Abstract:
A floating, semi-submersible offshore drilling vessel has a blowout preventer located on the semi-submersible instead of on the seabed. The mud return housing is attached directly to the blowout preventer without the use of a slip joint. This configuration allows for a significantly reduced drill floor height and, consequently, a lower vertical center of gravity for the rig.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS: 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 61/638,291 filed on Apr. 25, 2012. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
       [0002]    Not Applicable 
       BACKGROUND OF THE INVENTION 
       [0003]    1. Field of the Invention 
         [0004]    This invention relates to oil and gas well drilling. More particularly, it relates to floating offshore drilling rigs. 
         [0005]    2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98 
         [0006]    A variety of floating vessels can be used for offshore drilling and production operations. Semi-submersible vessels (“semis”) are a particularly popular vessel for drilling operations. A semi-submersible is a floating unit with its deck supported by columns to enable the unit to become almost transparent for waves and provide favorable motion behavior. The unit stays on location using dynamic positioning and/or is anchored by means of catenary mooring lines terminating in piles or anchors. A Semi-submersible Drilling Rig is fitted with drilling facilities and dynamic positioning system for deep-water drilling. 
         [0007]    “Mud” is a term that is generally synonymous with drilling fluid and that encompasses most fluids used in hydrocarbon drilling operations, especially fluids that contain significant amounts of suspended solids, emulsified water or oil. Mud includes all types of water-base, oil-base and synthetic-base drilling fluids. Drill-in, completion and work over fluids are sometimes called muds, although a fluid that is essentially free of solids is not strictly considered mud. 
         [0008]    Drilling fluid(s) comprise any of a number of liquid and gaseous fluids and mixtures of fluids and solids (as solid suspensions, mixtures and emulsions of liquids, gases and solids) used in operations to drill boreholes into the earth. Drilling fluid is synonymous with “drilling mud” in general usage, although some prefer to reserve the term “drilling fluid” for more sophisticated and well-defined “muds.” 
         [0009]    In a typical drilling operation, mud is pumped down the drill string and returns to the surface carrying the drill cuttings via the annulus between the drill string and the well bore (or casing). The mud is returned to a shale shaker—the primary and probably most important device on the rig for removing drilled solids from the mud. This vibrating sieve is simple in concept, but a bit more complicated to use efficiently. A wire-cloth screen vibrates while the drilling fluid flows on top of it. The liquid phase of the mud and solids smaller than the wire mesh pass through the screen, while larger solids are retained on the screen and eventually fall off the back of the device and are discarded. Obviously, smaller openings in the screen clean more solids from the whole mud, but there is a corresponding decrease in flow rate per unit area of wire cloth. Hence, the drilling crew may seek to run the screens (as the wire cloth is called), as fine as possible, without dumping whole mud off the back of the shaker. Where it was once common for drilling rigs to have only one or two shale shakers, modern high-efficiency rigs are often fitted with four or more shakers, thus giving more area of wire cloth to use, and giving the crew the flexibility to run increasingly fine screens. 
         [0010]    The “Christmas tree” (or just “tree”) in drilling operations is the set of valves, spools and fittings connected to the top of a well to direct and control the flow of formation fluids from the well. In offshore drilling operations, the tree may be either “wet”—i.e., located on the seabed or “dry”—i.e., located above the surface on the drilling vessel. 
         [0011]    A Deep-Draft Semi® drilling vessel with a dry tree operating in the Gulf of Mexico may be subjected to severe heave motions during a major hurricane or winter storm conditions. This can pose a challenge in the design of the associated drilling riser tensioner as well as the well-bay and drilling structure arrangement. A key to successful development of a dry tree semisubmersible is to balance the conflicting design requirements of stability and in-service motion. 
         [0012]    Due to the large heave motion of a dry-tree semi, the blowout preventer installed at the surface on a drilling riser above the drilling riser tensioner will have a large range of vertical motion relative to the semi in a major hurricane. With the drilling riser tensioner installed on the semi lower deck, the drill floor needs to be located high enough above the semi lower deck to accommodate the large range of estimated blowout preventer motions in a major hurricane. Consequently, the higher the drill floor the higher the vertical center of gravity of the drilling structure, which increases the moment arm of the wind load. Hence, it poses challenges to the platform stability and overall design. 
         [0013]    A typical drilling riser slip joint (or “travel joint”) provides one solution which accommodates the vertical motion of the dry-tree semi relative to the surface stationary blowout preventer. A slip joint is a telescoping joint at the surface in floating offshore operations that permits vessel heave (vertical motion) while maintaining a riser pipe to the seafloor. As the vessel heaves, the slip joint telescopes in or out by the same amount so that the riser below the slip joint is relatively unaffected by vessel motion. A slip joint may be installed between the diverter, located below the drill floor well center, and the top of the blowout preventer, to complete the mud system to direct returning mud back to the shakers/mud pits. A slip joint that can accommodate 35 ft. of vertical movement or stroke, estimated during a major hurricane, will have a minimum closed length of 48-50 ft. Allowing for a full 35 feet of stroke requires 65.5-67.5 ft. of open area between the blowout preventer and the drill floor. 
         [0014]    One way of reducing the drill floor height is for operational procedures to stipulate the removal of the slip joint in case of a major hurricane. The drill floor can be lowered enough to use a shorter slip joint that would still be needed to accommodate heave of up to 15 ft. expected in a hurricane originating in the Gulf of Mexico or normal winter storms. This requires the emergency abandonment procedures to allow sufficient time to remove the slip joint before a major hurricane. Moreover, slip joint removal is an operational step that always involves health, safety, security and environmental risks. 
         [0015]    U.S. Pat. No. 6,913,092 describes a system for the return of drilling fluid from a sealed marine riser to a floating drilling rig while drilling. A floating rig or structure for drilling in the floor of an ocean using a rotatable tubular includes a seal housing having a rotatable seal connected above a portion of a marine riser fixed to the floor of the ocean. The seal rotating with the rotating tubular allows the riser and seal housing to maintain a predetermined pressure in the system that is desirable in underbalanced drilling, gas-liquid mud systems and pressurized mud handling systems. The seal may be either an active seal or a passive seal. A flexible conduit or hose is used to compensate for relative movement of the seal housing and the floating structure. 
         [0016]    U.S. Pat. No. 6,244,359 describes a drilling head used to seal around a drill pipe while drilling in a subsea location. The drilling head has an inner body located within an outer body. At least one bearing is located between the outer body and the inner body for facilitating the rotation of the inner body relative to the outer body. A seal mounted to a lower portion of the inner body seals around the outer surface of the drill pipe. While lowering the drilling head to the wellhead, a support attached to the drill pipe is inserted into a skirt which surrounds a portion of the seal. The skirt and support are releasably connected using a J-slot mechanism. An inner annulus and an outer annulus are located between the inner and outer bodies, the annuluses containing a lubricating fluid. Helical vanes are located within the inner annulus and affixed to the inner body. The vanes rotate with the inner body for circulating the fluid through the inner and outer annuluses. 
       BRIEF SUMMARY OF THE INVENTION 
       [0017]    A dry-tree semi-submersible drilling rig comprises a drilling rotating control head attached to the top of a blowout preventer and a flexible hose attached to the pump and connected to fixed piping on the semi-submersible and in fluid communication with shakers, mud pits or diverter overboard lines at a remote location on the semi-submersible. This arrangement eliminates the need for a slip joint between the blow-out preventer and the rotary table. Elimination of the slip joint permits the drilling floor to be located at a lower elevation on the rig thereby improving the vessel&#39;s dynamics by lowering its vertical center of gravity. In certain embodiments, an in-line progressive cavity pump is attached to the rotating head. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S) 
         [0018]      FIG. 1  is a side view of a portion of a drilling vessel of the prior art having a drill floor 120 feet above deck bottom with a slip joint having a 35-foot stroke. 
           [0019]      FIG. 2  is a side view of a drilling vessel equipped with a lowered drill floor and a mud return system according to the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0020]    The invention may best be understood by reference to the exemplary embodiment(s) illustrated in the drawing figures. 
         [0021]    To permit lowering the drill floor of a semi-submersible, offshore drilling vessel, the present invention uses an alternative mud return system that does not require the use of a slip joint. Advantage is taken of technology that already exists in dual gradient drilling and land underbalanced drilling. In place of the slip joint and diverter under the drill floor, an industry proven drilling rotating control head is used on top of the blowout preventer. An optional, in-line progressive cavity pump that moves mud returns from the well to the shakers/mud pits may be in fluid communication with the mud return line coming off the rotating head. A flexible hose designed to the specifications for a diverter flow line may attached to the pump and connected to fixed pipe under the drill floor to direct mud returns to the shaers/mud pits or to diverter overboard lines separated by remotely-operated valves. 
         [0022]    Advantages of the System Include:
       Significantly reduced drill floor height with resulting reduction of the rig&#39;s vertical center of gravity;   More convenient construction and installation of the substructure and derrick; and,   Deletion of the operational requirement for removing the slip joint during emergency evacuation procedures.       
 
         [0026]    Referring now to  FIG. 1 , a drilling rig  10  of the prior art is depicted in side view.  FIG. 1  shows a portion of a floating, offshore drilling vessel such as a semi-submersible vessel. Drilling derrick  12  is mounted on drill floor  14  and drill floor skid base  16  which, in turn, is supported on main deck  20  by support structure  18  which, in the illustrated example, is a truss-type structure. 
         [0027]    Drilling riser  30  which extends to the wellhead on the seafloor (not shown) is connected near its upper end to riser tensioner  32 . In the illustrated drilling rig, riser tensioner  32  is a ram-type tensioner and is shown in its fully extended position. Also shown in  FIG. 1  are riser tensioners  32 ′ and  32 ″ in well bays to the left and right, respectively, of riser tensioner  32 . The riser tensioners are shown mounted in deck box  34  below main deck  20  of the offshore vessel. 
         [0028]    Drill string  22  (shown in  FIG. 1  as a dot-dashed line) is supported by derrick  12  at its upper end and extends through diverter  38 , slip joint  24 , blowout preventer  28  and drilling riser  30  to the drill bit (not shown) at its lower end. 
         [0029]    In operation, drilling fluid (“mud”) is pumped down the drill string  22  and is returned to the surface by way of the annulus between the drill string and the borehole, well casing and slip joint  24 . Diverter  38  is mounted at or near the top of slip joint  24  and directs the returning mud (containing drill cuttings) to drilling fluid processing equipment on the vessel. Processed drilling fluid is stored in mud tank  36  prior to being pumped back down the drill string. 
         [0030]    A diverter is equipment that is conventionally attached to the bell nipple on a wellhead or marine riser. The diverter can be closed to prevent fluids from flowing vertically and to divert the fluids out a blooey line, away from the rig. Two, common types of diverters are: a) a bag-type unit; and, b) a modified rotating blowout preventer. A control head, such as that described in U.S. Pat. No. 6,913,092 may be used in place of a diverter to channel drilling fluid flowing up the annulus between the drill string and the well casing to drilling fluid processing equipment on the rig. 
         [0031]    Riser  30  is a fixed length from the seafloor. Due to vessel heave in ocean tides, swells and waves, the vessel&#39;s decks (together with the equipment mounted thereon) moves vertically relative to riser  30  and equipment attached to riser  30  such as blowout preventer  28 . The maximum stroke of riser tensioner  32  is selected to accommodate the greatest heave the vessel is likely to encounter during drilling operations. 
         [0032]    In the drilling rigs of the prior art, the changing vertical distance between the top of the blowout preventer  28  and drill floor  14  is accommodated by telescoping slip joint  24  which is guided and stabilized by roller frame module  26 . Returning drilling fluid flows up the annulus between the slip joint and the drill string to diverter  38  and thence to mud processing equipment such as shale shakers and the like. 
         [0033]    As will be appreciated by those skilled in the art, drill floor  14  must be located a sufficient vertical distance above main deck  20  to accommodate the telescoping range of slip joint  24 . The higher this heavy equipment is located on the vessel, the higher the vertical center of gravity of the vessel. In general, a lower vertical center of gravity is preferable for vessel stability. Thus, improvements which permit the height of the drill floor to be lowered can favorably influence vessel stability. 
         [0034]    An illustrative embodiment of the invention is shown in  FIG. 2 . Drilling rig  40  shares many components of prior art drilling rig  10  (see  FIG. 1 ) and are correspondingly numbered. 
         [0035]    Drill floor  14  of rig  40  is supported on main deck  20  by modified drill floor skid base  42  which has a lesser vertical dimension than support structure  16  of drilling rig  10  ( FIG. 1 ). This lowered drill floor is made possible by an alternative mud return system which does not require a slip joint between the blowout preventer  28  and the drill floor. Drill floor  14  (and the equipment mounted thereon) may thus be lowered by up to the telescoping distance of the removed slip joint  24 . 
         [0036]    The elimination of slip joint  24  is made possible by providing diverter  44  at the upper terminus of blowout preventer  28 . Diverter  44  receives returning drilling mud from the annulus between the drill string  22  and the walls of the central axial passageway of blowout preventer  28  and directs the mud via flexible conduit  48  and rigid piping  50  to on-board mud processing equipment such as shale shaker  52 . 
         [0037]    In the illustrated embodiment, shale shaker  52  is mounted on main deck  20 . In other embodiments, mud processing equipment such as shale shaker  52  may be mounted in other locations such as within deck box  34 . This acts to further lower the vertical center of gravity of the vessel. 
         [0038]    Optional pump  46  may be provided to increase the return mud flow and may be necessary if mud processing equipment is located at a higher elevation on the rig. Optional pump  46  may be an in-line, progressive cavity pump. 
         [0039]    In certain alternative embodiments, element  44  may be a control head of the type described in U.S. Pat. No. 6,913,092 containing a rotatable seal. 
         [0040]    Although particular embodiments of the present invention have been shown and described, they are not intended to limit what this patent covers. One skilled in the art will understand that various changes and modifications may be made without departing from the scope of the present invention as literally and equivalently covered by the following claims.