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FIELD OF THE INVENTION 
     The present invention concerns a core barrel, in particular for oil exploration, the core barrel having a string, an external tube fixed to the front end of the string, from the point of view of forward travel of the core barrel in a formation, an annular core bit mounted on a front portion of the external tube, and an internal tube assembly, which is mounted in “wire line” mode, that is to say mounted in the external tube so as to be able to be retrieved and brought to the surface again through the string, and which has hydrodynamic means arranged to transform a core sampling fluid pressure flowing in the string into a force thrusting said assembly towards the front of the core barrel. 
     BACKGROUND OF THE INVENTION 
     There is a pressing need to improve this type of core barrel, particularly for its use in wells with portions inclined with respect to the vertical or more particularly with horizontal portions, all the more so when these portions are at a great distance from the vertical entry portion of the well. It is in fact known that, in this case, the string is subjected at least locally to compression and therefore to a risk of buckling amplified by an inclined or horizontal positioning. This situation causes the string to rub unnecessarily against the wall of the well, and therefore to wear, to be checked in its forward movement and, where applicable, in its rotation driving the bit. 
     The purpose of the present invention is to propose a solution to the problem set out above and thus to procure a core barrel on which it is possible for the bit not to be constantly pushed directly by the string, particularly in the horizontal position, for its forward movement in a formation but can be pushed particularly by the pressure of the coring fluids sent to the bottom of the well through the string, the bit however being able to be held up, if necessary, in its advance by this string. 
     SUMMARY OF THE INVENTION 
     To this end, the core barrel mentioned at the start has according to the invention the following particularities: the front portion of the external tube is mounted for axial sliding, limited by front and rear stops, in a rear portion of the same external tube and projects therefrom longitudinally; the internal tube assembly has a support stop arranged to selectively push forward the front portion of the external tube, and means are arranged between the internal tube proper and the said front portion of the external tube so that the latter can be driven in rotation about its longitudinal axis independently of said internal tube, the latter being able to be held fixed in rotation with respect to the formation during core sampling. 
     Through this arrangement of the core barrel of the invention, the bit is pushed by the internal tube assembly in the formation to be sampled as long as the front portion is not in abutment against one or other of the stops. Because of this, at least part of the string is not subjected to buckling and is substantially less pressed against the wall of the well being sampled, for example in a bend therein, which would not be the case in a normal core barrel configuration. 
     According to a preferred embodiment of the core barrel of the invention, the latter has, for rotating the front portion of the external tube and the bit, a motor mounted in a rear part of the internal tube assembly. Means are then arranged between the front and rear portions of the external tube so that one can be driven in rotation about its longitudinal axis independently of the other. The motor stator can be fixed in rotation with respect to the rear portion of the external tube, during core sampling, whilst the motor rotor is then fixed in rotation to the front portion of the external tube. 
     Through these measures, the string and the rear portion of the external tube are appreciably less subject to wear and to fatigue due to stresses of the type caused by alternating bending of the tubes turning in curves of the well. In addition, the evenness of the rotation of the bit thus driven by the motor is greater than that which would be provided by the string since there is no interference from a rubbing of the strings against the wall of the well. 
     Advantageously, the motor stator can be kept fixed in rotation with respect to the rear portion of the external tube by at least one assembly consisting of a catch and a corresponding longitudinal groove. Likewise, the rotor can be kept fixed in rotation with respect to the front portion of the external tube also by at least one assembly consisting of a catch and corresponding longitudinal groove. One or other or both of the catches can then be arranged so as to come into engagement in a corresponding groove or to be released therefrom automatically when the internal tube assembly is put in the core sampling position in the external tube or is respectively withdrawn therefrom. This design allows easier fitting and removal of the internal tube assembly in the external tube. 
     Other technically equivalent means can be used in place of the catch and groove assemblies, such as for example reciprocal flutes. 
     The motor is preferably of a type actuated by the core sampling fluid, for example a PDM (Positive Displacement Motor) or a turbine known in the art. In this case, the motor can participate in the aforementioned hydrodynamic means, at least for the part of said fluid which actuates it and which is therefore subjected to a pressure drop. 
     One advantage of a PDM or particularly of a turbine is to be seen in its high rotation speed, and therefore in an advantageous rate of advance, in particular in the case of hard rocks for which preferably bits of the impregnated type or concretion type are preferably used. However, given the small diameter imposed on this type of motor in order to be able to pass through the external tube of the core barrel of the invention, it may be appropriate to choose an extended type or one with several stages in order to obtain sufficient power at the bit. This does not however constitute a handicap since there is a means of easily organising space longitudinally for this purpose. 
     In one embodiment of the invention, the core barrel can have, to help the internal tube assembly to descend in the external tube, a joint system mounted on the internal tube assembly so as to substantially completely close the annular space between this assembly and the external tube and thus to receive, like a piston, the full pressure and full flow of drilling fluid. Then the joint system can have at least two flat circular joints, whose external edge is divided into a kind of petal, the two joints being arranged one on the other so that a petal on one joint covers a gap between two petals on the other joint. 
     The core barrel of the invention is thus advantageously arranged to function according to two modes. There is a decoupled mode in which the front portion of the external tube slides freely during core sampling with respect to the string and the rear portion, under the thrust of the internal tube assembly, with the advantages explained above and the additional advantage that variations in the progress of the string in the well are no longer transmitted to the bits. There is also a coupled mode in which the internal tube assembly pulls by means of stops on the rear external tube portion and therefore on the string. At least part of the string then being under traction, and this is advantageous from the point of view of wear and the directional behaviour of complete assembly in the well. 
    
    
     Other details and particularities of the invention will emerge from the accompanying claims and the description of the core barrel of the invention, given below by way of non-limitative example, making reference to the accompanying drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows in axial section a portion of a core barrel of the invention at a point of attachment of the means of retrieving the internal tube assembly. 
     FIG. 2 shows an axial section of a portion of the core barrel of the invention which follows that of FIG. 1, tying in with it at a transverse plane I—I, going towards the bit. 
     FIG. 3 shows in axial section, as a variant to that of FIG. 1, another type of portion of a core barrel of the invention at the point of attachment of the means of retrieving the internal tube assembly. 
     FIG. 4 shows a transverse section, at the cutting plane IV—IV in FIG. 2, seen in the direction of the arrows. 
     FIG. 5 shows in a plan view a joint system used in an embodiment of the invention. 
     FIG. 6 shows an axial section of a portion of the core barrel of the invention which follows that of FIG. 2, going towards the bit. 
     FIG. 7 shows in axial section a portion of a core barrel of the invention downstream of the one in FIG.  6  and at the bit. 
     FIG. 8 shows in axial section a variant of the end of the core barrel portion in FIG.  7 . 
     In the different figures, the same references designate similar or analogous elements. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The core barrel  1  of the invention has, as is known (FIGS.  1  and  3 ), a string  2 , an external tube  3  fixed to the front end of the string  2 , looking in a direction of advance S of the core barrel  1  in a formation  4  (FIG.  7 ), an annular core bit  5  mounted on a front section  3 A of the external tube  3 , and an internal tube assembly  8 , including amongst other things the internal tube  9  proper. This assembly  8  is designed to be lowered in the external tube  3  and to be brought to the surface again through the string  2  and has hydrodynamic means arranged to convert the pressure of the core sampling fluid flowing in the string  2  into a force thrusting the said internal tube assembly forwards. These hydrodynamic means consist amongst other things of the different surfaces of the internal tube assembly  8  subjected to different pressures which the core sampling fluid exhibits, both along this assembly  8  and at its ends, because of pressure drops which occur therein, as is known in the art, these different pressures decreasing as the bit  5  is approached. These pressure drops can be controlled in the assembly and can be regulated, for example by adapting an appropriate exchangeable nozzle  10  (FIG. 2) in a pipe  11  through which the fluid runs. 
     According to the invention, the front section  3 A (FIGS. 6 and 7) of the external tube  3  is mounted for axial sliding, limited by reciprocal stops, front  12 A,  12 P (FIG. 6 or  7 ) and rear  13 A,  13 P (FIG.  7 ), in a rear section  3 P of the same external tube  3 , and projects longitudinally therefrom. The stops  12 A and  13 A are fixed with respect to the front section  3 A whilst the stops  12 P and  13 P are fixed with respect to the rear section  3 P. The latter can form a kind of sheath for the front section  3 A, as shown in FIG.  7 . It goes without saying that an expert can choose to dispose the stops  12 A,  12 P either in accordance with FIG. 6 or in accordance with FIG. 7 or again according to any other corresponding embodiment. 
     In addition, the internal tube assembly  8  has a support stop  14  arranged so as to selectively push the front section  3 A of the external tube  3  forwards. In addition, means  15  described below are arranged between the internal tube proper  9  (intended to receive a core) and said front external tube section  3 A so that the latter can be driven in rotation about its longitudinal axis independently of said internal tube  9 , the latter being able to be kept fixed in rotation with respect to the formation  4 , during core sampling. 
     The bit  5  of the core barrel  1  can be driven in rotation by means of the external tube  3  and the string  2 . It is however preferred for the core barrel  1  to have a motor  17  for rotating the front section  3 A and the bit  5 . In this case, means, for example of the rotation and sliding bearing raceway  18  type (FIGS.  6  and  7 ), are arranged between said front section  3 A and the rear section  3 P of the external tube  3 , so that one section can be driven in rotation, about its longitudinal axis, independently of the other. The motor  17  is advantageously mounted in a rear part  19  (FIG. 6) of the internal tube assembly  8 . The stator  20  of the motor  17  can be fixed in rotation with respect to the rear section  3 P of the external tube  3 , during core sampling, whilst the rotor  21  of this motor  17  is fixed in rotation to the front section  3 A of the external tube  3 . 
     In the internal tube assembly  8 , the internal tube  9  proper can be supported by the rotor  21  of the motor  17 , by means  18  which can be a thrust ball bearing assembly  15 , to allow the independent rotation of the internal tube  9  with respect to the rotor  21 . 
     The stator  20  of the motor  17  can be kept fixed in rotation with respect to the rear section  3 P of the external tube  3  (FIGS. 2 and 4) by at least one assembly  26  consisting of catch  27  and corresponding groove  28 . The catch  27  is then advantageously arranged to engage in the groove  28  or to be automatically released therefrom when the internal tube assembly  8  is put in the core sampling position in the external tube  3  or is respectively withdrawn therefrom. Three assemblies  26  can advantageously be arranged around the longitudinal axis of the assembly  8  and the core barrel  1 . 
     For its part, the rotor  21  can be coupled in rotation to the front section  3 A of the external tube  3  (FIG. 6) by at least one assembly  31  consisting of catch  32  and corresponding groove  33 , the catch  32  is then advantageously arranged to engage in the groove  33  or to be automatically released therefrom during the same manoeuvre of putting in the core sampling position or respectively of withdrawal as above of the internal tube assembly  8 . The assemblies  31  can be disposed three in number, in a similar manner to the assemblies  26  in FIG.  4 . 
     The catches  27  and  32  can have springs, stops and bevels depicted in FIGS. 2 and 6, in order to facilitate their introduction into the grooves  28  and respectively  33  and their removal therefrom. The grooves  28  (FIG. 4) and  33  can for their part have a longitudinal face (for example  29 ) sloping in order to assist a gentle entry of the catches  26  and respectively  32  into the appropriate groove by a rotation of the assembly  8  or respectively of the rotor  21  in the clockwise direction in the rear section  3 P or respectively front section  3 A. 
     Cooperating means  34  for throttling the passage of core sampling fluid can be provided on the internal tube assembly  8  and in the external tube  3  (FIG.  3 ). These throttling means  34  are arranged so as to be inactive during normal core sampling and so as to throttle the said passage where the internal tube assembly  8  is pushed back in the external tube  3  in the opposite direction to the direction of forward travel S of the core sampling. For this purpose, these throttling means  34  are formed for example by an annular protrusion  35  on the internal tube assembly  8  and by a corresponding internal annular rim  36  on the external tube  3 . These throttling means  34  are designed to indicate for example an abnormal situation during core sampling, as is described below. 
     The motor  17  is advantageously of a type actuated by the core sampling fluid. In this case, the motor  17  can be arranged in the system of pipes  39  (FIGS. 2 and 6) for the passage of fluid so that it participates in the aforementioned hydrodynamic means, at least for the part of the said fluid which it actuates. 
     A bypass pipe  41  (FIG. 2) for core sampling fluid can be provided in the system of fluid pipes  39 , between an inlet  42  and an outlet  43  of the motor  17  for this fluid. Then, on the one hand, it is possible to equip the internal tube assembly  8  with a valve  45  mounted, in the bypass pipe  41 , so that it can slide longitudinally between two stop positions, a front one  46  and a rear one  47  on said assembly  8 . The front stop  46  can consist, for example, for a construction facility, of two half rings as suggested in FIG.  2 . On the other hand, in the same bypass pipe  41 , the rear section  3 P of the external tube  3  can be provided with a valve seat  48  which is fixed to said rear section  3 P, downstream of the valve  45 , and which is arranged to cooperate with the latter as follows. 
     On the one hand, the bypass pipe  41  is closed when the valve  45  is in the rear stop position  47  depicted in FIG.  2  and is applied at the same time against the said seat  48  by the weight of the internal tube assembly  8  and, where applicable, by the pressure of the fluid on this assembly  8  or when the valve  45  slides on this same assembly  8 , between the front  46  and rear  47  stop positions, but is applied at the same time against the said seat  48  by the pressure of the fluid upstream in the bypass pipe  41 . The motor  17  can then receive maximum pressure and throughput of the core sampling fluid and produce its maximum torque for driving the bit  5 . 
     Moreover, the bypass pipe  41  is open when the valve  45  is in the front stop position  46  and is moved away at the same time from the said seat  48  by the effect of the internal tube assembly  8  pushed upstream in the rear section  3 P of the external tube  3 . The motor  17  is then in some way put in short-circuit with regard to the fluid which actuates it and has consequently practically no more driving torque. 
     In the case of the example shown in FIG. 2, it can be seen that the rear stop  47 , the valve  45  and the valve seat  48  form a stop  49  which longitudinally positions, in the direction of forward travel S, the above-mentioned assembly  8  in the external tube  3  and more precisely in the rear section  3 P thereof. Another kind of stop can however be used for the same purposes. 
     Between a supply pipe  50  to the motor  17  and an outlet pipe  51  (FIGS. 2,  6  and  7 ) for the core sampling fluid to the bit  5 , a closure valve  54  can advantageously be mounted, arranged so as to open in the event of overpressure of the fluid at the inlet  42  to the motor  17 , this closure valve  54  preferably being a rated disc  54  which is pierced at a given overpressure. 
     In order to help the internal tube assembly  8  to descend in the external tube  3 , the core barrel  1  of the invention can also include (FIGS. 1 and 3) a joint system  55  mounted on the internal tube assembly  8  so that, in the active position, it substantially completely closes the annular space  56  between the external tube  3  and this assembly  8  and thus receives, like a piston, the full pressure of the core sampling fluid. 
     The joint system  55  can include at least two flat circular joints  57  and  58 , the external edge  57   a ,  58   a  (FIG. 5) of which is in each case divided into a kind of petal  57   b ,  58   b , the two joints  57 ,  58  being arranged one on the other so that a petal  57   b  or  58   b  on one joint covers a gap  58   c  or respectively  57   c  between two petals on the other joint. 
     The internal tube assembly  8  can advantageously have a sleeve  62  disposed downstream of the joint system  55  so as to be without action thereon during the descent of the internal tube assembly  8  in the external tube  3  and during core sampling, but so as to slide in advance upstream on the internal tube assembly  8  when the latter is withdrawn from the external tube  3 . During this sliding, the sleeve  62  comes into engagement with the joint system  55  and then moves it away from the external tube  3  in a position  55 R folded back against the internal tube assembly  8 , so that the fluid can once again flow through the annular space  56 , having a vastly reduced or practically zero effect on the joint system  55  and therefore on the said assembly  8 . 
     It should be understood that the present invention is in no way limited to the embodiments described above and that many modifications can be made without departing from the scope of the claims given below. 
     Thus it is advantageously possible to equip the core barrel  1  of the invention with a sensor  63  (FIG. 8) disposed in the internal tube proper  9 . A portion  64  of this sensor  63 , equipped with measuring means, not shown nor described in detail since they are known to experts, then projects from this internal tube  9  during the descent of the internal tube assembly  8  in the external tube  3  and in the core sampling position of the internal tube proper  9  with respect to the external tube  3  whilst the latter is still at a distance from a core sampling well bottom. The sensor  63  is also disposed in the internal tube  9  so as to be able to be pushed inside it, during core sampling, by the bottom of the well and/or by the top of the core. This type of sensor  63  can have means of recording the measurements made, so that they can be examined on the surface when the assembly  8  has been removed from the well and the sensor  63  has been connected to appropriate equipment. 
     Modes of functioning of the core barrel of the invention are described below by way of non-limitative examples. 
     The front section  3 A of the external tube  3  is introduced into the rear section  3 P, through the rear, before connecting external tube sections  3  to each other at  66  (FIG.  6 ). It is arranged so as to be able to slide therein between a position of suspension in the rear section  3 P, by the reciprocal effect of the stops  13 A,  13 P (FIG.  7 ), and an extreme pushed-in position, in the rear section  3 P, limited by the reciprocal effect of the stops  12 A,  12 P (FIG. 6 or  7 ). The bit  5  can then be mounted on the front section  3 A. The external tube  3  thus equipped can be fixed to the string  2  (FIG. 1 or  3 ) and be lowered into a well to be sampled. 
     The internal tube assembly  8  can be lowered in the external tube  3 , according to the known so-called wire-line technique, if necessary by means of a known attachment device  67  (FIG. 1 or  3 ) which the assembly  8  has. Core sampling fluid sent at this moment under pressure in the external tube  3  can bear on the joint system  55  in order to help in the lowering of the internal tube assembly  8 , especially if the external tube  3  does not only follow a vertical but takes a strong inclination with respect to the vertical, even as far as the horizontal. The internal tube assembly  8  can thus descend until it comes into abutment (FIG. 2) against the aforementioned stop  49 . Preferably, in this stopped position, the said assembly  8  is at the same time in abutment, during core sampling, against the support stop  14  (FIG. 6) through which it can act on the front section  3 A. 
     At this moment, the attachment device  67  is forced to push in, in the direction S, a connecting bar  68  (FIG. 1 or  3 ) in the said assembly  8 , causing an immobilisation pin  69  to break, until a face  70 P of a recess  70  in the bar  68  comes into abutment against a stop spindle  71 . Following the travel thus followed by the bar  68 , channels  72  and  73 , isolated from each other up till now by joints  74 , are put in communication and core sampling fluid can flow as from this moment from the inside of the string  2 , through the annular space  75  and the channels  72 ,  73 , into a pipe  76  (FIG. 1 or  3 , and FIG. 2) hollowed out in the connecting bar  68 , in pipes  77 , as far as the bit  5 . On this path, the fluid undergoes, from at least the top end of the internal tube assembly  8  as far as its exit from the bit  5 , a useable pressure drop. 
     In one mode of functioning, the front section  3 A is driven in rotation by the rear section  3 P by means, not shown but known to experts, which allow the aforementioned sliding. At the start of core sampling, the bit  5  can, where applicable, partially push the front section  3 A into the rear section  3 P until it is in abutment against the support stop  15  of the internal tube assembly  8 . The assembly  8 , subjected to the pressure of the fluid, offers to the front section  3 A a resistance determined by this pressure which depends amongst other things on the pressure drop produced by the nozzle  10 . 
     On the one hand, during core sampling, said front section  3 A, pushed forward by the assembly  8  subjected to the pressure of the fluid, can be pushed to a maximum extent out of the rear section  3 P (FIG. 7) against the formation to be sampled, the stops  13 A and  13 P not necessarily being against each other (this depending on the relative positions thereof and of the stop  49 ). Already before reaching this first extreme position, a braking of the advance of the string  2  on the surface can cause a retaining of the advance of the bit  5  in the formation by the action of the stop  49  on the assembly  8 . Advantageously then, the string  2  is at least partially under traction and therefore held more straight, even on the horizontal, which is not usually possible when it is completely under compression subject because of this to buckling. In addition, the bit  5  receives under these circumstances a weight which is appreciably more even than with a string in the buckling situation and subjected to significant friction against the wall of the well. The bit  5  can therefore progress in the formation  4  with more regularity. All this amongst other things promotes the ability to correctly direct the bit  5 . 
     If on the other hand the front section  3 A were pushed back to the maximum extent in the rear section  3 P, counter to the pressure on the assembly  8 , the stops  12 A,  12 P (FIG. 6 or  7 ) coming into contact, it would be possible to obtain a second extreme situation in which an advancement of the string  2  could force the penetration of the bit  5  into the formation  4 . 
     On the other hand, during core sampling, the front section  3 A can be maintained in the rear section  3 P, by the pressure on the assembly  8 , in positions in which the reciprocal stops  12 A,  12 P on the one hand and  13 A,  13 P on the other hand are not in respective contact. If in addition the assembly  8  is not in abutment against the valve  45  but the latter is held against its seat  48  by an upstream fluid pressure greater than the downstream one, what can be termed a decoupled functioning mode is obtained. The front  3 A and rear  3 P sections of the external tube  3  can then slide with respect to each other under the effect of the pressure on the assembly  8 . In this case, even if the string  2  advances only in jerks, for example following a temporary sticking to the walls of the well and/or following curves between vertical and horizontal portions of the well, the front section  3 A can for its part progress in a regular manner, according only to the action of the bit  5  in the formation  4  during core sampling. 
     The assembly  8 , the front section  3 A and the bit  5  can thus move longitudinally in one direction or the other, with respect to the rest of the string  2 , depending on whether the speed of the string  2  is less than or greater than that of the bit  5  in the formation  4 , and this can be adjusted from the surface by acting on the string  2 . 
     If at present the assembly  8  is pushed further into the external tube  3 , the front stop  46  which carries the assembly  8  comes into contact with the valve  45  and can push it away from the valve seat  48 . Because of this, the fluid, which up till then could pass only through the pipe  50  (FIG. 2) and the nozzle  10 , can now also pass between the valve  45  and its seat  48 , and this considerably reduces the pressure drop in the core barrel  1 , and therefore the pressure applied to the assembly  8  and consequently the force exerted by the bit  5  on the formation  4 . The core barrel  1  of the invention is thus automatically practically put out of service in the event of excessive force to be supplied, to the benefit of its constituents, through a direction action at these at the bottom of the well. 
     If the variant according to FIG. 3 is applied, it is possible to obtain another type of signalling of a process of functioning of the core barrel  1  of the invention. In this case, when the internal tube assembly  8  is pushed upstream, whether by a core which is blocked in the internal tube  9  (FIG. 7) and which continues to be cut by the bit  5  or whether it is because the front section  3 A is pushed into the rear section  3 P, the following is obtained at the cooperating throttling means  34  (FIG.  3 ). As the assembly  8  is pushed back in the external tube  3 , the annular protrusion  35  provided on the assembly  8  approaches the annular rim  36  on the external tube  3  and thereby the passage of fluid is throttled therein. This causes, upstream of the means  34 , an increase in the pressure of the fluid, which the operators can note and interpret. This can also accentuate the resulting force on at least the assembly  8  and possibly on the front section  3 A and on the bit  5  but it is then possible to limit this force from the surface by acting directly on the fluid flow or, preferably, on the advance of the string  2 . 
     When the front section  3 A and the bit  5  are not driven in rotation by the rear section  3 P but by a motor  17  arranged on the core barrel  1  of the invention, as described above, another operating mode can be obtained. During core sampling, the motor  17  is supplied with fluid arriving from the string  2  and running successively (FIG. 1 or  3 ) through the annular space  56 , the channels  72  and  73 , the pipe  76 , the pipes  77  (FIG.  2 ), the pipe  11  and for example the nozzle  10  disposed at the inlet  42  to the motor  17 . The fluid then leaves the motor  17  through its outlet  43  (FIG. 6) and is conveyed by various pipes  51  as far as the bit  5 . It can be seen that, advantageously, advantage is taken of the internal thrust bearing of the motor  17  (between the stator  20  and rotor  21 ) to transmit, to the front section  3 A, the thrust forces of the pressure of fluid on the assembly  8  and in the motor  17 , since these forces are transmitted precisely by the stator  20  and rotor  21 . The internal thrust bearings of known motors are in fact in a position to withstand the forces necessary for this purpose. 
     It will be noted that, although the string  2  must not cause the bit  5  to rotate, it can be rotated at slow speed in order for example to prevent the string  2  sticking to the wall of the well. 
     If, for any reason known to experts, the bit  5  and/or the front section  3 A oppose an excessive resisting torque at the motor  17 , this causes the pressure upstream of the latter to be increased. The rated disc  54  can then be pierced when the pressure there reaches a limit safety value for the motor  17 , and the fluid is diverted from the inlet  42  of the motor  17  to the conduit  51  and the outlet at the bit  5 , and the motor  17  stops. 
     If on the other hand the front section  3 A is pushed back for any reason in the section of external tube  3 P at the point that the valve  45  (FIG. 3) leaves the valve seat  48 , the fluid which arrives from the pipes  17  can escape between the valve  45  and the seat  48  to the pipe  51 . The motor  17  is thus practically discharged of all fluid pressure since the latter is at this moment substantially equal at the inlet  42  and outlet  43  of the motor  17 , so that at least the motor  17  and the bit  5  are no longer subjected to torques which could be harmful to them. 
     If the internal tube assembly  8  is pushed back upstream in the external tube  3 , whether by the effect of a core which has jammed therein or by the effect of the front section  3 A of the external tube pushing against the support stop  14 , the throttling means  34  (FIG. 3) can come into action and, by throttling the passage of fluid, cause an appreciable increase in the pressure upstream of these means  34 . This increase in pressure causes the warning explained above and the operator can once again adjust the flow of fluid and/or the advance of the string  2  accordingly. 
     As has been seen, three different protections for the motor  17  and bit  5  can be provided simultaneously or separately or in accordance with any combination of two amongst them. 
     If it is wished to raise the internal tube assembly  8 , it is gripped in the usual fashion by an attachment device  67  (FIG. 1 or  3 ) and it is pulled on in the direction of the surface. This causes a sliding of the connecting bar  68  until one face  70 A of the recess  70  comes into abutment against the stop spindle  71  fixed to the assembly  8 . During said sliding, the connecting bar  68  pulls the sleeve  62  with it, by means of pins  79  (FIG.  2 ), and it separates the joint system  55  from the external tube  3 , until it strikes against the assembly  8 , for example in the position shown at  55 R. Because of this, the fluid present in the external tube  3  makes practically no more obstacle to the raising of the internal tube assembly  8 . In addition, the fluid can still escape for this purpose, where applicable, through channels  80  (FIG. 1 or  3 ), not yet described, at this time put opposite the channels  72  by the sliding of the connecting bar  68  upstream, in the assembly  8 . 
     LEGEND TO FIGURES 
     S direction of advance of core sampling 
       1  core barrel 
       2  string 
       3  external tube 
       3 A a front section of  3   
       3 P rear section of  3   
       4  formation 
       5  annular core bit 
       8  internal tube assembly 
       9  internal tube proper 
       10  exchangeable nozzle 
       11  pipe 
       12 A front stop on  3 A 
       12 P front stop on  3 P 
       13 A rear stop on  3 A 
       13 P rear stop on  3 P 
       14  support stop 
       15  thrust ball bearing means/assembly 
       17  motor 
       18  rotation/sliding means/rings 
       19  rear part of  9   
       20  stator 
       21  rotor 
       26  catch and groove assembly 
       27  catch 
       28  groove 
       29  sloping longitudinal face of  28   
       31  catch and groove assembly 
       32  catch 
       33  groove 
       34  cooperating throttling means 
       35  annular protrusion on  9   
       36  internal annular rim on  3   
       39  fluid pipe system 
       40  external surface of  17   
       41  bypass pipe 
       42  inlet to  17   
       43  outlet from  17   
       45  valve 
       46  (position of) front stop on  9   
       47  (position of) rear stop on  9   
       48  valve seat 
       49  stop on  9   
       50  supply pipe 
       51  outlet pipe 
       54  closure valve/rated disc 
       55  joint system, active position 
       55 R joint system, folded back position 
       56  annular space 
       57  joint 
       57   a  external edge of  57   
       57   b  petals on  57   
       57   c  gaps on  57   
       58  joint 
       58   a  external edge of  58   
       58   b  petals on  58   
       58   c  gaps on  58   
       62  sleeve 
       63  sensor 
       64  portion of  63   
       66  point of connection of sections of  3   
       67  attachment device 
       68  connecting bar 
       69  immobilisation pin 
       70  recess 
       70 A front face of  70   
       70 P rear face of  70   
       71  stop pin 
       72  channel 
       73  channel 
       74  joint 
       75  annular space 
       76  pipe 
       77  pipes 
       79  connecting pins between  62  and  68   
       80  channel

Summary:
The invention concerns a core barrel, in particular for oil exploration, comprising a string, an external tube ( 2 ) fixed at the front end of the string, a core bit mounted on a front section ( 3 A) of the external tube, and an internal tube assembly ( 8 ) mounted in the external tube and comprising hydrodynamic means transforming a core sampling line pressure into a load thrusting said assembly towards the core barrel front, the front section of the external tube being mounted axially sliding, limited by front and rear stop elements ( 12 A,  12 P), in a rear section ( 3 P) of the same external tube to project therefrom longitudinally, the assembly comprising a support stop ( 14 ) arranged to thrust selectively forward the front section of the external tube, means being arranged between the inner tube ( 9 ) proper and said front section such that the latter can be driven in rotation about its longitudinal axis independently of said internal tube, a motor ( 7 ) capable of driving the core bit in rotation relative to the string.