Subsea equipment cleaning system and method

Abstract

The invention concerns a subsea water processing system comprising an electrochemical unit that uses raw or treated seawater to generate high pH and low pH solutions that are used to clean at least one subsea process apparatus during a cleaning cycle by circulation through the at least one subsea process apparatus via acid or base flow lines connecting the electrochemical unit with the subsea process apparatus on-site.

Claims

1. A subsea water processing system comprising: at least one underwater membrane separation module of a water injection system arranged in a feed line from a seawater or oil field produced water intake to a water injection pump; an electrochemical acid and base generation unit operable for on-site generation of high PH and low PH cleaning solutions using seawater or oil field produced water in an electrochemical process; and acid or base flow lines and valves connecting the electrochemical acid and base generation unit on-site with the at least one underwater membrane separation module to supply high PH and low PH cleaning solutions for circulation through the at least one underwater membrane separation module during a membrane cleaning cycle.

2. The system of claim 1, wherein the acid or base flow lines connect the electrochemical unit with at least one subsea process apparatus on-site, the at least one subsea process apparatus selected from the group consisting of heat exchangers, components of a hydrocarbon production system, pipelines, injection or production wells, pumps for injecting acid into a reservoir formation for reservoir stimulation, and water injection systems.

3. The system of claim 2, wherein the electrochemical acid and base generation unit can supply low PH solution for injection into a corresponding reservoir formation for hydrocarbon production stimulation.

4. The system of claim 1, wherein the electrochemical acid and base generation unit is operable to produce a high pH solution having a pH higher than 9.5, and the electrochemical acid and base generation unit is operable to produce a low pH solution having a pH lower than 4.

5. The system of claim 1, wherein the electrochemical acid and base generation unit is an electrolysis unit without membranes.

6. The system of claim 1, wherein the electrochemical acid and base generation unit is an electrolysis unit with membranes.

7. The system of claim 1, wherein the electrochemical acid and base generation unit is a bipolar membrane electro-dialysis unit.

8. The system of claim 1, wherein the electrochemical acid and base generation unit is a bipolar membrane electro-dialysis unit with a 2-compartment design or a 3-compartment design.

9. The system of claim 1, further comprising a nano-filtration stage and/or an ion exchange unit operable to treat feed water to the electrochemical acid and base generation unit such that the feed water to the electrochemical acid and base generation unit has a combined Ca2+ and Mg2+ ions level below 10,000 mg/L.

10. The system of claim 1, further comprising a nano-filtration stage operable to treat feed water to the electrochemical acid and base generation unit with nanofiltration and/or an ion exchange unit operable to treat the feed water to the electrochemical acid and base generation unit with ion exchange, to substantially remove Ca2+ and Mg2+ ions.

11. The system of claim 1, wherein the electrochemical acid and base generation unit feeds, via a hydraulic circuit hydraulically connected to a boosting pump, seawater or oil field produced water through the at least one underwater membrane separation module in normal operation.

12. The system of claim 11, wherein the electrochemical acid and base generation unit is a bipolar membrane electro-dialysis unit.

13. The system of claim 11, wherein the electrochemical acid and base generation unit is via a hydraulic circuit connectable on-site with at least one of the at least one underwater membrane separation module including any of an underwater coarse filter membrane, a multiple media filter membrane, a microfiltration filter membrane, an ultrafiltration filter membrane, a nanofiltration filter membrane or a reverse osmosis filter membrane.

14. The system of claim 11, wherein the electrochemical acid and base generation unit is via a hydraulic circuit connectable on-site to at least one of the at least one underwater membrane separation module including any of a single-bore or multi-bore hollow fiber membrane, a plate-and-frame membrane, a tubular membrane, or a spiral wound membrane.

15. The system of claim 1, wherein the electrochemical acid and base generation unit feeds, via a hydraulic circuit hydraulically connected to a boosting pump, seawater or oil field produced water through the at least one underwater membrane separation module in normal operation.

16. The system of claim 15, wherein the electrochemical acid and base generation unit further feeds, via the hydraulic circuit hydraulically connected to the boosting pump, acid or base cleaning solution through the at least one underwater membrane separation module during a membrane cleaning cycle.

17. The system of claim 15, wherein the electrochemical acid and base generation unit further feeds, via the hydraulic circuit hydraulically connected to the boosting pump acid or base cleaning solution through the at least one underwater membrane separation module during a membrane cleaning cycle.

18. The system of claims 1, wherein the electrochemical acid and base generation unit is via a hydraulic circuit connected on-site to at least one of the at least one underwater membrane separation module including any of a single-bore or multi-bore hollow fiber membrane, a plate-and-frame membrane, a tubular membrane, or a spiral wound membrane.

19. The system of claim 1, further comprising a nano-filtration stage and/or an ion exchange unit operable to treat feed water to the electrochemical acid and base generation unit such that the feed water to the electrochemical acid and base generation unit has a combined Ca2+ and Mg2+ ions level below 200 mg/L.

20. The system of claim 1, further comprising a nano-filtration stage and/or an ion exchange unit operable to treat feed water to the electrochemical acid and base generation unit such that the feed water to the electrochemical acid and base generation unit has a combined Ca2+ and Mg2+ ions level below 10 mg/L.

Description

BREIF DESCRIPTION OF THE DRAWINGS

(1) Embodiments and details of the present invention will be further discussed below with reference to the accompanying schematic drawings wherein:

(2) FIGS. 1-13 illustrate different embodiments of the subsea water processing system,

(3) FIG. 14A is a diagram showing the pH levels of acid and base streams generated by a lab-scale bipolar membrane electro-dialysis unit (BPED),

(4) FIG. 14B is a diagram showing the concentration of generated NaOH solution,

(5) FIG. 14C is a diagram showing the current efficiency of the lab-scale acid and base generation system. All data in FIG. 14A-C were obtained with 25° C. and synthetic seawater as feed,

(6) FIG. 15A is a diagram showing the pH levels of generated acid and base streams by the lab-scale BPED unit,

(7) FIG. 15B is a diagram showing the concentration of generated NaOH solution,

(8) FIG. 15C is diagram showing the current efficiency of the lab-scale acid and base generation system. All data in FIG. 15A-C were obtained with 4° C. and synthetic seawater as feed, and

(9) FIG. 16 is a schematic illustration of a BPED cell used in the lab experiment.

DETAILED DESCRIPTION

(10) With reference to FIG. 1 an embodiment of the subsea water processing system in the form of a water injection system 1 briefly comprises a submerged water filtration station 2, a pump 3 feeding seawater or produced water through the filtration station from an inlet 4 to a water injection pump 5 by which treated water is injected into an injection well 6 in an oil and/or gas-holding subterranean formation 7. The water injection system 1 can be controlled from a topside control station 8 via an umbilical 9. A subsea control module 10 may be included in the control of the water injection system 1.

(11) The submerged water filtration station 2 may comprise underwater membrane separation modules of successively finer grades as seen in the feed direction of water through the system 1. The filter stages may include a coarse filtration module 11 and a fine filtration module 12.

(12) In this context separation of particulate matter and microorganisms from seawater typically involves filtration in several stages using different types of filters or membranes. The range of separation membranes applied in seawater treatment processes covers filters or membranes included in subsea coarse filter (CF) modules and multiple media filters (MMF) modules, as well as membranes used in microfiltration (MF) modules, ultrafiltration (UF) modules, nanofiltration (NF) and reverse osmosis (RO) modules. In membrane filtration, pressure is used to force water against a semipermeable membrane capable of separating out substances from the water, mainly through size exclusion or solution-diffusion. The stages of filtration are not principally different from each other except in terms of the size of the pores and the size of species (e.g. particles, ions) they retain. In general terms the pore size or species size removal capacity of ultrafiltration membranes range from 0.005 to 0.1 micron, whereas the nanofiltration membranes range from 0.001 to 0.01 micron and the reverse osmosis membranes are capable of excluding species sizes ranging down to 0.0001 micron.

(13) The filter membranes included in the underwater separation modules referred to in the present disclosure are not limited to the exact figures and ranges mentioned here, which are introduced as a general illustration of the different stages of filtration which can be applied in the seawater injection system 1.

(14) For example, the coarse filtration stage 11 may be realized as a strainer or as a multiple media filter, whereas the fine filtration stage 12 may be composed of a number of ultrafiltration modules 13 disposed as indicated in the drawing of FIG. 1. The fine filtration stage may be supplemented by nanofiltration and/or desalination provided from a nanofiltration module 14 and/or a reverse osmosis module 15, if appropriate. In the drawing, reference RS refers to reject streams discharged from the membrane separation modules 11-15.

(15) In the water injection system 1, an acid-and-base generation unit 16 is hydraulically connected to at least one of the underwater separation modules 11-15 and driven for on-site generation of acid and base cleaning solutions using ambient seawater or treated produced water as input. In the drawing the seawater or produced water input to the acid-and-base generation unit 16 (herein also referred to as the acid/base generation unit 16) is indicated by reference number 17. The acid/base generation unit 16 is connectable to the one or more membrane separation modules via acid and base solution flow lines 18 and 19 and valves 20 and 21 for optional supply of acid and base cleaning solution during a membrane cleaning cycle. Circulation of the acid and base cleaning solutions through the membrane(s) may be driven by boosting pump 3 or via a dedicated pump 22 which for this purpose is hydraulically connected to the acid/base generation unit 16 via a hydraulic circuit 23 and valves 24, 25 and 26. The hydraulic circuit 23 may be extended to other membrane separation modules in the water injection system as illustrated by broken lines in FIG. 1.

(16) It should be noted that FIG. 1 only schematically illustrates the layout of a water injection system configuring an on-site operated and hydraulically connected acid and base generation unit 16. The hydraulic circuit which supplies acid and base cleaning solutions to the underwater separation modules may in practise include additional flow lines, directional control valves and check valves as appropriate. Supply of power and monitoring and control of the operation of the acid/base generation unit 16 and the membrane cleaning cycle may be accomplished via an umbilical 9 and the topside control module 8 or the subsea control module 10, as indicated in the drawing by the continuous line 27.

(17) Embodiments of the water injection system comprise one or more underwater membrane separation modules including any of an underwater coarse filter membrane (CF), a multiple media filter membrane (MMF), a microfiltration filter membrane (MF), an ultrafiltration filter membrane (UF), a nanofiltration filter membrane (NF) or a reverse osmosis filter membrane (RO).

(18) FIG. 2 shows a modified embodiment of the subsea water processing system 1 of FIG. 1. Similar to the embodiment of FIG. 1 the generated acids and bases are used for chemical cleaning of membranes. The embodiment of FIG. 2 comprises additional mixing or storage vessels 31 and 32. Vessels 31 are used for storage of generated acid and base cleaning solution. A nanofiltration (NF) permeate stream 35 is supplied to control the pH levels and concentration of the acid and base streams. Vessels 31 and 32 may be used for storage of CIP wastes before neutralization and discharge. In FIG. 2 reference numbers 33 and 34 point towards discharge valves.

(19) FIG. 3 shows a modified embodiment of the subsea water processing system 1 of FIG. 2. Similar to the embodiments of FIGS. 1 and 2 the generated acids and bases are used for chemical cleaning of membranes. The embodiment of FIG. 3 comprises use of an ultrafiltration (UF) permeate stream 37 to feed the acid and base generation unit 16 (as compared to using raw seawater or treated produced water to feed the acid and base generation unit as in the embodiments of FIGS. 1 and 2).

(20) FIG. 4 shows a modified embodiment of the subsea water processing system 1 of FIG. 3. Similar to the embodiments of FIGS. 1-3 the generated acids and bases are used for chemical cleaning of membranes. The embodiment of FIG. 4 comprises use of a nanofiltration (NF) permeate stream 38 to feed the acid and base generation unit 16 (as compared to using raw seawater or UF to feed the acid and base generation unit as in FIGS. 1-3).

(21) Because of the capacity of a nanofiltration membrane to remove divalent and reactive calcium and magnesium ions from the water to be supplied to the cells of the acid and base generation unit the problem of scaling and deposition of calcium carbonate CaCO.sub.3 and magnesium hydroxide Mg(OH).sub.2 crystals in an electrochemical cell or in downstream filters and membranes will be substantially reduced or completely avoided. Nanofiltrate is relatively free from divalent ions such as Ca.sup.2+ and Mg.sup.2+, species known to foul electrochemical cells. As a result, the nanofiltrate that is fed into the acid and base generation unit will be relatively free of Ca.sup.2+ and Mg.sup.2+ while being rich in useful halide salts such as sodium chloride.

(22) FIG. 5 shows a modified embodiment of the subsea water processing system 1 of FIG. 4. Similar to the previous embodiments the generated acids and bases are used for chemical cleaning of membranes. In the embodiment of FIG. 5 the NF permeate 38 is further filtrated by a second NF stage 40 to further reduce Ca.sup.2+ and Mg.sup.2+ concentration to prevent scaling in the acid and base generation unit 16.

(23) Raw seawater has the combined concentration of Ca.sup.2+ and Mg.sup.2+ ions of approximately ˜1500 to 2000 mg/L. After one NF stage, their combined concentration of 80 to 120 mg/L may still have the potential to foul electrodes and membranes. After a second NF stage, the combined concentration of Ca.sup.2+ and Mg.sup.2+ is in the range of 20-40 mg/L, a greatly reduced fouling potential of the electrochemical cells.

(24) The concentrate stream from the NF stage 40 can be either discharged, sent as RO feed, or be injected into wells for IOR.

(25) FIG. 6 shows a modified embodiment of the subsea water processing system 1 of FIG. 5. Similar to the previous embodiments the generated acids and bases are used for chemical cleaning of membranes. In the embodiment of FIG. 6 the NF permeate 38 is further filtrated by a third NF stage 41 to further reduce Ca.sup.2+ and Mg.sup.2+ concentration to prevent scaling in the acid and base generation unit 16.

(26) After a third NF stage the combined concentration of Ca.sup.2+ and Mg.sup.2+ ions are less than 10 mg/L, effectively mitigating the fouling of electrodes and membranes in the cells of the acid and base generation unit.

(27) The concentrate stream from NF stages 40 and 41 can be either discharged, sent as RO feed, or be injected into wells for IOR.

(28) FIG. 7 shows a modified embodiment of the subsea water processing system 1 of FIG. 6. Similar to the previous embodiments the generated acids and bases are used for chemical cleaning of membranes. In the embodiment of FIG. 7 a RO concentrate (i.e. reject) stream 39 (instead of NF permeate stream 38) is passed to three-stage NF membrane units 40-42 to remove Ca and Mg ions, and subsequently fed into the acid and base generation unit 16.

(29) An advantage of using RO concentrate stream to feed the acid and base generation unit is that this stream is rich in useful halide salts such as sodium chloride (2× of typical seawater) while it will be relatively free of Ca.sup.2+ and Mg.sup.2+, the fouling species after NF treatment.

(30) FIG. 8 shows an embodiment of the subsea water processing system in which only coarse and UF treated water is injected into oil wells, i.e. no NF and RO units are included in the main treatment train.

(31) In the embodiment shown, UF permeate 37 is further filtrated by a single stage, smaller NF membrane unit 40 to reduce Ca.sup.2+ and Mg.sup.2+ concentration to prevent scaling in the acid and base generation unit 16. As in previous embodiments the generated acids and bases are used for chemical cleaning of membranes. The concentrate stream from the NF stage 40 can be either discharged, sent as RO feed, or be injected into wells for IOR.

(32) FIG. 9 shows a modified embodiment of the subsea water processing system 1 of FIG. 8. Similar to the previous embodiments the generated acids and bases are used for chemical cleaning of membranes. In the embodiment of FIG. 9 the UF permeate 37 is further filtrated by two-stage NF membrane units 40 and 41 to reduce Ca.sup.2+ and Mg.sup.2+ concentration to prevent scaling in the acid and base generation unit 16.

(33) The concentrate stream from the NF stages 40 and 41 can be either discharged, sent as RO feed, or be injected into wells for IOR.

(34) FIG. 10 shows a modified embodiment of the subsea water processing system 1 of FIG. 9. Similar to the previous embodiments the generated acids and bases are used for chemical cleaning of membranes. In the embodiment of FIG. 10 the UF permeate 37 is further filtrated by three-stage NF membrane units 40-42 to reduce Ca.sup.2+ and Mg.sup.2+ concentration to prevent BPED scaling. The concentrate stream from units 40, 41, and 42 can be either discharged, sent as RO feed, or be injected into wells for IOR.

(35) FIG. 11 shows a modified embodiment of the subsea water processing system 1 of FIG. 8. Similar to the previous embodiments the generated acids and bases are used for chemical cleaning of membranes. In the embodiment of FIG. 11 the NF permeate 38 is further softened by an ion exchange unit 50 (instead of NF membrane unit) to reduce Ca.sup.2+ and Mg.sup.2+ concentration to prevent scaling in the acid and base generation unit 16.

(36) FIG. 12 shows a modified embodiment of the subsea water processing system 1 of FIG. 8. Similar to the previous embodiments the generated acids and bases are used for chemical cleaning of membranes. In the embodiment of FIG. 12 the UF permeate 37 is further filtrated by a combination of a NF membrane unit 40 and an ion exchange unit 50 to reduce Ca.sup.+ and Mg.sup.2+ concentration to prevent scaling in the acid and base generation unit 16.

(37) FIG. 13 shows a modified embodiment of the subsea water processing system 1 of FIG. 7. In the embodiment of FIG. 13 a subsea water processing system comprises a heat exchanger 60. Seawater passes through coarse filter 11, UF filter 13, NF filter 40, and is subsequently fed into the acid and base generation unit 16. The generated acids and bases are used for chemical cleaning of the heat exchanger 60.

(38) In embodiments of the subsea water processing system the acid/base generation unit is via the hydraulic circuit connectable on-site to at least one underwater membrane separation module including any of a single-bore or multi-bore hollow fibre membrane, a plate-and-frame membrane or a spiral wound membrane.

(39) Embodiments of the water injection system include an acid/base generation unit that is via the hydraulic circuit connectable on-site to at least one underwater membrane separation module arranged to operate at outside-in or at inside-out flow pattern and at either dead-end or cross-filtration mode.

(40) Further embodiments of the water injection system include the acid/base generation unit which via a hydraulic circuit is connectable on-site to at least one underwater separation module(s) located on the seabed, or underwater near the surface, or attached to a floating platform.

(41) Embodiments of the invention include applications wherein the acid/base generation unit is operated as stand-alone unit, or in assemblies where filtration elements are located upstream only to serve the acid/base generation unit. Examples include any combination of the acid/base generation unit and an upstream coarse filtration membrane, or upstream located coarse and ultrafiltration membranes, or coarse filtration, ultrafiltration and nanofiltration membranes all sequentially installed in the water feed upstream of the acid/base generation unit.

(42) It is further contemplated that the pump or pumps to feed seawater through the membranes or other subsea process apparatus can be installed at any desired location within the assembly. High pressure injection pumps may be required for reservoir stimulation. It is further contemplated that the acid/base generation unit can be equipped to produce highly concentrated or more dilute acid or base solutions at concentrations suitable for a particular process.

(43) Any of the above options may be available for on-site production of chemicals for subsea filter membranes, subsea heat exchangers, pipes and flow lines and other subsea process equipment, for injection and production wells etc.

(44) Experiments

(45) Proof-of-concept lab experiments have demonstrated the capability to produce acid and base from seawater on-site and on-demand. FIGS. 14 and 15 show lab-scale experimental results obtained using a bipolar membrane electro-dialysis unit (BPED) and synthetic seawater as feed at both room temperature and subsea temperature (4° C.).

(46) Salts for making synthetic seawater for this experiment include KCl, MgCl.sub.2, CaCl.sub.2, NaCl, and NaHCO.sub.3 (all from Sinopharm Chemical Reagent Co., Ltd). Acid and basic solutions were prepared from concentrated hydrochloric acid solution and solid NaOH (Sinopharm Chemical Reagent Co., Ltd).

(47) Experiments were conducted using a BPED stack obtained from GE Power & Water. FIG. 16 shows the schematic of one unit included in the BPED stack, which comprises 6 such units. The single BPED unit consists of one anion exchange membrane (AM), one cation exchange membrane (CM) and a composite bipolar membrane (BP) made of a cation exchange membrane and an anion exchange membrane. During operation, charging of the stack induces a water splitting reaction at the surface of the bipolar membranes and protons and hydroxide ions are generated. In addition, a reduction reaction occurs at the surface of the cathode as hydrogen and hydroxide ions are generated, and a corresponding oxidation reaction occurs at the surface of the anode as oxygen and protons are produced.

(48) For the lab-scale set-up an electrochemical charging system (Germany Digatron power electronics Co., Ltd) was connected to the two electrodes of the BPED stack to supply certain current and voltage. A SevenMulti pH and conductivity meter (Metler Toledo) was used to identify the pH level of generated alkali and acid. It was also used to identify the conductivity level of synthetic seawater during the experiment. Fresh synthetic seawater was added into the system when the conductivity dropped during the experiment. A pump (LongerPump) was used to pump synthetic seawater into the BPED stack and acid/base out of the BPED stack to form a recirculating system. A thermostatic water bath (Fisher Scientific) was used to keep synthetic seawater at a constant temperature. Finally a stirrer (Heidolph instruments) was used to keep the concentration of synthetic seawater uniform.

(49) The results shown in FIGS. 14A and 15A indicate that the pH of the base stream quickly reached 10-12 and higher; and the pH of the acid stream reached 2-3.5 or lower within minutes, adequate for membrane cleaning purpose. FIGS. 14B and 15B show that the concentration of the base can reach 2 to 18 g/L, or 0.2 to 1% (wt), depending on the reaction time, power density, and temperature. Increase in the charging current and the temperature accelerates alkali/acid generation rates due to stronger electrical field and faster ion mobility. The penalty with higher current density is higher energy consumption. 100, 250, and 400 mA/m.sup.2 were used in the experiments. The current efficiency was found to be in the range of 60% to 85% range for all experiments. At 4° C. and at higher current density, the current efficiency lowered slightly as compared with room temperature current efficiency due to the slower ion mobility.

(50) The lab experiments demonstrate the feasibility of using an electro-dialysis unit to generate base and acid on-demand, on-site, for CIP (cleaning in place) of membranes and other subsea process apparatus. Obviously, the recited lab experiment was conducted on a BPED unit with relatively small membrane area, and the system was in recirculation mode. Commercial BPED units provide considerably larger membrane areas such that the feed water usually is in one-through mode (as in contrast to recirculation), to produce acid and base directly, no recirculation is needed.

(51) The demonstrated on-site generation of acid and base from seawater offers great advantages when compared to marinization of conventional onshore CIP system. These advantages include eco-friendliness, much lower weight and footprint, greatly reduced CAPEX (capital expenditures) and OPEX (operation expenditures), and excellent robustness. In addition, the acid/base generation unit's modular design guarantees up-scalability.

(52) The scope of the present invention as disclosed above and in the drawings, is defined by the appended claims, covering the embodiments disclosed and modifications which can be derived therefrom without leaving the scope of the invention.

(53) This written description uses examples to disclose the invention, including the preferred embodiments, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.