I was a little bit wrong about the wind turbine explosion post from yesterday. I wrote:

1. The second blade crashes into the support tower, breaking both the tower and the blade.

The bit about “breaking the tower” isn’t quite right. Wind turbine towers are generally built from a tapered sections that are stacked together and that slip over one another. It’s a lot like stacking traffic cones or paper cups. This kind of construction is called a “monopole tower,” and it’s the same way that really tall street lights and signposts (like on the highway) are built.

Each section of a monopole tower is hollow and made of relatively thin sheet steel. The base is anchored to a solid platform (like a concrete bed), and then the upper pieces are lowered down onto it with a crane. Some combination of gravity and friction (from the sections compressing under their own weight) holds the thing together. There are rarely, if ever, welds, rivets, or bolts holding the sections in place. The overlap between the sections for a tower the size of the one in the video is usually on the order of 2m (or a bit less).

What (I think) actually happened to the tower in Denmark is that, when it got hit with the wind turbine blade, one of the tower sections crumpled and buckled near the overlap. That crumpling was enough to “squirt” the top section off of the one below it and ultimately cause it to fall over. If you look at the second video in the previous post, you’ll notice that there are no ragged edges or other signs of catastrophic failure on the part of the tower that’s still standing. The actual edge of the section that fell is pretty clean, too, but the area near the “break” is pretty severely caved in. Update: There’s now a much clearer view of the “break” courtesy of epn.dk.

Anyhow, I realize that what I’m talking about still probably fits most people’s definition of “breaking the tower,” but I wanted to clear up that a blunt fiberglass blade — even a really big one moving pretty fast — is unlikely to actually break through the steel wall of a tower.

Also, somebody in Reddit land was kind enough to post a translation of the Danish news coverage. My favorite line: “Vestas had technicians up in the mill this morning, when the brakes of the mill failed, and [they] got busy getting out,” writes jp.dk.