Separation Design and Operation Tools For Transfering Best Practices [PDF]

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SPE 36647 Separator Design and Operation : Tools for Transferring “Best Practice” P.N.E.Lawson and L.M.Little, BP Exploration and Operating Co.Ltd., London, UK.

Copyright 1996, Society of Petroleum Engineers, Inc. This paper was prepared for presentation at the 1996 SPE Annual Technical Conference and Exhibition held in Denver, Colorado, U.S.A., 6–9 October 1996. This paper was selected for presentation by an SPE Program Committee following review of information contained in an abstract submitted by the author(s). Contents of the paper, as presented, have not been reviewed by the Society of Petroleum Engineers and are subject to correction by the author(s). The material, as presented, does not necessarily reflect any position of the Society of Petroleum Engineers, its officers, or members. Papers presented at SPE meetings are subject to publication review by Editorial Committees of the Society of Petroleum Engineers. Permission to copy is restricted to an abstract of not more than 300 words. Illustrations may not be copied. The abstract should contain conspicuous acknowledgment of where and by whom the paper was presented. Write Librarian, SPE, P.O. Box 833836, Richardson, TX 75083-3836, U.S.A., fax 01-972-952-9435.

Abstract Maintaining flexibility in the design of separation systems is a key consideration in coping with the uncertainty in fluid properties, reservoir performance and production profiles. There is an increasing need to improve our understanding of how different separation schemes perform over a range of operating conditions. This paper outlines two recent approaches that BP Exploration have been developing in the area of separator design and operation. The first addresses the need to understand the ‘operating envelope’ of a separator. In contrast to the conventional method of sizing a separator for a number of design cases, the operating range of a separator can be presented as an operating envelope. This concept will be demonstrated as a potential method for taking account of reservoir uncertainty and as a means of identifying operating flexibility. A parallel development is the construction of a database which captures information on BP Exploration’s Production Separators. In addition to design and current operating data, ‘case histories’, incorporating details on retrofits and design changes, are being collected as a means of transferring ‘Best Practice’ in separator design and operation across the BP Exploration operating Assets world-wide.

Background Maintaining flexibility in the design of three phase separation systems is a key consideration in coping with the uncertainty in fluid properties, reservoir performance and production profiles. There is an increasing need to improve our understanding of separation schemes over a range of different operating scenarios. In common with other operators there is also a drive to debottleneck existing separation trains as the volumes of bulk fluids to be processed increase through the addition of tie-back production. The paper outlines the development of the separator Operating Envelope and its use to estimate the overall

performance of a separation scheme. This method can be used to estimate the performance of a separation scheme and how it will perform over a range of operating scenarios. A parallel activity has been the generation of a Separator Database which contains both process and design data for HP Production separators throughout BPX. The Database is a method by which design and performance data can be collated and a means by which operating experience can be transferred between Assets. The aim of this paper is to give an overview of these two activities and to facilitate further discussion on the potential for these developments.

Separator Rating Programs BP have developed two separator design and rating programs (PROSEPARATOR and SSA). PROSEPARATOR has been used to design separators for new facilities and to provide valuable information on the design/operation of existing separators. The SSA program is still under development although it has been used to aid our understanding of oil-water separation issues. These two programs have been used by BP and our consultants to • evaluate alternative production separation systems for new fields • check third party separator designs for new fields and retrofits • define the operating envelope of existing equipment (plant debottlenecking and troubleshooting studies). • extend the operating range of a plant The two programs are described in more detail below. It should be noted that the main aim of the programs is to provide a tool for the preliminary assessment of separator system designs and equipment configuration options.

PROSEPARATOR (Gas-Liquid Separation) The gas-liquid separator rating program, PROSEPARATOR, can be used to size horizontal and vertical two phase separators. The key difference between PROSEPARATOR and conventional separator rating methods such as API 12J, is that PROSEPARATOR accounts for the creation of liquid droplets at the inlet to the separator. Conventional sizing methods use a single critical droplet size of ca.100 to 140µm to estimate the separator

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diameter based on Stoke’s Law. In addition, these methods do not provide the user with any information on the performance of the separator. PROSEPARATOR continues to use similar critical droplet sizes for separator dimensioning but the main advance is the inclusion of a droplet size distribution. In practice, it is known that a range of liquid droplet sizes will exist in the separator due to the shear of the fluids at the inlet. PROSEPARATOR estimates the liquid droplet size distribution using an empirical correlation which takes into account the fluid flowrates, physical properties and the separator inlet dimensions and configuration. The estimated droplet size distribution is used to determine the gravity separation performance of the separator and the liquid carryover from the vessel. The program can also estimate the performance of separator secondary devices such as mesh pads, vane packs and multicyclone bundles. PROSEPARATOR has been widely used within BP’s operations for a number of years and has clearly demonstrated benefits for a number of Assets. One of the main areas in which the program has found increasing value is the conceptual design of new separators, including the assessment of third-party designs. The program allows a range of different separator configurations to be quickly assessed, and has demonstrated clear benefits in applications where there is a need to reduce the size/weight/number of separators. In addition, the ability to quantitatively assess a range of separator options proposed by third parties has been successfully applied for a number of Assets. This is a very powerful concept where the proposed designs can be reviewed and the likely separation performance estimated. The program has also been successfully applied to a number of debottlenecking applications where existing separators are experiencing operating problems or there is a requirement to estimate the effect of additional fluid throughputs. Operational data (e.g. measured liquid carryover) can be rationalized using the program and this provides a sound basis for forecasting the effect of future production rates or the impact of design changes. An exclusive license for PROSEPARATOR has been granted to AEA Technology - Petroleum Services at Harwell who are now supplying PROSEPARATOR to outside companies.

SSA (Oil-Water Separation) The traditional methods for designing oil water separators are typically based on either a residence time, which is a function of crude density and the operating temperature, or by using Stokes law and a single droplet size (100 to 140µm as with gas-oil separation). Whilst these methods provide vessel sizes, they are unable to predict the quality of the product streams, or how they will change as operating conditions change. To overcome this, BP have developed SSA (Separation Systems Analysis) to model oil-water separation systems. As with gas-liquid separation, the approach taken has been to model the droplet size distribution and use this to predict

SPE 36647

separator performance. Flowrates and physical properties are entered, together with a PFD describing the process, to allow the shear history of the oil/water emulsion to be modeled. Correlations developed by Hinze (for shear across valves/orifices) and Karabelas (for equilibrium droplet sizes in pipes) are used to generate a characteristic droplet size, and a Rosin-Rammler distribution is applied to this to give the droplet size distribution. The separator is modeled using Stokes law settling only, and droplet-droplet coalescence is not modeled at present. The model assumes that all droplets which reach the interface are separated. The effect of emulsion breaking chemicals is modeled by applying a factor to the calculated droplet size, based on bottle test results. Whilst the physical property data required for SSA can be generated using a process simulator, some test data are required to demonstrate that the operating conditions selected for the separator (i.e. temperature, dosing of demulsifier chemical) are sufficient for separation to take place. As with PROSEPARATOR, the program has been developed as a spreadsheet for ease of use. The system is built using macros, allowing the engineer to select the process building “blocks” (valves, pipes, pumps etc.) to mimic the process. This allows the changes in the droplet size distribution through the system to be modeled, and hence separator performance. The building block approach also allows the easy addition of new technologies to SSA as they become available. The absence of a droplet-droplet coalescence model has been recognized as a limitation for SSA. Coalescence is the subject of a JIP managed by IKU SINTEF, in which BP, Shell and Statoil are members. This is intended to develop a model for droplet-droplet coalescence.

Operating Envelope: Principle The conventional method of sizing a separator for a given application is to size the separator for a number of Design Cases which aim to cover the likely range of operating conditions (e.g. Maximum Oil, Maximum Gas etc.) over field life. The design is based around the results of bottle tests and/or extended well tests (i.e. a snapshot of early field behaviour). The aim of the operating envelope is to identify the operational range of a separator scheme in terms of the potential fluid throughputs for the required product specifications (e.g. liquid carry-over into the gas and oil-inwater specification). A logical next step that follows from the operating envelope concept is the suggestion that rather than designing a oneoff separator, fit-for-purpose, there could be substantial benefit in having a standard off-the-shelf unit with the separation performance described by its operating envelope. The development of the Operating Envelope for a typical separation scheme is illustrated below.

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SEPARATOR DESIGN AND OPERATION : TOOLS FOR TRANSFERRING "BEST PRACTICE"

Concept Development A typical ‘element’ of a gas/oil processing system was used to develop and illustrate the concept of the system operating envelope for gas-liquid separation (Figure 1). The separator sizes, process flow conditions and physical properties used in this exercise are typical of separators in current operation (Table 1). This study is based on gas-oil separation to illustrate a principle but it will be extended to gas-oil-water separation at a later date.

Construction of Operating Envelope The following describes the steps used to construct an Operating Envelope (OE) for the separation scheme detailed above. PROSEPARATOR was used to generate the required gas/oil separation performance data for both the horizontal separator and the downstream vertical scrubber.

Step 1 - HP Separator OE - Gravity Separation The first step is the construction of an OE for the HP separator. The curve (Figure 2) shows the gas handling capacity of the separator for feed oil flows of up to 150mbd. The main performance limit is that the oil carry-over from the gravity section of the separator can be adequately handled by the downstream secondary separation device (mesh pad).

Step 2 - HP Separator OE - Addition of Secondary Separation Device The next step considers the performance of the HP separator secondary separation device. Although there are many such devices available, a mesh-pad was chosen, given its wide spread application and the availability of established performance prediction methods. The main constraint of mesh-pads is a limiting gas velocity which is a function of the phase density ratio and the cross sectional area (CSA) of the mesh-pad; it is less sensitive to the inlet oil loading. If the inlet gas velocity is exceeded, the probability of meshpad liquid flooding increases significantly and results in a severe deterioration in separation performance. The HP separator OE from Step 1 was modified to include typical limits for the performance of a mesh-pad of appropriate cross sectional area (Figure 3). For this mesh pad flooding limit, it can be seen that the oil carry-over from the gravity section can always be handled by the downstream mesh-pad and meet the required liquid carry-over specification from the horizontal separator. The exercise can be repeated for the vertical scrubber on the gas outlet of the separator in order to generate an OE for the complete separation scheme. This is illustrated in the following worked example.

Worked Example To illustrate the operating envelope concept, the above methodology will be used to the determine the maximum oil production rate for the scheme shown in Figure 1. The GasOil Ratio (GOR) of the feedstream will be assumed to be 350scf/bbl and the main requirement is to ensure that the

liquid carry-over from the scrubber does not exceed the typical carry-over limit of 0.1USgal/MMscf. The GOR of the feedstream is plotted on the HP separator OE (Figure 4) and indicates that the limiting gas flow to achieve the required carry-over specification from the HP separator is 27MMscfd. This gas flow is equivalent to an oil production rate of 77mbd. The OE for the HP separator can be combined with that for the downstream scrubber on the gas outlet as shown in Figure 5. This shows the predicted gas handling capacity of the scrubber for inlet liquid flows of up to 70USgal/hr (note change in units). The main limit is to ensure that the liquid carry-over from the scrubber does not exceed the 0.1USgal/MMscf specification. In addition, the liquid carry-over rate from the HP Separator are plotted on the same axes. It can be seen that the limiting gas flow for scrubber operation (to the required gas outlet specification) is a much higher 37MMscfd. This indicates that at these conditions, there is spare separation capacity available in the gas scrubber. This could be utilized by relaxing the carry-over specification from the HP separator by increasing the gas flow until it matches the limiting gas flow of the scrubber to maintain the 0.1USgal/MMscf carry-over specification. For the current example this occurs at an inlet gas flow of 34MMscfd which gives an optimum oil production rate of 97mbd. The links between the HP separator and downstream scrubber operating envelopes is summarized in Figure 6. In order to maximize the benefits of this technique a composite system OE can be developed from the individual operating envelopes as shown in Figure 7. This can be used to identify the operating boundaries of both the HP separator and downstream scrubber for a required gas carryover specification. The impact of different feed GORs on the given separation scheme can be quickly identified and the system limits identified. This is a very powerful concept that allows different scenarios to be quickly screened and operating limits identified. Work is continuing on the development of the concept with the aim of developing a software method of generating operating envelopes for different separator designs and flow conditions.

Separator Operation: Separator Database Another tool currently being developed, aimed at improving the dissemination of data and operating experience across the Assets, is the Separator Database. The database is a detailed survey of BP Exploration Production Separators, recording the current designs, dimensions and internals of production separators on both existing and emerging platforms. The database is currently in spreadsheet format, and includes design and operating data for all of BP Exploration’s production separators. In addition, GORs, liquid levels, residence times and API gravities (Figure 8) are also being incorporated into the database, in order to establish trends and correlations across the Assets.

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The database provides an indication of the range of conditions over which the separator can operate, from design conditions, through current operating conditions, to assessing whether the separator can accommodate increased operating capacities. It can also be used to compare the performance and internals of similar-sized separating vessels on other Assets operating under similar conditions. The database also includes several case histories of selected separators. The case histories focus on separators that have been debottlenecked and capture the changes made to the design and operation of the separator. Details of when and why new internals, inlet devices and/or secondary devices were fitted are documented, together with relevant performance data after modifications. Any operating problems encountered during or after modifications are also recorded. The case histories are a valuable method of sharing operating experience and providing feedback on debottlenecking modifications and lessons learnt. This type of information is also of significant interest to the vendors of the internals and secondary devices. The vendors provide the equipment to meet the design specifications initially, but often receive little feedback except when there is a problem or the equipment requires debottlenecking. Vendors are recognizing the increasing importance of feedback on the performance of their equipment, and the need to increase the interaction and transfer of information between customer and vendor.

the data in the database provides some idea of typical separator dimensions, for a given set of flow conditions, together with some indication of the more common inlet devices, secondary devices and internals. Correlations can then be established, for example, between fluid throughput and GORs, inlet nozzle diameter and inlet momentum values, and separator performance with internals and secondary devices.

Conclusions The tools described above facilitate the transfer of performance data and operating experience across BP. The necessity to increase awareness (via Separator Database) is becoming increasingly important as more Assets are facing similar issues with separator debottlenecking. The Operating Envelope concept can be used to estimate separator performance over a range of operating conditions. The method can be applied to both existing separation equipment (identify operating flexibility) and to the specification and design of new facilities (taking account of reservoir uncertainty).

6.0 Acknowledgements The authors wish to thank BP Exploration and Operating Company Limited for permission to publish this paper. In addition we acknowledge the advice and assistance of Brian Oswald, Lisa Hunt and Alastair Sinker.

The database is also a valuable tool in developing the concept of a standard off-the-shelf separator. Analysis of TABLE 1 SEPARATOR GEOMETRY AND OPERATING CONDITION HP Separator Vessel Diameter Effective Length Inlet Nozzle Diameter Liquid Surface Level Mesh Pad Secondary Device Vertical Gas Scrubber Vessel Diameter Inlet Nozzle Diameter Mesh Pad Secondary Device Liquid Flow Range Operating Pressure Range Operating Temperature 40 API Crude

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2.5m 12.5m 12 inch 50%

3m 16inch

0 → 150mbd (no water) 5 bar 60°C

36647

SEPARATOR DESIGN AND OPERATION : TOOLS FOR TRANSFERRING "BEST PRACTICE"

Typical gas/liquid processing element Dimensions: HP separator Ø = 2.5m

L = 12.5m

Vertical gas scrubber Ø = 3m Liquid flow range 0 - 150mbd

Gas

Mesh Pad

Operating pressure 5bar Vertical Gas Separator

40API crude Mesh Pad Feed

NNF

HP Separator

Liquid

Fig. 1-Schematic of typical Separation Element

Inlet Gas Flow (M M scfd)

Constraints Liquid outlet hydraulic lim it

60

Liquid carry-over spec.

50

40

Liquid Carry-Over Spec

30

20

10 Max Liquid Capacity 0 0

20

40

60

80

100

120

140

160

Inlet Liquid Flow (mbd)

Fig. 2- Construction of HP Separator Envelope - Gravity Separation

Inlet G as Flow (M M scfd)

50

Constraints Liquid outlet hydraulic lim it Mesh pad flooding lim it Liquid carry-over spec.

40

M esh P ad Flooding Limit

30

Liquid Carry-Ov er S pec

60

20

10 M ax Liquid Capacity 0 0

20

40

60

80

100

120

140

Inlet Liquid Flow (m bd)

Fig. 3- Construction of HP Separator Envelope - Secondary Separation

160

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P.N.E. LAWSON, L.M.LITTLE

Inlet Gas Flow (M M scfd) 60

Input: 350 scf/bbl GOR 50

Requirem ent: m ax liquid production rate for gas/liquid separation elem ent and suitable liquid carry-over

40

Mesh Pad : Flooding Limit 27 MMscfd

30

Liquid Carry-Over Spec 1

20

10 Inlet GOR = 350 scf/bbl

Max Liquid Capacity

0 0

20

40

60

80

100

120

140

160

Inlet Liquid Flow (mbd)

Fig. 4- Impact of Feed GOR on Separator Operating Envelope

Inlet G as Flow (M M scfd) 60 Liquid Carry-Ov er = 0.1 US gal/M M scf 50 37 M M scfd 40

30

20 27 M M scfd 10

Inlet OGR = HP S ep Liq Carry-Ov er S pec

0 0

10

20

30

40

50

70

60

Inlet Liquid Flow (USgal/hr)

Fig. 5- Construction of Downstream Scrubber Operating Envelope

HP Separator - Operating Envelope

Gas Scrubber - Operating Envelope

Inlet Gas Flow

Inlet Gas Flow Increasing Liquid C/O Spec.

Increasing Liquid C/O Spec.

Inlet OGR

M atching Gas Flows Liquid C/O = 0.1 USgal/M M scfd

Optimised Oil Production

Inlet Liquid Flow

Inlet Liquid Flow

Example Case Optimised gas flow - 34 MMscfd Optimised oil production - 97 mbd (from 77mbd)

Fig. 6-Summary of Links between HP Separator and Scrubber Operating Envelopes

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SEPARATOR DESIGN AND OPERATION : TOOLS FOR TRANSFERRING "BEST PRACTICE"

Inlet Gas Flow (MMscfd) 60

Inlet GLR = 350 scf/bbl

50

40

System Liquid Carrry-Over = 0.1 USgal/MMscf 97mbd

30

Constraint :

Constraint :

HP Sep Mesh Pad Flooding Limit

Scrubber Performance

20

10 77mbd

Max Liquid Capacity

0 0

20

40

60

80

100

120

140

160

Inlet Liquid Flow (mbd)

Fig. 7-Composite Operating Envelope for Separation Scheme

Case histories.

Easily accessible.

Performance data.

Continually updated.

Separator : Asset, Tag No.

Inlet Device : Vendor, Type

Internals : Vendor, Type

Pooling in-house experience.

Secondary Device : Vendor, Type

Inlet Nozzle : Internal Diameter Separator Dimensions : I.D., Tan-Tan Current Operating Conditions : Gas, Oil, Water Flows Temperature, Pressure Gas, Oil Densities GOR API Gravity K Factor Slug Capacity Residence Time

Normal Operating Level

Antifoaming Chemicals : Name, Dosage

Fig. 8-Typical Content of Separator Database

Operating Target : e.g. 175 mbd by 1999

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