Patent ID: 12209960

DETAILED DESCRIPTION OF THE INVENTION

The system described herein is minimally invasive or even non-invasive for determining the properties present in a linear system and for supporting inferences into the characteristics of the linear system.

For example, the system exploits transverse waves propagating in a linear system. Sources of transverse waves in a cable being towed through a medium by a vessel include cable strum, waves from vessel motion, and waves transmitted to the cable from winch and sheave contact points. Cable strum includes transverse vibrations induced by vortex shedding.

Embodiments of a non-invasive tension measuring system include data communication with a transverse wave sensing structure mounted to and/or integrated with a cable under tension. The systems measure cable tension by measuring a propagation speed of transverse waves in the cable as provided by Equation (1):

c=Tm(1)
where “c” is the propagation speed of transverse waves in the cable, “T” is the cable tension and “m” is the cable mass per unit length. As an example, consider a one inch diameter steel cable with T=1000 Newtons (N) and m=1 kilogram per meter (kg/m). Using Equation (1), the propagation speed is approximately 31.6 meters per second.

Referring toFIG.1andFIG.2, a first embodiment of a non-invasive tension measuring system100tests a cable200on which accelerometers102and104are attached at a distance “d” between the devices. The accelerometers102and104measure vibrations due to cable strum.

In the event that no cable strum nor any similar measurable acceleration is present in the cable200; transverse waves can be introduced by using a mechanical shaker or by striking the cable.

Each accelerometer102and104affixed to the cable200comprises two accelerometers separated at ninety degrees around a circumference of the cable. This configuration enables each accelerator pair to detect a transverse wave of vertical and horizontal wave components.

A first end of the cable200connects with a sensor drive system300. A second end of the cable200may terminate physically at a weighted body400. Alternatively, the second end of the cable200may be at a point along the cable beyond which no additional spaced accelerators are positioned and, therefore, no transverse wave detection takes place. The transverse waves propagate along the cable200from the first end to the second end and/or vice versa; thereby, producing a time delay between the accelerometers102and104.

The time delay “t” is measured by the sensor drive system300to infer the propagation speed “c” and the tension “T” when the mass “m” per unit length is known, either by direct measurement (e.g., by weighing the cable section between the accelerometers102and104) or from manufacturer specifications.

Referring toFIG.3, a sensor drive system300has a computing resource302with a processor304and data storage306. The instructions of the sensor drive system300implement a control subsystem308, an analysis subsystem310and/or a sensor measurements database312that is stored in the data store306and retrieved by the processor304. The control subsystem308allows the processor304to operate a sensor interface314in order to transmit to and/or receive data from the accelerometer102and104.

The analysis subsystem310allows the processor304to record and retrieve transverse wave data using the sensor measurements database312and to determine respective time delays for sensed transverse waves propagating between adjacent pairs of accelerators and, by inference, respective cable tensions between adjacent accelerator pairs as a function of the propagation speed of the sensed transverse waves. For example, by solving Equation (1) for the cable tension “T”.

The analysis subsystem310is implemented with various algorithms to measure the time delay between adjacent accelerometers along a cable. For example, performing a cross-correlation of the time series returns an estimate of the time delay “t” between the signals. A one meter separation leads to a detectable time delay of approximately 0.03 seconds. The time delay is the same for transverse waves propagating in alternating directions

In embodiments that comprise multiple sensor pairs deployed along a cable; a series of time delays can be measured between pairs of sensors. The propagation speed estimate is then averaged to increase the accuracy of the cable tension estimate.

The sensor drive system300employs any or all of the control subsystem308, the analysis subsystem310, and/or the sensor measurements database312collocated upon the host computing resource302or distributed among two or more host computing resources.

Referring toFIG.4andFIG.5, an exemplary embodiment of a second non-invasive tension measuring system500comprises a cable600with an integrated optical fiber502. As used herein, Rayleigh scattering refers to the scattering of light or other electromagnetic radiation by particles much smaller than the radiation wavelength. Rayleigh scattered measurements from the optical fiber502determine strain along the cable600.

Measurement positions are non-invasively defined within the optical fiber502at a distance “d” to bound a measurement region for vibrations present in the cable600. Similar to the embodiment ofFIG.1, transverse waves can be proactively introduced.

In use, the transverse waves induce strain in the cable600. The present invention supports strain detection via Rayleigh-scattered measurements in a standard single mode or multimode telecommunications grade fiber502that is longitudinally coextensive with the cable600. InFIG.5, the optical fiber502is integral to the measured cable600.

Alternatively, the optical fiber502can mount either non-invasively or with minimally invasive connectivity to an outer surface of the cable600. The longitudinally-positioned optical fiber502detects a transverse wave aligned to an angle around the circumference of the cable600.

A first end of the cable600connects (e.g., physically and/or in optical communication) with an optical drive system700. A second end of the cable600may terminate physically at a weighted body800for aiding the cable to reach a desired position in a deployment environment. Alternatively, the second end may terminate at a processing bin positioned along the cable600beyond which no additional strain measurements are required.

The transverse waves propagate along the cable600from the first end to the second end, and/or vice versa to produce a time delay t=c/d between the two measurement positions. This time delay “t” is measured by the optical drive system700to infer the propagation speed “c” and, by extension, the tension “T” when the mass “m” per unit length is known, either by direct measurement (e.g., by weighing a section of the cable600) or from the manufacturer's specifications of the cable.

Referring now to the block diagram ofFIG.6; the optical drive system700includes a computing resource702with a processor704that executes computerized instructions. A data store706stores data and instructions used by the processor704.

The computerized instructions of the optical drive system700implement a control subsystem708, an analysis subsystem710, and/or some number of processing bins712that are stored in the data store706and are retrieved by the processor704. The control subsystem708allows the processor704to operate a light source714to transmit an optical signal into the cable600and to operate a receiver716that receives and interprets Rayleigh-scattered measurements from the measurement positions defined in the cable.

The analysis subsystem710allows the processor704to record and retrieve transverse wave data using the processing bins712. The analysis subsystem710determines respective time delays for sensed transverse waves propagating between adjacent pairs of measurement positions and, by inference, respective cable tensions between the adjacent measurement position as a function of the propagation speed of the sensed transverse waves.

Employing the adaptation of optical time domain reflectometry described forFIG.6; the optical drive system700collects time-of-flight measurements of backscattered light into the processing bins712defined for each distance “d” along the cable600. This leads to strain measurements corresponding to each processing bin712from which the analysis subsystem710infers the tension “T” along the entire cable600.

For the embodiments described, inference of the cable tension is accomplished by using a variety of deterministic approaches. Equation (1) is used to infer the tension “T” when “c” is first determined via a cross-correlation of signals between two adjacent accelerometers or processing bins. Alternatively, the cable tension “T” is inferred by measuring standing waves or by using a k-ω approach.

If standing waves are present in a linear system of interest due to cable strum, an alternative approach to inferring cable tension “T” involves measuring the amplitude of standing waves in the cable. This is a direct approach to estimate the wavelength, which leads to determination of the propagation speed using c=fλ, where “λ” is the wavelength and “f” is the strum frequency. Note that the wavelength will vary along the cable because the tension varies.

A k-ω plot is a two-dimensional fast Fourier transform (FFT) in the time dimension and the spatial dimension of time series signals from a line array of “n” receivers. If the frequency “ω” represents the vertical axis and the wavenumber k=ω/c represents the horizontal axis; then energy propagating at a fixed speed “c” will appear as a straight line with a slope c=dω/dk.

For the present invention, only waves propagating along the direction of the line array in both directions need be considered, which corresponds to the wavenumber k0=±ω/c0. The measurement error is governed by the wavenumber resolution, which is Δk=2π/L, where “L” is the length of the line array of receivers.

For the Rayleigh scattered measurements of strain described in the second embodiment, each measurement bin is one-half meter in length. Therefore, for two hundred bins, L=100 meters.

Cable strum is typically the dominant mode of transverse vibration, and the frequency is approximately 10 Hertz. The propagation speed of transverse waves is c=30 meters/second. Consequently, the error estimate is calculated by Equation (2):

Δ⁢kk0=2⁢π/Lω/c0=3⁢010·100=3⁢%(2)

Because cables may be a mile in length; the scope of measurement estimation could extend to receivers of n=1000 or more. However, the propagation speed variation due to the tension variation in the cable would exceed the error in the propagation speed estimate due to the wavenumber resolution.

Implementation of the optical drive system700may employ any or all of the control subsystem708, the analysis subsystem710, and/or the processing bins712collocated upon a single host computing resource702or distributed among at least two host computing resources.

The present disclosure uses computer instructions and/or system configurations that perform operations for determining cable tension. The instructions that include the control subsystem308,708, the analysis subsystem310,710, instructions of the sensor measurements database312, and/or the instructions of the processing bins712is not meant to be limiting in any way.

For example, the processors304,704may be in data communication with external devices and may be configured to direct input from the external devices to the data stores306,706for storage and subsequent retrieval. Therefore, an additional subsystem recorded to the data stores306,706enable the processors304,704to retrieve data from the data stores and to forward as output to various networked components.

Referring now toFIG.7, block diagram900illustrates the operational sequence and use of computer-assisted aspects of the present invention. Starting at Block902, the sensor drive system300or optical drive system700performs an initialization at Block904of known values to be used in tension measurement calculations.

The known values include a spacing distance between sensors that defines a unit length of the linear system200,600and a mass of the unit length of the linear system. If the embodiment of the present invention defines a plurality “n” of unit lengths of the linear system200,600; then initialization values for each unit length or measurement region are recorded. At Block906, the drive system300,700operates the sensors102,104,502to detect a transverse wave propagating through the linear systems200,600.

In Block906, the drive system300,700can continue to loop until a transverse wave is detected. In Block908, the drive system300,700determines a time delay(s) during which the transverse wave propagates the defined spacing distance(s) of the measurement region(s). At Block910, the drive system300,700uses the time delay(s) and spacing distance(s) to determine a propagation speed of the transverse wave for the employed measurement region(s).

At Block912, the sensor drive system300,700then uses the computed propagation speed and the retrieved mass of the unit length(s) of the linear system200,600to solve for the tension(s) of each measurement region of the linear system. Regarding Block912, if more than one measurement region is employed in the estimation calculation method900; then some subset of all determined tensions is averaged at Block914to determine a full tension estimate for the entire linear system200,600. At Block916, the drive system300,700delivers the full tension estimate by displaying the tension estimate to a user interface, and/or by forwarding the tension estimate to an automated application for further processing).

In Block918and if ongoing tension monitoring for the linear system200,600is required; the process may be repeated starting at the transverse wave detection loop of Block906. If not, operation of the tension measuring system ends at Block920.

InFIG.8, deployment of the present invention in environments that complicate wave propagation measurement with hydrodynamic loading are described. For example, when towing a cable200,600behind a vessel1000; a transverse propagation speed experienced by the cable in water differs from that in the air because of hydrodynamic loading (also known as added mass). The present invention can determine where the cable200,600enters the water also known as a cable air/water interface.

Hydrodynamic loading in the form of added mass alters the propagation speed of transverse waves. Thus, a difference in the propagation speed and the wavelength is detected when sections of the linear system transitions from air to water. A cable in water will have a lower speed of propagation for the same tension because of added mass. The formula for this scenario is represented in Equation (3):

c=Tm+ma(3)
where “ma” is the added mass per unit length, which is equal to the cable volume per unit length times the density of water.

Because the tension of the cable200,600is continuous across the air/water interface; a discontinuity is detected in the propagation speed between sensor points positioned in the air and adjacent sensor points positioned in the water. For example, the optical drive system700will identify the specific optical time-domain reflectometer measurement from the set of processing bins712containing the air/water interface. As a result, the air/water interface can be determined using the present invention with a precision of one-half meter along the cable600.

The present invention recognizes that transverse waves have propagation speeds that are governed by the cable tension. Thus, measuring those propagation speeds leads to an accurate measurement of the tension.

Embodiments of the present invention are likely less expensive and easier to implement than devices known in the art, especially for estimating tension in steel and Kevlar cables that are one inch and greater in diameter and that typically have operating tensions in the range of thousands of pounds.

In the first embodiment described above, the present invention uses accelerometers that mount onto a cable of interest without requiring any tie-points or rigging fixtures such as shackles that can be cumbersome and difficult to implement and/or that can induce failure points to the cable system. Depending on the device and the cable, the accelerometers can be clamped or even glued to the cable. Clamping or gluing to the cable has a negligible change in tension or contact stress compared to conventional apparatus for measuring tension in a cable.

The present invention is scalable in the diameter of the cable as well as the tension of cable; is safer due to being non-invasive and requiring no additional failure points, and is widely applicable for measuring nearly any cable at any tension.

In the second embodiment, an apparatus does not need to be attached to the cable. The measurements are made via Rayleigh scattered measurements in optical fibers integrated and/or surface-mounted with the cable of interest.

The measurement methods using accelerometers and Rayleigh-scattered measurement fibers are not the only approaches that can acquire the required time series data at two or more points on a cable of interest. Other measurement methods include magnetic pickups and Doppler velocity sensor measurements. The key inventive concept of magnetic pickups and Doppler velocity sensor measurements is based on the use of time series data from at least two locations on the cable of interest in which transverse waves that that propagate between these two locations are recognized. Magnetic pickups and Doppler velocity measurements can both generate time series data at the two locations that can enable to estimate a time delay between those locations.

The present invention eliminates the need for additional rigging equipment, overhead loading, compounding forces, and extraneous trigonometric parameters in dynamic environments to increase safety, accuracy and ease of implementation. The present invention is also scalable, unlike most prior art tension meters that have a fixed or scaled resolution and a maximum load limit.

Also, the tension measurement technology of the present invention is invariant to cable twist. Even torque-balanced cables twist significantly under tension. Because the present invention only relies on the propagation speed of transverse waves; there is no dependency on twist angles.

The foregoing description of the preferred embodiments of the invention has been presented for purposes of illustration and description only. It is not intended to be exhaustive nor to limit the invention to the precise form disclosed; and obviously many modifications and variations are possible in light of the above teaching. Such modifications and variations that may be apparent to a person skilled in the art are intended to be included within the scope of this invention as defined by the accompanying claims.