Type of towers – stiff, soft or soft soft?

In the last month I spent a lot of time discussing about “soft soft” towers.

But what does it exactly means?

Steel tower for wind turbine are classified as stiff, soft, or soft soft based on the relative natural  frequencies of tower, rotor and blades.

You obviously want to avoid that your tower is excited by dynamic loads and start resonant oscillations.

The primary sources of dynamic loads on the tower are the rotational speed of the rotor (usually indicated with P) and the blade passing in front of the tower. The blade passing speed will obviously be 3P. I think that it’s worth mentioning that rotational frequency loads will arise only when the blades are unbalanced.

We call “stiff” (or “stiff stiff”) a tower whose fundamental  natural frequency is higher than that of the blade passing frequency. This is a very good thing (the tower is unaffected by  the rotor) but a bigger mass is needed – therefore the cost can be very high. Additionally, a stiff tower tends to radiate less sound.

“soft” is a tower whose fundamental frequency is lower than the blade passing frequency, but above rotor frequency.

“soft soft” is a tower whose natural frequency is below BOTH rotor frequency and blade passing frequency.

“stiff stiff” design is not usual.

Currently, towers in  the market are either “soft stiff” or “soft soft”.

Soft towers are usually lighter (= cheaper) but require more dynamic analysis.