During the summer of 1996, the TransAlaska Pipeline System (TAPS) experienced a vibration problem near Thompson Pass which is 25 miles north of Valdez and is part of the Chugach Mountain Range, the most southerly of the three mountain ranges which the pipeline crosses on its route from Prudhoe Bay to the Port of Valdez. The vibrations could, on occasion, be detected by residents living near the pipeline at the bottom of the pass. Close to the source of the phenomena, small bushes could be seen moving in response to the seismic shocks and a noise similar to “mortars firing in the distance” could be heard.
Alyeska Pipeline Service Company initiated an extensive investigation and quickly determined that the seismic shocks were a result of pressure pulses originating near the slackline-packline interface. This only occurred when the slackline-packline interface was positioned near a terraced portion of the pipeline topography on the downstream side of the pass. This knowledge allowed Alyeska Pipeline to control the pulsations by backpressuring the pipeline and moving the slackline-packline interface well above the terrace location.