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

Table 5.3 shows annual reliability indices for different FDF values. FDF = 10 is used for fatigue critical details
which cannot be inspected. The annual reliability index is seen to be 4.3 corresponding to an annual
probability of failure equal to 10 -5 . This reliability level corresponds to that generally required for fatigue
critical details with large consequences of failure.
Table 5.3: Annual reliability index for different FDF values.
FDF 1 2 3 5 10
β
∆ 2.39 2.84 3.16 3.63 4.32
Steel substructure for offshore wind turbines – wave load
In Table 5.4 is shown examples of how to model the uncertainty related to estimation of stress concentration
factors, XSCF (partly based on [38]). Five values of COVSCF are used to model different levels of analysis
and complexity.
Table 5.4: Examples of COVSCF.
COVSCF Fatigue critical detail
0.00 Statically determinate systems with simple fatigue critical details (e.g. girth welds)
where FEM analyses are performed
0.05 Statically determinate systems with complex fatigue critical details (e.g. multiplanar
joints) where FEM analyses are performed
0.10 Statically in-determinate systems with complex fatigue critical details (e.g. doubler
plates) where FEM analyses are performed
0.15 2 dimensional tubular joints using SCF parametric equations
0.20 Tubular joints in structures where tubular stiffness is modeled by Local Joint
Flexibility (LJF) models and SCF parametric equations are used
Basically it is assumed that COVSCF = 0.10 and COVWave = 0.10. If the response is quasi-static then the
number of stress cycles is typically ν = 5.10 6 . It is assumed that the design lifetime is TL = 20 year.
For offshore wind turbines where the consequences of failure is typically less serious compared to offshore
oil & gas structures the maximum acceptable annual probability of failure is in the range ∆PF,max =
10 -4 - 10 -3 , corresponding to the reliability level for an unmanned fixed offshore structure, see above.
If a linear SN-curve with m = 3 is used then Table 5.5 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. The importance of using m = 5 is shown
below.
Table 5.5: Required FDF and corresponding partial safety factors γf γm in ( ) for given ∆βmin,FAT (∆PF,max,FAT). (–)
indicates that FDF ≤ 1.
P(COL|FAT) 3,1 (10 -3 ) 3,5 (2 10 -4 ) 3,8 (10 -4 )
1.0 2.40 (1.34) 3.38 (1.50) 4.32 (1.63)
0.5 1.98 (1.26) 2.88 (1.42) 3.73 (1.55)
0.1 1.11 (1.03) 1.87 (1.23) 2.60 (1.37)
0.01 (-) (-) 1.26 (1.08)
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