GB2212514A - Cathodic protection of subterranean pipes - Google Patents

Description

c It 22125 1'# CATHODIC PROTECTION OF SUBTERRANEAN PIPES This invention

relates to the cathodic protection of subterranean pipes, especially (but not exclusively) dast iron water and gas pipes.

It is known to reduce the rate of corrosion of subterranean iron pipes by connecting them electrically to a sacrificial anode buried nearby in the ground. In the case of new pipes, this cathodic protection can be provided when the pipes are laid, but where existing installations are concerned, excavation is necessary both to expose the pipe surface to connect a wire thereto, and to bury the sacrificial anode. Excavation large enough for man entry, eg.

17 2m x 1.5m x depth of pipe is relatively costly not only because of the volume of earth etc. to be moved but also because of the substantial cost of reinstatement when the work is complete. In fact, the cost of providing cathodic protection in this way generally equals or exceeds the cost of completely replacing the pipe, and so is usually not worthwhile. However, if cathodic protection for existing installations could be provided more economically, much use could with advantage be made of it.

In our European specification no. 0235917 we have described a method of tapping subterranean pipes wherein, in some embodiments, non-man entry small excavations are used to expose the pipe. In one such procedure, pins are brazed to the excavated pipe to locate a saddle on 0 - 2 on the pipe prior to tapping. These pins or studs can be joined to an electric wire connected to a sacrifical anode to provide cathodic protection. However, whilst substantial economies can be achieved in this way, i.e. by using small excavations eg. 25 to 30 cm in diameter., very difficult problems arise in making a good electrical connection to the pipe surface whilst working from ground level down a very narrow hole. For example, it is very difficult properly to clean a small area of the pipe surface to receive the connection, and the making of the connection whether by brazing or otherwise can also be difficult.

We have now found a way of overcoming or mitigating these problems whereby a good electrical connection can be reliably made, and cathodic protection provided at a cost substantially less than the cost of the pipe replacement.

Generally, the method of the invention comprises the following steps in sequence: (a) a small excavation is made to expose the top crown of the pipe, (b) the exposed crown is cleaned of soil and treated to prepare it it is de- scaled for receiving the electrical connection eg. and/or abraded, (c) an electrical conductor is affixed to the cleaned surface, and (d) connection is made with a buried sacrificial anode to complete the cathodic protection circuit.

in step (a) of the method, a small excavation is made from ground level down to the top of the pipe. By "small" we mean too small for man-entry and, indeed, as small in area as possible, consistent with the work to be done. In general, we have found that circular holes approximately 30cm in diameter are of adequate size in most cases. The hole may be excavated in various ways, for example using an auger, if necessary with bar to get down to a maximum depth of 1.5m or more. 35 Alternatively, vacuum excavation can be used. The an extension 0 excavation technique used will depend on the nature of the soil, as is well known. Preferably, the hole will have substantially vertical walls and the pipe will be exposed substantially centrally of the hole.

Step (b) involves cleaning and preparing the pipe surface for the electrical connection. Preferably, in the interests of economy, only sufficient of the surface will be cleaned as is required for the subsequent electrical connection. This cad be a very small area where, for example, the connection is to be made by brazing. Firstly, some earth on each side of the exposed pipe crown is removed so that the crown is clear of the surrounding soil, and the crown itself is wiped clean of soil. This can be effected manually from ground level using long-handled spades etc.

The next stage involves descaling and/or abrading the appropriate area of the pipe surface. It is extremely.difficult to descale and abrade the small area of pipe surface working downhole with elongate tools, from ground level. Thus, the tools wander over and off the crown surface and very easily become contaminated and/or clogged with soil. We have found that, in accordance with a highly preferred feature of the invention, this problem can be overcome by providing in the hole guide means for maintaining the working tools on the desired pipe surface area. In accordance with one 2S embodiment of the invention, the guide means comprises a frame or plate member having an aperture therein through which the tool passes. By maintaining the plate in an appropriate position in the hole above the pipe, the tool (being confined laterally by the aperture walls) is maintained with its working head on the desired pipe surface area. The height of the plate above the pipe can preferably be varied to suit each particular tool (it being intended that the aperture walls should not of course engage any moving parts of the working head of the tool).

The provision of guide means is an important aspect of the present invention. The guide means can be located in the excavation in various ways. In one particularly preferred arrangement of the invention, a liner sleeve is first inserted in the excavation, the sleeve including means for receiving and locating therein the guide means. Thus., the guide means are held relative to the sleeve which extends from the pipe at the bottom of the hole up to ground level (or above or slightly below). The sleeve will preferably be a relatively snug fit in the hole, but if not it can for example be temporarily jammed in position. The sleeve can not only provide (optionally) a way of locating the guide means but it will also serve to prevent earth falling from the sides of the hole onto the working area. Thus, even when the sleeve is not used to locate the guide means, it can be used to keep the working area clear of soil fall.

The liner sleeve is preferably a plastics sleeve (but it can be of other materials). It is normally removed from the excavation after making the electrical connection, and re-used. When it is to be used to locate the guide means, there will normally be some way of locking the guide means to the sleeve in one or more working positions, eg. by means of cams receivable in slots.

The amount and type of surface preparation necessary will depend on the nature of the proposed elec trical connection. However, although the connection can be made in a number of ways,, eg. by brazing, welding or by conductive adhesives, in general both descaling and abrasive cleaning of the pipe surface will be desirable.

For descaling purposes, we have found an air-powered needle descaler to be appropriate, fitted of course with an elongate handle and controls as necesssary for operation downhole under manual control from ground level. For abrasion, a sand disc can suitably be used in an air powered die grinder. Again, the tool requires a long handle and 1 C. ' - a remote control for operation downhole manually from ground level. Both these tools, and other similar tools, can be extremely difficult to use without employing guide means previou.sly described.

After the necessary cleaning of the area of the pipe surface, the electrical connection thereto can be made (step (c)). As stated above, we prefer to use brazing, i.e. to braze a pin to the pipe surface, the pin preferably already carrying an electric wire for electrical connection to the sacrificial anode. However, other techniques can be used as mentioned above, although they are not generally as advantageous as brazing.

Where brazing is to be effected, the brazing equipment used is generally standard except that the pins need to be modified for connection to the electric wire, and the pin brazing unit has to be mounted for operation downhole remote from the operator at ground level. Modification of the standard pins can be achieved by, for example, drilling a hole through the pin and soldering and crimping the electric cable thereto. During brazing, the earth connection can be made using for example, a magnet on a c= 3 1= long reach pole. Again, brazing is greatly facilitated by the use of a guide means as previously described.

Finally, the sacrificial anode (which may be of conventional design) is buried in an appropriate position, with the electrical cable connecting it to the brazed pin (or other connection) on the pipe. The anode may be in a separate nearby small excavation, or it may be placed in the main excavation during infill.

Depending on the spacing of the anode from the pipe, and the situation of the site, eg. roadway, verge or open country, a number of options are available as to the method of work, and attachment of anode to pipe. Thus:

if the holes are in open country, the anode may be attached (via the cable) to the brazing pin before installa- - 6 tool guide; tion. The pin would be brazed to the pipe, and the anode then lowered into position. The cable joining the pipe and anode could be laid in a shallow trench linking the two holes. ii) if the pipe and anode are to be placed further apart, or the installation is within a roadway, the connecting trench may not be desirable. A small connecting hole could be bored between the two holes, and if the anode were installed first, the brazing pin and cable could then be drawn through this small bore to the pin installation hole, the pin then being brazed as before.

iii) the pin and anode may be installed each with a lead attached but as yet unconnected. The wires can then be joined after installation either using a permanent solder joint. or a plu., and socket within a small inspection chamber to allow the anode to be checked periodically.

In order that the invention may be more fully understood, reference is made to the accompanying drawings, wherein:

FIG. 1 is a schematic sectional view of an excavation to a pipe, with the sleeve and tool guide in position; FIG. 2 is a schematic view of the cathodically protected pipe arrangement; FIG. ^5 is similar to Fig. 1 but illustrates the way in which the aperture in the guide means laterally confines the elongate tools; FIG. 4 is a top plan view of one embodiment of sleeve:

FIG. 4B is a section on the line X-X of Fig. 4A; FIG. SA is a top plan view of one embodiment of and FIG. 5B is a section on the line Y-Y of Fig. SA.

Referrina to the drawings. Fig. 1 shows a c= C- subterranean pipe 1 with a small excavation to reveal.

i 1 c 1 its crown area 2. A cylindrical sleeve 3 is snugly into the excavation, with its upper end 3a slightly ground level 10. Within sleeve 3 is located a tool comprising a plate 4 having an aperture 6, and a pair of 5 elongate control rods 5 extending out of the sleeve 3. Projections 7 are provided to selectively engage in corresponding recesses provided in the sleeve 3, to locate the plate 4 at the desired height above surface 2 for the particular tool (not shown) to be used. After cleaning surface 2 and, for example, brazing a pin on surface 2, the guide and sleeve 3 are withdrawn, and the excavation filled.

Fig. 2 shows the final result. 20 brazed to its crown 2, with a cable 21 connecting the f itted above P-uide Pine 1 has a nin pin to sacrificial anode 22 spaced from pipe 1.

Referring to Fig. 3, the arrangement shown is essentially as in Fig. 1 (like numerals indicate like parts), but the extreme lateral positions (not normally adopted) of two tools T1, T 2, are shown. As can be seen, even at these extremes, the working heads H,, H 2. crown area X where the work is required.

Figures 4A and 4B show in some detail a cylindrical sleeve 3. The sleeve is provided internally with two lengthwise ribs 30,31 extending parallel and being diametrically opposed. These ribs serve as guides to the tool guide of Fig. 5. Also, the sleeve 3 has a series of pairs of diametrically opposed slots 32,33,34,35 (only one of each pair is visible in Fig. 0) to receive cam plates of the guide of Figs. S. It is to be noted that each pair of slots 00 is at a different distance from lower end 36 of sleeve 3, so that the tool guide can be mounted at different heights in the sleeve.

The tool guide of Figs. 5 comprises a generally circular plate member 40 having two diametrically opposed peripheral recesses 41,42 in which ribs 30,31 are to be are still in rhe 1 1 - 8 is received as the guide is moved into the sleeve 3. The ribs thus locate the plate 40 with respect to the sleeve. Extending from the plate on one face are the two elongate control rods 5, from diametrically opposed positions of plate 40 close to the periphery of the plate and at positions about 90 to the positions of recesses 41,42. Each rod passes through a bore in the plate and is fixed (eg. by nut 45) eccentrically to a respective cam member 43,44 lying below the plate. Rotation of each rod causes the respective cam plate to turn. Each cam plate is shaped so that, in one position (as drawn) it does not project outwardly of the plate periphery but in another position it does project. In this way, the guide can be lowered down sleeve 3 with the cams retracted and then, upon rotating rods S, the cams can be caused to project and engage a chosen pair of slots 32-35, to fix the guide in the sleeve.

The plate 40 has an aperture 50 therein. Mounted to lie over parts of the aperture is an apertured plate S1.

Holes and fixings (52,53) are provided to locate the plate 51 in any of a series of positions. This enables the plate aperture position to be adjusted. This is important because, in use, the excavation may not expose the pipe crown exactly centrally of the liner sleeve 3. It is then important to be able to guide the working tools into an area (on the pipe crown) which is off-centre of the sleeve 3. This facility if provided by adjusting the position of plate 51 on plate 40 so as to present a through aperture "Z11 in exactly the right position. Thus, the working aperture Z is the 6^0 through aperture formed by apertured plate 51 and aperture 50 of guide plate 40.

C 1 1

Claims (10)

CLAIMS:

1. A method of providing cathodic protection to a subterranean pipe, which comprises the steps of (a) making a small excavation to expose the top crown surface of the pipe; (b) descaling and/or abrading the surface to receive an electrical connection thereon; (c) fixing an electrical conductor to the prepared surface; and (d) connecting the conductor to a sacrificial anode.

2. A method according to claim 1, wherein in step (b), guide means are provided in the excavation and a descaling tool and/or an abrading tool, is applied to the pipe surface and located by the guide means to operate on the pipe surface.

3. A method according to claim 2, wherein the guide means comprises a frame or plate member having an aperture therein, and wherein the or each tool passes through the aperture and is guided thereby to-work on the pipe surface.

4. A method according to claim 2 or 3, wherein the or each tool is constrained by said guide means to work on only a small area of the pipe surface.

5. A method according to claim 2,3 or 4, wherein, in step (a), a liner sleeve is inserted in the excavation, and in step (b) the guide means is held relative to the sleeve.

A method according to any preceding claim wherein in step (c), the conductor is fixed by brazing, welding or by a conductive adhesive.

7. A method according to claim 6, wherein in step (c) a pin, connected to an electric wire, is brazed to the pipe surface, and in step (d) the wire is electrically connected to the anode.

8. A method according to claim 6 or 7 when dependent on claim 2, wherein the or each tool utilised for fixing the conductor to the pipe surface is guided to the pipe surface by the guide means.

9. A method according to any preceding claim, wherein the subterranean pipe is a cast iron water or gas pipe.

10. A method of providing cathodic protection to a subterranean pipe substantially as herein described with reference to the accompanying drawings.

Published 1989 atThe Patent Office, StateHouse, 66.71 High Holborn, London WClR 4Tp. Further copiesmaybe obtained from The Patent Office. Sales Branch, St Mary Crky. Orping' on. Kent BR5 3RD. PrMted by multiplex techniques ltd, St Mary Cray, Kent. Con. 1187 i 1 1