The Industry Challenge:
Gas pipelines in Arctic regions pass through areas of continuous and discontinuous permafrost. The segments where the pipeline is buried in discontinuous permafrost are subject to freeze-thaw cycles that can cause uplift on the pipeline.
Where the ground transitions from discontinuous to permanently frozen ground, the uplift forces can cause severe bending loads that can exceed the yield strength of the pipe, resulting in permanent deformations.
If the pipe yields, the conventional stress-based design approach which aims to limit the stress to some fraction of the yield strength of the pipe, is not possible.
A strain-based design approach is required which has the goal of allowing some plastic deformation of the pipe without losing pressure integrity or seriously compromising the serviceability of the pipe due to large deformations.
How We Help:
C-FER uses a strain-based design approach which compares the strain demand imposed by the ground movement with the strain capacity of the pipe.
This requires detailed pipe-soil interaction models to evaluate the strain demand along the full pipe route while considering factors such as: soil conditions, severity of the freeze-thaw cycles and pipe geometry.
The compressive strain capacity is determined through a series of full-scale biaxial bend tests. The tensile strain capacity is determined through a series of curved wide plate and full-scale biaxial tension tests.
For large diameter pipelines, these full-scale tests require the construction of very large loading apparatuses with capacities as high as 72 MN (20 million pounds), as well as safety systems to ensure the tests are conducted safely.
Test results are used to qualify pipe configurations and weld procedures to determine critical crack sizes that are likely to result in rupture of the specimen when subjected to large tensile strain.