The civil and criminal proceedings stemming from the explosion of Deepwater Horizon and the resulting massive oil spill in the Gulf of Mexico began shortly after the April 20, 2010 incident and have continued since then. They have included an extensive claims settlement process for a guilty plea to criminal charges by BP, and an ongoing Clean Water Act lawsuit brought by the U.S. Department of Justice and other parties.

A federal judge, ruling on the Clean Water Act suit in September 2014, found that BP was primarily responsible for the oil spill as a result of its deliberate misconduct and gross negligence. The finding means that the company may be subject to $18 billion in penalties in addition to the $28 billion already paid out in claims and cleanup costs. In July 2015 BP settled the case and carries some extra penalties.

Litigation timeline

Litigation commenced almost immediately after the explosion and oil spill. By May 27, 2010, Transocean, which owned the Deepwater Horizon, said in testimony before the U.S. House Judiciary Committee that it was defendant in 120 lawsuits, of which more than 80 were class actions seeking payment for financial losses covered by the Oil Spill Pollution Act. The company said that most of these early plaintiffs were "fishermen, hotel operators, landowners, rental companies, restaurants and seafood processors, who claim a current or potential future loss of business in the aftermath of the oil spill."

At the same time UK media reported that over 130 lawsuits relating to the spill had been filed against one or more of BP, Transocean, Cameron International Corporation, and Halliburton Energy Services, although it was considered likely by observers that these would be combined into one court as a multidistrict litigation. Because the spill had been largely lingering offshore, the plaintiffs who claimed damages at that time were mostly out-of-work fishermen and tourist resorts that were receiving cancellations. BP and Transocean wanted the cases to be heard in Houston, seen as friendly to the oil business, but the plaintiffs requested the case be heard in Louisiana, Mississippi or Florida. Five New Orleans judges recused themselves from hearing oil spill cases because of stock ownership in companies involved or other conflicts of interest. BP has retained law firm Kirkland & Ellis to defend most of the lawsuits arising from the oil spill.

BP, which largely self-insures through its own Jupiter Insurance Ltd, was also the subject of early litigation from multiple Lloyd's of London underwriting syndicates and other underwriters seeking a declaration that they were not liable to BP under Transocean's insurance policy.

In June 2010, Hornbeck Offshore Services, joined by several dozen corporations with offshore interests, filed suit in U.S. District Court seeking to enjoin the U.S. Department of the Interior from imposing a ban on deep water drilling.

In July 2010, news reports claimed that BP had been attempting to hire prominent scientists from public universities around the Gulf Coast to aid its defense against the lawsuit that the federal government will bring as a result of the spill. BP attempted to hire the entire marine sciences department at one university, but the university declined because of confidentiality restrictions, however several other universities have accepted. In developing its case, the government will draw on the large amount of scientific research conducted by academic institutions along the Gulf, and many scientists being hired by BP serve at those same institutions. The contract prohibits the scientists from publishing their research, sharing it with other scientists or speaking about the data that they collect for at least the next three years, and it requires scientists to agree to withhold data even in the face of a court order if BP decides to fight the order. It stipulates that scientists will be paid only for research approved by BP. Robert Wiygul, who specializes in environmental law, said that he sees ethical questions regarding the use of publicly owned laboratories and research vessels to conduct confidential work on behalf of a private company. "This is not an agreement to do research for BP. This is an agreement to join BP's legal team. You agree to communicate with BP through their attorneys and to take orders from their attorneys." The contracts have the added impact of limiting the number of scientists who are able to work with federal agencies.

In August 2010, 77 cases, including those brought by state governments, individuals, and companies, in the U.S. District Court for the Eastern District of Louisiana under Multi-District Litigation docket MDL No. 2179, captioned In re: Oil Spill by the Oil Rig "Deepwater Horizon" in the Gulf of Mexico, on April 20, 2010, presided over by U.S. District Judge Carl Barbier.Staff, Environmental Law Institute. Deepwater Horizon Oil Spill Litigation Database Judge Barbier is trying the case without a jury, as is normal in United States admiralty law.Margaret Cronin Fisk & Laurel Brubaker Calkins for Bloomberg News. December 14, 2011. Cameron Appeals BP Gulf Spill Trial Plan, Wants Case Before Jury Day 1 Morning Session: Transcript of Nonjury Trial Proceedings Heard before the Honorable Carl J. Barbier United States District Judge

On December 15, 2010 The US Department of Justice filed a civil suit against BP and other defendants for violations under the Clean Water Act in the U.S. District Court for the Eastern District of Louisiana, this was consolidated with the other cases.US Department of Justice United States of America v. BP Exploration & Production Inc. et al., Civ. Action No. 2:10-cv-04536 captioned United States of America v. BP Exploration & Production Inc. et al., Civ. Action No. 2:10-cv-04536.John Wyeth Griggs. (2011) BP Gulf of Mexico Spill Energy Law Journal. Vol. 32:57-79

In April 2011, BP filed $40 billion in lawsuits against rig owner Transocean, cementer Halliburton and blowout preventer manufacturer Cameron International. The oil firm alleged failed safety systems and irresponsible behaviour of contractors had led to the explosion, including claims that Halliburton failed to properly use modelling software to analyze safe drilling conditions.

In October 2013, it was reported that BP could face tens of millions of dollars of new claims from British shareholders who lost money and claim that BP did not come clean about the scale of the disaster. The shareholders include the South Yorkshire Pensions Authority, Skandia Global Funds and GAM Fund Management.The Daily Telegraph, BP could be hit by new Deepwater Horizon claims after US court ruling

In April 2023, The Guardian reported that BP had engaged in "scorched earth" legal tactics in fending off litigation from cleanup workers and others impacted by the spill, many of whom are low-income and suffer from long-term health effects.

Department of Justice lawsuit

On December 15, 2010, The United States Department of Justice filed a civil and criminal suit against BP and its partners in the oil well, Transocean and Halliburton, for violations under the Clean Water Act in the U.S. District Court for the Eastern District of Louisiana. The plaintiffs included Gulf states and private individuals. The case was consolidated with about 200 others, including those brought by state governments, individuals, and companies under Multi-District Litigation docket MDL No. 2179, before U.S. District Judge Carl Barbier. The Justice Department sought the stiffest fines possible, and has said it would seek to prove that BP "was grossly negligent and engaged in willful misconduct in causing the oil spill." BP denied that, saying that gross negligence is a high bar that cannot be surmounted, and that the oil spill was a "tragic accident." The case was carefully watched, because a ruling of gross negligence would result in a four-fold increase in Clean Water Act penalties, and would leave the company liable for punitive damages for private claims.Richard Thompson for NOLA.com and The Times-Picayune. 16 April 2013 BP oil spill trial continues as demonstrators note upcoming 3-year anniversary of disasterDeon Daugherty for the Houston Business Journal. 5 April 2013 Judge dismisses latest BP injunction in oil spill settlement disputeAndrew Callus and Braden Reddall for Reuters. 20 February 2013 U.S. judge accepts BP collected 810,000 barrels in spill Any fines from gross negligence would hit BP's bottom line very hard, because they would not be tax-deductible. The company paid no federal income tax to the U.S. government in 2010 because of deductions related to the spill.

DOJ claims On August 31, 2012, the US Department of Justice (DOJ) filed papers in federal court in New Orleans blaming BP PLC for the Gulf oil spill, describing the spill as an example of "gross negligence and willful misconduct." BP rejected the charges, saying "BP believes it was not grossly negligent and looks forward to presenting evidence on this issue at trial in January." The DOJ also said Transocean, the owner and operator of the Deepwater Horizon rig, is guilty of gross negligence as well.

The DOJ brief strongly criticized officials for failing to rerun a "negative pressure test" when the first test revealed a pressure anomaly from the well. Despite acknowledging the pressure reading and agreeing that it was worrisome, BP's supervisors did not run the test again. Ordering a retest would have begun with the closing of the rig's blowout preventer, which would have stopped the flow before the blowout. DOJ attorneys state, "That such a simple, yet fundamental and safety-critical test could have been so stunningly, blindingly botched in so many ways, by so many people, demonstrates gross negligence."

The government cited "a culture of corporate recklessness" in their investigation of the events leading up to the blow out. The DOJ's attorneys brief includes several emails exchanged between John Guide, the Macondo well engineer, and David Sims, his boss. In one email Guide remarked that Macondo was a very difficult well, that the drilling crew was "flying by the seat of our pants" under a "huge level of paranoia" that was "driving chaos" and concluded that "the operation is not going to succeed if we continue in this manner." The government attorneys call Guide's email a "clarion cry of impending disaster" and question why Sims did no follow-up and why BP's internal investigators didn't mention it in their report on the events leading up to the disaster. In another email Sims notes that BP had sent out 15 extra "centralizers" to complete the well cementing, but Guide then ordered that they not be used. In an email quoted in the brief, Guide concluded, "But, who cares, it is done, end of story. Will probably be fine."

The DOJ also refused to accept BP's assertion that the Gulf's ecosystem has gone through a robust recovery and that the clean up is largely complete, saying that there is far more damage than meets the eye and more cleanup may need to be done. "The fact that a section of shoreline is no longer considered suitable for response action does not mean that it is not suffering continuing injury from the Spill. For example, an important consideration in deciding on appropriate response action is whether a cleanup technique will cause more harm than good."

First phase trial