This hardly ever happens. However ...
Last Friday, a Nordex N80 collapsed on Murley Mountain in County Tyrone. Images apparently show the tower buckled close to the base. The foundations and foundation bolts appear to be intact: there was a tower failure.
Back in 2013, in Donegal, a Vestas V52 failed with similar results; these are very unusual events.
Nordex, the manufacturer, will conduct an investigation and until it is completed, the cause of the failure is uncertain. Nordex are known for reliable and safe machines. What follows is a general technical speculation with no details of the event beyond media reports. The usual cautions apply.
With the published facts to hand and without visiting the site, what can we say?
On the day of the crash, the wind speeds across Northern Ireland may have peaked at about 30 m/s (58 knots, see Met Éireann's Malin Head data for January 2nd). This is about a whole gale. The Malin Head data is coastal and at a low altitude, about 60 km to the north of Fintona.
If the average wind speed over a few minutes rises above about 25 m/s, wind turbines will attempt to park until the wind speed eases. Although potentially dangerous, such wind speeds are not all that high in wind turbine design terms. All large wind turbines are built to the very demanding IEC 61400- series standards.
A simple structural analysis will likely show that the steel tower will tolerate thrust loads from an uncontrolled rotor up to wind speeds well in excess of those reported on January 2nd. It is straightforward to repeat the rough load calculations for the V52 for the N80.
The conclusion from the Donegal V52 failure investigation was that the turbine's control system failed, leaving the machine's blades in power generation configuration as the wind speeds increased. Normally blades pitch, or turn, parallel to the wind as the wind speeds increase to shed the increased load. A pitch system failure that leaves the machine in run mode is rare - pitch systems are designed to fail-safely - but if this happens, the forces acting on the blades increase very quickly with wind speed, as shown in the graph above.
If the turbine's ability to slow or stop is compromised, the rotor will accelerate to a high rate of rotation. This uncontrolled state is called overspeed. According to Vestas, the Donegal V52 tower collapse was caused by one of the blades striking the tower during overspeed. The unusually high loads may have pushed the blade out of its normal plane of rotation, i.e. into the tower. There seemed to be little evidence of this in photographs at the time but it did explain the tower failure.
As discussed in the earlier V52 note, you can see a recorded example of a large system failing in this way online; note how the tower does not fail before the blade strike - this 2008 control system failure was unique and the result of an unfortunate sequence of events. The wind speeds at Malin Head last Friday were higher than those in East Jutland on February 22nd 2008 and than those in Donegal in 2013 when the V52 failed. The Nordtank 600 kW (43 m diameter rotor) machine and the Vestas V52 (52 m) are both smaller than the Nordex N80 and of different design. At the same time, the behaviour of these turbines in overspeed is instructive: other things fail before the tower.
Looking at these photographs of the N80 in Tyrone, the tower has failed, as is typical for tower failures, close to the base. There is no sure evidence of a tower blade strike in the photographs but at least two blades appear destroyed. The ground close to the base is covered with what appears to be white debris, possibly from a blade strike while the tower was still vertical.
From eyewitness reports, the machine had been "groaning and whining" and making a noise like a "low flying helicopter" together with "banging" for some time before failure. According to the BBC, one witness reported the rotor spinning out of control. A wind turbine in overspeed sounds quite like a helicopter.
It's interesting to note that one blade is relatively intact aside from a broken tip. This blade hit something, possibly the tower. In wind speeds of 25 m/s, a wind turbine of this size in an uncontolled run mode will complete a revolution in a little more than a second. A damaged wind turbine of this size could collapse in under five seconds. It's possible to imagine a machine of this size falling but running, completing at least one full revolution before the blades make contact with the ground.
How does a wind turbine lose control? The 2008 Nordtank overspeed was a result of an extraordinary series of events during maintenance, including a catastrophic gearbox failure. The 2013 V52 failure involved loss of blade pitch control and, from the report, it seems that the pitch system failed unsafely, i.e. fixing the machine in run mode.
Most deployed large wind turbines use pitch control for primary power control. Aside from the pitch system, most deployed machines also use the generator and a disc parking brake for speed control. Since pitch systems are designed to be fail-safe, which has a specific technical meaning that if the pitch system fails, it will fail in a way that leaves the machine under control, and since the cost and size of disc brakes can be large, a correctly sized parking brake may be unable to fully stop the machine in high winds unaided. Additionally, the disc brake is usually positioned behind the gearbox relative to the rotor, i.e. braking is effected through the gearbox. This is done to reduce the brake size but there is a downside: the gearbox becomes part of the braking system.
If a wind turbine's pitch system failed unsafely and if the gearbox and/or brake failed under heavy loading, the rotor would free-wheel, i.e. operate at the extreme end of the over-speed envelope. With the gearbox intact and brake active, it's possible that the rotor would still accelerate into the overspeed range. In high winds, a generator's capability to stop a machine is limited and on many machines, generators are frequently disconnected during error states.
The failure may then proceed in the same way as the 2008 Nordtank and 2013 V52 failures.
In such a situation, the work then becomes to determine why the primary and secondary control systems failed.
The investigation into the specific event in Tyrone is in progress and it's possible that the failure resulted from something else entirely. Nevertheless, it highlights what happens when things go wrong and underlines the importance of robust control strategies and maintenance procedures.
Finally, although dramatic, this kind of failure is very rare. As of 2012, there were more than 225,000 wind turbines operating continuously around the world. The list of accidents is thankfully short and is the result of the high standard of engineering applied by manufacturers such as Nordex.