WO1999051850A2

WO1999051850A2 - Underground cable and pipelaying

Info

Publication number: WO1999051850A2
Application number: PCT/GB1999/001044
Authority: WO
Inventors: Christopher John Hamilton
Original assignee: Christopher John Hamilton
Priority date: 1998-04-06
Filing date: 1999-04-06
Publication date: 1999-10-14
Also published as: GB9807279D0; AU3339799A; WO1999051850A3

Abstract

The invention relates to a method of laying an underground elongate member such as a cable or pipe. The member is installed underground by directional drilling including being drawn through an unlined bore of larger cross section with lubricant mud being supplied to the space between the member and the bore. The lubricant mud is replaced by a product typically comprising bentonite and a setting agent such as cement. In time, the cement transforms the product into a solidified jacket around the elongate member. Preferably the product contains a pigment. The invention also relates to a two-stage apparatus for enlarging the hole and drawing the cable or pipe into the hole. The invention further relates to a material mix for use as the product.

Description

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Underground Cable and Pipelaving

This invention relates to underground cable and pipelaying. It provides a method of laying an elongate member, bore-enlargement apparatus for use with this method and a material for use with this method.

Conventionally cables and pipes have been laid underground by first digging a trench, laying the cable or pipe in the trench, backfilling, compacting and re-surfacing. This is laborious and expensive, disruptive of surface activity (such as the passage of vehicles or pedestrians), environmentally unfriendly, and it often leaves a bump or hollow in the road.

However, a trenchless technique has been developed which is also called directional drilling or guided horizontal boring. In the technique a series of rods is directed into the ground at one location and is pushed along its way underground to another location, forming a pilot bore. The path of the drill string may be straight, from one pit to another, but it is also possible to cause it to alter course underground, in a radial direction. There are known ways of monitoring its position and direction from above ground.

When the pilot bore has been formed, the steering head is removed and replaced by a large diameter reamer (hole opener). This is pulled back by the drill string along the pilot bore, enlarging it and itself pulling the cable or pipe into the bore via a series of shackles, swivels and towing hooks into the bore. The reamer is a rotary device that enlarges the pilot bore to a diameter greater by about a third than that of the cable or pipe. A fluid is pumped in through the drill string to emerge at the reamer in order to lubricate the annulus between the new bore and the cable or pipe, to help wash away the debris which is cut away by the reamer and to cool down the drilling head. The fluid is 2 not a pure liquid such as water alone, but is traditionally a low viscosity drilling mud known as bentonite, with polymer additives to give it the required lubricity.

A problem exists for electricity companies that wish to lay high voltage cables (11,000 volts and higher) underground. Due to health and safety issues resulting from the presence of such a cable, the electricity company is required to mark its position by providing some sort of indicator. Presently, the only means available are to overlay the cable with plastic marker tape or terracotta tiles. This requires an open excavation. Alternatively, if such an excavation is not possible, such as crossing motorways, rivers or railways, then the electricity company may use the directional drilling technique provided that they first install a plastics duct and then insert the cable inside.

The practice of laying HV cables inside ducts is generally kept to a minimum due to operational disadvantages. Apart from the additional cost factor of laying cable inside duct, the operational performance of the cable is significantly impaired, resulting in losses back as far as the generating station. This is because the plastics duct and the air void surrounding it are very poor conductors of heat.

The problem of the void and the lack of an adequate marker also applies to the laying of gas pipes, water mains and telecommunications cables (including fibre optics cables) when directional drilling is used. In some cases the immediate hazard to a subsequent excavator is less than in the case of electricity, but the inconvenience and expense caused by interruption of service is still substantial.

To sum up, the problems encountered when installing pipes and cables using directional drilling techniques are the potential air voids in the bore and the lack of a reasonable warning. Also the plastic duct is a poor conductor of heat away from an electric cable. The potential air voids could be continuous along the duct, and water could run along the void causing erosion and possible subsidence. 3 One proposed solution is given in "Protecting Cables Installed by Directional Drilling" at page 16 of the July 1998 edition of No-Dig International magazine. This is an extremely complex solution involving a plastics sheath laid above the cable. The sheath is inflated with grout. The process is to bore the pilot hole, to ream the hole and then to ream it a second time, at the same time pulling the cable and sheath into the hole. This is a very expensive process and there is no guarantee that the sheath will remain intact. The sheath will also be a poor conductor of heat. Air voids remain to the sides of the cable.

Warning is claimed to be provided in International Patent Application WO98/03764 in which a colouring material is added to the drilling mud during the drill operation. The intention is that the colouring material, iron oxide, colours the soil around the elongate member to warn subsequent excavators of the impending danger. The main problem with this technique is that the flow of ground water may leach away the colouring material over time leaving the cable without warning of the danger. Even before it has leached away, the Iron Oxide is at times virtually indistinguishable from some soils. Since the soil condition is not known along the bore, there is a risk that the warning will not be apparent at some locations. In addition, the degree of colouration is dependent upon an on-site mixing process which is susceptible to operator error. There can be no guarantee that the colourant would be present throughout the bore. It is also unlikely that the reamer disclose would operate successfully in anything other than clay soils. It is also likely that the plastics duct will float during installation and may at places be at the top of the hole. The colour, in this situation, will provide no warning.

It is the aim of this invention to reduce or virtually eliminate such extra time and trouble as is necessitated by known trenchless techniques.

According to a first aspect of the present invention there is provided a method of laying an underground elongate member such as a cable or pipe, wherein the member is installed underground by being drawn through an unlined bore of larger cross-section 4 with product being supplied to the space between the member and the bore, and wherein the product includes an agent that, in time, transforms the lubricant mud into a solidified jacket around the elongate member. The product replaces the drilling mud normally used in directional drilling.

Subsequent excavators will reach the solid jacket realise that an existing service is present. Excavation can then proceed more slowly or even by hand. The solid jacket ensures that cavities underground are avoided, thus preventing water erosion and subsequent subsidence.

Preferably the product contains a pigment. Once set the pigment will be captive in the solidified jacket.

Thus, someone digging near the elongate member will come across unusually coloured soil which will serve as a warning to proceed with great caution. The pigment must not be water-soluble so that it will leach away, nor should it be bio-degradable.

The pigment is preferably provided pre-mixed together with the product including the setting agent as this reduces mixing time and eliminates the risk that incorrect mixtures are used.

It is possible, even preferable, that some liquid from the product will permeate a short distance into the soil around the bore, due to the reaming and compacting process used.. This could provide an early warning that the original bore was being approached. A two-stage bore enlargement apparatus (reamer) and elongate member laying apparatus have been found to be particular successful.

According to a second aspect of the present invention, there is provided an arrangement for laying an underground elongate member along the path of a pilot hole comprising a reamer for enlarging the diameter of the pilot hole to a first diameter, a compactor coupled to, and arranged to follow, the reamer for compacting the ground to provide a 5 hole having a second diameter smaller that the first diameter, the compactor further comprising means for coupling to the elongate member and means for providing a liquid to surround the elongate member in the hole.

By fluting both stages this apparatus, a controllable portion of soil can be arranged to mix with the product. This may be required in some ground, and could to provide improved heat conduction away from a cable etc.

According to a third aspect of the present invention there is provided a material mix for use during drilling after mixing with water, the material mix comprising a drilling mud material and a setting agent.

This 'one-bag' solution provides the product in an accurate pre-determined ratio. Preferably only water need be added. There is a reduced risk of incorrect mixtures being used.

Cement has been found to be an adequate hardening agent.

The present invention will now be explained and described, by way of non-limiting example, with reference to the accompanying drawings, in which:

Figure 1 shows a diagrammatic side view of a combined reamer and compactor;

Figure 2 shows a sectional view of the combined reamer and compactor;

Figures 3 and 4 show a face view and a side, sectional view of a reamer;

Figures 5 and 6 shows a face view and a side sectional view of a compactor; and 6 Figure 7 to 11 show an actual example of the technique of the present invention.

In preparation for an embodiment of the invention a pilot hole is bored following known directional drilling techniques that will not be described further here. As is known, once the pilot hole is bored there will be a string of drill rods in the pilot hole. These may then be pulled back after attachment to a reamer of any suitable design to enlarge the bore. According to an embodiment of the invention the cable or pipe is installed by attaching it to the hole opener (reamer) and shackle assembly and pulling them through the enlarged hole together with supply of the product.

Only two traverses are conducted (the pilot bore and the pull back) which is facilitated by an arrangement shown in Figure 1. A drill string 12 is attached to a reamer head 14 which may be of any suitable design. The diameter of the reamer head will usually be 1/3 to 1/2 greater than the outer diameter of the jacket surrounding the cable or pipe to be installed. The product is injected via the jets 22 on the cutting face of the reamer head 14 to assist the reaming operation.

The reamer head (first stage) 14 is attached at 16 to a compactor tail (second stage) comprising a tapered portion 18 and a constant-diameter portion 19.

Behind the compactor tail a coupling 20 is provided to connect the arrangement of swivels and towing hooks to the cable or pipe to be installed and the product issues from the rear face of the compactor tail 24.

In some materials the soil which has been disturbed by the first stage reamer 14 is passed to the back via flutes in the reamer face and is re-compacted by the face of the second stage reamer and the bore stabilised by the body 19 (which may also contain flutes).

Figure 2 shows a side view of the soil after each stage of this two-stage process. It is assumed that the product 22 which issues from the front face of the reamer includes a 7 setting agent and a pigment. The rate of flow of the product and the mixture proportions thereof result in a mixture of approximately 3/4 cuttings and 1/4 product. This is then compacted by the compactor tail and this mixture forms an annulus around the periphery of the reamed tunnel (portion B).

More of the product 24 is then fed from the rear face of the compactor tail to give a mix comprising approximately zero to 1/4 cuttings and 3/4 to 4/4 product. This has two benefits. Firstly, the mixture within the annulus contains more setting agent and/or pigment for increased strength and colour intensity. Secondly, prior to setting the inner core of the hole contains a high proportion of product which allows a cable 26 to sink to the bottom of the inner core. This occurs because the specific gravity of the product is less than that of the cable. This gives the thickest possible protective jacket against subsequent excavation accidents. The proportion of setting agent is determined to give a strength of approximately 0.5 to 1.0 N/mm² to the protective jacket. This provides adequate strength to hold the pigment and give some mechanical protection without making it unduly difficult to break through the jacket carefully by hand to make subsequent connections to the cable.

When the present invention is applied only to avoid the generation of cavities in the ground it is not necessary to take steps to ensure that the elongate member is at the bottom of the compacted bore when the modified mud sets. Consequently the setting time can be reduced and no weighting of buoyant members is required. There may not in this instance be a requirement to add a pigment colourant.

Besides the reduction in complexity, the present technique has another major advantage over ducted systems and that relates to heat dissipation. Air, as is known, is a poor conductor of heat. When a cable is laid in a plastics duct only a very small proportion of its surface area will be in contact with the duct. Consequently, heat dissipation from the cable will be very poor, necessitating a de-rating of the cable. This, of course increases the cost as a large-capacity cable is required for each application. 8 Where the cable is not sufficiently de-rated then a loss of power occurs along the length of the cable. It is not unreasonable for such a loss to exceed 10%. In addition, hotspots may occur along the length of the cable at times of high current demand. The resultant thermal cycling of heating and cooling may cause premature failure of the cable.

The described technique, however, provides a good thermal path between a cable and the surrounding earth so that there is no need to de-rate the cable. By providing more efficient electricity distribution, less electricity-generating capacity may be required over the longer term with attendant environmental benefits.

Figures 3 to 6 show an embodiment of a reamer and compactor. Figure 4 shows a side view of a reamer head while Figure 3 shows a face-on view . In operation the reamer head moves from right to left as shown in Figure 4.

The reamer head 30 has a female coupling 32 with a thread size and type to match the couplings being used on the string of drill rods (not shown) to which it is connected in use. This is also connected to a central fluid channel 34 which passes through the length of the reamer head to a male coupling 36 for connection to the compactor (Figures 5 and 6).

The reamer head has a frusto-conical face cutter 38 which, together with the rear wall 40, defines a fluid chamber 42. The face cutter carries a number of jets 44 from which the product is injected during use. These correspond with the jets shown diagrammatically at 22 in Figure 1. The product is fed from the central fluid channel 34 via a number of side channels 46 and the fluid chamber 42 to the jets 44. The constitution and supply of the product will be described below.

While a particular pattern of jets 44 is shown, the number and arrangement of jets is variable to suit the conditions. The dimensions and the cutting elements (not shown) of the face cutter are also variable to suit the application and the prevailing conditions.

Claims

9 In some applications the reamer head and compactor may be provided with channels or fluting (not shown) around its periphery. The object of such fluting is to pass some soil to the rear of the reamer head to meet the compactor tail in use. This allows a proportion of soil to be compacted which both reduces the amount of modified drilling mud that needs to be supplied and provides improved thermal; conductivity between a cable and the surrounding ground. Such fluting will be needed in certain ground conditions where complete recompaction is not possible.The male coupling 36 (Figure 4) connects in use to a female coupling 52 on a first end of a compactor tail shown in Figures 5 and 6. A fluid-carrying central channel 54 passes through the length of the compactor 50. A second end of the compactor tail is provided with a male connector 56 for the assembly connecting the cable or pipe (not shown) to be installed. At that leading edge of the compact tail is a compactor face 58 which is frustoconical in shape and tapers outward from the female coupling 52 to a cylindrical outer wall of the compactor. The compactor face 58 carries four jets 70 for delivering the product to the face of the second stage reamer, for uses only in certain ground conditions where the first stage reamer is not required. The mud passes from the central channel 54 via the side channels 74 to a fore fluid chamber 66 and through the jets 70. The fore fluid chamber is defined by the central fluid channel, the compactor face 58 and an annular wall 62 extending between the central fluid channel and the cylindrical outer wall of the compactor. The size, number and layout of the jets 70 may be varied according to the application.The compactor 50 is also provided with a frustoconical rear face 78 which extends between the cylindrical outer wall and the male connector 56. The wall carries a number of jets 72 (for example four) which are arranged along the axis of the tunnel and around the cable or pipe being laid. The product is provided from the central channel 54 to the jets via side channels 76 and an aft fluid chamber 68. The fluid chamber 688 is defined by the central channel, the rear face and an annular wall between the central channel and the outer wall of the compactor. The number, size, and layout of the jets 72 may be varies according to the application. In common with the 10 reamer head, the compactor tail may be fluted to provide a proportion of soil to mix with the product from the jets 72..In use the reamer had and the compactor tail are coupled together and drawn along the path of a pilot bore by a string of drill rods. The speed at which the combination may be drawn through the soil is determined by the ground conditions, the amount of force that can be applied to the drill string and the ability of the product supply apparatus (not shown) to maintain supply of the product.The product is pumped via the drill string into the combination of reamer head and compactor tail. It is supplied by the pumping apparatus at a high pressure, for example 100 bar . This ensures that the cable or pipe is surrounded by the product leaving no cavities. Avoiding cavities is important if subsequent subsidence of the hole is to be avoided. Occasionally a fissure will be encountered which needs to be filled if this requirement is to be met. The pressure at the pumping apparatus will fall (an operator could be warned by way of an alarm). When this occurs the operator needs to stop the retraction of the drill string and wait until pressure is restored (indicating that the fissure has been filled). Retraction of the drill string can then recommence.The product will now be described. As a minimum, the product comprises sodium or calcium bentonite and a lubricant to which a cement has been added. Preferably, the product is provided pre-mixed as a one-bag product which is then mixed on-site with water. The following examples include a pigment in accordance with a preferred feature of the invention. Yellow has been used in this example because it provide a reliable warning, especially to those 5% to 10% of male who are red-green colour blind. Alternative colours may be chosen if desired, for example in accordance with the following code that indicates the service within the jacket:blue water red electricity yellow gas 11 white telephone purple fibre optic orange street lightingTwo examples of the product are

1. A mix of sodium bentonite (10 - 25%, preferably 17%) and cement (any type 75 - 90%), preferably 83%). The mix will contain a cement compatible lubricant and other additives such as an anti-foaming agent to assist performance.

2. A mix of calcium bentonite (25% - 60%, preferably 32% to 55%) and cement (any type 40 - 75%, preferably 45% to 68%). The mix will contain a cement compatible lubricant and other additives such as an anti-foaming agent to assist performance.

A coloured pigment (1 - 3%) can be added to either mix if differentiation from adjacent ground is required. For the electricity industry, the sodium bentonite mix with a pigment can provide a warning, mechanical protection and heat dissipation from the cable.

For the gas, water and telecommunications industries, either the sodium bentonite or the calcium bentonite mix fills the voids that would have been left if normal drilling mud has been used. Therefore it is a further aspect of this technique and product that a pigment-less version be provided to act as a ground stabiliser following the installation of ducts by the directional drilling technique. It is not the intention to add a pigment to provide a warning. This is because the duct may rise to the top of the hole during installation, and a warning cannot be guaranteed. However, a pigment may be provided to either mix of the utility company require it.

It is highly desirable that the pigment chosen to give colour to the product will give no problems if it passes into the water supply. For this reason water-soluble pigments are considered unsuitable. The requirement for a product that has a high colouring power, is unaffected by moderate heat and is non-bleeding into the surrounding materials. The 12 pigments chosen are regularly tested for compliance with a wide range a regulatory bodies' requirements. The yellow pigment used was Irgalite yellow BAWP available from Ciba Speciality Chemicals pic of Macclesfield, United Kingdom. Colours based on iron oxides have been found to be too dull and do not differentiate sufficiently with the surrounding ground materials.

The grey colour of the cement and bentonite has a tendency to dilute the colour of the pigment, for example, yellow tends to become green and red becomes pink. More pigment can be used to counteract this problem but an alternative solution is to use white cement.

Colour enhancement of the product containing a pigment has been found to be enhanced by using particles of pigment that are in the range lOμm to 20μm.

The shelf life of product containing sodium bentonite is enhance by using a cement having a reduced quantity of calcium ions.

Successful experiments have been conducted using blast furnace cement, Ordinary Portland Cement, white cement and pozzolanic cement.

The one-bag product is applicable to cables and pipes laid in the traditional manner using a trench.

To correctly mix the product a colloidal mixer should be used. The grout module incorporates a high shear colloidal mixer unit with a minimum rotation speed of 2000 rpm. Mixed grout is fed to a storage and agitation vessel. The unit incorporates a variable pressure, variable flow grout pump capable of maximum pressure of 100 bar. The pump is fitted with a variable speed adjustment system, a high output selector control and a device to measure and display the flow rate and record pumped volume. The unit is complete with its own water bath system. 13 Test results for the method described above have been excellent. A test portion of cable was laid using the vivid yellow dye and cement/ sodium bentonite mix identified above. After four weeks a test excavation was carried out by backhoe. The banksman could clearly see (Figure 7) the first traces of the coloured product and the machine operator could see (Figure 8) the traces of coloured product in the spoil.

Excavation then proceeded more slowly (Figure 9) and, once the presence of the cable was confirmed, proceeded by hand (figure 10). The solidified jacket resisted a shovel.

Removal of one side of the jacket (Figure 11) revealed that the cable has sunk happily to the lower part of the bore prior to the product setting and that there were no air voids between the cable and the surrounding earth. Consequently, there was no risk of water erosion, subsidence and no need to de-rate the cable.

While this technique has been developed primarily in view of the requirements of the electricity industry, it should be equally applicable to telephone cabling, and pipelines for water, gas and liquid fuels. It is envisaged that different pigments would be used by different services, so that the type of service being approached would be instantly recognisable.

14 CLAIMS:

1. A method of laying an underground elongate member such as a cable or pipe, wherein the member is installed underground by being drawn through an unlined bore of larger cross-section with lubricant mud being supplied to the space between the member and the bore, and wherein the lubricant mud includes an agent that, in time, transforms the lubricant mud into a solidified jacket around the elongate member.

2. A method as claimed in claim 1, wherein the lubricant mud also includes a pigment which colours the solidified jacket around the elongate member.

3. A method as claimed in claim 1 or claim 2, the unlined bore of larger cross section being formed by a reaming step followed by a compacting step conducted substantially simultaneously.

4. A method as claimed in claim 1, claim 2 or claim 3, wherein the agent that transforms the product into a solidified jacket is arranged to perform this operation in a period of substantially one day.

5. A method as claimed in any one of the claims 1 to 4, wherein the drawing of the member through the unlined bore is slowed or halted in response to a drop in pressure of the supply of the product.

6. A method as claimed in any one of the claims 1 to 5, wherein the product is provided by mixing a single-bag product with water.

7. A method as claimed in claim 6, wherein the mixing is carried out in a colloidal mixer.

8. An arrangement for laying an underground elongate member along the path of a pilot hole comprising a reamer for enlarging the diameter of the pilot hole to a first diameter, a compactor coupled to, and arranged to follow, the reamer for compacting 15 the ground to provide a hole having a second diameter smaller that the first diameter, the compactor further comprising means for coupling to the elongate member and means for providing a liquid to surround the elongate member in the hole.

9. An arrangement as claimed in claim 8, wherein the reamer and compactor are provide with fluting to allow a proportion of disturbed soil to pass behind the arrangement.

10. A material mix for use during drilling after mixing with water, the material mix comprising a drilling mud material and a setting agent.

11. A material mix as claimed in claim 10, wherein the setting agent is cement.

12. A material mix as claimed in claim 10 or claim 11, wherein the product contains sodium bentonite.

13. A material mix as claimed in claim 12 wherein the sodium bentonite comprises 10 - 25% by weight and the cement comprises 75 - 90% by weight.

14. A material mix as claimed in claim 10 or claim 11, wherein the product contains calcium bentonite.

15. A material mix as claimed in claim 14 wherein the calcium bentonite comprises 25 - 60%) by weight and the cement comprises 40 - 75% by weight.

16. A material mix as claimed in any one of the claims 10 to 15, further comprising a pigment.

17. A material mix as claimed in claim 16 wherein the size of the pigment constituents is between 10 μm and 20μm. 16

18. A material mix as claimed in any one of the claims 10 to 17, wherein the drilling mud is arranged to set to a compressive strength of around 0.5N/mm².

19. A material mix as claimed in any one of the claims 10 to 18, including a setting agent having a reduced level of free calcium ions.