Patent Publication Number: US-6668921-B2

Title: Providing a conduit for an instrumentation line

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This is a continuation of U.S. Ser. No. 09/849,588, filed May 4, 2001, now U.S. Pat. No. 6,568,481. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to deep wells, which are drilled into the ground for extraction of fluid or gaseous materials. The invention particularly relates to oil, gas or hydrocarbon wells. Most particularly, the invention relates to means for providing instrumentation in the depths of an oil, gas or production well. 
     BACKGROUND 
     In drilling an oil well, it is customary to commence with a wellhead, which provides a steel surface casing, generally around 46 cm (18 plus inches) in diameter. As drilling proceeds, successive sections of a steel intermediate casing are inserted, stage by stage, into the well bore, set in place with concrete slurry, and residual, set, internal concrete slurry plugs drilled out to continue the well bore down until a production zone, where hydrocarbon is found to be present in extractable quantities, is reached. Once contact has been made with the production zone, production tubing, of smaller diameter than the intermediate casing, is introduced down to the production zone, ready to extract hydrocarbon. A perforated production liner, intermediate in diameter (around 18 cm, otherwise 7″ or smaller) between that of the production tubing and that of the intermediate casing, may be extended beyond the end of the intermediate casing and the production tubing, allowing ingress of hydrocarbon into the production liner. The production liner allows hydrocarbon to flow into the production tubing but the intermediate casing is plugged, or sealed using a packer, against ingress of hydrocarbon from the production liner. 
     Fibre optic sensor line has been used, for some years, in the oil industry, to collect data from oil wells. The data collected primarily relates to temperature. Techniques exist whereby transmitted and backscattered light in a fibre optic line can be analysed to extract much useful information. Such techniques are not part of this invention. The instant invention is concerned, rather, with the introduction of a fibre optic line into an oil well. 
     Well data is of great economic importance, allowing the operator to give more effective surveillance to the well and thereby to enhance the productivity of the well. In these days of slimmer margins of economic viability in oil wells, and falling reserves, such data may be vital for the economy of the oil industry and, by extension, to the greater economy of the world, as a whole. 
     The fibre optic line is extremely fragile. It has a diameter, even with coating and sleeving, of no more than one millimetre. Its internal reflective properties can be compromised by surface contaminants. Being made of glass, it can shatter and break. It has a minimum radius of curvature below, which it certainly breaks. 
     The environment in an oil well is extremely hostile. Drill bits, capable of penetrating hard rock, are lowered into the well and rotated with great torque by heavy steel tubes. Heavy steel casings are lowered into the drill shaft to line the shaft. The drill shaft is filled with cement and mud slurries. Residual cement plugs, once a slurry has set, are drilled out. An oil well represents a very hazardous environment for a fibre optic line. 
     In order to protect the fibre optic line from mechanical damage or contamination, it is customary to use control line. Control line, in the oil industry, is remarkably like metal hydraulic tubing, as used in industrial, agricultural and building site machinery. It is tough, usually 0.6 cm (¼ inch) in outside diameter, able to sustain high pressures up to 15000 psi (100 Mega Pascals), thermally conductive, can be joined in lengths by couplings, and provides a protected, clear channel down which a fibre optic line or electrical cable can be fed. 
     Installing a continuous length of fibre optic line, in the current art, requires the use of a continuous length of control line. Currently, to investigate an oil well, lengths of control line are strapped to the outside of a string of steel casings which are passes down the well to reach and to cross the zone of interest, where measurements are required or desirable. Alternatively, the control line is run inside a protective oilfield tubing string, on the inside of the well bore, down to and across the zone of interest. 
     Should the zone of interest turn out to be the required producing interval, it is customary to complete an oil well by topping off the zone of interest with a set concrete casing and inserting a perforated production liner into and through the zone of interest. This creates a well with two separated strings of pipes, albeit concentric. 
     The completion of a well with a set concrete casing and a production liner precludes running a single length of fibre optic line, inside control line, down to and across the zone of interest, while maintaining the fibre optic line external to the well bore. The plug, through which the production liner passes, blocks off the end of the intermediate casing run, preventing the fibre optic line from passing out of the end of the intermediate casing and isolating the inside of the intermediate casing from the zone of interest. 
     When stimulating a well, a substantial advantage is gained by being able to gather distributed temperature data, without interfering with the near well bore area and without data being masked by the presence of a hydraulically isolated zone. When fibre optic line is installed on the inside of the well bore, the well bore becomes inaccessible to other tools. The control line and the (optional) protective tubing string reduce the room available for the tools. The fragility, even of a protective tubing string and control line protected fibre optic line, and the loss of room, mean that ancillary tools cannot be inserted or operated down a well bore where a fibre optic installation is maintained. Before ancillary tools are run down the well bore, it is necessary first to retrieve the fibre optic line. Stimulation of the well can then take place, or tools run, but without the gathering of data that could have a significant impact on well productivity. 
     With the fibre optic line in the well bore, any fluid flowing in the well bore can affect the fibre optic line. Its temperature readings no longer reflect, with accuracy, the temperature of the rock external to the well bore, but are altered or dominated by the fluid in the well bore. 
     An internally installed and maintained fibre optic line, in a string of protective tubing (pipes), restricts the flow of the well and requires a larger diameter well bore to accommodate the string of protective tubing/pipes and allow adequate flow. Well bores cost a great deal of money to create, and the price rises steeply with their diameter. 
     It is costly to install a control line across the producing interval. Therefore, a small diameter tubing, known as a “stinger”, is used to support the control line and lower it down the well bore into the region of interest or production zone. The present invention, as well as its other advantages, also seeks to provide means, which eliminate the cost, time, and well incapacity that results from the intrusive use of a “stinger”. 
     The present invention has, as its object, the provision of apparatus, method and means, capable of allowing the introduction and maintenance of a fibre optic line, passing into and across the zone of interest, with a portion thereof external to the wellhead, capable of being maintained in position while other operations are carried out in the well bore, unaffected by fluids flowing in the well bore and eliminating the need for a well bore of increased diameter. 
     SUMMARY 
     According to a first aspect, the present invention consists in an apparatus for providing a down-hole conduit for carrying an instrumentation line for use with a well bore in a substrate, the instrumentation line passing from the surface, towards the bottom of the well bore; said apparatus comprising: a hollow primary member, for insertion to extend into the well bore; said primary member comprising a first line of conduit on the outer surface thereof and primary coupling means for accepting the distal end of said first line of conduit; said apparatus further comprising a secondary member comprising a terminal conduit and secondary coupling means for accepting the free end of said terminal conduit; said secondary member being insertable through said hollow first member for said primary coupling means to couple with said secondary coupling means for the distal end of said first line of conduit to be coupled to said free end of said terminal conduit. 
     According to a second aspect, the present invention consists in method for providing a down-hole conduit for carrying an instrumentation line for use with a well bore in a substrate, the instrumentation line passing from the surface, towards the bottom of the well bore; said method including the steps of: inserting a hollow primary member to extend into the well bore; providing a first line of conduit on the outer surface of said primary member; providing primary coupling means for accepting the distal end of said first line of conduit; providing a secondary member comprising a terminal conduit and secondary coupling means for accepting the free end of said terminal conduit; and inserting said secondary member through said hollow first member for said primary coupling means to couple with said secondary coupling means for the distal end of said first line of conduit to be coupled to said free end of said terminal conduit. 
     The invention further provides for a method and apparatus wherein the primary member comprises a second line of conduit on the outside thereof; wherein the primary coupling means is operative to accept the distal end of the second line of conduit; wherein the terminal conduit is a loop of conduit; wherein the secondary coupling means accepts both free ends of the loop of conduit; and wherein the primary coupling means, on coupling with the secondary coupling means, couples the distal ends of the first and said second lines of conduit each to a respective one of the free ends of the loop of conduit; whereby the instrumentation line is passable through the loop of conduit back towards the surface. 
     The invention provides that the secondary member can be hollow and that the conduit loop is on the outside of the secondary member. 
     The invention further provides that the primary member and the secondary member, when coupled together, can form a continuous tube. 
     The invention further provides that the secondary member can be self locating on the primary member. 
     The invention further provides that the primary member can comprise a locating scoop, that the secondary member can comprise a locating tongue, and that the locating scoop and the locating tongue are co-operative to bring the primary coupling means and the secondary coupling means into angular registration for coupling as the secondary member is lowered through the primary member. 
     The invention further provides that the primary coupling means comprises one or the other of a coupling probe or a coupling socket and that the secondary coupling means comprises the other or one of the coupling probe or the coupling socket, and that the coupling probe and the coupling socket, on coupling, can form a sealed coupling between the distal end of one of the lines of conduit and one of the free ends of the loop of conduit. 
     The invention further provides a hollow modified member, the modified member having a secondary coupling means at its top end for accepting the proximal ends of two extension conduits, and having primary coupling means at its bottom end for accepting the distal ends of the two extension conduits, and provides that the modified member can be inserted through the primary member for the secondary coupling means on the modified member to couple with the primary coupling means on the primary member. 
     The invention further provides that a further modified member can be inserted through the modified member for the secondary coupling means on the further modified member to couple with the primary coupling means on the further modified member. 
     The invention further provides that the secondary member can be inserted through the modified member for the secondary coupling means on the secondary member to couple with the primary coupling means on the modified member. 
     The invention further provides that the secondary member can be inserted through the further modified member for the secondary coupling means on the secondary member to couple with the primary coupling means on the further modified member. 
     The invention further provides that the conduit can be control line and that the apparatus can be designed for use where the instrumentation line is a fibre optic line. 
     In the preferred embodiment, it is preferred that the primary member is set into the well bore with concrete or cement. It is further preferred that the well bore is part of an oil well. 
    
    
     The invention is further explained by the example given in the following description and drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A is a cross sectional schematic view, with shortened vertical scale, of an oil well incorporating the present invention, illustrating the manner in which a control line can be conducted into and down the hydrocarbon well using the primary and secondary members of the invention. 
     FIG. 1B is a similar diagram, and shows another embodiment of the invention where the control line can be conducted down the hydrocarbon well, around in a loop and back out of a hydrocarbon well using the primary and secondary members of the present invention. 
     FIG. 2 is a cutaway view, in greater detail, of the primary member of the present invention, installed within an intermediate casing. 
     FIG. 3 is a cutaway view of the secondary member of the present invention. 
     FIG. 4 is a cutaway view of the primary and secondary members of the present invention, coupled together in the oil well. 
     FIG. 5 is a detailed cross sectional view of the coupling elements of the primary and secondary members, lined up prior to coupling. 
     FIG. 6 is a detailed cross sectional view showing the coupling elements of FIG. 5, when coupled. 
     FIG. 7 is a cross sectional view, looking vertically, of either of the primary or secondary members of the present invention, illustrating how control line is held on their exterior. 
     FIG. 8 is an isometric projection of the open upper end of the primary member, illustrating the locating scoop whereby correct angular registration with the secondary member is assured. 
     FIG. 9 is a view, from below, of the secondary member, showing the angular disposition of a locating tongue, which engages the locating scoop of FIG.  8  and swings the secondary member into correct angular registration with the primary member. 
     FIG. 10 is a side view of FIG. 9 showing further detail of the locating tongue 
     FIG. 11 is a schematic view of a variant preferred embodiment, comprising a chain consisting in a primary member, modified secondary members to whatever number is required, and a secondary member proper. The chain can be extended into the well bore or zone of interest however far the user requires. 
    
    
     DETAILED DESCRIPTION 
     Attention is first drawn to FIG. 1A, showing a hydrocarbon well, in the form of an oil well incorporating the present invention. 
     A wellhead  10  is set into a well bore  12  and provides support, control and registration for further operations in a manner well known in the art. The well bore  12  descends, through the surrounding rock  13  to a zone of interest  15  wherefrom hydrocarbon is to be extracted. Intermediate casing  14  is then lowered into the well bore  12  with at least one or more parallel, adjacent, lines of control line  16  attached to the outer surface thereof. In this example a single conduit, in the form of a single control line  16  line is shown. The primary member  18  of the present invention is attached to the lower end of the intermediate casing  14  and carries the single control line  16  from a fibre optic connection module  19  to primary coupling  20  on the primary member. The primary member is hollow, allowing cement  21  slurry to be pumped into the intermediate casing  14  and forced up from the bottom of the well bore  12  between the intermediate casing  14  and the surrounding rock  13 . When the cement  21  has set, the single control line  16  is encased between the steel intermediate casing  14  and the rock  13  surrounding the well bore  12 . The primary coupling  20  is protected by a primary coupling protective sleeve  23 , a soft metal tube, on the inside of the primary member  18 , which prevents slurry  21  or other debris entering the primary coupling  20  and against damage from drilling operations. 
     The cement slurry  21  having set, a drill bit is lowered through the primary member and the residual cement plug at the bottom of the well bore  12  is drilled out. Downward drilling continues until a bore of sufficient depth has been achieved to accept the secondary member  22 . A tool, on a drilling string, is lowered into the primary member  18 , the primary coupling protective sleeve  23  is engaged, and is then removed by being drawn up the well bore  12  with the drilling string. The primary member  18  and the well bore  12  are, at this stage, ready to receive the secondary member  22 . 
     The secondary member  22  is of a smaller outer diameter than the hollow interior of the primary member  18  and passes through the primary member  18  for the top portion of the secondary member  22  to engage the top portion of the primary member  18  to effect coupling. The secondary member  22 , like the primary member  18 , is also hollow, allowing a clear path from the wellhead  10  to the zone of interest  15 . When the secondary member  22  is lowered into the intermediate casing  14 , it couples with the primary member  18 . 
     In coupling, the top portion of the secondary member  22  and the top portion of the primary member  18  automatically mechanically align. The primary coupling  20  comes together with a secondary coupling  24  on the secondary member. The secondary coupling  24  carries the end of a terminal control line  27 . When the secondary member  22  has self-located on the primary member, the single control line  16 , terminated at the top end of the primary member  18  at the primary coupling  20 , is mated, by the aligned engagement of the primary coupling  20  and the secondary coupling  24 , with the terminal control line (conduit)  27 , which is closed and sealed at its far end. The single control line  16 , and the terminal control line  27 , are thereby joined to form a continuous, sealed length of control line, passing from the fibre optic connector module  19  at the surface, down to the bottom of the well bore  12  and into and through the zone of interest  15 . A fibre optic line can thus be passed, from the fibre optic connector module  19 , through the control line  16   26 , down the single control line and down the terminal control line  27 . More than one fibre optic line, and even electrical devices can be passed into and through the zone of interest. Items can be replaced when damaged or when it is desired to measure a different parameter. All these actions can be accomplished from the surface, with no intervention in the well bore  12 . 
     The advantage of the invention extends further. So far, the description shows how a fibre optic line (or similar item) can be passed down to the zone of interest  15  without mechanical intervention in the well bore  12 . The invention also permits continuous monitoring of the zone of interest  15  while permitting other operations to take place in or via the well bore  12 . 
     In the example shown, secondary member  22  is attached to the top end of a production liner  28 , a perforated steel tube which allows ingress of oil. The terminal control line  27  is attached to the outside of the production liner  28  which extends through the zone of interest  15 . The terminal control line  27  thus extends right through the zone of interest. 
     The control line  16  is protected against mechanical activity in the well bore  12  by being on the outside of the intermediate casing  14 , and encased in concrete  21  between the surrounding rock  13  and the intermediate casing  14 . The terminal control line  27  is protected against mechanical activity in the well bore  12  and the zone of interest  15  by being on the outside of the production liner  28 . The terminal control line  27  is further protected against hazards from the rock surrounding the production liner  28  and the lower portion of the secondary member  22  by the presence of a terminal control line protective sleeve  29 . The protective sleeve  29  is a sturdy metal sleeve, preferably of steel or titanium, which runs down the outside of the secondary member  22  from at least where it exits the primary member  18  down to at least as far as the deepest point for the terminal control line  27 . It is thus possible to execute further drilling, or other activities, with the instrumentation (fibre optic line) in place. The primary member  18  and the secondary member  22 , both being hollow, permit tools, slurries and probes to be passed through them for operation. 
     In the example shown, the wellhead  10  is set for production by the introduction, into the zone of interest  15 , of production tubing  30  which allows oil to be pumped from the production liner  28  to the wellhead  10 . 
     The terminal control line  27 , being on the outside of the production liner  28 , is in intimate thermal contact with the contents of the zone of interest  15 , and is not affected by thermal effects of flow in the production liner  28 . The control line  16 , being on the outside of the intermediate casing  14 , is isolated from fluids and conditions in the well bore  12 , being in close thermal contact with the surrounding rock  13 . The present invention thus provides thermal fidelity for the fibre optic line. 
     These advantages are achieved in a well bore of normal dimensions. 
     Attention is drawn to FIG. 1B showing a second embodiment of the invention. The single control line  16  is replaced with a pair of control lines, each terminating in the first member  18  and each extending from the fibre optic connection module  19 . The terminal control line is replaced by a control line loop  26 , which loops down from the top of the secondary member, and extends, depth wise, the same amount as the terminal control line  27  would extend and is fixed and protected in just the same way. When the primary and secondary members  18   22  couple, the distal end of each of the pair of control lines  16  is coupled to a respective free end of the control line loop  26 . A continuous path is thus formed from the fibre optic connection module  19 , down a first one of the control lines  16 , around the control line loop  26 , and back up to the fibre optic connection module through the second of the pair of control line  16 . An instrumentation line can thus be looped, through the continuous path. 
     Attention is drawn to FIGS. 2,  3  and  4  showing, respectively, detailed, cutaway views of the primary member  18  alone, the secondary member  22  alone, and primary  18  and secondary  22  members coupled. 
     The invention is hereinafter described with a preferred embodiment like that shown in FIG. 1B, where a control line loop  26  is employed as the furthest element for carrying the instrumentation line. It is to be appreciated that, hereinafter, whenever a reference is made to a pair of control lines  16  (as in FIG.  1 B), reference is equally made to a single control line  16  (as in FIG.  1 A), and when reference is made to control line loop  26 , reference is equally made to a terminal control line  27 . It is also to be appreciated that, while just a single control line loop  26  (or terminal control line  27 ) is shown in FIGS. 1A and 1B, the present invention can be employed to provide a system having a plurality of control line loops  26 , a plurality of terminal control lines  27 , or a mixture of one or more of each kind. 
     Returning to FIGS. 2,  3  and  4 , the primary member  18 , attached to the intermediate casing  14 , is in the form of a tube having a central bore  32 , extended in diameter and shaped to form a locating scoop  34 , which assists in the angular registration and alignment between the primary  18  and secondary  22  members. At the bottom of the locating scoop  34 , inside the central bore  32 , the primary coupling  20  includes a coupling probe  36  at the end of one of the two control lines  16 , accepting the control line  16  from below and pointing upwards. The control line  16  is, in this example, wound around the outside of the primary member. 
     The secondary member  22  comprises a locating tongue  38  which co-operates with the locating scoop  34  to register and angularly align the primary  18  and secondary  22  members as they are brought into engagement. The secondary member  22  is also in the form of a hollow tube, having a hollow centre  40 . The locating tongue  38  is, in this example, integral with the secondary coupling  24 , which accepts one end of the control line loop  26 , from above, and presents it to a coupling socket  42 , facing downwards. A spring  44  is provided on the outside of the secondary member  22 , on the side opposite to and spanning the extent of the locating tongue  38 . 
     When the primary  18  and secondary  22  members are brought into engagement, the production liner  28 , or any other item intended to lie below the secondary member  22 , is passed through the central bore  32  of the primary member  18  until the top of the secondary member  22  approaches the top of the primary member  18 . The spring  44  on the secondary member  22  engages the inside of the central bore  32  of the primary member and urges the locating tongue  38  into the locating scoop  34 . The locating tongue  38  and the locating scoop  34  co-operate, as the secondary member  22  is further lowered, to rotate the secondary member  22  with respect to the primary member  18  to be in correct angular alignment for the coupling probe  36  to mate with the coupling socket  42 . When the primary  18  and secondary  22  members are fully engaged, the primary member  18  supports the secondary member  22  with the coupling probe  36  fully engaged with the coupling socket  42  to provide a continuous run of control line  16   26 . The joint between the control line loop  26  and the control line  16  is sealed against any pressure and ingress of outside contaminants, likely to be encountered, by the close mechanical seal achieved between the coupling probe  36  and the coupling socket  42 . The hollow centre  40  of the secondary member  22  provides continuity down the well bore  12  for further operations. 
     FIGS. 2,  3  and  4  show only one end of the control line loop  26  and one of the two lengths of control line  16  being joined. This is an artefact of the chosen view of the drawings. It is to be appreciated that at least two coupling probes  36  and coupling sockets  42  will be provided. 
     Attention is drawn to FIGS. 5 and 6, showing, in greater detail, the coupling portions of the primary  18  and secondary  22  members. 
     The end of the control line loop  26  terminates in a loop gland  46 , from the other side of which a secondary coupling tube  48  extends part way along a small diameter channel into the coupling socket  42 . The control line  16 , within the coupling probe  36 , terminates in a tube gland  50  from the other side of which a primary coupling tube extends a short way. When the coupling probe  36  is fully engaged in the coupling socket  42 , the ends of the primary coupling tube  52  and of the secondary coupling tube  48  meet exactly within the small diameter channel in coupling socket  42 . It is preferred that the coupling probe  36  and the coupling socket  42  are made of resilient material, such as hardened rubber or polymer, capable of making a tight seal against the environment in the well bore  12 . The invention also provides that any other form of seal, created on contact, could be used. 
     Attention is drawn to FIG. 7, showing a cross sectional view of a preferred manner of laying the control line  16  or the control line loop  26  on the outside of the primary member  18  or the secondary member  22 . The control line  16  or control line loop  26  is laid on the outer surface of the primary member  18  or the secondary member  22  and is held thereon by linearly spaced clamps  54 . The control line  16   26  is thus held firmly in place. This is a preferred arrangement, the control line  16   26  being laid in straight lines down the outside of the intermediate casing  14  and the production liner  28  as shown in FIGS. 2,  3  and  4 . The invention also permits the attachment of control line  16   26  by other means, such as clips, channels, tension wrapping, gluing or welding. 
     Attention is drawn to FIG. 8, showing an isometric projection of the top of the primary member  18 , and highlights the construction and function of the locating scoop  34 . 
     The locating scoop  34  is formed by a funnel shaped widening  56  of the central bore  32  of the primary member  18 , tapering down to the coupling probes  36 , which sit centrally and at the bottom thereof. The funnel shaped widening  56  extends around a portion of the angular extent of the top of the primary member  18 . In the preferred example shown, the angular extent of the locating scoop  34  is chosen as 120 degrees, but wider or smaller extents, right up to 360 degrees, allowing the locating tongue  38  to correct its angular registration, even if it is +/−180 degrees out, are within the invention. If the locating tongue  38  is not in the correct angular registration, as the primary  18  and secondary  22  members come together, the funnel shaped widening  56  urges the locating tongue  38 , under pressure from the spring  44 , towards the centre of the locating scoop  34 . 
     Attention is drawn to FIGS. 9 and 10. FIG. 9 shows a view, from below, of a cross section of the secondary member  22 , and FIG. 10 shows a side elevation of FIG. 9, looking directly onto the locating tongue  38 . The vertical scale of FIGS. 9 and 10 is compressed. In the preferred embodiment, the vertical extent of the locating scoop  34  and the locating tongue  38  are each in the region of 1 metre (3 feet) to 1.5 metres (4.5 feet), though the invention still covers other vertical extents. 
     The locating tongue  38  is provided on the exterior of the secondary member  22  and, at the lowest part thereof, provides the coupling sockets  42  for the control line loop  26  ends. The locating tongue  38  comprises a straight portion  58  for engaging the coupling probes  36 , together, for preference, with a shaped portion  60  for fully engaging the funnel shaped widening  56  in the locating scoop  34  to form a rugged seal. 
     Finally, attention is drawn to FIG. 11, showing, schematically, how the invention further provides for extension further into the zone of interest  15 , or deeper into the ground, by means of modified secondary members  22 . 
     A primary member  18  comprises a primary coupling  20  which mates a pair of control lines, in the above described way, with a secondary coupling  24  on a modified secondary member  22 A. Instead of supporting a control line loop  26 , the modified secondary member  22 A carries a pair of extension control lines  16 A to a primary coupling  20  at its far end. This, in turn, can mate with the secondary coupling at the top of further modified secondary members  22 A, until a sufficient depth has been reached. Two modified secondary members  22 A are shown in this example. Finally, a true secondary member  22  terminates the string by mating with the primary coupling  20  of the final modified secondary member  22 A. Each successive modified secondary member  22 A is of a smaller diameter than the preceding primary member  18  or modified secondary member  22 A. The whole assembly thus resembles a telescopic car antenna, stretching into the ground. 
     The invention has so far been explained by way of example and embodiments. The invention is further described by the following claims.