Environmental Engineering Reference
In-Depth Information
continuous varying loads over time. For a fatigue check it is therefore vital to
know the following details:
•
environmental loads over the lifetime of the structure
•
steel properties at the most severely loaded sections (typically: welds)
•
fatigue resistance of the details of these welds: empirical S-N curves.
The long-term environmental loads are usually gathered in tables listing the
simultaneous occurrence of wave height, period and direction, wind speed and
direction and potentially several other parameters. For the fatigue check of the
support structure the amount of data for a 20-year lifetime can accumulate to over
1000 load cases. To reduce these for the initial design stages, scatter diagrams are
used. Table 4 shows a typical scatter diagram for wave height and period.
Such a wave scatter diagram is available for every wind speed bin (0-2 m/s
wind, 2-4 m/s, 4-6). We then have the simultaneous occurrence of wave height,
period and wind speed in a 3D scatter diagram.
To calculate the fatigue of the structure, we ideally need to run all these load
cases through a wind turbine simulation program such as Bladed or Flex to
incorporate all wind, wave and structure interactions an fi nd the stress varia-
tions in each critical point of the structure. In the preliminary design stages, a
reduced amount of data can be used that represents the most commonly occur-
ring wind and wave conditions. Typically, the amount of data is reduced to 15
or 20 of these environmental states. An example for the Blyth turbines is shown
in Table 5 .
Table 4: Wave scatter diagram for
H
s
and
T
z
with occurrence in parts per thousand
for the OWEZ location.
Sum
H
s
(m)
T
z
(s)
0-1
1-2
2-3
3-4
4-5
5-6
6-7
7-8
6.5-7.0
0.0
6.0-6.5
0.1
0.1
5.5-6.0
0.1
0.1
0.2
5.0-5.5
0.1
0.1
0.2
4.5-5.0
1
1.0
4.0-4.5
4
4.0
3.5-4.0
4
5
9.0
3.0-3.5
19
0.1
19.1
2.5-3.0
0.1
38
38.1
2.0-2.5
27
43
70.0
1.5-2.0
0.1
115
5
120.1
1.0-1.5
6
220
1
227.0
0.5-1.0
236
145
1
382.0
0.0-0.5
1
1
113
14
0.1
129.2
Sum
1.0
0.0
1.0
355.1
521.1
111.1
10.4
0.3
1000
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