Abstract:
The invention relates to tubing handling apparatus for injecting tubing into and recovering the tubing from a wellbore. In an embodiment, the apparatus includes a storage reel of coiled tubing, a first injector head and a second injector head for injecting the tubing from the reel into the wellbore. The second injector head may be located between the storage reel and the wellbore, and the first injector head may be disposed between the storage reel and the second injector head. The first and second injector heads can control bending of a length of the coiled tubing from the reel which is passed between the first and second injector heads.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to tubing handling apparatus and associated methods. In addition, the invention relates to a gooseneck device for a tubing injector head. 
       BACKGROUND TO THE INVENTION 
       [0002]    Tubing is commonly used in oil, gas and water wells, to convey fluids between the well and the surface. Coiled tubing is normally supplied in continuous lengths wound onto a reel, and is unwound from the reel as it is injected into the wellbore. Normally the injection of the coiled tubing into the wellbore is controlled by an injector head. In offshore wells, the tubing is injected into the well through a riser, which is secured to the wellhead on the seabed, and which forms a rigid and stationary conduit between the platform or drill ship at the surface, and the wellhead on the seabed. When the platform is subject to heave, the platform and derrick therefore move relative to the injector head attached to the stationary riser. In order to accommodate this relative movement between the injector head and the derrick/platform, the injector head is typically supported by a motion compensator system on the derrick. The motion compensator system typically supports the injector head during the cycles of movement relative to the platform to compensate for the heave. This relative movement between the derrick/platform and the injector head is typically accommodated by a length of the tubing between the injector head on the derrick and the reel. Typically, the length of tubing may be managed by providing the length of tubing in a long smooth parabolic curve where the tubing may extend vertically upwards from the reel, curving smoothly through 180° over the derrick, and entering the injector head from above the derrick. This has drawbacks in relation to the deck space required. 
         [0003]    Alternatively, the length of tubing between the reel and the injector head can be managed by providing gooseneck devices on the derrick and keeping the uninjected line in tension by keeping a back tension on the reel motor. As the reel moves relative to the stationary injector head, the motor on the reel applying the back tension cycles to reel in and pay out tubing from the reel in order to keep the tension constant. 
         [0004]    Where space is limited, as typically encountered on offshore platforms/vessels, the length of tubing between the reel and the injector head may be arranged in a lazy loop. In this configuration, the length of tubing has a degree of slack between the reel and the injector head on the derrick to accommodate the relative motion, and gooseneck devices on the derrick may guide the length of tubing into the injector head and the well. 
         [0005]    However, this arrangement can be problematic since the lazy loop of tubing may tend to bend repeatedly due to the cyclical relative motion between the platform and the injector head. Bending of the tubing typically occurs in areas where the tubing separates or extends away from its supporting apparatus, for example, where the tubing comes off the reel, where it enters the injector head on the derrick, or where the tubing comes into contact with the gooseneck device. In those areas, the tubing may tend to bend or sag about its fixed support points. In addition, the tubing may deform plastically when it is bent and repeated plastic bending can lead to fatigue and damage to the tubing and significantly shorten its lifespan. 
       SUMMARY OF THE INVENTION 
       [0006]    According to a first aspect of the invention, there is provided tubing handling apparatus for injecting tubing into a wellbore, and recovering the tubing therefrom, the apparatus comprising 
         [0007]    (a) a storage reel of tubing; 
         [0008]    (b) a first injector head; 
         [0009]    (c) a second injector head for injecting the tubing from the reel into the wellbore, 
         [0010]    (d) the second injector head being located between the storage reel and the wellbore, 
         [0011]    (e) the first injector head being disposed between the storage reel and the second injector head, and 
         [0012]    (f) the first and second injector heads being together operable to control bending of a length of tubing passed between the first and second injector heads. 
         [0013]    Said length of tubing passed between the first and second injector heads may, at least in part, form a lazy loop of tubing therebetween. Thus, the length of tubing passed between the first and second injector heads may have a degree of slack or sag between the injector heads due to gravity. This may help to provide stability. 
         [0014]    Typically, the length of tubing passed between the first and second heads may be held within the elastic bend radius limits of the tubing. 
         [0015]    The second injector head may be provided with a gooseneck device for facilitating introduction of the tubing into a central injection point of the injector head. The gooseneck device provides typically a curved or arched track bed for receiving a section of tubing and/or for guiding the tubing toward the injection point. Thus, the gooseneck device may be adapted to help control bending of the length of tubing. The gooseneck device and/or track bed may have a first portion and a second portion having different radii of curvature. In particular, a first portion of the track bed closer to the injection point than the second portion may have a smaller radius than the second portion of the track bed. The track bed curvature may be selected so that when tubing is being guided by the second portion of the track bed, the tubing is kept within the elastic bend radius limits of the tubing by the second portion of the track bed. This may help to restrict plastic deformation of the tubing. The track bed may be adapted to provide contact with the length of tubing passed over the track bed/gooseneck along its length so as to support the tubing and control its bend radius from the point at which the tubing is brought onto the gooseneck to the injection point. The gooseneck may include clamps to maintain contact between the track bed and the tubing. 
         [0016]    The apparatus may include a synchronising device whereby the tubing is moved through the first and second injector heads at substantially the same rate. 
         [0017]    Controlling the rate of injection of tubing through the second injector head on the derrick to be the same as the rate of tubing drawn off the reel means that the length of the tubing between the main derrick injector head and the reel is maintained substantially constant, and the bend radius can thus be maintained within acceptable limits, within the elastic bend radius of the tubing, so that plastic deformation of the tubing is avoided or mitigated. Also, the reel does not need to cycle to maintain the bend radius of the tubing within the limits of elastic deformation, which reduces the fatigue on the tubing. 
         [0018]    The first portion of the track bed may be adapted to hold the tubing in contact with the track bed and guide curvature in a plastic deformation zone. The second portion of the track bed may be adapted to allow separation of the tubing and track and deviation of the tubing away from the fixed radius of curvature of the track, but the track bed may limit the minimum bend radius of tubing to keep the tubing within the elastic bend radius. The fixed radius of curvature of the track may therefore be sufficient to guide the tubing and keep it within the elastic bend radius limits. 
         [0019]    Typically the second injector head has a motion compensator system. 
         [0020]    The reel is typically back tensioned by reversing the motor on the reel against the force applied by the feed mechanism of the first injector device or injector head (typically on the frame or skid bearing the reel) so that the tubing between the reel and the first injector device is held taught, and the first injector device is pulling the tubing off the reel in the opposite direction from the reel motor, against the back tension. The tubing between the first and second injector devices is typically not tensioned, and is maintained in an arc, which is preferably maintained within the elastic bend radius of the tubing, so that the tubing between the first and second injector heads is not subjected to plastic deformation from which it cannot recover. During tubing recovery, the two injector heads are typically synchronised also, to maintain the arc between the injector heads within acceptable bend radius limits. The first injector head may be adapted to draw (e.g. against the back tension applied by the reel motor) a length of tubing from the reel to form a length of tubing, for example the lazy loop of tubing, between the first and second injector heads. 
         [0021]    The tubing is typically coiled tubing, and is typically injected into or recovered from a wellbore of an oil or gas well, typically an offshore oil or gas well. 
         [0022]    Typically the reel is mounted on a frame and can optionally rotate relative to the frame. The first injector is typically secured to the frame and is adapted to apply a traction force to the tubing on the reel. The first and/or second injector head can work in different directions, for example, to draw the tubing from the reel when the tubing is being injected into a well etc, or to wind the tubing onto the reel when the tubing is being recovered from the well. 
         [0023]    The first injector head can optionally be provided on a spooling device on the frame, such as a levelwind device. 
         [0024]    According to a second aspect of the invention, there is provided a method of handling reeled tubing, the method comprising the steps of: 
         [0025]    passing the tubing from the reel through a first injector head; 
         [0026]    passing the tubing from the first injector head through a second injector head to form a length of tubing passed between the first and second injector heads; 
         [0027]    passing the tubing from the second injector head into the wellbore; and 
         [0028]    controlling bending of said length of tubing passed between the two injector heads. 
         [0029]    The length of tubing passed between the first and second injector heads may form a lazy loop. The method may include moving the tubing into the well by controlling the injector heads. The method may include synchronising the movement of the tubing through the two injector heads. 
         [0030]    Typically the movement of the tubing through the injector heads is synchronised so that the tubing moves through the two injector heads at substantially the same rate, and typically so that the length of tubing between the two injector heads remains substantially constant. 
         [0031]    According to a third aspect of the invention, there is provided a method of configuring a tubing handling system, comprising the steps of: 
         [0032]    installing a tubing reel on a platform; 
         [0033]    installing a first injector head for paying out tubing from the reel; 
         [0034]    installing a second injector head for injecting tubing into a well; 
         [0035]    paying out a section of tubing to pass through the first and second injector heads; and 
         [0036]    engaging the first and second injector heads to control the bend of the section of tubing passed out between the injector heads. 
         [0037]    The first injector head and/or a spooling device may typically be used to pay out the tubing. 
         [0038]    According to a fourth aspect of the invention, there is provided a gooseneck device for a tubing injector head, the gooseneck device comprising: a guide member configured to receive thereon a section of tubing, and to present the tubing for insertion through an injector head, wherein the guide member has a first guide portion and a second guide portion, and the first and second guide portions have different radii of curvature. 
         [0039]    Typically, the tubing may be coiled tubing. 
         [0040]    Typically the first guide portion is attached to an injector head. Typically, the guide is aligned with a central aperture of the injector head. The coiled tubing may be adapted to lie against the guide along the length of the guide. The coiled tubing may be held against the guide along said length, for example using clamps provided on the gooseneck. 
         [0041]    The curved guide member may provide an arch for supporting the tubing. The guide member may be curved to control the bending of the tubing adapted to lie against the guide member. 
         [0042]    The first portion may have a radius of curvature greater than the radius of curvature of the second portion or vice versa. In an example, the curvature of the guide may increase and/or decrease gradually or stepwise from a first end toward a second end of the guide, for example, near the injector head to which the guide may be attached. The first portion may be joined to the second portion to provide a gradual or stepwise transition in curvature between the first and second portions. 
         [0043]    The radius of curvature of the first guide portion may be between in the region of around 25 to 40 ft (approximately 7.6 to 12.2 m). The radius of curvature of the second guide portion may be between around 90 to 120 inches (approximately 229 to 305 cm). 
         [0044]    The guide may be adapted to pivot into different rotational positions. The gooseneck device may incorporate a hydraulic cylinder for cushioning the guide. 
         [0045]    The tubing is typically coiled tubing, and is typically injected into or recovered from a wellbore of an oil or gas well, typically an offshore oil or gas well. 
         [0046]    Any one of the first to fourth aspects of the invention referred to above may include one or more feature(s) as defined in relation to any one or more of the other aspects of the invention where appropriate. 
         [0047]    In a further aspect, the invention provides a tubing handling system for injecting tubing into a wellbore, and recovering the tubing therefrom, the system comprising: 
         [0048]    a storage reel of tubing; 
         [0049]    a first injector head; 
         [0050]    a second injector head for injecting the tubing from the reel into the wellbore, 
         [0051]    the second injector head being located between the storage reel and the wellbore, and 
         [0052]    the first injector head being disposed between the storage reel and the second injector head. 
         [0053]    This aspect may include one or more feature(s) as defined in relation to any one of the first to fourth aspects of the invention where appropriate. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0054]    Embodiments of the present invention will now be described, by way of example only, and with reference to the accompanying drawings, in which, 
           [0055]      FIG. 1  is a side view of a tubing handling system; 
           [0056]      FIG. 2  is a side view of a reel in a frame on the tubing handling system of  FIG. 1 ; 
           [0057]      FIG. 3  is a side view of an injector head on the reel of  FIG. 2 ; 
           [0058]      FIG. 4  is a side view of a gooseneck for an injector head on the derrick in a further embodiment; and 
           [0059]      FIG. 5  is an overhead view of the gooseneck of  FIG. 4  showing different rotational positions. 
       
    
    
     DETAILED DESCRIPTION 
       [0060]    Referring now to the drawings,  FIG. 1  shows a tubing handling system for injecting coiled tubing into an oil or gas well. The system has a reel  5  of tubing  1 , rotatably mounted on its axis  5   a  on bearings in a frame  6 . The reel is cylindrical and is driven in rotation around the bearings by a motor  7 . The tubing  1  is spooled on and off the reel by a levelwind head  9 . The levelwind head  9  has a pair of sheaves on each end that engage respective rails  8  extending parallel to the axis of rotation of the reel  5 . The levelwind head  9  can move along the rails  8  parallel to the axis of the reel, to guide the tubing and distribute it onto and from the reel in an even manner. The lateral motion of the levelwind head along the rails  8  can be powered or free. 
         [0061]    The reel  5  is mounted on the deck of a drilling platform, over an oil or gas well. The platform has a derrick injector head  15  of known design which is suspended from the derrick D. The derrick injector head  15  controls the rate of injection of the tubing through a riser, which is rigidly connected to the wellhead on the seabed. Although the riser and the derrick injector head  15  are held stationary, the platform and derrick D move with the heave. The movement of the platform and the derrick D relative to the stationary derrick injector head  15  is accommodated by a conventional motion compensator system (not shown) from which the derrick injector head  15  is supported by a cable. The motion compensator system moves in opposite relation to the movement of the platform and derrick D, so that the movement of the platform and the derrick D during the heave cycles is isolated from the static derrick injector head  15 , the riser and the portion of tubing  1   d  below the derrick injector head  15 , which all remain static relative to the wellbore when the platform moves with the heave. Typically, the heave produces a cyclic relative motion of the platform fixtures such as the reel  5  and the stationary derrick injector head  15 . The derrick injector head  15  has a gooseneck  14  extending from its upper end to receive the tubing and to control its rate of bending and support it as it enters the top of the derrick injector head  15 . 
         [0062]    The tubing is spooled from the reel  5  through the levelwind head  9  to the gooseneck  14 , through the derrick injector head  15  and into the well below the platform. The tubing between the reel  5  and the levelwind head  9  is denoted  1   a , the tubing between the levelwind head  9  and an outer end of  14   e  of the gooseneck  14  is denoted  1   b , the tubing passed over the gooseneck  14  between its outer end  14   e  and the derrick injector head  15  is denoted  1   c , and the tubing between the injector head  15  and the wellbore is denoted  1   d.    
         [0063]    The tubing portion  1   b  between the levelwind head  9  and the injector head  15 , forms a lazy loop of tubing and is held in a predictable bend radius, which is maintained within the elastic bend stress limits of the tubing used (which varies according to the tubing design). 
         [0064]    For example, the characteristic bending stress (a) limits for a particular diameter of coiled tubing (above which plastic deformation can be expected to take place) is typically determined from the following relationships: 
         [0000]      σ( r   1 )= E ε( r   1 ), and 
         [0000]      ε=Δ L/L    
         [0000]    where ε=strain; r 1 =coiled tubing bend radius; E=Young&#39;s Modulus of the tubing material; ΔL is change in original length after bending; and L is the original length of tubing. 
         [0065]    The levelwind head  9  has an injector head  20  which grips the tubing to draw the tubing from the reel  5 , as described in more detail below with reference to  FIG. 3 . The injector heads  15 ,  20  are operated to maintain the combined length of tubing portions  1   b , 1   c  and to control the bend of the tubing portions  1   b ,  1   c . In particular, when the tubing portion  1   d  is not being injected into the well, the two injector heads  15 ,  20  keep the tubing  1  static between them, and the only movement of the tubing portion  1   b  between the two heads  15 ,  20  is caused by the motion compensator on the main derrick injector head  15  in response to the heave movement of the platform. As the injector head  15  moves relative to the deck, the overall length of the tubing portions  1   b ,  1   c  does not change, but the amount of slack in the tubing portion  1   b  between the two injector heads  15 ,  20  cycles with the movement of the injector head  15  relative to the stationary reel  5 . For example, as the derrick injector head  15  rises, the tubing portion  1   b  will tighten towards the upper limit  2   a  shown in  FIG. 1  decreasing the bend radius in the tubing portion  1   b . When the injector head  15  drops, the tubing portion  1   b  will sag towards the lower limit  2   b , increasing the bend radius. However, the extent of movement is kept within the elastic bend limits of the tubing and is spread over the whole of the tubing portion  1   b , so the tubing portion  1   b  is not permitted to extend outwith the elastic bend limits of the tubing  1 . Also, the bend cycling is limited to the tubing portion  1   b , and is kept within the elastic bend radius of the tubing by the outer portion of the gooseneck  14 , and neither the tubing portion  1   a  coming off the drum, nor the tubing portion  1   d  being injected into the well, experiences any cycling in bend radius. 
         [0066]    The gooseneck  14  is formed to control the deformation of the tubing portions  1   b , 1   c  as it passes over the gooseneck particularly so that deformation of tubing is controlled at the end point  14   e  at which tubing section  1   b  moves onto the gooseneck where the tubing may be susceptible to deflecting due to cyclic movement of the tubing section  1   b  produced due to motion of the sea. The gooseneck  14  is formed near the end  14   e  so that it keeps the tubing portion  1   b  in the elastic bend radius limits, preventing plastic flexure of the tubing at this point. 
         [0067]    As mentioned above, the levelwind head  9  has an injector head  20 , which is shown in more detail in  FIG. 3 . The injector head  20  has two pairs of sprockets  21   a  and  21   b . One of the sprockets  21  in each pair can be driven in rotation by a motor on the injector head  20  and the other can freely rotate. Each pair of sprockets is surrounded by a respective belt  22   a ,  22   b , optionally bearing resilient gripper blocks  23   a  and  23   b  on its outer surface, so that the belts  22  are moved around an endless track defined by the sprockets  21 . The sprockets  21   a  and  21   b  are driven by the motor in opposite directions, so that when the sprockets  21   a  are rotating anticlockwise, the sprockets  21   b  are rotating clockwise. The belts  22  are typically a tight fit on the sprockets  21 , which are optionally keyed into apertures on the belts  22  to rotate the belts  22  at the same speed as the sprockets  21 . Optionally the sprockets can be smooth and can engage with the inner surface of the belts through friction. In some embodiments one (or both) of the sprockets in each pair  21   a ,  21   b  can be mounted in a slot mechanism (not shown) that allows relative movement of the sprockets in each pair, to tighten and loosen the belts  22  on the sprockets  21  in order to ensure that the movement of the belts  22  corresponds with the movement of the sprockets  21 . The slot mechanism can also allow movement of the sprocket pairs so that the belts  22   a  and b move laterally away from one another to widen the space between the belts. The two pairs of sprockets  21   a  and  b  can optionally be driven by the same motor through appropriate gear linkages, so that they (and the belts) are driven at the same rate and there is no stutter between them. 
         [0068]    The two belts  22   a ,  22   b  are spaced apart from one another and extend side-by-side and parallel to one another, so that there is an elongate space between them. The elongate space accommodates the tubing  1 . The optional gripper blocks  23  typically grip the tubing while it is between the belts  21   b ,  22   b , and the gripping force exerted on the tubing  1  by the belts  21   b ,  22   b  can typically be adjusted by optional adjusting mechanisms such as skate adjusters  24   a ,  24   b , which comprise arcuate guide surfaces (or skates) mounted on hydraulic cylinders that push the arcuate skates towards and away from one another in order to change the path of the endless track taken by the belts  22  as they move over the outer surfaces of the skates. Thus the hydraulic cylinders can be extended in order to grip the tubing  1  tighter between the belts  22 , or can be retracted to loosen the grip of the belts  22  on the tubing  1 . The gripping blocks  23  can be omitted, and the belts  23  can optionally grip opposing surfaces of the tubing  1  by their outer opposing surfaces. 
         [0069]    The tubing  1  is gripped between the two belts  22 , and is driven by the belts  22  through the injector head  20  in one direction, towards the main derrick injector head  15  and off the reel  5 , when the tubing  1  is being injected into the well. When the tubing is being recovered from the well, the injector head  20  reverses the direction of rotation of the sprockets  21  to draw the tubing through the injector head  20  in the other direction, away from the main derrick injector head  15 , and onto the reel  5 . 
         [0070]    The injector heads  15 ,  20  are synchronised by a synchronisation mechanism  26 , connected to both heads  15 ,  20 , which controls the rate of the motors of the injector heads  20  and  15 , so that they both draw tubing  1  through at the same speed. 
         [0071]    In  FIG. 4 , there is shown another example gooseneck  114 , which may be used instead of the gooseneck  14  of the derrick injector head  15 . The gooseneck  114  provides support for the portion of coiled tubing  1   c  along its length and guides the tubing  1  into the injector head  15 . The gooseneck  114  has a bed of successive rollers  117  which are aligned and shaped to receive the tubing  1  thereon and which rotate to ease and control the movement of the tubing over the gooseneck  114  into the centre of the injector  15 . The tubing typically lies tight against the gooseneck  114  and follows the overall curved or arched contour of the gooseneck  114  as it enters the injector head  15 . 
         [0072]    Further, the gooseneck  114  has an outer section  119  and an inner section  121 , which define different angles of radius. The outer section defines an arc with a 33 ft (approximately 10.1 m) radius. The inner section radius defines an arc of a 110 inch (approximately 279 cm) radius. This presents an increase in curvature toward the derrick injector  15  which controls the extent of bending of the tubing as it passes over the gooseneck  114 . The set curvature of the gooseneck limits the maximum bend allowed for the tubing, and can therefore prevent the tubing from exceeding the elastic bend radius limits, even if the tubing may deflect away or separate from the track bed for example toward the outer end of the gooseneck  114  due to cyclic heave motion. 
         [0073]    Where the tubing is located in contact with the gooseneck  114 , the tubing is movable together with the gooseneck  114  and injector head  15  assembly, and cyclic movement of the tubing with respect to the gooseneck and injector head  15  is prevented. 
         [0074]    Where the tubing  1   c  passes over the outer section  119 , the high radius curvature of the gooseneck bed  117  ensures that the tubing is held within the limits of the elastic bend radius of the tubing. As the tubing  1   c  passes into the section  121  the curvature is increased, and the radius is reduced (due to the limited space available in typical work areas) and the tubing on the 110-inch radius section forces the tubing to exceed its elastic bend limit. Only as it enters this portion  121  therefore does the tubing experience plastic deformation before entering the injector  15 . In addition, the outer section  119  is formed so that the tubing section  1   b  is brought onto the gooseneck  114  at the outer end  119   e  at a gentle angle, avoiding plastic flexure of the tubing at the outer end  119   e . The gradual change in curvature and bend of the tubing  1  as it moves on the inner end toward the injector means that the tubing is moved from elastic to plastic deformation conditions in a controlled fashion. The tubing  1  is typically maintained in contact with the inner end by clamps provided to the gooseneck track bed (not shown), so a given point on the tubing  1  passing over the gooseneck  114  is exposed only to a single transition from elastic to plastic bending conditions on the approach to the injector  15 . 
         [0075]    The gooseneck  114  also has a two-part construction having an outer part  114   a , and an inner part  114   b . The outer part  114   a  is connected to the inner part  114   b  via a pin  123  allowing the outer part to be rotated from an operational position as shown in  FIG. 4 , to a stowed position (not shown) where the outer portion  114   a  is folded down alongside the injector head  15  in the direction of arrow  125 . The injector head  15  is provided with a hydraulic actuator  127  coupled at one end to a frame of the injector head  15  and at the other to the outer part  114   a . The actuator  127  operates to retract the outer part  114   a , via an extending arm of the actuator, when required. The actuator arm also acts as a support to hold the outer part  114   a  in position in the operational configuration as seen in  FIG. 4 , where the first and second parts together define a guide track for the tubing. The hydraulic actuator  127  acts to provide a degree of cushioning to the outer part  114   a  of gooseneck  114 , and to the tubing section  1   c  passed over the gooseneck  114 . This helps to damp vibrations and shocks, reducing wear on the tubing. 
         [0076]    With further reference now to  FIG. 5 , the gooseneck  114  can also take up different rotational positions  114   x,y,z  to present favourable rotational angles in relation to the tubing at it enters onto the gooseneck at the outer end  119   e . This further protects the tubing  1  from plastic flexure, i.e. it helps to ensure that as the section  1   b  of the tubing  1  comes onto the gooseneck  114  at the end  119   e , the tubing  1  is kept within its elastic bend radius limits. 
         [0077]    The gooseneck positions  114   x,y,z  shown in  FIG. 5  are spaced from each other by an angle of around 4°, providing an overall angle of rotation of around 8°. It will however be appreciated that in other examples, the gooseneck  114  could be configured to be rotated to a greater or lesser extent, as required. The gooseneck may also be configured to be locked in one or more of the rotational positions. 
         [0078]    The system is configured for use by first installing the injector  15  on the derrick. A gooseneck is selected and fitted to the injector with an appropriate curvature for the particular tubing to be used so as to facilitate operation within the elastic bend radius limits, and taking account of the space available on deck. The tubing is passed through the injector heads, and held by the injector head  15  on the derrick, whilst the injector head  20  on the reel is configured to freewheel to allow tubing to be freely moved through. The injector head  15  is moved to its highest elevation above the deck, causing the tubing to be gradually spooled off the reel  5  to accommodate the change in elevation. The tubing spooled out is thereby pulled taught from the injector head  15 . The injector heads  15 ,  20  are engaged so that the chain belts  22  clamp onto the tubing and grip the tubing and hold it in place. The reel injector  20  of the levelwind head  9  is then run, typically on its own against the back tensioned reel  5 , to pay out slack from the reel, so that the tubing portion that is paid out is within the elastic bend radius limits for all elevations of the injector head  15  relative to injector head  20 . The paid out tubing portion  1   b  forms a lazy loop of tubing between the gooseneck  14 ,  114  and the reel injector head  20  and ensures that the tubing  1  will remain within the elastic bend radius limits of the tubing for operation. The tension in the tubing that is passed between the injector heads is typically monitored using a load pin provided on the gooseneck  14 , 114 , to help check that it is not being overstressed, for example beyond the elastic deformation limits of the tubing. 
         [0079]    In use, the tubing  1  is fed from the reel  5  through the injector head  20 , to the gooseneck  14 ,  114  and into the main derrick injector head  15 . The reel  5  is driven by the hydraulic motor  7  in reverse to put a back tension on the reel tending to rotate it anticlockwise as shown in the figures. The injector head  20  grips the tubing  1  between the belts  22  and the motor of the injector head  20  exerts a force on the tubing  1  tending to pull it off the reel and feed it to the main derrick injector head  15 . The forces between the motor on the reel  5  and the motor on the sprockets  21  are balanced, so that the portion of the tubing  1   a  between the reel and the injector head  20  is held in tension, and is substantially static when the tubing portion  1   c  is not being inserted into or recovered from the well. 
         [0080]    When the tubing is being recovered form the well the injector heads  15  and  20  are driven in the opposite directions, and the tubing  1   c  is pulled from the well. The two injector heads  15  and  20  are again synchronised by the synchronisation mechanism, so that the length of the tubing portion  1   b  is preferably maintained substantially constant by driving the two injector heads  15 , at the same speed, thereby maintaining the bend radius in the portion  1   b  within acceptable limits. The reel  5  is again back tensioned, to keep the tubing portion  1   a  tight between the reel  5  and the levelwind injector head  20 , thereby preventing any cyclic bending of the tubing  1  in the portion  1   a.    
         [0081]    Modifications and improvements can be incorporated without departing from the scope of the invention.