Severe slugging and unstable flow in an S-Shaped riser has been investigated in three research themes - experiments, criterion modelling and transient code modelling. A series of experiments were carried out on the Cranfield University Three Phase Facility and Riser Rig using a 10 m high riser over the pressure range 2,4 and 7 bar(a). The collected information was used to characterise the unstable flows in terms of pressure cycling, riser liquid inventory and fluid production characteristics. From analysis of the flow behaviour, it was found that in terms of slug characteristics, transitional severe slugging and oscillation flows are as potentially problematic as 'classical' severe slugging. This is due to the magnitude of peak flow in excess of the average fluid throughput in the riser and the size of the liquid slugs generated. A criterion for the occurrence of unstable flows in an S-Shaped riser was developed based upon considerations of bubble penetration at the riser base. This proved to be successful at predicting the experimental results from this work and an independent source. Comparisons were made between the experimental results and a transient code. The code could predict the occurrence of 'classical' severe slugging however the detailed characteristics of the experiments were not predicted by the code. The results for transitional severe slugging and oscillation flow showed further differences between the code prediction and the experiments. Drawing all simulation results together it was concluded that the prediction of pipeline behaviour and the propagation of flow regimes local to the riser base and in the curved riser pipe were significant sources of error in the simulation.

No description available.

No description available.

There is a growing concern amongst offshore pipeline operators over the instability problem of lateral buckling and axial walking of offshore pipelines which is linked with elevated operating temperature and pressure. While some mitigating options are available to tackle this phenomenon, in most cases these are expensive and impracticable in deep waters, and none ofthem involves the modification of the ambient soil properties typically characterised by very low undrained shear strength (Cu) and high water content (w). In recent times, the use of engineered buckle solutions has become generally accepted as a cost effective and elegant solution. This option involves laying the pipeline in a snake configuration where some specific sections are designed to move during operation while others relatively stable. This option depends on accurate understanding of pipe-soil interactions which presently poses the greatest uncertainty in pipeline design. Furthermore, in order to ensure that the buckles are formed as predetermined, the ambient soil strength must be sufficient to resist the pipe motion at locations designed to be relatively stable or the entire design approach would be undermined. This dissertation presents laboratory investigations at both small and pilot scale directed at using the electro-kinetic phenomenon (EK) to treat the soil around a partially buried pipeline with the aim of increasing the pipeline stability to lateral buckling and axial walking. The influence of the EK treatment was assessed by evaluating the changes in the soil Cu, wand the soil resistance to vertical, lateral and axial displacements of pipe sections. Additionally, large-scale pipe-soil interaction studies were conducted to study the soil deformations, especially the real time study of the berm of soil formed and the development of the soil resistance during pipe motion. Preliminary results of the application of EK in geotechnical engineering to offshore pipelines show up to 600% increase in Cu, 14% decrease in w and 190% increase in the pull out force thus implying promising outcomes which could form the basis for subsequent research in this area.

The study looks at the rheology and pipeline flow properties of colloidal suspensions dispersed in electrolytes of different concentration and type. Fuso silica spheres (0.8pm) form the dispersed phase of the suspension, and KN03, KCI electrolytes the continuous phase. The strength of the particle-particle interaction is significantly influenced by the electrolyte concentration. An increase in the electrolyte concentration from 10 4M to IM results in the formation of aggregates, thus influencing the sedimentation, sediment bed structure and pipeline transportation properties. Silica aggregates formed in IM electrolytes are on average 5.75 times bigger than the primary particle. Pipeline transportation studies have shown the aggregated suspension to have a lower minimum transport velocity than the dispersed suspension. Such behaviour is believed to be related to interfloc flows within the aggregate, enhancing the level of fluid turbulence. The centre-line and near wall turbulence intensities are enhanced in the presence of aggregates, while dispersed suspensions have negligible effect on modulating the fluid turbulence. Measurement of the streamwise RMS shows two critical Reynolds numbers with increasing flow velocity. The first critical Reynolds number (Re = 5500) occurs when the RMS profile of the aggregated suspension diverges (exceeds) from the RMS profiles of the dispersed and single phase flows. This enhancement is thought to be related to interfloc flows. A second critical Reynolds number (Re = 8000) is identified when the RMS profile of the aggregated suspension begins to converge with the dispersed and single phase RMS profiles. Convergence of the RMS profiles is related to the break-up of aggregates once a critical fluid shear stress is exceeded. Aggregate break-up data is verified with results collected using a Bohlin CVO-R rheometer.

The work described in this thesis addresses the issue of flow assurance in hydrocarbon recovery systems. Deposition of wax and formation of hydrates are amongst the critical flow assurance challenges that need to be resolved to avoid serious reduction in hydrocarbon flows emanating from wells and passing through flow lines (which are often sub-sea) to processing facilities. The hydrocarbon streams need to be maintained at temperatures above those at which solids formation occurs. The work described in this thesis is focussed on active heating of the flow lines by arranging them in a tube bundle in which a heat source is provided by a hot fluid (typically water) flowing in a separate tube. The objective of the work was to develop a generic methodology for the prediction of such bundle systems using Computational Fluid Dynamics (CFD) to generate a heat transfer data base and to interpolate this data base using neural network methods. It was convenient to develop this methodology by focussing on a specific geometry (namely a 3 meter long bundle with 4 internal pipes - product, test, heat flow and heat return respectively). Use of this geometry allowed direct validation of the computational method since an experimental investigation of an identical geometry was carried out in a parallel research project. The CFD methodology was first used to investigate the design of the experiments; it was shown that changes were needed in rig flows and in the sensitivity of the temperature measurements to ensure that the experiments complied closely enough with the basic assumption the each of the surfaces was isothermal. Once changes were made, the experimental and computational results were directly compared and satisfactory agreement was observed. For the bundle in a horizontal orientation, a two-dimensional (2D) CFD solution was used and a data base of 1683 solutions (covering the expected range of surface temperatures) was created. A further set of 1053 three-dimensional (3D) cases were calculated for the situation where the bundle was inclined at up to 90o to the horizontal. Both sets of data were fitted by means of neural networks which allowed prediction of bundle behaviour for a wide range of conditions.

This thesis considers steady, fully-developed flows through weakly curved pipes using the extended Stokes series method. The Stokes series for pipes of circular cross-section is expanded in powers of the Dean number, K, to 196 terms by computer. Analysis shows that the convergence is limited by an imaginary conjugate pair of square-root singularities K = ±iKc. Contrary to previous analysis of this solution, analytic continuation of the series indicates that the flux ratio in a weakly curved pipe does not vary asymptotically as K−1/10 for large K. Using generalised Padé approximants it is proposed that the singularity at iKc corresponds to a symmetry breaking bifurcation, at which three previously unreported complex branches are identified. The nature of the singularity is supported in part by numerical consideration of the governing equations for complex Dean number. It is postulated that there exists a complex solution to the governing equations for which the azimuthal velocity varies asymptotically as K−1/2, and the streamfunction as K0 near K = 0. This is supported by the results from the generalised Padé approximants. Brief consideration is given to pipes of elliptic cross-section. The Stokes series for pipes of elliptic cross-section for various aspect ratios, λ, is expanded up to the K24 term by computer. For small K, it is found that the flux ratio achieves a minimum for aspect ratio λ ≈ 1.75. This, and the behaviour of the total vorticity, is in agreement with previous studies which found that the effect of the curvature is reduced in the limit of small and large aspect ratios. The convergence of the series solution is found to be limited by an imaginary conjugate pair of square-root singularities K = ±iKc(λ), which varies with λ.

Progressive cavity pumps are used in the transportion of slurries. The pumping element consists of a single helical rotor, which intermeshes with a double helical resilient stator, to create the moving cavities that transport the slurry. Both components suffer from weaL at different rates, due to relative sliding movement of the rotor to the stator, and the presence of the abrasives carried within the slurry. for a pump manufacturer to remain active in the market they must provide the customer with optimised material selection, for both wearing parts, at a competitive price. Wear is not an intrinsic material property and its value is dependent upon the conditions within each individual tribological system. In order to improve or optimise the wear life of a system, it is first vital to understand the complexity of the mechanisms that generate the material loss. This thesis achieves this goal, with specific reference to a pumping system, by analysing the wear mechanisms of the pumping element components in progressive cavity pumps and evaluating how the wear severity changes with the system parameters. The in-depth study has enabled a new wear model to be proposed which describes how the behaviour of the abrasive particles contribute to the wearing process, in the pumping element of a progressive cavity pump. Hard particle laboratory wear tests were reviewed and assessed to determine their suitability for assessing the wear performance of rotor and stator materials. It was concluded that no one standard laboratory test was suitable and recommendations are given for two tribometers which specifically meet the tribological needs of the pumping system

No description available.

The Gas-Liquid Cylindrical Cyclone (GLCC) separator is commonly used for the separation of oil and gas mixtures flowing from the well head. Similar to the design used by other separators, it has an inlet and two outlets for gas and liquid respectively. However, the inlet to the separator can either be single or dual type. The pipeline connection from the upstream preconditioning equipment (CFC) is inclined downwards and has a tangential inlet slot. The essence of having a downward inclination is to promote pre-separation of the fluid phases. On the other hand, a tangential inlet promotes circular fluid motion thereby inducing separation of the fluid phases by centrifugal forces. Due to the complex behaviour of the flow within the GLCC, liquid carry over (LCO) as drops into the gas phase pipeline and gas carry under (GCU) as bubbles into the liquid phase pipeline are inevitable. Both phenomena greatly reduce the purity of the fluid phases at the outlets. To overcome this challenge, it has been proposed from field experiments carried out by Chevron Energy Technology Company, to precondition the influent flow in an upstream vertical pipe before entrance to the GLCC. In order words, a suggested solution to overcoming liquid carry over (LCO) and gas carry under (GCU) is to precondition the oil/gas mixture by forcing small bubbles/drops of 3 - 5mm in diameter to coalesce in an upstream vertical pipe. The upstream vertical preconditioner is known as a Churn Flow Coalescer (CFC). This is because the churn flow regime is the most suitable for the coalescence of both liquid and gas phases. Therefore, it is in the scope of this research work to carry out detailed preconditioning experiments within an upstream vertical pipe that serves the purpose of a Churn Flow Coalescer (CFC). All experiments in this research work have been carried out in the Chemical and Environmental Engineering L3 laboratory at the University of Nottingham. Although, the churn flow regime is specifically the most suitable for the GLCC, the operational envelope for the initial set of experiments spans the bubble to churn regimes. This is because the experiments were performed with the aim of delineating the conditions for the inception of typical churn flow in a large diameter pipe. These set of experiments were conducted in a 121mm internal diameter, 5.3m in length vertical pipe using air and water as the operating fluids. In these experiments, slug flow characterised by a Taylor bubble and a liquid slug was not observed. The churn flow regime is made up of two sub-regimes namely: liquid bridging of the gas core and formation of huge waves. The former is a phenomenon that occurs when the liquid phase forms a bridge as a result of radial coalescence of the wave crests flowing about the pipe centreline and momentarily blocks the entire pipe cross-section. The huge waves occur when the liquid phase flows as waves on the inner walls of the pipe and about the pipe centreline having large amplitudes. Between bubble to churn flow regimes in these experiments, four regimes were observed namely, discrete bubbly flow and spherical cap bubbly flow which make up the bubbly flow regime, churn turbulent regime (transition region) and typical churn flow regime. These experiments paved way for detailed experiments to be carried within the churn flow regime. Detailed churn flow experiments were then carried out in a large scale closed loop facility having an internal diameter of 127mm and a longer vertical pipe of 11m. The rationale for performing the experiments in this facility is because the facility offered a wider range of conditions within the churn flow compared to the first experimental rig facility. Data was acquired at L/D = 2.4, 7.1, 30.7, 35.4 and 82.7 which represent different axial distances from the gas-liquid injection at the base of the test section. Air and water were also used as the operating fluids. The void fraction data acquired at different axial distance from the injection varies logarithmically with increase in axial distance. The flow can be considered to be developed at L/D = 82.7 based on the void fraction. In addition, the frequency of liquid bridging of the gas core decays with increasing distance from the injection (downstream) while the frequency of the huge waves and liquid structures entrained in the gas core increases downstream from the injection due to coalescence. Finally, the effect of viscosity in the churn flow regime was investigated using air-glycerol/ water as the fluid pairs in the same large scale loop facility. Two glycerol/water mixtures were used having viscosities of 12.2cP and 16.2cP respectively. The data was acquired at a suitable axial distance from the injection at L/D = 65.5. In this experimental campaign, the size and frequency of the liquid structures entrained in the gas core are larger compared to the liquid structures present when experiments were carried out using air-water as the operating fluids. As a result, this gives a bi-modal probability distribution for air-glycerol/ water compared to air-water. Similar to the air-water experiments, the liquid bridging operating condition gives a high degree of coalescence of both phases. The mechanism of entrained liquid structure formation has been proposed based on the comparative study to the air-water experiments. A model has also been developed that predicts the effective length of pipe for the Churn Flow Coalescer (CFC). Overall, the liquid bridging sub-regime of churn flow should be the prevailing condition in the CFC to enhance proper separation of gas and liquid phases in the downstream GLCC separator.