Patent Publication Number: US-2011073314-A1

Title: Riser termination

Description:
CROSS-REFERENCE 
     Applicant claims priority from U.S. provisional patent application No. 61/246,916 filed Sep. 29, 2009. 
    
    
     BACKGROUND OF THE INVENTION 
     In order to facilitate the explanation of the invention, applicant uses the term “vessel” to describe any type of structure that floats at the surface of a body of water. Applicant uses the term “sea” to refer to any salt or fresh water body of water. Applicant uses the term “moon pool” to describe a space at the height of a vessel hull where work can be done, which is usually a space surrounded by the hull but which can lie outside the hull. 
     During the drilling or servicing of a subsea well into a sea floor, a drill string or other string (group of series connected devices), extends down from a vessel or other structure that floats at the sea surface, and through a riser that extends vertically to the sea floor. The string may be hollow to carry fluid such as drilling mud down to a drill bit or other tool at the bottom of the string. Used fluids such as drilling mud then passes up through an annular space between the outside of the string and the inside of the riser, to a top termination at the top of the riser, that connects to the vessel where the fluids are processed for reuse. 
     While the riser extends at a fixed height above the sea floor and is stationary, the vessel translates and rotates, as by tilting in roll and pitch and moving vertically in heave as waves pass by. Tilting can be accommodated by flexible joints, but heave movement has adverse consequences. The top termination preferably lies in a vessel moon pool, with a work floor at the top of the moon pool. In the prior art, heave was accommodated by the vessel being connected to the top termination through a telescoping joint. The telescoping joint included a long inner pipe hanging from the vessel, that telescoped into a tall outer pipe extending upward from the top termination. The telescoping joint typically accommodates vertical heave of up to about 50 feet (15 meters) or more, and could be of a multi-barrel type. A taller space is required to accommodate the telescoping joint because one pipe must lie within the other pipe along a considerable length to produce a long stroke telescoping joint with limited function. The long length of telescoping pipes resulted in the need to leave a tall vertical space in the vessel (or outside it) between the water level and the underside of the work floor or the like, resulting in the need to locate the work floor (from which a string such as a drill string or other parts are launched) high above the vessel main deck. Also, it has been difficult to provide reliable seals along the telescoping joint, resulting in multiple seal packages which require frequent maintenance or replacement. 
     SUMMARY OF THE INVENTION 
     In accordance with one embodiment of the invention, an offshore hydrocarbon or other development system is provided that connects a top termination that lies at the top of a riser, to a floating unit such as a vessel, in a connection that avoids the need for a telescoping joint to accommodate motions of the vessel, especially heave. The system includes a plurality of flexible jumper hoses that each extends in a U-shaped or other curve between the top termination and a tank on the vessel in which fluid is collected. When the vessel translates and pivots, the jumper hose flexes to accommodate such motions. 
     The vessel supports the weight of the riser string through flexible tension lines or other means of tensioning that extend from the vessel to the top terminal. Except for the flexible lines and the jumper hoses, the top termination is free of connections to the vessel. This allows the vessel to undergo motion (e.g. roll and pitch) without comparable motions of the top termination, to minimize the need for flexible joints to connect the top termination to the vessel. 
     The novel features of the invention are set forth with particularity in the appended claims. The invention will be best understood from the following description when read in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side elevation view of a prior art offshore hydrocarbon or other development system which has a telescoping joint. 
         FIG. 1A  is a sectional view of a drill string penetrating the sea floor. 
         FIG. 2  is a side elevation view of an upper portion of an offshore development system of the present invention. 
         FIG. 3  is a side elevation view of the top termination of the riser and adjacent portion of the system of  FIG. 2 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  shows an offshore development system  10  of the prior art, such as a hydrocarbon development system, which includes a floating unit, or vessel  12  that floats at the sea surface  14  and connects to a riser  20  that extends from near (within 20% of the sea height) or at the sea floor  22  up to the vessel. The system can be used in a construction phase (to drill holes in the sea bed) as well as in a testing and maintenance phase (well services, workover, intervention) and an abandonment phase. The system also can be used for the development of mineral deposits or entities requiring a fixed connection between a location near the sea floor and the vessel. A variety of different types of floating units can be used, all of which are sometimes referred to herein as a vessel. In  FIG. 1 , the system includes a derrick  24  or other means of hoisting a string, that supports a string within the riser, to drill or service a well in the sea floor. The figure shows a subsea stack  30  at the sea floor, that includes a disconnection section  32  comprising a well control and (dis)connection system, and a blowout prevention section comprising a well control section and a connection/disconnection section  34 . 
       FIG. 1A  shows an assembly  112  that lies in the riser  20  and that is drilling a hole in the sea floor  22 . Drilling fluid is pumped down through the hollow center  36  of the string. Used fluid then flows upward through an annular region  38  for processing and reuse. 
     The riser  20  extends primarily vertically from the sea floor up to a coupling  40  that includes a riser tensioning ring that lies in a vessel moonpool  42  (in or beside a side or end of the vessel). The moonpool provides access to the riser bore. A moonpool in the vessel hull comprises an opening (usually square or otherwise rectangular) extending through the height of the vessel hull, that provides room for working around systems such as the upper part of the riser, and that extends up through the height of the bottom of the hull. A plurality of wires  44  or other means of tensioning extend up from a tensioning ring  46  to an upper portion of the vessel, to support the riser. The vessel undergoes motion (e.g. pitch and roll), but the riser is isolated from them by flexible couplings  40 ,  52  at the top of the telescopic joint and at the bottom of the riser. 
     The vessel  12  undergoes heave motion that may move the vessel vertically up or down by up to 15 meters or more. In the prior art system of  FIG. 1 , such vertical movement of the vessel relative to the riser is accommodated by a TJ (telescopic joint)  60 . In the TJ  60 , a long vertical inner pipe hangs from the vessel and extends into a long vertical outer pipe that lies in the tensioning ring. For a maximum anticipated vertical movement of the vessel, the TJ must be about 20% longer than the vertical movement. This is necessary so that adequate lengths of interfitting pipes remain even when the vessel is at its highest or lowest. The long lengths required result in the work floor  64  where the supported part of the derrick or other hoisting means lies and where string sections, tools, etc. are lowered into or retrieved from the riser, lying high above the bottom  66  of the moon pool. Also, seals that seal pipes that slide one within the other, are more difficult to access and maintain than devices that do not require seals to accommodate movement. 
       FIG. 2  shows a portion of a vessel  12 A of the present invention, and the upper portion of a riser  22 A of the present invention. The vessel  12 A is similar to that of  FIG. 1 . The top of the riser has a top termination  70  that lies on the rest of a riser and at or near the riser vertical axis  73 . The vessel has at least one fluid collecting tank  72  that receives used fluids (e.g. drilling mud) from the riser through a plurality of jumper hoses  74  that connect to the top termination  70 . The fluid passes to a fluid treatment module  41  that processes the fluid for reuse. Each jumper hose  74  has one rigid pipe section  76  with an inner end that connects to the upper end of the riser and with an outer end  82  that extends downward. Each jumper hose has an opposite rigid pipe section  84  with an inner end that connects to the collecting tank  72  and an outer end  85  that extends downward. Each jumper hose also has a flexible hose portion  90  with ends  86  that extend downward from the rigid pipe sections, and has a middle  87  that extends primarily in a half circle, or 180° loop. When the vessel and its collecting tanks move up and down during vessel heave, the flexible hose portion  90  bends to maintain the fluid connection. 
     The top termination  70  of the riser is supported by flexible tension lines  44 A that extend from a tensioning ring  46 A around sheaves to a tensioner  92 . The tension lines support the riser to locate it within the moonpool  66 A, and the tensioner elongates and shortens the tension lines when the vessel moves up and down. Other means of tensioning also can be used to support the upper end of the riser. Applicant notes that the riser  20 A terminates at the top termination  70  and is not connected to the vessel except through the flexible jumper hoses  74  and the flexible tension lines  44 A. As a result, vessel motions can be accommodated without the riser undergoing corresponding motions, because the tension lines  44  can bend, and without requiring the flexible pivot joint at  40  of  FIG. 1 . Other means for tensioning the riser upper end preferably provide tension while allowing movement of the riser upper end. Applicant notes that similar jumper hoses and flexible tensioning means can be used when the moonpool lies at or beyond the bow, stern, or side of the vessel instead of in a pool surrounded by the hull. 
       FIG. 3  shows additional details of the system of  FIG. 2 , showing that the top termination  70  can support blowout preventers, including double ram preventers  100 , single ram preventers  102  and annular preventers  104 . A rotating head blowout preventer  106  lies near the top of the termination. A dedicated flow divider  108  divides fluid flow between the two jumper hoses, and is used to change vertical flow to horizontal flow. A collector funnel  110  lies at the very top of the termination, and serves to guide components such as a drill string  112  into the riser  20 A. 
     Thus, the invention provides a hydrocarbon, mineral, or other development system which includes a riser that extends up from the sea floor to a top termination that lies in a space (e.g. moonpool) in a vessel hull or beside the hull, which allows the vessel to move relative to the riser and relative to a termination at the top of the riser, in a system of high reliability. The top termination is connected to one or more fluid collecting tanks on the vessel, through one or more jumper hoses that include flexible hose sections that maintain fluid connections of the riser to the vessel despite motions of the vessel. Such fluid connections are maintained without the need for a telescoping sliding connection. The top termination is connected to the vessel only through the jumper hoses or other fluid connection means and through flexible tension lines or other tensioning means, so the vessel can execute motions, especially heave and tilt (roll and pitch) without requiring the riser to execute corresponding motions. 
     Although particular embodiments of the invention have been described and illustrated herein, it is recognized that modifications and variations may readily occur to those skilled in the art, and consequently, it is intended that the claims be interpreted to cover such modifications and equivalents.