GB2439178A - A device which provides for the axial movement of a drill bit - Google Patents

Abstract

A device for a tool 1 for use in drilling with a rotary drill bit (4' fig 1), the purpose of the tool being to provide an axial movement 15 of the drill bit when the torque 14 exceeds a predetermined value. For this purpose the tool is made up of two cylindrical string parts 2 and 3 connected through guide elements and seals 5, 8, 10. The string parts 2 and 3 are interconnected through a rotationally rigid, axially movable toothing 9. Axial movement 15 is achieved when the pressure differential between an internal pressure 13 and an external pressure 22 generates a force across the differential area, between the periphery 16 of the string part 3 and the periphery 17 of the string part 2, which is greater than the pre-tensioned force of the spring 6.

Description

<p>A DEVICE FOR A TOOL FOR AXIAL DISPLACEMENT OF THE DRILL IT</p>

<p>OF P. DRILL STRING PROVIDED WITH A MUD MOTOR This invention relates to a device for a tool of the kind which is utilized at a rotary drill bit, in which the aim is to provide an axial movement of the drill bit when, for a shorter or longer time, the torque exceeds a value given in advance.</p>

<p>Devices of various designs, the purpose of which is to dampen vibratiorJ iid maximally limit the torque on the drill io string, are known from the oil industry among others. In a drill string several thousand metres long, vibrations may arise due to varying torque on the string. Such vibrations are most often caused by the drill bit hitting different rock formations, causing varying torque loads to be applied to the is drill string. When the torque is applied to the drill string from the drilling machine, large amounts of energy are stored, which are released when the drill bit suddenly breaks loose. Uncontrolled release of energy causes extreme strain on the drill string and drill bit. Today's controllable drilling systems are provided with a lot of electromechanical equipment1 which may be damaged by extreme shocks and vibra-tions.</p>

<p>In the present invention the object is to provide a simple, suitable and reliable embodiment ot a tool which gives the s drill bit an axial movement of retraction when the torque on the drill string exceeds a predetermined value. Such a device is particular]-y well suited for modern drill bits with fixed cutters, in which industrially produced diamonds of high hardness are used, but in which the structure is brittle arid sensitiVe to impacts.</p>

<p>Within the oil industry problems associated with overloading of the drill string are well known, and there has been con-tinuous work on different solutions to remedy the problems.</p>

<p>In this connection reference is made to the following pat-io ents: NO 315209 GB 5,083,623 US 4,281,726 EP 065 601 Al US 4,443,206 In principle the present invention has the same functionality as that of NO 315209, but the technical design is substan-tially different.</p>

<p>Relative to the state of the art, the object of the invention is to provide a device which gives the tool sufficient strength and robustness to withstand the extreme strain to which it is subjected. The tool is designed in such a way that in the event of torque overload it contracts so as to s reduce the axial load on the drill bit. Relative to the state of the art, the invention has significant advantages while at the same time it is simpler and more robust. Thereby the risk of breakdowns is reduced, which is extremely important in connection with carrying out costly drilling operations. In addition, substantially smaller costs for production and maintenance are achieved.</p>

<p>This is achieved in accordance with the invention by two cy-lindrical string parts being concentrically supported in each other and arranged for limited axial movement relative to each other. The string parts are torsionally rigidly con-s nected through toothing with axial movement. Additionally, the string parts are supported in guide elements and seal elements.</p>

<p>When drilling operations are performed with a mud motor, the torsion on the drill string and drill bit will be approxi-io mately proportional to the fluid pressure in the drill string in front of the mud motor. In the present invention this flu- id pressure is utilized to generate an axial force contract-ing, at a predetermined fluid pressure. the tool axially, thereby relieving the drill bit when the torque exceeds a preset value. A spring disposed centrally provides for reset-ting of the axial movement whenever the fluid pressure falls again. In this way the torque is kept approximately constant within an optimum range, and undesired vibrations and impacts damaging the shears on the drill bit are avoided. Dampening of vibrations and impacts also provides significantly in-creased drilling speed. A reduction in vibrations and impacts has a favourable effect on components in the measuring-while- drilling system (MWD), thereby obtaining increased reliabil-ity. All in all a tool with the described device gives great economiC gains-A tool with the described device is fitted into the drill string just behind a drilling-fluid- driven mo-tor In what follows is described a non-limiting example of a pre- ferred embodiment which is visualized in the accompanying drawings, in which: Figure 1 shows a system overview with the too]. located in the drill string; and Figure 2 shows the tool with a section made therein.</p>

<p>In figure 1, 1 denotes the tool fitted between a drill string and a drilling-fluid-driven motor 3 connected to the drill bit 4'. Behind the drill bit are placed pads 18 for direc-s tional control.</p>

<p>Figure 2 shows the tool 1 with a section made therein. Two cylindrical string parts 2 and 3 are concentrically supported relative to each other through guide elements and seals 5 and 8. On the string parts are arranged adapters 19 and 11 with threads for connection to the drill string 20 and the mud mo- tor 3. The string parts are also connected through a rota-tionally rigid toothing 9 with axial movement. The toothing provides for the torque 14 to be transferred through the tool. In the toothing part of the string part 2 is arranged a is screwed-on guide element 12. The toothing 9 is protected from contaminants with seals 8 and 10.</p>

<p>The spring 6 is placed concentrically and with abutment 21 on the adapter 11 and, at the opposite end, on the lugs 4 on the string part 2. The string part 2 is provided with holes 7 for o the fluid pressure to fill in the space to the inner periph-ery 16 of the string part 3.</p>

<p>Axial movement iS is achieved when the differential fluid pressure internally and externally of the tool 1 generates a force on the differential area between the surfaces 16 and 17 exceeding the pre-tensioned spring force of the spring 6.</p>

<p>When the mud motor has an approximately linear torque/pressure characteristic, the fluid pressure providing axial movement 15 will follow the torque. Thereby the desired torque range for axial movement can be determined by the o spring force and differential area between the circle periph-eries 16 and 17.</p>

Claims (2)

1. <p>Claims 1. A device for a tool (1) for use in drilling with a mud motor

   (3) and rotary drill bit (4), the aim of the tool being to provide an axial movement (15) of the drill bit (4) at a predetermined torque (14), characterized in that two cylindrical string parts (2) and (3) are concentrically supported with seals and guide elements (5), (8), (10) and that between the periphery (17) of the string part (2) and the periphery (16) of the string part (3) is arranged an area which is exposed to a pressure differential between an internal pressure (13) and an external re-turn pressure (22)
2. 2. The device as described in claim 1, c Ii a r a c - S t e r i z e d i n that the torque (14) is trans-ferred by axially movable toothing (9).</p>

   <p>3 The device as described in claims 1 and 2, c h a -r a c t e r i z a d ii that a pre-tensioned spring (6) is arranged, supported concentrically.</p>