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: 
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   Embodiments of the present invention relate generally to a method and an apparatus for a drilling operation. More particularly, the invention relates to a rotating control head. Still more particularly, the invention relates to the actuation and cooling of a rotating 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 drill pipe. Thereafter, a drive unit rotates the drill pipe through a drive member, referred to as a kelly as the 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. 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 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 active seal 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. Preferably, the pressure in the active seal arrangement is maintained at a greater pressure than the wellbore pressure. Typically, the hydraulic circuit receives input from the wellbore and supplies hydraulic fluid to the active seal arrangement to maintain the pressure differential. However, the hydraulic circuit in the conventional active seal rotating control head has a less than desirable response time to rapidly changing wellbore pressure. 
   During the drilling operation, the 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 are typically 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. 
   Additionally, the thrust generated by the wellbore fluid pressure and the radial forces on the bearing assembly causes a substantial amount of heat to build in the conventional rotating control head. The heat causes the seals and bearings to wear and subsequently require repair. The conventional rotating control head typically includes a cooling system that circulates oil through the seals and bearings to remove the heat. However, the oil based cooling system may be very expensive to implement and maintain. 
   There is a need therefore, for a cost-effective cooling system for a rotating control head. There is a further need therefore for a cooling system in a rotating control head that can be easily implemented and maintained. There is a further need for an effective hydraulic circuit to actuate the active sealing arrangement in a rotating control head and to maintain the proper pressure differential between the fluid pressure in the rotating control head and the wellbore pressure. There is yet a further need for an improved rotating control head. 
   SUMMARY OF THE INVENTION 
   The present invention generally relates to an apparatus and method for sealing a tubular string. In one aspect, a drilling system is provided. The drilling system includes a rotating control head for sealing the tubular string while permitting axial movement of the string relative to the rotating control head. The drilling system also includes an actuating fluid for actuating the rotating control head and maintaining a pressure differential between a fluid pressure in the rotating control head and a wellbore pressure. Additionally, the drilling system includes a cooling medium for passing through the rotating control head. 
   In another aspect, a rotating control head is provided. The rotating control head includes a sealing member for sealing a tubular string while permitting axial movement of the string relative to the rotating control head. The rotating control head further includes an actuating fluid for actuating the rotating control head and maintaining a pressure differential between a fluid pressure in the rotating control head and a wellbore pressure. 
   In another aspect, a method for sealing a tubular in a rotating control head is provided. The method includes supplying fluid to the rotating control head and activating a seal arrangement to seal around the tubular. The method further includes passing a cooling medium through the rotating control head and maintaining a pressure differential between a fluid pressure in the rotating control head and a wellbore pressure. 

   
     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 a rotating control head in accord with the present invention. 
       FIG. 2A  illustrates a rotating control head cooled by a heat exchanger. 
       FIG. 2B  illustrates a schematic view of the heat exchanger. 
       FIG. 3A  illustrates a rotating control head cooled by flow a gas. 
       FIG. 3B  illustrates a schematic view of the gas in a substantially circular passageway. 
       FIG. 4A  illustrates a rotating control head cooled by a fluid mixture. 
       FIG. 4B  illustrates a schematic view of the fluid mixture circulating in a substantially circular passageway. 
       FIG. 5A  illustrates the rotating control head cooled by a refrigerant. 
       FIG. 5B  illustrates a schematic view of the refrigerant circulating in a substantially circular passageway. 
       FIG. 6  illustrates a rotating control head actuated by a piston intensifier in communication with the wellbore pressure. 
       FIG. 7A  illustrates an alternative embodiment of a rotating control head in an unlocked position. 
       FIG. 7B  illustrates the rotating control head in a locked position. 
       FIG. 8  illustrates an alternative embodiment of a rotating control head in accord with the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Generally, the present invention 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 on the drilling rig.  FIG. 1  generally describes the rotating control head and  FIGS. 2–6  illustrate various methods of cooling and actuating the rotating control head. Additionally,  FIGS. 7 and 8  illustrate alternate embodiments of the rotating control head. 
     FIG. 1  is a cross-sectional view illustrating the rotating control head  100  in accord with the present invention. The rotating control head  100  preferably 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 are mounted for rotation on a plurality of bearings  125 . 
   As depicted, the active seal assembly  105  includes a bladder support housing  135  mounted on the plurality of bearings  125 . The bladder support housing  135  is used to mount bladder  130 . Under hydraulic pressure, as discussed below, bladder  130  moves radially inward to seal around a tubular such as a drilling pipe (not shown). In this manner, bladder  130  can expand to seal off borehole  185  through the rotating control head  100 . 
   As illustrated in  FIG. 1 , upper and lower caps  140 ,  145 , respectfully, fit over the upper and lower end of the bladder  130  to secure the bladder  130  within the bladder support housing  135 . Typically, the upper and lower caps  140 ,  145  are secured in position by a setscrew (not shown). Upper and lower seals  155 ,  160 , respectfully, seal off chamber  150  that is preferably defined radially outwardly of bladder  130  and radially inwardly of bladder support housing  135 . 
   Generally, fluid is supplied to the chamber  150  under a controlled pressure to energize the bladder  130 . The hydraulic control (not shown) will be illustrated and discussed in  FIGS. 2–6 . Essentially, the hydraulic control maintains and monitors hydraulic pressure within pressure chamber  150 . Hydraulic pressure P 1  is preferably maintained by the hydraulic control between 0 to 200 psi above a wellbore pressure P 2 . The bladder  130  is constructed from flexible material allowing bladder surface  175  to press against the tubular at approximately the same pressure as the hydraulic pressure P 1 . Due to the flexibility of the bladder, it also may conveniently seal around irregular shaped tubular string such as a hexagonal kelly. In this respect, the hydraulic control maintains the differential pressure between the pressure chamber  150  at pressure P 1  and wellbore pressure P 2 . Additionally, the active seal assembly  105  includes support fingers  180  to provide support to the bladder  130  at the most stressful area of the seal between the fluid pressure P 1  and the ambient pressure. 
   The hydraulic control may be used to de-energize the bladder  130  and allow the active seal assembly  105  to release the seal around the tubular. Generally, fluid in the chamber  150  is drained into a hydraulic reservoir (not shown), thereby reducing the pressure P 1 . Subsequently, the bladder surface  175  loses contact with the tubular as the bladder  130  becomes de-energized and moves radially outward. In this manner, the seal around the tubular is released allowing the tubular to be removed from the rotating control head  100 . 
   In the embodiment shown in  FIG. 1 , the passive seal assembly  110  is disposed below the active seal assembly  105 . The passive seal assembly  110  is operatively attached to the bladder 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 it utilizes pressure P 2  to create a seal around the tubular. The passive seal assembly  110  is constructed and arranged in an axially downward conical shape, thereby allowing the pressure P 2  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 and the passive seal assembly  110 . 
     FIG. 2A  illustrates a rotating control head  200  cooled by heat exchanger  205 . As shown, the rotating control head  200  is depicted generally to illustrate this embodiment of the invention, thereby applying this embodiment to a variety of different types of rotating control heads. A hydraulic control  210  provides fluid to the rotating control head  200 . The hydraulic control  210  typically includes a reservoir  215  to contain a supply of fluid, a pump  220  to communicate the fluid from the reservoir  215  to the rotating control head  200  and a valve  225  to remove excess pressure in the rotating control head  200 . 
   Generally, the hydraulic control  210  provides fluid to energize a bladder  230  and lubricate a plurality of bearings  255 . As the fluid enters a port  235 , the fluid is communicated to the plurality of bearings  255  and a chamber  240 . As the chamber  240  fills with a fluid, pressure P 1  is created. The pressure P 1  acts against the bladder  230  causing the bladder  230  to expand radially inward to seal around a tubular string (not shown). Typically, the pressure P 1  is maintained between 0-200 psi above a wellbore pressure P 2 . 
   The rotating control head  200  is cooled by the heat exchanger  205 . The heat exchanger  205  is constructed and arranged to remove heat from the rotating control head  200  by introducing a gas, such as air, at a low temperature into an inlet  265  and thereafter transferring heat energy from a plurality of seals  275  and the plurality of bearings  255  to the gas as the gas passes through the heat exchanger  205 . Subsequently, the gas at a higher temperature exits the heat exchanger  205  through an outlet  270 . Typically, gas is pumped into the inlet  265  by a blowing apparatus (not shown). However, other means of communicating gas to the inlet  265  may be employed, so long as they are capable of supplying a sufficient amount of gas to the heat exchanger  205 . 
     FIG. 2B  illustrates a schematic view of the heat exchanger  205 . As illustrated, the heat exchanger  205  comprises a passageway  280  with a plurality of substantially square curves. The passageway  280  is arranged to maximize the surface area covered by the heat exchanger  205 . The low temperature gas entering the inlet  265  flows through the passageway  280  in the direction illustrated by arrow  285 . As the gas circulates through the passageway  280 , the gas increases in temperature as the heat from the rotating control head  200  is transferred to the gas. The high temperature gas exits the outlet  270  as indicated by the direction of arrow  285 . In this manner, the heat generated by the rotating control head  200  is transferred to the gas passing through the heat exchanger  205 . 
     FIG. 3A  illustrates a rotating control head  300  cooled by a gas. As shown, the rotating control head  300  is depicted generally to illustrate this embodiment of the invention, thereby applying this embodiment to a variety of different types of rotating control heads. A hydraulic control  310  supplies fluid to the rotating control head  300 . The hydraulic control  310  typically includes a reservoir  315  to contain a supply of fluid and a pump  320  to communicate the fluid from the reservoir  315  to the rotating control head  300 . Additionally, the hydraulic control  310  includes a valve  345  to relieve excess pressure in the rotating control head  300 . 
   Generally, the hydraulic control  310  supplies fluid to energize a bladder  330  and lubricate a plurality of bearings  355 . As the fluid enters a port  335 , a portion is communicated to the plurality of bearings  355  and another portion is used to fill a chamber  340 . As the chamber  340  fills with a fluid, a pressure P 1  is created. Pressure P 1  acts against the bladder  330  causing the bladder  330  to move radially inward to seal around a tubular string (not shown). Typically, the pressure P 1  is maintained between 0 to 200 psi above a wellbore pressure P 2 . If the wellbore pressure P 2  drops, the pressure P 1  may be relieved through valve  345  by removing a portion of the fluid from the chamber  340 . 
   The rotating control head  300  is cooled by a flow of gas through a substantially circular passageway  380  through an upper portion of the rotating control head  300 . The circular passageway  380  is constructed and arranged to remove heat from the rotating control head  300  by introducing a gas, such as air, at a low temperature into an inlet  365 , transferring heat energy to the gas and subsequently allowing the gas at a high temperature to exit through an outlet  370 . The heat energy is transferred from a plurality of seals  375  and the plurality of bearings  355  as the gas passes through the circular passageway  380 . Typically, gas is pumped into the inlet  365  by a blowing apparatus (not shown). However, other means of communicating gas to the inlet  365  may be employed, so long as they are capable of supplying a sufficient amount of gas to the substantially circular passageway  380 . 
     FIG. 3B  illustrates a schematic view of the gas passing through the substantially circular passageway  380 . The circular passageway  380  is arranged to maximize the surface area covered by the circular passageway  380 . The low temperature gas entering the inlet  365  flows through the circular passageway  380  in the direction illustrated by arrow  385 . As the gas circulates through the circular passageway  380 , the gas increases in temperature as the heat from the rotating control head  300  is transferred to the gas. The high temperature gas exits the outlet  370  as indicated by the direction of arrow  385 . In this manner, the heat generated by the rotating control head  300  is removed allowing the rotating control head  300  to function properly. 
   In an alternative embodiment, the rotating control head  300  may operate without the use of the circular passageway  380 . In other words, the rotating control head  300  would function properly without removing heat from the plurality of seals  375  and the plurality of bearings  355 . This embodiment typically applies when the wellbore pressure P 2  is relatively low. 
     FIGS. 4A and 4B  illustrate a rotating control head  400  cooled by a fluid mixture. As shown, the rotating control head  400  is depicted generally to illustrate this embodiment of the invention, thereby applying this embodiment to a variety of different types of rotating control heads. A hydraulic control  410  supplies fluid to the rotating control head  400 . The hydraulic control  410  typically includes a reservoir  415  to contain a supply of fluid and a pump  420  to communicate the fluid from the reservoir  415  to the rotating control head  400 . Additionally, the hydraulic control  410  includes a valve  445  to relieve excess pressure in the rotating control head  400 . In the same manner as the hydraulic control  310 , the hydraulic control  410  supplies fluid to energize a bladder  430  and lubricate a plurality of bearings  455 . 
   The rotating control head  400  is cooled by a fluid mixture circulated through a substantially circular passageway  480  on an upper portion of the rotating control head  400 . In the embodiment shown, the fluid mixture preferably consists of water or a water-glycol mixture. However, other mixtures of fluid may be employed, so long as, the fluid mixture has the capability to circulate through the circular passageway  480  and reduce the heat in the rotating control head  400 . 
   The circular passageway  480  is constructed and arranged to remove heat from the rotating control head  400  by introducing the fluid mixture at a low temperature into an inlet  465 , transferring heat energy to the fluid mixture and subsequently allowing the fluid mixture at a high temperature to exit through an outlet  470 . The heat energy is transferred from a plurality of seals  475  and the plurality of bearings  455  as the fluid mixture circulates through the circular passageway  480 . The fluid mixture is preferably pumped into the inlet  465  through a fluid circuit  425 . The fluid circuit  425  is comprised of a reservoir  490  to contain a supply of the fluid mixture and a pump  495  to circulate the fluid mixture through the rotating control head  400 . 
     FIG. 4B  illustrates a schematic view of the fluid mixture circulating in the substantially circular passageway  480 . The circular passageway  480  is arranged to maximize the surface area covered by the circular passageway  480 . The low temperature fluid entering the inlet  465  flows through the circular passageway  480  in the direction illustrated by arrow  485 . As the fluid circulates through the circular passageway  480 , the fluid increases in temperature as the heat from the rotating control head  400  is transferred to the fluid. The high temperature fluid exits out the outlet  470  as indicated by the direction of arrow  485 . In this manner, the heat generated by the rotating control head  400  is removed allowing the rotating control head  400  to function properly. 
     FIGS. 5A and 5B  illustrate a rotating control head  500  cooled by a refrigerant. As shown, the rotating control head  500  is depicted generally to illustrate this embodiment of the invention, thereby applying this embodiment to a variety of different types of rotating control heads. A hydraulic control  510  supplies fluid to the rotating control head  500 . The hydraulic control  510  typically includes a reservoir  515  to contain a supply of fluid and a pump  520  to communicate the fluid from the reservoir  515  to the rotating control head  500 . Additionally, the hydraulic control  510  includes a valve  545  to relieve excess pressure in the rotating control head  500 . In the same manner as the hydraulic control  310 , the hydraulic control  510  supplies fluid to energize a bladder  530  and lubricate a plurality of bearings  555 . 
   The rotating control head  500  is cooled by a refrigerant circulated through a substantially circular passageway  580  in an upper portion of the rotating control head  500 . The circular passageway  580  is constructed and arranged to remove heat from the rotating control head  500  by introducing the refrigerant at a low temperature into an inlet  565 , transferring heat energy to the refrigerant and subsequently allowing the refrigerant at a high temperature to exit through an outlet  570 . The heat energy is transferred from a plurality of seals  575  and the plurality of bearings  555  as the refrigerant circulates through the circular passageway  580 . The refrigerant is preferably communicated into the inlet  565  through a refrigerant circuit  525 . The refrigerant circuit  525  includes a reservoir  590  containing a supply of vapor refrigerant. A compressor  595  draws the vapor refrigerant from the reservoir  590  and compresses the vapor refrigerant into a liquid refrigerant. Thereafter, the liquid refrigerant is communicated to an expansion valve  560 . At this point, the expansion valve  560  changes the low temperature liquid refrigerant into a low temperature vapor refrigerant as the refrigerant enters inlet  565 . 
     FIG. 5B  illustrates a schematic view of the vapor refrigerant circulating in the substantially circular passageway  580 . The circular passageway  580  is arranged in an approximately 320-degree arc to maximize the surface area covered by the circular passageway  580 . The low temperature vapor refrigerant entering the inlet  565  flows through the circular passageway  580  in the direction illustrated by arrow  585 . As the vapor refrigerant circulates through the circular passageway  580 , the vapor refrigerant increases in temperature as the heat from the rotating control head  500  is transferred to the vapor refrigerant. The high temperature vapor refrigerant exits out the outlet  570  as indicated by the direction of arrow  585 . Thereafter, the high temperature vapor refrigerant rejects the heat to the environment through a heat exchanger (not shown) and returns to the reservoir  590 . In this manner, the heat generated by the rotating control head  500  is removed allowing the rotating control head  500  to function properly. 
     FIG. 6  illustrates a rotating control head  600  actuated by a piston intensifier circuit  610  in communication with a wellbore  680 . As shown, the rotating control head  600  is depicted generally to illustrate this embodiment of the invention, thereby applying this embodiment to a variety of different types of rotating control heads. The piston intensifier circuit  610  supplies fluid to the rotating control head  600 . The piston intensifier circuit  610  typically includes a housing  645  and a piston arrangement  630 . The piston arrangement  630  is formed from a larger piston  620  and a smaller piston  615 . The pistons  615 ,  620  are constructed and arranged to maintain a pressure differential between a hydraulic pressure P 1  and a wellbore pressure P 2 . In other words, the pistons  615 ,  620  are designed with a specific surface area ratio to maintain about a 200 psi pressure differential between the hydraulic pressure P 1  and the wellbore pressure P 2 , thereby allowing the P 1  to be 200 psi higher than P 2 . The piston arrangement  630  is disposed in the housing  645  to form an upper chamber  660  and lower chamber  685 . Additionally, a plurality of seal members  605  are disposed around the pistons  615 ,  620  to form a fluid tight seal between the chambers  660 ,  685 . 
   The piston intensifier circuit  610  mechanically provides hydraulic pressure P 1  to energize a bladder  650 . Initially, fluid is filled into upper chamber  660  and is thereafter sealed. The wellbore fluid from the wellbore  680  is in fluid communication with lower chamber  685 . Therefore, as the wellbore pressure P 2  increases more wellbore fluid is communicated to the lower chamber  685  creating a pressure in the lower chamber  685 . The pressure in the lower chamber  685  causes the piston arrangement  630  to move axially upward forcing fluid in the upper chamber  660  to enter port  635  and pressurize a chamber  640 . As the chamber  640  fills with a fluid, the pressure P 1  increases causing the bladder  650  to move radially inward to seal around a tubular string (not shown). In this manner, the bladder  650  is energized allowing the rotating control head  600  to seal around a tubular. 
   A fluid, such as water-glycol, is circulated through the rotating control head  600  by a fluid circuit  625 . Typically, heat on the rotating control head  600  is removed by introducing the fluid at a low temperature into an inlet  665 , transferring heat energy to the fluid and subsequently allowing the fluid at a high temperature to exit through an outlet  670 . The heat energy is transferred from a plurality of seals  675  and the plurality of bearings  655  as the fluid circulates through the rotating control head  600 . The fluid is preferably pumped into the inlet  665  through the fluid circuit  625 . Generally, the circuit  625  comprises a reservoir  690  to contain a supply of the fluid and a pump  695  to circulate the fluid through the rotating control head  600 . 
   In another embodiment, the piston intensifier circuit  610  is in fluid communication with a nitrogen gas source (not shown). In this embodiment, a pressure transducer (not shown) measures the wellbore pressure P 2  and subsequently injects nitrogen into the lower chamber  685  at the same pressure as pressure P 2 . The nitrogen pressure in the lower chamber  685  may be adjusted as the wellbore pressure P 2  changes, thereby maintaining the desired pressure differential between hydraulic pressure P 1  and wellbore pressure P 2 . 
     FIG. 7A  illustrates an alternative embodiment of a rotating control head  700  in an unlocked position. The rotating control head  700  is arranged and constructed in a similar manner as the rotating control head  100  shown on  FIG. 1 . Therefore, for convenience, similar components that function in the same manner will be labeled with the same numbers as the rotating control head  100 . The primary difference between the rotating control head  700  and rotating control head  100  is the active seal assembly. 
   As shown in  FIG. 7A , the rotating control head  700  includes an active seal assembly  705 . The active seal assembly  705  includes a primary seal  735  that moves radially inward as a piston  715  wedges against a tapered surface of the seal  735 . The primary seal  735  is constructed from flexible material to permit sealing around irregularly shaped tubular string such as a hexagonal kelly. The upper end of the seal  735  is connected to a top ring  710 . 
   The active sealing assembly  705  includes an upper chamber  720  and a lower chamber  725 . The upper chamber  720  is formed between the piston  715  and a piston housing  740 . To move the rotating control head  700  from an unlocked position to a locked position, fluid is pumped through port  745  into an upper chamber  720 . As fluid fills the upper chamber  720 , the pressure created acts against the lower end of the piston  715  and urges the piston  715  axially upward until it reaches the top ring  710 . At the same time, the piston  715  wedges against the tapered portion of the primary seal  735  causing the seal  735  to move radially inward to seal against the tubular string. In this manner, the active seal assembly  705  is in the locked position as illustrated in  FIG. 7B . 
   As shown on  FIG. 7B , the piston  715  has moved axially upward contacting the top ring  710  and the primary seal  735  has moved radially inward. To move the active seal assembly  705  from the locked position to the unlocked position, fluid is pumped through port  755  into the lower chamber  725 . As the chamber fills up, the fluid creates a pressure that acts against surface  760  to urge the piston  715  axially downward, thereby allowing the primary seal  735  to move radially outward as shown on  FIG. 7A . 
     FIG. 8  illustrates an alternative embodiment of a rotating control head  800  in accord with the present invention. The rotating control head  800  is constructed from similar components as the rotating control head  100  shown on  FIG. 1 . Therefore, for convenience, similar components that function in the same manner will be labeled with the same numbers as the rotating control head  100 . The primary difference between the rotating control head  800  and rotating control head  100  is the location of the active seal assembly  105  and the passive seal assembly  110 . 
   As shown on  FIG. 8 , the passive seal assembly  110  is disposed above the active seal assembly  105 . The passive seal assembly  110  is operatively attached to the bladder support housing  135 , thereby allowing the passive seal assembly  110  to rotate with the active seal assembly  105 . The passive seal assembly  110  is constructed and arranged in an axially downward conical shape, thereby allowing the pressure in the rotating control head  800  to act against the tapered surface  195  and close the passive seal assembly  110  around the tubular. Additionally, the passive seal assembly  110  includes the inner diameter  190 , which is smaller than the outer diameter of the tubular to allow an interference fit between the tubular and the passive seal assembly  110 . 
   As depicted, the active seal assembly  105  includes the bladder support housing  135  mounted on the plurality of bearings  125 . The bladder support housing  135  is used to mount bladder  130 . Under hydraulic pressure, bladder  130  moves radially inward to seal around a tubular such as a drilling tubular. Generally, fluid is supplied to the chamber  150  under a controlled pressure to energize the bladder  130 . Essentially, a hydraulic control (not shown) maintains and monitors hydraulic pressure within pressure chamber  150 . Hydraulic pressure P 1  is preferably maintained by the hydraulic control between 0 to 200 psi above a wellbore pressure P 2 . The bladder  130  is constructed from flexible material allowing bladder surface  175  to press against the tubular at approximately the same pressure as the hydraulic pressure P 1 . 
   The hydraulic control may be used to de-energize the bladder  130  and allow the active seal assembly  105  to release the seal around the tubular. Generally, the fluid in the chamber  150  is drained into a hydraulic reservoir (not shown), thereby reducing the pressure P 1 . Subsequently, the bladder surface  175  loses contact with the tubular as the bladder  130  becomes de-energized and moves radially outward. In this manner, the seal around the tubular is released allowing the tubular to be from the rotating control head  800 . 
   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:
The present invention generally relates to an apparatus and method for sealing a tubular string. In one aspect, a drilling system is provided. The drilling system includes a rotating control head for sealing the tubular string while permitting axial movement of the string relative to the rotating control head. The drilling system also includes an actuating fluid for actuating the rotating control head and maintaining a pressure differential between a fluid pressure in the rotating control head and a wellbore pressure. Additionally, the drilling system includes a cooling medium for passing through the rotating control head. In another aspect, a rotating control head is provided. In yet another aspect, a method for sealing a tubular in a rotating control head is provided.