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

Introduction

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, R_t. For each case, I’m going to look at the effect of the variables on the remaining strength factor (RSF).