You are an expert at summarizing long articles. Proceed to summarize the following text:

You are an expert at summarizing long articles. Proceed to summarize the following text: 
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of co-pending U.S. patent application Ser. No. 11/123,329, filed on May 6, 2005 now U.S. Pat. No. 7,779,903, which is a continuation-in-part of U.S. patent application Ser. No. 10/285,336, filed on Oct. 31, 2002, now U.S. Pat. No. 7,040,394, the disclosures of which are herein incorporated by reference in their entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     Embodiments of the present invention generally relate to wellbore operation. More particularly, the invention relates to a method of use and an apparatus for sealing around a tubular. Still more particularly, the invention relates to a seal assembly for use in a control head. 
     2. Description of the Related Art 
     Drilling a wellbore for hydrocarbons requires significant expenditures of manpower and equipment. Thus, constant advances are being sought to reduce any downtime of equipment and expedite any repairs that become necessary. Rotating equipment is particularly prone to maintenance as the drilling environment produces abrasive cuttings detrimental to the longevity of rotating seals, bearings, and packing elements. 
     In a typical drilling operation, a drill bit is attached to a string of drill pipe. Thereafter, a drive unit rotates the string of drill pipe through a drive member, referred to as a kelly as the string of drill pipe and drill bit are urged downward to form the wellbore. In some arrangements, a kelly is not used, thereby allowing the drive unit to attach directly to the drill pipe. The length of the wellbore is determined by the location of the hydrocarbon formations. In many instances, the formations produce gas or fluid pressure that may be a hazard to the drilling crew and equipment unless properly controlled. 
     Several components are used to control the gas or fluid pressure. Typically, one or more blow out preventers (BOP) are mounted to the well forming a BOP stack to seal the mouth of the well. Additionally, an annular BOP is used to selectively seal the lower portions of the well from a tubular body that allows the discharge of mud through the outflow line. 
     An example of a BOP is disclosed in U.S. Pat. No. 4,440,232. The BOP in &#39;232 uses a spherical sealing element to seal the mouth of the well. The spherical sealing element is typically made from an elastomeric material and formed in a shape of a dome with a hole in the middle thereof wherein the inner diameter of the spherical sealing element is greater than an outer diameter of a tubular and greater than an outer diameter of a tubular joint. An upper end of the spherical sealing element is reinforced by a plurality of flanged steel inserts and a lower end of the spherical sealing element is supported by a movable tapered piston. In operation, fluid pressure wedges the tapered piston against the spherical sealing element, thus urging the spherical sealing element against the plurality of flanged steel inserts and causes the spherical sealing element to move radially outward into contact with the tubular to form a seal between the BOP and the tubular. Even though an effective seal is formed between the BOP and the tubular, the spherical element may be damaged as the tubular is rotated and tubular joints are stripped through a closed BOP. More specifically, as the spherical sealing element is urged against the plurality of flanged inserts, the sealing element tends to extrude under the noses of the flanged inserts where it is restricted from movement and forced into the path of the moving tool joint which results in damage to the spherical sealing element. 
     In many instances, a conventional rotating control head, also referred to as a rotating blow out preventor, is mounted above the BOP stack. An internal portion of the conventional rotating control head is designed to seal and rotate with the string of drill pipe. The internal portion typically includes an internal sealing element mounted on a plurality of bearings. The internal sealing element may consist of both a passive seal arrangement and an active seal arrangement. The active seal arrangement is hydraulically activated. Generally, a hydraulic circuit provides hydraulic fluid to the rotating control head. The hydraulic circuit typically includes a reservoir containing a supply of hydraulic fluid and a pump to communicate the hydraulic fluid from the reservoir to the rotating control head. As the hydraulic fluid enters the rotating control head, a pressure is created to energize the active seal arrangement. During the drilling operation, the string of drill pipe is axially and slidably forced through the rotating control head. The string of drill pipe is made up of individual drill pipes connected together at tool joints. The tool joints have a larger diameter than each individual drill pipe. In order to seal the mouth of the well, the active seal arrangement in the rotating control head must effectively maintain a seal around each drill pipe and the larger diameter joints between each drill pipe. However, the active seal arrangement in the conventional rotating control head has a tendency to leak at the seal as the string of drill pipe is axially forced through the rotating control head which may result in eventual failure of the rotating control head. 
     Additionally, as the string of drill pipe is axially and slidably forced through the rotating control head, the axial movement of the drill pipe causes wear and tear on the bearing and seal assembly and subsequently requires repair. Typically, the drill pipe or a portion thereof is pulled from the well and a crew goes below the drilling platform to manually release the bearing and seal assembly in the rotating control head. Thereafter, an air tugger in combination with a tool joint on the drill string is used to lift the bearing and seal assembly from the rotating control head. The bearing and seal assembly is replaced or reworked and thereafter the crew goes below the drilling platform to reattach the bearing and seal assembly into the rotating control head and operation is resumed. The process is time consuming and can be dangerous. 
     A need therefore exists for an improved active seal arrangement for a rotating control head. There is a further need for an active seal arrangement that can be efficiently removed from the rotating control for repair or replacement. 
     SUMMARY OF THE INVENTION 
     The present invention generally relates to an apparatus and method for sealing a tubular string. In one aspect, a seal assembly for use with a rotating control head is provided. The seal assembly includes a rotatable member and a cavity formed between the rotatable member and a tubular radially inwardly disposed from the rotatable member. The cavity having a first surface and a second surface. The seal assembly further includes a seal member having a first end and a second end disposed between the first surface and second surface of the cavity and sealable with the tubular between the first and the second ends due to deformation of the seal member. 
     In a further aspect, a method for sealing an annular space defined by a wellbore tubular and a seal housing is provided. The method includes providing a seal within a variable volume cavity, wherein the cavity is contained within the seal housing. The method further includes providing a wellbore tubular extending through the seal housing and presenting a variable diameter outer surface for engaging the seal. Additionally, the method includes automatically varying the volume of the cavity in response to a variation in diameter of the outer surface. 
     In yet a further aspect, a method for sealing a tubular in a control head is provided. The method includes providing a seal member contained within a substantially cylindrical volume and causing the seal member to deform radially by applying a compressive force to an end of the seal assembly from an end of the volume. Additionally, the method includes balancing the compressive force with a radial reforming force to allow an object applying the reforming force to pass axially through the seal member. 
     In one embodiment, an assembly for sealingly engaging a tubular may comprise a housing and a seal member disposed in the housing. A first chamber may be formed between an inner surface of the housing and an outer surface of the seal member. The assembly may comprise a piston coupled to the seal member that is moveable relative to the housing. The piston may comprise a second chamber, and a substantially constant fluid pressure may be supplied to the first and second chambers to force the seal member into sealing engagement with the tubular. 
     In one embodiment, a method for sealingly engaging a tubular may comprise moving the tubular through a seal assembly, wherein the seal assembly includes a housing, a seal member disposed in the housing, and a piston coupled to the seal member. The method may further comprise supplying fluid pressure to a first chamber formed between an inner surface of the housing and an outer surface of the seal member, and supplying fluid pressure to a second chamber formed by the piston. The method may further comprise forming a seal between the tubular and the seal member, and maintaining a substantially constant fluid pressure in the first and second chambers. 
     In one embodiment, a method for sealingly engaging a tubular may comprise moving the tubular through a seal assembly, wherein the seal assembly includes a housing, a seal member disposed in the housing, and a piston coupled to the seal member, wherein a first chamber is formed between the seal member and the housing, and wherein a second chamber is formed between the piston and the housing. The method may further comprise supplying a substantially constant fluid pressure to at least one of the first and second chambers, forming a seal between the tubular and the seal member, and adjusting a volume of the first and second chambers while maintaining the substantially constant fluid pressure and the seal between the tubular and the seal member. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. 
         FIG. 1  is a cross-sectional view illustrating the rotating control head in accordance with the present invention. 
         FIG. 2  is an enlarged sectional view of one embodiment of the active seal assembly. 
         FIG. 3  is a sectional view illustrating the tubular urged through the active seal assembly of the rotating control head. 
         FIG. 4  is a sectional view illustrating the tubular urged further through the active seal assembly of the rotating control head. 
         FIG. 5  is an enlarged sectional view of another embodiment of the active seal assembly. 
         FIG. 6  is an enlarged sectional view of another embodiment of the active seal assembly. 
         FIG. 7  is an enlarged sectional view of another embodiment of the active seal assembly. 
         FIG. 8  is an enlarged sectional view of another embodiment of the active seal assembly. 
         FIG. 9  is a cross-sectional view illustrating another embodiment of a rotating control head in accordance with the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     The present invention generally relates to a rotating control head for use with a drilling rig. Typically, an internal portion of the rotating control head is designed to seal around a rotating tubular string and rotate with the tubular string by use of an internal sealing element, and rotating bearings. Additionally, the internal portion of the rotating control head permits the tubular string to move axially and slidably through the rotating control head.  FIGS. 1 and 9  generally describe the rotating control head and  FIGS. 2-8  describe several embodiments of a sealing assembly. To better understand the novelty of the present invention and the methods of use thereof, reference is hereafter made to the accompanying drawings. 
       FIG. 1  is a cross-sectional view illustrating the rotating control head  100  in accordance with the present invention. The rotating control head  100  includes an active seal assembly  105  and a passive seal assembly  110 . Each seal assembly  105 ,  110  includes components that rotate with respect to a housing  115 . The components that rotate in the rotating control head  100  are mounted for rotation on a rotatable member such as a plurality of bearings  125 . 
     As depicted, the active seal assembly  105  includes a support housing  135  mounted on the plurality of bearings  125 . The active seal assembly  105  includes an annular cavity  160  also referred to as a substantially cylindrical volume for housing a seal member  130 . The cavity  160  is formed between a tubular  120  and a backing surface  145  of the support housing  135 . The cavity  160  is a variable volume cavity. More specifically, the cavity  160  includes a fixed end  155  and a movable wall portion in the form of a piston  180  at another end thereof. The piston  180  is movable within a chamber  170  and thereby permits the volume and the shape of the cavity  160  to change due to a change in the shape of the seal member  130 . The chamber  170  may include a pressure P 1 . Additionally, as illustrated in  FIG. 1 , a chamber  225  is formed between the housing  135  and the seal  130 . The chamber  225  may include a pressure P 2 . In one embodiment, the piston  180  is a compliant piston which means that the piston is movable to conform or to adapt to the change of shape of the cavity  160  due to a change in the shape of the seal member  130 . 
     The seal member  130  is typically made from a solid flexible material, such as an elastomer. As will be described herein, the application of a force on the seal member  130  causes the mid section of the seal member  130  at an inner surface  175  to contact and create a seal between the rotating control head  100  and the tubular  120 . The tubular  120  has a variable diameter outer surface. For instance, the tubular  120  includes a smaller diameter outer surface  140  and a larger diameter outer surface  185 . As shown in  FIG. 1 , the smaller diameter outer surface  140  is the outer surface of a single tubular and the larger diameter outer surface  185  is typically formed at a joint between two tubulars in the tubular string  120 . In one embodiment, the seal member  130  is arranged such that an inner diameter of the seal member  130  is slightly larger than the outer diameter surface  140  of the tubular  120  yet smaller than an outer diameter surface  185  of the tubular  120  to allow an interference fit therebetween. Furthermore, a wellbore pressure P 3  below the active seal assembly  105  may be utilized to assist the piston  180  in the formation of a seal between the seal member  130  and the tubular  120 . 
     In the embodiment shown in  FIG. 1 , the passive seal assembly  110  is disposed above the active seal assembly  105 . It should be understood, however, that the passive seal assembly  110  may be positioned below the active seal assembly without departing from principles of the present invention. The passive seal assembly  110  is operatively attached to the support housing  135 , thereby allowing the passive seal assembly  110  to rotate with the active seal assembly  105 . Fluid is not required to operate the passive seal assembly  110 , but rather the assembly  110  utilizes the wellbore pressure P 3  to create a seal around the tubular  120 . The passive seal assembly  110  is constructed and arranged in an axially downward conical shape, thereby allowing the wellbore pressure P 3  to act against a tapered surface  195  to close the passive seal assembly  110  around the tubular. Additionally, the passive seal assembly  110  includes an inner diameter  190  smaller than the outer diameter of the tubular to allow an interference fit between the tubular  120  and the passive seal assembly  110 . 
     The rotating control head  100  also includes a releasable member  250  for connecting the active seal assembly  105  to the housing  115 . If a component of the active seal assembly  105  requires repair or replacement, then the releasable member  250  is activated which allows the active seal assembly  105  to be released easily from the housing  115 . Due to the size of the active seal assembly  105 , the seal assembly  105  typically may be removed without having to use a crane to lift the rotating control head  100  and without disassembling portions of the drilling platform. After the component in the active seal assembly  105  is replaced or repaired, then the active seal assembly  105  may be once again easily attached to the housing  115  and secured into place by the releasable member  250 . An example of a high pressure rotating drilling head assembly with a hydraulically removable packer is disclosed in U.S. Pat. No. 6,547,002 and U.S. Pat. No. 6,702,012, both of which are incorporated herein in their entirety. 
       FIG. 2  is an enlarged sectional view of one embodiment of the active seal assembly  105 . As shown, the seal  130  has been urged radially inward into contact with the tubular  120 , thereby forming a sealing relationship between the tubular  120  and the rotating control head  100 . In this embodiment, the sealing relationship is formed by urging fluid through a port  205  into the chamber  225  formed between the housing  135  and the seal  130 . As fluid builds up in the chamber  225 , the fluid pressure P 2  urges the seal  130  toward the tubular  120  to form the sealing relationship therebetween. Thereafter, a hydraulic control (not shown) maintains and monitors the fluid pressure P 2  in the chamber  225 . In this embodiment, the fluid pressure P 2  is preferably maintained between 0 to 200 psi above the wellbore pressure P 3  and the piston pressure P 1  is maintained at atmospheric pressure. Additionally, as shown in  FIG. 2 , the end  155  of the cavity  160  includes an extension member  215  to support an end of the seal  130 . 
       FIG. 3  is a sectional view illustrating the tubular  120  urged axially through the active seal assembly  105  of the rotating control head  100 . As shown, a portion of the larger diameter outer surface  185  has moved through the seal assembly  105 , thereby causing the seal  130  to move toward the backing surface  145  of the housing  135  and reconfigure the shape of the cavity  160  by moving the piston  180  away from the end  155 . At the same time, the pressure P 1  increases as the volume in the chamber  170  decreases due to the movement of the piston  180 . Additionally, the pressure P 2  in the chamber  225  is monitored and adjusted accordingly by the hydraulic control unit. 
       FIG. 4  is a sectional view illustrating the tubular  120  urged axially further through the active seal assembly  105  of the rotating control head  100 . As shown, the smaller diameter surface  140  of the tubular  120  is again now in contact with the seal  130 , thereby allowing the seal member  130  to move away from the backing surface  145  of the housing  135  and reconfigure the shape of the cavity  160  by allowing the piston  180  to move away from the end  155 . At the same time, the pressure P 1  decreases as the volume in the chamber increases due to the movement of the piston  180 . Additionally, the pressure P 2  in the chamber  225  is monitored and adjusted due to the movement of the tubular  120 . 
       FIG. 5  is an enlarged sectional view of another embodiment of the active seal assembly  105 . For convenience, components in  FIG. 5  that are similar to components in  FIG. 2  will be labeled with the same number indicator. As shown, the seal  130  has been urged radially inward into contact with the tubular  120 , thereby forming a sealing relationship between the tubular  120  and the rotating control head  100 . In this embodiment, the sealing relationship is formed by urging fluid through the port  205  into the chamber  225  formed between the housing  135  and the seal  130  and by urging fluid through a port  210  into the chamber  170  formed between the housing  135  and the piston  180 . As fluid builds up in chamber  225  and chamber  170 , the fluid pressure P 2  and the fluid pressure P 1  urge the seal  130  toward the tubular  120  to form the sealing relationship therebetween. Thereafter, the hydraulic control maintains and monitors the fluid pressure P 2  in chamber  225  and the fluid pressure P 1  in chamber  170 . As the larger diameter outer surface  185  of the tubular  120  is urged through the seal assembly  105 , the seal  130  moves toward the backing surface  145  of the support housing  135  and subsequently reconfigures the shape of the cavity  160  by moving the piston  180  in the chamber  170 . In this embodiment, the fluid pressure P 1  is preferably maintained between 0 to 200 psi above the wellbore pressure P 3  and the fluid pressure P 2  is preferably maintained around 25% to 75% of P 1 . In another embodiment, the fluid pressure P 2  is preferably maintained between 0 to 200 psi above the wellbore pressure P 3  and the fluid pressure P 2  is preferably maintained around 25% to 75% of P 1 . In yet another embodiment, both the fluid pressure P 1  and P 2  are preferably maintained between 0 to 200 psi above the wellbore pressure P 3 . 
       FIG. 6  is an enlarged sectional view of another embodiment of the active seal assembly  105 . For convenience, components in  FIG. 6  that are similar to components in  FIG. 2  will be labeled with the same number indicator. As shown, the seal  130  has been urged radially inward into contact with the tubular  120 , thereby forming a sealing relationship between the tubular  120  and the rotating control head  100 . In this embodiment, the sealing relationship is formed by urging fluid through the port  210  into the chamber  170  formed between the housing  135  and the piston  180 . As fluid builds up in the chamber  170 , the fluid pressure P 1  urges the piston  180  towards the end  155  thus changing the volume of the cavity  160  and causing the seal  130  to move toward the tubular  120  to form the sealing relationship therebetween. Thereafter, the hydraulic control maintains and monitors the fluid pressure P 1  in the chamber  170 . As the larger diameter outer surface  185  of the tubular  120  is urged through the seal assembly  105 , the seal  130  moves toward the backing surface  145  of the support housing  135  and subsequently reconfigures the shape of the cavity  160  by moving the piston in the chamber  170 . In this embodiment, the fluid pressure P 1  is preferably maintained between 0 to 200 psi above the wellbore pressure P 3  and the pressure P 2  is maintained at atmospheric pressure. 
       FIG. 7  is an enlarged sectional view of another embodiment of the active seal assembly  105 . For convenience, components in  FIG. 7  that are similar to components in  FIG. 2  will be labeled with the same number indicator. As shown, the seal  130  has been urged radially inward into contact with the tubular  120 , thereby forming a sealing relationship between the tubular  120  and the rotating control head  100 . In this embodiment, the sealing relationship is formed by urging fluid through the port  205  into the chamber  225  formed between the housing  135  and the seal  130  and by urging fluid through a port  235  into the chamber  245  formed between the housing  135  and the seal  130 . As fluid builds up in the chamber  225  and the chamber  245 , the fluid pressure P 2  urges the seal  130  toward the tubular  120  to form the sealing relationship therebetween. Thereafter, the hydraulic control maintains and monitors the fluid pressure P 2  in the chamber  225  and the chamber  245 . As the larger diameter outer surface  185  of the tubular  120  is urged through the seal assembly  105 , the seal  130  moves toward the backing surface  145  of the support housing  135  and subsequently reconfigures the shape of the cavity  160  by moving the piston in the chamber  170 . In this embodiment, the fluid pressure P 2  is preferably maintained between 0 to 200 psi above the wellbore pressure P 3  and the pressure P 1  is maintained at atmospheric pressure. 
       FIG. 8  is an enlarged sectional view of another embodiment of the active seal assembly  105 . For convenience, components in  FIG. 8  that are similar to components in  FIG. 2  will be labeled with the same number indicator. As shown, the seal  130  has been urged radially inward into contact with the tubular  120 , thereby forming a sealing relationship between the tubular  120  and the rotating control head  100 . In this embodiment, the sealing relationship is formed by urging fluid through the port  205  into the chamber  225  and through the port  235  into the chamber  245  and through the port  210  into the chamber  170 . As fluid builds up in the chambers  225 ,  245 ,  170 , the fluid pressures P 2  and P 1  urge the seal  130  toward the tubular  120  to form the sealing relationship therebetween. Thereafter, the hydraulic control maintains and monitors the fluid pressure P 2  in the chambers  225  and  245  and the fluid pressure P 1  in the chamber  170 . As the larger diameter outer surface  185  of the tubular  120  is urged through the seal assembly  105 , the seal  130  moves toward the backing surface  145  of the support housing  135  and subsequently reconfigures the shape of the cavity  160  by moving the piston in the chamber  170 . In this embodiment, the fluid pressure P 1  is preferably maintained between 0 to 200 psi above the wellbore pressure P 3  and the fluid pressure P 2  is preferably maintained around 25% to 75% of P 1 . 
       FIG. 9  is a cross-sectional view illustrating another embodiment of a rotating control head  200  in accordance with the present invention. For convenience, components in  FIG. 9  that are similar to components in  FIG. 1  will be labeled with the same number indicator. As shown in  FIG. 9 , the rotating control head  200  includes the passive seal assembly  110  and the active seal assembly  105  in a similar manner as the rotating control head  100  in  FIG. 1 . The primary difference between the rotating control head  200  and the rotating control head  100  is the location of a movable wall portion in the form of a piston  280  and a corresponding chamber  270 . As illustrated, the piston  280  is located at an upper end of the active seal assembly  105 . Due to this arrangement, the wellbore pressure P 3  does not assist the piston  280  to form the seal between the seal member  130  and the tubular  120  and therefore the pressure P 1  in the chamber  270  must be maintained at higher pressure then the pressure P 1  in the chamber  170  in the rotating control head  100  of  FIG. 1 . Other than the location of the piston  280  and the corresponding chamber  270 , the active seal assembly  105  in rotating control head  200  in  FIG. 9  may be configured and operated in a similar manner as described and shown in  FIGS. 2-8 . 
     While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.

Summary:
A seal assembly for use with a rotating control head is provided. The seal assembly includes a rotatable member and a cavity formed between the rotatable member and a tubular radially inwardly disposed from the rotatable member. The cavity having a first surface and a second surface. The seal assembly further includes a seal member having a first end and a second end disposed between the first surface and the second surface of the cavity and sealable with the tubular between the first and the second ends due to deformation of the seal member.