The central question of this thesis is how and to what extent the representation ofudambient heat transfer in the calculation models contributes to observed deviationsudbetween modelled and measured flow parameters in natural gas transmission. The focusudis on the heat transfer occurring with buried pipelines. The research approach was audcombination of model studies with a large experimental component.udThe first 12 km of an export gas pipeline was instrumented and used for theudexperimental investigation of heat transfer behaviour and result verification. Thisudpipeline section contains both onshore and offshore sections. A high accuracy modeludwas made. Real data of the gas flow instrumentation present on the pipeline define theudfluid conditions at the model boundaries. At an onshore location, the pipeline andudsurrounding soil was instrumented. Measurements included pipe wall and soiludtemperatures, soil humidity and meteorological quantities.udA one-dimensional flow model was used to model the gas flow inside the pipe. Thisudmodel is coupled to three different external heat transfer models of the ambient domainud(pipe wall layers and soil) for comparison. These heat transfer models are 1D steadyudstate, 1D radial unsteady and 2D unsteady description of pipe wall layer and soil. Bothudconduction and convection heat transfer in the soil layers were investigated. The effectudof transient boundary conditions on heat transfer rates and flow parameter calculationsudwere quantified. The developed models were used to analyse and understand theudexperimental data, to study the effect of different external heat transfer models, theudrelevant importance of different heat transfer modes, and the boundary conditionudassumptions on the pipe flow calculations.udThe thesis addresses the following research objectives:ud- The spatial and temporal formulation of the heat transfer problem: how does theudchoice of external heat transfer model influence the calculation accuracy of theudflow parameters during transient flow? To what degree do the different modelsudcapture the physics around the pipelineud- Sensitivities for governing parameters: how do key governing ambientudparameters like air/seawater temperature, and the thermal properties of the soiludaffect the calculation accuracy of the flow parameters?ud- The effect of ground water convection and ambient boundary conditions onudcalculating the flow parametersudThe model verification, carried out over an extended period, and the sensitivity studiesudshow that including the heat storage term in the ambient model has the biggest impactudon the accuracy of the calculated gas temperatures. The accuracy of gas pressures isudmuch less sensitive for the choice of heat transfer model. A large improvement in theudcalculation accuracy of gas temperatures is obtained when using an unsteady heatudconduction model representing the pipe and soil in radial coordinates. This confirmudfindings from earlier published work that using such a so called 1D radial unsteady model of the pipe wall and soil, i.e. including the time dependent heat storage term inudthe governing heat conduction equation, leads to a major improvement of theudcalculation of gas temperatures during transient flow. This model was compared to theud2D unsteady model, based on coupling a FLUENT model to the flow model. The heatudtransfer response obtained with the 1D radial unsteady model was found very similar toudthe geometrically more accurate 2D unsteady model during transient flow conditions.udThe 1D radial unsteady model does lead to a gas temperature error in response to theudannual periodic ambient temperature. This error was found to be small for a typicaludexport gas pipeline, but can be significant for other configurations. The error introducedudby the definition of the ambient soil surface boundary condition was also found to beudsmall compared to the choice of heat transfer model.udThe results show that the gas temperature is sensitive to the values of soil thermaludconductivity, inner film coefficient and seawater temperature during transient flow. Theudsoil surface boundary conditions have a smaller influence. The sensitivity for theudgoverning parameters of the heat transfer model is strongly dependent on the flowudconditions; the resulting deviations in gas temperature are larger during transient flowudconditions resulting in large gas temperature fluctuations.udThe results also show that the effect of natural convection upon the heat transfer is smalludfor the studied case but that at higher intrinsic permeability of the soil, the role ofudnatural convection will play a significant role. The role of forced convection was foundudto have a negligible effect.udSoil thermal properties were determined using different methods. The resulting valuesudfor thermal conductivity and thermal diffusivity are in agreement with each other toudwithin the measurement accuracy. The measurements in the soil surrounding the pipeudshow that the thermal properties are mostly constant in time. Some temporal variationsudwere found, but these were not found to make a significant difference on the resultingudcalculated heat transfer rates and gas temperatures. The experimental results show thatudthe temporal development of soil temperature profiles around the pipe is asymmetricaludwhen comparing the left and right direction. The soil temperatures under the pipe closeudto the pipe wall were found to be lower than those above the pipe wall, which isudopposite of the expectation with a heat conduction model. Both the use of forced andudnatural convection heat transfer in the model could not explain this difference, but theudasymmetry was found not to affect the heat transfer rates significantly within theudaccuracy of the measurements and calculations.udComparison to experimental results during a longer time period, showed that using a 1Dudradial unsteady model leads to good overall agreement in gas temperature, pressure andudpipe wall heat transfer. However, incidental, significant, gas temperature and pressureuddeviations still occur in connection with transient flow conditions. A 1D radial unsteadyudheat conduction model with constant thermal properties, using air temperature as soiludsurface boundary condition, will for most practical purposes satisfactorily approximateudthe more complex physics of the heat and mass transfer in the soil in response to the gasudtemperature fluctuations and the ambient parameters.
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