Offshore drilling system, vessel and method

Abstract

An offshore drilling vessel includes a floating hull having a moonpool, a drilling tower positioned on the hull at or near the moonpool, a tubular string main hoisting device including a main hoisting winch and main cable connected to the main hoisting winch, a crown block mounted on the drilling tower, a travelling block suspended from the crown block in a multiple fall arrangement of the main cable, a mobile working deck and an integrated heave compensation system including a main cable heave compensation sheave in a path of the main cable between the main hoisting winch and the travelling block, a hydraulic sheave compensator connected to the main cable heave compensation sheave to provide a heave compensated motion of the travelling block, and a hydraulic deck compensator connected to the hull and to the mobile working deck to provide a heave compensated motion of the working deck relative to the hull within the heave compensation motion range.

Claims

1. An offshore drilling vessel for performing subsea wellbore related activities, wherein the drilling vessel comprises: a floating hull comprising a moonpool; a drilling tower positioned on said hull at or near said moonpool; a tubular string main hoisting device comprising: a main hoisting winch and main cable connected to said main hoisting winch, a crown block mounted on said drilling tower, and a travelling block suspended from said crown block in a multiple fall arrangement of said main cable, which travelling block is adapted to suspend a tubular string therefrom along a firing line through said moonpool; a mobile working deck positioned above the moonpool and movable with respect to the drilling tower along said firing line within a motion range including a heave compensation motion range; an integrated heave compensation system configured to provide a heave compensation of the travelling block as well as of the mobile working deck, wherein the heave compensation system comprises a main cable heave compensation sheave in a path of said main cable between said main hoisting winch and said travelling block, and wherein the heave compensation system comprises a hydraulic sheave compensator connected to said main cable heave compensation sheave to provide a heave compensated motion of the travelling block; wherein the heave compensation system comprises a hydraulic deck compensator, which hydraulic deck compensator is connected to the hull and to the mobile working deck to provide a heave compensated motion of the working deck relative to the hull within said heave compensation motion range; wherein heave compensation system is configured such that, in operation, said hydraulic deck compensator and said hydraulic sheave compensator move synchronously in order to provide heave compensation of both the travelling block and the mobile working deck.

2. The offshore drilling vessel according to claim 1, wherein said motion range of the mobile working deck comprises a lower stationary position, wherein the working deck is usable as a drill floor deck that is stationary with respect to the hull and aligned with a deck structure of the hull, and wherein the heave compensation motion range lies higher than said lower stationary position.

3. The offshore drilling vessel according to claim 1, wherein said hydraulic sheave compensator comprises a hydraulic cylinder having a piston rod, the main cable heave compensation sheave being connected to said piston rod.

4. The offshore drilling vessel according to claim 3, wherein said hydraulic cylinder of the hydraulic sheave compensator is connected to a hydraulic/gas separator cylinder, said hydraulic/gas separator cylinder having a chamber that is connected to a gas buffer.

5. The offshore drilling vessel according to claim 1, wherein the heave compensation system comprises a first main cable heave compensation sheave in a path of said main cable between a first main hoisting winch and said travelling block, said first main cable heave compensation sheave being connected to a first hydraulic cylinder, and wherein the heave compensation system comprises a second main cable heave compensation sheave in a path of said main cable between a second main hoisting winch and said travelling block, said second main cable heave compensation sheave being connected to a second hydraulic cylinder, and wherein said first hydraulic cylinder is connected to an active actuator configured to obtain active control of the heave compensation system.

6. The offshore drilling vessel according to claim 1, wherein said hydraulic deck compensator comprises at least one double acting hydraulic cylinder having a cylinder housing, a piston, and a piston rod, which piston subdivides the cylinder housing in a first cylinder chamber at a piston side and a second cylinder chamber at an opposite rod side of the piston, wherein said at least one double acting hydraulic cylinder further comprises a valve controlled by-pass channel and a by-pass valve which is configured to be opened or closed to respectively open or close the by-pass channel, wherein said by-pass channel is configured to interconnect said first cylinder chamber and said second cylinder chamber when said by-pass valve is open, wherein the piston has a piston area at a piston side of the piston that is to be pressurised in order to extend the piston rod out of the cylinder housing and wherein the piston has a ring-shaped piston area at a rod side of the piston that is to be pressurised in order to return the piston rod into the cylinder housing.

7. The offshore drilling vessel according to claim 6, wherein the piston area at the piston side of the piston and the ring-shaped piston area at the rod side of the piston are dimensioned such that, when the by-pass channel is closed by said by-pass valve, the piston side of the piston provides for an effective piston area that is a factor larger than an effective piston area provided by said piston side of the piston when the by-pass channel is opened by said by-pass valve, and wherein said tubular string main hoisting device comprises main cable sheaves each having an individual lower latching device that is configured to allow for connecting and disconnecting of said main cable sheave to and from the travelling block in order to adjust the multiple fall arrangement between a first arrangement with a first number of falls and a second arrangement with a second number of falls, wherein said second number of falls differs from the first number of falls by a factor that is the same as said factor between said effective piston areas in case said by-pass channel is closed or open.

8. The offshore drilling vessel according to claim 7, wherein said main cable sheaves each also have an upper latching device that is configured to allow for latching of the sheave to the crown block if the respective main cable sheave is disconnected from the travelling block.

9. The offshore drilling vessel according to claim 6, wherein the by-pass channel is connected, in between the by-pass valve and the second cylinder chamber, to an accumulator wherein a volume of hydraulic liquid is accumulated, and wherein said accumulator is valve controlled by an accumulator valve allowing to close said accumulator in case of said by-pass channel being open.

10. The offshore drilling vessel according to claim 7, wherein the by-pass channel is connected, in between the by-pass valve and the second cylinder chamber, to an accumulator wherein a volume of hydraulic liquid is accumulated, and wherein said accumulator is valve controlled by an accumulator valve allowing to close said accumulator in case of said by-pass channel being open.

11. The offshore drilling vessel according to claim 1, wherein the vessel is further provided with: a drill pipe storage rack, a pipe racker system that is adapted to move a drill pipe section between said drill pipe storage rack and a position in said firing line and between the mobile working deck and the travelling block, wherein the mobile working deck is provided with an opening therein aligned with said firing line and with a drill string slip device that is configured to suspend a drill string in said firing line and within a riser extending to a subsea well when the drill string has been disconnected from the travelling block in view of the connection of a new drill pipe section to the suspended drill string, wherein the pipe racker system is provided with a heave motion synchronization system that is adapted to bring a drill pipe section that has been retrieved from the drill pipe storage rack into a vertical motion that is synchronous with the heave motion of the mobile working deck and of drill string slip device provided thereon from which the drill string is suspended.

12. The offshore drilling vessel according to claim 1, wherein the vessel is further provided with a drillers cabin deck and a drillers cabin thereon, wherein a lower stationary position of the mobile working deck is at the level of said drillers cabin deck.

13. A method for drilling a subsea wellbore, comprising the step of using the offshore drilling vessel according to claim 1.

14. A method for drilling a subsea wellbore, comprising the step of using the offshore drilling drilling vessel according to claim 1, wherein a riser string is arranged between a subsea wellhead and the drilling vessel, which riser string includes a slip joint, wherein the slip joint includes an outer barrel and an inner barrel collapsible within said outer barrel, wherein the outer barrel is connected to a section of the riser string that extends to the subsea wellbore, and wherein the inner barrel extends to the mobile working deck, wherein the slip joint is provided with a locking mechanism that is adapted to lock the slip joint in a collapsed position of the inner barrel, wherein the method comprises using the mobile working deck in a lower stationary position thereof as a stationary drill floor deck with the slip joint being unlocked, or collapsing and locking the slip joint, providing an upper riser section that extends from the inner barrel of the collapsed and locked slip joint upward into said heave motion range of the mobile working deck, arranging said mobile working deck on the upper riser section, said mobile working deck performing compensated heave motion within said heave motion range, said travelling block performing a synchronized compensated heave motion by means of said integrated heave compensation system.

15. The method according to claim 14, wherein the mobile working deck is made to rest onto the upper riser section, said upper riser section being the sole vertical loads support of the mobile working deck.

16. The method according to claim 14, wherein said upper riser section comprises one or more of a rotating control device (RCD) configured to close off an annulus between said upper riser section and a tubular string extending through the riser, a mudline connector, a flowhead member, and annular BOP.

17. The method according to claim 15, wherein said upper riser section comprises one or more of a rotating control device (RCD) configured to close off an annulus between said upper riser section and a tubular string extending through the riser, a mudline connector, a flowhead member, and annular BOP.

18. A method for drilling a subsea wellbore, comprising the steps of: using the offshore drilling vessel according to claim 7; adjusting said multiple fall arrangement of the tubular string main hoisting device from said first arrangement with said first number of falls to said second arrangement with said second number of falls; and opening or closing said by-pass channel in response to said adjustment of said multiple fall arrangement of the tubular string main hoisting device.

19. A method for drilling a subsea wellbore, comprising the steps of: using the offshore drilling vessel according to claim 8; adjusting said multiple fall arrangement of the tubular string main hoisting device from said first arrangement with said first number of falls to said second arrangement with said second number of falls, and opening or closing said by-pass channel in response to said adjustment of said multiple fall arrangement of the tubular string main hoisting device.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) In the drawings:

(2) FIG. 1 shows schematically in vertical cross-section a drilling vessel according to the invention;

(3) FIG. 2 shows a portion of the vessel with the drilling mast with a compensator cylinder therein and a mobile working deck, as well as a slip joint;

(4) FIG. 3 shows in a perspective view a main hoisting device provided with a heave compensation system including two sheave compensators;

(5) FIG. 4 shows in a perspective view the heave motion system according to the invention, wherein a sheave compensator is hydraulically connected to a deck compensator;

(6) FIG. 5 shows the heave motion system of FIG. 4 in a schematic view;

(7) FIGS. 6A and 6B shows a valve controlled deck compensator to allow an adjustment in correspondence with an adjustment to a multiple fall arrangement of the main hoisting device;

(8) FIG. 7 illustrates the mast of the vessel of FIG. 1, with a mobile working deck, the mast being provided with a vertical rails whereon two mobile pipe racker arm units and a mobile iron roughneck device are mobile in heave compensation mode, and with pipe storage carousels mounted on the hull;

(9) FIG. 8 illustrates the assembly of a new drill pipe to the drill string in heave motion

(10) FIG. 9 shows in a cross sectional view a riser, a slip joint, a vessel including a riser tensioning system and a heave motion compensated working platform;

(11) FIG. 10 shows in a left sided view a deck compensator of the heave motion system in a lower position and in a right sided view a deck compensator of the heave motion system in an upper position.

DETAILED DESCRIPTION OF EMBODIMENTS

(12) With reference to the drawings an example of an offshore drilling system for performing subsea wellbore related activities, e.g. drilling a subsea wellbore, according to the invention will be discussed.

(13) As shown in FIG. 1 and FIG. 2, the system comprises a drilling vessel 1 having a floating hull 2 subjected to heave motion, the hull comprising a moonpool 5, here the moonpool having a fore portion 5a and an aft portion 5b.

(14) As is preferred the vessel 1 is a mono-hull vessel with the moonpool extending through the design waterline of the vessel. In another embodiment, for example, the vessel is a semi-submersible vessel having submergible pontoons (possibly an annular pontoon) with columns thereon that support an above-waterline deck box structure. The moonpool may then be arranged in the deck box structure.

(15) The vessel is equipped with a drilling tower 10 at or near the moonpool. In this example, as is preferred, the tower is a mast having a closed outer wall and having a top and a base. The base of the mast is secured to the hull 2. In this example the mast is mounted above the moonpool 5 with the base spanning the moonpool in transverse direction.

(16) In another embodiment the tower 10 can be embodied as a derrick, e.g. with a latticed derrick frame standing over the moonpool.

(17) The vessel 1 is provided with a tubular string main hoisting device, the tubular string for example being a drill string 15.

(18) The main hoisting device is further illustrated in FIG. 3 and FIG. 4.

(19) The main hoisting device comprises: a main hoisting winch, here first and second winches 20, 21, and a main cable 22 that is connected to said winches 20, 21, a crown block 23, here at the top end of the mast 10, and a travelling block 24 that is suspended from the crown block 23 in a multiple fall arrangement of the main cable 22. In FIG. 8, the travelling block 24 is suspended in a 12-fall arrangement from the crown block 23.

(20) As shown in FIG. 3 one or more main cable sheaves connected to the travelling block 24 have an individual lower latching device 25 allowing to connect and disconnect the individual sheave to and from the travelling block 24. Preferably these one or more sheaves also have an upper latching device 26 allowing to latch the sheave to the crown block if the sheave is disconnected from the travelling block. This splittable block arrangement is known in the art.

(21) The travelling block 24 is adapted to suspend a tubular string, e.g. the drill string 15, therefrom along a firing line 16, here shown (as preferred) with an intermediate topdrive 18 that is supported by the travelling block 24 and that is adapted to provide a rotary drive for the drill string.

(22) FIG. 5 shows in a schematic view, the main hoisting device which comprises a first main hoisting winch 20 and the second main hoisting winch 21, wherein the main cable 22 is connected at either end thereof to a respective one of the first and second main hoisting winches 20,21.

(23) The vessel 1 is provided with a heave compensation system adapted to provide heave compensation of the travelling block 24. This heave compensation system comprises a main cable heave compensation sheave, here two sheaves 30,31, one each in the path between each of the main hoisting winches 20, 21 and the travelling block 24. These sheaves 30, 31 are each connected to a passive and/or active heave motion compensator device, here including hydraulic cylinders, also called sheave compensators 32, 33, which are each connected to a respective main cable heave compensation cable sheave 30, 31.

(24) In the shown embodiment each sheave compensator comprises a hydraulic cylinder having a piston rod, the main cable heave compensation sheave 30,31 being connected to said piston rod. For example the compensator cylinders 32, 33 each have a stroke between 5 and 15 meters, e.g. of 6 meters. As is preferred, the cylinders 32, 33 are mounted within the mast in vertical orientation. FIG. 3 shows the lengthy cylinders 32, 33 including a fully extended position of the piston rods thereof which are preferably mounted vertically within the mast 10.

(25) As further shown in FIG. 5, as an example of a passive heave compensation system, each sheave compensator 32, 33 is connected to a hydraulic/gas separator cylinder A, B, C, D, one chamber thereof being connected to a gas buffer as is known in the art.

(26) As shown in FIG. 4, the heave compensation system is arranged to provide heave compensation of the travelling block 24 of the main hoisting device, but also to provide heave compensation of a mobile working deck 70. The mobile working deck 70 is positioned above the moonpool 5. The working deck 70 comprises equipment, like a slip tool 77, to carry out operational steps to a tubular string, in particular a drill string 15.

(27) The mobile working deck 70 is supported by a deck compensator. The deck compensator is connected to the vessel 1 and the mobile working deck 70. The deck compensator comprises at least one double acting hydraulic cylinder, here two hydraulic cylinders 61, 62 which are positioned below the mobile working deck. The hydraulic cylinders 61, 62 are positioned opposite each other. The hydraulic cylinders 61, 62 are positioned at opposite sides of the firing line 16. Here, the firing line 16 and the two hydraulic cylinders 61, 62 are positioned in a common plane which is oriented in a vertical direction. Advantageously, the arrangement of the deck compensator including two hydraulic cylinders 61, 62 contribute to the accessibility of the area below the working deck 70. The deck compensator e.g. allows access to the area for drilling equipment or a guidance of conduits.

(28) As shown in FIG. 4, the sheave compensator 32, 33 and the deck compensator 60 are hydraulically interconnected by a hydraulic conduit 65, 66. The first hydraulic sheave cylinder 32 is hydraulically connected via the first hydraulic conduit 65 to the first hydraulic deck cylinder 61 and the second hydraulic sheave cylinder 33 is hydraulically connected via the hydraulic conduit 66 to the second hydraulic deck cylinder 62. Advantageously, the hydraulic interconnection of the sheave compensator and the deck compensator provides a synchronous compensated heave motion of both the travelling block 24 and the working deck 70.

(29) The mobile working deck 70 is movable with respect to the vessel 1, in particular the drilling tower 10, along the firing line 16 within a motion range including a heave compensation motion range 72. The motion range is further illustrated and explained hereafter with reference to FIG. 10.

(30) FIG. 6A shows the embodiment of the hydraulic deck cylinder 61 in further detail. The hydraulic deck cylinder 61 comprises a cylinder housing 610, a piston 613 and a piston rod 614. The piston subdivides the cylinder housing 610 in a first cylinder chamber 611 at a piston side and a second cylinder chamber 612 at an opposite rod side of the piston 613. The hydraulic deck cylinder 61 further comprises a valve controlled by-pass channel 63 which allows to interconnect the first and second cylinder chamber 611, 612. The by-pass channel 63 comprises a by-pass valve 631 which can be opened or closed to respectively open or close the by-pass channel 63.

(31) FIG. 6A shows a closed by-pass channel 63. The first and second cylinder chamber 611, 612 inside the cylinder housing 610 are separated from each other by the piston 613. The by-pass valve 631 is closed.

(32) FIG. 6B shows an open by-pass channel 63 which interconnects the first and second cylinder chamber 611, 612 inside the cylinder housing 610. The by-pass valve 631 is open.

(33) In comparison with the configuration of the deck cylinder 61 as shown in FIG. 6B, the configuration of the deck cylinder 61 as shown in FIG. 6A includes a larger effective piston area 615 to operate the deck cylinder 61. The open by-pass channel 63 in FIG. 6B reduces the effective piston area 615 in that in operation a hydraulic pressure acts on a ring-shaped piston area at the rod side of the piston 613. In case of an open by-pass channel 63, the resulting effective piston area is equal to the piston area at the piston side minus the ring-shaped piston area at the rod side of the piston 613. In case of a closed by-pass channel 63, the resulting effective piston area is equal to the piston area as the piston side.

(34) Advantageously, the difference in between the effective piston area of a closed by-pass channel 63 and the effective piston area of an open by-pass channel 63 is equal to a factor which corresponds with a factor selectable in a multiple fall arrangement of a hoisting device which is coupled in heave motion to the working deck 70. When for example, a combination with a multiple fall arrangement of a hoisting device includes a selective 12-fall arrangement and an 8-fall arrangement, the factor of the selective fall arrangements is 1.5. In that case, this same factor is configured in the hydraulic deck cylinder 61, by configuring the piston area and ring-shaped piston area, such that an effective piston area 615 of a factor 1.5 results.

(35) The by-pass channel 63 is connected to an accumulator 64 for accumulating a volume of hydraulic liquid. Preferably, the accumulator 64 is valve controlled by an accumulator valve 641, such that the accumulator can be closed in case that the bypass channel is open and in that the accumulator can be opened in case that the bypass channel is closed. The accumulator 64 is fluidly connected to the bypass channel 63 at a position in between the second chamber of the hydraulic cylinder 61 of the deck compensator 60 and the accumulator valve 613. The accumulator allows an accumulation of hydraulic liquid from the second chamber in case that the bypass channel 63 is closed. In case that the bypass channel is open, the hydraulic liquid can flow out from the second chamber to the first chamber via the bypass channel 63, such that the accumulator can be closed.

(36) As shown in FIGS. 2, 9 and 10, the vessel is furthermore provided with a riser tensioning system that is adapted to connect to a riser 19 extending along the firing line 16 between a subsea wellbore, e.g. a BOP on the subsea wellhead, and the vessel 1. The riser tensioning system comprises a tension ring 40 and tensioner members 41 connected to the tension ring 40. In the depicted example a wire line tensioning system is shown, with the members 41 being wires that run from the ring 40 upward to sheaves 42 and then to a tensioning arrangement, e.g. including cylinders 43 and a gas buffer.

(37) The drawings further show the presence of a slip joint 50 having a lower outer slip joint barrel 51 and an upper inner slip joint barrel 52. As is known in the art the outer barrel 51 is adapted to be connected at its lower end, e.g. via bolts, to a fixed length section of the riser 19 extending to the seabed. As is known in the art and not shown in detail here the slip joint is provided with a locking mechanism 53, e.g. including hydraulically activated locking dogs, which is adapted to lock the slip joint in a collapsed position. As explained in the introduction the slip joint has a higher pressure rating when collapsed and locked that in dynamic stroking mode, e.g. as the locked position includes an operative metal-to-metal seal in the slip joint.

(38) As is known in the art the tension ring 40 of the riser tensioning system is adapted to be connected to the outer barrel 51 of the slip joint 50, thereby allowing to absorb the effective weight of the riser.

(39) FIG. 7-10 further show the vessel 1 provided with a vertically mobile working deck 70 that is vertically mobile within a motion range including a lower stationary position 71, wherein the working deck is used as a drill floor deck stationary with respect to the hull of the vessel in which the slip joint 50 is unlocked. The shown position of the working deck 70 in FIG. 9 is also called a regular position of the working deck in which the working deck is aligned with an overhead deck structure 12. The motion range further includes a heave compensation motion range 72 that lies higher than the lower stationary position 71. In this heave compensation motion range the working deck 70 can perform heave compensation motion relative to the hull of the vessel.

(40) For example the heave compensation motion range is between 5 and 10 meters, e.g. 6 meters. For example the average height of the working deck in heave motion above the driller cabin deck 73 with cabin 74 of the vessel is about 10 meters.

(41) The drawings show that the working deck 70 has an opening 75 therein that is aligned with the firing line 16, the opening 75 being dimensioned to at least allow for passage of the tubular string 15 that extends into and through the riser 19. The working deck is provided with a tubular string suspension device, e.g. a device known as a slip tool 77 in the drilling field.

(42) The working deck 70 may be provided with a rotary table.

(43) The system further comprises an upper riser section 80 that is mounted at the top of the riser, more in particular mounted on the inner barrel 52 of a slip joint 50, and extends upward from the inner barrel 52 of the slip joint 50 at least to above the lower stationary position 71 of the working deck 70, preferably to the heave compensation motion range of the deck 70, as visible in FIG. 10.

(44) In the depicted example in FIG. 10, the working deck 70 rests on the upper riser section 80 and this upper riser section 80 is the sole vertical loads support of the working deck 70.

(45) The upper riser section 80 forms a rigid connection between the actual end of the inner barrel 52 and a riser member 83 which extends upward to above the level 71, even in the lowermost heave motion situation depicted at a left side in FIG. 10. Above said riser member 83 equipment to be integrated with the riser top, such as preferably at least a rotating control device (RCD) 84, and a mudline connector 85 are mounted. For example other riser integrated equipment like an annular BOP 86 may be arranged here as well.

(46) As best seen in FIG. 8, the height of the riser above the drillers cabin deck 73 with the drillers cabin 74 allows for the drilling personnel in this cabin to have a direct view on equipment in the upper riser section 80 and all lines attached thereto when operated with the slip joint 50 in collapsed and locked position, with the working deck 70 in compensated heave motion in the elevated heave motion compensation range.

(47) The inventive system can also be embodied such that the working deck 70, in heave motion compensation mode, does not rest with its weight and, if present, any load thereon (e.g. from the drill string suspended from a slip device on the working deck 70 on or entirely on the upper riser section.

(48) The vessel is provided with a vertically mobile working deck 70 that is vertically mobile within a motion range including a lower stationary position 71, wherein the working deck is used as stationary drill floor deck with the slip joint unlocked, see FIG. 9, and the motion range further including a heave compensation motion range 72 that lies higher than said lower stationary position 71. Such positions of the working deck 70,70 are shown in FIG. 10. The vessel is provided with a drillers cabin deck 73 with a drillers cabin (not shown) thereon, and the lower stationary position of the working deck is at said drillers cabin deck level.

(49) As shown further in FIG. 7. the vessel is furthermore provided with a drilling tower, here embodied as a mast 10, of a closed hollow construction. The top section including the drawworks and topdrive 18 as already shown in FIG. 3 has been removed in the drawing of FIG. 7. Also shown are the storage racks 110, 111 for tubulars, e.g. drill pipes and casing, here multi-jointed tubulars. Such racks are also referred to as carousels.

(50) At the side of the mast 10 facing the firing line 16 the drilling system is provided with a pipe racker system, here comprising two tubular racking devices 140 and 140, each mounted at a corner of the mast 10. If no mast is present, e.g. with a latticed derrick, a support structure can be provided to arrive at a similar arrangement of the racking devices 140 and 140 relative to the firing line 16.

(51) In the shown embodiment of FIG. 8, each racking device 140, 140 has multiple, here three racker assemblies. Here a lower first tubular racker assembly 141, 141, a second tubular racker assembly 142, 142, operable at a greater height than the first tubular racker assembly, and a third tubular racker assembly 143, 143.

(52) Each set of racker assemblies is arranged on a common vertical rails 145, 145 that is fixed to the mast 10, here each at a corner thereof.

(53) In the embodiment of FIG. 8, a drill pipe multi-joint tubular may be held by racker assemblies 142 and 141 in the firing line above the well center 27, thereby allowing to connect the tubular to the upper riser section 80. Each of said assemblies 142 and 141 carries a tubular gripper member 142t and 141t at the end of the motion arm of the assembly.

(54) The lower racker assembly 143 of the other racker device 140 carries an iron roughneck device 150, optionally with a spinner thereon as well.

(55) According to a preferred embodiment of the invention, the pipe racker system is provided with a heave motion synchronization system, adapted to bring a drill pipe retrieved from a drill pipe storage rack into a vertical motion synchronous with the heave motion of the upper end of the riser, e.g. of the working deck resting thereon, thereby allowing the interconnect the drill pipe to a drill pipe string suspended from a slip device. Hence, in the shown embodiment, the two tubular racking devices 140 and 140, each with three racker assemblies, are mobile in heave compensation mode.

(56) It is both conceivable that the racker assemblies are mobile in heave compensation mode with respect to their common vertical rails 145, 145, and that the common vertical rails 145, 145 with the racker assemblies are mobile in heave compensation mode with respect to the mast 10.

(57) In FIG. 8 the assembly of a new drill pipe 15, held by the pipe racker system of FIG. 7 comprising racker assemblies mounted on vertical rails, which pipe racker system is provided with a heave motion synchronization system that brings the drill pipe 15 retrieved from a drill pipe storage rack (not shown in FIG. 8) into a vertical motion synchronous with the heave motion of the upper end of the riser, thereby allowing the interconnect the drill pipe 15 to a drill pipe string suspended from a slip device. to the drill string in heave motion is shown in a detailed perspective view. In FIG. 8, racker assemblies 143 and 143 and 141 and 141 are visible, wherein racker assembly 141 grips the drill pipe 15.

(58) In the depicted example of FIG. 10, the working deck 70 rests on the upper riser section 80 and this upper riser section 80 is the sole vertical loads support of the working deck 70.

(59) The upper riser section 80 comprises equipment to be integrated with the riser top, such as preferably at least a rotating control device (RCD) 84, and a mudline connector 85.

(60) The height of the riser above the drillers cabin deck 73 with the drillers cabin 74 allows for the drilling personnel in this cabin to have a direct view on equipment in the upper riser section 80 and all lines attached thereto, with the working deck 70 in heave motion in the elevated heave motion compensation range.

(61) Thus, the invention provides an offshore drilling system and method for drilling a subsea wellbore, wherein use is made of such a system. The offshore drilling system comprises a drilling tower 10, a tubular string hoisting device with a crown block 23 and a travelling block 24 suspended from said crown block in a multiple fall arrangement, a heave compensation system adapted to provide heave compensation of the travelling block 24. The heave compensation system comprises a hydraulic sheave compensator 32, 33. The system further comprises a mobile working deck 70 which is movable with respect to the drilling tower 10 within a motion range including a heave compensation motion range 72. The heave compensation system is further adapted to provide heave compensation of the mobile working deck 70 by a hydraulic deck compensator 60, which is hydraulically connected via a hydraulic conduit 65,66 to the hydraulic sheave compensator 32, 33, such that in operation the deck compensator 60 moves synchronously with the sheave compensator 32, 33 of the heave compensation system.

(62) TABLE-US-00001 Reference numbers: 1 vessel 5 moonpool 2 floating hull 5a moonpool fore portion 5b moonpool aft portion 72 heave compensation motion range 10 drilling tower 73 driller cabin deck 12 overhead deck structure 74 cabin 15 drill string 75 deck opening 16 firing line 77 slip tool 18 top drive 80 upper riser section 19 riser 82 collar 20 first winch 83 riser member 21 second winch 84 RCD (rotating control device) 22 main cable 85 mudline connector 23 crown block 86 BOP (blow out preventor) 24 traveling block 110 first storage rack 25 lower latching device 111 second storage rack 26 upper latching device 140 tubular racking device 27 well centre 140 tubular racking device 30 first sheave; main cable 141 lower first racker assembly heave compensation sheave 141b base 31 second sheave; main cable 141m motion arm heave compensation sheave 141m-1 first arm segment 32 first hydraulic cylinder; 141m-2 second arm segment heave motion compensator 141m-3 third arm segment 33 second hydraulic cylinder; 141t tubular gripper heave motion compensator 142 second tubular racker assembly 40 tension ring 143 third tubular racker assembly 41 tensioner member 145 common vertical rails 42 sheave 147 vertical axis bearing 43 cylinder 147a bearing housing 50 slip joint 148 connector 51 outer slip joint barrel 150 iron roughneck device spinner 52 inner slip joint barrel 156 connector pin 53 locking mechanism 161 pinion 60 deck compensator 162 motor 61 hydraulic deck cylinder 62 hydraulic deck cylinder 65 hydraulic conduit 66 hydraulic conduit 70 mobile working deck 71 lower stationary position