Location of the Deepwater Horizon
on April 20, 2010
Deepwater Horizon was a floating
semi-submersible drilling unit — a fifth-generation, ultra-deepwater,
dynamically positioned, column-stabilized drilling rig owned by
Transocean and built in
South Korea. The platform 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).
 Press releases from
Transocean state the platform had historically been used for deeper wells, including the deepest underwater gas and oil well in history at 35,055 feet (10,685 m) in 2009.
 The $560 million platform was built by
Hyundai Heavy Industries in South Korea and completed in 2001.
 It was owned by Transocean, operated under the
flag of convenience, and was under lease to
BP until September 2013.
 At the time of the explosion, the Deepwater Horizon was on
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.
 In March 2008, the mineral rights to drill for oil on the Macondo Prospect were purchased by BP at the
Minerals Management Service's lease sale.
 The platform commenced drilling in February 2010 at a water depth of approximately 5,000 feet (1,500 m).
 At the time of the explosion the rig was drilling an exploratory well.
 The planned well was to be drilled to 18,360 feet (5,600 m) below sea level, and was to be plugged and suspended for subsequent completion as a
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.
Transocean safety record
The rig owner, Transocean, had a "strong overall" safety record with no major incidents for 7 years.
 However an analysts' review "painted a more equivocal picture" with Transocean rigs being disproportionately responsible for safety related incidents in the Gulf and industry surveys reporting concerns over falling quality and performance. In the 3 years 2005 to 2007, Transocean was the owner of 30% of oil rigs active in the Gulf, and 33% of incidents that triggered a
Minerals Management Service (MMS) investigation were on Transocean rigs. However, in the 3 years from 2008 to February 15, 2010, it owned 42% of rigs but was the owner for 73% of incidents. Industry surveys saw this as an effect of its November 2007
merger with rival
GlobalSantaFe. Transocean "has had problems" previously with both cement seals (2005) and
blowout preventers (2006), which are the suspected cause of the Deepwater Horizon loss, although Transocean states that cementing is a third party task and that it has "a strong maintenance program to keep blowout preventers working".
 According to the
Wall Street Journal online:
- "In 2008 and 2009, the surveys ranked Transocean last among deep-water drillers for "job quality" and second to last in 'overall satisfaction'. For three years before the merger, Transocean was the leader or near the top in both measures. Transocean ranked first in 2008 and 2009 in a category that gauges its in-house safety and environmental policies. There were few indications of any trouble with the Deepwater Horizon before the explosion. The rig won an award from the MMS for its 2008 safety record, and on the day of the disaster, BP and Transocean managers were on board to celebrate seven years without a lost-time accident. A BP spokesman said rigs hired by BP have had better safety records than the industry average for six years running, according to MMS statistics that measure the number of citations per inspection. BP has been a finalist for a national safety award from the MMS for the past two years. Mr. Odone wouldn't comment on BP's relationship with Transocean after the Gulf disaster but said BP continues to use Transocean rigs."
Pre-explosion risks and precautions
In February 2009, BP filed a 52-page exploration and environmental impact plan for the Macondo well with the
Minerals Management Service (MMS), an arm of the
United States Department of the Interior that oversees offshore drilling. The plan stated that it was "unlikely that an accidental surface or subsurface oil spill would occur from the proposed activities".
 In the event an accident did take place the plan stated that due to the well being 48 miles (77 km) from shore and the response capabilities that would be implemented, no significant adverse impacts would be expected.
 The Department of the Interior exempted BP's Gulf of Mexico drilling operation from a detailed environmental impact study after concluding that a massive oil spill was unlikely.
 In addition, following a loosening of regulations in 2008, BP was not required to file a detailed blowout plan.
wellhead had been fitted with a
blowout preventer (BOP), but it was not fitted with remote-control or acoustically activated triggers for use in case of an emergency requiring a platform to be evacuated. It did have a
dead man's switch designed to automatically cut the pipe and seal the well if communication from the platform is lost, but it was unknown whether the switch was activated.
 Documents discussed during congressional hearings June 17, 2010 indicated that Transocean previously made modifications to the BOP for the Macondo site which increased the risk of BOP failure, in spite of warnings from their contractor to that effect. Regulators in both
Brazil generally require acoustically activated triggers on all offshore platforms, but when the Minerals Management Service considered requiring the remote device, a report commissioned by the agency as well as drilling companies questioned its cost and effectiveness.
 In 2003, the agency determined that the device would not be required because drilling rigs had other back-up systems to cut off a well.
Pre-explosion problems and warnings
US Coast Guard had issued pollution citations for Deepwater Horizon 18 times between 2000 and 2010, and had investigated 16 fires and other incidents. These incidents were considered typical for a Gulf platform and were not connected to the April 2010 explosion and spill.
 The Deepwater Horizon had other serious incidents, including one in 2008 in which 77 people were evacuated from the platform when it tilted and began to sink after a section of pipe was incorrectly removed from the platform's
American Bureau of Shipping last inspected the rig's failed blowout preventer in 2005.
Internal BP documents show that BP engineers had concerns as early as 2009 that the metal casing that BP wanted to use might collapse under high pressure.
 According to a number of rig workers, it was understood that workers could get fired for raising safety concerns that might delay drilling.
In March 2010, the rig experienced problems that included drilling mud falling into the undersea oil formation, sudden gas releases, a pipe falling into the well and at least three occasions of the blowout preventer leaking fluid.
 The rig's mechanic stated that the well had problems for months and that the drill repeatedly kicked due to resistance from high gas pressure.
 On March 10, a BP executive e-mailed the Minerals Management Service about a stuck pipe and well control situation at the drilling site, and stated that BP would have to plugback the well.
 A confidential survey commissioned by Transocean weeks before the explosion stated that workers were concerned about safety practices and feared reprisals if they reported mistakes or other problems.
 The survey raised concerns "about poor equipment reliability, which they believed was a result of drilling priorities taking precedence over maintenance." The survey found that "many workers entered fake data to try to circumvent the system. As a result, the company's perception of safety on the rig was distorted".
The blowout preventer was damaged in a previously unreported accident in late March.
 According to Transocean, workers had been performing standard routines and had no indication of any problems prior to the explosion.
By April 20 the operation was running five weeks late.
An April draft of a BP memo warned that the cementing of the casing was unlikely to be successful.
Halliburton has said that it had finished cementing 20 hours before the blowout, but had not yet set the final cement plug.
 A special
nitrogen-foamed cement was used which is more difficult to handle than standard cement.
BP Vice President of drilling, Patrick O'Bryan, was on the platform two hours prior to the explosion
 to celebrate seven years without a "lost-time incident" with the rig's crew.
 A BP official on board the rig directed the crew to replace the drilling mud with lighter seawater even though the rig's chief driller protested.
Preliminary findings from BP's internal investigation indicated several serious warning signs in the hours prior to the blowout.
 Equipment readings indicated gas bubbling into the well, which could signal an impending blowout.
 The heavy drilling mud in the pipes initially held down the gas.
 A House Energy and Commerce Committee statement in June 2010 noted that in a number of cases leading up to the explosion, BP appears to have chosen riskier procedures to save time or money, sometimes against the advice of its staff or contractors.