Riser tensioning construction

Abstract: The invention relates to a vessel (1) comprising a riser and/or tendon (3,4) tensioning construction. A connector, such as an arm or deck structure, is suspended from cables (12,13) or pivot arms on the vessel. The connector carries two or more risers and/or tendons (3,4) extending from a subsea structure to above water level. The free end of the cables (12,13) or pivot arms is attached to a weight (16,17) for exerting a tensioning force on the riser which is substantially decoupled from pitch, roll and heave motions of the vessel. One embodiment of the vessel according to the invention is characterized in that the free end of the tensioning member extends outside the hull 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. In one embodiment the connector is carried by cables and sheaves (19,20) that are supported on two spaced apart mounting arms (21,22) above deck level (24). 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. To further reduce movements of the weights, the cables or pivot arms can be attached to the connector at the side of the centre line of the vessel, opposite to the side of the tensioning weight.

Description: The invention relates to a vessel comprising a hull with a first side and a second side, the first and second sides being situated at opposite locations of a centre line, the vessel comprising a riser or tendon tensioning construction having at the first side at least one first guide element and a second guide element, a cable or chain comprising a first, substantially vertical section connected to a tensioning member, a second substantially horizontal section extending transversely to the vessel and a third section attached to a connector, the connector being attached to at least one riser or tendon, extending from a sub sea level to the connector. Such a riser tensioning construction is known from US patent nr. 4,423,983. In this construction, a petroleum production platform is shown in which a plurality of marine risers is supported from the main deck structure extending upward through a central opening in the deck structure. The marine riser system is supported by a tensioning construction allowing for vertical movement of the platform while maintaining tension on the riser system. The tensioning members comprise clump weights supported over the deck structure via a cable and sheave. The known freely swinging counterweights take up a relatively large space on the deck structure, and may form an obstruction for personnel on deck of the vessel. Furthermore, sudden movements of the vessel may result in large forces being exerted by the weights on the sheaves. From U.S. patent nr. 4,567,842 a mooring system for a floating production vessel is known comprising 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 counterweight 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 is 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. From U.S. patent nr. 3,681,928 a barge supporting a drilling rig is known, in which a platform is movably suspended from two mounting arms above deck level of the barge. The platform is connected to the seabed via two parallel cables, or tendons, which pass through openings in the platform and through a central well in the barge. By this construction the platform remains in a horizontal position and at a constant height above the seabed when the vessel moves vertically due to wave motion. Under the influence of the dependent counterweights, the cables are kept taut. This construction has as a disadvantage that upon movement of the barge relatively large inertia forces may be exerted on the sheaves by the counter weights, and that large forces are exerted on the cables by the swinging counterweights. Furthermore, the freely swinging counterweights may form an obstruction for personnel on deck of the drilling barge, and take up a lot of space as they should be clear from any structural parts of the barge, especially in situations of high seas. It is therefore an object of the present invention to provide a riser and/or tendon tensioning construction which can be used in deep waters using a dry production tree, using a relatively simple vessel which tensioning construction 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. The risers may convey hydrocarbon well production fluids to productions trees on the riser supporting deck, or alternatively be used to convey flow between the sub sea structure and the deck. The upper and lower connections of the risers may be rigid with bending taking by the pipe, or may include pivoting means. Thereto the riser tensioning construction according to the present invention is characterised in that the first cable or chain section extends along the first side of the hull of the vessel, the tensioning member is situated below sea level near the first side of the hull of the vessel the second guide member is situated near the second side of the hull, the second cable or chain section extends from the first guide member across the vessel to the second guide member, and the third cable or chain section is situated near the second side, the at least one riser or tendon extending along the second side of the hull of the vessel. By placing the counterweight below water level, the forces exerted by the counterweight on the vessel upon movement thereof are reduced and the motions of the counterweight are 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 the buoyancy. The riser and/or tendon tensioning construction according to the present invention is particularly useful in deep waters as it allows rigid risers to pass from great depths to the surface alongside of the vessel using only proven components that can take up considerable forces and external pressures. Pipes and flowthrough pivot joints are available for these pressures. No complex deck structure and vessel having a central well or moon pool are required. The tensioning member may comprise a cable having a clump weight suspended thereon or may comprise a cable that is attached to the seabed by anchoring means such as a clump weight, a suction anchor or a pile, for exerting a tensioning force on the risers and/or tendons, in which case it is preferred that the cable is elastic, such as for instance a polyester cable. It is furthermore possible that the suspension member and the tensioning member are formed by a single cable which continues along the cable guide means to extend towards the seabed. It is noted that from WO 98/18673 a mooring system is known in which a cable extends from the seabed towards deck level of the vessel to be directed around a sheave back to a counterweight freely suspended from the cable below sea level, for the reduction of mooring loads attributable to oscillating wave drifts. The tensioning system described therein uses for each mooring line a separate counterweight and is not flexible in case several risers or anchor lines need to be added to the vessel. In a further embodiment according to the present invention the vessel comprises on each of the first and second sides of the hull at least one riser or tendon, each being connected to a respective tensioning member on the opposite side of the hull. Placing the counterweights on each of the opposite sides of the ship with respect to the point in which the suspension member is attached to the connector, an angular compensation for the roll and pitch motions is achieved, which results in little to substantially zero vertical movements of the hanging weights. The motions of the vessel are decoupled from the risers. A substantially constant tensional force is exerted on the risers and/or tendons 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 for instance be formed by a support arm extending between the mounting arms in the length or width direction of the vessel. The 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 support arm can be effectively used as, or be part of 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 therein. 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. One embodiment 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 top view of an embodiment according to claim 1, wherein two riser supporting decks and their tensioning weights are located on opposite sides of the longitudinal centre line of the vessel; Figures 2-16 show different embodiments of riser tensioning systems in which: Figure 2 shows a schematic frontal view of a first embodiment of a vessel comprising the riser tensioning system according to the present invention; Figure 3 shows another embodiment of a vessel in the form of a tension leg platform comprising a supporting deck located over a moon pool of the vessel; Figure 4 shows an embodiment wherein the tensioning members are formed by cables having weights distributed along their length; Figure 5 shows an embodiment wherein the tensioning member comprises an elastic cable anchored to the seabed; Figure 6 shows an embodiment wherein the tensioning member is connected to the seabed and is provided with additional tensioning weights; Figures 7a and 7b show a side view and a plan view respectively of the tensioning members being connected to the seabed, the tensioning members being interconnected and provided with additional tensioning weights; Figure 8 shows an embodiment wherein the tensioning member comprises an additional spring member for damping oscillations of the tensioning weights; Figures 9a and 9b show a side view and frontal view respectively of a vessel wherein the connector comprises a riser supporting deck, multiple risers being attached on each side of the vessel; Figure 10 shows an embodiment wherein the riser supporting deck is suspended from two pivoting arms; Figure 11 shows an embodiment wherein the riser supporting deck is suspended by a combination of sheaves and pivoting arms; Figure 12a and 12b show a top view and a side view respectively of an embodiment wherein the riser tensioning weight and the attachment point of the tensioning cable to the riser supporting deck are located at opposite sides of the centre line of the vessel; Figure 13 shows an embodiment of a vessel comprising a riser tensioning construction extending through the turret, and Figures 14, 15 and 16 show different embodiments of weight guiding systems for preventing lateral movements of the counterweights. Figure 1 shows an embodiment according to the present invention wherein a vessel 90 carries two riser supporting decks 92, 93 which are each connected to respective counterweights 94, 95 and 96, 97 which are located on opposite sides of the longitudinal centre line 91 for reduction of the vertical motion of the tensioning weights caused by angular motion of the vessel. Placing the counterweights 94, 95, 96, 97 further away from the longitudinal centre line 91, further reduces variation in tension in the cables attached to the riser supporting decks 92, 93. Figure 2 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 semisubmersibles, 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 meters, 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 at one end attached to a connector such as a transverse support arm or a supporting deck 15 and being at their other end connected to a respective clump weight 16, 17. The cables 12, 13 are guided over the 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 and heaving of the vessel, the weights 16, 17 are lifted or lowered. In this way the position of the riser supporting deck 15 and the tensional forces on the risers remain substantially constant, independent of the movements of the vessel. The length of the cables 12, 13 may for instance be between 50 and 2000 metres. The mass of each weight 16, 17 may for instance be about 100 tonnes. Preferably the transverse supporting arm 15 is part of a supporting 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. Each mounting arm 21, 22 is long 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 supporting deck 15 and the hull 23. This relative movement would mainly be a combination of the hull response to waves, supporting deck set down due to horizontal drifting of the vessel and/or draft changes of hull 23 due to different loading conditions. Preferably drilling or production equipment 26 is mounted on the supporting deck 15. Flow and communication lines that need 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 hull 23 and supporting deck 15. Manned acces between hull 23 and supporting deck 15 will be provided with the flexibility to cope with the relative motions between the deck 15 and hull 23. In figure 3 an embodiment of a vessel 1 is shown which is attached to the seabed 29 via tethers or tendons 2' that are attached to a template 31. In figure 3 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 deck 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 32, 33, 34 of the weights and cables with one another. Guiding of the weight can also effectively control this dynamic swinging action. The embodiment of the vessel 1 that is shown in figure 3 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 upon installation thereof. Figure 4 shows a barge 1 wherein the tensioning members comprise cables 13, 14 and distributed along their length weights 10, 11. The cables 13, 14 are resting on the seabed 29. Upon motions of the barge 1 the cables 13, 14 will be lowered or raised to maintain the riser supporting deck 15 in a substantially horizontal position. Upon larger excursions of the vessel, the cables 13, 14 will be partially lifted from the seabed 29 so that a progressively increasing tensioning force is generated thereby. In the embodiment according to figure 5, the riser supporting deck 15 is connected to the seabed 29 via elastic cables or lines 12, 13. The cables 12, 13 can be attached to the seabed 29 via weights 16, 17, suction anchors, anchor piles and any other known means. The polyester lines 12, 13 can be combined with steel cables and/or chains. In the present embodiment, the anchoring function of the barge 1 is integrated with the tensioning function of the riser supporting deck 15. In the embodiment shown in figure 6 additional weights 14, 14' are connected to the elastic cables 12, 13 for providing an additional tensioning force on the riser supporting deck 15. As shown in the embodiments of figures 7a and 7b, the riser supporting deck 15 is connected to the seabed via four cables 13, 14. The cables are, at a depth of for instance 20 metres below keel level of the vessel 1, interconnected via connecting cables 18, 18', which may extend at angles of between 30 and 40 degrees with the horizontal. Weights 25, 25', which each may have a mass of for instance 200 tonnes, are suspended from cables 30, 30' which may have a length of about 100 metres. In the embodiment shown in figure 8, the tensioning cables 12, 13 are provided with spring members 35, 36, for instance elastic cable sections, for damping the upward and downward motions of the tensioning weights 16, 17. Figures 9a and 9b show an embodiment wherein on each side of the vessel 40 a multiplicity of risers 44 is suspended from the riser supporting deck 41. The riser supporting deck 41 is on each side suspended from two sheaves 42, 43 via tensioning cables 45, 46 and tensioning weights 47, 48. Figure 10 shows an embodiment of a vessel wherein the riser supporting deck 55 is suspended via cables from two pivot arms 51, 52. The pivot arms 51, 52 are connected to the vessel 50 via pivoting connections 53, 54 above deck level. The pivot arms can be tilted along two parallel pivot axes extending in the direction perpendicular to the plane of the drawing. The facing end parts 62, 63 of the pivot arms 51, 52 are connected to the riser supporting deck 55 via cables, whereas the second end parts 64, 65 of the pivot arms 51, 52 are connected to the seabed via elastic cables 56, 57 and anchoring weights 58, 59. Instead of elastic cables 56, 57 it is also possible to connect counter weights to the end parts 64, 65 of the pivot arms 51, 52. Compared to constructions wherein the supporting deck 55 is suspended from sheaves, the pivoting arms show relatively little wear and therefore have an increased lifetime and reduced maintenance. In the embodiment shown in figure 11, the riser supporting deck 67 is supported by cables which are connected to pivot arms 70, 71 via sheaves supported on mounting arms 68, 69. The pivot arms 70, 71 are with their free ends 77, 78 connected to the riser supporting deck 67 via cables, running along the sheaves. The pivot arms 70, 71 are on one side 79, 79' side connected to pivot points 75, 76 on the vessel (74) and may be comprised of A-frame type constructions to provide a tensioning force on the risers 72 and tendons 73 that are connected to the riser supporting deck 67. Figure 12a shows a top view of a vessel 80 wherein the riser supporting deck 81 is suspended from first and second sheaves 82, 83 that are located on opposite sides of the longitudinal centre line 84. The tensioning weights 85, 85' and the attachments points 86, 86' of the cables 87, 87' are located on opposite sides of the centre line 84 such that upon rolling of the vessel around the centre line 84 the motion of the weights 85, 85' is compensated by the movement of the sheaves 82, 83. Upon rolling of the vessel around the longitudinal centre line 84 in the direction of the sheave 82, the weight 85 is lowered such that the tension in cable 87 decreases. The tension in the opposite cable 87' will increase as the counterweight 85' is lifted such that the side of the riser supporting deck 81 that is attached to cable 87' will be raised. The side of the deck attached to cable 87 will be lowered and weight 85 will be raised over substantially the distance corresponding with the height by which the weight 85 was lowered because of downward motion of the sheave 82. As the construction according to figures 12a and 12b substantially reduces the motion of the weights 85, 85', wear of the cables 87, 87' and sheaves 82, 83 is strongly reduced as is the dynamic load on the riser supporting deck 81. Figure 13 shows another embodiment according to the present invention wherein the mounting arms 121, 122 carrying the sheaves 119, 120 are placed near a central well 128 extending through the hull of the vessel 111. The mounting arms 121, 122 may be mounted on a bearing structure 130 of a turret 133 that will allow the vessel to weathervane or rotate with respect to the mounting arms. The cables 112, 113 extend through the well 128 to below keel level of the vessel. The cables 112, 113 moving over sheaves 119, 120 may after a certain period require replacement. To not disrupt the workings of the riser tensioning system multiple cables 112, 113 and/or weights 116, 117 giving redundant stability to deck 115 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 such that replacement/failure does not affect the tensioning of deck 115. This also assures the unexpected failure of one or more cables 112, 113 does not cause a failure of the riser system. Figure 14 shows an embodiment wherein the cables 112, 113 extend close to the seabed 129. Two weight guiding elements 147, 148 such as for instance piles, are placed in the seabed and extend through holes in the weights 116, 117 such that these can vertically slide along the piles 147, 148. Hereby lateral movement of the weights 116, 117 is prevented such that they cannot contact the risers 113, 114. Figure 15 shows an embodiment wherein the weight guiding elements are formed by shafts or cages 149, 150 connected to the vessel 111 near keel level 123. The weights 116 and 117 can slide up and down in the shafts or cages 149,150. Figure 16 shows an embodiment wherein the weights 116 and 117 are provided with a throughbore and are placed around the risers 113, 114 to prevent lateral movement of the weights. Although is has been shown in the previous figures that the weights at the end of cables 112, 113 are clump weights, it is also envisaged that these weights 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 112, 113 may be formed by steel cables, wire rope cables, polyester lines, chains or combinations thereof. The riser and/or tendon tensioning construction according to the present invention can be easily installed by transporting the riser supporting deck on the vessel to the installation site, installation of the mooring lines (which is optional), suspending the deck from the vessel at the desired elevation above sea level, installation of the risers and/or tendons between the deck and the seabed, and tensioning the tensioning lines, for instance by connecting tensioning weights to these lines. Although the present invention has been illustrated in the exemplary drawings by means of an offshore hydrocarbon transport or production system, it can also be used to provide a stabilised deck structure for semi-submersible constructions, floating gangways, floating docks, floating airstrips, floating bridges, artificial islands etc.

Claims: Vessel (90) comprising a hull with a first side and a second side, the first and second sides being situated at opposite locations of a centre line (91), the vessel (90) comprising a riser or tendon tensioning construction having at the first side at least one first guide element and a second guide element, a cable or chain comprising a first, substantially vertical section connected to a tensioning member (94,95,96,97), a second substantially horizontal section extending transversely to the vessel and a third section attached to a connector (92,93), the connector being attached to at least one riser or tendon, extending from a sub sea level to the connector, characterised in that , the first cable or chain section extends along the first side of the hull of the vessel, the tensioning member is situated below sea level near the first side of the hull of the vessel, the second guide member is situated near the second side of the hull, the second cable or chain section extends from the first guide member across the vessel to the second guide member, and the third cable or chain section is situated near the second side, the at least one riser or tendon extending along the second side of the hull of the vessel. Vessel according to claim 1, the vessel comprising on each of the first and second sides of the hull at least one riser or tendon, each being connected to a respective tensioning member on the opposite side of the hull. Vessel according to claim 1 or 2, the tensioning member comprising a weight element (94, 95, 96, 97). Vessel according to any of the preceding claims, characterized in that , the connector comprises an arm or a deck structure, carrying the at least one riser and / or tendon. Vessel according to claim 4 comprising flexible piping and / or cabling for flow and communication between the deck structure and a hull (23) of the vessel. Vessel according to any of the preceding claims, characterized in that , the vessel comprises a turret around which the vessel can weathervane, the guide elements being located on the turret. Vessel according to any of the preceding claims, characterized in that , the riser comprises a rigid metal riser. Vessel according to any of the preceding claims, characterized 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 preceding claims, characterized in that , the riser and / or tendon tensioning construction comprises a weight guiding element for restraining the sideways movement of the weight. Vessel according to claim 9, characterized in that , the weight guiding element is attached to the vessel. Vessel according to claim 9, characterized in that , the weight guiding element is attached to the seabed. Vessel according to claim 9, characterized in that , the weight guiding element is formed by at least one of the risers. Vessel according to any of the preceding claims, characterized in that , the tensioning member comprises an elastic cable attached to the seabed. Vessel according to any of the preceding claims, characterized in that , the vessel comprises multiple tensioning members such that the riser tensioning construction remains functional upon failure or replacement of at least one tensioning member. Vessel according to any of the preceding claims, characterized in that , the cables and / or the weights are below water level mutually connected for reducing swinging motions of the weights.

Inventors: POLLACK JACK, POLLACK, JACK
Applicants: SINGLE BUOY MOORINGS, SINGLE BUOY MOORINGS INC.
Application year: 1999
Main classification: E21B19
IPC codes: E21B7, E21B19, B63B35