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FFS Forum: Using Fracture Mechanics to Lower Risk and Improve Plant Reliability - Part 3, Practical Applications

By Greg Garic, Managing Principal at Stress Engineering Services, Inc. This article appears in the January/February 2021 issue of Inspectioneering Journal.
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This article is part 3 of a 3-part series on Fracture Mechanics.
Part 1 Part 2 Part 3

Editor's Note: This article is the last of a 3-part series on the application of fracture mechanics in a plant environment.

Introduction

Part 1 of this series discussed the basics and theory of fracture mechanics.  Part 2 drilled down on the FAD and its interpretation.  In Part 3 I’ll touch on a few practical points regarding the application of fracture mechanics in a plant environment.    

Let’s start with safety factors…

Safety Factors

The 2007 edition of API 579 included a procedure for calculating partial safety factors (PSFs) in Part 9 fracture mechanics assessments.  Partial Safety Factors (PSFs) are safety factors applied to the inputs rather than a traditional safety factor applied to the output.  The PSF approach is advantageous because it can be directly tied to the probability distribution of the input variables.  

The procedure included PSFs on stress, flaw size, and fracture toughness.  The PSFs procedure was largely pulled from the 2016 edition because there were some errors in its development and it was overly conservative.  There are still a few references to PSFs in the 2016 edition, but it’s no longer in the Level 2 procedure.

I’ve heard several arguments for excluding PSFs in Part 9 analysis (i.e., all PSFs = 1), but personally, I believe that some safety factors should be included, with a few exceptions.   For example, if lower-bound toughness values are used, you don’t need an additional PSF on top of lower-bound values. 

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