Introduction
In fitness-for-service (FFS) assessments under the API 579 International Standard (API 579), there are many instances when it is necessary to evaluate if a flaw or defect is too close to another flaw or defect or some “feature” of the structure. In such cases, the assessment may not be allowed, or special consideration may be required. I’ll collectively call these “proximity limits.”
Over many years of teaching my API 579 short course, I’ve noticed that there can be a good bit of confusion regarding when and how to apply these proximity limits. There are proximity limits in most Parts of API 579. Many are identical in several Parts, while others vary slightly. I think it will be useful to collect all of these criteria in one place and to look for the common threads and logic as to why these requirements are important. This is intended to be something of a “quick reference” to remind you which Parts use which proximity limits. A summary table is also provided at the end for easy reference.
Types of Proximity Issues
There are essentially four types of proximity limits:
- MSD - Too close to a major structural discontinuity (MSD).
- Weld - Too close to a weld.
- Same type defect - Too close to another flaw or defect like itself (e.g., a crack near another crack or a local thin area (LTA) near another LTA).
- Different type defect – Too close to a different type of defect (e.g., a crack near an LTA).
The fourth type, interaction with different types of defects, is the most problematic. I wrote an in-depth article on this topic in the July/August 2020 issue of Inspectioneering Journal, entitled “FFS Forum: Dealing with Multiple Damage Mechanisms in an FFS Assessment.” I won’t be recapping that article here, but I strongly recommend you read it to get a better understanding of this fourth type of proximity limit.
In the following sections, I’ll briefly discuss the first three proximity limits for each Part of API 579 and provide a summary table for your reference (see Table 1).
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