Abstract
The application of thermal insulation in underground pipelines presents important operational benefits during normal flow, compared to those obtained by the operation of underground pipelines without thermal insulation. However, when an extended shutdown occurs, resulting in stagnant fluid in the pipeline, the thermal insulation applied does not present major operational benefits concerning temperature maintenance inside the pipeline. In this case, the fluid reaches lower temperature values after a determined shutdown period, in the initial sections of the pipeline, which may cause wax deposition, especially for heavy crude oils at very low velocities, and significant restart pressures to reach steady state conditions. The theoretical analysis of this behavior was developed by the application of a mathematical model using iterative calculations based on heat transfer theory, for a 10-inch nominal diameter pipeline, a 1.5-inch thermal insulation thickness (if applicable) and a 5.2-feet burial depth. These results were contrasted with those using commercial software for single-phase dynamic calculations. The most significant finding, contrary to what was expected, is concerned the cooling experienced by the static fluid in an insulated pipeline. This criterion shall be considered for designing buried pipelines which transport high viscosity fluids, especially in stagnant conditions (no fluid motion) and for restarting operations.
