Wind Energy

Base moment (10 6 Nm)
0.4
0.2
−0.2
2 Wave Loads on Wind-Power Plants in Deep and Shallow Water 11
1
0.8
0.6
0
−0.4
1900
Moment, comparison between linear and non-linear realisation
1910 1920 1930 1940 1950 1960
Time (s)
1970 1980
Linear
Non-linear
1990 2000
Fig. 2.4. Comparison of linear and non-linear overturning moment
Table 2.1. Accumulated fatigue damage for the whole lifetime of a structure using
a linear S–N curve with m =3 1 NL/L is the ratio between non-linear and linear
damage
1 2 3 4 5 6 Mean
Linear 0.55 0.50 0.57 0.49 0.56 0.55 0.54
Non-linear 0.71 0.63 0.63 0.58 0.72 0.67 0.66
No wave 0.08 0.09 0.07 0.07 0.09 0.09 0.08
NL/L 1.29 1.25 1.11 1.19 1.28 1.22 1.22
the difference is large. In Table 2.1 simulated fatigue damages – stress due to
overturning moment – to a wind turbine in 20 m water depth are listed for
linear waves, non-linear waves and wind only [7]. The six sets of simulations
give as a mean 20% larger damage for non-linear waves.
1 The stress amplitude, S, as a function of No of load cycles to failure, N:
log(N) = log(K) – m log(S), K and m are empirical material parameters.