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You are an expert at summarizing long articles. Proceed to summarize the following text: 
FIELD OF THE INVENTION 
     This invention relates to a drilling method. 
     BACKGROUND OF THE INVENTION 
     When drilling bores in earth formations, for example to access a subsurface hydrocarbon reservoir, the drilled bore will often include sections which deviate from the vertical plane; this allows a wide area to be accessed from a single surface site, such as a drilling platform. The drilling of such bores, known as directional drilling, utilises a number of tools, devices and techniques to control the direction in which the bore is drilled. The azimuth and inclination of a bore is determined by a number of techniques, primarily through the use of measurement-while-drilling (MWD) technology, most commonly in the form of an electromechanical device located in the bottomhole assembly (BHA). MWD devices often transmit data to the surface using mud-pulse telemetry. This involves the production of pressure pulses in the drilling fluid being pumped from surface to the drill bit, a feature of the pulses, such as the pulse frequency or amplitude, being dependent on a measured parameter, for example the inclination of the bore. Currently, three main mud-pulse telemetry systems are available: positive-pulse, negative-pulse, and continuous-wave systems. By analysing or decoding the pressure pulses at surface it is possible for an operator to determine the relevant measured bore parameter. 
     It is among the objectives of embodiments of the present invention to utilise the pressure pulses produced by MWD apparatus for uses in addition to data transfer. 
     BRIEF SUMMARY OF THE INVENTION 
     According to one aspect of the present invention there is provided a drilling method comprising: 
     producing pressure pulses in drilling fluid using measurement-while-drilling (MWD) apparatus; and 
     allowing the pressure pulses to act upon a pressure responsive device to create an impulse force on a portion of the drill string. 
     The impulse force resulting may be utilised in a variety of ways, including providing a hammer-drilling effect at the drill bit, and vibrating the BHA to reduce friction between the BHA and the bore wall. 
     The invention also relates to apparatus for implementing the method. 
     The pressure pulses produced by conventional MWD apparatus are typically up to around 500 psi. At this pressure it may be possible to produce a useful impulse force, however it is preferred that the pressure pulses are in the region of 700-1000 psi. Pressure pulses of this magnitude may be produced by modifying or varying the valving arrangements provided in conventional MWD apparatus, for example by modifying the valving arrangement such that the valve remains closed for a longer period. The greater magnitude of the pressure pulses will also facilitate detection at surface, particularly in situations where there may be relatively high levels of attenuation of the pulses, for example in extended reach bores or in under-balance drilling operations where the drilling fluid column may be aerated. The pressure pulses may be of any appropriate form, including positive pulses, negative pulses, and continuous waves of pulses, as are familiar to those of skill in the art. 
     The pressure responsive tool may be in the form of a shock tool, typically a tool forming part of a drill string which tends to axially extend or retract in response to changes in internal fluid pressure. The shock tool may be tubular and formed of two telescoping parts, with a spring located therebetween. One of the parts may define a piston, such that a rise in drilling fluid pressure within the tool tends to separate the parts and thus axially extend the tool. 
     The pressure responsive tool may be located above or below the MWD apparatus, and most preferably is above the MWD apparatus. The optimum location may be determined by the mud-pulse telemetry system being utilised. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     These and other aspects of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: 
     FIG. 1 is a schematic illustration of drilling apparatus in accordance with a preferred embodiment of the present invention; 
     FIG. 2 is a sectional view of a shock tool of the apparatus of FIG. 1; 
     FIGS. 3 and 4 are sectional views of the valve of the MWD apparatus of FIG. 1; and 
     FIG. 5 is a schematic illustration of drilling apparatus in accordance with a further embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Reference is first made to FIG. 1 of the drawings, which is a schematic illustration of drilling apparatus  10  in accordance with an embodiment of the present invention, shown located in a drilled bore  12 . 
     The apparatus  10  is shown mounted on the lower end of a drill string  14  and, in this example, comprises a shock tool  16 , an MWD tool  18 , a downhole motor  20  and a drill bit  22 . Of course those of skill in the art will recognise that this is a much simplified representation, and that other tools and devices, such as stabilisers, bent subs and the like will normally also be present. 
     During a drilling operation, drilling fluid is pumped from surface down through the tubular drill string  14 , and the string  14  may be rotated from surface. 
     The shock tool  16 , as illustrated in section in FIG. 2 of the drawings, is tubular and is formed of two telescoping parts  24 ,  25 , with a spring  26  located therebetween. One of the parts  25  defines a piston  28 , such that a rise in drilling fluid pressure within the tool  16  tends to separate the parts  24 ,  25  and thus axially extend the tool  16 . The internal spring  26 , and the weight-on-bit (WOB), tends to restore the tool  16  to a retracted configuration when the drilling fluid pressure falls. 
     The MWD tool  18  includes various sensors and a motorised valve  30  which opens and closes at a frequency related to the MWD apparatus sensor outputs. FIGS. 3 and 4 of the drawings illustrate the valve  30  in the open and closed positions. In the illustrated example the valve  30  is of a poppet type, and is pushed up onto a seat  32  by an actuator  34  below the valve  30 . The opening and closing of the valve  30  produces a variation in the flow area through the tool  18 , and thus creates corresponding pressure variations in the drilling fluid. As the valve  30  closes, the pressure of the drilling fluid above the tool  18 , including the fluid pressure in the shock tool  16 , rises to produce a pressure pulse. By measuring and monitoring the pressure pulses at surface, and by decoding the thus transmitted signal, it is possible to determine the condition being measured or detected by the tool sensors. 
     The motor  20  is a positive displacement motor (PDM) and is powered by the flow of drilling fluid therethrough. When drilling “straight ahead” the drill string is also driven to rotate the bit  22  from surface, however when the drilling direction is to be varied typically only the motor  20  will drive the bit  22 . 
     In use, the pressure pulses produced by the MWD tool  18  will act on the shock tool  16 , causing the tool  16  to expand and retract; this has a number of effects. Firstly, if the magnitude of the pressure pulses is sufficient, the expansion and retraction of the shock tool  16  will produce a percussion or hammer-drill effect on the bit  22 , and in certain rock types this will accelerate the rate of advancement of the bit  22 . Further, particularly when the bit  22  is being driven only by the motor  20 , the vibration of the tool  18 , motor  20 , and other tools and devices mounted on the string resulting from the extension and retraction of the string tends to reduce the friction between the string elements and the bore wall. This in turn facilitates the advance of the bit  22 . 
     From the above description, it will be apparent to those of skill in the art that the apparatus  10  utilises the data-transmitting signals generated by the MWD tool  18  to facilitate advancement of the bit  22 , in addition to carrying information to surface. 
     Those of skill in the art will also recognise that the above-described embodiment is merely exemplary of the present invention, and that various modifications and improvements may be made thereto, without departing from the scope of the invention. In particular, MWD tools take many different forms, and it should be noted that the illustrated MWD valve arrangement is merely one of a number of possible valves which may be utilised in the present invention. 
     Also, a MWD tool  118  may be provided above a shock tool  116 , as illustrated in the apparatus  110  of FIG. 5, in which the features common to the apparatus  10  described above are labelled with the same reference numbers, incremented by 100.

Summary:
A downhole drilling method comprises producing pressure pulses in drilling fluid using measurement-while-drilling (MWD) apparatus ( 18 ) and allowing the pressure pulses to act upon a pressure responsive device ( 16 ) to create an impulse force on a portion of the drill string.