Abstract:
A riserless mud recovery system including a mud return line secured by an anchor is disclosed. Some system embodiments for drilling a well bore in an offshore location having a water surface and a subsea formation include an offshore structure positioned on a platform at a water surface, a drill string for forming the well bore suspended from the offshore structure, a drilling fluid source on the platform for supplying drilling fluid through the drill string, a suction module for collecting the drilling fluid emerging from the well bore, a return pipe coupled to the suction module, a pump for receiving the drilling fluid from the suction module and pumping the drilling fluid through the return pipe to a location at the water surface, and an anchor for securing the return pipe, where the anchor is coupled to the return pipe and the subsea formation.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    Not applicable. 
       STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    Not applicable. 
       BACKGROUND OF THE INVENTION 
       [0003]    Embodiments of the invention relate to riserless mud return systems used in drilling subsea wells for the production of oil and gas. More particularly, embodiments of the invention relate to a systems and methods for riserless mud return using a mud return line secured to the sea floor by an anchor. 
         [0004]    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. 
         [0005]    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 mud. 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. 
         [0006]    For these reasons, systems for recycling drilling mud 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 mud from the borehole through a pipeline to a pump positioned on the sea floor. The pump, in turn, conveys the drilling mud 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. In certain applications, such as in deep water and strong currents, the use of a flexible return line may not be desirable. 
         [0007]    Thus, the 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 
       [0008]    Systems and methods for riserless mud return systems including a mud return line secured by an anchor, which is not a subsea pump or other mechanism that moves the fluid to the surface, are disclosed. Some system embodiments include an offshore structure positioned on a platform at a water surface, a drill string with a bottom hole assembly adapted to form the well bore and suspended from the offshore structure, and a drilling fluid source for supplying drilling fluid through the drill string to the bottom hole assembly. The drilling fluid exits from the bottom hole assembly during drilling and returns up the well bore. These system embodiments further include a suction module for collecting the drilling fluid emerging from the well bore, a return conduit coupled to the suction module, a pump for receiving the drilling fluid from the suction module and pumping the drilling fluid through the return conduit to a location at the water surface, and an anchor for securing the return conduit. The anchor is coupled to the return conduit and the sea floor. 
         [0009]    Some embodiments include driving a bit mounted at an end of a drill string to form a well bore in a subsea formation, injecting a drilling fluid into the drill string, collecting the drilling fluid after the drilling fluid passes through the drill string, returning the drilling fluid to a location at the water surface through a pipe using a subsea pump, and anchoring the pipe to the subsea formation. 
         [0010]    Some embodiments include a suction module for mounting over a well bore in sealed relation to the surrounding seawater to prevent leakage of drilling fluid from the well bore, a floating drilling vessel operable to supply a drilling fluid to a drill string disposed in the well bore, at least one pump module spaced from and connected to said suction module to effect a differential pressure therein for pumping drilling fluid from said sealing device upwardly to said floating drilling vessel, a return line providing fluid communication between said suction module and said floating drilling vessel, wherein said return line is in fluid communication with said pump module, and an anchor that couples said return line to the sea floor. 
         [0011]    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 
         [0012]    For a detailed description of the preferred embodiments of the invention, reference will now be made to the accompanying drawings in which: 
           [0013]      FIG. 1  is a representation of a drilling rig with a riserless mud return system comprising a mud return line secured by an anchor in accordance with embodiments of the invention; 
           [0014]      FIG. 2  is schematic representation of the anchor depicted in  FIG. 1 ; 
           [0015]      FIG. 3  is a schematic representation of an embodiment of the anchor depicted in  FIG. 2  but adapted for use in a firm seabed solid; 
           [0016]      FIG. 4  is a cross-sectional view of another anchor in accordance with embodiments of the invention; 
           [0017]      FIG. 5  is a cross-sectional view of yet another anchor in accordance with embodiments of the invention; and 
           [0018]      FIG. 6  is a cross-sectional view of still another anchor in accordance with embodiments of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0019]    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 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. 
         [0020]    In the following discussion and in the claims, the terms “including” and “comprising” are used in an open-ended fashion, and thus should be interpreted to mean “including, but not limited to . . . ”. Also, the terms “couple,” “couples”, and “coupled” used to describe any connections are each intended to mean and refer to either an indirect or a direct connection. 
         [0021]    The preferred embodiments of the invention relate to riserless mud return systems used in the recycling of drilling mud during top hole drilling. 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 present 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. 
         [0022]    Referring now to  FIG. 1 , drilling rig  5  comprises 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. 
         [0023]    Suction module  20  is coupled to jet casing wellhead  90 , which is positioned on the sea floor  25  above borehole  30 . Drill string  35 , including bottom hole assembly  95 , is suspended from drill floor  10  through suction module  20  and jet casing wellhead  90  into borehole  30 . Deployment and hang-off system  40  is positioned adjacent to moonpool  15  and supports return string  45 , which is secured to the sea floor  25  by anchor  50 . Return string  45  further comprises upper mud return line  55 , pump module  60 , docking joint  65 , lower mud return line  70 , and emergency disconnect  75 . 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. 
         [0024]    Upper and lower mud return lines  55 ,  70  are both preferably formed from drill pipe, but may be formed from other suitable material known in the industry, such as coiled or flexible tubing. Accordingly, reference herein will be made to drill pipe, but it should be understood that the invention is not so limited. Thus, mud return lines  55 ,  70  are formed from a series of individual lengths of drill pipe connected in series to form the continuous conduit. 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 . Preferably, pump module  60  is coupled to return string  45  below sea level  80  and above sea floor  25 . See U.S. patent application Ser. No. 11/833,010, entitled Return Line Mounted Pump for Riserless Mud Return System, which is hereby incorporated herein by reference in its entirety for all purposes. 
         [0025]    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 lower mud return line  70 . 
         [0026]    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 preferably 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. 
         [0027]    During drilling operations, drilling mud 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 mud 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 mud. Pump module  60  draws the mud through suction hose assembly  85 , lower mud return line  70 , and docking joint  65  and then moves 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. 
         [0028]      FIG. 2  is a schematic representation of a preferred embodiment of anchor  50 . Anchor  50  comprises suction anchor  200 , perforated guide tube for sliding mass  205 , sliding mass  230 , foundation plate  225 , drill collar to mass adaptor  228 , shackles  210 , return line elbow with hang-off pad  237  and hose swivel  218 . Suction anchor  200  is a hollow member further comprising open lower end. Guide tube  205  is coupled to suction anchor  200  by foundation plate  225  and further comprises open upper end  226 , a plurality of perforations  240  through the wall of guide tube  205 , and suction port with remotely operated vehicle (ROV) docking joint  215 . Sliding mass  230  is inserted into open upper end  226  of guide tube  205  and configured to slide upward and downward within guide tube  205 . Perforations  240  in guide tube  205  allow seawater to flow therethrough, thereby reducing resistance encountered by sliding mass  230  as sliding mass  230  translates within guide tube  205 . 
         [0029]    Sliding mass  230  is coupled via drill collar to mass adaptor  228  and shackles  210  to mud return line elbow hang-off pad  237  or an emergency disconnect  75  (shown in  FIG. 1 ). Preferably, hose swivel  218  couples suction hose assembly  85 , extending 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. Moreover, hose swivel  218  is configured to allow rotation of suction hose assembly  85  about the coupling of mud return line  70  and sliding mass tube  205 . 
         [0030]    Prior to installation, anchor  50  is assembled on drilling rig  5  and coupled to return mud line  70 , or emergency disconnect  75 . During installation, anchor  50  is lowered via mud return line  70  to the sea floor  25 . Due to its mass and open end  220 , suction anchor  200  imbeds into the soil upon landing on the sea floor  25 . An ROV docks to the suction anchor  200  at suction port  215  and pumps seawater from suction anchor  200  to achieve final penetration into the sea floor  25 . Suction hose assembly  85  may then be coupled to suction module  20  and to hose swivel  218  of anchor  50 . Once coupled to suction hose assembly  85 , hose swivel  218  makes manipulating suction hose assembly  85  easier. 
         [0031]    Once installed, anchor  50  limits displacement of the lower end of return string  45  relative to drill string  35  caused by surrounding water currents  130  and weather and sea state induced motions on drilling rig  5 . Anchor  50  substantially prevents lateral movement of return string  45 , thereby preventing return string  45  from displacing and contacting other submerged equipment and drilling rig  5 . At the same time, anchor  50  permits some vertical movement of return string  45  as sliding mass  230  translates within guide tube  205 . Additionally, perforations  240  in tube  205  further enable such vertical movement by allowing water, which may be contained in perforated guide tube  205 , to be forced out through perforations  240  as sliding mass  230  translates downward inside guide tube  205 . Thus, anchor  50  provides a flexible connection between return string  45  and the sea floor  25 , which alleviates wear to the other components of return string  45  caused by forces from changing water currents  130  and some drill rig  5  movements caused by sea state and weather, thereby increasing their service life. 
         [0032]    Moreover, hose swivel  218  enables lower stresses on the coupling of suction hose assembly  85  to mud return line  70 , or emergency disconnect  75 . As the mud return line  70  and suction hose assembly  85  move in response to surrounding currents  130  and some drill rig  5  movements caused by sea state and weather, hose swivel  218  allows rotation of suction hose assembly relative to mud return line  70  and sliding mass tube  205 , thereby reducing the stresses at this connection. This too permits increased service lives for the affected components. 
         [0033]      FIG. 3  is a schematic representation of an embodiment of anchor  50  depicted in  FIGS. 1 and 2 , but adapted for use in a firm seabed. In this exemplary embodiment, anchor  500  does not comprise suction anchor  200  ( FIG. 2 ). Instead, guide tube  205  is coupled to wedge anchor jet in manifold  505  by foundation plate  225 . Wedge anchor  505  further comprises suction port with ROV docking joint  215  and wedge anchor blades  510  preferably shaped to limit lateral movement of the return string  45  once the blades  510  are embedded in the sea floor  25 . Each blade  510  further comprises a nozzle  515  at its tip to enable embedding of blades  510  in the sea floor  25 . 
         [0034]    Assembly, installation and operation of anchor  500  are in most ways similar to that described above in reference to  FIG. 2  for anchor  50 . Anchor  500  can be assembled on drilling rig  5  and coupled to return mud line  70 , or emergency disconnect  75 . During installation, anchor  500  can be lowered via mud return line  70  to the sea floor  25 . Due to its mass and the shape of blades  510 , anchor  500 , or more specifically, blades  510  of manifold  510 , imbeds into the soil upon landing on the sea floor  25 . An ROV docks to the manifold  510  at suction port  215  and pumps seawater into manifold  510 . The injected seawater then flows through the manifold  510 , out of the nozzles  515  and into the seabed to liquefy the seabed. Softening of the seabed in this manner allows anchor  500  to achieve final penetration into the sea floor  25 . Once installed, anchor  500  limits displacement of the lower end of return string  45  relative to drill string  35  caused by surrounding water currents  130  and weather and sea state induced motions on drilling rig  5 . 
         [0035]      FIG. 4  is an enlarged cross-sectional view of another anchor in accordance with embodiments of the invention. Anchor  280  comprises pipe conduit  250 , housing  255 , and retainer  260 . Housing  255  further comprises opening  265 , cavity  270 , and tip  275  at its lower end. Retainer  260  is disposed within housing  255  and has an outer diameter that is larger than opening  265  in housing  255 . Conduit  250  is coupled to retainer  260  within cavity  270  and extends through opening  265  of housing  255 . The upper end of conduit  250  is connected to the lower mud return line  70  or an emergency disconnect  75  (shown in  FIG. 1 ). Retainer  260 , with attached conduit  250 , is free to translate along cavity  270  within housing  255 . 
         [0036]    Tip  275  of anchor  280  is preferably shaped so as to penetrate sea floor  25  as anchor  280  is lowered via return string  45  (shown in  FIG. 1 ). Upon reaching the sea floor  25 , anchor  280  is installed to secure return string  45  to the sea floor  25 . Anchor  280  will initially imbed itself in sea floor  25  due to its own weight. Anchor  280  can then be further set into sea floor  25  by repeatedly lifting and dropping return string  45 , causing retainer  260  to translate upward in cavity  270  and then downward to impact tip  275  within housing  255 . The impact of tip  275  by retainer  260  will drive tip  275  into the sea floor  25 . The lifting and dropping process is repeated until anchor  280  is driven to a desired depth in the sea floor  25 . 
         [0037]    Once installed, anchor  280  limits displacement of return string  45  caused by surrounding water currents  130 . Anchor  280  substantially prevents lateral movement of return string  45 , thereby preventing return string  45  from displacing and contacting other submerged equipment and drilling rig  5 . At the same time, anchor  280  permits some vertical movement of return string  45  as retainer  260 , with attached pipe  250 , translates within cavity  270  of housing  255 . Thus, anchor  280  provides a flexible connection between return string  45  and the sea floor  25 , which alleviates wear to the other components of return string  45  caused by forces from changing water currents  130 , thereby increasing their service life. 
         [0038]      FIG. 5  is a cross-sectional view of another anchor in accordance with embodiments of the invention. Anchor  300  comprises conduit  305  connected at its lower end to chain  310  by connector  315 . The upper end of conduit  305  is connected to lower mud return line  70  or emergency disconnect  75  (shown in  FIG. 1 ). Chain  310  is of sufficient weight to anchor return string  45  (shown in  FIG. 1 ) to the sea floor  25 . To achieve the necessary weight, chain  310  may comprise dense materials and/or have extensive length. Chain  310  is also flexible to permit limited displacement of conduit  305 . Moreover, chain  310  and connector  315  are capable of withstanding tension loads imparted to these components by movement of conduit  305  in response to surrounding water currents  130 . In some embodiments, chain  310  is a metal link chain, but may be made of any suitable material. 
         [0039]      FIG. 6  is a cross-sectional view of another anchor in accordance with embodiments of the invention. Embodiments of the anchor exemplified by  FIG. 6  are similar to those illustrated by  FIG. 5  with one primary difference. In embodiments exemplified by  FIG. 6 , a weight is used to anchor return string  45  to the sea floor  25 , rather than additional chain length. It should be appreciated that a portion of the chain  410  may also rest on the sea floor  25 . 
         [0040]    As shown in  FIG. 6 , anchor  400  comprises conduit  405  connected at its lower end to the upper end of chain  410  by connector  415 . The upper end of conduit  405  is connected to lower mud return line  70  or emergency disconnect  75  (shown in  FIG. 1 ). The lower end of chain  410  is connected to weight  420  by connector  425 . Weight  420  is of sufficient weight to anchor return string  45  (shown in  FIG. 1 ) to the sea floor  25 . Chain  410  is flexible to permit limited displacement of conduit  405 . Moreover, chain  410 , connector  415 , and connector  425  are capable of withstanding tension loads imparted to these components by movement of conduit  405  in response to surrounding water currents  130 . In some embodiments, chain  410  is a metal link chain, but can be made from any suitable material. 
         [0041]    Once installed, anchor  400  limits displacement of return string  45  caused by surrounding water currents  130 . Due to the weight of weight  420 , anchor  400  limits movement of return string  45 , thereby preventing return string  45  from displacing and contacting other submerged equipment and drilling rig  5 . At the same time, the flexible nature of chain  410  enables anchor  400  to provide a flexible connection between return string  45  and the sea floor  25 . The flexibility of anchor  400  alleviates wear to the other components of return string  45  caused by forces from changing water currents  130  and thus increases their service life. 
         [0042]    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 sliding mass tube and suction anchor in  FIG. 1  are not limited to the circular shapes shown, but may assume other physical forms. Similarly, the retainer and weight depicted in  FIG. 6  are also not limited to the shapes shown, but may assume other physical forms. Lastly, the chains depicted in  FIGS. 5 and 6  are not limited to the design configuration shown, but may assume other physical forms that are flexible and have sufficient strength and weight, and the housing, conduit, and tip of the anchor of  FIG. 4  may take any physical form. 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.