Patent Publication Number: US-9422035-B2

Title: Disconnectable turret mooring system

Description:
FIELD OF THE INVENTION 
     The present invention relates to a turret mooring system, a vessel comprising the turret mooring system and a method of mooring the vessel using the turret mooring system. 
     BACKGROUND OF THE INVENTION 
     Turret mooring systems are known from the prior art, for instance from WO2007/077126. 
     Turret mooring systems comprise a mooring buoy and a turret mooring structure. The mooring buoy is anchored to the seabed with anchoring legs. The turret mooring structure, provided on a vessel, has a receptacle for receiving the mooring buoy and one or more buoy locking devices for locking the mooring buoy in the receptacle. 
     The turret mooring structure may be an internal turret mooring structure or an external turret mooring structure. An internal turret mooring structure is provided inside the hull of the vessel, in a so-called moonpool of the vessel. The receptacle is formed as an opening at or near the bottom of the vessel, facing downwards. An external turret mooring structure is provided outside the hull of the vessel. The external turret is fixed with suitable connection members to the bow or stern of the vessel. 
     The mooring buoy may be moved up and down, i.e. from a storage position at a safe distance below the water surface (e.g. 30-200 meters) to a mooring position close to or at the surface of the water where it can be received by the receptacle. 
     The turret mooring structure itself is connected to the vessel, but is rotatable with respect to the vessel, allowing the vessel to weathervane under influence of wind, waves, currents and drifting ice. The turret mooring system may be disconnected and reconnected when needed, thereby providing a disconnectable turret mooring system. 
     The turret mooring system comprises a fluid transfer system to allow transportation of fluids, for instance by establishing a flow path between the vessel and a subsea well via the turret mooring system and the mooring buoy. 
     The turret mooring structure may comprise a first part of the fluid transfer system and the mooring buoy may comprise a second part of the fluid transfer system. 
     The turret mooring structure may comprise a turret manifold and the mooring buoy may comprise a buoy manifold, both manifolds each comprising at least one conduit. The turret and buoy manifold are matching such that conduits of the turret manifold can be connected to corresponding conduits of the buoy manifold of the fluid transfer system to establish a flow path. 
     During the connection of the mooring buoy to the turret mooring structure, the mooring buoy is locked in a fixed position with respect to the turret mooring structure. 
     According to state of the art in turret mooring systems, the centering has to be done using the locking of the mooring buoy into the receptacle of the turret mooring structure. 
     International patent applications WO1993011030-WO1993011035 disclose locking mechanisms of a turret mooring structure comprising a plurality of locking fingers distributed around an annular locking shoulder of the buoy, for releasable locking of the outer member of the mooring buoy in the receiving space. The mechanism comprises a pair of locking dogs which are actuated by a hydraulic system and are rotatable about horizontal axes at diametrically opposite sides of the receiving space. If desired, more than two locking dogs may be provided. The hydraulic actuators for operation of the locking dogs may for example be hydraulic cylinders. When activating the locking dogs these will pivot in a vertical plane to engagement with the downwards facing abutment edge of the upper cone member. 
     International patent application WO2001089919 discloses a mechanism for releasable locking of an element in relation to a base, especially for the locking of a buoy in a downwardly open receiving space in a floating vessel. The mechanism comprises a rotatably mounted locking arm which is pivotable between a release position and a locking position in which an abutment edge on the locking arm is in engagement with an abutment edge on the element to be locked, a linkage which is connected between the locking arm and the base and which, in the locking position, is in a self-locking over-centre position, and a driving means for actuation of the linkage. The linkage comprises a length-adjustable first link which, from an initial position with the locking arm close to its locking position, is arranged to be extended to thereby pivot the locking arm additionally to a final locking position, for achieving a desired preloading force in the engagement between the abutment edges of the locking arm and the element. The first link preferably is a hydraulic cylinder having a piston rod of which one end is connected to the locking arm. 
     However, in these systems, designs are not very compact and need a lot of space to be installed, further they do not provide a system that enables to center the buoy. It is therefore an object of the present invention to provide a turret mooring system that overcomes one or more of the disadvantages from the prior art. 
     SUMMARY OF THE INVENTION 
     Hereto, a turret mooring system is provided wherein the turret mooring system is arranged for receiving a mooring buoy, the turret mooring system comprising a turret mooring structure, and a buoy locking system for locking the mooring buoy, wherein the buoy locking system comprises at least one locking device, which in a first support frame comprises a locking dog and a further structural element, the first support frame being connected to the turret mooring structure; wherein the further structural element is arranged in the first support frame at a different vertical level than the level of the locking dog and the locking dog is rotatable around a horizontal shaft in the first support frame between an open and a closed position; wherein the further structural element is arranged to engage the surface of the mooring buoy. 
     Such an embodiment provides the advantage that centering of the mooring buoy and of the fluid transfer system within the buoy can be done during the locking of the mooring buoy into the turret mooring system. 
     Also, the turret mooring system allows an accurate setting of the concentricity of the turret and the buoy. The locking devices allow proper centering of the mooring buoy with respect to the turret, during the positioning of the top ring of the buoy into the turret. When approaching the locking devices, the top ring of the buoy, lifted-up by the winch, comes into contact vertically with the centering dogs that start rotating thanks to an articulation, and then, achieve the final centering of the buoy by pushing radially on the buoy top ring side. When the centering is completed, the locking dogs are closed and clamp the top ring of the buoy in the turret. 
     Another advantage is that the number of buoy locking devices can easily be changed, i.e. only requires replacing the intermediate connection member, without the need to adjust the design of the turret mooring structure. 
     According to an aspect, the present invention relates to a turret mooring system as described above, wherein the further structural element is an upper centering dog positioned above a level of the locking dog, wherein the centering dog is rotatable around a further horizontal shaft in the first support frame between an open and a clamping position; the centering dog having a supporting surface which when rotating from the open to the clamping position, is arranged to perform a translation in a horizontal direction for pushing a top ring of a received mooring buoy radially. 
     According to an aspect, the present invention relates to a turret mooring system as described above, wherein the locking dog is arranged to rotate around the horizontal shaft to a clamping position after the centering dog has reached the clamping position, the locking dog in the clamping position being arranged for clamping the top ring of the mooring buoy to the turret mooring structure. 
     According to an aspect, the present invention relates to a turret mooring system as described above, wherein the locking device comprises a second support frame and a hydraulic jack, wherein the second support frame is rotatably connected with the first support frame over the horizontal shaft and the hydraulic jack is connected between a further shaft of the second support frame and an extended arm of the locking dog, wherein the connection of the hydraulic jack and the extended arm of the locking dog comprises a hinge. 
     According to an aspect, the present invention relates to a turret mooring system as described above, wherein the locking device comprises a releasable connection between the first and the second support frames. 
     According to an aspect, the present invention relates to a turret mooring system as described above, wherein the releasable connection comprises a backup release jack attached to one of the first and second support frames comprising a release pin and a locking hole attached to the other of the first and second supporting frames, the locking hole arranged for releasably receiving the release pin. 
     According to an aspect, the present invention relates to a turret mooring system as described above, wherein the hydraulic jack comprises a dedicated hydraulic power unit which is integrated in the locking device. 
     According to an aspect, the present invention relates to a turret mooring system as described above, wherein a top part of the mooring buoy to be received in the turret mooring structure, comprises a ring portion provided with a first abutment surface and a second abutment surface placed at a distance from and in parallel with the first abutment surface; 
     the locking dog is arranged for engagement with the first abutment surface provided on the ring portion of the mooring buoy; the locking device further comprises an engagement member and the engagement member is arranged for engagement with the second abutment surface of the mooring buoy, wherein a distance between the locking dog in closed position and the engagement member corresponds with the distance between the second abutment surface and the first abutment surface on the mooring buoy. 
     According to an aspect, the present invention relates to a turret mooring system as described above, wherein the mooring buoy comprises a top part to be received in the turret mooring structure having a ring portion provided with a first abutment surface and a second abutment surface placed at a distance from and in parallel with the first abutment surface, wherein the further structural element is an engagement member; the locking dog being arranged for engagement with the first abutment surface provided on the ring portion of the mooring buoy and the engagement member being arranged for engagement with the second abutment surface of the mooring buoy wherein a distance between the locking dog in closed position and the engagement member corresponds with the distance between the second abutment surface and the first abutment surface on the mooring buoy. 
     According to an aspect, the present invention relates to a turret mooring system as described above, wherein the locking dog in the clamping position is arranged for clamping the first abutment surface and causes the engagement member to clamp on the second abutment surface. 
     According to an aspect, the present invention relates to a turret mooring system as described above, wherein the mooring buoy comprises a top part to be received in the turret mooring structure having a ring portion provided with a first abutment surface and a second abutment surface placed at a distance from and in parallel with the first abutment surface, wherein the further structural element is a lower centering dog, the locking dog being arranged for engagement with the first abutment surface provided on the ring portion of the mooring buoy and the lower centering dog being arranged for engagement with the second abutment surface of the mooring buoy wherein a distance between the locking dog in closed position and the lower centering dog corresponds substantially with the distance between the second abutment surface and the first abutment surface on the mooring buoy. 
     According to an aspect, the present invention relates to a turret mooring system as described above, wherein the locking dog in the clamping position is arranged for clamping the first abutment surface and causes the lower centering dog to clamp on the second abutment surface. 
     According to an aspect, the present invention relates to a turret mooring system as described above, wherein the lower centering dog is configured to rotate around a horizontal axis from an lower open position for receiving the second abutment surface to an upper clamping position for engaging the second abutment surface, wherein the upper position of the lower centering dog is a blocked position. 
     According to an aspect, the present invention relates to a turret mooring system arranged for receiving a mooring buoy, the turret mooring system comprising a turret mooring structure, and a buoy locking system for locking the mooring buoy, the mooring buoy comprising at least one buoy connection lug provided on an upper surface portion of the body of the mooring buoy; the connection lug comprising a passage in the lug for receiving a locking body, wherein the buoy locking system comprises at least one locking device, which in a first support frame comprises a rotatable locking dog, the first support frame being connected to the turret mooring structure; the rotatable locking dog having an extended body being arranged for engagement with the connection lug by entering the passage in the lug. 
     According to an aspect, the present invention relates to a turret mooring system as described above, wherein a locking movement of the locking dog of the at least one locking device is aligned with the orientation of the passage of the corresponding connection lug. 
     According to an aspect, the present invention relates to a mooring buoy arranged for coupling with a turret mooring system, wherein the mooring buoy comprises at least one buoy connection lug provided on an upper surface portion of the body of the mooring buoy; the lug comprising a passage in the lug for receiving a locking body of a locking device attached to the turret mooring system. 
     According to an aspect, the present invention relates to a mooring buoy as described above, wherein the connection lug is coupled to a top part of the buoy structure to be received in the turret mooring structure. 
     According to an aspect, the present invention relates to a mooring buoy as described above, wherein the connection lug is coupled to the buoy structure by a connector plate. 
     According to an aspect, the present invention relates to a mooring buoy as described above wherein the connector plate comprises a pivot pin. 
     According to an aspect, the present invention relates to a mooring buoy as described above, wherein the passage has a first width and first height and the extended body of the locking dog has a second width and second height, wherein the second width smaller than the first width, and the second height is smaller than the first height. 
     According to an aspect, the present invention relates to a mooring buoy as described above, wherein an upper portion of the passage has a curved internal surface with crosswise a first radius of curvature and the extended body of the locking dog has perpendicular to its length of the extended body a curved upper outer surface with a second radius of curvature, wherein the second radius is equal to or smaller than the first radius. 
     According to an aspect, the present invention relates to a mooring buoy as described above, wherein the connection lug has a curved internal surface in the direction of the opening of the passage, and the extended body has a substantially straight surface along its longitudinal direction. 
     Further the present invention relates to a vessel comprising a hull and a turret mooring system as described above, wherein the turret mooring system is rotatably suspended from the hull of the vessel. 
     The present invention relates to a method of mooring a vessel comprising a turret mooring system to a mooring buoy, the turret mooring system being arranged to receive the mooring buoy and the turret mooring system comprising a turret mooring structure being rotatably suspended from the vessel and at least one locking device, which in a first support frame comprises a locking dog and a further structural element, the first support frame being connected to the turret mooring structure; 
     wherein the further structural element is arranged in the first support frame at a different vertical level than the level of the locking dog, the method comprising: 
     receiving the mooring buoy into the turret mooring system, 
     contacting the mooring buoy with the locking dog in open position; 
     lifting the buoy further so as to let the further structural element engage the surface of mooring buoy. 
     Advantageous embodiments are further defined by the dependent claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments will now be described, by way of example only, with reference to the accompanying schematic drawings in which corresponding reference symbols indicate corresponding parts, and in which: 
         FIG. 1A  shows a schematic drawing of a turret mooring system provided on a vessel; 
         FIG. 1B  shows an enlargement of the selected box in  FIG. 1A ; 
         FIG. 2  shows a perspective view of an embodiment of a locking device according to the present invention; 
         FIG. 3  shows a cross-section of the locking device of  FIG. 2 ; 
         FIG. 4 a    schematically shows the locking device during a first stage of positioning of the buoy into the turret; 
         FIG. 4 b    schematically shows the locking device during a second stage of positioning of the buoy into the turret; 
         FIG. 4 c    schematically shows the locking device during a third stage of positioning of the buoy into the turret; 
         FIGS. 5 a  and 5 b    schematically show a locking device in accordance with an embodiment of the invention; 
         FIG. 6  schematically shows a locking device according to an embodiment of the invention; 
         FIGS. 7 a -7 d    shows detailed views of the locking device during the locked phase of the buoy and the turret. 
     
    
    
     DETAILED DESCRIPTION 
     In the following figures, the same reference numerals refer to similar or identical components in each of the figures. 
       FIGS. 1 a -1 b    show schematic drawings of a turret mooring system  1  provided on a vessel  2 , which for example could be a floating production unit (FPU) or floating production storage and offloading (FPSO) unit or floating storage and offloading (FSO) unit. The vessel  2  comprises a hull  16  having near a bottom  17  a moonpool  18 . A lifting device  26  is placed on the turret mooring structure  3  comprising a cable  19 , shown in  FIGS. 1 a - b    and  2 , that extends through a central shaft  24  provided in the mooring buoy  6 . In addition, the vessel  2  comprises a turret mooring system  1 , wherein the turret mooring system  1  is rotatably suspended from the hull  16  of the vessel  2 . The turret mooring system  1  comprises a turret mooring structure  3  within the moonpool  18 . 
     A turret bearing system  21  connects and aligns the turret mooring structure  3  with respect the vessel  2 . The turret mooring system  1  is as a whole rotationally suspended from the vessel  2 . The turret mooring system  1  can rotate with respect to the vessel  2  to allow the vessel  2  to weathervane after connection to the mooring buoy  6  or to orientate the turret mooring system  1  with respect to the mooring buoy  6 , without the need to reposition the vessel  2 . 
     In addition, the turret mooring system  1  may comprise an intermediate connection member  4 . Such an intermediate connection member  4  is arranged to be rotated together with the mooring buoy  6  with respect to the turret mooring structure  3 , i.e. after locking the mooring buoy  6  inside the receptacle  5 , so that the fluid piping of the turret manifold piping and the buoy manifold piping can be aligned. The intermediate connection member  4  is positioned in between the turret mooring structure  3  and the mooring buoy  6 , if present. After disconnecting the mooring buoy  6 , the intermediate connection member  4  remains attached to the turret mooring structure  3 . 
     The turret mooring structure  3  comprises a receptacle  5  for receiving a mooring buoy  6 . In an alternative embodiment (not shown), the receptacle  5  could be attached directly to the intermediate connection member  4 , if present. The mooring buoy  6  carries an anchoring system  27  which may comprise at least 1 anchoring leg  22  that are connected to a seabed  23 . The mooring buoy  6  is receivable in the receptacle  5  for coupling with the turret mooring structure  3 . 
     The turret mooring structure  3  comprises a buoy locking system  7 , comprising a number of buoy locking devices  10  for locking the mooring buoy  6  inside the receptacle  5 . An embodiment of the buoy locking device  10  is described with reference to  FIGS. 2-7 . The buoy locking system  7  is only shown schematically in  FIGS. 1 a   - b.    
     The turret mooring structure  3  comprises a turret manifold  8  that can be connected, after alignment, to a corresponding buoy manifold  9  to establish a fluid flow path between the turret mooring structure  3  and the mooring buoy  6 . 
     When the mooring buoy  6  enters the receptacle  5 , the mooring buoy  6  is pre-centered into the receptacle  5 , due to the conical shape and fenders  11  on the inside of the receptacle  5 , and due to the pulling tension in the reconnection winch cable of the lifting device  26 . 
     When approaching the locking devices, the top ring of the buoy, lifted-up by a winch, comes into contact vertically with the locking devices that will achieve a final centering of the buoy as will be described in more detail below. When the centering is completed, the locking devices are closed and clamp the top ring of the buoy in the turret mooring structure  3 . 
     After alignment of the fluid piping manifolds and the locking of the turret with regard to the connected buoy  6 , a fluid transfer path can be established between the turret and buoy manifold. 
       FIG. 2  shows a perspective view of an embodiment of a locking device according to the present invention. 
     The locking device  10  comprises a first support frame  100  and a second support frame  102 . Further the locking device  10  comprises a locking dog  106 , a centering dog  108 , and a main hydraulic jack  110 . 
     In this embodiment, the first support frame  100  is substantially T-shaped and connected to the turret mooring structure by fixed bottom and side supports  120  and  122  which are located at two substantially perpendicular ends of the T-shaped first support frame. The connection between the first support frame  100  and the fixed supports may be by shafts  121  and  123  respectively. 
     The second support frame is a substantially oblong frame which has a first end that is rotatably connected to the first support frame by a common shaft  104 , which is located in the first support frame at some vertical distance above the fixed bottom support  120 . 
     The locking dog  106  is rotatably connected to the first support frame  100  on the common shaft  104 . The centering dog  108  is connected to the first support frame  100  on a dedicated shaft  125  which is located at a vertical distance above the common shaft. 
     Both the locking dog  106  and the upper centering dog  108  are arranged on a free end  124  of the first support frame  100 , i.e., the end of the first support frame that is not connected to the fixed bottom or side supports  120 ,  122 . 
     A second end of the second support frame  102  is rotatably connected to one end of the main hydraulic jack  110  by means of a shaft  128 . 
     The opposite end of the hydraulic jack  110  is rotatably connected to the locking dog  106  through a hinge  132 . The arrangement of the hydraulic jack and the locking dog is described in more detail with reference to  FIG. 3 . 
     Additionally, the locking device comprises on the second support frame  102  a backup release (hydraulic) jack  140 , which provides a releasable connection between the second support frame and the first support frame at a location adjacent to the fixed side support  122 . 
     One or more of shafts  104 ,  121 ,  122 ,  125 ,  128  of the locking device  10  are preferably provided with low friction bushes. 
     Optionally the hydraulic jack on each locking device is operated with a dedicated HPU (Hydraulic Power Unit) with the HPU being integrated within the locking device structure (i.e. not connected to the jack only via a flexible). This has the advantage of having a stand alone locking device as a whole one piece assembly which is complete and only needs to be installed on site. 
       FIG. 3  shows a cross-section of the locking device of  FIG. 2 . 
     The hydraulic jack  110  is rotatably connected to an extension arm  109  of the locking dog  106  through the hinge  132 , such that a change of the length of the hydraulic jack causes a rotation of the locking dog  106  around common shaft  104 . In  FIG. 3 , the hydraulic jack is shown in its extended position, with the locking dog  106  in a clamping position of the mooring buoy (not shown). 
     The centering dog  108  is shown also in a clamping position of the mooring buoy. 
     A lower edge  108   a  of the centering dog  108 , extending over a portion of the length of the body of the centering dog  108  and that is arranged to contact the top ring of the buoy is positioned substantially horizontal. An upper edge  108   b  of the centering dog  108 , extending over substantially the full length of the centering dog body is parallel to the lower edge  108   a . The upper edge  108   b  is arranged to abut a stopper  126  on the first support frame  100  when in the clamping position. In addition to the lower edge  108   a  and upper edge  108   b  portions, the centering dog  108  comprises in a portion of the body close to the shaft  125  an angled edge  108   c  which is arranged under an oblique angle with the upper and lower edges  108   a ,  108   b . The angled edge  108   c  of the centering dog body is arranged to abut the locking dog  106  when the locking device is in a open position. 
     In between the lower edge  108   a  and the angled edge  108   c  the centering dog  108  comprises a supporting surface  108   d , that is substantially perpendicular to the lower edge  108   a.    
     The backup release jack  140  is shown in the connected position having a release pin  141  attached to the second support frame and positioned in a locking hole  142  attached to the first support frame to form a releasable connection  141 ,  142 . The backup release jack is arranged as a releasable lock that can break the releasable connection in case of a malfunction of the locking device (or an emergency) during the clamping position. By releasing the releasable connection the second support frame  102  can rotate with respect to the first support frame around the common shaft. Since the rotation axis (common shaft) of the second support frame coincides with the rotation axis  104  of the locking dog  106 , the locking dog will rotate accordingly to an open position so as to release the buoy. 
     In  FIG. 4 a    schematically the locking device  10  is shown during a first stage of positioning of the buoy  6  into the turret mooring structure  3 . 
     The mooring buoy  6  comprises a top ring portion  200  which has an upper edge or surface  201  and a lower edge  202 . 
     During this stage the upper surface  201  of the top ring  200  of the buoy is in contact with the lower edge  108   a  of the centering dog  108 . The lower edge of the centering dog  108  is in the open position under an oblique angle with respect to the top ring of the buoy. The centering dog starts rotating around the dedicated shaft  125  until it reaches its position wherein the upper edge  108   b  of the centering dog  108  abuts the stopper  126  on the first support frame. At the same time the supporting surface  108   d  is oriented towards the outer circumference of the top ring of the buoy such that during the rotation of the centering dog it pushes radially against the top ring of the buoy to displace the buoy horizontally. Since each of the locking devices on the turret mooring structure performs this rotation and radial push, the buoy will be translated with respect to the turret mooring structure to position centered with respect to the locking devices. 
       FIG. 4 b    schematically shows the locking device during a second stage of positioning of the buoy into the turret. The centering dog  108  now abuts the stopper  126  of the locking device  10 . Additionally, the locking dog  108  is in the clapping position. 
     The locking device is instrumented to monitor the open/clamped status with respect to the status of the mooring buoy, i.e. being either connected or disconnected. In an embodiment, a signaling system comprising a suitable monitoring device e.g. a processing unit and sensors on the locking device and/or the turret mooring system is installed. 
     Thus the present invention relates to a turret mooring system arranged for receiving a mooring buoy  6 , the turret mooring system comprising a turret mooring structure  3 , and a buoy locking system  7  for locking the mooring buoy  6 , wherein the buoy locking system  7  comprises at least one locking device  10 , which in a first support frame  100  comprises a locking dog  106  and a further structural element, the first support frame being connected to the turret mooring structure;
 
wherein the further structural element is arranged in the first support frame at a different vertical level than the level of the locking dog and,
 
the locking dog is rotatable around a horizontal shaft  104  in the first support frame between an open and a clamping position; wherein the further structural element is arranged to engage the surface of the mooring buoy, and wherein the further structural element is an upper centering dog  108  positioned above a level of the locking dog,
 
wherein the centering dog is rotatable around a further horizontal shaft  125  in the first support frame between an open and a clamping position; the centering dog having a supporting surface  108   d  which when rotating from the open to the clamping position, is arranged to perform a translation in a horizontal direction for pushing a top ring of a received mooring buoy radially.
 
       FIG. 4 c    schematically shows the locking device during positioning of the buoy into the turret in accordance with an embodiment of the invention. 
     In this embodiment, the mooring buoy  6  comprises a ring portion  200  at a top part of the buoy that is received in the turret. The ring portion is provided with a first abutment surface  202  and a second abutment surface  203  placed at a distance from and in parallel with the first abutment surface. 
     The buoy locking system  7  comprises at least one locking device  10  that in a first support frame  100  comprises a rotatable locking dog  106  and an engagement element  107 . Within the locking device  10  the locking dog  106  and engagement element  107  are positioned at a vertical distance from each other. In this embodiment, the engagement element  107  is positioned below the locking dog  106 . 
     The rotatable locking dog  106  is arranged for engagement with the first abutment surface  202  provided on the ring portion  200  of the mooring buoy  6 . The first abutment surface is collar shaped and positioned in an upper region of the ring portion. Thus a contacting surface of the locking dog can engage the first abutment surface by moving upwards. 
     The second abutment surface is arranged below the first abutment surface. The engagement member  107  of the locking device can engage with the second abutment surface  203  of the mooring buoy  6 , since the upward movement of the locking dog  106  enlarges the distance between the locking dog and the engagement element, thus effectively pulling the second abutment surface up to a contacting surface of the engagement element  107 . In this manner the buoy is clamped to the locking device  10 . In the clamped position a distance between the contacting surface of the rotatable locking dog  106  and the contacting surface of the engagement member  107  corresponds substantially with the distance between the second abutment surface  203  and the first abutment surface  202  on the mooring buoy. 
     The contacting surfaces of the locking device may exert some forces on the first and second abutment surfaces to generate a clamping force for holding the mooring buoy in position. 
     The engagement member may be embodied as a fixed bumper. 
     In an embodiment, the centering dog  108  may be omitted. Thus the invention relates to a turret mooring system arranged for receiving a mooring buoy  6 , the turret mooring system comprising a turret mooring structure  3 , and a buoy locking system  7  for locking the mooring buoy  6 , wherein the buoy locking system  7  comprises at least one locking device  10 , which in a first support frame  100  comprises a locking dog  106  and a further structural element, the first support frame being connected to the turret mooring structure; wherein the further structural element is arranged in the first support frame at a different vertical level than the level of the locking dog and, 
     the locking dog is rotatable around a horizontal shaft  104  in the first support frame between an open and a clamping position; wherein the further structural element is arranged to engage the surface of the mooring buoy and 
     the mooring buoy  6  comprises a top part to be received in the turret mooring structure having a ring portion  200  provided with a first abutment surface  202  and a second abutment surface  203  placed at a distance from and in parallel with the first abutment surface, wherein the further structural element is an engagement member  107 ,
 
the locking dog  106  being arranged for engagement with the first abutment surface  202  provided on the ring portion  200  of the mooring buoy  6 
 
and the engagement member  107  being arranged for engagement with the second abutment surface  203  of the mooring buoy  6  wherein a distance between the locking dog  106  in closed position and the engagement member  107  corresponds with the distance between the second abutment surface  203  and the first abutment surface  202  on the mooring buoy.
 
     For coupling the mooring buoy to the turret mooring structure using the above arrangement of mooring buoy and locking device, a method comprises the steps of 
     receiving the mooring buoy into the turret mooring system, with the locking dog being set in an open position, 
     leveling the first abutment surface and the locking device with the locking dog in open position; 
     at the same time, contacting the second abutment surface of the ring portion of mooring buoy and the engagement member; 
     bringing the locking dog to a clamped position on the first abutment surface so as to lock the ring portion of the mooring buoy to the locking device. 
       FIGS. 5 a  and 5 b    show a locking device according to an embodiment of the invention. 
     In this embodiment, the mooring buoy  6  comprises a ring portion  200  at a top part of the buoy that is received in the turret. The ring portion is provided with a first abutment surface  203  and a second abutment surface  204  placed at a distance from and in parallel with the first abutment surface. 
     The buoy locking system  7  comprises at least one locking device  10  that in a first support frame  100  comprises a rotatable locking dog  106  and a centering dog element  208 . Within the locking device  10  the locking dog  106  and a lower centering dog  208  are positioned at a vertical distance from each other. In this embodiment, the lower centering dog  208  is positioned below the locking dog  106 . The lower centering dog  208  is arranged for rotation around a horizontal axis between a lower position  209  and an upper position  210 . In the upper position  210  the lower centering dog  208  is configured to be blocked from further upward rotation. 
     The rotatable locking dog  106  is arranged for engagement with the first abutment surface  202  provided on the ring portion  200  of the mooring buoy  6 . The first abutment surface is collar shaped and positioned in an upper region of the ring portion. Thus a contacting surface of the locking dog  106  can engage the first abutment surface by moving upwards. 
     The second abutment surface is arranged below the first abutment surface for engaging the second abutment surface  203 . 
     Before contacting the second abutment surface  203  of the buoy  6 , the lower centering dog  208  is in an open position  209 , configured to contact and engage with the second abutment surface and after contact to rotate upward with the upward moving buoy  6 . 
     During the upward movement of the second abutment surface  203 , the lower centering dog  208  rotates upwards, until the upper position  210  is reached and further rotation is blocked. In that upper position the upward movement of the buoy surface is stopped. 
     The upward movement of the locking dog  106  may enlarge the distance between the locking dog and the lower centering dog  208 , thus effectively pulling the second abutment surface upto a contacting surface of the lower centering dog  208 . In this manner the buoy is clamped to the locking device  10 . 
     In the clamped position a distance between the contacting surface of the rotatable locking dog  106  and the contacting surface of the lower centering dog  208  corresponds substantially with the distance between the second abutment surface  203  and the first abutment surface  202  on the mooring buoy. 
     The contacting surfaces of the locking device may exert some forces on the first and second abutment surfaces  202 ,  203  to generate a clamping force for holding the mooring buoy  6  in position. 
     Thus the invention relates to a turret mooring system arranged for receiving a mooring buoy  6 , the turret mooring system comprising a turret mooring structure  3 , and a buoy locking system  7  for locking the mooring buoy  6 , wherein the buoy locking system  7  comprises at least one locking device  10 , which in a first support frame  100  comprises a locking dog  106  and a further structural element, the first support frame being connected to the turret mooring structure; wherein the further structural element is arranged in the first support frame at a different vertical level than the level of the locking dog and, the locking dog is rotatable around a horizontal shaft  104  in the first support frame between an open and a clamping position; wherein the further structural element is arranged to engage the surface of the mooring buoy, and wherein the mooring buoy  6  comprises a top part to be received in the turret mooring structure having a ring portion  200  provided with a first abutment surface  202  and a second abutment surface  203  placed at a distance from and in parallel with the first abutment surface, wherein the further structural element is a lower centering dog  208 ; the locking dog  106  being arranged for engagement with the first abutment surface  202  provided on the ring portion  200  of the mooring buoy  6  and the lower centering dog  208  being arranged for engagement with the second abutment surface  203  of the mooring buoy  6  wherein a distance between the locking dog  106  in closed position and the lower centering dog  208  corresponds substantially with the distance between the second abutment surface  203  and the first abutment surface  202  on the mooring buoy. 
     For coupling the mooring buoy to the turret mooring structure using the above arrangement of mooring buoy and locking device, a method comprises the steps of 
     receiving the mooring buoy into the turret mooring system, with the locking dog and the lower centering dog being set in an open position, 
     leveling the first abutment surface and the locking device with the locking dog in open position; 
     bringing the locking dog to a clamped position on the first abutment surface thus upward moving the first abutment surface of the mooring buoy relative to the locking device. 
     at the same time, contacting the second abutment surface of the ring portion of mooring buoy with a contacting surface of the lower centering dog; 
     during upward moving, rotating the lower centering dog to an upper blocked position, thus effectively locking the buoy with the first abutment surface above the locking dog at the clamped position of the locking dog on and the second abutment surface below the surface of the lower centering dog in the upper blocked position. 
       FIG. 6  schematically shows a locking device in accordance with an embodiment of the invention. 
     In this embodiment, the mooring buoy  6  comprises at least one buoy connection lug  300  provided on an upper surface portion of the body of the mooring buoy. The connection lug is provided with a passage  301  in the lug for receiving a locking body. 
     To this end, the buoy locking system  7  comprises at least one locking device  10 , which in the first support frame  100  that is connected to the turret mooring structure, and which comprises a rotatable locking dog  106 . 
     The rotatable locking dog  106  has an extended body being arranged for engagement with the connection lug  300  by entering the passage  301  in the lug. The extended body can be moved from vertical position to a horizontal position in which the extended body is positioned in the passage in the lug to couple the mooring buoy to the turret mooring structure. 
     The extended body may exert some upward force on the connection lug  300  to generate a clamping force for holding the mooring buoy  6  in position in the turret mooring structure. 
       FIGS. 7 a -7 d    show detailed views of the locking device during the locked phase of the mooring buoy and the turret. 
       FIG. 7 a    shows a schematic side view of the connection lug  300  during the locked phase of the mooring buoy  6  and the turret. The connection lug  300  is shown in a viewing direction parallel to the passage  301 . The connection lug is attached to the upper part of the mooring buoy. 
     In an embodiment, the connection lug  300  is connected to the mooring buoy  6  by a coupling pin  302 , which functions as a pivot, allowing the connection lug to pivot around the coupling pin. 
     In the upper part of the passage  301  the extended body of the locking dog  106  is shown in cross-section. The inner surface  303  of the passage  301  has a curved surface in its upper part. Similarly, the upper part  136  of the extended body that in the clamped position is in contact with the inner surface of the connection lug, has a rounded surface curved around the longitudinal axis of the extended body. In this manner it is achieved that while the inner surface  303  of the lug  300  and the upper surface  136  of the extended body of the locking dog  106  are in contact a rotation relative to each other is possible. Such a rotation a is shown in  FIG. 7 b   . Advantageously, this rotation can provide compensation for a torsional misalignment of the mooring buoy. In this situation the connection lug  300  may also rotate around the coupling pivot  302  on the mooring buoy. 
       FIG. 7 c    shows a cross-sectional view across the extended body of the locking dog  106  during the locked phase of the mooring buoy and the turret. The solid outline shows the extended body in horizontal coupled position. The dashed outline shows the extended body in vertical opened position. 
     In the longitudinal direction, the upper surface  136  of the extended body is substantially straight. When coupled with the extended body in horizontal position, the inner surface  303  of the passage  301  of the lug  300  adjacent to the upper surface  136  of the extended body is preferably rounded in the same direction as the length of the extended body. In this manner the connection lug  300  can move relative to the extended body of the locking dog  106 , allowing compensation of a radial motion β of the mooring buoy relative to the locking device. See  FIG. 7   d.    
     In an embodiment, the inner surface of the passage of the lug is double curved to allow rotation due to torsional misalignment and/or radial motion. 
     For coupling the mooring buoy to the turret mooring structure using the above connection lug and locking dog arrangement, a method comprises the steps of 
     receiving the mooring buoy into the turret mooring system with the locking device in an open position; 
     positioning the mooring buoy relative to the locking device to align the locking device with the connection lug; 
     moving the locking dog from the open position to a closed position for engagement with the connection lug by entering of the locking dog into the passage so as to couple the connection lug of the mooring buoy to the locking device. 
     The descriptions above are intended to be illustrative, not limiting. It will be apparent to one skilled in the art that modifications may be made to the invention as described above without departing from the scope of the claims set out below. 
     LIST OF PARTS 
     
         
           1 . Turret mooring system 
           2 . Vessel 
           3 . Turret mooring structure 
           4 . Intermediate connection member 
           5 . Receptacle 
           6 . Mooring buoy 
           7 . Buoy locking system 
           8 . Turret manifold 
           9 . Buoy manifold 
           10 . Locking device 
           11 . Fenders 
           12 . Radial bearing 
           13 . Upper axial bearing 
           14 . Lower axial bearing 
           15 . Driving member 
           16 . Hull 
           17 . Bottom of hull 
           18 . Moon pool 
           19 . Cable 
           20 . Annular ring 
           21 . Turret bearing system 
           22 . Anchoring legs 
           23 . Seabed 
           100 . First support frame 
           102 . Second support frame 
           104 . Common shaft 
           106 . Locking dog 
           107 . Engagement element 
           108 . Centering dog 
           108   a . Lower edge 
           108   b . Upper edge 
           108   c . Angled edge 
           108   d . Supporting surface 
           110 . Main hydraulic jack 
           120 ,  122 . Fixed bottom and side supports 
           121 ,  123 ,  125 ,  128 . Shaft 
           126 . Stopper 
           132 . Hinge 
           140 . Backup release jack 
           141 . Release pin 
           142 . Locking hole 
           200 . Top ring portion 
           201 . Upper edge or surface 
           202 . Lower edge, First abutment surface 
           203 . Second abutment surface 
           205 . Connection lug 
           206 . Passage 
           207 . Locking dog body 
           208 . Lower centering dog