Title:
DRILLING RIG AND METHOD FOR INTRODUCING ONE OR MORE DRILLING PIPES INTO AN UNDERBALANCED WELL USING SAME
Kind Code:
A1


Abstract:
A drilling rig includes a mast to be placed above or close to a well and a device connected to the mast for carrying drill pipes to be lowered into the well, which carrying device includes at least three carriers which are connected at different heights to the mast and displaceable in height direction therealong and which are adapted to exert a pulling force or a pressing force as required on the drill pipes.

The upper carrier can have a device for rotatingly driving the drill pipes, while the lower carrier, which can be connected releasably to the mast, can have a flexible device for connecting thereof to a blowout preventer placed on the well.

A method for introducing one or more drill pipes into an underbalanced well while making use of such a drilling rig includes suspending the drill pipe(s) from the carrying device and urging the pipe(s) downward into the well while exerting a pressing force.




Inventors:
De Wit, Lammert (De Kiel, NL)
Bergman, Cornelis (Emmen, NL)
Application Number:
12/067146
Publication Date:
08/27/2009
Filing Date:
09/15/2006
Primary Class:
Other Classes:
175/195, 175/203, 175/85
International Classes:
E21B19/08; E21B3/02; E21B7/00; E21B15/00; E21B19/00; E21B19/02
View Patent Images:
Related US Applications:



Primary Examiner:
GAY, JENNIFER HAWKINS
Attorney, Agent or Firm:
Hunton Andrews Kurth LLP/HAK NY (2200 Pennsylvania Avenue NW, Washington, DC, 20037, US)
Claims:
1. 1-17. (canceled)

18. A drilling rig, comprising: a mast adapted to be placed at least one of (a) above and (b) close to a well; and a carry device, connected to the mast, configured to carry drill pipes to be lowered into the well; wherein the carry device includes at least three carriers connected at different heights to the mast and displaceable in a height direction along the mast; and wherein the carriers are adapted to selectively exert at least one of (a) a pulling force and (b) a pressing force on the drill pipes.

19. The drilling rig according to claim 18, wherein at least an upper carrier includes a rotating drive device configured to rotatingly drive the drill pipes.

20. The drilling rig according to claim 19, wherein in that the rotating drive device of the upper carrier includes a top drive.

21. The drilling rig according to claim 18, wherein the carriers are adapted to co-act during a drilling operation when the well is overbalanced, an upper carrier adapted to lower drill pipes in a downward movement into the well, at least one of (a) a middle carrier and (b) a lower carrier adapted to take over and hold the drill pipes fixedly at an end of the downward movement.

22. The drilling rig according to claim 18, wherein a lower carrier is releasably connected to the mast.

23. The drilling rig according to claim 22, wherein the lower carrier includes a flexible connection device configured to connect to a blowout preventer placed on the well.

24. The drilling rig according to claim 23, wherein a middle carrier and a lower carrier are adapted to co-act during an operation when underbalancing prevails in the well, the middle carrier adapted to urge components in a downward stroke into the well, a lower carrier arranged to remain in place and adapted to hold the components fast and to transfer forces that occur into the mast.

25. The drilling rig according to claim 18, further comprising at least one platform connected to the mast and displaceable along the mast in the height direction.

26. The drilling rig according to claim 25, wherein the platform is connected to one of the carriers.

27. The drilling rig according to claim 18, further comprising a supply device configured to supply drill pipes and arranged to co-act with the carry device.

28. The drilling rig according to claim 27, wherein the supply device includes a device adapted to raise the drill pipes from a lying position and transfer the drill pipes to the carry device.

29. A method for introducing at least one drill pipe into an underbalanced well using a drilling rig including a mast adapted to be placed at least one of (a) above and (b) close to the well and a carry device, connected to the mast, configured to carry drill pipes to be lowered into the well, the carry device including at least three carriers connected at different heights to the mast and displaceable in a height direction along the mast, the carriers adapted to selectively exert at least one of (a) a pulling force and (b) a pressing force on the drill pipes, comprising: suspending the drill pipe from the carry device; and urging the drill pipe downward into the well while exerting the pressing force.

30. The method according to claim 29, further comprising flexibly connecting a lower carrier to a blowout preventer placed on the well and fixing the lower carrier to the mast prior to urging of the drill pipe into the well.

31. The method according to claim 30, wherein a middle carrier urges the drill pipe in a downward stroke into the well and the lower carrier remains in place, holds the drill pipe fast, and transfers forces which occur into the mast.

32. The method according to claim 31, wherein arranged between the middle carrier and the lower carrier is a system of telescopic sluice tubes, of which a stationary outer sluice tube is fixed to the lower carrier and an inner sluice tube is fixed to the middle carrier and moved by the middle carrier.

33. The method according to claim 29, further comprising picking up and presenting the drill pipe to a middle carrier by an upper carrier.

34. The method according to claim 29, further comprising rotatingly driving the drill pipe by an upper carrier.

35. A drilling rig, comprising: a mast adapted to be placed at least one of (a) above and (b) close to a well; a carry device, connected to the mast, configured to carry drill pipes to be lowered into the well; and at least one platform connected to the mast and displaceable along the mast in the height direction.

36. The drilling rig according to claim 35, wherein the carry device includes at least three carriers connected at different heights to the mast and displaceable in a height direction along the mast; and wherein the platform is connected to one of the carriers.

Description:

FIELD OF THE INVENTION

The invention relates to a drilling rig, which may include a mast to be placed above or close to a well and a device connected to the mast for carrying drill pipes to be lowered into the well.

BACKGROUND INFORMATION

Such a drilling rig is generally known and is used to form wells at locations where oil or gas is present in the ground.

The most characteristic component of a conventional drilling rig is the mast or derrick construction, generally a slender framework pyramid which is placed above the drilling location or erected thereon and which rests on a foundation adjacently of or around the drilling location. Arranged at the top of this mast is a hoisting gear with which drill pipes can be successively picked up from the ground at the front side of the mast and subsequently lowered in controlled manner into the well. The hoisting gear is situated some distance directly above the well.

The conventional drilling rig is further provided with a rig floor which forms an integral carrying component thereof and is rigidly connected to the mast. An opening is formed in this rig floor for passage of the drill pipes. A device is further present on this rig floor with which the drill pipes can be rotatingly driven, usually in the form of a rotary table having therein a displacing rod of non-round cross-section, the so-called kelly.

Finally, the drilling rig is further provided with a device for pumping a drill fluid downward through the drill pipes into the ground, whereafter this drill fluid comes up again through the hollow space between the drill pipes and the wall of the well.

The drilling rig is thus in fact a crane with multiple functions, i.e. the raising or lowering of the drill pipes, the rotation of the drill pipes during the drilling process, and storage of the drill pipes during changing of the drill head.

In modern drilling rigs use is in fact usually made, instead of a rotary table in the rig floor, of a so-called “top drive” at the top of the mast to rotate the drill pipes. Such a top drive consists of one or more motors connected via a transmission to a rotatable connecting piece to which one drill pipe at a time is attached. The top drive is suspended from the hoisting gear in the mast and can be moved up and downward thereby.

The basic functions of the drilling rig are therefore to move the drill pipes up and downward in controlled manner, to impart a rotating movement to the drill pipes if necessary, and to pump drill fluid. The controlled movement of the drill pipe includes compensating for the downward force exerted by the force of gravity on the drill pipes, so their weight.

Use is made for the drilling process of drill pipes of a limited length which, when screwed together, form the final drill string. Due to the physical limitation of the drill pipes and the screwing required, the process of constructing a drill string is an interrupted process of placing and screwing on new pipes and subsequently lowering the whole drill string. The drill string can herein move no further downward each time than the length of the newly placed drill pipe. During placing of each new drill pipe the drill string is stationary and is supported by a non-movable mechanical support in the form of the rig floor. The actual hoisting gear is used during this operation for placing of the new drill pipe to be screwed on.

During the whole drilling process the drill string is supported by the rig floor during the stationary part of the movement process and by the hoisting gear of the drilling rig during the dynamic part.

The above described process can also take place in reverse sequence, wherein the drill string is pulled up in steps by the hoisting gear.

In addition to the above described basic functions of the drilling rig, control of the pressures occurring in the well are an important factor in the overall drilling process. Controlling of the pressures occurring in the well, also referred to as well control, is based on the principle that the force exerted on the well by the column of liquid, the drill fluid, is equal to or slightly greater than the ground pressure present in the well. This is referred to as “balanced”, or even as “overbalanced”, drilling.

Mounted on the well are blowout preventers which can close the well if the balance were to be temporarily disrupted. If the ground pressure becomes greater than the back-pressure exerted by the liquid column, the liquid column will then be pressed upward out of the well. This is prevented by closing blowout preventers, whereafter the balance can once again be restored by increasing the weight of the liquid column, and the drilling process can hereby be resumed.

In order to be also able to maintain this overbalanced drilling process in rocks with a high permeability and porosity, two prerequisites necessary to be able to speak of a reservoir rock, the drill fluid is provided with solids. These solids serve to reduce the permeability of the reservoir rock by cementing, in order to give sufficient support to provide support for the liquid column required by the drilling process.

This cementing, which is required during the drilling process, has the effect of slowing production considerably during the production phase of the well. As the prevailing ground pressure decreases due to the extraction of oil or gas from the well during the production phase, an increasing amount of cementing will be necessary as time passes to provide a liquid column to be supported. The ultimate result hereof is a steadily increasing production loss.

In order to prevent this production loss immediately after the drilling phase as well as during later drilling activities, another method of well control has been developed. In this modified process use is no longer made of the basic principle that the liquid column must compensate the prevailing ground pressure, with all the side-effects this entails. Operation takes place instead with a liquid column which is lighter than the prevailing ground pressure, or a liquid column is even no longer used at all. This process, in which no liquid column or too light a liquid column is used, is often referred to with the term “underbalanced” drilling.

This fundamental change in the drilling process requires replacement measures which can compensate for the effect of removing (a part of) the liquid column.

The most important measure is that the blowout preventers must be and remain closed, since the ground pressure will no longer be balanced. This underbalanced situation is translated into a pressure in the well which can be detected at the surface. The drill string will also have to be internally safeguarded against the now prevailing well pressure, this being achieved by valves closing in one direction.

In addition to these direct modifications, the whole process of handling the drill pipes, thus the moving up and downward and rotating thereof, must be performed by the now permanently closed blowout preventers. Since the blowout preventers are going to be used in this situation not only in the case of very short-lived disturbances in balance but will serve instead as important component in the performing of a continuous process, additional blowout preventers, diverse of which are specially equipped for this task, must be placed on the well. This applies all the more because the process will also cause wear of the valves since the drill pipes are moved through them.

In addition to the above stated results, the well pressure caused by the process change has another consequence. A force which is uniform in all directions will be exerted by the prevailing well pressure, and thus also on the closed underside of the now closed drill string. Since only atmospheric pressure is exerted on the portion of the drill string protruding from the well, the difference in pressure inside and outside the well will result in an upward directed force on the drill string. As soon as this upward directed force becomes greater than the weight of the drill string, which does after all depend on the length of the drill string and so the number of drill pipes, the upward directed force will press the drill string out of the closed well. As the length of the drill string decreases, the upward directed differential force will become increasingly greater. Since a conventional drilling rig is only adapted to overcome the forces caused by the force of gravity, which are by definition directed downward, the upward directed forces caused by the well pressure cannot be controlled by the drilling rig.

In addition to the changed play of forces in the well and the consequences ensuing herefrom, it is no longer possible, due to the operation with closed blowout preventers, to move through the closed blowout preventer drill pipes with a form other than that for which the blowout preventer is designed. In order to be able to do this special techniques, so-called sluice techniques, have been developed which cannot be incorporated directly, and certainly not without consequences, in the normal operating pattern of a drilling rig. Specialist companies are usually called in for this purpose which have at their disposal suitable equipment, the so-called hydraulic workover units or snubbing units, which must co-act with the drilling rig already on site.

Making the necessary changes to the system of blowout preventers, arranging the hydraulic workover unit in the conventional drilling rig are not only time-consuming and so expensive, but also involve an increased safety risk.

SUMMARY

Example embodiments of the present invention provide a drilling rig with which, in addition to normal drilling, special operations in an underbalanced well can also be performed without use having to be made for this purpose of separate constructions and services of third parties. According to example embodiments of the present invention this is achieved in a drilling rig of the above described type in that the carrying device includes at least three carriers which are connected at different heights to the mast and displaceable in height direction therealong and which are adapted to exert a pulling force or pressing force as required on the drill pipes. By manipulating the drill pipes in suitable manner using these three carriers which are displaceable independently of each other and the lower of which in fact replaces the fixed rig floor of a conventional drilling rig, different functions can be carried out with one and the same drilling rig in both the overbalanced and underbalanced situation of the well. Because the drilling rig itself is in this manner made suitable to not only lower drill pipes in controlled manner into a well but to really press them therein, it is possible to dispense with the use of a separate unit such as a hydraulic workover unit or a snubbing unit when the pressure ratio in the well becomes underbalanced.

When at least the upper carrier has a device for rotatingly driving the drill pipes, it can be used in all operations, both drilling and workovers, in an overbalanced situation. In this case it is recommended that the rotating drive device of the upper carrier is formed by a so-called top drive.

The carriers in the drilling rig according to example embodiments of the present invention are advantageously adapted to co-act during a “normal” drilling operation when the well is (over)balanced, wherein the upper carrier is adapted to lower drill pipes in a downward movement into the well, and the middle or lower carrier is adapted to take over and hold the drill pipes fixedly at the end of the downward movement. The drilling rig can thus be used conventionally in an overbalanced situation.

The lower carrier is preferably connected releasably to the mast. This lower carrier can thus be removed and stored when it is not required, thus during normal use of the drilling rig, whereby the two other carriers can move unimpeded along the mast.

When the lower carrier has a flexible device for connecting thereof to a blowout preventer placed on the well, this prevents the construction of the well being loaded during operations in an underbalanced situation by downward directed forces occurring here.

The middle and lower carrier can be adapted to co-act during an operation when underbalancing prevails in the well, wherein the middle carrier is adapted to urged components in a downward stroke into the well, and the lower carrier remains in place and is adapted to hold the components fast and to transfer forces that occur into the mast.

The drilling rig is advantageously further provided with at least one platform connected to the mast and displaceable therealong in height direction. Tools and instruments used during drilling can be kept on this platform, which itself does not fulfil a supporting function in the drilling process, and personnel who must be present in the drilling rig can take up position thereon. Because the platform is height-adjustable, the process of assembling the drill string or removing drill pipes therefrom can take place at any desired height.

The platform can be connected to one of the carriers. The number of separate components of the drilling rig can thus be limited.

In an example embodiment the drilling rig is further provided with a device for supplying drill pipes which co-acts with the carrying device. The process of introducing or removing the drill pipes can hereby be for the greater part mechanized, so that a human contribution is not necessary here, or hardly so. This is important because handling of drill pipes on the rig floor is one of the most hazardous operations on and around a drilling rig. This supply device preferably has a device for raising the drill pipes from a lying position and transferring them to the carrying device. The pipes can thus be stored simply lying next to the drilling rig.

Example embodiments of the present invention further relate to a method for introducing one or more drill pipes into an underbalanced well. As stated, when a well is underbalanced, a separate hydraulic workover or snubbing unit has heretofore been placed adjacently of or in the drilling rig. This hydraulic workover or snubbing unit, which is usually operated by staff specially employed for that purpose, has provisions for urging drill pipes along or through the blowout preventers in the well without oil or gas escaping. The use of such units and the associated staff is expensive and results in long delays.

Example embodiments of the present invention provide a method wherein the use of separate installations or the employment of third parties can be dispensed with. According to example embodiments of the present invention this is achieved by making use of a drilling rig as described above for introducing one or more drill pipes into an underbalanced well, wherein the method includes suspending the drill pipe(s) from the carrying device and urging the pipe(s) downward into the well while exerting a pressing force.

As part of this method the lower carrier is preferably connected in flexible manner to a blowout preventer placed on the well and fixed to the mast prior to urging of the drill pipe(s) into the well. Due to the rigid attachment of the carrier to the mast and its flexible connection to the blowout preventer(s), the well is safeguarded from any downward directed load resulting from the processes carried out by the drilling rig.

The method is advantageously adapted such that the middle carrier urges the drill pipe(s) in a downward stroke into the well and the lower carrier remains in place, holds the drill pipe(s) fast and transfers forces which occur into the mast. The middle and lower carrier can be provided for this purpose with holding elements or “slips”, of which one engages on the drill pipe and one allows passage of the drill pipe in alternating manner, whereby these slips as it were pass the drill pipes to each other. Because the operations necessary to introduce the drill pipes are performed by the middle and lower carriers, the upper carrier can carry out other operations, such as supplying and fastening to each other or, conversely, taking apart drill pipes, while the space above the middle carrier remains available for temporary storage of the drill pipes.

In the method according to an example embodiment of the present invention there is arranged between the middle and lower carrier a system of telescopic sluice tubes, of which a stationary outer sluice tube is fixed to the lower carrier and an inner sluice tube is fixed to the middle carrier and moved thereby. Using such a system of sluice tubes, also referred to as telescopic deployment tubes (TDT), drill pipes of differing forms which cannot be pressed in any other manner into an underbalanced well can nevertheless be introduced under pressure into the well.

In this method the drill pipe(s) are advantageously picked up and presented to the middle carrier by the upper carrier. Because the operations necessary for introducing the drill pipes are performed by the middle and lower carriers, the upper carrier and the space above the middle carrier do after all remain available for other operations, such as manipulation of drill pipes using the top drive, fastening together or, conversely, taking apart drill pipes, etc. Operation can hereby take place efficiently and safely.

The drill pipe(s) can herein also be driven rotatingly by the upper carrier.

Example embodiments of the present invention are described in further detail below, wherein reference is made to the accompanying Figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic side view of a drilling rig according to an example embodiment of the present invention during normal operation, wherein an overbalanced pressure ratio prevails in the well,

FIG. 2 is a view corresponding with FIG. 1 of the drilling rig during underbalanced operation, thus when an underbalanced pressure ratio prevails in the well, and

FIG. 3 is a view corresponding with FIGS. 1 and 2 of the drilling rig used with a system of telescopic sluice tubes.

DETAILED DESCRIPTION

A drilling rig 1 (FIG. 1) which is erected at a drilling location DL includes a mast 2 which is placed eccentrically in the shown example embodiment, so adjacently instead of directly above a well 3 (of which only the upper part is shown). This mast 2 is placed on a support or substructure 4 around well 3. Connected to mast 2 are carrying device(s), e.g., in the form of three carriers 5, 6 and 7, of which the lower carrier 7 is releasable. Carriers 5, 6, 7, which are suspended directly above well 3 (as indicated schematically with a dot-dash line) are movable independently of each other in height direction along mast 2. Further suspended from the mast is a platform 8 which is displaceable in height direction. A cabin 9 is situated on this platform 8 on which operatives P can take position.

The upper carrier 5 is provided with a device 17 (shown schematically) for rotatingly driving drill pipes to be fastened thereto, in the form of a so-called top drive. The middle carrier 6 is also provided with a device for rotatingly driving the drill pipes, in this case in the form of a rotary table.

Also disposed at the drilling location DL is a supply device 10 with which drill pipes 29, which are stored lying on racks in the vicinity of drilling rig 1, can be carried to drilling rig 1 and raised. This supply device 10 is provided here with an arm 11 which has an engaging member 30 movable therealong which picks up drill pipes 29. By raising the arm 11 and simultaneously rotating engaging member 30 the drill pipe 29 is carried from the lying position to a vertical position at a determined position above platform 8, where drill pipe 29 is then placed. Arm 11 with engaging member 30 then returns to its starting position.

Well 3 is provided on the top side with a system 12 of blowout preventers, a so-called BOP stack. In the shown example embodiment, wherein the pressure in well 3 is greater than the pressure in the formation in which drilling takes place and there is therefore an overbalanced pressure ratio, the valve system 12 is opened. In the shown example embodiment this valve system 12 is formed by two ram BOPs 13, 14 and an annular BOP 15, while a further conduit 16 for drill fluid is connected to the underside of valve system 12.

Drilling rig 1 is used as follows in this configuration. Supply device 10 feeds one drill pipe at a time which is parked at a position assigned therefor on platform 8. Here the drill pipe is taken over by upper carrier 5 which for this purpose is first displaced along mast 2 to its lowest position 5′. The first carrier 5 is then moved upward, while the drill pipe is moved from its parked position to a position directly above well 3. The drill head is for instance first coupled to the bottom side of the first drill pipe, whereafter each subsequently supplied drill pipe is screwed onto the preceding one.

Once the drill pipe has thus been connected to either the drill head or the preceding drill pipe, the upper carrier 5 is moved downward in controlled manner along mast 2. The drill pipe herein descends into well 3 through an opening in middle carrier 6 and through the normally opened valve system 12. The speed at which upper carrier 5 lowers the drill pipe into well 3 depends on the conditions in well 3. Simultaneously with lowering thereof, the drill pipe—and with it the whole drill string suspended thereunder—can be set into rotation by top drive 17. Due to the combination of the weight of the drill string and the rotational movement thereof the drill head cuts into the bottom of well 3, which is thereby deepened still further.

When upper carrier 5 has reached its lowest position 5′, top drive 17 can be stopped and the drill pipe is engaged by middle carrier 6, or optionally lower carrier 7, which are provided for this purpose with an engaging device. Top drive 17 is then disconnected from the drill pipe which is then situated roughly at the position of platform 8. Upper carrier 5 then takes over a subsequent drill pipe from supply device 10 and moves upward in order to resume drilling.

When drilling is ended or interrupted, for instance in order to change the drilling head, the above described operations are performed in reverse sequence. The drill string is then lifted by upper carrier 5 per length of the drill pipes out of well 3 and held fast by middle carrier 6, whereafter the uppermost drill pipe is in each case removed and stored.

Should a situation occur during the drilling process in which the formation pressure in well 3 becomes greater than the hydrostatic pressure of the liquid column therein, and should it be desirable to allow this situation to persist, the pressure ratio in well 3 therefore becomes underbalanced. In that case the drilling process can be interrupted and, by making determined modifications, the drilling rig and the assembly 12 of blowout preventers can be made suitable for this new operating situation. Use is made for this purpose of the feature that the carrying devices of drilling rig 1 are adapted not only to exert pulling or lifting forces, but also provide the option of exerting pressing forces on the drill pipes.

The configuration of drilling rig 1 is first modified by suspending the lower carrier 7 from mast 2 (FIG. 2). This carrier 7 is mounted at a suitable height on mast 2 at the position of the top of valve system 12. This valve system 12 is usually first expanded, compared to the normal operating state of drilling rig 1, by adding one or more additional valves 18, 19, 20 which play a part in the controlled passage of drill pipes. Compared to the previous situation, valve system 12 is here further expanded with a bypass conduit 23 for drill fluid around the annular valve 15.

Due to the higher position of valve system 12 the lower carrier 7 thus comes to lie higher above the ground G than the middle carrier 6 during normal use of drilling rig 1 when the valve system 12 has fewer valves. That the height of lower carrier 7 can be adjusted is an advantage compared to conventional drilling rigs, where the rig floor is placed at a fixed height above the ground and the space below it is not usually sufficient for placing of the required additional blowout preventers and hydraulic workover unit. In conventional drilling rigs a part of this equipment must therefore be placed above the rig floor, which in turn has consequences for the normal operating process of the drilling rig. (Under these conditions the actual drilling rig can usually no longer be used at all, and all further operations are carried out using the hydraulic workover unit.)

Before the drill pipes can be pressed again into well 3 in this operating situation, lower carrier 7 is first connected to valve system 12. This carrier 7 is provided for this purpose with flexible, pressure-tight connecting device(s). Due to the flexibility of these connecting device(s) the well is disconnected mechanically from the drilling rig. Loads occurring in drilling rig 1 during operations under pressure are thus not transmitted into well 3, but are absorbed instead in the construction of carriers 6, 7 and mast 2. These loads are eventually transmitted directly from mast 2 to support 4 and the ground G.

For the purpose of pressing drill pipes into the underbalanced well 3 use is made according to example embodiments of the present invention of the middle and lower carriers 6, 7. Upper carrier 5 plays no part in this operation and remains free for use in other operations, such as supplying or removing drill pipes in co-action with supply device 10, rotating the drill pipes, etc. Drilling rig 1 can hereby also be used in the normal manner in this situation, so that no different procedures, with the associated safety hazards, need be introduced. It is therefore also possible to continue working efficiently and, above all, safely in this operating situation.

As stated, lower carrier 7 is fixed to mast 2 once it has been brought to the desired height. Middle carrier 6 performs all movements necessary for the process. In order to allow the drill pipes to be pressed through the valve system 12 into the well counter to the formation pressure, the middle and lower carriers 6, 7 are first provided with holding elements or slips 21, 22 which engage on the outer side of the drill pipes, and the diameter of which can be adjusted to that of the relevant drill pipe.

When a drill pipe is clamped between the middle and lower carriers 6, 7, the diameter of holding element 22 on lower carrier 7 is first enlarged so that the drill pipe can move freely therein. Middle carrier 6 is then moved downward so that the drill pipe is pressed through lower carrier 7 and through the valve system 12. Once the drill pipe has been pressed far enough downward, holding element 22 is tightened so that the drill pipe is fixed by the lower carrier. Holding element 21 on middle carrier 6 is then released so that the drill pipe can move freely therein, whereafter middle carrier 6 is moved upward again so as to engage this or a subsequent drill pipe at a higher point. By thus alternately tightening and loosening holding elements 21, 22 synchronously with displacements of middle carrier 6, the drill pipes are as it were passed between carriers 6, 7.

A similar method as specified above is applied when drilling rig 1 is used to introduce drill pipes of differing cross-sectional form or other components into the underbalanced well 3 using a system 24 of telescopic sluice tubes or TDT (FIG. 3). In this case the bottom side of an outer stationary tube 25 is fastened to lower carrier 7 while the inner tube 26 is fastened with its top side to middle carrier 6. While the lower carrier 7 thus holds fast the TDT and transfers forces that occur into mast 2, the middle carrier 6 moves up and downward synchronously with opening and closing of valves 27, 28 of the TDT in order to sluice drill pipes and other components through the TDT into well 3. It is also the case that upper carrier 5 is left free to carry out other operations.

Using the drilling rig design according to example embodiments of the present invention different operations occurring during a drilling process can thus be performed, wherein at least in respect of the procedures carried out above ground no distinction need be made between drilling in an overbalanced well and operation in an underbalanced well. It is also possible to dispense with the use of separate equipment and external staff, which results in considerable savings.

Although the present invention has been described above on the basis of example embodiments, it will be apparent that it is not limited thereto. All described and shown components of the drilling rig can thus be varied in form, dimensions and number. The different aspects of the described method and drilling rig can also be used in all manner of combinations, even together with elements of conventional methods or drilling rigs, while retaining the associated advantages.

The scope of the present invention is therefore defined solely by the following claims.