Severe Slug Flow Research Articles

Attenuating severe slug flow at large valve opening for increased oil production without feedback control signal

In the co-current flow of gas-liquid mixtures through pipeline-riser system, severe slugging is frequently encountered and manifests in significant fluctuation of flow and pressure. This can pose serious threat to production facilities. The most common method for its mitigation is by choking which unfortunately could negatively affect production. The objective of this study therefore is to develop a technique that can help stabilise the system and maximise production simultaneously.In this paper, a new general method for multiphase flow system stability analysis was proposed based on a new passive attenuation method – the intermittent absorber. A series of experiments were carried out in a 4″ pipeline-riser system which is 55m long with horizontal pipe inclined at −2° connected to 10.5m high catenary riser followed by 3m horizontal topside section. Air and water were used as experimental fluids. Numerical studies were also conducted on a 4″ pipeline-riser system to proof the concept.The results showed that the intermittent absorber possess potential for stabilising severe slug flow at larger valve opening and lower pressure. For the case studied, up to 9% reduction in the riser base pressure was reported which practically implies increased oil production.

Influence of separator control on the characteristics of severe slugging flow

Due to the special structure of offshore multiphase pipes, it is easy for severe slugging to occur in the riser at low gas-liquid velocity. Violent pressure fluctuations and dramatic changes of flow rate are the main characteristics of severe slugging, leading to the risk of serious damage. In this paper, the separator control is adopted to accurately control the separator liquid level and pressure under severe slugging flow conditions. This indicates that the separator liquid level control alone does not have a significant impact on the upstream flow, but it is beneficial for normal operation and pressure control of the separator. As the separator pressure increases, the peak pressure in the riser apparently diminishes, and the amplitude of pressure fluctuation gradually decreases, which means that severe slugging is inhibited. During the slug blowing out, the gas/liquid slipping in the riser intensifies. The long gas plug quickly flows through the riser, and then tends to morph into short and slowly flowing gas bubbles. The elimination effect of the pressure control strategy on severe slugging is related to the relative rate of the superficial gas/liquid flow.

Closed-loop model identification and PID/PI tuning for robust anti-slug control

Active control of the production choke valve is the recommended solution to prevent severe slugging flow at offshore oilfields. This requires operation in an open-loop unstable operating point. It is possible to use PI or PID controllers which are the preferred choice in the industry, but they need to be tuned appropriately for robustness against plant changes and large inflow disturbances. The focus of this paper is on finding tuning rules based on model identification from a closed-loop step test. We perform an IMC (Internal Model Control) design based on the identified model, and from this we obtain PID and PI tuning parameters. In addition, we find simple PI tuning rules for the whole operation range of the system considering the nonlinearity of the static gain. The proposed model identification and tuning rules show applicability and robustness in experiments on a test rigs as well as in simulations using the OLGA simulator.

Severe Slugging in a Free-Standing Riser

A series of experiments on air/water two-phase flow in a flowline riser have been performed. The riser has a free-standing form (7 m high) with the flowline of a 2° downward inclination toward the riser-base. Severe slugging flows are described in the free-standing riser (FSR). Then, experimental pressure comparison between the FSR and L-shaped riser is presented, and three differences are analyzed. Finally, the behavior of severe slugging flow in the FSR was simulated with OLGA software. In conclusion, it is difficult to do the theoretical simulation of severe slugging in a FSR.

Nonlinear model-based control of two-phase flow in risers by feedback linearization

Active control of the production choke valve is the recommended solution to prevent severe slugging flow conditions at offshore oilfields. The slugging flow constitutes an unstable and highly nonlinear system; the gain of the system changes drastically for different operating points. Although PI and PID controllers are the most widely used controllers in the industry, they need to be re-tuned for different operating conditions. The focus of this paper is to design a model-based nonlinear controller in order to counteract nonlinearities of the system. Feedback linearization based on the riser-base pressure and the topside pressure was used for the control design. Stability and convergence of the closed-loop system was verified in theory as well as by experiments on a test rig.

Experimental study on severe slugging mitigation by applying wavy pipes

Wavy pipes were installed in the pipeline for mitigating severe slugging in pipeline/riser systems. Experimental results have revealed that: with a wavy pipe applied, the operating region of severe slugging is reduced; the severity of severe slugging and oscillation flow is mitigated; the wavy pipe performs better with its outlet located upstream of the riser base. The wavy pipe is essentially reducing the slug length. For severe slugging the wavy pipe works by accelerating the movement of the gas in the pipeline to the riser base; for the oscillation flow it works by mixing the gas/liquid two phases.

Controllability analysis of severe slugging in well-pipeline-riser systems

Active control of the production choke valve is the recommended solution to prevent severe slugging flow conditions at offshore oilfields. The focus of this work is to find the structure of a simple, yet robust anti-slug control system. In order to find suitable control variables for stabilization, a controllability analysis of the system with different available measurements or different combinations of them was performed. Moreover, for including robustness and performance requirements at the same time, the controllability analysis was extended to a mixed sensitivity ℋ∞ optimization problem. Two case studies were considered; first, the controllability analysis was performed on a pipeline-rise system using a 4-state model for comparing the results to the previous works. Next, using a 6-state model, the results were extended to a more general well-pipeline-riser system. The controllability results were in accordance with the practical experience in anti-slug control.

Simplified Dynamical Models for Control of Severe Slugging in Multiphase Risers

AbstractIn order to prevent the severe slugging flow regime in multiphase transport pipelines, active feedback control is the recommended solution. Instead of elaborated models such as CFD and OLGA® models, a simple dynamical model with few state variables is required in a model-based control system. We propose a new simplified dynamical model for severe slugging flow in pipeline-riser systems. The proposed model, together with five other simplified models existing in the literature, is compared to the results from the OLGA® simulator. The proposed model could be able to maintain the main dynamics of the severe slugging flow regime.

Severe Slugging In a Catenary-shaped Riser: Experimental and Simulation Studies

: A series of experiments on two-phase, air-water flow in a flowline-riser geometry have been made. The riser has a catenary-shaped form (10.5 m high), and the flowline has a 2-degree downward inclination toward the riser-base (10.16 cm ID). Experimental data were used to characterize the severe slugging and unstable flows in terms of pressure cycling and fluid production characteristics. A model to study dynamic behavior of severe slugging flow in a pipeline-riser system was also developed under the environment of the OLGA2000 code, and experimental test results were compared with the OLGA model predictions. The motive for this work was to identify areas that pose substantial difficultly to the code for the simulation of severe slugging in a catenary-shaped riser. In addition, numerical experiments, exploring some of the reasons for difference between experimental and simulation results, have been presented in this article.

Active Feedback Control as a Solution to Severe Slugging

Summary Severe slugging in multiphase pipelines can cause serious and troublesome operational problems for downstream receiving production facilities. Recent results demonstrating the feasibility and the potential of applying dynamic feedback control to unstable multiphase flow like severe slugging and casing heading have been published.1–5 This paper summarizes our findings on terrain-induced slug flow.5 Results from field tests as well as those from dynamic multiphase flow simulations are presented. The simulations were performed with the pipeline code OLGA2000.* The controllers applied to all of these cases aim to stabilize the flow conditions by applying feedback control rather than coping with slug flow in the downstream processing unit. The results from simulations with feedback control show stable process conditions at both the pipeline inlet and outlet in all cases, whereas without control, severe slug flow is experienced. Pipeline profile plots of the liquid volume fraction through a typical slug flow cycle are compared against corresponding plots with feedback control applied. The comparison is used to justify the internal stability of the pipeline. In many cases, feedback control enables a reduced pipeline inlet pressure, which, again, means an increased production rate. This paper summarizes the experience gained with active feedback control applied to severe slugging. The focus is on extracting similarities and differences between the cases. The main contribution is demonstrating that dynamic feedback control can be a solution to the severe slugging problem.

Multiphase-Pump Field Trials Demonstrate Practical Applications for the Technology

Summary This paper presents the results of two multiphase-pump field trials. One field trial was conducted offshore on a platform in the Gulf of Mexico (GOM). It is a low-pressure boost (100 psi) application involving gas-lifted wells. The other field trial was conducted onshore in an oil field in Alberta, Canada. This multiphase pump was designed for a high-pressure boost (850 psi) capability with primarily rod-pumped wells feeding the suction of the pump. The offshore pump was sized to handle the flow from one well. By lowering the back pressure on the well, increased production was realized. The increased flow from one of the wells far surpassed the predicted quantity. Early problems with the double mechanical seal system were overcome and a new, simplified single mechanical seal system has been designed and installed. The onshore multiphase pump clearly demonstrated that a twin-screw pump can operate reliably in a field environment, even under severe slug flow conditions. The trial indicated that a considerable portion of the liquid in the recycle stream (required because of the high gas fraction of the multiphase fluid from the field) flashes into gas, which occupies more volume in the pump than if it remained liquid. This decreased the capability of the pump to handle net flow from the field. These conditions motivated a re-evaluation of the pump-sizing techniques. Performance data and lessons learned information are presented for both field trials.

Severe Slugging in Offshore Pipeline Riser-Pipe Systems

Abstract Severe slug flow (i.e., terrain-dominated slug flow) was studied in a simulated offshore pipeline riser-pipe system. Severe slug flow is characterized by extremely long liquid slugs generated at the base of the vertical riser. This phenomenon occurs at low gas and liquid flow rates and for negative pipeline inclinations. Slugging in some offshore platforms has required the use of operating procedures that drastically curtail production. Losses in flow capacity up to 50% have been reported. A hydrodynamic model has been developed for severeslug flow. The model's predictions agree with experimental data. The model can be used to design new pipeline riser-pipe systems or to adjust the operation of existing systems to prevent the occurrence of severe slug flow. Also, a flow-regime map is presented for predicting the severe slug flow regime, where the boundaries are determined analytically. Finally, additional methods are proposed to prevent the flooding of separation facilities by riser-pipe generated slugs. This study is an extension of Ref. 1, in which severe slug flow was introduced and was only partially modeled.

Experimental Study of Severe Slugging in a Two-Phase-Flow Pipeline - Riser Pipe System

Abstract Slug flow was studied in a simulated, offshore, pipeline-riser pipe system. Two distinct slug flow patterns were identified: severe slugging and normal slug flow. Severe slugging, characterized by generation of slugs ranging in length from one to several riser pipe heights, occurs at low gas and liquid flow rates and for negative pipeline inclinations. A mathematical model was developed for severe slugging. Results agree well with experimental data. Choking was found to be an effective method of eliminating severe slugging.