Abstract:
A method for laying an elongated element in a duct using a fluid injected under pressure through a fluid inlet in a pressure housing disposed at the elongated element inlet extremity of the duct. A mechanical drive placed before the pressure housing and the duct pushes the elongated element into the pressure housing and the duct. The method includes installing a first front end pulling pig at the front end of the elongated element and installing at least one subsequent pulling pig around the elongated element. The distance between each pulling pig is mainly constant. The method also includes applying the same maximum pressure drop to each pulling pig. The total pressure of the fluid injected being mainly equally shared by all the pulling pigs.

Description:
TECHNICAL FIELD 
       [0001]    The disclosure concerns a process for laying an elongated element, as per example a power or telecommunication cable, in a duct just as several embodiments of a pig and of a pressure housing used with this process. 
       BACKGROUND 
       [0002]    A typical method to install cables in ducts includes water is injected under pressure into the duct, exerting a pushing force at a pig attached to the front end of the cable, the pig exerting also a pulling force at the cable. At the same time the cable is pushed into the duct with water under pressure and with a mechanical drive, e.g. caterpillars or wheels, disposed at the rear end of the duct. 
         [0003]    One system generally includes blowing a cable with the help of pistons, without sealing rings, which are released from a pressure chamber at the cable entrance point at regular intervals. The pistons are just a little smaller in diameter then the inner diameter of the duct. The force over the piston is generated by the airflow. There is no other mechanism to control the force over the pistons. 
         [0004]    Another method generally includes using multiple pulling pigs. Here the intermediate pulling pig gets extra pulling force when the seal got stuck somewhere at the duct route (it does nothing when the duct is smooth). It is not intended here to increase cable installation length by distributing the available pushing force over more locations (and reduce the capstan effect), but to get the cable passing at positions where the cable got stuck. The front pig does not allow the passage of air, essential for functioning of the present invention. 
         [0005]    The methods described above are often referred to as 30 push/pull method. The advantage of using water instead of air is its buoyancy, reduces the effective weight of the cable. Also it cools the frictional heat. Furthermore the safety of water under pressure is less a problem than for compressed air. This makes the technique with water under pressure suitable for use with larger cables in larger ducts, e.g. high power electric cables. However, for smaller cables in smaller ducts, e.g. telecom cables (like fiber optic cables) the same method can also be used with compressed air (still called push/pull method). Advantages in this case are that no water needs to be fed or exhausted nor remains in the duct. 
       SUMMARY 
       [0006]    With push/pull methods long installation lengths can be achieved, especially in the case of water and balanced (ideally floating) cables. However, when a large pulling tension is built-up the so-called capstan effect takes over: the built-up tension pulls the cable against the duct wall in bends and undulations of the duct and causes extra friction (depending on the bends and undulations). Because this friction is proportional to the tension it causes an exponential force build-up that “explodes” soon after becoming dominant. This is especially a drawback when using water. The capstan effect dominates earlier. And then the advantage of the buoyancy of the water is completely lost. 
         [0007]    A method for laying an elongated element in a duct using a fluid injected under pressure through a fluid inlet in a pressure housing disposed at the elongated element inlet extremity of the duct. A mechanical drive placed before the pressure housing and the duct pushes the elongated element into the pressure housing and the duct. The method includes installing a first front end pulling pig at the front end of the elongated element and installing at least one subsequent pulling pig around the elongated element. The distance between each pulling pig is mainly constant. The method also includes applying the same maximum pressure drop to each pulling pig. The total pressure of the fluid injected being mainly equally shared by all the pulling pigs. 
         [0008]    The details of one or more implementations of the disclosure are set forth in the accompanying drawings and the description below. Other aspects, features, and advantages will be apparent from the description and drawings, and from the claims. 
     
    
     
       DESCRIPTION OF DRAWINGS 
         [0009]      FIG. 1  is a diagram showing the pushing force (Fpush) in regard to the location of the front end of the cable (xhead) according to a process of the prior art comprising only one front end pig, 
           [0010]      FIG. 2  is a diagram showing the same parameter of  FIG. 1 , according to an exemplary process including two pigs, 
           [0011]      FIG. 3  is a diagram showing the same parameter of  FIG. 1 , according to an exemplary process including three pigs, 
           [0012]      FIG. 4  shows a laying of a cable in a first step of an exemplary process, using a pressure housing, 
           [0013]      FIG. 5  shows a laying of a cable in a second step of the process, 
           [0014]      FIG. 6  shows a laying of a cable in a third step of the process, 
           [0015]      FIG. 7  shows a laying of a cable in a fourth step of the process, 
           [0016]      FIG. 8  shows a longitudinal view of a first embodiment of a dividable pig according to the invention, 
           [0017]      FIG. 9  shows a sectional view of the pig of  FIG. 8 , 
           [0018]      FIG. 10  shows a detail of the construction of the body of the pig of  FIG. 8 , 
           [0019]      FIG. 11  shows another detail of the construction of the body of the pig of  FIG. 8 , 
           [0020]      FIG. 12  shows an enlarged view of an exemplary pressure reducing means for the pig of  FIG. 8 , 
           [0021]      FIG. 13  shows a longitudinal view of an exemplary dividable pig, 
           [0022]      FIG. 14  shows a longitudinal view of an exemplary dividable pig, 
           [0023]      FIG. 15  shows an enlarged view of the pressure reducing means of  FIG. 14 , 
           [0024]      FIG. 16  shows a longitudinal view of an exemplary dividable pig, 
           [0025]      FIG. 17  shows a sectional view of a part of the pig of  FIG. 16 , 
           [0026]      FIG. 18  shows an enlarged view of a portion of the sealing means of the pig of  FIG. 16 , 
           [0027]      FIG. 19  shows a longitudinal view of an exemplary dividable pig, 
           [0028]      FIG. 20  shows another exemplary pig, 
           [0029]      FIG. 21  shows an exemplary pressure housing, and 
           [0030]      FIG. 22  shows another an exemplary pressure housing according to the disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0031]    According to the process of laying an elongated element of the present disclosure more than the one pulling pig at the front end of the elongated element, e.g. a cable is used. Also intermediate pigs are placed around the cable at regular intervals. The intervals are chosen such that just when the capstan effect starts to dominate a pulling pig is placed. Note that when using multiple pigs not the full working pressure is working over each pulling pig. The pulling pigs share this working pressure. The pulling pigs are designed such that they work with a specified pressure drop (a flow of the fluid through all pulling pigs is needed to obtain this). This pressure drop is chosen such that sum of the pressure drops over the number of pulling pigs planned is the maximum working pressure monitored at the cable injection side, where also the flow is controlled. 
         [0032]    In some implementations, the pressure at the pressure inlet at the cable insertion is increased by the pressure drop over a pig, each time when a pig is launched. Each pig can exert a pulling force on the section behind and a pushing force at the section in front of the pig. Said forces are e.g. divided 50%-50%. The front pig has no section in front, but the excess force is used to compensate extra friction that is generated at the cable head in bends and undulations of the duct trajectory due to its stiffness. 
         [0033]    However operation is also possible with the maximum constant pressure at the pressure inlet at the cable insertion location during the complete installation provided that one pig alone and its attachment to the cable can hold such a pressure. Note that it is one of the advantages of using multiple pigs that the forces acting on a pig are smaller than when using only one pig, which improves reliability of the seals and cable attachment. 
         [0034]      FIGS. 1 ,  2  and  3  illustrate the benefit of using multiple pulling pigs, each taking a part of the available pressure, according to the disclosure, instead of one pulling pig at the front end of the cable taking the full pressure as in the prior art. In the examples a cable with a diameter of 66 mm, weight 43 N/m and stiffness 1300 Nm 2  is installed in a duct with inner diameter of 102 mm that shows undulations with amplitude of 15 cm and period of 15 m (these parameters include effectively the few bends that might also be present along the trajectory). The cable is installed using water under a pressure of 12 bars that exerts a pushing force on one or multiple pulling pigs. In this water the buoyancy of the cable in the water causes the weight of the cable to drop to 9.44 N/m effectively. The coefficient of friction between the cable and the duct in this situation is taken as 0.2. At the entry side the cable is pushed into the duct with water under pressure with a force of 14000 N. Inside the duct this gross pushing force has dropped to net 9895 N, after overcoming the pressure drop over the cable inlet. 
         [0035]    The example of  FIG. 1  illustrates a laying with only one pig at the front end of the cable according to the prior art, the pushing force at the cable injection side being showed as a function of the location of the front end of the cable. 
         [0036]    With 12 bars over the pulling pig a pulling force of 9805 N is exerted on the front end of the cable (note that this force is seen at the cable injection side when the front end of the cable is still at the cable injection side, at the start of the installation). 
         [0037]    After a length of 2000 m the maximum net pushing force of 9895 N is reached at the cable injection side. 
         [0038]    The example of  FIG. 2  illustrates a laying with two pigs, each working with a drop of pressure of 6 bars, the pushing force at the cable injection side being showed as a function of the location of the front end of the cable. 
         [0039]    With 6 bars over each pulling pig a force of 4903 N is exerted over each pulling pig, at the front end, and after 1500 m. After a length of 3100 m the maximum net pushing force of 9895 N is reached at the cable injection side. 
         [0040]    The example of  FIG. 3  illustrates a laying with three pigs, each working with a drop of pressure of 4 bars, the pushing force at the cable injection side may be a function of the location of the front end of the cable. 
         [0041]    With 4 bars over each pulling pig a force of 3268 N is exerted over each pulling pig, at the front end, after 1200 m and after 2400 m. After a length of 3800 m the maximum net pushing force of 9895 N is reached at the cable injection side. 
         [0042]    Regarding  FIG. 4 , an elongated element, e.g. a cable  1  is installed from a drum  10  by pushing and fluid injection equipment, consisting of caterpillars  11  and pressure housing  12  with fluid inlet  13 . The pressure housing  12  is connected fluid tight to duct  14  into which cable  1  is installed. During installation a fluid flow  15  is injected into duct  14  through pressure housing  12  and fluid inlet  13 . At the front end of the cable a pulling pig  2   a  is connected. The pulling pig  2   a  is constructed such that a defined maximum pressure drop (e.g. 4 bars) is present over the pulling pig. When the pressure drop gets higher the fluid will flow through the pulling pig. It is also possible to use pulling pigs where the pressure drop over the pigs is controlled by adjusting the water flow. Several constructions, described below, are possible to obtain this functionality. During installation of the first length two dividable pulling pigs  2  are waiting. In some implementations, the pigs  2  are constructed with the same maximum pressure drop as the front pig  2   a.    
         [0043]    In some implementations of the pressure housing  12 , the fluid flow is interrupted, the pressure housing  12  is opened after installation of the first section of cable to enable placing the first dividable pulling pig  2 , see  FIG. 5 . The dividable pulling pig  2  is than shifted forwards a little over cable  1  before final tightening. This can e.g. be done by screwing a dividable nut at the rear end or by pushing an internal piston forward. The latter can be pushed back by reversed pressure, allowing retrieval of the pig  2  (not described here). Then the pressure housing  12  is closed again and the installation started again, see  FIG. 6 . In  FIG. 7  placement of a second dividable pulling pig  2  is shown, the same operation. This process can be continued, also with more dividable pulling pigs  2 , dependent on the maximum fluid pressure that can be used. In the described example with 3 pigs  2   a ,  2  and  2  with maximum pressure drop of 4 bars the total maximum pressure is 12 bars. 
         [0044]      FIGS. 8 to 12  represent an example of dividable pig  2  that can be used with the pressure housing  12  as described above. Pig  2  comprises essentially: a body  3 , sealing means  4 , guiding means  5 , gripping means  6  and pressure reducing means  7 . Pig  2  is mounted around the elongated element or the cable  1 , the body  3  comprising two parts  30   a  and  30   b  which can be held together by a sliding interlock joint  31 . An O-ring  32  is placed to get a fluid tight seal around the 25 cable. Said O-ring can be a length that can be glued to a ring when placed around the cable. 
         [0045]    In some implementations, the pulling pig  2  includes a central body  30 , constituted by the assembling of the two parts  30   a  and  30   b , with a radial walled part  33  onto which a flexible cylindrical part  34  is fixed. The flexible  30  cylindrical part  34  can e.g. be made of fiber reinforced plastic foil like Kapton. After sliding the two halves  30   a  and  30   b  of the pulling pig together around the cable  1 , the flexible cylindrical parts  34   a  and  34   b —see FIGS.  10  and  11 —can be connected by sliding another locking element  35  around the edges  34   c  and  34   d  of cylindrical parts  34   a  and  34   b . Strong locking edges  34   c  and  34   d  can be made by winding layers of reinforced foil  34   e  around wires  35   a  made of hard plastic and gluing this together with e.g. epoxy. 
         [0046]    Around the rear end of the flexible cylindrical part  34 , sealing means  4 , preferably a lipseal  40 , can be placed. Said lipseal can be glued together in the same way as done with the O-ring  32 . Said seal  40  is axially supported by support part  50 . Said support part  50  also serves to centre and guide the pulling pig  2  as secondary guiding means  5 . 
         [0047]    The guiding means  5  are here completed by a main guiding element comprising several guiding fingers  51  mounted attached to the central body  30 . 
         [0048]    The gripping means  6  comprise here a part  60  disposed at the rear end of central body  30  with screw-thread. Around this a dividable nut  61 , held together with the same kind of interlocking joint (not shown) as in  FIG. 9 , can be screwed. When screwing the nut  61   a  conical gripping part  62  (open aside for cable) with gripping teeth is squeezed against the cable  1 , axially securing the pulling pig  2  around the cable. Screwing the nut is done after the pig is slit far enough into the duct that the seals  40  seal off. 
         [0049]    Pressure reducing means  7  of  FIG. 8  are particularly visible on  FIG. 12 . In the radial walled part  33  at least one pressure reducer  70  is placed having a communication between an inlet cavity left (inside) functioning as high pressure chamber  73   a , and an outlet cavity  73   b  communicating with the cavity right (outside) of the radial wall part  33  at low pressure, in communication with the front end of the pig. Said pressure reducer  70  can be made of a piston  72  with unequal cross-sectional area at both sides (designed to the demanded pressure reduction) and at least one outlet  71   a - 71   b  communicating between the high pressure chamber  73   a  and the cavity  73   b  right of the radial walled part  33 . For proper functioning a low pressure chamber  73 , where the piston diameter changes, is reduced in pressure, to a pressure lower than the pressure in cavities  73   a  and  73   b , by venturi hole  74 . 
         [0050]    Placing more than one pulling pig  2 , of this kind (i.e. with only one seal) at one location of the cable allows passage of gaps in the internal duct path (e.g. at duct couplings where the duct ends not exactly abut). 
         [0051]    Alternatively, a pig used with the pressure housing described above is represented in  FIG. 13 . The body  3 , the sealing means  4  and the guiding means  5  are the same as described above. 
         [0052]    Here, the gripping means  6  comprise a “socket”  63 , made of wires (preferably non-metallic) that are counter-spiraled around the cable  1 . The tensed wires grip the cable by friction caused by pressing the wires against the cable because of the Capstan effect. 
         [0053]    The pressure reducing means  7  comprise an orifice  75  which regulates the pressure over the pig  2 . The orifice  75  is just a short opening of a size that for a certain flow (additional to the flow to follow cable and pigs) the required pressure difference is obtained. 
         [0054]    For a gas the dimensions of this hole can be found from sonic conditions, as described in US05197715 (1992). Note that the examples described in this invention are given with liquids (water) as a fluid, but that the invention is not limited to that, i.e. also gas (air) can be used (with the flow set e.g. by sonic conditions). For liquid the dimensions of said hole are found from the acceleration of the liquid in the hole  75  (Bernoulli law). When the speed of the liquid in the annulus between cable and duct can be neglected to the speed in orifice  75  the relation between volume flow Q, pressure difference Zip and cross-sectional area A of orifice  75  is given by: 
         [0000]    
       
         
           
             Q 
             = 
             
               
                 0.62 
                 · 
                 A 
               
                
               
                 
                   
                     2 
                      
                     Δ 
                      
                     
                         
                     
                      
                     p 
                   
                   ρ 
                 
               
             
           
         
       
     
         [0055]    Here ρ is the density (1 000 kg/m 3  for water) and 0.62 (approximate value for fully developed flow) a factor which takes into account the narrowing of the flow after the orifice. Example: a pressure difference of 3 bars over a hole with cross-sectional area of 1.1 cm 2  corresponds to (additional) volume flow of 1001/min (note that also frictional losses may not always be negligible; the right settings of the orifice(s) can be obtained by measuring). Therefore, the pressure differences over the multiple pigs can be regulated by the flow (monitored by the pressure at the inlet; the pressure difference will be equally shared by the pigs when they are equipped with the same orifices). 
         [0056]      FIG. 14  shows another example of a pig used with the pressure housing described above, the only difference with the pressure housing of  FIG. 13  being on the pressure reducing means  7  which are represented at enlarged scale on  FIG. 15 . 
         [0057]    The pressure reducing means  7  comprise an annular piston  76 , moving under the action of the pressure difference (sealed airtight by O-rings  76   a ; note that for an annular piston as in  FIG. 14  with diameter of around 70 mm a cross-sectional area of 1.1 cm 2  corresponds to an annular gap of 0.5 mm; this means that the O-rings  76   a  are often needed for correct functioning of the system) in radial walled part  33 , and balanced by resilient ring  76   b , made of natural or synthetic rubber (e.g. Nitrile, Silicone or natural rubber) or other material. Note that here an entirely non-metallic pig is chosen, to which this invention is not limited. In case that metal is allowed solutions with springs are possible. The pressure left of annular piston  76  is the pressure at the back of the piston, the pressure right of piston  76  the pressure at the front. The latter pressure is allowed through hole  77  in radial walled part  33 . When there is no pressure difference between the back and the front of the pig, annular piston  76  is pressed to the left by resilient ring  76   b . When the pressure at the back of the piston is higher than at the front the piston starts moving to the right. The system is developed such that when the demanded pressure difference is reached, annular piston  76  has moved to the position that hole  77   a  just starts opening. From that moment on air is bleeded from the back to the pig to the front, and further increase of the pressure difference is prevented. Advantage of this (and also the first) embodiment is that it can be made independent of the flow, avoiding overpressure by wrong operation. In fact also the combined advantage of maximum overpressure and some regulation of the pressure difference by the flow is possible, depending on the size of the holes  77   a  and also by selecting a proper annular gap for piston  76  (and not using O-rings  76   a ). 
         [0058]    Example: An annular piston  76  with inner diameter of 70 mm and a wall thickness of 4 mm has a working cross-sectional area of 9.3 cm 2 . A pressure difference of 3 bars will result in a force on this piston of 279 N. The rubber ring  76   b  is a little smaller (space is required for Poisson expansion), in this example 72 mm ID and 76 mm OD resulting in a working cross-sectional area of 4.65 cm 2 . A soft rubber with a Young&#39;s modulus E of 1 MPa is selected (these values can be obtained with silicone or natural rubber). According to Hooke&#39;s law the force for a 3 bars pressure difference will result in 60% compression of the rubber. For a 6 mm thick rubber ring this will be 3.6 mm (optionally this 3.6 mm can be partly pre-compressed). In the 3 bars position the holes  77   a  just start to open (for a pig designed for 3 bars pressure difference). At 4 bars the holes are opened 1.2 mm. For an almost continuous slit (instead of holes) this is amply more than the 0.5 mm needed for flowing through of 100 l/min (this is an example flow; pumps can often supply more and lower settings are also possible). It is also possible to drill holes, e.g. holes with a diameter of 10 mm. In the latter case 1.2 mm opening results in 0.054 cm 2 , so 19 holes will just do the job (and there is just place for 19 holes). Larger (and less) holes will result in more throughputs. 
         [0059]    The examples of  FIGS. 16 and 19  make use again of an orifice for pressure regulating, like the pig of  FIG. 13 . The design is made simple, with all parts dividable except the seal (this seal can easily be preinstalled over the cable, even when a large head is pre-mounted at the cable front end; optionally, in cases that an unplanned pulling pig needs to be placed gluing a split ring is still an option). 
         [0060]    The dividable body  3  of the pulling pig  2  represented at  FIG. 16  consists of 3 dividable sections  36   a ,  36   b  and  36   c . The front section  36   a  comprises the gripping means  6  having a firm grip on the cable  1  by means of teeth  64 . All dividable sections  36   a ,  36   b  and  36   c  are clamped together by bolts  36   d , the front section  36   a  tightly bolted. Between the sections  36   a ,  36   b  and  36   c  the sealing means  41  are placed (now 2 seals are present, i.e. one pulling pig is able to pass gaps in the internal duct path). They are preferably made of rubber-like material that makes a suction seal with both duct and cable. The seals are made of one piece, or can be cut at one place and glued around the cable when placed. The pressure reducing means  7  comprise at least one hole  78 , present in the seals—see FIG.  18 —(one hole is shown in the drawings for clear presentation, preferably at least 2 holes are present, allowing less costly design of the other parts because of symmetry). This hole  78  is again suction sealing to body sections  36   a ,  36   b  and  36   c , enhanced by the annular groove  43  in seal  41 , where the “lips” seal around pieces of pipe  36   e  extending from the sections  36   a ,  36   b  and  36   c . The holes  78  in the seal and cylindrical hollow portions  36   e  in the body sections  36   a ,  36   b  and  36   c , also provide correct aligning of all parts. The pieces of pipe  36   e  have a hole  78   a  inside them that extends through the hole body  3 . Halfway this hole  78   a , in body section  36   b , a cavity  78   b  is made to allow deceleration of the flowing medium. This, together with hole  78   c  that vents to the space in the duct, makes it possible to have three different pressure zones for the three body sections. 
         [0061]    In  FIG. 19  a similar design as in  FIG. 16  is shown. Only now the parts  37   a ,  37   b  and  37   c  of the body  3  are not bolted together, but are held together by a sliding interlock joint as on the embodiment shown on  FIG. 9 . The gripping means of front section  37   a  comprise two concentric tapered elements  37   a  and  64 .  37   a  houses the female taper. The clip  64  with internal gripping teeth forms the male taper. The clip  64  can be sufficiently open to engage radially on and embrace the cable  1 . The axial force caused by the pressure drop acts on the female taper of  37   a , creating thus a sufficient inward radial force on the teeth of the clip  64  to grip and push cable  1 . 
         [0062]    Part  37   a  again consists of 2 halves held by a sliding interlock joint. Also now two holes have been designed. And, contrary as the example of  FIG. 16 , there is no annular groove  43  now in seal  41 . For this it is required that the material of the seal is hard enough that the hole  78  is not “blown up” (there is then still suction sealing because the seals are pressed forward against body section  36   a  and  36   b ). Furthermore cavity  78   b  is not present anymore, but material is completely machined away such that the hole  79  immediately exhausts in the duct space sections  79   a  and  79   b . Finally the body is made a little smaller in diameter at the places where the seals will be pushed when the duct gets smaller, so 92 mm remains the smallest total diameter. 
         [0063]    A first or front end pulling pig  2   a , as illustrated in  FIGS. 4 to 7 , is constructed of the same manner as described above, the only differences being that the body  3  is not tubular but preferably of the form of a solid piston with a hemispherical or truncated head. The gripping means  6  are also designed to grip the front end of the elongated element or the cable. 
         [0064]    Several other implementations of a pressure housing are described below, these implementations not necessitating a dismantling of the laying device for the insertion of a new pig and also not necessitating to use dividable pigs. 
         [0065]    These pressure housings use a different pigs  2 , as shown in  FIG. 20 , than the pigs previously described. As the previous pig  2  comprises a tubular body  3 . Here the body  3  is constituted from only one tubular element, his inner diameter being a little greater than the external diameter of the cable  1 . The sealing means  4  are from one of the embodiments previously described. In this execution, the guiding means  5  are constituted by the sealing means  4 . The gripping means  6  are here constituted by a separate element, a gripping ring  65 , comprising on its inner surface facing the cable  1 , grips or teeth  66 . The gripping ring  65  comprises a tapered external surface  67  able to cooperate with a corresponding tapered front inner surface  38  of the tubular body  3 . On the upper part of  FIG. 20 , we can see the gripping ring  65  just lay down on the cable  1 . On the under part of the  FIG. 20 , the pig  2 , under action of a pressure exerting a force from the left to the right of the Fig rejoins the gripping ring  65 , the tapered front inner surface  38  of the tubular body  3  pressing against the tapered external surface  67  of ring  65 , the grips  66  hemming the cable  1 . According to this manner, pig  2  is firmly fixed on cable  1 . Pressure reducing means, schematically represented at  7 , are of one of the types described above. Alternatively, the pressure reducing mean  7  can be installed in the gripping ring  65  instead in the body  3 . 
         [0066]    Another example of a pressure housing  12  is represented on  FIG. 21 . Cable  1  is introduced in duct  14 , pressure housing  12  is connected to the duct  14  and pressure flow of water  15  is introduced in pressure housing  12  of the same manner as previously described. 
         [0067]    The pressure chamber  12  comprises a sleeve  120  surrounding the cable  1 , the diameter of the sleeve  120  being chosen as narrow as possible, but leaving enough margin to pass cable diameter fluctuations (usually specified). Pigs  2  are parked on sleeve  120 , each being held by a fork  121 . Inside the fork  121  there is a separate parking position  122  for the gripping ring  65 . This is needed for sliding the pig  2  over sleeve  120 , without the griping ring  65  being pressed to sleeve  120  by forward pressing of the pig  2  via the tapered conical surface  38  to the corresponding surface  67  of the griping ring  65 . Furthermore, sleeve  120  is made of hard material (e.g. steel) onto which the grips or teeth  66  of the griping ring  65  have no grip. 
         [0068]    Each fork  121  is connected to a rotating belt  123  pivoting around two wheels  123   a , at least one of these wheels being driven by driving means  123   b , motorized or manual. To release a pig  2 , the driving means  123   b  are activate, the forks  121  being moved in direction of the duct  14 . When the front fork  121 , the fork being closest of duct  14 , reaches the end of sleeve  120  it is lifted up by lifting means  124  or caused by the curvature of the belt  123  around front wheel  123   a , first to release the griping ring  65 , and looses it from the sleeve  120 . The gripping ring  65  snaps the cable  1  with its smaller diameter by resilient working of the hard-plastic material. A slit being provided inside griping ring  65  to allow this. After this, the fork  121  closest of duct  14  is lifted further to also release pig  2 . The resilient sealing means  4  of the pig  2  will make a suction seal to the cable  1  and, when inside, also to duct  14 . The water flow will move pig  2  forward until it meets up its griping ring  65 . Now the water pressure will result in the grips or teeth  66  pressing on cable  1 . Simultaneously the lifted fork  121  will be taken out of the way and the other forks  121  are moving forward by activating the driving means  123   b , until the pig  2  leaves sleeve  120  and surrounds the cable  1 . Optionally the lifting means  124  can comprise clutch means  125  to clutch the fork from the belt  123  permitting the parking of the used forks  121  in a parking portion  12   a  of the pressure chamber  12 . 
         [0069]    Another exemplary pressure housing  12  is shown on  FIG. 22 . As previously described, the pigs  2  are stocked on a sleeve  120 , each pig  2  being held by a fork  121 , a gripping ring  65  being held in the separate parking position  122 . In this example, the forks  121  are lifted, manually or by motorized means not represented on the figure, from outside the pressure housing  12 . Sealing fork means, e.g. an O-ring, (not represented) assume the sealing of each fork  121  through the pressure housing  12 . A rotating clamp  126  permits to hold the fork  121  on low position. When released, the pressure inside pressure housing  12  will cause the fork to move to high position and to park them in the parking portion  12   a  of the pressure housing  12 . When the fork  121  has been entirely lifted up, a mobile sleeve  127  is pushed backwards by motorized or magnetic means  128  disposed outside the pressure housing  12 , such that at least the first seal of the released pig  2  is enclosed by the mobile sleeve  127 . As the water flow can only go through mobile sleeve  127 , it will cause a pressure difference over the released pig  2 , moving it forward. As the pig  2  and the gripping ring  65  will slide over sleeve  120 , it is needed that sleeve  120  is made of hard material onto which gripping ring  65  has no grip. The mobile sleeve  127  has to be thin-walled permitting the easily pass of pig  2  by suction sealing, through the portion  129  of the pressure housing  12  with slightly larger diameter in the section immediately after the rear part of the mobile sleeve  127 . 
         [0070]    In case of high power electric cables, and when the pig  2  cannot be retrieved, the pig shall be made entirely out of non-metallic parts. In some examples, the pig  2  is radial symmetric. In order to reduce friction of the pulling pig also an asymmetric design is possible, allowing the cable to follow the bottom of the duct. For a symmetric design the friction can also be reduced by means of wheels facing against the duct wall. For retrieval of symmetrical pigs also wheel facing against the cable wall can be used. 
         [0071]    Many implementations of a pulling pig have been described above; it is to understand that a body  3  of one or other of the forms as described can be combined with a sealing means  4  of one or other of the forms as described, and with a guiding means  5  of one or other of the forms as described, and with a gripping means  6  of one or other of the forms as described, and with a pressure reducing means  7  of one or other of the forms as described. 
         [0072]    As described above the process according to the disclosure used with the pulling pigs and the pressure housing according to one or other of the described implementations are particularly designed for the laying of very long cables or longitudinal elements.