FFS Forum: Dealing with Multiple Damage Mechanisms in an FFS Assessment

What’s the Problem?

Assessment procedures in API 579 are organized by damage mechanism. For example, Part 3 is about brittle fracture, and Part 10 is about creep damage. Generally, each part deals with one type of flaw and/or damage mechanism.

Each Part has some underlying methodology or approach to the problem. For example, crack-like flaw assessment under Part 9 is fracture mechanics based, whereas local metal loss assessment under Part 5 uses a thickness averaging approach. Notice that these two methodologies are not variations on the same method; they are completely different methods based on largely unrelated engineering principals.

While a methodology gives us an approach to evaluating a damage mechanism, we also need an acceptance criterion to help us decide what’s acceptable. For example, our methodology for evaluating the axial load in a rod may be to calculate the stress using a simple load divided by area formula, σ = P/A. But we still need an acceptance criterion to tell us what stress is acceptable.

The problem is further complicated by the fact that a single Part may use different criteria for Levels 1, 2, and 3. For example, Part 8, Weld Misalignment and Shell Distortions, uses a remaining strength factor (RSF) based acceptance criteria for Level 2, but for Level 1 is uses the simple 1% ovality rule from Section VIII, Division 1 of the ASME Boiler and Pressure Vessel Code.

When different damage mechanisms are in play, the evaluation criteria may not be compatible. The evaluation criteria for a crack-like flaw is based on the failure assessment diagram (FAD), whereas the criteria for local metal loss is based on the Code minimum required thickness. You can’t easily combine or compare these dissimilar criteria. It’s like asking: “What is the sum of red + chicken?” There just isn’t a good answer.

Reviewing API 579, you’ll find that several Parts provide guidance for a few combinations of specific damage types, but this guidance doesn’t address all of the possible combinations.

Criteria for Damage Evaluation

The fact that different Parts have different methodologies and evaluation criteria significantly complicates the evaluation of multiple damage mechanisms. So, it’s probably worth the effort to review the evaluation criteria for the various Parts. For simplicity, I’m going to limit my discussion to criteria used in Level 2 assessments.

Looking at all of the Parts, you’ll find a number of different evaluation criteria. They are: