*A client operates a buried hot bitumen (hotbit) pipeline in northern Alberta and engaged Worley to provide detailed engineering for the replacement of a buried mainline block valve that had failed open.  The hotbit pipeline is NPS 24 Grade 483, designed to CSA Z245.1 with a maximum design temperature of 149^0^C.  This paper presents unique aspects of the valve replacement design such as its compact layout to meet restrictive space requirements of the right-of-way, and mechanical tensioning of the buried pipeline to return it to its original prestressed condition.*

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*An important concept in its original design was that it be “fully restrained” by the surrounding soil so that thermal expansion forces caused by high operating temperatures would be held in check by the restraining strength of the soil.  Because it is fully restrained, its high operating temperature causes large axial compressive stresses which pose a risk of upheaval buckling (UHB) and possible loss of containment if the backfill over the pipe is removed. To control the risk of UHB, the hotbit line was preheated to 90^0^C during original construction and allowed to expand freely prior to being backfilled and locked into the surrounding ground in its expanded state. * *When the pipeline was cut to install the replacement valve, the original prestress was released, and the cut ends of the pipe pulled back on either side of the valve. The lost prestress had to be reinstated to the same level it had when the pipeline was originally constructed so that when the pipeline was returned to hotbit service at 149°C, axial compressive stresses would not exceed the original design values.*

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* Because the valve may have failed open due to the large axial compressive stresses applied on it by the buried pipeline, an early design decision was to replace the failed valve with an aboveground valve and aboveground piping with sufficient flexibility to keep axial loads on the valve well within acceptable limits.  The design for the aboveground valve replacement featured a structural belowground component referred to as a “pipe prop” which connected* *the cut ends of the buried pipeline on either side of the valve.* *The purpose of the* *pipe prop was twofold.  Firstly, it included a unique bolt tensioning system to restore the original level of prestress in the buried portions of the pipeline; and secondly it prevents movement of the belowground pipeline due to changes in pressure and temperature, and therefore significantly reduces the flexibility required in the aboveground piping to accommodate such movements.  This permitted the use of a short offset distance in the aboveground piping which in turn reduced the footprint of the aboveground piping, making it small enough to fit within the restrictive boundaries of the site.*

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*Strain gauges were installed on the aboveground piping and on the buried piping upstream and downstream of the valve so that the changes in stress when the buried pipeline was first cut could be measured and the stress state of the buried pipeline could be calculated to determine if axial loads applied to the valve by the buried pipeline may have caused it to fail open.  The strain gauges were also used to measure the tension in the buried pipeline while it was being prestressed back to its original value by the bolt tensioning system built into the pipe prop.  The measurements confirmed that the tensioning was successful.  The aboveground piping was then cold sprung at ambient atmospheric temperature to keep stresses in the arms of the aboveground piping loop within the limits allowed by the Canadian Pipeline Standard CSA Z662.  The Hotbit pipeline was returned to hotbit service in August 2019 and has operated continuously since that time without further problems at the valve station.*


Hot Bitumen Pipeline Valve Replacement: Pipe Prop Anchoring Design With Mechanical Tensioning

Division / Sponsors: 
Pipeline Systems Division

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

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