WO2022096111A1

WO2022096111A1 - Dual mode operation of a drilling rig

Info

Publication number: WO2022096111A1
Application number: PCT/EP2020/081185
Authority: WO
Inventors: Morten Norderud-Poulsen; Michael Olesen; Henrik Kjaer
Original assignee: Maersk Drilling AS
Current assignee: Noble Drilling AS
Priority date: 2020-11-05
Filing date: 2020-11-05
Publication date: 2022-05-12
Anticipated expiration: 2023-05-05

Abstract

An off-shore drilling rig has a drill deck with two well centers adapted for performing drilling operations on a pair of well bores simultaneously. The sequence of drilling operations are performed by alternating between drilling a well stage and subsequently arranging a casing in the drilled well stage. According to the invention, the sequence of drilling operations performed by a first one of the two well centers is shifted in time a relatively to a sequence of drilling operations performed by the other of the two well centers, whereby only one of the two well centers is drilling a well stage at the same time.

Description

DUAL MODE OPERATION OF A DRILLING RIG

The present invention relates to operating an off-shore drilling rig having a drill deck with two second well centers. The invention also relates to an offshore drilling rig and a method of drilling a plurality of well bores by mean of an off-shore drilling rig.

BACKGROUND OF THE INVENTION

At some oilfields, it is beneficial to drill a plurality wells in close to each other. These wells may be arranged in a few meters distance. Due to many wells to be drilled, it is time consuming to drill such an oil field, and there is a need for speeding up the drilling process.

SUMMARY OF THE INVENTION

This purpose is according to the invention achieved by a method of operating an off-shore drilling rig having a drill deck with two well centers as defined in claim 1, an off-shore drilling rig as defined in claim 7, and a method of operating an off-shore drilling rig having a drill deck with two well centers as defined in claims 10 and 15. Preferred embodiments are defined in the dependents claims.

According to one aspect of the invention, there is provided a method of operating an off-shore drilling rig having a drill deck with two well centers adapted for performing drilling operations on a pair of well bores simultaneously. This method comprises performing a sequence of drilling operations including alternating between drilling a well stage and subsequently arranging a casing in the drilled well stage, and shifting in time a sequence of drilling operations performed by a first one of the two well centers relatively to a sequence of drilling operations performed by a second one of the two well centers, whereby only one of the two well centers is drilling a well stage at the same time.

In one embodiment, the method includes performing the sequence of drilling operations in a sequence of drilling operation windows, wherein the two well centers in one drilling operation window, in respective well bores, performs drilling the well stage and arranges the casing in the drilled well stage.

In one embodiment, the method includes completing one drilling operation window when both the two well centers have completed respective tasks for the one drilling operation window, the tasks includes performing drilling the well stage and arranging the casing in the drilled well stage. In one embodiment, the method includes performing the sequence of drilling operations in a sequence of drilling operation windows, each drilling operation window comprises for the two well centers either drilling the well stage or arranging the casing in the drilled well stage, wherein the well stages comprising either drilling for or arranging one of the following casing types: a surface casing, one or more intermediate casings, and one or more production casings.

In one embodiment, the method is used for drilling a plurality of well bores arranged as pairs, why the off-shore drilling rig is adapted to displace the drill deck with two well centers to another pair of well bores. In one embodiment, the method includes steps of skidding a cantilever carrying the drill deck relatively to the offshore rig.

According to a second aspect of the invention, there is provided an off-shore drilling rig comprising a drill deck, a first well center, and a second well center. The first well center and the second well center are adapted for performing, in a sequence of drilling operation windows, a sequence of drilling operations including alternating between drilling a well stage and subsequently arranging a casing in the drilled well stage. The first well center starts performing the sequence of drilling operation windows one drilling operation window ahead of the second well center.

In one embodiment, the rig has a cantilever carrying the drill deck, and the cantilever is skiddable relatively to the off-shore drilling rig to displace two well centers. In one embodiment, the distance between the first well center and the second well center corresponds to the distance between a pair of well bores.

According to a third aspect of the invention, there is provided a method of operating an off-shore drilling rig having a drill deck with two well centers adapted for performing drilling operations on a pair of well bores simultaneously. The method includes alternating between drilling a well stage and subsequently arranging a casing in the drilled well stage in a sequence of drilling operations in a sequence of drilling operation windows for each of the two well centers, and shifting in time the sequence of drilling operation windows of the two well centers, whereby only one of the two well centers is drilling a well stage at the same time.

In one embodiment, the method includes completing one drilling operation window when both the two well centers have completed respective tasks for the one drilling operation window, the tasks includes performing drilling the well stage and arranging the casing in the drilled well stage.

In one embodiment, each drilling operation window comprises for the two well centers either drilling the well stage or arranging the casing in the drilled well stage, and wherein the well stages comprising either drilling for or arranging one of the following casing types: a surface casing, one or more intermediate casings, and one or more production casings.

According to a fourth aspect of the invention, there is provided a method of operating an off-shore drilling rig having a drill deck with two well centers adapted for performing drilling operations on a pair of well bores simultaneously. The method includes performing a sequence of drilling operations including alternating between drilling a well stage and subsequently arranging a casing in the drilled well stage, and performing a sequence of drilling operations in a sequence of drilling operation windows, wherein a first one of the two well centers in a drilling operation window, performs drilling in a first one of said pair of well bores, and wherein a second one of the two well centers in the drilling operation window, arranges the casing in a second one of said pair of well bores.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in further details with reference to preferred aspects and the accompanying drawing, in which:

Fig. 1 illustrates schematically a Hydrocarbon Production Well with Multiple Casing Strings drilled by means of the method according to an embodiment of the invention;

Fig. 2 illustrates schematically one embodiment of a dual mode operation off-shore oil drilling rig according to the invention;

Fig. 3 illustrates schematically a bottom hole assembly in a well bore

Fig. 4 illustrates schematically the operation on multiple well bores by means of the off-shore oil drilling rig shown in fig. 2 and seen from above;

Fig. 5 a flow chart illustrating of performing drilling operations on a plurality of well bores according to the invention;

Fig. 6 illustrates dual mode operation in drilling operation windows of the two well centers.

DETAILED DESCRIPTION Referring to fig. 1, a well-known drilling process for building a well bore for oil or gas production. The steps are almost universally applied to the drilling of almost all wells. Exploration geologists generates an exploration well prospect, and development geologists creates plans for development wells locations and objectives. In collaboration with the geologists, drilling engineers develop some detailed drilling plans taking safety, environmental, and regulatory requirements into account. Additional geological surveils may be carried out to identify the locations of potential shallow gas hazards.

Offshore drilling takes place at water depths range typically up to approximately 150 meters for jack-up rigs and up to approximately 3.000 meters for semisubmersibles and drill ships. Drilling a single well may take from 2 weeks up to 12 months depending on the complexity of the project.

A conduit (also known as a riser or a conductor) made from sections of steel pipe permits drilling fluids to move between the rig and a well head.

Referring to fig. 3, a drill string 170 is used for drilling the well, and the drill string includes a plurality of connected slender steel pipes other tools used for the drilling. At the bottom of the drill string 170 is a hole-boring device called a drill bit 300. Drill collars 310 are heavy sections of pipe and are used for adding weight and stability to the drill bit 300.

While the drilling proceeds, the drilling crew adds new sections of drill pipe to the ever-lengthening drill string. Hydraulic devices keep constant tension on the drill string to prevent the motion of the rig and conductor from being transmitted to the drill bit 300.

The drill string is lowered through the conductor through a blowout preventer (BOP) 70 which is a stack of safety valves designed to contain natural high pressures encountered while drilling through the geological formations. The purpose of the blowout preventer 70 is to prevent a blowout, which is an uncontrolled eruption of oil, gas or other wellbore fluids into the sea 1 due to excessive natural pressure.

The blowout preventer 70 is mounted on a wellhead, and the wellhead provides a suspension point for the drill string and pressure seals for casing strings in the well bore. The wellheads may be welded onto the first string of casing, which has been cemented in place during drilling operations, to form an integral structure of the well.

In a first drilling sequence, a large diameter hole capable of accommodating the first string of casing (surface casing) is drilled. In one embodiment, the surface casing 20 may have a diameter in the range from 40 to 100 cm (18-40 inches). The first section of the well to be drilled is the section going down to the pre-determined surface casing depth.

The surface casing 20 does typically pass through the loose sediments and loose rocks to the bed rock. The objective of the surface casing 20 is to isolate the wellbore from the seabed to ensure that loose debris or sediments does not enter the well during early drilling operations.

When the surface casing point is reached, the surface casing 20 is run into the wellbore and cemented into place. First, the drill pipe is removed from the wellbore during cementing operations. Then, the surface casing 20 is lowed into the well bore and cement slurry is pumped down into the interior of the surface casing 20. The cement is forced, with drilling fluid, up into the annular space between the casing string and the wellbore, and the cement is allowing time to cure (harden).

The cementing of the surface casing 20 serves several purposes. The surface casing 20 act as a barrier separating liquids inside the casing from water outside the casing, and prohibits the fluids mixing up. The barrier also protects the well from shallow gas hazards. The blowout preventer 70 may then be installed on the surface casing string 20.

This drilling process then is continued to the next pre-determined casing point. The selection of these intermediate casing points is coming from the detailed drilling plans mentioned earlier. As the drilling string, drilling tools and intermediate casing 30 must pass through the spacing provided by the surface casing 20, it is obvious that intermediate casing 30 has an outer diameter being smaller than the inner diameter of the surface casing 20. The purpose intermediate casing 30 is to act as a barrier and to isolate the interior of the casing against over-pressured zones behind the barrier (prevent a kick) and to protect against unstable geological sections outside the barrier.

The process is repeated for each of the planned casing points. Successive casing strings are run and cemented into place, and smaller diameter tools and drill bit 300 must be used for the continued drilling operations.

During the drilling, the driller/person in charge may maximize the drilling speed or the Rate of Penetration (ROP) of the drill bit 300 by controlling drilling parameters like the weight-on-bit and the rotational speed of the rotary system 180.

Once the final intermediate casing 30 string is run and cemented, the drilling process is continued until the well reaches the Total Depth (TD) of the well. One or more production casing-strings 40 or liners may be applied to reach the Total Depth. The production casing string 40 is extending into the oil and gas reservoir 4. The production casing string 40 or production liner is used in the completion, and the production casing string 40 is run and cemented. The production casing string has perforations allowing fluids from the oil and gas reservoir 4 to collected in a production tubing 50.

Fig. 1 shows the architecture of the well bore 10 and well head 60. The surface casing 20 seals the bore in a layer with sediments and loose rocks in a layer 2 between the seabed and the bedrock 3. Two intermediate casings 30 and 35 act as barriers and to isolate the interior of the casing against over-pressured zones and unstable geological sections outside the intermediate casings 30 and 35. The production casing string 40 is extending into the oil and gas reservoir 4. The oil and gas reservoir 4 is a subsurface pool of hydrocarbons contained in porous or fractured rock formations. Inside the, the production tubing 50 brings pressurized hydrocarbons to a production rigs for further handling. A production packer 52 is used to isolate a section of a borehole to prevent hydrocarbons to escape the production casing string 40.

The seabed wellhead 60 provides a suspension point for the drill string and pressure seals for casing strings in the well bore.

The blowout preventer (BOP) 70 including the stack of safety valves protecting against blowouts is placed on top of the wellhead 60. In some embodiments, it is beneficial to be able to remove the blowout preventer (BOP) 70 when changing well center on the drilling rig for operation on the well. In these situations, a valve unit 65 may be integrated in the well head 60 or mounted on the well head 60. This valve unit 65 will be able to seal the well efficiently so the conductor and the blowout preventer (BOP) 70 may be removed safely not risking any leakage or sea water entering the well.

The blowout preventer (BOP) 70 is a part of the Well Control System on a drilling rig is the system that prevents the uncontrolled, and potentially catastrophic release of high-pressure formation fluids (oil, gas, or salt water) from subsea geological formations. These uncontrolled releases of formation fluids are referred to as Blowouts. Due to the explosive nature of oil and gas, any spark on the surface can result in the ignition of the fluids and an explosion on the rig.

The blowout preventer (BOP) 70 is basically a piece of equipment being operated hydraulically, meaning that pressurized fluids are used to operate pistons and cylinders to open or close the valves in the blowout preventer (BOP) 70 to isolate the well from the rig. These valves (not shown) are typically stacked and may include one or more of the following: annular preventers, blind rams, shear rams or combinations thereof.

The Annular preventer is a ring-shaped piece of equipment on the top of the valve stack of the blowout preventer (BOP) 70 and is used to prevent flow through the annular space between the drill string or casing and the annular preventer. The annular preventer may include a doughnut shaped bladder that when in the open position allows the drill pipe to rotate but in the closed position seals the annulus. Blind rams isolate both the pipe and the annular space by crushing the pipe and it pinching-off when closed. Shear rams isolate both the pipe and the annular space by shearing-off the pipe when closed. Blind shear rams isolate both the pipe and the annular space by shearing-off and crushing the pipe when closed. Pipe rams isolate the annular space by wrapping around the pipe when closed.

In some embodiments, the subsea well shown in fig. 1 can also be on a jacket.

Fig. 2 shows one embodiment of an offshore drilling rig 100 for carrying out the method according to the invention. The offshore drilling rig 100 illustrated is a so-called jack-up drilling rig. The jackup drilling rig has a hull 120 that can be jacked up above the sea level using legs 110 that can be lowered. These jack-up drilling rigs are typically used in water depths up to 120 meters or more depending on the legs 110. When the legs are resting on the seabed and the hull 120 is raised, the drilling operations may take place. The jack-up drilling rigs are designed to move from place to place, and then anchor themselves by deploying their legs 110 to the ocean bottom using a rack and pinion gear system (not shown) on each leg 110.

The hull 120 carries a cantilever 130 being a rigid structural element that extends horizontally hull 120 and being supported at only one end. In one embodiment, the cantilever 130 is displaceable from a first position fully within the horizontal contour of the hull 120 to a second position where the cantilever 130 extends horizontally e.g. 40 meters the contour of the hull 120. The cantilever 130 construction allowing an overhanging structure without additional support. In one embodiment, the displacement of the cantilever 130 takes place by skidding the cantilever 130 along a greased steel frame.

According to the invention, a first well center 140 and a second well center 141 are provided on a drill deck 131 on the cantilever 130. The two well centers 140 and 141 are placed as close as possible to each other, and due to the elements applied in the well centers 140, 141 when placed next to each other in the same plane or level, the minimum distance will be minimum 4-5 meters. The two well centers 140, 141 comprises a static mechanical structure called a derrick 150 and 151 or a mast. In some embodiment, the mechanical structure is provided as two separate two derricks 150 and 151 or masts, and in other embodiment the mechanical structure is provided as one coherent and common derrick or mast structure as shown in fig. 2.

Each of the derricks 150, 151 support a hoisting system 160 (only reference numbers for the derrick 150 for clarification reasons) for the first well center 140, and thereby the drill string handled by the well center 140. In some embodiments, the derricks 150, 151 is pyramidal in shape to achieve a good strength-to-weight ratio. A crown block 164 and a draw-work of the hoisting system 160 comprises a winch that reels the drilling line in or out on a drum 161 and causing a traveling block 162 to move up or down. The traveling block 162 is connected to a swivel 163 connecting the hoisting system 160 to a rotary system 180 and to a circulation system 190. The swivel 163 holds the weight of the drill string on the hoisting system 160, allow rotation of the drill string below it, and includes a Bottom-Hole Assembly (BHA) and the drill bit 300, and allow passage of the drilling fluid from the circulation system 190 to the rotating drill string,

In some embodiments, the draw-works is the component of the hoisting system 160 consuming most energy from a power system 200 on the rig. The draw-works includes a braking system used to control the Weight-on-Bit (WOB) during drilling. The weight-on-bit is achieved with the weight of the drill pipe and drill collars (the Bottom-Hole Assembly (BHA)). The braking system is used to take up some of the weight of the drill string, so that the weight-on-bit is only a fraction of the total weight.

The offshore rig 100 has a circulation system 190 allowing drilling fluid or mud to circulate down through the hollow drill string and up through the annular space between the drill string and wellbore. It is a continuous system of pumps, distribution lines, storage tanks, storage pits, and cleansing units. Drilling fluid is mixed in mud pits and pumped by mud pumps 192 through the swivel 163, a top drive 166, a rotary table 181, a diverter 165, through a blow out preventer (BOP) 169 and down through the hollow drill string 170 inside the conductor string 175. In the well, drilling fluid passes through holes in the drill bit 300, and transport cuttings through the annular space between drill string 170 and wall 305 of the well, through the annular space in the conductor string 175 towards the offshore rig 100. Via a diverter 165, the drilling fluid containing rock cuttings is led to a separation system 191. In the separation system 191, major cutting pieces are separated from the drilling fluid in a shale shaker (vibrating mesh screens) and hydro-cyclones (desander/desilter) using centrifugal forces to separate the smaller solids from the drilling fluid, where after the drilling fluid is recirculated. The circulation system 190 removes cuttings from bore hole, lubricates and cools the drill bit 300.

The mud pump 192 of the circulation system 190 and the hoisting system 160 are the two most power consuming systems in the drilling process.

According to the invention, drilling is only performed in one of the two well centers 140, 141 at a time, why the separation system 191 and the mud pump 192 may be common for the two well centers 140, 141. Appropriate valves may be used between two drilling operation windows (DOW) for coupling the separation system 191 and the mud pump 192 to the well center 140, 141 to drill.

The top drive 166 is large drill machine with a hole drilled through the middle for a fluid path and to transmit rotation to the drill string 170, while allowing the drill string 170 to be lowered or raised during rotation.

The rotary table 181 is a revolving or spinning section of the drill-floor providing power to turn the drill string 170.

In one embodiment, top-drive technology allowing continuous rotation of the drill string 170, has replaced the rotary table 181.

A cementing system is used for pumping cement into place in a wellbore for sealing the annulus after a casing string has been run, to seal a lost circulation zone, to set a plug in an existing well.

Fig. 4 illustrates schematically the operation on multiple well bores 10 by means of the off-shore oil drilling rig shown in fig. 2. The hull 120 and two legs 110 of the offshore drilling rig 100 of the jackup type, is shown. The cantilever 130 is skiddable in its longitudinal direction marked with an arrow, x, and advantageously also in the transverse direction marked with an arrow, y. In some embodiments the cantilever 130 is only skiddable in one direction. The drill deck 131 are provided with two well centers 140 and 141 marked with squares. Prior to drill start the offshore drilling rig 100 is placed relative to a pattern of projected wells 10 to be drilled. Then the well 10 can be reached by the two well centers 140 and 141 by skidding the cantilever 130 in the x-direction. The distance between the well centers 140 and 141 are marked as distance 142.

By performing drilling operations by two well centers 140 and 141 simultaneously in a plurality of drilling operation windows, the oil drilling rig 100 illustrated may carry out the method according to the invention. The oil drilling rig 100 may drill a bore, position the surface casing 20, continue drilling, position the one or more intermediate casings 30, 35, continue drilling and position the one or more production casings 40. Each step is carried out by one of the well centers 140 and 141 in separate drilling operation windows. When the drilling of the pair of wells 10 has been completed, the cantilever 130 may be skidded to prepare for drilling the next pair of wells 10.

In some embodiments, the wells 10 may be arranged in is distance corresponding to half the distance 142 between the well centers 140, 141. Then the well centers 140, 141 operates on a pair well bore with a third well bore in between.

In the illustrated embodiment the two well center 140, 141 each includes the hoisting system 160, the rotary system 180 as described above. The circulation system 190, and the cementing system may be common as these are used by both well centers 140, 141 in one drilling operation windows (DOW). Appropriate valves may be used between two drilling operation windows (DOW) for coupling the circulation system 190 and the cementing system to the appropriate fluid passages of the well center 140 and 141 needing mud or cement for the drilling operation.

The mud pump 192 of the circulation system 190 and the hoisting system 160 are the two most power consuming systems in the drilling process. Due to the dual mode processing of the two well centers 140, 141 it is possible to speed up the entire drilling process.

In one mode according to the invention, first well center 140 is drilling a well stage, while the second well center 141 is arranging a casing in a drilled well stage. When shifting to the next drilling operation window, the roles of the two well center 140 and 141 is swapped in a second mode according to the invention. As the mud pump 192 of the circulation system 190 and the draw-works of the hoisting system 160 are the two most power consuming systems in the drilling process, it is possible to operate in the two modes as explained and add an extra well center 141 to an existing offshore drilling rig without having to increase the storage capacity for cuttings and the power systems as only one of the well center 140, 141 will consume a high amount of power due to drilling and mud pumping.

In some embodiments, the drill floor 131 may skid relative to the cantilever 130. Then the displacement of the well centers 140, 141 may be obtained by skidding either the cantilever, the drill floor 131 or a combination thereof.

The invention will now be described with reference to the flow chart shown in fig. 5. The process starts in step 500 with placing the offshore drilling rig 100 relative to a pair of projected wells 10 to be drilled. The first well center 140 start drilling the first well stage drill bore in the first Drilling Operation Window DOW, while the second well center 141 remains idle. Once completed, the process proceeds to step 510 in which drilling operations for the next Drilling Operation Window DOW are prepared. This includes retracting the drilling equipment used by the first well center 140, handing it over to the second well center 141, and preparing surface casings for the first well center. The circulation system 190, and the cementing system is also coupled to the appropriate fluid passages of the well center 140 and 141 needing mud or cement for the drilling operation.

In step 520, the two well centers 140 and 141 are doing drilling operation in parallel, one is drilling, and one is placing casing. Normally it takes longer time to drill a well stage than to place casing in the same well stage. When drilling operations for the two well centers 140 and 141 has been completed, it is in step 530 investigated whether the last Drilling Operation Window DOW has been reached. If not, the process reverts to step 510 where tools and equipment is prepared for the next Drilling Operation Window DOW, and the roles or modes of the two well centers 140 and 141 are swapped. When the last Drilling Operation Window DOW, the first well center 140 has complete the first well while the second well center 141 still must arrange the production casing in the recently drilled bore, when this is in place the process finishes in step 540. The cantilever 130 may the be skidded to the next pair of well to be drilled.

Fig. 6 shows a Gantt chart illustrating the drilling operations for the two well centers 140 and 141. The drilling operations for each of the two well centers 140, 141 follows a sequence of drilling operations including alternating between drilling a well stage and subsequently arranging a casing in the drilled well stage. The illustrate process shows four stages: a surface stage, a first intermediate stage, a second intermediate stage and well completion stage. In the well completion stage, the final part of the bore is drilled at the production casing is placed. For each of the four stages a first step includes drilling the bore and a second step placing a casing in the drilled bore.

It is seen that the duration of the steps in the Gantt chart may vary as the well stages varies in depth or length. The Gantt chart illustrates a planned process, by in real time the geologic formation, and failures on or break down of the drilling equipment affects the actual timing.

By drilling two wells in parallel according to the invention, it is not possible to drill double as fast as drilling two wells successively by using a single well center. Instead of saving half of the operation time, approximately one third of the operation time may be saved during a drilling campaign for drilling a pair of well bores. The reason for this is that that placing of casings is faster that the actual drilling of the bore, this means that the two well centers 140 and 141 will be idle occasionally during the campaign for drilling the pair of well bores. This is actual quite beneficial as it permits maintenance service on one well center, while the other well center is busy. Hereby the life time of the well centers may be extended and the number of failures on the drilling equipment may be reduced.

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Claims

1. A method of operating an off-shore drilling rig having a drill deck with two well centers adapted for performing drilling operations on a pair of well bores simultaneously, and comprising: performing a sequence of drilling operations including alternating between drilling a well stage and subsequently arranging a casing in the drilled well stage; shifting in time a sequence of drilling operations performed by a first one of the two well centers relatively to a sequence of drilling operations performed by a second one of the two well centers, whereby only one of the two well centers is drilling a well stage at the same time.

2. The method according to claim 1 and comprising performing the sequence of drilling operations in a sequence of drilling operation windows, wherein the two well centers in one drilling operation window, in respective well bores, performs drilling the well stage and arranges the casing in the drilled well stage.

3. The method according to claim 2 and comprising completing one drilling operation window when both the two well centers has completed respective tasks for the one drilling operation window, the tasks includes performing drilling the well stage and arranging the casing in the drilled well stage.

4. The method according to claim 1 and comprising performing the sequence of drilling operations in a sequence of drilling operation windows, each drilling operation window comprises for the two well centers either drilling the well stage or arranging the casing in the drilled well stage, wherein the well stages comprising either drilling for or arranging one of the following casing types: a surface casing, one or more intermediate casings, and one or more production casings.

5. The method according to claim 1 and used for drilling a plurality of well bores arranged as pairs, wherein the off-shore drilling rig is adapted to displace the drill deck with two well centers to another pair of well bores.

6. The method according to claim 5 and comprising steps of skidding a cantilever carrying the drill deck relatively to the offshore rig.

7. An off-shore drilling rig comprising: a drill deck (131), a first well center (140), a second well center (141), wherein the first well center (140) being adapted for performing a first of drilling operation sequence in a first sequence of drilling operation windows, wherein the second well center (141) being adapted for performing a second of drilling operation sequence in a second sequence of drilling operation windows, wherein the first of drilling operation sequence and the second of drilling operation sequence being shifted in time, whereby only one of the first well center (140) or the second well center (141) is drilling a well stage at the same time.

8. The off-shore drilling rig according to claim 7, wherein the first well center (140) starts performing the sequence of drilling operation windows one drilling operation window ahead of the second well center (141).

9. The off-shore drilling rig according to claim 7, wherein a cantilever (130) carries the drill deck (131), and wherein the cantilever (130) is skiddable relatively to the off-shore drilling rig (100) to displace the first well center (140) and the second well center (141).

10. The off-shore drilling rig according to claim 7, wherein the distance between the first well center (140) and the second well center (141) corresponds to the distance between a pair of well bores (10).

11. A method of operating an off-shore drilling rig having a drill deck with two well centers adapted for performing drilling operations on a pair of well bores simultaneously, the method comprising: alternating between drilling a well stage and subsequently arranging a casing in the drilled well stage in a sequence of drilling operations in a sequence of drilling operation windows for each of the two well centers, and shifting in time the sequence of drilling operation windows of the two well centers, whereby only one of the two well centers is drilling a well stage at the same time.

12. The method according to claim 11 and comprising completing one drilling operation window when both the two well centers has completed respective tasks for the one drilling operation window, the tasks includes performing drilling the well stage and arranging the casing in the drilled well stage.

13. The method according to claim 11, wherein each drilling operation window comprises for the two well centers either drilling the well stage or arranging the casing in the drilled well stage, and wherein the well stages comprising either drilling for or arranging one of the following casing types: a surface casing, one or more intermediate casings, and one or more production casings.

14. The method according to claim 11 and used for drilling a plurality of well bores arranged as pairs, wherein the off-shore drilling rig is adapted to displace the drill deck with two well centers to another pair of well bores.

15. The method according to claim 14 and comprising steps of skidding a cantilever carrying the drill deck relatively to the offshore rig.

16. A method of operating an off-shore drilling rig having a drill deck with two well centers adapted for performing drilling operations on a pair of well bores simultaneously, and comprising: performing a sequence of drilling operations including alternating between drilling a well stage and subsequently arranging a casing in the drilled well stage, performing a sequence of drilling operations in a sequence of drilling operation windows, o wherein a first one of the two well centers in a drilling operation window, performs drilling in a first one of said pair of well bores, and o wherein a second one of the two well centers in the drilling operation window, arranges the casing in a second one of said pair of well bores.