In this paper, we investigate the problem of the start up of gelled waxy crude oil flows in a pipeline. Waxy crude oils are deemed to be weakly compressible, thixotropic and viscoplastic material. In severe situations, the waxy crude oil is fully gelled in the pipeline and the main issue concerning the restart of waxy crude oils relies on the estimation of the minimum pressure required at the pipe inlet. The objective of this study is to examine the possibility that the flow can restart for a pressure drop below the value predicted by the conservative relation Δp=4τ_yL/D, where τy denotes the yield stress, L the pipe length and D the pipe diameter, thanks to the combined effects of compressibility and thixotropy. Using a 1.5D numerical model, we evidence that there exists situations where, though the pressure drop is below the theoretical minimum pressure drop 4τ_yL/D, the flow restarts thanks to the combined effects of thixotropic and compressibility. From a parametric survey, we derived a new analytical relation able to predict more accurately the minimum pressure required to restart pipelines filled with a gelled waxy crude oil. This relation accounts for not only viscoplastic effects but also for compressibility and thixotropic effects.
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