GB2459739A

GB2459739A - A counterbalanced cantilever connector assembly for a vessel

Info

Publication number: GB2459739A
Application number: GB0808261A
Authority: GB
Inventors: Peter Graham Jay; Robin Stuart Colquhoun
Original assignee: SIGMA OFFSHORE Ltd
Current assignee: SIGMA OFFSHORE Ltd
Priority date: 2008-05-07
Filing date: 2008-05-07
Publication date: 2009-11-11
Anticipated expiration: 2028-05-07
Also published as: GB0808261D0; GB2459739B

Abstract

The invention relates to a connector assembly for connecting a vessel 12 to a mooring element 16 in an offshore environment, to a vessel comprising such a connector assembly, and to an offshore vessel mooring system comprising such a connector assembly. In an embodiment of the invention, a cantilever connector assembly 10 for connecting a vessel 12, such as an FPSO to a mooring element in the form of a mooring canister 16 is disclosed. The assembly comprises a cantilever mooring structure 20 for releasably connecting the assembly to the canister, a mounting arrangement 22 for releasably connecting the cantilever mooring structure to the vessel, the mounting arrangement adapted to be coupled to structural load bearing members (23, Fig 4) of the vessel for transferring loading on the cantilever mooring structure during use to the structural load bearing members, and a counterweight arrangement 24 for counterbalancing at least part of the loading on the cantilever mooring structure during use.

Description

CONNECTOR ASSEMBLY FOR VESSEL, VESSEL AND MOORING SYSTEM The present invention relates to a connector assembly for connecting a vessel to a mooring element in an offshore environment, to a vessel comprising such a connector assembly, and to an offshore vessel mooring system comprising such a connector assembly.

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) during 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 S... * . S...

Whilst some vessels are constructed and designed specifically for these purposes, many FPSOs and FSOs are conversions of existing trading tankers. Converted * . vessels of this type have usually functioned adequately, but there is a continuing need * ** for a substantial reduction in costs in order to improve the economics of prospective :.: development and production of oil and gas fields, particularly those which are *..S.S * 25 currently deemed to be marginal. Tankers used in the past have often required extensive conversion work to enable them to operate as an FPSO or FSO. The extent of conversion work required has depended upon factors including the particular circumstances under which the vessel is to be moored offshore.

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 they do not allow a vessel to weathervane continuously without restriction; 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/GB2005/003 766 (published as WO-20061037964), 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-20061037964 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-2006/03 7964 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. * .* * S *

It is now desired to yet further improve upon systems known prior to the development of WO-2006/03 7964. * SS * * * **. *

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

* *25 According to a first aspect of the present invention, there is provided a cantilever connector assembly for connecting a vessel to a mooring element in an offshore environment, the assembly comprising: a cantilever mooring structure for releasably connecting the assembly to the mooring element; a mounting arrangement for releasably connecting the cantilever mooring structure to the vessel, the mounting arrangement adapted to be coupled to structural load bearing members of the vessel for transferring loading on the cantilever mooring structure during use to the structural load bearing members; and a counterweight arrangement for counterbalancing at least part of the loading on the cantilever mooring structure during use.

Providing a connector assembly having a cantilever mooring structure and with a mounting arrangement for releasably connecting the mooring structure to the vessel permits rapid fitting of the connector assembly on the vessel, and also rapid release of the connector assembly from the vessel. This in turn facilitates, for example, rapid conversion of a vessel such as an existing tanker for use as an FPSO or FSO, and also rapid conversion of the vessel back to its original configuration. In particular, the invasive procedures often employed in the past in converting existing vessels can be avoided, or the degree of invasion required at least significantly reduced.

Furthermore, providing a counterweight arrangement to counterbalance at least part of the loading on the cantilever mooring structure may counteract external loading applied to the mooring structure, with consequent savings in terms of weight, materials and cost of the structure and in terms of increased safety.

The present invention may seek to avoid or minimise the traumatic invasion of the structure of a vessel which is implicit in many prior FPSO/FSO turret mooring systems and weathervaning mooring systems. The present invention may additionally or alternatively seek to minimise the time taken to install the cantilever connector assembly (in particular the cantilever mooring structure, which niay be prefabricated) * ** on the vessel, and to remove the structure on completion of a deployment campaign offshore. This may assist in miniinising the unproductive time of a, for example, **S * 25 tanker between assignments. The present invention may additionally or alternatively seek to create an assembly which offers as little impediment as possible to green water passing over the bow, thereby avoiding excessive hydrodynamic loading on the mooring structure. It will be understood that green water' is a term known in the art as referring to water intruding on to the main deck of a vessel, for example in poor or heavy sea conditions.

The connector assembly may be for connecting the vessel to a mooring element of a vessel mooring and riser inboarding system in which the mooring element takes the form of a mooring buoy, canister or other buoyant element. A typical such system is that disclosed in WO-20061037964, the disclosure of which is incorporated herein by way of reference. 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 riser in such a system may comprises or take the form of a fluid flow riser or flowline, and the system may comprise a transfer line adapted to be coupled to the riser, which may be a transfer flowline. Connection of the cantilever mooring structure to the mooring element may facilitate flow of fluid between the fluid flow riser, the transfer flowline and the vessel. The transfer flowline may be for the passage of fluid from the fluid flow riser into the transfer flowline and to the vessel, or vice-versa. Where the riser comprises or takes the form of a power and/or control cable, the transfer line may provide an electrical and/or hydraulic and/or other connection to the riser, or other such suitable, separate lines may be provided. This may facilitate power supply, data transmission and/or supply of hydraulic control fluid.

The connector assembly may comprise a rotatable coupling, which may facilitate rotation of the connector assembly about or with reference to the mooring element, * and thus movement of the vessel relative to the mooring element. The connector * : : * assembly may comprise an outer gimbal member, which may be mounted for rotation relative to the cantilever mooring structure. The connector assembly may also comprise an inner gimbal member mounted for rotation relative to the outer gimbal * * member. Where the connector assembly comprises a rotatable coupling, the rotatable i: coupling may facilitate rotation of the inner gimbal member relative to the mooring element. The rotatable coupling, inner gimbal member and outer gimbal member may * 25 together permit relative rotation between the vessel and the mooring element about three mutually perpendicular axes of rotation.

The connector assembly may further comprise a swivel arrangement comprising a swivel and a connector coupled to the swivel by an at least one transfer line, which may be flexible/deformable. The connector of the swivel arrangement may be adapted to be coupled to the mooring element, and may be adapted to be secured against rotation relative to the mooring element. In use, rotation of the connector assembly (and thus the vessel) about or relative to the mooring element may cause a relative rotation between the vessel and said transfer line. This relative rotation may be accommodated by the swivel. The connector assembly may comprise at least one communication line coupled to the swivel and thereby to the transfer line, and the communication line may be provided on the cantilever mooring structure. In this fashion, where the mooring element is coupled to a riser, the communication line may provide for connection between the vessel and the riser via the transfer line.

The cantilever mooring structure may define a mooring location where the structure is adapted to be connected to the mooring element, and the swivel of the swivel arrangement may be adapted to be located spaced from the mooring location. The swivel may be adapted to be located at an elevated position relative to the mooring location. In this fashion, and where the cantilever connector assembly is provided at or in the region of a bow of a vessel, the swivel may be raised above an anticipated green water intrusion zone in the bow area, reducing the likelihood of damage to the swivel. The connector assembly may comprise a tower coupled to the cantilever mooring structure, and the swivel may be coupled to the tower for mounting the swivel at a location spaced from the mooring location.

* *. The cantilever mooring structure may be a frame structure, and may be a space frame.

This may offer advantages in terms of weight and cost savings, optimised strength, and the reduction of wind and/or water (for example, green water) loading on the structure. The structure may comprise a plurality of tubular members.

* The cantilever mooring structure may be modular, and may comprise a plurality of cantilever mooring structure units which are coupled together to form the complete *0*I** * 25 structure. This may offer advantages in terms of facilitating transportation, lifting and assembly/disassembly of the structure. In particular, providing the structure in modular form may facilitate assembly of the structure on the vessel, and may enable use of lower lifting capacity cranes than would otherwise be the case. The units may be adapted to be releasably coupled together, and may comprise releasable couplings, which may be quick-release couplings.

The mounting arrangement may comprise a plurality of couplings, which may be quick-release couplings, for releasably connecting the cantilever mooring structure to the vessel. The couplings may be flanged, pin-jointed couplings, or may each comprise interengaging male and female coupling members.

The mounting arrangement may comprise at least one load transfer member, which may be adapted to be coupled to an at least one structural load bearing member of the vessel and to the cantilever mooring structure, for transferring loading on the cantilever mooring structure during use to the structural load bearing member or members.

The cantilever mooring structure may cOn1priSe at least one support, which may be a support column, and preferably comprises a plurality of supports. The cantilever mooring structure may be coupled to the vessel by the at least one support, through the mounting arrangement, and the support may therefore be coupled to the mounting arrangement. An at least one support may be located at a fulcrum line of the connector assembly. It will be understood that the fulcrum line may be a line about which the loads in the connector assembly (typically when coupled to the mooring element in a calm sea state) would be balanced in the absence of external loading such * *. as wind, wave, tidal and/or current loading. Said at least one support may be adapted to be coupled to a structural load bearing member or members of the vessel such that the fulcrum line is at or above the location of said member or members. * . * * * I** S

*. The counterweight arrangement may be provided at or adjacent an inboard!sternmost * * end of the connector assembly, and may be provided at an inboardlsternmost end of the cantilever mooring structure. In this fashion, the counterweight arrangement may *SsS*S act to resist tensile loading in the stem portion or region of the connector assembly (in particular in the mooring structure) in use, such as when the vessel heaves up relative to the mooring element. The counterweight arrangement may comprise a single counterweight or may be made up from a plurality of counterweights.

According to a second aspect of the present invention, there is provided a vessel comprising a cantilever connector assembly for connecting the vessel to a mooring element in an offshore environment, the cantilever connector assembly comprising: a cantilever mooring structure for releasably connecting the assembly to the mooring element, the cantilever mooring structure being releasably connected to the vessel by a mounting arrangement which is coupled to structural load bearing members of the vessel for transferring loading on the cantilever mooring structure during use to the structural load bearing members; and a counterweight arrangement for counterbalancing at least part of the loading on the cantilever mooring structure during use.

Further features of the cantilever connector assembly forming part of the vessel defined in the second aspect of the invention are defined above in relation to the first aspect of the invention.

According to a third aspect of the present invention, there is provided an offshore vessel mooring system comprising a mooring element adapted to be located in an offshore environment and a cantilever connector assembly for connecting a vessel to the mooring element, the cantilever connector assembly comprising: a cantilever mooring structure for releasably connecting the assembly to the mooring element; a mounting arrangement for releasably connecting the cantilever mooring structure to the vessel, the mounting arrangement adapted to be coupled to structural load bearing *.** members of the vessel for transferring loading on the cantilever mooring structure during use to the structural load bearing members; and a counterweight arrangement for counterbalancing at least part of the loading on the :.: : :* cantilever mooring structure during use.

* *, Further features of the cantilever connector assembly forming part of the offshore :.: 5 vessel mooring system of the third aspect of the invention are defined above in * relation to the first aspect of the invention.

According to a further aspect of the present invention, there is provided a cantilever space frame structure whose purpose is to provide connection between a bow mooring and riser inboarding system and a weathervaning offshore vessel, the frame comprising two fore-aft tubular trusses with interconnecting structural members; a counterweight or counterweights at the aft end of this space frame to act as kentledge so as to reduce the tension forces between the aft of the cantilever frame and the ship caused by the downward component of the load from the mooring system acting at the forward end of the cantilever frame in conjunction with the fulcrum part-way between the fore and aft ends of the cantilever frame; two (or possibly more) connections between the frame and the ship port and starboard at the fulcrum line and at the aft end of the cantilever frame, the ship side of these connections being the ends of vertical structural members pre-inserted into the ship and welded or otherwise secured in place, such pre-inserted members being anchored in the longship and thwartship directions at both focsle deck level and main deck level (if these levels are different) and anchored in the vertical direction at main deck level; such connections being designed in such a way as to be easily made or unmade so as to facilitate rapid installation of the cantilever frame on the vessel and rapid removal on completion of the campaign; the said trusses carrying between them at their forward end the mooring gimbals and mooring swivel or other mooring system interface devices transferring loads from the mooring system to the cantilever space frame; preferably also the cantilever space frame carrying at its forward end a lightweight tubular space frame tower supporting single or multi-path fluid swivel and electrical sliprings connected by flexible pipe and cable jumpers to the mooring swivel assembly or other interface devices with the mooring system.

* ::* An embodiment of the present invention will now be described, by way of example * . 20 only, with reference to the accompanying drawings, in which: **** : . Figs. 1, 2 and 3 are side, plan and perspective views, respectively, of a cantilever connector assembly, and of part of a vessel incorporating the cantilever connector * : assembly, in accordance with an embodiment of the present invention; * 25 * S*.

* Fig. 4 is an enlarged, transverse sectional view through a forward part of the vessel taken about line A-A of Fig. 1, illustrating the relationship between components of the connector assembly and structural members of the vessel; Fig. 5 is an enlarged perspective view of the connector assembly and the vessel of Fig. 1, taken from a different angle to the view of Fig. 3, and illustrating operation in calm weather; Fig. 6 is a view similar to Fig. 5 but illustrating operation under applied external loading such as wind, wave, tidal and/or current loading; and Fig. 7 is a sectional plan view of the vessel taken about line B-B of Fig. 1, and showing a typical disposition of supports of a cantilever mooring stmcture of the connector assembly standing on and connected to the structural members of the vessel.

Turning firstly to Figs. 1, 2 and 3, there are shown side, plan and perspective views, respectively, of a cantilever connector assembly indicated generally by reference numeral 10, and of part of a vessel incorporating the cantilever connector assembly 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 drawings. It will be understood 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 connector assembly 10 serves for connecting the FP SO 12 to a mooring element * .20 in the form of a buoyant mooring canister 16, which is of a type disclosed in WO-*...

2006/037964, the disclosure of which is incorporated herein by way of reference. The mooring canister 16 is shown located in an offshore environment such as a sea, but which may equally be an ocean, river, estuary or the like. In Fig. 1, the sea surface L:: level is indicated by reference numeral 18.

*...S.

* The connector assembly 10 comprises a cantilever mooring structure 20 for releasably connecting the assembly 10 to the mooring canister 16. A mounting arrangement 22 is provided for releasably connecting the cantilever mooring structure 20 to the FPSO 12, and the mounting arrangement 22 is adapted to be coupled to structural load bearing members in the form of main frames 23 of the FPSO 12, which are shown in the transverse sectional view of Fig. 4. As will be described in more detail below, this connection of the cantilever mooring structure 20 to the vessel main frames 23 serves for transferring loading on the cantilever mooring structure 20 during use to structural load bearing members of the frames 23. A counterweight arrangement 24 is also provided for counterbalancing at least part of the loading on the cantilever mooring structure 20 during use.

The mooring canister 16 forms part of an offshore vessel mooring system incorporating the connector assembly 10 and the canister 16 and, in the illustrated embodiment, serves for transferring reservoir fluids such as oil or gas from a subsea reservoir (not shown) to the FPSO 12. It will be understood that these fluids may, for example, be transferred directly from one or more subterranean reservoirs via wellbead trees or subsea production control equipment, or from one or more offshore storage platforms or other vessels. This is achieved via flow risers extending between the tree, production equipment, platform or vessel and the mooring canister 16, three of which are shown in Fig. 1 and given the reference numerals 26a, 26b and 26c.

The mooring structure 20 is located on a forecastle 25 at the bow 14 of the FPSO (Fig. 2), but may be located on a main deck of the FPSO 12 in the event that the FPSO does not have a raised forecastle area. The mooring structure is mounted via a number of sets of supports 27a, 27b and 27c, each set comprising three supports in the form of tubular support columns. The set of columns 27a are provided bowmost and the set 27c sternmost on the FPSO 12. These sets of support columns 27ac serve for *20 transmitting loading on the connector assembly 10 during use to the main frames 23 of the FPSO 12, as will be described in more detail below. The mooring canister 16 is inherently buoyant, and designed such that an upper portion 17 of the canister protrudes above the sea surface 18 with the fluid filled risers 27a-c suspended from * *. the canister. This enables hook-up to the connector assembly 20. However, the :.: 5 canister 16 may be arranged to reside below the sea surface, its location indicated by a **.*** * marker buoy (not shown), which may offer protection from certain external loading such as wind and wave.

The connector assembly 10 and the FPSO 12 are shown in a neutral position, referred to as mooring centre, in Figs. 1 to 3 and also in the enlarged perspective view of Fig. 5, which is taken from a different angle to Fig. 3. When the FPSO 12 is connected and thus moored to the mooring canister 16, external loading acting on the FPSO 12 (such as wind, wave, tidal and/or current loading) causes the FPSO 12 to pitch, roll, heave and/or surge, resulting in a shift off mooring centre. A condition in which the FPSO 12 has experienced external loading causing it to surge astern is illustrated in Fig. 6. This and other movements of the FPSO 12 are accommodated by inner and outer gimbals and a rotatable coupling, shown in the Figures, and which will be described in more detail below.

This movement of the FPSO 12 causes the mooring canister 16 to exert an additional load on the cantilever mooring structure 20, above the loads applied when the canister and the associated risers 26a-c are drawn up and suspended from the structure 20.

This additional loading is applied in the direction of the arrow C shown in Fig. 6, and results in a turning moment about a fulcrum line 34 (Figs. 2 and 7), as shown by the arrow D of Fig. 6. As a result, an additional compressive loading is applied to the bowmost support columns 27a, whilst the sternmost support columns 27c are placed in tension. The counterweight arrangement 24 on the mooring structure 20 acts to counteract such tensile loading, and thus to reduce the tensile loads borne by the support columns 27c, other parts of the mooring structure 20 and the FPSO main frames 23. The fulcrum line 34 is a line about which the loads in the connector assembly 10 (typically when coupled to the mooring element 16 in a calm sea state) would be balanced in the absence of external loading such as wind, wave, tidal and/or * current loading. Positioning the support columns 27a at the fulcrum line 34 offers advantages in terms of reducing loading borne by the connector assembly, particularly the mooring structure 20, during use. * .* * * * *** S

The counterweight arrangement 24 includes two sets of counterweight units 36a and * * 36b, each of which comprise a number of counterweights 38a and 38b, respectively.

:.: *:25 These counterweights 38a, 38b are typically of a low cost, dense material such as 5.5.5 * concrete, and can be bolted or otherwise coupled together to form the units 36a, 36b.

In this fashion, the units 36a, 36b can be assembled on the FPSO 12 with the counterweights 38a, 38b lifted individually on to the FPSO, if desired. The total mass of the counterweight arrangement 24 will depend upon factors including vessel size and the particular operating parameters. However, it is anticipated that the counterweight arrangement mass may be of the order of 150-200 metric tonnes.

The risers 26a-c pass up through a passage (not shown) in the canister 16, and are mated to a connector 28 which forms part of a swivel arrangement 30. In addition to the connector 28, the swivel arrangement 30 comprises a swivel 32, which is coupled to the connector 28 by fluid transfer lines or jumpers 35a, 35b and 35c corresponding to the risers 26a, 26b and 26c, respectively. The swivel 32 is of a type known in the art such as is disclosed in WO-2006/03 7964, and allows relative rotation between the FPSO 12 and the mooring canister 16 without either any consequent loading on the risers 26a-c, transfer lines 35a-c or the swivel. The swivel 32 is a multi-path swivel, allowing for connection with multiple risers, but may be single-path swivel depending upon circumstances. The swivel 32 also includes electrical sliprings 33 (Fig.3) for electrical connectionlcontrol.

Additionally, the connector assembly 10 includes communication lines 40 (two shown in Figs. 5 and 6 and designated 40a, 40b) corresponding to each transfer line 36a-c, which extend from the FPSO 12 to the swivel 32. These communication lines 40 provide for communication between the FPSO 12, transfer lines 35a-c and the risers 26a-c, and the swivel enables this communication to be maintained irrespective of external loading and consequent movement of the FPSO 12 relative to the canister 16.

Where the risers 26a-c are fluid risers and the transfer lines 35a-c are fluid transfer lines, the communication lines 40 will provide for fluid flow up through the risers 26, *.. transfer lines 35 and communication lines 40 into storage tanks (not shown) in the . 20 FPSO 12. * **

The mooring canister 16 is located offshore using a number of catenary mooring chains 42, which pass through pulley joints 44 provided towards a lower end 46 of the canister 16, and which are anchored to a fixed collar 48 on the upper portion 17 of the * : * : 25 canister. Coupling the chains 42 to the canister 16 in this fashion offers advantages in terms of restraining movement of the canister 16 under external loading. This is because the canister will tend to pivot about the pulleys 44 under external loading from the FPSO 12. Consequently, the tension in portions 43 of the chains 42 extending between the pulleys 44 and anchor points on the collar 48 will vary around the circumference of the canister. For example, in the displaced position shown in Fig. 7, the tension in the chain portions 43 located furthest away from the FPSO bow 14 will be greater than in chains located adjacent the bow 14. This increased tension will tend to exert a force on the upper portion 17 of the canister 16 tending to urge the canister back to mooring centre (Fig.1).

The mounting arrangement 22 of the connector assembly 10 will now be described in more detail, with reference also to Fig. 7, which is a sectional plan view of the FPSO 12 taken about line B-B of Fig. 1. As described above, the mooring structure 20 includes a number of sets of support columns 27a-c. The mounting arrangement comprises a number of beams 50a, 50b and 50c corresponding to the columns 27a, 27b and 27c, respectively, and upon which the columns are located. These beams 50a-c extend transversely of a deck 52 of the forecastle 25, and are located above and secured to the main frames 23a, 23b and 23 c respectively of the FPSO 12. The frames 23 are indicated in Fig. 7 in dashed outline. Typically the beams 50a-c will be located above the deck 25, to avoid invasive cutting into the deck structure. In this fashion, the support columns in each of the sets 27a-c are coupled to the FPSO main frames 23a-c, with the result that loading on the mooring structure 20 during use is transmitted through the columns 27 and beams 50 to the main frames 23.

Accordingly, the inherent strength of the frames 23, required and built into the vessel during construction, can be utilised to counteract loading on the connector assembly, in particular the mooring structure 20, during use. The connector assembly 10 can *.. therefore be mounted on the FPSO 12 without requiring an excessively invasive construction procedure such as required in certain prior systems. This enables both * quick assembly of the connector assembly 10, and quick disassembly for conversion of the FPSO to other duties. Thus, for example, where the FPSO 12 is a conversion of an existing tanker vessel, the tanker can be quickly converted for service as an FPSO L: and, if required, quickly converted back for service as a tanker, this is being achieved with minimum down-time. It will be noted that the main frames 23 of the FPSO 12 *.I * are of a conventional type, comprising a number of outer and inner columns 54 and 56 which are interconnected with upper and lower beams 58 and 60. However, strengthening members may be added to the structure, such as flange plates 62 and 64.

The support columns 27a-c are mounted on the transverse beams 50a-c typically via releasable couplings such as flanged couplings (not shown), although any suitable temporary coupling may be utilised. This facilitates removal of the mooring structure from the FPSO 12, when required. However, it will be understood that the support columns 27a-c may be welded if a long or permanent deployment is required.

With the mooring structure 20 mounted on the FPSO 12 as shown in the drawings, the mooring structure 20 extends overboard of the FPSO in a cantilever arrangement, for connection to the canister 16. This offers advantages in terms of providing for a bow surge of the FPSO 12 towards the canister 16 with minimal risk of the FPSO coming into contact with the canister. The mooring structure 20 itself is a space-frame structure primarily comprising spaced port and starboard trusses 66a, 66b which are coupled together via interconnecting arrangements 68a, 68b of tubular members, as best shown in Fig. 4. This enables the mooring structure 20 to be provided as a modular structure, which may be assembled on the FPSO 12. For example, the trusses 66a, 66b and the interconnecting arrangements 68a, 68b may be preassembled and lifted oil to the FPSO 12 separately, this permitting a crane having a lower lifting capacity than might otherwise be needed to be utilised. Although it will be understood that the modular components of the mooring structure 20 may be welded, it is preferred that the components be secured using suitable releasable connections, to facilitate removal of the mooring structure 20 from the FPSO 12. It will be noted that constructing the mooring structure 20 as a space frame offers advantages in comparison to prior mooring systems such as those employing plated mooring structures. This is because green water loading on the structure 20 will be * . 20 significantly lower, as a large portion of the green water will pass through the frame structure, thereby dissipating energy and reducing loading on the structure.

The mooring structure 20 is connected to the canister 16 at a mooring location 70 (Fig. 2), and carries a carries a support device 72 for the canister 16. The support device 72 is similar in construction and operation to that disclosed in WO-*..S.S * 2006/03 7964. However, in brief, the support device 72 comprises an outer gimbal ring 74, an inner gimbal ring 76 and a rotatable coupling 78. The outer gimbal ring 74 is mounted between the trusses 66a and 66b, and is rotatable about trunnions 80a, 80b.

Tile inner gimbal ring 76 is mounted for rotation relative to the outer ring 74 about trunnions (not shown) spaced 900 around the circumference of the outer ring 74 relative to tile trunnions 80a, b. The inner ring also carries the rotatable coupling 78, by which the canister 16 is coupled to the support device 72, and which allows for rotation of the inner gimbal ring 76 relative to the canister. In this fashion, rotation of the FPSO 12 relative to the canister 16 about three mutually perpendicular axes X, Y and Z (Fig. 3) is permitted. Accordingly, there are three degrees of freedom in movement of the FPSO 12 relative to the canister 16.

The swivel 32 of the swivel arrangement 30 described above is positioned at a raised position on the mooring structure 20 above the mooring location 70, on a relatively lightweight tower 82. The tower 82 can be lightweight in that it is isolated from the loading applied to the mooring structure 20 by the canister 16, and primarily only requires to support the load of the swivel 32. In a similar fashion to the mooring structure 20, the tower 82 is a space frame structure offering advantages in terms of dissipating green water energy. Furthermore, positioning the swivel 32 at a raised location moves the swivel, which is a complex and expensive piece of equipment, out of the green water surge zone in the region of the forecastle deck 52.

The connector assembly 10 also includes a winch 84 for pulling in the canister 16 for subsequent connection. In use, the FPSO 12 approaches the canister 16 which, as described above, typically protrudes from the sea surface 18. The FPSO is brought to a position where the canister 16 resides below the mooring location 70 of the mooring structure 20. Winch lines (not shown) are then attached to the canister in the region of * the upper canister portion 17, and the winches activated to raise the canister 16 and ***20 the risers 26a-c. The canister 16 is then locked into the rotatable coupling 78 using a **** * suitable locking mechanism, such as that disclosed in WO-2006/037964, and the :.: * canister 16 is then suspended from the cantilever mooring structure 20. The * connector 28 of the swivel arrangement 30 is then mated to the canister head, and the : risers 26a-c mated with receptacles (not shown) in the connector 28 for fluidly coupling the risers 26a-c to the jumpers 35a-c. Connection of the canister 16 to the *.I * . connector assembly 10 is completed by connecting any other control lines or cables (not shown) to control equipment through the electrical slipring 33, and the FPSO 12 is now moored to the canister 16 and ready to receive production fluids, Movement of the FPSO 12 off mooring centre is allowed for by the gimbals 74 and 76, rotatable coupling 78 and the swivel arrangement 30.

Various modifications may be made to the foregoing without departing from the spirit and scope of the present invention. For example, the connector assembly may be for use with many different types of mooring elements, and may follow the principles of other known types of mooring systems. The connector assembly may, for example, be modifiedlsuitable for use with mooring systems in which less than three degrees of freedom is provided for in movement of the vessel relative to the mooring element.

The mooring element may take the form of a mooring buoy or other buoyant element.

The risers and or transfer lines may be other types than fluid flow risers/transfer lines, andlor may provide for power supply, data transmission and/or supply of hydraulic control fluid.

Whilst the present invention has been described primarily in relation to the offloading of fluids, particularly reservoir fluids (oil andior gas) into a vessel, it will be understood that the principles of the present invention may be applied to the supply of fluids from a vessel into the risers coupled to the mooring element. Such fluids may comprise well fluids being transferred to a storage location such as on an offshore platform, or downhole such as buffer or treatment fluids. * a. * . * * SS *S*. * * **** * ** * * * S.. * ** * .* * S S S.. S

S..... * a

Claims

CLAIMS1. A cantilever connector assembly for connecting a vessel to a mooring element in an offshore environment, the assembly comprising: a cantilever mooring structure for releasably connecting the assembly to the mooring element; a mounting arrangement for releasably connecting the cantilever mooring structure to the vessel, the mounting arrangement adapted to be coupled to structural load bearing members of the vessel for transferring loading on the cantilever mooring structure during use to the structural load bearing members; and a counterweight arrangement for counterbalancing at least part of the loading on the cantilever mooring structure during use.
2. An assembly as claimed in claim 1, in which the cantilever mooring structure is a frame structure. 0)C

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3. An assembly as claimed in either of claims 1 or 2, in which the cantilever Q mooring structure is modular and comprises a plurality of cantilever mooring structure Ic) units which are coupled together to form the complete structure. Q20
4. An assembly as claimed in claim 3, in which the units are adapted to be releasably coupled together using releasable couplings.
5. An assembly as claimed in any preceding claim, in which the mounting arrangement comprises a plurality of couplings for releasably connecting the cantilever mooring structure to the vessel.
6. An assembly as claimed in claim 5, in which the couplings are flanged, pin-jointed couplings.
7. An assembly as claimed in claim 5, in which the couplings each comprise interengaging male and female coupling members.
8. An assembly as claimed in any preceding claim, in which the mounting arrangement comprises at least one load transfer member adapted to be coupled to an at least one structural load bearing member of the vessel and to the cantilever mooring structure, for transferring loading on the cantilever mooring structure during use to the structural load bearing member or members.
9. An assembly as claimed in any preceding claim, in which the cantilever mooring structure comprises at least one support column for coupling to the mounting arrangement to thereby couple the cantilever mooring structure to the vessel.
10. An assembly as claimed in any preceding claim in which at least one support is located at a fulcrum line of the connector assembly.
11. An assembly as claimed in claim 10, in which the at least one support is adapted to be coupled to a structural load bearing member or members of the vessel such that the fulcrum line is at or above the location of said member or members. L()

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12. An assembly as claimed in any preceding claim, in which the counterweight i.() arrangement is provided at or adjacent an inboard end of the cantilever mooring Q 20 structure.
13. An assembly as claimed in any preceding claim, in which the connector assembly is for connecting the vessel to a mooring element of a vessel mooring and riser inboarding system and in which the mooring element is buoyant.
14. An assembly as claimed in any preceding claim, in which the connector assembly comprises a rotatable coupling that facilitates rotation of the connector assembly with reference to the mooring element, and thus movement of the vessel relative to the mooring element.
15. An assembly as claimed in any preceding claim, comprising an outer gimbal member mounted for rotation relative to the cantilever mooring structure, an inner gimbal member mounted for rotation relative to the outer gimbal member and a rotatable coupling which facilitates rotation of the inner gimbal member relative to the mooring element.
16. An assembly as claimed in any preceding claim, comprising a swivel arrangement having a swivel and a connector coupled to the swivel by an at least one flexible transfer line, and wherein the connector is adapted to be coupled to, and secured against rotation relative to, the mooring element.
17. An assembly as claimed in claim 16, in which the cantilever mooring structure defines a mooring location where the structure is adapted to be connected to the mooring element, and in which the swivel of the swivel arrangement is adapted to be located spaced from the mooring location.
18. An assembly as claimed in claim 17, in which the swivel is adapted to be 0) located at an elevated position relative to the mooring location.C

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19. A vessel comprising a cantilever connector assembly for connecting the vessel Q to a mooring element in an offshore environment, the cantilever connector assembly IC) comprising: Q 20 a cantilever mooring structure for releasably connecting the assembly to the mooring element, the cantilever mooring structure being releasably connected to the vessel by a mounting arrangement which is coupled to structural load bearing members of the vessel for transferring loading on the cantilever mooring structure during use to the structural load bearing members; and a counterweight arrangement for counterbalancing at least part of the loading on the cantilever mooring structure during use.
20. A vessel, as claimed in claim 19, in which the cantilever connector assembly is a cantilever connector assembly as claimed in any one of claims 2 to 18.
21. An offshore vessel mooring system comprising a mooring element adapted to be located in an offshore environment and a cantilever connector assembly for connecting a vessel to the mooring element, the cantilever connector assembly comprising: a cantilever mooring structure for releasably connecting the assembly to the mooring element; a mounting arrangement for releasably connecting the cantilever mooring structure to the vessel, the mounting arrangement adapted to be coupled to structural load bearing members of the vessel for transferring loading on the cantilever mooring structure during use to the structural load bearing members; and a counterweight arrangement for counterbalancing at least part of the loading on the cantilever mooring structure during use.
22. A system as claimed in claim 21, in which the cantilever connector assembly is a cantilever connector assembly as claimed in any one of claims 2 to 18.
23. A cantilever space frame structure whose purpose is to provide connection between a bow mooring and riser inboarding system and a weathervaning offshore vessel, the frame comprising: two fore-aft tubular trusses with interconnecting structural members; IC) a counterweight or counterweights at the aft end of this space frame to act as Q kentledge so as to reduce the tension forces between the aft of the cantilever frame If) and the ship caused by the downward component of the load from the mooring system Q 20 acting at the forward end of the cantilever frame in conjunction with the fulcrum part-way between the fore and aft ends of the cantilever frame; two or more connections between the frame and the ship port and starboard at the fulcrum line and at the aft end of the cantilever frame, the ship side of these connections being the ends of vertical structural members pre-inserted into the ship and welded or otherwise secured in place, such pre-inserted members being anchored in the longship and thwartship directions at both focsle deck level and main deck level (if these levels are different) and anchored in the vertical direction at main deck level; such connections being designed in such a way as to be easily made or unmade so as to facilitate rapid installation of the cantilever frame on the vessel and rapid removal on completion of the campaign; the said trusses carrying between them at their forward end mooring gimbals and mooring swivel or other mooring system interface devices transferring loads from the mooring system to the cantilever space frame.
24. A structure as claimed in claim 23, in which the cantilever space frame carries at its forward end a lightweight tubular space frame tower supporting single or multi-path fluid swivel and electrical sliprings connected by flexible pipe and cable jumpers to the mooring swivel assembly or other interface devices with the mooring system. 0)C L()C L()C