This paper describes a 3rd party review of geometry pig inertial surveys of the ConocoPhillips Alaska, Inc.-operated Alpine 14-inch diameter oil and 12-inch diameter water pipelines at the horizontal directionally drilled (HDD) Colville River crossing. The geometry of these pipelines is periodically surveyed to monitor for potential thaw-induced differential settlement between thaw-stable and thaw-susceptible soils along the HDD profile under the river. Preliminary reviews of the elevation profiles from multiple surveys showed significant run-to-run variations within the HDD. However, considering the long distances between the reference elevation tie points, the observed elevation differences appeared to be within the position accuracy of the inertial survey specifications. As a follow up to the initial review, a more detailed 3rd party review of the data was performed. This paper describes how the elevation, pitch and bending strain data from four different inertial surveys performed for each pipeline over a five year period was processed to look for monotonic trends and/or locations of significant pipe deformation. It was determined that the bending strains for both pipelines were small compared to the allowable strains and that the most critical locations for both pipelines occurred in the vicinity of the end of the below grade casing and the adjacent aboveground pipe support. At each end of the HDD, the pig data signatures indicate the presence of a sagbend curvature lobe at/near the end of the casing and an overbend curvature lobe at/near the adjacent pipe support which is clearly indicative of relative movement between the below grade section of the pipe and the pipe support. The response at the pipe support was confirmed by visual inspection of the pipe where minor buckling damage to the external sheet metal insulation jacket at the ends of the saddle was observed. The results from this review were used to develop a recommended forward-looking geometry monitoring schedule.

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