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You are an expert at summarizing long articles. Proceed to summarize the following text: 
TECHNICAL FIELD 
       [0001]    This invention relates in general to drilling systems, and, in particular, to a horizontal directional drilling system. More specifically, but without restriction to the particular embodiments hereinafter described, this invention relates to a horizontal directional drilling system, wherein the drilling is fast, and the energy consumption may be optimized to be minimum. 
       PRIOR ART 
       [0002]    Horizontal directional drilling systems using percussive tools to make a borehole without disturbing the above ground surface are known in the art. The percussive tool provides percussive energy to a hammer. The drill bit is connected to the hammer and receives the percussive energy from the hammer. They are advantageous in many aspects but restricted to a limited range of operations, e.g. due to not being steerable. 
         [0003]    Conventional steerable horizontal drilling systems comprise a dual-member drill string and a drilling tool connectable with the dual-member drill string. The dual-member drill string comprises an inner member and an outer member, where the inner member is independently rotatable of the outer member. The inner member of the dual-member drill string drives operation of the drilling tool. 
         [0004]    U.S. Pat. No. 6,761,231 discloses a steerable drilling tool adapted to receive rotational energy from the inner member of a dual-member drill string. In a preferred embodiment the drilling tool has a hydraulic pump, driven by a drive member, to operate the drill assembly. In another preferred embodiment the drilling tool has a rotary-driven cam assembly adapted to mechanically operate the drill assembly. This invention provides steerable control in horizontal directional drilling operations, but presents numerous disadvantages, e.g. complex and costly interconnections. 
         [0005]    Use of hydraulic pump or a cam assembly to operate the hammer does not provide optimization of the process of making a borehole. Thus, horizontal directional drilling systems utilizing hydraulic pumps or cam assembly to operate the drilling are characterized by relatively high cost and relatively low productivity as the system is slow. 
       SUMMARY OF THE INVENTION 
       [0006]    It is an object of the invention to eliminate or at least minimize the above mentioned problem which is achieved by means of a horizontal directional drilling system in accordance with the appended claims. 
         [0007]    Thanks to the invention there is provided a horizontal directional drilling system that is surprisingly fast, easy to operate, relatively inexpensive and which at the same time provides the synergetic effect of consuming a relatively low amount of energy. 
         [0008]    Further advantages and aspects of the invention will be prepared in the following part of the description. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    Further objects of the present invention together with additional features contributing thereto and advantages accruing therefrom, will be apparent from the following description of the a preferred embodiment of the invention which is shown in the accompanying drawing with like reference numerals indicating like components throughout, wherein: 
           [0010]      FIG. 1  is a diagrammatic representation of a horizontal directional drilling system in accordance with an embodiment herein, 
           [0011]      FIG. 2  presents an enlargement of the encircled area shown in  FIG. 1 , 
           [0012]      FIG. 3  illustrates a front view of a drive shaft in accordance with a preferred embodiment herein, 
           [0013]      FIG. 4  illustrates a front view of a front casing in accordance with an embodiment herein, 
           [0014]      FIG. 5  illustrates a drive shaft arranged coaxially into a front casing in accordance with an embodiment herein, and 
           [0015]      FIG. 6  illustrates a hammer in more detail. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0016]    Various embodiments are described with reference to the drawings, wherein like reference numerals are used to refer to like elements throughout. In the following description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of one or more embodiments. It may be evident, however, that such embodiments may be practiced without these specific details. 
         [0017]      FIG. 1  illustrates a horizontal directional drilling system in accordance with an embodiment of the invention. The horizontal directional drilling system may be used to make a borehole  6 , without disturbing an above ground structure. The horizontal directional drilling system comprises rotary drive systems  4 ,  5  on a moveable base  13 , an earth anchor  11 , a dual string  1  and equipment  14 ,  15  to supply pressurized fluid. The dual string  1 , the rotary drive systems  4 ,  5  equipment  14 ,  15  to supply pressurized fluid are standard equipment known in the art. 
         [0018]    In a preferred embodiment, the dual string  1  comprises standardized sections of an inner string (not shown) coaxially fitted within an outer string  3 . The length of each section is normally about 4.5 m. The dual string  1  is operatively connected to the independent rotary drive systems  4 ,  5  at a first end. The rotary drive system  4  drives the inner string. The rotary drive system  5  drives the outer string  3 . 
         [0019]    A front casing  7  is fitted to the most distal section of the dual string  1 . The front casing  7  is operatively connected to a hammer  9 . The length  1   2  of the casing  7  is normally within the range of 0.5-2 m. The drill string  1  transmits torque and thrust to the hammer  9  to fracture the subterranean formation. The hammer  9  generates percussive energy using pressurized air supplied through an annular passage between the inner string and the outer string  3  of the dual string  1 . 
         [0020]    The earth anchor  11  is driven into the earth to stabilize a frame  13  against the axial force exerted by the rotary drive systems  4 ,  5  during advancement of the hammer  9 . 
         [0021]      FIG. 2  illustrates an enlargement of the encircled area shown in  FIG. 1 . As illustrated in the enlarged view of  FIG. 2 , the inner string  2  is in the form of a tube/pipe having a central bore  20  to enable transportation of working fluid there through to the hammer  9 . Each outer string  3  is connected to another string  3  by means of conical threads  30 ,  31 . The inner strings  2  are rotationally locked to each other by means of hexagonal couplings  21 ,  22 . The inner string  2  and the outer string  3  are connectable with the inner strings  2  and the outer strings  3  of adjacent dual string sections to form the dual string  1 . 
         [0022]    The interconnected inner string  2  and outer string  3  rotate independent of each other. The rotary drive system  4  used to rotate the interconnected inner string  2 , drives the hammer  9 . 
         [0023]    The front casing  7  is arranged to be easily fitted to the most distal outer string  3  with a kind of arrangement similar to the conical threads  31  used to connect each outer string  3  to another string  3 . The length  1   2  of the front casing  7  is about 1.2 m. At about half the length of the front casing  7  there is a bend  70  to arrange for an angle α, about 1-3° of an end part  71  of the front casing  7 . This angle α of the end part  71  permits easy steering of the hammer  9  through the borehole  6 . The length of the hammer  9  is normally in the range 1-2 m, which together with the bent portion of the casing will provide a considerable displacement of the hammer head  90  despite the use of a small angle α at the same time as it allows for straight forward drilling without problems. 
         [0024]    A drive shaft  8 , that extends in one integral part, is operatively engaged with the inner string  2  at a first end  22  with a kind of arrangement similar to the hexagonal connection  22  used to rotationally lock the inner strings  3  to each other. At the other end  71  of the integral shaft  8  there is a fitting for the hammer  9 . The working medium  10  is transported to the hammer  9  using a space between an external wall of the drive shaft  8  and the internal wall of the front casing  7 . 
         [0025]    Referring now to  FIG. 3 , there is shown a preferred embodiment of the drive shaft  8  engaged with the inner string  2 . The drive shaft  8  has an inlet channel  80  with a dimension for e.g. φ=13 mm, that is larger than the dimension of an inlet channel of a standard conventional drive shaft. Similarly, the outlet channel  82  has a dimension larger than the outlet channel of a standard conventional drive shaft, for e.g. φ=13 mm Further, three radially extending transfer holes  83 A,  83 B, and  83 C are positioned out of line to such as not to create a break line and are inclined (for e.g., about 45°) to eliminate disturbing turbulent flow within the outlet channel  82 . The diameter within the section  85  with the holes  83 A,  83 B, and  83 C is larger than similar section of a conventional standard drive shaft  8 , for e.g. 70 mm 
         [0026]    The inlet channel  80  comprises an opening  81  so that the working medium  10  entering the inlet channel  80  may move out of the inlet channel  80 . 
         [0027]      FIG. 4  illustrates the front casing  7  in accordance to a preferred embodiment herein. The bend  70  arranges for an angle α, about 1-3° of the end part  71  to enable steering of the hammer  9  through the borehole  6 . The front casing  7  comprises an opening  79  to receive a positioning sensor. Thus, the steering of the hammer  9  may be performed in a controller manner, as the positioning sensor provides the position/direction of the hammer  9  at an instance. 
         [0028]      FIG. 5  illustrates the arrangement of the drive shaft  8  and the front casing  7  in more detail. As illustrated, the drive shaft  8  is accurately positioned coaxially with the end part  71  of the front casing  7 , by means of a plurality of bearings  75 . As a consequence the drive shaft  8  will flex to be slightly curved within the front casing  7 , to achieve an ability to steer the direction R 1  of the hammer  9 , as shown in  FIG. 2 , when the outer string  3  is not rotated. 
         [0029]    Referring still to  FIG. 5 , a seal  76  is arranged to totally seal the bearings  75  from the influence of the working medium  10  within the space in the front casing  7 . A positioning sensor  77  is fixed on an opening  79  on the front casing  7 , through a sealed lid  78 . The drilling operation may be securely controlled by means of the positioning sensor  77 . The hexagonal connection  22  operatively engages the drive shaft  8  with the inner string  2 . 
         [0030]    The working medium supplied through the inlet channel  80  is transported to the spacing between drive shaft  8  and the front casing  7  through the opening  81 . The working medium  10  in the spacing between the drive shaft  8  and the front casing  7  enters the outlet channel  82  through the three radially extending transfer holes  83 A,  83 B, and  83 C. The working medium may be further supplied to the hammer  9  from the outlet channel  82 . 
         [0031]    Referring now to  FIG. 2 ,  FIG. 3 , and  FIG. 5 , during straight forward drilling, both the outer string  3  and the inner string  2  are rotated, which will form a straight line borehole, i.e., proceed in the direction of the dual string  1  centre R 2 . When a change of direction is desired, the outer string  3  is fixed in a position to have the direction R 1  of the hammer  9  to work in a desired direction, which is sensed and controlled by means of the position sensor  77 . 
         [0032]    During change of direction of the borehole  6 , the inner string  2  is continuously rotated and the hammer  9  is active. The outer string  3  is not rotated and only pushed together with the inner string  2 , until the desired direction is achieved. Thus, thereinafter both the inner string  2  and the outer string  3  are rotated. 
         [0033]      FIG. 6  illustrates the hammer  9  in more detail. The hammer  9  is a standard conventional product known per se (supplied by Alpha Laval) in a preferred embodiment. A conically threaded attachment part  92  connects the hammer  9  to the front casing  7 . By supplying pressurized air to the hammer  9 , the drill head  90  performs percussions/blows to improve drill speed. The percussions/blows from the drill head  90  are received by a drill tooth  91 . As a result the drill tooth  91  penetrates the soil by a certain increment of displacement. 
         [0034]    Thanks to the percussions/blows performed by the drill head  90  using the supply of pressurized air, the process of making the borehole  6  using the horizontal directional drilling system in accordance with the invention is optimized to obtain minimum energy consumption. Further, the drilling process in accordance with the invention is faster than conventional horizontal drilling methods. 
         [0035]    While this invention has been described in detail with reference to a (certain) preferred embodiments, it should be appreciated that the present invention is not limited to those precise embodiments. Rather, in view of the present disclosure which describes the current best mode for practicing the invention, many modifications and variations would present themselves, to those of skill in the art without departing from the scope and spirit of this invention. For instance, it is evident to the skilled person that the advantages of the principles of the invention may also be achieved when using other fluids than air to obtain the percussive action, e.g. water, nitrogen or any other suitable gas or liquid depending on circumstances. Also, despite the fact that merely horizontal drilling is described above, it is evident that the principles of the invention may be used for other kind of drilling operations. The scope of the invention is, therefore, indicated by the following claims rather than by the foregoing description. All changes, modifications, and variations coming within the meaning and range of equivalency of the claims are to be considered within their scope. As is evident for the skilled person the principle according to the invention may be applied within a wide range of different dimensions of the hammer and the casing. However, preferably a not too large drill hole is produced in merely one step. Instead a kind of standardized dimension for performing a first hole, in a first step, with the steerable drilling is chosen, e.g. using a diameter of the hammer within the range of 5-10 inches, more preferred using a hammer of about 6 inches. In a subsequent step a plurality of different kind of traditional equipment may be used to expand the drill hole to a larger, desired size. The advantages is that the same kind of steerable drill equipment may be used to produce many different sized drill holes and that a less complex drilling action (with or without dual string) can be used in the final drill step. The subsequent enlargement of the drill hole may be performed by the use of various types of traditional drill equipment and if a pneumatic hammer is used by a single line, which for example may be pulled backwards through the first drill hole. Also of course traditional hammers and/or drill tools may be used where a pushing action is used. As is also evident some kind of protruding steering device (centralizer), to exactly follow the first drill hole, may be applied to the drill tool that shall expand the first drill hole. In some cases it may be possible to in fact use the same drilling equipment to also pull the bigger drilling tool through the hole backwards, which of course then will provide some further advantages, i.e. easier logistics, etc.

Summary:
The invention relates to a horizontal directional drilling system, comprising a dual string ( 1 ), said dual string ( 1 ) comprising an inner string ( 2 ), and an outer string ( 3 ), said inner string ( 2 ) is rotatable independently of said outer string ( 3 ), a first rotary drive system ( 4 ) to drive said inner string ( 2 ), a second rotary drive system ( 5 ) to drive said outer string ( 3 ), a frame ( 11 ) to movably support said rotary drive systems ( 4, 5 ), a front casing ( 7 ) fitted to a distal end of said outer string ( 3 ), a drive shaft ( 8 ) coaxially positioned into said front casing ( 7 ) and connected with said inner string ( 2 ) and a hammer ( 9 ) connected to said front casing ( 7 ) and operable in response to rotation of said drive shaft ( 8 ), said hammer ( 9 ) comprising a drill head ( 90 ) and a drill bit ( 91 ) wherein said drill head ( 90 ) generating percussive energy using pressurized fluid, e.g. pressurized air, supplied through an annular passage between said outer string ( 3 ) and said inner string ( 2 ).