Method, apparatus and system for producing electric power by using the abandoned extraction wells of an offshore plant for producing hydrocarbons or derivatives thereof

Abstract

Method for producing electric power by using the abandoned extraction wells (4) of an offshore plant (3) for producing hydrocarbons or derivatives thereof comprising : a) having an offshore plant (3) for producing hydrocarbons or derivatives thereof comprising: at least one surface platform (5) located above an at least locally depleted underwater deposit (G) of hydrocarbons; at least one extraction well (4) abandoned due to depletion of the deposit (G) and which is formed therein and connected to the platform (5) with a tubular pipeline (6) covering the walls and extending continuously between it and the platform (5), wherein the tubular pipeline (6) has a lower end (6a) housed in the well (4) and provided with openings (7) which place the internal volume thereof in fluid communication with the deposit (G), an upper end (6b) open and coupled to the platform (5) and at least one inlet port formed at an underwater level for letting water from the aqueous layer (M) surrounding it into it; wherein there is a first minimum water level within the tubular pipeline (6), the inlet port being closed; b) while keeping the inlet port closed, sliding a floating body (8) along the tubular pipeline (6) falling by the action of its own weight with a descending rectilinear motion from an upper dead centre, close to the upper end (6b), to a lower dead centre close to the lower end (6a), transforming the descending rectilinear motion of the floating body (8) in a rotary motion for driving an electric power generator (12), as long as the floating body (8), having entered the water, receives a buoyancy force equal to its weight which stops the descending rectilinear motion thereof at the lower dead centre; c) opening the inlet port and allowing water to enter the tubular pipeline (6) filling it as long as the buoyancy force acting on the floating body (8) brings the latter towards the upper end (6b) with an ascending rectilinear motion; d) when the floating body (8) reaches a level corresponding to the upper dead centre, stopping the entry of water in the tubular pipeline (6) by closing the inlet port, thus stopping the ascending movement of the floating body (8), and blocking the latter in the upper dead centre thereof; e) allowing the water introduced during step d) and present in the tubular pipeline (6) to be absorbed by the deposit (G) until a minimum m-th water level is reached; f) reiterating said steps b) to e) 1 times, with i>l.

Description

[0001] METHOD, APPARATUS AND SYSTEM FOR PRODUCING ELECTRIC

[0002] POWER BY USING THE ABANDONED EXTRACTION WELLS OF AN

[0003] OFFSHORE PLANT FOR PRODUCING HYDROCARBONS OR DERIVATIVES

[0004] THEREOF

[0005] The present invention refers to a method, an apparatus ad a system for producing electric power by using the abandoned extraction wells of an offshore plant for producing hydrocarbons or derivatives thereof .

[0006] The present invention also refers to a method for converting an at least partially abandoned offshore plant for producing hydrocarbons or derivatives thereof in a plant for producing electric power .

[0007] The present invention concerns the field of the clean electric power production in the offshore sector, using the already "depleted" (exhausted) extraction wells located in offshore production platforms in the

[0008] "Oil & Gas" and / or Hydrogen field.

[0009] It is known that the activity of extracting raw materials in fluid state from the subsoil can cause the phenomenon known as "subsidence", meaning thereby the movement of the continental shelf or of the seabed which, as a result of the weight of the sediments that accumulate on it, tends to lower . The decrease in pressure that, following extractive activities, is generated in the geological layer of a deposit causes or accelerates such a phenomenon . Figure 6 is a diagram showing this "subsidence" phenomenon in the event of a depleted deposit : the first box A shows an initial situation before an activity of extraction of fluids from a deposit, the second box B shows the extraction activity and the third box C shows the subsidence phenomenon generated or in any case accelerated by the depletion of the deposit at the end of the extraction activity where a slippage and an overlap of the seabed can be noted.

[0010] It is also known that at the end of the operational life of offshore plants for producing hydrocarbons due to depletion of the related deposit, the problem of reclamation, dismantling and scrapping of the related structures arises .

[0011] To date, it is known that in the case of offshore plants, certain structures thereof, including in particular the so-called "casing" of the extraction wells, are not dismantled and removed, as it is not possible or in any case convenient to proceed in this regard. They therefore remain in a marine environment without use .

[0012] At present, systems for energy storage by using the force of gravity and systems for energy storage by using the buoyancy force are known ,

[0013] Thus, for example, the publication "Underwater

[0014] Energy Storage - Emphasis on Buoyancy Technique" (Kyle

[0015] Patrick Basset University of Windsor Canada

[0016] 08.03.2017) reports the study of a system for energy storage by using the buoyancy force (BBES Buoyancy Battery Energy Storage) . In tthhiiss ccaassee,, the energy is stored by dragging large floats underwater through the use of winches operated by an external force; in case of energy demand, the floats dragged and blocked underwater are allowed to rise to the surface doing useful work on the winch .

[0017] Systems aass for example described in the article "Energy Vault - energy storage made of concrete blocks and cranes" by the company Energy Vault are also known which provide for the storage of potential energy for the subsequent production of energy by using the force of gravity . In this case, weights consisting of concrete blocks are lifted and retained at a certain lifting level by using cranes operated by using excess energy produced from renewable sources . The potential energy thus stored is used in case of energy demand by letting the concrete blocks fall and the kinetic energy generated by the fall of these blocks is transformed into electric power .

[0018] In both cases, the energy consumed to drag the floats underwater or to lift the weights is significantly greater than the energy stored and that then actually produced. In other words, such known systems entail considerable energy expenditure which adversely affects their efficiency . Nevertheless, these known systems are positively acknowledged, as the need ttoo find energy storage systems is always felt . In particular, the need is felt to find energy storage systems with improved efficiency and reduced environmental impact .

[0019] In light of the above, aim of the present invention is to overcome on the one hand the problems of subsidence and on the other hand the problems of reclamation, dismantling and scrapping of the structures of an offshore plant for producing hydrocarbons or derivatives thereof resulting from the decommissioning of one or more of its "depleted" extraction wells due to depletion of the deposit, using them as an "alternative source" for the production of clean electric power .

[0020] A further aim of the present invention is therefore to provide a method, an apparatus and a system for producing electric power in a particularly efficient manner and with reduced environmental impact .

[0021] Another aim of the present invention is to realize a particularly simple aanndd functional apparatus and system for producing electric power with low costs .

[0022] These aims according to the present invention are achieved by realizing a method, an apparatus and a system for producing electric power by using the abandoned extraction wells of an offshore plant for producing hydrocarbons or derivatives thereof as set forth in the independent claims .

[0023] These aims are also achieved with a method for converting an at least partially abandoned offshore plant for producing hydrocarbons into a plant for producing electric power as set forth in claim 14. Further features are provided in the dependent claims .

[0024] The characteristics and the advantages of a method, an apparatus and a system for producing electric power by using the abandoned extraction wells of an offshore plant for producing hydrocarbons or derivatives thereof according to the present invention will become more apparent from the following exemplary and non-limiting description, referred ttoo the accompanying schematic drawings in which :

[0025] Figure 1 schematically represents an assembly view of a system for producing electric power according to the present invention;

[0026] Figure 2 shows oonn an enlarged scale a detail of Figure 1 relating to the surface components;

[0027] Figure 3 shows oonn an enlarged scale a detail of Figure 1 relating to underwater components;

[0028] Figure 4 schematically shows a floating body; Figure 4A is a detail on an enlarged scale of Figure

[0029] 4 ;

[0030] Figure 5 schematically shows a system for producing electric power according to the present invention;

[0031] Figure 6 is an illustrative diagram of the phenomenon of subsidence (prior art) .

[0032] With reference ttoo the accompanying figures, a system for producing electric power has been indicated with 1 , comprising an apparatus 2 for producing electric power installed on an offshore plant 3 for producing hydrocarbons or derivatives thereof comprising at least one extraction well 4 (hereinafter also referred to as

[0033] "well 4" and shown in Figure 5) abandoned due to depletion of the deposit G .

[0034] Hydrocarbons are understood to be hydrocarbons in the fluid state, in particular in the gaseous state such as, for example, methane gas .

[0035] The plant 3 comprises : at least one surface platform 5 located above an at least locally exhausted underwater deposit G of hydrocarbons, in the jargon of the sector also "depleted"; and

[0036] - at least one well 4 abandoned due to depletion of the deposit G .

[0037] The platform 5 is for example supported by pylons 50 anchored to the seabed F .

[0038] The well 4 is formed in the deposit G and is connected to the platform 5 by means of a respective tubular pipeline 6 that ccoovveerrss the walls thereof and extends continuously from the well 4 to the platform 5 .

[0039] The tubular pipeline 6 has :

[0040] - a lower end 6a housed in the well 4 and provided with openings 7 or holes which place the internal volume of the tubular pipeline 6 in communication with the deposit

[0041] G,

[0042] - an open upper end 6b leading at the platform 5 to which it is coupled.

[0043] The tubular pipeline 6, as is known, is formed by a plurality of tubular sections of variable diameter (decreasing aass the distance from the platform 5 increases) - known in the jargon of the field as "casing"

[0044] - which are installed during the drilling step of the well 4 and which, at the end of the exploitation of the deposit G, are left in place, since their recovery is difficult, if not impossible .

[0045] The tubular pipeline 6 thus connects the deposit G with the surface, reaching up to the platform 5 and, with specific reference to the schematized embodiment, to the bridge of the platform 5.

[0046] The openings 7 or holes formed at the lower end 6a of the tubular pipeline 6 place the deposit G in fluid communication with the volume inside the tubular pipeline 6.

[0047] The apparatus 2 comprises for each abandoned well 4 a respective electric power generation unit 20 which, in turn, comprises : a floating body 8 housed within the tubular pipeline

[0048] 6 slidingly along it in rectilinear motion in both sliding directions;

[0049] - at least one flexible body 9 having one end anchored to the floating body 8 and the opposite end fixed to a winch 10 about which the flexible body 9 can be wound; the winch 10 is in turn composed of a rotating drum and its own supporting structure and is anchored on a load- bearing structure 11 arranged and anchored to the platform 5; an electric power generator 12 that is coupled to the winch 10 with a coupling adapted to transmit the rotational motion from the winch 10 to the generator 12 and also arranged on the load-bearing structure 11; at least one shut-off valve 13 coupled to at least one water inlet port formed in the tubular pipeline 6 at an underwater level .

[0050] In the accompanying figures, the aqueous layer M that extends above the seabed F in which the deposit G is present has been schematized.

[0051] For the purposes of the present invention, water is intended to mean the water forming the aqueous layer M that extends above the seabed F in which the deposit G is present at which the offshore plant 3 is present . Generally it is seawater, which will be referred to for simplicity' s sake in the rest of the description, being evident to a person skilled in the art that it may in any case be wwaatteerr from the reservoir in which the offshore plant 3 is located.

[0052] The floating body 8 is configured and dimensioned to have a buoyancy force greater than its own weight .

[0053] The floating body 8 consists of a longitudinally developed body having a longitudinal axis A-A arranged coaxial to the central axis of the tubular pipeline 6.

[0054] The floating body 8 consists of an elongated cylindrical body closed at the opposite ends which is configured and dimensioned so as to be able to slide without interference inside the tubular pipeline 6 for the entire length thereof ,

[0055] Means for guiding the rectilinear motion of the floating body 8 internally to the tubular pipeline 6 are advantageously provided. These guide means are configured to keep the floating body 8 coaxial to the tubular pipeline 6. In the exemplified embodiment, said guide means comprise a plurality of rolling bodies 14, for example rollers, which are supported in a rotatable manner about respective rotation axes 14a on the side wall of the floating body 8. The rotation axes 14a belong to planes orthogonal to the longitudinal axis A-A of the floating body 8. The rolling bodies 14 are distributed between them spaced along different radial directions of a same cross section of the floating body 8 to form a respective series or battery . Advantageously, two or more series oorr batteries of rolling bodies 14 are distributed spaced apart from each other along the length of the floating body 8 .

[0056] The flexible body 9 may consist of a rope, a cable, a chain or the like . It has one end fixed to the winch

[0057] 10 and the opposite end anchored, hooked or otherwise fixed to the floating body 8, advantageously near the upper end thereof .

[0058] The winch 10 or hoist comprises, in a known manner, its own supporting ssttrruuccttuurree that support as drum rotatable about its own axis (rotation axis) and about which the flexible body 9 can be wound. The winch 10 as a whole is supported by the load-bearing structure 11 of the apparatus 2 .

[0059] The winch 10 is preferably coupled to a respective brake (not depicted) operable between at least a first position wherein it blocks the rotation of the winch 10 (i . e . drum) and a second position wherein it leaves the rotation of the winch 10 (i . e . drum) free . The winch 10 is coupled to a generator 12 by means of a coupling adapted to transmit the rotational motion from said winch 10 to said generator 12 or rather to the rotor of said generator 12. Such coupling can for example consists of a speed reducer, for example a gear reducer, and be realized in a plurality of different ways immediately understandable to the person skilled in the art .

[0060] The generator 12 consists of an alternator adapted to transform the mechanical energy provided to the winch 10 by the descending rectilinear motion of the floating body 8 into electric power . The generator 12 or alternator is in itself of a type known to the person skilled in the art and therefore not described in detail .

[0061] There are then one or more return pulleys 15 of the flexible body 9 along a respective sliding path that extends between the winch 10 and the tubular pipeline 6.

[0062] The pulleys 15 are supported by a frame 16 arranged and anchored on the platform 5 above the tubular pipeline 6.

[0063] The pulleys 15 are arranged to deflect the flexible body 9 during unwinding / winding on the respective winch 10 along a section of path coaxial to the corresponding tubular pipeline 6.

[0064] The frame 16 and the load-bearing structure 11 can be made separate from each other or in a single unit .

[0065] If the offshore plant 3 comprises a plurality of wells 4 abandoned due to depletion of the deposit G, the apparatus 2 comprises for oonnee or mmoorree of them, advantageously for each of them, a respective electric power generation unit 20 as shown in Figures 1 and 2.

[0066] In such a case, the load-bearing structures 11 of two or more electric power generation units 20 may be realized as sections or parts of a single load-bearing structure 111 . Similarly, the frames 16 of two or more power generation units 20 may be made as sections or parts of a single frame 160.

[0067] The underwater shut-off valve 13 is in itself of a type known to the person skilled in the art and therefore not described or depicted in detail . It can be actuated between at least a closed position wherein it isolates the internal volume of the tubular pipeline 6 with respect to the aqueous layer M that surrounds it, preventing the entry of water into the latter, and at least an open position wherein it places the internal volume of the tubular pipeline 6 in fluid communication with the aqueous layer M that surrounds it, allowing the entry of water (seawater) into the latter . One or more underwater shut-off valves 13 may be provided for each tubular pipeline 6.

[0068] As is known, when a hydrocarbon extraction well is put iinnttoo p prroodduuccttiioonn,, the so-called operation for "opening the levels" is carried out, during which the deposit G is placed in fluid communication with the interior of the extraction well by means of a tubular pipeline 6 of the type generically described above .

[0069] At the end of the productive life of a hydrocarbon extraction well, in particular of hydrocarbons in the gaseous state (e . g . methane gas) , the layer pressure (i . e . the pressure in the deposit G) is particularly low so that, by introducing water (in an offshore plant 3 seawater) inside it, the deposit G starts absorbing the gas and water present in the extraction well itself or rather in the respective tubular pipeline ("casing") . This happens because the hydrostatic pressure of the water column weighing in the extraction well is much greater than the pressure present in the deposit G . By stopping introducing wwaatteerr (e . g. seawater) , an equilibrium situation is reached when the water inside the extraction well m ather the relative tubular pipeline ("casing") reaches a level (charge) whose hydrostatic pressure equals the pressure of the deposit

[0070] G . The remaining part of the extraction well (i . e . the part of the respective tubular pipeline or "casing" above the water level / charge) remains therefore full of air, thus making it possible to lleett a body fall in the extraction well, i . e . along the respective tubular pipeline ((""casing"")) and produce energy by using the potential energy of that body .

[0071] This observation underpins the method for producing electric power by using the abandoned extraction wells 4 of an offshore plant 3 for producing hydrocarbons comprising the steps of : a) having an offshore plant 3 as described above, wherein inside the tubular pipeline 6 there is a first minimum water level, the respective seawater inlet port being closed; b) while keeping said inlet port closed, sliding a floating body 8 along the tubular pipeline 6 falling by the action of its own weight with a descending rectilinear motion from an upper dead centre, close to the upper end 6b of the tubular pipeline 6 to a lower dead centre close to the lower end 6a thereof, transforming the descending rectilinear motion of the floating body 8 in a rotary motion for driving an electric power generator 12, aass lloonngg aass the floating body 8, having entered the water, receives a buoyancy force equal to its weight which stops the descending rectilinear motion thereof at the lower dead centre; c) opening the water inlet port (seawater) and allowing water enter the tubular pipeline 6 through the inlet port thus opened, filling the tubular pipeline 6 as long as the buoyancy force acting on the floating body 8 brings said floating body 8 towards the upper end 6b of the tubular pipeline 6 with an ascending rectilinear motion; d) when the floating body 8 reaches a level corresponding to the upper dead centre thereof, stopping the entry of water (seawater) in the tubular pipeline 6 by closing the respective inlet port, thus stopping the ascending movement of the floating body 88,, and blocking the floating body 8 at the upper dead centre thereof; e) allowing the water introduced during step d) and present in the tubular pipeline 6 to be absorbed by the deposit G until a minimum water level is reached; f ) reiterating the steps b) to e) i times, with i>1.

[0072] Steps b) , c) , d) and e) are performed in succession one after the other .

[0073] The reiteration f) one or more times of steps b) to e) occurs if necessary when the production of electric power is required.

[0074] Step b) corresponds to a step of descent and production of electric power .

[0075] Steps c) and d) altogether correspond to a step of ascent and stop for the storage of energy (potential energy) .

[0076] Step e) corresponds to a step of emptying the well 4 or the respective tubular pipeline 6 thereof . During step b) , the descending rectilinear motion of the floating body 8 is controlled for it to take place at substantially constant speed and is used to generate electric power .

[0077] The upper dead centre at which the floating body 8 is blocked at the end of its ascending travel during steps c-d) is defined by the level at which the respective seawater inlet port is present along the tubular pipeline 6.

[0078] During the ascent step c) the flow rate of seawater entering the tubular pipeline 6 is greater than the flow rate of water simultaneously absorbed by the deposit G through the openings 7, the respective inlet port being configured, dimensioned and adjusted for this purpose .

[0079] If the offshore plant 3 comprises a plurality of abandoned extraction wells 4, the steps from b) to e) may b bee implemented for one or more of them, advantageously for each of them.

[0080] Such a method can be implemented with a system 1 as described above .

[0081] Steps b) , c) , d) and e) are detailed below with reference to a method for producing electric power implemented with a system 1 as described above with reference to a single well 4 .

[0082] Step b) of descent and production of electric power .

[0083] The level (charge) of the seawater present in the well 4 or rather in the respective tubular pipeline 6 is at a minimum, consequently substantially the entire internal volume of the tubular pipeline 6 is filled with air . The floating body 8 is located at the upper dead centre thereof which defines the starting point of its descending travel and is retained in that position (storage position) by a mechanical brake installed on the winch 10.

[0084] By releasing this brake, the floating body 8, as a result of its own weight, begins to slide falling along the tubular pipeline 6 with a descending rectilinear motion, during which the flexible body 9 unwinds from the winch 10 placing it in rotation .

[0085] The resistant torque generated by the generator 12 coupled to the winch 10 limits the acceleration of the floating body 8 by advancing it towards its lower dead centre at substantially constant speed.

[0086] During the entire descending travel of the floating body 8 in the tubular pipeline 6, the generator 12 produces electric power, its rotor being placed in rotation by the winch 10.

[0087] The descending rectilinear motion of the floating body 8 is controlled by the rolling bodies 14 which ensure the centring thereof within the tubular pipeline 6.

[0088] The floating body 8 slows down its speed once it enters the water in the tubular pipeline 6 (minimum level / charge) and stops once its weight is equal to the buoyancy force .

[0089] Steps c) and d) of ascent and stop (energy storage) .

[0090] The ascent step begins with the opening of the underwater shut-off valve 13 that allows the entry of water of the aqueous layer that surrounds it (seawater) into the tubular pipeline 6.

[0091] The interior of the tubular pipeline 6 is filled with seawater, the flow rate of seawater at the inlet being greater than that absorbed by the deposit G through the openings 7 . The shut-off valve 13 and the respective inlet port are configured, dimensioned and adjusted for this purpose .

[0092] As a result of the buoyancy force, the floating body 8 performs an ascending rectilinear motion towards the platform 5, being configured and dimensioned to have a buoyancy force greater than its own weight .

[0093] When the floating body 8 (i . e . the upper end thereof) arrives at the shut-off valve 13 (which defines the upper dead centre of its descending travel) , the mechanical brake coupled to the winch 10 is engaged and the shut-off valve 13 is closed ssoo aass to stop the ascending rectilinear motion of the floating body 8 and block it at the upper dead centre thereof, storing energy for a subsequent step b) of descent and production of electric power .

[0094] Step e) of emptying the well .

[0095] As a result of the pressure of the water column present in the tubular pipeline 6 at the end of the steps of ascent and stop c) and d) , the deposit G continues to absorb water and the well 4 or rather the respective tubular pipeline 6 is emptied until the water present in it reaches a ni-th minimum water level .

[0096] It is thus possible to repeat a new cycle (steps b) -e) ) .

[0097] The present invention also relates to a method for converting an abandoned offshore plant 3 for producing hydrocarbons into a plant for producing electric power, wherein the offshore plant 3 for producing hydrocarbons comprises :

[0098] - at least one surface platform 5 located above an at least locally depleted underwater deposit G of hydrocarbons;

[0099] - at least one extraction well 4 abandoned due to depletion of the deposit G and which is formed in the deposit G and connected to the platform 5 by means of a tubular pipeline 6 covering the walls thereof and extending continuously from the well 4 to the platform 5, wherein the tubular pipeline 6 has a lower end 6a housed in the well 4 and provided with openings 7 which place the internal volume thereof in fluid communication with the deposit G and an upper end 6b coupled to the platform 5; wherein the converting method comprises : disconnecting the tubular pipeline 6 from the extraction equipment of the offshore plant 3 so that the upper end thereof 6b is open; forming in the tubular pipeline 6 an inlet port for letting water of the aqueous layer surrounding it into it, said inlet port being formed at an underwater level, and coupling the inlet port thus formed to at least one respective underwater shut-off valve 13;

[0100] - arranging on the platform 5 at least one loadbearing structure 11 on which at least one winch 10 and at least one electric power generator 12 rotatably coupled to the winch 10 are to be mounted, wherein the winch 10 (or rather the respective drum) is rotatably supported about its own rotation axis and is fixed to an end of at least one respective flexible body 9 which is windable about it; anchoring at least one floating body 8 to the free end of the flexible body 9;

[0101] - arranging the floating body 8 within the tubular pipeline 6 such that it can slide along it in rectilinear motion in both sliding directions .

[0102] A brake as described above is coupled to the winch 10.

[0103] On the platform 5 there are then arranged pulleys

[0104] 15 rotatably supported by a respective supporting frame

[0105] 16 and arranged to guide the flexible body 9 along a winding / unwinding path on / from the winch 10 having at least one section coaxial to the tubular pipeline 6.

[0106] The method, the apparatus and the system according to the present invention allow to store energy and to produce electric power on demand by using both the force of gravity and the buoyancy force ooff the same body (floating body) .

[0107] The method, the apparatus and the system according to the present invention allow to store energy and to produce electric power on demand with a high efficiency, combining the exploitation of the force of gravity and of the buoyancy force of the same body without the substantial expenditures and energy consumptions that are instead typical of the systems according to the known technique described above .

[0108] The method, the apparatus and the system according to the present invention also allow to limit the phenomenon of subsidence thanks to the induced absorption of water by the depleted deposit and to use the existing structures of abandoned offshore plants, allowing ttoo produce electric power with reduced environmental impact .

[0109] The apparatus and the system for producing electric power by using the abandoned extraction wells of an offshore plant for producing hydrocarbons or derivatives thereof thus conceived aarree susceptible to numerous modifications and variants, aallll falling within the invention; moreover, all the details can be replaced by technically equivalent elements . In practice, the materials used, as well as their dimensions, can be of any type according to the technical requirements .

Claims

CLAIMS1 ) Method for producing electric power by using the abandoned extraction wells (4 ) of an offshore plant (3) for producing hydrocarbons or derivatives thereof comprising : a) having an ooffffsshhoorree plant (3) for producing hydrocarbons or derivatives thereof comprising :- at least one surface platform (5) located above an at least locally depleted underwater deposit (G) of hydrocarbons ;- at least one extraction well (4 ) abandoned due to depletion of said deposit (G) and which is formed in said deposit (G) and connected to said platform (5) by means of a tubular pipeline (( 66)) covering the walls thereof and extending continuously from said well (4 ) to said platform (5) , wherein said tubular pipeline ( 6) has a lower end (6a) housed in said well (4) and provided with openings (7 ) which place the internal volume of said tubular pipeline ( 6) in fluid communication with said deposit (G) , an upper end ( 6b) open and coupled to said platform (5) and at least one inlet port which is formed therein at an underwater level for letting water of the aqueous layer (M) surrounding it in said tubular pipeline ( 6) ; wherein there is a first minimum water level within said tubular pipeline ( 6) , said at least one inlet port being closed; b) while keeping said at least one inlet port closed, sliding a floating body (8) along said tubular pipeline ( 6) falling by the action of its own weight with a descending rectilinear motion from an upper dead centre, close to said upper end ( 6b) , to a lower dead centre close to said lower end ( 6a) , transforming the descendingrectilinear motion of said floating body (8) in a rotary motion for driving an electric power generator (12) , as long as said floating body (8) , having entered the water, receives a buoyancy force equal to its weight which stops said descending rectilinear motion thereof at said lower dead centre; c) opening said at least one inlet port and allowing water to enter said tubular pipeline ( 6) through said at least one inlet port thus opened, filling said tubular pipeline ( 6) with water as long as the buoyancy force acting on said floating body (8) brings said floating body (8) towards said upper end ( 6b) with an ascending rectilinear motion; d) wwhheenn ssaaiidd floating body (8) reaches a level corresponding to said upper dead centre, stopping the entry of water of said aqueous layer (M) in said tubular pipeline ( 6) by closing said at least one inlet port, thus stopping the ascending movement of said floating body (8) , and blocking said floating body (8 ) in said upper dead centre thereof; e) allowing the water introduced during said step d) and present in said tubular pipeline ( 6) to be absorbed by said deposit ((GG)) until a minimum n±-th water level is reached; f) reiterating said steps b) to e) i times, with i>1.2 ) Method for producing electric power according to claim 1 , wherein said descending rectilinear motion is controlled to occur at substantially constant speed.3) Method for producing electric power according to claim 1 or 2, wherein said upper dead centre is defined by the level at which there is said at least one inlet port .4 ) Method for producing electric power according to anyone of the preceding claims, wherein during said step c) the flow rate of seawater entering said tubular pipeline ( 6) is greater than the flow rate of water simultaneously absorbed by said deposit (G) .5) Method for producing electric power according to any one of the preceding claims, wherein said floating body(8) is configured to have a buoyancy force greater than its own weight .6) Method for producing electric power according to one or more of the preceding claims, wherein said offshore plant (3) comprises a plurality of said extraction wells(4 ) abandoned due to depletion of said deposit (G) , wherein said steps b) ttoo ff )) are performed for one or more of said wells (4 ) .7 ) Apparatus (2) for implementing a method for producing electric power according to one or more of the preceding claims by using the abandoned extraction wells (4) of an offshore plant (3) for producing hydrocarbons or derivatives thereof comprising :- at least one surface platform (5) located above an at least locally depleted underwater deposit (G) of hydrocarbons or derivatives thereof;- at least one extraction well (4) abandoned due to depletion of said deposit (G) and which is formed in said deposit (G) and connected to said platform (5) by means of a tubular pipeline ( 6) covering the walls thereof and extending continuously from said well (4) to said platform (5) , wherein said tubular pipeline (6) has a lower end ( 6a) housed in said well (4 ) and provided with openings ((77 )) which place the internal volume of said tubular pipeline ( 6) in fluid communication with said deposit (G) , an upper end ( 6b) open and coupled tosaid platform (5) and at least one inlet port which is formed therein at an underwater level; wherein said apparatus (2 ) comprises for said at least one abandoned extraction well (4) a respective electric power generation unit (20) comprising : a floating body (8) which can be housed within said tubular pipeline ( 6) slidingly along it in rectilinear motion in both directions; at least one flexible body ( 9) having an end anchored to said floating body (8) and the opposite end fixed to a winch (10) about which it can be winded, wherein said winch (10) comprises a drum which is rotatably supported about its own rotation axis by its own supporting structure and is mounted on a load-bearing structure (11 ) anchorable on said platform (5) ; an electric power generator (12 ) coupled to said winch (10) and mounted on said load-bearing structure( 11 ) ; at least one shut-off valve (13) couplable to said at least one inlet port .8 ) Apparatus (2 ) for producing electric power according to claim 7, characterised in that said electric power generation unit (20) comprises a brake coupled to said winch (10) and operable between aatt least a first position, wherein it blocks the rotation of said winch ( 10) , and a second position, wherein it leaves the rotation of said winch (10) free .9) Apparatus (2 ) for producing electric power according to claim 7 oorr 8, characterised in that said floating body (8) is configured to have a buoyancy force greater than its own weight .10) Apparatus (2) for producing electric power accordingto any one of claims 7 to 10, characterised in that it comprises means for guiding said rectilinear motion of said floating body ( 8 ) in said tubular pipeline (6) .11) Apparatus (2) for producing electric power according to claim 10, characterised in that said guiding means comprise a plurality of rolling bodies (14) which are mounted on the outer surface of said floating body (8) in a rotatable manner about respective rotation axes(14a) .12) Apparatus (2) for producing electric power according to one or more of claims 7 to 11 , characterised in that it comprises a plurality ooff ssaaiidd electric power generation units (20) , each of which is associable with a respective extraction well (4) of said offshore plant (3) abandoned due to depletion of said deposit (G) .13) System (1 ) for producing electric power comprising an apparatus (2) according to one or more of claims 7 to12 installed on said offshore plant ((33)) for producing hydrocarbons or derivatives thereof .14) Method for converting an at least partially abandoned offshore plant (3) for producing hydrocarbons or derivatives thereof iinn a system (1) for producing electric power, wwhheerreeiinn said offshore plant (3) for producing hydrocarbons or derivatives thereof comprises :- at least one surface platform (5) located above an at least locally depleted underwater deposit (G) of hydrocarbons;- at least one extraction well (4) abandoned due to depletion of said deposit (G) and which is formed in said deposit (G) and connected to said platform (5) by means of a tubular pipeline (( 66)) covering the walls thereof and extending continuously from said well (4) tosaid platform (5) , wherein said tubular pipeline (6) has a lower end (6a) housed in said well (4 ) and provided with openings (7) , which place the internal volume of said tubular pipeline ( 6) in fluid communication with said deposit (G) , and an upper end ( 6b) coupled to said platform (5) ; wherein said converting method comprises : disconnecting said at least one tubular pipeline ( 6) from the extraction equipment so that said upper end thereof (6b) is open; forming in said at least one tubular pipeline ( 6) and at an underwater level at least one inlet port for letting water of the aqueous layer (M) surrounding it in said at least one tubular pipeline ( 6) and coupling at least one respective underwater shut-off valve (13) to said at least one inlet port; arranging on said platform (5) at least one loadbearing structure (11 ) on which said at least one winch ( 10) and at least oonnee electric power generator ( 12) rotatably coupled to said winch (10) are to be mounted, wherein said winch (10) comprises a drum which is rotatably supported about its own rotation axis to its own supporting structure and is fixed to an end of at least one respective flexible body ( 9) which is windable about it ; anchoring said at least one floating body (8) to the free end of said at least one flexible body ( 9) ; arranging said at least one floating body (8) within said at least one tubular pipeline ( 6) such that it can slide along it in rectilinear motion in both sliding directions .15) Converting method according to claim 14, whereinsaid at least one winch (10) is coupled to a brake operable between at least a first position, wherein it blocks the rotation of said at least one winch ( 10) , and a second position, wherein it leaves the rotation of said at least one winch (10) free .

Images