Wind Energy

54
Benefits of Fatigue Assessment with Local
Concepts
P. Schaumann and F. Wilke
54.1 Introduction
Support structures of offshore wind energy converters for medium to high
water depths are supposedly carried out as braced or lattice constructions.
Due to the combined dynamic loads from wind and waves the fatigue design
is of special interest. The large variety in type and dimensions of tubular joints
requires a fatigue design with local approaches.
54.2 Applied Local Concepts
The authors gave an overview of expected types of support structures in [1].
State-of-the-art in current offshore standards is the structural stress approach.
More sophisticated concepts like the notch stress approach and notch strain
approach [2], here referred to as “local approaches”, require detailed FE modelling
of the mesoscale weld shape.
The applied notch stress approach is based on the recommendations in [3],
but with a modified slope of the S–N curve with m = 5 for ND > 5 ×
10 6 and without a cut-off limit. This modification fits well to the results
of [4] in the high-cycle fatigue range. A fictitious weld toe fillet with a radius
ρf=1 mm is applied to the FE model to account for microstructural support.
The notch strain approach uses stabilized cyclic material data according to
the uniform material law (UML). For the material in the heat-affected zone
(HAZ) an increased hardness is considered. To include the mean stress effects,
the damage parameter given by
PSWT = � (σa + σm)ɛaE (54.1)
has been used. Together with the equations by Manson, Coffin and Morrow
this leads to a relevant P –N curve. As the shear strains are low for the