Browsing by Author "Kara, Fuat"

Now showing 1 - 7 of 7
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    Investigation of slug mitigation: self-lifting approach in a deepwater oil field
    (Society for Underwater Technology, 2017-11-01) Adefemi, Israel; Kara, Fuat; Okereke, N. U.
    Slug flow is a flow assurance issue that staggers production and, in some cases, 'kills the flow' of the well. Severe slugging, a type of slugging which usually occurs at the base of the riser column, causes large amplitudes in the fluctuation of pressure within the riser column and consequently damages equipment placed topside. An adaptation of a novel concept to slug mitigation: the self-lifting model, is presented. This model presents variations to the internal diameter of the self-lift bypass to produce effective mitigation to severe slugging.
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    Numerical modelling of the electro-kinetic effect on pipe-soil interaction.
    (2017-10) Joshua, Hakuri Nwen; Kara, Fuat; Oakey, John
    The high pressure and high temperature (HP/HT) of operating fluid and soft soil content significantly account for the axial and lateral displacement of subsea pipelines. The ability of a pipeline to resist displacement depends on the soil strength. Due to low soil shear strength, the accumulation of displacement over a period of operating cycles leads to pipeline displacement. Increasing the pipe-soil interaction resistance will significantly reduce pipeline displacement. Soil contains solid, water and air particles. The levels of water, air and the solid particle size in the soil, govern its mechanical behaviour. Removal of pore water content from the soil voids may lead to consolidation with a resultant increase in effective stress. One of the methods used to increase soil strength is the Electro-Kinetic (EK) process. This is applied to increase the strength of onshore and offshore soil foundations. An important advantage is a reduction in time taken for the soil to consolidate. A Numerical model is developed for the determination of the EK effect on soft soil using the ABAQUS software tool. Two stages involved in the EK analyses are electro-osmotic consolidation and dynamic pipe-soil interaction. Three different test series each for electro-osmotic consolidation and dynamic analyses were built. The electro-osmotic consolidation analyses determined the soil consolidation followed by a dynamic pipe-soil interaction process. The electro-osmosis considered the effect of voltage variations, time taking for the soil to consolidate and variations with numbers of electrodes. The dynamic analyses consider the effect of electro-osmosis due to the displacement of pipeline in the vertical, axial, and lateral direction. From the electro-osmotic analyses, the soil settlement is due to its reduction in void volume as a result of the pore water pressure dissipation. A further interaction of pipeline on the (settled) consolidated soil indicates a significant improvement in the soil strength when compared with the non-treated soil.
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    Numerical prediction and mitigation of slugging problems in deepwater pipeline-riser systems
    (Cranfield University, 2015-11) Okereke, Ndubuisi Uchechukwu; Kara, Fuat; Oakey, John
    Slugging involves pressure and flowrate fluctuations and poses a major threat to optimising oil production from deepwater reserves. Typical production loss could be as high as 50%, affecting the ability to meet growing energy demand. This work is based on numerical simulation using OLGA (OiL and GAs) a one- dimensional and two-fluid equations based commercial tool for the simulation and analysis of a typical field case study in West Africa. Numerical model was adopted for the field case. Based on the field report, Flow Loop X1 consisted of well X1 and well X2, (where X1 is the well at the inlet and X2 is the well connected from the manifold (MF)). Slugging was experienced at Flow Loop X1 at 3000 BoPD; 4MMScf/D and 3%W/C. This study investigated the conditions causing the slugging and the liquid and gas phase behaviour at the period slugging occurred. The simulation work involved modelling the boundary conditions (heat transfer, ambient temperature, mass flowrate e.t.c). Also critical was the modelling of the piping diameter, pipe length, wall thickness and wall type material to reflect the field geometry. Work on flow regime transition chart showed that slugging became more significant from 30% water-cut, especially at the riser base for a downward inclined flow on the pipeline- riser system. Studies on diameter effect showed that increasing diameter from 8” – 32” gave rise to a drop in Usg (superficial velocity gas) and possible accumulation of liquids on the riser- base position and hence a tendency for slugging formation. Depth effect study showed that increasing depth gave rise to increasing pressure fluctuation, especially at the riser- base. Studies on the Self-Lift slug mitigation approach showed that reducing the internal diameter of the Self-lift by-pass pipe was effective in mitigating slug flow. S3 (Slug suppression system) was also investigated for deepwater scenario, with the results indicating a production benefit of 12.5%. In summary, the work done identified water-cut region where pipeline-riser systems become more susceptible to slugging. Also, two key up-coming slug mitigation strategies were studied and their performance evaluated in-view of production enhancement.
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    ItemOpen Access
    Numerical study of electro-osmotic consolidation effect on pipe-soil interaction
    (Elsevier, 2018-03-05) Joshua, Hakuri Nwen; Kara, Fuat
    Subsea pipelines are laid directly on seabed with further constraining measures to stabilise it against adverse effect of axial walking, upheaval buckling and lateral buckling. Costly mitigating measures are being employed and the need for further investigation to explore more option is considered. Stability of soil depends on the soil strength. Increasing the soil strength has been identified as a possible mitigation against pipeline displacement. Electro-osmotic consolidation process is currently being employed to increase soil strength around offshore and onshore structures, but the effect on pipe-soil interaction has not been fully investigated. This aspect received no attention on numerical model or detail experiment in this regard. The present study numerically investigates the effect of pipe-soil interaction using capabilities of commercial ABAQUS finite element software tool on both Electro-Kinetic (EK) treated and untreated soil to determine their behaviours. Results from this study when compared with non-EK treated soil, indicates remarkable developments, as the force required to displace pipeline increases significantly due to EK treatment.
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    Point absorber wave energy converters in regular and irregular waves with time domain analysis
    (SciDoc Publishers, 2016-12-15) Kara, Fuat
    A discrete control of latching is used to increase the bandwidth of the efficiency of the Wave Energy Converters (WEC) in regular and irregular seas. When latching control applied to WEC it increases the amplitude of the motion as well as absorbed power. It is assumed that the exciting force is known in the close future and that body is hold in position during the latching time. A heaving vertical-cylinder as a point-absorber WEC is used for the numerical prediction of the different parameters. The absorbed maximum power from the sea is achieved with a three-dimensional panel method using Neumann-Kelvin approximation in which the exact initial-boundary-value problem is linearized about a uniform flow, and recast as an integral equation using the transient free-surface Green function.The calculated response amplitude operator, absorbed power, relative capture width, and efficiency of vertical-cylinder compared with analytical results.
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    Steady state and transient liquid gas pipe flow models
    (Cranfield University, 2016-10) Oloruntoba, Olusola; Kara, Fuat; Oakey, John
    Two-phase ow analyses are critical to successful design and operations of liquid-gas pipe ow applications found in major industrial fields, such as petroleum, nuclear, chemical, geothermal and space industries. Due to diffculties in obtaining analytical solutions, approximate solutions have been applied to two-phase flows. However, several limitations still exist, and categorised into three prediction models, namely: ow regime, pressure gradient, and transient models. Previous studies show that existing ow regime models and maps for horizontal flows under-predicts transition from stratified to annular flow. Furthermore, there is requirement to include criteria for identifying mist and plug flows in unified flow regime model. In order to improve under-prediction in stratified to annular prediction, nondimensional liquid lm height in original criterion is replaced with nondimensional liquid holdup. This shifts stratified to annular transition line towards higher gas superficial velocity thus improving prediction. Using experimental data available in literature, a simple flow rate dependent criterion is proposed for identifying the existence of mist flow. Two criteria are proposed for identifying plug flow in horizontal and inclined flows. The first criterion is the exact criterion for identifying bubble flow in vertical flows. The second criterion is also based on bubble flow criterion but fitted to experimental data. Transition criterion for the existence of dispersed-bubble flow is also proposed, based on stability of gas bubble in liquid ow. These flow regime criteria are combined in a solution algorithm to obtain a unified flow regime model, which has been verified using existing unified flow regime models and map, and validated using experimental data. Mechanistic or phenomenal methods are generally applied in predicting pressure gradient in two-phase liquid-gas pipe flow. These methods relies on prior knowledge of prevalent flow regime, and subsequent application of flow regime specific pressure gradient model. This approach is susceptible to error should wrong flow regime be selected. In order to overcome this problem, a Single Equation Two-Phase Mechanistic (SETM) model is proposed. SETM is obtained by combining: liquidgas momentum equations, existing and modified flow regime criteria, and new flow regime boundaries at the initiation and completion of transition to annular flow. Thus, SETM implicitly determines pressure gradient and flow regime in liquid-gas pipe flow, and also captures liquid-gas interface transition from at to curved interface. SETM is applicable to all pipe inclination, and has been validated using experimental data available in literature. Further, prediction of flow characteristic features per ow regime, such as identified flow regime, liquid holdup in slug lm region, ratio of slug regions, and apparent liquid heights, have been verified against theoretical limits for different flow regimes. Alternative to SETM, modified homogeneous pressure gradient model is also proposed for liquid-gas pipe flow. Existing homogeneous models are applicable to dispersed bubble flow, and slug flow with low or negligible liquid-gas slip. The modified homogeneous model is obtained by correcting mixture fanny friction factor using error between experimental pressure gradient and unmodified homogeneous pressure gradient; observed error is particularly large at high liquid-gas slip values. The modified homogeneous model is therefore applicable to all flow regimes, including stratified, annular, and mist flows. The modified model has been verified against existing homogeneous model, and validated using published experimental data. Transient analysis is critical to liquid-gas pipe flow design. Rigorous analytical solution is generally not available. Alternative solution method is full numerical solution approach, which is subject to high demand on computational resources and time, especially for long pipelines. Hence simplified transient methods are sort. Existing simplified transient liquid-gas pipe flow models assume quasi-steady state conditions for liquid-gas momentum equations, thus neglecting convective terms in the momentum equations. The simplified transient liquid-gas pipe ow model proposed in this study include: (a) transient liquid-gas continuity equations, (b) transient convective terms of liquid-gas momentum equations, and (c) steady state pressure gradient terms of liquid-gas momentum equations. The proposed transient model captures gas and/or liquid flow variations at coarse pipe discretisation, and has been validated against published experimental data and verified with a proprietary program (OLGA).
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    Subsea controls future proofing: A systems strategy embracing obsolescence management
    (2013-11-01T00:00:00Z) Abili, Nimi; Onwuzuluigbo, Raluchukwu; Kara, Fuat
    The increasing world energy demands for enhanced oil and gas recovery in the offshore industry has led to new subsea technology developments with increased system functionality. Technologies such as subsea processing and instrumentations for subsea control data acquisition place a greater demand on bandwidth, power and capacity. Therefore, the legacy controls system becomes obsolete and proves unsustainable in supporting the increased functionality placed on it by the new subsea technologies. With the fast evolution of electronics, the replacement of the core components of the control system when they fail becomes difficult as they are no longer being produced or supported by the original component manufacturers (OCM). The present paper explores the different strategies in addressing obsolescence on the subsea controls system for offshore field developments.