[0001] This invention relates to casing cutters.
[0002] Hydrocarbon wells are commonly lined with steel tubing known as casing. Casing normally
has one of a number of standardised internal diameters. At any given depth in a lined
hydrocarbon well, there may be a single casing, or two or more concentrically nested
casings of different diameters. On occasion, it may be necessary to sever installed
casing, for example to enable its removal from the well. Cutters have been developed
in the form of a tool rotated by a string to cut casing by milling action. Such a
tool consists of three arms which can be pivoted outwards (like umbrella ribs) under
the control of mud pressure in the string, so as to bear against the casing. The arms
are coated with particles of tungsten carbide (secured by brazing), and mill through
the casing as the tool is rotated by turning the string. However, when cutting through
two or more nested casings at a given depth, such a tool requires to be pulled after
cutting through an individual casing, and re-equipped with fresh milling arms. This
procedure provides fresh cutting edges for each casing, but at the economic cost of
having to trip between the successive cuttings of the concentric casings, and of also
having to provide a set of new milling arms (or a complete replacement tool) for each
casing. It is therefore an object of the invention to provide a casing cutter which
obviates or mitigates these disadvantages.
[0003] According to a first aspect of the present invention there is provided a casing cutter
of the type comprising at least one milling arm which in use pivots outwards of the
cutter about a pivot axis to trace a substantially conical path during rotation of
the cutter about a rotation axis, the cone angle of the conical path with respect
to the rotation axis increasing with increasing diameter of a casing being cut in
use of the cutter, the or each milling arm having a plurality of effective cutting
edges each having a respective offset with respect to a notional axial reference line
of the arm which passes through the respective pivot axis of that arm, the offset
of each effective cutting edge being a distinct combination of circumferential offset
and radial offset from the respective notional axial reference line whereby a different
effective cutting edge is presented to each casing when the cutter is in use to cut
a plurality of concentric casings of different diameters.
[0004] Each effective cutting edge may be a substantially continuous cutting edge, or each
effective cutting edge may comprise a relatively large number of relatively small
individual cutters mounted in a row to present respective individual cutting edges
which collectively form the effective cutting edge. The effective cutting edge may
be substantially linear.
[0005] The casing cutter preferably has three milling arms which are preferably symmetrically
arranged on the cutter. The pivot axes of the milling arms are preferably each tangential
to a common circle co-axial with the rotation axis of the cutter. The cutter may contain
a piston which is axially movable in response to hydraulic pressure within a string
of which the cutter forms part in use of the cutter, the piston linking with each
milling arm such that axial movement of the piston in one direction forces each milling
arm to pivot outwards in concert.
[0006] In the preferred form of cutter wherein the pivot axes are tangential to a common
circle, the notional axial reference line of each arm will not only pass through the
respective pivot axis, but can also be selected to pass through the rotation axis
of the cutter by being positioned to be coincident with the point of tangency of the
pivot axis with the common circle. The reference lines of each arm will coincide on
the rotation axis if each arm is pivoted outwardly of the cutter by an equal angle.
With respect to such a selected notional axial reference line, the effective cutting
edges of each arm are preferably each parallel but otherwise asymmetrical whereby
the respective offset of each effective cutting edge on a given arm has the requisite
distinction from the offset of each other effective cutting edge on that arm.
[0007] According to a second aspect of the present invention there is provided a milling
arm for adapting a casing cutter to form a casing cutter in accordance with the first
aspect of the invention, the milling arm being as defined in any of the preceding
paragraphs relating to the first aspect of the invention.
[0008] Embodiments of the invention will now be described, by way of example, with reference
to the accompanying drawings, in which:-
Fig. 1 is a side elevation of a first form of milling arm for use in a casing cutter
in accordance with the invention;
Fig. 2 is a front elevation of the first form of milling arm as shown in Fig. 1;
Fig. 3 is two end elevations of the first form of milling arm as shown in Fig. 1;
Fig. 4 is a side elevation of a second form of milling arm for use in a casing cutter
in accordance with the invention;
Fig. 5 is a front elevation of the second form of milling arm as shown in Fig. 4;
Fig. 6 is three end elevations of the second form of milling arm as shown in Fig.
4; and
Fig. 7 is a half-sectioned elevation of a known form of casing cutter which can readily
be modified to form a casing cutter in accordance with the invention.
[0009] Referring first to Figs. 1 and 2, the first form of milling arm 10 in accordance
with the invention comprises a body 12 which incorporates and extends radially away
from an integral pivot bore 14. The arm 10 also integrally incorporates a spur-like
heel 16 which extends generally away from the body 12. In use of the arm 10, it will
be mounted (in a symmetrical array along with (usually) two other identical such arms)
on a respective pivot pin (not shown) passing through the pivot bore 14 on a pivot
axis which is tangential to a circle centered on the rotational axis of the cutter
as a whole. The heel 16 will lie inboard of the pivot bore 14 (radially inwards in
the cutter) when the body 12 is fully retracted into the cutter to lie with its long
axis generally parallel to the rotation axis of the cutter, and downward of the pivot
bore 14.
[0010] When the tip of the heel 16 is pressed downwards, the body 12 will tend to pivot
outwards of the cutter around the axis of the pivot bore 14 so as to bring the outer
face of the body 12 into contact with the inner surface of a surrounding casing. The
outer face of the body 12 carries two rows 18 and 20 of tungsten carbide "buttons"
or frusto-conical inserts 22. Each of the rows 18 and 20 is formed by a respective
channel milled longitudinally in the outer face of the body 12 (see especially the
lower end view in Fig. 3). The inserts 22 are secured in the respective channels to
the body 12 by brazing. Although each individual insert 22 presents only part of its
larger circular edge as a cutting edge in use of the milling arm 10, the inserts 22
in each of the rows 18 and 20 collectively form a respective cutting edge which is
substantially a straight line in its effect (analogous to the manner in which the
variously profiled and set teeth of a hacksaw blade together form an effectively straight
cutting edge).
[0011] Of particular significance is the fact that the rows 18 and 20 are asymmetrically
arrayed on the body 12 (see especially Figs. 2 and 3). Considering a hypothetical
reference line running along the length of the body 12 through the middle of its transverse
section and selected so as to intersect the axis of the pivot bore 14 at right angles,
it will be seen that the two effective cutting edges formed by the rows 18 and 20
of carbide buttons 22 are each parallel to this selected reference line, but with
different offsets from the selected reference line. In particular, the effective cutting
edge formed by the row 18 (see Fig. 3) is marginally radially inwards of the effective
cutting edge formed by the row 20, but the edge formed by the row 18 is circumferentially
(or angularly) at a greater distance from the selected reference line than is the
edge formed by the row 20. (Note that the terms "radial" and "circumferential" relate
to these effective cutting edges when the casing cutter is fully assembled with the
milling arms symmetrically mounted and fully retracted so that the selected reference
lines lie parallel to the rotation axis of the cutter). Thus the effective cutting
edge of the row 18 stands proud of the milling arm 10 when the arm 10 is at zero and
low pivot angles relative to the casing cutter (as will be the situation when cutting
smaller diameter casing) but the effective cutting edge of the row 20 will stand proud
of the arm 10 at higher pivot angles (as will be the situation when cutting larger
diameter casing). This changeover is illustrated as occuring from lower to upper views
in Fig. 3.
[0012] The placement of the rows 18 and 20 on the milling arm 10 can be designed so that
the changeover between instantaneously effective cutting edges occurs at an arm pivot
angle corresponding to a diameter intermediate standard casing diameters, as is schematically
illustrated in Fig. 1. Such a placement of the rows 18 and 20 will result in only
one effective cutting edge being presented to cut an inner casing of two concentric
casings, with a changeover to the other and hitherto unused effective cutting edge
when cutting the outer casing of the two concentric casings. This results in improved
cutting efficiency, and faster cutting of multiple casing. (These advantages are at
least partially retained even if the nested and nominally concentric casings are in
fact substantially eccentric).
[0013] Correctness of this design consideration (changeover of cutting edges between casing
diameters) can be checked by ensuring that the plane formed by the locus of points
equidistant from the two effective cutting edges intersects the plane including the
selected reference line and the rotation axis of the cutter at a line which lies intermediate
the lines corresponding to the lesser and greater diameters (respectively denoted
as the points 24 and 26 in Fig. 3 where the aforementioned planes and lines appear
as lines and points respectively owing to the effectively transverse sectional nature
of Fig. 3 with respect to the foregoing three dimensional geometric analysis).
[0014] Different forms of design (and correspondingly different methods of checking their
correctness) would apply if the effective cutting edges were not parallel to the selected
reference line, or were not effectively straight over their entire length.
[0015] The outboard end of the body 12 (remote from the pivot bore 14) can be coated with
a brazed-on layer 28 of tungsten carbide particles.
[0016] The inventive principle of providing the milling arms with multiple cutting edges
so arranged that a different cutting edge presents to each casing of a different diameter
can be extended from the two-edges form of milling arm shown in Figs. 1 to 3 to a
three-edges form of milling arm as shown in Figs. 4, 5, and 6. The three-edged form
of milling arm differs fundamentally from the two-edged form only in that three rows
of tungsten carbide inserts are brazed into respective channels in the outer face
of the milling arm. The channels are differently offset such that the three effective
cutting edges present in turn to casings of three different diameters, as shown in
Fig. 6.
[0017] Fig. 7 is a half-sectioned longitudinal view of a known form of casing cutter, being
the multi-string cutter of Tri-State Oil Tool Industries Inc. as described in their
Manual No. 7008 of April 1985. This multi-string cutter has three so-called "knifes"
30 which are pivoting arms whose outer edges are coated with a layer of brazed-on
tungsten carbide particles. To modify this known form of casing cutter to provide
a casing cutter in accordance with the invention, each of the three knifes of the
cutter is replaced by a milling arm, either as described above in Figs. 1 to 3 or
in Figs. 4 to 6, or designed in accordance with the principles described above.
[0018] Thus, the milling arms in accordance with the invention can be manufactured by simple,
standard procedures, and the casing cutters in accordance with the invention can be
provided by a simple adaptation of existing casing cutters.
[0019] While certain modifications and variations of the invention have been described above,
the invention is not restricted thereto, and other modifications and variations can
be adopted without departing from the scope of the invention.
1. A casing cutter of the type comprising at least one milling arm which in use pivots
outwards of the cutter about a pivot axis to trace a substantially conical path during
rotation of the cutter about a rotation axis, the cone angle of the conical path with
respect to the rotation axis increasing with increasing diameter of a casing being
cut in use of the cutter, the or each milling arm having a plurality of effective
cutting edges each having a respective offset with respect to a notional axial reference
line of the arm which passes through the respective pivot axis of that arm, the offset
of each effective cutting edge being a distinct combination of circumferential offset
and radial offset from the respective notional axial reference line whereby a different
effective cutting edge is presented to each casing when the cutter is in use to cut
a plurality of concentric casings of different diameters.
2. A casing cutter as claimed in Claim 1, wherein each effective cutting edge is a
substantially continuous cutting edge.
3. A casing cutter as claimed in Claim 1, wherein each effective cutting edge comprises
a number of individual cutters mounted in a row to present respective individual cutting
edges which collectively form said effective cutting edge.
4. A casing cutter as claimed in any preceding claim, wherein the effective casing
edge is substantially linear.
5. A casing cutter as claimed in any preceding claim, wherein the cutter has three
milling arms symmetrically arranged on the cutter.
6. A casing cutter as claimed in any preceding claim, wherein the pivot axes of the
milling arms are tangential to a common circle co-axial with the rotation axis of
the cutter.
7. A casing cutter as claimed in any preceding claim, and including a piston axially
movable in response to hydraulic pressure within a drill string of which a cutter
forms part in use of the cutter, the piston linking with each milling arm so that
axial movement of the piston in one direction forces each milling arm to pivot outwards.