Final report for WP4.3: Enhancement of design methods ... - Upwind

Steel substructure for offshore wind turbines – wind load
In this section reliability analysis and Fatigue Design Factors are performed for fatigue sensitive details in
a steel substructure for a single wind turbine and for a wind turbine in a wind farm.
The mean wind speed is assumed to be Weibull distributed with scale parameter = 10.0 m/s and shape
coefficient = 2.3. It is assumed that the reference turbulence intensity is Iref =0.14.
number
300
250
200
150
100
50
0
0 2000 4000 6000 8000 10000
44
M_x (kNm)
Figure 5.1: Number of load cycles in a 10 minutes period for mudline bending moment. Mean wind speed equal to 14
m/s.
Figure 5.1 shows a typical distribution of stress ranges for a pitch controlled wind turbine for mudline
bending moments, see [34]. Generally, the stress ranges can be modelled by a Weibull distribution. The
Weibull shape coefficient k is typically in the range 0.8 – 1.0. These results are for cases where the response
is dominated by the “background” turbulence in the wind load. The corresponding number of load
cycles per year is typically ν = 5.10 7 .
sig_M / sig_u
2400
2000
1600
1200
800
400
0
0 5 10 15 20 25
U (m/s)
Figure 5.2: σ∆σ(U)/σu(U) for mudline bending moment – pitch controlled wind turbine.
In Figure 5.2 is shown a typical example for a pitch controlled wind turbine of α∆σ(U)/z = σ∆σ(U)/σu(U), i.e.
the ratio between the standard deviations of stress ranges and turbulence at a given mean wind speed U.
The ratio is seen to be non-linear due to the effect of the control system.
Single wind turbine
The stochastic model shown in Table 5.2 is used with XWind LogNormal distributed with expected value = 1
and coefficient of variation, COVWind = 0.10. If a linear SN-curve with m = 3 is used then Table 5.11 shows
the required FDF values for ∆PF,max = 10 -4 , 2 10 -4 , 10 -3 and for P(COL|FAT) = 1.0, 0.5, 0.1 and 0.01. In
brackets is shown the corresponding values of the product of the load and material partial safety factors γf
γm. For a minimum annual reliability index equal to 3.5, FDF = 2.3 (and partial safety factor 1.31) is obtained
if the consequence of the fatigue failure is large.