Loadbearing structures and detailed design - Concrete Structures for Wind Turbines - page 158

Fig. 4.26 Classi cation of internal forces [70]

A total dynamic design of the support structure with the time series determined is only
carried out in exceptional circumstances because of the huge effort required. Normally,
a simplified calculation is performed, which involves evaluating the simulated load
cycles at the individual design cross-sections using a suitable counting method (e.g.
rainflow counting) and classifying them according to mean values and stress ranges.
The collective loads obtained in this way are combined together with the associated
number of load cycles, for example in Markov matrices, and extrapolated to the design
working life of the wind turbine (see Figure 4.26). The chronological relationship
between the different action effects is lost while doing this, which means that there may
be an unfavourable superposition of individual internal forces that do not occur
simultaneously within one collective load [67].
In a total dynamic analysis, which should be carried out on the combined structure
consisting of foundation, tower, wind turbine and three-dimensional wind field, the
design internal forces may be determined with a partial safety factor g F ¼ 1.00. If on

the other hand in a simplified calculation, collective loads from the aeroelastic simulation
are applied to the turbine/tower interface, then the collective loads must be multiplied by
the partial safety factors depending on the design load case groups of Tables 4.3 or 4.7.
Strength and stability analyses with the most unfavourable of all design load cases must
be carried out for groups N, (E,) A and Tat the ultimate limit state. On the other hand, the
analyses for fatigue failure are carried out with the design load cases for group F.
Analysing the limits to stresses and crack widths and decompression (see above) for the
serviceability limit state calls for the use of the design load cases to Tables 4.1 or 4.4. These
cases correspond to DIN 1045-1 [33] depending on the analyses of the rare, frequent or
quasi-permanent design load cases that have to be performed. For details see [9].

4.9.1.2 Actions due to waves and sea state
The design of support structures for offshore wind turbines must include, in particular, the
dynamic excitation due to wave loads (see Sections 2.4 and 2.5). Both the extreme wave
loads and the fatigue loads due to the sea state are critical for the design. Offshore structures
can be subjected to more than 10 8 wave load cycles over a design working life of 20 years.