EP3829967A1

EP3829967A1 - Semi-submersible

Info

Publication number: EP3829967A1
Application number: EP19755996.6A
Authority: EP
Inventors: Joop Roodenburg; Diederick Bernardus Wijning
Original assignee: Itrec BV
Current assignee: Huisman Equipment BV
Priority date: 2018-08-03
Filing date: 2019-07-25
Publication date: 2021-06-09
Anticipated expiration: 2039-07-25
Also published as: EP3829967B1; NL2021425B1; WO2020027655A1

Abstract

Semi-submersible for subsea wellbore related activities, wherein a floating structure comprises firstly an equipment deck, for storage of wellbore equipment located at a third level, higher than the upper deck, adjacent the moonpool, opposite the tower, and above the BOP storage deck. It secondly comprises a vertically mobile working deck, arranged above the moonpool and adjacent the towerandsupported by a compensator,such that itis movable between a lowered position at asecond levelandflush with an adjoining area of the upper deck of adeckbox structure, and a raised position at the third level andflush with an adjoining deck area of the equipment deck.

Description

Title: Semi-submersible

BACKGROUND OF THE INVENTION

This invention relates to a semi-submersible, more in particular to a semi-submersible comprising a deck box structure, having an upper deck, a BOP storage deck, and a moonpool.

FIELD OF THE INVENTION

The present invention relates to the field of semi-submersible drilling vessels. Generally the hull of a semi-submersible drilling vessel has a deckbox structure with an upper deck and a box bottom. Further the hull has one or more pontoons, e.g. two parallel pontoons or a ring pontoon, and multiple support columns that extend upward from the one or more pontoons and support thereon the deckbox structure. A semi-submersible drilling vessel further comprises a drilling installation with a drilling tower, e.g. a mast or a derrick, erected above the upper deck of the deckbox structure and adapted to perform drilling operations along at least one firing line through the moonpool in the deckbox structure.

With drilling vessels limited deck space is always a problem. This is in particular the case when the vessel also supports one or more BOPs (Blow Out Preventers). A typical BOP, when assembled, has a substantial footprint and a height larger than that of a hull or deck box of a drilling vessel. Storing a BOP thus takes up much deck space. In addition, due to the large mass and height, a BOP is preferably stored as low as possible, and to keep transport into and out of the firing line manageable, as close to the firing line as possible.

Thus it is for example known from the prior art W02009/102197 to provide a tower with a drilling side and a stand building side. At the drilling side, the tower is provided with tensioners for supporting a riser. The tensioners are located on lateral sides of the moonpool. At the stand building side, the tower is provided with equipment for building and lowering a riser. The BOP’s are stored at the stand building side, and are supported in carts located on lateral sides of the moonpool. When to be deployed, a riser supporting the BOP is assembled at the stand building side, and subsequently has to be skidded below the tower from the stand building side to the drilling side. At the drilling side, the riser and BOP are lowered and installed, the riser, at the top side thereof, being supported by the tensioners. The present invention aims to provide an improved semi-submersible vessel. According to a further aspect, the invention aims to provide a semi-submersible with an improved deck-lay out, more in particular a semis-submersible that allows for a more efficient use of deck space.

The present invention achieves this aim by providing a semi-submersible according to claim 1.

The invention provides a semi-submersible for performing subsea wellbore related activities, e.g. well drilling, workover activities, well maintenance.

The semi-submersible comprises:

- a floating structure, wherein the floating structure comprises:

- a deckbox structure, the deckbox having an upper deck, located on top of the deck box structure, a BOP (Blow Out Preventer) storage deck, located lower than the upper deck, and a moonpool, wherein the moonpool extends through the deckbox structure alongside the BOP deck and up to the upper deck, such that the BOP deck is located adjacent the moonpool on a first level and the upper deck is located, preferably on opposite lateral sides of the moonpool, on a second level;

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

- multiple support columns extending upward from the one or more pontoons and supporting thereon the deckbox structure;

- a firing line hoist system, wherein the firing line hoist system is supported by the deck-box structure, and wherein the firing line hoist system comprises:

- a tower, wherein the tower is erected above the upper deck of the deckbox structure, adjacent the moonpool and opposite the BOP storage deck, wherein tower has a top and a base, a first side and an opposite second side, and wherein the base of the tower is connected to the deck box structure such that the first side of the tower faces the moonpool; and

- a hoisting device, which hoisting device comprises a load attachment device moveable relative to the tower in a vertical direction along a firing line, which firing line extends on the outside of tower at the first side thereof, and which firing line extends through the moonpool, wherein the hoisting device is adapted for raising and lowering a tubular string with a BOP to the seabed;

- a BOP transport system, the BOP transport system comprising:

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

- a BOP handling cart, which is movably supported by the 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 above the moonpool and in the firing line and can be manipulated by the hoisting device;

- an equipment deck, for storage of wellbore equipment, such as workover or well maintenance equipment, wherein the equipment deck is located at a third level, adjacent the moonpool, opposite the tower, and, preferably in vertical projection, above the BOP storage deck;

- a vertically mobile working deck, wherein the mobile working deck is arranged in vertical projection above the moonpool and adjacent the tower, wherein the working deck is supported by a mobile working deck compensator such that the mobile working deck is vertically movable between:

- a lowered position, in which lowered position the mobile working deck is supported at the second level and is flush with at least an adjoining area of the upper deck of the deckbox structure, preferably wherein said working deck and said adjoining area of the upper deck of the deckbox structure are provided with rail tracks configured to transfer equipment over said rail tracks, e.g. equipment arranged on a skid pallet skiddable over said rail tracks, from the equipment deck onto the mobile working deck and vice versa; and

- a raised position, in which raised position the mobile working deck is supported at the third level and is flush with at least an adjoining deck area of the equipment deck.

According to the claimed invention, the vessel is provided with a mobile working deck that, in a lowered position, can be combined with the upper deck, located on top of the deck box structure, and, in a raised position, can be combined with the equipment deck, located above the BOP storage deck. The mobile working deck can thus on different levels be combined with different decks, which allows for an efficient use of deck space, more in particular allows for dedicating different levels to particular equipment and/or activities, e.g. dedicating the equipment deck to equipment for performing subsea wellbore related activities, e.g. well drilling, workover activities, well maintenance while dedicating the main deck to drilling activities.

In an embodiment the equipment deck is provided with one or more spoolable product coil devices, accommodating therein one or more coil devices, 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, e.g. an umbilical that is to be attached to the exterior of the riser.

Furthermore, the mobile working deck has one side facing the tower. The other three sides are free sides, that each can be positioned adjacent a deck area of which one is located at a different level. The invention thus provides access to the mobile working deck from all three free sides. This allows for a more flexible use of the mobile working deck.

The invention provides additional deck space in the form of an equipment deck that can be combined with the mobile working deck when in its raised position. The equipment deck is located at a third level, adjacent the moonpool, opposite the tower, and, preferably in vertical projection, above the BOP storage deck. The invention thus also allows for storing a BOP close to the firing line, adjacent the moonpool and opposite of the tower.

The invention thus provides an improved semi-submersible vessel. The equipment deck, that can be combined with the mobile working deck, provides the semi-submersible with an improved deck-lay out, more in particular allows for a more efficient use of deck space.

The working deck, in a raised position thereof, is flush with at least an adjoining area of the equipment deck. In an embodiment, the working deck and the adjoining area of the equipment deck are provided with rail tracks configured to transfer equipment over said rail tracks, e.g.

equipment arranged on a skid pallet skiddable over said rail tracks, onto and off the working deck.

The upper deck is located on top of the deck box structure and thus is an open air deck. In a preferred embodiment, the equipment deck is also 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 walls, for shielding against wind, and is open on the top to provide access for a crane.

It is submitted that the mobile working deck can also be used for moving equipment between the upper deck and the equipment deck.

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

In a further embodiment, walls are provided between the equipment deck and the upper deck of the deck box, such that the semi-submersible s provided with a BOP garage for fully sheltering a subsea BOP from the environment.

The invention further provides a semi-submersible having a buoyant hull, e.g. a semi-submersible drilling vessel according to claim 1 , wherein the buoyant hull has a moonpool, an upper deck, and a drilling installation with a drilling tower that is erected above the upper deck of the deckbox structure, which drilling installation is adapted to perform drilling operations along at least one firing line through the moonpool, wherein adjoining the moonpool a subsea BOP (Blow Out Preventer) garage is provided. The BOP garage comprises a BOP storage room located lower than the upper deck, such that it is enclosed by the deck box. The BOP garage comprises the BOP deck as a floor and the equipment deck as a roof. Walls are provided between the upper deck of the deck box and the equipment deck. Thus is provided a BOP garage for sheltering a subsea BOP from the environment. Furthermore, the mobile working deck, when in its’ raised position, is flush with at least an adjoining area of the roof of the BOP garage.

It is noted that of such a BOP garage, its’ lower part is recessed in the deck box structure, and is located lower than the upper deck, and its’ upper part extends outside the deck box structure, and is located higher than the upper deck.

The BOP storage deck is preferably in open communication with the moonpool and through one or more vent openings in the sidewalls of the garage, i.e. the walls formed by the deck-box and/or the side walls provided between the upper deck and the equipment deck, with the exterior, so as to allow for continuous venting of the moonpool. In an embodiment, the garage has a substantially rectangular layout, and is open towards the moonpool and walled off along the other three sides of the garage. Preferably, at least one of a drill string slip device, a riser spider device, and/or a diverter is supported by the 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 mobile working deck is a heave compensated working deck.

In an embodiment of a semi-submersible according to the invention, adjacent the BOP deck is provided a mezzanine deck, e.g. for storing a BOP-top section, which mezzanine deck preferably is flush with the upper deck of the deck box structure, and wherein the equipment deck extends over both the BOP storage deck and the Mezzanine deck.

In an embodiment of a semi-submersible according to the invention, a hoisting device, preferably a gantry crane, is mounted directly below the equipment deck, for transporting equipment over the BOP storage deck, preferably over the BOP storage deck and over a mezzanine deck located adjacent the Bop storage deck, and for moving equipment from the mezzanine deck to the BOP deck.

In an embodiment of a semi-submersible according to the invention, the working deck is configured to be elevated relative to the lowered position and to be movable within a motion range including a heave compensation motion range.

In an embodiment of a semi-submersible according to the invention, the mobile working deck compensator comprises multiple vertically mounted working deck compensator cylinders, which compensator cylinders are arranged between the deckbox structure and the mobile working deck, e.g. two sets of multiple compensator cylinders, e.g. two pairs, supporting the mobile working deck, wherein said working deck compensator cylinders are configured to provide a heave compensated motion of the working deck relative to the deckbox structure.

In an embodiment of a semi-submersible according to the invention, a first set of working deck compensator cylinders is arranged outward of a first BOP handling cart rail, relative to the moonpool, and a second set of working deck compensator cylinders 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 lowered position to allow for passage of the subsea BOP from the BOP storage room into the firing line.

In an embodiment of a semi-submersible according to the invention, working deck compensator cylinders are configured to provide a heave compensated motion of the mobile working deck, allowing for motion of the mobile working deck between a heave compensation maximum height position and a heave compensation minimum height position, wherein the heave compensation maximum height position is lower than the raised position of the mobile working deck, and the heave compensation minimum height position is higher lowered position of the mobile working deck.

In an embodiment of a semi-submersible according to the invention, the mobile working deck compensator further comprises a lifting device, preferably a lifting cylinder, that is configured to lift the mobile working deck out of the lowered position and into an elevated position, e.g. over a height between 1 and 6 meters, e.g. approximately 2 meters, wherein the elevated position is preferably is equal to, or higher than, the heave compensation minimum height position of the mobile working deck.

For example a single lift cylinder supports two heave compensation cylinders, e.g. the single lift cylinder in between the two heave compensation cylinders.

For example a working lift cylinder is secured with a rod directed downwards, and each heave compensation cylinder is secured with its cylinder body to the cylinder body of the lift cylinder, e.g. via a frame, and has its rod directed upward to the working deck.

This arrangement allows for a relatively reduced length of the heave compensation cylinder, or cylinders, as this cylinder(s) only has to have a stroke length attuned to the expected heave motion compensation. The lifting of the mobile working deck to the elevated position for example avoids any risk of the working deck reaching its stationary resting position during heave motion operation, and for example allows for passing of lines, pipes, etc. from underneath the working deck, e.g. from the diverter and/or a rotary control device (RCD) to locations outside of the moonpool, e.g. onto the upper deck.

The second aspect of the present invention also relates to a method for assembly of a riser from a vessel according to the second aspect of the invention, which method comprises moving the BOP from the BOP storage room to a position aligned with the firing line through the moonpool, wherein the BOP sticks out above the upper deck when stored, and possibly assembled, in the BOP storage room, wherein the method comprising lifting the mobile working in order to allow for travel of the BOP on the respective BOP handling cart towards the moonpool and underneath the working deck into a position aligned with the firing line.

In an embodiment of a semi-submersible according to the invention, the mobile working deck compensator comprises a lift cylinder for each working deck compensator cylinders, which lift cylinder and working deck compensator cylinder are provided in series, and wherein the lift cylinder is configured to lift the mobile working deck out of the lowered position and to move the mobile working deck between the lowered position and the elevated position.

In an embodiment of a semi-submersible according to the invention, the multiple vertically mounted working deck compensator cylinders are secured at a lower end thereof to a lower deck of the deckbox structure, e.g. the lowermost deck of the deckbox structure.

In an embodiment of a semi-submersible according to the invention, a set of vertically oriented wireline riser tensioner cylinders is arranged outward of the working deck compensator cylinders, relative to the moonpool.

In an embodiment of a semi-submersible according to the invention, the tower is provided with a mobile working deck fixating device, for fixing the mobile working deck when in its’ raised position and for preferably supporting the mobile working deck in that position.

In an embodiment of a semi-submersible according to the invention, the tower is provided with a mobile working deck support track, and the mobile working deck is coupled with said track, either directly or via a trolley, for guiding the working deck in the vertical direction and supporting the mobile working deck at least in the horizontal direction while being lifted and/or lowered by the mobile working deck compensator.

In an embodiment of a semi-submersible according to the invention, the mobile working deck when in the lowered position is supported by the deck box structure and not by the mobile working deck compensator.

In an embodiment of a semi-submersible according to the invention, a mobile work deck access walkway, preferably a telescopic access walkway is provided, for providing personnel access to the mobile working deck platform, which walkway at one end is pivotably supported by the mobile working deck, and at an opposite end is pivotably supported by an access platform, which access platform preferably is provided higher than the upper deck and lower than the equipment deck.

In an embodiment of a semi-submersible according to the invention, the 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 tower is embodied as a mast having a closed wall contour, e.g. over at least a major portion of the height of the tower, for example over a lower major portion with a top portion being embodied as a latticed structure or over the entire height of the tower.

In an embodiment the hoisting device comprising at least one winch and at least one winch driven cable, which hoisting device is adapted to suspend a load from said crown block structure via said at least one winch driven cable and to manipulate a suspended load in the firing line of the drilling installation, which firing line extends along the first side of a vertical mast structure of the tower, and wherein the tower 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 addition to the heave compensation cylinders and/or as an alternative the winch may be embodied as an active heave compensated winch as is known in the art.

In an embodiment the tower is provided at the first side thereof with one or more vertical guide rails and a travelling device, e.g. a trolley, is provided that is movable up and down along and outside of the first side of the tower and guided by the one or more vertical guide rails of the tower. The travelling device can for example be a trolley supporting a load attachment device. The travelling device may comprise one or more sets of guide rollers engaging the one or more guide rails. For example the travelling device is suspended from the winch driven cable and the travelling device is provided with a load attachment device to suspend a load from said travelling device.

In an embodiment the winch of the hoisting device is mounted on the tower, e.g. inside the closed wall contour or on a side of the tower, e.g. on a side opposite the first side, L the second side, and to the outside thereof. In the latter design the weight of the one or more winches may be employed as a counterweight for the load in the firing line of the tower. In another design the winch is mounted in the hull, e.g. in the deckbox structure. In an embodiment the vessel is provided with a top drive device as commonly used in drilling operations. For example the top drive device is connected or connectable to the travelling device on the tower. The top drive device comprises one or more motors to provide torque to a rotary output quill that is connectable to a drilling tubulars string as is known in the art.

In another embodiment the tower is embodied as a mast having a latticed structure, e.g. over at least a major portion of the height of the tower, e.g. over the entire height of the tower. For example, the first side facing the firing line is cladded with a panel so as to avoid any

entanglement of components and/or loads in the latticed structure during manipulation activities with the hoisting device.

In an embodiment the tower is embodied as a vertical mast structure erected above the upper deck of the deckbox structure and adjacent a side of the moonpool, the vertical mast structure being so as to allow for optimal movement of objects out of and into the moonpool.

Preferably a crown block structure is mounted on top of the vertical mast structure, e.g.

supporting a set of crown block sheaves that guide a winch driven cable from which a travelling block is suspended, the travelling block having a set of sheaves for said cable.

In an embodiment of a semi-submersible according to the invention, the tower is embodied as a vertical mast structure, and wherein a crown block structure is mounted on top of the vertical mast structure, and wherein the mast structure has an operative face directed towards the firing line through the moonpool,

and wherein the hoisting device comprises at least one winch and at least one winch driven cable, which hoisting device is adapted to suspend a load from said crown block structure via said at least one winch driven cable and to manipulate said suspended load in the firing line that extends along and outside of said operative face of the vertical mast structure.

In a further embodiment of a semi-submersible according to the invention, the BOP cart rails are arranged perpendicular to the operative face of the mast structure.

In an embodiment of a semi-submersible according to the invention, the mast structure 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, for example a drilling tubular engagement tool, e.g. said drilling tubular engagement tool being configured to assist in transfer of a riser joint between a position thereof aligned with the firing line and a position in a carousel.

In an embodiment of a semi-submersible according to the invention, the semi-submersible further comprises a drilling tubulars storage rack e.g. multi-joint drill pipe stands storage rack, e.g. a rotary storage rack, at a lateral side of the tower, which drilling tubulars storage rack is adapted for storage of drilling tubulars in vertical orientation therein, and wherein the semi-submersible, 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 tubulars storage rack is a 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 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.

In an embodiment of a semi-submersible according to the invention, 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 semi-submersible 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 semi-submersible, e.g. the tower, 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, e.g. wherein 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, 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 of a semi-submersible according to the invention, the firing line hoist system is a dual firing line hoist system, comprising a hoisting device on the second side of the tower, and preferably the moonpool on the second side of the tower is connected to the moonpool on the first side of the tower, and a track, preferably the BOP track extends from the first side of the tower up to and along the moonpool on the second side of the tower, such that a cart supported by the Bop track can be moved from the first side moonpool to the second side moonpool

In an embodiment the vertical tower is configured to perform subsea wellbore related operations and has a vertical operative face that is directed towards the moonpool, i.e. on the first side of the tower. On this operative face a pair of vertical guide rails is mounted and a travelling device, e.g. a trolley supporting the load attachment device, is provided that is movable up and down along and outside of this operative side of the tower and guided by these vertical guide rails of the tower. The tower is provided with a winch and a winch driven cable, which passes from a crown block structure with one or more cable sheaves at the top of the tower down along the firing line of the tower. In an embodiment the travelling device mostly serves to guide this cable and a load in the firing line, e.g. mating with the load attachment device as a load is lifted out of the water. The hoisting device, which comprises the winch and winch driven cable, is adapted to suspend a load from the vertical tower via the winch driven cable and to manipulate the suspended load the firing line of the tower that extends along and outside of said vertical operative face of the tower.

The invention furthermore provides 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 semi-submersible according to the invention. The invention furthermore relates to a method for performing wellbore related activities using one or more spoolable product coil devices, wherein the method comprises locating the spoolable product coil device on the equipment deck, and guiding the spoolable product into a riser supported by the mobile working deck, while the mobile working deck is heave compensated.

The present invention also relates to a method for assembly of a riser from a vessel, which method comprises moving the BOP from the BOP garage to a position aligned with the firing line through the moonpool, wherein the BOP sticks out above the upper deck when stored, and possibly assembled, in the BOP garage room, wherein the method comprising lifting the mobile working in order to allow for travel of the BOP on the respective BOP handling cart towards the moonpool and underneath the mobile working deck into a position aligned with the firing line.

In a further embodiment of said method discussed above, the working deck is used to retain a first riser joint to be joined on top of the BOP and to lower the working deck to mate said first riser joint to the BOP positioned in the firing line. As preferred the working deck is then raised to lift the BOP of the cart and the cart is moved back into the storage room. This allows for lowering the working deck into the lowered position thereof and subsequently for extension of the riser by adding riser joints, e.g. from a carrousel holding riser joints and located adjacent the tower, in a manner known in the art (e.g. the working deck supporting a riser spider device to support the riser during said assembly).

Advantageous embodiments of the radiation assembly according to the invention and the method according to the invention are disclosed in the sub claims and in the description, in which the invention is further illustrated and elucidated on the basis of a number of exemplary embodiments, of which some are shown in the schematic drawing. In the figures, components corresponding in terms or construction and/or function are provided with the same reference numbers.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be discussed with reference to the appended drawings. In the drawings: Fig. 1 shows in perspective view an example of a semi-submersible drilling vessel according to the invention;

Fig. 2 shows a plan view of the vessel of figure 1 near the moonpool, with an equipment deck located over a BOP storage deck;

Fig. 3 shows a plan view of the vessel of figure 1 in horizontal cross section below the equipment deck, i.e. without the equipment deck over a BOP storage deck; Fig. 4 shows a vertical cross-section of the vessel of figure 1 ;

Fig. 5 shows a vertical cross-section in close up of the vessel of figure 1 ;

Fig. 6 shows a plan view of the vessel of figure 1 in horizontal cross section below a tower, i.e. without the tower;

Fig. 7 shows a partial side view in close up of a mobile working deck and the equipment deck of the vessel of fig. 1 , with the mobile working deck in a raised position;

Fig. 8 shows a partial view in close up of the mobile working deck and the equipment deck of the vessel of fig. 1 , with the mobile during heave compensation;

Fig. 9 shows a partial view in close up of the mobile working deck and the equipment deck of the vessel of fig. 1 , with the mobile working deck in an intermediate position;

Fig. 10 shows a partial frontal view in close up of the mobile working deck and the equipment deck of the vessel of fig. 1 , with, from left to right, the mobile working deck in a heave compensation intermediate height position, the mobile working deck in a heave compensation minimum height position, the mobile working deck in a heave compensation maximum height position,

Fig. 11 shows a schematic view of a lay out of a hydraulic circuit comprising the heave compensation cylinders supporting the mobile working deck and heave compensation cylinders connected with a hoisting device; and

Figs. 12B, C illustrate a combination of a lift cylinder and heave compensation cylinders supporting the mobile working deck of the vessel of figure 1 in fully retracted state and in fully extended state.

DETAILED DESCRIPTION OF THE INVENTION

Whilst primarily presented for illustrative purposes with reference to one or more of the figures, any of the technical features addressed below may be combined with any of the independent claims of this application either alone or in any other technically possible combination with one or more other technical features.

The invention will now be elucidated with reference to an exemplary harsh environment semi- submersible drilling vessel 1 shown in the drawings.

The vessel 1 comprises:

- a deckbox structure 2 having an upper deck 3 and a box bottom 4,

- one or more pontoons 5, here two parallel pontoons, - multiple, here four, support columns 6 extending upward from the one or more pontoons 5 and supporting thereon the deckbox structure 3,

- a vertically mobile working deck 100, wherein the mobile working deck 100 is arranged in vertical projection above a moonpool 20 and adjacent a tower 30, wherein the working deck 100 is supported by a mobile working deck compensator 140 such that the mobile working deck is vertically movable;

- an equipment deck 10, for storage of wellbore equipment, such as workover or well

maintenance equipment, wherein the equipment deck 10 is located at a third level, higher than the upper deck 3 of the deckbox structure 2, opposite a tower 30, and, preferably in vertical projection above a BOP storage deck located in the deckbox structure 2.

In an embodiment, the height of the deckbox structure between the upper deck 3 and the box bottom is between 11 and 15 meters, e.g. about 12.5 meters.

The semi-submersible comprises a drilling installation with a drilling tower 30 erected above the upper deck 3 of the deckbox structure 2 and adapted to perform drilling operations along at least one firing line 30a of the drilling installation that vertically extends through the moonpool 20 into the sea.

The vessel 1 as shown and preferred has four supporting columns 6 in a rectangular or square arrangement. The moonpool is located, when seen in top view, at the center of the deckbox structure, between the columns 6.

The deckbox structure 2 comprises, adjoining the moonpool 20, one or more wireline riser tensioner equipment rooms 70 accommodating therein wireline riser tensioner equipment, e.g. comprising vertically oriented wireline riser tensioner cylinders 71. This equipment is configured to provide top tension to a riser supported in the moonpool of the vessel.

As preferred these rooms 70 are below the upper deck 3 and are located adjoining the moonpool 20, e.g. said area of the upper deck being flush with a mobile working deck 100 in its lower resting position, which deck 100 is arranged above the moonpool 20 as will be explained herein. So the invention provides for the option to have the wireline riser tensioner equipment 71 close to the moonpool 20, effectively above the storage space 14 for the riser joints 15, 16 in the caisson 10, and with the equipment deck also adjacent the moonpool, more in particular adjacent the mobile working deck located above the moonpool, not interfering with the riser tensioner equipment 71 and not with the BOP located on the BOP deck.

It will be appreciate that, in an embodiment, the vessel 1 may also, or as an alternative to wireline riser tensioner system, be equipped with a riser tensioner system with direct acting riser tensioner cylinders as is known in the art.

The deckbox structure 2 comprises, at a location that adjoins the moonpool 20, a subsea BOP (Blow Out Preventer) storage room 80, e.g. on a lower deck of the deckbox structure, here on the lowermost deck of the deckbox structure as preferred.

A set of BOP handling cart rails 81 , 82 is provided, e.g. skid rails, extending from the subsea BOP (Blow Out Preventer) storage room 80 towards and along opposed sides of the moonpool 20.

The vessel is provided with a BOP handling cart 83 travelling over said BOP handling cart rails 81 , 82 so as to allow for transfer of a subsea BOP 85 (Blow Out Preventer) between the BOP storage area 80 and a position aligned with the firing line 30a. The BOP 85 may be, as often seen in practice, a tall BOP with an upper portion thereof sticking out above the upper deck 3 of the deckbox 2. This is for example envisaged in combination with a vertically mobile working deck 100 over the moonpool 20, that in an elevated position thereof allows for bringing the tall BOP 85 underneath the elevated working deck 100 in alignment with the firing line 30a.

The BOP storage area 80 is in open communication with the moonpool 20.

The deckbox structure 2 comprises, in vertical projection above the BOP deck the equipment deck 100. On the equipment deck may be provided a spoolable product coil devices, see for example 4. In addition, or as an alternative spoolable product coils may also be provided on the upper deck, adjacent the moonpool. One or more coil devices 91 , each having a coil storing thereon a spoolable product, such as a (control) line, wireline, cable, hose, coiled-tubing, umbilical, etc. may thus be accommodated on the equipment deck and/or the upper deck.

The equipment deck and the upper deck being adjacent the moonpool, more in particular adjacent the mobile working deck, allowing to pass the one or more spoolable products from the respective coil device 91 towards the firing line 1 , e.g. an umbilical that is to be attached to the exterior of the riser, and for example is supported by the mobile working deck. The vessel 1 is provided with the mobile working deck 100 which is arranged in vertical projection above the moonpool 20, which working deck 100 is vertically movable, e.g. by one or more hydraulic cylinders arranged between the working deck 100 and the deckbox structure 2 as will be explained herein in more detail.

The working deck 100 is in a lower stationary resting position thereof flush with at least an adjoining area of the upper deck 3 of the deckbox structure 2. Herein the working deck 100 and the adjoining area of the upper deck 3 of the deckbox structure are provided with rail tracks 110 configured to transfer equipment over said rail tracks, e.g. equipment being arranged on a skid pallet skiddable over said rail tracks 110, onto and off the working deck 100.

The working deck 100 is configured to be elevated, preferably by an arrangement of cylinders 140, 141 , 142 between the upper deck 3 of the deckbox 2, relative to a lowered position, e.g. a resting position in which it is supported by the deck structure. In the lowered position, the deck surface of the mobile working deck is flush with an adjoining area of the upper deck 3, and to be movable within a motion range including a heave compensation motion range, as depicted in figure 10.

As preferred the heave compensation motion range of the working deck 100 is above an elevated position of the same working deck, and below a raised position of the mobile working deck. In said raised position, depicted in figure 4, the deck surface of the mobile working deck is flush with an adjoining area of the equipment deck 100.

For example the height of the elevated working deck 100 above the upper deck 3 is between 4 and 6 meters, e.g. 5 meters.

For example the heave compensation motion range has a height of between 7 and 12 meters, e.g. of approximately 10 meters.

For example the maximum height of the working deck 100 above the upper deck 3 is between 10 and 18 meters, e.g. approximately 15 meters.

The working deck 100 may be provided with a mobile work deck access walkway 105, depicted in figure 10, preferably a telescopic access walkway is provided, for providing personnel access to the mobile working deck platform, which walkway at one end is pivotably supported by the mobile working deck, and at an opposite end is pivotably supported by an access platform, which access platform preferably is provided higher than the upper deck and lower than the equipment deck.

At least one of a drill string slip device 125, a riser spider device, and/or a diverter 130 is supported by the mobile working deck 100. For example a diverter 130 is arranged on the underside of the working deck 100.

The drill string slip device 125, e.g. having mobile clamping jaws, is configured to support a suspended drill string within a riser.

The riser spider device is configured to support a suspended riser, e.g. during assembly and disassembly of a riser. For example the riser spider device has radially movable dogs that engage underneath a flange of a riser joint to support the weight of the riser string.

The diverter 130 is configured to divert a hydrocarbon and/or drilling mud stream from a subsea wellbore to the vessel. Commonly a hose or pipe connects the diverter 130 to a mud handling facility onboard the vessel 1 , e.g. located within the deckbox structure 2.

As shown multiple vertically mounted working deck compensator cylinders 140 are arranged between the deckbox structure 2 and the mobile working deck 100, here two sets of multiple compensator cylinders, e.g. two pairs, supporting the mobile working deck. Preferably the working deck compensator cylinders 140 are configured to provide a heave compensated motion of the working deck 100 relative to the deckbox structure.

A first set of working deck compensator cylinders 140 is arranged outward of a first BOP handling cart rail 81 , relative to the moonpool 20, and a second set of working deck compensator cylinders 140 is arranged outward of a second BOP handling cart rail 82, relative to the moonpool 20, so as to allow for passing a subsea BOP 85 on a BOP handling cart 83 in between the first and second set of working deck compensator cylinders 140.

For example the working deck compensator cylinders 140 are extendable to raise the mobile working deck 100 from its stationary resting position into an elevated position so as to allow for passage of the subsea BOP 85 from the BOP storage area 80 into the firing line 30a and underneath the working deck 100. As preferred working deck compensator cylinders 140 not only allow for raising and lowering of the working deck 100 but also for heave compensation motion of the working deck 100, e.g. with a riser connected via a telescopic joint 190 to the working deck 100. As is common the telescopic joint 190 has a cylinder body or barrel 191 and a piston part 192 that is telescopic relative to the cylinder body 191. The body 191 here is suspended via riser tension ring 74 from wireline 73 of riser tensioning equipment of the vessel.

A first set of vertically oriented wireline riser tensioner cylinders 71 is arranged outward of a first set of working deck compensator cylinders 140, relative to the moonpool, and a second set of vertically oriented wireline riser tensioner cylinders 71 is arranged outward of a second set of working deck compensator cylinders 140, relative to the moonpool. This allows for a compact arrangement of the compensator cylinders 140 and the wireline riser tensioner cylinders 71 , e.g. with sheaves 72 for the wirelines 73 to the riser tension ring 74 being arranged in proximity of the working deck compensator cylinders 140.

As is common a flex joint 128 may be provided in the riser string, above the telescopic joint 190, to allow for angular positions of the riser.

The multiple vertically mounted working deck compensator cylinders 140 are here secured at a lower end thereof to a lower deck of the deckbox structure, e.g. the lowermost deck of the deckbox structure in view of having maximum height for these cylinders.

In an embodiment, as shown here, the mobile working deck compensator 140 comprises in series:

a lift cylinder 141 that is configured to lift the mobile working deck 100 out of the stationary resting position and to move the mobile working deck between the lowered position and an elevated position, e.g. over a height between 4 and 6 meters, e.g. approximately 5 meters, and

a heave compensation cylinder 142 that is configured to provide a heave compensated motion of the mobile working deck 100 when lifted in the elevated position by the lift cylinder 141 , allowing for motion of the mobile working deck 100 between a heave compensation maximum height position and a heave compensation minimum height position.

This arrangement of the combination of a lift cylinder and a heave compensation cylinder allows for a relatively reduced length of the heave compensation cylinder 142, or cylinders, as this cylinder(s) 142 only has to have a stroke length attuned to the expected maximum heave motion compensation. The lifting of the mobile working deck 100 to the elevated position by a dedicated lift cylinder 141 for example avoids any risk of the working deck reaching its stationary resting position during a heave motion operation wherein the lift cylinder 141 remains extended and the heave compensation cylinder 142 performs the heave motion, and for example allows for reliably passing of lines, pipes, etc. from underneath the working deck, e.g. from the diverter 130 and/or a rotary control device (RCD) to locations outside of the moonpool, e.g. onto the upper deck.

For example, as illustrated in figures 12B and 12C a single lift cylinder 141 supports two heave compensation cylinders 142, e.g. the single lift cylinder 141 in between the two heave compensation cylinders 142.

As shown, for example, a working deck lift cylinder 141 is secured with the piston rod thereof directed downwards, and each heave compensation cylinder 142 is secured with its cylinder body to the cylinder body of the lift cylinder 141 , e.g. via a frame, and has its piston rod directed upward to the working deck (not shown in figures 12B,C).

Each of the lift cylinder 141 and heave motion compensation cylinder 142 may be embodied as a single acting hydraulic cylinder.

As is common in the field, the hydraulic heave motion compensation cylinder(s) 142 may be connected to a gas buffer, e.g. a nitrogen buffer, preferably via a medium separator as is known in the art.

The lift cylinder(s) 141 may be connected to a motorized pump that is connected to a tank containing hydraulic fluid.

The tower 30 is embodied as a vertical mast structure erected above the upper deck of the deckbox structure and adjacent a side of the moonpool 20, the vertical mast structure being located outside of the vertical projection of the moonpool 20 so as to allow for optimal movement of objects out of and into the moonpool. This in contrast to a derrick mounted with its derrick structure over the moonpool as in the mentioned Jack Bates vessel.

The mast 30 may for example have a height of 60 meters, e.g. in view of handling multi-joint drilling tubulars 165, also called stands, e.g. a stand having a length of between 25 and 35 meters, e.g. triple stands having a length of 96ft with the working deck 100 in heave motion compensation mode or quad stand when the working deck 100 is in its lower resting position.

The vertical mast structure 30 here, as preferred, is arranged in vertical projection above the storage space 14 of the riser storage caisson 10, so as to be close to the moonpool 20 which is favorable in view of the (bending) loads on the mast structure during hoisting of objects, e.g. a riser string with BOP 85 at the lower end thereof, in the firing line 30a.

A crown block structure 31 is mounted on top of the vertical mast structure, e.g. supporting a set of crown block sheaves 32 that guide a winch driven cable 33 from which a travelling block 34 is suspended, the travelling block having a set of sheaves for the cable 33.

The mast structure has an operative face 35 directed towards the firing line 30a through the moonpool 20.

The drilling installation further comprises a firing line 30a associated hoisting device comprising at least one winch (e.g. accommodated in the deckbox 2 or in the mast 30) and at least one winch driven cable 33, which hoisting device is adapted to suspend a load from a crown block structure 31 via said at least one winch driven cable 33 and to manipulate a suspended load in the firing line, e.g. that extends along and outside of an operative face of the vertical mast structure.

The riser joint transfer passage 8 is arranged in proximity of the vertical mast structure 30.

In an embodiment the riser joint transfer device is embodied as a crane 60 arranged on the vertical mast structure, the crane being configured to lift and lower a riser joint through the riser joint transfer passage.

In an embodiment the crane 60 comprises a cantilevered crane arm, having an inner end connected to the tower structure, e.g. via a base that is vertically movable along a vertical rail on the tower structure. A winch driven cable may then depend, e.g. in a multi fall arrangement, from the crane arm and be provided with a riser joint connector configured to connect the cable to a riser joint 15, 16.

The BOP handling rails 81 , 82, e.g. on a lower or lowermost deck of the deckbox structure, are here arranged perpendicular to the operative face of a vertical mast structure of the drilling installation.

The BOP 85, as shown here, may be composed of a lower stack assembly 85a, with one or more ram units, and an upper stack assembly 85b (often referred to as lower marine riser package). For example storage of multiple upper stack assemblies may be provided for, as shown here. The mast structure 30 is provided with a vertical motion arm assemblies rail 160, wherein at least one, here three, motion arm assemblies 161 , 162, 163 are mounted on this vertical motion arm assemblies rail.

Each motion arm assembly has a base that is vertically mobile along the vertical motion arm assemblies rail and an extensible, e.g. telescopic, arm that is mounted via a vertical axis slew bearing on the base so as to allow for extension and retraction of said arm as well as slewing motion of the arm about the vertical slew axis. The arm is adapted to support a tool at an end of the arm, for example a riser joint engagement tool.

The vessel 1 is provided with a drilling tubulars storage rack 170, e.g. multi-joint drill pipe stands storage rack, e.g. a rotary storage rack 170.

The drilling tubulars storage rack 170 is adapted for storage of drilling tubulars in vertical orientation therein, e.g. multi-joint drilling tubulars, e.g. triples and/or quads.

The vessel 1 , e.g. the mast structure 30, is provided with a racker system 180 that is adapted to move a drilling tubular between the storage rack 170 and a position aligned with the firing line 30a.

In an embodiment the racker system 180 comprises a vertical motion arm assemblies rail 181 , wherein at least one, here multiple, motion arm assemblies 182, 183, 184 are mounted on that vertical motion arm assemblies rail.

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

The working deck 100 is movable, e.g. vertically movable. As shown here the working deck 100 is guided along one or more vertical guide rails 37 that are here mounted to the operative face 35 of the mast 30. For example, as shown here, the working deck 100 is provided with roller assemblies 106 that engage the one or more vertical guide rails 37. As explained earlier herein, the working deck 100 may be guided over one or more vertical guide rails, e.g. on mast 30, and supported on compensator cylinders 140, e.g. on two sets of lift cylinder 141 and heave compensation cylinders 142 combined.

The mobile working deck 100, in a lower stationary resting position thereof, is flush with at least an adjoining area of the upper deck 3 of the deckbox structure. Locking devices may be provided to lock the working deck in said position relative to the deckbox structure.

As shown the working deck 100 and the adjoining area of the upper deck of the deckbox structure 2 are provided with rail tracks 110 configured to transfer equipment over the rail tracks 110, e.g. equipment arranged on a skid pallet skiddable over said rail tracks, onto and off the working deck 100.

In an embodiment the rail tracks comprise a section 110b that extends between the riser joint transfer passage 8 and the working deck 100. The vessel may comprise a riser joint cart that is configured to travel over this section 110b of the rail tracks 110 and that is configured to support a riser joint 15, 16 thereon in vertical orientation for transfer thereof between a position above the upper deck 3 and aligned with the riser joint transfer passage 8 on the one hand and a position aligned with the firing line 30a on the other hand. Herein the working deck 100 will be in its lowered resting position, flush with the upper deck 3. Once aligned with the firing line 30a, for example, the riser joint 15, 16 can then be connected to a riser lifting tool connected to the travelling block 34 and so taken over by the hoisting device so that the riser joint cart can be moved away and the riser joint connected to the upper end of the already assembled part of the riser string (which is for example held by a riser spider device arranged on the working deck 100).

The tower 30 is embodied as a vertical mast structure erected above the upper deck 3 of the deckbox structure and adjacent a side of the moonpool 20. The mast structure, e.g. at the operative face 35 thereof directed towards the firing line 30a through the moonpool 20, is provided with one or more vertical guide rails 37.

The depicted drilling installation comprises a travelling device 95 that is movable up and down along and outside of said operative face of the mast and guided by the one or more vertical guide rails 37 of the mast 30.

Here the travelling device 95, or trolley, is suspended from a winch driven cable 33, e.g.

suspended from a crown block structure 31 of the tower via travelling block 34, e.g. the travelling device being suspended from a travelling block 34, e.g. wherein the travelling device is adapted to suspend a load from said travelling device and/or to support the travelling block.

The tower 30 here, as preferred, is embodied as a singular vertical mast structure having closed wall contour, here as preferred, an octagonal cross-section, e.g. over at least a major portion of the height of the tower.

The vessel is provided with a top drive device 210 as commonly used in drilling operations. For example the top drive device is connected or connectable to the travelling device 95 on the tower. The top drive device 210 comprises one or more motors to provide torque to a rotary output quill that is connectable to a drilling tubulars string as is known in the art.

Claims

C L A I M S

1. Semi-submersible for performing subsea wellbore related activities, e.g. well drilling, workover activities, well maintenance, wherein the semi-submersible comprises:

- a floating structure, wherein the floating structure comprises:

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

- a BOP transport system, the BOP transport system comprising: - a BOP track with a set of parallel BOP handling cart rails, e.g. skid rails, extending from a BOP storage area on the BOP storage deck towards and along opposite sides of the moonpool; and

- a BOP handling cart, which is movably supported by the 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 above the moonpool and in the firing line and can be manipulated by the hoisting device; characterized, in that the floating structure further comprises:

- an equipment deck, for storage of wellbore equipment, such as workover or well maintenance equipment, wherein the equipment deck is located at a third level, higher than the upper deck, adjacent the moonpool, opposite the tower, and, preferably in vertical projection, above the BOP storage deck;

- a raised position, in which raised position the mobile working deck is supported at the third level and is flush with at least an adjoining deck area of the equipment deck, preferably wherein the working deck and said adjoining area of the equipment deck are provided with rail tracks configured to transfer well bore equipment over said rail tracks, e.g. equipment arranged on a skid pallet skiddable over said rail tracks, onto and off the working deck; wherein, preferably, at least one of a drill string slip device, a riser spider device, and/or a diverter is supported by the 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.

2. Semi-submersible according to claim 1 , wherein adjacent the BOP deck is provided a mezzanine deck, e.g. for storing a BOP-top section, which mezzanine deck preferably is flush with the upper deck of the deck box structure, and wherein the equipment deck extends over both the BOP storage deck and the Mezzanine deck.

3. Semi-submersible according to claim 1 or claim 2, wherein a hoisting device, preferably a gantry crane, is mounted directly below the equipment deck, for transporting equipment over the BOP storage deck, preferably over the BOP storage deck and over a mezzanine deck located adjacent the Bop storage deck, and for moving equipment from the mezzanine deck to the BOP deck.

4. Semi-submersible according to one or more of the preceding claims, wherein the working deck is configured to be elevated relative to the lowered position and to be movable within a motion range including a heave compensation motion range.

5. Semi-submersible according to one or more of the preceding claims, wherein the mobile working deck compensator comprises multiple vertically mounted working deck compensator cylinders, which compensator cylinders are arranged between the deckbox structure and the mobile working deck, e.g. two sets of multiple compensator cylinders, e.g. two pairs, supporting the mobile working deck, wherein said working deck compensator cylinders are configured to provide a heave compensated motion of the working deck relative to the deckbox structure.

6. Semi-submersible according to one or more of the preceding claims, wherein a first set of working deck compensator cylinders is arranged outward of a first BOP handling cart rail, relative to the moonpool, and a second set of working deck compensator cylinders 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 lowered position to allow for passage of the subsea BOP from the BOP storage room into the firing line.

7. Semi-submersible according to one or more of the claims 4-6, wherein working deck compensator cylinders are configured to provide a heave compensated motion of the mobile working deck, allowing for motion of the mobile working deck between a heave

compensation maximum height position and a heave compensation minimum height position, wherein the heave compensation maximum height position is lower than the raised position of the mobile working deck, and the heave compensation minimum height position is higher lowered position of the mobile working deck.

8. Semi-submersible according to one or more of the claims 4-7, wherein the mobile working deck compensator further comprises:

a lifting device, preferably a lifting cylinder, that is configured to lift the mobile working deck out of the lowered position and into an elevated position, e.g. over a height between 1 and 6 meters, e.g. approximately 2 meters, wherein the elevated position is preferably is equal to, or higher than, the heave compensation minimum height position of the mobile working deck.

9. Semi-submersible according to claim 8, wherein the mobile working deck compensator comprises a lift cylinder for each working deck compensator cylinder, which lift cylinder and working deck compensator cylinder are provided in series, and wherein the lift cylinder is configured to lift the mobile working deck out of the lowered position and to move the mobile working deck between the lowered position and the elevated position.

10. Semi-submersible according to one or more of the preceding claims, wherein the multiple vertically mounted working deck compensator cylinders are secured at a lower end thereof to a lower deck of the deckbox structure, e.g. the lowermost deck of the deckbox structure.

11. Semi-submersible according to one or more of the preceding claims, wherein a set of vertically oriented wireline riser tensioner cylinders is arranged outward of the working deck compensator cylinders, relative to the moonpool.

12. Semi-submersible according to one or more of the preceding claims, wherein the tower is provided with a mobile working deck fixating device, for fixing the mobile working deck when in its’ raised position and for preferably supporting the mobile working deck in that position.

13. Semi-submersible according to one or more of the preceding claims, wherein the tower is provided with a mobile working deck support track, and the mobile working deck is coupled with said track, either directly or via a trolley, for guiding the working deck in the vertical direction and supporting the mobile working deck at least in the horizontal direction while being lifted and/or lowered by the mobile working deck compensator.

14. Semi-submersible according to one or more of the preceding claims, wherein the mobile working deck when in the lowered position is supported by the deck box structure and not by the mobile working deck compensator.

15. Semi-submersible according to one or more of the preceding claims, wherein a mobile work deck access walkway, preferably a telescopic access walkway is provided, for providing personnel access to the mobile working deck platform, which walkway at one end is pivotably supported by the mobile working deck, and at an opposite end is pivotably supported by an access platform, which access platform preferably is provided higher than the upper deck and lower than the equipment deck.

16. Semi-submersible according to one or more of the preceding claims, wherein the 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.

17. Semi-submersible according to one or more of the preceding claims, wherein the tower is embodied as a vertical mast structure, and wherein a crown block structure is mounted on top of the vertical mast structure, and wherein the mast structure has an operative face directed towards the firing line through the moonpool,

18. Semi-submersible according to claim 17, wherein the BOP cart rails are arranged perpendicular to the operative face of the mast structure.

19. Semi-submersible according to one or more of the preceding claims, wherein the mast structure 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, for example a drilling tubular engagement tool, e.g. said drilling tubular engagement tool being configured to assist in transfer of a riser joint between a position thereof aligned with the firing line and a position in a carousel.

20. Semi-submersible according to one or more of the preceding claims, wherein the semi- submersible further comprises a drilling tubulars storage rack e.g. multi-joint drill pipe stands storage rack, e.g. a rotary storage rack, at a lateral side of the tower, which drilling tubulars storage rack is adapted for storage of drilling tubulars in vertical orientation therein, and wherein the semi-submersible, 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.

21. Semi-submersible according to claim 19 and claim 20, wherein 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.

22. Semi-submersible according to one or more of the preceding claims, wherein the firing line hoist system is a dual firing line hoist system, comprising a hoisting device on the second side of the tower, and preferably the moonpool on the second side of the tower is connected to the moonpool on the first side of the tower, and a track, preferably the BOP track extends from the first side of the tower up to and along the moonpool on the second side of the tower, such that a cart supported by the Bop track can be moved from the first side moonpool to the second side moonpool.

23. Semi-submersible according to one or more of the preceding claims, wherein walls are provided between the equipment deck and the upper deck of the deck box, such that the semi-submersible is provided with a BOP garage for sheltering a subsea BOP from the environment.

24. Semi-submersible according to one or more of the preceding claims, wherein the BOP storage deck is in open communication with the moonpool and through one or more vent openings in the sidewalls of the garage, i.e. the walls formed by the deck-box and/or the side walls provided between the upper deck and the equipment deck, with the exterior, so as to allow for continuous venting of the moonpool.

25. 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, the method comprising the step: using a semi-submersible according to one or more of the preceding claims.