Abstract:
The invention relates to a loading system for transfer of hydrocarbons between an installation on the sea bed ( 16 ) and a floating vessel ( 10 ) in areas exposed to drifting ice. The system comprises a submerged turret buoy ( 19 ), a flexible riser ( 18 ) extending from the sea bed installation ( 24 ) to the buoy ( 19 ) intended to be securely connected to a corresponding pipe on board the vessel ( 10 ), and a plurality of mooring lines ( 17 ) connected to the buoy ( 19 ) and extending outwards therefrom. The system further comprises: protective means ( 20 ) for protecting the riser ( 18 ) from impacts when the riser ( 18 ) is in an extended, load transferring mode, and a protective structure ( 24 ) located in or on the sea bed ( 16 ) for protection of the riser ( 18 ) when in a retracted position in a non-operative mode; that the protective structure ( 24 ) containing means ( 28 ) for storing the riser ( 18 ) in a protected position when disconnected and retracted from the vessel ( 10 ). The invention also relates to a method for mooring a vessel to a submerged turret buoy and a method for installing a riser protection means.

Description:
[0001]     The present invention relates to a submerged turret loading system (STL-system). More particularly, the present invention relates to a loading system for transfer of hydrocarbons from an installation on the sea bed to a floating vessel in areas exposed to drifting ice. Further, the invention relates to a method for mooring a vessel to a submerged turret buoy in ice infested waters, and to a method for installing a riser protection system in such waters. It is emphasized that the invention also can be used in areas where other types of dangers could harm a system for transfer of hydrocarbons, e.g. in the case of trawling or drifting timber.  
       BACKGROUND FOR THE INVENTION  
       [0002]     Oil exploration has moved into arctic waters. Motion of drifting ice is often a crucial problem when designing and planning an off-take loading and mooring system in ice infested waters. It is imperative to design systems and methods which eliminate the risks for pollution, caused by damage to the equipment due to impacts from the drifting ice.  
         [0003]     The drifting motion of ice is mainly governed by wind, waves, ocean currents and tidal forces. From analyses for the Eastern Barents Sea, it has been found that on a large time scale the ice drifting motion is clearly stochastic and with the exception of periods with rather straight lined movement, it resembles Brownian motion. Since ice floes are generally large and heavy, the direction and absolute value of their speed cannot change momentarily. Models predict steady motion of the ice, but occasionally the direction of the ice drift may change to the opposite direction in roughly half an hour. This is a major concern for the conventional loading concept where the tanker, say 90 000DWT, is staying in the “wake” behind a platform or a tower extending up above the sea level. If using a sub sea loading concept instead in waters subjected to drifting ice, allowing the tanker to ‘ice-vane’, advantages may be achieved.  
         [0004]     In ice-infested waters, however, bottom installations might be damaged by deep ice formations (ice ridges in the Pechora Sea, icebergs in some other places).  
         [0005]     Tests executed in 1997 and 2000 at the Hamburg Ship Model Basin (HSVA), Germany, testing the Submerged Turret Loading system, STL, in frozen seas, showed that under-keel installations will be in contact with ice as soon as the ice conditions worsen (interactions with ice ridges). Hence, the riser has to be protected from this hazard.  
       PRIOR ART  
       [0006]     US Patent Specification No. 5,820,429 describes an arrangement of a loading/unloading buoy for use in shallow waters wherein a buoy is arranged for introduction and releasable securement in a downwardly open receiving space in a floating vessel. The buoy comprises a bottom anchored centre member for the passage of fluid from or to a transfer line which is coupled to the underside of the center member. The buoy further comprises an outer member which is rotatably mounted on the center member to allow turning of the vessel about the center member when the outer member is secured in the receiving space. The buoy is provided with a bottom support structure which is connected to the center member of the buoy and arranged for support of the buoy at the sea bed when not in use. To the center member of the buoy there are connected a number of mooring lines extending outwards from the buoy a substantial distance along the sea bed. Such a system has an inherent elasticity allowing raising of the buoy from the sea.  
       SUMMARY OF THE INVENTION  
       [0007]     The object of the invention is to further develop a loading system which may operate safely in ice infested waters where the submerged turret system, including the riser may be stored in a completely sheltered manner when the system is not in use.  
         [0008]     A further object is to provide a protection system for the riser extending between the sea bed and the vessel.  
         [0009]     A still further object of the present invention is to provide a method for mooring a vessel to a submerged turret buoy in ice infested waters.  
         [0010]     A further object of the present invention is to achieve a system wherein the loading system may quickly be retracted to a completely protected position where the riser will not be exposed to impact by the drifting ice. Correspondingly, it is an object to achieve a loading system where the loading operation may be quickly aborted and the moored tanker may be quickly released from the mooring system.  
         [0011]     According to the present invention the objects are achieved by means of a loading system and methods as further defined in the claims. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]     The invention will be further described in detail below in connection with an exemplary embodiment with reference to the drawings, wherein:  
         [0013]      FIG. 1  shows modelled movement of the ice drift;  
         [0014]      FIG. 2  shows a typical prior art loading system;  
         [0015]      FIG. 3  shows the loading system according to the invention wherein the riser is connected to a vessel;  
         [0016]      FIG. 4  shows details of the riser protection means;  
         [0017]      FIG. 5  shows the loading system in a retracted, idle position on the sea bed;  
         [0018]      FIG. 6  shows the riser protection in the process of being lifted up from its retracted position towards the vessel; and  
         [0019]      FIG. 7  shows one step in the process of installing parts of the loading system on the sea bed. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0020]      FIG. 1  shows modelled movements of the ice drift. The increment between each dot on the graph represents a time lapse of 10 minutes. The Figure gives an impression of the movement during a 24-hour period. As indicated in the graph, the model predicts steady motion of the ice. Occasionally, however, the ice drift may change to the opposite direction in roughly half an hour. This is a major concern for the conventional loading concept where the tanker, say 90 000DWT, is staying in the “wake” behind a platform or a tower extending up above the sea level, as shown in  FIG. 2 .  
         [0021]     In  FIG. 2 a  tanker vessel  10  is moored to a platform  11  and fluids are transferred from the platform  11  to the vessel  10  through a flexible hose  12 . The flexible hose  12  is suspended from a rotatably arranged loading arm  13 . Since the vessel is only moored to the platform, the possibility of collision between the vessel  10  and the platform  11  is large if and when the drifting direction of the ice changes abruptly. In such case, the loading operation must stop immediately and the tanker  10  must quickly be released from its mooring system.  
         [0022]     In order to overcome such problems, a sub sea loading concept is required, reducing possible interference with the drifting ice, and still allowing the tanker  10  to ‘ice-vane’ depending on the movement of the drift ice.  
         [0023]      FIG. 3  shows in principle a preferred embodiment of a loading system according to the invention. As shown in  FIG. 3 , a vessel  10  is floating on the sea surface. The vessel  10  is equipped with a moon pool  15  and is rotatably moored to the sea bed  16  by means of a plurality of mooring lines  17 . A flexible riser  18  extends between the sea bed  16  and the vessel  10 . At its upper end the riser  18  is connected to a submerged turret buoy  19 . The mooring lines  17  are coupled to the submerged turret buoy  19 , allowing the vessel to weather vane. Such turret buoy may be of a type as is further detailed in the applicants US Patent Specification No. 5,820,429, the content of which hereby is included by reference. The upper end of the riser  18  is releasable connected to a corresponding pipe line onboard the vessel by means of a swivel joint (not shown).  
         [0024]     According to the invention, the riser  18  is protected by a riser protection means  20 . According to the embodiment shown in  FIG. 3  the upper end of the riser protection means  20  is suspended from the submerged turret buoy  19  by means of a plurality of chains, wires or the like  21 . The lower end of the riser protection means  20  is coupled to a riser socket  22 . According to a preferred embodiment of the invention the riser protection means  20  comprises a plurality of hollow, upwardly truncated conical elements  23 , having a smaller upper diameter and a larger lower diameter or vice versa.  
         [0025]     The loading system according to the invention comprises further a sea bed installation  24 . According to a preferred embodiment of the invention the sea bed installation  24  is formed by a silo that will store and protect the riser  18  and the riser protection means  20  when the loading system not is in use. The silo  24  is dug into the sea bed  16 , a top slab  25  of which being more or less flush with the sea bed  16 . Hence, a very small part of the system is exposed on the sea bed when the loading system is retracted to its protected position, ref.  FIG. 5 .  
         [0026]     The silo comprises two main parts; a cell  26  and a main chamber  27 . A riser reel  28  is located in the chamber  27 . The reel  28  rotates around a horizontal axis (not shown) and at least the lower end of the riser is reeled on to the reel  28 . The reel  28  may for example be driven by motor or the like. The lower end of the riser  18  is coupled to a pipeline  29  from an oil well or the like. The coupling between the pipeline  29  and the lower end of the riser  18  is provided with a swivel of any conventional type, allowing relative rotation between the pipeline  29  and the riser reel  28 .  
         [0027]     The top slab  25  may according to an embodiment of the invention be provided with an opening  30  having a shape and a size adapted to the shape and size of the riser socket  22 . The top slab  25 , at least when used in shallow waters may be equipped with a manhole  31 , allowing access for light maintenance.  
         [0028]     A vertical slot  33  is provided in the lower part of a wall  32  dividing the cell  26  and the chamber  27 . The height of the slot  33  exceeds the maximum expected heave amplitude of the vessel  10 . The width of the slot  33  exceeds the diameter of the riser  18 .  
         [0029]     In order to minimize soil intrusion into the silo  24  when the loading system is connected to the vessel, flexible deflectors  34  are arranged over the opening  30  for the riser  18  and its protection means  20 . Some yearly light maintenance can be performed to remove the soil deposited at the bottom of the silo. The system can also be modified to be soil intrusion proof, if needed.  
         [0030]      FIG. 4  shows the parts of the riser protection means  20 . As shown on the figure the protection means comprises a plurality of hollow, truncated, conical elements  35 . Each element is open ended at both ends. The elements  35  are suspended to each other by means of chains or wires  21 . The riser extends through the set of elements  35 .  
         [0031]     Such riser protection means  20  will resist dragging and impact loads from ice passing under the keel of the vessel. The design of the elements  35  in the riser protection means  20 , (ref.  FIGS. 3 and 4 ) will give the required bending capabilities due to suspended, separate elements, and will protect the riser from excessive bending.  
         [0032]     Since the elements  35  are suspended from each other, the elements  35 , when the riser protection means  29  is lowered, will be stacked into each other. This allows the riser protection means  20  always to have an adequate length. When the vessel is in its mean position, some elements  35  are stacked at the bottom of the riser protection means  20 , on top of the silo  24 . Consequently, the total length of the riser protection means  20  will be sufficiently long to follow the heave of the vessel  10 .  
         [0033]     The elements  35  are suspended independently of the riser  18 . The riser  18  will thus heave with the vessel  10  and is free to slide within the lower elements  35 .  
         [0034]     A possible design for the elements  35  is presented in  FIG. 4 . This design may be varied without deviating from the inventive idea and only shown to give an idea of the function of the elements  35 . On the drawing, chains  21  are used to link the elements  35 . It should be appreciated however, that wires or other type of links may be used. The drawing suggests further that four chains  21  are used to link the elements  35 . It should be appreciated that the number of chains may be varied, as for example three chains may be suitable.  
         [0035]     As further shown in  FIG. 4  the lower rim  36  of each element  35  may be provided with a stacking ridge  37  which also includes attachment eyes  38  for the chains  21 .  
         [0036]     Further,  FIG. 4  shows a schematic view of the riser socket  22 . As shown in the Figure the riser socket  22  is provided with locking means  39  intended to interact with corresponding recesses in the top slab  25 , thereby interlocking the top slab  25  and the riser socket  22  when in operational mode.  
         [0037]      FIG. 5  shows the riser protection means  20  in a retracted position, the riser protection means  20  being in an in-active position within the cell  26  in the silo  24 . Here the submerged turret buoy  19  is resting on the top slab  25 , while the riser socket  22  is released from its engagement with the top slab  25 , resting on a particularly adapted support  40  at the lower end of the cell  26 . In this position the elements  35  are stacked on top of each other, while substantially the entire length of the riser  18  is reeled on the reel  28  in the chamber  27 . As further shown on  FIG. 5  the sag bend of the riser  28  extends below the lower end of the slot  33 . The mooring lines  17  rest freely on the sea bed  16 .  
         [0038]      FIG. 6  shows the loading system in the process of being lifted up towards the vessel  10  by means of a wire  41 . As shown, the submerged turret buoy  19  is lifted off the top slab  25  and the riser socket  22  is in a locked position in the top slab  25 . The riser  18  is fed out from the riser reel  28  as the submerged turret buoy  19  is lifted upwards.  
         [0039]      FIG. 7  shows one stage in the installation process of the loading system according to the invention. Firstly, the silo  24  is installed on the sea bed  16 . Alternatively, the silo  24  may be embedded into the sea bed  16  as shown in  FIG. 7 . Mooring lines  17  may be preinstalled, resting on the sea bed  16 . A preferably prefabricated unit comprising the top slab  25  for the silo  24 , the reel  28  with the riser  18  and the riser protection means  20 , including the submerged turret buoy is lowered down onto the silo  24  and secured to silo  24 . Thereafter the swivel link at the lower end of the riser  18  is connected to the pipe line  29 . Further, the mooring lines  17  are attached to the submerged turret buoy  19 .  
         [0040]     The system operates in the following way:  
         [0041]     At first the elements  23  are stored in a stacked configuration in the cell  26  of the silo  24 . The vessel  10  comes into position over the silo  24  and connects to the system, ref. the situation shown in  FIG. 5 . It first lifts the buoy and the riser socket  22  off its lower support  40  and then lifts out the whole riser protection means  20  to a position as shown in  FIG. 6 . The riser socket  22  (details shown in  FIG. 4 ) is then fastened to the top slab  25  of the silo  24 , engaging the locking means  39  on the riser socket  22  with corresponding means on the top slab  25 . During this first lifting operation, the reel  28  is not rotated; the slack in the riser being sufficient to provide the required length. The vessel  10  then pulls the submerged turret buoy  19  upwardly into contact and locked engagement with the moon pool  15  on the vessel  10  ( FIG. 3 ). During this phase, the riser  18  is unreeled to a position where the slack in the riser  18  is sufficient to compensate for the heave of the vessel  10 . For this purpose a vertical slot  33  is provided in wall of the cell  26 , adjacent the reel  28 , allowing the riser  18  to move up and down. In  FIG. 3  two extreme positions of the riser  18  are shown by dotted lines. When the system is connected to the vessel  10 , the reel  28  is not intended to rotate, and consequently does not have to feed out or pull in the riser  18  to follow dynamically the motions of the vessel  10 .  
         [0042]     For the disconnection phase, the operations are the same in a reverse manner. The system can be designed as “self storable”. In case of an emergency disconnection, the whole system may retract automatically into the silo.  
         [0043]     For installation and for heavy maintenance, the top slab  25  may be unlocked from the silo and lifted up onboard a barge, vessel or the like.  
         [0044]     An important advantage of this system is its ability to operate in any ice condition. As long as the vessel  10  and the mooring can withstand the incoming sea ice, so will the riser  18 , as it is at least partly protected under the vessel  10 . The vertical elasticity of the system makes it able to cope with quite heavy seas. This loading system will thus have a very high operability rate.  
         [0045]     This transfer system is independent of the methods used for connection to the vessel  10 . It is very suitable for the STL system for example, but may also be employed in other systems. It could for example be adapted to be used as a Single Anchor Mooring loading system for light ice infested waters or waters where, for example, use of heavy trawl boards is taking place.  
         [0046]     The loading system according to the invention may be installed in different water depths, from very shallow waters (such as for example 20 m or less as met offshore Sakhalin in the Pechora Sea and the Northern Caspian Sea) to deeper water. For deeper waters, the riser protection means  20  does not need to cover the riser  18  along its entire length, only the upper part which may be subjected to ice loads. Limiting the riser protection means  20  to cover only the upper part of the riser  18  will allow the system still to be compact when stored on the sea bed  16 .