FIELD The disclosure generally relates to offshore drilling vessels such as drillships. BACKGROUND

Offshore drilling vessels are widely used for the exploration and exploitation of hydrocarbon reservoirs under the sea floor. One type of drilling vessel structure is a drillship, e.g. as described in WO 2014/108541 .

It is generally desirable to provide an offshore drilling vessel that allows for an efficient and flexible operation. It is further generally desirable to provide an offshore drilling vessel that facilitates operation with a high degree of safety. It is further generally desirable to provide drilling vessels that can perform drilling operations at large depths.

Typically offshore drilling vessels comprise a hull that defines a lower or main deck through which a moon pool extends downward towards the body of water in which the vessel operates. The vessel features one or more well centers which extend or are aligned through the moon pool. The well center defines an operational axis or region along which operations are performed. Such vessels typically further comprise an upper or drill floor deck that extends at a height above and across the moon pool. The space between the main deck and the drill floor is frequently used to store, assemble, prepare, maintain and/or to move subsea equipment to and from the well center such that the equipment can be raised and/or lowered from/towards the sea floor by means of a hoisting system of the vessel. One example of such subsea equipment is the so-called subsea Christmas tree, an assembly of valves, spools, and fittings used for an oil well to monitor and control flow and/or pressure. For the purpose of the present description and for the sake of brevity the subsea Christmas tree will merely be referred to as a Christmas tree. The subsea Christmas tree is sometimes also referred to as an Xmas tree or subsea tree. It is normally fixed to the wellhead of a well, such as a completed well.

As drilling is performed at increasing water depths and as control and monitoring requirements increase, it has become increasingly desirable to be able to deploy larger and, in particular, taller Christmas trees. Hence, it is desirable to provide a drilling vessel that can efficiently and safely accommodate and deploy larger Christmas trees. SUMMARY

An aspect of the disclosure relates to an offshore drilling vessel for operating in a body of water, comprising:

an upper deck;

a lower deck located below the upper deck;

a moon pool defining an opening in the lower deck;

a well centre; and

a Christmas tree carrier for positioning a Christmas tree for deployment on the well centre below the upper deck, the Christmas tree carrier defining a Christmas tree storage area, wherein the Christmas-tree storage area is moveable across the moon pool and is vertically movable between an upper position and a lower position to.

The Christmas-tree storage area may be moved vertically to provide a clearance height (e.g., an adjustable clearance height) over the Christmas-tree storage area at least at the position on the well centre. Such an arrangement may permit a clearance height to be provided over a stored Christmas tree at least at the position on the well centre. Another aspect of the present disclosure relates to a method of positioning a Christmas tree on a well centre of an offshore drilling vessel, the method comprising: locating the Christmas tree on a storage area of a Christmas tree carrier located adjacent a lower deck of the vessel and to a side of a moon pool of the vessel;

translating the storage area and the Christmas tree located thereon across the moon pool to position the Christmas tree on the well centre and below an upper deck of the vessel; and

vertically translating the storage area to provide a vertical clearance height over the Christmas tree at least at the position on the well centre. The step of vertically translating the storage area may be performed before, after or concurrently with the step of translating the storage area across the moon pool.

The well centre may be defined by the vessel as an axis or region along or in which well operations may be performed.

The Christmas tree carrier may thus be employed to carry a Christmas tree to the well centre for deployment, or indeed to recover a previously deployed Christmas tree. The ability to move the storage area vertically may facilitate safe handling of taller Christmas trees and accommodate restrictions in head space at certain locations above or around the lower deck and the moon pool. For example, at certain areas within the work space there will be apparatus, such as cranes or hoists, spiders and the like, mounted to or beneath the upper deck and extending downwards towards the lower deck or the moon pool. Also, the upper deck may span areas of the lower deck and the moon pool and structural elements of the upper deck may extend further downwards towards the centre of the span. A typical vessel will have a head space between the surface of the main or lower deck and the underside of the drilling or upper deck of 10 - 12 m. However, a larger Christmas tree may require, for example, 17 m clearance, which with an embodiment of the disclosure may be accommodated by lowering the storage area. Embodiments of the disclosure may thus facilitate operations involving deployment of taller Christmas trees to be carried out in and around the main deck area without requiring substantive modifications to vessels, such as the raising of the drilling deck, or substantive modifications to Christmas tree deployment procedures. The storage area may be movable between: a storage or loading position to a side of the moon pool adjacent the lower deck, and a deployment or unloading position on or adjacent the well centre. This may facilitate transfer of a Christmas tree from the lower deck onto the storage area, and then movement of the Christmas tree to the well centre for deployment.

A hoisting or lifting system may be aligned with the well centre, whereby in the deployment position the Christmas tree storage area is presented to the hoisting system. The Christmas tree may then be attached or secured to the hoisting system and then lowered into the water towards the seabed. A Christmas tree may also be recovered from the water and raised through the moon pool using the hoisting system. The storage area may then be positioned below the suspended Christmas tree, the tree lowered onto the storage area (or the storage area raised to support the tree), and the Christmas tree disconnected from the hoisting system. The tree may then be moved to a suitable storage or unloading position, for example adjacent the lower deck.

The Christmas tree carrier may include a platform and a frame, wherein the platform is vertically movable relative to the frame. The frame may be mounted to the hull of the vessel. The frame may be mounted to the lower deck.

In some examples the platform may be sized to accommodate a base of the Christmas tree. The platform may be arranged to accommodate a secondary support structure, such as a space frame or test-stump within which the Xmas tree is mounted or loaded. The platform may be configured to accommodate a width dimension of, for example, at least 0.3m (approx. 1 ft.), at least 0.6m (approx. 2 ft.), at least 1 .52m (approx. 5 ft.), at least 2.44m (approx. 8 ft.), at least 3m (approx. 10ft), or at least 4.6m (approx. 15 ft.). In some examples the moonpool of the vessel may be of a sufficient geometry and size to accommodate a lateral width or envelope of a Christmas tree. The frame may be translatable only in a horizontal plane. For example, the frame may be movable along rails or tracks mounted to the hull of the vessel. The rails or tracks may extend along sides of the moon pool. The Christmas tree carrier may span at least a portion of the moon pool. In some examples the tree carrier may extend across the moon pool, facilitating operator access from the lower deck onto the carrier and facilitating operations on or around the well centre. In other cases the carrier may extend only part-way across the moon pool.

When in the upper position, the storage area may be located relative to the lower deck so that a Christmas tree element may be skidded onto the storage area from the lower deck. The storage area may be flush with the lower deck. This may facilitate transfer of the tree from the deck to the storage area. Tracks, rails, skid beams or the like may extend from the deck onto the storage area. In other examples the upper position may be above or below the level of the lower deck.

The storage area may define a cut-out to permit the storage area to accommodate an elongate member, such as a riser joint, on the well centre. This may facilitate use of the Christmas tree carrier for other tasks, allowing the storage area to be located at the well centre even when a riser or the like extends through the moon pool. In some examples the cut-out may define a width of, for example, greater than 0.3m (approx. 1 ft.), greater than 0.6m (approx. 2 ft.), greater than 1 .52m (approx. 5 ft.), or greater than 2.44m (approx. 8 ft.).

The storage area may be lockable in a position on the well centre. This may provide enhanced safety and security while, for example, a Christmas tree is being coupled to or uncoupled from a hoist system. Of course the storage area may be locked in other positions, as required or desired.

The lower position of the storage area may be determined by a number of factors, for example the vessel configuration or the form of Christmas trees to be handled by the vessel. In the lower position, the storage area may be located below the lower deck. In the lower position the storage area may be above an operational waterline of the vessel, and may be within a splash zone of the vessel. In the lower position the storage area may be at least one of: no more than 3 m above the operation waterline, no more than 2 m above the operational waterline, no more than 1 m above the operational waterline, no more than 0.5 m above the operational waterline, and at or below the operational waterline. The storage area may be lowered from an upper position to a lower position by a distance determined by the same or other factors, for example by at least one of: 1 metre; 2 metres; 3 metres; 4 metres; 5 metres, or more.

The Christmas-tree carrier may be provided at any appropriate location on the vessel, but will typically be at least partly within a perimeter of the moon-pool.

A Christmas tree may be supplied to the vessel in a substantially fully assembled condition. However, in other instances the Christmas tree may be at least partially assembled on the vessel, and of course the Christmas tree may be assembled or disassembled on the vessel as part of a maintenance or repair program. At least part of this assembly may take place on the lower deck. Alternatively, some or at least a part of this assembly may take place on the carrier. The storage area may be lowered as the components are assembled, for example to ensure there is sufficient clearance above the components or to facilitate operator access to a current work area.

Also disclosed herein are embodiments of an offshore drilling vessel for operating in a body of water, comprising:

- a hull defining a deck, often referred to as a main deck;

- a moon pool defining an upwardly extending opening in the hull, the opening extending downwards from said deck and towards the body of water,

- a Christmas-tree carrier defining a Christmas-tree storage area for accommodating a Christmas tree.

According to one aspect, the Christmas-tree carrier is configured for movement of the Christmas-tree storage area between at least a lower position and an upper position, higher than the lower position; wherein the lower position is lower than a level of said deck.

According to a further aspect, the Christmas-tree carrier is configured for temporary or permanent positioning, during operation of the vessel at normal operational draughts, of the Christmas-tree storage area at a level lower than a level of the lower deck and no higher than 3 m above an operational waterline of the vessel, i.e. the waterline at operational draught, such that the Christmas-tree storage area is exposed to splashing sea water. According to this aspect, the Christmas-tree carrier may be arranged at a fixed height, or it may be movable between at least an upper and a lower position as described in connection with the previous aspects.

Hence, taller Christmas trees may be stored in or at the moon pool and transferred to/from a deployment position aligned with a well center. To this end, the Christmas- tree storage area may have, when the Christmas-tree carrier is positioned with the Christmas-tree storage area at its lower position, a free height that is larger than the free height of the moon-pool area. The free height of the moon pool area may be limited by the drill floor deck extending at a height across the moon pool and/or by structures, such as frames, cranes, pipes and/or other installations or equipment located below the drill floor deck.

At least temporarily positioning the Christmas tree during storage and/or transfer at a level below the level of the deck from which the moon pool extends downwards through the hull allows larger and in particular taller Christmas trees to be stored and/or deployed. In particular, this is achieved without the need for arranging the drill floor at an increased height above the moon-pool area. This is advantageous in many embodiments, as a higher position of the drill floor may negatively affect the stability of the vessel. Moreover, the inventors have realized that it is possible to position the Christmas- tree carrier with the Christmas tree at least temporarily at a level that lies close to the operational waterline and, in particular, within a splash zone that is exposed to splashing sea water even though these levels are normally not suitable as a working area for personnel. However, at least temporarily positioning the Christmas-tree carrier with the Christmas tree at such levels does not negatively affect the Christmas tree itself, since the Christmas tree is designed for subsea deployment and, hence, to withstand contact with sea water. The inventors have further realized that embodiments of the Christmas-tree carrier disclosed herein do not require structural changes to existing Christmas trees. When the Christmas-tree storage area of the Christmas-tree carrier is movable between an upper and lower position, the Christmas-tree storage area may be elevated out of the splash zone, e.g. so as to allow personnel to access the Christmas-tree storage area and lower portions of the Christmas tree.

In some embodiments, the lower position is no more than 3 m above the operational waterline of the vessel, e.g. within a splash zone above said operational waterline. For example, said lower position and/or said level lower than a level of said deck and no higher than 3 m above the operational waterline may be in close proximity to, or even at, the waterline at operational draughts, such as no more than 2 m above the operational waterline, such as no more than 1 m above the operational waterline, such as no more than 0.5 m above the operational waterline or even at or below the operational waterline. According to some embodiments, the upper position is chosen so as to allow equipment, such as a Christmas tree or a component thereof, to be skidded between the deck and the Christmas-tree storage area of the Christmas-tree carrier. In particular, when the Christmas-tree storage area is in its upper position, the Christmas-tree storage area may be flush with the deck level of said deck. To facilitate skidding, the Christmas-tree carrier and the deck may be provided with suitable skid beams that are in, or may be brought into, alignment with each other so as to allow skidding equipment between the deck and the Christmas-tree carrier. In particular, the Christmas-tree storage area defined by the Christmas-tree carrier may be provided with such skid beams.

The Christmas-tree carrier may be formed as, or comprise, a platform or similar structure defining the Christmas-tree storage area on which the Christmas tree may rest, even without having to modify existing Christmas trees, as no connectors or similar elements are required on the Christmas tree. Moreover, a platform may also be used as a working platform and/or for storage of other items or for transporting other items at least partly across the moon pool. In some embodiments, the Christmas-tree carrier may comprise a frame, trolley or similar support structure. In some embodiments, such a support structure may be arranged to support a platform that defines the Christmas-tree storage area and that may be configured to be raised or lowered relative to the support structure.

The Christmas-tree carrier may be positioned partly or completely within the perimeter of the moon pool or outside the perimeter of the moon pool, e.g. directly adjacent the periphery. In some embodiments, the Christmas-tree carrier may be positioned or at least have a parking position at the aft or at the forward end of the moon pool.

In some embodiments, the at least the part of the Christmas-tree carrier that defines the Christmas-tree storage area may be suspended within the perimeter of the moon pool, e.g. supported by a support structure supported on respective sides, e.g. respective lateral sides of the moon pool. The support structure may be movably supported, e.g. on tracks that may be positioned on the deck or recessed or elevated relative to the deck. For example, the support structure may comprise one or more elongated support members extending or extendable downward into the moon pool, e.g. at respective sides and/or corners of the Christmas-tree carrier. The support structure may further comprise one or more feet portions connected with the support members and being supported on respective sides of the moon pool. The movement of the Christmas-tree storage area between the upper and lower positions may be performed by any suitable elevation mechanism, e.g. hydraulically, by means of one or more winches and/or the like. The upward and downward movements may be performed by or along elongated support arms which may function as actuators and/or guides. To this end, they may comprise suitable tracks or other guide members. Generally, it will be understood that the Christmas-tree storage area may be positioned at more than two positions, e.g. an uppermost position which may be flush with or even above the deck level of said deck, a lowest position which may be within the splash zone, and one or more intermediate positions at levels between the uppermost and the lowest positions. In some embodiments, the Christmas-tree carrier is horizontally movable across at least a portion of the moon pool. In particular, the Christmas-tree carrier may be horizontally movable between at least a parking position at one end of the moon pool and a deployment position aligned with a well center. To this end, the offshore drilling vessel may comprise one or more guide members, e.g. one or more tracks, extending along at least one side of the moon pool wherein the Christmas-tree carrier is configured to be moved along said one or more guide members between the parking position and the deployment position. For example the track(s) may extend along the longitudinal direction on the starboard and/or the port side of the moon pool. Support members, e.g. feet portions of a support structure of the Christmas-tree carrier may thus be movably supported along the track(s). In some embodiments, the tracks extend on opposite sides of the moon pool, while other embodiments may only provide a single track on one side of the moon pool. In yet other embodiments other horizontal guide members may be used instead of or in addition to the one or more tracks. In some embodiments, the Christmas-tree storage area may be moved between the upper and lower positions both at the parking position and at the deployment position. Similarly, in some embodiments, the Christmas-tree carrier may be horizontally moved both when the Christmas-tree storage area is located at the lower position or at the upper position. It will further be understood that the vertical and horizontal movements of the Christmas-tree storage area may in some embodiments be performed concurrently.

In some embodiments a part of or the whole perimeter of the Christmas-tree storage area may comprise, or be configured to be provided with, a railing or other form of barrier. In some embodiments, at least a portion of the railing may be configured to be removed or otherwise selectively be brought in a blocking configuration where it functions as a protective barrier and a non-blocking configuration where equipment may be skidded or otherwise be moved between a deck area and the Christmas- tree storage area.

In some embodiments, the offshore drilling vessel comprises a crane that is operable to move one or more Christmas-tree components or other equipment between the Christmas-tree storage area and a location displaced from the Christmas-tree storage area, e.g. a location on said deck. For example, the crane may be a gantry crane, an overhead crane, a crane supported by the substructure supporting the drill floor, or another type of crane. The crane may be movable between a first crane position above the Christmas-tree storage area and a second crane position displaced from the first position. For example the first and second crane positions may be defined by the position of the hook or other attachment member of the crane to which a load to be carried is attachable. The crane may be movable under the drill floor deck or at least with a part of the crane extending under the drill floor deck. Accordingly, when the crane is positioned at the first crane position, parts of the crane being positioned above the Christmas-tree carrier may limit the free height above the Christmas-tree storage area.

In some embodiments, the Christmas-tree storage area is operable to be lowered to the lower position when the crane is positioned at the first crane position and to the upper position when the crane is positioned at the second crane position. Hence, larger Christmas trees may be accommodated in the moon-pool area under the drill floor deck. Moreover, even when the lower position is within the splash zone, the Christmas-tree storage area may be elevated such that it is positioned above the splash zone when the crane or other equipment movable below the drill floor deck is not positioned above the Christmas-tree storage area. This allows personnel to access the Christmas-tree storage area, e.g. for performing assembly, maintenance or other work in relation to the Christmas tree.

Generally, the vessel may be oblong having two ends - a bow and a stern. The vessel may comprise the hull and a vessel superstructure extending upwards above the hull. In some embodiments, the vessel comprises a midship portion between the ends. The hull of the vessel typically defines a main deck from which the moon pool extends downward. The drilling vessel may further comprise a drill floor, one or more drilling support structures such as a derrick, a mast and/or the like, and a hoisting system configured to raise or lower tubulars through a well center and/or through another work center in the drill floor. Examples of hoisting systems include draw-works hoisting systems and hydraulic hoisting systems. The drilling vessel may further comprise one or more top-drives and/or other equipment for imparting torque on a drill string.

The moon pool defines an opening in the hull to the sea through which equipment may be lowered from the drill floor towards the seafloor so as to allow drilling operation into the seabed for accessing of hydrocarbon reservoirs. In particular, the moon pool extends downwards from said deck, typically the main deck of the vessel, and towards the body of water, the moon pool defining a moon-pool perimeter in the deck and the deck comprising a moon-pool area adjacent to and at least partly surrounding the moon-pool perimeter. The hull may comprise upwardly extending walls that surround and define a periphery of the moon pool. The moon pool forms a hole in the deck and in all lower decks below said deck. The moon pool may be positioned at a central portion of the vessel, e.g. such that the centerline of the vessel extends through the moon pool and such that the longitudinal center of the vessel extends to the moon pool. It will be appreciated however that the hull may comprise lip portions that horizontally extend from a side wall of the moon pool across a portion of the moon pool. Hence, the width of the moon pool may vary at different heights above a bottom of the hull. The term well center generally refers to an axis or defined path extending downwardly through the vessel and along which the drilling rig is configured to lower tubulars towards the seabed and, in particular, through which tubulars may be lowered all the way to the seabed. One or more holes may be provided within one or more decks of the vessel which are aligned with (for example define) the well center. In some cases multiple well centers may be present. A well center is sometimes also referred to as a drilling center. It will be appreciated that, in addition to one or more well centers, the drill floor deck may comprise additional holes, e.g. one or more additional well centers and/or one or more other work centers through which tubulars may be lowered but through which tubulars may not necessarily be lowered all the way to the seabed. Such a work center may even comprise a bottom or base which prevents tubulars from inadvertently falling to the seabed. Examples of such work centers include foxholes and mouseholes that may e.g. be used for building stands of tubulars but through which the drilling rig cannot lower tubulars to the seabed and/or through which the drilling rig cannot perform drilling into the seabed e.g. by lacking a system arranged to rotate a drill string with sufficient force such as a top-drive or a rotary table. In some embodiments, such an additional hole is a hole in the drill floor deck through which the drilling rig cannot progress a drill string through a riser system. In some embodiments, a well center is differentiated from an additional hole by having a diverter and/or a diverter housing arranged below so that drill string passed through the well center extends through said diverter or diverter housing. In some embodiments, the drilling rig is a dual (or even multiple) activity rig where more than one main or auxiliary drilling operations may be performed through two or even more separate work centers, one, some or all of which may be well centers.

The part of the drill floor deck in immediate proximity of the well center is normally referred to as the drill floor, which is the primary work location for the rig crew and/or machines performing similar functions, such as iron roughnecks. The drill floor normally comprises a rotary table for rotating the drill string. For the purpose of the present description, the term drill floor deck includes the drill floor located directly under/next to the mast and surrounding the well center as well as deck areas on the same level as the drill floor, e.g. connected with the drill floor by uninterrupted floor area on the same level. The drill floor deck may thus include a deck area where human operators and movable equipment such as forklifts, equipment moved on skid beams, etc. can move around and to/from the well center.

The drill floor deck is typically the lowest deck above the diverter system. Diverter systems for offshore drilling vessels are typically provided beneath the drilling rig rotary table. Such a diverter system provides a vent line and ensures that the flow may be directed away from the drilling rig. Hence, in some embodiments, the offshore drilling vessel comprises a diverter system under the well center.

The present disclosure relates to different aspects including the offshore drilling vessel according to the various aspects described above and in the following, further aspects of a drilling vessel and corresponding methods and/or products. Each aspect may yield one or more of the benefits and advantages described in connection with one or more of the other aspects, and each aspect may have one or more embodiments with all or just some of the features corresponding to the embodiments described in connection with one or more of the other aspects and/or disclosed in the appended claims.

Embodiments of the Christmas-tree carrier described herein may also advantageously be used for other purposes. For example, the Christmas-tree carrier may be used to provide a working platform, e.g. for providing access to components along the drill string or components that are suspended above and/or into the moon pool in the vicinity of the well center. In one particular example, the Christmas-tree carrier may be utilized to maintain and/or remove and/or install a riser tension ring. During operation, the riser tension ring is typically attached at the lower ends of a number of riser tensioners that are suspended downwards into the moon pool. Embodiments of the Christmas-tree carrier disclosed herein may thus be horizontally moved towards a position aligned with a well center and under the suspended riser tension ring. The Christmas-tree storage area of the Christmas-tree carrier may then be raised so as to serve as a working platform that provides access to the riser tension ring. The Christmas-tree storage area may even be raised such that the riser tension ring comes to rest on top of the Christmas-tree storage area. The riser tension ring may then be disconnected from the riser tensioners and moved by the Christmas-tree carrier to a side of the moon pool where it can be moved away from the Christmas-tree carrier. For example, when the Christmas-tree storage area is positioned flush with the deck, the ring may be skidded off the Christmas-tree carrier. The reverse procedure may be employed to install a riser tension ring at the end of the riser tensioners. The Christmas tree carrier may be utilized to position a Christmas tree in positions which may not necessarily be aligned with a well center.

In some embodiments, the offshore drilling vessel further comprises a movable BOP carrier, different from the Christmas-tree carrier, for transporting a blow-out- preventer assembly (BOP) between a parking position, e.g. a position displaced from the moon pool, and a deployment position aligned with a well center. The BOP carrier may be formed as a frame or trolley into which a BOP may be hung using suitable connector members attached to a frame of the BOP. In some embodiments, the Christmas-tree carrier and the BOP carrier have parking positions at opposite ends of the moon pool, e.g. the forward and aft ends of the moon pool, respectively. In some embodiments, the BOP carrier and the Christmas- tree carrier may cooperate to transfer heavy equipment, such as BOP components, across the moon pool, e.g. between a position forward of the moon pool and a position aft of the moon pool. For example, this may be desirable in order to load such equipment from a supply vessel or onto the supply vessel using a deck crane having high lifting capacity and a landing area which may be positioned forward or aft of the moon pool. Consequently, only one such landing zone and high-capacity crane may be required.

In particular when transporting a component, e.g. a BOP component, from the BOP- parking end of the vessel to the Christmas-tree parking end, the component may be placed on the BOP carrier, e.g. by means of a BOP crane. For example, to this end, a platform or plate may be inserted or put on top of the BOP carrier frame. The BOP carrier with the component may then be moved to the well center where the component may be picked up by the main hoisting system of the drilling vessel. The BOP carrier may then be moved back to its parking position and the Christmas tree carrier may be moved to the well center. The main hoisting system may then set off the component on the Christmas-tree storage area of the Christmas-tree carrier which may then be moved to its parking position from where the component may be off-loaded, e.g. by skidding it off the Christmas tree carrier or by using an overhead crane. BRIEF DESCRIPTION OF THE DRAWINGS

The above and/or additional objects, features and advantages of embodiments and aspects of the present disclosure, will be further elucidated by the following illustrative and non-limiting detailed description with reference to the appended drawings, wherein:

FIGs. 1 -3 illustrate an embodiment of an offshore drilling vessel; and

FIGs. 4A-C illustrate an embodiment of a Christmas-tree carrier. DETAILED DESCRIPTION OF THE DRAWINGS

In the following description, reference is made to the accompanying Figures, which show by way of illustration how examples disclosed herein may be put into practice.

FIGs. 1 -3 illustrate an embodiment of an offshore drilling vessel. In particular FIG. 1 illustrates a cross sectional view along a centreline of an embodiment of an offshore drilling vessel, FIG. 2 illustrates a deck layout of a main deck of an embodiment of an offshore drilling vessel and FIG. 3 illustrates a transverse cross sectional view of an embodiment of an offshore drilling vessel.

The offshore drilling vessel of FIGs. 1 -3 is a drillship having a hull 101 , a moon pool 102, a main or lower deck 1 15, a drill floor or upper deck 107, and superstructures 197 and 106 extending above the hull and above the main deck. The superstructure 197 is located in the midship portion of the vessel and extends above the moon pool. Superstructure 197 serves as a substructure supporting the drill floor and a dual activity mast 104. In the example of FIGs. 1 -3, the forward and aft perimeter of the moon pool are formed by straight side-wall portions extending partly between the main deck 1 15 and the bottom of the hull. The side-wall portions include respective steps 1 13 and 120 protruding longitudinally into the moon pool at or under the waterline at operational draughts.

The drill floor deck 107 is located at a level above the main deck and it spans across the moon pool 102 that is formed in the hull of the drillship. The drill floor deck 107 comprises two holes defining well centers 123 located next to the dual activity mast 104. The dual activity mast 104 extends upwardly from the drill floor deck 107 and comprises two mast portions arranged side by side such that they are both located on the same side relative to the well centers. Each mast portion accommodates a hoisting system, each for lowering a drill string 128 through a respective one of the well centers 123 and through the moon pool 102 towards the seabed. In the example of FIGs. 1 -3, the hoisting system is a hydraulic hoisting system. In alternative embodiments, other hoisting systems may be used, e.g. a draw-works system comprising a draw-works motor/drum which may be positioned at a suitable location on the drilling rig. Each well center is located next to one of the mast portions and the corresponding hoisting system. The side-by-side configuration of the dual activity mast and well centers allows for efficient dual operations, easy access to both well centers, and convenient visual control of both well centers from a single driller ^' s cabin.

The drilling vessel comprises a setback area 1 12 or similar tubular storage structure for storing stands of tubulars such that the stored tubulars are located partly or completely below the level defined by the drill floor deck, i.e. below the uppermost platform of the superstructure 197, and partly covered by the drill floor deck 107. The setback area comprises a support framework supporting fingerboards having horizontally extending fingers between which tubulars may be stored. The setback area is positioned and arranged so as to allow stands to be moved to/from both well centers 123 from/to the setback area. To this end, one or more column rackers or similar vertical pipe handling equipment may be arranged to move stands into and out of the setback area 1 12. The setback area 1 12 further comprises stand building equipment configured to build stands from individual pieces of pipe. An example of such stand building equipment is described in WO 02/057593. Alternatively or additionally, stands may be built on the drill floor. The mast structure 104 is located on one side of the well centers, and the setback area 1 12 is located on the side of the mast opposite the well centers and/or behind the driller's cabin. Hence, the drill floor deck provides a large, unobstructed deck area on the side of the well centers opposite the mast. This area provides unobstructed access to both well centers and is free of pipe handling equipment. Consequently, these areas may be used as working area, e.g. for rigging up suspendable auxiliary equipment, and/or for positioning on-deck auxiliary equipment, e.g. wireline or coiled-tubing equipment.

The drilling vessel further comprises a pipe storage area 109 for storing pipes in horizontal orientation located aft of the moon pool, and a storage area 1 16 for storing marine riser joints located forward of the moon pool. In the present example, the storage areas 109 and 1 16 are located at a level below the drill floor deck; in particular, the storage area 1 16 is located on the main deck 1 15. One or more catwalk machines 108 or similar horizontal pipe handling equipment are arranged to feed tubulars from the storage area 109 to the well centers. To this end, the catwalk machine(s) may be aligned with the axis defined by the two well centers. Another catwalk machine may be used to provide riser joints, load the riser storage 1 16 below the drill floor deck and/or to provide the drill floor with other tubulars. One or each of the catwalk machines may be operable to service both well centers. The drilling vessel comprises other storage areas below the drill floor deck 107, e.g. on the main deck 1 15 and configured for storing a variety of equipment, such as replacement parts, e.g. for mud pumps, etc. The superstructure 106 is located in the forward part of the vessel and serves as an accommodation superstructure.

A part of the main deck 1 15 of the vessel is located beneath the drill floor deck and allows heavy subsea equipment, e.g. a BOP 130 and a Christmas tree 103 to be moved to the moon pool under the well centers so as to allow such equipment to be lowered toward the seabed. Consequently, the drill floor deck and, in particular, the part of that drill floor deck that is located in close proximity to the well center may be fixed in position and does not need to be hoisted or lowered for the subsea equipment to be lowered to the seabed.

In the illustrated embodiments the Christmas tree 103 may be placed on a Christmas-tree carrier 1 1 1 at a lower level than the main deck 1 15 and, in particular, in close proximity of the waterline at operational draught. This allows a larger Christmas tree 103 to be stored without having to increase the free height between the main deck 1 15 and the underside of the drill floor deck 107 which could otherwise negatively influence the stability of the vessel. The Christmas-tree carrier 1 1 1 is formed as, or comprises, a platform which may be horizontally moved from a parking or storage position at the aft end of the moon pool to a deployment position aligned with the well center so as to allow deployment of the Christmas tree. In some embodiments, the Christmas-tree carrier platform may also be moved up and down. In the example of FIG. 1 , the Christmas-tree carrier platform may be vertically moved and positioned at different heights between an uppermost and a lower position. In this embodiment the uppermost position is aligned with the main deck 1 15 so as to allow equipment to be skidded between the main deck and the Christmas-tree carrier. The lower position is on top of a lip portion 1 13 of the hull that protrudes from the aft end forward into the moon pool. Equipment, such as Christmas-tree components, may be skidded onto the Christmas-tree carrier from an area 1 14 of the main deck adjacent to the moon pool when the Christmas-tree carrier platform is vertically aligned with the main deck, or it can be positioned on the Christmas-tree carrier by means of an overhead crane 105. For example, this allows a Christmas tree to be assembled from multiple components on the carrier. To this end, a bottom component of the Christmas tree may be skidded onto the Christmas-tree carrier platform. Subsequently, additional components may be added to the top of the Christmas-tree by means of the overhead crane. If desired, the Christmas-tree carrier platform may be lowered so as to allow a taller Christmas tree to be assembled. This may be desirable so as to allow the overhead crane to pass above the Christmas tree. The assembled Christmas tree may then be moved by the Christmas-tree carrier from the parking position (sometimes referred to as a storage or loading position) to the deployment position aligned with the well center where the Christmas tree may be attached to a tubular string or other support and lowered towards the seabed using the hoisting system of the vessel.

The Christmas-tree carrier 1 1 1 may also be used to facilitate other operations. For example, it may be employed to mount or dismount the riser tension ring 1 17 that is attached to the lower ends of the riser tensioners 1 18 and which serves to hold a marine riser under suitable tension. For example, when the riser tension ring 1 17 needs to be dismounted, the Christmas-tree carrier 1 1 1 may be moved to a position under the riser tension ring 1 17 and then raised until the riser tension ring rests on top of the Christmas-tree carrier platform. Personnel may then access the Christmas-tree carrier platform and dismount the riser tension ring from the riser tensioners. The Christmas-tree carrier with the riser tension ring may then be moved to the parking position of the Christmas-tree carrier from where the riser tension ring may be moved further, e.g. skidded onto the main deck or moved by the overhead crane 105.

Another example of an operation facilitated by the Christmas-tree carrier may be the movement of BOP components from the BOP parking area forward of the moon pool to a position aft of the moon pool where the BOP component may be lifted by a high-capacity crane onto a supply vessel. To this end, the BOP component may initially be placed on a BOP carrier (not explicitly shown in FIG. 1 ) at the forward end of the moon pool.

FIGs. 4A-C illustrate an embodiment of a Christmas-tree carrier. In particular, FIG. 4A shows a three-dimensional view of the Christmas-tree carrier with its platform in its upper position, FIG. 4B shows a three-dimensional view of the Christmas-tree carrier with its platform in its lower position and FIG. 4C shows a cross-sectional view of the Christmas-tree carrier with its platform in its upper position. Although the Christmas-tree carrier in accordance with the disclosure may have any appropriate dimensions, a typical carrier may be 25 feet by 25 feet to provide an upper surface area of approximately 625 square feet. A typical Christmas tree to be carried may have a base with corner supports at 8 feet spacings and occupy or require a 20 feet by 20 feet clear area.

The Christmas-tree carrier, generally designated by reference numeral 1 1 1 , comprises a platform or storage area 421 , a frame 422 and an elevation mechanism that allows the height of the platform relative to the frame to be adjusted. In the example of FIGs. 4A-C, the elevation mechanism comprises four hydraulic actuators 424 but other embodiments may use additional or alternative devices, e.g. hydraulic actuators which extend or retract flexible supports, such as cables, roller, lift or leaf chains, a different number of hydraulic actuators, one or more winches, a rack and pinion mechanism, etc. or a combination thereof. The frame 422 is a U-shaped frame having a closed end, an open end and two side portions. The open end of the U-shaped frame faces the center of the moon pool when the Christmas-tree carrier is positioned at a parking position at one end of the moon pool, as illustrated in FIG. 4A. The side portions of the frame are movably supported on tracks 425 that run on both sides of the moon pool 102. The tracks may run on top of the main deck 1 15 or be recessed into or elevated from the main deck. The frame is shaped and sized such that it fills the entire lateral width of the moon pool and such that the upper surface of at least the bottom portion of the frame is flush with the main deck 1 15 surrounding the moon pool. The frame comprises four upwardly extending hydraulic actuators 424. However, other examples of Christmas-tree carriers may comprise a different number of actuators or other types of elevation mechanisms. The piston portions 426 of the cylinders extend through holes on the frame and the free ends of the pistons are attached to the platform 421 such that the platform is suspended from the frame by the actuators. Hence, when the pistons 426 are retracted into the actuators, the platform 421 is in its upper position (as illustrated in FIGs. 4A and 4C) and when the pistons are extended from the actuators the platform is lowered to its lower position (as illustrated in FIG. 4B). The platform 421 is shaped such that it fits into the central opening defined by the U- shaped frame, with the upper surface of the platform being flush with an upper surface of the frame. The main deck, the frame and the platform are provided with skid beams 427 that allow equipment to be skidded between the main deck and the platform when the platform is in its upper position.

The platform 421 comprises a cut-out 429 which allows the platform to be moved towards a position aligned with the well center such that lateral parts of the platform extend on either side of an elongate support extending downward through the well center, e.g. a marine riser string. The cut-out extends longitudinally from the edge of the platform that is aligned with the open end of the U-shaped frame and towards the center of the platform. The cut-out may be of any appropriate dimensions, and may be, for example, 60 inches wide.

At least parts of the periphery of the platform may be secured by a railing or other barrier so as to provide a secure working platform. In some embodiments the railing is detachably mounted. In some embodiments the carrier 1 1 1 may be locked in a particular configuration, for example when the carrier platform is on the well center and vertically positioned to allow an operation to be carried out on the platform, particularly an operation requiring operator intervention or the presence of a human operator on the platform.

Even though the above embodiments have been described in the context of a drillship, it will be appreciated that the described features may also be implemented in the context of a semi-submersible or other type of drilling vessel. Although some embodiments have been described and shown in detail, the disclosure is not restricted to them, but may also be embodied in other ways within the scope of the subject matter defined in the following claims. In particular, it is to be understood that other embodiments may be utilised and structural and functional modifications may be made without departing from the scope of the present disclosure.

For example, the described embodiments comprise two well centers, but it will be appreciated that alternative embodiments may comprise a single well center or a well center and additional work centers.

In the device claims enumerating several features, several of these features can be embodied by one and the same item of hardware. The mere fact that certain measures are recited in mutually different dependent claims or described in different embodiments does not indicate that a combination of these measures cannot be used to advantage.

It should be emphasized that the term "comprises/comprising" when used in this specification is taken to specify the presence of stated features, integers, steps or components but does not preclude the presence or addition of one or more other features, integers, steps, components or groups thereof. However, on the other hand the term "comprises/comprising" is intended to also include embodiments where the particular articles is formed entirely by the comprised features.