Inspectioneering Journal

FFS Forum: Local Thin Areas (LTAs) - How Bad is Too Bad?

By Greg Garic, P.E., Senior Staff Consultant at Stress Engineering Services, Inc. This article appears in the March/April 2023 issue of Inspectioneering Journal.


This is an article I’ve been thinking about for a long time. The reason being that in all the local thin area (LTA) assessments I’ve done over the years, I rarely see one that fails the assessment. To be clear, it happens, but just not very often.

I believe that this is the case for two reasons. First, the engineering mechanics of LTAs are very “forgiving.” That is to say, when you perform a rigorous analysis, you find that an LTA has to be pretty severe before it fails to meet the fitness-for-service (FFS) acceptance criteria. Second, I believe that the “qualitative risk assessment” performed routinely by plant personnel is far less forgiving than the rigorous assessment. I’m using “qualitative risk assessment” somewhat tongue-in-cheek to refer to the “gut check” – the point at which plant maintenance and reliability personnel just say “we need to fix that.”  It’s the situation where people aren’t discussing getting an FFS analysis; they’re discussing getting a welder. And generally, that’s a good thing – it’s driven by a desire to be safe, not cutting it too close, and an instinct for self-preservation.  

On the other hand, there are many circumstances where an LTA is the preferred repair solution. Welds can introduce new flaws and residual stresses. Blend grinding of a crack or gouge might be the best solution. Accepting an area of local corrosion based on an LTA assessment may be preferable to weld overlay. If an LTA assessment is performed and it passes the API 579, Part 5 (Local Thin Area), Fitness-for-Service (API 579) assessment criteria, it may be better to leave it alone.  

The problem I’ve noticed is that plants often don’t get to the LTA assessment because it doesn’t pass the gut check. So, my goal for this article is to recalibrate your gut. I want to show you some examples of LTAs that pass. I think you’ll be amazed at how significant an LTA can be and still pass an assessment.

A Few Case Studies

To illustrate the point, I analyzed a number of LTA cases. I performed a series of calculations in which I’ve varied several parameters including ID, thickness, d/T, and the remaining thickness ratio, Rt. For each case, I’m going to look at the effect of the variables on the remaining strength factor (RSF).

This content is free for registered users

Register today to read this article for free.

Create your free account and you'll also get access to:

  • Unlock one premium article of your choosing per month
  • Exclusive online content, videos, and downloads
  • Insightful and actionable webinars
Interested in unlimited access? VIEW OUR SUBSCRIPTION OPTIONS

Current subscribers and registered users can log in now.

Comments and Discussion

Posted by Rakshit BHATT on May 3, 2023
Very simple, thoughtful, to-the-point discussion.... Log in or register to read the rest of this comment.

Posted by Orlando Nieto on May 30, 2023
For operating units corrosion is an important... Log in or register to read the rest of this comment.

Add a Comment

Please log in or register to participate in comments and discussions.

Inspectioneering Journal

Explore over 20 years of articles written by our team of subject matter experts.

Company Directory

Find relevant products, services, and technologies.

Training Solutions

Improve your skills in key mechanical integrity subjects.

Case Studies

Learn from the experience of others in the industry.


Inspectioneering's index of mechanical integrity topics – built by you.

Industry News

Stay up-to-date with the latest inspection and asset integrity management news.


Read short articles and insights authored by industry experts.

Expert Interviews

Inspectioneering's archive of interviews with industry subject matter experts.

Event Calendar

Find upcoming conferences, training sessions, online events, and more.


Downloadable eBooks, Asset Intelligence Reports, checklists, white papers, and more.

Videos & Webinars

Watch educational and informative videos directly related to your profession.


Commonly used asset integrity management and inspection acronyms.