WO1999038763A1

WO1999038763A1 - An arrangement for a drilling/production vessel with geostationary anchoring

Info

Publication number: WO1999038763A1
Application number: PCT/NO1999/000020
Authority: WO
Inventors: Leiv Wanvik
Original assignee: Kværner Oil & Gas A.S
Priority date: 1998-01-28
Filing date: 1999-01-27
Publication date: 1999-08-05
Also published as: CA2318990A1; ZA99483B; DK200001142A; GB2349613B; GB0017416D0; NO305714B1; GB2349613A; NO980374A; AR012786A1; NO980374D0; DK174136B1; AU2191099A

Abstract

The invention relates to an arrangement for a drilling/production vessel (1) having geostationary anchoring, including a turret (4) in the form of a substantially hollow cylindrical body, which turret (4) is pivotally supported about a vertical axis in a vertical through opening (3) in the vessel (1), a vessel-supported drill floor (8) above the turret (4), guides means (17) for risers (18) in the turret (4), and a drag chain (20) for the transfer of production from the risers (18) to a deck (2) on the vessel. The invention is characterised by a vertical pipe rack (14) which extends down from the drill floor (8) and into the through opening in the turret (4), a lateral access opening (16) in the vertical pipe rack (14) on a level with the drag chain (20), and in that the drag chain (20) is designed to have a free circumferential area, or is constructed having an opening in a circumferential area (38) radially external to the said lateral access opening (16).

Description

1

AN ARRANGEMENT FOR A DRILLING/PRODUCTION VESSEL WITH GEOSTATIONARY ANCHORING

Several proposed arrangements for drilling for oil and gas from production ships are known. These arrangements can be put into several categories, and there are three main categories.

According to the first category, drilling takes place with the aid of conventional dynamically positioned or firmly moored drilling ships which are also equipped with brackets for the suspension of risers, preferably on the outside of the hull.

It is typical for this category that the arrangement solutions are used in areas where weather conditions are more moderate, and where the ship basically can lie in a fixed direction which is co-linear with the dominant wind direction. The ship may be firmly moored or based on dynamic positioning, a solution particularly relevant where great ocean depths are involved.

According to a second category, drilling takes place from a ship having a geostationary turret with production risers, and where the drilling centre is positioned outside the turret.

Such ships are also normally intended for use in areas where weather conditions are moderate, as they are chiefly meant to use the drilling package only when the ship is able to lie in certain fixed directions.

A third category in one wherein drilling is carried out from ships having geostationary turrets with production risers, and where the drilling takes place centrally in the turret.

An arrangement of this kind has the best potential for use in more exposed environments, such as the North Sea, west of Shetland and the Norwegian Sea. With this arrangement a ship can drill irrespective of the direction from which the forces of wind and weather come, and can adjust its position to the direction which gives least motion for the drilling operation. Thus far, no combined drilling/production ship having such an arrangement has been built, but information regarding several versions has been published. A recurrent feature in the proposed arrangements in that they are largely based on existing turret solutions for pure production purposes. A typical and well known solution in the last-mentioned category is to place the derrick and drill floor up on the actual turret, so that the drilling system is geostationary together with the moorings and the risers. Because it is then possible to use a swivel solution to transfer the production from the riser to the ship's deck, a transfer system in the form of a chain, referred to hereinafter for the sake of simplicity as "drag chain", which is located on and supported by the turret, is employed instead. Of course, this means that the actual turret will be large in size and will have great bearing forces, with the turret's own weight, mooring equipment and forces, riser weight and forces, the weight of the drag chain and the drilling package with hook load and riser forces all contributing.

One problem with an arrangement of this kind is that the drill floor constantly changes direction, so that pipe handling becomes problematic. Similarly, the supply of drilling mud from high-pressure pumps on the deck represents a problem (must go in the drag chain), whilst the return mud is a little easier to handle.

A related and somewhat improved variant of the last-mentioned arrangement is to lift the drilling package off the turret and support it directly from the ship's deck by means of a separate support structure. The gain here is that the turret is relieved of the associated loads, and that it becomes considerably easier to arrange the drilling mud process (as on a conventional exploration rig).

Nevertheless, the same problem remains as with the arrangement where the derrick and drill floor are arranged on the actual turret, namely the elevated height of the structure, the weight of the drag chain, and problems of clearance for the risers in the so-called moonpool, i.e., the vertical opening in the turret. The elevated height of the structure is due to the fact that the blow-out preventer (BOP) must be handled at a level above the drag chain, preferably at an upper turret level, and that the drill floor and derrick will be positioned many metres (as a rule about 15 metres) above this level. The high level of the drill floor causes problems as regards centre of gravity and not least as regards the clearance in the turret opening for the risers.

The invention relates to an arrangement of the last-mentioned type, and the object of the invention is to provide a new and improved arrangement for a combined drilling/production ship of the last-mentioned category. According to the invention, an arrangement is proposed for a drilling/production ship having geostationary anchoring, including a turret in the form of a substantially hollow cylindrical body, which turret is pivotally supported about a vertical axis in a through opening in the vessel, a vessel-supported drill floor above the turret, guides means for risers in the turret, and a drag chain for the transfer of production from the risers to a deck on the vessel, which arrangement according to the invention is characterised in that it includes a vertical pipe rack which extends down from the drill floor and into the through opening in the turret, a lateral access opening in the vertical pipe rack on a level with the drag chain, and that the drag chain is designed to have a free circumferential area, or is constructed having an opening in a circumferential area radially external to the said lateral access opening.

The drill floor is supported on the vessel's deck at such a height as to enable a BOP or other largish equipment units to be manoeuvred from the vessel's deck and into the turret through the lateral opening. If the blow-out preventer is to be capable of being brought inside the turret, the drag chain must not block the lateral opening. Such clearance is in theory completely possible by locating the starting point of the drag chain on the side of the turret and the end point of the drag chain on the deck of the vessel so that in a particular sector or particular circumferential section it is possible to cross the drag chain track when the vessel is lying in the dominant wind direction. The lateral access opening can also be cleared by breaking a circumferential area of the drag chain, i.e., its utilised support structure, naturally on the condition that the drag chain (the pipes) are not in this area in the particular vessel orientation.

The new arrangement is also especially distinctive because the volume or space below the drill floor is used for vertical storage of pipes, such as risers for drilling and drill pipes. As this vertical pipe rack is suspended from the drill floor or the drill floor support structure, the pipe rack will be stationary relative to the vessel and to the drill floor that is stationary relative to the vessel.

The vertical pipe rack may to advantage be incorporated in a cylindrical structure coaxial with the turret. This means that the available space or volume below the drill floor and on the inside of the drag chain is put to good use. 4

As mentioned, when the drag chain includes a support structure extending around the turret, this support structure may be designed to have a removable circumferential section opposite the lateral access opening.

If a drag chain is used which only extends along a part of the circumference of the turret, the arrangement may advantageously be constructed so that the lateral opening will be freely accessible from the vessel's deck, in the area not covered by the drag chain in the dominant wind direction.

An especially preferred embodiment of the arrangement according to the invention is one wherein the drag chain is "inverse", i.e., that it is supported by and moves on the vessel's deck, an inner part of the chain following the movement of the turret, whilst the outer part rests on the vessel's deck. This means that the inner part of the drag chain will follow the movement of the turret, and move relative to the vessel's deck (scrubber), whilst the outer part of the drag chain rests on the deck of the vessel.

In a "normal" and known embodiment of the drag chain, the drag chain is positioned on the actual turret and is set in motion with the aid of a securing structure (derrick) which projects up from the vessel's deck with fixed piping.

An especially favourable arrangement is obtained if the "bull nose" of the drag chain, i.e., the drag chain's turning arrangement includes a planet wheel unit which has driving engagement with the turret (sun wheel) and with a rotating drive ring in the vessel, direct or via the drag chain. A rotating bull nose of this kind will cause a "lifting" of the chain from the ship's deck and will place it on the turret. The planet wheel will to advantage be made so that the engagement with the chain is synchronous will the chain's travel from inner to outer track and vice versa, i.e., the same radius for the planet wheel as the turning radius of the chain. In this way, chain movement friction against the underlying surface will be avoided. In all positions, the drag chain will rest against the underlying surface, if the transverse movement which takes place when the chain enters and exits the planet wheel is disregarded.

The planet wheel solution will be especially advantageous if it is used simultaneously for dynamic braking of the turret relative to the orientation of the ship. Safety is increased considerably by controlling the planet wheel. A substantial reduction in capacity for operating the turret is obtained if the procedure is based on the use of forces (moment) from moorings and risers to turn the turret. This can be achieved by turning the ship actively with the aid of thrusters 7 - 15° beyond the desired turning. As the angle of friction is reached, a controlled movement will be obtained.

On the deck of the vessel there may to advantage be arranged a trackway leading in towards a lateral opening for transport of the blow-out preventer. This trackway intersects the drag chain path and runs towards a lateral opening in the turret for a particular vessel orientation. The trackway may, for example, be made as a recessed track which meets similar tracks in the turret. Inside the actual turret, a transport trolley running on the trackway will be capable of rolling onto tracks which are suspended in the support structure of the drill floor, so that the transport of the blow-out preventer can take place in the very centre of the turret, at the so-called cellar deck level. Here, the blow-out preventer (BOP) will be capable of being suspended from the drill floor and lowered down through the moonpool to the seabed by means of a drill/riser.

The planet wheel unit may to advantage be incorporated in a carriage, which carriage is moveable in a guide track around the turret.

In the carriage there may be provided two motors, each of which is drive-connected to a respective gear wheel which is in engagement with the turret or the rotating drive ring in the vessel.

Furthermore, a curved redirecting plate for the drag chain may to advantage be provided in the carriage.

The guide track is constructed as a type of circumferential trench and it can advantageously be covered by a grating or the like which is attached to the carriage.

The particular advantage obtained with the new arrangement according to the invention is a reduction in structural height, because the BOP can be brought in on the level of the vessel's deck. At the same time, the space under the drill floor is used in an expedient manner for a vertical pipe rack that is stationary relative to the drill floor.

The invention will now be explained in more detail with reference to the drawings, wherein: Fig. 1 is a perspective view of an arrangement according to the invention;

Fig. 2 is a perspective view of the arrangement in Fig. 1, from the opposite side;

Fig. 3 is a perspective view of a second feasible embodiment of the arrangement according to the invention, seen from the same side as in Fig. 2;

Fig. 4 is a schematic horizontal projection of the arrangement according to Figs. 1 and

2;

Fig. 5 is a schematic horizontal projection of the arrangement according to Fig. 3;

Fig. 6 is a cross-sectional view of a vessel equipped with an arrangement according to the invention as in Fig. 1 ;

Fig. 7 is a schematic section of a planet wheel unit used in the embodiment of Figs. 1, 2 and 4.

Fig. 8 is a schematic horizontal projection of an arrangement according to the invention, with a planet wheel unit carriage; Fig. 9 is a schematic sectional view through the rail area in the system in Fig. 8;

Fig. 10 is a schematic longitudinal sectional view through a planet wheel unit carriage;

Fig. 11 is a schematic horizontal projection of the carriage illustrated in Fig. 10;

Fig. 12 is a cross-sectional view through the carriage;

Fig. 13 is a schematic horizontal projection of a modified planet wheel embodiment; and

Fig. 14 is a schematic sectional view as in Fig. 7, of the modified embodiment shown in

Fig. 13.

The same reference numerals have been used in the drawings for the same or corresponding components in the two embodiments.

Figs. 1, 2 and 4 show a section of a vessel 1 having a deck 2. In the vessel 1 there is a vertical through opening 3 (see Fig. 6) where in a known manner there is arranged a turret 4. The turret 4 is made in the form of a hollow cylindrical body having an upper annular flange 5.

The vessel 1 is constructed having a hull that can take a large vertical opening 3, and the turret 4 on its part is equipped with a through opening or moonpool 6 which is as large as possible, so that it will not come into conflict with a drilling operation. The through opening 6 in the turret can without any difficulty be made so large that, for example, a 9 - 10° deviation of the drilling riser (not shown) will be allowable without coming into contact with the inner wall of the turret. In the exemplary embodiment, the turret is constructed as a hollow cylinder with a solid casing, but the turret may also, for example, be constructed as illustrated and described in Norwegian Patent Application P970508.

A support structure 7 for a drill floor 8 is provided on the deck 2 of the vessel. The support part 7 in this case has three legs 9, 10 and 11. This gives a stable structure with plenty of space between and on the inside of the support legs 9, 10, 11.

A rig or derrick 12, here only indicated schematically, extends upwards from the drilling deck 8.

A cylindrical structure 13 extends down from the underside of the drill floor 8. This cylindrical structure 13 extends down to the deck 2 of the vessel and advantageously some way down into the moonpool 6, as is shown in Fig. 6. The cylindrical structure 13 is open at the top and made in the form of a vertical pipe rack 14. To illustrate this some pipes 15 placed vertically in the pipe rack 14 have been indicated in Figs. 1 and 6. The pipe rack 14 takes up quite a large part of the circle, and in the centre there is an open space under the drill floor 8, for the carrying out of drilling operations and also for the lowering of necessary equipment, and especially the so-called blow-out preventer or BOP.

Here, the cylindrical structure 13 is shown for the sake of simplicity with a solid casing wall, but the structure may of course be made as a truss construction. Regardless of the casing embodiment, in the vertical pipe rack there is provided a laterally directed access opening 16 on a level with the vessel's deck 2. A BOP can be passed in through this opening 16 from the vessel's deck 2 and into the space under the drill floor 8, for lowering to the seabed.

In the turret 4 there are provided conductor casings 17 (see Fig. 6) for non-illustrated production riser 18 (see Fig. 1). These production risers 18 run up to valves 19 on the upper annular flange 5 of the turret. For the transfer of the production from the turret to the vessel a drag chain is used which consists of a pipe transfer system in the form of a chain and which is known in the industry as a drag chain. A drag chain 20 of this kind is indicated in Figs. 1, 2, 4 and 6. The illustrated drag chain 20 extends as shown along a part of the circumference of the turret and is arranged so that in the dominant wind direction of the vessel the side opening 16 will be freely accessible, as can be seen in particular from Figs. 2 and 4. The drag chain's so-called bull nose or chain nose is advantageously provided with a special planet wheel unit 21, shown in a schematic section on a larger scale in Fig. 7. The planet wheel unit 21 is positioned as a turning arrangement for the pipes 22 in the drag chain. In this case, the planet wheel unit 21 includes three gear wheels 23, 24 and 25. The two upper gear wheels 23, 24 have respective driving engagement with respectively the upper and lower elements 26, 27 of the drag chain. The lower gear wheel 25 has driving engagement with a gear rim 28 on the turret 4, 5 and an outer annular gear rim 29 which is attached to the vessel 1. Thus, in this case the turret 4, 5 functions like a sun wheel, whilst the gear wheel 25 functions as a planet wheel. This planet wheel unit thus forms the so-called "bull nose" or chain nose of the drag chain. When the rotating part 4 rotates relative to the ship 1 , the chain nose or planet wheel unit 21 will also be rotationally actuated and will "lift" the chain from the ship's deck 2 and over onto the turret, i.e., the track formed in the annular flange 5 of the turret. In this way motional friction against the underlying surface is avoided, because the drag chain will rest constantly against the underlying surface, apart from the transverse movement which takes place when the chain enters/exits the planet wheel.

In Fig. 7 roller bearings 30, 31 are provided for the turret 4, 5, whilst upper guide rails 32 for the drag chain 20 are indicated.

It will be appreciated at once that the arrangement in Figs. 1, 2, 4 and 6 gives free access to the opening 16 in the vessel's dominant direction, i.e., when the ship is lying in the dominant wind direction (that is calculated beforehand), so that a BOP can be brought in under the drill floor 8 with the aid of a non-illustrated trolley which runs on the indicated trackway 33.

In Figs. 3 and 5 it is shown how access to the side opening 16 can be obtained in those cases where the choice has been made to use a conventional drag chain 35. The drag chain 35 has in a known manner an inner and encompassing support structure 36, here only shown schematically, which extends around the whole circle. In Figs. 3 and 5, the so-called "bull nose" is indicated by means of the reference numeral 37. The drag chain 35 will not be described in more detail here as it is a well-known device. What is essential here is that a portion 38 of the support structure 36 (both inner and encompassing) can be taken away or removed when the vessel is lying in the dominant wind direction, so that access to the side opening 16 is thus cleared and a BOP can be moved into the space below the drill floor 8, for example, with the aid of a trolley which runs on the rails 33.

One embodiment of the invention, where the planet wheel unit is incorporated in a carriage is shown in Figs. 8-12.

This carriage 40 is shown schematically in Figs. 10, 11 and 12. In essence, the carriage 40 is constructed having a bottom plate 41 and a top plate 42. The planet wheel unit 21 is pivotally mounted in the top plate 42 and the bottom plate 41. In the vessel 1 there is around the rotating part 4, 5 provided an encompassing trench 43 as guide track for the carriage. As in the embodiment in Fig. 7, there is provided in the trench a gear rim 28 on the rotating part 4, 5 and an outer annular gear rim 29 attached to the vessel 1. The planet wheel unit 21 also includes in this case gear wheels 23, 24 in engagement with gear rims 26, 27 in the drag chain 20. Furthermore, the planet wheel unit has a lower gear wheel in engagement with gear rims 28 and 29, see Fig. 12.

In the carriage 40 there are provided two motors 44, 45 which are drive-connected to a respective drive or gear wheel 46, 47 which has driving engagement with respectively the gear rim 28 on the turret 5 and the gear rim 29 on the side of the vessel (see Figs. 10 and 1 1).

Support structures 48, 49 for the drag chain 20 are indicated in Fig. 12.

As shown in Figs. 10 and 11, a curved redirecting plate 50 for the drag chain 20 (indicated by means of a broken line in Figs. 10 and 11) is provided in the carriage.

The motors 44, 45 are used essentially as a brake to control the rotational speed of the turret 4, 5 in the vertical opening, and also for locking the turret.

A driving mechanism 66 for the turret 4, 5 is ship-mounted and indicated in broken lines in Fig. 8. In an arrangement with planet wheels there will be a sector of 90° where a drive wheel will not come into conflict with the planet wheel.

Advantageously, there is provided, for example, a grating or grid-like deck surface structure 51 over the trench 43, see Figs. 8 and 9. As shown, this grating is expediently divided into sections 52-58 which are hingedly interconnected 59 and are also hingedly connected 60, 61 to the carriage 40. The carriage 40 in the schematic view in Figs. 10 10 and 11 is shown having a rectangular form in horizontal projection, but the carriage 40 will in practice have the sector shape shown in Fig. 8.

A track 33 runs from the vessel 1 and into the rotating part, for the introduction of a BOP as described above. Supplementary rails are inserted in the open short rail sections 62, 63 and 64, see Fig. 9, when the track 33 is to be used.

As indicated in Fig. 9, the grating 51 is supported by suitable rollers 65 in the guide track or trench 43. The grating 51 can be made to maintain the form of the drag chain in its lower part, and this will largely ensure that the whole chain in the vertical plane maintains the desired form and stability.

Figs. 13 and 14 show how the planet wheel unit 21 can have indirect driving interaction with respectively the outer annular gear rim 29 attached to the vessel 1 and a gear rim 28 on the turret 4, 5, wherein a lower part of the drag chain 20 is in engagement with the annular gear rim 29 and the gear rim 28, and at the same time has driving engagement with a lower gear wheel 25' in the planet wheel unit. The respective gears 46, 47 also have indirect driving engagement via the drag chain.

Today's drag chains have inter alia a lower chain section wherein the chain links are hinged centrally. This must therefore be taken into account (the distance between the links diminishes in the "inner swing" when the chain runs in a curve around the planet wheel) when forming the teeth on the planet wheel, or the drag chain (its hinging) must be modified accordingly.

The invention provides a crucial advantage, namely that heavy and high equipment, and in particular a blow-out preventer (BOP) can be moved in on a level with the vessel's deck 2. This brings about a possibility for lowering the centre of gravity of the whole arrangement, with accompanying improved stability of the vessel.

It is also an advantage that a very expedient vertical pipe rack is provided which is stationary relative to the drill floor. A vertical pipe rack of this kind facilitates the handling of the pipes to and from the rack.

The whole arrangement allows drilling to take place from the ship's weather deck or main deck in the same way as drilling from the deck of a semi-submersible platform, 11 i.e., with the bracket joints of the riser just below the drill floor with underlying telescopic joints and with a good clearance from risers to other parts of the hull.

Claims

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

1.

An arrangement for a drilling/production vessel (1) having geostationary anchoring, including a turret (4) in the form of a substantially hollow cylindrical body, which turret

(4) is pivotally supported about a vertical axis in a vertical through opening (3) in the vessel (1), a vessel-supported drill floor (8) above the turret (4), guides means (17) for risers (18) in the turret (4), and a drag chain (20) for the transfer of production from the risers (18) to a deck (2) on the vessel, characterised by a vertical pipe rack (14) which extends down from the drill floor (8) and into the through opening in the turret (4), a lateral access opening (16) in the vertical pipe rack (14) on a level with the drag chain

(20), and in that the drag chain (20) is designed to have a free circumferential area, or is constructed having an opening in a circumferential area (38) radially external to the said lateral access opening (16).

2. An arrangement according to claim 1, characterised in that the vertical pipe rack (14) in incorporated in a cylindrical structure (13) coaxial with the turret.

3.

An arrangement according to claim 1 or 2, wherein the drag chain (35) includes an (inner and outer) support structure (36) which extends around the circumference of the rotating part (4), characterised in that the support structure (36) is constructed having a removable circumferential section (38) opposite the lateral access opening (16).

4. An arrangement according to claim 1 or 2, characterised in that the drag chain (20) only extends along a part of the circumference of the turret (4), so that the lateral opening (16) is freely accessible from the vessel's deck (2) when the vessel is lying in its dominant wind direction.

5.

An arrangement according to one of the preceding claims, characterised in that the drag chain (20; 35) is "inverse", i.e., that it is supported by and moves on the vessel's deck 13

(2), whilst an inner part of the chain follows the movement of the turret (4) and is preferably supported thereby.

6. An arrangement according to claim 4 or 5, characterised in that a turning arrangement in the drag chain (20) includes a planet wheel unit (21) which has driving engagement with the turret (4, 5) (sun wheel) and with a rotating drive ring (29) in the vessel (1).

7. An arrangement according to claim 6, characterised in that the planet wheel unit (21) has driving engagement with lower and upper parts (26, 27) in the drag chain (20).

8.

An arrangement according to one of claims 5 to 7, characterised in that the planet wheel unit (21) is designed to be able to brake the turret (4, 5) relative to the vessel (1).

9.

An arrangement according to one of the preceding claims, characterised by a trackway

(33) on the vessel's deck (2) leading in towards the said lateral opening (16).

10.

An arrangement according to one of claims 6 to 9, characterised in that the planet wheel unit (21) is incorporated in a carriage (40), which carriage (40) is moveable in a guide track (43) around the turret (4, 5).

11.

An arrangement according to one of claims 6-10, characterised by two motors (44, 45) in the carriage (40), wherein each motor (44, 45) is drive-connected to a respective gear wheel (46, 47) in engagement with respectively the turret (4, 5) and the rotating drive ring (29) in the vessel (1).

12.

An arrangement according to one of claims 6 to 11, characterised by a redirecting plate

(50) for the drag chain (20) in the carriage (40). 14

13.

An arrangement according to one of claims 6 to 12, characterised by a grating (51) or the like connected to the carriage which covers the trench-like guide track (43) of the carriage (40).