Upp í vindinn - 01.05.2017, Page 51

Háskóli íslands
Harnessing of wind energy using wind turbines is an essential part of de~
veloping a sustainable energy grid for the future. Some of the countries that are
leading in wind energy development, such as USA and China, are very seismical-
ly active and have known earthquake faults running along a large section of their
borders. With growing interest in utilizing this renewable energy, it is inevitable
that wind farms in some countries are installed close to earthquake sources. In
Iceiand, for example, the National Power Company, Landsvirkjun, is planning to
build a wind farm in the Búrfell area, which lies close to potential earthquake
faults in the South Iceland Seismic Zone. In the near-fault area, ground motion
is often affected by forward directivity effects. Such ground motions are known
to severely affect tall and flexible structures (see, for example, [7]). Wind turbine
towers are slender and tall structures and are more flexible than common build-
ings. It is therefore expected that near-fault ground motions would affect wind
turbine towers in a different way than they would affect the low-rise apartment
buildings that are traditionally built in that area. Existing guidelines for wind
turbine design [6,10] mostly rely upon building design codes (for example, [9])
which do not account for forward directivity effects. Response spectral shapes
specified in design codes are derived mostly from far-fault ground-motion data
and underestimate the effect of velocity pulses commonly observed in near-fault
ground motion. Ihe dominant period of near-fault velocity pulse is proportional
to earthquake size, with increasing pulse period, the ground motion becomes
more critical to structures with long fundamental period of vibration. The fun-
damental period of a typical wind turbine can be close to the pulse period of a 7
Mw earthquake, meaning that large seismic demands can be expected.
The model being used in this study is the one described by [2]. Ihe turbine is
a conventional three-bladed, upwind, variable-speed, with 5-MW rated power.
Ihe focus is on the tower structure; hence the modelling of the nacelle and rotor
is simplified as lumped masses. Ihe base of the tower is considered as fixed, as-
suming the structure is anchored to the engineering bedrock. Ihe tower itself is
a steel circular hollow-section with a diameter and thickness which decreases
linearly along the height. A finite element model of the tower is created by using
Figure 2 - Maximum horizontal nacelle
displacement due to each ground motion;
the results are divided into different
magnitude bins as indicated in the
legend. The horizontal axis represents
the predominant period of velocity
pulse [1] normalized by the fundamental
period of vibration of the stracture. b)
Average maximum nacelle displacement
in each bin computed from time histoiy
analysis and response spectral analysis.
Figure 2 - Maximum horizontal nacelle
displacement due to each ground motion;
the results are divided into different
magnitude bins as indicated in the
legend. The horizontal axis represents the
predominant period of velocity pulse [1]
normalized by the fundamental period
of vibration of the structure. b) Average
maximum nacelle displacement in each
bin computed from time history analysis
and response spectral analysis.
350
300
S 250
z
■S 200
Time hístory GM SRSS RR2011 SRSS EC8SRSS
Figure 3 - a) Overturning moment demand due to
each ground motion; the results are divided into
different magnitude bins as indicated in the legend.
b) Average overturning moment demand in each bin
computed from time history analysis and response
spectral analysis.
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