Boring or penetrating the earth – With tool shaft detail – Shaft carried guide or protector
Reexamination Certificate
2002-01-23
2004-08-17
Bagnell, David (Department: 3672)
Boring or penetrating the earth
With tool shaft detail
Shaft carried guide or protector
C175S325200, C166S241200
Reexamination Certificate
active
06776247
ABSTRACT:
The present invention relates to stabiliser tool and a wedge assembly for use in stabiliser tools that stabilise the operation of a drill string in a borehole drilling through the earth's surface.
BACKGROUND OF THE INVENTION
In operation boreholes are initially drilled as vertical holes and may then curve to an inclined or horizontal direction. Maintaining of complete control is very important but very difficult to achieve. A borehole drill may, for example, include three cutters or reamers, which are circumferentially spaced from one another. When the drill bit is inclined to the original axis, the space between the cutters can cause erratic or irregular operation in negotiating a curve.
So as to maintain control of the drill string, it is known to use a stabiliser tool having projecting wear pad assemblies. Generally a stabiliser tool includes recesses containing projecting pad assemblies arranged in a spaced configuration, with each pad assembly generally having two outer blocks and a central wedge block. The recesses have at least two side walls that diverge radially inwardly by an included angle providing an undercut recess surface. The outer blocks include diverging outer side walls of the same angle as the recess side walls. The outer walls are retained in abutment with the undercut recess surface by the wedge block acting on the inner walls of the outer blocks, the central wedge block having inwardly converging walls of the same angle as the diverging inner walls of the outer blocks. An appropriate bolt draws the central wedge into the gap between the outer blocks and causes them to move outwardly into firm abutment with the undercut recess thus locking them in place. The advantage of this configuration is that to repair or replace the wear pad assemblies one only needs to remove the central wedge block between the two outer blocks that then can be moved towards each other and away from the recess walls that locked them in place.
The difficulty with that configuration is that if the bolt locking the central wedge assembly in the recess were to break the central wedge may come away from the recess. This then allows the outer blocks to move away from the recess side walls and away from the recess. The end result is that the drill string may be jammed in a borehole, whereupon abandonment is unavoidable. In addition, not only is a multi-block wear pad assembly relatively expensive to manufacture, the manufacture of a recess with inwardly diverging side walls is also a complex task. Since the cost of the drill string is quite high it is an object of the present invention to provide improvements in stabiliser tools which overcome at least some of the abovementioned problems or provide a useful alternative.
BRIEF SUMMARY OF THE INVENTION
In one aspect of the invention a drill string stabiliser tool has a body having an outer surface symmetrical about an axis of rotation, and a plurality of recesses extending inwardly from said outer surface;
said recesses each including opposite surfaces which converge inwardly from said body outer surface;
a plurality of pad assemblies in respective said recesses, each said pad assembly including a wedge block with converging radially inwardly opposite side surfaces complementary abutted by said recess surfaces;
each said wedge block having a relatively hard outer wear surface located radially outwardly from said body surface, and a bolt securing said wedge block to said body;
characterised by the outer side surfaces of the wedge block converging inwardly at a pre-determined angle and the opposite surfaces of said recesses converging inwardly at the same pre-determined angle whereby said angle is so chosen to provide an interference fit between the outer side surfaces of said wedge block and opposite surfaces of said recesses.
It has been surprisingly found that, even under immense forces that may be imposed on the wedge (wear) blocks and that are transmitted to the inwardly converging recess surfaces, the interference fit locks the wedge blocks in their recesses and resists the tendency of them to be removed by compressive forces, turning moments, or both even if the central bolt locking the wedge were to break. This then allows the drill string to be removed from the borehole and the pad assembly to be repaired without having to be abandoned.
This is an unexpected result for intuitively it is thought that a recess whose sides are inwardly converging would need a locking mechanism, such as a bolt to hold the wedge block in the recesses. It has previously been assumed that an engaging member such as a bolt passing through the assembly locks the pad assemblies in place. Accordingly there have been a number of proposals for secondary locking mechanisms to prevent the loss of a drill string. It has however been the unexpected discovery by the present inventor, that such an interference fit provides the necessary locking force to keep the assembly locked in place and that the central bolt is essentially only required to urge the wedge block inwardly and outwardly to achieve or break the interference fit.
To assist in achieving an interference fit, the total size (or width) of the wedge block is preferably slightly larger (wider) than that of the recess. This ensures that an interference fit is properly achieved when the wedge block is urged into the recess. Generally one would calculate the width of the recess and the wedge block would then be manufactured to be of slightly larger dimension than the dimensions of the recess. In preference it has been found that if the wedge block is {fraction (3/1000)} of an inch greater in width than the remaining width of the recess an effective interference fit occurs.
Thus to assemble the pad assembly a bolt is used to draw the central wedge into the recess resulting in an interference fit locking the assembly in place. To remove the pad assembly it is also necessary for the bolt to urge the wedge block outwardly and thus break the interference fit. Accordingly in use even if the central bolt were to break the interference fit has been found to lock the assembly in place.
In preference the predetermined angle depends on the material from which the assembly is manufactured. If made from steel, an angle of some 2 degrees has been found to produce an interference fit. However a range of angles of between 1.5 to 3.5 degrees has also been found to achieve an interference fit if the angle is greater or smaller than that range it has been found that an interference fit is not achieved and the wedge block is then only held in place by the bolt. If this were to break in a borehole than this could lead to the pad assembly becoming loose and being jammed in the borehole. Preferably the angle is some 2 degrees.
Further in the invention, there may be provided hard tipped cutters (for example polycrystallised diamond cutters) projecting outwardly from the sides of the wedge block, and if these are located towards the top end of the stabiliser and above the reamers of a drill string, the wall of a small bore hole will be enlarged upon withdrawal, thereby reducing likelihood of the drill string being jammed and lost in the hole. A negative rake of the cutters will still provide a cutting action, but may be effective in firmly compacting loose earth or rock upon withdrawal and avoid exacerbating the resistance to withdrawal by unnecessarily adding loose material between the cutters and reamers.
Preferably the wear blocks are positioned in a helical or spiral pattern that ensures that the drill stem upwardly of the reamers remains centralised, and during drilling, being above reamers (when used) the blocks will engage borehole surfaces which are more regular both in size and in smoothness than would exist if the reamers were not present. The stiffness below the stabiliser body of a reamer body and the bit assembly does much to avoid the difficulties previously encountered due to the spacing between the bits and the bit assembly, and thereby much improves the ability of an operator to control the direction of the bit mov
Bagnell David
Gay Jennifer H
Gearhart Australia Ltd.
Nixon & Peabody LLP
Studebaker Donald R.
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