Marine chain condition monitoring

Abstract

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

Description

[Technical Field]

[0001] The present invention relates to monitoring the condition of chains, and in particular to monitoring the deterioration of chains used in marine applications, such as mooring chains that act as mooring lines or form sections of mooring lines. [Background technology]

[0002] Among other applications, mooring chains are used to secure floating offshore installations in the offshore oil and gas industry and the offshore renewable energy industry. Specific examples of offshore energy facilities secured 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 operational life, successive articulated links of the chain rotate and slide past one another. As such, the chain is prone to wear where there is repeated frictional contact between successive links, which can be accelerated by the effects of corrosion. Over time, deterioration and thinning of the links can reduce the load-bearing capacity of the chain and ultimately cause the chain to break.

[0004] Although floating offshore installations use multiple mooring chains for redundancy, a mooring chain break is unacceptable as it reduces the safety margin and increases stress on the remaining mooring chain. Effective monitoring of the degree of wear would allow preventative replacement of worn mooring chains before breakage occurs.

[0005] The phenomenon of chain degradation is well known and typical chain wear rates are well understood in the art. For example, a thorough discussion of chain degradation is provided in Research Report No. RR1098, published by the UK Health and Safety Executive (HSE), entitled "Degradation of mooring chains of floating offshore installations: chain measurement, estimation of wear, corrosion rates, and their effect on break load" (available at https: / / www.hse.gov.uk / research / rrpdf / rr1098.pdf).

[0006] Actual chain wear rates may be higher or lower than predicted because they depend on variables such as tolerances, material quality, link position along the chain, etc. Therefore, wear may not be uniform along the length of the chain, which may result in some links being closer to failure than others or closer to failure than predicted.

[0007] Traditionally, the risk of premature mooring chain failure has been mitigated by over-designing the links, but this comes at the expense of increased costs, particularly the weight of the chain. Long chains used in deep waters or large chains for large floating structures can become so heavy that they are difficult to handle, with reduced capacity due to tension caused by their own weight.

[0008] Where the size of the links in a mooring chain must be limited to reduce weight, it is common for the links to be inspected periodically and monitored intermittently or continuously.

[0009] In a comprehensive approach to chain monitoring, the overall geometry of the chain may be checked as a whole. For example, U.S. Patent No. 10,780,954 teaches a method for recording images of the chain, and WO 2020 / 164760 teaches a method for attaching optical fibers to the chain to monitor its geometry and joints. More generally, comprehensive chain monitoring can be performed continuously, for example, by attaching acoustic transducers to critical links, or intermittently, as in the RAMS system offered by Tritech International Limited (trademark approved). The RAMS system monitors the integrity of mooring lines from sonar heads deployed under the moored vessel, eliminating the need for additional sensors on the mooring lines. However, a comprehensive approach cannot detect potential breaks in individual links of the chain.

[0010] Chain link inspection typically relies on visual inspection, caliper measurements, or non-destructive testing (e.g., ultrasonic testing to detect cracks as disclosed in EP 2507583 or WO 2015 / 030600). Optical measurements or 3D photogrammetry may also be used to measure critical chain dimensions. However, non-destructive testing and measuring individual links is difficult, time-consuming, and costly, especially when performed at sea or underwater.

[0011] Chain link monitoring can rely on continuous mechanical measurement of link deformation, typically through the use of strain gauges or by measuring load using load cells as exemplified in U.S. Patent No. 10,078,025. Dimensional or stress deviations can indicate abnormal wear or fatigue. Similarly, U.S. Patent Application Publication No. 2013 / 279298 embeds strain gauges in shrouds connected to the chain links, acoustically transmitting signals representative of strain.

[0012] Retrofitting chains with sensor clamps is known in the art and is described, for example, in CN104330102, CN109029527, and CN110081921. The sensors proposed in these documents are also strain gauges. However, given the large dimensions of the chain links used to moor offshore energy facilities such as FPSOs, FOWTs, or WECs, especially in deep water, the standard approach of monitoring mooring chains using strain gauges has limitations.

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

[0014] US Patent Application Publication No. 2022 / 0003636 and WO 2009 / 044117 describe monitoring chains used in power transmission and motion control (e.g., in lifting or conveying applications). Summary of the Invention [Means for solving the problem]

[0015] It is against this background that the present invention has been devised. In one aspect, the present invention resides in a system for monitoring the condition of a chain, the system comprising: at least one transmitter in fixed relation to a first link of the chain, the transmitter configured to emit a signal; at least one receiver on any link of the chain configured to receive the signal; and a processor configured to measure the time lapse between emission and receipt of the signal to determine the distance between the transmitter and a link of the chain other than the first link.

[0016] Both the transmitter and receiver may be present on the first link of the chain, in which case an acoustic transducer may function as both the transmitter and receiver. The transmitter and receiver may each face an intermediate reflective surface on the second link of the chain. The reflective surface may be defined by a reflector fixed to the second link of the chain.

[0017] The transmitter may be configured to emit a signal toward an opposing reflective outer end face of a second link of the chain, with that face of the second link being located within the inner opening of the first link. Alternatively, the signal may be emitted toward an opposing reflective outer end face of the second link of the chain, with that face of the second link being located within the inner opening of an intermediate link that connects the first link to the second link. In that case, the transmitter may also be located within the inner opening of the intermediate link.

[0018] Conveniently, the transmitter and receiver may be mounted together in a sensor unit that is attached to or attachable to the first link of the chain, for example the sensor unit may straddle an inner opening of the first link from one longitudinal side to the other.

[0019] In a direct transmission scheme, the receiver may be in a fixed relationship to the second link in the chain. Nevertheless, the receiver on the second link may be located within an inner opening of the first link or within an inner opening of an intermediate link connecting the first link to the second link. In the latter case, the transmitter may also be located within an inner opening of the intermediate link. The processor may be in data communication with the transmitter and receiver via a wired or wireless connection extending between the first and second links.

[0020] The transmitter is preferably attached or attachable to the first link at a central longitudinal position along the first link. For example, the transmitter is attached or attachable to at least one longitudinally extending side of the first link. Alternatively, the transmitter can be located on an outer end face 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 emit a 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 for the transmitter to be laterally offset from the plane of the first link, in which case the transmitter can be oriented to emit a signal in a direction converging on the central longitudinal axis of the first link.

[0022] The first and second transmitters may be in opposite orientations. For example, the first and second transmitters may be in a fixed relationship to a first link, the first link being an intermediate link disposed between and joining a pair of outer links, each having a reflective outer end face opposite one of the transmitters and positioned within the inner opening of the first link.

[0023] At least a first link may be a stud link, with a stud dividing an internal opening, in which case the or each transmitter may be conveniently mounted on the stud, and the first and second transmitters, if present, may be located on opposite sides of each stud.

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

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

[0026] The system may include a reference reflector configured to reflect at least a portion of the signal emitted 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 emitted by the transmitter, and specifically, may be positioned 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 comprise a reference transmitter (in addition to the transmitter that emits the signal used to measure the distance to the chain link). In this case, if a reference reflector is also incorporated, the reference transmitter may be configured to emit an auxiliary signal (to the receiver), and the reference reflector may be configured to reflect at least a part of the auxiliary signal as a reference signal for use by the processor in calibrating the determined distance. In this example, the reference reflector is positioned obliquely with respect to the beam path of the signal emitted by the transmitter. The reference reflector in this case may take the form of a plate. Instead of a reference reflector, it is also possible to incorporate a reference receiver configured to receive the auxiliary signal (emitted by the reference transmitter) as a reference signal for use by the processor in calibrating the determined distance.

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

[0029] In this way, the processor can account for variations in the speed of sound in water due to changes in temperature, salinity, and / or depth of the water surrounding the monitored chain. By using the speed of sound in water appropriate to the surrounding conditions, the processor can calibrate the distance measurements to the chain links, thus obtaining more accurate distance measurements.

[0030] The system may include a sensor unit having first and second sensor modules in operative communication with each other. The emitter 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 inserted or insertable into an interior opening of a link of a chain, while the remaining electronic components of the sensor unit (in the second sensor module) may be separate and located adjacent to the chain, allowing the emitter and receiver to measure distance between chain links on a smaller scale.

[0031] If a reference transmitter is implemented, it may also be housed in the first sensor module. Optionally, a reference reflector (if 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 may optionally correspond to part of the housing of the second sensor module.

[0032] The inventive concept encompasses a corresponding method for monitoring the condition of a chain, which method comprises emitting a signal from a fixed emitting location relative to a first link of the chain, receiving the signal at a receiving location on any link of the chain, and measuring the time lapse between emitting and receiving the signal to determine the distance between the emitting location and a link of the chain other than the first link.

[0033] The signal may be reflected to the receiving location from either a second link in the chain, from a location within the inner opening of the first link, or from a location within the inner opening of an intermediate link that joins the first link to the second link. The signal may be emitted from within the inner opening of the intermediate link.

[0034] The receiving location may be fixed relative to the second link of the chain, but may also be fixed relative to the second link of the chain within the inner opening of the first link, or within the inner opening of an intermediate link joining the first and second links, in which case the signal may also be emitted within the inner opening of the intermediate link.

[0035] The signals can be emitted along the central longitudinal axis of the first link, in directions converging with the central longitudinal axis of the first link, from within an inner opening of the first link, or from an outer end face of the first link. The signals can also be emitted in opposite directions from first and second emission locations fixed relative to the first link. In this case, the respective signals can be emitted at different times, different phases, or different frequencies.

[0036] Data representing the distance may be generated at a link and communicated from that link to a receiving station remote from the link, for example, the distance data may be relayed along the chain to the receiving station or may be transmitted to the receiving station via an underwater vehicle deployed alongside the link.

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

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

[0039] A reference time lapse between transmitting a primary / auxiliary signal and receiving a 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, this distance can typically correspond to a known distance to a reference reflector or reference receiver. The determined distance between the transmitter and a link of the chain other than the first link can then be calibrated using the reference speed of sound in water.

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

[0041] In some embodiments, one or more sensor units of the present invention periodically measure and store the longitudinal distance between opposing adjacent links joined together through intermediate links. Because the nominal distance between opposing links in a new chain is known, it is possible to estimate the degree of wear even if a sensor unit of the present invention is attached to a chain late in its useful 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 distances measured 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 the data, and a corresponding modem can be installed on the vessel, rig, or buoy to receive the data. In another approach, a modem, such as an optical modem, can be installed on an underwater vehicle, such as an ROV, that moves past the sensor and downloads the stored data as needed.

[0043] The sensor unit of the present invention may include a transceiver including a transmitter, such as a piezoelectric transducer, a receiver, and a processing unit. In pulse-echo embodiments, ultrasonic acoustic pulses are transmitted from the transducer toward opposing links in the chain, and echoes reflected from the chain are 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, and then processed 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 link. The sensor can then be used to indirectly estimate how much metal has eroded from the contact interface between successive links behind the link being measured by the pulse-echo measurement.

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

[0045] The chain wear sensor unit of the present invention can be used in combination with a clamp that attaches, for example, a corrosion-preventing anode to a link of a mooring chain. Such clamps can be attached to multiple links of the mooring chain. The sensor unit of the present invention can be attached to such a clamp in addition to the anode, or it can be provided on a separate clamp.

[0046] Design criteria for pulse-echo ultrasound systems include the beam profile and pulse length. The beam profile, which depends on the transducer shape, size, and curvature as well as the excitation frequency, determines the degree to which the beam flares beyond the diameter of the transducer face as the distance from the transducer face increases.

[0047] In the context of the present invention, the ultrasonic beam should be wide enough to strike the opposing link even if the ultrasonic transducer is not perfectly aimed at the double-curved surface of the opposing link. In other words, the beam width should be sufficient to ensure reasonable robustness of the alignment. However, the beam should not be so wide that it could generate "false echoes," for example, by striking the intermediate link on which the sensor is attached.

[0048] In pulse-echo embodiments of the present invention, ultrasonic pulses are primarily used to detect the presence of steel structures on opposing links and measure round-trip time-of-flight to estimate the distance between the sensor and the nearest point on the doubly curved opposing surface of the link. Depending on the method chosen to detect the presence and arrival time of the reflected pulse, the length and shape of the pulse may be important. For example, the received pulse can be envelope detected and a comparator used to determine the arrival time of the pulse. However, in this case, the phase information of the reflected pulse is lost. A sharp leading edge of the transmitted pulse may be useful for accurate measurements.

[0049] Another approach is to digitize the received RF pulse with a high-speed analog-to-digital converter and cross-correlate the pulse shape with a fixed reference pulse stored in a processing unit. This method preserves the phase information of the reflected pulse, which may allow for a more robust and accurate time estimation. Alternatively, the received signal can be quadrature demodulated to baseband and digitized with an analog-to-digital converter that does not need to be as fast as the previous approach. Amplitude and phase information is preserved, and the processing unit can use this information to determine the time of arrival with high time resolution.

[0050] It is contemplated that the present invention can be implemented over a wide range of operating parameters. For example, the diameter or rectangular dimensions of the transducer's emitting face can typically range from a few millimeters to a few centimeters. The transducer frequency can range from a few hundred kilohertz to several megahertz, and the beam profile or aperture angle can range from a few degrees to, for example, 45 degrees.

[0051] There are necessarily trade-offs between these and other parameters and performance such as detection reliability, mounting robustness, and distance estimation accuracy. For example, higher ultrasound frequencies generally improve the ability to measure distance with high resolution, but may result in impractically narrow beam aperture angles. However, beam-widening solutions can be considered, such as curving the transducer surface and / or applying acoustic lenses. Arrays of multiple transducers are also possible.

[0052] While some chains have open links, known as studless or coiled chains, many mooring lines employ studded or stud-link chains, in which a crossbar or stud extends across the central opening of each link. The presence of studs may necessitate modifications to the ultrasonic sensor of the present invention. For example, a pair of ultrasonic transducers may be separated and placed one on each side of the stud. Alternatively, the transducers may be located in a single unit, but offset from the central longitudinal plane of the intermediate link and angled toward the opposing faces of the links joined by the intermediate link.

[0053] In some embodiments of the present invention, a monitoring device for measuring wear on a mooring chain can include a mount positioned on a first or second link connected to each other by an interconnecting link, or a mount 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 chain link, as determined from the arrival time of the received signal.

[0054] The data logger may include a storage device for the data. The data logger may include or be interfaced with a signal transmission system for transmitting a signal, for example, acoustically, wirelessly, electrically along a wire or cable, or optically along an optical fiber.

[0055] The mount may be permanently attached to the link, may be integral with the link, or may be attachable to the link, for example as a clamp.

[0056] The mount may be attached to one or both sides of the interconnecting link, for example, at a location approximately central to one side of the interconnecting link. If the interconnecting link is a stud link, the mount may be attached to the central bar or stud of the interconnecting link.

[0057] The monitoring device may include two longitudinally oriented acoustic transducers, namely a first and a second, facing in opposite directions, with the first acoustic transducer facing towards the first link and the second acoustic transducer facing towards the second link.

[0058] The signals emitted by the transducers may be emitted simultaneously, with a delay between them, or at different phases, and / or at the same frequency and / or at different frequencies. The data logger may be equipped with a filtering system to remove interference between the reflected signals of different transducers.

[0059] Thus, in accordance with the present invention, the condition of a chain is monitored by fixing at least one acoustic transducer to the first link of the chain. The transducer transmits an acoustic signal to a receiver located on any link of the chain. For example, the receiver may be located on the first link and receive the signal after reflection from a second link that may be interconnected with the first link or may be located across an intermediate link from the first link. Alternatively, in the case of a direct transmission system, the receiver may be located on the second link or another link.

[0060] By measuring the time of flight of the signal from the transducer to the receiver, the distance between the transducer and any link of the chain other than the first link is determined, with an increase in this distance indicating the degree of wear at one or more contact interfaces between successive links of the chain.

[0061] Also described is a system for measuring a distance characteristic of a marine element, the system comprising: at least one transmitter positioned in a fixed relationship relative to the marine element and configured to emit 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 perform a calibration or correction related to the distance characteristic using the reference signal. The processor may also be configured to measure the distance characteristic using the received signal (or the portion thereof not reflected by the reference reflector).

[0062] More specifically, the reference signal (and subsequent calibration) can be used to correct for or account for changes in environmental conditions surrounding the system that may cause inaccuracies in the measurement of distance characteristics. For example, changes in environmental conditions may affect the speed of sound in water, which is one of the main parameters used to measure distance characteristics, especially when the signal is emitted by an acoustic transducer and / or utilizes pulse-echo detection.

[0063] In some cases, at least one transmitter and at least one receiver are implemented as acoustic transducers. In this example, the reference reflector can reflect a portion of the same (main) signal emitted from the transducer and subsequently received by the transducer to measure the distance characteristic. Therefore, the reference reflector in this case may be referred to as a main beam reference reflector. The main beam reference reflector may correspond to any suitable reflector that can reflect a sufficient proportion of the emitted signal to generate a detectable reference signal, while allowing a sufficient proportion of the signal to pass through unreflected to enable accurate measurement of the distance characteristic. For example, the reference reflector can be implemented as a thin wire or disk suspended in the path of the signal emitted from the transmitter.

[0064] In some cases, the at least one transmitter and at least one receiver are implemented as multiple acoustic transducers. In such cases, a first transducer may be configured to emit and receive a first (primary) signal for measuring the distance characteristic, and a second transducer may be configured to emit a second (auxiliary) signal that is reflected by a reference reflector as a reference signal. From another perspective, one transducer may function primarily as a "reference" or "auxiliary" transducer, emitting and receiving the reference signal, while the remaining transducers of the multiple transducers may be used to emit and receive the "primary" signal used to measure the distance characteristic of the marine element. The reference reflector in this case may be referred to as an auxiliary beam reference reflector. As an example, such a reflector may be implemented as a reflector.

[0065] Alternatively, it is envisioned to implement a reference receiver in combination with a reference transmitter rather than a reference reflector. The reference transmitter may emit an auxiliary signal that may be received by the reference receiver as a reference signal.

[0066] In any of the above arrangements, the processor is configured to measure a reference time lapse before receiving the reference signal; and to use the reference time lapse to determine a reference speed of sound in water. For example, this determination may be made based on the reference time lapse and a distance traveled by the reference signal, which may typically correspond 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.

[0067] The system may include a sensor unit having first and second sensor modules in operable communication with each other. At least one transmitter and at least one 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 may be implemented. For example, if the marine element is a chain including links, the first sensor module may be sized and configured to be inserted or insertable into an inner opening of a link of the chain. In this manner, the transmitter and receiver may be located adjacent to the chain, separating the remaining electronic components of the sensor unit in the second sensor module, thereby enabling distance measurements between chain links at a smaller scale.

[0068] 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. When multiple transmitters and receivers are used (corresponding to multiple acoustic transducers), they can also be housed in the first sensor module. In such cases, the reference reflector can be housed within the first sensor module or alternatively connected to the first sensor module. In specific embodiments, the reference reflector can correspond to a portion of the housing of the second sensor module.

[0069] The above-described system is very flexible in its practical implementation and can be used to measure distance properties for a variety of different marine elements, which may correspond to any one of the following: a flexible elongated seabed element such as a rope, umbilical, or chain; a relatively rigid elongated seabed element such as a hydrocarbon transport riser or pipe, or a structural member of a subsea structure such as a rig; a subsea object such as a subsea wellhead (or subcomponent thereof); or a subsea attachment or item of subsea equipment such as a blowout preventer (BOP).

[0070] If the marine element is a chain including multiple links, the measured distance characteristic may correspond to the distance between the links of the chain to monitor wear on the links of the chain and / or to monitor or measure axial bending or load on the chain, which distance characteristic may be monitored over time.

[0071] If the marine element is a rope, umbilical, or other flexible elongated element, the measured distance characteristic may correspond to the distance between two locations on the marine element to determine the elongation of the marine element. If the marine element is a rigid elongated subsea element, the measured distance characteristic may correspond to the distance between two locations on the marine element to determine the axial bending and / or load of the marine element.

[0072] In the above example in which an elongated marine element is monitored, the system can further include a plurality of supports attachable to the element at respective spaced locations along the element. One of the supports carries at least one transmitter and 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. Such a system can also incorporate a reference reflector or reference receiver, for example, in association with the support carrying at least one transmitter and at least one receiver. Additionally or alternatively, another of the supports (not carrying at least one transmitter and at least one receiver) carries a reference reflector laterally offset from the element and positioned to reflect a reference signal to at least one receiver. Similarly, a reference receiver can be located on another of the supports.

[0073] If the reference reflector is located on a support different from the support on which the at least one transmitter and at least one receiver are located, the reference reflector may surround the element and may optionally be circumferentially continuous (for example, it may take the form of a ring-shaped or torus-shaped element). Alternatively, the reference reflector may be a spot reflector angularly aligned with the at least one transmitter. In some cases, multiple spot reflectors may be provided: one may function as a reference reflector and the rest of the multiple spot reflectors may be used to measure distance characteristics.

[0074] If the marine element is a subsea object or subsea attachment, such as a subsea wellhead (or component thereof) or BOP, the measured distance characteristic may correspond to the distance between components of the subsea object, or the distance between two subsea objects or attachments. In such cases, at least one transmitter may be mountable on one of the objects or components or attachments, while at least one receiver may be mountable on the other object or component or attachment. The reference reflector may be mounted or mountable on at least one transmitter or its corresponding object or component or attachment.

[0075] Also described is a method for measuring a distance characteristic of a marine element. The method includes transmitting a signal from a fixed transmitting location relative to the marine element; receiving the signal; reflecting at least a portion of the signal as a reference signal; receiving the reflected signal; and performing a calibration or correction for the distance characteristic using the reference signal. The method may also include measuring the distance characteristic using the received (unreflected) signal.

[0076] As mentioned above, the reference signal (and subsequent calibration) can be used to correct for or account for changes in the environmental conditions surrounding the system that may cause inaccuracies in the measurement of distance characteristics, particularly changes in the speed of sound in water due to changes in environmental conditions.

[0077] The method may include reflecting a portion of the same (main) signal emitted from the emission location to measure the distance characteristic, including reflecting a sufficient proportion of the emitted signal to produce a detectable reference signal while allowing a sufficient proportion of the emitted signal to be received unimpeded to allow accurate measurement of the distance characteristic.

[0078] Optionally, the method may further include transmitting an auxiliary signal that is reflected by the reference reflector as a reference signal. The auxiliary signal is transmitted in addition to a “main” signal that is transmitted for the primary purpose of measuring the distance characteristic. In this example, the main signal is not reflected to form the reference signal.

[0079] Alternatively, the method may further include transmitting an auxiliary signal that is received (eg, by a reference receiver) as the reference signal.

[0080] In any of the above methods, the method may further include measuring a reference time lapse that occurs 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; this distance may typically correspond 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.

[0081] As noted above, the marine element may correspond to any one 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 pipe for hydrocarbon transport, or a structural member of a subsea structure such as a rig; a subsea object such as a subsea wellhead (or a subcomponent thereof); or a subsea attachment, or an item of subsea equipment such as a blowout preventer (BOP).

[0082] If the marine element is a chain comprising multiple links, measuring the distance characteristic may correspond to measuring the distance between the links of the chain to monitor wear on the links of the chain and / or to monitor or measure axial bending or load on the chain. The steps of the method may be repeated to measure and monitor the distance characteristic as a function of time.

[0083] If the marine element is a rope, umbilical, or other flexible elongated element, measuring the distance characteristic may correspond to measuring the distance between two locations on the marine element to determine the elongation of the marine element. If the marine element is a rigid elongated subsea element, measuring the distance characteristic may correspond to measuring the distance between two locations on the marine element to determine the axial bending and / or load of the marine element. The steps of the method may be repeated to measure and monitor the distance characteristic as a function of time.

[0084] If the marine element is a subsea object or subsea attachment, such as a subsea wellhead (or component thereof) or a BOP, measuring the distance characteristic may correspond to measuring the distance between components of the subsea object, or the distance between two subsea objects or attachments. The steps of the method may be repeated to measure and monitor the distance characteristic as a function of time.

[0085] 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: [Brief explanation of the drawings]

[0086] [Figure 1] FIG. 2 is a schematic detailed side view of a link of a studless chain in an unworn state. [Figure 2] Corresponds to Figure 1 but shows the chain in a worn state. [Figure 3] 1, but showing one of the links of the chain fitted with a sensor unit of the present invention. [Figure 4] FIG. 4 is a top view of the chain and sensor unit shown in FIG. 3. [Figure 5]FIG. 5 is a schematic block diagram of the sensor unit shown in FIGS. 3 and 4, also showing ultrasonic beams emitted by the unit's ultrasonic transducers towards opposing links of the chain. [Figure 6] FIG. 6 is a schematic detailed side view of the variant of FIG. 5, in which a reflector facing the transducer is attached to the opposing link. [Figure 7] 1 is a schematic diagram of a mooring chain fitted with sensor units of the present invention, showing options for data communication from the sensor units to a monitoring station. [Figure 8] FIG. 10 is a timing diagram illustrating possible delays between pulses of each transducer in a sensor unit of the present invention. [Figure 9] 4 corresponds to FIG. 3, but shows a stud link chain to which a modified sensor unit of the present invention is attached. [Figure 10] FIG. 10 is a top view of the chain and sensor unit shown in FIG. 9. [Figure 11] 10, showing another modified example of the sensor unit of the present invention. [Figure 12] 5, but shows a variation of the invention that detects the time of arrival of the direct beam rather than the reflected beam. [Figure 13] FIG. 10 is a schematic detailed side view of a variation of the present invention in which a transducer attached to one link transmits a signal across an inner opening in an intermediate link to another opposing link. [Figure 14] 10(a) to 10(c) are top perspective views showing modified examples of the sensor unit of the present invention. [Figure 15] 14(c) is a top perspective view showing how a variation of the sensor unit of FIG. 14(c) can be used to monitor the links of the chain of FIG. 1. FIG. [Figure 16] 1A and 1B are top perspective views showing an example of a sensor unit of the present invention incorporating a reference reflector. [Figure 17] FIG. 10 is a side view showing another example of a sensor unit of the present invention incorporating a reference reflector. [Figure 18]2 is a top perspective view of a sensor unit used in a variant of the invention to monitor the load on the chain of FIG. 1; FIG. [Figure 19] 10(a)-10(c) are side perspective views each showing a variation of the invention in which at least one transmitter-reflector pair is used to determine the properties of an elongated undersea element. [Figure 20] FIG. 10 is a side view of a variation of the present invention in which the sensor unit is used to monitor the landing of a BOP at a subsea wellhead. DETAILED DESCRIPTION OF THE INVENTION

[0087] 1 and 2, these figures show three consecutive links of a chain 10: opposing outer links 12 and 14; and an inner intermediate link 16 that joins the outer links 12, 14. The outer links 12, 14 are thus oriented with their ends facing each other across an inner opening 18 in the intermediate link 16. The outer links 12, 14 have the same general orientation, while the intermediate link 16 is oriented generally perpendicular to the outer links 12, 14. Otherwise, the links 12, 14, 16 are all identical to each other.

[0088] 1 shows, by dotted lines, contact interfaces 20 where the inner end faces of the outer links 12, 14 contact the inner end face of the intermediate link 16. It is at these interfaces 20 that wear on the chain 10 is most likely to occur.

[0089] In Figure 1, the chain 10 is shown in an initial, unworn condition, so that the ends of the links 12, 14, and 16 all have their original full thickness. This determines the initial longitudinal distance D1 between the opposing ends of the outer links 12, 14.

[0090] Due to frictional wear during use, the metal of the links 12, 14, 16 erodes at the contact interface 20. As a result, the thickness of the ends of the links 12, 14, 16 decreases, as shown in an exaggerated manner in Figure 2. This causes the initial longitudinal distance D1 between the outer links 12, 14 to lengthen to a new, larger distance D2. Embodiments of the present invention are based on measuring the increase between D1 and D2 and thereby estimating the degree of wear between the links 12, 14, 16.

[0091] 3 and 4 of the drawings, the intermediate links 16 of the chain 10 are shown attached to or incorporating a sensor unit 22 of the present invention for measuring the longitudinal distance D between the opposing ends of the outer links 12, 14. For this purpose, the sensor unit 22 is positioned approximately midway along the length of the intermediate link 16. In this example, the sensor unit 22 extends perpendicular to the length of the chain 10 and straddles the inner opening 18 so as to bridge the gap between the opposing sides of the intermediate link 16. In other examples, the sensor unit 22 may cantilever from one side of the intermediate link 16.

[0092] The sensor unit 22 comprises a pair of ultrasonic transducers T1 and T2, each disposed about a central longitudinal axis 24 of the intermediate link 16. In this example, the transducers T1, T2 face in opposite directions aligned or parallel to the central longitudinal axis 24. As a result, the emitting face of transducer T1 faces one of the outer links 12, and the emitting face of transducer T2 faces the other outer link 14.

[0093] Transducers T1, T2 emit a signal 26 comprising a train of ultrasonic pulses that reflect off the opposing doubly curved outer end faces of links 12, 14 located within inner opening 18 of middle link 16 and return to sensor unit 22. Transducers T1, T2 may receive the reflected signal and thus function as a transceiver, or sensor unit 22 may support a separate receiver located next to transducers T1, T2. The round trip flight time of the pulses and associated reflections are proportional to the longitudinal distance between sensor unit 22 and the opposing ends of each outer link 12, 14, which indicates wear between the links 12, 14, 16.

[0094] 3, 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, while d0 represents the longitudinal distance between the transmitting 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 a constant and known value, while d1 and d2 are variables that are measured by sensor unit 22 to determine the degree of wear on links 12, 14, and 16.

[0095] 5 shows the main components of sensor unit 22. In addition to transducers T1 and T2, sensor unit 22 includes a power supply 28, a processor 30, a data store 32, and a communications module 34, all of which are sealed or enclosed within sensor unit 22 to ensure waterproofing.

[0096] Power source 28 may be a power input from a permanently wired or temporarily connected external power source, an on-board battery, or a wireless power receiver such as an induction loop or photocell that temporarily receives energy from an external power source such as a visiting ROV.

[0097] The processor 30 generates pulses to drive the transducers T1, T2, receives the reflected signals via the transducers T1, T2 or via a separate receiver, and processes those signals relative to the elapsed time to determine the time of flight and hence d1 and d2, from which D can be derived. The processor 30 outputs the resulting data to a data store 32, from which the data can be transferred periodically or continuously from the sensor unit 22 to a remote monitoring station.

[0098] Data is output from the sensor unit 22 via a communication module 34, which may be configured for wired or wireless data transmission by electrical, acoustic, electromagnetic, or optical means. The communication module 34 may also be configured as a repeater to forward data received from other sensor units 22 attached to other links in the chain 10. As a result, the communication module 34 may include an input or receiver in addition to an output or transmitter.

[0099] FIG. 5 also shows some parameters of one of the transducers, i.e., T2. Specifically, DT is the diameter of the transmitting face, and θ is the angle at which the signal 26, comprising a train of ultrasonic pulses, radiates from the transmitting face as a beam into a truncated cone-shaped volume. For example, DT may be 10 mm, and θ may be 15 to 20 degrees from a line parallel to the central longitudinal axis 24. The resulting beam of signal 26 is wide enough to impinge on the doubly curved outer end faces of the opposing outer links 14, the nearest point or apex of which is located a distance d2 from transducer T2. However, the beam of signal 26 is not wide enough to impinge on other structures, such as the sides of the intermediate links 16, and thus not produce spurious echoes.

[0100] 6 illustrates the possibility of attaching a reflector 36 to one or both of the outer links 12, 14 used for distance measurement in a pulse-echo arrangement, in this case the outer end of link 14 directly opposite transducer T2 along central longitudinal axis 24. For example, reflector 36 may be attached to link 14 by a magnetic clamp. Reflector 36 is oriented orthogonal to central longitudinal axis 24, thus reducing scattering that may occur when acoustic signal 26 comprising the beam of pulses impinges on the doubly curved outer end face of link 14.

[0101] Corrosion and marine product buildup can affect the measured values ​​of d1 and d2, and therefore D. In particular, metal loss due to corrosion at the location where the ultrasonic pulse signal 26 impinges on the links 12, 14 can lead to an overestimation of the distance D, while marine product buildup or deposition at the same location on the links 12, 14 can cause an underestimation of the distance D. Therefore, the reflector 36 can advantageously be formed from 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 antifouling compound to resist the buildup or deposition of marine product.

[0102] The reflector 36 or similar measures can be applied to either of the links 12, 14 from which the acoustic signal 26 is reflected in the pulse-echo arrangement. For example, in principle, the outer end surface of the link 14 could be treated, coated, and / or reshaped or flattened to improve the reflective quality without necessarily attaching a reflector 36.

[0103] FIG. 7 shows various options for transmitting data from the sensor units 22A-22D attached to the mooring chain 10 to a monitoring station 38.

[0104] Sensor unit 22A transmits data directly to monitoring station 38, for example, acoustically and by radio transmission, while sensor unit 22B transmits data indirectly to monitoring station 38 via ROV 40, which periodically visits and interrogates sensor unit 22B. ROV 40 can also temporarily power sensor unit 22B, for example, by electromagnetic induction or by illuminating a photocell in sensor unit 22B.

[0105] Sensor unit 22C is wired to monitoring station 38 and transmits data electronically or optically, while sensor units 22C and 22D illustrate the possibility of relaying data from one sensor unit 22 to another, in this case wirelessly.

[0106] 7 for sensor units 22C and 22D can involve most or all sensor units 22 on chain 10 to provide a system in which all sensor units 22 can relay data, e.g., acoustically, from other sensor units 22 located more distally along chain 10 to monitoring station 38. For example, sensor unit 22A can, in turn, report to monitoring station 38 via sensor units 22B, 22C, and 22D.

[0107] Because wear of the chain 10 is a gradual process, time-of-flight measurements of the signals 26 from transducers T1 and T2 can be taken at widely different times. Nevertheless, if more rapid, simultaneous, or near-simultaneous time-of-flight measurements are required, for example, in real time as the sensor unit 22 is interrogated by a visiting ROV 40, steps can be taken to avoid or mitigate interference between the ultrasonic emissions from transducers T1 and T2. One such step is illustrated in the timing diagram of FIG. 8, namely, delaying or offsetting the pulse train 42 emitted from one transducer T2 until the pulse train 42 of the other transducer T1 is emitted and a reflected signal 44 is received from the opposing link 12 of the chain 10. In this diagram, 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 times t1 and t2 indicate dimensions d1 and d2, respectively.

[0108] 9-11, these figures show a variation of the invention in which sensor unit 22 is adapted to a stud link chain 10 in which a central stud 46 straddles, bisects, and divides the inner opening 18. In each case, sensor unit 22 is positioned to surround stud 46 of intermediate link 16, although in other examples sensor unit 22 could be fixed to one side of stud 46, or, as in the preceding embodiment, to one or both sides of intermediate link 16.

[0109] 9 and 10, the transducers T1, T2 of the sensor unit 22 are centered about the central longitudinal axis 24 and face in opposite directions aligned with that axis 24. As a result, the transducers T1, T2 are located on opposite sides of the stud 46 of the intermediate link 16. To this end, the transducers T1, T2 are housed on respective peripheries of the sensor unit 22 that surround or embrace the stud 46.

[0110] In contrast, in FIG. 11, transducers T1 and T2 of sensor unit 22 are offset from central longitudinal axis 24 to one side of stud 46. Transducers T1 and T2 also face in opposite directions intersecting central longitudinal axis 24. Because transducers T1 and T2 are angled relative to central longitudinal axis 24, the ultrasonic beams from transducers T1 and T2 still impinge on opposite ends of outer links 12 and 14. This generates reflected signals that can be received by transducers T1 and T2 or other receivers in sensor unit 22, allowing calculation of d1, d2, and thus D, as shown in FIG. 3. Compensation for the fixed transducer beam angle is required.

[0111] While the preceding embodiments assume a pulse-echo arrangement, Figure 12 illustrates how the invention can alternatively be practiced 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. Thus, the signal 26 does not need to be reflected intermediately. In this example, the receiver 48 is fixed to the outer end of the outer link 14 opposite transducer T2. For example, the receiver 48 could be attached to the link 14 by a magnetic clamp.

[0112] 12, receiver 48 communicates with processor 30 of sensor unit 22 via wired or wireless link 50. Thus, after driving transducer T2 to emit signal 26 including a train of pulses, processor 30 measures the time of flight between the emission of the pulse from transducer T2 and its reception 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 d4 between the face of receiver 48 and the underlying outer end face of link 14 is known, determining d3 allows for the derivation of distance d2 between the emitting face of transducer T2 and the outer end face of link 14. Alternatively, because d4 is constant, the increase in d3 over time can be used to directly infer wear affecting link 14.

[0113] The direct transmission system shown in Figure 12 presents several challenges compared to pulse-echo systems, such as the need to install an additional receiver transducer, potentially requiring additional cables and fixation clamps. However, despite the more complex installation, direct transmission systems can be retrofitted into the chain. On the other hand, because they do not rely on reflection, direct transmission systems offer advantages over pulse-echo systems, such as more reliable transmission of the acoustic signal and potentially requiring less sensitive receivers. Because the surface roughness of the reflecting surface does not affect the acoustic signal, they are less susceptible to corrosion and, more generally, are insensitive to the orientation or curvature of the reflecting surface. Marine fouling can be managed by using antifouling materials on the transducers that function as transmitters or receivers.

[0114] In the example shown in FIG. 12, the transducer T2 and receiver 48 face each other along the central longitudinal axis 42. However, this embodiment allows for greater design flexibility because the signal 26 is not reflected. In particular, the signal 26 need not be constrained to travel solely within the inner opening 18 of the intermediate link 16 and impinge on one of the outer links 12, 14 joined by the intermediate link 16. Instead, the signal 26 can travel longitudinally along the chain 10 along a path sufficiently offset from the central longitudinal axis 24 to bypass the links. Thus, the signal 26 can travel between a transmitter and a time-synchronized receiver on the chain 10 separated longitudinally by two or more intermediate links 16. This allows the chain monitoring system of the present invention to measure average wear across successive groups, sets, or series of links in the chain 10.

[0115] Many other variations are possible within the scope of the present invention. For example, a single ultrasonic transducer could be used in a fixed relationship to a first link, with the transducer facing the outer end face of a second, adjacent link interlocked with the first link. This variation would only measure the effects of wear at one contact interface between two links, but could provide a useful indicator of the condition of the chain around that location. Such a variation could, for example, be achieved by removing transducer T2 from the sensor unit 22 of the preceding embodiment and measuring only the distance d1 between transducer T1 and the opposing end of the opposing link 12. This would increase distance d1 as the links 12, 16 wear at their contact interface 20, thereby detecting degradation of the links 12, 16.

[0116] 13, it may be possible to include transducer T1 at the outer end of first link 12 rather than intermediate link 16 and mount sensor unit 22 thereon. In this arrangement, signal 26 can be transmitted from transducer T1 along central longitudinal axis 14 within inner opening 18 of intermediate link 16 and reflected back from the opposing outer end face of second link 14.

[0117] The monitoring stations shown in Figure 7 can be replaced by relay stations, such as buoys, which can transmit data to remote monitoring stations that can be located on ships, offshore installations, or on land.

[0118] The transducers (emitters or receivers) used in the present invention can have active surfaces made of, treated with, coated with, or impregnated with corrosion-resistant and / or antifouling materials or compounds. In addition to these measures and the reflector shown in Figure 6, the effects of corrosion and marine deposits on pulse-echo systems can be mitigated if the ultrasonic beam profile is wide enough to produce reflections from an entire specific 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. On the other hand, if the system operates with a very narrow beam, e.g., only about 10 mm on the outer end face of the link, the effect of corrosion and marine deposits on the pulse-echo system ... 2 When the beam strikes an area, small local grooves in the steel due to corrosion or small formations due to the attachment of marine products (such as small mussels or barnacles) can cause large errors in the measurements.

[0119] Pulse-echo embodiments of the present invention rely on the specific reflectivity of the link surface on which the ultrasonic beam signal impinges when transmitted from the transducer. When an ultrasonic beam is directed toward the outer end face of the link, only a small area at the apex of its doubly curved surface is nearly perpendicular to the incident wavefront and is oriented to reflect the signal back to the transducer along the central longitudinal axis. In principle, the entire peripheral area of ​​the outer end face of the link reflects the signal at an angle relative to the central longitudinal axis that does not necessarily cause the signal to impinge on the transducer. This is particularly evident when the surface roughness of the reflecting surface is fine compared to the wavelength, a so-called mirroring effect. However, when the surface roughness is coarse compared to the wavelength, each small area of ​​the surface reflects the incident signal in many different directions, resulting in "diffuse scattering." Rust and other corrosion roughen the surface, so diffuse scattering can be achieved by selecting the appropriate ultrasonic frequency. In combination with a relatively wide beam, this can be beneficial in mitigating inaccuracies due to corrosion.

[0120] Other measures can be taken to avoid or reduce interference between the ultrasonic waves emitted by the transducers and the corresponding reflected signals. For example, the ultrasonic signals can be emitted from the transducers at different phases or different frequencies to help distinguish the reflected signals. Filtering downstream of the sensor unit processor or monitoring station can further reduce any interference that may occur between the reflected signals of the transducers.

[0121] FIG. 14 illustrates various exemplary embodiments contemplated for sensor unit 60, including components corresponding to those of sensor unit 22 illustrated in FIG. 5. While not shown in detail, the same major components are present in both sensor units: at least one transducer (T1 and / or T2); and key electronic components such as a power supply, processor, data storage, and communication module. These electronic components may be implemented in the form of a PCBA. As with sensor unit 22 illustrated in FIG. 5, all components are sealed or encapsulated within sensor unit 60 to ensure waterproofing. Therefore, any references to sensor unit 60 below should be considered to also apply to sensor unit 22 described above.

[0122] In Fig. 14(a), the sensor unit 60 comprises a housing 62 and a single transducer T1 or T2. All components of the sensor unit 60 are sealed within the housing 62. On the other hand, in Fig. 14(b), two transducers T1 and T2 are provided within the housing 62, and are arranged in an antiparallel arrangement or in opposite directions relative to the housing axis, with their signal beams radiating in opposite directions.

[0123] It is also contemplated that the sensor unit 60 may include two or more sensor (housing) modules or sub-units, with the sensor unit components split or distributed among these modules. In this regard, FIG. 14(c) illustrates a specific example in which a first module 62a houses transducers T1 and T2, while a second module 62b houses the remaining electronic components. An arm 62c extending between the two modules 62a, 62b connects them and includes electrical and communication connections between the transducers T1, T2 and the remaining electronic components. This arrangement advantageously allows the size of the first module 62a, which contains the transducers, to be reduced because the transducers are offset or separated from the other electronic components. As a result, the transducers T1, T2 enable distance measurements to be performed in a smaller space than would be possible using a sensor unit with only a single large housing 62.

[0124] 15 illustrates this advantage in the particular context of using sensor unit 60 to monitor links 12, 14 of chain 10. Module 62a containing transducer pair T1, T2 is inserted into inner opening 18 of intermediate link 16, while module 62b containing the remaining electronic components is positioned adjacent chain 10 at a position laterally offset from the longitudinal axis of chain 10.

[0125] As previously mentioned, in their most general sense, pulse-echo embodiments of the present invention measure the distance D traveled in water by a signal pulse emitted from and received by the sensor units 22, 60. W The distance to the target is estimated by calculating the speed of sound in water, C W and the flight time in water of the signal pulse to and from the target, T W This involves multiplying

[0126] The time of flight measured for a given signal corresponds to the flight time through the thin transducer protection layer (usually made of PEEK or polyetheretherketone), through the water column to the target, and back along the same beam path. The time of flight is the total flight time through the protection layer and water, T PW However, the flight time T P is not negligible and may need to be corrected for in subsequent distance calculations by the processor of sensor unit 60. This is done using the following formula: D W =C W / 2 * (T PW -2 * D P / C P ), where C P is the sound velocity in the protective layer, and D P is the thickness of the protective layer, which can also be expressed as: D W =GAIN * T PW +OFFSET, where C W / 2=GAIN and -(C W / C P * D P )=OFFSET.

[0127] Sound velocity in the protective layer C P and the thickness of the protective layer D P is known with very high accuracy during the sensor manufacturing process. This means that the OFFSET value is known and can be programmed into the processor of the sensor unit 22, 60. P and C P Both, especially (C P There may be slight tolerances due to temperature changes. These changes may affect the OFFSET value, but can be compensated for and calibrated, for example, by measuring the temperature in the sensor units 22, 60.

[0128] D WA particularly important parameter in the formula used to calculate W However, this parameter varies with water temperature, salinity, and depth. Of these variables, depth and salinity are unlikely to change significantly for a given sensor unit installed in a semi-permanent location, for example for wear monitoring of marine mooring chains. However, if the sensor unit is installed in a freshwater environment at one time and in a saltwater environment at another time, the change in salinity must be taken into account in this example once the sensor unit is moved. Also, the relationship between temperature and C W The sensitivity is small but not negligible: during an offshore campaign, the water temperature can change by several degrees. This must be taken into account to avoid possible measurement errors.

[0129] The salinity and temperature of the water around the sensor unit 60 are measured, and C is calculated using a known literature formula. W Alternatively, in the present invention, the effect of changes in salinity and temperature on C W It is envisioned that variations in can be directly corrected for using a reference measurement through calibration of the sensor unit 60. To obtain the reference measurement, the sensor unit 60 is placed at a known (or constant) distance D Ref A reference target is placed at the reference target position, and a pulse is irradiated from the transducer T1 or T2 in the sensor unit 60 toward the reference target. The time of flight T Ref Obtain a reference signal that indicates the (reference) sound speed in water, C W,Ref can be derived from the reference signal using the following equation: C W,Ref =2 * (D Ref / T Ref )

[0130] The sound speed in water, C, derived in this way W,Ref can be used to calibrate the processor in the sensor unit 60. The derived value C W,Refare input into the corresponding equation when determining the distance to the primary target (for example, a link of the chain 10 in the above embodiment).

[0131] The reference target can take one of several different forms. In its simplest form, the reference target may correspond to a receiver positioned a known or fixed distance from a transmitter (which may correspond to an acoustic transducer such as those shown and described above). The reference signal in this case may correspond to a signal emitted from the transmitter.

[0132] Alternatively, as exemplified by the arrangements shown in Figures 16 and 17, 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 emitted by the emitter (in the arrangements shown, one of transducers T1 or T2). The reference reflectors reflect at least a portion of the emitted signal incident on them, and the reflected signal is returned to the receiver (in the arrangements shown, transducer T1 or T2) to form the reference signal. The reference reflectors used can be arranged in a variety of ways.

[0133] In one configuration, the reference reflector is positioned in the path of the main beam, i.e., the signal beam that is directed toward the primary target (e.g., in the embodiment described above, a link of the chain 10) to obtain the desired distance measurement output. Thus, the reference reflector in this case may be referred to as a "main beam reference reflector." In this configuration, the reference reflector may be positioned to reflect only a portion of the incident signal back to the transducer as a reference "echo" signal (this may also be referred to as "partial reflection"). The remainder of the incident signal not reflected by the reference reflector can reach the primary target.

[0134] The main beam reference reflector must be large or wide enough to produce a detectable reference echo from the main beam directed at the main target, but narrow enough to maintain a useful signal reflected from the main target. Thus, the same transducer can be used to measure both the reference distance and the target distance. This is beneficial in that the reference distance can be obtained without the need for additional electronics, circuitry, and transducer implementation. All that is required is a few additional programming instructions for the processor to separate the two signals.

[0135] The main beam reference reflector thus takes the form of a relatively small or narrow structure inserted in the beam path of the "main beam", i.e., the signal emitted by transducers T1, T2 towards the main target. For example, it is a thin wire 66 shown in the single-transducer implementation of FIG. 16(a). Alternatively, a narrow disk (not shown) could be suspended in the beam path. A variation on this embodiment could include extending the outer casing of module 62a containing transducer T1 to also encompass the reference reflector, thereby avoiding potential deformation or displacement of the reference reflector during operation.

[0136] In another configuration, the reference reflector is positioned in the path of an "auxiliary beam," i.e., a signal beam emitted by the sensor unit's transmitter but not directed at the main target. The reference reflector in this case may therefore also be called an "auxiliary 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 (this latter scenario may also be called "total reflection").

[0137] In configurations using an auxiliary beam reference reflector, at least two transducers are used: one "reference" or "auxiliary" transducer emits an "auxiliary" or "reference" beam towards the reference reflector and receives a reference "echo" signal to determine the reference sound speed in water, C W,Ref (using the formula given above). The other "primary" transducer emits a "primary" beam towards the primary target and measures the distance to the primary target. In these arrangements, a greater percentage of the signal emitted towards the primary target is returned to the transducer, providing a reliable measurement of the distance to the primary target.

[0138] The auxiliary beam reference reflector can take various forms, as shown in Figures 16(b) and 17. In the two-transducer implementation of Figure 16(b), one transducer T1 corresponds to the main transducer and emits the main beam signal toward the main target. The other transducer T2 corresponds to the auxiliary transducer and emits the auxiliary beam signal toward a reference reflector positioned at a known or fixed location relative to the sensor unit 60. In this case, the reference reflector takes the form of a reflector 68. In the illustrated example of Figure 16(b), the reflector 68 is positioned to allow a portion of the auxiliary beam to pass through the plate (e.g., through a hole in its center); therefore, the reference signal in this example corresponds to only a partial reflection of the incident signal. That is, the auxiliary beam also functions as a "secondary" main beam, directed toward another "main" target further away and used to calculate another "main" distance measurement. Alternatively, it is contemplated that the reference reflector may reflect substantially all of the auxiliary beam signal to form the reference signal.

[0139] In the example of FIG. 17, three transducers T1, T2, and T3 are used. One transducer, T3, emits an auxiliary or reference beam toward a reference reflector and thus corresponds to an auxiliary transducer. The other two transducers, T1 and T2, emit main beams toward two different main targets and thus correspond to main transducers. This configuration is particularly useful for monitoring chain wear (as in the embodiment described above), because the distance to two different links 12 and 14 of the chain can be measured using the main transducers T1 and T2. In this arrangement, the auxiliary beam emitted by the auxiliary transducer T3 is emitted toward a second sensor module 62b containing electronic components, and a portion 70 of the housing of this sensor module 62b serves as the reference reflector. While a separate reference reflector, such as that shown in FIG. 16(b), could be used instead, using the portion 70 of the housing of the module 62b as the reference reflector allows for the utilization of existing components, thereby minimizing the number of components that need to be used. Furthermore, the distance between the two modules is defined by the length of the arm 62c and is therefore known or predefined (can be substantially fixed).

[0140] The use of a spherical concave reflector can be particularly beneficial when an auxiliary beam reference reflector is used, resulting in reflection of substantially the entire beam incident on the reference target. The beam emitted from the transducer propagates through water as a spherically expanding pressure wave. When a spherical concave reference reflector (with a radius of curvature equal to the distance to the transducer) is used, the entire wavefront strikes the receiving surface of the reference reflector simultaneously, and contributions from all points on the reflector surface return to the transducer in phase. This maximizes reflected signal strength and minimizes echo duration, resulting in sharp, well-defined echoes. This results in an improved, stronger signal.

[0141] Other characteristics of the reference reflector, such as acoustic impedance and surface roughness, are also factors to consider when selecting an appropriate reference reflector. Reflectors made of materials with acoustic impedances significantly different from that of water will result in strong reflections. Similarly, reflectors with rough surfaces will result in diffuse reflections, thereby reflecting increased energy back to the transducer. Therefore, particularly suitable materials for use as reference reflectors include thin metal wires (e.g., made of stainless steel or other corrosion-resistant materials) with a diameter of a few millimeters, or small, highly polished concave disks with a diameter equal to or smaller than the width of the ultrasonic beam emitted by the transducer.

[0142] It will also be appreciated that the general calibration system and method described above is applicable across a wide range of implementation scenarios where reflections of ultrasonic beam pulses are used for distance measurements and where it may be beneficial to account for variations in the speed of sound in water.

[0143] As mentioned above, this calibration method may be useful when measuring the distance to or between chain links to monitor wear on mooring chain links. It is also envisioned that this calibration method may be used when measuring the distance to or between marine chain links for other purposes, such as measuring the distance to or between chain links to monitor axial strain on a mooring chain, and thus indirectly monitoring the load on the mooring chain.

[0144] In this regard, Figure 18 illustrates one possible implementation scenario in which a sensor unit 60 comprising at least one ultrasonic transducer (not shown) is clamped to, clamped next 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 an acoustic signal beam emitted by the transducer. The time of flight of pulses of the acoustic signal beam to and from the transducer is proportional to the longitudinal distance between the transducer and the reflector 72.

[0145] Although not shown, reference reflectors 66, 68, 70 corresponding to any of the above examples may be incorporated into or used with sensor unit 60 substantially as described in any of the above examples. Reference reflectors 66, 68, 70 may be used to obtain a measurement of the speed of sound in the surrounding water, which may be used to calibrate the processor of sensor unit 60 during or before measuring the distance between sensor unit 60 and reflector 72.

[0146] 19 shows another possible use case of the calibration method described above, where the time-varying properties of an elongated subsea element 80 are derived using pulse-echo ranging. The elongated element 80 can take the form of a relatively rigid element such as a structural member of a rig, a riser or pipe for transporting hydrocarbons; or a relatively flexible element such as a mooring rope, cable or umbilical.

[0147] A pair of clamps 82a, 82b are attached to the elongated element 80 at respective spaced apart locations along the central longitudinal axis 84 of the elongated element 80. The elongated element 80 is depicted schematically as having a substantially circular cross-section and is therefore substantially cylindrical when upright. In these embodiments, one clamp 82a, shown on the left, is a reflector clamp, and the other clamp 82b, shown on the right, is a transmitter / receiver clamp. The clamps have opposing axially inwardly facing surfaces 86a, 86b. On its inwardly facing surface 86b, the transmitter / receiver clamp 82b supports one or more signal emitters, which are ultrasonic transducers that direct one or more beams of acoustic signals 88 toward at least one reflector 90, 92 attached to the opposing inwardly facing surface 86a of the reflector clamp 82a, as shown in FIGS. 19(a), 19(b), and 19(c), respectively. As noted above, the time of flight of pulses in the acoustic signal beam traveling to and from the transducer is proportional to the longitudinal distance between the transducer and the reflectors 90,92.

[0148] As shown in FIG. 19(a), when only a single discrete transducer-reflector pair is utilized, a simple one-dimensional distance measurement is obtained. This is suitable for measuring the elongation of the elongated element 80 over time. In this example, an additional reference reflector 66, 68, 70 can be incorporated for calibration measurement purposes. This can take the form of a main beam reference reflector, such as the thin wire 66 shown in FIG. 16(a), inserted in the main beam path of the signal emitted from the transducer toward the main target (reflector 90). This arrangement allows the beam emitted from a single transducer to also be used for calibration purposes. Alternatively, an additional "auxiliary" transducer can be incorporated into the transmitter / receiver clamp 82b, and an auxiliary beam reference reflector can be utilized for calibration. This auxiliary beam reference reflector could correspond to the plate 68 shown in FIG. 16(b) or the portion 70 of the housing of the sensor module 62a. Alternatively, a portion of the reflector clamp 82a could be used as the reference reflector if the material in question is suitably reflective.

[0149] It is contemplated that multiple discrete transducer-reflector pairs can be used, as shown in FIG. 19(b). As discussed in connection with FIG. 19(a), additional reference reflectors 66, 68, and 70 can be incorporated for calibration measurement purposes. When all transducers are used to obtain primary distance measurements, the reference reflector can take the form of a primary beam reference reflector, such as the thin wire 66 shown in FIG. 16(a), inserted in the primary beam path of the signal emitted from one of the transducers toward the primary target (corresponding reflector 90). Alternatively, one of the transducers can be used as an auxiliary transducer, and an auxiliary beam reference reflector can be incorporated for calibration purposes. This auxiliary beam reference reflector can correspond to the plate 68 or portion 70 of the sensor unit housing 62a shown in FIG. 16(b). The remaining transducers can still be used to emit signals toward the primary target and obtain distance measurements to the primary target. Ideally, at least three transducer-reflector pairs would be used to achieve accurate primary distance measurements.

[0150] Using multiple transducer-reflector pairs to derive the primary distance measurement means that changes in elongation along additional dimensions (not just along the central longitudinal axis of the elongated element) can be detected, which can indicate other properties of the elongated element. For example, if the distance measurements obtained by each of the transducer-reflector pairs at any given time are the same (within measurement error), this indicates that the elongated element was subjected to a pure axial load (i.e., no or minimal bending). If the distance measurements obtained by each of the transducer-reflector pairs are significantly different, but the average measurement across all sensors remains constant, this indicates that the elongated element was subjected to a pure bending load. If the distance measurements obtained by each of the transducer-reflector pairs and the average measurement across all pairs are significantly different, this indicates that the elongated element was subjected to a combination of bending and axial load.

[0151] It is envisaged that in some cases one transducer-reflector pair can be used for calibration purposes, i.e. one of the "primary" reflectors can be reused as a reference reflector, and the remaining transducer-reflector pair can be used to calculate the primary distance measurements.

[0152] An alternative configuration for measuring the axial and bending loads of the elongated element 80 is shown in FIG. 19(c). A single, substantially continuous reflector 92, e.g., in the form of a ring-, torus-, or donut-shaped reflector, is provided in the reflector clamp 82a. This continuous reflector 92 still allows for multiple echoes to be obtained, one for each transducer used, but may avoid the need for more precise alignment that may be required when using discrete pairs of transducers and reflectors. Furthermore, regardless of the degree of bending / twisting of the elongated element 80, the emitted pulse beam can still be reflected by the reflector 92. Furthermore, the implementation of a continuous reflector eliminates the need to correct for other characteristics of the reflector when processing the primary distance measurement.

[0153] As discussed in connection with Figure 19(b), additional reference reflectors 66, 68, 70 can be incorporated for calibration measurement purposes. This reference reflector can take the form of a main beam reference reflector if all transducers are used for "main" target distance measurements. Alternatively, if one of the transducers is used primarily for calibration purposes, the reference reflector can take the form of an auxiliary beam reference reflector. Alternatively, a portion of the continuous reflector 92 can be reused as a reference reflector.

[0154] It is further contemplated that the calibration methods described above may be suitable for distance measurements in other use cases. For example, as shown in FIG. 20 , these methods may be used when landing a BOP (blowout preventer) 100 on a subsea wellhead 102. In this case, the sensor unit 60 may be inserted between the two components 100, 102 as shown. Alternatively, subcomponents of the sensor unit 60 may be mounted on the BOP 100 and the subsea wellhead 102, respectively: for example, a transmitter or transducer may be mounted on the BOP 100 and a receiver or reflector may be mounted on the subsea wellhead 102, or vice versa. In this example, one or more reference reflectors 66, 68, 70 may be mounted on either the BOP 100 or the subsea wellhead 102, depending on which transducer is mounted thereto, and calibration may be achieved via any of the methods described above.

[0155] Alternatively (not shown), the sensor unit 60 can be used to monitor the periodic expansion of the subsea wellhead itself (i.e., between the subsea wellhead substructures). In this example, the emitter or transducer can be mounted on one of the substructures (e.g., via a magnetic clamp or other suitable mounting means) and the receiver or reflector can be mounted on another substructure, or vice versa. In this example, one or more reference reflectors 66, 68, 70 can be mounted on any of the substructures to which the transducers are mounted, and calibration can be achieved via any of the methods described above.

Claims

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

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

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

4. 4. The system of claim 2 or 3, wherein the transmitter and receiver each face 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 fixed to the second link of the chain.

6. 6. The system of claim 4 or 5, wherein the transmitter is configured to emit the signal toward an opposing reflective outer end face of the second link of the chain, that face of the second link being positioned within the inner opening of the first link.

7. 6. The system of claim 4 or 5, wherein the transmitter is configured to emit the signal toward an opposing reflective outer end face of the second link of the chain, that face of the second link being positioned within an inner opening of an intermediate link joining the first link to the second link.

8. The system of claim 7 , wherein the transmitter is also disposed within the inner opening of the intermediate link.

9. A system according to any one of the preceding claims, wherein the transmitter and receiver are mounted together in a sensor unit that is attached to or can be attached to the 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 longitudinal side to the other.

11. 2. The system of claim 1, wherein the receiver is in a fixed relationship to a second link in 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 that couples the first link to the second link.

14. The system of claim 13 , wherein the transmitter is also disposed within the inner opening of the intermediate link.

15. A system according to 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 and second links.

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. 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 is aligned with a central longitudinal axis of the first link.

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

20. A system according to any preceding claim, wherein the transmitter is located within an inner opening of the first link.

21. A system according to any preceding claim, wherein the transmitter is laterally offset from the plane of the first link.

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

23. A system according to any preceding claim, comprising first and second transmitters facing in opposite directions.

24. 24. The system of claim 23, wherein the first and second transmitters are in a fixed relationship to the first link, the first link being an intermediate link disposed between and joining a pair of outer links, each outer link having a reflective outer end face facing a respective one of the transmitters and disposed within an inner opening of the first link.

25. A system according to any one of the preceding claims, wherein at least the first link is a stud link comprising a stud dividing an internal opening.

26. 26. The system of claim 25, wherein the transmitter is attached to the stud.

27. 27. A system according to claim 26 when dependent on claim 23, wherein the first and second transmitters are located on opposite sides of each of the studs.

28. A system according to any preceding claim, wherein the transmitter is located on an outer end face of the first link.

29. 29. The system of any one of claims 1 to 28, further comprising a processor configured to determine a time of flight of the signal from the emitter to the receiver, and to generate distance data accordingly.

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

31. 31. The system of claim 29 or 30, further comprising a communications module for communicating said distance data to a monitoring or relay station.

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

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

34. 34. The system of any one of claims 1 to 33, further comprising a reference reflector configured to reflect at least a portion of the signal emitted by the transmitter to the receiver as a reference signal for use by the processor in calibrating the determined distance.

35. 34. A system according to any preceding claim, further comprising a reference reflector and a reference transmitter, the reference transmitter configured to emit an auxiliary signal, and the reference reflector configured to reflect at least a portion of the auxiliary signal to the receiver as a reference signal for use by the processor in calibrating the determined distance.

36. 34. A system according to any preceding claim, further comprising a reference receiver and a reference transmitter, the reference transmitter configured to emit an auxiliary signal, and the reference receiver configured to receive the auxiliary signal as a reference signal for use by the processor in calibrating the determined distance.

37. the processor: measuring a reference time lapse occurring prior to receiving said reference signal; determining a reference speed of sound in water using the reference time lapse; The system according to any one of claims 34 to 36, configured to:

38. 35. The system of claim 34, comprising first and second sensor modules in communication with each other, the transmitter and receiver being co-implemented in the first sensor module and the processor being implemented in the second sensor module.

39. 36. The system of claim 35, comprising first and second sensor modules in communication with each other, the transmitter, receiver and reference transmitter being co-implemented in the first sensor module and the processor being implemented in the second sensor module.

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

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

42. 1. A method for monitoring the condition of a mooring chain, comprising: transmitting a signal from a fixed transmitting location relative to a first link of the chain; receiving said signal at a receiving location on any link of said chain; measuring the time lapse between emission and reception of said signal to determine the distance between said emission location and a link of said chain other than said first link; A method comprising:

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

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

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

46. 46. ​​The method of claim 45, including launching the signal within the interior opening of the intermediate link.

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

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

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

50. 50. The method of claim 49, including launching the signal within the interior opening of the intermediate link.

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

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

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

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

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

56. 56. The method of claim 55, comprising emitting the signals at different times, different phases, or different frequencies.

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

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

59. 58. The method of claim 57, comprising communicating said distance data to said receiving station via an underwater vehicle deployed alongside said first link.

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

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

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

63. measuring a reference time lapse occurring prior to receiving the reference signal; determining a reference speed of sound in water using the reference time lapse; and 63. The method of any one of claims 60 to 62, further comprising:

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