**2. Field measurement**

#### **2.1. Overview of target and measurement setup**

The responses of structure were investigated using field data, which were measured at a wind turbine equipped in 2003 on the campus of Nihon University and along the Abukuma River in the Fukushima prefecture, Japan. The wind turbine specifications and the positions of the measuring equipment are shown in **Figure 1**. The rated power is 40 kW and rated wind speed is 11 m/s. The cut-in wind speed and cut-out wind speed are 2 and 25 m/s, respectively. This had been continuously operated even during the Great East Japan Earthquake in 2011, which resulted in no visible damage.

The accelerometers were attached at the top and middle of the tower horizontally. There are 60,000 data points for 5 min at a sampling frequency of 200 Hz. The data were recorded every hour when exceeding the acceleration threshold of ±0.7 m/s <sup>2</sup> . To investigate the transmission of vibration from the tower to the foundation, strain gauges were attached at the anchor bolts in the east, west, north, and south direction. The sampling conditions were the same as those in the accelerometer. Wind speed and direction were measured using an anemometer attached to the nacelle with a sampling frequency of 1.0 Hz. The measurements have been begun since May 2013.

Evaluation and Stability Analysis of Onshore Wind Turbine Supporting Structures http://dx.doi.org/10.5772/intechopen.75885 105

**Figure 1.** Overview and schematic of the apparatus used in measurements (based on [13]).

#### **2.2. Responses of the tower**

In particular, the structures supporting wind turbines, especially the foundation made of concrete, have been paid attentions in recent years. There are the reports that investigate cracks on foundation concrete [4–7]. It is not easy to identify the cause of cracks, while repeated action transferred from the tower is thought to be one of the causes. Therefore, fatigue of supporting structure made of concrete has become a main concern of researchers [7–10]. Even though the cracks are not always the trigger of structural collapse of wind turbine, further

Here, it is important, for practical design, to precisely analyze the responses of structures and to capture the action of wind. The response of existing wind turbine tower was analyzed using wireless system of accelerometers [11]. The development of a health monitoring system for the wind turbine tower-foundation system has been reported [12]. In addition, three-dimensional nonlinear finite element (FE) analyses for wind turbine tower-foundation

This chapter shows research of stability of supporting structure of onshore wind turbine foundations based on field measurements, laboratory experiment and FE analysis. In order to investigate the relation of action-response of tower-foundation system, long-term field measurements were carried out for an existing onshore wind turbine without piles for its foundations. Then, the model was built up for three-dimensional nonlinear FE analyses. The damage process of reaching failure was examined by FE models. In addition, limit state of foundation was defined by fatigue limit state of concrete. Consequently, the stress-number of cycle (S-N) diagram derived from laboratory experiment and analysis was discussed for the assessment of existing structure.

The responses of structure were investigated using field data, which were measured at a wind turbine equipped in 2003 on the campus of Nihon University and along the Abukuma River in the Fukushima prefecture, Japan. The wind turbine specifications and the positions of the measuring equipment are shown in **Figure 1**. The rated power is 40 kW and rated wind speed is 11 m/s. The cut-in wind speed and cut-out wind speed are 2 and 25 m/s, respectively. This had been continuously operated even during the Great East Japan Earthquake in 2011, which

The accelerometers were attached at the top and middle of the tower horizontally. There are 60,000 data points for 5 min at a sampling frequency of 200 Hz. The data were recorded every

sion of vibration from the tower to the foundation, strain gauges were attached at the anchor bolts in the east, west, north, and south direction. The sampling conditions were the same as those in the accelerometer. Wind speed and direction were measured using an anemometer attached to the nacelle with a sampling frequency of 1.0 Hz. The measurements have been

. To investigate the transmis-

investigations are required for safe and steady operation of power plant.

104 Stability Control and Reliable Performance of Wind Turbines

systems have been conducted using idealized static forces as input [13].

**2. Field measurement**

resulted in no visible damage.

begun since May 2013.

**2.1. Overview of target and measurement setup**

hour when exceeding the acceleration threshold of ±0.7 m/s <sup>2</sup>

**Figure 2** shows the maximum wind speed versus the maximum response of acceleration. Despite the scatter of the data, shown as black dots for operating wind speeds of 2–20 m/s, the maximum acceleration increased linearly with wind speed. The red dots represent data recorded while the generator was not operating. The difference between the two datasets suggests that the blade pitch control system dampened the acceleration response.

**Figure 2.** Max acceleration versus max wind speed in October 2013 [13].

The acceleration response of the tower in the time domain and trajectory of the tower displacement, which are derived through double integration of acceleration in the time domain are shown in **Figure 3**. To remove noise, a digital band pass filter with pass band between about 0.1 and 30 Hz was designed. The maximum displacement was about 0.5 cm at the top of the tower in the EW direction. Elliptical trajectories with different main axis were observed for each height in different scales when the wind turbine was operating. In particular, the trajectories of the top and middle of the tower were almost similar. This means that the predominant vibration mode was the primary mode.

frequency was 1.8 Hz in the primary mode and 13 Hz in the secondary mode based on the eigenvalue analysis and free vibration tests. The figure suggested the predominant vibration

Evaluation and Stability Analysis of Onshore Wind Turbine Supporting Structures

http://dx.doi.org/10.5772/intechopen.75885

107

The nonlinear finite element (FE) analysis code COM3D developed by Maekawa et al. [14, 15] are used in this study. The decrease of stiffness and the accumulation of plasticity of concrete subjected cyclic load are carefully formulated for concrete in the code. In particular, employing the logarithmic integral scheme that enabled to calculate fatigue damage of concrete is one of the advantages of this code. The properties for steel are expressed with bi-linear form. An overview of the FE model is shown in **Figure 5**. The model was modified from the model in the previous study [13] through further material investigation and verifications. Mechanical

In order to simplify structural model, the shape of nacelle and blades was not directly modeled. Alternatively, dead weight of them was applied to certain elements located at the top of with each material density. All the members except anchor bolts and the intermediate restraining reinforcements of the pedestal were modeled by solid element. Exceptions were expressed by line element; in particular, the torque on an anchor bolt was replaced by initial strain of the lines. The boundary condition between steel and concrete was modeled by joint element based on the Mohr–Coulomb theory with 0.6 as friction coefficient. Vertical displacement was restricted at the nodes of the footing bottom surface; however, confinement of sur-

rounding soil was not considered on the side of the footing as same in literature [16].

mode transmitted from the tower to the foundation was the primary mode.

properties of these constituent materials are summarized in **Table 1**.

**3. Finite element analysis**

**Figure 5.** Overview of FE model (based on [10]).

**3.1. Modeling**

#### **2.3. Action to the foundation transmitted from the tower**

The strain of nut for anchor bolt clearly depended on the acceleration variations, even though the value of the response was less than 1 μ. This can be explained that the location of strain gauges attached to anchor bolts was not consistent with wind direction which measured max wind speed.

When taking a long-term measurement, time varying character of the wind can be captured in a spectrum. The Fourier spectrum exhibited the waveform shown in **Figure 4**. The natural

**Figure 3.** Acceleration response of tower and trajectory of its displacement (based on [13]).

**Figure 4.** Fourier spectrum of acceleration of tower and strain on anchor bolt (based on [13]).

frequency was 1.8 Hz in the primary mode and 13 Hz in the secondary mode based on the eigenvalue analysis and free vibration tests. The figure suggested the predominant vibration mode transmitted from the tower to the foundation was the primary mode.
