Oswell et al. 324 Although the authors have presented an interesting and compelling paper on one of the risks associated with the construction and operation of northern pipelines, the discussers would like readers to consider several aspects of the paper. Several points of the discussion are presented: (1) Chilled pipelines that promote frost bulb growth and potentially heave beneath the pipe also contribute to strength improvement of the frozen backfill above and around the pipe. This improvement in backfill resistance and enlargement of the frozen soil mass may not have been adequately included by the authors in the assessment of resistance to upheaval. Summer operating conditions (thawed ground surface) may be a critical time for pipe upheaval in frost heave impacted areas. Longitudinal restraint to uplift is also provided by backfill adhesion to the pipe adjacent to the uplifting section. (2) It is considered that other factors besides frost heave are more likely to initiate upheaval and buckling, such as the design and control of operating temperatures and construction practices used to install the pipe. Pipes may have extra length or “slack” and associated compressive stresses after construction. Compressive stress already exists in the pipe, and any upheaval redistributes this stress. Hence, upheaval might actually improve pipe conditions through redistribution of stress and changes to pipe curvature. These factors are readily mitigated through design, construction, and operating methods and protocols. (3) Although it is dramatic when a pipe daylights above the ground surface, it does not always lead to a critical condition in terms of pipe strain or integrity, does not necessarily hamper pipe operation, and does not automatically becomes a major safety issue. Readers not familiar with the context of the technical issues being discussed and historical performance of North American pipelines may interpret the overall tone of the paper with undue alarm and pessimism. There are only two gas pipelines operating in the continuous permafrost region of North America (Trans-Alaska Oil Pipeline fuel–gas pipeline and Ikhil gas pipeline). There are numerous gas pipelines (laterals, gathering, and export), however, that have been constructed and operated in northern Alberta, British Columbia, and the southern Northwest Territories where significant seasonal frost penetration is experienced every winter. Under these conditions, seasonal frost heaving could be a very active process. The fact is, there are very occasional instances of upheaval buckling, and even those that the discussers are aware of have occurred in summer. The authors acknowledge that upheaval buckling occurs in nonpermafrost regions. Hence pipe upheaval, as an initiator of pipe buckling, is often unrelated to frost heave. Indeed, the discussers consider frost heave as one of many potential initiating mechanisms of upheaval buckling, but most importantly it is likely not the most critical or the most prevalent factor contributing to upheaval. Further, if other potential initiation mechanisms (e.g., design and control of pipe operating temperatures and proper construction practices) are correctly addressed in the design and construction, the likelihood of frost heave acting to initiate upheaval is small. Another mechanism is underestimation of the temperature differential (delta T) that the pipe will be subjected to during operations. In some cases, this underestimation is due to unexpectedly colder temperatures during burial, and in others it is due to a change in operating conditions of the pipeline. An example of this latter situation has been documented by Nixon and Burgess (1999) concerning the performance of the Norman Wells oil pipeline operated by Enbridge Pipelines (NW) Inc. In this specific case, the original design delta T was about 35 °C, but subsequent to the initiation of operations the summertime inlet temperature was increased by nearly 10 °C. This resulted in an applied delta T of 45 °C. The resulting additional thermal expansion, combined with some localized frost heaving, created a condition where the 324 mm diameter pipeline uplifted by over 1 m. In this case it is considered that the additional thermal expansion caused the uplifting, not frost heave, as the frost heaving had been present or developing for nearly a decade without initiating any upheaval prior to the change in temperature regime. The placement of rock fill over the uplifted section
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Aug 4, 2006
Mathieu Gelhaye + 3
Open Access