Subsea wellbore operations vessel and method

Abstract

A vessel adapted to perform subsea wellbore related operations involving a riser string that is assembled from releasably interconnected riser sections and extends between a subsea wellbore and the vessel. The riser string vertical handling system of the vessel comprises a controlled motion device that is adapted to displace the riser string lifting tool in at least one horizontal direction relative to the riser spider device at least whilst travelling between said elevated and lowered position thereof loaded by the riser string suspended from the riser string lifting tool, thereby allowing to establish an inclined travel path with selectively variable inclination of the riser string lifting tool relative to an imaginary vertical line through the riser string passage of the riser spider device, e.g. said inclined travel path having an inclination selected to correspond to an actual water current induced inclination of an upper portion of the riser string during the riser string assembly process.

Claims

1. A riser spider device comprising: a base; a gimballing part; and a riser dogs carrying part comprising multiple riser dogs, wherein the riser dogs are movable between a retracted position where the riser dogs are disengaged from a riser string, and an operative position where the riser dogs are engaged with the riser string and support the riser string, wherein the gimballing part is configured to allow for the riser dogs carrying part to be inclinable relative to the base, at least about one axis, thereby, in operation, allowing the riser dogs carrying part to have an inclination corresponding to an actual water current induced inclination of an upper portion of the riser string suspended from the riser spider device, and wherein the riser spider device is provided with a controlled inclination actuator that is arranged between the base and the riser dogs carrying part and that is configured to selectively vary the inclination of the riser dogs carrying part relative to the base while the riser spider device is disengaged from the riser string.

2. The riser spider of claim 1, wherein the gimballing part comprises resilient members and/or spring members that restore the dogs carrying part to a non-inclined, horizontal, position once the spider device is disengaged from the riser string.

3. The riser spider of claim 1, wherein the riser spider device is provided with an inclination sensor that measures an actual inclination of the riser dogs carrying part relative to the base.

4. The riser spider of claim 1, wherein the controlled inclination actuator of the riser spider device is adapted to cause controlled inclination of the riser dogs carrying part about solely a single horizontal axis.

5. The riser spider of claim 1, wherein the controlled inclination actuator is linked to a computerized control unit that is configured to select and set the inclination of the riser dogs carrying part to correspond to the actual water current induced inclination of an upper portion of the riser string during a riser string assembly process prior to engagement of the riser string with the riser dogs.

6. A vessel adapted to perform subsea wellbore related operations involving a riser string that is assembled on board said vessel floating at sea from releasably interconnected riser sections, the riser string extending between a subsea wellbore and said vessel, wherein the vessel is provided with the riser spider of claim 1.

7. The vessel according to claim 6, wherein said vessel comprises: a floating hull; a riser storage adapted to store therein multiple risers sections; a moonpool in said hull; a tower arranged at said moonpool and fixed to said hull; a riser string vertical handling system comprising: a travelling device that is movable up and down along the tower; a riser string lifting tool mounted on said travelling device and adapted to connect to an upper end of a riser section, said riser string lifting tool being embodied to support a weight of the riser string; and a hoisting device by which said travelling device and said riser string lifting tool is suspended from the tower and which hoisting device is adapted to move the riser string connected thereto via the riser string lifting tool up and down relative to the tower; and a riser spider device that is supported above the water in the moonpool and is adapted to temporarily suspend therefrom the riser string into the sea, said riser spider device having a riser string passage therein through which the riser string passes, wherein the riser string is temporarily suspended from the riser spider device and a riser section is connected to the riser string lifting tool and positioned above an upper end of the riser string and then connected thereto, after which the riser spider device is disengaged from the riser string and the riser string is lowered by the riser string vertical handling system, wherein the riser string lifting tool moves from an elevated position to a lowered position thereof allowing the upper end of the riser section to be engaged by the riser spider device, and wherein the vessel comprises a controlled riser spider displacement actuator that is adapted to displace, in at least one horizontal direction, the riser spider device or at least the riser dogs carrying part thereof relative to the hull of the vessel, and thereby relative to the riser string lifting tool at least whilst the riser spider device is disengaged from the riser string and whilst said riser string lifting tool travels between said elevated and lowered position thereof and the riser string is suspended from the riser string lifting tool, thereby allowing to establish an inclined travel path with selectively variable inclination of the riser string lifting tool relative to an imaginary vertical line through the riser string passage of the riser spider device.

8. The vessel according to claim 7, wherein the controlled riser spider displacement actuator is linked to a control unit that is also linked to one or more riser string centering sensors that are adapted to detect the actual position, in a horizontal plane, of the riser string relative to the riser string passage in the riser spider device, and wherein said control unit is adapted to control said horizontal direction displacement on the basis of output of said one or more riser string centering sensors.

9. A riser string assembly process onboard a vessel adapted to perform subsea wellbore related operations involving a riser string, wherein said riser string is assembled onboard said vessel floating at sea from releasably interconnected riser sections, wherein use is made of a riser spider device according to claim 1, and wherein the controlled inclination actuator is operated to bring the riser dogs carrying part in a selected inclined orientation.

10. The riser string assembly process of claim 9, wherein the controlled inclination actuator is operated to bring the riser dogs carrying part in a selected inclined orientation ahead of the landing of the riser string onto the riser dogs.

11. The riser string assembly process of claim 9, wherein the controlled inclination actuator is brought in a free motion mode once the riser string is suspended from the riser spider, so that this actuator does not interfere with the gimballing effect provided by the gimballing part.

12. The riser string assembly process of claim 9, wherein a measurement of an actual inclination of the riser string is done prior to a release of the riser string from the riser spider device, and wherein said measurement is used as input to steer the inclination actuator of the riser spider device ahead of a subsequent landing of the riser string onto the riser dogs.

13. A vessel adapted to perform subsea wellbore related operations involving a riser string that is assembled on board said vessel floating at sea from releasably interconnected riser sections, the riser string extending between a subsea wellbore and said vessel, said vessel comprising: a floating hull; a riser storage adapted to store therein multiple risers sections; a moonpool in said hull; a tower arranged at said moonpool and fixed to said hull; a riser string vertical handling system comprising: a travelling device that is movable up and down along the tower; a riser string lifting tool mounted on said travelling device and adapted to connect to an upper end of a riser section, said riser string lifting tool being embodied to support a weight of a riser string; and a hoisting device by which said travelling device and said riser string lifting tool is suspended from the tower and which hoisting device is adapted to move the riser string connected thereto via the riser string lifting tool up and down relative to the tower; and a riser spider device that is supported above the water in the moonpool and is adapted to temporarily suspend therefrom the riser string into the sea, said riser spider device having a riser string passage therein through which the riser string passes, wherein the riser string is temporarily suspended from the riser spider device and a riser section is connected to the riser string lifting tool and positioned above an upper end of the riser string and then connected thereto, after which the riser spider device is disengaged from the riser string and the riser string is lowered by the riser string vertical handling system, wherein the riser string lifting tool moves from an elevated position to a lowered position thereof allowing the upper end of the riser section to be engaged by the riser spider device, and wherein the vessel comprises a controlled riser spider displacement actuator that is adapted to displace, in at least one horizontal direction, the riser spider device or at least a riser dogs carrying part thereof relative to the hull of the vessel, and thereby relative to the riser string lifting tool at least whilst the riser spider device is disengaged from the riser string and whilst said riser string lifting tool travels between said elevated and lowered position thereof and the riser string is suspended from the riser string lifting tool, thereby allowing to establish an inclined travel path with selectively variable inclination of the riser string lifting tool relative to an imaginary vertical line through the riser string passage of the riser spider device.

14. The vessel according to claim 13, wherein the riser spider device comprises a base and a riser dogs carrying part comprising multiple riser dogs, wherein the riser dogs are movable between a retracted position where the dogs are disengaged from the riser string, and an operative position where the dogs are engaged with the riser string and support the riser string.

15. The vessel according to claim 13, wherein the controlled riser spider displacement actuator is linked to a control unit that is also linked to one or more riser string centering sensors that are adapted to detect the actual position, in a horizontal plane, of the riser string relative to the riser string passage in the riser spider device, and wherein said control unit is adapted to control said horizontal direction displacement on the basis of output of said one or more riser string centering sensors.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) In the drawings:

(2) FIG. 1 shows in longitudinal cross section a part of a vessel according to the invention,

(3) FIG. 2 illustrates a step in the assembly process of a riser string with the vessel of FIG. 1,

(4) FIG. 3 illustrates a further step in the assembly process of the riser string,

(5) FIG. 4 illustrates a further step in the assembly process of the riser string,

(6) FIG. 5 illustrates that the new riser section has been connected to the upper end of the riser string held by the riser spider device, which riser string has an inclination due to water current,

(7) FIG. 6 shows a portion of FIG. 5,

(8) FIG. 7 illustrates the engagement of the riser spider device on the riser string in the situation of FIG. 5,

(9) FIG. 8 illustrates an embodiment of a travelling device in a vessel according to the invention,

(10) FIG. 9 illustrates the riser spider device of the vessel of FIG. 5,

(11) FIG. 10 illustrates an embodiment of an inventive riser spider device,

(12) FIG. 11 illustrates an embodiment of an inventive riser spider device mounting on a vessel.

DETAILED DESCRIPTION OF EMBODIMENTS

(13) FIG. 1 shows a part of a mono-hull vessel 1 having a hull 2 with a bow, a stern 3, and a moonpool 5 that extends through the hull 1.

(14) The vessel 1 is adapted to perform subsea wellbore related operations involving a riser string between the subsea wellbore and the vessel, in particular drilling operations, e.g. for exploratory drilling. The vessel can also perform other subsea wellbore related operations, e.g. wellbore intervention.

(15) The moonpool 5 has, as is preferred, a generally rectangular shape with opposed lateral sides, a front side and a rear side.

(16) A front main deck extends between the moonpool 5 and the bow.

(17) A rear main deck 9 extends between the moonpool 5 and the stern 3 of the vessel.

(18) The vessel is equipped with a tower 10, which is, as is preferred, embodied as a hollow construction mast having a top 11 and having a base 12 that is integral with the hull 2. The base 12 extends between sections of the hull on opposed lateral sides of the moonpool 5 and the base 12 is spaced from each of the front side and the rear side of the moonpool, thereby forming a front moonpool area 5a forward of the mast 10 and a rear moonpool area 5b rearward of the mast 10.

(19) The tower 10 is fixed to the hull. The tower 10 is not inclinable relative to the hull.

(20) For example, drilling tubular racks, e.g. embodied as carrousel type racks, may be located adjacent the lateral sides of the mast 10, as is known in the art, e.g. for storage of drill pipe.

(21) At the rear moonpool area 5b, the vessel is provided with a working deck 15 arranged above the rear moonpool area 5b.

(22) As is preferred the working deck 15 is a mobile working deck, e.g. pivotable, slidable, and/or liftable, e.g. allowing to bring a blow-out preventer BOP above the rear moonpool area 5b. In view of assembly and disassembly of a riser string along a firing line 20 through the rear moonpool area 5b the vessel is equipped with a riser spider device 30 that is adapted to temporarily suspend therefrom a riser string 8 in the firing line 20 into the sea during the riser string assembly and disassembly process.

(23) As preferred, this riser spider device 30 is mounted on the working deck 15, thereby requiring that the working deck 15 is adapted to support the weight of the riser string 8, e.g. with a BOP and/or other subsea equipment at the lower end of the riser string.

(24) The vessel 1 has a riser string vertical handling system including a riser string lifting tool 25 which is movable up and down relative to the mast 10 and that is adapted to connect to an end of a riser section 8a, and is embodied to support the weight of a riser string 8 in the firing line 20 when released from the riser spider device 30.

(25) The riser string lifting tool 25 here is suspended from a travelling device 40 that is movable up and down along a side, here the rear side, of the mast 10 along one or more vertical rails 17.

(26) The hanger device 40 is suspended by one or more cables 28 from a sheave arrangement 29 at the top of the mast 10, which one or more cables 28 are connected to one or more winches 29a, e.g. arranged within the mast 10.

(27) It is noted that the firing line 20 is outside of the mast 10, here along the rear side of the mast 10, so that the firing line 20 can be reached without hindrance in the process of upending a riser section 8a or riser stand.

(28) In an alternative embodiment, the mast 10 is replaced by a derrick type tower having a latticed frame with corner posts that forms a frame extending over the moonpool 5. It is then envisaged that the riser storage is outside of the derrick type tower and the derrick is provided with a V-door or similar to allow passage of a riser section or riser stand into and out of the derrick.

(29) The vessel also has a second hoisting device having a load attachment device 50 which is movable up and down relative to the mast at a side opposed from the riser firing line 20, so as to allow for handling of items passing through the other moonpool area 5a along a second firing line 21 distinct and spaced from the first firing line 20 where the riser string assembly, also known as riser running, and disassembly, also known as tripping, takes place.

(30) The second firing line 21 extends through the front moonpool area 5a. Along this firing line 21 primarily drilling operations are performed.

(31) The second hoisting device is embodied as a drilling drawworks, and is provided with a topdrive 51 suspended from the load attachment device 50 to perform drilling operations. The load attachment device 50 is preferably embodied similar as the travelling device 40.

(32) A working deck 16 is arranged above the moonpool area 5a and may include a rotary table, iron roughneck machine, etc.

(33) The vessel 1 is thus capable of assembly of a riser string in firing line 20. For transfer of the riser string to the other firing line 21 a riser string support cart with a second riser spider device is provided. This cart is displaceable within the moonpool, e.g. skiddable over rails along the lateral sides of the moonpool 5.

(34) The vessel 1 has a riser storage, here embodied as a storage hold 40, here as is preferred, within the hull 2 aft of the moonpool 5. The riser storage 40 is embodied to store the riser sections 8a therein in horizontal position and parallel to a longitudinal axis of the vessel 1.

(35) The hold 40 is covered here by a roof that is formed by the rear main deck 9 so that in practice the riser sections in the hold 40 are not visible from above.

(36) The riser storage hold 40 is adapted to store therein, or has stored therein, multiple riser sections 8a, e.g. individual riser sections or pre-assembled riser stands, e.g. at least 25 riser sections 8a. A riser stand is assembled from multiple individual riser sections connected end-to-end. A riser stand may, as preferred, consists of two riser sections. Each riser section comprises a riser pipe and optionally one or more auxiliary pipes on the outside of and along the riser pipe as is known in the art. Each riser section comprises a connector fitting arrangement, e.g. including a flange, at each end thereof. Preferably, and as common in this field, riser sections comprise buoyancy members.

(37) The riser storage 40 comprises one or more riser storage racks 41 adapted to store therein multiple riser sections 8a in horizontal orientation.

(38) A riser section 8a may have a length that exceeds common riser sections, e.g. the common length being 75 ft. In this example the riser sections 8a each have a length of 150 ft. (45.72 m). The riser storage hold 40 is provided with an elongated riser stand transfer opening having a length and a width so as to allow for transfer, e.g. by lifting, of a single riser section 8a in horizontal or substantially horizontal (e.g. less than 30 degrees inclination) orientation via said riser stand transfer opening out of and into the riser storage hold 40.

(39) Within the hold 40 a handling crane 80 is provided, here embodied as a travelling overhead beam crane with crane rails transverse to the vessel and with a crane beam from which riser grippers or the like are suspended by means of winched cables. The crane 80 is adapted to lift and lower a riser section 8, e.g. for removing a riser stand from a stack and placing a riser stand back into the stack. The crane 80 allows for transverse transportation of a lifted riser section 8a within the hold 40 to a transfer station of the riser storage. This transfer station is provided with a transfer elevator that is adapted to raise and lower a riser section in horizontal orientation thereof so as to pass the riser section through the transfer opening. The opening is oriented with its length towards the moonpool 5, here, as preferred, along a central axis of the vessel 1.

(40) In order to bring the connector fitting arrangement at the lower end of the upended riser 62 onto the top end fitting of the launched riser string the hanger device 26 is somewhat lowered, but in an alternative the subframe 87 is used to effect this motion of the riser stand 62. The connection to the deployed riser string can now be made, e.g. by application of bolts and nuts as is known in the art to interconnect flanges of the riser sections to be joined.

(41) After the connection has been made and tested, the device 17 is released so that the entire riser string becomes suspended from the hanger device 26. Then the riser string is lowered by means of winch 29a and cable(s) 28 until the top end thereof can again be secured by means of device 17. In the process of this lowering of the riser string, the strongback 80 is moved towards its transfer position. Then the process of supplying and upending a riser stand can start anew.

(42) It will also be possible to lower the riser string such that it can become suspended from the cart 35 within the moonpool 5. The string can then be shifted to the firing line 21, e.g. to perform drilling operations with a drill string driven by topdrive 31, with the drill string within the riser string.

(43) In a preferred embodiment the cart 35 is provided with a gimballing riser string hanger device, to allow for relative motion between the riser string and the vessel.

(44) At the front moonpool area 5a a riser tensioner system may be provided to suspend the riser string during drilling operations or the like that are conducted through the riser string.

(45) In a slightly different sequence one can envisage that a new riser stand 62 is already moved upward through the opening 45 by means of the elevator 57 ahead of the strongback 80 being returned to its transfer position. So then the frame moves over the riser stand 62 and then the stand 62 is secured to the frame. This approach may be used to save time in the assembly process of the riser string.

(46) It will be appreciated that first a blow-out preventer can be positioned at the rear moonpool area 5b, so that the BOP is at the lower end of the riser string.

(47) Once riser handling is no longer required, the strongback frame together with the travelling carriage is lowered into the riser stand transfer opening 45 by means of the transfer elevator 57.

(48) The topside of the strongback frame, and here also of the carriage, is embodied as a deck portion that is flush with an adjacent deck area 9 when the strongback has been lowered into the opening 45 into its the docking position.

(49) It is envisaged that the vessel may be equipped with a mobile catwalk machine 135 that is embodied to handle single riser sections as is known in the art.

(50) It is envisaged that, e.g. for handling a first riser section to be connected to the top of a blow-out preventer, this catwalk machine 135 can be positioned on top of the topside of the strongback frame in its docking position, so that the riser section or other tubular can be supplied to the firing line 20. For example the catwalk machine 135 is moved by means of crane 130, but one can also envisage that the machine 135 is placed in line with the rails 85 of the strongback and moved in said direction from a stern-side non-operative position to an operative position near the moonpool 5. For example the machine 135 is then moved over the same rails 85 as the strongback.

(51) The vessel 1 may comprise a solid mass ballast device may be embodied to actif desiredas active roll damping mechanism, the device further including: a sensor detecting the rolling motion of the hull, and a drive and control system operable to cause and control the movements of the solid ballast in response to the detections of the sensor to provide roll stabilization. For example a winch and cable arrangement is provided to move the ballast masses, either continuously in synchronization with sea-motion of the vessel or to a desired position to obtain a balancing moment.

(52) The vessel also may comprise a water ballast system including ballast tanks for ballasting the vessel.

(53) In an embodiment of the riser assembly and/or disassembly process it is envisaged that, with the bow of the vessel facing a water current, the vessels water ballast system is used to tilt the hull of the vessel with its bow downwards, e.g. over an angle of at most 5, e.g. of about 1 or 2, therefor effectively inclining the tower 10 relative to a line vertically through the riser spider device 30. It will be appreciated that such an approach allows assist the operation of the discussed motion device in the establishing of the incline travelling path of the riser lifting tool 25, e.g. in order to enlarge the effective angular operating window of said motion device, e.g. allowing to deal with stronger currents. This inclination of the entire vessel 1 by use of the water ballast system may also be used as a complete alternative of the provision of the discussed motion device.

(54) The vessel 1 further comprises a riser horizontal handling system comprising a riser section handling catwalk machine 200.

(55) In the exemplary embodiment shown, the catwalk machine 200 is arranged on a deck 9 and above the riser storage hold 40. The catwalk machine 200 comprises a pair of horizontal catwalk machine rails 202, an elongated catwalk machine frame 204 movable over the catwalk machine rails, and a skate 206 that is movable supported by the frame.

(56) The catwalk machine frame 204 has a rear end and a front end, and is movable over the catwalk machine rails at least in a rearward loading position and a forward riser release position. In the rearward loading position a riser section 8a in horizontal orientation can be loaded onto the catwalk machine, and in the forward riser release position a riser section 8a to be lifted is connectable to the riser string lifting tool 25.

(57) The skate 206 comprises a riser end support to support thereon a rearward end of a riser section 8a. The skate 206 is supported by the catwalk machine frame 204, and may be movable by a drive motor along the length of the frame between a rearward skate position and a forward skate position.

(58) The riser horizontal handling system further comprises, as an optional feature thereof, a riser forward section auxiliary support device 220, that is distinct from the catwalk machine 200.

(59) The auxiliary support device 220 is arranged at a location along the catwalk machine rails 202 between the moonpool 5 and the catwalk machine frame 204, when in its rearward loading position.

(60) The auxiliary support device 220 is movable between an operative position and a retracted position. Both positions are depicted in FIG. 1. The auxiliary support device 220 has been depicted in its operative position in full lines, and in its retracted position in dashed lines.

(61) The riser forward section auxiliary support device 220 is adapted to, in its operative position, support a forward section of a riser section 78a that rests with its rear end on the skate 206 and that extends beyond the front end of the catwalk machine frame.

(62) The riser section 8a has been loaded in horizontal orientation onto the catwalk machine 200 whilst the latter was in its rearward loading position. Thus, with a riser section 8a loaded onto the catwalk machine 200 and also supported by the riser forward section auxiliary support device 220, the catwalk machine frame is movable along said catwalk machine rails towards the auxiliary support device 220 in which advancing motion the forward riser section is supported by said auxiliary support device. In the particular embodiment shown, this is possible whilst maintaining its horizontal orientation.

(63) The forward riser end can thus be brought near the riser string lifting tool 25 and allow for connection thereof to the forward riser end.

(64) The riser forward section auxiliary support device 220 can furthermore be moved into its retracted position, after connecting the forward riser end to the riser string lifting tool 25. Thus, the support device 220 disengages from the riser section 8a and allows the catwalk machine frame 204 to move further towards its forward position in the process of bringing the riser vertically into the firing line, wherein the forward end of the riser is lifted by the riser string vertical handling system.

(65) In the preferred embodiment shown the catwalk machine 200 is provided with a tailing-in arm device 210 that is mounted at the forward end of the catwalk machine frame 202. The tailing-in arm device thus moves along with the catwalk machine, and therefore forms no obstacle near the firing line when the catwalk machine is retracted, e.g. when not in use. In an alternative the tailing-in arm device is supported on the vessel in a different manner, e.g. mobile in the tower.

(66) The operation of the vessel according to the invention during the process of assembly of a riser string along the firing line 20 will be first discussed in general terms with reference to FIGS. 2-4.

(67) In FIG. 2 a launched portion of the riser string 8 is suspended from the riser spider device 30. A new riser section 8a has been retrieved from the storage 8 and placed on the catwalk machine 200 and auxiliary support device 220. The riser lifting tool 25 has been lowered to the level of the still horizontal riser section 8a, so that in a subsequent step the riser section 8a can be advanced to the firing line 21. Then the forward end of the riser section 8a is connected to the tool 25.

(68) In FIG. 3 it is depicted that the riser section 8a is being raised at one end thereof by means of the vertical handling system, whilst the catwalk machine skate 206 supports the other end of the riser section 8a. The lifting is continued until the riser section 8a becomes suspended from the tool 25.

(69) FIG. 4 illustrates that the tailing-in arm device 210 has been employed to guide the rear or now lower end of the riser section 8a as the riser section 8a reaches its vertical position in the firing line 20. In FIG. 4 the new section 8a is not yet connected to the upper end, commonly embodied as a flange, of the riser section at the top of the launched riser string 8.

(70) As explained, in practice, the launched riser string 8 may be subject to such a water current that the riser string 8 assumes an inclination. Commonly, especially if a significant current is present, the vessel 1 is directed with its bow into the current, so that the water current induced inclination is in the longitudinal plane of the vessel 1 with the launched riser string 8 diverging rearward from a line 23 that is vertically through the passage of the riser spider device 30. An example of this situation is depicted in FIG. 5. In practical terms this inclination may be one or just a few degrees, but as will be shown below even a seemingly small inclination may significantly impact the process of assembly of the riser string.

(71) FIG. 5 shows that the lower end of the new section 8a has been connected, e.g. bolted, to the upper end of the inclined riser string 8 held in the riser spider device 30. In order to keep the new riser section alignedas to the inclination thereofwith the upper part of the launched riser string 8 the lifting tool 25 has been moved closer to the tower 10 away from the line 23.

(72) For example, as shown in FIG. 8, the riser string lifting tool 25 is displaceable mounted on the travelling device so as to allow displacement of the riser string lifting tool relative to the travelling device in one horizontal direction, here in the longitudinal plane of the vessel so towards and away from the mast 10.

(73) A controlled motion device 90, here including one or more hydraulic cylinders 91, is arranged between the riser string lifting tool 25 and the travelling device 40.

(74) As the riser section is 150 ft. and in case the inclination is about 1 the horizontal position of the tool may be about 0.5 meter away from the line 23, closer to the tower. This shows that, when desiring to handle 150 ft. length riser sections, it is preferred for the motion device 90 to provide a motion range in at least one horizontal direction of at least 0.5 meter.

(75) In FIG. 6 it is illustrated that the riser lifting tool 25 is directly suspended from the cable 28, so that the device 40 merely serves to guide the tool 25 relative to the tower and does not support the weight of the riser string 8 during the assembly and/or disassembly process.

(76) FIG. 7 illustrates the riser spider device 30 in more detail. In this example, as is preferred, it is assumed that the riser spider device 30 is stationary mounted in or on the working deck 15 and maintains its position during the riser assembly process relative to the tower 10.

(77) The riser spider device comprises a base 31 and a dogs carrying part 32 comprising multiple dogs 33 that are movable between a retracted position wherein the dogs are disengaged from the riser string and an operative position wherein the dogs are engaged with the riser string and support the riser string.

(78) The riser spider device 30 also comprises a gimballing portion 34 between the base 31 and the dogs carrying part 32, in this example with resilient damper members 35 as is known in the art. Due to the presence of the gimballing portion 34 the riser dogs carrying part 32 is inclinable relative to the base 31, e.g. allowing the riser dogs carrying part to have an inclination corresponding to the actual water current induced inclination of an upper portion of the riser string 8.

(79) A riser passage 36 extends through the riser spider device, e.g. having smallest diameter of 60 inch for a 54 inch maximum diameter riser string 8.

(80) In FIG. 7 an annular bumper device 18 is shown at the underside of the working deck 15, below the riser spider device 30. As discussed it is envisaged that contact, or at least damaging contact, between the riser string (whilst inclined due to water current) and the riser spider 30 is avoided as the string 8 is lowered or raised through the disengaged riser spider device 30 in the process of assembly or disassembly of the riser string 8. The bumper device 30 has a diameter similar to the riser spider passage 36 and is designed to withstand such contact and has a flared or rounded lower region to avoid damage.

(81) FIG. 9 depicts the riser spider device 30 with dogs 33 both in retracted, here upwardly pivoted, position and in extended or deployed position supporting the riser string 8, here extending below a flange 8a1 of riser section 8a. The riser spider device 30 allows for a maximum inclination of the riser string of 5.

(82) The invention envisages that, starting from the initial position in FIG. 5, the riser spider device 30 is disengaged from the launched riser string 8, here by retraction of the dogs 36, so that the entire riser string including the new section 8a becomes suspended from the tool 25.

(83) Then, as in the prior art, the tool 25 is lowered by means of the cable 28 and winch 29a.

(84) In contrast with the prior art, the tool 25 is now not lowered in a vertical path, here parallel to the mast 10, but due to appropriate operation of the motion device 90 the riser string lifting tool 25 is moved in at least one, here just one, horizontal direction relative to the riser spider device 30 whilst travelling between the elevated position of FIG. 6 and a lowered position thereof. During this lowering the tool 25 supports the weight of the entire riser string 8 that is suspended from the riser string lifting tool.

(85) The motion device 90 is operated such that an inclined travel path for the lifting tool 25 towards its lower position is established having an inclination relative to the imaginary vertical line 23 through the riser string passage of the riser spider device 30. This inclination is depicted with a in FIG. 7.

(86) As explained this inclined travel path preferably has an inclination that is selected to correspond to the actual water current induced inclination of an upper portion of the riser string during the riser string assembly process. So, seen in horizontal plane, as the tool 25 is lowered the tool 25 also moves gradually closer to the center of the riser spider device passage so that in the end the top end of the riser string is rather precisely centered with respect to the riser spider device.

(87) This approach greatly contributes to the avoidance of potentially damaging collisions between the riser string 8 and the disengaged riser spider device 30. It will be appreciate that a similar approach can be used when tripping the riser string.

(88) The riser spider device 30 is provided with centering sensors 75 as discussed herein, e.g. mounted on the base 31.

(89) The riser spider 30 also has an inclination sensor 76 that senses the inclination of the part 32 relative to the base 31. For example the sensor 76 is mounted on the part 32 as can be seen in FIG. 10.

(90) FIG. 11 schematically depicts a mobile mounting of the entire riser spider device 30 relative to the hull 2, here on rails 38, so as to be displaceable in at least one, here just one, horizontal direction, e.g. parallel to be main longitudinal axis of the hull.

(91) Furthermore FIG. 11 illustrates schematically the provision of a controlled riser spider displacement actuator 135 which is linked to a control unit 136 that is also linked to one or more riser string centering sensors 75. For example the control unit 140 is adapted to control horizontal direction displacement of the entire riser spider device 30 on the basis of output of said one or more riser string centering sensors 75, e.g. in order to avoid contact between the riser string 8 and the riser spider 30 or riser dogs part 32 thereof during the passage of the riser string 8 through the riser string passage of the disengaged riser spider device 30.