Strain Based Design Versus Preheat for Hotbit Pipelines

The first buried hot bitumen (hotbit) pipeline is now operating successfully in the Alberta oil sands north of Fort McMurray and more are on the way. These hotbit pipelines are designed to transport raw, undiluted bitumen to a central refining plant at temperatures up to 140°C. They are constructed in winter when the ground is frozen allowing heavy construction equipment to travel across the watery muskeg terrain without sinking. Construction continues even when atmospheric temperatures fall as low as −30°C. Hotbit pipelines are buried with more than 1.2 m of cover, which can prevent them from expanding when they are heated from their locked-in installation temperature to their operating temperature of 140°C. Large longitudinal compressive stresses induced by this restrained thermal expansion combined with high hoop tensile stresses due to internal pressure produce stresses in the pipe wall that exceed the maximum allowable combined stress of 90%SMYS specified in North American pipeline codes (ASME B31.4 and CSA Z662). Two methods are available to handle these high combined stresses in hotbit pipelines. The first method is to expand the pipe during construction by preheating it to a temperature of approximately 90°C and then locking in the expansion by backfilling the pipeline trench before the pipe has had a chance to cool. By limiting the positive temperature differential between installation and operation to approximately 50°C, this method keeps thermally induced axial compressive stresses low enough that the combined stress does not exceed the allowable limit of 90%SMYS specified by pipeline codes. In the second method, the pipeline is still constructed in winter but without preheating. Temperature differentials and thermally induced axial compressive forces are much higher than in the first method and carefully engineered restraint is require to prevent the pipe from failing by pushing out of the ground at bends or by either lateral or upheaval buckling of long straight sections in muskeg swamps and bogs. This method requires strain-based design principles to show that, when the pipeline is first heated to its operating temperature, large thermally induced compressive stresses in the pipe wall are acceptable because they dissipate without causing failure when the pipe steel yields. Both methods are technically acceptable but require specialized pipeline engineering skills to implement them successfully. The first method incurs the cost of preheating and increased construction costs due to reduced pipe lay rates while the second method incurs the cost of more robust restraint systems, particularly at bends. Details of both methods are presented and discussed to determine which of the two methods has the least cost and the least risk.

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