In maintenance works in connection with underwater oil and gas wells, it ordinarily represents applied technique to move various kinds of tools down into the underwater well by means of a composed pipe string or a continuous coiled tubing.

If well maintenance is carried out while the well is under pressure, it is necessary to feed tools or other accessories

forcibly down into the well, against the well pressure, simultaneously as a packer is brought to seal externally against the pipe string.

When drilling an underwater well, this is filled with mud having a sufficiently high specific weight in order to balance the reservoir pressure. Therefore, in this condition, the underwater well is pressure-less, and a drilling rig is equipped with a vigorous hoisting winch capable of raising and lowering pipes within the well. Normally, a drilling rig is, however, not such equipped that it can feed pipes down into a pressurized well, as this operation requires a substantial downwardly directed force is supplied, in addition to the weight of pipe and accessory/tool.

It is known to use a socalled snubbing rig for the maintenance of wells under pressure. Such a snubbing rig comprises a vigorous supporting rack and a raising-lowering device adapted to deliver both upwardly and downwardly directed vertical forces to a pipe string or a coiled tubing being fed out of or into the well. Ordinarily, the snubbing rig is equipped with a hydraulically operated feeding device having two sets of catchers, one catcher set on a first, higher level and another catcher set on a lower level. A fixed set of catchers (usually a lower set of catchers) retains the pipe string in position while a vertically movable set of catchers is moved and positioned in relation to a top and a bottom position and, in one or the other of these positions, grips around the pipe string, whereafter the stationarily placed set of self-mobile catchers is released and opened, so that the vertically movable set of catchers can move the pipe string downwardly (from the top end position) or upwardly (from the bottom end position).

When the movable set of catchers has carried out one operational stroke in vertical direction, the fixed, set of catchers are then activated immediately.

Then, the movable set of catchers is released and brought into position to carry out a new operational stroke, whereafter the working operations are repeated in this sequence until the desired, total feeding distance for the pipe string has been carried out by means of the raising- lowering device.

This feeding device is single-acting, because the pipe is retained in position every other moving operation carried out by the movable set of catchers. As the movable catchers carry with them the pipe only upon every other movement, while it otherwise is retained in position, this known feeding device in a snubbing rig is time-consuming in use.

A snubbing rig can be mounted on the drilling deck in a drilling rig, but it is as usual to mount the snubbing rig directly on the well head, using various extension members, so that the snubbing rig will be positioned above the skidbase deck. The snubbing rig is usually braced opposite the skidbase deck and mounted on top of a blowout preventer mounted on top of the underwater well, several metres below the skidbase deck, a pipe through which the working operations are carried out through, extending between the snubbing rig and the blowout preventer.

Additionally, it is common practice to secure a snubbing rig by means of guy ropes. The functions of the snubbing rig are controlled from an operator panel board on the top of the snubbing rig. After having been used in connection with pipe feeding, the snubbing rig is disassembled and moved to another well.

The mode of operation in e. g. raising, with a starting point in a pipe retained in position by a lower catcher means is such: Upper catcher means is moved to the lowermost position thereof, where its hydraulic piston cylindres are entirely contracted, the piston rods being pulled into the cylinders,

the closed ends of the latter being stationarily positioned, but articulated to adjacent structure.

In this lowermost position of the upper catcher means, the catchers thereof, e. g. slips, are activated, causing it to grip firmly and securely around the pipe. Thereafter, the catchers of the lower catcher means, e. g. slips, are deactivated in order to release the pipe, now only being retained by the upper, movable catcher means, accommodating all vertical force from the pipe.

The hydraulic piston cylindres or other actuators of extendable/shortenable design assigned to the upper catcher means are caused to expand axially, thus lifting the upper catcher means and the pipe retained thereby. When the piston rods of the vertically orientated hydraulic piston cylinders are pushed maximum out from the latter, and the upper catcher means, thus, has reached an upper end position, the lower catcher means is reactivated while the upper catcher means is deactivated, so that the pipe again is retained by the lower catcher means only. While the upper catcher means is released and, thus, made relatively displaceable in relation to the pipe, the upper catcher means is moved by the piston cylinders back to its lower position, where it once more is caused to grip around the pipe, whereupon the succession of the working operations is repeated until the pipe has been triced up in a desired degree.

The mode of operation when a pipe is to be moved down into an oil/gas well is as follows: The upper catcher means is moved with inactivated catchers to its upper end position, while the lower catcher means retains the pipe in position. The upper catcher means is activated while the lower catcher means is released, so that the upper catcher means thereupon accommodates all vertical forces acting on the pipe. By means of the piston cylinders, the upper catcher means and, thus, the pipe is moved downwardly within the underwater well until

the upper catcher means reaches its lower end position and the lower catcher means is activated, whereupon the upper catcher means immediately is released, so that the pipe is retained by the lower catcher means alone, while the upper catcher means, by means of the piston cylindres, once more is moved to its upper end position, in order to get-at that position-a new grip around the pipe, the feed-down of the pipe within the well continuing upon the repetition of the above described operations until the desired length of pipe has been fed down into the well.

Upon every other displacement operation, the movable, upper catcher means makes its linear movement alone, namely in order to change its grip on the pipe. During these return movements of the upper catcher means, of course, no forward feeding of the pipe takes place.

In snubbing rigs of the kind concerned, such a single-acting pipe feeding is time-consuming.

A substantial object of the present invention has been to provide a more efficient pipe feeding device on a snubbing rig of the kind defined introductorily.

In a device as defined in the preamble of claim 1, this object is achieved in combination with the features appearing from the characterizing clause of claim 1.

In relation to prior art snubbing devices, the present invention consists in broad outline in that also the lower catcher means is movable, coaxially with the upper catcher means.

In such a device, it does not arise-as with snubbing rigs built up in accordance with the state of the art-pipe feeding stops-between each working stroke.

With a starting point at the upper, vertically displaceable catcher means at e. g. an upper end position, corresponding to fully pushed-out piston rods when the upper catcher means has been assigned piston cylinders in order to effect the displacement and alone retaining a pipe string, the piston cylindres are activated for full withdrawal of their piston rods therewithin, such that the upper catcher means-as known per se-is moved downwardly, taking with it the pipe string. As soon as this vertically downwardly directed displacement of the upper catcher means and the pipe string is initiated, the lower, vertically displaceable catcher means (having no contact with the pipe string) is activated for upwardly directed movement, preferably synchronized with the downwardly directed displacement movement of the upper catcher means, so that the lower catcher means preferably and at least approximately arrives into its upper end position simultaneously as the upper catcher means arrives into its lower end position. In these end positions, in which each catcher means, the upper catcher means from above and the lower catcher means from below, can strike against a common, stationary lateral frame, catchers of each catcher means are adapted, from a closed, operative position, to take an inoperative, open position.

I. e. when a situation arises wherein the two catcher means being moved towards each other, stop because they have arrived in one end position thereof, one set of catchers of the lower, vertically displaceable catcher means is adapted to become activated and grip retainingly around the pipe string, while one set of catchers of the upper catcher means immediately thereupon will be activated in order to make them release the pipe string.

As soon as the vertically displaceable, lower catcher means alone has taken over the pipe string and accommodates the weight thereof, new movements of both catcher means start; this time in opposite directions: The lower catcher means,

bringing along with it the pipe string, is moved downwardly, preferably with substantially the same speed as the speed with which the upper catcher means is moved upwardly, so that both catcher means reach second end positions thereof, the lower catcher means, together with the pipe string, its lower end position and the upper catcher means its upper end position, at least nearly simultaneously.

With the catcher means in these second end positions, one set of catchers of the upper catcher means is activated, so that it immediately can take over the support and displacement of the pipe string, the hitherto activated catchers of the lower catcher means being deactivated, releasing the pipe as soon as one set of catchers of the upper catcher means has obtained a secure grip around the pipe, in order to initiate a new feed-down of the pipe immediately.

In the pipe feeding method and device according to the invention, the only intervals in the pipe feeding are those time delays taking place every time the pipe is delivered from one catcher means to the other. Such insignificant delays are due to the fact that the set of catchers of one catcher means has to establish a firm grip around the pipe prior to the moment at which the corresponding set of catchers of the other catcher means can release and neutralize its grip around the pipe. These effects may take place almost simultaneously, dependent on the accuracy of synchronization.

Thus, the pipe feeding device is"double-acting"in relation to the previously described"single-acting"snubbing devices.

Each catcher means may have two sets of catchers, e. g. slips, situated at mutually differing levels.

One set is used when the pipe is to be moved down into a well, the other set being used when the pipe is to be moved out from a well.

When hauling up a pipe previously moved down into a well, one proceed in the opposite succession. The upper catcher means starts at its lower end position where it takes over the pipe from the lower catcher means which, then, takes its upper end position. While the upper catcher means pulls the pipe up on the way toward the upper end position thereof, the lower catcher means moves toward its lower end position, in which it takes over the pipe from the upper catcher means at the upper end position thereof. Thereafter, the lower catcher means moves, bringing with it the pipe, in an upward direction toward its upper end position, simultaneously as the upper catcher means moves toward its lower end position, in order to take over the pipe during its upwardly directed displacement, the succession in these displacement operations alternately effected by the upper catcher means and the lower catcher means being repeated until the pipe has been pulled out from the well or when a desired length of pipe has been displaced upwardly.

If pressurized fluid operated piston cylinders are used as actuators for effecting the vertical displacement upwardly or downwardly of the respective catcher means, the closed end of all cylinders, belonging to the lower catcher means as well as to the upper catcher means, can be mounted in upright positions on a lower lateral frame.

Reference is made to accompanying drawings which are further explained in the following, and wherein: Figure 1 shows in perspective a snubbing rig according to the invention, where substantial portions of two trusswork side walls have been cut away in order to illustrate the internal structure;

Figures 2A-2D show side elevational views, partially in vertical section II-II in figure 3, figure 2A showing the device while the two catcher means move in directions toward each other, the upper catcher means taking with it a pipe in the downward direction; in figure 2B, both catcher means are stationary while the pipe is transferred to the lower catcher means; figure 2C shows that the lower catcher means has taken over the pipe, the arrows indicating that the two catcher means immediately thereafter will get into displacement movements in opposite directions, and in figure 2D these movements have started; Figure 3 shows a lateral cross-section III-III in figure 2A.

As mentioned, in the perspective view in figure 1, the two nearest trusswork side walls have been cut away. Four such trusswork side walls are included in an elongate, three- dimensional frame carrying the pipe feeding device incorporated in the snubbing rig according to the invention.

The frame is built up from two lateral end frame members 10 and 12 interconnected with vertically extending corner poles 14 and diagonal stays 16. The frame of the snubbing rig may e. g. be attached as indicated in figures 2A-2D, where its lower lateral frame member 10 is screwed firmly to a wellhead on top of a conductor pipe 18.

The lower closed ends of cylinders 20a and 20b included in hydraulic piston cylinders comprising pistons 20a'and 20b', respectively, and piston rods 20a"and 20b", respectively, are attached to the lower lateral frame member 10 of the frame 10,12,14,16, in which the cylinders 20a in a number of four are placed at the vertical corners of the frame, thus bracing and strengthening the corner edges, while the cylinders 20b, in a number of two, strengthen and brace the frame's 10,12,14,16 vertically extending central portion.

Figure 3 is simplified and shows the positioning of the

piston cylinders in a horizontal plane only (frame structure has been removed for the sake of clarity).

To the four piston cylinders comprising the cylinders 20a there is assigned an upper catcher means, generally denoted at reference numeral 22, the piston rod ends thereof being connected to a holder 24 carrying the catcher means 22.

To the two piston cylinders comprising the cylinders 20b, a lower catcher means has been assigned, generally denoted at reference numeral 26. The piston rod ends have been connected to a holder 28 carrying the catcher means 26.

The catcher means 22,26, the lateral frame members 10,12 and the holders 24 and 28 for the catcher means 22 and 26, respectively, are all formed with a central, vertically through-going hole 30a, 30b, 30c, 30d, 30e, 30f for a pipe which, in the exemplary embodiment, has been shaped and designed as a composed pipe string 32.

According to the working operational sequence defined in figures 2A-2D, this pipe string 32 is moved downwardly in a substantially continuous rectilinear displacement movement; this in opposition to prior art technique where the displacement movement of the pipe takes place intermittently, such as previously described.

In the vertically through-going hole 30a, 30b of each catcher means 22 and 26, respectively, two sets of catchers are disposed, said sets of catchers being located at mutually differing levels. The upper set of catchers of the upper catcher means 22 is denoted at 34a and its lower set of catchers at 34a', and the upper set of catchers of the lower catcher means 26 is denoted at 34b and the lower set of catchers at 34b'. Each set of catchers 34a, 34a', 34b, 34b'

can be constituted by slips or they may consist of more claw- like catchers having reliable opening and closing functions.

The hydraulic piston cylinders for the two catcher means 22, 26 are adjusted/synchronized such that the two catcher means 22,26 take their respective end positions simultaneously or at least nearly simultaneously: Thus, the upper catcher means 22 is to reach its upper end position simultaneously as the lower catcher means 26 arrives at its lower end position, figure 2A. Likewise, the upper catcher means 22 shall reach its lower end position simultaneously as the lower catcher means 26 arrives at its upper end position, figures 2B and 2C.

From the end positions of the catcher means 22,26, figure 1, they are displaced approximately simultaneously in vertical directions toward each other, arrows 36,36', until they simultaneously arrive at their end positions in figure 2B and then, from the corresponding end positions in figure 2C, the two catcher means 22 and 26 are displaced in opposite directions, arrows 38,38', until the catcher means 22,26 again have taken their end positions according to figure 2A, where the lower set of catchers 34a'of the upper catcher means 22 takes over the pipe 32 from the lower set of catchers 34b'of the lower catcher means 26.

An intermediate position between positions shown in figure 2B and figure 2B/2C is shown in figure 2D, where the directions of motion of the upper and lower catcher means 22,26 away from each other have been indicated with arrows 40 and 40', corresponding to the arrows 38 and 38'.

If the pipe 32 is to be hauled up from the well 18 instead of being moved into the same, one may take as a starting point the position shown in figure 2A, where the upper catcher means 22 is taking its upper position and the lower catcher means 26 its lower position, the upper set of catchers 34b of

the latter being activated to grip around the pipe 32 and retain it by itself. Thus, while the lower catcher means 26 moves the pipe 32 upwardly to the upper end position thereof, the upper catcher means 22 moves down to the lower end position thereof (figures 2B and 2C), where the upper set of catchers 34a of the last-mentioned catcher means is activated, gripping around the pipe 32 and retaining it, the lower catcher means 26 immediately thereupon releasing the pipe from the upper catchers 34b thereof. Thereupon, the upper catcher means 22 moves upwardly, taking with it the pipe 32 up toward its upper end position, the directions of motion of the two catcher means 22,26 following the arrows 38,38'in figure 2C or the arrows 40,40'in figure 2D, respectively, where the last-mentioned figure represents a somewhat later phase of the catcher means displacement toward figure 2A than figure 2C. When a pipe 32 is hauled up from a lower position within the well, it is always the upper set of catchers 34a and 34b of the catcher means 22 and 26, respectively, which are brought into activity in alternate operations.

Each catcher means looks like a reel or bobbin and is provided with convex portions facing each other within the respective catcher means 22 and 26. The catcher means has its largest wall thickness midway of the height of the catcher means 22 or 26, respectively, where the through-going hole 30a or 30b, respectively, has its smallest lateral extent.

Such a design favours the mounting of slips 34a, 34a', 34b, 34b'or other catcher members, e. g. of the claw type, adapted to be brought to close themselves retainingly around the pipe 32 or to open and release the pipe 32 in accordance with a distinct pattern of motion adapted to the extension and contraction of the actuators 20a and 20b, based on synchronizing mechanisms (not shown) known per se.

In case underwater well maintenance, possibly drilling, is carried out while the well is under pressure, it is usual to dispose a packer (not shown), sealing externally against the pipe.