Underwater water treatment unit and method for cleaning said unit

Abstract

The invention relates to an underwater water treatment unit including specific cleaning means suitable for cleaning the filtration membranes under the unusual conditions associated with use at great or very great depths, as well as to a method for cleaning the water treatment membrane of the underwater unit.

Description

[0001] UNDERWATER WATER TREATMENT UNIT

[0002] AND METHOD FOR CLEANING SAID UNIT

[0003] TECHNICAL FIELD OF THE INVENTION

[0004] The present invention relates to the field of subsea applications, particularly in the oil industry. More specifically, the invention concerns a subsea water treatment unit with special cleaning means, as well as a method for cleaning the water treatment membrane of the subsea unit.

[0005] EARLIER ART

[0006] Water injection into hydrocarbon reservoirs is a well-known technique in the oil industry. It involves injecting water, commonly called injection water, into an oil well to recover hydrocarbons and prevent well collapse, which can occur due to pressure loss during hydrocarbon extraction.

[0007] The source of injection water generally depends on its availability and the constraints surrounding the hydrocarbon extraction site. For example, in offshore extraction, it is common practice to use seawater. However, treatment steps are usually essential to obtain seawater of sufficient quality for reintroduction into the underground formation. Injection water is always obtained through a filtration step, aimed at reducing or even eliminating suspended solids, and a deoxygenation step. Often, additional treatment in the form of desulfation is used if the underground formation content is such that a reduction of sulfate ions is necessary.

[0008] The injection water can also be aquifer water, river or lake water, and possibly domestic or industrial wastewater. Here too, treatment steps may be necessary to obtain water of a quality compatible with injection into the underground formation. When the injection water is seawater, the presence of sulfates is typically problematic if the underground formation contains barium, calcium, or strontium ions. Sulfate ions combine with barium, calcium, or strontium ions to form mineral deposits (known as "scaling") that hinder efficient hydrocarbon extraction. Furthermore, the presence of sulfates can lead to the generation of hydrogen sulfide (H2S) by bacteria, a toxic and corrosive gas that can corrode pipelines used for hydrocarbon recovery. Therefore, removing sulfates from the water before injection into the underground formation is often necessary.

[0009] A conventional method for removing sulfates from water involves nanofiltration membrane filtration, which retains multivalent ions and allows monovalent ions to pass through. Another conventional method for water desalination is reverse osmosis. Such processes are described, for example, in patent applications WO 2006 / 134367 and WO 2007 / 138327.

[0010] Generally, water treatment units are located near the hydrocarbon deposit. In the case of subsea deposits, these units are classically installed on the surface, on the offshore hydrocarbon extraction platform or on auxiliary floating platforms, commonly called FPSO units (acronym for "Floating Production, Storage and Offloading" according to Anglo-Saxon terminology, meaning floating production, storage and shipping unit).

[0011] One of the major problems associated with installing surface water treatment units is their size. These units take up a lot of space. However, space management on offshore platforms is challenging because space is limited and many installations are essential. Therefore, water treatment units that require minimal space on offshore platforms are needed.

[0012] A solution to this problem has already been proposed in the prior art. It consists of replacing existing treatment units with underwater units capable of operating underwater. In particular, international patent application WO 2009 / 122134 describes an underwater seawater treatment unit. Placing the unit underwater also offers an energy advantage: the system, located in an underwater environment, benefits from pressure hydrostatic pressure is approximately proportional to the depth at which it is located. US patent 7,600,567 and GB patent application 2,451,008 also describe an underwater water treatment unit that can be placed underwater at a depth of between 250 and 700 meters.

[0013] However, discoveries of large hydrocarbon deposits are currently becoming increasingly rare in shallow and medium-depth waters, that is, down to 500 meters. In order to ensure the renewal of reserves, it is necessary to develop new deposits located in the deep sea (that is, at a depth between 500 and 1,500 meters) and in the ultra-deep sea (that is, beyond 1,500 meters).

[0014] In shallow and medium depths, temperature, salinity, and aquatic fauna are not radically different from surface conditions. However, in the deep sea and ultra-deep sea, the water temperature is approximately 3°C to 5°C, and water viscosity increases with depth. Furthermore, pressure increases by 1 OMPa per 1000 meters. Therefore, the environmental conditions to which underwater water treatment units are subjected are quite unique.

[0015] Furthermore, the actual underwater environment cannot be defined solely by pressure and temperature conditions. It is a complex environment, with varying chemical compositions, and above all, it includes microorganisms specific to the deep sea and ultra-deep seabed.

[0016] These microorganisms tend to attach themselves and multiply on certain surfaces, and more specifically, to quickly foul any underwater equipment. This phenomenon of biological fouling is commonly referred to by the English term "biofouling." It is the phenomenon of degradation or deterioration of a surface or object left in an aquatic environment, such as typically the sea, due to the growth of living organisms such as bacteria, protozoa, algae, and crustaceans.

[0017] When deployed at great or ultra-great depths, underwater water treatment units, and in particular filtration membranes that they contain, are therefore subject to very specific environmental conditions.

[0018] Methods for reducing, or even eliminating, filter clogging are known to those skilled in the art. For example, patent application WO 2012 / 049618 describes a water treatment process that reduces the risk of membrane filter clogging by treating the retentate before recycling. Another example, in the previously cited US patent 7,600,567, describes backwashing of filtration membranes. In one embodiment, the backwash water can be enriched with gas. This embodiment is not suitable when the units containing the filtration membranes are located at great depths. Another embodiment involves adding a biocide and / or an anti-scaling agent to the backwash water. These chemicals are primarily organic molecules.At the end of the washing phase, they are released into the environment and are therefore likely to cause ecological problems, especially since the filtration membranes are located at great and ultra-great depths where the biological environment is still poorly understood.

[0019] It would therefore be advantageous to reduce the use of these conventional cleaning methods in the very particular environmental conditions to which units located in the deep sea and ultra-deep sea are subjected.

[0020] Furthermore, in the case of seawater treatment units intended for use as injection water in an oil well, these units should ideally remain operational for several months, or even several years, without any intervention. In addition, it is preferable for the unit to be as robust as possible in order to minimize the number of necessary maintenance interventions.

[0021] It is therefore necessary to have a processing unit adapted to these very particular environmental conditions of the deep sea and ultra-deep sea.

[0022] Furthermore, conventional methods for cleaning filtration membranes are described in the scientific publications of Davis et al. ("Membranes solve North Sea waterflood sulfate problems" OU & ​​Gas Journal, Nov. 25, 1996) and O'Donnell ("Membrane technology works on North Sea platform" OU & ​​Gas Journal, Dec. 2, 1996), and in US patents and patent applications 3,827,976, US 5 403 479 and US 2012 / 0090641. These documents do not present a solution to the particular problem posed here by environmental conditions.

[0023] It is in this context that the inventors discovered an alternative, environmentally advantageous solution for washing filtration membranes using acidic and / or alkaline solutions under underwater conditions. While these acidic and / or alkaline solutions are relatively harmless to the environment and used in highly diluted form, they can potentially be released into the environment after use. However, the invention also provides means to prevent the release of these solutions into the environment.

[0024] SUMMARY OF THE INVENTION

[0025] These objectives, among others, are addressed by the present invention, which relates to an underwater water treatment unit adapted for injection into a hydrocarbon reservoir, comprising:

[0026] - a means of supplying the underwater unit with energy;

[0027] - an underwater water treatment system comprising at least one filtration membrane, a water intake suitable for bringing into the underwater unit a flow of water taken from the environment of the system, a pipe connecting said water intake and said filtration membrane, a treated water discharge pipe, and at least one feed pump suitable for carrying out the water filtration;

[0028] - a hydraulic circuit for cleaning said membrane, forming a loop between the inlet and outlet of said membrane, and having a drain pipe;

[0029] - at least two water storage tanks, each water tank being connected to the cleaning circuit;

[0030] - at least one storage tank for a cleaning product chosen from an acid and a base, the cleaning product tank being connected to the cleaning circuit;

[0031] - at least one suitable cleaning pump to move fluids in the cleaning circuit; said filtration membrane being equipped with at least one heating means, and at least one of the water tanks being equipped with at least one tank heating means.

[0032] The invention further relates to a method for cleaning the filtration membrane of a submersible unit as defined above, comprising the steps of:

[0033] a) interrupt the flow of water into said filtration membrane;

[0034] b) inject water from one of the water storage tanks into the hydraulic cleaning circuit, and circulate this water in a loop through the hydraulic cleaning circuit using the cleaning pump;

[0035] c) heat the filter membrane using the heating means to a temperature between 15°C and 40°C;

[0036] d) inject a cleaning product contained in at least one storage tank into the hydraulic cleaning circuit;

[0037] e) evacuate the contents of the hydraulic cleaning circuit via the drain pipe;

[0038] f) inject water from one of the water storage tanks back into the hydraulic cleaning circuit, and circulate this water in a loop through the hydraulic cleaning circuit;

[0039] g) drain the contents of the hydraulic cleaning circuit again via the drain line.

[0040] DESCRIPTION OF THE FIGURES

[0041] Figure 1 is a schematic representation of one embodiment of the underwater water treatment unit according to the invention.

[0042] Figure 2 is a schematic representation of one embodiment of the cleaning means of a unit according to the invention.

[0043] DETAILED DESCRIPTION OF THE INVENTION

[0044] In this application, the expression "between and" should be understood as including the boundaries. The subsea unit according to the invention is designed to treat water so that this water can be injected into a hydrocarbon reservoir. The water that is treated is preferably seawater, and the resulting water is injection water.

[0045] To perform the water treatment, the underwater unit according to the invention first comprises a means for supplying the underwater unit with energy and an underwater water treatment system. Said water treatment system comprises at least one filtration membrane, a water intake adapted to bring into the underwater unit a flow of water taken from the environment of the system, a pipe connecting said water intake and said filtration membrane, a treated water discharge pipe, and at least one feed pump adapted to perform the water filtration.

[0046] Filtration membranes are devices well known to those skilled in the art that allow the separation of different components from a liquid stream. The nature of the separation is determined in part by the size of the membrane pores.

[0047] According to the IUPAC classification:

[0048] - a microfiltration membrane has macropores with a diameter greater than 50 nm,

[0049] - an ultrafiltration membrane has mesopores with a diameter between 2 nm and 50 nm,

[0050] - a nanofiltration membrane has micropores with a diameter of less than 2 nm.

[0051] On the other hand, reverse osmosis membranes are a special type of membrane, well known to those skilled in the art. A reverse osmosis membrane is a semi-permeable membrane that only allows the solvent (usually water) to pass through, and not other substances in solution.

[0052] The filtration membrane in the present invention can be a microfiltration membrane, an ultrafiltration membrane, a nanofiltration membrane, or a reverse osmosis membrane. Furthermore, according to a more advantageous embodiment, the filtration membrane in the present invention is a desulfation membrane. In the present invention, "desulfation membrane" means a membrane that allows the separation of sulfate ions from water. of a mode of embodiment particularly advantageous for the petroleum industry, because these membranes allow the removal of sulfates in seawater intended to be reinjected into underground formations containing hydrocarbons.

[0053] Among the nanofiltration membranes for seawater desulfation, we can mention, among others, the SR90 membranes from DOW Filmtec and the NANO-SW from Hydranautics.

[0054] The subsea water treatment system may include several filtration membranes, arranged in series and / or in parallel, which may be identical or different. In a preferred embodiment, the subsea water treatment system includes a nanofiltration membrane and an ultrafiltration membrane, the ultrafiltration membrane being located upstream of the nanofiltration membrane. The subsea water treatment system may then include, in order relative to the normal flow of water through the system:

[0055] - a water intake;

[0056] a pipe connecting said water intake and an ultrafiltration membrane;

[0057] said ultrafiltration membrane;

[0058] a conduit connecting said ultrafiltration membrane and a nanofiltration membrane;

[0059] said nanofiltration membrane; then

[0060] on the one hand, a discharge pipe downstream of said nanofiltration membrane; and on the other hand, a discharge pipe for the retentate from the nanofiltration membrane.

[0061] According to another embodiment, the underwater water treatment system includes a microfiltration membrane and / or an ultrafiltration membrane.

[0062] In addition, the underwater water treatment system may include other filters, which can be selected, for example, from a sieve and a pre-filter. A sieve is a grid with a more or less fine mesh or a surface perforated with holes. A sieve generally functions to separate solid particles of different sizes from a liquid stream; the size of the separated particles depends on the size of the mesh or the holes in the filter. The term A "pre-filter" is a filter capable of stopping relatively large solid particles, that is, those larger than 100 micrometers. The pre-filter provides coarse filtration.

[0063] Water enters the underwater water treatment system through a water intake. A water intake is a standard system with an opening that allows water to flow into the system. The intake may optionally be equipped with a strainer to trap large solid particles, thus preventing rapid clogging of the filtration system by these large items.

[0064] The location of the water intake determines the quality of the water entering the subsea water treatment system. The intake can advantageously be mobile, for example, using a telescopic system, which allows its location to be adjusted without altering the location of the subsea water treatment unit itself. Specifically, a telescopic system or a reel can be used to move the intake level, which is measured relative to the seabed or the surface.

[0065] In the underwater water treatment system of the present invention, the filtration membrane can be arranged in a specific module comprising two chambers: a first chamber upstream of the filtration membrane and a second chamber downstream of the filtration membrane, the filtration membrane forming a partition between the two chambers. The upstream chamber is equipped with at least one conduit for the water flow to enter. The downstream chamber is equipped with at least one discharge conduit. Optionally, the upstream chamber is also equipped with a second discharge conduit. The sizing of the upstream and downstream chambers can be determined by a person skilled in the art, particularly according to the type of filter and the nature and flow rate of the water to be filtered.

[0066] Typically, the water flow is brought into the first chamber of the filter via a feed pipe. The portion of the flow that passes through the filter and reaches the second chamber, downstream of the filter, is commonly called the filtrate. The portion of the flow that is retained by the filter is commonly called the retentate. Depending on the type of filter, the retentate can be solid, liquid, or a mixture of both. To circulate water through the filter, the underwater water treatment system according to the invention includes at least one pump adapted to perform water filtration. This pump may be located on the pipe connecting the water intake and the filter. The pump may be controlled by an electronic system. This electronic system may be controlled by a pre-recorded program that does not require operator intervention. Alternatively, it may be controlled by an operator, preferably stationed on the surface, and the information exchanged between the operator and the electronic system may be transmitted via a cable or electronically (for example, by waves, particularly radio waves in air or acoustic waves in water).

[0067] The discharge pipe downstream of the filter is used to remove the filtrate from the system. This filtrate constitutes the treated water suitable for use as injection water in a hydrocarbon reservoir. The filtrate discharge pipe is therefore preferably connected to a hydrocarbon extraction well.

[0068] Optionally, the underwater water treatment system according to the invention may include a discharge pipe for the filter retentate. Preferably, the retentate is not recovered and is therefore preferably released directly into the environment, near the underwater water treatment system.

[0069] To carry out the various treatment stages, and in particular to operate the feed pump used for water filtration, the underwater water treatment system, and more generally the entire underwater water treatment unit, requires a power source. Therefore, the underwater water treatment unit includes a means of supplying it with energy.

[0070] According to a first embodiment, the subsea water treatment unit is powered from the surface via an umbilical. An umbilical is a well-known device in the field of subsea oil production. The umbilical can conduct an electric current. It can therefore supply electricity to the subsea water treatment unit from the surface. In particular, the umbilical can connect the subsea water treatment unit to a control unit located afloat, typically a vessel or a floating platform, for example, directly above the submerged subsea water treatment unit, or in a satellite field several tens of kilometers from the surface unit. The same umbilical can perform several functions at once. Typically, the same umbilical can serve as a conduit for the power supply to the underwater water treatment unit and for the transmission of commands.

[0071] In a second embodiment, the underwater water treatment unit is electrically self-sufficient and powered by a battery located within the unit. The use of batteries is not preferred due to their size and weight. However, this embodiment has the advantage of making the underwater water treatment unit self-contained and easily movable.

[0072] The underwater water treatment unit is preferably compact, with the possible exception of the water intake and discharge pipe, which may extend beyond the structure. The dimensions of this structure can range from 8 m (length) x 5 m (width) x 5 m (height) to 20 m (length) x 10 m (width) x 15 m (height). The total weight of the unit above water can range from 100 tonnes to 800 tonnes.

[0073] The underwater water treatment unit is intended to be placed underwater at a depth of at least 500 meters, and in particular at least 800 meters. The depth of the underwater water treatment unit is preferably between 800 meters and 3000 meters, more preferably between 1000 meters and 3000 meters, and even more preferably between 1500 meters and 3000 meters.

[0074] Preferably, the underwater water treatment unit is placed in the sea, and the water flow taken from the environment is a seawater flow.

[0075] The subsea water treatment unit is subjected to an external pressure preferably between 80 and 300 bar, this pressure being of course a function of the depth at which the subsea water treatment unit is installed. The temperature at the location of the subsea water treatment unit also depends on the depth at which the unit is located, but also more generally on the geographical location of the subsea water treatment unit. For example, If the subsea water treatment unit is installed at a depth of 3000 meters in the Gulf of Guinea, the water temperature will be approximately 4°C. If the subsea water treatment unit is installed at a depth of 500 meters, particularly 800 meters, in the Arctic Sea, the water temperature will be approximately -2°C. Generally, the subsea water treatment unit is subjected to an outside temperature between 20°C and -2°C, preferably between 14°C and -

[0076] In the marine environment, it is well known that the lifespan and filtration capacity of filter membranes can be significantly reduced by biofouling, which causes filter clogging. However, conventional methods for cleaning filter membranes have proven unsuitable for the very specific environmental conditions encountered in deep-sea and ultra-deep-sea submersible water treatment units.

[0077] In situ cleaning reduces the frequency of intervention on the filtration membranes (either to replace them or to clean them).

[0078] Therefore, the underwater water treatment unit according to the present invention further includes special cleaning means.

[0079] The underwater water treatment unit includes:

[0080] - a hydraulic circuit for cleaning said membrane, forming a loop between the inlet and outlet of said membrane, and having a drain pipe;

[0081] - at least two water storage tanks, each water tank being connected to the cleaning circuit;

[0082] - at least one storage tank for a cleaning product chosen from an acid and a base, the cleaning product tank being connected to the cleaning circuit;

[0083] - at least one suitable cleaning pump to move fluids in the cleaning circuit.

[0084] In addition, the filtration membrane of the underwater water treatment system is equipped with at least one heating means, and at least one of the water tanks is equipped with at least one tank heating means. The hydraulic cleaning circuit according to the invention forms, by means of suitable pipes, a closed circuit that includes the filter membrane. The hydraulic cleaning circuit comprises a drain line, which can be controlled by a valve. As long as this drain valve is kept closed, a liquid flow can be circulated within the hydraulic cleaning circuit, and it flows continuously along this loop, passing continuously through the filter membrane. Opening the drain valve breaks the loop and allows the liquid flow to be discharged from the hydraulic cleaning circuit, for example, when the cleaning process is complete.

[0085] According to a preferred embodiment, the underwater water treatment system comprises a nanofiltration membrane and an ultrafiltration membrane, said ultrafiltration membrane being located upstream of said nanofiltration membrane. In this case, the hydraulic cleaning circuit according to the invention can advantageously form:

[0086] - a loop between the inlet and outlet of the nanofiltration membrane, or

[0087] - a loop between the inlet and outlet of the ultrafiltration membrane, or

[0088] - two loops between the inlet and outlet of the nanofiltration membrane on one hand and between the inlet and outlet of the ultrafiltration membrane on the other hand, with valves located on the loops to allow selection of the membrane whose cleaning is desired.

[0089] The choice of the number of loops in the hydraulic cleaning circuit can depend on the cleaning requirements of the subsea water treatment unit. These requirements can vary depending on the type of filtration membranes used, but also on the aquatic environment of the subsea water treatment unit, for example, water quality, temperature, pressure, chemical composition, and the microorganisms it contains.

[0090] Preferably, when the subsea water treatment system includes a nanofiltration membrane and an ultrafiltration membrane, the hydraulic cleaning circuit preferably forms two loops: one between the inlet and outlet of the nanofiltration membrane, and the other between the inlet and outlet of the ultrafiltration membrane on the other hand, valves being located on the loops to allow selection of the membrane whose cleaning is desired.

[0091] The means for cleaning the underwater unit according to the invention include a number of storage tanks. At least two storage tanks are intended for storing water. At least one storage tank is intended for storing a cleaning agent selected from an acid and a base.

[0092] The tanks are designed to fulfill their storage function in an aquatic environment, at the temperature and pressure to which the subsea water treatment unit is subjected. The tank sizes can be calculated by a person skilled in the art based on the quantity of water and cleaning agent required to clean the filtration membrane for the desired duration. Preferably, the tanks are sized to ensure the subsea unit is self-sufficient for at least six months. Since space is not a limiting factor for a subsea water treatment unit, robust equipment requiring minimal maintenance is preferred.

[0093] In one particular embodiment, the storage tank(s) are flexible, as a result of which they collapse or flatten when the fluid is withdrawn during the emptying of the tank.

[0094] In another particular embodiment, the flexible storage tank(s) are surrounded by a rigid shell enabling them to withstand low overpressures and allowing water to circulate within the rigid structure when the tank collapses.

[0095] Each tank is connected to the cleaning circuit. Preferably, a valve is placed at the outlet of each tank, so that the opening and closing of each tank can be controlled.

[0096] The acid used as a cleaning agent can be organic or mineral. It can be chosen from among the common compounds known to those skilled in the art. In particular, it can be selected from the group consisting of citric acid and hydrochloric acid. Acids are suitable for dissolving mineral deposits.

[0097] The base used as a cleaning product can be organic or mineral, preferably mineral. It can be chosen from the following compounds These are classic solutions known to those skilled in the art. They can be chosen from caustic soda or a detergent with a proprietary chemical formulation. The bases are specially formulated to eliminate organic growth or microbial residues. Cleaning products are preferably in the form of concentrated aqueous solutions, with concentrations ranging from 1% to 4%.

[0098] According to a preferred embodiment, the subsea unit includes at least two storage tanks for cleaning product, at least one of the tanks being a storage tank for an acid and at least one of the tanks being a storage tank for a base, the acid and base tanks being connected to the cleaning circuit.

[0099] In general, the underwater water treatment unit is subjected to specific pressure and temperature conditions due to its immersion. In particular, the underwater water treatment unit is subjected to an external temperature between 20°C and -2°C, preferably between 14°C and -2°C. The inventors observed that, under these conditions, the cleaning methods had to be adapted.

[0100] Therefore, the underwater water treatment unit also includes heating means. The filtration membrane of the underwater water treatment system is equipped with at least one heating means, and at least one of the water tanks is equipped with at least one tank heater.

[0101] These heating devices advantageously allow for cleaning the filter membrane at a temperature higher than the outside temperature. The heating devices can be positioned appropriately, near the filter membrane to heat it, and near or within the water tank to heat the water it contains.

[0102] The heating means can be powered by the power supply of the underwater unit.

[0103] For example, one can heat a point in the circulation loop and maintain a circulation of hot liquid through the filtration membrane.

[0104] Alternatively, the heating methods are such that they allow the water surrounding the filtration membrane to be heated rather than heating the circulation loop directly. In a preferred embodiment, the subsea water treatment unit comprises at least three water storage tanks, at least two of which are equipped with at least one tank heating means. The subsea water treatment unit further comprises at least one cleaning pump suitable for circulating fluids in the cleaning circuit. This cleaning pump may be powered by the subsea unit's power supply. The cleaning pump is preferably located on the hydraulic cleaning circuit.

[0105] Advantageously, the feed pump and the cleaning pump can be mounted on the same shaft. This embodiment increases the robustness of the submersible unit according to the invention. This is a significant advantage because maintaining the unit when submerged at great or ultra-great depths is complex and costly.

[0106] The underwater water treatment unit according to the invention can therefore be used to treat a stream of water drawn from the unit's aquatic environment, and to treat it so that it is usable as injection water. In normal operation, the water treatment process using the underwater unit according to the invention may include the steps of:

[0107] - to introduce a flow of water into the underwater water treatment unit via the water intake;

[0108] - bring this flow into the underwater water treatment system up to the filtration membrane;

[0109] - to obtain a filtrate downstream of said filtration membrane; and

[0110] - inject said filtrate, at least in part, into a hydrocarbon reservoir. The subsea processing unit may be placed underwater at least

[0111] 500 meters deep. In addition, the hydrocarbon reservoir into which the treated water is injected is preferably an offshore reservoir.

[0112] The filtration membrane can be cleaned from time to time to maintain its filtration capacity and prevent clogging. The present invention relates to a method for cleaning the membrane of a submersible unit as defined above, comprising the steps of:

[0113] a) interrupt the flow of water into the filtration membrane;

[0114] b) inject water from one of the water tanks into the hydraulic cleaning circuit, and circulate this water in a loop through the hydraulic cleaning circuit using the cleaning pump;

[0115] c) heat the membrane using the heating means to a temperature between 15°C and 40°C;

[0116] d) inject a cleaning product contained in at least one storage tank into the hydraulic cleaning circuit;

[0117] e) evacuate the contents of the hydraulic cleaning circuit via the drain pipe;

[0118] f) inject water from one of the water tanks back into the hydraulic cleaning circuit, and circulate this water in a loop through the hydraulic cleaning circuit;

[0119] (g) Discharge the contents of the hydraulic cleaning circuit again via the discharge pipe. Step (a) consists of interrupting the water supply to the filtration membrane. If it is desired not to interrupt the production of treated water for injection, it is possible to install several filtration membranes and / or several water treatment systems, arranged in parallel within the same subsea unit.

[0120] Once the membrane is isolated from the normal water treatment circuit, in step b), water contained in one of the water tanks is injected into the hydraulic cleaning circuit, and this water is circulated in a loop in the hydraulic cleaning circuit using the cleaning pump.

[0121] Preferably, the water injected into the hydraulic circuit during step b) is at the same temperature as the filtration membrane. The filtration membrane is therefore a priori at ambient outdoor temperature since it was in contact with water drawn from the outside environment during the normal water treatment process. The water injected into the hydraulic circuit during step b) is therefore preferably at The ambient outside temperature, which can be between 20°C and -2°C, preferably between 14°C and -2°C. This water can preferably be treated water obtained by the water treatment process using the underwater unit itself.

[0122] The duration of step b) can be determined by a person skilled in the art, so that the water flow in the hydraulic cleaning circuit is constant.

[0123] Step c) of the cleaning process according to the invention consists of heating the filter membrane using the heating means to a temperature between 15°C and 40°C. This step can be carried out by switching on the heating means located near the filter membrane. With the water circulating in the hydraulic cleaning circuit maintained, heating the filter membrane also heats the water circulating in the hydraulic cleaning circuit. Alternatively, this step can be carried out by heating a point in the circulation loop and maintaining hot liquid circulation through the filter membrane. Step c) is preferably completed when the filter membrane and the water circulating in the hydraulic cleaning circuit have reached a desired stable temperature.A person skilled in the art can determine the duration of this step (c) based, in particular, on the initial temperature of the membrane and the water, the volume, the water flow rate, and the heating power. The temperature to be reached during step (c) can be defined according to the nature of the cleaning product that is injected during the subsequent step (d).

[0124] When step c) is completed, a cleaning product from at least one storage tank is injected into the hydraulic cleaning circuit during step d). The injection can be controlled by means of a valve located at the inlet of the storage tank. The amount of cleaning product injected may depend on the nature of the product, its concentration in the storage tank, and the desired cleaning effect.

[0125] During step c), water circulation in the hydraulic cleaning circuit is maintained. The cleaning product is diluted or dispersed in the water. The heating element for the filter membrane is preferably kept in operation so that the temperature remains stable. The filter membrane is then traversed by a flow containing cleaning product. The circulation of this flow is maintained in the hydraulic cleaning circuit for a sufficient time to ensure effective cleaning of the filter membrane.

[0126] At the end of this cleaning process, the contents of the hydraulic cleaning circuit are discharged during step e) of the cleaning process via the discharge pipe. The contents of the hydraulic cleaning circuit can be discharged directly into the environment surrounding the subsea water treatment unit, particularly directly into the sea, provided the cleaning products used are not harmful to the local flora and fauna. Alternatively, if the discharge of cleaning products is likely to cause an environmental problem, the contents of the hydraulic cleaning circuit can be discharged into the water tanks emptied following step b). The discharge can be controlled by a valve located on the discharge pipe.

[0127] The unit may therefore also include a discharge pipe allowing the contents of the hydraulic cleaning circuit to be discharged into the water tank(s).

[0128] When the entire contents of the hydraulic cleaning circuit have been evacuated, the discharge pipe can be closed, and in step f), water from one of the water tanks is injected back into the hydraulic cleaning circuit and circulated in a loop. This step f) advantageously rinses the filter membrane and removes any remaining cleaning agent. The water injected into the hydraulic cleaning circuit in step f) can preferably be treated water obtained by the water treatment process using the subsea unit itself. The duration of the rinsing step f) can be determined by a person skilled in the art to ensure effective rinsing.

[0129] In one particular embodiment, steps e) and f) are carried out simultaneously.

[0130] At the end of step f), the contents of the hydraulic cleaning circuit are again discharged via the discharge pipe during step g). The contents of the hydraulic cleaning circuit are preferably discharged directly into the environment surrounding the subsea water treatment unit, in particular directly into the sea. The discharge can be controlled by means of a valve located on the discharge pipe.

[0131] Following step g), the filtration membrane can advantageously be put back into service. It can again be used in the water treatment process. The supply of water drawn from the environment can be reopened.

[0132] The cleaning process can be implemented on an ad hoc basis or, preferably, regularly over time. The cleaning process according to the invention can be implemented at a frequency of between once a week and once a year, more preferably between once a month and once every six months.

[0133] The cleaning process according to the invention is particularly well-suited to cleaning filtration membranes under atypical conditions related to deep or ultra-deep water use. The objectives of the invention, which are to achieve effective cleaning without causing environmental damage and without requiring complex and costly maintenance, are met by the process of the present invention, which includes, in particular, a step of heating the filtration membrane.

[0134] Some filtration membranes, particularly ultrafiltration membranes, can, however, be damaged by thermal shock. Thermal shock can occur when the membrane is heated during step c) of the process according to the invention, and / or when cleaning is completed and the membrane is put back into service to resume water treatment in the unit's water treatment system.

[0135] To prevent thermal shock, the temperature increase is preferably done slowly during step c) of the cleaning process according to the invention.

[0136] Furthermore, to avoid thermal shock due to temperature drops, the water injected into the hydraulic cleaning circuit during step f) is preferably heated to a temperature between 15°C and 40°C using the heating element in the water storage tank(s). The cleaning process may also include a step of heating the injected water. in the hydraulic cleaning circuit in step f) at a temperature between 15°C and 40°C by means of the heating means for the tank(s) containing the water. This step can be carried out by switching on the heating means located near or in said tank. Advantageously, the water temperature can be the same as the temperature of the filter membrane reached in step c). This heating step can be implemented at the appropriate time so that the water is at the desired temperature when step f is carried out.

[0137] When this rinse water circulates in a loop within the hydraulic cleaning circuit, the heating element for the filter membrane is preferably switched off. The rinsing step can then preferably be continued for a sufficient duration to allow the temperature of the filter membrane and the water circulating in the hydraulic cleaning circuit to decrease, preferably until they reach the ambient outside temperature. Cooling is preferably achieved slowly and naturally, through contact with the external environment.

[0138] Preferably, the cleaning agent injected into the hydraulic cleaning circuit during step d) is an acid, and more preferably, the cleaning agent injected into the hydraulic cleaning circuit during step d) is citric acid. Acid cleaning of the filter membrane dissolves mineral deposits that can clog the membrane. When the cleaning agent injected into the hydraulic cleaning circuit during step d) is an acid, the temperature reached during step c) is preferably between 20°C and 40°C, and more preferably between 25°C and 35°C.

[0139] Alternatively, the cleaning agent injected into the hydraulic cleaning circuit in step d) is a base. Cleaning at the base of the filter membrane removes organic and microbial matter. When the cleaning agent injected into the hydraulic cleaning circuit in step d) is a base, the temperature reached in step c) is preferably between 20°C and 40°C, more preferably between 30°C and 40°C, and even more preferably between 35°C and 40°C.

[0140] According to one embodiment of the present invention, the filtration membrane can be cleaned several times, successively, with different cleaning products. The cleaning process according to the invention may include, between step f) and step g), the steps of:

[0141] d) inject into the hydraulic cleaning circuit a cleaning product different from the product injected in step d), said product being contained in at least one storage tank;

[0142] e') evacuate the contents of the hydraulic cleaning circuit via the drain pipe; and

[0143] f) inject water from one of the water storage tanks back into the hydraulic cleaning circuit, and circulate this water in a loop through the hydraulic cleaning circuit.

[0144] Steps d') to f) correspond to steps d) to f), except that the cleaning product injected during step d') is different from the cleaning product injected during step d). Steps d') to f) can be as described previously for steps d) to f). In particular, the water injected into the hydraulic cleaning circuit during step f) can optionally be heated to a temperature between 15°C and 40°C by means of the heating means of the tank(s) containing the water.

[0145] According to a preferred embodiment, comprising steps a) to f), then d') to f), then g), the cleaning agent injected into the hydraulic cleaning circuit during step d) can be a base, and the cleaning agent injected into the hydraulic cleaning circuit during step d') can be an acid. This embodiment can be advantageous for achieving more effective cleaning than cleaning with a single cleaning agent. The cleaning frequency can therefore be increased.

[0146] The cleaning process may preferably include, subsequently after step g), the steps of:

[0147] (h) resume water treatment in the unit's water treatment system; and

[0148] i) replace the water that was used in steps b) and f), and possibly f), by injecting water obtained from the outlet of the treatment system into the water storage tanks. It is understood that, if the subsea water treatment system includes several filtration membranes, a cleaning process can be implemented for each of said membranes, each cleaning process preferably being adapted to each membrane. Furthermore, each cleaning process can be carried out at different frequencies.

[0149] One of the advantages of the cleaning process according to the invention is that it can be fully automated. Each step of the process can be triggered without direct human intervention, by electronic systems. This is particularly advantageous since the underwater water treatment unit is located in deep water. Any direct intervention requires either the use of underwater robots or bringing the unit to the surface, which in either case represents a lengthy, delicate, and costly operation. In one embodiment, the process can be remotely controlled by an operator, preferably stationed on the surface, and the information exchanged between the operator and the electronic system can be transmitted via a cable (for example, an umbilical) or electronically (for example, by radio waves).According to another embodiment, the electronic system can be controlled by a pre-recorded program that does not require operator intervention. Furthermore, the underwater water treatment unit according to the invention may include one or more other means for reducing, or even eliminating, clogging of the filtration membrane. These means may be selected from the group consisting of a recirculation circuit connecting the treated water discharge line to the line upstream of the filtration membrane, a backwash circuit, and one or more biocide tanks, at least one of these tanks being arranged to allow the injection of biocide into the line connecting the water intake and the filter, preferably near the water intake.

[0150] A recycling circuit connecting the treated water discharge pipe to the pipe upstream of the filtration membrane allows some of the filtrate collected at the membrane outlet to be returned to the water flow intended to pass through the filtration membrane. This recycling system dilutes the water flow intended to pass through the filtration membrane, which on the one hand increases the flow rate of the incoming water, and on the other hand improves the quality of the incoming water flow, thus facilitating its filtration.

[0151] Backwashing is a well-known method for cleaning filters. It involves circulating a flow of water in the opposite direction to the normal flow of water through the filter. This counter-current water can carry away particles of varying sizes that clog the filter. A backwashing step might include stopping the normal flow of water into the filter membrane, introducing a flow of water into the subsea water treatment system via a second intake, directing this flow to the filter membrane face (initially on the downstream side), recovering a flow upstream of the filter membrane, and discharging this flow into the environment through a second discharge pipe.Alternatively, when the subsea water treatment system includes several parallel filtration membranes, the backwashing step may consist of bringing the entire flow of water entering the subsea water treatment system to a single filtration membrane, recovering at least a portion of the filtrate from that filter and conveying that filtrate to the downstream face of a second filtration membrane located in parallel with the first via the normal filtrate discharge line of that second filtration membrane, then recovering that flow upstream of said filtration membrane and discharging said flow into the environment via a retentate discharge line of the second filtration membrane.

[0152] Injecting biocide into the water flow within the underwater water treatment system according to the invention reduces biofouling. Well-known biocides for this application include, for example, DBNPA (dibromo-nitrolo-propionamide) and isothiazoline.

[0153] Advantageously, the invention makes it possible to reduce the quantities of biocide used for cleaning membranes in prior art processes, or even to eliminate their use altogether. Other features and advantages of the present invention will become apparent from the description of particular embodiments shown in the figures. Figure 1 schematically represents one embodiment of the underwater water treatment unit according to the invention.

[0154] The underwater water treatment unit 1 comprises an underwater water treatment system 2, itself comprising a filtration membrane 3, a water intake 4 adapted to bring in a flow of water taken from the environment, a pipe 5 connecting the water intake 4 to the filtration membrane 3, a discharge pipe 6 and a feed pump 7, which is disposed on the pipe 5 connecting the water intake 4 to the filtration membrane 3.

[0155] The underwater water treatment unit 1 also includes a means 8 for supplying the underwater unit with energy, in particular to power the feed pump 7, and the cleaning pump 10.

[0156] In addition, the underwater water treatment unit 1 includes a hydraulic cleaning circuit 9, and a cleaning pump 10 arranged on the hydraulic cleaning circuit 9. The hydraulic cleaning circuit 9 forms a loop which includes the filtration membrane 3. The hydraulic circuit also includes a discharge pipe 11.

[0157] The underwater water treatment unit 1 also includes two water storage tanks 12 and 13 and a cleaning product storage tank 14. Each tank is connected to the hydraulic cleaning circuit 9.

[0158] Finally, in the embodiment shown in Figure 1, only the filtration membrane 3 and the water storage tank 13 are equipped with heating means 15 and 16.

[0159] Figure 2 is a schematic representation of one embodiment of the cleaning means of a unit according to the invention.

[0160] This figure shows two filtration membranes 20 and 21, membrane 20 can be an ultrafiltration membrane, and membrane 21 can be a nanofiltration membrane.

[0161] The hydraulic cleaning circuit 22 forms two loops: a first loop including the filter membrane 20, and a second loop including the filter membrane 21. The opening and closing of the fluid circulation in these two loops is controlled by valves 23 and 24. When it is necessary to clean the filter membrane 20, valve 23 is opened and the valve 24. Conversely, when you want to clean the filter membrane 21, you open valve 24 and close valve 23.

[0162] Each membrane is equipped with a heating element 25 and 26.

[0163] A cleaning pump 27 is disposed on the hydraulic cleaning circuit 22. In addition, the hydraulic cleaning circuit 22 includes a valve 28 which allows control of the flow circulation in the hydraulic cleaning circuit 22, and a discharge line 29, which has a valve 29'.

[0164] The cleaning equipment also includes three 30-liter water tanks,

[0165] 31 and 32, and two cleaning product tanks 33 and 34. Each tank is connected to the hydraulic cleaning circuit 22. Valves 35 and 36 are located at the inlet of the cleaning product tanks 33 and 34, to control the injection of cleaning product into the hydraulic cleaning circuit 22. The water storage tanks 30, 31, and 32 are equipped with valve pairs 37a / 37b, 38a / 38b, and 39a / 39b. These valve pairs advantageously control the injection of water into the hydraulic cleaning circuit 22, and also allow the filling of tanks 30, 31, and

[0166] 32 by a flow of water from pipe 40. Preferably, the water arriving via pipe 40 is water obtained from the outlet of the water treatment system. The filling of the tanks can be controlled by valve 41 and by valve 42.

[0167] Water storage tanks 31 and 32 are equipped with heating means 43 and 44.

[0168] As an example, a cleaning method according to the invention of the filtration membrane 20 is described below.

[0169] To clean the filter membrane 20, valve 23 is opened and valve 24 is closed. The filter membrane 20 is isolated from the water treatment system. Valves 28 and 42 are opened, while the drain valve 29' is closed. The hydraulic cleaning circuit 22 then forms a loop, which includes the filter membrane 20. In addition, valves 35, 36, 37a / 37b, 38a / 38b, and 39a / 39b are closed, as well as valve 41.

[0170] Water from reservoir 30 is injected into the hydraulic cleaning circuit 22 by opening valve 37b. The water is at ambient outside temperature. This water is circulated in the hydraulic cleaning circuit 22 using the cleaning pump 27.

[0171] The heating means 25 is switched on. This will raise the temperature of the filtration membrane 20 and of the water circulating in the hydraulic cleaning circuit 22 to a temperature between 15°C and 40°C.

[0172] When this temperature is reached, a cleaning product contained in the storage tank 33 is injected into the hydraulic cleaning circuit 22 by opening the valve 35.

[0173] When the membrane cleaning is sufficient, the contents of the hydraulic cleaning circuit 22 are evacuated via the evacuation line 29 by opening the valve 29'. Once the evacuation step is complete, the valve 29' is closed.

[0174] Next, water from tank 31 is injected back into the hydraulic cleaning circuit by opening valve 38b. The water in tank 31 is at the temperature of the filter membrane 20, i.e., between 15°C and 40°C. This temperature was reached by heating the tank using the heating device 43. This water is then circulated through the hydraulic cleaning circuit 22 by the cleaning pump 27.

[0175] When the rinsing of the filter membrane 20 is complete, the contents of the hydraulic cleaning circuit 22 are evacuated again via the discharge line 29 by opening the valve 29'. Once the evacuation step is finished, the valve 29' is closed.

[0176] Once the cleaning process is complete, water treatment can resume in the water treatment system using the filtration membrane 20. Furthermore, the used water can optionally be replaced by injecting water from the treatment system outlet into the water storage tanks 30 and 31 via pipe 40. This is done by opening valves 37a, 38a, and 41, and keeping valves 37b, 38b, and 42 closed. As an example, a second cleaning process for the filtration membrane 21 according to the invention is described below.

[0177] To clean the filter membrane 21, valve 24 is opened and valve 23 is closed. The filter membrane 21 is isolated from the treatment system of water. Valves 28 and 42 are open, while the drain valve 29' is closed. The hydraulic cleaning circuit 22 then forms a loop, which includes the filter membrane 21. In addition, valves 35, 36, 37a / 37b, 38a / 38b and 39a / 39b are closed, as well as valve 41.

[0178] Water from reservoir 30 is injected into the hydraulic cleaning circuit 22 by opening valve 37b. The water is at ambient outdoor temperature. This water is circulated through the hydraulic cleaning circuit 22 by means of the cleaning pump 27.

[0179] The heating means 26 is switched on. This will raise the temperature of the filtration membrane 21 and of the water circulating in the hydraulic cleaning circuit 22 to a temperature between 15°C and 40°C.

[0180] When this temperature is reached, a cleaning product contained in the storage tank 34 is injected into the hydraulic cleaning circuit 22 by opening the valve 36.

[0181] When the membrane cleaning is sufficient, the contents of the hydraulic cleaning circuit 22 are evacuated via the discharge line 29 by opening the valve 29'. Once the evacuation step is complete, the valve 29' is closed.

[0182] Next, water from tank 31 is injected back into the hydraulic cleaning circuit by opening valve 38b. The water in tank 31 is at the temperature of the filter membrane 21, i.e., between 15°C and 40°C. This temperature was reached by heating tank 31 using the heating device 43. This water is then circulated through the hydraulic cleaning circuit 22 by the cleaning pump 27.

[0183] When the rinsing of the filter membrane 21 is completed, the cleaning product contained in the storage tank 33 is injected into the hydraulic cleaning circuit 22 by opening the valve 35.

[0184] When the membrane cleaning is sufficient, the contents of the hydraulic cleaning circuit 22 are evacuated via the discharge line 29 by opening the valve 29'. Once the evacuation step is complete, the valve 29' is closed.

[0185] Next, water from tank 32 is injected back into the hydraulic cleaning circuit by opening valve 39b. The water in tank 32 is at the temperature of the filter membrane 21, i.e., between 15°C and 40°C. This temperature was reached by heating the tank using the medium heating 44. This water is always circulated in the hydraulic cleaning circuit 22 using the cleaning pump 27.

[0186] When the rinsing of the filter membrane 21 is complete, the contents of the hydraulic cleaning circuit 22 are evacuated again via the discharge line 29 by opening the valve 29'. Once the evacuation step is finished, the valve 29' is closed.

[0187] With the cleaning process now complete, water treatment can resume in the water treatment system using the filtration membrane 21. Furthermore, the used water is replaced by injecting water from the treatment system outlet into the water storage tanks 30, 31, and 32 via pipe 40. This is done by opening valves 37a, 38a, 39a, and 41, and keeping valves 37b, 38b, 39b, and 42 closed.

Claims

DEMANDS 1. Submarine water treatment unit adapted for injection into a hydrocarbon reservoir comprising: - a means of supplying the underwater unit with energy; - a subsea water treatment system comprising at least one filtration membrane, a water intake suitable for bringing into the subsea unit a flow of water taken from the environment of the system, a pipe connecting said water intake and said filtration membrane, a treated water discharge pipe, and at least one feed pump suitable for carrying out the water filtration; - a hydraulic circuit for cleaning said membrane, forming a loop between the inlet and outlet of said membrane, and having a drain pipe; - at least two water storage tanks, each water tank being connected to the cleaning circuit; - at least one storage tank for a cleaning product chosen from an acid and a base, the cleaning product tank being connected to the cleaning circuit; - at least one suitable cleaning pump to move fluids in the cleaning circuit; said filtration membrane being equipped with at least one heating means, and at least one of the water tanks being equipped with at least one tank heating means.

2. A subsea unit according to claim 1, characterized in that the filtration membrane is a microfiltration membrane, an ultrafiltration membrane, a nanofiltration membrane, or a reverse osmosis membrane.

3. A subsea unit according to claim 2, characterized in that the water treatment system comprises a nanofiltration membrane and an ultrafiltration membrane located upstream of said nanofiltration membrane, the hydraulic cleaning circuit forming: - a loop between the inlet and outlet of the nanofiltration membrane, or - a loop between the inlet and outlet of the ultrafiltration membrane, or - two loops between the inlet and outlet of the nanofiltration membrane on one hand and between the inlet and outlet of the ultrafiltration membrane on the other hand, with valves located on the loops to allow selection of the membrane whose cleaning is desired.

4. Submarine unit according to any one of claims 1 to 3, characterized in that it comprises at least two storage tanks for cleaning product, at least one of the tanks being a storage tank for an acid and at least one of the tanks being a storage tank for a base, the acid and base tanks being connected to the cleaning circuit.

5. Submarine unit according to any one of claims 1 to 4, characterized in that it comprises at least three water storage tanks, at least two of the water tanks being equipped with at least one tank heating means.

6. Submarine unit according to any one of claims 1 to 5, characterized in that the feed pump and the cleaning pump are mounted on the same shaft.

7. Submarine unit according to any one of claims 1 to 6, characterized in that it further comprises a discharge conduit enabling the contents of the hydraulic cleaning circuit to be discharged into the water storage tank(s).

8. A method for cleaning the filtration membrane of a subsea unit as defined in any one of claims 1 to 7, comprising the steps of: a) interrupt the flow of water into said filtration membrane; b) inject water from one of the water storage tanks into the hydraulic cleaning circuit, and circulate this water in a loop through the hydraulic cleaning circuit using the cleaning pump; c) heat the filter membrane using the heating means to a temperature between 15°C and 40°C; d) inject a cleaning product contained in at least one storage tank into the hydraulic cleaning circuit; e) evacuate the contents of the hydraulic cleaning circuit via the drain pipe; f) inject water from one of the water storage tanks back into the hydraulic cleaning circuit, and circulate this water in a loop through the hydraulic cleaning circuit; g) drain the contents of the hydraulic cleaning circuit again via the drain line.

9. Cleaning method according to claim 8, characterized in that the cleaning product injected into the hydraulic cleaning circuit during step d) is an acid.

10. A cleaning method according to either of claims 8 or 9, characterized in that it comprises, between step f) and step g), the steps of: d) inject into the hydraulic cleaning circuit a cleaning product different from the product injected in step d), said product being contained in at least one storage tank; e') evacuate the contents of the hydraulic cleaning circuit via the drain pipe; and f) inject water from one of the water storage tanks back into the hydraulic cleaning circuit, and circulate this water in a loop through the hydraulic cleaning circuit.

11. Cleaning method according to claim 10, characterized in that the cleaning product injected into the hydraulic cleaning circuit during step d) is a base and the cleaning product injected into the hydraulic cleaning circuit during step d') is an acid.

12. A cleaning method according to any one of claims 8 to 11, characterized in that it subsequently comprises the steps of: (h) resume water treatment in the unit's water treatment system; and i) replace the water that was used in steps b) and f), and possibly f), by injecting water obtained from the outlet of the treatment system into the water storage tanks.

13. A cleaning method according to any one of claims 8 to 12, characterized in that the water injected into the hydraulic cleaning circuit during step f), and optionally during step f), is heated to a temperature between 15°C and 40°C by means of the heating means for the water storage tank(s).

14. A cleaning method according to any one of claims 8 to 13, characterized in that the underwater water treatment unit is located underwater at a depth of at least 500 meters.

15. Cleaning method according to any one of claims 8 to 14, characterized in that the contents of the hydraulic cleaning circuit are evacuated into the water storage tank(s) emptied following step b).

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