During its nine years at sea, the Deepwater Horizon oil rig operated by BP suffered a series of spills, fires — even a collision — because of equipment failure, human error and bad weather. It also drilled the world’s deepest offshore well.

But Deepwater Horizon’s lasting legacy will undoubtedly be the environmental damage it caused after it exploded and sank, killing 11 crew and releasing an estimated 210,000 gallons of oil a day into the Gulf of Mexico.

What likely destroyed the rig in a ball of fire last week was a failure — or multiple failures — 5,000 feet below. That’s where drilling equipment met the sea bed in a complicated construction of pipes, concrete and valves that gave way in a manner that no one has yet been able to explain.

Oil services contractor Halliburton Inc. said in a statement Friday that workers had finished cementing the well’s pipes 20 hours before the rig went up in flames. Halliburton is named as a defendant in most of the more than two dozen lawsuits filed by Gulf Coast people and businesses claiming the oil spill could ruin them financially. Without elaborating, one lawsuit filed by an injured technician on the rig claims that Halliburton improperly performed its job in cementing the well, “increasing the pressure at the well and contributing to the fire, explosion and resulting oil spill.”

Remote-controlled blowout preventers designed to apply brute force to seal off a well should have kicked in. But they failed to activate after the explosion.

Scott Bickford, a lawyer for several Deepwater Horizon workers who survived the blast, said he believes a “burp” of natural gas rose to the rig floor and was sucked into machinery, leading to the explosion.

Halliburton said “it is premature and irresponsible to speculate on any specific causal issues.”

Before last week’s catastrophe, Deepwater Horizon’s most recent hiccup occurred in Nov. 2005, when the rig — under contract with BP — spilled 212 barrels of an oil-based lubricant due to equipment failure and human error. That spill was probably caused by not screwing the pipe tightly enough and not adequately sealing the well with cement, as well as a possible poor alignment of the rig, according to records maintained by the federal Minerals Management Service.

Following that spill, MMS inspectors recommended increasing the amount of cement used during this process and applying more torque when screwing in its pipes.

Experts say the number of safety incidents experienced by Deepwater Horizon isn’t unusual for an industry operating in harsh conditions. And it is difficult to draw any connections between those problems and last week’s deadly explosion, they say.

“These are big, floating cities,” said Tyler Priest, a historian of offshore oil and gas exploration. “You’re always going to have minor equipment failure and human error, and of course they’re operating in a hurricane-prone environment.”

Because vessels like the Deepwater Horizon operate 24 hours a day, Coast Guard officials said minor equipment problems appear frequently. But if they go unfixed, such incidents could mushroom into bigger concerns.

• In Feb. 2002, just months after the rig was launched from a South Korean shipyard, it spilled 267 barrels of oil into the Gulf after a hose failed, according to MMS records.

• In June 2003, the rig floated off course in high seas, resulting in the release of 944 barrels of oil. MMS blamed bad weather and poor judgment by the captain. A month later, equipment failure and high currents led to the loss of 74 barrels of oil.

• In January 2005, human error caused another accident. A crane operator forgot he was in the midst of refueling a crane, and 15 gallons of overflowing diesel fuel sparked a fire.

The rig was being used by BP during all of the above incidents.

The Coast Guard, which is supposed to ensure the vessels are seaworthy, keeps its own set of safety records on the Deepwater Horizon.

From 2000 to 2010, the Coast Guard issued six enforcement warnings and handed down one civil penalty and a notice of violation to Deepwater Horizon, agency records show.

On 18 different occasions during that period, the Coast Guard cited the vessel for an “acknowledged pollution source.” No further details about the type of pollution were immediately available.

The agency also conducted 16 investigations of incidents involving everything from fires to slip-and-fall accidents.

Steven Sutton, who oversees offshore drilling inspectors in the Coast Guard’s New Orleans office, said the number of accidents and incidents reported on the Deepwater Horizon didn’t strike him as unusual.

A collision with a towing vessel reported on June 26, 2003, could have created safety problems over the long term if the $95,000 damage to the rig’s hull wasn’t adequately repaired, Sutton said. The collision risked compromising the rig’s watertight integrity or weakening the structure that supports the drilling operation, he said.

Guy Cantwell, a spokesman for the rig’s owner Transocean Ltd., said Friday that the Swiss-based company planned to conduct its own investigation of what caused last week’s explosion.

“Any prior incidents were investigated,” he said. “Any speculation that they are related to the Deepwater Horizon incident is speculation.”

Document shows BP didn’t plan for major oil spill

MOUTH OF THE MISSISSIPPI RIVER — British Petroleum downplayed the possibility of a catastrophic accident at an offshore rig that exploded, causing the worst U.S. spill in decades along the Gulf coast and endangering shoreline habitat.

In the 52-page exploration plan and environmental impact analysis, BP repeatedly suggested it was unlikely, or virtually impossible, for an accident to occur that would lead to a giant crude oil spill and serious damage to beaches, fish, mammals and fisheries.

BP’s plan filed with the federal Minerals Management Service for the Deepwater Horizon well, dated February 2009, says repeatedly that it was “unlikely that an accidental surface or subsurface oil spill would occur from the proposed activities.”

And while the company conceded that a spill would “cause impacts” to beaches, wildlife refuges and wilderness areas, it argued that “due to the distance to shore (48 miles) and the response capabilities that would be implemented, no significant adverse impacts are expected.”

“Clearly, the sort of occurrence that we’ve seen on the Deepwater Horizon is clearly unprecedented,” BP spokesman David Nicholas told The Associated Press on Friday. “It’s something that we have not experienced before … a blowout at this depth.”

Robert Wiygul, an Ocean Springs, Miss.-based environmental lawyer and board member for the Gulf Restoration Network, said he doesn’t see anything in the document that suggests BP addressed the kind of technology needed to control a spill at that depth of water.

“The point is, if you’re going to be drilling in 5,000 feet of water for oil, you should have the ability to control what you’re doing,” he said.

— The Associated Press