Method for riser string handling and an offshore drilling vessel

Abstract

The present invention relates to a method for riser string handling on an offshore drilling vessel (1), the offshore drilling vessel comprising a multiple firing line hoist system, a riser tensioner system (50) arranged in the second firing line, and a suspended riser transfer device (60). The method comprising the steps of lowering a riser string in the first firing line, and simultaneously assembling and preparing a riser tensioner system in the second firing line, wherein the riser hang-off assembly displaces the riser string, leaving the top end of the riser string exposed, from the first firing line to the second firing line to be connected to the riser tensioner system.

Claims

1. A method for riser string handling on an offshore drilling vessel, the offshore drilling vessel comprising: a hull having a moonpool extending through the hull; a multiple firing line hoist system mounted on the hull at said moonpool, the multiple firing line hoist system comprising: a tower having a top side and a base connected to the hull of the drilling vessel; a first hoisting device supported by the tower and having a first load attachment device displaceable along a first firing line, which extends essentially parallel to the tower; the first hoisting device being adapted to build and lower a riser string in the first firing line; and a second hoisting device supported by the tower and having a second load attachment device displaceable along a second firing line, which extends essentially parallel to the tower; wherein a rotary drilling drive is provided in the second firing line, being adapted to assemble and disassemble a drill string and effect drilling in the second firing line; a riser tensioner system arranged in the second firing line, adapted to be connected to a top end of the riser string, in order to suspend the riser string from in the second firing line; and a suspended riser transfer device, comprising: a riser hang-off assembly being actuable between an open configuration in which the riser hang-off assembly is adapted to move around an upper end of the riser string, and a closed configuration in which the riser hang-off assembly engages and supports the riser string, leaving the top end of the riser string exposed to allow for connection to the riser tensioner system; a frame with a gimbal device configured to support the riser string in a gimballing manner, to which the riser hang-off assembly is mounted; and rails extending in a longitudinal direction along the moonpool between the first firing line and the second firing line, allowing to displace the frame in a suspended riser transfer path in the longitudinal direction of the moonpool while the riser string is supported by the gimbal device, the method comprising the following steps: building and lowering the riser string in the first firing line, and simultaneously assembling and preparing the riser tensioner system in the second firing line, suspending the riser string from the first load attachment device, positioning the frame with the gimbal device to which the riser hang-off assembly is mounted, with the riser hang-off assembly in the open configuration, in the first firing line, positioning the upper end of the riser string in the riser hang-off assembly and actuating the riser hang-off assembly to the closed configuration, transferring the weight of the riser string from the first load attachment device to the riser hang-off assembly, disconnecting the first load attachment device from the riser string, displacing the riser hang-off assembly in the suspended riser transfer path while the gimbal device supports the riser string from the first firing line to the second firing line, connecting the top end of the riser string to the riser tensioner system, transferring the weight of the riser string from the riser hang-off assembly to the riser tensioner system, actuating the riser hang-off assembly to the open configuration and thus disconnecting the riser hang-off assembly from the riser string, and displacing the frame with the gimbal device to which the riser hang-off assembly is mounted in the suspended riser transfer path from the second firing line to the first firing line.

2. The method according to claim 1, wherein the riser tensioner system of the offshore drilling vessel comprises: a riser tensioner ring, riser tensioners connected to the vessel and supporting the riser tensioner ring in the second firing line, a telescopic joint comprising an inner barrel and an outer barrel with a seal therebetween, wherein the riser tensioner ring is adapted to be connected to the top of the outer barrel, a landing joint, and wherein in the second firing line a clamp for the telescopic joint is provided above the riser tensioner ring, and wherein the method furthermore comprises the following steps: positioning the riser tensioner ring in the second firing line above the suspended riser transfer path, positioning the telescopic joint in the clamp in the second firing line, connecting the landing joint to the second load attachment device and the telescopic joint, lifting the telescopic joint with the landing joint by the second hoisting device to a level above the suspended riser transfer device, allowing the telescopic joint to be connected to the exposed top end of the riser string, displacing the riser hang-off assembly in the suspended riser transfer path while the gimbal device supports the riser string from the first firing line to the second firing line, lowering the telescopic joint with the landing joint by the second hoisting device and connecting the top end of riser string to the lower end of the outer barrel of the telescopic joint, transferring the weight of the riser string from the riser hang-off assembly to the telescopic joint, actuating the riser hang-off assembly to the open configuration and thus disconnecting the riser hang-off assembly from the riser string, displacing the riser hang-off assembly from the second firing line to the first firing line in the suspended riser transfer path, further lowering the telescopic joint with the landing joint by the second hoisting device and connecting the riser tensioner ring to the top of the outer barrel of the telescopic joint, further lowering the telescopic joint with the landing joint by the second hoisting device until the weight of the telescopic joint and suspended riser is supported by the riser tensioners, and raising the inner barrel of the telescopic joint with the landing joint by the second hoisting device, disconnecting the landing joint from the second load attachment device and from the telescopic joint.

3. The method according to claim 1, wherein the hull of the offshore drilling vessel is a monohull with a bow and a stern, wherein, an accommodation topside having crew quarters and a bridge is arranged on the hull at the bow, and wherein a main deck extends between the accommodation topside and the stern of the vessel.

4. The method according to claim 1, wherein the offshore drilling vessel is a semi-submersible having a deck box structure support by legs on parallel pontoons.

5. The method according to claim 1, wherein a first parking position for the frame of the suspended riser transfer device of the offshore drilling vessel is provided in the vicinity of the first firing line, in a direction opposed to the second firing line, and wherein the rails of the suspended riser transfer device extend beyond the first firing line to the first parking position.

6. The method according to claim 1, wherein a second parking position for the frame of the suspended riser transfer device of the offshore drilling vessel is provided in the vicinity of the second firing line, in a direction opposed to the first firing line, and wherein the rails of the suspended riser transfer device extend beyond the second firing line to the second parking position.

7. An offshore drilling vessel, comprising: a hull having a moonpool extending through the hull; a multiple firing line hoist system mounted on the hull at said moonpool, the multiple firing line hoist system comprising: a tower having a top side and a base connected to the hull of the drilling vessel; a first hoisting device supported by the tower and having a first load attachment device displaceable along a first firing line, which extends essentially parallel to the tower; the first hoisting device being adapted to build and lower a riser string in the first firing line; and a second hoisting device supported by the tower and having a second load attachment device displaceable along a second firing line, which extends essentially parallel to the tower; wherein a rotary drilling drive is provided in the second firing line, being adapted to assemble and disassemble a drill string and effect drilling in the second firing line; a riser tensioner system arranged in the second firing line, adapted to be connected to a top end of the riser string, in order to suspend the riser string from in the second firing line; and a suspended riser transfer device, comprising: a riser hang-off assembly being actuable between an open configuration in which the riser hang-off assembly is adapted to move around an upper end of the riser string, and a closed configuration in which the riser hang-off assembly engages and supports the riser string, leaving the top end of the riser string exposed to allow for connection to the riser tensioner system; a frame with a gimbal device configured to support the riser string in a gimballing manner, to which the riser hang-off assembly is mounted; and rails extending in a longitudinal direction along the moonpool between the first firing line and the second firing line, allowing to displace the frame in a suspended riser transfer path in the longitudinal direction of the moonpool while the riser string is supported by the gimbal device, the offshore drilling device being configured to: build and lower the riser string in the first firing line, and simultaneously assemble and prepare the riser tensioner system in the second firing line, suspend the riser string from the first load attachment device, position the frame with the gimbal device to which the riser hang-off assembly is mounted, with the riser hang-off assembly in the open configuration, in the first firing line, position the upper end of the riser string in the riser hang-off assembly and actuate the riser hang-off assembly to the closed configuration, transfer the weight of the riser string from the first load attachment device to the riser hang-off assembly, disconnect the first load attachment device from the riser string, displace the riser hang-off assembly in the suspended riser transfer path while the gimbal device supports the riser string from the first firing line to the second firing line, connect the top end of the riser string to the riser tensioner system, transfer the weight of the riser string from the riser hang-off assembly to the riser tensioner system, actuate the riser hang-off assembly to the open configuration and thus disconnect the riser hang-off assembly from the riser string, and displace the frame with the gimbal device to which the riser hang-off assembly is mounted in the suspended riser transfer path from the second firing line to the first firing line.

Description

(1) Preferred embodiments of the invention are discussed in the description with reference to the drawings. The invention will now be explained with reference to the appended drawings, in which:

(2) FIG. 1 shows an example of an offshore drilling vessel according to the invention in a mid ship longitudinal cross-section of the vessel, prior to carrying out the method of claim 1,

(3) FIG. 2 shows the moonpool of the vessel of FIG. 1 on a larger scale, in a longitudinal cross-section of the vessel,

(4) FIGS. 3A and 3B show a top view of the suspended riser transfer device of FIG. 1,

(5) FIGS. 4A-D shows the suspended riser transfer device of FIGS. 1-3 in detail, in various views,

(6) FIGS. 5A and B show the offshore drilling vessel of FIG. 1 in a transverse cross-section of the vessel at the second firing line, prior to (FIG. 5A) and after (FIG. 5B) carrying out the method of claim 1,

(7) FIG. 6 shows the offshore drilling vessel of FIG. 1, after having carried out the method of claim 1, in a mid ship longitudinal cross-section of the vessel,

(8) FIGS. 7A-7J show a detail of the mid ship longitudinal cross-section of the vessel, wherein the method steps according to the invention are shown in succession,

(9) FIGS. 8A and B show in a perspective view steps vi) and xi) of the method of the invention.

(10) In FIGS. 1-6 a preferred embodiment of a monohull offshore drilling vessel 1 that is suitable for offshore drilling, e.g. for oil and gas exploration, well servicing and/or other drilling related activities (e.g. servicing and/or placement of subsea equipment) is shown. The hull 2 has crew quarters and a bridge 3 on the bow side, here with helicopter platform 3a. Between the accommodation topside 3 and stern 6 a main deck 7 extends. In this example about halfway the length of the hull 2 the vessel 1 has a large moonpool 5.

(11) Effectively above this moonpool 5 a multiple firing line hoist system 10 is mounted on the hull 2 so thatas preferreda forward portion and a rear portion of the moonpool 5 are accessible at the front and the rear of the system 10. The multiple firing line hoist system 10 comprises: a tower, here embodied as a mast 11 having a top side and a base connected to the hull of the drilling vessel, wherein the mast 11 has a hollow construction with a first side 12 (in this example the rear side) and an opposed second side 13 (in this example the front side), a first hoisting device 14 supported by the mast and having a load attachment device 14b displaceable along a first firing line 14a, which extends on the outside of and adjacent to the first side of the mast 11; the first hoisting device being adapted to build and lower a riser string in the first firing line; a second hoisting device 15 supported by the mast and having a load attachment device 15b displaceable along a second firing line 15a, which extends on the outside of and adjacent to the second side of the mast. In the shown embodiment, the second hoisting device comprises a rotary drilling drive 15c in the second firing line being adapted to assemble and disassemble a drill string and effect drilling in the second firing line.

(12) The first and second hoisting devices 14, 15 here each include one or more cables and one or more associated winches to manipulate the position of each of the first and second load attachment devices 14b, 15b relative to the mast. The winches are preferably located in the mast, most preferably in the base of the mast, but other location are also possible. Details of the mast and the hoisting devices can be derived from U.S. Pat. No. 6,763,898.

(13) A BOP storage (not shown) is optionally present in the hull of the vessel adjacent the moonpool 5, for example at a lateral side of the moonpool. It is highly preferable that the first hoisting device 14 is adapted for raising and lowering the BOP to the seabed.

(14) A riser storage 71 extends into the hull 2 at the rear side of the vessel. A riser manipulator 72 is arranged adjacent the firing line 14a, said riser manipulator 72 being adapted to receive a riser, and raise the riser so that the upper end thereof arrives in the firing line 14a thus allowing the upper end to be connected to the load attachment means 14 of the first hoisting device for further handling of the riser by said first hoisting means.

(15) The vessel furthermore comprises a riser handling gantry crane 90. This riser handling gantry crane is preferably provided with riser hoisting device that allows to raise and lower a riser and displace said riser to and from the riser manipulator 72.

(16) A mobile working deck 30 is provided at the rear side of the mast 11, which in an active position covers a portion of the moonpool 5 at said rear side of the mast 11 while the first firing line 14a extends through said mobile working deck (the deck has an opening 31 that can be aligned with the firing line 14a), and which in a non-active position is pivoted upwards about pivot 32, as also visible in FIG. 1. In this non-active position, the deck is cleared from the first firing line 14a and e.g. the BOP is allowed to be brought in said first firing line and manipulated by the first hoisting device.

(17) The vessel 1 has a working deck 40, here a stationary working deck 40 at the front side of the mast 11. In the shown embodiment, the working deck 30 at the rear side of the mast is in its active position at substantially the same height as the working deck 40.

(18) As indicated above, a rotary drilling drive 15c is provided at the front side of the mast, being adapted to assemble and disassemble a drill string and effect drilling at the front side of the mast. The working deck 40 has an opening 41 (visible in FIGS. 1 and 5A) for the passage of tubulars, including a telescopic joint, that can be raised and lowered with the hoisting device 15 at the front side of the mast 11.

(19) The working deck 40 may be provided with a rotary table, an iron roughneck and/or a riser suspension device, e.g. a riser spider, allowing to suspend a riser string, most preferably with a BOP attached to the lower end of the riser string, from the deck 40. In the shown embodiment, two C-shaped clamps 42a, 42b are provided on a rail. Preferably, such a clamp 42a, 42b is suitable to support the telescopic joint. It is noted that a similar clamp is also provided in the mobile working deck 30. Furthermore, a catwalk machine is arranged in longitudinal direction to feed tubulars, e.g. drill pipes into the front firing line. A driller's cabin is arranged on the drill floor.

(20) The mobile working deck 30, as is preferred and as shown in more detail in FIG. 2, includes a suspension device 33 arranged at the opening 31 in the deck, said suspension device 33 being adapted to connect to and support the top end of a string of tubulars 72, most preferably a riser string 72 with a BOP 75 attached to the lower end of the riser string. This suspension device 33 may include a clamping device or similar to suspend a string of tubulars from the deck, e.g. a device known as a riser spider. It will be appreciated that in this preferred embodiment the mobile working deck 30, in its active position, is capable to support the weight of the suspended string of tubulars. Preferably, the suspension device 33 comprises a gimbal device to which the clamping device is mounted, allowing a gimballing movement of the suspended riser string. Preferably, the suspension device 33 is embodied as a spider/gimbal assembly.

(21) In a possible method, after the BOP has been lowered in the first firing line, the working deck 30 can be returned to its active position and used to suspend the BOP from the suspension device 33 of the working deck 30. Subsequently, a riser may then be connected to the top end of the BOP and the entirety lowered by means of hoisting device 14 into the sea, so that the riser top end is then suspended from the working deck 30. Then risers can be added in the manner known in the art.

(22) At the front firing line 15a a riser tensioner system 50 is provided, adapted to be connected to a top end of the riser string, in order to suspend the riser string from in the second firing line. In the shown embodiment, as shown in detail in FIGS. 2, 5a and 5b the riser tensioner system 50 comprises a riser tensioner ring 51 and riser tensioners 52 connected to the vessel and supporting the riser tensioner ring 51 in the second firing line 15a. In particular, the riser tensioners 52 comprise cables 52a, sheaves 52b and cylinders 52c, wherein the cables 52a extend from the riser tensioner ring 51 to the cylinders 52c via sheaves 52b. Here, the riser tensioner cylinders 52c are provided vertically, but a configuration wherein the riser tensioner cylinders are provided horizontally is also conceivable. The riser tensioner system 50 further comprises a telescopic joint 53, which is sometimes also referred to as a slip joint, shown in detail in FIG. 5b, comprising an inner barrel 53a and an outer barrel 53b with a seal 53c therebetween, wherein the riser tensioner ring 51 is adapted to be connected to the top of the outer barrel 53b. Transfer hoses 56 extend from the tensioner ring to the vessel, to transfer electronics, pneumatic fluids and other fluids.

(23) In this embodiment, the riser tensioner system 50 further comprises a top flex joint 54 above the inner barrel 53a of the telescopic joint, to provide lateral restraint and reduce rotation through elastomeric stiffness elements. Also a diverter 55 is located just above the top flex joint 54 and just below the drill floor 40 allowing mud with drill cuttings returning from the well through the riser to be dumped to a mud processing system.

(24) According to the invention, the vessel 1 is furthermore equipped with a suspended riser transfer device 60. The suspended riser transfer device 60 of the present invention, shown in detail in FIGS. 2-4, includes rails 65 extending in longitudinal direction along the moonpool 5 between the first firing line 14a and the second firing line 15a, allowing to displace a frame, possibly embodied as a skid cart, in a suspended riser transfer path P in the longitudinal direction of the moonpool while supporting the riser string in a gimballing manner, in particular a riser string of interconnected risers, optionally with a BOP attached to the lower end of the riser string, lowered into the sea, generally between the rear moonpool area and the front moonpool area, so as to pass underneath the base of the mast. According to a preferred embodiment of the invention, two frames 63, 64 are provided, each supporting both a riser hang-off assembly 63a, 64a and a gimbal device 63b, 64b, to which the riser hang-off assembly 63a, 64a is mounted. The gimballing movement of the riser string is shown schematically in FIG. 1, and in FIG. 2 it is visible that both frames 63, 64 allow a gimballing motion of the riser string. In particular, in FIG. 1 two positions of a riser string 72 are shown: supported by the suspension device 33 and in a gimballing manner supported by frame 63. In FIG. 2, in addition to these two positions, the riser string 72 is also shown in a gimballing manner supported by frame 64.

(25) Hence, when the riser string as being built in the rear firing line 14a has reached a sufficient length, the top end of the riser string may be lowered to the frame 63, so that the top end can be supported by the riser hang off assembly 63a on said frame 63. Then the frame with the suspended riser string can be moved to the front firing line 15a.

(26) A frame 63 is shown in detail in FIGS. 4A-4D. The frame 64 is configured similarly. The frame 63 supports a riser hang-off assembly 63a being actuable between an open configuration in which the riser hang-off assembly is adapted to move around an upper end of a riser string, and a closed configuration in which the riser hang-off assembly engages and supports the riser string, leaving the top end of the riser string exposed to allow for connection to the riser tensioner system. The riser hang-off assembly as shown comprises four rams 62c, e.g. hydraulically actuated rams, that engage the sides of a riser string, similar to a collar clamp, leaving the top end of the riser string exposed.

(27) The riser hang-off assembly 63a is mounted to a gimbal device 63b. The gimbal device 63b of the suspended riser transfer device allows for the angular motion of the riser string with respect to the riser hang-off assembly, to avoid undesirable stresses. The shown gimbal device 63b comprises multiple elastomeric dampeners 63d that absorb the loads induced by the various motions of the drilling vessel. Alternative configurations are also conceivable.

(28) As visible in particular in the top view of FIG. 4D, and also in FIG. 3, the configuration of the frame 63, gimbal device 63b and riser hang-off assembly 63a allows opening and closing of a lock 63e, which is actuable between an open configuration in which the riser hang-off assembly 63a is adapted to move around an upper end of a riser string, and a closed configuration in which the riser hang-off assembly engages and supports the riser string. The lock 63e of the shown embodiment is e.g. configured as a locking bar, or a door. In FIG. 3, the lock 64e of frame 64 is visible.

(29) In FIGS. 2 and 3A, frame 63 is shown in three different positions, indicated with reference numbers 63, 63, 63; and frame 64 is shown in two different positions, indicated with reference numbers 64, 64. The position indicated with reference number 63 is a first parking position for the frame 63, provided in the vicinity of the first firing line 14a, in a direction opposed to the second firing line 15a. The position indicated with reference number 63 is provided in the first firing line 14a, and the position with reference number 63 is provided in the second firing line 15a. The second frame 64 is moveable between the second firing line 15a, via a second parking position indicated with reference number 64, to a third parking position 64. Both the second and the third parking position are provided in the vicinity of the second firing line 15a, in a direction opposed to the first firing line 14a. Accordingly, the rails 65 of the suspended riser transfer device, in the shown embodiment a pair of parallel rails, extend beyond the first firing line 14a to the first parking position 63 and beyond the second firing line 15a to the second parking position 64.

(30) In FIGS. 3A and 3B, the moonpool 5 is shown from a top view. Where in FIG. 3A multiple positions of the frame 63 are indicated, in FIG. 3B the outline of the moonpool 5 is highlighted. Now it becomes apparent that in the shown embodiment, a string of tubulars may suspend from the frames 63, 64 in the positions 63 and 63, corresponding to the first firing line 14a and second firing line 15a, and in the first parking position 63 and second parking position 64. In the third parking position indicated with reference number 64, no string of tubulars is allowed to suspend from the frame.

(31) In the shown embodiment the vessel is a monohull vessel wherein a moonpool extends having a width in the transverse direction of the hull and a length in the longitudinal direction of the hull. Here, the moonpool 5 comprises a main moonpool area 5a in which both firing lines extend, and in addition thereto at least one parking area, here two parking areas 5b, 5c, provided centrally at the transverse ends of the main moonpool area, the width of which does not extend over the entire width of the moonpool. This is advantageous for the overall vessel properties, such as stiffness. These parking areas allow the frames 63, 64 to park including a gimballing string of tubulars. In embodiments wherein the offshore drilling vessel is a semi-submersible, the dimension of the moonpool is generally less of an issue, and an overall increase of the dimensions of the moonpool is possible without creating such recessed parking areas 5b, 5c.

(32) In an embodiment, not shown, a frame of the suspended riser transfer device may in addition have one or more actuable BOP support members, to directly support the BOP on the frame. This allows an alternative method, wherein it is not possible to suspend the BOP from the suspension device 33 of the working deck 30 immediately after the return of the working deck 30 to its active position, which is e.g. the case in alternative configurations of the working deck, e.g. a liftable working deck as disclosed in WO2009/102197. A direct support of the BOP on the support frame may be used for disconnecting the hoisting device 14 from the BOP after it has been lowered to be supported on the frame, so that the mobile working deck can then be returned to its active position. The BOP may then be reattached to the hoisting device and raised with its top end to the level of the working deck 30, so as to suspend the BOP from a suspension device of the working deck 30. Subsequently, similar to the above-indicated method, a riser may then be connected to the top end of the BOP and the entirety lowered by means of hoisting device 14 into the sea, so that the riser top end is then suspended from the working deck 30. Then risers can be added in the manner known in the art.

(33) According to the invention, the offshore vessel as described in relation to the drawings 1-6 allows to build and lower the riser string 72 in the rear firing line 14a with the first hoisting device. When the riser string 72 has reached a sufficient length, the top end of the riser string may be lowered to the frame 63, so that the top end can be supported by the riser hang off assembly 63a on said frame 63. Then the frame with the suspended riser string, preferably with a BOP is moved to the front firing line 15a, where it is connected to the riser tensioner system 50. The operational configuration wherein the riser string 72 with BOP 75 is suspended from the riser tensioner system 50 is shown in FIG. 6. In this operational configuration, drilling can be performed through the riser string 72.

(34) The method according to the invention is shown in detail in FIGS. 7A-7J and FIGS. 8A and 8B. Same parts are given same reference numbers as in FIGS. 1-6.

(35) In FIG. 7A, it is visible that a BOP 75 has been lowered in the first firing line 14a, and that the working deck 30 has returned to its active position and is used to suspend the BOP 75 from the suspension device 33 of the working deck 30. In the embodiment of FIG. 7A, it is visible that a riser section 75a is connected to the BOP. A riser 72 has been retrieved from riser storage 71 and positioned in the first firing line 14a, and has just been connected to the top end of the BOP. In FIG. 7B the entirety of BOP 75 and riser 72 has been lowered by means of hoisting device 14 into the sea, so that the riser top end is then suspended from the working deck 30. Then risers can be added in the manner known in the art, thus building and lowering a riser string a the first side of the mast, in the first firing line.

(36) In FIG. 7B, it can be discerned that in the second firing line, the riser tensioner system is being assembled and prepared. In particular, the riser tensioner ring 51 is positioned in the second firing line 15a above the rails 65 of the suspended riser transfer device, in particular above the suspended riser transfer path P, and in FIG. 7B also the picking up of a telescopic joint 53 is visible, by a catwalk or the like. In the first firing line, the riser string 72 is suspended from the first load attachment device 14b.

(37) In FIG. 7C, the telescopic joint 53 is positioned in the second firing line 15a. In the second firing line, a clamp 42a for the telescopic joint is provided above the riser tensioner ring, to support the telescopic joint 53. The telescopic joint 53 extends through the riser tensioner ring 51. Furthermore, in FIG. 7C it is visible that a landing joint 57 is connected to the second load attachment device 15b, and is placed in the second firing line 15a.

(38) The offshore vessel of the invention allows to perform actions in the first and second firing line simultaneously, and hence together with the preparation of the riser tensioner system in the second firing line, the riser hang-off assembly 63a is positioned in the open configuration in the first firing line 14a. An upper end of the riser string 72 is lowered to be positioned in the riser hang-off assembly 63a, which lowering is enabled by the mobile working deck 30 pivoting upwards and allowing the first hoisting device 14 to lower this top end of the riser string 72 to a level below that of the active position of the working deck, to the position in the hang-off assembly 63a. The riser hang-off assembly 63a is subsequently actuated to the closed configuration. Hereafter, the weight from the riser string 72 is transferred from the first load attachment device 14a to the riser hang-off assembly 63a.

(39) In FIG. 7D, the landing joint 57 is connected to the telescopic joint 53 in the second firing line 15a. In the first firing line 14a, the first load attachment device 14b is disconnected from the riser string 72. The mobile working deck 30 is allowed to pivot back. Subsequently, the riser hang-off assembly 63a with frame 63 is displaced in the suspended riser transfer path on the rails while it supports the riser string 72 in a gimballing manner from the first firing line 14a to the second firing line 15a.

(40) In FIG. 7E, in the second firing line 15a, the telescopic joint 53 with the landing joint 57 is lifted above the rails 65 of the suspended riser transfer device by the second hoisting device 15. Thereby, displacement of the riser hang-off assembly 63a with the riser string 72 into the second firing line 15a, below the telescopic joint 53 is allowed, as visible in FIG. 7E.

(41) Because according to the invention the riser hang-off assembly 63a engages and supports the riser string 72 leaving the top end of the riser string 72 exposed, no additional handling needs to be carried out, allowing the connection of the top end of the riser string 72 to the riser tensioner system immediately after positioning the riser string in the second firing line, as visible in FIG. 7F. In particular, the telescopic joint 53 with the landing joint 57 is lowered by the second hoisting device 15 and subsequently, the top end of the riser string is connected to the lower end of the outer barrel 53b of the telescopic joint 53.

(42) This is shown in a perspective view in FIG. 8A, wherein the top end of a riser 172 is supported by a hang-off assembly 163a. Hang-off assembly 163a is supported by a frame 163, which is displaceable along rails, not shown. The top end of the riser 172 is connected to the lower end of an outer barrel 153b of a telescopic joint 153. Hereby the riser 172 extends through the tensioner ring 151, which is connected via cables 152a and sheaves 152b to riser tensioner cylinders 152c.

(43) In FIG. 7G, the step of transferring the weight from the riser string 72 from the riser hang-off assembly 63a to the riser tensioner system has been performed. In particular, the weight from the riser string 72 is transferred from the riser hang-off assembly 63a to the telescopic joint 53, which is still supported via landing joint 57 by the second hoist assembly 15. After the weight has been transferred, the riser hang-off assembly 63a is actuated to the open configuration and thus allowing the disconnection of the riser hang-off assembly 63a from the riser string 72. Hereafter, the riser hang-off assembly 63a with frame 63 is displaced in longitudinal direction of the moonpool from the second firing line 15a to a parking position 63 beyond the first firing line 14a. In the second firing line 15a, the second hoisting device 15 lowers the telescopic joint 53 with the landing joint 57 further, and connects the riser tensioner ring 51 to a top of the outer barrel 53b of the telescopic joint. In FIG. 7G, the telescopic joint 53 is lowered even further with the landing joint 57 by the second hoisting device 15, until the weight of the telescopic joint 53 and the suspended riser 72 is supported by the riser tensioners 52. In particular, the taut cables 52a are visible in FIG. 7G.

(44) The same situation is depicted in a perspective view in FIG. 8B, in which the outer barrel 153b of the telescopic joint is connected to the riser tensioner ring 151, which is allowed to be lowered until the cables 152 of the riser tensioner system are taut. The telescopic joint is still connected to a landing joint 157.

(45) In FIG. 7H, it is visible that the inner barrel 53a of the telescopic joint 53 is raised out of the outer barrel 53b, with the landing joint 57 by the second hoisting device 15.

(46) In FIG. 7I, the landing joint 57 is disconnected from the second load attachment device 15 and from the telescopic joint 53. The weight of the telescopic joint 53 and the suspended riser 72 is fully supported by the riser tensioners 52. Now that the top end of the telescopic joint 53 is free, in the shown embodiment a flex joint 54 is mounted to the telescopic joint, as well as a diverter connection to be connected to the diverter 55, mounted in the vessel.

(47) The assembled operational configuration is shown in FIG. 7J. The second hoisting device is used to further lower the flex joint 54. The situation of FIG. 7J corresponds to the configuration as shown in FIG. 5B and FIG. 6.