Fatigue loads are important for the overall cost of energy from a
wind turbine. Loading on the tower is one of the more important
loads, as the tower is an expensive component. Consequently, it
is important to detect tower loads, which are larger than
necessary.

This paper deals with both fore-aft and sideways tower
oscillations. Methods for estimation of the amplitude and
detection of the cause for vibrations are developed. Good
results are demonstrated for real data from modern multi mega
watt turbines. It is shown that large oscillations can be
detected and that the method can discriminate between wind
turbulence and unbalanced rotor.