DK201470227A1 - An offshore drilling rig and a method of operating the same - Google Patents

Abstract

An offshore drilling rig comprising a drill deck, at least one primary well center and a diverter system arranged below the primary well center, a drilling support structure extending upwardly from the drill deck and above the primary well center and the other work center and a first and a second hoisting system supported by the drilling support structure and being adapted for raising or lowering a first and a second load carrier, respectively, and where the offshore drilling rig comprises a positioning sys-tem adapted for selectively positioning at least the first load carrier in at least a first or a second horizontal position different from the first horizontal position, where the first load carrier in the first horizontal position is positioned above the primary well center, and in the second horizontal position is positioned above the other work center.

Description

Title:

An offshore drilling rig and a method of operating the same.

The prior art:

The present invention relates to an offshore drilling rig comprising a drill deck and a drilling support structure extending upwardly relative to the drill deck, and where at least one or two work centers are arranged in the drill deck, at least one of the work centers being a primary well center meaning that it is equipped for drilling operations through a riser whereby mud and cuttings are returned to the surface. This is typically via a diverter that can be connected to a riser, such as a marine riser or a mud return conductor (which is sometimes also referred to as a high-pressure riser) being capable of conducting drilling mud from the sea floor to the offshore drilling rig.

Offshore drilling rigs of this kind are expensive to build and operate, and the continued development of this kind of rigs is therefore focused on providing a rig that will reduce the time required for production, meaning that the time for drilling and installing the necessary equipment for e.g. oil production is preferably be as short as possible preferably without significantly increasing the costs of building and operating the rig.

For this purpose many different embodiments of offshore drilling rigs have been proposed over time.

The lifting capacity of a rig is typically designed to the expected requirement imposed on the rig. Venturing into deeper waters, drilling deeper and/or setting longer casing strings imposes an increased requirement on the lifting capacity. However, for long periods of the operation far lower capacity may be required. A higher lifting capacity often imposes additional cost as well as, in many cases, slower operation because the equipment, such as the top drive, is heavier. In some cases the well design is affected and less optimal due to limitations imposed by the lifting capacity of the rig, e.g. forcing the well designer to use shorter casing strings than required by the formation. In some cases this limitation is imposed because equipment at sufficiently high load rating is not available at the time of construction of the rig.

Due to the high cost of the rig and the cost of operating the rig, down time due to equipment failure should be avoided.

On this background it is desirable to provide an offshore drilling facility that will increase the possibility of operating the rig, even when essential equipment is out of order, e.g. due to maintenance or breakdown. It is further desirable to provide a flexible way of occasionally increasing the lifting capacity without having to suffer some of the drawbacks of heavier lifting equipment in the majority of the operation.

Summary:

In some embodiments of the present invention a rig as mentioned in the introduction further has two hoisting systems (a first and a second hoisting system supported by the drilling support structure and each being adapted for raising or lowering a load carrier (a first and a second load carrier, respectively), a first top drive suspended from the first load carrier and a second top drive suspended from the second load carrier, a first vertical guide system connectable to the first top drive to guide said first top-drive in a position over said first work center and a second vertical guide system connectable to the second top drive to guide said second top drive in a position over said second work center; and by comprising a positioning system adapted for selectively positioning said second load carrier and, optionally, said first load carrier a) in a normal-operation position with the first and second load carriers positioned over said first and second work centers, respectively, to allow the first load carrier to lift a load in or out of the first work center via said first top drive connected to the first vertical guide system and to allow the second load carrier to lift a load in or out of the second work center via said second top drive connected to the second vertical guide system; and b) in a joint-operation position with the first and second load carriers positioned to allow the first and second load carrier to jointly lift a load in or out of the first work center wherein the second top drive is disconnected from the second vertical guide system.

Thereby, with the rig in joint-operation position, the two hoisting systems may operate to jointly lift a load in and out of the first work center also referred to as the first well center. Jointly lifting loads is also discussed below as synchronously lifting loads, typically via a connecting tool. Typical types of loads include strings of tubulars such as strings of casing pipe and, optionally, landing strings or riser strings with or without a Lower Marine Riser Package (LMRP) or a complete Blow-Out Preventer (BOP) attached at the distal end. Loads may also include heavy subsea equipment on its own such as a BOP. Typically such loads are not in themselves passed through the drill deck but rather connected to a landing string, drill pipe, riser joints or other tubulars below the drill deck and then lowered towards the sea bed from there. In the normal-operation position the two load carriers may be operated independently up and down such as to perform dual activity operations on two wells, on a single well or by one top drive working over a work center which is not a well center such as a fox hole. In some embodiments the second work center is omitted and the second top drive and hoisting system is available as spare equipment which can be utilized in the joint-operation position over the first well center and/or for redundancy over the first well center independently of the first top drive. The features discussed in relation to Fig. 19a-c may also be generally application to embodiments of the invention.

In principle the first and second load carriers may also be brought to a joint-opration position over a third work center with the first and second load carriers positioned to allow the first and second load carriers to jointly lift loads in or out of a third work center wherein the second top drive is disconnected from the second vertical guide system. Typically, in such an embodiment, the first top drive will also be disconnected from the first vertical guide system. Additionally or alternatively, a third vertical guide system may be arranged to be connected to one or both top drives individually or in connection. In general the third vertical guide system may be placed in any suitable location such as coaxially with one of the others or behind (the axis joining the first and second well centers next to) the well center for which it is arranged to guide.

Disconnecting a top drive from the load carrier is often a very time-consuming process; therefore, it is preferable to leave the top drives in place. The load carriers may, in some embodiments, lift via the top drives. In some embodiments, in the joint-operation position, some of the load, or even the entire load, is arranged to be transferred to the load carrier around the top drive e.g. via a beam or frame connecting the load carrier to the connecting tool. In some embodiments this frame is part of the connecting tool. Even so the weight of the top drives must typically also be guided particularly for drill ships and semi submersibles which may be subject to heave, pitch and roll motions. To this end, the second top drive is, in some embodiments, connectable to the first top drive, to the first vertical guide system and/or to a third vertical guide system, when positioned in the joint-operation position. This is also discussed in a general manner in relation to Fig. 19a-c. When the second top drive is connected directly to the first top drive or the first vertical guide system, the first vertical guide system must be dimensioned to handle the extra loads relative to guiding only the first top drive.

Alternatively or in combination with the inventions discussed above, the rig may be adapted to comprise a redundancy-operation position with the second load carrier positioned to allow the second load carrier to lift loads in or out of the first work center wherein the second top drive is disconnected from the second vertical guide system. In this way the second top drive may replace the first top drive and operate independently thereof in the first work center.

As discussed in relation to Fig. 19a-c the second top drive may remain connected to the service loop umbilical by which it operates above the second well center or be disconnected from this when operation in relation to the first well center. In some embodiments where the service loop remains connected to the second top drive, the second vertical guide system preferably lowers and raises a section of the service loop the second top drive is lowered or raised. In some embodiment the vertical guide system supports a point of the service loop (such as on the dolly) which is raised or lowered substantially with the second top drive such as with the top drive. In this way the section of the service loop extending across over the drill floor may be kept relatively short..

The first work center is typically a primary well center (defined and discussed below) because most of the requirements for very heavy lifting are found as part of the building of sections of the well drilled through a riser, while redundancy is often required because a large portion of the drilling operations is performed here. In some embodiments, the second work center is also a well center such as a primary well center. This provides flexibility and redundancy in the types of operations that can be performed.

In some embodiments the roles of the first and second systems may be reversed so that e.g. the rig comprises a. a second joint-operation position with the first and second load carriers positioned to allow the first and second load carrier to jointly lift loads in or out of the second work center wherein the first top drive is disconnected from the first vertical guide system, b. a second redundancy-operation position with the first load carrier positioned to allow the first load carrier to lift loads in or out of the second work center wherein the first top drive is disconnected from the first vertical guide system.

As it is clear from the above, several of the embodiments of the invention allow the rig to be adapted for different modes of operation, including a dual activity mode where independent operations, such as drilling, running a BOP and riser and/or stands-building are simultaneously performed using both the primary well center and the other work center, or in a dual lifting mode where both hoisting systems cooperates for the purpose of lifting heavy loads above one work center, or in a redundancy mode where one hoisting system for some reason is disabled, and the other hoisting system is moved from one position to another in order to replace the disabled hoisting system.

For the purpose of the present description, the term drilling support structure means any structure extending upwardly relative to the drill deck and being equipped for supporting a hoisting system for hoisting and lowering tubulars (such as drill strings, casings and/or risers towards the seabed) so that drilling into the seabed can be performed. The drilling support structure may extend from the drill deck or from a deck different to the drill deck. The hoisting system is in this relation any system that provides a lifting capacity above one or more of the work centers arranged in the drill deck. This may, in one embodiment of the invention, be in the form of a hydraulic hoisting system comprising upwardly extending cylinders for supporting the load to be hoisted or lowered typically via cable sheaves mounted on top of the cylinders or alternatively it may be in the form of a conventional draw works system. Exam- pies of a drilling support structure includes a derrick structure which are typically applied to support a draw works hoisting system and a mast structure which are typically applied to support a cylinder hoisting system.

The term work center refers to a hole in the drill deck through which the drilling rig is configured to lower tubular equipment towards the seabed and, in particular, through which tubular equipment may be lowered all the way to the seabed. A work center thus defines a downward passage extending through the drill deck through which tubular equipment may be lowered toward the seabed or even to the seabed. In this respect the term work center covers e.g. a well center, a mousehole, a rathole or a standbuilding foxhole, with or without different tools inserted into or supported from it, such as power slips or other equipment. A work center through which the drilling rig is configured to lower tubulars all the way to the seabed and/or through which the drilling rig can perform drilling into the seabed is often referred to as a well center. A well center is sometimes also referred to as a drilling center. It will be appreciated that the drill deck may comprise additional holes such as foxholes and mouse holes 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. The function, and hence the term assigned to a work center, may imply certain functionality implemented in the hole itself (typically slips, rotary table etc.), below the drill deck (typically diverter system, riser tensioners etc.) as well as above the deck (such as top drive and the load carrier of a hoisting system). In the present invention the top drive associated with a well center may be moved to another position away from the remaining elements of the well center. In some embodiments a work center is therefore considered a well center when the positioning system is arranged to provide the necessary functionality in combination with any functions fixed to the well center (such as, in some embodiments, a rotary table), to drill into the seabed. In some embodiments, a well center comprises a rotary table, slips or a similar device allowing a drill string to be suspended by, or hung off in, the well center; to this end, a well center may comprise power slips or other devices operable to engage tubular equipment and to support the weight of the tubular equipment and, in particular, a string of tubular equipment extending to the seabed, so as to prevent the tubular equipment from descending through the well center. In some embodiments the slips themselves may vary in size depending on the diameter and type of the tubular and therefore the well center may be suitable for receiving and operating variuous sizes of slips. A displaceable well center may comprise a displaceable rotary table or a similar displaceable element comprising a hole and defining a downward passage. Displaceable well centers are explained and laid out in codepending application PCT/EP2014/055312 where one or more parts of a well center such as a diverter, a rotary table, slips system in the drill deck or the like may be movable (see also Figures 1-18). In some embodiment embodiments the two work centers are mounted in a substantially horizontal track in the drill deck, and the drill deck comprises a work center positioning system adapted for selectively moving at least one of or each of the work centers in the horizontal track to the first or the second position in the drill deck. In this relation the horizontal track may preferably be linear at least along a part of it, and the diverter system may comprise at least one diverter over board tube having a first end being connected to the primary well center and the other end being supported and fixed with respect to the drill deck and having at least one telescopic section between the first and the second end, the telescopic section extending parallel to linear part of the horizontal track in the drill deck. Thereby the diverter over board tube, which may be directing well fluids under high pressure from the diverter and over board, is relatively easy to keep tight during drilling operations e.g. by using a hydraulic, pneumatic or mechanical packer to tighten and seal the telescopic section during drilling operations. The drilling rig may advantageously further comprise at least one riser tensioning arrangement below the drill deck, and where the riser tensioners are mounted on linear tracks for repositioning, and in parallel to the horizontal track in the drill deck, so that the riser tensioners can be positioned below either of the work centers and/or below either of the first or second positions. As noted above this further explained in co-depending application PCT/EP2014/055312. A primary well center is in this relation a well center being adapted for drilling operations comprising a mud return system using e.g. a mud return conduit such as a marine riser typically used with floating drilling rigs or a high pressure riser (also sometime referred to as a conductor pipe) which is typically used on stationary offshore drilling rigs such as jack-ups. In this relation a primary well center is therefore differentiated from other work centers by having a mud return system often including a diverter system including a diverter housing arranged below so that drill string passed through the primary well center extends through said diverter housing arranged for diverting e.g. blow outs to one side of the offshore drilling rig. Moreover, the drilling rig comprises a hoisting system, top drive and/or other equipment configured to operate through the primary well center and to perform drilling operations in the seabed. In some embodiments, the drilling rig may comprise a single primary well center or two or even more primary well centers. In addition to one or more primary well centers, the drilling rig may comprise additional work centers and or other additional holes in the drill deck through which the drilling rig cannot progress a drill string through a riser system or another mud return conduit. In some embodiment the drilling comprises a primary well center along with an auxiliary well center without mud return capability which is arranged to perform drilling operations into the seabed where mud return is not required.

Functioning as a well center requires lifting equipment, such as hoisting system arranged to lower and lift in and out of the hole (well center), respectively. It also requires structure for providing continuous rotation to a drill string such as a top drive which is a drilling machine suspended from the hoisting system with a shaft for attaching to a drilling string and providing rotation and conduits for pumping mud into the drill string as well as an IBOP suitable for controlling the connection to the mud system. The top-drive is typically mounted on rails or connected to a dolly travelling on rails (with or without an extend-and-retract capability) to keep the top drive from spinning when torque is applied to the string of tubulars suspended from the shaft such as the drill string. Furthermore, the well center may be operable to rotate the string with a device in or below the drill floor typically a rotary table. Depending on the design the rotary table may be able to provide continuous rotation or rotation over a limited angle to allow orientation of the string and/or for breaking out connections subsea. The mud return is often directed through a diverter having a mud return line and two overboard lines. In the context of the present invention the presence of a diverter is in some embodiment generalized to mean that as a minimum the well center is connected to mud return lines. For such embodiment a primary well center is a well center where the rig is arranged so that the functionality of a primary well center obtainable by moving the well center components into place in the well center, in some embodiments the top drive is replace by a pipe elevator and components for providing drilling at the end of the drill string such as connections for providing compressed mud for a mud motor, electrical power for a downhole electrical motor or laser or optical connections for providing laser light at the end of the drill string.

The offshore drilling rig may be a semi-submersible drilling rig, i.e. it may comprise one or more buoyancy pontoons located below the ocean surface and wave action, and an operation platform elevated above the ocean surface and supported by one or more column structures extending from the buoyancy pontoon to the operation platform. Alternatively the offshore rig may be of a different type, such as a jack-up drilling rig or a drill ship.

The term tubular equipment is intended to refer to tubular equipment that is advanced through the well center towards the sea floor during one or more stages of the drilling operation. The tubular equipment may be selected from drill pipes and/or other tubular elements of the drill string, risers, liners, landing strings and casings. Examples of tubular elements of the drill string include drill pipes, drill collars, heavy drill pipe etc.

For the purpose of this description, the term drill deck is intended to refer to the deck of an operating platform of an offshore drilling rig immediately above which joints of tubulars are assembled to form the drill string which is advanced through the well center towards the seabed. Hence the drill deck is the primary work location for the rig crew and/or machines performing similar functions, such as iron roughnecks. The drill deck normally comprises at least one rotary table for supporting the rotating drill string during drilling operations. For the purpose of the present description, the term drill 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 and connected with the drill floor area by uninterrupted floor area on the same level, i.e. the floor area where human operators and movable equipment such as forklifts, equipment moved on skid beams, trolleys, such as trolleys etc. can move around and to/from the well center; in some embodiments without having to climb/descend stairs or other elevations. The drill deck is typically the floor of a platform, e.g. the lowest platform, above the diverter system.

At least parts of the drill deck may be formed by the roof of a housing or enclosure accommodating mud mixing equipment and/or other operational equipment of the drilling rig, thus allowing for a compact and space-saving arrangement of equipment on the drilling rig. For example the drill deck may comprise a storage area for storing pipes, e.g. a storage area for storing pipes in horizontal orientation. The storage area may be located next to the between two horizontal pipe handling devices or, if this is movable, next to the path along which the horizontal pipe handling device may travel. In some embodiments, the pipe storage area and/or horizontal pipe handling equipment may be partially or completely surrounded by open drill deck area, e.g. drill deck area shaped and sized to allow vehicles or skidable items to be moved around the pipe storage area.

Generally, a skidable arrangement of equipment allows the displacement, in particular horizontal displacement, of the equipment, e.g. guided by tracks or a similar guide structure. In some embodiments the term skidable is synonymous with movable. In some embodiments skidable is synonymous with movably mounted such as mounted on a track.

While the invention is applicable to rigs with two or more work centers any embodiments described herein may also be applicable where only one work hole is available (or used) with the lifting yokes, hoisting systems or load carriers. In such embodiments the other work center from the primary well center is replaced by the absence of a work center e.g. a blank.

In some embodiments the present invention relates to an offshore drilling rig comprising two hoisting systems each comprising a lifting cable hanging over at least one cable crown being supported by the drilling support structure and each being adapted for raising or lowering a load carrier, and further comprising a positioning system adapted for selectively positioning each load carrier in a number of different horizontal positions comprising at least a first horizontal position above the primary well center, and a second horizontal position different from the first horizontal position, wherein the positioning system is adapted for positioning each cable crown above said first horizontal position above the primary well center, or said second horizontal position.

As will be recognized by the skilled person, a wide variety of solutions for the positioning system are available in the art for shifting the position of a cable crown. Examples include shifting the cable crown hydraulically, via a rack and pinion. In some embodiments the position system comprises a locking mechanism to lock the cable crown mechanically in positions e.g. via slips and remote activated pin coupling the track and the cable crown. The same applies for part of the position system arranged to skid the work center, well center or trip saver system for those embodiments where such as system is implemented.

Thereby the two hoisting systems may be used simultaneously for providing the combined lifting capacity of each hoisting system in order to lift a load otherwise too heavy for one hoisting system e.g. out of the primary well center.

The hoisting systems may in this relation be any suitable hoisting system such as based on a draw works or a hydraulic cylinder configuration. In this relation the term cable crown covers any device supported by the drilling support structure and being adapted for supporting one or more lifting cables hanging below the drilling support structure beneath the cable crown. This may be in the form of a single cable sheave adapted for supporting one or more cables, or a cluster of cable sheaves being independently rotatable so as to constitute a crown block or a cable sheave cluster e.g. supporting a travelling block beneath the crown block. Furthermore the term load carrier in this relation means any device adapted for being carried by the hoisting system, and allowing a load to be connected to the load carrier. Examples of load carriers may be a load carrying hook, yoke, shackle or a travelling block. In the present context a hoisting system is a system providing a lifting force to a lifting cable which can be independently controlled. Accordingly, in some embodiments, the first and second hoisting systems share one or more com ponents whereas, in other embodiments, they do not. In some embodiments the first and second hoisting system may be connected not only when a synchronous lift if required (such as when operating in the joint-operation position) e.g. via the dead-line such as describe in U.S. Patent US5762279 (see e.g. Fig. 2 as well the claims). This connection is upstream along the lifting cable toward the machine providing the primary mechanical forces such as a winch, drawworks or hydraulic cylinder. In some embodiments the link is imposed by coupling the sources of mechanical force (e.g. a first and second drawworks, winch or hydraulic cylinder) when operating in synchronous mode whereas operating them separately otherwise. For a winch or drawworks this may be accomplished by coupling the cable drums and on cylinder the cylinders may be coupled hydraulically and/or mechanically.

In one embodiment of the present invention, each of the load carriers are connected to a lifting cable hanging from a cable crown (such as a cable sheave cluster or a crown block) supported by the drilling support structure, and the positioning system is adapted for shifting at least one of or each of the cable crown to and from a first and a second horizontal position relative to that cable crown where the load carrier is positioned right above a selected one of the work centers. The position of the cable crown is measured by the position the load carrier would have if it is allowed to hang freely from the cable crown. An example where the position of the load carrier is offset e.g. by a dolly as described below. Flowever, due to the extensive loads requirements that may be imposed on hoisting capacity of the rig (i.e. the load safely carried by the load carrier sometime referred to a hook loads) it often advantageous that the position of the load carrier is not substantially offset in a horizontal direction from that it would have if it was freely suspended from the cable crown. In some embodiments (typically when the load carrier is or is extended from a travelling block) the load carrier will have a freely suspended position below the center of the cable crown (see e.g. Fig. 3) whereas in some embodiments the position will be aligned with the periphery of the sheave(s) of the cable crown (see e.g. 1). Also, by each of the cable crowns having at least two positions several configurations are possible including but not limited to the configurations where the rig having two working holes or three holes discussed below where the cable crowns have a common position in the middle and each have a second position to the side.

Typically a so-called elevator or pipe elevator is used to latch onto the upper end of the tubular string so that the weight of the string can be transferred to a load carrier. Typically the elevator is held by bails or links mounted on the top drive so that the weight of the string (i.e. the load) is transferred to the load carrier via the top drive. The pipe elevator may be designed to latch on specific types and diameters of pipes and thus may need to be replaced by another type of pipe elevator when the type of pipe is changed. In this way the bails and the elevator works as an extension of the top drive so in some embodiments bails are in the present context considered a part of the top drive even though they may be replaceable attached to the top drive. In other embodiments the load is transferred to the load carrier via another structure directly attached to the load carrier.

The drilling rig may further comprise a connecting tool having two opposite ends each being adapted for directly or indirectly connecting it to one of the load carriers, so that the connecting tool can be carried by two different load carriers, and where the connecting tool has an intermediate load carrier being arranged between said two opposite ends and being adapted for carrying a load. In this way, it is possible to mount the connecting tool so that it is hanging below and between two load carriers, and thereby it is possible to provide a lifting power being higher than the lifting power of each of the hoisting systems by using both hoisting systems to lift the same load via the connecting tool. Embodiments of such a connection tools are laid out in copending Danish patent application PA 2014 70131. In some embodiments the lifting tool is suspended from bails mounted on the two top drives. In some embodiments the connecting tool is arranged to carried by (i.e. transferring the weight to the load carrier holding the top drive via) at least one of the top drives such as both of them. In some embodiments via bails mounted to the top drives.

In this relation, the positioning system may advantageously further be adapted for shifting each of the two cable crowns to a position right next to the first position, and so that the two load carriers are positioned on opposite sides of the first position to allow them to carry each end of the connecting tool and the elevator (or other intermediate load carrier for latching on to the load) available to hold a load between them.

As noted above and in relation to Fig. 19a-c the top drive or both of the top drives are in some embodiments arranged to be rotated to allow the side normally connected to the vertical guide system to be facing the other top drive or its guide system arranged to guide it in the joint-operation position or the redundancy position. In some embodiments, the top drive is arranged to be connectable from both sides so that, in some embodiments, the invention relates to a top drive comprising a shaft or other connector for connecting the top drive to the upper end of a string of tubulars for drilling into the bed of the sea, carrying the load of said drill string and providing rotation to said string via a motor; said top drive being suitable for being raised or lowered by a hoisting system during normal operation wherein said top drive is adapted for selectively receiving a mechanical connection from a dolly or other vertical guide system on two opposite sides of the top drive, said dolly being arranged to keep the top drive from rotating as the rotation is applied to the string.

Typical features of a top drive are an element to be connected to the upper end of the string for rotation therewith and motor means for power rotating said element and the connected string; a powered torque wrench at the lower end of said top drive operable to apply torque in opposite directions to said string and said element; an elevator at the lower end of said assembly adapted to engage a section of said drill string at a location beneath the torque wrench and suspend said section for movement by the elevator; connecting means attaching said torque wrench and said elevator to said top drive assembly for movement upwardly and downwardly there with and in a relation suspending said torque wrench and said elevator and a section of the drill string carried by the elevator from said top drive assembly; said connecting means being constructed to enable movement of said torque wrench and elevator vertically relative to one another.

One of the advantages of the joint-operation position allowing a joint (synchronous) lift is that the lifting capacity in this position may exceed the load rating of the individual top drive (or hoisting system). In some embodiments, this advantage is used to apply lighter top drives of a lower rating to reduce the cost and/or weight of the top drive used for the majority of drilling operations (this may also increase the speed of the hoisting system). Accordingly, in some embodiments the load rating of the first and/or second top drive is 1500 tons or less, such as 1250 tons or less, such as 1000 tons or less, such as 750 tons or less, such as 500 tons or less, such as 250 tons or less. However, in some embodiments the aim is to obtain a combined load rating exceed the available load rating so that the load rating is 750 tons or more, such as 1000 tons or more, such as 1500 tons more, such as 1750 tons or more, such as 2000 tons or more. A vertical guide system is arranged to guide the top drive as it is lowered or raised during operation. This system typically has the function to keep the top drive from rotating as it applies rotation to a drill string but in particular to guide the substantial weight of the top drive particularly for drill ship and semi-submersible rigs exposed to roll and pitching motions which could otherwise cause the top drive to oscillate as a pendulum over the drill deck. For embodiments where the first and second top drives are coupled the vertical guide system must be dimensioned to the increased load caused by the two top drives. Typically the vertical guide system comprises a set of vertical rails often supported by the drilling support structure. The rails and top drive may be designed to engage with each other directly but often a dolly is used which spaces the top drive from the vertical rails. Often this dolly comprises an extend-and-retract function by which the horizontal position may be changed. In some embodiments, the vertical guide system comprises tracks or skids upwards in the support structure to allow vertical movement of a top-drive in a stable manner. The guide system may be configured to restrain the top drive against lateral movements and/or to counteract torque imparted by the motor of the top drive.

In order to reduce the horizontally induced load on the retractable dolly it is in this relation preferred that the cable crown carrying a top drive, and the retractable dolly connecting the same top drive to said vertically extending track are adapted to keep the lifting cable between the cable crown and the top drive substantially vertical. However, in some embodiments the dolly is used to position the top drive (may be said to be part of the positioning system) often for parts of the operation where the top drive is not carrying a load or at least not a large load. For instance, the dolly may be used in the redundancy-operation position to move the first top drive out of the way of the first work center to allow the second top drive to continue its work. In other embodiments the dolly system is designed to position the top drive even when carrying a load and may be said to be part of the positioning system.

Accordingly, the positioning system may comprise a retractable dolly arranged for each top drive, and the retractable dolly being adapted to connect the top drive to a vertically extending track preferably mounted on the drilling support structure, and to position the top drive in the first and the second horizontal position above e.g. two different work centers and being adapted for positioning the top drive at a distance from the track, so that it is positioned right above one of the work centers. In order to enable this function in practice it will be recognized that the retractable dolly may be adapted to reach a top drive at a substantial horizontal distance from the vertically extending track, even where such a distance exceeds 4 or even 5 meters, such as exceeds 6 or even 7 meters, such as exceeds 8 or even 9 meters, such as exceeds 10 or even 11 meters, such as exceeds 12 or even 15 meters.

Another function of the vertical guide system is that it supports the service loop typically going from a goose neck to the top drive via the guide system such as point on a dolly of the guide system. The service loop is a collection of cables and hoses which may comprise electrical cables for control and/or power, hydraulic hoses and hoses for connected to the mud system for pumping mud into the bore hole. In some the disconnected guide system moves up and down substantially with the disconnected top drive to allow the service loop to follow the disconnected top drive.

Especially in relation to rigs having a very large distance between the work centers, such as the above mentioned 10 meters the requirement to the reach provided by the retractable dolly is substantial. As such large dollies are not practical to operate for different reasons, but also because it is expensive to implement dollies providing a substantial reach, then the offshore drilling rig may advantageously comprise at least three work centers arranged in the drill deck horizontally spaced apart from each other’s in mutually different distances from the vertically extending track. Thereby it is still possible to provide the redundancy and increased lifting capacity even if the reach provided by retractable dolly connecting the top drive to the vertically extending track may be reduced so that it only allows the top drive to be vertically aligned above the two of the three work centers being closest to the vertically extending track. In some embodiments this is combined with one or more of the inventions according to the appended claims where the second top drive is disconnected from the guide system used during normal operation, repositioned relative to the first work center/well center and preferably mechanically attached to the first top drive (or any other part of the hoisting system related to that well center such as the load carrier) or a guide system which is either separate from or the same as that of the first top drive. In some embodiment the first and second well center are spaced apart a distance which exceeds 4 or even 5 meters, such as exceeds 6 or even 7 meters, such as exceeds 8 or even 9 meters, such as exceeds 10 or even 11 meters, such as exceeds 12 or even 15 meters.

It is understood that variations of the present invention may be made by applying a retractable dolly. Firstly, in some embodiments the retractable dolly comprises a load carrier for carrying loads extending into the work hole and the load carrier of the cable connected to the hoisting system is connected to the retractable dolly for providing vertical lifting power to the dolly. In combination of alternatively, the retractable dolly may be provided to offset the load carrier connected the cable from the position it would have based on the position of the cable crown and no retractable dolly. Typically, it is advantageous if these two positions are substantially aligned so the lift provided by the hoisting system via the cable is substantially vertical. However, by providing such offset via the dolly less movement may be necessary of the cable crown which may be a faster and/or simpler way of changing position of the load carrier. Accordingly, in some embodiments the position of the cable crown refers to the position of the load carrier as provided by the cable crown and the dolly in combination. In some embodiments the position of the cable crown refers to the position of the load carrier as it would have been without a retractable dolly influencing the horizontal position.

In a further preferred embodiment, two work centers may both be primary well centers.

In the context of this description the terms of moving, positioning, skidding shifting and so on is meant to include the process of displacing a component or part from one position to another, but also the necessary structure for holding or fixing the component or part at a selected position during operation of the drilling rig.

The drawing:

In the following one or more embodiments of the invention will be described in more detail and with reference to the drawing, where:

Fig. 1a and 1b: Are concept drawings showing two different operation situations of a dual activity drilling facility seen from one side, incorporating full redundancy for the intended drilling operation by incorporating skidding well centers/work center in the drill deck.

Fig. 2a and 2b: Are concept drawings showing two different operation situations of a cyclic operating hoisting system within the same drilling facility seen from one side, enabling both hoisting systems to work over the same work center individually or in turn for providing fast tripping of drill pipe, casing running or riser- and BOP running/retrieval operation.

Fig. 3: Is a concept drawing showing an alternative embodiment of the invention, where each hoisting system comprises a cable winch.

Fig. 4: Is a concept drawing showing an operation mode where the two hoisting systems are operated synchronously within the same drilling facility seen from one side, enabling both hoisting systems to work over the same work center in sync for providing a combined lifting capacity in the one work center.

Fig 5: Is a concept drawing showing an operation mode where the two hoisting systems are operated synchronously within the same drilling facility seen from one side, enabling both hoisting systems to work over the same work center in sync for providing a double lifting capacity in the one work center, but with only one hoisting system carrying a top drive, and with a single work center.

Fig. 6: Is a concept drawing showing a transferable diverter housing and mud return tubing system with telescoping diverter overboard lines according to the invention, seen from below.

Fig. 7: Is a sectional drawing showing a cross section through the center of a primary well center in the form of a rotary table supported on a transferable skid base on tracks arranged in the drill deck, with a diverter housing suspended from underneath the said transferable skidbase and with a riser supported by in-line hydraulic riser tensioners mounted on a separate tracks below the drill deck.

Fig. 8 is a concept drawing showing a transferable primary well centre, diverter system and riser tensioning system.

Fig. 9 is a concept drawing showing a transferable diverter system and riser tensioning system.

Figs. 10-18 illustrate another embodiment of an offshore drilling rig, wherein fig. 10 shows a side view of the drilling rig, figs. 11-14 show 3D views of parts of the drilling rig from different viewpoints, figs. 15-16 show horizontal cross-sectional views of the drilling rig, and figs. 17-18 show lateral cross sections of the drilling rig.

Fig. 19a-c shows an exemplary embodiment of the invention in the normal operating position Fig. 19a and the heave operating position Fig. 19c and the intermediary step from Fig. 19a to 19c. Elements 1903 a/b represent a first and second goose neck, respectively and 1904 a/b represent the first and second service loops respectively.

Description of exemplary embodiments:

Fig.1a, 1b, 2a, 2b and 4 all shows a drilling support structure 1 arranged above a drill deck 2 and three work centers 3a, 3b and 3c, where one is in the form of a primary well center 3a being equipped with a diverter housing 13. The three work centers 3a, 3b and 3c are supported on individual skid-bases on tracks 4 arranged in the drill deck 2, and the drilling support structure 1 carries two cable crowns 5a and 5b on fig. 1a, 1b, 2a, 2b and 4 in the form of a crown sheave cluster, and on fig. 3 in the form of a crown block, being skidably arranged on the top of the drilling support structure 1 on separate tracks.

From each of the crown sheave clusters 5a and 5b lifting cables 7a and 7b are running down and connecting to a load carrier 8a and 8b each carrying a top drive 9a and 9b at the end of the lifting cables 7a and 7b. Each of the top drives are connected via a retractable dolly 10a and 10b to a vertical track 11 a and 11 b arranged at the drilling support structure 1. The retractable dollies 10a and 10b are each adapted so that they can position and keep the top drives in different positions above the work centers 3a, 3b, 3c in the drilling deck 2.

In the embodiment shown on fig. 1a, 1b, 2a, 2b, and 4 each hoisting system has a linear actuator in the form of a hydraulic cylinder 28a, 28b, having its lowermost end 29a, 29b fixed with respect to the drill deck 2 and an upper travelling end 30a, 30b with a cable sheave 31a, 31b. At least one lifting ca- ble 7a, 7b has one end extending from another hydraulic cylinder 32a, 32b arranged for compensating heave during e.g. drilling operation, and over the travelling cable sheave 31a, 31b and further below a second cable sheave 33a, 33b being fixed with respect to the drilling support structure 1, and thereafter over the crown sheave cluster 5a, 5b. skidably mounted on the drilling support structure 1 on a track 26. In these figures only a single lifting cable is shown for each hoisting system, but in practice it is necessary in order to provide significant lifting capacity, as well as redundancy in case that one cable breaks, to have multiple mutually parallel lifting cables extending along with the lifting cables 7a, 7b.

Alternatively the drilling support structure 1 shown on fig. 3 has a hoisting system with two cable crowns 5a, 5b each in the form of a crown block being connected to a travelling block 34a, 34b via multiple cable loops hanging down from the crown block 5a, 5b, In this embodiment each travelling block is carrying a topdrive 9a, 9b. In this embodiment a single lifting cable 7a, 7b is providing the multiple cable loops, and thereby the necessary cable lifting capacity of the hoisting system, and therefore, in order to provide the necessary travelling length of the travelling block, a cable winch 27a, 27b is arranged for each hoisting system.

The embodiment shown in fig. 3 shows a dual cable winch 27a, 27b system for each of the two hoisting systems for providing better hoisting speeds and for adding redundancy to the active heave compensating winches. However, one of the cable winches on each or both of the two hoisting systems may be replaced by a deadline compensating system (not shown in fig. 3) at the dead end of the lifting cables 7a, 7b, so that it is not necessary to use the cable winches 27a, 27b for heave compensation.

The skilled person will, however, appreciate that the mere combination of the skidable crown sheave clusters 5a, 5b shown on figure 1a, 1b, 2a, 2b and 4 and the linear actuators 28, disregarding other features of the present invention, mutatis mutandis, provides both an efficient and safe lifting capacity, because each hoisting system may comprise multiple lifting cables 7a, 7b extending parallel to each other’s in order to carry the same load carrier 8a, 8b or the same topdrive 9a, 9b.

For the same reason each cable winch 27a, 27b shown in figure 3 may therefore comprise more than one cable drums arranged on the same axle, and each drum containing a cable so that two or more cables may be arranged parallel to each other, and so that one cable winch 27a, 27b simultaneously rotates all cable drums and thereby wind or unwind all the cables simultaneously. Thereby the hoisting system comprises multiple lifting cables 7a, 7b extending parallel to each other’s in order to carry the same travelling block, or the same topdrive 9a, 9b. thereby a similar cable redundancy is obtained with the cable winch based hoisting system shown on fig. 3 as is the case with the hoisting systems described in relation to the embodiments shown on fig. 1a, 1b, 2a, 2b, and 4.

In figure 4 and 5 a connecting tool 12 is connecting the load carriers 8a and 8b via the top drives 9a and 9b in fig. 4 and a single topdrive 9a in fig. 5. Thereby it is possible to connect a load to the connecting tool 12, so that it is possible to provide a lifting force by combining the lifting force of both hoisting systems lifting the two load carriers 8a and 8b.

Figure 6 shows the conceptual layout of a preferred embodiment of a diverter system attached to the primary well center 3a such as it is shown on fig. 7. This diverter system comprises a diverter housing 13 suspended from the skidbase 25 and supporting the rotary table 14 of the primary well center 3a.

The diverter housing 13 has at least two outlet ports 17a and 17b each being connected to telescopic overboard lines 18a and 18b. This allows the diverter housing 13 to be positioned at different positions along a line defined by the track 4 being parallel to the overboard lines. One such position is shown in fig. 6 in full line, and another is shown with dotted lines.

The diverter housing 13 also comprises a mud return outlet port 19 adapted for leading drilling mud from the diverter housing back to the mud process systems via the main mud return line system 20. The mud return line comprises a number of telescopic connectors 21 a 21 b and 21c arranged at selected positions in order to connect the mud return line to the mud return outlet port 19 on the diverter housing 13.

In figure 7 a primary well center 3a is shown I more detail comprising the above mentioned components and parts, and in this figure a riser tensioner system is also shown comprising skidding carriages 22 and hydraulic in-line tensioning cylinders 23 being skidably supported by a separate riser tensioner track 24 arranged parallel to and below the track 4 supporting the rotary table skidbase 25. Thereby the riser tensioners 23 may be moved along with the rotary table and diverter housing 13 or independently of the rotary table 14 and diverter housing 13.

In the following different modes of operating the drilling rig shown in the figures are disclosed in more detail with reference to the relevant figures. 1. Full redundancy:

With reference to fig. 1a and 1b especially, but not only, a fully redundant dual activity hoisting- and drilling facility is provided.

Full redundancy is achieved by having a transferable, riser-capable primary well center 3a, which may be positioned under either one of the top drives (e.g. 9a or 9b) and load carriers (8a and 8b) of the two fully rated main hoist ing- and drilling systems comprising the facility. In this relation the primary well center may be transferred and positioned as mentioned above, with or without one tubular or a string of tubulars 35, such as riser tubes, casings, drilling pipes or the like being supported and/or hanging down from the primary well center, and these tubulars may be either hanging freely down from the primary well center, or they may extend all the way to the sea floor and further extend into the well or be connected to the well at the sea floor. In the latter case a diverter system and a tensioning system as shown on figures 6 and 7 respectively may be employed along with other well control equipment.

The riser-capable primary well center 3a comprises a rotary table 14 supported by a horizontally transferable skid/trolley 25 that is sunk into a slot in the drill deck 2, so that the rotary table 14 top cover is substantially flush with the drill deck 2 level.

The transferable skid/trolley 25 is resting on horizontal skid beams forming a track 4 spanning the width between the two fully rated main hoisting- and drilling systems 9a and 9b. A diverter housing 13 with telescoping overboard lines (overboard tubing 18a and 18b) and a detachable main flowline (mud return tubing 20) is suspended from underneath the said transferable skid/trolley 25. A transferable/skidding riser tensioning system is arranged on horizontal skid beams suspended from underneath the drill deck structure, while spanning the full width between the two fully rated main hoisting- and drilling systems.

In the following, examples are given for the intended operation to ensure full redundancy e.g. in the case some of the equipment related to a well center in operation, such as fully rated main hoisting- and drilling system, suffers a main equipment breakdown: 1. While drilling, tripping drill pipe or running casing during the riserless top-hole sections of the well. 1.1 the drillpipe or casing string is hung off in the power-slips/casing-spider 1.2 the drillpipe or casing string, while being suspended from the pow-er-slips/casing-spider inside the rotary table, will be transferred to the opposite fully rated main hoisting- and drilling system 1.3 drilling, tripping drill pipe or casing running operation may resume on the opposite fully rated main hoisting- and drilling system

This method is applicable for a riser enabled skidable well center as well as well center without the ability to connect a riser or otherwise receive return mud.

2. while running or retrieving riser and BOP 2.1 the riser string and BOP is lowered and hung-off in the riser spider and gimbal, which is resting on top of the rotary table. 2.2 the riser string and BOP, while being suspended from the riser spider and gimbal, will be transferred to the opposite fully rated main hoisting- and drilling system 2.3 running of the riser may resume on the opposite fully rated main hoisting- and drilling system 3. while drilling, tripping drill pipe or running casing through the riser and BOP after this has been connected to the well and the riser has been put in tension 3.1 the drillpipe or casing string is hung off in the power-slips/casing-spider 3.2 the well is secured 3.3 the drillpipe or casing string, while being suspended from the pow-er-slips/casing-spider inside the rotary table, will be transferred to the opposite fully rated main hoisting- and drilling system 3.4 the diverter housing with telescoping overboard lines and detachable main flowline, suspended from below the transferable skid/trolley supporting the rotary table, will be transferred to the opposite fully rated main hoisting- and drilling system 3.5 in fully synchronous motion, the riser string, while being suspended from the riser tensioners will be transferred to the opposite fully rated main hoisting- and drilling system 3.6 drilling, tripping drill pipe or casing running operation may resume on the opposite fully rated main hoisting- and drilling system

The fully redundant dual activity hoisting- and drilling facility illustrated in figures 1 a and 1 b will allow for continued operation on either port or starboard side hoisting- and drilling system (by repositioning of the primary well center), while the opposite side is decommissioned for any extended period of time, e.g. for Class required Special Periodic Survey, breakdown or other reason.

Providing redundancy or flexibility, in general, by providing a skidable well-center is applicable to any configuration of hoisting equipment including any embodiments of the present text. For instance, for a drilling rig with a single hoisting system the invention allows for one work hole/well center to replace the primary well center such as a fox hole or another well center. This may be useful for rigging up equipment or performing drilling operations on another well while other operations are performed in the first well such as running coil tubing. In a rig with dual (or more) lifting systems the skidable well center may furthermore provide the flexibility of changing the position of the well center to one of the other hoisting systems. As mentioned, this may be beneficial during service and/or break down of equipment such as the hoisting system, top drive, service loop to the top drive (or any part of the system supplying the service loop) or the dolly for the top drive. 2. Cyclic Dual Hoisting:

With reference to fig. 2a and 2b especially, but not only, a cyclic dual hoisting facility is provided especially for Fast Tripping, Casing- or Riser Running:

Fast tripping, casing- or riser running operation is ensured by having two independent and fully redundant hoisting- and drilling systems (top drives 9a and 9b) working in cyclic operation over a common primary well centre 3a.

Each hoisting and drilling system is shown comprising a main hoisting system with a horizontally transferable (via the positioning system) cable crown in the form of a crown sheave cluster arrangement 5a, 5b, allowing the crown sheave cluster 5a, 5b to be horizontally transferred, aligned and locked into position over at least two independent positions/well centers 3a, 3b, 3c on the drill deck 2 below.

Each hoisting system supports a vertically travelling load carrier 8a, 8b arrangement, from which a topdrive 9a, 9b is suspended below on a horizontally extend-/retractable dolly 10a, 10b system for guiding of the topdrive 9a, 9b.

The extend-/retractable dolly 10a, 10b has a horizontal travel corresponding with the horizontal travel of the crown sheave cluster 5a, 5b arrangement above, while extending/retracting horizontally in synchronous motion with the skidding crown sheave cluster 5a, 5b arrangement above, ensuring that the hoisting system is kept in true vertical alignment with the cable sheave clus-ter/load carrier/yoke/hook arrangement and the topdrive 9a, 9b suspended underneath it.

In the following typical examples are given for the intended cyclic operation of the dual hoisting and drilling facility to provide fast tripping, casing or riser running operation: 1. Tripping/running in the hole: low setback and pipe racking system off-drill deck. 2. Tripping/running in the hole: setback and pipe racking system on-drill deck.

Fast running or retrieval of the riser and BOP may be performed in a similar cyclic operation, while employing suitable arrangements for facilitating handling of the riser joints to/from the primary well centre 3a with this being in the center position

An added benefit of the invention is the full redundancy provided within this facility in that each hoisting and drilling system will offer full redundancy for the other system in the center operating position, without repositioning of the primary well centre 3a. 3. Synchronous Dual Hoisting:

With reference to fig. 4 especially, but not only, a synchronous dual hoisting facility is provided for heavy duty well construction.

The synchronous hoisting facility is realized by utilizing the two independent and fully redundant hoisting- and drilling systems in a combined synchronous mode of lifting operation above the common primary well center 3a, by using a connecting tool 12.

In fig. 4 the synchronously hoisting facility comprises two topdrives, but as shown in fig. 5 it is possible to operate the hoisting systems synchronously even when only one topdrive 9a is used. In this situation the connecting tool 12 is carried by the topdrive 9a on one side, but is directly connected to the hoisting cable via the load carrier 8b at the other side.

This principle allows for extra heavy duty lifting operation without necessitating any of the two hoisting- and drilling systems to be rated beyond the current design loads of such equipment, where especially the load capacity of the topdrives 9a, 9b are limiting the load capacity of the hoisting systems.

Recurring requests for rigs capable of running extended sections of heavy wall casing strings through deep formations in ultra-deep water may require lifting facilities of 1500 metric tons SWL or beyond.

Current designs of hoisting and drilling systems are limited to approx. 1200 metric tons only, with systems and equipment currently under design and development for up to 1500 metric tons.

Consequently, the next generation of DW drilling rigs may only provide incrementally larger hoisting capacity compared with the current generation of rigs and will therefore restrict well designs to within the 1500 metric tons limit of the next generation of top drives.

However, this invention will allow for hoisting and lowering loads exceeding 2000 metric tons, limited only by the structural integrity and load carrying capacity of the casing and landing string tubulars, running- and handling tools. A generic ultra-deep subsalt and/or HPHT (High Pressure High Temperature) well development program in ultra-deep water might utilize all aspects of the invention to their full potential through the following steps and transitions between modes of operation: 1. Dual activity operation for concurrently drilling tophole sections, while running and cementing casing down to and including the first casing casing section (typically 18” but further casing section could in principle be set as well and other diameters may apply). 2. Dual activity operation for concurrently running riser and BOP, while cementing this first casing section, incorporating full redundancy. 3. Transferring the primary well center 3a with riser and BOP suspended to the center position with subsequent landing of the BOP. 4. Drilling, tripping and running casing in cyclic operation through riser with primary well center 3a in center position 5. Running and landing extra-long heavy casing sections in synchronous dual hoisting mode

Fig. 10-18 is a concept drawing showing a drill ship adapted for dual activity operation, and having a first and a second hoisting system supported by the drilling support structure 1, the first hoisting system and each being adapted for raising or lowering a first load carrier, and the second hoisting system being adapted for raising or lowering a second load carrier, respectively. A top drive 9a, 9b is suspended from each of the load carriers, and the offshore drilling rig comprises a positioning system adapted for selectively positioning at least the one load carrier or the primary well center selectively in at least a first or a second horizontal position different from the first horizontal position, where the first load carrier in the first horizontal position is positioned above the primary well center, and in the second horizontal position is positioned above the other work center.

In particular, Fig. 8 shows a part of the drill deck 2 including a displacable primary well center 3a. The well center is defined by hole of a rotary table 14 that is skidable along tracks 4 in the drill deck. In the example of Fig. 8 the rotary table has a top surface that is flush with the upper surface of the drill deck, and the drill deck defines a slot 40 having a width matching the size of the rotary table. In other embodiments, the slot may be narrower, e.g. by letting the drill deck surface extend partially across the rotary table. Alternatively or additionally, the slot 40 may be covered by plates or hatches which may be removed during the skidding of the well centre. The drilling rig of Fig. 8 further comprises a diverter system comprising a diverter housing 13 from which outlet ports 17 and 19 extend. The diverter housing is mounted below the well centre 3a and arranged to be skidable together with the well center 3a. For example, the diverter housing may be suspended from a skidbase supporting the rotary table, as described in connection with Fig. 6 above. The drilling rig of Fig. 8 further comprises a riser tensioning system comprising hydraulic in-line tensioning cylinders 23 being skidably supported by a separate riser tensioner track (not explicitly shown in Fig. 8) arranged parallel to the track 4 supporting the rotary table 14, e.g. as described in connection with Fig. 6. Thereby the riser tensioners 23 may be moved along with the rotary table and diverter housing 13. The riser tensioning cylinders are, at their lower end, connected to a tensioner ring 41 which engages a marine riser string 15 so as to control the tension on the riser string. In the example of Fig. 8, the rotary table 14, the diverter housing 13 and the riser tensioners 23 with the riser string 15 suspended from it may individually or jointly be skidded in the direction indicated by an arrow in Fig. 8. As is further illustrated in Fig. 8, the above components may even be jointly skidded while a string of pipe 35 is suspended in the rotary table 14 and extends downward through the riser 15.

Fig. 9 is a concept drawing showing a transferable diverter system and riser tensioning system. In the example of Fig. 9, the drilling rig comprises two stationary rotary tables 14a,b, each defining a work center 3a,b, respectively, that is operable as a primary well center. The drilling rig of Fig. 9 further comprises a diverter system comprising a diverter housing 13 from which outlet ports 17 and 19 extend. The diverter housing is mounted below the drill deck 2 and arranged to be skidable along tracks 95 extending between the rotary tables 14a,b. To this end, the diverter housing comprises carriages 94 movably attached to the tracks 95. The drilling rig of Fig. 9 further comprises a riser tensioning system comprising hydraulic in-line tensioning cylinders 23 being skidably supported by a separate riser tensioner track (not explicitly shown in Fig. 8) arranged parallel to the track 94 that supports the diverter housing 13, e.g. as described in connection with Fig. 6. Thereby the riser tensioners 23 may be moved along with the diverter housing 13 between positions under the respective work centers 3a,b. The riser tensioning cylinders are, at their lower end, connected to a tensioner ring 41 which engages a marine riser string 15 so as to control the tension on the riser string. In the example of Fig. 9, the diverter housing 13 and the riser tensioners 23 with the riser string 15 suspended from it may jointly be skidded in the direction indicated by an arrow in Fig. 9. Consequently, the work centers may selectively be operated as primary well center so as to obtain an increased redundancy, e.g. in case of a failure of a hoisting system that operates above one of the work centers.

Figs. 10-18 show another embodiment of a drilling rig, in this example of drill-ship having a hull 1501. In particular, Fig. 10 shows a side view of the drilling rig, Figs. 11 and 12 show views of the drill floor seen from the starboard side of the drillship, Figs. 13 and 14 show views of the drill floor seen from the port side of the drillship (a part of the hull of the ship is cut away in Fig. 14); Figs. 15 and 16 show horizontal cross sections in a plane above the drill deck and a plane below the drill deck, respectively; finally, Figs. 17 and 18 show lateral cross sections of the drill ship.

The drilling rig of the present embodiment comprises a drill deck 2 formed on top of a substructure 1597. The substructure comprises a platform supported by legs. The platform defines the drill deck and spans across a moon pool 2122 formed in the hull of the drillship. The drill deck 2 comprises two holes defining well centres 3a,b. The drilling rig comprises a drilling support structure in the form of a mast 1. In the present example, the well centres are located within the footprint of the mast 1. The mast includes two mast portions, each associated with, and adjacent to, one of the well centers. The dual activity mast 1 is supported by the substructure 1597 and extends upwardly from the drill deck 2. The mast comprises two mast portions arranged in a face-to-face configuration, i.e. the respective mast portions are located along the axis connecting the well centres such that both well centres are located between the mast portions. Each mast portion supports a hoisting system, each for lowering a drill string through a respective one of the well centres 3a,b towards the seabed. In the example of Figs. 10-19, the drilling rig comprises two well centres, one of which being operable as a primary well centre 3a and being equipped with a diverter housing 13. The primary well centre 3a is supported on a skidbase 25 on tracks 4 arranged below the drill deck (e.g. as shown in more detail in Figs. 7 and 8) so as two allow the well centre and the diverter housing 13 to be displaced along the direction connecting the two well centres. Alternatively, both work centres may be provided with a diverter and be operable as primary well centers. The skidbase extends across the moon pool and the tracks are mounted on opposite sides of the moonpool and extends along the direction connecting the well centers. The drilling rig may further comprise a skidable riser tensioning system as described in connection with Fig. 7 and 8. The primary well center 3a may be moved between a first, central horizontal position, as shown in figs. 12 and 13, and a second, peripheral position 1003c, where the first position is located on the axis connecting the second position 1003c and the work centre 3b. In the present example, the first position is positioned substantially in the centre between the second position 1003c and the work center 3b. The position not currently occupied by the displacable well centre (e.g. the second position 1003c in figs. 12 and 13) may be covered by floor plates or a similar cover 1584. In alternative embodiments, both well centres may be displacable. In yet another embodiment, the drilling rig may comprise three well centres, e.g. aligned along a common axis. Each of the two hoisting systems may be operable to lower tubulars selectively through a work centre at each of at least two horizontal positions, such as the central position (where the primary well center 3a is located in the example of fig. 12) and one of the peripheral positions (the position of the work center 3b and the second position 1003c). To this end, the mast 1 carries two cable crowns 5a, b, e.g. in the form of a crown sheave cluster or in the form of a crown block, being skidably arranged on the top of the mast on separate tracks, so as to enable that each of the cable crowns 5a, 5b may be shifted horizontally and allowing e.g. that a selected one, or both, of the cable crowns 51, 5b is positioned above one specific well center or work center 3a, 3b.

From each of the cable crowns lifting cables 7a,b are running down and connect to a corresponding top drive 9a,b which is suspended from a hook or other load carrier connected to the lifting cables. Each of the top drives is connected via a retractable dolly 10a,b to a vertical track arranged at the mast 1. The retractable dollies are each adapted so that they can position and keep the top drives in different positions above the well centers, as described herein.

Each hoisting system has one or more linear actuators in the form of a hydraulic cylinder 28a,b having its lowermost end fixed with respect to the drill deck and an upper travelling end with a cable sheave. At least one lifting cable has one end extending from another hydraulic cylinder arranged for compensating heave during e.g. drilling operation, and over the travelling cable sheave and further below a second cable sheave being fixed with respect to the mast, and thereafter over the cable crown. The hydraulic cylinders are displaced from the well centres along the direction connecting the well centres and positioned such that both well centres are located between the cylinders of the respective hoisting systems. As can be most easily seen on fig. 15, the cylinders of each hoisting system are further (optionally) arranged in two groups of cylinders positioned on either side of an axis connecting the well centres so as to form a gap through which a catwalk machine 1508 or other pipe handling equipment can travel and feed tubulars to one or both of the well centres. Each cable crown 5a,b defines an axis that is parallel to the direction connecting the two groups of cylinders of one of the hoisting systems.

As is most easily seen in fig. 12, both hoisting systems may cooperate so as together to lower or raise tubulars through the same well centre, e.g. the primary well center when located at a central position as illustrated in fig. 12. To this end, a connecting tool 12 may be arranged to connect the top drives 9a,b. In this example, the connecting tool is in the form of an elevator and bail sections connected to said elevator in one end and suitable for being lifted by second elevators each connect to a top drives 9a,b via bails in the conventional manner. A stabbing and circulation device (e.g. in the form a

Casing Fill-Up and Circulating System tools or FLOW BACK & CIRCULATION TOOLS FOR DRILL PIPE (CFT)) is mounted between the bail sections and further connected to a mud connection, preferably of one or both (as illustrated here) of the top drives. Thereby it is possible to connect a load to the connecting tool 12, so that it is possible to provide a lifting force by combining the lifting force of both hoisting systems lifting the connecting tool. To better support increased loads, the mast comprises diagonal beams 1578 forming an inverted V. In alternative modes of operation, the two hoisting systems may be operated above respective well centres or they may be operated in a cyclic dual hoisting mode over a single well centre, e.g. as described herein.

The drilling rig further comprises a pipe storage area 1509 for storing pipes in horizontal orientation and catwalk machines 1508 or other horizontal pipe handling equipment for transporting pipes between the storage area 1509 and the well centers 3a,b. To this end, the catwalk machines are aligned with the axis defined by the two well centres.

The drilling rig comprises a setback structure 1812 or similar pipe storage structure for storing stands of tubulars below the substructure 1597 and partly covered by the drill deck 2. The setback structure comprises a support framework 1890 supporting fingerboards having horizontally extending fingers between which tubulars may be stored. The setback structure is arranged so as to allow stands to be moved to/from both well centres from/to the setback. To this end, one or more column rackers 1891 or similar vertical pipe handling equipment may be arranged to move stands into and out of the setback structure 1812. The setback structure 1512 further comprises stand building equipment 1877 configured to build stands from individual pieces of pipe. The setback structure 1812 is located adjacent the moon pool 2122 laterally displaced from the axis defined by the well centers.

Moreover the drilling rig comprises one or more further catwalk machines 1876 configured to feed tubulars from the pipe storage area 1509 or from other storage areas on the opposite side of the mast (towards the aft of the ship) to the stand building equipment 1877. The stand building equipment 1877 may thus receive the pipes from the catwalk machine 1876, bring them in upright orientation, and connect them to other pieces so as to form stands. To this end the stand building equipment may comprise a mousehole through which the stand may be gradually lowered while it is made up until the lowermost end of the stand is at the lowermost level of the setback area 1812, while the uppermost end of the stand is below the drill floor level. The stands may then be received by pipe rackers 1891 and placed in the setback structure 1812 for future use. To this end the pipe rackers are operable to traverse across the setback area, e.g. in the direction parallel to the direction connecting the well centres.

The drilling rig comprises a number of slanted chutes 1892 each for feeding pipes from the setback area 1812 to one of the well centers. To this end the drilling rig may comprise one chute for each well center position, i.e. either the fixed well-center positions or the positions to which a skidable well center can be moved. Alternatively, the chutes may be displacable so as to be selectively aligned with respective well centres. Each chute 1892 receives pipes from one of the pipe rackers 1891 and feed the pipes in a slanted upward direction through a corresponding slit 1785 in the drill floor towards a respective one of the well centres 3a,b, where they are picked up at their uppermost end by the corresponding hoisting system and lifted through the slit 1785 until they are vertically suspended above the corresponding well center. To this end, the drilling rig further comprises pipe handling equipment 1786 operable to guide the pipes while they are being lifted through the slit 1785. The slits 1785 are elongated and point away from the axis connecting the well centers and towards the side where the setback area 1812 is positioned. To allow for the pipes to be presented in this fashion, the driller’s cabin 1534 is positioned at an elevated level above the slits 1785. One or more further pipe handling devices, such as iron roughnecks 1727, may be located between neighbouring slits and underneath the driller’s cabin, e.g. such that each iron roughneck may service two well center positions.

The drilling rig comprises another storage area 1515 below the drill deck 2 and configured for storing risers in a vertical orientation. The riser storage area 1515 is located adjacent the moon pool 2122, e.g. on the side of the moon pool opposite the setback structure 1812. The risers may then be moved, e.g. by means of a gantry crane 2298 and respective chutes 2294 or other suitable pipe feeding equipment through holes 1681 in the drill deck floor. The riser feeding holes 1681 may be covered by removable plates, hatches or similar covers, as illustrated in e.g. Figs. 13 and 15. The riser feeding holes are displaced from the axis connecting the well centers.

As the stands of tubulars and the risers are stored below the drill deck, and since the cat walk machines 1508 extend towards opposite sides from the well centers, and since the mast structure 1 is located on one side of the well centres, 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 both well centres 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. Moreover, at least parts of the setback structure 1812 may be covered by a platform 1788 so as to provide additional storage or working area.

Detachable vertical guide system for synchronous lifting and/or redundancy

Fig. 19a-c illustrates embodiments where the vertical guide system for one or both top drives is detachably mounted (typically a rail system directly attached to the top drive or a dolly 10a, 10b on a vertical track or guide rail 11 a, 11b). The embodiments of Fig. 19a-c may be combined with any of the embodiment portrayed in any of the previous figures. In some embodiments the rig comprises one or more skidable well center and/or 1,2, 3 or more fixed work centers for added redundancy or flexibility as above. The advantages of cyclic dual hoisting may be enjoyed for any well center within reach of two dollies such for example a third well center (not shown) between the first well/work center 3a, 1901a and the second well/work center 3b, 1901b. The advantages and details of the synchronous lift may also be applied to these embodiments (see Fig. 19c illustrating a synchronous lift with a connecting tool 1908).

Fig. 19a illustrates a dual activity rig similar or corresponding to any of Fig. 1-4 and/or 10-18. The configuration of Fig. 5 may also be applied to the Fig. 19a-c be removing one of the top drive 9a or 9b. The lifting capacity of the hoisting system will in this case be increased as the load carrier 8 (8a or 8b) no longer has to carry the top drive 9 and potentially further increased if the lifting capacity is otherwise limited by the load rating of the top drive. On the other hand removing a top drive is often a time consuming and cumbersome process and therefore often advantageously avoided. The first and second well centers 3a and 3b may be skidable/movable as in Fig. 1-4 or fixed. They are preferably both primary well centers but may also be one primary well centre and one other type of work center. As with the previous discussed embodiments, several types of hoisting systems may be applied such as cylinder, draw works or winch based. Further to the previously discussed features the service loops 1904a,b are outlined along with their goosenecks 1903a,b. A service loop is an umbilical supplying the top drive with electrical, hydraulic and or compressed air power (see e.g. US Patent 7,918,270). In the present example the service loops runs via the goosenecks 1903a,b to the dollies 10a and 10b and from there to the top drives 9a and 9b.

Fig. 19b illustrates a step in the method of combining the two top drives to connect the second top drive to the vertical guide system of the first top drive. In general, some embodiments of the present invention relates to a method of operating a drilling rig comprising: a. operating the drilling rig in normal-operation position (such Fig. 19a) and lowering into and/or raising tubulars out of the first work center 3a and optionally also the second work 3b center (simultaneously and/or sequentially) via the first and optionally the second top drive 9a 9b, respectively; b. placing the first and second load carriers 8a and 8b in the joint-operation position by i. disconnecting the second top drive 9b from the second vertical rail system 10b, 11b; ii. lower the first top drive 9a; iii. shifting the second top drive 3b via the positioning system to a position next to the first top drive 9a (see Fig. 19b where the top drive 9b is being moved to the right); iv. mechanically fixing the second top drive to the first top drive (via the clamp 1906) and/or connecting the second top drive to a vertical rail system different from the second vertical rail system (not shown); c. operating the second hoisting systems so that the second load carrier lift load in or out of the first work center, such as operating the first and second hoisting systems so that the first and second load carriers jointly lift loads in or out of the first work center (see Fig. 19c).

In Fig. 19c the second top drive 9b is connected to the first top drive 9a via a clamp 1906. This clamp may be any suitable mechanical connection between the two top drive but should preferably be strong enough to handle the forces imposed by the relatively heave top drive (similar to the second vertical guide system) - particularly if the rig is a exhibits heave, pitch and roll during operation which is typically the case for drill ships and semi submersibles.. Furthermore, the first vertical guide system (i.e. the dolly 10a and track 11a) should be arranged to handle the forces from both top drives. In combination or alternatively another vertical guide system (not shown) may be arranged to guide the second top drive. This third vertical guide system may for example be arranged coaxially with the second vertical guide system and comprises a dolly for connecting to said second top drive around the first dolly.

If the second top drive is moved over to the first well center 3a to replace the first top drive 3a and corresponding hoisting system instead, the second top drive may be either connected to (i) the first top drive as in Fig. 19c but instead aligned directly over the first well center (as opposed to next to the first well center to allow spacing for the connection tool typically used in the synchronous lift), (ii) connected to a third vertical guide system with the first top drive preferably moved out of the way or (iii) to the first vertical guide system after the first top drive has been disconnected. The second top drive may stay connected to its own service loop 1903b or be connected to the service loopl 903a of the first top drive.

Depending on the design of the top drive the top drive may be suitable for connecting to the vertical guide system from either side; the top drive and the clamp 1906 are preferably made so that the connection between the top drive and the clamp is similar to that of the respective vertical guide system (dolly). In this way both top drives can conveniently be connected to each other or to each of the vertical guide systems. The load carriers, service loop and/or top drives are in some embodiments arranged to allow the top drive to rotated 180 degrees so that the side normally connected to the vertical guide system can be made to face the other top drive or the other vertical guide system.

As previously mentioned, it may be time consuming to disconnect a top drive and a part of this is the disconnection of the service loop. In some embodiments (not shown) the second top drive may be disconnected from its service loop 1903b and connected to another service loop at first well center. This may require that also the control systems for the drilling equipment is arranged to allow such a disconnect/connect operation. On the other hand, for the case where the second top drive replaces the first top drive it may be preferable that the control previously used to controls the first top drive are now controlling the second top drive over the first well center. The service loop is shown connected to the second top drive via the dolly and the dolly 10b is arranged to move up and down in coordination with second top drive 9b to keep the service loop hanging over the drill floor relatively short.

To control the relatively large weight of the second top drive as it traverses toward the first well center 3a, the second top drive is preferable lowered onto a cart 1907 on the drill deck. The cart is preferable arranged to receive and carry at least part of the weight of the second top drive. The cart may have wheels or other mounted onto a track in the drill deck. Once the weight of the second top drive is controlled by the cart the crown block 5b is shifted towards the right along and the cart 1907 follows. An optional second cart (not shown) may be used to receive and hold the first top drive when it is connected to the second top drive but this cart is often omitted when the first top drive is still connected to its vertical guide system.

The first top drive is also shifted to the right hand (i.e. in a direction on the other side of the first work center relative to the second top drive) side to allow room for the connecting tool 1908 to aligned with the first work/well center 3a. The positioning system may shift the cable crown 5a along with retracting the arm of the dolly 10a correspondingly, but this shift could in principle also be imposed by the dolly alone. The connecting tool 1908 was dis cussed in relation to the other figures (see e.g. Fig. 4, 5, and 12) as well as in co-pending Danish patent application PA 2014 70131.

Although some embodiments have been described and shown in detail, the invention 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 utilized and structural and functional modifications may be made without departing from the scope of the present invention.

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.

Claims (40)

1. An offshore drilling rig comprising; - a drill deck; - a first work center and a second work center horizontally spaced apart from each other, - a drilling support structure extending upwardly relative to the drill deck; - a first and a second hoisting system supported by the drilling support structure, the first hoisting system being adapted for raising and lowering a first load carrier, and the second hoisting system being adapted for raising and lowering a second load carrier; - a first top drive suspended from the first load carrier and a second top drive suspended from the second load carrier; - a first vertical guide system connectable to the first top drive to guide said first top-drive in a position over said first work center and a second vertical guide system connectable to the second top drive to guide said second top drive in a position over said second work center; and where the offshore drilling rig is arranged with a positioning system adapted for selectively positioning said second load carrier and, optionally, said first load carrier a) in a normal-operation position with the first and second load carriers positioned over said first and second work centers, respectively, to allow the first load carrier to lift a load in or out of the first work center via said first top drive connected to the first vertical guide system and to allow the second load carrier to lift a load in or out of the second work center via said second top drive connected to the second vertical guide system; and b) in a joint-operation position with the first and second load carriers positioned to allow the first and second load carrier to jointly lift a load in or out of the first work center wherein the second top drive is disconnected from the second vertical guide system.

2. The offshore drilling rig of claim 1 wherein the positioning system is configured to place the first and second load carriers proximal to and on either side of the first work center when positioned in the joint-operation position.

3. The offshore drilling rig of claim 1 or 2 wherein the drilling rig is arranged so that, when positioned in the joint-operation position, the first and second load carrier are configured to jointly lift a load in or out of the first work center via said first and second top drive.

4. The offshore drilling rig of one or more of the preceding claims wherein the second top drive is connectable to the first top drive and/or the second vertical guide system and/or a third vertical guide system when positioned in the joint-operation position.

5. An offshore drilling rig comprising; - a drill deck; - a first work center and a second work center horizontally spaced apart from each other, and wherein at least said first work center is a primary well center; - a drilling support structure extending upwardly relative to the drill deck; - a first and a second hoisting system supported by the drilling support structure, the first hoisting system being adapted for raising and lowering a first load carrier, and the second hoisting system being adapted for raising and lowering a second load carrier; - a first top drive suspended from the first load carrier and a second top drive suspended from the second load carrier; - a first vertical guide system connectable to the first top drive to guide said first top-drive in a position over said first work center and a second vertical guide system connectable to the second top drive to guide said second top drive in a position over said second work center; and where the offshore drilling rig is arranged with a positioning system adapted for selectively positioning said second load carrier and, optionally, said first load carrier a) in a normal-operation position with the first and second load carriers positioned over said first and second work centers, respectively, to allow the first load carrier to lift loads in or out of the first work center via said first top drive connected to the first vertical guide system and to allow the second load carrier to lift loads in or out of the second work center via said second top drive connected to the second vertical guide system; and b) in a redundancy-operation position with the second load carrier positioned to allow the second load carrier to lift loads in or out of the first work center wherein the second top drive is disconnected from the second vertical guide system.

6. The offshore drilling rig of one or more of the preceding claims wherein said first work center is a primary well center;

7. The offshore drilling rig of one or more of the preceding claims wherein the second work center is a well center, such as a primary well center.

8. The offshore drilling rig of one or more of the preceding claims wherein the rig is arranged to allow operation in the normal-operation position, the joint-operation position and the redundancy-operation position.

9. The offshore drilling rig of one or more of the preceding claims wherein each of the hoisting systems further comprises a lifting cable hanging over at least one cable crown being supported by the drilling support structure and each being adapted for raising or lowering one of said load carriers and wherein positioning of said load carriers by said positioning system comprises positioning of said cable crown.

10. The offshore drilling rig of one or more of the preceding claims further comprising, when operating in the joint-operation position, a connecting tool having two opposite ends, each being adapted for being directly or indirectly connected to one of the load carriers, and where the connecting tool has an intermediate load carrier arranged between said two opposite ends and being adapted for carrying a load.

11 .The offshore drilling rig of one or more of the preceding claims wherein the second top drive is mechanically attached directly to the first top drive in the joint-operation position.

12. The offshore drilling rig of one or more of the preceding claims wherein the first top drive is connected to the first vertical guide system in the joint-operation position.

13. The offshore drilling rig of one or more of the preceding claims wherein the second top drive is connected to the first vertical guide system in the joint-operation position.

14. The offshore drilling rig of one or more of the preceding claims wherein the second top drive is connected to a third vertical guide system in the joint-operation position.

15. The offshore drilling rig of claim 14 wherein the third vertical guide system is arranged coaxially with the second vertical guide system and comprises a dolly for connecting to said second top drive.

16. The offshore drilling rig of one or more of the preceding claims where said first and/or second vertical guide systems comprises a dolly for connecting to a top drive.

17. The offshore drilling rig of claim 16 where said dolly comprises an extend and retract function.

18. The offshore drilling rig of claim 17 wherein at least three work centers are arranged in the drill deck horizontally spaced apart in mutually different distances from the vertically extending guide, and where the retractable dolly connecting the top drive to the vertically extending guide is adapted so that it only allows the top drive to be vertically aligned above the two of the three work centers being closest to the vertically extending guide.

19. The offshore drilling rig of one or more of the preceding claims further comprising alignment system for keeping the first and second load carriers aligned.

20. The offshore drilling rig of one or more of the preceding claims comprising one or more of a. a second joint-operation position with the first and second load carriers positioned to allow the first and second load carrier to jointly lift loads in or out of the second work center wherein the first top drive is disconnected from the first vertical guide system, b. a second redundancy-operation position with the first load carrier positioned to allow the first load carrier to lift loads in or out of the second work center wherein the first top drive is disconnected from the first vertical guide system.

21 .The offshore drilling rig of claim 20 wherein the reference to first and second is interchangeable in any of claims 1 to claim 19.

22. An offshore drilling rig of one or more of the preceding claims, wherein at least one work center is mounted in a substantially horizontal guide in the drill deck, and where the drill deck comprises a work center positioning system adapted for selectively moving and positioning each of the work centers in the horizontal guide to a first or a second or a third position in the drill deck.

23. An offshore drilling rig according to claim 22, wherein the horizontal guide is linear at least along a part of it, and where the diverter system is connected to the primary well center and comprises at least one diverter over board tube having a first end being connected to the primary well center and the other end being supported and fixed with respect to the drill deck and having at least one telescopic section between the first and the second end, the telescopic section extending parallel to linear part of the horizontal guide in the drill deck.

24. An offshore drilling rig according to claim 22 or 23, and comprising at least one riser tensioning system arranged below the drill deck, and where the riser tensioners are mounted on a linear riser tensioner guide being arranged below, and in parallel to the horizontal guide in the drill deck, so that the riser tensioner selectively can be positioned below a selected work center.

25.An offshore drilling rig comprising; - a drill deck; - a first work center and a second work center horizontally spaced apart from each other, and wherein at least said first work center is a primary well center; - a drilling support structure extending upwardly relative to the drill deck; - a first and a second hoisting system supported by the drilling support structure, the first hoisting system being adapted for raising and lowering a first load carrier, and the second hoisting system being adapted for raising and lowering a second load carrier; - a first top drive suspended from the first load carrier and a second top drive suspended from the second load carrier; - a first vertical guide system connectable to the first top drive to guide said first top-drive in a position over said first work center and a second vertical guide system connectable to the second top drive to guide said second top drive in a position over said second work center; and where the offshore drilling rig is arranged with a positioning system adapted for selectively positioning said second load carrier and, optionally, said first load carrier a) in a normal-operation position with the first and second load carriers positioned over said first and second work centers, respectively, to allow the first load carrier to lift loads in or out of the first work center via said first top drive connected to the first vertical guide system and to allow the second load carrier to lift loads in or out of the second work center via said second top drive connected to the second vertical guide system; and b) in a joint-operation position with the first and second load carriers positioned to allow the first and second load carrier to jointly lift loads in or out of a third work center wherein the second top drive is disconnected from the second vertical guide system.

26. The offshore drilling rig of claim 25 comprising a third vertical guide system arranged to guide and support the first and second top drive in the joint-operation position.

27. The offshore drilling rig of claims 25 or 26 further comprising any of the features of claim 1 to 21

28. A method of operating a drilling rig according to any claims 1 to 27 comprising a. operating the drilling rig in normal-operation position and lowering into and/or raising tubulars out of the first work center and optionally also the second work center (simultaneously and/or sequentially) via the first and optionally the second top drive, respectively; b. placing the first and second load carriers in the joint-operation position by i. disconnecting the second top drive from the second vertical rail system; ii. lower the first top drive; iii. shifting the second top drive via the positioning system to a position next to the first top drive; iv. mechanically fixing the second top drive to the first top drive and/or connecting the second top drive to a vertical rail system different from the second vertical rail system; c. operating the second hoisting systems so that the second load carrier lifts a load in or out of the first work center, such as operating the first and second hoisting systems so that the first and second load carriers jointly lift loads in or out of the first work center or so that the second load carrier lifts loads in or out of the first work center.

29. The method of claim 28 further comprising lowering the second top drive onto a cart on the drill deck;

30. The method of claim 28or 29 further comprising shifting the first top drive via the positioning system to a position in a direction on the other side of the first work center relative to the second top drive.

31 .The method of claim 29 wherein the first top drive is connected to a dolly of the first vertical rail system and the shift in vertical position of the first top drive is performed solely by a retract function of the dolly.

32. The method of claim 29 wherein the first top drive is connected to a dolly of the first vertical rail system and the shift in position is performed by a retract function of the dolly in combination with other parts of the positioning systems, such as a shift in cable crown of the first hoisting system.

33. The method of one or more of claims 28 to 32 comprises lowering said first top drive onto a carrier on the drill deck prior to connecting the second top drive.

34. The method according to one or more of claims 28 to 33, wherein the two load carriers are connected by a connecting tool (such as a connecting tool connecting the first and second top drives), and where the two load carriers are raised or lowered synchronously when carrying a load via the connecting tool.

35. A top drive comprising a shaft for connecting to the upper end of a string of tubulars for drilling into the bed of the sea, carrying the load of said drill string and provide rotation to said string via a motor said top drive being suitable for being raised or lowered by a hoisting system during normal operation wherein said top drive is adapted for receiving a mechanical connection from a dolly on two opposite sides of the top drive said dolly being arranged to keep the top drive from rotating as the rotation is applied to the string.

36. The top drive of claim 0 where the load capacity of the topdrive is so that the string of tubulars may weigh up to 750 tons or more, such as 1000 tons or more, such as 1500 tons more, such as 1750 tons or more, such as 2000 tons or more.

37. The top drive of claims 0 or 36 further comprising an element to be connected to the upper end of the string for rotation therewith and motor means for power rotating said element and the connected string; a powered torque wrench at the lower end of said top drive operable to apply torque in opposite directions to said string and said element; an elevator at the lower end of said assembly adapted to engage a section of said drill string at a location beneath the torque wrench and suspend said section for movement by the elevator; connecting means attaching said torque wrench and said elevator to said top drive assembly for movement upwardly and downwardly there with and in a relation suspending said torque wrench and said elevator and a section of the drill string carried by the elevator from said top drive assembly; said connecting means being constructed to enable movement of said torque wrench and elevator vertically relative to one another.

38. An offshore drilling rig comprising; - a drill deck; - a first work center wherein said first work center is a primary well center; - a drilling support structure extending upwardly relative to the drill deck; - a first and a second hoisting system supported by the drilling support structure, the first hoisting system being adapted for raising and lowering a first load carrier, and the second hoisting system being adapted for raising and lowering a second load carrier; - a first top drive suspended from the first load carrier and a second top drive suspended from the second load carrier; - a first vertical guide system connectable to the first top drive to guide said first top-drive in a position over said first work center; and where the offshore drilling rig is arranged with a positioning system adapted for selectively positioning said first and second load carriers a) in a normal-operation position with the first positioned over said first to allow the first load carrier to lift loads in or out of the first work center via said first top drive connected to the first vertical guide system; and b) in a redundancy-operation position with the second load carrier positioned to allow the second load carrier to lift loads in or out of the first work center.

39.The offshore drilling rig of claim 38 wherein the position system is further arranged to selectively positioning said first and second load carriers in a joint-operation position with the first and second load carriers positioned to allow the first and second load carrier to jointly lift loads in or out of the first work center wherein the second top drive is disconnected from the second vertical guide system.

40. The offshore drilling rig of claim 38 or 39 further comprising any of the features of any of claims 1 to 27 optionally excluding the second vertical guide system and the second work center.