EP0945587A1

EP0945587A1 - Riser tensioning construction

Info

Publication number: EP0945587A1
Application number: EP98200984A
Authority: EP
Inventors: Jack Pollack
Original assignee: Single Buoy Moorings Inc
Current assignee: Single Buoy Moorings Inc
Priority date: 1998-03-27
Filing date: 1998-03-27
Publication date: 1999-09-29

Abstract

The invention relates to a vessel (1) comprising a riser tensioning construction. A tensioning member (9) is movably carried on a support frame of the vessel, one side of the tensioning member carrying a riser (3,4) extending from a subsea structure to above water level. The other end of the tensioning member is attached to a weight (16,17) for exerting a tensioning force on the riser. The vessel according to the invention is characterised in that the second end of the tensioning member (9) extends outside the hull (1) of the vessel or in a well in the vessel such as to be located below water level. Hereby the movements of the weight upon pitch, roll or heave motions of the vessel are limited and additional space on the vessel is made available for production or processing equipment. One embodiment comprises a support frame which is carried by cables and sheaves that are supported on two spaced apart arms above deck level. The risers (3,4) are attached to the support frame (15) such that a substantially equalized tensional force on the risers is maintained during movements of the vessel. Weight guiding elements may be provided on the vessel or on the seabed or may be formed by the risers themselves for preventing lateral movement of the weights and preventing damage to the risers. Other embodiments comprise flexible lines as a tensioning member connected to the seabed.

Description

The invention relates to a vessel comprising a riser tensioning construction having a support frame attached to the vessel, a tensioning member movably carried by the frame, the tensioning member being with a first end attached to an upper end of a riser extending from a subsea structure to the tensioning member and being with a second end attached to a weight for exerting a tensioning force on the riser.
From U.S. patent nr. 4,567,842 a mooring system for a floating production vessel is known using a riser which is tensioned by a weight type motion compensating system. Herein the tensioning construction comprises a pivotable frame at the bow of the vessel which at one end is provided with a large counter weight near deck level. The known system has as a disadvantage that it takes up a lot of space and that during roll, pitch or heave movements of the vessel the large mass of the tensioning construction can give rise to an unbalance and exerts large forces on the supporting frame structure.
From U.S. patent nr. 4,272,059 a riser tensioning system is known wherein a riser, such as a drilling riser, is at its upper end provided with a tension ring which is connected via cables to sheaves on the drilling vessel. The sheaves are mounted on the free ends of piston rods of hydraulic cylinders, the second end of the cables being attached to the vessel. Upon heave, roll or pitch of the vessel, the tensional forces on the riser are maintained generally constant by movement of the piston rods against the hydraulic pressure in the cylinders. This system has as a disadvantage that the tensional forces exerted on the riser will vary with the buoyancy of the vessel. In order to obtain a relatively large stroke of the cylinders the cylinders should be relatively long and therefore take up a lot of space, which in view of the moving nature of the cylinders cannot be effectively used. Furthermore, the hydraulic system is relatively complex.
It is therefore an object of the present invention to provide a riser tensioning construction which can be used in deep waters using a dry production tree, which consumes relatively little space and which is stable under different motions of the vessel. It is a further object of the present invention to provide a riser tensioning construction which allows attachment of multiple risers while maintaining a substantially equalised tensional force on the risers upon movements of the vessel. It is a further object of the present invention to provide a riser tensioning system which can also function as a stable support platform for production or drilling equipment.
Thereto the riser tensioning construction according to the present invention is characterised in that the second end of the tensioning member extends outside the hull of the vessel or through a well in the vessel such as to be located below water level. By placing the counterweight below water level, the forces exerted by the counterweight on the vessel upon movement thereof are reduced and the motion of the counterweight is damped. Furthermore, location below water level of the counterweight provides for an easy way of varying the tensional force exerted thereby, not only by varying the mass thereof but also by varying its buoyancy for instance by use of compressed air. Placing the counterweight below water level allows additional use of the space needed for drilling and/or production equipment on the vessel.
The tensioning member may be a rigid arm but is preferably formed by a cable. The tensioning member according to the present invention can be placed alongside the hull, through a well in the hull or can be located below keel level of the vessel.
In one embodiment according to the present invention the support frame comprises at least two spaced apart mounting arms each carrying a cable guide means and a respective cable, a connector being supported by the first ends of the cables, at least one riser being attached to the connector. In this embodiment the motions of the vessel are completely decoupled from the riser. A substantially constant tensional force is exerted on the riser upon heave, pitch or roll of the vessel. As the mounting arms according to the present invention remain stationary, they do not form an obstruction for the drilling and production equipment on the vessel.
The connector can be formed for instance by a transverse support arm extending between the mounting arms. The transverse support arm, preferably supporting multiple risers, is lowered or raised a small amount that is determined by the elasticity of the risers, at the respective side at which the tension in the riser increases or decreases by lifting or lowering of the counterweights. Besides dynamic forces acting on the counterweights and frictional forces in the cable guide means, the tensional forces on the risers remain substantially constant and are substantially independent of the movements of the vessel. Furthermore, the transverse support arm can be effectively used as a stable deck structure for supporting drilling or production equipment, as it will be maintained in a substantially horizontal position by the tensional forces of the risers acting thereon.
The riser tensioning construction according to the present invention may be mounted on a turret structure of a vessel around which the vessel can weathervane, at deck level or at keel level thereof. It is also possible to use the present riser tensioning construction in a vessel wherein the cables and counterweights extend in a central well, for instance through the turret.
To prevent lateral motions of the tensioning weight it is possible to provide a weight guiding element on the vessel, for instance near keel level or near the seabed. It is also possible to guide the tensioning weights along the risers, in case a rigid steel casing is used.
Finally an embodiment is envisaged wherein the tensioning member comprises a relatively elastic cable which is at it second side attached to the seabed, which in this case forms the tensioning weight. The tensioning member can be a rope such a polyester line which is held under a certain pretension and which therefore acts as a spring to tension the risers.
Some embodiments of the riser tensioning construction according to the present invention will, by way of example, be explained in detail with reference to the accompanying drawings. In the drawings:
Figure 1 shows a schematic frontal view of a first embodiment of a vessel comprising the riser tensioning system according to the present invention;
Figure 2 shows another embodiment of a vessel in the form of a tension leg platform comprising a support deck located over a moon pool of the vessel;
Figure 3 shows an embodiment of a vessel comprising a riser tensioning system extending through the turret;
Figure 4 shows an embodiment wherein the riser tensioning system is placed at keel level on a turret structure;
Figures 5, 6 and 7 show different embodiments of weight guiding systems for preventing lateral movements of the counterweights, and
Figure 8 shows a riser tensioning system comprising elastic cables connected to the seabed.
Figure 1 shows vessel 1, such as for instance a floating storage and production vessel which is moored to the seabed via catenary anchor lines 2. As used herein the word "vessel" is intended to mean any floating construction such as semi-submersibles, floating production vessels, tension leg platforms, barges etc. The vessel can be anchored to the seabed via anchor lines or ropes or via tendons or tethers. Within the scope of the present invention also vessels are comprised which are connected to the seabed only via one or more risers for the supply of hydrocarbons from the subsea structure to the vessel.
From a subsea well head, which may be at a depth of for instance 1000 or 2000 metres two hard casing steel risers 3,4 extend up to above water level 5 and are supported by the buoyancy of the vessel 1. The upper ends 7,8 of the risers 3,4 are attached to a tensioning member 9 comprising two cables 12,13 being on one side attached to a transverse support arm or a support deck 15 and being on their other ends connected to a respective clump weight 16,17. The cables 12,13 are guided over fixed position sheaves 19,20 which are supported on vertical mounting arms 21,22. The arms 21,22 are located near the sides of the hull 23 of the vessel 1 such that the cables 12,13 extend alongside the vessel to below water level 5. Upon rolling of the vessel, the weight tensioned risers 12,13 will cause the respective weights 16,17 to be lifted or lowered. In this way the tensional forces on the risers remain substantially equalised. The length of the cables 12,13 may for instance be between 50 and 2000 metres. The weight of each weight 16,17 may for instance be about 100 tonnes.
Preferably the transverse support arm 15 is part of a support deck, for which at least three mounting arms, including the arms 21,22 and a further mounting arm, which is not shown in the drawing, are provided. The length of each mounting arm 21, 22 is high enough to space the sheaves 19, 20 and the deck 15 far enough from deck level 24 to avoid contact upon relative movements of the support deck 15 and the hull 23. This relative movement would mainly be a combination of: hull response to waves, transverse support arm set down due to horizontal drifting of vessel and/or draft changes of hull 23 due to different loading conditions. Preferably drilling or production equipment 26 is mounted on the support deck 15. Flow and communication lines needed to pass from the hull 23 to the support deck 15 will be formed by piping or cabling capable of handling the relative movements between the hull 23 and the support deck 15. Manned access between hull 23 and support deck 15 will be provided with the flexibility to cope with the relative motions between the deck 15 and hull 23.
In figure 2, an embodiment of a vessel 1 is shown which is attached to the seabed via tethers or tendons 2'. In figure 2, the elements corresponding to those in figure 1 have been given identical reference numerals. The risers 3, 4 and tendons 2' extend through a central well or moon pool 28 in the vessel to be pivotably connected to the supporting deck structure 15. On the deck structure 15 the production trees 37 at the end of the risers are supported. Supported on the deck structure 15 are piping and manifolds 15', the drilling area 15'' being located centrally over the trees 37. The parts 23' of the hull 23 located on both sides of the moon pool 28 can be used for oil or gas storage. The decks areas 24' and 24'' located below the sheaves 19, 20 can be used for accommodation and processing equipment respectively.
Weights 16,17 when hanging without guides from cables 13,14 can swing due to dynamic excitation. A reduction in this swinging can be achieved by interconnections 55,56,57 of the weights and cables with one another. Guiding of the weight can also effectively control this dynamic swinging action.
In figure 2, an embodiment of a vessel 1 is shown which is attached to the seabed via tethers or tendons 2'. The system shown in figure 2 can also have a lateral mooring system 2 of the kind that is shown in figure 1 for controlling horizontal motions. The tethers or tendons 2' are primarily used to fix the deck structure 15 at its horizontal position above the hull 23. In this way the deck structure can be initially supported without any riser being attached.
Figure 3 shows an other embodiment according to the present invention wherein the mounting arms 21,22 carrying the sheaves 19,20 are placed near a central well 28 extending through the hull of the vessel 1. The mounting arms 21,22 may be mounted on a bearing structure 30 of a turret 33 that will allow the vessel to weathervane or rotate with respect to the mounting arms. The cables 12,13 extend through the well 28 to below keel level of the vessel.
The cables 12,13 moving over sheaves 19,20 may after a certain period require replacement. To not disrupt the workings of the riser tensioning system multiple cables 12,13 and/or weights 16,17 giving redundant stability to deck 15 would be used in a way that temporary removal of one weight for cable maintenance/replacement does not greatly affect the stability or tension of the riser system. Multiple cables can also be connected to the same weight so cable replacement/failure does not affect the tensioning of deck 15. This also assures the unexpected failure of one or more cables 12,13 does not cause a failure of the riser system.
Figure 4 shows an embodiment wherein the riser tensioning construction 32 is mounted on a turret 33 of a vessel 34. Near keel level 35 the turret 33 carries a chain table 36 and is attached to the seabed 29 via anchor lines 38,39. A sheave 40 is suspended from a mounting arm 41 and carries a cable 42 on one end connected to a weight 43 and on the other end being connected to the upper end of a riser 44. The upper end of the riser 44 is connected to a swivel 45 on the vessel via a flexible riser 46 allowing for heave of the vessel 34 with respect to the riser 44.
Figure 5 shows an embodiment wherein the cables 12,13 extend to close to the seabed 29. Two weight guiding elements 47,48 such as for instance piles, are placed in the seabed and extend through holes in the weights 16,17 such that these can vertically slide along the piles 47,48. Hereby lateral movement of the weights 16,17 is prevented such that they can not contact the risers 3,4. Figure 6 shows an embodiment wherein the weight guiding elements are formed by shafts or cages 49,50 connected to the vessel 1 near keel level 29. The weights 16 and 17 can slide up and down in the shafts or cages 49,50.
Figure 7 shows an embodiment wherein the weights 16 and 17 are provided with a throughbore and are placed around the risers 3,4 to prevent lateral movement of the weights.
Finally figure 8 shows an embodiment wherein the riser tensioning construction comprises two flexible lines, such as polyester lines 52,53 which are attached to the seabed 29 for instance via weights 54,55. Upon rolling of the vessel 1, the polyester cables 52,53 will act as springs keeping the support deck 15 in a horizontal position and maintaining a substantially equal tension force in both risers 3,4.
Although it has been shown in the previous figures that the weights at the end of cables 12,13 are clump weights, it is also envisaged that these weight may be formed by other means, such as for instance chain parts which may be 500 metres long, or other types of weights. Furthermore, the cables 12, 13 may be formed by steel cables, wire rope cables, polyester lines, chains or combinations thereof.

Claims

Vessel (1) comprising a riser tensioning construction having a support frame (19, 20, 21, 22) attached to the vessel, a tensioning member (12, 13, 15) movably carried by the frame, the tensioning member being with a first end attached to an upper end (7, 8) of a riser (3, 4) extending from a subsea structure to the tensioning member and being with a second end attached to a weight (16, 17, 53, 54) for exerting a tensioning force on the riser, characterised in that, the second end of the tensioning member extends outside the hull (23) of the vessel or through a well (28) in the vessel such as to be located below water level.
Vessel according to claim 1, characterised in that, the tensioning member comprises a cable (13, 14), the support frame comprising a cable guide means (19, 20) along which the cable can be moved relative to the frame.
Vessel according to claim 2, wherein the support frame comprises at least two spaced apart mounting arms (21, 22) each carrying a cable guide means (19, 20) and a respective cable (12, 13), a connector (15) being supported by the first ends of the cables (12, 13), the riser (3, 4) being attached to the connector (15).
Vessel (1) comprising a riser tensioning construction having a support frame (19, 20, 21, 22) attached to the vessel, a tensioning member (12, 13, 15) movably carried by the frame, the tensioning member being with a first end attached to an upper end (7, 8) of a riser (3, 4) extending from a subsea structure to the tensioning member and being with a second end attached to a weight (16, 17, 53, 54) for exerting a tensioning force on the riser, characterised in that, the support frame comprises a mounting arm (21, 22) carrying a cable guide means (19, 20) and a cable (12, 13), a connector (15) being supported by the first end of the cable (12, 13), the riser (3, 4) being attached to the connector (15).
Vessel according to claim 4, characterised in that, the support frame comprises at least two spaced apart mounting arms (21,22), each carrying a cable guide means (19,20) and a respective cable (12,13).
Vessel according to claims 3, 4 or 5 characterised in that, the mounting arms (21, 22) extend above deck level (24) of the vessel (1).
Vessel according to claims 3, 4, 5, or 6, characterised in that, the vessel is has a central well (28) wherein the cables (12, 13) and the weight (16, 17) are placed.
Vessel according to claims 3, 4, 5, 6, or 7, characterised in that, the connector (15) comprises a transverse arm or a deck structure, carrying at least two risers.
Vessel according to claim 8 comprising flexible piping and/or cabling for flow and communication between the deck structure (15) and the hull (23) of the vessel.
Vessel according to claims 8 or 9 comprising flexible access ways between the deck structure (15) and the hull (23) of the vessel.
Vessel according to claims 8, 9 or 10, characterised in that, the vessel comprises a storage tank (23') and a connection between the riser (3, 4) on the transverse arm or deck structure (15) and the storage tank (23') for passing flow from the deck structure (5) to said storage tank (23').
Vessel according to claim 2, characterised in that, the support frame (41) is located near keel level (35) of the vessel (34), the cable guide means (40) being suspended from the frame.
Vessel according to claim 12, characterised in that, the riser comprises a flexible end part (46) extending to the vessel.
Vessel according to any of the previous claims, characterised in that, the vessel comprises a turret (33) around which the vessel can weathervane, the support frame (41) being connected to the turret.
Vessel according to any of the previous claims, characterised in that, the riser comprises a rigid metal riser.
Vessel according to any of the previous claims, characterised in that, the weight comprises adjustable buoyancy or weight means for varying the buoyancy or the mass of the weight.
Vessel according to any of the previous claims, characterised in that, the riser tensioning construction comprises a weight guiding element (47, 48, 49, 50) for restraining the sideways movement of the weight.
Vessel according to claim 17, characterised in that, the weight guiding element (49, 50) is attached to the vessel.
Vessel according to claim 17, characterised in that, the weight guiding element (47, 48) is attached to the seabed.
Vessel according to claim 17, characterised in that, the weight guiding element is formed by at least one of the risers.
Vessel according to any of the previous claims, characterised in that, the tensioning member comprises an elastic cable (52, 53) attached to the sea bed (29).
Vessel according to any of the previous claims, characterised in that, the vessel comprises multiple cable guide means (19, 20) and multiple cables (13, 14) such that the riser tensioning construction remains functional upon failure or replacement of at least one cable guide means (19, 20) and/or cable.
Vessel according to any of the previous claims, characterised in that, the cables and/or the weights are below water level mutually connected for reducing swinging motions of the weights.