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CROSS-REFERENCE TO RELATED APPLICATIONS 
     Not Applicable. 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not Applicable. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to downhole tools that control thrust generating members. More particularly, the present invention relates to an apparatus that absorbs the thrust generated by a downhole tool having a mud motor and/or a propulsion system. 
     2. Description of the Related Art 
     It is known that the recovery of subterranean deposits of hydrocarbons requires the construction of wells having boreholes hundreds, perhaps thousands, of feet in depth. One known system configured for well construction activities includes a bottom hole assembly (BHA) that is tethered to surface support equipment by a flexible umbilical. This BHA may be a self-propelled system that forms a borehole using a bit adapted to disintegrate the earth and rock of a subterranean formation. One such system is described in U.S. Pat. No. 6,296,066, entitled “Well System,” issued Oct. 2, 2001, hereby incorporated herein by reference for all purposes. This system preferably includes a bit, a downhole means to rotate the bit, and a downhole means to thrust the bit against the bottom of the borehole. An exemplary arrangement utilizes a positive displacement motor (e.g., a “mud motor”) to rotate the bit and a tractor to generate thrust or weight on bit (WOB). In these systems, high pressure drilling mud is conveyed to the BHA through the umbilical. After passing through the BHA, the drilling mud exits through nozzles located in the bit and the drilling mud with returns flows back to the surface via an annulus formed between the umbilical and the borehole wall. The mud motor and tractor use the drilling fluid flowing through the umbilical as their power source. 
     A system wherein two or more components share a common hydraulic fluid supply have certain drawbacks. Referring now to FIG. 1, there is schematically shown an exemplary hydraulic circuit that is susceptible to these drawbacks. The hydraulic circuit includes a fluid line  10 , a tractor  11  having a pressure chamber  12  and piston head  13 , a mud motor  14  having a power section  18  that includes a rotor  15 , a stator  19 , and a bit  16 . Drilling fluid flows through fluid line  10  and mud motor  14  to bit  16 . A portion of the drilling fluid is diverted via line  17  to tractor  11 . When drilling fluid enters pressure chamber  12 , piston head  13  drives bit  16  into the formation. The drilling fluid flowing through mud motor  14  induces rotation of power-section rotor  15  and connected bit  16 . Thus, mud motor  14  uses the pressure differential across power-section rotor  15  to induce bit  16  to rotate whereas tractor  11  uses the pressure in chamber  12  to drive piston head  13  and bit  16  into the formation. 
     Because tractor  11  and mud motor  14  draw from a common hydraulic fluid line  10 , an unstable operating condition in mud motor  14  may cause a corresponding instability in tractor  11 , and vice versa. For example, during drilling operations, the BHA may encounter a formation having earth and rock that is particularly difficult to disintegrate. A bit  16  forced against this hard to drill formation tends to increase the torque required to turn the drill bit against the formation. The bit torque increase causes a resultant increase in the differential pressure across power section  18  of mud motor  14 . As the pressure differential across mud motor  14  increases, the pressure of the drilling fluid in fluid line  10  upstream of mud motor  14  also increases. Tractor  11  receives this higher pressure drilling fluid from line  17  which is connected to fluid line  10 . Because drilling fluid pressure and tractor thrust are directly related, this increased pressure causes tractor  11  to drive the bit  16  even harder against the formation and at a faster rate. This increase in tractor rate of advancement further contributes to the increase in the torque required to turn the bit  16 , thereby creating a feed-back effect which may ultimately cause the bit to stall or shorten the operating life of BHA components such as mud motor  14 . 
     Some systems incorporate shock absorbers or dampeners in BHAs just above the mud motors. These shock absorbers or dampeners are sometimes Belleville springs that reduce the spring rate of the BHA between the motor and the tools above. However, having the springs just above the mud motors increases the length of the drillstring and also requires extra connections. An additional spline for transmitting torque load is also required. Additionally, the tractor still pushes the bit by weight on bit and can have the same problems discussed above. The tractor, having dampeners on each anchor allows for each dampener to be reset whenever its anchor disengages the hole wall so that additional length of dampening movement can allow tractor rate of advancement to slow down to drilling rate. Also directional control ability of drill bit below is reduced due to lower bending rigidity, and also circumferential looseness of spline connections. 
     The present invention addresses these and related deficiencies in prior art systems discussed above. 
     SUMMARY OF THE INVENTION 
     The present invention features a thrust absorber interposed between a thrusting means and an anchoring means. Normally, the thrusting means and the anchoring means cooperate to axially displace a tube. In a preferred embodiment, the thrust absorber includes an enclosure that is fixed to the anchoring means and a retainer connecting to the thrusting means. Disposed within the enclosure is a biasing member that is configured to absorb thrust energy when a predetermined condition occurs. Particularly, the thrusting means can encounter an overthrust condition when the thrusting means imparts a thrust force to the tube, but the tube is not substantially axially displaced. When an overthrust condition occurs, the biasing member is compressed by the tube, and thereby absorbs the thrust that otherwise would have been imparted to the tube. Also, by absorbing the thrust, the pressure increase is substantially reduced. The reduction in pressure increase reduces the tractor advancement rate increase so that the tractor rate is modulated and makes the system more stable. Furthermore, for a bottom hole assembly having more than one thrusting means, a thrust absorber may be provided for each such thrusting means. 
     In a first and second alternative embodiment, the thrust absorbers additionally comprise two different configurations that restrict the speed of movement of the thrust absorbers. The thrust absorbers are especially restricted once the external load across the absorber is relaxed. 
     In a third alternative embodiment, the thrust absorber additionally comprises a second biasing member disposed within the enclosure. Particularly, the second biasing member restricts movement of the thrust absorber when the tube is displaced in a direction opposite that of the intended forward direction of the tractor. The second biasing member allows most of the length of the thruster stroke to be realized by preventing loss of stroke length due to movement of the thrust absorber. 
     The present invention comprises a combination of features and advantages which enable it to overcome various problems of prior devices. The various characteristics described above, as well as other features, will be readily apparent to those skilled in the art upon reading the following detailed description of the preferred embodiments of the invention, and by referring to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more detailed description of the present invention, reference will now be made to the accompanying drawings, wherein: 
     FIG. 1 is a schematic diagram of a prior art hydraulic circuit that includes a tractor, a mud motor, and a bit constructed in accordance with a preferred embodiment; 
     FIG. 2 is a schematic diagram of a bottom hole assembly constructed in accordance with the preferred embodiment disposed in a well bore; 
     FIG. 3A is a cross-sectional view of a tractor incorporating a forward thrust controller constructed in accordance with the preferred embodiment; 
     FIG. 3B is a cross-sectional view of a tractor incorporating an aft thrust controller constructed in accordance with the preferred embodiment; 
     FIG. 4A is a cross-sectional view of a forward thrust controller constructed in accordance with the preferred embodiment; 
     FIG. 4B is a cross-sectional view of an aft thrust controller constructed in accordance with the preferred embodiment; 
     FIG. 5A is a top-half cross-sectional view of a first alternative embodiment of a forward thrust controller; 
     FIG. 5B is a top-half cross-sectional view of a first alternative embodiment of an aft thrust controller; 
     FIG. 6A is an enlarged cross-sectional view of a thrust controller retainer orifice in a first position constructed in accordance with the first and second alternative embodiments; 
     FIG. 6B is an enlarged cross-sectional view of a thrust controller retainer orifice in a second position constructed in accordance with the first and second alternative embodiments; 
     FIG. 7A is a top-half cross-sectional view of a second alternative embodiment of a forward thrust controller; 
     FIG. 7B is a top-half cross-sectional view of a second alternative embodiment of an aft thrust controller; 
     FIG. 8A is a top-half cross-sectional view of a third alternative embodiment of a forward thrust controller; and 
     FIG. 8B is a top-half cross-sectional view of a third alternative embodiment of an aft thrust controller. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     While the present invention may be used in a variety of situations, a preferred embodiment of the present invention may be used in conjunction with a well tool adapted to form a well bore in an subterranean formation. It should be appreciated, however, that the below-described arrangement is merely one of many for which the present application may be advantageously applied. 
     Referring initially to FIG. 2, a bottom hole assembly (BHA)  20  is shown disposed in a well bore  22  formed in a formation  24 , the well bore  22  having a wall  26  and a well bottom  28 . Arrangements for exemplary BHA&#39;s are discussed in U.S. Pat. No. 6,296,066, issued Oct. 2, 2001, entitled “Well System”, and in U.S. patent application Ser. No. 09/467,588 filed Dec. 20, 1999 entitled “Three Dimensional Steering System”, both hereby incorporated herein by reference for all purposes. BHA  20  may include a bit  30 , instrumentation  32 , a mud motor  34 , a tractor  36 , and other auxiliary equipment  38 , such as telemetry systems or data processors. An umbilical  40  connects BHA  20  to the surface. For convenience, movement of BHA  20 , or any of its components, in direction “D” is intended to denote movement of BHA  20  towards well bottom  28  (downhole). Movement of BHA  20 , or any of its components, in direction “U” is intended to denote movement of BHA  20  away from well bottom  28  (uphole). 
     The various devices and mechanisms of BHA  20  may be energized using high pressure drilling fluid (i.e., “mud”) pumped from the surface through umbilical  40 . Under ordinary operations, this drilling fluid flows through the umbilical  40 , through BHA  20 , and exits at bit  30  through nozzles (not shown). The drilling fluid returns uphole through the annulus  25  formed by well bore wall  26  and umbilical  40  and carries with it the cuttings of earth and rock that have been created by the cutting action of bit  30  against well bottom  28 . Drilling mud pumped downhole is normally under very high pressure. This high pressure can be converted into energy by BHA  20  components, such as the tractor  36  and mud motor  34 , that use hydraulically actuated mechanisms. 
     Referring now to FIGS. 2,  3 A and  3 B, there is shown a preferred arrangement of forward and aft thrust controllers  130 ,  160  mounted on each end of tractor  36 . Tractor  36  is configured to convert the hydraulic pressure of the drilling fluid into a thrusting force for urging bit  30  against well bottom  28  (FIG.  2 ). The thrust developed by tractor  36  is controlled by a forward thrust controller  130  and an aft thrust controller  160 . The details of tractor  36 , the valve control circuitry (not shown) and other related mechanisms are discussed in U.S. Pat. No. 6,003,606 Puller-Thruster Downhole Tool, hereby incorporated herein by reference for all purposes. Tractor arrangements are also disclosed in U.S. Pat. No. 3,180,437, also hereby incorporated herein by reference for all purposes. Accordingly, only general reference will be made to the structure and operation of tractor  36 . 
     A exemplary tractor  36  may include a forward anchor  60 , an aft anchor  70 , a forward thruster  80  and an aft thruster  100 , all disposed on a mandrel or center tube  50 . These components are energized using high pressure drilling fluid that is directed through tractor  36  by valve circuitry (not shown) and associated piping (not shown). The valve circuitry and associated piping will be referred to generally as valve circuitry hereinafter. Valve circuitry can be programmed to cause tractor  36  to deliver a thrust force to bit  30  and/or propel BHA  20  through well bore  22  (FIG.  2 ). 
     Tube  50  transmits the thrust generated by forward and aft thrusters  80 ,  100  to bit  30 . Tube  50  includes a medial portion  52  and first and second end portions  56 ,  58  and with a flowbore  54  extending therethrough. First and second end portions  56 ,  58  include connection interfaces for adjacent components in the bottom hole assembly  20 . For example, first end portion  56  may link tractor  36  with mud motor  34 . Second end portion  58  may link tractor  36  with auxiliary equipment  38 . Flowbore  54  provides a channel for conveying drilling fluid through tractor  36  to bit  30 . Tube medial portion  52  telescopically reciprocates within tractor  36  as forward and aft thrusters  80 ,  100  alternately deliver their respective thrust forces to tube  50  in a manner described below. 
     Forward anchor  60  holds forward thruster assembly  80  stationary relative to borehole wall  26  while forward thruster  80  urges tube  50  and aft thruster assembly  100  downhole towards well bottom  28  (i.e., direction “D”). Forward anchor  60  includes borehole retention assemblies  62  and a housing  64 . The tractor  36  valve circuitry directs high pressure drilling fluid into and out of actuation assemblies which are a part of borehole retention assemblies  62 . Borehole retention assemblies  62  may include wedge members that extend radially or expandable bladder-like grippers. The introduction of drilling fluid causes borehole retention assemblies  62  to extend/inflate and engage borehole wall  26 . Borehole retention assemblies  62  disengage borehole wall  26  when the valve circuitry discharges the drilling fluid into the annulus  25 . In a similar manner, aft anchor  70  engages borehole wall  26  while aft thruster  100  urges tube  50  downhole towards well bottom  28 . Like forward anchor  60 , aft anchor  70  includes borehole retention assemblies  72  and a housing  74 . 
     Forward thruster  80  generates a thrusting force that urges bit  30  downhole against the well bottom  28 . Forward thruster  80  includes a cylinder member  82 , a piston head  90 , a closure member  92  and a valve assembly (not shown). Cylinder member  82  surrounds and freely slides along tube  50  and is a barrel-shaped member having a forward end  83 , an interior chamber  84 , and an aft end  85 . Closure member  92  is received within forward end  83  of cylinder member  82  to seal interior chamber  84 . Piston head  90  is fixed onto tube medial portion  52  and is positioned within chamber  84  to divide chamber  84  into a power section  86  and a reset section  88 . Piston head  90  begins its stroke within chamber  84  next to cylinder aft end  85  and completes its stroke next to cylinder forward end  83 . The valve circuitry initiates a stroke by injecting or “spurting” pre-determined amounts of drilling fluid into the power section  86  for a finely controlled rate of advancement. When piston head  90  completes its stroke, i.e., reaches forward end  83 , the valve assembly directs drilling fluid into reset section  88  to urge piston head  90  back to its original position. 
     Aft thruster  100  generates the thrusting force that urges bit  30  downhole against the well bottom  28  in generally the same manner as forward thruster  80 . Aft thruster  100  includes a cylinder  102 , a piston head  110 , a closure member  112 , and associated valve assemblies (not shown). Cylinder member  102  surrounds and freely slides along tube  50 . Cylinder member  102  is a barrel-shaped member having an forward end  103 , an interior chamber  104 , and an aft end  105 . Closure member  112  is received by aft end  105  of cylinder member  102  to seal interior chamber  104 . Piston head  110  mounts directly onto tube medial portion  52  and is positioned within chamber  104  to divide chamber  104  into a power section  106  and a reset section  108 . Piston head  110  begins its stroke within chamber  104  next to cylinder aft end  105  and completes its stroke next to cylinder forward end  103 . The valve assembly initiates a stroke by directing drilling fluid into the power section  106 . When piston head  110  has completed its stroke, i.e., reached forward end  103 , the valve assembly directs drilling fluid into reset section  108  to urge piston head  110  back to its original position. 
     Referring now to FIGS. 3A and 4A, forward thrust controller  130  controls the thrust generated by forward thruster  80 . Forward thrust controller  130  includes a housing  132 , a retainer  134  and at least one spring  136 . Housing  132  includes first end  138 , a back shoulder  140  forming an annular area  142  with tube  50 , and a cavity  144 . The cavity  144  is not sealed and although it initially preferably contains a high temperature grease, fluids such as annular drilling fluids may enter the cavity  144  during operation. Housing first end  138  is attached to forward anchor housing  64  (FIG. 3A) via a threaded connection or other suitable means. Retainer  134  transmits thrust between forward thruster  80  and spring  136 . Retainer  134  includes a sleeve  146  and a collar  148  which are disposed around tube  50  and within housing cavity  144  in a piston-cylinder fashion. Sleeve  146  is generally a tubular member having a first end  143  and a second end  145  having collar  148 . Sleeve  146  presents an outer surface  151  that is adapted to seat spring  136 . First end  143  of sleeve  146  extends through the annular area  142  of back shoulder  140  and is attached to closure member  92  of forward thruster  80 . Spring  136  on sleeve  146  is disposed between back shoulder  140  and collar  148 . 
     When hydraulic pressure is applied on piston head  90  in power section  86 , tube  50 , which is attached to piston head  90 , moves within thruster  80 . Cylinder member  82 , which is attached to forward anchor  60  via forward thrust controller  130 , remains stationary as tube  50  moves within thruster  80 . Should the bit  30  attached to tube  50  become stalled such as due to torque demand on the bit and mud motor, tube  50  will stop its forward movement. Also, tube  50  may stop its forward movement due to an excessive amount of “U” direction drag force from borehole wall  26  on tube  50 . Because piston head  90  no longer can move, the hydraulic pressure will cause cylinder member  82  to move in a direction generally away from bit  30 . As cylinder member  82  moves relative to forward anchor  60 , collar  148  on sleeve  146  slides towards back shoulder  140  and compresses spring  136  between back shoulder  140  and collar  148 . 
     Spring  136  absorbs the energy associated with an undesired increase in the thrust developed by forward thruster  80 . Spring  136  is disposed about sleeve  146  and is compressed against back shoulder  140  by collar  148 . The capacity of spring  136  to absorb energy depends, in part, on the spring constant of the material forming the spring, the number of springs, and the diameter of the springs. It will be appreciated that springs, such as Belleville springs, are a relatively reliable and inexpensive biasing mechanism capable of absorbing bursts of increased thrust. Other methods utilizing coiled springs, compressible fluids, or other means may also be used in other circumstances. 
     It can be seen that a resilient connection is established between forward borehole retention assembly  62  and cylinder member  82 . Under normal operating conditions, this connection has a first state wherein a substantially solid connection is provided. Under overthrust conditions, this connection becomes resilient and allows cylinder member  82  to slide axially relative to forward borehole retention assembly  62  provided that the spring force of spring  136  is overcome. 
     Referring now to FIGS. 3B and 4B, aft thrust controller  160  modulates the thrust generated by aft thruster  100 . Similar to the construction of forward controller  130 , aft thrust controller  160  includes a housing  162 , a retainer  164 , and at least one spring  166 . Housing  162  includes a first end  167  forming a first shoulder  168 , and a second end  169  forming a second shoulder  170  that forms an annular area  171  with tube  50 , and a cavity  172 . The cavity  172  is not sealed and although it initially preferably contains a high temperature grease, fluids such as annular drilling fluids may enter the cavity  172  during operation. Housing first end  167  is connected with aft anchor housing  74  (FIG. 3B) via a threaded connection or other suitable means. Retainer  164  transmits thrust to and from aft thruster  100  and spring  166 . Retainer  164  includes a sleeve  174  and a collar  176  which are disposed around tube  50  and within housing cavity  172  in a piston-cylinder fashion. Sleeve  174  is generally a tubular member having a first end  178  and a second end  180  having collar  176 . First end  178  of sleeve  174  extends through the annular area  171  and is connected to closure member  112  of aft thruster  100 . 
     When hydraulic pressure is applied on piston head  110  in power section  106 , tube  50 , which is attached to piston head  110 , moves within aft thruster  100 . Cylinder member  102 , which is attached to aft anchor  70  via aft thrust controller  160 , remains stationary as tube  50  moves within aft thruster  100 . Should the bit  30  attached to tube  50  become stalled such as due to encountering slow drilling formation or formation that requires higher torque to rotate the bit or an excessive amount of drag force, tube  50  will stop its forward movement. Because piston head  110  can no longer move, the hydraulic pressure will cause cylinder member  102  to move in a direction generally away from bit  30 . As cylinder member  102  moves relative to aft anchor  70 , collar  176  on sleeve  174  slides towards first shoulder  168  and compresses spring  166  between first shoulder  168  and collar  176 . 
     Spring  166  is formed in substantially the same manner as spring  136  of forward controller  130  and will not be discussed in further detail. 
     It can be seen that a resilient connection is established between aft borehole retention assembly  72  and cylinder member  102 . Under normal operating conditions, this connection has a first state wherein a substantially solid connection is provided. Under overthrust conditions, this connection becomes resilient and allows cylinder member  102  to slide axially relative to aft borehole retention assembly  72  provided that the spring force of spring  166  is overcome. 
     Referring again to FIGS. 2,  3 A, and  3 B, under one mode of operation, the valve circuitry sequentially energizes the components of tractor  36  to impart a thrust on tube  50 . The sequence of this thrusting action has a first step wherein the forward anchor  60  and thruster  80  are energized and a second step wherein the aft anchor  70  and thruster  100  are energized. 
     During the first step, the valve circuitry directs hydraulic fluid into forward anchor  60  to actuate borehole retention assembly  62 . While forward anchor  60  engages borehole wall  26  (FIG.  2 ), valve circuitry injects hydraulic fluid into power section  86  of forward thruster  80 . Under normal conditions, the hydraulic pressure in power section  86  works against piston head  90  to drive piston head  90  and connected tube  50  downhole in direction “D.” Once piston head  90  completes its stroke within chamber  84 , the valve circuitry de-actuates forward borehole assembly  62  and directs drilling fluid into reset section  88  to reset piston head  90  within chamber  84 . 
     The second step, which may overlap with the conclusion of the first step, begins with actuating aft anchor  70  causing borehole retention assembly  72  to engage borehole wall  26 . At the same time, the valve circuitry injects fluid into power section  106  of aft thruster  100 . With aft anchor  70  engaged, the hydraulic pressure in power section  106  drives piston head  110  and connected tube  50  downhole in direction “D.” Once piston head  110  completes the stroke within chamber  104 , hydraulic fluid is directed into reset section  108  to reset piston head  110  within chamber  104  and the actuator assembly of borehole retention assembly  72  of aft anchor  70  to disengage from borehole wall  26 . Thereafter, the operation repeats in substantially the same steps. 
     In the preferred embodiment, controllers  130  and  160  are actuated when tube  50  encounters difficulty in moving downhole in direction “D.” This can happen when attempting to drill through a particularly slow drilling formation or formation that causes an increase in the torque required to turn the drill bit  30  or when there is an excessive amount of drag force on tube  50 . In either situation, the mud motor may unintentionally and nearly instantaneously raise the upstream differential pressure. 
     As described above, during the first step of the tube movement cycle, forward anchor  60  engages borehole wall  26  (FIG. 2) while high pressure drilling fluid is directed into power section  86 . The drilling fluid injected into power section  86 , however, has a pressure higher than the desired operating pressure. Although the increased hydraulic pressure in power section  86  cannot urge tube  50  downhole in direction “D,” the resilient connection between cylinder  82  and controller housing  132  enables the hydraulic pressure in power section  86  to urge cylinder  82  uphole in direction “U.” The axial motion of cylinder  82  and connected retainer  134  causes collar  148  to impart a compressive force on spring  136 . If the hydraulic pressure in power section  86  exceeds the spring force of spring  136 , then cylinder  82 , retainer  134  and collar  148  will be displaced uphole in direction “U,” causing the spring  136  to be compressed against back shoulder  140 . This compression continues until the hydraulic pressure in power section  86  is absorbed by spring  136 . Thus, it can be seen that the excess thrust, which is attributable to the increase in hydraulic pressure, that would have normally been transmitted to bit  30  via tube  50  has been redirected into spring  136 . 
     It will be appreciated that spring  136  maintains a WOB on bit  30  until tube  50  can slide downhole in direction D. That is, while thruster  80  is energized, but not moving, spring  136  urges collar  148  downhole in direction D. Collar  148  transmits this thrust via sleeve  146  through closure member  92  to cylinder  82 . This thrust is delivered through the generally non-compressed hydraulic fluid in chamber  86  to piston head  90  and ultimately through tube  50  to bit  30 . Thus, the thrust delivered to bit  30  by tube  50  is that which is stored in spring  136 , and not moving thruster  80 . 
     Aft controller  160  operates in substantially the same manner as forward controller  130 . In the event that tube  50  is prevented from movement downhole in direction “D” when hydraulic fluid is directed into power section  106 , cylinder  102  is driven uphole in the “U” direction by the hydraulic pressure in power section  106 . The movement of cylinder  102  also forces retainer  164  to move uphole in direction “U.” This movement by retainer  164  causes collar  176  to compress spring  166  against housing interior shoulder  168 . As before, the spring  166  remains compressed until the thrust generated by the hydraulic pressure in power section  106  is reduced. The hydraulic pressure is reduced either due to bit drill-off where the rate the hole is drilled is faster than tractor rate of advancement or due to the end of the stroke. 
     Preferably, springs  136  and  166  incorporate a certain level of pre-compression that urges sleeves  146 ,  174  and thrusters  80 ,  100  downhole in direction D. This pre-compression is preferably enough to minimize any type of play or axial movement of retainers  134 ,  164  within their respective housings. This pre-compression may also provide a limited amount of compression of the spring from WOB during normal operating conditions. Preferably, springs  136 ,  166  are sized to have the capacity to absorb as much thrust as can be generated in instances where an unusually slow drilling formation or formation that requires higher torque to rotate the bit is encountered by bit  30  or where there is an excessive amount of drag force on tube  50 . 
     Referring now to FIGS. 5A and 5B, thrust controllers  130 ,  160  constructed in accordance with a first alternative embodiment will now be described. With the exception of the material discussed below, the first alternative embodiment comprises the same elements and operates in the same manner as the preferred embodiment discussed above. The first alternative embodiment thrust controllers  130 ,  160 , however, additionally comprise a dampener with orifices  510 ,  560  located in the collars  148 ,  176  of the forward and aft thrust controller retainers  134 ,  164 , respectively. Cavities  144  and  172  are filled with oil or other fluid. In operation, increased loading across the thrust controllers  130 ,  160  allows movement between the thrusters  80 ,  100  and the borehole retention assemblies  62 ,  72 . Once the borehole retention assemblies  62 ,  72  release their grip on the borehole, however there is no external force across thrust controllers  130 ,  160 . For example, with borehole retention assembly  62  no longer engaging borehole wall  26 , spring  136 , acting on back shoulder  140  of housing  132  connected to borehole retention assembly  62  and on collar  148  of retainer  134  connection to thruster  80 , causes thruster  80  and borehole retention assembly  62  to move together as spring  136  de-compresses. Further, with borehole retention assembly  72  no longer engaging borehole wall  26 , spring  166 , acting on first shoulder  168  of housing  162  connected to borehole retention assembly  72  and on collar  176  of retainer  164  connected to thruster  100 , causes thruster  100  and borehole retention assembly  72  to move apart as spring  166  de-compresses. Thrusters  80 ,  100  and borehole retention assemblies  62 ,  72  thus move in accordance with the force stored in the springs  136 ,  166 . The orifices  510 ,  560  restrict the movement of the borehole retention assemblies  62 ,  72  by requiring the fluid to pass through the orifices  510 ,  560 . The orifices  510 ,  560  thereby restrict movement so that borehole retention assemblies  62 ,  72  will not slam against the thrusters  80 ,  100  whenever the borehole retention assemblies  62 ,  72  release their grip on the borehole. 
     Referring now to FIGS. 6A and 6B, the orifices  510 ,  560  in collars  148 ,  176  respectively of the first alternative embodiment will now be discussed. Both of the orifices  510 ,  560  work in the same manner so that a description of orifice  510  in the forward thrust controller  130  will also describe orifice  560  in aft thruster controller  160 . The orifice  510  has two positions, one maximum flow through orifice  510  and the other minimal flow therethrough. Flow through orifice  510  is maximized when spring  136  is being compressed to absorb energy and then is minimized when spring  136  is being de-compressed after borehole retention assembly  62  disengages borehole wall  26 . This is done so that whenever the thruster  130  moves the tractor  36  down against the bit  30  during drilling, the movement of the thruster controller  130  and its ability to absorb load is not hampered by the orifice  510 . 
     The orifice  510  is biased toward the minimal flow position. The orifice  510  can be biased several ways and still remain within the spirit of the first alternative embodiment. One way is to have a spring biased piston  710  with a hole  720  through its center axis. A spring  730  loads the piston head  740  against a shoulder  750  that is the transition between diameters in a through hole  760  in the thrust controller collar  148 . Fluid flow in the direction  770  that increases the thrust controller cavity  144  in volume causes the piston head  740  to seat more securely against the through hole inside shoulder  750 . This allows flow only through the small hole  720  through its center axis. This is shown in FIG.  6 A. Fluid flow in the direction  780  that maximizes flow through orifice  510  pushes against the head of the piston  740  and biasing spring  730 , moving the piston head  740  away from the shoulder  750 , thereby increasing the flow area. This is shown in FIG.  6 B. 
     Referring now to FIGS. 7A and 7B, thrust controllers  130 ,  160  constructed in accordance with a second alternative embodiment will now be described. With the exception of the material discussed below, the second alternative embodiment comprises the same elements and operates in the same manner as the preferred embodiment discussed above. The second alternative thrust controllers  130 ,  160 , however, also comprise a dampener with orifices  510 ,  560  similar to those discussed above in the first alternative embodiment. The second alternative embodiment thrust controllers  130 ,  160  additionally comprise collar seals  610 ,  660  on the forward and aft retaining collars  148 ,  176 , respectively. The collars  148 ,  176  are sealed so that movement between the forward and aft thrusters  80 ,  100  and the forward and aft borehole retention assemblies (not shown) forces fluid flow through the orifices  510 ,  560 . The second alternative thrust controllers  130 ,  160  also comprise housing seals  615 ,  665  on the exterior portions  616 ,  666  of the forward and aft housings  64 ,  74 . Thus, unlike the preferred embodiment, the cavities  144 ,  172  are sealed to the outside environment inside the borehole  26 . Preferably, the cavities  144 ,  172  are filled with a hydraulic fluid or high temperature grease, both fluids with low viscosity. Thrust controllers  130 ,  160  additionally comprise forward and aft biased volume compensator pistons  620 ,  670  located in enlarged diameter portions of the ends of forward and aft housings  64 ,  74  respectively. These pistons  620 ,  670  are biased by springs  625 ,  675  located in compensator cavities  630 ,  680  between the compensator pistons  620 ,  670  and the forward and aft compensator cavity shoulders  635 ,  685 . The compensator cylinders  620 ,  670  are sealed with compensator seals  640 ,  645 ,  690 ,  695  to prevent fluid flow into the compensator cavities  630 ,  680 . Retainer rings retain pistons  620 ,  670  in the enlarged diameter portions. 
     The housing seals  615 ,  665 , collar seals  610 ,  660 , and compensator seals  640 ,  645 ,  690 ,  695 , form closed systems within the thrust controller cavities  144 ,  172 . As closed systems, the volume in cavities  144 ,  172  remains somewhat constant. With a constant volume, movement of retaining collars  148 ,  176  changes the pressure in the volumes on either side of the collars  148 ,  176  that hinders movement of the retaining collars  148 ,  176 . This is because the fluid in controller cavities  144 ,  172  is not able to stabilize through the orifices  510 ,  550  quickly enough to balance the changes in volume and pressure on either side of the collars  148 ,  176 . To relieve the hindrance of these volume changes, the compensator pistons  620 ,  670  adjust to account for the changes in volume on either side of the collars  148 ,  176 . So as to not hinder movement of the compensator pistons  620 ,  670  with a similar pressure, the compensator cavities  630 ,  680  communicate with the environment outside the housings  64 ,  74  through ports  647 ,  697 . 
     Referring now to FIGS. 8A and 8B, forward and aft thrust controllers  130 ,  160  constructed in accordance with a third alternative embodiment will now be described. With the exception of the material discussed below, the third alternative embodiment comprises the same elements and operates in the same manner as the preferred embodiment discussed above. The third alternative thrust controllers  130 ,  160 , however, also comprise dampeners similar to those discussed above in the first or second alternative embodiments. The third alternative thrust controllers  130 ,  160  additionally comprise secondary biasing elements  810 ,  860 . The first secondary biasing element  810  is located in the forward thrust controller cavity  144  between retainer collar  148  and the end  65  of housing  64 . The second secondary biasing element  860  is located in the aft thrust controller cavity  172  between the collar  176  and the end  169  of housing  162 . These secondary biasing elements  810 ,  860  are preferably springs that have limited movement, but can be other configurations without leaving the spirit of the third alternative embodiment. 
     When the tractor  36  is moving in the reverse direction U, or coming out of the borehole  22 , fluid volume in the reset section  88  of the interior chamber  84  of the forward thruster  80  and in the reset section  108  of the interior chamber  104  of the aft thruster  100  is increased. This added volume places pressure on the forward and aft thruster pistons  90 ,  110 , moving them and the tube  50  in the direction U. This operation moves the tube  50  out of the borehole  22  in the exact opposite method as was used to insert the tube  50  into the borehole  22 . As with inserting the tube  50  into the borehole  22 , the tube  50  incurs opposing forces as it moves out of the borehole  22 . These forces work in the opposite direction as those discussed above that create an overthrust condition. With opposing forces on the tube  50  during the removal cycles of each thruster  80 ,  100 , the forward and aft thrusters  80 ,  100  move in opposite directions than they would under overthrust conditions while moving the tube  50  into the borehole  22 . Thus, when the elements are not preloaded by the secondary biasing elements, the forward thruster  80  moves closer to the forward housing  64  and the aft thruster  100  moves further away from the aft housing  74 . This movement prevents the tractor  36  from realizing the full length of the thruster stroke due to movement between the thrusters  80 ,  100  and the housings  64 ,  74  under load. With the secondary biasing elements  810 ,  860 , however, when the tractor  36  is moving in the reverse direction or coming out of the borehole  22 , most of the length of the thruster strokes is realized in tractor  36  movement out of the borehole  22 . This is because the secondary biasing elements  810 ,  860  reduce the total spring rate in upward direction but at minimal amount of movements so that the thruster strokes are not significantly reduced. The secondary biasing elements also reduce the total spring rate to protect the borehole retention assemblies (not shown) from high impact loads. 
     It should be understood that the present invention may be adapted to nearly any arrangement of devices. Although the present invention has been described as applied to a tractor having two thrusters, the present teachings may be, as an example, advantageously applied to a BHA arrangement that includes only one thruster. Further, the terms “U”, uphole, “D”, downhole, forward, and aft are terms merely to simplify the discussion of the various embodiments of the present invention. These terms, and other such similar terms, are not intended to denote any required movement or orientation with respect to the present invention. 
     While preferred embodiments of this invention have been shown and described, modifications thereof can be made by one skilled in the art without departing from the spirit or teaching of this invention. The embodiments described herein are exemplary only and are not limiting. Many variations and modifications of the system and apparatus are possible and are within the scope of the invention. Accordingly, the scope of protection is not limited to the embodiments described herein, but is only limited by the claims that follow, the scope of which shall include all equivalents of the subject matter of the claims.

Summary:
A thrust absorber is interposed between a thruster and an anchor that cooperate to axially displace another member. The thrust absorber includes an enclosure fixed to the anchor and a retainer connected to the thruster. A biasing member is operably associated with the retainer. During an overthrust condition, the thruster imparts a thrust force to the member, but the member is not substantially axially displaced. In such a condition, the biasing member absorbs the thrust that the thruster would otherwise impart to the member. A dampener is also included to dampen the movement of the thruster and anchor when the anchor is no longer anchoring the thruster.