Offshore apparatus and method

An offshore platform for use in combination with an offshore rig comprises an upper deck defining an upper deck aperture which is alignable with a rig aperture of the offshore rig, when said offshore rig extends over the upper deck, to permit objects to extend between the offshore rig and through the upper deck. The offshore platform further includes a sheath arrangement aligned with the upper deck aperture and extending upwardly relative to the upper deck to be alignable with the rig aperture, wherein the sheath arrangement defines a confinement zone for confining objects which pass downwardly from the offshore rig through the rig aperture.

FIELD

The present invention relates to offshore apparatus and methods, for example for use in the exploration and extraction of mineral resources from subterranean reservoirs.

BACKGROUND

In the offshore oil and gas industry wellbores are drilled below the seabed using specialised drilling platforms or vessels, known colloquially as drilling “rigs”. Multiple types of drilling rig exist, such as fixed platforms, jack-up rigs, mobile offshore drilling units (MODUs), semi-submersibles, drilling ships and the like. The particular type of rig used can depend on a number of factors, such as water depth, rig availability, expected longevity of the associated reservoir, and the like.

Once a well has been drilled and appraised, it will be completed with the appropriate downhole infrastructure to permit production (and/or injection), and then capped at the wellhead with a production tree, known as a X-mas tree. Such completion operations may be achieved from the same drilling rig, or may be achieved using a different service rig. Further, during the life of the well it may be necessary to perform workover or intervention operations, which again could be supported by the drilling rig, or alternatively with a different service or workover rig.

The production tree may be located on a subsea wellhead, with a tie back to a surface production facility. In alternative arrangements the tree may be located at surface, on a surface or wellhead platform. Multiple wellbores will typically be present, such that a cluster of trees are provided on the wellhead platform.

Surface tree systems will typically require rigs, such as drilling rigs, service rigs, workover rigs and the like to operate alongside the wellhead platform. For example, portions, such as cantilever portions, of a rig may extend over the wellhead platform. Like in all offshore operations, safety is paramount, and risk to personnel, the environment and equipment must be minimised. In many circumstances there is a risk of objects being dropped from height, for example between separate decks of a wellhead platform, or indeed from a rig operating alongside a wellhead platform. For safety reasons some operators may establish no-entry zones for personnel, for example in defined drop zones, such as below a rig cantilever portion. This may restrict or delay certain operations which cannot otherwise be performed while the rig cantilever portion is in position, or while inter-deck operations are being performed. Also, irrespective of personnel access, there will always be a risk of damage to equipment, such as production trees, from dropped objects and the like. This may be compounded by the close proximity of equipment in tree clusters on the production platform.

SUMMARY

An aspect or embodiment relates to an offshore platform for use in combination with an offshore rig, comprising:an upper deck defining an upper deck aperture which is alignable with a rig aperture of the offshore rig, when said offshore rig extends over the upper deck, to permit objects to extend between the offshore rig and through the upper deck; anda sheath arrangement aligned with the upper deck aperture and extending upwardly relative to the upper deck to be alignable with the rig aperture, wherein the sheath arrangement defines a confinement zone for confining objects which pass downwardly from the offshore rig through the rig aperture.

In use, any objects passing, for example intentionally passing and/or accidentally passing (i.e., dropped), from the offshore rig may be confined within the confinement zone defined by the sheath arrangement. Such confinement provided by the sheath arrangement may thus minimise or eliminate the extent of a drop zone on the upper deck. This may provide a number of advantages. For example, the risk to personnel may be reduced, and may permit personnel to perform operations on the upper deck while the offshore rig is extended over the upper deck. Further, the risk of damage to equipment may be reduced, such as the risk of damage to wellheads and associated equipment (such as production trees) accommodated on the offshore platform.

In normal operations objects may extend form the offshore rig, through the upper deck of the offshore platform, to perform operations associated with the installation, intervention and/or decommissioning of an offshore well. Objects passing from the offshore rig may include tubing, conductor pipe, casing strings, liners, risers, drill strings, bottom hole assemblies, completion equipment, intervention equipment, tools and the like. While many objects may be intentionally passed, in some instances objects may be dropped from the offshore rig, such as tools, equipment and the like, and as noted above, the sheath arrangement assists to minimise risk from such dropped objects.

The offshore rig may define a drilling rig. The offshore rig may be operable adjacent, for example alongside, the offshore platform. The offshore rig may comprise a cantilever portion which is extendable over the offshore platform. The cantilever portion may comprise the rig aperture. The cantilever portion may comprise a BOP (Blow Out Preventor) enclosure.

The offshore rig may define a service rig, workover rig or the like.

The offshore rig may define a drill centre, wherein operations supported by the offshore rig, such as drilling, running equipment and the like, are provided along the drill centre. Accordingly, the rig aperture is provided on the drill centre. The offshore rig may be positioned over the offshore platform such that the drill centre is aligned with the upper deck aperture.

Although the term “drill centre” is used, this is not intended to be limited only to drilling operations, or indeed to limit the rig to a drilling rig.

The offshore platform may define a wellhead platform. In such an arrangement the offshore platform may accommodate a terminating end of one or more surface wells, along with associated equipment, such as wellheads, production trees and the like.

Generally, a wellhead platform is a structure or structures, which support the upper end (opposite of the reservoir) of the well including any superstructures, one or more well processing stations or similar. Such a wellhead platform is typically a structure (such as a jacket based or gravity based platform) resting on the seabed ranging from very basic configurations to complex facilities. The offshore wellhead platform may comprise one or more well-processing stations. Alternatively, the offshore wellhead platform does not comprise any well-processing stations. In such cases, well-processing tasks such as drilling may be performed by a drilling rig placed next to the wellhead platform.

The wellhead platform typically fulfils one or more of the following functions in supporting a well:(i) shield the well from accidental impacts from ships and vessels;(ii) keeping a completed surface well from otherwise tipping over;(iii) provide structure where pipes can be mounted for connecting to a valve assembly or production tree mounted on each well and interfacing these pipes with various equipment or manifolds on and/or off the platform, such as pumps and storage tanks;(iv) supporting production trees so that they are substantially static relative to the platform (at least during production) as the platform and/or well is exposed to forces from current, wind and wave.

The sheath arrangement may define a circumferential confinement zone. The sheath arrangement may comprise one or more tubulars.

The sheath arrangement may circumscribe the upper deck aperture. The sheath arrangement may extend upwardly from the level of the upper deck. That is, no gap between the upper deck and the sheath arrangement may be present. Such an arrangement may assist to minimise risk of objects being dropped from the upper deck and through the upper deck aperture downwardly to a lower deck.

The sheath arrangement may be securable around a peripheral region of the upper deck aperture. The sheath arrangement may be securable around a peripheral region of the rig aperture.

The sheath arrangement may extend through the upper deck aperture. This may assist to minimise the risk of objects being dropped from the upper deck and through the upper deck aperture downwardly to a lower deck.

The sheath arrangement may extend through the rig aperture.

At least a portion of the sheath arrangement may be rigid.

At least a portion of the sheath arrangement may be flexible. At least a portion of the sheath arrangement may comprise a flexible material, such as a chain-link material. The sheath arrangement may define a sheath curtain. Such an arrangement may provide advantages in terms of installation, storage, and the like.

At least a portion of the sheath arrangement may be formed or comprise a metal or metal alloy material, composite material, plastic material or the like.

At least a portion of the sheath arrangement may be supported by the upper deck. At least a portion of the sheath arrangement may be secured to the upper deck. At least a portion of the sheath arrangement may be supported by the offshore rig. At least a portion of the sheath arrangement may be secured to, for example suspended from, the offshore rig.

At least a portion of the sheath arrangement may be provided on the offshore rig. For example, a portion of the sheath arrangement may be provided on the offshore rig, and a remaining portion of the sheath arrangement may be mated or otherwise arranged relative to the portion provided on the rig.

At least a portion of the sheath arrangement may be extended, for example lowered, from the offshore rig, for example through the rig aperture, during deployment of the sheath arrangement. The sheath arrangement may be deployed entirely from the offshore rig, for example by extending downwardly towards the platform.

At least a portion of the sheath arrangement may be extended, for example raised, from the offshore platform an upwardly towards the offshore rig during deployment of the sheath arrangement. The sheath arrangement may be deployed entirely from the platform, for example by extending upwardly towards the rig.

The sheath arrangement may comprise a load point or support on an outer surface thereof. The load point or support may permit connection of a load to the sheath arrangement. This may permit the sheath arrangement to at least partially support a connected load.

The sheath arrangement may comprise a support arrangement on an outer surface thereof. This may permit the sheath arrangement to be used as a structural support to facilitate offline (i.e., offset from the drill centre) operations such as slickline and wireline operations, hoisting operations such as during installation of production trees and the like. The support arrangement may comprise one or more arms, for example cantilever arms. The support arrangement may comprise one or more pulleys, sheaves or the like. The support arrangement may comprise a winch assembly.

The sheath arrangement may comprise a unitary component. Alternatively, the sheath arrangement may comprise multiple components. In one embodiment the sheath arrangement may comprise a first component connected or mounted to the offshore platform, and a second component connected or mounted to the offshore rig.

The sheath arrangement may be telescopic. For example, the sheath arrangement may comprise telescoping tubular structures. This may facilitate storage of the sheath arrangement when not in use. This may facilitate improved deployment of the sheath arrangement. Such a telescoping arrangement may provide a degree of movement compensation, for example to accommodate relative movement between the offshore platform and offshore rig.

The sheath arrangement may extend downwardly relative to the upper deck towards a first lower deck. The sheath arrangement may therefore define a confinement zone for confining objects passing between the upper and first lower decks. This may minimise or eliminate the extent of a drop zone on the first lower deck.

The offshore platform may accommodate equipment or apparatus associated with surface wells within the space defined between the first lower deck and upper deck. Such equipment or apparatus may comprise wellheads, production trees, pipework and the like. The provision of a confinement zone between the first lower and upper decks, in addition to reducing risk to personnel, may also reduce risk of damage to such sensitive equipment.

The sheath arrangement may define a continuous confinement zone both above and below the upper deck.

The first lower deck may define a first lower deck aperture aligned with the upper deck aperture.

The sheath arrangement may define an upper sheath portion extending upwardly relative to the upper deck and a lower sheath portion extending downwardly relative to the upper deck. At least part of the upper and lower sheath portions may be integrally formed.

The sheath arrangement may terminate at the first lower deck. The sheath arrangement may terminate above the first lower deck. The sheath arrangement may extend through a lower deck aperture. The sheath arrangement may extend below the first lower deck, for example towards a second lower deck.

The sheath arrangement, or portions thereof may define a permanent installation.

The sheath arrangement, or portions thereof, may be removable.

The offshore rig may be used to support a number of procedures or operations. For example, rig operations may include deploying and installing a conductor, drilling through a conductor, re-entering an existing well, deploying completion equipment and the like.

The offshore platform may accommodate multiple wells arranged in a cluster around the drill centre. The upper ends of the wells may be moveable to be aligned with the drill centre of the offshore rig. This may permit multiple wells to be established and accessed from a common drill centre. Such movement/alignment of the wells may be performed in accordance with desired operator procedures, such as those described in the applicant's co-pending patent applications DK PA2015 00668 and GB 1522856.2, the disclosure of which is incorporated herein by reference.

In one example, the offshore platform may be configured to selectively allow access to the wells. For example, the sheath arrangement itself, or indeed the platform, may comprise a plate arrangement. Such a plate arrangement may be configured to form a flange, when associated with the sheath arrangement, to selectively prevent objects or equipment from accessing the wells, (e.g. those wells not at the drill centre).

In some examples, the plate arrangement may comprise an operable hatch or otherwise opening. Such a hatch may permit, when opened, access to the wells (e.g. to permit intervention operations to be carried out on the other wells when the sheath arrangement is at the drill centre.) The plate arrangement may be configured to allow access to other wells when drilling operations are occurring through the sheath arrangement (e.g. even at times when the plate arrangement is associated with the sheath arrangement).

In some examples, the plate arrangement may comprise at least one separation plate. Such a separation plate may be configured to extend downwardly from the plate arrangement. Such a separation plate may be configured to inhibit objects passing through the hatches to adjacent regions at the platform (e.g. adjacent wells). Further, such a separation plate may be configured to come into contact with a deck as the sheath arrangement is lowered, which may reduce or avoid impact of the sheath arrangement on wellheads.

The sheath arrangement may comprise an access hatch. The access hatch may be arranged such that, when in situ, the access hatch is user accessible from a deck (e.g. the upper deck). Such an access hatch, when opened, may permit access to the equipment passing within the sheath arrangement.

The sheath arrangement may be configured to flex compliantly in order to accommodate relative movement between the offshore rig and upper deck, when in use. For example, the sheath arrangement, or at least a portion thereof, may be axially and/or longitudinally compliant in order to accommodate relative movement between the offshore rig and upper deck, when in use. In some examples, the sheath arrangement may comprise specific compliant portions, for example one or more flexible joints.

An aspect or embodiment relates to a method for confining objects extending between an offshore rig and an offshore platform, comprising extending a sheath arrangement upwardly relative to an upper deck of the offshore platform, wherein the sheath arrangement is aligned with a rig aperture of the offshore rig such that the sheath arrangement defines a confinement zone for confining objects which pass downwardly from the offshore rig through the rig aperture.

An aspect or embodiment relates to a sheath arrangement for use between an upper deck of an offshore platform and an offshore rig positioned over the offshore platform, wherein the sheath arrangement is alignable with a rig aperture provided on the rig and defines a confinement zone for confining objects which pass downwardly from the offshore rig through the rig aperture.

The features defined in relation to one aspect may be provided in combination with any other aspect.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1diagrammatically illustrates an offshore platform, specifically an offshore wellhead platform, generally identified by reference numeral10, which extends above a sea surface12. The platform10accommodates a number of well structures14which extend from their terminating upper ends16, through the sea and into the seabed, to intercept a subterranean formation. As is known in the art, the well structures14will typically include an outer conductor pipe and a number of concentrically arranged casing strings cemented within the conductor pipe. Each well structure14terminates at a wellhead18and is capped with a production or X-mas tree20.

The platform10includes a number of decks, including an upper deck22, a first lower deck24and a second lower deck26arranged one above the other. Multiple terms are used in the art to define or classify each deck. For example, the upper deck22may be known as a weather deck. The first lower deck24may be defined as a wellhead or tree deck, in that, at least in the present example, the wellheads18and production trees20are located generally at the level of the first lower deck24. The second lower deck26may be defined as a cellar deck.

The upper deck22defines an upper deck aperture28, and the first lower deck24defines a first lower deck aperture30, wherein the respective apertures28,30are aligned on a platform axis32. The second lower deck26may also include a corresponding aperture, although this is not illustrated. In the example illustrated the wells14are arranged in a cluster around the platform axis. Whenever well access is required, for example during well construction, intervention or the like, the upper end16of the well14is moved into line with the platform axis32, resulting in bending of the well14, as exemplified by well14a. Such movement may be performed as described in the applicant's co-pending patent applications DK PA2015 00668 and GB 1522856.2, the disclosure of which is incorporated herein by reference.

The offshore platform10includes or is associated with a sheath arrangement34which is aligned on the platform axis32and includes an upper portion36which extends upwardly relative to the upper deck22, and a lower portion38which extends downwardly relative to the upper deck22towards the lower deck24. The lower portion38in the illustrated example terminates just above the first lower deck24. The sheath arrangement34thus defines a confinement zone40above both the upper and first lower decks22,24.

In typical offshore operations a rig will be operated alongside a wellhead platform, to perform operations such as drilling, installing well infrastructure, performing workover or intervention operations, and the like. InFIG. 1a rig cantilever42is illustrated alongside the offshore platform10. The rig cantilever42includes a BOP enclosure44which includes a rig floor46(which may be defined as a drill floor), with a rig aperture48defined in the rig floor46. The rig cantilever42defines a drill centre50which is aligned with the rig aperture48.

The rig cantilever42will be moved over the offshore platform10, as illustrated inFIG. 1B, such that the drill centre50of the rig cantilever42is aligned with the platform axis32. When the rig cantilever42is in place, the sheath arrangement34is thus aligned with the rig aperture48, such that all operations from the rig cantilever42are performed through the sheath arrangement34, specifically within the confinement zone40defined by the sheath arrangement34, thus improving safety on the upper and first lower decks22,24. In the example illustrated inFIG. 1Ba drilling operation is being performed through well14a, using typical drilling equipment such as a drilling BOP52, drilling riser54and the like.

In addition to normal operations through the sheath arrangement34, the confinement zone40may also function to confine objects which are accidentally dropped through the rig aperture48. In addition to improved personnel safety, this can also assist to minimise risk of damage to equipment, such as the wellheads18and production trees20accommodated on the platform10.

In the embodiment illustrated inFIGS. 1A and 1Bthe sheath arrangement includes one or more rigid tubular bodies, which may be made from metal, such as steel. However, in other examples other material may be used, such as polymers, composite material or the like.

In a modified example, as illustrated inFIG. 2, the sheath arrangement34may include a second lower section56(shown in broken outline) which extends below the first lower deck24towards the second lower deck26.

Further, the sheath arrangement34, and specifically the upper portion36of the sheath arrangement34includes supports in the form of cantilever support arms60. Although two arms60are illustrated inFIG. 2more or less arms may be provided. The arms60may be used for a number of support operations, effectively permitting the sheath arrangement to act as a structural component for supporting load. InFIG. 2one arm60is illustrated in use as a hoisting point, permitting manipulation of a load, such as the illustrated production tree20a, in combination with a wire62and associated winch64.

Reference is now made toFIG. 3Ain which a further modified example of an offshore platform110is illustrated. The platform110is similar in many respects to platform10ofFIGS. 1A and 1Band as such like features share like reference numerals, incremented by 100. Thus, the platform110includes an upper deck122, a first lower deck124and a second lower deck126, wherein the upper deck122defines an upper deck aperture128, and the first lower deck124defines a first lower deck aperture130.

In the present example a sheath arrangement134is also provided. However, in this example a lower section138of the sheath arrangement134is mounted on the upper deck122, extending through the upper deck aperture128towards the first lower deck124, with a short stub portion138aextending upwardly from the upper deck122. The sheath arrangement134includes a separate upper section136, which inFIG. 3Ais shown being passed downwardly from a rig cantilever142, through a rig aperture148formed in a rig floor146.

When fully deployed from the rig cantilever142, as illustrated inFIG. 3B, the upper sheath section136is received over the stub portion138aof the lower sheath section138, thus creating a confinement zone140for confining objects passing downwardly through the rig aperture148.

A further modified example of an offshore platform210is illustrated inFIG. 4A. The platform210is similar in many respects to platform110ofFIGS. 3A and 3Band as such like features share like reference numerals, incremented by 100. Thus, the platform210includes an upper deck222, a first lower deck224and a second lower deck226, wherein the upper deck222defines an upper deck aperture228, and the first lower deck224defines a first lower deck aperture230.

In the present example a sheath arrangement234is also provided, with a lower section238of the sheath arrangement234mounted on the upper deck222, extending through the upper deck aperture228towards the first lower deck224, with a short stub portion238aextending upwardly from the upper deck222. The sheath arrangement234includes a separate upper section236, which is provided in the form of a flexible curtain suspended from the floor246of a rig cantilever242, around the periphery of a rig aperture248. The flexible curtain may be formed from a chain-link material, for example.

InFIG. 4Athe upper sheath section236is shown in a retracted, or folded, state. When fully deployed from the rig cantilever242, as illustrated inFIG. 4B, the upper sheath section236is received over the stub portion238aof the lower sheath section238, thus creating a confinement zone240for confining objects passing downwardly through the rig aperture248. The lower end of the flexible upper sheath section236may be tethered or otherwise secured to the upper deck222.

In the examples provided above the rig cantilever includes a single drill centre. However, in other examples an offshore platform may be provided which is operational with a rig cantilever which includes multiple drill centres, such as in the example illustrated inFIG. 5. In this example the offshore platform310includes a number of decks, including an upper deck322, a first lower deck324and a second lower deck326arranged one above the other. The upper deck322defines two upper deck apertures328a,328b, and the first lower deck324defines two first lower deck apertures330a,330b, wherein the respective apertures328a,328b,330a,330bare aligned on first and second platform axes332a,332b. In the example illustrated separate clusters of wells14are arranged around the platform axes332a,332b.

A rig cantilever342is operated over the platform310, wherein the rig cantilever342includes first and second drill centres350a,350bwhich are aligned with the respective first and second platform axes332a,332b. The rig cantilever342includes first and second rig apertures348a,348bprovided on the respective first and second drill centres350a,350b.

A first sheath arrangement334ais aligned on the first platform axis332a, and includes an upper portion336awhich extends upwardly relative to the upper deck322, and a lower portion338awhich extends downwardly relative to the upper deck322towards the lower deck324. The first sheath arrangement334athus defines a first confinement zone340aabove both the upper and first lower decks322,324.

A second sheath arrangement334bis aligned on the second platform axis332b, and includes an upper portion336bwhich extends upwardly relative to the upper deck322, and a lower portion338bwhich extends downwardly relative to the upper deck322towards the lower deck324. The second sheath arrangement334bthus defines a second confinement zone340babove both the upper and first lower decks322,324.

The provision of the separate drill centres350a,350bmay permit multiple operations to be performed simultaneously. For example, in the illustrated example ofFIG. 5a drilling operation is being performed along the first drill centre350a, whereas a conductor pipe80is being deployed along the second drill centre350b.

Thus, during operations any objects passing, for example dropped, from the rig cantilever342will be confined within the first and second confinement zones340a,340bdefined by the respective first and second sheath arrangements334a,334b.

Reference is now made toFIGS. 6A, 6B and 6Cin which a further example of a sheath arrangement434is illustrated. The sheath arrangement434is similar in many respects to the sheath arrangement34shown in relation to platform10ofFIGS. 1A and 1Band as such like features share like reference numerals, incremented by 400. Thus, the platform (not shown) in which sheath arrangement434is to be used includes an upper deck422, a first lower deck424and a second lower deck426, wherein the upper deck422defines an upper deck aperture428, and the first lower deck424defines a first lower deck aperture430. The new features shown in relation to the sheath arrangement434in the example of the offshore platform inFIGS. 6A, 6B and 6Care equally applicable to the offshore platform110,210and310of previous examples.

FIG. 6Ashows a cross sectional view through a plate arrangement470ofFIG. 6B(as will be explained). The plate arrangement470is configured to allow, or indeed prevent, access to a cluster of wells414around a platform axis432. In this example, six wells414are shown but in other embodiments there could be more or fewer. In a similar manner to that explained forFIG. 1, well414ahas been bent into line with the platform axis432and is thus located centrally within the cluster of wells (e.g. at the drill centre). The sheath arrangement434(not shown) is aligned with the platform axis432and is also located centrally within the cluster of wells414. In this example, the plate arrangement470is located at the bottom of the sheath arrangement434. However in other examples, the sheath arrangement434can also extend below the plate arrangement470.

Here, the plate arrangement470is located on the outside of the sheath arrangement434and can be considered to extend horizontally outwardly from the sheath arrangement434to form a flange that covers the area above the wells414. While in some cases, the plate arrangement470may be positioned relative to the sheath arrangement434when use is desired, in this example the plate arrangement470may be considered to be attached to the sheath arrangement434. The plate arrangement470is intended to be located between upper deck422and a first lower deck424, i.e. directly above production trees420located above the first lower deck424. In other embodiments, the plate arrangement470may be located above upper deck422or at another position along the sheath arrangement434. In other embodiments, the plate arrangement470may cover only some of the wells414.

A perspective view of the sheath arrangement434and the plate arrangement470is shown inFIG. 6Bwith the sheath arrangement434extending both above and below the horizontal plane of the plate arrangement470. Here, the plate arrangement470is provided with operable hatches472(shown inFIG. 6B), which are openable and closable. The hatches472can be raised and lowered to open and close respectively by retaining members475. In this example, there are six hatches472, one for access to each well414. In other embodiments, there may be fewer or more than six hatches472provided in the plate arrangement470. When opened, the hatches472provide plate apertures473in the plate arrangement470. The hatches472allow operations to be carried out on the other wells414through the apertures473when the sheath arrangement434is above well414a. The hatches472also allow equipment to be passed through the plate arrangement470(e.g. when needed). For example, intervention operations (e.g. wireline operations) can be carried out in one or more wells414through the plate apertures473while drilling is occurring through the sheath arrangement434in well414a.

Further, referring now toFIG. 6C, separation plates474are provided in the example, which extend downwards from the plate arrangement470. The separation plates474can also be seen inFIG. 6A. Six separation plates474are provided in this example but more or fewer could be provided depending on the number of wells414. The bottoms of the separation plates474come into contact with the first lower deck424before the plate arrangement470contacts the wellheads of the wells14when the sheath arrangement434is lowered. This may reduce impact on the wellheads of the wells414.

The separation plates474are configured to assist with safety by further confining objects that may be accidentally dropped through hatches472. That is, objects dropped through the hatches472at an angle to the vertical will impact the separation plates474and thus be confined to an area between the separation plates474and those adjacent wells. In this embodiment, the separation plates474are straight and extend outwardly from the sheath arrangement434but, in other embodiments, they could be any other suitable shape. There are spaces between the separation plates474to allow movement of the wells414to the platform axis432.

Further shown here is an access hatch476, which is provided in the sheath arrangement434as shown by dotted lines inFIG. 6B. The access hatch476allows access to the confinement zone440of the sheath arrangement434. Although the access hatch476is intended in this example to be above the upper deck422, it may be located in any position along the sheath arrangement434, e.g. between the first lower deck424and the second lower deck426. However, the access hatch476may be arranged such that, when in situ, the access hatch476is user accessible from a deck (e.g. the upper deck). Such an access hatch476, when opened, may permit access to the equipment passing within the sheath arrangement434.

It will readily be appreciated that in some examples it may be helpful for the sheath arrangement434to flex compliantly in order to accommodate relative movement between the offshore rig and upper deck422, when in use. Compliance, in this regard may be considered to mean, for example, the sheath arrangement434may be axially and/or longitudinally compliant in order to accommodate some relative movement between the offshore rig and upper deck422, when in use.

In some examples, that compliance may be provided by the structure of the sheath arrangement434itself. Otherwise, and in the example shown, the sheath arrangement434may comprise compliant portions, (e.g. one or more flex joints (not shown)). Here, the flex joint is provided by a portion of the sheath arrangement434that is flexible. That is, the flex joint is not rigid and can bend etc. to accommodate the difference in motion between the platform410and the rig cantilever (not shown).

While in some of the above examples, the various embodiments have been described using the sheath arrangement434together with the rig cantilever, it will be appreciated that the sheath arrangement434may be readily useable when also performing intervention operations, e.g. using wireline line setups. That is, while drilling is occurring in one well structure, wireline operations can be carried out in another well structure. A skilled reader will readily be able to implement these embodiments accordingly.

It should be understood the examples described above are indeed merely exemplary and that various modifications may be made thereto without departing from the scope of the invention.