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CLAIM OF PRIORITY 
     This application is a U.S. National Stage of International Application No. PCT/US2013/073150, filed Dec. 4, 2013. 
     TECHNICAL FIELD 
     This disclosure generally relates to a tool and method for damping lateral vibration in a drilling string. 
     BACKGROUND 
     In the recovery of hydrocarbons from the earth, wellbores are generally drilled using any of a variety of different methods and equipment selected according to the particular drilling site and objectives. When drilling a well, a drill bit is rotated in axial engagement against the formation to remove rock, to thereby form the wellbore to a desired depth. The drill bit is typically rotated via the rotation of a drill string to which the drill bit is coupled and/or by the rotary force imparted to the drill bit by a subsurface drilling motor. 
     Downhole vibrations and shocks (referred to collectively and/or interchangeably herein as “shock loads”) are induced by interactions between downhole tools and formations along the wellbore. Shock loads induced at points along the drill string are in turn transmitted to other components of the drill string and bottom hole assembly. Lateral shock loads imparted on the drill string can diminish the life of its interconnected members by accelerating the process of fatigue. Lateral shock loads may also cause damage to the wellbore itself, such as when lateral vibrations cause the drill string to contact the walls of the wellbore, for example. Additionally, excessive shock loads can cause spontaneous downhole equipment failure, wash-outs and a decrease in penetration rate. 
    
    
     
       DESCRIPTION OF DRAWINGS 
         FIG. 1  is a diagram of an example drilling rig for drilling a wellbore. 
         FIG. 2A  is a perspective exploded view of an example vibration damper assembly. 
         FIG. 2B  is a cross sectional view of the example vibration damper piston assembly of  FIG. 1A . 
         FIGS. 3A-3D  are various views of an example piston assembly used in the vibration damper assembly of  FIG. 2A . 
         FIGS. 4A-4D  are various views of the example vibration damper assembly with a collection of damper pistons in a retracted configuration. 
         FIGS. 5A-5D  are various views of the example vibration damper assembly with a collection of damper pistons in an extended configuration. 
         FIGS. 6A-6C  are various views of an example vibration damper assembly with an electrical interface assembly. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a diagram of an example drilling rig  10  located at a drilling site. A drill string  20  is positioned in a wellbore  60  below a surface  12  at the drilling site. The drill string  20  includes any number of segments of drill pipe  21  interconnected end-to-end to reach a desired drill depth. The surface equipment  14  on the drilling rig  10  is used to drill the wellbore  60  to the desired drill depth by controllably rotating and lowering the drill string  20 . The drill string  20  includes a downhole power section  22 . The downhole power section  22  may include a positive displacement motor, such as a Moineau type motor having a rotor  26  that is rotatable relative to a stator  24  in response to the controlled delivery of pressurized fluid to the power section  22 . 
     The drill string  20  also includes a “tool string”  40  and a drill bit  50 . When the drill string  20  is rotated, power and torque are transferred to the drill bit  50  and other downhole equipment coupled to a lower end of the drill string  20 , such as to the “tool string”  40  attached to a longitudinal output shaft  45  of a downhole positive displacement motor. The drill bit  50  may alternatively rotated by the downhole positive displacement motor when the drill string  20  is not being rotated from the surface  12 . 
     After drilling the wellbore  60 , the wellbore  60  may be reinforced by a cementing operation with a casing  34  and a cement sheath  32  in the annulus between the casing  34  and the borehole. 
     During drilling, the surface equipment  14  pumps drilling fluid (i.e. drilling mud)  62 , down the drill string  20  and out ports in the bit  50 . The drilling mud then flows up the annulus  64  between the drill string and borehole wall. The surface equipment rotates the drill string  20 , which in the implementations shown is coupled to the stator  24  of the downhole motor in the power section. The rotor  26  is rotated due to pumped fluid  62  pressure differences across the power section  22  relative to the stator  24  of a downhole positive displacement motor. 
     While drilling, the tool string  40  and/or the drill bit  50  may transmit vibrations that can travel along the drill string  20 . For example, the drill pipe  21  may flex and contact the wellbore  60  or a wellbore wall  61 , sending vibrations along drill string  20 . A vibration damper assembly  100  is included along the tool string  40  to reduce the amount of vibration that is propagated along the tool string  40 . 
       FIG. 2A  is a perspective exploded view of the example vibration damper assembly  100 . The vibration damper assembly  100  includes a collection of piston assemblies  200  arranged about the axial length and about the outer circumference of the generally cylindrical body of a tubular housing  102 . The tubular housing  102  has a longitudinal passageway  103  including several bore sections. Each of the piston assemblies  200  occupies a corresponding transverse passageway  104  formed in the tubular housing  102  and extending radially from the longitudinal passageway  103 . Each of the transverse passageways  104  includes a smooth bore section  106  and a threaded bore section  108 . 
     The piston assembly  200  will now be described, referring to both the exploded view provided by  FIG. 2A  and the view provided by  FIG. 2B , which is a cross sectional view of the example piston assembly  200 . Each of the piston assemblies  200  includes a collection of seals  202   a - 202   i . In some embodiments, the seals  202   a - 202   i  can be O-rings, D-rings, square seals, or combinations of these or other appropriate seal types. 
     A piston cap  210  is formed with an outer surface  212 , an outer peripheral surface  214 , and a threaded section  216 . The outer surface  212  is semi-cylindrical in shape, with a radius and curvature that approximates that of the tubular housing  102 . The outer peripheral surface  214  is formed with a diameter that substantially fills the smooth bore section  106  of a corresponding one of the transverse passageways  104 . The outer periphery of the threaded section  216  is formed with circumferential threads that threadably mate with threads formed upon the inner circumference of the threaded bore section  108  of the corresponding one of the transverse passageways  104 . A pair of spanner holes  218  is formed in the outer surface  212 . In some implementations, the spanner holes  218  can accept the pins of a spanner wrench to assist in the assembly and disassembly of the transverse passageway  210  with the tubular housing  102 . 
     A spring  220  is located about an upper section  232  of a damper piston  230 . The upper section  232  is a generally cylindrical body that is formed to pass through an upper bore portion  240  formed radially through the piston cap  210 . The upper section  232  is separated from a lower section  236  of the damper piston  230  by a circumferential ring  234 . The circumferential ring  234  is formed about the outer periphery of the damper piston  230 . The circumferential ring  234  has a diameter that substantially fills a lower bore portion  242  of the piston cap  210 . The lower bore portion  242  is radially larger than and along the same axis as the upper bore portion  240  through the housing cap  210 . The lower bore portion  242  has a diameter sized to slidably receive the circumferential ring  234 . The lower bore portion  242  is formed partly through a radial section of the piston cap  210  opposite the outer surface  212 . 
     The spring  220  rests against the circumferential ring  234  and becomes constrained axially about the upper section  232  within a spring chamber  244 . The spring chamber  244  is defined between the circumferential ring  234  and the piston cap  210  and the lower bore portion  242  in the assembled form of the piston assembly  200 . A fluid reservoir  246  is defined by the opposite side of the circumferential ring  234 , the lower bore portion  242 , and a support plate  250 . The support plate  250  is formed as a disk with an outer diameter larger than that of the lower bore portion  242 , and a central bore  252  formed to accommodate the lower section  236 . The support plate  250  is removable, fastened to the piston cap  210  by a collection of fasteners  260 , e.g., bolts, screws. 
       FIGS. 3A-3D  are a perspective view, side view, cross section side view, and end view of the example damper piston  230  of  FIG. 2A . Visible in these views are the upper section  232 , the circumferential ring  234 , and the lower section  236 . Also visible in  FIGS. 3A ,  3 C, and  3 D are a collection of apertures  302 . With particular reference to  FIG. 3C , the apertures  302  are axial bores formed through the circumferential ring  234 . In the assembled form of the vibration damper  100 , the apertures fluidly connect the spring chamber  244  of  FIG. 2A  with the fluid reservoir  246 . 
       FIGS. 4A-4D  are a side view, perspective view, side cross section view, and end cross section view of the example vibration damper assembly  100  with a collection of the damper pistons  230  in a retracted configuration. The damper pistons  230  are considered to be retracted when their respective upper sections  236  do not protrude substantially beyond the outer periphery of the tubular housing  102 . With reference to  FIGS. 4C and 4D , each damper piston  230  is urged into its retracted configuration by the spring  220  exerting a spring force against the transverse passageway  210  and the circumferential ring  234 . 
       FIGS. 5A-5D  are a side view, perspective view, side cross section view, and end cross section view of the example vibration damper assembly  100  with a collection of the damper pistons  230  in an extended configuration. The damper pistons  230  are considered to be extended when their respective upper sections  236  protrude substantially beyond the outer periphery of the tubular housing  102  by a radial distance  502 . 
     With reference to  FIGS. 5C and 5D , each damper piston  230  is urged into its extended configuration by applying a pressurized fluid, such as drilling fluid, within the bore  103 . The fluid exerts a fluid pressure upon a lower surface  504  of the lower section  236  of the damper piston  230 . When a predetermined amount of fluidic force is provided, the biasing retractable force of the spring  220  is overcome and urges the upper portion  232  to protrude beyond the outer periphery of the tubular housing  102 . 
     Extension and retraction of the upper portion  232  of the damper piston  230  is damped by fluidic action. Referring back to  FIG. 2B , a fluid such as hydraulic oil substantially fills the spring chamber  244  and the fluid reservoir  246 . As the damper piston  230  is urged from the retracted position to the extended position, fluid in the spring chamber  244  is displaced through the collection of apertures  302  to the fluid reservoir  246 . The apertures  302  restrict the flow of the fluid from the spring chamber  244  to the fluid reservoir  246 , resisting the extensile movement of the damper piston  230 . Similarly, as the damper piston  230  is urged from the extended position to the retracted position, e.g. when the upper portion  232  contacts the wellbore wall  61 , hydraulic fluid in the fluid reservoir  246  is displaced through the collection of apertures  302  to the spring chamber  244 . The apertures  302  restrict the flow of the fluid from the fluid reservoir  246  to the spring chamber  244 , compliantly resisting the retraction of the damper piston  230 . 
     This resistance that is developed by the flow of fluid through the apertures  302  dampens the speed of the damper piston  230  in response to changes in the pressure of fluids provided within the bore  103  and/or to external forces acting upon the upper portion  232 , e.g. when the upper portion  232  contacts the wellbore  60 . In some embodiments, the apertures  302  can be configured to provide a predetermined amount of damping. For example, the quantity and/or bore sizes of the apertures  302  can be selected to provide various damping rates. In another example, check valves or other directional flow assemblies can be included in the damper piston  230  to provide a first damping rate during extension and a different damping rate during retraction of the upper portion  232 . In yet another example, other appropriate assemblies may be included in the damper piston  230  to provide speed-dependent, e.g., progressive, damping rates during extension or retraction of the upper portion  232 . 
       FIGS. 6A-6C  are cross sectional, end, and exploded perspective views of the example vibration damper assembly  100  with an electrical interface assembly  600 . In general, the electrical interface assembly  600  provides one or more electrically conductive pathways to transmit power and/or electrical signals from one end of the assembly  100  to the other. For example, the electrical interface assembly  600  can be used to provide power and/or communications between equipment at the surface  12  and measuring while drilling (MWD) or logging while drilling (LWD) tools positioned below the damper assembly  100 . 
     The electrical interface assembly  600  includes one or more electrical conductors  602 . The electrical conductors  602  extend from an electrical connector  604   a  located at a first end  110   a  of the assembly  100  to an electrical connector  604   b  located at a second end  110   b  of the assembly  100 . The electrical conductors  602  are routed through a conduit  606 . In some embodiments, the conduit  606  can be electrically and/or mechanically isolated from the bore  103 . For example, the conduit  606  may be electrically insulating, and/or protect the electrical conductors  602  from fluids within the bore  103 . 
     The electrical connector  604   a  is supported by a bracket  610   a , and the electrical connector  604   b  is supported by a bracket  610   b . The brackets  610   a ,  610   b  position and orient the electrical connectors  604   a ,  604   b  relative to the tubular housing  102 . For example, the brackets  610   a ,  610   b  can align the electrical connectors  604   a ,  604   b  with the central axis of the vibration damper assembly  100 , and electrical contact can be made between the electrical connectors  604   a ,  604   b  and similar electrical connectors in adjacent tool string components when the adjacent tool string components are threaded into the vibration damper assembly  100 . 
     A collection of seals  620  provide sealing contact between the tubular housing  102  and the brackets  604   a ,  604   b . A collection of fasteners  630 , such as bolts or screws, removably secures the bracket  604   a  to the first end  110   a  and the bracket  604   b  to the second end  110   b.    
     While drilling, a vibration damper assembly  100  is inserted in the drill string  20 . A first end portion of the spring  220  is contacted with at least a portion of the spring chamber  244  and a second end portion of the spring  220  is contacted with the circumferential ring  234 , biasing the damper piston  230  in a retracted position such as the position shown in  FIGS. 4A-4D . The drill string  20  and vibration damper assembly  100  are inserted into the wellbore  60 . The fluid  62  is flowed down the drill string  20  and exerts fluid pressure on the lower surface  504  of the lower portion  236  of the damper piston  230 . The fluidic pressure acting on the lower surface  504  creates a fluidic force sufficient to overcome a biasing retractable force of the spring  220 . The fluidic force extends the damper piston  230  longitudinally until the upper portion  232  contacts the wellbore wall  61 . 
     Although a few implementations have been described in detail above, other modifications are possible. For example, the process flows described herein do not require the particular order shown, or sequential order, to achieve desirable results. In addition, other steps may be provided, or steps may be eliminated, from the described flows, and other components may be added to, or removed from, the described systems. Accordingly, other implementations are within the scope of the following claims.

Summary:
A method includes inserting a vibration damper tool in a drill string, the damper includes a tubular housing having an exterior surface and a longitudinal passageway, and at least one fluid actuated piston assembly. The piston assembly includes an extendable piston, a transverse passageway, a spring chamber in the transverse passageway, and at least one spring disposed in the spring chamber. The spring biases the piston in a refracted position. The drill string and dampening tool are inserted into a wellbore, fluid is flowed down the drill string and exerts pressure on a proximal end of the piston, and creates a fluidic force sufficient to overcome a biasing retractable force of the spring to extend the piston longitudinally until a distal end of the piston contacts a sidewall of the wellbore.