The present invention relates to a semi-submersible floating offshore vessel. Such a vessel commonly has a deckbox structure, one or more buoyant pontoons, e.g. two parallel pontoons or a ring pontoon, and multiple buoyant support columns projecting upwards from the one or more pontoons and supporting thereon the deckbox structure.

Commonly known configurations include vessels having three buoyant support columns supporting a generally triangular deckbox structure. Most common, however, are vessels with four buoyant columns, supporting a rectangular or square deckbox structure. Other known vessels have, for example, six buoyant columns to support a rectangular deckbox structure on two parallel pontoons.

The deckbox structure has an upper or main deck and a lower deck, possibly with one or more tween decks between the upper and lower decks.

A moonpool extends vertically through the deckbox structure, which moonpool has a lower moonpool opening.

The performing wellbore related activities, e.g. drilling, well intervention, etc., known vessels also have a drilling installation with a drilling tower that is erected above the upper deck of the deckbox structure and adapted to perform wellbore related operations along one or multiple firing lines through the moonpool.

In known semi-submersible vessels, the deckbox structure has a main deckbox bottom that extends underneath the lower deck. In common designs the main deckbox bottom is generally horizontal and planar and extends to the buoyant support columns of the vessel. The main deckbox bottom is generally closed, so impervious to waves, e.g. apart from a lower moonpool opening and possibly a mouse hole opening or other equipment (e.g. ROV) related openings therein.

An important design parameter of this type of vessel, is the so-called air gap. Generally, the air gap is the vertical distance between the mean waterline of the vessel and the main deckbox bottom. The air gap serves to provide a reasonable clearance between the deckbox bottom and a wave crest in rough weather conditions. In general a low air gap may lead to undue wave impact, e.g. excessive slamming onto the bottom of the deckbox and/or excessive side impact on the sides of the deckbox structure. One could therefore seek to increase the air gap, yet an excessive air gap will increase the cost of the vessel, decrease the stability, reduce payload capacity, etc. Determination of the correct air gap usually involves the use of complex numerical models to predict the instantaneous air gap between wave crests and the deckbox structure, e.g. based on sets of environmental conditions, e.g. different sea states, wave shapes, etc. It commonly also involves performing model tests, wherein the model tests and numerical models may benefit from one another to achieve the optimal design of the vessel.

Reference is made to the Andersson et al. document “Flush Drill Floor, A conceptual design that lowers the vertical center of gravity on semi-submersible offshore drilling rigs”, Chalmers University of Technology, Joakim Andersson & Mikael Ahlstedt, Goteborg, Sweden, 2014, Report No. E2014:019.

URL: http://publications.lib.chalmers.se/records/fulltext/202824/ 202824.pdf

As discussed in Andersson et al. it is common for the lower deck of the deckbox structure to include a part in vicinity of the moonpool that is called the cellar deck. In Andersson rails for a BOP (Blow Out Preventer) cart are arranged on this cellar deck, allowing for travel of a BOP between a storage area and a position aligned with the firing line. Also it is common for the cellar deck to include a BOP test stump to allow for testing and maintenance of a BOP, e.g. the lower stack thereof.

Andersson et al. also discuss that a flush deck design, so a design where the drill floor is at the same height as the upper deck, places significant constraints on the layout, compared to known designs where the drill floor is elevated above the upper deck, e.g. by some 10 meters. In general, lowering the bottom of the deckbox structure in order to increase height within the deckbox structure reduces the air gap, which reduces the operational window of the vessel. Keeping the bottom at a certain height and raising the upper deck of the deckbox structure, and thus everything mounted on the upper deck, in order to increase the height within the deckbox structure, significantly impacts the stability of the vessel.

The present invention aims to provide an improved semi-submersible floating offshore vessel or at least an alternative for existing vessels.

The present invention aims to provide a semi-submersible floating offshore vessel having the benefit of stability, yet combined with advantageous behavior when it comes to wave impact, in particular the effects of wave crests on the vessel in rough weather conditions, in particular on the deckbox bottom side and any equipment in said zone.

The present invention aims to provide a semi-submersible floating offshore vessel having the benefit of height within the deckbox in vicinity of the moonpool.

The present invention, according to a first aspect thereof, provides a semi-submersible floating offshore vessel according to claim 1.

As in known vessels of this type, seen in plan view, so from above onto the vessel, multiple, e.g. three or four, buoyant support columns are arranged around the moonpool. Herein each pair of adjacent buoyant support columns define an intercolumn space between them.

In the inventive vessel, the deckbox structure comprises a recessed cellar deck structure that protrudes below the main deckbox bottom, which recessed cellar deck structure has a cellar deck bottom wall that is closed and wave impact resistant and which recessed cellar deck structure has a peripheral wall that is closed and wave impact resistant, wherein the peripheral wall extends between an outer perimeter of the cellar deck bottom wall and the main deckbox bottom of the deckbox structure, wherein the moonpool extends through the recessed cellar deck structure, wherein the cellar deck bottom wall delimits the lower moonpool opening, and wherein a cellar deck of the recessed cellar deck structure adjoins the moonpool, wherein, seen in plan view, the recessed cellar deck structure comprises a number of pointed wave crest splitting sections corresponding to the number of intercolumn spaces defined by the buoyant support columns arranged around the moonpool, wherein each pointed wave crest splitting section has, seen in plan view, a tip remote from moonpool and diverging peripheral wall sections, wherein each pointed wave crest splitting section is directed with the tip thereof to a respective intercolumn space.

The invention thus seeks to lower the cellar deck compared to, for example, the mentioned vessel of Andersson et al, where the cellar deck is at the same level as the lower deck of the deckbox structure. The downwardly protruding cellar deck structure has an air gap, which is less than the air gap of the main deckbox bottom.

In practical embodiments, the recessed cellar deck structure protrudes below the main deckbox bottom over a height between 2 and 5 meters, preferably between 3 and 4 meters, e.g. about 3.5 meters.

In practical embodiments, the air gap of the recessed cellar deck structure is between 12 and 15 meters, e.g. between 13 and 14 meters, e.g. about 13.5 meters.

So, for example, the air gap of the main deckbox bottom to the mean waterline of the vessel is 17 meters in absence of waves and the air gap of the recessed cellar deck structure to the mean waterline of the vessel is between 12 and 15 meters, e.g. between 13 and 14 meters, e.g. about 13.5 meters.

The downwardly protruding cellar deck structure may in rough circumstances find itself in the path of waves crests. For said reason the protruding cellar deck structure has a cellar deck bottom wall that is closed and wave impact resistant and a peripheral wall that is closed and wave impact resistant. This peripheral wall extends between an outer perimeter of the cellar deck bottom wall and the main deckbox bottom of the deckbox structure, and therefore a well-protected cellar deck zone is created that may be accessible for personnel. The moonpool extends through the recessed cellar deck structure, and the cellar deck bottom wall delimits the lower moonpool opening. In embodiments, the lower moonpool opening is the only sizable opening in the cellar deck bottom wall.

In view of the potential of being side impacted by incoming wave crests, the inventive downwardly protruding cellar deck structure has a number of pointed wave crest splitting sections corresponding to the number of intercolumn spaces defined by the buoyant support columns arranged around the moonpool. Therefore, for a three-columns vessel, the cellar deck structure will have three pointed wave crest splitting sections. For a four-columns vessel, the cellar deck structure will have four pointed wave crest splitting sections. In a six- columns vessel, the moonpool is generally offset from the center of the vessel, so four of the columns are located around the moonpool, with the other two columns being more remote. In said case, the cellar deck structure will have four pointed wave crest splitting sections.

Each pointed wave crest splitting section has, seen in plan view, a tip remote from moonpool and diverging peripheral wall sections. In addition, each pointed wave crest splitting section is directed with the tip thereof to a respective intercolumn space. In rough weather conditions, a wave crest of a wave entering via said intercolumn space under the deckbox structure may be split by said pointed wave crest splitting section.

Due to this design, an incoming wave crest will in many circumstances be split by the pointed wave crest splitting section, thereby reducing the side impact on the downwardly protruding cellar deck structure as well as generally diffusing the energy of the wave crest and thereby any other impact thereof on the vessel.

In an embodiment, the vessel has four buoyant columns supporting the deckbox structure vessel, and the cellar deck structure has four pointed wave crest splitting sections. For example, the cellar deck structure is, seen in plan view, square or diamond shaped, so the peripheral wall sections of adjacent pointed wave crest splitting sections merging in a side of the square or diamond. The sides of the square or diamond shape can be rectilinear, but other shapes, e.g. convex, concave, or combinations thereof, when seen in plan view, are also contemplated.

In an embodiment, the buoyant support columns of a four-columns vessel each are square or rectangular in horizontal cross-section, at least over a height thereof impacted by wave action, with said four cross-sections being located within an imaginary rectangle or square defined by said four cross-sections combined. In this arrangement, the corner of a column effectively splits a wave that comes in from at said corner of the imaginary rectangle/square, a so-called quartering wave, which compensates for the four pointed wave crest splitting sections not being truly effective for said direction of incoming waves.

In another embodiment, the buoyant support columns of a four-columns vessel each are triangular in horizontal cross-section, at least over a height thereof impacted by wave action, with said four cross-sections being located within an imaginary rectangle or square defined by said four cross-sections combined. In this arrangement, the apex of a column effectively splits a wave that comes in from said corner of the imaginary rectangle/square, a so-called quartering wave, which compensates for the four pointed wave crest splitting sections not being truly effective for said direction of incoming waves.

In an embodiment, the moonpool is offset from the geometrical center of the deckbox structure. ln an embodiment of a four-columns vessel, the moonpool and the cellar deck structure are, seen in plan view, offset from the geometrical center of the four columns, e.g. towards a bow of the vessel.

In an embodiment, the vessel has three buoyant columns supporting the deckbox structure vessel, and the cellar deck structure has three pointed wave crest splitting sections. For example, the cellar deck structure is, seen in plan view, triangular, so the peripheral wall sections of adjacent pointed wave crest splitting sections merging in a side of the triangle. The side of the triangle can be rectilinear, but other shapes, e.g. convex, concave, or combinations thereof, when seen in plan view, are also contemplated.

In an embodiment, the buoyant support columns of a three-columns vessel each are triangular in horizontal cross-section, at least over a height thereof impacted by wave action, with said three cross-sections being located within an imaginary triangle defined by said three cross-sections combined. In this arrangement, the apex of a column effectively splits a wave that comes in from at said corner of the imaginary triangle, which compensates for the three pointed wave crest splitting sections not being truly effective for said direction of incoming waves.

In an embodiment, the cellar deck structure may have a central portion extending at some distance around the moonpool opening, e.g. of circular or rectangular/square shaped central portion when seen in plan view, wherein the pointed wave crest splitting sections extend outward from the central portion, e.g. like a pointed star configuration.

In an embodiment, the tip of one or more, e.g. each, of pointed wave crest splitting sections has a forward rake. In another embodiment, the tip is vertical.

In an embodiment, diverging peripheral wall sections of one or more, e.g. each, of the wave crest splitting sections have an outward flare, so outwardly inclined relative to vertical from the lower edge of the wall section to the top edge thereof that adjoins the main deckbox bottom. In another design these diverging peripheral wall sections are, for example, vertically oriented.

In an embodiment, each wave crest splitting section has a substantially planar bottom wall portion forming part of the cellar deck bottom wall, e.g. horizontally oriented, to which the diverging peripheral wall sections join along their lower edges. ln an embodiment, the main deckbox bottom is double walled.

In an embodiment, the cellar deck bottom wall is double walled.

In an embodiment, a column is configured as a paired column.

In an embodiment, the vessel comprises a BOP transport system, the BOP transport system comprising:

- a BOP track arranged on the cellar deck and having a set of parallel BOP handling cart rails, e.g. skid rails, extending from a BOP storage area towards and along opposite sides of the moonpool; and

- a BOP handling cart, which is movably supported by the BOP handling cart rails, for travelling over said rails so as to allow for transfer of a subsea BOP between a BOP storage position, in which the BOP is located at the BOP storage area, and a BOP launch position, in which the BOP is located in the moonpool and aligned with a firing line.

In an embodiment, the recessed cellar deck structure supports a BOP test stump, e.g. a vertically mobile test stump, in the BOP storage area, at a location outside of and adjacent the BOP track.

In an embodiment, the vessel comprises vertically oriented wireline riser tensioner cylinders that are mounted in vicinity of the moonpool and arranged between the upper deck and the recessed cellar deck bottom wall. In an embodiment, these cylinders have a cylinder body that is mounted to the upper deck construction, and a piston extendable in downwards direction.

In an embodiment, the vessel comprises a first set of vertically oriented wireline riser tensioner cylinders arranged along one side of the moonpool, e.g. outward of a BOP handling cart rails extending along said one side of the moonpool, and a second set of vertically oriented wireline riser tensioner cylinders arranged along an opposite side of the moonpool, e.g. outward of a BOP handling cart rails extending along said opposite side of the moonpool.

In an embodiment, the vessel is provided with a vertically mobile working deck which is arranged in vertical projection above the moonpool, which working deck is vertically movable. In an embodiment, multiple vertically mounted working deck compensator cylinders are provided on the vessel. For example, these cylinders are arranged between the deckbox structure and the mobile working deck. The working deck compensator cylinders are configured to provide a heave compensated motion of the working deck relative to the deckbox structure.

In an embodiment, the working deck compensator cylinders are mounted at a lower end thereof to the recessed cellar deck structure.

In an embodiment, the vertically mobile working deck, in a lower stationary resting position thereof, is flush with at least an adjoining area of the upper deck of the deckbox structure. In an embodiment, the working deck and an adjoining area of the upper deck of the deckbox structure are each provided with rails configured to transfer equipment over said rails, e.g. equipment arranged on a skid pallet skiddable over said rails, onto and off the working deck.

In an embodiment, the vertically mobile working deck is configured to be elevated, relative to a lower stationary resting position, that e.g. is flush with the upper deck, and to be movable within a motion range including a heave compensation motion range by a heave motion compensation system, e.g. by said working deck compensator cylinders,.

In an embodiment, the mobile working deck when in the stationary resting position is supported by the deckbox structure, e.g. flush with the upper deck. This avoids that the working deck is then supported by any mobile working deck compensator cylinders.

In an embodiment, the vessel is provided with a mobile work deck access walkway, preferably a telescopic access walkway, for providing personnel access to the mobile working deck in an elevated position thereof, e.g. whilst in heave compensating motion, which walkway at one end is supported by the mobile working deck, and at an opposite end is supported by an associated access platform, which access platform preferably is provided higher than the upper deck and lower than the equipment deck.

In an embodiment, at least one first working deck compensator cylinder is arranged outward of a first BOP handling cart rail, relative to the moonpool, and at least one second working deck compensator cylinder is arranged outward of a second BOP handling cart rail, relative to the moonpool, so as to allow for passing a subsea BOP on the BOP handling cart in between the first and second set of working deck compensator cylinders, e.g. said working deck compensator cylinders being extendable to raise the working deck from its stationary resting position to allow for passage of the subsea BOP from the BOP storage room into the firing line.

In an embodiment, at least one of a drill string slip device, a riser spider device, and/or a diverter is supported by the vertically mobile working deck, wherein said drill string slip device is configured to support a suspended drill string within a riser, wherein the riser spider device is configured to support a suspended riser, e.g. during assembly and disassembly of a riser, and wherein the diverter is configured to divert a hydrocarbon and/or drilling mud stream from a subsea wellbore to the vessel.

In an embodiment, the vessel comprises an equipment deck that is located higher than the upper deck of the deckbox structure, e.g. in vertical projection above the BOP storage area, wherein the equipment deck is configured for storage thereon of wellbore related equipment, e.g. workover or well maintenance equipment, e.g. a coiled tubing device.

In an embodiment, the equipment deck is configured such that the vertically mobile working deck of the vessel, in a raised position thereof, adjoins the equipment deck allowing for transfer of wellbore related equipment between the equipment deck and the vertically mobile working deck. In an embodiment, the working deck and the equipment deck are both provided with rails that align in said raised position of the mobile working deck, wherein these rails are configured to transfer equipment over the rails, e.g. equipment arranged on a skid pallet skiddable over said rails, from the equipment deck onto the vertically mobile working deck and vice versa.

It is envisaged that the equipment deck may be used for arranging thereon one or more spoolable product coil devices, each device having a coil storing thereon a spoolable product. Examples of a spoolable product are a (control) line, wireline, cable, hose, coiled- tubing, umbilical, etc..

In an embodiment, the equipment deck is an open-air deck. Being an open air deck allows for using a crane for placing equipment on the deck, and for removing equipment from the deck.

In an embodiment, the equipment deck is provided with raised weather shielding walls, for shielding against wind, etc.. The equipment deck is, preferably, open on the top to provide access for a crane. The vertically mobile working deck can also be used for moving wellbore related equipment between the upper deck and the equipment deck, e.g. such equipment being placed on or provided with a skid pallet that is skiddable over rails on the upper deck, then onto rails on the working deck, which working deck is then lifted to align with the equipment deck, where the equipment is skidded from the working deck onto the equipment deck.

In an embodiment, the equipment deck, located at a height above the upper deck of the deckbox structure, forms a roof covering the BOP storage area. In an embodiment, when seen in plan view, the plan of the equipment deck is similar to the BOP storage deck. Thus the BOP deck is at least partially sheltered from the environment.

In a further embodiment, weather walls are provided between the equipment deck and the upper deck of the deckbox structure, such that the semi-submersible is provided with a BOP garage fully sheltering subsea BOP equipment in said BOP storage area.

In an embodiment, a BOP handling crane, e.g. an overhead travelling beam crane, is provided in the BOP storage area, e.g. mounted below the equipment deck. The BOP handling crane allows, for example, moving a BOP or BOP stack between a test location having a test stump and the BOP cart.

In an embodiment, a BOP guide device is mounted under the protruding cellar deck structure, the BOP guide device being configured to provide guidance for the BOP during displacement along the firing line below the protruding cellar deck structure, e.g. to avoid or reduce sway motion of the BOP during such displacement.

In an embodiment, the vessel is provided with a shaker room within the deckbox structure and in proximity of, e.g. adjacent to, the moonpool, e.g. wherein the shaker room is on the lower deck.

In an embodiment, the vessel is provided with a drill cuttings handling room arranged below the shaker room, e.g. on the cellar deck and below the shaker room arranged on the lower deck.

In an embodiment, the vessel is provided with a mud pump room within the deckbox structure and in proximity of, e.g. adjacent to, the shaker room, e.g. on the lower deck. In an embodiment, the drilling tower is embodied as a single vertical mast structure erected above the upper deck of the deckbox structure and adjacent a side of the moonpool, the vertical mast structure being located outside of a vertical projection of the moonpool.

In an embodiment, a crown block structure is mounted on top of the vertical mast structure.

In an embodiment, the mast structure has an operative face directed towards the moonpool, for example one of more vertical guide rails being mounted on the mast structure at the operative face.

In an alternative embodiment, for example, the tower is embodied as a derrick that is arranged over the moonpool.

In an embodiment, the drilling installation is embodied as a RamRig design.

In an embodiment, the drilling installation comprises a hoisting device comprising at least one winch and at least one winch driven cable, which hoisting device is adapted to suspend a load from a crown block structure via said at least one winch driven cable and to manipulate said suspended load in the firing line, e.g. from a vertical mast structure erected above the upper deck of the deckbox structure and adjacent a side of the moonpool such that the firing line extends along and outside of an operative face of the vertical mast structure.

In an embodiment, BOP cart rails are arranged perpendicular to the operative face of the mast structure.

In an embodiment, the mast structure, e.g. at an operative face thereof directed towards the firing line through the moonpool, is provided with one or more vertical guide rails.

In an embodiment, the vertically mobile working deck is guided along the one or more vertical guide rails on the mast structure.

In an embodiment, the drilling installation comprises a travelling device that is movable up and down along and outside of said operative face of the mast structure and guided by said one or more vertical guide rails of said mast structure, e.g. wherein said travelling device is suspended from a winch driven cable, e.g. suspended from a crown block structure of the tower, e.g. the travelling device being suspended from a travelling block, e.g. wherein the travelling device is adapted to suspend a load from said travelling device and/or to support the travelling block.

In an embodiment, the tower is provided with a vertical motion arm assemblies rail, wherein at least one, e.g. multiple, motion arm assembly is mounted on said vertical motion arm assemblies rail, each motion arm assembly having a base that is vertically mobile along said vertical motion arm assemblies rail and an extensible, e.g. telescopic, arm that is mounted via a vertical axis slew bearing on said base so as to allow for extension and retraction of said arm as well as slewing motion of said arm about said vertical slew axis, wherein said arm is adapted to support a tool at an end of said arm .

In an embodiment, the vessel is provided with a drilling tubulars storage rack, e.g. multi-joint drill pipe stands storage rack, e.g. a rotary storage rack, which drilling tubulars storage rack is adapted for storage of drilling tubulars in vertical orientation therein, and wherein the vessel, e.g. the mast structure, is provided with a racker system that is adapted to move a drilling tubular between the storage rack and a position aligned with the firing line.

In an embodiment, the racker system comprises a vertical motion arm assemblies rail, wherein at least one, e.g. multiple, motion arm assembly is mounted on said vertical motion arm assemblies rail, each motion arm assembly having a base that is vertically mobile along said vertical motion arm assemblies rail and an extensible, e.g. telescopic, arm that is mounted via a vertical axis slew bearing on said base so as to allow for extension and retraction of said arm as well as slewing motion of said arm about said vertical slew axis, wherein said telescopic arm is adapted to support a tubulars gripper tool at an end of said arm, so as to allow for gripping of a drilling tubulars by means of the tubular gripper tool.

In an embodiment, the vessel is provided with a drilling tubulars storage rack that is mounted on the deckbox structure, e.g. multi-joint drill pipe stands storage rack, e.g. a rotary storage rack, which drilling tubulars storage rack is adapted for storage of drilling tubulars in vertical orientation therein, and wherein the vessel, e.g. the mast structure, is provided with a racker system that is adapted to move a drilling tubular between the storage rack and a position aligned with the firing line, and wherein the racker system is heave compensated and is configured to bring a drilling tubular removed from the storage rack in a heave compensation motion that is synchronized with the heave compensation motion of the mobile working deck. For example, the racker comprises a vertical motion arm assemblies rail, wherein at least one, e.g. multiple, motion arm assembly is mounted on said vertical motion arm assemblies rail, each motion arm assembly having a base that is vertically mobile along said vertical motion arm assemblies rail by a drive configured to provide said heave compensation motion that is synchronized with the heave compensation motion of the mobile working deck. For example, each motion arm assembly further having an extensible, e.g. telescopic, arm that is mounted via a vertical axis slew bearing on said base so as to allow for extension and retraction of said arm as well as slewing motion of said telescopic arm about said vertical slew axis, wherein said arm is adapted to support a tubulars gripper tool at an end of said arm, so as to allow for gripping of a drilling tubulars by means of the tubular gripper tool.

In an embodiment, the vessel comprises a drilling tubulars rotary storage rack that is rotatable about a vertical axis and has storage slots for storage of multiple drilling tubulars in vertical orientation, the drilling tubulars rotary storage rack including a drive to rotate the drilling tubulars storage rack about its vertical axis. For example, the drilling tubulars rotary storage rack comprising a central vertical post and multiple discs at different heights on the post, at least one disc being a fingerboard disc having tubulars storage slots, each slot having an opening at an outer circumference of the fingerboard disc allowing to introduce and remove a tubular from the storage slot.

In an embodiment, the drilling tower is embodied as a singular vertical mast structure having closed wall contour, e.g. an octagonal cross-section, e.g. over at least a major portion of the height of the tower.

In an embodiment, the drilling installation further comprises a main hoisting device comprising at least one winch and at least one winch driven cable, which hoisting device is adapted to suspend a load from a crown block structure via said at least one winch driven cable and to manipulate a suspended load in the firing line, which firing line preferably extends along and outside of an operative face of a vertical mast structure of the tower, and wherein the vessel, e.g. the tower, e.g. in the mast, is provided with one or more heave compensation cylinders acting on one or more cable sheaves along with the winch driven cable passes in order to provide heave compensation functionality for the load suspended in the firing line.

In an embodiment, the vessel has a catwalk machine arranged on the upper deck and configured to feed and remove drilling tubulars to and from the firing line through the moonpool.

A second aspect of the invention relates to a vessel according to the preamble of claim 1. According to the second aspect of the invention the vessel is characterized in that the deckbox structure comprises a recessed cellar deck structure that protrudes below the main deckbox bottom, which recessed cellar deck structure has a cellar deck bottom wall that is closed and wave impact resistant and which recessed cellar deck structure has a peripheral wall that is closed and wave impact resistant, wherein the peripheral wall extends between an outer perimeter of the cellar deck bottom wall and the main deckbox bottom of the deckbox structure, wherein the moonpool extends through the recessed cellar deck structure, wherein the cellar deck bottom wall delimits the lower moonpool opening.

Due to the presence of the recessed cellar deck structure the deckbox structure has an increased effective height at said location, e.g. allowing for enhanced integration of a mobile working deck, and/or BOP storage and handling, as discussed herein with reference to the first aspect of the invention.

The vessel of the second aspect of the invention may be provided with one or more features discussed herein with reference to the vessel according to the first aspect of the invention.

A third aspect of the invention relates to a semi-submersible floating offshore vessel, said vessel comprising:

- a deckbox structure;

- one or more buoyant pontoons, e.g. two parallel pontoons or a ring pontoon,

- multiple buoyant support columns projecting upwards from the one or more pontoons and supporting thereon the deckbox structure; wherein the deckbox structure comprises an upper deck and a lower deck, and a main deckbox bottom, wherein the main deckbox bottom extends underneath said lower deck, which main deckbox bottom is closed and wave impact resistant, which main deckbox bottom connects to the buoyant support columns, wherein a moonpool extends through said deckbox structure, said moonpool having a lower moonpool opening, wherein, seen in plan view, multiple, e.g. three or four, buoyant support columns are arranged around the moonpool, wherein each pair of adjacent buoyant support columns define an intercolumn space between them, characterized in that the deckbox structure comprises a recessed cellar deck structure that protrudes below the main deckbox bottom, which recessed cellar deck structure has a cellar deck bottom wall that is closed and wave impact resistant and which recessed cellar deck structure has a peripheral wall that is closed and wave impact resistant, wherein the peripheral wall extends between an outer perimeter of the cellar deck bottom wall and the main deckbox bottom of the deckbox structure, wherein the moonpool extends through the recessed cellar deck structure, wherein the cellar deck bottom wall delimits the lower moonpool opening, and wherein a cellar deck of the recessed cellar deck structure adjoins the moonpool, wherein, seen in plan view, the recessed cellar deck structure comprises a number of pointed wave crest splitting sections corresponding to the number of intercolumn spaces defined by the buoyant support columns arranged around the moonpool, wherein each pointed wave crest splitting section has, seen in plan view, a tip remote from moonpool and diverging peripheral wall sections, wherein each pointed wave crest splitting section is directed with the tip thereof to a respective intercolumn space.

The vessel may be embodied as an offshore drilling vessel, e.g. as discussed herein.

The vessel could also be a production platform linked to one or more subsea hydrocarbon reservoir.

The vessel could be a pipelaying vessel, e.g. equipped with a pipe lay tower having a pipe lay firing line passing through the moonpool, e.g. for J-lay of pipe on the seabed. The vessel of the third aspect of the invention may be provided with one or more features discussed herein with reference to the vessel according to the first and/or second aspect of the invention.

A fourth aspect of the invention relates to a deckbox structure configured for integration in a floating semi-submersible offshore vessel, wherein the vessel has one or more buoyant pontoons, e.g. two parallel pontoons or a ring pontoon, and has multiple buoyant support columns projecting upwards from the one or more pontoons and configured for supporting thereon the deckbox structure, wherein each pair of adjacent buoyant support columns define an intercolumn space between them, wherein the deckbox structure comprises an upper deck and a lower deck, and a main deckbox bottom, wherein the main deckbox bottom extends underneath said lower deck, which main deckbox bottom is closed and wave impact resistant, which main deckbox bottom is configured to connects to the buoyant support columns, wherein a moonpool extends through said deckbox structure, said moonpool having a lower moonpool opening, wherein the deckbox structure comprises a recessed cellar deck structure that protrudes below the main deckbox bottom, which recessed cellar deck structure has a cellar deck bottom wall that is closed and wave impact resistant and which recessed cellar deck structure has a peripheral wall that is closed and wave impact resistant, wherein the peripheral wall extends between an outer perimeter of the cellar deck bottom wall and the main deckbox bottom of the deckbox structure, wherein the moonpool extends through the recessed cellar deck structure, wherein the cellar deck bottom wall delimits the lower moonpool opening, and wherein a cellar deck of the recessed cellar deck structure adjoins the moonpool, wherein, seen in plan view, the recessed cellar deck structure comprises a number of pointed wave crest splitting sections corresponding to the number of intercolumn spaces defined by the buoyant support columns arranged around the moonpool, wherein each pointed wave crest splitting section has, seen in plan view, a tip remote from moonpool and diverging peripheral wall sections, wherein each pointed wave crest splitting section is oriented so that, when the deckbox structure has been integrated with the columns, each pointed wave crest splitting section is directed with the tip thereof to a respective intercolumn space.

The fourth aspect of the invention also relates to a method for building a floating semi- submersible offshore vessel, wherein the deckbox structure is placed as a unit onto the columns, e.g. in a float-over, a heavy lift, skidding, or in a dock.

The aspects of the invention each also relate to a method of performing a subsea wellbore related operation, e.g. a drilling and/or wellbore intervention operation and/or installation of wellbore related subsea equipment, wherein use is made of a vessel as described herein.

The invention will now be described with reference to the drawings. In the drawings:

Fig. 1 shows a perspective view an example of a four-columns semi-submersible drilling vessel according to the invention,

Fig. 2 shows the vessel of figure 1 from a different angle,

Fig. 3 shows the vessel of figure 1 in a perspective view from above,

Fig. 4 shows an area of figure 3 on a larger scale,

Fig. 5 shows the vessel of figure 1 in front view,

Fig. 6 shows the vessel of figure 1 in side view,

Fig. 7 shows a cross-section of the vessel of figure 1 over the centerline,

Fig. 8 shows an area of figure 7 on a larger scale,

Fig. 9 shows another cross-section of the vessel of figure 1 in a plane parallel to the centerline,

Fig. 10 shows a transverse cross-section of the vessel of figure 1,

Fig. 11 shows an area of figure 10 on a larger scale,

Fig. 12 shows in plan view the layout of the deckbox structure at the level of the lower deck, Fig. 13 shows in plan view the layout of the deckbox structure at the level of the recessed cellar deck,

Fig 14 shows in a horizontal cross-section, the layout of the vessel at the level of the upper deck, including the vertically mobile working deck in lower stationary resting position,

Fig. 15 illustrates an example of a three-columns semi-submersible vessel according to the invention.

With reference to figures 1 - 14 now an exemplary semi-submersible floating offshore drilling vessel 1 according to the invention will be discussed. All figures 1 - 14 are to scale.

The vessel 1 has a floating hull comprising: - a multi-deck level deckbox structure 2, - one or more buoyant pontoons, here two parallel pontoons 3,

- multiple, here four, buoyant support columns 4 projecting upwards from the pontoons and supporting thereon the deckbox structure 2. Here two columns are mounted on each pontoon 3.

In the depicted example, the deckbox structure 2 is generally rectangular in plan view.

Burner booms 240 are mounted at the stern of the vessel 1.

Engine exhausts 245 are mounted at the stern of the vessel 1.

A storage area 270 for horizontal storage of drilling tubulars and/or riser joints is provided on upper deck 5, here at the stern of the vessel 1. A crane 271 for handling drilling tubulars and/or riser joints is provided on the vessel 1 in association with the storage area 270.

Reference numeral 280 denotes a drillers cabin on the upper deck 5 in vicinity of firing line 9 and, as preferred, in vicinity of a stand-building site.

Figures 1 - 3 show a telescopic joint 250 and riser string 260 suspended from the vessel 1 along a firing line 9, e.g. as often in drilling operations.

The deckbox structure comprises an upper deck 5, a tween deck 6, and a lower deck 7.

The deckbox structure 2 comprises side walls 8 around the periphery of the structure. In practice, at least a lower region of these side walls 8 will be closed and wave impact resistant, in view of potential wave impact in rough weather conditions.

The deckbox structure 2 comprises a main deckbox bottom 10 that extends horizontally underneath the lower deck 7. The main deckbox bottom is closed and wave impact resistant, e.g. in view of waves slamming onto the bottom 10. The bottom 10 may be double-walled.

The main deckbox bottom 10 extends up to and connects to the buoyant support columns 4.

The deckbox structure 2 comprises a recessed cellar deck structure 20 that protrudes below the main deckbox bottom 10. The recessed cellar deck structure 20 has a cellar deck bottom wall 21 that is closed and wave impact resistant.

In practical embodiments, the recessed cellar deck structure 20 protrudes below the main deckbox bottom 10 over a height between 2 and 5 meters, preferably between 3 and 4 meters. In the depicted embodiment, this height is about 3.5 meters.

The recessed cellar deck structure 20 further has a peripheral wall 22 that is closed and wave impact resistant. The peripheral wall 22 extends between an outer perimeter of the cellar deck bottom wall 21 and the main deckbox bottom 10 of the deckbox structure 2.

A moonpool 40 extends through the deckbox structure 2, from the upper deck 5 through the recessed cellar deck structure 20. The cellar deck bottom wall 21 delimits a lower moonpool opening 41.

The recessed cellar deck structure 20 provides for a well-protected cellar deck zone, that is readily accessible for personnel. As shown here, it is preferred for the lower moonpool opening 41 to be the only sizable opening in the cellar deck bottom wall 21.

A cellar deck 23, which lies at a lower level than the lower deck 7 of the deckbox structure 2, is part of the recessed cellar deck structure 20 and is accessible for personnel. This cellar deck 23 adjoins the moonpool 40.

Seen in plan view, here four, buoyant support columns 4 are arranged around the moonpool 40. Herein each pair of adjacent buoyant support columns 4 defines an intercolumn space between them.

The vessel comprises a drilling installation with a drilling tower 60, here embodied as a single mast, that is erected above the upper deck 5 of the deckbox structure and that is adapted to perform wellbore related operations along one or multiple firing lines 9 through the moonpool 40.

Seen in plan view, the recessed cellar deck structure 20 comprises a number, here four, of pointed wave crest splitting sections 23, 24, 25, 26 corresponding to the number of intercolumn spaces defined by the buoyant support columns 4 arranged around the moonpool. As shown by way of example, each pointed wave crest splitting section has, seen in plan view, a tip 23a that is located remote from moonpool 40. Each pointed wave crest splitting section further has diverging peripheral wall sections adjoining at the tip 23a. For example, diverging peripheral wall sections of section 23 have reference numerals 23c, d in figures 1,

2.

As illustrated, each pointed wave crest splitting section 23, 24, 25, 26 is directed with the tip 23a thereof to a respective intercolumn space.

In rough weather conditions a very high wave having a crest that comes fairly close to the bottom 10, and enters via the intercolumn space between two columns to under the deckbox structure 2, is likely to be split by said pointed wave crest splitting section.

As illustrated, the tip 23a of each pointed wave crest splitting sections has a forward rake. In another embodiment, the tip is vertical.

As illustrated, the diverging peripheral wall sections 23c, d b of one or more, e.g. each, of the wave crest splitting sections have an outward flare, so outwardly inclined relative to vertical from the lower edge of the wall section to the top edge that adjoins the main deckbox bottom. As illustrated, each wave crest splitting section 23, 24, 25, 26 has a substantially planar bottom wall portion 23e forming part of the cellar deck bottom wall, here horizontally oriented, to which the diverging peripheral wall sections 23c, d join along their lower edges.

As preferred, the main deckbox bottom 10 is double walled.

As preferred, the cellar deck bottom wall 21 is double walled.

The pointed wave crest splitting sections 23, 24, 25, 26 will generally reduce wave impact on the downwardly protruding cellar deck structure as well as generally diffuse the energy of the wave crest and thereby any other impact thereof on the vessel, e.g. on the bottom wall 10. The splitting may reduce side impact loading on the peripheral wall 22, but may also serve to reduce more upward oriented impacts, e.g. slamming of waves from below onto the bottom of the deckbox structure 2.

The provision of the recessed cellar deck structure 20 and pointed wave crest splitting sections may serve to reduce wave impact on other parts of the vessel, e.g. on the bottom 10 or the columns 4, e.g. at locations where a column 4 adjoins the bottom 10. As explained, the exact design of the vessel 1 in view of wave impact is commonly done by design routines that include mathematical modelling and model testing, e.g. based on data related to the environment where the vessel will be operated and the operational conditions of the vessel. The relative location, general dimensions, shape, any flaring of the peripheral wall, the strength of the bottom wall and peripheral wall, etc. of the recessed cellar deck structure 20 may result or benefit from such an approach.

As illustrated here, the vessel has four buoyant columns 4 supporting the deckbox structure 2, and the cellar deck structure 20 has four pointed wave crest splitting sections. Seen in plan view, square or diamond shaped, so the peripheral wall sections of adjacent pointed wave crest splitting sections merging in a side of the square. The sides of the square or diamond shape are shown to be rectilinear, but other shapes, e.g. convex, concave, or combinations thereof, when seen in plan view, are also contemplated.

As illustrated here, the buoyant support columns 4 each are generally square or rectangular in horizontal cross-section, at least over a height thereof impacted by wave action. The four cross-sections or the columns 4 are located within an imaginary rectangle or square defined by said four cross-sections combined. In this arrangement, the vertical corner of a column 4 effectively splits a wave that comes in at said corner of the imaginary rectangle/square, a so- called quartering wave, which compensates for the four pointed wave crest splitting sections not being truly effective for said direction of incoming waves.

It is noted that the vessel 1 may be operated whilst at anchor, using anchors 15, that prevent the vessel 1 from orienting the bow 16 into the incoming sea. Already for said reason it is preferred to have four wave crest splitting sections 23, 24, 25, 26.

The vessel 1 may also be operated whilst using a dynamic positioning system of the vessel including thrusters 17 to maintain location and heading, e.g. over a subsea wellhead. In such situation, the dynamic positioning can be operated to keep the bow 16 of the vessel oriented into the incoming seas during rough weather conditions.

As illustrated here, the vessel 1 comprises a BOP transport system, the BOP transport system comprising:

- a BOP track arranged on the cellar deck 23 and having a set of parallel BOP handling cart rails 70, e.g. skid rails, extending from a BOP storage area 71 towards and along opposite sides of the moonpool 40; and - a BOP handling cart 72, which is movably supported by the BOP handling cart rails 70, for travelling over said rails so as to allow for transfer of a subsea BOP 73 between a BOP storage position, in which the BOP is located at the BOP storage area 71, and a BOP launch position, in which the BOP is located in the moonpool 40 and aligned with the firing line.

As illustrated here, the recessed cellar deck structure 20 supports a BOP test stump 26, e.g. a vertically mobile test stump, e.g. having a vertical drive cylinder that is mounted in the bottom 21 , in the BOP storage area 71 , at a location outside of and adjacent the BOP track 70.

As illustrated here, the vessel 1 comprises vertically oriented wireline riser tensioner cylinders 80 that are mounted in vicinity of the moonpool 40 and are arranged between the upper deck 5 and the recessed cellar deck bottom wall 21. It is shown that the cylinders 80 each have a cylinder body that is mounted to the upper deck construction, and a piston extendable in downwards direction. The piston carriers a sheave assembly for the wirelines of the riser tensioner system as is known in the art.

As illustrated here, the vessel 1 comprises a first set of vertically oriented wireline riser tensioner cylinders 80 arranged along one side of the moonpool 40, here outward of a BOP handling cart rails 70 extending along said one side of the moonpool, and a second set of vertically oriented wireline riser tensioner cylinders 80 arranged along an opposite side of the moonpool, here outward of a BOP handling cart rails 70 extending along said opposite side of the moonpool 40.

As illustrated here, the vessel 1 is provided with a vertically mobile working deck 90 which is arranged in vertical projection above the moonpool 40, which working deck is vertically movable.

As illustrated here, multiple vertically mounted working deck compensator cylinders 91, 92 are arranged between the deckbox structure and the mobile working deck 90. In this example, two hydraulic cylinders 91 , 92 support the deck 90. For example, the cylinders are multi-stage telescopic cylinders.

The working deck compensator cylinders 91 , 92, are configured to provide a heave compensated motion of the working deck relative to the deckbox structure, e.g. as disclosed in WO2016/062812. As illustrated here, the working deck compensator cylinders 91, 92 are mounted at a lower end thereof to the recessed cellar deck structure 20. This allows to make optimum use of the increase height afforded by the structure 20.

The vertically mobile working deck 90, in a lower stationary resting position thereof, is flush with at least an adjoining area of the upper deck 5 of the deckbox structure.

As illustrated here, the working deck 90 and an adjoining area of the upper deck 5 of the deckbox structure are each provided with rails 98, 5a that are configured to transfer equipment over said rails, e.g. equipment arranged on a skid pallet 5b, 5c skiddable over said rails, onto and off the working deck 90 in the lower stationary resting position.

As illustrated here, the vertically mobile working deck 90 is configured to be elevated, by said working deck compensator cylinders 91, 92, relative to a lower stationary resting position and to be movable within a motion range including a heave compensation motion range, e.g. as disclosed in WO2016/062812.

As illustrated here, the mobile working deck 90 when in the stationary resting position is supported by the deckbox structure 2 and not by the mobile working deck compensator cylinders 91, 92.

As illustrated here, the vessel 1 is provided with a mobile work deck access walkway 100, here a telescopic access walkway, for providing personnel access to the mobile working deck 90 in an elevated position thereof, e.g. whilst in heave compensating motion. The walkway is at one end supported by the mobile working deck 90, and at an opposite end supported by an associated access platform 101, which access platform preferably is provided higher than the upper deck and lower than an equipment deck 120.

As illustrated here, at least one first working deck compensator cylinder 91 is arranged outward of a first BOP handling cart rail 70, relative to the moonpool, and at least one second working deck compensator cylinder 92 is arranged outward of a second BOP handling cart rail 70, relative to the moonpool, so as to allow for passing a subsea BOP on the BOP handling cart 72 in between the first and second set of working deck compensator cylinders 91 , 92. It is shown that the working deck compensator cylinders 91 , 92 are extendable to raise the working deck from its stationary resting position to allow for passage of the subsea BOP from the BOP storage room into the firing line. Herein the assembled BOP has a height that exceeds the height between the cart 72 and the upper deck 5, so the BOP sticks out above the upper deck 5.

As illustrated here, at least one of a drill string slip device 95, a riser spider device 96, and/or a diverter 97 is supported by the vertically mobile working deck 90, not necessarily all at once. For example, only the riser spider device 96 is arranged on the deck 90 when tripping a riser, whereas the drill string slip device 95 as well as the diverter 97 are present when performing drilling operations.

In figures 10, 11 it is shown that a BOP 73 is suspended via collapsed and locked telescopic joint 250 from riser spider device 96 arranged on the mobile working deck 90 that is in a raised position thereof.

As illustrated here, the vessel 1 comprises an equipment deck 120 that is located higher than the upper deck 5 of the deckbox structure 2.

The equipment deck 120 is located at least in part in vertical projection above the BOP storage area 71.

The equipment deck 120 is configured for storage thereon of wellbore related equipment, e.g. workover or well maintenance equipment, e.g. a coiled tubing injector device 125.

As illustrated here, the equipment deck 120 is configured such that the vertically mobile working deck 90, in a raised position thereof, adjoins the equipment deck 120 allowing for transfer of wellbore related equipment 125 between the equipment deck 120 and the vertically mobile working deck 90.

As illustrated here, the working deck 90 and the equipment deck 120 are both provided with rails 98, 128 that align in said raised position of the mobile working deck. These rails 98, 128 are configured to transfer equipment 125 over these rails, e.g. equipment arranged on a skid pallet skiddable over said rails, from the equipment deck onto the vertically mobile working deck and vice versa.

As illustrated here, the equipment deck 120 is provided with one or more spoolable product coil devices 125, each having a coil storing thereon a spoolable product, such as a (control) line, wireline, cable, hose, coiled-tubing, umbilical, etc., allowing to pass the one or more spoolable products from the respective coil device to the firing line. As illustrated here, the equipment deck 120 is an open air deck. Being an open air deck allows for using a crane 200 for placing equipment on the deck, and for removing equipment from the deck 120.

As illustrated here, the equipment deck is provided with walls 123, for shielding against wind, and is open on the top to provide access for a crane 200.

The vertically mobile working deck 90 can also be used for moving equipment between the upper deck 5 and the equipment deck 120.

As illustrated here, the equipment deck 120, located at a height above the upper deck 5 of the deck box structure, forms a roof covering the BOP storage area 71. Seen in plan view, the plan of the equipment deck 120 is similar to the BOP storage deck 71. Thus, the BOB deck is at least partially sheltered from the environment.

Weather walls 74 are provided between the equipment deck 120 and the upper deck 5 of the deckbox structure, such that the semi-submersible is provided with a BOP garage 75 sheltering subsea BOP equipment in said BOP storage area.

As illustrated here, a BOP handling crane 76, here an overhead travelling beam crane, is provided in the BOP storage area 71, here mounted below the equipment deck 120. The BOP handling crane 76 allows, for example, moving a BOP or BOP stack between a test location having a test stump 27 and the BOP cart 72.

As illustrated here, a BOP guide device 170 is mounted under the protruding cellar deck structure 20. The BOP guide device 170 is configured to provide guidance for the BOP during displacement along the firing line below the protruding cellar deck structure 20, e.g. to avoid or reduce sway motion of the BOP during such displacement, e.g. when passing through the splash zone.

As illustrated here, the vessel 1 is provided with a shaker room 130 within the deckbox structure 2 and in proximity of, e.g. adjacent to, the moonpool 40. It is shown that the shaker room 130 is on the lower deck 7.

As illustrated here, the vessel 1 is provided with a cutting handling room 135 that is arranged below the shaker room 130. Here the room 135 is within the structure 20, here on the cellar deck and below the shaker room 135 that is arranged on the lower deck 7. This facilitates transfer of cuttings to be handled, e.g. examined, from the shaker room 135 to the room 130.

As illustrated here, the vessel 1 is provided with a mud pump room 140 that is within the deckbox structure 2 and in proximity of, e.g. adjacent to, the shaker room 130, e.g. on the lower deck 7 as shown here.

As illustrated here, the drilling tower 60 is embodied as a vertical mast structure erected above the upper deck 5 of the deckbox structure and adjacent a side of the moonpool 40. The vertical mast structure is located outside of a vertical projection of the moonpool 40, instead of over the moonpool as with a traditional derrick design of the tower.

A crown block structure 61 is mounted on top of the vertical mast structure, and the mast structure has an operative face 62 directed towards the moonpool 40.

The drilling installation further comprises a main hoisting device 150 comprising at least one winch 151 , e.g. within the mast 60, and at least one winch driven cable 152. The hoisting device is adapted to suspend a load from the crown block structure 61 via said at least one winch driven cable 152 and to manipulate said suspended load in the firing line.

As illustrated here, the BOP cart rails 70 are arranged perpendicular to the operative face 62 of the mast structure 60.

The mast structure, at operative face 62 thereof directed towards the firing line through the moonpool, is provided with one or more vertical guide rails 63.

As illustrated here, the vertically mobile working deck 90 is guided along the one or more vertical guide rails 63 on the mast structure.

The drilling installation comprises a travelling device, e.g. a trolley 160, e.g. a top drive trolley with a top drive 161, that is movable up and down along and outside of said operative face 62 of the mast structure and guided by said one or more vertical guide rails 63 of said mast structure. The travelling device is suspended from the winch driven cable 152, e.g. suspended from a crown block structure 151 of the tower, e.g. the travelling device being suspended from a travelling block 153, e.g. wherein the travelling device is adapted to suspend a load from said travelling device and/or to support the travelling block. As illustrated here, the tower 60 is provided with a vertical motion arm assemblies rail 63, wherein at least one, e.g. multiple, motion arm assembly 180 is mounted on said vertical motion arm assemblies rail. Such an arrangement is, for example, disclosed in W02014/182160, WO2015/133895, and in WO2018/199754. Each motion arm assembly has a base that is vertically mobile along said vertical motion arm assemblies rail and an extensible, e.g. telescopic, arm that is mounted via a vertical axis slew bearing on said base so as to allow for extension and retraction of said arm as well as slewing motion of said arm about said vertical slew axis, wherein said arm is adapted to support a tool at an end of said arm.

The vessel 1 is provided with a drilling tubulars storage rack, e.g. multi-joint drill pipe stands storage rack, here a single rotary storage rack 190, at a lateral side of the mast structure 60. This drilling tubulars storage rack is adapted for storage of drilling tubulars in vertical orientation therein. Motion arm assemblies 180 are configured to act as part of a racker system that is adapted to move a drilling tubular between the storage rack and a position aligned with the firing line.

Each motion arm assembly 180 in the racker system has a base that is vertically mobile along said vertical motion arm assemblies rail 63 and an extensible, e.g. telescopic, arm that is mounted via a vertical axis slew bearing on said base so as to allow for extension and retraction of said arm as well as slewing motion of said arm about said vertical slew axis, wherein said telescopic arm is adapted to support a tubulars gripper tool at an end of said arm, so as to allow for gripping of a drilling tubulars by means of the tubular gripper tool.

As preferred, the racker system is heave compensated and is configured to bring a drilling tubular removed from the storage rack in a heave compensation motion that is synchronized with the heave compensation motion of the mobile working deck.

For example, each motion arm assembly 180 has a base that is vertically mobile along said vertical motion arm assemblies rail by a drive configured to provide said heave compensation motion that is synchronized with the heave compensation motion of the mobile working deck.

As illustrated here, the vessel comprises a drilling tubulars rotary storage rack 190 that is rotatable about a vertical axis and has storage slots for storage of multiple drilling tubulars in vertical orientation, the drilling tubulars rotary storage rack including a drive to rotate the drilling tubulars storage rack about its vertical axis, for example said drilling tubulars rotary storage rack comprising a central vertical post and multiple discs at different heights on the post, at least one disc being a fingerboard disc having tubulars storage slots, each slot having an opening at an outer circumference of the fingerboard disc allowing to introduce and remove a tubular from the storage slot.

As illustrated here, the drilling tower 60 is embodied as a singular vertical mast structure having closed wall contour, e.g. an octagonal cross-section, e.g. over at least a major portion of the height of the tower.

As illustrated here, the vessel 1 has a catwalk machine 230 arranged on the upper deck 5 configured to feed and remove drilling tubulars to and from the firing line of the tower. Drilling tubulars can be transferred between storage area 270 and catwalk machine 230 by means of crane 271.

It is shown that, as an optional feature, stand-building can be done at a dedicated stand building line 300, at a site remote from the firing line 9 in line 9b yet within reach of yet another assembly 180. The vessel 1 has in the stand-building line 9b a tube catcher device 310 that extends through and projects below the structure 2. A dedicated stand-building catwalk machine 231 is provided. The tower 60 has another crown block 66, opposite crown block 61 and corresponding equipment suspended from the crown block 66 for stand building. The stands, once assembled, at then loaded into the storage device 190 for later use in the firing line 9.

Figure 15 illustrates an example of a three-column semi-submersible vessel T according to the invention. This vessel has three buoyant columns 4’ supporting the deckbox structure 2’ of the vessel.

The recessed cellar deck structure 20’ has three pointed wave crest splitting sections 23’,

24’, 25’. The cellar deck structure is, seen in plan view, triangular, so the peripheral wall sections of adjacent pointed wave crest splitting sections merging in a side of the triangle. The side of the triangle can be rectilinear, but other shapes, e.g. convex, concave, or combinations thereof, when seen in plan view, are also contemplated.

The buoyant support columns 4’ of the three-columns vessel T each are triangular in horizontal cross-section, at least over a height thereof impacted by wave action, with said three cross-sections being located within an imaginary triangle defined by said three cross- sections combined. In this arrangement, the apex of a column effectively splits a wave that comes in from at said corner of the imaginary triangle, which compensates for the three pointed wave crest splitting sections not being truly effective for said direction of incoming waves. It will be appreciated that the three-columns vessel 1’ may have one or more features as discussed herein with reference to the four-columns vessel 1.