Abstract:
A method and a facility for manufacturing rigid tubular pipes having a double casing for hydrocarbon transport. The pipes include an inner rigid tube ( 42, 242, 342, 442 ) that is inserted into an outer rigid tube ( 16, 216, 316, 416 ). The facility includes a supporting frame ( 12, 212, 312, 412 ) for an inlet end of the outer rigid tube and a movable apron ( 22, 222, 322, 422 ). The apron includes a catch ( 38, 238, 338, 438 ) for catching the rigid tube and inserting it in the outer rigid tube. The apron includes storage members ( 68, 268, 368, 468 ) for receiving stored heating cables ( 69, 269, 369, 469 ) and the driving force of the apron makes it possible to deploy a portion having the length of the stored heating cables ( 69, 269, 369, 469 ) so as to be able to apply the deployed portion, having the length of the heating cables, along the inner rigid tube ( 42, 242, 342, 442 ).

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a 35 U.S.C. §§371 national phase conversion of PCT/FR2010/051446, filed Jul. 8, 2010, which claims priority of French Application No. 0903540, filed Jul. 17, 2009, the contents of which are incorporated by reference herein. The PCT International Application was published in the French language. 
     The present invention relates to a method for manufacturing rigid tubular pipes having a double casing and a facility dedicated to manufacture of said pipes. The invention also concerns the rigid tubular pipe having a double casing obtained by said method of manufacture. 
     These rigid tubular pipes, made up of two rigid tubes engaged one within the other, are intended for the transportation of hydrocarbons, notably on the seabed, and enable the hydrocarbons to be maintained at a given temperature in order that the paraffins that they contain do not set and block passage of hydrocarbons through the pipe. In fact, the hydrocarbon is initially at a relatively high temperature, when it is beneath the seabed, and is significantly cooled when it is conveyed in the pipes between the seabed and the surface. Insulating the double wall of the rigid pipe thus enables the hydrocarbon to be maintained at a temperature close to its original temperature. However, when hydrocarbon extraction is effected at a great depth, where the temperature at the level of the seabed is below 5° C., for example, the paraffins in particular set and form plugs that block the pipe. Under these conditions the insulated double casing is not sufficient by itself to prevent that occurrence with the paraffin. 
     To increase the temperature of the hydrocarbon flowing in the pipe, it has been envisaged to introduce additional thermal energy into it. Thus the document U.S. Pat. No. 6,564,011 discloses a rigid pipe including a rigid tube and a heating cable installed around the rigid tube, and the assembly being covered with a plastic material sheath. Such a device undoubtedly enables thermal energy to be supplied to the hydrocarbon flowing inside the rigid tube, but on the other hand is extremely costly because part of this thermal energy escapes to the exterior of the pipe via the plastic material sheath and is lost. 
     It has also been envisaged to cause an electrical current to flow between the two rigid tubes engaged one in the other of rigid tubular pipes having a double casing to heat the rigid tubes by the Joule effect. See in particular the document GB 2 084 284, which discloses one such device. However, this necessitates the provision on the one hand of an electrical insulator placed in the annular gap between the two rigid tubes to prevent the formation of electrical arcs and on the other hand of electrically conductive spacers to enable current to flow between the internal and external pipes. Furthermore, such a rigid tubular pipe consumes a great quantity of electrical energy. 
     SUMMARY OF THE INVENTION 
     Thus a problem that arises and that the present invention aims to solve is to provide a rigid tubular pipe that enables hydrocarbons to be conveyed on the seabed at great depth without plugs, in particular paraffin plugs, forming inside it, and that consumes little energy. The present invention also aims to provide a method of manufacture and an installation for using that method to manufacture a rigid tubular pipe adapted to convey hydrocarbons on the seabed at great depth. 
     With the aim of solving this problem, and in a first aspect, the present invention proposes a method of manufacturing rigid tubular pipes having a double casing for transporting hydrocarbons, said method being of the type in which there are provided an internal rigid tube and an external rigid tube adapted to be engaged one in the other, there is provided a frame for holding an inlet end of said external rigid tube, there is provided an apron equipped with activatable grasping means for grasping said rigid internal tube, said apron being mobile in translation between a position near said holding frame and a position far from said holding frame, said activatable grasping means are activated and said apron is driven toward said near position to engage said internal rigid tube in the interior of said external rigid tube leaving an annular space between the rigid tubes, and said activatable grasping means are then deactivated to release said internal rigid tube when said apron is driven in reverse toward said far position. According to the invention there are further provided heating cables for enabling a lengthwise heating cable portion to be applied along said internal rigid tube when said apron is driven in movement during the driving of said apron toward said near position and said lengthwise heating cable portion applied along said internal rigid tube is engaged in the interior of said annular space. 
     Thus one feature of the invention resides in the use of a heating cable inside the annular space between the two rigid tubes of the rigid pipe during its manufacture, without greatly modifying the existing installations for manufacturing pipes having a double casing, on the one hand, and without increasing the manufacturing time of those pipes. Thus this heating cable is installed virtually in masked time inside the annular space of the rigid pipe when the internal rigid tube is threaded into the interior of the external rigid tube. As explained in more detail hereinafter, the lengthwise heating cable portion is applied along said internal rigid tube either simultaneously when said apron is driven in movement or sequentially. 
     Until now, no simple and economical method has been developed for inserting a heating cable into the interior of the annular space of a rigid pipe having a double casing. Furthermore, the heating cable is advantageously applied to the internal tube and is covered with an insulative material before inserting the assembly into the interior of the external tube. Use of such a cable covered with thermal insulation enables significant reduction of the electrical energy consumption to power the heating cable and to provide the necessary thermal energy to the hydrocarbon conveyed in the pipe. 
     In one particular embodiment of the invention, said lengthwise heating cable portion is applied in a spiral around said internal rigid tube. Thus, on the one hand, the length of heating cable around the internal tube is increased and the transfer of heat to the hydrocarbon is more homogeneous, and, on the other hand, deterioration caused by elongation of the heating cable when the pipe is laid by the rigid paying out method is avoided. 
     Furthermore, a plurality of lengthwise heating cable portions are advantageously applied simultaneously, which enables the quantity of thermal energy that may be supplied to the hydrocarbon to be further increased. 
     Said heating cables are preferably bent and stored, and said lengthwise portion of stored heating cables is deployed to apply said deployed lengthwise heating cable portion along said internal rigid tube. Accordingly, as explained in more detail hereinafter, the bent and stored heating cables are easier to drive in movement around the internal rigid tube. 
     In one particular embodiment said bent heating cables are driven in rotation about said internal rigid tube to enable application of said lengthwise heating cable portion in a spiral. It will be noted that the plurality of lengthwise heating cable portions may also be deployed while turning them about the internal pipe in order to wind them substantially parallel to each other into a spiral. 
     According to a first variant of the invention, a lengthwise heating cable portion is applied along said internal rigid tube when said apron is driven in movement toward said near position. Thus the lengthwise heating cable portion is applied around the internal rigid tube as the latter is engaged in the interior of the external rigid tube. 
     According to a second variant, a lengthwise heating cable portion is applied along said internal rigid tube when said apron is driven in reverse toward said far position. Thus the heating cable is paid out and applied against the internal tube when the apron is driven in reverse to be able to seize the internal tube again and engage another length of internal tube in the interior of the external tube. As a result, the heating cable is applied while the apron is moving in reverse toward a loading position that is necessary to produce the rigid pipe. Furthermore, the spiral deployment may be effected simply, again while the apron is driven in reverse, i.e. in masked time. 
     Moreover, in this second variant, said bent heating cables are advantageously stored on said mobile apron, which greatly facilitates application of the lengthwise portion of heating cable. In fact, thanks to this feature, the installation for implementing the method of the invention is more compact. And what is more, if a plurality of lengthwise portions of heating cable is applied simultaneously, there is no risk of the lengthwise portions becoming entangled. 
     Again in the case of this second variant, but according to another embodiment described in more detail in the remainder of the description, the lengthwise heating cable portion is applied along the rigid tube independently of the apron moving in reverse toward said loading position. It may move in reverse simultaneously with, before or after applying the heating cable along the rigid tube. 
     Moreover, the internal rigid tube is advantageously clamped radially to grasp said internal rigid tube in such manner as to protect it from deformation and to preserve its integrity. 
     According to the second variant of the invention, and when the bent heating cables are stored to the rear of the apron, opposite the holding frame, said heating cables are preferably and advantageously guided relative to said apron when said apron is driven in reverse, in such manner as to facilitate their sliding relative to the apron and also to apply them directly to the internal pipe. Moreover, said stored lengthwise heating cable portion is deployed via said activatable grasping means when they are inactive and the apron is driven in reverse. 
     According to a second aspect, the present invention proposes an installation for manufacturing rigid tubular pipes having double casing for the transport of hydrocarbons, said tubular pipes comprising an internal rigid tube engaged in the interior of an external rigid tube, said installation comprising a frame for holding an inlet end of said external rigid tube and an apron mobile in translation between a position near said holding frame and a position far from said holding frame, said apron comprising means for grasping said rigid internal tube to enable grasping of said rigid internal tube and engagement of said internal rigid tube in the interior of said external rigid tube leaving an annular space between the two rigid tubes when said apron is driven toward said near position, said grasping means being adapted to release said internal rigid tube when said apron is driven in reverse toward said far position; according to the invention the installation further comprises storage members for storing stored heating cables and application means for enabling application of a lengthwise portion of stored heating cables along said internal rigid tube when said apron is driven in movement, and during the driving of said apron toward said near position said lengthwise heating cable portion applied along said internal rigid tube is engaged in the interior of said annular space. 
     Accordingly, the pipe manufacturing installation is itself relatively simple to implement starting from a conventional installation for manufacturing pipes having a double casing. The stored lengthwise heating cable portion is applied along the internal rigid tube and the apron is driven in movement either simultaneously or sequentially. 
     Said storage members of said stored heating cables are advantageously mounted on said application means, which enables the installation to be erected in a relatively restricted space, as explained hereinafter. 
     In a first embodiment of the installation, said application means are mounted around said internal rigid tube and at a particular distance from said holding frame. As a result, the application means are mounted in a fixed position relative to the holding frame in a direction parallel to the internal rigid tube, while the apron is mobile in translation relative to the application means. Thus this first embodiment of the installation enables use of the method of the invention in accordance with the aforementioned first variant. 
     In a second embodiment of the installation, said application means are mounted on said apron and are therefore mobile in translation with the apron relative to the holding frame. This second embodiment enables the method of the invention to be used in accordance with the second variant specified hereinabove and described in more detail hereinafter. 
     Thus if the heating cable storage members are also mounted on the apron, the pipe manufacturing installation is relatively simple and based on a standard installation for manufacturing pipe having a double casing. Consequently, the additional cost of manufacturing these rigid tubular pipes is substantially equivalent to the cost of the heating cable. 
     Said grasping means are advantageously fastened to said application means, which are themselves fastened to the apron. Moreover, in one particular embodiment of the invention, said application means comprise a drum mounted to rotate about an axis substantially coinciding with said internal rigid tube. Thus, when said storage members of said heating cables are mounted on the drum, the latter drum enables them to be driven around the internal rigid tube whilst allowing deployment of a lengthwise portion of heating cables. 
     As a result, in the second embodiment of the installation, the drum may be driven in rotation when the apron is driven in reverse and thus enable winding of the lengthwise portion of heating cable in a spiral around the internal rigid tube. Thus the heating cable is wound in a spiral in a simple manner, with a lay that may be adjusted by adjusting the relative speeds of rotation of the drum and of reverse movement in translation of the apron. From these are defined the helix angle, which must be in the range 10 to 30°, for example 15°. 
     It will be noted that a drum equipped with storage members for the heating cable may advantageously also be installed between the holding frame and the apron, as in the first embodiment of the invention. Moreover, in this particular embodiment, the installation for manufacturing rigid tubular pipes further comprises a carriage mobile in translation between said apron and said holding frame and said application means are mounted on said mobile carriage. As a result, the heating cables are applied along the internal rigid tube by means of the mobile carriage that is driven in translation between a position near the holding frame and a position far from it. The apron situated upstream of the mobile carriage is driven in translation independently of the latter carriage, either simultaneously or sequentially. 
     Moreover, said storage members advantageously comprise a plurality of spools mounted to rotate about their respective axis on said application means in such manner as to enable simultaneous application in spirals of a plurality of lengthwise heating cable portions. As a result, the spools on which the heating cables are wound enable the cables to be paid out as the apron is driven in translation, because the heating cables already paid out are trapped between the two rigid tubes. Thus there is no need to provide any particular driving of these spools. 
     Moreover, according to another advantageous feature, said grasping means comprise activatable jaws adapted to clamp the internal rigid tube radially to grasp said internal rigid tube. 
     The mode of operation of these activatable jaws is explained in more detail hereinafter. 
     In a particularly advantageous manner, said activatable jaws are installed on said drum in such manner as to enable adjustment and guidance of the heating cable between the activatable jaws without damaging it when the apron is driven in reverse. 
     According to a further advantageous embodiment of the invention, said apron includes a central passage to allow the free passage in translation of said rigid internal tube, and in the event of use of the second variant, also to allow the passage of said deployed lengthwise heating cable portion. Accordingly, the apron may be driven in translation without interfering with the internal pipe or the heating cables. What is more, the apron advantageously being constituted of a thick rectangular plate installed substantially perpendicularly to the pipe, it is easy to provide at least two hydraulic actuators for driving this apron on either side of the central passage. As a result, the activatable jaws are installed facing the central passage and the thrust produced by the two hydraulic actuators is applied directly to the internal tube so as to be able to force it inside the external tube as explained in more detail hereinafter. 
     Said apron preferably comprises guide means for guiding said deployed lengthwise heating cable portion in translation through said apron in such manner as on the one hand not to damage the heating cables and on the other hand to be able to apply them precisely against the internal tube. 
     According to a further aspect, the present invention concerns a rigid tubular pipe obtained by the aforementioned method. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other features and advantages of the invention emerge on reading the description given hereinafter of one particular embodiment of the invention, provided by way of nonlimiting illustration, given with reference to the appended drawings, in which: 
         FIG. 1  is a detailed diagrammatic view in axial section of an installation conforming to a first embodiment of the invention for manufacturing rigid tubular pipes; 
         FIG. 2  is a diagrammatic view in axial section of the installation conforming to the first embodiment of the invention in a first working position; 
         FIG. 3  is a diagrammatic view in axial section of the installation shown in  FIG. 2  in a second working position; 
         FIG. 4  is a diagrammatic detail view in axial section of an installation conforming to a second embodiment of the invention for manufacturing rigid tubular pipes; 
         FIG. 5  is a diagrammatic detail view in axial section of an installation conforming to a third embodiment of the invention for manufacturing rigid tubular pipes; 
         FIG. 6  is a diagrammatic detail view in axial section of an installation conforming to a fourth embodiment of the invention for manufacturing rigid tubular pipes; and 
         FIGS. 7A and 7B  are diagrammatic detail views in axial section of an installation conforming to a fifth embodiment of the invention for manufacturing rigid tubular pipes. 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     First elements of the installation  10  of the invention for manufacturing rigid tubular pipes are described first with reference to  FIG. 2 . The installation  10  includes a holding frame  12  adapted to hold the inlet end  14  of an external rigid tube  16  in a fixed position. The holding frame  12  includes a thick plate and first clamping means  17  for clamping the external rigid tube  16 , as described in detail hereinafter. The external rigid tube  16  extends longitudinally on the downstream side  18  of the holding frame  12 , while on the opposite side extends longitudinally a chassis  20  inside which is a sliding apron  22  adapted to be driven. The chassis  20  includes a stop plate  24  opposite the holding frame  12  at a distance in the range 5 to 20 m, for example. The stop plate  24  and the apron  22  are connected by two peripheral actuators  26 ,  28 , the body  30  of these peripheral actuators being fastened to the stop plate  24 , while the end of their rod  32  is fastened to the apron  22 . The latter apron includes a central passage  34 , a drum  36  installed through the central passage  34 , and the second clamping means  38  mounted on the drum  36  facing the holding frame  12 . Furthermore, the drum  36  also includes a central space  40  enabling the passage of a rigid internal tube  42  adapted to extend longitudinally inside the chassis and also to pass freely through the stop plate  24 . 
     On the downstream side  18  and the upstream side  44 , the external rigid tube  16  and the internal rigid tube  42 , respectively, are produced by assembling tube sections. Obviously, the diameter of the external rigid tube  16  is greater than that of the internal rigid tube  42  so that it is possible to engage the latter in the former. Thus the second clamping means  38  enable clamping of the internal rigid tube  42 , which is then fastened to the apron  22 , and the latter apron is adapted to be driven in translation toward the holding frame  12  by means of the peripheral actuators  26 ,  28  to force the internal rigid tube  42  longitudinally into the interior of the external rigid tube  16 . 
     The installation conforming to the invention is described in more detail next with reference to figure  1 .  FIG. 1  includes the frame  12  equipped with its first clamping means  17  and the apron  22  provided with its drum  36  and its second clamping means  38  fastened to the drum  36 . The inlet end  14  of the external rigid tube  16  is held in a fixed position clamped by the first clamping means  17 , while the internal rigid tube  42  is held clamped by the second clamping means  38 . 
     The drum  36  includes two opposite flanges  46 ,  48  connected together by a ring  50  mounted through the central passage  34  to rotate freely by means of a bearing  52 . The ring  50  extends through the apron  22  and the flange  48  situated outside the apron faces the frame  12 , while the other flange  36  faces the frame  12 , also outside the apron  22 . Accordingly, the drum  36  is free to rotate about a longitudinal axis A substantially corresponding to the axis of the internal rigid tube  42  and that is substantially perpendicular to the apron  22 . However, on the one hand means for locking the drum  36  in rotation relative to the apron  22  are provided but are not shown here and on the other hand rotation drive means are also provided, for example an electric motor that is not shown. The second clamping means  38  are installed precisely on the flange  48  facing the frame  12 . These second clamping means  38  comprise a first frustoconical ring  54  centered on the ring  50  which includes a frustoconical internal wall  56  open toward the holding frame  12 . To the interior of this frustoconical ring  54  are applied jaws  58  having an inclined wall  60  adapted to come to bear against the frustoconical internal wall  56 . The jaws  58  have opposite their inclined wall a clamping wall  62  adapted to come to be engaged against the internal tube  22 . The jaws  58  are retained by a drive disk  64  installed coaxially in front of the first frustoconical ring  54  and is connected to the latter ring by hydraulic coupling actuators  66 . Accordingly, when the coupling actuators  66  are retracted in an axial direction, the drive disk  64  drives the jaws  58  axially toward the interior of the first frustoconical ring  54  so that their inclined wall  60  comes to slide against the frustoconical internal wall  56  of the first ring  54  which then forms a ramp and thus causes radial movement toward each other of the clamping walls  62  and, as a consequence of this, clamping of the internal rigid tube  42 . 
     On the opposite side, the other flange  46  is equipped here with two diametrically opposite spools  68  onto which heating cables are wound. Storing cables by winding them around a spool is an extremely practical way of bending them so as to be able to deploy them afterwards. These two spools  68  are installed eccentrically on the other flange  46  by means of a yoke the rotation shaft  72  of which is substantially perpendicular to the longitudinal axis A. The spools  68  are free to rotate about their shaft  72 . On the other hand, means not shown enable them to be locked against rotation. The heating cables  69  are adapted to extend through the opposite flanges  46 ,  48  and through the ring  50  and the apron  22  inside the central passage  34  around the internal rigid tube  42 . They are also adapted to extend through the first frustoconical ring  54  and between the jaws  58 . To prevent them from being damaged, roller type guide means, not shown, are provided on the drum  36  along the path of the heating cables  69 . The drum  36  then constitutes means for application of the heating cables  69  around the internal rigid tube  42 , while the spools  68  constitute means for storing the heating cables  69 . 
     Moreover, the apron  22  is provided with a bearing abutment  74  to the rear of the flange  48  facing the frame  12 , to retain the drum  36  axially when the apron is driven in axial translation toward the holding frame  12 . Moreover, spring means  76  enable damping of the bearing of the flange  48  against the bearing abutments  74 . In fact, the forces that are exerted on the drum  36  when the internal rigid tube  42  is forced into the external rigid tube  16  are relatively high and lead to slight axial relative movement of the drum  36  relative to the apron  22  that the bearing  52  is not able to absorb on its own. 
     On the opposite side, the first clamping means  17  are constituted of elements analogous to the second clamping means  38  and are activated from the outset and deactivated only when the rigid tubular pipe is finished. 
     Thus the structural elements have been defined, so consider now  FIG. 2 , showing the starting point of the method of installing the heating cables  69  around the rigid internal tube  42 . Thus, in this position, the first clamping means  17  are activated while the second clamping means  38  are placed in a deactivated position, i.e. the hydraulic coupling actuators  66  shown in  FIG. 1  are brought into an extended position and the clamping wall  62  of the jaws  58  is moved away from the internal rigid tube  42  in order to release it. Moreover, the heating cables engaged in the annular gap between the external rigid tube  16  and the internal rigid tube  42  extend through second clamping means  38  and through the drum  36  and the apron  22  to return to the spools  68 . Moreover, the means for locking the drum  36  against rotation are deactivated so that the latter drum may be driven in rotation. 
     The peripheral actuators  26 ,  28  are then commanded to retract them to drive the apron  22  in translation in a direction away from the holding frame  12  and the drum  36  is simultaneously driven in rotation, while the spools  68  on which the heating cables are wound are free to rotate. Consequently, the double movement in rotation and translation enables two particular lengths of the two diametrically opposite heating cables  69  to be deployed and applied in a spiral around the internal rigid tube  42 . The movement in rotation of the drum  36  is then stopped as soon as the apron  22  is at a given distance from the stop plate  24 . 
     Refer now to  FIG. 3  in which the apron  22  is stopped and is situated at said given distance from the stop plate  24 , facing the holding frame  12 . This figure shows heating cables  69  wound around the internal rigid tube  42 . 
     Moreover, in this position spacers  80  are installed around the internal rigid tube  42 , holding the heating cables  69  so that they bear against the external surface of the rigid internal tube  42  and the latter is covered with insulation  82  that extends in cylindrical manner around the internal rigid tube  42  and axially between the spacers  80 . Then, and conversely, the means for locking the drum  36  against rotation and the second means  38  for clamping the internal rigid tube  42  are activated. Then the peripheral actuators  26 ,  28  are caused to extend in such manner as to drive the apron  22  in translation toward the holding frame  12 . As a result, a given length of the rigid internal tube  42  equipped with two particular lengths of heating cables  69 , the spacers  80  and the insulation  82  are forced into the interior of the external rigid tube  16 . This given length of the rigid internal tube  42  corresponds to the stroke of the apron  22  between the stop plate  24  and the holding frame  12 . 
     As a result, the heating cables  69  are applied around the internal rigid tube  42  in masked time, which enables production of rigid tubular pipes having a double casing equipped with heating cables at a relatively low cost, since the manufacturing time is not affected compared to conventional installations. 
     The invention also concerns a rigid tubular pipe having a double casing obtained by the method described above. These pipes having a double casing are laid by the so-called rigid paying out method. Thus to avoid damaging the heating cable by stretching caused by flexing of the pipe when it is wound onto the shipboard storage spools, there is provision for an increase in the length of the cable. The use of helical heating cables in the aforementioned method is then particularly advantageous. 
     Nevertheless, if an elastically deformable cable is used, it is no longer necessary to add an additional length of heating cable, in particular if the pipe is installed using the rigid paying out method, and laying the cable parallel to the internal rigid tube may then be envisaged. To do this the drum is locked against rotation. 
     There is provision for equipping the rigid tubular pipe having a double casing produced in this way with a three-phase electrical power supply. At least one multiple of three cables is laid that may be carried by a single spool. The storage capacity of the spool depends on the available space. It is then necessary to provide for reloading the spools splicing the cables by welding to ensure continuity. 
     A second embodiment of the invention is described with reference to  FIG. 4 . Elements common to the previous embodiment have the same reference preceded by the digit “2” for second. Thus  FIG. 4  shows a second installation  210  including a second holding frame  212  for holding a second external rigid tube  216 . The latter has second first clamping means  217 . Second chassis  220  comprises a second apron  222  shown in its two extreme positions, one position  a  near the second holding frame  212 , the other position  b  far from the latter. The second chassis  220  has a second stop plate  224  and the second apron  222  is connected thereto by way of two second peripheral actuators  226 ,  228 . The second apron  222  has a second central passage  234  to enable passage of a second internal rigid tube  242  and a second drum  236  installed not through the central passage  234  but on the face of the second apron  222  facing the second holding frame  212 . Second second clamping means  238  are also installed on the second apron  222 . The second drum  236  is then equipped with four second spools  268  of heating cables diametrically opposite in pairs and arranged around the second internal rigid tube  242 . The second drum  236  is adapted to be driven in rotation by an electric motor  237  connected to a variable speed drive. These driving elements are obviously fastened to the second apron  222  in such manner that they may be driven in translation with it. 
     The mode of operation of the second installation  210  is then analogous to the preceding one. On the other hand, a second chassis  220  is provided that is significantly longer to attain a stroke of the second apron  222  of approximately 4.50 m without being impeded by the second spools  268 . 
     Refer now to  FIG. 5  showing a third embodiment of the invention in which the references of common elements are preceded by the digit “3” for third. Thus a third installation  310  comprises a third holding frame  312  for holding a third external rigid tube  316  and includes third first clamping means  317 . A third chassis  320  comprises a third apron  322  shown in its two extreme positions. The third chassis  320  includes a third stop plate  324  and the third apron  322  is connected thereto by way of two third peripheral actuators  326 ,  328 . The third apron  322  has a third central passage  334  to allow the passage of a third internal rigid tube  342  and a third drum  336  installed on the face of the third apron  322  facing the third holding frame  312 . The third drum  336  is adapted to be driven in rotation by a third electric motor  337 . Third second clamping means  338  are also installed on the third drum  336 . On the other hand, the third installation  310  is equipped with four third spools  368 , successively installed around the internal rigid tube  342  in such manner that the latter tube extends along their rotation axis. Thus the third internal rigid tube  342  is threaded through each of these third spools  368 . Each of the third spools  368  is then equipped with a rotary arm, not shown, that picks up the heating cable to feed it along the internal rigid tube  342 . The four arms are oriented at 90° to each other and are constrained to rotate together. The heating cables then pass through the third jaws  358  installed on the third drum  336 . 
     Obviously, in the second and third embodiments described above, the electric motor coupled to the reducer is also equipped with a clutch and brake system. The speed of the reducer is also variable. Thus the driving in rotation of the drums  236 ,  336  may be rendered independent of the movement in translation of the aprons  222 ,  322 . However, depending on the mode of operation, the movement of the apron then causes the drum to rotate. 
     A fourth embodiment of the invention is described next with reference to  FIG. 6 . On the principle adopted above, the references of elements common to the previous embodiments are preceded by the digit “4” for fourth. 
     A fourth installation  410  comprises a fourth holding frame  412  for holding a fourth external rigid tube  416  and includes fourth first clamping means  417 . A fourth chassis  420  comprises a fourth apron  422  also shown here in its two extreme positions  a ,  b . The fourth chassis  420  includes a fourth stop plate  424  and the fourth apron  422  is connected thereto by way of two fourth peripheral actuators  426 ,  428 . The fourth apron  422  includes a fourth central passage  434  to allow the passage of a fourth internal rigid tube  422 . On the other hand, fourth clamping means  338  are installed on the face of the fourth apron  422  facing the fourth holding frame  412  around the fourth central passage  434  and in fixed manner relative to the apron  422 . Moreover, a fourth drum  436  is installed, no longer fastened to the apron  422 , but in a fixed position relative to the fourth holding frame  412  and in a direction parallel to the axis of the fourth internal rigid tube  442 , and also between the fourth apron  422  and the fourth holding frame  412 . The fourth drum  436  is then equipped with four fourth spools  468  of heating cables  469  diametrically opposite in pairs and arranged around the fourth internal rigid tube  442 . Only two of these four spools  468  are shown here. The fourth drum  436  is intended to be driven in rotation by a fourth electric motor  437  also coupled to an adjustable variable speed drive. 
     Accordingly, in this fourth embodiment of the invention, lengthwise heating cable portions  469  are applied along the fourth internal rigid tube  442 , not now when the fourth apron  422  is driven in reverse toward a position  b  away from the holding frame  412  but, to the contrary, when it is driven in translation toward the near position  a  of the holding frame  412 . Consequently, the fourth drum  436  is commanded to rotate when a fourth apron  422  is driven toward the holding frame  412  in such manner as simultaneously to apply the four heating cable  469  portions in a spiral around the internal rigid tube  442 . The internal rigid tube  442  equipped with the heating cables  469  is then forced into the interior of the external rigid tube  416 . 
     A fifth embodiment of the invention is described next with reference to  FIGS. 7A and 7B . Again the references of the elements common to the previous embodiments are preceded by the digit “5” for fifth. 
     A fifth installation  510  comprises a fifth holding frame  512  for holding a fifth external rigid tube  516  and includes fifth first clamping means  517 . A fifth chassis  520  comprises a fifth apron  522  shown in  FIG. 7A  in one of its two extreme positions and in  FIG. 7B  in the other of its two extreme positions. The fifth chassis  520  includes a fifth stop plate  524  and the fifth apron  522  is connected thereto by way of two fifth peripheral actuators  526 ,  528 . The fifth apron  522  includes a fifth central passage  534  to enable the passage of a fifth internal rigid tube  542 . On the other hand, fifth clamping means  538  are installed on the face of the fifth apron  522  around the fifth central passage  534  and in fixed manner relative to the apron  522 . 
     Moreover, a longitudinal working space  584  extends between the fifth apron  522  and the fifth holding frame  512  and the fifth drum  536  is installed on a carriage  586  mobile in translation inside this longitudinal working space between the fifth apron  522  and the fifth holding frame  512 , in a direction parallel to the axis of the fifth internal rigid tube  542 . Also, in this fifth embodiment of the invention it is necessary for the apron  522  and the holding frame  512  to be installed at a distance from each other significantly greater than the distance between them in the aforementioned first embodiment. The carriage  586  further includes attachment means  588  for connecting it in translation to the fifth internal rigid tube  542 . 
     The carriage  586  is then mobile in translation between a position near the holding frame  512  as shown in  FIG. 7A  and a position far from the holding frame  512  but near the fifth apron  522 , as shown in  FIG. 7B . 
     The fifth drum  536  is then equipped with four fifth spools  568  of heating cables  569  diametrically opposite in pairs and arranged around the fifth internal rigid tube  542 . Only two of these fifth spools  568  are shown here. The fifth drum  536  is also driven in rotation by a fifth electric motor  537  coupled to an adjustable variable speed drive. 
     Thus in this fifth embodiment of the invention lengthwise heating cable portions  569  are applied along the fifth internal rigid tube  542 , independently of the fifth apron  522 , by way of the carriage  586  when the latter is driven in reverse from the position near the holding frame  512  as shown in  FIG. 7A  toward the position far from the holding frame  512  as shown in  FIG. 7B . 
     While the heating cables  569  are deployed around the internal rigid tube  542 , or after that, the apron  522  is driven in rotation toward the stop plate  524 . Furthermore, after the heating cables  569  have been deployed, the corresponding internal rigid tube  542  portion is equipped with insulation  582  and spacers  580 . 
     The fifth second clamping means  538  of the retracted fifth apron  522  and the attachment means  588  of the carriage  586  are then activated. The fifth peripheral actuators  526 ,  528  are then actuated to drive in translation the fifth apron  512  and consequently the internal rigid tube  542  equipped with the heating cables  569  and the carriage  586  that is fastened to it. The internal rigid tube  542  equipped with the heating cables  569  is then forced into the interior of the external rigid tube  516 , while the carriage  586  returns to its position near the holding frame  512  as shown in  FIG. 7A .