WO2002082151A1 - Cable laying and apparatus therefor - Google Patents

Abstract

Cable is laid in a duct, especially a long duct without access to intermediate points and with a height too small to deploy personnel in, by passing through the duct a vehicle bearing a supply of cable and laying cable from that supply in a manner not requering any substantial longitudinal movement of the cable after it is taken from the supply. Attachments on the vehicle engage at least one cable previously laid loose in the duct, lift it clear of the walls of the duct, guide it through the vehicle, and re-deposit it together with the new cable being laid, so avoiding the risk that the previously laid cable will obstruct the movement of the vehicle or become entangled with the one currently being laid. It is contemplated that the duct may be as small as 100 mm or even 50 mm in diameter and up to a few hundred kilometres long when the cable is very small (e.g. a single optical fibre with protective coating).

Description

Cable Laying and Apparatus Therefor This invention relates to a method of, and apparatus for, laying cables (possibly together with other long flexible bodies, such as pipes and tubes) in ducts. It is mainly applicable to cables of small cross-section and therefore primarily to optical communication cables, though small "metallic" communication cables or small energy cables are not excluded. The term "cable" is used in this application in a broad sense, to include even a single optical fibre with sufficient protection to withstand the handling stresses inherent in a laying process as well as fibre ribbons and fibre bundles which are sometimes distinguished from cables.

While electric energy and conventional metallic communication cables, once installed, are expected to remain serviceable and in service for 30 years or more, modern communication systems are rapidly evolving, and installations may need to be modified in a relatively short time (say around 2 years) to enlarge capacity and/or keep up with technical developments . This is more readily achieved with cables laid in ducts, so that they can be supplemented or replaced with minimal civil engineering work by comparison with cables directly buried in the ground or the sea bed. However, the length of duct into which cables can be installed by conventional pulling techniques is quite limited (in practice to a few hundred metres) , because even if the duct is perfectly straight, the cable will rest on the bottom of it for much of its length, and the frictional resistance to movement rapidly approaches the maximum tension the cable will withstand without damage. Strengthening the cable is of limited benefit, since it is always going to be accompanied by an increase in mass and so in friction. The difficulty is compounded when the duct is sinuous or otherwise curved (and in varying degrees virtually every duct is) , since the cable will be pulled taut across the convex side of each curve, whereby the tension in the cable itself contributes to an increase in the frictional drag it experiences, and/or if a cable is already present in the duct with which the new cable may become entangled.

Some improvement can be obtained by selection of lo - friction materials for the cable and duct, by lubrication, by pushing from the entry end and/or by passing a current of air or other fluid through the duct during the laying operation, but at best a length of only around 2 or 3 km can be installed without needing access at intermediate points . For some installations (on sea crossings, for example) such access is entirely impracticable, and in others (on or alongside busy highways, for example) it causes considerable inconvenience to the public. In such cases, there is an option to make the duct of such dimensions that a supply of cable can be advanced through the duct, unwinding as it goes, so that the cable is laid in an essentially stationary manner without any substantial need to slide longitudinally. This is entirely practicable if the duct is a tunnel to be shared with a railway, a road or a watercourse of a major hydroelectric installation, since the cable can be laid from a suitable vehicle and personnel deployed to secure it in a chosen position on the wall of the tunnel and to make joints if necessary; but the cost of providing a duct big enough for this to be done (or simulated by a robot) for the sole purpose (or for the sole additional purpose) of installing cables is excessive, and so sea crossings (in particular) continue to be made with direct-buried cables with the inevitable result that the whole cost of installation is incurred anew whenever an additional or substitute cable is needed.

There is thus a clear need for a technique by which cables can be laid in ducts too small for persons to enter (say less than about 0.8 to 1 m high, depending on the extent to which space is occupied by cables or other hardware, and if possible much smaller) and/or otherwise hazardous (for example because they are gas or water pipes) without substantial risk of damaging, or being detrimentally affected by the presence of, one or more previously installed cable lying loose in the duct in unpredictable and possibly varying position(s) . The method in accordance with the invention comprises passing through the duct a vehicle bearing a supply of cable and laying cable from that supply in a manner not requiring any substantial longitudinal movement of the cable after it is taken from the supply and is characterised by attachments on the vehicle engaging at least one cable previously laid loose in the duct, lifting it clear of the walls of the duct, guiding it through the vehicle, and re-depositing it together with the new cable being laid.

The invention includes a cable-laying vehicle suitable for operating while travelling through a duct and characterised by attachments for the purposes just stated. In all cases the previously laid cable may be guided between the supply of new cable and the wall of the duct, but when the supply of new cable is on one or more than one annular package having axes coincident with or parallel to the duct axis, it may alternatively be passed through the centre (s) of the annular package (s), in which case the new cable will be helically laid about the previously laid one. If the previously laid cable is in service, this will require the annular package (s) to be wound in si tu in the vehicle - the vehicle itself may be de-mountable or may be a permanent part of the installation - but has the advantage that the resulting laid assembly of cables may be more robust than a loose bundle of nominally parallel cables. Preferably the vehicle includes apparatus for controlling the tension in the previously laid cable (s), at least in the direction ahead of the vehicle, to avoid the presence of slack that might risk entanglements. This may include provision for taking up a limited length of the previously laid cable into the vehicle when tension tends to fall, and releasing it when it rises; alternatively slack may simply be passed to the rear of the vehicle - this need not result in undue accumulated movement, since successive operations of the vehicle may be in opposite directions. To minimise problems with slack in subsequent operations, we prefer the cable to be laid under a small degree of tension. In the case of an optical cable, the maximum allowable tension will normally be set by the strain imposed on the fibres; current fibres are considered capable of withstanding a strain of up to 0.2% in service (and temporarily up to 0.3% in installation process) . Conventional tension control mechanisms can be used, or for small cables it may be practicable and convenient to control tension by supplying the cable in coils having a controlled degree of adhesion between the turns (US 4,950,049 describes one way of making a coil of this kind) .

The invention remains applicable if the existing cable is laid in, and the new cable is to be laid into, a slotted flexible tube itself laid loosely in the main duct: in such cases, the flexible tube needs to be taken up into the vehicle .

Usually the vehicle will need wheels (or tracks, in the sense of a track laying vehicle, sometimes called a "caterpillar" vehicle) for engagement with the walls of the duct or with rails, racks or other suitable formations in the duct to enable it to travel through the duct in a reliable manner.

It may be powered by an electric motor supplied by batteries, or possibly by power externally supplied to current rails installed in the duct at the time of its construction using conventional sliding pick-up devices, or by an internal (or external) combustion engine supplied with liquid or gaseous fuel from corresponding tanks on the vehicle . Alternatively, it may be powered by a sufficient flow of fluid along the duct (preferably in the same direction in which the vehicle travels) . At least in the case where the fluid is a liquid and the duct is not too long, it may be feasible for the vehicle simply to nearly occlude the cross- section of the duct so that it is advanced by the direct combined effect of the pressure drop between its ends and the drag of fluid leaking around it; but in many cases it will be preferably for the vehicle to include an appropriate motor (a turbine, for instance) which drives the vehicle through reduction gearing or an electric or other indirect drive, so that the vehicle travels slowly compared with the fluid.

At least in cases where the vehicle is advanced, whether directly or indirectly, by flow of fluid through the duct, it may be desirable to provide a governor mechanism to limit speed, for example by applying a brake or opening a valve to reduce the pressure-drop across the vehicle.

In exceptional cases it may be possible for the vehicle to be advanced by pulling from the end of the duct using a rope or other tensile member provided in the duct at the time of its installation. Since the tensile member will be subject to frictional forces in the same way as described for cables and there will be a need to draw in a replacement tensile member, our perception is that this will only apply when the cable is very small and the duct relatively short. It is to be expected that the vehicle will in practically every case be unmanned. Following the terminology of the pipeline industries, it will be sometimes be referred to as a "pig" , or when it is articulated to enable it to negotiate appreciable curves in the duct, a "pig train". In most cases a pig train, often with several articulations, will be required to hold a sufficient supply of cable. The invention will be further described, by way of example, with reference to the accompanying drawings in which: Figure 1 is a schematic longitudinal cross-section of a simple form of pig train in accordance with the invention; Figure 2 is a diagrammatic end view of one unit of the pig trains shown in Figure 1;

Figure 3 is a composite diagram illustrating how the pig may negotiate curves in a duct;

Figure 4 is a schematic drawing corresponding to Figure 1 and illustrating modifications to the pig train;

Figure 5 is a diagrammatic longitudinal view of an alternative design of pig train incorporating a simple mechanism for controlling tension in a previously laid cable; Figure 6 shows a form of pig powered by a motor and carrying an energy supply;

Figures 7 and 8 show (in side and end view respectively) a self-tensioning cable supply unit;

Figure 9 shows an active tension control mechanism for the cable being laid; and Figures 10 and 11 illustrate the option of using a flexible tube .

Figures 1 and 2 show a simple design of pig train in accordance with the invention for installing a small optical cable into a circular duct 1. The duct is too small for it to be practicable to secure cables in set positions on its walls; it is probably less than 0.5 in diameter, often less than 0.3 m, sometimes less than 0.2 m and may be as small as 100 mm or even 50 mm if the cable is small and flexible enough; its length is unlikely to be less than 1 km and may be several tens of kilometres or even a few hundred kilometres, again if the cable is small and flexible enough. The smallest diameters and greatest lengths may only be achievable when the cable is a single optical fibre with a thin protective coating. The pig train comprises a leading unit 2, a variable number of cable supply units and optionally a trailing unit 3. Two cable supply units 4 and 5 are shown in figure 1, but in some cases one may be sufficient and any reasonable number can be used if the length of the duct and the size of the cable to be installed requires. The optional trailing unit is identical with the leading unit, and will become the leading unit if the train is reversed to lay a another cable, but it may also have other functions: for example, it may include conventional means for cleaning the duct and/or may re-position the previously laid cable as further described below.

Each of the units 2-5 is supported and broadly centred in the duct by three pairs of wheels 6, each mounted on a cantilever and biased outwardly; the wheels are spaced circumferentially at 120° (Figure 2) and axially (Figure 1) in convenient positions, a consideration being to avoid lengthening the train. The units are connected and spaced by flexible bellows 7, which are capable of transmitting both tensile and compressive forces without major change in their lengths. This allows the train to negotiate substantial curvature of the duct, in any plane, as illustrated by Figure 3.

The leading unit 2 occludes a large proportion of the cross-sectional area of the duct, leaving just a small central passage 8 and a peripheral gap 9, and so it and the whole train can be advanced though the duct by passing water (possibly sea-water) or another suitable fluid through the duct in the direction of desired movement. The gap 9 may be minimised by adding a flexible collar 10 if it is considered that the benefit of reduced leakage and faster movement outweighs the disadvantage of frictional engagement with the duct wall .

The coated optical fibre 11 or other cable to be laid is wound in the cable supply units (4, 5) in a pattern designed to be withdrawn from the centre in an axial direction, optionally with a torsional pre-stress to partially or completely neutralise the one twist per turn that accompanies this mode of withdrawal . To avoid the need for an active tension control, the turns of the winding are lightly adhered together to resist separation but release before the tension reaches a level that would extend the fibre (s) by 0.2%. If it is not possible for the cable to be continuous from one unit to the next, splices 12 (with adequate tensile characteristics) are pre-formed in the length of cable between the units . The forward end 13 of the passage 8 through the leading unit 2 is flared, and in accordance with the invention it receives one or more previously laid cable 14, which will usually be lying on the bottom of the duct but might at some places be anywhere within the duct cross-section, especially if it is slack and/or torsionally stressed. For simplicity, the following description will refer to a single previously laid cable, but whatever is done with one may be done with two or more. If the previously laid cable is not already threaded through the leading unit, typically because it was laid by the train and the train remained in place, the leading unit will need to be longitudinally divided to the extent required to allow it to be positioned around the cable. As the train advances, the previously laid cable is lifted clear of the walls of the duct (unless it is already clear) , guided through the pig train, and re-deposited together with the new cable to the rear of the vehicle . In one option (shown in solid lines) the previously laid cable is engaged with a guide pulley 15 which positions it consistently on the bottom of the duct while the train passes over it and the new cable 11, secured at the starting end, is pulled from the cable supply units and simply laid in the duct nominally alongside the previously laid cable; in another option, shown in chain lines, the previously laid cable 14 is passed axially through the whole train and the new cable 11 will become helically laid around it as it is pulled from the supply. The second option may allow better management of the position the cables occupy in the duct, especially if the number of cables is large; but it will normally require the cable to be wound into the cable supply units in si tu in the pig train.

Figure 4 illustrates how the units of the pig train may be linked by rods 17 of appropriate slight flexibility instead of the bellows 7 shown in Figure 1. It also illustrates possible alternative paths for the already-laid cable 14: as shown at 14a, it may remain on the bottom of the duct 1 and pass under the trailing unit 3 instead of passing through it; or more radically, it may pass axially through all the units of the pig train, as shown at 14b, with the result that (unless arrangements were made to rotate the supply of cable at a rate corresponding to the speed at which it is withdrawn) the newly laid cable 11 will be helically applied around the cable 14 that was previously installed.

Where a previously laid cable has substantial slack (excess cable length over duct length) there may be a risk that slack will be pushed ahead of the pig train and eventually accumulate in one location sufficiently to risk entanglement . This risk may be countered by providing a tensioning mechanism for the previously laid cable. Figure 5 shows a simple mechanism that might be used, in which an additional unit 20 in the pig train supports two groups of coaxial free-rotating pulleys 21 and 22 which are biased away from one another, as indicated diagrammatically by a spring 23, and around which the cable 14 is wound (the pulley groups are preferably supported by cantilever shafts to facilitate threading) . The pulleys tend to move apart and so maintain adequate tension when slack is encountered and to move together when the cable is tensioned. It may be noted that at 14c the cable is in front of the duct axis and so clears the cable-supply unit 4.

There are a variety of reasons why it may not be acceptable or satisfactory for the pig train to be driven by acting as a piston in the manner so far described: among others the cable may not be sufficiently resistant to contact with a suitable fluid; the duct may be in use for transporting an unsuitable fluid and interruption of flow or the risk of contamination with another fluid unacceptable; or the train may just be too massive for this mode of driving to be reliable. In such cases, the leading unit may be modified, as shown diagrammatically in Figure 6, to incorporate at least one motor 24 and its fuel supply 25 (for instance an electric motor and a battery pack or an internal combustion engine and a liquefied or compressed fuel gas bottle) . The motor (s) may drive one or more of the wheels 6 (for instance via drive belts 26 , or one or more than one auxiliary wheel or endless track. In other cases, it may not be desirable for the pig train to be burdened with a fuel supply, and in this case power may be obtained by using one or more turbines powered by relatively rapid flow of an acceptable fluid (such as air or the fluid being transported through the duct) . The turbine may drive a wheel or track through a mechanical, electrical or hydraulic coupling, and in such cases the speed of the pig train will be small compared with that of the fluid.

The tension in the cable being laid needs to be kept within acceptable limits, and it is.plainly desirable to do this without adding substantially to the mass, volume or complexity of the pig train. Figures 7 and 8 illustrate a particularly attractive technique that requires no mechanism at all: the cable is supplied wound in an orderly manner in a container 27 so as to be capable of unwinding from the inside by simply pulling on the cable, and the turns of the cable are lightly bonded together by an adhesive: this ensures that the cable will unwind only when the tension applied to it reaches a level sufficient to break the bond between the turns (and will unwind rapidly if that level is exceeded) . If this technique is found inadequate, or is undesirable for any reason, tension may be controlled by engaging the cable with pinch rolls 28 as shown in Figure 9 (or with a capstan, not shown) that are passively braked or actively driven to maintain a desired tension.

Figures 10 and 11 illustrate the possibility that cables may be loosely laid in a slotted flexible tube 30 within the duct; in this case the whole tube 30 needs to be lifted and guided through the pig train, while the new cable 11 needs to be inserted through the slot 31 into the pipe.

Any discussion of the background to the invention herein is included to explain the context of the invention . Where any document or information is referred to as "known", it is admitted only that i t was known to at least one member of the public somewhere prior to the date of this application . Unless the content of the reference otherwise clearly indicates, no admission is made that such knowledge was available to the public or to experts in the art to which the invention relates in any particular country (whether a member-state of the PCT or not) , nor that it was known or disclosed before the invention was made or prior to any claimed date. Further, no admission is made that any document or information forms part of the common general knowledge of the art ei ther on a world-wide basis or in any country and it is not believed that any of it does so .

Claims

CLAIMS :

1 A method of laying a cable in a duct comprising passing through the duct a vehicle bearing a supply of cable and laying cable from that supply in a manner not requiring any substantial longitudinal movement of the cable after it is taken from the supply, characterised by attachments on the vehicle engaging at least one cable previously laid loose in the duct, lifting it clear of the walls of the duct, guiding it through the vehicle, and re-depositing it together with the new cable being laid.

2 A method as claimed in claim 1 in which the previously laid cable is guided between the supply of new cable and the wall of the duct .

3 A method as claimed in claim 1 in which the supply of new cable is on one or more than one annular package having axes coincident with or parallel to the duct axis and in which the previously laid cable is passed through the centre (s) of the annular package (s) and the new cable is helically laid about the previously laid one. 4 A method as claimed in any one of claims 1-3 comprising controlling the tension in the previously laid cable (s), at least in the direction ahead of the vehicle.

5 A method as claimed in any one of claims 1-5 in which the cable is an optical cable and is laid under tension but such that no optical fibre is exposed to strain in excess of 0.3%.

6 A method as claimed in any one of claims 1-5 in which the cable is an optical cable and is laid under tension but such that no optical fibre is exposed to strain in excess of 0.2%.

7 A method as claimed in any one of claims 1-6 in which tension is controlled by supplying the cable in coils having a controlled degree of adhesion between the turns.

8 A method as claimed in any one of claims 1-7 when used to lay a cable in a duct less than 0.8 in height.

9 A cable-laying vehicle suitable for operating while travelling through a duct and characterised by attachments for engaging at least one cable previously laid loose in the duct, for lifting it clear of the walls of the duct, for guiding it through the vehicle, and for re-depositing it 5 together with the new cable being laid.

10 Apparatus as claimed in claim 9 including means for guiding the previously laid cable between the supply of new cable and the wall of the duct.

11 Apparatus as claimed in claim 9 comprising a supply of 10 new cable on one or more than one annular package having an axis or axes coincident with or parallel to the duct axis including means for guiding the previously laid cable through the centre (s) of the annular package (s) and so helically laying the new cable about the previously laid one. 15 12 Apparatus as claimed in any one of claims 9-11 including apparatus for controlling the tension in the previously laid cable (s), at least in the direction ahead of the vehicle.

13 Apparatus- as claimed in any one of claims 9-12 having wheels or tracks for engagement with the walls of the duct or

20 with rails, racks or other suitable formations in the duct.

14 Apparatus as claimed in claim 13 including means for driving the wheels or tracks selected from an electric motor, an internal (or external) combustion engine, or a motor powered by flow of fluid through the duct .

25 15 Apparatus as claimed in any one of claims 9-13 which nearly occludes the cross-section of the duct so that it can be advanced by fluid passing though the duct without use of a motor.

16 Apparatus as claimed in any one of claims 9-15 having a 30 diameter less than 0.8 m.

17 Apparatus as claimed in any one of claims 9-15 having a diameter less than 0.2 m.

18 Apparatus as claimed in any one of claims 9-15 having a diameter about 100 mm or less.

35 19 Apparatus for laying cable in a duct substantially as described with reference to the drawings . 20 A method of laying cable in a duct substantially as described with reference to the drawings.