Few Americans have ever heard of Robert Kaluza and Donald Vidrine. They have spent much of their lives offshore, drilling holes in the seafloor for the oil giant BP. Over many decades in the business, each rose to the rank of well site leader, a job commonly known on a rig as “the company man.”

Their lives changed irreversibly on April 20, 2010, a calm, clear day in the Gulf of Mexico. Kaluza and Vidrine took turns supervising the drilling operation on a rig named Deepwater Horizon.

Now they are criminal defendants, facing their first court appearance Wednesday before a federal judge in New Orleans. After 21 / 2 years of investigation by the Justice Department, a federal grand jury this month indicted Kaluza, 62, and Vidrine, 65, on multiple felony counts of manslaughter in the fiery blowout that killed 11 crewmen and caused the worst oil spill in American history.

The charges against Kaluza and Vidrine are part of the complex and contentious legal aftermath of the disaster. The legal battles will probably take years to resolve. So far, Kaluza and Vidrine are the only individuals facing criminal charges for the blowout itself.

Their attorneys say their clients are scapegoats and will plead not guilty.

“Every investigative report has said there were multiple failures at multiple levels at multiple companies,” said Shaun Clarke, Kaluza’s attorney. “So it is difficult for me to understand why the federal government decided to single out two guys working on a rig.”

Vidrine’s attorney, Bob Habans, said of his client: “It is almost inconceivable that any fair-minded person would blame this hardworking and diligent man for one of the most catastrophic events in the history of the oil business.”

The blowout of the Macondo well sent an estimated 4.9 million barrels of oil into the gulf, polluted beaches and marshes, killed wildlife and roiled the region’s economy. Culpability remains a matter of robust litigation and legal maneuvering among powerful companies.

At the corporate level, BP has agreed to plead guilty to multiple counts of manslaughter, admitting to simple negligence but not gross negligence — a key distinction in the legal battle. The plea deal was reached after extensive negotiations between BP attorneys and the Justice Department’s criminal division, as well as negotiations with the Securities and Exchange Commission.

BP will pay about $4.5 billion in fines as part of the deal, which still requires court approval.

But even with the criminal case against BP apparently resolved, there remains a multibillion-dollar civil case against the company that is grinding toward a Feb. 25 trial date. That is a multi-jurisdictional case involving federal and state claims and more private lawsuits, with U.S. District Judge Carl Barbier presiding over the entire affair in New Orleans.

Under the Clean Water Act, BP can be fined $1,100 to $4,400 per barrel spilled, depending on the degree of negligence determined by the court. The government estimates that about 4 million barrels of oil were never captured, skimmed or burned — an estimate BP has disputed — and thus BP could face Clean Water Act civil penalties of nearly $18 billion if found guilty of gross negligence.

The company has apologized for its role in the disaster and has agreed to a roughly $7.8 billion settlement with a large group of private plaintiffs. But the company vows to vigorously defend itself in the remaining civil cases.

Although the presidential Oil Spill Commission concluded that an overarching “failure of management” was at the root of the spill, no one working onshore for BP faces any charge regarding the design of the well or the protocols for the drilling. Nor have charges been filed against anyone working for BP’s contractors, which included Transocean, the owner of the rig, and Halliburton, which cemented the Macondo well.

The plea deal with BP does not preclude further criminal charges against individuals, and the Justice Department has said the criminal investigation is not closed. The department would not comment on the assertion by the attorneys for Vidrine and Kaluza that their clients were being scapegoated.

The grand jury has also charged one BP executive, David Rainey, with obstruction of Congress for allegedly misleading a congressional subcommittee about the well’s flow rate. He also faces arraignment Wednesday and will plead not guilty, his attorneys have said.

The manslaughter charges against Kaluza and Vidrine — and BP’s agreement to plead guilty to manslaughter — hinge entirely on a single procedure that occurred on the day of the disaster. It was called “the negative test.”

The well had supposedly been plugged at the bottom with a modest quantity of cement, 60 barrels. The negative test essentially tempted oil and gas to invade the well. The drill team removed much of the heavy drilling mud that normally choked the well, to see whether that would cause pressure to build up on the drill pipe. If the cement had effectively plugged the bottom of the well, there should not have been any pressure.

But the initial result was troubling, according to testimony at the government inquiry conducted by the Coast Guard and what was then called the Minerals Management Service. When the drill pipe was opened at the top, fluid surged up onto the rig. When crewmen closed the drill pipe, pressure rose again.

People on the rig that day testified that one of the Transocean employees, tool pusher Jason Anderson, suggested that the troubling pressure reading was due to what he called the “bladder effect.” This has remained a fuzzy element of the disaster narrative. The testimony indicated that Anderson believed that heavy mud still in the well could have been creating the pressure seen on the drill pipe and that the pressure was not caused by oil and gas invading the well bore.

Anderson never had a chance to testify, because he was killed in the subsequent explosion as he and his co-workers desperately tried to seal the blown-out well.

BP’s well site leaders, Kaluza and Vidrine, were part of the discussions during the negative test and had ultimate authority to decide how to proceed. According to testimony and BP internal investigatory notes, Vidrine decided to conduct another negative test, this time looking for pressure on a flexible line that was connected to the bottom of the well.

This second test showed no pressure. That looked like a good result — but pressure on the drill pipe remained.

“It appears they began with the assumption that the cement job had been successful and kept running tests and proposing explanations until they convinced themselves that their assumption was correct,” concluded the 2011 report of the chief counsel to the Oil Spill Commission.

The drill team continued to pump heavy mud out of the well. But then the well began spewing mud uncontrollably, followed by gas and then the explosion.

Attorneys for Kaluza and Vidrine would not discuss what their strategy will be at trial, but one possible argument is that the two men were trying to interpret the negative test without full information about potential problems with the well and the cement job.

“BP’s onshore team should have, and easily could have, alerted the well site leaders and rig crew that cement failure at Macondo might be more likely than normal and instructed them to be extra vigilant regarding any odd pressure readings,” the chief counsel’s report stated.

The Oil Spill Commission concluded that multiple risk factors played a role in the disaster, including a flawed recipe for the cement slurry; procedures that included unnecessary removal of mud from the well bore; inadequate supervision of late-stage decisions in the well design; and missed signals from pressure gauges suggesting that gas had entered the well. The commission also found feeble regulatory oversight by the government. “Each of the mistakes made on the rig and onshore by industry and government increased the risk of a well blowout,” the commission concluded.

In a bizarre twist to the onshore-offshore dichotomy, two of Kaluza and Vidrine’s bosses happened to be on the rig at the time of the disaster. The two BP executives, Pat O’Bryan and David Sims, had flown by helicopter to the rig that afternoon along with two Transocean executives. The four executives took a tour and then held a meeting to congratulate the rig crew on its record of safe operations. The executives were not asked for input into the interpretation of the negative test.

Neither Kaluza nor Vidrine testified during earlier investigatory hearings. BP records obtained by the government show that Kaluza e-mailed two colleagues days after the blowout:

“I believe there is a bladder effect on the mud below an annular preventer as we discussed,” Kaluza wrote.

The grand jury indictment charged that Kaluza and Vidrine “accepted a nonsensical explanation for the abnormal readings, again without calling engineers onshore to consult.”

Both men remain employed by BP but have been on leaves of absence since the disaster.

Kaluza’s attorney, Clarke, said his client is being blamed for an accident that was the result of a collective failure. Kaluza had been on Deepwater Horizon only four days when the disaster happened. He had replaced another company man who had gone to shore to take classes in blowout prevention.

“Right now he’s devastated. He’s with family. But he feels very strongly that he did not cause the deaths of his 11 co-workers and he did not cause the most monumental environmental disaster in the country’s history,” Clarke said of his client. “And he has every intention of proceeding to trial.”