Sensor systems for controlling elongated details and methods for controlling elongated details.

Abstract

The invention relates to a sensing system for controlling a tapered part. Specifically, a sensing system for controlling the elongation or bending of a tapered part such as a rope comprising at least two supports which can be attached to the part at corresponding positions located along the part. At least one of the supports bears a transmitter for emitting a signal and at least one of the supports bears a receiver for receiving a signal. The transmitter and receiver are offset laterally to the part along the signal path which extends along and beside the part. A processor is configured to determine the distance between the supports by measuring the transmission time of the signal from the transmitter to the receiver along the signal path. The invention relates to a method for controlling a tapered part.

Description

[0001] Monitoring elongate subsea elements

[0002] This invention relates to monitoring elongate subsea elements such as mooring lines, cables or umbilicals. The invention is particularly concerned with measuring elongation of flexible subsea elements that remain under tension over long timescales and so could extend significantly in length over their working life. Examples are ropes that serve as mooring lines or that form sections of hybrid mooring lines in series with chains, as used in various marine applications.

[0003] The term ‘rope’ will be used hereinafter to encompass elongate elements or cables made from any natural, polymeric or metallic fibres, wires or filaments. Examples are spiral strand elements, wire ropes or synthetic ropes.

[0004] Mooring lines are commonly used to anchor floating offshore installations in the subsea oil and gas industry and in the offshore renewable energy industry, including floating production, storage and offloading vessels (FPSOs), floating platforms, floating offshore wind turbines (FOWTs) and wave-energy converters (WECs). Such installations typically remain on station for several years, with the mooring lines remaining under tension throughout their long operational life.

[0005] Other elongate subsea elements that operate under tension over long periods include cables and umbilicals that sustain tensile loads of self-weight, for example when hanging as catenaries in the water column between the surface and the seabed.

[0006] Over time, elongate elements will tend to lengthen or creep under continuous tensile loading. Elongation makes it challenging to monitor the loadings and geometry of a system that includes such elements. Also, excessive or rapid elongation may be indicative of structural degradation and potential failure of an elongate element.

[0007] Whilst floating offshore installations employ multiple mooring lines, excessive elongation of any one mooring line would reduce the margin of safety and increase stress on other mooring lines coupled to the same installation. If the degree of elongation of each mooring line can be measured effectively, the behaviour of the system as a whole can be monitored and modelled more accurately. Also, an elongated mooring line can be shortened, re-tensioned, reinforced or replaced as a preventative measure to forestall potentially greater problems. Ongoing inspection or monitoring is necessary to understand which mooring line requires attention, what action is required and when that action will be required.

[0008] Inspection and monitoring of elongate subsea elements tends be difficult, time-consuming and expensive in view of the inherent challenges of marine operations, which are heightened when performed far offshore and deep underwater. Monitoring devices can be incorporated into or subsequently added to an element for remote measurement of elongation but such devices can complicate the structure of the element and hinder its installation. Notably, adding monitoring devices may lie on the critical path of an operation to install the element, for example when paying out a mooring line from a reel aboard an installation vessel. If a monitoring device is instead designed to be incorporated into an element, it cannot typically be retro-fitted.

[0009] EP 4045883 describes a monitoring arrangement for a mooring line in which various sensors are incorporated into end fittings of a fibre rope. The sensors are capable of monitoring tension in the rope and the condition of fibre-optic elements extending along the rope but they do not directly monitor elongation of the rope. More generally, optical fibres woven into ropes are common for condition monitoring, as exemplified also by US 6999641 and US 5182779.

[0010] US 7441464 proposes an alternative to optical fibres by disclosing a strain gauge system that includes a sensor target assembly mounted on a rope and sensor devices for sensing the sensor target assembly magnetically. The sensor target assembly has at least one pair of magnets spaced apart by a gauge length. Relative longitudinal motion between the magnets of the sensor target assembly and the sensor devices generates position signals that can be processed to determine the gauge length, in comparison to the original gauge length, and therefore to infer the overall strain in the rope. However, the system proposed in US 7441464 is a complex arrangement whose application is restricted by its dependence upon relative movement.

[0011] As exemplified in US 7441464, elongation of an element can be inferred by measuring an increase in the distance between two points that are spaced apart along the length of the element and that are each in fixed relation to the element. The greater the distance between those two points, the greater the accuracy of overall elongation measurement may be. However, this presents a challenge when measuring elongation of long, flexible elements such as ropes and umbilicals where the apparent distance between the two points can vary due to twisting about and / or bending along the longitudinal axis of the element.

[0012] US 2018 / 0163532 describes a system for monitoring a mooring line in which a plurality of sensing nodes are attached to mooring line at pre-selected positions, and can communicate with one another. The distance between nodes can be determined using time-of-flight ultrasound signals. WO 2024 / 017612 relates to a device for measuring elongation of an anchor line.

[0013] It is against this background that the invention has been devised. In one sense, the invention resides in a sensor system for monitoring elongation or bending of an elongate element such as a rope. The system comprises at least two supports that are attachable to the element at respective positions spaced along the element. At least one of the supports carries an emitter for emitting a signal and at least one of the supports carries a receiver for receiving the signal. The emitter and the receiver are offset laterally from the element on a signal transmission path that extends along and beside the element. A processor is configured to determine the distance between the supports by measuring time of flight of the signal from the emitter to the receiver along the signal transmission path.

[0014] Each of the supports may be penetrated by an aperture for accommodating the element. For example, the apertures may be disposed centrally within the respective supports, which can be disc-shaped plates that lie in respective planes orthogonal to the longitudinal axis of the element. The element, which can be generally cylindrical like a rope, a cable or an umbilical, may be an interference fit within the respective apertures. More generally, each of the supports may comprise at least two parts that are movable relative to each other and that are arranged to clamp the element between them.

[0015] In some embodiments, one of the supports carries the emitter and the receiver and the other of the supports carries a reflector that is offset laterally from the element and is positioned to reflect the signal back toward the receiver along the signal transmission path. Such a reflector can surround the element and may be circumferentially continuous. Alternatively, the reflector may be a spot reflector that is angularly aligned with the emitter about a longitudinal axis of the element. In either case, a reflective surface of the reflector can lie in a plane that is substantially orthogonal to a longitudinal axis of the element.

[0016] In other embodiments, one of the supports carries the emitter and another of the supports carries the receiver. In that case, a link can convey time-of-flight information between the supports.

[0017] At least one of the supports may carry first and second emitters that are angularly spaced about the element to emit respective signals along respective signal transmission paths that extend along and beside the element. For example, the first and second emitters may be disposed on mutually-opposed sides of a longitudinal axis of the element. This allows the processor to determine a difference between times of flight of the respective signals along the respective signal transmission paths, and thereby to determine or to compensate for bending of the element based upon that time difference.

[0018] In some embodiments, the system may comprise at least three of said supports. Of these, a central support carrying emitters can be disposed at a longitudinal position between outer supports carrying respective reflectors or receivers. The emitters can then be oriented to transmit signals along respective signal transmission paths that extend in mutually opposed longitudinal directions from the central support toward the outer supports.

[0019] Correspondingly, the inventive concept embraces a method of monitoring an elongate element. The method comprises: emitting at least one signal from an emitter attached to the element; conveying the or each signal along a signal transmission path that extends along and beside the element from the emitter to a datum position spaced along the element from the emitter; receiving the signal at a receiver attached to the element; and determining a distance between the emitter and the datum position by measuring a time of flight of a signal from the emitter to the receiver along the signal transmission path.

[0020] The emitted signal can be received at the datum position or can be reflected from a reflector that is attached to the element at the datum position. The receiver can then receive the reflected signal. In that case, the emitter and the receiver can be supported by a common support that is attached to the element. The emitted signal can impinge on the reflector at different, and potentially unlimited, angular positions relative to the element as the length of the element extending between the emitter and the reflector twists about a longitudinal axis. Nevertheless, the time of flight of the signal from the emitter to the receiver need not change.

[0021] Two or more signals can be transmitted along respective signal transmission paths that are angularly spaced about the element, for example disposed on mutually-opposed sides of a longitudinal axis of the element.

[0022] In summary, embodiments of the invention envisage a pulse echo ultrasound system for monitoring elongation in offshore fibre mooring ropes that are subject to stretch or elongate after some weeks or months under tension. Thus, there is often a need to tighten up a rope to keep an anchored structure in place. The invention measures elongation of the rope in a segment of, for example, one metre or a few metres in length to estimate and quantify elongation of the entire rope. Simulations indicate that rope elongation may increase in the order of 1% for a load increase of 10% due to non-linear effects..

[0023] The proposed approach also addresses challenges of twisting and bending of the rope. In particular, a twist in the rope could bring a receiver or reflector target out of axial alignment with an ultrasound beam emitted by a transducer or transceiver. In that case, the strength of the received or reflected signal will tend to decrease and the measured distance will tend to increase, hence overestimating elongation of the rope. Conversely, a bend in the rope along its length may affect the distance measurement, overestimating elongation on the outer side or extrados of the bend and / or underestimating elongation on the inner side or intrados of the bend.

[0024] Twisting of the rope may be countered by using a circular or rotationally symmetrical target that does not affect the distance of ultrasound pulse propagation if the target rotates around the longitudinal axis of the rope. An example of such a target is a substantially toric reflector surface, more particularly the double-curved surface of a toroid of semi- ellipical or semi-circular section. In other words, the reflector is shaped like a ring doughnut or bagel that is sliced in half in a plane orthogonal to the longitudinal axis of the rope. Bending of the rope may be countered by increasing the number of ultrasound beams from one to more than one, for example two, three or four independent beams that are angularly spaced from each other about the longitudinal axis of the rope. Then measurements from at least two locations that are angularly spaced or mutually opposed about the longitudinal axis of the rope can be compared or averaged. For example, if one beam registers an increased distance due to being on the outer side of a bend in the rope, a beam on the opposite side of the rope should register a similar decrease in distance. Consequently, the average length measurement should be unaffected by the bend.

[0025] Moreover, two or more distance measurements taken at locations spaced angularly around a rope can, in principle, be used to estimate the curvature of a rope bend by calculating the difference between those measurements. Rope bend curvature may be serve as a parameter of mooring analysis.

[0026] Embodiments of the invention provide a sensor for monitoring an elongate element, the sensor comprising at least two collars attached to the elongate element at some distance apart from each other, where at least one collar carries at least one signal emitter such as a transducer and at least one collar carries a signal receiver. The sensor can thereby monitor the distance between the collars to infer longitudinal extension of the portion of the elongate element that extends between them.

[0027] The signal may be an ultrasound signal, a light signal such as laser signal, or another transmissible wave.

[0028] The signal emitter and the receiver can be mounted on different collars but may nevertheless be connected together for power and / or signal transmission. The signal emitter and the receiver can instead be mounted on the same collar or combined in a transceiver, in which case the other collar may comprise an opposed reflector. The reflector could be a spot reflector, which may be angularly aligned with the transceiver about the longitudinal axis of the elongate element. The reflector could instead be a circumferential ring extending around the elongate element. Such a ring could be circumferentially complete and continuous or could be a circumferential array of individual reflectors that may adjoin, abut or be angularly separated from each other. The elongate element could be a mooring line, a cable, an umbilical, a fibre rope or any element comprising spiral strand, wire or synthetic line.

[0029] The collars may be clamped to, bonded to or otherwise engaged with the elongate element before or during installation of the element under water or as a retrofit after installation. For example, the collars could be flexible metal collars or articulated clamps configured to surround and embrace the elongate element, and could be secured by fastenings such as integral or separate clips or bolted flanges. The collars may be discshaped and may have an outer diameter more than twice an inner diameter that corresponds to the diameter of the elongate element.

[0030] The invention therefore also contemplates a method for detecting elongation of an elongate element by measuring time of flight of a signal transmitted along the element, for example using a pulse-echo ultrasound technique. Over a long period, tension may cause elongation of the element. Consequently, the time of flight of the signal between two points mutually spaced along the element will increase over time, and elongation of the element can be inferred from that increase.

[0031] Periodically, one or more sensor units of the invention can measure and store the longitudinal distance between points mutually spaced along the element. For example, a sensor unit of the invention can store such distance measurements in an internal memory when taking measurements at regular intervals. A time series or a statistical summary of the measured distance over time could be made available to service personnel via a wireless modem or other data communication device. For example, an acoustic modem could be coupled to the sensor to transmit data and a counterpart modem could be located in a vessel, on a rig or in a buoy to receive data. In another approach, a modem such as an optical modem could be carried by an underwater vehicle such as an ROV that travels past the sensor to download stored data from time to time.

[0032] A sensor unit of the invention may comprise a transceiver comprising a transmitter such as a piezoelectric transducer, a receiver and a processing unit. In pulse-echo embodiments, an ultrasound acoustic pulse is emitted from the transducer toward an opposed reflector and an echo reflected from the reflector is detected by the receiver. The received signal may be processed, for example by a rectifier and low-pass filter, i.e. an envelope detector, followed by a comparator of the processing unit. The time of flight from the transducer and back to the receiver is used to determine the distance between the transducer and the reflector. The sensor is thereby used to estimate changes in the length of the element.

[0033] Sensor units of the invention could, for example, be used in conjunction with clamps that attach corrosion-protecting anodes to an elongate tensile element. A sensor unit of the invention could be attached to such a clamp in addition to an anode or could be provided on a separate clamp.

[0034] Design criteria for a pulse-echo ultrasound system include beam profile and pulse length. Beam profile depends on the shape, size and curvature of the transducer as well as excitation frequency and determines the extent to which the beam flares beyond the diameter of the transducer face with increasing distance from that face. In the context of the invention, the ultrasound beam should be wide enough to impinge on an opposed reflector even if the ultrasonic transducer is not aimed perfectly toward the reflector. In other words, the beam width should be sufficient to ensure reasonable robustness of alignment. However, the beam should not be so wide that it could generate false echoes.

[0035] In pulse-echo embodiments of the present invention, the ultrasound pulse is used principally to measure time of flight back and forth to estimate the distance between the sensor and the reflector. In that context, pulse length and shape may be important, depending on the selected method for detecting the presence of, and time of arrival of the reflected pulse. For example, the received pulse may be envelope-detected and a comparator may be used to determine time of arrival of that pulse. In that case, however, phase information in the reflected pulse is lost. A steep leading edge of the outgoing pulse may be beneficial for a precise measurement.

[0036] In other approaches, a received RF pulse may be digitized by a fast AD-converter, and the shape of the pulse can be cross-correlated with a fixed reference pulse stored in the processing unit. This method could provide a more robust and accurate time estimate because the phase information in the reflected pulse is maintained. Alternatively, the received signal may be quadrature-demodulated to the base band and digitized by an AD- converter that does not need to be as fast as in the preceding approach. Amplitude and phase information are maintained, and the processing unit can use this information to determine the time of arrival with high time resolution. It is envisaged that the present invention can be realised with a broad range of operating parameters. For example, the diameter of the radiator face of a transducer can typically range from a few millimetres to a few centimetres. Transducer frequency could range from a few hundred kilohertz to several megahertz and the beam profile or opening angle could range from a few degrees to, for example, forty-five degrees.

[0037] There will inevitably be trade-offs between these and other parameters and performance such as detection reliability, robustness to mounting, accuracy in distance estimation and so on. For example, a high ultrasound frequency will generally increase the ability to measure distance with high resolution, but the beam opening angle may become impractically narrow. However, solutions to widen the beam can be considered such as curving the transducer surface and / or applying an acoustical lens. Providing an array of multiple transducers is also possible.

[0038] In some embodiments of the invention, a monitoring device for measuring elongation of an elongate element may comprise a mount to be positioned on the element. At least one acoustic transducer may be oriented to send an acoustic signal generally longitudinally toward a reflector also mounted on and spaced along the element. The transducer, or another receiver, receives the signal directly or after reflection. A data logger records the distance between the transducer and the reflector.

[0039] The data logger may comprise storage for data. The data logger may comprise or be interfaced with a signal transmission system, for example for transmitting a signal acoustically, wirelessly, electrically along a wire or a cable, or optically along a fibre optic.

[0040] The signals emitted by two or more transducers may be emitted simultaneously or with a delay between them or with different phases, and / or with the same frequency and / or with different frequencies. The data logger may comprise a filtering system to eliminate interference between the reflected signals of the different transducers.

[0041] The mount may be permanently attached to or integrated with the elongate element or may be attachable to the element, for example as a clamp. Thus, the length of an elongate element is monitored in accordance with the invention by fixing at least one acoustic transducer to a first location along the element. The transducer emits an acoustic signal toward a receiver that is also fixed to the element. For example, the receiver could be at the first location to receive the signal after reflection from a reflector spaced apart along the element. Alternatively, in a direct transmission system, the receiver could be at a second location spaced apart along the element.

[0042] By measuring time of flight of the signal from the transducer to the receiver, the distance between the first and second locations is determined. An increase in that distance indicates elongation of the element.

[0043] In order that the invention may be more readily understood, reference will now be made, by way of example, to the accompanying drawings, in which:

[0044] Figure 1 is a perspective view of a first embodiment of the invention being an apparatus for measuring elongation of an elongate element;

[0045] Figure 2 is a side view of the apparatus shown in Figure 1 ;

[0046] Figure 3 is a cross-sectional view on line Ill-Ill of Figure 2;

[0047] Figure 4 is a cross-sectional view on line I V-l V of Figure 2;

[0048] Figure 5 is a perspective view of a second embodiment of the invention;

[0049] Figure 6 is a side view of the apparatus shown in Figure 5;

[0050] Figure 7 is a cross-sectional view on line VII-VII of Figure 6;

[0051] Figure 8 is a cross-sectional view on line VI I l-VI 11 of Figure 6;

[0052] Figure 9 is a schematic diagram of a sensor unit that can be used in embodiments of the invention; Figure 10 corresponds to Figure 6 but shows the elongate element twisted about its central longitudinal axis;

[0053] Figure 11 corresponds to Figure 6 but shows the elongate element bent along its central longitudinal axis;

[0054] Figure 12 is a side view of a third embodiment of the invention;

[0055] Figure 13 is a side view of a fourth embodiment of the invention; and

[0056] Figure 14 is a schematic side view of a mooring line underwater, fitted with measuring apparatus of the invention.

[0057] Referring firstly to Figures 1 to 8, these drawings show two embodiments of the invention in which like numerals are used for like features. In each case, a section of rope 10 forming part of a mooring line is shown fitted with two plate-like clamps 12, 14 of the invention, being examples of collars or supports that are spaced apart from each other along the central longitudinal axis 16 of the rope 10. The rope 10 is represented schematically as being of substantially circular cross-section, hence being substantially cylindrical when straight. In these embodiments, one clamp 12, shown here to the left, is a reflector clamp and the other clamp 14, shown to the right, is an emitter / receiver clamp.

[0058] The clamps 12, 14 have axially inward faces 18 in mutual opposition, those inward faces extending orthogonally with respect to the central longitudinal axis 16. On its inward face 18, the emitter / receiver clamp 14 supports one or more signal emitters being ultrasonic transducers that direct one or more beams of acoustic signals 20 toward a reflector 22 mounted on the opposed inward face 18 of the reflector clamp 12, as shown in Figures 1 and 5. Where there is more than one beam of acoustic signals 20, those beams are preferably substantially parallel as shown in Figure 5. The reflector 22 serves as a datum that reflects the or each signal 20 back to the inward face 18 of the emitter / receiver clamp 14, which also supports one or more ultrasonic receivers.

[0059] In these examples, the transducers and the receivers supported by the emitter / receiver clamp 14 are combined as transceivers 24. The transceivers 24 emit signals 20 comprising trains of ultrasonic pulses that are reflected back to the transceivers 24 from the opposed reflector 22. The transceivers 24 and the reflector 22 are offset or spaced laterally from the rope 10 on a signal transmission path that extends along and beside the rope 10. The time of flight of the pulses and corresponding reflections out and back along the signal transmission path is proportional to the longitudinal distances between the transceivers 24 and the reflector 22. In other examples, the transducers and the receivers could be separate components that lie beside each other on the inward face 18 of the emitter / receiver clamp 14.

[0060] Each clamp 12, 14 is penetrated by a central aperture 26 that is undersized relative to the diameter of the rope 10 and therefore accommodates the rope 10 as an interference fit. Further, each clamp 12, 14 has a generally circular disc shape. The mutually-opposed inward faces 18 of the clamps 12, 14 are therefore annular, being circumferentially continuous and extending radially from the central aperture 26 to the outer edge 28 of each clamp 12, 14. In these examples, the outer diameter of each clamp 12, 14 defined by the circular outer edge 28 is more than twice the diameter of the central aperture 26 that is concentric with the outer edge 28.

[0061] Each clamp 12, 14 comprises parts that are brought together about the rope 10 to clamp onto the rope 10, hence conveniently enabling the clamps 12, 14 to be fitted or retrofitted to the rope 10. In these examples, each clamp 12, 14 is in two parts that are joined together on one side by a hinge 30 and that have respective flanges 32 on the diametrically opposed side. The parts can be clamped together around the rope 10 by tightening a bolt (not shown) extending through both flanges 32. Other separate or integral fastenings for holding together the parts of the clamps 12, 14 are also possible.

[0062] In the first embodiment shown in Figures 1 to 4, a transceiver 24 is paired with an individual spot reflector 22. There may be more than one such pair of transceivers 24 and reflectors 22 but only one pair is shown in this embodiment. The transceiver 24 and the reflector 22 are disposed in axial and angular alignment parallel to and about the central longitudinal axis 16.

[0063] In another variant of the first embodiment, there could instead be one or more separate but associated transducers and receivers on the emitter / receiver clamp 14 opposed to a respective reflector 22 on the reflector clamp 12, that reflector 22 being oriented to receive a signal 20 from the transducer and to reflect the signal back to the associated receiver. In the second embodiment shown in Figures 5 to 8, multiple transceivers 24 on the emitter / receiver clamp 14 are opposed to a common reflector 22 on the inward face 18 of the reflector clamp 12. The transceivers 24 are angularly spaced around the central longitudinal axis 16. Conversely, the reflector 22 encircles the rope 10 in concentric relation about the central longitudinal axis 16. In this example, the reflector 22 is a toroid of semi-circular section whose double-curved acoustically-reflective surface faces toward the inward face 18 of the emitter / receiver clamp 14.

[0064] In variants of the second embodiment, there could instead be only one transceiver 24 opposed to the reflector 22 or one or more separate but associated transducers and receivers opposed to the reflector 22. In the latter case, the arrangement is such that the reflector 22 receives a signal 20 from the or each transducer and reflects the signal 20 back to the associated receiver.

[0065] Figure 9 shows two of the transceivers 24 in addition to other components within the body of the emitter / receiver clamp 14 of the second embodiment. In addition to the transceivers 24, the emitter / receiver clamp 14 contains a power source 34, a processor 36, a data store 38 and a communication module 40. The power source 34, the processor 36, the data store 38 and the communication module 40 are sealed or encapsulated within the body of the emitter / receiver clamp 14 for water tightness.

[0066] The power source 34 could be a power input from a permanently wired or temporarily connected external source, an onboard battery, or a wireless power receiver such as an induction loop or a photocell that receives energy transiently from an external source such as a visiting ROV.

[0067] The processor 36 generates pulses to drive the transceivers 24, receives reflected signals via the transceivers 24 or via separate receivers, and processes those signals against elapsed time to determine time of flight, from which the distance between each transceiver 24 and the reflector 22 can be derived. The processor 36 outputs the resulting data to the data store 38 from which the data can be transferred or output periodically or continuously from the emitter / receiver clamp 14 to a remote monitoring station. At least some processing of signals or data could take place remotely from the emitter / receiver clamp 14. Data is output from the emitter / receiver clamp 14 via the communication module 40, which may be configured for wired or wireless data transmission by electrical, acoustic, electromagnetic or optical means. As will be explained with reference to Figure 14, the communication module 40 could also be configured as a relay to receive and forward data received from other emitter / receiver clamps 14 mounted at other locations spaced along the rope 10. Consequently, the communication module 40 could comprise an input or receiver in addition to an output or transmitter.

[0068] The features of the second embodiment are advantageous if the rope 10 extending between the clamps 12, 14 twists about the central longitudinal axis 16, as shown in Figure 10. Here, it can be seen that twisting of the rope 10 has displaced the emitter / receiver clamp 14 and its transceivers 24 angularly with respect to the reflector clamp 12. However, as the clamps 12, 14 remain in mutually parallel planes, this twisting does not affect the distance D derived from the time of flight from the transceivers 24 to the closest points of the reflector 22 and back again. This is because the reflective surface of the reflector 22 lies in a plane that is orthogonal to the central longitudinal axis 16 and that surface is circumferentially extensive, at least to a greater angular extent than the degree of torsion. More preferably, as exemplified here, the apex of the curved reflective surface is circumferentially continuous or uninterrupted.

[0069] Even if there is only one transceiver 24 on the opposed clamp 14, the annular reflector 22 of the second embodiment copes with twisting of the rope 10 and avoids the need for angular alignment of the transceiver 24 with the reflector 22 during installation.

[0070] The features of the second embodiment are also advantageous if the rope 10 extending between the clamps 12, 14 is bent along the central longitudinal axis 16, as shown in Figure 11. Bending that intermediate portion of the rope 10 tilts the clamps 12, 14 relative to each other out of their initial parallel relation. Thus, it can be seen in Figure 11 that the sides of the clamps 12 on the intrados of the bend, on the upper side of the rope 10 as illustrated, converge with each other whereas the sides of the clamps 12 on the extrados of the bend, on the lower side of the rope 10 as illustrated, diverge from each other.

[0071] As the transceivers 24 are angularly spaced and, in this example, disposed on mutually- opposed sides of the rope 10, it follows that the time of flight from the transceivers 24 to the closest points of the reflector 22 and back again will vary depending upon the positions of the transceivers 24 on the emitter / receiver clamp 14. That variation will result in a distance Di on the intrados of the bend that is smaller then the distance D2 on the extrados of the bend. However, the processor 36 within the emitter / receiver clamp 14 can compare the times of flight associated with the respective transceivers 24. This enables the processor 36 to determine an averaged or otherwise compensated time-of-flight value that is representative of the distance between the clamps 12, 14 but with the effect of bending of the rope 10 filtered out.

[0072] Figures 12 and 13 show third and fourth embodiments in which, again, like numerals are used for like features.

[0073] In Figure 12, a single bi-directional emitter / receiver clamp 14 is disposed between mutually-opposed reflector clamps 12 to increase the sensitivity of the system to elongation of the rope 10. For this purpose, the emitter / receiver clamp 14 has transceivers 24 on both faces, facing in mutually-opposed directions toward the reflectors 22 of the reflector clamps 12. In this example, the reflectors 22 have the circumferentially continuous toroidal configuration of the second embodiment but they could instead be configured differently, for example as a circumferential array of separate reflectors.

[0074] Whilst the preceding embodiments contemplate pulse-echo arrangements, Figure 13 exemplifies how the invention could instead be implemented by measuring time of flight of a signal transmitted directly from an emitter to a time-synchronised receiver. Thus, the signal need not be reflected in the interim. To achieve this, the reflector clamp 12 and the emitter / receiver clamp 14 of the first and second embodiments have been replaced in Figure 13, respectively, by a receiver clamp 42 serving as a datum and an emitter clamp 44. Alternatively, in principle, a bi-directional emitter clamp 44 could be disposed between mutually-opposed receiver clamps 42 in an arrangement akin to that of the third embodiment.

[0075] The emitter clamp 44 shown in Figure 13 supports a circumferential array of acoustic transducers 46, like the transceivers 24 of the second embodiment, whereas the receiver clamp 42 supports a circumferential array of acoustic receivers 48. The receiver clamp 42 is connected to the emitter clamp 44 by a wired or wireless link 50 that extends along or parallel to the rope 10. The link 50 conveys time-of-flight information that enables a processor aboard either clamp 42, 44 to compare the time of transmission of an acoustic pulse from a transducer 46 with the time of reception by a receiver 48 to determine the time of flight and thus the distance D between the radiator face of the transducer 46 and the opposed face of the receiver 48. Such a processor need not be housed in a clamp 42, 44 but could instead be housed elsewhere.

[0076] The direct transmission system exemplified in Figure 13 presents some challenges compared with a pulse-echo system, for example the need to implement the link 50, but is still retrofittable to a rope 10. Conversely, as it does not rely on reflection, the direct transmission system may offer advantages over a pulse-echo system such as more reliable transmission of acoustic signals, and may require a less sensitive receiver. There is no sensitivity to the orientation or curvature of a reflective surface and less susceptibility to corrosion or marine growth.

[0077] As Figure 13 shows, there could be more receivers 48 than transducers 46. In this example, the receivers 48 are in close proximity to each other in a circumferential array and indeed could adjoin or abut with neighbouring receivers 48 in the array. This would increase the receiving surface area and reduce vulnerability to twisting or bending of the rope 10.

[0078] Turning finally to Figure 14, this illustrates various options for conveying data to a monitoring station 52 from emitter / receiver clamps 14A to 14D mounted on a mooring rope 10 together with associated reflector clamps 12. The emitter / receiver clamps 14A to 14D and the associated reflector clamps 12 may be like those of the first and second embodiments but could be instead be like those of the third embodiment or, analogously, could be replaced by the emitter and receiver clamps 42, 44 of the fourth embodiment.

[0079] The emitter / receiver clamp 14A transmits data directly to the monitoring station 52 by wireless transmission, for example acoustically. In contrast, the emitter / receiver clamp 14B transmits data to the monitoring station 52 indirectly via an ROV 54 that periodically visits and interrogates the emitter / receiver clamp 14B. The ROV 54 could also, transiently, provide power to the emitter / receiver clamp 14B, for example by electromagnetic induction or by illuminating a photocell of the emitter / receiver clamp 14B. The emitter / receiver clamp 14C has a wired connection 56 to the monitoring station 52 to convey data electronically or optically. Conversely, the emitter / receiver clamps 14C and 14D illustrate the possibility of relaying data from one emitter / receiver clamp 14 through another, in this case wirelessly.

[0080] More generally, the relay function shown for the emitter / receiver clamps 14C and 14D could involve most or all of the emitter / receiver clamps 14 on a rope 10, providing a system in which all the emitter / receiver clamps 14 can relay data, for example acoustically, from other emitter / receiver clamps 14 that are at more distal positions along the rope 10 with respect to the monitoring station 52. For example the emitter / receiver clamp 14A can report to the monitoring station 52 via the emitter / receiver clamps 14B, 14C and 14D in sequence.

[0081] The monitoring station 52 shown in Figure 14 could be substituted by a relay station such as a buoy. The relay station could, in turn, convey data to a remote monitoring station that could be on a vessel, on an offshore installation or on land.

[0082] Many other variations are possible within the inventive concept. For example, transducers and / or reflectors used in the invention, whether transceivers, emitters or receivers, could have active faces made of, treated with, coated with or impregnated with corrosionresistant and / or anti-fouling materials or compounds.

[0083] Measures could be taken to avoid or to mitigate interference between ultrasonic emissions from the transducers and the corresponding reflected signals. For example, ultrasonic signals could be emitted from the transducers with different phases or with different frequencies to assist in discriminating between their reflected signals. Filtering performed in the processor, or downstream in a monitoring station, can further mitigate such interference as may arise between the reflected signals of the transducers.

Claims

Claims1. A sensor system for monitoring an elongate element, the system comprising: at least two supports that are attachable to the element at respective positions spaced along the element, where at least one of the supports carries an emitter for emitting a signal and at least one of the supports carries a receiver for receiving the signal, the emitter and the receiver being offset laterally from the element on a signal transmission path that extends along and beside the element; and a processor that is configured to determine a distance between the supports by measuring a time of flight of the signal from the emitter to the receiver along the signal transmission path.

2. The system of Claim 1 , wherein each of the supports is penetrated by an aperture for accommodating the element.

3. The system of Claim 2, wherein the apertures are disposed centrally within the respective supports.

4. The system of Claim 2 or Claim 3, wherein the element is an interference fit within the respective apertures.

5. The system of any preceding claim, wherein each of the supports comprises at least two parts that are movable relative to each other and are arranged to clamp the element between them.

6. The system of any preceding claim, wherein the element is generally cylindrical.

7. The system of Claim 6, wherein the element is a rope, a cable or an umbilical.

8. The system of any preceding claim, wherein one of the supports carries the emitter and the receiver and the other of the supports carries a reflector that is offset laterally from the element and is positioned to reflect the signal back toward the receiver along the signal transmission path.

9. The system of Claim 8, wherein the reflector surrounds the element.

10. The system of Claim 9, wherein the reflector is circumferentially continuous.

11. The system of Claim 8, wherein the reflector is a spot reflector that is angularly aligned with the emitter about a longitudinal axis of the element.

12. The system of any of Claims 8 to 11 , wherein a reflective surface of the reflector lies in a plane that is substantially orthogonal to a longitudinal axis of the element.

13. The system of any of Claims 1 to 7, wherein one of the supports carries the emitter and another of the supports carries the receiver.

14. The system of Claim 13, further comprising a link for conveying time-of-flight information between the supports.

15. The system of any preceding claim, wherein at least one of the supports carries first and second emitters that are angularly spaced about the element to emit respective signals along respective signal transmission paths that extend along and beside the element.

16. The system of Claim 15, wherein the first and second emitters are disposed on mutually-opposed sides of a longitudinal axis of the element.

17. The system of Claim 15 or Claim 16, wherein the processor is configured to determine a time difference between times of flight of the respective signals along the respective signal transmission paths.

18. The system of Claim 17, wherein the processor is configured to determine or to compensate for bending of the element based upon said time difference.

19. The system of any preceding claim and comprising at least three of said supports, of which a central support carrying emitters is disposed at a longitudinal position between outer supports carrying respective reflectors or receivers, the emitters being oriented totransmit signals along respective signal transmission paths that extend in mutually opposed longitudinal directions from the central support toward the outer supports.

20. The system of any preceding claim, wherein the or each signal emitter is configured to emit an ultrasound signal, a light signal or another transmissible wave.

21. A method of monitoring an elongate element, the method comprising: emitting at least one signal from an emitter attached to the element; conveying the or each signal along a signal transmission path that extends along and beside the element from the emitter to a datum position spaced along the element from the emitter; receiving the signal at a receiver attached to the element; and determining a distance between the emitter and the datum position by measuring a time of flight of a signal from the emitter to the receiver along the signal transmission path.

22. The method of Claim 21 , comprising reflecting the emitted signal from a reflector attached to the element at the datum position before the receiver receives the reflected signal.

23. The method of Claim 22, wherein the emitter and the receiver are supported by a common support attached to the element.

24. The method of Claim 22 or Claim 23, comprising causing the emitted signal to impinge on the reflector at different angular positions relative to the element as the element extending between the emitter and the reflector twists about a longitudinal axis but without changing the time of flight of the signal from the emitter to the receiver.

25. The method of Claim 24, comprising allowing the emitted signal to impinge on the reflector at unlimited angular positions relative to the element.

26. The method of Claim 21 , comprising receiving the signal at the datum position.

27. The method of Claim 26, comprising conveying time-of-flight information along or beside the element between the datum position and the emitter.

28. The method of any of Claims 21 to 27, comprising transmitting two or more signals along respective signal transmission paths that are angularly spaced about the element.

29. The method of Claim 28, wherein the signal transmission paths are disposed on mutually-opposed sides of a longitudinal axis of the element.

30. The method of Claim 28 or Claim 29, comprising determining a time difference between times of flight of the respective signals along the respective signal transmission paths.

31. The method of Claim 30, comprising using the time difference to determine or compensate for bending of the element along its length between the emitter and the datum position.

32. The method of any of Claims 21 to 31 , wherein the element is a rope, a cable or an umbilical.

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