buried pipes stress analysis by Meena Rezkallah, P.Eng.
In every process industry, there is a significant part of the piping system which runs underground. Oil and gas pipelines are buried in the ground for 360-degree protection and support. However, buried or underground piping is generally used to carry fluids for long miles.
These buried pipelines experience a significant load because of relative ground displacements along their length. Piping stress analysis offers a helping hand for underground piping to address the static as well as dynamic loading, which results from the temperature changes, effects of gravity, internal and external pressures. This pipe stress analysis ensures the safety of piping, piping components, connected equipment and supporting structure.
The analysis of a buried pipeline is entirely different from plant piping stress analysis. The unique characteristics of this kind of pipeline require some unique problems involvement like code requirements and techniques in the underground pipeline stress analysis. However, the analysis elements include anchorage force, pipe movements, lateral soil force, soil friction, and soil pipe interaction. Piping flexibility analysis, as well as the stress analysis for the underground piping system, is carried out using CAESAR II software. It provides adequate flexibility to absorb thermal expansion, displacement incurred in the buried piping system, and code compliance for stresses.
It is essential to distinguish the pipeline from plant piping to acknowledge the pipe code requirements and envision the problems involved in the buried pipe stress analysis.
Here we are listing some of the unique characteristics a pipeline possesses:
- High Allowable Stress: A pipe has a circular shape and often runs several miles before turning. Hence, the calculation of stress is through simple static equilibrium formulas, considered as the most reliable ones. As the produced stresses are predictable, the permissible stress for the plant piping is considerably higher.
- High Yield Strength Pipe: The first obstacle is to raise the allowable yield strength. Although the pipeline that operates above the yield strength does not create the problems of structural integrity, it can cause undesirable deformation and chances of strain follow up. Therefore, for the construction of pipelines, a high-test line with the very high ratio of yield to ultimate strength is used. All the allowable stresses have yield strength as the only basis.
- High-pressure Elongation: The movement of a pipeline is generally because a very long line’s expansion has a low-temperature difference. The pressure elongation is negligible in a plant piping, contributing much to the total movement and hence should comprise the pipe stress analysis.
- Soil-pipe Interaction: As a huge part of a pipeline is buried underground, any movement in the pipe has to overcome the soil force, generally divided into two categories:
(i) Friction Force Created by Sliding, and
(ii) Pressure Force Resulting from Pushing. The most crucial task of underground pipe stress analysis is to investigate the soil-pipe interaction, which does not matter in the plant piping analysis.
The analytical study of a buried piping system is mostly done using ASME 31.1, ASME B31.3, ASME B31.4, ASME B31.8, CSA Z662, the piping and pipelines codes whereas the CAESAR II platform helps in modeling and analyzing the piping system. Modified layouts can be then checked through CAESAR II for the same operating and environmental conditions to select the most optimized piping system layout.