Environmental Engineering Reference
In-Depth Information
12.4.3.3 Seismically Excited Onshore Reference Wind Turbine
with Tuned Mass Damper
In the following paragraph, an example of seismic analyses of wind turbines
is presented. These were carried out with the aid of the software FAST-Seismic
[
26
,
27
], which offers the possibility to consider the interaction between earth-
quake loadings and aerodynamics of wind turbines.
The FAST-Seismic is a modified version of FAST tool. In FAST-Seismic, it is
possible to apply base shaking to the turbine model, which still includes all the
feature of the normal FAST program. In FAST-Seismic, a damped oscillator of
stiffness k
act
is connected to the base of the turbine model. This damped actuator is
used to translate specified motions into required seismic forces. The actuator
stiffness k
act
is calculated as
k
act
¼
m
tf
ð
2pX
act
Þ
ð
12
:
19
Þ
where m
tf
is the mass of turbine and foundation, and X
act
is the actuator frequency.
At each time step, the damped oscillator is subjected to seismic forces, calculated
as
f
g
ð
t
Þ¼
k
act
ð
x
g
;
seis
x
g
;
0
Þþ
c
act
ð
x
g
;
seis
x
g
;
0
Þ
ð
12
:
20
Þ
where c
act
is the actuator damping coefficient, x
g,0
and x
g
;
0
are the realized foun-
dation motions, x
g,seis
and x
g
;
seis
are the desired seismic foundation motions.
Tuning frequency and damping, the realized motions get closer to the desired
seismic motions.
FAST-Seismic provides routines to modify the user-defined or synthetically
generated input motions. The user can also manipulate the motions through baseline
corrections and target response spectrum adjustments. For the calculations, five
recorded accelerograms are applied in the fore-aft and side-to-side tower directions
equally. The following historic earthquakes are used (Fig.
12.19
):
El Centro Earthquake: 1979, recorded in Bonds Corner
Takochi Oki Earthquake: 1968, recorded in Hachinohe
Kobe Earthquake: 1995, recorded at KJMA
Kocaeli Earthquake: 1999, recorded in Düzce
Northridge Earthquake: 1994, recorded in Tarzana.
The time step is set to 0.005 s. The acquired results are presented for each
earthquake in Figs.
12.20
,
12.21
,
12.22
,
12.23
, and
12.24
. Similar to the previous
calculations, from the 1,000 s total simulation time, a 600 s part of it is analyzed.
Earthquakes started at 100 s. Depending on the excited frequencies, most of the
earthquakes cause a significant tower vibration, which can be clearly separated
from the wind-induced vibrations.
As shown in Figs.
12.25
,
12.26
, and
12.27
, TMD causes an important
improvement of the tower dynamics. Compared to previous results, the vibration
