Flexible riser system

Abstract

Riser tower system connecting a fixed subsea installation to a floating surface unit, comprising at least one flexible pipe arranged in a first catenary and extending between the surface installation and a submerged buoy; at least one riser arranged in a second catenary between the buoy and the seabed, the buoy being anchored to the seabed by a tether device comprising at least two taut tethers, at least two moorings in a catenary and on which there are provided a return device which exerts on the buoy a return force that depends on the lateral movement of the buoy. It has oil industry applications for offshore operation.

1 - 10 . (canceled)  
   
   
       11 . A riser system for connecting a fixed subsea installation to a floating surface unit, comprising: 
 a submerged buoy;    at least one flexible pipe arranged in a first catenary and extending between the surface installation and the submerged buoy;    at least one riser also arranged in a respective second catenary and extending between the buoy and the seabed;    a tether device anchoring the buoy to the seabed, the tether device comprising at least two taut tethers between the buoy and the seabed;    at least two moorings attached to the buoy and each in a third catenary; a respective return device on each third catenary positioned to exert a return force on the buoy that is dependent on the lateral movement of the buoy with respect to the seabed.    
   
   
       12 . A riser system according to  claim 11 , wherein each mooring has a lower portion toward the seabed, and 
 the return device comprises at least one ballast weight arranged on the lower portion of each mooring.    
   
   
       13 . A riser system according to  claim 12 , wherein 
 each mooring catenary has an end at the seabed and has a touchdown point spaced from the end at which the mooring touches down on the seabed when the buoy is not displaced from a first lateral position; and    the ballast weight on the mooring is distributed on each side of the mooring from the touchdown point.    
   
   
       14 . A riser system according to  claim 13 , wherein a part of the ballast weight is situated above the touchdown point of the mooring and provides the return force on the buoy.  
   
   
       15 . A riser system according to  claim 11 , further comprising a bridle connecting each mooring to the buoy, the bridle being arranged under the buoy and the bridle is positioned for preventing the buoy from rotating.  
   
   
       16 . A riser system according to  claim 11 , wherein the taut tethers lie approximately in the mid-plane passing through a longitudinal axis of the buoy.  
   
   
       17 . A riser system according to  claim 11 , wherein the buoy is of variable buoyancy.  
   
   
       18 . A riser system according to  claim 17 , wherein the buoy is comprised of several parts each selectively operable to be buoyant, whereby the buoy is of variable buoyancy.  
   
   
       19 . A riser system according to  claim 12 , wherein the buoy has opposite lateral ends; and 
 each of the moorings is connected to one lateral end of the buoy and the riser is connected to the other lateral end.    
   
   
       20 . A riser system according to  claim 12 , wherein each mooring is connected to the submerged buoy at a point lying approximately on the midplane of the buoy.  
   
   
       21 . A riser system for connecting a fixed subsea installation to a floating surface unit, comprising: 
 a submerged buoy;    at least one flexible pipe arranged in a first catenary and extending between the surface installation and the submerged buoy;    at least one riser also arranged in a respective second catenary and extending between the buoy and the seabed;    a tether device anchoring the buoy to the seabed;    a mooring attached to the buoy and in a third catenary; a respective return device on the third catenary positioned to exert a return force on the buoy that is dependent on the lateral movement of the buoy with respect to the seabed.    
   
   
       22 . A riser system according to  claim 21 , wherein the mooring has a lower portion toward the seabed, and 
 the return device comprises at least one ballast weight arranged on the lower portion of the mooring.    
   
   
       23 . A riser system according to  claim 22 , wherein 
 the mooring catenary has an end at the seabed and has a touchdown point spaced from the end at which the mooring touches down on the seabed when the buoy is not displaced from a first lateral position; and    the ballast weight on the mooring is distributed on each side of the mooring from the touchdown point.    
   
   
       24 . A riser system according to  claim 23 , wherein a part of the ballast weight is situated above the touchdown point of the mooring and provides the return force on the buoy.  
   
   
       25 . A riser system according to  claim 21 , wherein the buoy is of variable buoyancy.  
   
   
       26 . Method for producing a riser system between a subsea installation and a floating surface installation and for restoring a position of the floating surface installation, the method comprising: 
 submerging a buoy, connecting the buoy to the surface installation by at least one flexible pipe and selecting the pipe length and the position of the buoy so that the pipe is arranged in a first catenary;    anchoring the buoy to the seabed by a taut tether;    arranging at least one riser in such a position and of such length as to define a second catenary between the buoy and the seabed, and connecting the riser to the at least one flexible pipe;    exerting on the buoy a return force which depends upon the lateral movement of the buoy from a predetermined lateral position.    
   
   
       27 . The method of  claim 21 , further comprising attaching a mooring between the buoy and the seabed and forming the mooring of a length and attaching the mooring to the seabed such that a third catenary is defined; and 
 wherein the return force is exerted by applying the return force on the mooring attached to the buoy, wherein the return force on the mooring moves the buoy laterally.    
   
   
       28 . The method of  claim 22 , wherein the mooring meets the seabed at a touchdown point, and the return force is applied increasingly as the mooring is lifted from the touchdown point on the seabed by lateral movement of the buoy.