Abstract:
A jet cutting device having a cutter head provided with a nozzle for ejecting a stream of fluid against a body so as to create a selected cut in said body. The cutter head is provided with a deflector having a deflection surface arranged to deflect the stream of fluid ejected by the nozzle into a selected direction in accordance with the position of said cut to be created.

Description:
FIELD OF THE INVENTION 
     The invention relates to a jet cutting device comprising a cutter head provided with one or more nozzles for ejecting a stream of fluid against a body so as to create a cut in the body. The jet cutting device can be applied, for example, in the industry of machining work pieces or in the industry of rock cutting during drilling of boreholes into the earth formations. 
     BACKGROUND OF THE INVENTION 
     WO 00/66872 discloses a rock cutting device whereby a stream of drilling fluid containing abrasive particles is ejected against the borehole bottom or borehole wall by a nozzle provided at a cutter head of the device. 
     A problem of the known device is that the direction of the ejected stream cannot be as optimal as desired in view of limitations regarding the position of the nozzle at the cutter head. For example in certain applications it is desirable that the ejected stream passes close to, and substantially parallel to, the borehole wall in order to accurately cut the borehole circumference. However, the position of the nozzle inwardly from the outer radius of the cutter head prevents such stream direction. 
     SUMMARY OF THE INVENTION 
     In accordance with the invention there is provided a jet cutting device, comprising a cutter head provided with at least one nozzle for ejecting a stream of fluid against a body so as to create a selected cut in said body, wherein, for each nozzle, the cutter head is provided with a deflector having a deflection surface arranged to deflect the stream of fluid ejected by the nozzle into a selected direction in accordance with the position of said cut to be created. 
     It is thereby achieved that the ejected stream can be deflected in directions other than the direction of ejection of the stream from the nozzle. 
     The jet cutting device is attractive for wellbore drilling, as it allows to drill a central part of the borehole by a portion of the stream not deflected by the deflector, and to drill a radial outer part of the borehole by a portion of the stream deflected by the deflector positioned close to the borehole wall thus allowing the outer circumference of the borehole to be accurately cut. 
     To focus the stream and to increase the cutting efficiency, the deflector suitably has a concave deflection surface onto which the stream impacts. Alternatively, when it is desired to diverge the stream, the nozzle can be arranged to eject the stream against a convex deflection surface of the deflector. 
     Since for most applications the intensity of the impact force from the stream on the deflection surface varies somewhat along the surface, suitably the deflection surface has an erosion resistance which varies along the deflection surface in accordance with the variation of the impact force so that the deflection surface is substantially uniformly eroded by the stream. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be described hereinafter in more detail and by way of example with reference to the accompanying drawings in which 
         FIG. 1  schematically shows a longitudinal section of a jet cutting device according to an embodiment of the invention; 
         FIG. 2  schematically shows a detail of the embodiment of  FIG. 1 ; 
         FIG. 3  schematically shows a longitudinal section of a drilling assembly including the jet cutting device of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIG. 1  there is shown a drilling assembly including a drill string  1  extending into a borehole  2  formed in an earth formation  3  and a jet cutting device  5  arranged at the lower end of the drill string  1  near the bottom  7  of the borehole  2 , whereby an annular space  8  is formed between the drilling assembly  1  and the wall of the borehole  2 . The drill string  1  and the jet cutting device  5  are provided with a fluid passage  9 ,  9   a  for drilling fluid to be jetted against the borehole bottom, as is described hereinafter. The jet cutting device  5  has a cutter head  5   a  provided with a mixing chamber  10  having a first inlet in the form of inlet nozzle  12  in fluid communication with the fluid passage  9 ,  9   a , a second inlet  14  for abrasive particles and an outlet in the form of jetting nozzle  15  directed towards a deflector  16  which is described hereinafter in more detail. A longitudinal extension  5   c  of cutter head  5   a  is provided to keep the jetting deflector  16  a selected distance from the borehole bottom  7 . A recess  17  is arranged in the cutter head  5   a  at the side surface thereof, which is in fluid communication with the mixing chamber  10  and with the second inlet  14 . 
       FIG. 2  shows a perspective view of the recess  17  whereby a semi-cylindrical side wall  18  of the recess  17  has been indicated. A cylinder  19  rotatable in direction  20  (cf.  FIG. 1 ; in  FIG. 2  the cylinder has been removed for clarity purposes) is arranged in the recess  17 , the diameter of the cylinder being such that only a small clearance is present between the cylinder  19  and the side wall  18  of the recess  17 . The outer surface of the cylinder  19  has been magnetised, whereby a number of N and S poles alternate in circumferential direction. The second inlet  14  and the mixing chamber  10  each have a side wall formed by the outer surface of the cylinder  19 . Furthermore, the second inlet  14  has opposite side walls  22 ,  24  which converge towards the mixing chamber  10  and which extend substantially perpendicular to the side wall  18 . 
     The deflector  16  extends into a lower recess  26  of the cutter head  5   a  in a manner allowing movement of the deflector  16  relative to the cutter head  5   a . A control means in the form of actuator  28  is arranged in the lower recess  26  to support the deflector  16  and to control movement of the deflector  16  relative to the cutter head  5   a . The deflector  16  is arranged so that during operation of the jet cutting device  5  a stream of fluid  30  ejected by the nozzle  15  impacts onto inner surface  32  of the deflector at a selected angle  34 . The inner surface  32  is preferably made of an erosion resistant material like Tungsten Carbide. 
     The actuator  28  is capable of moving the deflector in opposite directions  36   a ,  36   b  which are substantially parallel to the deflector inner surface  32  and opposite directions  38   a ,  38   b , which are substantially perpendicular to the deflector inner surface  32 . Furthermore the actuator  28  is capable of rotating the actuator so as to change the angle  34  at which the stream  30  impacts on the deflector inner surface  32 . 
     During normal operation of the drilling assembly  1 , a stream of drilling fluid initially containing abrasive particles is pumped via the fluid passage  9 ,  9   a  and the inlet nozzle  12  into the mixing chamber  10  employing pump means  41  as schematically shown in  FIG. 3 . The abrasive particles include a magnetically active material such as martensitic steel, and typical abrasive particles are martensitic steel shot or grit. The stream flows through the jetting nozzle  15  in the form of a jet stream  30  against the deflector  16  which deflects the stream  30  to form deflected stream  40  impacting against the borehole bottom  7 . The direction of deflected stream  40  is determined by the angle of impact  34 , the deflector shape and the deflector orientation. 
     After all abrasive particles have been pumped through the fluid passage  9 ,  9   a , drilling fluid which is substantially free of abrasive particles is pumped through the passage  9 ,  9   a  and the inlet nozzle  12  into the mixing chamber  10 . 
     By the impact of the jet stream  40  against the borehole bottom  7 , rock particles are removed from the borehole bottom  7 . The drill string us simultaneously rotated about longitudinal axis A (see  FIG. 3 ) so that the borehole bottom  7  is evenly eroded resulting in a gradual deepening of the borehole. The rock particles removed from the borehole bottom  7  are entrained in the stream which flows in upward direction through the annular space  8 . As the stream passes the cylinder  19  the abrasive particles are attracted by the magnetic forces induced by cylinder  19 , which magnetic forces thereby separate the abrasive particles from the stream and move the particles onto the outer surface of the cylinder  19 . The cylinder  19  is induced to rotate a) due to frictional forces exerted to the cylinder by the stream of drilling fluid flowing into the mixing chamber, b) due to frictional forces exerted to the cylinder by the stream flowing through the annular space  8 , and c) due to the high velocity flow of drilling fluid through the mixing chamber  10  which generates a hydraulic pressure in the mixing chamber  10  significantly lower than the hydraulic pressure in the annular space  8 . The abrasive particles adhered to the outer surface of the cylinder  16  thereby move through the second inlet  14  in the direction of the mixing chamber  10 . The converging side walls  22 ,  24  of the second inlet  14  guide the abrasive particles into the mixing chamber  10 . Upon arrival of the particles in the mixing chamber  10  the stream of drilling fluid ejected from the inlet nozzle  12  removes the abrasive particles from the outer surface of the cylinder  19  whereafter the particles are entrained into the stream of drilling fluid. 
     The remainder of the stream flowing upwardly through the annular space  8  is substantially free of abrasive particles and continues flowing upwardly to surface where the drill cuttings can be removed from the stream. After removal of the drill cuttings the drilling fluid is pumped through the fluid passage  9 ,  9   a  and the inlet nozzle  12 , into the mixing chamber  10  so as to entrain again the abrasive particles, etc. 
     When the area of deflector surface  32  where the stream  30  impacts becomes worn, the actuator  28  is induced to move the deflector  16  either in direction  36   a  or  36   b  so as to displace said area away from the location of impact and to position a new area of deflector surface  32 , not worn, at the location of impact. In this manner it is achieved that the life time of the deflector is increased. 
     When it is desired to change the direction of the deflected stream  40 , the actuator  28  is induced to rotate the deflector so as to change the angle  34  at which the stream  34  impacts on the deflector. 
     Furthermore when it is desired to increase the diameter of the borehole  2  drilled, the actuator  28  is induced to move the deflector  16  in the direction  38   b  thereby increasing the distance between the deflector  16  and the stream  30 . Conversely, when it is desired to decrease the diameter of the borehole  2  drilled, the actuator  28  is induced to move the deflector  16  in the direction  38   a  thereby decreasing the distance between the deflector  16  and the stream  30 .