Patent Publication Number: US-2021190234-A1

Title: Coiled tubing system

Description:
FIELD 
     This relates to a coiled tubing system for deployment into a conduit, in particular but not exclusively, a coiled tubing system for deployment into a fluid conduit for transporting hydrocarbons such as an extended reach horizontal wellbore. 
     BACKGROUND 
     In the oil &amp; gas exploration and production industry, in order to access hydrocarbons from a formation, a well borehole (“wellbore”) is drilled from surface. The wellbore is then lined with sections of bore-lining metal tubulars, known as casing, and production infrastructure installed to facilitate the ingress of hydrocarbons into the wellbore and transport them to surface. 
     The development of directional drilling techniques has facilitated the creation of high angle and horizontal wellbores (referred to below collectively as horizontal wellbores) which deviate from vertical and thus permit the wellbore to follow the hydrocarbon bearing formation to a greater extent. Amongst other things, horizontal wellbores beneficially facilitate increased production rates due to the greater length of the wellbore which is exposed to the reservoir. 
     In view of the benefits of horizontal wellbores, e.g. in increasing production rates, there is a continuing desire to extend the length or “reach” of horizontal wellbores. However, the operation of extended reach horizontal wellbores nevertheless poses a number of significant challenges. 
     For example, in order to perform an intervention or workover operation in a horizontal wellbore, the intervention tools and equipment must be capable of being advanced along the horizontal portion of the wellbore, which may define a tortuous path over several kilometres. 
     In the case of mechanical intervention tool systems, the ability to transmit push and/or pull forces to the intervention tool may be severely limited in the horizontal portion of the wellbore, for example due to frictional losses between the intervention tool system and the low side of the wellbore and/or the capstan effect at the heel of the wellbore (that is, the transition from the vertical to the horizontal sections of the wellbore). 
     Coiled tubing intervention systems—which employ a long continuous length of metal piping wound on a spool—provide the advantage over mechanical intervention tools in that coiled tubing facilitates the transportation of fluid downhole, for example as a cleaning or jetting fluid in a wellbore cleaning operation, as a power fluid to operate fluid-powered downhole tools, or as a treatment fluid, e.g. fracturing fluid, chemical wash operations, or the like. Coiled tubing systems are also particularly, but not exclusively, suited to offshore operations where it is necessary to direct intervention equipment through a flexible marine riser. 
     However, while coiled tubing intervention systems are used effectively in numerous applications, there are drawbacks with conventional coiled tubing systems. For example, friction between the coiled tubing and the wellbore caused by the weight of the coiled tubing lying against the low side of the wellbore, axial tension or compression forces in the coiled tubing when it transitions around curves in the wellbore and/or axial compressive force in the coiled tubing causing it to helically buckle, typically limit the extent to which coiled tubing can be pushed along the wellbore. Moreover, as coiled tubing systems are deployed from a reel they are not suitable for rotation, which may traditionally be used to reduce frictional effects during deployment. 
     These and other factors therefore typically limit the extent to which coiled tubing can be utilised in longer extended reach wellbores. 
     SUMMARY 
     According to a first aspect, there is provided a coiled tubing system for deployment into a conduit, comprising: 
     a tubing string comprising a first tubing portion and a second tubing portion configured for coupling to a proximal end of the first tubing portion, the first tubing portion comprising composite coiled tubing and the second tubing portion comprising metallic coiled tubing; and 
     a fluid discharge apparatus configured for location at a distal end of the tubing string, the fluid discharge apparatus comprising: 
     a body comprising an axial flow passage configured for fluid communication with an axial flow passage of the tubing string; 
     a lateral flow passage disposed through the body; and 
     a valve arrangement, 
     wherein the fluid discharge apparatus is configured to move between a first configuration in which the valve arrangement obturates fluid communication through the lateral flow passage and a second configuration in which the valve arrangement permits fluid in the axial flow passage to be discharged through the lateral flow passage, said discharge generating a pressure pulse which facilitates transport of the tubing string along the conduit. 
     Beneficially, the provision of a coiled tubing system having a fluid discharge apparatus in combination with a “hybrid” tubing string comprising composite coiled tubing and metallic coiled tubing facilitates the deployment of coiled tubing systems to a distance (or “reach”) not previously attainable using conventional coiled tubing systems. The composite coiled tubing has a lower coefficient of static friction than conventional metallic coiled tubing, e.g. 0.15 compared to 0.25, such that for the same input force the reach of the tubing string can be extended. Moreover, each pressure pulse breaks the contact between the tubing string and the conduit, such that the coefficient of friction between the tubing string and the conduit is changed from a static coefficient of friction to a dynamic coefficient of friction, e.g. 0.1, thereby facilitating further reach along the conduit, such as by an injector arrangement at surface or the like. As will be described further below, the pressure pulse may also provide a local thrust force which acts to pull the tubing string along the wellbore. The fluid discharge apparatus may be configured to cycle between the first configuration and the second configuration. For example, but not exclusively, the fluid discharge apparatus may be configured to cycle between the first configuration and the second configuration between 4 and 10 times per minute. 
     In use, cycling the fluid discharge apparatus between the first configuration and the second configuration may create discrete phases of fluid build-up and discharge, thereby creating a series of distinct pressure pulses. 
     As described above, the tubing string includes a first tubing portion comprising composite coiled tubing and a second tubing portion comprising metallic coiled tubing. 
     The first tubing portion may define a distal portion of the tubing string. Thus, on locating the system in the conduit, the first tubing portion may define a downhole or downstream portion of the tubing string. 
     The second tubing portion may define a proximal portion of the tubing string. Thus, on locating the system in the conduit, the second tubing portion may define an uphole or upstream portion of the tubing string. 
     It will be understood that the term proximal means closer to surface and that the term distal means further from surface. 
     In instances where the conduit comprises a horizontal wellbore, the system may be configured for deployment into the wellbore, with the first tubing portion disposed in the horizontal section of the wellbore and the second tubing portion disposed in the vertical section of the wellbore. The second tubing portion may also be disposed partially in the horizontal section of the wellbore. For example, the system may be deployed into the wellbore until the second tubing portion is disposed around the heel of the wellbore and into the horizontal section of the wellbore. 
     As described above, the tubing string is configured for deployment into the conduit. 
     The first tubing portion may be configured for storage on and deployment from a reel. By constructing the first tubing portion from composite coiled tubing, the first tubing portion may be configured to be stored on and deployed from the reel. 
     The second tubing portion may be configured for storage on and deployment from a reel. By constructing the second tubing portion from metallic coiled tubing, the second tubing portion may be configured to be stored on and deployed from the reel. 
     In use, the fluid discharge apparatus may be disposed at a distal end of the first tubing portion at surface, the first tubing portion and the fluid discharge apparatus then being deployed into the conduit by unreeling the first tubing portion from the reel. In instances where the conduit comprises a horizontal wellbore, for example, the system may be configured to deploy the fluid discharge apparatus and the first tubing portion into the horizontal section of the wellbore. The first tubing portion may comprise a single run of composite coiled tubing. Alternatively, where required the first tubing portion may comprise a plurality of runs of composite coiled tubing coupled together. The first tubing portion may thus be deployed into the conduit to an initial deployment location, at which the distal end of the tubing string is disposed in the conduit and the proximal end of the first tubing portion is at surface. The second tubing portion may then be coupled to the proximal portion of the first tubing portion, the second tubing portion then being unreeled from the reel. 
     As an alternative, the first and second tubing portions may be coupled together at surface and deployed into the conduit together. 
     The first tubing portion may take a number of different forms. 
     The first tubing portion may comprise a base pipe. 
     The base pipe may be constructed or formed from a polymeric material. 
     The polymeric material may be a thermoplastic material. 
     The thermoplastic material may be at least one of: polyaryletherketone (PAEK); polyarylketone (PAK); polyetherketone (PEK); polyetheretherketone (PEEK); polycarbonate (PC) or the like. 
     In particular embodiments, the base pipe is constructed or formed from polyetheretherketone (PEEK). 
     The composite coiled tubing may comprise a composite laminate disposed around the base pipe. 
     The composite laminate may comprise a matrix. 
     The matrix may comprise a polymeric material. 
     The matrix may, for example, comprise a thermoplastic material. 
     The matrix may comprise at least one of: polyaryletherketone (PAEK); polyarylketone (PAK); polyetherketone (PEK); polyetheretherketone (PEEK); polycarbonate (PC) or the like. 
     The composite laminate may comprise a plurality of reinforcing elements disposed within the matrix. 
     The reinforcing elements may be embedded in the matrix. 
     The reinforcing elements may comprise fibres, strands, filaments, nanotubes or the like. 
     For example, the reinforcing elements may comprise glass fibres, carbon fibres or the like. 
     In particular embodiments, the reinforcing elements comprise carbon fibres. 
     The first tubing portion may have a diameter of between 25 mm (1 inch) and 83 mm (3.25 inches). 
     The second tubing portion may take a number of different forms. 
     The second tubing portion may be constructed or formed from steel. 
     The second tubing portion may have a diameter of between 25 mm (1 inch) and 83 mm (3.25 inches). 
     The system may comprise a connector arrangement for connecting the first tubing portion and the second tubing portion. 
     As described above, the fluid discharge apparatus is configured for location at a distal end of the tubing string. 
     The fluid discharge apparatus may, for example, be configured for coupling to the distal end of the first tubing portion. 
     The system may comprise a connector for coupling the fluid discharge apparatus to the first tubing portion. The connector may comprise a connector sub. The connector may, for example, comprise a threaded connector or the like. Alternatively or additionally, the connector may be coupled to the first tubing portion and the fluid discharge apparatus by a bond, e.g. an adhesive bond or the like. 
     As an alternative to the connector, the first tubing portion may be configured to be directly coupled to the fluid discharge apparatus. For example, the first tubing portion may be configured to be directly coupled to the fluid discharge apparatus by a threaded connection. Alternatively or additionally, the first tubing portion may be configured to be directly coupled to the fluid discharge apparatus by a bond, e.g. an adhesive bond or the like. 
     As a further alternative to the connector, the body of the fluid discharge apparatus may be integrally formed with the first tubing portion. 
     As described above, the fluid discharge apparatus comprises a valve arrangement, wherein the fluid discharge apparatus is configured to move between a first configuration in which the valve arrangement obturates fluid communication through the lateral flow passage and a second configuration in which the valve arrangement permits fluid in the axial flow passage to be discharged through the lateral flow passage, said discharge generating a pressure pulse which facilitates transport of the tubing string along the conduit. 
     The valve arrangement may be disposed in the body. 
     In particular embodiments, the valve arrangement takes the form of a cartridge valve arrangement. The valve arrangement may comprise a valve body disposed in the body of the fluid discharge apparatus, in particular the axial flow passage of the fluid discharge apparatus. 
     The valve arrangement may comprise a valve member. 
     The valve member may comprise or take the form of a valve stem. 
     The valve arrangement may be configured to move between the first configuration and the second configuration in response to fluid pressure. 
     The valve arrangement may be configured to move between the first configuration and the second configuration passively, that is without external intervention or operation. 
     The first configuration of the fluid discharge apparatus may define a closed configuration. In the first configuration, the valve arrangement may be configured to obturate the lateral flow passage, such that fluid pressure builds up in the axial flow passage. 
     The second configuration of the fluid discharge apparatus may define an open configuration. In the second configuration, the valve arrangement may be configured to permit fluid communication through the lateral flow passage, such that fluid pressure in the axial flow passage is discharged into the annulus between the fluid discharge and the conduit. 
     The apparatus may be configurable in at least one intermediate configuration. 
     The at least one intermediate configuration may define a partially open configuration. 
     The apparatus may be configurable in a plurality of intermediate configurations. 
     The valve arrangement may comprise a piston. 
     The piston may be disposed in the body. 
     The piston may be axially movable relative to the body. 
     The piston may comprise or take the form of a sleeve or spool member. 
     The valve member, e.g. valve stem, may be disposed in the piston. 
     The piston may be configured move axially with respect to the valve member. 
     The valve member, e.g. valve stem, may comprise one or more flow apertures. 
     The flow apertures of the valve member may be radially oriented. 
     The piston may comprise one or more flow apertures. 
     The flow apertures of the piston may be radially oriented. 
     The fluid discharge apparatus may comprise a chamber. 
     The fluid discharge apparatus may comprise a seal. 
     The seal may be a circumferential seal around the piston of the valve arrangement. 
     Alternatively or additionally, the seal may be a circumferential seal between the piston of the valve arrangement and the body. 
     The seal may comprise or take the form of a metal-to-metal seal. 
     The fluid discharge apparatus may comprise a biasing member. 
     The biasing member may be arranged to urge the valve arrangement to a positon which obturates the lateral flow passage. 
     The biasing member may, for example, comprise one or more springs. 
     The springs may, for example, comprise washer springs. In particular embodiments, the springs comprise Belleville springs. 
     The piston may define an active piston area in the first configuration of the fluid discharge apparatus. 
     In use, pressure acting on the first active piston area may move the piston. 
     The piston may be configured to move when pressure acting on the first active piston area exceeds a predetermined opening pressure. 
     The piston may comprise a second active piston area, and pressure acting on the second active piston area may be operable to move the piston from a partially open position towards the fully open condition. 
     The second active piston area may be larger than the first active piston area. 
     The first piston area may be formed on a face on an interior profile of the piston. 
     The second piston area may be formed on a face on an exterior profile of the piston. 
     At least one of the piston areas may be annular faces, e.g. conical annular surfaces. 
     The fluid discharge apparatus may comprise one or more nozzles. 
     The nozzle, or where a plurality of nozzles are provided at least one of the nozzles, may be radially arranged. 
     In use, the one or more nozzles may be configured to direct the flow of fluid discharged through the lateral flow passage. 
     For example, the nozzle, or where a plurality of nozzles are provided at least one of the nozzles, may be arranged to direct fluid discharged from the lateral flow passage in an uphole or upstream direction. Beneficially, this provides a local thrust force urging the tubing string along the conduit. 
     As described above, the fluid discharge apparatus is configured to generate a pressure pulse which facilitates transport of the tubing string along the conduit 
     The tubing system may comprise a tubing injector. The tubing injector may be configured to apply a push force on the tubing string which urges the tubing string along the conduit. 
     The tubing system may comprise a pressure control arrangement. The pressure control arrangement may be interposed between the injector and the tubing string. 
     The tubing string may be disposed through the pressure control arrangement, the pressure control arrangement configured to prevent loss of containment. 
     As described above, the coiled tubing system facilitates the deployment of coiled tubing systems to total depth in a conduit. 
     The conduit may take a number of different forms. 
     The conduit may comprise a wellbore. 
     In particular embodiments, the conduit comprises an extended reach horizontal wellbore. 
     Alternatively or additionally, the conduit may comprise one or more of: a pipeline, e.g. a hydrocarbon production or transportation pipeline; a riser, e.g. a marine riser; and an umbilical. 
     Beneficially, the provision of a coiled tubing system having a fluid discharge apparatus in combination with a “hybrid” tubing string comprising composite coiled tubing and metallic coiled tubing facilitates the deployment of coiled tubing systems to total depth in a conduit, such as an extended reach horizontal, high angle or deviated wellbore (referred to hereinbelow as the horizontal wellbore); in contrast to conventional coiled tubing systems which for the reasons outlined above cannot reach total depth in extended reach horizontal wellbores. 
     A second aspect relates to a method for deploying a tubing string into a conduit using the coiled tubing system of the first aspect. 
     The method may comprise coupling the fluid discharge apparatus to a distal end of the first tubing portion. 
     The method may comprise running the first tubing portion and the fluid discharge apparatus into the conduit. 
     Running the first tubing portion into the conduit may comprise unreeling the first tubing portion from a reel. 
     The first tubing portion may be run into the conduit using the injector. 
     The method may comprise coupling the second tubing portion to the proximal end of the first tubing portion. 
     The method may comprise running the second tubing portion into the conduit. 
     Running the second tubing portion into the conduit may comprise unreeling the second tubing portion from a reel. 
     The second tubing portion may be run into the conduit using the injector. 
     The method may comprise flowing fluid through the tubing string. 
     The method may comprise applying a fluid pressure via the tubing string. 
     The method may comprise pumping fluid from a fluid source through the tubing string to the fluid discharge apparatus. 
     It will be understood that the features defined above or described below may be utilised in isolation or in combination with any other defined feature. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other aspects will now be described, by way of example, with reference to the accompanying drawings, of which: 
         FIG. 1  shows a schematic view of a coiled tubing system; 
         FIG. 2  shows a sectional view of the fluid discharge apparatus of the tubing system shown in  FIG. 1 ; 
         FIG. 3  shows an enlarged view of part of the fluid discharge apparatus shown in  FIG. 2 ; and 
         FIGS. 4 to 12  show operation of the fluid discharge apparatus. 
     
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
     Referring first to  FIG. 1  of the accompanying drawings, there is shown a coiled tubing system, generally depicted at  10 , for use in deploying a tubing string  12  into a conduit  14 . 
     In the illustrated system  10 , the conduit  14  takes the form of an extended reach horizontal wellbore having a vertical section  16  and a horizontal section  18 , the conduit  14  being accessible from intervention vessel  20  via a marine riser  22  coupled to wellhead  24  disposed at the seabed S. 
     As shown in  FIG. 1 , the system  10  includes the tubing string  12  and a fluid discharge apparatus  26 , the fluid discharge apparatus  26  disposed at a distal end of the tubing string  12 . 
     The tubing string  12  includes a first tubing portion  28  comprising composite coiled tubing and a second tubing portion  30  comprising metallic coiled tubing, the fluid discharge apparatus  26  coupled to a distal end of the first tubing portion  28 . 
     In the illustrated system  10 , the first tubing portion  28  has a base pipe constructed or formed from polyetheretherketone (PEEK) and has a composite laminate disposed around the base pipe, the composite laminate constructed or formed from a plurality of carbon fibre reinforcing elements disposed within a matrix constructed or formed from polyetheretherketone (PEEK) and the second tubing portion  30  is constructed or formed from steel. 
     In use, and as will be described further below, the system  10  is configured to deploy the tubing string  12  and the fluid discharge apparatus  26  into the conduit  14 , the fluid discharge apparatus  26  operable to generate pressure pulses which facilitate transport of the tubing string  12  along the conduit  14 . 
     Beneficially, the provision of a coiled tubing system  10  having a fluid discharge apparatus  26  in combination with a “hybrid” tubing string  12  comprising composite coiled tubing and metallic coiled tubing facilitates the deployment of coiled tubing systems to a distance not previously attainable using conventional coiled tubing systems. 
     In use, the tubing string  12  is deployed from a reel  32  using an injector apparatus  34 . Before passing into the riser  22 , the tubing string  12  passes through a pressure control arrangement  36  which, in the illustrated system  10  comprises a stripper  38  and a blowout preventer  40 . 
     Referring now also to  FIG. 2  of the accompanying drawings, there is shown an enlarged longitudinal section view of a distal end portion of the system  10 . 
     As shown in  FIG. 2 , the fluid discharge apparatus  26  comprises a body  42  which is coupled to the distal end of the first tubing portion  28  by a connector  44 . The body  42  defines an axial flow passage  46  which in the illustrated system  10  takes the form of a bore. The axial flow passage  46  communicates with an axial flow passage  48  of the tubing string  12 . 
     A lateral flow passage  50  is provided through the body  42 . In the illustrated fluid discharge apparatus  26 , the lateral flow passage  50  take the form of a plurality of radial ports extending through the body  42 . 
     A valve arrangement  52  is disposed within the bore  46  of the body  42 . In the illustrated fluid discharge apparatus  26 , the valve arrangement  52  takes the form of a cartridge valve arrangement disposed within the body  42  of the fluid discharge apparatus  26 . 
     As will be described further below, the fluid discharge apparatus  26  is configured to move between a first configuration in which the valve arrangement  52  obturates fluid communication through the lateral flow passage  50  and a second configuration in which the valve arrangement  52  permits fluid in the axial flow passage  46  to be discharged through the lateral flow passage  50 , said discharge generating a pressure pulse which facilitates transport of the tubing string  12  along the conduit  14 . 
     As shown in  FIG. 2 , the valve  52  comprises a piston  54 . The piston  54  is disposed within the body  42  and is configured to move axially relative to the body  42 . 
     A valve stem  56  is disposed within the piston  54 . The valve stem  56  defines an internal cavity  58  which receives pressurised fluid from the tubing string  12 . The valve stem  56  comprises radial apertures  60  which provide fluid communication between the cavity  58  and the outer surface of the valve stem  56 . The radial apertures  60  are distributed axially and circumferentially. The piston  54  comprises radial apertures  62 , distributed circumferentially around the piston  54 . Thus, it can be seen that the cavity  58  is in fluid communication with the exterior of the piston  54  via the apertures  60 , 62 . 
     As shown in  FIG. 2 , a biasing member  64  is disposed in the body  42 , the biasing member  64  engaging an end of the piston  54 . The biasing member  64  biases the piston  54  towards the position shown in  FIG. 2  (to the left as shown in  FIG. 2 ) in which the piston  54  obturates the lateral flow passage  50  and thus maintains the fluid discharge apparatus  26  in its first configuration. In the illustrated fluid discharge apparatus  26 , the biasing member  64  takes the form of a plurality of spring washers. 
     A nozzle arrangement  66  is disposed around the body  42 , and more particularly around the lateral flow passage  50 . The nozzle arrangement  66  comprises a number of nozzles  68 . The nozzle arrangement  68  facilitates fluid jetting from the fluid discharge apparatus  26 , for example to perform a cleaning operation, fracturing operation, chemical wash operation or the like. The nozzle arrangement  26  is configurable at surface by varying nozzle sizes and/or blanking off one or more of the nozzles  68 , e.g. with threaded inserts (not shown). 
     Referring now also to  FIG. 3  of the accompanying drawings, there is shown an enlarged view of the part of the fluid discharge apparatus  26  shown in  FIG. 2 . For ease of reference, the nozzle arrangement  66  is not shown. 
     As shown in  FIG. 3 , the piston  54  takes the form of a cylindrical sleeve with internal and external stepped profiles  70 ,  72 . The internal stepped profile  70  comprises a first portion  74  with a first inner diameter, and a second portion  76  with a second, reduced inner diameter. Between the first and second portions  70 , 72  is a conical face  78  which defines a first active piston area of the piston  54 . The external stepped profile  72  comprises a first portion  80  with a first outer diameter and a second portion  82  with a second, greater, outer diameter. Between the first and second portions  80 , 82  is an exterior conical face  84  which provides a second active piston area of the piston  54 . As shown in  FIG. 3 , a chamber  86  is formed between the piston  54  and the body  42 , the chamber  86  communicating with the lateral flow passage  50 . The conical face  84  also provides a sealing surface for a metal-to-metal seal between the piston  54  and a shoulder  88  in the body  42 . The exterior conical face  84  is selected to be larger than the interior conical face  78 , such that the second active piston area is larger than the first active piston area. 
     Seal elements  90 , which in the illustrated fluid discharge apparatus  26  take the form of elastomeric seals, are provided between the piston  54  and the body  42 . Seal elements  92 , which in the illustrated fluid discharge apparatus  26  take the form of elastomeric seals, are provided between the valve stem  56  and the piston  54 . 
     A port  94  is provided in a distal end portion of the body  42 , the port  94  pressure balancing the bore and spring cavity. Ports  96  in the body  42  pressure balance the fluid discharge apparatus  26 , and allow the piston  54  to move between its respective operating positions, as will be described below. 
     In use, the fluid discharge apparatus  26  is configured to discharge fluid via the nozzles  68  into the conduit  14  when a pressurised fluid is provided to the fluid discharge apparatus  26  via the tubing string  12 . 
     The discharged fluid creates fluid jets which are directed towards material such as wax, scale, and/or other deposits in the fluid conduit to physically dislodge or loosen it from the fluid conduit. 
     The nozzles may also be directed towards the rear of the tool to provide forward thrust to move the assembly forwards in the pipeline. 
     Operation of the fluid discharge apparatus  26  will now be described with reference to  FIGS. 4 to 12  of the accompanying drawings. For clarity, some components of the fluid discharge apparatus  26  are not shown. 
       FIG. 4  shows the fluid discharge apparatus  26  in the first, closed, configuration in which the valve arrangement  52  prevents fluid flow through the lateral flow passage  50 . The cavity  58  is in fluid communication with the tubing string  12  which receives fluid pumped from a fluid source at an elevated pressure (higher than the ambient pressure in the pipeline). Fluid pressure acts on the face  78  which defines the first active piston area. However, in this configuration the fluid pressure acting on the face  78  is insufficient to overcome the bias of the biasing member  64 . 
       FIG. 5  of the accompanying drawings shows the fluid discharge apparatus  26  in a condition in which the pressure has increased to a level at which the pressure force on the active piston area overcomes the bias of the biasing member  64 . As shown in  FIG. 5 , the piston  54  has moved axially relative to the body  42  and the valve stem  56  (to the right as shown in  FIG. 5 ). In this position, the metal-to-metal seal is broken. 
       FIG. 6  of the accompanying drawings shows the fluid discharge apparatus  26  in a partially open condition. The applied fluid pressure has reached a threshold required to move the piston  54  to a position at which the radial apertures  60  are in direct fluid communication with chamber  86 . At this stage, the fluid flow rate from the cavity  58  to the lateral flow passage  50  increases rapidly. Chamber  864  becomes pressurised, resulting in an overall drop in pressure within the fluid discharge apparatus  26 . However, with the chamber  86  pressurised, the second active piston area (defined by surface  84 ) becomes active. The force on the piston  54  is sufficient to continue to move the piston against the biasing member  64  (to the right as shown in the drawings), providing that the internal pressure does not drop below a ‘set close’ pressure threshold. In this condition, the flow rate is initially small and the internal pressure is initially high. 
       FIG. 7  of the accompanying drawings shows the fluid discharge apparatus  26  in a partially open condition, in which the piston  54  has moved further against the biasing member  64 . As the piston  54  moves, the rate of change of the flow rate decreases, and the internal pressure reduces. The radial flow path through the fluid discharge apparatus  26  increases as the apertures  62  moves into alignment with the chamber  86 . 
       FIG. 8  of the accompanying drawings shows the fluid discharge apparatus  26  in a fully open condition. The fluid discharge apparatus  26  remains in this condition while the internal pressure exceeds a critical ‘set close’ pressure, at which the hydrostatic load on the piston faces  78 , 84  is less than the bias of the biasing member  64 . The flow rate in this condition is relatively high. 
       FIG. 9  of the accompanying drawings shows the fluid discharge apparatus  26  in a condition at which the critical ‘set close’ pressure has been reached. In this condition, the bias of the biasing member  64  now exceeds the hydrostatic load on the piston faces  78 , 84 , and the piston  54  begins to move towards the valve stem  56  (to the left as shown in the drawings) as the pressure continues to drop. The radial flow path area from the cavity  58  to the exterior of the fluid discharge apparatus  26  begins to reduce and become more restricted, while the flow rate is still relatively high. This results in an increasing pressure drop across the piston  54  between the cavity  58  and the pressure acting on the piston area defined by face  84  in the chamber  86 , reducing the relative pressure acting on the piston area defined by face  84 . Conversely, the relative pressure acting on the piston area defined by face  78  increases, relative to the pressure in the chamber  86 . However, as the active second piston area defined by face  84  is significantly larger than the first active piston area defined by face  78 , the net hydrostatic force on the piston  54  continues to reduce, and the bias of the biasing member  64  continues to move the piston  54  towards the valve stem  56 . As the piston  54  moves towards the valve stem  54  fluid exits through ports  94 . 
       FIG. 10  of the accompanying drawings shows the fluid discharge apparatus  26  in a partially closed condition, in which the apertures  62  of piston  54  are not in direct fluid communication with the chamber  86 . The chamber  86 , the lateral flow passage  50  and the nozzle arrangement  66  discharge to the ambient pipeline pressure. In this condition, the flow rate is initially high, and the internal pressure is initially low compared to the condition shown in  FIG. 6  in the opening phase of the cycle. The fluid continues to be pumped from the fluid source, and although the pressure in the tubing string  12  and cavity  58  increases, the pressure only acts on the piston area of face  78 . This piston area is relatively small, and is overcome by the bias of the biasing member  64 . 
       FIG. 11  of the accompanying drawings shows the fluid discharge apparatus  26  in a partially closed condition. As the piston  54  moves towards the valve stem  56  and towards a closed position, the flow path between the chamber  86  and the lateral flow passage  50  is obturated. This provides a degree of hydraulic dampening or cushioning as the piston  54  approaches the point of contact of the metal-to-metal seal, to reduce the impact on the metal seal and increase its longevity. Ports  94  are also restricted to provide hydraulic dampening between the piston  54  and the valve stem  56 . 
       FIG. 12  of the accompanying drawings shows the fluid discharge apparatus  26  in its fully closed condition, at the end of the cycle. This condition is the same as the condition in  FIG. 4 , and no flow is permitted through the fluid discharge apparatus  26 . Fluid continues to be is pumped from the fluid source, and the cycle begins again with the internal pressure in the cavity  58  increasing as described with reference to  FIG. 4 . 
     As described above, the provision of a coiled tubing system  10  having a fluid discharge apparatus  26  in combination with a “hybrid” tubing string  12  comprising composite coiled tubing and metallic coiled tubing facilitates the deployment of coiled tubing systems to a distance (or “reach”) not previously attainable using conventional coiled tubing systems. The composite coiled tubing has a lower coefficient of static friction than conventional metallic coiled tubing, e.g. 0.15 compared to 0.25, such that for the same input force the reach of the tubing string can be extended. Moreover, each pressure pulse breaks the contact between the tubing string  12  and the conduit  14 , such that the coefficient of friction between the tubing string and the conduit is changed from a static coefficient of friction to a dynamic coefficient of friction, e.g. 0.1, thereby facilitating further reach along the conduit, such as by an injector arrangement at surface or the like. The pressure pulse may also provide a local thrust force which acts to pull the tubing string  12  along the conduit  14 . 
     Referring again to  FIG. 1  of the accompanying drawings, in use, the fluid discharge apparatus  26  is disposed at a distal end of the first tubing portion  28  at surface, the first tubing portion  28  and the fluid discharge apparatus  26  then being deployed into the conduit  14  by unreeling the first tubing portion  28  from a reel. In the illustrated system  10 , the fluid discharge apparatus  26  and the first tubing portion  28  are deployed into the horizontal section  18 . The first tubing portion  28  may comprise a single run of composite coiled tubing. Alternatively, where required the first tubing portion  28  may comprise a plurality of runs of composite coiled tubing coupled together. The first tubing portion  28  is thus deployed into the conduit  14  to an initial deployment location, at which the distal end of the first tubing portion  28  is disposed in the conduit  14  and the proximal end of the first tubing portion  28  is at surface. The second tubing portion  30  is then coupled to the proximal portion of the first tubing portion  28 , the second tubing portion  30  then being unreeled from reel  32  to further deploy the tubing string  12  into the conduit  14 . 
     During running, the fluid discharge apparatus  26  is operated to facilitate transport of the tubing string  12  to a distance not previously attainable using conventional coiled tubing systems. 
     As described above, in addition to facilitating transport of the tubing string  12 , the system  10  is operable to perform an intervention operation, such as a cleaning operation, fracturing operation or chemical wash or the like. 
     It will be recognised that the system  10  described above is merely exemplary and that various modifications may be made without departing from the scope of the claimed invention as defined by the appended claims.