FFS Forum: Assessment of Dents and Gouges Using API 579, Part 12

Introduction

Level 1 and 2 assessments under Part 12 are pretty simple. They involve checking a few simple calculations relating primarily to the depth of a dent or material toughness of a gouge. Under Level 2, there is also a simple and very useful fatigue calculation for systems in cyclic service. I’m going to discuss these assessment methods, but I’m also going to discuss the many limitations, exclusions, particular vocabulary, and more subtle points that one may not readily catch on the first reading.

A quick note - in case you’re not already aware, API 579 uses the nomenclature “Parts” instead of “Chapters.” So, for example, when I reference Part 12, this refers to what in other contexts might be the “Chapter” on dents and gouges.

An API 579 Vocabulary Lesson

A dent is a certain type of distortion in the shell. However, there are several different types of shell distortions addressed in API 579. Other than Part 12 (the subject of this article), most other types of shell distortions are found in Part 8, “Assessment of Weld Misalignment and Shell Distortion.” Part 8 includes discussion of bulges, general shell distortions, and out-of-round.

Let’s start by getting the vocabulary straight and making sure we understand the difference between these various damage mechanisms.

What is a “Dent”?

A dent is:

“An inward deviation of a cross-section of a shell member from an ideal shell geometry that is characterized by a small local radius or notch.” [from API 579 Figure 12.2 and §12.2.1(a)]

Dents compare to other forms of shell distortions as follows:

• Bulges
• An outward deviation of the shell
• Characterized by a local radius and angular extent
• Whereas “dent” was defined as a “small local radius or notch”
• Bulge geometry may be cylindrical or spherical in its general shape
• General shell distortions
• a more generalized deformation than a simple bulge
• characterized by “multiple local curvatures” that may be in the longitudinal or circumferential direction (or a combination thereof)
• Out-of-Round (OOR)
• The cross-section is not round, and the OOR is constant for a distance along the length of the cylinder
• The OOR can be just an ovalization of the cross-section or may be more arbitrary and random in the circumferential direction

A “kinked dent” is a dent in which the “…local radius of curvature (in any direction) of the sharpest part of the dent is less than or equal to five times the wall [thickness].” Figure 1 illustrates the difference between a regular dent and a “kinked” dent. Kinked dents cannot be analyzed using Level 1 or 2 analysis (discussed more below).