GB2568689A

GB2568689A - Control apparatus and method

Info

Publication number: GB2568689A
Application number: GB1719413.5A
Authority: GB
Inventors: Goel Abhinav
Original assignee: Individual
Current assignee: Individual
Priority date: 2017-11-22
Filing date: 2017-11-22
Publication date: 2019-05-29
Anticipated expiration: 2037-11-22
Also published as: WO2019102196A1; GB201719413D0; GB2568689B

Abstract

An automatic slugging control apparatus is provided, the apparatus comprising, a conduit arranged to convey a multiphasic mixture of materials; a first measuring unit in communication with the conduit and arranged to provide a first measurement, the first measurement being characteristic of a first pressure upstream of a predetermined slugging zone; a valve in communication with the conduit and located downstream of the first measuring unit and a predetermined slugging zone, the valve having a valve position; a second measuring unit in communication with the conduit and arranged to provide a second measurement, the second measurement being characteristic of a second pressure downstream of a predetermined slugging zone and upstream of the valve; a third measuring unit in communication with the conduit and arranged provide a third measurement, the third measurement being characteristic of a third pressure downstream of the valve; a fourth measuring unit in communication with the conduit and arranged to provide a fourth measurement, the fourth measurement being characteristic of a gas fraction of said multiphasic mixture of materials; a controller in communication with the first, second, third and fourth measuring units and the valve, the controller being arranged to receive the first, second, third and fourth measurements and calculate at least one valve adjustment variable; the controller being further arranged to provide an output signal to a valve adjuster, the output signal comprises at least one valve adjustment variable; wherein the valve adjuster is arranged to adjust the position of the valve to a desired valve position according to the output signal received from the controller. Preferably the apparatus of the present invention enables the automatic detection of slugging events and adjusts a valve position accordingly, in order to mitigate the risks of continuous slugging events on the life time of a conduit system. A method of controlling a valve to control slugging using the apparatus is also claimed.

Description

The present invention relates to a control apparatus and method of use thereof, particularly to a control apparatus for use in the detection of slugging and automatic adjustment to reduce the effects of slugging and a method of carrying out the same.

Background to the Invention

Slugging

Slugging refers to unsteady flow behaviour observed in a multiphase flow through a conduit. Slugging can be triggered in certain conditions due to the different physical properties of more than one phases flowing in a single conduit simultaneously. This phenomenon is widely studied in upstream oil and gas industry. Certain forms of slugging are discussed below.

Terrain slugging: Terrain / severe slugging can affect a production facility at any point in its life. It is a natural phenomenon well known and studied in the upstream industry. It is the cyclical build-up and blow-out of liquid (oil and water) at the low-point of a riser or well. It is normally observed at low throughputs in a flow-line where a lower inclined conduit meets a upward inclined conduit. It can severely impact the operation of a receiving facility and lead to decreased asset utilisation. Subsurface impact can be equally severe with wide fluctuations in the bottom-hole pressure leading to potential long-term well I reservoir damage.

Hydrodynamic slugging: Hydrodynamic slugging is a natural property of the underlying flow conditions and is typically seen at high liquid and gas flowrates. Slugs are formed due to wave instabilities on the gas-liquid interface. Depending on the topography of the conduit, the slugs will grow or shrink as they travel along the conduit, but hydrodynamic slugging can be observed at flat terrains as well. For a horizontal conduit, hydrodynamic slugging will cause lower pressure oscillations at the inlet of the flowline. In case the horizontal conduit is followed by a vertical conduit, the slugging characteristics in the horizontal are amplified by the flow dynamics across the vertical conduit.

Operation induced slugging: Certain operations like start-up, ramp-up, introduction of Gas Lift, Gas lift trip, displacement, pigging etc. can induce transients in the system causing high pressure fluctuation in the conduit (which might propagate to well downhole for low potential wells). Operational induced slugging like pigging can also lead to high surge volumes in the downstream production separators, causing trips due to high level. Pigging causes high surge volumes in gas-condensate production systems where slip effects between gas and liguid can lead to high liguid inventories.

Slug Control

Slugging needs to be mitigated in order to avoid detrimental effects of slugging on operation facilities while provide smooth operation, avoiding trips and production loss. Two types of slug mitigation mechanism are often utilised in oil & gas industry with varying effectiveness.

Passive Slug Control: Passive slug control is manually choking (closing) the choke downstream of the slugging zone. This helps in controlling incipient slugging by either regulating the flowrate through the choke or by providing additional back-pressure to prevent the gas bubble in vertical riser from accelerating. The flow regulation is achieved when there is sufficient pressure drop across the choke for choked flow conditions to persist. In case of severe slugging however, the choke has to be closed to a very low value - giving a very high pressure drop across it for it to be effective. This might become untenable given the physical constraints of the system - reservoir pressure, frictional and hydrostatic pressure drops. Passive slug control is normally not employed in the upstream industry due to the risk of tripping the plant at high riser top pressure and it’s ineffectiveness in controlling severe/terrain slugging in the physical constraints of the system.

Active Slug control: Active slug control mitigates slugging by actively modulating the riser top choke opening, while controlling either the farthest (or riser base) pressure (WO 02/46577) or the hydrostatic head (WO 2009/133343) in the slugging zone. Active slug controller is thus able to mitigate slugging without giving high-back pressures as needed for a passive slug control. Active slug control has found to be very effective in mitigating the adverse impact of slugging for terrain and operational induced slugging.

Automation of Slug Controller: Automation of the slug control has been suggested in an earlier patent (WO 2006/120537). In this they aim to continuously modulate the set point based on the measured pressure drop across riser top choke to maintain it at a fixed value. This is however done only during steady state. When the pipeline is unstable, this set point is not used. This technology requires a manual operator (or an external system) provided status of the flowline - ‘stable I unstable’.

The technology (WO 2006/120537) works by taking action if there is expected liquid blockage in the pipeline, when the system is ‘unstable’. When ‘unstable’ is selected by operator, the expected liquid blockage is inferred with a sudden pressure drop across the riser top choke/ pipeline inlet and /or through a mass balance across the system. The action taken by it is to increase the riser top choke valve position to prevent sustained low flowrate production. The output of this calculation block can be used completely independent of the active slug controller (WO 02/46577) or in conjunction with it. When it is used in conjunction with active slug controller (WO 02/46577) its output is added on to the active slug controller output.

Limitations of existing technology for slug control

Limitations of stand-alone active slug controllers (WO 02/46577; WO 2009/133343):

Constraint of set point: Operationally in order to maximise the facility production, the facilities need to be run with minimum pressure drop in the multiphase conduit. This in a slugging system is achieved by reducing the set point of the active slug controller. However, the minimum value (of pressure drop) that can be achieved without destabilising the system is not understood presently. This results in operators either providing a very high set point (causing unnecessary pressurisation of the system), or too low-causing destabilisation.

Manual Set Point: The active slug controller needs to be provided a set point manually by operators. In a dynamic production system, the flowrates, GOR, WC keep changing with time. The active slug control set points would thus have to be altered manually. It is difficult to provide operators with a set point of every plausible case in the matrix of variable parameters.

Manual Entry of low / high limits: high and low limits need to be changed by operators in transient events like start-up of the facility. Incorrect low/high limits can hinder the slug controller performance or lead to violation of the physical constraints of the production system, leading to trip of the production facility.

Controllability: There is no mechanism in place with prior art active slug controllers to bring the choke valve in controllability range (when the system is slugging). If the choke valve is outside the controllability range it could be rendered ineffective. This could reduce the effectiveness of active slug controllers in mitigating hydrodynamic slugging (or amplification of the hydrodynamic slugging due to riser).

Continuous Operation: The active slug controllers are currently utilised (switched ON’) when slugging is observed in field or in case it is expected to occur (start-up). They are not typically kept continuously in operation, due to the manual nature of its operation. Thus unforeseeable transient events - like gas lift trip, which induce slugging, cannot be automatically handled by them.

Gas condensate field / Operation induced slugging: In gas condensate fields, operational induced slugging like pigging I ramp-up can cause significantly large surge volumes, which are higher than the available volume of downstream separator I slug-catcher. Prior art active slug-controllers are generally unable to control this as significant change in inlet pressure I differential hydrostatic head (if the terrain is flat) of the slugging zone is not observed for these operations.

Limitation of prior art automation of slug-controllers (WO 2006/120537)

Automation of Slug Controller: The earlier patent (WO 2006/120537) requires a manual operator (or an external system) provided status of the flowline - ‘stable I unstable’. If ‘unstable’ is selected by operator - ‘set point is not calculated by the block’. This thus cannot provide a set point to the master controller during slugging or any other transient scenario (start-up, gas-lift trip etc.) automatically. Moreover, the technology requires a human interface (or an external system) to determine the stability of the pipeline.

As described above, there are many problems facing the currently available technology. It is therefore desirable to provide an apparatus for automation of slug-control that can overcome the aforementioned problems and which does not require manual intervention to operate.

Summary of the invention

In accordance with the invention, a container is provided as outlined in the accompanying claims.

In accordance with a first aspect of the present invention, there is provided an automatic slugging control apparatus comprising, a conduit arranged to convey a multiphasic mixture of materials; a first measuring unit in communication with the conduit and arranged to provide a first measurement, the first measurement being characteristic of a first pressure upstream of a predetermined slugging zone; a valve in communication with the conduit and located downstream of the first measuring unit and a predetermined slugging zone, the valve having a valve position; a second measuring unit in communication with the conduit and arranged to provide a second measurement, the second measurement being characteristic of a second pressure downstream of a predetermined slugging zone and upstream of the valve; a third measuring unit in communication with the conduit and arranged provide a third measurement, the third measurement being characteristic of a third pressure downstream of the valve; a fourth measuring unit in communication with the conduit and arranged to provide a fourth measurement, the fourth measurement being characteristic of a gas fraction of said multiphasic mixture of materials; a controller in communication with the first, second, third and fourth measuring units and the valve, the controller being arranged to receive the first, second, third and fourth measurements and calculate at least one valve adjustment variable; the controller being further arranged to provide an output signal to a valve adjuster, the output signal comprises at least one valve adjustment variable; wherein the valve adjuster is arranged to adjust the position of the valve to a desired valve position according to the output signal received from the controller.

The present invention was created with a view for it to preferably be suitable for all manner of transient slugging scenarios for, for example, oil fields and gas-condensate fields etc. The present invention preferably prevents trips of production facilities by significantly reducing the peak surge volumes observed in, for example, downstream phase separating members and slug-catchers, while preferably minimising the incremental back-pressure caused by slugging events.

Preferably the at least one valve position variable comprises a first variable characteristic of a preliminary valve position; wherein the first variable is calculated using at least the first, second, third and fourth measurements.

The controller is preferably further arranged to receive measurements characteristic of at least one selected from the range: flow through the valve; valve characteristic; average valve position; mixture density; gas density; liquid density; a predetermined optimum pressure drop across the valve.

A preferable objective of the present invention is to provide an apparatus for mitigating slugging, such that it can continuously operate and diminish slugging while maximising the flow of the multiphasic mixture of materials through the apparatus. Another preferable objective is for the slug control apparatus to retain its effectiveness in all types of fields - oil fields and gas-condensate fields, in all expected transient scenarios like terrain slugging, pigging, start-up, ramp-up etc. This is preferably achieved through the details mentioned below.

Controlling Differential Head across choke (valve downstream of slugging zone - but upstream of production separator)

It is proposed that for the active automatic slugging control apparatus of the present invention to work effectively in all transient scenarios and for all types of multiphase production fields, some minimum differential head preferably needs to be maintained across the choke (valve), which is located downstream of the predetermined slugging zone, and preferably upstream of a production separator acting as a phase separating member.

This can be preferably explained mathematically through the eguation of flow through a valve. The active automatic slugging control apparatus of the first aspect of the present invention preferably works with low differential pressures across it, and thus the flow of said multiphasic mixture of materials is preferably generally sub-critical. Said flow through the valve in steady state is preferably expressed as:

Q = K x Z x [(P_u - P_d-)/Pm\°·⁵

- Eguation(1)

Where,

Q - Flow through the valve

K - Valve characteristic (constant over a small range of valve movement)

Z - Average valve position

P_u - Pressure upstream of choke

P_d - Pressure downstream of choke p_m - Mixture Density

The mixture density can be further written as:

Pm — Pg X Gy + p₀ X (1 — Gy)

- Eguation(2)

Where, p_m - Mixture Density p_g- Gas Density

Po - Liquid Density

Gf- Gas volume fraction

Substituting Equation (2) in Equation (1) and differentiating Equation (1) with respect to time (assuming pressure downstream of choke (valve) to be constant with time), we get:

dQ_ ₌ (P_u - fiA^{0 5} dz O.Sx/CxZ dP_udt \ p_m J dt l(P_u-P_d) X P_m]°-^5>< dt

0.5 x K x Z x (P_u - P_d)⁰·⁵

^x (Po ~ Pg)

- Equation (3)

The first term on the right hand side of the Equation (3) indicates the contribution to dynamics by the valve movement (active part of automatic slugging control apparatus). The second term on the right hand side indicates the amount of acceleration of said multiphasic mixture of materials through the choke (valve) due to change in upstream pressure across the choke (valve). The third term on the right hand side indicates the change in dynamics contributed by the change in gas fraction ofthe multiphasic mixture fluid stream.

The Equation (3) can be rearranged to give:

[(P_M-P_d) x p_m]⁰·⁵ dt 0.5 x K x Z ^X dt 0.5 x Z * dt p_m ^X V’ ^X dt

- Equation (4)

Equation (4) gives the increase in upstream pressure of a choke (valve), during transient fluid flow of said multiphasic mixture of materials through the choke (valve). The first term on the right hand side (3Q/3t) shows that an acceleration of said multiphasic fluid flow rate through the choke (valve) would increase the upstream pressure. The second term on the right hand side indicates that if the valve is closed (dZ/dt < 0), then again the pressure upstream of the choke (valve) increases. The third term similarly signifies that if the gas fraction through the choke (valve) reduces (3G_f/3t < 0), say due to arrival of a liquid slug, then the pressure upstream of the choke (valve) would increase.

Controlled Variable

In preferable embodiments, the calculation of the at least one valve position variable further comprises a measurement characteristic of at least one selected from the range: flow through the valve; valve characteristic; average valve position; mixture density; gas density; liquid density.

Equation (4) and the need for maintaining constant head through the riser top choke (valve) results in defining the control variable, which we need to maintain at a constant value in a standard feedback control system:

CV = β±χ P_in + β₂ x (P_u — P_d) + β₃ x Gf

- Equation (5)

Where,

CV - Controlled variable

Pm - Pressure at inlet of slugging zone

P_u - Pressure upstream of choke (valve - downstream of slugging zone)

P_d - Pressure downstream of choke

Gf - Gas volume fraction upstream of the choke βι, β2, β3 - weightage I tuning factors derived partly from Equation (4) and are system specific.

Set Point

Preferably the at least one valve position variable comprises a second variable characteristic of a target valve position; wherein the second variable is calculated by the controller using at least the first and fourth measurements and the measurement characteristic of the optimum pressure drop across the valve.

In a heavily slugging system, neither of the three variables mentioned in the control variable (Equation (5)) are constant with time. The intention of the slug-controller is to stabilise the system at an average value of the observed fluctuations. The set point is thus defined as:

SP= β_± ^x Pin-avg + /?2 ^X OPT_Dp + /?₃ ^x Gf-avg

- Equation (6)

Where,

SP - Set Point

Pin-avg - Average pressure at inlet of slugging zone

OPT_DP - User specified optimum differential pressure across choke

Gf-avg- Average gas volume fraction upstream of the choke βι, β2, β3 - weightage I tuning factors derived partly from Equation (4) and are system specific.

Filtering Inlet Pressure and gas fraction

In a heavily slugging system, the intention of the apparatus of the present invention is to preferably stabilise the system around an average value. The intention to have an average of the system (rather than user defined values), is to preferably have an adaptive system, which would automatically move to a new steady state when flow rate, water cut, gas oil ratio etc, change over the life of a field. The aim here is not to filter out the noise, but to preferably obtain a moving average value of the system when the value is fluctuating quite considerably at a low frequency as observed during slugging. The filter time constant would then be specific to the application of the apparatus (but very high as compared to filter time constants used for filtering out noise).

Pin-avg can be determined using the equation for 1st order low pass filter as:

pn ^rin-avg

Pⁿ~^{r r}in-avg

- Equation (7)

Where,

Pⁿm-avg - is the filtered value of Pressure at current time step. Pⁿ-¹in- avg - is the filtered value of Pressure at previous time step. Pⁿ - is the current value of Pressure.

T_s - Sample time.

T_f - Filter time constant.

Similarly Gf._avg can be determined using the equation for 1st order low pass filter as:

2k) ^Tf) X Gf-a_Vg +

- Equation (8)

Where,

Gⁿf-_avs- is the filtered value of Gas fraction at current time step.

G ⁿ~\-avg - is the filtered value of Gas-fraction at previous time step.

Gⁿ _f - is the current value of Gas fraction.

T_s - Sample time.

T_f - Filter time constant.

The filtered value of inlet pressure and gas fraction would preferably change dynamically based on the change in operating conditions during transient events like start-up, ramp-up, ramp-down, gas lift trip etc. Thus the resultant ‘set point’ (Equation (6)), would preferably automatically change to an average value during transient events and would also preferably change in an operating system over the life of a field due to variety of reasons, like change in flow rate, gas:oil ratio, water cut etc.

Feedback Control

The at least one valve position variable preferably comprises an error variable; wherein the error variable is calculated by the controller using at least the first variable and the second variable.

The at least one valve position variable, in accordance with preferable embodiments, comprises a preliminary valve position variable; wherein the preliminary valve position variable is calculated by the controller using at least the error variable.

From the set point calculated (Equation (6)) and controlled variable (Equation (5)), an error signal is preferably created which is then fed to a standard feedback PID (proportionalintegral-derivative) controller to produce a controller output. This output preferably regulates the position of a choke (valve position), modulating it with time.

_En _ _cyn _ _spn

- Equation (9)

Where,

Eⁿ - is the error at current time step.

CVⁿ - is the controlled variable at current time step.

SPⁿ - is the set point at current time step.

Preferably the at least one valve position variable comprises a desired valve position variable; wherein the desired valve position variable is calculated by the controller using at least the preliminary valve position variable.

The controller output is determined through a standard PID controller:

T, j ^{c s υ} dt

- Equation (10)

Where,

Co - is the controller output.

Kc - is the gain of the controller.

K_s - is the scale factor of the controller.

T| - is the integral time constant of the controller.

T_D - is the derivative time constant of the controller.

Low Limit of controller output signal

The controller output generated from Equation (10), needs to be further limited to a low limit in view of the physical constraints of the production facility on which the automatic slugging control apparatus is installed. High pressure upstream of the choke (valve) could cause the production facility to “trip”. Moreover, adjustment of said valve position resulting in high valve closure could cause high back-pressure during transient events, causing the low potential wells to back-flow. In order to prevent this from happening, a low limit is preferably applied to the controller output signal as part of the present invention.

A dynamic low limit on said controller output signal is preferably achieved by measuring the pressure at the second measuring unit, preferably located at a riser top, upstream of the choke (valve). Said pressure (second measurement) is then compared with a fixed, maximum (user-defined) value of the riser top pressure. When adjustment of the valve position constitutes negative valve movement, this is preferably negated when the pressure upstream of the riser (second measurement) reaches this user-defined maximum value. When adjustment of the valve position constitutes negative valve movement, this is also preferably negated when the current valve position reaches a user defined minimum value of a low limit.

The proposed method thus preferably works at different throughputs through the system, by dynamically limiting the adjustment of the valve position, preventing excessive riser top pressure (second measurement) without user input.

High Limit of Controller Output signal

The controller output generated from Equation (10), needs to be further limited to an high limit in view of the physical constraints of the production facility on which the automatic slugging control apparatus is installed. A dynamic high limit preferably prevents adjustment of the valve position when such adjustment would constitute excessive opening of the valve and prevent loss of controllability of the system. When adjustment of the valve position constitutes opening of the valve to a high value, this could also cause the level in a said phase separating member to rise to a high value causing plant trip, especially when a liquid slug arrives at the production facility.

A dynamic high limit is preferably achieved by negating the adjustment of the valve position when constituting positive valve movement, when the level in the separator reaches a (userdefined) high value. When adjustment of the valve position constitutes positive valve movement, this is also negated when the differential head across the choke reduces to a (user defined) low value. A minimum value of the high limit is also preferably provided (user input), to preferably prevent the high limit from reaching very low values, for example, during shutdown of the system (zero flow rate).

The apparatus preferably further comprises a phase separating member located downstream of the valve, and arranged to separate the said multiphasic mixture of materials into one or more substantially single phase materials. More preferably the phase separating member is arranged to accommodate a portion of the multiphasic mixture of materials prior to separation of said portion into one or more substantially single phase materials. Preferably said portion is separated into substantially single phase materials by the phase separating member. Still more preferably, the apparatus further comprises a fifth measuring unit, the fifth measuring unit arranged to provide a fifth measurement characteristic of an amount of multiphase mixture of materials accommodated within the phase separating member. Most preferably, the controller is in communication with the fifth measuring unit, the controller being further arranged to use at least the fifth measurement from the fifth measuring unit to calculate the at least one valve adjustment variable.

In accordance with a second aspect of the present invention there is provided a method of automatic adjustment of a valve position in a conduit system suitable for automatic mitigation of slugging effects, the method comprising the steps of:

a) measuring the pressure upstream of a predetermined slugging zone using a first measurement unit;

b) measuring the pressure downstream of the slugging zone and upstream of a valve using a second measurement unit;

c) measuring the pressure downstream of the valve using a third measurement unit;

d) measuring the fraction of gas/liquid in the slugging zone using a fourth measurement unit;

e) using the measurements of a), b) c) and d) to obtain a first variable characteristic of a preliminary valve position;

f) using the measurements of a) and d) obtained over time in conjunction with a predetermined variable characteristic of an optimum pressure drop across the valve to determine a second variable characteristic of a target valve position;

g) determining an error variable between the second variable and the first variable;

h) using the error variable to determine a preliminary valve position;

i) adjusting the preliminary valve position according to a predetermined upper valve position limit and a predetermined lower valve position limit to give a desired valve position; and

j) transmitting the desired valve position to a valve adjuster arranged to adjust the valve to the desired valve position.

Preferably there is provided a method according to the second aspect of the present invention, wherein steps a) to j) are performed using an apparatus according to the first aspect of the present invention.

Detailed Description

Specific embodiments will now be described by way of example only, and with reference to the accompanying drawings, in which:

FIG. 1: Shows the slugging map of a typical oil and gas production pipeline. This is for fixed valve opening representing passive slug control;

FIG. 2: Shows the operating envelope for the active slug controller, with a preferred operating line for the adaptive slug controller (default optimum DP across riser top choke as 5 bar). This operating envelope is with respect to the riser top choke opening for a typical oil and gas production pipeline;

FIG. 3: Shows the operating envelope for the active slug controller, with a preferred control line (default optimum DP across riser top choke as 5 bar). This operating envelope is with respect to the differential pressure across the riser top choke for a typical oil and gas production pipeline;

FIG. 4: Shows a high level block diagram of how the present invention integrates with production facility;

FIG. 5: Shows the high level block diagram for the present invention software calculation with inputs and output of sub-blocks;

FIG. 6: Shows the high level block diagram for the ‘Basic Slug Controller Calculation Block’;

FIG. 7: Shows a flowchart for the ‘Set Point Calculation’ block;

FIG. 8: Shows a flowchart for the ‘Controlled Variable Calculation’ block;

FIG. 9: Shows a block diagram for the inputs and output of ‘Low Limit Calculation block;

FIG. 10: Shows a flowchart for the ‘Low Limit Calculation’ block;

FIG. 11: Shows a block diagram for the inputs and output of ‘High Limit Calculation’ block;

FIG. 12: Shows a flowchart for the ‘High Limit Calculation’ block;

FIG. 13: Shows the results from transient simulation of a pipeline experiencing terrain slugging. This is then controlled using the present invention. The comparative riser bottom pressure values are shown for the uncontrolled and controlled simulation cases;

FIG. 14: Shows the results from transient simulation of a pipeline which is started with a liquid filled riser and the bean-up rate for the well is very small causing slugging for 40 hours. This case is then controlled using the present invention. The comparative riser bottom pressure values are shown for the uncontrolled and controlled simulation cases;

FIG. 15: Shows the results from transient simulation of a pipeline which is slugging and then the flowrate is ramped-up by beaning up the well. The pipeline remains in slugging envelope at higher flowrate as well. This case is then controlled using the present invention. The comparative riser bottom pressure values are shown for the uncontrolled and controlled simulation cases; and

FIG. 16: Shows the results from transient simulation of a gas-condensate pipeline in which is pigging is carried out. The high liquid surge volume in pigging causes flooding of production slug-catcher (level goes beyond LAHH). This case is then controlled using the present invention. The comparative levels of slug-catcher and pig-velocity etc. are shown for the uncontrolled and controlled simulation cases.

The present invention consists of measuring certain field production data, which is generally available at a site - pressure upstream and downstream of choke, pressure upstream of the slugging zone (generally pressure at the inlet of the pipeline or riser bottom pressure), level in the separator and gas fraction at the inlet of the separator. Gas fraction at inlet of separator is generally not available at production facility and a non-intrusive gas-fraction meter needs to be installed at site. The measured data is then processed in calculation blocks - which can be configured in software. The output of these calculation blocks is a controller signal - which regulates the choke valve opening in order to control slugging.

The present invention integration with an oil & gas production facility is shown in FIG. 4. The overall configuration of the software embedded calculation blocks along with the production system I inputs it takes is shown in FIG. 5. The calculation consists of broadly 3 blocks ‘Basic Slug controller calculation block’, ‘low limit calculation block’ & ‘high limit calculation block’. The ‘Basic Slug controller calculation block’ calculates the ‘set point’ and ‘controlled variable’ and using these two, generates a controller output. The ‘low’ & ‘high’ limit calculation blocks, further limit this output to a low or high value dynamically depending on system constraints. The details of the various elements of the software calculations are given below.

Basic Slug Controller Calculation Block: The inputs taken by the ‘basic slug controller calculation block’ are given in FIG. 6. The block further consists of 3 sub-blocks: ‘Set point calculation block’, ‘controlled variable calculation block’, ‘standard PID controller block’.

Set Point Calculation block: ‘Pressure upstream of slugging zone’ and ‘gas fraction’ are continuous signals provided to this block. Optimum DP across riser top choke’ is fixed user provided value (set to a default value), which would be changed infrequently. The output of this block is ‘set point’ which is continuously provided to the PID controller block.

The flowchart for the ‘set point calculation block’ is shown in FIG. 7. The pressure upstream the slugging zone is filtered through a high filter time constant to get a moving average and multiplied with a factor (βϊ). Similarly gas fraction is filtered through a high filter time constant to get a moving average and multiplied with a factor (β₃). The user provided ‘optimum differential pressure’ is also multiplied with a factor (β₂). The three factors are then added to produce the set point. This value is passed through a dead band with user defined dead band range. The output of this is limited to min/max constraints based on the physical limitations of the production system. This is then passed on as the set point to the PID block.

Control Variable Calculation block: ‘Pressure upstream of slugging zone’, ‘Pressure upstream of choke’, ‘Pressure downstream of choke’ and ‘gas fraction’ are continuous signals provided to this block. The output of this block is ‘control variable’ which is continuously provided to the PID controller block.

The flowchart for the ‘set point calculation block’ is shown in FIG. 8. The pressure upstream the slugging zone is multiplied with a factor (β^. Similarly gas fraction is multiplied with a factor (β₃). The ‘Pressure upstream of choke’ is subtracted from ‘Pressure downstream of choke’ to obtain the differential pressure across the choke and is then multiplied with a factor (β₂). The three factors are then added to produce the control variable. The output of this is limited to min/max constraints based on the physical limitations of the production system. This is then passed on as the ‘control variable’ to the PID block.

PID block: the output of the controller is determined using set point and control variable using Equation (9) and Equation (10). Standard PID blocks are also available in control vendor DCS systems.

Low Limit Calculation Block: The inputs taken by the low limit calculation block are given in FIG. 9. ‘Pressure upstream of riser top choke’, Output of Slug Controller (velocity form)’ and ‘Slug Controller output from memory (positional form)’ are continuous signals provided to this block. ‘Maximum Pressure upstream of riser top choke’ and ‘Minimum Low Limit’ are fixed user provided values, which would be changed infrequently. The output of this block is ‘slug controller output - velocity form (low output limited)’ which is continuously provided to the high limit calculation block.

The flowchart for the low limit calculation block calculation is shown in FIG. 10. The pressure upstream the riser top choke is subtracted from the maximum pressure limit at upstream choke. If this difference is less than (or equal to) zero and differential valve movement is negative, the differential valve movement is set to zero using Boolean operators. Current positional form of the output is computed by adding the velocity form of output with the previous time step positional output of the slug controller. This is subtracted from the minimum value of the slug controller output. If this difference is less than (or equal to) zero and differential valve movement is negative, the differential valve movement is set to zero using Boolean operators. The velocity form of output after multiplication with these Boolean operators is passed on as the final velocity output of this block (low output limited).

High Limit Calculation Block: The inputs taken by the low limit calculation block are given in FIG. 11. ‘Pressure upstream of riser top choke’, ‘Pressure downstream of riser top choke’, Output of Slug Controller (velocity form - Low output limited)’ and ‘Level in the separator’ are continuous signals provided to this block. ‘Maximum differential pressure across choke’ and ’Maximum level in separator’ are fixed user provided values, which would be changed infrequently. The output of this block is ‘slug controller output - velocity form (high output limited)’ which is continuously provided to the riser top choke.

The flowchart for the high limit automation block calculation is shown in FIG. 12. The ‘Pressure upstream of choke’ is subtracted from ‘Pressure downstream of choke’ to obtain the differential pressure across the choke. The maximum differential pressure across choke is then subtracted from it. If this difference is less than (or equal to) zero and differential valve movement is positive, the differential valve movement is set to zero using Boolean operators. The level in the separator is subtracted from the maximum level limit in the separator. If this difference is greater than (or equal to) zero and differential valve movement is positive, the differential valve movement is set to zero using Boolean operators. The velocity form of output after multiplication with these Boolean operators is passed on as the final velocity output of this block (high output limited).

Benchmarking:

The current invention was tested for various transient scenarios as given in FIG. 13, FIG. 14, FIG. 15 and FIG. 16. These transient scenarios are simulated with a transient multiphase simulator for a typical operating systems in deep-water oil & gas production system having a multiphase flowline followed by a long vertical riser (and thus have pronounced slugging affects). Operational induced slugging is simulated in a typical gas-condensate production system.

FIG. 13 shows a terrain (severe) slugging scenario, where there is alternating intermittent flow of gas and liquid causing high pressure and flow variations in the entire system. The uncontrolled system (riser top choke not modulated and left open at 100% opening) exhibits the unstable behaviour as observed in the cyclical pressure variation of riser bottom pressure. Pressure fluctuations of 120 bar with frequency of 0.3 cycles/hr are observed. This scenario when simulated with the proposed invention controls slugging (stable riser bottom pressure) and takes the riser bottom pressure to low values - thus increasing production through it. This is with default parameters, i.e. without the need for any operator intervention.

FIG. 14 shows a start-up scenario. The system (uncontrolled case - riser top choke 100% open) is shutdown at start with a fully liquid filled riser. The well is started with a very low bean-up rate consequently resulting in heavy slugging. This scenario when simulated with the proposed invention controls slugging (stable riser bottom pressure). It takes the riser bottom pressure to lowest possible values based on the physical constraints and inherent stability of the system. This is with default parameters, i.e. without the need for any operator intervention.

FIG. 15 shows a ramp-up scenario in the slugging envelope. The system (uncontrolled case - riser top choke 100% open) is slugging heavily. The well is ramped-up at 24 hours causing higher flowrate through the system although the slugging persists as the flowrate is not high enough to be outside the slugging envelope. This scenario when simulated with the proposed invention controls slugging (stable riser bottom pressure) during the initial stage and also after the well is ramped up. It takes the riser bottom pressure to lower values after the well is ramped-up as lower riser bottom pressures are expected at higher flowrates in gravity dominated region. This is with default parameters, i.e. without the need for any operator intervention

FIG. 16. Shows an operational induced pigging scenario in a gas-condensate field. The system (uncontrolled case - riser top choke 100% open) when pigged causes high surge volumes in the downstream slug-catcher causing it to reach high-high level values, which would have tripped the production facility. This scenario when simulated with the proposed invention, prevents high surge volumes, as it slows down the pig when it arrives. The liquid surge ahead of the pig is detected by the gas-fraction meter, causing the riser top choke to close and slow down the pig, causing lower peak surge volumes in the slug-catcher.

It will be appreciated that the above described embodiments are given by way of example only and that various modifications thereto may be made without departing from the scope of the invention as defined in the appended claims. Example applications of the present invention are detailed below:

Application of the present invention to different types of slugging

Terrain slugging

Terrain slugging (and cyclical blow out of liquid) is caused by the Taylor bubble rising up a predominantly liquid filled vertical column. As it moves up, it sees reduced back-pressure on it (as hydrostatic head reduces) and accelerates, pushing the liquid ahead of it. This results in a blow out of liquid into the downstream facilities.

The described invention works by increasing the back-pressure on this Taylor bubble, during its movement upwards. This helps in decelerating the Taylor bubble, preventing the blow-out of liquid.

In light of the discussion above, Equation (4) represents the rate of increase of pressure upstream of the choke due to any acceleration of the fluid through it. In order for the slug controller to be effective, or the choke to be in controllability range, there should a significant increase in pressure upstream of the choke to any given acceleration (to provide increased back-pressure to the Taylor bubble to slow it down). In other words - whenever the Taylor bubble starts to accelerate - if there is an increase in system resistance downstream of it, the Taylor bubble would be prevented from accelerating.

As can be observed from Equation (4), when the Taylor bubble rises up - there is predominantly liquid in front of this. The gas fraction meter indicates a predominantly liquid stream and the gas fraction is higher than the average. This causes the 3Gf/3t < 0 and causes higher differential pressure across the choke (3rd term on right hand side of Equation (4)). Moreover, if the Taylor bubble starts to accelerate, this causes a higher liquid flowrate through the choke, causing 3Q/3t > 0, causing further increase in pressure upstream of the riser top choke (1st term on right hand side of Equation (4)). The slug-controller further manipulates the valve position (3Z/dt < 0), causing further backpressure and resultant deceleration of the Taylor bubble.

The combined effect of all the three factors contribute to making the invention highly effective in mitigating terrain induced slugging.

Hydrodynamic slugging

Hydrodynamic slugging as stated earlier can happen even in relatively flat terrain. The cyclical variation in inlet pressure due to hydrodynamic slugging is not significant for the conventional active slug controllers (WO 02/46577; WO 2009/133343) to be effective in mitigating this. The proposed invention works by measuring the gas fraction at the inlet of the separator and indicating the liquid slug front arrival at the separator. This causes the 3Gf/3t < 0 and causes higher differential pressure across the choke (3rd term on right hand side of Equation (4)). Conversely when the liquid slug front is followed by a gas bubble, it reduces the differential pressure across it, preventing the next hydrodynamic slug from building up. The hydrodynamic induced terrain slugging effects are controlled in a fashion similar to the terrain slugging described above.

Operation-induced slugging

As stated earlier, operator induced slugging is very severe in gas-condensate systems when pigging operation is carried out. The proposed invention works in a similar fashion as described for ‘hydrodynamic slugging’, where the arrival of liquid front in front of the pig is detected by the ‘gas fraction meter’, causing the choke to close and subsequently cause the pig to slow down. The pigging liquid slug front is decelerated and received more gradually in separator / slug-catcher, preventing production facility trip due to high level.

Worked example

For the case presented in FIG. 13:

K = 0.0002 m²/%

OPT_DP (Pu-Pd) = 5 bar

Density Gas (p_g) = 10 kg/m³

Density Oil (p₀) = 850 kg/m³

Average Flow (Q) = 500 m³/hr

Average Gas Fraction (Gf) = 0.5

From Equation (2), the gas mixture density can be worked out as:

Mixture Density (p_m) = 430 kg/m³

From Equation (1), the average valve position (at steady state) to get the required optimum DP (5 bar) can be worked out as:

Z_ss = 20.36 %

Now from Equation (4), assuming 3Q/3t = 0 and 3Gf/3t = 0, β₂ can be worked out to be:

0.5 X Zee β = —----21 = 2.0365 ^P (P_u ~ Pa)

Similarly, from Equation (4), assuming 3Q/3t = 0, β₃ can be worked out to be: β₃=β₂Χ (^po~^p^ x (R_u - RJ = 19.89

Pm

As can be observed from the above calculations, β₂ and β₃ will keep on dynamically changing with a change in the ‘flowrate’ in the system and the Optimum DP’ specified by a user, βτ is normally kept as ‘T, however this can vary between ‘0’ to ‘T depending on the 5 specific application. This is determined by studying the system dynamically with active feedback control.

Claims

1. An automatic slugging control apparatus comprising, a conduit arranged to convey a multiphasic mixture of materials;

a first measuring unit in communication with the conduit and arranged to provide a first measurement, the first measurement being characteristic of a first pressure upstream of a predetermined slugging zone;

a valve in communication with the conduit and located downstream of the first measuring unit and a predetermined slugging zone, the valve having a valve position; a second measuring unit in communication with the conduit and arranged to provide a second measurement, the second measurement being characteristic of a second pressure downstream of a predetermined slugging zone and upstream of the valve;

a third measuring unit in communication with the conduit and arranged provide a third measurement, the third measurement being characteristic of a third pressure downstream of the valve;

a fourth measuring unit in communication with the conduit and arranged to provide a fourth measurement, the fourth measurement being characteristic of a gas fraction of said multiphasic mixture of materials;

a controller in communication with the first, second, third and fourth measuring units and the valve, the controller being arranged to receive the first, second, third and fourth measurements and calculate at least one valve adjustment variable;

the controller being further arranged to provide an output signal to a valve adjuster, the output signal comprises at least one valve adjustment variable;

wherein the valve adjuster is arranged to adjust the position of the valve to a desired valve position according to the output signal received from the controller.

2. An automatic slugging control apparatus according to claim 1, wherein the at least one valve position variable comprises a first variable characteristic of a preliminary valve position; wherein the first variable is calculated using at least the first, second, third and fourth measurements.

3. An automatic slugging control apparatus according to claim 2, wherein the controller is further arranged to receive measurements characteristic of at least one selected from the range: flow through the valve; valve characteristic; average valve position; mixture density; gas density; liquid density; a predetermined optimum pressure drop across the valve.

4. An automatic slugging control apparatus according to claim 3 wherein the calculation of the at least one valve position variable further comprises a measurement characteristic of at least one selected from the range: flow through the valve; valve characteristic; average valve position; mixture density; gas density; liquid density.

5. An automatic slugging control apparatus according to claim 4, wherein the at least one valve position variable comprises a second variable characteristic of a target valve position; wherein the second variable is calculated by the controller using at least the first and fourth measurements and the measurement characteristic of the optimum pressure drop across the valve.

6. An automatic slugging control apparatus according to claim 5, wherein the at least one valve position variable comprises an error variable; wherein the error variable is calculated by the controller using at least the first variable and the second variable.

7. An automatic slugging control apparatus according to claim 6, wherein the at least one valve position variable comprises a preliminary valve position variable; wherein the preliminary valve position variable is calculated by the controller using at least the error variable.

8. An automatic slugging control apparatus according to claim 7, wherein the at least one valve position variable comprises a desired valve position variable; wherein the desired valve position variable is calculated by the controller using at least the preliminary valve position variable.

9. An apparatus according to any one of the preceding claims, wherein the apparatus further comprises a phase separating member located downstream of the valve, and arranged to separate the said multiphasic mixture of materials into one or more substantially single phase materials.

10. An apparatus according to claim 9, wherein the phase separating member is arranged to accommodate a portion of the multiphasic mixture of materials prior to separation of said portion into one or more substantially single phase materials.

11. An apparatus according to claim 10, wherein the apparatus further comprises a fifth measuring unit, the fifth measuring unit arranged to provide a fifth measurement characteristic of an amount of multiphase mixture of materials accommodated within the phase separating member.

12. An apparatus according to claim 11, wherein the controller is in communication with the fifth measuring unit, the controller being further arranged to use at least the fifth measurement from the fifth measuring unit to calculate the at least one valve adjustment variable.

13. A method of automatic adjustment of a valve position in a conduit system suitable for automatic mitigation of slugging effects, the method comprising the steps of:

d) measuring the fraction of gas or liquid in the slugging zone using a fourth measurement unit;

h) using the error variable to determine a preliminary valve position;

14. A method according to claim 13, wherein steps a) to j) are performed using an apparatus according to any one of claims 1 to 12.