Improvements in or relating to fibre optic cables

Abstract

Fibre optic cables (100), for use in a data communications network are adapted to be laid, along at least part of their length within a pipe (or conduit) (1) carrying a fluid and comprise fibres adapted for sensing the condition within or in the vicinity of the pipe (1). The cables (100) comprise one or more data fibre bundles (11) and armour (32) in the form of a layer of protective wires provided around the data fibre bundles (11). The cable (100) is additionally provided with outer sensing fibres (14) which replace one or more wires in the armour (32). The cables (100) are optionally adapted to remain stably positioned in the invert (2) of the pipe (1) by suitable selection of stiffness and relative density compared to the fluid. The cables (100) are adapted to remain stably positioned in the invert (2) of the pipe (1) by the provision of surface formations to reduce lift and / or drag forces experienced by the cable as a result of fluid flow.

Description

[0001] IMPROVEMENTS IN OR RELATING TO FIBRE OPTIC CABLES

[0002] Technical Field of the Invention

[0003] The present invention relates to improvements in or relating to fibre optic cables. In particular, the present invention relates to improvements in or relating to fibre optic data cables suitable for use in a data communication network and to a data communication network incorporating such cables. In some embodiments, the invention may further relate to fibre optic data cables adapted to be laid in fluid pipes to enable monitoring of events within or near, and / or conditions of fluid pipes and / or transmission of power alongside data.

[0004] Background to the Invention

[0005] Many modem services rely upon pipes to carry or distribute fluids. Examples of fluids that can be distributed in this manner include fresh water, waste water and sewage, and fuels such as oil or gas or associated distillates, additionally in industrial uses which may include mining and similar. It is common to monitor the operation of the fluid system and the condition of pipes by using dedicated sensors at suitable fixed locations around the system. In this manner, blockages, leaks or other issues can be identified and scheduled for repair.

[0006] 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, sporadic pressure / audio sensors or the like may be utilised to detect vibrations of the pipe and thereby provide information on conditions within a pipe. Alongside other forms of sensing it is possible to monitor many aspects of the fluid or pipe using distributed acoustic sensing (DAS) (otherwise referred to as distributed vibration sensing (DVS)). 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. By corresponding analysis of back scattered light, it is also possible to carrying out distributed strain sensing (DSS) and / or distributed temperature sensing (DTS) using the same sensing fibre or additional sensing fibres. DSS can help identify a change in strain on a pipe indicative of a pipe failure or potential for future failure. DTS may help identify temperature change indicative of a leak or other issue.

[0007] Whilst DAS is effective in many situations for monitoring flow within a pipe and detecting leaks, additional information on the condition of the pipe can be obtained by carrying out DSS and / or DTS using a sensing fibre. DTS may help identify adiabatic cooling in a gaseous fluid in the vicinity of a leak candidate and / or other temperature change indicative of a flow of heat between a pipe and surrounding ground of a different temperature or a likely additional strain on the pipe. DSS may help identify changes in strain along the length of a pipe indicative of a potential for future failure of the pipe.

[0008] In recent times, there has been an increase in demand for bulk data communications, for instance broadband internet and the increasing density of cells for mobile data services such as 5G. Data communication networks which provide such capacity typically comprise one or more local connections to individual homes or business or cellular base stations to a local junction box / node which feed into one or more trunk data communication cables. The data cables each comprise a bundle of typically single mode optical fibres. To protect such data cables from damage said cables are preferentially laid within a conduit. Providing a dedicated conduit that does not impede other utilities can be difficult and expensive. Accordingly, some recent data networks include data cables laid may be within fluid pipes. This can provide a convenient way for data cables to be routed utilising an existing conduit without impacting other utilities.

[0009] Once laid in a fluid pipe, there can be interaction between the fluid and the cable. In many pipes, the cable will be positioned within the invert of the pipe to experience minimised fluid flow velocity, and maximised distance from any appurtenances (such as air release valves) or spurs off the pipe. Nevertheless, interaction between the cable and the flow can lead to friction or turbulence within the flowing fluid. Additionally, the cable may experience lift and / or yaw forces which can cause the cable to move or flutter within the flow. These effects can introduce noise into any DAS, DSS or DTS signals from sensing fibres and may also put an undesirable strain on the cable. Whilst this can be partially addressed by pinning the cable into position using one or more brackets, this adds complexity and expense to cable installation. The brackets may also create a snagging hazard and / or pockets of standing fluid within the pipe. Standing fluid is a particular issue in fresh water supply pipes since this creates an opportunity for biological growth.

[0010] Within a cable containing both data fibres and sensing fibres, it is desirable to provide multiple sensing fibres for redundancy, such fibres optimised for differing sensing modes or sensitivity, and to accommodate for complex tree & branch routing. It is also desirable that sensing fibres are provided at locations with good acoustic coupling to the exterior of the cable, typically closer to the exterior of the cable. Similarly, it is desirable that sensing fibres are positioned within the cable so as not to limit sensitivity in particular directions. Nevertheless, there is a possibility that in some locations (for instance close to a pump) background noise may be high and therefore some shielding or reduced sensitivity is desirable. These requirements also have to be balanced against the desire to allow the majority of fibres to be used for carrying data.

[0011] Where the fluid distribution system comprises dedicated sensors other than sensing fibre, these sensors are typically powered by a local low power / low voltage source such as a battery. Nevertheless, to reduce the frequency of the need to replace batteries typically sensor data is stored locally and only transmitted to the system controller periodically. For instance, in many such systems the sensors may be set to transmit data only once in every 24 hour period. Whilst periodic transmission allows for a significant extension of battery life for each sensor this also limits the possibility of using real-time sensor data to monitor and / or control fluid distribution system operation.

[0012] It is an object of the present invention to provide a cable which at least partially overcomes or alleviates at least some of the above problems.

[0013] Summary of the Invention

[0014] According to a first aspect of the present invention, there is provided a fibre optic cable suitable for use in a fluid pipe, the cable comprising: one or more elongate data fibre bundles within an outer sheath wherein each data fibre bundle comprises multiple optical fibres configured for data transmission, wherein the outer sheath has an external surface upon which are provided one or more surface formations adapted to mitigate friction or turbulence between the cable and flowing fluid and / or mitigate lift or yaw forces generated by interaction between the cable and the flowing fluid.

[0015] The cable of the present invention is thereby adapted to reliably lie within the invert of a fluid pipe during fluid flow due to the surface formations mitigating lift and / or yaw forces generated by the fluid flow. Any sensing fibres within the cable may experience reduced noise due to reduction in turbulence between the cable surface and the fluid flow and / or the reduction in movement of the cable within the pipe.

[0016] In the context of the present application, the term ‘fluid’ in relation to a fluid conduit may refer to any material, liquid or gaseous, including and fuels such as oil or gas or associated distillates, additionally in industrial uses which may include mining and similar, the invention may more specifically refer to a primarily water-based fluid, such as potable water, pre-treatment water, wastewater or water-based slurries.

[0017] Similarly, in the context of the present application, the term ‘pipe’ or ‘conduit’ may refer to any fluid conduit used to convey a fluid (gas, liquid or a mixture including a slurry) between two points, spaced apart at or below local ground level. In particular, the ‘pipe’ or ‘conduit’ may traverse at a relatively regular displacement from local ground level, without requiring the ‘pipe’ or ‘conduit’ to be horizontal.

[0018] In some embodiments, the outer sheath has an external surface upon which are provided one or more surface formations adapted to mitigate friction or turbulence between the cable and flowing fluid and / or mitigate lift or yaw forces generated by interaction between the cable and the flowing fluid.

[0019] The surface formations may comprise a repeating pattern of surface formations. The repeating pattern may run substantially parallel to or helically around the axis of the cable. The surface formations may comprise projecting features. The surface formations may comprise recessed features. The surface formations may comprise both projecting and recessed features.

[0020] Projecting features may comprise pimples or ribs. Pimples may be substantially round, substantially oval or any other suitable shape. Pimples may have a symmetrical or asymmetrical cross-sectional profile parallel to cable axis. Pimples may have a symmetrical or asymmetrical cross-sectional profile perpendicular to the cable axis.

[0021] Ribs may be aligned parallel to the axis of the cable. Ribs may be provided in a helical formation relative to the axis of the cable.

[0022] Recessed features may comprise dimples or grooves.

[0023] Dimples may be substantially round, substantially oval or any other suitable shape. Dimples may have a symmetrical or asymmetrical cross-sectional profile parallel to cable axis. Dimples may have a symmetrical or asymmetrical cross-sectional profile perpendicular to the cable axis.

[0024] Grooves may be aligned parallel to the axis of the cable. Grooves may be provided in a helical formation relative to the axis of the cable.

[0025] In some embodiments, pimples or dimples may have a width or length in the region of 0.25mm-4mm. In some such embodiments, pimples or dimples may have a width or length in the region of 0.5mm- 1mm, 1mm to 2mm or 2mm to 3.5mm

[0026] In some embodiments, pimples may have a height or dimples may have a depth in the region of 0.05mm to 1mm. In some such embodiments, pimples may have a height or dimples may have a depth in the region of 0.05mm to 0.1mm, 0.1mm to 0.25mm or 0.25mm to 1mm.

[0027] In some embodiments, the separation between neighbouring pimples or dimples may be in the region of 0.05mm to 10mm. In some such embodiments, the separation between neighbouring pimples or dimples in the region of 0.1mm- 1mm, 1mm to 5mm or 5mm to 10mm.

[0028] In some embodiments, pimples may have a height to width / length ratio or dimples may have a depth to width / length ratio in the region of 2:1 to 1:100. In some such embodiments, pimples may have a height to width / length ratio or dimples may have a depth to width / length ratio in the region of 1:1 to 1:10, 1:5 to 1:20, or 1:20 to 1:100. In some embodiments, pimples or dimples may have a width / length to separation ratio in the region of 5:1 to 1:400. In some such embodiments, pimples or dimples may have a width / length to separation ratio in the region of 2:1 to 1:2, 1:1 to 1:10, 1:5 to 1:20, or 1:20 to 1:100.

[0029] In some embodiments ribs or grooves have a width in the region of 0.25mm- 4mm. In some embodiments, ribs may have a height or grooves may have a depth in the region of 0.05mm to 1mm. In some embodiments, the pitch of helical rib or groove in the region of 1mm to 100mm. In some embodiments, the separation between neighbouring ribs or grooves is in the region of 0.05mm to 10mm.

[0030] The cable may have specific gravity in the region of 1.925-2.95. Specific gravity may be defined as the relative density of the cable compared to the fluid in the pipe.

[0031] The cable may have stiffness in the region of 0.0775 - 0.2175 Nm2. The cable stiffness may be beam stiffness which can be defined by measuring the drop between a supported cable end and an unsupported cable end of unit length due to an applied unit force N.

[0032] In particular embodiments, the cable may have the above specific gravity and stiffness combined with a diameter in the range of 5-15mm, in the range of 6-10mm or in the range of 10- 16mm. Such dimensions and properties may be well suited to pipes where the fluid comprises water.

[0033] A cable having either or both of the above properties is adapted to reliably lie within the invert of a fluid pipe during fluid flow due to the density and stiffness of the cable. Nevertheless, the cable is sufficiently flexible to be readily stored on standard drums and readily introduced into the pipe by pulling. Such a cable is also not unduly heavy for manual handling. Any sensing fibres within the cable may experience reduced noise due to reduction in movement of the cable within the pipe. This may beneficially increase the effective sensitivity of the sensing fibres.

[0034] In some embodiments, the cable specific gravity may be in any one of the ranges 2-2.85, 2.2-2.75, 2.35-2.55, 2.4-2.5, 1.95-2.5, 2.05-2.4, 2.15-2.3, 2.2-2.25, 2.4-2.9, 2.5- 2.8, 2.6-2.7, or 2.65-2.7. The cable specific gravity may be selected to fall within a desired range by incorporating any one or more of suitable strength members, conductive members, armour or by suitable selection of the outer sheath material or outer sheath thickness.

[0035] In some embodiments, the cable stiffness may be in any one of the ranges 0.1- 0.2 Nm2, 0.12-0.18 Nm2, 0.14-0.16 Nm2, 0.15-0.155 Nm20.0775-0.16 Nm2, 0.1-0.145 Nm2, 0.11-0.13 Nm2, 0.12-0.125 Nm20.14-0.2175 Nm2, 0.16-0.2 Nm2, 0.18-0.19 Nm2, or 0.18-0.185 Nm2. The cable stiffness may be selected to fall within a desired range by incorporating any one or more of suitable strength members, conductive members, armour or by suitable selection of the outer sheath material or outer sheath thickness.

[0036] The cable may be provided with armour between the one or more elongate data fibre bundles and the outer sheath. The cable may be provided with armour comprising multiple elongate wires between the one or more elongate data fibre bundles and the outer sheath, wherein one or more sensing fibres are provided within the armour. In some embodiments, one or more of the sensing fibres may replace a wire in the wire armour formation. This provides a particularly simple and efficient way of providing sensing fibres within the armour.

[0037] The armour may be provided between the outer sheath and the data fibre bundles. The armour may be provided between the outer sheath and the data fibre bundles and sensing fibres. The armour therefore provides additional protection for the data fibre bundles and sensing fibres from abrasion and impact during storage, installation and use.

[0038] The armour may be formed from a suitable metal or a suitable polymer. A suitable metal may be steel or stainless steel. A suitable polymer may be aramid or Kevlar. The armour may comprise a single layer of wires or multiple layers of wires.

[0039] The wires run substantially parallel to each other. In such embodiments, the wires may be arranged in a helical formation around the axis of the cable. If the data fibre bundles are provided in a helical formation, the pitch of the helical wire formation may match the pitch of the data fibre bundles. The pitch of the helical wire formation may be in the opposite direction to the pitch of the data fibre bundles. This can reduce coil memory compared to armour formed from wires with a helical formation in the same direction as the data fibre bundles. The wires may be arranged in a coaxial weave. The coaxial weave may comprise multiple wires in interwoven helical formations of opposite directions, each centred around the axis of the cable. If the data fibre bundles are provided in a helical formation, the pitch of the coaxial weave helical formations may match the pitch of the data fibre bundles. The pitch of the coaxial weave helical formations may be less than the pitch of the data fibre bundles. Armour formed from a coaxial weave may be more flexible than and have a reduced coil memory compared to armour formed from wires running substantially parallel to each other.

[0040] The coaxial weave may be defined by an optical coverage value. The optical coverage value may be defined as the proportion of the underlying cable obscured by the wires forming the coaxial weave. In some embodiments, the coaxial weave may have a high optical coverage value. In this context, a high optical coverage value may be 70% or greater, 75% or greater, 80% or greater, or 85% or greater.

[0041] An insulating film may be wrapped around the armour. The insulating film may help hold the armour formation together and / or help prevent the ingress of water to the cable interior in the event that the outer sheath is damaged. The insulating film may comprise a tape wrapped around the data fibre bundles and sensing fibres.

[0042] The insulating film may be formed from a suitable polymer. A suitable polymer may be mylar. The insulating film may be provided with a foil on one or both surfaces. The foil may be a conductive foil.

[0043] In some embodiments, in a direction transverse to the cable axis the data fibre bundles are regularly packed across the cable. The data fibre bundles may be packed in a regular array. The regular array may take any suitable form. In one embodiment the regular array may be a hexagonal close packed (HCP) array.

[0044] The cable may additionally comprise one or more sensing fibres provided alongside the data fibre bundles. In some such embodiments, one or more sensing fibres are provided in the interstitial spaces between data fibre bundles within or on the outer edge of the regular packing of the data fibre bundles. In the context of the present invention, interstitial spaces include internal interstitial spaces defined by surrounding data fibre bundles and edge interstitial spaces where the space is defined by at least two fibre bundles. Such interstitial edge space would have a further defined edge if the regular packing extended further.

[0045] The cable may comprise any suitable number of sensing fibres. In some embodiments, the cable comprises at least 2 sensing fibres, at least 3 sensing fibres, at least 4 sensing fibres, at least 5 sensing fibres, at least 6 sensing fibres, at least 7 sensing fibres, at least 8 sensing fibres, at least 9 sensing fibres, at least 10 sensing fibres, at least 11 sensing fibres or at least 12 sensing fibres. The skilled person will appreciate that the particular number of sensing fibres provided may be determined based on the number required to fulfil specific sensing and / or routing needs.

[0046] Each sensing fibre may be a single mode optical fibre. In some embodiments, each sensing fibre within may be provided with an individual sheath. Each individual sensing fibre sheath may be opaque.

[0047] In some embodiments, each sensing fibre may have a substantially equivalent sensitivity. In other embodiments, some sensing fibres may vary in inherent sensitivity. This may be achieved by selecting different fibre constructions or processing sensing fibres prior to installation.

[0048] In some embodiments, sensing fibres are only provided in internal interstitial spaces. In other embodiments, sensing fibres are only provided in edge interstitial spaces. In such embodiments, the shorter distance between the exterior of the cable and the sensing fibres can effectively improve the sensitivity of the sensing fibres. In some embodiments, sensing fibres are provided in edge interstitial spaces and inner sensing fibres are provided in internal interstitial spaces. This beneficially provides a difference in sensitivity and / or frequency response between the sensing fibres and inner sensing fibres, which may be beneficial in environments with significant background noise.

[0049] The cable may comprise any suitable number of data fibre bundles. In some embodiments, the cable comprises at least 4 data fibre bundles, at least 5 data fibre bundles, at least 6 data fibre bundles or at least 7 data fibre bundles. The skilled person will appreciate that the particular number of data fibre bundles provided may be determined based on the estimated data capacity required. Each data fibre bundle may comprise only single mode fibres, only multi-mode fibres or a combination of single mode and multi-mode fibres. Each data fibre bundle may comprise any suitable number of fibres. Each data fibre bundle may be the same size. Each data fibre bundle may comprise the same number of fibres. In some embodiments, each data fibre bundle comprises at least 12 or at least 24 fibres. In other embodiments, some data fibre bundles may be of different sizes. This can provide for more efficient packing.

[0050] In some embodiments, each fibre within a data bundle may be provided with an individual sheath. Each individual data fibre sheath may be opaque. In some embodiments, each data fibre bundle may be provided with a bundle sheath. The bundle sheath may be opaque.

[0051] The outer sheath may be opaque. The outer sheath maybe formed from a polymer. The polymer may be any suitable polymer. In particular, the choice of polymer may be determined by the fluid within which the cable will lie in use. For instance, in water pipe, the polymer may be a water safe polymer. Suitable polymers include but are not limited to polyethylene (PE), polyurethane (PU), polyvinyl chloride (PVC), polytetrafluoroethylene (PTFE), Polyvinylidene fluoride (PVDF) or the like.

[0052] The cable within the outer sheath may be filled with a packing fluid. This can improve the sensitivity of the sensing fibres by improving acoustic coupling between the exterior of cable and the sensing fibres and may also improve temperature conduction between the exterior of cable and the sensing fibres. The packing fluid may be a non-aqueous and / or low viscosity fluid. In one embodiment, the packing fluid is a silicone fluid.

[0053] In some embodiments, the data fibre bundles and sensing fibres may be provided in a linear formation within the cable. In such embodiments, each data fibre bundle and sensing fibre may lie substantially parallel to the cable axis. In other embodiments, the data fibre bundles and sensing fibres may be provided in a helical formation centred on the cable axis. A helical formation can improve the structural stability of the cable. Furthermore, a helical formation beneficially ensures that individual sensing fibres have exposure to detectable events in all directions around the cable over the length of the cable. This can ensure that a failure in a single sensing fibre does not unduly hamper sensing operations.

[0054] The helical formation of the cable may have a pitch, in the region of 6 to 30 times greater than the cable diameter. In some embodiments suitable for use in water pipes, the cable may have a diameter in the range of 5- 15mm, in the range of 6- 10mm or in the range of 10- 16mm. The skilled person will appreciate that other ranges of cable diameters may be necessary to accommodate the required data fibre unit count in particular circumstances. In such embodiments, the pitch might be in the region of 100- 250mm.

[0055] The cable may be provided with a strength member. The strength member provides additional axial strength and stability for the cable. This can help ensure the cable is not damaged by axial forces, for instance, pulling forces during installation. The strength member can also increase the density of the cable and / or the stiffness of the cable in a direction transverse to the axis. This can help the cable reliably lie within the invert of a fluid pipe during fluid flow. Reduction in movement of the cable within the pipe can reduce excessive noise detected on the sensing fibres in use.

[0056] The strength member may be provided along the cable axis. In alternative embodiments, the strength member may be provided offset form the cable axis. In some embodiments, multiple strength members may be provided.

[0057] The strength member may be formed from a suitable metal or a suitable polymer. The polymer may be any suitable polymer. In particular, the choice of polymer may be determined by the fluid within which the cable will lie in use. For instance, in water pipe, the polymer may be a water safe polymer. A suitable metal may be steel or stainless steel. A suitable polymer may be aramid or Kevlar or a glass reinforced polymer (GRP). The strength member comprise a single elongate rod may comprise a rope comprising multiple elongate wires.

[0058] The strength member may be of substantially equivalent dimensions to the data fibre bundles. This allows the strength member to effectively pack alongside said data fibre bundles. In such embodiment, the strength member may effectively replace a data fire bundle in the regular array. In some embodiments, the strength member may have different dimensions to the data fibre bundles. In some such embodiments, the strength member may be narrower than the data fibre bundles. In other such embodiments, the strength member may be wider than the data fibre bundles. The strength member having different dimensions to the data fibre bundles can provide larger inner interstitial spaces for a sensing fibre.

[0059] A packing film may be wrapped around the data fibre bundles and sensing fibres. The packing film may help to hold the data fibre bundles and sensing fibres together and / or limit abrasion between the data fibre bundles and sensing cables and the armour or outer sheath, as appropriate. The packing film may also help prevent the ingress of water to the data fibre bundles and sensing fibres in the event that the outer sheath is damaged. The packing film may comprise a tape wrapped around the data fibre bundles and sensing fibres.

[0060] The packing film may be formed from a suitable polymer. A suitable polymer may be mylar. The packing film may be provided with a foil on one or both surfaces. The foil may be a conductive foil. The foil may have a high water permeation coefficient.

[0061] In embodiments comprising both an insulating film and a packing film, the insulating film may be thicker than the packing film.

[0062] The cable may be provided with one or more electrically conductive members. In some embodiments, a single conductive member may be provided along the cable axis. In alternative embodiments, the two or more conductive members may be provided offset from the cable axis.

[0063] Each conductive members may be formed from a suitable metal. A suitable metal may be copper. Each conductive member comprise a single elongate rod may comprise a rope comprising multiple elongate or stranded wires.

[0064] Each conductive member may be of substantially equivalent dimensions to the data fibre bundles. This allows conductive members to effectively pack alongside said data fibre bundles. In such embodiment, each conductive member may effectively replace a data fire bundle in the regular array. The skilled person will however appreciate that conductive members may have differing dimensions to data fibre bundles, as required or appropriate to accommodate particular electrical characteristics. In such embodiments, the packing of the data fibre bundles and conductive members may be varied as necessary.

[0065] In embodiments comprising two conductive members, the respective conductive members may provide outbound and return paths for electrical current along the cable. In embodiments comprising metal armour and a single conductive member, outbound and return paths for electrical current may be provided by the conductive member and the metal armour. In such embodiments, the cable can be used to supply electrical power between devices at either end of the cable in addition to transmitting data via the data fibre bundles and providing sensing capacity via the sensing fibres. In such embodiments, an electrical power supply device may be provided at one end of the cable and an electrically powered device may be provided at the other end of the cable. The electrically powered device may comprise any one or more of a sensor, transmitter, receiver, transceiver, light, display, user interface or any other suitable device. In general embodiments, voltages deployed on the conductive elements for use in cables provided in fluid pipes would be limited to low voltages as set by regulations in the appropriate territory.

[0066] In embodiments comprising two conductive members and metal armour, the metal armour may provide a path for residual or leakage current to travel along in the event of a fault. In embodiments comprising two conductive members and a foiled packing film or a foiled insulating film, the foil of the packing film or insulating film may provide a path for residual or leakage current to travel along in the event of a fault. In embodiments comprising metal armour, a single conductive member and a foiled packing film or a foiled insulating film, the foil of the packing film or insulating film may provide a path for residual or leakage current to travel along in the event of a fault.

[0067] In embodiments providing a leakage current path, the cable may be connected to a safety device configured to cut off a power supply to the cable in response to detection of a residual or leakage current. This can ensure the cable is safe for operation in fluid pipes despite the present of live electrical conductors. Suitable safety devices include but are not limited to residual-current device (RCD), residual-current circuit breaker (RCCB), ground fault circuit interrupter (GFCI), earth-leakage circuit breaker (ELCB) or the like. Such safety devices may be fitted at easier end of a cable. In some embodiments, safety devices are provided at an end of the cable where an electrical power supply device is provided.

[0068] In some embodiments, the cable additionally comprises one or more power fibres. In such embodiments, the power fibres may comprise optical fibres configured for transmitting light for powering a device connected to the cable.

[0069] Each power fibre may comprise a single multi-mode optical fibre. In some embodiments, power fibres may be provided as a bundle of multi-mode optical fibres. Suitable power fibres may have a relatively large aperture or diameter compared to sensing fibres of data fibres. Each power fibre may be of graded index construction wherein the refractive index increases towards the edges of the fibre. In other embodiments, power fibres may be of step index format with a defined refractive index transition.

[0070] Each power fibre or each power fibre bundle may be provided within an optically dense opaque sheath. This inhibits light from within the power fibre leaking into a data fibre bundle or sensing fibre.

[0071] Individual power fibres may be provided in interstitial spaces within the regular packing of the data bundles. In such embodiment, power fibres may be provided in inner interstitial spaces, edge interstitial spaces or both inner interstitial spaces and edge interstitial spaces. In such embodiments, power fibres may be of substantially equivalent dimensions to sensing fibres.

[0072] Power fibre bundles may be of substantially equivalent dimensions to the data fibre bundles. This allows the power fibre bundles to effectively pack alongside said data fibre bundles. In such embodiment, each power fibre bundle may effectively replace a data fire bundle in the regular array.

[0073] In such embodiments, a light emitter may be provided at one end of the cable and a photovoltaic unit may be provided at the other end of the cable. The photovoltaic unit may be used to power one or more devices comprising any one or more of a sensor, transmitter, receiver, transceiver, light, display, user interface or any other suitable device.

[0074] According to a second aspect of the present invention, there is provided a fibre optic cable suitable for use in a fluid pipe, the cable comprising: one or more elongate data fibre bundles within an outer sheath wherein each data fibre bundle comprises multiple optical fibres configured for data transmission, wherein the cable has specific gravity in the range 1.925-2.95 and stiffness in the range 0.0775 - 0.2175 Nm2.

[0075] The cable of the present invention is thereby adapted to reliably lie within the invert of a fluid pipe during fluid flow due to the density and stiffness of the cable. Nevertheless, the cable is sufficiently flexible to be readily stored on standard drums and readily introduced into the pipe. Such a cable is also not unduly heavy for manual handling. Any sensing fibres within the cable may experience reduced noise due to reduction in movement of the cable within the pipe.

[0076] According to a third aspect of the present invention, there is provided a fibre optic cable suitable for use in a fluid pipe, the cable comprising: one or more elongate data fibre bundles within an outer sheath wherein each data fibre bundle comprises multiple optical fibres configured for data transmission, wherein the cable has specific gravity in the range 1.925-2.95 and stiffness in the range 0.0775 - 0.2175 Nm2.

[0077] The cable of the present invention is thereby adapted to reliably lie within the invert of a fluid pipe during fluid flow due to the density and stiffness of the cable. Nevertheless, the cable is sufficiently flexible to be readily stored on standard drums and readily introduced into the pipe. Such a cable is also not unduly heavy for manual handling. Any sensing fibres within the cable may experience reduced noise due to reduction in movement of the cable within the pipe.

[0078] According to a fourth aspect of the present invention, there is provided a fibre optic cable suitable for use in a fluid pipe, the cable comprising: one or more elongate data fibre bundles within an outer sheath wherein each data fibre bundle comprises multiple optical fibres configured for data transmission, wherein the cable has specific gravity in the range 1.925-2.95. According to a fifth aspect of the present invention, there is provided a fibre optic cable suitable for use in a fluid pipe, the cable comprising: one or more elongate data fibre bundles within an outer sheath wherein each data fibre bundle comprises multiple optical fibres configured for data transmission, wherein the cable has a stiffness in the range 0.0775 - 0.2175 Nm2.

[0079] According to a fifth aspect of the present invention, there is provided a fibre optic cable suitable for use in a fluid pipe, the cable comprising: one or more elongate data fibre bundles within an outer sheath wherein each data fibre bundle comprises multiple optical fibres configured for data transmission, the cable further provided with armour comprising multiple elongate wires between the one or more elongate data fibre bundles and the outer sheath, wherein one or more sensing fibres are provided within the armour.

[0080] The cable of the fifth aspect of the present invention provides a convenient and efficient cable construction. By providing sensing fibres in the armour good quality coupling can be achieved between the sensing fibre and the fluid around the cable. This also can maximise the space within the cable for data fibre bundles.

[0081] According to a sixth aspect of the present invention, there is provided a data communication network comprising one or more cables according to the first aspect of the present invention.

[0082] The network of the sixth aspect of the invention may include any or all features of the cables of the previous aspects of the present invention, as desired or as appropriate.

[0083] In some embodiments, the data cables are laid for at least part of their length in fluid pipes. In such embodiments, if said pipes are provided with are provided with one or more system sensors and / or one or more system devices said cables may be configured to carry data for said system sensors and / or system devices alongside network data. In embodiments wherein the cables comprise one or more conductive members and if said pipes are provided with are provided with one or more system sensors and / or one or more system devices, the one or more system sensors and / or one or more system devices may be supplied with electrical power via said cables. In some such embodiments, wherein the cable is adapted to provide a leakage current path, the cable may be connected to a safety device configured to cut off a power supply to the cable in response to detection of a residual or leakage current. In embodiments wherein the cables comprise one or more power fibres and if said pipes are provided with are provided with one or more system sensors and / or one or more system devices, the one or more system sensors and / or one or more system devices may be supplied with power via said cables.

[0084] According to a seventh aspect of the present invention, there is provided a fluid distribution system comprising one or more system sensors and / or system devices; at one or more points within the fluid distribution system; and a system controller wherein the one or more system sensors and / or system devices are connected to the system controller by one or more cables according to the first aspect of the present invention or by a network according to the second aspect of the present invention.

[0085] The system of the seventh aspect of the present invention may incorporate any or all features of the previous aspects of the present invention, as desired or as appropriate. In particular, the system may comprise one or more system devices configured to vary the operation of the system in response to a suitable input signal.

[0086] According to an eighth aspect of the present invention, there is provided an apparatus for monitoring a fluid pipe comprising: a cable according to the first aspect of the present invention; a light emitter for introducing light pulses into the one or more sensing fibres of the cable; and a light detector module configured to detect backscattering of the said light pulses; and a processing unit for processing the detected backscattered light so as to obtain information about the condition of the pipe.

[0087] According to a ninth aspect of the present invention, there is provided a method for monitoring a fluid pipe, the method comprising the steps of: installing a cable according to the first aspect of the present invention: providing a sensing fibre within the pipe; introducing light pulses into a sensing fibre of the cable; detecting backscattered from the fibre; and processing the backscattered light so as to obtain information about the condition of the pipe and / or events occurring within or in the vicinity of the pipe. The apparatus of the eighth aspect and the method of the ninth aspect of the present invention may include any or all features of the previous aspects of the invention, as desired or as appropriate.

[0088] The light emitter of the apparatus may be a laser. The method may include the step of introducing laser light pulses into the sensing fibre. The apparatus or method may be adapted to carry out distributed acoustic sensing (DAS) (otherwise referred to as distributed vibration sensing (DVS)), distributed strain sensing (DSS) and / or distributed temperature sensing (DTS). Each of DAS, DSS and DTS involves the detection of backscattering of light pulses introduced into an optical fibre. In some embodiments, dedicated fibres may be provided for each technique. In other embodiments, each technique may be carried out on the same sensing fibre.

[0089] Detailed Description of the Invention

[0090] In order that the invention may be more clearly understood one or more embodiments thereof will now be described, by way of example only, with reference to the accompanying drawings, of which:

[0091] Figure 1 is a schematic cross-sectional view of a fibre optic cable;

[0092] Figure 2 is a schematic cross-sectional view of an outer part of a fibre optic cable comprising (a) an outer sheath and (b) an outer sheath and a film;

[0093] Figure 3 is a schematic cross-sectional view of an outer part of a fibre optic cable comprising (a) parallel wire armour, (a) parallel wire armour and an insulating film, (c) coaxial weave armour and (d) coaxial weave armour and an insulating film;

[0094] Figure 4 is a schematic cross-sectional view of an inner part of a fibre optic cable comprising (a) a single data fibre bundle, (b) data fibre bundles, sensing fibres and an axial strength member, and (c) data fibre bundles, sensing fibres a thinner axial strength member;

[0095] Figure 5 is a schematic cross-sectional view of an inner part of a fibre optic cable comprising (a) a single conductive member, (b) two conductive members, (c) a single conductive member and sensing fibres, and (b) two conductive members and sensing fibres;

[0096] Figure 6 schematically illustrates the supply of electrical power using (a) a fibre optic cable comprising one conductive element, (b) a fibre optic cable comprising a pair of conductive elements and a wrap between the armour and the outer sheath;

[0097] Figure 7 schematically illustrates the supply of power using a fibre optic cable according to the present invention comprising one or more power fibres;

[0098] Figure 8 is a schematic cross-sectional view of an inner part of a fibre optic cable comprising (a) two power fibres, (b) one power fibre alongside sensing fibres;

[0099] Figure 9 is a schematic cross-sectional view of an outer part of a fibre optic cable according to the present invention comprising (a) sensing fibres within the wire armour, and (b) sensing fibres within the wire armour and a film;

[0100] Figure 10 schematically illustrates (a) a fibre optic cable laid within the invert of a fluid pipe, (b) the forces experienced by a fibre optic cable laid within the invert of a fluid pipe, (c) the definition of beam stiffness as applied to a fibre optic cable, and (d) the necessity for the cable to bend when entering the invert of a pipe;

[0101] Figure 11 schematically illustrates embodiments of a sheath for a fibre optic cable according to the present invention having surface formations to inhibit lift, drag or yaw forces on the cable when laid in a flowing fluid, the surface formations comprising (a) round pimples (b) round dimples, (c) oval pimples, (d) oval dimples, (e) oval pimples with an asymmetrical height profile; (f) oval pimples with an asymmetrical depth profile; and

[0102] Figure 12 schematically illustrates embodiments of a sheath for a fibre optic cable according to the present invention having surface formations to inhibit lift, drag or yaw forces on the cable when laid in a flowing fluid, the surface formations comprising (a) a helical rib, (b) a helical groove, (c) longitudinal ribs, and (d) longitudinal grooves.

[0103] The present invention relates to various embodiments of fibre optic cables 100, adapted for use in a data communications network. As illustrated in figure 10a, the cables 100 are adapted to be laid, along at least part of their length within a pipe (or conduit) 1 carrying a fluid. Furthermore, the cables 100 are adapted to comprise fibres adapted for sensing the condition within or in the vicinity of the pipe 1. When laid in pipes 1, the cables 100 are optionally positioned in the invert 2 of the pipe 1, the cable 100 optionally being adapted to lie stably in this position. This allows the cable 100 to experience minimised fluid flow velocity, and maximised distance from any appurtenances (such as air release valves) or spurs off the pipe 1. The cables 100 can enter and exit the pipe 1 by any suitable means including bespoke fluid tight fittings.

[0104] As illustrated by a schematic cross-section in figure 1, a typical cable 100 comprises an outer part 80 and an inner part 90. A number of exemplary compositions for the outer part 80 will be described in further detail below with reference to figures 2, 3 and 9 which schematically illustrate cross-sections of different embodiments of outer parts 80. A number of exemplary compositions for the inner part 90 will be described in further detail below with reference to figures 4, 5 and 8 which schematically illustrate cross-sections of different embodiments of inner parts 90.

[0105] In each embodiment, the outer part 80 comprises a cable outer sheath 31. The cable outer sheath 31 can help hold the bundles 11 together and may provide some protection for the bundles 11, for instance from the surrounding fluid and / or from wear / impact events during storage, installation or use. The cable outer sheath 31, if opaque, can also block the escape of light from within the cable 101. The sheath 31 may be formed from polyethylene (PE), polyurethane (PU), polyvinyl chloride (PVC), polytetrafluoroethylene (PTFE), Poly vinylidene fluoride (PVDF) or any other suitable polymer. The skilled person will appreciate that the specific sheath material may be selected with reference to the nature of the fluid within which the cable will be deployed.

[0106] In figure 2a, the outer part 80 comprises only a cable outer sheath 31. In figure 2b, another embodiment of an outer part 80 is shown. In this figure, the common components are labelled with the same refence numbers, the outer part 80 additionally comprises an insulating film 34. Typically, the insulating film 34 is in the form of a tape wrapped around the armour 32. The insulating film 34 may be formed of mylar or other suitable polymer. In some embodiments, the insulating film 34 may be provided with a metal foil on the outer surface. The insulating film 34 helps to hold the outer part 80 in position around the inner part of 90. In embodiments having with a foil on the outer surface of insulating film 34, the insulating film 34 can help prevent the ingress of water to cable interior. In particular embodiment incorporating conductive members (see figure 5 below) the foil can provide a path for residual or leakage current to travel along in the event of a fault.

[0107] In figure 3, further embodiments of the outer part 80 are shown, each comprising armour. In each figure, common components are labelled with the same refence numbers. In figure 3a and figure 3b the armour is in the form of a layer of parallel wires provided around the inner part 90. Whilst a single layer of wires forms the armour 32 illustrated in figure 3a and figure 3b, the skilled person will appreciate that additional layers of wires may be provided as desired or as appropriate.

[0108] As illustrated in figures 3a and 3b, the armour 32 is provided within the outer sheath 31. The armour wires 32 may be wrapped around the inner part 90 in a helical formation centred on the cable axis. If the inner part is in a helical formation, the pitch of the armour 32 would typically match that of the inner part 90.

[0109] The armour 32 is formed from a suitable metal or polymer. For instance, a suitable metal may be steel or stainless steel and suitable polymers may be aramid or Kevlar.

[0110] In figure 3c and figure 3d the armour is in the form of a coaxial weave 35. The coaxial weave 35 comprises multiple wires in interwoven helical formations of opposite directions. The coaxial weave 35 can provide all the benefits of the armour 32 formed from wires wrapped to form a single direction helix. Over and above these benefits, the coaxial weave can provide for more effective guidance of radio frequency signals along the cable. Additionally, the coaxial weave 35 is generally more flexible and has a reduced coil memory compared to a single helix armour layer 32. This can be beneficial in cable installation and for ensuring cable stability after installation in a pipe. If the inner part is in a helical formation, the pitch of the interwoven helical formations of the coaxial weave 35 would typically match that of the inner part 90.

[0111] In figures 3b and 3d, an insulating film 34 corresponding to the insulating film 34 in figure 2b is provided. In such examples, the insulating film 34 may help to hold the armour 32, 35 in position around the inner part 90.

[0112] Turning now to figures 4 to 6, each embodiment of the inner part 90 comprises one or more data fibre bundles 11, each of which comprises multiple optical fibres configured for data transmission. Each data fibre bundle 11 typically comprises a plurality of single mode optical fibres. The skilled person will appreciate that each bundle can alternatively contain multi-mode optical fibres and / or a mixture of single mode optical fibres and multi-mode optical fibres. The bundles may comprise 12, 24 or any other suitable number of individual fibres. Each bundle 11 may optionally be provided with an individual bundle sheath (not shown) to hold the bundle together, provide abrasion protection and / or, if opaque, block the capture or escape of stray light. The single mode fibres within the data bundle might typically match a known communications standard such as G.652x or G.657x.

[0113] In figure 4a, the inner part comprises a single data fibre bundle 11. In figure 4b common components to the embodiment of figure 4a are labelled with the same refence numbers. The embodiment of figure 4b differs from that of figure 4a in the inner part 90 comprises seven data fibre bundles regularly packed in a hexagonal close packed (HCP) array or a quasi-HCP array. This provides for efficient packing of data fibre bundles 11 of the same size within the cable 101. The skilled person will appreciate that other packing schemes or array formations may be used as alternatives. Similarly, the skilled person will appreciate that data fibre bundles 11 of a range of different sizes may be used to make up the inner part 90.

[0114] The data fibre bundles 11 can be provided in a linear formation, each substantially parallel to the cable axis. More typically, the data fibre bundles 11 and are provided 12 in a helical formation centred on the cable axis. This improves the structural stability of the cable 101. Such a helical formation is characterised by the pitch (sometimes referred to as lay length), defined as the axial distance over which a fibre bundle 11 or sensing fibre 12 completes a full turn around the axis. A suitable pitch might be in the region of 6 to 30 times greater than the cable diameter.

[0115] Whilst the overall structure of the cable 100 can be scaled to any appropriate size. For example, the diameter of cable 100 might be in the region of 5-15mm, in the region of 6- 10mm or in the region 10- 16mm. In such embodiments, the pitch might be in the region of 100-250mm. These exemplary dimensions would render the cable suitable for use in a typical water supply pipe. The skilled person will appreciate that different cable dimensions may be suitable in different contexts.

[0116] Turning to figure 4c, this embodiment of the inner part 90 differs from the embodiment shown in figure 4b in that it additionally comprises six sensing fibres 12 (only one of which is labelled in figure la for clarity), a strength member 21 which replaces the data fibre bundle 11 aligned with the cable axis, and a packing film 33 wrapped around the outside of the inner part 90. The sensing fibres 12 are typically single mode optical fibres but can be multi-mode optical fibres in some embodiments. The sensing fibres are adapted to be used for carrying out distributed acoustic sensing (DAS) (otherwise referred to as distributed vibration sensing (DVS)), distributed strain sensing (DSS) and / or distributed temperature sensing (DTS). Each of DAS, DSS and DTS involves the detection of b ackscattering of light pulses introduced into an optical fibre. In some embodiments, some of the sensing fibre 12 will be dedicated fibres for one technique. More typically, each sensing fibre 12 is adapted to be used for each technique either simultaneously or successively.

[0117] Each sensing fibre 12 is typically an individual single mode optical fibre. Each sensing fibre 12 may optionally be provided with an individual sheath (not shown) to provide abrasion protection and / or, if opaque, block the capture or escape of stray light. Each single mode sensing fibre 12 might typically match a known communications standard such as G.652x or G.657x. Optionally, the sensing fibres may vary in inherent sensitivity. This may be achieved by selecting different fibre constructions or processing sensing fibres prior to installation. As illustrated in figure 4c, the sensing fibres 12 are provided in edge interstitial spaces 19 (only one labelled in figure la for clarity) between data fibre bundles 11 at the outer edge of the regular packing of data fibre bundles 11. Nevertheless, the skilled person will appreciate that it is possible for sensing fibres to be provided within other interstitial spaces in the HCP array or any other packing array. For instance, it may be possible in some embodiments to provide sensing fibres in internal interstitial spaces 18 (only one of which is labelled in figure la for clarity).

[0118] A particular benefit of the provision of sensing fibres 12 in edge interstitial spaces 19 is that it provides for regular spacing of sensing fibres 12 within the cable 100. This permits the cable 100 to be installed in any orientation without compromising potential sensing performance. It also ensures that the sensing fibres 12 can provide even coverage of detectable events displaced from the cable in any direction. If the inner part is provided in a helical formation, this also beneficially ensures that individual sensing fibres 12 have exposure to detectable events in all directions around the cable over the length of the cable 100. This can ensure that a failure in a single sensing fibre 12 does not unduly hamper sensing operations.

[0119] The provision of the strength member provides additional strength and stability for the cable 100 in the axial direction. This can help ensure the cable 100 is not damaged by axial forces, for instance if the cable 102 is pulled along a pipe during installation. The strength member 21 can also increase the density of the cable 100 and / or the stiffness of the cable in a direction transverse to the axis. By suitable selection of the density and stiffness, the cable 100 may be adapted to reliably lie within the invert of a fluid pipe during fluid flow, without preventing the cable 100 from being readily stored on standard drums, readily introduced into a pipe and manual handled if necessary. Reduction in movement of the cable within the pipe can reduce excessive noise detected on the sensing fibres 12 in use.

[0120] The strength member 21 is formed from a suitable metal or a suitable polymer. For instance, a suitable metal may be steel or stainless steel and suitable polymers may be aramid or Kevlar or glass reinforced polymer. The strength member 21 may be a single elongate rod as illustrated in figure lb or may be a rope comprising multiple elongate wires. As illustrated in figure lb, the strength member 21 can be of substantially equivalent dimensions to the data fibre bundles 11 so that it may be readily packed alongside said data fibre bundles 11. The skilled person will appreciate that additional strength members could be provided if required or apriority an / or that the strength member 21 need not replace the central data fibre bundle, if required or desired. The skilled person will also appreciate that the strength member 21 need not match the dimensions of the data fibre bundles 11, one such example being discussed in relation to figure 4d below.

[0121] The packing film 33 is typically in the form of a tape wrapped around the data inner part 90. The film may be formed from mylar or other suitable polymer. In some embodiments, the packing film 33 may be provided with a metal foil on one or both surfaces.

[0122] The packing film 33 helps to hold the inner part 90 together. The packing film 33 can help hold these components in a helical formation as discussed above. The packing film 33 can also limit abrasion between the data fibre bundles 11 in the inner part 90 and outer part 80. In embodiments having a packing film 33 with a foil on one or both surfaces, this can help prevent the ingress of water to cable interior.

[0123] Turing to figure 4d, this an embodiment of the inner part 90 which differs from the embodiment of figure 4c in that a thinner strength member 21a is provided. In particular, the strength member 21a is much thinner than the data fibre bundles 11. This provides larger internal interstitial space 18, which is filled by a set of inner sensing fibres 13 (only one of which is labelled in figure 4d for clarity) as an addition to the sensing fibres 12 disclosed in the previous embodiments. This provides additional sensing capability within cable 103. Whilst the inner sensing fibres 13 are further from the exterior of the cable 103 and therefore of lower sensitivity, this could still be useful, especially in environments where there is significant background noise as the sensing response of higher sensitivity sensing fibres 12 could be saturated. Similarly, a potential difference in frequency response between inner sensing fibres 13 and sensing fibres 12 could provide useful information.

[0124] Turning now to figure 5, embodiments of the inner part 90 adapted to carry electrical power alongside data fibre bundles 11 are shown. As with the previous embodiments common components are labelled with the same refence numbers. Turning to figure 5a, the inner part is provided with a conductive member 22. The conductive member 22 shown replaces the central data fibre bundle 11 in the inner part 90. The conductive member 22 can be formed from metal (or metal rope). One suitable metal for forming the conductive member 22 would be copper. The conductive member 22 can thus provide one path for electrical current along cable 100. Typically, an inner part with a single conductive member 21 would be provided with an outer part 80 comprising metal armour 35, 35 to provide a return path for the electrical current. This can therefore enable a device at one end of the cable 106 to be supplied with power in addition to enabling data to be transmitted to / from the powered device (or other devices / equipment) along the cable 106. If the associated outer part 80 additionally comprises a foiled insulating film 34, this can provide this can provide a path for residual or leakage current to travel along in the event of a fault.

[0125] Turning to figure 5b, the inner part 90 differs from that in figure 5a in that it is provided with a pair of conductive members 23 replacing data fibre bundles 11 on opposing sides of the inner part 90. As with conductive member 22, the pair of conductive members 23 may be formed from a suitable metal or metal rope. Accordingly, the conductive members 23 can thus provide an outbound and return path for electrical current along the cable 108. In the cable 108, if the packing film 33 is provided with a metal foil on one or both surfaces, this can provide a path for residual or leakage current to travel along in the event of a fault. Alternatively, the armour 32 can provide a path for residual or leakage current to travel along in the event of a fault.

[0126] Turning to figures 5c and 5d the inner parts 90 differ from those in figures 5a and 5b respectively in that each inner part of figure 5c and 5d is additionally provided with sensing fibres 12 as discussed above.

[0127] Turning now to figure 6, there is a schematic illustration of how power might be supplied along a cable 100 incorporating an inner part 90 adapted to carry electrical power alongside data fibre bundles 11 as shown in figure 5.

[0128] In this context, figure 6a is a schematic illustration of powering a device 49 using a cable 100 having two conducive members 23 as shown in the inner parts 90 of figures 5b or 5d and using armour 32, foiled film 33, 34 or a coaxial weave 35 as shown in the inner parts 80 of figure 2b or figure 3, to act as a residual or leakage current path. A power supply 41 is connected to a junction box 42. The junction box 42 is connected to conductive members 23 at one end of cable 100. In turn the conductive members 23 are connected to device 49 at the other end of cable 100. The junction box 42 also comprises an internal residual current circuit breaker which is connected to armour 32, foiled film 33, 34 or a coaxial weave 35, as appropriate. The internal residual current circuit breaker is configured to actuate a switch within the junction box 42 to disconnect power supply from conductive elements 23 in the event that a residual or leakage current is detected on armour 32, foiled film 33, 34 or a coaxial weave 35.

[0129] Figure 6b in contrast is a schematic illustration of powering a device 49 using a cable such as cable 100 having a single conducive member 22 as shown in the inner parts 90 of figures 5a or 5c and armour 32, 35 as shown in figure 3 adapted to provide an outbound and return path for electrical current. In this instance foiled insulating film 34 as shown in figures 3b and 3d is adapted to act as a residual or leakage current path. As with figure 6a, power supply 41 is connected to a junction box 42. In figure 6b, junction box 42 is connected to conductive member 22 and armour 32 at one end of cable 107. In turn the conductive member 22 and armour 32 are connected to device 49 at the other end of cable 107. The junction box 42 also comprises an internal residual current circuit breaker which is connected to foiled insulating film 34. The internal residual current circuit breaker is configured to actuate a switch within the junction box 42 to disconnect power supply from conductive elements 23 in the event that a residual or leakage current is detected on foiled insulating film 34.

[0130] Turning now to figure 7, in an alternative to (or in addition to) supplying electrical power along a cable 100, it can be possible to provide a power supply to a far end of a cable 100 (described in more detail below with reference to figure 8 below) using light transmitted along a power fibre 15 within the cable 100. The power fibre 15 comprises an optical fibre (or bundle of optical fibres) configured for transmitting light for powering a device connected to the cable 100. As illustrated schematically in figure 7, such power fibres 15 can be connected to a light emitter 51 at one end of the cable 100 and to a photovoltaic unit 52 at the other end of the cable. Accordingly, light emitted by the light emitter 51 can be converted into electrical current by the photovoltaic unit 52 to power one or more devices connected to the photovoltaic unit 52. Whilst figure 7 shows four power fibres 15 within the cable 100, there can be one or more power fibres in such cables as required or as appropriate.

[0131] The power fibres 15 can comprise a single multi-mode optical fibre (or a bundle of multi-mode optical fibres). Suitable fibres may have a relatively large aperture or diameter, say of the order of 62.5 pm. Suitable fibres may also be of graded index construction or step index construction wherein the refractive index increases towards the edges of the fibre.

[0132] The single multi-mode optical fibre (or a bundle of multi-mode optical fibres) is typically provided within an optically dense opaque sheath. This inhibits light from within the power fibre 15 leaking into a data fibre bundle 11 or sensing fibre 12. Accordingly, the prospect of interference with data transmission or sensing along the cable 114-120 is reduced.

[0133] Turning now to figure 8 a, an inner part 90 adapted to incorporate two power fibres 15 is shown. As illustrated, the inner part 90 of figure 8a, comprises seven data fibre bundles 11 packed closely in a hexagonal close packed array similar to the earlier inner part embodiments and a pair of power fibres 15 in two of the edge interstitial spaces 19. The inner part is also provided with a packing film 33. The skilled person will however appreciate that if additional power transmission capacity is required further power fibres 15 can be provided. Additionally or alternatively, the skilled person will appreciate that a bundle of power fibres 15 can replace one or more data fibre bundles 11.

[0134] The inner part 90 of figure 8b is that of figure 8a but differs in that only one power fibre 15 is provided. In the remaining edge interstitial spaces 19 sensing fibres 12 are provided.

[0135] In some the above embodiments, the inner portion 90 may be filled with a packing fluid. This can improve the sensitivity of the sensing fibres 12 by improving acoustic coupling between the exterior of cable 100 and the sensing fibres 12. It may also improve temperature conduction between the exterior of cable 100 and the sensing fibres 12. The packing fluid is typically a non-aqueous and / or low viscosity fluid. An example of a suitable packing fluid is a silicone fluid. Turing to figure 9, embodiments of the outer part 80 are adapted to comprise outer sensing fibres 14 (only one of which is labelled in figure 7a and figure 7b for clarity) within the armour. The outer sensing fibres 14 replace one or more wires in the armour 32. The outer part 80 embodiment of figure 9b differs from that of figure 9a in that it also comprises an insulating film 34.

[0136] As illustrated in the example of figure 9, one in every four wires within the armour 32 are replaced by sensing fibres 14. The skilled person will however appreciate that a different proportion of wires in the armour 32 can be replaced, as required or as appropriate. Providing outer sensing fibres 14 within the armour 32can enable the sensing outer sensing fibres 14 to provide greater sensitivity than the sensing fibres 12 (or inner sensing fibres 13, if provided). This is because the shorter distance between the sensing fibre 14 and the exterior of cable 105 allows for improved acoustic coupling and / or temperature conduction between the exterior of cable 105 and the sensing fibres 14. Similarly, a potential difference in frequency response between outer sensing fibres 14 and any sensing fibres 12 (or inner sensing fibres 13, if provided) could provide useful information.

[0137] The skilled person will of course appreciate that outer sensing fibres 14 may also be provided within a coaxial weave 35 or replacing one or more wires within a coaxial weave 35.

[0138] As illustrated in figure 10a, each cable 100 is laid, along at least part of their length within a pipe (or conduit) 1 carrying a fluid. The cables 100 are positioned in the invert 2 of the pipe 1, to experience minimised fluid flow velocity, and maximised distance from any appurtenances (such as air release valves) or spurs off the pipe 1. Nevertheless, even in this position, the cables 100 experience lift forces Lf from interaction between the cable 100 and the fluid flow F. As illustrated in figure 10b, these forces can case the cable to lift away from the invert 2 and twist, pitch or yaw Y about the cable axis. As a result, the cable can experience strain, making an impact on any strain measurements using the sensing fibres. Variation in flow friction can also create variations in noise and temperature measured using the sensing fibres. As such a position is typically not stable over time, the cable will tend to oscillate or vibrate within the flow, which can introduce further undesirable noise into measurements. In the present invention, to combat these issues, the cables 100 can be manufactured with a particular range of cable stiffness. In this context, the stiffness of a cable 100 is assessed as illustrated in figure 10c by measuring the drop D between a supported cable end A and an unsupported cable end B of unit length L due to an applied unit force N. Additionally or alternatively, the cables 100 can be manufactured with a suitably selected specific gravity. The specific gravity of the cable 100 may be defined relative to the density of fluid within the pipe 1.

[0139] Considering cable stiffness, increasing the cable stiffness increases the lift force required to bend the cable hence reducing pitching, twisting or yawing of the cable 100 within the invert. Nevertheless, there is a limit to the stiffness of a cable 100 that is suitable for practical use. This is due to the requirement that the cable must be sufficiently flexible to be readily stored prior to deployment. Similarly, the cable 100 must be sufficiently flexible to be readily introduced into the pipe 1 during deployment. Furthermore, after deployment, the optimum cable 101 would traverse from an entry / exit fitting to the invert of the pipe at an orientation close to perpendicular to the fluid flow but must be sufficiently flexible to bend to a position parallel to the fluid flow within the invert 2, as illustrated in figure lOd.

[0140] It has been found that cables 100 having cable stiffness in the region of 0.0775- 0.2175 Nm2are particularly well suited to minimising cable movement within the invert during fluid flow. Within this broader range, improved cable performance can be observed within the ranges 0.1-0.2 Nm2, 0.12-0.18 Nm2, 0.14-0.16 Nm2, 0.15-0.155 Nm20.0775-0.16 Nm2, 0.1-0.145 Nm2, 0.11-0.13 Nm2, 0.12-0.125 Nm20.14-0.2175 Nm2, 0.16-0.2 Nm2, 0.18-0.19 Nm2, or 0.18-0.185 Nm2.

[0141] Considering cable specific gravity, which is defined as the relative density of eth cable to the surrounding fluid, increasing the cable density increases the lift force required to move the cable hence reducing pitching, twisting or yawing of the cable 100 within the invert. Nevertheless, there is a limit to the density of a cable 100 that is suitable for practical use. In particular, an excessively dense cable 100 may be too heavy for manual handling and / or for using certain deployment equipment. An excessively heavy cable may also damage pipe 1. It has been found that cables 100 selecting a cable for use in a fluid pipe having specific gravity in the region of 1.925-2.95 are particularly well suited to minimising cable movement within the invert during fluid flow. Within this broader range, improved cable performance can be observed within the ranges 2-2.85, 2.2-2.75, 2.35- 2.55, or 2.4-2.5. Further improved performance can be observed in the ranges 1.95- 2.5, 2.05-2.4, 2.15-2.3, or 2.2-2.25. Additionally, improved performance can be observed in the ranges 2.4-2.9, 2.5-2.8, 2.6-2.7, or 2.65-2.7.

[0142] Providing a cable 100 for use in a fluid pipe having both stiffness and specific gravity in one of the preferred ranges disclosed above has especially improved performance with relation to cable stability, without having unduly negative impact on cable storage, handling, deployment or operation.

[0143] Optical fibres alone or in bundles of a size for practical use data transmission within fluid pipes, would not necessarily have intrinsic stiffness or specific gravity within the optimum ranges outlined above. Accordingly, the cable 100 stiffness and density can be increased by suitable variations in cable construction.

[0144] In one example, a cable 100 may be provided with a strength member 21, as illustrated in figures lb, 1c, 2a, 2b or 8b. By suitable selection of the material used for the strength member and the relative size of the strength member to the cable 100 as a whole, the stiffness and / or specific gravity of the cable can be significantly increased.

[0145] In another example, a cable 100 may be provided with a conductive member 22 or a pair of conductive members 23, as illustrated in figures 3a-3d, 4a, 4b, 5a, 5b, 8c or 9a-9d. By suitable selection of the material used for the strength member and the relative size of the strength member to the cable 100 as a whole, the stiffness and / or specific gravity of the cable can be significantly increased.

[0146] In another example, a cable 100 may be provided with armour 32, as illustrated in figures 2a, 2b,3a-3d, or 9a, 9d. By suitable selection of the material used for the armour 32 and the relative size of the armour to the cable 100 as a whole, the stiffness and / or specific gravity of the cable can be significantly increased.

[0147] In another example, a cable 100 may be provided with a co-axial weave 35, as illustrated in figures 4a, 4b, 9c or 9d. By suitable selection of the material used for the co-axial weave 35 and the relative size of the armour to the cable 100 as a whole, the stiffness and / or specific gravity of the cable can be significantly increased.

[0148] The stability of a cable 100 within a pipe invert can be further improved by providing one or more surface formations 60 on an external surface of cable outer sheath 31. By providing suitable surface formations, friction or turbulence between the cable and flowing fluid can be reduced. Additionally or alternatively, suitable surface formations, may mitigate drag, lift or yaw forces generated by interaction between the cable and the flowing fluid. Suitable formations 60 may also minimise noise created by fluid flow at the cable surface and thereby increase signal-to-noise ratio for sensing.

[0149] Examples of surface formations 60 that could be provided are illustrated in figures I la- I lf and 12a- 12d. These figures schematically illustrate the exterior surface of the sheath 31 of a cable 100. Turning to figure Ila, the sheath 31 is provided with a plurality of circular pimples 61 (only a subset of which are labelled for clarity) projecting from the exterior surface. In this example, the circular dimples 61 are arranged in regular array on the surface of sheath 31. Interaction between the flowing fluid and the pimples 61 reduces drag and / or lift and / or yaw forces experienced by the cable 100 as a result of the fluid flow.

[0150] Figure 11b is similar to figure I la but the circular pimples 61 are replaced by circular dimples 65 (only a subset of which are labelled for clarity). Once again, interaction between the flowing fluid and the pimples 61 reduces drag and / or lift and / or yaw forces experienced by the cable 100 as a result of the fluid flow.

[0151] The skilled person will appreciate that different shapes of pimple / dimple can be used where desired or appropriate. For instance, in figure 11c the circular pimples 61 of figure I la are replaced by oval pimples 62 (only a subset of which are labelled for clarity). Similarly, in figure l id the circular dimples 65 of figure 11b are replaced by oval dimples 66 (only a subset of which are labelled for clarity).

[0152] Whilst the embodiments of figures 11 a- l id have pimples / dimples that have a symmetrical cross-sectional profile parallel to the cable axis or direction of flow, this is not strictly necessary. Examples are shown in figures 1 le and 1 If. Figure 1 le is similar to figure I la but the circular pimples 61 are replaced by oval pimples 63 (only a subset of which are labelled for clarity) with an asymmetrical cross-sectional height profile. In this particular example, the pimples 63 are orientated such that they have a steeper slope in the direction of incoming flow F. Similarly, figure I lf replaces the circular dimples 65 of figure 1 lb with are replaced by oval dimples 67 (only a subset of which are labelled for clarity) with an asymmetrical cross-sectional depth profile. In this particular example, the dimples 67 are orientated such that they have a steeper slope in the direction of incoming flow F. Such asymmetric profiles can provide improved performance in fluid flow compared to symmetrical profiles. Nevertheless, to take advantage of this benefit does require that a pipe 1 has a consistent primary flow direction and that the cable 100 is installed in the correct orientation relative to the primary flow direction.

[0153] In the above examples, the pimples 61, 62, 63 or dimples 65, 66, 67 are all of the same size. The skilled person will however appreciate that pimples 61, 62, 63 or dimples 65, 66, 67 of different sizes may be provided if desired. In the above examples, the pimples 61, 62, 63 or dimples 65, 66, 67 are each provided in a regular rectangular array on the exterior surface of the sheath 31. The skilled person will appreciate that alternative spacing or arrays of pimples 61, 62, 63 or dimples 65, 66, 67 positions may be provided.

[0154] Whilst pimples 61, 62, 63 or dimples 65, 66, 67 may be formed in any suitable size to be effective in a given fluid and for a given cable diameter, some size ranges are well suited to cables of dimensions suitable for use in water pipes. In the present examples, pimples 61, 62, 63 or dimples 65, 66, 67 have a width or length in the region of 0.25mm-4mm. Similarly, in the present examples, pimples 61, 62, 63 or dimples 65, 66, 67 have a height or depth in the region of 0.05mm to 1mm. Further in the present examples the separation between neighbouring pimples 61, 62, 63 or dimples 65, 66, 67 is in the region of 0.05mm to 10mm.

[0155] Turning now to figure 12, alternative surface formations to pimples 61, 62, 63 or dimples 65, 66, 67 are illustrated. In figure 12a, the sheath 31 is provided with a projecting helical rib 71. In the alternative embodiment of figure 12b, the projecting helical rib 71 is replaced by a helical groove 75. As with the pimples 61, 62, 63 or dimples 65, 66, 67, interaction between the flowing fluid and the helical rib 71 or helical groove 75 reduces drag and / or lift and / or yaw forces experienced by the cable 100 as a result of the fluid flow.

[0156] In the alternative embodiment of figure 12c, the sheath 31 is provided with a series of projecting longitudinal ribs 72 (only a subset of which are labelled for clarity). In the further alternative embodiment of figure 12d, the projecting longitudinal ribs 72 replaced by a series of longitudinal grooves 76 (only a subset of which are labelled for clarity). As with the helical rib 71 or helical groove 75, interaction between the flowing fluid and the series of longitudinal ribs 72 or longitudinal grooves 76 reduces drag and / or lift and / or yaw forces experienced by the cable 100 as a result of the fluid flow.

[0157] Whilst ribs 71, 72 or grooves 75, 76 may be formed in any suitable size to be effective in a given fluid and for a given cable diameter, some size ranges are well suited to cables of dimensions suitable for use in water pipes. In the present examples, ribs 71, 72 or grooves 75, 76 have a width in the region of 0.25mm-4mm. Similarly, in the present examples, ribs 71, 72 or grooves 75, 76 have a height or depth in the region of 0.05mm to 1mm. Further in the present examples the pitch of helical rib 71 or groove 75 or the separation between neighbouring ribs 72 or grooves 76 is in the region of 1mm to 100mm.

[0158] Bearing in mind the above examples, the skilled person will appreciate that other further embodiments can be provided with alternative pimple or dimple forms, alternative rib or groove forms or combinations of such forms.

[0159] The one or more embodiments are described above by way of example only. Many variations are possible without departing from the scope of protection afforded by the appended claims.

Claims

CLAIMS1. A fibre optic cable suitable for use in a fluid pipe, the cable comprising: one or more elongate data fibre bundles within an outer sheath wherein each data fibre bundle comprises multiple optical fibres configured for data transmission, wherein the outer sheath has an external surface upon which are provided one or more surface formations adapted to mitigate friction or turbulence between the cable and flowing fluid and / or mitigate lift or yaw forces generated by interaction between the cable and the flowing fluid.

2. A fibre optic cable as claimed in claim 1, wherein the surface formations comprise a repeating pattern of surface formations.

3. A fibre optic cable as claimed in claim 2, wherein the repeating pattern runs substantially parallel to or helically around the axis of the cable.

4. A fibre optic cable as claimed in any preceding claim, wherein the surface formations comprise projecting features, recessed features or both projecting and recessed features.

5. A fibre optic cable as claimed in claim 4, wherein the projecting features comprise pimples.

6. A fibre optic cable as claimed in claim 5, wherein the pimples have a width or length in the region of 0.25mm-4mm, or a height in the region of 0.05mm to 1mm, or a separation between neighbouring pimples in the region of 0.05mm to 10mm.

7. A fibre optic cable as claimed in one of claims 4 to 6, wherein the projecting features comprise ribs aligned parallel to the axis of the cable.

8. A fibre optic cable as claimed in any one of claims 4 to 7 wherein the projecting features comprise ribs provided in a helical formation relative to the axis of the cable.

9. A fibre optic cable as claimed in claim 7 or claim 8, wherein the ribs have a width in the region of 0.25mm-4mm, or a height in the region of 0.05mm to1mm, or the separation between neighbouring ribs is in the region of 0.05mm to 10mm.

10. A fibre optic cable as claimed in any one of claims 4 to 9, wherein the recessed features comprise dimples.

11. A fibre optic cable as claimed in claim 10, wherein the dimples have a width or length in the region of 0.25mm-4mm, or a depth in the region of 0.05mm to 1mm, or a separation between neighbouring dimples in the region of 0.05mm to 10mm.

12. A fibre optic cable as claimed in any one of claims 4 to 11, wherein the recessed features comprise grooves aligned parallel to the axis of the cable.

13. A fibre optic cable as claimed in any one of claims 4 to 12, wherein the recessed features comprise grooves provided in a helical formation relative to the axis of the cable.

14. A fibre optic cable as claimed in claim 12 or claim 13, wherein the grooves have a width in the region of 0.25mm-4mm, or a depth in the region of 0.05mm to 1mm, or the separation between neighbouring grooves is in the region of 0.05mm to 10mm15. A fibre optic cable as claimed in any preceding claim, wherein the cable has stiffness in the region of 0.0775 - 0.2175 Nm2.

16. A fibre optic cable as claimed in any preceding claim, wherein the cable is provided with armour between the one or more elongate data fibre bundles and the outer sheath.

17. A fibre optic cable as claimed in claim 16, wherein the armour comprises multiple elongate wires wherein one or more of the sensing fibres may replace a wire in the wire armour formation.

18. A fibre optic cable as claimed in claim 17, wherein an insulating film formed from a polymer is wrapped around the armour, the insulating film optionally provided with a conductive foil on one or both surfaces.

19. A fibre optic cable as claimed in any preceding claim, wherein the cable additionally comprises one or more sensing fibres provided alongside the data fibre bundles.

20. A fibre optic cable as claimed in any one of claim 19, wherein the data fibre bundles and sensing fibres are provided in a helical formation centred on the cable axis.

21. A fibre optic cable as claimed in any one of claims 19 to 21, wherein a packing film formed from a polymer is wrapped around the data fibre bundles and sensing fibres.

22. A fibre optic cable as claimed in any preceding claim, wherein the cable is provided with a strength member, the strength member provided along the cable axis.

23. A fibre optic cable as claimed in any preceding claim, wherein the cable is provided with a single conductive member along the cable axis or with two or more conductive members offset from the cable axis.

24. A fibre optic cable as claimed in claim 23, wherein the metal armour provides a path for residual or leakage current to travel along in the event of a fault.

25. A fibre optic cable as claimed in claim 23 or claim 24, wherein if the cable comprises a foiled packing film or a foiled insulating film, the foil of the packing film or insulating film may provide a path for residual or leakage current to travel along in the event of a fault.

26. A fibre optic cable as claimed in any preceding claim, wherein the cable additionally comprises one or more power fibres, each power fibre comprising one or more optical fibres configured for transmitting light for powering a device connected to the cable27. A data communication network comprising one or more cables according to any one of claims 1 to 26.

28. A data communication network as claimed in claim 27, wherein the data cables are laid for at least part of their length in fluid pipes.

29. A data communication network as claimed in claim 28, wherein if said pipes are provided with are provided with one or more system sensors and / or one or more system devices said cables are configured to carry data for said system sensors and / or system devices alongside network data.

30. A data communication network as claimed in claim 28 or claim 29, wherein if the cables comprise one or more conductive members and if said pipes are provided with are provided with one or more system sensors and / or one or more system devices, the one or more system sensors and / or one or more system devices are supplied with electrical power via said cables.

31. A data communication network as claimed in claim 30, wherein if the cable is adapted to provide a leakage current path, the cable is connected to a safety device configured to cut off a power supply to the cable in response to detection of a residual or leakage current.

32. A data communication network as claimed in claim 30, wherein if the cables comprise one or more power fibres and if said pipes are provided with are provided with one or more system sensors and / or one or more system devices, the one or more system sensors and / or one or more system devices are supplied with power via said cables33. A fluid distribution system comprising one or more fluid system sensors; at one or more points within the fluid distribution system; and a system controller wherein the one or more fluid system sensors are connected to the system controller by one or more cables according to any one of claims 1 to 26 or by a network according to any one of claims 27 to 32.

34. A fluid distribution system as claimed in claim 33, wherein the system comprises one or more system devices configured to vary the operation of the system in response to a suitable input signal.

35. An apparatus for monitoring a fluid pipe comprising: a cable according to any one of claims 1 to 26; a light emitter for introducing light pulses into the one or more sensing fibres of the cable; and a light detector module configured to detect backscattering of the said light pulses; and a processing unit forprocessing the detected backscattered light so as to obtain information about the condition of the pipe36. A method for monitoring a fluid pipe, the method comprising the steps of: installing a cable according to any one of claims 1 to 26: providing a sensing fibre within the pipe; introducing light pulses into a sensing fibre of the cable; detecting backscattered from the fibre; and processing the backscattered light so as to obtain information about the condition of the pipe and / or events occurring within or in the vicinity of the pipe.

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