WO2004080566A1

WO2004080566A1 - Multiphase flow handling

Info

Publication number: WO2004080566A1
Application number: PCT/NO2004/000062
Authority: WO
Inventors: Alf Reidar Kluge
Original assignee: Statoil ASA
Current assignee: Equinor ASA
Priority date: 2003-03-12
Filing date: 2004-03-05
Publication date: 2004-09-23
Anticipated expiration: 2005-09-12
Also published as: NO20031131L; NO319645B1; NO20031131D0

Abstract

Control system and assembly for flow stabilization, pre-separation of gas from liquid and hindrance of gas passage for a fluid flow from a pipeline for which liquid is the dominant phase, wherein a compact cyclone based inline degasser with a downstream connected compact multiphase inlet separator are arranged either at the outlet, inlet or both inlet or inlet of the pipeline, distinguished in that the control system is comprising: means for automatic control of liquid drainage from the degasser and inlet multiphase separator, means for automatic control of gas takeoff from the degasser and inlet multiphase separator, and protective functions.

Description

Multiphase flow handling

Field of the invention

The present invention relates to flow stabilization, pre-separation of gas from liquid and hindrance of gas passage for a fluid flow for which liquid is the dominant phase, such as a multiphase pipeline that in substance conducts liquid. A compact cyclone based degasser is used combined with a downstream connected compact multiphase inlet separator, combined with a control system. Both the degasser and the downstream connected compact multiphase inlet separator are designed and constructed according to pipe code specifications, and the units are arranged either at the outlet, inlet or both at the outlet and inlet of the pipeline. With the present invention a beneficial technical effect is achieved with respect to weight, volume and investment for the equipment, and downstream arranged separation equipment can be eliminated or downscaled significantly.

Background of the invention and prior art

With respect to multiphase pipelines that in substance conduct liquid, but also contain gas, it is currently required to use relatively expensive and voluminous equipment to achieve flow stabilization and pre-separation of gas from the liquid, which constitutes a problem, particularly for surface installations offshore.

Therefore a demand exists for equipment that to a significant degree reduces the above-mentioned problem.

Compact equipment exists for degassing, which would be very beneficial to utilize. In patent publication PCT/NO00/00224 a compact inline-degasser is described, which unfortunately exhibits problems with slow response, reduced degree of separation and tendency of flooding with liquid. In patent publication NO 2002 5841 and particularly in patent publication PCT/NO04/00<?53 further developed improved degassers are described, which very advantageously can be used in connection with the present invention. Each of said preferable degassers are a compact inline cyclone based device for separation of gas from a multiphase fluid flow that flows through a pipeline.

The degasser with control means according to PCT/NO04/00/)33is comprising: a pipe formed separation chamber with an upstream end where fluid that is passed in by use of a spin element in the upstream end is set into rotation and separated into; a heavier fraction that essentially is accumulated along the inner pipe wall of the separation chamber and is taken out through an outlet in a downstream end of the separation chamber, and; a lighter fraction essentially accumulated along the longitudinal axis of the separation chamber, from where an outlet pipe is arranged for delivery of the lighter fraction to; a control separator arranged to separate out any entrained heavier fraction from the lighter fraction, which entrained heavier fraction is taken out through an outlet pipe from a bottom zone filled with heavier fraction in the control separator, preferably for delivery thereof to the heavier fraction from the separation chamber, while the lighter fraction is taken out from the control separator through a separate outlet pipe, said degasser being distinguished by: an orifice/nozzle with differential pressure transmitter for transmitting a differential pressure over the orifice/nozzle arranged in the outlet pipe for the lighter fraction from the separation chamber to the control separator, said differential pressure being utilized as basis for controlling a valve arranged in said outlet pipe.

Reference is made to the above-mentioned patent publications for further description of the particularly advantageous degassers. A demand exists for further improved compact equipment for flow stabilization, pre-separation and hindrance of gas passage in the liquid conduit towards downstream liquid treatment equipment.

Summary of the invention

With the present invention the above-mentioned demand is met by providing a control system and an assembly for flow stabilization, pre-separation of gas from liquid and hindrance of gas passage for a fluid flow from a pipeline for which liquid is the dominant phase, by having a compact cyclone based inline degasser with a downstream connected compact multiphase inlet separator arranged either at the outlet, inlet or both outlet and inlet of the pipeline. ,

The control system is distinguished by comprising: devices for automatic control of liquid drainage from the degasser and inlet multiphase separator, devices for automatic controlled gas takeoff from the degasser and inlet multiphase separator, and protective functions.

The control system according to the present invention is particularly beneficial if it is connected to transmitters for flow information in the pipeline, such as forward fed transmitters for flow rate, flow composition and flow pressure.

The degasser includes a control separator that provides a utility volume with respect to control of the separation effect, represented by the drainage rate from the control separator.

The objective of the multiphase inlet separator downstream the degasser is to avoid gas passage out from the liquid outlet, represented by the liquid outlet from the multiphase inlet separator to downstream located liquid treatment equipment. This is achieved by holding a sufficient liquid volume in the multiphase inlet separator such that a valve in the liquid outlet makes it to be completely closed before gas passage occurs. The function is completely independent from the volume of the control separator of the degasser. With the present invention both a compact pre-separation, flow stabilization and hindrance of gas passage are achieved to a significantly reduced investment in equipment, particularly as the functionality is achieved at high pressure in equipment designed according to the pipe code specification, which have very different requirements with respect to safety functions and flaring than equipment designed according to the pressure vessel code specifications.

In a preferable embodiment a deliquidizer is arranged in the gas pipeline between the control separator and the multiphase inlet separator, to increase the separation efficiency. A deliquidizer as described in the patent publication NO 2000 6656 can preferably be utilized.

Another preferable embodiment has a further degasser arranged in the liquid pipeline from the degasser to the multiphase inlet separator, for increased separation efficiency.

Drawings

The invention is illustrated with two figures, of which:

Figure 1 is a principle illustration of the control system with degasser, multiphase inlet separator and field instruments.

Figure 2 illustrates the most preferred embodiment of the control system according to the present invention.

Detailed description

Firstly reference is made to Figure 1, where the degasser, with separation chamber 1 and control separator 2, and a multiphase inlet separator 3 are illustrated, with field instruments. A part of the outlet pipe for the lighter fraction from the separation chamber is illustrated arranged as a riser separating the separation chamber from the control separator. In the riser an orifice is arranged with a differential pressure transmitter DPT 100 for transmitting the differential pressure over the orifice, said differential pressure being utilized as basis for adjustment of a valve FV 100 in the riser to regulate the flow rate of the lighter fraction from the separation chamber to the control separator. As the density of gas is far lower than the density of liquid, entrainment of liquid with the lighter phase from the separation chamber will result in a dramatic increase in differential pressure over the orifice, which dramatically increased differential pressure results in the valve FV 100 in the riser being choked to avoid entrainment of liquid.

The capacity for gas handling of the degasser can be exceeded at start-up of the pipeline, at setting into operation or after a shut-down, or when the flow rate through the pipeline is increased, which can result in flow transients and problems with gas voids while the flow is adjusted from one steady-state condition to another. Therefore the multiphase inlet separator 3 is arranged. The multiphase inlet separator is far smaller than, conventional separation equipment. The dimensioning of the multiphase inlet separator is according to the volume to be sufficient to achieve a desired residence time for liquid that has flown in, such that valve FV 600 in the liquid outlet line makes it to be opened to avoid flooding and closing/choking to avoid emptying, and in addition valve FV 850 in the gas outlet pipe must make it to be closed before flooding of the multiphase inlet separator.

The liquid drainage from the degasser is primarily controlled by valve FV 200 in the outlet pipe for liquid from the control separator, and the liquid drainage from the multiphase inlet separator is primarily controlled by valve FV 600 in the outlet pipe for liquid from the multiphase inlet separator. Gas takeoff from the degasser and multiphase inlet separator is controlled primarily by valve FV 850 in the outlet pipe for gas from the multiphase inlet separator.

To protect downstream equipment and the control system per se, protective functions are arranged to hinder passage of liquid into the gas outlet and passage of gas into the liquid outlet. The primary control of the protective functions is to valve XV 100 in the gas outlet from the multiphase inlet separator and valve XV200 in the liquid outlet for the multiphase inlet separator, respectively.

In the following the control system will be described in further detail, and reference is made to Fig. 2 that illustrates a fully equipped embodiment that represents the most preferred embodiment of the invention.

The retroactive flow controller FC 100 manipulates valve FV 100 to maintain a steady-state multiphase flow into the axial outlet pipe from the separation chamber, which is achieved based on measured differential pressure over the orifice. By a dramatic increase of differential pressure set-point for FC 100 has to be adjusted, which can be undertaken automatically. Optimization of the gas takeoff, which means maximum gas takeoff without significant liquid entrainment, must be considered in connection with control of the liquid level in the control separator. To avoid emptying of the control separator drained water must be replaced such that a liquid balance is maintained. Therefore a level controller LC 100 A is arranged, connected to level transmitter LT 100 in the control separator and flow controller FC 100, as illustrated on Fig. 2. LC 100 A will automatically compensate for a decreasing level L 100 in the control separator, by increasing the set-point for FC 100, which would result in increased quantity of gas and increased quantity of entrained liquid to the control separator.

Liquid drainage from the control separator takes place via valve FV 200 and control thereof, which in principle is taking place by flow transmitter FT 200 connected to flow controller FC 200 that again controls valve FV 200. Drainage of liquid from the control separator results in a decreased liquid level L 100 in the control separator, why the previously described level controller LC 100 A will cause opening of valve FV 100 such that an increased flow rate of liquid arrives to the control separator and the level in the control separator is re-established. The flow rate of liquid drained from the control separator will typically be in the range 5 % to 10 % of the total flow rate of liquid through the liquid outlet from the degasser. At steady-state condition of flow rate of liquid through the degasser, also the liquid drainage F 200 from the control separator can be maintained at steady state. If the total flow rate of liquid F 300 through the degasser varies, F 200 can be controlled in relation to F 300, such as indicated with connections between the flow transmitters FT 300 and FT 200 on Fig. 2.

The pressure in the control separator must be kept sufficiently high so liquid can be passed from the control separator to the outlet pipe for the heavier phase from the degasser, and not the opposite way. This is achieved by regulating valve FV 850 so that it is sufficiently choked for the pressure to at least be equal to the pressure in the outlet pipeline for the heavier phase from the inlet multiphase separator.

The level control for liquid drainage from the inlet multiphase separator takes place by the level L700 being controlled by level controller LC700A, which acts on valve FV 600 via flow controller FC600, as illustrated on Fig 2.

The inlet multiphase separator has compensation of the level measurement because of prevailing density variations, such as illustrated by f(x) connected to LC700A. The liquids arriving to the inlet multiphase separator may vary in density from pure glycol to water to pure condensate, in all mixing ratios. The level L700 will be influenced by said variation if the level measurement is undertaken according to differential pressure.

The differential pressure DP 100 over the orifice of the degasser is preferably used as feed forward to the liquid outlet from the inlet multiphase separator, based on the ratio control principle, to promote an increasing liquid drainage before an increased level L700 in the inlet multiphase separator is detected. The differential pressure signal is manipulated to achieve appropriate linearization.

The gas takeoff from the inlet multiphase separator is controlled automatically. In principle the control is achieved by having the valve FV850 in the gas outlet from the inlet multiphase separator to be controlled based on the pressure in the inlet multiphase separator, provided by pressure transmitter PT 750. However, the control is compensated to account for the mass balance, illustrated by the feed forwards as apparent on Fig. 2. The mass balance of the pipeline is in general maintained by adjusting FC850 set-point as required, to keep the variation of the pipeline pressure within acceptable limits. By summarizing the liquids arriving the inlet multiphase separator with the gas flow rate, providing the sum as a process value, the total mass flow functions as a feed forward to PC750, which again is used to adjust the internal mass balance of the pipeline to keep the outlet pressure of the pipeline within acceptable limits as defined by the operational set- points.

In addition to the above mentioned stabilization of the pipeline flow a further stabilization is achieved by installing a device for flow information, such as pressure controller PC800, upstream in the pipeline, for example near well heads, at high or low points, such as in the bottom of risers and at the top of raised parts of the pipeline route. PC800 can be connected to PC750 as illustrated on Fig. 2,, or alternatively the connection can be directly to FC900 or FC850.'

The protective functions for the control system is comprising protection against passing liquid into the gas outlet from the inlet multiphase separator and passing gas into the liquid outlet from the inlet multiphase separator. So protection of downstream arranged equipment is achieved, protection of the control system per se, in addition to said protective functions being essential to ensure automatic control, in particular automatic start and stop.

By flooding of the inlet multiphase separator the retroactive level controller LC700B will override the above-mentioned control loops, and valve FV850 will be choked, and by further flooding the gas outlet isolation valve XVI 00 will be closed, after which also FV850 will be closed to protect the connected units of the control system.

A leakage proof isolation valve XV200 is arranged in the outlet pipe for liquid to achieve further safety against passage of gas into the liquid outlet than achievable with the valve FV600 alone, as illustrated on Figure 2. By closing XV200 also FV600 will be closed to protect connected units of the control system.

If downstream arranged equipment must be shut down, isolation valve for gas XVI 00 and isolation valve for liquid XV200 are closed, respectively, as illustrated with the symbols PSD on Fig. 2.

The most preferred embodiment of the control system according to the present invention is the embodiment illustrated on Fig. 2. However, the control system can be adapted to the prevailing process requirements. Preferably all variables are monitored because equipment for such monitoring is standard functionality on commonly used SAS (Safety and Automation System).

The cycle time for the control functions implemented in SAS is preferably less than 0.5 second. All control units and control functions are preferably provided with bumpless transfer between the different control-modes manual, automatic and cascade.

Claims

C l a i m s

1. Control system and assembly for flow stabilization, pre-separation of gas from liquid and hindrance of gas passage for a fluid flow from a pipeline for which liquid is the dominant phase, wherein a compact cyclone based inline degasser with a downstream connected compact multiphase inlet separator is arranged either at the outlet, inlet or both outlet and inlet of the pipeline, characterized in that the control system is comprising: means for automatic control of liquid drainage from the degasser and the inlet multiphase separator, means for automatic control of gas takeoff from the degasser and the inlet multiphase separator, and protective functions. .

2. Control system according to claim 1, characterized in that the means for automatic control of liquid drainage from the degasser and inlet multiphase separator are based on level signals from said units, said control being adjusted according to feed forward measured values for density and flow rate.

3. Control system according to claim 1, characterized in that the means for automatic control of gas takeoff from the degasser and inlet multiphase separator are based on pressure control that is adjusted according to feed forward measured values for density and flow rate.

4. Control system according to claims 1-3, characterized in that it is comprising at least one automatic protective function against passing liquid out through the gas outlet.

5. Control system according to claims 1-4, characterized in that it is comprising at least one automatic protective function against passing gas out through the liquid outlet.

6. Control system according to claim 1, characterized in that a part of the outlet pipe for the lighter fraction from a separation chamber in the degasser is arranged as a riser separating the separation chamber from a control separator in the degasser, in which riser an orifice with differential pressure transmitter is arranged for transmitting the differential pressure over the orifice, said differential pressure being utilized as basis for controlling a valve in the riser to control the flow rate of the lighter fraction from the separation chamber to the control separator.

7. Control system according to claims 1-6, characterized in that one or more transmitters for flow information is arranged in the pipeline, such as forward feeding transmitters for flow rate, flow composition and flow pressure.

8. Control system according to claims 1-7, characterized in that the response time for any control loop of the control system is less than 0.5 second.

9. Control system according to claims 1-8, characterized in that both the degasser and the downstream connected compact multiphase inlet separator are designed and constructed according to pipe code specifications.

10. Control system according to claims 1-9, characterized in that a deliquidizer is arranged in the gas pipe between the control separator and the multiphase inlet separator, to increase the separation efficiency.

11. Control system according to claims 1-10, characterized in that a further degasser is arranged in the liquid line from the degasser to the multiphase inlet separator, for increased separation efficiency.