'Lumping' of Fatigue Load Cases

Garrad Hassan & Partners Ltd.
Mungo Morris
Consequently the equation for the DEL mean wave height is:
Hs =
⎛
⎜
⎝
1
N
N
∑ i = 1
In contrast, for the wave period, the fatigue damage is approximately proportional to the
number of cycles, and the appropriate mean is therefore the arithmetic mean of the wave
periods within each bin * . In addition, the mean wind speed within each bin was not calculated
– all values that fell between 5 and 7m/s, for example, were considered to have a mean wind
speed of 6m/s. This is consistent with the normal approach for simulating wind speed
distributions for onshore turbines.
Damage equivalent loads were calculated at key points in the turbine structure for inverse SN
slope values of 4, 6, 8 and 10 and assuming a frequency of 1Hz.
Having calculated the initial fatigue loads using the probability distribution of the 204
simulations, the same process was then repeated for increasingly larger bin sizes of wave
height and period. In all cases, the wind speed bin size was kept at 2m/s to reflect the current
practice for onshore wind distribution simulation. The full range of bin sizes covered are
shown in Figure.1.
Hs
m
i
1
m
⎞
⎟
⎠
3
2.5
Wave period bin size [s]
2
1.5
1
0.5
0
0 0.5 1 1.5 2 2.5 3
Significant wave height bin size [m]
Figure 1. Bin size combinations
When comparing the fatigue loads generated by these ‘coarser’ bin sizes with the original
base-line loads, it became apparent that the reduction in resolution in bin sizes had very little
effect on the final fatigue loads generated.
* In Hindsight it would have been more correct to have averaged (1/Period), but this would not change
the conclusions significantly.