Movement Due to Heave and Thaw Settlement of a Full-Scale Test Chilled Gas Pipeline Constructed in Fairbanks Alaska

Abstract

Abstract In this paper the authors report the heave and thaw settlement properties ofa test chilled gas pipeline. A full-scale field experiment of the chilled gaspipeline system was conducted in Fairbanks Alaska from 1999 to 2005. The lengthof the test pipeline was 105m and the diameter was 0.9m. The circulated chilledair was -10 °C. One-third of the pipeline was buried in permafrostand the rest of it was placed in non-permafrost. At the end of July 2003, circulation of the chilled air ceased, however, monitoring of the thawsettlement properties of the test pipeline continued until the middle of April,2005. The results obtained include:As the frost-bulb around the pipeline innon-permafrost section formed, the test pipeline in the non-permafrost sectionmoved upward, resulting in bending of the pipeline at the boundary.Insummers, overburden frozen ground of the pipeline became thinner due to thedevelopment of active layer above. The pipeline buried in permafrost sectionmoved upward abruptly, fracturing the thinning overburden frozen ground.Thephenomenon mentionedoccurred successive summer, and the pipeline uplift inpermafrost section continued in summers.In relation withthe upwardmovement in non-permafrost section was confirmed by frost heaving of the pipefoundation.Settlement of the test pipeline was also confirmed by thawsettlement of the foundation.During the thawing process, the temperature ofthe thawing frost bulb became 0 °C at first and then thawed rapidlyin summer together with the development of active layer. As a result, settlement of the pipeline happened rapidly in summer. Introduction In the existing natural gas production field in permafrost regions such asWest Siberia, gas pipelines float in water or are exposed in ditchs as shown inthe photos of Fig. 1. However the gas pressure at the present time has droppedconsiderably comparing to the one in the initial production days. This pressuredrop enables the damaged pipeline system to survive. However, those initiallyburied gas pipelines are now mostly exposed and lost the structural stabilitiesand security reliability. As for the natural gas pipeline installation in permafrost regions, theburied system has been recommended for security reasons. In order to preventthawing of the permafrost shown in Fig. 1, the gas must be chilled fortransportation in permafrost regions. On the other hand, even with the chilledgas pipeline system, miner problems may still happen in limited sections of thepipeline where frost heave damage occurs when the pipeline freezessurrounding soils in non-permafrost section (Talik). Two primary chilled pipeline test experiments are discussed in theliterature: the Calgary Frost Heave Facility and the Caen, France experiment. Athird chilled pipeline experiment was conducted at the Fairbanks Frost HeaveFacility, but the data remains unavailable to the public. The Caen, France chilled pipeline experiment is well documented in public literature byGeotechnical Science Laboratories (1983, 1986a, 1986b, 1988) and Dallimore andWilliams (1985). The purposes of the Caen experiment were toinvestigate differential heave resulting from the abrupt transitionbetween two different lithologic soils (Caen silt and SNEC sand) with varyingfrost susceptibilities and the associated stresses incurred by the pipeline andthe soil mass. Abrupt lithologic transition zones are common in thenatural environment such as the transition between active fluvial graveldeposits and silt overbanks deposits.