AU2011369881B2 - Mooring assembly for a vessel - Google Patents

Description

Mooring assembly for a v ssel

The invention relates to a mooring assembly for a vessel, comprising a moonpooi defined in the vessel, a tur^¬ ret rotatably mounted within the moonpooi by means of an up^¬ per main bearing for transmitting horizontal and vertical ioads between the vessel and turret and by means of a lower secondary bearing for substantially transmitting horizontal ioads between the vessel and the turret, and mooring lines attached to the lower region of the turret .

Ideally the lower secondary bearing transmits the major part of the mooring forces (which are caused by the mooring lines) in a horizontal direction to the vessel, Practically,, however, it is difficult to avoid that a substantial part of these forces are reacted by the upper main bearing, causing severe stresses, particularly in the turret and in the upper main bearing.

One suggested solution for this problem is to decrease the diameter of at least a lower part of the turret such as to reduce its resistance against bending. As a re^¬ sult the turret may bend under horizontal mooring forces, causing the lower secondary bearing to transmit most of the horizontal loads. A disadvantage, however, is that such a reduction of diameter of the turret reduces the available space for risers (or other elements such as, for example, a disconnectable buoy member) . Further such a. reduction of di- ameter may decrease the ability of the turret to transmit circuxnferentiai loads .

Document EP-A-0 656 293 illustrates in figures 13- 15 embodiments of a mooring system with a turret provided with lower pivotal or flexible arms having free ends to which mooring lines are connected and which carry bearing pads that can move into contact with a vessel bearing ring. The shown embodiments of said system however have a complicated structure and require a careful design to provide suf- ficient ability to transmit loads in a c rcumferential di^¬ rection .

It is an object of the present invention to pro^¬ vide a improved mooring assembly of the above type.

Thus, in accordance with the present invention the mooring assembly is characterized in that the turret at its lower region comprises a number of circumferent.ial.Iy spaced torsion box like torsion members each extending substan^¬ tially vertically between an upper end attached to the tur^¬ ret and a lower free end, which torsion members have a re^¬ sistance against bending that in a circumferential direction is greater than in a radial direction, wherein the mooring lines are attached to said torsion members near said lower free ends and wherein the lower secondary bearing is defined by outwardly facing bearing members attached to said torsion, members and an inwardly facing bearing ring attached to the moonpool, and wherein in a situation in which a torsion member is not loaded, horizontally by a respective mooring line its bearing member does not engage the bearing ring.

The torsion members offers a simple design, how^¬ ever combined with the provision of a flexibility which, at one hand allows the torsion, members to bend outwardly into contact with the bearing ring (avoiding the transmission of horizontal loads to the upper main bearing) and at the other hand offers sufficient rigidity in the circumferential di^¬ rection to transmit circumferential loads to the turret (and to whatever constructional and/or operational parts sup^¬ ported thereby) . Because, further, this design does not require a reduction of the diameter of any part of the turret, the available space for risers is optimised. The torsion box design results in a torsion stiffness which ensures full contact between the bearing members and effectively avoids undesired phenomena, such as fretting between the bearing members and the bearing ring due to dynamic stick-slip ef- fects. As a result the bearing members and bearing ring are maintained in full engagement .

The gap present between a bearing member and the bearing ring in a situation in which a torsion member is not loaded horizontally by a respective mooring line moreover allows hogging^' and sagging of the vessel with a resulting change of the gap dimension without effecting the horizontal load distribution over the upper main bearing and the lower secondary bearing.

In one embodiment of the mooring assembly accord^¬ ing to the present invention the torsion members taper in a vertical direction, for example upwardly. Such a taper al^¬ lows the mooring assembly, and specifically the turret, to be optimised, both in a constructional and spatial sense.

In another embodiment of the mooring assembly the ratio between the resistance of the torsion members against bending in a circumferential direction and in a radial direction is at least five.

There are several possibilities for controlling the manner in which the torsion members will bend when loaded. For example it is possible that the spacing between successive torsion members have different lengths and/or widths .

In yet another embodiment of the mooring assembly according to the present invention the bearing members are frict ion pad.s .

In an em.bodim.ent of the mooring assembly the lower secondary bearing is located at the lower free ends of the torsion members wherein the position where the mooring lines are attached to the turret substantially is located at the same level. Thus the mooring lines engage the torsion members near to the lower secondary bearing, such that mooring loads are almost directly directed to the lower secondary bearing without loading the turret in an excessive manner. However, alternatively it is possible that the lower secondary bearing is located above the lower free ends of the torsion members at a higher level than the position where the mooring lines are attached to the turret. In such a case the torsion members must be sufficiently flexible to be moved radially outward when loaded, but also sufficiently rigid to prevent an engagement between the lowermost part of a torsion member (below said secondary bearing) and the rrioonpooi .

It is also possible that the torsion members have different cross sections, for example when one expects the mooring assembly to be loaded in an asymmetrical manner (caused, for example, by a predominant direction of a sea current or wind) .

Finally an embodiment of the mooring assembly is mentioned in which the turret comprises a lower turret section and an upper turret section and wherein the lower turret section is detachable from the upper turret section. For example the lower turret section may be part of or may define a detachable buoy member which may support the risers.

Hereinafter the invention will be elucidated while referring to the drawing, in which

Figure 1 scheinatica11y shows an embodiment of a mooring assembly according to the present invention in a side elevat ional view, and

Figure 2 schematically shows a partial cross section according to ΙΊ-ΙΪ in figure 1.

Firstly referring to figure 1 a mooring assembly for a vessel 1 is illustrated schematically. It comprises a moonpool 2 defined in the vessel, a turret 3 rotatably mounted within the moonpool by means of an upper main bearing 4 for transmitting horizontal and vertical loads between the vessel and turret and by means of a lower secondary bearing 5 for substantially transmitting horizontal loads between the vessel and the turret . As may appear more clearly from figure 2, the tor- ret 3 at its lower region comprises a number of circumferen- tially spaced torsion box like torsion members 7 each extending substantially vertically between an upper end attached to the turret 3 and a lower free end. These torsion members are designed in. such a manner that they have a resistance ag inst bending that in a circumferential direction is greater than in a radial direction. These resistances may differ at least a factor five.

Mooring lines 6 are attached to said torsion members 7 near said lower free ends of the torsion members 7.

The lower secondary bearing 5 is defined by outwardly facing bearing members 8 attached to said lower free ends of the torsion members 7 and an inwardly facing bearing ring 9 attached to the wall of the moonpool 2, Preferably the bearing members 8 are friction pads. In a situation in which a torsion member 7 is not loaded horizontally by a respective mooring line 6 (as illustrated in figures 1 and 2} its bearing member 8 does not engage the bearing ring 9, Under a sufficiently large horizontal {outward; load, however, such a flexible torsion member 7 will bend outwardly and its bearing member 8 will engage the bearing ring 9, thus activating the lower secondary bearing 5 and keeping the resulting load on the upper main bearing 4 as small as possible.

Tors on of the to sion members 7 is avoided because of the torsion box design. This maintains a full engagement between the bearing members 8 and bearing ring 9,

As appears clearly from figure 1 the spacing 10 between two successive torsion members 7 extends substantially vertically but conceivable are other orientations and shapes too.

The length of the spacing 10 may differ from what has been illustrated. Moreover some spacings may have different lengths and/or widths, The torsion members 7, which define torsion boxes comprising among others (in a known manner and thus not further elucidated here) opposite wall sections and internal stiffening members, may taper vertically, for example up™ wardly. Further their cross section may differ between different torsion members 7.

The turret 3 may comprise a lower turret section and an upper turret section wherein the lower turret section is detachable from the upper turret section. For example such a lower turret section may be part of or may define a detachable buoy member supporting risers 11 in a manner known per se.

The invention is not limited to the embodiments described above which ma be varied widely within the scope of the invention as defined by the attached claims.

Claims (1)

1. 1. Mooring assembly for a vessel, comprising a moonpool defined in the vessel, a turret rotatably mounted within the moonpool by means of an upper main bearing for transmitting horizontal and vertical loads between the ves™ sel and turret and by means of a lower secondary bearing for substantially transmitting horizontal loads between the ves^¬ sel and the turret, and mooring lines attached to the lower region of the turret, char cter!zed in that the turret at its lower region comprises a number of oircumferentially spaced torsion box like torsion members each extending sub^¬ stantially vertically between an upper end attached to the turret and a lower free end, which torsion members ave a resistance against bending that in a circumferential direction is greater than in a radial direction, wherein the mooring lines are attached to said torsion members near said lower free ends and wherein the lower secondary bearing is defined by outwardly facing bearing members attached to said torsion members and an inwardly facing bearing ring attached to the moonpool, and wherein in a situation in which a tor- sion member is not loaded horizontally by a respective mooring line its bearing member does not engage the bearing ring„

   2, Mooring assembly according^' to claim 1, wherein the torsion members taper in a vertical direction.

   3. Mooring assembly according to claim 2, wherein the torsion members taper upwardly.

   4. Mooring assembly according to any of the previous claims , wherein the ratio between the resistance of the torsion members against bending in a circumferential direc- tion and in a radial direction is at least five,

   5, Mooring assembly according to any of the previ^¬ ous claims, wherein the bearing members are friction pads.

   6. Mooring assembly according to any of the previ^¬ ous claims, wherein the lower secondary bearing is located at the lower free ends of the torsion members and wherein the position whe e the mooring lines are attached to the turret substantially is located at the same level.

   7. Mooring assembly according to any of the claims 1-5, wherein the lower secondary bearing is located above the lower free ends of the torsion members at a higher level than the position where the mooring lines are attached to the turret.

   8. Mooring assembly according to any of the previ^¬ ous claims, wherein the torsion members have different cross sections .

   3. Mooring assembly according to any of the previ^¬ ous claims, wherein the turret comprises a lower turret section and an upper turret section and wherein the lower tur^¬ ret section is detachable from the upper turret section.

   10. Mooring assembly according to claim 9, wherein the lower turret section is part of or defines a detachable buoy member.