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Deepwater Horizon oil spill
The Deepwater Horizon oil spill began on April 20, 2010,
in the Gulf of Mexico on the BP-operated Macondo Prospect. Killing eleven people,
it is considered the largest marine oil spill in the history of the petroleum industry
and estimated to be 8% to 31% larger in volume than the previous largest, the Ixtoc I oil spill.
The US Government estimated the total discharge at. After several failed efforts
to contain the flow, the well was declared sealed on September 19, 2010.
Reports in early 2012 indicated the well site was still leaking. A massive response ensued
to protect beaches, wetlands and estuaries from the spreading oil utilizing skimmer ships,
floating booms, controlled burns and of Corexit oil dispersant. 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. In Louisiana,
of oily material was removed from the beaches in 2013, over double the amount collected in 2012.
Oil cleanup crews worked four days a week on of Louisiana shoreline throughout 2013. 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.
In 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.
One study released in 2014 reported that tuna and amberjack that were exposed to oil
from the spill developed deformities of the heart and other organs that would be expected
to be fatal or at least life-shortening
and another study found that cardiotoxicity might have been widespread in animal life exposed
to the spill. Numerous investigations explored the causes of the explosion
and record-setting spill. Notably, the U.S. government's September 2011 report pointed
to defective cement on the well, faulting mostly BP, but also rig operator Transocean
and contractor Halliburton. 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".
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 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 US government. BP and the Department of Justice agreed
to a record-setting $4.525 billion in fines and other payments. As of February 2013,
criminal and civil settlements and payments
to a trust fund had cost the company $42.2 billion. In September 2014, a U.S.
District Court judge ruled that BP was primarily responsible for the oil spill,
because of its gross negligence and reckless conduct. In July 2015, BP agreed
to pay $18.7 billion in fines, the largest corporate settlement in U.S. history.
Deepwater Horizon drilling rig
 [^]  The Deepwater Horizon was a 10-year-old semi-submersible, mobile, floating,
dynamically positioned drilling rig that could operate in waters up to deep. Built
by South Korean company Hyundai Heavy Industries and owned by Transocean,
the rig operated under the Marshallese flag of convenience, and was chartered to BP from March 2008
to September 2013. It was drilling a deep exploratory well, below sea level,
in approximately of water.
The well is situated in the Macondo Prospect in Mississippi Canyon Block 252 of the Gulf of
Mexico, in the United States' exclusive economic zone.
The Macondo well is located roughly off the Louisiana coast. BP was the operator
and principal developer of the Macondo Prospect with a 65% share, while 25% was owned
by Anadarko Petroleum Corporation, and 10% by MOEX Offshore 2007, a unit of Mitsui.
Explosion
 [^]  At approximately, on 20 April 2010, high-pressure or methane gas
from the well expanded into the drilling riser and rose into the drilling rig, where it ignited
and exploded, engulfing the platform. At the time,
126 crew members were on board: seven BP employees, 79 of Transocean,
and employees of various other companies.
Eleven missing workers were never found despite a three-day U.S. Coast Guard search operation
and are believed to have died in the explosion. Ninety-four crew were rescued by lifeboat
or helicopter, 17 of whom were treated for injuries.
The Deepwater Horizon sank on the morning of 22 April 2010.
 Volume and extent of oil spill 
 [^]  [^]   [^]   [-]  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. The oil flowed for 87 days.
BP originally estimated a flow rate of.
The Flow Rate Technical Group estimated the initial flow rate was.
The total estimated volume of leaked oil approximated  with plus or minus 10% uncertainty,
including oil that was collected, making it the world’s largest accidental spill.
BP challenged the higher figure, saying that the government overestimated the 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.
The company argued that government figures do not reflect over of oil that was collected
or burned before it could enter the Gulf waters. According to the satellite images,
the spill directly impacted of ocean, which is comparable to the size of Oklahoma.
By early June 2010, oil had washed up on of Louisiana's coast and along the Mississippi, Florida,
and Alabama coastlines. Oil sludge appeared in the Intracoastal Waterway and on Pensacola Beach
and the Gulf Islands National Seashore. In late June, oil reached Gulf Park Estates,
its first appearance in Mississippi. In July, tar balls reached Grand Isle
and the shores of Lake Pontchartrain.
In September a new wave of oil suddenly coated of Louisiana coastline
and marshes west of the Mississippi River in Plaquemines Parish. In October,
weathered oil reached Texas. As of July 2011, about of coastline in Louisiana, Mississippi,
Alabama and Florida were contaminated by oil and a total of had been oiled since the spill began.
As of December 2012, of coastline remain subject to evaluation and/or cleanup operations.
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. 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.
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%. That means that over remained in the Gulf.
As of January 2011, tar balls, oil sheen trails, fouled wetlands marsh grass
and coastal sands were still evident. Subsurface oil remained offshore and in fine silts.
In April 2012, oil was still found along as much as of Louisiana coastline and tar balls continued
to wash up on the barrier islands. 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.
In 2013, more than of "oiled material" was removed from the Louisiana coast. 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.
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
off the Tampa Bay region. According to researchers, there is
"some evidence it may have caused lesions in fish caught in that area".
 Short-term efforts 
 [^]   [^]  First BP unsuccessfully attempted to close the blowout preventer valves on the wellhead
with remotely operated underwater vehicles. Next it placed a 125-tonne 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.
Pumping heavy drilling fluids into the blowout preventer
to restrict the flow of oil before sealing it permanently with cement also failed.
BP then inserted a riser insertion tube into the pipe
and a stopper-like washer around the tube plugged the end of the riser
and diverted the flow into the insertion tube. The collected gas was flared
and oil stored on board the drillship Discoverer Enterprise. Before the tube was removed,
it collected of oil. On 3 June 2010, BP removed the damaged drilling riser
from the top of the blowout preventer and covered the pipe by the cap which connected it
to another riser. On 16 June a second containment system connected directly
to the blowout preventer began carrying oil and gas to service vessels,
where it was consumed in a clean-burning system.
The United States government's estimates suggested the cap and other equipment were capturing less
than half of the leaking oil. On 10 July the containment cap was removed to replace it
with a better-fitting cap. Mud and cement were later pumped in through the top of the well
to reduce the pressure inside it which didn't work either. A final device was created
to attach a chamber of larger diameter than the flowing pipe with a flange that bolted
to the top of the blowout preventer and a manual valve set to close off the flow once attached.
On July 15 the device was secured and time was taken closing the valves
to ensure the attachment under increasing pressure until the valves were closed completing the
temporary measures.
 Considerations of using explosives 
In mid-May, United States Secretary of Energy Steven Chu assembled a team of nuclear physicists,
including hydrogen bomb designer Richard Garwin
and Sandia National Laboratories director Tom Hunter.
Oil expert Matthew Simmons maintained that a nuclear explosion was the only way BP could
permanently seal the well and cited successful Soviet attempts to seal off runaway gas wells
with nuclear blasts. A spokesperson for the U.S. Energy Department said that
"neither Energy Secretary Steven Chu nor anyone else" ever considered this option.
On 24 May BP ruled out conventional explosives, claiming that if blasts failed to clog the well,
"we would have denied ourselves all other options."
 Well declared "effectively dead" 
Transocean's Development Driller III started drilling a first relief well on 2 May.
GSF Development Driller II started drilling a second relief on 16 May. On 3 August, first test oil
and then drilling mud was pumped at a slow rate of approximately per minute into the well-head.
Pumping continued for eight hours, at the end of which time the well was declared to be
"in a static condition." On August 4, BP began pumping cement from the top,
sealing that part of the flow channel permanently.
On September 3 the 300-ton failed blowout preventer was removed from the well
and a replacement blowout preventer was installed. On 16 September,
the relief well reached its destination and pumping of cement to seal the well began.
On 19 September, National Incident Commander Thad Allen declared the well "effectively dead"
and said that it posed no further threat to the Gulf.
Recurrent or continued leakage
 [^]  In May 2010, BP admitted they had "discovered things that were broken in the sub-surface"
during the "top kill" effort. Oil slicks were reported in March and August 2011, in March
and October 2012, and in January 2013.
Repeated scientific analyses confirmed that the sheen was a chemical match for oil
from Macondo well. The USCG initially said the oil was too dispersed to recover
and posed no threat to the coastline, but later warned BP
and Transocean that they might be held financially responsible for cleaning up the new oil.
USGS director Marcia McNutt stated that the riser pipe could hold at most,
because it is open on both ends, making it unlikely to hold the amount of oil being observed.
In October 2012, BP reported that they had found and plugged leaking oil
from the failed containment dome, now abandoned about from the main well. In December 2012,
the USCG conducted a subsea survey; no oil coming from the wells or the wreckage was found
and its source remains unknown. In addition, white, milky substance was observed seeping
from the wreckage. According to BP and the USCG it is "not oil and it's not harmful."
In January 2013, BP said that it was continuing to investigate possible sources of the oil sheen.
Chemical data implied that the substance might be residual oil leaking from the wreckage.
If that proves to be the case, the sheen can be expected to eventually disappear.
Another possibility is that it is formation oil escaping from the subsurface,
using the Macondo well casing as flow conduit, possibly intersecting a naturally occurring fault,
and then following that to escape at the surface some distance from the wellhead. If it proves
to be oil from the subsurface,
then that could indicate the possibility of an indefinite release of oil.
The oil slick was comparable in size to naturally occurring oil seeps and was not large enough
to pose an immediate threat to wildlife.
 Containment 
 [^]  Containment booms stretching over were deployed, either to corral the oil or as barriers
to protect marshes, mangroves, shrimp/crab/oyster ranches or other ecologically sensitive areas.
Booms extend above and below the water surface and were effective only in relatively calm
and slow-moving waters. Including one-time use sorbent booms, a total of of booms were deployed.
Booms were criticized for washing up on the shore with the oil, allowing oil to escape above
or below the boom, and for ineffectiveness in more than three to four-foot waves.
The Louisiana barrier island plan was developed to construct barrier islands
to protect the coast of Louisiana. The plan was criticised for its expense and poor results.
Critics allege that the decision to pursue the project was political with little scientific input.
The EPA expressed concern that the booms would threaten wildlife.
 Use of Corexit dispersant 
 [^]  The spill was also notable for the volume of Corexit oil dispersant used and
for application methods that were "purely experimental." Altogether, of dispersants were used;
of this were released at the wellhead. Subsea injection had never previously been tried, but due
to the spill's unprecedented nature BP together with USCG and EPA decided to use it.
Over 400 sorties were flown to release the product. Although usage of dispersants was described as
"the most effective and fast moving tool for minimizing shoreline impact", the approach continues
to be investigated. A 2011 analysis conducted by Earthjustice
and Toxipedia showed that the dispersant could contain cancer-causing agents, hazardous toxins
and endocrine-disrupting chemicals.
Environmental scientists expressed concerns that the dispersants add to the toxicity of a spill,
increasing the threat to sea turtles and bluefin tuna. The dangers are even greater
when poured into the source of a spill, because they are picked up by the current
and wash through the Gulf. According to BP and federal officials,
dispersant use stopped after the cap was in place; however,
marine toxicologist Riki Ott wrote in an open letter
to the EPA that Corexit use continued after that date and a GAP investigation stated that
"[a] majority of GAP witnesses cited indications that Corexit was used after [July 2010]."
According to a NALCO manual obtained by GAP, Corexit 9527 is an “eye and skin irritant. Repeated
or excessive exposure. may cause injury to red blood cells, kidney
or the liver.” The manual adds: “Excessive exposure may cause central nervous system effects,
nausea, vomiting, anesthetic or narcotic effects.” It advises, “Do not get in eyes, on skin,
on clothing,” and “Wear suitable protective clothing.” For Corexit 9500 the manual advised,
“Do not get in eyes, on skin, on clothing,” “Avoid breathing vapor,”
and “Wear suitable protective clothing.” According to FOIA requests obtained by GAP,
neither the protective gear nor the manual were distributed to Gulf oil spill cleanup workers.
Corexit EC9500A and Corexit EC9527A were the principal variants.
The two formulations are neither the least toxic, nor the most effective,
among EPA's approved dispersants, but BP said it chose to use Corexit,
because it was available the week of the rig explosion. On 19 May, the EPA gave BP 24 hours
to choose less toxic alternatives to Corexit from the National Contingency Plan Product Schedule,
and begin applying them within 72 hours of EPA approval
or provide a detailed reasoning why no approved products met the standards. On 20 May,
BP determined that none of the alternative products met all three criteria of availability,
non-toxicity and effectiveness. On 24 May, EPA Administrator Lisa P. Jackson ordered EPA
to conduct its own evaluation of alternatives and ordered BP to reduce dispersant use by 75%.
BP reduced Corexit use by per day, a 9% decline. On 2 August 2010,
the EPA said dispersants did no more harm to the environment than the oil
and that they stopped a large amount of oil from reaching the coast by breaking it down faster.
However, some independent scientists and EPA's own experts continue
to voice concerns about the approach.
Underwater injection of Corexit into the leak may have created the oil plumes which were
discovered below the surface. Because the dispersants were applied at depth,
much of the oil never rose to the surface. One plume was long, more than wide and deep.
In a major study on the plume, experts were most concerned about the slow pace
at which the oil was breaking down in the cold, water at depths of. In late 2012, a study
from Georgia Tech
and Universidad Autonoma de Aguascalientes in Environmental Pollution journal reported that
Corexit used during the BP oil spill had increased the toxicity of the oil by 52 times.
The scientists concluded that "Mixing oil with dispersant increased toxicity to ecosystems"
and made the gulf oil spill worse."
 Removal 
 [^]   [^]  The three basic approaches for removing the oil from the water were: combustion,
offshore filtration, and collection for later processing.
USCG said of tainted water was recovered, including of oil. BP said had been recovered
or flared. It is calculated that about 5% of leaked oil was burned at the surface
and 3% was skimmed. On the most demanding day 47,849 people were assigned on the response works.
From April to mid-July 2010, 411 controlled in-situ fires remediated approximately.
The fires released small amounts of toxins, including cancer-causing dioxins. According
to EPA's report, the released amount is not enough to pose an added cancer risk to workers
and coastal residents,
while a second research team concluded that there was only a small added risk.  [^]  Oil was collected
from water by using skimmers. In total 2,063 various skimmers were used. For offshore, more
than 60 open-water skimmers were deployed, including 12 purpose-built vehicles.
EPA regulations prohibited skimmers that left more than 15 parts per million of oil in the water.
Many large-scale skimmers exceeded the limit. Due to use of Corexit the oil was too dispersed
to collect, according to a spokesperson for shipowner TMT. In mid-June 2010,
BP ordered 32 machines that separate oil and water, with each machine capable of extracting up
to. After one week of testing, BP began to proceed and by 28 June, had removed.
After the well was captured, the cleanup of shore became the main task of the response works.
Two main types of affected coast were sandy beaches and marshes.
On beaches the main techniques were sifting sand, removing tar balls,
and digging out tar mats manually or by using mechanical devices. For marshes,
techniques such as vacuum and pumping, low-pressure flush, vegetation cutting,
and bioremediation were used.
 Oil-eating microbes 
Dispersants are said to facilitate the digestion of the oil by microbes. Mixing dispersants
with oil at the wellhead would keep some oil below the surface and in theory, allow microbes
to digest the oil before it reached the surface. Various risks were identified and evaluated,
in particular that an increase in microbial activity might reduce subsea oxygen levels,
threatening fish and other animals.
Several studies suggest that microbes successfully consumed part of the oil. By mid-September,
other research claimed that microbes mainly digested natural gas rather than oil. David L.
Valentine, a professor of microbial geochemistry at UC Santa Barbara,
said that the capability of microbes to break down the leaked oil had been greatly exaggerated.
Genetically modified Alcanivorax borkumensis was added to the waters to speed digestion.
The delivery method of microbes to oil patches was proposed by the Russian Research
and Development Institute of Ecology and the Sustainable Use of Natural Resources.
 Access restrictions 
On 18 May 2010, BP was designated the lead "Responsible Party"
under the Oil Pollution Act of 1990,
which meant that BP had operational authority in coordinating the response.
The first video images were released 12 May, and further video images were released
by members of Congress who had been given access to them by BP.
During the spill response operations, at the request of the Coast Guard,
the Federal Aviation Administration implemented a temporary flight restriction zone
over the operations area. Restrictions were to prevent civilian air traffic from interfering
with aircraft assisting the response effort.
All flights in the operations' area were prohibited except flight authorized
by air traffic control; routine flights supporting offshore oil operations; federal, state, local
and military flight operations supporting spill response; and air ambulance
and law enforcement operations. Exceptions
for these restrictions were granted on a case-by-case basis dependent on safety issues,
operational requirements, weather conditions, and traffic volume. No flights,
except aircraft conducting aerial chemical dispersing operations, or for landing and takeoff,
were allowed below. Notwithstanding restrictions, there were 800 to 1,000 flights per day
during the operations. Local and federal authorities citing BP's authority denied access
to members of the press attempting to document the spill from the air, from boats,
and on the ground, blocking access to areas that were open to the public.
In some cases photographers were granted access only
with BP officials escorting them on BP-contracted boats and aircraft. In one example, the U.S.
Coast Guard stopped Jean-Michel Cousteau's boat and allowed it
to proceed only after the Coast Guard was assured that no journalists were on board.
In another example, a CBS News crew was denied access
to the oil-covered beaches of the spill area. The CBS crew was told by the authorities:
"this is BP's rules, not ours," when trying to film the area.
Some members of Congress criticized the restrictions placed on access by journalists.
The FAA denied that BP employees or contractors made decisions on flights and access,
saying those decisions were made by the FAA and Coast Guard.
The FAA acknowledged that media access was limited to hired planes or helicopters,
but was arranged through the Coast Guard. The Coast Guard
and BP denied having a policy of restricting journalists;
they noted that members of the media had been embedded with the authorities and allowed
to cover response efforts since the beginning of the effort, with more
than 400 embeds aboard boats and aircraft to date. They also said that they wanted
to provide access to the information while maintaining safety.
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