Offshore drilling rig and a method of operating the same

Abstract

An offshore drilling rig includes a drill deck, at least one work center arranged in the drill deck; a diverter system operatively connectable at least to a riser extending towards the seafloor; a hoisting system adapted for raising or lowering tubular equipment through the work center; wherein the offshore drilling rig includes a first positioning system configured for positioning at least the work center and/or the diverter system and/or a riser tensioning system selectively at a first horizontal position and a second horizontal position, different from the first horizontal position; and wherein the hoisting system is operable to raise or lower tubular equipment through the work center when said work center is positioned at said first horizontal position.

Claims

1. An offshore drilling rig comprising: a drill deck; a first work center arranged in the drill deck; a first hoisting system adapted for raising or lowering tubular equipment through the first work center; a second work center arranged in the drill deck horizontally spaced apart from the first work center; a second hoisting system adapted for raising or lowering tubular equipment through at least the second work center; a riser tensioning system operable to provide tension to said riser; wherein the offshore drilling rig comprises a first positioning system configured for positioning at least the riser tensioning system selectively at a first horizontal position and a second horizontal position, different from the first horizontal position; and wherein the first and second work centers are both primary well centers, each comprising a diverter system arranged below the primary well center, each diverter system operatively connectable to a riser extending towards the seafloor, and the first hoisting system is operable to raise or lower tubular equipment through the said riser at least when said riser tensioning system is positioned at said first horizontal position and the drilling rig is operable to raise or lower tubular equipment through the said riser by means of at least the second hoisting system when the riser tensioning system is positioned at said second horizontal position.

2. An offshore drilling rig according to claim 1, wherein the drilling rig is operable to raise or lower tubular equipment through the first work center and said riser by means of at least the first hoisting system when the riser tensioning system is positioned at said second horizontal position.

3. An offshore drilling rig according to claim 1, wherein the drilling rig is arranged so that the first and second hoisting systems can operate simultaneously to raise or lower tubulars in and out of the first and second work centers, respectively, while the second work center occupies the second horizontal position.

4. An offshore drilling rig according to claim 1, wherein the drilling rig is operable to raise or lower tubular equipment through the second work center and said riser by means of at least the second hoisting system when the riser tensioning system is positioned at said second horizontal position.

5. An offshore drilling rig according to claim 1; wherein the second hoisting system is operable to raise or lower tubular equipment through the diverter system by means of at least the second hoisting system when the diverter system is positioned at said second horizontal position.

6. An offshore drilling rig according to claim 1; wherein the first positioning system is configured to selectively position the diverter system at the first and the second horizontal positions while the second work center is positioned at a third horizontal position, different from the first and second horizontal positions.

7. An offshore drilling rig according to claim 1; wherein the first and second hoisting systems are operable to separately or together raise or lower tubular equipment through said first work center at said first horizontal position.

8. An offshore drilling rig according to claim 1, wherein the first and second work centers are mounted in a substantially horizontal track, and where the first positioning system is adapted for selectively moving and positioning each of the work centers in the horizontal track to a first or a second or a third position in the drill deck.

9. An offshore drilling rig according to claim 8, wherein the horizontal track is linear at least along a part of it, and where its respective diverter system is connected to the first work center and comprises at least one diverter having a first end being connected to the first work center and the other end being supported and fixed with respect to the drill deck and the diverter having at least one telescopic section between the first and the second end, the telescopic section extending parallel to a linear part of the horizontal track in the drill deck.

10. An offshore drilling rig according to claim 1, wherein at least one of the diverter systems is displaceable.

11. An offshore drilling rig according to claim 1; wherein the first hoisting system is configured to raise or lower a first load carrier, and the second hoisting system is adapted to raise or lower a second load carrier; and wherein the drilling rig comprises a second positioning system configured for positioning at least a displaceable one of the first and second load carriers at the first horizontal position and the second horizontal position.

12. An offshore drilling rig according to claim 1, wherein the first work center is displaceable between the first and second horizontal positions and wherein the first work center and a first load carrier are displaceable along parallel paths.

13. An offshore drilling rig according to claim 1, wherein the first and second hoisting systems comprise respective first and second lifting cables hanging from respective first and second cable crowns each supported by a drilling support structure, and where the second positioning system is adapted for shifting at least one of or each of the cable crowns so as to move at least a displaceable load carrier to and from the first and the second horizontal position.

14. An offshore drilling rig according to claim 1, comprising at least one top drive suspended from the first load carrier.

15. An offshore drilling rig according to claim 1, wherein each of the first and second hoisting systems comprises at least one substantially vertically extending linear actuator, such as a hydraulic cylinder, having a stationary end being fixed with respect to the drill deck, and a travelling end comprising at least one cable sheave.

16. An offshore drilling rig according to claim 1, wherein the diverter system is mounted on a track extending between the first and second horizontal positions.

17. An offshore drilling rig according to claim 1, wherein the riser tensioning system and the first positioning system are configured so that the riser tensioning system can be moved from the first horizontal position to the second horizontal position, and vice versa, while the diverter system is connected to the riser extending towards the seafloor.

18. A method of performing drilling operations by using an offshore drilling rig, the drilling rig comprising: a drill deck; a first work center and a second work center arranged in the drill deck horizontally spaced apart from each other, both work centers being operable as a primary well center each comprising a diverter system arranged below the primary well center, each diverter system operatively connectable to a riser extending towards the seafloor, and; a first hoisting system adapted for raising or lowering tubular equipment through at least the first work centers; a second hoisting system adapted for raising or lowering tubular equipment through at least the second work centers; and a riser tensioning system; and wherein the method comprises: positioning the riser tensioning system at a first horizontal position; performing drilling operations through the riser center by means of at least the first hoisting system; moving the riser tensioning system from the first to a second horizontal position, different from the first horizontal position; and continuing drilling operations through the riser work center by means of at least the second hoisting system.

19. A method according to claim 18, wherein said drilling operations are performed using the first hoisting system is performed through the first work center.

20. A method according to claim 18, wherein said drilling operations performed using the second hoisting system are performed through the second work center.

21. A method according to claim 18, wherein a riser is connected to the riser tensioning system when the riser tensioning system is moved from said first horizontal position to said second horizontal position.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) In the following one or more embodiments of the invention will be described in more detail and with reference to the drawing, where:

(2) FIGS. 1a and 1b are concept drawings showing two different operational situations of a dual activity drilling facility seen from one side, incorporating full redundancy for the intended drilling operation by incorporating skidding well centers/work centers in the drill deck.

(3) FIGS. 2a and 2b are concept drawings showing two different operational situations of a cyclic operating hoisting system within the same drilling facility seen from one side, enabling both hoisting systems to work over the same work center individually or in turn for providing fast tripping of drill pipe, casing running or riser- and BOP running/retrieval operation.

(4) FIG. 3 is a concept drawing showing an embodiment of the drilling rig disclosed herein, where each hoisting system comprises a cable winch.

(5) FIG. 4 is a concept drawing showing an operational mode where the two hoisting systems are operated synchronously within the same drilling facility seen from one side, enabling both hoisting systems to work over the same work center in sync for providing a combined lifting capacity in the one work center.

(6) FIG. 5 is a concept drawing showing an operational mode where the two hoisting systems are operated synchronously within the same drilling facility seen from one side, enabling both hoisting systems to work over the same work center in sync for providing a double lifting capacity in the one work center, but with only one hoisting system carrying a topdrive, and with a single work center.

(7) FIG. 6 is a concept drawing showing a transferable diverter housing and mud return tubing system with telescoping diverter overboard lines, seen from below.

(8) FIG. 7 is a sectional drawing showing a cross section through the center of a primary well center in the form of a rotary table supported on a transferable skidbase on tracks arranged in the drill deck, with a diverter housing suspended from underneath the said transferable skidbase and with a riser supported by in-line hydraulic riser tensioners mounted on a separate tracks below the drill deck.

(9) FIG. 8 is a concept drawing showing a transferable primary well center, diverter system and riser tensioning system.

(10) FIG. 9 is a concept drawing showing a transferable diverter system and riser tensioning system.

(11) FIGS. 10-18 illustrate another embodiment of an offshore drilling rig, wherein FIG. 10 shows a side view of the drilling rig, FIGS. 11-14 show 3D views of parts of the drilling rig from different viewpoints, FIGS. 15-16 show horizontal cross-sectional views of the drilling rig, and FIGS. 17-18 show lateral cross sections of the drilling rig.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

(12) FIGS. 1a, 1b, 2a, 2b and 4 all show a drilling support structure 1 arranged above a drill deck 2 and three work centers 3a, 3b and 3c, where one is in the form of a primary well center 3a being equipped with a diverter housing 13. The three work centers 3a, 3b and 3c are supported on individual skidbases on tracks 4 arranged in or below the drill deck 2, and the drilling support structure 1 carries two cable crowns 5a and 5b, in FIGS. 1a, 1b, 2a, 2b and 4 in the form of a crown sheave cluster, being skidably arranged on the top of the drilling support structure 1 on separate tracks 26. Generally, a skidable arrangement allows the displacement, in particular horizontal displacement, e.g. guided by tracks or similar guide means.

(13) From each of the crown sheave clusters 5a and 5b lifting cables 7a and 7b are running down and connecting to a load carrier 8a and 8b each carrying a top drive 9a and 9b at the end of the lifting cables 7a and 7b. Each of the top drives is connected via a retractable dolly 10a and 10b to a vertical track 11a and 11b arranged at the drilling support structure 1. The retractable dollies 10a and 10b are each adapted so that they can position and keep the top drives in different positions above the work centers 3a, 3b, 3c in the drilling deck 2.

(14) In the example of FIGS. 1a, 1b, 2a, 2b, and 4, all three work centers are displaceable. In particular, the primary well center 3a can be positioned in three horizontal positions: a central position and two peripheral positions on respective sides of the central position. The other work centers 3b,c may be positioned on respective peripheral positions and on respective parking positions located further away from the central position so as to allow the primary well center to be positioned at the corresponding peripheral position. The retractable dollies 10a and 10b are each adapted such that each top drive can be positioned above the central position and a respective one of the peripheral positions. It will be appreciated that, in alternative embodiments, the drilling rig may comprise only two work centers, e.g. work centers 3a and 3b, where the work center 3b is positioned at one of the peripheral positions while the primary well center 3a may be positioned at the central position and at the other peripheral position not occupied by the well center 3b. In such an alternative embodiment, the well center 3b may be stationary or displaceable, e.g. so as to allow the primary well center 3a to be moved to each of the peripheral positions.

(15) In the embodiment shown on FIGS. 1a, 1b, 2a, 2b, and 4 each hoisting system has a linear actuator in the form of a hydraulic cylinder 28a, 28b, having its lowermost end 29a, 29b fixed with respect to the drill deck 2 and an upper, travelling end 30a, 30b with a cable sheave 31a, 31b. At least one lifting cable 7a, 7b has one end extending from another hydraulic cylinder 32a, 32b arranged for compensating heave during e.g. drilling operation, and over the travelling cable sheave 31a, 31b and further below a second cable sheave 33a, 33b being fixed with respect to the drilling support structure 1, and thereafter over the crown sheave cluster 5a, 5b, skidably mounted on the drilling support structure 1 on a track 26. In these figures only a single lifting cable is shown for each hoisting system, but in practice, in order to provide significant lifting capacity as well as redundancy in case one cable breaks, multiple mutually parallel lifting cables extending along with the lifting cables 7a, 7b are typically used.

(16) FIG. 3 shows a drilling rig similar to the drilling rig of FIGS. 1a, 1b, 2a, 2b and 4, but comprising a different type of hoisting system. In particular, the drilling support structure 1 shown on FIG. 3 has a hoisting system with two cable crowns 5a, 5b each in the form of a crown block being connected to a travelling block 34a, 34b via multiple cable loops hanging down from the crown block 5a, 5b, In this embodiment each travelling block is carrying a topdrive 9a, 9b. In this embodiment a single lifting cable 7a, 7b is providing the multiple cable loops, and thereby the necessary cable lifting capacity of the hoisting system, and therefore, in order to provide the necessary travelling length of the travelling block, a cable winch 27a, 27b is arranged for each hoisting system. As in the previous embodiments, the drilling rig of FIG. 3 comprises three displaceable work centers. However, as discussed in connection with the previous embodiment, the drilling rig of FIG. 3 may alternatively be provided with only two work centers, one or both of which may be displaceable.

(17) The skilled person will, however, appreciate that the mere combination of the skidable crown sheave clusters 5a, 5b shown on FIGS. 1a, 1b, 2a, 2b and 4 and the linear actuators 28, disregarding other features of the present invention, mutatis mutandis, provides both an efficient and safe lifting capacity, because each hoisting system may comprise multiple lifting cables 7a, 7b extending parallel to each other in order to carry the same load carrier 8a, 8b and the same top drive 9a, 9b.

(18) In FIGS. 4 and 5 a connecting tool 12 is connecting the load carriers 8a and 8b via the top drives 9a and 9b in FIG. 4 and via a single topdrive 9a in FIG. 5. Thereby it is possible to connect a load to the connecting tool 12, so that it is possible to provide a lifting force by combining the lifting force of both hoisting systems lifting the two load carriers 8a and 8b. The drilling rig of FIG. 4 comprises three displaceable work centers. However, as discussed in connection with the previous embodiments, the drilling rig of FIG. 4 may alternatively be provided with only two work centers, one or both of which may be displaceable. The drilling rig of FIG. 5 is shown with a single displaceable work center thus allowing the top drive 9a to operate above the primary well center 3a while the dolly 10a is retracted and to operate above the primary well center 3a together with the other load carrier 8b and the connecting tool 12 when the primary well center is positioned in the central position as shown in FIG. 5. However, as discussed in connection with the previous embodiments, the drilling rig of FIG. 5 may alternatively be provided with two or even three work centers, one or several of which may be displaceable.

(19) FIG. 6 shows the conceptual layout of a preferred embodiment of a diverter system attached to the primary well center 3a such as it is shown on FIG. 7. This diverter system comprises a diverter housing 13 suspended from the skidbase 25 and supporting the rotary table 14 of the primary well center 3a.

(20) The diverter housing 13 has at least two outlet ports 17a and 17b each being connected to telescopic overboard lines 18a and 18b. This allows the diverter housing 13 to be positioned at different positions along a line defined by the track 4 being parallel to the overboard lines. One such position is shown in FIG. 6 in full line, and another is shown with dotted lines.

(21) The diverter housing 13 also comprises a mud return outlet port 19 adapted for leading drilling mud from the diverter housing back to the mud process systems via the main mud return line system 20. The mud return line comprises a number of telescopic connectors 21a 21b and 21c arranged at selected positions in order to connect the mud return line to the mud return outlet port 19 on the diverter housing 13. Alternatively or additionally, the mud return line 20 may comprise a telescopic section extending parallel with the track 4.

(22) In FIG. 7, a displaceable primary well center 3a is shown in more detail comprising the above mentioned components and parts, and in this figure a riser tensioner system is also shown comprising skidding carriages 22 and hydraulic in-line tensioning cylinders 23 being skidably supported by a separate riser tensioner track 24 arranged parallel to and, in this example, below the track 4 supporting the rotary table skidbase 25. Thereby the riser tensioners 23 may be moved along with the rotary table and diverter housing 13 or independently of the rotary table 14 and diverter housing 13.

(23) In the following different modes of operating the drilling rig shown in the figures are disclosed in more detail with reference to the relevant figures.

(24) 1. Full Redundancy:

(25) With reference to FIGS. 1a and 1b especially, but not only, a fully redundant dual activity hoisting- and drilling facility is provided.

(26) Full redundancy is achieved by having a transferable, riser-capable primary well center 3a, which may be positioned under either one of the top drives (e.g. 9a or 9b) and load carriers (8a and 8b) of the two fully rated main hoisting- and drilling systems comprising the facility. In this relation the primary well center may be transferred and positioned as mentioned above, with or without one tubular or a string of tubulars 35, such as riser tubes, casings, drilling pipes or the like being supported and/or hanging down from the primary well center, and these tubulars may be either hanging freely down from the primary well center, or they may extend all the way to the sea floor and further extend into the well or be connected to the well at the sea floor. In the latter case a diverter system and a tensioning system as shown on FIGS. 6 and 7 respectively may be employed along with other well control equipment.

(27) The riser-capable primary well center 3a comprises a rotary table 14 supported by a horizontally transferable skid/trolley 25 that is sunk into a slot in the drill deck 2, so that the rotary table 14 top cover is substantially flush with the drill deck 2 level. Alternatively, the drill deck floor may partially extend above the rotary table.

(28) The transferable skid/trolley 25 rests on horizontal skid beams forming a track 4 spanning the width between the two fully rated main hoisting- and drilling systems.

(29) A diverter housing 13 with telescoping overboard lines (overboard tubing 18a and 18b) and a detachable main flowline (mud return tubing 20) is suspended from underneath the said transferable skid/trolley 25.

(30) A transferable/skidding riser tensioning system is arranged on horizontal skid beams suspended from underneath the drill deck structure, while spanning the full width between the two fully rated main hoisting- and drilling systems.

(31) In the following, examples are given for the intended operation to ensure full redundancy e.g. in the case that the one (active) fully rated main hoisting- and drilling system suffers a main equipment breakdown: 1. While drilling, tripping drill pipe or running casing during the riserless tophole sections of the well. 1.1 the drillpipe or casing string is hung off in the power-slips/casing-spider. 1.2 the drillpipe or casing string, while being suspended from the power-slips/casing-spider inside the rotary table, will be transferred to the opposite fully rated main hoisting- and drilling system. 1.3 drilling, tripping drill pipe or casing running operation may resume on the opposite fully rated main hoisting- and drilling system.
Hence, in this example, only the rotary table/work center may need to be moved without requiring the diverter housing or riser tensioner system to be displaceable. 2. while running or retrieving riser and BOP 2.1 the riser string and BOP is lowered and hung-off in the riser spider and gimbal, which is resting on top of the rotary table. 2.2 the riser string and BOP, while being suspended from the riser spider and gimbal, will be transferred to the opposite fully rated main hoisting- and drilling system. 2.3 running of the riser may resume on the opposite fully rated main hoisting- and drilling system.
Hence, in this example, only the rotary table/work center and, optionally, the spider and gimbal may need to be moved without requiring the diverter housing or riser tensioner system to be displaceable 3. while drilling, tripping drill pipe or running casing through the riser and BOP after this has been connected to the well and the riser has been put in tension. 3.1 the drillpipe or casing string is hung off in the power-slips/casing-spider. 3.2 the well is secured. 3.3 the drillpipe or casing string, while being suspended from the power-slips/casing-spider inside the rotary table, will be transferred to the opposite fully rated main hoisting- and drilling system. 3.4 the diverter housing with telescoping overboard lines and detachable main flowline, suspended from below the transferable. skid/trolley supporting the rotary table, will be transferred to the opposite fully rated main hoisting- and drilling system. 3.5 in fully synchronous motion, the riser string, while being suspended from the riser tensioners will be transferred to the opposite fully rated main hoisting- and drilling system. 3.6 drilling, tripping drill pipe or casing running operation may resume on the opposite fully rated main hoisting- and drilling system.

(32) Hence, in this example, the rotary table/work center and the diverter housing and riser tensioners may need to be moved.

(33) The fully redundant dual activity hoisting- and drilling facility illustrated in FIGS. 1a and 1b will allow for continued operation on either port or starboard side hoisting- and drilling system (by repositioning of the primary well center), while the opposite side is decommissioned for any extended period of time, e.g. for Class required Special Periodic Survey, breakdown or other reason.

(34) 2. Cyclic Dual Hoisting:

(35) With reference to FIGS. 2a and 2b especially, but not only, a cyclic dual hoisting facility is provided especially for Fast Tripping, Casing- or Riser Running:

(36) Fast tripping, casing- or riser running operation is ensured by having two independent and fully redundant hoisting- and drilling systems including top drives 9a and 9b working in cyclic operation over a common primary well center 3a.

(37) Each hoisting system comprises a horizontally transferable cable crown in the form of a crown sheave cluster arrangement 5a, 5b, allowing the crown sheave cluster 5a, 5b to be horizontally transferred, aligned and locked into position over at least two independent positions/well centers 3a, 3b, 3c on the drill deck 2 below.

(38) Each hoisting system supports a vertically travelling load carrier 8a, 8b arrangement, from which a topdrive 9a, 9b is suspended below on a horizontally extend-/retractable dolly 10a, 10b system for guiding of the topdrive 9a, 9b.

(39) The extend-/retractable dolly 10a, 10b shall have a horizontal travel corresponding with the horizontal travel of the crown sheave cluster 5a, 5b arrangement above, while extending/retracting horizontally in synchronous motion with the skidding crown sheave cluster 5a, 5b arrangement above, ensuring that the hoisting system is kept in true vertical alignment with the cable sheave cluster/load carrier/yoke/hook arrangement and the topdrive 9a, 9b suspended underneath it.

(40) In the following typical examples are given for the intended cyclic operation of the dual hoisting and drilling facility to provide fast tripping, casing or riser running operation:

(41) 1. Tripping/running in the hole: low setback and pipe racking system off-drill deck.

(42) 2. Tripping/running in the hole: setback and pipe racking system on-drill deck.

(43) Fast running or retrieval of the riser and BOP may be performed in a similar cyclic operation, while employing suitable arrangements for facilitating handling of the riser joints to/from the primary well center 3a with this being in the center position.

(44) An added benefit of the invention is the full redundancy provided within this facility in that each hoisting and drilling system will offer full redundancy for the other system in the center operating position.

(45) 3. Synchronous Dual Hoisting:

(46) With reference to FIG. 4 especially, but not only, a synchronous dual hoisting facility is provided for heavy duty well construction.

(47) The synchronous hoisting facility is realized by utilizing the two independent and fully redundant hoisting- and drilling systems in a combined synchronous mode of lifting operation above the common primary well center 3a, by using a connecting tool 12.

(48) In FIG. 4 the synchronously hoisting facility comprises two topdrives, but as shown in FIG. 5 it is possible to operate the hoisting systems synchronously even when only one topdrive 9a is used. In this situation the connecting tool 12 is carried by the topdrive 9a on one side, but is directly connected to the hoisting cable via the load carrier 8b at the other side.

(49) This principle allows for extra heavy duty lifting operation without necessitating any of the two hoisting- and drilling systems to be rated beyond the current design loads of such equipment, where especially the load capacity of the topdrives 9a, 9b are limiting the load capacity of the hoisting systems.

(50) Recurring requests for rigs capable of running extended sections of heavy wall casing strings through deep formations in ultra-deep water may require lifting facilities of 1500 metric tons SWL or beyond.

(51) Current designs of hoisting and drilling systems are limited to approx. 1200 metric tons only, with systems and equipment currently under design and development for up to 1500 metric tons.

(52) Consequently, the next generation of DW drilling rigs may only provide incrementally larger hoisting capacity compared with the current generation of rigs and will therefore restrict well designs to within the 1500 metric tons limit of the next generation of top drives.

(53) However, this invention will allow for hoisting and lowering loads exceeding 2000 metric tons, limited only by the structural integrity and load carrying capacity of the casing and landing string tubulars, running- and handling tools.

(54) A generic ultra-deep subsalt and/or HPHT well development program in ultra-deep water might utilize some or all aspects of embodiments of the drilling rig disclosed herein, e.g. through the following steps and transitions between modes of operation:

(55) 1. Dual activity operation for concurrently drilling tophole sections, while running and cementing casing down to and including the 18 casing section (e.g. using a configuration as illustrated in FIG. 1a).

(56) 2. Dual activity operation for concurrently running riser and BOP, while cementing the 18 casing section, incorporating full redundancy (e.g. using a configuration as illustrated in FIG. 1a).

(57) 3. Transferring the primary well center 3a with riser and BOP suspended to the center position with subsequent landing of the BOP (e.g. using a configuration as illustrated in FIG. 2a).

(58) 4. Drilling, tripping and running casing in cyclic operation through riser with primary well center 3a in center position (e.g. using a configuration as illustrated in FIG. 3).

(59) 5. Running and landing extra-long heavy casing sections in synchronous dual hoisting mode (e.g. using a configuration as illustrated in FIG. 4).

(60) FIG. 8 is a concept drawing showing a displaceable primary well center, diverter system and riser tensioning system. In particular, FIG. 8 shows a part of the drill deck 2 including a displaceable primary well center 3a. The well center is defined by hole of a rotary table 14 that is skidable along tracks 4 in the drill deck. In the example of FIG. 8 the rotary table has a top surface that is flush with the upper surface of the drill deck, and the drill deck defines a slot 40 having a width matching the size of the rotary table. In other embodiments, the slot may be narrower, e.g. by letting the drill deck surface extend partially across the rotary table. Alternatively or additionally, the slot 40 may be covered by plates or hatches which may be removed during the skidding of the well center. The drilling rig of FIG. 8 further comprises a diverter system comprising a diverter housing 13 from which outlet ports 17 and 19 extend. The diverter housing is mounted below the well center 3a and arranged to be skidable together with the well center 3a. For example, the diverter housing may be suspended from a skidbase supporting the rotary table, as described in connection with FIG. 6 above. The drilling rig of FIG. 8 further comprises a riser tensioning system comprising hydraulic in-line tensioning cylinders 23 being skidably supported by a separate riser tensioner track (not explicitely shown in FIG. 8) arranged parallel to the track 4 supporting the rotary table 14, e.g. as described in connection with FIG. 6. Thereby the riser tensioners 23 may be moved along with the rotary table and diverter housing 13. The riser tensioning cylinders are, at their lower end, connected to a tensioner ring 41 which engages a marine riser string 15 so as to control the tension on the riser string. In the example of FIG. 8, the rotary table 14, the diverter housing 13 and the riser tensioners 23 with the riser string 15 suspended from it may individually or jointly be skidded in the direction indicated by an arrow in FIG. 8. As is further illustrated in FIG. 8, the above components may even be jointly skidded while a string of pipe 35 is suspended in the rotary table 14 and extends downward through the riser 15.

(61) FIG. 9 is a concept drawing showing a displaceable diverter system and riser tensioning system. In the example of FIG. 9, the drilling rig comprises two stationary rotary tables 14a,b, each defining a work center 3a,b, respectively, that is operable as a primary well center. The work centers 3a,b thus define respective first and second horizontal positions. The drilling rig of FIG. 9 further comprises a diverter system comprising a diverter housing 13 from which outlet ports 17 and 19 extend. The diverter housing is mounted below the drill deck 2 and arranged to be skidable along tracks 95 extending between the rotary tables 14a,b. To this end, the diverter housing comprises carriages 94 movably attached to the tracks 95. The drilling rig of FIG. 9 further comprises a riser tensioning system comprising hydraulic in-line tensioning cylinders 23 being skidably supported by a separate riser tensioner track (not explicitly shown in FIG. 8) arranged parallel to the tracks 95 that supports the diverter housing 13, e.g. as described in connection with FIG. 6. Thereby the riser tensioners 23 may be moved along with the diverter housing 13 between positions under the respective work centers 3a,b. The riser tensioning cylinders are, at their lower end, connected to a tensioner ring 41 which engages a marine riser string 15 so as to control the tension on the riser string. In the example of FIG. 9, the diverter housing 13 and the riser tensioners 23 with the riser string 15 suspended from it may jointly be skidded in the direction indicated by an arrow in FIG. 9. Consequently, the work centers may selectively be operated as primary well center so as to obtain an increased redundancy, e.g. in case of a failure of a hoisting system that operates above one of the work centers.

(62) FIGS. 10-18 show another embodiment of a drilling rig, in this example of drillship having a hull 1501. In particular, FIG. 10 shows a side view of the drilling rig, FIGS. 11 and 12 show views of the drill floor seen from the starboard side of the drillship, FIGS. 13 and 14 show views of the drill floor seen from the port side of the drillship (a part of the hull of the ship is cut away in FIG. 14); FIGS. 15 and 16 show horizontal cross sections in a plane above the drill deck and a plane below the drill deck, respectively; finally, FIGS. 17 and 18 show lateral cross sections of the drill ship.

(63) The drilling rig of the present embodiment comprises a drill deck 2 formed on top of a substructure 1597. The substructure comprises a platform supported by legs. The platform defines the drill deck and spans across a moon pool 2122 formed in the hull of the drillship. The drill deck 2 comprises two holes defining well centers 3a,b. The drilling rig comprises a drilling support structure in the form of a mast 1. In the present example, the well centers are located within the footprint of the mast 1. The mast includes two mast portions, each associated with, and adjacent to, one of the well centers. The dual activity mast 1 is supported by the substructure 1597 and extends upwardly from the drill deck 2. The mast comprises two mast portions arranged in a face-to-face configuration, i.e. the respective mast portions are located along the axis connecting the well centers such that both well centers are located between the mast portions. Each mast portion supports a hoisting system, each for lowering a drill string through a respective one of the well centers 3a,b towards the seabed. In the example of FIGS. 10-18, the drilling rig comprises two well centers, one of which being operable as a primary well center 3a and being equipped with a diverter housing 13. The primary well center 3a is supported on a skidbase 25 on tracks 4 arranged below the drill deck (e.g. as shown in more detail in FIGS. 7 and 8) so as two allow the well center and the diverter housing 13 to be displaced along the direction connecting the two well centers. Alternatively, both work centers may be provided with a diverter and be operable as primary well centers. The skidbase extends across the moon pool, and the tracks are mounted on opposite sides of the moonpool and they extend along the direction connecting the well centers. The drilling rig may further comprise a skidable riser tensioning system as described in connection with FIGS. 7 and 8. The primary well center 3a may be moved between a first, central horizontal position, as shown in FIGS. 12 and 13, and a second, peripheral position 1003c, where the first position is located on the axis connecting the second position 1003c and the work center 3b which is located at a third horizontal position. In the present example, the first position is positioned substantially in the center between the second position 1003c and the work center 3b. The position not currently occupied by the displaceable well center (e.g. the second position 1003c in FIGS. 12 and 13) may be covered by floor plates or a similar cover 1584. In alternative embodiments, both well centers may be displaceable. In yet another embodiment, the drilling rig may comprise three well centers, e.g. aligned along a common axis. Each of the two hoisting systems may be operable to lower tubulars selectively through a work center at each of at least two horizontal positions, such as the central position (where the primary well center 3a is located in the example of FIG. 12) and one of the peripheral positions (the position of the work center 3b and the second position 1003c). To this end, the mast 1 carries two cable crowns 5a,b, e.g. in the form of a crown sheave cluster or in the form of a crown block, being skidably arranged on the top of the mast on separate tracks.

(64) From each of the cable crowns lifting cables 7a,b are running down and connect to a corresponding top drive 9a,b which is suspended from a hook or other load carrier connected to the lifting cables. Each of the top drives is connected via a retractable dolly 10a,b to a vertical track arranged at the mast 1. The retractable dollies are each adapted so that they can position and keep the top drives in different positions above the well centers, as described herein.

(65) Each hoisting system has one or more linear actuators in the form of a hydraulic cylinder 28a,b having its lowermost end fixed with respect to the drill deck and an upper travelling end with a cable sheave. At least one lifting cable has one end extending from another hydraulic cylinder arranged for compensating heave during e.g. drilling operation, and over the travelling cable sheave and further below a second cable sheave being fixed with respect to the mast, and thereafter over the cable crown. The hydraulic cylinders are displaced from the well centers along the direction connecting the well centers and positioned such that both well centers are located between the cylinders of the respective hoisting systems. As can be most easily seen on FIG. 20, the cylinders of each hoisting system are further (optionally) arranged in two groups of cylinders positioned on either side of an axis connecting the well centers so as to form a gap through which a catwalk machine 1508 or other pipe handling equipment can travel and feed tubulars to one or both of the well centers. Each cable crown 5a,b defines an axis that is parallel to the direction connecting the two groups of cylinders of one of the hoisting systems.

(66) As is most easily seen in FIG. 12, both hoisting systems may cooperate so as together to lower or raise tubulars through the same well center, e.g. the primary well center when located at a central position as illustrated in FIG. 12. To this end, a connecting tool 12 may be arranged to connect the top drives 9a,b. In this example, the connecting tool is in the form of an elevator and bail sections connected to said elevator in one end and suitable for being lifted by second elevators each connect to a top drives 9a,b via bails in the conventional manner. A stabbing and circulation device (e.g. in the form a Casing Fill-Up and Circulating System tools or FLOW BACK & CIRCULATION TOOLS FOR DRILL PIPE (CFT)) is mounted between the bail sections and further connected to a mud connection, preferably of one or both (as illustrated here) of the top drives. Thereby it is possible to connect a load to the connecting tool 12, so that it is possible to provide a lifting force by combining the lifting force of both hoisting systems lifting the connecting tool. To better support increased loads, the mast comprises diagonal beams 1578 forming an inverted V. In alternative modes of operation, the two hoisting systems may be operated above respective well centers or they may be operated in a cyclic dual hoisting mode over a single well center, e.g. as described herein.

(67) The drilling rig further comprises a pipe storage area 1509 for storing pipes in horizontal orientation and catwalk machines 1508 or other horizontal pipe handling equipment for transporting pipes between the storage area 1509 and the well centers 3a,b. To this end, the catwalk machines are aligned with the axis defined by the two well centers.

(68) The drilling rig comprises a setback structure 1812 or similar pipe storage structure for storing stands of tubulars below the substructure 1597 and partly covered by the drill deck 2. The setback structure comprises a support framework 1890 supporting fingerboards having horizontally extending fingers between which tubulars may be stored. The setback structure is arranged so as to allow stands to be moved to/from both well centers from/to the setback. To this end, one or more column rackers 1891 or similar vertical pipe handling equipment may be arranged to move stands into and out of the setback structure 1812. The setback structure 1812 further comprises stand building equipment 1877 configured to build stands from individual pieces of pipe. The setback structure 1812 is located adjacent the moon pool 2122 laterally displaced from the axis defined by the well centers.

(69) Moreover the drilling rig comprises one or more further catwalk machines 1876 configured to feed tubulars from the pipe storage area 1509 or from other storage areas on the opposite side of the mast (towards the aft of the ship) to the stand building equipment 1877. The stand building equipment 1877 may thus receive the pipes from the catwalk machine 1876, bring them in upright orientation, and connect them to other pieces so as to form stands. To this end the stand building equipment may comprise a mousehole through which the stand may be gradually lowered while it is made up until the lowermost end of the stand is at the lowermost level of the setback area 1812, while the uppermost end of the stand is below the drill floor level. The stands may then be received by pipe rackers 1891 and placed in the setback structure 1812 for future use. To this end the pipe rackers are operable to traverse across the setback area, e.g. in the direction parallel to the direction connecting the well centers.

(70) The drilling rig comprises a number of slanted chutes 1892 each for feeding pipes from the setback area 1812 to one of the well centers. To this end the drilling rig may comprise one chute for each well center position, i.e. either the fixed well-center positions or the positions to which a skidable well center can be moved. Alternatively, the chutes may be displaceable so as to be selectively aligned with respective well centers. Each chute 1892 receives pipes from one of the pipe rackers 1891 and feed the pipes in a slanted upward direction through a corresponding slit 1785 in the drill floor towards a respective one of the well centers 3a,b, where they are picked up at their uppermost end by the corresponding hoisting system and lifted through the slit 1785 until they are vertically suspended above the corresponding well center. To this end, the drilling rig further comprises pipe handling equipment 1786 operable to guide the pipes while they are being lifted through the slit 1785. The slits 1785 are elongated and point away from the axis connecting the well centers and towards the side where the setback area 1812 is positioned. To allow for the pipes to be presented in this fashion, the driller's cabin 1534 is positioned at an elevated level above the slits 1785. One or more further pipe handling devices, such as iron roughnecks 1727, may be located between neighbouring slits and underneath the driller's cabin, e.g. such that each iron roughneck may service two well center positions.

(71) The drilling rig comprises another storage area 1515 below the drill deck 2 and configured for storing risers in a vertical orientation. The riser storage area 1515 is located adjacent the moon pool 2122, e.g. on the side of the moon pool opposite the setback structure 1812. The risers may then be moved, e.g. by means of a gantry crane 2298 and respective chutes 2294 or other suitable pipe feeding equipment through holes 1681 in the drill deck floor. The riser feeding holes 1681 may be covered by removable plates, hatches or similar covers, as illustrated in e.g. FIGS. 13 and 15. The riser feeding holes are displaced from the axis connecting the well centers.

(72) As the stands of tubulars and the risers are stored below the drill deck, and since the cat walk machines 1508 extend towards opposite sides from the well centers, and since the mast structure 1 is located on one side of the well centers, the drill deck provides a large, unobstructed deck area on the side of the well centers opposite the mast. This area provides unobstructed access to both well centers and is free of pipe handling equipment. Consequently, these areas may be used as working area, e.g. for rigging up suspendable auxiliary equipment, and/or for positioning on-deck auxiliary equipment. Moreover, at least parts of the setback structure 1812 may be covered by a platform 1788 so as to provide additional storage or working area.

(73) Although some embodiments have been described and shown in detail, the invention is not restricted to them, but may also be embodied in other ways within the scope of the subject matter defined in the following claims. In particular, it is to be understood that other embodiments may be utilized and structural and functional modifications may be made without departing from the scope of the present invention.

(74) The mere fact that certain measures are recited in mutually different dependent claims or described in different embodiments does not indicate that a combination of these measures cannot be used to advantage.

(75) It should be emphasized that the term comprises/comprising when used in this specification is taken to specify the presence of stated features, integers, steps or components but does not preclude the presence or addition of one or more other features, integers, steps, components or groups thereof.