Determination Of Thermal Conductivity Of Soils: A Need For Computing Heat Loss Through Buried Submarine Pipelines
Authors: Rawat P.C. , Agarwal S.L. , Malhotra A.A. , Gulhati S.K. , Rao G.V. 
Published: October 2008

An important parameter required for computing heat loss through buried submarine pipelines transporting crude oil is the thermal conductivity of soils. This paper describes an apparatus designed for determination of the thermal conductivity of soils at the desired moisture-density condition in the laboratory under steady state conditions. Experimental results on the three soils studied show that the thermal conductivity increases as dry density increases at a constant moisture content and that it increases as water content increases at constant dry density. These results confirm the trends isolated earlier by Kersten. Further the experimental results are compared with the available empirical relationships. Kersten's relation is observed to reasonably predict the thermal conductivity of these soils. The predictions by Makowski and Mochlinski's relation are not good but improve if the sum of silt and clay fractions is treated as clay fraction in the computation.

INTRODUCTION

Submarine pipelines are extensively used for transporting crude oil from a location offshore to others offshore or onshore. These pipelines are normally steel pipes covered with a coating of concrete. They are often buried some depth below the mud-line. The rheological properties of different crude oils vary. The viscosity of all of them increases upon a decrease in temperature. Below some temperature the liquid oil tends to gel. For efficient transportation it is necessary, therefore, that the crude be at a relatively high temperature so that it has a low viscosity. The temperature of the soil-water system surrounding a submarine pipeline is usually lower than the temperature of oil. This temperature difference induces heat to flow from the oil to the environment and the temperature of the oil reduces as it travels along the length of the pipeline. One must ensure that this temperature reduction does not exceed desirable limits dictated by the rheological properties of oil and the imperatives of efficient economic transportation. The analytical aspect of this engineering problem is thus one of being able to predict the temperature of crude in the pipeline some distance away from the input station. In order to do so one requires knowledge of the overall heat transfer coefficient for the pipeline. For which, in turn it is necessary to know the thermal conductivities of the oil, the pipeline materials and its coating and the soil. This paper presents thermal conductivities of soils determined in the laboratory under steady state conditions and also presents a comparison of the test results on three soils with values determined using existing empirical relationships.

LITERATURE REVIEW

Heat moves spontaneously from higher to lower temperature. In a perfectly dry porous body, transmission of heat can take place not only by conduction through the solid framework of the body and the air in the pores but also by convection and radiation between the walls of a pore as also by macro and micro-distillation.