Conductive and Convective Heat Transfer in Inductive Heating of Subsea Buried Pipelines

Abstract

Inductive heating with high-voltage cables reduces the risk of hydrate formation by raising the temperature of the production fluid in pipelines. Heating the pipeline results in losing a certain fraction of the heat to the surrounding soil through conduction- or convection-dominated flow through the soil. However, the amount of heat lost in conduction versus convection and the transition from conduction- to convection-dominated heat loss remains unknown. Soil permeability, temperature gradient between cable and mudline, and burial depth all influence the mode of heat transfer and the amount of heat lost. We studied the dominant mode of heat transfer in pipelines with inductive heating using 2D finite difference analysis under different soil and environmental conditions. Low permeability soils primarily exhibit conductive heat transfer, thus losing minimum heat to the surrounding soil. In contrast, in highly permeable soils convective flow drives a significant fraction of the heat away from the pipeline and towards the ground surface, barely heating the fluid in the pipe. We identified a critical Rayleigh-Darcy number (1) as the controlling value separating conduction- and convection-dominated heat transfer. An increase in burial depth reduces the heating efficiency in convection-dominated high permeability soils, while it has no effect in conduction-dominated low permeability soils.