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BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to an agitator tool for introducing axial movement in a downhole tool, e.g., of a drill string in a bore hole, where the agitator tool comprises: 
         [0000]    a housing configured to be positioned in the bore hole, wherein the housing has a first open end connected to a second open end via at least an inner surface, wherein the housing is configured to guide at least a portion of a drilling fluid through the housing via the first and second open ends;
 
a drive unit, e.g., a fluid activated drive unit, configured to drive the agitator tool, wherein the drive unit is configured to be coupled to a first moveable element arranged inside the housing; and
 
wherein the first element is configured to move in an axial direction relative the housing for introducing axial movement in an external downhole tool coupled to the agitator tool when the drive unit drives the first element.
 
         [0003]    2. Description of Related Art 
         [0004]    The present invention also relates to the use of such an agitator tool. 
         [0005]    Today, bore holes (also called wellbores) comprise an upper hole section coupled to a movable or stationary drilling rig for reaching the desired drilling depth and a lower hole section reaching the desired reservoirs, e.g., an oil or gas reservoir, in the underground. The bore hole is typically a vertical hole that turns or branches off into one or more horizontal holes in which the drill string is subjected to various loads, such as gravity, pore pressure of the surrounding material, fluid density, and pressure/torque/weight from the moving drill parts. The drilling fluid, e.g., mud, is typically circulated inside the drill string, then through the bit and into the annulus. The mud then lifts the cuttings to the surface and thereby cleans the hole. It is well-known that the production output and thus the profit of the bore hole is often determined by the length of the reservoir section. It is known to use an agitator tool to introduce movement in a downhole tool in a drill string which reduces friction between the drill string and the sidewalls of the bore hole and allows the length of the bore hole to be increased. 
         [0006]    An example of such an agitator tool is the NOV agitator (U.S. Pat. No. 8,167,051 B2) from the company NOV which comprises a mud motor with a rotor and stator configuration coupled to a valve arrangement which in turn is often coupled to a shock sub. The mud motor drives the valves which expand the drill string under increased pressure. This configuration has the disadvantage that it generates a high fluctuating pressure drop that interferes with the data transmission through the drilling fluid. This configuration also has a temperature problem as the stator elastomer is limited by temperature. 
         [0007]    U.S. Pat. No. 8,162,078 B2 discloses another agitator tool comprising an annular turbine that rotates relative to an output opening in the outer housing. As the turbine rotates, the pressure in the drilling fluid will continuously increase and decrease as an inner opening in the turbine passes the output to the annulus. This increased internal pressure causes the drill string to expand which will interfere with the data transmission through the drilling fluid. 
         [0008]    Both of the above described solutions require a shock absorbing tool to be placed behind the agitator tool in order to absorb the reactive movement that follows the forward movement, as described in Newton&#39;s third law of equal and opposite forces. The valves and openings formed in these tools are likely to plug due to the particles, solids and LCM in the drilling fluid which may cause the tool to fail due to the increasing internal pressure. 
         [0009]    U.S. Patent Application Publication 2012/0186878 A1 describes an agitator tool comprising a mud motor driving a rotating shaft having an offset end which controls the flow of the fluid to a reciprocating piston. The piston is in turn coupled to a movable mass which is brought into contact with a drill bit when the piston is moved forward. The valve openings formed in this tool are also likely to plug due to the particles and solids in the drilling fluid which may cause the tool to fail due to the increasing internal pressure. This configuration has a relative complex structure with a lot of wearable parts which need to be cleaned or serviced at regular basis. 
         [0010]    U.S. Pat. No. 5,601,152 discloses an agitator tool comprising a rotating spindle subassembly coupled to a vibrational subassembly that moves a lower sub-assembly in a reciprocating manner in an axial direction. The drill string rotates the spindle assembly which rotates a main body part of the vibrational subassembly. The main body part drives a first shaft having a radial extending pin. A second shaft is pivotal coupled to the radial extending pin and is connected to a T-shaped element of the lower sub-assembly. As the first shaft is rotated the second shaft is pivoted around the radial pin for each revolution. This leads to a reciprocating movement of the lower assembly in the axial direction. The agitator tool has a relative complex configuration with a large number of components that increase the risk of one or more components failing during operation, particularly the pivoting components. The pivotal shaft provides a limited axial movement of the lower assembly, thus reducing the effect of the agitator tool. This configuration delivers a hammer action to the system interfering with the data transmission through the fluid and provides very narrow passageway for the fluid to pass through the tool increasing the risk of blockages. 
         [0011]    International Patent Application Publication WO 2012/120403 A1 discloses a downhole tool comprising an axial moveable mass coupled to a rotatable drive axle by means of a wobble plate and a connecting rod. The connecting rod is located on the end surface of the mass where the free end is coupled to the periphery of the wobble plate located on a side surface of the drive axle. A protective spring is arranged in or relative to the downhole tool to protect the wobble plate when applying a hammer effect. 
       SUMMARY OF THE INVENTION 
       [0012]    An object of the invention is to provide an agitator tool that reduces friction between the drill string and the inner wall of the bore hole and improves weight transfer from the drill string to the drill bit. 
         [0013]    An object of the invention is to provide a forward movement of an oscillating weight without an equal backward movement through the design of sinus shaped curves. 
         [0014]    An object of the invention is to provide an agitator tool that has a simple configuration and less wearable parts, and which has a relative constant pressure drop during operation. 
         [0015]    An object of the invention is to provide an agitator tool that does not interfere with the data transmission trough the drilling fluid. 
         [0016]    An object of the invention is to provide an agitator tool that can be used for various purposes, such as fishing for objects, pulling items, or moving and landing tube or casings. 
         [0017]    An object of the invention is achieved by an agitator tool characterized in that a second moveable element is arranged inside the housing, wherein the second element is configured to be coupled to the drive unit and to move, e.g., rotate, in a lateral direction relative to the first element, wherein the first and second elements are arranged relative to a common center axis; and 
         [0000]    wherein the first element is coupled to the second element via mechanical coupling means for converting the lateral movement of the second element into the axial movement of the first element. 
         [0018]    This provides an agitator tool suitable for the use in bore holes in which drilling fluid, such as drilling mud, are pumped through the drill string and into the bore hole, e.g., a bore hole for natural gas, such as shale gas. This agitator tool is configured to be driven by the drilling fluid being pumped through the agitator tool and out to the drill bit. This agitator tool has a simple structure and comprises very few moving and thus wearable parts, unlike other agitator tools which have a complex structure and a lot of wearable parts. The oscillating movement between the two elements in the housing causes a weight imbalance that introduces an axial movement in the downhole tools coupled to the agitator tool. This reduces the friction between the drill string and the inner wall of the bore hole and allows weight from the drill string behind the agitator to be transferred to the drill bit. This reduces the maintenance time and increases the operation time since it does not comprise any valves or narrow flow paths. The length of the drill string may be increased horizontally up to 12 kilometers or more. 
         [0019]    The housing may comprise a support element in the form of one or more taps or an annular protrusion located near the first open end of the housing for supporting the parts arranged inside the housing. This allows the parts of the agitator tool to hang freely from the support element which eliminates the need for any supporting bearings located in the opposite end of the housing. A support stack may be placed on a contact surface of the support element. The stack may comprise one or more bearings, such as a radial bearing and/or a thrust bearing, and optionally damping means in the form of one or more spring elements, e.g., Belleville springs. 
         [0020]    The first open end may comprise coupling means in the form of a screw thread with internal or external threads for coupling to another housing or downhole tool with a mating coupling. The second open end may additionally or alternatively comprise coupling means in the form of a screw thread with internal or external threads for coupling to another housing or downhole tool with a mating coupling. The agitator may be placed after the drill bit or a measuring unit or at any other position in the drill string. 
         [0021]    The coupling means comprises a first coupling element located on a first surface of the first element which is configured to engage a second coupling element located on a second and opposite facing surface of the second element, and wherein the first coupling element is configured to move along the second coupling element when the drive unit drives the second element. 
         [0022]    The two moveable elements are coupled together via a mechanical coupling that converts the rotating movement of the second element into an axial movement of the first element. This eliminates the need for any valve arrangements and/or pistons to drive the first element which in turn reduces the number of parts in the agitator tool and provides a configuration that is more resilient to wear during operation. This also eliminates the need for a valve arrangement that would cause fluctuations in the pressure drop over the agitator tool. 
         [0023]    According to one embodiment, the first coupling element is a pin extending out from a first surface of one of the elements and the second coupling element is a groove, e.g., a curved and/or straight groove, arranged on a second surface of the other element, wherein the groove is configured to at least partly receive the free end of the pin. 
         [0024]    The mechanical coupling may in a simple embodiment be a pin and grove arrangement where the groove has a configuration that allows the pin to move along the grove when the two elements move relative to each other. The groove is shaped to receive the free end of the pin where the thickness or diameter of the pin more or less corresponds to the width of the groove. The width of the groove may be increased to allow for a more loose fit around the pin. This allows for a looser travel of the pin and allows it to compensate for any tolerances between the outer surfaces of the groove and pin. The pin may form part of the element for a stronger coupling or be coupled to the element via fastening means, such as screws, bolts, nuts, or a threaded coupling, for easy assembly. The pin may be inserted through a mounting hole in the outer surface of the first element during assembly. 
         [0025]    The coupling means may have any other configuration, such as a cam and follower system where a rotating cam contacts and moves a follower. The rotating second element may comprise a drum or cylindrical shaped cam or be coupled to a drum or cylindrical shaped cam element where the cam has a contact surface for contacting a contact surface on the axial moving of the first element. The first element may comprise a mating drum or cylindrical shaped cam or be coupled to a mating drum or cylindrical shaped cam element. The first element may instead comprise or be coupled to a roller follower having at least one rotating element. The second contact surface is located on the cam or the rotating element of the roller follower. 
         [0026]    According to one embodiment, the second coupling element forms at least a first guiding section for forward axis movement of the first coupling element, where the first guiding section is connected to at least a second guiding section for backward axis movement of the first coupling element, and wherein the first and second sections form a guiding loop for the axial movement of the first coupling element. 
         [0027]    The groove forms a closed guiding loop along the surface of that element that allows the first element to move forward and backward in an oscillating manner. The groove has at least one groove section with a predetermined amplitude, pitch and length that provides a forward movement and at least one other groove section with a predetermined amplitude, pitch and length that provides a backward movement. The groove may be configured so that the first element performs one cycle per one revolution of the second element. The speed and number of cycles per revolution may be increased by arranging more than two groove sections on the surface. 
         [0028]    According to a special embodiment, the first and second guiding sections has a symmetric shape, e.g., a sinus shaped groove, or at least one of the guiding sections has a modified shape, e.g., a predetermined amplitude, pitch and/or period, that differs from its symmetric shape for accelerating or de-accelerating the axis movement of the first coupling element. 
         [0029]    The two groove sections may form a sinus curve or any other shape having a predetermined amplitude, frequency/period and pitch. The shape of each groove section may be symmetrically shaped around a peak section connecting the two groove sections thereby allowing for a uniform movement of the first element. At least one of the groove sections may have a modified shape where the amplitude, pitch and/or period of that groove section differ from its symmetric values. One or both peak sections may be modified accordingly. This allows the movement of the first element to be accelerated and/or de-accelerated between the peaks and provides a fast and/or slow stop at the peaks or allows the total movement to be neutral. The entire length of the groove section may have a curved shape or at a part thereof may have a straight shape. The amplitude, frequency/period and pitch of the groove sections may be determined based on various desired criteria, e.g., flow rate, number of cycles per revolutions, type or weight of drilling fluid, viscosity of fluid, size of drill string, or the like. 
         [0030]    One of the groove sections may have an unmodified shape while the other groove section has a modified shape thus providing an aggressive movement. At least one of the peak sections between the groove sections may be shaped to provide a fast or slow stop, i.e. have a straight or flat curvature. This allows the speed of the forward and backward movement to differ. The groove sections may be designed according to Newton&#39;s third law so that the agitator tool provides an equal action and reaction or an increased action or reaction, e.g., slow down the reaction of the stroke. 
         [0031]    According to a special embodiment, one of the sections has a guiding subsection which has a third shape that differs from the remaining shape of that section for a third axial movement, e.g., a stroke movement, of the first coupling element. 
         [0032]    At least one of the groove sections may have a sub-suction that has a different shape than the rest of the groove section. The width of the groove in this sub-section may be increased, e.g., by forming a curved recess and/or protrusion in one of the side surfaces of the groove. This allows the first element to perform a positive (forward) or negative (backward) stroke movement every time the pin passes that sub-section. This allows the agitator tool to perform more than one stroke movement per cycle. 
         [0033]    According to one embodiment, the first element is a cylinder and the second element is a shaft and wherein the cylinder has a first surface facing the shaft and the shaft has a second surface facing the cylinder, where the shaft preferably extends at least partly into a cavity of the cylinder. 
         [0034]    The first element is preferably configured as a weight element having a predetermined mass and weight. The weight element may in a preferred embodiment be shaped as a cylindrical element. The second element is preferably configured as an activating element which activates or drives the second element. In a preferred embodiment, the activating element is shaped as a shaft that is configured to be coupled to the drive unit. The shaft may extend through the weight element or into a cavity of the weight element. 
         [0035]    The pin may be shaped as a single elongated pin or an L- or T-shaped pin extending out from a surface, and the groove may be arranged on an opposite facing surface where the free end(s) of the pin is placed in the groove. The pin may be located on an inner surface or end surface of the first element facing the second element, and the groove may be located an outer surface or end surface of the second element facing the first element, or vice versa. The pin may be mounted on a bearing for reducing friction in the groove. 
         [0036]    According to one embodiment, guiding means is arranged between an outer surface of the second element and the inner surface of the housing for restricting the movement of the first element to an axis movement relative to the second element. 
         [0037]    Guiding means in the form of a spline system may be arranged between the housing and the first element where a first spline element is coupled to the inner surface of the housing and a second spline element is coupled to the outer surface of the first element. The two spline elements may be shaped as elongated guiding protrusions where the two sets of protrusions are offset relative to each other. The width between two adjacent protrusions may more or less correspond to the width of an opposite mating protrusion. The width between two adjacent protrusions may be increased to allow the opposite protrusion to move freely wherein. The spline system may be configured to guide the first element along a first path and guide it backward along a second path. This prevents the first element from rotating with second element. 
         [0038]    According to one embodiment, at least a first sealing system is arranged at the first open end, wherein at least one of the elements, e.g., the second element, extends through the sealing system and comprises at least one inlet opening connected to a fluid path which is turn is connected to at least one outlet opening. 
         [0039]    The housing may be sealed off at both open ends using a sealing system in the form of a circular or ring shaped seal coupled to contact the inner surface of the housing and the outer surface of the drive unit or second element. The shaft forming the second element may extend through the seal at the first open end and comprise one or more inlets for leading the drilling fluid into a flow path arranged inside the shaft. The flow path extends through the shaft and is connected to one or more outlets at the second open end for leading the drilling fluid out to the drill bit. This allows a guide wire to be guided through the hollow shaft and thus through the agitator tool. 
         [0040]    A seal in the form of a pressure compensating system may be used to seal off the second open end thus forming a closed chamber in which the first and second elements are arranged. The first and second elements may be submerged in another suitable fluid, e.g., oil or water, for reducing friction of the moveable elements. The drilling mud may be used instead. 
         [0041]    The housing may have a cylindrical shape and an inner diameter that is greater than the outer diameter of the first cylindrical element. A gap at either end between the seals and the first element and a gap between the first element and the housing allow the first element to move freely and displace the second fluid in the chamber. 
         [0042]    According to a special embodiment, a pressure compensating system is arranged at the second open end for compensating for a pressure difference between the fluid inside the housing and the fluid outside the housing. 
         [0043]    The pressure compensating system may be a moveable balance piston having a sealing element for contacting the inner surface of the housing and a second sealing element for contacting the outer surface of the first or second element. The pressure compensating system seals off the second open end while regulating the pressure inside the chamber based on the pressure outside the open end. The pressure compensating element is positioned relative to the first sealing system so that the first element is able to move freely within the amplitude of the groove or cam of the second element even at the maximum allowable pressure difference caused by hydrostatic column, pump pressure or weight of the drilling fluid. 
         [0044]    According to one embodiment, at least one protrusion is arranged on the inner surface of the housing and comprises a first contact surface for contacting a second surface on the first element when the first element moves in an axial direction. 
         [0045]    A protrusion in the form of one or more taps or an annular protrusion may be located at or near the open end. The protrusion may comprise a contact surface facing the first element for contacting a mating contact surface on the first element. The protrusion is located relative to the support element so that the first element impacts the protrusion during the forward movement thus providing a hammer or anvil effect. The groove may at or around the point when the first element contacts the protrusion have a greater width than the remaining part of the groove which allows the pin to move freely relative to the groove during the impact. The sub-section may be used to provide the impact with the protrusion. 
         [0046]    According to one embodiment, at least another moveable first element is coupled to the second element by another set of coupling means, wherein the set of coupling means comprises a third coupling element configured to move along a fourth coupling element when the drive unit drives the second element. 
         [0047]    Two or more first elements may be coupled to the same second element where both first elements are coupled to the second element via two mechanical couplings in the form of a pin and groove system and/or a cam and follower system. The weight of the first elements may be adapted to the desired application, dimensions of the agitator tool, or the force of forward movement or hammer effect. The weight of each first element may differ from each other as well as the amplitude, frequency and pitch of each mechanical coupling. This allows the frequency and effect of the movement to be adapted to the desired application and use. 
         [0048]    According to one embodiment, the drive unit is configured as a turbine or progressive cavity pump, wherein the drive unit comprises at least one blade arranged on a shaft for leading at least a part of the drilling fluid through the drive unit, and wherein preferably flow regulating means are arranged in front of the drive unit. 
         [0049]    The use of any type of turbines to drive the agitator tool provides a more stable pressure drop which does not interfere with the data transmitted through the drilling fluid, such as MWD (Measurement While Drilling) or other pressure conveyed information. This also eliminates the temperature problem since it does not comprise a stator with an elastomer. The turbine may comprise a plurality of turbine blade arranged on a shaft which is configured to be coupled to the second element via coupling means in the form of a screw thread. The turbine shaft may have internal threads for coupling to external threads of the second element, or vice versa. The turbine blade may be configured to rotate the second element in a clockwise or counterclockwise direction. This allows the drive unit to be configured as a separate unit that can be easily coupled to the second element. The drive unit alternatively may be a conventional progressive cavity pump. The progressive cavity pump may be any type of stator/rotor configuration rating from half-lobe to multi-lobe and multi-stage systems. The drive unit may be arranged in a second housing which is coupled to the first open end of the first housing. This second housing may comprise coupling means in the form of a screw thread with internal or external threads for coupling to another downhole tool with a mating coupling. 
         [0050]    The turbine may at the opposite side of the coupling means be coupled to a flow restrictor for regulating the amount of the fluid passing through the turbine blades and the flow path in the shaft. The flow restrictor may have a static configuration where the flow is set to a predetermined rate during assembly or may have a dynamic configuration that allows the flow rate to be adjusted during operation, e.g., via an external control unit. 
         [0051]    A third housing for protecting the outlets of the second element may be coupled to the second open end of the first housing. This third housing may comprise coupling means in the form of a screw thread with internal or external threads for coupling to another downhole tool with a mating coupling. 
         [0052]    The embodiments of the agitator tool allow it to be used for any one of the following applications: drilling bore holes, e.g., horizontal bore holes; moving items, e.g., casings or tubes, in a bore hole; fishing for objects in a bore hole or/and installing and removing monopole foundations. 
         [0053]    The agitator tool may be used when drilling a bore hole to introduce forward movement in a drill bit. The oscillating internal weight elements allow the agitator tool to also be used to push or pull other item in a bore hole, such as casing, tubes, packers, pumps, screens, or the like. The forward movement force and the hammer effect may also be used to fish for lost or stuck item in the borehole where the agitator tool may be used to “vibrate” the item and retrieve the item. In a special embodiment, the size of the agitator tool may be increased and/or the second housing may configured to be coupled to or placed on the upper end of a monopole foundation, e.g., for wind turbines or other offshore units. The oscillating internal weight elements are then used to install and then loosen the monopole foundation from the seabed. 
         [0054]    The invention is described by example only and with reference to the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0055]      FIG. 1  shows a first exemplary embodiment of an agitator tool according to the invention; 
           [0056]      FIG. 2  shows a second exemplary embodiment of the agitator tool; 
           [0057]      FIG. 3  shows a first embodiment with an unmodified groove according to the invention; 
           [0058]      FIG. 4  shows a second embodiment with a modified groove; 
           [0059]      FIG. 5  shows a another embodiment of the modified groove; and 
           [0060]      FIG. 6  shows a further embodiment of the modified groove. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0061]    In the following text, the figures will be described one by one and the different parts and positions seen in the figures will be numbered with the same numbers in the different figures. Not all parts and positions indicated in a specific figure will necessarily be discussed together with that figure. 
         [0062]      FIG. 1  shows a first exemplary embodiment of an agitator tool  1  for introducing axial movement in a downhole tool of a drill string in a bore hole (not shown). The agitator tool  1  may comprise a first housing  2  configured to be placed in the bore hole and which may have a cylindrical shape. The housing  2  has an outer surface  3  facing the inner surface of the bore hole and an inner surface  4  facing at least one moveable element  5  arranged inside the housing  2 . The housing  2  may comprise a first open end  6  connected to a second open end  7  via the sides of the housing  2 . The housing  2  may be made of a metal, such as steel, iron or another suitable material. The length and outer diameter of the housing  2  is adapted to the desired application of the agitator tool  1 . 
         [0063]    The moveable element  5  is in the form of a weight element that configured to be moved in an axial direction (marked with arrow  8 ) relative to the housing  2 . A second moveable element  9  may be arranged inside the housing  2  and coupled to the first element  5 . The second element  9  is in the form of an activation element and is configured to be driven by a fluid activated drive unit  10 . The second element  9  is configured to rotate in a lateral (circumferential) direction (marked with arrow  11 ) relative to the housing  2 . The first and second elements  5 ,  9  may be coupled together via a mechanical coupling  12  configured to convert the lateral movement of the second element  9  into the axial movement of the first element  5 . The elements  5 ,  9  may be of metal, such as steel, iron, lead or another suitable material. The mechanical coupling  12  may comprise a pin  12   a  and a groove  12   b  configured to at least partly receive the pin  12   a  and guide it along the groove  12   b  when the second element  9  is rotating. 
         [0064]    A support element  13 , in the form of one or more taps, may be arranged on the inner surface  4  of the housing  2  and may be coupled to the housing by fastening means, such as bolts or welding, or may form part of the housing  2 . A stack  14  may be placed on a contact surface of the support element  13  and rotatable coupled to the element  5 . The stack  14  may comprise a thrust bearing  14   a,  a radial bearing  14   b,  and one or more spring elements  14   c  for dampening axial movements of the second element  9  and suspending the elements  5 ,  9 . 
         [0065]    The first element  5  may be a cylinder having an outer surface  15  facing the inner surface  4  of the housing  2  and an inner surface  16  facing the second element  9 . A first open end  17  faces the first open end  6  of the housing  2  and is connected to a second open end  18  facing the second open end  7  of the housing  2  via the sides of the cylinder  5 . The second element  9  may be a shaft having an outer surface  19  facing the inner surface  16  of the cavity in the first element  5 . The second element  9  may extend through the first element  5 , as shown in  FIG. 1 , and towards the open ends  6 ,  7 . A through-hole  20  may be arranged in the second element  9  for leading at least a portion of a drilling fluid (marked with arrow  21 ) through the agitator tool  1 . The through-hole  20  may be connected to one or more inlets  22  located at the open end  6 , e.g., in front of the drive unit  10 , and one or more outlets  23  located at the open end  7 . This allows the through-hole  20  to act as a flow path for the drilling fluid  21 . 
         [0066]    A sealing system  24  in the form of a deformable element may be arranged between the stack  14  and the support element  13  or on the opposite side of the stack  14 . Another sealing system  25  in the form of a moveable pressure compensating system may be arranged near or at the open end  7 . The systems  24 ,  25  form together with the inner surface  4  a closed chamber  26  filled with a second fluid, such as oil. The pressure compensating system  25  may be configured to move freely between a first end position and a second end position for regulating the pressure of the fluid located inside the chamber  26 . A gap  27  is arranged between the first element  5  and the inner surfaces of the chamber  26  so that the element  5  is able to move freely inside the chamber  26 , even when the system  25  is positioned in one of the end positions. A second inlet and outlet (not shown) are coupled to the chamber  26  for leading the second fluid in and out of the chamber  26 . A locking system  28  may be arranged at the end of the second element  9  and define one of the end positions. 
         [0067]    Guiding means  29  in the form of a spline system may be arranged between the housing  2  and the first element  5 . The spline system  29  may comprise a first spline element  29   a  coupled to the inner surface  4  and configured to be guiding along a second spline element  29   b  coupled to the outer surface  15 . The guiding means  29  is configured to restrict the first element  5  to an axial movement relative to the second element  9 . A bearing system  30  may be located between the outer surface  15  and the inner surface  4  for centering of the element  5 . 
         [0068]    One or more protrusions  31 , formed as taps may be arranged on the surface  4  at the opposite end of the support element  13 . The protrusion  31  comprises a contact surface  31   a  for contacting a contact surface  31   b  on the first element  5 . The protrusion  31  may be arranged relative to the first element  5  so that the contact surfaces  31   a,    31   b  are brought into contact with each other when the first element  5  moves forward. 
         [0069]    The drive unit  10  may be a turbine having a plurality of turbine blades  32  arranged on a turbine shaft  33 . The turbine blades  32  may be orientated in a clockwise or counterclockwise direction. The shaft  33  may comprise a coupling element  33   a  in the form of a screw thread for coupling to a mating coupling element  33   b  on the element  9 . One or more secondary inlets  34  may be located between the turbine blades  32  and the coupling element  33   a  and may be connected to the through-hole  20 . A flow regulating system  35  may be arranged in the front of the drive unit  10  for regulating the flow to the turbine blade  32  and to the through-hole  20 . The flow regulating system  35  may have a static configuration, e.g., a cone or funnel shaped element, with an inlet  35   a  for leading a portion of the fluid  21  into the flow regulating system  35  and an outlet  35   b  for leading the fluid  21  into the through-hole  20 . 
         [0070]      FIG. 2  shows a second exemplary embodiment of the agitator tool  1 ′ where the first element  5 ′ differs from the first element  5  shown in  FIG. 1  by extending past the pressure compensating system  25 . The sealing system  25 ′ is configured to move relative to an outer surface  15   a  of the element  5 ′. A sealing system  36  may be arranged between the outer surface  19  of the second element  9  and an inner surface  16   a  of the first element  5 ′. 
         [0071]    A second housing  37  may be coupled to the first housing  2  at the open end  6 . The housing  37  may at one end comprise a first coupling element  38   a  in the faun of a screw thread for coupling to a mating coupling element  38   b  at the open end  6  for protecting the drive unit  10 . A third housing  39  may be coupled to the first housing  2  at the open end  7  for protecting the ends of the elements  5 ,  9 . The housing  39  may at one end comprise a first coupling element  40   a  in the form of a screw thread for coupling to a mating coupling element  40   b  at the open end  7 . The housings  37 ,  39  may comprise couplings elements  41   a,    41   b  for coupling to mating coupling elements of another housing or an external downhole tool (not shown). 
         [0072]      FIG. 3  shows a first exemplary embodiment of the mechanical coupling  12  in the agitator tool  1  where the pin  12   a  is omitted. The groove  12   b  may form a closed loop  42  defining a first groove section  43   a  for forward movement of the first element, i.e. towards the open end  7 , and a second groove section  43   b  for backward movement of the first element, i.e., towards the open end  6 . The groove sections  43  are connected via a first and a second unmodified peak sections  44   a,    44   b.  The sections  43 ,  44  may form an unmodified sinus shaped groove. The groove sections  43  form at least one cycle with a predetermined amplitude, frequency/period and pitch which introduce a neutral oscillating movement in the agitator tool  1 . 
         [0073]      FIG. 4  shows a second exemplary embodiment of the closed loop  42 ′ where the sections  43 ,  44  form a modified sinus shaped groove. In this embodiment, the second groove section  43   c  may be modified (pitch increased) so that the backward movement of the first element  5  is accelerated. The peak section  44   c  connected to the groove sections  43   a,    43   c  may be modified so that the movement of the first element  5  is slowly stopped (pitch decreased). The peak section  44   d  connected to the groove sections  43   a,    43   c  may be modified so that the movement of the first element  5  is quickly stopped (pitch increased). The amplitude and/or frequency of the cycle may be the same as shown in  FIG. 3 . 
         [0074]      FIG. 5  shows a third exemplary embodiment of the closed loop  42 ″ where the sections  43 ,  44  form a modified sinus shaped groove. This embodiment differs from the embodiment of  FIG. 4  in that the peak section  44   e  may be modified (pitch increased) so that the movement of the first element  5  is quickly stopped (Pitch increased). The second groove section  43   d  may be modified (pitch increased) so that the backward movement of the first element  5  is accelerated. The peak section  44   a  is not modified which means that the frequency of the cycle is increased. The amplitude of the cycle may differ from the one shown in  FIG. 3 . 
         [0075]      FIG. 6  shows a fourth exemplary embodiment of the closed loop  42 ″&#39; where the sections  43 ,  44  form a modified sinus shaped groove. The second groove section  43   e  may comprise a sub-section  45  located towards the peak section  44   a  or the peak section  44   b.  The groove sub-section  45  may be shaped so that the first element  5  performs a second and smaller cycle, i.e., stroke movement, during the backward movement. The groove subsection  45  may alternatively be located on the first groove section  43   a.  The amplitude, frequency and/or pitch of the remaining cycle may be the same as shown in  FIG. 3 . The groove  12   b  may at the point where the first element  5  contacts the protrusion  31  have a greater width than the width of the remaining part of the groove  12   b,  as shown in  FIG. 6 . 
         [0076]    The configuration of the groove  12   b  is not limited to the embodiments shown in  FIGS. 3-6  and may form any desired shape. The groove  12   b  may be configured so that the first element  5  performs any number of cycles per revolution of the second element  9 , preferably one, two, three, four or more. The size, length and configuration of the agitator tool  1  is not limited to the embodiments shown in  FIGS. 1-2  and the elements  5 ,  9  may be adapted to the desired application. Any number of first elements  5  may be arranged along the length of the second element  9 , preferably one, two or more, and the mechanical coupling  12  between the second element  9  and each of the first elements  5  may differ.

Summary:
An agitator tool ( 1 ) for introducing forward and backward axial movement in a downhole tool of a drill string, where the agitator tool has a first axially moveable element ( 5 ) coupled to a second laterally moveable element ( 9 ) arranged inside a housing ( 2 ) via mechanical coupling. The mechanical coupling may be a pin and groove arrangement where the groove forms a modified or unmodified sinus shaped guiding loop that allows the first element ( 5 ) to oscillate within the housing ( 2 ). The second element ( 9 ) is driven by a turbine unit which may have flow regulating means. This provides an agitator tool comprising with very few moving and wearable parts. The shape of the groove sections allows an accelerated and/or de-accelerated forward and backward movement of the first element ( 5 ) that allows it to be used for various applications in a bore hole.