Abstract:
A stripper assembly for sealing around a drill pipe includes an outer housing having a lateral outlet. The outer housing is mounted at an upper end of a well for receiving an upward flow of drilling fluid and diverting the drilling fluid through the lateral outlet. An inner member is rotatably mounted in the outer housing. A rigid cartridge housing is mounted to the inner member for rotation therewith. The cartridge housing is open to drilling fluid. An annular elastomeric seal member is located in the cartridge housing. The cartridge housing limits upward, downward and outward movement of the seal member as it deforms against the drill pipe due to drilling fluid pressure.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims the priority of U.S. Provisional patent application Ser. No. 60/332,076 filed Nov. 21, 2001. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates generally to rotating drilling head systems in which an elastomer seals around and grips a rotating drill pipe during drilling operations. 
   2. Description of the Related Art 
   Oil and gas wells are typically drilled by use of a rotating drill pipe with a drill bit at the lower end. Drilling fluids are pumped down the drill pipe and out the drill bit. The drilling fluid returns to the surface, along with cuttings, through the annulus around the drill pipe. In many cases, the pressure at the upper end of the drill pipe annulus is atmospheric. The weight of the drilling fluid is controlled to provide a hydrostatic pressure at the earth formations that is greater than the formation pressure to prevent blowouts. 
   In some cases, however, it is advantageous to isolate the pressure at the upper end of the drilling fluid column from atmospheric pressure. For example, in highly deviated well, a lightweight drilling fluid may be used that is not heavy enough to prevent upward flow in the well due to formation pressure. A drilling head at the upper end of the well controls the pressure. Drilling head systems use an elastomeric element to seal the drilling head against the rotating drill pipe during drilling operations. In some rotating drilling head systems, the seal is formed by the natural resiliency of the elastomeric element against the drill pipe while others use hydraulic pressure to deform the seal element. In U.S. Pat. No. 6,016,880, hydraulic pressure to energize an elastomeric gripper element that is located above an elastomeric primary seal. The gripper grips the drill pipe to cause the gripper and primary seal to rotate with the drill pipe. The gripper also serves as a secondary seal in the event of leakage of the primary seal. 
   The primary seal of the &#39;880 patent and in other prior art normally comprises an elastomeric seal with a tapered exterior that is exposed to drilling fluid pressure. The drill string has enlarged tool joint sections at the end of each drill pipe that must pass through the interior of the seal. The drilling fluid pressure and movement of the drill pipe through the seal causes extrusion of the seal, which limits the life of the seal. 
   SUMMARY OF THE INVENTION 
   A stripper assembly for sealing around a drill pipe has an annular elastomeric seal member with an inner passage for receiving drill pipe. The seal member has an upper end, a lower end and an outer sidewall. A rigid outer support member extends around and is bonded to an exterior portion of the sidewall of the seal member. An annular rigid lower support member bonded to the lower end of the seal member around the inner passage. The seal member is exposed to drilling fluid pressure, causing a lower portion of the sidewall to deform the seal member inwardly around a drill pipe. 
   In the preferred embodiment, the seal member, along with the support members, is mounted inside a cartridge housing. The housing has upper and lower ends an a cylindrical outer wall. The outer wall has at least one hole for admitting drilling fluid. The upper end and lower ends of the seal member engage the upper and lower ends of the cartridge housing. A portion of the outer sidewall of the seal member engages the outer wall of the housing. 
   Preferably, the seal member is configured to define a cavity at upper portion of its outer sidewall. The cavity spaces part of the seal member inward from the cartridge housing while not under drilling fluid pressure. The seal member deforms into this cavity while under drilling fluid pressure. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     So that the manner in which the described features, advantages and objects of the invention, as well as others which will become apparent, are attained and can be understood in detail, more particular description of the invention may be had by reference to the embodiments thereof that are illustrated in the drawings, which drawings form a part of this specification. It is to be noted, however, that the appended drawings illustrate only typical preferred embodiments of the invention and are therefore not to be considered limiting of its scope as the invention may admit to other equally effective embodiments. 
       FIG. 1  is an orthogonal view of a rotating drilling stripper constructed in accordance with the present invention. 
       FIG. 2  is a cross section of the rotating drilling stripper of FIG.  1 . 
       FIG. 3  is an orthogonal view of the housing of the rotating drilling stripper of FIG.  2 . 
   

   DETAILED DESCRIPTION 
     FIG. 1  shows a rotating drilling stripper  10  constructed in accordance with the present invention. Stripper  10  is used in drilling operations and is preferably a lower portion of cartridge  12  (partially shown) of a rotating drilling head. Stripper  10  rotates with rotating portion of cartridge  12 , but the present invention would permit a rotational connection between them. In the embodiment of  FIG. 1 , cartridge  12  and stripper  10  are generally located very near the drilling rig floor. The primary function of stripper  10  is to provide a seal near the upper end of the well annulus through which drilling fluids return. 
   The drilling head includes a drilling head housing  13  that is coupled to well casing (not shown) that extends some distance below the surface into the well bore, as well as some distance above the surface, approximately to the drilling rig floor. Cartridge  12  and stripper  10  are inside housing  13 . Housing  13  forms the outer boundary of the well annulus where housing  13  is present. Cartridge  12  has a rotatable inner sleeve  14  and a stationary outer sleeve  17 . 
   A gripper element (not shown), such as shown in U.S. Pat. No. 6,016,880, is mounted to inner sleeve  14  above seals  15  and, when supplied with hydraulic fluid pressure, will grip drill pipe  26  to cause inner sleeve  14  and stripper  10  to rotate with drill pipe  26 . Seals  15  seal between inner and outer sleeves  14 ,  17 . Lubricant is circulated via passages  19 . Lateral outlet  21  of housing  13  below seal  15  is in fluid communication with the annulus to return the drilling fluid from the annulus to the pump (not shown) for recirculation. 
   Stripper  10  mounts to cartridge  12  below seals  15  by conventional means. For example, stripper  10  can be attached to cartridge  12  by passing threaded bolts through a flanged end of cartridge  12  into threaded holes (not shown) in housing  16  of stripper  10 . 
     FIG. 2  shows a stripper  10  having a cartridge housing  16 . Housing  16  must be constructed of very strong material such as steel to withstand large mechanical loads.  FIG. 3  shows housing  16  comprises a cylindrical wall  18  and upper and lower ends  20 ,  22 , respectively. Ends  20 ,  22  each have an axial opening  24  of sufficient diameter to accommodate a drill pipe  26 , including the connecting portion  28  of drill pipe  26 , referred to as tool joints  28 . Cylindrical wall  18  has holes  30  along its lower portion to allow passage of fluids into the lower interior region of housing  16 . 
   Stripper  10  further comprises a seal unit  32 , as shown in FIG.  2 . Seal unit  32  comprises a rigid upper support or retainer  34 , a seal  36 , and a rigid lower support or retainer  38 . Upper retainer  34  is a structural support element onto which seal  36  is secured, such as by bonding. The upper retainer  34  shown in  FIG. 2  generally conforms to the shape of the upper portion of housing  16 . Upper retainer  34  has a cylindrical shell  40  of slightly smaller diameter than wall  18 , and, similar to housing  16 , has an upper end cap  42  with an axial opening  24  to accommodate drill pipe  26 . The portion of end cap  42  nearest drill pipe  26  is slightly thicker than the other portion of end cap  42 , forming a circular support shoulder  44 . Shell  40  extends down along the interior of wall  18 , but stops short of holes  30 . The lowermost end of shell  40  tapers quickly to an edge  46  terminating on the interior of wall  18  above holes  30 . Upper retainer  34  is attached to housing  16  using conventional means such as screws or bolts (not shown). Housing  16  preferably can be conveniently opened and closed to permit access to its interior region, permitting installation or replacement of seal unit  32 . This can be done using various conventional means such as a flange (not shown) connecting end  20  or end  22  to wall  18 , or by placing such a flange in the midsection of wall  18  above holes  30 . 
   Seal  36  is preferably made from an essentially incompressible elastomer such as cast urethane or treated natural rubber. Although incompressible, seal  36  is deformable. The embodiment of seal  36  in  FIG. 2  is cylindrically symmetric, but has many facets that are most easily described by tracing the cross sectional perimeter of the surface of seal  36 . Beginning at edge  46  of shell  40  and extending upward nearly to end cap  42 , the outermost surface of seal  36  abuts and is bonded to the inner surface of shell  40 . The outer surface of seal  36  stops short of end cap  42 , however, and turns radially inward before continuing upward again until it meets and bonds to end cap  42 . This forms an annular cavity or recessed area  48  having an approximately rectangular cross section bounded by seal  36 , shell  40 , and end cap  42 . 
   Continuing along the cross sectional perimeter of seal  36 , the upper end of seal  36  extends radially inward along end cap  42  until it meets shoulder  44 . The upper end of seal  36  extends down and then radially inward to wrap around and conform to shoulder  44 . Where the surface of seal  36  abuts shell  40 , end cap  42 , and shoulder  44 , it adjoins and is held fast by bonding material. 
   From shoulder  44 , the surface of seal  36  tapers simultaneously downward and inward to form an upper transition surface  50 . At the inward end of upper transition surface  50 , the surface of seal  36  turns and extends downward nearly the entire length of seal  36  to form a cylindrical sealing surface  52 . Cylindrical sealing surface  52  is slightly smaller in diameter than drill pipe  26 . At the downward end of sealing surface  52 , the surface tapers simultaneously downward and outward to form lower transition surface  54 . Lower transition surface  54  terminates in abutting contact with end  22  of housing  16 . For additional structural support, lower retainer  38  is bonded to seal  36  with bonding material along the lowermost portion of lower transition surface  54 . Lower retainer  38  has an inner diameter greater than the inner diameter of seal  36  and slightly greater than the outer diameter of the connecting joints  28  of drill pipe  26 . 
   The remaining portion of the surface of seal  36  extends a very short length outward along end  22  before quickly turning upward and continuing outward until it intersects tip  46 , thus returning to our beginning point. The sloped length of seal  36  from end  22  to tip  46  forms a tapered bearing surface  56 . Bearing surface  56  presents a frustoconical surface to the drilling fluid. 
   Stripper  10  effects a seal through a friction fit between sealing surface  52  and the drill pipe  26  that passes through stripper  10 . Energy to maintain the seal is provided by upwardly-directed flowing fluids that enter housing  16  through openings  30 . In conventional drilling, drilling fluids are forced down through the hollow interior of drill pipe  26  to the drill bit and into the well bore, whereupon the fluid, still under pressure, returns to the surface in the annular region between the drill pipe  26  and the well bore. 
   While the present invention can be used in such conventional drilling operations, the more modern trend, at least for geologic formations that may be damaged by the pressure exerted by the drilling fluid, is to use underbalanced drilling. Underbalanced drilling relies on overburden pressure to supply the impetus for fluids within the well bore to rise to the surface. Thus, in underbalanced drilling, fluids may rise through the interior of drill pipe  26  as well as the annular region between the drill pipe  26  and the well bore. The present invention is particularly suited for application in underbalanced drilling. In underbalanced drilling, as in conventional drilling, pressurized fluid enters housing  16  through openings  30 . 
   Sealing surface  52  is the portion of seal  36  that actually effects the seal against drill pipe  26  in response to the pressure from the drilling fluid impinging on bearing surface  56 . The pressurized fluid that enters into the lower portion of the interior region of housing  16  through holes  30  bears against bearing surface  56 . There is a functional relationship between the pressure bearing on bearing surface  56  and the pressure transferred across sealing surface  52 . The greater the area of bearing surface  56 , the greater the pressure transferred across sealing surface  52 . 
   However, one cannot simply maximize the area of bearing surface  56  to produce the maximum sealing pressure on sealing surface  52 . The drill pipe  26  passing through stripper  10 , and particularly a tool joint  28 , tends to tear seal  36  along or adjacent to sealing surface  52 , often at the intersection of sealing surface  52  and upper transition surface  50 . Excess sealing pressure exacerbates the problem because sealing surface  52  tends to deform into the region between the drill pipe  26  and shoulder  44 , or the drill pipe  26  and lower retainer  38 . During those periods in which drill pipe  26  is rapidly removed or inserted (tripping in or tripping out), the frictional force between the drill pipe  26  and sealing surface  52  can cause sealing surface  52  to heat up and weaken. As the tool joint  28  passes by, it tends to lop off the extruded portion, ruining the sealing surface  52 . Transition surfaces  50 ,  54  are designed to assist the passage of the drill pipe  26 , particularly the tool joints  28 , by allowing the tool joints  28  to impinge on a tapered surface, giving seal  36  an opportunity to deform out of the path of the drill pipe  26  and tool joints  28  as they pass through stripper  10 . 
   Cavity  48  provides a chamber into which seal  36  can deform when pressure is applied to it. By deforming into cavity  48 , seal  36  is less likely to deform into the region between the drill pipe  26  and shoulder  44 , or the drill pipe  26  and lower retainer  38 , and be lopped off or torn by the passing drill pipe  26  or tool joint  28 . Thus, as bearing surface  56  transfers the pressure from the pressurized fluid into seal  36 , seal  36  may change its shape, but its volume is essentially constant and there is no significant energy loss through seal  36 . 
   If the expected fluid pressure for a given drilling program is known in advance, such as in an exploitation field, one can select a stripper  10  having a bearing surface  56  just large enough to form an effective seal between sealing surface  52  and the drill pipe  26 . By using just enough pressure to form an effective seal, and no more, the detrimental effects of overpressuring seal  36  are minimized and the life of seal  36  is extended. 
   The present invention offers many advantages over the prior art. Placing seal unit  32  inside housing  16  allows for the pre-assembly of strippers having variously sized seals  36  for different drilling environments. It allows for regulating the amount of surface area exposed to the drilling fluid by changing the dimensions of bearing surface  56 . Thus, pressures can be regulated by choosing a seal with a bearing surface  56  optimally sized to accommodate expected drilling pressures. By reducing the pressure applied by the sealing surface  52  onto the drill pipe  26 , the frictional force between them and unwanted extrusion is reduced. That increases the useful lifetime of seal  36 . The useful lifetime of seal  36  is also increased by incorporating a cavity around seal  36 , thereby reducing the likelihood of seal  36  deforming into the region between the drill pipe  26  and shoulder  44 , or the drill pipe  26  and lower retainer  38 , and being lopped off or torn by the passing drill pipe  26  or tool joint  28 . 
   While the invention has been particularly shown and described with reference to a preferred and alternative embodiments, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention. For example, the outer sidewall retainer  34  and upper shoulder  44  need not be connected together. Upper shoulder  44  and lower retainer  38  could be formed in the interior of cartridge housing  16 , and the outer sidewall of seal  36  could be bonded to the interior of housing  16 . However, such would not allow housing  16  to be readily reused with a different seal member.