GB2229549A

GB2229549A - Hydraulic cable installation system

Info

Publication number: GB2229549A
Application number: GB9009263A
Authority: GB
Inventors: David Lancelot Walters
Original assignee: STC PLC
Current assignee: STC PLC
Priority date: 1986-05-17
Filing date: 1990-04-25
Publication date: 1990-09-26
Anticipated expiration: 2006-05-17
Also published as: GB2190457A; GB8612063D0; GB2229549B; GB9009263D0; GB2190457B

Abstract

A method of installing an optical fibre cable (26) comprises installing a ductlet (25) in e.g. a water main pipeline (24) and carrying the cable (26) through the ductlet (25) using liquid pumped from one end and of similar density to the cable, the friction between the flowing liquid and the cable and the buoyancy of the cable, carrying the cable through. (Fig.4) <IMAGE>

Description

HYDRAULIC CABLE INSTALLATION SYSTEM This invention relates to installing cables in pipes or ducts, particularly but not exclusively optical fibre cables in pipes or ducts containing liquid.

British Patent Specification 2122367A discloses a method of laying a cable in a liquid pipeline and also discloses a cable design suitable for laying in a pipeline. This technique has been sown to be successful but where the length of the cable becomes excessive in a pipeline n which ne liquid is flowing, then significant longitudinal stress can be suffered by the cable after installation due to the frictional drag of the flowing licuic. on the surface of the cable.

It is an object of the present invention to provide a method of installing a telecommunications cable in a pipeline which does or will maintain a flowing iquid and in which the length of cable in the pipeline 5 substantial e.g. more than one kilometre.

According to the present invention there is provided a method of installing a telecommunications cable in a duct or pipe which will in use carry a flowing liuid, the method comprising providing within and along the length of the first duct or pipe a second pipe whose diameter is smaller than the first one but large enough to enable the cable to be drawn through the second pipe by means of an hydraulic fluid, and drawing the cable through the second pipe by said hydraulic fluid.

According to another aspect of the present invention there is provided a method of installing an optical fibre telecommunications link, comprising providing an optical fibre cable which contains a plurality of optical fibres within a liquid-tight sheath, providing a liquid-tight ductlet along the route to be followed by the link, and carrying the cable through the ductlet by hydraulic fluid pumped from one end of the ductlet.

Preferably the second pipe has a diameter of about 20mm or less and is clipped to the top or side of the first pipe. The first pipe may have an internal diameter of the order of 2.5m.

Preferably the cable has near-neutral buoyancy in the hydraulic fluid.

Preferably te second pipe is an extruded plastics pipe of e.g. Nylon (RTM) or polyethelene.

In order that te invent Ion can be clearly uncerstood reference will now be made to the accompanying crawlngs i which: Fig. 1 shows in cross section a first cable suitable for hydraulic Insertion through a pipe according to a first embodiment of the invention; rig. 2 shows in cross section a second cable which is an alternatIve to that shown in Fig. 1; Fig. 3 shows diagramatically a pipe-shaft-station system at a water pumping station and in which an end of an installed cable according to an ambodiment of the Invention is terminated;; Fig. 4 shows schematIcally part of a water pipe system in which the cable of Fig. 1 or Fig. 2 has been installed, and Fig. 5 shows diagramatically an installation system for the cable of Figs. 1 and 2.

Referring to Fig. 1 of the drawings the cable there shown comprises an extruded core 1 in which are embedded a plurality of acrylate coated fibres 2 around a central member 3. This can be made in a number of ways but an advantageous way is the technique described in our co-pending patent application 8235740 (L.R. Spicer 32).

Around the central core 1 is provided a Kevlar strength member 4 which comprises woven threads of Kevlar and this provides significant tensile strength to the cable. Around the Kevlar strength member is extruded an outer low-density polymer jacket 5 of for example polyethelene.

The overall density of the composite cable will be in the order of unity but within the range 0.85 to 1.15.

The overall diameter of the cable just described is cf the order of 6mm.

As an alternative to the cable embodiment shown in Fig. 1 an alternative arrangement is possible and this is shown in Fig. 2.

eferring to Fig. 2 a central strength member comprises a central core 10 of giass reinforced polymer surrounded by a polyamide layer 11 of for example Kevlar and this is oversheathed wit polyethelene layer 12.

Secondary coated optical fibres 13 are stranded helically around the central strength member and this. is then oversheathed with a low-densIty thermoplastic, for example polyethelene.

The interstices between the fibres are filled with a blocking material 14 to prevent the ingress of water.

The bend performance of this cable will probably be superior to that of the cable shown in Fig. 1 because the fibres are stranded around the central member. The flexural modulus can be optimised by selecting the correct proportions of glass reinforced polymer 10 to the polyamlde layer 11. However this design of cable is more expensive than that shown in Fig. 1 and it has a lower fibre count than the Fig. 1 for the same size of cable although it is pointed out that the same number of fibres are shown in each cable. However the outer diameter of the cable of Fig. 2 would be of the order of 6mm to 10 mm.

Referring now to Fig. 3 there is shown part of a water main pipe system in which a cable according to Fig.

1 cr Fig. 2 has been installed.

Referring to Fig. 3 a pumping station 20 houses the tcp end of a surge shaft 21 communicating with the junction region 22 between main water pipes 23 and 24.

Valves such as 25 are provided in the pipeline to control, open and close off the flow of water as appropriate.

Pipes 23 and 24 have an internal diameter of the ormer of 2.5 metres and inside pipe 24 is shown a second pi 25 having a diameter at least an order of magnitude smaller tan that of the main pipe 24. A schematic perspective view of a pipe Incorporating the secondary pipe no te optical fibre cable is shown more clearly in As shown In Fig. 4 the secondary pipe 25 is secured to the wali of the main water pipe 24 by means cf clips 27 so that the secondary pipe 25 is secured fast wit te wall of the main pipe 24. This will ensure that with te flow of water along the main pipe 24 no movement o the cable 26 wil occur since it is isolated by means o secondary tube 25 from the moving water in the pipe 24.

One technique for installing the cable is shown n Fig. 5. Before pipeline 24 is filled with water t is proposed to run the secondary pipe 25 into the pipeline 24 and secure it to the walls cf the pipeline 24 with clips such as 27. This can be achieved as the pipeline 24 is erected because it consists of spun concrete sections 2.5 metres in diameter and about 2 metres long. However it can be installed after the pipeline has been erected if the pipeline is big enough for workmen to subsequently install the secondary pipe 25 and secure it to the walls.

Then the cable 26 is carried into the pipe 25 using a hydraulic liquid and this is shown more clearly in Fig. 5. Referring to Fig. 5 the pipeline 25 at its in let end 25a is sealed to a housing 31 its outlet is spaced apart from it inlet by a distance in the range 1.m to 10 m. The housing 31 has a variable aperture 32 in the form of a cable gland through which the cable 26 is pushed by a drive unit 33 comprising a pair of tyred wheels 34 and 35 with torque monitor and control system 36. Wheels 34 and 35 squeeze the cable 26 and push the cable through the cable gland 32.

The variable aperture can be formed by a hydraulicly pressurise diaphragm whose aperture is variable in size by varying the hydraulic pressure applied. Thus the entry of the cable through the diaphragm can be adjusted to just fit the cable 26 to minimise leakage of liquid out of the pipeline.

The housIng 31 has connected to it an hydraulic 37 which pumps hydraulic fluid e.g. water via a flow meter 38 into the housIng 31. A pressure gauge 30 monitors the pressure in the housing.

t is found that with the cable substantially neutrally buoyant the frictional effect of the hydraulic fluid, for example water, on the cable is sufficient to draw it through pipeline 25 over distances of up to 10 kilometres. It is a particular advantage in this applIcation which is envisaged because the main pipeline 24 will run substantially horizontally so that there will be lIttle if any static Induced pressure heads of liquid within the long distance travelled by the secondary pipe 25.Although of course there will be a static head where the pipe rises up to the pumping station 20 such as is illustrated in Fig. 3. thus it is envisaged that the pipeline shown in Fig. 4 will extend over a distance of up to 10 kilometres between two pumping stations each as shown in Fig. 3 with the cable being pushed and carried from one cable gland in one pumping station and retrieved up to 10 kilometres away at the next pumping station.

Repeaters for repeating the signals in the optical fibre cable would be installed in the pumping station. The leading end of the cable will be protected with a watertight sock to prevent water ingress to the front end of the cable.

Example Cable diameter dl 6 x 10 m Ductlet diameter d2 20 x 10 m Liquid (water) density # 10 kg m- Density difference cable-liquid #p 10 kg m-3 Coefficient of friction between cable and duct # # 0.3 Liquid viscosity @ 1.2 x 10- NS m- Liquid velocity V (ms-1) TBD Cable velocity Vc (ms-1) TBD t. is expected that In order to achieve a reasonable drag force, the fluid flow will be turbulent.

The liquid velocIty is therfore not expected to vary significantly at different points within the ductlet, except when passing from that part of the ductlet containing the cable to the empty par The drag force per metre on the cable is: FCD = 0.5 . CFC . p . (V- Vc) . II . d. . k (1) where CFC is the skin friction coefficient between cable and liquid and k is the proportion of the cable circumference which experiences the drag (0.5 < k < 1).

The drag force per metre which the liquid exerts on the ductlet (pipeline 25) is: FDD - 0.5 . CFD p p . V2 . II . d (2) where C FD is the skin coefficient between ductlet and liquid.

At the flow rates likely within this system, the skin coefficients CF will vary with Reynolds number Re roughly in accordance with the Blasius law Cf - 0.079 Re -0.25 (3) where

i.e. the CF is a slowly varying function of the system parameters, and will be typically of the order of 0.01 (dimensionless).

The total force per metre exerted by the water in the (pipeline 25) ductlet is therefore: F tot (liquid) - FCD +FDD (5) and tis force, divided by the area of the ductlet containing the water, gives the pressure drop per metre

The total force on the cable is the drag force CD plus an additional force due to the pressure drop x cable area

Ftot.cable = C | SI d1 2 dP (7 CD ~~~~ 4 dl This force, Ftot.cable' will tend to move the cable through the ductlet. It will be opposed by the frictional force between the cable and the ductlet.

Ignoring bends for te moment (see section d) this frictional force per metre is given by:

d, = I; d1 2 . g g (8) 4 and the cable will move along the ductlet provided Ftot.cable > Ffrict. (9) In practise, the pressure or flow would be adjusted so that Ftot. cable is greater, but not very much greater than Ffrict. so that the net force on the cable during installation is not excessive.

Simplification Equations (1) to (9) describe completely the relationships between forces, flow-rates, pressures etc.

for the installation of a cable in a straight ductlet.

They can be solved as they stand, using an iterative computer method, but it is useful to make somesimplifying assumptions.

Assuming k = 1, V V0 and CFC = CFD = CF, equations (1), (2) and (6) simplify to:

Substituing (11) into (7) we obtain: Ftot.cable = dP . II . d d2 (12) dl 4 Using (12), (8) and (9), the criterion for movement becomes:

dp > (d1/d2). #p . # . g (13) dl Using the nominal numerical values quoted in sectIon 2(a), the pressure gradient necessary for movement is 0.088 MPa/km (#12psi/km). This corresponds to a flow velocity in the part of the ductlet containing the cable of 0.24 m/s, or a flow rate of 70 ml/s. A density mismatch of 10% has been assumed.In practise is likely that cable and liquid densities can be matched much cre closely than that. Nevertheless, it can be see that with the figures assumed installations over many kT are achievable at quite moderate pressures.

Effect of Bends in the Duct For a cable under a tension T rounding a bend of e radians the tension change is: 4 m = t (1 ~ e ) (14) This will be the only tens ion change due to the bend. provided that the overall flexural modulus of the cable IS sufficiently low.

It can be seen In equation (14) that as the tension in the cable decreases, so does the tension cane at the bend. In the limiting case, if the tension at the "feed" end dropped to zero (cable slack), the frictional effect at the bend would also be zero, so that cable movement due to flow would still take place.

The effect of bends In the duct may therefore be to slow down the rate of cable installation, but they will not stop it.

During installation there will be a flow of water out of the far end of the ductlet 25 for some considerable time before emergence of the cable. In some environments this water flow could be a serious practical problem. It is, however, unlikely to be a problem for situations such as a Water Main.

It is to be expected that the flow rates and pressures actually used during installation will be significantly higher than those minimum quantities calculated above.

While in principle this installation technique could be applied to copper-wire cables provided they met the density requirement, these cables are not generally installed in long lengths, they can tolerate high strains (1%), and can be jointed relatively easily. It is therefore with optical fibre cables that the main advantages of this installation method becomes apparent, and its use is envisaged almost entirely with this type of cable. It is not necessarily limited to any particular optical cable type (tight construction, loose tube or open channel).

For installation of long lengths the overall cable SG should be as close as possible to that of the impelling fluid, anc the density, or SG, mismatch should certainly not be greater than 10%.

Unless ductlet sizes and pumping rates are increased to higher values than those considered in section (2), the cable diameter will typically be 6mm + 4mm).

Assuming a pressure crop of 0.1 Mpa/km, equation (1: gives a drag force cf 9 N/km. Assuming a maximum installed length of lokm, with a stationery cable and neglIgible friction, this gives a maximum force of 90 N (say 100 N). If this corresponds to 0.2% fibre strain ana there is negligible strain relief in the cable, it corresponds to a cable load for 1% strain of 500 N.

The cable flexural modulus needs to be suffIciently high for the cable to be driven through the inler gland, but sufficiently low to have negligible effect at any bends in the ductlet.

Claims

C'AIMS:

1. A cable for installation by a method as claimed in any of claims 5 to 11, said cable comprising a plurality of optical fibres tightly held within the cable structure, a flexible tensile strength member, and an outer low-density polymer sheath.

2. A cable as claimed in claim 1 wherein the tensile strength member comprises a high tensile layer of plastic threads.

3. A cable as claimed in claim 2, wherein the tensile strength member incorporates glass reinforced polymer.

4. A cable for installation in a pipeline by hydraulic fluid friction, substantially as hereinbefore described with reference to and as illustrated in Fig 1 or Fig 2 of the accompanying drawings.

5. A A method of installing a telecommunications cadge In a duct or pipe which will in use carry a flowing licuid, the method comprising providing within and along the length of te first duct or pipe a second pipe witose diameter Is smaller than the first one but large enough to enable the cable to be drawn through the second pipe by means of a hydraulic fluid and the frictional effect between the hydraulic fluid and the external surface of the cable, and drawing the cable through the second pipe by said hydraulic fluid.

ó. A method as claimed in claim 5 wherein the secondary pipe has a diameter of about 20mm or less.

7. A method as claimed in claim 5 or claim 6, wherein the secondary pipe is clipped to the top or the side of the first pipe or duct.

8. A method as claimed in any of claims 5, 6 or 7, wherein the first pipe or ducts has an internal diameter of te order of 2.5 metres.

9. A method as claimed in any of claims 5 to 8, wherein the cable has a near neutral buoyancy in the hydraulic fluid.

10. A method as claimed in any of claims 5 to 9, wherein the second pipe comprises an extruded plastics pipe.

11. A method of installing a telecommunications cable in a duct or pipe which will in use carry a flowing liquid, substantially as hereinbefore described with reference to and as illustrated in the accompanying drawincs.

12. A method of installing an optical fibre teleconmunicatlons link, comprising providing an optical fibre cable which contains a plurality of optical fibres within a liquid-tight sheath, providing a liquid-tight ductlet along the route to be followed by the link, and carryIng the cable through the ductlet by hydraulic fluid pumped from one end of the ductlet.

13. A method as claimed in claim 12, wherein said fluid nas a density sirnllar to that of the cable.

14. A method as claimed in claim 13, wherein the density of the liquid i slightly greater than that of the cab

15. A method as claimed in claims 12 to 14 wherein the liquid is pumped at a linear velocity in the range 1 to lOr/sec.