An NPS 24 inline gate valve on a buried hot bitumen pipeline operating at temperatures up to 149°C failed open. The valve is on the north bank of the Steepbank River in northern Alberta and is equipped with an actuator that can automatically close the valve to protect the river in case of an emergency. It was therefore important to replace the valve as soon as practical.
Worley was engaged to provide detailed engineering services for replacing the valve. Engineering objectives covered safety concerns associated with high operating temperatures and large axial compressive force in the pipeline, minimization of downtime, development of the best long-term valve replacement solution, and return of the pipeline to service with the same resistance to upheaval buckling it had when it was originally designed and constructed.
Because the pipeline is buried and therefore restrained by the surrounding soil, an important goal of the original design was to prevent upheaval buckling and possible loss of containment by controlling thermal expansion forces due to its high operating temperature. Control was achieved during the original construction in two ways. Firstly, thermal compressive forces were reduced by heating the line to 90°C with forced air and locking it into the surrounding soil in its expanded state, and secondly, restraint was increased by using good backfill compaction, increased depth of burial, and imported fill wherever necessary.
The high axial compressive force on the inline buried valve was identified as a possible cause of failure, and an early decision was made to replace it using an aboveground valve with sufficiently flexible aboveground piping to minimize or eliminate compressive forces on the valve.
When the pipeline was cooled and cut to install the new valve, the original prestress was released, and the cut ends of the pipe pulled back on either side of the valve. The lost prestress was reinstated to the level specified in the original design using an innovative custom designed load bearing strut and tensioning system, referred to here as a Pipe Prop, that was installed between the cut ends of the buried mainline after the failed valve and fittings had been removed.
The Pipe Prop also prevented differential axial movement between the cut ends of the buried pipeline due to changes in the operating pressure and temperature. This reduced the need for flexibility in the aboveground piping and allowed a short offset to be used between the new valve and the buried mainline, which reduced the footprint of the aboveground piping enough to fit within the restrictive boundaries of the site.
Strain gauges were installed on the pipeline adjacent to the failed valve and upstream and downstream of the valve site. The gauges measured changes in stress when the buried pipeline was first cut, and allowed the stress state of the buried pipeline to be calculated to find if the cause of failure had been large axial loads imposed on the valve by the pipeline. The strain gauges also measured strain in the buried pipeline while using the tensioning system built into the Pipe Prop to re-establish the design level of prestress. Permanent strain gauges were also installed on the new aboveground piping adjacent to the replacement valve.
The pipeline was returned to hotbit service in August 2019 and has operated continuously since that time without further problems at the valve station.