Velocity-Dependent Soil Resistance in Finite Element Analysis of Pipeline Walking

Abstract

Soil resistance to pipeline axial displacement plays a key role in the ratcheting process known as “pipeline walking.” Still, it is not yet fully understood. New frameworks to address the different geotechnical aspects involved have recently been published. However, the current practice has been to lump all the time-dependent effects back into a single “equivalent” friction factor, based on a representative pipeline velocity. This paper argues that defining a single velocity as representative of the pipeline expansion (or contraction) is not trivial. While the pipeline ends might move a couple of meters in the few hours it takes to heat up, somewhere close to the middle it will move a few millimeters only. As a result, different levels of soil drainage, for example, are observed along the same pipeline, during the same loading. This paper presents the results of “true” velocity-dependent pipeline walking analyses and compares them to those obtained using constant equivalent friction factors. For the particular cases analyzed, the difference between the results obtained with the two approaches ranged from negligible up to about 30%. Examples show that the results of velocity-dependent pipeline walking analyses are significantly influenced by how the temperature changes over time along the pipeline length. The velocity-dependent model employed describes the axial soil resistance as a hyperbolic function of the pipe velocity. Additional aspects which are expected to influence the soil response (e.g., consolidation time between movements, progressive compression, and consolidation hardening) have been neglected.