FFS Forum: What is a Major Structural Discontinuity?

Every part of API 579 has rules addressing the case where a defect is too close to a “Major Structural Discontinuity” (MSD). But I’ve noticed in the API 579 training I’ve conducted for the last 15+ years, that most of the people in the course are unclear on exactly what constitutes an MSD. In this column, I’ll discuss the MSD: what an MSD is, and why it is so important in fitness-for-service (FFS) assessments.

Let’s Start with a Few Definitions

API 579-1 (2016), paragraph 1A.55 says:

“Major Structural Discontinuity - A source of stress or strain intensification that affects a relatively large portion of a structure and has a significant effect on the overall stress or strain pattern of the structure as a whole.”

The ASME Boiler & Pressure Vessel Code (BPVC), Section VIII, Division 2 uses the exact same definition, except that it calls them “Gross” structural discontinuities, instead of “Major” structural discontinuities. We know that “Gross” and “Major” structural discontinuities are the same because API 579-1 provides another definition that clarifies that point:

API 579-1 (2016), paragraph 1A.39 says:

“Gross Structural Discontinuity – Another name for a Major Structural Discontinuity.”

A slightly different definition is provided by the nuclear code, BPVC, Section III, Division 1, “Rules for Construction of Nuclear Facilities.”

Section III, Division 1, paragraph NB-3213.2 says:

“Gross structural discontinuity is a geometric or material discontinuity which affects the stress or strain distribution through the entire wall thickness of the pressure retaining member. Gross discontinuity type stresses are those portions of the actual stress distribution that produce net bending and membrane force resultants when integrated through the wall thickness.”

These definitions of “Major” or “Gross” structural discontinuities can be compared to the definition of “Local” structural discontinuities (LSDs) defined in Section VIII, Division 2 and API 579.

API 579-1 (2016) & BPVC Section VIII, Division 1 have identical wording and say:

“Local Structural Discontinuity – A source of stress or strain intensification that affects a relatively small volume of material and does not have a significant effect on the overall stress or strain pattern, or the structure as a whole.”

Comparing the definitions of “Major” or “Gross” to “Local” structural discontinuities, the main difference is that MSDs are defined as affecting a “relatively large” portion of the vessel, whereas LSDs affect a “relatively small” portion of the vessel.

Personally, I find this definition a bit too imprecise for engineering purposes. I prefer to think of this in terms of the definition from Section III which says the MSD “…affects the stress or strain distribution through the entire wall thickness.” That’s easier to think about, and quantify.