WO1998034829A1

WO1998034829A1 - Geostationary anchoring arrangement for a vessel

Info

Publication number: WO1998034829A1
Application number: PCT/NO1998/000041
Authority: WO
Inventors: Leiv Wanvik
Original assignee: Kvaerner Oil and Gas AS
Current assignee: Kvaerner Oil and Gas AS
Priority date: 1997-02-05
Filing date: 1998-02-05
Publication date: 1998-08-13
Anticipated expiration: 1999-08-05
Also published as: NO970508D0; AU6231298A; NO306334B1; NO970508L

Abstract

A geostationary anchoring arrangement for a vessel comprises a rotating part which is rotatably supported about a vertical axis in a well (2) in a vessel (1), and mooring lines (7) extending from the rotating part to a seabed. The rotating part consisting of a plurality of separate elements (3) arranged in a circular-cylindrical garland around the vertical axis of rotation, which elements (3) are flexibly interconnected so that they can move in the well (2) as a virtually united rigid annular body.

Description

GEOSTATIONARY ANCHORING ARRANGEMENT FOR A VESSEL

The invention relates to a geostationary anchoring system for a vessel, comprising a rotating part that is rotatably supported about a vertical axis in a well in a vessel and mooring lines extending from the rotating part to a seabed.

It is known to anchor, e.g., a production ship for the production of hydrocarbons with the aid of a geostationary rotating part which is recessed into the ship's hull, usually in the foreship. Mooring lines run out from the rotating part preferably in a radial pattern and down to anchors on the seabed. As a rule, the rotating part is supported at weather deck level with separate bearings for vertical forces and radial lateral forces. Owing to the great stresses from dynamics, wave pressure variation, dead weight, mooring and riser forces, the load on the points of support of the rotating part against the ship and the accompanying friction will be so great that the rotating part normally will not be capable of rotating relative to the ship in a controlled manner. It is therefore common to have systems which actively turn the rotating part.

Today's art with respect to a geostationary anchoring arrangement of this kind is encumbered by a number of problems as regards design, production, operation and maintenance.

In addition to mooring lines, risers conveying the production are led up from the seabed. When a plurality of functions are combined in a single structure, the rotating part will be large, and a net weight of 4 to 5000 tonnes is not unusual.

As a ship in the sea has dynamic deflections, the bearing arrangement for the rotating part must be designed and proportioned therefor. Great demands are therefore made on the bearings' structural details, capacity, and size, global and local construction tolerances, and also maintenance.

Since the external forces from mooring or anchoring and risers act on the bottom of the rotating part, a large torque arm arises with enormous loads on varying parts of the support. It is feasible to have the support in the lower portion of the rotating part, but normally this is confined in practice to arrangements which take extreme forces. Great forces are required to actively rotate the rotating part. As a pure consequence of great weight and large bearing loads, jacking systems of considerable size are used which grip toothing in the periphery of the turret.

The risers, as a rule flexible risers, are often attached to a swivel arrangement which transfers the production to an on-deck processing plant. When a swivel solution is used, very expensive limits of tolerance are set for the global dimensions of the rotating part and the associated points of support provided in the ship's hull. "Eccentricity" in the centre of the rotating part will cause large forces in pipelines which conduct fluids under great pressure up to the swivel arrangement. Today's swivel solutions are dependent upon these forces being small since otherwise wear on gasket surfaces and leaks would easily occur. The mooring lines are passed into the rotating part via guide wheels secured to the lower portion of the rotating part. Access for inspection and replacement of such guide wheels in the bottom region of the rotating part is very limited, but a replacement technique with the use of careening is known.

In some cases it will be desirable to place a drilling rig on the rotating part or turret to enable drilling or other work through an opening in the centre of the rotating part. In practice this means an additional load in the range of 2000 to 2500 tonnes, which represents a considerable increase in the support forces and an associated increase in complexity.

The object of the present invention is to provide an arrangement which will contribute to a better solution of the above-identified weaknesses.

According to the invention, there is proposed a geostationary anchoring arrangement for a vessel, comprising a rotating part which is rotatably supported about a vertical axis in a well in a vessel, and mooring lines extending from the rotating part to a seabed, which anchoring arrangement is characterised in that the rotating part consists of a number of separate elements arranged in a circular-cylindrical garland around the vertical axis of rotation, which elements are interconnected flexibly so that they can move in the well as a virtually united rigid annular body.

A division or segmentation of this kind into single elements with flexible interconnection makes it possible to absorb deviations from the defined theoretical geometry (tolerances/deflections/wear) without there being any danger of the rotating part being exposed to locking forces as a whole body or between the elements. The individual elements impart a direct transfer of riser and mooring forces limited to the immediately assigned support on the ship. The individual support of force which is due to the division into individual elements means that coercive forces as a result of the deformation of the vessel are largely avoided. The weight of the rotating part will also be capable of being reduced drastically, and a realistic estimate is a weight reduction of 30%, perhaps as much as 50% and more compared with the classical, known rotating part embodiments.

Each element may to advantage be in the form of an elongatedly constructed element which is flexibly interconnected to adjacent elements at the top and the bottom.

The very fact that the elements are interconnected at the top and bottom means that an integrated system is produced which will be capable of taking up variations in the loads.

According to the invention, the flexible interconnection may be in the form of a true swivel joint.

According to an advantageous embodiment of the invention, the flexible interconnection at the top may be in the form of welded or interconnected plates, which form horizontal plating.

According to the invention, each element may include an upper part, a lower part and one or more vertically extending connecting parts therebetween.

It is particularly expedient for the upper part to constitute a bearing component for interaction with a bearing surface provided around the well, whilst the lower part constitutes a second bearing component for interaction with a bearing surface provided around the well.

An especially advantageous embodiment according to the invention is one where there is provided between the adjacent elements a jacking device with which the elements can be forced apart, so that the annular body can generate friction against a surrounding cylindrical surface in the well for controlled locking of relative tangential movement.

It is particularly advantageous if the bearing surface inside the well (Fig. 12) is given a conical design having an angle not exceeding the angle of friction with a view to obtaining additional progressive and geometrically conditioned increase in frictional force against the bearing raceway in the event of extreme vertical loads on the elements, collectively or individually.

According to the invention, the individual elements may have two-sided upper support in the well.

It is especially advantageous if each element can be an elongatedly constructed element having two or more parallel, vertical tubular parts which are connected to one another at the top and at the bottom.

In an advantageous embodiment, the elements at the top are interconnected by means of plating, each element under this plating having a support structure with separate bearing pads for contact with a rotary bearing around the well, where the support structure is designed to function as a transmission device for distributing discrete loads from each one of the guide pipes out to a plurality of bearing pads in such manner as to avoid uneven loading or overloading of the individual bearing pads.

The invention will now be described in more detail with reference to the drawings, wherein:

Fig. 1 is a schematic vertical section through a geostationary anchoring arrangement according to the invention;

Fig. 2 is a plan view of the arrangement in Fig. 1 ;

Fig. 3 is a section along the line III-III in Fig. 1 ; Fig. 4 is a plan view of a modified geostationary anchoring arrangement according to the invention;

Fig. 5 is a schematic vertical section through another embodiment of the invention;

Fig. 6 is a perspective section of the embodiment according to Fig. 4;

Fig. 7 is a plan view of the embodiment in Fig. 6, without plating; Fig. 8 is a perspective section, seen from below, of an arrangement as in Figs. 4, 6 and

7;

Fig. 9 is a schematic vertical section through a geostationary anchoring arrangement according to the invention, with swivel connection for the risers;

Fig. 10 is a schematic vertical section through a geostationary anchoring arrangement according to the invention, with a drag chain arrangement for the risers;

Fig. 11 is a schematic vertical section through a geostationary anchoring arrangement according to the invention, during a possible drilling operation; and Fig. 12 is a schematic vertical section through an embodiment having a conical bearing surface in the well.

All the drawings are schematic and show only those components necessary for the understanding of the invention.

In Fig. 1 a part of a ship's hull is indicated by means of the reference numeral 1. In the ship's hull 1 there is a vertical through-going well 2. In the well 2 there is provided a plurality of separate, elongate elements 3. As shown in Figs. 2 and 3, these elements 3 are arranged in a circular-cylindrical garland along the well wall.

In the exemplary embodiment in Figs. 1 - 3, each element 3 is constructed having an upper plate 4, a lower plate 5, and pipes 6 extending therebetween, for guiding mooring lines 7, and pipes 8 for guiding risers 9.

The upper plates 4 are connected to each other with the aid of joints 10 having horizontal swivelling axes. The lower plates 5 are interconnected by means of joints 11 , these also having horizontal swivelling axes.

In the case of the upper plates 4, the joints 10 are made in the form of fish plates 12 which are pivotally connected to the respective adjacent plate 4 so that each joint 10 is a fished joint having two swivelling axes.

In the case of the lower plates 5, the individual joints 11 are made in the form of pin- and-lug hinges.

As shown in Figs. 1 - 3, in this way there is provided a rotating part in the well 2 consisting of a plurality of separate elements 3 arranged in a circular-cylindrical garland around a vertical axis (the vertical centre line of the well 2). The elements 3 are interconnected by means of the joints 10, 11 so that they can move in the well 2 as a virtually united rigid, annular body.

An annular horizontal bearing raceway 14 for the upper plates 4 is provided on the ship's 1 deck 13. An annular vertical bearing raceway 15 is provided for the plates 5 in the well 2. Bearing pads here are indicated by means of reference numeral 16. The connecting joints arranged between the elements 3 in the cylindrical garland or rotating part may be constructed in many other ways, provided that they function in a satisfactory manner to couple the elements so that they can move in the well 2 as a virtually united rigid unit. It may thus be a favourable embodiment to collect the upper plates 4 into a single ring, as is shown in Fig. 4. In the embodiment outlined in Fig. 4, the plates 4 are welded together as indicated at 17, so that there is a rotating part which at the top consists of plating from where pipes 6, 8 extend down to the hinge-connected 11 lower plates 5. The upper plating, which is formed by the upper plates 4, can be proportioned and made so that the plating will be capable of following the deformation motions of the ship's hull, and especially deformation motions in the deck 13, whether they are horizontal (ovalisation of the well opening) or vertical (out of co-planar position). Of course, the plating may be constructed in a manner other than that shown by lateral welding of the plates 4. The plating per se could be a uniform plating body, or for example, pairs of the upper plates 4 could be joined and made in the form of uniform bodies, with an accompanying reduction in the number of weld joints 17. What is essential is that the upper plating is so flexible that it can follow the motions of the ship and thus the bearing raceway 14 and give the individual elements 3 the necessary support and relative freedom.

The parts designated as upper and lower plates 4, 5 could in practice be constructed plates, for example, chest or box structures, and the upper and lower plates 4, 5 may of course be connected to one another in a way other than that illustrated. For instance, instead of the pipes 6, 8, a single pipe may be provided, or a truss-like construction may be used.

The rotating part may to advantage be constructed so that some of the elements 3 serve as guides for and fixing of mooring lines 7, whilst others serve as guides for risers 9 and others again merely act as structural components of the rotating part.

It will be possible to give the individual elements 3 and the rotating part made thereof respectively an upper two-sided support in the well 2. This is shown in Fig. 5. On the ship's deck 13 there is located a gantry 18 where there hangs a circular-cylindrical structure 19 which at the bottom is made having or supports a bearing raceway 20 for the upper plates 4. The rotating part will thus have support on an outer bearing raceway 14 and an inner bearing raceway 20. In other respects, the embodiment is as described above in connection with Figs. 1 - 4. The segmentation of the rotating part will be assured even if the rotating part uppermost emerges as a fully welded or interconnected structure, see for example the welded embodiment that is outlined in Fig. 4. An especially preferred embodiment in this connection is shown in Figs. 6 - 8. Here, the upper plates 4 have been given a structural form which provides each element with a great torsional pliancy. As in Fig. 4, plating of welded 17 plates 4 is constructed. The individual pipes 6, 8 run through these plates. Under each of the welded plates 4 there is constructed a separate support structure 21, see Figs. 7 and 8 (where the plates 4 have been omitted!) . Peculiar to this support structure 21, is that it is made having, e.g., four bearing pads 22 - 25 designed for bearing interaction with the annular bearing raceway 14 (see Fig. 1). The purpose of the illustrated design is to achieve a load of maximum conformity on all the load pads 22 - 25 along the periphery of the whole element through a transmission system of supporting beams 21. For instance, pure vertical loads in one of the pipes 6 will be distributed quite equally between bearing pads 22, 23 or 24, 25. Similarly, loads in the pipe 8 will be distributed between the four bearing pads 22 - 25. Much of the same distribution function is achieved also during asymmetric loading. If only one of the pipes 6 is loaded, deformations in the supporting beams 2 will result in adjacent pipe 6 being drawn with it by means of structural coupling and transferring forces to adjacent bearing pads. This load distribution function also takes place when the bearing raceway on the ship's deck 13 goes out of coplanar form. A vertical offset between bearing pads 22 and 23 will be filtered away through torsion in the supporting beam into the pipe 6. Similarly, relative deformation or inclination between element and bearing raceway will be filtered or balanced between the pipes 6 and 8 in each individual element. As regards horizontal deformation of the annular plate 4, this is made having large rounded cut-outs in the radial direction in order to obtain a limited and controlled level of tension on ovalisation owing to the ship's motions.

This ability to take up deformation is an essential feature of the proposal as the aim is a rotating body that gives a minimum of reaction forces from the rotating body to the ship when deformations occur in the hull girder. This same line of thought has given the basis for the design of the annular body 5 down in the well.

In the exemplary embodiment in Fig. 8 it can be seen how the lower plates 5 optionally may be made - in the form of a box - or a chest structure having bearing pads 26 for bearing interaction with the annular bearing raceway 15 in the well 2 (see Fig. 1). Fig. 9 shows how the invention can be accomplished in connection with so-called swivel connection 26 of risers 9, in a known way per se, and Fig. 10 shows the invention implemented in connection with a drag chain device 27, which is also known per se.

Fig. 11 shows the invention effected as a rotating part, where a drilling operation can be carried out through the well 2. A drilling derrick 28 is located on a gantry or bridge structure 29 on the ship's deck 13. A drilling string is indicated by means of the reference numeral 30. A tension adjusting device for the mooring line 7 is shown at 31.

The flexibility of the rotating part according to the invention allows a change in diameter in the lower support area (at 15 in Fig. 1) so that the rotating part can be checked or locked against rotation. Such locking is especially favourable when vessel and rotating part move in the sea, in order to avoid small movements which cause wear (chafing) on parts in contact with one another. Such change in diameter may, for example, be carried out by inserting a jacking device between adjacent elements 3 as shown in Fig. 3, where suitable jacks 32 are indicated between an element 3 and the two adjacent elements 3. Instead of expanding the diameter in the lower area of the rotating part as mentioned, the annular circumference may also be constricted by means of the jacking device to facilitate rotation. A net force contact against one side of the well 2 where the mooring forces act will also be obtained. Friction from said net force contact can be reduced considerably by passing lubricating water under high pressure to the bearing surfaces 16 through suitable piping from the rotating part.

A large increase in tangential and vertical locking capacity can be obtained if a conical bearing surface 15' is used between the rotating part and the well wall as shown in Fig. 12. This angle should be smaller than the angle of friction so that self-locking occurs on the use of clamp bodies 32 and also that the deformation of the rotating part in a vertical direction results in geometrically progressive locking of the rotating body, so that a controlled distribution of force is obtained between the lower rotating part and the other portions of the rotating part which rest on the horizontal rotary raceway on the ship's deck. This can be accomplished by using co-rotating flexible bearing pads, shown as 16 in Fig. 3, with a desired composition of axial and shear deformation rigidity.

Measures are preferably taken which allow controlled shifting or turning of the rotating part. This may, for example, take place with the aid of a toothed-rim drive gear device (not shown) or using other known means. There might be a need for lubrication of the bearings. A water-high pressure system, attached, for instance, to the bearing pads 22 - 25, would be a favourable solution in this respect.

The segmentation of the rotating part makes it possible to remove, e.g., one element and replace it with another in connection with maintenance or faults.

Claims

P a t e n t c l a i m s

1.

A geostationary anchoring arrangement for a vessel, comprising a rotating part which is rotatably supported about a vertical axis in a well (2) in a vessel (1), and mooring lines (7) extending from the rotating part to a seabed, characterised in that the rotating part consists of a plurality of separate elements (3) arranged in a circular-cylindrical garland around the vertical axis of rotation, which elements (3) are flexibly interconnected so that they can move in the well (2) as a virtually united rigid annular body.

2.

A geostationary anchoring arrangement according to claim 1, characterised in that each element (3) is an elongatedly constructed element which is flexibly interconnected (10, 11, 17) with adjacent elements (3) at the top and the bottom.

3.

A geostationary anchoring arrangement according to claim 1 or 2, characterised in that the flexible interconnection is in the form of a swivel joint (10, 11).

4.

A geostationary anchoring arrangement according to claim 2, characterised in that the top flexible interconnection is in the form of welded or interconnected plates (4) which form horizontal plating (Fig. 4).

5.

A geostationary anchoring arrangement according to one of the preceding claims, characterised in that each element (3) comprises an upper part (4) and a lower part (5) and one or more vertically extending connecting parts (6, 8) therebetween.

6.

A geostationary anchoring arrangement according to claim 5, characterised in that the upper part (6) constitutes a bearing component for interaction with a bearing surface (14) provided around the well (2) and the lower part (5) constitutes a second bearing component for interaction with a bearing surface (15) provided around the well (2).

7.

A geostationary anchoring arrangement according to claims 1-6, characterised in that between adjacent elements (3) there is provided a jacking device (32) with which the elements (3) can be forced apart so that the annular body can generate frictional force against a surrounding surface (15) in the well (2) for controlled locking of relative tangential movement.

8.

A geostationary anchoring arrangement according to claim 7, characterised in that the surface (15') in the well is constructed with a conical surface having an angle not exceeding the self-locking angle with a view to achieving additional self-locking of the elements (3) on the occurrence of extreme vertical forces which exceed initial frictional force provided by jacking device (32).

9.

A geostationary anchoring arrangement according to claims 1-8, characterised in that the individual elements (3) have two-sided upper support (14, 20) in the well (2).

10. A geostationary anchoring arrangement according to claims 1-9, characterised in that each element (3) is an elongatedly constructed element having two or more parallel, vertical tubular parts (6, 8) which are interconnected at the top (4) and at the bottom (5).

11. A geostationary anchoring arrangement according to claims 1-10, characterised in that the elements (3) at the top are interconnected by means of plating (4, 4, 17) and that each element (3) under this plating has a support structure (21) with separate bearing pads (22 - 25) for contact with a rotary bearing (14) around the well (2).