Extending API 579-1 to the Hydropower Industry: How Coupling Advanced Ultrasonics and Computational Fracture Mechanics Can Overcome Limitations

Abstract The API 579-1/ASME FFS-1 Fitness-For-Service (API 579-1) Part 9 procedures are commonly used to assess crack-like flaws within components in hydroelectric power facilities. However, these components are often geometrically complex and may not be amenable to geometric idealizations that enable the use of standard crack driving force solutions provided in API 579-1 for a Level 2 assessment. The approximation of crack-like flaws using Level 2 methods can greatly overestimate the crack driving forces, thereby influencing the assessment outcomes. In contrast, Level 3 procedures can leverage computational fracture mechanics to enable direct computation of the crack driving forces by embedding the crack features within a finite element analysis (FEA). This approach is especially beneficial for non-uniform components, such as centerline pressure components (i.e., spiral casings, head covers, etc.), where the stiffness and behavior of the component also varies along the length of the crack. Level 3 assessments provide the greatest benefit when detailed mapping of the crack front is available. However, advanced ultrasonic technology and creative/bespoke approaches to scan procedures are required to achieve this benefit. This is particularly true when working with poor access and complex component geometry such as that seen in the hydropower industry. This paper presents an example of a coupled inspection and assessment performed for a hydropower turbine. Phase coherence imaging (PCI) was used to provide detailed crack dimensions along an 11 m crack-like flaw. The advantages of PCI over traditional ultrasonics for this application are discussed and compared. This detailed crack profile is then incorporated into an FEA of the component to enable calculation of detailed crack driving forces for a Level 3 analysis. A comparison of the outcomes from a Level 2 and Level 3 analysis is made, highlighting the limitations to applying Level 2 procedures on hydropower equipment.