Abstract:
A device for diverting liquid from a pipeline is described having a first conduit; a second conduit connected to the first conduit at at least a first location and a second location; with the second conduit having a first section to collect diverted liquid and a second elongated section adapted to contain at least parts of the diverted liquid with a hydrostatic head or level reactive to a pressure drop between the first and second location thus controlling flow of liquid from the first conduit through the second conduit by balancing the pressure drop with the hydrostatic head or level.

Description:
FIELD OF THE INVENTION 
   This invention relates to a device for diverting fluid from a pipeline and is particularly concerned with separating oil and water from a multi-phase flow of gas, oil and water. 
   BACKGROUND TO THE INVENTION 
   In the oil industry, the water-liquid-ratio (wlr) is an important measurement of the output flowing from a well and for wells producing a three-phase flow (gas, oil and water) separator systems are used to separate the gas, oil and water into individual streams to simplify the wlr measurement. However separator systems are heavy, bulky, costly and prone to failure and obtaining three separate streams for each component can be complicated and costly. 
   Where the liquid, i.e. water and oil, is not separated and metering is performed on the multi-phase flow, the measurement of the wlr can be very difficult, particularly when the volume fraction of gas in the line is high. For example, where the wlr is 10% in a multi-phase flow with 90% gas, then 1% of the total volume is water, 9% is oil and 90% is gas. Measurement of the wlr in a multi-phase flow requires detecting the presence of the 1% by volume of water, whereas if all the gas is removed leaving an oil-water flow, measurement of the wlr requires detecting the presence of the 10% by volume of water. Removing gas from the multi-phase stream to produce a liquid-rich stream therefore makes measurement of the wlr easier. 
   Hydrocyclones are used to produce a liquid-rich stream from a multi-phase flow. However these systems tend to be large and result in the liquid and gas phases travelling in opposite directions which can cause problems with pipe layout. 
   It is an aim of the present invention to produce a device for obtaining a liquid-rich stream without the disadvantages associated with the prior art, and also aims to provide a device for retrofitting to existing flow meters to increase their range of operation. 
   SUMMARY OF THE INVENTION 
   In accordance with one aspect of the present invention, there is a device for diverting liquid from a pipeline, the device comprising a first conduit, a second conduit connected across at least part of the first conduit, and means for controlling flow of liquid from the first conduit through the second conduit by use of hydrostatic pressure. 
   Such a device is particularly applicable for surface pipelines transporting gas-rich flows from wells, the flow being of around 90% gas, with the remaining percentage by volume consisting of oil and water. The device is suitable for use as a flowmeter when used in combination with appropriate gauges, and also is such as to allow sampling as liquid can be tapped off separately to gas. The device is also intended for retrofitting to existing flowmeters to increase the range over which flows can be measured, with the device acting to provide an offset to these meters, or the device can be incorporated into the flowmeter during manufacture. 
   In use, the device is typically connected to a pipeline transporting a multi-phase flow which is predominantly made up of gas, with the first conduit arranged to be substantially vertical relative to the earth&#39;s surface. 
   Therefore the first conduit in use is preferably connected between two sections of an existing pipeline. This particularly requires the pipeline flow to be stopped whilst the pipeline is cut to allow the placing of the device. 
   The means for controlling flow of liquid from the first conduit through the second conduit is preferably provided by an S-shaped section within the second conduit. Thus the second conduit may incorporate the means for controlling the flow of liquid. By using an S-shaped bend and arranging the second conduit to provide a bypass route across a length of the first conduit, the pressure drop across the bypassed length of the first conduit will be balanced by a difference in the height of fluid in the two curved sections forming the S-shaped bend. As a result, if more liquid is introduced into the second conduit, the level of fluid in the S-shaped bend alters to remain in equilibrium with the pressure drop across the length of the first conduit, as a result of hydrostatic pressure, and thus new fluid introduced into the second conduit will force fluid out of the S-bend and into a return section of the second conduit thereby to return to the first conduit. 
   The second conduit may further comprise a collecting means placed at least partly within the first conduit, and thus the second conduit preferably further comprises an annulus extending inwards from an inner wall of the first conduit and a lip extending upwards from an inner circumference of the annulus, and acting to trap liquid travelling along the walls of the first conduit and direct liquid into the second conduit. 
   The second conduit may comprise an elongate lip attached to the annulus or collar, with the elongate lip forming a baffle plate to act to separate liquid from the gas flow. 
   Preferably the first conduit is provided with an inlet at substantially right angles to the first conduit. This tangential inlet ensures that in use fluid passing into the first conduit gains a certain degree of centrifugal force to further assist with separation of liquid components from the gas. 
   The collecting means may further comprise receptacle means in communication with the first conduit and the annulus. This allows a volume of liquid to be stored before the liquid enters the S-shaped section, and so provides time for gas inadvertently trapped in the liquid to escape the liquid whilst it is held in the receptacle means prior to entry into the S-shaped section, the gas then returning to the first conduit. 
   The receptacle means is preferably placed adjacent to the annulus and the first conduit, and connected thereto by first and second passages. 
   Alternatively the receptacle means surrounds the annulus and at least part of the first conduit with two spaced apart apertures in an encased wall of the first conduit providing communication between the receptacle means, annulus and first conduit. 
   The second conduit may further comprise an elongate section extending from an end of the S-shaped section furthest from the first conduit, the elongate section providing a generally downward path and joining with the first conduit at a distance below an inlet to the second conduit. 
   The second conduit may comprise a delay section leading from the collecting means and joining with a first end of the S-shaped section, the delay section comprising a hollow cylinder of tapering cross section which is wound around the first conduit to form a spiral. 
   The device is suitable for use with surface pipes, but may be adapted for use on a vertical well pipe such as a borehole. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
     The invention is now described, by way of example, with reference to the following drawings in which: 
       FIG. 1  shows a section through a first embodiment of a device in accordance with the present invention; 
       FIG. 2  shows a sectional view on the line II—II of  FIG. 1 ; 
       FIG. 3  shows a section through a second embodiment of a device in accordance with the present invention; 
       FIG. 4  corresponds to  FIG. 3  and is used to explain operation of the device; 
       FIG. 5  is a graph illustrating the amount of liquid extracted as a percentage of liquid input for the device shown in  FIG. 3 ; 
       FIG. 6  shows a section through of a third embodiment of a device in accordance with the present invention; 
       FIG. 7  shows a section through a fourth embodiment of a device in accordance with the present invention; 
       FIG. 8  shows a section through a fifth embodiment of a device in accordance with the present invention; and 
       FIG. 9  shows a schematic diagram of a flowmeter incorporating a device in accordance with the present invention. 
   

   DESCRIPTION 
   A device  10  in accordance with the present invention is illustrated in  FIG. 1 . Typically the device is inserted into a surface pipeline carrying a multi-phase flow of gas, oil and water from a well. To insert the device, the flow in the pipeline is stopped, and the pipeline cut and modified so that an upstream portion of pipeline feeds into an inlet  12  of the device with a downstream portion of the pipeline connected to an outlet  14  of the device. The device is placed at right angles to ground level. 
   The device  10  is made from metal and comprises a first conduit  20  and a second conduit  22 . The first conduit  20  has a circular cross-section of typically the same size as the cross-section of the pipeline, whilst the second conduit  22  has generally a substantially smaller circular cross-section than the first conduit. The walls of the first and second conduits are of a suitable thickness to withstand the pressures associated with multi-phase production flows, and thus are typically of a thickness that will withstand 5000 psi. 
   The second conduit  22  is connected across a length L of the vertical first conduit  20  so providing a path along which liquid can be temporarily diverted from the main conduit, entering inlet  24  before returning to the main conduit at outlet  26 . The second conduit  22  comprises a collecting means  30  joined to the inlet  24  and which sits within the first conduit  20 , a first elongate section  32  of pipe attached between the inlet  24  and a first end  34  of an S-shaped section  36 , a second elongate section  38  joined to a second end  40  of the S-shaped section, and a downwardly slanting section  42  leading from the second elongate section to the outlet  26  and joining to the first conduit  20 . The collecting means  30  comprises an annulus  44 , of the same outer diameter as the first conduit, and a lip  46  extending upwards from an inner edge  50  of the annulus  44 . 
     FIG. 2  shows a sectional view along line II—II of  FIG. 1  from which can be seen the cross-section of the second conduit  22 , excluding the collecting means, and the cross-section of the first conduit  20  are circular, with the diameter of the first conduit  20  being substantially greater than the diameter of the second conduit  22 . 
   When a multi-phase flow travels in a pipeline, the liquid in the flow, i.e. oil and water, predominantly travels along the walls of the pipeline as a result of frictional effects. Thus by placing an annulus  44  with a lip  46  within the first conduit  20 , the liquid portion of the gas-liquid flow is channelled into the second conduit  22 . In the embodiment shown in  FIG. 1 , some gas will pass with this liquid into the second conduit  22 , but, as will be explained later, due to the residence time of liquid within the S-shaped section  36 , much of the gas will return to the first conduit  20 . 
   A second embodiment of the device is shown in  FIG. 3 , and comprises a main conduit  50  and a secondary conduit  52  incorporating a more complex collecting means  54  than that of the first embodiment. Instead of flow from the pipeline travelling down into the first conduit, i.e. with gravity, a tangential inlet  56  to the main conduit  50  is provided. The second collecting means  54  comprises an annulus  58  with an elongate lip  60  which extends up beyond the tangential inlet  56  so as to act as baffle plate, and a substantially enclosed cylinder  62 . The cylinder has a lower inlet  64  which joins with the first conduit  50  and so connects to the annulus  58  as well, and an upper inlet  66  placed further up the wall of the first conduit. The remainder of the second conduit comprises a first elongate section  70 , an upper end  72  of which extends up and into the cylinder  62  and is open to receive liquid, with a lower end  74  attached to a first end of an S-shaped section  80 . A flared elongate section  82  joins to a second end of the S-shaped section  80  and leads into a downwardly slanting section  84  which in turn connects with a horizontal portion  86  of pipe meeting the first conduit  50  at a position below the collecting means. An anti-siphon line  90  is provided between the flared section  82  and an uppermost end  92  of the first conduit  50  so as to ensure that automatic siphoning of the liquid through the system does not occur. The dimensions of the device are approximately 1200 mm (high)×500 mm×500 mm. 
   This embodiment has enhanced gas rejection over the first embodiment as the passage time of the liquid through the device is increased due to the increased volume of the collecting means. In addition, the tangential inlet imparts a certain degree of centrifugal force to the fluid as it enters the first conduit and this produces a swirling effect in the flow which assists with separation of liquid from gas. The baffle plate also acts to increase separation of liquid from gas as, when the multi-phase flow hits the baffle plate, the passage of liquid is abruptly halted causing the liquid to fall to the base of the collecting annulus. However the gas is not so affected and passes along the length of the first conduit. 
   The operation of the device will now be described with reference to  FIG. 4 , which uses common reference numerals to  FIG. 3  where appropriate. The device uses the principle of hydrostatic pressure to provide a controllable passage of liquid through the system, eventually to return to the first conduit. The device thus avoids the need for any moving parts or any external control for the device to operate. When a multi-phase flow enters the first conduit  50  via the tangential inlet  56 , liquid incident on the baffle plate  60  falls down to the collecting annulus  58  and, due to the interconnection of the annulus, cylinder  62  and S-shaped bend  80 , will pass into all these parts of the second conduit. Gas in the multi-phase flow is largely unaffected by the presence of the baffle plate  60  and generally will simply continue flowing along the length of the first conduit, although some gas will be trapped in the liquid falling into annulus  58 . 
   A pressure drop exists in the first conduit  50  due to gravity, with gas at the upper end  92  of the conduit being at a lower pressure P 1  than gas at a lower end  94  of the conduit which is at pressure P 2 . The pressure difference, P 1−P   2 , of around 100 mbar is balanced by the head of liquid in the S-shaped bend  80 , i.e. the pressure exerted by the liquid h liquid −h 0 . Thus in the equilibrium position where fluid has been introduced into the second conduit but where, for example, flow has then stopped, the height of liquid in the S-shaped bend is greater than the height of the liquid in the collecting means by an amount that balances the pressure difference. 
   As more liquid is introduced into the collecting portion of the conduit, the system moves out of equilibrium. Thus the level of liquid in the cylinder and annulus will be such that the head of liquid does not balance the pressure drop ΔP. The system will act to restore the equilibrium state and thus increase the level of fluid in the S-bend to h level  so as to ensure that the head balances the pressure drop. Liquid is thus forced up and out of a vertical portion  96  of the S-bend and into the flared portion  82  to return to the first conduit, as the system continuously acts to restore equilibrium as liquid flows into the second conduit. The maximum liquid extraction flow rate is a function of the dimensions of the device, but for a device of dimension 1200 mm×500 mm×500 mm is typically 8 m 3  an hour. 
   To explain in more detail, the equilibrium state is thus when no liquid is extracted and the hydrostatic head, ρg(h Level −h 0 ) is balanced by the pressure drop P 1−P   2 :
 
Δ P=P   1   −P   2   =ρg ( h   Level   −h   0 )  (1)
 
where ρ is the liquid density, h Level  is the greatest height of liquid in S-shaped section  80  of diameter d, and h liquid  is the height of liquid in the cylinder which has diameter D.
 
   For a liquid velocity of ν 1  in diameter D, the velocity ν 2  in diameter d is 
               v   2     =       v   1     ⁢       D   2       d   2                 (   2   )             
 
   When h Liquid &gt;h 0  then liquid flows through the device and with a liquid velocity in diameter D of ν 1 , the liquid velocity ν 2  in diameter d, can be written as 
                 v   2     =         (     2   ρ     )     ⁢       (       Δ   ⁢           ⁢   P     -     P   Losses     -     ρ   ⁢           ⁢     g   ⁡     (       h   Level     -     h   Liquid       )           )       (     1   -       d   4       D   4         )             ⁢     
     ⁢   where           (   3   )                 P   Losses     =       2   ⁢           ⁢   f   ⁢           ⁢   ρ   ⁢           ⁢     v   2   2     ⁢     L   Losses       d             (   4   )             
 
and
 
 f=aRe   −b  Blasius formula,  a= 0.079 , b= 0.25  (5)
 
and 
             Re   =       ρ   ⁢           ⁢     v   2     ⁢   d       η   liquid               (   6   )             
 
where P Losses  is the pressure loss in diameter d, L Losses  the equivalent straight pipe length diameter d, and η liquid  the viscosity of the liquid.
 
   When ν 2 =0 then P Losses =0, and ΔP is given by equation (1). h 0  should be chosen to be large enough so that no liquid enters the liquid leg, in which case 
                 v   2     =         (     2   ρ     )     ⁢       (       ρ   ⁢           ⁢     g   ⁡     (       h   Liquid     -     h   0       )         -     P   Losses       )       (     1   -       d   4       D   4         )             ⁢     
     ⁢         For   ⁢           ⁢     v   2       &gt;   0     ,         then   ⁢           ⁢     P   Losses       &lt;     ρ   ⁢           ⁢     g   ⁡     (       h   Liquid     -     h   0       )           =   0               (   7   )             
 
   The maximum value of ν 2  (or equivalently the maximum liquid extracted=ν 2 πd 2 /4) is driven by h Liquid  and this determines the total height of the device. 
   Thus if there is only gas in the main flow line then the equilibrium state is when the hydrostatic pressure difference between h liquid  and h 0  equals the pressure difference between the top of the first conduit and where the second conduit returns to join the first conduit. If the hydrostatic pressure of the liquid head is less than ΔP, then fluid will flow over the top bend of the S-shaped section and return to the main flow line via sections  82 ,  84 ,  86 . 
   This system is self regulating in that liquid will only flow out of the S-bend section when liquid is in the annulus, cylinder and S-shaped section and the hydrostatic head, h, is too small to balance the pressure difference ΔP. Hence a heavy liquid phase will flow through the device, if there is only gas in the main flow line there will be no flow through the device, and the device rejects gas. 
   As mentioned previously, some gas will be trapped with the liquid when it is collected from the first conduit, and to ensure that the wlr measurement is easy to perform, as much gas as possible needs to be returned to the main conduit. A delay time, or lag, before liquid enters the S-shaped section is desirable so that gas caught within the liquid can escape. 
   There are a variety of ways of producing such a delay, with the second embodiment achieving this by increasing the volume of liquid waiting to pass into the S-shaped section. 
   The increased residence time of the liquid in the collection means allows gas bubbles trapped within the liquid to have an extended time in which to rise to the surface of the liquid and return to the first conduit by means of the first conducting passageway  66 . There are other ways of increasing the residence time, and these are discussed with reference to  FIGS. 6 and 7 . Ideally the residence time is around 5 s or such that the time for gas to rise to the surface of liquid in the collection means is less than the time for fluid to pass from the collection means to the S-shaped bend. The residence time needed depends on the distance the gas has to travel through the liquid to reach a liquid-gas interface and the velocities of the gas and liquid phases. 
   The devices discussed herein selectively divert liquid from a multi-phase flow so that the wlr can readily be measured, without a large proportion of gas being associated with the liquid and interfering with the measurement. The devices have many uses in that they allow direct sampling of the liquid by placing valves at positions A and B, and easy measurement of liquid and gas flow rates by placing a gas flow meter at position C and a liquid flow and/or wlr meter at position D. The devices can also be used as a sandtrap by placing a valve at E to draw sand out of the base of cylinder  62 , and the devices can be used to provide liquid removal from a flow by pumping liquid out of the device from any point in return path  82 ,  84 ,  86  before the liquid returns to the main conduit. The device can also be used as a compact separator of liquid for multi-phase flows. 
   In situations where a representative liquid sample is required the collection means is positioned in an area of high mixing of gas and liquid. 
   The devices can also be used for measurement of oil shrinkage, cleaning of the system by fluid injection, and calibration of any meter positioned at D by injecting fluids of known properties at known velocities. 
   Local heating can also be used to increase the flow of viscous fluids through the device. 
   Enhanced liquid removal can be achieved by careful design of the flow conditions upstream of the device and by positioning two or more devices in series. 
   With a device such as shown in  FIG. 3 , it is possible to extract around 90% of the liquid in a liquid gas flow. This is illustrated by the graph of  FIG. 5  which plots the “liquid extracted” against “the liquid input into the first conduit” for a variety of different water liquid ratios ranging from glr (gas volume rate/liquid flow rate) less than 10 and gvf (gas volume rate/total volume) less than 0.91, up to glr in the range of 200–1000 and gvf in the range 0.995–0.999. The liquid extracted has less than 1% gas entrained. 
     FIG. 6  shows a further embodiment of a device in accordance with the invention where a cylinder  100  surrounds a portion of a first conduit  102  with upper  110  and lower  112  apertures in plate  114  providing communication paths for gas and liquid between a collecting annulus  116  and the first conduit  102 . 
     FIG. 7  illustrates another embodiment of the present invention, where to increase residence time of fluid in the device, a tapering cross-section pipe  120  is wound around a first conduit  122  to lead into an S-shaped bend  124 . The diameter of this pipe  120  and the pitch of the winding are such that the flow in this pipe is stratified with the liquid on the lower surface of the pipe. In this case, any gas in the liquid has to travel a distance equal to the thickness of the liquid stratified layer before exiting via the collecting means  126 . Having a pipe with a decreasing diameter enhances this effect. This ensures that the liquid collected within the S-bend is to a large extent gas free. 
   A further embodiment is illustrated at  FIG. 8 , this being a device for use in an operational well  130  with fluid flowing up to surface as shown by arrow  132 . By providing low pressure at one end  134  of the device, liquid collection can be achieved in a similar manner as aforesaid. 
   A device in accordance with the present invention can also be used to modify existing flowmeters so as to extend their range of operation, and this is shown in  FIG. 9 . 
   Multi-phase flowmeters are used in the oil industry to measure the flow rates of oil, water and gas in a pipeline without separating the phases. These meters have an operational range with a lower limit that can generally only be modified by a change in dimensions or by use of additional meters. One way of lowering the operating range of a multi-phase meter therefore involves a second multi-phase meter in series with the first. In the cases where the oil, water and gas phases are separated into individual streams, lowering the range involves adding additional meters to each flow line: this is costly as the number of meters needed is doubled and each meter is expensive. However in accordance with another aspect of the present invention, a device in generally the same form as that discussed previously is fitted to, either on manufacture or by retrofitting, a multi-phase meter so as to increase flow by a known amount that allows the meter to function over an increased operating range. 
   Such a modified flow meter  138  is illustrated schematically in  FIG. 9 . The basic flow meter is described in Atkinson, I., Berard, M. B-V Hanssen, G Segeral: “New Generation Multiphase Flowmeters from Schlumberger and Framo Eng.AS.,” 17 th  International North Sea Flow Measurement Workshop, Oslo, Norway, October 1999. A multi-phase flow  140  is fed into an input  142  of the flowmeter and passes through the meter to outlet  144 . A device  150  in accordance with the invention, such as that depicted in  FIG. 3 , is inserted in the outlet, or downstream, path of the meter and the device used to divert liquid from the multi-phase flow, the liquid passing along path  152  to a liquid storage tank  154 . Any gas contained in the liquid is returned along line  156  to the outlet path. The liquid in the liquid storage tank  154  is fed back along line  158  to the inlet  142 , and upstream of the metering section  159 , by pump  160 , and the amount of liquid fed back is monitored by liquid flow meter  162 . 
   This increases the total liquid flow through the multi-phase meter by a measured amount such that the flow through the meter is within the original operating range. The actual flow in the main pipeline is computed from the difference between the flow measured by the multi-phase meter and that measured by the meter in the liquid line. The flow ‘returned’ is only liquid phases to simplify the metering and pumping operation as then metering can be performed with a liquid meter and standard liquid pump. The accuracy of the liquid flow rate will be decreased slightly as a result of using two meters, for example, if the accuracy of the multi-phase meter is ±5% and the liquid meter is ±2% then the final accuracy of the combined meter will be ±5.4% (using RMS method). 
   The size of the tank  154  depends upon the efficiency of the device  150 , the liquid volume rate required to be pumped to get the multi-phase meter within its operating range, and the measurement time of the multi-phase meter. 
   The above-described embodiments are illustrative of the invention only and are not intended to limit the scope of the present invention.