WO1992007109A1

WO1992007109A1 - Device for compensation of varying electric potential in a medium surrounding a pipeline disposed in the medium

Info

Publication number: WO1992007109A1
Application number: PCT/SE1991/000676
Authority: WO
Inventors: Uno Jonsson; Dan Karlsson
Original assignee: Statens Vattenfallsverk
Priority date: 1990-10-12
Filing date: 1991-10-08
Publication date: 1992-04-30
Also published as: SE9003258L; AU8725291A; SE9003258D0; SE466160B

Abstract

A pipeline (1) buried in soil or in water is provided with equipment for cathodic corrosion protection (3). To compensate for, for example, potential variations along the pipeline caused by ground currents from d.c. plants, a circuit (4, 81, 82) insulated from ground is connected to the pipeline at two points (A, B). The circuit comprises a controllable current source (4). The potential difference in the ground between these points is determined, and the current source is controlled in dependence thereon such that a voltage drop is generated in the pipeline corresponding to the difference in ground potential.

Description

Device for compensation of varying electric potential in a medium surrounding a pipeline disposed in the medium

TECHNICAL FIELD

The present invention relates to a device for compensation of varying electric potential in a medium which surrounds a metallic pipeline disposed in the medium, the pipeline being surrounded by an electrically insulating material and provided with cathodic corrosion protection.

BACKGROUND ART

Metallic structures disposed in soil or in water are often protected with the aid of so-called cathodic protection. A potential difference is then maintained between the metal structure and the surrounding medium with the aid of one or more current or voltage units which maintain the metal structure at a negative potential of a suitable magnitude in relation to the surrounding medium. The suitable potential depends, inter alia, on the material in the pipeline. The invention relates to pipelines disposed in soil or in water, for example oil, water or gas pipelines. These are usually made of steel tubes, and a suitable potential in such pipelines has proved to be between about -900 mV and -1100 mV. If the potential is higher than about -900 mV, a risk of corrosion arises, and if the potential is lower than about -1100 mV, a risk of hydrogen embrittlement of the tube material arises.

Pipelines of the kind referred to here are provided with a protective coating or a protective casing of an electrically insulating material, for example a plastic material. Damage to the protective coating (casing) , which is unavoidable in practice, will, however, result in the above-mentioned risk of corrosion arising. From, for example, Swedish patent specification 134 075 and German published patent application 2 007 347, equipment for cathodic protection is known, in which an adjustable or con¬ trollable current or voltage source is connected between a point on the pipeline and an anode disposed in the sur¬ rounding medium. In this way, the pipeline may be main¬ tained at such a potential in relation to the surrounding medium as to reduce or prevent corrosion of the pipeline.

However, it has proved that the potential of the surrounding medium in certain cases varies along a pipeline. Such variations may be due to variations in the condition of the soil in which the pipeline is disposed, or to ground currents, flowing in the soil or in water, for example from d.c. plants for power transmission or for power supply for railway systems, giving rise to potential differences along the pipeline. In these cases, equipment for cathodic protection cannot maintain the pipeline at the correct potential in relation to the surrounding medium other than at one single point, and at all other points on the pipeline a risk of corrosion or hydrogen embrittlement arises. Arranging a plurality of systems for cathodic protection distributed along the pipeline has proved not to remove this risk, inter alia because of the necessary, relatively high currents through the equipment and their ground electrodes.

US Patent 2 584 623 describes a solution to the problems arising because of varying soil conditions along a buried elongated, uninsulated metallic object, such as an un- insulated pipeline, a cable sheath or the like. A d.c. source is connected to two points on the object, and the current through the object gives an electric potential gradient along the pipeline which counteracts or eliminates the currents which cause corrosion and which would otherwise flow between the surface of the object and the surrounding ground. Since .one of the poles of the d.c. source is grounded, the circuit connected to the object will not be free-floating in relation to ground and a relatively high current will flow through the ground' electrode, which gives rise to high power consumption and rapid electrode con¬ sumption. Further, the proposed device is only intended to counteract corrosion arising because of varying soil condi- tions. It is unable to counteract the corrosion which is caused by such variations in the soil or water potential as are caused by ground currents from d.c. plants. These ground currents, and hence the potential gradients in the ground caused thereby, vary rapidly and considerably and in an unpredictable manner with time in dependence on the load and other operating conditions in the d.c. plant.

SUMMARY OF THE INVENTION

The present invention aims to provide a device of the kind described in the introductory part of the this specifica¬ tion, with the aid of which efficient corrosion protection of a pipeline can be maintained in a simple manner, and with a small set of apparatus and a low power consumption, when potential gradients, varying in an arbitrary manner in time and space, occur along the pipeline.

What characterizes a device according to the invention will be clear from the appended claims.

The device according to the invention can be composed in a simple manner of standard components and can be designed with a low rated power and a low power consumption. It provides the intended effect even at potential gradients along the line which vary considerably in time and even if these potential gradients change signs in an unpredictable manner. By providing a pipeline with a plurality of pairs of connection points and connecting to each one of these ^• pairs a controllable circuit, insulated from ground, accor- ding to the invention, corrosion can be completely prevented even if the potential along the line varies in an arbitrary manner - and thus not only in the case where the potential changes linearly along the line. BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in the following with reference to the accompanying Figures 1-8. Figure 1 shows a pipeline, buried in the ground, with known equipment for cathodic corrosion protection. Figure 2 shows a circuit diagram of the device according to Figure 1. Figure 3 shows an example of the variation of the ground potential along the line. Figure 4 shows an example of equipment according to the invention. Figure 5 shows how the ground potential and the potential of the pipeline vary along the line. Figure 6 shows an alternative location of the electrodes for determining the ground potential. Figure 7a shows equipment according to the invention with a plurality of connec- tion points and a plurality of controllable circuits, connected therebetween, according to the invention, and Figure 7b shows the potential conditions along the pipeline. Figure 8a shows how sections of the pipeline at a distance from each other are provided with controllable circuits, and Figure 8b shows the potential conditions along the pipeline.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Figure 1 shows a pipeline 1 which is buried in the ground designated 2. The ground surface is designated 21. Figure 1 shows a section through the pipeline, which consists of a steel tube 11 provided with a protective coating 12 of, for example, epoxy resin. The pipeline is provided with equip¬ ment 3 for cathodic protection of the line, and this equip- ment is connected to the pipeline via a connecting conductor 13 and to a ground electrode 31.

Figure 2 shows in the form of a circuit diagram how the protective equipment is connected between a point C on the pipe and the electrode 31 serving as an anode. In a manner known per se, the equipment 3 may consist of a rectifier which delivers a direct voltage and has its positive pole connected to the electrode 31 and its negative pole connec- ted to the pipeline. The direct voltage is adapted such that the pipeline is maintained at a negative potential of a suitable magnitude, for example about 1 volt, relative to the surrounding ground. The rectifier may suitably be con- trollable and provided with control equipment which main¬ tains the potential of the pipeline at the desired value independently of variations in the operating conditions.

Figure 3 shows the variation of the potential along the pipeline 1. Curve a shows how the ground potential may vary along the pipeline, for example due to the effect of ground currents from a d.c. plant. Curve b shows the potential of the pipeline. The equipment 3 maintains the potential of the pipeline at point C at such a level that the line poten- tial is lower than the potential of the ground at this point by the desired value du. The metallic pipeline then has this potential along its length. This means that the potential difference between the pipeline and ground only at point C will assume the desired negative value du. At, for example, the point A shown in Figure 2, the potential diffe¬ rence is so small that a risk of corrosion arises, and at point B the potential difference is so great that a risk of hydrogen embrittlement arises.

Figure 4 shows a pipeline provided with equipment according to the invention. To the pipeline there is connected at point C, in the manner described above, equipment 3 for cathodic protection of the pipeline. The equipment accor¬ ding to the invention is connected to the pipeline at two connection points A and B at a distance from each other. The equipment comprises a controllable rectifier 4, for example a phase-angle controlled thyristor rectifier, which is fed via a transformer 5 from an alternating voltage net¬ work 6. By means of a smoothing inductor 7, a current re- verser 8 and electrically insulated cables 81, 82, the d.c. outputs of the rectifier are connected to the pipeline at points A and B. The rectifier is assumed to be a simple rectifier which can only deliver direct current of a certain polarity. The current reverser is therefore necessary to make possible current of arbitrary polarity through the pipeline and may, for example, consist of contactors in a known manner. The smoothing inductor smooths the direct current of the rectifier and is suitably selected so large as to obtain a well smoothed direct current. The rectifier 4, the cables 81 and 82 and the members arranged on the d.c. side of the rectifiers, for example the inductor 7 and the current reverser 8, are electrically insulated from ground. The members mentioned thus form a controllable circuit which is free-floating in relation to ground and which is connec¬ ted to the pipeline at the points A and B. This circuit in¬ cludes a current source - the rectifier 4 - which drives a current I through the circuit, and this current will tra- verse the pipeline and give rise to a voltage drop therein.

For control of the voltage of the rectifier, and hence of its current and of the voltage drop in the pipeline, the rectifier is provided with automatic control equipment. Two ground electrodes 10A and 10B are arranged in the ground in the vicinity of the connection points A and B of the pipe¬ line. The electrodes will lie at the potential of the ground at the respective point, u_gr__ and u_gr©, respectively. The electrodes are connected to an amplifier 91, included in the control equipment, which forms a signal du'_gr corres¬ ponding to the difference du_gr between the two ground poten¬ tials. A current measuring device 96 delivers a signal Im corresponding to the direct current I. The voltage drop du_&B in the pipeline between the points A and B is proportional to the direct current, and the proportionality constant con¬ sists of the resistance of the pipeline between the points, which resistance may be determined in a simple manner once and for all by measurement. In an amplifier 93, the signal Im is multiplied by this proportionality constant, and from the amplifier a signal duΑs is obtained which corresponds to the voltage drop in the pipeline between the points A and B. This signal is compared in an amplifier or regulator 92 with the signal du'_gr, obtained from the amplifier 91, which corresponds to the difference in ground potential between the two points. The difference d between the two signals mentioned is supplied to an absolute value-generating circuit 94. From the circuit 94 the signal |d| is obtained, which corresponds to the absolute value of the difference signal d and which is supplied to the control input of the rectifier 4 and hence influences the direct voltage of the rectifier. The units 96, 93, 92, 94, 4 form a closed-loop control system which strives automatically to maintain the amount of the sensed quantity du' _B equal to the amount of the reference value du'_gr supplied to the control system. From a sign-generating circuit 95, which is supplied with the signal du_gr from the amplifier 91, the signal SGN(du'_gr) is obtained, which indicates the sign of the signal du'_gr and controls the current reverser so as to obtain the correct polarity of the current I.

The control system now described will thus automatically control the direct current I with respect to magnitude and direction such that the voltage drop d _AB iⁿ the pipeline between points A and B becomes equally great as the diffe¬ rence du_gr in the ground potential between points A and B. The result is shown in Figure 5. Point C is maintained, as in Figures 1-3, at the desired negative potential du rela- tive to the ground potential at this point. In this way, the potential differences of the pipeline in relation to the surrounding ground at points A and B - d and dus - will closely correspond to the desired value du. If the ground potential varies linearly between A and B, the correspon- dence will be exact. In reality, of course, the ground potential does not vary linearly along the pipeline, but at a certain variation of the ground potential, the distances of the connection points from each other can always be chosen so small that the potential difference between the pipeline and ground at no point between the connection points deviates by more than a certain maximum, acceptable magnitude, for example ±0.1 V, from the desired value du. It has proved that the necessary potential gradients along a pipeline may be achieved with the aid of currents of a mode¬ rate magnitude, typically of the order of magnitude of some 10-50 amperes. Therefore, equipment according to the inven- tion may be designed with low rated power and low power con¬ sumption. It operates in the desired manner independently of how the ground potential varies with time and how it varies along the line, and it provides the desired effect also in case of rapid, considerable and unpredictable varia- tions of the ground potential.

It is not necessary that the electrodes 10A and 10B for sen¬ sing of the ground potential are mounted at the connection points A and B. As shown in Figure 6, for example, one of the electrodes 10B may be mounted displaced towards the point A and possibly relatively close thereto. The sensed difference between the potentials of the electrodes must then be corrected in some suitable way to provide a quantity which corresponds to the difference in ground potential between points A and B. For example, the latter quantity may be obtained by multiplying the sensed value by a con¬ stant L/l, where L is the distance between points A and B and 1 is the distance between electrodes 10A and 10B. Alternatively, the value of the constant may be determined empirically by measurement. If considered suitable, of course, both electrodes 10A and 10B may be displaced rela¬ tive to the connection points A and B.

According to an embodiment of the invention, as shown in Figure 7a, a plurality of controllable circuits with the current sources 4, 41, 42, 43 etc. may be connected to a plurality of pairs A-B, B-C, C-D etc. of connections on the pipeline. Each one of the circuits, for example 4, will then, in the manner described above with reference to Figures 4 and 5, maintain a potential difference between its two connection points - A, B - which corresponds to the difference between the ground potentials at the two points . In case of a ground potential variation according to curve a in Figure 7b, the potential of the pipeline shown by curve b is, for example, obtained. In this embodiment, it may be advantageous from an economic point of view to arrange the current sources of two adjacent circuits in a common unit.

If the ground potential is sufficiently constant between two connection points, for example points B and C in Figure 7, the free-floating circuit - 41 -, connected between these two points, may be omitted.

The circuits of adjacent sections may then be controlled so as to obtain a certain overcompensation. Figures 8a and 8b show how, with the aid of the current sources 4 and 42, the potential differences in the pipeline between points A and B and between points C and D are controlled so as to be greater than the ground potential differences between the corresponding points . In this way, a good adaptation of the tube potential to the ground potential can be obtained with a smaller number of controllable circuits .

The invention has been described above relating to a pipeline buried in the ground, but the invention may be applied, with the same advantage, to a pipeline disposed in water.

The conventional cathode protection equipment 3 in, for example, Figure 4 may be located at an arbitrary location along the pipeline. A plurality of such equipment may possibly be connected to the pipeline at separate locations. In order not to disturb the measurement, however, the elec¬ trodes of this plurality of equipment should be placed at a certain distance from the measuring electrodes - 10A, 10B - which are used in equipment according to the invention.

The current source and the control equipment described above with reference to Figure 4 are only examples, and equipment according to the invention may be designed in a plurality of other ways. For example, another current source than a phase-angle controlled rectifier may be used. A rectifier or other current source which can provide current in both directions may be used, thus eliminating the need of the current reverser. This need is also eliminated in those cases where, based on experience, the ground potential difference between the connection points in question never change signs.

The control system shown in Figure 4 may be designed in other ways within the scope of the invention. Instead of the proportional amplifier shown in Figure 4, an integrating regulator may be used. Similarly, the reference value which corresponds to the ground potential difference may be ob¬ tained in other ways than with the aid of measuring elec- trodes. In those cases where the ground potential gradients are primarily caused by ground currents from convertor or other d.c. plants, the potential differences will vary approximately proportionally to these currents. The refe¬ rence value for a certain pipe section between two connec- tion points may therefore be obtained by measuring the ope¬ rating current of the plant which causes the ground currents and multiplying the measured value by a constant which can be determined empirically by measuring during the commis¬ sioning of the equipment. The measured value or the refe- rence value can then be transferred from the plant to the compensation equipment according to the invention by cable, the telecommunication network, or by wireless communication. The ground currents and hence the ground potential gradients may also be due to the mode of operation of the plant, for example single- or two-pole operation in a bipolar plant for power transmission by means of high-voltage direct current, and the dependence of the reference value on the measured operating current of the plant may then be adjusted in dependence on the mode of operation of the plant which, at each moment, is known.

The above description is directed to an embodiment in which the actual value of the control system - the potential difference in the pipeline between two connection points - is obtained by measurement of the direct current of the current source. This method is simple, provides satisfac¬ tory results, and demands no extra measuring cables between the control system and the connection points. Alternative¬ ly, however, this actual value may be obtained in other ways, for example by direct measurement of the voltage between the connection points.

In the foregoing an embodiment has been described in which a closed-loop control system is arranged for control of the potential difference in the pipeline in accordance with the reference value supplied to the control system. In many cases, satisfactory accuracy can be obtained with the aid of an open-loop control system, that is, a system in which the direct voltage is controlled proportional to the reference value without any feedback occurring.

In the above the invention has been described with reference to a pipeline in which the ground potential differences are primarily caused by ground currents occurring in the form of direct currents. Since equipment according to the invention automatically ensures that the potential difference in the pipeline follows variations in the ground potential diffe- rence, such equipment with a suitable design may also be used in such cases where the ground currents are alternating • currents and thus the ground potential differences vary periodically with time. Since alternating currents, at least such with high amplitude, may cause corrosion, equip- ment according to the invention affords advantages also in these cases.

Claims

1. A device for compensation of varying electric potential in a medium (2) which surrounds a metallic pipeline (11) disposed in the medium, said pipeline being surrounded by an electrically insulating material (12) and being provided with equipment (3) for cathodic corrosion protection, characterized in that the device comprises

a) members (10A, 10B, 91) for forming a first quantity (du'_gr) which corresponds to the difference between the potentials (u_grA, u _rs) in the medium at two connection points (A, B) of the pipeline,

b) a current circuit (4, 81, 82) insulated from ground, said circuit comprising a controllable current source (4) and being connected to said connection points (A, B) to bring a current (I) to traverse the pipeline and thus cause a potential difference (du β) between the connection points,

c) control members (92, 94, 95) for control of the current source in dependence on said first quantity.

2. A device according to claim 1, characterized in that the current source (4) consists of a d.c. source.

3. A device according to claim 2, characterized in that the current source (4) consists of a rectifier with con¬ trollable direct voltage.

4. A device according to claim 3, characterized in that the rectifier is provided with members (7) for smoothing of the direct current (I) delivered by the rectifier.

5. A device according to any of the preceding claims, characterized in that the control members (92, 94, 95) are adapted to control the current source (4) such that the potential difference (du_AB) between said two connection points (A, B) corresponds to said first quantity (du'_gr) .

6. A device according to claim 5, characterized in that it comprises members (93, 96) adapted to form a second quantity (duΑβ) which corresponds to the potential difference between said two points (A, B) of the pipeline, and that the control members are adapted to be supplied with said first and second quantities and to control the current source in dependence on the difference (d) between said first and second quantities.

7. A device according to any of the preceding claims, characterized in that said members for forming the first quantity comprises two measuring electrodes (10A, 10B) arranged at spaced-apart points along the pipeline in the medium as well as a difference-generating member (91) con¬ nected to said measuring electrodes.

8. A device according to any of the preceding claims, characterized in that the pipeline has a plurality of pairs of connection points (AB, BC, CD) and that circuits (4, 41, 42), insulated from ground, are connected to at least certain of the pairs of connection points.