Abstract:
A traveling slip assembly of a snubbing unit is supported directly through mechanical fastening of two or more hydraulic actuators to one or more load members extending from a tubular snubbing structure for transfer of snubbing loads to a wellhead. A stationary slip assembly is supported by the snubbing structure and is substantially independent from the hydraulic actuators. This arrangement ensures proper alignment of the traveling and stationary slip assemblies and the tubular snubbing structure enabling ease of assembly, precise alignment and a lighter overall structure. Preferably, the load members are part of load plate which is mechanically fastened to an outer flange on the snubbing structure, and separate therefrom, thereby being isolated from the wellbore fluids and permits the load plate&#39;s material properties to be selected without consideration of corrosion restrictions and thereby allowing a stronger and lighter load plate to be employed.

Description:
FIELD OF THE INVENTION  
       [0001]     The invention relates to snubbing units for manipulating tubing in and out of a well under pressure. More particularly, specific arrangements of the load-bearing components minimize weight, stress and exposure to well effluent.  
       BACKGROUND OF THE INVENTION  
       [0002]     Snubbing units are known in the oil and gas industry for facilitating access to a well which is under pressure including, for example, well operations such as well completions. A snubbing unit manipulates various tubular components such as pipe, tubing, and bottom hole assemblies (BHA) in and out of a well while controlling the well under pressure. Wellhead components are not conventionally equipped to deal with the manipulation of tubular components therethrough. Therefore, additional hardware in cooperation with the snubbing unit provides additional sealing and physical handling components which are required to handle tubular components, which may be either heavy tubular components which tend to fall into the well, termed “pipe heavy”, or which can be upwardly energized for ejection from the well under pressure, termed “pipe light”.  
         [0003]     Generally, a snubbing unit employs stationary (lower) and traveling (upper) slip assemblies, opposingly oriented, to releasably and controllable shift tubulars into and out of the well through a wellhead despite the possibility of either heavy tubular loads which urge the pipe to fall into well or the pressure-generated forces on the tubulars which urge the tubular out of the well. The snubbing unit is installed above an existing wellhead seal and incorporates its own seals to seal the tubulars as they are introduced or removed from the wellhead. Applicant is aware of early snubbing units such as that set forth in U.S. Pat. No. 3,096,075 to Brown, and those using similar principles, which employ hydraulic actuators to raise and lower the traveling slip assembly relative to the stationary slip assembly supported by the wellhead. Hydraulic cylinders are clamped between upper and lower head plates, secured together by a plurality of stud bolts and forming a frame to the wellhead. The stationary slip assembly is supported on the upper head plate of the frame and on the cylinders acting in part as support columns in compression. Loads on the stationary slip assembly are transferred through the frame. The base plate of the frame is a unitary member providing connection to the wellhead and for housing a stripper which sealingly engages the tubulars passing therethrough.  
         [0004]     The Brown unit and in more contemporary snubbing units are characterized by similar frame structures. A variety of challenges result, including manufacturing imprecision and variability in alignment of the hydraulic actuators and limitations in selections of materials. One dramatic result of residual stresses and misalignment of the actuators in conventional units is termed “walking of the jack” wherein the lifting actuators or jacks tend to operate independently in small alternating jerks, due to slight misalignment in the structure, imposing greater wear on components such as hydraulic actuator seals, glands and rods. This “walking”, if severe, can compromise the safety of the snubbing operation which may include failure of the slips “bite” on the tubing being manipulated.  
       SUMMARY OF THE INVENTION  
       [0005]     In one aspect, a snubbing unit is provided in which tubular manipulation loads are transferred through a snubbing structure to the wellhead without the need for an intermediary frame. The present frameless snubbing unit makes use of the structural strength, integrity, and precision inherent in hydraulic actuators to both support and manipulate the traveling slip assembly. The stationary slip assembly is supported by the snubbing structure and substantially independent from the hydraulic actuators. The snubbing unit employs fewer load bearing components which are machined and bolted together allowing the structure to be constructed in less time and with greater accuracy.  
         [0006]     In one embodiment of the invention, the snubbing loads are now transferred directly from the traveling slip assembly and through the hydraulic actuators to load members such as a load plate secured to the snubbing structure, thereby avoiding the need for a frame or intermediary structure which is subject to stress, fatigue and misalignment. Further the arrangement of the hydraulic actuators and the snubbing structure results in a stronger assembly which minimizes misalignment. Precision machining and bolted assembly minimizes misalignment and “walking” of the traveling slip assembly which reduces or eliminate stresses on the snubbing unit in use. Further, minimizing misalignment virtually eliminates the walking phenomenon and associated wear. The structure becomes much stiffer and stronger by using the hydraulic cylinders themselves as the load transfer structural components. This embodiment of a snubbing unit has a much higher safety factor in its design than the prior art style of utilizing a frame-type of structure. The unit assembly time can be reduced by about half with significant reduction in weight.  
         [0007]     The hydraulic actuators transfer the load through the load plate into the snubbing structure. The flanges on the hydraulic cylinders are mounted in a position which allows for maximum stroke with a minimum overall height of the unit and are designed to provide the best stability therefor.  
         [0008]     Further, an arrangement wherein the load plate is mechanically affixed to the snubbing structure avoids restrictions on materials of construction and thereby enables the use of lighter, stronger engineered materials. Without the need for welding, which eliminates the natural stresses inherent in a welded structure, the hydraulic cylinders, as the sole load bearing structure, remains true and square. Further, where the load plate is bolted to the structure, additional stresses and misalignment are eliminated which are inherent with welded structures. Moreover, the load plate can be located externally from the snubbing structure, thereby isolating the load plate from well effluents like H 2 S and allowing higher strength, lighter engineered materials to be used.  
         [0009]     In a broad aspect of the invention, a snubbing unit comprises: a lower connection adapted to a wellhead and a tubular snubbing structure above the lower connection and having a working bore having a common axis with the wellbore. A stationary slip assembly is supported on the tubular snubbing structure, and a traveling plate is movable independently of the tubular snubbing structure and has a traveling slip assembly supported thereon for imparting or receiving snubbing loads. One or more load members are supported by and extend laterally from the tubular snubbing structure At least two actuators are positioned parallel to the common axis and secured to the load members, each comprising a cylinder and a hydraulically-actuated rod moveable therein, each cylinder being secured intermediate its length to the one or more load members and each rod having a lower end actuable in the cylinder and an upper end secured to the traveling plate wherein, actuation of the rods moves the traveling plate and snubbing loads are transferred through the rods, to the cylinders, to the load members to the tubular snubbing structure.  
         [0010]     Preferably the load members are fastened to the snubbing structure through an outer flange isolated from the wellbore and can therefore be formed of materials, such as higher strength materials, which are selected independent of the wellbore fluid characteristics. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]      FIG. 1  is a side view of one embodiment of the snubbing unit atop a wellhead and having personnel access and safety railings illustrated thereon;  
         [0012]      FIG. 2  is a partial, side, cross-sectional view of the embodiment  FIG. 1  with the personnel access and safety railing structure removed for clarity;  
         [0013]      FIG. 3  is a perspective top-down view of the snubbing unit of  FIG. 1  with a hydraulic cylinder retainer illustrated exploded from the load plate;  
         [0014]      FIG. 4  is a perspective top-down and exploded view, in isolation from the remainder of the snubbing unit, of a pair of hydraulic actuators, the load plate extending from the load spool, and hydraulic cylinder clamps;  
         [0015]      FIG. 5  is a perspective bottom-up view of the snubbing unit of  FIG. 3 ;  
         [0016]      FIG. 6  is a view of the snubbing unit of  FIG. 4  with a separate and external mounting of the load plate with a fanciful illustration of the load plate displaced from the unit&#39;s tubular structure so as to better expose the load plate port and load spool bolting flange;  
         [0017]      FIG. 7  is a perspective top-down and exploded view, in isolation from the remainder of the snubbing unit of  FIG. 6 , the separate load plate being loose and illustrated prior to bolting to the load spool flange and with one hydraulic actuator exploded from the load plate and the other hydraulic actuator having the cylinder and rod exploded with the cylinder being displaced from both the rod, bearing the traveling plate, and the load plate;  
         [0018]      FIG. 8  is a plan view of an alternate load plate equipped for four hydraulic actuators;  
         [0019]      FIG. 9  is a broken-length, cross-sectional view of one of a pair of hydraulic actuators for raising and lowering the traveling plate; and  
         [0020]      FIGS. 10   a  and  10   b  are cross-sectional views of the snubbing unit with the traveling plate and upper slip assembly in the raised and lowered positions respectively.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0021]     With reference to  FIG. 1 , a snubbing unit  10  comprises a tubular snubbing structure  11  secured atop a wellhead  12  at a lower connection  13  such as an industry standard 3000 or 5000 psi flanged connection. The top of the wellhead  12  is represented by a conventional spool, however those of skill in the art understand that other components may exist, or could be provided as part of the snubbing unit  10 . The wellhead  12  normally blocks the wellbore of a well (not shown) under pressure. Before nippling up to the wellhead  12 , various procedures are performed as necessary to ensure site safety such as to install a plug in any tubing extending into the wellbore, and to ensure that a tubing hanger is secured in the casing bowl for sealing well fluids in the wellbore. It is understood that the term fluid used herein includes either liquid or gases.  
         [0022]     The snubbing unit  10  employs a lower, stationary slip assembly  14  and an upper, traveling slip assembly  15  for releasably and controllably shifting tubulars (not shown) along a common axis through the tubular snubbing structure  11  and the wellhead  12  below. The tubular snubbing structure  11  has a working bore which is open to communicate with the wellbore. Typically, the tubular snubbing structure  11  also incorporates a stripping annular BOP  16  for sealing about the tubulars which are manipulated therethrough.  
         [0023]     The traveling slip assembly  15  is supported on a traveling plate  21  which is raised and lowered on at least two hydraulic actuators  30 . A pair of hydraulic actuators  30  is shown, however, three actuators positioned at 120° (not shown), or four positioned at 90° (See  FIG. 7 ), or even more can be used. The hydraulic actuators  30  impart or receive snubbing loads from the traveling plate  21  to the tubular snubbing structure, and to the wellhead  12 . Flanges  28  at the top of hydraulic actuators  30  can be employed to support an operator&#39;s platform and safety railing  29 .  
         [0024]     Typically the tubular snubbing structure  11  further incorporates tongs  17  and additional operational features located between the wellhead  12  and the stationary slip assembly  14  comprising: primary well control BOP&#39;s  18 , a stripping pipe ram  19 , and a working or load spool  20 . The height of the load spool  20  is sized to provide working space between the stripping annular BOP  16  above the stripping pipe ram  19  for housing and removal of tubulars such as a BHA from the wellhead  12 .  
         [0025]     With reference also to  FIG. 2 , each of the hydraulic actuators  30  comprise a cylinder  31  and a rod  32 . In this embodiment, the cylinders  31  are spaced laterally from and parallel to the common axis of the tubular snubbing structure  11 . The cylinders  31  are mounted to one or more load members  60  which extend from the load spool  20  of the tubular snubbing structure  11 , for example, one load member  60  could extend laterally across the common axis and support two diametrically opposed and parallel hydraulic actuators  30 . Similarly, two or more load members  60  could extend, individually or as a combined structure or unitary plate, generally and radially outwards from the common axis to support a plurality of hydraulic actuators  30 . The lateral extension of the one or more load members  60  is sufficient to space the hydraulic actuators  30  from the various components along the common axis of the snubbing structure  11  including the stripping annular BOP  16  and the slip assemblies  14 , 15 .  
         [0026]     The rods  32  are mounted to the traveling plate  21 . Hydraulics raise and lower the rods  32  relative to the cylinders  31 , thereby raising and lowering the traveling plate and traveling slip assembly  21 , 15  relative to the stationary slip assembly  14 .  
         [0027]     The cylinders  31  are mounted to the load members  60  somewhere intermediate along their length, and preferably about midway therealong, thereby positioning the traveling slip assembly  15  in close proximity to the stationary slip assembly  14  when fully retracted to a lowered position. Further, such spacing enables maximum stroke with a minimum overall height of the snubbing unit  10  while maintaining suitable stability.  
         [0028]     The mounting of the hydraulic actuators  30  through the load members  60  to the tubular snubbing structure  11  minimizes the introduction of manufacturing stress and misalignment. In particular, the cylinders  31  are removeably secured through mechanical fastening, not welding, to the load members  60 . As shown, the cylinders  31  can be bolted to the load members  60  using flanges  34  or alternative clamping arrangements (not shown).  
         [0029]     Use of the hydraulic actuators  30  alone as the sole load-transferring structure for raising and lowering the traveling slip assembly  15  results in fewer load bearing components which can be machined with great accuracy (typically 0.005 of an inch) and also through mechanical fastening of the hydraulic actuators  30  to the load members  60 , the snubbing unit can be accurately aligned and assembled in about one half the usual time. Without the need for welding, which eliminates the natural stresses inherent in a welded structure, the resulting structure remains true and square.  
         [0030]     With reference to  FIGS. 1, 3  and  4 , the hydraulic actuators  30  are illustrated as being mechanically fastened to the load members  60 . As shown, one or more load members  60  can be formed and be integrated into a unitary load plate  33  which is connected in some secure fashion to the load spool  20 . Two or more load members  60  are now provided in a single unitary load plate  33 .  
         [0031]     Generally, the load members  60  can be integral with the tubular snubbing structure  11 , such as extending as an oversized flange from the load spool  20  or can be a member or plate sandwiched between adjacent flanges. In both instances, at least a portion of the load members  60  are exposed to the working bore. In other instances the load members  60  are separated from and secured external to the tubular snubbing structure  11 ; in such cases the load members  60  being isolated from the working bore.  
         [0032]     More particularly, with reference to  FIGS. 2, 6 ,  7 , and in another embodiment, the load plate  33  is optionally separate, as more clearly illustrated in the identified drawings, from the load spool  20  and, as such, is mechanically fastened to the tubular snubbing structure  11  at flange  35 . Separation of the load plate  33  as an independent structure from the load spool  20  provides greater options in selecting material properties for the load plate  33 . As discussed, the working bore of the tubular snubbing structure  11  can be in communication with wellbore fluids which dictate selection and restriction of the materials of construction to account of corrosion and related service conditions. Due to the possibility of the presence of hydrogen sulphide (H 2 S) in the well fluids, the materials of construction of components exposed thereto are typically restricted, and regulated to be conventional carbons steel. For example, all piping and components are firstly to ASTM and ASME standards and secondly those exposed to hydrogen sulfide (H 2 S) bearing hydrocarbon service are provided accordance with the appropriate requirements of the National Association of Corrosion Engineers (NACE) such as those current set forth in NACE/MR0175.  
         [0033]     Accordingly, components which transfer the loads to the wellhead  12 , and which are in contact with well fluids, are typically constructed of conventional materials and are limited to the strength of such suitable materials. Conventionally therefore, greater load carrying capacity is only obtained by using thicker, heavier components. However, material selection can be expanded by removing the structure materials from exposure to the wellbore fluids.  
         [0034]     With reference to  FIGS. 6 and 7 , in this embodiment, the load members  60  extend from or are incorporated into one or more load plates  33  which are discrete components from the tubular snubbing structure and, thus, which are isolated from the wellbore fluids. The one or more load plates are mechanically connected to the outer flange  35  of the load spool  20 .  
         [0035]     In one arrangement, a single load plate  33  is formed having a central port  37  sized to fit about a tubular body portion of the load spool  20  and positionable adjacent the outer flange  35  for bolting connection thereto. The port  37  of the load plate  33  is fit about the load spool  20  prior to final fitting of spaced-apart, welded flanges, one of which is the flanged connection  35 .  
         [0036]     The use of the flanged connection  35  to the load plate  33  in the present snubbing unit  10  of the present invention enables clear isolation of the wellbore fluids in the working bore of the load spool  20  and the load plate  33 . Thus, the material properties of the load members  60  of the load plate  33  can be engineered to best suit the loads and service life required. For example, load members  60  of conventional mild steel, such as that integral with the load spool  20 , may be in the order of 4 inches in thickness so as to accommodate traveling slip assembly loads in the order of 200,000 pounds. However, in the case of a separate, higher-strength load plate  33 , bolted exterior to the tubular snubbing structure  11 , a suitable required thickness would only be about 2 to 2-½ inches thick with commensurate weight savings of up to 35 to 50%.  
         [0037]     With reference to  FIG. 7 , the flanged connections  34 ,  35  of the hydraulic actuators  30  and load plate  33  respectively both aid the parallel alignment of the hydraulic actuators  30  and the tubular snubbing structure  11 . In particular, the cylinder flanges  34  can be made in controlled manufacturing conditions with appropriate jigs and subsequent stress relief. Thus, the hydraulic actuators  30  can be secured to the load plate  33  and relative to the tubular snubbing structure  11  with precision, thereby minimizing residual stresses, misalignment and resultant operational difficulties such as any detrimental interaction between the hydraulic actuators  30 , such as walking of the traveling plate  21 .  
         [0038]     The hydraulic actuators  30  also are readily assembled and removed for maintenance through load plate clamps or retainers  41 . Best shown in  FIGS. 3 and 5 , the load plate  33  is formed with slotted openings  40  for receiving the cylinders  31 . After each cylinder  31  is set into position in it respective slotted openings  40  in the load plate  33 , the retainer  41  is mechanically secured to sandwich the cylinder  31  therebetween and thereby fix the cylinder&#39;s lateral position. Flanges  34  secure the vertical position of the cylinders  31  to the load plate  33 . Alternative fitting of the cylinders  31  to the load plate  33  comprise fitting actuator ports in the load plate  33 , such as those resulting where the retainer is permanently fixed in the load plate  33 , and sliding the cylinders axially through the actuator ports for mechanical fastening to the load plate  33  through the flange  34  or other clamping means.  
         [0039]     With particular reference to  FIG. 9 , and as discussed earlier, each hydraulic actuator  30  comprises the cylinder  31  and the rod  32 . The cylinders  31  can comprise an outer barrel  50  within which the rod  32  and a piston  51  can reciprocate. As described, a cylinder and rod actuator also represent fairly conventional hydraulic actuators which are also suitable for the present snubbing unit  10 . However, in a preferred case, the rod  32  is hollow and the actuators  30  further comprise an inner barrel  52  which is sealable within the rod  32 . The use of the inner barrel  52  reduces the actuator&#39;s weight significantly due to the removal of the metal mass from the inside the rod  32 . Further, less hydraulic oil is required to lift the rod  32 , increasing the speed and volume displacement between a rod side and a blind side of the cylinder  31  can be more equally matched. Also, the diameter of the rod  32  and of the cylinder  31  can be increased resulting in a more stable actuator  30  with a higher safety factor while maintaining a similar weight of that of a smaller conventional hydraulic actuator having similar lifting force capabilities.  
         [0040]     With reference to  FIGS. 10   a,    10   b,  hydraulic actuators  30  of conventional or hollow design are operated to reciprocate the traveling slip assembly  15  between a raised position ( FIG. 10   a ) and a lowered position ( FIG. 10   b ).  
         [0041]     As a result of incorporating the structure of the raising and lowering hydraulic actuators  30  as an integral part of the structural design, the snubbing unit  10  has fewer load-bearing components and those few components can be machined with great accuracy (typically 0.005 of an inch). The components can be bolted together allowing the unit to be constructed in less time, reducing the assembly to about one half the conventional time. Without the need for welding, which eliminates the natural stresses inherent in a welded structure, the resulting structure remains true and square. Further, by avoiding combining structural requirements with wellbore-exposed components, and thus avoiding wellbore fluids, a higher strength and thus thinner and lighter engineered material can be used to bear the loads.