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CROSS REFERENCE TO RELATED APPLICATIONS 
     Not applicable 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not applicable 
     BACKGROUND OF THE INVENTION 
     Embodiments of the invention relate to riserless mud return systems used in the oil production industry. More particularly, embodiments of the invention relate to a novel system and method for riserless mud return using a subsea pump suspended along a rigid mud return line. 
     Top hole drilling is generally the initial phase of the construction of a subsea well and involves drilling in shallow formations prior to the installation of a subsea blowout preventer. During conventional top hole drilling, a drilling fluid, such as drilling mud or seawater, is pumped from a drilling rig down the borehole to lubricate and cool the drill bit as well as to provide a vehicle for removal of drill cuttings from the borehole. After emerging from the drill bit, the drilling fluid flows up the borehole through the annulus formed by the drill string and the borehole. Because, conventional top hole drilling is normally performed without a subsea riser, the drilling fluid is ejected from the borehole onto the sea floor. 
     When drilling mud, or some other commercial fluid, is used for top hole drilling, the release of drilling mud in this manner is undesirable for a number of reasons, namely cost and environmental impact. Depending on the size of the project and the depth of the top hole, drilling mud losses during the top hole phase of drilling can be significant. In many regions of the world, there are strict rules governing, even prohibiting, discharges of certain types of drilling fluid. Moreover, even where permitted, such discharges can be harmful to the maritime environment and create considerable visibility problems for remote operated vehicles (ROVs) used to monitor and perform various underwater operations at the well sites. 
     For these reasons, systems for recycling drilling fluid have been developed. Typical examples of these systems are found in U.S. Pat. No. 6,745,851 and W.O. Patent Application No. 2005/049958, both of which are incorporated herein by reference in their entireties for all purposes. Both disclose systems for recycling drilling fluid, wherein a suction module, or equivalent device, is positioned above the wellhead to convey drilling fluid from the borehole through a pipeline to a pump positioned on the sea floor. The pump, in turn, conveys the drilling fluid through a flexible return line to the drilling rig above for recycling and reuse. The return line is anchored at one end by the pump, while the other end of the return line is connected to equipment located on the drilling rig. 
     Positioning the pump on the sea floor requires that the pump be designed and manufactured to withstand hydrostatic forces commensurate with the depth of the sea floor. Also, positioning the pump on the sea floor may be undesirable in certain conditions due to the time needed to retrieve the pump in the event that the pump needs maintenance or bad weather occurs 
     Thus, embodiments of the invention are directed to riserless mud return systems that seek to overcome these and other limitations of the prior art. 
     SUMMARY OF THE PREFERRED EMBODIMENTS 
     Systems and methods for drilling a well bore in a subsea formation from an offshore structure positioned at a water surface and having a drill string that is suspended from the structure and including a bottom hole assembly adapted to form a top hole portion of the well bore. A drilling fluid source on the offshore structure supplies fluid through the drill string to the bottom hole assembly where the fluid exits from the bottom hole assembly during drilling and returns up the well bore. A suction module is disposed at the sea floor and collects the fluid emerging from the well bore. A pump module is disposed on a return line, which is in fluid communication with the suction module, at a position below the water surface and above the sea floor. The pump module is operable to receive fluid from the suction module and pump the fluid through the return pipe to the same or a different offshore structure, 
     Thus, embodiments of the invention comprise a combination of features and advantages that enable substantial enhancement of riserless mud return systems. These and various other characteristics and advantages of the invention will be readily apparent to those skilled in the art upon reading the following detailed description of the preferred embodiments of the invention and by referring to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a detailed description of the preferred embodiments of the invention, reference will now be made to the accompanying drawings in which: 
         FIG. 1  is a schematic representation of a drilling rig with a riserless mud return system comprising a subsea pump suspended along a rigid mud return line in accordance with embodiments of the invention; 
         FIGS. 2A and 2B  are schematic representations of the docking joint depicted in  FIG. 1 ; and 
         FIG. 3  is a schematic representation of the subsea pump module depicted in  FIG. 1 , 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Various embodiments of the invention will now be described with reference to the accompanying drawings, wherein like reference numerals are used for like parts throughout the several views. The drawing figures are not necessarily to scale. Certain features of the invention may be shown exaggerated in scale or in somewhat schematic form, and some details of conventional elements may not be shown in the interest of clarity and conciseness. 
     Preferred embodiments of the invention relate to riserless mud return systems used in the recycling of drilling fluid. The invention is susceptible to embodiments of different forms. There are shown in the drawings, and herein will be described in detail, specific embodiments of the invention with the understanding that the disclosure is to be considered an exemplification of the principles of the invention and is not intended to limit the invention to that illustrated and described herein. It is to be fully recognized that the different teachings of the embodiments discussed below may be employed separately or in any suitable combination to produce desired results. 
     Referring now to  FIG. 1 , drilling rig,  5  includes drill floor  10  and moonpool  15 . An example of an offshore structure, drilling rig  5  is illustrated as a semi-submersible floating platform, but it is understood that other platforms or structures may also be used. For example, offshore structures include, but are not limited to, all types of rigs, barges, ships, spars, semi-submersibles, towers, and/or any fixed or floating platforms, structures, vessels, or the like, 
     Suction module  20  is positioned on the sea floor  25  above borehole  30 . Drill string  35  is suspended from drill floor  10  through suction module  20  into borehole  30 . Deployment and hang-off system  40  is disposed adjacent to moonpool  15  and supports the return string  45 , which is secured to the sea floor  25  by anchor  50 . Although this exemplary embodiment depicts return string  45  coupled to drilling rig  5 , it is understood that, in other embodiments, return string  45  may be coupled to and supported by the same or another offshore structure and can return fluid to the same offshore structure as coupled to the drill string  35  or to a second offshore structure. Return string  45  further includes upper mud return line  55 , pump module  60 , docking joint  65 , lower mud return line  70 , and emergency disconnect  75 . 
     Upper and lower mud return lines  55 ,  70  are both formed from pipe, such as drill pipe or other suitable tubulars known in the industry. Mud return lines  55 ,  70  are preferably formed from a series of individual lengths of pipe connected in series to form the continuous line. In preferred embodiments, mud return lines  55 ,  70  are rigid, having only inherent flexibility due to their long, slender shapes. As it is used herein, the term “rigid” is used to describe the mud return lines as being constructed from a material having significantly greater rigidity than the coiled tubing or flexible hose conventionally used in mud return lines. In other embodiments, mud return lines  55 ,  70  may be non-rigid or flexible, for example coiled tubing, flexible hose, or other similar structures. 
     Upper mud return line  55  is connected at its upper end to deployment and hang-off system  40  and at its lower end to docking joint  65 , which is located below sea level  80 . Pump module  60  is releasably connected to docking joint  65 . Lower mud return line  70  runs from docking joint  65  and is secured to the sea floor by anchor  50 . In certain embodiments, emergency disconnect  75  may releasably couple lower mud return line  70  to anchor  50 . Suction hose assembly  85  extends from suction module  20  to lower mud return line  70  so as to provide fluid communication from the suction module to the mud return line. 
     Prior to initiating drilling operations, return string  45  is installed through moonpool  15 . Installation of return string  45  includes coupling anchor  50  and emergency disconnect  75  (if desired) to lower mud return line  70 . Anchor  50  is lowered to sea floor  25  by adding individual joints of pipe that extend the length of lower mud return line  70 . As return string  45  is installed, docking joint  65  and upper mud return line  55  are added. Pump module  60  may be run with return string  45  or after the string has been completely installed. Upon reaching the sea floor  25 , anchor  50  is installed to secure return string  45  to the sea floor  25 . Return string  45  is then suspended from deployment and hang-off system  40  and drilling operations may commence. 
     During drilling operations, drilling fluid is delivered down drill string  35  to a drill bit positioned at the end of drill string  35 . After emerging from the drill bit, the drilling fluid flows up borehole  30  through the annulus formed by drill string  35  and borehole  30 . At the top of borehole  30 , suction module  20  collects the drilling fluid. Pump module  60  draws the mud through suction hose assembly  85 , lower mud return line  70 , and docking joint  65  and then pushes the mud upward through upper mud return line  55  to drilling rig  5  for recycling and reuse. During operation, anchor  50  limits movement of return string  45  in order to prevent the return string from impacting other submerged equipment. 
       FIGS. 2A and 2B  are schematic representations of one embodiment of a docking joint  65  as depicted in  FIG. 1 . As shown in  FIG. 2A , docking joint  65  includes housing  100 , inlet line  105 , outlet line  110 , isolation valves  115 ,  120 , and upper connecting pipe  122 . Housing  100  includes fluid outlet port  125  at its upper end  128  and a fluid inlet port  130  at its lower end  132 . Housing  100  includes a first internal passage that provides fluid communication between fluid inlet port  130  and inlet line  105  and a second internal passage that provides fluid communication between outlet line  110  and fluid outlet port  125 . Housing  100  may be formed from a single block of material or may be constructed from separate pieces as a fabricated assembly. 
     Inlet line  105  further includes inlet  140  that is coupled to housing  100 , outlet  145  that connects to pump module  60 , and flowbore  150  providing fluid communication therebetween. Similarly, outlet line  110  further includes inlet  155  that connects to pump module  60 , outlet  160  coupled to housing  100 , and a flowbore  165  providing fluid communication therebetween. Isolation valves  115 ,  120  are positioned along flowbore  150 ,  165 , respectively, in order to selectively allow fluid communication along inlet line  105  and outlet line  110 . 
     Mud return line  70  is coupled to housing  100  at lower end  132  via a threaded connection or other suitable type of connection. Upper connecting pipe  122  couples mud return line  55  to housing  100  at upper end  128  via threaded connections or other suitable type of connections known in the industry. Referring now to  FIG. 2B , connecting pipe  122  further includes helix  138 , which is configured to align pump module  60  with docking joint  65 . Cover  170  provides a surface  180  on which pump module  60  is seated when pump module  60  is installed. Cover  170  further includes cut-outs  175 , which permit pump module  60 , when installed, access to isolation valves  115 ,  120 , inlet line  105  and outlet line  110 . 
       FIG. 3  illustrates one embodiment of a subsea pump module  60  that is operable to interface with docking joint  65 , as shown in  FIGS. 2A and 2B . Pump module  60  includes pump assemblies  200 , flowlines  205 , and isolation valves  210 , all assembled and contained within frame  215 . Pump assemblies  200  are arranged in series so that flowlines  205  provide fluid communication through pump module  60  that allows fluid from return line  70  to be successively pressurized by each pump assembly  200 . Valves  210  allow for the flow to be directed to the pump assemblies  200  as desired for a particular application. Pump assemblies  200  are illustrated as disc or, alternatively, centrifugal pump units but it is understood that any type of pump can be used in pump module  60 . Power for pump-motor assemblies  200  may be provided by electrical wiring from drilling rig  5 . In some embodiments, isolation valves  210  may be electrically actuated also via electrical wiring from drilling rig  5 . Additionally, isolation valves  210  may be manually actuated during operations involving ROVs. 
     Frame  215  protects pump assemblies  200  and their piping components and provides attachment points for lifting pump module  60  and facilitating the installation and retrieval of the module. Frame  215  includes an opening  220 , which permits pump module  60  to be inserted over mud return line  55  (see  FIGS. 1 and 2A ) and lowered along mud return line  55  to docking joint  65  during installation. Frame  215  is also configured to interface with helix  138  so as to align pump module  60  with docking joint  65  during installation of the pump module. 
     As described above in reference to  FIG. 1 , docking joint  65  is installed with mud return lines  70 ,  55  to form return string  45 . Prior to the installation of pump module  60 , isolation valves  115 ,  120  on lines  105 ,  110  of docking joint  65  may be closed to prevent circulation of seawater into return string  45 . Pump module  60  may then be installed along return string  45  with docking joint  65  or independently of docking joint  65 . 
     During normal deployment procedures, pump module  60  may be installed with docking joint  65 . In this scenario, pump module  60  is coupled to docking joint  65  and the two components are then lowered to the desired depth. To enable these procedures, docking joint  65  is designed to allow pick-up of pump module  60  without breaking return string  45 . Installation of pump module  60  with docking joint  65  in this manner is less time consuming than conventional methods because it is not necessary to break return string  45 . Retrieval of pump module  60  using docking joint  65  is also more efficient for this same reason. 
     Alternatively, during maintenance and/or emergency procedures, pump module  60  may be installed independently of docking joint  65 . For example, when pump module  60  requires maintenance and/or bad weather approaches, it may be necessary to retrieve pump module  60  while return string  45 , including docking joint  65 , remains in place. After maintenance of pump module  60  is completed or the bad weather has passed, pump module  60  may be lowered along return line  55  to engage docking joint  65 . 
     In either scenario, installation of pump module  60  preferably includes inserting mud return line  55  into opening  220  and lowering pump module  60  over the mud return line  55  to docking joint  65 . As pump module  60  is lowered over connecting line  122  of docking joint  65 , pump module  60  engages helix  138 , causing pump module  60  to rotate as pump module  60  descends toward docking joint  65  such that when pump module is seated on docking joint  65 , pump module  60  is aligned with cover  170  and engaged with inlet line  105  and outlet line  110 . Aligning pump module  60  with cover  170  allows pump module  60  access, via cut-outs  175 , to isolation valves  115 ,  120 . 
     In some embodiments, seating pump module  60  on docking joint  65  automatically actuates isolation valves  115 ,  120  from closed positions to open positions. Conversely, unseating pump module  60  from cover  170  of docking joint  65  actuates isolation valves  115 ,  120  to closed positions. In other embodiments, seating and unseating of pump module  60  in this manner may not actuate isolation valves  115 ,  120 . Rather, a signal transmitted to the isolation valves  115 ,  120  from a remote location, erg drilling rig  5 , actuates isolation valves  115 ,  120 . Additionally, isolation valves  115 ,  120  may be manually actuated during operations involving ROVS. 
     After pump module  60  is installed and isolation valves  115 ,  120  are opened, a fluid flowpath is established through pump module  60 . Once pump module  60  is operational and top hole drilling operations begin, drilling fluid is permitted to flow from mud return line  70  into docking joint  65  through fluid inlet port  130 . The drilling fluid then passes through inlet line  105 , entering at inlet  140  and exiting at outlet  145 . Upon exiting inlet line  105 , the drilling fluid flows through pump module  60  to outlet line  110  at inlet  155 . After exiting bypass line  110  through outlet  160 , the drilling fluid then flows from docking joint  65  through fluid exit port  125 , upward through connecting line  122 , and into mud return line  55 . 
     As described above, top hole drilling operations may commence after pump module  60  is installed. While operational, pump assemblies  200  of pump module  60  draw drilling fluid from the suction module  20  through suction hose assembly  85 , mud return line  70 , and bypass line  110  of docking joint  65 . Pump-motor assemblies  200  preferably then push the mud through flowlines  205 , through bypass line  110  of docking joint  65 , and upward through return line  55  to drilling rig  5  for recycling and reuse. Isolation valves  210  are actuated, as needed, to direct the flow of the drilling fluid through flowlines  205  and back into docking joint  65 . 
     In the event that pump module  60  requires maintenance and/or bad weather occurs necessitating the retrieval of pump module  60 , drilling operations cease. The flow of drilling fluid through pump module  60  is discontinued, and isolation valves  115 ,  120  are actuated to closed positions. Pump module  60  is then disengaged from docking joint  65  and returned to drill floor  10  of drilling rig  5 , either for maintenance or safe stowage. Closure of isolation valves  115 ,  120  prevents drilling fluid from dispersing into the surrounding water after pump module  60  is disengaged from docking joint  65 . 
     Retrieval of pump module  60  in this manner is expedited for at least two reasons. First, pump module  60  may be disengaged from docking joint  65  without the need to break the return string  45 . Second, pump module  60  is suspended above the sea floor  25 , rather than seated on it. Once maintenance has been performed on pump module  60  and/or bad weather has passed, pump module  60  may be redeployed by lowering pump module  60  along return string  45  to docking joint  65  where, again, pump module  60  engages docking joint  65 , as described above. Subsequent redeployment of pump module  60  is also expedited for these same reasons. 
     The terms “couple,” “couples,” and “coupled” and the like include direct connection between two items and indirect connections between items. 
     While preferred embodiments have been shown and described, modifications thereof can be made by one skilled in the art without departing from the scope or teachings herein. The embodiments described herein are exemplary only and are not limiting. Many variations and modifications of the systems are possible and are within the scope of the invention. For example, the relative dimensions of various parts, the materials from which the various parts are made, and other parameters can be varied. In particular, the subsea pump module may comprise fewer or more pump-motor assemblies as needed to convey drilling fluid from the suction module through the return string to the drilling rig. Accordingly, the scope of protection is not limited to the embodiments described herein, but is only limited by the claims that follow, the scope of which shall include all equivalents of the subject matter of the claims.

Summary:
Systems and methods for drilling a well bore in a subsea formation from an offshore structure positioned on a water surface with a drill string that is suspended from the structure and includes a bottom hole assembly adapted to form a top hole portion of the well bore. A drilling fluid source on the offshore structure supplies drilling fluid through the drill string to the bottom hole assembly where the drilling fluid exits from the bottom hole assembly during drilling and returns up the well bore. A suction module is disposed at the sea floor and collects the drilling fluid emerging from the well bore. A pump module is disposed on a return line, which is in fluid communication with the suction module, at a position below the water surface and above the sea floor. The pump module is operable to receive drilling fluid from the suction module and pump the drilling fluid through the return pipe to the same offshore structure or a different offshore structure.