GB2549258A - Drillship - Google Patents

Abstract

A drillship comprises a drill deck 104 and a hoisting system for raising and lowering a string of tubular equipment along a vertical hoisting axis. The hoisting structure has a load carrying structure (hydraulic cylinders 116) that carries a load of the string of tubular equipment and transfers it downwards into or through the drill deck. The drill deck has a single well centre 211, through which the string of tubular equipment is advanced. The load carrying structure transfers the load predominantly on a hoisting side (fore or aft) of the well centre. The side opposite the hoisting side may be unobstructed to allow a deck crane (512, fig 5) to position and pick up items at the well centre. There may be a setback area 107 in which stands of tubulars are stored, which may be below the level of the drill deck.

Description

Drillship Technical Field

The invention generally relates to drillships.

Background

Drillships are widely used in the exploration and exploitation of hydrocarbon reservoirs under the sea floor.

It is generally desirable to provide a drillship that allows for an efficient and flexible operation. It is further generally desirable to provide an offshore drilling rig that facilitates operation with a high degree of safety. It is also generally desirable to provide a drillship that has relatively low construction costs.

Drillships typically have an oblong hull which imposes certain limitations on the layout of the operational parts of the drillship such as storage areas, well centres, hoisting systems etc. Arranging these components so as to provide a stable ship, high utilisation of available space and efficient drilling operations is far from a trivial task. Several proposals for drillship designs have been proposed in the past.

For example, WO 2014/108542 discloses a drillship with a dual activity mast laterally located on the starboard side of a pair of well centres. The drillship further comprises a setback structure such that the mast is positioned between the well centre and the setback area. The port side of the well centres thus provides an open drill floor area allowing easy access to the well centres and efficient and safe drilling operations to be performed. However, feeding tubulars from the setback structure to the well centre needs to be performed through the mast structure.

It is thus desirable to provide alternative or even improved drillships.

Summary

According to a first aspect, disclosed herein are embodiments of a drillship configured for drilling a well into a subsea reservoir of hydrocarbons. The drillship comprises: - a drill deck; - a hoisting system adapted for raising and/or lowering a string of tubular equipment along a vertical hoisting axis; the hoisting system comprising a load carrying structure extending upwardly relative to the drill deck and configured to carry a load of the string of tubular equipment and to transfer said load downwards into or through the drill deck; - a single well centre on the drill deck, wherein the drillship is configured for constructing a subsea well by advancing the string of tubular equipment through the well centre and into the seabed; wherein the load carrying structure transfers the load into or through the drill deck predominantly on a hoisting side of the well centre, said hoisting side being fore or aft of the well centre.

Embodiments of the drillship disclosed herein are relatively inexpensive to manufacture as the drillship only comprises a single well centre. Consequently, the drillship may only need to have a single hoisting system configured to advance a tubular string to and into the seafloor.

The well centre is typically arranged above a moon pool and close to, or even on, a longitudinal centre line of the drillship. Embodiments of the drillship disclosed herein may thus be designed to have a high degree of stability, as the load is transferred to the ship's substructure fore or aft of the well centre, i.e. close to the centre line of the ship and since the hoisting system and the well centre may be aligned along the longitudinal direction of the drillship. These and/or other advantages are achieved while still allowing for a large open drill floor area with unobstructed access to the well centre from at least the side opposite the hoisting side. Moreover, as the load carrying structure and any associated mast structure is located fore or aft of the well centre, the areas laterally adjacent the moon pool may be conveniently utilised for e.g. storage of tubulars in an upright orientation such that the load carrying structure or any associated mast structure does not interfere with these storage structures and/or with associated devices for handling of these tubulars. Consequently any such storage structure may be designed with a high storage capacity and/or with efficient handling of tubulars to/from and/or within the storage structure.

In some embodiments the hoisting side is fore of the well centre and the side opposite the well centre is aft of the well centre. In alternative embodiments the hoisting side is aft of the well centre and the side opposite the well centre is forward of the well centre.

In some embodiments, the load carrying structure is configure to transfer at least 70%, such as at least 80%, such as at least 90% of the load on the hoisting side. Hence, the side of the well centre opposite the hoisting side may substantially or even completely be kept free of load carrying structures and further support structures associated with the load carrying structure, such as a part of a mast or the like. Accordingly, a relatively or even completely unobstructed access to the well centre from the side opposite the hoisting side is facilitated.

In particular, when the access to the well centre from the side opposite the hoisting side is sufficiently free of fixedly installed structures - in particular free of any load carrying structure of the hoisting system and any associated support structures of the hoisting system -, an access path having a relatively large free height may be provided from the side opposite the hoisting side. This is particularly useful when advancing long tubulars towards the well centre. Accordingly, in some embodiments, the drillship defines a sufficiently large free height on the side of the well centre opposite the hoisting side so as to allow tubular equipment having a length of at least 70ft, such as at least 90ft, such as at least 120ft, such as at least 150ft , such as at least 170ft, such as at least 190ft, to be horizontally advanced from the side opposite of the hoisting side towards the well centre with a first end of the tubular equipment facing towards the well centre, and to be lifted at said first end by the hoisting system into an upright orientation aligned with the well centre. In some embodiments, the tubular equipment is raised to vertical by a horizontal to vertical machine at or prior to arriving at the well center. A partially or completely unobstructed access from the side opposite the hoisting side may also facilitate crane access to the well centre and the drill floor area immediately surrounding the well centre. Accordingly, in some embodiments, the drillship comprises a deck crane, wherein the drillship defines a sufficiently unobstructed area on the side opposite the hoisting side so as to allow said deck crane to position and pick up items at/from the well centre. The deck crane may be a knuckleboom crane or another type of crane. The crane may be positioned on the port or the starboard side of the drillship.

The load carrying structure generally refers to the part of the hoisting system that transfers the loads of the drill string suspended by the hoisting system into the drill deck and the substructure under the drill deck. The type of load carrying structure typically depends on the type of hoisting system. In some embodiments, the hoisting system is a hydraulic hoisting system comprising upwardly extending cylinders or other actuators for carrying the load to be hoisted or lowered typically via large sheaves mounted on top of the cylinders. The load carrying structure of a cylinder rig thus comprises the cylinders and the one or more sheaves which may be in the form of a sheave cluster. The cylinders may extend from the drill deck or from a foundation below or elevated above the drill deck. The loads exerted on the hoisting system during lowering or hoisting of equipment in or out of the well centre can be said to be at least predominantly transferred to the drilling rig via the cylinders.

In alternative embodiments, the hoisting system may be a draw-works system. For such systems the sheaves are carried by other suitable support members, such as upwardly extending columns and/or other load-bearing parts. The load carrying structure of such a hoisting system thus comprises the sheaves and the support members that carry the sheaves and any load suspended from the sheaves. In yet other embodiments, the hoisting system is a direct winch hoisting system.

In other words, generally, the load carrying structure transfers substantially the entire weight of the hoisting system and of the load suspended from it to parts of the drillship at or below the drill deck.

The hoisting system may comprise a hoisting line and in some embodiment multiple lines which is typical for cylinder and winch based hoisting systems, whereas a drawworks based hoisting system typically applies a single line. One end of the hoisting line may be anchored at one side of the load carrying structure, e.g. opposite the side on which the well centre is located, e.g. such that the forces transferred via the sheaves and the cylinders, or other support members carrying the sheaves, are substantially vertical. In some embodiments, the hoisting lines may be anchored to the drillship via a number of anchoring members such as compensators, e.g. a number of cylinders.

In some embodiments, the drillship comprises a mast structure configured for supporting at least a part of the load carrying structure against lateral forces; wherein the mast structure is predominantly or even entirely located on the hoisting side of the well centre. Hence, the side opposite the hoisting side may be kept substantially or even completely free of any mast structure that might otherwise obstruct the free access to the well centre, e.g. by limiting the free height of any access path to the well centre from the side opposite the hoisting side and/or by limiting, or even preventing, crane access from said side opposite the hoisting side. The mast structure may extend upward relative to the drill deck. The mast may extend from the drill deck or from a deck or foundation different from the drill deck. When the hoisting system is a hydraulic hoisting system, the mast structure predominantly supports the load carrying structure of the hoisting systems against horizontal forces while the load is carried by the cylinders. In a drawworks hoisting system a mast structure may form the load carrying structure as well as providing support against lateral forces.

According to a second aspect, disclosed herein are embodiments of a drill-ship configured for drilling a well into a subsea reservoir of hydrocarbons, the drillship comprising: - a drill deck; - a hoisting system adapted for raising and/or lowering a string of tubular equipment along a vertical hoisting axis; the hoisting system comprising a load carrying structure extending upwardly relative to the drill deck and configured to carry a load of the string of tubular equipment and to transfer said load downwards into or through the drill deck; - a single well centre on the drill deck, wherein the drillship is configured for constructing a subsea well by advancing the string of tubular equipment through the well centre and into the seabed; wherein the load carrying structure comprises hydraulic cylinders positioned on the port and on the starboard side of the well centre.

In the following, further embodiments of the first and/or second aspects are summarised.

In some embodiments, the drillship comprises a setback area for storing tubular equipment in an upright orientation, each tubular equipment having a lower end and an upper end when stored upright in the setback area. The setback area may be recessed relative to the drill deck such that at least some tubulars are stored with their respective lower ends positioned at a level below the drill deck while drilling operations are performed through the well centre. Consequently, the centre of gravity of the drillship may be lowered. As the load carrying structure of the hoisting system is positioned fore or aft of the well center space is freed up for the setback area which is not obstructed by parts of the load carrying structure of the hoisting system or by any mast structure associated with the hoisting system. This may be particularly advantageous when long tubulars are stored in the setback such that the upper ends of the tubulars extend above the drill deck.

Generally, a setback area is a storage area, typically in direct communication with one or more well centres (via pipe handling equipment), where tubulars can be stored ready for use and where they can be quickly retrieved to be lowered towards the seabed. Similarly, tubulars can conveniently be set back into the storage area for later use. In many cases tubulars will be transported to another storage area or off the drilling rig during transit due to the substantial weight and/or higher centre of gravity of the setback area relative to the motions of the drilling rig in the sea. In the setback area, tubulars are typically stored vertically as this is convenient because the tubulars do not require a complete rotation before being run; however, tubulars may in principle be stored in horizontal position or both. Tubulars may also be stored in the setback area when tripping tubulars out of the well centre. Typically, a setback area is suitable for storing and receiving stands (i.e. 2 or more joined single tubulars, such as triplets) of drill pipe and/or casing in varying diameters. Storing tubulars in stands saves time in a drilling operation because fewer connections have to be made when running the tubulars through the well center and broken when tripping the tubulars out of the well centre. In such cases vertical storage is preferable because storing stands (which are typically very long) horizontally requires a large footprint. The setback will comprise equipment for holding the pipes, such as fingerboard systems for storing vertical tubulars, which will be designed to accommodate one or more diameters of tubulars. It is therefore typical that the setback will have designated areas for holding e.g. drill-pipe and casings depending on diameter. Accordingly, in some embodiments a setback area is a tubular storage arranged to store stands of drill pipe, casing (optionally of varying diameters) or both. Such stands may be stands of 2 or more, such as of 3 or more, such as 4 or more. In some embodiments the setback further comprises pipe handling equipment (such as a pipe racker) arranged to deliver stands to and from the holding equipment of the setback area and present the stand to the hoisting system (typically the hook or top-drive) or hand over the stand to another machine in the pipe handling equipment (such as a chute or a further pipe racker machine) which performs the presentation. A setback area is also typically combined with stand building equipment which is arranged to receive singles and connect those into a stand after which the stand is set back in the setback area ready to be run through the well center by the hoisting system.

When the setback area is located starboard or port of the well centre, the setback area does not obstruct access to the well centre from the side opposite the hoisting side, e.g. from the aft or fore of the well centre. Moreover, when the load carrying structure of the hoisting system is not positioned between the setback area and the well centre, tubulars may be transferred between the setback area and the well centre without having to pass through or under the load carrying structure or any supporting mast structure associated with the load carrying structure. Moreover, any mast structure laterally supporting the load carrying structure may extend on the hoisting side of the well centre away from the well centre without interfering with the setback area. In some embodiments, the mast structure is independent and separate from any storage structure of the setback area.

In some embodiments, the setback area defines a top surface located above the respective upper ends of the tubular equipment and operable as storage and/or landing area. Such an area may thus be arranged such that it remains obstructed by any load carrying structure or by any associated mast structure of the hoisting system.

In some embodiments the drillship comprises pipe transfer equipment for moving upright tubular equipment selectively along a longitudinal direction of the drillship and/or along a lateral direction of the drillship and for placing the upright tubular equipment at selected storage positions of the setback area. Examples of such pipe transfer equipment includes pipe rackers or column rackers that can perform linear movements along the longitudinal or lateral direction of the drillship, or linear movements along the longitudinal and lateral direction of the drillship, respectively.

In some embodiments, the load carrying structure is arranged to transfer said load symmetrically port and starboard of the well centre.

The term well centre refers to a hole in the drill deck through which the drill-ship is configured to lower tubulars towards the seabed and, in particular, through which tubulars may be lowered all the way to the seabed and into a bore hole in the seafloor. A well centre is sometimes also referred to as a drilling centre.

It will be appreciated that, in some embodiments, the drill deck may comprise additional holes such as 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 centre 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 centre extends through said diverter or diverter housing.

In some embodiments, the drillship defines a centre line connecting a bow and a stern of the drillship and wherein the well centre is located on the centre line. The single well centre may be the only hole located along a longitudinal axis through the well centre which is configured to allow tubulars to be advanced downwardly through the moon pool such that the tubulars extend below the drillship. In some embodiments, the single well centre is the only hole located along a longitudinal axis through the well centre which is configured to allow tubulars to be advanced downwardly through the moon pool and into a borehole in the seabed.

In some embodiments, the drillship comprises a hull defining a main deck; wherein the drill deck is elevated relative to the main deck, e.g. supported by a substructure. The space below the drill deck, e.g. the space between the main deck and the drill deck, may be used for storing and/or setting up a blow-out-preventer. In some embodiments, the blow-out-preventer may be movable to a position below the drill deck and vertically aligned with the well centre such that the blow-out-preventer can be lowered towards the seafloor by the hoisting system. Accordingly, the drill deck and, in particular, the part of the drill deck that is in direct proximity to the well centre, may be kept stationary without the need to hoist or lower parts of the drill deck so as to allow running (i.e. lowering) the blow-out preventer (BOP) and/or other heavy subsea equipment (e.g. the Christmas tree).

The term tubular equipment is intended to refer to tubular equipment that is to be advanced through the well centre towards the sea floor during one or more stages of the drilling operation. In particular, the term tubular equipment refers to straight tubular elements that can be joined to form a string of tubular equipment. The tubular equipment may be selected from drill pipes and/or other tubular elements of the drill string, risers, landing strings, liners and casings. Examples of tubular elements of the drill string include drill pipes, drill collars, etc. For the purpose of the present description, tubular equipment will also generally be referred to as tubulars. Tubulars may have varying lengths and diameters. Drill pipe typically has a length between 15’ to 45’ and diameters of up to 19” or even 20” when considering stabilisers, bits, casing et. Prior to advancement through the well centre, drill pipes are normally assembled to stands of two, three or even more stands of drill pipe, so-called doubles, triples etc but can also be run as singles. The building of stands is performed by dedicated stand-building equipment and/or by a hoisting system. The drillship may thus comprise stand-building equipment operable to interconnect two or more tubular joints so as to form a stand made up of multiple, e.g. 2, 3, or even more, individual joints. The stand-building of drill pipes and/or casings may thus be performed at a position laterally displaced relative to the well centre. Once assembled, the stands are normally stored in the set-back area, typically in upright position supported by e.g. fingerboards. Riser joints typically have lengths between 50’ and 90’ (pup joints may be shorter) and diameters of up to 70”.

In some embodiments, the drillship comprises first tubular advancing equipment configured to advance tubular equipment towards the well centre from a side opposite the hoisting side. Alternatively or additionally, the drillship may comprise second tubular advancing equipment configured to advance tubular equipment towards the well centre from the hoisting side.

The second tubular advancing equipment may be operable to move tubular equipment at least partly underneath the load carrying structure of the hoisting system and/or through a gap formed in said load carrying structure. In some embodiments, the load carrying structure and, optionally, a mast structure associated with the load carrying structure, are configured to allow tubular equipment to be moved towards the well centre from the hoisting side and along a longitudinal direction of the drillship. The hoisting side and the side opposite the hoisting side may be distinguished by a lateral axis through the well centre extending between a port and a starboard side of the drill ship and, in particular, by a lateral axis that is orthogonal to a longitudinal axis of the drillship where the longitudinal axis extends between the bow and the stern of the drillship.

In some embodiments, the first and/or second tubular advancing equipment comprises horizontal pipe handling equipment for handling horizontally oriented tubular equipment, e.g. along a longitudinal axis of the tubular equipment. In some embodiments the first and/or second tubular advancing equipment comprises vertical pipe handling equipment for handling at least vertically oriented tubular equipment. Examples of horizontal pipe handling equipment include catwalk machines while examples of vertical pipe handling equipment include pipe rackers. Consequently, the first and/or second tubular advancing equipment may allow tubulars from different types of stor-age/setback areas for storing tubulars at different orientations to be moved to the well centre and/or between one or more storage areas. Alternatively or additionally, the tubular advancing equipment may comprise devices for changing the orientation of tubulars and/or equipment for advancing tubulars in a slanted direction e.g. upwards from a recessed storage location. Such equipment may comprise a slide, a specially designed pipe racker with a travelling upper arm for raising pipes or chute. The first and/or second tubular advancing equipment may be configured to advance the tubular equipment towards the well centre along the longitudinal direction of the drillship or along an axis defining an angle relative to the longitudinal axis of the drillship. In some embodiments the first and/or second tubular advancing stations may be able to raise tubulars from horizontal to vertical.

In some embodiments, tubular equipment may thus be moved to the well centre from both sides of the well centre, e.g. along the longitudinal direction of the drillship, thus allowing tubular equipment from multiple storage areas, e.g. a storage area aft of the well centre and another storage area forward from the well centre. To this end, the load carrying structure and, optionally, the mast structure may define an opening through which tubular equipment is movable towards the well centre from the hoisting side. Alternatively or additionally the tubular equipment may be at least partly moved underneath the load carrying structure. For example, in some embodiments risers and/or another type of tubulars may be stored on the hoisting side of the well centre, while other types of tubulars, such as drill pipes and/or casings, may be stored and/or assembled to stands on the side opposite the hoisting side or vice versa.

In some embodiments, the drillship comprises a storage area for storing tubulars located below the drill deck, and pipe handling equipment for feeding tubulars from said storage area through an opening in the drill deck towards the well centre, e.g. along a chute or otherwise along a sloping direction.

Brief description of the drawings

The above and/or additional objects, features and advantages of embodiments and aspects of the present invention, will be further elucidated by the following illustrative and non-limiting detailed description with reference to the appended drawings, wherein: FIGs. 1 and 2 show schematic cross sections of an embodiment of a drillship. FIGs. 3-5 show schematic top views of an embodiment of a drillship. FIG. 6 illustrates the required free height for advancing a tubular to the well centre of an embodiment of a drillship.

Detailed description FIGs. 1 and 2 show schematic cross sections of an embodiment of a drillship. In particular, FIG. 1 shows a cross section seen from the bow end of the drill-ship while FIG. 2 shows a cross section seen from the port side of the drill-ship. The drillship comprises an elongated hull 101 having a bow 214 and a stern 215 and defining a longitudinal axis extending between the bow and the stern. The hull defines a main deck 102 and the drillship comprises a drill deck 104 formed on top of a substructure 105 and elevated relative to the main deck. The substructure 105 comprises a platform supported by legs. The platform defines the drill deck and spans across a moon pool 103 formed in the hull of the drillship. As the drill deck 104 is elevated relative to the main deck 102, heavy subsea equipment, e.g. BOPs and Christmas trees, may be moved to the moon pool under the well centre so as to allow such equipment to be lowered toward the seabed by means of the hoisting system. Consequently, the drill deck and, in particular, the part of that drill deck that is located in close proximity to the well centre may be stationary and does not need to be hoisted or lowered for the subsea equipment to be lowered to the seabed.

The drill deck 104 comprises a hole defining a well centre 211 located next to a mast 106 and next to cylinders 116. The cylinders form part of the load carrying structure of a hydraulic hoisting system. The hoisting system further comprises a top drive 208 suspended by a hoisting line 217 via a sheave 218 that is carried by the cylinders 116 such that the top drive can be raised and lowered relative to the drill deck by means of the cylinders 116. In the present example, the hoisting system comprises two cylinders, but it will be appreciated that other embodiments may comprise more than two cylinders. Alternatively, other hoisting systems such as a drawworks hoisting system or a direct winch hoisting system may be used instead of a hydraulic hoisting system.

The drillship comprises only a single well centre and a single hoisting system. The well centre and the mast are positioned proximal to, or even aligned with, the longitudinal axis of the drillship, thus providing a high degree of stability. The well centre 211 is located aft of the mast and aft of the load carrying structure of the hoisting system, thus providing an open, unobstructed drill deck area aft of the well centre. In other embodiments, the well centre may be located forward of the mast. In any event, the side of the well centre on which the mast and the load carrying structure of the hoisting system are located is referred to as the hoisting side of the well centre; the side opposite the hoisting side may serve as open drill deck area unobstructed by any load carrying structures of the hoisting system and/or by any associated mast structures.

The drillship further comprises pipe storage areas and associated catwalk machines 209 and 220 located aft and forward of the well centre, respectively. The catwalk machines 209 and 220 are configured to advance tubulars towards the well centre along a longitudinal direction of the drillship. Catwalk machine 220 may advance tubulars to the well centre through a suitably sized gap in the mast 106 and the load carrying structure of the hoisting system. The pipe storage areas fore and aft of the well centre may be configured for storing pipes in horizontal orientation and/or for storing pipes in vertical orientation. For examples, pipes may be stored in a vertical orientation under the drill deck, e.g. as described in WO 2014/108541 in the context of storing riser joints under the drill deck. In particular, as the drillship only comprises a single well centre and since there need not be other holes for stand-building or similar auxiliary operations arranged along the longitudinal axis of the drill ship, tubulars may be advanced towards the well centre 211 from both the front and from the aft, as can be best seen in FIG. 2. In alternative embodiments, tubulars are fed along the longitudinal axis of the drillship only from the aft or from the front of the well centre. For example, in such an embodiment, tubulars may only be advanced to the well centre from the setback area and from either the aft or the front of the well centre. Alternatively or additionally, tubulars may be advanced towards the well centre from a different direction, e.g. from a lateral side opposite the setback area. For example, tubulars, may be stored in upright orientation on a lateral side of the moon pool, opposite the setback area, e.g. at a level below the drill deck. These tubulars may then be advanced to the hoisting system by means of a chute extending laterally from the side of the drillship towards the hoisting system. In yet other embodiments, tubulars may be advanced from the aft or the front of the well centre along a chute rather than using a catwalk machine. This may be desirable when the tubulars are stored in upright orientation at a level below the drill deck.

The drilling rig comprises a setback area 107, or a similar pipe storage structure, for storing stands of tubulars such that the stored tubulars are located partly or completely below the level defined by the drill deck, i.e. below the uppermost platform of the substructure 105. The setback area may even be at least partly covered by the drill deck 104. The setback area comprises a support framework 119 supporting fingerboards having horizontally extending fingers between which tubulars may be stored. The setback area is positioned and arranged laterally next to the well centre and laterally next to the moon pool so as to allow stands to be conveniently moved between the well center and 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. Tubulars may be presented to the hoisting system from a lateral direction, e.g. along a direction orthogonal to the longitudinal axis of the drillship or along a direction having a different angle relative to the longitudinal axis, e.g. along a diagonal direction.

In some embodiments, e.g. in case of stands of drill pipe or casings, the tubulars may be so tall that, when they are stored in the setback area in an upright orientation, their uppermost ends may extend above the drill deck.

When feeding them to the well centre they may be laid into a chute. Alternatively, the setback structure may extend from the drill floor deck upwards. The handling of tubulars within the setback area may be performed by vertical pipe rackers or the like. The setback structure 107 may further comprise stand building equipment configured to build stands from individual pieces of pipe. Alternatively or additionally, stands may be built on the drill floor, e.g. using the main hoisting system.

The setback area and any support structure associated with the setback area, e.g. for supporting the fingerboards and/or a column racker and/or a stand-building equipment, may be arranged independently of the mast structure supporting the hoisting system. In particular, as the setback area is located laterally displaced from the well centre on the starboard or port side of the well centre, the mast 106 can extend towards the bow of the ship so as to provide sufficient stability against lateral forces without having to interfere with the setback area. This allows the top of the setback area to be used as a storage and/or landing area. To this end, the top of the setback area may be provided with a platform 210.

It will be appreciated that the setback area may be located on the port and/or on the starboard side of the well centre, as is illustrated in FIGs. 3 and 4, respectively. As in the example of FIG. 1 and 2, the drillships shown in FIGs. 3 and 4 comprise a single well centre 211 which is located close to or even on the longitudinal axis 321 of the drill ship and aft of the mast 106 and the load carrying structure of the hoisting system. The mast 106 and the load carrying structure are positioned symmetrically relative to the longitudinal axis 321. The setback area 107 is laterally displaced relative to the well centre 211 and extends along a lateral side of the drillship from the mast, e.g. towards the aft. In the example of FIG. 3 the setback area is located at the port side of the drillship while, in the example of FIG. 4, the setback area is located at the starboard side. Regardless of the location of the setback area 107, the examples of the drillship of FIGs. 3 and 4 may comprise some or all of the features of the drillship of FIGs. 1 and 2.

As the mast structure 106 is located fore or aft of the well centre, and since the setback area is located on a lateral side of the well centre, the drill deck provides a large, unobstructed deck area on the side of the well centres opposite the mast. This area provides unobstructed access to the well centre. Consequently, this area may be used as working area, e.g. for rigging up auxiliary equipment, and/or for positioning on-deck auxiliary equipment. Moreover, deck cranes on both sides of the vessel may conveniently access the well centre and the drill floor area surrounding the well center, as is illustrated in FIG. 5. FIG. 5 schematically illustrates a top view of a drillship where the well centre 2011 is located aft of the mast 106 and the setback area 107 is located on the port side of the well center and extends towards the aft relative to the mast 106. Deck cranes 512 and 513 are positioned at the port and starboard side, respectively, of the drill ship and aft of the well centre, i.e. on the side opposite to the side where the mast is located. These cranes can thus conveniently access the well centre. It will be appreciated that the well centre may also be positioned forward of the mast and with a setback area extending towards the front of the drillship.

The open deck space fore or aft of the well centre also allows very long tubulars to be advanced towards the well centre along the longitudinal direction of the drillship, e.g. by a catwalk machine, and then to be raised to an upright orientation by the hoisting system. This is schematically illustrated in FIG. 6 which illustrates the mast 106 of one of the drillships described in connection with FIGs. 1-5, and a catwalk machine 209 arranged for feeding a tubular 613A towards the well centre from the aft of the drillship. One end of the tubular 613A may then be connected to the top drive 208A when the top drive is lowered towards the drill deck. When the top drive is raised to an upper position, as illustrated by reference numeral 208B in FIG. 6, the tubular is gradually lifted into an upright orientation, as illustrated by reference numeral 613B. Such an operation requires a relatively large free height along the axis of approach of the tubular towards the well centre - in particular when advancing long tubulars, such as tubulars having a length of at least 70ft, such as at least 90ft, such as at least 120ft, such as at least 150ft, such as at least 170ft, such as at least 190ft.

Claims (20)

1. A drillship configured for drilling a well into a subsea reservoir of hydrocarbons, the drillship comprising: - a drill deck; - a hoisting system adapted for raising and/or lowering a string of tubular equipment along a vertical hoisting axis; the hoisting system comprising a load carrying structure extending upwardly relative to the drill deck and configured to carry a load of the string of tubular equipment and to transfer said load downwards into or through the drill deck; - a single well centre on the drill deck, wherein the drillship is configured for constructing a subsea well by advancing the string of tubular equipment through the well centre and into the seabed; wherein the load carrying structure transfers the load into or through the drill deck predominantly on a hoisting side of the well centre, said hoisting side being fore or aft of the well centre.

2. A drillship according to claim 1; wherein the load carrying structure is configure to transfer at least 70%, such as at least 80%, such as at least 90% of the load on the hoisting side.

3. A drillship according to any one of the preceding claims; wherein the drill-ship defines a sufficiently high free height on a side of the well centre opposite the hoisting side so as to allow tubular equipment having a length of at least 70ft, such as at least 90ft, such as at least 120ft, such as at least 150ft, such as at least 170ft, such as at least 190ft, to be horizontally advanced from the side opposite of the hoisting side towards the well centre with a first end of the tubular equipment facing towards the well centre, and to be lifted at said first end by the hoisting system or by a horizontal to vertical machine into an upright orientation aligned with the well centre.

4. A drillship according to any one of the preceding claims; comprising a deck crane, wherein the drillship defines a sufficiently unobstructed area on the opposite of the hoisting side so as to allow said deck crane to position and pick up items at/from the well centre.

5. A drillship according to any one of the preceding claims, further comprising a mast structure configured for supporting at least a part of the load carrying structure against lateral forces; wherein the mast structure is predominantly located on the hoisting side.

6. A drillship configured for drilling a well into a subsea reservoir of hydrocarbons, the drillship comprising: - a drill deck; - a hoisting system adapted for raising and/or lowering a string of tubular equipment along a vertical hoisting axis; the hoisting system comprising a load carrying structure extending upwardly relative to the drill deck and configured to carry a load of the string of tubular equipment and to transfer said load downwards into or through the drill deck; - a single well centre on the drill deck, wherein the drillship is configured for constructing a subsea well by advancing the string of tubular equipment through the well centre and into the seabed; wherein the load carrying structure comprises hydraulic cylinders positioned on the port and on the starboard side of the well centre.

7. A drillship according to any one of the preceding claims; comprising a setback area for storing tubular equipment in an upright orientation each tubular equipment having a lower end and an upper end when stored upright in the setback area.

8. A drillship according to claim 7; wherein the setback area is recessed relative to the drill deck such that at least some tubulars are stored with their respective lower ends positioned at a level below the drill deck.

9. A drillship according to claim 7 or 8; wherein the setback area is located starboard or port of the well centre.

10. A drillship according to any one of claims 7 through 9; wherein the setback area defines a top surface located above the respective upper ends of the tubular equipment and operable as storage and/or landing area.

11. A drillship according to any one of claims 7 through 10; further comprising a mast structure configured for supporting at least a part of the load carrying structure against lateral forces; wherein the setback area comprises a storage structure separate from the mast structure.

12. A drillship according to any one of claims 7 through 11; further comprising a pipe transfer equipment for moving upright tubular equipment selectively along a longitudinal direction of the drillship and a lateral direction of the drill-ship and for placing the upright tubular equipment at selected storage positions of the setback area.

13. A drillship according to any one of the preceding claims; wherein the load carrying structure is arranged to transfer said load symmetrically port and starboard of the well centre.

14. A drillship according to any one of the preceding claims wherein the drill ship defines a centre line connecting a bow and a stern of the drillship and wherein the well centre is located on the centre line.

15. A drillship according to any one of the preceding claims; wherein the single well centre is the only hole located along a longitudinal axis through the well centre which is configured to allow tubulars to be advanced downwardly through the moon pool and into a borehole in the seabed.

16. A drillship according to any one of the preceding claims; comprising a hull defining a main deck; wherein the drill deck is elevated relative to the main deck and supported by a substructure.

17. A drillship according to claim 16; configured for lowering a blow-out-preventer by the hoisting system towards the seafloor.

18. A drillship according to any one of the preceding claims; comprising first tubular advancing equipment configured to advance tubular equipment towards the well centre from a side opposite the hoisting side.

19. A drillship according to any one of the preceding claims; comprising second tubular advancing equipment configured to advance tubular equipment towards the well centre from the hoisting side.

20. A drillship according to any one of the preceding claims; comprising a storage area for storing tubulars located below the drill deck, and pipe handling equipment for feeding tubulars from said storage area through an opening in the drill deck towards the well centre.