Turret bearing structure for vessels

A radial bearing structure for radially supporting a turret (14) rotatably carried by a vessel (10) including a plurality of arcuately spaced radial bearing assemblies (74) for radially supporting a rail (38) about the circumference of the turret (14). Each radial bearing assembly (74) has a roller (104) and a follower (94). A pair of concentric spring assemblies (92, 98) continuously urge follower (94) and roller (104) toward rail (38) so that roller (104) remains in contact with the rail (38) at all times. As shown in FIG. 7, inner springs (92) have reached maximum radial travel toward rail (38) and do not exert a radial force against follower (94) and roller (104). During maximum lateral movements of the turret (14), outer coil spring (98) forces follower (94) and roller (104) a distance (D) relative to and beyond inner springs (92) to maintain a spring loading against roller (104) for maintaining contact of roller (104) against rail (38). As shown in FIG. 8, the inner spring assembly may comprise an elastomer spring (92A) having a plurality of elastomeric pads (93A) separated by metal discs (95A).

REFERENCE TO RELATED PROVISIONAL APPLICATION 
This application claims the benefit of Provisional application Ser. No. 
60/033,669 filed Dec. 18, 1996. 
FIELD OF THE INVENTION 
This invention relates to a turret bearing structure for vessels such as 
offshore drilling or production vessels, and more particularly to such a 
turret bearing structure having radial bearings for reacting horizontal 
loads between the turret structure and the vessel. 
BACKGROUND OF THE INVENTION 
Heretofore, vessel mooring systems have been provided in which a vessel may 
weathervane about a turret which is normally anchored to the sea floor. 
The turret is supported for rotation on bearing structures secured to the 
vessel. Both horizontal and vertical loads are transferred between the 
turret and the fixed structure of the vessel. Vertical and radial bearing 
structures have been provided heretofore for transferring the horizontal 
and vertical loads between the turret and the vessel. For example, U.S. 
Pat. No. 5,240,446 dated Aug. 31, 1993, shows a mooring system which 
includes a rotatable turret mounted on the vessel. The turret is supported 
in a well of the vessel and has a radial bearing structure or assembly. A 
plurality of radial bearing assemblies are spaced about the outer 
periphery of the turret and have bearing blocks engaging a support ring 
about the outer periphery of the turret. The bearing blocks may pivot to 
fit any uneven adjacent surfaces of the turret. 
U.S. Pat. No. 5,359,957 dated Nov. 1, 1994 likewise discloses a turret for 
a vessel with the turret having fixed radial arms which are supported by 
both radial and axial bearings. The radial bearings includes rollers or 
wheels which run against a rail fixed to the vessel. A wedge device moves 
the rollers in and out in a radial direction. The rollers are mounted in 
very close relation to each other to limit flexure in the rail section. 
Radial bearings react horizontal loadings from the turret structure to the 
ship structure. Radial flexures and radial deformations must be 
compensated by the radial bearings. Such radial bearings heretofore have 
also included Belleville type springs to compensate for radial flexures or 
deformations in radial bearings for turrets in which rollers on a turret 
side against a rail fixed to the structure of the vessel. The Belleville 
springs absorb a substantial horizontal loading with only a relatively 
small deflection required. However, a disadvantage in the utilization of 
Belleville springs occurs when a relatively large horizontal loading is 
being reacted from only one side of the turret with the rollers and rail 
on such one side in contact, while the rollers and rail on the opposite 
side of the turret are out of contact as a result of the relatively small 
travel or deflection provided by the Belleville springs. As a result, once 
the turret returns or travels back to its neutral center, the horizontal 
rail and rollers on the opposite side recontact each other but the rollers 
have to "climb" up the crown of the rail until the rail and rollers return 
to their original radially centered position. During return to the 
original radially centered position, the bearing system undergoes or 
experiences an increased rolling resistance which requires an increased 
torque in order for the vessel to weathervane about the turret. Thus, a 
need exists for a spring assembly to take up any slack occurring when a 
relatively large horizontal loading is being reacted from only one side of 
the turret which may result in a spacing between the rollers and rail on 
the opposite side of the turret. 
It is an object of the present invention to provide a spring urged radial 
bearing structure for a turret which remains in contact with the turret 
during maximum lateral excursions of the turret. 
Another object of this invention is the provision of a spring structure for 
a radial bearing structure including a pair of spring device with one 
spring device being effective after the other spring device is ineffective 
thereby to maintain the radial bearing structure in contact with the 
turret during maximum lateral excursions of the turret. 
SUMMARY OF THE INVENTION 
The present invention is directed to a radial bearing arrangement for 
radially supporting a substantially vertically aligned cylindrical turret 
rotatably mounted within a vertical opening or well in the vessel. The 
radial bearing arrangement includes a rail secured to the outer 
circumference of the turret and a plurality of radial bearing assemblies 
mounted on the structure of the vessel and spaced at arcuate intervals 
about the outer circumference of the turret for engaging the rail. 
Each radial bearing assembly includes a roller frame having a roller 
rotatably mounted thereon for bearing contact against the bearing rail on 
the turret. A pair of concentric spring assemblies continuously urge a 
follower against the roller frame in an inward radial direction for 
forcing the roller into riding contact with the turret rail. A fixed guide 
shaft is mounted within an outer cylindrical housing and concentric spring 
assemblies fit within the cylindrical housing about the shaft for 
contacting the follower which also fits about the guide shaft. The inner 
concentric spring assembly includes a plurality of Belleville disc springs 
fitting about the shaft and absorbing horizontal loads from the turret 
rail and contacting roller in an outward radial direction with a 
relatively small radial travel. The outer concentric spring assembly 
includes a coil compression spring contacting the follower and likewise 
absorbing horizontal radial loads from the turret rail and contacting 
roller with a relatively large radial travel. The outer coil compression 
spring is effective to maintain contact relation between the roller and 
turret rail even though the inward horizontal radial travel of the roller 
may be greater than the inward horizontal radial travel of the Belleville 
springs thereby rendering ineffective the Belleville springs. However, the 
radial travel of the outer coil compression spring is of an amount 
sufficient to maintain a relatively strong and continuous bias against the 
follower and roller at all times thereby preventing "untracking" of the 
rollers from the turret on the rail. 
Other features and advantages of the invention will be apparent from the 
following specification and drawings.

DESCRIPTION OF THE INVENTION 
Referring now to the drawings for a better understanding of this invention, 
and more particularly to FIG. 1, the bow of a vessel 10 is shown having a 
well or moon pool 12 extending through the hull of vessel 10. Mounted 
within well 12 is a turret generally indicated at 14 about which vessel 10 
may weathervane. Well 12 is normally of a circular cross section and 
turret 14 is of a cylindrical shape to fit within well 12. Anchor legs 16 
connected to turret 14 are secured to the sea bed by suitable anchors to 
restrict rotation of turret 14. Risers 18 extend to subsea wellheads or 
distribution facilities on the sea floor and are connected to turret 12. A 
manifold deck 20 is supported on the upper end of turret 14 and includes 
valves connected to risers 18 by flexible hoses. A swivel stack shown 
generally at 22 is secured to vessel 10 and extends upwardly from turret 
14 and manifold deck 20. Swivel stack 22 is connected by bearing 24 to 
turret 14 and permits rotation or weathervaning of vessel 10 about turret 
14 and risers 18. 
As shown particularly in FIGS. 2 and 3, turret 14 includes a cylindrical 
body 28 having an upper radial extension or arm 27 including a pair of 
horizontal annular flanges or plates 30, 32 extending outwardly from 
cylindrical body 28. A vertical annular web 34 in concentric relation to 
body 28 reinforces annular flanges 30 and 32. Transverse reinforcing webs 
29 are secured between cylindrical body 28 and annular web 34. Extending 
outwardly horizontally from web 34 is an annular T-shaped rail support 36. 
Mounted on rail support 36 is an annular rail 38 which extends about the 
outer circumference of rail support 36 to provide a radial bearing member 
about the outer periphery of turret 14 for horizontal loads exerted by 
turret 14. 
Upper plate 30 has an upper rail 40 and lower plate 32 has a pair of lower 
rails 42 to provide bearing members on turret 14 for vertical loads 
exerted by turret 14. A turret support structure generally indicated at 44 
is fixed to a depressed deck portion 46 of vessel 10 to permit 
weathervaning of vessel 10 about turret 14. A pair of concentric rings 48 
and 50 extend upwardly vertically from deck portion 46. An upper annular 
cover plate 52 extends over concentric rings 48 and 50. Horizontal 
stiffeners 54 and 56 are secured between rings 48 and 50. 
A T-section 58 extends upwardly from deck portion 46 and a pair of parallel 
lower rails 60 are secured thereto in spaced relation to opposed rails 42 
on turret 14. A pair of rollers 62 are mounted between rails 42 and 60 for 
transferring vertical loads between rails 42 and 60 from turret 14 to 
vessel 10. Rollers 62 have shafts extending between plates 64. An upper 
rail 66 is secured by brackets 68 to stiffener 54 and extends downwardly 
therefrom in a vertically spaced relation to upper rail 40 on turret 14. 
Rollers 70 mounted between plates 72 are positioned between rails 40 and 
66 and assist in maintaining turret 14 in axially aligned relation with 
well 12. 
Radial Bearing Arrangement 
Forming an important part of this invention is the radial bearing 
arrangement as shown generally in FIGS. 2 and 3. A plurality of radial 
bearing assemblies 74 are arcuately spaced about the outer periphery of 
turret 14 with each bearing assembly 74 extending in a radial direction 
and having an outer cylindrical housing 76 secured to rings 48 and 50. As 
shown further in FIGS. 4-7, an outer end cap 78 is removably secured to a 
mounting flange 80 on cylindrical housing 76 by suitable nut and bolt 
combinations 82. 
An internally threaded block 83 is secured to end cap 78. A shaft 84 having 
external threads 86 is threaded within block 83 and may be adjusted 
longitudinally by a suitable tool engaging flats on end 88. An end nut 90 
is threaded onto shaft 84. A follower 94 has an opening receiving shaft 84 
and is mounted about shaft 84 for relative longitudinal movement. A pair 
of spaced arms 100 are secured to follower 94 and support a shaft 102 
therebetween. A roller 104 is mounted for rotation on shaft 102 and is in 
engagement with rail 38 on turret 14. Follower 94 thus moves with roller 
104. 
An inner spring housing 96 has a front end plate 97 about shaft 84 and 
abutting follower 94 when turret 14 and rail 38 are in a centered position 
within well 12 as shown particularly in FIG. 4. The rear end flange 99 of 
inner spring housing 96 is mounted for sliding movement with housing 96 
along end nut 90 threaded onto shaft 84. A plurality of Belleville washers 
or springs 92 are mounted about shaft 84 between nut 90 and end plate 97 
of inner spring housing 96. An outer concentric spring housing 112 is 
secured to follower 94 by front end flange 106 for movement with follower 
94. Housing 112 has a rear end flange 108. A coil compression spring 98 is 
mounted about inner spring housing 96 and is compressed between follower 
94 and rear end flange 99 of inner spring housing 96. 
Operation of Radial Bearing Arrangement 
Referring to FIG. 4, turret 14 and rail 38 are shown in a centered position 
in which the horizontal loading is being reacted by rail 38 generally 
equally about the outer circumference of turret 14. In this position, 
follower 94 urges roller 104 into contact with rail 38 under a 
predetermined compression of Belleville springs 92 and coil spring 98. The 
compression of Belleville springs 92 may be adjusted as desired by 
rotation of shaft 84 and movement of nut 90. 
Referring to FIG. 6, turret 14 and rail 38 are shown in a position in which 
unequal horizontal forces from an opposite side of turret 12 moves turret 
12 and roller 104 in the direction of arrow 115. Corresponding movement of 
roller 104 and follower 94 compresses Belleville springs 92 against nut 
90. Coil spring 98 remains under its predetermined compression of FIG. 4 
as inner spring housing 96 moves with follower 94 in the direction of 
arrow 115. 
Referring to FIG. 7, turret 14 and rail 38 are shown in a lateral position 
in which unequal horizontal forces from the side of the turret shown in 
FIG. 7 moves turret 14 and rail 38 in the direction of arrow 117 from the 
centered position shown in FIG. 4. In this position follower 94 is spaced 
a distance D from end plate 97 of inner spring housing 96 and Belleville 
springs 92 are in a non-compressed relation. In this position, Belleville 
springs 92 are not effective to maintain contact of roller 104 against 
rail 38 as the deflection of Belleville springs 92 is limited. However, 
coil spring 98 is effective to maintain a horizontal force against 
follower 94 sufficient to maintain contact between roller 104 and rail 38 
under all conditions of operation. Coil spring 98 is sized to provide a 
predetermined loading effective to maintain a predetermined force against 
rail 38 for maximum lateral movement of excursions of turret 14 and rail 
38. Thus, coil spring 98 is effective to take up any slack occurring when 
a relatively large horizontal loading is being reacted from only one side 
of the turret. 
Referring to FIG. 8, a modified spring arrangement is shown in which an 
elastomer type spring 92A is used in lieu of the Belleville springs. 
Elastomer spring 92A includes a plurality of elastomeric pads 93A 
separated by a plurality of metal plates or discs 95A. Shaft 84 extends 
through elastomer spring 92A and spring 92A functions in a manner similar 
to Belleville springs 92 in the embodiments of FIGS. 1-7. Under certain 
conditions and for some turret bearing systems, elastomer springs 92A 
could be utilized without coil spring 98. 
While preferred embodiments of the present invention have been illustrated 
in detail, it is apparent that modifications and adaptations of the 
preferred embodiments will occur to those skilled in the art. However, it 
is to be expressly understood that such modifications and adaptations are 
in the spirit and scope of the present invention as set forth in the 
following claims.