Numerical Pipe/Soil Interaction Modelling: Sensitivity Study and Extension to Ice Gouging

Abstract In predicting the geotechnical constraint against pipeline movement usingfinite element methods, the treatment of the pipe/soil interface contactbehavior is of utmost importance, especially in the tangential direction. Thisstudy focuses on the interpretation of soil resistance to axial pipe movementin cohesive soil material for oblique loading, specifically the effect ofchanging the interface shear stress limit and friction coefficient. The mainfinding of the present study is that the incorporation of a shear stress limitin the definition of tangential shear behavior has a considerable effect on theaxial pipeline reaction forces. Without the shear stress limit, the maximumaxial forces due to oblique pipe movement are effectively doubled in comparisonto a limit equal to half of the undrained shear strength. A simple analyticalmethod is provided to estimate the maximum oblique axial soil resistance inundrained conditions. The effect of changing the assumed frictional behavior isalso discussed with respect to predicting the soil reaction forces acting on anice keel during an undrained gouging event in cohesive soil. Introduction Pipeline systems, that are surficial or buried, may be subject to obliquegeotechnical loads as a result of large deformation geohazards, pipelineout-of-straightness and uneven seabed terrain. The geotechnical restraintagainst oblique pipe movement is a function of soil type and strength, burialdepth, oblique displacement angle, contact mechanics, and soil strainlocalization among other parameters. Properly accounting for the mechanicalbehavior at the pipe/soil contact interface is a critical aspect in estimatingthe forces exerted on the pipe during oblique displacement. In the context ofice keel/soil/pipeline interaction, it is equally important to capture thecorrect behavior at the ice keel/soil interface, as this will affect thecharacterization of the geotechnical loads imposed on the pipeline system. Further complexity is introduced as the pipeline mechanical response isdependent on the ice keel bearing pressure, subgouge soil deformation and axialpipeline feed-in. This study focuses on pipeline/soil interaction inthree-dimensional continuum form to examine the effect that the definition ofinterface contact properties has on axial interaction forces; the effect onlateral interaction forces has been previously studied (Pike and Kenny, 2011b,2012a). In this study, the Arbitrary Lagrangian Eulerian (ALE) method(Abaqus/Explicit) is used to study oblique lateral-axial pipe/soil interaction. The coupled Eulerian Lagrangian (CEL) method is used to simulate ice keel/soilinteraction. The geotechnical constraint in each case is discussed with respectto the applied contact interface properties. This paper presents some preliminary analysis suggesting that there isuncertainty in predicting the axial geotechnical restraint against obliquelateral-axial pipe movement. The results show that the inclusion of a shearstress limit at the pipe/soil interface can reduce the interaction forcessignificantly. Further work is necessary to calibrate and validate thenumerical modeling procedures. A physical testing program will be carried outto assess relatively simple pipe/soil interaction scenarios to examine contactmechanics and interface behavior in greater detail. The outcomes from thislaboratory testing and physical modelling program will be used to assess andadvance the capabilities of advanced numerical methods such as ALE to accountfor contact mechanics, strain localization and soil failure mechanisms.

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