Patent Publication Number: US-9428975-B2

Title: Drilling fluid pump module coupled to specially configured riser segment and method for coupling the pump module to the riser

Description:
BACKGROUND 
     The disclosure relates generally to the field of wellbore drilling using a pump to lift drilling fluid out of the wellbore so as to maintain a selected wellbore pressure. More specifically, the disclosure relates to mud return pumps and methods for connecting such pumps to a drilling riser. 
       FIG. 1  shows an example “mud lift” drilling system using a drilling fluid (“mud”) return pump when drilling from a platform (drilling unit) at the water surface. Typically, a conductor is first driven into the water bottom in marine drilling operations. When drilling a borehole  15  from the drilling device, drilling fluid is pumped through a drill string  16  down to a drilling tool, usually including a drill bit (not shown). The drilling fluid serves several purposes, one of which is to transport drill cuttings out of the borehole. Efficient transport of drill cuttings is conditioned on the drilling fluid being relatively viscous. The drilling fluid flows back through an annulus  30  between the borehole wall, a liner  14 , which is typically coupled to a riser  12  at a wellhead (not shown) proximate the water bottom and the drill string  16 , and up to the drilling unit, where the drilling fluid is treated and conditioned before being pumped back down to the borehole. In many cases, this will result in a head of pressure that is undesirable. 
     By coupling a pump  20  to the liner  14  near the water bottom or to a drilling riser  12  at a selected level above the water bottom the returning drilling fluid can be pumped out of the annulus  30  and up to the drilling rig. The annular volume in the riser  12  above the drilling fluid may be filled with a riser fluid. Preferably, the density of the riser fluid is less than that of the drilling fluid. 
     The drilling fluid pressure at the water bottom may be controlled from the drilling unit by selecting the inlet pressure to the pump  20 . The height H 1  of the column of drilling fluid above the water bottom depends on the selected inlet pressure of the pump, the density of the drilling fluid and the density of the riser fluid, as the inlet pressure of the pump is equal to: P=H 1 γb+H 2 γs, wherein γb=the density of the drilling fluid, H 2 =the height of the column of riser fluid, and γs=the density of the riser fluid. 
     In order to prevent the drilling fluid pressure from exceeding an acceptable level (e.g. in the case of a pipe trip), the riser may be provided with a dump valve. A dump valve of this type can be set to open at a particular pressure for outflow of drilling fluid to the sea. 
     The following describes a non-limiting example of a method and device illustrated in the accompanying drawings, in which, as noted above,  FIG. 1  is a schematic view of a fixed drilling rig provided with a pump for the returning drilling fluid, the pump being coupled to the riser section near the seabed and the riser section being filled with a fluid of a different density than that of the drilling fluid. 
     Reference number  1  denotes a drilling unit comprising a support structure  2 , a deck  4  and a derrick  6 . The support structure  2  is placed on the water bottom  8  (or the support structure may be affixed to flotation devices as is well known in the art) and projects above the surface  10  of the water. The riser section  12  of the liner  14  extends from the water bottom  8  up to the deck  4 , while the liner  14  runs further down into a borehole  15 . The riser section  12  is provided with required well head valves (not shown). 
     The drill string  16  projects from the deck  4  and down through the liner  14 . A first pump pipe  17  is coupled to the riser section  12  near the water bottom  8  via a valve  18  and the opposite end portion of the pump pipe  17  is coupled to a pump  20  placed near the seabed  8 . A second pump pipe  22  runs from the pump  20  up to a collection tank  24  for drilling fluid on the deck  4 . 
     A tank  26  for a riser fluid communicates with the riser section  12  via a connecting pipe  28  at the deck  4 . The connecting pipe  28  has a volume meter (not shown). Preferably, the density of the riser fluid is less than that of the drilling fluid. 
     The power supply to the pump  20  may be via an electrical or hydraulic cable (not shown) from the drilling unit  1 . The pressure at the inlet to the pump  20  is selected from the drilling unit  1 . The pump  20  may be electrically driven, or may be driven hydraulically by means of oil that is circulated back to the drilling unit or by means of water that is dumped in the sea from the pump power outlet. 
     The drilling fluid is pumped down through the drill string  16  in a manner that is known in the art, returning to the deck  4  via an annulus  30  between the liner  14  and the drill string  16 . When the pump  20  is started, the drilling fluid is returned from the annulus  30  via the pump  20  to the collection tank  24  on the deck  4 . Using such a system it is possible to achieve, for example a significant reduction in the pressure of the drilling fluid in the borehole  15 . 
     A particular issue with such systems is possibility of collapse of the first pump pipe  17  as a result of differential pressure between the hydrostatic pressure of the water at the depth of the pump pipe  17  and the internal pressure of the first pump pipe, depending on the pressure desired to be maintained in the wellbore. This is particularly an issue when the first pump pipe is made of flexible material, such as rubber hose. Such flexible materials are used so that the location of the pump  20  may be moved to suit the particular conditions in the water or proximate the water bottom  8 . 
     What is needed is a pump system that excludes the use of a lengthy first pump pipe between the riser outlet and the pump inlet. 
     SUMMARY 
     One aspect of the invention is a pump module for a drilling riser. A pump module according to this aspect of the invention includes at least one pump mounted to a structure. The structure includes features to couple the it to a segment of a riser. A fluid inlet is affixed to the pump module. The fluid inlet is in fluid communication with an intake of the at least one pump. The fluid inlet has features to make fluid tight hydraulic connection to a fluid outlet of the riser segment when the frame is coupled thereto. 
     Other aspects and advantages of the invention will be apparent from the description and claims which follow. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows an example wellbore drilling system using a pump to lift fluid from the wellbore annulus so as to maintain a selected pressure in the wellbore. 
         FIG. 2  shows an example of a horizontally oriented pump module in plan view. 
         FIG. 3  shows the example module of  FIG. 2  in side view with a mud return line. 
         FIG. 4  shows an example vertically oriented pump module in side view docked to the riser, which special riser joint having a fluid outlet line. 
         FIG. 5  shows another example vertically oriented pump module. 
         FIG. 6  shows the module of  FIG. 5  in plan view. 
         FIG. 7  shows the special riser joint of  FIG. 5  in more detail and shows a pump module landing structure. 
         FIG. 8  shows details of the special riser joint and module landing structure. 
         FIG. 9  shows a plan view of the landing structure. 
         FIG. 10  shows an oblique view of the landing structure. 
         FIG. 11  shows locking pins that mount the pump module to the landing structure. 
         FIG. 12  shows an upper pump module retaining structure. 
         FIGS. 13 and 14  show two different views of a BOP cart (trolley) and an insert therefor to enable using the BOP cart to move one embodiment of a pump module. 
         FIGS. 15 and 16  show a “soft landing” structure to enable the pupm to make wet connections to the structure. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 2  shows one example of a pump module  40  that can be used with a drilling system such as shown in  FIG. 1 . The pump module  40  may be assembled to the riser ( 12  in  FIG. 1 ) below the drilling platform ( 4  in  FIG. 1 ), either in the body of water or in the “moon pool” of a floating drilling platform to a specific riser segment (explained below) that has features for mating the pump module  40  both hydraulically and mechanically thereto. The pump module  40  may have one or more (three shown in  FIG. 2 ) pumps  42  that are in fluid communication on an inlet side thereof with a fluid outlet (see  FIG. 8 ) disposed in or forming part of the specific riser segment. An outlet of the pumps is shown in  FIG. 3  at  43  and returns drilling fluid to the drilling unit ( FIG. 1 ). An outlet of the pumps may in other examples be connected to one or more of the auxiliary lines associated with the riser, e.g., lines shown at  12 A and  12 B in  FIG. 7 . Such connection would require minor reconfiguration of the pump outlet ( 43  in  FIG. 3 ) to conform to a lower end coupling of the auxiliary line(s) on the riser joint immediately above the pump module  40 . The pumps  42  may be mounted on a platform or plate structure  41  that may include a semi-circular opening on one side ( FIG. 3 ) to enable engagement with a mating feature (not shown) on the specific riser segment (described below). Features such as an externally mounted ring (not shown) may be provided on the specific riser segment to hold the structure  41  in a selected axial position along the riser segment. A possible advantage of the configuration of the pump module  40  shown in  FIGS. 2 and 3  is that its weight may be more evenly circumferentially distributed around the riser ( 12  in  FIG. 1 ) thus reducing lateral stresses on the riser ( 12  in  FIG. 1 ). 
       FIGS. 4 and 5  show two different examples of a vertically mounted pump module,  50  and  50 A, respectively, each coupled to the specific segment  46  of the riser  12 . The respective pump modules  50 ,  50 A each may include one or more pumps, shown at  42  in  FIG. 5 , mounted in a structure  51 . The structure  51  may be generally in the shape of an open rectangular box and which may include features (described below) to couple the structure  51  to the riser segment  46 , and to make hydraulic connection between the pump(s)  42  fluid inlet and a riser fluid outlet. Below each pump module  50 ,  50 A the specific riser segment  46  may include a riser fluid outlet  48  in the form of a pipe that exits the riser segment  46  laterally and may turn vertically to couple to the pump(s) fluid inlet when the frame  51  is coupled to the riser segment  46 . The fluid outlet  48  may be a metal forging having the capacity to withstand high external differential pressure (e.g., in excess of 600 psi) without crushing. As will be further explained, an upper end of the riser fluid outlet  48  may include a feature to enable easy connection of the pump module pump inlet ( FIG. 11 ) to the upper end of the fluid outlet  48 . 
       FIG. 6  shows a plan view of the pump module  50 A of  FIG. 5  from above coupled to one side of the riser  12 , and showing three pumps  42 , although the number of such pumps in any example module is not intended to limit the scope of the disclosure. A possible advantage of using the vertical configuration shown in  FIGS. 4 through 6  is that such a pump module (either  50  in  FIG. 4 or 50A  in  FIG. 5 ) may be mounted to the specific riser segment (called a “joint”  46  in  FIGS. 4 and 5 ) using a modified blowout preventer (BOP) cart disposed under the platform ( 4  in  FIG. 1 ), but still above the water surface, i.e., within the confines of the drilling unit ( 1  in  FIG. 1 ). An example of such configuration will be explained below with reference to  FIGS. 13 and 14 . 
       FIG. 7  shows the riser segment  46  in more detail, including the fluid outlet  48 , the previously described pipe  48 B, which may be a forged component, optional control valves  48 E and a spool piece  48 D leading from the control valves  48 E to a docking structure  48 C coupled to the riser segment  48 . The foregoing components are shown in more detail in  FIG. 8 . A plan view of the docking structure  48 C is shown in  FIG. 9 . 
     The modified riser segment including outlet  48  and docking structure  48 C may be configured such that it will pass through the rotary table of the drilling unit. 
     An enlarged view of the docking structure  48 C is shown in  FIG. 10 . The opening to the spool piece ( 48 D in  FIG. 8 ) is shown at  49 , and mates with a corresponding device coupled hydraulically to the intake of the pumps ( 42  in  FIG. 6 ). Receptacles  49 A are provided for guide and locking pins to be received to engage the pump module (e.g.,  50  in  FIG. 4 ) to the docking structure  48 C. 
     An enlarged view of one of the guide and locking pins  51  approaching the corresponding receptacle  49  in the docking structure  48 C is shown in  FIG. 11 . The pins  51  may form part of or be affixed to the pump module frame  50 B. 
     Finally, in  FIG. 12 , an upper pump module frame support  54  is shown clamped to the riser  12 . The upper support  54  may be affixed to the riser  12  after the pump module ( 50  in  FIG. 4 ) is received in the docking structure ( 48 C in  FIG. 11 ) and moved so that it is effectively parallel to the riser  12 . Corresponding pins (not shown) on the upper end of the pump module frame ( 50 B in  FIG. 11 ) may mate with openings  54 A in the upper frame support  54 . 
       FIGS. 13 and 14  show two views of a BOP cart or trolley  60  typically used just below the platform ( 4  in  FIG. 1 ) of the drilling unit ( 1  in  FIG. 1 ) to assemble a blowout preventer (“BOP”—not shown) to the bottom end of a lower marine riser package (not shown) during assembly of the riser  12 . The cart  60  may include an insert  62  having dimensions selected to fit within or attach to the cart  60  and retain the frame ( 51  in  FIGS. 4 and 5 ) of the pump module  50 A within or on the cart  60 . During assembly of the riser  12  the specific riser segment  46  as explained above is coupled into the riser  12 . The riser  12  may be lowered by the drilling unit ( 1  in  FIG. 1 ) until the specific riser segment  46  is below the platform ( 4  in  FIG. 1 ) and is at the same elevation on the drilling unit as the BOP cart  60 . The BOP cart  60  may be moved laterally until the frame  51  of the pump module  50 A is in contact with the specific riser segment or joint  46  as explained above. Mechanical and hydraulic connections to the pump module may be made as explained above, and the riser  12  assembly may then continue as is ordinarily performed. 
     In some examples, the pipe  48 B, valves  48 E and spool piece  48 D may be omitted. The riser segment  46  may include an opening (not shown) in the wall thereof that mates to a corresponding feature hydraulically connected to the fluid intake of the pump(s) when the pump module (e.g.,  40  in  FIG. 2, 50  in  FIG. 4 or 50A  in  FIG. 5 ) is coupled to the riser segment  46 . Such opening and pump module feature form a pressure tight seal when the pump module ( 50  or  50 A in  FIG. 4 or 5 ) is assembled to the riser segment  46 . 
     It will also be appreciated by those skilled in the art that any of the foregoing embodiments of a pump module may be disconnected from the riser ( 12  in  FIG. 1 ) and retrieved to the drilling unit ( 1  in  FIG. 1 ) in the event of component malfunction. Such operation may be performed with the riser ( 12  in  FIG. 1 ) fully assembled from the drilling unit to the wellhead (not shown) of the wellbore, typically proximate the water bottom. The pump module may be removed from the riser, for example by a remotely operated vehicle (ROV) and lifted by a winch to the drilling unit for repair or replacement. During such retrieval operation, the wellbore operator may or may not remove the drill string from the wellbore, but the wellbore operator may close one or more of the inflatable annular elements or “rams” on the BOP (not shown) for safety reasons, e.g., to prevent wellbore pressure from escaping through the opening in the riser segment. 
     In another example, and referring to  FIGS. 15 and 16 , one or more “soft landing” elements may be affixed to the docking structure ( 48 C in  FIG. 7 ) or to the upper landing structure ( 54  in  FIG. 12 ). The soft landing structure(s) may include one or more guide posts  70  affixed to either the docking structure or the upper landing structure. A cylinder  72  having ports  73  in an upper end, and a spring  74  and piston  75  coupled to the spring  74  may be affixed to the pump structure. Such soft landing structures may slow or cushion the rate of engagement of the pump structure to the docking structure rt upper landing structure, thereby reducing the possibility of damage and enabling wet coupling of the pump and lines. 
     A pump module and corresponding mating riser segment (joint) according to the various aspects of the invention may make assembly of a subsea pump to a fluid return system more efficient, and may reduce the possibility of collapse of the intake pipe to the subsea pump as a result of differential pressure. 
     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.