Abstract:
A safety valve for use in a wellbore formed in an earth formation, comprising a valve body having a fluid passage for passage of a stream of hydrocarbon fluid flowing from the earth formation via the wellbore to the earth surface, a closure member movable relative to the valve body between an open position in which the fluid passage is open and a closed position in which the closure member closes the fluid passage, and an activating device for selectively subjecting the closure member to a drag force of selected magnitude, the drag force being exerted by the stream of fluid and inducing the closure member to move from the open position to the closed position thereof. 
     The safety valve further comprises control means for controlling the activating device to subject the closure member to said drag force.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a safety valve for use in a wellbore formed in an earth formation, comprising a valve body having a fluid passage for passage of a stream of hydrocarbon fluid flowing from the earth formation via the wellbore to the earth surface, a closure member movable relative to the valve body between an open position in which the fluid passage is open and a closed position in which the closure member closes the fluid passage. The safety valve serves to shut down production from the wellbore either by control from surface or automatically in case of undesirable flow conditions. The latter situation occurs for example in case of increased hydrocarbon fluid flow rate as a result of an accident at a surface production facility. Therefore it is generally aimed to design a safety valve for a wellbore such that the valve closes upon the flow rate in the wellbore reaching a selected threshold flow rate. 
     However, experience has shown that conventional safety valves generally have a low accuracy with regard to the flow rate at which the valve closes. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide an improved downhole safety valve which overcomes the drawbacks of the conventional downhole safety valves. 
     In accordance with the invention there is provided a safety valve for use in a wellbore formed in an earth formation, comprising 
     a valve body having a fluid passage for passage of a stream of hydrocarbon fluid flowing from the earth formation via the wellbore to the earth surface; 
     a closure member movable relative to the valve body between an open position in which the fluid passage is open and a closed position in which the closure member closes the fluid passage; 
     an activating device for selectively subjecting the closure member to a drag force of selected magnitude, the drag force being exerted by the stream of fluid and inducing the closure member to move from the open position to the closed position thereof; and 
     control means for controlling the activating device to subject the closure member to said drag force. 
     By selectively subjecting the closure member to the drag force it is achieved that a step-change in the resulting force acting on the closure member is created rather than a gradual change as in conventional safety valves. As a result the closure member closes the fluid passage in response to the step-change of force. It is to be understood that the drag force can act directly onto the closure member or onto a drag surface connected to the closure member. 
     Suitably the activating device is operable between a first mode in which flow of the stream of fluid against the closure member is substantially prevented, and a second mode in which flow of the stream of fluid against the closure member is allowed. 
     In a preferred embodiment the closure member is arranged in a conduit and wherein in said first mode the activating device substantially prevents flow of the stream of fluid into the conduit, and in said second mode the activating device allows flow of the stream of fluid into the conduit. 
     Preferably the activating device includes a flapper valve having a flapper element arranged upstream the closure member, which flapper element in said first mode substantially closes the conduit and in said second mode substantially leaves the conduit open. 
     The invention will be described hereinafter in more detail and by way of example with reference to the accompanying drawings in which: 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A better understanding of the present invention may be obtained by reference to the following drawings when read in conjunction with the specification. 
     FIG. 1 schematically shows a longitudinal cross-section of an embodiment of a downhole safety valve according to the invention; 
     FIG. 2 schematically shows cross-section  2 — 2  of FIG. 1; 
     FIG. 3 schematically shows cross-section  3 — 3  of FIG. 2; 
     FIG. 4 schematically shows side view  4 — 4  of FIG. 2; 
     FIG. 5 schematically shows a detail of alternative embodiment of a downhole safety valve according to the invention; and 
     FIG. 6 is a top view of FIG.  5 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In FIG. 1 is shown a wellbore  1  formed into an earth formation  2  for the production of a stream of hydrocarbon gas. The wellbore  1  is provided with a casing  3  fixed in the wellbore by a layer of cement  4 . A downhole safety valve  6  according to the invention is concentrically arranged in the casing  3  and fixed to the casing by a packer  8  which prevents hydrocarbon gas from bypassing the safety valve  6 . The direction of flow of the gas is indicated by arrows  9 . 
     The safety valve  6  includes a conduit in the form of tubular valve body  10  having a plurality of gas inlets in the form of slots  12  provided in the tubular valve body and a gas outlet  14  in fluid communication with the slots  12  via a valve opening  16 . A valve seat  17  extends around the valve opening  16  at the upstream side thereof. A closure member  18  is arranged in the valve body  10 , the closure member having a front surface  20  matching the valve seat  17 . The closure member  18  is movable in axial direction of the valve body  10  between an open position (as shown in FIG. 1) in which the front surface  20  is located away from the valve seat  17  and a closed position in which the front surface  20  contacts the valve seat  17  and thereby closes the valve. A small radial clearance  22  is present between the outer surface of the closure member  18  and the inner surface of the valve body  10 . A spiral tension spring  24  is provided in the tubular valve body  10 , one end of the spring  24  being connected to the closure member  18 , the other end to a stop member  26  arranged in the valve body. The stop member  26  is adjustable in axial direction of the valve body  10  in order to adjust the tension force of the spring  24 . When in rest position, the spring  24  holds the closure member  18  in the open position thereof. 
     Referring further to FIG. 2, the upstream end part of the valve body  10  is provided with an activating device in the form of a flapper valve  30  operable between a closed mode in which flow of the stream of gas against the  18  closure member is substantially prevented, and an open mode in which flow of the stream of gas into the valve body  10  and against the closure member  18  is allowed. The flapper valve  30  includes a flapper body  31  arranged inside the tubular valve body  10  and having a flow opening  33 , and a flapper element  32  connected to a rotatable shaft  34  which divides the flapper element  32  in portions  36 ,  37  of different surface areas. To illustrate this arrangement, the eccentricity between axis of symmetry  39   a  of the flapper element and the longitudinal axis  39   b  of the shaft  34  has been indicated in FIG. 2 by reference sign  38 . 
     Referring further to FIG. 3, the rotatable shaft  34  extends through a chamber  40  provided in the valve body  31 . The shaft  34  has a cam surface formed by a flat surface portion  43  of the shaft, the flat surface portion  43  extending parallel to the flapper element  32  and being located in the chamber  40 . The remaining surface of the axis  34  has a circular cross-section. A leaf spring  42  is arranged in the chamber  40  so that both ends of the leaf string  42  are fixed to the walls of the chamber  40  and that a central portion of the leaf spring  42  fully engages the flat surface portion  43  of the shaft  34  when the flapper  32  is in its closed position. 
     The leaf spring  42  and the shaft  34  form a system for counter-acting initial rotation of the flapper element  32  from the closed position to the open position thereof as a result of flow of the stream of fluid against the surface portions  36 ,  37  of different surface areas. 
     Referring further to FIG. 4 the shaft  34  extends into a recess  44  provided at the outside of the valve body  10 . A vane element  46  is arranged in the recess  44  and fixedly connected to the shaft  34  in a manner that the vane element  46  extends at a selected angle α relative to the direction of flow  9  when the flapper element  32  is in its closed position (the flapper element  32  and the leaf spring  42  are indicated in phantom lines in FIG.  4 ). 
     The vane element  46  forms a trigger means for triggering said initial rotation of the flapper element against the action of the system for providing said torque when the flow rate of the stream exceeds a selected threshold flow rate. 
     The shaft  34  is furthermore provided with a spiral spring  48  (FIG. 2) which biases the shaft  34  to the position in which the flapper element  32  is in the closed position thereof. 
     During normal operation a stream of hydrocarbon gas produced from the earth formation flows at a normal flow rate through the casing  3  in the direction  9  to the safety valve  6 . The flapper element  32  is in its closed position by the action of the spiral spring  48  and the action of the leaf spring  42 , and the closure element  18  is in its open position by the action of the tension spring  24 . 
     The flapper element  32  prevents flow of the stream of gas into the valve body  10  and against the closure element  18 . Furthermore, the stream of gas exerts a drag force to the vane element  46  acting so as to align the vane element with the stream and to cause thereby initial rotation of the flapper element  32 . However such alignment is countered by the action of the leaf spring  42  as long as the flow rate of the stream does not exceed the threshold flow rate. 
     The stream of gas flows from via the slots  12  to the valve opening  16  and from there to the gas outlet  14 . From the gas outlet  14  the gas flows further through the casing  3  in the direction  9  to a processing facility (not shown) at surface. 
     If the flow rate of the stream exceeds the threshold flow rate, for example due to an undesired pressure drop at the processing facility at surface, the drag force exerted to the vane element increases and causes the vane element to align with the stream and thereby to rotate the axis  43  and the flapper element  32  against the action of the leaf spring  42  biasing against the shaft  34 . As the axis  34  rotates, the leaf spring  42  increasingly bends until the leaf spring  42  becomes engaged against the cylindrical portion of the hinge axis  34 . Further rotation of the hinge axis  34  is then no longer counter-acted by the leaf spring  42 , and the flow of the stream against the flapper element  32  provides a turning moment causing the flapper element  32  to rotate to its open position. With the flapper  32  in its open position, the stream is allowed to flow into the valve body  10  and against the closure member  18 . As a result the closure member  18  becomes subjected to a hydraulic force applied by fluid flow  9 , which causes the closure member  18  to move to the closed position thereof against the action of the spring  24 . Any further flow through the safety valve  6  is thereby prevented. In the absence of flow, the vane element  46  is no longer subjected to a drag force thereby allowing the spiral spring  48  to bias the flapper  32  back to its closed position. The closure member  18  will retain its closed position as long as a pressure difference across the closure member  18  prevents returning of the closure member  18  to its open position. 
     When production is to be resumed the gas pressure at the surface facility is raised so that the spring force of spring  24  urges the closure member  18  again to its open position. 
     Referring to FIGS. 5 and 6, there is shown a detail of an alternative embodiment of a safety valve according to the invention, which alternative embodiment is largely similar to the embodiment described with reference to FIGS. 1-4, except that the trigger means is a fluidic trigger device  49  instead of the vane element referred to hereinbefore. In FIG. 5 is shown the upstream end part of the alternative embodiment including the valve housing  10 , the flapper valve  30 , the valve body  31 , the flapper element  32 , the leaf spring  42  and the spiral spring  48 . 
     The fluidic trigger device  49  includes a fluid inlet  50 , a first fluid outlet  52  and a second fluid outlet  54 . A port  56  provides fluid communication between the exterior of the valve housing  10  and the junction between the first outlet  52  and the second outlet  54 . The second outlet  54  is arranged so that a fluid stream leaving the second outlet  54  flows against the larger one of the surface portions  36 ,  37 . The inlet  50 , the outlets  52 ,  54 , and the port  56  are so arranged that if the flow rate of the stream of gas does not exceed the threshold flow rate, a sub-stream of the stream of gas entering the inlet  50  leaves the device  49  through the first outlet  52 , and that if the flow rate of the stream of gas exceeds the threshold flow rate, the sub-stream leaves the device  49  through the second outlet  54 . The sub-stream is diverted into the second outlet  54  by virtue of a decreased pressure in port  56  at the higher flow rate of the stream of gas. 
     Normal operation the alternative embodiment is similar to normal operation of the embodiment described with reference to FIGS. 1-4, except that instead of initial rotation of the flapper element being triggered by the vane element, such initial rotation is being triggered by the fluidic trigger  49 . 
     Instead of the leaf spring biasing against the cam surface of the shaft so as to counter-act initial rotation of the flapper element, a solenoid activated element can be biased against the cam surface so as to counter-act initial rotation of the flapper element. Preferably the solenoid activated element is biased against the cam surface if electric power is provided to the solenoid, and retracted from the cam surface if no power is provided to the solenoid.