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BACKGROUND  
       [0001]     This invention relates generally to drilling of wells and production from wells.  
         [0002]     Generally, wells are drilled in a slightly over-balanced condition where the weight of the drilling fluid used is only slightly over the pore pressure of the rocks being drilled.  
         [0003]     Drilling mud is pumped down the drill string to a drill bit and used to lubricate and cool the drill bit and remove drilled cuttings from the hole while it is being drilled. The viscous drilling mud carries the drilled cuttings upwardly on the outside and around the drill string.  
         [0004]     In a balanced situation, the density of the mud going downwardly to the drill bit and the mud passing upwardly from the drill bit is substantially the same. This has the benefit of reducing the likelihood of a so-called kick. In a kick situation, the downward pressure of the drilling mud column is not sufficient to balance the pore pressure in the rocks being drilled, for example of gas or other fluid, which is encountered in a formation. As a result, the well may blowout (if an effective blow out preventer (BOP) is not fitted to the well) which is an extremely dangerous condition.  
         [0005]     In underbalanced drilling, the aim is to deliberately create the situation described above. Namely, the density or equivalent circulating density of the upwardly returning mud is below the pore pressure of the rock being drilled, causing gas, oil, or water in the rock to enter the well-bore from the rock being drilled. This may also result in increased drilling rates but also the well to flow if the rock permeability and porosity allowed sufficient fluids to enter the well-bore.  
         [0006]     In this drilling environment it is general practice to provide a variety of blowout preventers to control any loss of control incidents or blowouts that may occur.  
         [0007]     A variety of techniques have been utilized for underbalanced or dual gradient drilling. Generally, they involve providing a density lowering component to the returning drilling mud. Gases, seawater, and glass beads have been injected into the returning mud flow to reduce its density.  
         [0008]     In deep subsea applications, a number of problems may arise. Because of the pressures involved, everything becomes significantly more complicated. The pressure that bears down on the formation includes the weight of the drilling mud, whereas the pressure in the shallow formations is dictated by the weight of seawater above the formation. Because of the higher pressures involved, the drilling mud may actually be injected into the formation, fracture it and may even clog or otherwise foul the formation itself, severely impairing potential hydrocarbon production. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]      FIG. 1  is a schematic depiction of one embodiment of the present invention;  
         [0010]      FIG. 2  is an enlarged schematic depiction of the subsea shut-off assembly shown in  FIG. 1  in accordance with one embodiment of the present invention;  
         [0011]      FIG. 3  is an enlarged, schematic, cross-sectional view of the spool  34  shown in  FIG. 2  in accordance with one embodiment of the present invention; and  
         [0012]      FIG. 4  is a schematic cross-sectional view of the rotating head shown in  FIG. 1  in accordance with one embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0013]     In some embodiments of the present invention, both drilling and production of fluids from a formation may occur in an underbalanced condition. As used herein, “underbalanced” means that the weight of the drilling mud is less than the pore pressure of the formation. As used herein, “dual gradient” refers to the fact that the density of fluid, at some point along its course, moving away from a drill bit, is lower than the density of the fluid moving towards the drill bit. Dual gradient techniques may be used to implement underbalanced drilling. The creation of a dual-gradient or underbalanced condition may be implemented using any known techniques, including the injection of gases, seawater, and glass beads, to mention a few examples.  
         [0014]     Referring to  FIG. 1 , a drilling and production apparatus  11  may include a rotating head  10  which rotates a string for purposes of drilling a well in a subsea formation SF. The rotating head  10  rotates the string through a surface blowout preventer (BOP) stack  12 . The surface blowout preventer stack  12  may include annular blowout preventers that control the flow of fluid moving upwardly from the wellhead to the overlying floating rig  14 .  
         [0015]     The rig  14  may be tensioned using ring tensioners  16 , coupled by a pulleys  54  to hydraulic cylinders  56  to create a tensioning system  50 . The tensioning system  50  allows the upper portion of the apparatus  11  to move relative to the lower portion, for example in response to sea conditions. The system  50  allows this relative movement and adjustment of relative positioning while maintaining tension on the casing  22 , which extends from the floating rig  14  downwardly to a subsea shutoff assembly  24 .  
         [0016]     The surface portion of the apparatus  11  is coupled by a connector  20  to the casing  22 . The casing  22  is connected to the lower section of the apparatus  11  via a disconnectable latch  72  located below the sea level WL. The latch  72  may be hydraulically operated from the surface to disconnect the upper portion of the apparatus  11  from the lower portion including the subsea shutoff assembly  24 .  
         [0017]     Also provided on the rig  14  is a source of fluid that is of a lower density than the density of mud pumped downwardly through the casing  22  from the surface in one embodiment of the present invention. The lower density fluid may be provided through the tubing  60 .  
         [0018]     A hanger system  58  includes a tensioner  58  that rests on a support  56 . The hanger system  58  tensions the tensioned tubing  26  that extends all the way down to a disconnectable subsea latch  74  above the subsea shutoff assembly  24 . Like the latch  72 , the latch  74  may be remotely or surface operated to sever the tubing  26  from the subsea shutoff assembly  24 . In one embodiment, the support  56  may include hydraulic ram devices that move like shear ram blowout preventers to grip the tubing  26 .  
         [0019]     The rate of lower density fluid flow through the tubing  26  from the surface may be controlled from the surface by remotely controllable valving in the subsea shutoff assembly  24 , in one embodiment. It is advantageous to provide this lower density fluid from the surface as opposed to attempting to provide it from a subsea location, such as within the subsea shutoff assembly  24 , because it is much easier to control and operate large pumps from the rig  14 .  
         [0020]     The subsea shutoff assembly  24  operates with the surface blowout preventer stack  12  to prevent blowouts. While the surface blowout preventer stack  12  controls fluid flow, the subsea shutoff assembly  24  is responsible for cutting off or severing the wellhead from the portions of the apparatus  11  thereabove, using shear rams  30   a  and  30   b  as shown in  FIG. 2 . Thus, the casing  22  may be coupled by connector  28   a  to the shear ram  30   a . The shear ram  30   a  is coupled by a spool  34  with flanges  32   a  and  32   b  to the shear ram  30   b . The shear ram  30   b  may be coupled through the flange  38  to a wellhead connector  28   b , in turn connected to the wellhead.  
         [0021]     As shown in  FIG. 2 , the tubing  26  connects to a remotely controlled valve  36  that controls the rate of lower density fluid flow through the tubing  26  to the interior of the spool  34 . The inlet from the tubing  26  to the spool  34  is between the two shear rams  30   a  and  30   b.    
         [0022]     The injection of lower density fluid, as shown in  FIG. 3 , makes use of the remotely controlled valve  36  on a spool  34 . The spool  34  may have drilling mud, indicated as M IN , moving downwardly through the casing  22 . The returning mud, indicated as M OUT , extends upwardly in the annulus  46  surrounding the string  40  and annulus  44 . Thus, lower density fluid may be injected, when the valve  36  is opened, into the returning mud/hydrocarbon flow to lower its density.  
         [0023]     An underbalanced situation may be created as a result of the dual densities of mud in one embodiment. Namely, mud above the valve  36  may be at a lower density than the density of the mud below the valve  36 , as well as the density of the mud moving downwardly to the formation. The valve  36  may include a rotating element  37  that allows the valve  36  to be opened or controlled. As an additional example, the valve  36  may be a pivoted gate valve with a hydraulic fail safe that automatically closes the valve in the event of a loss of hydraulics. The valve  36  may enable the extent of underbalanced drilling to be surface or remotely controlled depending on sensed conditions, including the upward pressure supplied by the formation. For example, the valve  36  may be controlled acoustically from the surface.  
         [0024]     Thus, in some embodiments of the present invention, flow control may be done most effectively at the surface, whereas shutoff control is done on the seafloor bed. The pumping of the lower density fluid is also done on the surface, but its injection may be done at the subsea shutoff assembly  24 , in one embodiment between the shear rams  30   a  and  30   b.    
         [0025]     The rotating head  10 , shown in more detail in  FIG. 4 , is coupled to the surface blowout preventer stack  12  at a joint  70 . Returning fluid, indicated as M OUT , is passed through a valve  68  to an appropriate collection area. The collection area may collect both mud with entrained debris, as well as production fluids such as hydrocarbons. The production fluids may be separated using well known techniques.  
         [0026]     The upward flow of the fluid M OUT  is constrained by a packer  62 . In one embodiment, the packer  62  is a rubber or resilient ring that seals the annulus around the string  40  and prevents the further upward flow of the fluids. At the same time, the packer  62  enables the application of a rotating force in the direction of the circular arrow from the rotating head  66  to the string  40  for purposes of drilling. Seals  65  may be provided between a telescoping joint  64  and the rotating head  66  as both drilling and production may be accomplished in an underbalanced situation.  
         [0027]     Thus, in some embodiments of the present invention, a subsea shutoff assembly  24  may be provided to cut off the string in the event of a failure, such as a blowout. At the same time, surface annular blowout preventers control fluid flow. Dual gradient drilling may be achieved through the provision of fluid from the surface through a side inlet into the region between the upper and lower ram type shear blowout preventers  30 . Through the provision of the separate tubing  26  with a remotely operable latch  74 , appropriate volumes of fluid can be achieved that would not be available with conventional kill and choke lines. The tubing  26  for providing the density control fluid may be both tensioned and latched. As a result, dual gradient production and drilling may be achieved in some embodiments of the present invention.  
         [0028]     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:
Underbalanced production and drilling may be achieved by a system which uses a rotating head coupled to surface blowout preventer stack for fluid flow control. A casing connects these surface components to a subsea shutoff assembly with a pair of ram shear devices to cut off the string to the wellhead. Both the casing and an alternate line may be latched so that they may be released if necessary. The alternate line may provide fluid from the surface to the subsea shutoff assembly for purposes of varying the density of the returning mud. The rotating head may include a rubber packer to prevent upward flow of drilling fluid and production hydrocarbons and, at the same time, provide rotation to the drill string.