During the summer of 1996, the TransAlaska Pipeline System (TAPS) experienced vibrations near Thompson Pass, about 25 miles north of Valdez, Alaska. 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 a “mortar 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 back pressuring the pipeline to move the slackline-packline interface well above the terrace location.
One key aspect of the project was an extensive analytical investigation of a dented and ovalled section of the pipeline near the origin of the pressure pulses. The main concern at this location was that, as each pressure pulse passes, the ovalled and dented pipe section tends to “reround” causing the pipe wall to flex a small amount. Since the pulses occurred frequently under certain flow conditions, there was a concern for possible fatigue damage to the pipe steel. The locations of maximum stress range were estimated to be near the 6:00 position on the outside surface of the pipe at the periphery of the dents in the X65 base metal (Stations 40959+28 and 40959+40 in Alyeska terminology).
This paper describes the “design” and “decision” S-N relationships that were developed during the Thompson Pass investigation to estimate fatigue damage. These curves are a combination of the AWS A fatigue curve and an adjusted version of the ASME Section VIII, Division 2 design fatigue curve. These curves were used together with a multi-axial fatigue model to compute fatigue damage due to pipe stresses caused by pressure pulse cycling as well as that due to operational startup and shutdown cycles.