HOUSTON (Reuters) - A cascade of human and mechanical failures likely caused last month’s deadly offshore rig explosion and an undersea oil gusher that could be the worst U.S. environmental disaster, according to data gathered by congressional investigators and reviewed by experts.

Around 9:53 p.m. CDT (0253 GMT) on April 20, Swiss-based Transocean Ltd’s Deepwater Horizon rig exploded while it was drilling a well a mile beneath the Gulf of Mexico under contract for London-based BP Plc. Eleven rig workers are presumed dead.

Hours before the explosion, set off by flammable methane gas that surged up the drill pipe, forces were already in motion on the drill deck and beneath the ocean surface that opened the door to catastrophe.

“There was a series of failures -- human and mechanical,” said Satish Nagarajaiah, professor in civil and mechanical engineering at Rice University in Houston. “This was the perfect storm.”

According to information from BP, Transocean, and rig contractor Halliburton Co gathered by investigators from the House Energy and Commerce Committee, rig workers pressed ahead to put the finishing touches on the well despite potentially alarming test results that signaled a buildup of gas pressure deep in the well’s reservoir.

And on the ocean floor, a 450-ton series of valves and pipes called a blowout preventer -- meant to be the last line of defense in the event of a blowout -- was disabled after being significantly modified.

“This catastrophe appears to have been caused by a calamitous series of equipment and operational failures,” Representative Henry Waxman, chairman of the committee, said at a hearing on the spill on Wednesday.

“NOT SATISFACTORY”

Drilling an oil well deep into the earth involves waging a delicate battle against a torrent of highly pressurized oil and methane gas unleashed when a hydrocarbon reservoir is penetrated.

To prevent an uncontrolled release of oil and gas, known as a blowout, the industry has developed multiple systems to hold at bay the extreme pressures within the well.

On the Deepwater Horizon, three of those systems -- the blowout preventer, the metal casing within the well and the cement that held it in place -- all likely failed, according to testimony from company officials and data gathered by investigators from the House Energy and Commerce Committee.

BP and Transocean made a decision late on April 20 to begin removing mud from within the drill pipe despite pressure tests from within the well that a BP official described as “not satisfactory” and “inconclusive,” Waxman said on Wednesday.

Drilling mud is a mixture of synthetic ingredients that is pumped into the well to exert downward hydrostatic pressure and prevent a column of oil and gas from rushing up the pipe.

Earlier in the day, well pressure tests showed an imbalance between the drill pipe and kill and choke lines running from the drill deck to the blowout preventer. The pressure in the drill pipe was 1,400 pounds per square inch (PSI), while the choke and kill lines read zero PSI, Waxman said.

“They knew there was something wrong because the pressure in the kill and choke lines was not correct,” Nagarajaiah said. “That should have alerted them.”

But according to Waxman, workers performed additional tests and at 8 p.m. CDT (0100 GMT) “company officials determined that the additional results justified ending the test and proceeding with well operations.”

“I’m a little shocked that they proceeded at that point,” said Philip Johnson, a petroleum engineering professor at the University of Alabama.

“It sounds like they never got an adequate low pressure test and someone decided to go ahead and displace the mud,” Johnson said. “That sounds like a pretty serious mistake.”

Once the well exploded in a green flash, rig workers tried to activate the blowout preventer on the ocean floor, designed as a fail-safe to choke off the well.

But officials from Cameron International Corp, which manufactured the device, told committee staff that a key hydraulic system meant to supply emergency power was disabled.

And another key device component designed to clamp down around the drill pipe and seal any leak -- known as a variable bore ram -- had been replaced by a useless test ram, according to Representative Bart Stupak, chairman of the Energy and Commerce Committee’s investigations subcommittee.

With oil gushing into the sea, BP sent remote robots to the ocean floor to attempt to activate the ram. “An entire day’s worth of precious time had been spent engaging rams that closed the wrong way,” Stupak said.