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TheDeepwater Horizon oil spill (also referred to as the "BP oil spill") was an environmental disaster which began on 20 April 2010, off the coast of the United States in the Gulf of Mexico on the BP-operated Macondo Prospect,[6][7][8][9] considered the largest marine oil spill in the history of the petroleum industry and estimated to be 8 to 31 percent larger in volume than the previous largest, the Ixtoc I oil spill, also in the Gulf of Mexico. Caused in the aftermath of a blowout and explosion on the Deepwater Horizon oil platform, the United States federal government estimated the total discharge at 4.9 MMbbl (210,000,000 US gal; 780,000 m3).[3] After several failed efforts to contain the flow, the well was declared sealed on 19 September 2010.[10] Reports in early 2012 indicated that the well site was still leaking.[11][12] The Deepwater Horizon oil spill is regarded as one of the largest environmental disasters in world history.

A massive response ensued to protect beaches, wetlands and estuaries from the spreading oil utilizing skimmer ships, floating booms, controlled burns and 1,840,000 US gal (7,000 m3) of oil dispersant.[13] Due to the months-long spill, along with adverse effects from the response and cleanup activities, extensive damage to marine and wildlife habitats and fishing and tourism industries was reported.[14] In Louisiana, oil cleanup crews worked four days a week on 55 mi (89 km) of Louisiana shoreline throughout 2013. 4,900,000 lb (2,200 t) of oily material was removed from the beaches in 2013, over double the amount collected in 2012.[15] Oil continued to be found as far from the Macondo site as the waters off the Florida Panhandle and Tampa Bay, where scientists said the oil and dispersant mixture is embedded in the sand.[16] In April 2013, it was reported that dolphins and other marine life continued to die in record numbers with infant dolphins dying at six times the normal rate.[17] One study released in 2014 reported that tuna and amberjack exposed to oil from the spill developed deformities of the heart and other organs which would be expected to be fatal or at least life-shortening; another study found that cardiotoxicity might have been widespread in animal life exposed to the spill.[18][19]

Numerous investigations explored the causes of the explosion and record-setting spill. The United States Government report, published in September 2011, pointed to defective cement on the well, faulting mostly BP, but also rig operator Transocean and contractor Halliburton.[20][21] Earlier in 2011, a White House commission likewise blamed BP and its partners for a series of cost-cutting decisions and an inadequate safety system, but also concluded that the spill resulted from "systemic" root causes and "absent significant reform in both industry practices and government policies, might well recur".[22]

In November 2012, BP and the United States Department of Justice settled federal criminal charges, with BP pleading guilty to 11 counts of manslaughter, two misdemeanors, and a felony count of lying to the United States Congress. BP also agreed to four years of government monitoring of its safety practices and ethics, and the Environmental Protection Agency announced that BP would be temporarily banned from new contracts with the United States government. BP and the Department of Justice agreed to a record-setting $4.525 billion in fines and other payments.[23][24][25] As of 2018[update], cleanup costs, charges and penalties had cost the company more than $65 billion.[26][27]

In September 2014, a United States District Court judge ruled that BP was primarily responsible for the oil spill because of its gross negligence and reckless conduct.[28] In April 2016, BP agreed to pay $20.8 billion in fines, the largest environmental damage settlement in United States history.[29]

Deepwater Horizon was a 10-year-old[30] semi-submersible, mobile, floating, dynamically positioned drilling rig that could operate in waters up to 10,000 ft (3,000 m) deep.[31] Built by South Korean company Hyundai Heavy Industries[32] and owned by Transocean, the rig operated under the Marshallese flag of convenience, and was chartered to BP from March 2008 to September 2013.[2] It was drilling a deep exploratory well, 18,360 ft (5,600 m) below sea level, in approximately 5,100 ft (1,600 m) of water. The well is situated in the Macondo Prospect in Mississippi Canyon Block 253 (MC253) of the Gulf of Mexico, in the United States' exclusive economic zone. The Macondo well is found roughly 41 mi (66 km) off the Louisiana coast.[33][34] BP was the operator and principal developer of the Macondo Prospect with a 65% share, while 25% was owned by Anadarko Petroleum, and 10% by MOEX Offshore 2007, a unit of Mitsui.[35]

At approximately 7:45 pm CDT, on 20 April 2010, high-pressure methane gas from the well expanded into the marine riser and rose into the drilling rig, where it ignited and exploded, engulfing the platform.[36][37] Eleven missing workers were never found despite a three-day U.S. Coast Guard (USCG) search operation and are believed to have died in the explosion.[38][39] Ninety-four crew members were rescued by lifeboat or helicopter, 17 of whom were treated for injuries.[36][40] The Deepwater Horizon sank on the morning of 22 April 2010.

The oil leak was discovered on the afternoon of 22 April 2010 when a large oil slick began to spread at the former rig site.[41] The oil flowed for 87 days. BP originally estimated a flow rate of 1,000 to 5,000 barrels per day [bbl/d] (160 to 790 m3/d). The Flow Rate Technical Group (FRTG) estimated the initial flow rate was 62,000 bbl/d (9,900 m3/d).[42][43][44] The total estimated volume of leaked oil approximated 4.9 MMbbl (210,000,000 US gal; 780,000 m3) with plus or minus 10% uncertainty,[3] including oil that was collected,[45] making it the world's largest accidental spill.[6][46] BP challenged the higher figure, saying that the government overestimated the prefaced volume. Internal emails released in 2013 showed that one BP employee had estimates that matched those of the FRTG, and shared the data with supervisors, but BP continued with their lower number.[47][48] The company argued that government figures do not reflect over 810,000 bbl (34 million US gal; 129,000 m3) of oil that was collected or burned before it could enter the Gulf waters.[45]

According to the satellite images, the spill directly affected 70,000 sq mi (180,000 km2) of ocean, comparable to the area of Oklahoma.[4][49] By early June 2010, oil had washed up on 125 mi (201 km) of Louisiana's coast and along the Mississippi, Florida, and Alabama coastlines.[50][51] Oil sludge appeared in the Intracoastal Waterway and on Pensacola Beach and the Gulf Islands National Seashore.[52] In late June, oil reached Gulf Park Estates, its first appearance in Mississippi.[53] In July, tarballs reached Grand Isle and the shores of Lake Pontchartrain.[54][55] In September a new wave of oil suddenly coated 16 mi (26 km) of Louisiana coastline and marshes west of the Mississippi River in Plaquemines Parish.[56] In October, weathered oil reached Texas.[57] As of July 2011[update], about 491 mi (790 km) of coastline in Louisiana, Mississippi, Alabama and Florida were contaminated by oil and a total of 1,074 mi (1,728 km) had been oiled since the spill began.[58] As of December 2012[update], 339 mi (546 km) of coastline remain subject to evaluation and/or cleanup operations.[59] The reported 3.19 million barrels of spilled oil was not the only effect of this disaster. A report detailed the release of thousands of tons of hydrocarbon gases (HC) into the atmosphere.[60]

Concerns were raised about the appearance of underwater, horizontally-extended plumes of dissolved oil. Researchers concluded that deep plumes of dissolved oil and gas would likely remain confined to the northern Gulf of Mexico and that the peak impact on dissolved oxygen would be delayed and long-lasting.[61]Two weeks after the wellhead was capped on 15 July 2010, the surface oil appeared to have dissipated, while an unknown amount of subsurface oil remained.[62] Estimates of the residual ranged from a 2010 NOAA report that claimed about half of the oil remained below the surface to independent estimates of up to 75%.[63][64][65]

That means over 10010^6 US gal (380 Ml) (2.4 million barrels) remained in the Gulf.[59] As of January 2011[update], tar balls, oil sheen trails, fouled wetlands marsh grass and coastal sands were still evident. Subsurface oil remained offshore and in fine silts.[66] In April 2012, oil was still found along as much as 200 mi (320 km) of Louisiana coastline and tar balls continued to wash up on the barrier islands.[67] In 2013, some scientists at the Gulf of Mexico Oil Spill and Ecosystem Science Conference said that as much as one-third of the oil may have mixed with deep ocean sediments, where it risks damage to ecosystems and commercial fisheries.[68]

In 2013, more than 4,600,000 lb (2,100 t) of "oiled material" was removed from the Louisiana coast.[15][69] Although only "minute" quantities of oil continued to wash up in 2013, patches of tar balls were still being reported almost every day from Alabama and Florida Panhandle beaches. Regular cleanup patrols were no longer considered justified but cleanup was being conducted on an as-needed basis, in response to public reports.[70]

It was first thought that oil had not reached as far as Tampa Bay, Florida; however, a study done in 2013 found that one of the plumes of dispersant-treated oil had reached a shelf 80 mi (130 km) off the Tampa Bay region. According to researchers, there is "some evidence it may have caused lesions in fish caught in that area".[16][71]

First, BP unsuccessfully attempted to close the blowout preventer valves on the wellhead with remotely operated underwater vehicles.[72][73] Next, it placed a 125-tonne (280,000 lb) containment dome over the largest leak and piped the oil to a storage vessel. While this technique had worked in shallower water, it failed here when gas combined with cold water to form methane hydrate crystals that blocked the opening at the top of the dome.[74] Pumping heavy drilling fluids into the blowout preventer to restrict the flow of oil before sealing it permanently with cement ("top kill") also failed.[75][76]

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