of Defense, DHS, DOC, DOI and EPA, among others.

On April 23, 2010, the Unified Area Command was formally stood up in Robert, La., after three days of informal operations and planning. The U.S. Coast Guard announced that the  Deepwater Horizon rig had been found upside down approximately 1,500 feet northwest of the  blowout preventer at the wellhead. At 5 p.m., the Coast Guard suspended the search for the 11 missing workers.

On April 29, Department of Homeland Security Secretary Janet Napolitano declared the event a Spill of National Significance (SONS), indicating that the government would designate more forces to contain the spill.  This designation also led to the naming of U.S. Coast Guard Commandant Admiral Thad Allen as the National Incident Commander on May 1, 2010, which provided additional authority and oversight in leveraging government assets to combat the spill.

In the weeks following the spill, a number of government probes were announced into the Deepwater Horizon explosion and oil spill.  These included probes by the Marine Board of Investigation (Coast Guard and Minerals Management Service), Interior Department Outer Continental Shelf Safety Board, National Academy of Engineering, House Energy and Commerce Committee, House Oversight and Government Reform Committee, and House Natural Resources Committee.

Reorganization of the Minerals Management Service

In the wake of the Deepwater Horizon accident, the Minerals Management Service came under heavy criticism for alleged conflicts of interest among it competing missions. The agency was tasked with collecting royalties from oil and gas produced on federal lands and issuing energy leases; at the same time it also is responsible for policing offshore drilling and setting regulations for the industry. Indeed, in the wake of the spill President Barack Obama criticized a "cozy relationship" between federal regulators at the MMS and the industry they police.

On May 19, Interior Secretary Ken Salazar announced a plan for breaking the MMS into three separate bureaus:

• The Bureau of Ocean Energy Management, which would be responsible for development of conventional and renewable energy resources on the outer continental shelf.
• The Office of Natural Resources Revenue, which would be responsible for collecting and distributing royalties from oil and gas produced on federal lands and waters.
• The Bureau of Safety and Environmental Enforcement, which would be tasked with broadly overseeing energy production and imposing safety and environmental regulations on all offshore energy activities. 

Endnotes

¹ This section is drawn directly from a National Research Council (2005) report. 

Sources

• New York Times, Gulf of Mexico Oil Spill (2010), Accessed 18 May 2010.
• New York Times, Tracking the Oil Spill. 
• New York Times, Scientists Fault Lack of Studies Over Gulf Oil Spill, Accessed 20 May 2010.
• Offshore-technology.com, Offshore Deepwater Horizon: A Timeline of Events, Accessed 18 May 2010
• Houston Chronicle, BP falls short in estimate of oil spill rate, Accessed 21 May 2010.
• Transocean Ltd., Fleet Specifications: Deepwater Horizon, Accessed 18 May 2010.
• NOAA Fisheries Service, BP Oil Spill: Fisheries Information, Accessed 18 May 2010.
• Skytruth, BP/Gulf Oil Spill - Video of Main Leak Supports SkyTruth Estimates - Nearly 30 Million Gallons Spilled So Far, Accessed 18 May 2010.
• Wall Street Journal, Disaster Plans Lacking at Deep Rigs, Accessed 18 May 2010.
• National Public Radio, Gulf Spill May Far Exceed Official Estimates, Accessed 18 May 2010.
• Committee on Understanding Oil Spill Dispersants: Efficacy and Effects, Ocean Sciences Board, National Research Council of the National Academies. 2005. Oil Spill Dispersants: Efficacy and Effects (Washington, D.C., The National Academies Press). 
• Committee on Oil in the Sea: Inputs, Fates, and Effects, Ocean Studies Board and Marine Board, Divisions of Earth and Life Studies and Transportation Research Board, National Research Council of the National Academies. 2003. Oil In the Sea III: Inputs, Fates and Effects, (Washington, D.C., The National Academies Press).
• Schrope, Mark. 2010 Oil cruise finds deep-sea plume, Nature 465: 274-275, doi:10.1038/465274a
• U.S. House of Representatives, Committee  on Energy and Commerce, Subcommittee on Oversight and Investigations, Hearing on the Inquiry into the Deepwater Horizon Gulf Coast Oil Spill, Wednesday, May 12, 2010.
• National Institute for Undersea Science and Technology (NIUST), Oil Spill Updates, Accessed 20 May 2010.
• Wereley, Steve, Oil Flow Rate Analysis Deepwater Horizons Accident, U.S. House of Representatives, Committee  on Energy and Environment, Briefing on "Sizing up the BP Oil Spill: Science and Engineering Measuring Methods, Wednesday, May 19, 2010.
   

 Further Reading