Morten Handberg, Uptime's blade whisperer, returns to the show to tackle leading edge erosion. He covers the fatigue physics behind rain erosion, why OEMs offer no warranty coverage for it, how operators should time repairs before costs multiply, and what LEP solutions are working in the field.
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Allen Hall: Morten, welcome back to the program.
Morten Handberg: Thanks, Allen. It's fantastic to be back on on, on the podcast. Really excited to, uh, record an episode on Erosion Today.
Allen Hall: Wow. Leading as erosion is such a huge worldwide issue and. Operators are having big problems with it right now. It does seem like there's not a lot of information readily available to operators to understand the issue quite yet.
Morten Handberg: Well, it, I mean, it's something that we've been looking at for the, at least the past 10 years. We started looking at it when I was in in DONG or as it back in 2014. But we also saw it very early on because we were in offshore environment, much harsher. Uh, rain erosion conditions, and you were also starting to change the way that the, the, uh, the coatings [00:01:00]that were applied.
So there was sort of a, there was several things at play that meant that we saw very early on, early on offshore.
Allen Hall: Well, let's get to the basics of rain erosion and leading edge erosion. What is the physics behind it? What, what happens to the leading edges of these blades as rain? Impacts them.
Morten Handberg: Well, you should see it as um, millions of, of small fat, uh, small fatigue loads on the coating because each raindrop, it creates a small impact load on the blade.
It creates a rail wave that sort of creates a. Uh, share, share loads out on, uh, into the coating that is then absorbed by the coating, by the filler and and so on. And the more absorbent that your substrate is, the longer survivability you, you're leading into coating will have, uh, if you have manufacturing defects in the coating, that will accelerate the erosion.
But it is a fatigue effect that is then accelerated or decelerate depending on, uh, local blade conditions.
Allen Hall: Yeah, what I've seen in the [00:02:00] field is the blades look great. Nothing. Nothing. You don't see anything happening and then all of a sudden it's like instantaneous, like a fatigue failure.
Morten Handberg: I mean, a lot of things is going on.
Uh, actually you start out by, uh, by having it's, they call, it's called mass loss and it's actually where the erosion is starting to change the material characteristics of the coating. And that is just the first step. So you don't see that. You can measure it in a, um, in the laboratory setting, you can actually see that there is a changing in, in the coating condition.
You just can't see it yet. Then you start to get pitting, and that is these very, very, very small, almost microscopic chippings of the coating. They will then accelerate and then you start to actually see the first sign, which is like a slight, a braided surface. It's like someone took a, a fine grain sandpaper across the surface of the plate, but you only see it on the leading edge.
If it's erosion, it's only on the center of the leading edge. That's very important. If you see it on the sides and further down, then it's, it's [00:03:00] something else. Uh, it's not pure erosion, but then you see this fine grain. Then as that progresses, you see more and more and more chipping, more and more degradation across the, the leading edge of the blade.
Worse in the tip of it, less so into the inner third of the blade, but it is a gradual process that you see over the leading edge. Finally, you'll then start to see the, uh, the coating coming off and you'll start to see exposed laminate. Um, and from there it can, it can accelerate or exposed filler or laminate.
From there, it can accelerate because. Neither of those are actually designed to handle any kind of erosion.
Allen Hall: What are the critical variables in relation to leading edge erosion? Which variables seem to matter most? Is it raindrop size? Is it tip speed? What factors should we be looking for?
Morten Handberg: Tip speeds and rain intensity.
Uh, obviously droplet size have an impact, but. But what is an operator you can actually see and monitor for is, well, you know, your tip speed of the blade that matters. Uh, but it is really the rain intensity. So if you have [00:04:00] sort of a, an average drizzle over the year, that's a much better condition than if you have like, you know, showers in, in, in, in a, in a few hour sessions at certain points of time.
Because then, then it becomes an aggressive erosion. It's not, it's, you don't, you get much higher up on the. On the, on the fatigue curve, uh, then if it's just an average baseline load over long periods of time,
Allen Hall: yeah, that fatigue curve really does matter. And today we're looking at what generally is called VN curves, velocity versus number of impacts, and.
The rain erosion facilities I've seen, I've been able to, to give some parameters to, uh, provide a baseline or a comparison between different kinds of coatings. Is is that the, the standard as everybody sees it today, the sort of the VN curve
Morten Handberg: that is what's been developed by this scientific, uh, community, these VN curve, that that gives you some level of measure.
I would still say, you know, from what we can do in a rain erosion tester to what is then actually going on [00:05:00] the field is still very two very, very, very different things you can say. If you can survive a thousand hours in a rain erosion tester, then it's the similar in the field that doesn't really work like that.
But there are comparisons so you can do, you know, uh, a relationship study, uh, between them. And you can use the VN curves to determine the ERO erosion aggressiveness. Field. We did that in the bait defect forecasting that we did in wind pile up with DCU back in 2019, uh, where we actually looked at rain erosion across Europe.
Uh, and then the, uh, the actual erosion propagation that we saw within these different sites, both for offshore and for onshore, where we actually mapped out, um, across Europe, you know, which areas will be the most erosion prone. And then utilize that to, to then mo then, then to determine what would be the red, the best maintenance strategy and also, uh, erosion, uh, LEP, uh, solution for that wind farm.
Allen Hall: Oh, okay. Uh, is it raindrop size then, or just [00:06:00] quantity of raindrops? Obviously drizzle has smaller impact. There's less mass there, but larger raindrops, more frequent rain.
Morten Handberg: If you have showers, it tends to be larger drops. Right. So, so they kind of follow each other. And if it's more of a drizzle. It will be smaller raindrops.
They typically follow each other. You know, if you've been outside in a rainstorm before we just showered, you would have sense that these are, these are much higher, you know, raindrop sizes. So, so there is typically an a relation between raindrop size and then showers versus a drizzle. It's typically more fine, fine grain rain drops.
Allen Hall: And what impact does dirt and debris mixed in with the rain, uh, affect leading edge erosion? I know a lot of, there's a lot of concern. And farm fields and places where there's a lot of plowing and turnover of the dirt that it, it, it does seem like there's more leading edge erosion and I, I think there's a little bit of an unknown about it, uh, just because they see leading edge [00:07:00]erosion close to these areas where there's a lot of tilling going on.
Is it just dirt impact worth a blade or is it a combination of dirt plus rain and, and those two come combining together to make a worse case. Uh, damage scenario.
Morten Handberg: Technically it would be slightly worse than if it were, if there is some soil or, or sand, or sand contamination in the raindrops. But I mean, logically rain typically, you know, comes down from the sky.
It doesn't, you know, it doesn't mix in with the dirt then, you know, it would be more if you have dirt on the blades. It's typically during a dry season where it would get mixed up and then blown onto the blades. Honestly, I don't think that that is really what's having an impact, because having contamination in the blade is not something that is, that would drive erosion.
I think that that is, I think that is, that is a misunderstanding. We do see sand, sand erosion in some part of the world where you have massive, uh, sand, uh, how do you say, sandstorms [00:08:00] coming through and, and that actually creates an, an abrasive wear on the plate. It looks different from rain erosion because it's two different mechanisms.
Uh, where the sand is actually like a sandpaper just blowing across the surface, so you can see that. Whereas rain is more of this fatigue effect. So I think in the, theoretically if you had soil mixed in with rain, yes that could have an impact because you would have an a, a hardened particle. But I do, I don't think it's what's driving erosion, to be honest.
Allen Hall: Okay, so then there's really two different kinds of failure modes. A particle erosion, which is more of an abrasive erosion, which I would assume be a maybe a little wider, spread along the leading edge of the blade versus a fatigue i
