2 Volumes Final Proceedings - Washington 1984.pdf - IARC Research

103a. b. time. Later on, once the near-surface perlayer has thawed, the newly formed freeze bulb andand relic permafrost below the pipeline would beginto degrade. Thus, the pipeline could undergo"""-LOWlERM THlW-first heaving and then subsidence. To overcomethese problems, the pipeline rmst be designed tobe insensitive to the particular geometry of permafrostboundaries..Figure 7 shows an example of a mitigativemode for permafrost boundaries based on the use offree-standing heat pipes placed in a row on eitherFIGURE 6 Schematic cross sections showing theside of the buried cold pipeline. The function ofthermal effects of an insulated, cold, buriedthe heat pipes is to freeze any thawed ground inpipeline at permafrost boundaries (a> in warm per- the vicinity of the pipeline and to keep initiallymafrost and (b) in mixed frozedthawed soil.frozen ground frozen. The main direction of heatHEAT- PIPEflow is horizontal, radially into the heat pipes,RAOllTORso the potential for frost heave is greatly reduced.This conclusion is supported by experiencefrom the trans-Alaska oil pipeline where heavingof thermal VSM's (vertical support members) hasnot been a significant problem; however, some minorheave has occurred (Table 1). Depending on thetiming of heat pipe installation and pipelinestartup, it may be possible to eliminate pipelineinsulation in this construction mode. Longitudinalspacing of the heat pipes, shown in the sideview (Fig. 7a), would be rmch closer than in thedesign for the trans-Alaska oil pipeline, perhapsevery 3 m or so, since a continuous freeze bulb isa.b.desired under the pipeline. The total number ofheat pipes should not be excessivthis special pipeline mode would be required onlyFIGURE 7 Schematic cross section and side viewsporadically for short distances--perhaps 30 m--onshowing the thermal effects of an insulated, cold, either side of a permafrost boundary.buried pipeline with heat pipes (mitigative pipe-Transition problems also occur for warm pipelinemode) at permafrost boundary at (a) time of lines. These problems were avoided on the transinstallationand (b) long-term.Alaskan oil pipeline by going from the buried tothe elevated mode before entering frozen, thaw-unstablesoil. The elevated mode was also used inCOLD PERMAFROST WARY PERMAFROST THAWED Sol1areas of mixed frozen and thawed soil.Figure 8 shows a summary of the pipeline constructionmodes that have been described: threefor the warm pipe (Figs. Ea, 8b, and 8c), andWARMthree for the cold pipe (Figs. Bd, 8e, and 8f).PIPEThe modes shown for the warm pipeline are the onesused for the trans-Alaska oil pipeline: simplePERMAFROST PIMAFROST BOUNDARY THAWED SOILelevation on cold permafrost, elevation with heatpipes in warm permafrost, and conventional burialin thawed soil. The three buried modes for a coldpipeline are conventional burial in permafrost,heat pipes at permafrost boundaries, and insulatedpipe in thawed soils. An extended gravel pad,shown in Figure Sf, Is used to take full advantageof the surface disturbance for preventing theFIGURE 8 Schematic cross sections showing exampgrowthof a freeze bulb. For the same reason, relesof mitigative pipeline modes for both warm andvegetation would be limited. These measurescold pipelines under various soil conditions.would, of course, not be required in areas wherethe thawed soil is not frost-susceptible, or wheregive reliable support. Accurate predictions ofit can be shown that the expected amount of frostheave would not over-stress the pipe.thaw bulb geometry and pipe deformation are diffi-Some of the more important design consideracultto make in this case. Groundwater flow could tions for warm offshore pipelines in areas of iceleadto additional thaw. A more complex situation bonded subsea permafrost are shown in Figure 9.with alternating layers of frozen and thawed soilSubsea permafrost is warm permafrost, not muchis shown in Figure 6b, In this case, the uppercolder than the overlying seawater, typlcallyfrozen layer would shield the soil below the pipe- about -1 to -3'C. In that regard, it Is similarline from the effect of the surface disturbanceto the warm permafrost in central Alaska, but theduring the early years of pipeline operation. A similarity ends there; there is no active layertemporary freeze bulb could be formed during this and no organic layer, and therefore construction