WO2000045148A1

WO2000045148A1 - Corrosion sensors contained within the thermally insulating member of a metal pipe

Info

Publication number: WO2000045148A1
Application number: PCT/GB2000/000196
Authority: WO
Inventors: Graham Peter Quirk
Original assignee: Integriti Investments Ltd.
Priority date: 1999-01-26
Filing date: 2000-01-26
Publication date: 2000-08-03
Also published as: GB9901572D0; GB2363201A; GB0117957D0; AU2116300A

Abstract

A thermally insulating member for a metal structure like a pipe is disclosed, the insulating member being provided with at least one electrode assembly. The electrode assembly is used to indicate the presence of an electrolyte in the vicinity of the electrode assembly, the presence of an electrolyte indicating that at least a portion of the insulating member may contain an electrolyte. The electrode assembly may be coupled to an indicator (e.g. a light emitting diode or other means) to give an indication that the insulating member contains an electrolyte. The electrode assembly might comprise an electrochemical corrosion cell and the instrumentation might comprise an electrochemical noise measuring apparatus or linear polarisation resistance measuring apparatus.

Description

CORROSION SENSORS CONTAINED WITHIN THE THERMALLY INSULATING MEMBER OF A METAL PIPE

1 2

3 The present invention relates to apparatus and a

4 method, and particularly, but not exclusively, to

5 apparatus and method for the detection of corrosion-

6 causing or corrosion-favouring conditions beneath the

7 external lagging of metal structures such as process or

8 production plant. 9

10 Metal structures are lagged primarily to provide

11 thermal insulation, but they may also be lagged to

12 protect the metal structure from direct atmospheric

13 attack when sited, for example, in a corrosive

14 environment. The lagging of pipe-work and vessels is

15 common in industrial plant, the lagging typically

16 consisting of wrappings or pre-shaped structure-hugging

17 portions, the lagging often being attached to the pipe-

18 work or the like using external strapping. 19

20 Piping or other metal structures which are lagged

21 typically corrode due to a hot and wet environment

22 which may exist under the lagging. The lagging typically comprises a synthetic, plastic, closed-cell foam with an external layer of aluminium sheet acting as an insulating outer skin. The piping or other metal structure under the lagging may become wet when, for example, the outer aluminium sheet insulation becomes damaged, thus allowing water or other corrosion- inducing fluids to come into contact with the lagging and the piping or other metal structure . Damage to the outer skin frequently occurs where the lagged metal structure (e.g. pipework) is situated in an area where personnel or vehicles pass regularly, and where the insulation is therefore more prone to accidental damage. Damage to the insulating layer of the lagging in these areas is more easily observed. However, inspection of more inaccessible locations is more difficult and damage to the insulation can often go unnoticed for considerable periods of time.

Visual inspection of the lagging tends to rely on the observation of damage to the aluminium outer skin of the lagging which, since it is aluminium, corrodes to a lesser extent, and an inspector looking for small holes or lifted seams in the lagging may not notice this type of damage.

In situations where the lagged metal structures are in inaccessible areas, or where the lagging has been damaged, it is difficult to detect any corrosion to the metal structure under the lagging. When the lagging becomes damaged allowing the ingress of fluids (e.g. seawater) the environment under the lagging, which is typically hot, becomes wet resulting in a high corrosion rate. Furthermore, the moisture and the corrosion products are retained under the lagging, thereby compounding the problem. On an offshore pipe, for example, if the lagging becomes damaged and there is a leakage of seawater through the lagging, highly corrosive ferric chloride solutions can develop typically producing very high corrosion rates. Coatings, for example epoxy paints, provided on the metal structure under the lagging typically offer negligible protection in such situations, and sometimes pipewall temperatures can be too high for coating protection. In these cases, there is usually a plentiful supply of oxygen and electrolyte (e.g. seawater) for replenishment through the damaged portion of the aluminium insulation outer layer to maintain high corrosion rates.

Corrosive conditions may arise because of the ingress of atmospheric liquid or moisture-condensate as a result of physical damage to the lagging, or capillary transfers at an exposed interface, or may arise as a result of leakage or deliberate venting of fluids from within the metal structure. Merely the presence of liquid at the interface between a lagging layer and the underlying metal structure does not prove the existence of corrosive conditions but will be a requirement for corrosion. Thus, nominally pure water condensate might not in itself induce corrosion of ferrous metal structures, but the presence, for example of electrolytes such as sodium chloride, may cause the corrosion rate of the structure to increase.

Conventional inspections of the condition of the lagging and metal structures underneath are time- consuming, difficult in inaccessible areas and normally result in a portion of the lagging around the metal structure being stripped off to allow visual inspection of the structure underneath the lagging to ascertain whether it has corroded.

Other conventional inspection methods include thermography, where the lagging becomes a poorer insulator when wet and thus conducts heat to the outer insulating skin where the increase in heat can be detected by an increase in surface temperature. This method is not reliable when the lagged structure forms part of a complex installation where a number of other structures in the proximity give off their own heat signatures, as these increase the background readings.

Also, some corrosion detecting instruments use a low- level neutron generator which projects a beam of neutrons from a radioactive source at the lagging and the underlying structure. The neutrons are energetic enough to penetrate the aluminium outer skin. If the lagging is dry, the neutrons tend to be reflected by the underlying structure and this reflection can be detected by a neutron detector provided as part of the portable instrument. If the lagging becomes wet, the water or other moisture absorbs the neutrons or slows them down due to neutron-proton scattering interactions with the hydrogen atoms in the water, so that the neutrons lose energy and are not energetic enough to be reflected back through the aluminium outer skin. Thus, a low neutron reading indicates a wet portion of lagging and hence a probable failure in the outer skin. However, use of this type of apparatus has a number of drawbacks, particularly concerning safety requirements, and the intensive labour required for inspecting a large amount of insulation (sometimes in the order of miles) .

Certain embodiments of this invention are particularly directed at those situations where liquid accumulation between the lagging layer and the metal structure lagged by it is, or may become, an electrolyte such that electrochemical activity (e.g. corrosion) involving the metal structure may occur.

According to a first aspect of the present invention, there is provided an insulating member for a metal structure, the member having at least one electrode assembly, the electrode assembly comprising a first electrode spaced apart from the metal structure, and a contact member for engaging the metal structure, the first electrode and the contact member being insulated from one another, and being adapted to be coupled to an indicator for indicating the presence of an electrolyte.

According to a second aspect of the present invention, there is provided an insulating member for a metal structure, the member having at least one electrode assembly, the electrode assembly comprising first and second electrodes, the electrodes being spaced apart from the metal structure, the first and second electrodes being insulated from one another, and being adapted to be coupled to an indicator for indicating the presence of an electrolyte.

Optionally, a contact member for engaging the metal structure may be provided.

According to a third aspect of the present invention, there is provided an insulated metal structure, the insulation having at least one electrode assembly, the electrode assembly comprising a first electrode spaced apart from the metal structure, and a contact member for engaging the metal structure, the first electrode and the contact member being insulated from one another, and being adapted to be coupled to an indicator for indicating the presence of an electrolyte.

According to a fourth aspect of the present invention, there is provided an insulated metal structure, the insulation having at least one electrode assembly, the electrode assembly comprising first and second electrodes, the electrodes being spaced apart from the metal structure, the first and second electrodes being insulated from one another, and being adapted to be coupled to an indicator for indicating the presence of an electrolyte.

Optionally, a contact member for engaging the structure may be provided.

The indicator typically indicates the presence of electrolyte between the first electrode and the metal structure. Alternatively, the indicator may indicate the presence of an electrolyte between the first and second electrodes.

The insulating member may have an external indicator or may have an external connector for connection to a remote indicator. Any electrolyte present between the first electrode and the metal structure completes a circuit between them to indicate the presence of electrolyte in the member. In the second aspect, any electrolyte between the first and second electrodes completes a circuit between them to indicate the presence of electrolyte in the member.

Activation of the indicator typically gives an indication that an electrolyte is present in at least a portion of the insulating member. The indicator typically comprises a light emitting diode (LED) . Alternatively, the indicator may comprise an electrochromic display, the display changing colour when current passes therethrough. Alternatively or additionally, the indicator may comprise an audible alarm, such as a buzzer, siren or the like. This would give a user an audible indication of the presence of an electrolyte. A visual indicator, e.g. an LED, may be used, in addition to the audible indication, to indicate to the user the location of the electrolyte in the insulating member.

The external connector typically allows for the connection of a remote indicator. The remote indicator may comprise a light emitting diode (LED) , the LED being located remotely from the insulating member (e.g. as part of a control board in a monitoring station or the like) . Alternatively, the remote indicator may comprise an electrochromic display, the display being located at a location remote from the insulating member. Alternatively or additionally, the remote indicator may comprise an audible alarm, such as a buzzer, siren or the like. This would give a user an audible indication of the presence of an electrolyte. A visual indicator, e.g. an LED, may be used, in addition to the audible indication, to indicate to the presence of an electrolyte.

In other embodiments, corrosion detecting and/or measuring and/or monitoring instrumentation may be electrically coupled to the external indicator. The corrosion detecting and/or measuring and/or monitoring instrumentation typically comprises a galvanic instrument e.g. a zero resistance ammeter (ZRA) . The display of the galvanic instrument can indicate that a current is flowing thus indicating the presence of an electrolyte. The galvanic instrument may be periodically coupled to the junction for periodic checking, or may be permanently coupled for constant monitoring. The galvanic instrument is typically calibrated so that the amount of current flowing gives a qualitative indication of the amount of electrolyte in the insulating member. Alternatively or additionally, the corrosion detection and/or measuring and/or detecting apparatus may comprise electrochemical noise measuring apparatus, linear polarisation resistance measuring apparatus, or other such apparatus .

The first electrode typically comprises a zinc metal electrode or a magnesium alloy electrode. Alternatively or additionally, the first electrode may comprise a carbon steel electrode. The first and/or second electrodes may be in the form of a metal strip or sheet. As a further alternative, the first and/or second electrodes may be formed of a metal mesh sheet. The mesh sheet may be wrapped around the metal structure or attached to an inner surface of the insulating member. Concentrically wrapped mesh electrodes increase sensitivity of any electrochemical measurements through increased electrode surface area and increases measurement coverage around the whole of the inner circumference of the insulating member.

The first electrode is typically isolated from an inner surface of the insulating member using a first insulating pad. The first electrode is typically isolated from and/or spaced apart from the metal structure by a second insulating pad. The first electrode is typically sandwiched between the first and second insulating pads. The or each insulating pad is typically attached to the first electrode using an adhesive. The or each insulating pad is preferably porous to liquid.

The second electrode may be of a metal which is the same or similar to the lagged metal structure. The contact member typically comprises a spring-loaded pin or the like, the pin being located in a sleeve that passes through the insulating member perpendicular to the longitudinal axis of the metal structure. Alternatively, the contact member may comprise a screw or the like which is threadedly engaged in a sleeve which passes through the insulating member perpendicular to the longitudinal axis of the metal structure.

The insulating member is typically in the order of 12 feet (3.66 metres) in length. Thus, a portion of insulating member may be provided along the metal structure, e.g. a pipeline, each portion being provided with one or more electrode assemblies for monitoring the entire structure. For example, a number of electrode assemblies may be provided along the length of the insulating member, the insulating members then being provided along the length of the metal structure . This allows for monitoring along the length of the metal structure e.g. pipelines. The insulating member may be spaced apart along the length of the metal structure (with insulating members not having electrode assemblies in between) allowing monitoring of the entire length of the metal structure. This is advantageous where the metal structure comprises a long pipeline e.g. in the order of miles.

The insulating member may optionally be pre-formed into a shape that conforms to the shape of the metal structure being insulated.

The insulating member typically includes an outer surface, the outer surface typically comprising an insulating outer skin. The outer surface is typically of aluminium.

The electrode assembly optionally includes a third electrode, typically arranged in a similar fashion to the first electrode. In this embodiment, the first and third electrodes are arranged so that the two electrodes form an electrochemical corrosion cell. The second electrode may optionally be used as a reference electrode in a three-cell arrangement. In one embodiment, the present invention provides a prefabricated lagging section for insulating, or screening from its external environment, a metal structure (e.g. a production or process plant or fluid conveying pipe work) wherein the section comprises a pre-shaped layer of lagging material that has an inner (structure - confronting) surface of predetermined (e.g. planar or curved) conformation, an outer surface (which may be that of covering sheet material such as aluminium sheet when the lagging is a laminate structure) , and elements of electrochemical circuitry attached to the layer of lagging material at chosen locations, said elements including an electrode presenting exposed electrode area open to the inner surface environment of the lagging section but arranged so as to be spaced from the lagged metal structure when the lagging section is firmly installed, a contact member of electrically conducting metal capable of being pressed into contact with the lagged metal structure when the lagging section is installed, either automatically as by resilient compression or in response to manual advancement, and electrical leads to the electrode and the contact member passing through the layer of lagging material and presenting couplings at the outer surface of the lagging section for attachment of means indicating the presence or absence of electrochemical activity at the surface of the metal structure, the electrode and the contact member being insulated from each other, other than through external coupling of their electric leads and other than through electrically conducting liquid bridging the space between the electrode and the metal structure which may accumulate after installation of the lagging section

The electrode may be in strip or sheet form, positioned at, for example, the 6 O'clock potion in the lagging section, or as a mesh sheet extending around the inner circumference of the lagging section.

If the lagging section were intended for use on ferrous metal structure, such as common plant vessels or pipes, the electrode may suitably be a zinc or magnesium alloy electrode . The contact member may be for example a spring loaded block or pin that becomes pressed firmly against any surface against which the lagging is affixed, as by clamping or wrapping. Alternatively, the contact member may be designed to be pressed into contact with a lagged metal structure, after the lagging is in place and in this case screw - threaded advancing of the contact member from a retracted position in the lagging may be preferred.

In a further embodiment of the present invention, there is provided a modification or development of the aforementioned specific embodiment wherein there is provided instead of, or alternatively in addition to, the contact member, a second electrode arranged in the lagging section analogously to the first mentioned electrode and electrically similarly insulated, so that two electrodes are available for use in an electrochemical corrosion cell, (whether or not a contact member is also present so as to permit the metal structure being lagged to serve as a reference electrode of a three - electrode cell arrangement.)

The or each electrode might conveniently be formed of metal strip or sheet . The metal of a second electrode may be chosen to be similar or electrochemically related to the metal of the metal structure for which the lagging is intended, e.g. carbon steel.

It is preferred that adequate electrode surface be exposed adjacent to the interfacial region between the lagging and the lagged metal structure. If the electrode (s) were formed of strip metal exposed edge surfaces can suffice and in such case a simple way of incorporating the electrode (s) in the lagging would be to affix the strips to the main body of the lagging, say by a double sided thin adhesive pad, and to prevent contact of the strip with the lagged metal structure by overlaying an adherent thin insulating pad. If the electrode (s) were formed of thin sheet material such that exposed face were required, then the sheet could be adhered to the main body of the lagging and overlaid with insulating material that still leaves sufficient electrode facial surface unmasked. The insulating material must not, of course, seal off the electrode so as to prevent ingress of liquid from surrounding regions, else its purpose is defeated. Similarly, if the electrodes were formed of a mesh, the first electrode could be positioned on an adhesive pad or sheet which is affixed to the inner surface of the lagging. A double-sided adhesive insulating sheet is typically positioned over the first electrode, and a second electrode, typically in the form of a mesh, is positioned on top of the insulating sheet . A second insulating sheet may then be positioned over the second (mesh) electrode to insulate it from the metal structure.

In another embodiment of the present invention, there is provided a lagged metal structure, e.g. a lagged vessel or section of pipe-work for a process or production plant, wherein the lagging is a pre-formed layer structure and includes an electrode and a contact member positioned and arranged as above described, or two electrodes and optionally a contact member positioned and arranged as described above.

The choice of electrode material (s) and the electrochemical corrosion methods they are used for, separately or in combination and with or without the metal structure serving as a reference, are matters for choice by the skilled corrosion engineer and are not limited by this invention in any of its aspects. A very simple arrangement would enable a corrosion engineer, or indeed a plant - operator, to determine if a chosen region for inspection contained an electrolyte or not under the lagging. An instrument might be connected across the electrode / contact member outer terminals at the outer lagging surface (conveniently in a sunk recess) to detect electrochemical activity or an external visible light - emitting diode (LED) or an electrochromic indicator might be hard - wired in to complete the external circuit so that visual inspection will show if there is electrochemical activity. More sophisticated measurements include linear polarisation resistance monitoring and electrochemical noise monitoring. These measurements might be made through permanent coupling to remote instrumentation or by bringing portable equipment to the relevant lagging sections and coupling in the electrode (s) / contact member to the portable equipment. In the sequence of data handling steps from measurement and storage, followed by manipulation or mathematical processing and finally display or interpretation, there is wide scope for integrated or stepwise treatments at site and/or remotely.

This invention may further include detecting corrosion promoting or causing conditions under lagging sections by detecting, or monitoring electrochemical activity beneath lagging sections affixed to a metal structure the lagging sections or the lagged structure being as described in relation to the first, second or third aspects of the invention. In a first preferred embodiment, the is provided a portion of lagging, as a pre-formed shaped layer structure designed for bringing into close abutment with metal structure to be lagged and having an inner surface and outer surface, characterised in that there is included in the lagging an electrode for an electrochemical, corrosion-detecting, cell of which a surface thereof is exposed at, but set back from, the innermost structure contacting surface of the lagging at the location of the electrode and a contact member capable of being brought into engagement to make electrical contact with the metal structure when the lagging is in place, the electrode and the contact member being insulated from each other electrically other than when in use through electrochemical cell circuitry, connected to them through the lagging from the outer surface of the lagging, and other than through any liquid electrolyte in contact with the electrode surface and adjacent surface of the metal structure with which the contact member is in engagement.

In a second preferred embodiment of the invention, there is provided a modification or development of the first preferred embodiment, wherein there is provided instead of, or alternatively in addition to, the contact member, a second electrode arranged in the lagging analogously to the first mentioned electrode and electrically similarly insulated, so that two electrodes are available for use in an electrochemical corrosion cell, (whether or not a contact member is also present so as to permit the metal structure being lagged to serve as a reference electrode of a three - electrode cell arrangement.)

In a third preferred embodiment of the invention, there is provided a lagged metal structure e.g. a lagged vessel or section of pipe-work for a process or production plant, wherein the lagging is a pre-formed layer structure and includes an electrode and a contact member positioned and arranged as specified in the first preferred embodiment, or two electrodes and optionally a contact member positioned and arranged as specified in the second preferred embodiment.

A method of detecting corrosion promoting or causing conditions under lagging by detecting (or alternatively by measuring) electrochemical activity wherein lagging sections are deployed that are as claimed in the first preferred embodiment or in the second preferred embodiment and the said electrodes and any contact member used constitute elements of the detection (or measurement) electrochemical circuitry by which electrochemical activity is detected (or measured) .

According to a fifth aspect of the present invention, there is provided a method of detecting the condition of a portion of insulation attached to a metal structure, the method comprising the steps of providing the insulation with at least one electrode assembly, the electrode assembly comprising a first electrode spaced apart from the metal structure, and a contact member for engaging the metal structure, the first electrode and the contact member being insulated from one another, and being adapted to be coupled to an indicator for indicating the presence of an electrolyte; and monitoring the indicator to determine the presence (and optionally the amount and/or the extent and/or the concentration) of electrolyte in the insulation.

According to a sixth aspect of the present invention, there is provided a method of detecting the condition of a portion of insulation attached to a metal structure, the method comprising the steps of providing the insulation with at least one electrode assembly, the electrode assembly comprising first and second electrodes spaced apart from the metal structure, the first and second electrodes being insulated from one another, and being adapted to be coupled to an indicator for indicating the presence of an electrolyte; and monitoring the indicator to determine the presence (and optionally the amount and/or concentration) of electrolyte in the insulation.

The indicator typically comprises a light emitting diode (LED) . Alternatively, the indicator may comprise an electrochromic display, the display changing colour when current passes therethrough. Alternatively or additionally, the indicator may comprise an audible alarm, such as a buzzer, siren or the like. This would give a user an audible indication of the presence of an electrolyte.

The method typically includes the additional step of checking the indicator to ascertain whether it has been activated.

The method may optionally include the additional step of determining qualitatively the amount and/or concentration of electrolyte in the insulation.

Embodiments of the present invention shall now be described, by way of example only, with reference to the accompanying drawings, in which: -

Fig. 1 is a general view of a length of pre-formed pip lagging; Fig. 2 is an enlarged view of part of the lagging of Fig. 1 incorporating a strip electrode and a contact member; Fig. 3 is an enlarged view of part of the lagging of Fig. 1 incorporating a sheet metal electrode; Fig. 4 shows schematically in transverse section the lagging of Fig. 2 with electrical connections exposed; and Fig. 5 shows schematically in transverse section the lagging of Fig. 2 coupled to a pipe in use. Referring to the drawings, Fig. 1 shows a portion of pip lagging 1, which is typically pre-formed into a half-circular cylinder so that two such portions may be coupled together to fully enclose and insulate a pipe (not shown) . Lagging 1 may suitably be formed of a synthetic plastic closed-cell foam with an external layer of aluminium sheet. The portion of lagging 1 is typically in the order of 12 feet (approximately 3.66 metres) in length. It will be appreciated that lagging 1 may have a number of different forms. For example, lagging 1 may be a flat sheet of insulation to be wrapped to otherwise attached to the pipe or other metal structure, or may be pre-formed into a variety of shapes to conform to that of the structure to be insulated.

Fig. 2 is an enlarged view of part of lagging 1 showing an electrode assembly 2 attached to an inner surface of lagging 1. The electrode assembly 2 typically comprises three layers; the first layer is a double- sided adhesive pad 2a; the second layer is a strip- metal electrode 2b, for example of zinc; and the third layer is an electrically- insulating porous pad 2c which is attached to the metal electrode 2b. As shown in Fig. 2, the lateral edges of metal electrode 2b are exposed to the surrounding environment, the upper and lower surfaces thereof (the terms "upper" and "lower" being used with reference to Fig. 2) being in contact with the porous pad 2c and the adhesive pad 2a respectively. The porous pad 2c and the adhesive pad 2a are typically absorbent and may be, for example, made from the type of material used as draft excluders.

The electrode assembly 2 includes a protruding contact member 3, shown schematically in Fig. 1, the member 3 being used to engage a steel pipe under the lagging 1. The insulating pads 2a, 2c may extend around the whole inner circumference of the pipe, and optionally along the entire length of the lagging 1. The electrode 2b may be a mesh sheet of metal, e.g. zinc, which also extends over the whole inner circumference of the lagging 1 between the insulating pads 2a, 2c. Protruding contact member 3 may still be used with mesh electrodes by passing the contact member 3 through one of the holes in the mesh to make contact with the pipe.

In use, the porous pad 2c insulates the electrode 2b from contact with a lagged pipe 10 (Fig. 5) . That is, when the lagging 1 is attached using any conventional means to pipe 10, for example, the porous pad 2c is located between an external surface 10s of the pipe 10 and metal electrode 2b (Fig. 5) . Contact member 3 is positioned within the lagging 1 so that it is insulated by the body of the lagging 1 from electrode 2b. Contact member 3 is in electrical contact with the lagged pipe 10 using any convenient manner, e.g. a spring-loaded pin or threadedly engaged with a sleeve (not shown) provided in the lagging 1, the screw, pin or the like contacting the outer surface 10s of the lagged pipe 10, as shown in Figs 4 and 5. It should be noted that in Fig. 5, the contact member 3 is shown schematically. Electrical connections from the electrode assembly 2 to the outside of the lagging 1 are not shown in Fig. 1.

Fig. 3 shows an alternative electrode assembly 4 attached to the lagging 1, in which a sheet metal electrode layer 4b is attached to the lagging interior (e.g. using adhesive, not illustrated). The electrode layer 4b is overlaid by a reticulated insulation layer 4c providing a number of windows 4w (eight shown in Fig. 3) where the surface of electrode layer 4b is exposed. The exposed surface regions of electrode layer 4b are set back from the innermost surface of the lagging structure by the depth of the insulation layer 4c, the innermost surface at that location being provided by the exposed top surface regions of insulation layer 4c. Thus, insulation layer 4c allows liquid to enter into the spaces between the electrode windows 4w and the metal structure, from the surrounding region. With the arrangement shown in Fig. 3, the electrode assembly 4 is capable of detecting whether the lagging 1 contains an electrolyte, for example, complex insulation pieces (e.g. joints or valves etc) where fluid may get caught above the 6'o'clock position.

The electrode assembly 2 shown in Fig. 2 may be positioned at the lowest (6 o'clock) position of the assembled lagging, whereas the electrode assembly 4 shown in Fig. 3 may extend, as a band, over the whole curvature of the lagging 1 so that two such lagging portions (when coupled together) would provide for corrosive monitoring around the entire inner circumference of the lagged pipe. The electrode assembly 4 may also extend along the full length of lagging 1 to provide for monitoring along the entire length of the lagged pipe. When the electrode assembly 2 is positioned at the lowest position in the lagging 1, the assembly 2 will only detect that the lagging 1 contains an electrolyte when the electrolyte (e.g. water) reaches the bottom of the lagging 1.

Fig. 4 shows in cross-section an arrangement similar to that shown in Fig. 2. The contact member 3 is illustrated as a pin 5 loaded in a sleeve 6. Pin 5 is typically spring-loaded in sleeve 6, or it may be inserted into sleeve 6 after the portions of lagging 1 are in place by screw-threaded engagement with sleeve 6.

Fig. 5 shows schematically a similar arrangement to that of Fig. 4, with a portion of lagging 1 being attached to a pipe 10 using any conventional means (not shown) .

Referring to Figs 4 and 5, the electrode 2b and the pin 5 are electrically coupled using circuitry which typically comprises unit 7 attached at an outer surface of lagging 1, and two electrical leads; a first lead 8a electrically couples unit 7 to electrode 2b; and a second lead 8b electrically couples unit 7 to contact member 3 (e.g. pin 5 in Fig. 4) . Care should be taken when fitting the electrical leads 8a, 8b and the contact member 3 (for example by using leak-tight fixings) to prevent the ingress of liquid (e.g. electrolyte) along the paths of the leads 8a, 8b or the sleeve 6.

When the insulation (i.e. lagging 1) is dry, there is no electrolyte to create a circuit between the electrode 2b and the metal (e.g. carbon steel) of the lagged pipe 10 (via contact member 3) , and thus no current would flow. However, if the insulation (i.e. lagging 1) were to contain an electrolyte, this would cause preferential corrosion of the (zinc) electrode 2b, and the electrolyte (e.g. seawater) completes the electrolytic circuit between the electrode 2b and the lagged pipe 10.

Thus, when the insulation around the bottom of the pipe 10 contains an electrolyte, a current would flow, as described above, with a voltage approximating y₂ to 1 volt through to unit 7. In a simple embodiment, unit 7 may be an indicator device, such as a low voltage light-emitting diode (LED) . The current flowing would light the LED giving a visual indication of the condition of the lagging. It should be noted that unit 7 (e.g. the LED) may be positioned at any convenient location on the lagging 1 so that it can easily be seen during a visual inspection.

The LED or other indicator device may be located remotely from the insulating member, e.g. as part of a control board in a monitoring station. This has the advantage that the detection of an electrolyte within the lagging 1 can be indicated to a user without having to inspect the entire length of the lagged pipe.

Alternatively, or additionally, unit 7 may comprise a small semiconductor plastic indicator which changes colour when current flows through it (e.g. an electrochromic indicator), or other such display means, so that a visual indication is given of the condition of the lagging 1 (i.e. whether it contains an electrolyte or not) . Appropriate electronic components may be used in the circuit to provide a voltage of sufficient magnitude to activate the indicator when current flows due to corrosion.

Alternatively, unit 7 may comprise a connection point, coupling or junction for remote or portable corrosion detecting/measuring/monitoring instrumentation. The connector would typically be left open so that an inspector could periodically plug in a galvanic instrument (e.g. a zero-resistance ammeter (ZRA) ) . A semaphore relay may also be used. Any current shown on the display of the instrument would indicate that the lagging 1 contains an electrolyte. If required, a small ZRA may be permanently coupled to the lagging 1 (or in close proximity thereto) to give continuous monitoring of the condition of the lagging 1. The ZRA may be calibrated so that the current flowing through the ZRA could indicate whether a large or small portion of the lagging 1 was affected.

Where a galvanometer is used, these devices tend to respond to very low currents, and move the position of a tell-tale (e.g. an indicator arm on the display) . When current is detected, the tell-tale is moved to indicate the level of current. The tell-tale may be held in the new position by gravity or by the use of a restraining mechanism (e.g. a small hair spring). The movement of the tell-tale may allow the presence of an electrolyte (typically indicative of corrosion) in the period since the last inspection to be identified, as the tell-tale will have moved from the previous position indicating that a current has been detected, the restraining mechanism being used to hold the tell- tale in this new position. The tell-tale can be reset at any time either manually by the inspector, or by the use of a magnet, for example.

If desired, of course, the electrode layer 2b (or 4b where the electrode assembly 4 shown in Fig. 3 is used) may be positioned in recessed regions of the lagging 1, in which case the insulation layer 2c, for example, may not be required because surrounding regions of the main body of the lagging 1 would prevent the electrode 2b contacting the lagged pipe.

It should be noted that the apparatus and method described herein gives an indication of whether the lagging 1 contains an electrolyte. The presence of an electrolyte within the lagging is indicative of corrosion, and thus the apparatus and method may also be used to give an indication of whether or not the metal structure under the lagging is corroding. If the electrolyte remains within the lagging 1 for a period of time, the metal structure under the lagging will eventually corrode. If it is desired to measure corrosion of the metal structure, a carbon steel strip may be used in place of, or in addition to, the (zinc) electrode 2b and unit 7 may comprise a suitable connector so that electrochemical noise, linear polarisation resistance (LPR) , or any other technique can be used to identify and/or classify the corrosion, by coupling the appropriate instrumentation to unit 7 during, for example, a walk-by inspection.

Modifications and improvements may be made to the foregoing without departing from the scope of the present invention.

Claims

CLAIMS 1. An insulating member for a metal structure, the member having at least one electrode assembly, the electrode assembly comprising a first electrode spaced apart from the metal structure, and a contact member for engaging the metal structure, the first electrode and the contact member being insulated from one another, and being adapted to be coupled to an indicator for indicating the presence of an electrolyte.

2. An insulating member according to claim 1, wherein the indicator indicates the presence of electrolyte between the first electrode the metal structure.

3. An insulating member according to either preceding claim, wherein the contact member comprises a spring- loaded pin to contact the metal structure.

4. An insulating member according to claim 1 or claim 2, wherein the contact member comprises a screw.

5. An insulating member according to claim 3 or claim 4, wherein the pin or screw is located in a sleeve passing through the insulating member substantially perpendicular to the longitudinal axis of the metal structure.

6. An insulating member for a metal structure, the member having at least one electrode assembly, the electrode assembly comprising first and second electrodes, the electrodes being spaced apart from the metal structure, the first and second electrodes being insulated from one another, and being adapted to be coupled to an indicator for indicating the presence of an electrolyte to indicate.

7. An insulating member according to claim 6, wherein the indicator indicates the presence of an electrolyte between the first and second electrodes.

8. An insulating member according to claim 6 or claim 7, wherein the second electrode is of a metal which is the same or similar to the metal structure.

9. An insulating member according to any preceding claim, wherein the first electrode comprises a zinc metal electrode, a magnesium alloy electrode or a carbon steel electrode.

10. An insulating member according to any preceding claim, wherein the first electrode comprises a metal mesh.

11. An insulating member according to any preceding claim, wherein the first electrode is isolated from an inner surface of the insulating member using a first insulating pad.

12. An insulating member according to any preceding claim, wherein the first electrode is isolated from and/or spaced apart from the metal structure by a second insulating pad.

13. An insulating member according to claim 12, wherein the first electrode is located between the first and second insulating pads.

14. An insulating member according to claim any preceding claim, wherein the electrode assembly includes a third electrode.

15. An insulating pad according to claim 14, wherein the first and third electrodes are arranged so that the two electrodes form an electrochemical corrosion cell.

16. An insulating member according to claim 15, wherein a second electrode is used as a reference electrode in a three cell arrangement.

17. An insulating member according to any preceding claim, wherein the indicator comprises a light emitting diode (LED) .

18. An insulating member according to any one of claims 1 to 16, wherein the indicator comprises an audible alarm.

19. An insulating member according to any one of claims 1 to 16, wherein the indicator comprises an electrochromic display.

20. An insulating member according to any preceding claim, wherein the insulating member has an external connector for connection to the indicator.

21. An insulating member according to any preceding claim, wherein the indicator is located remotely from the insulating member.

22. An insulating member according to any preceding claim, wherein corrosion detecting and/or measuring and/or monitoring instrumentation is electrically coupled to the electrode assembly.

23. An insulating member according to claim 22, wherein the corrosion detecting and/or measuring and/or monitoring instrumentation comprises a galvanic instrument.

24. An insulating member according to claim 23, wherein the galvanic instrument comprises a zero resistance ammeter (ZRA) .

25. An insulating member according to any one of claims 22 to 24, wherein the instrumentation determines whether a current is flowing, indicating the presence of an electrolyte.

26. An insulating member according to any one of claims 22 to 25, wherein the instrumentation is adapted to be connected and disconnected from the electrode assembly.

27. An insulating member according to any one of claims 22 to 26, wherein the instrumentation is calibrated to give a qualitative indication of the amount of electrolyte present.

28. An insulating member according to any one of claims 22 to 27, wherein the instrumentation comprises electrochemical noise measuring apparatus or linear polarisation resistance measuring apparatus.

29. An insulating member according to any preceding claim, wherein the insulating member is pre-formed into a shape that conforms to the shape of the metal structure being insulated.

30. A method of detecting the condition of a portion of insulation attached to a metal structure, the method comprising the steps of providing the insulation with at least one electrode assembly, the electrode assembly comprising a first electrode spaced apart from the metal structure and a contact member for engaging the metal structure, the first electrode and the contact member being insulated from one another, and being adapted to be coupled to an indicator for indicating the presence of an electrolyte between the first electrode and the metal structure; and monitoring the indicator to determine the presence of electrolyte in the insulation.

31. A method of detecting the condition of a portion of insulation attached to a metal structure, the method comprising the steps of providing the insulation with at least one electrode assembly, the electrode assembly comprising first and second electrodes spaced apart from the metal structure, the first and second electrodes being insulated from one another, and being adapted to be coupled to an indicator for indicating the presence of an electrolyte; and monitoring the indicator to determine the presence of electrolyte in the insulation.