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BACKGROUND 
       [0001]    This relates generally to dual activity drilling from a drilling ship. 
         [0002]    Generally, when drilling in deep water environments, drilling mud is forced down from a drilling ship into a subsurface formation. As used herein, the term “drilling ship” encompasses a floating platform capable of propulsion on its own or by being towed, pushed or pulled, and includes semi-submersible and self-propelled vessels. 
         [0003]    When the drilling mud pressure is high, the possibility of fracture and leakage of the formation increases. When the drilling mud pressure is low, the possibility of blowout when the drilling mud pressure is less than the pore pressure arises. Generally, the mud pressure increases with depth. Thus, the deeper the formation, the more prone the formation is to fracture and the more shallower portions of the formation may be more prone to blowout. Thus, the pore pressure is higher the deeper the borehole goes. This means that mud pressure must be increased for well control. In such case, it is necessary to isolate that higher mud pressure from the shallower portions of the formation using casings. 
         [0004]    With depth, the pore pressure in the rock and the fracture pressure in the rock begin to diverge. The physics of the subsurface makes it impossible to drill a hole through this transition zone as increased equivalent circulating density through friction of returning drilling mud and the open hole limits the depth the hole can be drilled before exceeding the fracture pressure of the rock. Casing, therefore, is set and cemented. 
         [0005]    Therefore, in subsurface situations where there are drilling hazards, such as shallow water flow, it is desirable to drill the top holes using the “pump and dump” drilling method and to set and cement the casing at a depth where drilling can be formed with an equivalent circulating density less than the fracture pressure. 
         [0006]    Often, several strings of casing are necessary, including a 36 inch conductor, a 30 inch casing, and a 24 inch casing, which are set and cemented before the 20 inch casing is set, enabling the subsurface blowout preventer and marine riser to be installed on the wellhead. 
         [0007]    With the pump and dump drilling technology, the drilling mud is water based and environmentally acceptable to dump on the seabed. The drilling mud needs to have the appropriate rheological properties to assure a stable well bore is maintained. In deep water drilling areas, like the Gulf of Mexico, it is not uncommon to use and lose up to 30 to 40 thousand barrels of mud while drilling these top holes. This may create logistical problems replenishing mud stocks on the rig. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  is a schematic cross-sectional view of one embodiment of the present invention; 
           [0009]      FIG. 2  is a partial, cross-sectional view at an earlier stage to that shown in  FIG. 1  in accordance with one embodiment; 
           [0010]      FIG. 3  is a partial, cross-sectional view at a stage subsequent to that shown in  FIG. 2  in accordance with one embodiment; 
           [0011]      FIG. 4  is an enlarged, cross-sectional view of a wellbore in accordance with one embodiment of the present invention; 
           [0012]      FIG. 5  is an enlarged, cross-sectional view at a subsequent stage to that shown in  FIG. 3  in accordance with one embodiment; 
           [0013]      FIG. 6  is an enlarged, cross-sectional view of another embodiment of the present invention; 
           [0014]      FIG. 7  is an enlarged, cross-sectional view at a subsequent stage in accordance with one embodiment of the present invention; and 
           [0015]      FIG. 8  is an enlarged, cross-sectional view at a subsequent stage in accordance with one embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0016]    Rather than using a pump set on the seabed, a submersible pump may be run from a dual activity drilling ship, including a main well center that drives a submersible pump. Then a secondary well center may be used for actually drilling the well. 
         [0017]    Referring to  FIG. 1 , a floating platform or multiple operation drilling ship  10  is shown in position over a formation in a seabed D under the ocean E. The ship  10  may include a single derrick, which may include multiple levels for different operations. In some embodiments, more than one derrick may be utilized. The ship  10  may include a secondary well center  14  and a main well center  12 . 
         [0018]    The main well center  12  supports a submersible pump  26  located in the ocean E, proximate to the seabed D. The main well center  12  is anchored on the seabed D using an anchor  30  and a heave compensator  28  coupled to the pump  26 . A pump cable  24  extends from the pump  26  through a reel  22 . The main well center may be supported by a load ring  20  that hangs off of compensators (not shown) on the main well center  12 . 
         [0019]    The secondary well center  14  supports the drill pipe  16 , which, in one embodiment, may be a 20 inch conductor. The drill pipe  16  may be rotated, as indicated by the arrow A to drill the formation using a drill bit  38 . In one embodiment, mud flow is provided from the ship  10  downwardly through the drill pipe  10 , as indicated by the arrows B, into the formation. 
         [0020]    The drill pipe  16  is supported within a funnel  34  and a drilling guide base  32  in one embodiment. The drilling guide base and funnel are positioned on the seabed D prior to initiation of the drilling operation. The guide based running foot profile is indicated at  33 . 
         [0021]    The drilling mud, after circulating through the drill bit  38  and annulus, passes upwardly between the formation and the drill pipe  16 . Then it passes through a fitting and into a flexible hose  40 . From the flexible hose  40  it passes out through another fitting and into the pump  26 . The pump  26  forces the drilling mud upwardly, as indicated by the arrow C, back to the drilling ship through the casing  18  of the main well center  12 . In one embodiment, the casing  18  may be a 9⅝ inch casing. 
         [0022]    The guide base  32  is placed on the seabed with a large hole in the guide base&#39;s center. There is a funnel  34  on top of the guide base  32  to guide drilling tools and large casings into the well, to provide a side outlet to connect the well to the submersible pump through the flexible hose, and to provide the ability to view the well with a remotely operated vehicle (ROY) so drilling levels can be regulated at the seabed by speeding up or slowing down the pump  26 . 
         [0023]    Below the drill string  16  may be casings (not shown in  FIG. 1 ) that are set based on anticipated fracture gradient below the seabed and the hydraulic friction created by the drilling fluids while drilling. So the depth will vary based on local geological and pore pressure knowledge. The gradient in the well is related to the gradient of the drilling fluid in the hole plus the gradient of the sea water from the seabed back to the ship  10 . 
         [0024]    Dual gradient drilling may be accomplished using the pump  26 . The speed of a pump on the ship and the pump  26  may be synchronized so that fluid volume in and out are equal so that the mud level in the annulus remains constant at the seabed. 
         [0025]    The anchor  30  may be as simple as a probe stuck into the seabed, if the seabed conditions allow, or as sophisticated as a suction pile anchor, to mention two examples. The compensator  28  may be a pressure or scope joint, such as a compensator bumper sub to cater for rig heave, again, to give a couple of examples. 
         [0026]    Referring to  FIG. 2 , the sequence of drilling operations begins when the ship  10  arrives at the drilling site. Upon arriving at the site, the casing  18  is extended down to the seabed floor with the anchor  30  and compensator  28  and pump  26  attached. The structure is then anchored on the seabed floor D, as indicated in  FIG. 2 . Of course, the anchor  30  is set adjacent to the site of the intended well. The secondary well center may have the drill pipe  16  hung off, but not yet extended to the seabed. 
         [0027]    Next, the guide base  32  and funnel  34  are positioned from the secondary well center  14 , as indicated in  FIG. 3 . Then, the flexible pipe  40  is coupled from the funnel  34  to the pump  26  using the fittings as illustrated. This may be done by a remotely operated vehicle (ROV). In one embodiment, the casing  18  may be 9 inch casing to reduce the total weight carried by the ship  10 . 
         [0028]    Then, referring to  FIG. 4 , a well  48  is drilled and set into the seabed D using the secondary well center  14  and the drill pipe  16 . The setting of casing  42  and drilling is done under dual gradient conditions on the secondary well center. When this drilling operation is completed, the casing  18  and pump  26  may be removed on the main well center  12 . 
         [0029]    Then the guide base  32  and funnel  34  are pulled and casing  42  is run and cemented using the secondary well center while picking up a blowout preventer  46  and running riser  44  on the main well center  12 , as shown in  FIG. 5 . Then the ship  10  is moved to the left, to position the secondary well center  12  over the well  48  and the blowout preventer  46  is run and landed over the well  48 . 
         [0030]    In accordance with another embodiment, shown in  FIGS. 6-8 , instead of using two separate well centers, a single well center  12   a , with a trolley  60 , may be utilized from a ship  10   a . The trolley  60  rides on a track  62 . 
         [0031]    Initially, the well center  12   a  is used to run casing  18  with a pump  26  and anchor  30 , as indicated in  FIG. 6 . Then the trolley  60  is moved into position to connect to and hang off the casing  18 , pump  26 , and anchor  30 , while they are still anchored in the seabed D. Then the ship  10   a  may be moved, as indicated by the arrow F in  FIG. 7 , while the core  18  remains stationary. The well center  12   a  is then positioned to the side of the casing  18 , pump  26 , and anchor  30 , as depicted in  FIG. 8 . Then drill pipe  16  may be run to the seabed D and attached to a base  32 . From the base  32 , which may include a funnel (not shown), a hose  40  may be connected to the pump  26 , as described previously. Then, drilling may proceed as previously described. 
         [0032]    References throughout this specification to “one embodiment” or “an embodiment” mean that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one implementation encompassed within the present invention. Thus, appearances of the phrase “one embodiment” or “in an embodiment” are not necessarily referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be instituted in other suitable forms other than the particular embodiment illustrated and all such forms may be encompassed within the claims of the present application. 
         [0033]    While the present invention has been described with respect to a limited number of embodiments, those skilled in the art will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover all such modifications and variations as fall within the true spirit and scope of this present invention.

Summary:
Dual gradient drilling may be performed by anchoring drilling tubulars from a drilling ship on the seabed. The drilling tubulars may include an inline pump for pumping mud through another set of tubulars that actually drill the well. Then dual gradient drilling may be instituted by controlling the pressure by controlling the operation of the pump.