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Title: The development of a knowledge-based wax deposition, three yield stresses model and failure mechanisms for re-starting petroleum field pipelines : building on Chang and Boger's yield stresses model, Bidmus and Mehrotra's wax deposition and Lee et al.'s adhesive-cohesive failure concepts to better underpin restart operation of waxy crude oil pipelines
Author: Fakroun, Abubaker A.
ISNI:       0000 0004 7660 533X
Awarding Body: University of Bradford
Current Institution: University of Bradford
Date of Award: 2017
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Twenty years ago, Chang et al. (1998) introduced the three-yield stresses concept (dynamic, static and elastic limits) to describe yielding of waxy crude oils cooled below the wax appearance temperature (WAT). At the time, the limits in rheological instruments were such that they never actually measured the elastic-limit, a key fundamental property. Using modern instruments, this research succeeds in recording for the first time the entire yielding process down to stresses of 10-7 Pa and shear rate of 10-6 min-1 as a function of temperature, cooling rate and stress loading rate using two waxy oils of different origins and wax content. A four-yield stress model is established using derivative data (dynamic fluidity and failure acceleration). In addition, calorimetry (DSC) and microscopy (CPM) helped extract WAT, the gel and pour points and link gel crystal structure and its yielding and breakage to rheological properties. The yielding stresses measured rheologically were tested in laboratory pipelines at two diameter scales, 6.5mm and 13.5mm to compare stresses in uniform and non-uniform cooling. It is demonstrated that rheological instruments can only predict gel breaking pressure when the cooling rate is low, i.e. yielding at the pipe wall. A complementary heat transfer study was performed on a section of pipe statically cooled, both experimentally and theoretically to predict the gel front-liquid oil interface that develops in industrial pipeline where gel breaking occurs. This key information together with rheological data provide the means to predict accurately restart pressures of shut gelled pipelines that have eluded previous research.
Supervisor: Not available Sponsor: Ministry of Higher Education of the Libyan Government
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Elasticity ; Yield stresses model ; Wax deposition ; Effective yield stress ; Waxy crude oil ; Failure acceleration ; Pipeline restart ; Adhesive-cohesive failure ; Crystallisation ; Morphology