Movements of pigs in gas pipelines are subject to more stringent parameters than that in liquid pipelines, predominantly due to the compressibility of gas. This is accentuated when the pig has to negotiate an upward inclination in the section of the pipeline, where the gravity force due to its weight can compromise the driving pressure drop across it. On a downward slope, a pig can accelerate to a velocity higher than the maximum required for the proper operation the instrumentation (which is typically around 5 m/s). On the other hand, in-line inspection tools often face challenges at wall thickness transitions or bends. The ability to accurately predict the functional performance of pigs is vital in the design and operation of pipelines and their associated pigging programs. The present paper provides a general formulation for the motion of pigs in an inclined pipeline section, taking into account effects of gas properties, wall friction, by-pass flow for speed control, differential pressure across the pig, seal efficiency, and gap flows, among other parameters. Comparison between model prediction and actual data from pigging a 158 km NPS 18 gas pipeline on TransCanada’s pipeline system in Alberta, Canada is presented. The elevation profile along this pipeline contains both positive (upward) and negative (downward) slopes. This is a lateral line which features 28 gas receipt points along the line, all were feeding in gas during the pigging program. Good agreement between model prediction and field data is demonstrated within ± 8% of St. Deviation. Example of a problem occurring at wall thickness transition at a valve section is demonstrated by a sudden stop of an MFL tool followed by a shootout at a higher velocity once the pressure is built up behind it.

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