Frameless snubbing unit
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's material properties to be selected without consideration of corrosion restrictions and thereby allowing a stronger and lighter load plate to be employed.
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 INVENTIONSnubbing 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”.
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.
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 INVENTIONIn 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.
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.
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.
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 H2S and allowing higher strength, lighter engineered materials to be used.
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.
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
With reference to
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.
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
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'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.
With reference also to
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.
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.
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).
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.
With reference to
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.
More particularly, with reference to
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.
With reference to
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.
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%.
With reference to
The hydraulic actuators 30 also are readily assembled and removed for maintenance through load plate clamps or retainers 41. Best shown in
With particular reference to
With reference to
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.
Claims
1. A snubbing unit for a wellhead having a wellbore comprising:
- a lower connection adapted to the wellhead;
- a tubular snubbing structure above the lower connection and having a working bore having a common axis with the wellbore;
- a stationary slip assembly supported on the tubular snubbing structure;
- a traveling plate movable independently of the tubular snubbing structure and having a traveling slip assembly supported thereon for imparting or receiving snubbing loads;
- one or more load members supported by and extending laterally from the tubular snubbing structure; and
- a least two actuators positioned parallel to the common axis, 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.
2. The snubbing unit of claim 1 further comprising a tubular load spool incorporated into the tubular snubbing structure for transferring snubbing loads from the one or more load members.
3. The snubbing unit of claim 2 wherein the load spool further comprises an outer flange isolated from the working bore, the snubbing unit further comprising one or more load plates mechanically connected to the outer flange and wherein the one or more load members extend from or are integral with the one or more load plates.
4. The snubbing unit of claim 3 wherein the one or more load plates are formed of high strength materials.
5. The snubbing unit of claim 1 wherein the one or more load members is a plate sandwiched in the tubular snubbing structure for transferring snubbing loads thereto.
6. The snubbing unit of claim 1 wherein each hydraulic actuator further comprises a mounting flange positioned intermediate along the cylinder for mechanical connection to the one or more load members.
7. The snubbing unit of claim 6 wherein the one or more load members further comprise:
- slotted opening in the load member for receiving each cylinder therein,
- a corresponding retainer fit to each slotted opening for removeably securing each cylinder to the load member; and
- mechanical fasteners for removeably securing each mounting flange to the load member.
8. The snubbing unit of claim 6 wherein the one or more load members further comprise:
- an actuator port formed in the load member for receiving each cylinder therein, and
- mechanical fasteners for removeably securing each mounting flange to the one or more load members.
9. The snubbing unit of claim 1 wherein each cylinder is secured to one or more load members at about midway along each cylinder's length.
10. The snubbing unit of claim 3 wherein each hydraulic actuator further comprises a mounting flange positioned intermediate along the cylinder for mechanical connection to the one or more load plates.
11. The snubbing unit of claim 10 wherein the one or more load plates further comprise:
- slotted opening in the load member for receiving each cylinder therein,
- a corresponding retainer fit to each slotted opening for removeably securing each cylinder to the one or more load plates; and
- mechanical fasteners for removeably securing each mounting flange to the one or more load plates.
12. The snubbing unit of claim 10 wherein the one or more load plates further comprise:
- an actuator port formed in the one or more load plates for receiving each cylinder therein, and
- mechanical fasteners for removeably securing each mounting flange to the one or more load plates.
13. The snubbing unit of claim 3 wherein each cylinder is secured to the one or more load plates at about midway along each cylinder's length.
14. The snubbing unit of claim 4 wherein each hydraulic actuator further comprises a mounting flange positioned intermediate along the cylinder for mechanical connection to the one or more load plates.
15. The snubbing unit of claim 14 wherein the one or more load plates further comprise:
- slotted opening in the load member for receiving each cylinder therein,
- a corresponding retainer fit to each slotted opening for removeably securing each cylinder to the one or more load plates; and
- mechanical fasteners for removeably securing each mounting flange to the one or more load plates.
16. The snubbing unit of claim 14 wherein the one or more load plates further comprise:
- an actuator port formed in the one or more load plates for receiving each cylinder therein, and
- mechanical fasteners for removeably securing each mounting flange to the I one or more load plates.
17. The snubbing unit of claim 4 wherein each cylinder is secured to the one or more load plates at about midway along each cylinder's length.
Type: Application
Filed: Dec 3, 2004
Publication Date: Jun 8, 2006
Inventor: Gerald Haakenson (Edmonton)
Application Number: 11/002,277
International Classification: E21B 19/18 (20060101);