FFS Forum: Old Vessels vs New NDE

Introduction

I’ve written in the past about the problems associated with inspecting older pressure equipment with current inspection technology (see FFS Forum, November/December 2018, “The Problem with Inspection of Older Vessels”). The fundamental problem is that the more sophisticated inspection methods available today typically reveal flaws that were not detected (or detectable) at the time of fabrication 30+ years ago. While that may sound like a good thing, it’s not necessarily so.

My interest in this article was prompted by a recent conversation with a client. There were no indications of damage or flaws in the pressure vessel being discussed, but work on a fracture-mechanics-based minimum pressurization temperature (MPT) curve raised the possibility of reducing the assumed initial flaw size (i.e., the reference flaw) by performing more detailed ultrasonic inspection of the long and girth seams. Reducing the initial flaw size below the API 579 reference flaw would require a new weld inspection to validate the new, lower flaw size assumption. This inspection would likely be a phased array ultrasonic (PAUT) inspection or similar.

My recommendation was that global inspection of the vessel weld seams should be considered only as a last resort. My reasoning was that this usually results in numerous indications and opens Pandora’s box of fitness-for-service evaluations. At which point someone asked:

“So, we shouldn’t look because we might find something?”

That’s an entirely reasonable question and, of course, “because we might find something” would be a terribly irresponsible reason not to inspect.

Thus, in this edition of Fitness-for-Service Forum, I want to discuss the rationale for being cautious when inspecting older vessels with modern NDE technologies and how to decide when such inspection is appropriate.

Let’s start by looking at an analogy from the medical profession.