Design of the Ruby Pipeline at the Wasatch Fault Crossing, Northern Utah, USA

Abstract

The Ruby Pipeline is a 1,067-mm (NPS 42) diameter natural gas transmission pipeline extending 1,086 km from southwest Wyoming to southern Oregon in the northwestern United States. The alignment crosses or projects to within 0.1 km of 31 traces of active or potentially active normal faults. It crosses the Wasatch fault, the largest active fault in Utah, at a location approximately 3 km north of Brigham City and about 150 m east of a state highway. A maximum vertical displacement of 2.1 to 2.4 m is expected from a moment magnitude (Mw) 7.1 earthquake rupturing the 37-km-long fault (Brigham City) segment of the Wasatch fault. The resilience of the pipeline in response to fault displacement was investigated using a three-dimensional, finite-element stress analysis following recommendations in seismic guidelines developed for the Pipeline Research Council International, Inc. The pipeline was modeled over a length of 300 m, which included the state highway crossing and two three-dimensional bends. The finite-element analysis was performed by applying 4 m of vertical displacement oriented at a 60° dip angle in 120 steps with computed longitudinal strains extracted at eight equally-spaced circumferential positions for each model element at every step. The displacement capacity was defined by a tension strain of 4.0% or compression strain of 3.6%. For the 22.0-mm pipe wall thickness at the fault crossing, the finite-element analysis showed that the fault displacement capacity was greater than 4 m, which corresponds to more than 1.7 times the expected 2.4-m maximum fault displacement in a single Mw 7.1 earthquake event.