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Engineering Critical Assessment Draft
Engineering Critical Assessment (ECA) is a fitness-for-service (FFS) procedure by which the acceptable flaw size of a welded structure can be determined. ECAs take into account the material properties and expected stress history of the weld in order to determine a flaw acceptance criteria which will ensure that welds will not fail during the construction or service life of the welded structure. ECAs are completed on structures in a wide variety of situations, including throughout the energy, manufacturing, and infrastructure industries. ECAs are based heavily on fracture mechanics principles, and offer various improvements over traditional methods of weld quality assurance.

During welding, defects or flaws can develop. In some cases, these flaws could potentially affect the integrity of the weld, resulting in failure by fatigue, creep, brittle fracture, or yielding. Therefore, codes to determine weld quality must be developed. Traditionally, welding codes have been based off workmanship criteria. These criteria are determined empirically, typically by estimating the level of weld quality expected from a skilled welder. While these criteria have been reliable historically, improvements over time in welding technology and material properties are not considered in these criteria. As a result, over time, workmanship criteria become increasingly conservative. This conservatism results in unnecessary repairs, which increase construction costs and can lead to undesirable residual stresses at the location of the repair weld.

Beginning in the late 1970s to early 1980s, engineering critical assessments began to emerge as an alternative to traditional workmanship criteria. These ECAs relied heavily on recent developments in the field of fracture mechanics. Where workmanship criteria were developed with a limited understanding of material characteristics, and considered only the length of a given weld flaw, these ECA-based flaw acceptance criteria considered additional factors such as: Together, these additional factors allow for more generous flaw acceptance criteria, resulting in fewer unnecessarily repaired welds.
 * the stress history of the weld, including all cyclic and static stresses expected throughout the life cycle of the welded structure,
 * the strength of the base metal and weld material ,
 * the fracture toughness (measured via Crack tip opening displacement testing or Charpy impact testing of the base metal and the weld material, and
 * the flaw orientation (e.g., measurement of flaw length and flaw height, as opposed to flaw length only).

In order to comply with the flaw acceptance criteria developed during ECA, some sort of non-destructive examination must be utilized. Ultrasonic testing is typically used due to its high accuracy, ability to identify flaw size and orientation, and its ability to provide feedback instantly.

ECA for Oil & Gas Pipelines

ECA-based flaw acceptance criteria are particularly well-suited to the oil & gas industry, especially when used to qualify girth welds of cross-country oil & gas transmission pipelines. This is because the girth welds are typically uniform from weld to weld, allowing construction contractors to utilize mechanized welding, which increases productivity over manual welding techniques. Mechanized welding allows for better, more uniform control over weld characteristics than manual welding. ECA-based flaw acceptance criteria are able to take advantage of the high strength and toughness of the welds produced to develop more generous flaw acceptance criteria. In addition, the flaws created during mechanized welding are different from those developed from manual welding, necessitating the need for flaw acceptance criteria considering flaw orientation, rather than just flaw length.

Several standards specific to ECAs for oil & gas pipelines have been developed. The most commonly used of these are API 1104 Appendix A, CSA Z662 Annex K, and BS 7910. The standard used depends primarily on the location of the pipeline being installed. The standards differ in methodology and can result in significantly different flaw acceptance criteria. However, due to the high strength and toughness of pipeline steels, the criteria are sufficiently generous that practical differences between the standards are relatively small.

Potential Additions

Maybe talk about ECA case study

Maybe talk in further depth about standards (API 1104, CSA Z662, BS 7910)

Possibly talk briefly about stress analysis for ECA (but more likely just slight expansion of already created section).