Axial Soil Stiffness for Deepwater Pipeline Walking Analysis-Paper

Paper

Pdf Summary

The paper examines how axial pipe–soil stiffness, particularly the displacement needed to mobilize full soil resistance, affects predictions of deepwater pipeline walking. Walking is the gradual movement of a pipeline caused by repeated start-up and shut-in thermal and pressure cycles. Although current practice often uses a simple bilinear soil-friction model, its mobilization displacement can substantially influence predicted walking rates.<br /><br />The authors develop a nonlinear analytical model for axial pipe–soil load–displacement behavior in clay, extending earlier elastic solutions to account for nonlinear soil stress–strain response. They compare its predictions with laboratory and in-situ measurements and existing guidance, finding significant discrepancies. Mobilization displacement appears sensitive to factors including drainage, pipe movement velocity, cyclic history, and soil consolidation.<br /><br />To translate nonlinear responses into the bilinear model used in structural analyses, the paper proposes an energy-based method: the equivalent bilinear response is selected to absorb the same energy as the nonlinear response. Applying this method to 53 cases from analytical predictions and test data produced mobilization displacements ranging from 1.3% to 21% of the pipe’s outer diameter. The median was 6.2%; the 5th and 95th percentiles were 1.4% and 20%, respectively.<br /><br />The authors recommend using 6.2% of the outer diameter as the best estimate when project-specific assessments are unavailable, with approximately 1.3% and 20% as low and high sensitivity values. They advise using the energy-based method when suitable nonlinear response data exist. Further testing and research are needed to clarify the effects of drainage, consolidation, and cyclic loading, and to improve the statistical basis for design recommendations.

Keywords

deepwater pipeline walking

axial pipe-soil stiffness

mobilization displacement

bilinear soil-friction model

nonlinear analytical model

clay pipe-soil behavior

thermal pressure cycles

energy-based method

equivalent bilinear response

pipeline design recommendations