52nd AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics and Materials Conference<BR> 19th
4 - 7 April 2011, Denver, Colorado
AIAA 2011-1946
MEASURING GLOBAL RESPONSE OF A WIND TURBINE TO
SIMULATED EARTHQUAKE SHAKING ASSISTED BY POINT
TRACKING VIDEOGRAMMETRY
Ian Prowell∗
Missouri University of Science and Technology, Rolla, MO 65409-0030, USA
Tim Schmidt†
Trilion Quality Systems, Richmond Hill, NY 11418, USA
Ahmed Elgamal‡ and Chia-Ming Uang§
University of California, San Diego, La Jolla, CA 92093-0085, USA
Hal Romanowitz¶ and J. Edward Duggank
Oak Creek Energy Systems, Escondido, CA 92025, USA
Experiments investigating earthquake response of structures traditionally use conventional wired instruments such as strain gauges, displacement transducers, and accelerometers deployed at key areas of interest
throughout structure. For wind turbines the rotor is one of these key areas, but due to rotation it is not possible to use wired instruments without a special slip ring. In a recent experiment conducted using the Network
for Earthquake Engineering Simulation (NEES) Large High Performance Shake Table (LHPOST) at the University of California, San Diego (UCSD) the global response of a full scale 65-kW turbine (22 m hub height)
was monitored using a novel combination of traditional instrumentation in conjunction with point tracking
videogrammetry. Using this approach, conventional strain, displacement, and acceleration instruments monitored the response of the turbine tower and the rotor was monitored by observing 16 target points placed on
each blade which successfully provided insight not previously available for the global response of a turbine
to earthquake shaking while spinning. Excellent correlation was observed between the conventional measurements and results from videogrammetry. This paper presents the methodology used and key response parameters. Such information is extremely valuable for validation of numerical simulation of combined earthquake
and wind loads for wind turbines.
I. Introduction
growth of wind turbine installations coupled with seismic provisions of wind turbine certification guidelines 1,2
has lead to an increased interest in addressing the related earthquake loading considerations (Figure 1). Recognizing that experimental validation is currently scarce, a full scale test was planned and conducted at the University
of California, San Diego (UCSD) to expand what was learned from earlier testing. 3 An actual 65-kW wind turbine
was subjected to base excitation using the Network for Earthquake Engineering Simulation (NEES) Large High Performance Outdoor Shake Table (LHPOST). 4 This experiment uses previous results as a basis for design and expands
investigation into additional aspects of seismic loads for wind turbines, including the relative orientation of the rotor
and earthquake loads as well as level of shaking.
Capturing structural response is a primary concern of any experimental dynamic test. In most cases this can be
challenging and often requires surrogate measurements to allow calculation of structural response parameters. In the
T
HE
∗ Assistant
Professor, Department of Civil, Architectural, and Environmental Engineering
President
‡ Professor, Department of Structural Engineering
§ Professor, Department of Structural Engineering
¶ President
k Executive Vice President
† Vice
1
American Institute of Aeronautics and Astronautics
Copyright © 2011 by Ian Prowell. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission.
Figure 1. Wind Turbine Configuration
Figure 2. Mounted in Configuration 2
case of full scale shake table testing this is further complicated by specimen scale. Conventional instruments, including
strain gauges, accelerometers, string potentiometers, and linear voltage differential transformers, provide a basis for
instrumentation of the tested turbine. These conventional instruments lack the ability to directly track displacement
and structural response of the turbine blades. In the past, Global Positioning System (GPS) displacement gauges
have been used to directly track displacement for shake table testing. 5 Encouraged by successful use of point tracking
videogrammetry for measurement of strain and displacement of an operating turbine, 6 the research team deployed
such a system to augment the conventional instrumentation and characterize the rotor response.
II. Test Procedure
A full scale test was conducted at UCSD to explore the response of a turbine due to base excitation. For that
purpose, Oak Creek Energy Systems of Mojave, California donated a 65-kW turbine that was erected directly on the
LHPOST platen (Figure 2).
A.
Description of Test Specimen
The tested 65-kW turbine (Figure 2) was manufactured in Denmark by Nordtank. In the early 1980s, this Nordtank
turbine and its contemporaries were installed in large numbers for utility scale power generation in California. 7 By
1985, Danish machines accounted for approximately 40 percent 7 of the turbines installed throughout California.
These early turbines are often employed beyond the original design life, with retired machines frequently sold on
the secondhand market. In comparison to modern megawatt-level machines, the tested unit is relatively small, but
represents the canonical configuration (Figure 1) of a tubular steel tower topped with a nacelle that actively yaws to
orient the rotor into the wind. A summary of the pertinent engineering properties of the test turbine is presented in
Table 1.
B.
Test Facility
The LHPOST 4 is an outdoor facility built to impart uni-axial horizontal excitation, with a platform of 7.6 m by
12.2 m in size (Figure 2) and a stroke of ±0.75 m. Notable features include a peak horizontal velocity of 1.8 m/s,
a horizontal force capacity of 6.8 MN, and a vertical peak payload capacity of 20 MN. The overturning moment
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Table 1. Properties of the 65-kW Wind Turbine
Property
Rated power
Rated wind speed
Rotor diameter
Rotor hub height
Tower weight
Nacelle weight
Rotor weight (with hub)
Value
65-kW
9.4 m/s
16.0 m
22.6 m
60.1 kN
26.2 kN
16.1 kN
capacity is 50 MN-m (for a nominal specimen configuration with a mass of 200 metric tons at an effective height of
10 m, and acceleration of 2.5 g). Motions containing frequencies up to 33 Hz can be imparted. The LHPOST is the
first outdoor facility of its kind and adds a significant new capability to U.S. testing facilities. Full scale wind turbine
experimentation, such as the work described here is now possible with no overhead space and lifting constraints.
C.
Instrumentation
Prior to erection of the turbine, a detailed description of the instrumentation layout was prepared. To optimize test
setup, the turbine was instrumented with accelerometers and strain gauges prior to placement on the platen. In all,
59 - MEAS model 4000A accelerometers were installed on the adapter plate as well as throughout the turbine tower
and nacelle. Seven rosette and eight linear strain gauges were installed throughout the turbine tower to identify yielding
as well as estimate shear and moment demand. In addition to acceleration and strain measurements, displacement of
the tower was measured through string potentiometers attached from the turbine tower to a fixed instrumentation tower.
Visual targets were placed on the tower and all three blades to track motion through video photogrammetry. At the
tower base, linear voltage differential transformers (LVDTs) were used to capture possible base rocking and sliding
relative to the table platen.
A standard Cartesian coordinate system originating at the center of the tower base was used to describe instrument
location and orientation. The X axis was parallel to the direction of shaking with positive values to the east. Normal
to the table surface, the Z axis values increased with elevation. Oriented to the north, the Y axis completed a right
handed coordinate system.
Cameras were placed on tripods approximately 9 m away from each other, and approximately 41 m away from the
tower of the wind turbine, resulting in a camera angle of about 13 degrees. Sunlight provided adequate illumination
for all tests despite variations from time of day and cloud cover. The accuracy of the photogrammetry system depends
on the size of the field of view and the number of pixels on the cameras. The targets had a diameter of 178 mm in order
to meet or exceed the 10 pixel diameter requirement for accurate determination of the center-point coordinates. After
targets were attached they were laminated for moisture protection. Lamination did not detrimentally influence target
identification and facilitated survival in outdoor conditions for the entire testing testing program (over 1 month). In
this setup, with a 18 m field of view and 2,450 pixels across that width, the nominal accuracy is expected to be ± 1 mm
for the out-of-plane measurements (nominally parallel to the Y axis) and ± 0.33 mm for the in-plane measurements.
D.
Experimental Test Program
Over 90 test runs were conducted to systematically evaluate the influence of motion characteristics, relative orientation
of shaking, and operational state of the turbine. This paper focuses on the novel combination of traditional and optical
measurement techniques to characterize the structural response of the turbine to earthquake motion. All experiments
were conducted in one of two test configurations. In Configuration 1, not discussed in detail here, the shaking was
imparted parallel to the rotor’s axis of rotation with the northern blade oriented horizontally. Based on geometry and
mass distribution, little or no eccentricity is expected in this configuration. In contrast, eccentricity is expected in
Configuration 2 (Figure 2) where the shaking was imparted perpendicular to the rotor’s axis of rotation. Selected
results are presented for reproductions of the 1992 Northridge earthquake scaled from the recorded motion at 14145
Mulholland Drive located in Los Angeles, California to have a peak ground acceleration of 1.18 m/s2 (Figure 3). The
1992 Northridge earthquake had a moment magnitude of 6.7, with a peak acceleration of 4.1 m/s2 in the original
recording.
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American Institute of Aeronautics and Astronautics
2
Accel. (m/s )
1
0
-1
0
5
10
15
20
25
30
35
Time (s)
Figure 3. Reproduction of Northridge Earthquake
III. Raw Results
A.
Results from Conventional Instruments
30
0
-30
Base
Moment
(kN-m)
Tower Top
Absolute
Accel.
(m/s2)
Tower Top
Relative
Disp. (mm)
For normal civil structures conventional instrumentation is capable of capturing key results of interest such as displacement, acceleration, shear, and base moment. A sample of these parameters are shown in Figure 4. These parameters
provide valuable insight into structural demand for the turbine tower. However, the conventional instrumentation provides little information about the turbine rotor, a key component for which demand parameters must be understood.
3
0
-3
300
0
-300
Base
Shear
(kN)
25
0
-25
0
5
10
15
20
25
30
Time (s)
Figure 4. Key Results from Conventional Instrumentation for Northridge Earthquake
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35
B.
Results from Point Tracking Videogrammetry
Displacement (mm)
By placing optical targets along each of the blades and at the top of the turbine tower, the conventional instrumentation
was augmented to provide valuable information about the turbine rotor when subjected to earthquake shaking. As
shown in Figure 5, the point tracking videogrammetry system was capable of capturing displacement in three orthogonal directions for each instrumented point. This information can be used directly to understand displacement of the
rotor for parked and operational states when subjected to the simulated earthquakes. Displacement measurements of
the rotor blades allows investigation of the influence of earthquake shaking on key demand parameters, such as blade
root bending moment.
50
25
0
-25
-50
X axis translation
50
25
0
-25
-50
Y axis translation
50
25
0
-25
-50
Z axis translation
0
2
4
6
8
10
Time (s)
12
14
16
18
Figure 5. Sample Videogrammetry Results of a Single Target for Northridge Earthquake
IV.
Synchronization of Comparison of Results
Due to logistical constraints it was not possible to capture video and other measurements using the same clock,
thus post processing is required to synchronize the signals. Of particular interest to this approach is an optical target
located at the top of the tower (Figure 6), which coincides with the point of attachment for the upper most string
potentiometer. Using the known initial geometry of the string potentiometer and known horizontal displacement of
the turbine base, a corrected horizontal displacement can be calculated from the inclined measurement. Since this
measurement coincides with the optical measurement, the time shift between the two measurements is determined
by minimizing the norm of the error between the two signals. Figure 7 shows a comparison of absolute horizontal
displacement captured using a traditional string potentiometer and the video photogrammetry. A very high level of
agreement between the two methods is apparent with the optical results showing resolution of higher frequencies not
transmitted through the long cable of the string potentiometer.
V.
Translation from Global to Local Coordinates
Point tracking videogrammetry inherently records data referenced to a global coordinate system and requires processing to understand component local displacement. Of interest for this test is the blade local flap deflection to assess
the displacement demand, as a surrogate for root bending moment, of each blade in response to earthquake shaking.
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Displacement (mm)
Figure 6. Rotor Orientation for Northridge Earthquake
40
20
0
-20
-40
String Potentiometer
40
20
0
-20
-40
Video Photogrammetry
0
2
4
6
8
10
Time (s)
12
14
16
18
Figure 7. Comparison of Tower Top X Direction Absolute Displacement for Northridge Earthquake
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A coordinate system which translates and rotates with the plane of the rotor is considered to convert recorded global
coordinates to describe the blade local flap displacement.
First, the rigid body translation of the rotor was calculated by averaging the displacement of the three innermost
points on each of the blades in each of the three recorded axes. Next, in a similar manner the rotation was calculated
about the center of the rotor. At each time step, the displacement component due to the calculated rigid body translation
and rotation was removed from each recording point.
Displacement (mm)
Observation of the translation (Figure 8) and rotation (Figure 9) time histories that, as expected, are clearly influenced by the tower top displacement. The rigid body translation in the X axis (parallel to earthquake shaking) of the
rotor is very similar to the tower top displacement (Figure 7). The translation in the Y axis (horizontal and normal to
earthquake shaking) shows displacement at the same frequency, but out of phase with the X direction displacement.
Vertical displacement (Z axis) is minimal in comparison to horizontal components. Unlike translation, rigid body
rotation of the rotor is relatively minor (Figure 9). The main observed rotation is about the Z axis, out of phase with
the X axis displacement.
40
20
0
-20
-40
X axis translation
40
20
0
-20
-40
Y axis translation
40
20
0
-20
-40
Z axis translation
0
2
4
6
8
10
Time (s)
12
14
16
18
Figure 8. Resulting Rigid Body Rotor Translation
VI. Analysis of Flap Deflection
The derived coordinate system that follows the rigid body motion of the rotor has a Y axis oriented parallel to flap
vibration of the rotor blades (parallel to the wind). Figure 10 shows the raw (as recorded) and corrected (with rigid
body components removed) flap deflection of each of the three rotor blades (Figure 6). Blade 1 is oriented vertically,
blade 2 is oriented down to the left (positive X), and blade 3 is oriented down to the right (negative X). It is seen that the
flap vibration is relatively minor, especially for blades 2 and 3. Blade 1, shows the greatest flap vibration, about 30 mm
peak to peak. This is very minor considering that the blade length is almost 8 meters. The small displacement demand
observed experimentally supports simulation results suggesting that seismic loading has relatively little influence on
blade moment demand. 8
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Rotation (deg)
0.4
0.2
0
-0.2
-0.4
0.4
0.2
0
-0.2
-0.4
0.4
0.2
0
-0.2
-0.4
Rotation about X axis
Rotation about Y axis
Rotation about Z axis
0
2
4
6
8
10
Time (s)
12
14
16
18
Displacement (mm)
Figure 9. Resulting Rigid Body Rotor Rotation
30
15
0
-15
-30
Blade 1
30
15
0
-15
-30
Blade 2
30
15
0
-15
-30
Blade 3
0
2
Raw
Corrected
4
6
8
10
Time (s)
12
Figure 10. Blade Local Flap Deflection
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American Institute of Aeronautics and Astronautics
14
16
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VII. Discussion
As with any new testing methodology challenges were encountered. The primary challenge in testing was the
recording time frame possible for optical measurements. Earthquake tests were typically around one to two minutes
with white noise diagnostic runs 9 lasting as long as five minutes. Optical recordings were limited to approximately
30 seconds. Complicating this limited recording time was the lack of integration between the table control system and
optical recording. Initiation of recordings was accomplished by manually starting the system, which occasionaly lead
to triggering earlier or later than desired and further reduced the effective recording time. A third, but relatively minor
challenge was the optical field of vision. Due to the field of vision, it was not possible to observe the entire tower, but
just the tower top and rotor.
Despite these challenges, the test results are very encouraging for use of this technique in further testing of wind
turbines in various situations, 6 as well as other large structures. 5 The system was able to successfully identify small
angle rotation (Figure 9), which would not have been possible using traditional techniques. Further, the system was
mounted on conventional tripods at a distance from the test setup. This reduces the influence of local vibrations and
eliminates the need to build large rigid instrumentation structures, which are costly to construct and move (large red
towers shown in Figure 2). Subsequent to the data acquisition for this project, further camera and computer advances
have enabled increases in available recording duration, so that 3 minutes is readily available and 10 minutes or more
can be achieved when needed. With this important step and ongoing rapid advancements in technology, it is foreseeable
that this approach will rapidly become cost effective and highly accurate for large scale structural testing.
VIII. Conclusion
This publication presents a novel approach to capturing dynamic response of a wind turbine subjected to simulated
earthquake loading. By applying optical techniques, the research team was able to directly track displacement of
the turbine tower and rotor. Optical measurement techniques proved to be beneficial over previous attempts with
string potentiometers and GPS displacement instruments by providing more accurate displacement measurements
with higher frequency resolution. Further benefits were due to the ease of instrumentation and number of discrete
locations where measurements were captured. The 48 discrete points on the rotor would have been overly burdensome
to instrument with GPS units and impossible with string potentiometers. The simplicity and robustness of the targets
allowed successful measurement in an outdoor, large-scale test where other more intricate optical targets have proved
to be too delicate for the difficult measurement conditions at the LHPOST. Unlike acceleration based measurements,
the final reading is relatively noise free and does not require special processing to remove centrifugal acceleration from
the records. This approach was extremely successful in capturing displacement data not possible through the use of
conventional instruments.
Acknowledgments
The authors extend their gratitude to the National Science Foundation (NSF) for funding this work as part of a
George E. Brown Jr. NEES Research project (NSF NEESR-II grant No. CMMI 0830422). Contributions, in the
form of donation of the tested turbine, guidance, and man-hours, from Oak Creek Energy Systems (Hal Romanowitz,
J. Edward Duggan, Vaughn Johnson, Michael Burns, and many others) are most appreciated. Shake table testing would
not have been possible without the hard work of the NEES@UCSD team (Robert Beckley, Lawrence Berman, Andrew
Gunthardt, Darren McKay, Richard Whalen, Alex Sherman, Michael Dyson, Lawton Rodriguez, Paul Greco, and Chris
Latham) headed by Dan Radulescu. Thanks are also due to Dr. Paul Veers and the Sandia National Laboratories Wind
Energy Technology Department for providing Ian Prowell with a summer internship.
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American Institute of Aeronautics and Astronautics
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Structural Engineering, University of California, San Diego, 2011.
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