WO1996030254A1

WO1996030254A1 - Turret device

Info

Publication number: WO1996030254A1
Application number: PCT/SE1996/000425
Authority: WO
Inventors: Holger Z. Nordin; Ivan Kores
Original assignee: GVA Consultants AB; Kockums AB
Current assignee: GVA Consultants AB; Saab Kockums AB
Priority date: 1995-03-31
Filing date: 1996-03-29
Publication date: 1996-10-03
Anticipated expiration: 1997-09-30
Also published as: NO974496D0; GB2313820B; SE9501206D0; GB2313820A; SE510221C2; SE9501206L; AU5293696A; NO974496L; GB9720795D0

Abstract

Turret device (2) for mooring an offshore vessel for oil/gas recovery, comprising a hollow cylinder body (3) journalled in a cylindrical well (4) which extends vertically through the hull (1) of the vessel, wherein the hull (1) is adapted for rotation about a vertical axis through said well (4), and said cylinder body (3) at its upper end being provided with a radially outwardly extending collar flange (9). The cylinder body (3) is journalled by a plain bearing (16) beneath the collar flange (9), said plain bearing (16) comprising a ring-shaped slide surface (17) which extends continuously around the periphery of the flange (9), and a number of opposed individual slide surfaces (18) placed at a predetermined distance from each other along discrete sections of the periphery. The separate slide surfaces are in the form of bearing blocks (21-24) made from an elastic material.

Description

TURRET DEVICE

TECHNICAL FIELD:

The present invention relates to a device for mooring an offshore-vessel, more precisely a so-called turret device for mooring an offshore-vessel for oil/gas recovery. The turret device comprises a hollow cylinder body journalled in a cylindrical well which extends vertically through the hull of the vessel. In this manner, the vessel is moored above a fixed point on the sea bed by means of anchor lines extending from the cylinder body of the turret device, whereby the hull of the vessel can rotate thereabout. The hollow cylinder body also constitutes an entrance for conduits (so-called riser pipes) between, for example, a well head on the sea bed and a treatment plant on board the vessel.

The underlying principle for a turret device of the above mentioned type is previously known from US-3,191,201 (Richardson) . With the help of this solution, an offshore vessel having an onboard treatment plant can be moored above a fixed point on the sea bed at the same time that the hull of the vessel can be freely rotated about the turret device with help of thruster units so that the vessel can be oriented in a desired manner with respect to prevailing wind conditions, etc. In its simplest form, said thruster unit is a conventional bow propeller.

A considerable problem with known turret devices is, how- ever, that the rotation of the vessel about the cylinder body is impeded or completely prevented by those deformations which occur in the hull construction as a result of movement of the vessel in the sea. One example of such deformation is the out-of-roundness of the cylindrical well which arises with "hogging" and "sagging" movements of the hull of the vessel . The deformations cause the bearings in the known turret devices to seize and this prevents the rotation, something which can have dire consequences in severe weather conditions out at sea.

Said deformations resulting from movements of the vessel, together with the static and dynamic loads which arise when moored, thus place high demands on the design of the bearings of the turret device in the cylindrical well of the vessel. The bearings are thus a weak point in previously known turret devices.

The turret device in US-3, 191,201 is solely journalled under a radially outwardly extending collar flange at the upper end of the turret device.

This bearing traditionally extends in its entirety continuously around the cylinder body which forms the turret device. This also applies for the section of the bearing which is fixedly attached to the hull of the vessel. In practice, this solution has been shown to be sensitive to deformations of the hull, which has created difficulties for the rotation of the vessel about the turret device. Since the turret device is only journalled at its upper edge (about the collar flange), oblique loads are imparted which further hinder the rotation.

SUMMERY OF THE INVENTION:

An object of the present invention is thus to overcome the above-mentioned problems by providing a turret device which by simple means permits rotation even under large deformations of the hull of the vessel.

The invention thus provides a turret device for mooring an offshore vessel for oil/gas recovery, comprising a hollow cylinder body journalled in a cylindrical well which extends vertically through the hull of the vessel, wherein the hull is adapted for rotation about a vertical axis through said well, and said cylinder body at its upper end being provided with a radially outwardly extending collar flange. The cylinder body is journalled both by a plain bearing beneath the collar flange, said plain bearing comprising a ring-shaped slide surface which extends continuously around the periphery of the collar flange, and a number of opposed individual slide surfaces placed at a predetermined distance from each other along discrete sections of the periphery, said continuous ring-shaped slide surface being either attached to the underside of the collar flange while said opposed individual slide surfaces are attached to the hull of the vessel, or vice versa, and about a mid-section of the cylinder body positioned at a height corresponding to the neutral axis of the hull of the vessel .

Distinguishing for the invention is that said individual lower slide surfaces are in the form of bearing blocks made from an elastic material and oriented diagonally about the periphery of the collar flange. The bearings blocks are preferably rectangular, though they may also be shaped as segments of a circle.

The bearing blocks, in an advantageous embodiment, further consist essentially of an elastic material, such as rubber or elastomer, to accommodate deformations.

BRIEF DESCRIPTION OF THE DRAWINGS:

One embodiment of the invention will be described below with reference to the attached drawings, in which

Fig. 1 schematically shows a cross section through an offshore vessel provided with the turret device according to the invention and in which the "hogging" deformation of the vessel has been greatly exaggerated;

Fig. 2 is a plan view of the vessel of Fig. 1, and

Fig.3 is an enlarged sectional view of the vessel taking along lines III-III in Fig. 2.

DESCRIPTION OF PREFERRED EMBODIMENTS: In Fig. 1, reference numeral 1 denotes a hull of an offshore vessel for oil/gas recovery. The vessel is shown schematically having a greatly exaggerated so-called "hogging" deformation. The vessel is provided with a so- called turret device 2 which comprises a hollow cylinder body 3 journalled in a cylindrical well 4 which extends vertically through the hull 1 of the vessel. The well 4 has a lower diverging funnel-shaped region 5 adapted for the outwardly angled anchor lines (not shown) .

The hull 1 of the vessel is adapted for rotation about a vertical axis 7 through the cylindrical well 4 in such a manner that the cylinder body 3 is moored above a fixed point on the sea bed and the hull of the vessel can freely rotate about the cylinder body 3 to a desired position with respect to prevailing weather conditions. The cylinder body 3 is further provided at its end with a radially outwardly extending collar flange 9.

In order to accommodate static and dynamic loads during mooring and high seas, the cylinder body 3 is journalled both along the underside 10 of the collar flange 9 and about a mid-section 12 of the cylinder body. The mid- section 12 where the cylinder body is journalled is positioned at a height corresponding to the neutral axis 14 of the hull, which axis is indicated by a chainline in Figs. 1 and 3. As a result of this placement of the mid- bearing, the bearing is exposed to the least possible deformation during movements of the hull 1. This implies a bearing without tendencies to seize and to resist rotation during deformations of the hull 1 of the vessel. The bearing about the mid-section 12 is, in the shown embodiment, in the form of a plain bearing. Other bearing types can however also be used, even though plain bearings are preferred.

The bearing around the underside 10 of the collar flange 9 is in the form of a plain bearing 16. The plain bearing 16 comprises a ring-shaped slide surface 17 which, in the shown embodiment, is attached to the underside 10 of the collar flange 9 as shown in Fig. 2. It is apparent from the drawings that the ring-shaped slide surface 17 extends continuously about the underside 10 of the collar flange 9. The plain bearing 16 further comprises a number of individual sliding surfaces 18 which are opposed to said ring-shaped slide surface 17 and intended for sliding contact therewith. In the shown embodiment, the individual slide surfaces 18 are in the form of four bearing blocks 21, 22, 23 and 24 which are diagonally oriented relative to the longitudinal direction of the hull 1 of the vessel in order to ensure the bearing function even during large deformations which arise during stretching of the hull 1 during, for example, so-called "hogging" and "sagging" movements . This diagonal placement of the four bearing blocks 21, 22, 23 and 24 has been shown to be effective for eliminating the hindering effects of said deformations on the functioning of the bearing. It is to be noted that the continuous ring-shaped slide surface 17 can either be attached to the underside 10 of the collar flange 9 whilst said opposed individual slide surfaces 18 are attached to the hull 1, as is the case in the shown embodiment, or vice versa. The bearing blocks 21, 22, 23 and 24 are preferably substantially rectangular-shaped as shown in Fig. 2. The rectangular shape implies low manufacturing costs and simplified handling during maintenance work and exchange of bearing blocks. It falls within the scope of the invention, however, that the bearing blocks be shaped as segments of a circle, even though this does of course imply higher manufacturing costs. It is also to be noted that the preferred embodiment makes use of four bearing blocks, though the number of bearing blocks can also be greater. Preferably, though, the bearing blocks are mounted in a relatively low, even number of diametrically opposed pairs about the cylindrical well 4.

In the preferred embodiment, the total area of the individual slide surfaces 18 correspond to about 40% of the continuous ring-shaped slide surface 17.

Horizontal displacements of the cylinder body 3 are accommodated by the bearing about the mid-section 12, though this may also be complemented by a buffer (not shown) about the periphery of the collar flange 9. In such case, the buffer may also consist of a plurality of individual bearing blocks which are diagonally oriented in the manner described above.

In Fig. 3 there is shown a sectional view on an enlarged scale along the line III-III in Fig. 2 and from which the construction of the turret device 2 is clearly apparent. The anchor lines which can consist of both chains and wire are indicated in part by chainlines 20.

The ring-shaped slide surface 17 of the bearing under the collar flange 9 consists of stainless steel which, thanks to the low surface pressure as a result of relatively large bearing surfaces, can have relatively low hardness. The bearing blocks 21, 22, 23 and 24 are made of a rubber or elastomer material and dimensioned for low surface pressure. The bearing blocks may further display relatively stiff characteristics. By division into individual blocks, an optimal geometry for a satisfactory spring constant of the blocks can be chosen. The slide surfaces 18 can be made with layers of a material with a lower coefficient of friction than the rest of the blocks, for example doped polyethylene. The elastic material in the bearing blocks serves to accommodate smaller deformations in the hull 1 and any irregularities or deformations in the continuous ring-shaped slide surfaces 17 which, as mentioned above, is in the form of a relatively thin ring of stainless steel in the shown example. Other materials, such as bronze, are however also conceivable, even though the combination of stainless steel and elastomeric material has been shown to be particularly suitable. The placement of the bearing blocks allows simple accessibility and checking even during operation.

In order to facilitate servicing, inspection or exchange of the bearing blocks, there is a possibility to lift the entire cylinder body with the aid of (not shown) hydraulic means placed in the hull 1 of the vessel. During lifting, the cylinder body 3 is rotated so that lifting surfaces on the collar flange 9 correspond to the hydraulic means.

The invention is not restricted to the embodiments described above and shown in the drawings, but can be freely varied within the scope of the appended claims.

Claims

CLAIMS:

1. Turret device (2) for mooring an offshore vessel for oil/gas recovery, comprising a hollow cylinder body (3) journalled in a cylindrical well (4) which extends vertically through the hull (1) of the vessel, wherein the hull (1) is adapted for rotation about a vertical axis through said well (4), and said cylinder body (3) at its upper end being provided with a radially extending collar flange (9), whereby the cylinder body (3) is journalled both by a plain bearing (16) beneath the collar flange (9), said plain bearing (16) comprising a ring-shaped slide surface (17) which extends continuously around the periphery of the flange (9), and a number of opposed individual slide surfaces (18) placed at a predetermined distance from each other along discrete sections of the periphery, said continuous ring-shaped slide surface (17) being either attached to the underside (10) of the collar flange (9) while said opposed individual slide surfaces (18) are attached to the hull (1) of the vessel, or vice versa, and about a mid-section (12) of the cylinder body positioned at a height corresponding to the neutral axis (14) of the hull (1) of the vessel, characterized in that said individual lower slide surfaces (18) are in the form of bearing blocks (21, 22, 23, 24) made from an elastic material and oriented diagonally about the periphery of the collar flange.

2. Turret device (2) according to claim 1, characterized in that said bearing blocks (21 ,22, 23, 24) are substantially rectangular.

3. Turret device (2) according to claim 2, characterized in that said bearing blocks (21, 22, 23, 24) are shaped as segments of a circle.

4. Turret device (2) according to claim 2, characterized in that said bearing blocks (21, 22, 23, 24) consist essentially of rubber or elastomer to thereby accommodate deformations.

5. Turret device (2) according to claim 2, characterized in that said bearing blocks (21, 22, 23, 24) are four in number.

6. Turret device (2) according to claim 4, characterized in that the slide surface (18) of the bearing blocks (21, 22, 23, 24) is a layer having a lower coefficient of friction that the elastic material, for example doped polyethylene.