GB2417438A

GB2417438A - A subsea well fluid processing system

Info

Publication number: GB2417438A
Application number: GB0519343A
Authority: GB
Inventors: Jarle Michaelsen; Paal J Nilsen
Original assignee: Vetco Aibel AS
Current assignee: Aibel AS
Priority date: 2002-02-11
Filing date: 2003-02-07
Publication date: 2006-03-01
Anticipated expiration: 2023-02-07
Also published as: GB0519343D0; GB2417438B

Abstract

A subsea well fluid processing system comprises: first and second 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 lighter component line connected to the lighter component outlet of each of the separators for delivering the lighter components to a surface processing facility; first and second heavier component lines connected to the heavier component outlets of the first and second separators, respectively; a pump having an intake connected to each of the heavier component outlets; a disposal line connected to an outlet of the pump and leading to a disposal location for pumping the heavier components to the disposal location; first and second bypass conduits connected between the outlet of the pump and to the first and second heavier component outlets of the separator, selectively; and first and second bypass valves in the first and second bypass conduits, respectively, such that opening the first bypass valve and closing the second bypass valve causes at least some of the heavier components being pumped by the pump to flow back into the heavier component outlet of the first separator for backflushing while the heavier components from the second separator continue to flow through the second heavier component line to the inlet of the pump.

Description

24 1 7438

SUBSEA PRODUCTION SYSTEM

FIELD OF THE INVENTION

This invention relates in general to well fluid processing systems and in particular to a subsea system.

BACKGROUN1) OF TlIE INVENTION 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.

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.

Also, separators commonly require maintenance because of sand accumulation and mineral deposits on the components. Once installed subsea, maintenance becomes difficult because of sea depths. Further, shutting down a separation system for maintenance would normally require shutting off well flow, which is expensive.

SUMMARY OF THE INVF,NTION

In accordance with a first aspect of the present invention, there is provided a subsea well fluid processing system, comprising: first and second 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 lighter component line connected to the lighter component outlet of each of the separators for delivering the lighter components to a surface processing facility; first and second heavier component lines connected to the heavier component outlets of the first and second separators, respectively; a pump having an intake connected to each of the heavier component outlets; a disposal line connected to an outlet of the pump and leading to a disposal location for pumping the heavier components to the disposal location; first and second bypass conduits connected between the outlet of the pump and to the first and second heavier component outlets of the separator, selectively; and first and second bypass valves in the first and second bypass conduits, respectively, such that opening the first bypass valve and closing the second bypass valve causes at least some of the heavier components being pumped by the pump to flow back into the heavier component outlet of the first separator for backflushing while the heavier components from ] O the second separator continue to flow through the second heavier component line to the inlet of the pump.

The system may further comprise a choke located downstream of each of the separators for limiting the flow rate through the separators.

Preferably, each of the separators comprises: a cylindrical chamber; a coalescing unit in the chamber, having a plurality of tubes to which an electrical potential is applied to cause water droplets in the well fluid flowing through the tubes to coalesce into larger droplets; and a dielectrophoresis unit in the chamber, having 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.

In this case, the dielectrophoresis unit may be located downstream from the coalescing unit within the separator.

In accordance with a second aspect of the present invention, there is provided a method of processing well Buid from first and second subsea wells, comprising: (a) with a first separator, separating heavier and lighter components of well fluid flowing from the first well; (b) with a second separator, separating heavier and lighter components of well fluid Rowing from the second well; (c) delivering the lighter components of each of the separators to a surface processing facility; (d) delivering the heavier components from a heavier component outlet of each of the separators to a pump; (e) with the pump, delivering the heavier components to a disposal location; and (f) selectively, causing at least a portion of the heavier components being pumped by the pump back to the heavier component outlet of the first separator to backflush the first separator while continuing receive heavier components from the heavier component outlet of the second separator.

Preferably, the method further comprises: ceasing step (f) and causing at least a portion of the heavier components being pumped by the pump back to the heavier. component outlet of the second separator to backflush the second separator while continuing to receive heavier components from the heavier component outlet of the first separator.

The method may further comprise: placing a choke downstream of each of the separators and limiting the nowrate through the separators with the chokes.

Steps (a) and (b) may comprise: applying an electrical potential to a plurality of tubes to cause water droplets in the well fluid flowing through the tubes to coalesce into larger droplets; and applying an electrical potential to a pair of undulating sheets spaced close to each other to force the water droplets in the well fluid into predetermined passage portions between the sheets to form high water content sections of liquid.

In this case, the tubes may be located upstream of the sheets.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure I is a schematic of a production system for a wellhead assembly.

Figure 2 is a schematic sectional view of one of the separators shown in Figure I. Figure 3 is an enlarged schematic sectional view of the separator ot Figure 2, taken along the line 31-31 of Figure 2, illustrating the coalescence separator portion.

Figure 4 is an enlarged schematic view of a dielectrophoresis separator portion of the separator of Figure 2.

Figure 5 is an enlarged schematic sectional view of the separator of Figure 2, taken along the line 33-33 of Figure 2, illustrating the dielectrophoresis separator portion.

DETAILED DESCRIPTION OF THE INVENTION

Figure l 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 Figure 2, comprises a horizontal vessel 253 that locates on the sea floor. Generally, greater water depths will require a higher Nvellhead pressure with corresponding lower actual gas Yolurnes when separation takes place at the sea floor. Lower gas Volumes are beneficial for oilater separation because feeler gas bubbles will move vertically and disturb the horizontal flow pattern generated by the oil and water flowing through the separator vessel 253. The lone gas percentage also allows more of the separator vessel to be utilized for oil/water separation.

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 cause by the fact that high pressure causes the liquid hydrocarbon fraction to be lighter, hence increase the density difference between updater 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 alloy 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 vistand a selected pressure. A 5 meter diameter cylinder will withstand the same pressure with a wall thickness of 25 millimeters. Consequenl:ly, separator 253 has a relatively small diameter, preferably no mole than 1/lOth its length.

Separator 251 may be of various types for separating water and oil. In this embodiment, separator 259 employers a coalescent unit 259. Coalescent unit 259 has a plurality of passages 261 within it. Figure 3 shows the large number of separate passages 261 located within vessel tubes 53. An electrostatic field is applied to the oil and water mixture at the tubes or passages 261. By exposing the mixture of avatar and oil to an electrostatic field, the dipolar water droplets contained in the oil phase Fill be oriented in a wall Slat 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 dianeter/size to be reduced.

As shown ill Figure 3, preferably loci voltage supplied subsea is routed through low voltage wires 263 into the interior of separator vessel 253. A plurality of tra,sfonnets 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 Keith control of each subsea well 11.

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 audit 267. Unit 267 also uses an electrostatic field, however the field is confi,ured to force the water droplets into designated sections of the separator arid thereby form streams of water.

Electrode sheets 269, as shown in Figures 4 and 5, have undulations. Electrode sheets 269 are closely spaced and arranged with the constrictive portions where taco 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 halo to five times greater than the gravity force.

This phenomenon is Vised to guide the Nvater droplets into these predetermined sections, where they form continuous sections of water for use in separation. s

Dielectrophoresis unit 267 reduces the time normally needed for a conventional gravity separator.

Refemog again to Figure 2, a bulkhead 271 extends upward front separator vessel 253 neat 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.

Refemng back to Figure 1, a choke 275 is located downstream of oil outlet 257. Choline 275 is a conventional device that provides a Yarialle orifice for increasing pressure tpstrearm and decreasing flow. One of the clamors 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.

flowline jumper 277 connects choke 275 to manifold 279. Clone 275 could be incorporated as pelt of Bowline jumper 277 such that it is lowered and installed with jumper 277. Alternately, choice 275 could be mounted to manifold 279.

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 As therein. All of the various separators 251 lead to manifold 279.

The separated water outlet 273 conriects to a flowline 284, which leads to a valving module 285. The various flovlines 2B4 join each other in module 285, with the combined flow leading to an intake line 286 of a subsea pump 287. Lines 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 \vater and in molly cases of sand that has been produced from the formation. Tile higher content of sand flows through output line 291. The free water 293 flows back to a second separator 295 that leads to flovline 291 for injecting into a well. The second vortex separator 295 separates any remairrg oil from the water and delivers the oil tlrough 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 resumed aria line 298 to manifold line 983. Vortex separator 989 thus separates sand fToin liquid, while vortex separator 295 separates any remaining oil from the water.

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 Grater to flow backboards through one of the water outlet lines 284 iiito the water outlet 273 for backflushing, Sand and other deposits accumulate in the subsea separation vessel 253, These sands and/or deposits are removed Tom separator 251 by the bacl;flushing injection through line 284. Tile injection of evader creates turbulence within separator vessel 253 to cause the sand and other deposits to flow out with the produced oil out manifold lines 281 and 283.

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 dielechophoresis unit within a small diameter separators provides a compact subsea processing unit. The backflusrig capability reduces maintainence.

.X,.ile the ir,vention has been shover' ill 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 front the scope of the invention.

Claims

Claims: 1. A subsea well fluid processing system, comprising: first and

second separators for separating heavier and lighter components of well fluid, each ofthe separators having a heavier component outlet and a lighter component outlet; a lighter component line connected to the lighter component outlet of each of the separators for delivering the lighter components to a surface processing facility; first and second heavier component lines connected to the heavier component outlets of the first and second separators, respectively; a pump having an intake connected to each of the heavier component outlets; a disposal line connected to an outlet of the pump and leading to a disposal location for pumping the heavier components to the disposal location; first and second bypass conduits connected between the outlet of the pump and to the first and second heavier component outlets of the separator, selectively; and first and second bypass valves h1 the first and second bypass conduits, respectively, such that opening the first bypass valve and closing the second bypass valve causes at least some of the heavier components being pumped by the pump to flow back into the heavier component outlet of the first separator for backRushirlg while the heavier components from the second separator continue to flow through the second heavier component line to the inlet of the pump.
2. The system according to claim I, further comprising a choke located downstream of each of the separators for limiting the flow rate through the separators.
3. The system according to either of claims I and 2 wherein each of the separators comprises: a cylindrical chamber; a coalescing unit in the chamber, having a plurality of tubes to which an electrical potential is applied to cause water droplets in the well fluid flowing through the tubes to coalesce into larger droplets; and a dielectrophoresis unit in the chamber, having 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.
4. The system according to claim 3, wherein the dielectrophoresis unit is located downstream from the coalescing unit within the separator.
5. A method of processing well fluid from first and second subsea wells, comprising: (a) with a first separator, separating heavier and lighter components of well fluid Rowing from the first well; (b) with a second separator, separating heavier and lighter components of well fluid flowing from the second well; (c) delivering the lighter components of each of the separators to a surface processing facility; (d) delivering the heavier components from a heavier component outlet of each of the separators to a pump; (e) with the pump, delivering the heavier components to a disposal location; and (f) selectively, causing at least a portion of the heavier components being pumped by the pump back to the heavier component outlet of the first separator to backflush the first separator while continuing receive heavier components from the heavier component outlet of the second separator.
6. The method according to claim 5, further comprising: ceasing step (0 and causing at least a portion of the heavier components being pumped by the pump back to the heavier. component outlet of the second separator to backflush the second separator while continuing to receive heavier components from the heavier component outlet of the first separator.
7. The method according to either of claims 5 and 6, further comprising: placing a choke downstream of each of the separators and limiting the flowrate through the separators with the chokes.
8. The method according to any of claims 5 to 7, wherein steps (a) and (b) comprise: applying an electrical potential to a plurality of tubes to cause water droplets in the well fluid flowing through the tubes to coalesce into larger droplets; and applying an electrical potential to a pair of undulating sheets spaced close to each other to force the water droplets in the well fluid into predetermined passage portions between the sheets to form high water content sections of liquid.
9. The method according to claim 8, wherein the tubes are located upstream of the sheets.