GB2124382A

GB2124382A - Determining the level of protection provided by a submarine cathodic protection system

Info

Publication number: GB2124382A
Application number: GB08219551A
Authority: GB
Inventors: Geoffrey Howard Backhouse; Richard John Holt
Original assignee: SUBSPECTION Ltd
Current assignee: SUBSPECTION Ltd
Priority date: 1982-07-06
Filing date: 1982-07-06
Publication date: 1984-02-15
Also published as: GB2124382B

Abstract

In a method for determining the level of protection provided by a cathodic protection system to a sub- sea structure, e.g. a pipeline, at an area of the structure which is buried beneath the sea-bed, in addition to measuring the potential of the sea adjacent the bed above the area relative to the (reference) potential of the sea remote from the protection system, measurements are made of the electrical field gradient of the sea- water at the test area, and of the electrical resistivities of the sea-water and mud at that area, whereby a more accurate calculation of the potential at the surface of the buried structure can be made. Shown is a probe suitable for such measurements and including two spaced identical silver/silver chloride electrodes 44, 46 in brine 48 and two stainless steel electrodes 40, 42 between which alternating current may be passed for resistivity measurements. Porous plug 52 between brine 48 and the seas carried in a flexible pressure balancing diaphragm enabling close rectifying of electrodes 44, 46 to be maintained. A separate silver/silver chloride brine reference Rolf cells similarly constructed. <IMAGE>

Description

SPECIFICATION Method and apparatus for surveying a sub-sea structure protected by a cathodic protection system This invention relates to the surveying of subsea structures, particularly but not exclusively submarine pipelines and flowlines, which are protected against corrosion by cathodic protection systems, in order to determine the level of protection which is being provided by such systems.

As is well known, it is conventional to protect metallic structures which are partially or completely immersed in sea water against electrochemical corrosion by so-called cathodic protection systems which serve to inhibit the corrosive process by lowering the normal electrical potential of the metallic structure being protected, and thereby retard the dissolution of the metal in the sea water. Cathodic protection systems are generally of two kinds, namely the sacrificial anode system and the impressed current system.In the sacrificial anode system there are attached to the structure to be protected electrodes of a metal which is more electropositive than that of the structure itself, so that there is formed an electrochemical cell in which the attached electrodes are the anodes from which dissolution of cations preferentially takes place, the structure to be protected forms the cathode, and the electrolyte is, of course, the sea water. Typically, as applied to a sub-sea pipeline, the pipeline will be provided with sacrificial anodes made from zinc or aluminium alloys in the form of encircling bracelets placed approximately every 1 30 metres along the length of the pipeline.

The impressed current system works on the same general principles but as its name suggests, the electrical potential of the structure is depressed by directly connecting the structure to the negative terminal of a direct current supply. The present invention is applicable to both kinds of cathodic protection systems.

It is a common practice to survey cathodically protected sub-sea structures periodically, in order to check the effectiveness of the protection system, and indeed in many countries it is now a statutory requirement that such surverys be carried out at regular intervals. One known survey method involves measuring the electrical potential difference between an electrode placed close to the underwater structure and a second electrode placed in the sea water at a point which is sufficiently remote from the test structure to be substantially free from the electrical influence of its cathodic protection system. To this value is added the potential of the remote electrode relative to the structure itself, which is periodically determined whenever direct metal contact with the underwater structure can be made.The value thus determined of the potential of the structure at the test point is compared to the predetermined value for effective protection (an industry standard is a negative potential of 800 millivolts relative to the standard silver/siiver chloride electrode), and in this way it is determined whether adequate cathodic protection is being maintained at the test point.

In such a survey method, the measured value of the electrical potential of the test point on the underwater structure is only accurate when it is possible to make metal-to-metal contact between the structure and one terminal of the metering device. However, often this is not possible.

Particularly in the case of submarine pipelines considerable lengths of the pipeline become buried to a greater or lesser depth in the seabed, and in such cases the measured potential at the test point, which is then in the water above the buried pipeline, may be significantly different from the true electrical potential of the pipeline itself. A false indication of the level of protection which is being provided by the cathodic protection system being surveyed is then given.

The present invention seeks to provide a survey method, and apparatus for use therein, which enables a more accurate determination of the effectiveness of cathodic protection systems to be obtained even though the underwater structure is at least partially buried in the mud beneath the sea floor.

In accordance with one aspect, the present invention provides a method of determining the level of protection provided by a cathodic protection system to a sub-sea structure at an area of the structure which is buried in the seabed which comprises: (a) deriving an electrical signal which is a function of the electrical potential of the sea water at a point adjacent the seabed above said area, relative to the potential of the sea water in the region substantially free from the electrical influence of said system; (b) deriving an electrical signal which is a function of the electrical field gradient of the sea water in the vicinity of said point; (c) deriving electrical signals which are a function of the electrical resistivity of the sea water and seabed, respectively, in the vicinity of said area; and (d) using said signals to determine the electrical potential at said area of said structure.

In order that the electrical signals (a)-(c) which are derived in the vicinity of the test area of the buried structure can be used in step (d) to determine the actual electrical potential at the test area, it is necessary to determine the electrical potential of the structure itself relative to the sea water in a region which is substantially free from the electrical influence of its cathodic protection system so as to provide proper calibration for the measured values. This requires that there should be at least one measurement of electrical potential with direct metal contact with the structure. In the case of a pipeline, such direct measurements preferably are made whenever an exposed area of the pipeline is encountered, which however may only be every 10 km or so along its length.

An important feature of the survey method of the present invention is that, unlike the known survey method discussed above, it takes into account that there may be relatively large ohmic potential drops across the mud lying above a buried structure. Whilst the existence of this factor has previously been known, we are not aware that it has higherto been appreciated that the resistivity, and hence the electrical potential drop across, the mud can differ significantly from region to region, even along the path of a single pipeline. Moreover, the resistivity of sea water can very from place to place.By making measurements which depend upon the actual, rather than any assumed value of the electrical resistivity of the sea water and mud in the region of the test area of the buried structure, as well as of the electrical field gradient in the water in that region, it becomes possible to make a more accurate determination of the true electrical potential of the buried structure. However, when making a continuous survey of a large buried structure such as a pipeline, it is not normally necessary at every point to measure the electrical resistivity of the sea water and mud, since it usually is satisfactory to assume that such resistivity will not alter significantly over a relatively small area. For example, if a pipeline is being surveyed, it would usually be adequate-to measure the resistivity of the mud only every 1000 metres, and of the sea water only every 5000 metres.

The determination of the electrical potential difference between a remote point (that is, one which is to all intents and purposes away from the electrical influence of the cathodic protection system), and a point in the water near the sea bed, above the buried structure i.e. signal (a), may be carried out by conventional procedures.

Preferably, therefore, this value is derived by measuring the potential between a lower probe electrode of silver/silver chloride in pure brine electrolyte and a remote electrode of a similar kind. If surveying a pipeline, it would be normal to make a continuous recording of this potential difference along the complete length of the pipeline. The information which this measurement provides is of the variation of potential which exists at given points in the vicinity of the pipeline as a result of its cathodic protection system, but unless the probe electrode is able to make metal to-metal contact with the pipeline, this potential will not represent the actual potential at the surface of the pipeline at that point, referred to the silver/silver chloride electride.

It is to enable the true potential to be more accurately calculated even if the pipeline, or other structure, is buried in the mud, that the further electrical signals (c) and (d) as defined above are generated, in accordance with the survey method of the present invention. The first of these additional signals may be conveniently derived by measuring the potential between the silver-silver chloride probe electrode and a second silver/silver chloride electrode spaced a short distance, e.g.

about 0.1 to 0.5 metres above the probe electrode. Measurement of this potential difference enables calculation of the current flowing to the buried structure at the test point, which is one of the parameters required in order that the true potential of the buried structure can be ascertained. It is desirable, in order to ensure optimum accuracy, that the two silver/silver chloride electrodes should be as closely matched as is possible, preferably being within 10 microvolts of each other. We have found that it is not normally possible to obtain such a close match from commercially available silver/silver chloride electrodes, and we have therefore prepared our own electrodes in order to achieve the desired matching.

As will be appreciated, the two, closely spaced, silver/silver chloride electrodes may also be used to measure the output currents of sacrificial anodes, thereby allowing their life expectancy to be calculated.

The second important parameter which is required in order to enable the potential of a buried structure to be accurately determined is the resistivity of the sea water and of the mud, respectively, at the test point. In a preferred embodiment of this invention, signals representative of these values may be obtained, whenever required, by passing a constant alternating current between a third electrode, for example of stainless steel, disposed above the upper silver/silver chloride electrode, for example by a distance between 0.1 and 0.3 metres, and a fourth electrode, for example of steel, disposed below the lower silver/silver chloride electrode, and measuring the resulting alternating potential difference developed between the two silver/silver chloride electrodes, whilst all four electrodes are either located in the sea water, to determine sea-water resistivity, or plunged into the mud, to determine mud resistivity. The measured alternating potential difference is directly proportional to the resistivity of the sea water or mud, as the case may be.

The fourth i.e. lowermost, electrode may conveniently be formed by the tip of an electrode probe which carries all four electrodes.

The measurements which are rnade in the survey method of this invention enable an accurate determination of the electrical potential at a test area of a buried structure to be made. As will be understood, these measurements do not lead directly to the desired potential value, but this can readily be calculated from the measured data in conjunction with data, previously known or measured at the same time at the test area, concerning the spacing of the electrodes, both relatively to each other, and to the sea bed, the depth beneath the sea bed of the buried test structure, and the geometry of the test structure, e.g. the radius in the case of a buried pipeline.In practice, it is desirable that the electrical signals which are derived as a result of the measurements made in the survey method be fed to a suitably programmed computer for processing and permanent recording.

The present invention also provides apparatus for use in determining the level of protection provided by a cathodic protection system to a sub-sea structure at an area of the structure which is buried in the sea-bed, which comprises: (a) means for deriving an electrical signal which is a function of the electrical potential of the sea-water at a point adjacent the sea-bed above said area, relative to the potential of the sea-water in a region substantially free from the electrical influence of said system; (b) means for deriving an electrical signal which is a function of the electrical field gradient of the sea water in the vicinity of said point; and (c) means for deriving electrical signals which are a function of the electrical resistivity of the sea-water and sea-bed, respectively, in the vicinity of said area.

As a specific feature of this invention, we provide a novel electrode probe useful in the method and apparatus of the present invention.

This probe carries four electrodes spaced apart along its length, namely upper and lower electrodes, for example of steel, and two intermediate silver/silver chloride reference electrodes which, as mentioned above, are preferably very closely matched to each other.

Preferably, each silver/silver chloride reference electrode is contained within a housing providing electrolytic connection between the brine of the electrode and the ambient sea water through a porous ceramic, e.g. alumina, plug mounted by a flexible diaphragm e.g. of rubber serving to equalise the hydrostatic pressure on either side of the ceramic plug. This expedient helps to prevent access of sea water impurities into the brine electrolyte of the silver/silver chloride electrodes, and thus enable the close matching of the two reference electrodes to be maintained.

Preferably, the remote silver/silver chloride electrode which may be used in the generation of electrical signal (a) is likewise housed.

We also provide a novel system for providing a constant alternating current source, useful in making the measurements of electrical potential which are a function of the electrical resistivities of the sea water and mud at the test point. This system will be described in detail below.

Reference will now be made to the accompanying drawings, in which: Figure 1 is a schematic diagram illustrating how a survey method of the present invention may be carried out; Figure 2 is a schematic diagram showing in cross-section the construction of an electrode probe of the present invention; Figure 3 is a schematic diagram showing in cross-section the construction of a remote electrode of the present invention; and Figure 4 is a circuit diagram of a constant AC current source useful in this invention.

Referring first to Fig. 1, there is shown a submersible vehicle 10, which may be manned or remotely controlled, moving along the sea-bed 12 beneath which is buried a pipeline 14 being surveyed. Although not shown, the pipeline 14 is provided with a cathodic protection system, either of the sacrificial anode or impressed current type.

Normally, the pipeline 14 would have an external insulating coating, for example of coal tar enamel which in turn would be surrounded by a cladding oi reinforced concrete.

The vehicle 10 has a manipulator arm 16 to which is attached an electrode probe 1 8 carrying four electrodes, and which is described in greater detail below. A remote electrode 20, also described in more detail below, is attached to an umbilical cable 22 leading from the vehicle 10 to a support vessel 24 which contains the electronic equipment to which the electrical signals generated during the survey are fed. The remote electrode 20 is sufficiently removed from the pipeline 14 to be, for all practical purposes, outside the influence of the electrical field generated by its cathodic protection system.

Alternatively, the remote electrode 20 may be held on a buoy (not shown), which is preferred when the vehicle 10 is manned rather than controlled remotely.

The vehicle 10 houses a signal processing unit 26 which is powered by an electric current source, not shown. The unit 26 receives signals from, and sends signals to, the electrode probe 18, and is connected to a data transmitter/receiver 28 on board support vessel 24. As diagrammatically shown, the transmitter/receiver 28 is linked to computer 30 which controls data-storage and display units 32, 34 respectively.

In operation, the support vessel 24 and submersed vessel 10 move along the line of the pipeline 14, which although shown as being buried in the mud in Fig. 1 would typically have exposed lengths as well, making both continuous and periodic measurements of potential, as described in more detail below.

The electrode probe 1 8 is shown in more detail in Fig. 2. An internal metal tube 36, e.g. of steel, is surrounded by an insulating sleeve 38 of plastics material e.g. polyvinyl chloride, and carries four spaced electrodes, and their associated conductors. The uppermost electrode 40, i.e. the left-most electrode as shown in Fig. 2, and the lowermost electrode 42, which is located in the tip of the probe 18, are provided primarily for passing an alternating current during the measurements which provide information on the electrical resistivity of the sea water above the pipeline or of the mud in which the pipeline is buried, and therefore may be formed of any suitable metal, for example stainless steel. The two intermediate electrodes, 44, 46 are silver/silver chloride electrodes in an electrolyte 48 of pure brine.Although only electrode 44, and its housing, are shown in detail, the second silver/silver chloride electrode 46, and its housing, are of identical arrangement, and as already noted above, the two electrodes 44, 46 are preferably matched at least two within 10 microvolts of each other.

As is shown for electrode 44, each electrode 44, 46 is contained within a housing 50 which can be unscrewed from the probe for repair or replacement of the electrode should this become necessary. Electrolytic connection between the brine electrolyte 48 and the sea-water outside the probe is provided by a porous ceramic plug 52, preferably alumina, which is carried by a diaphragm 54 of rubber or other flexible material which acts as a closure member for the housing 50. The provision of such a flexible closure results in the hydrostatic pressure on either side of the porous ceramic plus 52 being equalised, thereby helping to prevent the access of sea-water impurities to the being electrolyte 48 which would destroy the close matching of the electrodes 44, 46.

The lowermost electrode 42 can also be used to make occasional calibration contacts with exposed areas of the sub-sea structure being surveyed, or of its sacrificial anodes where these are present. Such calibration contacts serve to provide absolute readings of the electrical potential of the structure, which are useful for calibrating the results obtained for buried areas of the structure.

All four electrodes 40, 42, 44, 46 are connected by insulated cables, which pass through the interior of the tuve 36, to an underwater connector 56 which is mounted on the upper end of the probe, and shrouded for protection.

The electrode probe is mounted on the arm 1 6 of the submersed vehicle 10 by means of a shockabsorbing bracket 58.

Fig. 3 illustrates the construction of the remote electrode 20. This also is a silver/silver chloride electrode 60 with a pure brine electrolyte 62, and like the silver/silver chloride electrodes 44, 46 in the probe, is contained within a housing having a pressure-balancing flexible closure diaphragm 64 provided with a porous ceramic plug 66 for electrolytic connection between the brine electrolyte and the ambient sea water.

Attachments 68 are used to clip the electrode to the umbilical cable 22.

The four electrodes of the probe 1 8 and the remote electrode 20 are connected to the imput circuits in the signal processing unit 26 via underwater cables. The unit 26 measures the potentials which appear at its input channels and converts them into coded signals which are sent to the transmitter/receiver 28 at the surface on demand, for processing and recording by the computer-controlled data logging system.

When a survey is in progress, the signal processing unit 26 can measure the following electrical potentials: (1) the potential between the lower silver/silver chloride reference electrode 44 of the probe and the remove electrode 20. This provides the potential existing in the sea water at point A (Fig.

1), just above the sea-bed 12. When surveying a large structure such as a pipeline, this potential value is normally measured continuously, in order to give a continuous potential "profile" of the structure.

(2) The potential between the lower silver/silver chloride reference electrode 44 of the probe and the lowermost electrode 42 thereof when the latter electrode is able to make direct metal-to-metal contact with exposed areas of the structure. This gives calibration potentials, which are used for calibrating the other measured potentials.

(3) The potential between the two silver/silver chloride electrodes 44, 46 of the probe. This gives the value of the electrical field gradient between points A and B (Fig. 1) in the sea-water, which is one of the parameters which enables the potential at test point X (Fig. 1) of the buried structure to be more accurately determined. This measurement of electrical field gradient can also be used to measure the output currents of sacrificial anodes, thereby permitting their life expectancy to be calculated. The measurement of the electrical field gradient between points A and B would normally be made continuously when a continuous survey is being performed.

(4) The alternating potential developed between the two silver/silver chloride electrodes 44, 46 of the probe whilst a constant alternating current is passed between the upper and lower electrodes 40, 42 thereof. This measurement, which is normally made only periodically if a large structure such as a pipeline is being surveyed, provides a measurement of the electrical resistivity of the sea-water in the vicinity of the test point X, if the four electrodes are located in the sea-water, or if the probe is plunged into the mud it similarly provides a measurement of the electrical resistivity of the mud in the vicinity of point X.Thus, the measured alternating potential is directly proportioned to the electrical resistivity of the medium (sea-water or mud) in which the measurement is made, as shown by the equation: VAC=KplAC where VAC is the measured alternating potential, IAC is the applied constant alternating current, p is the electrical resistivity, and K is a constant, whose value in any particular case depends on the geometry of the measuring electrodes, i.e. of the electrodes 40-46 in the illustrated embodiment.

This measurement of potential which is a direct function of the electrical resistivity of the seawater or mud is a further parameter enabling the potential at test point X to be accurately determined. However, as already mentioned above, it is not usually necessary to make this measurement on a continuous basis, when a continuous survey is being conducted, since the resistivities of the mud, and particularly of the sea-water, may be safely assumed not to vary significantly over a relatively small area. In any event, the AC current source must not be applied continuously, since the resulting disturbance would cause errors in the normal cathodic protection potential measurements. Accordingly, the current source is adapted to be switched on and off at will by a signal from the controlling computer 30.

In addition to the potential measurements discussed above, in order to enable the electrical potential at test point X to be accurately determined, it is also necessary, as already mentioned, to have knowledge of, or as required to make measurements of, the spacing of the electrodes, both relatively to each other and to the seabed, the depth beneath the seabed of the buried test structure, the radius of the test structure, and for calibration purposes the electrical potential of the structure relative to the sea water in a resgion which is substantially free from the electrical influence of its cathodic protection system.

A circuit diagram of a constant alternating current source which we have developed for use in the present invention is shown in Figure 4. In this system, an AC oscillator (1) drives a power AC amplifier (3) via a voltage controlled attenuator (2). The output stage is transformer coupled (4) to the driven electrode (5) i.e.

electrode 40 of Figure 2 and electrode 6, i.e., electrode 42 of Figure 2 via a current transformer (7), and DC isolation capacitor (8). The current transformer signal is fed to a peak-to-peak detector circuit (9) which then produces a DC signal directly proportional to the AC output current. The peak-to-peak detector output is then compared against a preset voltage level which is provided by a potential divider network (10). The voltage comparator (11) gives an error signal which controls the attenuator (2) so closing the control loop.

In this way the output AC current is controlled by the preset voltage level on (10), only, within the limits of the output voltage of the output stage (3).

The measuring circuit consists of an AC buffer amplifier (12) connected to the matched silver/silver chloride reference electrodes (electrodes 44, 46 of Fig. 2).

The output of the buffer feeds a peak-to-peak detector circuit (13), which gives a DC output proportional to the peak-to-peak voltage measured across the electrodes. The AC voltage measured between A and B is dependent on the electrode geometry, the AC current, and the electrolyte resistivity. As the geometry and AC current are both constant, the AC voltage measured across A and B is then directly proportional to electrolyte resistivity.

The automatic constant AC source is contained in the signal processing unit 26. Also one of the signal input channels is designed to respond to the absolute value of the alternating potential between the matched silver/silver chloride electrodes.

In order to maintain optimum accuracy for small signals, the computer 30 is programmed to command the signal processing unit 26, so that the most sensitive signal channel is used for reading small signals. In this way, a high dynamic range can be achieved.

Claims

1. A method of determining the level of protection provided by a cathodic protection system to a sub-sea structure at an area of the structure which is buried in the sea-bed, which comprises: (a) deriving an electrical signal which is a function of the electrical potential of the seawater at a point adjacent the sea-bed above said area, relative to the potential of the sea-water in a region substantially free from the electrical influence of said system; (b) deriving an electrical signal which is a function of the electrical field gradient of the sea water in the vicinity of said point; (c) deriving electrical signals which are a function of the electrical resistivity of the seawater and sea-bed, respectively, in the vicinity of said area; and (d) using said signals to determine the electrical potential at said area of said structure.

2. Apparatus for use in determining the level of protection provided by a cathodic protection system to a sub-sea structure at an area of the structure which is buried in the sea-bed, which comprises: (a) means for deriving an electrical signal which is a function of the electrical potential of the sea-water at a point adjacent the sea-bed above said area, relative to the potential of the sea-water in a region substantially free from the electrical influence of said system; (b) means for deriving an electrical signal which is a function of the electrical field gradient of the sea water in the vicinity of said point; and (c) means for deriving electrical signals which are a function of the electrical resistivity of the sea-water and sea-bed, respectively, in the vicinity of said area.