## 5 Conclusions  
In this study, we developed a health-monitoring methodology and software for the high-temperature steam pipes of operating power plants. The proposed monitoring software can evaluate fatigue and creep life for the actual power-plant system in real time. To evaluate stress in the high-temperature stream pipes, we adopted three different methods--boiler code, design stress, and operating stress. As a theoretical approach, stress according to the pipe shape (i.e., straight, bent, and T/Y piece) was estimated by the German boiler standard TRD code, and allowable stress (or design stress) was presented as a function of operating temperature. In addition, the operating stress was calculated by the pipe-stress analysis software called CAESARII under the measured displacements by 3 DDMS and operating temperature as boundary conditions. When developing a new methodology  
Figure 5: Display of life-assessment software for piping systems: (**a**) basic information, (**b**) 3 DDMS importing and temperature calculation, (**c**) stress calculation, and (**d**) life prediction.  
under laboratory conditions, separate validation is required for field application, but in this study, data from a 3D displacement system, installed in an actual power plant boiler located in Manjung Malaysia, was used for methodology development. Based on the actual operating stress data, we proposed a response surface model to calculate the operating stress according to the measured displacement by 3 DDMS in real time. Finally, the creep life and the low-cycle fatigue life were investigated by the Larson-Miller parameter equation and the Smith, Hirschberg, and Manson equations, respectively. Through this method, a health-monitoring methodology that can predict stress in real time was developed, and a stress-based life-evaluation-monitoring analysis software was completed.  
The health-monitoring methodology proposed in this study provides real-time stress and theoretical stress evaluation equations for the high-temperature steam pipes of operating power plants, and based on this, creep and fatigue life evaluations are possible. With the surrogate model, stress and life can be quickly calculated using only displacement information, making it easy to analyze the health of power-plant piping systems online and offline. The proposed monitoring system has already been applied an operating power plant in the Manjung power plant, and we believe this is a practical and meaningful study in that our method is currently being used for power-plant maintenance.  
Conceptualization, W.C. and J.H.; methodology, W.C. and J.H.; software, W.C.; validation, W.C. and J.H.; visualization, W.C. and J.H.; data curation, W.C.; investigation, W.C. and J.H.; writing--original draft preparation, W.C. and J.H.; review and editing, W.C and J.H.; supervision, J.H.; All authors have read and agreed to the published version of the manuscript.  
This research was supported by the \"Research Base Construction Fund Support Program\" funded by Jeonbuk National University in 2021, the KEPCO Research Institute (No. R08 GH21), and the National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIT) (No.2020R1F1A1060442).  
Not applicable.  
The authors declare no conflict of interest.