100m Turbine collapse: Not bolts this time – weld failure in the main tower

Tallbloke’s Talkshop

De Bilt reports on yet another serious turbine failure. This time the 100m structure folded at a weld line some 30m off the ground. This is of great concern. Whereas most catastrophic failures are due to installation errors at the base bolts, this is a problem with the specification or execution of the construction of the monocoque structure – the main tower.

remainder of turbine tower

There’s a fundamental problem here. When steel is welded, the structure of the material either side of the joint, and the weld itself is different to the rolled steel sheet. This inevitably creates a location where stress gets concentrated more as the structure ages.

This is because the material at the joint is more prone to ‘work hardening’, a process which occurs, for example, when stresses are repeatedly applied and relaxed by the resonant motion of the structure in the wind. Wind turbines are particularly prone to this because of the heavy weight on the top of the structure and the wind-exposed location they are placed in.

Work hardened steel becomes brittle, and brittle materials crack. Cracks are huge stress raisers and can propagate faster than the speed of sound during catastrophic failure. This is the cause sharp, loud whip-crack like noise you sometimes hear when something breaks – a miniature sonic boom. You can see from the photo above that the crack has propagated right around the main tower and caused the catastrophic failure. You can see the aftermath in the photo below. Good job no-one was driving a tractor or walking a dog under this when it came down.

collapsed turbine on the ground

We can expect catastrophic failures such as this one to occur more frequently as the wind turbine fleet ages. So what programme of inspections is in place to regularly inspect wind turbine towers for cracks resulting from work hardening of the weld joints? I don’t know, because the industry is secretive about problems, but when did you last see inspectors with magnifying glasses spraying crack revealing dye penetrant on weld joints 20-100m off the ground up the side of a wind turbine?

Time for the government’s ‘Elf and Safety’ executive to step up and give a report on the state of wind industry oversight.