The invention relates to a machining unit for milling material off an, in the position of application, surrounding pipe wall in a borehole in an underground structure, wherein the machining unit is connected to a rotary-drive unit arranged to provide a controlled peripheral speed in a milling unit, is connected to an axial-drive unit arranged to provide a controlled axial displacement of at least the milling unit, and is connected to a conduit for the supply of fluid.

When a hydrocarbon well is to be abandoned and left, it must be plugged in such a way that the wellhead and other installations above the wellbore can be removed without there being any risk of formation fluids leaking out of the wellbore. Plugging usually presupposes that a portion of casing bounding the borehole against the surrounding underground formation will have to be removed so that the plug may come into direct contact with the formation. The removal of the casing portion is carried out with a chipping tool, for example a radially expanding cutting tool which, by rotation and axial displacement, gradually removes the casing material and possible cement that has filled the annulus between the casing and the structure. A well-known problem with today's milling methods attaches to the removal of milled chips from the milling area. This is due to the fact that chips that are milled according to the turning principle, that is to say with cutting tools that, when rotated around the centre axis of the wellbore, are in continuous cutting engagement with the casing, become entangled because of their great length and form accumulations of chips that cannot be transported away by means of circulating liquid. The invention has for its object to remedy or reduce at least one of the drawbacks of the prior art or at least provide a useful alternative to the prior art.

The object is achieved through the features, which are specified in the description below and in the claims that follow.

A machining unit comprising a milling unit which, by radial displacement of at least two rotatable milling heads, can engage with a pipe wall encircling the machining unit is provided. Each milling head is connected to a driving motor. The milling head has a diameter, which is considerably smaller than the pipe to be worked. Thereby material is milled off the pipe wall in short cuts, which results in short chips being formed during the milling. The machining unit is provided with or connected to a rotary-drive unit which may set at least the milling unit into rotation around its centre axis so that the milling heads are moved in the peripheral direction of the pipe wall, and the machining unit is connected to an axial-drive unit which ensures that the milling heads achieve a displacement in the axial direction of the pipe, both the rotational speed and the feed rate of the milling heads in the radial, peripheral and axial directions of the pipe being controlled The rotary-drive unit may be arranged at or in the machining unit or remotely therefrom, as the machining unit may be connected to a cable or coiled tubing or a rotatable pipe string. At least when the machining unit is connected to a cable, the rotary unit is provided with engagement means, which are adapted to absorb the reaction forces, arising when the machining unit is set into rotation, by said engagement means engaging with the pipe wall in a rotation-preventing manner. By connection to a rotatable pipe string, the machining unit may be set into rotation by the rotation of the pipe string when this is connected to a rotary table, a top drive drilling machine or the like at the surface, for example on a drilling rig.

The displacement of the machining unit in the axial direction of the pipe is provided by means of the axial-drive unit which may be a feeding unit for the pipe string, coiled tubing or cable arranged on the surface, or the axial-drive unit may be arranged at or integrated in the machining unit and may move, by the abutment of driving wheels against the pipe wall, in the axial direction of the pipe. In an advantageous embodiment, the axial-drive unit is provided with a feeding device of the type described in the applicant's own NO patent 336371 , in which several feeder wheels are radially displaceable between a retracted, inactive position and an extended, active position in which the feeder wheels are resting against the casing. The centre axes of the feeder wheels are slanted relative to the centre axis of said machining unit. When the machining unit is rotated around its own centre axis, the slant of the feeder wheels will have the effect of making the feeder wheels follow a helical line, so that the machining unit is moved in the axial direction without a push force having been applied to the pipe string. A suitable slant being chosen for the feeder wheels, the machining unit may thereby achieve the desired feed rate.

The radial displacement of the milling unit(s) of the machining unit may be brought about by each milling unit being supported on a projecting first end portion of an arm, which is supported, at a second end portion, in the machining unit and is pivotable in the radial direction of the machining unit. The arm is connected to an actuator providing the radial displacement of the milling unit. The actuator may be a hydraulically operated linear actuator, preferably provided with an annular piston with a working direction coinciding with the axial direction of the machining unit, and all the milling units are preferably displaced by the same actuator. The conversion of the axial movement of the actuator into the radial rotational movement of the arm may be carried out in various ways, for example by the axially movable element of the actuator and the arm being interconnected by a linking arm, or by a portion of the axially movable element of the actuator being moved along a ramp surface facing radially inwards on the arm. The retraction of the arm may be provided by the actuator or by means of other means, for example one or more resilient elements. In a preferred embodiment, the annular piston is returned by means of a helical spring resting against the piston. Each milling unit is connected via transmission elements to the driving motor, which provides rotation of the milling units. All the milling units may be connected to the same driving motor, for example via a driveshaft arranged centrally. In a preferred embodiment, the driveshaft is a tubular one, encircling a centre bore extending at least through a portion of the machining unit. On account of the balancing of the reaction forces to which the machining unit is subjected by the milling of the material of the pipe wall by the milling units, it is an advantage if the milling units are evenly distributed around the periphery of the machining unit.

One or more of the driving motors of the milling heads, the rotary-drive unit and the axial-drive unit of the machining unit is/are preferably driven by a pressurized fluid, typically a liquid, which is sup- plied to the machining unit through a conduit connected to a reservoir and a pump on the surface installation.

It is an advantage if the milling heads are supplied with a cooling and lubricating liquid. The liquid may be parts of the amount of pressurized liquid that has been supplied to the machining unit for the operation of its motors. Milled material may be deposited near the milling area, typically by means of gravity, which provides for the milled material to sink and be deposited, or possibly it to can be transported out of the pipe together with the return flow of liquid from the machining unit via an annulus between the machining unit and the pipe wall.

The invention is defined by the independent claim. The dependent claims define advantageous embodiments of the invention.

The invention relates, more specifically, to a machining unit for milling material off an, in the position of application, surrounding pipe wall in a borehole in an underground structure, wherein the machining unit is connected to a rotary-drive unit arranged to provide a controlled peripheral speed in a milling unit, is connected to an axial-drive unit arranged to provide a controlled axial displace- ment of at least the milling unit, and is connected to a conduit for the supply of fluid, characterized by the milling unit being provided with at least two rotatable milling heads that are evenly distributed around the periphery of the machining unit and which are displaceable, by means of a milling- head actuator, in the radial direction of milling unit, and which is connected via a transmission to a pressure-fluid-driven motor arranged in the machining unit. The milling head may be supported in a freely projecting first end portion of a pendulum arm, which is provided with a hinge joint at a second end portion and is pivotably mounted in the milling unit.

The pendulum arm may be connected to the milling-head actuator in the form of a linear actuator, which is connected to the pendulum arm via a link arm. The machining unit may be connected to a rotatable drill-pipe string, the rotary-drive unit of the machining unit being a drill-string rotary unit arranged on a surface installation.

The axial-drive unit may be arranged in the machining unit and may be provided with several feeder wheels lying in a plane which is slanted relative to a plane that is perpendicular to the centre axis of the machining unit, the feeder wheels being displaceable, by means of an actuator, between a retracted, inactive position and an active position in which they rest against the pipe wall.

A fluid-supply conduit may be arranged downstream of the pressure-fluid-driven motor and is provided with one or more first outlets arranged near the milling unit.

The fluid-supply conduit may be provided with one or more second outlets arranged remotely from the milling unit.

A settable packer may surround a portion of the machining unit, it being arranged to separate a milling area in the wellbore from an annulus forming a flow path for return fluid from the machining unit.

A fluid-return conduit may extend out from the milling unit, it being provided, remotely from the mill- ing unit, with one or more inlets arranged to hold back material milled by the milling unit.

The fluid-return conduit may be connected to one or more return outlets discharging into an annulus forming a flow path for return fluid from the machining unit.

In what follows, an example of a preferred embodiment is described, which is visualized in the accompanying drawings, in which: Figure 1 shows a side view of a machining unit according to the invention while working a portion of a cased wellbore, in which an upper portion of a rotatable drill string and also an associated surface installation are shown on a smaller scale;

Figure 2 shows an axial section, on a larger scale, through the machining unit, in which the milling heads of a milling unit are in an inactive, retracted position; Figure 3 shows a ground plan, on a larger scale, of the machining unit with the milling heads in an active, extended position; and

Figure 4 shows an axial section of the milling unit with the milling heads in an active, extended position.

Reference is first made to figure 1 , in which a machining unit 1 according to the invention is arranged in an end portion of a pipe string 2 and is placed in a borehole 4 which is bounded in a manner known per se against an underground structure 42 by a casing 41 . In an annulus between the casing 41 and the underground structure 42, cement 43 has been filled in in a manner known per se. The casing 41 forms an internal pipe wall 41 1 . The machining unit 1 is connected to a rotary-drive unit 31 shown here as a top-drive drilling machine arranged on a surface installation 3 and connected to the machining unit 1 via the pipe string 2. Reference is now made to figures 2, 3 and 4. A milling unit 1 1 is provided with three milling heads 1 1 1 , which are each arranged in a first end portion 1 12a of a respective pendulum arm 1 12 attached in a rotationally rigid manner to an axle 1 1 1 a which is rotatably supported in the pendulum arm 1 12. The pendulum arms 1 12 are attached, at a second end portion 1 12b, by a hinge joint 1 12c to an end portion of a housing 19, which, for reasons of installation, is sectioned, several housing sections 19a-19l being joined together in a releasable manner.

The milling heads 1 1 1 are of a considerably smaller diameter than the casing 41 , typically in the order of 1/3-1/4 of the diameter of the casing 41 .

Each pendulum arm 1 12 is connected at its first end portion 1 12a to a milling-head actuator 12 via link arms 125, a piston-rod head 124, and several piston rods 123, which are attached to a linear actuator 121 , shown here as an annular piston 121 arranged in an actuator annulus 127 in the housing 19. The piston 121 pushes the milling heads 1 1 1 into an active position when pressurized fluid is supplied to the actuator annulus 127 through a port 128. Retraction of the piston heads 1 1 1 is effected by the pressurized fluid being drained from the actuator annulus 127 and the piston being pushed back by a return spring 122. The piston-rod head 124 is displaceable along a guide, which, in the embodiment shown, is a fluid- return pipeline 17 arranged centrically in the milling unit 1 1 and projecting from the milling unit 1 1 .

The milling heads 1 1 1 are connected to a driving motor 13 via a transmission, shown here as a tubular driveshaft 131 , on which a first toothed wheel 132 is fixed, meshing with a second toothed wheel 133 fixed on the axle 1 1 1 a of each milling head 1 1 . The toothed wheels 132, 133 are formed in such a way that they are in sufficient engagement with each other in all the possible angular positions of the pendulum arms 1 12. The motor 13 is driven by pressurized fluid, for example a drilling fluid, which is supplied from a fluid plant 32 on the surface installation 3 (see figure 1) via a fluid-supply conduit 21 , shown here as a centre bore in the pipe string 2.

An axial-drive unit 15 is provided with several feeder wheels 151 lying in a plane which is slanted relative to a plane that is perpendicular to the centre axis of the machining unit 1 , the feeder wheels 151 being displaceable, by means of an actuator 152, between a retracted, inactive position and an active position in which they are resting against the pipe wall 41 1 . The axial-drive unit 15 is preferably of a design as described in WO 2013/129938, which is hereby incorporated by reference.

In the machining unit 1 , in the extension of the driving motor 13, a fluid-supply conduit 16 is provid- ed, shown here as an annulus encircling a portion of the tubular driveshaft 131 . The fluid-supply conduit 16 is provided with several first outlets 161 arranged on the periphery of the milling unit 1 1 near the milling heads 1 1 1 . The fluid-supply conduit 16 is also provided with several second outlets 162 discharging at the periphery of the milling unit 1 1 upstream of the first outlets 161 .

A settable packer 18 encircles a portion of the milling unit 1 1 between the first and second outlets 161 , 162. The packer 18 separates a milling area 44 from an annulus 45 (see figure 1 ) which functions, in a manner known per se, as a return flow path for fluid from the machining unit 1 . The second outlet 162 functions as a weir for fluid, the driving motor 13 usually having a greater need for driving fluid than what the milling heads 1 1 1 have for cooling and lubricating fluid.

From a lower portion of the tubular driveshaft 131 , a tubular fluid-return conduit 17 extends out of the milling unit 1 1 . In a freely projecting end portion, several return inlets 172 are arranged, which are designed to hold back milled metal chips etc. deposited in the wellbore 4 below the machining unit 1 , when fluid which has been supplied to the milling heads 1 1 1 is entering the fluid-return conduit 17. The return inlets are typically provided with a suitable filter (not shown). The fluid-return conduit 17 discharges inside the tubular driveshaft 131 , there being several openings 134 arranged in the driveshaft, which are in fluid communication with several return-fluid outlets 173 (see figure 1) arranged in the milling unit 1 1 on the annulus side of the packer 18.

It should be noted that all the above-mentioned embodiments illustrate the invention, but do not limit it, and persons skilled in the art may construct many alternative embodiments without departing from the scope of the attached claims. In the claims, reference numbers in parentheses are not to be regarded as restrictive.

The use of the verb "to comprise" and its different forms does not exclude the presence of elements or steps that are not mentioned in the claims. The indefinite article "a" or "an" before an element does not exclude the presence of several such elements.

The fact that some features are indicated in mutually different dependent claims does not indicate that a combination of these features cannot be used with advantage.