# Numerical Evaluation of Structural Safety for Aged Onshore Wind Foundation to Extend Service Life
## 2 Service Life Extension of Onshore Wind Foundation
### Decision Making Process for Structural Safey Evaluation  
The structural safety evaluation of the aged foundation of this study was conducted on the in-service wind farm of service life 16 years. Figure 3 shows the procedure for structural safety evaluation and the decision-making process for the service life extension or re-use of the aged foundation. In this paper, only the structural safety of the anchor ring type foundation was evaluated, and the retrofit method of the aged foundation was not included.  
In order to evaluate the structural safety of the onshore wind foundation in-service, it is necessary to first review the design documents. However, in-service onshore wind farms were built in the early days of wind power generation, and most of the design documents are not available. In addition, it is known that, in the early days of onshore wind power, design methodology for wind foundations, FE fatigue analysis tools for concrete members, etc., were not properly established. Therefore, fatigue life was also not evaluated for the foundations. It was considered general technical common sense at that time that the concrete members do not require fatigue life evaluation because the concrete members are highly resistant to fatigue; namely, there is almost no fatigue damage.  
Next, in order to evaluate the structural safety of an aged foundation to decide the service life extension, the material aging effect (deterioration in strength, etc.) of concrete is evaluated. Based on this, it is necessary to evaluate the structural safety of the foundation against extreme turbine loads. This is because, depending on the material aging effect (deterioration in strength, etc.) of the foundation concrete during the in-service period, it is determined whether the ultimate strength required for the service life extension is satisfied. Also, it is necessary to evaluate the fatigue life of the foundation to decide whether it is possible to extend service life, since, as described above, it is known that the fatigue life evaluation of the foundation was not performed in the early days of the onshore wind farm. In addition, a review of overturning and sliding, which are structural design items, should be evaluated.  
Figure 2: Service life extension and re-use methods of onshore wind foundation: (**a**) service life extension; (**b**) partial-repowering; (**c**) full-repowering.  
Based on the results of the structural safety evaluation, a decision is made on the possibility of service life extension of in-service wind foundation. If the structural safety result satisfies the current design criteria for the onshore wind foundation, it is possible to extend the service life, and if not, an appropriate structural retrofit or decommission is decided. Another problem is that there are no documents on the design method applied to the design of the aged wind foundation 16 years ago. The design methods of the onshore wind foundation include the allowable stress design method, the strength design method, and the limit state design method, and the limit state design method is widely used at present. Therefore, a structural safety evaluation of this study was performed in accordance with the current limit state design method.