GB2489107A

GB2489107A - In-situ self-calibrating strain sensor with fixed reference magnet

Info

Publication number: GB2489107A
Application number: GB1204562.1A
Authority: GB
Inventors: Pei An
Original assignee: Pulse Structural Monitoring Ltd
Current assignee: Pulse Structural Monitoring Ltd
Priority date: 2011-03-15
Filing date: 2012-03-15
Publication date: 2012-09-19
Also published as: GB201104345D0; GB201204562D0

Abstract

Self-calibrating sensor 1 for monitoring strain in an underwater structure 4 (e.g. a deepwater riser) comprises a magnetostrictive linear displacement detector 110 that includes elongate rod 116 containing magnetostrictive waveguide 122 with magnets 50, 52 positioned along its length. Reference magnet 50 is fixed to the rod, defining fixed reference position 60. Interaction between the magnetic fields of the magnets and current pulses 123 applied to the waveguide induces torsional strain waves therein in dependence on magnet positions. Attached to the structure are reference mount 70 (fixed to the detector at a fixed distance 80 from the reference position) and measurement mount 72. Measurement magnet 52 is fixed to the measurement mount and free to move along the rod so that the distance 82 between reference mount and measurement magnet varies depending on the strain in the structure. Strain wave propagation times thus allow calculation of magnet positions and strain in the structure. Changes in the measured reference magnet position owing to aging related drift are used to add or subtract an offset to the determined measurement magnet position to correct these values against accuracy drift.

Description

In-Situ Self-Calibrating Strain Sensor

BACKGROUND

a. Field of the Invention

This invention relates to a self-calibrating strain sensor for remotely monitoring strain in structures, and in particular for in situ monitoring of a loaded underwater structures such as a deepwater riser or conductor, or an oil or gas drilling or production platform.

b. Related Art Risers and conductors are long tubular structures assembled from shorter lengths of steel pipe typically 12 m long. In service they are subjected to high dynamic loads and since service lives as long as 25 years are often required fatigue is an important design issue. Offshore oil and gas platforms are often constructed from tubular steel elements that are welded or bolted together. Platforms are also intended to remain in service for many years.

With all such structures, fatigue damage is an ever present concern. Corrosion can also be a long term problem. As material is lost to corrosion, the strain will increase for a given load. It is necessary to monitor over many years the strain in underwater structures used in the oil and gas industry. Historically strain gauges or sensors have been used to monitor strain in underwater structures. These sensors, once installed, are not able to be calibrated in service. There is a need for strain gauges to be ultra reliable as well as ultra stable over decades, as the reliability of the information provided by such sensors is input to a planned integrity management program for underwater structures.

To be ultra stable, the output from a sensor, for example an output voltage from an analogue sensor or a digital output from a digital sensor, must not drift over time at a constant environmental temperature. If the drift due to changes in the performance of electronic circuitry associated with the sensor or the drift in performance of a transducer component used in the sensor is not quantifiable, then the sensor will fail to measure a key performance characteristic of the underwater structure.

In this specification, the term deep water means water depths of approximately 250 m (1000 feet) up to about 1250 m (5000 feet) and ultra deep water means depths in excess of 1250 m. The invention is particularly concerned with strain sensors required to operate in ultra deep water, for example at pressures of 40 MPa (400 bar), equivalent to the water pressure at a depth of 4000 m, but which may also be useful in deep water situations. The harsh requirements of deep and ultra deep water render many otherwise suitable sensors, such as capacitive and optical sensors, which normally operates in air at I atmospheric pressure, incapable of practical use. Clever packaging and water sealing methods which work well in shallow water applications become progressively impractical in deep water applications and effectively impossible in ultra deep water applications.

Prior art subsea strain sensors are based on displacement measurement. A typical displacement sensor used in shallow water applications is a piston that extends from a fixed housing towards a measurement mount or block. The separation of the housing and block determines the extension of the piston, which is monitored by circuitry within the housing. Both the housing and block are fixed to a section of the underwater structure in which change in strain is to be monitored. If the initial separation between the mounting points of the housing and block is S0 and at a later time the strain in the structure has changed such that the separation is Si then the change in separation AS = S -So divided by the initial separation is a measure of the change in strain within the structure: AS/S0 Although strain gauges are widely used in land-based application, unfortunately, the reliability of strain gauges in deep water has been poor due to water ingress.

Strain gauges may also suffer from long term calibration drift with a consequent drift in monitored strain. Although, recalibration of sensors is a standard practice in land-based workshops or in laboratories, because strain gauges need to be fixed to the structure being monitored, which may be in deep water, it can be difficult or impossible to calibrate a strain gauge in situ or to replace a strain gauge suspected of not being reliable.

Other types of sensor (non-contact displacement sensors) can be used to monitor an underwater structure, however, these all known sensor suffer a common problem of long-term drift, for example due to aging of electronic components.

Recalibration is also not practical or possible for this type of sensor.

It is an object of the invention to provide a more reliable and convenient self-calibrating strain sensor for remotely monitoring strain in underwater structures that carry either dynamic or static loads, particularly when used in deep water and ultra deep water situations.

SUMMARY OF THE INVENTION

According to a first aspect of the invention, there is provided a self-calibrating strain sensor apparatus for monitoring strain in a section of an underwater structure, said apparatus comprising: -a magnetostrictive linear displacement detector, said detector comprising a sensor module and a magnetostrictive transducer, said transducer comprising an elongate rod and within the rod a magnetostrictive waveguide, the rod and said waveguide extending away from the sensor module, the sensor module comprising means for providing to said waveguide current pulses and the waveguide being configured to carry torsional strain waves induced in the waveguide when current pulses are applied to said waveguide and the sensor module comprising means for receiving said strain waves; -a plurality of magnets each of the magnets being positioned in proximity with the magnetostrictive waveguide at different positions along the length of the elongate rod, the generation of said torsional strain waves being dependent on the position of said magnets along the length of the elongate rod; -a plurality of mounts, each of the mounts being configured to be fixed to said section of said underwater structure, the mounts comprising a reference mount and at least one measurement mount, the reference mount being fixed to a mounting portion of said detector, and the magnets comprising a reference magnet and at least one measurement magnet, the reference magnet being fixed to the elongate rod to define a fixed reference position such that the distance between said reference mount and reference position is fixed and the or each measurement magnet being fixed to a corresponding one of said measurement mounts and being free to move relative to the length of the elongate rod such that, in use, when both the reference mount and the or each measurement mount are fixed to said section of said underwater structure, the distance between the reference mount and the or each measurement magnet varies depending on the strain in said section of said underwater structure; and -A processing means; wherein the processing means is operable to detect the time of arrival of said strain waves received by the sensor module relative to said current pulses as a measure of the position of each of said magnets along the elongate rod and to calculate from said measured positions of each of said magnets a measured strain in said section of said underwater structure, the processing means further being operable to use changes in said measure of the position of the reference magnet relative to the fixed reference position owing to aging-related drift of the sensor module to add or subtract an offset to said measured position of said at least one measurement magnet in order to correct said measured strain against said aging-related drift.

The apparatus preferably comprises additionally a memory. As the offset is the difference between the current measure of the position of the reference magnet and an initial measure of the position of the reference magnet, the initial measure of the position of the reference magnet may then be stored in the memory.

According to a second aspect of the invention, there is provided a strain monitoring system for in situ monitoring of strain in a section of an underwater structure, the system comprising a self-calibrating strain sensor apparatus and an underwater structure, the apparatus being according to the first aspect of the invention, each of said plurality of mounts being fixed to said section of said underwater structure, the processing means being operable to use said changes in said measure of the position of the reference magnet relative to the fixed reference position owing to aging-related drift of the sensor module to add or subtract an offset to said measured position of said at least one measurement magnet in order to correct said measured strain against said aging-related drift while the apparatus is in situ with said section.

The initial measure of the position of the reference magnet is then determined after fixing of the self-calibrating strain sensor apparatus to the section of the underwater structure where strain is to be monitored.

The underwater structure may be carry a static load, or may be dynamically loaded with different loads depending on its operation.

As the electrical signal is carried by the waveguide from the head end to the foot end, a magnetic interaction between each of the magnets and the magnetostrictive waveguide causes torsional strain waves to be induced in the waveguide. The torsional strain waves are also called sonic strain pulses.

In general, the elongate rod will have a head end and a foot end. In a preferred embodiment of the invention, the head end of the elongate rod is fixed to the sensor module such that the elongate rod extends away from the sensor module towards the foot end of the elongate rod, the mounting portion of said detector being proximate said head end.

If there are two reference magnets, one proximate the head end and one proximate the foot end, with at least one measurement magnet in between to define two fixed reference positions, then the measure of the position of each of said magnets can be used to deduce the relative contributions of both constant and percentage drifts. In this case the distance between the first and second reference magnets should be fixed, for example, by attaching the reference magnets directly to one another with a fixed distance between the reference magnets. Then, the reference distance from the head end to the first reference magnet can be used to deduce an offset or constant correction needed to correct aging-related drift of the sensor module, and the reference distance between the head end and the second reference magnet can be used to deduce a percentage change correction against drift.

A percentage charge in the measured distance values can occur when temperature changes cause components in the apparatus, for example the rod, to expand or contract. Temperature changes could also cause the section being measured to expand or contract. In applications where the temperature of the unit and measured section will be essentially constant, for example in deep-water measurement of strain in an oil or gas exploration or production rig or riser pipe, there will be little or no expected percentage drift over time to be compensated. In such applications, it is only necessary to correct against aging related drift of the sensor unit, which will require a positive or negative offset to be added to the measured distance to the or each measurement magnet.

The offset will change slowly with time, for example changing appreciably on the timescale of at least months or years. The offset will also be a constant offset, irrespective of the distance being measured. The offset required to correct the distance measurement for the measurement magnet will therefore be expected to be the same as that for the reference magnet. As the initial distance measurement for the reference magnet can determined and stored, for example in a memory of a processor within the sensor module, the offset is then the difference between the initial reference distance measurement and the current reference distance measurement. When there is more than one measurement magnet, the same distance offset (which will be either positive or negative in magnitude) can be applied to each measured distance in order to correct aging-related drift of the sensor module.

The torsional waves will, in general, spread in both directions along the waveguide. Therefore, a reflection termination is provided at the foot end to reflect the waves at the foot end. A damping termination is provided at the head end to ensure that the energy from the torsional waves is conveyed to a torsional motion sensor at the head end which generates an electrical indication of the torsional motion within the magnetostrictive waveguide induced by passage of the electrical excitation at the position of the reference and measurement magnets. The displacement is determined from the interval of time between the detection of the torsional motion travelling directly from each magnet and the detection of the torsional motion reflected from the reflection termination. Each electrical pulse therefore results in two detected torsional waves generated at the point of each magnet, one wave having travelled directly to the torsional motion sensor and the other having been reflected from the reflection termination.

In a preferred embodiment of the invention, the mounting portion is a portion of the sensor module nearest the head end of the elongate rod. However, it would, alternatively be possible for the mounting portion to be a portion of the elongate rod nearest the sensor module.

In one embodiment of the invention the at least one measurement mount comprises a first measurement mount, and the at least one measurement magnet comprises a first measurement magnet. The first measurement magnet is then fixed to the first measurement mount, with the first measurement mount being farther away from the sensor module than the reference mount. The reference mount may, however, be mounted to a mounting portion anywhere along the detector. For example, the first measurement mount may be closer to the sensor module than the reference mount.

In a preferred embodiment of the invention, the magnetostrictive waveguide is an elongate conductor such as a wire. Other forms of elongate conducting waveguide may alternatively be used, for example tubular wavegu ides.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be further described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 shows a known magnetostrictive linear displacement detector having a sensor module and a magnetostrictive transducer, the transducer comprising an elongate sensing rod for sensing the position of a measurement magnet; Figure 2 is a schematic diagram illustrating how the magnetostrictive linear displacement detector of Figure 1 works; Figure 3 is a schematic diagram of a subsea strain sensor apparatus for Is remotely monitoring strain in a section of a loaded underwater structure according to a first preferred embodiment of the invention, comprising a magnetostrictive linear displacement detector including one reference magnet and one measurement magnet; and Figure 4 is a schematic diagram of a subsea strain sensor apparatus for remotely monitoring strain in a section of a loaded underwater structure according to a second preferred embodiment of the invention, comprising a magnetostrictive linear displacement detector including one reference magnet and two measurement magnets.

DETAILED DESCRIPTION

Magnetostriction is a property of ferromagnetic materials such as iron, nickel, and cobalt. When placed in a magnetic field, these materials change size and/or shape. The ferromagnetic materials used in magnetostrictive linear displacement sensors are transition metals such as iron, nickel, and cobalt. When a material has positive magnetostriction, it enlarges when placed in a magnetic field. With negative magnetostriction, the material shrinks. The amount of magnetostriction in base elements and simple alloys is small. Changes in volume are of the order of 1 06.

Applying a magnetic field to a magnetostrictive material therefore causes stress that changes the physical properties of the material. The converse also holds: applying stress to a magnetostrictive material changes its magnetic properties, for example its magnetic permeability, and this effect is called the Villari effect.

Normal magnetostriction and the Villari effect are both used in the operation of a magnetostrictive linear displacement sensor. An important characteristic of an elongate conductor such as a wire or tube made of a magnetostrictive material is the Wiedemann effect. The Wiedemann effect describes the twisting due to an axial magnetic field applied to such a ferromagnetic conductor that is carrying an electric current. When an axial magnetic field is applied to the conductor, and a current is passed through the conductor, a twisting occurs at the location of the axial magnetic field. The twisting is caused by interaction of the axial magnetic field, usually from a permanent magnet, with the magnetic field along the conductor, which is present due to the current in the conductor.

The current is applied as a short-duration dc pulse, for example I to 2 ps. The minimum current density is along the centre of the conductor and the maximum at the conductor surface. This is due to the skin effect. The magnetic field intensity is also greatest at the conductor surface. This aids in developing the waveguide twist. Since the current is applied as a pulse, the mechanical twisting travels in the wire as an ultrasonic wave. The magnetostrictive conductor is therefore called the waveguide. The wave travels at the speed of sound in the waveguide material, which is typically about 3000 mIs.

Figure 1 shows a known magnetostrictive linear displacement detector 10 having a sensor module 12 and a magnetostrictive transducer 14. The sensor module contains electronic components (not shown) to which connections are made via a -11 -connector 13. The transducer comprises an elongate sensing rod 16 for sensing the position 18 of a measurement magnet 20.

The operation of the magnetostrictive linear displacement sensor is shown schematically in Figure 2. The rod 16 is illustrated cut away to show an elongate waveguide 22, which in this example is a taut magnetostrictive wire running axially through the rod. The rod 16 forms a protective housing for the wire 22. The rod 16 has a head end 24 nearest the module 12 and a foot end 26 farthest from the module used to complete a circuit so that the magnetostrictive wire 22 can be pulsed electrically 23. A return wire 21 formed in a non-magnetostrictive material such as copper is used to complete a circuit through the wire.

A current pulse is provided to the wire 22 generates a circular magnetic field 26 around the wire. Magnetostriction in the wire in the magnetic field formed by the combination of a longitudinal magnetic field 28 from the measurement magnet 29 and the circular magnetic field 26 created by the current pulse 23 generates a strain pulse 30, 31 that travels in both directions down the waveguide wire. The strain pulse 30 moving towards the head end is detected by a coil 32 in a torsional motion sensor 35. The other strain pulse 31 encounters an end termination 38 to the wire at the foot end 26 of the rod 16. The end termination can be either a damping termination to minimise reflections, or a reflection termination to reflect the stain wave so that the torsional motion sensor 35 senses two pulses for each current pulse. A torsion wave is a wave motion in which the vibrations of the medium are periodic rotational motions around the direction of propagation.

The location of the measurement magnet 20 is determined by first applying the current pulse 23 to the waveguide 22. At the same time, a timer is started. The current pulse causes a sonic wave to be generated at the location of the measurement, which travels along the waveguide until it is detected by the torsional motion sensor. This stops the timer. The elapsed time indicated by the timer then represents the distance between the measurement magnet 20 and the torsional motion sensor 35.

The torsional motion sensor 35 makes use of the Villari effect. A small piece of magnetostrictive material 33, called the tape, is welded to the waveguide 20 near one end 34 of the waveguide. This tape 33 passes through the coil 32 and is magnetized 37 by a small permanent magnet 36 called the bias magnet. When a sonic wave propagates down the waveguide and then down the tape, the stress induced by the wave causes a wave of changed permeability in the tape owing to the Villari effect. This in turn causes a change in the tape magnetic flux density, and thus a voltage output pulse is produced from the coil 32, owing to the Faraday effect. The voltage pulse is detected by electronic circuitry and conditioned into the desired output. The tape helps to damp torsional movement in the waveguide to prevent back reflection of the torsional wave into the waveguide.

The invention will now be described with reference to Figures 3 and 4. A subsea strain sensor apparatus 1, 101 is shown schematically, mounted to a section 2 of an underwater structure 4, which may be either statically loaded or dynamically loaded. The apparatus comprises a magnetostrictive linear displacement detector 110, 210 comprising a sensor module 112 and a magnetostrictive transducer 114 comprising an elongate sensing rod 116 and within the rod a magnetostrictive waveguide 122 which is electrically connected to a return conductor 123. The rod and waveguide extending away from the sensor module 112, from a head end 124 towards a foot end 126. The sensor module comprises a pulse generator 40 for providing to the waveguide current pulses 123. As explained above, the waveguide is configured to carry torsional strain waves induced in the waveguide when current pulses are applied to the waveguide in the presence of an external longitudinal magnetic field applied to the waveguide by an external magnet. The sensor module as comprises means for receiving said strain waves such as a torsional motion sensor 135 similar to that described above.

The apparatus 1, 101 also comprises a plurality of magnets 50, 52, 150, 152, 154 each of which is positioned in proximity with the magnetostrictive waveguide 122 at different positions 60, 62, 160, 162, 164 along the length of the elongate rod 116. The timing of the generation of the torsional strain waves following the application of the current pulse 123 is dependent on the position of the magnets along the length of the elongate rod.

The apparatus comprises a plurality of mounts 70, 72, 170 172, 174, each of which is configured to be fixed to the section 2 of the underwater structure 4 to be monitored. The mounts span the section to be monitored and include a reference mount 70, 170, and at least one measurement mount 72, 172, 174 including a first measurement mount 72, 172. The reference mount is fixed to a mounting portion 76, 176 of the detector, which in this example is a part of the sensor module nearest the head end 124 of the rod 122. The reference mount defines a reference mount position 71, 171 on the section 2 being monitored. The reference mount may, however, be fixed to any convenient part of the detector. The magnets comprise a reference magnet 50, 150 and at least one measurement magnet 52, 152 154, including a first measurement magnet 52, 152. The reference magnet is fixed to the elongate rod to define a fixed reference position such that the distance (SRQ) 80, 180 between the reference mount 70, 170 and reference position 60, 160 is fixed. The, or each, measurement magnet 52, 152, 154 is fixed to a corresponding one 72, 172, 174 of the measurement mounts and is free to move relative to the length of the elongate rod 116 such that, in use, when both the reference mount and the or each measurement mount are fixed to the section to be monitored 2, the distance (SM) 82, 182, 184 between the reference mount and the or each measurement magnet varies depending on the strain in that section of the underwater structure.

The or each measurement magnet is fixed to a corresponding measurement mount to form a measurement assembly 92, 192, 194, the first measurement magnet 52, 152 therefore being fixed to the first measurement mount 72, 172 to form a first measurement assembly 92, 192, the elongate rod not being fixed to the or each measurement assembly so that, in use, the or each measurement magnet is free to move 96, 196, 198 relative to the elongate rod owing to changes in distance between the or each corresponding measurement mount and the -14 -reference mount position 71, 171.

The apparatus also comprises a processing means 49, for example, a microprocessor based computational unit, and may include other associated electronic components, for example a power supply unit 46. External data and power connections 48, 47 can be made through a suitable connector.

The processing means 49 is operable to detect the time of arrival of the strain waves received by the sensor module 112 relative to the 123 current pulses as a measure of the position 60, 62, 160, 162, 164 of each of the magnets along the length of the rod and to calculate from this a measured strain in the monitored section 2 of the underwater structure 4. A circuit or logic triggered in the processing means by the transmitted pulse 123 and detects the time of arrival of the received pulse which, after further processing produces a digital output. The processing means 49 is further operable to use changes in the measure of the position of the reference magnet relative to the fixed reference position 60, 160 owing to drift in the accuracy the measured positions to correct the measured strain against said drift.

The use of two or more magnets, one of which is fixed at a fixed location permits the apparatus to be used in a self-calibrating manner. As the distance (SRQ) 80, to the reference magnet 50, 150 is fixed, any change in the measured distance (SRI) 86, 186 to the reference magnet will be due to drift in the performance of the apparatus. Depending on the type of drift that is to be expected, the measurement of the distance to the measurement magnet(s) can be corrected to account for the measured drift as determined from the reference magnet. For example, if the drift results in a constant shift of all determined time measurements of the return sonic pulse relative to the electrical pulse, which will be expected to be the case for aging-related drifty of electronic components within the sensor module, then the difference between the initial time (or distance) measurement TRO (or SRQ) and the current time (or distance) measurement TRI (or SR1) for the reference magnet will represent the drift, and this difference AT = TRI -TRO (or AS = SRi -SRO) can be subtracted or added to all determined time (or distance) measurements TM1, TM2, etc. (or SMI, SM2, etc.) for the measurement magnets to generate time or distance measurements corrected for drift as follows: TN=TMN-AT SN=SMN-LIS where N is the number of measurement magnets N = 1, 2, etc. In this way a constant offset, either negative or positive, can be applied to the measured positions to correct measured strain against drift.

If the expected drift results in a constant percentage change in all determined time measurements of the return sonic pulse relative to the electrical pulse (for example, due to thermal expansion or contraction of the rod), then the difference between the initial time (or distance) measurement and the current time (or distance) measurement for the reference magnet divided by the initial time (or distance) measurement AT/TRI = (TRI -TRQ)/TRI (or AS/SRI = (SRi -SR0)ISRI) will represent the drift, and this percentage difference can be multiplied to all determined time (or distance) measurements TM1, TM2, etc. (or SM1, SM2, etc.) for the measurement magnets to generate time or distance measurements corrected for drift as follows: TN = TMN X (1+ (AT/TRN)) SN = SMN X (I + AS/SRN)) where N is the number of measurement magnets N = 1, 2, etc. In this way a percentage offset can be applied to the measured positions to correct measured strain against drift.

Of course, if more than one component in the apparatus is expected to change over time, then the drift may result in a combination of both a constant offset and a percentage offset, and as long as the drift is characterised, then it will be possible to apply the a correction to the measured time and distance values, for example by using the following equations: TN = TMN x (A(1+ (LXT/TRN)) + B(1+ (LXT/TRN))) SN = SMN x (A(1 + LISISRN)) + B(1 + ASISRN))) where A and B are parameters that match the expected contributions from constant and percentage contributions to drift.

In deep water measurement situations, only the first type of aging related drift will be present, due to aging-related changes in the performance (i.e. drift) of the sensor unit. In such situations, when using conventional strain sensors, this type of drift is difficult to deal with, as it is often necessary to raise and recalibrate a measurement unit. By calculating and then adding or subtracting an offset value to measured values, the strain sensing apparatus of the invention is self-calibrating in situ, (i.e. while mounted at the section where strain is to be measured) over the operating lifetime of the apparatus.

The apparatus may be powered by a remote source of electrical power by means of a cable, but the apparatus of the invention consumes relatively little electrical power and so is well suited to being powered by means of a battery power source attached or within the sensor module. To conserve power, distance measurements may be taken an extended time intervals, for example, once per day. Results may then be communicated by conventional means to the surface, for example by cable or by acoustic data transmission through water.

The rod length will vary from 300 mm to 500 mm. The pulse rate is not critical for the operation of the apparatus but may be of the order of 0.1 s to 1 s when measurements are being taken, which may be at intermittent intervals depending on the structure being monitored. The overall accuracy after self-calibration is typically 2 micro strains with an initial distance of 1000 mm.

The length of the rod and waveguide may be selected to suit the size of the section being monitors, but will, in most cases, be between about 300 mm and 500 mm long, although in certain applications may be up to I m long. The overall accuracy in the sensed position is better than 2 x 1 06, equivalent to 2 pm if with an initial distance is 1000 mm over a period of operation in excess of 25 years.

The self-calibrating strain sensor apparatus described above is therefore reliable and convenient, for example when used as a subsea strain sensor for remotely monitoring strain in a loaded underwater structures, and particularly when used in deep water and ultra deep water situations where aging related drift is expected to be present in the sensor module and where drift due to temperature changes is negligible or unimportant.

Claims

CLAIMS1. A self-calibrating strain sensor apparatus for monitoring strain in a section of an underwater structure, said apparatus comprising: -a magnetostrictive linear displacement detector, said detector comprising a sensor module and a magnetostrictive transducer, said transducer comprising an elongate rod and within the rod a magnetostrictive waveguide, the rod and said waveguide extending away from the sensor module, the sensor module comprising means for providing to said waveguide current pulses and the waveguide being configured to carry torsional strain waves induced in the waveguide when current pulses are applied to said waveguide and the sensor module comprising means for receiving said strain waves; -a plurality of magnets each of the magnets being positioned in proximity with the magnetostrictive waveguide at different positions along the length of the elongate rod, the generation of said torsional strain waves being dependent on the position of said magnets along the length of the elongate rod; -a plurality of mounts, each of the mounts being configured to be fixed to said section of said underwater structure, the mounts comprising a reference mount and at least one measurement mount, the reference mount being fixed to a mounting portion of said detector, and the magnets comprising a reference magnet and at least one measurement magnet, the reference magnet being fixed to the elongate rod to define a fixed reference position such that the distance between said reference mount and reference position is fixed and the or each measurement magnet being fixed to a corresponding one of said measurement mounts and being free to move relative to the length of the elongate rod such that, in use, when both the reference mount and the or each measurement mount are fixed to said section of said underwater structure, the distance between the reference mount and the or each measurement magnet varies depending on the strain in said section of said underwater structure; and -a processing means; wherein the processing means is operable to detect the time of arrival of said strain waves received by the sensor module relative to said current pulses as a measure of the position of each of said magnets along the elongate rod and to calculate from said measured positions of each of said magnets a measured strain in said section of said underwater structure, the processing means further being operable to use changes in said measure of the position of the reference magnet relative to the fixed reference position owing to aging-related drift of the sensor module to add or subtract an offset to said measured position of said at least one measurement magnet in order to correct said measured strain against said aging-related drift.
2. A self-calibrating strain sensor apparatus as claimed in Claim 1, in which the elongate rod has a head end and a foot end, the head end of the elongate rod being fixed to the sensor module such that the elongate rod extends away from the sensor module towards the foot end of the elongate rod, the mounting portion of said detector being proximate said head end.
3. A self-calibrating strain sensor apparatus as claimed in Claim 2, in which the mounting portion is a portion of the sensor module nearest the head end of the elongate rod.
4. A self-calibrating strain sensor apparatus as claimed in Claim 2, in which the mounting portion is a portion of the elongate rod nearest the sensor module.
5. A self-calibrating strain sensor apparatus as claimed in any preceding claim, in which said at least one measurement mount comprises a first measurement mount, and said at least one measurement magnet comprises a first measurement magnet, the first measurement magnet being fixed to the first measurement mount, the first measurement mount being farther away from the sensor module than the reference mount.
6. A self-calibrating strain sensor apparatus as claimed in any of Claims 1 to 4, in which said at least one measurement mount comprises a first measurement mount, and said at least one measurement magnet comprises a first measurement -20 -magnet, the first measurement magnet being fixed to the first measurement mount, the first measurement mount being closer to the sensor module than the reference mount.
7. A self-calibrating strain sensor apparatus as claimed in any preceding claim, in which the magnetostrictive waveguide is an elongate conductor.
8. A self-calibrating strain sensor apparatus as claimed in any preceding claim, in which said apparatus comprises additionally a memory, and the offset is the difference between a current measure of the position of the reference magnet and an initial measure of the position of the reference magnet, the initial measure of the position of the reference magnet being stored in said memory.
9. A strain monitoring system for in situ monitoring of strain in a section of an underwater structure, the system comprising a self-calibrating strain sensor apparatus and an underwater structure, the apparatus being as claimed in any preceding claim, each of said plurality of mounts being fixed to said section of said underwater structure, the processing means being operable to use said changes in said measure of the position of the reference magnet relative to the fixed reference position owing to aging-related drift of the sensor module to add or subtract an offset to said measured position of said at least one measurement magnet in order to correct said measured strain against said aging related drift while the apparatus is in situ with said section.
10. A strain monitoring system as claimed in Claim 9, when dependent from Claim 8, in which said initial measure of the position of the reference magnet is determined after fixing of the self-calibrating strain sensor apparatus to said section of the underwater structure.
11. A self-calibrating strain sensor apparatus for remotely monitoring strain in a section of an underwater structure, substantially as herein described, with reference to or as shown in Figures 3 or 4 of the accompanying drawings.-21 -
12. A strain monitoring system for in situ monitoring of strain in a in a section of an underwater structure, the system comprising a self-calibrating strain sensor apparatus fixed to a loaded underwater structure, substantially as herein described, with reference to or as shown in Figures 3 or 4 of the accompanying drawings.