Method and Apparatus for Monitoring the Condition of A Pressure-Containing Vessel

Abstract

In a method and apparatus for monitoring the condition of a pressure-containing vessel / pipe 4, a force sensitive sensor (3) is applied to the pipe 4 externally, outer surface 4o, of the pipe. The stre

Description

This disclosure relates to a method and apparatus for monitoring the condition of a 5 pressure-containing vessel such as a fluid-containing pipe. SUMMARY The invention provides a method for monitoring the condition of a pressure-10 containing vessel, the method comprising: applying a sensor to the pressure-containing vessel externally of the pressurecontaining vessel; measuring stress applied to the pressure-containing vessel by monitoring output of the sensor; 15 measuring the internal pressure of the pressure-containing vessel by monitoring LO output of a pressure measurement system; CM determining the wall thickness of the pressure-containing vessel using the measured T- stress applied to the pressure-containing vessel and the measured internal pressure of the pressure-containing vessel, wherein the sensor is secured on the pressure 20 containing vessel by a fixing member and is compressed between the fixing member CM and the pressure-containing vessel during monitoring of the output of the sensor, and wherein compression applied to the sensor is reduced or removed between measurements of the output of the sensor.. 25 The pressure-containing vessel optionally comprises an outer surface. The outer surface of the pressure-containing vessel can optionally be covered in a coating, which may optionally be a protective coating and which may optionally be an insulation material. The sensor may optionally be applied to the outer surface of the pressure-containing vessel or optionally to an outer surface of the coating. 30 Optionally the sensor is in contact with the outer surface of the pipe, or its coating. Optionally the sensor is disposed radially underneath one of a layer or a coating which may be a rubber protective layer or coating or insulation. Optionally the method monitors the condition of a pressure-containing vessel by monitoring corrosion, optionally by measuring the degree of corrosion on the pressure-containing vessel. 5 Optionally the sensor covers a localised area of an outer surface of the pressurecontaining vessel. Optionally the measured stress applied to the pressure-containing vessel is stress applied across the localised area. Optionally the determined wall thickness is the wall thickness across the localised area. 10 Optionally the output of the sensor is generated in response to compression of the sensor, optionally between the fixing member and the pressure-containing vessel. The fixing member optionally comprises a clamp and optionally a housing shell. Optionally the housing shell is compressed between the pipe and the clamp. Optionally the housing shell is engaged with the sensor and the clamp is optionally 15 engaged with the housing shell. LD CM Optionally the Young’s modulus of the fixing member is larger than the Young’s T— modulus of the pressure-containing vessel. 20 The sensor is optionally a force sensitive sensor, such as a force sensitive resistor or CM a force sensitive capacitor. In other words, the output of the sensor is dependent on the amount of force (typically compressive force) applied to said sensor. Optionally stress applied to the pressure-containing vessel is measured by 25 determining a force applied to a force sensitive sensor. Optionally the force applied to the force sensitive sensor is a compression force between the fixing member and the pressure-containing vessel. Optionally the force applied to the force sensitive sensor is determined by calibrating an output signal of the force sensitive sensor to calibration data associated with the force sensitive sensor. 30 Optionally the output signal of the sensor is one or more of the voltage across the sensor, the electrical resistance of the sensor, the electrical capacitance of the sensor or the force / pressure applied to the sensor. Optionally the output signal of the sensor varies with stress on the pressure-containing vessel, e.g. hoop stress, 35 radial stress, etc. Optionally the sensor detects the stress applied to the pressure containing vessel as a result of pressure changes within the fluid in the pressurecontaining vessel, and generates an output signal which varies with the stress detected by the sensor. 5 Optionally the sensor is part of an array of sensors. Optionally, the sensors in the array are arranged circumferentially spaced from one another, e.g. arranged circumferentially in at least one row e.g. at a common axial position. Optionally the sensors are axially spaced from one another in the array, e.g. arranged in at least one line, e.g. parallel with an axis of the vessel. Optionally more than one row 10 and / or more than one line of sensors can be provided. Optionally the sensors are arranged in a cylindrical array. Optionally the array surrounds a circumference of the pressure-containing vessel, e.g. sensors can be placed at regular intervals around the entire circumference. 15 The method optionally comprises the step of adjusting the output of the sensor, LO optionally to bring the output of the sensor within an operable range. Optionally the CM relationship between the force applied to a force sensitive sensor and the output of T— the force sensitive sensor is linear within the operable range. Optionally the operable range is within 20%-80% of the maximum output of the sensor. Optionally 20 the output of the sensor is adjusted using an adjustment mechanism optionally to CM adjust the force (e.g. a preload force) applied to a force sensitive sensor. Optionally the pressure-containing vessel is magnetic (optionally ferromagnetic). Optionally the pressure-containing vessel is formed of or comprises a ferrous metal. 25 Optionally the adjustment mechanism comprises an electromagnet. Optionally the electromagnet is configured to apply a force (i.e. the preload force) to the force sensitive sensor. Optionally the output of the sensor (i.e. the force sensitive sensor) is adjusted using the electromagnet optionally to adjust the force applied to the force 30 sensitive sensor. Optionally the force applied to the force sensitive sensor is dependent on (i.e. proportional to) the current supplied to the electromagnet. Optionally the electromagnet is disposed (i.e. compressed) between the force sensitive sensor and the fixing member. Optionally the sensor is disposed between the adjustment mechanism and the pressure-containing vessel. 35 Optionally the method comprises the step of applying a preload force to the force sensitive sensor to bring the output of the sensor within the operable range. Optionally the preload force is applied using the adjustment mechanism (i.e. using the electromagnet). Optionally the preload force applied to the force sensitive sensor 5 is a compressive force (i.e. between the electromagnet and the pressure-containing vessel). Optionally the method comprises the step of unloading (i.e. either partially or completely) the preload force on the sensor. Having the sensor continuously pre-loaded throughout the monitoring process may 10 attenuate the signal from the sensor thus increasing maintenance requirements. Therefore, in certain examples it is desirable reduce or remove the preload force, at least while the measurements are taken of the output from the sensor. Some examples unload and subsequently reload the preload force applied by the adjustment mechanism in-between measurements of the stress applied to the 15 pressure-containing vessel. Advantageously, in some examples, the preload can be LO applied during the measurements, and released or reduced between the CM measurements. For example, where the adjustment mechanism comprises the T— electromagnet, this allows a user to unload and reload the preload force in-between said measurements more easily and more consistently by adjusting the current 20 supplied to the electromagnet, e.g. by switching off or reducing current between the CM measurements. Optionally the pressure-containing vessel is a pipe, optionally a cylindrical pipe. Optionally the pressure containing vessel can have a circular cross section, but this 25 is not essential. Optionally the method comprises the further step of connecting the sensor and optionally the pressure measurement system to a sensor node wherein the sensor node optionally supplies power to the sensor and optionally to the pressure 30 measurement system. Optionally measurements of the stress applied to the pressure-containing vessel and optionally measurements of the internal pressure of the pressure-containing vessel are stored, optionally in the sensor node. Optionally the method includes processing calculations using the measured stress applied to the pressure-containing vessel and optionally the measured internal pressure of the 35 pressure-containing vessel, optionally in the sensor node. Optionally the method includes transmitting output signals from the sensor (e.g. from the sensor node) and optionally the pressure measurement system to a storage system, optionally a cloud storage system or a portable data reader which may optionally be a handheld data reader, such as a mobile phone, a tablet device or other handheld computing device. 5 Transmission is optionally done by the sensor node. Optionally outputs from the pressure measurement system can be transmitted to a separate control system such as a distributed control system. The control system is optionally connected to the storage system such that outputs from the control system 10 can be transmitted to the storage system. The method optionally comprises the step of determining a change in stress applied to the pressure-containing vessel. Optionally the change in stress applied to the pressure-containing vessel is determined by calculating the difference between the 15 measured stress applied to the pressure-containing vessel and a previously LO measured stress applied to the pressure-containing vessel. Optionally the measured CM stress applied to the pressure-containing vessel used to determine the wall thickness T- of the pressure-containing vessel is greater than an average measured stress applied to the pressure-containing vessel. Optionally the change in stress applied to 20 the pressure-containing vessel determined by using the measured stress applied to CM the pressure-containing vessel used to determine the wall thickness of the pressure containing vessel is greater than an average change in pressure. The method optionally comprises the further steps of continuously measuring (e.g. 25 sampling at regular intervals) the stress applied to the pressure-containing vessel; optionally continuously measuring the internal pressure of the pressure-containing vessel and optionally storing said measurements of the stress applied to the pressure-containing vessel and internal pressure of the pressure-containing vessel. The method optionally comprises the further step of continuously determining the 30 wall thickness optionally using the continuous measurements of the stress applied to the pressure-containing vessel and optionally using the continuous measurements of the internal pressure of the pressure-containing vessel. The method optionally comprises the further step of determining a change in wall thickness optionally using the continuous determinations of the wall thickness, e.g. by comparing stored measurements with more recent measurements. 5 The method optionally comprises the further step of determining the rate of change of the wall thickness optionally using the continuous determinations of the wall thickness. The method optionally comprises the further step of alerting an operator, optionally 10 when the wall thickness of the pressure-containing vessel is less than a critical wall thickness. Optionally the operator is alerted when the change in wall thickness is greater than a critical change in wall thickness. Optionally the operator is alerted when the rate of change of the wall thickness is greater than a critical rate of change of the wall thickness. 15 LO The invention also provides a monitoring system for monitoring the condition of a CM pressure-containing vessel, the monitoring assembly comprising: T— a sensor assembly; wherein the sensor assembly comprises: 20 at least one sensor adapted to be applied external to the pressure-containing vessel; CM the sensor being secured on the pressure containing vessel by a fixing member such that it is compressed between the fixing member and the pressure-containing vessel during monitoring of the output of the sensor;wherein the at least one sensor is adapted to emit an output signal indicative of stress being applied to the pressure-25 containing vessel; and a pressure measurement system adapted to emit an output signal of the internal pressure of the pressure-containing vessel; wherein the monitoring system is adapted to determine the wall thickness of the pressure-containing vessel using the measured stress applied to the pressure-30 containing vessel and the measured internal pressure of the pressure-containing vessel, and wherein the system is configured such that compression applied to the sensor is reduced or removed between measurements of the output of the sensor. Optionally the monitoring system comprises an adjustment mechanism, optionally 35 adapted to adjust the output of at least one sensor within the sensor assembly. Optionally the adjustment mechanism is adapted to adjust the force applied to a force sensitive sensor. Various aspects of the invention will now be described in detail with reference to the 5 accompanying figures. Still other aspects, features, and advantages of the present invention are readily apparent from the entire description thereof, including the figures, which illustrates a number of exemplary aspects and implementations. The invention is also capable of other and different examples and aspects, and its several details can be modified in various respects, all without departing from the 10 spirit and scope of the present invention. Accordingly, each example herein should be understood to have broad application, and is meant to illustrate one possible way of carrying out the invention, without intending to suggest that the scope of this disclosure, including the claims, is limited to that example. Furthermore, the terminology and phraseology used herein is solely used for descriptive purposes and 15 should not be construed as limiting in scope. In particular, unless otherwise stated, LO dimensions and numerical values included herein are presented as examples CM illustrating one possible aspect of the claimed subject matter, without limiting the T- disclosure to the particular dimensions or values recited. All numerical values in this disclosure are understood as being modified by "about". All singular forms of 20 elements, or any other components described herein are understood to include CM plural forms thereof and vice versa. Language such as "including", "comprising", "having", "containing", or "involving" and variations thereof, is intended to be broad and encompass the subject matter listed 25 thereafter, equivalents, and additional subject matter not recited, and is not intended to exclude other additives, components, integers or steps. Likewise, the term "comprising" is considered synonymous with the terms "including" or "containing" for applicable legal purposes. Thus, throughout the specification and claims unless the context requires otherwise, the word “comprise” or variations thereof such as 30 “comprises” or “comprising” will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers. Any discussion of documents, acts, materials, devices, articles and the like is 35 included in the specification solely for the purpose of providing a context for the present invention. It is not suggested or represented that any or all of these matters formed part of the prior art base or were common general knowledge in the field relevant to the present invention. 5 In this disclosure, whenever a composition, an element or a group of elements is preceded with the transitional phrase "comprising", it is understood that we also contemplate the same composition, element or group of elements with transitional phrases "consisting essentially of”, "consisting", "selected from the group of consisting of, “including”, or "is" preceding the recitation of the composition, element 10 or group of elements and vice versa. In this disclosure, the words “typically” or “optionally” are to be understood as being intended to indicate optional or non-essential features of the invention which are present in certain examples but which can be omitted in others without departing from the scope of the invention. 15 References to directional and positional descriptions such as upper and lower and LO directions e.g. “up”, “down” etc. are to be interpreted by a skilled reader in the CM context of the examples described to refer to the orientation of features shown in the T— drawings, and are not to be interpreted as limiting the invention to the literal interpretation of the term, but instead should be as understood by the skilled 20 addressee. CM BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 shows a schematic of a monitoring system applied to a pipe; 25 Fig. 2 shows a detailed view of a first embodiment of a sensor assembly; Fig. 3 shows a view of the first embodiment of the sensor assembly applied to a Pipe; Fig. 4 shows a detailed view of a second embodiment of a sensor assembly; Fig. 5 shows a cross sectional end view of an adjustment mechanism of the second 30 embodiment of the sensor assembly; Fig. 6 shows an illustration of sensors applied to a pipe and arranged in a cylindrical array; Fig. 7 shows an illustration of a force sensitive sensor applied to an outer surface of a pipe and fixed in place with a fixing member; Fig. 8a shows a circuit diagram of a voltage divider comprising a force sensitive resistor and a fixed value resistor; Fig. 8b shows a circuit diagram of a voltage divider comprising a force sensitive capacitor and a fixed value capacitor; 5 Fig. 9 shows a method for preparing a sensor assembly for use in a monitoring system; Fig. 10 shows a method for monitoring the condition of a pipe by utilising a monitoring system to determine the wall thickness of the pipe; Fig. 11 shows a validation process that each digital integer reading taken in step 304 10 of the method of Fig. 10 may undergo in step 305 of the method of Fig. 10; Fig. 12 shows an output signal of a sensor applied to a pipe can change over time as the pipe undergoes corrosion; Fig. 13 shows a cross sectional side view of an adjustment mechanism of a third embodiment of the sensor assembly; 15 Fig. 14 shows a modified version of the method of Fig. 10 capable of utilising LO advantages of the third embodiment of the sensor assembly; and CM Fig. 15 shows how the force applied to a force sensitive resistor (FSR) of the third T- embodiment of the sensor assembly can change over time. 20 DETAILED DESCRIPTION CM A monitoring system 1 is typically used to monitor the occurrence of corrosion within a pipe 4. Fig. 1 shows a schematic of a monitoring system 1 with a sensor assembly 2 applied to an outer surface 4o of a pipe 4, a pressure measurement system 30, a 25 distributed control system 31 and a storage system 50 The sensor assembly 2 is connected to a sensor node 20 such that the outputs of the sensor assembly 2 can be relayed to the sensor node 20. In this example, the sensor node 20 is connected to a storage system 50 such that the outputs of the sensor node 20 can be relayed to the storage system 50. The pressure measurement system 30 is connected to a 30 distributed control system 31 such that outputs from the pressure measurement system can be relayed to the distributed control system 31. The distributed control system 31 is connected to the storage system 50 such that outputs from the distributed control system 31 can be relayed to the storage system. Fig. 2 shows a detailed view of a first embodiment of the sensor assembly 2. In this embodiment, the sensor assembly comprises an array of sensors 3 housed within a fixing member 10. The fixing member 10 comprises a housing shell 11 and a clamp 12. The sensors 3 are attached to an inner surface 11 i of the housing shell 11 and 5 the clamp 12 is engaged with an outer surface 11o of the housing shell 11. Fig. 3 shows a view of a segment of a pipe 4 with the first embodiment of the sensor assembly 2 applied such that the sensors 3 are in contact with an outer surface 4o of the pipe 4. In other examples of the invention, the pipe 4 may be covered in a 10 coating (i.e. insulation) and the sensors 3 are in contact with an outer surface of the coating. Fig. 3 also shows a sensor node 20 applied to the outer surface 4o of the pipe 4. A second clamp 21 is used to fix the sensor node 20 to the outer surface 4o of the pipe 4. 15 The pipe 4 is adapted to contain a flowing fluid such as a liquid or a gas. In some LO examples, the pipe 4 can be made from metal such as steel. In this example, the CM pipe 4 is a cylindrical steel pipe with a circular cross section but other embodiments T- may be used on pipes with different cross sections (i.e. square, rectangular or elliptical), or on pipes made from different materials. 20 CM The fixing member 10 fixes the sensors 3 to the pipe 4 in order to prevent relative movement (axial and rotational) between them. Figs. 2-3 show a housing shell 11 comprising a partial ring of material concentric with 25 the pipe 4. Typically the housing shell 11 is made from either a plastic or a metal, and typically is more resilient to radial compression than the pipe. The housing shell 11 may be applied to the outer surface 4o of the pipe 4 either by wrapping the housing shell 11 around the outer surface 4o of the pipe 4 or by sliding the housing shell 11 over an open end (not shown) of the pipe 4 (after which the pipe 4 can be 30 connected to another pipe in order to form a pressure-containing vessel). The clamp 12 typically comprises a band 12b that is engaged with the outer surface 11o of the housing shell 11 and a fastener 12f adapted to adjust the force the band 12b applies to the housing shell 11, sensors 3 and pipe 4. The pressure measurement system 30 measures the internal pressure of the pipe 4, typically from within the pipe 4 and typically via a pressure tapping or an intrusive valve on the pipe 4. The pressure tapping or the intrusive valve may be on a different pipe that is connected to the pipe 4 such that pressure within the fluid in the pipe 4 is 5 communicated to the other pipe. The pressure measurement system 30 is adapted to emit an output signal indicative of the internal pressure of the pipe 4. The output signals of the pressure measurement system 30 in this example are transmitted to the distributed control system 31. The pressure measurement system 30 may be a pressure transducer or a pressure gauge. 10 The sensors 3 are adapted to emit an output signal indicative of the stress being applied to the pipe 4, e.g. hoop stress and / or radial stress. The stress is typically due to internal pressure resulting from fluid flowing in the pipe 4. The output signals of the sensors 3 are relayed to the sensor node 20 via their respective shared 15 connections. LD CM In the first embodiment of the sensor assembly 2, the sensors 3 are force sensitive T— sensors 3, specifically force sensitive resistors (FSRs) 3 (such as FlexiForce™ sensors offered by Tekscan, Inc. of South Boston, MA 02127, USA, but any other 20 suitable FSR 3 could be used instead). The resistance of a FSR 3 is dependent on CM the force, pressure or mechanical stress applied to it. Force sensitive capacitors (FSCs) 3 (such as SingleTact micro force sensors offered by PPS UK Ltd of Glasgow, UK but any other suitable FSC 3 could be used instead) are force sensitive sensors 3 where the capacitance of the sensor 3 is dependent on the force, pressure 25 or mechanical stress applied to it and can optionally be used in some examples of the invention. Fig. 7 shows a schematic of a force sensitive sensor 3 applied to an outer surface 4o of a pipe and fixed in place with a fixing member 10. An increase in internal 30 pressure within the pipe 4 causes it to expand, increasing the radius of the pipe 4. In turn, the compression force between the force sensitive sensor 3 and the fixing member 10 increases as the sensor 3 is compressed between the pipe 4 and the housing shell 11. This force is at least partially experienced by the force sensitive sensor 3, causing it to deform and reduce in thickness. This causes a decrease in 35 the resistance (or capacitance) across the force sensitive sensor 3. When the pipe 4 undergoes corrosion and its wall thickness decreases, the pipe 4 becomes more reactive to changes in the internal pressure of the pipe 4. This causes the radius of the pipe 4 and the compression force between the force sensitive sensor 3 and the fixing member 10 to increase more than it would in an un-corroded pipe. 5 Typically each sensor 3 and the pressure measurement system 30 are directly connected to the sensor node 20. The sensor node 20 typically comprises a suitable integrated circuit (not shown) typically adapted to gather the measurements from the sensors 3; process the gathered data and transmit the gathered and processed data. 10 The sensor node 20 may also be adapted to control the power supplied to the sensors 3. The sensor node 20 is typically connected to a power supply (not shown) that supplies power to the sensor node 20 and in turn the sensors 3. The power supply is typically a suitable onboard battery and / or any suitable external power supply. The power supply may also supply power in turn to the pressure 15 measurement system 30 but the power supply may also be connected to another LO power supply independent to the sensor node 20. The data gathered and processed CM by the sensor node 20 can be transmitted to the storage system 50 either through T— wireless transmission means (such as Bluetooth connection and / or LoRa connection and / or wireless LAN connection) or physical transmission means (such as HART 20 connection and / or Ethernet LAN connection and / or suitable wires / cables). For CM example, the data from the sensor node 20 can be transmitted wirelessly to a cloud storage system. Data transmitted to the storage system 50 may then undergo further processing within a software package (e.g. used in calculations of the wall thickness of the pipe 4). 25 Typically, each force sensitive sensor 3 is connected in series with a fixed value component 5 in order to create a voltage divider (see Figs. 8a-8b). In examples using a force sensitive resistor 3, the fixed value component 5 is a fixed value resistor 5 with a fixed resistance (see Fig. 8a). In examples using a force sensitive 30 capacitor 3, the fixed value component 5 is a fixed value capacitor 5 with a fixed capacitance (see Fig. 8b). The sensor node 20 converts an analog measurement of the voltage across the fixed value component 5 into a digital integer reading from 0 to 1023 using an analog-to-digital converter (not shown) comprised within the sensor node 20. The digital integer reading is then optionally transmitted to the storage 35 system 50, from which it can be converted to a digital voltage reading by mapping the digital integer range of the force sensitive sensor 3 to the possible range of analogue voltage readings (typically, taken as OV to the voltage supplied across the force sensitive sensor 3 and the fixed value component 5). 5 The digital integer reading can then be converted to a digital voltage reading using the following equation: Vo = (So Smln)(VR — Vmin) / Sr 10 Where Vo is the digital voltage reading across the fixed value resistor 5; So is the digital integer reading, Smm is the minimum possible digital integer reading; Vr is the range of possible analog readings of the voltage; Vmm is the minimum possible analog reading of the voltage and Sr is the range of possible digital integer readings. 15 In examples using a FSR 3, the digital voltage reading across the fixed value resistor LO 5 can then be converted to a digital resistance reading of the FSR 3 by using the CM following equation: O RFSr = ((Vs - Vo) RFIx) / Vo 20 CM Where Rfsr is the digital resistance reading of the FSR 3; Rfix is the resistance of the fixed value resistor 5; Vs is the total voltage supplied across the FSR 3 and the fixed value resistor 5 and \ / o is the digital voltage reading across the fixed value resistor 5. 25 In examples using a FSC 3, the digital voltage reading can then be converted to a digital capacitance reading of the FSC 3 by using the following equation: Cfsc - (Vo Cfix) / (Vs — Vo) 30 Where Cfsc is the digital capacitance reading of the FSC 3; Cfix is the capacitance of the fixed value capacitor 5; Vs is the total voltage supplied across the FSC 3 and the fixed value capacitor 5 and \ / o is the digital voltage reading across the fixed value capacitor 5. The pressure measurement system 30 may take analog measurements indicative of the internal pressure within the pipe 4. These analog measurements may be converted to digital readings using an analog-to-digital converter (not shown) comprised within the pressure measurement system 30.The digital readings are then 5 optionally transmitted to and stored in the distributed control system 31. The stored digital readings are then optionally converted into digital internal pressure readings that are then typically transmitted to the storage system 50 from which the digital internal pressure readings can be further processed (e.g. used in calculations of the wall thickness of the pipe 4). 10 The material of the clamp 12 typically has a Young’s modulus greater than that of the pipe 4. Increasing the Young’s modulus of the clamp 12 relative to the Young’s modulus of the pipe 4 increases the amount of force measured by a force sensitive sensor 3. The specific material of the clamp 12 is dependent on the material of the 15 pipe 4 (for example, a stainless steel clamp may be used for a cast iron pipe). LD CM Fig. 2 shows the sensors 3 arranged in a circular array and equidistantly spaced T— around the circumference of the pipe 4. The sensors 3 may also be arranged in a cylindrical array with circular arrays of sensors 3 repeating along the axial length of 20 the pipe 4 (as illustrated in Fig. 6). Some examples of the sensor assembly 2 may CM comprise a single sensor 3. The specific number of sensors 3 used and their size can vary depending on the type of corrosion expected in the pipe 4. After the sensor assembly 2 is applied to the pipe 4, insulation lagging can be 25 applied over the sensor assembly 2 and the pipe 4. Fig. 4 shows a detailed view of a second embodiment of a sensor assembly 102 in accordance with the present invention. This second embodiment of the sensor assembly 102 is relatively similar to the first embodiment of the sensor assembly 2 30 described in Figs. 2 and 3, and thus similar components and features to those of the first sensor assembly 2 are indicated with the same reference numeral but with the addition of 100 and unless otherwise subsequently described, operate in a similar manner and therefore may not be described again. Like the first embodiment described in Figs. 2-3, the second embodiment of the sensor assembly 102 can be utilised in a monitoring system 1 as described in Fig. 1 and comprises an array of sensors 103 housed within a fixing member 110. The fixing member 110 comprises a housing shell 111 and two clamps 112. The sensors 5 103 are attached to an inner surface 111i of the housing shell 111 and the clamps 112 are engaged with an outer surface 111o of the housing shell 111. The second embodiment of the sensor assembly 102 is adapted to be applied to an outer surface 4o of a pipe 4 in a similar way as the first embodiment of the sensor assembly 2 is in Fig. 3. 10 The housing shell 111 comprises an array of protrusions extending from an outer circumference of the housing shell 111. Each protrusion is adapted to house an adjustment mechanism 140. The adjustment mechanisms 140 are arranged in a circular array that is azimuthally aligned with the circular array of sensors 103. 15 Accordingly, each sensor 103 has a corresponding adjustment mechanism 140. LD CM The two clamps 112 are positioned on the outer surface 111o of the housing shell T- 111 such that there is one clamp 112 at either axial end of the array of adjustment mechanisms 140. 20 CM The housing shell 111 comprises a plurality of partial ring segments. Adjacent partial ring segments are coupled together using hinges 113. In operation, the housing shell 111 is wrapped around the outer surface 4o of the pipe 4 before being fixed in place with the clamps 112. 25 Fig. 5 shows a cross sectional end view of an adjustment mechanism 140 in accordance with the second embodiment of the sensor assembly 102. In Fig. 5, the housing shell 111 and the sensor 103 are engaged with the outer surface 4o of the pipe 4 and are fixed in place using the clamps 112 (not shown in Fig. 5). An 30 adjustment screw 141 is passed through a threaded bore passing through the housing shell 11 and engaged with a pressure pad 142. The pressure pad 142 is engaged with an outer surface of the sensor 103 and typically covers the whole outer surface area of the sensor 103. Passing the adjustment screw 142 further through the housing shell 111 increases the force applied to the sensor 103. The pressure pad 142 is adapted to distribute the force applied from the adjustment screw across the outer surface of the sensor 103. A monitoring system 1 is used to monitor the condition of a pipe 4 by monitoring the 5 degree of corrosion within the pipe 4. Typical corrosion mechanisms include microbiologically influenced corrosion and erosion corrosion. Corrosion can occur in relatively localised areas within the pipe 4 or across relatively wide areas within the pipe 4. Corrosion causes the wall thickness of the pipe 4 to reduce. By determining the wall thickness of the pipe 4, the degree to which the pipe 4 has corroded can be 10 gauged. Fig. 9 illustrates a method for preparing a sensor assembly 2, 102 for use in a monitoring system 1. The method shown in Fig. 9 will be described in detail below as if the first embodiment of the sensor assembly 2 as described above specifically 15 utilising FSRs 3 is being prepared for operation. All calculations described in this LO method are to be carried out using SI units. CM T— At step 201, an operator may apply the FSRs 3 with the housing shell 11 to a test pipe in a way similar to that shown in Fig. 3 and use the clamp 12 to fix the FSRs 3 20 and housing shell 11 in place. The test pipe is typically connected to a system CM adapted to vary the internal pressure of the test pipe. The test pipe may share some physical properties with the pipe 4 such as wall thickness, Young’s modulus, and radius. The test pipe should be free of any corrosion. 25 At step 202, the force applied to the FSRs 3 is adjusted such that the output from each FSR 3 is within a desired operational range (e.g. from 20% to 80% of the maximum output of the FSR 3). The desired operational range may be where there is a linear relationship between the resistance of the FSR 3 and the force applied to the FSR 3. When using the first embodiment of the sensor assembly 2, this may be 30 done by using the clamp fastener 12f to adjust the contact force between the sensors 3 and the clamp 12. At step 203, a relationship between the internal pressure of the test pipe and the resistance of each FSR 3 is determined. In examples of the method using FSCs 3 35 instead of FSRs 3, a relationship between the internal pressure of the test pipe and the capacitance of the FSR 3 is determined. For each FSR 3, this relationship is determined by taking measurements of the digital output of the FSR 3 over a range of known internal pressures (typically from OPa to the maximum operational pressure of the pipe 4 the sensor assembly is being prepared for). The digital output 5 measurements are converted to digital resistance readings as described above. The collected data from each FSR 3 can be fitted to a straight line, resulting in a calibration formula where the internal pressure of the test pipe is dependent on the resistance of the FSR 3. The calibration formula typically follows the following form: 10 P(R) = aR + b Where Pi(R) is the calculated internal pressure, R is the measured resistance of the FSR 3 and a and b are constant values respectively determined by fitting the 15 collected data. LD CM At step 204, a relationship between the pressure applied to an FSR 3 and the T— resistance of an FSR is determined. This can be determined using the following equation: 20 CM PS(R) = Pi(R) rp2 Ec tc / (rp2 Ec tc + rc2 Ep tp) Where PS(R) is the pressure applied to the FSR 3; P,(R) is the internal pressure determined using the calibration formula determined in step 203; rp is the original 25 radius of the test pipe without any applied internal pressure; Ec is the Young’s modulus of the clamp 12; tc is the thickness of the clamp and the ; rc is the original radius of the clamp 12 applied to the FSRs 3; Ep is the Young’s modulus of the test pipe and tp is the wall thickness of the test pipe. 30 The above formula can typically be simplified as: PS(R) = cR + d Where c and d are constant values determined when converting Pi(R) to PS(R). After completing step 204, the sensor assembly 2 is prepared to be used in the monitoring system 1 and the method ends. The method described in Fig. 9 may also be carried out using the second 5 embodiment of the sensor assembly 102 utilising FSRs 103 as described above. The steps of the method using the second embodiment of the sensor assembly 102 are relatively similar to the method using the first embodiment of the sensor assembly 2 described above and thus unless subsequently described, are carried out in a similar manner and may not be described again. 10 At step 201, the two clamps 112 of the second embodiment of the sensor assembly 102 are used to fix the FSRs 103 and housing shell 111. Each clamp 112 is made from the same material as the other. Using their respective fasteners 112f, the force applied to the FSRs 103 by each clamp 112 is adjusted such that the force is evenly 15 distributed between each axial end of the FSRs 103. LD CM At step 202, the adjustment mechanisms 140 of the second embodiment of the T- sensor assembly 102 allow the force applied to the FSRs 103 to be adjusted individually. Using the adjustment mechanisms 140, the force applied to each FSR 20 103 is adjusted such that they share an equal output as well as being within the CM desired operational range. Adjusting the force applied to each FSR 103 such that they share an equal output results in a more accurate determination of the relationships in steps 203 and 204. 25 Additionally at step 204, Ec, tc and rc are respectively taken as the Young’s modulus, thickness and radius shared across the two clamps 112. Fig. 10 illustrates a method for monitoring the condition of a pipe 4 by utilising a 30 monitoring system 1 to determine the wall thickness of the pipe 4. The method shown in Fig. 10 will be described in detail below as if it is utilising a monitoring system 1 utilising the first embodiment of the sensor assembly 2 as described above. Additionally, it is described as if the sensor assembly was prepared using the method outlined in Fig. 9 and described in detail above. All calculations described in 35 this method are to be carried out using SI units. At step 301, an operator may apply the FSRs 3 with the housing shell 11 to a pipe 4 in a way similar to that shown in Fig. 3 and use the clamp 12 to fix the FSRs 3 and housing shell 11 in place. The pipe 4 may be a pipe connected to a system different 5 to that of the test pipe used in the method of Fig. 9. It may also be the test pipe used in the method of Fig. 9. At step 302, the clamp fastener 12 can be used to adjust the force applied to the FSRs 3 such that the digital integer reading from each FSR is within the desired 10 operational range (i.e. from 20% to 80% of the maximum output of the FSR 3). When the pipe 4 shares an equal wall thickness, Young’s modulus and radius with the test pipe used in the method outlined in Fig. 9, the force applied to the FSRs 3 may be adjusted such that the output of the FSRs 3 match or are close to the output associated with the pipe 4 at its current internal pressure. This is done using the 15 calibration formula determined in step 203. LD CM At step 303, a measurement from each FSR 3 within the sensor assembly 2 is taken. T— Each measurement is then relayed from the FSRs 3 to the sensor node 20 where it is converted to a digital integer reading that is typically then transmitted to a storage 20 system 50 (such as a cloud storage system or a portable data reader which may CM optionally be a handheld data reader, such as a mobile phone, a tablet device or other handheld computing device) from which the reading can be further processed. At step 304, a measurement from the pressure measurement system 30 is taken. 25 This measurement is then converted into a digital internal pressure reading that is then typically transmitted to the storage device from which it can be further processed if desired. At step 305, each digital integer reading taken during step 303 undergoes a 30 validation process. The validation process determines whether or not a digital integer reading meets a desired validation criteria. Digital integer readings that meet the validation criteria outlined move on to step 306 while digital integer readings that do not meet the validation criteria are discarded. If every digital integer reading fails to meet the validation criteria, the method goes back to step 303. Measurements from 35 the FSRs 3 may be taken and subsequently converted to corresponding digital integer readings continuously between a given interval of time. The interval of time may be 30 minutes, one hour, two hours or any other suitable length of time. Step 305 may instead be carried out using digital integer readings converted into either digital resistance readings or readings of the pressure applied to the FSR 3. If so, 5 step 306 is bypassed. At step 306, the digital integer readings that meet the validation criteria of step 305 are converted to digital resistance readings. These digital integer resistance readings are then subsequently converted to readings of the pressure applied to the FSR 3 10 using the calibration formulae determined in step 204. At step 307, the digital readings of the pressure applied to the FSR 3 determined in step 306 are used to determine the wall thickness of the pipe 4 by using the following equation: 15 LO tp = (P-Ps(R))rp2tcEc / Ps(R)rc2Ep CM T- Where PS(R) is the pressure applied to the FSR 3; Pi is the internal pressure measured using the pressure measurement system 30; Ec is the Young’s modulus of 20 the clamp 12; tc is the total thickness of the clamp 12 and the housing shell 11; rc is CM the radius of the clamp 12 applied to the FSRs; Ep is the Young’s modulus of the pipe 4 and tp is the wall thickness of the pipe 4. At step 308, it is determined whether the operator wishes for the method to continue, 25 failing which, the method ends. If the method is to continue the method returns to step 303. This allows for continuous determinations of the wall thickness of the pipe 4 to be made, e.g. taking samples at regular intervals. Continuous determinations of the wall thickness of the pipe 4 can be used to 30 determine a change in wall thickness of the pipe 4 and a rate of change in the wall thickness of the pipe 4. An operator with access to the storage system may be alerted when a determined wall thickness is less than a predetermined critical wall thickness; when a 35 determined change in wall thickness is greater than a predetermined critical change in wall thickness or when the rate of change in wall thickness is greater than a predetermined critical rate of change. The method described in Fig. 10 may also be carried out using the second 5 embodiment of the sensor assembly 102 utilising FSRs 103 as described above. The steps of the method using the second embodiment of the sensor assembly 102 are relatively similar to the method using the first embodiment of the sensor assembly 2 described above and thus unless subsequently described, are carried out in a similar manner and may not be described again. 10 At step 301, the two clamps 112 of the second embodiment of the sensor assembly 102 are used to fix the FSRs 103 and housing shell 111. Each clamp 112 is made from the same material as the other. Using their respective fasteners 112f, the force applied to the FSRs 103 by each clamp 112 is adjusted such that the force is evenly 15 distributed between each axial end of the FSRs 103. LD CM At step 302, the adjustment mechanisms 140 of the second embodiment of the T- sensor assembly 102 allow the force applied to the FSRs 103 to be adjusted individually. Using the adjustment mechanisms 140, the force applied to each FSR 20 103 is adjusted such that they share an equal output as well as being within the CM desired operational range. Being able to individually adjust the force on each FSR 103 results in a more accurate adjustment to the output associated with the pipe 4 at its current internal pressure. 25 The adjustment mechanisms 140 of the second embodiment of the sensor assembly 102 allow for a more accurate determination of whether a determined wall thickness is less than a predetermined critical wall thickness; a determined change in wall thickness is greater than a predetermined critical change in wall thickness and the rate of change in wall thickness is greater than a predetermined critical rate of 30 change. Fig. 11 illustrates a validation process that can be carried out for each measurement taken in step 305. The method is described below as if digital integer readings determined from the measurements from the FSRs 3 are used. Although, the method may instead be carried out using digital integer readings converted into either digital resistance readings or readings of the pressure applied to the FSR 3. At step 401, the change in measurement is determined by calculating the difference 5 between the digital integer reading taken at step 304 and a previous digital integer reading (typically a digital integer reading taken directly previous to the digital integer reading taken at step 304). At step 402, it is determined whether the measurement constitutes an increase in the 10 force applied to an FSR 3. This can be carried out by comparing the digital integer reading taken in step 304 to an average digital integer reading of the FSR 3. The average digital integer reading of the FSR 3 is calculated as the mean of the digital integer readings taken over a given time period. The given time period may be the past day, the past three hours, the past one hour or any other suitable period of time. 15 If the digital integer reading taken at step 304 is greater than the average digital LO integer reading, it proceeds to step 404 of the method. Otherwise, the digital integer CM reading is determined to not meet the validation criteria and proceeds to step 403 of T— the method where it is discarded and the method subsequently ends. 20 At step 404, it is determined whether the change in measurement determined in step CM 401 constitutes a reduction in the wall thickness of the pipe 4. This can be carried out by comparing the change in digital integer reading taken in step 401 to an average change in digital integer reading. The average change in digital integer reading is calculated as the mean of previously determined changes in digital integer 25 readings that constitute an increase in force applied to the FSR 3. This value is determined over a given period of time. The given period of time may be the past day, the past three hours, the past one hour or any other suitable period of time. If the change in digital integer reading taken at step 401 is greater than the average change in digital integer reading, it is determined to constitute a reduction in wall 30 thickness and ultimately meet the validation criteria of step 305. The digital integer reading taken in step 304 proceeds to step 405 and is to be used in step 306. If the change in digital reading taken at step 401 is not greater than the average change in digital integer reading, the digital integer reading is determined to not meet the validation criteria and proceeds to step 403 where it is discarded and the method 35 subsequently ends. Fig. 12 shows how an output signal (i.e. readings of the resistance of a sensor 3) of a sensor 3 applied to a pipe 4 can change over time as the pipe 4 undergoes corrosion. Each resistance reading corresponds to a determination of the wall 5 thickness of the pipe 4 (i.e. the baseline resistance value Rb corresponds to the wall thickness of an un-corroded pipe). Resistance readings may be converted to determinations of the wall thickness of the pipe 4 using the calibration equation described above in step 307 of the method of Fig. 10. 10 In Fig. 12, the pipe 4 begins to corrode at a linear rate at time ti. Correspondingly, the resistance of the sensor begins to decrease from a baseline resistance Rb. This reduction in resistance corresponds to a reduction in the wall thickness of the pipe 4. At time tent, the resistance has fallen to a critical resistance reading Rcrit that corresponds to a critical wall thickness (e.g. 50% of the wall thickness of an un- 15 corroded pipe). The wall thickness of the pipe falling below the critical wall thickness LO indicates that the pipe 4 is in danger of bursting and in need of repair. Once the CM resistance reading has fallen to Rcrit, an operator is alerted that the wall thickness of T— the pipe has reached the critical wall thickness and that they need to repair the pipe O 4. 20 CM Fig. 13 shows a schematic and not-to-scale cross sectional side view of an adjustment mechanism 540 of a third embodiment of a sensor assembly 502. This third embodiment is relatively similar to the first embodiment of the sensor assembly 2 described in Figs. 2 and 3 and the second embodiment of the sensor assembly 25 102 described in Figs. 4 and 5. Therefore, similar components and features to those of the first and second sensor assemblies 2, 102 are indicated with the same reference numeral but with the addition of 500 with respect to the first embodiment and unless otherwise subsequently described, operate in a similar manner and therefore may not be described again. 30 Similar to the second embodiment of the sensor assembly 102, a housing shell 511 is engaged with the outer surface 4o of the pipe 4 and fixed in place using at least one clamp (not shown) such as a jubilee clip, which extends around the housing shell 511 in a similar manner as described for the sensor assembly 102 and applies 35 a radial force which resists movement of the housing shell 511 relative to the pipe. Optionally the housing shell 511 and the clamp are relatively rigid, and each typically has a relatively high Young’s modulus as compared with the pipe 4 as in previous examples. In this example, the sensor 503 is held in place against the outer surface of the pipe 4o in a cavity of the housing shell 511, and is optionally at least partially 5 covered on its radially outer surface by a pressure pad 542. In this example, the adjustment mechanism differs from the sensor assembly 102, in that in this example the adjustment mechanism is in the form of an electromagnet 541 rather than a screw thread. The electromagnet 541 is disposed within the same 10 cavity as the sensor 503 and pressure pad 542, and lightly compressed between the inner surface of the housing shell 511 and the outer surface of the pressure plate 542 by one or more resilient devices such as springs 543 held in compression between the inner surface of the housing shell 511 and the outer surface of the electromagnet 541. The electromagnet 541 (and optionally the pressure pad 542) 15 optionally cover the majority of the outer surface of the sensor 503. In other LO examples, the electromagnet 541 covers the whole of the outer surface of the sensor CM 503. In this example, the pipe 4 is made from a magnetic (i.e. ferromagnetic) T- material (i.e. iron). Any suitable electromagnet 541, such as the DFR0794 offered by DFRobot) may be used. The function of the pressure pad 542 is to distribute the 20 force of the electromagnet onto the outer surface of the sensor 503. CM The springs 543 are relatively weak, and serve to keep the stack of the electromagnet 541, the pressure pad 542 and the sensor 503 in place against the outer surface 4o of the pipe, without applying significant radial force to the sensor 25 503. Optionally the components of the stack (e.g. the electromagnet 541, the pressure pad 542 and the sensor 503) can be fixed together, e.g. glued. In this example, at least the electromagnet 541 (and optionally the pressure pad and the sensor 503) can move radially within the cavity relative to the housing shell 511 either radially outwards away from the pipe 4 (compressing the springs 543) or 30 radially inwards towards the pipe 4 (extending the springs 543 and compressing the sensor 503 against the pipe 4). The springs 543 typically urge the stack of the electromagnet 541, pressure pad 542 and sensor 503 in a radial direction back into a relatively neutral position in the absence of any other forces. The pressure pad 542 is optional, and could be omitted in certain other examples. Fig 13 is schematic and not to scale and the radial thickness of the sensor 503 and optionally the pressure pad 542 could be much thinner than is shown in the drawings. In this example, supplying the electromagnet 541 with current causes the 5 electromagnet 541 to generate a magnetic field. This generates an attractive force between the electromagnet 541 and the magnetic pipe 4 which in turn causes compression of the sensor 503 between the electromagnet 541 and the pipe 4 as the electromagnet 541 moves towards the pipe 4 as a result of the electromagnetic attraction between the two. Increasing the current supplied to the electromagnet 541 10 increases the magnitude of the magnetic field. This increases the attractive force between the electromagnet 541 and the pipe 4 and therefore increases the compressive force applied to the sensor 503. Also, the compressive force applied to the sensor 503 can be decreased by reducing the current supplied to the electromagnet 541 (i.e. either partially or completely). 15 LO The method described in Fig. 9 may also be carried out using the third embodiment CM of the sensor assembly 502 utilising FSRs for the sensors 503 as described above. T- The steps of the method using the third embodiment of the sensor assembly 502 are relatively similar to the methods using the first and second embodiments of the 20 sensor assembly 2, 102 described above and thus unless subsequently described, CM are carried out in a similar manner and may not be described again. At step 201, the housing shell 511 is clamped to the pipe 4, holding the FSRs 503 against the outer surface 4o of the pipe 4. 25 At step 202, the adjustment mechanisms 540 of the third embodiment of the sensor assembly 502 typically allow the force applied to the FSRs 503 to be adjusted individually at each FSR 503 by altering the current supplied to each respective electromagnet 541. Using the adjustment mechanisms 540, the force applied to each FSR 503 is typically adjusted such that they share an equal baseline output as well 30 as being within the desired operational range. Advantageously, the electromagnets 541 allows an operator to more easily and more precisely execute step 202 than if they were using the second embodiment of the sensor assembly 102 as it is possible to adjust the current supplied to each 35 electromagnet 541 using a suitable computer program rather than manually turning each adjustment screw 141 of each adjustment mechanism of the second embodiment of the sensor assembly 102. Also, this permits the adjustment to be performed remotely. 5 Fig. 14 illustrates a modified version of the method of Fig. 10 capable of utilising advantages of the third embodiment of the sensor assembly 502. The steps of Fig. 14 are relatively similar and are carried out in a relatively similar manner to the steps of the method of Fig, 10. Steps corresponding to those described in Fig, 10 are indicated with the same reference numeral but with the addition of 300 (i.e. step 301 10 of Fig. 10 corresponds to step 601 of Fig. 14) and unless subsequently described, are carried out in a similar manner and therefore may not be described again. At step 601, the housing shell 511 is clamped to the pipe 4, holding the FSRs 503 against the outer surface 4o of the pipe 4. 15 LO At step 602, the adjustment mechanisms 540 of the third embodiment of the sensor CM assembly 502 allow the force applied to the FSRs 503 to be adjusted individually by T- altering the current supplied to each respective electromagnet 541. Using the adjustment mechanisms 540, the force applied to each FSR 503 is adjusted such 20 that they share an equal output as well as being within the desired operational range. CM Typically, when the output of the FSR is deemed too high, the output is decreased by decreasing the current supplied to the electromagnet and when the output of the FSR 503 is deemed too low, the output is increased by increasing the current supplied to the electromagnet 541. The final current supplied to each electromagnet 25 541 at this stage may be recorded as a respective preload current. At step 603, a series of measurements (typically three) from each FSR 503 within the sensor assembly 502 is taken. Typically there is an interval of around 15 seconds between each measurement within each respective series of 30 measurements. For each FSR 503, the current supplied to the respective electromagnet 541 is typically reduced (typically completely) after the final measurement of the respective series of measurements in order to unload the respective FSR 503. Each series of measurements is then relayed from the respective FSRs 503 to a sensor node (not shown) where each series of 35 measurements is converted to a series of digital integer readings that is typically then transmitted to a storage system (not shown) from which the reading can be further processed. At step 603a, each series of digital integer readings undergoes a maintenance check 5 process. The maintenance check process determines whether or not a series of measurements indicates that the respective FSR 503 requires maintenance (i.e. recalibration, replacement due to excessive wear, etc.). In this example, the maintenance check process comprises calculating the variation across each digital integer reading within each respective series of digital integer readings. If said 10 variation exceeds a predetermined threshold value, the operator is indicated that the respective FSR 503 requires maintenance and the method proceeds to step 603b, in which the operator may cease operation of the monitoring system 1, detach the sensor assembly 503 from the pipe 4 and carry out the necessary maintenance. Afterwards, the method returns to step 601. If no maintenance is required, the 15 method proceeds from step 603a to step 604. LD CM Step 605 is carried out in the same manner as step 305 of Fig, 10 with the exception T- that average digital integer readings (calculated from respective series of digital integer readings) undergo the validation process of step 305. Step 605 may also be 20 carried out using the validation process of Fig. 11. CM Step 606 is carried out in the same manner as step 306 of Fig, 10 with the exception that each average digital integer readings calculated in step 605 are converted into a digital resistance reading. 25 At step 608, it is determined whether the operator wishes for the method to continue, failing which, the method ends. If the method is to continue, the method returns to step 602. On return to step 602, each electromagnet 541 is supplied with the previously recorded preload current. 30 For each cycle of the method of Fig. 14, force is only applied to each FSR 503 during steps 602 and 603. This is in contrast to the method of Fig. 10, in which force is applied to each FSR 3, 103 continuously throughout each cycle. This continuous application of force may cause excessive wear to the FSRs 3, 103, meaning that the 35 FSRs 3, 103 require maintenance more frequently. Having each adjustment 24 01 25 mechanism 540 comprise the electromagnet 541 allows an operator to more easily and more consistently unload and reload the force applied to each FSR 503 by simply adjusting the current supplied to each respective electromagnet 541. 5 Fig. 15 illustrates how the force applied to a FSR 503 of the third embodiment of the sensor assembly 502 can change over time. At time ta, the respective preload current is supplied to the electromagnet 541 in order to preload the respective FSR 503. Typically starting from ta, a first series of measurements A, comprising measurements Ai, A2 and A3, is taken from the FSR 503. Typically, there is a period 10 of around 15 seconds (or any other suitable length of time) between each measurement Ai, A2 and A3 of the first series of measurements A. After the final measurement A3 of the first series of measurements A is taken at time tb, the preload current is cut from the electromagnet 541, unloading the FSR 503. The FSR 503 remains unloaded until time tc, at which the electromagnet 541 is again supplied with 15 the preload current and the FSR 503 is preloaded. The time between tb and tc may be 30 minutes, one hour, two hours or any other suitable length of time. Similarly to the first series of measurements A, a second series of measurements B comprising measurements Bi, B2 and B3 are taken between tc and td. 20 Yet further modifications and improvements may be made to the embodiments of the apparatus and methods hereinbefore described without departing from the scope of the invention.

Claims

1. A method for monitoring the condition of a pressure-containing vessel, the method comprising:5 applying a sensor to the pressure-containing vessel externally of the pressurecontaining vessel;measuring stress applied to the pressure-containing vessel by monitoring output of the sensor;measuring the internal pressure of the pressure-containing vessel by monitoring 10 output of a pressure measurement system;determining the wall thickness of the pressure-containing vessel using the measured stress applied to the pressure-containing vessel and the measured internal pressure of the pressure-containing vessel, wherein the sensor is secured on the pressure containing vessel by a fixing member and is compressed between the fixing member15 and the pressure-containing vessel during monitoring of the output of the sensor, LO and wherein compression applied to the sensor is reduced or removed between CM measurements of the output of the sensor.

2. A method as claimed in claim 1, wherein the Young’s modulus of the fixing20 member is larger than the Young’s modulus of the pressure-containing vessel.CM3. A method as claimed in claim 1 or 2, wherein the sensor is a force sensitive sensor.25 4. A method as claimed in claim 3, wherein the force sensitive sensor is a forcesensitive resistor or a force sensitive capacitor.

5. A method as claimed in any one of claims 1-4, including the step of preloading the sensor to adjust the output of the sensor to within a range where the30 relationship between the force applied to the force sensitive sensor and the output of the force sensitive sensor is linear.

6. A method as claimed in any preceding claim, wherein stress applied to the pressure-containing vessel is measured by determining a compression force applied 35 to the sensor.

7. A method as claimed in any preceding claim, including calibrating an output signal of the sensor to calibration data associated with the force sensitive sensor.5 8. A method as claimed in any preceding claim, wherein the sensor detectsstress applied to the pressure-containing vessel as a result of pressure changes within the fluid in the pressure-containing vessel, and generates an output signal which varies relative to the stress detected by the sensor.10 9. A method as claimed in any preceding claim, wherein the sensor comprisesan array of sensors spaced from one another along and around at least one axis.

10. A method as claimed in any preceding claim, including recording measurements of the stress applied to the pressure-containing vessel and 15 measurements of the internal pressure of the pressure-containing vessel and LO processing calculations using the stored values of measured stress applied to the CM pressure-containing vessel and the stored values of measured internal pressure ofT- the pressure-containing vessel.20 11. A method as claimed in any preceding claim, including the step ofCM determining a change in stress applied to the pressure-containing vessel.

12. A method as claimed in any preceding claim, including sampling at regular intervals the stress applied to the pressure-containing vessel and the internal 25 pressure of the pressure-containing vessel; storing said samples of the stress applied to the pressure-containing vessel and internal pressure of the pressurecontaining vessel, and storing sequential determinations of the wall thickness obtained from the stress and internal pressure samples.30 13. A method as claimed in claim 12, including determining a change in wallthickness by comparing wall thickness values determined at different times.

14. A method as claimed in claim 12 or 13, including determining the rate of change of the wall thickness of the pressure-containing vessel.

15. A method as claimed in any one of claims 12-14, including generating an alert when the wall thickness of the pressure-containing vessel is less than a critical wall thickness and / or where the rate of change of the wall thickness is greater than a critical rate of change of wall thickness.

516. A monitoring system for monitoring the condition of a pressure-containing vessel, the monitoring assembly comprising: a sensor assembly;wherein the sensor assembly comprises:10 at least one sensor adapted to be applied external to the pressure-containing vessel, the sensor being secured on the pressure containing vessel by a fixing member such that it is compressed between the fixing member and the pressure-containing vessel during monitoring of the output of the sensor;wherein the at least one sensor is adapted to emit an output signal indicative of 15 stress being applied to the pressure-containing vessel; andLO a pressure measurement system adapted to emit an output signal of the internal CM pressure of the pressure-containing vessel;T- wherein the monitoring system is adapted to determine the wall thickness of thepressure-containing vessel using the measured stress applied to the pressure-20 containing vessel and the measured internal pressure of the pressure-containing CM vessel, and wherein the system is configured such that compression applied to thesensor is reduced or removed between measurements of the output of the sensor.

17. A monitoring system as claimed in claim 16, including an adjustment 25 mechanism adapted to adjust the output of at least one sensor within the sensor assembly.

18. A monitoring system as claimed in claim 16 or 17, wherein the adjustment mechanism is adapted to adjust a preload compression force applied to a force 30 sensitive sensor.

19. A monitoring system as claimed in claim 17 or 18, wherein the adjustment mechanism is remotely actuable.24 01 2520. A monitoring system as claimed in claim 16-19, wherein the sensor is in compression and in contact with an outer surface of the pressure-containing vessel or its coating.5 21. A monitoring system as claimed in any one of claims 16-20, wherein thesensor is adapted to detect stress applied to the pressure-containing vessel as a result of pressure changes within the fluid in the pressure-containing vessel, and is adapted to generate an output signal which varies relative to the stress detected by the sensor.1022. A monitoring system as claimed in any one of claims 16-21, wherein the sensor comprises an array of sensors spaced from one another along and around at least one axis.15 23. A monitoring system as claimed in any one of claims 16-22, adapted torecord measurements of the stress applied to the pressure-containing vessel and measurements of the internal pressure of the pressure-containing vessel and process calculations using the stored values of measured stress applied to the pressure-containing vessel and the stored values of measured internal pressure of20 the pressure-containing vessel.

24. A monitoring system as claimed in any one of claims 16-23, adapted to generate an alert when the wall thickness of the pressure-containing vessel is less than a critical wall thickness and / or where a rate of change of the wall thickness is 25 greater than a critical rate of change of wall thickness.

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