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CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     Not applicable.  
       STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
       [0002]     Not applicable.  
       BACKGROUND OF THE INVENTION  
       [0003]     1. Field of the Invention  
         [0004]     The invention relates generally to the field of floating petroleum drilling and production systems. More specifically, the invention relates to methods for retrieving part of a riser connecting a wellbore to a floating platform such that the platform can be moved in preparation for adverse weather conditions.  
         [0005]     2. Background Art  
         [0006]     Floating structures are known in the art for drilling wellbores in Earth formations located below the ocean floor, and for producing petroleum from such wellbores. The wellbores are typically drilled using fluid pressure control equipment, called a “blowout preventer” (BOP) affixed to the top of a casing cemented into a relatively shallow portion of the wellbore. A “riser”, which is a pipe formed from segments coupled end to end, is affixed to the top of the BOP and extends therefrom to the floating platform. The riser provides a conduit for fluids to move from the wellbore upwardly to the floating platform. Therefore a riser as used in wellbore drilling forms a conduit for drilling fluid and drill cuttings to be returned to the floating platform for processing and recirculation into the wellbore.  
         [0007]     A riser is assembled to the wellbore from the floating platform by coupling together segments, called “joints” of riser, and moving the assembled “string” of joints of riser downward from the floating platform as successive riser joints are coupled to the string on the platform. The foregoing procedure continues until the riser is long enough to reach the wellbore from the floating platform, whereupon the lowermost end of the riser is coupled to the BOP.  
         [0008]     During the approach of severely adverse weather conditions, such as a tropical cyclone (hurricane or typhoon), safety considerations require preparing the well for the possibility that the floating structure will be moved from its location, either by, or intentionally to avoid, the force of such weather conditions. In preparation for such weather conditions, it is necessary to retrieve the riser from the wellbore. Retrieving the riser includes lifting the riser and consecutively disassembling joints from the remaining riser string as it is suspended from the floating platform in the water. The disassembly continued until all the riser is disassembled and is retrieved from the water. Retrieving the riser is a time consuming and therefore costly procedure, the time and cost of which is related to the depth of water in which the well is being operated. Further, because of the amount of time needed to retrieve the riser, it is necessary to begin retrieval thereof at such a time prior to the expected arrival of such weather conditions as to make it more likely that the adverse weather conditions do not in fact approach the location of the floating platform. Thus, in a number of instances, retrieving the riser proves to be unnecessary.  
         [0009]     A riser disconnect system and method disclosed in U S. Patent Application Publication No. 2004/0173356 A1 filed by Dore et al. provides a way to move a floating drilling structure from a well location by enabling disconnecting the riser from the well. Such system and method have particular application for wells drilled from floating drilling structures where the BOP is located at the top of the riser on the floating drilling structure. However, the method and system disclosed in the Dore et al. publication requires that a disconnect device for the riser be located proximate the water bottom in order to prevent the riser from collapsing under its own weight when tensile force from the floating drilling structure is thus removed. Therefore, the method and system disclosed in the Dore et al. publication still require retrieval of a substantial length of riser below the floating drilling structure, which is time consuming as explained above.  
         [0010]     A riser system having a BOP near the water bottom and including a riser disconnect is shown in U.S. Pat. No. 5,657,823 issued to Kogure et al. The system shown in the &#39;823 patent provides a way to disconnect the riser at a relatively shallow depth in the water, described as 50 to 500 feet, to enable relatively quickly moving the floating structure away from the well area if a storm approaches. The system shown in the &#39;823 patent includes buoyancy devices such as canisters that are affixed to the lower portion of the riser in the event the upper portion riser is to be disconnected from the floating platform. When the upper portion of the riser is disconnected from the lower portion, the buoyancy devices support the lower portion of the riser in tension. It can be time consuming and difficult to affix buoyancy devices to a riser when it is deployed in the water.  
         [0011]     What is needed is an improved method and system to reduce the amount of time needed to make adverse weather preparations for a floating drilling or production platform.  
       SUMMARY OF THE INVENTION  
       [0012]     One aspect of the invention is a method for preparing to move a floating structure away from a wellbore drilled below the bottom of a body of water. A method according to this aspect of the invention includes filling a riser extending from the bottom of the body of water to the floating structure with water. The water in the riser is displaced with gas to a selected first depth below the water surface. The riser is sealed at a second selected depth shallower than the first selected depth. The portion of the riser above the second selected depth is withdrawn onto the floating structure.  
         [0013]     Other aspects and advantages of the invention will be apparent from the following description and the appended claims.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]      FIG. 1  shows a floating drilling platform drilling a wellbore in the Earth&#39;s subsurface below the bottom of a body of water including a riser disconnect disposed at a selected depth below the water line.  
         [0015]      FIG. 2  shows a more detailed view of the riser disconnect, and a special tool for sealing the riser to fluid pressure. 
     
    
     DETAILED DESCRIPTION  
       [0016]     Various embodiments of the invention are explained herein in the context of drilling operations from a floating drilling platform. However, it should be clearly understood that methods and systems according to the invention are also applicable to floating production systems, and thus, application of the method according to the present invention to drilling is not a limitation on the scope thereof.  FIG. 1  shows a floating drilling platform  10 , such as a semisubmersible drilling rig or a drill ship, on the surface of a body of water  11  such as the ocean as the platform is used for drilling a wellbore  16  in subsurface Earth formations  17  below the bottom  11 A of the body of water  11 . The wellbore  16  is drilled by a drill string  14  that includes (none of which shown separately) segments of drill pipe threadedly coupled end to end, various stabilizers, drill collars, heavy weight drill pipe, and other tools, all of which may be used to turn a drill bit  15  disposed at the bottom end of the drill string  14 . As is known in the art, drilling fluid is pumped down the interior of the drill string  14 , exits through the drill bit  15 , and is returned to the platform  10  for processing. A riser  18  connects the upper part of the wellbore  16  to the floating platform  10  and forms a conduit for return of the drilling fluid. Wellbore fluid pressure control equipment, collectively referred to as a blowout preventer (BOP) and shown generally at  20  includes sealing elements (not shown separately) to close the wellbore  16  below the BOP  20  in the event closing the wellbore  16  becomes necessary. The BOP  20  is controlled from the platform  10  by suitable control lines  20 A known in the art.  
         [0017]     In the present embodiment, the riser  18  may include a booster line  22  coupled near the BOP end thereof or to the BOP  20  selectively opened and closed by a booster line valve  22 A. The booster line  22  forms another fluid path from the platform  10  to the wellbore  16  at a depth proximate the BOP  20 . The purpose for the booster line  22  and valve  22 A as related to the invention will be further explained below. The riser  18  also includes therein a riser disconnect  24  of any type well known in the art, such as may be obtained from Cooper Cameron, Inc., Houston Tex. The disconnect  24  is disposed in the riser  18  at a selected depth below the water surface. The riser disconnect  24  is preferably located at the shallowest depth in the water that is substantially unaffected by action of storms on the water surface. Such depth is presently believed to be about 500 feet. As will be further explained below, when storm preparations are made, the riser  18  may be uncoupled at the riser disconnect  24 , hydraulically sealed, and the upper section of the riser  18 , from the disconnect  24  to the surface may be retrieved onto the floating platform  10 , whereupon the platform  10  may be moved from the location for safety.  
         [0018]     In a method according to the invention, the riser  18  can be partially filled with gas to a selected depth. Such depth is preferably selected such that the gas in the riser  18  will reduce the weight of the riser  18  in the water  11  so as to provide the portion of the riser  18  below the disconnect  24  with sufficient buoyancy to support the weight of the riser  18  below the disconnect  24 . Such support may require additional buoyancy, such as can be provided by air cans (not shown) or other buoyancy device known in the art, however the principle is that the additional buoyancy provided by gas displacement within the riser  18  will enable disconnection from the riser  18  of the tensile force otherwise provided by the platform  10  to the riser  18 . Where the riser disconnect  24  is located about 500 feet below the water surface, it is contemplated that the displacement of the liquid in the riser  18  with gas should take place to a depth of about 2,500 feet from the water surface. If the riser disconnect  24  is placed about 500 feet below the water surface, the displacement depth would thus be about 2,000 feet below the riser disconnect  24 . The particular depth to which gas should displace liquid in the riser  18  will depend on the weigh in water of the particular riser and the length of the riser below the disconnect  24 .  
         [0019]     In one embodiment of a method according to the invention, to prepare the wellbore  16  and the riser  18  for moving the platform  10  from the location, first the drill string  14  is lifted out of the wellbore  16  such that it is above the BOP  20 . The BOP  20  is then closed to seal the wellbore  16  below the BOP  20 . When the BOP  20  is closed, the control line  20 A may be retrieved therefrom and withdrawn to the platform  10 . Then, the drilling fluid in the riser  18  is displaced with sea water. In one implementation, the sea water is pumped into the interior of the drill string  14  so as to displace all the drilling fluid in the riser  18  up the annular space between the drill string  14  and the riser  18 . By displacing all the drilling fluid, which may have a specific gravity of as much as 2.2 or more, the weight of the riser  18  in the water  11  is reduced. After the drilling fluid in the riser  18  is displaced, the drill string  14  is withdrawn from the riser  18  and a riser fluid displacement and riser sealing tool can be coupled to the drill string. A more detailed view of the riser  18  near the position of the disconnect  24 , and the disconnect  24 , itself are shown in  FIG. 2  along with one embodiment of a riser fluid displacement and sealing tool that may be used with a method according to the invention.  
         [0020]     Referring to  FIG. 2 , the displacement and riser sealing tool in one embodiment may include a running tool, shown generally at  30 , that may be threadedly coupled to the lower end of the drill string ( 14  in  FIG. 1 ). A sealing element, shown generally at  32 , may be itself threadedly coupled to the running tool  30 . The sealing element  32  may include external threads  32 A thereon to mate with corresponding internal threads  24 B in a lower portion  24 A of the riser disconnect  24 . The sealing element  32  includes an opening  39  in the bottom thereof to enable passage of fluids therethrough. The opening  39  is sealed by a flapper valve  38  or the like that closes automatically when the running tool  30  is withdrawn from the sealing element  32 . The running tool  30  may include a J-slot engagement device for coupling to the sealing element  32  to the running tool  30  during the procedure of inserting the sealing element  32  into the riser  18 . The J-slot engagement device is formed from two components, one shown on the running tool at  36 A and the other shown on the sealing element  32  at  36 B. During insertion of the sealing element  32 , the sealing element  32  is affixed to the running tool  30  by the J-slot device  36 A,  36 B. The running tool  30  and sealing element  32  are lowered into the lower portion  24 A of the riser disconnect  24  at the end of the drill string ( 14  in  FIG. 1 ) and the sealing element  32  is threadedly engaged to the lower portion  24 A using the corresponding threads  32 A,  24 B by rotating the drill string ( 14  in  FIG. 1 ). In the present embodiment, the corresponding threads  32 A,  24 B are left handed, the reason for which will be explained below.  
         [0021]     The interior of the drill string ( 14  in  FIG. 1 ) may then be charged with gas, such as nitrogen, under pressure contemporaneously with opening the booster line valve ( 22 A in  FIG. 1 ). The gas flows through the drill string ( 14  in  FIG. 1 ), outward from the opening  39 , and by action of the gas pressure displaces the water in the riser  18  (which upon displacement travels upwardly through the booster line  22 ) downwardly to a selected depth, which as previously explained may be about 2,000 feet below the riser disconnect  24 . The water leaves the riser  18  through the booster line ( 22  in  FIG. 1 ) and moves back toward the platform ( 10  in  FIG. 1 ). The booster line valve ( 22 A in  FIG. 1 ) is then closed, and the drill string ( 14  in  FIG. 1 ) can be withdrawn from the riser  18  by disengaging the running tool  30  from the sealing element  30 . Such disengagement may be performed by suitable axial and rotational movement thereof to disengage the J-slot device  36 A,  36 B. When the running tool  30  is withdrawn from the sealing element  32 , the flapper valve  38  closes, thus hydraulically sealing the riser  18 . The riser disconnect  24  may then be operated in the manner known in the art, and the portion of the riser  18  above the riser disconnect  24  can be withdrawn from the water and placed on the platform ( 10  in  FIG. 1 ).  
         [0022]     After any storm danger has passed, the upper portion of the riser  18  may be reengaged with the lower portion thereof by engaging the components of the riser disconnect  24  in the manner known in the art. The sealing element  32  may be removed from the riser disconnect  24  by again affixing the running tool  30  to the drill string ( 14  in  FIG. 1 ), and moving the running tool  30  such that it engages the sealing element  32 . In removing the sealing element  32 , the J-slot devices are not used, but instead, corresponding threads  34 A,  34 B on the running tool  30  and the sealing element  32 , respectively, are engaged by rotating the drill string ( 14  in  FIG. 1 ) clockwise. The corresponding threads  34 A,  34 B are right-handed, and thus opposite-handed to the threads  24 B on the lower portion  24 A of the riser disconnect  24  and the corresponding threads  32 A on the sealing element  32 . Therefore, continuing drill string rotation in the direction (clockwise in the present embodiment) that engages the threads  34 A,  34 B between the running tool  30  and sealing element  32  disengages the corresponding threads  32 A,  24 B between the sealing element  32  and the riser disconnect  24 . The drill string ( 14  in  FIG. 1 ) may then be withdrawn from the riser  18 , with the running tool  30  and sealing element  32  attached at the bottom end thereto, and normal drilling operations may resume.  
         [0023]     In water depths of several thousand feet or more, it is therefore only necessary to remove the length of riser  18  to a depth just below that to which storm effects descend. As an example, only the topmost 500 feet of riser is expected to be withdrawn to the platform ( 10  in  FIG. 1 ), leaving the entire lower portion of the riser self-suspended by enough buoyancy to avoid collapse thereof. Such procedure is expected to take only a few hours, as contrasted with the several days it may take to withdraw the entire riser from the water.  
         [0024]     The embodiment explained above with reference to  FIG. 2  in which the riser is displaced by pumping gas therein is only one method for displacement. In other embodiments, suitable “swab” devices may be affixed to the lower end of the drill string such that the water in the riser may be lifted out of the riser by motion of the drill string.  
         [0025]     Methods according to the invention can substantially reduce the time necessary to prepare a floating platform for storm evacuation, and can reduce the number of times the preparations are made unnecessarily by shortening such preparation time.  
         [0026]     While the invention has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the invention as disclosed herein. Accordingly, the scope of the invention should be limited only by the attached claims.

Summary:
A method for preparing to move a floating structure away from a wellbore drilled below the bottom of a body of water includes filling a riser extending from the bottom of the body of water to the floating structure with water. The water in the riser is displaced to a selected first depth below the water surface. The riser is sealed at a second selected depth shallower than the first selected depth. The portion of the riser above the second selected depth is withdrawn onto the floating structure.