US patent specification No. 5,551,509 and International patent application PCT/GB95/02267, publication number WO 96/09460, disclose the use of a rotary milling tool which is induced to cut a window in a well casing by pressing a rotating mill downwards along the slant surface of a whipstock which is fixed within the well casing at the location where the window is to be milled. UK patent application GB 2,306,985 discloses a mill which is pressed radially into the casing to be milled by means of hydraulic pistons which protrude in a radial direction from the bit motor housing and which are actuated by the elevated pressure of the fluid which simultaneously actuates the hydraulic bit motor. The fluid is injected via a coiled tubing which is also used to pull or push the mill in longitudinal direction through the casing during the window cutting operation.

Disadvantages of the known window milling devices are that the shape and position of the window cut thereby is rather imprecise, that the window cutting operations are time consuming and generate a significant amount of sometimes coarse cuttings which are difficult to remove from the well.

A further disadvantage of the known milling devices is that the rotating mill exerts a high and variable tangential torque to the device which induces the device to run away in tangential direction. The known devices

therefore need to be guided by guide means, such as a whipstock, which is fixed firmly inside the casing to avoid the device to run away during the milling operations.

It is an object of the present invention to alleviate the disadvantages of the known milling devices.

Summary of the Invention The device according to the present invention thereto comprises a stator housing, a toppling head which is pivotably secured to the housing and equipped with a number of rotatable mills, a drive mechanism for rotating each mill relative to the toppling head, and a pivot mechanism for pivoting the toppling head relative to the housing between a contracted position in which each mill is substantially aligned with the stator housing and an extended position in which each mill protrudes at an angle in a lateral direction away from the housing.

Preferabiy the device is equipped with a pair of contra-rotatable mills that exert opposite tangential torques to the device which neutralise each other.

Such contra-rotating mills are able to cut in the casing or other well tubular a well-defined window having well-polishe rims which can be sealed off easily.

Preferably, the stator housing defines a longitudinal central plane of the device and the drive mechanism comprises a flexible drive shaft which is rotatable about an axis of rotation located in said central plane and which forms an input shaft of a gearwheel mechanism within the toppling head, which gearwheel mechanism comprises a pair of contra-rotating output shafts on which the mills are mounted.

It is also preferred that the pivot mechanism allows the toppling head to pivot relative to the stator housing about a pivot axis which is substantially orthogonal to said central plane, that the output shafts are rotatable

about axes of rotation which are parallel to each other and which are located at substantially equal distances from the central plane of the device and that the mills comprise substantially cylindrical bodies on which helical cutting teeth are mounted such that the helix angles of the cutting teeth are identical but opposite to each other and the cutting teeth of adjacent mills face each other at or near said central plane.

Suitably the drive mechanism comprises a hydraulic Moineau-type motor having a rotor which is rotatably mounted at the upper end of the stator housing and which is secured to the upper end of the flexible drive shaft and the pivot mechanism comprises a hydraulic piston which is slidably arranged within the stator housing between the outlet of the Moineau-motor and a slant upper surface of the toppling head such that in use the elevated pressure of the driving fluid discharged by the motor presses the piston against said slant upper surface of the toppling head thereby inducing the toppling head to pivot towards the extended position thereof.

In that case it is preferred that the hydraulic piston is slidably arranged around the flexible shaft and the flexible shaft is made of a resilient material and exerts a residual torque to the toppling head which torque induces the toppling head to pivot towards its contracted position in the absence of hydraulic pressure exerted to the piston.

It is also preferred that if the device is used as a casing window milling device a cuttings collecting box is secured to the housing beneath the mills and in use the hydraulic driving fluid emerging from the motor flushes cuttings from the mills into the cuttings collecting box.

Brief description of the drawings These and other features, objects and advantages of the milling device according to the present invention

will bye mande apparent by the following detailed description with reference to the drawings, in which: Fig. 1 is a side view of the milling device according to the invention with a pair of mills in a retracted position; Fig. 2 is a schematic longitudinal sectional view of the milling device of claim 1; Fig. 3 shows the milling device of Fig. 2 wherein the mills are extended and are cutting a window in a well casing; and Fig. shows an alternative embodiment of the device according to the invention where the device is used as a wellbore underreamer.

Detaile : Description of the Invention Referring to Fig. 1 there is shown a milling device having a stator housing 1, a toppling head 2 and a pair of contra-rotating twin mills 3 and 4. The twin mills 3 and 4 face each other at a central plane of symmetry of the device which is orthogonal to the plane of drawing of Fig. 1 and which intersects the plane of the drawing at central axis 5.

The toppling head 2 is pivotably suspended within the stator housing 1 by a pair of hinge pins 6 which permit the toppling head 2 to rotate about an axis of rotation 7 which is substantially orthogonal to the central axis 5 and the central plane of symmetry of the device.

The toppling head 2 comprises a gear box (not shown) having a flexible input shaft 9 and a pair of parallel output shafts 8 which carry the mills 3 and 4 and which are rotated relative to the toppling head 2 at equal rotational speeds but in opposite directions, which are illustrated by arrows 10, in response to rotation illustrated by arrow 12 of the flexible input shaft 7 relative to the stator housing 1 by means of a

Moineau-~ype or other hydraulic or electric motor (not shown).

The mills 3 and 4 comprise substantially cylindrical bodies on which helical cutting teeth 11 are mounted at similar but opposite pitch angles which teeth intermesh at the central plane of symmetry of the device.

When the milling device is lowered through the well the toppling head 2 and the mills 3 and 4 are in a retracted transportation mode in which the mills 3 and 4 are substantially parallel to the central axis 5 of the device and do not protrude from the stator housing 1, as illustrated in Figs. 1 and 2. Fig. 2 shows that in the transportation mode the flexible shaft 9 obtains a curved shape and that the toppling head 2 has a slant upper surface 13 which has a top 14 that engages the lower end of a piston 15 which is slidably arranged inside the stator housing 1.

The piston 14 and slant upper surface 13 of the toppling head together form a pivoting mechanism which causes the toppling head 2 to rotate towards its extended position which is shown in Fig. 3.

Fig. 3 shows that liquid discharged by the hydraulic motor flows through an annular opening 16 between the upper end of the piston 15 and the flexible shaft 9 into the interior 18 of the piston 15, as illustrated by arrows 19.

As a result of the flow restriction of the annular opening 16 G hydraulic pressure difference is created between the cavity 20 above the piston 15 and the piston interior 18 which induces the piston to slide downwards through the stator housing 1 as illustrated by arrows 21.

The downward motion of the piston also pushes the top 14 of the slant upper surface 13 of the toppling head 2 downwards which causes the toppling head 2 to pivot about pivot axis-as illustrated by arrow 31.

The mills 3 and 4 pivot together with the toppling head 2 about the pivot axis 7 towards their extended position and are simultaneously rotated by the shafts 8 and thereby mill a window 22 in a casing 23 surrounding the device.

The outward pivoting of the mills 3 and 4 continues until the entire slant upper surface 13 of the toppling head 2 engages the lower end of the piston 15, in which position the mills 3 and 4 protrude at a predetermined orientation from the stator housing 1. Liquid is discharged from the interior 18 of the piston via an outlet opening 24 which injects the liquid at high speed towards the mills 3 and 4 thereby cooling the cutting teeth 11 thereof and flushing away cuttings 25. The liquid is then circulated through a cuttings collecting box 26 which is suspended at the bottom of the stator housing 1 as illustrated by arrows 27. A guide plate 28 in the collecting box 26 serves to reduce the speed of the liquid so that the cuttings are allowed to be separated by gravity and/or magnetic forces from the liquid and are deposited at the bottom of the box, whereas the liquid is discharged via an opening 29 at the top of the box 26 into the annulus surrounding the device.

While the mills 3 and 4 are cutting the window 22 in the well casing 23 the device is pulled upwards through the casing as illustrated by arrows 30 by means of a piston (not shown) above the device until the window 22 has the desired length. It is preferred that the device is suspended within the casing 23 from a coiled tubing or drill string (not shown) and that the coiled tubing or drill string is gradually pulled up during the milling operations whilst the piston above the device ensures that the device is moved up through the casing 23 at an

accurately defined speed and over an accurately defined distance.

The balanced forces exerted by the mills 3 and 4 to the stator housing allow the device to mill a straight and well defined window in the well casing 23 in a quick and accurate manner and without leaving cuttings in the casing interior. It will be understood that, if desired, the mills may have a non-cylindrical shape, such as a frusto-conical shape. In such case it is preferred that the axes of rotation of the mills intersect each other at the central plane of the device.

Fig. 4 shows an embodiment of the device according to the invention where the device is used for underreaming of an uncased underground borehole.

The upper part of the device shown in Fig. 4 is similar to the upper part of the device shown in Figs. 1-3 and similar reference numerals denote similar components.

The device shown in Fig. 4 however contains no cuttings collection box and has a stator housing 1 which is rotated about the central axis 5 as is illustrated by arrow 40.

The device of Fig. 4 is equipped with a single mill 4 which underreams the wellbore 41 as a result of the simultaneous rotation of the stator housing and of the mill 4 relative to the toppling head 2. The toppling head 2 is extended and retracted in the same way as described with reference to Figs. 1-3.

The flexible shaft 9 is connected to the output shaft 8 via a gearbox in the toppling head 2, or may alternatively be integrated with the output shaft 8. If desired the underreamer device shown in Fig. 4 may be equipped with a pair of contra-rotatable twin mills 4 similar to those shown in Figs. 1-3.