Monitoring conditions of marine chains

Abstract

The condition of the chain is monitored by at least one acoustic transducer secured to a first link of the chain. The transducer transmits an acoustic signal to a receiver on any chain link of the chain. A receiver may be on the first link to receive a signal after reflection from the second link. The second link may be interconnected with the first link, or may be opposed to the first link across the intermediate link. In the direct transmission option, the receiver may be on a second link or on another link. A distance between the transducer and a chain link of the chain other than the first chain link is determined by measuring a time of flight of a signal from the transducer to the receiver. The increase in the distance indicates a degree of wear at one or more contact interfaces between consecutive links of the chain.

Description

Technical Field

[0001] The present invention relates to monitoring the condition of a chain. In particular, the present invention relates to monitoring the degradation of a chain used in marine applications, such as a mooring chain used as a mooring line or forming a mooring line section. Background Art

[0002] Among other applications, mooring chains are used to anchor floating offshore installations in the subsea oil and gas industry and 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).

[0003] As a mooring chain bends along its length under tension during its service life, the chain's successive articulated links pivot and slide relative to each other. Consequently, with repeated frictional contact between successive links, the chain is susceptible to wear, which can be exacerbated by the effects of corrosion. Over time, the chain's load-bearing capacity decreases as the links degrade and thin, until eventually the chain may fail.

[0004] While floating offshore facilities employ multiple mooring chains for redundancy, a mooring chain failure is unacceptable as it reduces safety margins and increases stress on the surviving mooring chains. If wear levels can be effectively monitored, worn mooring chains can be replaced preventively before failure occurs.

[0005] The phenomenon of chain degradation is well known, and typical chain wear rates are well known in the art. For example, chain degradation is thoroughly discussed in the UK Health and Safety Executive's research report RR1098, "Degradation of mooring chains for floating offshore installations: measurement of chain wear, estimation of corrosion rates and their impact on breaking loads" (available at https: / / www.hse.gov.uk / research / rrpdf / rr1098.pdf).

[0006] Actual chain wear rates can be greater or less than predicted because they depend on variables such as tolerances, material quality, and the location of the links along the chain. Consequently, wear may not be uniform along the length of the chain, causing some links to be closer to failure than others, or closer to failure than predicted.

[0007] Traditionally, reducing the risk of premature mooring chain failure has involved overdesigning the chain links; however, this increases costs and, in particular, the weight of the chain. Long chains used in deep water and large chains used to moor large floating facilities can have reduced capacity due to the tension generated by their own weight and can become too heavy to handle.

[0008] Where the size of mooring chain links has to be limited to save weight, the links are usually checked regularly or monitored intermittently or continuously.

[0009] In a holistic approach to chain monitoring, the overall geometry of the chain as a whole can be checked. For example, US10780954 teaches recording images of the chain, while WO 2020 / 164760 teaches attaching optical fibers to the chain to monitor its geometry and articulations. More generally, holistic chain monitoring can be performed continuously, for example by installing acoustic transducers on key links, or intermittently, as in the RAMS system provided by Tritech International Limited (trademark recognized). In a RAMS system, the integrity of the mooring line is monitored by a sonar head deployed below the moored vessel, without the need for additional sensors on the line. However, the holistic approach cannot detect potential failures of individual links of the chain.

[0010] Chain link inspection typically relies on visual inspection, caliper measurement, or nondestructive testing, such as ultrasound, to detect cracks, as disclosed in EP 2507583 or WO 2015 / 030600. Optical measurement or 3D photogrammetry can also be used to measure critical chain dimensions. However, nondestructive testing and measurement of individual links is difficult, time-consuming, and expensive, especially when performed offshore and underwater.

[0011] Monitoring of chain links can rely on continuous mechanical measurement of link deformation, typically using strain gauges or by measuring load using load cells as exemplified in US10078025. Deviations in dimensions or stress can indicate abnormal wear or fatigue. Similarly, in US2013 / 279298, strain gauges are embedded in shields attached to the chain links, and signals representing strain are acoustically transmitted.

[0012] Retrofitting chains with sensor fixtures is also known in the art, as described, for example, in CN 104330102, CN 109029527, and CN 110081921. Again, the sensors proposed in these documents are strain gauges. However, given the large size of the chain links used to moor offshore energy devices such as FPSOs, FOWTs, or WECs, particularly in deep water, standard methods for monitoring mooring chains with strain gauges are limited.

[0013] GB 2415256 describes another method of chain monitoring, namely stimulating the chain links by, for example, hammering them to propagate a signal through the links, which is then received and processed to determine the structural integrity of the links.

[0014] US 2022 / 0003636 and WO 2009 / 044117 describe the monitoring of chains used in power transmission and motion control, for example in lifting or conveying applications. Summary of the Invention

[0015] It is in this context that the present invention was devised. In one sense, the present invention consists in a system for monitoring the status of a chain, the system comprising: at least one transmitter in fixed relation to a first link of the chain, the transmitter being configured to transmit a signal; at least one receiver on any link of the chain being configured to receive the signal; and a processor configured to measure the time lapse between the transmission and reception of the signal to determine the distance between the transmitter and a link of the chain other than the first link.

[0016] The transmitter and receiver may both be on the first link of the chain, in which case the acoustic transducer may serve as both transmitter and receiver. The transmitter and receiver may each be positioned opposite an intermediate reflective surface on the second link of the chain. The reflective surface may be defined by a reflector secured to the second link of the chain.

[0017] The transmitter can be configured to transmit a signal toward an opposing reflective outer end surface of a second link of the chain, the surface of the second link being disposed within an interior opening of the first link. Alternatively, the signal can be transmitted toward an opposing reflective outer end surface of a second link of the chain, the surface of the second link being disposed within an interior opening of an intermediate link that couples the first link to the second link. In this case, the transmitter can also be disposed within the interior opening of the intermediate link.

[0018] Conveniently, the transmitter and receiver may be implemented together in a sensor unit that is attached or attachable to the first link of the chain.For example, the sensor unit may span the inner opening of the first link from one longitudinally extending side to the other longitudinally extending side.

[0019] In a direct transmission method, the receiver can be in a fixed relationship with the second link of the chain. Nevertheless, the receiver on the second link can be positioned within the interior opening of the first link or within the interior opening of an intermediate link coupling the first link to the second link. In the latter case, the transmitter can also be positioned within the interior opening of the intermediate link. The processor can communicate data with the transmitter and receiver via a wired or wireless connection extending between the first and second links.

[0020] The transmitter is suitably attached or attachable to the first link at a central longitudinal position along the first link. For example, the transmitter may be attached or attachable to at least one longitudinally extending side of the first link. The transmitter may alternatively be disposed on an outer end surface of the first link.

[0021] The transmitter can be aligned with the central longitudinal axis of the first link, in which case the transmitter can be oriented to transmit the signal in a direction substantially parallel to the central longitudinal axis of the first link. More generally, the transmitter can be disposed within an interior opening of the first link. It is also possible that the transmitter is laterally offset from the plane of the first connector. In this case, the transmitter can be oriented to transmit the signal in a direction converging with the central longitudinal axis of the first link.

[0022] The first and second emitters can be in opposite orientations relative to each other. For example, the first and second emitters can be in fixed relation to a first link, the first link being an intermediate link disposed between and connected to a pair of outer links, each pair of outer links having a reflective outer end surface that is opposite a corresponding one of the emitters and disposed within an interior opening of the first link.

[0023] At least the first link may be a stud link comprising a stud defining an internal opening. In this case, the or each launcher may be conveniently mounted on the stud. Where a first launcher and a second launcher are present, they may be arranged on respective, mutually opposing sides of the stud.

[0024] The system may further include a processor configured to determine a time of flight of a signal from the transmitter to the receiver. The processor may thereby generate distance data that may be stored in a data memory and / or transmitted by the communication module to a monitoring station or a relay station.

[0025] The or each transmitter may, for example, be configured to transmit the signal as a beam having a beam angle of up to 45°, or having a pulse frequency of 0.1 to 5 MHz.

[0026] The system may include a reference reflector configured to reflect at least a portion of the signal transmitted by the transmitter to the receiver as a reference signal for use by the processor in calibrating the determined distance. In other words, the reference reflector may be positioned along the beam path of the signal transmitted by the transmitter, and specifically between the transmitter and the receiver. The reference reflector may take the form of a thin length of wire or a disk.

[0027] Additionally or alternatively, the system may include a reference transmitter (in addition to the transmitter that transmits the signal used to measure the distance to the chain link). In this case, and also in the case of incorporating a reference reflector, the reference transmitter may be configured to transmit an auxiliary signal (towards the receiver), and the reference reflector may be configured to reflect at least a portion of the auxiliary signal as a reference signal for use by the processor in calibrating the determined distance. In this case, the reference reflector is positioned at an angle to the beam path of the signal transmitted by the transmitter. In this case, the reference reflector may take the form of a plate. Instead of the reference reflector, a reference receiver may be incorporated that is configured to receive the auxiliary signal (transmitted by the reference transmitter) as a reference signal for use by the processor in calibrating the determined distance.

[0028] The processor can be configured to measure a reference time lapse that occurs before receiving the reference signal, and use the reference time lapse to determine a reference speed of sound in water. For example, this can be based on the determined reference time lapse and a distance traveled by the reference signal, which typically corresponds to a known distance to a reference reflector or reference receiver. Furthermore, the processor can be configured to use the reference speed of sound in water to calibrate the determined distances between the transmitter and links of the chain other than the first link.

[0029] In this way, the processor can account for any changes in the speed of sound in the water caused by changes in the temperature, salinity, and / or depth of the water surrounding the monitored chain. The processor can calibrate the distance measurement to the chain link by using the speed of sound in the water appropriate to the surrounding conditions. This allows for more accurate distance measurements.

[0030] The system may include a sensor unit having a first sensor module and a second sensor module in operable communication with each other. The transmitter and receiver may be housed together in the first sensor module, and the processor may be housed in the second sensor module. This configuration provides flexibility in the scenarios in which the sensor unit can be implemented. For example, the first sensor module may be sized and configured to be insertable into or into an internal opening of a link of a chain, thereby allowing the transmitter and receiver to measure distances between links on a smaller scale, as the remaining electronic components of the sensor unit (in the second sensor module) may be separated and located near the chain.

[0031] Where a reference emitter is implemented, it may also be housed in the first sensor module. In some cases, a reference reflector (where appropriate) may also be housed in the first sensor module. Additionally or alternatively, the reference reflector may be connected to the first sensor module and, in some cases, may correspond to a portion of the housing of the second sensor module.

[0032] The present invention includes a method for monitoring the status of a chain, comprising: transmitting a signal from a transmitting position fixed relative to a first link of the chain; receiving the signal at a receiving position on any link of the chain; and measuring the time lapse between transmitting and receiving the signal to determine the distance between the transmitting position and a link of the chain other than the first link.

[0033] The signal can be reflected from the second link of the chain to the receiving location, or reflected from a location within the interior opening of the first link, or reflected from a location within the interior opening of an intermediate link coupling the first link to the second link. The signal can be transmitted from within the interior opening of the intermediate link.

[0034] The receiving position can be fixed relative to the second link of the chain, but still within the inner opening of the first link or within the inner opening of the intermediate link connecting the first link to the second link. In this case, the signal can also be emitted within the inner opening of the intermediate link.

[0035] The signal can be transmitted along the central longitudinal axis of the first link, in a direction converging with the central longitudinal axis of the first link, from within an internal opening of the first link, or from an outer end surface of the first link. The signal can also be transmitted in mutually opposite directions from first and second transmitting positions fixed relative to the first link. In this case, the corresponding signals can be transmitted at different times, with different phases, or at different frequencies.

[0036] The data representing the distance may be generated at the link and transmitted from the link to a receiving station remote from the link. For example, the distance data may be relayed along the link to the receiving station or transmitted to the receiving station via an underwater vehicle located next to the link.

[0037] At least a portion of the (primary) signal transmitted by the transmitter may be reflected to the receiver as a reference signal for use by the processor in calibration of the determined distance.

[0038] Additionally or alternatively, an auxiliary signal may be transmitted, and at least a portion of the auxiliary signal may be reflected to the receiver as a reference signal for use by the processor in calibrating the determined distance. Alternatively, at least a portion of the auxiliary signal may be received as a reference signal (e.g., by a reference receiver) for use by the processor in calibrating the determined distance.

[0039] A reference time lapse between transmitting the primary / auxiliary signal and receiving the reference signal can be measured, and the reference time lapse can be used to determine a reference speed of sound in water. For example, based on the determined reference time lapse and the distance traveled by the reference signal, which typically corresponds to a known distance to a reference reflector or reference receiver, the reference speed of sound in water can then be used to calibrate the determined distances between the transmitter and links of the chain other than the first link.

[0040] In summary, the present invention contemplates a technique for detecting chain wear by measuring the time of flight of a signal transmitted between chain links, for example using pulse-echo ultrasound technology. Over a period of months or years, corrosion and frictional movement between successive chain links will lead to metal loss at the interfaces between these links. Consequently, the longitudinal distance between opposing chain links, as represented by the signal's time of flight, will increase over time, and this increased distance can be used to infer the effects of chain link degradation.

[0041] In some embodiments, one or more sensor units of the present invention periodically measure and store the longitudinal distance between adjacent, opposing links connected to each other via intermediate links. Because the nominal distance between opposing links of a new chain is known, the wear level can be estimated even if the sensor units of the present invention are installed on a chain later in its operating life.

[0042] The sensor unit of the present invention can periodically store distance measurements in its internal memory, for example, when measurements are taken at regular intervals. A time series or statistical summary of the measured distances over time can be made available to service personnel via a wireless modem or other data communication device. For example, an acoustic modem can be coupled to the sensor to transmit data, and a corresponding modem can be located in a vessel, on a drilling rig, or in a buoy to receive the data. In another approach, a modem, such as an optical modem, can be carried by an underwater vehicle, such as an ROV, which travels past the sensor to download the stored data from time to time.

[0043] The sensor unit of the present invention may include a transceiver comprising a transmitter, such as a piezoelectric transducer, a receiver, and a processing unit. In a pulse-echo embodiment, an ultrasonic sound pulse is transmitted from the transducer to the opposing link of the chain, and the echo reflected from the chain is detected by the receiver. The received signal may be processed, for example, by a rectifier and a low-pass filter, i.e., an envelope detector, followed by a comparator in the processing unit. The time of flight from the transducer back to the receiver is used to determine the distance between the sensor and the chain link. The sensor is thus used to estimate, by indirect measurement, how much metal has eroded at the contact interface between consecutive links following the link undergoing the pulse-echo measurement.

[0044] Two miniature pulse-echo systems of this design can be attached to the middle link of a chain to measure the distance to two opposing links connected by the middle link. These pulse-echo systems can be integrated into a single combined unit that is attached to the chain link, or they can be embodied in separate units that can be attached to the same chain link.

[0045] The chain wear sensor unit of the present invention may for example be used in combination with a clamp that connects the corrosion resistant anodes to the links of a mooring chain.

[0046] Several links of a mooring chain may be equipped with such a clamp.In addition to the anode, the sensor unit of the invention may be attached to such a clamp or may be provided on a separate clamp.

[0047] Design criteria for pulse-echo ultrasound systems include beam profile and pulse length. The beam profile depends on the shape, size, and curvature of the transducer and the excitation frequency, and determines how much the beam extends beyond the diameter of the transducer face as the distance from the transducer face increases.

[0048] In the context of the present invention, the ultrasonic beam should be wide enough to impinge on the opposing link, even if the ultrasonic transducer is not perfectly aligned with the hyperboloid 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 an intermediate link on which the transducer is mounted.

[0049] In the pulse-echo embodiment of the present invention, ultrasonic pulses are primarily used to detect the presence of steel structures relative to the chain link. The round-trip time is measured to estimate the distance between the sensor and the closest point on the hyperboloidal facing surface of the chain link. In this case, pulse length and shape can be important, depending on the method chosen to detect the presence and arrival time of the reflected pulse. For example, envelope detection can be performed on the received pulse, and a comparator can be used to determine its arrival time. However, in this case, the phase information in the reflected pulse is lost. A steep leading edge in the output pulse can be beneficial for accurate measurement.

[0050] In other approaches, received RF pulses can be digitized using a fast D-converter, and the pulse shape can be cross-correlated with a fixed reference pulse stored in a processing unit. This approach can provide more robust and accurate time estimates because the phase information in the reflected pulse is preserved. Alternatively, the received signal can be quadrature-demodulated to baseband and digitized using an A / D converter, which does not need to be as fast as in the aforementioned approach. Amplitude and phase information is preserved, and the processing unit can use this information to determine the time of arrival with high temporal resolution.

[0051] It is contemplated that the present invention can be implemented with a wide range of operating parameters. For example, the diameter or rectangular dimensions of the radiator face of the transducer can typically range from a few millimeters to a few centimeters. The transducer frequency can range from a few hundred kilohertz to a few megahertz, and the beam profile or angle can range from a few degrees to, for example, 45 degrees.

[0052] There will inevitably be trade-offs between these and other parameters and performance, such as detection reliability, robustness to installation, accuracy of distance estimation, etc. For example, high ultrasonic frequencies will generally increase the ability to measure distances with high resolution, but the beam angle may become impractically narrow. However, solutions to widen the beam, such as curving the transducer surface and / or applying an acoustic lens, can be considered. It is also possible to provide an array of multiple transducers.

[0053] While some chains have open links, hence the name "nailless links" or "screw links," many mooring lines employ nailed or studded links, in which a transverse rod or stud extends through the central opening of each link. The presence of studs may require modifications to the ultrasonic transducers of the present invention. For example, a pair of ultrasonic transducers could be separated, with one placed on each side of a stud. Alternatively, the transducers could be located in a single unit, but offset from the central longitudinal plane of the intermediate connector and angled toward the opposing faces of the connectors connected by the intermediate connector.

[0054] In some embodiments of the present invention, a monitoring device for measuring wear of a mooring chain can include a mounting member positioned on a first or second link connected to each other by an interconnecting link, or positioned on the interconnecting link itself. At least one acoustic transducer can be oriented to transmit 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 link, which is assessed based on the time of arrival of the received signal.

[0055] The data logger may comprise a memory for the data.The data logger may comprise or interface with a signal transmission system, for example for transmitting signals acoustically, wirelessly, electrically along a wire or cable, or optically along an optical fiber.

[0056] The mounting may be permanently attached to or integral with the link, or may be attached to the link, for example as a clamp.

[0057] The mounting piece may be attached to one or both sides of the interconnecting link, such as at a position approximately centered relative to one side of the interconnecting link. If the interconnecting link is a stud link, the mounting piece may be attached to a center rod or stud of the interconnecting link.

[0058] The monitoring device may comprise two or first and second acoustic transducers oriented longitudinally in mutually opposite directions, the first acoustic transducer facing the first link and the second acoustic transducer facing the second link.

[0059] The signals emitted by the transducers may be emitted simultaneously, or with delays between them, or with different phases, and / or with the same frequency and / or with different frequencies. The data logger may include a filtering system to eliminate interference between reflected signals from different transducers.

[0060] Therefore, according to the present invention, the condition of a chain is monitored by attaching at least one acoustic transducer to the first link of the chain. The transducer transmits an acoustic signal to a receiver located at any link of the chain. For example, the receiver may be located at the first link to receive the signal after reflection from the second link, which may be interconnected or opposite the first link across an intermediate link. Alternatively, in a direct transmission system, the receiver may be located at the second link or at another link.

[0061] By measuring the time of flight of the signal from the transducer to the receiver, the distance between the transducer and a link in the chain other than the first link is determined. An increase in this distance indicates the extent of wear at one or more contact interfaces between successive links of the chain.

[0062] A system for measuring a distance property of a marine feature is also described. The system includes at least one transmitter positioned in a fixed relationship relative to the marine feature and configured to transmit a (primary) signal; at least one receiver configured to receive the signal; a reference reflector positioned in a fixed relationship relative to the at least one transmitter and configured to reflect at least a portion of the signal to the at least one receiver as a reference signal; and a processor configured to use the reference signal to perform calibration or compensation with respect to the distance property. The processor can also be configured to use the received signal (or the portion thereof not reflected by the reference reflector) to measure the distance property.

[0063] More specifically, the reference signal (and subsequent calibration) can be used to compensate for or account for changes in environmental conditions surrounding the system that may result in inaccuracies in the measurement of the distance characteristic. For example, changing environmental conditions can affect the speed of sound in water, which is one of the primary parameters used to measure the distance characteristic, particularly where the signal is emitted by an acoustic transducer and / or detected using pulse echoes.

[0064] In some cases, the at least one transmitter and the at least one receiver are implemented as acoustic transducers. In this case, the reference reflector can reflect a portion of the same (primary) signal transmitted from the transducer and subsequently received by the transducer to measure the distance characteristic. The reference reflector in this case can therefore be referred to as a primary beam reference reflector. The primary beam reference reflector can correspond to any suitable reflector that can reflect a sufficient proportion of the transmitted signal to generate a detectable reference signal while allowing a sufficient proportion of the signal to pass unreflected to enable accurate measurement of the distance characteristic. For example, the reference reflector can be implemented as a thin length of wire or disk suspended in the path of the signal transmitted by the transmitter.

[0065] In some cases, at least one transmitter and at least one receiver are implemented as multiple acoustic transducers. In this case, the first transducer can be configured to transmit and receive a first (primary) signal to measure the distance characteristic, and the second transducer can be configured to transmit a second (auxiliary) signal reflected by a reference reflector as a reference signal. Considering another approach, one transducer can be primarily used as a "reference" or "auxiliary" transducer to transmit and receive the reference signal, while the remaining transducers in the plurality of transducers can be used to transmit and receive the "primary" signal for measuring the distance characteristic of the ocean feature. In this case, the reference reflector can be referred to as an auxiliary beam reference reflector. As an example, such a reflector can be implemented as a reflector plate.

[0066] Alternatively, it is envisaged that the reference receiver may be implemented in conjunction with a reference transmitter, rather than a reference reflector. The reference transmitter will transmit an auxiliary signal which will be received by the reference receiver as a reference signal.

[0067] In any of the above arrangements, the processor is configured to: measure a reference time lapse before receiving the reference signal; and use the reference time lapse to determine a reference speed of sound in water. For example, this determination can be based on the reference time lapse and a distance traveled by the reference signal; the traveled distance typically corresponds to a known distance to a reference reflector or reference receiver. The reference speed of sound in water can then be used when measuring the distance characteristic.

[0068] The system may include a sensor unit having a first sensor module and a second sensor module that are in operable communication with each other. The at least one transmitter and the at least one receiver can be housed together in the first sensor module, and the processor can be housed in the second sensor module. This configuration provides flexibility in the scenarios in which the sensor unit can be implemented. For example, where the marine component is a chain comprising links, the first sensor module can be sized and configured to be inserted into or into an internal opening of a link of the chain. Because the remaining electronic components of the sensor unit in the second sensor module can be separated and located near the chain, the transmitter and receiver can measure the distance between the links on a smaller scale.

[0069] In some cases, particularly when only a single transmitter and receiver are used, the reference reflector can also be housed in the first sensor module. In the case of using multiple transmitters and receivers (corresponding to multiple acoustic transducers), they can also be housed in the first sensor module. In this case, the reference reflector can be housed within the first sensor module or can alternatively be connected to the first sensor module. In a specific implementation, the reference reflector can correspond to a portion of the housing of the second sensor module.

[0070] The above system is very flexible in its actual implementation and can be used when measuring distance attributes of a variety of different marine features. For example, the marine element can correspond to any of the following: a flexible, elongated subsea element, such as a rope, umbilical, or chain; a relatively rigid, elongated subsea element, such as a riser or pipeline used for hydrocarbon transportation or a structural member of a subsea structure such as a drilling rig; a subsea object, such as a subsea wellhead (or subcomponents thereof); or a subsea appendage or item of subsea equipment, such as a blowout preventer (BOP).

[0071] In case the marine element is a chain comprising a plurality of links, the distance property measured may correspond to the distance between the links of the chain to monitor wear of the links, and / or to monitor or measure axial bending or loads in the chain. The distance property in question may be monitored over time.

[0072] In case the marine element is a rope, an umbilical cord or another flexible elongated element, the measured distance characteristic may correspond to the distance between two locations of the marine element to determine the elongation of the marine element. In case the marine element is a rigid elongated seabed element, the measured distance characteristic may correspond to the distance between two locations of the marine element to determine the axial bending and / or loading of the marine element.

[0073] In the above example of monitoring an elongated marine element, the system may further comprise a plurality of supports attachable to the element at respective locations spaced apart along the element. One of the supports carries the at least one transmitter and the at least one receiver, and another of the supports carries at least one reflector, each reflector corresponding to or paired with one of the transmitters. A reference reflector or reference receiver may also be incorporated into such a system, for example associated with the support carrying the at least one transmitter and the at least one receiver. Additionally or alternatively, another of the supports (not carrying the at least one transmitter and the at least one receiver) carries a reference reflector that is laterally offset from the element and positioned to reflect a reference signal back to the at least one receiver. Similarly, a reference receiver may be located on another of the supports.

[0074] In the case where the reference reflector is located on a different support than the support on which the at least one transmitter and the at least one receiver are located, the reference reflector can surround the element and optionally can be circumferentially continuous (for example, it can take the form of a ring or torus-shaped component). Alternatively, the reference reflector can be a point reflector that is angularly aligned with the at least one transmitter. In some examples, a plurality of point reflectors can be provided: one serves as the reference reflector, and the rest of the plurality of point reflectors can be used to measure the distance characteristic.

[0075] In the case where the marine element is a subsea object or subsea appendage, such as a subsea wellhead (or component thereof) or a BOP, the measured distance attribute may correspond to a distance between components of the subsea object or between two subsea objects or appendages. In such a case, the at least one transmitter may be attachable to one of the objects, components, or appendages, and the at least one receiver may be attachable to the other object, component, or appendage. A reference reflector may be attached or attachable to at least one transmitter or its corresponding object, component, or appendage.

[0076] A method for measuring a distance property of an ocean feature is also described. The method includes transmitting a signal from a fixed transmission position relative to the ocean feature; receiving the signal; reflecting at least a portion of the signal as a reference signal; receiving the reflected signal; and performing calibration or compensation related to the distance property using the reference signal. The method may also include using the received (unreflected) signal to measure the distance property.

[0077] As described above, the reference signal (and subsequent calibration) can be used to compensate for or account for changes in the environmental conditions surrounding the system that may cause inaccuracies in the distance characteristic measurements. In particular, changes in the speed of sound in water due to changing environmental conditions.

[0078] The method may include reflecting a portion of the same (primary) signal transmitted from the transmitting location to measure the distance characteristic. This involves reflecting a sufficient proportion of the transmitted signal to produce a detectable reference signal while allowing a sufficient proportion of the transmitted signal to be received unimpeded to allow accurate measurement of the distance characteristic.

[0079] Optionally, the method may further include transmitting an auxiliary signal that is reflected by the reference reflector to serve as the reference signal. The auxiliary signal is transmitted in addition to a "primary" signal, which is transmitted for the primary purpose of measuring the distance characteristic. In this case, the primary signal is not reflected to form the reference signal.

[0080] Alternatively, the method may further comprise transmitting an auxiliary signal, the auxiliary signal being received (eg, by a reference receiver) as a reference signal.

[0081] In any of the above methods, the method may further include: measuring a reference time lapse occurring before receiving the reference signal; and using the reference time lapse to determine a reference speed of sound in water. For example, this determination may be based on the reference time lapse and a distance traveled by the reference signal; the traveled distance typically corresponds to a known distance to a reference reflector or reference receiver. The reference speed of sound in water may then be used when measuring the distance characteristic.

[0082] As described above, the marine element may correspond to any of the following: a flexible, elongated subsea element, such as a rope, umbilical or chain; a relatively rigid, elongated subsea element, such as a riser or pipeline for hydrocarbon transport or a structural member of a subsea structure such as a drilling rig; a subsea object, such as a subsea wellhead (or subcomponents thereof); or a subsea attachment or item of subsea equipment, such as a blowout preventer (BOP).

[0083] In the case where the marine element is a chain comprising a plurality of links, measuring the distance characteristic may correspond to measuring the distance between the links of the chain to monitor wear of the links, and / or to monitor or measure axial bending or loads in the chain. The steps of the method may be repeated to measure and monitor the distance characteristic as a function of time.

[0084] In the case where the marine element is a rope, an umbilical cord, or another flexible elongated element, measuring the distance characteristic may correspond to measuring the distance between two locations of the marine element to determine the elongation of the marine element. In the case where the marine element is a rigid elongated seabed element, measuring the distance characteristic may correspond to measuring the distance between two locations of the marine element to determine the axial bending and / or loading of the marine element. The steps of the method may be repeated to measure and monitor the distance characteristic as a function of time.

[0085] In the case where the marine element is a subsea object or subsea appendage such as a subsea wellhead (or component thereof) or a BOP, measuring the distance attribute may correspond to measuring the distance between components of the subsea object or between two subsea objects or appendages. The steps of the method may be repeated to measure and monitor the distance characteristic as a function of time. BRIEF DESCRIPTION OF THE DRAWINGS

[0086] In order that the present invention may be more readily understood, reference will now be made, by way of example, to the accompanying drawings, in which:

[0087] Figure 1 is a schematic detailed side view of a link of a spikeless chain in an unworn condition;

[0088] Figure 2 Corresponding to Figure 1 But the chain is shown in a worn state;

[0089] Figure 3 Corresponding to Figure 1 , but showing one of the links of the chain equipped with a sensor unit of the invention;

[0090] Figure 4 for Figure 3 a top view of the chain and sensor unit shown;

[0091] Figure 5 yes Figure 3 and Figure 4 a schematic block diagram of a sensor unit shown, also showing ultrasonic beams emitted by ultrasonic transducers of the unit towards opposite links of a chain;

[0092] Figure 6 yes Figure 5 Schematic detail side view of a variant in which opposing links are equipped with reflectors facing the transducer;

[0093] Figure 7 is a schematic diagram of a mooring chain equipped with a sensor unit of the present invention, illustrating options for communicating data from the sensor unit to a monitoring station;

[0094] Figure 8 is a timing diagram illustrating the possibility of delays between pulse transmissions of the various transducers in the sensor unit of the present invention;

[0095] Figure 9 Corresponding to Figure 3 , but showing a modified column link chain equipped with a sensor unit of the present invention;

[0096] Figure 10 yes Figure 9 A top view of the chain and sensor unit shown in ;

[0097] Figure 11 Corresponding to Figure 10 , but shows another variant of the sensor unit of the present invention;

[0098] Figure 12 Corresponding to Figure 5 , but showing a variation of the invention in which the arrival time of a direct beam rather than a reflected beam is detected;

[0099] Figure 13 is a schematic detail side view of a variation of the present invention in which a transducer mounted to one link radiates a signal to another link opposite across an interior opening of an intermediate link;

[0100] Figure 14 14a, 14b and 14c are top perspective views showing variations of the sensor unit of the present invention;

[0101] Figure 15 is a top perspective view showing Figure 14 How can the variations of the sensor unit in c be used to monitor Figure 1 the links of the chain;

[0102] Figure 16 16a and 16b are top perspective views each showing an example of a sensor unit of the present invention incorporating a reference reflector;

[0103] Figure 17 is a side view showing another example of a sensor unit of the present invention incorporating a reference reflector;

[0104] Figure 18 A variation of the present invention is shown for monitoring Figure 1 A top perspective view of a load sensor unit in a chain;

[0105] Figure 19 19a, 19b and 19c are side perspective views each showing a variation of the invention in which at least one emitter-reflector pair is used to determine a characteristic of an elongated subsea element; and

[0106] Figure 20 is a side view showing a variation of the invention in which a sensor unit is used to monitor the landing of a blowout preventer on a subsea wellhead. DETAILED DESCRIPTION

[0107] First reference Figure 1 and Figure 2, these figures show three consecutive links of chain 10, namely, outer links 12 and 14 that face each other, and an inner intermediate link 16 that couples outer links 12, 14 to each other. Thus, outer links 12, 14 face each other end-to-end across an inner opening 18 of intermediate link 16. Outer links 12, 14 have the same general orientation as one another, while intermediate link 16 is oriented generally perpendicular to outer links 12, 14. Otherwise, links 12, 14, 16 are all identical to one another.

[0108] Figure 1 The contact interfaces 20 are shown in dashed lines, where the inner end faces of the outer chain links 12, 14 bear against the inner end face of the center chain link 16. Wear of the chain 10 is most likely to occur at these interfaces 20.

[0109] exist Figure 1 , the chain 10 is shown in its initial, unworn state. Thus, the ends of the links 12, 14, 16 all have their original full thickness. This determines the initial longitudinal distance D1 between the opposite ends of the outer links 12, 14.

[0110] Due to wear caused by friction over a period of use, the metal of the links 12, 14, 16 will be eroded at the contact interface 20. As a result, the thickness of the ends of the links 12, 14, 16 will decrease, as shown in FIG. Figure 2 This causes the initial longitudinal distance D1 between the outer links 12, 14 to lengthen to a new, greater distance D2. Embodiments of the present invention are based on determining the increase between D1 and D2 and thereby inferring the degree of wear experienced between the links 12, 14, 16.

[0111] Next, go to the attached Figure 3 and 4 , the middle link 16 of the chain 10 is shown as being equipped with or incorporating a sensor unit 22 of the present invention for determining the longitudinal distance D between the opposing ends of the outer links 12, 14. To this end, the sensor unit 22 is positioned approximately centrally along the length of the middle link 16. In this example, the sensor unit 22 spans the interior opening 18, extending in a direction orthogonal to the length of the chain 10 to bridge the gap between the opposing sides of the middle link 16. In other examples, the sensor unit 22 may be cantilevered from one side of the middle link 16.

[0112] Sensor unit 22 includes a pair of ultrasonic transducers T1 and T2, each centered on a central longitudinal axis 24 of intermediate link 16. In this example, transducers T1 and T2 face in opposite directions aligned with or parallel to central longitudinal axis 24. Thus, the radiator face of transducer T1 faces toward one of outer links 12, and the radiator face of transducer T2 faces toward the other outer link 14.

[0113] Transducers T1, T2 transmit signals 26 comprising ultrasonic pulse trains that are reflected from the opposing, hyperbolic outer end surfaces of respective links 12, 14, disposed within interior opening 18 of intermediate link 16, back to sensor unit 22. Transducers T1, T2 may also receive the reflected signals, thereby acting as transceivers, or sensor unit 22 may support separate receivers disposed alongside transducers T1 and T2. The time of flight of the pulses and the corresponding outward and backward reflections are proportional to the longitudinal distance between sensor unit 22 and the opposing end of each outer link 12, 14, which in turn indicates wear between links 12, 14, 16.

[0114] exist Figure 3 In Figure 2, d1 represents the longitudinal distance between transducer T1 and the opposing outer link 12, and d2 represents the longitudinal distance between transducer T2 and the opposing outer link 14. Conversely, d0 represents the longitudinal distance between the radiator faces of transducers T1 and T2, which approximates the overall thickness of sensor unit 22. The longitudinal distance D between the opposing outer ends of outer links 12, 14 is equal to d1 + d2 + d0. Dimension d0 is constant and known, while d1 and d2 are variable and are measured by sensor unit 22 to determine the degree of wear on links 12, 14, 16.

[0115] Figure 5 There are shown the main components of the sensor unit 22. In addition to the transducers T1 and T2, the sensor unit 22 contains a power supply 28, a processor 30, a data storage 32 and a communication module 34. All of these are sealed or encapsulated within the sensor unit 22 to achieve water tightness.

[0116] The power source 28 may 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 photocell that receives energy instantaneously from an external source such as a visiting ROV.

[0117] Processor 30 generates pulses to drive transducers T1, T2, receives reflected signals via transducers T1, T2 or via separate receivers, and processes these signals with respect to elapsed time to determine the time of flight, and thus d1 and d2, from which D can be derived. Processor 30 outputs resulting data to data storage 32, from which the data can be periodically or continuously transmitted from sensor unit 22 to a remote monitoring station.

[0118] Data is output from the sensor unit 22 via the communication module 34, which can be configured for wired or wireless data transmission by electrical, acoustic, electromagnetic, or optical means. The communication module 34 can also be configured as a repeater to receive and forward data received from other sensor units 22 mounted on other links of the chain 10. Thus, in addition to an output or transmitter, the communication module 34 can also include an input or receiver.

[0119] Figure 5 Also shown are some parameters of one of the transducers, namely, T2. Specifically, DT is the diameter of the radiator face, and θ is the beam angle at which signal 26, comprising a series of ultrasonic pulses, emanates from the radiator face as a beam within a frusto-conical volume. For example, DT may be 10 mm, and θ may be angled 15° to 20° from a line parallel to central longitudinal axis 24. The resulting signal beam 26 is wide enough to impinge on the double-curved outer end surfaces of opposing outer links 14, with its closest point or apex at a distance d2 from transducer T2. However, signal beam 26 is not so wide that it impinges on other structures, such as the sides of intermediate links 16, which would otherwise generate false echoes.

[0120] Figure 6 The possibility of attaching a reflector 36 to the outer end of one or both of the outer links 12, 14 for distance measurement in a pulse-echo arrangement is shown, in which case the link 14 is directly opposite the transducer T2 along the central longitudinal axis 24. For example, the reflector 36 can 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 thus reduces scattering that would occur when the acoustic signal 26 comprising the pulse beam impinges on the hyperbolic outer end face of the link 14.

[0121] Corrosion and marine growth can affect the measurement of d1 and d2, and thus D. In particular, metal loss due to corrosion caused by the ultrasonic pulse signal 26 impinging on the links 12 and 14 can lead to an overestimation of the distance D, while marine growth or deposition at the same location on the links 12 and 14 can lead to an underestimation of the distance D. Advantageously, therefore, the reflector 36 can be made of a corrosion-resistant material, such as a polymer, to minimize the effect of corrosion on the reflected signal. For similar reasons, the reflector 36 can be treated, coated, or impregnated with an anti-fouling compound to resist marine growth or deposition.

[0122] The reflector 36 or similar measures can be applied to any chain link 12, 14 from which the acoustic signal 26 is to be reflected in a pulse-echo arrangement. For example, in principle, the reflection quality can also be improved by treating, coating and / or reshaping or flattening the outer end surface of the chain link 14 without having to attach the reflector 36 thereto.

[0123] Figure 7Various options for transmitting data from the sensor units 22A to 22D mounted on the mooring chain 10 to the monitoring station 38 are shown.

[0124] Sensor unit 22A transmits data directly to monitoring station 38 via wireless transmission, e.g., acoustically. In contrast, sensor unit 22B transmits data indirectly to monitoring station 38 via ROV 40, which periodically visits and interrogates sensor unit 22B. ROV 40 may also momentarily provide power to sensor unit 22B, e.g., via electromagnetic induction or by illuminating a photocell that illuminates sensor unit 22B.

[0125] Sensor unit 22C has a wired connection to monitoring station 38 to transmit data electronically or optically. Conversely, sensor units 22C and 22D illustrate the possibility of relaying data from one sensor unit 22 to another, in this case wirelessly.

[0126] More generally, Figure 7 The relay functionality of sensor units 22C and 22D shown in FIG may involve most or all sensor units 22 on chain 10, providing a system in which all sensor units 22 can, for example, acoustically relay data from other sensor units 22 that are located further along chain 10 relative to monitoring station 38. For example, sensor unit 22A may report to monitoring station 38 via sensor units 22B, 22C, and 22D in sequence.

[0127] Since wear of the chain 10 is a slow process, time-of-flight measurements of the signals 26 from the transducers T1, T2 may be made at widely varying times. However, if faster, simultaneous, or near-simultaneous time-of-flight measurements are required, such as when the sensor unit 22 is interrogated by a visiting ROV 40, steps may be taken to avoid or mitigate interference between the ultrasonic emissions from the transducers T1, T2. Figure 8 One such measure is shown in the timing diagram of FIG, which delays or offsets the pulse train 42 transmitted from one transducer T2 until the pulse train 42 of the other transducer T1 has been transmitted and the reflected signal 44 has been received from the opposite link 12 of the chain 10. In this illustration, the round-trip flight time from the start of the pulse train 42 to the start of the corresponding reflected signal 44 is t1 for transducer T1 and t2 for transducer T2. These time periods t1 and t2 represent dimensions d1 and d2, respectively.

[0128] Next go to Figures 9 to 11, these figures show a variation of the invention in which a sensor unit 22 is adapted for use with a stud link chain 10 in which a central stud 46 spans, bisects, and divides the interior opening 18. In each case, the sensor unit 22 surrounds the stud 46 of the intermediate link 16, but in other examples, the sensor unit 22 may be secured to one side of the stud 46, or indeed to one or both sides of the intermediate link 16, as in the previously described embodiments.

[0129] exist Figure 9 and Figure 10 In the embodiment, the transducers T1, T2 of the sensor unit 22 are kept centered on the central longitudinal axis 24 and face in mutually opposite directions aligned with this axis 24. Thus, the transducers T1, T2 are arranged on mutually opposite sides of the stud 46 of the intermediate link 16. To this end, the transducers T1, T2 are respectively housed in the branches of the sensor unit 22 surrounding or surrounding the stud 46. In contrast, in Figure 11 , the transducers T1, T2 of the sensor unit 22 are offset from the central longitudinal axis 24 to one side of the stud 46. Moreover, the transducers T1, T2 face in mutually opposite directions that converge with the central longitudinal axis 24. By virtue of the tilt 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 produces a reflected signal that can be received by the transducers T1, T2 or by other receivers of the sensor unit 22 to enable calculation of d1, d2 and thus the energy distribution of the outer links 12, 14. Figure 3 D shown. Compensation is required for the fixed transducer beam angle.

[0130] Although the foregoing embodiments consider pulse echo devices, Figure 12 The present invention is illustrated by measuring the time of flight of a signal 26 transmitted directly between a transmitter, such as transducer T2, and a time-synchronized receiver 48. Therefore, no reflections of signal 26 are required during this time. In this example, receiver 48 is affixed to the outer end of outer link 14, opposite transducer T2. For example, receiver 48 can be attached to link 14 using a magnetic clamp.

[0131] exist Figure 12In the illustrated example, receiver 48 communicates with processor 30 of sensor unit 22 via a wired or wireless link 50. Thus, after driving transducer T2 to emit signal 26 comprising a series of pulses, processor 30 measures the time of flight between the emission of the pulses by transducer T2 and their receipt by receiver 48, thereby determining the distance d3 between the emitting face of transducer T2 and the opposing face of receiver 48. Because the spacing between the surface of receiver 48 and the lower, outer end surface of link 14 is known, distance d2 between the emitting face of transducer T2 and the outer end surface of link 14 can be inferred by determining d3. Alternatively, wear affecting link 14 can be directly inferred from the increase in d3 over time, since d4 is a constant.

[0132] Compared with pulse echo systems, Figure 12 The direct transmission system exemplified in [1] presents some challenges, such as the need to install an additional receiver transducer and potentially additional cables and fixing clamps. However, even though installation is more cumbersome, a direct transmission system can still be retrofitted to the chain. Conversely, because it does not rely on reflections, a direct transmission system can offer advantages over pulse-echo systems, such as more reliable transmission of the acoustic signal and the potential need for a less sensitive receiver. Because the surface roughness of the reflecting surface has no effect on the acoustic signal, there is less sensitivity to corrosion and, more generally, no sensitivity to the orientation or curvature of the reflecting surface. Marine growth can be managed by using anti-fouling materials in the transducers used as transmitters or receivers.

[0133] exist Figure 12 In the example shown, transducer T2 and receiver 48 face each other along central longitudinal axis 42. However, because signal 26 is not reflected in this embodiment, greater design freedom is afforded. Specifically, signal 26 need not be constrained to travel only within interior opening 18 of intermediate link 16 in order to strike one of outer links 12, 14 connected by intermediate link 16. Instead, signal 26 can travel longitudinally along chain 10, along a path sufficiently offset from central longitudinal axis 24 to bypass the links. Thus, signal 26 can propagate between a transmitter and a time-synchronized receiver on a chain 10 longitudinally separated by two or more intermediate links 16. This allows the chain monitoring system of the present invention to determine the average wear across a continuous group, set, or series of several links of chain 10.

[0134] Many other variations are possible within the present invention. For example, it is possible to use only one ultrasonic transducer in fixed relation to a first link, with the transducer facing the outer end surface of a second, adjacent link that is interengaged with the first link. This variation is only able to measure the effects of wear at one contact interface between the two links, but still effectively indicates the condition of the chain around that location. For example, such a variation could be constructed by removing sensor T2 from sensor unit 22 of the aforementioned embodiment and measuring only the distance d1 between sensor T1 and the facing end of the opposing link 12. This would detect degradation of links 12, 16, as distance di would increase with wear at contact interface 20 between links 12, 16.

[0135] Similarly, if Figure 13 As shown, it is possible to mount sensor unit 22 including transducer T1 to the outer end of first link 12 rather than intermediate link 16. In this arrangement, signal 26 can be emitted from transducer T1 along central longitudinal axis 14 within interior opening 18 of intermediate link 16 to be reflected from the opposite outer end surface of second link 14.

[0136] Figure 7 The monitoring stations shown in FIG can be replaced by relay stations such as buoys. The relay stations can in turn transmit data to a remote monitoring station, which can be on a ship, an offshore facility, or on land.

[0137] The transducers used in the present invention, whether transmitters or receivers, may have active surfaces made of, treated with, coated with or impregnated with corrosion-resistant and / or anti-fouling materials or compounds. Figure 6 In addition to the reflectors shown, the effects of corrosion and marine growth in pulse-echo systems can be mitigated if the ultrasonic beam profile is wide enough to produce a reflection from an area of ​​the link so that the reflected signal represents an average of echoes from many smaller spots, points, or sub-areas on the link.

[0138] In contrast, if the system operates with a very narrow beam, for example illuminating only about 10 mm 2 On the outer end faces of the chain links, small localized grooves in the steel caused by corrosion or small formations caused by marine growth (e.g. small mussels or barnacles) can lead to significant errors in the measurement.

[0139] The pulse-echo embodiment of the present invention relies on a certain reflectivity of the chain link surface, upon which the ultrasonic beam signal impinges when emitted from the transducer. When the ultrasonic beam is directed at the outer end surface of the connector, only a small area at the vertex of this hyperboloid is substantially perpendicular to the incident wavefront and, therefore, oriented to reflect the signal back to the transducer along and around the central longitudinal axis. In principle, all peripheral areas on the outer end surface of the chain link will reflect the signal at such an angle to the central longitudinal axis that the signal will not necessarily impinge on the transducer. This is particularly true if the surface roughness of the reflecting surface is fine compared to the wavelength, a phenomenon known as the mirror effect. However, if the surface roughness is coarse compared to the wavelength, each small surface area will reflect the incident signal in many different directions, thus producing "diffuse scattering." Since rust and other corrosion conditions can lead to rough surfaces, diffuse scattering can be achieved by selecting an appropriate ultrasonic frequency. Combined with a relatively wide beam width, this can help reduce inaccuracies caused by corrosion.

[0140] Other measures can be taken to avoid or mitigate interference between ultrasonic emissions from the transducers and the corresponding reflected signals. For example, ultrasonic signals can be emitted from the transducers at different phases or frequencies to help distinguish their reflected signals. Filtering performed in the sensor unit's processor or downstream at a monitoring station can further mitigate such interference that may occur between the transducers' reflected signals.

[0141] Figure 14 Various exemplary embodiments are contemplated for the sensor unit 60, which includes a Figure 5 2. Although not shown in detail, the same main components are present in both sensor units, namely, at least one transducer (T1 and / or T2); and key electronic components such as power supply, processor, data storage and communication module. These electronic components can be implemented in the form of a PCBA. Figure 5 As with the illustrated sensor unit 22, all components are sealed or encapsulated within the sensor unit 60 to achieve water tightness. Therefore, subsequent references to the sensor unit 60 should be considered applicable to the previously described sensor unit 22.

[0142] exist Figure 14 In (a), the sensor unit 60 includes a housing 62 and a single transducer T1 or T2. All components of the sensor unit 60 are sealed within the housing 62. In contrast, in Figure 14 In (b), two transducers T1 and T2 are disposed within a housing 62 in an anti-parallel configuration relative to one another or oriented opposite one another around the housing, with their signal beams radiating in opposite directions to one another.

[0143] It is also contemplated that the sensor unit 60 may comprise two or more sensor (housing) modules or sub-units, wherein the components of the sensor unit are divided or distributed between the modules. Figure 14 (c) shows a specific example in which a first module 62a contains transducers T1 and T2, and a second module 62b contains the remaining electronic components. An arm 62c extending between the two modules 62a, 62b connects them together and contains electrical and communication connections between transducers T1, T2 and the remaining electronic components. This arrangement—whereby the transducers are offset or separated from the other electronic components—advantageously enables the size of the first module 62a containing the transducers to be reduced. As a result, transducers T1, T2 can be used to measure distances in a smaller space than would be possible using a sensor unit comprising only a single, larger housing 62.

[0144] Figure 15 This benefit is illustrated in the specific case of using sensor unit 60 to monitor links 12, 14 of chain 10. Module 62a containing transducer pair T1, T2 can be inserted into interior opening 18 of intermediate link 16, while module 62b containing the remaining electronic components remains adjacent to chain 10, laterally offset from its longitudinal axis.

[0145] As previously mentioned, in their most general sense, the pulse-echo embodiments of the present invention operate by calculating the distance D traveled in the water by the signal pulses transmitted and received by the sensor units 22, 60. w to estimate the distance to the target. This calculation involves substituting the speed of sound in water, C w Multiply by the time T that the signal pulse takes to travel to and from the target in water w .

[0146] The time of flight measured for any given signal will correspond to the time of flight through the thin transducer cover (typically made of PEEK or polyetheretherketone), through the water column to the target, and back along the same beam path. The time of flight takes into account the total flight time T through the cover and water. PW , but the flight time through the protective layer is T P is not negligible and will need to be compensated for in the subsequent distance calculations performed by the processor in the sensor unit 60. This is done by the following formula:

[0147] D w =C w / 2*(T Pw -2*D p / C p ),

[0148] Among them C p is the speed of sound in the protective layer, and D Pis the thickness of the protective layer. This can also be expressed as:

[0149] D w =Gain*T PW +Offset,

[0150] Among them C w / 2 = gain and -(C w / C P *D P ) = offset.

[0151] During the manufacturing process of the sensor, the protective layer C will be known with a fairly high accuracy. P The speed of sound and the protective layer D P This means that the offset value will be known and can be programmed into the processor of the sensor unit 22, 60. P and C P Small tolerance variations in both are to be expected, especially (in C P These changes may affect the offset value, but can be compensated and calibrated, for example, by temperature measurements in the sensor units 22, 60.

[0152] To calculate D w The key parameter in the formula is the speed of sound in water, C w However, this parameter varies depending on temperature, salinity, and water depth. Of these variables, depth and salinity are unlikely to change significantly for any given sensor unit implemented in a semi-permanent location, for example, to monitor wear on marine mooring chains. However, if the sensor unit is implemented at one time in a freshwater environment and at a different time in a seawater environment, then changes in salinity in this case need to be accounted for once the sensor unit is moved. In addition, C w The sensitivity to temperature is small but not negligible: in offshore activities, the water temperature may vary by several degrees. This needs to be taken into account to avoid possible measurement errors.

[0153] The salinity and temperature of the water surrounding the sensor unit 60 can be measured and then the response to C can be estimated using known textbook formulas. w Alternatively, in the present invention, it is assumed that the C w The variation of can be directly compensated by calibration of the sensor unit 60 using reference measurements. To obtain a reference measurement, a reference target is placed at a known (or constant) distance D Ref The pulses from the sensor unit 60 and from the transducer T1 or T2 in the sensor unit 60 are directed to the reference target to obtain an indication of the time of flight T RefThe reference signal is between the transducer and the reference target. The (reference) speed of sound in water, C, can then be derived from the reference signal using the following formula w, Ref:

[0154] C w,Ref =2*(D Ref / T Ref )

[0155] The speed of sound in water C derived in this way is w,Ref It can be used to calibrate the processor in the sensor unit 60: when determining the distance to the primary target (eg, a link of the chain 10 in the above embodiment), the value C is derived. w,Ref Enter into the corresponding formula.

[0156] The reference target can take one of several different forms. In its simplest form, the reference target can correspond to a receiver (which can correspond to an acoustic transducer such as those shown and described above) positioned at a known or constant distance from the transmitter. In this case, the reference signal will correspond to the signal transmitted by the transmitter.

[0157] Optionally, and as Figure 16 and 17 As exemplified by the arrangement shown in , the reference target can take the form of one or more reference reflectors: components made of reflective material and placed in the path of the signal beam transmitted by the transmitter (in the arrangement shown, one of the transducers T1 or T2). The reference reflector will reflect at least some of the transmitted signal incident on it; the reflected signal is returned to the receiver (in the arrangement shown, also transducer T1 or T2) to form the reference signal. The reference reflectors used can be arranged in different ways.

[0158] In one configuration, the reference reflector is positioned in the path of the primary beam, i.e., the signal beam directed toward the primary target (e.g., in the embodiment described above, a link of chain 10), to obtain the desired distance measurement output. Thus, the reference reflector in this case may also be referred to as a "primary beam reference reflector." In this configuration, the reference reflector is arranged so that it reflects only a portion of the incident signal back to the transducer as a reference "echo" signal (this may also be referred to as a "partial reflection"). The remaining portion of the incident signal that is not reflected by the reference reflector is able to reach the primary target.

[0159] The main beam reference reflector should be large or wide enough to produce a detectable reference echo from the main beam, which is also directed toward the primary target, but narrow enough so that the signal reflected from the primary target remains useful. Thus, the same transducer can be used to measure both the reference range and the target range. This is beneficial because it enables the reference measurement to be obtained without the need to implement any additional electronics, circuitry, or any additional transducers. All that is required is some additional programming instructions for the processor to separate the two signals.

[0160] Thus, the main beam reference reflector takes the form of a relatively small or narrow structure that is inserted into the "main beam," which is the beam path of the signal transmitted by the transducers T1, T2 toward the main target. Figure 16 A thin wire 66 is shown in the single-transducer embodiment of FIG. Alternatively, a narrow disk (not shown) can be suspended in the beam path. A variation of this embodiment can involve extending the housing of module 62a containing transducer T1 to also enclose the reference reflector, thereby avoiding potential deformation or displacement of the reference reflector during operation.

[0161] In another configuration, the reference reflector is positioned in the path of a "secondary beam," i.e., a signal beam emitted by the sensor unit's transmitter but directed away from the primary target. Thus, in this case, the reference reflector may also be referred to as a "secondary beam reference reflector." In this configuration, the reference reflector may reflect at least a portion of the incident signal back to the transducer as a reference "echo" signal, but typically reflects most or substantially all of the incident signal back to the transducer as a reference signal (the latter case may also be referred to as "total reflection").

[0162] In an arrangement using an auxiliary beam reference reflector, at least two transducers will be used. One "reference" or "auxiliary" transducer will provide the ability to determine the reference speed of sound in water, C, by transmitting an "auxiliary" or "reference" beam towards the reference reflector and receiving a reference "echo" signal. w,Ref The primary function of the primary transducer (using the formula described previously) is to transmit a primary beam toward the primary target to measure the distance to the primary target. In these arrangements, a more robust measurement of the distance to the primary target can be obtained because a greater proportion of the transmitted signal directed toward the primary target is returned to the transducer.

[0163] The auxiliary beam reference reflector can take various forms, such as Figure 16 b and 17. Figure 16In the two-transducer embodiment of b, one transducer T1 corresponds to the main transducer and transmits a main beam signal toward the main target. The other transducer T2 corresponds to the auxiliary transducer and transmits an auxiliary beam signal to a reference reflector that is positioned at a known or constant position relative to the sensor unit 60. In this case, the reference reflector takes the form of a reflective plate 68. Figure 16 In the example shown in FIG. 6b , the reflective plate 68 is arranged so that a portion of the auxiliary beam can pass through the plate (e.g., through a hole in its center); therefore, the reference signal in this particular example corresponds to only a partial reflection of the incident signal. This means that the auxiliary beam can also be used as a "secondary" primary beam—it can be directed toward another "primary" target at a greater distance and used to calculate another "primary" distance measurement. Alternatively, it is contemplated that the reference reflector can reflect substantially all of the auxiliary beam signal to form the reference signal.

[0164] exist Figure 17 In the example of , three transducers T1, T2, T3 are used. One transducer T3 emits an auxiliary or reference beam towards a reference reflector and thus corresponds to an auxiliary transducer. The other two transducers T1, T2 emit a main beam towards two different main targets and thus correspond to main transducers. This configuration is particularly advantageous when monitoring the wear of a chain (as in the embodiment described above) because it allows the distance to two different links 12, 14 of the chain to be measured using the main transducers T1, T2. In this arrangement, the auxiliary beam emitted by the auxiliary transducer T3 is emitted towards the second sensor module 62b containing the electronics and a part 70 of the housing of this sensor module 62b serves as a reference reflector. Although it is possible to use a second sensor module such as Figure 16 b, but using a portion 70 of the housing of module 62b as the reference reflector utilizes existing components, thereby minimizing the number of components that need to be used. In addition, the distance between the two modules is known or predefined (and can be substantially fixed) because it is defined by the length of arm 62c.

[0165] The use of a spherical concave reflector can be particularly beneficial when using an auxiliary beam reference reflector and reflection of substantially the entire beam incident on the reference target occurs. The beam emitted by the transducer will propagate in the water as a spherically expanding pressure wave. When using a spherical concave reference reflector (with a radius of curvature equal to the distance from the transducer), the entire wavefront will simultaneously strike the receiving surface of the reference reflector, and contributions from all points on the reflector surface will be returned to the transducer in phase. This maximizes the reflected signal strength and minimizes the duration of the echo, resulting in a sharp and clear echo. An improved, stronger signal is thus obtained.

[0166] Other properties of the reference reflector—such as acoustic impedance and surface roughness—are also factors to consider when selecting an appropriate reference reflector. A reflector made of a material with an acoustic impedance significantly different from that of water will result in strong reflections. Similarly, a reflector with surface irregularities will achieve diffuse reflections, thereby reflecting increased energy back to the transducer. Therefore, some particularly suitable materials for use as a reference reflector can include a thin metal wire with a diameter of several millimeters (for example, made of stainless steel or other corrosion-resistant materials), or a small, highly polished concave disk that is equal to or smaller than the width of the ultrasonic beam emitted by the transducer.

[0167] It will be appreciated that the general calibration system and method described above is also applicable to a wide range of embodiments where reflections of ultrasonic beam pulses are used to measure distance and where accounting for variations in the speed of sound in water would be beneficial.

[0168] As described above, the calibration method is useful when measuring the distances between or to monitor wear on mooring chain links. It is also envisioned that the calibration method could be used when measuring the distances between or to monitor marine chain links for other purposes. One such example involves measuring the distances between or to monitor axial strain in a mooring chain, and thus indirectly monitoring the load in the mooring chain.

[0169] in this regard, Figure 18 One possible embodiment is shown in which a sensor unit 60 including at least one ultrasonic transducer (not shown) is clamped onto, adjacent to, or otherwise associated with one link 12 of the chain 10. A reflector 72 is similarly associated with another link 14 of the chain 10, such that an uninterrupted beam path B exists between the sensor unit 60 and the reflector 72. This beam path B is parallel to and laterally offset from a central longitudinal axis 74 extending through the chain 10. The reflector 72 reflects the acoustic signal beam emitted by the transducer. The time of flight of a pulse in the acoustic signal beam to and from the transducer is proportional to the longitudinal distance between the transducer and the reflector 72.

[0170] Although not shown, reference reflectors 66, 68, 70 corresponding to any of the above examples can be incorporated into or used with the sensor unit 60, substantially as described in any of the above examples. The reference reflectors 66, 68, 70 can be used to obtain a measurement of the speed of sound in the surrounding water, and this value can be used to calibrate the processor of the sensor unit 60 during or before measuring the distance between the sensor unit 60 and the reflector 72.

[0171] Figure 19Another possible use case for the calibration method described above is shown in which a pulse-echo distance measurement method is used to derive a time-varying characteristic of an elongated subsea element 80. The elongated element 80 may take the form of a relatively rigid element, such as a structural member of a drilling rig, or a riser or pipeline for transporting hydrocarbons, or a relatively flexible element, such as a cable, rope, or umbilical.

[0172] A pair of clamps 82a, 82b are attached to the elongated member 80 at respective locations spaced apart from one another along a central longitudinal axis 84 of the elongated member 80. The elongated member 80 is schematically represented as having a substantially circular cross-section and therefore being substantially cylindrical when straight. In these embodiments, one clamp 82a, shown here on the left, is a reflector clamp and the other clamp 82b, shown on the right, is a transmitter / receiver clamp. The clamps have axially inwardly facing surfaces 86a, 86b that oppose each other. On its inwardly facing surface 86b, the transmitter / receiver clamp 82b supports one or more signal transmitters, which are ultrasonic transducers that direct one or more acoustic signal beams 88 toward at least one reflector 90, 92 mounted on the opposing inwardly facing surface 86a of the reflector clamp 82a, as shown. Figure 19 As described above, the time of flight of a pulse in the acoustic signal beam to and from the transducer is proportional to the longitudinal distance between the transducer and the reflector 90,92.

[0173] like Figure 19 As shown in Figure 1, a simple one-dimensional distance measurement can be obtained if only a single discrete transducer-reflector pair is used. This is suitable for measuring the elongation of an elongate element 80 over time. In this case, additional reference reflectors 66, 68, 70 may be incorporated for the purpose of calibrating the measurement. This may take the form of a main beam reference reflector - for example Figure 16 The thin wire 66 shown in Figure 8a is inserted into the main beam path of the signal transmitted from the transducer towards the main target (reflector 90). This arrangement will allow the beam transmitted from a single transducer to also be used for calibration purposes. Alternatively, an additional "auxiliary" transducer can be incorporated into the transmitter / receiver fixture 82b and calibration can be performed using an auxiliary beam reference reflector. This auxiliary beam reference reflector can correspond to Figure 16 b shows a plate 68 or a portion 70 of the housing of the sensor module 62a. Alternatively, a portion of the reflector fixture 82a may be used as a reference reflector if the material in question is suitably reflective.

[0174] like Figure 19 As shown in b, it is envisaged that multiple discrete transducer-reflector pairs can be used. Figure 19As noted above, additional reference reflectors 66, 68, 70 may be incorporated for the purpose of calibrating the measurement. Where all transducers are used to obtain the primary distance measurement, the reference reflector may take the form of a primary beam reference reflector - e.g. Figure 16 The thin line 66 shown in a is inserted into the main beam path of the signal emitted from one of the transducers towards the main target (the corresponding reflector 90). Optionally, one of the transducers can be used as an auxiliary transducer and an auxiliary beam reference reflector can be incorporated for calibration purposes. The auxiliary beam reference reflector can correspond to Figure 16 b. The plate 68 or portion 70 of the sensor unit housing 62a is shown. The remaining transducers will still be used to transmit signals to the primary target and obtain a distance measurement to the primary target. Ideally, at least three transducer-reflector pairs will be used to provide an accurate primary distance measurement.

[0175] Using multiple transducer-reflector pairs to derive the primary distance measurement means that changes in elongation along additional dimensions (rather than just along the central longitudinal axis of the elongated element) can be detected. This provides an indication of other characteristics of the elongated element. For example, if the distance measurements obtained by each transducer-reflector pair at any given time are the same (within measurement error), this indicates that the elongated element has been subjected to a pure axial load (i.e., no bending or minimal bending). If the distance measurements obtained by each of the transducer-reflector pairs are significantly different, but the average measurement of all sensors remains constant, this indicates that the elongated element has been subjected to a pure bending load. If the distance measurements obtained by each transducer-reflector pair and the average measurement of all pairs are significantly different, this indicates that the elongated element has been subjected to a combined bending and axial load.

[0176] In some cases, it is envisaged that one transducer-reflector pair may be used for calibration purposes, ie, one of the "primary" reflectors may instead be reused as a reference reflector, while the remaining transducer-reflector pair may be used to calculate the primary distance measurement.

[0177] Figure 19 An alternative configuration for measuring axial and bending loads on elongated element 80 is shown in Figure c. A single, substantially continuous reflector 92 is provided on reflector fixture 82a, for example in the form of a ring, donut, or annular reflector. This continuous reflector 92 still enables the acquisition of multiple echoes, one for each transducer used, but avoids the need for the more precise alignment required when using discrete pairs of transducers and reflectors. Furthermore, regardless of the degree of bending and twisting of elongated element 80, the emitted pulsed beam will still be reflected by reflector 92. Furthermore, the continuous reflector implementation avoids the need to correct for any other properties of the reflector when processing the primary distance measurement.

[0178] As about Figure 19 As noted in Figure 2, additional reference reflectors 66, 68, and 70 may be incorporated for the purpose of calibration measurements. If all transducers are used for "primary" target distance measurements, this reference reflector may take the form of a main beam reference reflector. Alternatively, if one of the transducers is primarily used for calibration purposes, the reference reflector may take the form of a secondary beam reference reflector. Alternatively, a portion of the continuous reflector 92 itself may be reused as a reference reflector.

[0179] It is also conceivable that the above calibration method can be applied to distance measurement in other use cases. Figure 20 As illustrated in FIG, these methods can be used when a BOP (Blowout Preventer) 100 is lowered onto a subsea wellhead 102. In this case, the sensor unit 60 can be inserted between the two components 100, 102, as shown. Alternatively, subcomponents of the sensor unit 60 can be mounted on each of the BOP 100 and the subsea wellhead 102: for example, a transmitter or transducer can be mounted on the BOP 100 and a receiver or reflector can be mounted on the subsea wellhead 102, or vice versa. In this case, one or more reference reflectors 66, 68, 70 can be connected to either the BOP 100 or the subsea wellhead 102, where the transducer is mounted, and calibration can be achieved using any of the methods described above.

[0180] Alternatively (and not shown), the sensor unit 60 can be used to monitor cyclic expansion of the subsea wellhead itself (i.e., between the substructures of the subsea wellhead). In this case, the transmitter or transducer would be attached to one of the substructures (e.g., by a magnetic clamp or other suitable attachment means), and the receiver or reflector could be mounted to the other substructure; or vice versa. In this case, one or more reference reflectors 66, 68, 70 could be attached to any substructure to which the transducer is mounted, and calibration could be achieved by any of the methods described above.

Claims

1. A system for monitoring the condition of a mooring chain, the system comprising: at least one transmitter in fixed relation to a first link of the mooring chain, the transmitter configured to transmit a signal; at least one receiver on any link of the chain, configured to receive the signal; as well as A processor is configured to measure a time lapse between transmitting and receiving the signal to determine a distance between the transmitter and a link of the chain other than the first link.

2. The system of claim 1, wherein the transmitter and the receiver are on a first link of the chain.

3. The system of claim 2, wherein an acoustic transducer is used as both the transmitter and the receiver.

4. A system according to claim 2 or claim 3, wherein the transmitter and the receiver are each opposite an intermediate reflective surface on a second link of the chain.

5. The system of claim 4, wherein the reflective surface is defined by a reflector secured to a second link of the chain.

6. A system according to claim 4 or claim 5, wherein the transmitter is configured to transmit the signal towards an opposite reflective outer end surface of a second link of the chain, the surface of the second link being disposed within the interior opening of the first link.

7. A system according to claim 4 or claim 5, wherein the transmitter is configured to transmit the signal toward an opposite reflective outer end surface of a second link of the chain, the surface of the second link being disposed within an interior opening of an intermediate link that couples the first link to the second link.

8. The system of claim 7, wherein the transmitter is further disposed within an interior opening of the intermediate link.

9. System according to any of the preceding claims, wherein the transmitter and the receiver are embodied together in a sensor unit, which is attached or attachable to a first link of the chain.

10. The system of claim 9, wherein the sensor unit spans the interior opening of the first link from one side to the other of the longitudinal extension.

11. The system of claim 1 , wherein the receiver is in fixed relationship with the second link of the chain.

12. The system of claim 11, wherein the receiver is disposed within an interior opening of the first link.

13. The system of claim 11, wherein the receiver is disposed within an interior opening of an intermediate link coupling the first link to the second link.

14. The system of claim 13, wherein the transmitter is further disposed within an interior opening of the intermediate link.

15. The system of any one of claims 10 to 14, wherein the processor is in data communication with the transmitter and the receiver via a connection extending between the first link and the second link.

16. A system according to any preceding claim, wherein the transmitter is attached or attachable to the first link at a central longitudinal position.

17. The system of claim 16, wherein the transmitter is attached or attachable to at least one longitudinally extending side of the first link.

18. The system of any preceding claim, wherein the transmitter and central longitudinal axes of the first link are aligned.

19. The system of claim 18, wherein the transmitter is oriented to transmit the signal in a direction substantially parallel to a central longitudinal axis of the first link.

20. The system of any preceding claim, wherein the transmitter is disposed within an interior opening of the first link.

21. The system of any one of claims 1 to 17, wherein the transmitter is laterally offset from the plane of the first link.

22. The system of claim 21, wherein the transmitter is oriented to transmit the signal in a direction converging with a central longitudinal axis of the first link.

23. A system according to any preceding claim, comprising first and second emitters in mutually opposing orientations.

24. The system of claim 23, wherein the first and second emitters are in fixed relation to the first link, the first link being an intermediate link disposed between and connected to a pair of outer links, each of the pair of outer links having a reflective outer end surface that is opposite a corresponding one of the emitters and disposed within an interior opening of the first link.

25. The system of any one of the preceding claims, wherein at least the first link is a stud link comprising a stud that defines an interior opening.

26. The system of claim 25, wherein the transmitter is mounted on the stud.

27. A system according to claim 26, when dependent on claim 23, wherein the first and second transmitters are provided on respective mutually opposing sides of the stud.

28. The system of any preceding claim, wherein the transmitter is provided on an outer end face of the first link.

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

30. The system of claim 29, further comprising a data storage device for storing the distance data.

31. The system according to claim 29 or 30, further comprising a communication module, wherein the communication module is configured to transmit the distance data to a monitoring station or a relay station.

32. A system according to any preceding claim, wherein the or each transmitter is configured to transmit the signal as a beam having a beam angle of up to 45°.

33. A system according to any preceding claim, wherein the or each transmitter is configured to transmit the signal at a pulse frequency of 0.1 to 5 MHz.

34. The system of any of the preceding claims, further comprising a reference reflector configured to reflect at least a portion of the signal transmitted by the transmitter to the receiver as a reference signal for use by the processor in calibration of the determined distance.

35. The system of any one of claims 1 to 33, further comprising a reference reflector and a reference transmitter, wherein the reference transmitter is configured to transmit an auxiliary signal, and the reference reflector is configured to reflect at least a portion of the auxiliary signal to the receiver as a reference signal for use by the processor in calibration of the determined distance.

36. The system of any one of claims 1 to 33, further comprising a reference receiver and a reference transmitter, wherein the reference transmitter is configured to transmit an auxiliary signal and the reference receiver is configured to receive the auxiliary signal as a reference signal for use by the processor in calibration of the determined distance.

37. The system of any one of claims 34 to 36, wherein the processor is configured to: measuring a reference time lapse occurring prior to receipt of the reference signal; and The reference time lapse is used to determine a reference speed of sound in water.

38. The system of claim 34, comprising a first sensor module and a second sensor module in communication with each other, wherein the transmitter and the receiver are implemented together in the first sensor module, and the processor is implemented in the second sensor module.

39. The system of claim 35, comprising a first sensor module and a second sensor module in communication with each other, wherein the transmitter, the receiver, and the reference transmitter are implemented together in the first sensor module, and the processor is implemented in the second sensor module.

40. The system of claim 39, wherein the reference reflector corresponds to a portion of a housing of the second sensor module.

41. The system of any one of claims 38 to 40, wherein the first sensor module is configured to be inserted or insertable into an interior opening of the first link.

42. A method for monitoring the condition of a mooring chain, the method comprising: transmitting a signal from a transmitting position fixed relative to a first link of the chain; receiving the signal at a receiving location on any link of the chain; as well as The time lapse between transmitting and receiving the signal is measured to determine the distance between the transmitting location and a link of the chain other than the first link.

43. The method of claim 42, comprising reflecting the signal from a second link of the chain to the receiving location.

44. The method of claim 43, comprising reflecting the signal from a location within an interior opening of the first link.

45. The method of claim 43, comprising reflecting the signal from a location within an internal opening of an intermediate link coupling the first link to the second link.

46. ​​The method of claim 45, comprising transmitting the signal within an interior opening of the intermediate link.

47. The method of claim 42, wherein the receiving location is fixed relative to the second link of the chain.

48. The method of claim 47, comprising receiving the signal within an interior opening of the first link.

49. The method of claim 47, comprising receiving the signal within an interior opening of an intermediate link coupling the first link to the second link.

50. The method of claim 49, comprising transmitting the signal within an interior opening of the intermediate link.

51. A method according to any one of claims 42 to 50, comprising transmitting the signal along a central longitudinal axis of the first link.

52. A method according to any one of claims 42 to 50, comprising transmitting the signal in a direction converging with a central longitudinal axis of the first link.

53. A method according to any one of claims 42 to 52, comprising transmitting the signal from within an interior opening of the first link.

54. A method according to any one of claims 42 to 52, comprising transmitting the signal from an outer end face of the first link.

55. A method according to any one of claims 42 to 54, comprising transmitting signals in mutually opposite directions from first and second transmitting positions fixed relative to the first link.

56. A method according to claim 55, comprising transmitting the signals at different times, at different phases or at different frequencies.

57. A method according to any one of claims 42 to 56, comprising generating data representing the distance at the first link, and transmitting the distance data from the first link to a receiving station remote from the first link.

58. The method of claim 57, comprising relaying the distance data to the receiving station along the chain.

59. The method of claim 57, comprising transmitting the distance data to the receiving station via an underwater vehicle located alongside the first link.

60. A method according to any one of claims 42 to 59, comprising reflecting at least a portion of the signal transmitted by the transmitter to the receiver as a reference signal for use by the processor in calibration of the determined distance.

61. A method according to any one of claims 42 to 59, comprising transmitting an auxiliary signal and reflecting at least a portion of the auxiliary signal to the receiver as a reference signal for use by the processor in calibration of the determined distance.

62. A method according to any one of claims 42 to 59, comprising transmitting an auxiliary signal, and receiving at least part of the auxiliary signal by a reference receiver as a reference signal for use by the processor in calibration of the determined distance.

63. The method according to any one of claims 60 to 62, further comprising: measuring a reference time lapse occurring prior to receiving the reference signal; as well as The reference time lapse is used to determine a reference speed of sound in water.

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