Electrostatic technology system and process to dehydrate crude oil in a crude oil storage tank of a floating production storage and offloading installation

Abstract

A process train for a floating production storage and offloading installation includes a crude oil storage tank equipped with at least one set of electrostatic internals. The set of electrostatic internals are arranged to provide a treatment flow path within the crude oil storage tank oblique to a longitudinal centerline of the crude oil storage tank and through an electric field provided by the set of electrostatic internals. Employing these electrostatic internals within the tank permits an allowable inlet water content into the tank of up to 80%, significantly reducing the required topside processing equipment. The process and system also includes, upstream of the tank, two separator vessels arranged in parallel so each receives a portion of an incoming oil-and-water stream, a flash vessel arranged downstream of the two separator vessels, and a degasser vessel. Downstream of the crude oil storage tank is an electrostatic treater.

Claims

1. A method for dehydrating crude oil, the method comprising: routing a produced stream containing oil and water to at least one cargo tank housing electrostatic internals comprising a plurality of legs; routing the produced stream along a treatment path within the at least one cargo tank, wherein the treatment path is oriented obliquely to a longitudinal centerline of the at least one cargo tank; and dehydrating the produced stream within the at least one cargo tank utilizing the electrostatic internals, wherein dehydrating comprises at least separating the produced stream into an oil dominant flow and an opposite water dominant flow along the treatment path defined by the plurality of legs.

2. A method according to claim 1, wherein the electrostatic internals define an electrostatic section.

3. A method according to claim 1, further comprising: routing an outlet stream of the at least one cargo tank to an electrostatic treater.

4. A method for dehydrating crude oil, the method comprising: routing a produced stream containing oil and water to at least one storage tank housing electrostatic internals comprising a plurality of legs within an electrostatic section of the at least one storage tank; dehydrating the produced stream by subjecting the produced stream to an electric field within the electrostatic section, and separating the produced stream into an oil dominant flow and an opposite water dominant flow along a treatment path defined by the plurality of legs within the at least one storage tank.

5. A method according to claim 4, wherein at least one of the legs is oriented obliquely to a longitudinal centerline of the at least one storage tank.

6. A method according to claim 4, further comprising: routing an outlet stream of the at least one storage tank to an electrostatic treater.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a block flow diagram of a prior art topside process train.

(2) FIG. 2 is a block flow diagram of a preferred embodiment of a topside process train that practices the system and method of this invention.

(3) FIG. 3 is a view of the crude oil storage tank of FIG. 2 equipped with electrostatic internals.

ELEMENTS AND NUMBERING USED IN THE DRAWINGS AND DETAILED DESCRIPTION

(4) 10 Process train 15 Produced oil-and-water stream 20 High pressure (HP) separator/degasser vessel 25 Outlet stream from 20, inlet stream to 30 30 Intermediate pressure (IP) flash vessel 35 Outlet stream from 30, inlet stream to 40 40 Low pressure (LP) degasser vessel 45 Outlet stream from 40, inlet stream to 50 50 Crude oil storage tank (cargo tank) 51 Inlet section or inlet (produced water) volume of 50 52 Oil-and-water mixture inlet 53 Partition isolating 51 from 67, 70 55 Oil outlet section or oil storage volume of 50 57 Baffle 59 Oil outlet 60 Electrostatic internals 61 Longitudinal centerline of 50 65 Outlet stream from 50, inlet stream to 100 67 Water outlet section or water storage volume of 50 69 Water outlet 70 Electrostatic separator section or volume of 50 71 Electrostatic internals (high water cut separators) 72 Elongated cylindrical pipe, vessel or leg 73 Inlet 74 Closed top end 75 Upflow end or portion of 72 76 Open bottom end 79 Oil outlet 80 Longitudinal center line (treatment flow path) 81 Oil-dominant flow or stream 85 Downflow end or portion of 72 86 Water outlet (open end) 91 Water-dominant flow or stream 100 Electrostatic treater 105 Outlet stream

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(5) A preferred embodiment of a crude oil storage tank includes an electrostatic separator section having at least one set of electrostatic internals arranged to provide a treatment flow path oblique to a longitudinal centerline of the crude oil storage tank and isolated from a surrounding volume of the electrostatic separator section. The set of electrostatic internals is arranged for communication with an oil-and-water inlet section of the crude oil storage tank, a water outlet section of the crude oil storage tank, and an oil outlet section of the crude oil storage tank. An oil-and-water stream or mixture entering the set of electrostatic internals travels along the treatment flow path and is subjected to an electric field. The treatment flow path is in an upwardly direction toward the oil outlet section and in a downwardly opposite direction toward the water outlet section of the tank. Dehydrated oil exits the electrostatic internals and can reside outside of the electrostatic internals in the electrostatic separator section and in the oil outlet section. In either case, the oil floats above the water residing in the water outlet section or volume of the tank.

(6) The set of electrostatic internals can include a first leg and a second leg, the legs being cylindrical in shape and elongated. The legs may be arranged parallel to one another with the second leg e located downstream of the first leg and a bottom end of at least one of the legs lying closer to the oil-and-water inlet section of the crude oil storage tank than does a top end of the leg. The size or length of the legs is determined based upon tank size and the requirements of a particular application.

(7) The first leg has a closed top end and an open bottom end, with the open bottom end located in the water outlet section of the crude oil storage tank. The inlet to the first leg is connected to an outlet of the oil-and-water inlet section and is located toward the bottom end of the leg. The second leg has an open bottom end, again located in the water outlet section and, if the second leg is the last leg in the set, an open top end exposed to the surrounding volume of the electrostatic separator section. An outlet of the first leg is an inlet to the second leg, with the inlet being located along a downflow portion of the second leg.

(8) The crude oil tank includes a partition that isolates the oil-and-water inlet section from the electrostatic separator section. Baffles can be placed where appropriate. In a preferred embodiment, a baffle is located between the electrostatic separator section and the oil outlet section and there is a baffle located in the water outlet section.

(9) For the purpose of comparison to the preferred embodiments, the prior art topside process train of FIG. 1 is used. This prior art train includes a high pressure separator, an intermediate pressure flash vessel (mounted on the pre-treater), a pre-treater, a low pressure degasser vessel (mounted on the electrostatic treater), and an electrostatic treater (see Table 1). The train also includes a crude oil storage tank located downstream of the electrostatic treater.

(10) TABLE-US-00001 TABLE 1 Prior art process train. HP Separator Operating Pressure: 2,000 kPa (a) Operating Temperature: 40 C. Slug volume: 20 m.sup.3 Maximum Water Content in Crude Outlet: 25% vol Size: 4.4 m ID 16.0 m T/T Horizontal IP Flash Vessel (mounted on Pre-treater) Operating Pressure: 770 kPa (a) Operating Temperature: 90 C. Liquid Hold Up: 2 Minutes Size: 3.3 m ID 10.0 m T/T Hori- zontal mounted on Pre-treater Pre-Treater Operating Pressure: 770 kPa (a) Operating Temperature: 90 C. Maximum Water Content in Crude Outlet: 1% vol Size: 3.3 m ID 10.0 m T/T Horizontal LP Degasser (mounted on Electrostatic Treater) Operating Pressure: 250 kPa (a) Operating Temperature: 75-80 C. Liquid Hold Up: 2 Minutes Size: 3.3 m ID 10.0 m T/T Hori- zontal mounted on Pre-treater Electrostatic Treater Operating Pressure: 250 kPa (a) Operating Temperature: 75-80 C. Maximum Water Content in Crude Outlet: 0.5% vol Maximum Salt content in Crude Outlet: 285 mg/l

(11) The train is designed to achieve a maximum total liquids flow rate of 24,000 m.sup.3/day (150,000 bpd), a maximum oil flow rate of 24,000 m.sup.3/day (150,000 bpd), a maximum produced water flow rate of 19,100 m.sup.3/day (120,000 bpd), and maximum gas flow rate of 6,000,000 m.sup.3/day. The incoming crude oil properties are 27 API; viscosity 47.1, 30.3 and 19.9 cSt at 30, 40, and 50 C. respectively; initial paraffin deposit temperature of 40 C. first event and 20 C. second event; pour point of 12 C.; and severe foaming. The oil temperature on arrival to the train is in a range of 20 C. (maximum oil cases) to 40 C. (maximum water cases).

(12) The size and weight requirements of the train are shown in Table 2. The train, not including the crude oil storage tank, requires a plot area of 275 m.sup.2, with a dry weight of 353.4 tonnes. Note the amount of water-in-oil decreases at each successive processing step, with 25% water content in the outlet stream of the high pressure separator to a 1% water content in the inlet streams to the pre-treater and low pressure degasser vessel.

(13) TABLE-US-00002 TABLE 2 Size and weight requirements of prior art process train. Equipment Technology & Size Plot Area Weights Item Performance Residence Requirements (ID T/T) (m.sup.2) (Tonnes) HP Separator Existing 2/3-Phase 2 Phase: 4.4 m 16 m 95 117.5 Dry Separator 2 Mins Liquids @ 1255 Horizontal 302.5 Op. Inlet: m.sup.3/hr = 41.83 m.sup.3 20 - 80% Water, 3 Phase: 80 - 20% Oil 5 minutes Oil @ 1061 Outlet: m.sup.3/hr = 88.4 m.sup.3 25% Water-in-Oil 5 minutes Water @ 794 m.sup.3/hr = 66.2 m.sup.3 20 m.sup.3 Surge both 2 & 3 phase IP Flash Existing Design Inlet: 2 minutes @ 1346 3.3 m 10 m Inc. 39.4 Dry, Vessel 20.8 - 28% Water-in-Oil m.sup.3/hr = 44.8 m3 Horizontal Below 92.4 Op Pre-Treater Existing Electrostatic As Required for Spec 4.2 m 16 m 90 83.8 Dry, Technology Outlet: Horizontal 286.8 Op 1% Water-in-Oil LP Degasser Existing Design Inlet: 2 minutes @ 1159 3.2 m 10 m Inc. 30.5 Dry, 1% Water-in-Oil m.sup.3/hr = 38.6 m3 Horizontal Below 77.5 Op. Electrostatic Existing Electrostatic As Required for Spec 4.2 m 16 m 90 82.2 Dry Treater Technology Desalter + Horizontal 288.2 Op. 100 m.sup.3/hr Wash Water Outlet: 0.5% BS&W 285 mg/l Salt Total 275-.sup. 353.4 Dry 1047.4 Op

(14) Referring now to FIG. 2, a preferred embodiment of a topside process train 10 includes a first and second high pressure separator/degasser vessel vessels 20A & B arranged to receive an incoming oil-and-water mixture 15, an intermediate pressure flash vessel 30 arranged downstream of the vessels 20A & B, a low pressure degasser vessel 40 arranged downstream of the flash vessel 30, a crude oil storage tank (cargo tank) 50 arranged downstream of the degasser vessel 40 and equipped with electrostatic internals 60, and, finally, an electrostatic treater 100 arranged downstream of the crude oil storage tank 50. Compared to the prior art train of FIG. 1, the process train of FIG. 2 has plot area of 75 m.sup.2, thereby reducing space requirements by 198 m.sup.2 (a 72% reduction), and a dry weight of 83.3 tonnes, thereby reducing the total weight by 270.1 tonnes (a 76% reduction, with operating weight reduced by 82%).

(15) TABLE-US-00003 TABLE 3 Preferred embodiment of the process train. Equipment Technology & Residence Size Plot Area Plot Area Weights Weight Item Performance Rqmts. (ID T/T) (m.sup.2) Reduction (Tonnes) Reduction HP CONSEPT 30 Two (2) 20 75 46.0 Dry 71.5 Dry Separator Compact Design Seconds 2.5 m 10.0 m 56.0 Op. 246.5 Op 20A & B Inlet: liquid Vertical Two (2) 20-80% Water, 80- retention 50% 20% Oil with cyclonic degassing IP Flash CONSEPT Cyclonic 1.6 m 5.3 m 6 +6 1.4 Dry, 38.0 Dry, Vessel 30 Compact Design degassing Vertical 3.4 Op. 89.0 Op Inlet: with no 20-80% Water, 80- slug 20% Oil volume rqmt. LP Degasser CONSEPT Cyclonic 1.6 m 5.3 m 6 +6 1.4 Dry, 29.1 Dry, Vessel 40 Compact Design degassing Vertical 3.4 Op. 74.1 Op. Inlet: with no 20-80% Water, 80- slug 20% Oil volume rqmt. Pre-Treater n/a n/a n/a n/a 90 n/a 83.8 Dry, 286.8 Op Cargo Tank Electrostatic Existing Existing Existing n/a n/a n/a 50 internals 60 Inlet: 20-80% Water, 80- 20% Oil Outlet: 3-10% Water in Oil Electrostatic Hi-flux DUAL As 4.2 m 6.1 m 45 45 34.5 Dry 47.7 Dry Treater 100 FREQUENCY Required Horizontal 123.3 Op. 164.9 Op. electrostatic desalter + for Spec 100 m.sup.3/hr Wash Water Outlet: 0.5% BS&W 285 mg/l Salt Total 77 198 83.3 Dry 270.1 Dry 186.1 Op 860.3 Op

(16) The prior art train requires the use of horizontal separator, flash and treater vessels because the amount of water entering the flash vessel cannot exceed 30% and the amount entering the electrostatic treater cannot exceed 10%. Therefore, produced streams having high water content require an appropriate amount of processing equipment volume to handle the water content so that the flash vessel does not see water volumes of 30% or more. Process train 10, on the other hand, can pass larger volumes of water from one piece of equipment to the other ahead of the crude oil storage tank 50 because the electrostatic internals 60 of the tank 50 can handle water volumes up to 80% and reduce those volumes to 3 to 10% prior to the electrostatic treater 100.

(17) The separator/degasser vessel vessels 20 are each vertical vessels arranged parallel to one another so that each receives a 50% split of an incoming produced oil-and-water stream 15. Typically, the percentage of water in the stream 15 can be in a range of 20% to 80% by volume, with the percentage of oil by volume varying accordingly. In some cases, the water is 95% by volume. Each vessel 20A & B is a high pressure separator suitable for an operating pressure of about 2,000 kPa (atm) at an operating temperature of 40 C. Preferably, each vessel 20A & B is equipped with an inlet having a cyclonic distributor.

(18) By way of example, a CONSEPT inlet cyclonic distributor (Cameron Solutions, Inc., Houston, Tex.) is a suitable (and preferred) inlet device. This device includes an inlet section, a dissipating volume, and a number of gas-liquid cyclones. After the inlet flange, the inlet velocity is retarded in a nozzle housing guiding vanes and then dissipated into the distribution chamber.

(19) The outlet stream 25 A & B from each vessel 20 A & B respectively, is combined and routed to the flash vessel 30. Flash vessel 30 can be an intermediate pressure flash vessel suitable for an operating pressure of 770 kPa (atm) at an operating temperature of 90 C. The flash vessel 30 is preferably equipped with an inlet having a cyclonic distributor. The CONSEPT inlet cyclonic distributor is a suitable (and preferred) inlet device.

(20) The outlet stream 35 from the flash vessel 30 is routed to the degasser vessel 40. Degasser vessel 40 is a vertical degasser vessel suitable for an operating pressure of 250 kPa (atm) at an operating temperature of 75 to 80 C. Similar to the vessels 20 and 30, the degasser vessel 40 is preferably equipped with an inlet having a cyclonic distributor such as the CONSEPT inlet cyclonic distributor.

(21) The outlet stream 45 from the degasser vessel 40 is routed to the cargo tank 50 equipped with electrostatic internals (see e.g. U.S. Pat. No. 8,591,714 B2 and U.S. Pat. No. 9,095,790 B2 to Sams et al., the subject matter of which is incorporated by reference herein, provide examples of electrostatic internals that can be modified for use in tank 50). Employing electrostatic internals within the crude oil storage tank 50 permits the allowable inlet water content of stream 45 to increase above 30% and up to 80%, significantly reducing the required topside processing equipment.

(22) Referring now to FIG. 3, in a preferred embodiment tank 50 includes an inlet section 51 (which receives the outlet stream 45 from the degasser vessel 40), an electrostatic separator section 70, a water outlet or storage section 67, and an oil outlet or storage section 55. The exact volume V of each section 51, 55, 67, 70 is determined by the requirements of a particular application. A partition 53 separates the electrostatic separator section 70 from the inlet section 51. A baffle (or sets of baffles) 57 lie between the separator section 70 and the oil outlet section 55 and divides the water outlet section 67.

(23) Within the electrostatic separator section 70 is one or more sets of electrostatic internals 71 with two or more legs 72 oriented vertically, horizontally, or at an incline. Each leg 72, which can be a cylindrical pipe, defines an elongated treatment path 80 for the incoming oil-and-water stream 45, separating the stream 45 into an oil dominant flow 81 and a water dominant flow 91. (The elongated treatment path 80 is represented by the longitudinal centerline of the leg 72.) A vertical orientation has logistical advantages, a horizontal orientation has fluid flow advantages, and an inclined orientation blends the advantages of the two. Preferably, the treatment path 80 is oblique to a longitudinal centerline 61 of the tank 50.

(24) In a preferred embodiment, the set of electrostatic internals 71 includes at least two legs 72, a first leg 72A and a second leg 72B located downstream of the first, arranged parallel to one another and at an incline. Each leg has an upflow end or portion 75 and a downflow end or portion 85. The downflow end 85 of both legs 72A & B is open at its bottom 86 and passes a water dominant flow 91 to the water outlet section 67. However, upflow end 75 of the first leg 72A is a closed end 74 whereas the upflow end 75 of the second leg 72B is an open end 76. The upper ends 76 of the legs 72A & B lie closer to the oil outlet section 55 of the tank 50 than does the bottom ends 86.

(25) An electrode (not shown) may be placed in the inlet 73, the upflow end 75 of the leg 72, or in both the inlet 73 and upflow end 75 (see e.g. U.S. Pat. No. 8,591,714 B2 and U.S. Pat. No. 9,095,790 B2). The electrode, which is connected to a voltage source (not shown) establishes a treatment field within the leg 72. As the incoming stream 45 is exposed to the electric field, the largest water droplets coalesce and flow downward and the drier oil flows upward.

(26) The oil-and-water stream 45 entering the inlet section 51 is distributed to an inlet 73 of the first leg 72A and initial separation occurs as the stream 45 is subjected to the treatment field, with an oil dominant flow 81 flowing upwardly and a water dominant flow 91 flowing downwardly. Preferably, inlet 73 is located toward the open bottom end 86 of the first leg 72A. The water dominant flow 91 exits the open bottom end 86 of the leg 72A and the oil dominant flow 81 exits an outlet 79 of the first leg 72A, which serves as an inlet 73 to the second leg 72B. Preferably, the inlet 73 to the second leg 72 is located along the downflow end 85.

(27) The stream is once again subjected to a treatment field within the second leg 72B. Another water dominant flow 91 exits the open bottom end 86 and a now-drier oil dominant flow 81 exits the open end 76 of the leg 72B. The oil dominant flow 81 then enters the oil outlet section 51 of the tank where it is removed as outlet stream 65. Stream 65 contains about 3 to 10% water-in-oil.

(28) In this way, the tank 50 serves as the separation vessel with its electrostatic internals 71 performing the fluid separation.

(29) The outlet stream 65 from the crude oil storage tank 50 is routed to the electrostatic treater 100. The electrostatic treater 20 is of a type well known in the art and provides an outlet stream 105 suitable for downstream processes or uses. Preferably, the BS&W content of stream 105 is no greater than 0.5% BS&W and 285 mg/l salt. Alternatively, employing DUAL FREQUENCY technology (Cameron Solutions, Inc., Houston, Tex.) within the treater 100 is viable for water cuts approaching 50%. The DUAL FREQUENCY technology is described in U.S. Pat. No. 6,860,979 B2 and U.S. Pat. No. 7,351,320 B2 to Sams, the contents of which are herein incorporated by reference. This technology uses an AC voltage to establish an electric field within the electrostatic treater at a selected frequency F1 and modules an intensity of the AC voltage at a selected frequency F2, F1>F2. The higher (high) base frequency F1 provides strong electrostatic forces. The lower (or low) frequency F2 amplitude modulation of the electrostatic field provides improved conditions for water droplet coalescence, which further improves the dehydration and desalting efficiency.

(30) The preferred embodiments provide examples of the system and method. The following claims define the scope of the invention, including the full range of equivalency to the recited elements.