# Numerical Evaluation of Structural Safety for Aged Onshore Wind Foundation to Extend Service Life
## 4 Conclusions  
In order to extend the service life or re-use of an onshore wind foundation, a structural safety evaluation using an engineering method is firstly performed on the wind foundation. In this paper, for the case of \"service life extension\" with the same capacity of the wind turbine, the structural safety evaluation was carried out to determine whether to extend the service life of the foundation. The structural safety evaluation of this study was conducted on the in-service wind farm of service life 16 years with the anchor ring foundation. The structural safety evaluation was carried out on six items which are essential for confirming the safety of foundation, such as material strength, ultimate strength, fatigue life, soil resistance, overturning, and sliding.  
As a result of structural safety evaluation for the onshore wind foundation, it was found that the existing foundation satisfies the structural safety of the ultimate strength, fatigue life, and serviceability. Although the in-service period has been over 16 years, it has been shown that the material properties of the concrete have exceeded the design strength and no significant material deterioration has occurred. Also, the structural safety metrics, such as the ultimate strength and fatigue life, could be evaluated  
\begin{table}
\begin{tabular}{l c c c c c c c} \hline \hline \multirow{2}{*}{**Items**} & **1. Material** & **2. Ultimate** & \multirow{2}{*}{**3. Fatigue Life**} & \multirow{2}{*}{**4. Soil Resistance**} & **5. Over** & **6. Sliding** \\  & **(MPa)** & & **(MPa)** & & **(yr.)** & **(kN/m2)** & **(kN-m)** \\ \hline
**Members** & **Concrete** & **concrete** & **steel** & **concrete** & **steel** & **74.1** & **10,473** & **266** \\ \hline Evaluation & 40.6 & 13.4 & 93.2 & \(>\) 40 yr. & \(>\) 20 yr. & 200.0 & 21,881 & 2722 \\ Criteria & 30.0 & 40.6 & 240.0 & \(>\) 20 yr. & \(>\) 40 yr. & 0.37 & 0.48 & 0.10 \\ Ratio & 1.35 & 0.33 & 0.49 & \(>\) 40 yr. & \(>\) 40 yr. & **10.35** & **10.35** \\ \hline \hline \end{tabular}
\end{table}
Table 5: Summary of the structural integrity evaluation of the onshore wind foundation.  
more realistically based on actual concrete properties. In particular, the fatigue life is the most important item to determine the service life extension of the foundation, because the foundation is continuously damaged by turbine loads depending on the service life. As a result of evaluating the fatigue life of the turbine load, it has been shown that it has a fatigue life of 40 years or more, so that service life can be extended.  
In addition, it is expected that the methodology used in this paper will be useful not only for safety evaluation of the foundation in service but also for decision-making for extending the service life. At present, when the technical data in this area is insufficient, the appropriate method is to determine the service life of the foundation depending on whether it satisfies the current design criteria. However, a more technical approach should be explored by many researchers in the future, and this paper has carried out one of those efforts.  
[PERSON] is a PI of the research project who planned most of the structural safety evaluation program and is a main writer of the paper; [PERSON] performed fatigue life evaluation; [PERSON] carried out ultimate strength and serviceability evaluation; [PERSON] assisted with the material test and FE analysis. All authors have read and agreed to the published version of the manuscript.  
This research was funded by [the Ministry of Trade, Industry, and Energy of South Korea] grant number [20183010025110].  
This study was supported by the Ministry of Trade, Industry, and Energy of South Korea, Project No: 20183010025110 (Development of repowering total technology to improve availability of old wind farm).  
The authors declare no conflict of interest.