Abstract:
The transfer system includes two generally vertically oriented duct sections which are placed at an angle with the vertical. These two sections are connected to a substantially horizontal third member, for instance a third duct section. Near the connection points of the vertically oriented duct sections and the horizontal member, a tensioning weight is provided such that a tensioning force in the horizontal duct section is created. Hereby bending/kinking and/or buckling due to currents or floating systems dynamics is reduced. A relatively long horizontal duct section can be used which is preferably made of hard pipe, having a reduced swing.

Description:
FIELD OF THE INVENTION 
     The invention relates to a transfer system for transfer of fluids from a first floating or fixed structure to a second floating structure, the transfer system comprising a first and second duct section connected to the first and second sores respectively, and a substantially horizontal, submerged, third duct section interconnecting the first and second duct sections. 
     BACKGROUND OF THE INVENTION 
     It is known to connect two floating offshore structures via a transfer duct system for conveying hydrocarbons from one structure to the other. One floating structure may be a production or storage structure such as a spar buoy, a semi-submersible structure, a fixed tower or a mooring buoy whereas the second structure may comprise a floating production storage and offloading vessel (FPSO), a shuttle tanker and the like. Such a system is described in Dutch patent application NL-A-8701849. In the known configuration, a production platform is anchored to the seabed via radial taut mooring lines, the platform being connected to a subsea well head via a riser. The production platform is connected to a mooring buoy via flexible duct sections. The duct sections are anchored to the seabed via tethers. The mooring buoy is connected to the seabed via a cable carrying at the end thereof a clump weight. The clump weight is anchored to the seabed via an anchor chain. The mooring buoy can freely drift within an area that is defined, by the length of the anchor chain between the clump weight and the sea bed. The taker that is moored to the buoy can weathervane around the buoy and is subject to drift in accordance with prevailing wind and current conditions. 
     From U.S. Pat. No. 4,339,002 a discharge manifold system is known wherein a flexible conduit extends vertically downwards from a production platform to below waterlevel, continues horizontally and extends vertically upward towards a mooring buoy which is anchored to the seabed. 
     The known systems have as a disadvantage that the duct sections may be subjected to bending/kinking or buckling due to currents which may displace the system sideways. In view of the connection of the shuttle tanker to the freely moving mooring buoy, the influence of the floating system dynamics on the transfer ducts, is limited but the system is relatively complex in view of the additional mooring buoy being required. Furthermore, in view of the freedom of movement of the tanker, there is a risk of the tanker damaging the transfer pipes. 
     An alterative option to connect two floating structures is to run the transfer pipes down to the seabed and back up in order to avoid curt and floating system-induced forces. Such a system however is not practical in deep water, for instance at depths of 1000 meters below sea level or more. 
     SUMMARY OF THE INVENTION 
     It is therefore ea object of the present invention to provide a transfer system in which the bending or buckling due to currents and floating system dynamics is reduced and which has a relatively small swing. It is another object of the present invention to provide a transfer system which can bridge a large distance between the interconnected structures. It is a further object of the present invention to provide a transfer system which can be produced in an economic manner. 
     Hereto the transfer system according to the present invention is characterized in that the horizontal member is near its ends provided with tensioning members oriented in a substantially vertical direction at least one tensioning member being inclined at an angle (α) with respect to the vertical, a tensioning weight being connected at or near the ends of the horizontal member for providing a tensioning force on the third duct section. 
     Because of the inclination of at least one of the vertically positioned tensioning members, the ballast weight exerts a horizontal component on the substantially horizontal third duct section. Hereby it is kept from bending or buckling and has a reduced swing due to the restoring force created by the counterweight when it is offset from its equilibrium position. Furthermore, the system according to the present invention does not require additional mooring constructions and allows to use relatively long, substantially horizontal duct section, having a length of for instance 3000 meters. 
     With “substantially horizontal” it is meant that the third duct section does not make a larger angle with the horizontal than at most 45°. 
     According to the invention it is possible to either integrate the tensioning member in either one of the first or second duct sections or embodying the tensioning member as a separate article. 
     In the first embodiment because of the tension, the related first or second duct section will generally extend according to a straight line. In the second embodiment the first and/or second duct section can have any shape. 
     This is dependent from its length relative to the length of the tensioning member as well as its weight. For example the related first or second duct section can comprise three parts, one substantial vertical part and other substantial horizontal part connected by a transitional part. 
     In one embodiment both first and second tensioning members are inclined with respect to the vertical, a tensioning weight being provided at or near each connecting point of the first and second duct sections with the third duct section. By using two tensioning weights, one at each end of the horizontal duct section, an even tension force can be applied on the horizontal duct section. 
     Preferably the first and second duct sections and/or tensioning members are attached to the third duct section via an articulation joint, such as for instance a flex joint or a pivoting joint. In one embodiment the duct sections are made of hard pipe which allows for a relatively economic manufacture. The use of hard pipe in this case is possible as the bending and buckling in the present system is reduced due to the tensioning effect of the weights. When hard pipe is used, the system of the present invention may be used in relatively large water depths such as 100-150 meters below sea level and deeper. It is possible to use however a combination of hard and flexible duct sections. Multiple transfer systems of the present invention may extend in a radial manner from a single floating structure, such as the spar buoy, to respective FPSO-tankers or buoys for export. The buoyancy of the tensioning weights may be adjustable for instance by ballasting the counter weights wit water or deballasting using compressed air. Additional weight could also be added or removed. The third duct section may be provided with buoyancy such as to have a neutral or even positive buoyancy in water. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Embodiments of the transfer system according to the present invention will, by way of example, be described in detail with reference to the accompanying drawings. In the drawings: 
     FIG. 1 shows a side view of the transfer system according to a first embodiment of the present invention; 
     FIG. 2 shows a top view of the system of FIG. 1 in the absence of a sideways current: 
     FIG. 3 shows a top view of the system of FIG. 1 wherein the horizontal duct section is displaced by a sideways current and 
     FIG. 4 shows schematically the side view of a fiber transfer system according to a further embodiment of the present invention. 
     FIG. 5 shows an embodiment wherein the horizontal duct section is connected to a floatation member, and 
     FIG. 6 shows an alternative tensioning construction. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 shows a mid-depth transfer system  1  according to the present invention connecting a spar buoy  2  to a floating production storage and offloading (FPSO) vessel  3 . 
     The spar buoy  2  is anchored to the seabed  4  via anchor lines  5 . One or more risers  6  connect the spar buoy to a subsea hydrocarbon well. The vessel  3  comprises a geostationary turret  7 . The turret  7  is via chain table, which extends near keel level of the vessel  3 , connected to the seabed  4  via mooring lines  8 . The vessel  3  can weathervane around the turret  7 . 
     From the production tree at deck level of the spar buoy  2 , one or more pipes  9  extend, for instance via a guide  10  at the outer perimeter of the spar buoy, to an inclined duct section  11 . The inclined duct section  11  is connected to a horizontal duct section  12  which at its other end is connected to a second inclined duct section  13 . The inclined duct section  13  is connected to the turret  7  of the vessel  32 . The inclined duct sections  11 ,  13  are connected to the spar buoy  2  and the vessel  3  respectively via flexible joints  21 ,  22 . 
     The horizontal duct section  12  is connected to the inclined duct sections  11 , 13  via pivot joints or flexible joints  14 , 15 . At or near the joints  14 , 15  tensioning weights  16 , 17  are attached via cables  18 , 19 . The tensioning force exerted by each weight  16 , 17  is proportional to sin α, wherein α equals the angle of inclination of the substantial vertical duct sections  11 , 13 . Although it is shown in FIG. 1 that the angles α of the duct sections  11 , 13  are equal, tis is not necessary and different inclinations may be used when differing weights  16 , 17  are used. Furthermore, it is not necessary that the duct section  12  is exactly horizontal but it may be offset from the horizontal. The horizontal duct section  12  may be located from a few meters, up to 150 meters or more below sea level  20 . 
     The angle of inclination a may for instance be about 30°. The height H 1  between the flexible joints  21 , 22  and the attachment point of the weights may be fox instance 115 meters. The horizontal distance between the flexible joints  21 , 22  may be about 2173 meters whereas the length of the horizontal duct section  12  may be about 2000 meters. The length of each inclined duct scion  11 ,  13  is about 173 meters. The weight of each tensioning weight  16 ,  17  can be for instance 100 t. The diameter of the ducts  11 ,  12  and  13  may be for hard pipe for instance 0,5 meter. 
     As the dynamic motions of floating vessels during storms can be large, the vertical motion transferred to duct  12  by way of duct  11  or  13  may cause unacceptable bending stresses near the ends of duct  12 . To alleviate this bending, an additional articulated pivot or flex joint  20 ,  21  may be installed perhaps 10 to 100 m from the flexible joints  14 ,  15 . 
     As shown in FIG. 2, in the absence of sideways current all duct sections  11 ,  12  and  13  will extend along a substantially straight line. 
     Due to a sideways current in the direction of the arrow c, as shown in FIG. 3, the horizontal duct section  12  is somewhat displaced and the distance L between the two tensioning weights  16 ,  17  is decreased compared to the distance L in the absence of a current, which has been indicated with the dashed lines in FIG.  3 . Hereby the horizontal duct section  12  will assume a curved or bend shape. The distance L of the section  12  can for instance be between 1000 and 10.000 meters. 
     As the tensioning weights  16 ,  17  exert a tensional force on the horizontal duct section  12 , the amount of buckling remains limited. Furthermore, the excursion of the horizontal duct section from its straight position will be limited due to the additional tensional restoring force of the tensioning weights  16 ,  17  when they are placed in their offset position, as shown in FIG.  3 . For the distance L of 2173 meters, the amount of sideways deflection B may be about 300 meters at a sideways current of about 1 m/s. In this case the angle of inclination a will increase from 30° to about 35°. The horizontal tensioning forces in the horizontal duct section  12  amount to about 52 tons whereas the vertically directed component of the tensioning weight  16 ,  17  amounts to about 31 t. 
     FIG. 4 shows a further embodiment of the invention wherein the mid-depth transfer system is referred to by  31  and connects two vessels  32  and  33 . From the production tree at deck level of vessel  32  one or more pipes [ 39 ] extend to a duct section  41 . This duct section  41  is connected to a horizontal duct section  42  comprising a long multiple pipe bundle which can either be rigid or flexible. This horizontal duct section  42  is at its other end connected to a second duct section  43  being connected to vessel  33 . From vessel  32  and  433  tension members  34 ,  35  extend to a connection  36 ,  37  on the third duct section. These tension members can comprise a chain cable or any other tension member known in the art. The long multiple pipe bundle  42  is provided with further connections  38 ,  39  to which weights  46  and  47  are connected. 
     Each of the first and second duct sections is no longer tensioned as in the embodiments according to FIGS. 1-3 wherein the tension member is integrated in the first and second duct section. Because of that the first and second duct section will have the shape as shown, i.e. comprising a first substantially vertical part and a third substantial horizontal part connecting to the third duct section. Inbetween is a transisitional part. 
     The person skilled in the art will understand that al alternatives given with regard to the embodiment discussed relating to FIGS. 1-3 can be introduced in the embodiment of FIG.  4  and vice versa. Furthermore further changes of the structure discussed above are possible being obvious for the person skilled in the art without leaving the scope of protection which is conferred by the appended claims. 
     In the embodiment of FIG. 5, the horizontal, third member comprises an elongated buoyancy element  50 , with several chambers which is tensioned by tension members  34 ,  35  and weights  46 ,  47 . The buoyancy element  50  serves as a structional support for the ducts  42 . 
     In the embodiment of FIG. 6 the tension members  34 ,  35  comprise polyester cables, attached to the seabed. Cable  35  is connected to a winch  51  and the vessel  33 .