Abstract:
Methods and apparatus for making connections to pipes and pressure vessels while they are under pressure employs a first drive mechanism for delivering a cutting tool to the pipe or vessel wall and a second separate drive mechanism for powering the movement of the cutting tool through the wall. The long linear travel associated with the transit of the cutter to the wall is divorced from the short cutter travel required to perform the actual cuffing operation. This allows the two long and short travel systems to be designed independently of one another, and engineering compromises between them can be avoided.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims benefit of PCT International application number PCT/GB02/02008filed on May 2, 2002, entitled “Making connections to pipes under pressure”, which claims the benefit of British application serial number 0110821.6, filed on May 3, 2001. 
   This invention relates to methods and apparatus for making connections to pipes and pressure vessels while they are under pressure, a process commonly known as hot tapping. 
   DESCRIPTION OF THE RELATED ART 
   Hot tapping is a method of providing a branch connection to an existing pipe line or pressure vessel under operating conditions typically as a method of flowing new product into or out of that line or introducing tooling into a pressurised environment. 
   The process typically involves the installation of a welded or mechanical connection to the target pipe or vessel and the installation of one or more isolation valves on the connection. A hot tapping assembly is then installed on the outboard end of the isolation valve and a window is cut in the pipeline or vessel using a mechanical hole saw. The hole saw, and coupon of pipeline wall removed, are then withdrawn through the isolation valve(s) and the valve(s) closed. The newly cut hole is then used to extract product from the pipe or to introduce other tooling, such as line stop plugs or corrosion coupons, into the target pipeline or vessel. 
   Current hot tapping technology has been developed to meet the requirements of the onshore oil and gas industry where installation of branches to existing trunk lines or pressure vessels is a relatively common event. 
   Current hot tapping machines are normally based on a rotating boring bar fitted with a hole saw type cutting head which is advanced by an internal lead screw mechanism through sets of chevron type seals and the isolation valves into the pressurised environment. 
   The resultant axial loads and bending moments generated by the line pressure, seal friction and cutting operation are resolved within the boring bar which is in turn restrained by the seal assemblies and lead screw mechanism. 
   SUMMARY OF THE INVENTION 
   According to the present invention there is provided apparatus for tapping into a pipe or vessel, the apparatus comprising a cutting tool for cutting through the wall of the pipe or vessel, a first drive mechanism for delivering the cutting tool to the vessel wall and a second drive mechanism for powering the movement of the cutting tool through the wall. 
   The invention also provides a method of cutting through a wall of a pipe or vessel, the method comprising moving a cutting tool into contact with the wall of the pipe or vessel by means of a first drive mechanism, and cutting through the wall by means of a second drive mechanism. 
   Typically the first and second drive mechanisms have different gearings, and are preferably entirely different, with the first drive mechanism being adapted for long travel linear movement along the axis of the cutting tool, and the second drive mechanism being adapted for short travel linear movement through the wall of the pipe or vessel. 
   The first drive mechanism is preferably a linear drive mechanism and typically does not rotate. The second drive mechanism is preferably a rotary mechanism such as a screw thread or worm drive, and is preferably a short travel mechanism. 
   The invention also provides apparatus for tapping into a pipe or vessel comprising a rotary cutting tool, a non-rotating tool-bearing member, and a drive mechanism for driving the cutting tool through the wall to be cut. 
   A stuffing box is typically provided for moving the non-rotating tool-bearing member close to the wall. 
   The non-rotating pressure housing is typically a sealed shaft which can optionally be configured with anyone or more of drive, feed, cutting and centralising assemblies to perform cutting operations by means of the cutting tool. 
   The tool shaft is typically fed into the pressurised environment through a stuffing box assembly using linear actuators typically acting through a collar on the tool shaft. The linear actuators are typically hydraulically powered, and in preferred embodiments can comprise hydraulic pistons. However, other types of linear actuator can be used. 
   One advantage of certain embodiments is that the long linear travel associated with passing through the isolation valve(s), typically 1,000-2,000 mm, is divorced from the short cutter travel, typically 100-150 mm, required to perform the actual cutting operation. This allows the two long and short travel systems to be designed independently of one another, and engineering compromises between them can be avoided. 
   The cutting device can typically comprise a pilot device such as a pilot drill or cutter, and a hole saw or hole cutter. The drive mechanisms for the pilot drill and hole cutter drive can optionally drive both the feed and rotary movements for the pilot drill and hole cutter, but in preferred embodiments the feed and rotary mechanisms are separate. 
   Typically the shafts of the cutter and the outer housing are sealed and the cutter shaft can preferably be centralised within the outer housing to enhance rigidity of the assembly when the cutter is fully extended into the pipe. 
   The speed of rotation and rate of feed for the pilot drill and hole cutter can typically be adjusted independently of each other enabling optimum feed and speed rates to be used for the various phases of the cutting operation. 
   The long-travel first drive mechanism can typically comprise one or more pistons that stroke the shaft of the outer housing into the stuffing box. Typically two or more pistons are used, as this guides the shaft on the desired axis more accurately. The pistons can be hydraulic or pneumatic, but can also be of other designs, or can be substituted for other types of linear drive device, such as worm drives etc. 
   The first mechanism is typically adapted for rapidly moving the cutter device towards the wall to be cut so that the cutter can then be driven by the second mechanism more slowly, accurately and with a higher degree of control than is necessary for the delivery of the cutter to the face of the wall. Therefore, the first drive mechanism can have very different characteristics from the second drive mechanism without a compromise between the two affecting the accuracy or efficiency of the system. 
   The invention also provides apparatus for tapping into a pipe or vessel, the apparatus comprising a cutting tool for cutting through the wall of the pipe or vessel, a first shaft to which the cutting tool is attached, and a second shaft co-axial with the first shaft with an annulus between the first and second shafts, wherein one or more control or signal lines are disposed in the annulus. 
   The first shaft with the cutting tool is typically inside the second shaft. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     An embodiment of the present invention will now be described by way of example and with reference to the accompanying drawings, in which: 
       FIG. 1  is a sectional view of a machine tool assembly; 
       FIG. 2  is a sectional view of an injector assembly, a stuffing box, and a valve interface spool; 
       FIG. 3  is a sectional view of a system for tapping into pressurised pipes embodying the  FIG. 1  and  FIG. 2  assemblies; 
       FIGS. 4 and 5  are side and sectional views of the  FIG. 1  machine tool assembly; 
       FIG. 6  is a side sectional view of a drive unit of the  FIG. 1  machine tool assembly; 
       FIG. 7  is a sectional view through a feed unit of the  FIG. 6  drive unit; 
       FIG. 8  is a sectional view through  FIG. 7  at line A-A; 
       FIG. 9  is a sectional view through the duplex tool shaft of the  FIG. 1  machine tool; 
       FIGS. 10 and 11  are side views of a bore centraliser; 
       FIG. 12  is a plan view of the  FIG. 10  bore centraliser in use; 
       FIG. 13  is a series of views of a pilot drill and hole cutter used in the  FIG. 1  machine tool assembly; 
       FIG. 14  is a side sectional view through a stuffing box and valve interface spool as shown in  FIG. 2 ; 
       FIG. 15  is an exploded view of the  FIG. 14  stuffing box; 
       FIG. 16  is a side view of a seal cartridge used in the  FIG. 15  stuffing box; 
       FIG. 17  is a side sectional view through an injector assembly; 
       FIG. 18  is a plan sectional view through line B-B of  FIG. 17 ; 
       FIG. 19  is a plan view through line C-C of  FIG. 17 ; 
       FIG. 20  is a schematic representation of a tapping system; 
       FIGS. 21 and 22  are side views of a synchronisation pump; 
       FIGS. 23 and 24  are schematic views of the  FIG. 21  pump with the piston in first and second configurations respectively; and 
       FIG. 25  is a series of views showing the sequence of operation of the tapping system. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring now to the drawings, a tapping apparatus has a cutter assembly  2  mounted on the end of a duplex shaft comprising an outer non-rotating shaft  10  and an inner rotating shaft  11  driven by a rotary drive unit  15 . A feed unit  16  controls the axial movement of the cutter assembly  2  mounted on the rotating shaft  11  through the wall being cut. Optionally a centraliser  20  can be provided to centralise the outer shaft  10  in the T piece of the pipe. 
   The shaft  10  and cutter assembly  2  is advanced towards the wall of the pipe through a stuffing box  26  and valve interface spool  27  by an injector assembly  25 , shown in  FIG. 2 . 
   The drive unit  15  is typically located on the outboard end of the apparatus and provides the rotary drive for the drilling operation through the wall. The drive unit  15  typically comprises a body  15   b  housing the drive train components and connected to the feed unit  16 . Where practical, all of the apparatus bodies can be oil-filled and pressure-compensated for subsea operations. A primary drive gear  15   g  is retained within drive unit body  15   b  by bearing assemblies and a retaining ring  15   r , and has an internal spline which engages on a corresponding spline on the rotary drive shaft  11 . The drive gear  15   g  is driven by two hydraulic motors  15   m  mounted on the body  15   b  and coupled to the drive gear  15   g  via spur gears. The hydraulic motors  15   m  coupled to the drive gear  15   g  generate the torque that is transmitted to the shaft  11  via the internal spline on the drive gear  15   g . A pocket  15   p  on the top of the drive unit housing  15   b  protects the shaft  11 , and allows space for the shaft  11  to move axially as will be described. The hydraulic motors  15   m  can be hydraulically coupled either in series or in parallel enabling the torque and speed to be switched within differing speed and torque bands. An operator can monitor the speed and direction via a surface or subsea readout from proximity sensors (not shown) located within the unit. 
   The feed unit  16  is housed within a top cap  16   c  on the outboard end of the duplex shaft and provides a variable axial feed mechanism for the rotary shaft  11  within the outer shaft  10 . The feed unit  16  has a thrust collar  16   t  mounted on the shaft  11  and driven towards the inboard end of the tool (i.e. downwards as shown in  FIG. 7 ) by a thread on its outside diameter engaging with an internal thread on a drive sleeve  16   s . Two thrust washers acting back to back and a locking nut retain the collar  16   t  on the drive shaft  11 . The collar  16   t  is prevented from rotating by the action of several tie-rods  16   r  that pass through the collar  16   t  and are anchored within the top cap  16   c.    
   The drive sleeve  16   s  pushes the thrust collar  16   t  forward via an internal thread and is driven by a drive gear  16   g  formed on the outside diameter of the sleeve  16   s . The sleeve  16   s  is retained within the top cap by two thrust roller bearings and a radial roller bearing. The drive gear  16   g  is secured to the OD of the drive sleeve  16   s  and transmits torque from a drive worm  16   w  to the sleeve. The drive worm  16   w  engages with the drive gear  16   g  and is driven by the motor  16   m  and reduction gear. The worm shaft is retained in the tool shaft top cap  16   c  by two end caps and a pair of bearing assemblies. Tie rods  16   r  pass through the thrust collar  16   t  and prevent the collar  16   t  from rotating under the influence of the drive sleeve  16   s . The tie rods  16   r  are secured at the outboard end by an anchor plate and on their inboard end by engaging in corresponding holes in the top cap  16   c . A locking ring  16 L secures the drive train components within the top cap  16   c . The feed motor  16   m  drives the worm gear  16   w  through a reduction gear box and provides the motive power for the axial movement of the shaft  11 . 
   The feed unit  16  provides the finely controlled linear motion necessary to control the axial movement of the drive shaft  11  during the cutting operation. The drive shaft  11  is connected to the thrust collar  16   t  which is advanced or retracted under the influence of the threads on its outside diameter engaging with the threads on the inside diameter of the drive sleeve  16   s . The drive sleeve  16   s  is driven by a worm and wheel arrangement  16   w , which is in turn driven by a reduction gearbox and hydraulic motor  16   m . This arrangement enables the rate of feed of the drive shaft  11  to be finely controlled for all stages of the cutting operation independently of the rotation of the pilot drill and cutter assembly  2 . Proximity sensors can optionally provide telemetry for accurate monitoring of drive ring speed and direction of movement. 
   The duplex tool shaft connects via the outer shaft  10  to the inboard end of the feed unit top cap  16   c  (e.g. by welding) and transmits the rotary cutting power to the cutter assembly  2  whilst maintaining high levels of sealing redundancy on the annular seal path within the outer shaft  10 . A control line bundle  5  within the annulus between the shafts  10 ,  11  provides fluid power for control functions such as actuating tooling, seal flushing, pressure monitoring, centraliser and ancillary function operation, as well as signal lines from sensors and other measuring instruments. The cap  16   c  provides the mechanical linkage between the drive unit  15  and the outer shaft  10 . The cap  16   c  also provides the locating shoulder and attachment, via a threaded locking collar, to an injector cross head  25   h  to be described. The cap  16   c  is oil-filled and pressure-compensated for subsea operations. 
   The outer shaft  10  houses the inner drive shaft  11  and the annular hydraulic lines  5  and provides the external sealing surface for stuffing box  26  sealing elements (to be described). The shafts  10 ,  11  are oil-filled and pressure-compensated for subsea operations. The inner drive shaft  11  connects the external drive and feed assemblies, outside the pressurised envelope, to the pilot and cutter assemblies  2 , inside the pressurised envelope. It has a splined section where it passes through the drive unit  15 , to transmit torque, and an upset shoulder for engagement of the feed system thrust collar  16   c . The shaft  11  is connected to the cutter  2  by two keys and to the pilot drill  1  via a threaded arrangement. A hydraulic control line (not shown) can pass through its centre for actuation of the pilot drill coupon retention mechanism. Alternatively, the mechanism can be actuated by means of hydraulic pressure applied direct through the bore of the shaft  11 , without a hydraulic line. 
   Upper  13   u  and lower  131  seal cartridges are retained by rings  7 , 9  within the outer shaft  10  and house rotary seal elements that seal against the inner rotary shaft  11 . The annular space between the lower cartridge  131  and the outer shaft  10  can be monitored and flushed independently of the annular cavity within the outer shaft  10  via one of the control lines. A bottom cap  3  houses the lower seal cartridge  131  which is retained therein by a retainer ring  9  and optionally has through-porting to connect a hydraulic line e.g. to a bore centraliser or for other functions. 
   The duplex tool shaft  10 , 11  provides the rotary drive connection between the drive  15  and feed  16  units, located outside the pressurised environment, and the pilot drill and cutter assembly  2 , located inside the pressurised environment, whilst maintaining high levels of sealing redundancy. The outer shaft  10  also contains the control lines and supports the bore centraliser. 
   The outer shaft  10  outside diameter is hardened and polished providing a sealing surface for stuffing box seals  26   s  and has a top cap assembly which connects to the drive unit  15  and injector system cross head assemblies. 
   The drive shaft  11  is captive within the outer shaft  10  and passes through seal cartridges  13  at either end of the outer shaft which provide the primary and secondary environmental barriers on the OD of the drive shaft  11 . Keyways and a cutter interface on the inboard end of the drive shaft  11  transmit the power generated by the drive unit  15  to the pilot drill and cutter  2 . 
   A bore centraliser  20  is located on the inboard end of the outer shaft  10 . The bore centraliser  20  comprises a body  20   b  housing radial pistons  20   r  retained by guide rods with spring assemblies to retract the pistons  20   r . The pistons  20   r  can extend and retract to adjust the position of the centraliser  20  with respect to the bore of the hot tap T piece in which the tool is deployed. The pistons  20   r  are typically extended by applying hydraulic pressure to the hydraulic lines  5  and are typically withdrawn under spring action with pressure assistance from the pressurised environment surrounding the centraliser  20 . Adjustable pads  20   p  are optionally fitted to the pistons  20   r  allowing the assembly to be configured for varying bore widths. The body  20   b  also houses sealing assemblies  20   s  and is typically rigidly held to the outer shaft  10  by cone head screws  20   c  locating into a groove (not shown) in the outer shaft  10 . Three sets of seals  20   s  on the inside diameter of the body  20   b  provide a seal between the body  20   b  and the outside diameter of the tool shaft  10 , and between the hydraulic conduits to the pistons  20   r.    
   The pilot drill and cutter assembly  2  is located on the inboard end of the outer shaft  10  and provides the means of cutting and (optionally) retaining the section of pipeline wall (coupon). The assembly  2  comprises a pilot drill  1  and a cutter  4 . The drill comprises a drill body  1   b  having a removable tip or bit it, one or more coupon retention pawls  1   r  and a piston assembly  1   p . The cutter  4  has a circular cutter blade  4   b  having brazed tip inserts on its cutting face, and is attached to the end of the shaft  11  by a collar  4   c  which has two internal keys to transmit the torque from the shaft  10  to the cutter  4 . The teeth on the cutter blade  4   b  are arranged in a staggered pattern thus minimising the power required to perform the primary hole cutting operation. 
   During the cutting operation the drill  1  and cutter  4  are advanced through the pipeline wall by the drive shaft  11  as it advances under the influence of the feed unit  16 . 
   The coupon retention pawls  1   r  are held in the withdrawn position shown in  FIG. 13   a  by a spring is acting between the body  1   b  and the piston  1   p  until pressure is applied to the back of the piston  1   p  via the port through the centre of the drive shaft  11 , which then strokes forward moving the pawls  1   r  outward and locking them in the extended or “Coupon Locked” position shown in  FIG. 13   b  once the pawls  1   r  have passed the coupon, allowing the coupon to be retained behind the pawls as the drill bit it is recovered. 
   The stuffing box  26  and valve interface spool  27  assemblies provide the primary mechanical and pressure interface to the pipeline isolation valve. The stuffing box  26  provides fully redundant environmental sealing and lateral guidance for the outer tool shaft  10 . 
   The stuffing box  26  is mounted on the outboard end of the valve interface spool  27  and contains the primary and secondary environmental seals and guide bushings  26   b . The stuffing box  26  houses seal cartridges  26   s  and is attached to the valve interface spool  27  via a series of retaining bolts. A port  26   p  in the body of the stuffing box  26  allows the pressure in the cavity between the seal cartridges  26   s  to be monitored during cutting operations. An ‘O’ ring type seal  26   o  is disposed between the assembled stuffing box  26  and the valve interface spool  27 . 
   A seal cartridge  26   s  seals between each end of the stuffing box  26  and the outer tool shaft  10 . Each cartridge  26   s  contains two inner annular sealing elements  26   i , which seal around the tool shaft  11 , and two outer body sealing elements  26   t  to seal against the stuffing box body  26 . A retaining ring  26   r  secures the outboard seal cartridge  26   s  within the stuffing box  26 . 
   A pressure monitoring port  26   p  is attached to the external diameter of the stuffing box body  26  and provides a means of monitoring the pressure between the two seal cartridges  26   s.    
   The stuffing box  26  is sandwiched between the base of the injector assembly  25  and the valve interface spool  27 , and provides guidance and annular sealing on the OD of the outer shaft  10 . The outer tool shaft  10  passes through the two sealing cartridges  26   s  where in each assembly two independent sealing elements (a spring energised lip type seal and an ‘O’ ring energised polymer type seal) provide redundant sealing on the annular leak path around the outer tool shaft  10 . Two guide bushings  26   b  within each seal cartridge  26   s  impart a high degree of rigidity to the tool shaft  10  as it passes through the stuffing box  26 . 
   The valve interface spool  27  is mounted on the outboard end of the isolation valve and provides a mechanical and pressure linkage between the tapping system and the isolation valve. The spool  27  acts as pressure chamber to house the machine tool components on the inboard end of the tool shaft  10  during deployment and recovery. The stuffing box  26  is mounted on the outboard end of the spool  27 . 
   Porting within the body of the spool  27  enables pressure testing and flushing of the isolation valve prior to hot tapping and also enables the spool  27  to be depressurised on completion of the tapping operation. 
   The injector base  25   b  anchors two hydraulic piston cylinders  25   c  to the stuffing box  26  and spool  27  and transmits the tool shaft torque and axial loading through to these assemblies. 
   A cross head  25   h  attaches the hydraulic piston rods  25   r  to the machine tool shaft top cap  16   c  via a threaded collar arrangement  16   h . The head  25   h  is a beam type structure and is designed to provide maximum rigidity for transmission of the axial load from the cylinders  25   c  to the machine tool. The hydraulic pistons generate axial force to overcome end load on the tool shaft  10  due to pipeline pressure and cutting loads. Optional linear motion transducers can provide positional feedback to the control system to determine tooling position. 
   A control system shown conceptually in  FIG. 20  provides control and monitoring functions enabling the system to be operated by a surface operator. As a secondary function the system can provide a level of automatic system shut down in the event of a loss of primary control functions. A hydraulic power pack provides hydraulic power for some of the subsea elements of the system. Two independent fluid supplies are provided; a pressure regulated HP supply for the injector assembly and a flow regulated supply to the machine tool drive motors. 
   The split supplies ensure that safety critical components and systems that may be contaminated by line fluids are separated from purely tooling requirements. A surface control panel provides the surface operator with the means to control and monitor the hydraulic supplies to the subsea equipment as well as providing tooling information such as speed of cut and tooling position. An electro-hydraulic umbilical and reel connects the surface equipment to the subsea assemblies. 
   A subsea valve pack housing the hydraulic control valves musters the electrical signals from the various sensors on the subsea assemblies. The valve pack can provide a level of automated system shut down in the event that the surface control and power is lost. Shut down is achieved via an accumulator and secondary pilot operated control valves that can operate the primary hydraulic valves within the system in a predetermined sequence if the surface umbilical is lost or disconnected. 
   In use, a branch connection B is made to the target pipeline P, by either hyperbaric welding or a mechanical tee fitting, and one or more isolation valves  40  are installed on the branch B. The orientation of the branch B is not a constraint; a vertically orientated branch B is shown in  FIG. 23  for illustrative purposes, but other orientations can be equally effective. 
   The tapping system is conveyed to the sea bed either on guide wires or as an independent tooling package and can be positioned by a divers or a Remotely Operated Vehicle (ROV). The system latched via a handling frame F onto guide posts  41  or stabbed into receptacles on the isolation valve handling frame. The connection between the interface spool  27  and the isolation valve V is made secure to provide a pressure tight seal between the valve V and the tapping system. The interface spool cavity is pressurized via porting in the spool body to prove the integrity of the connection and the sealing systems on the tapping system. 
   The isolation valve(s) V are then opened, and the hydraulic cylinders  25   c  then stroke the duplex shaft  10 ,  11  inward until the pilot drill  1  is at a suitable stand-off from the pipeline wall—typically 5 mm. 
   The seawater in the valve and spool cavities is displaced using an inert fluid, then the cavity is pressurised until it is 1 to 2 bars above the pipeline operating pressure. 
   The cutting operation is completed in five distinct stages; 
   1) The bore centraliser pistons  20   r  are activated to centralise the duplex shaft  10  (optional step). 
   2) The drive and feed units  15 ,  16  are activated and the pilot drill  1  is advanced by the feed unit  16  to drill a pilot hole through the pipeline wall. The pilot drill pawl-extension piston  1   p  is then activated to extend the pawls  1   r  ensuring that the pipe wall coupon to be cut is retained on the drill  1  on completion of the cutting. 
   3) The rotating cutter  2   b  is advanced forward using the feed unit  16  until cutting of pipeline wall is completed. 
   4) The drive unit  15  is then stopped and the feed unit  16  reversed to retract the cutter  2   b , and the retained pipeline coupon, from the pipeline. 
   5) The bore centraliser  20  is de-activated ready for withdrawal of the duplex shaft  10 . 
   The injector assembly hydraulic cylinders  25   c  are then operated to retract the tool shaft  10  until the cutter and pilot drill assembly  2  is within the interface spool  27 . The isolation valve(s) V are then closed and interface spool cavity de-pressurised via the spool test ports. The connection between the interface spool  27  and the isolation valve V is released, and the system unlatched from the isolation valve frame  41 . 
   All of the above stages can be accomplish remotely supported as required by a Remotely Operated Vehicle (ROV). 
   An optional synchronisation pump  30  provides a means of synchronising the stroke of the two hydraulic cylinders  25   c  in the injector system  25  to a precise degree. 
   The synchronisation pump  30  comprises a piston  31  having an axial rod with a central annular shoulder. The axial rod has opposite end faces of equal area to the end faces of the annular shoulder. The piston  31  is annularly sealed within a housing comprising a central cylinder  32  with end caps  32   e . The housing is divided into three chambers  32   a ,  32   b  &amp;  32   c ), and has ports  33   a,b  on one side of the central annular shoulder, and ports  34   a,b  on the opposite side of the shoulder; each port is separated from the other by an annular seal such as an o-ring against which the piston  31  seals in the assembled pump  30 . Thus the left hand end port  33   a  is disposed at the left hand end of the pump  30  in  FIG. 21 , within the end chamber  32   a , and with an o-ring seal between it and the next port  33   b , which is spaced axially along the cylinder  32  from the end port  33   a , and is located in the mid chamber  32   b . The port  34   a  is also located in the mid chamber  32   b  and is spaced axially along from the port  33   b ; in the assembled pump  30  the central annular shoulder of the piston is disposed between the two ports  33   b  and  34   a , and is sealed to the ID of the cylinder  32  by o-rings or the like. The port  34   b  is located in the end chamber  32   c  and is spaced axially along the cylinder  32  from the port  34   a  with an o-ring seal disposed between them. 
   Thus each port is disposed within a separate chamber  32   a ,  32   b ,  32   c  enclosed within seals, with the central chamber  32   b  being divided by the central annular shoulder of the piston  31 . Because of the matching annular and piston areas the swept volume of these three chambers is equal, resulting in fixed and equal volumes of fluid being displaced from outlet ports on each stroke of the pump  30 , irrespective of the direction of the stroke. 
   In use the ports  33  and  34  are connected to a separate valve block  40  which switches the connections from a fluid supply intermittently between the two pairs of ports  33  and  34 , so as to shuttle the piston  31  from one side of the pump  30  to the other, and drive identical amounts of fluid from the pump sequentially through the pairs of ports  33  and  34 . 
   In operation, pressure is applied initially to one set of ports  33   a ,  33   b through the valve block  40  with the configuration shown in  FIG. 23 . This results in the piston  31  moving towards the right as shown in  FIG. 23 . The annular and piston areas of the piston  31  are equal resulting in equal amounts of fluid being displaced from the two outlet ports  34   a ,  34   b . When the piston  31  reaches the end of the stroke, the valve block  40  is switched manually or automatically to the configuration shown in  FIG. 24 , where the connections are reversed and pressure is applied to the opposite set of ports  34   a ,  34   b  resulting in the piston  31  moving to the left and ejecting fluid through the other ports  33   b ,  33   a . These ports are now linked through the valve block  40  to the cylinders  25   c  and extend the pistons by a further quantum amount. Meter in and out is achieved by reversing the tank and pressure lines. 
   Repeated strokes of the pump are used to deliver metered volumes of fluid to the hydraulic cylinders resulting in matched movement of the cylinders. 
   A piston of this design forms another part of the invention. Accordingly the invention also provides a hydraulic piston assembly comprising a chamber and a piston sealed to the chamber and movable therein, wherein the chamber has first and second ports for entry and exit of fluid, the ports being isolated by the piston seals to form separate compartments within the assembly, wherein the volume of fluid swept from each of the compartments by movement of the piston in the chamber is substantially equal. 
   Typically the areas of the piston sweeping the various compartments are substantially equal. The areas can be end faces of piston rods or annular upsets on the rods. 
   Preferably there are two pairs of ports, and each port can typically function as an entry port or an exit port. 
   In certain embodiments of the invention, it is beneficial to divorce the long linear travel of the assembly through the isolation valves V towards the wall to be cut, from the short travel needed to advance the cutter accurately through that wall. 
   A non-rotating tool shaft  10  and separate rotating drive shaft  11  separates the role of the axial seals around the tool shaft  10  from the role of the rotary seals around the drive shaft  11  and enables them to be selected for optimum performance. 
   With the duplex shaft, hydraulic (and other) control lines can be deployed within the annulus between the shafts to facilitate operation of the bore centraliser  20 , to flush seals, and to operate ancillary tooling. 
   The use of wholly independent drive and feed mechanisms  15 ,  16  enables the speed of the cutting assemblies and the rate of axial feed to be varied independently of each other, thereby allowing greater control of the delicate cutting operation. 
   The actuable bore centraliser  20  enables the cutting head to be centralised and restrained allowing the hole size to be optimised for a given application. 
   A synchronisation pump to advance or retract the cylinders  25   c  accurately ties the motion of the two drive cylinders to each other, facilitating the accurate delivery of the cutter to the wall of the pipe. 
   A subsea valve pack with piloted control valves coupled to sequenced control valve enables the system to be returned to a safe condition on the loss of surface control. 
   Hydraulically actuated pawls as a coupon retention device within the pilot drill, actuated independently of the drive shaft rotation or other rotary shaft, are also an advantage. 
   Modifications and improvements can be incorporated without departing from the scope of the invention.