Abstract:
Vessel comprising a hull having near a bottom a cavity for receiving a mooring buoy, a lifting device being placed on the hull comprising a cable that extends through the cavity to a weight that is situated below the bottom of the vessel, the mooring buoy being attached to the cable, the mooring buoy carrying mooring lines that are connected to a sea bed and being receivable in the cavity for coupling with the turret, the mooring buoy comprising a central shaft through which the cable passes, the buoy being movable relative to the cable in a length direction of the cable, wherein the weight is located on the cable at or below the buoy, a stopper being provided on the cable for engaging with the buoy and for blocking relative movement of the buoy and the cable, the stopper being fixed to the cable near an upper or a lower end of the buoy.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a disconnectable turret mooring system for a vessel. Such systems comprise a mooring buoy member and a turret structure mounted in a moonpool of the vessel, the mooring buoy member being anchored to the seabed with mooring lines and having a plurality of passages each adapted to receive a riser, the turret structure having a receptacle for receiving the buoy member and one or more locking devices for locking the buoy member in the receptacle, the turret structure accommodating a plurality of conduits to be connected to risers installed in passages of the buoy member, wherein the turret structure is rotatably supported in the moonpool of the vessel by means of at least one bearing assembly mounted above sea level. 
       BACKGROUND OF THE INVENTION 
       [0002]    A disconnectable mooring system of the above-mentioned type is described in patent publication U.S. Pat. No. 5,823,131. This patent discloses a disconnectable riser buoy for supporting only risers or riser lines, but with no mooring lines attached to it. This riser buoy can be docked within a rotatable turret placed in a moonpool of a floating vessel and carries risers that are connected to flow paths which are removably coupled to vessel product lines at a position above sea level. When the riser buoy is disconnected from the turret, it is maintained at a submerged depth in the sea by a weight attached to a buoy anchor leg which can be lowered down to the sea floor or raised within the turret. The turret is directly anchored to the sea floor via multiple mooring lines that are connected to the lower turret. When the riser buoy is released, the weight connected to the riser buoy, once resting on the sea floor, will moor the riser buoy and as such limit the excursions of the risers within acceptable limits. Further as the mooring legs are directly connected to the turret, the riser buoy has only sufficient buoyancy to support the risers. Another aspect of the known riser buoy is that in order to dock this riser buoy, a retrieval line is pulled upwardly via a winch until the weight contacts the buoy. Then, buoy and weight are hooked up together, the weight being in contact with the bottom of the riser buoy and both riser buoy and weight are placed within the moonpool of the vessel. The main purpose of this system is to allow for hook-up of a pre-installed riser buoy before installation of a vessel and the connection of the mooring lines to the turret. It does not function as a quick disconnectable system to be used in cyclone areas or ice infested waters as the mooring legs stays connected to the turret. Also hook-up of both the riser buoy and the weight together is only possible for relatively small buoys and weights and not for large buoys with large connected weights, as this would require a winch capacity that is larger than the capacity of presently available winches. Furthermore, in the known mooring system there is a risk of creating large snap-loads in the hauling-in line that connects the buoy and the winch. 
         [0003]    In these known mooring system, the capability to reconnect a buoy to a turret is mainly limited by the sea state and winch capacity. 
         [0004]    Another disconnectable mooring system is disclosed in US2007155259. This system includes a buoy member that is provided with a conical outer casing and a receptacle of the turret structure has a cone shape corresponding to the conical outer casing of the buoy member. The turret structure includes a turntable carrying conduits to be connected to the risers, wherein the turntable is supported on a bearing assembly in a manner allowing rotation with respect to the turret structure to align the conduits with the risers only after the buoy member is received and locked in the receptacle of the turret structure. In this patent publication it is shown that the turntable is supported by a main turret upper roller ball bearing assembly only. This bearing assembly includes three mutually movable parts that are directly interconnected to each other. In fact, this upper turret bearing assembly consists of 2 roller ball bearings that are directly placed on top of each other and interconnected via one common inner bearing housing member. This known upper bearing assembly has therefore become a critical and essential part of the turret-moored weathervaning system. A disadvantage of integrating the turret bearing and the manifold bearing into a single bearing structure is that if one or more roller balls fails, the complete bearing assembly has to be replaced, meaning that the turret system can no longer function as a weathervaning system. This replacement cannot be carried out under offshore conditions, so that the vessel needs to be transported to a on shore location for repair. 
         [0005]    Another disadvantage of the known system is that the very heavy turntable with several manifold and swivels decks is at all times supported by this very sensitive rolling bearing assembly. Consequently all the large static and dynamic forces both in a radial and in an axial direction, and moments resulting form the turntable deck structure and the environment, are directly transferred to this critical roller ball bearing system, which is known in the industry to be very sensitive to wear and fatigue. Another disadvantage of this integrated roller bearing system is that the inner diameter due to the fabrication limitations, is limited to a maximum dimension of only about 8 meters, so that it not suitable for large disconnectable turret-buoy systems which comprise for example 20 or more risers connected to the buoy. 
         [0006]    Another patent publication that describes a disconnectable mooring system that is provided with two separate bearing systems, one of which is used only for rotating a turntable in order to align the manifold pipe ends with the riser ends of a connected buoy, is U.S. Pat. No. 5,651,708. This patent shows a disconnectable buoy that is provided with a bearing system, which stays with the boy after it is disconnected. The buoy is rotatably connected to the moonpool of a vessel under the waterline without the use of a turret. An additional upper bearing system is disclosed at deck level, which supports a turntable with manifold, so that after the buoy is connected directly to the moonpool of the vessel, the turntable can be aligned with the risers of the connected buoy. The turntable is supported by the bearing system, so that even during production when hydrocarbons are received through the flexible piping connecting the manifold and the buoy, the turntable can be rotated at all times and be aligned with the buoy. When the twisting angle in the flexible piping between the buoy and the turntable is exceeded, the turntable is rotated by means of a connected motor driven pinion to a new position neutralizing the twisting. This system is therefore not suitable for disconnectable turret-buoy systems sized to receive larger numbers of risers, and cannot be used when using only hard piping to connect the risers and manifold. 
         [0007]    It is therefore an object of the present invention to provide a rapidly disconnectable and easy connectable mooring buoy system that is able to support a large numbers of risers, for example at least 20 risers and 10 umbilicals. It is a further object of the present invention to provide a turret mooring system that is reliable in operation and can be easily and safely be disconnected and reconnected without the risk of large snap-load occurring. It is again an object of the present invention to provide a disconnectable turret-mooring buoy design with an improved reconnection capability of the system even in severe sea states of for example up to 6 m significant wave height. Therefore, the system according to the present invention has the advantage to ensure a high availably of the system in all weather conditions and minimize the down time before reconnection even considering the constant severity of the environment. 
       SUMMARY OF THE INVENTION 
       [0008]    Hereto, a vessel according to the invention comprises a hull having near a bottom a cavity for receiving a mooring buoy a lifting device being placed on the hull comprising a cable that extends through the cavity to a weight that is situated below the bottom of the vessel, the mooring buoy being attached to the cable, the mooring buoy carrying mooring lines that are connected to a sea bed and being receivable in the cavity for coupling with the turret, the mooring buoy comprising a central shaft through which the cable passes, the buoy being movable relative to the cable in a length direction of the cable, wherein the weight is located on the cable at or below the buoy, a stopper being provided on the cable for engaging with the buoy and for blocking relative movement of the buoy and the cable, the stopper being fixed to the cable near an upper or a lower end of the buoy. 
         [0009]    The weight that is acting on the mooring buoy during disconnection, does not act on the buoy during the connection and hook-up. According to the invention, the buoy is not directly pulled in by the hauling in cable. The buoy is allowed to rise upwards to the cavity due to its own buoyancy once the weight is lifted from the buoy via the hauling in cable connected to a winch that is placed on the vessel. Hence, the winch cable directly lifts the weight, while the buoy slides along the hauling in cable that only functions as a guiding element for the buoy when it rises. When the mooring buoy rises, the vertical motions of the buoy can be controlled and restrained by the stopper that is fixed to the hauling-in cable. Hereby, the loads on the cable are largely reduced and a large riser buoy can be connected to the vessel using relatively modest size winches. 
         [0010]    In one embodiment the buoy comprises a buoyancy compartment filled with a buoyancy material. The buoyancy compartment may be open to the environment to obtain a variable buoyancy, which increases with decreasing depth, or may contain for instance compressed air cylinders. 
         [0000]    The stopper may be attached to the buoy via a spring member in order to obtain a further damping of the loads on the hauling cable due to heave movements of the vessel. 
         [0011]    According to the present invention, there are may be two separate bearing assemblies, preferably near or at deck level of the vessel. The first bearing assembly is the turret bearing system that could for instance consist of a large diameter upper bogie bearing system (with or without radial guiding wheels) and optionally a lower radial low friction pad bearing system to connect the turret rotatably to the moonpool of the vessel. The use of bogie wheel bearings allows for large diameter turrets and consequently allows for a large diameter mooring buoy to be connected to this turret. 
         [0012]    The second bearing assembly according to the invention is situated between the turntable with manifold and the turret, allowing for independent rotation of the turntable with regard to the turret and the connected mooring buoy, so that fluid lines of both the manifold and the mooring buoy can be aligned after the buoy is connected to the turret. This procedure is very important as in severe conditions it is needed to connect the buoy as quickly as possible to the turret without first aligning the fluid line ends of the buoy and turret, as this is a very time consuming operation. This second bearing system or turntable bearing system is not directly connected to the turret bearing system but placed at a radial and preferably an axial distance from the turret bearing system, so that specific forces and moments are taken up by each specific bearing. The turntable bearing system is preferably a bogie wheel bearing system. 
         [0013]    Because the turntable bearing system supports the turntable only temporarily in a rotary manner during the alignment of the piping ends on the manifold and buoy and supports the turntable directly onto the turret in a non-rotating manner in all other situations, such as during production of hydrocarbons from the wells via the risers and connected piping, the turntable bearing is not subject to large forces, resulting in reduced wear and fatigue-related defects. 
         [0014]    Providing a separate turret and turntable bearing assembly in accordance with the invention has several advantages because the design of each bearing assembly can be optimised for its specific function. An additional advantage is that the maintenance and repair activities for these two bearing systems now becomes easier as each bearing system can be inspected, maintained and repaired independently, keeping the other one in place and in function. Also when bogie wheel bearings are used, the wheels can be replaced under offshore conditions independently from each other while the bearing systems as such remain functional, which results in reduced costs while a safe functioning of the overall system is better controlled. The turntable bearing system is only used at the moment when the manifold pipe ends need to be aligned with those of the buoy connected to the turret, and is in fact a temporary bearing system; once the alignment procedure is completed this bearing system does not need to be active anymore and does not transfer any loads and moments during the hydrocarbon production process. 
         [0015]    In order to place the manifold support structure into its rotational position, it may be lifted relative to the turret (and to the turret bearing) over a small distance in the axial direction via the displacement device, where after the bearing members of the manifold bearing are lowered into rotating contact with the turret. The displacement device then lowers the manifold support structure such that its weight is supported on the turret via the bearing members in a rotating manner. In the non-rotational position the manifold support structure may rest on the turret, while the bearing members are retracted to a non load-bearing position. 
         [0016]    Alternatively, the manifold support structure may be placed in its non-rotational position by the displacement device lifting the manifold support structure such that the bearing members are disengaged from the turret, the displacement device supporting the manifold support structure in a non-rotating manner on the turret. The manifold support structure is brought into its rotational position by lowering the support structure such that it is supported on the turret via the bearing members in a rotating manner. 
         [0017]    The displacement device for lifting the manifold support structure may comprise one or more hydraulic cylinders situated between the turret and the manifold support structure having a relatively small stroke, such as for instance a few mm. In case the bearing members of the manifold bearing are formed by bogie wheel bearings, the displacement device may be integrated with the bogie wheels of the bearing, wherein lowering the wheels from the support structure against the turret in an axial direction causes the manifold support structure to be lifted from the turret to its rotational position. 
         [0018]    In one embodiment, the turret bearing is placed at an axial distance and a at a radial distance of at least 0.5 m from the manifold bearing, wherein a radial distance from a turret center line is larger for the turret bearing than for the manifold bearing. By placing the manifold bearing closer to the center line of the turret than the turret bearing and at an axial distance above or below the turret bearing, the forces acting on the manifold bearing while aligning the manifold and connected conduits on the turret with the riser couplings on the buoy, can be effectively decoupled form the forces acting on the turret bearing. The diameter of the turret bearing may range from for instance 15 to 30 m or more, whereas the diameter of the manifold bearing may be at least 1 m smaller. Rotation of the manifold support structure may be effected by a drive member, such one or more electrical motors, hydraulic drive members or any other suitable actuator. 
         [0019]    In a preferred embodiment, at least the turret bearing comprises a bogie wheel bearing. The bogie wheel bearing of the turret allows for constructing a large diameter turret comprising a large number of risers. Maintenance on one or more wheels of the bogie wheel bearing can be carried out under offshore conditions while the turret remains operational. The manifold bearing may be comprised of an axial-radial precision bearing with forged or segmented raceways, having a diameter not larger than about 8 m, but preferably is a bogie wheel type bearing. 
         [0020]    In one embodiment the vessel according to the invention comprises a lifting device that is placed on the hull with a cable that extends through the cavity to a weight that is situated below a bottom of the vessel, a mooring buoy being attached to the cable, the mooring buoy carrying mooring lines that are connected to a sea bed and being receivable in the cavity for coupling with the vessel, the mooring buoy comprising a central shaft through which the cable passes, the buoy being movable relative to the cable in a length direction of the cable, which weight is located on the cable at or below the buoy, a stopper being provided on the cable for engaging with the buoy and for blocking relative movement of the buoy and the cable, the stopper being fixed to the cable near an upper or a lower end of the buoy. 
         [0021]    The weight added to the buoy will cause the buoy to sink to a specific predetermined depth below the water surface upon disconnection from the vessel, for instance upon approach of an iceberg in ice-infested waters. Lifting the buoy towards the vessel is carried out via hauling in the weight suspended from the cable while allowing the buoy to rise by its own buoyancy towards the cavity for connection. By lifting only the weight that is suspended from the buoy without exerting a direct puling force on the buoy, the buoy will rise to the surface due to its own buoyancy once the weight is lifted from the buoy via the hauling in the cable connected to a winch on the vessel. This causes the buoy to slide along the hauling in cable that only functions as a guide element for the buoy while it rises. When the mooring buoy rises, the vertical motions of the buoy can be controlled and restrained by a stopper/spring assembly that is fixed to the hauling-in cable. This system allows to reduce the loads on the cable under heavy sea states and allows large size riser buoys carrying a large number of risers an mooring lines to be lifted by a winch an cable of limited size. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0022]    Some embodiments of a vessel comprising a disconnectable turret mooring system according to the invention will be explained in detail with reference to the accompanying drawings. In the drawings: 
           [0023]      FIG. 1  shows a sectional view of a disconnectable turret mooring system according to the present invention, 
           [0024]      FIG. 2  shows a three-dimensional view of the system of  FIG. 1 , 
           [0025]      FIG. 3  shows an enlarged detail of the upper part of the turret mooring system including the turret bearing and the manifold bearing, 
           [0026]      FIG. 4  shows a submerged mooring buoy with a soft buoyancy chamber according to one embodiment of the present invention, 
           [0027]      FIG. 5  schematically shows a submerged riser buoy during disconnection according to one embodiment of the present invention, 
           [0028]      FIG. 6  schematically shows the lifting of a submerged riser buoy according to one embodiment of the present invention, 
           [0029]      FIGS. 7   a  and  7   b  show alternative embodiments of the disconnection and connection of a submerged riser buoy according to the present invention, and 
           [0030]      FIGS. 8   a  to  8   e  show different embodiments of a submerged riser buoy and associated weight according to the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0031]      FIG. 1  shows a sectional view of a disconnectable turret mooring system according to the present invention. The system comprises a cylindrical turret structure  1  located within a cylindrical moonpool  2  integrated into the hull  3  of a vessel  14  which for example could be a FPU or FPSO. A turret bearing system  4  connecting and aligning the turret to the moonpool of the vessel, comprises a large diameter top bogie bearing situated near deck level  40  and (optionally) a bottom low friction pad radial bearing  5 , situated near bottom  41  of the hull  3 . 
         [0032]    A large multi-deck superstructure  6  is located on top of the turret  1  and houses installation and production equipment, piping manifolds  7  and the fluid/gas swivel stack  8  for incoming production fluids, exported fluids and control/chemical umbilicals. 
         [0033]    The manifolds  7  and swivel stack  8  are supported on a manifold support structure or turntable  31 . The turntable  31  is rotatably supported on the turret  1  via a manifold bearing  32 , as can be seen in  FIG. 3 . A steel framework  9  is positioned above and around the superstructure. The framework  9 , which is connected to the vessel, supports the piping extending from the fluid swivel stack  8  to the FPU, provides access to the turret  1  from the vessel, drives the rotating part of the swivel and supports the wintering panels. The turret design allows for maintenance and repair in operation, which maximizes its availability over the full field design life. 
         [0034]    A conical mooring buoy  11  is received into a cavity  42  near the bottom  41  of the hull  3  and is locked into the cavity  42  in a non-rotating manner. The buoy  11  is anchored to the sea bed via anchor legs  10  and carries risers  12  extending from a sub sea hydrocarbon well, such as production risers or umbilical risers. 
         [0035]    The upper end of each anchor leg  10  is directly connected to a low friction articulated universal joint  10 ′ on the hull of the buoy  11 . When the mooring buoy  11  is connected to the vessel  14  or FPU, the buoy risers&#39; deck  50  at a top part  49  of the buoy, is elevated above the maximum vessel draft level  43 . This will ensure that in all conditions, all piping equipments are kept permanently dry to ease access and maintenance. 
         [0036]    The mooring buoy  11  has two different functions. Firstly, when the vessel  14  is connected to the buoy  11 , the buoy transfers the mooring loads of the anchor lines  10  which are connected to its outer shell. Secondly, when the vessel  14  is disconnected from the mooring buoy  11 , the mooring buoy descends down to a depth at a predetermined distance below sea level and supports the anchor legs  10  and risers  12  at this submerged position. The pre-determined depth can for example be 30-50 meters below water level so that the disconnected buoy stabilizes under the wave active zone. In ice and iceberg infested waters the buoy  11  can be stabilized at a distance of even more than 100 m below water level to avoid any contact with icebergs. 
         [0037]    The mooring buoy  11  comprises a stiffened cylindrical shell with watertight internal bulkheads which divide the buoy into compartments. The center of the buoy incorporates a thick walled inner cylinder  44  to house and guide a hauling in or connecting cable  17  that is attached to a winch  45 . The top part of the buoy  11  is fitted with the annular connecting ring  46  on which the structural connector ratchets  26  that are placed within the turret, can be locked. I-tubes  47  are fitted in the center of the buoy for receiving risers and sub-sea umbilicals  12  and are terminated at a bottom end  48  in a flange to support the riser/umbilical bell-mouths. Risers bend stiffeners and bell-mouths are protected from ice drifting under the vessel hull by a conical skirt  13  at the bottom of the mooring buoy  11 . Alternatively, there also can be protection means against ice like a skirt or fence placed at the bottom  3  of the vessel  14  to protect the moonpool  2  against ingress of ice when the vessel is disconnected from the buoy  11  or to protect the buoy  11  and risers  12  when the mooring buoy is connected to the turret. 
         [0038]    The buoyancy required for keeping the buoy  11  supporting risers  12  and anchor legs  10  at the specified depth level in the disconnected state is provided by central compartments and compartments fitted on the buoy periphery, as can be seen from  FIG. 2 . The structural arrangement is such that it minimizes the contact between the buoy periphery and the turret parts during disconnection, so that there is no risk of accidental flooding. Nevertheless, the watertight buoy is compartmented in order to ensure sufficient buoyancy in case of accidental flooding of one compartment. 
         [0039]    The connection and disconnection procedure of the mooring buoy  11  to the cavity  42  of the turret  1  shown in  FIG. 1 , is carried out as follows: 
         [0040]    For reconnection, the vessel  14  will slowly approach the submerged mooring buoy  11  until a floating pick-up line that remains attached to the buoy can be grappled. Two sections of the hauling in line  17 , of which the upper section is wound around the winch  45 , are then shackled together and the pick-up line is removed. In case of reconnection in ice-covered waters, the connection of the hauling in line will be carried out directly in the dry part of the turret moonpool  2 . In this situation, the vessel  14  is in effect moored to the submerged buoy. The traction winch  45  will be operated and the mooring buoy  11  is slowly lifted below the vessel  14  and into the cavity  42  of the moonpool  2  until the buoy top flange with connecting ring  46  will be in contact with a structural connector centralizer. The clamps  25  of the structural connector will be closed and mechanical locks are activated. The vessel  14  is now securely reconnected and moored via the turret  1  to the anchor legs  10  of the mooring buoy  11 . 
         [0041]    Next, the manifold support structure or turntable  31  will be unlocked and lifted over a small distance in the axial direction (e.g. over a distance of a few mm) via a hydraulic jack  33 . The bearing members  32  are lowered such that they support the turntable  31  in a rotatable manner. Then a turntable orientation motor, schematically indicated at reference numeral  52 , is started. By slowly rotating the turntable  31 , the correct orientation of the manifold  7  will be achieved when manifold pipe ends  53 , 54  are brought in line with the mooring buoy riser ends  55 , 56 . This operation is monitored from the control panel of the motor  52  and will in fact be controlled from the manifold lower deck. Once the correct turntable orientation has been achieved, the turntable  31  will be automatically locked and the temporary turntable bearing system  32  is deactivated by displacing the bearings  32  hydraulically upward in a vertical direction (e.g. over a distance of a mm) so that the lifted and orientated turntable  31  again comes to rest on the turret in a non-rotational manner. The flowlines, down stream their fluid connector that interconnects pipe ends  53 ,  54  and riser ends  55 ,  56 , will be lowered back to their operating positions. The fluid connectors will be closed and leak tested. Once the isolation valves are opened production can recommence. The umbilicals are connected using a similar procedure. 
         [0042]    As can be seen from  FIG. 2 , the rotational link between the weathervaning vessel  14  and the mooring buoy  11  comprises multiple sets of bogie wheels  4  for axial loads and radial wheels  30 . This bearing system  4  is designed for both axial and radial loads. 
         [0043]    The turret  1  shown in  FIG. 2  consists of two main parts, a lower turret and an upper turret that includes the manifold decks for swivels, piping and equipment. The lower turret extends from near bottom level  41  to the upper bogie wheel bearing  4 . The lower turret is formed by a cylindrical/conical shell structure with ring stiffeners, designed to resist water and explosion pressures and prevailing mooring forces. The upper section of the lower turret structure provides the support for the bogie wheel bearing system  4  and consists of two subassemblies, the outer support structure connected to the vessel  14  via a cone and the inner support structure onto which the bogie rails  29  are bolted. 
         [0044]    The weight of the turret  1  and the vertical loads from the anchor legs  10 , risers  12 , and umbilicals are transmitted through the upper bogie wheel bearing  4  and then via the bogie wheel bearing to the outer support structure mounted on the vessel moonpool  2 . 
         [0045]    Multiple structural connectors  25  of the clamping type establish the connection of the mooring buoy  11  to the turret  1 . The structural connectors  25  are designed to transmit moments, vertical and horizontal loads. Hydraulic cylinders  26  drive the connectors  25  and the screw/motor-reductor system is used as mechanical locking system. Each connector can be individually activated when the buoy is connected for inspection, maintenance and repair. 
         [0046]    Reconnection of the buoy  11  to the cavity  42  is achieved by lifting the mooring buoy  11  with the installation cable  17 , which passes through the hollow steel guide piece  44  in the center of the manifold chamber  7 . The mooring buoy  11  is connected without any specific attention as to its orientation. Only after the vessel  14  has safely been moored to the buoy  11 , the turntable  31  with the complete turret manifold  7  is rotated to match the piping orientation on the buoy. The fact that the complete manifold  7  can be orientated with regard to the turret  1  will avoid performing the alignment of the manifold piping with the mooring buoy piping at the critical stage of the reconnection when the buoy  11  is supported from the connection winch  45  and is not yet securely moored to the turret  1 . 
         [0047]    The center of the turret  1  forms a receptacle for the mooring buoy  11  and is at the bottom terminated by a cylindrical hollow structure which holds the lower turret bearing assembly. This lower bearing assembly comprise of a set of low-friction bearing pads  5  made of self-lubricating material mounted on the lower turret outer box and radial stoppers  28 . The bearings  5  are arranged in a radial pattern to resist the horizontal forces of the mooring system while permitting the turret  1  to rotate inside the moonpool  2 . The pads are self oriented and can be inspected and removed in situ via the access in the lower turret. 
         [0048]      FIG. 3  shows the upper bearing system  4  and the turntable bearing system  32  of the present invention in more detail. Reference number  31  shows the turntable that supports the upper turret manifold decks and swivel stack in a rotatable way. The turntable can be hydraulically lifted up (few mm) by means of a hydraulic jack  33  so that bearing system  32  can be activated and support the turntable on the turret in a rotatable way which is only needed for alignment of the manifold with the piping of the already connected buoy. To rotate the turntable  31  for alignment, the turntable motor drive system  52  is for example formed by a rack and pinion system of a type that is similar to known driving systems for turret rotation. This temporarily activated turntable bearing system  32  preferably comprises a bogie wheel bearing having at least 3 sets of hydraulically vertically displaceable bogie wheels, but can also comprise any other known bearing system including ball bearing systems, slide pads etc. After alignment, the turntable can again be lowered (by a few mm) onto the turret by deactivating the hydraulically vertically displaceable bogie wheels, and turntable  31  and turret can be locked and secured together in that position via hydraulic jacks  33 . 
         [0049]      FIG. 4  illustrates a subsea buoy  11  according to one embodiment of the present invention. The aim of the mooring system according to this embodiment is to ease the lifting of the submerged buoy  11 , used as a disconnectable mooring system for a vessel  14 , by fitting it with a variable buoyancy tank  15 . The variable buoyancy function is achieved by the use of a compressible substance such as air, lighter than water and with a smaller bulk modulus. The substance contained in the tank  15  equalizes its pressure with the hydrostatic pressure either by direct contact (by being contained in a tank that is in open contact with the sea) or through a deformable membrane, air filled backs, a piston etc. Being more compressible than water, the volume of the substance, and therefore the displacement volume of the tank  15 , depends on the depth at which the tank is located. The initial amount of substance to be placed in the tank  15  is determined as to fully or partially compensate for the anchoring/risers system variation of suspended weight with regards to depth. When the buoy is disconnected from the vessel and sinks, the hydrostatic pressure acting on the tank increases and the volume of the substance is reduced, as it is schematically illustrated in  FIG. 4 . The buoyancy of the tank  15  becomes smaller and the buoy continues to sink until equilibrium is reached with the other vertical forces acting on the buoy (buoy weight, anchoring/risers system suspended weight). 
         [0050]    When the buoy is lifted from its disconnected rest position, the pressure exerted on the substance decreases and the substance volume inflates, inducing more buoyancy and reducing the pulling effort required to lift the buoy and its anchoring/risers system. 
         [0051]    Hence, the loads acting on the connection winch are reduced compared to conventional systems, allowing for reconnection of the mooring buoy with less dynamics and in higher sea-states. 
         [0052]    A large pretension in the reconnection cable at reconnection is due to the variation in suspended chains/risers weight over the course of reconnection. This variation can be in the range of for instance 600 tons. Having a tank  15  with variable buoyancy in the buoy  11  according to the present invention allows to reconnect the buoy and to maintain the buoy in the connected state at a reduced pretension. The variation in suspended weight can therefore be compensated by the change in volume. 
         [0053]    However, this variable buoyancy tank  15  fitted into the buoy  11  may not be sufficient to ensure the buoy will sink deep enough and fast enough for example to avoid an iceberg. The present invention therefore proposes also to attach a weight  16  via a cable  17  to the buoy  11 . 
         [0054]      FIG. 5  illustrates the system during disconnection according to one embodiment of the present invention. The system according to the present invention is especially designed to be disconnected in the event of approach of an iceberg. Following a subsequent reconnection or, after the initial installation, the turret has to be prepared for a disconnection. When it is determined that the iceberg is on a direct course for the site and the decision is taken to disconnect, the flowlines will be flushed after which the valves upstream and downstream of the fluid connector will be closed. The short length of the piping downstream the fluid connector will be lifted after the upper connection point has been released in order to get clearance between the buoy and its receiving cavity in the turret. The umbilical will be simultaneously disconnected using a similar procedure. On the final decision to disconnect, the structural connector mechanical locks will be released and the structural connector will be opened. The mooring buoy  11  will then be released from the vessel and sink slowly to the predetermined water depth. The vessel can now sail away from the approaching iceberg and the buoy  11  placed at a sufficient depth (e.g. approximately 100 meters below the surface) to avoid contacting the iceberg. 
         [0055]    In the embodiment shown in  FIG. 5  everything is predetermined in such a way that the buoy is submerged at the desired depth, the buoyancy of the buoy  11 , the mass of the weight  16  required underneath it at disconnection and the length of the cable  17 . In the embodiment shown, the length of the cable  17  is adjusted so that the buoy  11  reaches its target depth when the weight  16  touches the seabed  19 , although other embodiments are envisaged in which the weight  16  remains free from the seabed. This configuration guarantees an excellent stability in the disconnected state and the pretension in the connected state (with the weight attached) enables to drop the buoy within the short time allowed. In this configuration, the use of compressive tank  15  fitted within the buoy  11  can be maintained to adjust the buoyancy both in connected and disconnected condition. 
         [0056]      FIG. 6  shows the system during connection according to one embodiment of the present invention. A traction winch  20  is located on the centerline of the turret  1  on the manifold structure  7 . The winch  20  is be used to haul in the buoy  11  inside the turret moonpool  2  during the reconnection. A storage winch can be located adjacent to the traction winch to receive the buoy reconnection line (not shown). The winch with associated sheave is also used for the hook-up of risers  12  and umbilicals. 
         [0057]    To (re)connect the buoy  11 , the weight  16  is lifted by the connection winch  20 , instead of the buoy  11  as is done for conventional systems. By the effect of its positive buoyancy, the buoy  11  is free to rise by the same amount the weight  16  has been lifted. The “lift” of the buoy  11  is therefore controlled by the lift of the weight  16 . In fact, the winch cable  17  lifts directly the pretension weight  16 , while the buoy  11  slides along the cable  17 . The vertical motions of the buoy are controlled and restrained by a stopper  21  fixed onto the winch cable  17 . The contact between the stopper  21  and the buoy  11  is either direct (see  FIG. 8   a ) or smoothened by springs  22  in order to ensure a smooth load transfer between the winch  20  and the buoy  11  as illustrated in  FIGS. 8   b  to  8   d.    
         [0058]    Theoretically, hauling in the in the buoy  11  is approximately similar to only lifting the weight  16 . When the vessel  14  heaves up, the weight  16  and the stopper  21  follow. The buoy is free to rise by means of its own positive buoyancy. When the vessel  14  goes down, the stopper  21  comes in contact with the buoy transferring the load of the winch  20  onto the buoy, controlling its lift. If the winch  20  becomes slack in the process, the peak load when the vessel goes up again is of limited amplitude because it is only restrained by the mass of the weight  16 . The configurations in which the weight  16  is attached to the buoy  11  via spring members ( FIGS. 8   b  to  86   d ) help limiting the amplitude of the snatch loads by transferring smoothly the loads between the winch  20  and the buoy  11 . It can be shown that the “decoupling” becomes effective when the stiffness of the spring  22  is small compared to the winch cable  17 . Sensitivity studies with regards to spring stiffness have shown that a ratio of 1/10 (i.e. ˜1000 kN/m for a winch cable of 10000 kN/m) is a workable order of magnitude. 
         [0059]    By lifting only the relatively small pretension weight  16  during reconnection of the buoy  11 , the present invention provides a very efficient way to reconnect a buoy having an important size carrying a relatively large number of risers and allows to use a main winch that has a lifting capacity that is comparable with that of drum winches that are known and available in the art. 
         [0060]    The system according to the present invention can also be provided with spring buoys  18  to lighten the anchor legs  10  weight and that can also be used as “drop stoppers”, to control the drop and stabilizing depth of the disconnected mooring buoy. 
         [0061]    Another advantage of this system is that it decouples the buoys hydrodynamic from the winch loads, giving precedence to functional sizing over hydrodynamics optimization. 
         [0062]      FIGS. 7   a  and  7   b  shows alternative embodiments of the disconnection/connection of the buoy  11  according to the present invention. The main difference over the known solutions is that it is not needed for the weight  16  to touch the sea bed  19  and to moor the disconnected riser buoy because in case of the invention the mooring buoy is already provided with mooring legs  10  that keep a disconnected buoy in place horizontally. The weight  16  is just to ease, control and simplify the connection and disconnection procedure of the mooring buoy. Additional weights  23  can be added on each mooring line  10  as is shown in  FIGS. 7   a  and  7   b . In this embodiment, the length of the cable  17  can be adjusted so that the buoy  11  reaches its target depth when weights  23  touch the seabed  19 . This configuration guarantees the same advantages as the one shown in  FIG. 5 . The reconnection of the buoy  11  according to this embodiment is following the same procedure as shown in  FIG. 6 . 
         [0063]    As stated earlier,  FIGS. 8   a  to  8   e  show different embodiments of the disconnectable buoy with its associated pretensioned weight  16 . 
         [0064]      FIGS. 8   a  and  8   e  illustrate cases when the contact between the stopper  21  and the buoy  11  is direct. In  FIG. 8   e  the weight  16  is suspended underneath the buoy  11  and has the form a long heavy chain  24 .  FIGS. 8   b  to  8   d  illustrate cases in which the contact between the stopper  21  and the buoy  11  is smoothened by springs  22  in order to ensure a smooth load transfer between the winch  20  and the buoy  11 . In  FIG. 8   b  the spring  22  is located at the top of the buoy  11 , between the stopper  22  and the buoy  11 . In  FIG. 8   c  the spring is located within the buoy in the hollow passage where the cable  17  also passes through the buoy  11 . In this configuration the stopper  21  is located underneath the buoyl  1 . In  FIG. 8   d , the weight  16  is not hanging underneath the buoy as illustrated in  FIGS. 8   a ,  8   b ,  8   c  and  8   e , but is located inside the buoy with spring means  21  above and underneath the weight, the stopper  21  being located underneath the buoy  11 . These embodiments are not limitative and any combination of these shown embodiments could also be realized. For instance the embodiment shown in  FIG. 8   e  could be modified by smoothening the contact between the stopper  21  and the buoy  11  by springs.