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Three phase slug flow in complex terrain (THREEPLEX)

Deliverables

New gamma and X-ray instrumentation has been developed to address the crucial issues of flow development and phase distributions in gas-liquid and gas-liquid-liquid slug flow. The gamma instruments were of four types; namely traversing beam; single beam and triple beam dual energy densitometers and a novel dismountable single beam; single energy device (DISGAM). The dual energy devices allowed average and time varying phase fractions (oil; water and gas) to be determined. The single beam unit; operated in a fixed chordal position; whereas the triple beam allowed simultaneous measurement along three chords in a given cross section. The traversing beam gamma densitometer was used for successive measurements along 15 chords; conditional sampling allowed measurements to be made of slug body holdup distributions. The DISGAM units were used primarily for tracking slugs along the test section. An X-ray tomography system was acquired which has two X-ray beams each passing through the test section and onto a 256 diode line array. This system has been included in the Imperial College high-pressure flow facility and calibration tests completed. In parallel; extensive work was carried out on the development of tomographic algorithms for interpretation of the data.
Innovative gamma-ray tomography methods were developed for obtaining distributions of the phases (air; oil and/or water) in the slug body for slug flows in both horizontal and deviated ('V') test sections. The tomographs reveal (for the first time) regions of mixing and separation between the phases. It was observed that the slug could be approximated as having a two-layer structure; with a lower layer consisting of unaerated liquid and an upper layer consisting of a gas-liquid mixture. As the water flow fraction was increased; the height of the liquid layer and the void fraction in the upper region also decreased. It was observed that the oil tended to separate near the pipe walls. For flows in a "V"-section; tomographs obtained at the end of the uphill leg illustrated greater mixing with a more even distribution in the phases being observed. The results obtained in this study offer an explanation as to why models that assume a homogeneous mixture in the slug body are unable to predict the liquid hold-up in the slug body for a wide range of conditions. Calculations have shown that treating the slug as having two distinct layers gives rise to better predictions.
The first unambiguous detailed measurements of slug initiation and development in both horizontal and V-geometries; giving novel insights into the nature of interfacial growth leading to the formation of slugs. Slugs were tracked at 15 positions for air-water tests and 7 positions for air-oil-water tests. In horizontal flow; high slug frequency is obtained near the inlet of the pipe in both two-and three-phase tests and this reduces to an asymptotic value with increasing distance. In three-phase flow; slugs are initiated nearer to the inlet section with increasing water cuts and the final slug frequency is higher for a more viscous liquid mixture. These results contrast with the best available prediction methods; which do not indicate such a maximum near the entrance. In the three-phase slug flow terrain study; a V-configuration was used where the downward section was 14 m and the upward section was 20 m. Three detector systems were placed before the dip and four placed after. Differences in flow regime before and after the dip were observed for a variety of water cuts. For low water cuts; stratified flow is predominant in the downhill leg. For high water cuts; slugs can be seen to initiate in the downhill leg.

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