Environmental Engineering Reference
In-Depth Information
Table 5: Summary of 15 environmental states for Blyth.
H
s
(m)
T
z
(s)
V
w
(m/s)
% of occurrence
1
0.25
2.0
5.0
20.47
2
0.25
5.2
4.9
3.73
3
0.25
4.0
11.8
21.76
4
0.25
5.6
15.7
3.85
5
0.25
5.8
20.6
1.00
6
0.75
3.4
6.7
8.62
7
0.75
5.3
5.8
13.25
8
0.75
5.5
11.7
5.58
9
1.25
5.2
8.8
10.66
10
1.25
8.0
8.5
1.25
11
1.75
6.0
9.9
4.83
12
1.75
6.7
16.2
0.55
13
2.4
6.8
12.8
3.54
14
3.4
7.8
14.5
0.77
15
3.3
9.7
18.7
0.14
100%
Stress range
histogram
Stress history
S-N curve
Miner sum
n
S
=
∑
i
D
n
i
fat
N
N
i
i
S
i
Filter stress
ranges
Figure 34: Flowchart of fatigue calculation due to variable stress ranges using
S
-
N
curve and Miner sum.
When the stress signal is determined for each location that needs to be checked,
the fatigue calculation can be performed. Figure 34 shows the calculation steps: the
stress history is converted to stress ranges via the rainfl ow counting method.
The stress ranges are then checked against the
S
-
N
curve for the detail under
consideration and the fatigue damage due to the load case is calculated using the
Miner sum.
When the Miner sum is determined for each load case, it is multiplied by the
percentage of occurrence during the design life of 20 years. The total fatigue dam-
age is then found by adding the damage of all individual load cases together.
Should a detail not pass the fatigue check, changing the wall thickness will
reduce the amount of stress and a re-calculation of the fatigue can be performed.
To check the full fatigue of a monopile design requires several hours of computa-
tion time with current industry standard software. New methods of calculating the
fatigue in the frequency domain show promising results and they have found their
way to preliminary design calculations.
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