Wind turbine tower cleaning: where the streaks come from and what can be done about them

Of all the cleaning orders a wind contractor receives, the tower is the most frequent. Not because it is the dirtiest part, but because it is the one visible from the access track, from the road and from the photo attached to the audit. And what is visible is almost always dark streaks running down from the nacelle.

Where the streaks come from

A nacelle has oil and grease in many places: gearbox, pitch and brake hydraulics, bearings, seals. Over the years some of that oil ends up outside, through a weeping seal, a breather or a small leak. The wind carries it and gravity takes it down the shaft, where dust sticks to the grease and the sun bakes it. The result is the black streak everybody recognises.

That means two things. First: a stained tower is a clue, not just a cosmetic problem. It says something up there is losing oil. Second: cleaning the tower without looking at the nacelle is cleaning the symptom.

Why towers get cleaned

• To see. With the tower clean you know whether the leak is still active: if a streak reappears within weeks, somebody has to go up and repair. It is the most useful reason and the least talked about.
• Before a repair or a repaint. No surface treatment goes on over grease.
• Before an audit, an end-of-warranty inspection or the sale of the site. The condition of the machines gets photographed, and the tower is the first thing in the frame.
• For image. Sites visible from towns or roads, or with visitors.

How it is cleaned

With rope access, a technician abseils from the nacelle and works down the shaft. With a drone, the crew sets up at the foot of the machine and the drone works down the tower with pressurised water and detergent, fed by hose from the ground. The tower alone is the quickest of the four scopes, which is why it is sometimes ordered on its own; the usual approach, though, is to do it within a full turbine clean, because mobilisation is already paid for.

A tower clean needs the same as any other: machine stopped and in maintenance position, a window without strong wind or rain and, with a drone, an aviation permit applied for about 3 weeks in advance.

What remains afterwards

Here it pays to be exact. What does not come off after cleaning is not rust or corrosion: it is bonded grime, old oil baked by the sun and fixed to the coating. It is greatly reduced, and the before-and-after photos of each machine show it, but removing it completely without treating the surface is not possible. A contractor who promises to leave the tower as on day one is selling a retouched photo. We prefer to show the real one: our case study page has the tower and nacelle photo pair from a real job.

What to ask for

A price per machine and scope, photographic evidence of every tower before and after, and a note in the report on where the oil appears to come from. That note is worth more than the cleaning: it is what turns a cosmetic job into maintenance.