Abstract:
A device for loading and/or unloading of flowable media includes a buoy connected to at least one riser and a vessel provided with a pick-up space for the buoy. The pick-up space ends at the bottom of the vessel, the buoy being provided with a buoyancy device and including a first part connected to the at least one riser and a second part which can be rotated with respect to the first part. The second part is arranged for locking to the vessel. The buoy comprises a connected for connecting the at least one riser to a swivel for transfer of the flowable medium. When the vessel is connected to the buoy, its position is maintained by a dynamic positioning system.

Description:
BACKGROUND OF THE INVENTION 
   (1) Field of the Invention 
   The present invention regards a loading buoy for loading and unloading liquids and gases in open sea. 
   (2) Description of Related Art 
   Several buoys of this type are known. The prior art buoys support one end of one or more risers that extend from storage tanks on the seabed, e.g. at a nearby production facility. The buoy is moored to the seabed and floats a distance below the surface of the sea, due to a positive buoyancy. When used for loading or unloading, the buoy is pulled up into a pick-up space in a vessel and held there. A swivel connected to loading and unloading lines aboard the vessel is coupled to the top of the buoy and connected to the tops of the risers from the buoy. Loading or unloading can then commence. 
   The buoy comprises an outer and an inner part which are interrotatable, the outer part being kept rotationally fixed relative to the vessel, and the inner part being substantially rotationally fixed relative to the seabed, by way of the mooring. Thus the vessel can rotate under the influence of wind and current without affecting the loading or unloading process. 
   A common feature of known solutions such as described in inter alia U.S. Pat. Nos. 4,490,121, 4,604,961, NO 176 131, NO 175 419, NO 175 420 and NO 175 421 is that of the buoy being moored to the seabed. Mooring the buoy may seem natural, as it is desired that the buoy stay substantially stationary. The mooring also serves as mooring for the vessel when connected to the buoy. This allows the vessel to weather a storm without there being any risk of it coming adrift and without wind and current affecting the loading and unloading operation. 
   However, mooring the buoy also carries drawbacks. The mooring requires the buoy to absorb all the forces exerted by the mooring line. These forces can get very large when there are heavy currents affecting a moored vessel. Consequently the buoy must be dimensioned to take up these forces. Heavy dimensioning means a high steel weight. A high steel weight requires the buoy to have a relatively large buoyancy chamber in order to float at the correct depth. This means that the buoy will be quite large. A large buoy requires a large pick-up space in the vessel. A large pick-up space has implications for the support structure of the vessel and therefore can not be placed just anywhere in the vessel. Thus the pick-up space has been located at the bow, which is already built to take up large stresses and strains. 
   However, all the above factors will impose certain limitations on the design of both the buoy and the vessel. The present invention aims to eliminate or at least bring about a substantial reduction in these limitations. 
   BRIEF SUMMARY OF THE INVENTION 
   This invention goes to the extreme measure of completely removing the mooring of the buoy to the seabed, thus leaving the buoy to be suspended from the risers only. Eliminating the need for the buoy to take up mooring forces allows it to be dimensioned only to support the risers. In turn, the risers are not subjected to any significant stress from the buoy, due to the small size and weight of this. 
   Thus the invention provides a light buoy of relatively small dimensions. 
   According to the invention there is provided a device for loading and/or unloading of flowable media, comprising a buoy which is connected to at least one riser and a vessel provided with a pick-up space for the buoy, which space ends at the bottom of the vessel, the buoy being equipped with buoyancy means and comprising a first part connected to the at least one riser and a second part which is interrotatable with the first part, the second part being arranged for locking to the vessel, the buoy comprising coupling means for coupling the at least one riser to a swivel for transfer of the flowable medium. The invention is characterized in that, when connected to the buoy, the vessel is kept in position by a dynamic positioning system. 
   As the buoy of the invention can not be used to moor the vessel to the seabed, other means must be employed to keep the vessel at the correct position. This is achieved by use of so-called dynamic positioning (DP). DP has been used since 1960 to position floating facilities, e.g. for oil and gas production. 
   In a DP system, a navigational system such as GPS is connected to a controller that monitors deviations in vessel position from the desired position. The controller is further connected to several thrusters. The thrusters return the vessel to the desired position if the deviation exceeds a certain value, and will also maintain a correct orientation of the vessel relative to wind and current. 
   With the dimensions of the buoy being smaller than for existing buoys, the dimensions of the pick-up space in the vessel can also be reduced significantly. Due to the reduced weight of the buoy, the buoy support arrangements onboard the vessel can also made much simpler. This saves space onboard the vessel and means that the pick-up space can be located where most appropriate, e.g. amidships. 
   In a preferred embodiment the first part of the buoy comprises most of the buoyancy means and is of a considerably larger volume than the second part of the buoy. Thus the buoyancy is provided by another part than that which is to be attached to the vessel. This means that the part which is to be attached to the vessel can be relatively small, and its support can be made simpler. 
   Connecting the buoy according to a preferred embodiment with the seabed via only the at least one riser will completely avoid mooring of the buoy. 
   Arranging the upper part of the buoy to be kept dry in the pick-up space allows easy access for inspection and repairs without taking any special measures. 
   Securing the buoy to the vessel by means of only a downward facing shoulder allows the devices that hold the buoy to be of a simple design, and makes it possible to disconnect the buoy in a short time. Moreover, it simplifies the support of the part of the buoy which is attached to the vessel. 
   A substantially cylindrical moon pool with a supporting structure makes it possible to alter existing vessels without excessive interventions. 
   Having the moon pool open at the top and bottom allows for easy access for installation and maintenance. 
   Active rotation of the buoy with the means of active rotation connected to the directional control of the vessel makes it easy to maintain the geostationary part of the buoy at the correct orientation. 
   Arranging the pick-up space close to the centre of motion of the vessel avoids large movements in the buoy. 
   Arranging the buoy hanging deck close to the neutral axis of the vessel places it at the location of the lowest stress concentrations. 
   In a preferred embodiment the device comprises a buoy pick-up tool which comprises a shield arranged for placement over the upper end of the buoy, and which is provided with a guide funnel for a gripper. This allows safe gripping of the buoy without risk of damage. 
   A significant share of the weight of the buoy is carried by its buoyancy, thus reducing the strain on the locking arms securing it to the vessel. 
   The buoy comprises means of adjusting its buoyancy by charging it with air or water after the buoy has docked in the vessel, thus making it easy to adjust the buoyancy of the buoy to the weight which the buoy is to support. 
   The buoyancy tanks can be ballasted individually, allowing the buoyancy of the buoy to be adjusted relative to risers and variations in weight, fouling and stability, and thus the ballasting of the buoy can be optimized. 
   The buoy comprises a central buoyancy chamber and peripheral ballasting chambers, thus allowing optimization of the weight distribution. 
   Gathering the connections to all the risers in a multibore connector makes for quick and easy connection and disconnection. 
   Providing the device with means of flushing out the connector in the case of an emergency disconnect avoids discharges of pollutants. 
   All hydraulic operations in the moon pool are carried out from the deck of the vessel, and no electric or hydraulic control systems are required in the moon pool. This means that EX requirements are met without any special measures being introduced. 
   Descriptive terms hereinafter, such as lower and upper parts and similar terms, as applied to among other things the buoy and the pick-up space, refer to the orientation of these elements in the operative mode. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S) 
     The invention will now be explained in greater detail by means of an exemplary embodiment illustrated in the accompanying drawings, in which: 
       FIG. 1  is a sectional and perspective view of a currently preferred embodiment of a buoy according to the invention; 
       FIG. 2  is a top view of the buoy of  FIG. 1 ; 
       FIG. 3  is a sectional view of the buoy of  FIG. 1 ; 
       FIG. 4  shows a section B of the buoy of  FIG. 3 , 
       FIG. 5  shows a section C of the buoy of  FIG. 3 ; 
       FIG. 6  is a perspective view of a section of a vessel connected to a buoy according to the invention; 
       FIG. 7  shows a section of the vessel and the upper part of the buoy according to the invention; 
       FIG. 8  shows the buoy and a part of a vessel just before the buoy is to be picked up into the vessel; 
       FIG. 9  shows the buoy being hoisted up into the pick-up space in the vessel 
       FIG. 10  shows the buoy after it has been hoisted all the way into the pick-up space; 
       FIG. 11  shows the buoy locked in place in the pick-up space and about to be connected to a swivel; and 
       FIG. 12  shows the buoy connected up and ready for the loading/unloading operation. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Reference is first made to  FIGS. 1 ,  2  and  3 . In  FIG. 1 , the buoy is shown generally to comprise a lower buoyancy part  1  and an upper fixation part  11 . A central tube  2  extends through these, defining an inner cavity  3 . Through the inner cavity there extend several risers  6  which are connected to a transport line on the seabed. These risers  6  are held by a lower cover  7  and an upper cover  8  at the lower  4  and upper  5  ends of the buoy, respectively. The covers  7 ,  8  also serve to seal the cavity  3  against the seawater. Thus the cavity  3  can function as a buoyancy chamber. Optionally the cavity  3  can be filled with seawater. The cover  8  is provided with a water inlet/outlet  17  and an air inlet/outlet  18  to allow water and air to be pumped in. 
   The upper ends of the risers are provided with connecting pieces  9 . Shutoff valves (not shown), preferably automatic ones, are also provided here. 
   The buoyancy part  1  comprises several ballast chambers  10  which can be filled with air or water to adjust the buoyancy. These chambers  10  can also be connected to the inlets/outlets  17 ,  18 . The diameter of the buoyancy part is slightly larger at the lower end  19 . The purpose of this will be explained in detail below. 
   The upper fixation part has a frusto-conical shape with a conical face  12 . On this face there are provided impact elements  13  shaped as strips of a rubber material or another shock absorbing material. The fixation part  11  is rotatably arranged on the central tube  2 , and a slide bearing system is provided between the fixation part  11  and the central tube  2 , to be explained in more detail below. 
   The fixation part  11  may also include ballast chambers  16 ; however, these will be of a much smaller size than the ballast chambers  10  in the buoyancy part  1 . 
   The portion of central tube  2  extending through the fixation part  11  carries internal reinforcing ribs  20 . 
   At the upper end of the fixation part  11  there is provided a locking ring  21 . This has a downward facing shoulder  22  encircling the ring  21 . The locking ring  21  is used to lock the fixation part  11  to the vessel, as will be described hereinafter. 
   As can best be seen in  FIG. 4 , which shows a section B of  FIG. 3 , the locking ring  21  is provided with a guide/retaining ring  24  projecting into a groove  25  formed in a reinforced portion  26  which forms the connection between the central tube  2  and the cover  8 . Between the guide/retaining ring  24  and the groove  25  there is a slide bearing system  27  consisting of a lower thrust bearing  27   a , an upper thrust bearing  27   b  and an upper radial bearing  27   c . The periphery of the cover  8  has a gear rim  23 . The function of this gear rim  23  will be explained in more detail below. 
     FIG. 5  shows a section C at the lower end of the fixation part  11 , where it borders on the central tube  2 . Here is provided a lower thrust bearing in the form of a sliding ring  60  attached to the central tube  2  and a sliding ring  61  attached to the fixation part  11 . This area of the central tube  2  comprises a reinforced section. 
     FIG. 6  shows a section of a vessel  30  with a deck  33 . A circular opening  70  defines the upper end of the moon pool  31 . Inside the moon pool  31  there is provided a structure  34  constructed from tray rings  72  and braces  73 , and the inside of which defines a pick-up space  77 . The structure  34  has an upper face  35  which also serves as a hanging deck, on which there is provided a system of locking arms which is to be used to secure the buoy to the structure  34 . These locking arms  36  are known per se, and thus will not be explained in greater detail herein. 
   In the moon pool  31  there is also provided a system  74  of rails to hold a swivel  48 . The system  74  of rails generally consists of a pair of rails  75  and a carriage  76  which can be moved along the rails  75 . The carriage is provided with hydraulic actuators (not shown) arranged to raise and lower the swivel. 
     FIG. 7  is a side view of the upper part of the structure  34  and the swivel  48 . 
   Reference is now made to  FIG. 8 , where the buoy of  FIG. 1  is shown schematically, floating under the surface of the sea. Here, the buoy is equipped with a coupling member  28  comprising among other things a shoulder for landing the swivel  48 . The coupling member is a multibore connector which allows all the risers  6  to be connected to the swivel  48  through one connection. A vessel  30  is positioned over the buoy. This positioning is primarily achieved by means of a navigational system such as GPS, where the initial position of the buoy is known. Once the vessel has reached this position, a transponder system will be activated to allow the exact position of the buoy to be determined. 
   As previously mentioned, the vessel  30  includes a moon pool  31 . Preferably this extends from the bottom  32  of the vessel to the deck  33 , but it is possible to construct the moon pool only in the lower part of the vessel. 
   As previously mentioned, a structure  34  is provided in the moon pool  31 , the inside shape of which is largely complementary to the outside shape of the buoy. 
   A gripper  37  is lowered to bring the buoy up into the pick-up space  31 . This gripper may be a gripper of the Ballgrab® type, which is marketed by BSW Ltd. The gripper  37  is suspended from a wire  38  which is controlled by a winch  39 . The wire passes via a reversing pulley  44  and over a pulley  40  which is connected to a hydraulic heave compensator  41 . The wire  38  then passes over a pulley  42  arranged in a rack over the pick-up space  31 . 
   On the gripper there is provided a transponder which emits signals that a transponder on the buoy responds to. This allows the exact position of the buoy to be determined. The gripper may also be provided with a camera to allow the final alignment and guiding of the gripper into the buoy receiving port to be performed visually. Optionally, the connection can be made using only visual control. 
   A cover (not shown) may be lowered before or possibly simultaneously with the gripper and be placed over the coupling member  28  to ensure that the gripper contacts the buoy in the correct manner and to prevent it from causing damage to the buoy or the coupling member  28 . The cover is provided with a conical funnel which, upon correct placement of the cover, guides the gripper down into the correct receiving port. 
   When the gripper  37  has been connected to the buoy, the buoy may be hoisted up.  FIG. 9  shows the buoy about to be hoisted into the pick-up space. In the course of this hoisting the risers that extend to the seabed, and which are normally curved or S-shaped, will straighten out slightly. The risers are sufficiently flexible that this straightening and a certain amount of drift in the vessel or the buoy will be of no consequence. 
   In  FIG. 10  the buoy has come all the way into the pick-up space, and the conical face  12  of the fixation part  11 , with the impact elements  13 , abuts a downward facing complementary face  45  on the structure  34 . Here, the buoy is locked into place by guiding the locking arms  36  into the groove  22  in the locking ring  21 . 
   In this position, the upper end of the buoy is above the surface of the sea. The water surface will be between a lower level  46  and an upper level  47 , depending the vessel loading. Thus the connecting pieces  9  for the risers  6  and the locking ring  21  are dry, making it easy to inspect and repair these parts. It is also quite easy to connect up air and water hoses to the inlet/outlet  17 ,  18  of the buoy, for adjusting the buoyancy of the buoy. Sealing elements (not shown) are provided on the buoy or on the walls of the pick-up space  77  to avoid seawater splashing up between the buoy and the walls of the pick-up space  77 . 
   The buoy is suspended only from the locking ring  21 . However, the outer diameter of the lower portion  19  of the buoy is matched to the diameter of the pick-up space, forming a lower point of support at an overlap area between the lower portion  19  of the buoy and a cylindrical part  78  of the pick-up space  77 . The clearance between the rest of the buoy and the cylindrical portion  78  of the pick-up space  77  is designed such that fouling on the buoy will be scraped off to a certain extent when the buoyancy part  1  of the buoy rotates in the pick-up space  77 , but also such that the buoy will move easily into the pick-up space. 
   When the buoy is locked to the structure  34  the gripper  37  may be released. A swivel  48  is connected up in its place, as shown in  FIG. 11 . The swivel is guided onto the rails  75  and lowered onto the coupling member  28  by means of hydraulic actuators. One or more loading/unloading lines (not shown) are connected to the swivel  48 . The swivel  48  is attached to the upper part of the buoy by a system of gripping arms  49  which are arranged to grip around a shoulder on the buoy coupling member  28 . The swivel generally consists of two parts, a lower part which is stationary relative to the buoy coupling member  28  and an upper part which is stationary relative to the vessel. The construction of such swivels is generally known to a person of skill and will not be explained in greater detail herein. However, in this particular case the buoy coupling member  28  and the swivel  48  are multibore, allowing all the risers  6  to be connected to the swivel  48  and the loading/unloading lines on the vessel  30  through one connection. Both the swivel  48  and the coupling member  28  are provided with valves that can be closed automatically to allow a quick disconnect in an emergency. 
   In addition to the swivel  48 , a plurality of (preferably three) rotary motors  50  are also brought into engagement with the gear rim  23  on the buoy. These motors  50 , which are rigidly mounted to the vessel  30 , will actively rotate the buoyancy part  1  of the buoy relative to the fixation part  11 , in dependence on the rotation of the vessel. The rotation of the vessel may be detected by means of GPS. This avoids twisting of the risers extending to the seabed. 
   In  FIG. 12  the swivel  48  is coupled to the buoy coupling member  28  and is ready for loading or unloading. 
   The present buoy allows retrofitting of more risers as the oil/gas field is developed. New risers may be pulled into the buoy while it is connected to the vessel  30 . 
   If an emergency situation arises when loading or unloading via the buoy, where the situation demands that the vessel must quickly leave the buoy, the device of the present invention makes it possible to perform a controlled disconnect in about a minute. As the swivel is connected to the buoy via a multibore connector and the coupling is held in place by a set of co-operating gripping arms  49 , this coupling can be released in a matter of seconds. Both the swivel  48  and the buoy coupling member  28  are equipped with isolating valves that will close immediately in the case of a rapid drop in pressure. Once the valves have closed it is possible to flush out the connector  28  to prevent any oil being discharged into the sea. The washwater can be collected by collecting means on the face  35 . After that the locking arms  36  release the buoy, which quickly falls into the sea. 
   A clump weight is suspended from the buoy in a rope, so as to make the buoy sink quickly but without subjecting the buoy to excessive strain after reaching the desired immersion depth. As long as the clump weight is above the seabed it will exert a force on the buoy. This force or weight will cause the buoy to sink rapidly away from the vessel. When the clump weight reaches the seabed this weight will be relieved and the buoy is kept floating by the carefully ballasted buoyancy. 
   When releasing the buoy in the normal manner, a cover will be installed over the coupling member  28  following the disconnection of the swivel  48 , to protect it against damage and fouling.