As part of pipeline integrity management, there were on average more than 2900 integrity digs per year in Canada during the decade of 2008–2018, according to the Canadian Energy Pipeline Association. During a pipeline excavation, additional pipe stress and deflection can be produced due to altered soil support beneath the exposed pipe, which might bring in additional integrity concerns. Classical beam theories and soil-spring modeling are inadequate for the complex pipe-soil interactions and boundary conditions. The objective of the present study was to develop a computational model that can be used to predict pipe stress and deflection during an axial strain-relief excavation.

The present study was motivated by a recent 40-m NPS 24 pipe strain-relief dig in Alberta, which occurred at a farm land underneath a foreign pipeline crossing. Within the vicinity of crossing location, the peak strain event of 0.16% is reported and suspected to increase the likelihood for occurrence of CSCC. Until the progressing excavation to 12 m, the deflection for exposed section continued to increase due to loss of soil support beneath while the overburden was removed. Eventually, the pipe deflection was reduced at 24-m excavation after abnormal soil constraints were eliminated.

The pipe-soil interaction was modeled with 3D finite elements using surface-to-surface contact approximation in ABAQUS. The pipe was assumed to be partially exposed with a minimum 20-m buried length on either end to mimic a buried pipeline under excavation. Both static and soil consolidation analyses were performed. Surprisingly, theoretically when the soil support below the buried section was assumed to be 3 m deep, the calculated pipe deflection was one order of magnitude smaller than the measured, even relatively a small soil modulus of 15 MPa was used. The deflection was reduced to 50% if the soil modulus was 50 MPa. However, the calculated deflection increases drastically with the increased soil depth. For the pipe with 12-m exposed length, the model predicted a maximum deflection of 62 mm, comparing to 120 mm in measurement, when 8.4-m deep soil support was considered, which was reasonable for the farmland.

In conclusion, the depth of soil beneath is a dominant factor in predicting the pipe stress and deflection during an integrity excavation. Furthermore, soil consolidation could increase the pipe deflection by 20-40% in 24 hours, which is also important because an excavation takes a couple of days. The model may be used to estimate the pipe stress and deflection prior to an integrity dig based on the soil type and depth.

Keywords: Pipe-soil interaction, soil consolidation, finite element modeling, measurement, intergrity dig


Pipe Stress and Deflection During an Integrity Dig

Division / Sponsors: 
Pipeline Systems Division

Paper Proceedings: https://asmedigitalcollection.asme.org/IPC/IPC2020/volume/84447

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