Conventional hydraulics modeling tools typically apply sophisticated calculations to accurately determine pipeline pressure profiles, product throughput rates, chemical composition effects, gas/liquid phase partitioning, and other such quantities. Incongruously, these same tools use very simplified assumptions and workarounds for modeling pipe-soil heat energy exchange for buried pipelines. These simplifications can result in inaccurate pipe temperature predictions which may have important consequences for the design of strain-based design buried pipelines. This paper presents a brief history of thermal-hydraulics model development with three specific arctic pipeline design scenarios which are addressed using these tools.
Pipeline hydraulics modeling is essential for pipeline design. Calculated pipeline temperatures, pressures, and product throughput, are dependent on many input variables and several physical processes including pipe-soil heat exchange, heat transfer through the soil, and heat exchange at the ground surface.
In the case of buried arctic gas pipelines, accurate pipe and soil temperatures are also necessary for several aspects of strain-based design including but not limited to assessing strain-demand from frost heave, assessing possible pipe strain relief scenarios, and avoiding pipeline operating temperatures that exceed the pipe minimum design metal temperature (MDMT).
After a historical review of hydraulics modeling for pipeline design, aspects of arctic pipeline strain-based design which benefit from the application of thermal-hydraulics modelling are discussed.
Accurate calculation of pipeline temperatures, pressures, and flow rates requires a reasonable representation of the heat transfer between the pipe and its surroundings. For buried pipes, this requires calculation of the heat transfer in the surrounding soil.
Historically, and even to this day, most pipeline hydraulics models rely on closed-form analytical solutions for estimating soil heat transfer. The genesis of such analytical solutions can be traced as far back as 1893 when A.E. Kennelly first investigated heat transfer around buried electrical cables to determine their amperage capacity (ampacity) limits which are governed by the maximum cable temperature reached when carrying current. Kennelly developed an analytical solution to calculate the steady-state heat transfer rate through the soil surrounding a cable and thus the maximum cable amperage corresponding to the maximum allowable cable temperature (Kennelly, 1893).
