OA17814A - Method for injecting fluids into an underwater facility. - Google Patents

Abstract

The invention relates to a method for injecting fluid into an underwater facility. A fluidstorage system comprising at least one storage vessel with rigid walls containing a functional fluid is lowered to the bottom of the water. At least one portion of the functional fluid is injected into the underwater facility. After a time of operation, the fluid-storage system, or a portion of the latter, including the storage vessel, is raised back up to the surface.

Description

The invention relates to techniques for storing and injecting functional fluids into underwater facilities.

Technological background

These techniques can in particular be used to inject chemicals, such as for example demulsifiers, biocides, corrosion inhibitors, hydrate formation inhibitors, deposit inhibitors or hydraulic control fluids on offshore sites for the exploitation of hydrocarbon deposits.

In the case of an underwater exploitation of a deposit of hydrocarbons the chemicals are transported to the well heads and other underwater equipment by umbilical ducts from a platform, a ship located on the surface or a facility located on land.

Due to the substantial depth of certain underwater deposits, which can reach depths exceeding 3000 métrés, and where these facilities sometimes comprise satellite sites distributed over a wide surface area, the transport of functional fluids by means of umbilical ducts can generate substantial manufacturing and maintenance costs due to the lengths and the sections of the umbilicals. Indeed, an umbilical duct generally comprises ail at once a cable system for the electrical power of the underwater facility, ducts for conveying hydraulic control fluids, ducts for conveying chemicals, and also cables for communication, control and supplying the devices located in deep water.

In addition to substantial production and maintenance costs, such umbilical ducts are subjected to the environmental constraints of the bottom of the sea, for example thermal constraints and pressure constraints, which requires regular monitoring.

The umbilical ducts must furthermore be connected to the facility, and require the presence of a platform or of a ship on the surface in order to provide the injecting and the receiving of fluids. Moreover, the storage, on a ship, of chemicals such as methanol, demulsifiers, biocides or products that prevent corrosion or the formation of deposits in the ducts of the facility, hâve a non-negligible environmental risk.

Alternatives to umbilical ducts are therefore sought to convey fluids, in particular chemicals, in an underwater facility.

Devices and methods for extracting and storing hydrocarbons using underwater facilities hâve been proposed. For example, FR 2 776274 Al and DE 25 38419 Al propose i

METHOD FOR INJECTING FLUID INTO AN UNDERWATER FACILITY

Technical field

The invention relates to techniques for storing and injecting functional fluids into underwater facilities.

Technological background

These techniques can in particular be used to inject chemicals, such as for example demulsifiers, biocides, corrosion inhibitors, hydrate formation inhibitors, deposit inhibitors or hydraulic control fluids on offshore sites for the exploitation of hydrocarbon deposits.

In the case of an underwater exploitation of a deposit of hydrocarbons the chemicals are transported to the well heads and other underwater equipment by umbilical ducts from a platform, a ship located on the surface or a facility located on land.

Due to the substantial depth of certain underwater deposits, which can reach depths exceeding 3000 métrés, and where these facilities sometimes comprise satellite sites distributed over a wide surface area, the transport of functional fluids by means of umbilical ducts can generate substantial manufacturing and maintenance costs due to the lengths and the sections of the umbilicals. Indeed, an umbilical duct generally comprises ail at once a cable system for the electrical power of the underwater facility, ducts for conveying hydraulic control fluids, ducts for conveying chemicals, and also cables for communication, control and supplying the devices located in deep water.

In addition to substantial production and maintenance costs, such umbilical ducts are subjected to the environmental constraints of the bottom of the sea, for example thermal constraints and pressure constraints, which requires regular monitoring.

The umbilical ducts must furthermore be connected to the facility, and require the presence of a platform or of a ship on the surface in order to provide the injecting and the receiving of fluids. Moreover, the storage, on a ship, of chemicals such as methanol, demulsifiers, biocides or products that prevent corrosion or the formation of deposits in the ducts of the facility, hâve a non-negligible environmental risk.

Alternatives to umbilical ducts are therefore sought to convey fluids, in particular chemicals, in an underwater facility.

Devices and methods for extracting and storing hydrocarbons using underwater facilities hâve been proposed. For example, FR 2 776274 Al and DE 25 38419 Al propose underwater storage vessels comprising a déformable réservoir made of elastomer located at the bottom of the water, connected to an underwater facility on the one hand, and to a ship located on the surface using an umbilical on the other hand. Document US 2012/0085276 Al also proposes an underwater storage vessel with déformable walls located at the bottom of the water. These documents however do not propose any method making it possible to avoid an umbilical duct connected to the surface in order to convey functional fluids to the underwater facility, and do not propose any solution allowing for the safe storage of these fluids at the bottom of the water for an exploitation duration of several months. No method allowing for the safe and controlled injection of these fluids is proposed.

Consequently, a method is sought that allows for the effective and safe storing and injecting of functional fluids, such as chemicals, into underwater facilities using vessels located at the bottom of the water.

Disclosure of the invention

In order to overcome the problems mentioned hereinabove, this invention proposes a method for injecting fluid into an underwater facility, with the method comprising:

- lowering a fluid-storage system at the bottom of the water, with the fluid-storage system comprising at least one storage vessel with rigid walls containing a functional fluid; and

- injecting at least a portion of the functional fluid into the underwater facility; and

- raising back up to the surface at least one portion of the fluid-storage system, the portion comprising the storage vessel.

This method has in particular the advantage of not requiring an umbilical duct and platform or ship on the surface in order to inject a functional fluid. This functional fluid is stored in the vicinity of the facility in a vessel with rigid walls maintained at the bottom of the water during the entire duration of injection of the fluid. The method is particularly advantageous for the exploitation of facilities that hâve many satellite fields, wherein the lengths of umbilical ducts required for the connection of the various well heads can reach many tens of km, substantially increasing the operating costs.

The method also makes it possible to maintain a fluid-storage vessel for a period of several months at the bottom of the water, with the fluid-storage system comprising vessels with rigid walls, withstanding better the constraints of the bottom of the sea and the constraints of chemical compatibility with the functional fluids than vessels made of elastomer.

The invention can also comprise a plurality of fluid-storage vessels containing the same functional fluid. Using at least two storage vessels containing the same functional fluid, namely a first vessel and a second vessel, allows for a continuons operation of the facility. Indeed, by injecting the functional fluid only of a single fluid-storage vessel at a time, when the functional fluid of the first vessel is depleted, the additional vessel can constitute a reserve of functional fluid that ensures the continuons injection of the fluid into the facility while the first vessel is being filled again.

The lowering and the raising of the storage system can advantageously be carried out by means of a crâne or any other device for hoisting from a platform or a ship located on the surface.

The expression functional fluid can designate a gas as well as a liquid, for example hydrocarbons or a chemical such as a demulsifier, a biocide, a corrosion inhibitor, a hydrate formation inhibitor, a deposit inhibitor or a hydraulic control fluid.

Advantageously, the storage vessel can comprise a partition forming a mobile piston that séparâtes the function fluid from the sea environment, and the injecting of the functional fluid can comprise a translation of the partition.

By maintaining only a simple mobile partition between the sea environment and the functional fluid, the method object of the invention can hâve the advantage of being able to be carried out in a hydrostatic configuration. Indeed, when the mobile partition is stopped, the pressure in the portion of the vessel containing the functional fluid can be equal to the pressure in the sea environment. In this way, it is possible to manufacture a fluid-storage vessel comprising lighter rigid walls, which do not require a résistance to the pressure différences greater than a few thousand hPa.

According to an advantageous embodiment, the storage system can comprise a pump connected to the storage vessel by a first duct and to said underwater facility by a second duct, and the injecting of the functional fluid can comprise the putting into operation of the pump.

Using a pump to cause the displacement of the mobile partition of the storage vessel can make it possible to overcome a mechanical actuator exerting a pressure on the mobile partition of the fluid-storage vessel. The pump causes the displacement of the

s.

mobile partition and the circulation of the functional fluid by creating a pressure différence between the sea environment and the functional fluid.

Using a pump makes it possible to separate from the vessel the element that actuates the displacement of the piston of the fluid-storage vessel. In particular, it is possible to provide on the storage system several independent modules connected to each other. As such, a module can comprise the pumps, while another module can comprise the fluid-storage vessels. The pumps hâve an operating duration that exceeds that of the cycle of use of the fluid-storage vessels. They can remain at the bottom of the water for more than a year, while the fluid-storage vessels are advantageously raised back up to the surface when ail of the functional fluid that they contain has been depleted, for example after a few months.

Using pumps can add an additional refining in the implémentation of the method. In particular, the storage system can comprise an accumulation vessel connected to the pump and to the underwater facility, and can comprise at least one valve on a portion of the third duct connecting the accumulation vessel to the underwater facility. Injecting the functional fluid can comprise:

- filling the accumulation vessel with a volume of functional fluid; and

- injecting a portion of the volume into the underwater facility by opening the valve.

In this way, it is possible to more finely control the quantity of functional fluid injected into the underwater facility, in particular when the quantifies to be injected are less than the volumes injected by the actuating of the pumps. In this embodiment, the accumulation vessel can be used as an intermediate réservoir, maintained at a slightly higher pressure, typically greater by a millibar to around ten bars, than the pressure of the underwater facility, and of which the contents can be injected into the underwater facility using a précisé control of the opening and of the closing of a valve.

According to an embodiment of the invention, the injecting of the functional fluid can take place over a duration between one month and one year before the raising back to the surface of said portion of the storage system.

A duration greater than one month has a substantial advantage in terms of the cost of maintaining and replacing used vessels. Indeed, if the fluid-storage system and the vessels that it contains are able to withstand the constraints présent at the bottom of the water, the intervention of a platform or of a ship on the surface in order to raise the fluidstorage vessels or other modules of the system, can be carried out at a lesser frequency.

The injection of functional fluid can be carried out continuously at a rate of a few tens of litres per hour, for chemicals such as for example demulsifiers, corrosion inhibitors, hydrate formation inhibitors, deposit inhibitors. However, certain other products may not be injected continuously, such as biocides which can be injected sporadically at a rate, for example, of about two hundred litres per hour, five hours per week.

Advantageously, a system for measuring the functional fluid contained in the storage vessel can be included.

In this way, it is possible to know the volume of functional fluid présent in the fluid-storage vessel which makes it possible to anticipate a replacement or a filling of the latter. It is as such also possible to provide a control of the quantity of functional fluid injected into the underwater facility.

According to an embodiment of the invention, the fluid-storage system can comprise at least one replenishing duct of the storage vessel, and the method can further comprise:

- lowering a replenishing vessel containing functional fluid at the level of the replenishing duct, and

- filling the storage vessel with the functional fluid of the replenishing vessel, and

- raising the replenishing vessel back up to the surface.

In this way, it may no longer be necessary to raise the fluid-storage vessels back up to the surface. Using a replenishing vessel makes it possible to lighten the structure to be manipulated from a platform or a ship for filling the fluid-storage vessel.

According to an embodiment, the fluid-storage system can comprise at least one replenishing umbilical connected to the storage vessel on the one hand, and to a réservoir of functional fluid on the surface on the other hand, with the replenishing umbilical carrying out a filling of the storage vessel.

This embodiment makes it possible to maintain permanent control over the quantity of functional fluid contained in the storage vessel. A filling of the storage vessel from the surface makes it possible to maintain the latter longer at the bottom of the water, and to raise it back up to the surface only for maintenance operations.

Description of the figures

The method object of the invention, will be better understood when reading the following description of example embodiments presented for the purposes of information, in no way limiting, and when observing the drawings hereinafter wherein:

- figure 1 shows as a diagrammatical perspective an underwater storage system in the vicinity of a well for the production of hydrocarbons in deep water; and

- figure 2 diagrammatically shows as a top view a system for storing and for injecting fluid according to an embodiment; and

- figure 3 diagrammatically shows as a top view a module comprising a vessel for storing and injecting fluid; and

- figure 4 diagrammatically shows as a cross section a vessel for storing and injecting fluid; and

- figure 5 diagrammatically shows a front view of a portion of a system for storing and for injecting fluid according to an embodiment.

For reasons of clarity, the dimensions of the various éléments shown in these figures are not necessarily in proportion with their actual dimensions. On the figures, identical references correspond to identical éléments.

Detailed description

According to the invention, a method for injecting fluid into an underwater facility comprises a first step, consisting in lowering a system for storing and for injecting fluid at the bottom of the water. As the objective of the method is to inject a functional fluid, the system for storing and injecting comprises at least one storage vessel filled, initially, with a functional fluid. This fluid-storage vessel is connected by the intermediary of a duct to a facility. Typically, this facility consists of a hydrocarbon exploitation well head. However other facilities can benefit from the invention presented here.

Once the system has been lowered and installed at the bottom of the water, an exploitation phase can take place, during which the functional fluid contained in the storage vessel is injected, continuously or intermittently, into the facility. Various means can be implemented to inject the functional fluid. It can in particular be considered to use a pump, connecting the fluid-storage vessel to the facility.

Figure 1 shows an example of an underwater storage system in the vicinity of a hydrocarbon production well in deep water. A well head 100 located at the bottom of the water is supplied by ducts 6 with functional fluid coming from a system for storing and injecting fluids 1. This system for storing and injecting fluids 1 contains among other things storage vessels 21. The extracted hydrocarbons can typically be conveyed to the surface from the well head 100 by a duct 106.

Once the injecting of the functional fluid has been carried out, the injecting is no longer necessary or when the fluid-storage vessel has been emptied of its contents, a final step consists in raising back up to the surface the storage system or a portion of the latter that at least includes the fluid-storage vessel.

The various éléments that participate in the implémentation of this method of injecting functional fluid shall now be described in more detail.

In a particular embodiment of the invention shown in figure 2, the system for storing and injecting fluids 1 comprises several modules 11. In this example, eight modules 11 are distinguished each comprising a fluid-storage vessel 21-24, and four modules each comprising two pumps 5.

The arrangement of these various modules 11 can differ from the one shown in figure 2, in particular in terms of bringing the pumps 5 doser to the fluid-storage vessels 21-24, or for adapting to the configuration of the bottom of the sea in the vicinity of the facility 100. It can also occur that a module 11 contains only one pump 5.

An example of a module 11 is shown in figure 3, showing a storage vessel 21 provided with a connecting element 70 for connecting to a duct. The module 11 comprises attaching means 14 for attaching to a hoisting device. These attaching means 14 can for example hâve the form of openings made in a métal framework, intended to receive a hook, a snap hook or any other system for attaching, for example to the cable of a crâne or to a winch.

Advantageously, the system for storing and injecting 1 comprises at least two fluid-storage vessels for the same type of functional fluid 30, in such a way as to be able to replace an empty storage vessel without having to interrupt the injection of said fluid into the facility 100.

First ducts 7 connecting the fluid-storage vessels 21-24 to the pumps 5 are provided. The connection to these first ducts 7 is carried out by techniques that are well known, and can for example comprise the use of a remote-controlled robot.

Second ducts 6 are provided to connect the pumps 5 to the underwater facility 100. The lengths of the first 7 and second 6 ducts are reduced in such a way as to reduce the risks of damage to these ducts 6, 7. These ducts 6 and 7 can be either of the rigid type, or of the flexible type.

As shown in figure 2, a module 11 containing two pumps 5 comprises a collecter 260 provided on a framework of the storage system 1. The second duct 6 extends from the collecter 260 to the underwater facility 100. The connection between the pump 5 and the collecter 260 is carried out by means of an intermediate duct 60. This intermediate duct 60 can be either of the rigid type, or of the flexible type.

In this way, the storage system 1 makes it possible to reduce the direct interaction with the éléments of the underwater facility 100. The utilisation of an intermediate duct 60 coupled to a collecter 260 makes it possible to raise a defective pump 5 back up to the surface for maintenance without having to disconnect the second duct 6 from the facility 100. As shown in figure 2, it is possible to provide an accumulation vessel 8, connected to the second duct 6 on the one hand, and to a third duct 16 on the other hand, of a smaller section, connecting the accumulation vessel 8 to the facility 100. A valve 9, for example a solenoid valve, is placed on the third duct 16.

In this way, with the valve 9 being closed, the pump 5 can in a first step fîll the accumulation vessel 8. This accumulation vessel 8 is able to withstand a high extemal pressure, for example a pressure exceeding one hundred thousand hPa. Then, the controlling of the flow rate of the functional fluid 30 injected into the facility 100 can be carried out by controlling the opening of the valve 9.

Injecting the functional fluid 30 is ordinarily carried out by actuating a pump 5, which then créâtes a vacuum in the functional fluid 30 of the fluid-storage vessel 21-24 to which the pump 5 is connected. However, the flow rate of the functional fluid 30 injected is difficult to control and primarily dépends on the power and on the operating mode of the pump 5. For applications that require greater finesse in the controlling of the flow rate of the injected functional fluid 30, the adding of the accumulation vessel 8 described hereinabove authorises the injection of functional fluid 30 with a low flow rate.

When the level of functional fluid 30 contained in a fluid-storage vessel 21-24 reaches a critical level or when the fluid is depleted, three options are considered for replacing the functional fluid that is missing in the storage vessel in question.

A first possibility consists in raising the empty fluid-storage vessel 21-24 back up to the surface using a crâne or any other hoisting device provided on a platform or a ship.

To do this, the attaching means 14 are arranged on the four corners of a removable module of the system 1 as shown in figures 2 and 3.

In this way, it is possible to ensure a stability of the system for storing and injecting 1 during the manipulation by the hoisting device.

A second possibility consists in using a replenishing vessel 270 lowered by a hoisting device such as, for example, a crâne from a platform or a ship. For small volumes, this replenishing vessel 270 can also be fixed to a remote-controlled robot. This replenishing vessel 270, can contain a volume of functional fluid less than that of a fluidstorage vessel 21-24, and can be used to at least partially fill an empty storage vessel. As shown in figure 2, the replenishing vessel advantageously comprises a system for attaching 271 to a container 211 provided on the storage system 1. The system for attaching 271 nests advantageously into the container 211, and ducts, not shown, make it possible to transfer the content from the replenishing vessel 270 to the fluid-storage vessel 21-24. It is possible to provide a container 211 on each module that comprises a fluidstorage vessel 21-24.

Such a means for filling a fluid-storage vessel without raising it can be advantageous when the storage vessel in question is the only one présent in the storage system 1. It can also hâve the advantage of not requiring a disconnecting of the first ducts

7.

A third possibility consists in deploying, from a replenishing ship, a replenishing umbilical that is connected to the container 211. The replenishing with product is then carried out by directly transferring the product from the replenishing ship to the at least partially empty fluid-storage vessel 21-24 via the replenishing umbilical. This method allows for the transfer of large volumes of products. The replenishing umbilical has either a line dedicated to each product, or a single line shared by ail of the products. In this latter case, this line can be cleared and cleaned between the replenishing of each product.

Figure 4 shows an example of a fluid-storage vessel 20 containing a functional fluid 30. Various shapes can be considered for the storage vessel 20, with a cylindrical shape being particularly advantageous. Such as shown in figure 4, the fluid-storage vessel 20 has an upper surface 202 that has openings 203. This upper surface 202 allows the sea environment 3 to penetrate into the storage vessel 20 up to a mobile partition 40. The presence of such a perforated surface 202 intégral with the fluid-storage vessel 20 makes it possible in particular to facilitate the handling thereof by limiting the movement of the mobile partition 40, and facilitâtes the installation of a sensor 204 intended to measure the volume of functional fluid 30 présent in the fluid-storage vessel 20.

The sensor 204 can advantageously be a device of the sonar type, that measures the position of the mobile partition 40 in the fluid-storage vessel 20, as such making it possible to deduce from it the remaining volume of functional fluid 30.

Moreover, the surface 202 comprises fastening means 201 for fastening to a framework of the system for storing and injecting 1. These means can for example be openings or notches provided to nest with éléments with a complementary shape on the storage system 1.

The mobile partition 40 advantageously has a height that is sufficient to guarantee a translation with no risk of inclination of the partition 40 in the storage vessel 20. As an example, a height of fifty centimètres satisfies this criterion. The height of the mobile partition 40 can be smaller. For example, when the storage vessel has a cylindrical shape, the mobile partition 40 has less risk of undergoing an over-centring by sliding in the vessel. A height of less than forty centimètres for the mobile partition 40 can then be provided.

The partition 40 comprises at least one guiding system 41. These guiding Systems 41 hâve a shape that matches that of the transversal section of the fluid-storage vessel 20. In figure 4, the guiding system 41 shown is a seal, in contact with the wall of the storage vessel 20.

The guiding system 41 makes it possible to favour a straight displacement of the partition 40 in the storage vessel 20.

The mobile partition 40 comprises at least one seal 42, made of a material that is compatible with the nature of the functional fluid 30 stored in the vessel 20. This seal can fulfil the rôle of the guiding system.

This seal 42 prevents the functional fluid 30 from mixing with the sea environment 3 despite the displacement of the partition 40. That way, it provides the seal of the storage vessel 20, and effectively séparâtes the sea environment 3 from the functional fluid 30.

The fluid-storage vessel 20 shown in figure 4 opérâtes in a quasi-hydrostatic configuration, i.e. the pressure in the functional fluid 30 is equal to or in the neighbourhood of that of the sea environment 3. The différence in pressure between the two environments does not exceed, for example, a thousand hPa.

As shown in figure 4, the fluid-storage vessel 20 comprises, in its lower portion, an opening and an element for connecting 70. Such as shown in figure 5, the connector 70 is intended to be connected to a duct 71. By placing such an opening in the lower portion of the fluid-storage vessel 20, the functional fluid 30 can escape from the storage vessel 20 via the element for connecting 70 until the mobile partition 40 abuts against the lower surface of the fluid-storage vessel 20.

Figure 5 diagrammatically shows a front view of an embodiment of the system for storing and for injecting fluid 1, comprising in particular a module 11 comprising a rigid ffamework provided with means for fastening 14 to a device for hoisting that makes it possible to raise at least one portion of the system 1. In this diagrammatical représentation, the system comprises two rows of fluid-storage vessels 20, with the latter being provided each with means for fastening 201 between them and to the ffamework of the storage system 1. The module 11 of figure 4 comprises two removable storage vessels 20.

The ffamework of the module 11 is configured in order to facilitate the insertion of the fluid-storage vessels 20 on the storage system 1 and to provide the stability of the storage vessels 20 in the module 11.

The module 11 comprises on its ffamework the ducts 71, required for the transfers of fluid, connected to a dispatcher 710.

This dispatcher 710 makes it possible to switch between injecting the functional fluid 30 ffom a replenishing vessel 270 by the container 211, and injecting the functional fluid 30 ffom a storage vessel 20 to the facility 100.

The invention is not limited to the embodiments presented hereinabove as examples and can comprise other équivalent embodiments.

For example, the storage system 1 may comprise only a single fluid-storage vessel 20 for a given type of functional fluid 30. The ffamework 11 can also be an intégral part of such a fluid-storage vessel 20.

Although they hâve been described as providing a function of injecting functional fluid 30, the fluid-storage vessels 20 can also be considered for the purposes of recovering fluid in order to store it then raise it back up to the surface. Alternatively, the storage vessels can be maintained at the bottom of the water in the event of an interruption in the exploitation of a deposit, in order to later be reused quickly when exploitation résumés.

The shape of the fluid-storage vessels may not be cylindrical, in particular for the purpose of optimising the storage of the vessels, optimising the use of the space within a module 11 and facilitating the installation of the storage vessels at the bottom of the water.

The Systems for guiding and sealing described hereinbefore can hâve forms other than seals. Other means can make it possible to provide the fonctions of guiding the sliding mobile partition into the storage vessel and maintaining a seal between the functional fluid of the storage vessel and the sea environment. For example a métal ring 5 with a diameter slightly less than the diameter of the vessel can be provided. Several castors or pads, for example three or four castors, distributed along the edge of the piston can also folfil these fonctions.

Claims (8)

1. L Method for injecting fluid into an underwater facility (100), the method comprising:

   - lowering a fluid-storage vessel (1) to the bottom of the water, the fluid-storage system comprising at least one storage vessel (20-24) with rigid walls containing a functional fluid (30); and

   - injecting at least a portion of the functional fluid into the underwater facility (100); and

   - raising at least one portion of the fluid-storage system back up to the surface, said portion including the at least one storage vessel.
2. 2. Method according to claim 1, wherein the at least one storage vessel (20-24) comprises a partition (40) forming a mobile piston that séparâtes the functional fluid (30) from the sea environment (3), and wherein the injecting of the functional fluid comprises a translation of said partition (40).
3. 3. Method according to any of one of claims 1 or 2, wherein the storage system (1) comprises a pump (5) connected to the at least one storage vessel (20-24) by a first duct (7) and to said underwater facility (100) by a second duct (6), the method comprising:

   - putting the pump into operation in order to inject at least a portion of the functional fluid into the underwater facility.
4. 4. Method according to claim 3, wherein the fluid-storage system (1) comprises an accumulation vessel (8) connected to the pump (5) and to the underwater facility (100), and comprises at least one valve (9) over a portion of the third duct (16) connecting the accumulation vessel to the underwater facility, and wherein injecting the functional fluid comprises:

   - filling the accumulation vessel (8) with a volume of functional fluid (30); and

   - injecting a portion of said volume into the underwater facility (100) by opening said valve (9).
5. 5. Method as claimed in any one of the preceding claims, wherein injecting of the functional fluid takes place over a duration between one month and one year before raising said portion of the storage system back to the surface.
6. 6. Method as claimed in any one of the preceding claims, characterised in that injecting of the functional fluid comprises:

   - measuring a volume of functional fluid (30) contained in the at least one storage vessel (20-24).
7. 7. Method as claimed in any one of the preceding claims, wherein the fluid-storage system (1) comprises at least one replenishing duct of the at least one storage vessel (20-24), with the method further comprising:

   - lowering a replenishing vessel (270) containing functional fluid (30) at the level of the replenishing duct;

   - filling the at least one storage vessel (20-24) with the functional fluid of the replenishing vessel; and

   - raising the replenishing vessel back up to the surface.
8. 8. Method as claimed in any one of the preceding claims, wherein the fluid-storage system (1) comprises at least one replenishing umbilical connected to the at least one storage vessel (20-24) on the one hand, and to a réservoir of functional fluid on the surface on the other hand, with the method comprising:

   - filling the storage vessel by means of the replenishing umbilical.