Patent Publication Number: US-2021172559-A1

Title: Pig drive, pig drive system and method of using same

Description:
TECHNICAL FIELD 
     The present invention relates to pigging of pipes, in particular a pig drive and a pig drive system for feeding a pig into a pipe. 
     BACKGROUND 
     Related to production and transportation of hydrocarbons, there is extensive use of pipes, both for unprocessed well stream (in the following referred to as flow lines), and for sales quality fluids (in the following referred to as trunk lines). For flow lines there may be a need for frequent pigging or removal of wax and liquid slugs. A purpose designed device, referred to as Pipeline Inspection Gauge (PIG), is pushed through the flowline or trunk line by the well stream or production fluid to clean out wax or slugs. 
     A pig may be any movable pipe tool which e.g. can be used for physical separation of fluids in a pipeline, internal cleaning of pipelines, inspection of pipelines, and recording geometric information of pipelines. 
     To run a pig into a pipe or pipeline, a subsea automated pig launcher (SAPL) is typically used. Such pig launchers comprise a kicker unit, a SAPL main unit and a running frame. The running frame is used to deploy and replace pig cassettes and is therefore temporarily installed subsea. The kicker unit and the SAPL main unit are permanently installed. 
     The kicker unit has: a first inlet connected to a flowline, riser, pipe, pipeline or similar; an outlet connected to a flowline, riser, pipe, pipeline or similar; and a second inlet connected to the SAPL main unit. Furthermore, the kicker unit has valves and flow loops providing and preventing fluid communication between the inlets and outlet. If a pig is fed into the kicker unit via the second inlet, connected to the SAPL main unit, the pigging can be initiated by operating the valves of the kicker such that the fluid flow entering the kicker unit through the first inlet forces the pig out through the outlet of the kicker unit. 
     The SAPL main unit comprises a pipe that is connected to the kicker unit in a first end and connectable to the running frame in a second end. 
     The running frame is used to transport a pig cassette from topside to the SAPL main unit and then to stroke the pig cassette into the pipe of the SAPL main unit. The pig cassette contains a train of pigs and a pig drive. From the pig cassette located inside the pipe of the SAPL main unit, the pig drive feeds the pigs into the kicker unit by means of a fluid pressure. 
     When all pigs have been fed into the kicker unit, the pig drive remains in the pig cassette. The pig cassette can then be pulled out of the SAPL main unit by means of the running frame and retrieved to topside where it can be reloaded with pigs. 
     The pig drive has a nose interfacing the last pig to be fed into the kicker unit. The nose ensures the last pig is fed sufficiently far into the kicker unit. After feeding the last pig, the pig drive protrudes outside the pig cassette. 
     Pig launcher systems comprising a kicker unit, a SAPL main unit and a running frame has a significant footprint. 
     Significant costs are associated with the production and installation of each of the three above-mentioned subsea units. 
     The present invention is directed to a leaner pig launcher system and pig drive than the above-described and a method for using same. A leaner design reduces the number of units such that the footprint can be reduced, pig capacity increased, cost saved, operational robustness improved, and transportation simplified. 
     SUMMARY OF THE INVENTION 
     The invention provides a device, a system and a method for feeding a pig into a pig kicker section of a pipe as set forth in the independent claims. Preferred embodiments are set forth in the dependent claims. 
     It is disclosed a pig drive for feeding a pig into a pig kicker section of a pipe , wherein the pig drive comprises: a piston with a radial direction and an axial direction; a rod connected to the piston and extending in the axial direction of the piston, wherein the rod has a distal end adapted to interface a pig; a retainer slidably connected to the rod and adapted to interface a retainer abutment; and a biasing device arranged to bias the retainer towards the distal end of the rod. 
     It is achieved a pig drive with an extended state and a retracted state. The pig drive is operated towards the extended state when the retainer is retained and the piston is applied a force, by means of a fluid pressure, exceeding a biasing force of the biasing device. The pig is automatically operated back to the retracted state when the applied force on the piston, by means of a fluid pressure, goes below the biasing force of the biasing device. 
     The extended and retracted states of the pig drive enable the rod to temporarily extend a given length beyond the retainer. In this way the pig drive may temporarily extend outside e.g. a pig cassette. This given length of extension may be adapted to displace the last pig from a pig cassette into a kicker line of a pipe. 
     An advantage of this pig is that it, in the extended state, may displace the last pig from a pig cassette into a kicker line of a pipe and, in the retracted state, not obstruct the disconnection and retrieval of the pig cassette. 
     The biasing device stores energy in response to a translation of the retainer towards the piston. This energy is released in response to a translation of the retainer towards a distal end of the rod, i.e. away from the piston. The biasing device may be a damper, a compression spring, a tension spring, a sleeve made of a resilient material such as rubber, or any combinations thereof. The pig drive may comprise a plurality of biasing devices. 
     The rod and piston may be produced out of one piece or the rod and piston may be produced out of several pieces. 
     The pipe may be a tube, a pipeline, a conduit or any other pressure housing suitable for conveying a pig. 
     The biasing device in the form of a spring or sleeve may be arranged around the rod, i.e. concentric with the rod. A biasing device arranged in this manner doesn&#39;t require attaching to the piston or the retainer. 
     The biasing device in the form of a spring or sleeve may be arranged between the piston and the retainer or between the retainer and the distal end of the rod. 
     The retainer is not adapted to seal; however, it may comprise at least one through hole for pressure equalization. 
     The pig drive may further comprise a spacer, wherein the spacer is: arranged around the rod and between the piston and the retainer, and/or adapted to ensure a given minimum distance between the piston and the retainer. 
     The spacer may be adapted to ensure a given minimum distance between the piston and the retainer by means of its axial length, wherein the axial extension of the spacer corresponds to the minimum distance between the piston and the retainer. 
     The distal end of the rod comprises a perpendicularly protruding portion limiting a maximum distance between the retainer and the piston. 
     The perpendicularly protruding portion prevents the retainer and/or the biasing device from sliding off the rod. 
     When the biasing device is arranged between the piston and the retainer, the perpendicularly protruding portion enables the biasing device to apply a force on the piston in the retracted state of the pig drive. The force applied on the piston in the retracted state of the pig drive depends inter alia on the axial extension of the biasing device relative to the axial extension of the rod. The skilled person would know how to manipulate this biasing force. 
     It is disclosed a pig drive system for feeding a pig into a pig kicker section of a pipe, wherein the pig drive system comprises: a pig drive according to the above disclosure, wherein a translation of the retainer towards the distal end of the rod operates the pig drive towards a retracted state; and wherein the translation of the retainer towards the piston operates the pig drive towards an extended state; at least one pig; and a cylinder adapted to house the pig drive and the at least one pig, wherein the cylinder comprises: a first end portion adapted for sealing connection to the pig kicker section of a pipe, a second end portion sealed during operation of the pig drive, and a retainer abutment; wherein the rod is arranged to extend outside the first end portion of the cylinder, when the pig drive is in the extended state and the retainer is in contact with the retainer abutment; wherein the rod is arranged to be within the cylinder when the pig drive is in the retracted state and the retainer is in contact with the retainer abutment. 
     The cylinder may be any pressure housing suitable for conveying a pig, such as a pig cassette. 
     The retainer abutment may be any type of inward restriction of the cylinder such as a load shoulder. The retainer abutment can be continuous or intermittent. The retainer abutment and the retainer preferably has mating geometry, e.g. a coned angle. 
     The bringing of the retainer into contact with the retainer abutment is dampened by means of the biasing device in that the biasing device will store some of the energy. Thus, the retainer is prevented from wedging to the retainer abutment. 
     It is achieved a pig drive system, i.e. a pig launcher, with a simplified design wherein large portions of the permanently installed main unit of the prior art pig launchers is made superfluous. Instead of connecting the running frame to the main unit and stroking the cylinder from the running frame into the main unit, the disclosed cylinder is adapted to be directly connected to the pipe. Hence, stroking of the cylinder is not required. 
     It is achieved a pig drive system that has significantly reduced installation and operational failure risk, which is essential for subsea operations. 
     It is achieved a more compact pig drive system that allows a larger number of pre-loaded pigs, which is more cost efficient and practical than the prior art. Alternatively, the footprint of the system may be reduced. 
     It is achieved a leaner system with reduced weight and size. 
     It is achieved a less complicated system providing improved operational robustness. 
     It is achieved a simplified transportation, loading, handling and installation. 
     An advantage of this pig drive system is reduced production cost. 
     A further advantage of this pig drive system is a simplified offshore installation campaign, which in turn reduces cost. 
     A further advantage of this pig drive system is a simplified offshore operation during deployment, replacement and retrieval of the cylinder, comprising fever steps than the prior art systems in that no stroking of the cylinder is required. 
     A further advantage is reduced operational failure risk. In cases where the last pig has been fed into the pipe, the automatic operation of the pig drive into the retracted state facilitates the replacement and retrieval of the cylinder in that:
         a shorter horizontal movement of the pig drive system is required to go clear of the pipe; and   the risk of damaging the seal surface of the pipe during retrieval of the cylinder is reduced.       

     The cylinder has an axial direction and a radial direction, and may comprise at least one internal groove for equalizing a pressure across the piston; wherein the at least one internal groove: is oriented in the axial direction of the cylinder; extends an axial length of the cylinder greater than an axial extension of the piston; and is arranged to allow the retainer to contact the retainer abutment and the pig drive to enter the extended state. 
     When the piston reaches the location of the internal groove, it will no longer seal against the entire inner wall of the cylinder. The fluid pressure will thus be equalized across the piston. The arrangement of the internal groove will therefore define a piston travel termination point. 
     The distance between the internal groove and the retainer abutment relative to the axial extension of the rod will define the maximum length of the temporary extension of the rod beyond the retainer, and thus also the maximum temporary extension of the rod outside the cylinder. 
     An advantage of the internal groove is that it contributes to the prevention of the retainer being wedged to the retainer abutment. 
     When a pressure is equalized across the piston, the retainer may see an increase of pressure. The retainer is not adapted to seal against the retainer abutment. The pressure will therefore also be equalized across the retainer. The equalization across the retainer may be improved by means of the at least one through hole in the retainer. In case of a sudden pressure increase on one side of the retainer, the retainer may be wedged to the retainer abutment. The at least one through hole in the retainer contributes to the prevention of the retainer being wedged to the retainer abutment in case of a sudden pressure increase on one side of the retainer. 
     The cylinder has an axial direction and a radial direction, and may comprise at least one by-pass conduit for equalizing a pressure across the piston; wherein the at least one by-pass conduit: comprises a first end and a second end, the two ends are connected to and in fluid communication with the cylinder: is arranged with the first end at a position which the piston will pass during operation of the pig drive towards the extended state if the retainer is in contact with the retainer abutment; and is arranged with the second end at a position spaced apart the first end in an axial direction towards the first end portion of the cylinder, wherein the spacing is greater than an axial extension of the piston. 
     The by-pass conduit is an alternative to the internal groove which also provides the above-mentioned effects and advantages. 
     A further advantage of the by-pass conduit is that it doesn&#39;t compromise the wall thickness of the cylinder. The by-pass conduit can therefore be made to allow a greater volume flow than the internal groove. 
     The by-pass conduit extends beyond the cylinder wall, while the internal groove doesn&#39;t extend beyond the cylinder wall. 
     The system may further comprise: an operable conduit comprising a first end connected to and in fluid communication with the first end portion of the cylinder and a second end connected to and in fluid communication with the second end portion of the cylinder; a valve arranged in the operable conduit, for control of equalization of a fluid pressure across the piston; wherein the pig drive is arranged closer to the second end portion of the cylinder than the pig, and with the retainer facing the retainer abutment. 
     The operable conduit may serve the same purpose as the by-pass conduit. However, the operable conduit can be closed by means of the valve, and thus not equalize a pressure across the piston at all. Because of the valve, the connection points of the ends of the operable conduit can be positioned such that a pressure equalization across the piston will occur regardless of the piston position whenever the valve is opened. Such connection points of the ends of the operable conduit may be at the first and second end portions of the cylinder. 
     The operable conduit may have a plurality of ends connected to and in fluid connection with the cylinder. The operable conduit may be provided with valves arranged between each end or between selected ends. The plurality of ends may be distributed across the entire axial extension of the cylinder. 
     The system may further comprise a distance piece arranged to provide a volume between the piston and a termination of the second end portion of the cylinder. 
     The distance piece may be part of or connected to the pig drive. The distance piece may be part of or connected to the second end of the cylinder. The distance piece may be formed as a dome, concave, shoulder, protrusion or any other geometry providing a partial gap between the piston and the termination of the second end portion of the cylinder, into which a fluid pressure can be supplied. 
     The cylinder may further comprise a supply port arranged in the second end portion of the cylinder, for supply of a fluid pressure on the piston. 
     The supply port may preferably be a supply port adapted for subsea connection, such as a hotstab receptacle. 
     It is disclosed a method of feeding a pig into a pig kicker section of a pipe using a pig drive system as described above. Wherein the method comprises the steps of arranging the pig drive and the at least one pig inside the cylinder; connecting the first end portion of the cylinder to the pig kicker section of a pipe; connecting a fluid pressure supply to the supply port; and moving the pig drive towards the pig kicker section of a pipe by means of applying fluid pressure on the piston, such that the movement of the pig drive causes the pig to move into the pig kicker section of a pipe. 
     The method may further comprise the step of: moving the pig drive until the retainer meets the retainer abutment of the cylinder; operating the pig drive into the extended state by means of applying fluid pressure on the piston, allowing the rod to extend into the pig kicker section of a pipe; operating the pig drive into the retracted state by means of reducing the fluid pressure on the piston, allowing the rod to retract from the pig kicker section of a pipe; disconnecting the fluid pressure supply from the supply port; and disconnecting and recovering the pig drive system. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The invention will now be described with reference to the exemplifying non-limiting embodiments shown in the accompanying drawings, wherein: 
         FIG. 1 a    shows a cross-section of a pig drive for feeding a pig into a pig kicker section of a pipe according to an embodiment of the invention, wherein the pig drive is in a retracted state; 
         FIG. 1 b    shows a cross-section of a pig drive for feeding a pig into a pig kicker section of a pipe according to an embodiment of the invention, wherein the pig drive is in an extended state; 
         FIG. 2 a    shows a cross-section of a pig drive for feeding a pig into a pig kicker section of a pipe according to an embodiment of the invention, wherein the pig drive is in a retracted state; 
         FIG. 2 b    shows a cross-section of a pig drive for feeding a pig into a pig kicker section of a pipe according to an embodiment of the invention, wherein the pig drive is in an extended state; 
         FIG. 3 a    shows a cross-section of a pig drive system for feeding a pig into a pig kicker section of a pipe according to an embodiment of the invention, wherein the pig drive is in a retracted state; 
         FIG. 3 b    shows a cross-section of a pig drive system for feeding a pig into a pig kicker section of a pipe according to an embodiment of the invention, wherein the pig drive is in an extended state and the pig is in a pig kicker section of the pipe; 
         FIG. 4 a    shows a cross-section of a pig drive system for feeding a pig into a pig kicker section of a pipe according to an embodiment of the invention, wherein the cylinder has an internal groove; 
         FIG. 4 b    shows a cross-section of a pig drive system for feeding a pig into a pig kicker section of a pipe according to an embodiment of the invention, wherein the cylinder has an internal groove and the internal groove equalizes a pressure across the piston; 
         FIG. 5 a    shows a cross-section of a pig drive system for feeding a pig into a pig kicker section of a pipe according to an embodiment of the invention, wherein the cylinder has a by-pass conduit; 
         FIG. 5 b    shows a cross-section of a pig drive system for feeding a pig into a pig kicker section of a pipe according to an embodiment of the invention, wherein the cylinder has a by-pass conduit and the by-pass conduit equalizes a pressure across the piston; 
         FIG. 6  shows a cross-section of a pig drive system for feeding a pig into a pig kicker section of a pipe according to different embodiments of the invention, wherein the cylinder has a distance piece; 
         FIG. 7  shows a cross-section of a pig drive system for feeding a pig into a pig kicker section of a pipe according to an embodiment of the invention, wherein the cylinder has an operable conduit; 
         FIGS. 8 a -8 c    show cross-sections of a pig drive system for feeding a pig into a pig kicker section of a pipe according to an embodiment of the invention, wherein the cylinder has a supply port; and 
         FIGS. 9 a -9 c    show cross-sections of a pig drive system for feeding a pig into a pig kicker section of a pipe according to an embodiment of the invention, wherein the cylinder is loaded with a plurality of pigs. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1 a    shows a cross-section of a pig drive  110 . The pig drive  110  can be used in a pig drive system  100  where it serves the purpose of feeding a pig  120  into a pig kicker section of a pipe  200 . Once the pig  120  is in the kicker section of the pipe  200 , it will be launched and pushed through the pipe  200  by means of a fluid flow in the pipe  200 . 
     The pipe  200  is typically permanently installed equipment and may in combination with the pig drive system  100  form a pig launcher. 
     The pig drive  110  comprises a piston  111 , a rod  112 , a retainer  113  and a biasing device  114 . The pig drive  110  may additionally comprise a spacer  115  and a distance piece  116 . 
     The piston  111  has a radial direction and an axial direction. The piston  111  is adapted to seal against the inside of a cylinder  130  wall and move along a longitudinal direction of the cylinder  130  in response to a differential pressure over the piston  111 . Pistons are well known, and the skilled person would know how to adapt the piston  111  to the expected fluid exposure as well as expected temperature and pressure ranges. 
     The rod  112  has a radial direction and an axial direction. The radial extension of the rod  112  is shorter than the radial extension of the piston  111 , and the axial extension of the rod  112  will typically be greater than the axial extension of the piston  111 . The rod  112  is connected to the piston  111  and the axial direction of the rod  112  is aligned with the axial direction of the piston  111 . When both the piston  111  and the rod  112  have circular cross-sections, the two will typically be concentrically arranged. The rod  112  may terminate at the piston  111  or extend on both sides of the piston  111 . 
     The rod  112  has a distal end pointing away from the piston  111 . This distal end of the rod  112  is adapted to interface a pig  120 . The distal end of the rod  112  may further comprise a perpendicularly protruding portion. 
     The retainer  113  has a radial direction and an axial direction. The retainer  113  is slidably connected to the rod  112 , typically by means of a central through hole adapted to encircle the circumference of the rod  112 . The axial direction of the retainer  113  is aligned with the axial direction of the rod  112 . The retainer is not required to seal against the rod  112  or the inside of any cylinder  130 . The perpendicularly protruding portion of the rod  112  may serve to prevent the retainer  113  from sliding off the rod  112 . 
     The retainer  113  is adapted to interface a retainer abutment  132  of the cylinder  130 . The interface of the retainer  113  may typically be circular and have a chamfered corner of the circumference. 
     The retainer  113  may comprise at least one through hole for pressure equalization across the retainer  113 . The retainer  113  may also have a radial extension dimensioned to provide a gap between the retainer  113  and the wall of the cylinder  130  to prevent a pressure build up on either side of the retainer  113 . By providing a small gap between the retainer  113  and the wall of the cylinder  130 , the retainer  113  may function as a centralizer, centralizing the pig drive  110  during its movement through the cylinder  130 . 
     The biasing device  114  provides a biasing force. The biasing device  114  is arranged to bias the retainer  113  towards the distal end of the rod  112 . The biasing means  114  may be arranged to abut the retainer  113  and the piston  111 , as illustrated in  FIG. 1   a,  or alternatively to abut the retainer  113  and the perpendicularly protruding portion of the rod  112 . 
     The biasing device  114  may be a helical compression spring, as illustrated in  FIG. 1   a,  or alternatively a helical tension spring, a resilient sleeve or a hydraulic damper. 
     If a helical spring is used as the biasing device  114 , it can preferably be arranged around the rod  112 , as illustrated in  FIG. 1   a.  Arranging the helical spring around the rod  112  will be space efficient and apply the biasing force on the retainer  113  in an evenly distributed manner. A compression spring will typically be arranged to abut the retainer  113  and the piston  111 . A tension spring will typically be arranged to abut the retainer  113  and the perpendicularly protruding portion of the rod  112 . 
     If a resilient sleeve is used, it can preferably be arranged around the rod  112 , between the piston  111  and the retainer  113 . 
     If one or several hydraulic dampers are used, it/they can be arranged between the piston  111  and the retainer  113 , offset the rod  112 . 
     The spacer  115  may be adapted to ensure a given minimum distance between the piston  111  and the retainer  113  by means of its axial extension, wherein the axial extension of the spacer  115  corresponds to the minimum distance between the piston  111  and the retainer  113 . 
     The spacer  115  may be arranged around the rod  112  and between the piston  111  and the retainer  113 . 
     The distance piece  116  may be a part of the rod  112 , when the rod  112  extends on both sides of the piston  111 . 
     The pig drive  110  illustrated in  FIG. 1 a    is in a retracted state. When the biasing device  114  is not affected by external forces, the biasing device  114  will operate the pig drive  110  towards the retracted state and/or maintain the pig drive  110  in the retracted state. In the retracted state, the distance between the retainer  113  and the distal end of the rod  112  is typically shorter than the distance between the retainer  113  and the piston  111 . When the retainer  113  moves away from the piston  111  and towards the distal end of the rod  112 , it is operated towards the retracted state. 
       FIG. 1 b    shows a cross-section of the same pig drive  110  as in  FIG. 1   a.  The pig drive  110  illustrated in  FIG. 1 b    is in an extended state. When the biasing device  114  is affected by an external force oppositely directed and exceeding the biasing force of the biasing device  114 , the pig drive  110  will be operated towards the extended state and/or maintained in the extended state. In the extended state, the distance between the retainer  113  and the distal end of the rod  112  is not necessarily greater than the distance between the retainer  113  and the piston  111 . However, the distance between the retainer  113  and the piston  111  is shorter in the extended state of the pig drive  110  than in the retracted state of the pig drive  110 . Accordingly, the distance between the retainer  113  and the distal end of the rod  112  is greater in the extended state of the pig drive  110  than in the retracted state of the pig drive  110 . When the retainer  113  moves towards the piston  111  and away from the distal end of the rod  112 , it is operated towards the extended state. 
     As illustrated in  FIG. 1 a   - b,  the biasing device  114  may be a compression spring. When a compression spring is compressed until the coils come in contact with each other, then the spring is said to be solid. The solid length of a spring is the product of total number of coils and the diameter of the wire. The free length of a compression spring is the length of the spring in the free or unloaded condition. 
     If a biasing device  114  in the form of a compression spring is used in combination with a spacer  115 , the axial extension of the spacer  115  should be greater than the solid length of the compression spring. 
     If a biasing device  114  in the form of a compression spring is used, the free length of the compression spring should be equal or greater than the axial extension of the rod  112 . 
       FIG. 2 a    shows a cross-section of the pig drive  110  in a retracted state. The pig drive of  FIG. 2 a    differs from the pig drive  110  of  FIG. 1 a    in that it has a biasing device  114  in the form of a tension spring. The tension spring is arranged around the rod  112  and between the retainer  113  and the distal end of the rod  112 . 
     The pig drive  110  using a tension spring will have a greater distance between the retainer  113  and the distal end of the rod  112  in its retracted state as compared to the pig drive using a compression spring. 
       FIG. 2 b    shows a cross-section of the same pig drive  110  as  FIG. 2 a   . The pig drive  110  illustrated in  FIG. 2 b    is in an extended state. 
     The same principles apply to the retracted and extended state of the pig drive  110  regardless if a compression spring or a tension spring is used. 
       FIGS. 3 a  and 3 b    show cross-sections of the same pig drive system  100 . The pig drive system  100  can be used in a pig launcher where it serves the purpose of feeding a pig  120  into a pig kicker section of a pipe  200 . Once the pig  120  is in the kicker section of the pipe  200 , it will be launched and pushed through the pipe  200  by means of a fluid flow in the pipe  200 . 
     The pig drive system  100  comprises a pig drive  110 , at least one pig  120  and a cylinder  130 . 
     The pig drive  110  illustrated in  FIG. 3 a    is in the retracted state. The pig drive  110  illustrated in  FIG. 3 b    has be operated towards the extended state. The pig drive  110  used in the pig drive system  100  may be any of the above-described pig drives  110 . 
     The at least one pig  120  used in the pig drive system  100  may be any type of pig  120 . 
     The cylinder  130  is adapted to house the pig drive  110  and the at least on pig  120 . The cylinder  130  has radial direction and an axial direction and comprises a first end portion  138  and an opposite second end portion  139 . 
     The first end portion  138  of the cylinder  130  is adapted for sealing connection to the pig kicker section of the pipe  200 . This connection may e.g. be a standard tie-in connection using a metal-to-metal seal in combination with a connector. Other types of connections available to the skilled person could also be used. The illustrated cylinder  130  is horizontally connected to the pipe  200 . The first end portion  138  may comprise a sealingly connectable cap or at least a protective cap. Such caps may inter alia protect the inside of the cylinder  130  from contamination, restrain the loaded pigs  120  and protect any seal surfaces of the first end portion  138 . Caps must be removed before connecting the first end portion  138  to the pipe  200 . 
     The second end portion  139  of the cylinder  130  is adapted to be sealed off, at least during operation of the pig drive  110 . The second end portion  139  may be permanently sealed off, but preferably comprises a cap or similar that can be removed to gain access to the cylinder  130 . If the second end portion  139  comprises a cap, the cap must be adapted to seal off the second end portion  139  of the cylinder  130 . When loading the cylinder  130  with the pigs  120  and the pig drive  110 , access through the second end portion  139  of the cylinder  130  may be required if the first end portion  138  of the cylinder  130  is too narrow for the pig drive  110  to enter. Access to the cylinder  130  through both the first end portion  138  and the second end portion  139  may be beneficial for maintenance purposes. 
     The cylinder  130  must be able to withstand the applicable wellhead pressure, wellhead pressure is dependent on the reservoir pressure. 
     Typical applicable wellhead pressure is up to 13.8 MPa (2000 psi), 20.7 MPa (3000 psi), 34.5 MPa (5000 psi), 68.9 MPa (10000 psi), 103 MPa (15000 psi), 138 MPa (20000 psi), 207 MPa (30000 psi). 
     The applicable wellhead pressure may be in the range from 13.8 MPa (2000 psi) to 207 MPa (30000 psi). 
     The cylinder  130  further comprises a retainer abutment  132 . The retainer abutment  132  illustrated in  FIG. 3 a    is arranged in the first end portion  138  of the cylinder  130 . The retainer abutment  132  is adapted to interface and restrain the retainer  113  of the pig drive  110 . The retainer  113  and the retainer abutment  132  have mating geometry, preferable in the form of a coned angle. The retainer abutment  132  should be shaped so that the pig  120  is not obstructed by it and such that it does not impose unacceptable stress concentration loads on the cylinder. Typically, the slope of the retainer abutment is in the range of 1:6 to 1:3. 
     When loading the cylinder  130  through the second end portion  139 , the required number of pigs  120  are loaded into the cylinder  130  prior to the pig drive  110 . The pig drive  110  is arranged in the cylinder  130  with the piston  111  oriented towards the second end portion  139  and the retainer  113  oriented towards the first end portion  138 . After the cylinder  130  has been loaded, the second end portion  139  of the cylinder  130  must be sealed off, such that a fluid pressure can build up in a chamber defined by the piston  111  of the pig drive  110  and an inside of the second end portion  139  of the cylinder  130 . 
     To provide the chamber defined by the piston  111  of the pig drive  110  and an inside of the second end portion  139  of the cylinder  130 , the cylinder  130  may comprise a distance piece  136 . 
     By sufficiently pressurizing the chamber defined by the piston  111  of the pig drive  110  and an inside of the second end portion  139  of the cylinder  130 , the pig drive  110  will start to travel towards the first end portion  138  of the cylinder  130 . Pigs  120  will be pushed out of the first end portion  138  of the cylinder  130  in response to the pig drive  110  moving towards said first end portion  138 . Pigs  120  pushed out of the first end portion  138  of the cylinder  130  will be fed into the connected pipe  200 . 
     When the last pig  120  has been pushed out of the cylinder  130 , the pig drive  110  will have reached the first end portion  138  of the cylinder  130 . In the first end portion  138  of the cylinder  130 , the retainer  113  of the pig drive  110  will come into contact with the retainer abutment  132 . When the retainer  113  of the pig drive  110  comes into contact with the retainer abutment  132 , the pig drive  110  will normally be in its retracted state as illustrated in  FIG. 3 a   . Some of the impact forces caused by the retainer  113  contacting the retainer abutment  132  may be absorbed by the biasing device  114  of the pig drive  110 . 
     When the pig drive  110  is in the retracted state and engaged with the retainer abutment  132 , the rod  112  of the pig drive  110  should not extend outside the cylinder  130 . 
     When the last pig  120  has been pushed out of the cylinder  130  and the pig drive  110  has engaged the retainer abutment  132 , said last pig  120  may not have fully reached the kicker section of the pipe  200 . The pig drive  110  is therefore adapted to extend out of the cylinder  130  and into the pipe  200  to make sure that the last pig  120  entering the pipe  200  also fully reach the kicker section of the pipe  200 . 
     The pig drive  120  can be operated towards its extended state when arranged in the cylinder  130  and in contact with the retainer abutment  132  by means of pressurizing the compartment defined by the second end portion  139  of the cylinder  130  and the piston  111  of the pig drive  110 . The force provided by the fluid pressure applied on the piston  111  must exceed the biasing force provided by the biasing device  114  for the pig drive  110  to be operated towards the extended state. 
     When the pig drive  110  is in the extended state and engaged with the retainer abutment  132 , the rod  112  of the pig drive  110  should extend outside the cylinder  130  and at least a given length into the pipe  200  connected to the first end section  138  of the cylinder  130 , as illustrated in  FIG. 3 b   . Said length of the rod  112  should be sufficiently long to push the last pig  120  entering the pipe  200  into the kicker section of said pipe  200 . 
     The pig drive  120  can be operated towards its retracted state when arranged in the cylinder  130  and in contact with the retainer abutment  132  by means of depressurizing the compartment defined by the second end portion  139  of the cylinder  130  and the piston  111  of the pig drive  110 . The force provided by the fluid pressure applied on the piston  111  must be reduced below the biasing force provided by the biasing device  114  for the pig drive  110  to be operated towards the retracted state. 
       FIGS. 4 a  and 4 b    show cross-sections of a pig drive system  100  like the pig drive system of  FIGS. 3 a  and 3 b   . The pig drive system  100  of  FIGS. 4 a  and 4 b    may comprise all the same features as the pig drive system of  FIGS. 3 a  and 3 b   . An additional feature of the pig drive system  100  illustrated in  FIGS. 4 a  and 4 b    is that the cylinder  130  comprises at least one internal groove  131 . The at least one internal groove  131  is adapted for equalizing a fluid pressure across the piston  111 . The depth of the internal groove  131  should not exceed a wall thickness of the cylinder  130 . 
     In  FIG. 4 a    the pig drive  110  is in the retracted state, and in  FIG. 4 b    the pig drive  110  is operated towards the extended state. 
     The internal groove  131  extends in the axial direction of the cylinder  130 . The extension of the internal groove  131  is greater than the axial extension of the piston  111 . In this way a piston  111  will not be able to travel past the internal groove  131  because the fluid pressure moving it would be equalized when the piston  111  reaches a point where the internal groove  131  extends on both sides of the piston  111 . 
     The internal groove  131  is arranged to allow the retainer  113  to contact the retainer abutment  132  while the pig drive  110  enters the extended state, or at least operates a given distance towards the extended state, before the piston  111  reaches the internal groove  131 . 
     The internal groove  131  may extend the entire  360  degrees of the internal circumference of the cylinder  130 . Alternatively, a plurality of internal grooves  131  may be distributed around the internal circumference of the cylinder  130 . The at least one internal groove  131  not extending the entire internal circumference of the cylinder  130  may be arranged helically, straight-lined, etc. 
       FIG. 5 a    and  FIG. 5 b    show cross-sections of a pig drive system  100  like the pig drive system of  FIGS. 3 a  and 3 b   . The pig drive system  100  of  FIGS. 5 a  and 5 b    may comprise all the same features as the pig drive system of  FIGS. 3 a  and 3 b   . An additional feature of the pig drive system  100  illustrated in  FIGS. 5 a  and 5 b    is that the cylinder  130  comprises at least one external by-pass conduit  133 . The at least one by-pass conduit  133  is adapted for equalizing a fluid pressure across the piston  111 . 
     In  FIG. 5 a    the pig drive  110  is in the retracted state, and in  FIG. 5 b    the pig drive  110  is operated towards the extended state. 
     The by-pass conduit  133  has at least two connection points with the cylinder  130  which are in fluid communication with the inside of the cylinder  130 . The distance between the two connection points of the by-pass conduit  133  extends in the axial direction of the cylinder  130 . Said extension is greater than the axial extension of the piston  111 . In this way a piston  111  will not be able to travel past the by-pass conduit  133  because the fluid pressure moving it would be equalized when the piston  111  reaches a point where it has one connection point of the by-pass conduit  133  on each side. 
     The by-pass conduit  133  is arranged to allow the retainer  113  to contact the retainer abutment  132  while the pig drive  110  enters the extended state, or at least operates a given distance towards the extended state, before the piston  111  reaches the by-pass conduit  133 . 
     The inner diameter of the by-pass conduit  133  may be selected independently of the cylinder  130  wall thickness. Hence, the inner diameter of the by-pass conduit  133  can be selected freely. By increasing the inner diameter of the by-pass conduit  133  the risk of clogging will be reduced. 
       FIG. 6  shows a cross-section of a pig drive system  100  like the pig drive system of  FIGS. 3 a  and 3 b   . The pig drive system  100  of  FIG. 8 a - c    may comprise all the same features as the pig drive system of  FIG. 3 a   - b,    FIG. 4 a - b    and  FIG. 5 a - b   . An additional feature of the pig drive system  100  illustrated in  FIG. 8 a - c    is that the cylinder  130  comprises at least one operable conduit  134  provided with a valve  135 . 
     The operable conduit  134  has at least two connection points with the cylinder  130  which are in fluid communication with the inside of the cylinder  130 . A valve  135  is provided between said connection points to prevent or allow a fluid flow through the operable conduit  134 . The operable conduit  134  comprising more than two connection points may also comprise more than one valve  135 . The operable conduit  134  may e.g. 
     comprise valves  135  between all adjacent connection points. 
     As illustrated in  FIG. 6  one connection point of the operable conduit  134  may be arranged in the first end portion  138  of the cylinder  130 . Preferably the one connection point is arranged such that it provides fluid communication to the volume defined by the piston  111 , the retainer  113  and the first end portion  138  of the cylinder  130  when the retainer  113  is engaged with the retainer abutment  132  and the pig drive  110  has been operated towards the extended state. 
     As illustrated in  FIG. 6  a second connection point of the operable conduit  134  may be arranged in the second end portion  138  of the cylinder  130 . Preferably the second connection point is arranged such that it provides fluid communication to the volume defined by the piston  111 , and the second end portion  138  of the cylinder  130  when the retainer  113  is engaged with the retainer abutment  132  and the pig drive  110  is in the retracted state. 
     The second connection point may alternatively be arranged in the cap connected to the second end portion  139  of the cylinder  130 . 
     The operable conduit  134  may comprise a series of intermediate connection points arranged between the above-mentioned first and second connection points. 
       FIG. 7  shows a cross-section of a pig drive system  100  like the pig drive system of  FIGS. 3 a  and 3 b   . The pig drive system  100  of  FIG. 7  may comprise all the same features as the pig drive system of  FIG. 3 a   - b,    FIG. 4 a   - b,    FIG. 5 a - b    and  FIG. 6 . An additional feature of the pig drive system  100  illustrated in  FIG. 7  is that the cylinder  130  comprises at least one supply port  137  for supply of a fluid pressure into the volume defined between the second end portion  139  of the cylinder  130  and the piston  111 . 
     The supply port  137  may comprise a hot stab receptacle or any type of fittings. 
       FIG. 8 a   ,  FIG. 8 b    and  FIG. 8 c    show cross-sections of a pig drive system  100  like the pig drive system of  FIGS. 3 a  and 3 b   . The pig drive system  100  of  FIG. 8 a - c    may comprise all the same features as the pig drive system of  FIG. 3 a   - b,    FIG. 4 a   - b,    FIG. 5 a   - b,    FIG. 6  and  FIG. 7 . An additional feature of the pig drive system  100  illustrated in  FIG. 8 a - c    is that the cylinder  130  comprises at least one distance piece  136 . The at least one distance piece  136  is arranged to provide a volume defined between the piston  111  and a termination of the second end portion  139  of the cylinder  130 . 
     If the supply port  137  or the operable conduit  134  is connected to the cylinder  130  wall of the second end portion  139 , i.e. not in the cap connected to the second end portion  139 , the volume provided by the distance piece  136  ensures fluid communication between said volume and the supply port  137  and/or the operable conduit  134 . This fluid communication enables a fluid pressure to be applied in said volume and thus also on the piston  111 . 
     The distance piece  136  may be any kind of protrusion arranged in the termination of the second end portion  139 , e.g. a dome on the cap or a shoulder on the cylinder  130  wall. 
     In  FIG. 8 a    the cylinder has a distance piece  136  in the form of a dome provided on the inside of the cap connected to the second end portion  139  of the cylinder  130 . 
     In  FIG. 8 b    the cylinder has a distance piece  136  in the form of a protrusion provided on the inside of the cap connected to the second end portion  139  of the cylinder  130 . 
     In  FIG. 8 c    the cylinder has a distance piece  136  in the form of a shoulder provided on the inside of the cylinder  130  wall in the second end portion  139  of the cylinder  130 . 
       FIG. 9 a   ,  FIG. 9 b    and  FIG. 9 c    show cross-sections of a pig drive system  100  like the pig drive system of  FIGS. 3 a  and 3 b   . The pig drive system  100  of  FIG. 9 a - c    may comprise all the same features as the pig drive system of  FIG. 3 a   - b,    FIG. 4 a   - b,    FIG. 5 a   - b,    FIG. 6 ,  FIG. 7  and  FIG. 8 a   - c.    
     In the pig drive system  100  illustrated in  FIG. 9 a   - c,  the cylinder  130  is loaded with a plurality of pigs  120  and a pig drive  110 . 
     In  FIG. 9 a    a first cap is arranged on the first end portion  138  of the cylinder  130  and a second cap is arranged on the second end portion  139  of the cylinder  130 . The pigs  120  and the pig drive  110  may preferably be loaded into the cylinder  130  before installing said caps on the cylinder  130 . The pig drive  110  is adapted to abutting a retainer abutment  132  in the cylinder  130 , i.e. the pig drive  110  may not travel past this retainer abutment  132 . When loading the pig drive  110  into the cylinder  130 , the pig drive  110  must be loaded from the opposite side from the retainer abutment  132 , e.g. from the second end portion  139  of the cylinder  130  when the retainer abutment  132  is arranged in the first end portion  138  of the cylinder  130 . 
     The pig drive system  100  is ready for storage, transportation or deployment when all pigs  120  and the pig drive  110  are loaded into the cylinder  130  and the caps are installed, as illustrated in  FIG. 9   a.    
     In  FIG. 9 b    the first cap is removed from the first end portion  138  of the cylinder  130 . This cap must be removed before connecting the cylinder  130  to the pipe  200 . The second cap may preferably stay arranged on the second end portion  139  of the cylinder  130  during installation. 
     In  FIG. 9 c    the cylinder  130  is connected to the pipe  200 . The first end portion  138  of the cylinder  130  is connected to the pipe  200  e.g. by means of a clamp connector. 
     The cylinder  130  may be provided with a structure for arranging and supporting the cylinder  130  during and after subsea installation. The structure of the cylinder  130  may be adapted to interface a structure arranged to the pipe  200 , as illustrated in  FIG. 9   c.    
     REFERENCE LIST 
     
         
         
           
               100 —Pig drive system
             110 —Pig drive
                 111 —Piston     112 —Rod     113 —Retainer     114 —Biasing device     115 —Spacer     116 —Distance piece (of pig drive)   
                 120 —Pig     130 —Cylinder
                 131 —Internal groove     132 —Retainer abutment     133 —By-pass conduit     134 —Operable conduit     135 —Valve     136 —Distance piece (of cylinder)     137 —Supply port     138 —First end portion     139 —Second end portion   
               
         
               200 —Pipe