Abstract:
A method and system for lifting drilling mud from subsea to a drilling vessel, which uses a pump having a body with a chamber, and a bladder in the chamber. The bladder attaches to the body and defines water and mud sides in the chamber. A mud inlet valve allows mud into the mud side of the chamber; which moves the bladder into the water side and urges water in the water side from the chamber and through a water exit valve. Pressurized water enters the chamber through a water inlet valve, which in turn pushes the bladder and mud from the chamber through a mud exit valve. The bladder separates the mud and water as it reciprocates in the chamber. The travel of the bladder in the chamber is controlled to prevent damage from contact with the chamber.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of Invention 
         [0002]    The present disclosure relates in general to a modular system for pumping drilling mud from subsea to above the sea surface. 
         [0003]    2. Description of Prior Art 
         [0004]    Subsea drilling systems typically employ a vessel at the sea surface, a riser connecting the vessel with a wellhead housing on the seafloor, and a drill string. A drill bit is attached on a lower end of the drill string, and used for excavating a borehole through the formation below the seafloor. The drill string is suspended subsea from the vessel into the riser, and is protected from seawater while inside of the riser. Past the lower end of the riser, the drill string inserts through the wellhead housing just above where it contacts the formation. Generally, a rotary table or top drive is provided on the vessel for rotating the string and bit. Drilling mud is usually pumped under pressure into the drill string, and is discharged from nozzles in the drill bit. The drilling mud, through its density and pressure, controls pressure in the well and cools the bit. The mud also removes formation cuttings from the well as it is circulated back to the vessel. Traditionally, the mud exiting the well is routed through an annulus between the drill string and riser. However, as well control depends at least in part on the column of fluid in the riser, the effects of corrective action in response to a well kick or other anomaly can be delayed. 
         [0005]    Fluid lift systems have been deployed subsea for pressurizing the drilling mud exiting the wellbore. Piping systems outside of the riser carry the mud pressurized by the subsea lift systems. The lift systems include pumps disposed proximate the wellhead, which reduce the time for well control actions to take effect. 
       SUMMARY OF THE INVENTION 
       [0006]    Disclosed herein are examples of a system and method of lifting drilling fluid from a subsea wellbore to above the sea surface. In one example, disclosed is a system for lifting the drilling fluid from a subsea wellbore that includes a drilling riser having a return flow of the drilling fluid, a subsea module coupled with the drilling riser and having piping, and that is transportable to the drilling riser on a vessel having a capacity that is less than a capacity of a rig used in conjunction with the drilling riser. Also included is a riser module coupled with the drilling riser and having controls, and that is transportable to the drilling riser on the vessel, and a pump module coupled with the drilling riser. The pump module has a pump that is in fluid communication with the drilling fluid in the drilling riser via the piping in the subsea module and that is in communication with the controls in the riser module. The pump module is transportable to the drilling riser on the vessel. The pump module can be a first pump module, in this example the system further includes a second pump module that is symmetric and interchangeable with the first pump module. Further in this example, each pump module includes three pumps. Each pump may have a housing, a water space in the housing, a mud space in the housing that is in pressure communication with the water space, a bladder mounted in the housing having a side in contact with the water space and an opposing side in contact with the mud space, and that defines a barrier between the water and mud space. Optionally, the pump module, the subsea module, and the riser module each have a weight less than 50 metric tons. In an optional embodiment, the piping in the subsea module includes a portion for bypassing the pump module. 
         [0007]    Also disclosed herein is an example method of lifting drilling fluid from subsea that includes providing a pump module having a series of pumps, providing a riser module having controls for the pump module, providing a subsea module having piping, forming a mud pump kit by coupling together the pump module, riser module, and subsea module, coupling the mud pump kit with a subsea riser, flowing mud from the riser to the pump module via the subsea module, and lifting the mud to above sea surface by pressurizing the mud with the pumps in the pump module. Alternatively, a second pump module can be included that is symmetric with the first pump module, so that the first and second pump modules can be interchangeable. In an example, the pump module, the subsea module, and the riser module are transported individually to a drilling riser on the sea surface with a vessel having a limited capacity. A spare pump module can be optionally provided, where the method further includes replacing the pump module with the spare pump module. The pump module can be controlled with the controls from the riser module. In one alternative, mud flow is bypassed around the pump module and through the subsea module. 
         [0008]    Another example method of lifting drilling fluid from subsea includes providing first and second pump modules that are symmetric to one another. In this example, each of the pump modules has a series of pumps. The method further includes providing a riser module having controls for the pump modules, providing a subsea module having piping; and transporting the first pump module, the second pump module, the riser module, and subsea module on a vessel and to an offshore rig. Here, each of the pump modules, the riser module, and subsea module are individually transported to the rig on the vessel. A mud pump kit is formed by coupling together the pump modules, riser module, and subsea module on the offshore rig, and the mud pump kit is coupled with a subsea riser that is operated in conjunction with the offshore rig. Mud is flowed from the riser to the pump modules via the subsea module, and the mud is lifted to above sea surface by pressurizing the mud in the pump modules. A spare pump module can optionally be provided; where the spare pump module is used to replace one of the first or second pump modules. In one alternative, the controls in the riser module include a processor and hydraulic power units, the method of this example can further include using the processor to selectively open and close valves provided with the pump modules. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0009]    Some of the features and benefits of the present invention having been stated, others will become apparent as the description proceeds when taken in conjunction with the accompanying drawings, in which: 
           [0010]      FIG. 1  is a side sectional view of an example of a subsea drilling system in accordance with the present invention. 
           [0011]      FIGS. 2 and 3  are partial side sectional views of an example of a subsea pump for use with the drilling system of  FIG. 1  in different pumping modes and in accordance with the present invention. 
           [0012]      FIG. 4  is a side view of an embodiment of an example of a lift pump assembly in accordance with the present invention. 
           [0013]      FIG. 5  is a side view of an alternate embodiment of the drilling system of  FIG. 1  and in accordance with the present invention. 
           [0014]      FIG. 6  is a perspective view of a portion of the drilling system of  FIG. 6 , and in accordance with the present invention. 
       
    
    
       [0015]    While the invention will be described in connection with the preferred embodiments, it will be understood that it is not intended to limit the invention to that embodiment. On the contrary, it is intended to cover all alternatives, modifications, and equivalents, as may be included within the spirit and scope of the invention as defined by the appended claims. 
       DETAILED DESCRIPTION OF INVENTION 
       [0016]    The method and system of the present disclosure will now be described more fully hereinafter with reference to the accompanying drawings in which embodiments are shown. The method and system of the present disclosure may be in many different forms and should not be construed as limited to the illustrated embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey its scope to those skilled in the art. Like numbers refer to like elements throughout. 
         [0017]    It is to be further understood that the scope of the present disclosure is not limited to the exact details of construction, operation, exact materials, or embodiments shown and described, as modifications and equivalents will be apparent to one skilled in the art. In the drawings and specification, there have been disclosed illustrative embodiments and, although specific terms are employed, they are used in a generic and descriptive sense only and not for the purpose of limitation. 
         [0018]    Shown in  FIG. 1  is a side partial sectional view of an example embodiment of a drilling system  10  for forming a wellbore  12  subsea. The wellbore  12  intersects a formation  14  that lies beneath the sea floor  16 . The wellbore  12  is formed by a rotating bit  18  coupled on an end of a drill string  20  shown extending subsea from a vessel  22  floating on the sea surface  24 . The drill string  20  is isolated from seawater by an annular riser  26 ; whose upper end connects to the vessel  22  and lower end attaches onto a blowout preventer (BOP)  28 . The BOP  28  mounts onto a wellhead housing  30  that is set into the sea floor  16  over the wellbore  12 . A mud return line  32  is shown having an end connected to the riser  26  above BOP  28 , which routes drilling mud exiting the wellbore  12  to a lift pump assembly  34  schematically illustrated subsea. Within the lift pump assembly  34 , drilling mud is pressurized for delivery back to the vessel  22  via mud return line  36 . 
         [0019]      FIG. 2  includes a side sectional view of an example of a pump  38  for use with lift pump assembly  34  ( FIG. 1 ). Pump  38  includes a generally hollow and elliptically shaped pump housing  40 . Other shapes for the housing  40  include circular and rectangular, to name a few. An embodiment of a flexible bladder  42  is shown within the housing  40 ; which partitions the space within the housing  40  to define a mud space  44  on one side of the bladder  42 , and a water space  46  on an opposing side of bladder  42 . As will be described in more detail below, bladder  42  provides a sealing barrier between mud space  44  and water space  46 . In the example of  FIG. 2 , bladder  42  has a generally elliptical shape and an upper open space  48  formed through a side wall. Upper open space  48  is shown coaxially registered with an opening  50  formed through a side wall of pump housing  40 . A disk-like cap  52  bolts onto opening  50 , where cap  52  has an axially downward depending lip  53  that coaxially inserts within opening  50  and upper open space  48 . A portion of the bladder  42  adjacent its upper open space  48  is wedged between lip  53  and opening  50  to form a sealing surface between bladder  42  and pump housing  40 . 
         [0020]    A lower open space  54  is formed on a lower end of bladder  42  distal from upper open space  48 , which in the example of  FIG. 2  is coaxial with upper open space  48 . An elliptical bumper  56  is shown coaxially set in the lower open space  54 . The bumper  56  includes upper and lower segments  58 ,  60  coupled together in a clamshell like arrangement, and that respectively seal against upper and lower radial surfaces on the lower open space  54 . The combination of sealing engagement of cap  52  and bumper  56  with upper and lower open spaces  42 ,  54  of bladder  42 , effectively define a flow barrier across the opposing surfaces of bladder  42 . Further shown in the example of  FIG. 2  is an axial rod  62  that attaches coaxially to upper segment  56  and extends axially away from lower segment  58  and through opening  50 . 
         [0021]    Still referring to  FIG. 2 , a mud line  64  is shown having an inlet end connected to mud return line  32 , and an exit end connected with mud return line  36 . A mud inlet valve  66  in mud line  64  provides selective fluid communication from mud return line  32  to a mud lead line  68  shown branching from mud line  64 . Lead line  68  attaches to an annular connector  70 , which in the illustrated example is bolted onto housing  40 . Connector  70  mounts coaxially over an opening  72  shown formed through a sidewall of housing  40  and allows communication between mud space  44  and mud line  64  through lead line  68 . A mud exit valve  74  is shown in mud line  64  and provides selective communication between mud line  64  and mud return line  36 . 
         [0022]    Water may be selectively delivered into water space  46  via a water supply line  76  ( FIG. 1 ) shown depending from vessel  22  and connecting to lift pump assembly  34 . Referring back to  FIG. 2 , a water inlet lead line  78  has an end coupled with water supply line  76  and an opposing end attached with a manifold assembly  80  that mounts onto cap  52 . The embodiment of the manifold assembly  80  of  FIG. 2  includes a connector  82 , mounted onto a free end of a tubular manifold inlet  84 , an annular body  86 , and a tubular manifold outlet  88 , where the inlet and outlet  84 ,  88  mount on opposing lateral sides of the body  86  and are in fluid communication with body  86 . Connector  82  provides a connection point for an end of water inlet lead line  78  to manifold inlet  84  so that lead line  78  is in communication with body  86 . A lower end of manifold body  86  couples onto cap  52 ; the annulus of the manifold body  86  is in fluid communication with water space  46  through a hole in the cap  52  that registers with opening  50 . An outlet connector  90  is provided on an end of manifold outlet  88  distal from manifold body  86 , which has an end opposite its connection to manifold outlet  88  that is attached to a water outlet lead line  92 . On an end opposite from connector  90 , water outlet lead line  92  attaches to a water discharge line  94 ; that as shown in  FIG. 1 , may optionally provide a flow path directly subsea. 
         [0023]    A water inlet valve  96  shown in water inlet lead line  78  provides selective water communication from vessel  22  ( FIG. 1 ) to water space  46  via water inlet lead line  78  and manifold assembly  80 . A water outlet valve  98  shown in water outlet lead line  92  selectively provides communication between water space  46  and water discharge line  94  through manifold assembly  80  and water outlet lead line  92 . 
         [0024]    In one example of operation of pump  38  of  FIG. 2 , mud inlet valve  66  is in an open configuration, so that mud in mud return line  32  communicates into mud line  64  and mud lead line  68  as indicated by arrow A Mi . Further in this example, mud exit valve  74  is in a closed position thereby diverting mud flow into connector  70 , through opening  72 , and into mud space  44 . As illustrated by arrow A U , bladder  42  is urged in a direction away from opening  72  by the influx of mud, thereby imparting a force against water within water space  46 . In the example, water outlet valve  98  is in an open position, so that water forced from water space  46  by bladder  42  can flow through manifold body  86  and manifold outlet  88  as illustrated by arrow A Wo . After exiting manifold outlet  88 , water is routed through water outlet lead line  92  and into water discharge line  94 . 
         [0025]    An example of pressurizing mud within mud space  44  is illustrated in  FIG. 3 , wherein valves  66 ,  98  are in a closed position and valves  96 ,  74  are in an open position. In this example, pressurized water from water supply line  76  is free to enter manifold assembly  80  where as illustrated by arrow A Wi , the water is diverted through opening  50  and into water space  46 . Introducing pressurized water into water space  46  urges bladder  42  in a direction shown by arrow A D . Pressurized water in the water space  46  urges bladder  42  against the mud, which pressurizes mud in mud space  44  and directs it through opening  72 . After exiting opening  72 , the pressurized mud flows into lead  68 , where it is diverted to mud return line  36  through open mud exit valve  74  as illustrated by arrow A Mo . Thus, providing water at a designated pressure into water supply line  76  can sufficiently pressurize mud within mud return line  36  to force mud to flow back to vessel  22  ( FIG. 1 ). 
         [0026]    As illustrated in  FIGS. 2 and 3 , bumper  56  travels axially within housing  40 , and has end strokes proximate to the inner surface of housing  40 . An optional controller  100  ( FIG. 1 ) may be provided for limiting travel of bladder  42  and bumper  56  to avoid collisions of bladder  42  or bumper  56  with the inner surface of housing  40 . In an embodiment, controller  100  includes an information handling system, and receives or contains instructions to selectively operate valves  66 ,  74 ,  78 ,  98 . Optionally, valves  66 ,  74 ,  78 ,  98  can include actuators (not shown) in communication with and/or controlled by controller  100 , that manipulate the valves  66 ,  74 ,  78 ,  98  to limit travel of the bumper  56 . The controller  100  can be set based upon an increase or decrease in fill volume that alters velocity of flow in one of the chambers  44 ,  46 . User defined set points can be input to the controller  100  for establishing limits of travel of the bladder  42 . This can be manifested via control of the valves  66 ,  74 ,  96 ,  98  so that they open and close at designated times and sequences so that travel of bladder  42  and/or bumper  56  prevents or avoids collision with housing  40 . Moreover, a set bias may be included with commands in the controller so that the control system automatically adjusts the set points to a higher or lower value to bring bladder travel within a safe range and thereby avoid any damaging contact. Examples exist wherein volume in one of the chambers  44 ,  46  at a maximum stroke ranges from about 15 gallons to about 55 gallons. By setting the set points with an included bias, the set points are adjusted during use so that in a subsequent cycle of pumping, the extent of bladder travel is decreased to avoid any overshoot from a designated position. 
         [0027]      FIG. 4  is a schematic illustration of an example of a lift pump assembly  34  having pumps  38 A-C arranged in parallel. In this example, and similar to that of  FIG. 2 , mud flows to pumps  38 A-C respectively from mud lines  64 A-C that each have an inlet end connected to mud return line  32 . Outlet ends of the mud lines  64 A-C discharge into mud return line  36 . Leads  68 A-C respectively communicate mud flow between pumps  38 A-C and lines  64 A-C, where valves  66 A-C,  74 A-C respectively regulate flow through lines  64 A-C. In similar fashion, water from water supply line  76  flows to pumps  38 A-C via water inlet lead lines  78 A-C and manifold assemblies  80 A-C; and water from pumps  38 A-C is delivered to water discharge line  94  via manifold assemblies  80 A-C and water outlet lead lines  92 A-C. Water to and from pumps  38 A-C is controlled by valves  96 A-C and  98 A-C, which are shown respectively in lines  78 A-C and lines  92 A-C. Optionally, one or more of valves  66 A-C,  74 A-C,  96 A-C,  98 A-C,  106 A-C,  108 A-C may be in communication with a controller  100  for selective opening and/or closing the valves, or throttling flow through the valves. 
         [0028]    The lift pump assembly  34  of  FIG. 4  is equipped with a pressure balance circuit  102  for minimizing a pressure differential across valves  96 A-C. In the example of  FIG. 4 , pressure balance circuit  102  includes pressurization tubing  104 A-C, each having inlets respectively connected to water inlet lead lines  78 A-C. Optionally, pressurization tubing  104 A-C can connect directly to water supply line  76 . Pressurization valves  106 A-C are provided within each run of pressurization tubing  104 A-C. Each run of tubing  104 A-C includes depressurization valves  108 A-C downstream of pressurization valves  106 A-C. Tubing leads  110 A-C branch respectively from pressurization tubing  104 A-C in the portions between pressurization valves  106 A-C and depressurization valves  108 A-C. The ends of tubing  110 A-C distal from pressurization tubing  104 A-C connect to water inlet lead lines  78 A-C downstream of inlet valves  96 A-C. In an example of operation, when water is being discharged from pumps  38 A-C, outlet valves  98 A-C are in the open position, and inlet valves  96 A-C are in the closed position, a pressure differential can exist across inlet valves  96 A-C that can approach pressure in water supply line  76 . Further in this example, opening valves  106 A-C, while valves  96 A-C and  108 A-C are in a closed position, communicates pressure from line  76  through pressurization tubing  104 A-C, tubing leads  110  A-C, and into inlet lead lines  78 A-C downstream of valves  96 A-C. In this example embodiment, fluid in lines  78 A-C upstream and downstream of valves  96 A-C is in pressure communication with line  76 , thereby minimizing pressure differential across valves  96 A-C. 
         [0029]    Downstream of valves  108 A-C, pressurization tubing  104 A-C connects to a tubing header  112 , through which water in the pressure balance circuit  102  can be discharged to ambient. In the example of  FIG. 4 , pumps  38 A-C and the associated piping disclosed herein are referred to as a pump module  114 A. Example embodiments exist wherein the lift pump assembly  34  includes two or more modules. As such, a water discharge line  116  from another module  114 B, that is substantially similar to module  114 A. Block valves  118 ,  120  are respectively provided in discharge lines  94 ,  116  for isolating water flow from modules  114 A,  114 B. Also in line  94  is an optional block valve  122  downstream of the intersection of line  116  with line  94 ; and a control valve  124  and flow meter  126  downstream of block valve  122 . An optional bypass line  128  connects tubing header  112  to water discharge line  94  between control valve  124  and flow meter  126 . A block valve  130  is shown in tubing header  112  downstream of bypass line  128 , and a block valve  132  is provided in bypass line  128 . In an alternative embodiment, block valves  130 ,  132  are in communication with controller  100 . 
         [0030]    Still referring to the example of  FIG. 4 , line  94  discharges to ambient downstream of control valve  124 , thus depending on the flow rate of fluid in line  94 , pressure in line  94  downstream of control valve  124  is substantially equal to ambient pressure. In the illustrated embodiment, control valve  124  and flow meter  126  are shown in communication with one another, so that a flow area through control valve  124  automatically adjusts in response to a flow rate detected by flow meter  126  to “throttle” flow across control valve  124 . Optionally as shown, control valve  124  is in communication with controller  100 , so that the amount of throttling can vary based on operating conditions of the lift pump assembly  34 . As such, a pressure differential can be generated across control valve  124  so that pressure in line  94  upstream of control valve  124  is greater than pressure at ambient and introduces a backpressure in line  94 . Where the backpressure in line  94  suppresses flow rate spikes in lines  92 A-C, which in turn reduces cycling forces on components of pumps  38 A-C during pumping operations. 
         [0031]    In some examples of use, pumps  38 A-C operate under “managed pressure drilling operations” where mud flow rates are reduced, but pressure of the mud to the pumps  38 A-C is increased. During these conditions, the flow path to ambient through the pressure balance circuit  102  and from lines  78 A-C can allow pressure in pumps  38 A-C to drop below a threshold value so that pumps  38 A-C will uncontrollably fill with mud during a subsequent pumping cycle. One example of operation to address the unacceptable pressure drop includes diverting flow in tubing header  112  that is being discharged from pressure balance circuit  102  through bypass line  128 . In this example, block valve  130  is set into a closed position and block valve  132  is open. In an optional example, controller  100  delivers instructions for opening/closing of the block valves  130 ,  132 . As indicated above, bypass line  128  terminates into water discharge line  94  upstream of control valve  124 , which is maintained at a pressure sufficiently above ambient so that a backpressure can be exerted onto pressure balance circuit  102 . In the example of  FIG. 4 , the backpressure on the pressure balance circuit  102  communicates to the water side  46  ( FIG. 2 ) of each pump  38 A-C; which maintains a minimum pressure in the water side  46  of each of the pumps  38 A-C to avoid an uncontrolled influx of mud flow into the pumps  38 A-C. 
         [0032]    Referring now to  FIG. 5 , an alternate embodiment of drilling system  10 A is shown in side partial sectional view and wherein lift pump assembly  34 A includes a mud pump kit  134  mounted integral onto riser  26 A. In this example, mud pump kit  134  includes a subsea module  136  shown circumscribing riser  26 A and that includes mud distribution manifold (not shown) and other flow control devices for selectively diverting flow to desired destinations. A riser module  137  is illustrated mounted on an upper surface of subsea module  136 , which also circumscribes riser  26 A. Riser module  137  of  FIG. 4  includes controls for operation of the pump kit  134 , such as a processor  138  having hardware and software for controlling operation of components of pump kit  134 . Also included in riser module  137  are hydraulic power units  139  for providing pressurized hydraulic fluid, which in an example is used for actuating devices subsea, such as valves in pump kit  134 . Riser module  137  also includes hydraulic control systems connection hardware for mounting mud pump kit  134  to riser  26 A. Pumps  38  ( FIG. 2 ) are housed in pump modules  140 ,  142  shown set on riser module  137 . In an embodiment, pump modules  140 ,  142  each include three pumps  38 . In an example, included in the subsea module  136  is piping  143  that provides connectivity, and communication of mud flow, between the riser  26 A and pump modules  140 ,  142 . Piping  143  further alternatively includes a circuit that connects to riser  26 A, but bypasses pump modules  140 ,  142 . Examples of operation where pump modules  140 ,  142  are bypassed include situations where pressure in the mud flow is sufficient for flowing to surface, or where pump module(s)  140 ,  142  are not in service. In an example, valves  144  (that can be part of a valve kit) in the pump modules  140 ,  142  are actuated and/or controlled by processor  138 , and may optionally be powered by hydraulic fluid supplied from hydraulic power unit  139 . 
         [0033]    A solids recovery unit (SRU)  145  is shown above the pump modules  140 ,  142 , and a subsea rotating device (SRD)  146  attaches to an upper end of SRU  145 . An upper end of SRD  146  flangedly attaches to a riser joint  148 ; where in one example a substantial portion of the riser  26 A between SRD  146  and vessel  22  ( FIG. 1 ) is made up of stacked riser joints  148 . 
         [0034]    In the example of  FIG. 4 , mud exiting drill string  20  flows upward in an annulus  150  defined between drill string  20  and wellbore  12 , and which extends further upward between drill string  20  and riser  26 A. The mud flows past mud pump kit  134  and SRU  145  within annulus  118  and into SRU  146  where a packer (not shown) blocks the mud. In an embodiment, the annulus  118  above packer is filled with sea water or other fluid. Mud within annulus  118  below packer is diverted to SRU  145  where cuttings or other solids are removed or particulated. After being processed in the SRU  145 , the mud is directed to the pump modules  140 ,  142  where it is pressurized so it can flow back to vessel  22 . Processing the mud in the SRU  145  can prevent damage to the pumps  38  ( FIG. 2 ) in the modules  140 ,  142 . 
         [0035]    In an example, modules  136 ,  137 ,  140 ,  142  are modular elements that can be transported separately to the vessel  22  ( FIG. 1 ) on site, where the pump kit  134  is assembled. Still referring to  FIG. 1 , a vessel  152 , which in an example is smaller than vessel  22 , is shown transporting a module  136  to vessel  22 . Optionally, embodiments exist where none of the modules  136 ,  137 ,  140 ,  142  weigh in excess of 50 metric tons. A maximum weight of 50 metric tons is advantageous as this is the upper weight capacity of most barges. In alternatives where the weight of each of modules  136 ,  137 ,  140 ,  142  does not exceed 50 metric tons, any one of modules  136 ,  137 ,  140 ,  142  can be individually transported to vessel  22  with vessel  152 . A significant time savings is one advantage of the modularity of modules  136 ,  137 ,  140 ,  142 . Due to the weight of the pump kit  134 , and that the pump kit  134  asymmetrically loads an offshore rig or vessel  22 , which requires anchoring and stabilization, loading a fully assembled pump kit  134  onto a vessel  22  is impractical. Whereas vessel  152  can transport the modules  136 ,  137 ,  140 ,  142  individually, and vessel  22  can accommodate individual modules  136 ,  137 ,  140 ,  142  on site and without becoming unstable. 
         [0036]    In an optional embodiment, pump modules  140 ,  142  are individually detachable from the pump kit  134 , and thus further enhancing modularity of the pumping system. Dedicated piping (not shown) may be routed from SRU  145  and separately to each module  140 ,  142  so that one of the modules  140 ,  142  can remain operational while the other is removed or otherwise out of service. Further, spare modules can be kept on site for one or both modules  140 ,  142 , and can installed in place of a one of the modules  140 ,  142  with little or no stoppage of operation of pumping mud to the vessel  22 . 
         [0037]    Yet further optionally, BOP  28 A is a BOP stack, whose upper portion includes an annular blowout preventer and is part of a lower marine riser package (LMRP). Additionally, LMRP can include controls, a multiplexer unit, and pods. In an embodiment, modules  136 ,  137 ,  140 ,  142 , SRU  145 , SRU  146 , BOP  28 A, and riser joints  116  are delivered to the vessel  22  ( FIG. 1 ) while on site and disposed above wellbore  12 . While on the vessel  22 , modules  136 ,  137 ,  140 ,  142  are attached together to form mud pump kit  134  which is coupled with BOP  28 A. SRU  145  and SRU  146  are attached onto mud pump kit  134 ; while suspended from riser joints  116  the assembled unit is lowered subsea onto wellhead housing  30 . 
         [0038]    Referring now to  FIG. 6 , shown in a perspective view is a schematic example of a mud pump kit  134  mounted onto a riser  26 A. As shown, pump modules  140 ,  142  are stacked respectively on starboard and port sides of riser  26 A and on top of riser module  137 ; where riser module  137  stacks on top of subsea module  136 . As illustrated in  FIG. 6 , mud return line  36  shown including a fitting  156  between where mud return line  36  couples with riser  26 A and the pumps  38 A-C. Fitting  156  can be a flanged surface, a valve, or any other device for fluidically coupling sections of a line. In this example pump modules  140 ,  142  are symmetric to one another so that pump module  140  can be switched out for pump module  142  (and vice versa). Thus the corresponding mud return line (not shown), provided with pump module  142  and for pumps  38 D-F, is oriented to mate with fitting  156 . The symmetric/mirror image configuration of pump modules  140 ,  142  allows one of the pump modules  140 ,  142  to be switched out for the other without rearranging any piping. An advantage of this design is that only a single spare pump module  160  need be stored onsite, which can be used to replace either of pump module  140  or pump module  142 . 
         [0039]    The present invention described herein, therefore, is well adapted to carry out the objects and attain the ends and advantages mentioned, as well as others inherent therein. While a presently preferred embodiment of the invention has been given for purposes of disclosure, numerous changes exist in the details of procedures for accomplishing the desired results. These and other similar modifications will readily suggest themselves to those skilled in the art, and are intended to be encompassed within the spirit of the present invention disclosed herein and the scope of the appended claims.