Stress Engineering: Fault Rupture – Pipeline Finite Element Analysis

Failure in pipelines has catastrophic economic and environmental impacts and can even be deadly. Buried energy pipelines in seismically active regions are especially vulnerable to failures due to permanent ground displacements resulting from fault movements. An active fault can generate large displacements across these regions generating high strain demand on the pipeline material. Stress Engineering Services Canada (SESC) specializes in the analysis of these difficult problems and can assist operators by completing detailed finite element analysis of the pipe-soil interaction.

Finite Element Modelling

Classic analytical methods to analyse loading in pipelines due to fault displacement lack the necessary accuracy. Simplified computer techniques such as 2D pipe elements supported by bi-linear soil springs cannot capture potential failure modes such as pipe cross-sectional ovalization or local buckling.  To capture the true nature of the soil-structure interaction at fault crossings, three-dimensional finite element analyses is required and provides the most comprehensive results.

The generated computer model is only as accurate as the available information and requires a geotechnical investigation and site-specific soil mapping.  The resulting model uses a Mohr-Coulomb yield criterion and defines the shear strength at a plane as a function of normal stress and soil cohesion.

To simulate a worst-case scenario, pipe ends at the fault-crossing are assumed to be fully constrained.  The moving side of the fault is iteratively displaced to the fault displacement values based on historical displacement records and design life.    The resulting analysis incorporates the response of the supporting soil as well as the operating pipeline.  A typical displacement plot is shown in Figure 1.0.

Figure 1 Displaced Pipeline Across a Fault Line

The resulting pipe condition can be obtained from the model and the expected pipe ovality, buckling, and strains can be further analyzed to ensure survival of the line.  This work can also include an engineering critical assessment to confirm integrity of the girth welds.

Figure 2 Material Strain Across the Fault

 

 

For more information about Stress Engineering, please visit: www.stress.com

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