AU656038B2

AU656038B2 - Retractable coupon

Info

Publication number: AU656038B2
Application number: AU28405/92A
Authority: AU
Inventors: Elio Monzu
Original assignee: STATE ENERGY COMMISSION OF WES
Current assignee: Dbngp (wa) Nominees Pty Ltd
Priority date: 1992-03-18
Filing date: 1992-11-16
Publication date: 1995-01-19
Anticipated expiration: 2012-11-16
Also published as: AU2840592A

Description

Regulation 3.2

AUSTRALIA

PATENTS ACT 1990 COMPLETE SPECIFICATION FOR A STANDARD PATENT r Name of Applicant: Address of Applicant: Actual Inventor: Address for Service: STATE ENERGY COMMISSION OF WESTERN AUSTRALIA 363 365 Wellington Street PERTH W.A. 6000 ELIO MONZU GRIFFITH HACK CO.

Patent Trade Mark Attorneys 256 Adelaide Terrace PERTH W.A. 6000 r r

I

Standard Complete Specification for the invention entitled: RETRACTABLE COUPON Details of Associated Provisional Applications: PL1426 filed 18 March, 1992 by THE STATE ENERGY COMMISSION OF W.A.

Details of Parent Application for Divisional Applications: The following is a full description of this invention, including the best method of performing it known to me:- 2 RETRACTABLE COUPON The present invention relates to a retractable coupon for below ground corrosion monitoring and relates particularly, though not exclusively to a retractable coupon suitable for monitoring and assessing the effectiveness of a cathodic protection system of the type used on underground pipelines.

SECWA operates and maintains an underground gas pipeline from Dampier to Bunbury for the transmission of natural gas. A cathodic protection (CP) system has been in operation on the Dampier to Bunbury pipeline for several years and must be regularly monitored to ensure effective corrosion protection is being maintained. The purpose of the CP system is to inhibit corrosion of the pipeline material at the site of any defects which may exist or develop in the protective coating on the exterior surface of the pipeline. A coupon is an electrical probe which is intended to simulate a coating defect and thus provide information about the corrosion specific characteristics of the pipeline, and hence the effectiveness of the CP system, in a limited area around the coupon.

A conventional coupon typically comprises a metal probe which is buried in close proximity to the pipeline and which is electrically connected to the pipeline. The coupon function is to establish the actual polarisation level of the pipe defect Epip, with respect to the surrounding environment.

The current practice is to measure Vpipe using a Cu/CuS0 4 cell above the ground as a reference potential. Hence, the pipe and the cell are typically 1.0 to 1.2 metres apart. This means that the measurement of the pipe-to-soil potential, Vpipe, actually includes an IR component due to current flow through the intervening soil. This may be expressed as follows V. =E IR pipe Epipe IR where Vpipe pipe potential measured Eppe pipe defect surface potential IR voltage drop created by current flow and the soil resistivity 3 Cu/CuS04 copper/copper sulphate reference half cell In order to remove the IR component from Vpipe it has been the practice to deactivate the CP system by switching OFF the transformer/rectifier which applies a DC polarisation potential to the pipeline. An instantaneous OFF potential measurement is made in an attempt to isolate the actual Epipe.

Epipe is the potential of the polarisation film at the steel interface. Typical potential values are Vpipe 1.1V, Epipe 0.8V and IR 0.3V. However, in practice, it is difficult to obtain a true "current free" Epipe potential measurement.

Furthermore, the transformer/rectifier may be located many hundreds of kilometres from the location of a coupon under test presenting problems of logistics and extended down-time when trying to obtain an instantaneous OFF potential measurement.

15 A further disadvantage of conventional coupons is that it is necessary to excavate the coupon in order to perform a visual inspection and laboratory analysis for signs of the extent and nature of corrosion. Furthermore, once the coupon is removed, it normally cannot be replaced due to damage sustained during the process of excavation. A further problem *associated with conventional coupons is the difficulty of ensuring good soil-to-coupon electrical contact. Voids frequently occur between the surface of the steel coupon and the surrounding soil which result in inaccurate operation since 25 the measurement is heavily reliant on good soil contact.

The present invention was developed with a view to producing an improved coupon which can overcome one or more of the abovementioned disadvantages of conventional coupons.

According to one aspect of the present invention there is provided a retractable coupon assembly for monitoring corrosion of a below ground facility such as a buried pipeline, the assembly comprising: an elongate outer casing of tubular construction adapted for locating in the ground adjacent the below ground facility; and, an elongate inner member adapted to be retractably PALIj, received within said outer casing, and carrying a coupon at one S I j\ end for monitoring corrosion; 4 wherein said outer casing has an opening at one end through which said inner member can be withdrawn and has a second opening at its other end through which said coupon can be exposed to soil whereby, in use, said coupon can be visually inspected for corrosion by withdrawing the inner member from the outer casing.

Typically said coupon can be used for monitoring the performance of a cathodic protection system for a below ground facility, said assembly further comprising means for providing an electrical connection between the coupon in situ and measurement apparatus located above ground, whereby, in use, an electrical potential of said coupon can be measured.

Advantageously said inner member is hollow and is provided with an opening at one end through which a reference a 1 °o 15 cell may be slidably received therein whereby, in use, said reference cell can be located below ground internal of said inner member in close proximity to said coupon to facilitate more accurate measurement of the coupon potential. Preferably .46. 0, 0 said inner member is provided with another opening at its other 20 end through which said internal reference cell can be brought e:e0 0 into electrical connection with said soil. Typically there is provided an electrically conductive medium within said hollow o inner member for providing an electrical c -nnection between gem° asaid internal reference cell and the soil.

S 25 In a preferred embodiment said inner member is also provided with perforations adjacent said coupon through which 0 liquid accumulated within a cavity between the outer casing and the inner member can drain into the soil adjacent the second opening whereby, in use, the electrical conductivity of the soil can be enhanced in the region of the coupon.

Advantageously said coupon is in the form of a disc which is carried at said one end of the inner member with the plane of the disc lying substantially perpendicular to the longitudinal axis of the inner member in such a manner that, in use, only one face of the disc is exposed to the soil.

Typically said disc is provided with an aperture therethrough $F.LAA, whereby, in use, an internal reference can be electrically connected to the soil through said aperture. Preferably said 5 aperture is filled with electrically conductive material, said material being electrically insulated from the disc by an insulating washer. Advantageously said electrically conductive material is a porous, sintered zircon ceramic plug and said insulating washer comprises a sleeve of plastics material.

In order that the nature of the present invention can be better ascertained a preferred embodiment of the retractable coupon will now be described in detail, by way of example only, with reference to the accompanying drawings, in which: Figure 1 illustrates a preferred form of the retractable coupon in which an outer casing and inner member are illustrated in side elevation in their disassembled condition; Figure 2 is a part section view through the 15 retractable coupon assembly of Figure 1, showing the outer •Tie :casing and the inner member in their assembled positions; and, SiFgrFigure 3 is an end view of the assembly illustrated in Figure 2 in the direction of arrow A.

Figure 1 illustrates a preferred embodiment of the retractable coupon assembly according to the invention comprising an elongate outer casing 10 and an elongate inner member 12 adapted to be retractably received within the outer casing 10. In its preferred form the inner member 12 carries a coupon 14 (not visible in Figure 1) proximate one end of the member 12 whereby, in use, the coupon 14 can be located below o ground adjacent a facility to be monitored for corrosion.

Outer casing 10 is of tubular construction having an opening 16 at one end through which the inner member 12 can be inserted and withdrawn. In this embodiment, the outer casing 10 is also provided with a second opening 18 at its other end through which the coupon 14, carried on inner member 12, in its assembled condition can be exposed to soil.

In use, the outer casing 10 of the retractable coupon assembly is located in the ground in an area to be monitored with the second opening 18 in close proximity to the below ground facility, for example, a gas pipeline. Periodically, the inner member 12 can be withdrawn from the outer casing to enable visual inspection and/or laboratory analysis of the 6 coupon for corrosion. The retractability of the inner member 12 thus obviates the need to excavate the coupon as with the prior art.

Figures 2 and 3 illustrate in greater detail the construction of the preferred embodiment of the retractable coupon assembly illustrated in Figure 1. Most of the components of the retractable coupon assembly are standard ultraviolet treated polyvinylchloride (UPVC) water pipe and fittings glued together, where necessary, using PVC cement.

This greatly simplifies the construction and manufacture of the assembly. Outer casing 10 comprises an 80mm diameter UPVC pipe provided with a necked coupling 22 at the first opening 16, and a second coupling 24 at the other end for holding a bush 26 of reduced internal diameter forming the second opening 18 "'oo 15 of outer casing *Inner member 12 comprises a length of 25mm diameter UPVC pipe 30 provided with a faucet socket 32 and a threaded Splug 34 at one end thereof. Plug 34 is used to seal the hollow interior of inner member 12 from soil and other contaminants.

At the other end, the inner member 12 is provided with a bush 36 having an external diameter slightly smaller than the internal diameter of the bush 26 of the outer casing 10. A minimum clearance is provided between bush 26 and bush •36. Bush 36 is designed to retain the coupon 14 on the end of the UPVC pipe 30 forming the inner member 12. In thi s embodiment, coupon 14 is in the form of a metal disc of grade pipeline steel.

As can be seen most clearly in Figure 3, coupon 14 is in the form of an annulus having an outer diameter of 33.8mm and an inner diameter of 16.4mm. The exposed face of the coupon 14 has a surface area of approximately 7.3cm 2 The steel coupon 14 is fixed within the bush 36 with a specially machined PVC sleeve 38. The central aperture of coupon 14 and the end of the pipe 20 of the inner member 12 is fitted with a porous, sintered zircon ceramic plug 40 adapted to provide an electrical connection between the interior of the inner member 12 and the surrounding soil immediately adjacent the coupon 14. The steel coupon 14 is electrically insulated from 7 the porous, sintered zircon ceramic plug 40 by a specially machined insulating washer 42 of plastics material which completely covers the inner face of the coupon 14 and its aperture.

Advantageously, the bush 36 is provided with a plurality of perforations forming drain holes 44 through which liquid accumulated within the cavity 45 between the outer casing 10 and the inner member 12 can drain into the soil adjacent the second opening 18 whereby, in use, the electrical conductivity of the soil can be enhanced in the region of the coupon. Thus, in use, the cavity 45 can be filled with water which seeps through the holes 44 and helps to bed the coupon into the soil and minimise the possibility of voids occurring :between the exposed surface of the steel coupon 14 and the o 15 surrounding soil. The necked coupling 22 is provided with a too* pour hole 47 for pouring water into cavity 45. The hole 47 is normally closed with a cap.

In the illustrated embodiment, an internal reference cell can be slidably received within the interior of pipe S 20 of the inner member 12 whereby, in use, the reference cell (not o "shown) can be lcated below ground in close proximity to the coupon 14 to facilitate more accurate measurement of the coupon potential. The porous ceramic plug 40 provides an electrical S" connection between the internal reference cell and the S 25 surrounding soil. Advantageously, the intcrior of the UPVC pipe 20 may be partly filled with an electrically conductive liquid medium such as, for example, water or SURGEL to enhance the electrical connection between the Cu/CuSO 4 internal reference and the surrounding soil. The UPVC pipe 20 of inner member 12 effectively acts as a salt bridge and is shielded from external variables by the material of the inner member 12 and outer casing 10. Having the Cu/CuSO 4 reference inside the inner member 12 effectively extends the measuring circuit via the salt bridge to provide a potential measurement within Imm from the coupon steel. Clearly, the closer the internal reference cell is to the steel coupon 14 the lower the IR component.

8 Preferably the potential measurement is taken with a high impedance voltmeter which reduces the current flow in the measuring circuit to minimise voltage drop within the circuit. Since the measuring circuit is all contained within the inner member 12 the IR component is practically reduced to zero, ie.

VP E IR

SP

If IR equals zero then VP Ep Therefore measured V (coupon potential) Ep (actual steel interface potential).

Therefore by measuring the potential of the coupon inside the retractable coupon assembly, the true potential above-grade measurement can be obtained. By eliminating the e 15 IR factor from the potential measurement, the retractable coupon not only provides improved measurement of the polarisation aspect of the coupon but also assists pipeline *0 operators in monitoring the effects of external interference and telluric activities.

As can be seen in Figure 2, electrical wiring in the form of a 1.5mm 2 stranded single core PVC sheathed cable 46 provides an electrical connection between the steel coupon 14 in situ and measurement apparatus (not shown) located above a ground. The cable 46 is connected to a side face of the coupon a 14 and passes through one of the drain holes 44, along the length of the inner member 12 through a second bush 48 to a miniature junction box 50 located adjacent the faucet socket 32 of the inner member 12. The junction box 50 is secured to pipe 30 eithei oy electrical cable ties or stainless steel self-tapping screws. The second bush 48 has an outer diameter slightly smaller than the inner diameter of the necked portion of the coupling 22 forming the first opening at the end of outer casing 10. The bush 48 is provided with a bore through which the cable 46 extends. Preferably two electrical connections to the steel coupon 14 are provided in the form of a pair of cables 46 (only one cable is visible in Figure 2) located on diagonally opposite sides of the inner member 12.

Typically the cables are connected to the coupon by 9 silver/soldering and a heat shrink sleeve may be inserted for added insulation. An epoxy sealant is prefeably applied to the inner faces of the coupon, ensuring that all connections are completely covered and isolated from any variables within the assembly.

The illustrated embodiment of the retractable coupon assembly according to the invention has a number of advantages, including the following: The true polarisation potentials of the coupon can be measured free of IR component, regardless of whether the transformer/rectifier system is ON AC potentials may be measured It can be used for retuning CP systems in active telluric activity areas Polarisation levels can be monitored under AC voltage interference The coupon is fully retractable without disturbing 0 00 the surrounding environment The coupon can be visually examined and analy-id in 20 the laboratory to determine corrosion characteristics The same coupon or a new coupon can be installed in the same location simply and effectively Any internal reference may be used within the retractable inner tube.

Now that a preferred embodiment of the retractable coupon has been described in detail, it will be apparent to persons skilled in the relevant arts that the retractable coupon is not limited in its application to monitoring the performance of CP systems, but can be used for monitoring corrosion of any below ground facility. Furthermore, numerous variations and modifications may be made to the illustrated embodiment without departing from the basic inventive concepts.

For example, it is by no means essential to the invention to have an internal reference cell provided within the inner member 12, since the principle of having the coupon retractable is applicable with or without an internal reference. However, 10 having an internal reference is preferable in order to minimise the IR component. All such variations and modifications which nevertheless still embody the basic principles of the invention are to be considered within the scope of the presen.t invention, the nature of which is to be determined from the foregoing description and the appended claims.

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Claims

1. A retractable coupon asPFnbly for monitoring corrosion of a below ground facility such as a buried pipeline, the assembly comprising: an elongate outer casing of tubular construction adapted for locating in the ground adjacent the below ground facility; and, an elongate inner member adapted to be retractably received within said outer casing, and carrying a coupon at one end for monitoring corrosion; wherein said outer casing has an opening at one end through which said inner member can be withdrawn and has a second opening at its other end through which said coupon can *b.e be exposed to soil whereby, in use, said coupon can be visually s. 15 inspected for corrosion by withdrawing the inner member from the outer casinr. a*

2. A retractable coupon assembly as defined in claim 1, further comprisin means for providing an electrical connection between the coupon in situ and measurement apparatus located 20 above ground, whereby, in use, an electrical potential of said :coupon can be measured. S.O.

3. A retractable coupon assembly as defined in claim 2, wherein said inner member is hollow and is provided with an "S opening at one end through which a reference cell may be slidably received therein whereby, in use, said refeience cell can be located below ground internal of said inner member in close proximity to said coupon to facilitate more accurate measurement of the coupon potential.

4. A retractable coupon assembly as defined in claim 3, wherein said inner member is provided with another opening at its other end through which said internal reference cell can ,-jTLr' be brought into electrical connection with said soil. 12 retractable coupon assembly as defined in claim 4, wherein there is provided an electrically conductive medium within said hollow inner member for providing an electrical connection between said internal reference cell and the soil.

6. A retractable coupon assembly as defined in claim wherein said inner member is also provided with perforations adjacent said coupon through which liquid accumulated within a cavity between the outer casing and the inner member can drain into the soil adjacent the second opening whereby, in use, the electrical conductivity of the soil can be enhanced in the region of the coupon. GOOD GO.O 7. A retractable coupon assembly as defined in claim 2, 9 wherein said coupon is in the form of a disc which is carried at said one end of the inner member with the plane of the disc :OO 15 lying substantially perpendicular to the longitudinal axis of the inner member in such a manner that, in use, only one face of the disc is exposed to the soil. A retractable coupon assembly as defined in claim 7, D wherein said disc is provided with an aperture therethrough 9.9. S 20 whereby, in use, an internal reference can be electa,: illy connected to the soil through said aperture. 0 9. A retractable coupon assembly as defined in claim 8, wherein said aperture is filled with electrically conductive material, said material being electrically insulated from the disc by an insulating washer. A retractable coupon assembly as defined in claim 9, wherein said electrically conductive material is a porous, sintered zircon ceramic plug and said insulating washer comprises a sleeve of plastics material. 13

11. A retractable coupon a; .oly for monitoring corrosion of a below ground facility such as a pipeline, the assembly substantially as herein described with reference to and as illustrated in the accompanying drawings. Dated this 28th day of October 1994. THE STATE ENERGY COMMISSION OF WESTERN AUSTRALIA By Its Patent Attorneys: GRIFFITH HACK CO. Fellows Institute of Patent 10 Attorneys of Australia. 0* 0 06.: *so e V 1' ABSTRACT A retractable coupon assembly for below ground corrosion monitoring, the assembly comprising an elongate outer casing 10 and an elongate inner member 12 adapted to be retractably received within the outer casing 10. A coupon 14 is carried proximate one end of the inner member 12. Outer casing 10 is of tubular construction having an opening 16 at one end through which the inner member 12 can be inserted and withdrawn. The outer casing 10 is also provided with a second opening 18 at it's other end through which in use the coupon 14 can be exposed to soil. The outer casing 10 of the retractable coupon assembly is located in the ground with the second opening 18 in close proximity to the below ground facility to be monitored for corrosion, for example, a gas pipeline. Periodically, the inner member 12 can be withdrawn from the outer casing 10 to enable visual inspection and/or laboratory analysis of the coupon 14 for corrosion. Drawing suggested to accompany abstract: Fioure 2 0o 0 0 o