INTRODUCTION:

Northwest Alaska and Fluor are presently designing the Alaskan Segment of the ANGTS, a system which has the potential of transporting 3.4 BCF/D to West Coast and Midwest Markets. The Alaskan Segment is unique in that it will be the first high pressure large diameter chilled gas pipeline ever built. The operation of the pipeline below 32°F will protect the pipeline from any thaw settlement in the ice-rich and enviromentally sensitive permafrost. As presently proposed, large refrigeration plants at each compressor station will remove the heat of compression before the gas re-enters the pipeline, thus maintaining sub-freezing pipe temperatures throughout Alaska. Due to the potential for frost heave in non-permafrost areas, and the need for reliable predictions of compression and refrigeration loads, Northwest has spent considerable effort in analyzing the thermal interaction between a buried pipeline and its environment. Today's higher costs for fuel and capital necessitate improved modeling accuracy whether the system being analyzed is a conventional West Texas pipeline, a two phase trunk line in Wyoming or the Alaska gas pipeline.

STEADY THERMAL - HYDRAULICS

Most hydraulic studies assume isothermal flow. For the majority of applications this is justified, but with the demand for improved accuracy in critical situations and the present trend toward more exotic pipelining techniques the assumptions of constant fluid temperature should be re-examined on an individual basis. Northwest Alaska began investigating heat transfer between a buried pipe and the surrounding soil based on work performed by A.E. Kennelly in 1893. He developed an analytical steady state solution to the temperature rise of a buried power cable using the method of images (Fig. 1). As addressed by J.H. Neher in 19492, the equation is directly applicable to steady state heat flux between a buried pipe and the soil if the soil has a uniform thermal conductivity and constant surface temperature. Efforts by C.E. Schorre3, J. A. Forrest 4, and D.M. Coulter and M.F. Bardon5 incorporated simplistic heat transfer models such as the Kennelly equation, with a closed form hydraulic approximation which can be used to estimate flowing gas temperatures for steady-state operation, (Fig. 2 and Appendix A.) Northwest's first efforts in developing a steady-state thermal hydraulic model utilized the Kennelly equation coupled with a fourth order numerical intergration of the partial differential equations for flow, as suggested by A.P. Buthod, et al 6 (Fig. 3). Fluid properties, compressibility and enthalpy are determined using the Starling-Han-BWR equations of state7. The later two refinements were felt to be necessary for accuracy due to the non-idealities of the dense phase gas at high pressure and low temperature (Fig. 4). With the availability of some preliminary geothermal field data, questions over the appropriate input data for the Kennelly equation were raised. The Kennelly equation was derived to be used with the soil surface temperature and yet at low flow rates and heat flux values, the gas temperature should approach the soil temperature at the centerline depth and not the surface temperature.

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