Vital warning systems on the Deepwater Horizon oil rig were switched off at the time of the explosion in order to spare workers being woken by false alarms, a federal investigation has heard.

The revelation that alarm systems on the rig at the centre of the disaster were disabled – and that key safety mechanisms had also consciously been switched off – came in testimony by a chief technician working for Transocean, the drilling company that owned the rig.

Mike Williams, who was in charge of maintaining the rig's electronic systems, was giving evidence to the federal panel in New Orleans that is investigating the cause of the disaster on 20 April, which killed 11 people.

Williams told the hearing today that no alarms went off on the day of the explosion because they had been "inhibited". Sensors monitoring conditions on the rig and in the Macondo oil well beneath it were still working, but the computer had been instructed not to trigger any alarms in case of adverse readings.

Both visual and sound alarms should have gone off in the case of sensors detecting fire or dangerous levels of combustible or toxic gases.

The evidence of deliberate dilution of the rig's safety mechanisms is likely to have wide ramifications for BP and Transocean, the world's largest offshore drilling company. It switches the spotlight of blame away from BP and towards the subcontractor which took the decisions. Of the 126 crew on board the rig on 20 April, seven worked for BP and 79 for Transocean.

Williams said he discovered that the physical alarm system had been disabled a full year before the disaster. When he asked why, he said he was told that the view from even the most senior Transocean official on the rig had been that "they did not want people woken up at three o'clock in the morning due to false alarms".

Williams' testimony will raise questions about whether lives could have been saved had the alarms been properly set and the disaster mitigated.

He also revealed that a crucial safety device, designed to shut down the drill shack in the case of dangerous gas levels being detected, had been disabled, or bypassed as it is called.

When he saw that the system had been bypassed, Williams protested to a Transocean supervisor, Mark Hay, who dismissed his concerns. Hay responded: "Damn thing been in bypass for five years. Matter of fact, the entire [Transocean] fleet runs them in bypass."

In a third significant disclosure, Williams also revealed that a computer system used to monitor the drill shack was constantly freezing up, and on one occasion even produced wrong information. The system failed to indicate that a vital valve inside the blowout preventer, the device designed to shut down the well in case of problems, had been damaged.

Pressure is now likely to mount on Transocean to explain the discrepancies.

The New York Times reported earlier this week that a survey of workers carried out by Transocean shortly before the blast suggested key safety practices had not been followed.

Workers said that, while they were aware of unsafe practices on the rig, they were afraid to report mistakes for fear of reprisals.

A BP spokesman said last night: "The investigations continue to demonstrate that a range of things went wrong and that responsibility lies with a whole load of different companies."