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Interim Report on Pressure Effect on Waxy-Crude Pipeline-Restart Conditions Investigated by a Model System Yield Phenomenon and Pipeline Restart. Yield stress is the threshold stress required to initiate flow of a viscoplastic ᤙ... Interim Report on Pressure Effect on Waxy-Crude Pipeline-Restart Conditions Investigated by a Model System
Yield Phenomenon and Pipeline Restart. Yield stress is the threshold stress required to initiate flow of a viscoplastic fl uid. Waxy crude-oil gels are viscoplastic in nature and exhibit time-dependent Bingham plastic flow behavior during restart under imposed constant pumping pressure.
The yielding process of a gel is a complicated process， and different models have been proposed to explain this phenomenon. Wardhaugh and Boger (1991) defined yield stress as the “shear stress at which the gelled oil ceases to behave as a Hookean solid.” Chang et al. (1998) adopted a three-yield stress concept: elastic limit (which defines the onset of viscoelastic creep)， static (where fracture occurs)， and dynamic yield stress (which describes the broken down structure after yielding). Among these three， only the static yield stress is used to design the restart pump for initiation of flow after shut-in.
Several types of apparatus and different techniques (direct and indirect) have been used for determining the yield stress. The results obtained and conclusions regarding their validity and usefulness differ widely. Setups used are capillary U-tubes， large pilot pipeline facilities， lab-scale model pipelines， controlled-stress or -shear rotational rheometry experiments (CSR).
The capillary tube and pilot-scale pipeline measurement tech-niques have been criticized as being unsuitable for understanding the yielding behavior of statically cooled waxy crude oils because of nonlinear pressure distribution， pipe compressibility effects， porosity of wax structure， and contraction effects (Wardhaugh and Boger 1991). However， the authors added that an exception to their conclusion was the work by Ronningsen (1992)， where reasonable estimates of the yield stress as compared to model pipeline results were obtained with a constant stress rheometer. Recently， Lee et al. (2007) also reported the consistency between the concentric rheometer and a model pipeline system. In fact， model pipeline test is the most common laboratory method resorted to in gel strength measurement because of its geometrical resemblance to the real pipeline. In such tests， gel is formed under controlled conditions (cooling rate， aging temperature， etc.) and pressure is then slowly applied on one end of the gel until flow is observed. The gel strength (τ) is then calculated by:
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