**1. Introduction**

Wind power is growing a major industry. At the year of 2006, the total amount of installed wind power capacity reached 74.1 GW in the world [1], while it has reached 486.8 GW in the world at the year of 2016 [2] due to a rapid installation of wind turbine in this decade. In particular, increasing wind energy production has been even more actively touted against the background of the Great East Japan earthquake and the resultant nuclear power plant accident. However, in the past, wind turbines have had troubling experiences of structural collapse as well as failure of blade [3].

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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

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

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.

**2.2. Responses of the tower**

**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 sug-

Evaluation and Stability Analysis of Onshore Wind Turbine Supporting Structures

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gests that the blade pitch control system dampened the acceleration response.

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

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