Drilling system

Details Drilling system Drilling system

2001-10-17

2004-06-15

Bagnell, David (Department: 3672)

Boring or penetrating the earth

Means traveling with tool to constrain tool to bore along...

C175S293000, C175S061000, C175S019000

Reexamination Certificate

active

06749031

ABSTRACT:

FIELD OF THE INVENTION
The invention concerns a drilling system having a drilling head fixed to a drill string which comprises an outer pipe and a percussion string inserted therein, wherein the percussion string comprises a plurality of rods which bear against each other with their end faces.
DESCRIPTION OF THE RELATED ART
A drilling system of that kind is known from EP 0 387 218 B1. This involves a rock drilling arrangement for producing straight boreholes for receiving anchors for buildings or explosive charges for carrying out rock blasting operations. In that case the cylindrical shank of the drilling bit is mounted axially displaceably to the front end of the outer pipe by way of a cylindrical guide which is several centimetres long and which is in contact with a small clearance. The same applies in regard to the free end of the rear drill rod against which a hammer or percussion piston strikes to apply the percussion forces. Each individual rod is guided in the region of two bushes at two positions on its length. Provided in the region of the guides for the percussion rod are axially extending ducts for passing therethrough a flushing medium, which make it possible for a flushing medium to be conveyed towards the drilling head from the rear end of the drill string through the intermediate space between the outer pipe and the percussion string or through the axially extending ducts between the outer pipe and the percussion string. The end faces of the individual rods of the percussion string, which bear against each other, extend in the radial direction so as to afford a maximum effective surface area for transmission of the axially acting percussion forces.
The arrangement described in EP 0 387 218 B1 has some major advantages which are essentially that the inner percussion string comprises various individual rods which bear against each other without screwing. The individual short rod has a natural frequency which is very much higher than a long screwed percussion string. Thus, in terms of transmission of the percussion force, very much harder and undamped transmission of the percussion force is afforded by way of a plurality of short rods which bear against each other without a screw connection. Added to that is greater ease of handling during the drilling operation. After the drilling arrangement is advanced by the length of an outer pipe section or an inner rod, the rotary and percussion drive is separated from the drill string and a fresh inner rod and a fresh outer pipe is introduced into the drill string. That situation involves time savings by virtue of the fact that the inner rod to be inserted does not have to be screwed in place.
The arrangement known from the above-quoted document is however suitable by virtue of its structure only for making bores which extend precisely in the axial direction of the drill string.
The object of the present invention is to provide a drilling system which permits a greater variation in the drilling direction.
In accordance with the invention that object is attained in that the outer pipe is adapted to be deformable along its longitudinal axis and the end faces of two rods which bear against each other are such that they bear against each other substantially in surface contact upon inclined positioning of the axes of the two rods relative to each other.
Drilling systems with elastically bendable outer pipes—so-called directional drilling systems—are known from the state of the art, for example from DE 196 12 902 A1. That publication states that a drill string having a drilling head which produces a curved borehole configuration is used for directional drilling. In straight-line drilling the drilling head is rotated at a uniform, generally low angular speed so that the force deflecting the drilling head is uniformly distributed to the entire periphery of the drilling head and is thus cancelled out. For drilling a radius, the drilling head remains in a given angular position without drilling drive so that it follows the curved path which is predetermined by virtue of its structural features. In that case the drilling heads may be of very different configurations. The drill string is usually mounted on a rail-guided sliding carriage connected to a linear drive and has a rotary or rotary-percussion drive with which the string can be caused to rotate and possibly also driven into the ground. In the previously known directional drilling systems the outer string was in principle used for transmission of the percussion force. Besides the above-described problem that the long outer string has a low natural frequency and is of a high mass, that gave rise to an additional problem that the wall friction of the outer string which is guided in the curvedly extending borehole in the earth nullifies a considerable proportion of the percussion energy. Furthermore, in addition to the mass of the outer pipe, the mass of the flushing medium contained in the outer pipe also has to be accelerated by the percussion drive. Finally, a hammer blow on the rear end of a curved pipe produces not only axial acceleration but also a bending force. In practice it has been found that the percussion force acting on the rear end of the drill string scarcely arrives in the region of the drilling head.
The inner string which can be found for example in FIGS. 6 and 7 of DE 196 12 902 A1 could not be used for percussion force transmission purposes. Either it was proposed that the individual elements of the inner string are connected together by way of universal joints which are destroyed by ongoing percussion forces. Alternatively, it was proposed that the universal joints be omitted, if the inner string is sufficiently flexible. With a high degree of flexibility however, it is not possible to achieve a sufficiently great percussion force transmission effect.
SUMMARY OF THE INVENTION
The proposal in accordance with the present invention, to provide a drilling system with rods which bear against each other in unscrewed relationship as a directional drilling system with a flexible outer pipe permits the transmission of percussion force by way of the inner percussion string if the end faces of two rods, which bear against each other, are so designed that they bear against each other substantially in surface contact even upon inclined positioning of the axes of the two rods. In other words, based on the drilling system described in the opening part of this specification and disclosed in EP 0 387 218 B1, end faces which depart from the flat radial shape had to be proposed, so as to ensure effective transmission of percussion forces even in a situation involving bending of the outer pipe which results in inclined positioning of the longitudinal axes of two drill rods relative to each other.
In comparison with the previously known transmission of percussion forces in directional drilling systems by way of the outer pipe, percussion force transmission by way of an inner percussion string has the crucial advantage that the percussion force cannot be reduced by virtue of friction of the percussion string against the wall of the borehole. As a general rule a flushing medium is passed between the outer pipe and the inner string, the flushing medium comprising for example water with swellable clay (bentonite). The aqueous swellable clay is of a viscous to pasty consistency and produces relatively slight frictional resistances upon movement of the percussion string with respect to the outer pipe. In that respect the flushing medium itself is not accelerated by the hammer blows and cannot absorb any percussion energy.
The hammer blows are transmitted by short straight rod sections of the inner string, in which respect no bending forces can occur as the individual rods of the inner string are not curved.
An essential feature of the invention provides that, in the case of the inner percussion string of the directional drilling system according to the invention, no fixed connection exists between the ends of the individual rods of the percussion string. In particular, screwing of the

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