Abstract:
Force balancing adjusts hydraulic fluid pressure in an upper piston area of a Rotating Pressure Control Device (RPCD) that has an inner housing rotatably engaged within an outer housing by an upper bearing and a lower bearing. The hydraulic fluid pressure is adjusted to balance net force in a upper piston area and a lower piston area. The fluid pressure adjustment creates a force differential that balances the total load transmitted through the upper bearing and the lower bearing and thereby extends the life of the sealing element and bearings. Additionally, a wear indicator signals the end of the useful life of the drill pipe sealing element.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a divisional of application Ser. No. 11/757,892, filed Jun. 4, 2007, status allowed. 
     The present invention is related to the subject matter of U.S. patent application Ser. No. 10/922,029. 
    
    
     FIELD OF THE INVENTION 
     The present invention is directed generally at drilling blowout preventers used in drilling oil and gas wells, and specifically to a rotating pressure control device for use in both under-balanced drilling applications and managed pressure drilling applications. 
     BACKGROUND OF THE INVENTION 
     When the hydrostatic weight of the column of mud in a well bore is less than the formation pressure, the potential for a blowout exists. A blowout occurs when the formation expels hydrocarbons into the well bore. The expulsion of hydrocarbons into the well bore dramatically increases the pressure within a section of the well bore. The increase in pressure sends a pressure wave up the well bore to the surface. The pressure wave can damage the equipment that maintains the pressure within the well bore. In addition to the pressure wave, the hydrocarbons travel up the well bore because the hydrocarbons are less dense than the mud. If the hydrocarbons reach the surface and exit the well bore through the damaged surface equipment, there is a high probability that the hydrocarbons will be ignited by the drilling or production equipment operating at the surface. The ignition of the hydrocarbons produces an explosion and/or fire that is dangerous for the drilling operators. In order to minimize the risk of blowouts, drilling rigs are required to employ a plurality of different pressure control devices, such as an annular pressure control device, a pipe ram pressure control device, and a blind ram pressure control device. If a “closed loop drilling” method is used, then a rotating pressure control device will be added on top of the conventional pressure control stack. Persons of ordinary skill in the art are aware of other types of pressure control devices. The various pressure control devices are positioned on top of one another, along with any other necessary surface connections, such as the choke and kill lines for managed pressure drilling applications and nitrogen injection lines for under balanced drilling applications. The stack of pressure control devices and surface connections is called the pressure control stack. 
     One of the devices in the pressure control stack can be a rotating pressure control device also referred to as a rotating pressure control head. The rotating pressure control head is located at the top of the pressure control stack and is part of the pressure boundary between the well bore pressure and atmospheric pressure. The rotating pressure control head creates the pressure boundary by employing a ring-shaped rubber or urethane sealing element that squeezes against the drill pipe, tubing, casing, or other cylindrical members (hereinafter, drill pipe). The sealing element allows the drill pipe to be inserted into and removed from the well bore while maintaining the pressure differential between the well bore pressure and atmospheric pressure. The sealing element may be shaped such that the sealing element uses the well bore pressure to squeeze the drill pipe or other cylindrical member. However, some rotating pressure control heads utilize some type of mechanism, typically hydraulic fluid, to apply additional pressure to the outside of the sealing element. The additional pressure on the sealing element allows the rotating pressure control head to be used for higher well bore pressures. 
     The sealing element on all rotating pressure control heads eventually wear out because of friction caused by the rotation and/or reciprocation of the drill pipe. Additionally, the passage of pipe joints, down hole tools, and drill bits through the rotating pressure control head causes the sealing element to expand and contract repeatedly, which also causes the sealing element to become worn. Other factors may also cause wear of the sealing element, such as extreme temperatures, dirt and debris, and rough handling. When the sealing element becomes sufficiently worn, it must be replaced. If a worn sealing element is not replaced, it may rupture, causing a loss of hydraulic fluids and control over the well head pressure. 
     Currently, visual inspections or time based life span estimates are used to determine when to replace a worn sealing element. Visual inspections are subjective, and may be unreliable. Time based estimates may not take into account actual operating conditions, and be either too short or too long for a particular situation. If the time based estimate is too conservative, then sealing elements are replaced too frequently, causing unnecessary expense and delay. If the time based estimate is too aggressive, then the risk for rupture may be unacceptable. 
     U.S. patent application Ser. No. 10/922,029 (the &#39;029 application) discloses a Rotating Pressure Control Head (RPCH) having a sealing element in an inner housing where the inner housing is rotatably engaged to an outer housing by an upper bearing and a lower bearing. The RPCH of the &#39;029 application offers many improvements over the prior art including a shorter stack size, a quick release mechanism for inner unit change out, and a reduction in harmonic vibrations. Further improvements can be sought in ways to extend the life of the components. Wellbore fluid pressure, pressurized hydraulic fluid, and pipe friction against the sealing element exert a net upward or downward force on the inner housing that translates into a load on the upper and lower bearings. The load on the upper and lower bearings generates heat which is the most significant factor in bearing wear and life expectancy. A need exists for a way to balance the net force on the inner housing in order to reduce heat and wear on the bearings. Additionally, a need exists for an objective way to determine when a sealing element is sufficiently worn and needs to be replaced, without causing waste from early replacement, and without increasing the risk of rupture. 
     SUMMARY OF THE INVENTION 
     A Rotating Pressure Control Device (RPCD) uses pressure balancing so that a force transmitted through the bearings from an inner housing to an outer housing is balanced, thereby increasing the service life of the bearings. 
     The RPCD comprises an upper body and a lower body that form an outer housing. An inner housing rotates with respect to the outer housing. The inner housing has a sealing element that constricts around the drill pipe, and bearings are placed between the inner housing and outer housing to allow rotation of the inner housing within the outer housing. 
     An upper dynamic rotary seal is located between the inner housing and the outer housing and above the sealing element. A middle dynamic rotary seal is located between the inner housing and the outer housing and below the sealing element. A lower dynamic rotary seal is located between the inner housing and the outer housing below the middle dynamic rotary seal. 
     An upper piston area is created between the inner housing and the outer housing by the upper dynamic rotary seal and the middle dynamic rotary seal. A lower piston area is created below the expanded sealing element between the outside of the drill pipe and the lower dynamic rotary seal. 
     Wellbore fluid pressure, pressurized hydraulic fluid, and pipe friction against the sealing element cause a net upward or downward force on the inner housing with respect to the outer housing. These net upward or downward forces cause wear to the bearings. By adjusting hydraulic fluid pressure in the upper piston area, users can adjust the amount of downward force exerted by the upper piston area to compensate for the upward force exerted by the lower piston area. In addition, such adjustments also compensate for forces caused by friction between the drill pipe and sealing element. The reduction in force on the inner housing achieved by pressure balancing results in reduced bearing heat and wear. 
     Additionally, the RPCD has an electrically conductive wear indicator integrated with the drill pipe sealing element. A conductive strip is embedded inside the sealing element. The conductive strip makes electrical contact with a first electrode of an electrical indicator. A second electrode of the electrical indicator is in electrical contact with the drill pipe. When the sealing element is worn down to a pre-determined depth, exposing the embedded conductive strip, a closed circuit is formed from the electrical indicator through the first electrode, the embedded conductive strip, the drill pipe, and the second electrode, causing a signal on an electrical indicator, alerting users of the RPCD that it is time to replace the sealing element. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself, however, as well as a preferred mode of use, further objectives and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein: 
         FIG. 1  is a cross sectional view of the RPCD; 
         FIG. 2  is a cross sectional view of the RPCD with the sealing element in an expanded position; 
         FIG. 3  is a perspective view of the RPCD; 
         FIG. 4  is a cross sectional view of the RPCD with a wear indicator top plate; 
         FIG. 5  is a detail view of a conductive bolt; 
         FIG. 6  is detail view of a conductive pin; and 
         FIG. 7  is a cross sectional view of the RPCD with a closed circuit caused by a worn sealing element. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       FIG. 1  is a cross sectional view of pressure balanced rotating pressure control device  500 . Upper body  200  and lower body  100  form outer housing  150 . Inner housing  300  rotates inside outer housing  150 . Inner housing  300  contains sealing element  340  adapted to constrict around a drill pipe. Upper bearing  332  and lower bearing  334  affixed to inner housing  300  provide vertical and lateral support between inner housing  300  and outer housing  150 . 
     Input port  204  allows hydraulic fluid to enter outer housing  150  to reach channel  338 , cavity  330 , and spaces between inner housing  300  and outer housing  150 . Alternate input port  202  is capped with input plug  210 . Output port  208  allows hydraulic fluid to exit outer housing  150 . Alternate output port  206  is capped with output plug  212 . Wellbore fluid enters RPCD at input  102  and exits through output  104 . 
     Upper dynamic rotary seal  322  is located between inner housing  300  and outer housing  150  and above sealing element  340  and upper bearing  332 . Upper dynamic rotary seal  322  is shown here as two separate dynamic rotary seals. 
     Middle dynamic rotary seal  324  is located between the inner housing  300  and outer housing  150 , below sealing element  340 , and below lower bearing  334 . Middle dynamic rotary seal  324  has a wider diameter than upper dynamic rotary seal  322 . 
     Lower dynamic rotary seal  326  is located between the inner housing  300  and outer housing  150  below middle dynamic rotary seal  324 . 
     Vent port  106  allows open space between middle dynamic rotary seal  324  and lower dynamic rotary seal  326  to remain at atmospheric pressure. In addition, vent port  106  serves as a leak detection system because in the event that middle dynamic rotary seal  324  or lower dynamic rotary seal  326  begin to leak, fluid will drain from vent port  106  revealing the leak. 
     Pair of o-rings  312  sit between upper body  200  and lower body  100 . Upper sealing element o-ring (or upper alternate sealing element)  315  and lower sealing element o-ring (or lower alternate sealing element)  313  sit between sealing element  340  and inner body  300 . 
       FIG. 2  is a cross sectional view of pressure balanced rotating pressure control device  500  with sealing element  340  in an expanded position around drill pipe  400 . 
     Pressurized hydraulic fluid  440  enters outer housing  300  through input port  204 . Alternate input port  202  is capped with input plug  210 . Pressurized hydraulic fluid  440  expands sealing element  340  around drill pipe  400 . Hydraulic fluid  440  permeates the area between inner housing  300  and outer housing  150  between upper dynamic rotary seal  322  and middle dynamic rotary seal  324 . Hydraulic fluid  440  lubricates upper bearing  332  and lower bearing  334 . Pressurized hydraulic fluid  440  exits outer housing through output port  208  for recirculation. Alternate output port  206  is capped by output plug  212 . 
     Upper piston area  520  is defined by the equation A(up)=(π×(D(s) 2 −D(us) 2 )/4 where D(ms)=middle dynamic seal ring  324  outer diameter, and where D(us)=upper dynamic rotary seal  322  outer diameter. Hydraulic fluid  440  is induced into upper piston area  520  to expand sealing element  340  around drill pipe  400 , when hydraulic fluid  440  is so induced, it acts upon upper piston area  520  to create a downward force on inner housing  300 . Force on upper piston area  520  is defined by the equation F(up)=A(up)×P(h) where P(h)=induced hydraulic pressure. Pressurized hydraulic fluid  440  energizes upper piston area  520  exerting a downward force on inner housing  300 . Upper piston area  520  remains constant. 
     Lower piston area  510  is defined by the equation A(lp)=(π×(D(b) 2 −D(p) 2 )/4 where D(b)=the outer diameter of lower dynamic rotary seal  326  and where D(p)=the outer diameter of drill pipe  400 . Thus, a smaller diameter pipe results in a larger cross sectional area for lower piston area  510 . Pressurized wellbore fluid  410  acts upon lower piston area  510  to create an upward force on inner housing  300 . Force on lower piston area  510  is defined by the equation F(lp)=A(lp)×P(wb) where P(wb)=wellbore pressure. Wellbore fluid  410  exerts an upward force on inner housing  300  as it presses upward into lower piston area  510 . Lower piston area  510  does not remain constant and varies in size due to drill pipe diameter changes as the drill pipe is lowered, or raised, through RCPH  500 . 
     Vented area  345  is defined as an area between the outer diameter of middle dynamic rotary seal  324  and the outer diameter of lower dynamic rotary seal  326 . Vent port  106  allows vented area  345  to remain at atmospheric pressure. By keeping vented area  345  at atmospheric pressure, a pressure imbalance is created such that upper piston area  520 , when it is energized by pressurized hydraulic fluid  440 , creates a force opposite that of lower piston area  510  when it is energized by wellbore fluid  410 . 
       FIG. 3  is a perspective view of RPCH  500  showing upper piston area  520  and lower piston area  510 . Upper piston area  520  is an area between the outer diameter of middle dynamic seal ring  324  and the outer diameter of upper dynamic rotary seal  322  defined by the upper piston area formula set forth above. Lower piston area  510  is an the area between the outer diameter of lower dynamic seal element  326  and the outer diameter of drill pipe  400  defined by the lower piston area formula set forth above. 
     The upward and downward forces on inner housing  300  are also affected by the frictional drag of the pipe moving through the collapsed sealing element  340 , as described by the equation: F(f)=(π×D(p)×L)×P(h)×u where L=length of pipe  400  in contact with sealing element  340 , and where u=coefficient of drag between pipe  400  and sealing element  340 . 
     The sum of the total forces on inner housing  300  is calculated with the equation F(sum)=F(lp)−F(up)+/−F(f). The sign for the friction force F(f) depends on whether drill pipe  400  is moving upwards or downwards. If drill pipe  400  is moving upwards, F(f) is positive. If drill pipe  400  is moving downward, F(f) is negative. A positive F(sum) indicates a net upward force on inner housing  300 , the bearings and seals. A negative F(sum) indicates a net downward force on inner housing  300 , the bearings and seals. 
     Pressure balanced rotating pressure control device  500  allows drillers to use pressurized hydraulic fluid  440  to compensate for upward and downward forces on inner housing  300 . By compensating for differences in upward and downward forces on inner housing  300 , heat and/or wear on upper bearing  332  and lower bearing  334  will be reduced and the life of upper bearing  332  and lower bearing  334  will be expanded. 
     A wear indicator is used to signal when it is time to replace the drill pipe sealing element.  FIG. 4  is a cross sectional elevation view of a wear indicator on pressure balanced RPCD  500 . Upper body  200  and lower body  100  form outer housing  150 . Inner housing  300  rotates inside outer housing  150 . Inner housing  300  contains sealing element  340  adapted to constrict around drill pipe  400 . Top plate  700  is attached to the top of RPCD  500 , which is electrically insulated from the top plate  700 . 
     Conductive strip  710  is embedded axially in sealing element  340  at a depth where, when worn down, sealing element  340  should be replaced. Conductive ring  720  contacts the top end of conductive strip  710 . Conductive strip  710  and conductive ring  720  are electrically isolated from inner housing  300  and other conductive surfaces by sealing element  340 . 
     Bolt  730  (described in  FIG. 5  below) connects conductive ring  720  to first electrode  770  with brush  738 . First electrode  770  passes through top plate  700 . First electrode  770  leads to indicator  790 . 
     Second electrode  780  connects indicator  790  to pin  750  (described in  FIG. 6  below). Pin  750  is located inside of top plate  700 . Spring  752  holds pin  750  against drill pipe  400  creating an electrical contact through conductor  758 . 
       FIG. 5  shows a cross-sectional detail of bolt  730 . Bolt  730  is a special insulated bolt having conductor  732  running axially through the center of bolt  730  which is electrically insulated from the body of the bolt  730 . Bolt conductor  732  extends below bolt  730  creating contact point  734 . Spring loaded electric brush  738  is located at top end  736  of bolt  730 . Spring loaded electric brush  738  is attached to bolt conductor  732  and is electrically isolated from the body of bolt  730 . 
     No alignment is required when installing sealing element  340  in RPCD  500 . Once sealing element  340  is installed inside inner housing  300 , bolt  370  is threaded through the upper portion of inner housing  300 , driving the contact point  734  into sealing element  340 . The location of bolt  730  is such that the contact point  734  will pierce conductive ring  720  establishing an electric circuit from conductive strip  710  in sealing element  340 , through conductive ring  720  and into bolt  730 . Note that bolt  730  rotates with inner housing  300  as drill pipe  400  is turned. 
     Commutator ring  772  on top plate  700  is aligned such that spring loaded electric brush  738  remains in contact with commutator ring  772  as inner housing  300  rotates with turning drill pipe  400 . Thus, an insulated electrical conductor path is established from conductive strip  710  in sealing element  340 , through conductive ring  720 , through bolt conductor  732  in bolt  730 , through spring loaded electric brush  738 , through commutator ring  772 , and out first electrode  770 . 
       FIG. 6  shows a detail of pin  750  mounted inside top plate  700 . Pin  750  is spring loaded inside top plate  700 , through outer aperture  702  and inner aperture  704 . Spring  752  exerts force between top plate  700  and rib  756  on pin  750 . Pin conductor  754  passes through pin  750  connecting pipe contactor  758  to second electrode  780 . Pin  750  is electrically insulated from top plate  700 . 
     Pin  750  is retracted as drill pipe  400  is lowered through RPCH  500  and is then allowed to spring against drill pipe  400 . Spring  752  keeps pipe contactor  758  in contact with drill pipe  400  as tool joints and other such changes in drill pipe  400  outside diameter pass through RPCH  500 . Thus, an electrical circuit is established from drill pipe  400 , through pipe contactor  758 , through pin conductor  754  inside pin  750 , and out through second electrode  780 . 
       FIG. 7  is a cross sectional elevation view of pressure balanced rotating pressure control device  500  with a closed circuit caused by worn sealing element  340 . Whenever sealing element  340  wears down, exposing conductive strip  710 , drill pipe  400  makes physical and electrical contact with conductive strip  710 . A closed circuit is formed from indicator  790  through first electrode  770 , brush  738 , bolt  730 , conductive ring  720 , conductive strip  710 , drill pipe  400 , conductor  758 , pin  750 , and second electrode  780 , causing a reading on indicator  790 . The reading on indicator  790  after the circuit is closed alerts users of RPCD  500  that it is time to replace sealing element  340 . 
     Persons skilled in the art are aware that a normally closed circuit could also be employed. With a normally closed circuit, the electrically conductive path is in place at all times until wear of the sealing element causes conductive strip  710  to sever, opening the circuit and causing indicator  790  to alert users of RPCD  500  that it is time to replace sealing element  340 . In other words, during normal operation, an indicator light would be on, and when the circuit is broken, the indicator light would turn off. 
     With respect to the above description, it is to be realized that the optimum dimensional relationships for the parts of the invention, to include variations in size, materials, shape, form, function, manner of operation, assembly, and use are deemed readily apparent and obvious to one of ordinary skill in the art. The present invention encompasses all equivalent relationships to those illustrated in the drawings and described in the specification. The novel spirit of the present invention is still embodied by reordering or deleting some of the steps contained in this disclosure. The spirit of the invention is not meant to be limited in any way except by proper construction of the following claims.