WO2009156751A2

WO2009156751A2 - Offshore vessel mooring and riser inboarding

Info

Publication number: WO2009156751A2
Application number: PCT/GB2009/050694
Authority: WO
Inventors: Peter Graham Jay; Robin Stuart Colquhoun
Original assignee: Sigma Offshore Limited
Priority date: 2008-06-25
Filing date: 2009-06-18
Publication date: 2009-12-30
Also published as: GB0811606D0; WO2009156751A3

Abstract

The invention relates to an offshore vessel mooring and riser inboarding system, and to an associated method. In an embodiment of the invention, an offshore vessel mooring and riser inboarding system (10) is disclosed. The system comprises a support assembly (16) mounted on a vessel such as an FPSO (12); a first mooring portion (20) located in an offshore environment and moored such that rotation of the first mooring portion is restricted; a second mooring portion (28) rotatably coupled to the first mooring portion and secured to the support assembly for rotation therewith relative to the first mooring portion; a plurality of risers (30a, 30b, 30c) coupled to the second mooring portion such that the risers can twist together on rotation of the second mooring portion relative to the first mooring portion; and a rotatable coupling (32a, 32b, 32c) associated with each riser and by which the risers are coupled to the second mooring portion, to thereby facilitate rotation of the risers relative to the second mooring portion.

Description

OFFSHORE VESSEL MOORING AND RISER INBO ARDING

The present invention relates to an offshore vessel mooring and riser inboarding system, and to an associated method. In particular, but not exclusively, the present invention relates to an offshore vessel mooring and riser inboarding system in which a plurality of risers are coupled to a mooring portion of the system.

In the oil and gas exploration and production industry, there have been movements towards the use of Floating Production Storage and Offloading Vessels (FPSOs) and Floating Storage and Offloading Vessels (FSOs) for the exploitation of offshore oil and gas fields. An FPSO is moored in an offshore location and is typically coupled to a number of producing wells, for the temporary storage of produced well fluids, which are periodically exported to shore by tankers. FPSOs typically include facilities for separating recovered well fluids into different constituents (oil, gas and water), so as to stabilise the crude oil for onward transport by tanker. FSOs are similarly moored and allow for the storage of recovered well fluids, and may either be disconnected from their moorings for travel to an offloading location, or the recovered fluids may similarly be exported by tanker

A number of different systems have been developed for mooring vessels such as FPSOs and FSOs. These systems have been found to suffer from a number of disadvantages, including: that the ability of the vessel to weathervane is overly restricted; that they are difficult to install and hook up in the field; that they have an uncertain ability to allow the vessel to disconnect rapidly, reliably and safely from fluid risers; and that they provide a relatively restricted sea state capability.

A more recently developed system, disclosed in international patent application no.PCT/GB2005/003766 (published as WO-2006/037964) was designed to address the problems associated with such prior systems. This was achieved by providing a system including a connector assembly, for connecting a vessel to a mooring element, in which relative rotation between the vessel and the mooring element about three mutually perpendicular axes of rotation is permitted. By permitting such movement between the vessel and the mooring element, the system of WO-2006/037964 facilitates movement of the vessel under external loading during use, and reduces forces transmitted to/borne by the vessel and the mooring and riser system components. Accordingly, it has been found that the connector assembly in the system of WO-2006/037964 is not required to support the relatively large loads found in prior systems. In addition, the system of WO-2006/037964 permits all likely ranges of movement of the vessel relative to the mooring element without excessive wear or damage to components either of the system or to the vessel itself.

It is now desired to provide a further system which improves upon those known prior to the development of that disclosed in WO-2006/037964.

It is therefore amongst the objects of embodiments of the present invention to obviate or mitigate at least one of the foregoing disadvantages.

According to a first aspect of the present invention, there is provided an offshore vessel mooring and riser inboarding system, the system comprising: a support assembly mounted on a vessel; a first mooring portion located in an offshore environment and moored such that rotation of the first mooring portion is restricted; a second mooring portion rotatably coupled to the first mooring portion and secured to the support assembly for rotation therewith relative to the first mooring portion; a plurality of risers coupled to the second mooring portion such that the risers can twist together on rotation of the second mooring portion relative to the first mooring portion; and a rotatable coupling associated with each riser and by which the risers are coupled to the second mooring portion, to thereby facilitate rotation of the risers relative to the second mooring portion.

According to a second aspect of the present invention, there is provided an offshore vessel mooring and riser inboarding system, the system comprising: a support assembly adapted to be mounted on a vessel; a first mooring portion adapted to be located in an offshore environment and to be moored such that rotation of the first mooring portion is restricted; a second mooring portion rotatably coupled to the first mooring portion and secured to the support assembly for rotation therewith relative to the first mooring portion; a plurality of risers adapted to be coupled to the second mooring portion such that the risers can twist together on rotation of the second mooring portion relative to the first mooring portion; and a rotatable coupling associated with each riser and by which the risers can be coupled to the second mooring portion, to thereby facilitate rotation of the risers relative to the second mooring portion.

Providing an offshore vessel mooring and riser inboarding system comprising a plurality of risers coupled to a second mooring portion such that the risers can twist together on rotation of the second mooring portion relative to a first mooring portion of the system permits weathervaning of the vessel about the first mooring portion without excessive loading on the risers.

It will be understood that rotation of the first mooring portion is restricted in that the mooring portion is restrained against such rotation, typically using appropriate mooring lines and anchors. However, a certain amount of rotation may nonetheless occur under external loading such as wind, wave, tidal or current loading, or indeed loading exerted on the mooring portion by the vessel such as under heavy seastate conditions. The first mooring portion may be located in an offshore environment and moored such that rotation of the first mooring portion about its own axis is restricted. The rotatable coupling associated with each riser may facilitate rotation of the risers relative to the second mooring portion about their own axes. The rotation may facilitate reduction of torsion loading in the risers.

Typically, the risers are coupled to the second mooring portion at locations spaced around the second mooring portion, and the risers may be coupled to a lower end of the second mooring portion. The risers may be equidistantly spaced around the second mooring portion, which may encourage controlled twisting together of the risers. The risers may be arranged such that, in use, they twist together in a helixing arrangement/helix pattern. Accordingly, it will be understood that the risers may helix on rotation of the second mooring portion relative to the first mooring portion. The risers may be spaced equidistantly around the second mooring portion in that the risers may be located at common distances from a central axis of the second mooring portion, and/or may be arranged on a circular path extending around the second mooring portion and spaced equidistantly around the circumference of the circular path. The risers may be at common angular spacings.

Providing an offshore vessel mooring and riser inboarding system comprising rotatable couplings associated with each riser and by which the risers are coupled to the second mooring portion offers advantages in terms of reducing torsional loading in the risers when the vessel weathervanes relative to the first mooring portion, and thus when the second mooring portion rotates relative to the first mooring portion.

It will be understood that the first mooring portion axis may be a main axis of the first mooring portion extending in a direction along a length of the first mooring portion, and thus that the first mooring portion may be moored such that rotation about said axis is restricted. In a similar fashion, the axes of the risers may be main axes of the risers extending in a longitudinal direction thereof, the risers being rotatable about said axes, permitted by the rotatable couplings.

The first mooring portion may be an outer mooring portion, and the second mooring portion may be an inner mooring portion located within the outer mooring portion. The second mooring portion may be restricted against translation relative to the first mooring portion, that is movement relative to the first mooring portion in a direction along a length of the first mooring portion, when the second mooring portion has been rotatably coupled to the first mooring portion. This may be achieved utilising a locking arrangement or the like for securing the first and second mooring portions against relative longitudinal movement following location of the second mooring portion within the first mooring portion. The first and second mooring portions may be mooring canisters, and may be generally tubular. The first and second mooring canisters may be elongate, and may be substantially cylindrical tubular canisters.

The rotatable couplings on the risers may be swivels, and may be in-line swivels. The second mooring portion may comprise a plurality of riser connectors, one for each rotatable coupling, the riser connectors serving for connecting the rotatable couplings (and thus the risers) to the second mooring portion. The riser connectors may each be provided at or adjacent a lower end of the second mounting portion, and the second mooring portion may comprise a plurality of connecting tubes extending along a passage within the second mooring portion, and which may each be coupled to a respective riser to facilitate fluid flow between the risers and the second mooring portion. The system may further comprise a plurality of transfer lines which may be jumpers or jumper lines, and which may facilitate fluid flow between the risers, the second mooring portion and the vessel.

References herein to inboarding of a riser and to a riser inboarding system are to the bringing inboard or onboard of a riser to a vessel and to such a system. The risers may be fluid flow risers; it will then be understood that the system may be for the passage of fluid from the fluid flow risers into the second mooring portion and to the vessel, or vice-versa. The risers may comprise or take the form of power and/or control cables, which may facilitate power supply, data transmission and/or supply of hydraulic control fluid.

The first mooring portion may comprise a plurality of mooring arrangements for mooring the first mooring portion in the offshore environment and restricting rotation of the first mooring portion. Each mooring arrangement may comprise a mounting bracket or stop by which a mooring line can be secured to the first mooring portion, and a mooring line guide adapted to receive the mooring line for guiding the line to the bracket. The mooring line guides may be adapted to permit movement of the risers relative thereto, which may occur in use of the system. Each mounting bracket may be provided above a respective mooring line guide, and the mooring line guides may be pulleys or pulley joints. Coupling the mooring lines to the first mooring portion in this fashion offers advantages in terms of restraining movement of the second mooring portion under external loading. This is because the first mooring portion will tend to pivot about the mooring line guides under external loading from the vessel, in certain seastate conditions. Consequently, the tension in portions of the mooring lines extending between the mooring line guides and the mounting brackets will vary around a circumference or perimeter of the first mooring portion. Increased tension in certain mooring lines relative to other lines will exert a force on the first mooring portion tending to urge the portion back to a substantially vertical, rest position. Each mounting bracket may be provided at or adjacent an upper end of the first mooring portion, and each mooring line guide may be provided at or adjacent a lower end of the first mounting portion. This may facilitate maximisation of the restorative force exerted on the second mooring portion in use. It will be understood that references herein to upper and lower parts of the system, and/or to upper and locations on parts of the system, are typically relative to the system in its normal orientation and in a rest or normal or neutral state, and should be interpreted accordingly.

The support assembly may comprise a securing arrangement for releasably securing the second mooring portion to the support assembly and thus to the vessel. The securing arrangement may take the form of a latch ring arrangement for releasably gripping the second mooring portion, and the latch ring arrangement may comprise a plurality of latch ring segments which together form a latch ring. The latch ring segments may be movable between release and latching positions for releasably gripping the second mooring portion. The latch ring arrangement may comprise a hydraulic actuator for moving the latch ring segments between the release and latching position. The securing arrangement may also comprise at least one locking element for locking the latch ring segments in their latching positions, which may provide a mechanical backup to the hydraulic actuator.

The system may comprise at least one riser spacer coupled to the risers, which may facilitate controlled twisting of the risers and/or control of spacings between the risers at least in the region of the risers spacers. Preferably, the system comprises a plurality of riser spacers, which may be riser spiders, spaced at intervals along the length of the risers.

The support assembly may comprise a rotatable support, which may facilitate rotation of the vessel about or with reference to the second mooring portion. The rotatable support may comprise an outer gimbal member, which may be mounted for rotation relative to a support structure of the support assembly and which support structure may be mounted on the vessel. The rotatable support may also comprise an inner gimbal member mounted for rotation relative to the outer gimbal member. Pivot points about which the outer gimbal member rotates relative to the support structure, and about which the inner gimbal member rotates relative to the outer gimbal member, may be spaced such that axes of rotation of the gimbal members are perpendicular. The outer and inner gimbal members may together permit relative rotation between the vessel and the second mooring portion about two mutually perpendicular axes of rotation. The rotatable support may further comprise a second mooring portion rotatable coupling, which may facilitate rotation between the inner gimbal member and the second mooring portion. This rotatable coupling may be selectively activatable to permit rotation of the second mooring portion relative to the inner gimbal member. This may be desirable if weathervaning of the vessel about the first mooring portion results in significant twisting together of the risers and it is desired to restrict further twisting, for example, to minimise further torsional loading on the risers. The system may then comprise a multi-path swivel for connecting the risers to the vessel. Such swivels are known for example from WO-2006/037964, the disclosure of which is incorporated herein by way of reference.

According to a third aspect of the present invention, there is provided a vessel comprising an offshore vessel mooring and riser inboarding system according to the first or second aspect of the invention.

According to a fourth aspect of the present invention, there is provided a method of mooring a vessel in an offshore environment, the method comprising the steps of: locating a first mooring portion in an offshore environment and mooring the first mooring portion such that rotation of the first mooring portion is restricted; rotatably coupling a second mooring portion to the first mooring portion; securing the second mooring portion to a support assembly on the vessel for rotation therewith relative to the first mooring portion; coupling a plurality of risers to the second mooring portion such that the risers can twist together on rotation of the second mooring portion relative to the first mooring portion; and rotatably coupling each riser to the second mooring portion using respective rotatable couplings, such that the risers can rotate relative to the second mooring portion.

Further features of the method of the fourth aspect of the present invention may be derived from or with respect to the system of the first and/or second aspect of the invention defined above. According to a fifth aspect of the present invention, there is provided an offshore vessel mooring and riser inboarding system, the system comprising: a support assembly mounted on a vessel; a first mooring portion located in an offshore environment and moored such that rotation of the first mooring portion is restricted; a second mooring portion rotatably coupled to the first mooring portion and secured to the support assembly for rotation therewith relative to the first mooring portion; a plurality of risers coupled to the second mooring portion such that the risers can twist together on rotation of the second mooring portion relative to the first mooring portion.

Further features of the system of the fifth aspect of the present invention may be derived from or with respect to the system of the first and/or second aspect of the invention defined above.

An embodiment of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 is a perspective view of an offshore vessel mooring and riser inboarding system, and of part of a vessel incorporating the system, in accordance with an embodiment of the present invention;

Figure 2 is an enlarged view of the system shown in Figure 1, taken from a different angle to the view of Figure 1 and shown without the vessel;

Figures 3 and 4 are further enlarged side and front views, respectively, of the system shown in Figures 1 and 2;

Figures 5 and 6 are views of the system of Figures 1 and 2 showing the system in a rest position prior to the vessel weathervaning relative to a first mooring portion of the system, and following weathervaning, respectively; Figures 7 to 9 are perspective, partial cross-sectional side and plan views, respectively, of part of a connector assembly and upper portions of first and second mooring portions of the system shown in Figures 1 and 2; and

Figure 10 is an enlarged side view of a portion of the connector assembly shown in Figures 7 to 9.

Turning firstly to Figure 1, there is shown a perspective view of an offshore vessel mooring and riser inboarding system 10, and of part of a vessel incorporating the system 10 and indicated generally by reference numeral 12, in accordance with an embodiment of the present invention. The vessel 12 takes the form of an FPSO, and only a bow 14 of the FPSO is shown in the drawing. It will be understood however that the vessel may be of any other suitable alternative type and may, for example, be an FSO, an offtake tanker or a buffer tanker. The vessel 12 will typically be for engagement in a life-of- field production procedure or an extended well test (EWT) procedure.

The system 10 is also shown in the enlarged view of Figure 2, which is taken from a different angle to the view of Figure 1 and which is shown without the FPSO 12. Additionally, the system 10 is shown in the further enlarged side and front views of the Figures 3 and 4, respectively.

The offshore vessel mooring and riser inboarding system 10 generally comprises a support assembly in the form of a cantilever connector assembly 16, part of a frame 18 of the connector assembly 16 being shown in the figures. The connector assembly 16 is generally of a type shown and described in WO-2006/037964, the disclosure of which is incorporated herein by way of reference. The connector assembly 16 is mounted on the FPSO 12 in the region of the vessel bow 14. The system 10 also comprises a first mooring portion in the form of an outer tubular mooring canister 20, which is located in an offshore environment such as a sea or ocean 22. The outer mooring canister 20 is moored using a number of catenary mooring chains 24, which restrict rotation of the outer mooring canister 20 about its own axis 26. It will be noted that the mooring chains 24 have been left out of the view of Figure 3, for ease of illustration. The system 10 also comprises a second mooring portion in the form of an inner tubular mooring canister 28, which is rotatably coupled to the outer mooring canister 20, and which is secured to the connector assembly 16 for rotation with the connector assembly 16 (and thus the FPSO 12) relative to the outer mooring canister 20. A number of risers 30 are coupled to the inner mooring canister 28 and, in the illustrated embodiment, three risers 30a, 30b and 30c are provided and are coupled to the inner mooring canister 28. The risers 30a to 30c are coupled to the inner canister 28 such that the risers can twist together in a helixing pattern or arrangement on rotation of the inner mooring canister 28 relative to the outer mooring canister 20. The risers 30a, 30b and 30c are each coupled to the inner mooring canister 28 by respective rotatable couplings in the form of in-line swivels 32a, 32b and 32c, which facilitate rotation of the risers 30a, 30b and 30c relative to the inner mooring canister 28 about their own axes 34a, 34b and 34c (Figure 4). Typically, the risers 30 are fluid flow risers for the flow of production fluids (oil and gas) from a number of subsea wellheads, production facilities or the like (not shown) to storage tanks (also not shown) on the FPSO 12. However, in certain circumstances, such as in a well stimulation or treatment activity where it is desired to inject fluid into a formation, fluid may be supplied from the FPSO 12 downhole. Thus fluid may be transferred from the FPSO 12 down through the risers 30.

The outer mooring canister 20 is located in the ocean 22 and moored, for example, to a seabed shown schematically at 36, using the catenary mooring chains 24 and suitable anchors (not shown). The arrangement of the mooring chains 24 and anchors restrict rotation of the outer mooring canister 20 about its axis 26 under external loading. The FPSO 12 is moored to the outer mooring canister 20 by means of the frame 18 of the cantilever connector assembly 16, which is secured to the inner mooring canister 28, as will be described in more detail below. Accordingly, as the inner mooring canister 28 is rotatably mounted within the outer mooring canister 20, the FPSO 12 may weathervane about the outer mooring canister 20 under external loading including wind, wave, tidal and/or current loading. For example, in the event that a prevailing wind acting on the FPSO changes from a direction towards the bow 14 of the FPSO 12 to a direction A (Figure 1 ) across the beam of the FPSO, the FPSO will weathervane so that the bow 14 once more points in the direction of the prevailing wind. Such movement occurs without rotation of the outer mooring canister 20 about its axis 26, by virtue of the inner canister 28 rotating within the outer canister. During this movement of the FPSO 12, the risers 30a to 30c begin to twist about one another in a helix pattern, through their connection with the inner mooring canister 28. Accordingly, weathervaning movement of the FPSO 12 is allowed for without excessive loading on the risers 30a to 30c, by allowing the risers to twist together.

Typically, the risers 30a to 30c will be arranged in a buoyant wave arrangement known in the art, to absorb movement of the risers 30a to 30c and to prevent damage to the risers at a touch down point on the seabed 36. The lazy wave can be located at depth with, for example, 500 metres of riser provided between the FPSO 12 and the lazy wave and which may be allowed to helix. Typically, up to 1800° of rotation of the FPSO 12 about the outer mooring canister 20 in either rotational direction from a static, rest position (prior to commencement of weathervaning) will be allowed for, equivalent to five full 360° weathervaning movements of the FPSO 12 in either rotational direction. Test analyses indicate that the risers 30a to 30c will helix around each other in a smooth curve, with a profile similar to that of a three strand rope. The risers 30a to 30c will typically be coated with a hard wearing, low friction material such as Rislan, to account for contact between the risers 30a to 30c during helixing.

The system 10 will now be described in more detail, with reference also to Figures 5 and 6, which are side views similar to Figure 3 showing the system prior to weathervaning of the FPSO 12, and following commencement of weathervaning, respectively. The mooring chains 24 have been left out of the views of Figures 5 and 6, for ease of illustration. Reference is also made to Figures 7 to 9 which are perspective, partial cross-sectional side and plan views, respectively, of part of the connector assembly 16 and upper portions of the outer and inner mooring canisters, 20, 28. The cross-sectional side view of Figure 8 is taken about the line X-X shown in Figure 9.

The outer mooring canister 20 includes a number of mooring arrangements, one for each of the mooring chains 24. The mooring arrangements each include a mounting bracket in the form of a mooring chain stopper 40 to which an end of the respective chain 24 is secured. The mounting arrangements also include guides 42 for the chains 24, each of which comprises a pair of spaced plates 44 coupled together by a pin 26. The chains 24 pass up along a passage 48 defined between the two plates 44 inwardly of the pin 46 to the stoppers 40, which allow the chains 24 to be drawn up through the guides 42 and secured to the stoppers 40. Coupling the chains 24 to the outer canister 20 in this fashion offers advantages in terms of restraining movement of the outer canister 20 under external loading. This is because the outer canister 20 will tend to pivot about the guides 42 under external loading from the FPSO 12 and/or loading due to prevailing weather/sea conditions. Consequently the tension in portions 50 of the chains 24 extending between the guides 42 and the stoppers 40 will vary around the circumference of the outer canister 20. For example, in a displaced position in which the outer mooring canister 20 has been tilted from the vertical position shown in the Figures due to the FPSO 12 surging astern, the tension in chain portions 50 located furthest away from the FPSO bow 14 will be greater than in chain portions 50 located adjacent the bow 14. This increased tension will tend to exert a force on an upper portion of the outer canister 20 tending to urge the canister back to the rest position (typically known as mooring centre) shown in the Figures.

The inner canister 28 is mounted within the outer canister 20 by upper and lower vertical bearings (not shown), which are located within housings 52, 54 of the outer canister 20. The bearings typically support both radially and axially directed loading. The bearing housings 52 and 54 also facilitate engagement of the inner canister 28 to the outer canister 20 so as to restrict translation of the inner canister 28 relative to the outer canister 20 in use of the system 10. Typically, the bearings in the bearing housings 52 and 54 will be provided as part of a locking arrangement (not shown) for securing the inner and outer canisters 28 and 20 together. The locking arrangement secures the canisters 20, 28 against relative longitudinal motion following location of the inner canister 28 fully within the outer canister 20, as shown in the Figures.

The connector assembly 16 includes an outer gimbal 56 which is mounted for rotation relative to the frame 18 about an axis 58 by trunnions 60, which permit relative rotation in the direction of the arrows B-B' (Figure 3). The connector assembly 16 also includes an inner gimbal 62, best shown in Figures 7 to 9, which is mounted for rotation relative to the outer gimbal 56 about an axis 64 by means of trunnions 66 (one shown), thereby permitting rotation in the direction of the arrows C-C (Figure 4). The inner and outer gimbals 62, 56 thus allow for relative movement between the FPSO 12 and the outer mooring canister 20 about the two axes 58 and 64, allowing for external loading on the FPSO tending to move the vessel off mooring centre and thus away from the rest position shown in the Figures. For example, a large tidal surge force acting on the bow 14 of the FPSO 12 may result in a significant surge of the vessel astern. This movement is allowed for by rotation between the outer gimbal 56 and the inner gimbal 62, and results in the outer canister 20 pivoting generally about the guides 42 in the direction B to a position inclined from the vertical (not shown). Similarly, a heavy seastate may result in the FPSO 12 rolling about a main axis (not shown) of the vessel, this being allowed for by rotation of the inner gimbal 62 relative to the outer gimbal 56 in the direction C-C. Once again, the outer canister 20 pivots generally about the guides 42 to an inclined position.

The inner mooring canister 28 is secured to the inner gimbal 62 and thus held against rotation relative to the inner gimbal. This is achieved using a securing arrangement which takes the form of a latch ring arrangement 66. The latch ring arrangement 66 comprises two latch ring segments in the form of latch half rings 68a and 68b, which are arranged to grip an upper end 70 of the inner canister 28. The latch half rings 68a 68b are mounted on the inner gimbal 62 and are movable from a release position towards one-another in the direction of the arrows D and D', to grip the upper end 70 of the inner canister 28, and in opposite directions E and E', to release the inner canister 28. The latch ring arrangement 66 includes two pairs of hydraulic actuators 72a, 72a' and 72b, 72b' associated with each latch half ring 68a and 68b, respectively, for moving the half rings between their release and latching positions. These actuators 72 each comprise a respective piston 74 mounted in a cylinder 76 which controls the position of the pistons 74.

Figure 10 is a view of the actuators 72a and 72b, which will now be described. However, it will be understood that the actuators 72a' and 72b' are of similar construction. Thus turning now to Figure 10, it will be seen that the piston 74b has a piston head 78b which is shaped to engage a corresponding shaped piston head 80a on the piston 74a. With the latch half rings 68a, 68b in their locking position shown in the Figures, apertures in the piston heads 78b, 80a align to define a bore 82 in which a locking pin 84 can be located, to lock the half rings together and thus restrain the half rings in their latching positions. A locking bar 85 is coupled to the pin 84 and located around part of the piston head 80a following location of the pin 84 in the aligned apertures, to restrain the pin against release and thus to lock the latch rings 68a, 68b in the latching position. In the latching position, the half rings 68a, 68b engage in a circumferential recess 86 below the inner canister upper end 70, to both restrict the inner canister 28 against rotation to the inner gimbal 62, and to resist separation of the inner canister from the gimbal.

Connecting tubes 88a, 88b and 88c extend through an internal passage of the inner mooring canister 28 between the upper end 70 and a lower end 92, terminating in connectors in the form of respective flanges 94a, 94b and 94c at the upper end and 96a, 96b and 96c at the lower end (Figures 4 and 5). The flanges 94a to 94c provide for connection to transfer lines or jumpers (not shown) extending to the FPSO 12, whilst the lower flanges 96a to 96c serve for connection to the in-line swivels 32a to 32c on the risers 30a to 30c. The swivels 32a to 32c permit rotation of the risers 30a to 30c about their respective axes 34a to 34c, as described above. Accordingly, during twisting together of the risers 30a to 30c, torsional forces in the risers can be reduced or minimised, thereby permitting a greater degree of twisting together of the risers 30a to 30c, and thus a greater number of potential weathervaning rotations of the FPSO 12 about the outer canister 20, than may otherwise be the case.

A number of riser spacers in the form of spiders are provided spaced at intervals along the length of the risers 30a to 30c, one of which is shown and given the reference numeral 98 (Figure 5). These riser spiders 98 facilitate in maintaining a spacing between the risers 30a to 30c, and in guiding the risers during helixing. As noted above, the system 10 is shown in a position prior to weathervaning of the FPSO 12 in Figure 5, and following commencement of weathervaning in Figure 6. In particular, Figure 6 shows the system 10 following a weathervaning of the FPSO 12 where the vessel has rotated in the direction F about the outer canister axis 26 (Figure 1). This has resulted in the risers 30a to 30c commencing to twist together, as shown in the Figure 6. The system is used to moor an FPSO 12 and commence production as follows. The inner canister 28 is first located within and locked against translation relative to the outer canister 20, as described above. Typically this is performed in a workshop onshore, but may be performed offshore on a suitable vessel. The mooring chains 24, carrying suitable anchors, are then secured to the connected canisters 20 and 28, and the chains and canisters deployed into the ocean 22. The chains 24 are each anchored to the seabed 36 in locations appropriate for positioning the connected canisters 20, 28 at a desired location on the ocean surface 100.

The risers 30a, 30b and 30c are connected to fixed subsea pipes extending from trees (not shown) on respective wellheads, run up to surface and the swivels 32a, 32b and 32c on the risers connected to the respective lower flanges 96a, 96b and 96c of the inner canister 28. It will be understood that the risers 30a-c may alternatively be connected directly to the trees. The FPSO 12 to be moored is then brought up to a position where the inner gimbal 62 is approximately above the canisters 20, 28 and a winch cable (not shown) is coupled to the upper end 70 of the inner canister 28. The canisters 20, 28 are then winched up to the FPSO 12, the inner canister upper end 70 entering the inner gimbal 62. The hydraulic actuators 72 are then actuated to move the latch half-rings 68a, 68b to grip the inner canister 28, as best shown in Figures 7 to 9.

The inner and outer canisters 28, 20 are now securely latched to the FPSO 12, and the transfer lines can be coupled to the upper flanges 94a, 94b and 94c of the inner canister 28. Following appropriate testing of the flow lines, flow of production fluids through the risers 30a-30c, connecting tubes 88a to 88c and the transfer lines to the storage tanks on the FPSO 12 can now commence. When it is desired to move the FPSO 12 off station, such as at the end of production or in an emergency disconnect, the above procedure is generally followed in reverse.

Various modifications may be made to the forgoing without departing from the spirit and scope of the present invention. For example, the system may comprise any desired number of risers depending upon factors including relative dimensions of the first and second mooring portions and the diameters of the risers.

Where the risers are fluid flow risers, they may be for the passage of fluid from the vessel into the second mooring portion and thus into the fluid flow risers. Accordingly, the system may be used for injecting fluid into a well/formation such as in a well treatment or flow stimulation operation. The risers may comprise or take the form of power and/or control cables, which may facilitate power supply, data transmission and/or supply of hydraulic control fluid.

The system may comprise a second mooring portion rotatable coupling, which may facilitate rotation between the inner gimbal member and the second mooring portion. This rotatable coupling may be selectively activatable to permit rotation of the second mooring portion relative to the inner gimbal member. This may be desirable if weathervaning of the vessel about the first mooring portion results in significant twisting together of the risers and it is desired to restrict further twisting, for example, to minimise further torsional loading on the risers. The system may then comprise a multi-path swivel for connecting the risers to the vessel. Such swivels are known for example from WO-2006/037964.

Claims

1. An offshore vessel mooring and riser inboarding system, the system comprising: a support assembly mounted on a vessel; a first mooring portion located in an offshore environment and moored such that rotation of the first mooring portion is restricted; a second mooring portion rotatably coupled to the first mooring portion and secured to the support assembly for rotation therewith relative to the first mooring portion; a plurality of risers coupled to the second mooring portion such that the risers can twist together on rotation of the second mooring portion relative to the first mooring portion; and a rotatable coupling associated with each riser and by which the risers are coupled to the second mooring portion, to thereby facilitate rotation of the risers relative to the second mooring portion.

2. A system as claimed in claim 1, wherein the first mooring portion is moored such that rotation of the first mooring portion about its own axis is restricted.

3. A system as claimed in either of claims 1 or 2, wherein the rotatable coupling associated with each riser facilitates rotation of the risers relative to the second mooring portion about their own axes.

4. A system as claimed in any preceding claim, wherein the risers are arranged such that, in use, they twist together in a helix pattern.

5. A system as claimed in any preceding claim, wherein the first mooring portion is an outer mooring portion, and the second mooring portion is an inner mooring portion located within the outer mooring portion.

6. A system as claimed in any preceding claim, wherein the second mooring portion is restricted against translational movement relative to the first mooring portion.

7. A system as claimed in any preceding claim, wherein the first and second mooring portions are elongate tubular mooring canisters.

8. A system as claimed in any preceding claim, wherein the rotatable couplings associated with the risers are in-line swivels, and wherein the second mooring portion comprises a riser connector for each rotatable coupling, which riser connectors serve for connecting the risers to the second mooring portion.

9. A system as claimed in claim 8, wherein the riser connectors are each provided at a lower end of the second mounting portion, and wherein the second mooring portion comprises a plurality of connecting tubes extending along a passage within the second mooring portion, each of which is coupled to a respective riser to facilitate fluid flow between the risers and the second mooring portion.

10. A system as claimed in any preceding claim, wherein the first mooring portion comprises a plurality of mooring arrangements for mooring the first mooring portion in the offshore environment and for resisting movement of the first mooring portion away from a neutral position in which a main axis of the first mooring portion is in a vertical orientation, each mooring arrangement comprising a mounting bracket by which a mooring line can be secured to the first mooring portion, and a mooring line guide adapted to receive the mooring line for guiding the line to the bracket.

11. A system as claimed in claim 10, wherein the mooring line guides are adapted to permit movement of the risers relative thereto, and each mounting bracket is provided above a respective mooring line guide.

12. A system as claimed in claim 1 1 , wherein each mounting bracket is provided at an upper end of the first mooring portion, and each mooring line guide at a lower end of the first mounting portion, to facilitate maximisation of a restorative force exerted on the second mooring portion by the mooring lines in use of the system.

13. A system as claimed in any preceding claim, wherein the support assembly comprises a securing arrangement for releasably securing the second mooring portion to the support assembly and thus to the vessel, and wherein the securing arrangement takes the form of a latch ring arrangement for releasably gripping the second mooring portion.

14. A system as claimed in claim 13, wherein the latch ring arrangement comprises a plurality of latch ring segments which together form a latch ring, and wherein the latch ring segments are movable between release and latching positions for releasably gripping the second mooring portion.

15. A system as claimed in any preceding claim, comprising at least one riser spacer coupled to the risers, which facilitates controlled twisting of the risers and/or control of spacings between the risers.

16. A system as claimed in any preceding claim, wherein the support assembly comprises a rotatable support which facilitates rotation of the vessel about the second mooring portion, and wherein the rotatable support comprises: an outer gimbal member mounted for rotation relative to a support structure of the support assembly and which support structure is mounted on the vessel; and an inner gimbal member mounted for rotation relative to the outer gimbal member, the outer and inner gimbal members together permitting relative rotation between the vessel and the second mooring portion about two mutually perpendicular axes of rotation.

17. A system as claimed in claim 16, wherein the rotatable support further comprises a second mooring portion rotatable coupling which facilitates rotation between the inner gimbal member and the second mooring portion, and wherein the rotatable coupling is selectively activatable.

18. A vessel comprising an offshore vessel mooring and riser inboarding system according to any one of the preceding claims.

19. A method of mooring a vessel in an offshore environment, the method comprising the steps of: locating a first mooring portion in an offshore environment and mooring the first mooring portion such that rotation of the first mooring portion is restricted; rotatably coupling a second mooring portion to the first mooring portion; securing the second mooring portion to a support assembly on the vessel for rotation therewith relative to the first mooring portion; coupling a plurality of risers to the second mooring portion such that the risers can twist together on rotation of the second mooring portion relative to the first mooring portion; and rotatably coupling each riser to the second mooring portion using respective rotatable couplings, such that the risers can rotate relative to the second mooring portion.

20. A method as claimed in claim 19, comprising arranging the risers such that they twist together in a helix pattern.

21. A method as claimed in either of claims 19 or 20, comprising reducing torsion in the risers using rotatable couplings in the form of in-line swivels, and coupling the risers to the second mooring portion by means of a riser connector, there being a riser connector for each rotatable coupling.

22. A method as claimed in any one of claims 19 to 21, comprising arranging the mooring lines to provide a restorative force on the second mooring portion following movement away from a neutral position, to urge the second mooring element towards the neutral position.

23. A method as claimed in any one of claims 19 to 22, comprising mooring the first mooring portion using a plurality of mooring lines and mooring arrangements, each mooring line received by a mooring line guide which passes the mooring line to a mounting bracket by which the mooring line is secured to the second mooring portion.

24. A method as claimed in any one of claims 19 to 23, comprising releasably securing the second mooring portion to the support assembly and thus to the vessel using a securing arrangement in the form of a latch ring arrangement.

25. A method as claimed in claim 24, comprising releasably gripping the second mooring portion using the latch ring arrangement.

26. A method as claimed in any one of claims 19 to 25, comprising coupling at least one riser spacer to the risers to facilitate controlled twisting of the risers and/or control of spacings between the risers.

27. An offshore vessel mooring and riser inboarding system, the system comprising: a support assembly mounted on a vessel; a first mooring portion located in an offshore environment and moored such that rotation of the first mooring portion is restricted; a second mooring portion rotatably coupled to the first mooring portion and secured to the support assembly for rotation therewith relative to the first mooring portion; and a plurality of risers coupled to the second mooring portion such that the risers can twist together on rotation of the second mooring portion relative to the first mooring portion.