"A Fail safe fails"

A news item in the The Wall street Journal reports the following:
"BP PLC came within 1.4 inches or less of preventing the worst offshore oil spill in U.S. history, say engineers studying the safety device that failed in last year's Gulf of Mexico disaster.
The device, known as a blowout preventer, was a massive set of valves that sat on the sea floor nearly a mile beneath the Deepwater Horizon drilling rig, which floated on the surface. It was equipped with powerful shears designed to cut through pipe and seal off the well in an emergency. Why the device failed has been one of the central mysteries of last year's disaster.
In a report released Wednesday, engineers hired by U.S. investigators say they have solved it: The force of the blowout bent the drill pipe, knocking it off-center and jamming the shears. Rather than seal the well, the blades got stuck 1.4 inches or less apart, leaving plenty of space for 4.9 million barrels of oil to leak out.
The investigators concluded the blowout preventer failed as a result of a design flaw, not because of misuse by BP or any of the other companies involved, and not because of poor maintenance. The fail-safe device, the last line of defense against a disaster, wasn't designed to handle a real-world blowout, according to investigators, who called for further study of the devices.
The report doesn't address what caused the blowout itself. That has been the subject of several other major inquiries, which all have found that a series of decisions by BP and its contractors set the disaster in motion.
Even if the device had worked, it wouldn't have saved the lives of the 11 rig workers killed in the accident. That's because no one even tried to activate the shears until after massive explosions killed the men and crippled the rig. But the device could have mostly prevented the oil spill that began when the Deepwater Horizon sank two days after the initial explosion".
How sure are you that your fail safe devices will work as intended? Today there are systems available to conduct a online partial stroke test of critical valves. But the test is done during normal operating conditions. In the case of a major upset or incident, conditions may prevent the fail safe devices from working. 
Read the article in this link.