WO1996019377A1

WO1996019377A1 - Swivel assembly

Info

Publication number: WO1996019377A1
Application number: PCT/GB1995/003022
Authority: WO
Inventors: Alastair Cameron Rennie
Original assignee: Alastair Cameron Rennie
Priority date: 1994-12-22
Filing date: 1995-12-22
Publication date: 1996-06-27
Also published as: GB9425969D0; GB2302073B; GB9617833D0; GB2302073A

Abstract

A multi-path swivel, for example for transfer of gas, liquids, power and control signals between two rotating parts (3, 4), has multiple flexible lines (5) for each duty which are arranged in a concentric circle of constant radius (28) and deflect into a helical shape when the swivel is rotated by a limited amount either way from the central position where the lines are straight. A low stress helical shape is maintained by movement, for example by rams (11), of one part (9) towards the other (3) in ratio to the rotation and individual line torsion is removed by individual line swivels (8). When applied to a heavy duty marine situation the swivel may form part of a mooring buoy or built-in turret, the upper part (4) being free to rotate a designed amount relative to a lower part (3) fixed to the seabed by moorings (2) and the main loads between the two parts may be transferred by roller and friction bearings (15, 16).

Description

SWIVEL ASSEMBLY

This invention relates to a swivel assembly, for example for use in the transfer of fluids and gases in pipes and power and signals along cables between structures which are rotatable with respect to each other.

The invention is particularly applicable to turret- moored monohull vessels for transferring fluids, electricity etc., from equipment on the seabed to equipment on the vessel. The vessel is rotatable about an axis of rotation within the turret which is usually fixed. Currently available swivel assemblies for such applications include two general types. One uses individual swivels spaced out along the rotation axis. For fluids these comprise tori with concentric circumferential seals, and for electrical paths a sliding contact about the common axis. This arrangement can allow unrestricted rotation in either direction. The second type uses a drag chain such that flexible lines are bent in a plane between end points rotating about a common centre as the vessel rotates with respect to the turret. The drag chain limits the amount of rotation. In both methods the rotation axis can be used for typically one line. For this central line there are very many swivel types available.

Currently available systems achieve good results in the application where a floating vessel is moored by a turret system. In a typical application three to twelve anchors are secured to the seabed and joined by mooring lines to a mooring structure on or near the surface. This structure is supported vertically by its own buoyancy or suspended from the vessel. It has limited horizontal and rotational freedom of movement due to the mooring line arrangement and the vessel is secured to the structure when moored. As well as moorings, other lines, such as pipelines and electrical lines, are required to be connected from the vessel to the seabed. Where conditions require that the vessel has to be able to "weathervane" or rotate around its mooring then these lines usually have to come through the centre of the mooring structure. The rotating part of the mooring structure is its turret. The turret rotates about an axis passing through the turret and the mooring structure (the "common axis") . If the structure is a buoy then the vessel may be secured by a yoke mechanism or mooring line. In other cases the mooring structure may be directly coupled or built into the vessel, eliminating relative motion except rotation, and the mooring structure is the turret itself which is integral with the vessel.

The limitation of the present multi-path swivel is the practical size of the toroidal swivel, less than 2m, and thus the number of paths. This leads to manifolding of lines and other compromises, and the turret/mooring structure being congested and complicated. The limitation of the drag chain turret is the large size of the installation and the limited rotation. Economically rotation may be limited to + and - 270 degrees and when the vessel weathervanes to near a limit it is turned 360 degrees about the mooring by a tug or its own tangential thrusters.

There are two basic turret types, small or large diameter, with large diameter turrets generally used for the larger multi-path systems to give room inside the turret.

The part of the vessel turret mooring addressed by this invention is the relative rotation of lines between the vessel and the seabed as they pass through the turret and mooring structure. It does not include mooring and external loads as these are transmitted via separate structural and bearing arrangements.

The present invention allows use of a compact turret and therefore structural choice and flexibility for the parts, and simpler pipework. It does have limits on rotation, but provision for extra rotation is more economical than for the drag chain.

The present invention provides a swivel assembly including first and second members, one of which is rotatable with respect to the other about an axis passing through both members and at least one flexible line fixed to the first and second members at points spaced from the axis of rotation and spaced from each other along the axis of rotation, the or each cable being constrained to limit its displacement from the axis of rotation. Preferably the assembly includes a plurality of flexible lines fixed to the first and second members at different points. The word "line" is intended to encompass hollow tubes forming parts of pipelines; electrical cables, either individually or in bundles; and so called "umbilicals" in which a number of individual lines are grouped, to name a few examples. As the first and second members rotate, the relative rotation between ends of an individual line can be cancelled by an individual swivel in or on one end of that individual line. An individual in-line swivel enables part of a line to rotate about its longitudinal axis with respect to another part. An end- swivel enables a line to rotate about its longitudinal axis with respect to the member to which it is fixed. Also as rotation occurs the increase in distance between the lower and upper support points for each flexible line can be allowed for by movement of one support point.

The result is that the only loads on the flexible line are dynamic bending which are readily predicted and part of current manufacturing ability. The individual line swivel is also standard low speed equipment, for example for control lines as used in reel slip rings and high pressure gas seals.

In order that the invention may be more fully understood embodiments of it will be described by way of example only with reference to the following drawings which are schematic only.

FIGURE 1 shows an elevation with in part a section of a vessel with an integral turret mooring;

FIGURE 2 shows an outline in isometric view of the basic geometry of a single line as rotation occurs; FIGURE 3 shows a vertical part section of one possible arrangement of a typical individual line with a swivel in the upper turret and line length adjustment in the lower turret;

FIGURE 4 shows a similar embodiment as described for Figure 3 but with an individual line swivel in the lower turret and line length adjustment in the upper turret.

Referring to the drawings (in all of which like numbers refer to like parts) there is shown in Figure 1 a vessel 10 including a turret structure comprising a generally stationary part 3 and rotatable part 4 which surrounds part 3 and rotates about an axis 17 passing centrally through part 3. The turret structure enables the vessel to "weathervane", or rotate using thrusters 12, about the axis 17. Vessel 10 is moored by lines 2 to the turret structure 3,4 through which pass a number of individual cables or lines 1,7 such as oil, water or gas pipelines and control lines including in them power, signal, hydraulic, and control cores joining seabed and vessel equipment 13,14 of various sorts each side of the turret 3,4.

In Figures 3 and 4 the line portion between the line parts 1 and 7 passing through the turret structure is shown as line part 5. The external loads on the lines are transferred as hang-off points 6. Each line part 5 has a swivel 8 for rotation about the lines' own centre line or longitudinal axis.

In Figure 3 the swivel 8 is located in the rotatable turret part 4 at the top of the vertical flexible line 5. (If there was a case where the torsional strength of the flexible line part 5 was not suitable for the rotational stiffness of the swivel 8 then a simple gear arrangement could be incorporated to rotate the relevant part of the swivel 8 to keep it fixed relative to the turret part 4 so that there would be no torsional stress along the length of the flexible 5.) When the rotation around the centre 17 approaches the design limits, say, for example only either + or - 400 degrees from the point where line 5 is vertical, then the operational procedure is that the vessel 10 is rotated back 360 degrees so that design parameters are not exceeded.

The geometry shown in Figure 2 is simplified for the purpose of explanation, for example by ignoring local end stiffening. It is also assumed that the spacings of the ends 21,22 of the line from the axis of rotation are the same, whereby the ends describe circles of equal diameter 20, but they do not have to be the same. The parameters of overall height 23, radius of the line "R" 28 and spacing 26 between adjacent lines can be varied to suit the requirements of the individual implementation and to suit the normal characteristics of known dynamic lines such as the allowable loads when bent about radius "r" 25. Control of the deflection of the flexible line 5 is by use of the manufactured characteristics of the individual line 5 and control of axial load or by sliding, for example on a smooth cylindrical surface inside the circle of lines 5. Note that concentric arrangements of the circles of lines 5 are envisaged by the invention.

The lines are capable of adopting a straight line position in a common plane or parallel with the axis of rotation or, as shown in Figure 2, a helical path in either direction about the axis of rotation, depending on the relative rotation of the two members.

In Figure 2 the dimension 23 is the distance between the points 21 and 22 at zero rotation. If the plane of rotation of point 22 is kept a constant distance from point 21 then the length of line 24 may be increased with increasing rotation by a sliding sleeve mechanism or elasticity in the line 5. Alternatively or in addition the dimension 24 is kept constant in which case the distance between the planes of rotation of the ends of line portion 55 may be varied with increasing rotation.

Figures 3 and 4 shows an arrangement for varying the dimension 23, indicated as 18 in these figures. As shown m Figures 3 and 4, fixed turret part 3 includes a movable support 9 to which the lines are attached. The support may be moved to vary the dimension 18. The movement is an amount related to the length 24, radius "R" 28 and rotation 27. Arrangements of multiple concentric circles of lines 5 may use one or more intermediate supports 9.

The intermediate support 9 is rotationally fixed and movable in the axial direction only with respect to the associated turret part. It will be appreciated that a movable intermediate support could be provided on either turret part 3 or 4. Additional swivels or pivot arrangements might be necessary between the intermediate support and its associated turret part.

The arrangements of Figures 3 and 4 serve to control the axial loads on the flexible l ne. The hang off or intermediate support 9 is moved decreasing the vertical dimension 18 in strict proportion to the absolute relative rotation 27 of the turret parts 3,4 for either direction of rotation from zero. The means 11 to achieve this movement use components from known art and may include mechanical, hydraulic and electrical components. For example a ring gear fixed to one part of the turret 3 or 4 could drive a hydraulic pump fixed to the other part, with flow reversal valves tripped at the position of zero degrees rotation, supplying hydraulic fluid to say three vertically mounted hydraulic cylinders acting between the support frame 9 and not allowing rotation relative to its support structure 3 or 4 as shown respectively in Figure 3 and Figure 4. The transfer of forces between the turret parts 3,4 are primarily by bearings 15,16 whose design and layout are known art and not part of this invention.

In certain circumstances it is desirable to ensure that there is less curvature of a line at its fixing points, for example to reduce strain on the fixings themselves. This may be achieved through the use of local clamps or bend stiffeners which are well known in this art.

Where multiple intermediate supports are used, separate lifting means may be necessary. Alternatively, the geometry of the line lengths and fixing points may be selected such that a single lifting mechanism may be used.

In Figure 4 the line 19 above the support 9 accommodates the vertical movement, for example as shown in this embodiment, by bending of the line 19 along the unconstrained length. Alternatively the line portion 19 might include additional swivel or pivot arrangements to avoid the need for the line to bend. In the embodiment in Figure 3 the changes in length below the support 9 are accommodated in the movement of the risers or flexible lines 1. A particular advantage of the present invention as compared to some known swivel assemblies is the fact that bending of the lines is minimised whereby, in the case of hollow lines, pigs may be sent down the lines for cleaning purposes without getting stuck.

Whilst the above embodiments have coincident and vertical axes for the turret parts 3 and 4 it is envisaged that axes 17 through the centres of circles of line ends shown in Figure 2 may be at an angle to each other and may not be coincident.

Claims

CLAIMS :

1. A swivel assembly including first and second members, one of which is rotatable with respect to the other about an axis passing through both members and at least one flexible line fixed to the first and second members at points sp&ced from the axis of rotation and spaced from each other along the axis of rotation, the or each cable being constrained to limit its displacement from the axis of rotation.

2. A swivel assembly as claimed in claim 1 including a plurality of flexible lines fixed to the first and second members at different points.

3. A swivel assembly as claimed in claim 2 in which the fixing points on the first member and fixing points on the second member are arranged in circles.

4. A swivel assembly as claimed in claim 3, including a plurality of lines arranged with their fixing points to at least one of the members in concentric circles about the axis of rotation.

5. A swivel assembly as claimed in any preceding claim in which at least a portion of the or each line is rotatable about its longitudinal axis with respect to one or both of the first and second members.

6. A swivel assembly as claimed in any preceding claim in which the or each line has at least one portion which is rotatable about its longitudinal axis with respect to another portion of the same line.

7. A swivel assembly as claimed in any preceding claim in which the length of the line portion(s) between the fixing points is variable in response to the relative rotation of the first and second members.

8. A swivel assembly as claimed in any preceding claim in which the first and second members include means for varying the axial distance between line fixing points measured along the axis of rotation in response to relative rotation of the first and second members.

9. A swivel assembly as claimed in claims 4 and 8 including separate means for each concentric arrangement of lines.

10. A swivel assembly as claimed in claim 8 including a plurality of lines, in which the fixing points of the cables and/or the lengths of the lines are chosen to enable the use of a single distance-varying means for all lines.

11. A swivel assembly as claimed in any preceding claim in which radial movement of the or each line is constrained by a cylindrical or conical surface at least partially enclosing the cables.

12. A swivel assembly as claimed in any preceding claim in which radial movement of the or each line is constrained by a cylindrical or conical surface between the line and the axis of rotation.