GB2586530A - Sensing element for monitoring condition of an article - Google Patents

Abstract

A monitoring device for an article (6, fig 13), for example a pipeline or turbine, includes a sensing element 1 with an array of resistive conductors 2, 3 forming a plurality of paths which may interconnect. The resistance across the paths is measured to provide status information about the array and infer the propensity to erosion or corrosion of the article. The ends of the conductors 2, 3 may be connected to terminals 4. There may be multiple sensing elements 1 applied to or within the article, and measured data may be collected by data collection hubs (14, fig 15), transmitted to analysis stations (15, fig 15) and then sent to a remote management station or device (17, fig 15). In another embodiment the resistive conductors 2, 3 comprise a material at least as susceptible to corrosion as the article 6. In a further embodiment the sensing element 1 includes bypass resistive conductors (60, fig 9) through which current passes upon failure of a resistive conductor 2,3.

Description

I

SENSING ELEMENT FOR MONITORING CONDITION OF AN ARTICLE

Field of the Invention

This invention concerns monitoring the propensity to erosion or corrosion of an article.

More particularly, hut not exclusively, this invention can he used for the remote continuous real-time monitoring of corrosion under insulation (CUI) in pipelines or the erosion of surface materials on static or moving bodies [e.g. wind turbines].

Background of the Invention

Testing methods used to evaluate the properties of an article, for example, a material or component, are usually either destructive or non-destructive. Destructive testing methods typically involve the application of a mechanical load to understand the response behaviour of an article. Examples of types of destructive test methods include tensile testing, compression testing, impact testing and bend testing. Destructive testing methods that induce permanent damage to an article are not suitable, however, for monitoring the condition of an article in service. Non-destructive testing methods do not typically cause damage to an article and are more suitable, therefore, for monitoring the condition of an article in service. Examples of types of non-destructive test methods include ultrasonic, magnetic-particle, liquid penetrant and visual testing. Since non-destructive testing methods are not always deployable in hard-to-reach areas, continuous real-time monitoring is not easily achieved.

Pipes are used in a variety of applications to carry fluids, and in the oil and gas sectors are of critical importance. As well as their use in refineries to carry fluids between vessels, tanks, heat exchangers, reactors, and condensers, pipes are used in the form of pipelines to carry fluid for extended distances, and are often buried underground or submerged in the sea. The following description illustrates the invention by reference to pipelines, but the invention is not so limited Pipelines are just one example of articles that would benefit from continuous real-time monitoring due to their susceptibility to corrosion, erosion and subsequent failure, and their location in hard to reach areas. [Surface material integrity of other articles can also be monitored using this system. For example, wind turbine blades are generally laminated composite structures that can experience surface erosion through water impact. Damage to the surface may render the blades liable to further erosion and eventually permit access of water to the interior of the blade.] Typically pipelines comprise an inner pipe (e.g. of metal) to contain fluid within the pipeline, overlaid with one or more insulating/protective layers to protect the inner pipe. In the pipeline industry, approximately 26% of all pipeline failures occur due to corrosion. Approximately 60% of pipeline leakages, and approximately 40 to 60% of overall pipeline maintenance costs, are accountable to corrosion under insulation (CUI) (corrosion damage to the inner pipe normally initiated between the one or more insulating/protective layers and the inner pipe). Typically, CUI initiates on an outer surface of the inner pipe due to moisture being present under the insulating/protective layers in the presence of oxygen.

The difficulty in monitoring CUI damage in a pipeline results in poor asset integrity management, and has led to significant costs for the energy sector as preventative replacement may be required based on a worst case analysis rather than a real-time understanding of the condition of the pipeline.

The present invention provides methods and systems for remote continuous real-time 20 monitoring of the propensity to erosion or corrosion of an article, for example, CUI damage in a pipeline, but can also be used to monitor the changing condition of other properties of other articles in other applications.

Summary of the Invention

The present invention provides a method for monitoring the propensity to erosion or corrosion of an article, comprising the steps of:- * measuring a variable indicative of electrical resistance across a plurality of paths formed by at least one array of resistive conductors in a sensing element applied to or embedded within the article, to provide status information concerning the state of the array * using the status information to infer propensity to erosion or corrosion of at least part of the article.

The invention further provides articles and sensing elements for use in such methods. Further details of the invention are evident in the appended claim and in the following nonlimitative description.

In the present application the term "conductor" should be read as meaning an electrical conductor and encompasses both metallic and non-metallic conductors and encompasses resistive elements, but not electrical insulators.

Brief Description of the Drawings

The invention is illustrated by way of example with reference to the drawings in which: Fig. 1: is a schematic view of an array of resistive conductors for use in a sensing element; Fig. 2: is a schematic view of a crossed pair of arrays for use in a sensing element; Fig 3: is a schematic view of a crossed and interconnected pair of arrays for use in a sensing element; Fig. 4: is a schematic view of an alternative crossed and interconnected pair of arrays for use in a sensing element; Fig. 5: is a schematic view of crossed and interconnected resistive conductors forming in a star-type layout; Fig. 6: is a schematic view of crossed and interconnected resistive conductors forming a radial array; Fig. 7: is a view of a sensor comprising a 2-D an-ay of resistors on a printed circuit board; Fig. 8: comprises a pair of images of sensing elements comprising a crossed and interconnected pair of arrays and corresponding maps showing resistance; Fig. 9: illustrates a printed circuit with a high resistance bypass that is layer 1 of a printed corrosion / erosion sensor; Fig. 10: shows layer 2 of a printed corrosion / erosion sensor comprising a low resistance sacrificial nodal resistor; Fig. 11: Shows the third layer of protective encapsulation and the windows allowing the exposure of the sacrificial sensor nodes to the envinmment; Fig. 12: shows a design for a piezoelectric based printed erosion sensor; Fig, 13: is a schematic view showing a portion of an article covered in sensing elements; Figs. 14 and 15: are schematic views showing the disposition of sensing elements on a pipeline;

Detailed Description

In the following, the same reference numerals are applied to like integers across the embodiments except where context requires otherwise for clarity.

The present disclosure is of a method for monitoring the condition of an article using a sensing element applied to or embedded within the article and comprising at least one array of resistive conductors. Current is passed across a plurality of paths formed by the array of resistive conductors to provide data indicative of electrical resistance of the paths, and the electrical resistance data is processed to provide status information concerning the state of the array. The status information is used to infer one or more characteristics concerning the condition of the article.

A plurality of sensing elements may he applied to an article allowing comparison between sensing elements to provide additional information concerning the article.

The sensing element The sensing element comprises at least one array of resistive conductors. The resistive conductors may be metallic or non-metallic, but conveniently comprise conductive particles dispersed in a carrier. Details of the resistive conductors are given below under the heading "Conductor materials".

Fig. 1 shows, a plurality of individual resistive conductors 2 connected to optional conductive terminals 4 disposed in a non-interconnected one dimensional array.

in an alternative arrangement, arrays as shown in Fig. I cross each other in an unconnected arrangement as in Fig. 2 to provide crossing but electrically isolated resistive conductors In a further arrangement. Fig. 3 shows a planar view of a sensing element 1 comprising an array of interconnected resistive conductors 2, 3 in a square layout with the resistive conductors 2,3 connected to optional conductive terminals 4.

Measuring resistance between terminals 4 of the array of Fig. 2 permits the relative resistances of the individual resistive conductors to be determined (and permits measurement of changes in resistance) but provides no information enabling localisation of changes in resistance along the resistive conductors 2. A difference between individual resistive conductors can show a change in array properties in the X-X direction but not the Y-Y direction. A plurality of such one-dimensional arrays (funning a two dimensional array) may provide additional information in the Y-Y direction (see Fig. 7).

Overlaying the arrays of Fig. 1 in an unconnected but crossing arrangement as in Fig. 2 enables the condition of one array to be measured in the X-X direction and the other in the Y-Y direction, so permitting localisation of changes that affect both arrays. However, as the crossing arrays are not in the same plane they may not be experiencing the same conditions and the location of changes in resistance of a conductor in one array can only he inferred once a conductor in the second array changes in resistance.

Measuring resistance with the array of Fig. 3 enables the location of increased resistance to be inferred as damage to any node, or path between nodes, will result in a change of resistance. Further detail of the measuring method is given below in the section "Resistance measurement" Alternative arrangements of intersecting resistive conductors can be used, as exemplified, in non-limiting manner, in Figs. 4-6, with different default resistances and algorithms required to deal with each.

Fig. 4 shows an array of interconnected resistive conductors forming a rectangular grid with varying conductor spacing (and hence resolution) across the grid. In Fig. 4, a higher spatial resolution is achieved in area 7 than in area 8. Arrangements with differing resolution across an array of interconnected resistive conductors may be useful where particular areas of an article require greater resolution of monitoring than other areas. In common with the other interconnected arrays disclosed herein, damage to an individual conductor 9, or intersection point (node) 10 will cause a change in the pattern of resistance.

The present disclosure is not limited to square/rectangular arrays. Nor is the present disclosure limited to regularly spaced arrays. Any array of interconnected resistive conductors could be used.

Fig. 5 shows an example of an alternative embodiment of the sensing element 1 comprising a square-shaped sensing element whereby the number and position of terminals are equivalent to that of the sensing element 1 shown in Figures 1 and 2, however, the array of interconnected resistive conductors 2 are in a star-type layout. Thus, the array of interconnected resistive conductors 2 produce a symmetrical layout while the spatial resolution is increased at the centre of the sensing element 1.

Fig. 6 shows a radial arrangement of resistive conductors.

The above described arrays of resistive conductors are disclosed with uniform resistive conductors, but the resistive conductors could be of different thicknesses/resistivities.

Each arrangement of interconnected resistive conductors will have its own default pattern of resistances characteristic of an undamaged array of interconnected resistive conductors.

Deviations from that default pattern of resistances provide information on deviations from the default state of the array.

An alternative to providing interconnected arrays is to provide an array of discrete resistive conductors and to compare the resistances of the discrete resistive conductors. Similar to the arrangements with interconnected resistive conductors, such sensing elements will have their own default pattern of resistances characteristic of an undamaged array of resistive conductors.

As mentioned above, the non-interconnected array of Fig. 1 permits a change in array properties in the X-X direction to he detected but not the Y-Y direction. The non-interconnected array of Fig. 7 solves this problem Fig. 7 shows an array of resistors 21-40 formed by convoluted resistive conductors 41 deposited on a printed circuit hoard to connect with conductors 42 and thereby connect with terminals 43. Measurements indicative of the resistance of each of resistive conductors 2140 can be taken and saved. An initial calibration enables a nominal value for an undamaged resistor to be obtained to serve as a reference for data monitored while the sensor is running Data acquisition may comprise recording acquired data on the resistive conductors to a file, and then compiling with the 1st set of resistance values (determined from the timestamp for the data set) for each resistor 21-40 to be taken as the reference value. All other resistance values for each resistor 21-40 acquired after this may then be evaluated against the reference generating a real time picture of changes of resistance occurring on the sensor where corrosion present on the resistive tracks 41 leads to a gradual increase in resistance. Variation of resistance between resistors 21-40 enables localisation of where erosion is present.

The resistors 21-40 are shown as square resistors presented in a rectangular array, but other geometries are within the scope of this disclosure (e.g. hexagonal outline resistors in a hexagonal array).

Conductor materials The disclosure encompasses use of wires, but conveniently the resistive conductors comprise conductive particles dispersed in a carrier to form an electrically resistive conductor.

Where the article to which the conductor is applied is or comprises a part susceptible to corrosion / erosion, the array conductor materials may comprise a material at least as susceptible to corrosion / erosion as said part. For example, where the article comprises a metal, the conductor may comprise dispersed particles of said metal. Dispersed particles of a metal are generally more prone to corrosion than the bulk metal, and so corrosion will preferentially take place on the conductor rather than on the article, so providing early warning of potential attack on the part susceptible to corrosion.

As an example

316 stainless steel powder (Atomized) with a polymer binding agent and solvent (e.g. benzyl alcohol) may be formed into an ink/paste. This is a typical conductor for corrosion monitoring on deep sea pipe lines. This paste may he printed onto a substrate surface and cured with multiple layers to ensure resistance sensitivity can be achieved as well as ensuring dielectric to cover the rest of the sensor so only the sensing elements is exposed. A common substrate material would he polyamide which is an example of a flexihle suhstrate capable of being wrapped around the surface of the pipe including around bends in the pipe or other high stress inducing areas of the pipelines where if conosion takes places there is a higher risk of severe failure.

The resistive conductors need to have a resistance appropriate for the measuring method used and typically resistances in the range 1000-20k0 are convenient for enabling measurement with materials with minimal-moderate thickness (02-1 5mm) as the corrosion taking place causes minimal change in the resistance values hence this sensitivity of the resistance will depend on the application. Approximately 10p m loss of material due to ionic corrosion a month relates to very small changes in the conductor's resistance values thus the resistance will be tailored to the application to ensure the sensitivity will be appropriate for detecting corrosion. Values much larger than this range provide such a small change in resistance that accurately determining the resistance change through corrosion is not viable as the sensitivity for the readings would require a large amount of material loss to confirm corrosion compared to the range mentioned.

Application of the resistive conductors to articles The resistive conductors may be applied directly to an article and secured in place. For example where the resistive conductors are in the form of conductive particles dispersed in a carrier they may be printed on the surface of the article, and optionally embedded within a protective coat. If the article is electrically conductive, an insulating/dielectric layer between the conductive article and the printed circuitry will ensure the resistive conductors are not short circuited.

Any forming method appropriate to the required conductor layout may be used, for example through printing or deposition techniques. Examples of such suitable techniques include screen printing, stereolithography, digital light processing, fused deposition modelling, selective laser sintering, selective laser melting, electronic beam melting, laminated object manufacturing, binder jetting, material jetting, sputtering.

Resolutions as low as e.g. 0.2mm may be achieved (or even finer with some techniques), but in the majority of applications far lower resolution will be adequate.

To ensure that the resistive conductors have appropriate conductivity, multiple layers of conductor material may be applied. 3D electrostatic printing is an inexpensive technique that permits the controlled deposition of conductive and non-conductive materials and may permit the deposition of different materials in different places. For example materials comprising conductive particles dispersed in a carrier may be deposited to form resistive conductors in some areas and electrically conductive inks may be deposited in other areas to provide conductors and terminals to permit connection to resistance measuring equipment.

An alternative to direct application to the article, is to provide the array of resistive conductors on a carrier (advantageously a flexible substrate) 5 as shown (for example) in Figs. 4 and 6. The carrier 5 carrying resistive conductors 2 can then be applied and secured to the article. Once secured to the article, the sensing elements with their carrier can optionally he embedded within a protective coat.

As shown by Fig. 7 the resistive conductors may be formed on a rigid substrate (e.g. printed circuit board) and then secured to or within the article to be monitored.

Resistance measurement Current passing through a conductor is proportional to voltage and inversely proportional to resistance. The current can be measured in any convenient manner. The data that is stored can be any data that is indicative of resistance and need not be a calculated resistance value. For example stored data may comprise (but is not limited to), resistance, conductance, current (for a fixed voltage), voltage (for a fixed current), or any other variable indicative of resistive conductor resistance.

Data acquisition may comprise recording acquired data indicative of resistance to a file, and then comparing with the 1st measured set of resistance values which may be taken as the reference value. All other resistance values acquired after this may then be evaluated against the reference generating a real time picture of the differences in, and changes of, the resistance of the resistive conductors.

For the interconnected arrays of Figs. 3-6, the problem of determining the resistance between two nodes in an infinite grid of resistors of identical resistance is a known problem for which solutions exist (see for example https://www.mathpages.conn/home/kmath668/kmath668.htm).

For a grid of finite size the boundary conditions will differ but in like manner, for a symmetric array, a symmetric distribution of resistances will result. However, damage to a node, or to a resistor, will result in a disturbance to the distribution of resistances, enabling damage to the grid to be determined, and to be localised if necessary.

Theoretically the optimal algorithm for localising damage compares the nominal resistances for each node before installation in the environment thus all subsequent measurements after installation show the relative change of the resistance values acquired for every node which is isolated from one another so every node is independent and entirely unique along the sensing element so each node corresponds to a small (e.g. square) area coordinate along the surface of the sensor.

At each interval of time the resistance for all the nodes is measured to build up a large data array to form a graphical map of rate of change of resistance vs time for every nodal position on the sensor based on a large set of data acquired at a user specified frequency. A heat map presentation then shows via a temperature/colour scale the smallest to largest deviations from each respective node's recorded nominal resistance value, this rate of change of resistance is monitored and will continually update with each new set of data.

Statistical analysis of the data set may be used to identify any trends that are indicative of corrosion / erosion via the change in area resulting from the loss of material through ionic corrosion / mechanical erosion. As each node is isolated from one another the matrix will localise where damage is occurring along the sensing element by the comparison of the nodes with damage on them relative to other nodes without damage and careful analysis of the data looking for the highest rate of change to determine the nodal position with corrosion present relative to the rest of the sensing element.

For the sensing element as a whole, a difference in the pattern of resistance between resistive conductors will be indicative of a change in the sensing element. Fig. 8 shows a sensing element comprising an 8 x 8 grid of crossed resistive conductors that have been damaged, and corresponding heat maps 20 graphically illustrating areas corresponding to a relatively higher resistance. In the upper sensing element a single intersection 18 has been damaged and in the lower sensing element an extended region of damage 19 is shown. The heat maps 20 show that the sensors are able to distinguish between localised and extended regions of damage.

Figure 9 illustrates a circuit with a resistive bypass 60 that may be used as a base layer of a corrosion / erosion sensor and which includes circuit connections 61. Figure 10 shows the next print layer, comprising an array of sacrificial resistors 62 which are in parallel to the bypass resistor 60 beneath. Figure 11, shows a protective coating 63 covering the resistive bypass 60 and circuit connections 61, leaving part at least of the sacrificial resistors 62 exposed. If desired, the sacrificial resistors 62 may be covered by a covering having a greater propensity to erosion than the protective coating 63, so that erosion takes place preferentially over the sacrificial resistors 62.

The operating principle of the sensor is that the sacrificial resistors 62 are in use exposed to the environment. As each sacrificial resistor 62 corrodes / erodes the resistance across the node will increase, showing incremental change. The effective resistance of the parallel sacrificial resistor 62 and bypass resistor 60 will thus tend towards the resistance of the bypass resistor 60 as the sacrificial resistor 62 corrodes / erodes.

Once the sacrificial resistor has corroded / eroded completely, current will flow through the 10 pathway provided by the resistive bypass 60, with a significant increase in measured resistance. The 'failure' of the sensor's printed material will be characterised by a step response in resistance to that of the bypass pathway.

Although this principle will work when the sacrificial resistor 62 has a higher resistance than the bypass resistor 60, preferably the resistance of the sacrificial resistor 62 has a lower resistance than the bypass resistor 60, as then the difference between the resistance of the parallel sacrificial resistor 62 and bypass resistor 60, and the bypass resistor 60 on its own, is increased. This is exemplified in the table below showing resistance in arbitrary units as the principle works over all resistance ranges Resistance (arbitrary units) Bypass (Rb) 20 40 Sacrificial resistor (Rn) 40 20 Resistance Rp of parallel sacrificial resistor and bypass Rp= 1/(1/Rb + 1/Rn) 13.3 13.3 Resistance Rb after loss of sacrificial resistor 20 40 % change in resistance 50% 200% Voltage measurement Alternative sensors may provide changes in output voltage in response to environmental factors. Figure 12 illustrates a design utilising a layer of piezoelectric material 70 (e.g. PZT) with bottom and top electrodes (71,72) encapsulated in the surface material of a structure. As the piezoelectric material is excited by environmental impacts (such as rain drops) on the rigid surface above, it will generate a measurable voltage, indicative of the propagation of shockwaves through the material. Analysis of the wave patterns and voltage may be used to measure the changing condition of the material. Such sensors may be used alone or in addition to the resistive sensors described above.

Arrays of sensing elements While a single (perhaps large) sensing element may be appropriate for many applications, there will be applications where to achieve required coverage and/or resolution requires an array of sensing elements.

For an array of such sensing elements, determination of which sensing elements have changed provides spatial resolution on the scale of the sensing element separation.

Within a sensing element determining which resistive conductor has experienced a change may provide spatial resolution on the scale of the conductor separation.

Fig. 13 shows a plurality of sensing elements I applied to an article 6, and arranged to cover a portion of the article 6. As shown by the expanded portion of the drawing, the individual sensing element provides resolution "r" within the sensing elements. In contrast the performance of adjacent sensing elements provides resolution R. It is therefore possible to provide both gross and fine detail of the plurality of sensing elements.

Communication with sensing elements Data from the sensing elements can be analysed proximate the sensing elements or remotely.

Fig. 14 shows schematically part of a pipeline in which data collection hubs 14 collect data from an array of sensing elements 1. The data collection hubs might be spaced, for example, 1-10m (e.g. 2m) apart. The data collection hubs may process the raw data to provide information as to which of the sensing elements 1 are showing signs of damage, and optionally the location of that damage within the sensing element. The data collection hubs act as a storage system for the sensor to store each subsequent measurement and time stamp for each data set acquired.

Data from the data collection hubs is transmitted via wired or optical communication bus 16 to analysis stations 15 and then on to a remote management station. Analysis stations 15 might be spaced, for example, between 100m and 10 km (e.g. 1km) apart. The analysis stations can undertake more sophisticated analysis, including the statistical analysis of the data to generate a real time heat map of the rate of change from the resistance values measured via the sensing element and determine where according to the highest rate of change of the sensors nodal positions corrosion is occurring.

Fig. 15 shows a similar arrangement, in which data s retrieved from the analysis stations wirelessly by devices 17 The present disclosure is not limited to any particular means of communication with the sensing elements. Use of a distributed form of data analysis is not required, but is advantageous.

Corrosion/erosion Detection of a change in resistance in a giving sensing element may indicate corrosion or erosion of the sensing element and so be indicative of potential for (or actual) corrosion in the underlying article. Corrosion and erosion are generally slow acting and so changes may take place over a long timescale. In addition corrosion and erosion may extend over a large area, resulting in changes to several sensing elements at once, For monitoring for potential or actual corrosion a relatively low resolution may be adequate, for example 5mm to 20 cm, e.g. 10cm resolution could provide appropriate monitoring.

Fracturing Detection of a sudden change in resistance in a giving sensing element may indicate fracturing of the article in which the sensing element is embedded, or to which the sensing element is applied. A crack in an underlying article may cause a change in resistance to one or more resistive conductors in the sensing element, or it may cause one or more resistive conductors to cease conducting where a resistive conductor is severed.

In addition, fracturing may start small and grow. Detection of mechanical erosion in early stages may provide early warning before cracks initiate in the article: and detection of small cracks provides an early warning before cracks grow. As the spatial resolution of the sensing elements of the present invention is determined by the distance between the resistive conductors, a desired spatial resolution can he controlled by adoption of appropriate manufacturing techniques.

Other variables Resistivity of materials varies with temperature. It may he advantageous to include temperature sensors [even as a resistive temperature sensor] in the sensing elements, or distributed among sensing elements, so that changes in resistance of the sensing elements consequent on temperature change can he compensated for.

Pipelines As indicated above, pipelines typically comprise an inner pipe to contain fluid within the pipeline, overlaid with one or more insulating/protective layers to protect the inner pipe. The sensing elements of the present disclosure may he provided within the insulating/protective layers proximate (but not necessarily immediately adjacent to) the inner pipe such that corrosion of the sensing elements indicates potential or actual corrosion of the inner pipe.

In addition, sensing elements may be provided at varying depth within the insulating/protective layers so that sensing elements remote the inner pipe can provide advanced warning of the potential for corrosion, and sensing elements nearer the inner pipe can provide a further warning, so indicating progress of humidity through the insulating/protective layers.

One or more self-repairing layers having a higher propensity to corrosion but forming protective coatings may be provided between the sensing elements and the inner pipe (e.g. an Al or Zn layer -both of which form coherent protective layers on exposure to sea water).

Detection of corrosion in a sensing element is indicative of the presence of humidity, and may be used to trigger pumps to evacuate moisture from within the insulating/protective layers.

The articles chimed herein this include pipelines.

Wind turbine systems As mentioned above, wind turbine blades are generally laminated composite structures that can experience surface erosion through water impact. Damage to the surface may render the blades liable to further erosion and eventually permit access of water to the interior of the blade where more damage may occur. The method now claimed permits the propensity of a turbine blade to erosion, to he assessed from erosion of the sacrificial resistors of the sensor. In addition the ability of the sensors to detect fractures or cracks at their earliest stage, and of the piezoelectric sensors to detect changes in the mechanical properties of the blade, provides added early warning of potential failure The articles claimed herein thus include wind turbine blades.

Other uses Use of the sensing elements in a pipeline has been shown above. Other uses to which the sensing elements may be applied to detect corrosion and/or mechanical erosion include (without limitation) vehicle bodies (including vehicles for nautical, land, or aerospace use); and building installations, including wind turbine masts and integers other than the blades (e.g. hubs).

Other uses to which the sensing elements may be applied include (without limitation) armour, particularly body armour. The presence of small fractures indicates the ability of the armour to resist impact, and detecting propensity to erosive damage to the surface of the armour may provide early warning of surface damage that may lead to crack initiation.

The sensing elements of the present disclosure are applicable in many applications and are not limited to those mentioned above.

Claims (20)

1. CLAIMSA method for monitor ng the propensity to erosion or corrosion of an article, comprising the steps of:-a) measuring a variable indicative of electrical resistance across a plurality of paths formed by at least one array of resistive conductors in a sensing element applied to or embedded within the article, to provide status information concerning the state of the array b) using the status information to infer the propensity to erosion or corrosion of at least part of the article. 2. 3. 4. 5. 6. 7.
2. A method, as claimed in Claim 1, in which the at least one array of resistive conductors comprises a plurality of resistive conductors disposed in a two dimensional array.
3. A method, as claimed in Claim 1 or Claim 2, in which the an-ay of resistive conductors comprises an interconnected array of resistive conductors and the plurality of paths comprise crossing and electrically connected paths.
4. A method, as claimed in any of Claims 1 to 3, in which the variable indicative of electrical resistance is processed to determine the location of at least one portion of the array having a higher resistance than other portions of the array.
5. A method, as claimed in any of Claims 1 to 4, in which a change in status information is used to infer mechanical damage to the article.
6. A method, as claimed in any of Claims 1 to 5, in which a plurality of the sensing elements are applied to or embedded within the article.
7. A method, as claimed in Claim 6, in which at least one of the sensing elements is embedded at a different depth within the article from at least one other of the sensing elements.
8. 8. A method, as claimed in Claim 6 or Claim 7, in which the status information determined for two or more of the sensing elements is used to infer one or more characteristics concerning the condition of the article between the two or more of the sensing elements.
9. A method, as claimed in any of Claims 6 to 8, in which data from the sensing elements are gathered and processed in data collection hubs mounted to or embedded within the article.
10. A method, as claimed in Claim 9, in which data from the data collection hubs is transmitted to analysis stations and then on to a remote management station.
11. A method, as claimed in Claim 10, in which data is transmitted from the data collection to the analysis stations by wired or optical communication bus or by wireless communication.
12. A method, as claimed in Claim 10 or Claim 11, in which data is transmitted from the analysis stations to the remote management station by wired or optical communication bus or by wireless communication.
13. An article comprising at least one sensing element comprising at least one array of resistive conductors, the sensing element being applied to or embedded within the article and connectable to permit measurement of a variable indicative of electrical resistance to be measured across a plurality of paths formed by the array, the article comprising a part at least susceptible to corrosion and/or erosion and the resistive conductors comprising a material at least as susceptible to corrosion and/or erosion as said part.
14. 14. An article as claimed in Claim 13, in which the resistive conductors comprise conductive particles dispersed in a carrier.
15. 15. An article as claimed in Claim 13 or Claim 14, in which the sensing element further comprises bypass resistive conductors in parallel to respective resistive conductors, whereby 10. 1L 12. 13.o in operation current passes through the resistive conductors and bypass resistive conductors, but o on failure of a resistive conductor, current passes through the respective bypass resistive conductor.
16. 16. An article as claimed in Claim 15, wherein the bypass resistive conductors have a higher resistance than the resistive conductors.
17. 17. An article as claimed in any of Claims 13 to 16, in which the material at least as susceptible to corrosion or erosion as said part comprises conductive particles dispersed in a carrier, the conductive particles comprising conductive particles of the same chemical composition as said part.
18. 18. A sensing element comprising at least one array of resistive conductors providing a plurality of paths and mounted to a substrate, and conductors disposed to permit a variable indicative of electrical resistance to he measured across the plurality of paths, the sensing element further comprising bypass resistive conductors in parallel to respective resistive conductors, whereby * in operation current passes through the resistive conductors and bypass resistive conductors, hut * on failure of a resistive conductor, current passes through the respective bypass resistive conductor.
19. 19. A sensing element as claimed in Claim 18 wherein the bypass resistive conductors have a higher resistance than the resistive conductors.
20. 20. A sensing element as claimed in Claim 18 or Claim 19, wherein at least one of the array of resistive conductors comprises an interconnected array of resistive conductors and the plurality of paths comprise crossing and electrically connected paths. 21. 22. 23. 24. 2iA sensing element as claimed in any of Claims 18 to 20, in which some at least of the bypass resistive conductors are on a first layer of the sensing element and some at least of the resistive conductors are on a second layer of the sensing element.A sensing element as claimed in Claim 22, in which circuit connections in the first layer are in electrical communication with resistive conductors on the second layer.A sensing element as claimed in any of Claims 18 to 22, in which a protective coating covers the bypass resistive conductors and does not cover at least part of the resistive conductors.A sensing element as claimed in Claim 23, in which the part of the resistive conductors not covered by the protective coating is covered by a second protective coating having a lower resistance to erosion and/or corrosion than the protective coating.A sensing element as claimed in any of Claims 18 to 24, or an article as claimed in any of Claims 13 to 17, further comprising a piezoelectric sensor.