Filed of the invention

The invention relates to a corrosion monitoring system for detecting corrosion due to ingress of water into an insulation system of a metal body such as a pipe, and to a method of corrosion monitoring.

Background

It is conventional to insulate metal pipelines with an insulation system that may comprise a mantle of thermal insulation material and a jacket around the insulation material. Ingress of water into the insulation system can cause gradual corrosion of the pipeline. When this continues over a long enough period, e.g. one or more years, this corrosion can cause pipeline defects.

WO2017129983 describes a method of getting advance warning of such corrosion. An electromagnetic waveguide in the form of a coaxial cable or a wire pair cable is used in the insulation system. The outer conductor of the coaxial cable or at least one of the wires of the wire pair cable, which is exposed to water in the insulation system (if any) is made of the same material as the pipe, or a less corrosion resistant material than the material of the pipe.

When water enters the insulation system, the water can reach both the pipe and the exposed part of the cable. Thus ingress of water infiltration will start corrosion processes in both the pipe and the cable. The effect on the cable is that it changes the impedance of at least the exposed conductor of the cable. This effect can be detected by means of electrical test signals. By means of time delay measurements the position of such corrosion along the pipe can be determined, making it possible to repair or replace the insulation system before the ingress of water infiltration has caused excessive damage to the pipeline.

In the embodiment that uses a coaxial cable, both the outer conductor and the inner conductor of the coaxial may be of copper, with a dielectric insulator in between. In addition, a perforated layer such as plastic netting is present around the outer conductor of the coaxial cable to keep the outer conductor electrically separated from the pipe, and allowing the water to reach the outer conductor of the coaxial cable. In this way corrosion of the outer conductor of the coaxial cable can be used as an indicator of a risk of corrosion of the pipe.

For reliable detection, corrosion of the cable due to water that has penetrated the insulation system of the pipe should proceed as fast as corrosion of the pipe and preferably faster. WO2017129983 describes that this can be realized by making the exposed conductor of the cable of a material (e.g. Cu) that corrodes faster than the material of the pipe (e.g. A1 or steel).

Summary Among others, it is an object to provide for a system and method for monitoring a risk of corrosion of a metal body by means of an electric cable in an insulation system of the metal body that makes it possible to detect a risk of corrosion of the metal body sooner.

A system for monitoring corrosion of a metal object according to claim 1 is provided. Herein, an electric cable is used in the mantle with a first electrical conductor having a smaller surface area per unit length than the second electrical conductor, characterized in that the first and second electrical conductor are made of a first and second material respectively, the first material being less corrosion resistant in the system than the second material when water penetrates the mantle. The metal object may be a pipe for example. Because a small, less corrosion resistant first conductor is used together with a larger, more corrosion resistant conductor, the effect of corrosion on the first conductor sooner becomes detectable.

Preferably a DC galvanic connection between the first and second electrical conductor is present in the system at least during long periods wherein no measurements are performed. This affects the difference between the corrosion resistance of the materials in the system and makes it easier to create a system wherein there is a significant difference between the corrosion resistance of the first and second material. For example, when the first electrical conductor is of carbon steel and the second conductor is of stainless steel, without galvanic connection the carbon steel would corrode about 0.2-0.5 mm per year, and the corrosion of the stainless steel would be negligible compared to this. But when a galvanic connection between the first and second electrical conductor is present, the carbon steel would corrode five to ten times more per year.

Preferably a coaxial cable is used, with the inner conductor as the first conductor of the less corrosion resistant material, and a water permeable electrical insulator and outer conductor surrounding the first electrical conductor. In this way reproducible relative positions of the conductors can easily be ensured, so that corrosion effects can easily be measured on the inner conductor in a well controlled context.

The water permeability may be achieved by using braided strands of conductor and/or isolation material. This provides for a strong cable that can easily be manufactured. The water may permeate between the strands. Optionally water permeable strands may be used, to increase the overall permeability.

Preferably a removable short circuit between the first and second electrical conductor is used at an end of the electrical cable, and preferably at both ends. When water leaks in between the conductors this provides for an electrical circuit loop, which increases the corrosion rate of the conductors, due to galvanic corrosion, thus making it possible to detect corrosion sooner. When water permeates, both material are exposed to the same electrochemical solution and they are in electric contact and this will cause galvanic corrosion where the water is present.

According to another aspect a method of monitoring a system for water infiltration is provided that comprises keeping the first and second electrical conductor short circuited to each other during periods of normal use of the metal object;

- removing the short circuit to create an interruption between these periods and

- measuring reflections of electrical signals sent along the cable during the interruption

When short circuits are used at both ends of the cable, at least one and preferably both are removed for the measurement.

Brief description of the drawing

These and other objects and advantageous aspects will become apparent from a description of exemplary embodiments with reference to the following figures

Figure 1 shows a cross-section of a pipe and a cable

Figure 2 shows a detailed cross-section of a cable

Figures 3a, b show side views of the metal pipe

Figures 4 shows a measurement configuration

Figures 5 shows a detailed cross-section of an alternative cable

Detailed description of exemplary embodiments

Figure 1 shows a system for monitoring corrosion of a metal object in cross section. The system comprises a metal pipe 10 and an insulation system 12 surrounding metal pipe 10. The insulation system 12 may comprise a mantle 120, e.g. of thermal insulation material, surrounding metal pipe 10 and an insulation jacket 122 surrounding the mantle. An electric cable 14 is provided in mantle of insulation material. Although only one electric cable 14 is shown in insulation system 12, it should be

appreciated that a plurality of electric cables may be provided in insulation system 12. Although a preferred example with a metal pipe 10 is shown, it should be appreciated that instead another type of metal object with an insulation system 12 may be used, such as a tank or a vessel. Insulation system 12 may entirely surround the metal object or only around an axial direction, as shown for metal pipe 10.

Figure 2 shows a cross-section of the cable in more detail. The cable includes a core electrical conductor (also referred to as first electrical conductor 20), a water permeable inner electrical insulator 22 around first electrical conductor 20, a water permeable surrounding electrical conductor 24 (also referred to as second electrical conductor) around inner electrical insulator 22 and a water permeable outer electrical insulator 26 around second electrical conductor 24. The electrical insulator 22, second electrical conductor 24 and outer electrical insulator 26 are arranged so that water from mantle 120 can reach the core electrical conductor (first electrical conductor 20) through inner electrical insulator 22 and second electrical conductor 24 and if necessary via outer electrical insulator 26. Preferably, the diameter of first electrical conductor 20 is considerably smaller than that of second electrical conductor 24, preferably no more than 0.5, 0.2 or 0.1 times the diameter of the coaxial cable.

Outer electrical insulator 26 is optional. It has the advantage that it prevents direct contact between second electrical conductor 24 and metal pipe 10, which reduces possible corrosion of metal pipe 10. But when second electrical conductor 24 is fixed in mantle 120 so that second electrical conductor 24 cannot come into contact with metal pipe 10, outer electrical insulator 26 may not be needed for this. Outer electrical insulator 26 may similarly not be necessary if the increase of corrosion of metal pipe 10 due to contact is acceptable. Outer electrical insulator 26 may also make it easier to use electric signals to detect corrosion

Inner electrical insulator 22 and optional outer electrical insulator 26 may be made of braided electrically insulating strands. Similarly, second electrical conductor 24 may be made of braided electrically conductive strands. The braiding of inner electrical insulator 22, second electrical conductor 24 and outer electrical insulator 26 makes inner electrical insulator 22, second electrical conductor 24 and outer electrical insulator 26 water permeable. Optionally water permeable strands may be used, to increase the overall permeability. Instead of braided material other water permeable arrangements of materials may be used in electrical insulator 22, second electrical conductor 24 and outer electrical insulator 26, such as netted material woven material, perforated material etc. Electrical insulator 22 and outer electrical insulator 26 are made of electrically insulating material.

The outer surface of pipe 10 may suffer corrosion damage when insulation system 12 suffers from ingress of water from outside. The cable is used to make it possible to detect the ingress. When water permeates insulation system 12, the water is able to reach the core electrical conductor (first electrical conductor 20) of the coaxial cable because inner electrical insulator 22, second electrical conductor 24 and outer electrical insulator 26 are water permeable.

First and second electrical conductor 20, 24 are of electrically conductive material, e.g. of metal. First electrical conductor 20 may be of the same metal as metal pipe 10, or of a material with less corrosion resistance than metal pipe 10, or at least of the same metal as the outer surface of metal pipe 10, or of a material with less corrosion resistance than the outer surface of metal pipe 10. Preferably, second electrical conductor 24 is of a more corrosion resistant material than first electrical conductor 20. Second electrical conductor 24 may be of the same material as metal pipe 10 (e.g. of carbon steel). Preferably, the material of second electrical conductor 24 is more corrosion resistant than that of pipe 10. Use of a high corrosion resistant material in second electrical conductor 24 helps to ensure that measurements using the cable will substantially be unaffected by corrosion of the second electrical conductor 24 (the outer conductor in the case of the coaxial cable). But for some measurements this may not be needed, and in some cases the material of second electrical conductor 24 may be the same as that of pipe 10.

In one example, the first electrical conductor is of carbon steel and the second conductor is of stainless steel. In this case, without galvanic connection the carbon steel would corrode about 0.2-0.5 mm per year, and the corrosion of the stainless steel would be negligible compared to this. When a galvanic connection between the first and second electrical conductor is present, the carbon steel would corrode five to ten times more per year.

It can be determined whether a material is more or less corrosion resistant than another material by comparing the corrosion rates of the material. The corrosion rate of a material is the thickness reduction of the uncorroded material in a time unit, e.g. the thickness reduction of a flat layer or a cylinder during one month or one year. The difference between the corrosion rates of materials in the system can be advantageously affected when a galvanic connection is used between the first and second electrical conductors. When materials with different electrochemical potentials are kept galvanically connected, the corrosion rate of the material with the lowest electrochemical potential will be increased relative to that of the material with the highest electrochemical potential. Generally, a material with a higher electrochemical potential is already more corrosion resistant than a material with a low electrochemical potential even without galvanic connection.

Preferably, the material of the second electrical conductor is chosen from materials that have a negligible corrosion rate, which is usually the case for materials with an electrochemical potential of at or above -0.2 Volt (as used herein, all values of the electrochemical voltages are relative to the electrochemical potential of a Saturated Calomel Electrode. Saturated Calomel Electrode is merely used to defined the reference potential level: no Saturated Calomel Electrode need be present in the system). The material of the second electrical conductor is chosen from materials that a

significantly higher corrosion rate, which is usually the case for materials with an electrochemical potential at or below -0.4 Volt.

Preferably, the electrochemical potential of the first electrical conductor (e.g. the core of the coaxial cable) is at least 50 millivolt, and more preferably at least 100 milhvolt less than that of the second electrical conductor. For example, stainless steel 430 or copper are more corrosion resistant than low alloy steel or low alloy steel cast irons. This correlates with the fact the electrochemical potentials of stainless steel 430 and copper are respectively -0.2 V and -0.3 volts, which are higher than the

electrochemical potentials of low alloy steel and low alloy steel cast iron, which are respectively -0.6 V and -0.7V. The material of the first electrical conductor is considered to be less resistant to corrosion than the material of the second electrical conductor if the material of the first electrical conductor is further down such a list, preferably with at least 50 millivolt lower electrochemical potential.

A more extensive list of materials in order of decreasing resistance to corrosion and electro chemical potential in Volts under typical

circumstances, from which materials with relatively more and less corrosion resistance can be selected according to the their order in the list is

Graphite 0,25, Platinum 0,2, Gold 0,2, Ni-Cr-Mo Alloy C 0, Titanium 0, Ni-Cr-Mo-Cu-Si Alloy G 0, Ni-Fe-Cr Alloy 825 0V, Alloy 20 stainless steel 0. Materials with increasingly negative electrochemical potential are

Stainless steel 316-317 minus 0, 1 Ni-Cu Alloy 400 k-500 minus 0, 1 Stainless steel 304, 321, 347, minus 0, 1 Silver minus 0, 1, Nickel 200 minus 0, 1, Silver-bronze alloys minus 0,1, Nickel-chromium Alloy 600 minus 0, 1,

Nickel aluminum bronze minus 0,2, 70-30 copper nickel minus 0,2,

Lead minus 0,2, Stainless steel 430 minus 0,2, 80-20 copper-nickel minus 0,2, 90-10 copper-nickel minus 0,2, Nickel silver-0,2, Stainless steel type 410, 416 minus 0,2, Tin bronze (G&M) minus 0,2, Silicon bronze minus 0,2, Manganese bronze minus 0,2, Admiralty brass, aluminum brass minus 0,3, 50 Pb-50 Sn minus 0,3, Copper minus 0,3, Tin minus 0,3, Naval brass, yellow brass, red brass minus 0,3, Aluminum bronze minus 0,3, Austenitic nickel cast iron minus 0,5, Low-alloy steel minus 0,6, Low-alloy steel cast iron minus 0,7, Cadmium minus 0,7, Aluminum alloys minus 0,9, Beryllium minus 1, Zinc minus 1, Magnesium minus 1,6.

When water permeates insulation system 12 and reaches the core electrical conductor (first electrical conductor 20) of the coaxial cable, this will cause corrosion of the core electrical conductor (first electrical conductor 20). First electrical conductor 20 will corrode faster than that of metal pipe 10. Even if the material of the conductors is the same, the relative corrosion of first conductors 20 due to galvanic current will be larger than that of second conductor 24, because the latter has a larger surface area, and this is all the more so when the material of second conductor 24 has a higher electrochemical potential than that of first conductor 20. Due to the coaxial arrangement of the conductors, the surface area of first electrical conductor 20 is much smaller than the surface area of second electrical conductor 24 that is exposed to the water. This is already so because the diameter of second electrical conductor 24 is larger than that of first electrical conductor 20, and even more because both the inner and outer surface of second electrical conductor 24 are exposed to water. As used herein, the term system for monitoring corrosion of a metal object refers to the suitability of the system for corrosion detection in the form of the dependence of properties of the cable, as used in the system, e.g. in a system with or without galvanic connection between the conductors, on the same water infiltration in the mantle that also affects the metal object within the mantle. As used herein the term does not necessarily include measurement equipment to measure these properties. A galvanic connection between the conductors increases the corrosion rate of first electrical conductor 20 compared to that of metal pipe 10.

Figures 3a, b show side views of metal pipe 10. In figure 3a, electric cable 14 extends in parallel with the axial direction of metal pipe 10. A removable short circuit 30, which galvanically connects first and second electrical conductor 20, 24 is provided at first end of electric cable 14.

Preferably, a second removable short circuit (not shown), which galvanically connects first and second electrical conductor 20, 24 at a second end of electric cable 14 (not shown) is used as well. In figure 3b, electric cable 14 runs helically around the central axis of metal pipe 10. Although a removable short circuit 30 at an end of the cable is shown, it is noted that instead, or in addition, one or more removable short circuits at one or more locations remote from the ends may be used.

The effect of removable short circuit 30 is that core electrical conductor (first electrical conductor 20) corrodes at least partly due an electrolytic corrosion process fed by electrical current from second electrical conductor 24 that occurs where water has permeated into inner electrical insulator 22, This current flows in a circuit through removable short circuit 30, first and second electrical conductor 20, 24 and the water that has permeated into inner electrical insulator 22 between first and second electrical conductor 20, 24. As may be noted, such a corrosion process causes more corrosion of first electrical conductor 20 in proportion to the size of first electrical conductor 20 than of second electrical conductor 24 in proportion to its size. The surface area per unit cable length of first

electrical conductor 20 is much smaller than that of second electrical conductor 24.

Figure 4 shows a testing configuration for using the system for monitoring corrosion of the metal object. In the testing configuration, short circuit 30 has been removed and a test device 40 has been connected to electric cable 14. Test device 40 may be configured to apply a time

dependent voltage between first and second conductor 20, 24 and to measure a resulting current and/or reflected electromagnetic signal that arrives through electric cable 14. Test device 40 may be configured to operate as described in WO2017129983, e.g. by using time domain

reflectometry to measure an effect and location of corrosion, or by means of measurement of the impedance of the cable.

Before test device 40 is connected to electric cable 14, electric cable 14 may be kept in a configuration like that shown in figure 3a, b, with a short circuit between first and second conductor 20, 24 for an extended period of time (e.g. more than a week, a month or even a year). After test device 40 has been used to perform measurements, e.g. after a few minutes and generally less than a day, electric cable 14 is preferably returned to a configuration like that shown in figure 3a, b.

As will be appreciated, when water permeates into electric cable 14 while the cable is in the configuration of figures 3a, b, a subsequent measurement in the configuration of figure 4 will be affected mostly by the effect of corrosion of the core electrical conductor (first electrical conductor 20) of the coaxial cable. The effect on the core electrical conductor develops much faster than on metal pipe 10. This provides for early detection of a risk of corrosion.

Instead of a galvanic coupling of test device 40 that is connected to electric cable 14, a magnetic coupling may be used. In this embodiment the short circuit may be replaced by a coupling loop that is galvanically connected between the first and second electrical conductor of the cable.

Such a coupling loop may act as a DC galvanic connection instead of the removable short circuit. The coupling loop may have one or more windings, which may form an inductor coil. During testing, the coupling loop remains connected and a further loop, which may also form an inductor coil, that is magnetically coupled to test device 40, e.g. through the air or with the aid of a magnetic core that extends through both the coupling loop and the further loop, may be used to send and detect AC signals into the cable via the coupling loop. Preferably, such a loop is coupled to an end of the cable. Both ends of the cable may be coupled to a respective coupling loop, or one end may be coupled to coupling loop and the other to a short circuit.

Alternatively, one or more coupling loops may be connected between the first and second electrical conductor of the cable at a distance from the end. Thus a long cable may be used, and tests may be performed at different places along the cable by using the coupling loops. As used herein the term“electric signal” is used to refer to both signals that are directly applied to the cable and signals that are applied via loops, using magnetic coupling.

Although an embodiment has been shown wherein a coaxial cable is used, it should be appreciated that instead a wire pair cable may be used. Here too the electrical conductor with the smaller surface area will corrode relatively faster than second conductor 24, already if both are of the same material and even more so if the latter has a higher electrochemical potential.

Figure 5 shows a cross section of such a wire pair, with a first electrical conductor 50, a second electrical conductor 52 and a water permeable electrical isolator 54 around first electrical conductor 50. In this embodiment first electrical conductor 50 of the wire pair cable is made of a material that has less corrosion resistance in the system than that of the metal object that is monitored (not shown). The effect of the corrosion of first conductor 50 is made relatively greater by making the diameter of the first electrical conductor 50 of the wire pair cable smaller than that of second electrical conductor 52 of the wire pair cable, e.g. no more than half or even no more than 0.2 or 0.1 of the diameter of second electrical conductor 52. The relatively thin first electrical conductor 50 of the wire pair cable may circle helically around a thicker second electrical conductor 52 of the wire pair cable. The second electrical conductor 52 is made of a material that has more corrosion resistance. Although a water permeable electrical isolator layer 54 is shown around first electrical conductor 50, it should be appreciated that a water permeable electrical isolator layer may also or be provided around the second electrical conductor 52, or around both conductors 50, 52. This makes it easier to avoid direct or indirect galvanic contact between first electrical conductor 50 and the pipe, which might undesirably speed up corrosion of the pipe.

However, a coaxial cable with a water permeable outer conductor and a water permeable electrical isolator layer between the outer conductor and the core conductor is preferred. This makes it easier to avoid additional corrosion due to contact between the first electrical conductor and the pipe. It also makes it easier to use a smaller first electrical conductor.