Monitoring the Condition of Marine Chains

Abstract

The condition of a chain is monitored by at least one acoustic transducer (ultrasonic transducers) T1, T2 on sensor unit 22 that is fixed to a first link of the chain (for example intermediate link 16

Description

This invention relates to monitoring the condition of chains. The invention is particularly concerned with monitoring degradation of chains used in marine applications, such as mooring chains that serve as mooring lines or form sections of mooring lines. Among other applications, mooring chains are used to anchor floating offshore installations in the subsea oil and gas industry and in the offshore renewable energy industry. Specific examples of offshore energy equipment anchored by chains include floating production, storage and offloading vessels (FPSOs), floating platforms, floating offshore wind turbines (FOWTs) and wave-energy converters (WECs). As a mooring chain bends along its length under tension during its operational life, successive articulated links of the chain pivot and slide relative to each other. The chain is therefore prone to wear where there is repetitive rubbing contact between the successive links, which may be exacerbated by the effects of corrosion. Over time, the load-bearing capacity of the chain diminishes with degradation and thinning of the links until, eventually, the chain could fail. Whilst floating offshore installations employ multiple mooring chains for redundancy, failure of a mooring chain is unacceptable because it would reduce the margin of safety and increase stress on the mooring chains that survive. If the degree of wear could be monitored effectively, preventative replacement of a worn mooring chain could be performed before failure occurs. The phenomenon of chain degradation is well known and typical chain wear rates are well understood in the art. For example, there is a thorough discussion of chain degradation in Research Report No. RR1098 published by UK’s Health and Safety Executive entitled Degradation of mooring chains of floating offshore installations: chain measurement, estimation of wear, corrosion rates, and their effect on break load (available at https: / / www.hse.gov.uk / research / rrpdf / rr1098.pdf). Actual chain wear rates can be greater or lesser than predicted as they depend on variables such as tolerances, material quality and the position of links along a chain. Thus, wear may not be uniform along the length of a chain, with the result that some of the links could be closer to failure than others, or closer to failure than predicted. 20 02 25 Conventionally, mitigating the risk of premature failure of a mooring chain involves over-engineering the links; however, this adds cost and especially weight to the chain. Long chains for use in deeper water and large chains for mooring huge floating facilities could suffer from reduced capacity due to tension arising from self-weight and could become too heavy to handle. Where the size of mooring chain links has to be limited to save weight, it is common to inspect the links periodically or to monitor them intermittently or continuously. In a holistic approach to chain monitoring, the overall geometry of a chain may be checked as a whole. For example, US 10780954 teaches recording an image of a chain whereas WO 2020 / 164760 teaches attaching a fibre optic to a chain to monitor its geometry and articulations. More generally, holistic chain monitoring can be performed continuously, for example by mounting acoustic transducers on critical links, or intermittently, as in the RAMS system offered by Tritech International Limited (trade marks acknowledged). In the RAMS system, the integrity of mooring lines is monitored from a sonar head deployed beneath a moored vessel, without additional sensors on the lines. However the holistic approach cannot detect potential failure of individual links of a chain. Inspection of chain links generally relies upon visual inspection, measurement by callipers or non-destructive testing, for example by ultrasound to detect cracks as disclosed in EP 2507583 or WO 2015 / 030600. Optical measurement or 3D photogrammetry may also be used to measure critical chain dimensions. However, non-destructive testing and measurement of individual links is difficult, time-consuming and expensive, especially when performed offshore and underwater. Monitoring of chain links may rely upon continuous mechanical measurement of deformation of a link, typically by using a strain gauge or by measuring load using a load cell as exemplified in US 10078025. Deviations in dimensions or stress can indicate abnormal wear or fatigue. Similarly, in US 2013 / 279298, strain gauges are embedded into a shroud connected to the chain links and signals representing strain are transmitted acoustically. Retrofitting a chain with a sensor clamp is also known in the art, for example as described in CN 104330102, CN 109029527 and CN 110081921. Again, the sensors proposed in those documents are strain gauges. However, in view of the large 20 02 25 dimensions of chain links used to moor offshore energy equipment such as FPSOs, FOWTs or WECs, especially in deep water, the standard approach of monitoring mooring chains with strain gauges is limited. GB 2415256 describes another approach to chain monitoring, namely exciting a chain link by, for example, hammering the link to propagate a signal through the link and then receiving and processing the signal to determine the structural integrity of the link. It is against this background that the invention has been devised. In one sense, the invention resides in a system for monitoring the condition of a mooring chain for use in marine applications, that system comprising: at least one emitter on a first link of the chain, the emitter being configured to emit a signal; at least one receiver on the first link of the chain, configured to receive the signal; and a processor configured to measure a time lapse between emitting and receiving the signal to determine a distance between the emitter and a link of the chain other than the first link. The emitter and the receiver are each opposed to an intermediate reflecting surface on a second link of the chain, and the emitter is configured to emit the signal toward an opposed reflecting outer end surface of the second link of the chain, that surface of the second link being disposed within an inner opening of the first link. An acoustic transducer could serve as the emitter and as the receiver. The reflecting surface could be defined by a reflector that is fixed to the second link of the chain. Conveniently, the emitter and the receiver can be implemented together in a sensor unit that is attached or attachable to the first link of the chain. For example, the sensor unit could span the inner opening of the first link from one longitudinally-extending side to another. In a direct transmission approach, the receiver could be in fixed relation to a second link of the chain. Nevertheless, a receiver on the second link could still be disposed within an inner opening of the first link or within an inner opening of an intermediate link that couples the first link to the second link. In the latter case, the emitter can also be disposed within the inner opening of the intermediate link. A processor may be in data communication with the emitter and the receiver via a wired or wireless connection that extends between the first and second links. 20 02 25 The emitter is suitably attached or attachable to the first link at a central longitudinal position along the first link. For example, the emitter could be attached or attachable to at least one longitudinally-extending side of the first link. The emitter could instead be disposed on an outer end surface of the first link. The emitter can be aligned with a central longitudinal axis of the first link, in which case the emitter can be oriented to emit the signal in a direction substantially parallel to the central longitudinal axis of the first link. More generally, the emitter could be disposed within an inner opening of the first link. It is also possible for the emitter to be offset laterally from a plane of the first link. In that case, the emitter may be oriented to emit the signal in a direction converging with the central longitudinal axis of the first link. First and second emitters may be in mutually-opposed orientation. For example, the first and second emitters can be in fixed relation to the first link being an intermediate link disposed between and conjoining a pair of outer links each with a reflecting outer end surface that is opposed to a respective one of the emitters and is disposed within an inner opening of the first link. At least the first link may be a stud link that comprises a stud dividing an inner opening. In that case, the or each emitter can conveniently be mounted on the stud. Where there are first and second emitters, they could be disposed on respective mutually-opposed sides of the stud. The system may further comprise a processor that is configured to determine time of flight of the signal from the emitter to the receiver. The processor may thereby generate distance data, which can be stored in a data store and / or conveyed by a communication module to a monitoring or relay station. The or each emitter may, for example, be configured to emit the signal as a beam with a beam angle of up to 45°, or with a pulse frequency of 0.1 to 5 MHz. The inventive concept embraces a corresponding method for monitoring the condition of a chain. That method comprises: emitting a signal from an emission location on a first link of the chain; receiving the signal at a receiving location on the first link of the chain; and measuring a time lapse between emitting and receiving the signal to determine a distance between the emission location and a link of the chain other than 20 02 25 the first link. The signal is reflected to the receiving location from a second link of the chain, from a location within an inner opening of the first link. The receiving location could be fixed relative to a second link of the chain but still within an inner opening of the first link or within an inner opening of an intermediate link coupling the first link to the second link. The signal can also be emitted within the inner opening of the intermediate link in that case. The signal can be emitted along a central longitudinal axis of the first link, in a direction converging with the central longitudinal axis of the first link, from within an inner opening of the first link, or from an outer end surface of the first link. It is also possible to emit signals in mutually-opposed directions from first and second emission locations that are fixed relative to the first link. In that case, the respective signals could be emitted at different times, with different phases or with different frequencies. Data representative of said distance may be generated at a link and communicated from that link to a receiving station remote from that link. For example, the distance data could be relayed along the chain to the receiving station or conveyed to the receiving station via an underwater vehicle stationed beside that link. In summary, the invention contemplates a technique for detecting chain wear by measuring time of flight of a signal transmitted between chain links, for example using a pulse-echo ultrasound technique. Over a period of months or years, corrosion and frictional movement between successive links will cause metal at the interfaces between those links to disappear. Consequently, the longitudinal distance between opposed links, represented by time of flight of the signal, will increase over time and that increasing distance can infer the effects of degradation of the links. In some embodiments, one or more sensor units of the invention periodically measure and store the longitudinal distance between opposed, adjacent links that are joined to each other via an intermediate link. As the nominal distance between opposed links of a new chain is known, it is possible to estimate the level of wear even if a sensor unit of the invention is mounted on a chain late in the working life of the chain. A sensor unit of the invention can store distance measurements in its internal memory periodically, for example when taking measurements at regular intervals. A time series or a statistical summary of the measured distance over time could be made available to 20 02 25 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. 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 link of a chain and an echo reflected from the chain 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 sensor and the link. The sensor is thereby used to estimate, by indirect measurement, how much metal has been eroded from the contact interface between successive links, behind the link that is subject to pulse-echo measurement. Two miniature pulse-echo systems of this design could be attached to an intermediate link of a chain to measure the distances to the two opposed links that are joined by the intermediate link. Those pulse-echo systems could be integrated into a single combined unit attached to a link or could be embodied in respective individual units that may be attached to the same link. Chain wear sensor units of the invention could, for example, be used in conjunction with clamps that attach corrosion-protecting anodes to links of a mooring chain. Several links of a mooring chain could be equipped with such clamps. 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. 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 link even if the ultrasonic transducer is not aimed perfectly toward the 20 02 25 double-curved surface of that link. 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’, for example by impinging on the intermediate link on which the sensor is mounted. In pulse-echo embodiments of the present invention, the ultrasound pulse is used principally to detect the presence of the steel structure of an opposed link and to measure time of flight back and forth to estimate the distance between the sensor and the closest point on the double-curved facing surface of the link. 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. In other approaches, the 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 or the rectangular dimension of the radiator face of the 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. 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 20 02 25 such as curving the transducer surface and / or applying an acoustical lens. Providing an array of multiple transducers is also possible. Whilst some chains have open links and so are referred to as studless or coil chains, many mooring lines employ studded or stud-link chains in which a transverse bar or stud extends across the central opening of each link. The presence of the stud may require an ultrasound sensor of the invention to be modified. For example, a pair of ultrasonic transducers could be separated and placed one on each side of the stud. Alternatively, the transducers could be located in a single unit, but offset from a central longitudinal plane of an intermediate link and angled toward the opposed faces of the links that are joined by the intermediate link. In some embodiments of the invention, a monitoring device for measuring wear of a mooring chain may comprise a mount positioned on a first link or a second link that are connected to each other by an interconnecting link, or positioned on the interconnecting link itself. At least one acoustic transducer may be oriented to send an acoustic signal generally longitudinally toward the first or second link. The transducer, or another receiver, receives the signal directly or after reflection. A data logger records the distance between the transducer and the first or second chain link evaluated from the time of arrival of the received signal. 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. The mount may be permanently attached to or integrated with a link or may be attachable to a link, for example as a clamp. The mount may be attached to one or both sides of the interconnecting link, for example at a position approximately central with respect to a side of the interconnecting link. If the interconnecting link is a stud link, the mount could be attached to the central bar or stud of the interconnecting link. The monitoring device may comprise two, or first and second, acoustic transducers oriented longitudinally in mutually-opposed directions, the first acoustic transducer 20 02 25 facing toward the first link and the second acoustic transducer facing toward the second link. The signals emitted by the 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. Thus, the condition of a chain is monitored in accordance with the invention by fixing at least one acoustic transducer to a first link of the chain. The transducer emits an acoustic signal toward a receiver on any link of the chain. For example, the receiver could be on the first link to receive the signal after reflection from a second link, which could be interconnected with the first link or could be opposed to the first link across an intermediate link. Alternatively, in a direct transmission system, the receiver could be on the second link or on another link. By measuring time of flight of the signal from the transducer to the receiver, the distance between the transducer and a link of the chain other than the first link is determined. An increase in that distance indicates a degree of wear at one or more contact interfaces between successive links of the chain. 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: Figure 1 is a schematic detail side view of links of a studless chain in an unworn state; Figure 2 corresponds to Figure 1 but shows the chain in a worn state; Figure 3 corresponds to Figure 1 but shows one of the links of the chain fitted with a sensor unit of the invention; Figure 4 is a top view of the chain and sensor unit shown in Figure 3; Figure 5 is a schematic block diagram of the sensor unit shown in Figures 3 and 4, also showing an ultrasonic beam emitted by an ultrasonic transducer of the unit toward an opposed link of the chain; 20 02 25 Figure 6 is a schematic detail side view of a variant of Figure 5 in which the opposed link is fitted with a reflector facing the transducer; Figure 7 is a schematic view of a mooring chain fitted with sensor units of the invention, showing options for communication of data from the sensor units to a monitoring station; Figure 8 is a timing diagram illustrating the possibility of a delay between pulse emissions of respective transducers in a sensor unit of the invention; Figure 9 corresponds to Figure 3 but shows a stud-link chain fitted with a variant of a sensor unit of the invention; Figure 10 is a top view of the chain and sensor unit shown in Figure 9; Figure 11 corresponds to Figure 10 but shows another variant of a sensor unit of the invention; Figure 12 corresponds to Figure 5 but shows a variant of the invention that detects time of arrival of a direct beam rather than a reflected beam; and Figure 13 is a schematic detail side view of a variant of the invention in which a transducer fitted to one link radiates a signal toward another link opposed across an inner opening of an intermediate link. Referring firstly to Figures 1 and 2, these drawings show three successive links of a chain 10, namely outer links 12 and 14 in mutual opposition and an inner, intermediate link 16 that couples the outer links 12, 14 to each other. Thus, the outer links 12, 14 face each other end-to-end across the inner opening 18 of the intermediate link 16. The outer links 12, 14 have the same general orientation as each other, whereas the intermediate link 16 is oriented generally orthogonally to the outer links 12, 14. Otherwise, the links 12, 14, 16 are all identical to each other. Figure 1 shows, in dotted lines, the contact interfaces 20 where the inner end surfaces of the outer links 12, 14 bear against the inner end surfaces of the intermediate link 16. It is at those interfaces 20 that wear of the chain 10 is most likely to occur. 20 02 25 In Figure 1, the chain 10 is shown in its initial unworn state. Consequently, the end portions of the links 12, 14, 16 all have their original full thickness. This determines an initial longitudinal distance Di between the opposed facing ends of the outer links 12, 14. With wear due to friction over a period of use, metal of the links 12, 14, 16 will erode away at the contact interfaces 20. Consequently, the thickness of the end portions of the links 12, 14, 16 will reduce as shown, exaggeratedly, in Figure 2. This causes the initial longitudinal distance Di between the outer links 12, 14 to lengthen to a new, greater distance D2. Embodiments of the invention are predicated upon determining the increase between Di and D2 and thereby inferring the degree of wear suffered by and between the links 12, 14, 16. Turning next, then, to Figures 3 and 4 of the drawings, the intermediate link 16 of the chain 10 is shown fitted with or incorporating a sensor unit 22 of the invention for determining the longitudinal distance D between the opposed facing ends of the outer links 12, 14. For this purpose, the sensor unit 22 is positioned approximately mid-way along the length of the intermediate link 16. In this example, the sensor unit 22 spans the inner opening 18, extending in a direction orthogonal to the length of the chain 10 to bridge the gap between opposed sides of the intermediate link 16. In other examples, a sensor unit 22 could be cantilevered from one side of the intermediate link 16. The sensor unit 22 comprises a pair of ultrasonic transducers T1 and T2 that are each centred on the central longitudinal axis 24 of the intermediate link 16. In this example, the transducers T1, T2 face in mutually-opposed directions aligned with or parallel to the central longitudinal axis 24. Consequently, a radiator face of transducer T1 faces toward one of the outer links 12 and a radiator face of transducer T2 faces toward the other outer link 14. The transducers T1, T2 emit signals 26 comprising trains of ultrasonic pulses that are reflected back to the sensor unit 22 from the opposed double-curvature outer end surfaces of the respective links 12, 14 disposed within the inner opening 18 of the intermediate link 16. The transducers T1, T2 may also receive the reflected signals and therefore serve as transceivers, or the sensor unit 22 may support separate receivers disposed beside the transducers T1 and T2. The time of flight of the pulses and 20 02 25 corresponding reflections out and back is proportional to the longitudinal distances between the sensor unit 22 and the opposed facing end of each outer link 12, 14, which in turn are indicative of wear between the links 12, 14, 16. In Figure 3, di represents the longitudinal distance between transducer T1 and the opposed outer link 12 and d2 represents the longitudinal distance between transducer T2 and the opposed outer link 14. Conversely, do represents the longitudinal distance between the radiator faces of the transducers T1 and T2, which approximates to the overall thickness of the sensor unit 22. The longitudinal distance D between the opposed facing outer ends of the outer links 12, 14 equates to di + d2 + do. The dimension do is constant and known whereas di and d2 are variable and measured by the sensor unit 22 to determine the degree of wear of the links 12, 14, 16. Figure 5 shows the main components of the sensor unit 22. In addition to the transducers T1 and T2, the sensor unit 22 contains a power source 28, a processor 30, a data store 32 and a communication module 34. All are sealed or encapsulated within the sensor unit 22 for water tightness. The power source 28 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. The processor 30 generates pulses to drive the transducers T1, T2, receives reflected signals via the transducers T1, T2 or via separate receivers, and processes those signals against elapsed time to determine time of flight and hence di and d2, from which D can be derived. The processor 30 outputs the resulting data to the data store 32 from which the data can be transferred periodically or continuously from the sensor unit 22 to a remote monitoring station. Data is output from the sensor unit 22 via the communication module 34, which may be configured for wired or wireless data transmission by electrical, acoustic, electromagnetic or optical means. The communication module 34 could also be configured as a relay to receive and forward data received from other sensor units 22 mounted on other links of the chain 10. Consequently, the communication module 34 could comprise an input or receiver in addition to an output or transmitter. 20 02 25 Figure 5 also shows some parameters of one of the transducers, namely T2, in particular DT being the diameter of the radiator face, and 8 being the beam angle at which a signal 26 comprising a train of ultrasonic pulses flares away from the radiator face as a beam within a frusto-conical volume. For example, DT could be 10mm and 8 could be 15° to 20° from a line parallel to the central longitudinal axis 24. The resulting beam of the signal 26 is broad enough to impinge on the double-curvature outer end surface of the opposed outer link 14, the closest point or apex of which is at distance cfe from the transducer T2. However, the beam of the signal 26 is not so broad as to impinge on other structures such as the sides of the intermediate link 16, which could otherwise produce false echoes. Figure 6 shows the possibility of attaching a reflector 36 to the outer end of either or both of the outer links 12, 14 used for distance measuring in a pulse-echo arrangement, in this case the link 14 that is directly opposed to the transducer T2 along the central longitudinal axis 24. For example, the reflector 36 could be attached to the link 14 by a magnetic clamp. The reflector 36 is oriented orthogonally with respect to the central longitudinal axis 24 and therefore reduces scattering that would arise when an acoustic signal 26 comprising a beam of pulses impinges on a double-curved outer end surface of the link 14. Corrosion and marine growth can affect the measurements of di, d2 and hence D. In particular, loss of metal due to corrosion where the ultrasound pulse signal 26 impinges on a link 12, 14, can lead to an overestimation of distance D whereas marine growth or deposition at the same location on a link 12, 14 could cause an underestimation of distance D. Advantageously, therefore, the reflector 36 could be made of a corrosionresistant material such as a polymer to minimise effects of corrosion on the reflected signal. For similar reasons, the reflector 36 could be treated, coated or impregnated with anti-fouling compounds to resist marine growth or deposition. A reflector 36 or similar measures could be applied to any link 12, 14 from which an acoustic signal 26 is to be reflected in a pulse-echo arrangement. For example, in principle, reflection quality could also be improved by treating, coating and / or reshaping or flattening the outer end surface of the link 14 without necessarily attaching a reflector 36 to it. Figure 7 illustrates various options for conveying data to a monitoring station 38 from sensor units 22A to 22D mounted on a mooring chain 10. 20 02 25 The sensor unit 22A transmits data directly to the monitoring station 38 by wireless transmission, for example acoustically. In contrast, the sensor unit 22B transmits data to the monitoring station 38 indirectly via an ROV 40 that periodically visits and interrogates the sensor unit 22B. The ROV 40 could also, transiently, provide power to the sensor unit 22B, for example by electromagnetic induction or by illuminating a photocell of the sensor unit 22B. The sensor unit 22C has a wired connection to the monitoring station 38 to convey data electronically or optically. Conversely, the sensor units 22C and 22D illustrate the possibility of relaying data from one sensor unit 22 through another, in this case wirelessly. More generally, the relay function shown for sensor units 22C and 22D in Figure 7 could involve most or all of the sensor units 22 on a chain 10, providing a system in which all sensor units 22 can relay data, for example acoustically, from other sensor units 22 that are at more distal positions along the chain 10 with respect to the monitoring station 38. For example the sensor unit 22A can report to the monitoring station 38 via the sensor units 22B, 22C and 22D in sequence. As wear of a chain 10 is a slow process, time-of-flight measurements of signals 26 from the transducers T1, T2 can be taken at widely different times. Nevertheless, measures could be taken to avoid or to mitigate interference between ultrasonic emissions from the transducers T1, T2 if quicker, simultaneous or near-simultaneous time-of-flight measurements are ever required, for example in real time when a sensor unit 22 is interrogated by a visiting ROV 40. One such measure is illustrated in the timing diagram of Figure 8, namely to delay or offset the pulse trains 42 emitted from one transducer T2 until the pulse trains 42 of the other transducer T1 have been emitted and reflected signals 44 have been received from the opposed link 12 of the chain 10. In this illustration, the out-and-back time of flight from the start of the pulse train 42 to the start of the corresponding reflected signal 44 is ti for transducer T1 and t2 for transducer T2. Those time periods ti and t2 imply the dimensions di and d2 respectively. Turning next to Figures 9 to 11, these drawings show variants of the invention in which sensor units 22 are adapted for stud-link chains 10 in which a central stud 46 spans, bisects and divides the inner opening 18. In each case, the sensor unit 22 surrounds 20 02 25 the stud 46 of the intermediate link 16 but in other examples, the sensor unit 22 could be fixed to one side of the stud 46, or indeed fixed to one or both sides of the intermediate link 16 as in the preceding embodiment. In Figures 9 and 10, the transducers T1, T2 of the sensor unit 22 remain centred on the central longitudinal axis 24 and face in mutually-opposed directions aligned with that axis 24. Consequently, the transducers T1, T2 are disposed on mutually-opposed sides of the stud 46 of the intermediate link 16. For this purpose, the transducers T1, T2 are housed in respective limbs of the sensor unit 22 that encircles or embraces the stud 46. In Figure 11, in contrast, the transducers T1, T2 of the sensor unit 22 are offset from the central longitudinal axis 24 to one side of the stud 46. Also, the transducers T1, T2 face in mutually-opposed directions that converge with the central longitudinal axis 24. By virtue of the inclination of the transducers T1, T2 relative to the central longitudinal axis 24, the ultrasonic beams from the transducers T1, T2 still impinge on the facing ends of the outer links 12, 14. This generates reflected signals that can be received by the transducers T1, T2 or by other receivers of the sensor unit 22 to enable calculation of di., ch and hence D as illustrated in Figure 3. Compensation for the fixed transducer beam angles will be required. Whilst the preceding embodiments contemplate pulse-echo arrangements, Figure 12 exemplifies how the invention could instead be implemented by measuring time of flight of a signal 26 transmitted directly between an emitter such as the transducer T2 and a time-synchronised receiver 48. Thus, the signal 26 need not be reflected in the interim. In this example, the receiver 48 is fixed to the outer end of the outer link 14 opposed to the transducer T2. For example, the receiver 48 could be attached to the link 14 by a magnetic clamp. In the example shown in Figure 12, the receiver 48 communicates with the processor 30 of the sensor unit 22 via a wired or wireless link 50. In this way, after driving the transducer T2 to emit a signal 26 comprising a train of pulses, the processor 30 measures the time of flight between emission of the pulses from the transducer T2 to reception of the pulses by the receiver 48, and thereby determines the distance ds between the radiator face of the transducer T2 and the opposed face of the receiver 48. As the spacing between the face of the receiver 48 and the underlying outer end surface of the link 14 is known, the distance d2 between the radiator face of the transducer T2 and the outer end surface of the link 14 can be deduced by determining 20 02 25 ds. Alternatively, wear affecting the link 14 can be inferred directly from an increase of ch over time because d4\s constant. The direct transmission system exemplified in Figure 12 presents some challenges compared with a pulse-echo system, for example the need to mount additional receiver transducers and potentially also additional cables and fixation clamps. However, even if more cumbersome to mount, the direct transmission system is still retrofittable to a chain. 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 less susceptibility to corrosion because surface roughness of a reflective surface has no impact on the acoustic signal, and more generally there is no sensitivity to the orientation or curvature of a reflective surface. Marine growth can be managed by using anti-fouling materials in transducers serving as emitters or receivers. In the example shown in Figure 12, the transducer T2 and the receiver 48 face each other along the central longitudinal axis 42. However, as the signal 26 is not reflected in this embodiment, there is greater design freedom. In particular, the signal 26 need not be constrained to travel only within the inner opening 18 of an intermediate link 16 so as to impinge on one of the outer links 12, 14 joined by the intermediate link 16. Instead, the signal 26 could travel longitudinally along a chain 10 on a path that is offset sufficiently from the central longitudinal axis 24 to bypass the links. Thus, the signal 26 could travel between an emitter and a time-synchronised receiver on the chain 10 that are separated longitudinally by two or more intermediate links 16. This would allow a chain monitoring system of the invention to determine average wear across a continuous group, set or series of several links of a chain 10. Many other variations are possible within the inventive concept. For example, it would be possible to use only one ultrasonic transducer in fixed relation to a first link, that transducer facing toward an outer end surface of a second, adjoining link interengaged with the first link. This variant would only be capable of measuring the effect of wear at one contact interface between the two links but could still be usefully indicative of the condition of a chain around that location. Such a variant could be constructed by, for instance, removing transducer T2 from the sensor unit 22 of the preceding embodiments and only measuring distance di between transducer T1 and the facing end of the opposed link 12. This would detect degradation of the links 12, 16 because distance di will increase with wear at the contact interface 20 between the links 12, 16. 20 02 25 Similarly, as shown in Figure 13, it would be possible to mount a sensor unit 22 comprising a transducer T1 to an outer end of a first link 12 rather than to an intermediate link 16. In this arrangement, the signal 26 can be transmitted from the transducer T1 along the central longitudinal axis 14 within the inner opening 18 of the intermediate link 16 to be reflected back from the opposed outer end surface of the second link 14. The monitoring station shown in Figure 7 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. Transducers used in the invention, whether emitters or receivers, could have active faces made of, treated with, coated with or impregnated with corrosion-resistant and / or anti-fouling materials or compounds. In addition to these measures and the reflector illustrated in Figure 6, the effects of corrosion and marine growth in a pulse-echo system can be mitigated if the ultrasound beam profile is wide enough to generate reflections from across a certain area of a link so that the reflected signal represents an average of echoes from many smaller spots, points or sub-areas on the link. In contrast, if the system operated with a very narrow beam, for example impinging on an area of only about 10mm2 on an outer end surface of a link, a small local groove in the steel caused by corrosion or a small formation arising from marine growth, such as a small mussel or barnacle, could cause a significant error in the measurements. Pulse-echo embodiments of the present invention rely upon a certain reflectivity of the link surface upon which the ultrasound beam signal impinges when transmitted from a transducer. When the ultrasound beam is aimed toward an outer end surface of a link, only one small area at the apex of that double-curved surface is substantially perpendicular to the incoming wavefront and so is oriented to reflect the signal back along and around the central longitudinal axis to the transducer. In principle, all peripheral areas on the outer end surface of the link will reflect the signal back at such an angle to the central longitudinal axis that the signal will not necessarily impinge on the transducer. This is especially the case where surface roughness of the reflective surface is fine compared to the wavelength, being the so-called mirroring effect. However, if the surface roughness if coarse compared to the wavelength, each small area of the surface will reflect the incoming signal in many different directions, hence producing ‘diffuse scattering’. As rust and other corrosion will result in a rough surface, diffuse scattering may be obtained by selecting an appropriate ultrasound frequency. In combination with a relatively wide beam, this may be beneficial to reduce inaccuracy arising from corrosion. Other 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 of a sensor unit, or downstream in a monitoring station, can further mitigate such interference as may arise between the reflected signals of the transducers. 20 02 25 20 02 25

Claims

1. A system for monitoring the condition of a mooring chain for use in marine applications, the system comprising:at least one emitter on a first link of the chain, the emitter being configured to emit a signal;at least one receiver on the first link of the chain, configured to receive the signal; anda processor configured to measure a time lapse between emitting and receiving the signal to determine a distance between the emitter and a link of the chain other than the first link;wherein:the emitter and the receiver are each opposed to an intermediate reflecting surface on a second link of the chain; andthe emitter is configured to emit the signal toward an opposed reflecting outer end surface of the second link of the chain, that surface of the second link being disposed within an inner opening of the first link.

2. The system of Claim 1, wherein an acoustic transducer serves as the emitter and as the receiver.

3. The system of Claim 1 or Claim 2, wherein the reflecting surface is defined by a reflector that is fixed to the second link of the chain.

4. The system of any preceding claim, wherein the emitter and the receiver are implemented together in a sensor unit that is attached or attachable to the first link of the chain.

5. The system of Claim 4, wherein the sensor unit spans the inner opening of the first link from one longitudinally-extending side to another.

6. The system of any preceding claim, wherein the receiver is in fixed relation to a second link of the chain.20 02 257. The system of Claim 6, wherein the receiver is disposed within an inner opening of the first link.

8. The system of Claim 6, wherein the receiver is disposed within an inner opening of an intermediate link coupling the first link to the second link.

9. The system of Claim 8, wherein the emitter is also disposed within the inner opening of the intermediate link.

10. The system of any of Claims 5 to 9, further comprising a processor in data communication with the emitter and the receiver via a connection that extends between the first and second links.

11. The system of any preceding claim, wherein the emitter is attached or attachable to the first link at a central longitudinal position.

12. The system of Claim 11, wherein the emitter is attached or attachable to at least one longitudinally-extending side of the first link.

13. The system of any preceding claim, wherein the emitter is aligned with a central longitudinal axis of the first link.

14. The system of Claim 13, wherein the emitter is oriented to emit the signal in a direction substantially parallel to the central longitudinal axis of the first link.

15. The system of any preceding claim, wherein the emitter is disposed within an inner opening of the first link.

16. The system of any of Claims 1 to 12, wherein the emitter is offset laterally from a plane of the first link.

17. The system of Claim 16, wherein the emitter is oriented to emit the signal in a direction converging with the central longitudinal axis of the first link.

18. The system of any preceding claim, comprising first and second emitters in mutually-opposed orientation.20 02 2519. The system of Claim 18, wherein the first and second emitters are in fixed relation to the first link being an intermediate link disposed between and conjoining a pair of outer links each with a reflecting outer end surface that is opposed to a respective one of the emitters and is disposed within an inner opening of the first link.

20. The system of any preceding claim, wherein at least the first link is a stud link that comprises a stud dividing an inner opening.

21. The system of Claim 20, wherein the emitter is mounted on the stud.

22. The system of Claim 21 when dependent on Claim 18, wherein the first and second emitters are disposed on respective mutually-opposed sides of the stud.

23. The system of any preceding claim, wherein the emitter is disposed on an outer end surface of the first link.

24. The system of any preceding claim, further comprising a processor configured to determine time of flight of the signal from the emitter to the receiver and to generate distance data accordingly.

25. The system of Claim 24, further comprising a data store for storing the distance data.

26. The system of Claim 24 or Claim 25, further comprising a communication module for conveying the distance data to a monitoring or relay station.

27. The system of any preceding claim, wherein the or each emitter is configured to emit the signal as a beam with a beam angle of up to 45°.

28. The system of any preceding claim, wherein the or each emitter is configured to emit the signal with a pulse frequency of 0.1 to 5 MHz.

29. A method for monitoring the condition of a mooring chain for use in marine applications, the method comprising:emitting a signal from an emission location on a first link of the chain;20 02 25receiving the signal at a receiving location on the first link of the chain;reflecting the signal, from a location on a second link of the chain and within an inner opening of the first link, to the receiving location; andmeasuring a time lapse between emitting and receiving the signal to determine a distance between the emission location and a link of the chain other than the first link.

30. The method of Claim 29, wherein the receiving location is fixed relative to a second link of the chain.

31. The method of Claim 30, comprising receiving the signal within an inner opening of the first link.

32. The method of Claim 30, comprising receiving the signal within an inner opening of an intermediate link coupling the first link to the second link.

33. The method of Claim 32, comprising emitting the signal within the inner opening of the intermediate link.

34. The method of any of Claims 29 to 33, comprising emitting the signal along a central longitudinal axis of the first link.

35. The method of any of Claims 29 to 33, comprising emitting the signal in a direction converging with the central longitudinal axis of the first link.

36. The method of any of Claims 29 to 35, comprising emitting the signal from within an inner opening of the first link.

37. The method of any of Claims 29 to 35, comprising emitting the signal from an outer end surface of the first link.

38. The method of any of Claims 29 to 37, comprising emitting signals in mutually opposed directions from first and second emission locations fixed relative to the first link.

39. The method of Claim 38, comprising emitting the signals at different times, with different phases or with different frequencies.

40. The method of any of Claims 29 to 39, comprising generating data representative of said distance at the first link and communicating that distance data from the first link to a receiving station remote from that first link.

41. The method of Claim 40, comprising relaying the distance data along the chain to the receiving station.

42. The method of Claim 40, comprising conveying the distance data to the receiving station via an underwater vehicle stationed beside the first link.20 02 25

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