OFFSHORE DRILLING SYSTEM, VESSEL AND METHOD

Abstract

An offshore drilling system includes a drilling tower, a tubular string hoisting device with a crown block and a travelling block suspended from the crown block in a multiple fall arrangement. A heave compensation system is adapted to provide heave compensation of the travelling block. The heave compensation system includes a hydraulic sheave compensator. The system further includes a mobile working deck which is movable with respect to the drilling tower within a motion range including a heave compensation motion range. The heave compensation system is further adapted to provide heave compensation of the mobile working deck by a hydraulic deck compensator, which is hydraulically connected via a hydraulic conduit to the hydraulic sheave compensator, such that in operation the deck compensator moves synchronously with the sheave compensator of the heave compensation system.

Claims

1-17. (canceled)

18. An offshore drilling system for performing subsea wellbore related activities, wherein the drilling system comprises: a drilling tower having a top and a base, which drilling tower is to be positioned at or near a moon pool of a floating body; a riser tensioning system adapted to connect a riser extending along the firing line between the subsea wellbore and the floating body; a tubular string main hoisting device, said tubular string main hoisting device comprising: a main hoisting winch and main cable connected to said winch; and a crown block 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; a heave compensation system adapted to provide heave compensation of the travelling block, the heave compensation system comprising a main cable heave compensation sheave in the path between said main hoisting winch and the travelling block, a hydraulic sheave compensator connected to said main cable heave compensation sheave; and a mobile working deck movable with respect to the drilling tower along the firing line within a motion range including a heave compensation motion range; wherein the heave compensation system is adapted to provide heave compensation of the mobile working deck, wherein the heave compensation system further comprises a hydraulic deck compensator, which deck compensator is connected to the floating body and to the mobile working deck to provide a heave compensated motion of the working deck relative to the floating body, and wherein the deck compensator is hydraulically connected via a hydraulic conduit to the hydraulic sheave compensator of the heave compensation system, such that in operation the deck compensator moves synchronously with the sheave compensator of the heave compensation system.

19. The system according to claim 18, wherein the 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 the hydraulic cylinder further comprises a valve controlled by-pass channel which allows to interconnect the first and second cylinder chamber, wherein the piston has a piston area at the piston side to be pressurised to extend the piston rod out of the cylinder housing and a ring-shaped piston area at the rod side to be pressurised to return the piston rod into the cylinder housing, in which an effective piston area at the piston side is in case of a closed by-pass channel a factor larger than an effective piston area in case of an open by-pass channel, and wherein said factor is configured in accordance with a multiple fall arrangement of the main hoisting device which is coupled in heave motion and which hoisting device comprises one or more main cable sheaves connected to the travelling block which have an individual lower latching device allowing to connect and disconnect an individual sheave to and from the travelling block to adjust a fall arrangement, of the hoisting device by the same factor.

20. The system according to claim 19, wherein the one or more sheaves also have an upper latching device allowing to latch the sheave to the crown block if the sheave is disconnected from the travelling block.

21. The system according to claim 19, wherein the by-pass channel is connected to an accumulator for accumulating a volume of hydraulic liquid.

22. The system according to claim 21, wherein the accumulator is valve controlled by an accumulator valve, such that the accumulator can be closed in case of an open by-pass channel.

23. The system according to claim 18, wherein the hydraulic deck compensator comprises a pair of hydraulic cylinders positioned at opposite sides of the firing line and being spaced apart to allow for a passage of the riser section in the firing line and between said pair of hydraulic cylinders.

24. The system according to claim 18, wherein the hydraulic deck compensator is arranged in its stationary operative position at a level below the working deck.

25. The system according to claim 18, wherein the motion range includes a lower stationary position and wherein the heave compensation motion range lies higher than said lower stationary position.

26. The system according to claim 18, wherein the first hydraulic compensator is connected to an active actuator to obtain an active control of the first heave compensation system.

27. The system according to claim 18, further comprising at least one of the following features: the floating body further comprising a drillers cabin deck and a drillers cabin thereon, with the lower stationary position of the working deck being at said drillers cabin deck level; the crown block and the travelling block suspended from said crown block in a multiple fall arrangement of said main cable, which travelling block is adapted to suspend the tubular string therefrom along the firing line, with an intermediate topdrive adapted to provide a rotary drive for a drill string; the effective piston area at the piston side is in case of a closed by-pass channel a factor 1.5 larger than the effective piston area in case of an open by-pass channel; the fall arrangement is adjustable from a 12-fall to a 8-fall arrangement; the riser tensioning system comprises a tension ring and tension members connected to said tension ring; a piper racker system is provided with a heave motion synchronisation system adapted to bring a drill pipe retrieved from a drill pipe storage rack into a vertical relative motion synchronous with a relative motion of the upper end of the riser, thereby allowing to interconnect the drill pipe to a drill pipe string suspended from a slip tool; a slip joint including an outer barrel and an inner barrel, wherein the outer barrel is adapted to be connected to a fixed length section of a riser extending to the subsea wellbore, and wherein the slip joint is provided with a locking mechanism adapted to lock the slip joint in a collapsed position; an upper riser section that is mounted on the inner barrel of the slip joint and which upper riser section extends upwards from the slip joint at least to above the lower stationary position of the working deck; an upper riser section that is mounted on the inner barrel of the slip joint and which upper riser section extends upwards from the slip joint at least to above the lower stationary position of the working deck to the heave compensation motion range; the working deck is adapted to rest onto the upper riser section with said upper riser section being the sole vertical loads support of the working deck; the tension ring of the riser tensioning system is adapted to be connected to the outer barrel of the slip joint; a rotating control device (RCD) to close off an annulus between an upper riser section and a tubular string extending through the riser and including at least one flowhead member to allow a connection of at least one hose for transferring an annular fluid flow to the floating body; and a rotating control device (RCD) above the slip joint and below the working deck to close off an annulus between an upper riser section and a tubular string extending through the riser in the course of managed pressure drilling, and including at least one flowhead member to allow a connection of at least one hose for transferring an annular fluid flow to the floating body.

28. An offshore drilling vessel comprising a floating hull subjected to heave motion, the hull comprising a moonpool further comprising the drilling system according to claim 18.

29. A method for drilling a subsea wellbore, comprising the step of using the system according to claim 18.

30. The method according to claim 29, further comprising the step of: arranging a riser string between a subsea wellhead and the floating body, which riser string includes a slip joint, wherein in one mode the slip joint is unlocked, and wherein in another mode the slip joint is collapsed and locked, such that the heave compensation system is operative.

31. The method according to claim 30, wherein the floating body has a mobile working deck that rests on the riser, and wherein the working deck performs a compensated heave motion relative to the floating body in a heave motion compensation range as the slip joint is locked.

32. The method according to claim 29, further comprising the steps of: adjusting a multiple fall arrangement of the tubular string main hoisting device; and opening or closing a by-pass channel of the hydraulic deck compensator in accordance with an adjustment of said multiple fall arrangement of the main hoisting device.

33. The method according to claim 32, wherein a suspension of the travelling block is adjusted from a 12-fall arrangement to an 8-fall arrangement.

34. The method according to claim 33, wherein the by-pass channel is opened in response to an adjustment of the main hoisting device from the 12-fall arrangement to the 8-fall arrangement.

35. The system according to claim 20, wherein the by-pass channel is connected to an accumulator for accumulating a volume of hydraulic liquid.

36. The system according to claim 19, wherein the hydraulic deck compensator comprises a pair of hydraulic cylinders positioned at opposite sides of the firing line and being spaced apart to allow for a passage of the riser section in the firing line and between said pair of hydraulic cylinders.

37. The system according to claim 19, wherein the hydraulic deck compensator comprises a pair of hydraulic cylinders which are positioned at opposite sides of the firing line and being spaced apart to allow for a passage of the riser section in the firing line and between said pair of hydraulic cylinders.

Description

[0092] The aspects of the invention will now be explained with reference to the drawings. In the drawings:

[0093] FIG. 1 shows schematically in vertical cross-section a drilling vessel according to the invention;

[0094] 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;

[0095] FIG. 3 shows in a perspective view a main hoisting device provided with a heave compensation system including two sheave compensators;

[0096] 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;

[0097] FIG. 5 shows the heave motion system of FIG. 4 in a schematic view;

[0098] 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;

[0099] 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;

[0100] FIG. 8 illustrates the assembly of a new drill pipe to the drill string in heave motion

[0101] 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;

[0102] 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.

[0103] 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.

[0104] 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.

[0105] 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.

[0106] 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.

[0107] In another embodiment the tower 10 can be embodied as a derrick, e.g. with a latticed derrick frame standing over the moonpool.

[0108] The vessel 1 is provided with a tubular string main hoisting device, the tubular string for example being a drill string 15.

[0109] The main hoisting device is further illustrated in FIG. 3 and FIG. 4.

[0110] The main hoisting device comprises: [0111] a main hoisting winch, here first and second winches 20, 21, and a main cable 22 that is connected to said winches 20, 21, [0112] 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.

[0113] 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.

[0114] 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.

[0115] 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.

[0116] 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.

[0117] 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.

[0118] 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.

[0119] 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.

[0120] 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.

[0121] 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.

[0122] 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.

[0123] 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.

[0124] 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.

[0125] 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.

[0126] 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.

[0127] 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.

[0128] 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.

[0129] 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.

[0130] 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.

[0131] 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.

[0132] 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.

[0133] 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.

[0134] 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.

[0135] The working deck 70 may be provided with a rotary table.

[0136] 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.

[0137] 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.

[0138] 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.

[0139] 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.

[0140] 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.

[0141] 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.

[0142] 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.

[0143] 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.

[0144] 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′.

[0145] 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.

[0146] 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 142′t and 141′t at the end of the motion arm of the assembly.

[0147] The lower racker assembly 143 of the other racker device 140 carries an iron roughneck device 150, optionally with a spinner thereon as well.

[0148] 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.

[0149] 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.

[0150] 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.

[0151] 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.

[0152] 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.

[0153] 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.

[0154] 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.

REFERENCE NUMBERS

[0155] 1 vessel [0156] 2 floating hull [0157] 5 moonpool [0158] 5a moonpool fore portion [0159] 5b moonpool aft portion [0160] 10 drilling tower [0161] 12 overhead deck structure [0162] 15 drill string [0163] 16 firing line [0164] 18 top drive [0165] 19 riser [0166] 20 first winch [0167] 21 second winch [0168] 22 main cable [0169] 23 crown block [0170] 24 traveling block [0171] 25 lower latching device [0172] 26 upper latching device [0173] 27 well centre [0174] 30 first sheave; main cable heave compensation sheave [0175] 31 second sheave; main cable heave compensation sheave [0176] 32 first hydraulic cylinder; heave motion compensator [0177] 33 second hydraulic cylinder; heave motion compensator [0178] 40 tension ring [0179] 41 tensioner member [0180] 42 sheave [0181] 43 cylinder [0182] 50 slip joint [0183] 51 outer slip joint barrel [0184] 52 inner slip joint barrel [0185] 53 locking mechanism [0186] 60 deck compensator [0187] 61 hydraulic deck cylinder [0188] 62 hydraulic deck cylinder [0189] 65 hydraulic conduit [0190] 66 hydraulic conduit [0191] 70 mobile working deck [0192] 71 lower stationary position [0193] 72 heave compensation motion range [0194] 73 driller cabin deck [0195] 74 cabin [0196] 75 deck opening [0197] 77 slip tool [0198] 80 upper riser section [0199] 82 collar [0200] 83 riser member [0201] 84 RCD (rotating control device) [0202] 85 mudline connector [0203] 86 BOP (blow out preventor) [0204] 110 first storage rack [0205] 111 second storage rack [0206] 140 tubular racking device [0207] 140′ tubular racking device [0208] 141 lower first racker assembly [0209] 141b base [0210] 141m motion arm [0211] 141m-1 first arm segment [0212] 141m-2 second arm segment [0213] 141m-3 third arm segment [0214] 141′t tubular gripper [0215] 142 second tubular racker assembly [0216] 143 third tubular racker assembly [0217] 145 common vertical rails [0218] 147 vertical axis bearing [0219] 147a bearing housing [0220] 148 connector [0221] 150 iron rougnneck device spinner [0222] 156 connector pin [0223] 161 pinion [0224] 162 motor