Vinay, Guillaume (2005) Modelling of the restart of a gelled waxy crude oil flow in pipelines. PhD thesis Mécanique Numérique, ENSMP - CEMEF Centre de Mise en Forme des Matériaux, ENSMP.

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Abstract

Pipelining crude oils that contain large proportions of paraffins can cause many specific difficulties. These oils, known as waxy crude oils, usually exhibit high ``pour point'', where this temperature is higher than the external temperature conditions surrounding the pipeline. During the shutdown, since the temperature decreases in the pipeline, the gel-like structure builds up and the main difficulty concerns the issue of restarting. This PhD attempts to improve waxy crude oil behaviour understanding thanks to experiment, modelling and numerical simulations in order to predict more accurately time and pressure required to restart the flow.
Using various contributions to the literature, waxy crude oils are described as viscoplastic, thixotropic and compressible fluid. Strong temperature history dependence plays a prevailing role in the whole shutdown and restart process. Thus, waxy crude oils under flowing conditions correspond to the non-isothermal flow of a viscoplastic material with temperature-dependent rheological properties. Besides, the restart of a waxy crude oil is simulated by the isothermal transient flow of a weakly compressible thixotropic fluid in axisymmetric pipe geometry. We retain the Houska model to describe the thixotropic/viscoplastic feature of the fluid and compressibility is introduced in the continuity equation. The viscoplastic constitutive equation is involved using an augmented Lagrangian method and the resulting equivalent saddle-point problem is solved thanks to an Uzawa-like algorithm. Governing equations are discretized using a Finite Volume method and the convection terms are treated thanks to a TVD (Total Variation Diminishing) scheme. The Lagrangian functional technique usually used for incompressible viscoplastic flows, is adapted to compressible situations.
Several numerical results attest the good convergence properties of the proposed transient algorithm. The non-isothermal results highlight the strong sensitivity of the flow pattern to the temperature changes in terms of yielded/unyielded regions. Then, the combined effects of compressibility and thixotropy have beneficial influence on the restart issue. In fact, a thixotropic flow, not able to start up in compressible situations, could be restarted thanks to compressibility. At last, comparison between numerical and experimental results allows to validate the numerical code.

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