Full Scale Testing for Investigation of Wind Turbine Seismic Response

Ian Prowell*, Marc Veletzos*, and Ahmed Elgamal*†

Abstract

The earthquake response of wind turbines is a topic of interest, relevant to installations in

seismic regions. In recent years, researchers and practitioners have approached this problem through application of existing code for building structures as well as numerical and analytical modeling of wind turbines. The Network for Earthquake Engineering Simulation Large High Performance Outdoor Shake Table at the University of California, San Diego has opened the possibility of full-scale shake table tests of tall structures, such as wind turbines. In November 2004, researchers at the University of California, San Diego successfully conducted a full-scale test on a 65 kW turbine. The turbine was excited perpendicular to the axis of the rotor with a seismic base shaking record scaled to various levels. This paper presents an overview of the experimental program and associated preliminary analytical results.

1 Introduction The worldwide installed wind power continues to grow rapidly. The year 2007

saw a significant increase in installed wind power (WWEA, 2008). Contrary to historical trends where Europe dominates the market for wind power, much of this growth was concentrated in North America and Asia. Both regions periodically experience strong earthquakes that may impact the final turbine design. As the installed wind power in earthquake prone regions grows, so does the importance of appropriate consideration of seismic hazards. Under-predicting this hazard exposes the operators and the communities dependent on wind power to undue risk. On the other extreme, over-prediction of earthquake influence may lead to costly designs that place unjustifiable pressure on the economical feasibility of wind power. Thus, rational prediction of seismic considerations will maintain and enhance the ability of wind power to economically compete with other energy sources.

The estimation of wind forces developed on the turbines is a mature field. It is based on extensive experimentation, practical experience, theoretical predication, and numerical modeling. In contrast, the process of estimating seismic forces on turbines is relatively new. Current practices for seismic loading vary greatly, but generally fall into one or both of two categories: numerical (finite element) analysis; and analysis based on building codes such as the 1997 Universal Building Code (ICBO, 1997). Additionally, there are situations where the estimation of seismic loads is simply not preformed. It is widely recognized that the dynamic behavior of wind turbines is distinct from that of other building structures. Despite this difference, code based procedures for seismic forces rely on the available principals that have been developed for buildings. While finite element methods allow specific consideration of wind turbines, they currently lack the extensive body of experimental results that exists for buildings to provide validation of predicted behavior.

It is imperative that wind farms remain in operation immediately following an earthquake to provide power for rescue and recovery efforts. In contrast to a city comprised of many different structures, a wind farm consists of few types of unique structures. This homogeneity raises the problem that an earthquake with unfavorable characteristics may damage most of the turbines at a given wind farm. The last * University of California, San Diego, Department of Structural Engineering, La Jolla, CA 92093-0085 † Corresponding author, Tel: (858) 822-1075, Email: elgamal@ucsd.edu

decade has seen an increase in the consideration of seismic loading for wind turbines (Bazeos et al., 2002; Lavassas et al., 2003; Zhao and Maisser, 2006). Unlike buildings, certification guidelines call for turbines to behave elastically (GL, 2003) and sustain no damage during an earthquake to ensure operability. Reliable methods for understanding seismically induced load on wind turbines will decrease the likelihood of damage due to seismic events without excessive cost.

Seeing this challenge, researchers at the University of California, San Diego (UCSD) are experimentally investigating wind turbines to gain an understanding of expected earthquake forces. This paper presents the experimental setup for a full scale shake table test of a wind turbine, along with a brief summary of the results. The data is analyzed using simple, yet effective, procedures to provide insight into the observed structural damping of the wind turbine.

2 Earthquake Loading on Wind Turbines The growth of wind turbine installations in seismic regions has spurred interest

in the consideration of earthquake loads on wind turbines (Riso, 2001; Agbayani, 2002; Bazeos et al., 2002; Lavassas et al., 2003; Witcher, 2005; Zhao and Maisser, 2006; Zhao et al., 2007). The main concerns for such loading are the same regardless of the type of structure being considered:

1. Seismic risk a. Anticipated level of shaking b. Recurrence of shaking

2. Local soil properties 3. Structural properties

a. Structural frequency b. Structural ductility c. Structural damping

Seismic risk, which includes the anticipated level of shaking and the recurrence of shaking, is associated with the geographic location. Building codes will use geographic location to assign a level of seismicity. In some cases, a site specific analysis is conducted to asses the anticipated level of seismicity. Historically, turbines were installed in regions of low seismicity throughout Europe, where the risk level was not high enough to warrant consideration. As wind turbine installation expands into new regions, seismicity is becoming a concern and must be considered. For example, some installations in California are located in regions of high seismicity and may even have earthquake fault lines that traverse the wind farm. The damage experienced in the 1995 Kobe earthquake is a graphic example of a near source seismic event, centered only 20 km from downtown Kobe (Horwich, 2000).

Local soil properties are another important consideration for seismic loading of civil structures. Foundation and super-structure design are strongly influenced by local soil properties. Many wind farms are situated along ridge lines and have stiff soils that present fewer design challenges. Other installations are located in coastal regions or alluvial deposits that frequently contain soft or loose soils. Such situations require special consideration and warrant careful attention during design. Additionally, the local soil properties can influence the level of shaking through amplification. The most graphic example of local soil amplification was observed in the 1985 Mexico City earthquake (Singh et al., 1988). Unlike Kobe, the earthquake was centered over 300 km from Mexico City, but local soil conditions amplified

motion that was hardly perceivable outside of the lakebed where the city is located to levels that caused widespread damage.

The natural frequencies of a structure also strongly influence the response to harmonic loading. If the earthquake contains energy at any of the lower natural frequencies, significant forces will develop in the structure. Major components of modern turbine designs commonly have natural frequencies in the range of up to 15 Hz (Bazeos et al., 2002; Zhao and Maisser, 2006; Zhao et al., 2007), which is approximate frequency range of interest for earthquake loading (ICBO, 1997; ICC, 2006). This loading phenomenon, known as resonance, is common to all turbine dynamic loading scenarios. Currently, careful attention is given to ensure that natural frequencies of major structural components, such as the tower and blades of the turbine do not overlap under normal operating conditions.

These factors, seismic risk, local soil properties, and structural frequencies, will strongly influence the amount of energy that a civil structure must handle to resist damage in an earthquake. During vibration, damping is a measure of the ability to dissipate dynamic energy, and is highly influenced by characteristics, such as material properties and construction details of the structure. A structure with high levels of damping will be able to efficiently dissipate energy imparted from an earthquake or other dynamic sources. A common method of economically increasing the influence of damping in civil structures is by providing ductility, the ability of the structure to deform and sustain damage without collapsing. However, as previously mentioned, wind turbines should not sustain damage in earthquakes precluding the reliance on this mechanism of increased damping.

3 Existing Work for Earthquake Forces As highlighted in a paper published in the proceedings from the Structural

Engineering Association of California (SEAOC) 2002 convention (Agbayani, 2002) design engineers currently struggle with the lack of guidelines specific to wind turbines for considering earthquake loads. Due to the lack of such guidelines, engineers sometimes utilize standard design methodologies from existing building codes, the Uniform Building Code (UBC) (ICBO, 1997) or the International Building Code (IBC) (ICC, 2006). Designers find, as turbines increase in size and weight, that according to these codes, seismic forces may be larger than the expected forces from other sources such as wind (Agbayani, 2002) leading to an increased interest in the earthquake response characteristics. In 2001, Riso National Laboratory (Riso, 2001) presented a simple process for estimating seismic loading of a wind turbine based on the first natural frequency in conjunction with a response spectrum to estimate seismic demand. In 2002, one of the first attempts to quantify the dynamics of wind turbines due to seismic demands was published (Bazeos et al., 2002) and presented extensive finite element modeling of a prototype turbine with a 38 meter tall steel tower designed for installation in Greece. Similar to the procedure suggested by Riso, this investigation modeled the response of the turbine tower to earthquake loads by representing the mass of the rotor and nacelle at a single point atop the tower. In 2003, an investigation of a 44 meter tall turbine with a steel tower designed for installation in Greece was published that included a more detailed model of the tower (Lavassas et al., 2003). The seismic loading in this investigation was based on Eurocode 3 with seismic zone II and rocky soil. For this relatively low level of seismicity, seismic forces were 60% lower compared to those potentially developed by wind loading. Again, the model placed the mass of the rotor and

nacelle at a single point at the top of the tower instead of reproducing the actual geometry.

More recently, two methods for integrating the impact of the rotor and the nacelle into seismic design of wind turbines were published. In response to demand for estimation of loading at seismically active sites, Garrad Hassan added a seismic module to GH Bladed (Witcher, 2005), their software product for wind turbine design. This software uses a finite element model of the wind turbine combined with artificial earthquake records that conform to code requirements for estimating forces caused by seismic loading at a particular site. In addition, a theoretical model has been developed (Zhao and Maisser, 2006; Zhao et al., 2007) that is capable of modeling aerodynamic loading, base shaking, and effects due to rotation of the rotor. Both developments provide promising computational models for evaluating wind turbine seismic loading. However, the value of these models will increase greatly through experimental verification to calibrate the predictions with real world observations.

4 Experiment Description To provide much needed data for validation of existing and future models of

wind turbine behavior in response to earthquakes, researchers at UCSD conducted a full scale test of a wind turbine. A 65 kW donated by Oak-Creek Energy Systems of Mojave, CA, was mounted on the Network for Earthquake Engineering Simulation (NEES) shake table located at UCSD and subjected to base shaking that simulated an actual earthquake. Instruments attached to the turbine recorded the response of the tower. This data is of great value to assist in the development and calibration of methods for predicting forces experienced by a turbine in an actual earthquake.

4.1 Physical Description of Test Turbine The tested unit (Figure 1) is typical of many Danish turbines installed in

California, characterized by high reliability and simple operation (Hau, 2006). This reliability and ease of operation has resulted in use beyond their design life, and these units are still sold on the second-hand market. In comparison to modern turbines, the unit tested is small, but represents the most common turbine configuration, a thin walled tubular steel tower topped with a nacelle that yaws to orient the rotor into the wind. A summary of the engineering properties of the turbine is presented in Table 1.

4.2 Shake Table Facility The shake table tests (Figure 2) were conducted on the UCSD, NEES Large

High Performance Outdoor Shake Table (LHPOST). The NEES shake table is a uni-axial table that is 7.6 m by 12.2 m in size with a stroke of ±0.75 m, a peak horizontal velocity of 1.8 m/s, a horizontal force capacity of 6.8 MN, an overturning moment capacity of 50 MN-m (for a 400 ton specimen), and a vertical peak payload capacity of 20 MN. The testing frequency range is 0-20 Hz. As such, this table is the largest worldwide in terms of load capacity and the first outdoor facility of its kind anywhere. The facility adds a significant new capability to existing United States testing facilities with no overhead space and lifting constraints, which is essential for full scale wind turbine testing.

Property Value Rated power 65 kW Rated wind speed 33.8 km/H (21 MPH) Rotor diameter 16.0 m (628 inches) Tower height 21.9 m (864 inches) Lower section length 7.9 m (313 inches) Lower section diameter 2.0 m (80 inches) Middle section length 7.9 m (312 inches) Middle section diameter 1.6 m (62 inches) Top section length 6.0 m (238 inches) Top section diameter 1.1 m (42 inches) Tower wall thickness 5.314 mm (0.2092 inches) Rotor hub height 22.6 m (888 inches) Tower mass 6400 kg (14.1 kips) Nacelle mass 2400 kg (5.2 kips) Rotor mass (with hub) 1900 kg (4.1 kips)

Table 1: Turbine Properties

4.3 Experimental Test Program In November 2004, the turbine described above was mounted on the NEES

shake table and subjected to numerous base shaking events (with the rotor axis perpendicular to the imparted motion). The rotor was oriented with one blade down, parallel to the main tower for all test motions (Figure 1). To capture the tower’s lateral response, uni-axial accelerometers were installed on the turbine and table as indicated in Figure 2. One accelerometer was located on top of the shake table. Four others were located on the turbine tower, one at the base, one at the lower joint, one at the upper joint, and one on at the top of the nacelle (Figure 2).

Excitation used for the tests was derived from a recording of the Desert Hot Springs (DHS) East-West component (0.15 g peak ground acceleration) of the June 28th, 1992 strike-slip Landers Earthquake (moment magnitude Mw=7.3). DHS is a California Strong Motion Instrumentation Program (CSMIP) station situated on deep alluvium located 23 km from the fault trace where the Landers Earthquake occurred. To remove any superfluous offset as well as high frequency noise, the earthquake record was filtered with a band pass of 0.05 Hz to 25 Hz. The record was scaled at 50%, 100%, 143%, and 200% for the shake table tests. As such, this was the first full scale base excitation test for a wind turbine.

Figure 1: Shake Table Setup

Figure 2: Accelerometer Locations

4.4 Summary of Results In this initial phase of experimentation, all tests were completed with no

damage to the turbine. As expected, the results showed essentially linear behavior (Table 2) across the scaling range of 50% - 200%. Video of the tests is publically available online at the UCSD NEES website, http://nees.ucsd.edu. In addition to the longitudinal shaking, first hand accounts as well as the video footage of the test indicate that some rotational motion occurred due to the mass distribution of the rotor and nacelle. This torsion suggests that consideration of the mass distribution of the rotor and nacelle may be of importance in understanding forces due to earthquakes. The team at UCSD is in the process of fully analyzing the results to advance the state of the art and provide further insights for seismic design.

Test Name Max Input Acceleration (g)

Max Response Acceleration (g)

Turbine Landers 50% 0.07 0.19 Turbine Landers 100% 0.12 0.28 Turbine Landers 143% 0.17 0.52 Turbine Landers 200% 0.24 0.70

Table 2: Brief Summary of Experimental Results using a Scaled Landers Earthquake Ground Motion

4.5 Observed Damping As previously mentioned, damping is an important factor in predicting the

ability of a structure to dissipate earthquake energy. Thus, it is important to use an appropriate level of damping to ensure applicable results when using analytic and numeric procedures to estimate earthquake induced loading. Specific estimates

used for wind turbines vary from values as low as 0.5% of critical damping (Bazeos et al., 2002) to 2% (Agbayani, 2002). These values are lower than the 5% assumed by the 2006 International Building Code (ICC, 2006). Damping due to energy radiation back into the ground influences the above seismic damping estimates, and must also be further investigated. This relatively wide range in damping values will affect the estimates of seismically induced forces, necessitating closer scrutiny of this important factor.

Experimental results from the shake table tests were used to estimate damping present in the turbine structure at the first natural frequency, by averaging the value obtained applying the log decrement method (Chopra, 2001) over the record of free vibration phase of the experimental results. Additionally, the half power method (Meirovitch, 1997) was used to estimate damping from the transfer function of the input motion to the motion at the tip of the nacelle. Due to numerical precision, the half power method represents an upper bound for the actual damping in the system. The results (Table 3) for both the log decrement and half-power methods show reasonable agreement across the 4 conducted tests, with the exception of the log decrement method for the 50% level test. Given the consistency of the other results, the log decrement 50% estimate is considered an anomaly and the data supports the conclusion that the structural damping is approximately 0.6% for the turbine structure (i.e., without the soil-structure interaction radiation damping influence).

Test Name Log Decrement Damping Half Power Damping Turbine Landers 50% 2.00 % 0.60% Turbine Landers 100% 0.86 % 0.64% Turbine Landers 143% 0.43 % 0.66% Turbine Landers 200% 0.41 % 0.52%

Table 3: Observed Damping

Since the turbine was not operating during the shake table tests, the values reported above do not also represent the level of aerodynamic damping that may be in effect during operation (Hansen, 2004). An experimental investigation for tall buildings suggests that such aerodynamic damping is directional (Murakawa et al., 1996). For vibration parallel to the wind direction, the aerodynamic damping of a scale model was positive and increased with wind speed. In contrast, aerodynamic damping for vibration perpendicular to the wind direction depended on many factors and was even found to become negative for particular combinations of geometry and wind speed. Unlike vibrations caused by wind, which are predominantly in the direction of the wind flow, it is difficult to forecast if the predominant direction of shaking from an earthquake and it is unlikely to coincide with the wind direction.

Furthermore, in both the 1997 Uniform Building Code (ICBO, 1997) and the 2006 International Building Code (ICC, 2006) wind loading and earthquake loading are not considered simultaneously. In the absence of better information, a reasonable solution is to neglect the impact of aerodynamic damping when considering earthquake loading. The research team at UCSD hopes to conduct in-situ experiments for operating wind turbines to quantify the directivity of aerodynamic damping, and its potential influence on seismic response.

If aerodynamic damping is neglected, the results reported here support the use of low damping (less than 1 percent) at the first natural frequency, for modeling

turbine response to strong ground motion (Bazeos et al., 2002). As mentioned above, in a field installation, effects such as soil structure interaction (Kramer, 1996) and aerodynamics may contribute additional damping. These and other effects may warrant the consideration of higher damping for in-situ damping levels. Nevertheless, the suggested range of damping (Newmark and Hall, 1987) for welded steel structures (5-7%) may be on the high side for many situations.

5 Conclusion This paper presents an introduction to an experimentally based program for

consideration of the impact of earthquakes on wind turbines. This area is of growing importance as wind power expands beyond its European roots into earthquake prone regions throughout the Americas, Asia, and elsewhere. Existing publications from both the academic community as well as the wind industry show that this field is growing and is of interest.

The experimental investigation summarized here shows that full scale seismic testing of wind turbines is possible and can provide valuable insight into dynamic behavior of wind turbines. The results obtained are an important first step in the process of developing a more accurate picture of how wind turbines are impacted by earthquakes. As shown by the low observed super-structure damping reported above, the data provides a valuable basis for calibration and further development of verified design procedures. Further work in this field is needed to develop a mature understanding of the impact of earthquakes on wind turbines.

6 Acknowledgments The authors extend thanks to all the organizations, corporations, and

individuals who contributed to this investigation. Oak Creek Energy Systems (Hal Romanowitz and J. Edward Duggan) generously donated the 65 kW turbine for shake table testing. The authors are grateful to NEES, the National Science Foundation (NSF), and others that have provided funding for the UCSD Englekirk Structural Engineering Center without which this research would not have been possible.

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