THERMAL ASSESSMENT OF PASSIVE COOLED FOUNDATION SOILS BENEATH THE TRANS-ALASKA PIPELINE AT ATIGUN PASS

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1 THERMAL ASSESSMENT OF PASSIVE COOLED FOUNDATION SOILS BENEATH THE TRANS-ALASKA PIPELINE AT ATIGUN PASS Keith F. Mobley 1, Mike R. Fitzpatrick. 2, John E. Ferrell 3 1. Shannon & Wilson, Inc Fairbanks Street, Suite 3 Anchorage, Alaska Michael Baker, Jr., Inc Business Park Blvd Anchorage, Alaska Alyeska Pipeline Service Co S. Bragaw Anchorage, Alaska Introduction The Trans-Alaska Pipeline crosses the Brooks Range in northern Alaska at Atigun Pass (See Figure 1). The upper portions of the pass consist of steep sided valleys with rock slopes rising about 800 to 1000 meters above the bottom. Due to avalanche and rock fall hazard, the pipeline was constructed in the below-ground mode, despite the presence of thaw unstable permafrost. To mitigate the stability issues, the pipeline was constructed within insulated boxes to protect two thaw unstable permafrost zones. In 1979, it was discovered that portions of the soil beneath the insulated boxes had begun to thaw. Thermosyphons were installed as a part of the effort to restore the permafrost. Abstract The foundation of a section of the Trans-Alaska Pipeline was stabilized in 1981 using thermosyphons. Over the past 16 years, some of the thermosyphon radiators were damaged from avalanches and rock fall. A large avalanche in 1994 did significant damage to about 1/3 of the array. This potential reduction in the cooling capacity of the array raised concern about the long-term stability of the pipeline in this area. A field and modeling study was conducted to assess the current cooling capacity of the array and to predict the future thermal environment of the foundation soils. Thermistor strings were installed in existing vacant cased borings. A twodimensional finite element thermal analysis was done, with the soil parameters calibrated to match the known temperatures. A multi-year model was run to help predict the future performance of the remaining portion of the array. Results of the modeling indicate that the remaining thermosyphons have sufficient capacity to maintain the foundation in a frozen state. The heat pipes located on the south side of Atigun Pass have been damaged as a result of several avalanches, rock falls, and ice expansion within the casings. The damage has reduced the cooling capacity of the system. The purpose of this study was to evaluate the current conditions and to determine if the stability Figure 1. Location of Atigun Pass. Keith F. Mobley, et al. 739

2 of the pipeline in this area was at risk, and to provide recommendations for further work, if necessary. The scope of work included a limited field effort to determine current temperature conditions and inventory of the heat pipe physical conditions, and a thermal analysis via finite element computer modeling. Two different scenarios representing full and partial heat pipe capacity were run. History During construction of the pipeline in Atigun Pass, two portions of the alignment, one each on the north and south sides of the pass, were recognized as being thaw unstable. However, the pipeline was constructed below ground based on the assessment of avalanche and rockfall hazards. To mitigate the thaw unstable foundation, each thaw unstable portion of the pipe was placed in an insulated box that rests on a continuous 3 m wide, 0.3-m thick concrete slab. The box on the north side of the pass is about 550 meters long, and the south side box is about 1300 meters long. In 1979, about 2 years after oil flow commenced, an oil leak was discovered on the north side of the pass. Investigation revealed that ice within a predominately rock rubble matrix had melted, allowing the pipe to settle. The investigation was expanded to look at the entire Atigun Pass section of the pipeline to see if other areas had experienced similar settlements. Several other portions of the pipe had settled, including a 122-m section near the north end of the southern insulated box, which settled up to 1.5 m. Field studies conducted from 1979 through 1981 (WCC, 1981) indicated that the major reason for the settlement was the presence of warm water flowing along the pipeline trench backfill and creating a large thaw bulb below the insulated box. In addition, the groundwater infiltrated into the insulated box, degrading the pipe coating, the grout sleeve and the insulation. Degradation of the insulation further exacerbated the thaw problems. Repair work for the south side insulated box included two grout curtains to control the water flow under the insulated box, pressure grouting to lift the pipe into a better curvature condition, construction of two surface drainage channels to divert runoff away from the pipe, drilling wells to remove existing groundwater along the pipeline corridor; and mechanical and passive ground freezing to restore the thermal regime, thereby restabilizing the foundation soils. The mechanical freezing was accomplished by drilling 268 refrigeration casings at about 1 m on centers on both sides of the insulated box to a depth of about 10 m, generally about 1.5 to 2.1 m from the pipe centerline. Once all of the casings were in place, refrigeration piping was placed in each, manifolded together and a brine coolant circulated through a refrigeration plant. At the conclusion of the mechanical refrigeration in October 1980, all of the refrigeration piping was removed from the casings. Ammonia (NH 3 ) filled, free standing heat pipes were installed in mid to late October 1980 to the bottom of about 1/2 of the mechanical refrigeration casings, generally alternating in the existing refrigeration casings. The heat pipes were designed to cool the ground enough to maintain frozen ground above the thaw unstable portion of the foundation. The remaining refrigeration casings were left open at the ground surface and were filled with a glycol, water and rust inhibitor mixture. Since 1980, there have been several avalanches and rock falls that have damaged a number of the heat pipes. In addition, ice expansion, jacking and splitting have damaged other heat pipes and casings. Field work In October 1996, one set of ground temperature profiles extending to 5 to 10 m were obtained by inserting a 12-point thermistor string to the bottom of an open refrigeration casing. The casings used still contained the antifreeze and corrosion inhibitor solution that was added at the end of the 1980 construction season. The thermosyphon array was inventoried to determine the approximate portion of cooling capacity remaining. In January 1997, an infrared study was conducted by Alyeska to assess the performance of the thermosyphons. Infrared readings were taken of each radiator for qualitative indications of heat transfer. Thermal analysis A finite element computer program titled TEMP/W was used to analyze ground thermal conditions with the heat pipe array at full, undamaged capacity and to simulate conditions with only one-third of the heat pipe capacity operating. TEMP/W is a two-dimensional finite element, geothermal computer program developed by Geo-Slope International to model ground thermal responses to environmental or physical changes. The governing set of equations used in TEMP/W states that the difference between the heat flux entering and leaving an elemental volume of soil at a point in time is equal to the change in the stored heat energy. The program allows for complete modeling of material types; properties such as thermal conductivity, heat capacity and volu- 740 The 7th International Permafrost Conference

3 Table 1 Model Input Parameters metric water content; and boundary conditions including changes of temperature with time, nodal heat fluxes and heat flux changes with time. See Table 1 for input parameters. A near step function was employed to model the latent heat removal or addition during the phase change of water within the material matrix. A finite element mesh was created and the various material properties and boundary conditions applied. The model does not take into account convective heat transfer. Because the subsurface water flow was controlled during the 1980 repair efforts, eliminating the convective portion of the heat transfer was considered acceptable. monthly temperature profile from the 1 day time steps of the model for one year to bring the model into equilibrium, then for an additional 18 months, for a total of 2.5 years of modeling. Each individual heat pipe was determined to have a heat extraction capacity of 0.56 kcal/min-m of evaporator embedment when there was a 1.7 C temperature differential between the radiator fins and the condenser pipe (WCC, 1980). For the model, we assumed that when the temperature differential was less than 1.7 C, the heat pipe had no cooling capacity over the portion The initial values for thermal properties of the soil were obtained from published data (Andersland and Anderson, 1978; Lunardini, 1981). The thermal conductivity for wet insulation was based on previously completed experimental data (Shannon and Wilson, 1994). ANALYTICAL MODEL The physical conditions of the model include the finite element geometry and boundary conditions, the infinite boundary, pipeline, wet insulation, nodes that represent the heat pipes, and the soil types. A half space (axis-symmetric) model was used in the analysis in order to reduce the calculation time required by the computer, see Figure 2. An infinite boundary was placed at the left edge. Input parameters for the pipeline, soil, heat pipe and air were developed based on our knowledge of the surrounding conditions, ground temperature profiles, and pipeline operating conditions. The model was developed to output a Figure 2. Finite Element Half-Space Model Keith F. Mobley, et al. 741

4 of the evaporator that failed the temperature differential criteria. It is possible for the heat pipe to be functional even if only the bottom foot of the heat pipe has a 1.7 C temperature differential. The heat flux functions were determined by trial and error. Trial and error was required as the heat removal capacity of the heat pipes is a function of the ground temperature and the air temperature, while the ground temperature is a function of the heat removed by the heat pipes. This results in one equation and two unknowns. The heat pipes generally begin to function at air temperatures of Ð2 C and below. These values are dependent on the pressure and concentration of the NH 3 in the heat pipe. It was also assumed that only 5 m of the 6 m of heat pipe length was active in extracting heat, with the top meter of the pipe in the ground maintaining very close to the air temperature. Results Temperatures read during the October 1996 fieldwork were mostly below freezing, with a range from about -8 to just above 0 C. Only one thermistor point recorded a temperature above freezing. The heat pipes had probably been operational for two to three weeks. Based on the model, the ground temperatures get so cold that thawing at the bottom of the concrete support slab probably does not begin until late May or June. Thaw does not reach the bottom of the thermosyphons until late August or September, just before the thermosyphons will again become active. The thermal model was adjusted to provide results at the end of the first year equilibrating run that closely matched the temperatures measured during the fieldwork. The assumptions used for the adjusted air temperature, soil parameters and heat pipe capacity, although most likely not equal to actual conditions, are close enough that individual errors are for the most part canceling each other. At 100% heat pipe capacity, the ground around the oil pipeline within the heat pipe array would be getting colder each year, as shown in Figure 3. Our model indicates that at 1/3 of the installed heat pipe capacity, it is marginal to keep the ground beneath the pipeline in frozen thermal equilibrium. See Figure 4. With approximately 2/3 of the original installed capacity still available, the heat pipe array in its current condition is more than sufficient to keep the ground frozen, even during extreme weather event years where there is a warm, snowy winter followed by a warm, wet summer. Conclusion The Trans-Alaska Pipeline within the south side insulated box is not in danger of settling due to thawing of the foundation soils. With the reduced oil temperatures (47¼C in 1980 and 34¼C in 1996) and the groundwater and surface water flow diverted away from the warm pipe upstream of the insulated box, the reduced capacity of the heat pipes is keeping the soil temperature in equilibrium. Approximately 1/2 of the heat pipes are physically damaged. The infrared study conducted by Alyeska in January 1997 indicates that most of the damaged heat pipes are still nearly fully functional. The north half of the array is in excellent condition with about 94% of the pipes still intact. Some of the pipes are bent and a few may have leaked ammonia, but it is likely that the cooling capacity is at or above 80% of the original installed capacity. The south half of the array is in much worse condition, with only about 57% of the heat pipes likely functional. Again some of the remaining pipes are bent and a few may have leaked ammonia, so the cooling capacity is probably near 50% of the original capacity. The average capacity of the entire array is therefore likely at or above 65% of the original installed capacity. The most important area to keep frozen is the north half, where the heat pipes are in good condition. Keeping the northern edge frozen effectively diverts any upstream groundwater away from the thaw unstable zones. Without moving water, even with greatly reduced cooling capacity, the ground will likely not thaw very much, and remain cooled due to the ambient air temperatures. If the annual, late summer thermal monitoring in the future shows a trend of increasing thaw, Alyeska would have time to implement a repair to refreeze the ground. Over most of the array, more than 7 m of ground below the concrete slab has to rethaw (the first time was in 1979 and 1980) before reaching new thaw unstable (never thawed) ground. Based on the model, replacement of most of the damaged, non-functional heat pipes would refreeze the ground in one winter. Acknowledgments The authors wish to express their appreciation to Alyeska Pipeline Service Company for their permission to present this information. 742 The 7th International Permafrost Conference

5 Figure % heat pipe capacity 0¼C Isotherm Figure 4. 33% heat pipe capacity 0¼C Isotherm References Andersland, O.B. and Anderson, D.M. (1978). Geotechnical Engineering for Cold Regions. McGraw Hill Inc., New York ( Lunardini, Virgil J. (1981). Heat Transfer in Cold Climates. Van Nostrand Reinhold Inc., New York (731 pp) Shannon & Wilson, Inc. (1994). Insulation Conductivity Study. Report to Alyeska Pipeline Service Co. Woodward Clyde Consultants, Inc. (1980). Atigun Pass, MP Pipe Stabilization and Instrumentation Volumes 1 and 2. Report to Alyeska Pipeline Service Company Woodward Clyde Consultants, Inc. (1981). Atigun Pass, MP Pipe Stabilization and Instrumentation, Report to Alyeska Pipeline Service Company Keith F. Mobley, et al. 743