Patent Description:
Many modem services rely upon a network of pipes to carry or distribute fluids. Examples include fresh water, waste water and sewage, and fuels such as oil or gas. It is common to monitor the operation of the network and the condition of pipes. In this manner, blockages, leaks or other issues can be identified and scheduled for repair.

Where pipes are provided above ground, monitoring may be achieved by visual inspection of the pipe exterior. In many cases, pipes are not accessible to visual inspection, being buried underground. Accordingly, pressure/audio sensors or the like may be utilised to detect vibrations of the pipe and thereby provide information on conditions within a pipe.

If a pipe operator has sufficient budget, pressure/audio sensors can be permanently fitted periodically along the pipe, the sensor spacing being determined by a combination of sensitivity, expense and convenience. Typically, for convenience, these sensors might be located adjacent to pipe access points such as valve, junction or pump spaces. This has the benefit of enabling sensor signals to be readily relayed away from the pipe and for enabling ready access to fit and check the operation of such sensors. Arrangements of this type are able to provide an indication that there is a problem, for instance a leak. However sensors are widely spaced and it can therefore be difficult to identify a problem. Additionally, where it is possible to identify a problem it may be difficult to identify the location of the problem more precisely than between two fixed sensors or an approximate distance along the pipe from a sensor location. For instance, in water pipes existing sporadically located pressure/audio sensors might only be able to define the location of a problem with very poor locational accuracy or certainty.

In addition to or in place of permanently fixed sensors, it is possible to monitor the condition of a pipe by using a drone or the like introduced into the pipe. The submersible is fitted with one or more sensors and moves along a length of pipe between suitable access points. Accordingly, such a method is only applicable where the pipe does have access points suitable for introducing a submersible into the pipe. The submersible may move along the pipe under its own power or in response to the fluid flow within the pipe. Since the submersible is moving within the pipe, flow noise associated with this movement or noise associated with powering the submersible may mask sensing of relatively minor problems. Specifically a submersible is only able to define the location of a problem if it occurs when the submersible is actually passing by the event, the detection is transitory and such that it is not distributed or real-time.

Assuming either monitoring arrangement above can determine the location of a leak relative to a sensor location or access point, the pipe will still need to be accessed at that location to be repaired. Where pipes are buried underground this may still present a significant challenge. For instance, when considering water supply pipes in rural areas, whilst the location of valve chambers are typically well catalogued, the precise route that the pipe takes between these points may not be recorded. Accordingly, if a leak is suspected at a distance of <NUM> from a valve, a repair team may seek to find the pipe by digging a pilot hole <NUM> from the valve along the suspected route of the pipe. If such a hole does not locate the pipe, a series of additional exploratory holes within a wide arc must be dug until the pipe is located. This technique is known as 'pot-holing' and can be highly time-consuming, costly and disruptive. In the event that the pipe takes a meandering route and/or where the location of the leak along the pipe known only to a few tens or hundreds of meters, it may still be necessary to dig further holes to access the actual leak location. Such difficulties add considerably to the expense of pipe repair. In some instances, they may lead to decisions being taken to tolerate minor leaks for some time rather than attempt repairs immediately, and, leaks particularly in metal pipes (such as corrosion holes) may deteriorate very rapidly without corrective action.

In view of the above issues, distributed acoustic sensing (DAS) has been used for monitoring pipes. DAS involves the detection of backscattering of light pulses introduced into an optical fibre. The time of arrival and intensity of the backscattered light is measured for each pulse, the time at which the backscattered light is detected being related to the distance along the fibre the light has travelled before being scattered. Subsequent changes in the reflected intensity of successive pulses from a common region of the fibre correspond to variations in the strain applied to the fibre at that region, for instance due to vibrations experienced by the region of fibre. In this manner, the DAS fibre can act as a plurality of virtual microphones along the length of the fibre and can locate events causing acoustic signals down to an accuracy of around <NUM> meter.

DAS has been applied to above ground pipes (and alongside buried pipes when installed at the time of constructing the pipe) by affixing fibres directly to the exterior of such pipes. This provides good acoustic coupling between the fibres, the pipe and the fluids within the pipe. For above ground pipes it also enables ready installation and ready access for maintenance. Nevertheless, such exterior fibres are exposed both to accidental damage and to intentional damage.

As accessing the exterior of an existing buried pipe is difficult DAS has been applied to buried pipes by burying a DAS fibre in close proximity (typically <NUM>-<NUM>) to the pipe. Even so, works required for burying a DAS fibre can be expensive and highly disruptive, particularly where the buried pipe runs through an urban area. Additionally, separately buried DAS fibres suffer from limitations in sensitivity, particularly where ground conditions provide poor or highly variable acoustic coupling to the pipe. Furthermore, such separately buried DAS fibres are still susceptible to damage.

<CIT> discloses a fibre surveillance system for monitoring a pipeline comprising one or more optical fibres acoustically coupled to the pipeline to detect acoustic signals associated with vibrations or other activity near or from the pipeline. Optical energy is injected into the optical fibre and an optical detector receives an optical return-signal. An analyser is configured to determine operating information about the pipeline based on the optical return-signal.

<CIT> discloses methods and apparatus for distributed acoustic sensing using a fibre optic distributed acoustic sensing (DAS) apparatus comprising at least one optical fibre deployed to monitor the acoustic response of a cavity to incident acoustic signals. The cavity is dimensioned such that the cavity resonates at a desired frequency and thus the relevant sensing portions of the DAS sensor show an enhanced response to acoustic signals which excite resonance in the cavity.

<CIT> discloses a method for monitoring for seismic events by interrogating an optic fibre which forms part of an existing communications infrastructure to provide distributed acoustic sensing (DAS).

It is an object of the present invention to provide methods and apparatus which at least partially overcome or alleviate at least some of the above problems.

According to a first aspect of the present invention as defined in claim <NUM> there is provided a method of monitoring a fluid pipe, the method comprising the steps of: providing a distributed acoustic sensing (DAS) fibre within the pipe; introducing coherent light pulses into the fibre; detecting backscattered light from the fibre; and processing the backscattered light so as to obtain information about the condition of the pipe, wherein the step of providing the DAS fibre within the pipe comprises: transporting a microduct along the pipe, aided by the provision of a sail structure attached to an end of the microduct and pulled along by a fluid flow within the pipe, and, following introduction of the microduct, subsequently blowing the DAS fibre along the microduct.

The invention is further defined in the appended claims.

By providing a DAS fibre within a pipe, good acoustic coupling between the DAS fibre and the pipe is assured. Accordingly, DAS fibre can be used to detect pipe condition information including pressure waves, temperature changes, flow noise, orifice noise or the like. Where location or sensitivity permits, the DAS fibre may also detect and monitor vibrations from sources outside the pipe such as nearby traffic. Furthermore, a fibre within the pipe is more resistant to accidental damage or intentional damage than an external fibre. Additionally, introducing a fibre within an existing buried pipe can be less disruptive and less expensive than burying a DAS fibre in close proximity to the pipe.

The pipe may be a pipe carrying any suitable fluid. In particular, suitable fluids might include but are not limited to: water, waste water, sewage, and fuel such as oil, gas, distillates or the like and chemical or mining products.

The DAS fibre may be a single fibre. The DAS fibre may be a dedicated fibre within a bundle of fibres. The bundle of fibres may form a multicore fibre cable. The DAS monitoring preferably operates from one end only. As such the DAS fibre may be a single ended fibre.

Preferably a barrier is provided between the DAS fibre and the fluid. This can protect the DAS fibre from damage from the fluid or debris within the fluid. The barrier may comprise a coating or cover provided over the DAS fibre. In the event that the DAS fibre is provided within a multicore fibre, the barrier may comprise a coating or cover provided over the multicore fibre. The barrier comprises a microduct within which the fibre is provided. The method includes the step of introducing the microduct to the pipe and subsequently blowing the DAS fibre along the microduct.

In the event that there is a gap between the DAS fibre and the barrier, the gap may be filled with gel. This can improve acoustic coupling between the DAS fibre and the pipe or fluid within the pipe. The method may include the step of introducing a gel between the DAS fibre and the barrier.

The method may include the step of installing the DAS fibre in the pipe. The fibre may be installed temporarily. In such instances the method may include the step of removing the DAS fibre from the pipe after use.

Introducing the DAS fibre may include the steps of forming an aperture in the pipe wall and introducing a DAS fibre through the aperture. The DAS fibre can then be run along the interior of the pipe to a desired end point or to a desired exit point. Similarly, where the fibre is required to exit the pipe, the method may include the steps of forming an aperture through the pipe wall and removing the DAS fibre from the pipe.

The aperture may be provided with a fitting operable to provide a seal between the DAS fibre and the edges of the aperture. The fitting may be adapted to enable the formation of an aperture.

The method may comprise the steps of depressurising the fluid pipe before forming the aperture and installing a leak tight coupling around the fibre at each aperture. In other embodiments, the aperture may be formed and the DAS fibre introduced without depressurising the pipe. Numerous such 'hot tap' techniques are known in the art.

The microduct is transported along the interior of the pipe by the fluid flow within the pipe. This transport is aided by the provision of a sail structure attached to the microduct.

The sail structure may be collapsible. This facilitates introduction/removal through the aperture and withdrawal of the DAS fibre against the fluid flow.

The DAS fibre is blown along the microduct.

The DAS fibre may run at any suitable position within the cross-section of the pipe. In some embodiments, the DAS fibre runs within the fluid separated from the pipe walls. The DAS fibre may have a neutral buoyancy. This helps to retain a position separated from the pipe walls.

In other embodiments, the DAS fibre may lie alongside a pipe wall. This may be achieved by the DAS fibre having a positive buoyancy or negative buoyancy as appropriate. Alternatively the DAS fibre may be secured to the pipe walls. In some such examples not forming part of the claimed subject matter, the DAS fibre may be secured to the sides or top of the pipe. In other such examples not forming part of the claimed subject matter, the DAS fibre may be secured to the base of the pipe. This location may be particularly suitable for pipes that do not carry a full fluid load at all times, for example gravity flow systems, as this ensures that the DAS cable is immersed in the fluid and can thus monitor the fluid flow.

In some examples not forming part of the claimed subject matter, the DAS fibre may run within a microduct integrally formed within a pipe liner. In such examples, the method may include the additional steps of installing a pipe liner incorporating an integral microduct. In such examples not forming part of the claimed subject matter, the method may include the additional steps of blowing DAS fibre along the integrated microduct. In such examples not forming part of the claimed subject matter, where a particular buoyancy is required the microduct and DAS fibre combination are adapted to provide the required buoyancy.

A pipe liner with an integral microduct may comprise an elongate duct formed from multiple laminated layers of thermoplastic material, where heating means are provided within the pipe liner, the liner adapted to provide a cable duct between two laminate layers and wherein the cable duct is formed from a thermoplastic material having a higher transition temperature than the thermoplastic material forming the laminate layers. Such a pipe liner may be installed by the method of inserting the pipe liner into the pipe; heating the pipe liner; and subsequently pressing the pipe liner against the interior surface of the pipe. Such a liner and method of installation is disclosed in our earlier application published as <CIT>.

Substantially the full length of the DAS fibre or at least the full length of the DAS fibre used for sensing may be within the pipe. In other examples not forming part of the claimed subject matter, the DAS fibre may include lengths within the pipe and lengths outside the pipe. In particular, the DAS fibre may lie within the pipe in unencumbered sections of the pipe and may exit and re-enter the pipe on either side of pipe machinery. In this context, pipe machinery may include, but is not limited to valves, pumps, junctions or the like as well as related building or land assets.

In some embodiments, the DAS fibre may be used to monitor multiple pipe segments. In some examples not forming part of the claimed subject matter, the DAS fibre may be spliced to create a continuous link for sensing.

In some examples not forming part of the claimed subject matter where DAS fibre leaves the pipe, the lengths of fibre outside the pipe may be utilised for monitoring activity outside the pipe. In one example, DAS fibre may exit the pipe at pipe machinery and lie buried around the pipe machinery before re-entering the pipe. In this manner, the buried section of the DAS fibre may be used for monitoring activity at or around the pipe machinery. In particular, this might include monitoring access to the pipe machinery, especially in the case of building or land assets, by detecting vehicles or individuals crossing the buried DAS fibre.

The light emitter may be a laser. The emitted light may any suitable wavelength for transmission along and backscattering within the DAS fibre. The light emitter and light detector may be integrated into a light transceiver unit.

Light emission may be controlled in order to vary any one or more of: pulse frequency, pulse length and pulse intensity of the emitted light. Detected backscattered light may be processed to determine vibration amplitudes and frequencies experienced by particular scattering points on the fibre and hence particular locations along the pipe. The method may include the step of filtering the received vibration signals. The filtering may be in respect of time of receipt (and hence location along the DAS fibre) or in respect of vibration frequency, vibration amplitude, or a combination thereof. In particular, the combination may include matching vibration frequency and amplitude against expected signatures of particular events. This can enable the method to be focussed on detecting or excluding particular sources of vibration. In one example, this could be orifice noise caused by fluid leaking from the pipe. In other examples, this could be pressure waves within the pipe or temperature variations within the pipe. In still further implementations, the processing unit may be operable to detect vibrations associated with activity outside the pipe. For instance, this may include the detection of traffic on roads overlying or close to a buried pipe.

The processing and/or filtering of detected light signals may be carried out by a processing unit. The processing unit may be in communication with the light detector. The processing unit may be in communication with the light emitter. In such embodiments, the processing unit may be operable to control the light emitter. The processing unit may be provided locally to the light detector and the light emitter.

In some embodiments, the light emitter, light detector and processor may be integrated into a pipe sensor unit. Such a sensor unit may be provided with a user interface. The user interface may enable a user to control operation of the pipe sensor unit and/or review indications relating to the condition of the monitored pipe.

The examples not forming part of the claimed subject matter may include the additional step of transmitting data along the DAS fibres. The data may be simplex data. The data may be transmitted in a non-contact fashion. In one example not forming part of the claimed subject matter, data may be encoded and transmitted by applying vibrations to the pipe, the fluid or to the DAS fibre. The vibrations may be applied by a vibrator unit. Preferably, the vibrator unit is directly coupled to the fibre. The applied vibrations may be encoded using a dual tone multiple frequency (DTMF) scheme. Such schemes provide a robust and reliable decoding of signals.

The vibrator unit may be connected to a processing device or to a sensor operable to sense the condition of the pipe, the condition of pipe machinery, or the presence of personnel. In the case of pipe machinery, this may include sensors monitoring the condition of physical access portals such as doors, gates, lids or the like. The encoded data may relate to the output of the processing device or sensor.

The method may include the additional step of sporadically transmitting a keep- alive signal from the vibrator unit. This can be used to verify continuing correct operation of the vibrator unit. This thereby helps overcome the limitations of simplex data transmission in such circumstances.

The method may include the additional steps of locating the route of an underground pipe. This may be achieved by successively tamping the ground surface at a number of locations in the vicinity of the suspect route of the pipe; processing the backscattered light so as to determine variations in the magnitude of vibrations due to the tamping at each location and thereby determining the route of the pipe.

The tamping may be carried out using any suitable tamping device, including both manual and powered devices. The method may include the step of filtering backscattered light to select vibrations at frequencies corresponding to those caused by the tamping or having acoustic signatures characteristic of tamping.

The tamping may be carried out at a series of regularly spaced locations along a line lying across the expected route of the pipe. In some implementations two or more parallel lines of tamping may be carried out. In some implementations, the tamping may be carried out at locations defined by a pre-set grid.

According to an example not forming part of the claimed subject matter, there is provided a method of locating the route of an underground pipe, the method comprising the steps of: providing a DAS fibre within the pipe; introducing coherent light pulses into the fibre; detecting backscattered light from the fibre; successively tamping the ground surface at a number of locations in the vicinity of the suspect route of the pipe; and processing the backscattered light so as to determine variations in the magnitude of vibrations due to the tamping at each location and thereby determining the route of the pipe.

The method of this example may incorporate any or all features of the first aspect of the present invention.

According to another example not forming part of the claimed subject matter, there is provided a method of transmitting data along a pipe, the method comprising the steps of: providing a DAS fibre within the pipe; introducing coherent light pulses into the fibre; detecting backscattered light from the fibre; applying vibrations to the DAS fibre, the vibrations encoding data; processing the backscattered light so as to detect the applied vibrations.

According to another example not forming part of the claimed subject matter, there is provided an apparatus for transmitting data along a pipe, the apparatus comprising: a distributed acoustic sensing (DAS) fibre provided within the pipe; a light emitter for introducing light pulses into the fibre; a light detector for detecting backscattering of the said light pulses; and a vibrator unit acoustically coupled to the DAS fibre, the vibrator unit operable to apply vibrations to the DAS fibre, the vibrations encoding data.

In order that the invention may be more clearly understood one or more and examples, not falling within the scope of the claimed invention, thereof will now be described, by way of example only, with reference to the accompanying drawings, of which:.

Turning now to <FIG>, a fluid pipe <NUM> is monitored using a distributed acoustic sensing (DAS) fibre <NUM>. In the description below, the invention is primarily described in terms of monitoring of a pipe carrying water. Nevertheless, the skilled man will appreciate that the invention may be applied to pipes <NUM> carrying other fluids including, but not limited to, waste water, sewage or fuels such as oil, gas distillates or the like and chemical or mining products.

The DAS fibre is coupled at one end to a light emitter <NUM>, typically a laser and a light detector <NUM>. The light emitter <NUM> emits light pulses into the DAS fibre. The light detector <NUM> detects backscattered light from the DAS fibre, the time of arrival of the backscattered light following the emission of a pulse relating to the location of the backscattering site along the DAS fibre. Vibrations propagating through the fluid around the DAS fibre <NUM> result in corresponding vibration of the DAS fibre <NUM>. Vibrations of the fibre cause variation in the backscattering that occurs from each backscattering site. Accordingly, these variations can be used to provide an indication of the vibration experienced by each section of the DAS fibre. A processing unit <NUM> may be provided to process the detected light and thereby provide an output indicative of vibrations imposed upon the DAS fibre <NUM> along its length. The processing unit <NUM> will typically be local to the light emitter <NUM> and light detector <NUM> but may be alternatively provided at a remote location. In the latter case, a communication unit (not shown) would be operable to communicate remotely with the processing unit <NUM>.

The processing unit <NUM> may be operable to identify vibrations as being characteristic of particular pipe events. This may be achieved by determining the frequencies or amplitudes of vibrations or by filtering selected frequencies of vibration. Common pipe events that might be detected beyond orifice noise and negative pressure waves indicative of leaks include flow noise, pressure waves indicative of operation of pipe machinery (valves, pumps or the like) or the change of fluid temperature. Where sensitivity permits, events external to the pipe may also be detected and monitored.

Turning now to <FIG>, the DAS fibre <NUM> is provided within pipe <NUM>. In the event that the pipe <NUM> has a leak <NUM>, vibrations <NUM> characteristic of orifice noise will travel through the fluid until they impinge on the DAS fibre <NUM>. Subsequent operation of the processing unit will determine the occurrence of vibrations of the DAS fibre <NUM> and the position along the DAS fibre <NUM> at which these vibrations occur. Accordingly, the position of the leak <NUM> along the length of pipe <NUM> can also be determined.

As shown in <FIG>, the pipe <NUM> runs under a road <NUM>. Vehicles <NUM> travelling along the road <NUM> generate vibrations <NUM> which can travel though the ground to the pipe <NUM>. The vibrations <NUM> can also be detected on DAS fibre <NUM>. Accordingly, the DAS fibre <NUM> can be used to monitor traffic flow on road <NUM>. In other situations, the DAS fibre <NUM> can be used to monitor other activity external to the pipe <NUM>.

In the example shown, the DAS fibre <NUM> is a single dedicated fibre in a multicore cable <NUM> formed from a plurality of fibres. The multicore cable <NUM> may be provided with a protective exterior coating (not shown). Furthermore, the multicore cable <NUM> is provided within a microduct <NUM>. The microduct <NUM> forms a barrier between the cable <NUM> and the fluid within pipe <NUM>.

The microduct <NUM> is introduced into the pipe <NUM> through an aperture (not shown) in the pipe wall. As shown in <FIG> and <FIG>, typically, this aperture will be provided with a suitable fitting <NUM> that provides a seal between the microduct <NUM> and the edges of the aperture. Such a fitting <NUM> can be installed whilst the pipe <NUM> is drained of fluid. Alternatively, the fitting <NUM> can enable the formation of an aperture and the subsequent introduction of a microduct <NUM> using so called 'hot tap' techniques known in the art that do not require the pipe <NUM> to be drained. Hot tap techniques are particularly suitable in instances where the DAS fibre <NUM> is installed temporarily, for instance to establish the location of a suspected leak. Non-limiting examples of fittings and techniques for introducing fibres and/or microducts to pipes are also disclosed in our prior patent applications <CIT>, <CIT>, <CIT>, <CIT> and <CIT>.

After introduction, the microduct <NUM> is transported along the pipe <NUM>. The microduct <NUM> is pulled along by fluid flow F within the pipe <NUM>. As is shown in <FIG>, this is aided by the provision of a sail structure <NUM> attached to the end of the microduct <NUM>. Where the microduct <NUM> is introduced temporarily, the sail structure <NUM> may be collapsible. This can aid in subsequently withdrawing the microduct <NUM> through the fitting. In alternative examples not forming part of the claimed subject matter, the DAS fibre <NUM> or microduct <NUM> is towed by a powered submersible introduced into the pipe <NUM>.

Following introduction of the microduct <NUM>, the DAS fibre <NUM> is blown along the microduct <NUM>. The microduct <NUM> may be filled with acoustic gel (not shown). This can improve acoustic coupling between the microduct <NUM> and the DAS fibre <NUM>.

In the event that a pipe <NUM> to be monitored incorporates pipe machinery such as valves or pumps or associated buildings or land assets, the DAS fibre <NUM> may exit and re-enter the pipe <NUM> on either side of the machinery. An example of such a situation is shown in <FIG>, where pipe <NUM> is fitted with an in-line stop valve <NUM>. As shown, microduct <NUM> exits the cable on one side of the valve <NUM> via a fitting <NUM> providing a seal between the microduct <NUM> and the edges of an aperture in the pipe <NUM>; and re-enters the pipe <NUM> on the other side of the valve <NUM> via a corresponding fitting <NUM>. In such examples, the DAS fibre <NUM> may run continuously around the valve <NUM>. Alternatively, such a bypass may provide a convenient point at which to provide a splice <NUM> connecting together different sections of DAS fibre <NUM>.

In order to ensure monitoring is confined to the sections of DAS fibre <NUM> within the pipe <NUM>, the processing unit <NUM> may be operable to disregard backscattered light where the time of detection indicates that it was backscattered from the section of DAS fibre <NUM> outside the pipe <NUM>. In other embodiments, the processing unit <NUM> may be operable to separately process light backscattered from the section of DAS fibre <NUM> outside the pipe <NUM>. This can allow separate monitoring of activity outside the pipe <NUM>. In some examples, such as those shown in <FIG>, an extended section <NUM> of DAS fibre <NUM> may be provided. The extended section <NUM> may be buried around the perimeter <NUM> of accessible pipe machinery comprising a building or land asset such as a pumping station or the like. Monitoring light backscattered from the extended section <NUM> can detect vibrations <NUM> characteristic of the crossing of perimeter <NUM> by persons or vehicles. This can enable unauthorised perimeter crossings, which may indicate unauthorised access, to be detected.

The position of the DAS fibre <NUM> (or microduct <NUM>) within the pipe <NUM> may be varied as appropriate. In <FIG>, the microduct <NUM> containing the DAS fibre <NUM> runs substantially along the centre of the pipe <NUM>. This position is advantageous in that it equally exposed to leaks from all sides of the pipe <NUM>. It is also relatively simple to allow DAS fibre <NUM> (or microduct <NUM>) to assume this position within the fluid on introduction, via a neutral buoyancy. Accordingly, this positioning is convenient for temporary installations.

In some cases it may be desirable to position the DAS fibre <NUM> or microduct <NUM> in an alternative position such as close to a wall of the pipe <NUM>. This can be achieved by the DAS fibre and/or microduct having positive or negative buoyancy, or by the use of suitable brackets or manifolds. Such positioning may be employed in order to minimise the effect of the microduct <NUM> on fluid flow or so as to ensure that the microduct <NUM> remains immersed in fluid, for instance in gravity fed systems. , Additionally, this portion of the pipe <NUM> remains immersed in fluid in most conditions, thereby improving the acoustic coupling between the pipe and the DAS fibre <NUM> or microduct <NUM>.

Turning now to <FIG>, this illustrates an example not forming part of the claimed subject matter where the DAS fibre <NUM> is provided within a microduct <NUM> integrally formed between two layers of a pipe <NUM> or of a pipe liner <NUM> installed within the pipe <NUM>. Pipe liners <NUM> of this type and methods for installing such pipe liners <NUM> are disclosed in our prior patent <CIT>.

In the example of <FIG>, the pipe liner <NUM> is orientated such that the integral microduct <NUM> is provided at the base of the pipe <NUM>. Nevertheless, the skilled man will appreciate that alternative orientations of pipe liner <NUM> are possible.

<FIG> also illustrates the case of a pipe <NUM>, such as a sewer, which is not always filled with fluid. In such instances, where there is a fluid level <NUM> partway up the pipe <NUM>, better performance can be obtained by laying the DAS fibre <NUM> along the base of pipe <NUM> as vibrations <NUM> are more readily coupled to the DAS fibre in this position.

Turning now to <FIG>, this illustrates use of a DAS fibre <NUM> in a pipe <NUM> in order to locate the route of a buried pipe <NUM>. This is achieved by successively tamping the ground surface at a number of locations A-E in the vicinity of the suspect route of the pipe <NUM>. The tamping generates tamping vibrations <NUM>. The tamping may be carried out by any suitable item, for example a manually operated or powered device.

The backscattered light associated with each tamping location A-E is processed so as to determine variations in the magnitude of tamping vibrations detected from each location A-E. The tamping is carried out at a series of regularly spaced locations A-E along a line lying across the expected route of the pipe. Comparing the tamping vibrations detected from each location A-E allows a determination of the route of the pipe <NUM> to be made.

As shown in vibration detection level graphs a-e of <FIG>, tamping at locations A and E furthest from the route of pipe <NUM> results in the detection of relatively weak tamping vibrations <NUM>. Tamping at locations B and D closer to the route of pipe <NUM> results in the detection of stronger tamping vibrations <NUM> and tamping at location C directly above the route of the pipe <NUM> results in the detection of the strongest tamping vibrations <NUM>. To improve performance, the processing unit <NUM> may filter the output of the light detector <NUM> to preferentially select frequencies characteristic of tamping vibrations <NUM>.

In instances where two tamping locations result in the detection of similar maximum strength tamping vibrations <NUM>, it may be deduced that the route of the pipe <NUM> lies between these locations. If this does not provide sufficient clarity on the pipe <NUM> route, then additional tamping can be carried out at a series of additional locations between the two locations.

Where the extended route of a pipe <NUM> is to be determined, the tamping locations may be arranged in two or more rows or a grid over the suspected route of the pipe <NUM>.

Turning now to <FIG>, this illustrates to use of a DAS fibre <NUM> within a pipe <NUM> for the simplex transmission of data. In this example, a vibrator unit <NUM> is coupled to the DAS fibre <NUM> via microduct <NUM>. The vibrator unit <NUM> may be connected to a processing device or to a sensor (not shown) operable to sense the condition of the pipe <NUM>, the condition of pipe machinery, or to identify the presence of personnel working on a section of pipe, pipe machinery or within an asset. The vibrator unit <NUM> is operable to receive data from the processing device or sensor and encode the data into vibrations <NUM> applied to the DAS fibre. As shown in table <NUM> in <FIG>, the vibrator unit <NUM> may encode data using a dual tome multi-frequency (DTMF) scheme to provide robust and reliable communications. To overcome the limitations of simplex data transmission, a keep-alive signal may optionally be broadcast from each vibrator unit <NUM>, on a sporadic basis, to verify continuing correct operation of the vibrator unit <NUM>.

Claim 1:
A method of monitoring a fluid pipe (<NUM>), the method comprising the steps of:
providing a distributed acoustic sensing (DAS) fibre (<NUM>) within the pipe (<NUM>);
introducing coherent light pulses into the fibre (<NUM>);
detecting backscattered light from the fibre (<NUM>); and
processing the backscattered light so as to obtain information about the condition of the pipe (<NUM>),
characterized in that
the step of providing the DAS fibre within the pipe comprises:
transporting a microduct (<NUM>) along the pipe, aided by the provision of a sail structure (<NUM>) attached to an end of the microduct (<NUM>) and pulled along by a fluid flow within the pipe, and, following introduction of the microduct (<NUM>), subsequently blowing the DAS fibre along the microduct.