EP3414154B1

EP3414154B1 - Slim turret

Info

Publication number: EP3414154B1
Application number: EP17704002.9A
Authority: EP
Inventors: Arne Smedal; Kåre SYVERTSEN; Jan Vidar Aarsnes; Ragnar THUNES
Original assignee: Cefront Technology As
Current assignee: Cefront Technology As
Priority date: 2016-02-10
Filing date: 2017-02-09
Publication date: 2019-10-09
Anticipated expiration: 2037-02-09
Also published as: NO20160222A1; NO341161B1; WO2017137492A1; CN108602548B; CN108602548A; EP3414154A1

Description

Technical Field:

The present invention relates to a turret assembly for mooring a storage and production vessel to a seabed and for transferring fluids from a subsea flowline to the vessel, wherein the turret assembly comprises notably a geostationary inner turret unit displaying at least one through-going channel extending axially along a longitudinal center axis of the inner turret unit. The present invention also relates to a vessel comprising said turret and a method for installing said turret onto such a vessel.

Background and prior art:

The function of a turret is to moor a weathervaning vessel to a seabed without creating excessive forces on the mooring lines. This is achieved by use of a swivel and bearings that keeps the turret geostationary during vessel rotations. Turrets on storage or production vessels must also provide space for pipelines extending along an axial center of the turret. The swivel enables the transfer of fluid flows. communication signals, hydraulics and any power between the geostationary turret and the weathervaning vessel.
Turrets for ensuring safe mooring of weathervaning vessels have been known for decades. As an example, patent publication GB1189758A discloses a conventional turret extending between the deck and the keel of a vessel. The turret is kept geostationary by a deck positioned rotatable table. A more recent example of a turret is disclosed in patent publication WO 98/56650 where a relatively small sized turret is located near the keel level of the vessel. In contrast to the turret disclosed in GB1189758A the rotatable chain table is attached below the vessel. In this way the chain table can adequately take up bending moments exerted by e.g. the mooring chains. The arrangement also enables easy removal of the lower part of the turret wall from the vessel in order to transform the vessel back to a transport tanker.
EP 1886914 A1 discloses a vessel with a mooring system comprising a turret anchored to the seabed and a turret casing being part of the vessel, wherein the turret and turret casing are interconnected by a connecting element comprising a bearing arrangement. The connecting element has a first end connected to the turret casing and an opposite second end connected to the turret. The connection between the connecting element and the vessel has a symmetric arrangement of the fixing means.
Other examples of conventional turrets are patent publication US 5782197 A disclosing a turret device for anchoring a vessel to the seabed and to transport fluids through pipes, patent publication WO 93/24733 A1 disclosing a system for use in offshore petroleum production comprising a submerged buoy and patent publication EP 2778041 A1 disclosing an assembly for transferring fluids between a vessel and a turret structure mounted on the vessel. However, none of these publications describes a turret support structure rotationally surrounding the entire inner turret unit.
Furthermore, common for all conventional turrets is that they have a construction where the forces from the turret to the vessel are transferred via radial and axial bearings capable of handling horizontal and vertical forces. The bearings situated at the lower part of the turret experience typically the largest radial loads, but some horizontal loads may also be transferred to the upper radial layer in order to balance any bending moments from the mooring line forces. Axial forces from the mooring lines and the weight for the turret construction must be carried by axial bearings (or any other arrangements capable of both transferring forces and to rotate). A turret of conventional design typically has a bearing integrated into the vessel structure with a diameter equal to the largest diameter of the turret construction.
Pipes for fluid flow, signal, etc., as well as mooring lines, must be placed within these bearings. An example of such a design is disclosed in WO 2015/063262 .
The challenges with these conventional turret constructions are numerous. When bearings are integrated into the vessels structure, deformations caused by the external loads on the vessel are transmitted to the bearings, setting higher demands on bearing tolerances, flexibility, etc. Such vessel deformations are particularly challenging for axial bearings transmitting high loads over a large bearing diameter. The mean forces within the axial bearings are high while the dynamic loads are weaker. In order to accommodate horizontal forces, conventional turret constructions also comprises upper and lower radial bearings, where the main part of the horizontal forces are absorbed at the lower radial bearing. The upper radial bearing is normally situated at the same level/area as the axial bearing. The horizontal forces normally undergo large dynamic variations relative to the mean forces. Hence, the forces that generate moments responsible for limiting the rotation of the turret construction are therefore mainly due to frictional forces within the axial bearing. This rotational moment is proportional to the size of the force, the friction coefficient and the bearing diameter. If sliding bearings are used the moments set up by mooring lines may not be sufficient for turning the turret. Active rotation of the turret may then be required, i.e. by turret rotating motors.
Another disadvantage with the above described prior art is that the turret will be affected by any deformation force or external force acting on the hull of the vessel, resulting in greater demands for bearing size and tolerances.
In addition, none of the above mentioned prior art discloses a solution where a complete turret assembly, i.e. a turret construction comprising all components necessary to establish a satisfactory mooring of a storage or production vessel, may be installed onto the vessel while being in dock.
Thus, there is an object of the invention is to provide a turret assembly for mooring a production vessel to a seabed and for transferring fluids from a subsea flowline to the vessel, which mitigate at least some of the above mentioned short-comings.
A particular object of the invention is to provide a turret assembly that is not significantly affected by any deformation of the vessel hull.
A second particular object of the invention is to provide a turret assembly that may accept less severe requirements for tolerance, flexibility, etc., than conventional turret assemblies.
A third particular object of the invention it to enable turret rotation at low friction moments.
A fourth particular object of the invention is to provide a turret assembly that has a size and a shape that allows mounting of the turret assembly when the vessel in question is in dock.

Summary of the invention:

The present invention is set forth and characterized in the independent claims, while the dependent claims describe other characteristics and embodiments of the invention.
In particular, the present invention relates to a turret assembly suitable for mooring a production vessel to a seabed and suitable for transferring fluids from a subsea flowline to the vessel. The turret assembly comprises a geostationary inner turret unit displaying at least one through-going channel extending axially along a longitudinal axis of the inner turret unit, a turret supporting structure rotationally surrounding the inner turret unit and a bearing arrangement situated between the inner turret unit and the turret supporting structure allowing the inner turret unit to rotate relative to the turret supporting structure and to transfer forces from the inner turret unit to the turret supporting structure.
The turret supporting structure furthermore comprises fixing means configured to fix the turret supporting structure to a hull structure of the vessel in such a way that, in use, any deformation force acting on a hull of the vessel has insignificant impact on the turret assembly. Said fixation should preferably be a rigid fixation.
The bearing arrangement between the geostationary inner turret unit and the turret supporting structure permits the vessel to pivot around the inner turret in response to changes in wind, waves, and currents without disturbing any seabed extending equipment being guided through the through-going channels, for example risers, cables, umbilicals, etc. The inventive turret assembly may optionally be equipped with auxiliary equipment's such as bending limiters for ensuring an operation effective guiding of the seabed extending equipment through the turret assembly from the vessel to the seabed at operation site.
The bearing arrangement preferably comprises at least one sliding bearing.
The rotational axis of the turret supporting structure is concurrent with the longitudinal center axis of the geostationary inner turret unit.
Hereinafter, the term 'insignificant impact' signifies an impact that has no operational influence on the above stated objects of the invention.
In one advantageous embodiment an exterior contact surface of the turret supporting structure is designed such that, when installed in a vertical extending shaft of a vessel, both horizontal and vertical forces are transferred between the hull of the vessel and the turret supporting structure. An example of such a designed may be a wedge-shaped contact surface.
In another advantageous embodiment the fixing means is, relative to the longitudinal center axis of the inner turret unit, located at or above the level of the upper bearing.
In another advantageous embodiment of the invention, the fixing means is arranged asymmetric relative to a longitudinal (vertical) center plane of the inner turret unit. For example, the fixing means may be fixed at the exterior of the turret supporting structure at one side of the longitudinal center plane running through the longitudinal axis and directed perpendicular to a direction of motion of the vessel during use and/or at one side of the longitudinal center plane running through the longitudinal axis and directed parallel to the direction of motion of the vessel during use.
In another advantageous embodiment of the invention, the bearings arrangements comprising an upper bearing situated at an upper part of the turret supporting structure, and a lower bearing situated at a lower part of the turret supporting structure having a larger diameter than the diameter of the upper bearing. The lower bearing(s) has/have a diameter that allows chain cables to pass at the inside of the bearing surface(s), for example the diameter of at conventional turret assembly. Since the movements of the vessel causes large variations of the horizontal load that accumulates in the lower radial bearing(s), the frictional forces in this / these bearing(s) do not have large impact on the turret rotation relative to the vessel. Typically, the rotation takes place when the horizontal loads are small.
The upper bearing has a diameter that should accommodate guide tubes 14 for seabed extending equipment 16, but not including mooring equipment 8,11 such as chain cables 8. Preferably, the diameter of the upper bearing may be less than 75 % of the diameter of the lower bearing, more preferably less than 70 %, even more preferably less than 65 %, even more preferably less than 60 %, for example 50%. Even smaller diameters of the upper bearings compared to the lower bearing are foreseeable.
The upper part of the turret supporting structure is herein defined as the part constituting less than 20 % of the total longitudinal length of the structure measured from the structure's top, for example approximately the vertical extension of the transverse beam at the structure's upper end. The lower part is defined as the part constituting less than 40 % of the total longitudinal length of the structure measured from the structure's base, for example from the base up to the start of the narrowing of the diameter of the inner turret unit. Any other parts of the turret supporting structure may be defined as the structure's mid part.
In another advantageous embodiment of the invention, the exterior contact surfaces of the turret supporting structure are, relative to the longitudinal center axis, located within the lower part, at or above the lower bearings.
In another advantageous embodiment of the invention, the upper bearing comprises an axial upper bearing accommodating loads predominantly parallel to the longitudinal center axis of the turret assembly and a radial upper bearing accommodating loads predominantly perpendicular to the longitudinal center axis of the turret assembly.
In another advantageous embodiment of the invention, the lower bearing comprises a radial lower bearing accommodating loads predominantly perpendicular to the longitudinal center axis of the turret assembly.
In another advantageous embodiment of the invention the turret assembly comprises at least one chain cable extending at least partly between the inner turret unit and the turret supporting structure, wherein the end of the at least one chain cable is fixed at a position situated between the upper bearings and the lower bearing. Preferably, the end of the at least one chain cable is guided through at least one anchor winch, where said at least one anchor winch is fixed to the inner turret unit below the upper bearing. Further, the end may be fixed to a respective chain locker of the inner turret unit.
In another advantageous embodiment of the invention the turret assembly further comprises at least one pivotable fairlead for guiding and control of the chain cable. Preferably, the at least one pivotable fairlead is fixed at a position located below the lower bearing of the inner turret unit in order to ensure that the chain cable may be pulled in/out and may rotate during operation at varying loads and load directions. The fairleads are further designed such that the total radial diameter set up by the at least one fairlead and the inner turret unit is changeable between a radial diameter being larger than the inner diameter of the base of the turret supporting structure and a radial diameter being smaller than the inner diameter of the base of the turret supporting structure. In this way the turret assembly (including the fairleads) may be arranged fully into the vessel's shaft during transport and be expanded during mooring.
Conventional fairleads may be used for the inventive turret assembly. Instead of conventional fairleads one may use alternative solutions such as fairlead arrangements for locking chains at the outside of the fairlead, or where the fairlead has been replaced with a rotating arm having integrated chain stoppers. The two latter alternatives may be particularly useful for the chain cables since they will experience a significantly reduces out-of-plane bending, and at the same time avoid being situated in the splash zone where there are higher requirements in respect of corrosion rate.
The invention also concerns a production vessel suitable for producing hydrocarbons from an offshore reservoir. The production vessel comprises a hull displaying at least one well or shaft extending vertically within the hull. The well is open in at least at its lower end. The vessel further comprises a turret assembly in accordance with any one of features mentioned above which is adapted to, and mounted within, the well.
The invention also concerns a method suitable for mooring a vessel to a seabed. The method comprises the steps of prefabricating, while in dock, a turret assembly comprising a geostationary inner turret unit displaying at least one through-going channel extending mainly parallel to a longitudinal center axis of the inner turret unit and a turret supporting structure rotationally surrounding the inner turret unit, transferring the prefabricated turret assembly into a well or shaft located within the vessel such that a base of the inner turret unit is situated in a transport position at or above a keel or base of the vessel, moving the vessel to an offshore mooring site, vertically lowering the prefabricated turret assembly to a mooring position where the base of the inner turret unit is located below the keel of the vessel and locking the prefabricated turret assembly into the mooring position.
In one advantageous embodiment of the invention the method further comprises the step of unlocking the prefabricated turret assembly from its mooring position, vertically raising the prefabricated turret assembly from its mooring position to the transport position prior to moving the vessel from the offshore mooring site.
In another advantageous embodiment of the invention the step of prefabricating includes mounting a plurality of fairleads at or near the base of the inner turret unit, the plurality of fairleads being pivotably mounted near the base enabling pivoting of the fairleads towards the exterior wall of inner turret unit. The term 'near the base" is defined as the part constituting less than 20 % of the total longitudinal length of the inner turret unit measured from the base and in relation to the lowest part of each fairleads. For example, the lowest part of each fairlead may be flush with the base of the inner turret unit.
In another advantageous embodiment of the invention, the prefabricated turret assembly mentioned in the method is in accordance with any of the features mentioned above.
The above mentioned turret assembly has thus a configuration that reduces the effect of deformations in the vessel hull, as well as reducing the required rotational moment in order to rotate the relevant part of the turret assembly.
The term "geostationary" is made in reference to a weathervaning vessel.
The term fairlead is in the following referred to a device for guiding a chain line, rope or cable around an object, out of the way and/or to hinder lateral movements. The prior art fairlead may be a separate piece of hardware or simply a hole into a structure. The fairlead constituting part of the inventive turret assembly is a separate piece of hardware that may rotate during operation.
The terms "upper part" and "lower part" of the turret assembly is referred the upper (deck) and lower (keel) parts of a vessel.
In the following description, numerous specific details are introduced to provide a thorough understanding of embodiments of the claimed turret assembly, vessel and method. One skilled in the relevant art, however, will recognize that these embodiments can be practiced without one or more of the specific details, or with other components, systems, etc. In other instances, well-known structures or operations are not shown, or are not described in detail, to avoid obscuring aspects of the disclosed embodiments.

Brief description of the drawings:

Fig. 1: shows a perspective view of the turret assembly according to the present invention,
Fig. 2: shows a turret supporting structure according to the present invention, where fig. 2 (a) shows a cross sectional side view and fig. 2 (b) shows a cross sectional view along section marked C-C in fig. 2 (a),
Fig. 3: shows a longitudinal cross section view of the geostationary inner turret unit according to the present invention,
Fig. 4: shows a top view of the turret supporting structure and fixing means according to the present invention,
Fig. 5: shows the turret assembly according with the invention in a transport position within a vertical extending shaft of a vessel,
Fig. 6: shows the turret assembly of fig. 5, seen from below,
Fig. 7: shows the turret assembly in an operable position,
Fig. 8: shows the turret assembly of fig. 7 seen from below, and
Fig. 9: shows a perspective view of the turret assembly according to the invention in the operable position.

Detailed description of the invention

Various objects, features, aspects and advantages of the inventive subject-matter will become more apparent from the following detailed description of preferred embodiments, along with the accompanying figures in which equal numerals represent equal or similar components.
Figure 1 shows an embodiment of a turret 1 for mooring a production vessel to a seabed (not shown). The turret 1 comprises a geostationary unit 5 comprising guide tubes 14 which extends in the direction of the unit's 5 (and thus turret's) center axis 15. The plurality of guide tubes 14 are configured to guide risers, umbilical's or cables 16 through the axial length of the turret 1 (see figure 9). By connecting the geostationary unit 5 of the turret 1 rotationally to the vessel 100, the vessel 100 may pivot around the geostationary unit 5 in response to changes in wind, waves, and currents without disturbing either the risers 16 or the mooring lines 8,11. The inventive turret 1 further comprises a weathervaning unit 6 that rotationally surrounds the axial circumference of the geostationary unit 5 and which is fixed to the geostationary unit 5 and to its surrounding hull / vessel structure 110 by bearings 2,2',3 and by fixing devices 7a,7b, respectively. As better illustrated in figure 4, the positions of these fixing devices 7a,7b are arranged asymmetrically around the central axis 15 of the turret 1. More specifically, the four fixing devices 7a,7b in figure 4 rigidly connects the weathervaning unit 6 to its surrounding hull 110 at one radial side of the center axis 15 only, for example at one side of the center axis 15 in the longitudinal direction of the vessel 100. The reason for this particular arrangement is that any distortion / deformation of the hull 110 will not be transmitted to the weathervaning unit 6, but instead cause a displacement of the weathervaning unit 6 in direction away from the fixing devices 7a,7b that avoids destructive built-up of stresses within the turret 1. I.e., any hull distortions have insignificant impact on the operation of the turret 1.
Below the guide tubes 14 there may be installed auxiliary equipment such as bending limiters. Such equipment may be mounted prior to setting up the vessel at the production field.
As seen in figure 1 the turret 1 comprises a small diameter upper bearing 2,2' situated at an upper part of the weathervaning unit 6, and a large diameter lower bearing 3 (i.e. larger than the upper bearings 2,2') situated at a lower part of the weathervaning unit 6. The upper bearing 2,2' may comprise both an axial upper bearing 2 and a radial upper bearing 2', while the lower bearing 3 may comprise a radial lower bearing 3 only.
Figure 7 also shows a plurality of chain cables 8 being connected to respective anchor winches 10. The anchor winches 10 are fixed to the geostationary unit 5 below the upper bearings 2,2'. Each of the chain cables 8 are guided through respective chain stoppers 9 situated on the geostationary unit 5, in figure 1 seen supported on a projecting block 18 below the upper bearings 2,2'. The chain cables 8 are pulled in by the dedicated anchor winches 10. This arrangement renders pull-in of mooring lines 8,11 possible without forcing the vessel 100 to keep a fixed heading during the operation. In an alternative embodiment, the anchor winches 10 may be arranged on a deck 108, for example at the same level as the upper bearing(s) 2,2', or arranged on or above the main deck 108 of the vessel 100. Anchor winches 10 may be installed for every group of anchor lines 8,11 and/or for each of the anchor lines 8,11. If only one anchor winch 10 for each anchor line 8,11 group is used, a chain cable 8 will after pull-in be locked into position by the chain stopper 9 and the chain cable 8 is cut of. The end of the chain 8 situated above the chain stopper 9 may advantageously be suspended in a chain hang-off padeye placed on the geostationary unit 5. By use of one anchor winch 10 per anchor line 8,11 it is no longer necessary to cut the chain cable 8.
The turret 1 further comprises a plurality of fairleads 13 into which the chain cables 8 are guided. The pivotable fairleads 13 are fixed to the lower end of the geostationary unit 5 below the lower bearing 3 such that the fairlead's outermost radial position (that is, in the outermost position in the direction perpendicular to the center axis 15) may be switched between positions outside and inside the weathervaning unit 6. The latter positions of the fairleads 13, pivoted to have outermost radial positions laying within the smallest cavity diameter of the weathervaning unit 6, are useful when the turret 1 is to be lifted into the vessel 100, either in parts or in it's entirely, for example during transport from docking to production site.
Figures 2 (a) and (b) show cross section views of the weathervaning unit 6 viewed from the side (a) and at section C-C (b), respectively. Three fixing devices 7a,7b are shown, arranged to form the mentioned asymmetric configuration.
In both figures 1 and 3 a plurality of guide tubes 14 are seen extending through the geostationary unit 5 with an angle from the center axis 15 slightly larger than zero, for example between 1-10°. The tubes 14 are configured to contain various equipment 16 intended to be guided down to the seabed during operation, for example risers, umbilical's or cables (see figure 9). Due to the bearings 2,2',3 between the geostationary unit 5 and the weathervaning unit 6, the vessel 100 is allowed to pivot around the geostationary unit 5 in response to changes in wind, waves, and currents without disturbing above mentioned equipment 16 or the mooring lines 8,11.
Figures 5 and 6 shows an embodiment where the turret 1 is arranged in a transport position inside a dedicated vertical extending shaft 4 within a hull 110 of a docked production vessel 100 comprising a vessel deck 108, a keel 102, derrick 104 with winches and a winch locker, an access trunk 107 (to access mid part of the turret 1 from the deck 108) and thrusters 103. The vessel 100 is situated on supporting blocks 101 on the docking base 105. A water line of the vessel 100 is indicated with reference 109. As in figures 1 and 3 a plurality of fairleads 13 are shown pivotably fixed to the lower end 6' of the turret 1. Since the turret 1 in figures 5 and 6 is in its vertically retracted position (transport position) the fairleads have been pivoted to one side, thereby reducing the total diameter sufficiently to fit into the relevant diameter of the shaft 4. Figures 5 and 6 shows a particular fairlead configuration at a vertically retracted position seen from the side and from below, respectively. The vertical displacement of the turret 1 is achieved by use of the derrick 104. Also shown in figure 6 are six through-going channels 14 for guiding seabed extending equipment 16 such as risers, umbilical's or cables (se figure 9).
Figure 7-9 shows the turret 1 arranged inside the dedicated shaft 4 of a production vessel 100 submerged in water 109, and where the turret 1 is in an operational position, i.e. vertically lowered by the derrick 104 to a level where the fairleads 13 may be pivoted to such a degree that their diameter exceeds the outer diameter of the shaft's 4 lowest part. In this operational position any seabed extending equipment 16 may be guided through the plurality of guide tubes 14 and any seabed extending mooring lines 8,11 may be arranged as seen for conventional turrets. In figure 8, a total of six guide tubes and a total of 12 mooring lines are illustrated as an example. Each mooring lines 8,11 are in the embodiment of figure 8 composed of a chain cable 8 extending from the anchor winch to a position below its respective fairlead 13 and an anchor line 11 (for example made of polyester) extending from the submerged end of the chain cable 8 to the seabed. Figure 9 shows in perspective the turret 1 in its operational position within the vessel 100 and with a swivel arrangement 20 enabling the introduction of fluid into some or all of the seabed extending equipment 16 during vessel rotation / weathervaning.
As described earlier, the axial and radial bearings 2,2' are transferring forces from the geostationary unit 5 to the weathervaning unit 6 (and vice versa). Furthermore, fixing devices 7a,7b situated between the weathervaning unit 6 and the vessel structure / hull 110 are transferring the forces on to the vessel structure 110. The fixing devices 7a,7b preferably set up a rigid fixing, creating a "single" moveable unit. In a particular configuration the lower part 6' of the weathervaning unit 6 has contact faces that are configured such that both horizontal and vertical forces may be transferred to the vessel structure 110. Further, the fastening point at the upper part 6" of the weathervaning unit 6 is configured such that the deformation of the vessel structure 110 is not transferred to the turret 1. This means that any deformation force acting on the vessel structure 110 has insignificant impact on the turret 1. This feature is herein achieved by fastening only one side of the upper part 6" of the weathervaning unit 6 to the vessel structure 110. Normally the fastening of the weathervaning unit 6 should be arranged to the side of the turret 1 having a component dominantly in the vessel's 100 direction of motion. However, if the vessel 100 experiences transverse deformations it may be beneficial to fasten the weathervaning unit 6 with a component dominantly in the vessel's 100 transverse direction.
In order to lower the requirements of tolerances and to minimize the moments needed to rotate the turret 1, the diameter of the upper axial bearing(s) 2 is significantly reduced compared to a conventional turret. This diameter reduction is achieved by letting only seabed extending equipment 16 such as risers and/or umbilicals pass inside the upper axial bearing(s) 2. In this way the diameters of both the upper axial bearing(s) 2 and the upper radial bearing(s) 2' will be reduced, thereby lowering the friction moments. This again will significantly simplify the demands for bearing tolerances etc.
The bearing surface(s) of the upper axial bearing(s) 2 and the upper radial bearing(s) 2' may be locked to the weathervaning unit 6, for example at or near its rim. Since the fixing(s) is/are only on one side (in the longitudinal and/or transverse direction) the deformations from the vessel 100 will not be transferred to the bearing surfaces. This ensures that the bearings 2,2',3 will have an optimum contact face being at least close to independent of outer strains.
As a consequence of the method of lifting / lowering the turret 1 (figure 5 and 7), the main (or even the entire) mounting procedure may take place when the vessel 100 is still in dock. By suspending part or the entire turret 1 in a position where the lower part of the turret 1 are located in line with, or above, the keel 102, conventional docking is possible where the vessel 100 is supported on supporting blocks 101 having a conventional height. When the vessel 100 is out of the dock, for example over a hydrocarbon producing area, at least part of the turret 1 may be lowered and locked into the operational / mooring position. In an equivalent way the at least part of the turret 1 may be lifted up to a transport position prior to further transport and/or docking.

In the preceding description, various aspects of the assembly, the vessel and the method according to the invention have been described with reference to the illustrative embodiment. For purposes of explanation, specific numbers, systems and configurations were set forth in order to provide a thorough understanding of the invention and its workings. However, this description is not intended to be construed in a limiting sense. Various modifications and variations of the illustrative embodiment, as well as other embodiments of the system, which are apparent to persons skilled in the art to which the disclosed subject matter pertains, are deemed to lie within the scope of the present invention.

List of reference numerals / letters:

1	Turret assembly / turret
2	Upper axial bearing
2'	Upper radial bearing
3	Lower radial bearing
4	Turret receiving space / vertically extending shaft / well
5	Geostationary inner turret unit / Geostationary unit
5'	Base of inner turret unit
5"	Top of inner turret unit
6	Turret supporting structure / weathervaning unit
6'	Lower part of the outer turret unit
6"	Upper part of the outer turret unit
7a	First fixing means (for fixing of outer turret unit to vessel traverse of vessel's direction of motion), fixing device
7b	Second fixing (for fixing of outer turret unit to vessel along vessel's direction of motion), fixing device
8	Chain cable
9	Chain stopper
10	Anchor winch
11	Anchor line / mooring line
13	Fairlead
14	Through-going channels / guide tube
15	Center axis of turret assembly / turret
16	Equipment (risers, umbilical, cables, etc.), seabed extending equipment
18	Projecting block
20	Swivel arrangement
100	Turret assembly vessel
101	Supporting block
102	Keel / vessel base
103	Thruster (as part of a dynamic positioning system of the vessel)
104	Lifting device / derrick
105	Docking base
106	Vessel anchor
107	Access trunk (to access mid part of turret 1 from vessel deck 108)
108	Vessel main deck
109	Water / water line
110	Vessel hull / vessel structure

Claims

A turret assembly (1) for mooring a production vessel (100) to a seabed (105) and for transferring fluids from a subsea flowline to the vessel (100), wherein the turret assembly (1) comprises:
- a geostationary inner turret unit (5) displaying at least one through-going channel (14) extending axially along a longitudinal center axis (15) of the inner turret unit (5),

- a turret supporting structure (6) rotationally surrounding the inner turret unit (5), the rotational axis of the turret supporting structure (6) being equal with the longitudinal center axis (15) of the geostationary inner turret unit (5), the turret supporting structure (6) comprising fixing means (7a,7b) configured to fix the turret supporting structure (6) to a hull structure of the vessel (100), the fixing means (7a,7b) being arranged asymmetric around the longitudinal center axis (15) of the inner turret unit (5), and fixed onto the exterior of the turret supporting structure (6) either;

- at one side of a center plane running through the longitudinal center axis (15) and directed perpendicular to a direction of motion of the vessel (100) during use, or

- at one side of a center plane running through the longitudinal center axis (15) and directed parallel to the direction of motion of the vessel (100) during use, or

- a combination thereof,
so that, in use, any deformation force acting on a hull (110) of the vessel (100) has insignificant impact on the turret assembly (1), and

- a bearing arrangement (2,2',3) situated between the geostationary inner turret unit (5) and the turret supporting structure (6) allowing the inner turret unit (5) to rotate relative to the turret supporting structure (6) and to transfer forces from the geostationary inner turret unit (5) to the turret supporting structure (6).
The turret assembly (1) in accordance with claim 1, wherein an exterior contact surface of the turret supporting structure (6) is designed such that, when installed in a vertical extending shaft (4) of a vessel (100), both horizontal and vertical forces are transferred between the hull (110) of the vessel (100) and the turret supporting structure (6).
The turret assembly (1) in accordance with claim 1 or 2, wherein the fixing means (7a,7b) is, relative to the longitudinal center axis (15), located at or above the level of the upper bearing (2,2').
The turret assembly (1) in accordance with any one of the preceding claims, wherein the bearings arrangements (2,2',3) comprising;
- an upper bearing (2,2') situated at an upper part (6") of the turret supporting structure (6) and

- a lower bearing (3) situated at a lower part (6") of the turret supporting structure (6) having an diameter being larger than the diameter of the upper bearings (2,2').
The turret assembly (1) in accordance with claim 4, wherein the diameter of the upper bearing (2,2') is less than 75 % of the diameter of the lower bearing.
The turret assembly (1) in accordance with any one of claims 4 or 5, wherein the upper bearing (2,2') comprises
- an axial upper bearing (2) accommodating loads predominantly parallel to the longitudinal center axis of the turret assembly (1) and

- a radial upper bearing (2') accommodating loads predominantly perpendicular to the longitudinal center axis of the turret assembly (1).
The turret assembly (1) in accordance with any one of claims 4 to 6, wherein the lower bearing (3) comprises a radial lower bearing (3) accommodating loads predominantly perpendicular to the longitudinal center axis of the turret assembly (1).
The turret assembly (1) in accordance with any one of the claims 4-7, wherein the turret assembly (1) comprises at least one chain cable (8) extending at least partly between the inner turret unit (5) and the turret supporting structure (6), wherein the end of the at least one chain cable (8) is fixed at a position between the upper bearings (2,2') and the lower bearing (3).
The turret assembly (1) in accordance with claim 8, wherein the end of the at least one chain cable (8) is guided through at least one anchor winch (10), the at least one anchor winch (10) being fixed to the inner turret unit (5) below the upper bearing (2,2').
The turret assembly (1) in accordance with claim 8 or 9, wherein the end of the at least one chain cable (8) is fixed in a chain locker (17) of the inner turret unit (5) via at least one anchor winch (10).
The turret assembly (1) in accordance with any one of claim 8 to 10, wherein the turret assembly (1) further comprises at least one pivotable fairlead (13) for guiding and control of the chain cable (8).
The turret assembly (1) in accordance with claim 11, wherein the at least one pivotable fairlead (13) is fixed below the lower bearing (3) on the lower end of the inner turret unit (5) and designed such that the total radial diameter set up by the at least one fairlead (13) and the inner turret unit (5) may change between a radial diameter being larger than the base diameter of the turret supporting structure (6) and a radial diameter being smaller than the base diameter of the turret supporting structure (6).
A production vessel (100) for producing hydrocarbons from an offshore reservoir, the production vessel (100) comprising a hull (110) displaying at least one well (4) extending vertically within said hull (110) and having an lower open end, wherein the vessel (100) further comprises a turret assembly (1) in accordance with any one of claims 1-12 mounted within the well (4).
A method for mooring a vessel (100) to a seabed, wherein the method comprises the steps of:
- prefabricating, while in dock, a turret assembly (1) according to any one of claims 1-12,

- transferring the prefabricated turret assembly (1) into a well (4) located within the vessel (100) such that a base (5') of the inner turret unit (5) is situated in a transport position at or above a keel (102) of the vessel (100),

- moving the vessel (100) to an offshore mooring site,

- vertically lowering the prefabricated turret assembly (1) to a mooring position where the base (5') of the inner turret unit (5) is located below the keel (102) of the vessel (100), and

- locking the prefabricated turret assembly (1) into the mooring position.
The method in accordance with claim 14, wherein the method further comprises the step of:
- unlocking the prefabricated turret assembly (1) from its mooring position,

- vertically raising the prefabricated turret assembly (1) from its mooring position to the transport position prior to moving the vessel (100) from the offshore mooring site.
The method in accordance with claim 14 or 15, wherein the step of prefabricating includes mounting a plurality of fairleads (13) near the base (5') of the inner turret unit (5), the plurality of fairleads (13) being pivotably mounted near the base (5') enabling pivoting of the fairleads (13) towards the exterior wall of inner turret unit (5).
The method in accordance with any one of claims 14 to 16, wherein the prefabricated turret assembly (1) is in accordance with any one of claims 1 to 12.