Abstract:
An actuatable apparatus, such as a mixer or flow splitter, is described that forms part of a multiphase pumping station. An outer tank has an upper inlet and an actuatable inner vessel disposed within the outer vessel. Multiphase fluid can pass from the outer vessel into the inner vessel though large upper openings. The inner vessel is configured to be actuatable such that the inner vessel moves in a vertical direction, thereby altering the size of an annular opening between the bottom of the inner vessel and the outer vessel. In some cases, the annular opening is adjusted to alter the operating envelope of a mixer. In other cases, the annular opening is opened to allow for sand cleaning. In yet other cases the apparatus is a downstream flow splitter, and the annulus is shut off to prevent loss of liquid phase during the startup of a dead field.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     Not applicable. 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not applicable. 
     BACKGROUND 
     1. Field of the Invention 
     The present disclosure relates to fluid conditioning systems. More particularly, the present disclosure relates to systems for conditioning multiphase fluids, such as mixing and/or splitting such fluids in connection with a subsea and/or topside multiphase pumping system. 
     2. Background of the Technology 
     In fluid processing systems, such as multiphase pumping stations, a multiphase pump can be combined with an upstream multiphase mixer and downstream flow splitter. The multiphase mixer utilizes a large volume tank and it is advantageous to design the mixer for long-term installation on the sea floor. Because the multiphase mixer is often intended to be deployed for long periods of time, up to the lifetime of the field, it should be designed to deal with a relatively wide operating envelope in terms of both flow rate and gas volume fraction (GVF). However, the larger the operating envelope, the greater the compromise in mixer performance. For example, the mixer may be designed larger than ordinary in order to handle hydrodynamic slugging, while at the same time the annulus clearance may be designed smaller in order to push the operating envelope up toward higher GVF levels. 
     Another challenge to multiphase mixers that are designed for long-term deployment is the accumulation of sand and other solid debris. Recent findings show that in some fields, accumulations of sand and solid debris are possible within the volume of multiphase subsea upstream mixers. Additionally, because the differential pressure across the liquid flow path in the mixer and splitter is substantially less than the pressure differences across the pump, it is easier to block a mixer compared to a pump, assuming the same size of clearance. 
     Yet another challenge in multiphase pumping stations is starting up a dead field. The pumping station might be designed such that the flow splitter is self-draining into the bypass header, which in turn is self draining into the flow line. There is hence a risk, when starting up a dead field where there are no free flowing wells. In such cases, the pump station can be quickly emptied of liquid if there is some amount of gas in the flow line upstream of the pump station. No or limited amounts of liquid inside the station will reduce the available draw down and hence limit the ability to start a dead field. 
     BRIEF SUMMARY OF THE DISCLOSURE 
     This summary is provided to introduce a selection of concepts that are further described below in the detailed description. This summary is not intended to identify key or essential features of the claimed subject matter, nor is it intended to be used as an aid in limiting the scope of the claimed subject matter. 
     According to some embodiments, an actuatable apparatus is described for receiving a multiphase fluid from a source thereof and for conditioning the multiphase fluid (e.g. mixing the fluid and/or splitting the fluid flow). The apparatus includes: an outer vessel configured to condition the multiphase fluid; an upper inlet configured to receive the multiphase fluid from a source and introduce the multiphase fluid at an upper location into the outer vessel; an actuatable inner vessel disposed within the outer vessel comprising one or more upper openings at an upper location of the inner vessel such that multiphase fluid can pass from the outer vessel into the inner vessel. The inner vessel has a lower opening at a lower location of the inner vessel such that multiphase fluid can pass out of the inner vessel. The inner vessel is configured to be actuatable such that the inner vessel moves in a vertical direction with respect to the outer vessel. The apparatus also includes a lower outlet configured to discharge the conditioned multiphase fluid from the outer vessel. The lower outlet is in fluid communication with the lower opening of the inner vessel such that multiphase fluid passing out of the inner vessel is discharged from the outer vessel. 
     According to some embodiments, the apparatus is a mixer and is configured to be deployed upstream of a multiphase pump or any equipment that benefits from reduced slugging (e.g. compressors, multiphase flow meters, centrifugal separators, etc.). According to some embodiments, a lower annulus is formed between an outer lower portion of the inner vessel and an inner lower portion of the outer vessel. Actuating the inner vessel in the vertical direction can alter the size of the lower annulus, thereby altering an operating envelope of the apparatus. According to some embodiments, actuating the inner vessel upward facilitates cleaning of solid debris collected in the outer vessel for example by flushing into a dummy pump module. 
     According to some embodiments, the apparatus is a flow splitter positioned downstream of an outlet of a multiphase pump or wet gas compressor, etc. The apparatus can include a liquid-rich outlet from the outer vessel configured to draw liquid-rich flow from the multiphase fluid and to recirculate the liquid rich flow back into the multiphase pump. The lower annulus formed between inner vessel and a wall of the outer vessel can be reduced in size by actuation of the inner vessel, thereby increasing ability to draw liquid-rich fluid from the liquid-rich outlet. 
     According to some embodiments, a method of conditioning a multiphase fluid is described. The method includes: introducing the multiphase fluid from a source into an upper inlet of an outer vessel; flowing at least a portion of the multiphase fluid into an actuatable inner vessel disposed within the outer vessel through one or more upper openings at an upper location of the inner vessel; flowing at least a portion of the multiphase fluid through the inner vessel, outward through a lower opening of the inner vessel, and outward through a lower outlet of the outer vessel; and flowing a second portion of the multiphase fluid from the outer vessel through a lower annulus formed between an outer lower portion of the inner vessel and an inner lower portion of the outer vessel, and outward through the lower outlet of the outer vessel, thereby bypassing the inner vessel. According to some embodiments, the inner vessel is actuated in a vertical direction with respect to the outer vessel, thereby altering a size of the lower annulus. By actuating the inner vessel in an upward direction, the lower annulus is enlarged and solid debris collected in the outer vessel can be flushed outward through the annulus and outward through the lower outlet. According to some embodiments, the inner vessel is removed from the outer vessel, and accumulated solid debris is flushed from the outer vessel using an external fluid source. According to some embodiments, actuating the inner vessel in the vertical direction alters the size of the lower annulus, thereby altering an operating envelope of the mixing. 
     According to some embodiments, the multiphase fluid is split into a first flow path leading toward a surface location (or toward another downstream flow line) and a second liquid-rich flow path recirculating back into an upstream pump. Actuating the inner vessel in a vertical direction with respect to the outer vessel can close the lower annulus and cause most or all of the liquid phase to recirculate back into the upstream pump. Flow from one or more wells that had been previously non-producing can thus be initiated. When one or more of the wells are producing sufficient liquid phase, the inner vessel can be re-actuated, thereby re-opening the lower annulus. 
     According to some embodiments, a method is described for starting up one or more dead wells using a multiphase pumping station. The method includes: in a subsea location, pumping a multiphase fluid with a subsea multiphase pump; introducing the pumped multiphase fluid into a flow splitter disposed downstream of the pump. The flow splitter includes an upper gas-rich outlet, a lower liquid rich outlet, and a main outlet. A first valve is closed, thereby shutting off the main outlet of the splitter. A second valve is opened, thereby allowing gas-rich fluid from the upper gas-rich outlet of the splitter to flow out of the pumping station. A third valve is opened, thereby allowing liquid-rich fluid from the lower liquid-rich outlet of the splitter to flow back into the pump. After sufficient liquid phase is being drawn from one or more previously non-producing wells into the pump, the first valve is opened and the the second valve is closed. 
     According to some embodiments, one or more of the described systems and/or methods can be used in topside or subsea fluid processing equipment in an analogous fashion. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The subject disclosure is further described in the detailed description, which follows, in reference to the noted plurality of drawings by way of non-limiting examples of embodiments of the subject disclosure, in which like reference numerals represent similar parts throughout the several views of the drawings, and wherein: 
         FIG. 1  is a diagram illustrating a subsea environment in which a multiphase fluid processing system may be deployed, according to some embodiments; 
         FIG. 2  is a diagram illustrating some aspects of a multiphase pumping station, according to some embodiments; 
         FIGS. 3A and 3B  are cross sections illustrating further detail of a multiphase mixer having an adjustable operating envelope, according to some embodiments; 
         FIGS. 4A and 4B  show aspects of a technique for cleaning accumulated debris from a multiphase mixer, according to some embodiments; 
         FIGS. 5A and 5B  show aspects of a technique for cleaning accumulated debris from a mixer according so some other embodiments; 
         FIGS. 6A and 6B  are cross sections showing a flow splitter having an adjustable central pipe, according to some embodiments; and 
         FIG. 7  is a diagram illustrating some aspects of a multiphase pumping station, according to some other embodiments. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The particulars shown herein are by way of example, and for purposes of illustrative discussion of the embodiments of the subject disclosure only, and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the subject disclosure. In this regard, no attempt is made to show structural details of the subject disclosure in more detail than is necessary for the fundamental understanding of the subject disclosure, the description taken with the drawings making apparent to those skilled in the art how the several forms of the subject disclosure may be embodied in practice. Further, like reference numbers and designations in the various drawings indicate like elements. 
       FIG. 1  is an example diagram illustrating a subsea environment in which a multiphase fluid processing system is deployed, according to some embodiments. On sea floor  100 , a station  120  is that which is downstream of several wellheads  110  being used, for example, to produce hydrocarbon-bearing fluid from a subterranean rock formation. Station  120  includes a subsea multiphase pump  130 . The station  120  is connected to one or more umbilical cables, such as umbilical  132 . The umbilicals in this case are being run from a floating production, storage and offloading unit (FPSO)  112  through seawater  102 , along sea floor  100  and to station  120 . In other cases, the umbilicals may be run from some other surface facility such as a platform, or a shore-based facility. In addition to pump  130 , the station  120  can include various other types of subsea equipment. The umbilical  132  is used to supply barrier fluid for use in the subsea pump or compressor. Umbilical  132  also provides electrical power to station  120 , and often also include lines for control fluid in order to operate actuated valves, as well as lines for various chemicals (e.g. for wax-, scale, corrosion inhibitors etc.). Also visible in  FIG. 1  is ROV  142 , tethered using main lift umbilical  146  and tether management system  144 . 
       FIG. 2  is a diagram illustrating some aspects of a multiphase pumping station, according to some embodiments. In this simplified diagram, pumping station  120  is shown with multiphase pump  130 . Multiphase fluid from wells enters from flow line  250 . In the case where valves  202  (V 1 ),  204  (V 2 ) and  208  (VR) are open and bypass valve  206  (V 3 ) is closed, the multiphase fluid flows first into mixer  210 . Multiphase fluid enters mixer  210  via mixer inlet  212 . The mixer, which will be described in further detail, infra, mixes the multiphase fluid to form a more homogeneous mixture of liquid and gas phases. The homogenized multiphase fluid mixture exits the mixer  210  via mixer outlet  214  and enters the suction port  232  of pump  130 . The multiphase fluid exits the pump  130  via port  234  and the fluid enters splitter  220  via splitter inlet  222 . Splitter  220  splits the flow. In normal operation, most of the flow exits splitter  220  via main outlet  224  while a small fraction (or none, if desired) of fluid-rich flow exits splitter  220  via fluid-rich outlet  226 . The fluid from the main outlet  224  flows through valve  204  and toward the surface via flowline  252 . The fluid exiting the fluid-rich port  226  recirculates back through valve  208  and back into mixer  210  via inlet  216 . According to some embodiments, the fluid exiting port  226  and through valve  208  is routed back to another location upstream of mixer  210  and/or to multiphase pump  130 . According to some embodiments, valve  208  can be closed when recirculation is not desired. According to some embodiments, an inlet strainer  240  can be included upstream of mixer  210 . In some embodiments, the inlet strainer  240  includes a back flush system (not shown) configured to push debris into, for example, a bypass header and further toward the topside so as to alleviate clogging issues. 
       FIGS. 3A and 3B  are cross sections illustrating further detail of a multiphase mixer having an adjustable operating envelope, according to some embodiments. The mixer  210  has an extended operating range both in terms of flow rate and gas volume fraction (GVF). This might be desirable, for example, with subsea components such as subsea mixers, which are installed for long periods of time and therefore may be expected to perform during changes occurring during the field&#39;s lifetime. 
     Multiphase fluid enters from mixer inlet  212  into the large volume  300  of the main mixer tank. Two main fluid paths exist from volume  300  to outlet  214 . First fluid can pass into central pipe  310  either through upper openings  312  or through smaller holes  314  along the side wall of pipe  310 . Fluid in pipe  310  flows downward and out through the outlet  214 . Fluid enters central pipe through a relatively circuitous path—upward along the inside of sleeve  320  or through the plurality of small holes  314 . A second fluid path exits through an annular opening  302  between the lower edge of pipe  310  and the tapered inner wall of mixer housing. For further details of operation and/or variations in design for mixer  210 , according to some embodiments, please see e.g. European patent application nos. EP0379319A2, EP2425890A1, and U.S. Pat. Nos. 5,135,684; 6,280,505; 6,284,023; 6,284,024; 6,699,308; and 7,018,451, each of which is hereby incorporated by reference herein. 
     The central pipe  310  is equipped with an actuator  332  that can be used to lower or raise the central pipe  310 , hence changing the cross sectional area of annular opening  302 . Note that although actuation of central pipe is shown and described as using an “actuator,” according to some embodiments, other forms of actuation can be used, such as an ROV turning a handle, or by remote operation in a similar fashion as is known with remote valve operation. Changing the area of annular opening  302  in turn changes the size of the liquid flow path through opening  302  and hence changes the operating envelope of the mixer. In general, operating at higher GVF values uses a smaller annular opening  302 . Increasing the flow rate will shift the operating envelope toward a higher GVF, while decreasing the flow will shift the operating envelope toward a lower GVF. 
     According to some embodiments, further details of cleaning accumulated debris from multiphase mixers will now be provided. Over time, it has been found that multiphase mixers, such as mixer  210 , that are upstream of a multiphase pump can accumulate debris such as sand, gravel and other solid matter.  FIGS. 4A and 4B  show aspects of a technique for cleaning accumulated debris from a multiphase mixer, according to some embodiments.  FIG. 4A  shows mixer  210  in which volume  300  is partially filled with accumulated debris  400 . According to the design shown in  FIGS. 4A and 4B , the central pipe  310  is removable. In particular, the central pipe  310  can be removed by opening a clamp connector. An ROV (such as ROV  142  shown in  FIG. 1 ) with a pumping skid can then be used to jet the debris  400  out of the mixer  210 . In  FIG. 4B , a jet nozzle  410  is shown inserted into the central pipe opening and is being used to clean out debris  400  from volume  300  of mixer  210 . The jet nozzle  410  is being fed pressurized liquid via hose  412  from a pumping skid attached to an ROV. Note that ROV is also used to divert fluid and debris passing through the outlet  214  to be gathered downstream, for example, by a dummy pump that forms part of the pumping system, or is installed by the ROV. An example of a dummy pump  260  is shown in  FIG. 2  and discussed in more detail, infra. According to some embodiments, the debris is simply pushed directly into the flowline  252 . 
       FIGS. 5A and 5B  show aspects of a technique for cleaning accumulated debris from a mixer according so some other embodiments. In this case, the position of the central pipe  310  can be adjusted by either an ROV override or by activating actuator  332 . The central pipe can be moved to an upper position where the inlet of the central pipe (large holes  312 ) is blocked, hence routing the total flow through the liquid path (annulus  302 ), thus using the production flow to wash out the sand. 
     As in the case of  FIGS. 4A and 4B , the sand/debris cleaning is combined with a dummy pump in order to avoid producing large amounts of sand through the downstream multiphase pump  130  (shown n  FIG. 2 ). A dummy pump  260  (shown in  FIG. 2 ) could also be designed as a sand trap if it is desirable to avoid pushing the sand into the flow line. According to some embodiments, dummy pump  260  is a spool piece connected to the inlet and outlet flanges where another pump could ordinarily be installed. The sand can be flushed out through the dummy pump  260  into the flow line  252  directly or it could be designed as a “container” in order to collect the sand/debris removed from the mixer  210 . According to some embodiments, the dummy pump  260  is positioned in parallel with pump  130 , and valves (e.g. valve  262 ) are used to route debris into the dummy pump  260  instead of through the pump  130 . 
     According to some embodiments, a differential pressure across an inlet strainer, if installed (e.g. see strainer  240  in  FIG. 2 ), can indicate if the wells are producing large amounts of sand/solids without damaging the pump station. The inlet strainer  240  can also give a clear indication to the operator of what problem is occurring, in such cases. 
     According to some embodiments, the small holes  314  in central pipe  310  can be configured differently in order to reduce sand collecting within mixer  210 . In one example, instead of 4 columns of small holes  314  such as shown in the figures, each perforated section of the central pipe  310  of the mixer  210  has one or two larger diameter holes. By having a fewer number of larger holes, the sand collecting ability of mixer  210  is reduced. In order to maintain symmetry, the pattern of holes can be staggered in a spiral pattern along the length of central pipe  310 . In general, enlarging the diameter of holes  314  will allow more sand to be produced through the central pipe  310 , which will reduce collection of sand that fills the mixer. The diameter of the holes  314 , according to some embodiments, should be larger than the radial clearance of the annulus  302 . One reason for increasing the diameter and reducing the number of holes  314  may be to reduce clogging of the holes (e.g. from wax, sand, asphaltenes, and/or scale) as clogging may decrease the operating envelope of the mixer. 
     Further details of a downstream splitter will now be provided, according to some embodiments.  FIGS. 6A and 6B  are cross sections showing a flow splitter having an adjustable central pipe, according to some embodiments. Multiphase fluid enters flow splitter  220  from inlet  222  into the large volume  600  of the main splitter tank. Two main fluid paths exits from volume  600  to outlet  224 . First fluid can pass into central pipe  610  either through large upper openings  612  or through small holes  614  along the sidewall of pipe  610 . Fluid in pipe  610  flows downward and out through the outlet  224 . Fluid enters central pipe  610  through a relatively circuitous path—upward along the inside of sleeve  620  or through the plurality of small holes  614 . A second fluid path exits through an annular opening  602  between the lower edge of pipe  610  and the tapered inner wall of splitter  220  housing. 
     According to some embodiments, the flow splitter  220  located downstream of the pump is equipped with an adjustable central pipe  610  that can be used to close the annular opening  602 . According to some embodiments, actuator  632  is used to move central pipe  610  in a vertical direction. The liquid content inside the station  120  can be maintained and even increased by injecting, for instance, MeOH (via optional MeOH supply  270  shown in  FIG. 2 ) while gas from upstream of the station  120  is allowed to escape to downstream of the station through the central pipe  610  via outlet  224 . Note that although MeOH supply  270  is shown feeding into inlet  216  of mixer  210 , according to some embodiments, several injection points can be provided, and any injection point located such that the liquid goes through the pump  130  can be used. The capability of shutting off the liquid-rich path in flow splitter  220  allows for the retention of liquid in pump station  120  while gas is still produced. This might be useful in starting up a dead field, because in such cases, there is a risk of producing most or all of the available liquid in station  120  very quickly, which reduces the draw down of pump  130 . Once the wells are started, the central pipe  610  is raised up again such that liquid can be produced normally. 
       FIG. 7  is a diagram illustrating some aspects of a multiphase pumping station, according to some other embodiments. In this simplified diagram, similar in many ways to  FIG. 2 , supra, pumping station  120  is shown with multiphase pump  130 , upstream mixer  210  and downstream flow splitter  220 . However, using the additional flow path and valve  700  (V 2 *) a similar functionality can be accomplished for retaining liquid while expelling gas as was described with respect to  FIGS. 6A and 6B , supra. By closing valve  204  (V 2 ) and having a second outlet pipe and isolation valve  700  (V 2 *) from upper outlet  228  of the flow splitter  220 , a gas-rich exit flow path is created that bypasses valve  204  (V 2 ). The gas from upstream the station will in this case be produced through the bypass valve  700  (V 2 *) until the wells are started. Valve  204  (V 2 ) is then opened and valve  700  (V 2 *) closed. 
     While the subject disclosure is described through the above embodiments, it will be understood by those of ordinary skill in the art that modification to and variation of the illustrated embodiments may be made without departing from the inventive concepts herein disclosed. Moreover, while some embodiments are described in connection with various illustrative structures, one skilled in the art will recognize that the system may be embodied using a variety of specific structures. Accordingly, the subject disclosure should not be viewed as limited except by the scope and spirit of the appended claims.