Data processing: structural design – modeling – simulation – and em – Simulating nonelectrical device or system – Mechanical
Reexamination Certificate
1998-09-24
2001-06-12
Teska, Kevin J. (Department: 2763)
Data processing: structural design, modeling, simulation, and em
Simulating nonelectrical device or system
Mechanical
C703S001000, C706S929000, C702S009000
Reexamination Certificate
active
06246974
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The Invention relates to rotary drag-type drill bits for use in drilling holes in subsurface formations. In particular, the invention is a method for determining operating characteristics of a rotary drag-type drill bit due to forces acting on its cutting elements.
2. Description of Related Art
The invention is particularly, but not exclusively, applicable to drill bits in which some or all of the cutters are preform cutters formed, at least in part, from polycrystalline diamond or other superhard material. One common form of cutter comprises a tablet, usually circular or part-circular, made up of a superhard table of polycrystalline diamond, providing the front cutting face of the cutter, bonded to a substrate which is usually of cemented tungsten carbide.
The bit body may be machined from solid metal, usually steel, or may be molded using a powder metallurgy process in which tungsten carbide power is infiltrated with a metal alloy binder in a furnace so as to form a hard matrix.
The cutters on the drill bit have cutting edges which, together, define an overall cutting profile which defines the surface shape of the bottom of the borehole which the bit drills. Preferably the cutting profile is substantially continuous over the leading face of the bit so as to form a comparatively smooth bottom hole profile.
The contribution which an individual cutter makes to the cutting action of the bit, and, in particular, to the forces acting on the bit, is subject to a number of variables. For example, such factors will vary according to the axial and radial position of each cutter relative to the other cutters. Thus, if a cutting element is radially located on the bit so that its path of movement partly overlaps the path of movement of a preceding cutter, as the bit rotates, it will be subject to lower forces than would be the case if it were radially positioned so that such overlapping did not occur, or occurred to a lesser extent, since the leading cutter will already have removed some material from the path swept by the following cutter.
Similarly, a cutter which is axially positioned so that it projects further than another similar cutter from the surface of the bit body may be subject to higher forces than said cutter. In practice the action of each cutter may be affected by the action of a number of other cutters which are at adjacent relative radial and axial positions. It will be appreciated that such cutters will not necessarily be directly adjacent one another on the actual bit body but may well be angularly displaced circumferentially from one another by a considerable distance.
In order to determine the forces acting on a particular drill bit in use, such as the effect of the cutters on weight-on-bit, torque, and any out of balance force and out of balance angle for the bit, it is desirable to be able to make an analysis of the contribution to such forces by individual cutters. This enables the force characteristics of a particular bit design to be determined and the effect of modification of the design, for example by re-positioning cutters, to be studied.
It is common practice to use computers to model and analyze bit designs and various methods of analysis have been proposed. It will be appreciated that such analysis may conveniently be carried out by constructing a computerized model or representation of a particular bit design, certain operating characteristics of the bit then being determined or estimated by a computer program which performs a series of steps on the computerized model of the bit.
The present invention sets out to provide a novel and improved method of determining characteristics of a drill bit design, and particularly for estimating the effect of cutter placement on the forces acting on the bit in use.
The method will be defined by a series of analytical steps and, for convenience and to assist understanding, such steps will be described as if being applied to physical elements. However, it will be appreciated that in practice such methods lend themselves to performance using a computer and the described steps will normally in practice be embodied in a computer program.
SUMMARY OF THE INVENTION
According to the invention there is provided a method of determining characteristics of a rotary drag-type drill bit of the kind comprising a plurality of cutters mounted on a bit body having an axis of rotation, the method comprising the steps of:
(a) creating a representation of the shapes of said cutters and their locations and orientations with respect to the bit axis;
(b) creating a plane which is fixed in relation to a selected one of said cutters;
(c) projecting on to the fixed plane the shape of said selected one of the cutters;
(d) overlaying the projection of the selected cutter with a two-dimensional array of two-dimensional cells which are smaller in area than the projection;
(e) assigning a first marker to those cells of the array which overlie the projection of the selected cutter;
(f) rotating the cutters about the bit axis until all the other cutters have passed through said plane at least once;
(g) moving the cutters axially while being rotated about the bit axis so as to represent the axial movement of the bit during drilling;
(h) projecting the shapes of said other cutters on to said plane, as they pass through the plane;
(i) assigning a second marker to those cells of the array which overlie both the projection of the selected cutter and the projections of any of the other cutters;
(j) determining one or more parameters of the region of the array which remains defined by cells having only said first marker; and
(k) estimating from said parameter or parameters one or more forces which will act at the location of said selected cutter in an actual drill bit.
Said plane intersects the selected cutter and may pass through the axis of rotation of the bit.
In the case where the plane passes through the axis of rotation of the bit, the projection of the shape of the selected cutter, and the projections of the shapes of the other cutters, will usually be normal to said plane. However, methods are possible where the direction of projection is not normal to the plane, as will be described.
The two-dimensional cells may be of any shape but are preferably rectangular. For example the cells may be square.
In step (e) of the method, said second marker may be assigned to cells of the array which do not overlie the projection of the selected cutter.
In any of the methods according to the invention the cutters are moved axially while being rotated about the bit axis so as to simulate the axial movement of the bit during drilling. Preferably the cutters are rotated about the bit axis in a direction which corresponds to reverse rotation of the bit, and are moved axially in a direction which corresponds to withdrawal of the bit from a borehole being drilled.
Preferably rotation of the cutters is continued until no projection of the other cutters overlies the projection of the selected cutter as the other cutters pass through the plane.
Preferably the steps of the method are carried out for all of the cutters, each being the selected cutter in turn.
The parameters which are determined of the region of the array which remains defined by cells having only said first marker may be selected from the cut area, shear length, moments of area, and second moments of area defined by said cells. The calculation of such parameters will be described in further detail below.
Preferably the method includes the further step of combining the forces acting at the respective cutters to estimate force parameters for the drill bit as a whole. For example, said force parameters may be selected from weight-on-bit, torque, out of balance force and out of balance angle.
In some forms of analysis it may be assumed that the cutters rotate about the central axis of the bit. However, as is well known, bits are sometimes subject to “bit whirl” where the rotating bit precesses around the walls of the borehole, as the bit rotates, with the re
Jelley David John
Wilcox Nigel Shaun
Camco International (UK) Limited
Daly Jeffery E.
Sergent Douglas W.
Teska Kevin J.
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