Understanding the Key Differences Between Folias Factor and Bulging Factor in Pipeline Integrity Assessments

In pipeline integrity assessments, both the Folias factor and the bulging factor are used to evaluate the impact of corrosion defects on the pipeline's strength, but they differ in their approach and complexity.

Folias Factor:

Purpose: The Folias factor is a correction factor used in calculations to account for the influence of a longitudinal surface flaw (such as a crack or corrosion defect) on the overall strength of a pressurized pipe.

Origin: It comes from a formula developed by A. Folias in the 1960s, used to account for the increased stress concentration around a defect in a cylindrical shell (like a pipeline) under internal pressure.

Application: This factor is included in simplified assessment methods such as the original and modified ASME B31G guidelines for evaluating the remaining strength of pipelines with corrosion defects.

Assumption: The Folias factor assumes small or moderate deformation of the pipe wall due to defects and does not account for more severe bulging that might occur in cases of large defects or deep corrosion.

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Bulging Factor:

Purpose: The bulging factor is a more advanced, refined correction factor that accounts for large deformations and non-linear behavior of the pipe wall around a defect, especially when the defect causes significant local bulging of the pipe wall under pressure.

Origin: The bulging factor was developed using non-linear, large deformation finite element analysis to provide a more accurate representation of the pipeline's structural response to larger, more severe defects.

Application: The bulging factor is used in more modern defect assessment methods, such as those developed by BG Technology and Det Norske Veritas (DNV), as a replacement for the Folias factor in specific cases of smooth corrosion defects and more complex loading conditions (e.g., bi-axial loading).

Assumption: Unlike the Folias factor, the bulging factor accounts for significant deformations that occur around larger defects, providing a more precise measure of the pipeline’s ability to withstand pressure.

Key Differences:

• Applicability: Folias factor: Suitable for smaller, simpler defects and linear elastic behavior of the pipeline wall. Bulging factor: Used for more severe or large defects that result in significant bulging, requiring non-linear analysis to accurately assess the wall deformation.
• Approach: Folias factor: Based on linear elastic theory and simpler analytical methods, appropriate for small to moderate defects. Bulging factor: Developed through advanced non-linear finite element analysis, which can model the actual deformation behavior of the pipe wall under large defects.
• Accuracy: Folias factor: Provides a conservative estimate of the pipeline’s strength with defects but may be less accurate for large defects. Bulging factor: Provides a more accurate and less conservative assessment for large or severe defects, as it accounts for real deformation behaviors.

In summary, while the Folias factor provides a simplified approach to assessing the effect of defects on pipeline strength, the bulging factor offers a more advanced and precise method for assessing defects that cause significant wall deformation, especially under high-pressure conditions.