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

Figure 5.4: Annual reliability indices obtained by the SN-approach and a fracture mechanics model (calibrated).
Further, it is assumed that inspections are performed with three different levels of reliability modelled by
exponential POD curves with λ = 2, 5 and 10 mm (surface cracks lengths). The inspections are assumed
to be performed with equidistant times, see Figure 5.3.
Table 5.21 to Table 5.25 show required FDF values for cases with wind / wave loads only, single wind turbine
/ wind farm and linear / bi-linear SN-curves. It is assumed that ∆βmin,FAT = 3.5 and PCOL|FAT =1. It is
seen that significant reductions in required FDF values can be obtained if a good inspection quality is used
and e.g. 3 inspections are performed during the design lifetime.
The costs of inspections and possible repairs should be compared to the cost reductions in the initial material
costs due to lower FDF values. It is noted that due to correlations between similar fatigue critical details
in substructures in wind farms information from inspection of one substructure can be used to update
the reliability assessment of nearby substructures.
The results in this section can be considered as representative, but more examples should be considered
before implementation in standards. Especially examples where both wind and wave loads are important
should be investigated.
Table 5.21: Required FDF and corresponding partial safety factors γf γm in ( ). Single wind turbine and linear SNcurve
(m = 3). Wind load only.
POD: λ λ 2 mm 5 mm 10 mm
No inspections
1 inspection 1.92
(1.24)
2 inspections 1.65
(1.18)
3 inspections 1.46
(1.13)
50
2.15
(1.29)
1.94
(1.25)
1.82
(1.22)
2.20
(1.30)
2.11
(1.28)
2.05
(1.27)
2.27
(1.31)
Table 5.22: Required FDF and corresponding partial safety factors γf γm in ( ). Wind farm and linear SN-curve (m =
3). Wind load only.
POD: λ λ 2 mm 5 mm 10 mm
No inspections
1 inspection 2.15 2.40 2.49
(1.29) (1.34) (1.36)
2 inspections 1.83 2.17 2.35
2.54
(1.37)