Session: MF-05-02 Fitness-For-Service and Failure Assessment-2

Paper Number: 124960

124960 - Using Computational Fracture Mechanics With the Api 579-1/asme Ffs-1 Annex 9h Procedure 

The 2021 edition of API 579-1/ASME FFS-1 (API 579-1) includes a new annex, Annex 9H which contains a procedure to reduce conservatism in the calculation of the toughness ratio, K_r, in a Fitness-For-Service (FFS) Assessment of a crack-like flaw to API 579 1 Part 9. The Annex 9H procedure incorporates constraint effects (based on T‑stress, a measure of crack front constraint) to improve the fracture toughness, K_mat. By applying Annex 9H, the K_r value is reduced in the Failure Assessment Diagram (FAD) used to determine if the crack-like flaw is acceptable or unacceptable.

The procedures in this annex is limited in its application. Annex 9H can only be applied to surface-breaking semi-elliptical crack-like flaws in flat plate and cylindrical shell geometries, when the stress distribution across the flaw plane can be accurately represented by a linearization.

Through the use of computational fracture mechanics, the procedures in Annex 9H can be expanded, and applied to the general case of a surface-breaking crack-like flaw in a component, under any stress distribution. For cases where Annex 9H is applicable, yet the necessary geometry idealization to apply this annex leads to an overly conservative result, computational fracture mechanics can also be beneficial in obtaining a more accurate solution.

This paper details a study where three-dimensional (3D) crack meshes were created to compute T-stress at the crack front by Finite Element Analysis (FEA). T-stress is computed using a linear-elastic FEA, and provides the values needed to apply Annex 9H procedures to other geometry and load cases.

Comparisons are made between FFS Assessments carried out following the typical Part 9 procedures, to those incorporating Annex 9H, with and without the use of computational fracture mechanics. Different component configurations are evaluated, such as plates, cylinders and nozzles, covering both simplified cases where Annex 9H can be directly applied, and cases outside the applicability of these annexes where FEA T-stress results are used. The application of the procedures in WRC Bulletin 590, which provides a method that essentially expands on the Annex 9H procedure for nonlinear stress distributions, is also compared. The improvement in K_mat, and thus the final FFS outcome, through the application of these procedures is compared and reviewed. Applying the Annex 9H procedure shows a reduction in the K_r value on the FAD plot, leading to larger acceptable flaw sizes compared to the standard Part 9 assessment. Recommendations are provided on how the procedures in Annex 9H can be implemented in an FFS Assessment carried out using computational fracture mechanics 3D crack meshes with FEA.

Presenting Author: Daniel Blanks Quest Integrity

Presenting Author Biography: Daniel is a Senior Structural Integrity Engineer with Quest Integrity, and has over 10 years' experience in conducting Fitness-for-Service assessments for assets in the Power, Pipeline and Process industries. Daniels key focus is in the application of computational fracture mechanics to solving challenging fracture mechanics problems.

Authors:

Daniel Blanks Quest Integrity