TURRET FOR DRILLING OR PRODUCTION SHIP

Abstract: A turret for a vessel such as a drilling or production vessel for recovery of oil offshore, is installed in such a manner that it can rotate in a throughgoing opening or well in the hull of the vessel, and includes bearing arms (15) which are equipped with axially and radially fitted bearing elements which act against corresponding bearing elements on the vessel. The bearing arms (15) are connected to a substructure in the turret which provide individual suspension to and can absorb unevennesses and deformations in the bearing. Further, the axial bearing track is disposed on a pedestal-like elevated area (30, 47, 56, 57, 58) which is rigid in the axial direction. The pedestal-like elevated area is connected with the hull, mainly at the level of the neutral axis of the vessel, and the radial bearing element on the vessel is in the form of a band-like structure (52, 61).

Description: Turret for drilling or production ship The present invention concerns a turret for vessels such as drilli or production vessels for recovery of oil offshore, said turr being erected so as to allow rotation in a throughgoing opening well in the hull of the vessel, and having suspension arms whi are equipped with axially and radially provided bearing element which operate in relation to corresponding bearing elements on th vessel. A turret of the above entioned type is normally fitted with bearin elements with spring devices to assure an even distribution of th bearing forces. The suspension arrangements have a fairly larg slack, partly to absorb elongation in the vessel, and are ofte jointed to handle angular deformation and to even out loads. I order to achieve the best possible control of suspension forces an deformation in bearing, vessel and rotary tower, complicate mechanical or hydraulic solutions are often used. A hydrauli solution is shown in EP patent application no. 0.207.915. I consists of an upper radial bearing, an axial bearing and a lowe radial bearing. Each of these bearings consists of a large numbe of hydraulic piston/cylinder devices which are each mounted on bearing element. One major disadvantage with these solutions is that they ar complicated, and therefore expensive to build and maintain. further disadvantage is that the bearing surfaces are subject t wear as a result of relative movements and constructional deviation which are due to the suspension/wheel arrangement and movements i the vessel. With regard to the wheel arrangement, because of th large relative momve ents, pommelled wheels have to be used. Thes pommelled wheels have limited bearing capacity, and in the case o large, heavy rotary bearing structures, slide bearings therefor have to be used, or a combination of wheel and slide bearings. One disadvantage with slide bearings, however, is that larg machinery is required to turn the turret, and special, expensiv precautions have to be taken to protect the bearings against the corrosive environment on board vessels at sea. Norwegian patent application no. 87511 shows a bearing system a turret in which an attempt is made to eliminate the wear and t on the radial bearing by using structural suspension. However, t structural suspension has limited independent suspensi particularly in the case of large, heavy turret, which are necess to maintain a satisfactory load distribution without using spec mechanical or hydraulic springs in connection with the axial bear elements. The wear and tear on the radial bearing surfaces is t not quite eliminated by this solution either. Further, with regard to the aforementioned Norwegian pat application, mechanical suspension is also used in the rad bearing of the turret, and this suspension is as previous mentioned costly to build and maintain, and will cause wear and te on the axial bearing. One objective with the present invention has been to provide turret for vessels in which the wear and tear on the axial a radial bearing of the turret is virtually eliminated, but which nevertheless cheaper to build and maintain than existing solution Another objective has been to provide a bearing design for th turret in which vessel-induced stresses and elongations do n induce undesired reaction forces on the bearing and the rota tower. A third objective has been to reduce displacements in t turret due to the external forces which act on it. A four objective has been to provide a turret solution in whi unevennesses in the bearing tracks etc. are absorbed by t substructure of the rotary tower and/or the bearing trac themselves. Last, but not least, one major objective was to provi at a solution which can be used on large, heavy rotary towers whi are subject to large forces. In accordance with the invention, there is provided a turret whic is characterised in that the bearing arms are connected with substructure in the turret which permits the bearing arms t individually absorb irregularities in the bearing surfaces, that th foundation for the bearings for the turret is disposed basically a level with the neutral axis of the ship, that the axial beari is disposed on a pedestal which is rigid in the axial direction, a that the radial bearing is made up of a band-like structure whi is designed to absorb displacements in the radial direction, specified in Claim 1. Claims 1-10 defines advantageous features of the invention. The invention will now be described in more detail by way of examp and, with reference to the drawings, where: Fig. 1 shows a longitudinal section of a turret with respect the invention installed in a vessel. Fig. 2 shows in perspective a section through the substructu of the turret as shown in Fig. 1. Fig. 3 shows in a larger scale a section through the actu bearing device for the turret. Fig. 4 shows the same bearing device, seen from above. Figs 5 and 6 show two alternative bearing designs Fig. 7 shows the principles for a ballast system for the turre The turret shown in Fig. 1 is mounted in a throughgoing opening well 3 in the hull 2 of the vessel. The lower part 4 of the turre the substructure, consists of a largely cylinder-formed structur while the upper part 1 of the turret, the manifold chamber, consis of a circular deck 5 which provides space for pipe systems a equipment. The oil and gas riser 9 is led through a guide pipe up to a choke and manifold system (not shown) . A swivel coupling 2 with a set of pipes 21 connects the flow of produced oil and g from the turret to the vessel's process equipment via a fra structure 22. The vessel may be dynamically positioned or anchored via moori lines connected with the turret. In the example shown here, t mooring lines 8 are led via a guide wheel 11 on the outside of t turret and are attached to stoppers 12 which are fitted inside th turret at the top. Mooring line lifters 13 mounted on the deck o winches (not shown) mounted on the turret are used to tighten th mooring lines over the guide wheel 10. Alternatively, there ca chain-stoppers instead for a guide wheel 11 at the lower part of turret. The guide wheels/chain stoppers 11 should preferably mounted high (in relation to the base line of the vessel) to red the capsizing moment due to the line elongation, and to simpl docking of the vessel. The turret bearings 28, 30 are arranged in an extended upper p 41 of the well 3 along the neutral axis of the vessel. By arrang the bearings mainly on a level with the neutral axis of the vess the hull-induced movement in the surfaces of the bearings reduced. The capsizing effect is also reduced, i.e. the dista between the bearings 28, 30 and the guide wheels 11 will be as sh as possible. The turret can be rotated by means of the cable lifters 13 via drive chains (not shown in detail) arranged along the circumferen of the turret, or a separately rigged rotary device can be us which includes a gear 24 driven by a motor 23. The gear 24 engag with a toothed wheel rim 6 on the turret. The lower part of the turret consists of a solid, ring-formed bo bearer 35. This forms the foundation for the guide wheel 11 of t mooring lines. The ring-bearer has a chamber 34 which preferably m be divided into separate tanks by radial bulkheads. With the aid a ballast system (see later section) these tanks can be filled emptied as desired (depending on the stretch in the mooring line to reduce the capsizing moment of the turret. Fig. 2 shows the substructure 4 of the turret. It compris vertical bearers 16 radial arms 15, a basically cylindrical colu 36, the ring-formed box-bearer 35 and a top plate 32. The radi arms 15 are fastened to the vertical be'arers 16, which in turn a connected to the box-bearer 35. In Fig. 2, the vertical bearers a of the T-bearer type, but they can with advantage be H-bearers, box shaped or some other appropriate type. The plate structure 36 between the vertical bearers 16 is largel shear-rigid in the vertical plane, but preferably flexible in radial direction. The top plate 32 is shear-rigid, and can be reinforced with a fla ring 29 or something similar in order to achieve adequate rad rigidity. Apart from that, it is mainly stiff in the horizon plane, but preferably flexible in the lateral plane. The plate also provided with openings 33 for the risers' guide pipe 19 ( Fig. 1) . As mentioned above, each of the radial arms 15 is fixed to vertical bearer 16. Moment loading induced by the arm will ca rotation at the point where the arm is fixed, and the verti bearer will deflect without affecting the adjacent bearers. This possible because the structure (plate/εtiffeners) between vertical bearers have an insignificant stiffness to deformations the radial direction of the turret. With the substructure given above for the turret, one achieves independent, structural suspension for each of the arms, which necessary to absorb unevennesses in the bearing tracks. Wi structural suspension, it will also be advantageous to use whee in the radial bearing, since major cross-movements of the beari element will be avoided in case of large loads. Use of wheels in t axial bearing (also the radial bearing) also reduces the rota moment when the turret rotates. The proposed substructure thus represents a second important featu of the invention, since it is substantially cheaper than the kno solutions which, as mentioned above, use hydraulic or mechanic suspension to absorb the same unevennesses. In this connection should be mentioned that in Norwegian patent application no. 87511 a turret with radial arms is used, but ' the arms here are connect to a torque box. This torque box provides flexibility against axi loads which act on the whole turret, since all the arms are fix to a common box structure. But it does not contribute much to independent deflection, which is necessary to absorb unevenness in the bearing race. Figs 3 and 4 show on a larger scale the bearing arrangement of turret. As mentioned previously, the bearing arrangement is larg aligned with the neutral axis of the vessel, to reduce hull-indu movements and loads on the bearings. The bearing arrangement consists of a radial wheel bearing 28 an axial bearing 31. Bogies attached to each of the arms 15 of turret are used for the axial bearing. The bogie wheel pairs 41 fastened to each end of a tangential girder 42. These girders 42 a supplied with a wide, lower flange or shear plate 43 which is rig to radial loads from the radial arm 15. The tangential girders a designed mainly to be rigid to loads in the axial direction, but allow rotation in relation to the radial arm 15. This assures th the tangential girders are rigid to radial and axial deformation but nevertheless allow an evening out of the load between the fo wheels 45 in the bogie. It is important that the girder 42 flexible enough to tolerate downward bending of the arm 15 witho this producing too great reaction loads in the wheels 4 Alternatively, the arms can be built with a certain pre-load ang which is opposite to the downward bending when the turret is subje to maximum loading, the object being that the loads on the whee are as even as possible when the rotary tower is subject to extre loads. The wheels 45 are mounted in a shear-rigid frame 46, so that t wheels are rigid in relation to one another. The wheels 45 ca therefore to advantage be made with a cylindrical surface. A slid bearing should preferably be used in the hub of the wheels t achieve a suitable resistance to rolling and at the same time allo the wheels to slide axially along its axis, in order to absor relative, radial deformations between the radial bearing and th axial bearing, and to absorb deviations due to construction betwee the position of the radial bearing and the rails. Using wheels with plane surfaces has the advantage over pommelle wheels that they have a considerably greater bearing capacity. The bogie 40 for the axial bearing rolls on a double rail system 4 and the rails in turn rest on a pedestal-like foundation consist of two cylindrical columns 30, and a torque box 47. Between the 47, the column 30 with the necessary bracing and the deck 48, th is no structure which would allow the two shells (columns) to freely deformed in a radial direction. The upper torque box can a be regarded as an upper rigid ring which ensures that the bear tracks retain their shape locally in the radial plane, while columns absorb the global relative displacements between the bear tracks and deck support. The position number 37 shows openings the plate structure 36 which are designed to allow air to p through. The columns are rigidly supported in the structure of the vess well 3 and a support in the deck of the vessel respectively, so t the axial position of the two rails in principle remains at the s elevation when the hull of the vessel is subjected to loads a elongations. One major advantage with the present foundation design is thus th the radial elongations in the hull of the vessel are filtered o by means of the flexible spacer (the columns 30) between the de 48 of the ship and the bearing tracks 44. This substantially reduc wear on the surfaces of the bearing compared with known solution In order to further reduce wear and tear and possibly increase t suspension of the bogies in relation to the arm, a rubber filler may be inserted between the bogies 40 and the arms 15. These fille will also eliminate sliding movements in the bearings of the whee in the bogies 40, and will help to even out the load on the whee 45. The radial bearing encompasses wheels 49 fitted close together a rim 50 which is connected with the radial arms 15. The wheels run against a radial bearing rail 51 which is fixed to a cylindric band 52. The band 52 and the rail 51 have a substantial tangenti tensile strength, but have local flexibility to minor deviations the establishment of the mutual radial position of the rail 51 a the wheels. As regards the radial position of the wheels, this c be secured by means of a wedge device 27 which moves the wheels or out in relation to the rim 50, or a kind of cam axle arrangem can be used. The upper band consists here of a column shell which extends f the lower edge of the rail up and a bit past the rail. This brea is determined by the necessary tangential strength and rad flexibility of the rail. The band can be strengthened with ex ring-bracers 53 which are placed a certain distance from the rai The foundation for the radial bearing shown here consists of column which is an extension of the band 52 down to the deck 48. can to advantage be made of a thin shell plate 54. The radial load from the closely mounted radial wheels transferred to the rail/band as tangential forces around the beari band. The elongation in the band is transferred to the deck of t ship in the range of 45-135 degrees in relation to the lo direction, via the lower part 54. The radial displacement of t turret is therefore limited. The wheels are mounted in a rigid rim on the turret, while t bearing band must be sufficiently flexible to compensate for defec in the rail and wheel. The wheels have to be mounted so clo together that limited flexion is caused in the rail/band section. The advantage of a radial bearing design of the kind described he is that the band has enough structural suspension to compensate fo local tolerances (unevennesses) in the rail and wheel mounting. Th ovalisation of the deck around the well is absorbed in th foundation 54 and/or by means of a certain clearance between rai and wheel, so that the radial bearing is maximally loaded as result of the vessel's elongations in heavy seas. The band 52 and the foundation 54 are also in principle so flexibl in relation to radial deformations, that global ovalisation (defects) in the turret do not affect the bearing reaction force to any significant degree. Alternatively, the band 52 and the foundation 54 may be connecte together by means of a coupling 55. The purpose of this coupling to give the column limited supplementary flexibility in relation the deck, whereby radial deformations of the well 3 reduce forces in the radial bearing, and that the reaction forces in radial bearing should be less affected by an ovalised turret. Fig. 5 shows an example of an alternative design, where the structure 56 for the axial bearing is provided directly on the de 48, i.e. without a flexible connection between the deck and the b structure. With this solution, a somewhat lower building height obtained for the substructure, but there will be somewhat more we and tear on the bearing surfaces. Fig. 6 shows a further bearing solution in which axial bearing a radial bearing are provided on a common pedestal 57, and in whi the box structure 58 provides the support for both the rails 59 the vertical bearing and the rail 60 of the radial bearing. T internal plate 61 acts in a manner similar to the band (52, Fig. mentioned above, since it is designed to compensate for min unevennesses in the wheels and rail (the plate is not braced) . The difference between this bearing and the bearing shown in Fi 3 and 4 is that a separate column for the radial bearing eliminated, and the radial bearing is provided at a lower level which helps to make the capsizing moment from the horizontal forc which act on the turret smaller, and less steel foundation needed. Fig. 7 is a sketch showing the principles of the ballast system f the turret, according to the invention. The lower part of the turr consists, as mentioned previously, of a solid, ring-formed bo bearer 35 which can be divided into separate tanks 6, 7 in t circumferential direction of the turret. With the aid of a pu system and pipelines 61 between the tanks, ballast can be pump from one or more tanks on one side to one or more tanks on t opposite side to reduce the loads on the bearings and reduce t capsizing moment of the turret. The pumps can to advantage controlled by an electronic control unit based on signals fr tension detectors 14 on the mooring lines. The above refers to an example of a turret solution in which w mounting is used for both the axial and the radial bearing. Howe within the frame of the invention, as it is defined in the cla a slide bearing can also be used, or a combination of slide bear and roller bearing.

Claims: Claims Turret for vessels such as drilling or production vessels recovery of oil and gas offshore, which turret is installed a manner allowing it to rotate in a throughgoing opening well in the hull of the vessel, and includes bearing arms ( which are equipped with axial and radially arranged bear elements which act on corresponding bearing elements on vessel, c h a r a c t e r i s e d i n t h a t the bearing arms (15) are connected to a substructure in turret which provides individual springing or flexibility f the bearing arms, so that they can absorb unevennesses a deformations in the bearing, that the axial bearing track mounted on a pedestal-like elevated area (30, 47, 56, 57, 5 which is rigid in the axial direction, that the pedestal-li elevated area is connected with the hull preferably at t level of the vessel's neutral axis, and that the radial beari element on the vessel consists of a band-like structure (5 61) . Turret according to claim 1, c h a r a c t e r i s e d i n t h a t the rigid box (47) is connected with the deck (48) by one more ring-formed columns (57) . Turret according to claim 1, c h a r a c t e r i s e d i n t h a t the rigid box (58) is mounted directly on the deck (48) Turret according to claim 1, c h a r a c t e r i s e d i n t h a t the band-like structure for the radial bearing consists of ring-formed column shell (52) which is connected with the de (48) by means of a foundation in the form of a thin shell pla (54). 5. Turret according to claim 4 , c h a r a c t e r i s e d i n t h a t the column shell (52) and the foundation (54) are connec together by means of a coupling (55) . 6. Turret according to claims 4 and 5, c h a r a c t e r i s e d i n t h a t the column shell is reinforced with ring-bracers (53) . 7. Turret according to claim 1, c h a r a c t e r i s e d i n t h a t the foundation for the axial bearing and the radial bear consists of an integrated unit in which the rigid box (58) connected with the deck (48) by means of two ring-form columns, and the band-formed structure (61) for the radi bearing consists of an internally ring-formed plate in t rigid box (58) . 8. Turret according to claims 1-7 c h a r a c t e r i s e d i n t h a t both the axial bearing and the radial bearing are roll bearings, whereby the axial bearing comprises a four-whe bogie (40) mounted on each arm (15) , with the wheels runni on two parallel rails (59) mounted on the box (47, 58) , a that wheels (49) for the radial bearing are mounted clo together in a rigid rim (50) which is connected with the ar (15). 9. Turret according to the aforementioned claims, c h a r a c t e r i s e d i n t h a t the substructure consists of a basically cylinder-shaped colum (36) , inside the column (36) vertically mounted bearers (16 which the arms (15) are connected to, an upper top plate (32) and a lower, ring-formed box-bearer (35) . 10. Turret according to the aforementioned claims c h a r a c t e r i s e d i n t h a t the lower box-bearer (35) is divided into tanks by means tight bulkheads, whereas on the basis of the tension in mooring lines, the tanks can be filled and emptied by means a pipe and pump system (61) .

Inventors: ASKESTAD SIGMUND, ASKESTAD, SIGMUND
Applicants: NORSK HYDRO AS, NORSK HYDRO TECHNOLOGY, NORSK HYDRO A.S, NORSK HYDRO TECHNOLOGY B.V.
Application year: 1992
Main classification: B63B21
IPC codes: B63B21