Applicant: Archer Oiltools AS, Sandnes, Norway.

Inventors: Andreas Fliss and Trond Skjeie, both employed.

Introduction

The present invention relates to a casing cutter employed in the petroleum well industry.

Background art and problems related thereto

Petroleum wells are provided with several casing pipes of progressively reduced diameter and increasing length, each casing extending from the surface and down to a casing shoe at their lower end. Casing pipes not extending from the surface are usually called liners. A well is generally provided with a first casing pipe of larger diameter, say 18 1/2", extending to a first borehole depth where it is cemented to the surrounding rocks, and then the well is drilled further with a second diameter which is less than the first drilled section, and provided with a casing pipe of lesser diameter than the first one, say 14 " or 13 3/8 ", extending generally concentrically from the surface and past the first casing shoe and to near the bottom of the smaller diameter borehole, where it is cemented in its annulus to the smaller borehole wall. The process may continue by drilling a further extension of a smaller diameter borehole section with an even smaller diameter casing pipe, say 103/4 " or 95/8 ", lining the third borehole section, also this third casing pipe extending from the surface. From this third casing one may drill a production section through a reservoir rock and provide it with a so-called production liner of diameter 7 5/8 " which is hung up in a so-called liner hanger fixed in the lower end of the third casing. This production liner is perforated at the desired production zones, and provided with a completion with a petroleum production pipe, and the production pipe is extended to the surface, through a wellhead and provided with blow-out preventer valves, and extends via a production riser to production and control valves.

During well intervention operations wherein the well shall be modified for renewed drilling or lateral section drilling, it is often required to remove one of the 14 " / 13 5/8 ", or 103/4 " / 9 5/8 " casings from the surface and down to a desired depth. The casing may be surrounded in its annulus by Baryte debris which may cause the severed casing to stick in the borehole or surrounding second casing.

Casings are usually cut using a rotating casing cutter tool operated at the lower end of a rotating drill pipe string. We may wish to cement the remaining casing below the severed section after the cutter tool has cut through the casing wall, and such cementing of the remaining casing below the cut is usually done in a separate later ran, after the cutting tool is removed. Tripping out a cutting tool, running in a cementing stinger, cementing, and circulating out excess cement and tripping out the cementing stinger requires several hours or days of expensive rig time.

Casing cutter tools

One casing cutting tool is described in McGarian WO2019016523, wherein is described a cylindrical main body with a through bore is provided with a return spring- loaded piston pipe which is arranged for moving longitudinally within the main body when blocked by a drop ball in the main bore, and provided with an array of circular grooves forming cogs which engage cogs of pivoting cutter arms which extend when the piston pipe moves. The ball-blocked piston moves due to increased pressure in the drilling mud.

There are some significant disadvantages of actuating a cutter tool using a drop ball:

Firstly, the drop ball blocks the main bore and requires drilling fluid or cement to pass only via bypass apertures back to the main bore of the moving piston. So if cementing of a plug in the casing bore shall be done, it must be done ahead of activating the cutter tool with the ball.

Secondly, the dropping of the ball requires a separate ball drop apparatus with a fluid bypass and control valves is required, the ball drop apparatus integrated in the drill pipe string extending above deck. The present invention requires no such ball drop apparatus and takes less space above the work deck.

Thirdly, the circulation of the ball along with drilling fluid down to its intended ball seat at the cutting depth requires many minutes or even several hours, depending on depth and circulation speed: about 45 minutes at 2000 m, to about 180 minutes at 6000 m, only for activating the tool. McGarian's tool allows cementing before the cut is made, which cement may only reside in the casing bore, not the casing annulus.

Fourthly, when we shall pull a drillpipe string with a cutting tool out of a well, we may circulate in a so-called "slug" to better drain fluids from the tubing string as we pull it from the wellbore. If using a cutting tool with narrow bypass holes or channels around a dropped ball, then when we pump slug, this mud block the small ball bypass passages when we pull out of hole between cuts or when the cut is done and string pulled out of hole. This result in pulling drill string "wet", i.e. filled with drilling fluid, for each time during tripping out we break another sectioned stand of drill pipes from the drill pipe string, the drilling fluid in the drill pipe stand will fall out of the stand and flow out on the drilling deck, for several kilometers of tripped out drill pipe string. GB2559353 A Ardyne Technologies Ltd., describes a cutter tool wherein a casing cutter is activated by pumping a ball, a sponge ball, through the toolstring and landing the sponge ball in a basket-like ball- catcher forming a bottom in the lower end of a piston sleeve. The ball then covers the entire central bore and pressure may be built up through the drill pipe string to translate a sleeve to open bypass channels internally. Dropping one more ball will block the central bore once more and allow pressure buildup which will displace the piston sleeve which is linked to rotate and extend cutter arms.

Loss of circulation

A significant disadvantage of the McGarian and Ardyne cutter tools is that in order to land a ball in the tool in order to activate it, one must have circulation. If circulation is lost, it is not possible to circulate in a drop ball, thus not possible to activate the cutter tool. With the present invention it is possible to activate the cutter tool despite loss of circulation, as long as we are able to pressurize the drill pipe string.

Pressure activation of bypass

Intelligent Drilling Tools GB patent GB2544136 has described drill pipe string conveyed circulation subassembly with a commercial name of "AVS". The patent describes a piston sleeve arranged in the main bore of a cylindrical body, the piston sleeve moveable in a downward direction by the pressure in the main bore generated by fluid through the drill pipe string. The piston sleeve is moveable due to its top end exposed area exposed to the central bore pressure being larger than its lower end area, not due to the ball valve. So it is the differential pressure in the central bore of the piston sleeve which moves the piston sleeve. The tool is provided with adjustable stroke stoppers, also called delimiters, arranged to be electromechanically controlled to stop the piston sleeves at three selectable positions:

- a first position for a purely flow-through position of the piston sleeve with lateral ports closed,

- a second position for a partial bypass position of the bypass sleeve, wherein the lateral ports in the sleeve are aligned with corresponding lateral ports in the main body, and

- a third position for a full bypass position wherein a ball is closed mechanically by means of a full abutting of the piston sleeve bottoming in its axial stroke, and larger bypass ports are alignment- opened for a full bypass of the flow from the main bore to the tool annulus is provided.

The tool described in the cited document is arranged to divert part of the drilling fluid to a bypass from the main bore and out to the drill string annulus, in order to improve the carrying capacity of debris in a widened section about the drill pipe string, while allowing drilling mud to pass also downwardly through the main bore to the drill bit. In this way, more drilling mud may be circulated than without the circulation sub, and there is a reduced risk of loss of carrying capacity for the debris, a reduced risk of getting stuck, and a reduced risk of inadvertently exceeding the rock's pressure capacity.

Flash cementing It may be possible to circulate in cement through a cutter tool If the cement is pumped through apertures in bypass channels past an otherwise piston-blocking ball in the knife-actuating piston sleeve. However, as such apertures in bypass channels inadvertently are of much smaller diameter than the full bore of the piston sleeve, there is a risk of local heating and flash setting of cement in such narrow throats as bypass channels, which would permanently prevent from cementing through the tool.

Brief summary of the invention

The invention is defined in the attached claim 1. Embodiments of the invention are defined in the dependent claims.

Brief Figure Captions

Embodiments of the invention are illustrated in the attached drawings. The invention shall not be limited by the drawings. Conversely, features and combinations of features shown in the drawings constitute embodiments of the invention.

Fig. 1 illustrates an embodiment of the invention wherein a cutter tool provided with cutter arms with casing cutting knives are folded in into recesses of a rotatable cutter tool body. A motor-controlled delimiter (14) controls the stroke of a piston sleeve (14) which upon abutment actuates a ball valve (6) in the main bore of the piston sleeve. The ball valve (6) is open for passage of drilling fluid to pass axially. Figure captions:

The heading on filing date read: "iCutter IB: differential pressure actuates piston - further actuates ball valve "

(01): recess.

(1): control module deactive mode (sleeve inu. pos.), controls delimiter (14) which controls stroke length which actuates ball valve operation.

(2): return spring in deactive pos.

(3): cutter knife arm in retracted pos.

(4): cogged actuation piston sleeve, retracted.

(5): bypass flow port, closed

(6): ball valve, open

(11): motor

(12): sensor unit

(121): pressure sensor

(122): accelerometer sensor (14): delimiter (40): central bore in the piston sleeve/ tool (13, 131):pressure signal from surface,

(13, 132): acceleration signal such as rotation controlled from surface

Fig. 2 illustrates the same embodiment of the invention wherein the pressure has been set in the drilling fluid from topsides, while the control module has activated the delimiter (14) to a retracted position by using a motor (11), thus allowing a pressure differential between the top and the bottom of the piston sleeve (11) to stroke so as for shutting the ball valve in the actuation piston sleeve, and the piston sleeve has moved to force out the cutter knife arms to their extended positions. Figure captions:

(1): control module controls motor 11 to move delimiter 14 to allow sleeve stroke to shut ball valve to "extended knives" mode (with extended knives and sleeve in lower position)

(2): return spring in active compressed pos.

(3): cutter knife arm, extended pos.

(33): cutting edge

(4): cogged actuation piston sleeve, axially translated downw.

(5, 51) : bypass flow ports, aligned and thus opened, fluid diverted (6): ball valve, closed, pump pressure set (11): motor

(14): delimiter in long stroke position (31): pivot axle

(41): cog array

(42): seal

Fig. 3 illustrates time series of pressure of the drilling fluid in the main bore of the apparatus of the present invention, rotational speed, and a logical parameter called "operational mode" for the tool of the invention. The logical parameter may comprise three modes:

- "listening" mode, wherein the tool listens for an activation signal,

- "ready to extend [cutter arms]" mode, wherein an activation signal has been received, and

- "extended knives" mode, wherein pressure has exceeded a working pressure to overcome a return spring force and the knives are extended.

Fig. 4 illustrates an embodiment of the cutter tool arranged on a drill pipe string in a cased well, when the cutter tool is in the "listening mode" or the "ready to extend" mode. Figure Captions:

(1): control module is first in deactive mode "listen" mode.

(1): then the control module commands motor 11 to control delimiter (14) to full stroke length of piston sleeve (4), increased pressure moves piston to full stroke to shut ball valve (3): "ready to extend" mode. (3): cutter knife arms in retracted pos.

(5): bypass closed

(6): ball valve, open

Fig. 5 illustrates an embodiment of the cutter tool similar to Fig. 4, with the cutter tool set in the "extended knives" mode with the knives extended and forced against the casing inner wall utilizing the piston force generated by the drill pipe supplied fluid pressure, and rotatingly cutting away material from the casing wall. Swarf resulting from the cutting, and heat generated by the cutting and friction is transported away by the circulating fluid. Figure captions:

(3): cutter knife arms extended to cutting pos.

(5): bypass open

Fig. 6 is an illustration of a subsequent situation wherein a cut has been made at an above level in the well in addition to the present cut, and the ball valve has been opened, and wash fluid is flushed through the central bore of the tool to be released near the cut. This may wash out and force out annular baryte from behind the casing, aiding to free the casing and enabling it to yield to an axial force from a so-called spear tool to retrieve the severed casing section from the well. Figure captions: (1): control module is back in deactive mode "listen" mode (3): cutter knife arms back in retracted pos.

(5) : bypass closed again

(6): ball valve, open.

"settled infill": Baryte settled infill in the casing annulus "annular wash-out": Baryte anular wash-out between cuts in casing

Description of embodiments of the invention

The invention is a drill pipe string (10) - conveyed casing cutter tool. The cutter tool comprises a cylindrical main body (0) provided with casing cutter knife arms (3). The knife arms (3) are arranged rotatable on pivot axles (31) and provided with rotating cogs (32) in a first (inner) end of said knife arms (3). In their passive state, said knife arms (3) are pivotally rotated back and thus retracted into recesses (01) in said main body (0) by a return spring (2). There may be two, three or more knife arms (3) distributed about the axis of the cylindrical main body (0). A number of three arms (3) will contribute to axial stability when rotating and cutting.

Further there is arranged a piston sleeve (4) which is spring loaded by the return spring (2). The piston sleeve has a central bore (40). The piston sleeve (4) is axially arranged in said cylindrical main body (0). The piston sleeve (4) is provided with seals (42) in two different bore diameters in the main body (0), the larger bore on top. The piston sleeve may be moved if an axially directed force generated by a force differential between its top and bottom end overcomes the spring load. The force differential AF is proportional to the difference between the axial force on the top of the piston which is

F ^P((R,-R _4O ) ² -(RL-R4O) ² ) wherein R ₄ o is the radius of the piston sleeve bore (40), R is the radius of the top of the piston, and The rotating cogs (32) of the knife arms are engaged with a corresponding linear cog array (41) on said piston sleeve (4) and arranged for extending said knife arms (0) with a cutting edge (33) out from said recess (01).

A novel feature of the invention is that said piston sleeve (4) is provided with a ball valve (6), and further that said ball valve (6) is actuated by the piston sleeve (4) being subject to the differential pressure and thus stroked in the axial direction. A motor (11) controlled by a control module (1) controls delimiter (14) to retract to allow the piston sleeve (4) to stroke to close the ball valve (6). and when the ball valve (6) is shut, it closes said central bore (40), so as for when pressure is set from the surface through said drill pipe string (10), said piston sleeve (2) will also actuate said knife arms (3) utilizing not only the differential pressure, but also the pressure on the closed ball, to a cutting position, which will increase the force on the piston sleeve, for rotation cutting an inner wall of a casing when said drill pipe string (10) is rotated. The actuating force from the piston sleeve increases from F =KP((Rt-R ₄₀ ) ² -(R _L -R ₄ o) ² ) to F =KPR ² when the ball valve (6) is closed by the initial force of the translating sleeve.

Advantages of the invention

An advantage of setting the pressure through the drill pipe string is that the high pressure provided by mud pumps at the surface is high enough both to move the piston sleeve (4) and further to actuate closure of the ball valve (6) and increase the force to engage the cutting edges (33) with high enough force to sever the casing efficiently. Such high forces required to extend the cutter arms would otherwise be generated through the use of a downhole motor that would require much energy and high torsional moment from such a motor, for which there is insufficient space.

The present invention is advantageous over McGarian' casing cutter which utilizes a separately, through the drill-pipe dropped ball to actuate a piston sleeve. Using that casing cutter, the operator cannot undo the presence of the ball in the main bore. Using the McGarian casing cutter requires any cementing to be done before the ball is dropped, and prevents cementing through the tool after the cutting process is finished. According to the present invention the main bore may be re-opened in its central bore for pumping cement through the main bore, after the cutting process has been conducted. An advantage of the tool is that having a ball valve with an aperture of generally the same diameter as the bore of the piston sleeve, we may have a high circulation rate both before the cutting, and also after the cutting tool has severed the casing and when cleaning circulation is required for getting swarf and debris out of the well after cutting. High circulation cleaning through the main bore is not possible with a solid ball in the main bore according to the background art, except possibly allowing a small lateral flow through the tool's wall.

Another advantage of the tool with a full bore ball valve in the piston sleeve is that when cementing through the tool, there are no local narrow throats for the cement to pass, thus the risk of flash setting of the cement is reduced, and cementing may be conducted after cutting.

An advantage of the invention is the possibility to do more than one cut in one ran in the well. Further advantageously, the invention enables us to reduce the pressure and open up the ball valve, disable the cutters by setting the delimiter (14) to prevent stroke of the piston sleeve (14), increase pressure and fluid flow to flush to clean out swarf and debris after one cut before displacing the tool along the well to make another cut in the same run. This use of the ball valve enables us to flush with high cleaning fluid pressure and volume to do a full cleanup before the next cut.

The cutting tool of the invention has, due to utilizing the pump pressure from the surface, and by utilizing the piston sleeve translation - actuated ball valve in the main bore of the piston sleeve, sufficient force to, when the ball valve is rotated to further increase the pressure and force out the casing cutter knife arms with a sufficient radial force component to force the cutting edge towards the casing with a sufficient force to sever the casing efficiently. When the casing is severed, it may be retrieved to the surface using a so-called casing spear.

Further, the time it takes to activate the tool according to the invention is no longer dependent on any drop ball travel time while being circulated in. The activation time according to the invention is short, it almost only requires the time it takes to send a signal (13) to the controller (1) to activate the stroke delimiter (14) to allow displacement of the piston sleeve to eventually shut the ball valve (6). The activation time according to an embodiment of the invention implies the predefined time of nearconstant rotation rate of e.g. 50 or 60 RPM +/- 1 RPM during e.g. 120 seconds, to activate the tool to enter "ready to extend [the knives]" mode. Then we increase the fluid pressure in the drill string to displace the piston sleeve and close the ball valve (6) and fully extend the cutter arms to the casing inner wall increase the rotation rate to a rotation rate which is sufficient to efficiently cut the casing, and then cut and sever the casing. The actuation mechanism of GB2 544 136 Intelligent Drilling Tools and the actuation mechanism of the present invention mutually resemble because there is a piston sleeve, bypass ports, and a ball valve involved in both. In an embodiment of the present invention there may also be a lateral port opening from the piston sleeve to the outside of the tool while the cutter arms are extended, cutter arms which GB2544 136 do not provide, but our lateral ports are of relatively small aperture and are generally for lubricating and cooling the knives. Although Intelligent Drilling Tools AVS diverter tool described in GB2 544 136 has sufficient force to open a lateral port of the tool, the axial force would be insufficient for actuating and forcing out our casing cutter knife arms with a radial force component strong enough to force their cutting edges towards the casing to sever it efficiently, because the second, fully closed ball-valve- diversion flow of GB2544 136 is intended to divert the total flow to carry out debris while drilling, and such a large diverter flow would reduce the pressure and the axial force on the closed ball valve too much to reliably build up an axial force on the sleeve sufficient to extend and force our cutter knife arms of the present invention against a casing wall and efficiently cut the wall. The bypass flow ports (5, 51) of the embodiment of the present invention are small compared to the main bore but sufficiently large to create a lubricating and cooling flow for the cutting process using our knives, but our bypass ports are not intended to produce a bypass flow for lifting out the large volume of debris associated with drilling, as we shall only transport swarf of relatively modest volume away from the cut in the casing, and cool and lubricate the knives. In the cited patent document it is described an electronically controlled stroke length pressure-displaced piston which abuts at its ultimate stroke length (if allowed by the electronically controlled motor- activated delimiters), and eventually gets its ball valve closed, and a similar function for closing the ball valve of the present invention.

Another advantage of the present invention is the fact that when we pull the cutting tool out of the well, the drilling fluid is continuously allowed to drain via the full-bore open ball valve of the cutter tool, avoiding pulling the drill pipe string wet out of hole. This avoids unnecessary pouring of drilling fluid from the broken out drill pipe stands to the drilling deck, and reduces the loss of drilling fluid, and cleaner and better working conditions on deck.

There is, with the present invention, no need for a bypass in the piston sleeve, because there is no drop ball, and the ball valve element may be returned to open position upon command.

With the invention no ball catcher is needed. Ball catchers require more length of the tool string and is undesirable. Ball catchers have limited capacity and sets thus a limit to the number of cuts in one run. With the invention allowing the ball valve to remain open during tripping or RIH, there occurs no swab of well bore when POOH because the drill pipe string fluid may be pumped in axially while the tool is retrieved upwardly.

Further embodiments of the invention

In an embodiment of the invention, said control module (1) is provided with a first sensor unit (12) arranged for detecting an activation signal (13) from surface. The activation signal may comprise one or more of an acceleration signal and a pressure signal, please see Fig. 3. The details of the operation are given below:

There is much noise in a well during intervention operations, both acoustic noise, acceleration noise due to a varying rotation rate and vertical movements, and pressure noise. The activation signal for the cutter tool must be clear and consistent in order to avoid activating the cutter tool inadvertently. In an embodiment of the invention, said control module (1) is provided with an accelerometer sensor (122) which is at least arranged for measuring rotation related accelerations. In this embodiment said activation signal (13) comprises an acceleration signal (132) generated upon sensing rotation of the accelerometer sensor with the tool in the Earth's gravity field, e.g. such as a stable predefined rotation rate of e.g. 50 or 60 RPM over a predefined period of time of e.g. 60 or 120 seconds, e.g. 60 RPM and with a duration of at least 120 s, please see the RPM curve with a continuous line in Fig. 3. This stable rotation signal may be combined with a pressure signal, the dashed P line in Fig. 3, to change the operating mode from "listening mode" to "ready to extend [knife arms]" mode, please also see below.

In an embodiment of the invention, said first sensor unit (12) comprises a pressure sensor (121) and said activation signal (13) comprises a pressure signal (131) sent from surface and measured due to the pressure in the drill pipe string, the pressure generated e.g. by signalling using mud pumps. The pressure signal (131) to contribute to activate the cutter tool may require simply that the pressure is increased above an initial minimum knife activation pressure threshold (PL).

In a further embodiment, whereupon said first sensor unit (12) has received said activation signal (13), makes the control module (1) register the tool as being in an active "ready for extending knife arms" mode, please see the stepped "mode" line in Fig. 3.

When said control module (1) has registered the operation mode as "ready to extend [knife arms]", it may command said motor (11) to enable the delimiter (14) to release from its blocking position which prevents the piston sleeve (4) to stroke. Thus the increased pressure will move the piston sleeve (4) and a link mechanism may upon abutting a ball rotation mechanism rotate and close said ball valve (6) to intermittently block said central bore (40) of the tool, please see Fig. 4. When "ready to extend" mode with the ball valve closed, the operator increases the drill pipe string internal pressure P > PL to overcome spring initial force, to translate the piston sleeve axially and eventually extend the knife arms to the casing wall. A pressure or force or a proximity sensor may sense when P > Pmax when there is mechanical resistance from the casing wall, and the control module changes to "extended knives" mode, please see Figs. 3 and 5.

The tool is arranged for being set under pressure in said central bore (40), and when said piston sleeve (4) is subject to a differential pressure between its top and bottom, to close said ball valve (6) as described above,, and the driller increases the pump pressure to above said predefined pressure threshold (PL) required to overcome the spring force of said return spring (2),

- thereby extending said cutter arms (3) to a fully extended position to the casing wall, please see Fig.

2 and Fig. 5, whereby the knives will start cutting the casing. The operator controls rotation of the drill pipe string ahead of extending the knives.

- While said control module (1) pressure sensor (121) registers a pressure above said pressure threshold (PL), the control module (1) registers the operation mode as "extended knives" mode, please see Fig. 3.

The knives now are forced against the inner wall of the casing, please see Fig. 5, and rotation of the drill pipe string will cut and eventually sever the casing. The pressure used is according to the discretion of the operator in order to force the cutter knives (3) cutting edges towards the inner casing wall while rotating at a desired cutting speed. The cutter knives will make a circular groove by cutting away material from the casing wall and produce swarf and heat, see Fig. 5, and eventually sever the casing pipe completely at the selected level. Swarf produced by the knives may be flushed out with circulating bypassed drilling mud, or flushed out using axial flow after the ball valve has been opened, please see below.

When having cut through the casing wall, while the tool is still in "extended knives" operation mode, we may reduce the pressure in the drill pipe string, thereby retracting said cutter arms (3) from their extended position to stop cutting. The pressure sensor (121) registers a pressure drop to below said pressure threshold (PL), while said control module (1) is arranged to change and register the operation mode from "extended knives" mote back to "ready to extend" mode, please see Fig. 3.

When the cutting tool has severed the casing and the pressure in the drill pipe string is decreased to below said pressure threshold (PL), said control module (1) registers the pressure below said pressure threshold (PL) while in "ready to extend" mode, the reduced pressure on the piston sleeve will eventually shift it back to open said ball valve (6) and allow flow through said central bore (40), said control module (1) commands said motor (11) to disable the delimiter (14) thus disallowing displacement of the piston sleeve thus denying closure of the ball valve (6), and the control module may change the operation mode back to "listen mode". The tool may now be conveyed further into the casing to start all over to make another cut, or being kept in the same position to flush out swarf and possibly flush out the casing annulus, or be retracted for a subsequent operation to start, such as engaging a spear in an upper part of the severed-off casing section in order to pull out the severed-off casing section from the well.

It is not desirable that the tool changes back from "ready to extend" - to "listen" mode inadvertently, e.g. due to brief interruptions below the pressure required to keep the arms extended. It takes some time and a precisely controlled rotation rate to revert to "ready to extend" mode. A more substantial signal to end the "ready to extend" mode is desirable. In an embodiment of the invention, wherein said control module (1) registers a pressure below said pressure threshold (PL) while in "ready to extend" mode, for more than a second predefined duration, e.g. 120 s, please see Fig. 3. Said control module (1) then commands said motor (11) to disable said delimiter (14) thus not enabling closure of said ball valve (6). Flow is then again allowed through said central bore (40), and the control module (1) changes the operation mode back to "listen mode". In order to re-enter into the "ready to extend" mode, one will have to start over again form the left side of the scheme in Fig. 3, which is easy to do, quick compared to prior art, and repeatable for as many times as desirable.

In an embodiment of the invention the tool is arranged to flush wash fluid such as drilling fluid through said ball valve (6) in said central bore (40) for one or more of:

- transporting out swarf and debris via the drill pipe string annulus to surface, please see Fig. 3, right part indicated as a U-shaped arrow, or

- washing out baryte, cement, or other particles from an annulus about the casing of the severed casing pipe.

Lateral flush ports may, according to an embodiment of the invention, be activated to divert drilling mud under high pressure out to the tool annulus to near the cutting knives when working, please see Fig. 5. Thus the knife extension mechanism may be kept clean, the knives are cooled and lubricated.

In an embodiment of the invention, said piston sleeve (4) is provided with a lateral passage (51) arranged for aligning with a bypass port (5) in the main body (0) when said piston sleeve (4) is translated by the drill pipe string differential pressure into an actuating position and extending said knife arms (3), in order to divert drilling fluid out to the tool's annulus when cutting, and for closing said bypass flow ports (5) when said piston sleeve (4) is retracted from said actuating position, please see Figs. 1 and 2. Running procedure

In an embodiment of the invention, The procedure for running the tool is in an embodiment as follows: i) Make up the tool at surface and perform a surface test of functionality. ii) Run-in-hole of the tool on a drill pipe string to the target cutting depth. iii) Apply the activation signal rotation rate for a predetermined time, to initiate and activate the control module (1) with the predefined input parameters (pressure above predefined level, rotation at predefined speed and duration, or string movement, etc.). iv) Apply rotation to the drill pipe string. v) Apply flow and pressure via the drill pipe string and inside the tool / borehole assembly BHA. vi) Build up pressure to shift down the activation sleeve as follows: a. The piston sleeve (2) is moved axially by the pressure, the piston sleeve abuts a closing mechanism which closes the ball valve (6). b. The moving piston sleeve (2) activates and extends the knives to the casing inner wall. c. The piston sleeve (2) also compresses the return spring when moved. d. The piston sleeve (2) lateral apertures (51) also aligns with flow ports (5) between the level of the knives and ball valve. vii) Let the rotating tool cut the casing until casing is completely severed around the entire circumference. The deviated fluid from flow ports (51) will cool and lubricate the knives. viii) Reduce the flow and pressure inside the borehole assembly. ix) Stop the rotation of the drill pipe string. x) The control module (1) is automatically deactivated by predefined parameters (flow and rotation) as described above. xi) The piston sleeve is now shifted upwards by the reduced pressure and the return spring: a. The returning action of the sleeve will open the ball valve. b. The return spring is relaxing. c. The knives are retracting. d. The lateral flow ports are closed. xii) The tool may be moved to a next cutting position and the procedure may be repeated as often as desired, limited by battery power of the control module (1), and knife wear.

Alternative embodiments

What is described as a ball valve (6) arranged for closing or opening the central bore (40) of the piston sleeve (4), may be replaced by a flapper valve or other valve mechanism closing or opening the central bore (40), actuated by a mechanism similar to the mechanism of the present invention.