Abstract:
A device for applying tension to a string of coiled tubing for the purpose of its insertion into or withdrawal from a wellbore. Specifically, the device is used with a wheel-type tensioner, which contacts and supports the tubing over an arc length of less than 180°. The device enhances the tractive ability of the tensioner by applying forces normal to the tubing to press it into more intimate contact with the surface of the wheel, thereby increasing the normal force and attendant frictional force between the tubing and the wheel.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a method and apparatus for applying tension to a string of coiled tubing during its injection into and withdrawal from a well bore. More particularly, the invention relates to a device used with a wheel-type tensioner, which contact and supports the tubing over an arc length of less than 180°. 
   2. Description of the Related Art 
   Currently wheel-type tensioners are commonly used with coiled tubing employed in servicing petroleum wells. For such devices, the tubing is stored on a large reel and is then passed over a tensioning wheel before entering the wellbore. The tubing only contacts the wheel of the tensioner over an arc length of less than 180° and typically less than 90°. 
   The wheel provides a turning function for the tubing so that it can be routed between the well bore and the storage reel, but the wheel can also provide tensioning capability if motive torque is applied. However, when the tubing must be forced into the well through a sealing gland and against well pressure or when the tubing must be withdrawn from the well, the ability of the wheel to apply motive tension to the tubing is limited. 
   The tension which can be applied by the simple wheel tensioner is controlled by the length of arc contact, the tubing tensions entering and leaving contact with the wheel, and the coefficient of friction of the tubing with the wheel. These limitations are particularly important when only a short length and weight of tubing are in the well. 
   A need exists for a means of enhancing the radial contact forces and resultant frictional forces between the tubing and the wheel of the tensioner. A further need exists for a simplified and cost-effective means for enhancing the radial contact forces and resultant frictional forces between the tubing and the wheel of the tensioner. 
   SUMMARY OF THE INVENTION 
   This invention pertains to a device for applying tension to a string of coiled tubing for the purpose of its insertion into or withdrawal from a wellbore. Specifically, the device is used with a wheel-type tensioner, which contacts and supports the tubing over an arc length of less than 180°. The device enhances the tractive ability of the tensioner by applying forces normal to the tubing to press it into more intimate contact with the surface of the wheel, thereby increasing the normal force and attendant frictional force between the tubing and the wheel. 
   One embodiment of the present invention includes a wheel tensioner assembly for applying tension to a string of coiled tubing during its injection into and withdrawal from a wellbore, the wheel tensioner assembly comprising: a selectably rotatable tensioner wheel having a circumferential contact surface for interacting with a first side of the coiled tubing; a holddown device having a plurality of contact rollers that interact with a second side of the coiled tubing; and a biasing mechanism for urging said contact rollers radially inwardly to bear against the second side of the coiled tubing and press the coiled tubing against the circumferential contact surface of the tensioner wheel. 
   A second embodiment of the present invention includes a wheel tensioner assembly for applying tension to a string of coiled tubing during its injection into and withdrawal from a wellbore, the wheel tensioner comprising: a tensioner wheel having a circumferential contact surface for interacting with a first side of the coiled tubing; a rotation mechanism for selectably applying torque to the tensioner wheel; a holddown device attached to the tensioner wheel, the holddown device having a plurality of contact rollers positioned radially outward of the circumferential contact surface, wherein the contact rollers interact with a second side of the coiled tubing; and means for urging the contact rollers radially inwardly to apply a force normal to the second side of the coiled tubing. 
   Another embodiment of the present invention includes a wheel tensioner assembly for applying tension to a string of coiled tubing during its injection into and withdrawal from a wellbore, the wheel tensioner comprising: a tensioner wheel rotatably mounted on a wheel pedestal, wherein the tensioner wheel has a rim, a shaft, a wheel sprocket mounted on the shaft, and a circumferential arcuate contact surface for interacting with a first side of the coiled tubing; a tensioner drive motor; a tensioner drive chain positioned about a circumference of the wheel sprocket and driven by the tensioner drive motor, wherein the drive chain causes the tensioner wheel to rotate whenever the drive chain is driven by the drive motor; a holddown device attached to the shaft of the tensioner wheel, the holddown device having a plurality of roller mounting structures, wherein each roller mounting structure mounts a contact roller and positions the contact roller radially outward of the circumferential contact surface; and a biasing mechanism for urging said contact rollers radially inwardly to bear against a second side of the coiled tubing and press the coiled tubing against the circumferential contact surface of the tensioner wheel. 
   Yet another embodiment of the present invention includes a wheel tensioner assembly for applying tension to a string of coiled tubing during its injection into and withdrawal from a wellbore, the wheel tensioner comprising: (a) a selectably rotatable tensioner wheel having an exterior circumferential contact surface for interacting with a first side of the coiled tubing; and (b) a holddown device attached to a shaft of the tensioner wheel, the holddown device comprising a plurality of roller mounting structures, wherein each roller mounting structure mounts a contact roller and positions the contact roller radially outward of the circumferential contact surface and wherein a first end sheave is mounted on each roller mounting structure on a first side of the contact roller and a second end sheave is mounted on an opposed side of each contact roller; and a selectably engaged biasing mechanism for urging said contact rollers radially inwardly to bear against a second side of the coiled tubing and press the coiled tubing against the circumferential contact surface of the tensioner wheel, wherein the biasing mechanism includes a first tensioning cable in communication with the first end sheave and a second tensioning cable in communication with the second end sheave. 
   The foregoing has outlined rather broadly several aspects of the present invention in order that the detailed description of the invention that follows may be better understood and thus is not intended to narrow or limit in any manner the appended claims which define the invention. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and the specific embodiment disclosed may be readily utilized as a basis for modifying or designing of the structures for carrying out the same purposes as the invention. It should be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
       FIG. 1  is an oblique view of a coiled tubing unit employing the tension enhancement means of the present invention. 
       FIG. 2  is a left side profile view of the coiled tubing unit of  FIG. 1 . 
       FIG. 3  is an oblique right side view of the coiled tubing unit of  FIG. 1 . 
       FIG. 4  is an oblique rear view of the coiled tubing unit of  FIG. 1 . 
       FIG. 5  is a left side profile view of the wheel tensioner employing the tension enhancement means of the present invention. 
       FIG. 6  shows an oblique rear view of the wheel tensioner of  FIG. 5 . 
       FIG. 7  is an exploded oblique rear view of the wheel tensioner of  FIGS. 5 and 6 . 
       FIG. 8  is an oblique view of the tension enhancement means of the present invention showing its interior side which contacts the tubing. 
       FIG. 9  is an oblique view of the frame of the tension enhancement means, with the mounting studs for its static sheaves shown. 
       FIG. 10  is an oblique view of the movable portions of the tension enhancement means and the static sheaves, with the components shown in their working positions. 
       FIG. 11  is an oblique view of a radial forcing unit of the tension enhancement means. 
       FIG. 12  is an exploded view of the radial forcing unit of  FIG. 11 . 
       FIG. 13  is a transverse sectional view of the radial forcing unit of  FIG. 11 , with the section being taken on a radial plane for the tensioner wheel. 
       FIG. 14  is a side profile view of the tensioning cable showing its geometry when engaged with both the static sheaves and the sheaves of the radial forcing units of the tension enhancement means. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   As a note, the use of the terms “invention”, “present invention” and variations thereof throughout the subject patent application (and headings therein) are intended to refer or relate to one or more embodiments of the present application, not necessarily every embodiment or claim of the application. 
   Referring now to the drawings, it is noted that like reference characters designate like or similar parts throughout the drawings. The figures, or drawings, are not intended to be to scale. For example, purely for the sake of greater clarity in the drawings, wall thicknesses and spacings are not dimensioned as they actually exist in the assembled embodiments. 
   The materials of construction of the structural components of the trailer, the storage reel, the wheel tensioner, and the tension enhancement device of the present invention are generally steel, but other materials may be used in certain cases. The support pads for the tensioner wheel are also typically made of steel, but other materials, such as high durometer polyurethane, can be substituted. Bearings used for the device will generally be ball, cylindrical, or spherical roller bearings. 
   Coiled Tubing Unit 
   Referring to  FIGS. 1 through 4 , the arrangement of the coiled tubing unit  10  employing the wheel tensioner  60  and its integral tension enhancement means of the present invention are shown in various views. The coiled tubing unit  10  typically is trucked to a well location and parked on the ground surface aligned with a well  11 . The well  11  generally is an oil or gas or injection well, but it could also be a water well. The well  11  is cased with one or more strings of tubular casing and normally can contain some internal pressure. The well  11  may also have tubing in place substantially coaxial with the casing. 
   When the well  11  is being serviced by the coiled tubing unit  10 , a set of one or more blowout preventers  13  and a pressure-containing stripping gland  14  are mounted on the upper end of the well, with the gland mounted above the blowout preventers. The stripping gland  14  is of conventional oilfield construction, and has a steel housing and an annular rubber element mounted therein. The rubber element of stripping gland  14  is biased to bear against the outer cylindrical surface of any tubing  16  inserted and axially reciprocated through its bore. 
   The coiled tubing unit  10  has a storage reel  30  and the wheel tensioner  60  mounted on the deck  23  of a trailer  20 . The wheel tensioner  60  is at the rear end of the trailer  20 , and the storage reel  30  is near the front end. Trailer  20  is supported at its front end by selectably vertically extensible and lockable jacks  21  when it is not supported by the fifth wheel of a truck (not shown). At its rear end, trailer  20  is supported by two spring mounted axles  22  with tires. Although it is not shown in the figures, supplementary vertically extensible and lockable jacks may also be used to support the rear end of trailer  20  on the ground  12  so that it is more rigidly mounted when high coiled tubing tensions are to be expected. 
   A unitized power unit  24 , including an engine and a hydraulic pump with a reservoir, is mounted at the forward end of deck  23  of the trailer  20 . The hydraulic power from the pump of the power unit  24  is used to drive the hydraulic motors  40  and  80 , which respectively drive the storage reel  30  and the wheel tensioner  60 , and the level wind hydraulic cylinder  26 . For clarity, the controls and hydraulic connections are omitted from the drawings herein, but are well understood by those skilled in the art of oilfield equipment. 
   Storage Reel 
   The storage reel  30  is translatable laterally across the deck  23  of the trailer  20  in order to provide levelwinding of the tubing  16  stored on the reel. Right circular cylindrical rods  25  extending athwart the trailer  20  and mounted at their distal ends by vertical plates attached to the trailer deck  23  serve as levelwind guide rails for the storage reel  30 . A transversely mounted levelwind hydraulic actuation cylinder  26  is attached to the deck  23  of the trailer  20 . The rod of cylinder  26  has a rod end attachment  27  which in turn is attached to the base  37  of the reel pedestal  36  of the storage reel  30 . 
   The storage reel unit  30  consists of a reel  31 , a reel pedestal  36 , and rotational drive means. The reel  31  is symmetrical about its axis of rotation and has a hollow right circular plate core upon which the end of the coiled tubing string  16  is anchored. The core (not shown) provides a surface upon which the base layer of the tubing  16  can be levelwound. 
   At the transverse ends of the reel core are located two opposed reel flanges  32  having planar inner faces. The reel flanges  32  have circumferential rims and may be fitted with radial stiffeners, although that is not shown herein. The reel flanges extend inwardly to where they are welded to a right circular cylindrical reel shaft  33 . The reel shaft  33  extends outwardly of the reel flanges  32  and has a large fixedly mounted concentric driven chain sprocket  34  on one side. The width of the reel  31  is usually slightly less than half of the width of the trailer  20 , so that laterally translating the reel by levelwinding permits the tubing to be controllably wound on the reel core over its entire width. 
   The reel pedestal  36  serves to support the reel  31 . The reel pedestal  36  consists of a horizontal base  37  with two opposed parallel antisymmetrical trapezoidal vertical side buttresses extending upwardly at its sides. The gap between the side buttresses is sufficient to accommodate the reel  31 . Coaxial holes penetrate each of the side buttresses perpendicular to the side buttresses in two places and serve to journal the levelwind guide rails  25 . 
   The reel pedestal  36  is mounted so that the side buttresses extend parallel to the sides of the trailer  20 . The upper horizontal surface of each of the trapezoidal side buttresses serves as a mounting surface for a pillow block  38  with an integral roller bearing. The pillow blocks  38  journal the reel shaft  33  of the reel  31  and are attached to the reel pedestal  36  by mounting bolts  39 . Selectably extending and retracting the rod of the hydraulic levelwind actuation cylinder  26  causes the storage reel unit  30  to traverse the guide rails  25  in order to effect levelwinding of the tubing  16  on the reel  31 . 
   The reel  31  is caused to rotate by a rotary hydraulic reel drive motor  40  attached to a reel drive motor mount  41 , which is in turn mounted to the reel pedestal base  37 . The shaft of the reel drive motor  40  is perpendicular to the side of the trailer  20  and mounts a reel drive chain sprocket  43 . The reel drive chain sprocket  43  is coplanar with the reel driven sprocket  34  mounted on the reel  31 . A heavy duty reel drive roller chain  42  is looped around and engaged with both the reel drive sprocket  43  and the reel driven sprocket  34 . This permits rotation of the reel drive motor  40  to be transmitted to the reel  31 . The levelwinding action of the levelwind cylinder  26  for the storage reel unit  30  is coordinated with the reel rotation either manually or with automatic controls. 
   Wheel Tensioner 
   The wheel tensioner  60  for the coiled tubing unit  10  mounted on the trailer  20  is shown assembled in  FIGS. 5 and 6 , while  FIG. 7  shows an exploded view of the wheel tensioner. The wheel tensioner  60  consists of a wheel subassembly  61 , a pedestal subassembly  72 , rotational drive means, and a tubing holddown device  90 . The wheel tensioner  60  has its narrow wheel  61  subassembly supported on its wheel pedestal  72 , with the transverse midplane of the wheel subassembly fixedly located on the longitudinal centerline of the trailer  20 . 
   The wheel tensioner  60  is positioned so that the coiled tubing  16  can pass smoothly between the wheel  61  and the storage reel  30 , making tangential contact with both. The wheel tensioner is also positioned so that the coiled tubing  16  can pass vertically between the wheel tensioner  60  and the pressure retaining gland  14  mounted on the blowout preventers  13  and the wellhead  11 . In order to provide room for the blowout preventers  13 , the tangential entry/exit point on the rear side of the tensioner wheel  60  is positioned to the rear of the trailer  20 . The tubing holddown device  90 , which does not rotate and serves as a tension enhancement device for the wheel tensioner  60 , is mounted directly to the shaft  64  of the wheel  61  of the wheel tensioner  60 . 
   The wheel subassembly  61  is symmetrical about its axis of rotation and has a U-shaped cross-section wheel rim  62  with the U opening radially outwardly and having small outwardly extending circumferential outer flanges. The wheel rim  62  is symmetrical about the transverse midplane of the wheel  61 . 
   An axially symmetric regularly spaced pattern of rim holes  65  are positioned offset from and parallel to the wheel axis so that they coaxially penetrate the transverse sides of the U of the wheel rim  62 . The rim holes  65  permit the mounting of tubing support blocks  69 . At the opposed outer transverse sides of the U of the rim  62  are located two flat circular annular plate wheel side plates  63 , which are welded to the sides of the rim  62 . The side plates  63  have a regularly spaced pattern of large lightening holes cut between their inner bore and outer periphery. The side plates may be fitted with radial stiffeners, although that is not shown herein. 
   A right circular cylindrical elongated wheel shaft  64  is closely fitted coaxially and welded to the center hole in each of the side plates  63 . The wheel shaft  64  extends axially farther outwardly on one side than the other in order to accommodate the attachment of the driven chain sprocket  66  for the wheel subassembly. 
   A set of arcuate tubing support blocks  69  have a close fit to the outwardly opening U of the wheel rim  62 . A tubing support block  69  has a semicircular outwardly opening annular tubing support groove  70  cut in its outer cylindrical face so that it can provide a loose fit to the tubing  16  to be accommodated by the wheel tensioner  60 . Each tubing support block  69  has one or more holes positioned offset from and parallel to the axis of symmetry of the block so that they can be mated with the rim holes  65  of the wheel rim  62  when the block is nested in the U of the wheel rim. A bolt and nut set  71  is then engaged through each rim hole  65  to retain the set of tubing support blocks  69 . 
   The wheel pedestal  72  serves to support the wheel  61 . The wheel pedestal  72  consists of a horizontal base with two opposed parallel antisymmetrical trapezoidal vertical side buttresses extending upwardly at its sides. The gap between the side buttresses is sufficient to accommodate the wheel  61 , the holddown device  90 , and the driven wheel sprocket  66  attached to the wheel shaft  64 . 
   The wheel pedestal  72  is mounted so that the side buttresses extend parallel to the sides of the trailer  20 . The upper horizontal surface of each of the trapezoidal side buttresses  73  serves as a mounting surface  74  for a pillow block  77  with an integral roller bearing. The pillow blocks  77  journal the wheel shaft  64  of the wheel  61  and are attached to the wheel pedestal  72  by pillow block mounting bolts  78  engaged in drilled and tapped bearing mounting holes  75  in the mounting surfaces  74 . On the rearward inclined faces of each of the side buttresses  73  of the wheel pedestal  72  are located a regular pattern of drilled and tapped holes  76  which serve to mount a knee  120  to each of the buttresses. 
   The wheel  61  is caused to rotate by a rotary hydraulic tensioner drive motor  80  attached to a tensioner drive motor mount  81  mounted to a horizontal tensioner motor mount spacer  82 , which is in turn mounted to the deck  23  of the trailer  20  forward of the wheel assembly  60 . The shaft of the tensioner drive motor  80  is perpendicular to the side of the trailer  20  and mounts a tensioner drive chain sprocket  84 . The tensioner drive chain sprocket  84  is coplanar with the wheel driven sprocket  66  mounted on the wheel  61 . A heavy duty tensioner wheel drive roller chain  83  is looped around and engaged with both the tensioner drive sprocket  84  and the wheel driven sprocket  66 . This permits rotation of the tensioner drive motor  80  to be transmitted to the wheel  61 . 
   Holddown Device 
   The holddown device  90  and its constituent components are shown in  FIGS. 7 through 14 . The holddown device functions to cause the coiled tubing passing over the tensioner wheel  61  to be more forcefully pressed against the annular groove of the tubing support blocks  69 . This additional radially inward force thereby enhances the possible magnitude of the circumferential frictional shear forces developable between the support blocks  69  and the tubing  16 , in turn permitting higher forces to be applied to the tubing going into or emerging from the pressure retaining gland  14 . 
   The holddown device  90  consists of two interconnected mirror image side frames  91  mounting a series of pulldown roller assemblies  140  and means for biasing the pulldown roller assemblies radially inwardly towards the rotational axis of the wheel tensioner. The holddown device  90  has one of its side frames on each side of the wheel  61  of the wheel tensioner  60 . The pulldown roller assemblies  140  are set radially outwardly from the contact blocks of the wheel  61 . The holddown device does not rotate, but is journaled about the wheel shaft  64  of the wheel  61  of the wheel tensioner  60 . 
   Referring to  FIG. 9 , the interconnected side frames  91  of the holddown device  90  can be seen without most of the other constituent items of the holddown device. A side frame  91  is constructed with a core element which is a circularly arcuate segment of radially extending flat side plate  92 . The side plate  92  has welded onto its outer side multiple equispaced radially extending transverse plate ribs  93 . The radial ribs  93  extend between a transverse circumferential stiffening plate rim  94  welded onto the outer side of the side plate  92  and a hub  95 . Two drilled and tapped holes are provided approximately midway on the length of the radial rib which is on the radial side of the side plate  92  opposed to the tangential extension  98  of the side plate. 
   The hub  95  is a thick wall axially short right circular cylindrical sleeve concentric with the center axis of the circularly arcuate side plate  92  and attached to the side plate so that it extends outwardly from the inner face of the side plate. A plain sleeve bearing  97  is mounted by being pressed into the bore  96  of the hub  95  of the side plate  91 . The bore of the sleeve bearing  97  is a rotational close slip fit to the cylindrical shaft  64  of the wheel subassembly  61 . 
   Extending coplanarly with the side plate  92  on one side is a tangential plate extension  98 . Tangential extension  98  has one edge which is tangent to the outer arcuate face of the side plate  92  and typically has a radial thickness of approximately 25% of the radial thickness of the side plate. The projection of the tangential extension  98  is approximately twice its radial thickness. Drilled and tapped holes  99  are located in the tangential extension  98  so that the tension cylinder body  116  can be attached by machine screws  181 . Tangential extension  98  is provided with a narrow transverse interior stiffener rib  100  mounted on its interior side surface so that it is parallel to the tangent outer edge of the extension. The transverse stiffener plate  100  is provided with two drilled and tapped holes to permit attachment of an inlet end cross tie plate  109  by means of a pair of tie plate attachment screws  110 . 
   Lapped onto the interior face of each side frame  91  are an array of multiple trapezoidal radial guide plates  104  and an array of end radial guide plates  106 . The guide plates  104  are used in the center of the array, while the guide plates  106  are used on the ends of the array. The guide plates  104  and  106  are configured so that when they are installed by welding onto a side plate  92  in a regularly spaced pattern, their adjacent inclined trapezoidal sides are parallel and equispaced. The width between adjacent plates  104  and between adjacent plates  104  and  106  is slightly more than the width between the outside of the cross plates  141  of the pulldown roller assemblies  140 , thereby permitting the pulldown roller assemblies to have a slip fit in the resulting radial slots. 
   Multiple drilled and tapped sheave mounting stud mounting holes  105  are regularly spaced so that they are perpendicular to and penetrate both the side plate  92  and the guide plates  104 . The sheave mounting stud mounting holes  105  penetrate the guide plates  104  on their radial midplanes and serve to mount the threadedly engaged static sheave mounting studs  112 . End guide plates  106  are coped so that the end sheave stud mounting holes only penetrate the side plate  92 . One extra hole  105  is provided on the side of the side plate  92  opposed to the tangential extension  98 . This extra hole serves as a mounting for a cable end anchor stud  113 . 
   A radially oriented and inwardly extending radial tab  107  having a pair of centrally located drilled and tapped holes is positioned adjacent the outer periphery of the side plate  92  on the radial edge of the side plate opposed to the edge with the tangential extension  98 . Rectangular outlet end cross tie plate  108  and inlet end cross tie plate  109  are respectively attached to the tabs  107  and the interior stiffening ribs  100  of the two opposed side plates  92  by tie plate attachment screws  110 , thereby spacing apart the mirror-image side plates so that their substantially flat interior sides are opposed. 
   The static sheave mounting studs  112  and the cable end anchor studs  113  are shoulder screws having upset heads, right circular cylindrical shanks, and reduced diameter threaded sections. The static sheave mounting studs  112  mount rotatable small static sheaves  180  on their cylindrical shanks and are threadedly engaged with the sheave stud mounting holes  105  so that the sheaves are mounted on the exterior side of the side plates  92 . The static sheaves  180  are relatively thin grooved sheaves which are sized for use with the tension cable  170 . The static sheaves can rotate, but they cannot translate. Although it is not shown herein, the static sheaves  180  may be provided with either plain bearings or needle bearings to reduce their rotational friction. 
   A cable end anchor stud  113  is threadedly engaged with each of the sheave stud mounting holes  105  which are adjacent the radial edge opposed to the tangential extension  98  side of each side plate  92 . The cable end anchor stud  113  is first engaged with an anchored end eye  172  of a tension cable  170  before it is inserted into its hole  105  from the outside. 
   The body  116  of a double-acting single end hydraulic tension cylinder is threadedly mounted on the outside of each tangential extension  98  of a side plate by machine screws  181  engaged through tension cylinder mounting holes  99 . The rod end of the cylinder body  116  is oriented so that the rod is tangential to the center of the groove of the closest static sheave  180  on the side of the sheave closest to the hub  95  of the side plate  92 . The rod  117  of the tension cylinder is selectably reciprocable within the body  116  of the cylinder and has a distal clevis fitting  118  having a clevis pin  119 . The clevis pin  119  is engaged with a cylinder end eye  171  of a tension cable  170 . 
   The cylinders  116  are provided with hydraulic fluid by utilizing output from the power unit  24  on the trailer  20 . The cylinders are mounted in a parallel hydraulic circuit and are controlled by the same directional valve (not shown). To apply tension to the cable attached to each cylinder, the rod end of each cylinder body  116  is pressurized. To release tension, the pressure on the rod end is vented. 
   A knee  120  is provided to extend between a rear inclined side of the tension wheel pedestal  72  and the radial rib  93  on the side opposed to the tangential extension  98  of each side frame  91 . The knee  120  is constructed of a rectangular cross-section segment of structural tube having rectangular end plates which are inclined relative one plane of symmetry of the tube. One end plate projects outwardly beyond a first side of the tube, and the other end plate projects outwardly beyond the second side of the tube. 
   When the holddown device  90  is installed on the shaft  64  of the tension wheel, the radial ribs  93  which are on the side of the side frames  91  opposed to the tangential extensions  98  of the side plates  92  are maintained in a horizontal position by the knees  120 . The two knees  120  are almost antisymmetrical, but the one on the lefthand side of the trailer  20  extends farther outwardly on its lower end and is strengthened by a gusset. This dissymmetry is to accommodate the larger distance of the lefthand wheel pedestal side buttress  73  from the centerline of wheel  61  due to the presence of the driven sprocket  66  on the wheel shaft  64 . Each knee  120  is attached by knee attachment screws  121  both to the rearward inclined face of a side wall  73  of the wheel pedestal  72  at knee mounting holes  76  and to a horizontal rib  93  of a side frame  91  at knee mounting holes  114 . This prevents the side frame  91  from rotating relative to the pedestal  72 , while the wheel  61  of the wheel tensioner  60  can still rotate. 
   The pulldown roller assembly  140  is shown in  FIGS. 11 ,  12 , and  13 . The pulldown roller assemblies  140  are reciprocable in the gaps between opposed pairs of either radial guide plates  104  or between a radial guide plate  104  and an end radial guide plate  106  on opposed inner faces of the side frames  91  of the holddown device  90 . Because of the alignment of the guide plates  104  and  106 , any reciprocation of the pulldown roller assemblies  140  will be radial with respect to the wheel  61  of the wheel tensioner  60 . 
   The frame of a pulldown roller assembly  140  is assembled by welding from a pair of cross plates  141 , a pair of middle radial plates  142 , a pair of end radial plates, and a cap plate  144 . The cross plate  141  has a wide tee shape, with the stem of the tee being relatively very wide and the cross bar of the tee having a height of approximately one quarter of the overall plate height. The width of the cross plate  141  is somewhat less than the gap between the side frames  91  of the holddown device  90 . The middle radial plate  142  is an elongated rectangle plate having rounded lower corners and two vertically spaced apart holes  148  and  149  located in its vertical midplane and penetrating through the plate thickness. The middle radial plate  142  is longer than the cross plate  141  is tall. The upper hole is the sheave shaft hole  148 , while the lower hole is the tubing roller shaft hole. 
   The end radial plate  143  is a rectangular plate having rounded lower corners and a single sheave shaft hole  147  located on its vertical midplane and penetrating through the plate thickness. Holes  147  and  148  have the same diameter. The width of the end radial plate  143  is equal to the width of the middle radial plate  142  plus twice the thickness of the cross plate  141 . The cap plate  144  is an elongated rectangular plate which has a length equal to the width of the cross plate  141  plus twice the thickness of the radial end plate  143 . The width of the cap plate  144  is the same as that of the radial end plate  143 . The length of the cap plate is such that it has a close slip fit to the gap between the inner sides of the radial side frames  91  of the holddown device  90 . 
   The pulldown roller assembly  140  has two transverse planes of symmetry. The cross plates  141 , the middle radial plates  142 , and the end radial plates  143  are all mounted with their top edges flush so that they can be abutted against and welded to the cap plate  144 . The middle radial plates  142  are spaced apart perpendicular to the flats of the plates by slightly more than the width of the tubing roller  160 . For the middle radial plates  142 , the planes of their plates are normal to both the cap plate  144  and the cross plates  141 . The end radial plates  143  are parallel and mounted at the outer ends of the cap plate so that the planes of their plates are perpendicular to both the cap plate  144  and the cross plates  141 . The ends of the tees of the cross plates  141  are welded to the inner faces of the end radial plates  143  after they are abutted onto the long sides of the middle radial plates  142 . The cross plates  141  are also welded to the middle radial plates  142  where they abut. 
   When assembled, the sheave shaft holes  148  of the middle radial plates  142  and the sheave shaft holes  147  of the end radial plates  143  are coaxial. The sheave shaft  150  is an elongated right circular cylindrical shaft which is a close slip fit to the sheave shaft holes  147  and  148 . The sheave shaft  150  is slightly longer than the cap plate  144 . A snap ring groove is located adjacent each of the ends of the sheave shaft, and Spirolox™ snap rings  155  can be inserted in to the grooves to abut the outer faces of the end radial plates  143  of the frame of the holddown device  140  and thereby retain the shaft. Two elongated right circular cylindrical sheave shaft spacer sleeves  154  are a close slip fit to the sheave shaft  150 . The end sheaves  156  are relatively thin grooved sheaves which are rotatably mounted on the sheave shaft  150 . Although it is not shown herein, the end sheaves  156  may be provided with either plain bearings or needle bearings to reduce their rotational friction. Both the end sheaves  156  and the static sheaves  180  are sized for use with the tension cable  170 . 
   When assembled, an end sheave  156  is mounted on the shaft  150  inwardly of each of the inner faces of the end radial plates  143 . A spacer sleeve  154  in mounted inboard of each of the end sheaves  156  and abutted against the outer face of its corresponding middle radial plate  142  in order to constrain the end sheaves to remain close to the end radial plates  143 . For the assembled frame of the pulldown roller assembly  140 , the two tubing roller shaft holes  149  in the opposed middle radial plates  142  are coaxial. 
   Elongated right circular cylindrical tubing roller shaft  157  is a close slip fit to the holes  149  and serves as an axle about which tubing roller  160  can rotate. Tubing roller shaft  157  has distal diametrical holes through which tubing roller shaft keeper pins  158  can be inserted in order to retain the shaft in place in its mounting bores. The tubing roller  160  is an annular right circular cylindrical tube with a semicircular groove cut at midlength to provide a tube contact face  161 . As shown herein, the bore of tubing roller  160  is a close rotating slip fit to the tubing roller shaft  157 , but as may be well understood by those skilled in the art, the tubing roller may be provided with needle bearings or other suitable types of bearing in order to improve roller operation. 
   The resulting structure of the pulldown roller assembly  140  is rigid and able to transfer forces from the end sheaves  156  to the tubing roller  160  and thence to the wall of the coiled tubing  16  which the tubing roller may contact. As mentioned previously, each of the pulldown roller assemblies  140  is inserted into a parallel sided slot formed on the interior side of a side frame  91  between either an adjacent pair of radial guide plates  104  or between an adjacent radial guide plate  104  and an end radial guide plate  106 . Because the side frames  91  are parallel and are mirror images, each pulldown roller assembly  140  is then restrained in the corresponding slot on the opposed side frame  91 . Because the length of the pulldown roller assembly  140  is a slip fit between the parallel side frames  91 , the pulldown roller assemblies are constrained to reciprocate in a radial direction with respect to the wheel  61  of the wheel tensioner assembly  60 . The grooves  161  of the tubing rollers are coplanar with the grooves  70  of the tubing support blocks  69  in the midplane of the wheel  61  due to the symmetry about the midplane of the wheel, the holddown device  90 , and the pulldown roller assemblies  140 . 
   Referring to  FIG. 10 , the arrangement of the pulldown roller assemblies  140  relative to the static rollers  180 , the tension cable  170 , and the tension cylinder (components  116 ,  117 ,  118 , and  119 ) can be seen. The relationship of the cable  170  to the static rollers  180  and the end sheaves  156  is further clarified by reference to  FIG. 14 . Cable  170  is provided with a cylinder end eye  171  at its first end and an anchor end eye  172  at its second end. On each half of the holddown device  90 , the cylinder end eye  171  is engaged with the tension cylinder clevis pin  119 , while the anchor end eye  172  is engaged with the cable end anchor stud  113 . The cable  170  extends from the cylinder clevis pin  119  to the nearest static sheave, which it contacts over an approximately 90° arc and then is looped in an alternating pattern of approximately 180° loops  174  and  173  around the end sheaves  156  and the static sheaves  180 . After passing over the last end sheave  156  of the pulldown roller assembly  140  farthest from the cylinder, the cable anchor end eye  172  is anchored by the cable end anchor stud  113 . 
   Cable  170  acts on each of the mirror image end sheaves  156  to apply a radial load to each individual end sheave  156  of about twice the local tension in the cable. Thus, the overall radial load on an individual pulldown roller assembly  140  is slightly less than four times the tension applied to an individual cable since substantially the same pressure is applied to each tension cable  170 . Approximately the same tension is applied through the cylinder rod  117  of each cylinder  116 , so that the tension applied to the cylinder end eye  171  of the two tension cables  170  is about the same. This approximate equality of force applied to the each end sheave  156  is necessary to avoid cocking the pulldown roller assembly  140  between the radial plates  92  of the opposed side frames  91 . 
   OPERATION OF THE INVENTION 
   When the coiled tubing  16  is being inserted into or being withdrawn from the well, high tractive forces must be applied to the tubing by a combination of the storage reel  30  and the wheel tensioner  60 . It is highly desirable to avoid having the storage reel  30  provide much of the tractive force to the tubing, since this increases the crushing force on the reel hub and also tends to damage the tubing where the wraps cross over each other. 
   For a wheel tensioner without the holddown device, having some tubing tension on both sides of the wheel tensioner is necessary in order to develop frictional forces between the wheel tensioner and the tubing. In such a case, higher tubing tension on the side of the wheel tensioner where the tube is incoming markedly enhances the tractive forces which may be applied to the tubing by the wheel. However, in actual coiled tubing operations, it is often necessary to force the tubing  16  to enter the well against pressure retained by the pressure retaining gland  14 . A wheel tensioner without a holddown device in such a case does not function well. Provision of a holddown device for a wheel tensioner overcomes this operational difficulty. 
   The holddown device  90  functions by causing an arcuate array of tubing rollers  160  to press radially inwardly against tubing  16  deployed in the grooved tubing support blocks  69 , thereby inducing an increased reactive normal force of the tubing support blocks against the tubing. The increased normal force between the tubing  16  and the tubing support blocks  69  of the wheel tensioner  60  increases the amount of static frictional force which can be developed between the support blocks and the tubing. 
   Tension is induced in the tubing  16  passing over the wheel tensioner  60  by these static frictional forces. Neglecting frictional and material cyclical hysteric losses in the bearings  78  and support blocks of the wheel tensioner  60  and the rollers  160  and their rotational mountings in the holddown device  90 , the torque applied to the tensioner wheel  60  is expressed at the contact zone between the tubing  16  and the support blocks  69  as the product of the change of tension in the tubing  16  and the radius of the tubing over the tensioner wheel. 
   The tubing rollers  160  of the holddown device  90  are each mounted in an individual symmetrical pulldown roller assembly  140 . The radially inward forces applied to the pulldown roller assemblies  140  are applied by a pair of tensioned cables  170 , one of which is looped around and acts on each of the two end sheaves  156  located on the distal end of each of the pulldown roller assemblies proximal the two end plates  143 . The radial force on each end sheave  156  is equal to the radial inward component of the vector sum of the tensions of the cable  170  on the two sides of the unsupported cable tangent to that end sheave. The overall radial inward force on each pulldown roller assembly  140  and hence its tubing roller  160  is the sum of the radial inward forces on the two end sheaves  156  of the assembly. 
   To insert the tubing  16  between the tubing support blocks  69  of the wheel  61  and the tubing rollers  160  of the holddown device  90  of the wheel tensioner  60 , the rod  117  of the tension cylinder body  116  is extended so that the cables  170  are not tensioned. This permits the end of the tubing to displace the pulldown roller assemblies  140  radially outwardly, thereby permitting passage of the tubing. 
   In order to enhance the tractive forces applicable to the tubing  16  by the tensioner wheel  61 , the rod  117  of the tension cylinder body  116  is retracted sufficiently to cause the tubing rollers  160  of the pulldown roller assemblies  140  to contact and bear on the tubing. Increasing the rod end pressure on the retracting cylinder increases the tension in the cable  170 , causing each pulldown roller assembly  140  to be forcibly urged into contact by the combination of twice the local cable tensions around each of the end sheaves  156 . This contact force of the tubing rollers  156  on the tubing  116  in turn enhances the total contact force between the tubing and the tubing support blocks  69 . It is noted herein that the tubing rollers  160  do not exert tractive forces on the tubing  16  except for substantially negligible rotating frictional resistances. 
   While the axial tension in the tubing  16  adjacent the zone of contact of the tubing roller  160  with the tubing strongly impacts in an additive way the local radial contact load between the tubing and the support blocks  69 , in most cases it is necessary to have the holddown forces applied by the cables  170  and the pulldown roller assemblies  140  in order to produce sufficient tractive force on the tubing from its contact with the wheel tensioner  61 . The tractive force which may be applied to the tubing  16  is equal to the integral over the contact arc length of the product of the contact force multiplied by the coefficient of friction between the tubing support block  69  and the tubing. Since only the tensioner wheel  61  is rotationally powered, only it can transfer tension into the tubing  16  by frictional shear forces. 
   The reeving of the tension cable  170  in alternating loops  173  and  174  over the static sheaves  180  and the end sheaves  156 , respectively, along with the ability of the cable to be tensioned by a single cylinder provides an economical and highly efficient means of applying radially inward contact forces to the tubing  16 . These forces then increase the contact force between the tubing  16  and the tubing support blocks  69  of the tensioner wheel  61 , thereby enhancing the frictional forces applicable as tractive force to the tubing. 
   The sections of the cables  170  tangent to each of the end sheaves  156  extend approximately radially relative to the wheel tensioner  60  and the holddown device  90 . With the relatively small diameter of the end sheaves relative to the tensioner wheel  61  and the close spacing between the pulldown roller assemblies  140 , the force on each end sheave  156  is slightly less than twice the local tension in the cable. 
   The construction and assembly of the holddown device is simple and inexpensive. Assembly and servicing of the holddown device  90  is also eased by its simple construction and accessibility. Provision of good rotary bearings for both the tubing rollers  160  and the sheaves  156  and  180  greatly reduces the frictional losses of cable tension between the cylinder  116  and the cable end anchor studs  113 . 
   The configuration of various elements of the holddown device of the present invention can be varied without departing from the spirit of the invention. For example, the sheaves and tubing contact rollers can be provided with needle bearings and the structure of the side plates altered without departing from the spirit of the invention. Similarly, the combination of the holddown device and a wheel tensioner can be applied to cables or solid rods or elongate material of a variety of cross-sections and materials of construction. Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims.