Session: HT-01-01 Joe Kapp Memorial Session: Design and Analysis of High-Pressure Equipment

Paper Number: 61075

Start Time: Wednesday, July 14, 2021, 08:00 PM

61075 - Thick-Wall Cylinder Reference Stress Crack Solutions for API 579-1/ASME FFS-1 Annex 9C 

The objective of this paper is to describe a new set of thick-wall cylinder reference stress crack results computed for the API 579-1/ASME FFS-1 standard in Annex 9C for a recent ASME Standards & Technology project. The Failure Assessment Diagram (FAD) method, described in API 579 Part 9, uses a reference stress solution to compute the Lr value on the FAD x-axis to evaluate cracks for failure, along with the stress intensity and toughness used to compute the Kr value on the FAD y-axis. These new reference stress solutions for Annex 9C complement the thick-wall cylinder stress intensity K solutions available in API 579 Annex 9B, so that both the Lr and Kr values can be computed for an FAD assessment point to evaluate cracks in thick-wall cylinders.

The current reference stress solutions in API 579 are applicable to thin-wall cylinder geometries, typically for a radius to thickness ratio greater than one. New reference stress geometry factor solutions for thick-wall cylinders have been computed for 523 geometry cases, which are described by ratios for the radius, thickness, crack depth, and crack length. The crack locations include axial, circumferential, internal, and external cracks. The FAD-based method to compute reference stress geometry factors is described in API 579 Annex 9G.4 and uses 3-D crack meshes with elastic-plastic stress-strain material data. Using that method, crack front J-integral results versus pressure load determine a nominal load to compute the non-dimensional geometry factor, which gives the reference stress result for each thick-wall cylinder crack case. The FAD-based methodology is briefly reviewed in this paper using results from this project.

Since elastic-plastic Finite Element Analysis (FEA) is used with 3-D crack meshes, a stress-strain curve was selected to represent typical material used for high-pressure components. The computed reference stress was shown to be dependent on the yield strength to tensile strength ratio, and a range of ratios was examined. The preliminary results helped select a yield strength value of 90% of the tensile strength, which is also typical for high-pressure components.

Typical geometry factor results are shown in 3-D surface plots to compare the computed geometry factor versus the crack depth and crack length ratios. The plots show the geometry factor increases for longer and deeper cracks, and the geometry factor decreases as the cylinder thickness increases.

Some internal shallow cracks showed unexpected behavior in the J-integral results trend, which gave insufficient J-integral results to provide enough values to compute the geometry factor. Examining the results showed the thicker cylinders begin yielding on the entire inner surface as the internal pressure is increased, which leads to erratic J-integral results. The entire ligament ahead of the shallow crack fully yields before the J-integral results reach a value sufficient to compute the nominal load. An alternative method was developed to use the maximum converged pressure load value to provide a nominal load, similar to a plastic collapse load. An overlap case where the nominal load from the J-integral and maximum converged pressure were nearly equal is used to examine the J-integral behavior and compare the geometry factor trends. Using the maximum converged pressure alternate method allowed geometry factor results to be obtained for all the cylinder crack cases in this study.

Engineers will benefit from these reference stress results when solutions are needed for thick-wall cylinders that contain cracks, or for evaluations of postulated cracks in design of high-pressure components.

Presenting Author: Greg Thorwald, Ph.D. Quest Integrity USA, LLC

Authors:

Greg Thorwald, Ph.D. Quest Integrity USA, LLC
Lucie Parietti Quest Integrity