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This application is a division of Application No. 09/936,750, filed on Sept. 17, 2001 now U.S. Pat. No. 6,499,418. Application No. 09/936,750 is the national phase of PCT International Application No. PCT/EP00//02363 filed on Mar. 16, 2000 under 35 U.S.C. §371. The entire contents of each of the above-identified applications are hereby incorporated by reference. 
    
    
     BACKGROUD OF INVENTION 
     The invention relates to a method of connecting two or more elongate connection members, such as risers or tendons, between the seabed and a floating vessel. The invention also relates to a vessel for carrying out this method and to a vessel carrying a connector suspended from the vessel by at least two spaced apart suspension members for relative displacement of the connector with respect to the vessel, one or more elongate connection members, such as risers or tendons, being attached with one end to the connector and with their other end to the seabed. 
     From U.S. Pat. No. 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 end of piston rods of hydraulic cylinders, the second end of the cables is 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 it carries only a single riser and that 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, these 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. Pat. No. 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 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 dependant counterweights, the tendons are kept taut. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a method and a vessel for attaching multiple risers or tendons to the seabed and to the vessel. It is especially an object of the present invention to provide a method of drilling a subsea hydrocarbon well and attaching multiple risers with one end to the wellhead and with their other end to the vessel. Thereto the method is characterised in that it comprises the steps of: 
     a. attaching one or more connection-members to the connector, 
     b. increasing the tensioning force on the tensioning means, or vice versa, and 
     c. repeating steps a and b until the connection members are installed between the vessel and the seabed. 
     The present invention is based on the insight that a constant tensioning force can be easily maintained on the tendons or risers when they are being connected to the wellhead one after the other, by a stepwise increase of the tensioning force of the tensioning means. Each time after a drilling operation, one or more additional risers are attached to the connector. In case of hydraulic tensioning means, it is envisaged that the oil pressure in the hydraulic system or the air pressure in case of a pneumatic system, is stepwise increased when the number of risers and tendons attached to the connector increases. In case of the connector being suspended from a cable or rod, the tensioning force can be increased in a stepwise manner by adding extra tensioning weights or buoyancy members to the second end of the cable or rod. The increase in the tensioning force according to step b above can be carried out either prior to or after connection of additional connection members to the connector. 
     The method of the present invention of stepwise increase of the tensioning force is especially suitable for drilling methods in deep water. In this case the risers may have a length of 1000 meters or more and may have a weight of between 40 and 60 tons in submerged conditions. Each time one or more risers are installed, the force exerted by the connector of the present invention is increased a little over 40-60 tons, or a multiplicity thereof corresponding to the number of risers or tendons that are added, such that a substantially constant tension is maintained in the risers or tendons at all times. A further advantage is that, depending on the prevailing current, the tensioning force can be varied to compensate for the current factor and prevent the risers from buckling. 
     In one embodiment, the connector may be formed by a deck structure suspended from two or more oppositely located pivoting arms, which on their free ends are provided with connection means for attaching a multiplicity of separate weight elements to the free end of the arms. 
     The connector may also be suspended from cables running along sheaves the free end of the cables carrying a counterweight. Additional counterweight elements may be added around the cable to be suspended from the free end to increase the tensioning force. As is used herein, the term “cable” comprises ropes, wires, chains, lines, combinations thereof and any equivalent means. 
     In one preferred embodiment, the cables have a first section extending vertically downward front the vessel to below water level and a second section which extends in a loop back upwards from the first section to a pulling device on the vessel, tensioning weights being comprised on the first and second cable sections. The tensioning force on the connector can be varied by varying the length of the first and second cable sections via the pulling device. When the loop is made large, the tensioning weight will be placed on the first cable section and will act completely on the connector. When the second cable section is shortened, the tensioning weight will be placed along the second cable section such that less weight is dependant from the connector via the first cable section. The tensioning weight may be comprised of clump-weights added to the cable or may be formed by the weight of the cable section itself, which may for instance be formed by a chain. 
     In another embodiment, the cables are guided along a cable guide means, such as a sheave, and are with one end attached to the seabed. A take-up device is connected to the cables for varying the cable length. The cable may be comprised of an anchor line such as a polyester line, the tension of which can be varied by the take-up device, which can for instance be formed by a winch. In another embodiment the cables may comprise a chain part carrying clump weights. By varying the length of the cable, the clump weights may be lifted from the seabed one after the other, such that the tensioning force on the connector is increased. The take-up device may be comprised of a winch and chain stopper assembly of the type known in the state of the art. 
     The varying tensioning force on the connector can in another embodiment be exerted by a buoyancy tank which is attached to the free end of cables from which the connector is suspended. The cables may be guided via a sheave to a cable guide means located below keel level, such that the upward buoyancy force acts on the cable. The buoyancy tanks may for instance exert a maximum upwards force of 600 tons each, three buoyancy tanks being attached to the connector. Pneumatic lines may be attached to the buoyancy tanks for ballasting or deballasting the tanks. In another embodiment, the position of the cable-guide means with respect to keel level of the vessel can be varied such that the tensioning force is increased or decreased. 
     In a further alternative embodiment, the tensioning force on the suspension members can be varied by movement of the pivot arms from which the risers or tendons are suspended, with respect to pivot points and/or by movement of a counterweight along the pivot arms. The counterweight may be moved for instance by means of a rack and pinion construction. 
     A vessel according to the present invention, which comprises a connector suspended from at least two spaced apart suspension members, is characterised in that a supporting deck is situated above the connector, a first section of a connection member extending from the seabed to the connector and being detachably connected to a second section of the connection member via coupling device, the second section extending to the supporting deck. Preferably the connector carries a blow-out preventor, the supporting deck carrying a drilling rig. The drill string that is attached to the drilling rig, may be disconnected from rig during high seas, when drilling is suspended. The casing is suspended from the blow-out preventor. When the relative motions between the connector and the deck are again within certain limits, the drill string can be reconnected to the drilling rig and drilling may be resumed. In this way it is not necessary to dismantle the total drill string and drilling riser, because the drill string and drilling riser can after disconnection move independently from the drilling rig. In this way drilling down time in stormy conditions is reduced compared to constructions in which the blow-out preventor is situated on the seabed and disconnection of the drilling riser is effected near the seabed, at the position of the blow-out preventor. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Different embodiments of the method and vessel according to the present invention will be explained in detail with reference to the accompanying, non-limiting drawings. In the drawings: 
     FIG. 1 shows a vessel comprising a riser tensioning deck attached to pivoting arms, to which weight elements can be added, 
     FIGS. 2 and 3 show a side view and a plan view, respectively, of a drilling barge comprising four riser tensioning decks, each deck being suspended from three mounting-arms and from three sets of three cables each, 
     FIG. 4 shows an embodiment wherein a varying tensioning force is exerted by a loop configuration of a cable carrying tensioning weights, 
     FIGS. 5 and 6 show an embodiment wherein the tensioning weight is formed by a chain section which is placed in a loop configuration, in a side view and a plan view, respectively, 
     FIG. 7 shows an embodiment wherein the tensioning weight is formed by clump-weights which are lifted from the seabed, 
     FIG. 8 is a detail of FIG. 7 showing the take up device, 
     FIG. 9 shows buoyancy means for exerting a tensioning force, 
     FIG. 10 shows an embodiment wherein the varying tensioning force is exerted by displacement of a counterweight along pivot arms, 
     FIG. 11 shows an embodiment wherein a supporting deck is mounted above the connector, carrying a drilling rig, with a disconnectable drill string, 
     FIG. 12 shows an embodiment of a vessel of the same type as shown in FIG. 11, which is moored to the seabed via a turret having full weathervaning capacities, and 
     FIG. 13 shows an embodiment comprising hydraulic tensioning means. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows a drilling barge  1  with a supporting deck  2 , carrying a drilling rig  3 . From the drilling rig  3 , a drilling riser, schematically indicated with the dash and dot line  4 , is introduced into the seabed for drilling a hydrocarbon well. The risers  5 ,  6  which are connected to the wellheads, are suspended from a connector, or riser tensioning deck  7 . The riser tensioning deck  7  is supported from cables or rods  8 ,  9  connected to the free ends of pivot arms  10 ,  11 , which are supported on pivot mountings  10 ′,  11 ′. At their second end the pivot arms  10 ,  11  are provided with weight elements  12 ,  12 ′,  13 ,  13 ′. The weight elements can be individually connected to or detached from the arms  10 ,  11 , each time a riser is connected to or is disconnected from the deck  7 . Tensioning deck  7  maintains a substantially constant tension in the risers  5 ,  6  during wave-induced motions of the barge  1 , by pivoting movements of the arms  10 ,  11 . When the risers  5 ,  6  are successively connected to the tensioning deck  7 , the total number of risers connected to the deck varies between 2 and 50 and the weight of each riser, which can have a length of about 1000-3000 meters, varies between  40  and 180 tons, the number of weight elements  12   12 ′,  13 ,  13 ′at the end of the pivoting arms  10 ,  11  is increased. 
     A blow-out-preventer  16  is supported from the supporting deck  2  for closing of the drilling riser upon a certain pressure increase. Positioning the blow-out-preventer in an accessible location on the supporting deck  2 , above water level, facilitates repair and change out of parts. 
     FIG. 2 shows a side view of an embodiment wherein the riser tensioning deck  20  is supported from a suspension member comprising sheaves  21 ,  22  along which cables  23 ,  24  are guided. The cables  23 ,  24  are with one end connected to the riser tensioning deck  20  and with the other end to tensioning weights  25 ,  26 . The weights  25 , 26  can slide up and down in guide shafts  27 ,  28 . The weights  25 ,  26  may be provided with rolling guide elements such as wheels, which contact the walls of the guide shafts  27 ,  28 . As can be seen in FIG. 3, the barge  29  comprises four riser tensioning decks  20 ,  20 ′,  20 ″,  20 ′″. Each deck is suspended from three sheaves  21 ,  22 ,  30 . Each sheave carries three cables for increased safety. 
     FIG. 4 shows a barge  30  carrying a drilling rig  31  and a riser tensioning deck  32 . In figure 4 two risers  33 ,  34  are connected to the riser tensioning deck  32 . The riser tensioning deck  32  is suspended from cables  35 ,  36 . Each cable has a first cable section  37 ,  38  extending through a well in the barge to below keel level. Each cable  35 ,  36  comprises a loop  39 ,  40  and a second cable section  41 ,  42  extending upwards from the loop  39 ,  40  through a well in the barge to a pulling device, such as a winch  43 ,  44 . At the position of the winches  43 ,  44  a chain stopper may be provided. When the length of cable sections  37 ,  38 ,  41 ,  42  is increased, the weight elements  45 ,  46  and  47 ,  48  can be lowered and can all be placed on the first cable section  37 ,  38 . In this way, the tensioning force on the riser tensioning deck  32  is increased when more risers are added to the tensioning deck  32  in addition to risers  33 ,  34 . By shortening the first and second cable sections  37 ,  38  and  41 ,  42 , the weight on the tensioning deck  32  can be decreased as the weights will then be distributed along the second cable sections  41 ,  42 . The weight elements  45 ,  46 ,  47 ,  48  may be combined with flushable buoyancy elements for varying the weight thereof. 
     FIG. 5 shows an embodiment of a barge  50  wherein the cables  51 ,  52  are each comprised of relatively long chains sections with a length of between 100 and 1000 meters. The chains  51 ,  52  are guided via sheaves  53 ,  54  projecting beyond the perimeter of the barge  50 , and are via a pulling device (not shown in the figure) collected in chain lockers  55 ,  56 . 
     As can be seen from FIG. 6, the riser tensioning, deck  57  is placed over a central well  58  in the vessel and is supported from eight sheave combinations  59 . 
     FIG. 7 shows a barge  62  in which the riser tensioning deck  63  is supported from two lines  64 ,  65 . The first ends  66  or  67  of each line are attached to the riser tensioning deck  63 , a second end  68 ,  69  being placed on the seabed  70 . Each line  64 ,  65  is attached to a take-up device  71 ,  72 , such as a winch and chain stopper combination. The tensioning force on the riser tensioning deck  63  can be increased by shortening the lines  64 ,  65  via the take-up device  71 ,  72  to lift clumpweights  73 ,  74 , attached to the second ends  68 ,  69  of the lines  64 ,  65  from the seabed  70 . By paying out the lines  64 ,  65 , the clumpweights  73 ,  74  will come to rest on the seabed, such that the tension in the lines  64 ,  65  is decreased. The upper part  75  of the lines  64 ,  65  may be formed by a steel cable part. The middle section  76 , even as the lower ends  68 ,  69  may be comprised of a chain. An intermediate section  77  may be formed by a cable or polyester mooring line part. 
     FIG. 8 shows an enlarged detail of a part of the barge  62  of FIG. 7 showing the upper section  75  of line  65  which is guided along a sheave  78 ,  78 ′ and which is attached to the riser tensioning deck  63 . At one end of upper section  75 , a chain stopper  71  is connected. By pulling the middle chain section  76  through a well  80  in the barge  62  and through chain stopper  71  and storing it in a chain locker  79 , the clump weights  73  are lifted from the seabed  70 . 
     FIG. 9 shows barge  82  wherein the riser tensioning deck  83  is suspended from cables  84 ,  85 . The cables  84 ,  85  extend along cable guide means  86 ,  87  at the end of pivot beams  88 ,  89 . Buoyancy tanks  90 ,  91  are attached to the end of cables  84 ,  85 . Before installation, the pivot beams  88 ,  89  may be placed into the position which is indicated with the dash and dot lines. When the weight of the risers attached to the deck  83  increases, the pivot beams  88 ,  89  may be lowered to the position shown in FIG. 9 with the solid lines, for increasing the tension in the cables  84 ,  85 . In an alternative embodiment, the arms  88 ,  89  are fixed and the buoyancy tanks  90 ,  91  are ballastable and flushable. 
     FIG. 10 shows an embodiment of a barge  92  wherein the riser tensioning deck  93  is suspended from cables  99 ,  100  which are attached to the ends of pivot arms  94 ,  95 . The pivot arms  94 ,  95  are hingeingly connected to the barge  92  via hinges  94 ′,  95 ′. Counterweights  96 ,  97  are movable along the arms  94 ,  95  for instance by a rack  98 ′and pinion  98  construction for varying the tensioning force on the cables  99 ,  100 . 
     FIG. 11 shows an embodiment of a barge  106  wherein the blow-out preventer  101  is supported on the riser tensioning deck  102 . A drilling rig  107  is placed on a supporting deck  108 . The riser tensioning deck  102  is suspended from cables which are tensioned by weights  109 ,  109 ′, which may be ballastable for varying the tensioning force. Via a coupling member, which may be a usual coupling of a drill string segment  103 , the drill string  104  may be detached from the drilling rig  107 . During stormy conditions, the drill string  104  is detached from the rig  107 , while the lower part of the drill string and drilling riser  105  are hung off the inside of the blow out preventer  101 . When the sea conditions return to within specific limits, the drill string  104  is reconnected to the drilling rig  107 , without the need to dismantle the total drill string and drilling riser ( 105 ) and with a minimum down time. 
     FIG. 12 shows a barge  110  which comprises a turret  111  which is anchored to the seabed via a chain table  112 . The riser tensioning deck  113  is suspended above or within the turret  111 . The supporting deck  114  and drilling rig  115  are connected to the vessel which can weather vane fully around the turret  111 . This construction provides full weathervaning and drilling capacities, such that the barge  110  can adjust its position according to prevailing wind and current conditions. 
     FIG. 13 shows an embodiment wherein two risers  120 ,  121  are suspended from hydraulic cylinders  122 ,  123 , that are connected via a flow line  124 . The upward force exerted by the cylinders can be increased by increasing the pressure or by increasing the number of interconnected cylinders. Any of the tensioning methods described above can be used either seperately or in combination with one or more other tensioning methods that are illustrated. Furthermore, active tensioning systems using winches or hydraulic pressure may also be used, either as an alternative to, or in combination with the passive tensioning methods described above.

Summary:
A method of connecting two or more elongate connection members ( 105 ) between the seabed and a floating vessel ( 106 ) carrying a connector ( 102 ) which is suspended from the vessel by at least two spaced apart suspension members for relative displacement of the connector with respect to the vessel, at least one suspension member being connected to tensioning elements ( 109, 109 ′) for exerting an upward force on the connector. The method comprises the steps of: a) attaching one or more connection members to the connector, b) increasing the tensioning force of the tensioning elements, or vice versa, and c) repeating steps a and b until the connection members are installed between the vessel and the seabed.