Method and apparatus for simultaneous inductive excitation and locating of utilities

A locating system is presented. In some embodiments, the locating system includes a first platform, the first platform including a transmitter capable of inducing a current in a line; a second platform, the second platform including a receiver capable of detecting the current in the line; and a processor coupled to the first platform and the second platform, the processor directing the first platform and the second platform to control their motion over the line and collecting location data of the line.

BACKGROUND

Technical Field

Embodiments of the present invention are directed towards utility location and, in particular, to the simultaneous inductive excitation and location of utilities.

Discussion of Related Art

The position of underground and underwater cable and pipeline utilities is routinely measured via the magnetic field induced when alternating current flows through the utility. This approach is used for various purposes, such as utility surveys, monitoring of depth of cover, construction and installation, dredging preparation, and fault locating. The alternating current may be injected into the utility through a direct electrical connection at an access point, may be injected through an inductive current clamp or an inductive antenna, or may be a part of the operation of the utility itself, as in the case of live power cables.

In some cases, the electrical properties of the utility may be such that current does not flow very far from the point of injection. Examples of this include utilities with strong capacitive or resistive coupling to ground and pipelines that are explicitly grounded at regular intervals for cathodic protection. In other cases, conditions may make it impractical or undesirable to inject current far from the measurement point. Examples of the former include underwater surveys that require an inductive antenna, but where both positioning and retrieval of the antenna are difficult. Increased calls for increased efficiency in covering long distances are examples of a case where positioning and retrieval of an inductive antenna is inefficient.

Solutions that currently exist for solving the problem where current is difficult to induce use pulse induction, such as that described in “Metal Detector Basics and Theory,” Minelab.com; and “Laying pipes and cables and meeting the challenge of finding them again afterwards,” MaritimeJournal.com, 2015. In these systems, a transmitter transmits an electromagnetic pulse in the direction of the utility, listens for a response, and analyzes the response to derive the utility position. However, these methods generally suffer from limited accuracy in cable and pipeline positioning applications due to distortion and other undesirable effects.

Therefore, there is a need for systems for better location of underground cables or pipelines.

SUMMARY

In accordance with aspects of the current invention, a location system is presented. A locating system according to some embodiments includes a first platform, the first platform including a transmitter capable of inducing a current in a line; a second platform, the second platform including a receiver capable of detecting the current in the line; and a processor coupled to the first platform and the second platform, the processor directing the first platform and the second platform to control their motion over the line and collecting location data of the line.

In accordance with some embodiments, a method of operating a locating system that comprises a first platform with a transmitter and a second platform with a receiver over a line, includes propelling the first platform over the line; propelling the second platform over the line such that the receiver detects presence of the line; and directing direction of propulsion of the first platform and the second platform according to the detected presence of the line.

These and other embodiments are further discussed below with respect to the following figures.

DETAILED DESCRIPTION

In the following description, specific details are set forth describing some embodiments of the present invention. It will be apparent, however, to one skilled in the art that some embodiments may be practiced without some or all of these specific details. The specific embodiments disclosed herein are meant to be illustrative but not limiting. One skilled in the art may realize other elements that, although not specifically described here, are within the scope and the spirit of this disclosure.

This description and the accompanying drawings that illustrate inventive aspects and embodiments should not be taken as limiting—the claims define the protected invention. Various changes may be made without departing from the spirit and scope of this description and the claims. In some instances, well-known structures and techniques have not been shown or described in detail in order not to obscure the invention.

Elements and their associated aspects that are described in detail with reference to one embodiment may, whenever practical, be included in other embodiments in which they are not specifically shown or described. For example, if an element is described in detail with reference to one embodiment and is not described with reference to a second embodiment, the element may nevertheless be claimed as included in the second embodiment.

FIGS. 2 and 5illustrate some embodiments of the present invention. As illustrated inFIG. 2, system200includes sensors204and206as well as transmitter202mounted on a rigid frame208. A processor/driver210can be coupled to control sensors204and206and transmitter202on rigid frame208. Although two sensors204and206are illustrated inFIG. 2, system200may include any number of sensors. Furthermore, sensors may be oriented relative to one another in order to detect magnetic fields having particular directionality. For example, detectors204and206may be oriented in orthogonal directions.

As illustrated inFIG. 5, system500includes a transmitter518and a receiver520. Transmitter518includes a transmitter502mounted on a frame504. Transmitter502is driven by a processor/driver506. Receiver520includes sensors510and512mounted on a second rigid frame514. A processor/driver516is coupled to driver receivers510and512. Processor/driver506can be in communication with processor/driver516, either by physical connection or wirelessly. As is illustrated inFIG. 5, frame508with transmitter502can be kept a minimum distance from frame514with sensors510and512. Again, sensor520can have any number of sensors, of which sensors510and512are illustrated. Furthermore, the sensors on sensor520can be oriented to detect magnetic fields in particular directions relative to one another. For example, sensors510and512may be orthogonally placed relative to each other.

As is illustrated in the embodiments ofFIGS. 2 and 5, one direct approach to addressing the issues of locating utilities is to continuously induce current from the same or different platforms that carry the positioning system itself. For example, such platforms for underwater applications include remotely operated vehicles (ROV), underwater trenchers and autonomous underwater vehicles (AUV), and all-terrain vehicles (ATV) for underground applications. In accordance with some embodiments, therefore, the position of a conducting linear structure (a cable or pipeline) can be measured using a continuous-wave induction system with one or more transmitters and one or more sensors mounted on a single rigid frame or multiple rigid frames working in relation to one another

The effects of direct coupling between transmitter202and sensors204and206, or between transmitter502and sensors510and512in the embodiment ofFIG. 5, can be modeled and subtracted from the actual set of measurements in order to derive the position of the structure from the residual fields. The direct coupling model can be based on physical laws for magnetic induction or be empirically based on measurements. In some embodiments, the phase of the direct coupling can be measured with the sensors and the phase aligned with the phase of other sensors, using the out-of-phase component of the result to derive the position.

Continuous-wave induction often offers improved positioning accuracy, but suffers from strong direct coupling between transmitter and receiver when the two are close to each other. This coupling distorts the field measured by the receiver and significantly affects accuracy if left unaddressed. Some embodiments of the present invention provide methods for the receiver to compensate for this direct coupling by extracting the magnetic field emanating from the utility from the total measured field.

As discussed above,FIG. 2illustrates a sensing system200according to some embodiments of the present invention. As illustrated inFIG. 2, sensors204and206and transmitters202are mounted on a rigid frame208. The rigid frame208provides for separation of the sensors204and206from transmitters202and also provides for certainty in the relative locations of transmitters202and sensors204and206. As is illustrated inFIG. 2, a processor/driver210is coupled to the transmitters202and the sensors204and206. The processor/driver210includes electronics for driving the transmitters202and receiving signals from the sensors204and206. Further, the process/driver210includes data processing capability sufficient to process the data received as described below. In some embodiments, the processor/driver210may be incorporated on the rigid frame208. In some embodiments, the process/driver210may be separated from the rigid frame208and electronically coupled to transmitters202and receivers204and206mounted on the frame208.

Another direct approach to addressing the issues of locating utilities is to separate the inducing transmitter and the positioning system by sufficient distance so that any effects of direct coupling are negligible. In this approach, the transmitter can be kept close enough to the target line to induce a measurable level of continuous current while also remaining close enough to the positioning system so that said current does not significantly diminish before reaching the positioning system. Such a system is illustrated by system500ofFIG. 5.

In most applications, the appropriate separation distance is larger than what can be practically supported by a single platform, requiring separate platforms for the transmitter and the positioning system. In some applications, the transmitter can remain stationary while the positioning system tracks the line position for some distance, but in other applications, the transmitter has to move continuously with the positioning system. As discussed above, system500includes transmitters502on a first platform504while sensors510and512are mounted on a second platform514. Platforms504and514can be kept separated by a minimum distance while each is positioned to interact with a utility.

In some applications, the location of the underground utility is not known a priori, so suitable transmitter locations or paths can be determined dynamically based on the positioning results.

FIG. 5illustrates a system500according to some embodiments where inductive transmitter502and a positioning system520with sensors510and512are mounted on separate platforms504and514, respectively, each operated by processor/driver506,516, respectively. One of the processor/drivers506or516may also provide navigational guidance to one or both platforms506or516in order to simultaneously keep the transmitter502sufficiently close to the target utility for efficient induction and reduce the direct path coupling to an insignificant level.

Magnetic Induction

An inductive loop antenna generates a magnetic field that is proportional to the magnitude of the current flowing in the loop and the loop area, and which predominantly behaves like a magnetic dipole anywhere except close to the loop itself (See David C. Jiles, “Introduction to Magnetism and Magnetic Materials (2 ed.)”, CRC 1998; I. S. Grant and W. R. Phillips, “Electromagnetism (2nded.),” Manchester Physics, John Wiley & Sons, 2008).

Accordingly, the magnetic field at a position in free space defined by a vector r relative to the antenna center can be described by the following equation:

HD⁡(r)=14⁢π⁢(3⁢⁢r⁡(m·r)r5-mr3)(Eq.⁢1)
Here, the vector m is the magnetic moment of the antenna, with magnitude equal to the product of the loop current and the loop area and direction along the primary antenna axis, and HDis magnetic field in units of A/m.

Equivalent equations can be derived for the magnetic field in a conductive medium such as seawater, but those equations are not included in this description.

Since the methods described in this document are based on continuous-wave excitation at one or more distinct frequencies, it is convenient to view both m and HDas phasors, each described by a vector of three complex values that captures the amplitude and phase of the continuous-wave field along each coordinate axis. Each component of these vectors has the same phase, but a superposition of two or more such vectors may not, as will be shown later.

As described by Faraday's law of induction (David C. Jiles, “Introduction to Magnetism and Magnetic Materials (2 ed.),” CRC 1998), the time-varying magnetic field HDwill induce an electromotive force ε in any closed circuit that is equal to the negative of the time rate of change of the magnetic flux enclosed by the circuit:

ɛ=-μ0⁢ddt⁢∫∫A⁢HD⁡(r,t)·dA(Eq.⁢2)
In (Eq. 2), dA is an incremental unit of area enclosed by the circuit, μ0is the vacuum permeability, equal to 4π×10−7Vs/(Am), and the integration is taken over the entire area enclosed by the circuit. The time-dependence of HDhas been emphasized here for clarity.

Then considering that the target utility can be considered an infinitely long straight conductor and the primary antenna axis can be considered to be perpendicular to the conductor, the expression provided in (Eq. 2) may be simplified as:

ɛ=μ0⁢ω⁢m4⁢π⁢⁢r(Eq.⁢3)
Here, ω=2π f, where f is the frequency of the excitation signal, and r is the shortest distance from the antenna center to the conductor. A straightforward modification can be made to factor in any rotation of the antenna with respect to the conductor.

If the closed circuit has total impedance Z, then the induced current in the target conductor is given by:

IL=ɛZ=μ0⁢ω⁢m4⁢π⁢⁢rZ(Eq.⁢4)
It should be noted that the impedance Z may include capacitive and inductive effects, so the phase of the current ILmay be different from the phase of the magnetic field HD.

Current will also be induced in other closed circuits present in the environment, including so-called eddy currents that are induced within metallic objects and other conductive materials that are in the vicinity of the transmitters.

The induced currents, including both ILand eddy currents, will in turn generate their own magnetic fields according to the Biot-Savart law. (See, e.g. David C. Jiles, “Introduction to Magnetism and Magnetic Materials 2ed.),” CRC 1998; I. S. Grant and W. R. Phillips, “Electromagnetism (2nded),” Manchester Physics, John Wiley & Sons, 2008. Consequently, a magnetic field sensor placed in the environment will measure the complex superposition of the magnetic fields emanating from these multiple sources, as well as the direct coupling from the transmitter.

The Biot-Savart law as applied to the target utility in free space is as indicated in (Eq. 5), where ILis the current on the utility, r is the measurement position vector, dx is a length element of the conductor and x is the position of that element on the conductor.

HL⁡(r)=IL4⁢π⁢∫C⁢dx×(r-x)r-x3(Eq.⁢5)
As before, HLis a time-varying quantity at one or more distinct frequencies and can be represented by a complex three-dimensional vector at each frequency, with each component describing the magnitude and phase of the magnetic field along a coordinate axis.

As before, equivalent equations can be derived for the magnetic field in a conductive medium such as seawater, but those equations are not included in this description.

(Eq. 5) can be simplified to a two-dimensional field that decays as the inverse of the distance r to the conductor, which may be represented in the conductor's coordinate system with the X-axis pointing along the conductor as shown inFIG. 4and indicated in (Eq. 6).

HL⁡(r)=IL4⁢π⁡[0-z/ry/r](Eq.⁢6)
Here, r is the shortest distance to the conductor and y and z may be chosen as the horizontal and vertical offsets from the conductor, respectively.
Performance

Embodiments of a receiver system can substantially distinguish between the magnetic field emanating from the utility from other effects, including the direct coupling, eddy-current effects and environmental noise. The effects of unstructured environmental noise may be partially removed by appropriately filtering the measured signals, but the other effects are highly structured and may be removed by other methods.

For accurate positioning of a target utility the undesirable effects can be reduced to well below the strength of the magnetic field emanating from the utility. In some embodiments, a level of reduction of 20 dB below the target field, or to 1/10th of its strength, may be sufficient. As described in the previous section the latter depends on the position and orientation of both the transmitter antenna and the receiver's sensors as well as the impedance of the utility. Therefore, positioning of the system with respect to the conductor becomes more difficult as distance between the combined system and the utility increases and as the impedance increases.

The impedance of cable or pipeline utilities varies significantly in practice. The internal resistance and ground capacitance of high-voltage power cables are typically on the order of 0.1-1 Ω/km and 0.1-1 μF/km, respectively (see, e.g. “Nexans Submarine Power Cables,” Nexans, 2013), and their ground impedance can be expected to be a few Ω. At a typical induction frequency of a few kHz and for a cable a few km long this results in a few Ω total impedance. Large pipelines may have significantly lower impedance, especially if grounded at regular intervals through protective anodes or similar methods, while utilities with higher internal resistance or poor grounding may have higher total impedance.

When a single platform, such as system200illustrated in the embodiment ofFIG. 2, is targeted for this use, its size also varies. As an example, a cable-laying trencher may be as long as 8 m, allowing for 10 m separation between antenna and sensors, while a work-class ROV only allows a 4 m separation. The former may also allow smaller separation between the inductive antenna and the cable.

The graphs illustrated inFIG. 1shows the maximum total impedance addressable with different performance levels at two different separation distances in order to maintain a 20 dB difference between the desired and undesired magnetic fields. The performance level is indicated by the parameter δ as a percentage of the full strength of the direct coupling between the transmit antenna and the receiver, with δ=10% indicating that 10% of the direct coupling remains, for example.

When separate platforms are used for the transmitter and the positioning system the minimum distance between the two that effectively eliminates the effects of direct coupling is typically on the order of 100 m, but this depends on both the strength of the antenna and the sensitivity of the sensors.

The rate of decay of the induced current depends on both the electrical properties of the target utility and the frequency of the inductive antenna. A fast decay may require that the two platforms be kept relatively close to each other, but slower decay may allow the separation distance to vary over a wider range.

Positioning Methods

This section outlines four methods for removing the undesirable effects of direct coupling and induced eddy currents, and three methods for deploying separated transmission and measurement. The first two are based on explicitly characterizing the distortion and then subtracting it from the aggregate measured signal, the next two rely on indirectly estimating the contribution of the target utility to the aggregate signal, and the last three focus on how a transmitter can be placed or moved while providing sufficient continuous current for the measurements to be used to locate or track a target cable or pipeline. It should be understood that a cable may be either composed of a single conductor or multiple conductors, such as bundled bipolar HVDC or bundled three-phase HVAC cables. One skilled in the art may recognize other methods from the ones described herein that may also be used for removing the undesirable effects or positioning a transmitter.

In the first four cases both transmitter and receiver are assumed to be mounted on one rigid frame and therefore are useful with system200as illustrated inFIG. 2. As discussed above, transmitter202and sensors204and206are typically at opposite ends of rigid frame208in order to maximize the separation between the two. Depending on the application, system200may utilize multiple transmitters, any number of sensors204and206, and may operate at multiple signal frequencies. Nonlinear solution methods such as the Levenberg-Marquardt algorithm (R. Fletcher, “Practical Methods of Optimization,” Wiley, 1987) or an iterative method such as a Kalman filter (Grewal and Andrews, “Kalman Filtering,” 2ndedition, Wiley, 2001) may then be applied to the resulting residual measurement to derive the position of the utility. This is described in more detail in U.S. Pat. No. 7,356,421 by T. Gudmundsson and J. Waite, “Precise Location of Buried Metallic Pipes and Cables in the Presence of Signal Distortion,” which is herein incorporated by reference in its entirety.

In the latter three cases the direct coupling is ignored and the position of the utility is derived via more conventional methods, such as the ones described in U.S. Pat. No. 7,356,421.

These latter three methods all rely on finding a suitable initial location for the transmitter platform, whether it be the rigid frame208ofFIG. 2or platform504of transmitter518, where the transmitter or transmitters are close enough to the target line to induce measurable current on it. There are multiple ways of accomplishing this depending on a priori knowledge of the line location, line conditions, available equipment and other factors.

In some embodiments, the platform may be placed near an accessible terminal, at landfall for a subsea cable, or at a section of the line that is visible.

In other embodiments, the transmitter platform504of transmitter518and the sensor platform514of sensor520may be used cooperatively to find a suitable location. If the orientation of the target line is known, cooperate use of transmitter518and520may be done by moving both platforms504and514simultaneously across the target line, noting when the largest measurement is obtained by the sensors and using that as the initial position for the transmitter platform. If the direction is not known the same procedure can be repeated at different angles until such a maximum is found.

Model-Based Estimation of Direct Coupling

When the primary cause for distortion is the direct coupling between the transmitter and receiver and other effects such as eddy-current induction are minimal, the direct coupling may be estimated directly from (Eq. 1) above or other equivalent equations. If the separation between the antenna and a sensor at position r is rD, then the measured signal H(r,rD) is the sum of the direct coupling signal HDand the induced signal HLas indicated in (Eq. 7).
H(r,rD)=HL(r)+HD(rD)  (Eq. 7)
An estimate of HDcan be obtained using (Eq. 1) and subtracted from the measurement to get an estimate of HL.
Empirical Estimation of Direct Coupling

In some applications, it may be possible to move the entire system far enough away from the target utility to remove its effects in the measurements for a period of time. Measurements made during this period may then be used to compose an empirical model of the aggregate distortion, including amplitudes and signal phases for each sensor.

Let HAbe the aggregate distortion signal. Then the measured signal can be described by (Eq. 8) below.
H(r,rD)=HL(r)+HA(rD)  (Eq. 8)
The empirical estimate of HAcan be subtracted from the measured signal to obtain an estimate of HL.
Differential Adjustment for Direct Coupling

In some cases, system200may be mounted on a single vehicle that is capable of navigating through turns and altitude changes, such as on an ROV, AUV or ATV. In those cases, a path can be chosen that continuously changes the relationship between the system and the target. The path may be predetermined or adjusted dynamically.

While the geometric relationship between the system and the target utility can be changed this way, the relationship between the transmitter antenna202, the rigid platform208, and the receiver sensors204and206does not change. Therefore, the distortion caused by direct coupling and eddy currents induced on the platform itself remains constant, at least for short periods of time, and subtracting measurements from two separate time instances and locations will largely cancel the effect. The remaining differential measurement can be used to derive the position of the target, for example by modeling the measured magnetic field as a function of both the sensor and antenna positions relative to the target, and solving for the latter.

FIG. 3shows an example path304over a target utility302that can be used with system200. The difference between measurements at points P1306and P2308illustrated inFIG. 3can be used for positioning. For reference,FIG. 4illustrates a coordinate system that can be used during calculations, with the X direction being along the target conductor and the Y and Z direction being orthogonal to the target conductor.

Let the two measurements at points P1and P2be as follows for each sensor in system200, where HLis the magnetic field induced by the current in the target utility, HAis the aggregate of the direct coupling and any fields induced by eddy currents on the platform, r1and r2are the two measurement positions, rDis the position of the sensor relative to the antenna center, and t1and t2are the two measurement times, resulting in the measured magnetic fields H1and H2as described in (Eq. 9).
H1(r1,rD,t1)=HL(r1,t1)+HA(rD,t1)
H2(r2,rD,t2)=HL(r2,t2)+HA(rD,t2)  (Eq. 9)

The two HAterms will only differ in phase if the time difference between the measurements is small enough to avoid any significant time-varying effects. If the transmitter and receiver are synchronized, the phase difference may be removed by simply subtracting the two measurements, leaving dependence only on the desired signal from the target utility.
H2(r2,rA,t2)−H1(r1,rA,t1)=HL(r2,t2)−HL(r1,t1)  (Eq. 10)
If the transmitter and receiver are not synchronized the phase difference between the two measurement times can be tracked and used to correct one of the equations before the subtraction.

If the change in the platform's position and orientation between the two positions is known, either from the vehicle's navigation system or another independent positioning mechanism, the right-hand side can be modeled in terms of a single target position using (Eq. 4) and (Eq. 6).

Using Signal Phase to Cancel Direct Coupling

In some applications such as cable installation the orientation of the magnetic field sensors relative to the target cable may be easily controlled, allowing a sensor to be placed perpendicular to the cable. This sensor will measure only the distortion from direct coupling and induced eddy currents and can therefore be used as a phase reference for that aggregate signal.

(Eq. 9) and (Eq. 6) may be used to describe this in more detail. Let the sensor in question be placed so that it aligns with the X axis of (Eq. 6) so that it will only measure the contribution of the HAcomponent of (Eq. 9). A measurement made by other sensors in the same location partially or fully aligned with the Y or Z axis of (Eq. 6) can be separated into two components, one phase-synchronous with X-axis measurement and the other 90° out of phase with it. If the phase of HLis substantially different than the phase of HAand the phase of HAis uniform for all directions, then this separation will result in an out-of-phase signal on the Y and Z sensors that only originates on the target utility and may be therefore be used for positioning.

As an example,FIG. 4shows the 2-dimensional field402emanating from the cable302and three sensor axes. The X-axis is perpendicular to the field and can be used as a phase reference for the distortion signal while the out-of-phase components of the measurements made by sensors oriented along the Y and Z axes can be used for positioning.

In cases where the distortion signal includes multiple eddy-current sources in addition to the direct coupling the signal received by sensors in different locations may not be phase-synchronous. In those cases, the out-of-phase components from different sensor locations are not balanced in amplitude and cannot be combined directly in a positioning method. However, if the Y and Z components from each location are themselves balanced, then the ratio of the two components from (Eq. 6) may still be utilized for positioning along with knowledge of the separation between sensor locations.

Separated Platforms with Stationary Transmitters

If the electrical properties of the target utility line are such that the induced current can travel a substantial distance before decaying below an acceptable level, the transmitter platform504may be left in place while the sensor platform514is used to track the location of the line. The transmitter platform504can be moved to a new location periodically, for example when the measured signal level has decayed beyond an acceptable level.

A suitable initial location for the transmitter platform504may be found using one of the approaches described above. Subsequent locations can either be chosen in the same way or by using the line position measured by the sensor520. The latter can be done either automatically based on the quality of that measurement, depth of burial, or other factors. Placement may also be accomplished manually by an operator.

Separated Platforms with a Predetermined Transmitter Path

If the electrical properties of the target utility line do not allow the induced current to travel very far, the transmitter518and sensor520may need to stay close to each other at all times, in some embodiments even as close as the minimum separation distance allows. In this case, both platforms504and514move in tandem, with the transmitter518either leading or following the sensor520within a range of separation distances.

If the horizontal position of the line is well known and if the platforms can be guided properly, this may be done by having the transmitter518follow a predetermined path, for example defined by a sequence of waypoints. Communication between the transmitter518and receiver520or between the transmitter518, receiver520and a central controller may be necessary to moderate and control the speed of one or both.

Separated Platforms with a Dynamically Adjusted Transmitter Path

If the location of both platforms is known sufficiently well the measurements done by the positioning system on the sensor platform may be used to dynamically guide the transmitter platform, where the latter follows the former within a range of separation distances.

In some embodiments, the positioning system500continuously measures both the horizontal position and the depth of the target line and guides the transmitter518so that it follows at a set distance while remaining directly above the line and as low as possible.

FIG. 6illustrates an example of this method, where the measurement area is viewed from above and the target utility line is shown as a thick blue line. The sensor platform520traces a line604while tracking the signal induced by transmitter on platform518. System500uses the measurement of the line position to guide transmitter518along a line close to target line302, resulting in the path602illustrated inFIG. 6. The transmitter518follows the sensor520at a distance that is large enough so that effects of direct path coupling are not significant but close enough that current induced in target302is detectable by receiver520.

Tracking of Underwater or Underground Pipelines or Cables

In some embodiments, pipelines can be located. In some cases, coupling of radiation into the target pipeline can be inefficient because of bleed-off over long distance or because of grounding through cathodic protection anodes, coating defects, or by other methods. Tracking methods that use a continuous-wave alternating current inductive transmitter are described in patent application 20150226559, but in some cases, the accuracy of such methods is limited by either direct magnetic coupling from the transmitter to the sensors used by the tracking system or by bleed-off over long distances or through grounding, especially on subsea pipelines. Methods for acquiring and tracking power lines is described in U.S. Pat. No. 9,285,222, some of which are similar to what is described in this document. However, those methods do not address pipeline tracking, particularly when pipelines are equipped with anodes for cathodic protection or are subject to leakage because of coating defects.

In some embodiments, methods by which multiple platforms can cooperate, some carrying inductive transmitters and others carrying sensors, may be an efficient way to locate pipelines or cables, for example. In some embodiments, one transmitter platform and one sensor platform can be used. However, more than one transmitter platform and/or more than one sensor platform may be used in some embodiments.

FIG. 7illustrates an example embodiment of a system700according to the present invention. As is illustrated inFIG. 7, system700includes platforms702and704. A sensor520is mounted on a platform704while transmitter518is mounted on a platform702. As is illustrated, platform704includes one or more magnetic field sensors (sensors510and512are illustrated) while platform702includes one or more inductive transmitters, where transmit coil502is illustrated. In some embodiments, transmitter518may provide a continuous wave transmission to induce a current in pipeline706. In some embodiments, transmitter518may provide a pulsed wave transmission to induce a current in pipeline706.

As further illustrated inFIG. 7, platform704travels ahead of platform702, tracking the position and depth of pipeline706via the induced current. The positions obtained from this tracking are used to guide platform702so that it can remain close to pipeline706and thereby induce current efficiently. In this description, although pipeline706is being used as an example, it should be understood that these embodiments can also operate to locate a cable.

FIGS. 8A and 8Billustrate examples of platform702and platform704, respectively. As is illustrated inFIG. 8A, transmitter coil502is mounted on a platform702. Platform702includes a processor802that controls both the propulsion and platform702and transmitter502. As such, processor802can be any processing unit capable of executing instructions for controlling the motion of platform702, communicating with platform704, and controlling the transmitter. Processor802can include one or more microprocessors coupled to sufficient volatile and non-volatile memory to hold data and instructions for controlling motion, controlling the output of transmit coil502, and logging data regarding position and generated field strengths.

As is illustrated inFIG. 8A, processor802can be coupled to a navigation unit804. Navigation unit804may include GPS locators and inertial navigation systems for determining the position and orientation of platform702. Further, processor802is coupled to drive unit810, which controls the propulsion and steerage of platform702. For example, propulsion814for underwater applications may include any marine drive (e.g., one or more propellers, one or more jet drives) as well as any control surfaces (e.g., one or more rudders, one or more planer surfaces, or directional controls on the marine drive) to control pitch, roll, yaw, speed, and direction of travel. Platform702can also be encapsulated within a waterproof hull that is shaped to provide smooth motion through water. Further, propulsion814may include buoyancy controls to help control the depth of platform702.

Processor802is further coupled to TX driver808that drives transmit coil502, which is mounted on platform702. Although transmit coil502is illustrated inFIG. 8Aas being mounted from the stern of platform702, transmit coil502can be mounted anywhere on platform702where it is capable of inducing current in a pipeline706as illustrated inFIG. 7.

Processor802is further coupled to communication806, which allows for communications with platform704. In some embodiments, communication806may further be in communications with operators on the surface so that both platform702and platform704can be controlled externally. In some embodiments, processor802receives instructions from platform704regarding its motion so that platform702can be guided by platform704.

FIG. 8Billustrates an example of platform704, which includes receiver520. As is illustrated inFIG. 8B, sensors510and512are mounted on platform704. Similar to platform702, platform704includes a processor822that controls both the propulsion of platform702and the receiving functions of platform702. As such, processor822can be any processing unit capable of executing instructions for controlling the motion of platform704, communicating with platform702and other entities, and processing data from receive sensors510and512. Processor822can include one or more microprocessors coupled to sufficient volatile and non-volatile memory to hold data and instructions for controlling motion, controlling the receiver functions, and logging data regarding the position of platform704and the location of pipeline706.

As is illustrated inFIG. 8B, processor822can be coupled to a navigation unit824. Navigation unit824may include GPS locators and inertial navigation systems for determining the position and orientation of platform704. Further, processor822is coupled to drive unit830, which controls the propulsion and steerage of platform704. Drive unit830controls propulsion834, which, for underwater applications, may include any marine drive (e.g., one or more propellers, one or more jet drives) as well as any control surfaces (e.g., one or more rudders, one or more planer surfaces, or directional controls on the marine drive) to control pitch, roll, yaw, speed, and overall direction of travel. Platform704can also be encapsulated within a waterproof hull that is shaped to provide smooth motion through water. Further, propulsion834may include buoyancy controls to help control the depth of platform704.

Processor822is further coupled to RX sensors832that receives data from sensors510and512mounted on platform704. Although sensors510and512is illustrated inFIG. 8Bas being mounted extending from the bow of platform704, sensors510and512can be mounted anywhere on platform704where the electromagnetic field from pipeline706can be detected. Further, processor822is configured to, from the data received from sensors510and512, determine the location of pipeline706relative to platform704and the direction in which both platform704and platform702should travel to maintain contact with pipeline706and map out the location of pipeline706.

Processor822is further coupled to communication826, which allows for communications with platform702. In some embodiments, communication826may further be in communications with operators on the surface so that both platform702and platform704can be controlled externally. In some embodiments, processor822provides instructions to platform702to guide platform702over pipeline706.

Platforms702and704are powered by power units812and828, respectively. Although other power systems may be used, in some embodiments power units812and828are battery systems that can be charged prior to use.

As illustrated inFIG. 7, platform702can receive direction based on detected data from platform704. In some embodiments, platform702can further communicate its location, speed, direction of travel, and orientation to a controlling entity. Platform704may also communicate its location, speed, direction of travel, and orientation.

As is illustrated inFIG. 9, platform702and platform704communicate between a data link902. Data link902can be, for example, a physical tether (i.e., a cable connecting platform702and platform704). Alternatively, any method of transferring data between platform702and platform704can be used. In such an example, platform702and platform704can cooperate to coordinate the operation of locating pipeline706and storing the data regarding that location.

Alternatively, as is illustrated inFIG. 10A, platform702and platform704can each communicate with a host1002. Again, data link1004between host1002and platform702as well as data link1006between host1002and platform704can be a physical tether or any other data link that allows for communications of data. In some embodiments, host1002may receive sensor data from platform704and provide navigational instructions to both platform702and platform704to locate pipeline706.

FIG. 10Billustrates an example embodiment of a host1002. Host1002can be similar to platform702and platform704as illustrated inFIGS. 8A and 8B. As such, host1002can include a processor1012, which as discussed above can include one or more microcontrollers combined with sufficient volatile and non-volatile memory to analyze data and provide navigational instructions to platform702and platform704. Processor1012can be coupled to a navigation unit1014, which may be a GPS unit and/or inertial navigation. Further, processor1012is coupled to drive1020that controls propulsion and steerage unit1024, as described withFIGS. 8A and 8B. Host1002is powered by a power unit1018.

Processor1012is further coupled with communication1016, which allows for communication with both platform702and platform704. In some embodiments, host1002may receive sensor data from platform704and determine the location of pipeline706. Further, host1002may determine which direction platform704and platform702should move to continuously track pipeline706and provide instructions to platform702and platform704accordingly.

Whether platform702and platform704are in direct communication, or whether they communicate through host1002, a separation distance between platform702and platform704can be maintained within a certain range to be effective, as is illustrated inFIG. 11. A minimum distance can be set so that the direct coupling from the transmitter518to the sensors520is insignificant, and the maximum distance can be set so that the current induced in pipeline706by the transmitter518will not bleed off significantly over the distance between platform702and platform704.

The separation range between platform702, which includes transmitter518, and platform704, which includes receive sensors520, may not be fixed and can be balanced against the overall tracking performance as needed. For example, platform702and platform704may move closer together to reduce bleed-off, but compensate for stronger direct coupling by slowing down and moving closer to the target pipeline706. Alternatively, platform702and platform704may move faster to increase the tracking efficiency, but compensate by moving further apart in order to reduce the direct coupling between transmitter518and receiver system520.

Bleed-off is primarily caused by capacitive or inductive coupling of the target pipeline706to seawater, which gradually reduces the current as the distance to an injection point increases. In addition, many pipelines may have ground sites1202, for example where they are equipped with cathodic-protection anodes, such as that shown inFIG. 12. Cathodic protection anodes on a pipeline which effectively grounds pipeline706at discrete points. Grounding the line, such as at ground sites1202, leave little or no current at the side of the anode opposite to the injection point to be detected by receive sensors520in platform704. Groundings at ground sites1202may also occur through defects or deterioration in a pipeline coating that provides contact between the pipeline and the surrounding soil or seawater. In between such grounding locations such as ground sites1202, or when pipeline706is not grounded, sufficient current should flow for a few hundred meters or more, depending on the electrical characteristics of the target706and its coupling to soil or seawater. Consequently, platform702can be operated so that it remains within the separation range. In other words, platform702is at least the minimum distance separation from platform704that will substantially reduce or eliminate the effects of direct coupling while platform702is separated from platform704by less than the maximum distance of the separation range to provide for reliable tracking, as is illustrated inFIG. 11.

However, when passing ground site1202, as is illustrated inFIG. 12, most or all of the current induced on one side will generally bleed off before reaching the other side. This will largely make tracking impossible when platform702is on one side of ground site1202while platform704is on the opposite sides of ground site1202. As is illustrated inFIG. 12, very little field may be detected by platform704because the field induced in pipeline706by platform702has been grounded at ground site1202. Subsequently, a segment of pipeline706will be left without measurements between the time platform704passes ground site1202and when platform702passes ground site1202, not only causing the pipeline position and depth to be unavailable, but also forcing platform704to fly unguided over that segment. Cathodic-protection anodes, which may form ground sites1202, are typically separated by 100-300 m, so this can leave significant portions of the pipeline706unobserved, especially when the minimum separation distance of the separation range between platform702and platform704is large.

In a cable system, the same effect as ground site1202in pipeline706can occur if the cable is broken, damaged, or shorted to ground. As such, signals induced in the cable may not transmit, or the transmission is substantially curtailed, across the damaged portion of the cable. In this case, platform702and platform704can be operated around damage in a cable, or a break in pipeline706, similarly to that described below with respect to ground site1202.

FIGS. 13A and 13Billustrate one approach to the lack of measurements after each ground site1202crossing can be handled in various ways. As is illustrated inFIG. 13A, a location system1300includes platform1302and platform1304, each of which can function as a transmitter and a receiver. In other words, both platform1302and platform1304are provided with a transmitter518and a receiver520. Platform1310, which can be either one of platform1302or platform1304, is illustrated inFIG. 13C.

As is illustrated inFIG. 13C, platform1310includes a transmit platform518as well as a receive platform510. Further, platform1310includes a processor1302as described above with platforms704and706. As discussed above, processor1302is coupled to a navigation platform1304and a propulsion drive control1318to control the location and the motion of platform1310by driving propulsion1314. Further, platform1310is coupled to a communication system1306that communicates with another platform1310so that mapping of pipeline706is accomplished with a system that includes two platforms1310. Furthermore, processor1302is coupled to RX sensors1316, which receives data from sensors510and512, as well as TX driver1308, which drives transmit coil502. Platform1310further includes a power source1312.

System1300as illustrated inFIGS. 13A and 3Bincludes a pair of platforms1310, platform1302and platform1304. As described with respect toFIG. 13C, platform1304is equipped with both a transmitter and a receiver and platform1302is also equipped with both a transmitter and a receiver. In some embodiments, transmit platform518of platform1302is configured to operate at a different frequency than transmit platform518of platform1304. Receiver520of platform1302is configured to detect radiation at the frequency generated by transmit platform518of platform1304and receiver520of platform1304is configured to detect radiation at the frequency generated by transmit system518of platform1302. Consequently, both platform1302and platform1304can both transmit and receive simultaneously, allowing both to track pipeline706simultaneously.

Consequently, both platform1302and platform1304can collect tracking data until ground site1202is reached, at which point platform1304is unable to collect tracking data. As soon as platform1304passes ground site1202, platform1302is unable to collect tracking data until it passes ground site1202, at which point it begins to collect tracking data again when sensors510and512are beyond ground site1202. Platform1304will again be able to collect tracking data when transmitter coil502of platform1302passes ground site1202and again induces current in pipeline706on the same side of ground site1202as is platform1304. This allows platform1302to collect tracking data as soon as it passes the ground site1202, since now that side of ground site1202is excited by transmitter coil502of platform1304, as is illustrated inFIGS. 13A and 13B. In some systems, one of platform1302and platform1304leads and provides instructions for the other of platform1302and platform1304in order to track the length of pipeline706.

Alternatively, instead of operating transmitters and receivers continuously in both platform1302and platform1304, platform1302and platform1304can be switched between a TX mode and a RX mode as the pair transit over ground site1202. As shown inFIG. 13A, platform1302can be in TX mode to induce current in pipeline706and platform1304can be in RX mode to detect radiation from pipeline706until detectors510and512pass ground site1202. When ground site1202is detected, for example due to reduced radiation from pipeline706, platform1302can be switched to RX mode and platform1304can be switched to TX mode. As such, platform1304collects tracking data on pipeline706up to ground site1202and platform1302collects tracking data on pipeline706after ground site1202. In some embodiments, whichever of platform1302and1304is tracking data can act as lead, providing instructions to whichever of platform1302and platform1304is in TX mode. If another ground site1202is reached, platform1302and platform1304will each switch modes again as it is detected.

A similar result can be achieved without platforms with both transmitters and receivers as is illustrated inFIGS. 13A through 13C.FIGS. 14A through 14Cillustrate operation with system700having platform702and platform704in the process of passing by ground site1202. Platform702and platform704are discussed above and illustrated inFIGS. 8A and 8B.

As shown inFIG. 14A, platform702and platform704operate as discussed above until sensor510and512of platform704reach ground site1202. As illustrated inFIG. 14B, when platform704detects ground site1202, platform704comes to a stop and pauses over ground site1202. At this point, platform702can bypass platform704and positions itself with the separation range ahead of platform704. As illustrated inFIG. 14C, platform704and platform702can then proceed to collect tracking data with platform704being behind platform702in the process. In some embodiments, this configuration will continue until there is enough room behind platform704for platform702to take station within the separation range. At which point, platform702can reposition itself behind platform704so that the pair can proceed as is illustrated inFIG. 14A.

In some embodiments, as shown inFIGS. 15A, 15B, and 15C, the survey direction can be reversed once platform704has surveyed enough distance (for example twice the minimum separation distance) after passing ground site1202to instruct platform702where to start. As shown inFIGS. 15A and 15B, platform702bypasses platform704when ground site1202is passed. When enough distance has been placed between platform704and ground site1202, platform702and platform704reverse direction of travel as is illustrated inFIG. 15C. Platform704then retraces its steps while platform702flies to the starting position identified by platform702, both continue in the reverse direction until platform704passes ground site1202again, and then both return to their normal configuration, with platform704traveling along its original direction and platform702trailing behind as directed by platform704, to continue collecting tracking data of pipeline706.

The lack of guidance after platform704passes ground site1202is not easily handled. The measurement range of system700is expected to be large enough to allow some deviation, for example at least 1-2 m, from the path of pipeline706, so simple dead-reckoning may suffice in most cases. However, platform704may still lose contact with the pipeline in extreme cases, at which time platform704must reacquire the pipeline706.

Acquisition of Target Pipeline or Cable

The combination of platform702and platform704needs to find pipeline706, both at the start of a survey and when contact with the target pipeline706has been lost during the survey. If any position of pipeline706is known, this can be used as a starting position for platform702, which carries the transmitter. Positioning platform702over pipeline706will induce current on pipeline706, allowing platform704to then sweep horizontally across pipeline706until a strong signal is detected, as is illustrated inFIG. 16. During this process, the separation distance between platform702and704may in some cases be below the minimum since platform704only needs to detect a change in signal strength in order to position itself about pipeline706. Once positioned, the separation between platform702and platform704may be adjusted to fall within the separation range.

When the position of pipeline706is not known well enough to place platform702in its starting position, then both platform702and704cooperate to locate and lock onto it, as is illustrated inFIGS. 17A and 17B. In this case, platform702and platform704may line up and then both move sideways towards the expected location of pipeline706. An increase in signal strength measured by platform704will occur if both cross the pipeline at approximately the same time, allowing platform702and platform704to establish contact. If no such contact is established another pass can be made at a slightly different heading to locate pipeline706.FIGS. 17A and 17Billustrate platform702and platform704making two passes along different directions to locate pipeline706. Again, the minimum separation distance may need to be satisfied in some cases, but not others. Once pipeline706is located, then the separation between platform702and platform704may be adjusted to fall within the separation range.

The above detailed description is provided to illustrate specific embodiments of the present invention and is not intended to be limiting. Numerous variations and modifications within the scope of the present invention are possible. The present invention is set forth in the following claims.