Patent Publication Number: US-6220359-B1

Title: Pump through safety valve and method

Description:
This application claims the benefit of U.S. Provisional Application No. 60/106,722, filed Nov. 2, 1998—entitled “Pump Through Safety Valve and Method” 
    
    
     TECHNICAL FIELD 
     The invention generally relates to valves for downhole use in oil and gas wells. More particularly, the invention relates to downhole safety valve. 
     BACKGROUND OF THE INVENTION 
     In oil and gas wells, uncontrolled downhole fluid pressures can cause a dangerous situation. Depending on the downhole conditions, it is usually possible to predetermine a threshold pressure differential between a higher fluid pressure in an area exterior of the downhole tubing string adjacent where a safety-valve is to be positioned, and a lower fluid pressure interior of the tubing string that is considered to present an undesirable or possibly dangerous situation. For example, excessive fluid pressure in a production zone that is unbalanced by fluid pressure interior of the tubing string adjacent the production zone may present a highly dangerous situation in which hydrocarbons and gases may be driven at uncontrolled pressures uphole through the tubing string. In extreme cases, this can result in a blow-out of the well. A safety valve is intended to automatically actuate when such a condition occurs. 
     Another situation where this type of tool is valuable is if a leak develops in the tubing string above the valve that allows fluid pressure to communicate into the annular, where annulus pressure is harder to control. In this situation, it would be advantageous to have a valve that would automatically shut in the well. 
     It would be advantageous to have a safety valve that could be re-opened in response to a sufficient increase in interior fluid pressure over the exterior fluid pressure, whereby the downhole fluids can be pumped through the safety valve back into the formation. 
     It would also be advantageous to have a safety valve that would open a circulating valve after closing the fluid flow through the safety valve, whereby fluid can be circulated between the tubing string and the annulus while the flow through the safety valve is closed. 
     To operate successfully in the harsh and remove downhole environments, a simple and reliable safety valve structure and method would be highly advantageous. 
     SUMMARY OF THE INVENTION 
     According to the invention, a safety valve is provided for downhole use in a well. In general, the safety valve includes a housing having an interior fluid flow passage therethrough and at least one circulating port in the housing for providing fluid communication between the interior fluid flow passage and the exterior of the housing. A piston is operatively positioned within the housing for movement between a first position and a second position in response to a fluid pressure differential between the interior fluid flow passage and the exterior of the housing. A slip is operatively connected to the piston, whereby over a first range of the movement of the piston the slip moves with the piston, and over a second range of the movement of the piston the slip does not move with the piston. A first valve is positioned within the housing for opening and closing the interior fluid flow passage of the housing, and a second valve is positioned within the housing for opening and closing the circulating port. An operative connection is provided between the slip and either one of the first and second valves, whereby the first range of the movement of the piston operates the one valve. An operative connection is provided between the piston and the other one of the first and second valves, whereby the second range of the movement of the piston operates the other valve. 
     The valves for controlling flow and for controlling circulation can be connected to be operated sequentially by the piston. For example, the range of movement of the piston is preferably structured such that when the piston moves from the first position to the second position, the first valve closes and then the second valve opens. When the piston moves in the reverse direction from the second position to the first position, the second valve closes and then the first valve opens. The second valve for controlling the circulation port is preferably located uphole of the first valve, which is normally expected to be the lower pressure side of the safety valve. 
     According to the invention of the safety valve, a break-away release is used that breaks in response to a predetermined force related to the fluid pressure exterior of the housing. The break-away release is adapted to break before the fluid pressure exterior of the housing can cause the piston to move. The break away release is preferably a rupture disk operatively positioned between the piston and the fluid pressure exterior of the housing. 
     The invention also includes a method of operating a downhole safety valve. The method includes the step of operatively mounting a piston in a housing for movement between a first position and a second position. The method also includes the step of operatively connecting a slip to the piston, whereby over a first range of the movement of the piston the slip moves with the piston, and over a second range of the movement of the piston the slip does not move with the piston. A first valve is positioned within the housing for opening and closing an interior fluid flow passage of the housing, and a second valve is positioned within the housing for opening and closing a circulating port between the interior fluid flow passage and the exterior of the housing. A slip is operatively connected to one of the first and second valves, whereby the first range of the movement of the piston operates the one valve. The piston is operatively connected to the other one of the first and second valves, whereby the second range of the movement of the piston operates the other valve. The step of exposing the piston to a fluid pressure differential between the interior fluid flow passage and the exterior of the housing causes to move the piston between the first position and the second position, thereby operating the first and second valves. 
     The steps of constructing and operating the downhole safety valve can be performed such that when the piston moves from the first position to the second position, the first valve closes and then the second valve opens; and when the piston moves in the reverse direction from the second position to the first position, the second valve closes and then the first valve opens. When the valve is oriented for downhole use, the second valve is preferably located uphole of the first valve. 
     The inventive method includes the step of operatively positioning a break-away release that breaks in response to a predetermined force related to the fluid pressure exterior of the housing. The break-away release is adapted to break before the fluid pressure exterior of the housing can cause the piston to move. The break away release is preferably a rupture disk operatively positioned between the piston and the fluid pressure exterior of the housing. 
     These and other aspects and advantages of the invention will become apparent to persons skilled in the art from the following drawings and detailed description of a presently most preferred embodiment of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The accompanying drawings are incorporated into and form a part of the specification to provide illustrative examples of the present invention and to explain the principles of the invention. The drawings are only for purposes of illustrating preferred and alternate embodiments of how the invention can be made and used. It is to be understood, of course, that the drawings are not to engineering scale, but are merely intended to represent and illustrate the concepts of the invention. The drawings are not to be construed as limiting the invention to only the illustrated and described examples. Various advantages and features of the present invention will be apparent from a consideration of the accompanying drawings in which: 
     FIGS. 1A-B illustrate a schematic vertical section view of a representative safety valve according to the invention, which is shown in the initial position; 
     FIGS. 2A-B illustrate a schematic vertical section view of the representative safety valve shown in FIGS. 1A-B, after excessive exterior fluid pressure has caused the rupture disks to rupture and then at least assists in driving the operating piston assembly, acting through the slip assembly, to an intermediate position and close the flow valve assembly; and 
     FIGS. 3A-B illustrate a schematic vertical section view of the representative safety valve shown in FIGS. 1A-B, after the flow valve assembly has been completely closed as shown in FIGS. 2A-B, at which point the slip assembly performs its slipping function to allow the exterior fluid pressure to continue to at least assist in driving the operating piston assembly to a second position at which an interior port and an exterior port of the circulating valve assembly are in fluid communication. The exterior fluid pressure communicates with the interior of the safety valve on the up-hole side of the safety valve. 
     By pumping fluid from up-hole to increase the fluid pressure at a faster rate than can be bled off through the interior port and exterior port of the circulating valve assembly, the increased fluid pressure drives the operating piston assembly back to the initial position, thereby selectively re-closing the circulating valve and then re-opening the flow valve. 
    
    
     DETAILED DESCRIPTION OF A PRESENTLY MOST PREFERRED 
     EMBODIMENT AND BEST MODE OF PRACTICING THE INVENTION 
     General Structure of a Safety Valve According to Invention 
     FIGS. 1A-B illustrates a preferred embodiment of a safety valve according to the invention, which is generally referred to by the reference numeral  100 . As will be appreciated by persons skilled in the art, the safety valve  100  is generally tubular and shown in partial section. The safety valve  100  encloses an interior fluid flow conducting passage  102  extending through the safety valve. The flow conducting passage  102  is intended to be a section of a flow conducting passage through a tubing string extending upward and/or downward from the safety valve  100 . 
     In general, a preferred embodiment of a safety valve  100  according to the invention can be described and characterized as including the following cooperative structures: a housing  200 , a piston assembly  300 , a break-away release  400 , a slip assembly  500 , a flow valve assembly  600 , and a circulating valve assembly  700 . According to a presently most preferred embodiment of the invention, the safety valve  100  also includes a biasing assembly  800  and a shear pin assembly  900 . 
     It is to be understood, however, that merely because a structure or feature is conveniently described or referenced herein as being part of a general structure or function is not necessarily to be construed as a limitation on the invention thereby. The various structures and features described herein with respect to the presently most preferred embodiment of the invention interact to form an operative safety valve  100  according to the principles of the invention. These structures could be described with different terminology according to a different organizational structure and with different reference numerals without departing from the scope of the invention as defined by the appended claims. 
     As may be appreciated by reference to the drawings, connections of the various components are often complimented by the use of O-rings or other conventional seals. The use of such seals is well known in the art and, therefore, will not be discussed in detail. 
     The safety valve is shown in a vertical orientation, based on a typical orientation at a downhole location. It is to be understood, of course, that the safety valve can be in any orientation down-hole, even completely upside down relative to the drawings herein. The terms “upper,” “lower,” “up-hole,” “down-hole,” “above,” and “below” and other such relative terms are only for the purpose of convenient reference to the drawings, and are not to be construed as limiting the use or operation of the safety valve  100 . Similarly, terms such as “inner,” “outer,” “inward,” and “outward” are used to indicate radial location or direction with reference to the interior of the safety valve  100 . 
     As used herein, the term “tubing” generally includes casing, liner, tubing, or production tubing and strings of any such tubing. 
     As used herein, the term “exterior” of the housing can be the well formation or it can be an annular space outside the housing defined by the outer wall surface of the housing and the inner wall surface of an exterior tubing string, such as in the case where the safety valve is used in a production tubing string within a casing string. 
     Housing  200   
     The exterior housing  200  can be designed in many different structural embodiments to accommodate engineering considerations such as component manufacturing, assembly, and safety valve maintenance. The general functions of the housing  200  are to be the structural body of the safety valve  100  and to isolate the interior flow conducting passage  102  of the safety valve from the exterior of the safety valve. If desired, and in the normal case, the housing  200  can also be structurally engineered to function as a section of a downhole tubing or tubing string. 
     According to a presently most preferred embodiment, the housing  200  defines a substantially continuous outer cylindrical wall surface, which is adapted to match the outer diameter of the tubing string in which the safety valve  100  is to be employed. The overall length of the safety valve  100  is at least sufficient to accommodate a piston assembly  300 , a slit assembly  500 , a flow valve assembly  600 , and a fluid circulating assembly  700 . 
     The housing  200  is preferably formed from several components, which accommodates engineering considerations such as assembly and maintenance of the safety valve  100 . According to the presently most preferred embodiment of the invention, commencing at the top of FIGS. 1A-B and working down, the housing  200  includes an upper connector  210 , a upper housing section  220 , a lower housing section  230 , and a lower connector  250 . It is to be understood, of course, that the fewer or additional housing sections can be used to form a housing suitable for use in a safety valve according to the invention. For example, lower housing section  230  may advantageously be formed from two sections having a threaded and sealed connection to further facilitate assembly of the safety valve  100 . The upper connector  210  and lower connector  250  are not necessary to the housing  200  or to the practice the invention, but are merely the presently most convenient structure for integrating the safety valve  100  into a downhole tubing string. 
     The upper connector  210  can be either a male or female threaded connector. According to the presently most preferred embodiment illustrated in the drawings, the upper connector  210  is a female or bell connector of conventional engineering design having interior female threads  212  at the upper end thereof, whereby the safety valve  100  can be connected into a tubing string having a mating male or pin connector (not shown). As will hereinafter be described in detail, the upper connector  210  protrudes at its lower end  214  between the upper end of upper housing section  220  and the upper end of the piston assembly  300 . 
     The upper housing section  220  is connected to the upper connector  210  at threaded connection  222 . An O-ring seal  224  seals the threaded connection  222 . According to the presently most preferred embodiment of the invention, the upper housing section  220  includes structures that can be more conveniently described in detail with respect to the piston assembly  300 , break-away release  400 , the fluid circulating assembly  700 , the biasing assembly  800 , and the shear pin assembly  900 . 
     The lower housing section  230  is connected to the upper housing section  220  at threaded connection  232 . An O-ring seal  234  seals the threaded connection  232 . According to the presently most preferred embodiment of the invention, the lower housing section  230  includes structures that can be more conveniently described in detail with respect to the slip assembly  500  and the flow valve assembly  600   
     The lower connector  250  can be either a male or female threaded connector. According to the presently most preferred embodiment illustrated in the drawings, the lower connector  250  is a male or pin connector of conventional engineering design having exterior male threads  252  at the lower end thereof, whereby the safety valve  100  can be connected to a tubing string having a mating female or box connector (not shown). 
     According to the presently most preferred embodiment of the invention, and as will hereinafter be described in detail with respect to the flow valve assembly  600 , the lower connector  250  is connected to the lower housing section  230  through the flow valve assembly  600 . The flow valve assembly  600  is retained within the lower housing section  230  and the lower connector  250  is connected to the flow valve assembly  600  at threaded connection  254 . An O-ring seal  256  seals the threaded connection  254 . The lower connector  250  protrudes at its upper end  258  between the lower end of lower housing section  230  and the lower end of flow valve assembly  600 . Another O-ring seal  260  seals the junction  262  between the lower housing section  230  and the lower connector  250 . It is to be understood, of course, that the lower connector  250  can be connected directly to the lower housing section  230  by a threaded connector and O-ring seal. The connection of the lower connector  250  through the flow valve assembly  600 , however, provides a structure that facilitates the assembly of the safety-valve  100 . 
     Based on the invention disclosure herein, a person of skill in the art would be able to adapt the principles of the present invention and the details of the presently most preferred embodiment to employ a different housing design without departing from the scope of the present invention. 
     Piston Assembly  300   
     The piston assembly  300  is adapted to employ the principle of a piston to provide the selective driving movements to operate the flow valve assembly  600  and the circulating valve assembly  700 . According to the presently preferred embodiment of the invention, a fluid pressure exterior of the housing is used to drive the piston assembly in a first direction to first close the flow valve assembly  600  and then, after the flow valve assembly is completely closed, to open the circulating valve assembly  700 . A fluid pressure interior of the housing is used to drive the piston assembly in a second or reverse direction to first re-close the circulating valve assembly  700 , and then re-open the flow valve assembly  600 . 
     According to the presently most preferred embodiment of the invention, the piston assembly  300  comprises a operating mandrel  310  captured for slidable movement within the housing  200 , and more particularly for movement within the upper housing section  220  of the housing  200 , a mandrel piston  320 , and an expansion chamber  330 . 
     The upper end  312  of the operating mandrel  310  is adapted to be stopped by the shoulder  314  formed on the upper connector  210  of the housing  200 , thereby limiting the upward slidable movement of the operating mandrel  310 . The upper end  312  of the operating mandrel  310  has an O-ring seal  316 , which seals the upper end of the operating mandrel for slidable movement against an inner landing surface  318  of the upper connector  210 . 
     The outer wall of the operating mandrel  310  has a circumferential outwardly extending mandrel piston  320  formed thereon. The mandrel piston  320  has an upwardly-facing circumferential surface  322 , a downwardly-facing circumferential surface  324 . An O-ring seal  326  seals the mandrel piston  320  for slidable movement against the inner wall surface of the upper housing section  220 . 
     An expansion chamber  330  is defined by the inner wall surface of the upper housing section  220  and the outer wall surface of the operating mandrel  310 , the upper end of which is defined by the lower portion  214  of the upper connector  210 , and the lower end of which is defined by the upwardly-facing circumferential surface  322  of the mandrel piston  320 . The expansion chamber  330  can be a channel or, more preferably, is an annular chamber. One or more ports  322  formed in the upper housing section  220  can provide fluid communication between the expansion chamber  330  and the exterior of the housing  200 . Additional ports  332  (not shown) can be spaced around the upper housing section  220  in positions that communicate with the annular expansion chamber  330 . 
     Downwardly facing surfaces such as the surface  324  on the mandrel piston  320  and/or other such surfaces formed on the operating mandrel  310  or translated through to the operating mandrel  310  that are exposed to the fluid pressure within the interior flow conducting passage  102  and/or other biasing forces can be used as piston surfaces to drive the operating mandrel  310  of the piston assembly  300  back upwards. To do so, such pressures and/or other biasing forces need to be sufficient to overbalance the exterior fluid pressure exerted on the upwardly facing surface  322  of the mandrel piston  320 . 
     Thereby, the operating mandrel  310  of the piston assembly  300  reciprocates in response to differential fluid pressures between the interior flow conduction passage  102  and the exterior of the housing. 
     Based on the invention disclosure herein, a person of skill in the art would be able to adapt the principles of a piston assembly  300  according to the present invention and the details of the presently most preferred embodiment described herein and illustrated in the drawings to employ alternative embodiments for the piston assembly  300  and mandrel piston  320  without departing from the scope of the invention. 
     Break-Away Release  400   
     According to the invention, a break-away release  400  is provided that ruptures, shears, or otherwise breaks free in response to a force related to an exterior fluid pressure. After the break-away release breaks, the fluid pressure exterior of the housing can act to drive the mandrel piston  320  of the piston assembly  300 . According to the presently most preferred embodiment of the invention, the break-away release  400  is a rupture disk  420  positioned in each of the one or more ports  332 . The physical characteristics of the rupture disk  420  are adapted such that the rupture disk will rupture in response to a predetermined pressure differential across the rupture disk  420 . Upon rupturing, the external fluid pressure will be communicated through the port  332  into the expansion chamber  330  and be exerted against the upwardly-facing circumferential surface  322  of mandrel piston  320 , which will tend to drive the entire operating mandrel  310  to slide downward in the housing  200 . 
     Slip Assembly  500   
     According to a preferred embodiment of the invention, a first range of reciprocal movement provided by the piston assembly  300  is preferably used to quickly operate the flow valve assembly  600  and close the safety valve  100 , and then, after the flow valve assembly  600  is fully closed, the slip assembly  500  slips such that a second range of reciprocal movement provided by the piston assembly  300  is not further transferred to the flow valve assembly  600 . The second range of reciprocal movement provided by the piston assembly  300  is preferably used to operate the circulating valve assembly  700  and open fluid circulating ports, which will allow for fluid circulation and gradually equalize the interior and exterior fluid pressures between the interior and exterior of the housing  200  on one side of the flow valve assembly  600 . 
     According to the presently most preferred embodiment of the invention, the slip assembly  500  generally includes a landing  510 , a mandrel extension  520 , and a collet  550 . 
     The landing  510  can be formed on the upper end of the lower housing section  230 . The landing  510  includes an upwardly-facing upper shoulder  512 , an inwardly-facing landing surface  514 , and a ramped lower shoulder  516 . 
     The mandrel extension  520  is connected to the lower end of the operating mandrel  310  at threaded connection  522 . An O-ring seal  524  is preferably used to seal the threaded connection  522 , which is advantageous if the safety valve  100  includes a biasing assembly  800  as hereinafter described in detail. The mandrel extension  520  essentially is a downward extension of the operating mandrel  310  and moves with the operating mandrel  310 . The threaded connection  522  for the mandrel extension  520  is intended to facilitate the assembly of the safety valve  100 . 
     The downwardly-facing circumferential surface  324  of the mandrel piston  320  preferably is adapted to be stopped by a shoulder. As will hereinafter be described in more detail, according to the presently most preferred embodiment of the invention, such a shoulder is preferably an upwardly-facing circumferential surface  712  of a circumferential inwardly-extending first isolating rib  710  formed on the inner wall surface of the upper housing section  220 . The mandrel extension  520  alternatively or additionally can have downwardly-facing surface  526  adapted to be stopped by the upwardly-facing upper shoulder  512  of the landing  510  formed on the inner wall of the upper housing section  220  of the housing  200 , thereby limiting the downward slidable movement of the operating mandrel  310  and mandrel extension  520 . 
     Accordingly, the mandrel extension  520  to the operating mandrel  310  operatively connects the slip assembly  500  to the piston  320  of the piston assembly  300 . 
     The mandrel extension  520  has an outer diameter that is less than the inner diameter of the circumferential landing  510 . The outer wall surface  528  of the mandrel extension  520  has a circumferential recess  530  formed therein for engaging with the collet  550 . 
     The downwardly facing surfaces  526  and also  532  at the bottom of the mandrel extension  520  can be used as piston surfaces to move the mandrel extension  520  and the operating mandrel  310  back upwards, as will hereinafter be described in detail. 
     The collet  550  includes a plurality of collet fingers  560  extending upwardly from a lower sleeve portion  570 . Each of the collet fingers  560  has an knuckle  562 . The plurality of collet fingers  560  are separated from one another by appropriately sized radial gaps  564 . 
     As will be appreciated by persons skilled in the art, the recess  530  in the mandrel extension  520  has ramped surfaces thereon for interacting with and guiding corresponding ramped surfaces on the knuckles  562  of the fingers  560  of the collet  550 . The plurality of collet fingers  560  are adapted to resiliently grip and engage the mandrel extension  520  such that when the knuckles  562  of the collet fingers  560  are vertically aligned with the recess  530 , the fingers  560  are urged inwardly to engage the recess  530 . The finger knuckles  562  have various ramped surfaces formed thereon to facilitate the one-way slip function of the collet assembly  500 . 
     In the initial position shown in FIGS. 1A-B of the drawing, the collet fingers  560  are trapped in the annular collet space  540  between the outer wall surface  528  of the mandrel extension  520  and an inner wall surface  514  of the landing  510  formed on the housing  200 , thereby creating a secure latch. When the operating mandrel  310  begins to slide downward, the mandrel extension  520  and the collet  550  are also moved downward. 
     After being moving downward into the position shown in FIGS. 2A-B to the point that the finger knuckles  562  pass the lower shoulder  516  of the landing  510  formed on the valve housing  230 , the finger knuckles  562  are no longer trapped between the inwardly-facing landing surface  514  of the landing  510  and outer wall surface  528  of the mandrel extension  520 . At that position, the downward sliding movement of the collet  550  is stopped by the end of the closing motion of the flow valve assembly  600 , as will hereinafter be described in detail. The continued downward movement of the operating mandrel  310  and the mandrel extension  520  creates an urging force against appropriately ramped surfaces between the recess  530  and the finger knuckles  562 , urging the collet fingers  560  to spread radially apart from each other and disengage the recess  530  formed in the mandrel extension  520 , which position is illustrated in FIGS. 2A-B. 
     After spreading apart and disengaging the recess  530  in the mandrel extension, the recess  530  of the mandrel extension  520  is free to slip past the finger knuckles  562  of the stopped collet  550 , such that the operating mandrel  310  and the mandrel extension  520  are free to continue moving downward to the third position shown in FIGS. 3A-B. 
     A substantially circumferential space is defined between the lower surface  526  and the upper surface  512 . This space can remain in fluid communication with the interior passage  102  of the safety valve  100  via the gaps  564  between the collet fingers  360 . As will hereinafter be described in detail, if the biasing assembly  800  is sealed, fluid pressure within the interior fluid flow conducting passage  102  can be used to exert an upward driving force against the lower shoulder  526  of the mandrel extension  520  to drive the mandrel extension  520  and the operating mandrel  310  upward, whereby the flow valve assembly  600  can be re-opened as will hereinafter be described in detail. If the biasing assembly  800  is not sealed, the pressure above and below the surface  526  is expected to be equal. 
     Flow Valve Assembly  600   
     The flow valve assembly  600  is adapted to close the interior fluid flow conducting passage  102  in response to an operating movement provided by the piston assembly  300  and transferred through the slip assembly  500  to the flow valve assembly  600 . 
     According to the presently most preferred embodiment of the invention, the flow valve assembly  600  is a ball-valve assembly. It is to be understood, of course, that while a ball-valve is presently believed to be particularly advantageous, reliable, and secure for use in safety valve applications. In certain applications or with appropriate adaptations, other types of closing valves are contemplated as being useful according to the principles of the present invention, including, for example, a flapper valve. Based on the invention disclosure herein, a person of skill in the art would be able to adapt the principles of the present invention and the details of the presently most preferred embodiment to employ a different flow valve assembly such as a flapper valve. 
     Accordingly, the valve assembly  600  can include, for example, two ball operating arms  610  (only one of which is illustrated in the drawings), ball housing  630 , and valve ball  690 . The flow valve assembly can be selectively opened and closed to permit fluid flow through the central flow conducting passage  102  of the safety valve. 
     An intermediate coupling  612  for connecting the ball operating arms  610  to the collet sleeve portions  570  can be integrally formed with the collet sleeve  570  or can be connected to the collet sleeve portion  570  at a threaded connection. The coupling  612  includes outwardly extending flanges  616  and  618  at the lower end thereof, which define an exterior annular recess  620  there between. 
     The ball operating arms  610  include inwardly extending flanges  622  and  624  at the upper end thereof. When assembled in the lower housing section  230  of the housing  200 , the outwardly extending flanges  616  and  618  at the lower end of the intermediate coupling  612  interlock with the inwardly extending flanges  622  and  624  of the ball operating arms  610 . Intermediate coupling  612  and the ball operating arms  610  are maintained in engagement by their location in annular recess  626  between the inner wall surface of lower housing section  230  and the ball housing  630 . 
     Accordingly, the ball operating arms  610  of the flow valve assembly  600  are operatively connected to the slip assembly  500  through the intermediate coupling  612 . 
     Each of the two ball operating arms  610  has an inwardly protruding lug  628  for engaging and rotating the valve ball  690 . 
     Ball housing  630  is of substantially tubular configuration having an interior fluid flow path there through  632  defined by the inner wall surface  634  of the ball housing  630 . As will be described in more detail, the ball housing  630  preferably has an upper portion  636  and a lower portion  638 . Ball housing  630  has two windows  640  in the wall thereof to accommodate the inward protrusion of lugs  628  on each of the two ball operating arms  610 . The windows  640  in the ball housing  630  are defined by the downwardly-facing shoulder  642  of the upper portion  636  and the upwardly-facing shoulder  644  of the lower portion  638 . 
     On the exterior of the ball housing  630 , two longitudinal channels (the location of which is shown by phantom arrow  646 ) of arcuate cross-section and circumferentially aligned with windows  640 , extend from shoulder  648  downward to shoulder  642 . Ball operating arms  610 , which are of substantially the same arcuate cross section as channels  646  and lower portion  638  of ball housing  630 , lie in channels  646  and windows  640 , and are maintained in place by the interior wall surface of the lower housing section  230  and the exterior wall surface of the lower portion  638  of the ball housing  630 . 
     The interior of ball housing  630  has an upper annular seat recess  650 , within which an annular upper ball seat  652  is disposed. Upper ball seat  652  has an arcuate metal-to-metal sealing surface  654 , which provides a sliding seal with the exterior surface  692  of valve ball  690 . 
     The interior of ball housing  630  also has a lower ball recess  656 , within which an annular lower ball seat  658  is disposed. Lower ball seat  658  has an arcuate metal-to-metal sealing surface  660  that slidingly seals against the exterior surface  692  of valve ball  690 . 
     Upper and lower ball seats  652  and  660  are preferably biased by a ring spring  662  into sealing engagement with the valve ball  690 . 
     Exterior annular shoulder  664  on ball housing  630  is captured against the upper ends  666  of splines  668  (shown in phantom line) on the interior wall of lower housing section  230 , whereby the flow valve assembly  600  of ball operating arms  610 , ball housing  630 , including the upper ball seat  652 , the ring spring  654 , and the lower ball seat  660 , and the valve ball  690  positioned between the upper and lower ball seats  652  and  660  are all maintained in vertical position inside of lower housing section  230  of housing  200 . Splines  668  engage splines  670  on the exterior wall of ball housing  630 , and, thus, rotation of the ball housing  630  within lower housing section  230  is prevented. 
     The windows  640  in the ball housing  630  include windows formed in the upper portion  636  between downwardly-facing shoulder  644  and upwardly-facing shoulder  672  of the downward circumferential extension  674  of the upper portion  636 . The downward circumferential extension  674  of the upper portion  636  is threaded to the lower portion  638 , below the upwardly-facing shoulder  672 , thereby forming the ball housing  630 , and the windows  640  defined by the downwardly-facing shoulder  542  of the upper portion  636  and the upwardly-facing shoulder  644  of the lower portion  638 . 
     Lower connector  250  protrudes at its upper end  256  between ball housing  630  and lower housing section  230  when made up with the lower portion  638  of ball housing  630  at threaded connection  254 . 
     Valve ball  690  has a diametrical bore  694  therethrough of substantially the same diameter as bore  652  of ball housing  630 . Two lug recesses  696  extend from the exterior surface  692  of valve ball  690  to bore  694 . 
     When valve ball  690  is in its open position, as shown in FIG. 1B, a “full open” condition of the interior flow conducting passage  102  extends throughout the length of the safety valve  100 , providing an unimpeded path for the movement of formation fluids and/or perforating guns, wireline instrumentation, etc. When valve ball  690  is in a closed position, as shown in FIGS. 2B and 3B, a “full closed” condition is created by the rotation of the valve ball  690  such that the bore  694  is no longer aligned with the interior fluid flow conducting passage  102  of the safety valve  100 . 
     Circulating Valve Assembly  700   
     The circulating valve assembly  700  allows for selective circulation of well fluids between the interior fluid flow conducting passage  102  of the housing  200  and the exterior of the housing. According to the presently most preferred embodiment of the invention, circulating valve assembly  700  uses the principle of moving the orifices of inner and outer ports across a gate into and out of fluid communication. In general, this type of valve is referred to herein as a gate valve. 
     The inner wall surface of the upper housing section  220  has a circumferential inwardly-extending first isolating rib  710  formed thereon. The first isolating rib  710  has an upwardly-facing circumferential surface  712  and a downwardly-facing circumferential surface  714 . An O-ring seal  716  seals the inner wall surface of the operating mandrel  310  below the mandrel piston  320  for slidable movement of the operating mandrel  310  against the first isolating rib  710 . Essentially, the first isolating rib  710  functions as a valve gate. 
     The inner wall of the upper housing section  220  has a circumferential inwardly-extending second isolating rib  720  formed thereon. The second isolating rib  720  has an upwardly-facing circumferential surface  722  and a downwardly-facing circumferential surface  724 . An O-ring seal  726  seals the inner wall surface of the operating mandrel  310  below the mandrel piston  320  for slidable movement of the operating mandrel  310  against the second isolating rib  720 . 
     An isolating chamber  730  is a space defined by the inner wall of the upper housing section  220  and the outer wall of the operating mandrel  310 , the upper end of which is defined by the downwardly-facing circumferential surface  714  of first isolating rib  710  and the lower end of which is defined by the upwardly-facing circumferential surface  722  of the second isolating rib  720 . Isolating chamber  730  can be a channel, but more preferably is an annular space. 
     One or more housing ports  740  can be formed in the upper housing section  220  of the housing  200  at a location between the first isolating rib  710  and the second isolating rib  720 . The housing port  740  allows fluid communication between the isolating chamber  730  and the exterior of the housing  200 . If the isolating chamber  730  is annular in shape, then preferably, a plurality of housing ports  740  are spaced around the housing  200  to communicate exterior fluid pressure into the isolating chamber  730 . 
     One or more mandrel ports  750  can be formed in the operating mandrel  310 . The mandrel port  750  is located to be separated from the expansion chamber  330  over the full operating range of the operating mandrel  310 . In the presently most preferred embodiment of the invention, the mandrel port  750  is located below the mandrel piston  320 . If the isolating chamber  730  is annular in shape, then preferably, a plurality of mandrel ports  750  are spaced around the operating mandrel  310  to communicate interior flow passage fluid pressure into the isolating chamber  730 , as will hereinafter be described in detail. 
     When the piston assembly  300  of the safety valve  100  is in the initial position shown in FIGS. 1A-B, the mandrel port  750  is positioned above the first isolating rib  710 . When the piston assembly  300  slides to the position shown in FIGS. 2A-B, the mandrel port is still positioned above the first isolating rib  710 . It is not until the piston assembly  300  is moved to the third position shown in FIGS. 3A-B that the mandrel port  750  slides past the first isolating rib  710  and provides fluid communication between the isolating chamber  730  and the interior flow passage  102  of the safety valve. Thus, the operating mandrel  310  and the first and second isolating ribs  710  and  720  isolate the housing port  740  from the interior flow passage  102  of the safety valve  100  until the piston assembly moves into the position shown in FIGS. 3A-B, whereby the mandrel port  750  is aligned with the isolating chamber  730 . 
     Of course, when aligned in the position shown in FIGS. 3A-B, the housing port  740  and the mandrel port  750  provide relatively restricted fluid flow between the interior passage  102  and the exterior of the housing  200 , which effectively creates a fluid choke. This effect can be advantageous used to gradually equalize the pressure between the interior passage  102  and the exterior of the housing. 
     The choking function can also be taken advantage of for the purpose of rapidly pumping up the fluid pressure in the interior fluid conducting passage  102 , whereby a pressure drop is obtained across the fluid circulating valve assembly  700  between the interior and the exterior of the housing  200 . Before the pressure is bled off through the open fluid circulating valve assembly  700 , the interior fluid pressure can be developed sufficiently to overcome the exterior fluid pressure to drive the piston assembly  300  to close the fluid circulating valve assembly  700  and then re-open the flow valve assembly  600 . 
     Biasing Assembly  800   
     The biasing assembly  800  creates a biasing force that facilitates the reliable operation of a safety valve  100  according to the invention. The biasing assembly  800  can be used to establish the amount of differential pressure between the interior flow conducting passage  102  and the exterior of the housing necessary to operate the safety valve  100 . For example, the biasing assembly  800  preferably creates a biasing force that biases the piston assembly  300  of the safety valve  100  to move into a position that closes the flow valve assembly  600 . 
     According to the presently most preferred embodiment of the invention, the biasing assembly  800  is a spring capturing subassembly  810  and a spring  850 . In general, the spring capturing assembly  810  is for capturing and storing potential energy of the spring  850 , which stored potential energy can be used to bias the operation of the piston assembly  300 . The biasing assembly  800  preferably uses structural features already in the other assemblies of the safety valve  100 . 
     For example, the spring capturing subassembly  810  can include the inner wall surface of the upper housing section  220 , the outer wall surface of the operating mandrel  310  and the mandrel extension  520 . A spring capturing rib  820  can be positioned on the outer wall of the mandrel extension  520 . The spring capturing rib  820  has an upwardly-facing circumferential surface  822 , a downwardly-facing circumferential surface  824 , which function as the surface  526 ). An O-ring seal  826  seals the mandrel extension  520  for slidable movement against the inner wall surface of the piston housing  220 . The chamber  830  is defined by the spring capturing subassembly  810 . 
     If the spring  850  is preferably a mechanical spring as shown in the drawings. The spring  850  can be a gas spring, although a mechanical spring is expected to be simpler to design, build, and implement in a safety valve application. For example, if a gas spring is employed, it is necessary to seal and pressurize the spring chamber  830 . In this regard, an O-ring seal  524  can be used to seal the threaded connection  522 , as mentioned above. 
     Shear Pin Assembly  900   
     The shear pin assembly  900  is preferably employed with the biasing assembly  800  to prevent the biasing assembly from initially closing the flow valve assembly  600  of the safety valve during normal handling and assist in setting the initial position of the valve  100 . According to the presently most preferred embodiment of the invention, the shear pin assembly  900  includes a shear pin receiving hole  910  formed in the lower end  214  of connector  210 , shear pin port  912  formed in the upper end  312  of operating mandrel  310 , and shear pin  914 . As will be appreciated by those skilled in the art, the shear pin port  912  does not go all the way through the operating mandrel  310  to avoid the possibility of a fluid leak. The shear pin assembly  900  is adapted to shear or break when a predetermined amount of force is applied. According to the invention, one or more such shear pin assemblies  900  can be adapted for this purpose. 
     Operation of the Preferred Embodiment 
     Based on the foregoing description of the safety valve  100  and the accompanying drawings, the operation of the safety valve  100  according to the invention will be apparent to persons skilled in the art. 
     FIGS. 1A-B illustrate a safety valve  100  according to the presently most preferred embodiment of the invention in an initial position. 
     FIGS. 2A-B illustrate the safety valve shown in FIGS. 1A-B, after an excessive exterior fluid pressure has caused the rupture disks  420  to rupture, shearing the shear pins  914 , and then at least assists in driving the operating piston assembly  300 , acting through the slip assembly  500 , to an intermediate position and close the flow valve assembly  600 . 
     FIGS. 3A-B illustrate the safety valve  100  shown in FIGS. 1A-B, after the flow valve assembly  600  has been completely closed as shown in FIGS. 2A-B, at which point the slip assembly  500  performs its slipping function to allow the exterior fluid pressure to continue to at least assist in driving the piston assembly  300  through to a second position at which an interior port and an exterior port of the circulating valve  700  are in fluid communication. The exterior fluid pressure communicates with the interior of the safety valve  100  on the up-hole side of the flow valve assembly  600 . 
     By pumping fluid from up-hole to increase the fluid pressure at a faster rate than can be bled off through the interior port and exterior port of the circulating valve assembly  700 , the increased fluid pressure drives the operating piston assembly  300  back to the initial position, thereby selectively re-opening the circulating valve  700  and then re-closing the flow valve  600 . 
     Scope of Invention Not Limited to Preferred Embodiments 
     The invention is described with respect to presently preferred embodiments, but is not intended to be limited to the described embodiments. It will be readily apparent to one of ordinary skill in the art that numerous such modifications may be made to the invention without departing from the spirit and scope of it as claimed.