DETECTING SYSTEM IN A SOIL DRILLING MACHINE AND RELATED METHOD

Abstract

A system and method detect at least one mechanical locking position of a drill string in a soil drilling machine. According the system and method, there are: a sensor apparatus which is configured to detect first data indicating the position assumed by the top of the drill string and second data indicating the position assumed by the bottom of the drill string. A control unit calculates the estimated length of the drill string based on the data and to compare the estimated length with a reference length value. A pair of kelly rods is in a mechanical locking position; and a signaling apparatus which is configured to signal when the estimated length corresponds to a reference length value.

Claims

1. A system for detecting at least one mechanical locking position of a drill string in a soil drilling machine; said machine comprising: a mast; a rotary which is mounted so as to slide along said mast; a winch around which a flexible pulling member is wound; a drill string which is rotated by said rotary, which crosses said rotary, and which hangs from said flexible pulling member; said drill string comprises at least one pair of kelly rods which are axially slidable inside one another so as to be moved to at least one mutual mechanical locking position, in which said drill string is at least partially supported by said rotary and said at least one pair of kelly rods is moved from a decoupled condition, in which a transmission of a downward thrust between said kelly rods is prevented and a mutual sliding is allowed, to a coupled condition, in which a transmission of a downward thrust between said kelly rods is allowed substantially without mutual sliding; movement from said decoupled condition to said coupled condition occurring when said rotary causes said drill string to rotate in a drilling rotation direction; said system comprising: sensor means for detecting first data indicating a position assumed by the top of the drill string and second data indicating a position assumed by the bottom of said drill string; a control unit which is configured to calculate an estimated length of said drill string based on said data and to compare said estimated length with at least one reference length value, wherein said at least one pair of kelly rods is in said at least one mechanical locking position; and signaling means which are configured to signal when said estimated length corresponds to said at least one reference length value.

2. The system according to claim 1, wherein said sensor means comprise a first sensor which is configured to detect a first information representing a position assumed by said rotary along said mast, and a second sensor which is configured to detect an information representing that said drill string is at least partially supported by said rotary.

3. The system according to claim 1, wherein said sensor means comprise a winding sensor which is configured to detect an information representing a length of the flexible pulling member that is unwound from the winch.

4. The system according to claim 1, wherein said control unit is configured to calculate the difference between said estimated length and said at least one reference length value; said signaling means being configured to provide a perceivable signaling of said difference.

5. The system according to claim 1, wherein said sensor means comprise a vibration sensor which is configured to detect vibration data representing vibrations propagated through said drill string; said control unit being configured to correct said estimated length determining that a respective pair of kelly rods has reached a maximum mutual distance position, said maximum mutual distance position being determined based on the estimated length of said drill string and whether said vibration data exceed a threshold value.

6. A soil drilling machine comprising a system according to claim 1.

7. A method for detecting at least one mechanical locking position of a drill string in a soil drilling machine; said machine comprising: a mast; a rotary which is mounted so as to slide along said mast; a winch around which a flexible pulling member is able to be wounded wound; a drill string which is capable of being caused to rotate by said rotary, which crosses said rotary, and which hangs from said flexible pulling member; said drill string comprises at least one pair of kelly rods which are axially slidable inside one another so as to be moved to at least one mutual mechanical locking position, in which said drill string is at least partially supported by said rotary and said at least one pair of kelly rods is moved from a decoupled condition, in which a transmission of a downward thrust between said kelly rods is prevented and a mutual sliding is allowed, to a coupled condition, in which a transmission of a downward thrust between said kelly rods is allowed substantially without mutual sliding; the movement from said decoupled condition to said coupled condition occurring when said rotary causes said drill string to rotate in a drilling rotation direction; said method comprising the following operating steps: detecting first data indicating a position assumed by the top of the drill string and second data indicating a position assumed by the bottom of said drill string; calculating the estimated length of said drill string based on said data and comparing said estimated length with at least one reference length value, wherein said at least one pair of kelly rods is in said at least one mechanical locking position; signaling when said estimated length corresponds to said at least one reference length value.

8. The method according to claim 7, wherein said first data comprise a first information representing a position assumed by said rotary along said mast and a second information representing that said drill string is at least partly supported by said rotary.

9. The method according to claim 7, wherein said second data comprise a winding information representing a length of the flexible pulling member that is unwound from the winch.

10. The method according to claim 7 comprising, furthermore, the operating steps of calculating a difference between said estimated length and said at least one reference length value; and providing a perceivable signaling of said difference.

11. The method according to claim 8 and comprising, furthermore, the operating steps of: detecting vibration data representing vibrations propagated through said drill string; correcting said estimated length determining that a respective pair of kelly rods has reached a maximum mutual distance position, said maximum mutual distance position being determined based on the estimated length of said drill string and whether said vibration data exceed a threshold value.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0053] FIG. 1A is a perspective view of an outer kelly rod according to the prior art and belonging to a drill string of a soil drilling machine.

[0054] FIG. 1B is a perspective view of an inner kelly rod according to the prior art and belonging to a drill string of a soil drilling machine.

[0055] FIG. 2A is a side elevation view of a soil drilling machine according to the prior art and comprising a drill string including the kelly rods shown in FIG. 1A and in FIG. 1B.

[0056] FIG. 2B is a view similar to the one of FIG. 2A, wherein the machine is shown in a different operating condition.

[0057] FIG. 3A is a side elevation view of a soil drilling machine provided with a system for detecting a plurality of mechanical locking positions of a drill string, wherein the system is structured according to an explanatory embodiment of the invention.

[0058] FIG. 3B is a view similar to the one of FIG. 3A, wherein the machine is shown in a different operating condition.

[0059] FIG. 4 is a schematic representation through block diagrams of a system for detecting a plurality of mechanical locking positions of a drill string in a soil drilling machine, wherein such system is structured according to an explanatory embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0060] With reference to FIGS. 3A and 3B, number 1 indicates, as a whole, a soil drilling machine.

[0061] Details and elements that are similar toor fulfill a similar function asthe ones of the known drilling machine, which is indicated with number 100 and was previously described with reference to FIGS. 1A, 1B, 2A and 2B, are associated with the same alphanumeric references. Therefore, for the sake of brevity, for machine 1 according to the invention reference is made to the description above concerning prior art machine 100, which is considered as part of the following detailed description.

[0062] Unlike machine 100 described above, machine 1 according to the invention is provided with a system for detecting at least one mechanical locking position of drill string 20, said system being manufactured according to an explanatory embodiment of the invention.

[0063] With reference in particular to FIG. 4, the system comprises a sensor apparatus S, which is configured to detect and provide: [0064] first data D1 indicating the position assumed by the top of drill string 20, in particular by the top of inner kelly rod 20A, and [0065] second data D2 indicating the position assumed by the bottom of drill string 20, in particular by the bottom of inner kelly rod 20C to which drilling toll 15 is to be fixed.

[0066] The system further comprises a control unit 31, which is configured to calculate an estimated length L20 of drill string 20 based on the first data D1 and the second data D2 detected by sensor apparatus S. In the embodiment shown, control unit 31 is configured to calculate the distance between the top of drill string 20 and the bottom of the drill string.

[0067] Furthermore, control unit 31 is configured to compare estimated length L20 with one or more reference length values. In particular, each one of the reference length values corresponds to a length assumed by drill string 20, in which a respective pair of the kelly rods (namely, in the embodiment shown, the first pair 20A, 20B or the second pair 20B, 20C) is in a mechanical locking position (in which the engagement portione.g. the recessesof the kelly rod of the aforesaid pair of kelly rods having the smallest diameter and the interlocking portione.g. the stringsof the kelly rod of the aforesaid pair of kelly rods having the largest diameter can be moved from the decoupled condition to the coupled condition).

[0068] Furthermore, the system comprises a signaling apparatus 34, which is configured to signal when estimated length L20 corresponds to a reference length value.

[0069] In this way, given the signal provided by signaling apparatus 34, an operator controlling machine 1 is aware of the fact that a pair of kelly rods (namely, in the embodiment shown, the first pair 20A, 20B or the second pair 20B, 20C) is in the respective mechanical locking position. In the way, the operator is capable of moving the kelly rods from the decoupled condition to the coupled condition, causing rotary 10 to make a rotation in the drilling direction.

[0070] Preferably, sensor apparatus S comprises a first sensor 35, which is configured to detect a first information representing the position assumed by rotary 10 along mast 5. Furthermore, the sensor apparatus comprises a second sensor 33, which is configured to detect a second information representing the fact that drill string 20 is at least partly supported by rotary 10. Operatively, when the second sensor 33 detects that drill string is at least partly supported by rotary 10, control unit 31 uses the first information representing the position assumed by rotary 10 along mast 5 as data D1 indicating the position assumed by the top of drill string 20. Indeed, in this case, the position of rotary 10 is substantially comparable with the position assumed by the top of drill string 20, namely by the top of outer kelly rod 20A.

[0071] Preferably, the first sensor 35 is configured to detect the axial position of rotary 10 along mast 5, in particular relative to an end of mast 5 or of head 7.

[0072] For example, the first sensor 35 is a laser distance measurer. Preferably, the laser distance measurer is installed in a fixed position at an end of mast 5 or on head 7. In case a laser sensor is used, it is preferably installed at a height above the upper limit stop position of rotary 10 along mast 5 or at a height below the lower limit stop position of rotary 10 along mast 5.

[0073] In the embodiment shown in FIGS. 3A and 3B, the first sensor 35 is installed on head 7 and is oriented so as to emit a laser beam directed towards the body of rotary 10 and, in particular, to a proper target surface present on rotary 10, which, for example, can be a reflecting surface. In this way, the first sensor 35 can measure all the translation movements of rotary 10 relative to a zero reference height, which can be set during the calibration and the zero setting of the sensor. For example, the zero height can be referred to the upper end of mast 5, as shown in FIGS. 3A and 3B, but, in an equivalent manner, it can relate to the lower end of mast 5 or to one of the limit stop positions of rotary 10 along mast 5. Anyway, different constructive variants are possible, which use different types of first sensor 35 to measure the position of rotary 10. For example, the first sensor 35 can be a wire sensor, in which the end of the wire is connected to the body of the rotary so that the wire is wound and unwound from the sensor based on the movement of the rotary.

[0074] In further embodiments, in case of a machine 1 with a pulling-pushing system 11 having a hydraulic cylinder, the first sensor 35 can be a sensor of the stroke of the rod of the cylinder, which moves rotary 10.

[0075] In further embodiments, in case of a machine 1 with a winch pulling-pushing system 11, the first sensor 35 can be an encoder installed on the winch of pulling-pushing system 11, which measures the rotations of the winch in order to determine the movements of rotary 10.

[0076] However, the use of a first laser sensor 35 is anyway preferable because it ensures a greater precision and can be used in the same manner both in cylinder pulling-pushing machines and in winch pulling-pushing machines.

[0077] As already mentioned above, the values measured by the first sensor 35 are then processed by control unit 31 in order to calculate the position of rotary 10 along mast 5.

[0078] Preferably, the second sensor 33 is configured to detect when outer kelly rod 20A of the drill string is in the lower limit stop position relative to rotary 10. As already mentioned above, this condition corresponds to the fact that drill string 20 rests on or is at least partly supported by rotary 10.

[0079] In the embodiment shown, the limit stop position corresponds to the situation in which outer kelly rod 20A has completely crossed rotary 10, causing its upper flange 24A to strike against the abutment surface of said rotary 10. In the example shown, hence, the second sensor 33 fulfills the function of detecting the lower limit stop of the outer kelly rod 20A.

[0080] Preferably, the second sensor 33 is a proximity sensor installed on the casing of rotary 10, in particular close to the passage for drill string 20. When the outer kelly rod 20A of the drill string is completely lowered relative to rotary 10, for example like in FIGS. 3A and 3B, upper flange 24A ends up in front of the second sensor 33.

[0081] In a variant of the invention, upper flange 24A is advantageously made of a metal material and the second sensor 33 is capable of detecting the presence of the flange and of sending a signal to control unit or CPU 31 of machine 1. In other constructive variants, the second sensor 33 could consists, for example, of a micro-switch, which is operated by the passage of flange 24A when it reaches the striking position against the abutment surface of rotary 10, when outer kelly rod 20A has reached the lower limit stop.

[0082] Let's consider a drilling configuration with drill string 20 completely closed, namely in a minimum length configuration, and with drill string 20 in an intermediate position relative to rotary 10, namely in a position different from the lower limit stop of drill string 20. In this drilling configuration, control unit 31 of machine 1 is capable of recognizing that drill string 20 is completely closed thanks to the fact that the second sensor 33 does not detect the limit stop position. Indeed, in the embodiment shown, the second telescopic kelly rod 20B and the following ones can start telescopically sliding out only when the outer kelly rod 20A rests with its upper flange 24A on the rotary. In this way, the outer telescopic kelly rod 20A does not hang any longer from the rope and rests on the recovery flange 27C.

[0083] When outer kelly rod 20A reaches the lower limit stop condition with drill string 20 completely closed, also visible in FIG. 3A, control unit 31 recognizes that estimated length L20 is equal to the minimum value and gets ready to measure the following variations of length L20. In this condition, the value of the quantity of rope unwound from the winch, which is measured by sensor apparatus S (in particular, by winding sensor 30 which will be described hereinafter), can be set as equal to zero.

[0084] In the embodiment shown, estimated length L20 referred to the drill string can be interpreted, for example, as the distance between upper flange 24A of outer kelly rod 20A and rod recovery flange 27C arranged at the lower end of inner kelly rod 20C.

[0085] In the embodiment shown, as already mentioned above, winding sensor 30 is configured to measure the quantity of flexible pulling member, such as rope 9, unwound from winch 8.

[0086] Preferably, winding sensor 30 is an encoder mounted on winch 8 and capable of measuring the rotations of the drum of winch 8.

[0087] Control unit 31, based on the signal of winding sensor 30 and based on the awareness of the characteristic sizes of the drum of winch 8 and of rope 9, is configured to calculate the quantity of rope unwound from the winch.

[0088] In a constructive variant, winding sensor 30 could be an encoder mounted on one of the pulleys of head 7 or it could be an optical sensor installed on a fixed structure close to rope 9 and capable of detecting the linear sliding of the rope.

[0089] In the embodiment shown, control unit 31 comprises a memory unit where all the geometric data concerning all drill strings 20 that can be installed in machine 1 are pre-loaded. Said rill strings 20 can vary in terms of length of kelly rods 20A, 20B, 20C, number of the kelly rods making up drill string 20 and diameter of the kelly rods. By way of non-limiting example, upon starting of machine 1, orif necessaryupon installation of machine 1, the operator must select, preferably from a list displayed on a display available in the cabin, telescopic drill string 20 installed in machine 1. When the selection is made by the operator, control unit 31 loads all the data concerning that drill string 20, for example the minimum length of the drill string in a completely contracted configuration and the maximum length of the drill string in a completely extended configuration, the length of each single kelly rod, the position of the recesses on each single kelly rod of the drill string, etc.

[0090] Since rope 9 is directly connected to the upper end of inner kelly rod 20C and since the length of inner kelly rod 20C is known to control unit 31, the measure of the quantity of rope unwound from the winch gives an indication of the position of drilling tool 15. Based on the initial zero setting position of the position of drilling tool 15 to be used, the measure carried out by sensor 30 can indicate the position of tool 15 relative, for example, to the ground level or relative to rotary 10 or relative to any reference surface.

[0091] In order for the value measured by winding sensor 30 to always be precise and reliable, it is preferable that the rope always is pulled, as a loosening thereof could alter the detections. Indeed, when drilling tool 15 reaches a resting surface stopping its downward movement, if winch 8 continued unwinding rope 9, winding sensor 30 would continue measuring an unwinding, but this unwinding of rope 9 would not correspond to a translation of drilling tool 15.

[0092] In order to avoid the aforesaid problem of the loosening of rope 9, machine 1 is provided with a known pulling device 32, which is configured to keep rope 9 pulled. For example, this pulling device 32 can act through a manual control element in the cabin, properly controlling the motor associated with winch 8, andif necessarythere can also be a loosening detection device (not shown) associated with rope 9 so as to always keep said rope 9 pulled.

[0093] Preferably, signaling apparatus 34 is a display configured to visually represent the reaching and/or the approaching of estimated length 20 to one of the reference length values.

[0094] Preferred and optional details of the system according to an embodiment of the invention will be described hereinafter.

[0095] Control unit 31, based on the values detected by sensors 30, 33, 35 and based on the geometric data of drill string 20 pre-loaded in the memory of control unit 31 itself, is capable of calculating, in every drilling phase of machine 1, actual length L20 of drill string 20. Indeed, as soon as machine 1 reaches the configuration shown in FIG. 3A, in which drill string 20 is completely retracted and, by sliding downward, has just struck against rotary 10 by means of upper flange 24A, control unit 31 recognizes that drill string 20 has an estimated length L20 with a minimum value. Starting from this minimum value, control unit starts calculating the variations of length L20 due to the operation of winch 8 or due to the operation of the pulling-pushing system moving rotary 10. Then, as the drilling goes on in order to reach, for example, the drilling condition shown in FIG. 3B, control unit 31 updates estimated length L20, adding or subtracting a length equal to the quantity of rope 9 unwound from or wound on winch 8 as measured by winding sensor 30. In the same way, control unit 31 adds to or subtracts from estimated length L20 a length equal to the translation made by rotary 10 and measured by the first sensor 35.

[0096] Control unit 31 keeps calculating the actual length of drill string 20 as long as detection sensor 33 detecting the lower limit stop of outer kelly rod 20A keeps detecting that upper flange 24A rests on the abutment surface of rotary 10. When the flange 24A lifts relative to rotary 10 and is not detected any longer by sensor 33, this means that drill string 20 is in a completely retracted condition, thus having a known length L20, and, in these conditions, the operator is capable of visually checking, from his/her control position in the cabin, where the recesses of the outer kelly rod 20A are located.

[0097] Since, as already mentioned above, in drill string 20 kelly rods 20A, 20B, 20C slide out and withdraw according to a precise sequence established by mechanical abutments, by knowing estimated length L20 in a given instant it is possible to determine the mutual position of all kelly rods 20A, 20B, 20C of drill string 20. The forced sequence of sliding out and return of kelly rods 20A, 20B, 20C further makes sure that each mutual mechanical locking position of the kelly rods, namely each position in which the inner strips of each kelly rod are suited to be coupled to a recess of the kelly rode arranged immediately on the inside, corresponds to a precise length L20 of the drill string.

[0098] By storing in control unit 31 all the reference length values at which there can be a correct mechanical locking of kelly rods 20A, 20B, 20C, control unit 31 is capable of comparing, in each instant, the actual length L20 with the table of the reference length values corresponding to the mechanical locking positions. Hence, control unit 31 can control the signaling to the operator of the reaching of a mechanical locking position, adapted to carry out the passage from the decoupled condition to the coupled condition, and/or control unit 31 can give indications for the reaching of said mechanical locking position.

[0099] With reference to FIG. 3B, intermediate kelly rod 20B is in an intermediate position between two possible mechanical locking positions between outer kelly rod 20A and intermediate kelly rod 20B. In the same way, estimated length L20 of drill string 20 has an intermediate value between two reference length values corresponding to two consecutive possible mechanical locking positions. In other words, inner strips 26A of outer kelly rod 20A are in an intermediate position between two contiguous recesses 23B and 22B (not visible) of the second kelly rod 20B. Therefore, in order to reach a possible locking condition, it is necessary to reduce estimated length L20 by a distance LR2 or to increase estimated length L20 by a distance LR1.

[0100] In particular, distance LR2 equals the distance of inner strips 26A from the first reachable recess 23B arranged at a lower height than strips 26A, whereas distance LR1 equals the distance of inner strips 26A from the first reachable recess 23B arranged at a higher height than strips 26A.

[0101] Preferably, control unit 31, based on the values measured by sensor apparatus S, is capable of calculating distances LR1 and LR2 and of showing them to the operator by displaying them on display 34 in the cabin. The operator, who wants to obtain a mechanical locking of kelly rods 20A, 20B, 20C, can operate pulling-pushing system 11 so as to cause rotary 10 to slide on mast 5 by a quantity equal to distance LR1 or LR2. The operator preferably chooses to make the smallest movement between LR1 and LR2 in order to accelerate the maneuver. During this translation, values of LR1 and LR2 are updated in real time.

[0102] In an embodiment of this invention, control unit 31 can also check whether, starting from the current position, the pulling-pushing system has a travel that is sufficient to permit both movements or whether one of the two movements is impossible, for example because rotary 10 is already too close to one of the two limit stop positions on mast 5. In case one of the two movements cannot be made, control unit 31 signals to the operator that the corresponding mechanical locking position cannot be reached. In a further variant, display 34 can show, in a graphic manner, the current position of the inner strips relative to the position of the two most adjacent recesses, thus updating the information displayed when the kelly rods are subjected to relative translations.

[0103] In a simplified constructive solution, control unit 31 could not indicate the two heights LR1 or LR2 to the operator on display 34, but it could operate a sound or light warning device when the value of length L20 of drill string 20 corresponds to mechanical locking position in which it is possible to shift from the decoupled position to the coupled condition.

[0104] Preferably, sensor apparatus S further comprises a vibration sensor 36, which is configured to detect vibration data V representing the vibrations propagated through said drill string 20. Control unit 31 is configured to correct the estimated length L20 determining that a respective pair of kelly rods (namely, the first pair 20A, 20B or the second pair 20B, 20C) as reached a maximum mutual distance position, based on estimated length L20 and when vibration data V exceed a threshold value.

[0105] In a constructive variant, in order to increase the precision of the detecting system in terms of mutual position of the kelly rods 20A, 20B, 20C, it is possible to install the vibration sensor 36, in particular, on the frame of the rotary 10.

[0106] In those applications in which great-depth drilling operations must be carried out, the use of drill strings 20 consisting of a large number of kelly rods 20A, 20B, 20C can lead to a condition in which, when drilling tool 15 is close to the bottom of the drilling site, the length of the free segment of rope 9 extending from the pulleys of head 7 up to upper end of the inner kelly rod 20C is more than a hundred meters long. In this situation, if the weight of one or more kelly rods 20A, 20B, 20C and, if necessary, of the material collected by drilling tool 15 weighs on rope 9, there can be elastic elongations of rope 9 to an extent that cannot be neglected. These elongations cannot be detected by winding sensor 30, for example because they do not cause additional rotations of winch that can be detected by an encoder. The elongation of the suspended rope segment causes at least inner kelly rod 20C and, if necessary, all the other kelly rods resting on the rod recovery flange to be subjected to a relative sliding movement with respect to the kelly rods that are completely extended and are suspended from and supported by rotary 10. As already mentioned above, this relative sliding is not detected by winding sensor 31 cooperating with control unit 31. Since the axial clearance present between the inner strips and the recesses, namely the difference between the length of the inner strips and the length of the recesses, can range from a few centimeters to some decimeters, a non-detected relative sliding that is equal to or greater than these clearances could cause errors in the calculation of the correct mechanical locking positions of kelly rods 20A, 20B, 20C.

[0107] With every filling cycle of drilling tool 15, when kelly rods 20A, 20B, 20C of drill string 20 telescopically slide out to reach the bottom of a drilling site created during the previous drilling cycles, the operator maintains a very high downward speed of kelly rod 20A, 20B, 20C through a quick unwinding of rope 9, so as to reduce the working times of the cycle. During the downward movement, when one of kelly rods 20B, 20C reaches its lower limit stop position, there is a collision between said kelly rod 20B, 20C and the preceding kelly rod sliding adjacent thereto (in this case 20A, 20B), in the area of the upper flange 24B, 24C of said kelly rod and the inner strips of the preceding kelly rod, which causes the kelly rod to stop. Given the high speeds and the significant weight of the kelly rods, the extent of the collision taking place every time a kelly rod reaches the lower limit stop is remarkable and is transmitted to all the kelly rods arranged at a higher height than the kelly rod subjected to the striking, thus reaching the body of rotary 10 from which the outer kelly rod 20A hangs. This collision generates a vibration peak which can easily be recognized as it has a greater intensity than the background vibrations generated during drilling operations. This peak can be detected by vibration sensor 36, preferably an accelerometer, installed on the body of rotary 10. Vibration sensor 36 can then send a signal to control unit 31, when it detects that there is a collision generated by the limit stop of a kelly rod.

[0108] Since, as already mentioned above, the kelly rods of drill string 20 slide out with a set sequence, from the one arranged more on the outside (20A) to the one arranged more on the inside (20C), it is possible to identify, starting from a condition with a completely retracted drill string 20, every time sensor 36 detects a collision during the downward movement of the kelly rods, which and how many kelly rods have reached the limit stop position. In the moment in which sensor 36 detects the collision of a kelly rod, it is possible to increase by one the number of kelly rods that have completely slid out, whereas the remaining ones are completely contracted. Since the length of the completely contracted drill string 20 is known and stored in control unit 31 and since, in the same way, the length an the travel of each kelly rod of the drill string are known and stored in control unit 31, every time vibration sensor 36 detects the collision, due to the limit stop of a kelly rod, it is possible to determine with precision length L20 of the drill string. By using sensor 36, operators can obtain an exact value of length L20 only when the following kelly rod reaches the limit stop generating a new collision and a new vibration peak.

[0109] In this way, then, vibration sensor 36 can be used by control unit 31 to correct the measure of length L20 calculated based on the data detected by sensors 30, 33, 35 and to compensate the elastic elongation effect of rope 9. Indeed, starting from a condition with completely contracted kelly rods and holding rotary 10 still, if the kelly rods are caused to move downward into the drilling site by unwinding rope 9, control unit 31 increases the calculated length L20 by adding the quantity of unwound rope 9 measured by means of sensor 30, which measures, for example, the rotations of winch 8. Since the quantity of unwound rope 9 is added to the elastic elongation, which is not measured by the sensors 30, 33 and 35, when great depths are reached, operators can check whether the actual position of drilling tool 15 is at a lower height than the calculated one. In the constructive variant in which vibration sensor 36 is also used (in addition to sensors 30, 33, 35), every time that, during the downward movement of drilling tool 15, a kelly rod reaches the limit stop, sensor 36 sends a signal to control unit 31, which corrects the value of length L20 assigning to this length a given corrected value corresponding to the number of kelly rods that have slid out and are in a mutual maximum length condition. As the downward movement goes on, control system 31 keeps increasing the calculated length L20 based on the parameters detected by sensor 30 until another slidable kelly rod reaches its limit stop, thus generating a new collision. When this collision is detected, control system 31 corrects again the calculated length L20 of drill string 20, assigning a correct value thereto. Therefore, thanks to the use of vibration sensor 36, the error in the calculation of the position of the kelly rods is greatly reduced, as it is affected only by the elastic elongations of rope 9 taking place during the extension of one single kelly rod, and not any more by the elastic elongation of rope 9 taking place during the extension of all the kelly rods of drill string 20.

[0110] Naturally, the principle of the invention being set forth, embodiments and implementation details can be widely changed relative to what described above and shown in the drawings as a mere way of non-limiting example, without in this way going beyond the scope of protection provided by the accompanying claims.