WO1999036596A2

WO1999036596A2 - Marine stock enhancement process

Info

Publication number: WO1999036596A2
Application number: PCT/GB1999/000097
Authority: WO
Inventors: Anthony Maxwell
Original assignee: Anthony Maxwell
Priority date: 1998-01-13
Filing date: 1999-01-13
Publication date: 1999-07-22
Also published as: AU2065599A; WO1999036596A3

Abstract

A method of and structure for enhancing stocks of marine life or fish. The method includes the steps of providing a two component structure of which one component is electrically conductive and the other component is electrically insulating or semi-conducting; locating the two component structure under water; and passing an electric current through the water and using the electrically conducting component as an electrode. Minerals are accreted on the electrode to form a reef. The two component structure may comprise a mesh formed of interleaved layers of copper and polymer mesh, and may be wrapped around a submerged oil rig jacket. The oil rig jacket may act as a sacrificial electrode.

Description

MARINE STOCK ENHANCEMENT PROCESS

This invention enables the transformation of a steel or concrete underwater structure into an artificial reef using modulated dc current to create a multiple crevice coating on or around a steel or concrete matrix such as a decommissioned oil or gas platform jacket or production structure to provide protective habitats for fingerling and juvenile fish species and crustaceans, thereby increasing marine stock holding capacity of the reef and life expectancy of its inhabitants.

In one aspect the present invention relates to a marine stock enhancement process. The process involves the use of an underwater structure to which young fish and the like can be introduced. The structure has various features, at least some of which are optional, which protect the fingerling fish from predators and which act to attract the fish to the structure. The structure does not of itself physically restrain the fish and its primary intended use is to enhance natural fish stocks rather than act as a fish farm - although it could be used in that capacity with the addition of means in and around the structure to retain the fish.

According to one aspect of the present invention there is provided a method of enhancing stocks of marine life or fish, including the steps of: providing a two component structure of which one component is electrically conductive and the other is electrically insulating or semi-conducting; locating the two component structure under water; and passing an electric current through the water using the electrically conductive component as an electrode.

According to another aspect of the present invention there is provided an accretion enhancing mesh, for use in the method of the present invention, comprising two

3ST-TUTE SHEET (RULE 26) components of which one component is electrically conductive and the other is electrically insulating or semi-conducting.

According to another aspect of the present invention there is provided a submerged metal structure, wherein said structure is wrapped in one or more layers of the mesh of the present invention.

Embodiments of the invention will now be described by way of example only and with reference to the accompanying drawings, in which:

Figure 1 is a diagrammatic partial cross-section through a structure used in accordance with one embodiment of the invention.

Figure 2 is a perspective view of a structure used in accordance with another embodiment of the present invention.

Figure 3 is an illustration of growth and time-space constraints for a crevice dwelling organism.

The process of electrically stimulated accretion in sea water has been known for many years. The process has been used to create solid underwater structures, as described for example in US Patent 4,246,075. As in US Patent 4,246,075, the electrodeposition of minerals from sea water has been used to provide a solid building material. In contrast, the present invention deliberately seeks to provide an accreted structure having a large number of crevices, cavities and the like. The crevices, cavities and the like provide secure havens for young fish and other marine life. Manufacture of the desired structural arrangement is greatly assisted by use, in the preferred embodiment, of the above mentioned perforated polymer sheet in association with the cathodic copper wire mesh. The randomly spaced and sized perforations in the polymer sheet result in a non-uniform quantity distribution of accreted material on the copper mesh. This results in the desired crevices, cavities and the like in the final structure. Of course, the electrically stimulated accretion is discontinued once the desired cavity ridden structure has been formed. Further, natural accretion will occur. This does not present a significant problem, for example closure of the cavities, due to the very slow rate of natural accretion. Generally, the rate of natural accretion in sea

SΪT! ITE SHEET RULE 26) water will be at least one and possibly two or more orders of magnitude slower than the rate of electrically stimulated accretion. Indeed, the inventor hereof has worked with electrically stimulated accretion rates very substantially less than the rates in excess of 1 cm per 100 hours quoted in US Patent 4,246,075. Even so, the contrast with natural accretion is enormous, with the natural process generally requiring at least several of decades, and often much longer, in order to accrete an amount of material corresponding to that accreted in several months in accordance with this invention.

The rate of accretion, natural or stimulated, is influenced by a wide range of factors including water temperature, salinity, light and the like. The rate of electrically stimulated accretion is influenced by factors such as voltage/current, electrode materials and the like. Factors affecting the rate of accretion are employed in various embodiments of the present invention in order to provide or assist with the formation of the desired non-uniform quantity distribution of accreted material. In one, simple, example electric current density is varied spatially and/or temporally with respect to different portions of the copper wire mesh. In another simple example, the material and or quantity of the electrically conductive mesh is varied in a non-uniform manner. Other features of the electrically stimulated accretion process can also be used to advantage in the present invention. For example, use can be made of the gasses released as a result of the electrolysis. As described in US Patent 4,241,015, gasses such as chlorine and oxygen can be released in significant quantities. The gases are released primarily at the anode. Thus, by arranging anodes vertically beneath the cathode mesh, the rise of bubbles of gas can be used to interfere with the accretion of solids at the cathode and thus assist in providing the desired non-uniform quantity distribution of accretion. Advantageously, the gases could be collected above the structure, compressed and stored so as to provide an additional commodity resulting from the accretion process.

Embodiments of the invention therefore provide an enhanced structural crevice surface created by mineral electrodeposition in a saltwater environment perforated with circular and irregular niches distributed at random over the surface without overlap. Thus, there is created an irregular size frequency of hole radii necessary to complete the life cycle of infaunal organisms with a given mortality and growth schedule and a constant occupancy rate.

Figure 3 illustrates growth in length and time-space constraints for a crevice dwelling organism. In the figure, the time duration of four arbitrary growth stanzas in their appropriate crevice radii is represented by the length of the fish, parallel to the time axis. Thus, it can be seen that younger small fish, having relatively rapid growth, may occupy together the same crevice in sequence in any one given year, while surviving larger fish could spend even longer than one year in the much fewer large crevices available. It will be clear from the diagram that an irregular reef having a proportionately higher number of smaller crevices is desirable when providing protective habitats for fingerling and juvenile fish species.

Determining a priori crevice size and frequency is achieved by us of conductive and non-conductive physically non-aligned pH or biodegradable polymer and metal mesh of two or more layers prelaminated to form a blanket cathode which is wrapped around the matrix structure such as a steel jacket of a decommissioned oil or gas platform.

Figure 1 illustrates an embodiment of the structure comprising the electrically conductive and insulating components used in the accretion process. The illustrated arrangement comprises a perforated polymer sheet 10 sandwiched between two copper mesh cloths 12 and 14. Mineral deposits formed by electrically stimulated accretion are shown diagrarnmatically and are indicted by the reference numeral 16. As indicated by reference numerals 18, a range of irregularly sized and shaped cavities and crevices are formed as a result of accretion on to the structure. Such cavities and crevices afford protection to marine life particularly (a) juvenile pelagic fish and (b) fingerling fish such as cod and halibut which are introduced to the underwater structure. Artificial supplies of food may be provided in order to assist in the stock enhancement process. Although a three layer structure is illustrated, any desired multiple of layers may be used. An alternative to the use of a polymer sheet or mesh is the use of a spray-on polymer, applied so as to produce the desired irregularity of accretion. The polymers used may be pH and/or biodegradable.

The preferred arrangement of copper mesh and polymer sheet can be provided as a pre-formed multiple layer mesh blanket and conveniently secured to a rigid underwater object. Figure 2 illustrates this embodiment of the preferred structure, in which the preformed multiple layer mesh blanket may comprise two or more layers of polymer mesh (20, 22) interleaved with layers of metal, preferably copper, mesh (24, 26).

Ideal for use as the above mentioned underwater objects are the so-called jackets of decommissioned oil and gas rigs, cut from their bases and toppled. The disposal of larger decommissioned oil and gas platforms has become a very severe problem, with costs of more than one hundred million dollars (US) per platform being common. The subject of decommissioning oil and gas rigs is complex and has for a number of years attracted a great deal of attention from national and international governmental bodies as well as environmental organisations. There is a growing consensus that there is no viable alternative to simply toppling and leaving the rigs on the sea bed. Larger platforms may be toppled in situ. Smaller platforms, although commonly removed to shore for recycling, might be used in a similar manner. In any event, with the present invention platform jackets are transformed and toppled at remote government approved sheltered loch or fjord locations or even transformed in situ to assist in valuable marine stock enhancement. It is to be noted that the oil rig jackets do not simply become so-called "artificial reefs" in the same manner as previous known attempts to dispose of discarded automobile shells or the like. Use of the mesh and polymer blanket of the present invention, together with other features as described, result in an active marine life environment rather than the barren "artificial reefs" previously known. Because of their open structure steel oil platform jackets are ideal for use with the present invention, but use can also be made of the base storage tanks and riser columns of large concrete gravity based structures.

The oil rig jackets can have the copper and polymer blanket secured thereto before or after toppling or sinking. Indeed, such components could even form an integral part of the original manufacture of new structures. In this respect, it is noted that cathodic protection of oil and gas platforms is known. When used according to the present invention, the blanket wrapped structure acts as the cathode and one or more anodes are placed in close proximity to the structure. A DC voltage is applied between the electrodes. Particularly suitable materials for the anodes are carbon, aluminium or zinc. Electrode separation and voltage values suitable for accretion are discussed in US Patent 4,246,075. However, although not limiting, it is generally preferred in the present invention that the DC voltage is in the range 6v to 24v and that the electrode separation is of the order of two meters. If appropriate, the anodes can be suspended from the structure itself. The power supply can be suitably sealed and placed underwater or can be placed on-land, on a pontoon or the like. Clearly, anticipated weather conditions, cable power loss, local shipping and other such factors need to be taken into consideration when determining the most appropriate form and location of the power supply. With appropriate local conditions; wind, wave and/or solar power may well be suitable energy sources.

The known accretion processes make use of a simple DC voltage applied between the electrodes. In the present invention it has been determined that more advantageous arrangements result from imposing a very low frequency signal on to the DC voltage. The very low frequencies are preferably compatible in frequency and timing with naturally occurring "teluric" currents and earth electromagnetic variations. The signals can be continuous or pulsed, can take the form of DC amplitude modulation and can be of constant frequency or time frequency. Establishing a direct electrical current between electrodes in sea water and sequentially modulating the dc current with periods of polarity change creates irregular electrodeposition on and within the cathodic laminated structure forming the desired niches and crevices.

A strong accreted material coating is not required therefore a high current density provides a rapid build-up of brucite, calcite and aragonite on the conductive mesh, resulting in the non-conductive pH degradable polymer mesh, which separates the conductive mesh, breaking down as the laminated mesh surface pH rises above pH 9 and the accreted material replaces the polymer.

The application of such a low frequency signal can significantly increase the attraction offish, particularly juvenile fish, to the underwater structure and increase the tendency for the fish to remain in the vicinity of the structure.

Another feature which has been found to attract fish and increase the tendency for the fish to remain in the vicinity of the structure is the addition of rope streamers. Polypropylene streamers attached to the wire mesh and having a diameter of about 5mm and a length between 300mm and 500mm have been found to be effective.

Drilling of the sea bed is carried out during the operational life of an oil or gas rig. Such drilling produces large deposits of cuttings which are usually left in piles on the sea bed. Such piles may be as high as 15m to 20m. Although practices have now changed, for many years diesel and low toxicity based oils were used to assist lubrication and drill cuttings removal from the drilling process. Thus, many of the older piles of cuttings contain significant quantities of hydrocarbons. In relatively shallow water, less than say 80m, wave action and underwater currents act to disperse the piles. This does not occur in deeper water. Environmental groups demand that all cuttings piles be removed, but in many cases there is no practicable and economic method of doing so. As a solution to these problems, one aspect of the present invention provides for the encapsulation of the cuttings pile using electrically stimulated accretion. The accretion process is generally as described above except that the irregular quantity distribution of accreted material is not necessarily required and therefor the electrically conductive cathode can be used without the electrically insulating or semi-conducting component. Hydrocarbons within the encapsulated piles may be broken down by anaerobic bacteria and it is believed that the process can be enhanced by virtue of the encapsulation. Pre-prepared cathodic blankets, connecting cables, anodes and battery power supply, i.e., a complete system, can be offered for installation by divers. Similarly, such a complete system could be installed on and around containers and activated before the containers are sunk in very deep waters. This may be advantageous in assisting with the deep sea disposal of more toxic wastes, perhaps even low level nuclear wastes where the temperature release from the material could be arranged to power or assist in powering the accretion process.

By way of variation of the arrangements discussed above, there is very considerable advantage in the previously unrecognised possibility of using a steel platform jacket as a sacrificial anode to dispose of the jacket. Preferably, the jacket is surrounded by a wire mesh cathode, which may be of a cage-type form. The wire mesh acts as described above so as to form an artificial reef and enhance marine stocks.

To render a decommissioned sub-sea oil or gas platform steel jacket as a sacrificial anode, it is of course necessary to establish a suitable electric power supply and path. In this respect, a plurality of cables can be used to connect the metal jacket to a surface power supply. The power supply may be housed on shore or on a supply vessel moored adjacent the sunken jacket. Of course, in determining the number and location of the cables, as well as their other properties and those of the power supply, consideration must be given to the electrical resistance of the various portions of the jacket and, for example, any welded joints between the various portions. Incidentally, if it is considered desirable to inhibit accretion and or corrosion at the sub-sea point of connection of the power supply cables; it can be effective to cover the connection with silicon grease.

Preferably, for the jacket to act as a sacrificial anode it is wrapped or encapsulated with a wire mesh which is subsequently connected to a surface power supply. The mesh can be electrically insulated from the jacket using spacers which may, for example, separate the mesh from the jacket by a distance of the order of 100mm. A mesh size of 2cm square can be effective. An appropriate number of power cables are connected to the mesh.

As mentioned above, emissions of carbon dioxide and other gases during electrolysis can assist with crevice formation in the materials accreted on the mesh.

Over a period of time the metal jacket will disintegrate, leaving only the encrusted wire mesh which is the original matrix of the new artificial reef.

Claims

1. A method of enhancing stocks of marine life or fish, including the steps of: providing a two component structure of which one component is electrically conductive and the other is electrically insulating or semi-conducting; locating the two component structure under water; and passing an electric current through the water using the electrically conductive component as an electrode.

2. A method as claimed in claim 1, comprising the step of securing the two component structure to a rigid underwater object.

3. A method as claimed in claim 1 or claim 2, comprising the step of connecting the mesh at various points to an electrical dc power supply.

4. An accretion enhancing mesh for use in the method of claim 1 , comprising two components of which one component is electrically conductive and the other is electrically insulating or semi-conducting.

5. An accretion enhancing mesh as claimed in claim 4, wherein the electrically insulating or semi-conducting component comprises a polymer.

6. An accretion enhancing mesh as claimed in claim 5, wherein the polymer is in the form of an irregular mesh or in the form of a sheet having irregularly spaced and sized perforations therein.

7. An accretion enhancing mesh as claimed in claim 5 or claim 6, wherein said polymer is pH or biodegradable.

8. An accretion enhancing mesh as claimed in any of claims 4 to 7, wherein the electrically conductive component comprises an interleaved copper wire mesh or cloth.

9. An accretion enhancing mesh as claimed in any one of claims 4 to 7, wherein the electrically conductive component comprises a conductive polymer mesh or cloth.

10. An accretion enhancing mesh as claimed in any one of claims 4 to 9, wherein said mesh is in the form of sheets or rolls or blankets.

11. An accretion enhancing mesh as claimed in any one of claims 4 to 10, wherein polypropylene cords are attached to the mesh to simulate marine plant growth.

12. A submerged metal structure wrapped in one or more layers of the mesh according to any one of claims 4 to 11.

13. A submerged metal structure according to claim 12, wherein said structure is a decommissioned oil or gas drilling, production or processing platform jacket.