US20080053661A1

US20080053661A1 - Pipe guides and methods of guiding pipes in snubbing units

Info

Publication number: US20080053661A1
Application number: US11/512,198
Authority: US
Inventors: Kelly Funk
Original assignee: Individual
Current assignee: Individual
Priority date: 2006-08-30
Filing date: 2006-08-30
Publication date: 2008-03-06
Also published as: CA2560229A1

Abstract

An hydraulic snubbing unit has a pair of hydraulic cylinders for moving travelling snubbing slips towards and away from stationary snubbing slips to snub a pipe through the snubbing unit, the hydraulic cylinders having spaced, parallel cylinder rods. A pipe guidance apparatus has pipe guides between the stationary snubbing slips and the travelling snubbing slips, the pipe guides each having a pair of cylinder rod openings spaced apart from one another and slidably receiving the rods of the hydraulic cylinders and a pipe guidance opening located between the cylinder rod openings and slidably guiding the pipe, and a pipe guide spacer having extendable and contractible interconnections between the pipe guides. In a contracted condition of the pipe guide spacer, the interconnections are contracted and the pipe guides are moved into proximity to one another by the simultaneous movement of the cylinder rods slips towards the stationary slips, and in an expanded condition, the interconnections are extended and the pipe guides are moved apart from one another by the simultaneous movement of the cylinder rods away from the stationary slips to provide lateral support of the pipe at intervals along the pipe.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention
The present invention relates to pipe guides for use in snubbing units for snubbing pipes into and from live underground well bores, to snubbing units provided with pipe guides and to methods of guiding pipes through snubbing units.
2. Description of the Related Art
Snubbing units, which are used to “snub” or feed pipes into and from underground wells, employ stationary snubbing slips or grippers and traveling snubbing slips to alternately grip and release a pipe as the traveling snubbing slips are moved towards and away from the stationary snubbing skips, so that the pipe is moved upwardly or downwardly by a “hand-over-hand” gripping of the pipe.
Traditional pipe guidance apparatus on snubbing units used today typically employ two interlocking and telescoping tubes positioned between the stationary snubbing slips and the traveling snubbing slips. Inherent problems with this type of guiding system include the space and weight of the pipe guidance systems and the heights which they add to the snubbing units. Also, approximately one half of the effective stroke of the snubbing unit is lost and snubbing operations therefore become much less efficient than would be the case if full snubbing strokes could be taken.
A pipe that is being snubbed into or out of a live well against the forces (compressive loads), which are generated from the well pressure over the cross-sectional area of the pipe being removed from or inserted into the well bore, can only support a limited amount of compressive force before the pipe buckles. The longer the length of exposed pipe between the point at which the traveling snubbing slips grip the pipe and the point at which the pipe is exposed outside of the well bore determines the allowable compressive load before buckling takes place. For example a 2-foot long section (window) of exposed and unsupported pipe under a compressive load can withstand a far greater compressive force, before major axis buckling occurs, than a 10-foot unsupported section subject to the same compressive load.
There are two types of bucking that can occur when a pipe is under a compressive load:—

- 1. Major Axis buckling occurs without of yielding the pipe and is therefore known as elastic buckling. This happens when compressive loads on the pipe are as such that the pipe buckles outwardly, at an exposed, i.e. laterally unsupported, section of the pipe as a result of not being supported laterally.
- 2. Local buckling requires the pipe to yield and is therefore known as inelastic buckling. This happens when the pipe may very well be supported laterally but the compressive load is as such that the pipe is effectively crushed vertically.

When snubbing against well pressures without pipe guidance apparatus, a reasonable safety factor to prevent buckling can be obtained by not exposing a section of pipe longer than the pipe can support from the generated forces with friction related to the seal point maintaining the well pressure. Additional factors, such as the speed at which the pipe is inserted into or removed from the well, are all part of typical pre-job calculations which are needed to predetermine the maximum allowable length of pipe that can be exposed while manipulating the pipe without possible bucking. Once these calculations have been made, a snubbing operator uses short strokes to move the pipe into or out of the well, never allowing more unsupported exposure than calculated.
With this type of snubbing operation, a safety factor is only maintained if the snubbing operator stays within the calculated parameters. The operation is therefore subject to human error in the event that the operator uses too long a stroke of the pipe, which could result in the pipe rocketing out sideways at an exposed, i.e. laterally unsupported, section of the pipe. This type of error, although not common, can easily occur, since operators are under constant pressure to complete snubbing operations as quickly as possible and they therefore generally push the limits by attempting to take strokes which are as big as possible. Often, an operator will take a stroke long enough to see the pipe bow as it is being snubbed into or out of the well. This bowing occurs just short of the pipe reaching its compression limit. The result of bucking of the pipe can be catastrophic, with potential loss of control of the pipe and the well and loss of life. The short-stroking operation is not only dangerous but slows the operation down as compared to longer strokes of exposed pipe that could be taken if the pipe was supported externally by means of a pipe guidance apparatus.
Hydraulic snubbing units generally comes in two configurations, namely a self-contained, stand-alone configuration, and a rig-assist snubbing unit.
The self contained stand-alone configuration works well for live wells, where the overall height of the equipment is not all that important because there is typically no spatial limitations in order to handle the pipe that is to be inserted or removed from the live well.
With a rig assist-snubbing unit, however, the overall height is critical because the snubbing unit must work within the height restraint of the top of a derrick forming part of a rig in conjunction with which the snubbing unit is being operated. The distance between the top of the snubbing unit and the inside working area from the top of the rig derrick in many cases eliminates the option of having pipe guidance apparatus, because the additional height that the apparatus would add to the snubbing unit would not allow sufficient space for lengths of pipe to be handled. Consequently, the operators have resorted to performing the dangerous short stroking operation on high-pressure wells, as described above.

BRIEF SUMMARY OF THE INVENTION

According to the present invention, there is provided a pipe guidance apparatus for guiding a pipe during snubbing of the pipe, the pipe guide having a plurality of pipe guides, each with a pair of jack rod openings spaced apart from one for sliding engagement with a pair of parallel, spaced hydraulic jack rods, which laterlly restrain the pipe guides when the apparatus is in use, and a pipe guidance opening located between the jack rod openings for slidable guiding engagement with the pipe. The jack rod openings are mutually aligned and the pipe guidance openings are also mutually aligned. The pipe guidance apparatus includes a pipe guide spacer comprising extendable and contractible interconnections between the pipe guides. The pipe spacer has a contracted condition, in which the interconnections are contracted and the pipe guides are moved towards one another, and an expanded condition, in which the interconnections are extended and the pipe guides are moved apart from one another.
In a preferred embodiment of the present invention, the extendable and contractible interconnections comprise two parallel sets of tubular members, the tubular members of each of the sets being telescopically extensible and retractable relative to one another and each of the sets of tubular members being provided at one end of the set with a connecting member, e.g. a rod, for telescopically extending the tubular members.
Also in accordance with the present invention, there is provided an hydraulic snubbing unit having stationary snubbing slips, traveling snubbing slips and a pair of spaced, parallel hydraulic jacks for moving the traveling snubbing slips towards and away from the stationary snubbing slips to snub a pipe through the snubbing unit, the hydraulic jacks comprising cylinders having jack rods. The hydraulic snubbing unit is provided with a pipe guidance apparatus for guiding a section of the pipe between the stationary snubbing slips and the travelling snubbing slips, and the pipe guidance apparatus comprises a plurality of pipe guides between the stationary snubbing slips and the travelling snubbing slips, the pipe guide guides each having a pair of jack rod openings spaced apart from one another and slidably receiving the jack rods, and a pipe guidance opening located between the cylinder rod openings and slidably guiding the pipe. The pipe guidance apparatus includes a pipe guide spacer comprising extendable and contractible interconnections between the pipe guides, the pipe spacer having a contracted condition, in which the interconnections are contracted and the pipe guides are moved into proximity to one another by the movement of the traveling slips towards the stationary slips, and an expanded condition, in which the interconnections are extended and the pipe guides are moved apart from one another by the movement of the traveling slips away from the stationary slips.
In the preferred embodiment of the invention, the extendable and contractible interconnections comprise two parallel sets of tubular members, the tubular members of each of the sets being telescopically extensible and retractable relative to one another, and each including a connecting member for telescopically extending and contracting the tubular members, the connecting members each having one end connected for movement with the traveling slips and an opposite end extending into the tubular members.
Also according to the present invention, there is provided a method of guiding a pipe as the pipe is being snubbed through snubbing unit having stationary snubbing slips, traveling snubbing slips and spaced, parallel hydraulic jacks for moving the traveling slips towards and away from the stationary slips, the hydraulic jacks comprising hydraulic cylinders having jack rods connected to the traveling snubbing slips, the method comprising the steps of gripping the pipe by the traveling snubbing slips during movement of the pipe through the snubbing unit, extending and retracting the hydraulic cylinders to displace the traveling snubbing slips relative to the stationary snubbing slips and thereby to displace the pipe through the snubbing unit, and guiding a section of the pipe extending between the stationary snubbing slips and the travelling snubbing slips by laterally restraining the pipe section at intervals spaced apart along the section of the pipe.
Prefereably, the pipe is guided by pipe guides spaced apart along the section of the pipe, and the pipe guides are moved towards one another on movement of the travelling slips towards the stationary slips and apart from one another on movement of the travelling slips away from the stationary slips by sliding engagement of the pipe guides with the jack rods of the hydraulic cylinders.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be more readily understood from the following description of an embodiment thereof given, by way of example only, with reference to the accompanying drawings, in which:—

FIG. 1 shows a view in elevation of a pipe snubbing unit provided with an extendable and contractible pipe guidance apparatus according to the present invention, with the pipe guidance apparatus in a contracted condition;

FIG. 2 shows a view corresponding to that of FIG. 1 but with the pipe guidance apparatus in an extended condition;

FIG. 3 shows a plan view of a pipe guide forming part of the pipe guidance apparatus of FIGS. 1 and 2;

FIG. 4 shows a view in side elevation of the pipe guide of FIG. 3;

FIGS. 5 and 6 show views in vertical cross-section through a set of tubular members forming part of the pipe guidance apparatus of FIGS. 1 and 2 in a contracted condition and an extended condition, respectively: and

FIG. 7 shows a broken-away view in perspective of a connection between one of the tubular members of FIGS. 5 and 6 and an associated guide plate.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring firstly to FIGS. 1 and 2 of the accompanying drawings, there is shown a pipe snubbing unit, indicated generally by reference numeral 10, for snubbing a vertical pipe 12 into or from an underground well (not shown).
The snubbing unit 10 has stationary snubbing slips or pipe grippers 14, provided with an underlying blow-out preventor 16, travelling snubbing slips or pipe grippers 18 and a pair of hydraulic jacks 20 for moving the travelling snubbing slips 18 vertically towards and away from the stationary snubbing slips 14.
The jacks 20 each comprise an hydraulic cylinder 22 from which extends an jack rod 24, the lowermost end of the hydraulic cylinder 22 being mounted on a middle plate 26, and the uppermost end of the jack rod 24 being connected to a travelling plate 28 on which the travelling snubbing slips 18 are mounted.
In operation of the snubbing unit 10, the jack rods 24 move the travelling snubbing slips 18 vertically up and down relative to the stationary snubbing slips 14, while the stationary snubbing slips 14 and the travelling snubbing slips 18 alternately grip the pipe 12. In this way, the pipe 12 is fed into or from its well by the snubbing slips 14 and 18 by a “hand-over-hand” operation of those slips.
The snubbing unit 10, as described above, and its operation are well known in the art and, therefore, are not described herein in greater detail.
To provide lateral support for a section 30 of the pipe 12 extending between the stationary snubbing slips 14 and the traveling snubbing slips 18 when the traveling snubbing slips 18 are displaced by the hydraulic jack rods 24 from the stationary snubbing slips 14, the snubbing unit 10 is equipped with a pipe guidance apparatus indicated generally be reference numeral 32 in FIG. 1, the components of which are shown in greater detail in FIGS. 3-7.
The pipe guidance apparatus 32 includes four pipe guides 34, one of which is shown in FIGS. 2 and 3.
Each pipe guide 34 includes an elongate guide plate which, as can be seen from FIG. 4, has a pair of parallel flat end portions 38, a central portion 40 which is parallel to the flat end portions 38, and a pair of intermediate portions 42 which extend between and at an inclination to the end portions 36 and the central portion 38. Each intermediate portion 42 is provided, in one of its edges, with a pair of threaded holes 45.
Each of the end portions 38 is formed with a circular jack rod opening 44, and each circular jack rod opening 44 is provided, at opposite sides of its respective end portion 38, with an annular slide member 46, which is secured to the end portion 38 by screws 48.
The circular jack rod openings 44 slidably receive the jack rods 24, as shown in FIGS. 1 and 2, so that the pipe guides 34 are guided and retained laterally of the jack rods 24 by slidable engagement with the jack rods 24 and are free to slide longitudinally of the jack rods 24. The jack rods 24 hold the pairs of jack rod openings 44 in mutual alignment and thereby also hold pipe openings 50 in mutual alignment.
The pipe openings 50, which are provided with annular slide members 52 secured to the central portion 40 by screws 54, slidably receive, guide and laterally restrain the pipe section 30, extending between the stationary snubbing slips 14 and the traveling snubbing slips 18, so that the pipe section 30 is prevented from buckling.
Referring now to FIGS. 2, 5 and 6, the pipe guides 34 are connected to one another by pipe spacers indicted generally by reference numeral 55 in FIG. 2. The pipe spacer 55 includes two sets of interconnections which are formed by two sets of telescopically extensible and retractable tubular members 54 a-54 d, as shown in greater detail in FIGS. 5 and 6. The pipe guidance apparatus therefore has a collapsible ladder-like assembly, formed by the rung-like pipe guides 34 and the stile-like sets of tubular members 54 a-54 d.
The pipe spacer 55 has a contracted condition, in which the interconnections comprising the sets of tubular members 54 are contracted and the pipe guides 34 are moved into proximity to one another by the movement of the traveling snubbing slips 18 towards the stationary snubbing slips 14 as shown in FIG. 1, and an expanded condition, in which the sets of tubular members 54 are telescopically extended relative to one other and the pipe guides 34 are moved apart from one another, as the traveling snubbing slips are moved away from the stationary snubbing slips 14 by the hydraulic jacks rods 24, as shown in FIG. 2. In the expanded condition of the pipe spacer 55, in which the pipe guides 34 are spaced apart along the pipe section 30, longitudinally of the pipe 12, for laterally supporting the pipe section 30 at these spaced apart positions as shown in FIG. 2.
The two sets of tubular members 54 a-54 d are each provided with a connecting member in the form of a rod 56 for telescopically extending and contracting the tubular members 54 a-d. Each of the rods 56 has an upper end 58 connected to the traveling plate 28, and thereby connected for movement vertically with the traveling snubbing slips 18, and a lower end 60 extending into the innermost tubular member 54 a. When the traveling snubbing slips 18 are displaced upwardly, away from the stationary slips 14 by extension of the jack rods 24 from their cylinders 22, the rods 56 pull the associated tubular members 54 a-54 d so as to telescopically extend the latter.
As shown in FIGS. 5 and 6, the lower end 60 of the rod 56 has an a metal annular anchor member 62 a, which is secured to the lower end 60 by a nut 64 in threaded engagement with the lower end 60. The anchor member 62 a includes an annular rubber cushion 66, which is provided on the lower end 60 at the side of the anchor member 62 opposite from the nut 64.
The tubular members 54 a-54 d have caps 68 a-68 d in threaded engagement with the upper ends of the respective tubular member 54 a-54 b. Each of the caps 68 b-68 d is formed with a circular opening 70 b-70 d for slidably receiving a respective one of the tubular members 54 b-54 d, while the cap 68 a of the innermost tubular member 54 a has a circular opening 70 a for slidably receiving the rod 56. Each of the tubular members 54 a-54 c has an annular anchor member 62 a-62 c, welded to its exterior at a spacing from its lower end, the annular anchor members 62 a-62 c being provided with annular cushions 62 a-62 c of resilient material, e.g. rubber.
Since the caps 68 a-68 d are secured by threaded engagement with the tubular members 54 a-54 d, the caps 68 a-68 d can be readily unscrewed and removed from the tubular members 54 a-54 d to allow replacement of the annular cushions 62 and 62 a-62 c and for other maintenance and repairs.
The caps 68 a-68 d serve as end abutments on the tubular members 54 a-54 d, so that when the rod 56 is drawn upwardly relative to the innermost tubular member 54 a, by upward displacement of the traveling snubbing slips 18 as described above, the rubber cushion 66 of the anchor member 62 is moved into abutment with the end cap 68 a, which causes the innermost tubular member 54 a to be telescopically extended from the next tubular member 54 b.
The cushion 66 b of the tubular member 54 b is then moved upwardly into abutment with the end cap 68 a, which causes the tubular member 54 b to be telescopically extended from the tubular member 54 c. The tubular member 54 c is then telescopically extended in a similar manner from the outermost tubular member 54 d, thereby extending the pipe guide spacer 55 into the extended condition shown in FIG. 2.
The lower end of the tubular member 54 d is closed by a circular plate 72, which is welded to the lower end of the tubular member 54 d. When the traveling snubbing slips 18 reverse their direction of movement and move towards the stationary snubbing slips 14, the rod 12 acts as a push rod and is telescopically retracted into the innermost tubular member 54 a. The tubular members 54 a-54 c are then telescopically retracted into the outermost tubular member 54 d until the lower ends of the tubular members 54 a-54 c abut the circular plate 72, as shown in FIG. 6.
FIG. 7 shows a connection between one of the guide plates 34 and its associated tubular member 54 a. This connection is provided by a metal strip 76, which is secured, by screws 77 in threaded engagement with the holes 45 (FIG. 4), to an edge of one of the inclined portions 42 of the guide plate 34. The metal strip 76 is connected to the cap 68 a by a metal plate 78, which is welded to the metal strip 76 and to the cap 68 a.
The remaining tubular members 54 b-54 d are connected to their guide plates 34 by connections which are similar to that shown in FIG. 7, and which have metal plates which correspond to the metal plate 78 of FIG. 7 but which are of different dimensions to accommodate the different diameters of the tubular members 54 a-54 d.
When the above-described pipe guidance apparatus 32 is in use, the section 30 of the pipe 12 which is exposed between the stationary snubbing slips 14 and the traveling snubbing slips 18 is automatically guided and laterally restrained by the mutually spaced pipe guides 34 when the traveling snubbing slips 18 are moved away from the stationary snubbing slips 14 by the hydraulic jack rods 24, which causes the pipe spacer 55 to become extended without any intervention by an operator. Therefore, the maximum allowable stroke by which the pipe 12 is snubbed can be employed but no judgement with respect to the length of stroke nor any other intervention by an operator is required to prevent buckling of the pipe section 30. Human error in determining the stroke length is therefore avoided and, consequently, highly efficient snubbing of the pipe 12 without buckling of the pipe 12 can be achieved. Also, the height of the snubbing unit 10 is not affected by the provision of the snubbing unit 10 with the pipe guidance apparatus 32.
The anchor members 62 a-62 c are staggered along their tubular members 54 a-54 c, as shown in FIGS. 5 and 6, to compensate for the heights of the caps 68 a-68 c, so that the tubular members 54 a-54 c are extended by equal lengths when the pipe spacer 55 is in its extended condition.
The pipe guides 34 shown in the drawings are each formed with a depressed central portion 40 to conform to the shapes of the slips between the stationary snubbing slips 14 and the travelling snubbing slips 18 when the snubbing unit is collapsed and thereby to avoid adding additional height to the snubbing unit. Otherwise, the pipe guides could, for example, be flat.
The tubular members are provided, at their lower ends, with annular metal rings 74 a-74 d, which are welded to the exteriors of the tubular members 54 a-54 d and which serve to provide internal stabilization of the apparatus.
As will be apparent to those skilled in the art, various modifications may be made in the above-described embodiment of the present invention within the scope of the appended claims.

Claims

1. A pipe guidance apparatus for guiding a pipe during snubbing of the pipe, comprising:—

a. a plurality of pipe guides;

b. the pipe guides each having a pair of jack rod openings spaced apart from one and a pipe guidance opening located between the jack rod openings for slidably guiding the pipe;

c. the jack rod openings being mutually aligned and the pipe guidance openings being mutually aligned; and

d. a pipe guide spacer;

e. the pipe guide spacer comprising extendable and contractible interconnections between the pipe guides; and

f. the pipe spacer having a contracted condition, in which the interconnections are contracted and the pipe guides are moved towards one another, and an expanded condition, in which the interconnections are extended and the pipe guides are moved apart from one another.

2. A pipe guidance apparatus as claimed in claim 1, wherein the extendable and contractible interconnections comprise two parallel sets of tubular members, the tubular members of each of the sets being telescopically extendable and retractable relative to one another and each of the sets of tubular members being provided, at one end of the set, with a pull member for telescopically extending the tubular members.

3. A pipe guidance apparatus as claimed in claim 2, wherein an outermost one of the tubular members of each set of tubular members, at an opposite end of the set, has an abutment for retaining the remainder of the set on contraction of the set.

4. A pipe guidance apparatus as claimed in claim 2, wherein each of the pull members has, at one end thereof, an anchor member located within an innermost one of the tubular members of the respective set of tubular members, said innermost tubular member having an end abutment engageable by the anchor member to telescopically extend said respective set of tubular members on extension of the pull member from the innermost tubular member.

5. A pipe guidance apparatus as claimed in claim 4, wherein each of the tubular members has, on one end thereof, a further anchor member and at an opposite end thereof a further end abutment, the further end abutment of each of the tubular members being engageable by the further anchor member of another of the tubular members to effect the telescopic extension of the tubular members.

6. A pipe guidance apparatus as claimed in claim 5, wherein the further end abutments are formed by end caps on the tubular members.

7. A pipe guidance apparatus as claimed in claim 5, wherein the further anchor members comprise annular collars on the tubular members.

8. In an hydraulic snubbing unit having stationary snubbing slips, travelling snubbing slips and a pair of spaced, parallel hydraulic jacks for moving the travelling snubbing slips towards and away from the stationary snubbing slips to snub a pipe through the snubbing unit, the hydraulic jacks comprising hydraulic cylinders and jack rods extending from the cylinders, the improvement comprising:

a pipe guidance apparatus for guiding a section of the pipe between the stationary snubbing slips and the travelling snubbing slips;

the pipe guidance apparatus comprising:—a plurality of pipe guides between the stationary snubbing slips and the travelling snubbing slips;

the pipe guide guides each having a pair of cylinder rod openings spaced apart from one another and slidably receiving the jack rods of the hydraulic cylinders and a pipe guidance opening located between the cylinder rod openings and slidably guiding the pipe; and

a pipe guide spacer;

the pipe guide spacer comprising extendable and contractible interconnections between the pipe guides; and

the pipe spacer having a contracted condition, in which the interconnections are contracted and the pipe guides are moved into proximity to one another by movement of the travelling slips towards the stationary slips, and an expanded condition, in which the interconnections are extended and the pipe guides are moved apart from one another by movement of the travelling slips away from the stationary slips.

9. An hydraulic snubber unit as claimed in claim 8, wherein the extendable and contractible interconnections comprise two parallel sets of tubular members, the tubular members of each of the sets being telescopically extendable and retractable relative to one another, and each including a connecting member for telescopically extending and contracting the tubular members, the connecting members each having one end connected for movement with the travelling slips and an opposite end extending into the tubular members.

10. An hydraulic snubber unit as claimed in claim 8, wherein each of the connecting members has at one end thereof an anchor member located within an innermost one of the tubular members of the respective set of tubular members, said innermost tubular member having an end abutment engageable by the anchor member to telescopically extend said innermost tubular member.

11. An hydraulic snubber unit as claimed in claim 10, wherein each of the tubular members of each set, other than an outermost one of the tubular members, has on one end thereof a further anchor member and each of the tubular members has at an opposite end thereof a further end abutment, the further end abutment being engageable by one of the further anchor members to effect the telescopic extension of the tubular members.

12. An hydraulic snubber unit as claimed in claim 8, wherein the extendable and contractible interconnections comprise two parallel sets of telescopically extendable and retractable tubular member connecting the pipe guides in a ladder-like assembly.

13. A method of guiding a pipe as the pipe is being snubbed through snubbing unit having stationary snubbing slips, traveling snubbing slips and hydraulic jacks for moving the traveling slips towards and away from the stationary slips, the hydraulic jacks comprising hydraulic cylinders and jack rods extending from the hydraulic cylinders, the jack rods being connected to the traveling snubbing slips, the method comprising the steps of:—gripping the pipe by the travelling snubbing slips during movement of the pipe through the snubbing unit;

extending and retracting the hydraulic jacks to displace the travelling snubbing slips relative to the stationary snubbing slips and thereby to displace the pipe through the snubbing unit; and

guiding a section of the pipe extending between the stationary snubbing slips and the travelling snubbing slips by laterally restraining the pipe section at intervals spaced apart along the section of the pipe.

14. A method as claimed in claim 13, in which the step of guiding a section of the pipe comprises guiding the pipe by pipe guides spaced apart along the section of the pipe, and which further includes moving the pipe guides towards one another on movement of the travelling slips towards the stationary slips and moving the pipe guides apart from one another on movement of the travelling slips away from the stationary slips.

15. A method as claimed in claim 14, which includes guiding the pipe guides along the pipe by sliding engagement of the pipe guides with the jack rods of the hydraulic cylinders.