Wind Energy

294 P. Schaumann and F. Wilke
analysed joints and loads, only strains and stresses normal to the weld toe
are taken into account (critical plane approach).
As a benefit of both local concepts fatigue design can be carried out uncoupled
from the standard S–N curves for given structural details and the
application to any local weld geometry is possible. At the same time the
number of influencing parameters increases which is shown in the following
chapter.
54.3 Comparison of Fatigue Design for a Tripod
The bottom part of the reference structure and the most important data are
shown in Fig. 54.1. For the scatter diagram of the chosen Baltic Sea location
see [1]. The relatively small turbine leads to dominating fatigue damage from
wave loading.
Figure 54.2 shows the resulting member S–N curve for a fictitious pulsating
nominal stress in one brace of the top tripod node. According to [5] three
different engineering approaches for the notch strain concept have been applied
(1. Heat-affected zone material without residual stresses and R = −1; 2.
heat-affected zone material with R = 0; 3. base material (BM) with residual
stresses in the order of the yield strength and R = −1). The notch stress curve
has been shifted to a probability of failure Pf= 50% (γ=1.38) to provide a
basis for comparison with the results obtained through the use of the UML.
In high cycle regions beyond N =2×10 6 , interesting for offshore structures,
the compliance is acceptable. This leads to the conclusion that the notch
stress approach seems to provide cycles up to crack initiation for high plate
thicknesses and notch dominated problems. Figure 54.2 also shows an obvious
handicap of the notch strain approach. Besides the usual scatter in the local
weld geometry it is hard to determine the parameters which govern the design
without experiments. Because of the higher robustness regarding the influencing
parameters the notch stress approach (NSA) is used for further parametric
studies.
Figure 54.3 (left) shows the comparison of cycles to failure for a sinusoidal
pressure of 5 N mm −2 on one brace of the top tripod node for a large variety
Water depth: 25 m
Hub height: 80 m
Turbine mass: 100 t
Main tube diam.: 4000 mm
Diam. of diagonals: 2000 mm
Angle of diagonals: 458
max t: 80 mm
Material: S 355
Fig. 54.1. Bottom part of the reference tripod