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Countermeasures for bending and abrupt uplift of a full-scale test chilled gas pipeline observed at boundary between frozen ground and talik

In this paper the authors report the vertical bending properties of a test chilled gas pipeline and the countermeasures of the bending. A full-scale field experiment of the chilled gas pipeline system was conducted in Fairbanks, Alaska from 1999 to 2005. The length of the test pipeline was 105m and the diameter was 0.9m. The circulated chilled air was –10 o C. One-third of the pipeline was buried in frozen ground and the rest of it was placed in talik. At the end of July 2003, circulation of the chilled air ceased, however, monitoring of the thaw settlement-related properties of the test pipeline continued until the middle of April 2005. The following results were presented at the 2nd ATC: 1) As the frost-bulb around the pipeline in talik section formed, the test pipeline in the talik section moved upward, resulting in bending of the pipeline at the boundary. 2) In summers, frozen overburden soil of the pipeline became thinner due to thawing of active layer above. The pipeline buried in frozen section moved upward abruptly, fracturing the thinning frozen overburden ground. 3) The phenomenon mentioned in 2) occurred in successive summers, and the pipeline uplift in frozensection continued. 4) In relation with 1), the upward movement in talik section was confirmed by frost heaving of the pipe foundation. In this report the bending behavior of the test pipeline is described and the several methods to deal with the bending are proposed. Introduction In the existing natural gas production fields in permafrost regions such as West Siberia, segments of gas pipelines could become floating in water or exposed in ditches. The inline gas pressure at the time of events might have dropped considerably from the initial production pressure. This pressure drop had enabled the damaged pipeline system to survive. As time progressed, many of those initially buried gas pipelines became exposed and lost the structural stabilities and security reliability. As for the natural gas pipeline installation in permafrost regions, the buried system has been recommended for security reasons. In order to prevent thawing of permafrost, the gas must be chilled for transportation in permafrost regions.On the other hand, even with the chilled gas pipeline system, minor problems can still happen in limited sections of the pipeline where frost heave damage occurs as the pipeline freezes surrounding soils in talik. Two primary chilled pipeline test experiments are discussed in literature: the Calgary Frost Heave Facility and the Caen, France experiment. A third chilled pipeline experiment was conducted at the Fairbanks Frost Heave Facility, but the data remains unavailable to the public. The Caen, France chilled pipeline experiment is well documented in public literature by Geotechnical Science Laboratories (1983, 1986a, 1986b, 1988) and Dallimore and Williams (1985). The purposes of the Caen experiment were to investigate differential heave resulting from the abrupt transition between two different lithologic soils (Caen silt and SNEC sand) with varying frost susceptibilities and the associated stresses incurred by the pipeline and the soil mass. Abrupt lithologic transition zones are common in the natural environment such as the transition between active fluvial gravel deposits and silt overbanks deposits. The Calgary Frost Heave Experiments, described in detail by Slusarchuk et al. (1978) and Foothills (1981), included six separate test sections with each consisting of 12.2-m long, 1.22-m diameter pipe. The experiments were conducted under field conditions within a highly frost susceptible thick glacial-lacustrine deposit with an average ground water table between 2.3m and 2.6m. In 1974 and 1978, total six sections were constructed. In order to propose a design method for pipelines construction in permafrost regions, which prevents the frost heave related engineering concerns, it is necessary to research the interaction of permafrost and burred chilled pipeline. A full-scale