Liyun Lao

Production Potential of Severe Slugging Control Systems

Abstract The application of riser top valve choking in severe slugging control has shown that feedback control stabilises slug flow with a valve opening larger than manual choking, resulting in an increased oil production. However, the reason and ultimate potential for an active slug control system to increase oil production, as well as how to achieve this potential are still unclear. A systematic method based on the pressure bifurcation map of a riser system is proposed in this work to analyse the production and pressure loss relationship at the different operating points resulting from the various slug control strategies. It is shown that for a given unstable riser production system with known inlet and outlet boundary conditions, production loss or gain due to operation in stable or unstable operating conditions could be predicted by using a pressure dependent dimensionless variable known as production gain index (PGI). This gives a clear indication of the ultimate potential to increase oil production through feedback control. This analysis has been successfully applied to an industrial riser system modeled in the commercial multiphase flow simulator, OLGA. Production predicted by using the PGI agrees with actual simulated production. The analysis is based on the understanding that the closed-loop stable operating point must match the corresponding open loop unstable equilibrium point. This result is very significant in planning and implementing suitable control strategy for stabilizing unstable riser-pipeline production systems with the aim of achieving stability and ensuring increased productivity, especially for brown fields.

ANUMET: A Novel Wet Gas Flowmeter

Wet gas metering is rapidly establishing itself as a special problem area within the more general area of multiphase flow metering. In fact, while attempts are being made across the Oil and Gas Industry to define the boundaries between humid gas, gas-condensate and high GVF (gas volume fraction) multiphase systems, a few commercial wet gas meters are already available for specific field applications. A major interest of Imperial College has been the development of metering schemes for three-phase (oil-gas-water) flows in oil-field production systems. Imperial College has already developed a flow meter (MIXMETER)1, which performs satisfactorily in the range of gas volume fractions of 0 to 90%. Imperial College has therefore been recently developing a new concept (ANUMET), which shows some promise for measuring two-phase flows at much higher gas volume fractions. The technical description of the ANUMET wet gas meter is outlined in this paper. Preliminary experiments on this new metering concept have been carried out with air-water flows in a 32 mm diameter tube in the LOTUS (LongTUbeSystem) facility at Imperial College. Encouraging results have been obtained and they are presented here.

Gas Injection for Hydrodynamic Slug Control

Abstract Gas injection as an effective method to mitigate hydrodynamic slug has been studied using OLGA simulation. Different control strategies have been investigated to reduce the amount of injected gas required to mitigate slugs. The control strategies are based on using a PI controller to control the valve opening of gas injection through various riser measurements used as controlled variables. The results show that the holdup transmitter at the riser top as the controlled variable is the best control strategy, followed by the differential pressure across the riser. It is also concluded that using riser top choking reduces the requirement of injection gas.

Effect of 180° bends on gas/liquid flows in vertical upward and downward pipes

Experimental investigation has been carried out on upward and downward vertical pipes with 180° bends to study gas-liquid two-phase flow behaviours in pipes with serpentine configuration. Wire mesh sensor (WMS) is installed at top and bottom positions of upward and downward sections in order to identify the void fraction distributions. Film thickness probes are employed to obtain circumferential profile of the liquid film thickness at different axial positions along both sections. Further features such as flow patterns are identified by examining the time trace and probability density function (PDF) data. All measurements are conducted for different superficial gas velocities, while superficial liquid velocity is fixed at 1.0 m/s. The study identified that the centrifugal force present in 180° bends caused a flow maldistribution in the adjacent straight sections. It is noted from the time trace and PDF results that the superficial gas velocity has obvious effects on the flow development along different positions of the pipes, where the flow regime varied over whole velocity ranges tested. These results are confirmed by the cross-sectional view and sliced stack images (longitudinal view) of the void fraction distributions. The results also showed that the flow behaviour in upward and downward pipes is affected by bends, although to varying degrees.

Physical properties of water-oil mixtures involving waxing

Non-Newtonian fluids exist extensively in the oil and gas industry and possess distinct physical properties that differ from those of Newtonian fluids. The multiphase flow involving non-Newtonian fluids have posed a serious challenge to the industry; however, comprehensive information expressing a wider knowledge of the mixture properties have not been well acquired. The focus of this paper is on water-oil mixtures involving petroleum wax, and seeks to highlight the waxing issue which is frequently encountered in offshore oil and gas transport pipelines. Experimental tests are carried out to investigate the behaviour of the water-oil mixture (with wax composition) and examine their fundamental physical and rheological properties. Furthermore, the effect of changes in temperature and varying water volume fraction on the shear rate – shear stress characteristics of the mixture are also considered. Results from this study show that the mixture properties depend significantly on temperature changes, fluid composition and the water content. Investigations also suggest that the phase inversion phenomenon has a significant impact on the shear rate-shear stress characteristics of the non-Newtonian oil/water mixture.

On-line mixing and emission characteristics of diesel engine with DME injected into fuel pipeline

This article presents a new on-line dimethyl ether/diesel mixing method, researches its blend characteristics, and also validates combustion and emission effects on a light-duty direct injection engine. This new blend concept is that dimethyl ether is injected into the fuel pipeline to mix with local diesel as the injector stops injection, and this mixing method has some advantages, such as utilization of the original fuel system to mix dimethyl ether with diesel intensively, flexibility on adjustable mixing ratio varying with the engine operating condition, and so on. A device was designed to separate dimethyl ether from the blends, and its mixing ratios and injection quantity per cycle were also measured on a fuel pump bench. The results show that compared with the injected diesel, the percentages of dimethyl ether injected into fuel pipeline are 13.04, 9.74, 8.55, and 7.82% by mass as the fuel pump speeds increase, while dimethyl ether injected into fuel pipeline are 45.46, 35.53, 31.45, and 28.29% of wasting dimethyl ether. The power outputs of engine fueled with the blends are slight higher than those of neat diesel at low speeds, while at high speeds, its power outputs are a little lower. Smoke emissions of the blends are lower about 30% than that of neat diesel fuel at medium and high loads with hardly any penalty on smoke and NOx emissions at light loads. The NOx and HC emissions of the blends are slight lower than that of neat diesel fuel at all loads.