A Study on the Multiphase Flow Characteristics Inside ...

A STUDY ON THE MULTIPHASE FLOW CHARACTERISTICS INSIDE SUBSEA SEPARATION SYSTEM

Nam-Sub Woo Korea Institute of Geoscience

and Mineral Resources Daejeon 305-350, Korea

[email protected]

Young-Ju Kim Korea Institute of Geoscience

and Mineral Resources Daejeon 305-350, Korea [email protected]

Sang-Mok Han Jeollabuk-do Branch Institute,

KOMERI Gunsan 573-540, Korea

[email protected]

Jae-Ki Kwon Korea Institute of Geoscience

and Mineral Resources Daejeon 305-350, Korea

[email protected]

Sang-Shik Kim Gyeongsang National University

Chinju, Gyeongnam, Korea [email protected]

Hee-Hak Ahn DK-LOK Corporation

Gimhae 621-842, Gyeongnam, Korea

[email protected]

ABSTRACT The implementation of subsea separation and liquid

boosting is becoming a common development scheme for the exploration of deep water fields. Subsea separation system should be reliable to ensure successful operation in a wide range of 3-phase flow regime, without need for developments. A subsea separator can avoid or simplying costly surface platforms of floating vessels, as well as being an efficient tool to enhance hydrocarbon production. One solution of interest is the separation and re-injection of water at the seabed to avoid bringing the water up to the surface facility. In this study, multiphase flow characteristics inside subsea separation system are investigated for the design of subsea separation system.

1. INTRODUCTION

In recent years, subsea processing, and more specially subsea separation, has been recognized as one of the most promising technology developments in the offshore industry.

Performing a part of the required separation process at the seabed, can enable a more effective production and reduce need for topside processing capacity. Subsea separation has already been applied for both a greenfield development as well as a retrofit solution for a field in production. The solution can

contribute to a better environment and higher revenue. For maturing fields, the production and recovery can increase cost-efficiently by improving and prolonging the use of existing infrastructure. Separation and re-injection of bulk water will free up space in flowlines and risers and eliminate the need for topside equipment upgrades.

Currently, subsea processing projects can be found in nearly every major offshore oil and gas region in the world, with the North Sea and offshore Brazil (Campos and Espirito Santo basins) experiencing the most activity to date. While still an emerging market, with the number of installed systems relatively small, recognition of the potential returns, and hence the level of acceptance of subsea processing technologies, continues to increase. Subsea processing is now appearing in development options for many industry projects (Michaelsem, 2003).

Subsea separation and processing systems development and field application projets are relatively common in a few developed countries and many studies have been conducted previously (Di et al., 2011; Bymaster et al., 2011; Hannisdal et al., 2012; Rune et al., 2004), but it is still a new field in Korea and this is the motivation of this study.

In this study, vertical type subsea separation system[2] simulation study is carried out as a process of subsea separation system development especially, focused on the investigation of gas/liquid mixture flow characteristics inside separators.

Proceedings of the ASME 2014 International Mechanical Engineering Congress and Exposition IMECE2014

November 14-20, 2014, Montreal, Quebec, Canada

IMECE2014-36952

1 Copyright © 2014 by ASME

2. SUBSEA SEPARATING SYSTEM

2.1 SUBSEA PROCESSING TECHNOLOGY Raw petroleum products consist of a variable mixture of

oil, condensate (light oils), natural gas, formation water, and formation solids (sands). Many of the technological challenges associated with petroleum production in the deep sea are a result of the mixture of these components.

In particular, the presence of water along with gas in the product stream increases the potential for hydrate formation and subsequent clogging of the pipelines. Figure 1 shows the general subsea processing solution together with multiphase pumping and subsea separating.

There are a number of reasons why operators may choose to install subsea processing equipment. First of all, most subsea processing will increase the recovery from the field, thus increasing profits. Additionally, by enhancing the efficiency of flowlines and risers, subsea processing contributes to flow management and assurance. Also, subsea processing enables development of challenging subsea fields, while reducing topside expenditures for equipment. Furthermore, subsea processing converts marginal fields into economically viable developments (Simon, 2010).

Subsea processing can encompass a number of different processes to help reduce the cost and complexity of developing an offshore field. The main types of subsea processing include subsea water removal and re-injection or disposal, single-phase and multi-phase boosting of well fluids, sand and solid separation, gas/liquid separation and boosting, and gas treatment and compression.

Saving space on offshore production facilities, separation of water, sand and gas can now be performed subsea. Subsea separation reduces the amount of production transferred from the seafloor to the water's surface, debottlenecking the processing capacity of the development.

In the configuration of subsea processing system, four types of subsea processing technology can be classified. Technologies that are currently being implemented in deep water include multiphase pumps (Type 1) and partial separation with pumping (Type 2). Multiphase pumping systems are proven technologies, wherease Type 2 systems have seen limited use. Technologies currently being developed for future applications of separators, scrubbers, and pumps that allow complete separation of production stream at the seabed (Type 3). The most advanced systems (Type 4) are likely to include multistage separation and fluid treatment with the production of export quality oil and gas. The key environmental isuse involved in the implementation of these technologies is the handling and disposal of the produced waters and sands (Thomas et al., 2010).

2.2 SUBSEA SEPARATION SYSTEM Initially subsea production involved fluid flow through

wellhead, valves and pipework. Subsea production is

Fig. 1. Subsea processing system (Seadiscovery.com)

significant mind set change in exploiting reserves close to existing infrastructure economically. In increasingly deeper water depths, with no nearby infrastructure, more processing is required locally.

Separating oil from one or all of the co-produced products including water, solids, and gas is the basis for future subsea separation. Water separation has attracted the highest priority due to the largest benefits to be gained in the short term. Solids separation is increasingly important especially in locations where solids production probability is high. Gas separation allows for a totally integrated separation system, bringing topsides to the sea floor.

Figure 2 shows that the subsea separation system is based on either a two-phase (gas/liquid) or three-phase (oil/gas/water) separation process. These systems have the potential to significantly reduce costs on offshore platforms by placing the equipment necessary to separate reservoir products on the seafloor. By placing the equipment on the seafloor, the capacity to process oil on the platform should increase and the need to separate potentially large volumes of produced water on the platform is eliminated.

Separation and removal of produced water is especially important near the end of the reservoir’s life span when the water fraction increases. Separating the produced water on the seafloor also keeps produced water from entering the riser and flowline system, which in turn reduces the back pressure on the well and allows for an increase in oil production.

Produced water can be a major contributing factor to hydrate and wax formation in flow lines. Separation of the produced water will help to control the formation of hydrate and wax in the oil flow lines. Reducing the volume of produced water entering the flowlines may also lead to reductions in the amount of chemicals used to control hydrate and wax formation in flow lines.

Especially, gas and liquid separation which is to separate the liquid phases, with oil and water, from the gas phase at low pressure to ensure the circulating fluids to stay out of the hydrate formation area (Di et al., 2011).


Fig. 2. Subsea separation system (Thomas et al., 2010)

With this configuration, the separator can be located at the

riser base or at the wellhead, depending on the pressure available. The goal is to mitigate the hydrate risk after the separation, to remove gas so as to reduce heat losses and to achieve efficient liquid boosting.

2.3 VERTICAL TYPE SUBSEA SEPARATOR The multi-pipe gas/liquid separator distribute the separation

volume that is normally allocated to a single vessel into an array of parallel vertical pipes whose relatively reduced diameter allow a more efficient mechanical design to sustain the external hydrostatic pressure (Di et al., 2011).

In this study, vertical type subsea separator was selected as a benchmarking model for CFD simulation test. Simulation test was performed according to the test condition of the study of Di et al. (2011). The multiphase flow is vertically raised through a central vertical pipe and is distributed into each separator pipe where the gas/liquid separation take place (Fig. 3). The whole separation system and the associated piping network is conceived to provide a continuos flow path.

Fig. 3. Multi-pipe subsea separator principle (Di et al., 2011)

For the separator design, 11 m height separator pipes were sized with an internal diameter of 76.2 cm and a wall thickness of 77 mm. For the environmental conditons, water depth is 2,500 m and a design pressure of 690 bar while the normal internal oprerating pressure of the separator is defined at 30 bar.

3. NUMERICAL CALCULATION

3.1 LEVEL SET MOTHOD Level set method has the advantage of being able to predict

more correct solution in boundary surface of 2 phase flow. So this method can be applied to interpret the slug flow in a riser system. In this study, this method was used in the CFD simulation for the 2 phase flow of liquid and gas (Djamel, 2011).

In the level set method, for the interface processing of the second phase, the approximation is added on the basis of the base-fluid equation. The interface tracking is performed by using the continuity equation, momentum equation, and energy equation which involve variable properties.

Smooth signed-distance function, Ф(x,y,z,t) is used in the

calculation around phase area. That is, Ф(x,y,z,t) is the function which means the shortest distance to the interface surface. Figure 4 explains the basic concept of level set model.

Fig. 4. Level set model

This approach calculates the hyperbolic equation, Eq. (1) in the Fixed Eulerian grid system in order to track the interface. As shown in Fig. 4, (+) value is one phase and (-) value is another phase. An interface surface is located in the Ф(x,y,z,t) = 0.

0=¶¶

+¶¶

j

j xu

t

ff (1)

In the calculation of the equation, to compensate the

physical properties such as density, viscosity, and thermal


conductivity, Heaviside function, H(Ф) is defined as shown in Eq (2).

îíì >-<

=...tanhsin,:)/(

10)(

dfdfdf

ff

oriforH (2)

At this time, each physical properties is applied as Eq (3).

))(1.(,..)(.,..,.. frfrr HHGL -+= (3)

3.2 CFD SIMULATION The software PVTsim and ANSYSF Fluent was used for

generation of the mesh, numerical solution of the governing equations and analysis of the results. PVTsim is a versatile PVT simulation program developed for reservoir, flow assurance and process engineers, etc. Gas and liquid are inflowed through the inlet of the separator. Topside of the separator is set as outlet condition. The mesh size was 2,800,000.

In the simulation of case 1, the gas and liquid volumetric flow rates are 0.0980 and 0.0787 m3/s, respectively, as shown in Table 1. The physical properties and flow rates obtained with the conservation of the Froude number (under the condition that the liquid-gas mixture flow can be separated by gravity effects) in the vertical multiphase feed pipe flow are presented in the Table 1 below (Di et al., 2011).

Table 1: Physical properties of fluids

Flowrates Fluid properties

Am3/s ρ(kg/m3) μ(cP) σ(mN/m)

Case 1

Gas 0.0980 22.5 0.0137 21.2

Liquid 0.0787 873.3 5.6

Case 2

Gas 0.0735 22.5 0.0137 50.7

Liquid 0.0782 895.5 13.5

Case 3

Gas 0.0490 22.5 0.0137 30.7

Liquid 0.0776 918.4 33

There are few simulation studies in this area because of the

difficulty of interpreting. So in this study, case 1 is only simulated and further simulation studies will be continued until the conference.

4. RESULTS AND DISCUSSION Unsteady simulation was performed by PVTsim for case 1,

as shown Fig. 5, the behavior of the 2 phase boundary surface

(a) t=0 sec

(b) t=5 sec

(c) t=10 sec

(d) t=20 sec

Fig. 5. Density plot with time for case 1 simulation


and the segregation of the liquid and gas could bel analyzed through the simulation.

At first, the gas(blue) and liquid(red) is separated clearly inside the tank and pipe as an initial condition as shown in Fig. 5 (a).

In the case of t=5 sec (Fig. 5 (b)), a mixture of gas and liquid is inflowed through inlet pipe. At this time, the gas flowed through the center of pipe as a slug flow pattern.

After a little more time (Figs. 5(c) and 5(d)), gas stream

become thin and continuously flowed into the vertical type separating tank in a irregular pattern.

Velocity of gas was calculated by Fluent and shown in Fig.

6. The moving pattern of the gas can be examined roughly. But was not easy to compare the results of the two software. Additional studies will be continued.

(a) t=3 sec

(b) t=6 sec

(c) t=9 sec

Fig. 6. Velocity of gas with time for case 1 simulation

5. CONCLUSIONS In this study, vertical type subsea separation system

simulation study is carried out as a process of subsea separation system development. The behavior of the two phase boundary surface and the segregation of the liquid and gas could bel analyzed through the unsteady simulation .

In the case of t=5 sec, a mixture of gas and liquid is inflowed through inlet pipe. At this time, the gas flowed through the center of pipe as a slug flow pattern. After a little more time, gas stream become thin and continuously flowed into the vertical type separating tank in a irregular pattern.

In the future, simulation study will be progressed for all cases of Table 1 and key factors are verified in the subsea separator design from thes study

ACKNOWLEDGMENTS This research was supported by the Development of

500Mpa URF & SIL 3 Manifold and Subsea System Engineering for Deepsea Field Project (13-9806) of the KIGAM funded by the Ministry of Trade, Industry and Energy of Korea.

REFERENCES [1]Michaelsen, J., 2003, “Innovative Technology for Ultra deepwater Gravity-Based Separators”, Offshore Technology Conference 15175. [2]Di, S. R., Abrand S., Shaiek, S., Butin, N., Riou, X., and Decrin, M. K., 2011, “A Novel Gas/liquid Separator to Enhance Production of Deepwater Marginal Fields”, Offshore Technology Conference 21394. [3]Bymaster, A. S., Olson, M. D., Grave, E. J., Svedeman, S. J., Viana, F., Mikkelsen, R., and Akdim, R., 2011, “High-Pressure Gas-Liquid Separation: An Experimental Study on Separator Performance of Natural Gas Streams at Elevated Pressures”, Offshore Technology Conference 21781. [4]Hannisdal, A., Westra, R., Dkdim, R., Bymaster, A., Grave, E., and Teng, D., 2012, “Compact Separation Technologies and Their Applicability for Subsea Field Development in Deep Water”, Offshore Technology Conference 23223. [5] Rune, F., Toine, H., and Robert, C., 2004, “Compact subsea separation system with integrated snad handling”, Offshore Technology Conference 16412. [6]Simon, D., 2010, “Statoil’s experience with and plans for subsea processing”, Deep Offshore Technology International. [7]Thomas M. G., et al., 2008, Effects of Subsea Processing on Deepwater Environments in the Gulf of Mexico, OCS Study, MMS 2008-022.


[8]Djamel, L., 2011, New Trends in Multiscale and Multi-physics Simulation of Transport Phenomena in Novel

Engineering Systems, Journal of Applied Fluid Mechanics, Vol. 4, No. 2, Issue 1, pp. 121-127.


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