GB2198407A

GB2198407A - Single point buoy

Info

Publication number: GB2198407A
Application number: GB08614889A
Authority: GB
Inventors: Peter William George Burridge; Edward Alexander Cremer
Original assignee: Individual
Current assignee: Individual
Priority date: 1986-06-19
Filing date: 1986-06-19
Publication date: 1988-06-15
Anticipated expiration: 2006-06-19
Also published as: GB8614889D0; GB2198407B

Abstract

An intermediate bearing element is introduced between two rows of rolling elements, which in combination support and separate the inner race and the outer race in a large bearing assembly. The intermediate bearing element is expressly actuated by a gear drive to provide adequate and continuous rotation of the adjacent rolling elements. Such rotation creates the dynamic conditions in the lubricating fluid of the bearing assembly required to maintain a lubricating film around the rolling elements. <IMAGE>

Description

SINGLE POINT BUOY OR ANCHOR LEG BEARINGS INTERMEDIATE ELEMENT(S) INVENTION FOR MAINTAINING A LUBRICANT FILM FOR SWIVEL CONNECTION BEARINGS OF A SINGLE POINT BUOY OR ANCHOR LEG MOORING 1,0 BACKGROUND TO INVENTION 2.0 SUMMARY OF INVENTION 3.0 THE EFFECTS OF ENVIRONMENT 4.0 DETAILED DESCRIPTION OF THE INVENTION 1.0 BACKGROUND TO THE INVENTION The nature of a swivel connection of a single point buoy or anchor leg mooring is to provide unrestricted rotation for any arc of movement of a ship shape vessel connected to it.

The present designs of swivel connections of a single point buoy or anchor leg mooring(s) have proved to have frequent bearing failures due to fretting, as the ball and bearing race relative speeds approach zero, and short periods of metal to metal contact occur as the rolling speed becomes too low to maintain a separation film of lubricant.

So far due to the constraints of the structure designs, a redesign of the bearings has not been endeavoured and higher viscosity lubricants have been investigated and used.

For the concepts used so far a bearing failure has lead to a considerable downtime period with consequential loss of production time and revenue. Repairs in U.K. North Sea environment reported as some t20 million.

The invention addresses this problem.

2.0 SUMMARY OF INVENTION Accordingly, the invention comprises in one embodiment a design to maintain relative speeds above normally associated movements at which fretting occurs and to eliminate the ball bearing speeds reducing to zero at the ends of travel in large arc oscillations.

Trials have already been carried out which have subjected the present bearing designs to movements of about -150 of arc with a race track of about 6 MM and a radial load. After about 40 x 106 oscillations at a rate of 400 cycles and 800 cycles per minute, the extremities of the arc over which the balls travelled showed signs of fretting.

3.0 THE EFFECTS OF THE ENVIRONMENT It is acknowledged that the North Sea is a harsh environment in which to operate machinery, particularly if it is in the open exposed to wind and weather and the salty atmosphere.

Instances of bearing failures caused by false brinelling have occurred on several occasions. This is the classic fretting related phenomenon where the protective film or metal is broken by continually repeated impacts usually in the presence of corrosive agents. In the cases reported, the moist and salt laden atmosphere has contributed to accelerating the brinelling process.

Wave induced slamming forces in cases of offshore single buoy or anchor leg applications, with wind over turning moments, and vibrations through the structure connecting a ship-shape vessel moored to a swivel connection, coupled with the slow or zero speeds with traditional designs of bearings have aggravated these failures.

4.0 DETAILED DESCRIPTION OF THE INVENTION In order to disclose the invention in greater detail the following description with figures 1, 2, 3 and 4 serve to illustrate the principle elements and their interaction.

Fundamentally, to maintain the relative speeds above those normally associated movements creating fretting conditions, a third element designed to continuously rotate, actuated by an external power source is placed to separate the rotationally stationary buoy or anchor leg from the motion from the ship-shape vessel mooring yoke.

Figure 1 illustrates the concept as an arrangement.

Figure 2 highlights the intermediate element(s) locations.

Figure 3 illustrates the invention (intermediate element(s).

Figure 4 illustrates the invention (intermediate element).

Referring to Figure 1, the stationary riser buoy 10, anchored to the sea floor, serves as a spindle for a turntable 16 mounted in a rigid yoke 18 connected to the surface vessel 22.

Referring to Figure 2, the turntable 16 provides the housing for a tapered bearing 24 and a cylindrical bearing 26, which support and align the turntable 16 relative to the spindle 28 attached to the top of buoy or riser 10. Rigid yoke 18 is permanently connected to the gimbal housing 16 through articulated joint 30 with hinge 32, cylindrical roller bearing 34 and tapered roller bearing 36, thereby allowing the moored vessel unrestricted movement in any direction around the riser buoy.

Referring to Figure 3, the essential elements of the tapered roller bearing assembly are the inner bearing races 40 and 56 (attached to riser spindle wall 28), the outer bearing race 48 (attached to turntable wall 16), and two sets of roller bearings. One set consists of bearing rollers 42 and 46 separated by intermediate bearing race 44, and the second set consists of bearing rollers 50 and 54 separated by intermediate bearing race 52. All rollers and intermediate races are tapered to suit the geometric configuration of the bearing assembly. The inner ends of the intermediate bearing races are machined to be driven by conical gear drive 58, which is powered by an electromotor not shown. The opposite ends of said intermediate bearing races are supported by rolling elements in contact with auxiliary bearing races 38 amd 60. Said auxiliary bearing races also serve as clearance control rings and carry dynamic seals to contain lubricant inside the bearing assembly.

Referring to Figure 4, the essential elements of the cylindrical roller bearing assembly are the inner bearing race 62 (attached to spindle wall 28), the other bearing race 70 (attached to turntable wall 16), and a set of bearing rollers 64 and 68 separated by intermediate bearing race 66. The top end of the intermediate bearing race 66 is machined to be driven by splined gear drive 72, which is powered by an electromotor and gearbox assembly, not shown. Auxiliary bearing race 74 and rollers 76 provide support and alignment to the intermediate bearing race 66.

Claims

1. Fundamentally, to maintain relative speeds between roller and race at all times, irrespective of movements which normally would create fretting conditions within existing (traditional) designs of bearings.

2. Sub 1. is achieved by introducing a special intermediate element (race), designed to continuously rotate, actuated by an external power source.

3. To separate the rotationally stationary buoy or anchor leg from the ship-shape vessel mooring yoke irrespective of position or speed of oscillation and maintain a lubricant film between bearing parts.

4. The invention can be applied to both designs of ball and roller bearings.

5. The lubricant film will not under normal design conditions breakdown at the mooring yoke inner race face of contact with roller, irrespective of the arc of oscillation direction of the yoke relative to the continuously moving element and rollers.

6. This invention claims as a requirement and to maintain the speed of the intermediate element driving the roller, that it be powered such as to rotate at speeds greater than the yoke, irrespective of its direction or rate of motion.

7. The power source can be sized within practical limits to drive the element.

8. Will substantially reduce maintenance downtime of offshore installations of existing designs.

9. This is a single invention encompassing a novel and non-obvious concept linking all designs of tapered roller, three-roller and single-cylindrical roller designs used for offshore yoke connections of ship-shape vessels to single buoy or anchor leg moorings Amendments to the claims have been filed as follows CLAIMS The embodiments of the invention in which an exclusive property or priviledge is claimed are defined as follows: 1.A mooring assembly for connecting a surface vessel to a fixed anchoring device, said assembly comprising: a riser spindle, adapted for securement to a floating buoy anchored to the sea bed, or to a vertical buoyant column pivoted in a base secured on the sea bed; a turntable assembly, mounted for rotation on said riser spindle; a rigid yoke or mooring arm, attached at one end with an articulated joint to the turntable, and at the other end via a hinge arrangement to the bow or stern of a surface vessel; 2.A turntable assembly according to Claim 1 wherein at least one rolling element bearing assembly is provided to allow said yoke and surface vessel to weathervane around said riser spindle; said turntable assembly further comprising two clearance control rings, mounted on the inside of said turntable at locations above and below said bearing assembly, said clearance control rings carrying dynamic seals to contain lubricant within said bearing assembly; 3.A rolling element bearing assembly according to Claim 2 for providing unrestricted rotational movement between yoke mounted turntable and riser spindle, said assembly comprising: at least one bearing race attached to the outside of said riser spindle, supporting an inner row of rolling elements and at least one bearing race attached to the inside of said turntable, supporting an outer row of rolling elements; an intermediate bearing element, placed between and supporting both the inner row and the outer row of rolling elements, one end of the intermediate bearing element to be machined to be gear driven, the opposite end to be in supporting contact with an auxiliary bearing assembly; and a motor driven gear assembly, mounted optionally on the inside of the riser spindle or on the outside of the turntable, said gear assembly causing constant motion of the intermediate bearing element relative to the structure on which said gear drive assembly is mounted, the shaft of the driving gear being sealingly supported by said mounting structure, and said constant motion providing the dynamic fluid condition necessary for the creation and maintenance of a lubricating film around the rolling elements of said bearing assembly.