Deep Anode Bed For Cathodic Protection

Abstract

The method and apparatus for placing an impressed current or sacrificial anodes in a deep bed to provide cathodic protection of underground metallic structures and for easily replacing anodes which have been expended.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to the protection of metallic structures and relates particularly to the reducing or preventing of corrosion of underground metallic structures by impressing electrical potentials thereon to make such structures cathodic with respect to the surrounding soil which functions as an electrolyte.

2. Description of the Prior Art

It has been known that underground metallic structures, such as pipe lines or the like, have corroded because such structures normally contained both anodic and cathodic areas. In the anodic areas, an electric current was flowing away from the pipe line into the surrounding electrolyte such as soil or water and caused corrosion in such anodic areas. Where electric current was flowing from the electrolyte onto the metallic structure in the cathodic areas, the surface of the structure did not corrode. Therefore, it became obvious that if the exposed metal on the surface of the structure could be made to collect current, it would not corrode because the entire surface would then be cathodic. When the amount of current is adjusted properly, it overpowers corrosion current discharged from all anodic areas of the structure and there is a net current flow onto the pipe surface at all points. The entire surface then will be cathodic and the protection will be complete. In other words, cathodic protection does not necessarily eliminate corrosion; however, it does remove corrosion from the structure being protected and concentrates the corrosion at another known location.

The use of surface and deep well ground beds or anode beds to introduce protection currents into the earth from an external source to equalize the galvanic potential between an anode and a steel pipe or other underground metallic structure has been known in the prior art. Formerly the most widely used material for ground beds was sacrificial metals such as scrap steel rails, steel or cast iron pipe, automobile engine blocks, aluminum, magnesium and their alloys, and manhole covers. These delayed corrosion for a few years but had to be replaced relatively frequently.

In recent years, many different designs have been tried with deep well ground beds with one of the simplest being a heavy wall steel pipe extending from the surface to a depth of approximately 200 to 250 feet. A source of electrical energy was connected to the pipe and the upper portion of the pipe was coated to prevent current discharge. The lower portion of the pipe would act as a remote ground bed to which an impressed potential was provided so that a current would flow through the earth from the deep well to the structure being protected. In this type of structure, the life of the ground bed is dependent on the weight of the pipe used and the current discharged from the bed, as well as the type of soil in which the bed is located. Normally it is from 5 to 8 years before the pipe column separates at one or more places because of localized corrosion. After the original pipe had corroded and sectionalized so that it no longer discharged the design current, a new pipe column was inserted inside the original pipe and the new pipe was connected to the positive terminal of the electrical energy source. This type of structure has not been satisfactory, particularly where earth of moderate resistivity was interspersed with layers of very low resistivity since the current being discharged was caused to concentrate at the low resistivity strata and the pipe column would separate at these areas before the bulk of the steel was consumed.

Some efforts have been made to use a full length pipe with the inside of the pipe being filled with graphite or high silicon cast iron anodes and a carbonaceous backfill. In this type of ground bed, during the early life of the installation the pipe column discharged current and was consumed while the internal anodes remained essentially dormant. After the pipe became sectionalized and ineffective, the anodes and backfill were intended to keep the ground bed in operation. Experience with this type of installation has shown that resistance remains reasonably stable as long as the pipe column remains intact. After the pipe had corroded to the point where it began to separate and the anodes started to take over, a pattern of increasing resistance was noted since the resistance through the backfill was not as low as resistance through the original pipe column. This caused a higher current flow density to be required.

Some deep well ground beds have been built with anodes and carbonaceous backfill installed in a drilled hole and without a full length steel pipe. This has not been satisfactory since the anodes must be replaced every few years and due to the collapsing walls of the holes the anodes could not be easily replaced and therefore new holes had to be drilled.

Some examples of previous efforts to provide cathodic protection of underground metallic structures are the U.S. Pats. to Mudd No. 2,552,208, Dorr No. 3,458,643, Anderson No. 3,527,685, and Section 11 Chapter 5 of Gas Engineers' Handbook published in 1965 by the Industrial Press, 93 Worth St., New York, N.Y.

SUMMARY OF THE INVENTION

The present invention includes a method and apparatus for forming a deep well anode ground bed and includes a non-metallic casing which, in most cases, has a metal end portion. One or more anodes are located within the casing and a backfill of low resistance carbonaceous material, such as calcined petroleum coke, metallurgical coke or graphite, is provided in the annulus between the hole and the casing for a substantial distance up from the bottom of the hole and in the interior of the casing substantially to ground level. The structure of the casing is such that any gas created by the discharge of electrical energy from the anodes will not be trapped. By using the present apparatus and method of forming the ground bed, the anodes can be easily replaced after they have deteriorated to the point where they no longer discharge the required electrical potential.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a vertical section illustrating one application of the invention.

FIG. 2 is a fragmentary section of the upper portion of the structure of FIG. 1 and illustrating a modified form thereof.

FIG. 3 is a vertical section illustrating a casing being inserted into a bore hole in the first step of making a deep ground bed.

FIG. 4 is a vertical section illustrating the removal of mud from the bore hole.

FIG. 5 is a vertical section illustrating the introduction of petroleum coke into the annulus between the bore hole and the casing.

FIG. 6 is a vertical section illustrating the introduction of anodes into the casing.

FIG. 7 is a vertical section illustrating the introduction of petroleum coke into the casing and surrounding the anodes.

FIG. 8 is a vertical section illustrating the first step in removing the anodes for replacement.

FIG. 9 is a vertical section illustrating the casing with the anodes removed.

FIG. 10 is a vertical section illustrating the introduction of new anodes into the casing.

FIG. 11 is an enlarged fragmentary side elevation of a casing with portions broken away for clarity.

FIG. 12 is an enlarged section on the line 12--12 of FIG. 11.

FIG. 13 is an enlarged section on the line 13--13 of FIG. 11.

FIG. 14 is a section on the line 14--14 of FIG. 13.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

With continued reference to the drawings, a steel pipeline or other metallic underground structure 20 is provided which must be protected from corrosion to increase the life of such structure, as well as to reduce maintenance thereon. In order to prevent or reduce corrosion on the pipeline 20, a deep well ground bed is formed by drilling a bore hole 21 to any desired depth, although normally between 200 and 250 feet has been found satisfactory. Since each ground bed will protect a well coated pipeline 20 for a distance of 10 miles or more, it is desirable that the bore hole 21 be drilled in earth having low resistance to the passage of electrical energy.

Normally in the drilling of a bore hole, a drill bit of a desired size (not shown) is forced into the earth while rotating. While the drilling process is going on, mud 22 is being introduced into the bore hole to lubricate and cool the drill bit and simultaneously to cause the material being removed by the bit to flow to the surface and be discharged from the bore hole.

After the bore hole has been drilled to a desired depth, the bit is removed and a casing 25 of a diameter less than the diameter of the bore hole is lowered into such bore hole. The casing 25 is of a length to rest on the bottom of the bore hole, extend the full length thereof, and terminate slightly above the surface of the ground. The casing 25 includes a tubular base portion 26 constructed of iron, steel, or other metal having a bottom wall 27 and with the upper end being open. As illustrated in FIG. 11, the bottom wall 27 of the base portion is provided with an opening 29 in which a check valve 30 is received. Such check valve includes a ball 31 normally engaging a seat 32 by means of a spring 33. A plurality of slots 34 around the lower portion of the check valve permits material to be discharged through the check valve into the area surrounding the casing 25. The upper portion of the check valve 30 which extends into the base portion 26 is provided with threads 35 for a purpose which will be described later.

The open upper end of the metallic base portion 26 receives and is connected to a reduced end 36 of a lower pipe section 37 of the casing. As illustrated best in FIG. 14, the lower pipe section is provided with a plurality of openings 38 with each of the openings being angularly disposed from a lower outer position to an upper inner position and extending entirely through the wall thickness of the lower pipe section. A plurality of imperforate upper pipe sections 39 are connected to the lower pipe section and extend upwardly to a position above the surface of the ground. The lower pipe section 37 and each of the upper pipe sections 39 are constructed of an inert material which is chemically stable in the presence of oxygen, hydrogen, chlorine, strong acids and strong bases, and is not subject to deterioration from concentrated electric fields. Examples of such material include rigid thermoplastic sheet material such as cellulose acetate, cellulose acetate butyrate, methyl acrilate, vinyl polymers and copolymers, polystyrene, ABS terpolymer, polyethylene and the like. The pipe sections can be joined together in any conventional manner, as by welding, solvents, adhesives or the like.

A cap 40 is fixed to the upper end of the casing 25 and such cap includes a vent 41 and an electrical conduit inlet 42. A sleeve 43, which preferably is constructed of sheet steel or other conductive material, is disposed about the lower pipe section 37 in a position to initially cover the openings 38 to substantially prevent the ingress of foreign material into the casing.

As the casing is lowered into the bore hole, such casing will displace a substantial quantity of the mud and cause the mud to be discharged from the top of the bore hole. After the casing 25 is in place at the bottom of the bore hole 21, a wash pipe 45 is threadedly connected to the threads 35 of the check valve 30 so that such wash pipe extends entirely through the casing 25.

As illustrated in FIG. 4, when the casing 25 is in position, one end of a hose 46 is connected to the upper end of the wash pipe 45 and the opposite end is connected to a source of clean water under pressure so that such water is introduced into the wash pipe. Water under pressure will open the check valve 30 and will be discharged through the slots 34 into the bottom of the bore hole, and such clean water will carry the remaining mud upwardly and discharge the same from the top of the bore hole. The clean water will continue to be pumped into the bottom of the bore hole until the fluid being discharged at the top is substantially clear and most of the mud has been removed.

When the water being discharged from the bore hole 21 is substantially clear, the hose 46 is disconnected from the water supply and is connected to a hopper (not shown) containing water in which carbonaceous material is suspended or fluidized. The slurry of water and carbonaceous material is introduced under pressure into the wash line 45 and is discharged through the check valve 30 into the area between the bore hole 21 and the casing 25. The injection of carbonaceous material continues until the upper level of such material is located approximately 100 to 120 feet above the bottom of the bore hole. The hose 46 is disconnected from the carbonaceous material supply hopper and is connected to a source of water under pressure so that the carbonaceous material within the wash line 45 will be discharged exteriorly of the casing 25.

When the carbonaceous material has all been discharged from the wash line 45, such wash line is disconnected from the check valve 30 and is separated therefrom by a few feet. The check valve 30 will prevent the carbonaceous material and water located exteriorly of the casing from entering the interior thereof.

Clear water under pressure then is introduced into the wash line 45 to remove any mud or foreign matter which has seeped into the casing, after which the wash line 45 is removed. A plurality of anodes 50 of high silicon cast iron, graphite, carbon or steel material are mounted on a support line 51 of an inert material such as nylon or the like having poor electric current carrying qualities. The anodes 50 are connected by well insulated electrical conduits 52 to the positive side of a rectifier 53. The negative side of the rectifier is connected to the pipe line 20. The rectifier 53 is connected to a suitable source of AC power and is adapted to rectify the AC power to provide a direct current to the anodes 50. Although a rectifier has been illustrated and described, it is noted that any conventional source of DC power, such as a storage battery or the like, could be used. Also, it is noted that the support line 51 could be omitted in which case the anodes would be supported by the electrical conduits 52.

Each of the anodes 50 preferably is provided with one or more centering devices 54 constructed of any desired material such as mild steel or the like, to maintain the anodes 50 substantially along the vertical axis of the casing 25. Such anodes are lowered into the casing 25 until the lowermost anode reaches a position slightly above the base portion 26 of the casing. When the anodes are in position, fluidized carbonaceous material is introduced into the casing to completely fill the interior thereof. The carbonaceous material has a specific gravity greater than 1 and, therefore, such material will settle rapidly to the bottom.

As the carbonaceous material 48 is being introduced into the casing, gravel 55 is introduced into the upper annulus between the bore hole 21 and the casing 25 and above the carbonaceous material located at the bottom of the bore hole. Gravel is not a good conductor of electric current and, therefore, the current discharged by the anodes 50 will not be dissipated to the surface. After the interior of the casing 25 has been filled to the desired level, the support line 51 is connected to the cap 40 and the carbonaceous material is permitted to settle for approximately 24 hours, after which the anodes are energized by the rectifier 53. The carbonaceous material provides a constant downward pressure to decrease resistance. As an example, the hydrostatic pressure of the earth at 200 feet below the surface is approximately 87 pounds per inch squared. Due to the specific gravity of the carbonaceous material, downward pressure at the same level is substantially higher.

The corrosion rate or effective life of the anodes depends upon the current discharged by the bed. The theoretical weight loss for graphite or carbon anodes is approximately 2 pounds per ampere per year, and the theoretical weight loss for high silicon cast iron anodes is less than 1 pound per ampere per year. Normally, these weight losses are caused by electrolytic discharge occurring at the anode surface and are based upon a definite limit of 1.0 ampere of current density per square foot at the anode surface when surrounded by carbonaceous material. The increased pressure of the carbonaceous material at the anode surface causes more intimate contact between the carbonaceous material and the anode to reduce resistance to current flow and substantially reduce anode weight loss. Current from the anodes 50 flows through the low resistance carbonaceous material and through the openings 38 in the lowermost pipe section and then through the sleeve 43 to the exterior carbonaceous material and earth surrounding the bore hole. Simultaneously current flows to the base portion 26 of the casing and is discharged to the surrounding earth. The low resistance earth functions as an electrolyte to transmit the current to the pipe line 20 which is being protected.

The flow of electrical energy from the anodes causes the sleeve 43 and the lower steel section 26 to deteriorate and substantially disappear, after which the carbonaceous material on the interior of the casing engages the material on the exterior to maintain a low resistance to the flow of current.

Ground water within the bottom of the bore hole will decrease the resistance to the travel of electric current; however, high current flow densities through such water may cause bubbles of hydrogen, oxygen or other gases to be formed on the lower surfaces of the casing. If permitted to remain in the casing, the bubbles of gas may form an insulating barrier which increases resistance to the flow of current. However, due to the porosity of the carbonaceous material, as well as the fact that the openings 38 in the casing wall are disposed at an angle, such gas bubbles will pass upwardly through the carbonaceous material and gravel and will be discharged to atmosphere.

The flow of an electric current from the impressed current or sacrificial anodes causes minute particles to be detached therefrom in the form of corrosion so that such anodes must be replaced normally at periods from 5 to 20 years, or when the anodes have deteriorated to the point where they no longer discharge the designed current. When this condition occurs, electrical service to the rectifier 53 is interrupted and the cap 40 is removed from the top of the casing 25. Thereafter, as illustrated in FIGS. 8-10, a hose 56 is connected to a source of liquid or gaseous fluid under pressure (not shown) and is regulated to a desired flow. The discharge end or nozzle 57 of the hose is inserted in the carbonaceous material within the casing 25. The flow of fluid from the hose 56 fluidizes or suspends the granular carbonaceous material 48 in a fluid bath so that the hose will sink by gravity into such material and will continue to fluidize the carbonaceous material for the entire length of the casing.

During this operation the flow of fluid is somewhat critical since there must be enough fluid being discharged from the hose to fluidize the carbonaceous material but not so much fluid that such material will be discharged from the top of the casing 25. As an example, water was introduced into a 2 inch pipe having a 3/4 inch anode therein surrounded by carbonaceous material and such material became fluidized when the flow rate of the water was approximately 0.4 pounds per minute or approximately 3 gallons per hour. Generally, the flow rate for larger casings varies as the square of the diameter of the casing. Also, although a flexible hose 56 has been illustrated and described, it is noted that a pipe (not shown) could be permanently installed within the casing for fluidizing the carbonaceous material.

When substantially all of the carbonaceous material within the casing has been fluidized, an upward lifting force is applied to the support line 51 or the conduits 52 to pull the remaining portions of the expended anodes from the casing. After the expended anodes have been removed, fresh anodes are placed within the casing and such anodes sink by gravity through the fluidized carbonaceous material until the anodes are located in their designed position. Thereafter, the flow of fluid through the hose 56 is interrupted and the hose is withdrawn from the casing. After a period of approximately 24 hours to permit the material to settle, the flow of electricity is re-established to the rectifier and the new anodes are placed in service. The entire operation of removing expended anodes and replacing the same requires approximately 4 man hours' work.

Due to the specific gravity of the carbonaceous material, as well as the pressure head of such material within the casing 25, the material, particularly at the bottom of the casing, will be compacted so that the resistance to the flow of electrical energy will be low and a greater portion of the anode surface will be in engagement with the carbonaceous material to reduce anode weight loss. As illustrated in FIG. 2, in order to further reduce electrical resistance, the cap 40 is secured to the upper portion of the casing in any desired manner (not shown) and such cap supports a fluid cylinder 58 having a piston rod 59 extending through the cap into the upper portion of the casing. A compression head 60 is fixed to the lower end of the piston rod 59 and such head is of a size to be slidably received within the inner diameter of the casing 25. Fluid under pressure is introduced into the cylinder 58 to cause the piston rod 59 to be extended and compress the carbonaceous material 48 within the casing. In this modification the support line 51 is connected to the head 60 and the conduits 52 are connected to opposite ends of a conductor bar 61 which extends through such head. The application of compression forces to the internal column of carbonaceous material causes compaction of the granules and further reduces the resistance to the flow of electrical energy.

In the operation of the device, the casing 25 is placed within the bore hole 21 after which granular carbonaceous material 48 is introduced into the lower portion of the bore hole exteriorly of the casing so that such material extends for a predetermined distance above the bottom of the bore hole. Additional carbonaceous material is introduced into the interior of the casing to substantially completely fill the same after the anodes 50 are placed in position. Due to the angularity of the openings 38, the material on the interior of the casing completely fills the openings and provides good contact with the sleeve 43 which surrounds the openings. After the sleeve has deteriorated, the carbonaceous material on the interior of the casing intimately engages the material on the exterior thereof. When the anodes are in position and the carbonaceous material has settled, the rectifier 53 is energized to transmit a direct current to the anodes which in turn discharge the current into the carbonaceous material and into the earth surrounding the deep well. The electrical current from the anodes travels through the earth to the pipeline 20 or other underground metallic structure so that the entire surface of the pipe 20 becomes cathodic. Since the pipe 20 is connected to the rectifier 53, a circuit is completed. The discharge of electrical current from the anodes 50 causes minute particles of the anodes to become detached so that after a period of time the anodes are no longer capable of discharging the required current. When this occurs, the carbonaceous material within the casing 25 is fluidized either by means of a fluid hose 56 or by a fluid pipe located within the casing and connectable to a source of fluid under pressure. When the material within the casing is fluidized or in suspension in the fluid which has been introduced into the casing, an upward lifting force is applied to the support line 51 or the conduits 52 to pull the expended anodes 50 from the casing. Thereafter, fresh anodes are introduced into the casing where such anodes will sink by gravity through the fluidized material. When the anodes are in place, the flow of fluid through the hose 56 is interrupted and the carbonaceous material is permitted to settle. After approximately 24 hours, the rectifier 53 is re-energized so that an electric current again will be discharged from the anodes 50. The anodes can be replaced as often as necessary in a minimum of time and with minimum effort. If the current being discharged from the anodes is sufficient to maintain the pipeline 20 in a cathodic condition, such pipeline should resist corrosion substantially indefinitely and thereby greatly reduce the maintenance required for such pipeline.

Claims

I claim:

1. The method of making a deep anode bed for the cathodic protection of underground metallic structures comprising the steps of: drilling a deep bore hole in the earth, inserting an elongated hollow casing having a generally tubular wall of relatively rigid chemically inert non-conductive material into said bore hole, the lower portion of said casing wall having a plurality of openings therethrough, filling the annulus between the bore hole and the exterior of said casing with granular electrically conductive material to a predetermined level above the bottom of the bore hole and at least above the level of said openings, attaching at least one anode to a support means, introducing said anode and at least a portion of said support means into said casing, substantially completely filling the interior of said casing with granular electrically conductive material after said anode is in place so that the conductive material within said casing is in intimate engagement with said anode and communicates with the conductive material exteriorly of said casing through said openings, and electrically connecting said anode to a source of direct electrical energy, whereby electrical energy flows from said anode through said interior and exterior conductive material and through the earth to the metallic structure so that the underground metallic structure becomes cathodic.

2. The method of claim 1 including the steps of introducing fluid under pressure into the granular electrically conductive material within said casing to fluidize said material after the anode has been substantially expended, applying an upward force to said support means to remove any remaining portions of the expended anode, placing a new anode within said casing so that said new anode sinks by gravity into the fluidized granular material within said casing.

3. The method of claim 1 including the step of introducing gravel into the annulus between the exterior of said casing and said bore hole and on top of the granular electrically conductive material located exteriorly of said casing.

4. The method of claim 1 including the step of compressing the granular electrically conductive material within said casing to reduce resistance to the flow of electrical energy.

5. In a deep anode bed for cathodic protection of underground metallic structures in which the bed includes a bore hole, at least one anode supported at a predetermined depth within said bore hole, granular electrically conductive material substantially filling said bore hole and providing communication between said anode and the earth surrounding the bore hole, and means for supplying electrical energy to said anode so that electrical energy flows from the anode through the conductive material and the earth to the underground structures: the improvement comprising the method of replacing an expended anode including the steps of; introducing fluid under pressure into said granular conductive material to fluidize the same, controlling the flow of fluid so that granular conductive material is not discharged from the top of the bore hole, applying an upward force to the anode support to remove an expended anode from the fluidized granular material, placing a new anode within the fluidized granular material so that the new anode sinks by gravity into the bore hole, and interrupting the flow of fluid under pressure into said granular conductive material so that said granular material settles into intimate engagement with said anode and the earth surrounding the bore hole.

6. Apparatus for cathodically protecting underground metallic structures comprising an elongated hollow tubular rigid casing for reception within a deep bore hole, said casing having at least an upper portion constructed of substantially rigid chemically inert non-conductive material with a plurality of openings adjacent to the lower end only, at least one anode, means for suspending said anode within said casing in the area of said openings, granular electrically conductive material substantially filling said casing and intimately engaging said anode and filling the lower portion of the bore hole exteriorly of said casing at least to a level above said openings, and means for supplying direct electrical energy to said anode, whereby electrical energy flows from said anode through said first and second conductive materials and through the earth to the underground metallic structures to cause the underground structures to become cathodic and thereby substantially prevent corrosion of such structures.

7. The structure of claim 6 in which said casing initially includes a metallic portion connected to one end.

8. The structure of claim 6 including sheet metal sleeve means covering said openings.

9. The structure of claim 6 including centering means for locating said anode generally axially of said casing.

10. The structure of claim 6 in which said means for supplying electrical energy to said anode includes a conductor connecting said anode to a rectifier.

11. The structure of claim 6 in which said openings are slanted at an inward and upward angle relative to the axis of said casing to reduce gas accumulation and to provide greater pressure on said granular electrically conductive material.