Abstract:
A well fluid processing system has a separator for separating heavier and lighter components of well fluid flowing from a subsea well and directing the lighter components to flow to a surface processing facility. The separator has a cylindrical chamber having a length at least ten times its diameter. A coalescing unit located in the chamber causes water droplets in the well fluid flowing through the tubes to coalesce into larger droplets. A dielectrophoresis unit having undulating sheets spaced close to each other is also located in the chamber. The sheets of the unit are supplied with an electrical potential to force the water droplets into predetermined passage portions to form high water content sections of liquid. Bypass valves allow backflushing of one of the separators while others continue to operate.

Description:
[0001]     This application is a continuation of application Ser. No. 10/360,387, filed Feb. 7, 2003, which claimed the benefit of provisional application Ser. No. 60/356,108, filed Feb. 11, 2002 and provisional application Ser. No. 60/425,377, filed Nov. 12, 2002. 
     
    
     FIELD OF THE INVENTION  
       [0002]     This invention relates in general to well fluid processing systems and in particular to a subsea system.  
       BACKGROUND OF THE INVENTION  
       [0003]     Oil and gas wells typically produce a well fluid that requires separation to remove formation water from the flow stream. With subsea wells, the separation typically takes place on a production platform or vessel. This usually requires pumping the well fluid, including the formation water, to the surface production facility. In deep water installations, thousands of feet deep, the energy required to pump the water is extensive.  
         [0004]     Locating the separation unit subsea has been proposed and done on at least one occasion. The environment of a subsea separation unit and a surface unit differs because of the high hydrostatic forces imposed on the separation vessels. While vessels can be made stronger, generally this results in a larger size and weight. Larger size and weight increase the difficulty of deploying the units.  
         [0005]     Also, separators commonly require maintenance because of sand accumulation and mineral deposits on the components. Once installed subsea, maintenance becomes difficult because of the sea depths. Further, shutting down a separation system for maintenance would normally require shutting off well flow, which is expensive.  
       SUMMARY OF THE INVENTION  
       [0006]     In this invention, a choke located downstream of the separator for limiting the flow rate of well fluid from the subsea well. The placement of the choke allows higher operating pressures in the separator, which facilitates separation. The separator has a cylindrical chamber, preferably with a length at least ten times its diameter. A coalescing unit having a plurality of tubes to which an electrical potential is applied is located in the chamber to cause water droplets in the well fluid flowing through the tubes to coalesce into larger droplets. Also, preferably a dielectrophoresis unit is located in the chamber downstream of the coalescent unit. The dielectrophoresis unit has a pair of undulating sheets spaced close to each other, the sheets being supplied with an electrical potential to force the water droplets in the well fluid into predetermined passage portions between the sheets to form high water content sections of liquid.  
         [0007]     In the preferred embodiment, the system has a number of separators for separating heavier and lighter components of well fluid, each of the separators having a heavier component outlet and a lighter component outlet. A pump has an intake connected to each of the heavier component outlets of the separators. A disposal line is connected to an outlet of the pump and leads to a disposal location for pumping the heavier components to the disposal location. Bypass conduits are connected between the outlet of the pump and the heavier component outlets of the separators. When actuated, bypass valves in the bypass conduits cause at least some of the heavier components being pumped by the pump to flow back into the heavier component outlet of the one of the separators for backflushing while the heavier components from the other separators continue to flow through the second heavier component line to the inlet of the pump.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]      FIG. 1  is a schematic of a production system for a wellhead assembly.  
         [0009]      FIG. 2  is a schematic sectional view of one of the separators shown in  FIG. 1 .  
         [0010]      FIG. 3  is an enlarged schematic sectional view of the separator of  FIG. 2 , taken along the line  3 - 3  of  FIG. 2 , illustrating the coalescence separator portion.  
         [0011]      FIG. 4  is an enlarged schematic view of a dielectrophoresis separator portion of the separator of  FIG. 2 .  
         [0012]      FIG. 5  is an enlarged schematic sectional view of the separator of  FIG. 2 , taken along the line  5 - 5  of  FIG. 2 , illustrating the dielectrophoresis separator portion.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0013]      FIG. 1  illustrates schematically a subsea processing system for the various wells  11  within a field. The subsea processing systems separates water and sand. The system includes a plurality of separators  251 . A single separator  251  may be utilized with each subsea well assembly  11 , or more than one well  11  may feed into a separator  251 . Separator  251 , as shown in  FIG. 2 , comprises a horizontal vessel  253  that locates on the sea floor. Generally, greater water depths will require a higher wellhead pressure with corresponding lower actual gas volumes when separation takes place at the sea floor. Lower gas volumes are beneficial for oil/water separation because fewer gas bubbles will move vertically and disturb the horizontal flow pattern generated by the oil and water flowing through the separator vessel  253 . The low gas percentage also allows more of the separator vessel to be utilized for oil/water separation.  
         [0014]     In addition to the issue described above, higher pressure in itself within separator vessel  253  will impact the separation. Preliminary results show that separation occurs easier at higher pressures. This can be caused by the fact that high pressure causes the liquid hydrocarbon fraction to be lighter, hence increase the density difference between water and oil. The oil fraction becomes lighter because lighter hydrocarbon fractions are liquefied at the higher pressure, hence if combined with the heavier fractions, the combination can reduce the overall density of the liquid hydrocarbon phase. Separator vessel  253  is designed to withstand the high external pressure due to the very deep water. Also, conservative design does not allow one to reduce the designed pressure differential due to internal pressure. Generally, smaller diameters will give thinner wall thickness for the same external pressure. For example, a 2.8 meter diameter cylinder requires 140 millimeters wall thickness to withstand a selected pressure. A 2.5 meter diameter cylinder will withstand the same pressure with a wall thickness of 25 millimeters. Consequently, separator  253  has a relatively small diameter, preferably no more than 1/10 th  its length.  
         [0015]     Separator  251  may be of various types for separating water and oil. In this embodiment, separator  253  employs a coalescent unit  259 . Coalescent unit  259  has a plurality of passages  261  within it.  FIG. 3  shows the large number of separate passages  261  located within vessel tubes  261 . An electrostatic field is applied to the oil and water mixture at the tubes or passages  261 . By exposing the mixture of water and oil to an electrostatic field, the dipolar water droplets contained in the oil phase will be oriented in a way that makes them collide or coalesce with each other. This causes the water droplets to grow to bigger droplets. Generally, bigger droplets move and separate faster than smaller droplets. Consequently, a first separation from water and oil takes place in coalescent unit  259 . This reduces the required retention time to get the water content out of the oil produced, allowing the separator vessel  253  diameter/size to be reduced.  
         [0016]     As shown in  FIG. 3 , preferably low voltage supplied subsea is routed through low voltage wires  263  into the interior of separator vessel  253 . A plurality of transformers  265  transform the low voltage to high voltage that is required for providing the electrostatic field. The same low voltage power supply is utilized for other functions, such as operating the solenoids and sensors involved with control of each subsea well  11 .  
         [0017]     If coalescent unit  259  is not adequate to reach the desired water content, a second stage could be employed. A second stage could be another coalescent unit  259  or it could be a unit of a different type, such as dielectrophoresis unit  267 . Unit  267  also uses an electrostatic field, however the field is configured to force the water droplets into designated sections of the separator and thereby form streams of water. Electrode sheets  269 , as shown in  FIGS. 4 and 5 , have undulations. Electrode sheets  269  are closely spaced and arranged with the constrictive portions where two valleys are separated by the widened portions where two peaks are spaced across from each other. Sheets  269  force the water droplets to move towards the stronger section of the electrostatic field with stronger field gradients. The forces imposed by the gradient field are in the order of magnitude two to five times greater than the gravity force. This phenomenon is used to guide the water droplets into these predetermined sections, where they form continuous sections of water for use in separation. Dielectrophoresis unit  267  reduces the time normally needed for a conventional gravity separator.  
         [0018]     Referring again to  FIG. 2 , a bulkhead  271  extends upward from separator vessel  253  near its downstream end. Bulkhead  271  divides a section for collecting higher water concentrations. A water outlet  273  is located upstream of bulkhead  271 . Oil and water inlet  255  is located on an upper side of the upstream end of separator vessel  253 . Oil outlet  257  is located on the downstream end of separator vessel  253  on the lower side.  
         [0019]     Referring back to  FIG. 1 , a choke  275  is located downstream of oil outlet  257 . Choke  275  is a conventional device that provides a variable orifice for increasing pressure upstream and decreasing flow. One of the chokes  275  is typically located on the tree of each of the subsea wells  11 . Choke  275  is adjusted to create a higher pressure within separator  251  to enhance separation, as previously mentioned.  
         [0020]     A flowline jumper  277  connects choke  275  to manifold  279 . Choke  275  could be incorporated as part of flowline jumper  277  such that it is lowered and installed with jumper  277 . Alternately, choke  275  could be mounted to manifold  279 .  
         [0021]     Manifold  279  is a conventional unit that has a pair of lines  281  and  283  that lead to the surface for delivery of the separated oil and any entrained gas therein. All of the various separators  251  lead to manifold  279 .  
         [0022]     The separated water outlet  273  connects to a flowline  284 , which leads to a valving module  285 . The various flowlines  284  join each other in module  285 , with the combined flow leading to an intake line  286  of a subsea pump  287 . Flowlines  281  and  283  lead back to a surface processing unit for transporting the oil. Water pump  287  discharges through a line  288  into a vortex separator  289 . Vortex separator  289  has an output  291  that leads back to an injection well for injecting the separated water. The output is a mixture of water and in many cases of sand that has been produced from the formation. The higher content of sand flows through output line  297 . The free water  293  flows back to a second separator  295  that leads to flowline  291  for injecting into a well. The second vortex separator  295  separates any remaining oil from the water and delivers the oil through line  296  back to manifold  279  for commingling with the other oil being produced through lines  281  and  283 . Similarly, any oil washed from the sand and sand collection vessel  292  is filtered and returned via line  298  to manifold line  283 . Vortex separator  289  thus separates sand from liquid, while vortex separator  295  separates any remaining oil from the water.  
         [0023]     A valve  301  is connected to a line  303  that leads from the output of pump  287 . Line  303  branches into separate lines, each connected to one of the lines  284  leading from one of the separators  251 . Each line has a valve  305 . Opening valves  301  and  305  enables water to flow backwards through one of the water outlet lines  284  into the water outlet  273  for backflushing. Sand and other deposits accumulate in the subsea separation vessel  253 . These sands and/or deposits are removed from separator  251  by the backflushing injection through line  284 . The injection of water creates turbulence within separator vessel  253  to cause the sand and other deposits to flow out with the produced oil out of manifold lines  281  and  283 .  
         [0024]     The invention has significant advantages. Locating the choke downstream of the separator allows higher operating pressures in the separator. The combination of a coalescence unit and a dielectrophoresis unit within a small diameter separators provides a compact subsea processing unit. The backflushing capability reduces maintainence.  
         [0025]     While the invention has been shown in only one of its forms, it should be apparent to those skilled in the art that it is not so limited but is susceptible to various changes without departing from the scope of the invention.