BP's 2010 Deepwater Horizon explosion that killed 11 workers and fouled the Gulf of Mexico for months is finally headed to court. On Monday, a federal judge in New Orleans is scheduled to gavel the case to order.

The defendants are well-known to the public: BP, Britain's top energy company; Anadarko Petroleum, one of the largest natural gas companies in the U.S., Triton, an oil field service company and Transocean, which operated the ill-fated rig.

Halliburton is being sued by BP in a separate suit to ensure, BP says, that every company involved maintains some level of responsibility for the spill.

The U.S. Department of Justice has charged the defendants with civil penalties under the Clean Water Act and unlimited liability under the Oil Pollution Act for spilling 5 million barrels of crude oil for weeks into the Gulf of Mexico between April 20 and July 2010.

Under the Clean Water Act, defendants can be fined as much as $1,100 per barrel of oil for negligence. If U.S. District Judge Carl Barbier finds BP and the other defendants were grossly negligent, they could be fined $4,300 per barrel of oil spilled.

That could bring the total payout to as much as $21.5 billion.

Besides costing the lives of 11 employees of Transocean, the spill was a catastrophe for marine life in the Gulf, spoiling livelihoods of the fishing, tourism and travel sectors from Florida to Texas. The spill was probably the worst ever for an industry now ambitiously targeting the world's offshore oil and gas fields.

The previous catastrophe most likened to the BP Horizon disaster was the Exxon Valdez tanker spill of 1989, which spilled 32 million gallons into Alaskan waters.

The administrator of the Sept. 11, 2001, settlement funds, Kenneth Feinberg, was named head of funds that so far have paid out an estimated $6 billion to Gulf spill victims.

How liable are BP, Anadarko, Transocean, and Triton Asset Leasing, an oil field service company, for the millions of gallons of crude that spewed into the Gulf between April 20 and July 2010?

The U.S. government asserts they are liable for civil and federal penalties. What each party might have to pay will be determined as the trial progresses. That is unless the defendants broker a last-minute settlement.

David Guest, head of Earth Justice's Florida office who initially filed environmental claims against BP before the environmental group was removed from the legal proceedings, said he'd surprised if the companies didn't settle before Monday.

Guest estimated a total settlement could be valued at $40 billion.

Whether or not that money includes the $20 billion already put aside for claims remains to be seen, the lawyer said.

As of Dec. 31, BP reported assets exceeding $272 billion, of which $18.5 billion is cash. Some of that is being used to pay lawyers, including Chicago's Kirkland and Ellis, one of the nation's biggest corporate firms.

The companies are charged with violations under the Clean Water and Oil Pollution acts stemming from various failures as outlined by the Department of Justice.

The charges allege defendants had not taken sufficient precautions to keep the Macondo Well under control because they failed to use the best available and safest drilling technology to monitor it, provide constant surveillance and maintain and use equipment designed to ensure the rig's safety.

Other organizations such as the Sierra Club tried suing BP, but were blocked by the court.

The trial - barring last-minute settlements by the defendants - should end what essentially has been an ocean of litigation.

Barbier, a 13-year court veteran appointed by President Bill Clinton, has been assigned the giant case. It will be tried without a jury.

On Thursday, the judge ruled BP and Anadarko liable for civil penalties on top of the federal claims brought against them by the U.S. government.

That ruling could expose the defendants to potentially billions of dollars in civil damages beyond what they've already contributed to Feinberg's fund.

If they are found grossly negligent, Barbier could award punitive damages to businesses and property owners under maritime law, Bloomberg reported.

In November, Barbier ruled Alabama and Louisiana could sue BP under maritime law. In December BP reached a settlement with blowout preventer maker Cameron International. Cameron agreed to pay $250 million to BP. The money was placed in the $20 billion trust BP put aside to pay for the spill.

Next, on Feb. 17, Mitsui MOEX Offshore, another defendant, settled with the government for $90 million.

The Department of Justice has not wavered in its commitment to hold all responsible parties fully accountable for what stands as the largest oil spill in U.S. history, Attorney General Eric Holder said in a statement following the settlement.

 This landmark settlement is an important step - but only a first step - toward achieving accountability and protecting the future of the Gulf ecosystem by funding critical habitat preservation projects, Holder added.

BP, Transocean and Anadarko, the leading players in the ownership of and operation of the Deepwater Horizon rig and Macondo oil well, each filed various petitions for summary judgments. In them, the defendants held the others accountable while offering their own different interpretations of which is more liable under the law.

On April 20, 2010, a natural gas blowout on the Deepwater Horizon oil rig traveled back up the wellbore and ignited, causing a fire  that burned for two days before the rig sank. As it did, an underwater pipe that connected the rig to the well broke, allowing an estimated 206 million gallons of crude to spew into the Gulf of Mexico for close to three months.

Eleven people were killed in the explosion and resulting fire.

Despite the large sums of money the companies stand to lose, Wall Street seems unperturbed, with their stocks trading at stable levels.

In Friday trading, BP shares fell 4 cents  to$47.23, about $2 below their 52-week high.

Anadarko shares were at $87.67, only 50 cents below their 52-week high.

Transocean shares were at $50.88, more than $35 below their 52-week high of $85.98.