GB2548334A

GB2548334A - Electrochemical reduction of metallic structures

Info

Publication number: GB2548334A
Application number: GB1603977.8A
Authority: GB
Inventors: Leonard Clive Ludlow Jeremy
Original assignee: 3-Sci Ltd
Current assignee: 3-Sci Ltd
Priority date: 2016-03-08
Filing date: 2016-03-08
Publication date: 2017-09-20
Anticipated expiration: 2036-03-08
Also published as: GB2548334B; GB201603977D0

Abstract

A method is described for cutting, or removal through dissolution, metal structures 1 in a conducting fluid 2 by connecting the metal structure, by means of a remote electrode 5, to the positive terminal 3 of a DC power supply 4 and immersing in close proximity to the metal, immersed in said conducting fluid, a cathode 7, connected to the negative terminal 6 of said power supply. The metal structure may be iron, steel, steel alloy, or other metallic alloy. The method is suitable for dismantling oil and gas installations and structures above and below the sea bed, or other metallic structures that are immersed or can be forced to be immersed in an ionic fluid. The method can be applied to bulk, large volume, dissolution or preferential smaller volume dissolution of the metal structure. A non-conducting element 8 may be provided within the structure and is disposed so as to be between the two electrodes and thereby act as in the manner of a barrier. In circumstances of a subsea marine structure where the structure is disposed in sea water which is a conducive fluid.

Description

Electrochemical Reduction of Metallic Structures

The invention relates to a method of disposal, reduction or removal of metal forming a part of a metal structure. The present invention is particularly, but not exclusively, applicable to techniques for the removal of metal from the surface of a metal structure, such as a subsea marine structure, located or buried in, or below, the sea bed.

In the past, operators of marine installations, for example offshore oil and gas installations and associated infrastructure such as well casings, have not been required to consider details, such as costs, engineering difficulties and environmental implications of the disposal of the installations when they have reached the end of useful life. Currently, and in the future, practice in the long-term abandonments of platforms and wells, including environmental considerations and the elimination of risks such as hydrocarbon leakages, will be more strictly enforced by obligatory guidelines and legislation. Therefore, safe, environmentally-acceptable dismantling of such installations and infrastructures will be required. Occasionally, such structures are very difficult to access and remove, such is the case, for example, of steel pipes sitting below the surface in abandoned wells. Often such pipes, in the form of well ‘casings’, are cemented in place and it is desired that they be partially removed at certain locations in order for the well to be effectively sealed against the rock formations by deposition of large quantities of cement.

Existing methods of disposal and dismantling of subsea structures and infrastructure often involve explosives, mechanical, abrasive, thermal or aggressive chemical methods to sever the metallic structures prior to removal. In general, these methods serve to enable cutting and subsequent removal of large volumes of the structure to land for further dismantling. All methods are considered to have advantages and disadvantages, both technically, commercially and environmentally. Explosives pose dangers in transportation and, in the open ocean, pose a threat to marine life. Aggressive chemicals can cause harm to the wider environment and their release in the open ocean is strictly controlled. Direct mechanical and abrasive cutting require delivery of complex hydraulic or machinery services to the cutting site and cannot be used very easily in locations that are difficult to access such as in deep well bores. As yet, no fully satisfactory method has been proposed to enable cutting of structures. such as well casings sitting deep within well bores below the sea bed, in such a way that allows an optimum engineering and environmental outcome.

It is known to preferentially remove metal as a surface finishing technique (US 4,483,755) or by electrochemical machining (US 3,909,388; US 4,098,659; US 4,287,033; US 4,614,571). In such surface finishing processes, the intention is to remove, for example, production marks, from the surface or to create high-tolerance shapes with tightly controlled dimensions. The amount of metal removed is often minimal and the process is usually required to ensure that the item conforms to exacting production tolerances, or for aesthetic reasons. Such techniques are not applicable to the bulk removal of a large structures, or the disposal thereof, as they would prove prohibitively time-consuming, costly and, because they often require the use of specialised chemicals which would be environmentally unacceptable.

In previously considered techniques which involve the electrochemical cutting of subsea structures (for example, as disclosed in IBC Conference Decommissioning and removal of offshore structures, held London 19-20 April 1989, paper 11 [7 p., 10 ref., 10 fig.]), small holes (typically only 7cms in diameter) have been created in steel structures through combining electrochemical cutting with mechanical abrasion. These techniques have included the application of a direct electrical contact to the steel structure as well as a mechanical grinding action, supplied by hydraulic forces and supply lines, in order to accelerate the cutting rates.

It is also known to protect marine structures, especially those constructed of steel, by coupling to a more active material, such as a zinc, or aluminium alloy which becomes a sacrificial metal that corrodes instead of the protected metal. A method known as impressed current cathodic protection utilises an external DC power source, whereby the structure to be protected from corrosion is made a cathode by connection to the negative terminal of the power supply and an inert anode is connected to the positive terminal of the supply.

By reversing the polarity and the direction of current flow, it is possible to accelerate the corrosion of a steel structure by connecting the structure by a remote electrode to the positive terminal of a power supply and by using a separate cathode connected to the negative terminal of the power supply. In other words, the structure to be removed is made the anode in an electrochemical circuit. A conductive fluid is also required to enable the electrochemical effect.

Thus, techniques are known in which electrical current is supplied to and removed from the metal structure by electrodes fed by a power supply. According to known electrochemical material-removal techniques carried out for dissolution of say, a steel structure/object/element, the negative cathode is suspended in the conducting fluid without making contact with the steel. Furthermore, the positive terminal of the power supply is connected directly to the steel so that the steel itself becomes the anode.

This direct contact is readily achieved by clamping, bolting or welding the positive terminal connection to the steel. However, this usually requires prior cleaning of the steel to allow a good electrical contact and direct contact can be difficult to achieve in environments where the steel surfaces are heavily contaminated, for example with marine growth, corrosion product or other surface contaminants. Such techniques are especially limited in difficult in circumstances where access to the metal surface is difficult.

So, this application of electrochemical dissolution has been known by those skilled in the art for many years, although techniques for optimising or improving the rate of material removal at high electrical power levels are not well-understood. In particular, previously considered techniques are ineffective or impractical in circumstances where it is desired to remove or reduce metallic material in tightly-confined regions where the insertion of moving, mechanical components required for carrying out the known methods proves difficult or impossible.

Thus, although numerous electrochemical techniques are known for dissolving metal structures, there is still a need to alleviate the problems associated with known techniques. In particular, there is a need to provide a method of electrochemical removal of metal that is suited for the removal of metal from the surface of a metal structure, such as a subsea marine structure, located or buried in, or below, the sea bed.

Preferred embodiments of the present invention seek to provide a method of disposal of metal structures in a marine environment that allows comparatively inexpensive, safe and environmentally-sound cutting and dissolution ‘in-situ’.

According to a first aspect of the present invention there is provided a method of removing material from the inner surface of a metal structure, the inner surface(s) of the metal structure defining an inner volume that contains a conductive fluid, the method comprising: providing first and second electrodes respectively connected to the positive and negative terminals of a DC power supply, wherein the electrodes are immersed within the conductive fluid and wherein the first electrode is disposed so as to be physically separate from the inner surface of the metal structure, operating the DC power supply such that current flows from the first electrode to the metal structure through the conductive fluid and then from an inner surface of the metal structure to the second electrode through the conductive fluid.

According to embodiments of this aspect, the method is applied to an inner surface of a metal structure since the electrodes are disposed or suspended within a volume of conductive fluid within the metal structure. For example, the metal structure may comprise a metal pipe, or tube, or other structure having an opening at one or both ends, and the inner surface(s) of the structure can be considered to define an inner volume that contains a conductive fluid such as sea water. Thus, as a consequence of the first electrode being disposed so as to be physically separate from the inner surface of the metal surface, current flows between the electrodes and the metal structure through the conductive fluid - i.e. indirectly, rather than by direct contact between the electrodes and the metal structure. A distinct advantage of this arrangement is that it is not required for the first, positive electrode, to be secured in contact with the metal structure and, rather, electrical contact between the positive electrode and the metal structure is achieved indirectly, through the conductive fluid. Indeed, the method of the present invention advantageously enables electrochemical dissolution of a metal without making physical connections to the structure to enable the structure to be dissolved.

Preferably, the method further comprises: providing one or more non-conductive elements within the inner volume, the/each non-conductive element being disposed between the first and second electrodes so as to inhibit current flowing directly between the first and second electrodes in the conductive fluid.

Thus, by providing one or more non-conductive elements it is possible to inhibit direct current flow between the electrodes so that a greater proportion of the current flows through the metal structure. Furthermore, it will be appreciated that the rate of removal of material from the inner surface of the metal structure as a result of current emanating towards the negative electrode is related to the charge density on the surface of the metal structure. Thus, the provision of one or more conductive elements can be seen to effectively reduce the surface area of the inner surface(s) that will experience electrochemical dissolution during the operation of the DC current, thereby increasing the charge density on that surface and increasing the rate of dissolution.

According to a preferred embodiment the method may comprise the step of dividing the inner volume into a plurality of sub-volumes by means of one or more non-conductive elements, such that the second electrode is disposed within one of the sub-volumes. Preferably, the/each non-conductive element is provided within the inner volume of the metal structure when in a first configuration, the method further comprising the step of operating the non-conductive element such that it changes from the first configuration to a second configuration wherein, in the second configuration, the non-conductive element extends between opposing points on the inner surface of the metal structure.

According to a second aspect of the present invention there is provided an apparatus for removing material from the surface of a metal structure the apparatus comprising: first and second electrodes respectively connected to the positive and negative terminals of a DC power supply; at least one non-conductive element that is operable to be changed from a first configuration to a second configuration, the first configuration being more compact that the second configuration.

Thus, according to this aspect of the present invention there is provided an apparatus or electrochemical cutting tool that is operable to cut the surface of a metal structure or remove material from the surface of a metal structure.

According to a third aspect of the present invention there is provided a method of removing material from the surface of a metal structure immersed in a conductive fluid, the method comprising: providing first and second electrodes respectively connected to the positive and negative terminals of a DC power supply, wherein the electrodes are immersed within the conductive fluid and are both disposed so as to be physically separate from the surface of the metal structure, operating the DC power supply such that current flows from the first electrode to the metal structure indirectly, through the conductive fluid and then from a surface of the metal structure to the second electrode through the conductive fluid.

The electrochemical material-removal technique according to embodiments of the present invention can be applied on either the gross volumes of structure or on smaller selected regions of the structure. In the case of the latter, isolation from the rest of the structure of the preferred region will be created by the use of one or more non-conductive elements and/or by design features of the electrochemical cutting tool itself so that no other intervention is required to achieve isolation.

It will be appreciated by those skilled in the art that the methods embodying the present invention can be applied equally to cutting from the inside or the outside of a metal structure.

According to embodiments of the present invention, the structure, such as a pipe or cylindrical tubular, may be cut or dissolved from the outside (e.g. as in the case of an oil platform jacket leg) or from the inside (e.g. a well casing or similar structure buried below the sea bed). Whilst the present invention is applicable to either situation, it is considered that it will provide significant benefit to the challenge of cutting steel tubulars from the inside, where the available space restricts the use of some other approaches.

Features of any given aspect may be combined with the features of any other aspect and the various features described herein may be implemented in any combination in a given embodiment.

The invention will now be described, by way of example, to the accompanying drawings in which:

Figure 1 illustrates a first embodiment of the present invention;

Figure 2a and 2b illustrates a second embodiment of the present invention

Figure 1 illustrates an example of an embodiment of the present invention. As shown in Figure 1, a metal structure 1, which is to be dissolved, is placed in contact with an electrolyte or conductive fluid 2, or is already in contact with an electrolyte/conductive fluid, such as a salt water solution. It will be appreciated that the conductive fluid may be provided so as to fully or partially fill the interior volume of the metal structure 1. Alternatively, the metal structure may already be immersed in a conductive fluid, e.g. in circumstances of a subsea marine structure where the structure is disposed in sea water which is a conducive fluid.

As shown in Figure 1, the metal structure is connected to the positive terminal 3 of a DC electrical power supply 4 by an electrode 5 which is operable to supply current to the structure through the conductive fluid. The electrode 5, which becomes an anode, is remote from - i.e. not in direct physical contact with - the structure 1. The negative terminal of the power supply 6 is connected to an electrode 7, which becomes a cathode, and which is also immersed in the conducting fluid. The electrode 6 is also remote from - i.e. not in direct physical contact with - the structure 1. The cathode 7 and anode 5 are located close to the surfaces of the metal structure and are therefore not in intimate/physical contact with the structure. The application of a direct current across the two electrodes gives rise to an electrical circuit in which current flows from the positive anode 5 to the negative cathode 7, through the conductive fluid 2 and the metal structure 1,10. The application of an external direct current therefore results in an ion exchange on the surface of the metal structure in region 10, thereby corroding/removing material from the surface of the metal structure.

Optionally, the upper and lower electrodes are physically separated by a nonconducting element 8. Thus, as shown in Figure 1, a non-conducting element 8 is provided within the structure and is disposed so as to be between the two electrodes and thereby act as in the manner of a ‘barrier’. This non-conducting barrier 8 serves to inhibit the flow of current occurring directly between the electrodes through the conductive fluid 2, thereby increasing the current flowing through the metal structure. This beneficially serves to increase the rate of dissolution of the metal for the electrical energies supplied. The non-conducting element 8 can be made of any suitable nonconducting material. The outer surface of non-conducting barrier 8 is preferably in direct contact with the inner surface of the steel 1. Preferably, the element is shaped and sized so as to generally conformal with the inner surface of the metals structure to be treated. However, the non-conducting element does not necessarily need to make direct intimate in order to effectively inhibit significant electrical current flow directly between the electrodes.

In operation, the following reactions occur with a steel structure:

At the anode:

equation 1

At the cathode: 4e' + 2H2O (+O2) 40H' equation 2

The iron at the anode converts to iron hydroxide and then iron oxide (rust)

Fe^·' + 20H' 2Fe(OH)2 equation 3

Fe(OH)2 + 02-^ Fe(0H)3 equation 4

Fe(0H)3 Fe203.nH20 (rust) equation 5

This follows the normal corrosion process, however, as a result of using an external electrical current as described, the corrosion rates are significantly accelerated.

In practical implementation, the geometries and sizes of the electrodes may be beneficially varied to suit the specific features of the steel to be dissolved. Preferably, the anode needs to be of sufficient area and volume to not act as a significant electrical resistance and to enable the desired levels of electrical current to be passed through the fluid and into the steel. The cathode may be of a number of sizes or geometries and may be presented as a plate, coil or sleeve, shaped to conform to the shape of the metallic object to optimise the electrochemical processes across the surfaces.

Locating the cathode close to the structure and shaping it to conform to the shape of the structure allows higher efficiencies of the process to be achieved. In Figure 1, material is lost from the inner surface of the casing over a relatively large surface area 10, at any point below non-conducting barrier 8. Thus, the position of the element 8 within the metal structure defines an interior region 10 of proposed removal of metal where metal is removed from the interior surface of the metal structure.

Figures 2a and 2b show a second embodiment of the present invention and illustrate how the direction of electrical currents and location of ion exchanges may be controlled to preferentially dissolve a more specific region of steel and therefore increase the through-thickness cutting rate. The rate of dissolution of the steel is directly related to the electrical current density at the steel surface. In order to maximise, or increase, the rate of steel removal through the thickness of the steel, it is therefore proposed to minimise, or reduce, the steel surface area to be treated. Figures 2a and 2b illustrate a method for reducing the area of steel through which the electrical current flows through use of additional non-conducting barriers 9 that serve to isolate the steel area to be cut. Preferably, but not essentially, non-conducting barriers 9 are configured to be in contact with adjacent steel. This may be achieved by any number of processes or mechanisms that allow the outer surfaces of non-conducting elements 9 to contact, and substantially conform to, the inner surface of the steel 1. Such mechanisms may include, for example, elements which may be laterally expanded so as to abut the inner surface of the metal structure. This may be achieved, for example, by hydraulic, spring-loaded or motor-driven actuation, as those skilled in the art, for example, of isolation of structures downhole will be familiar.

To illustrate the case of cutting or removing material from the inner surface of a steel pipes. Figure 2a shows non-conducting elements 9 in a first condition which allows them to be conveniently inserted into the steel pipe. This configuration would preferably be established prior to establishing electrical current from the power supply 4. In Figure 2b it is shown that the non-conducting barriers 9 have been "actuated" to achieve a second configuration and thereby to be placed in intimate or close contact with the inner surface of the steel. The arrows of Figure 2b then show the direction of travel of the electrical currents from the power supply in this configuration. In the region 10 between the two non-conducting barriers 9, the ion exchange is illustrated as occurring in a confined region of the steel 10. Under these conditions, the steel in this region is dissolving at a rate which is influenced by both the magnitude of the electrical currents being driven by the power supply 4 and the surface area of steel 10 exposed to the electrolyte. Specifically, it will be appreciated that the rate of removal of material is inversely proportional to the surface area of metal on which ion exchange occurs when the DC current is applied by the supply 4. For the same electrical current emanating from the power supply, the through-thickness cutting rate illustrated in Figure 2b is far greater than the through-thickness cutting rate of Figure 1.

It will be appreciated that that there could be a number of different arrangements of non-conducting elements or barriers disposed so as to selectively isolate regions of a metal structure such as a steel pipe or element. For example, it is envisaged that non-conductive, or “isolation” elements could be arranged in a variety of different orientations such that steel region to be treated could be cut, or divided, in the horizontal or vertical planes. That is, cuts or material reduction methods could be performed that are axially or circumferentially aligned in any multiple of configurations. Electrochemical tools can therefore be designed on these principles, which may have modular features and any number of configurations of electrodes and barriers to allow metallic structures to be cut simultaneously into a large number of different shapes - a feature that is not readily achieved by any alternative cutting approach. Embodiments of the present invention may find particular application in seeking to remove well bore casings. Embodiments of the present invention also represent a convenient way of segmenting metal structures, such as metal or steel tubes, so that they can be more easily removed to the land, or so that the steel casing segments can be allowed to simply drop to the bottom of the well.

It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. The word “comprising” does not exclude the presence of elements or steps other than those listed in a claim, “a” or “an” does not exclude a plurality, and a single feature or other unit may fulfil the functions of several units recited in the claims. Any reference signs in the claims shall not be construed so as to limit their scope.

Claims

1. A method of removing material from the inner surface of a metal structure, the inner surface(s) of the metal structure defining an inner volume that contains a conductive fluid, the method comprising: providing first and second electrodes respectively connected to the positive and negative terminals of a DC power supply, wherein the electrodes are immersed within the conductive fluid and wherein the first electrode is disposed so as to be physically separate from the inner surface of the metal structure, operating the DC power supply such that current flows from the first electrode to the metal structure through the conductive fluid and then from an inner surface of the metal structure to the second electrode through the conductive fluid.

2. A method as claimed in claim 1, further comprising: providing one or more non-conductive elements within the inner volume, the/each non-conductive element being disposed between the first and second electrodes so as to inhibit current flowing directly between the first and second electrodes through the conductive fluid.

3. A method as claimed in claim 1 or 2, further comprising the step of dividing the inner volume into a plurality of sub-volumes by means of one or more non-conductive elements, such that the second electrode is disposed within one of the sub-volumes.

4. A method as claimed in claim 2 or 3, wherein said non-conductive element is provided within the inner volume of the metal structure when in a first configuration, the method further comprising the step of operating the non-conductive element such that it changes from the first configuration to a second configuration wherein, in the second configuration, the non-conductive element extends between opposing points on the inner surface of the metal structure.

5. A method of removing material from the surface of a metal structure immersed in a conductive fluid, the method comprising: providing first and second electrodes respectively connected to the positive and negative terminals of a DC power supply, wherein the electrodes are immersed within the conductive fluid and are both disposed so as to be physically separate from the surface of the metal structure. operating the DC power supply such that current flows from the first electrode to the metal structure indirectly, through the conductive fluid and then from a surface of the metal structure to the second electrode through the conductive fluid.

6. An apparatus for removing material from the inner surface of a metal structure, the inner surface(s) of the metal structure defining an inner volume that contains a conductive fluid, the apparatus comprising: first and second electrodes respectively connected to the positive and negative terminals of a DC power supply, wherein in use the electrodes are immersed within the conductive fluid, at least one non-conductive element that is operable to be changed from a first configuration to a second configuration, the first configuration being more compact that the second configuration.

7. A method of disposal of metal structures immersed in a conductive fluid comprising either large volume or preferential area dissolution of the metal structure by connecting said structure to the positive terminal of a DC power supply through a remote electrode and a cathode connected to the negative terminal of said power supply in said conducting fluid.

8. A method as claimed in claim 7 or any one of claims 1 to 5 wherein said conductive fluid is sea water.

9. A method as claimed in claim 7 or claims 1 to 5 wherein said conductive fluid is a fluid other than sea water.

10. A method as claimed in claims 7, 8 or 9 or any one of claims 1 to 5 wherein said structure is any metal, iron or steel, including alloy steel.

11. A method as claimed in any one of claims 7 to 10 or any one of claims 1 to 5 wherein said structure is an oil and gas installation, including well casings.

12. A method as claimed in any one of claims 7 to 11, or any one of claims 1 to 5 wherein the electrodes are remote from, and not directly contacting the structure to be dissolved.

13. A method as claimed in any one of claims 7 to 12, or any one of claims 1 to 5 wherein a combination of physically attached and remote anodes are used.

14. A method as claimed in any one of claims 7 to 13, or any one of claims 1 to 5, wherein part of the structure is isolated from the remainder of the structure.

15. A method as claimed in any one of claims 7 to 14, or any one of claims 1 to 5, wherein a multiple of non-conducting barriers can be used to isolate regions of steel to be preferentially dissolved.

16. A method/apparatus substantially as hereinbefore described with reference to the accompanying drawings.