Norman Wells pipeline settlement and uplift movements

Abstract

The Norman Wells oil pipeline has been operating successfully since 1985. The pipe was designed to operate as an ambient-temperature pipeline and accommodate up to 0.8 m of thaw settlement in inorganic terrain. The pipeline has settled close to this amount in some areas, without excessive straining of the pipe. An average thaw strain for the soil back-calculated from the thaw depth and resulting thaw settlement at several sites gives average values of 16-20%. At one location (kilometre post 5.2), the pipeline has experienced uplift of 1.1 m or more. The mechanism for pipe movement is likely a combination of high axial stresses and some small initial frost heave, which triggered uplift buckling of the pipe. Low-density thawed soils contributed to this behavior. An internal profiling device (Geopig) has been run through the pipe in recent years. Analysis of the profiles indicates excellent agreement with manual surveys at the site. The pipe is experiencing about 0.3% bending strain in the uplift zone, and about 0.4% strain in a settling area immediately to the north. Pipe strain analysis using a structural model indicates that about 0.2 m of frost heave would be required to initiate uplift buckling over a critical heave length of 22-25 m.