Downhole tractor comprising an improved hydraulic system

Abstract

A downhole tractor having a hydraulic system for driving a plurality of hydraulic cylinders and a plurality of hydraulic motors. The system comprises: a hydraulic power pack; a first hydraulic supply line for supplying hydraulic fluid to the plurality of hydraulic cylinders; a second hydraulic supply line for supplying hydraulic fluid to plurality of hydraulic motors; a valve section comprising a respective part of first hydraulic supply line and a further respective part of second hydraulic supply line, valve section further comprising an inlet for receiving hydraulic fluid, and a set of valves, and a hydraulic bypass supply line coupled to hydraulic power pack for supplying hydraulic fluid directly to the inlet of valve section bypassing at least part of first hydraulic supply line and second hydraulic supply line. The first and the second hydraulic supply line each comprise two parts connected via respective part in the valve section. Each respective part is connected to a respective sub-set of plurality of hydraulic components. The valve section configured for individually controlling flow of hydraulic fluid into each respective part of hydraulic supply lines.

Claims

1. Downhole tractor comprising a hydraulic system for driving a plurality of hydraulic cylinders and a plurality of hydraulic motors, wherein the hydraulic system comprises: a hydraulic power pack; a first hydraulic supply line for supplying hydraulic fluid to the plurality of hydraulic cylinders; a second hydraulic supply line for supplying the hydraulic fluid to the plurality of hydraulic motors; a valve section comprising a respective part of the first hydraulic supply line and a further respective part of the second hydraulic supply line, and the valve section further comprising an inlet for receiving the hydraulic fluid, and a plurality of valves, and a hydraulic bypass supply line coupled to the hydraulic power pack for supplying the hydraulic fluid directly to the inlet of the valve section bypassing at least part of the first hydraulic supply line and the second hydraulic supply line; wherein the first hydraulic supply line comprises a first part and a second part that are connected via the respective part in the valve section, wherein the first part of the first hydraulic supply line is connected to a sub-set of the plurality of hydraulic cylinders, wherein the second part of the first hydraulic supply line is connected to a further sub-set of the plurality of hydraulic cylinders, wherein the second hydraulic supply line comprises a first part and a second part that are connected via the further respective part in the valve section, wherein the first part of the second hydraulic supply line is connected to a sub-set of the plurality of hydraulic motors, wherein the second part of the second hydraulic supply line is connected to a further sub-set of the plurality of hydraulic motors, wherein the valve section is configured for individually controlling flow of the hydraulic fluid into each respective part of the hydraulic supply lines.

2. The downhole tractor in accordance with claim 1, wherein the valve section comprises a pressure-setting valve provided in between the respective part of the first hydraulic supply line and the respective part of the second hydraulic supply line, the pressure-setting valve being configured for feeding excess hydraulic fluid in the first hydraulic supply line to the second hydraulic supply line.

3. The downhole tractor in accordance with claim 1, wherein the valve section comprises valves placed in said hydraulic supply lines for allowing individual control of the flow of the hydraulic fluid into each respective part of the hydraulic supply lines.

4. The downhole tractor in accordance with claim 1, wherein the valve section comprises a relief valve for dumping hydraulic fluid from the respective part of the second hydraulic supply line to a tank.

5. The downhole tractor in accordance with claim 1, wherein the first hydraulic supply line is terminated at a side facing the hydraulic power pack for allowing hydraulic fluid only to flow to the plurality of hydraulic cylinders via the hydraulic bypass supply line.

6. The downhole tractor in accordance with claim 2, wherein the second hydraulic supply line is coupled to the hydraulic power pack via a check valve (CV) for allowing hydraulic fluid to be directly fed into second hydraulic supply line via the hydraulic power pack in a first operational mode of the downhole tractor and for preventing hydraulic fluid to flow back into the hydraulic power pack in a second operational mode of the downhole tractor, wherein hydraulic fluid flows to the second hydraulic supply line via the hydraulic bypass supply line and the pressure-setting valve.

7. The downhole tractor in accordance with claim 1, wherein the first hydraulic supply line comprises a bidirectional hydraulic supply line.

8. The downhole tractor in accordance with claim 1, wherein the second hydraulic supply line comprises a unidirectional hydraulic supply line.

9. The downhole tractor in accordance with claim 1, wherein each hydraulic cylinder is configured for actuating at least one tractor arm or hydraulic actuator.

10. The downhole tractor in accordance with claim 1, wherein each hydraulic motor is configured for rotating at least one tractor wheel or hydraulic rotatable tool.

Description

BRIEF INTRODUCTION OF THE DRAWINGS

(1) In the following is described examples of embodiments illustrated in the accompanying drawings, wherein:

(2) FIG. 1 shows a downhole tractor having a hydraulic system as known from the prior art;

(3) FIG. 2 shows an embodiment of a downhole tractor in accordance with the invention;

(4) FIG. 3 shows the hydraulic system of the downhole tractor of FIG. 2, and

(5) FIGS. 4 to 9 illustrate what happens in the hydraulic system in different operational modes of the downhole tractor.

DETAILED DESCRIPTION OF THE EMBODIMENTS

(6) Various illustrative embodiments of the present subject matter are described below. In the interest of clarity, not all features of an actual implementation are described in this specification. It will of course be appreciated that in the development of any such actual embodiment, numerous implementation-specific decisions must be made to achieve the developers' specific goals, such as compliance with system-related and business-related constraints, which will vary from one implementation to another. Moreover, it will be appreciated that such a development effort might be complex and time-consuming, but would nevertheless be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure.

(7) The present subject matter will now be described with reference to the attached figures. Various systems, structures and devices are schematically depicted in the drawings for purposes of explanation only and so as to not obscure the present disclosure with details that are well known to those skilled in the art. Nevertheless, the attached drawings are included to describe and explain illustrative examples of the present disclosure. The words and phrases used herein should be understood and interpreted to have a meaning consistent with the understanding of those words and phrases by those skilled in the relevant art. No special definition of a term or phrase, i.e., a definition that is different from the ordinary and customary meaning as understood by those skilled in the art, is intended to be implied by consistent usage of the term or phrase herein. To the extent that a term or phrase is intended to have a special meaning, i.e., a meaning other than that understood by skilled artisans, such a special definition will be expressly set forth in the specification in a definitional manner that directly and unequivocally provides the special definition for the term or phrase.

(8) Hydraulic tractor systems are typically built up as follows:

(9) 1. An electronics unitThis unit communicates with the surface panels, transfers sensor data (such as pressure data) from the instrumented part of the tractor and controls activation and propulsion of the tractor system, including steering and control of the signals on the electric cables that are distributed throughout the tractor.

(10) 2. A motor unitThis unit includes the electromotor, the hydraulic pump and the manifold for generating and distributing the hydraulic propulsion pressure.

(11) 3. Driving sectionsThis unit includes two or more arm systems per driving section.

(12) 4. A compensatorThis unit regulates the internal pressure in relation to the ambient pressure of the system, and feeds hydraulic oil to compensate for leakage.

(13) There exist hydraulic systems having only one hydraulic circuit (a hydraulic supply line with a return line). In these systems the single hydraulic circuit activates both the pivotable arms as well as the wheels. A bypass line in such systems may also extend the functionality in a limited manner switching on and off sections but would not allow independent control of drive wheels.

(14) The invention primarily aims at improving hydraulic systems having two hydraulic circuits, i.e. two hydraulic supply lines, wherein one hydraulic supply line activates the pivotable arms and the other hydraulic supply line activates the wheels.

(15) The hydraulic system uses hydraulic power generated by an electric motor and a pump. The pump generates volume and pressure to two different separated hydraulic circuits, whereas the manifold that is coupled to the pump regulates the pressure as well as the on- and off-function.

(16) At the start-up of the hydraulic system the hydraulic pressure is generated by the motor/pump system and provided to the arm and wheel propulsion. The arm pressure is held constant at a predefined level and any excess volume is fed to the tank. The biggest (hydraulic) volume is fed to the wheel motors for propulsion of the system. The whole hydraulic circuit is optimized for minimal friction and for maximal use of the available hydraulic power. The wheel motors, the gearing ratio from the motors to the wheels and the wheel size are all calculated and dimensioned to provide the desired propulsion force.

(17) Less friction as well as a larger generated hydraulic volume in the hydraulic circuit will increase the speed of the wheel propulsion system.

(18) FIG. 1 shows a downhole tractor having a hydraulic system as know from the prior art. The hydraulic system shows a hydraulic power pack 100 that is configured for driving two hydraulic supply lines 200, 300 (each forming a respective hydraulic circuit together with a supply line to (hydraulic) tank 99). The hydraulic power pack 100 comprises a motor/pump block 105 that comprises an electric motor M that drives two pumps P, and a manifold block 110 coupled to said pumps P as illustrated. The manifold block 110 comprises respective parts of said hydraulic supply lines 200, 300. The first hydraulic supply line 200 is a bidirectional hydraulic supply line and is connected to a hydraulic cylinder 160 for driving a tractor arm (not shown). The second hydraulic supply line 300 is a unidirectional hydraulic supply line and is connected to a hydraulic motor 170 for driving a tractor wheel 180. In FIG. 1 only two drive sections 150, meaning two hydraulic cylinders 160-1, 160-2, two hydraulic motors 170-1, 170-2, and two tractor wheels 180-1, 180-2 have been drawn. However, in practise this may be any other number of drive sections. A typical number of drive sections 150 for a downhole tractor is eight.

(19) The manifold block 110 comprises a (3-port) sequence valve 112 and a relief valve 114 connected to the first hydraulic supply line 200 as illustrated. The relief valve 114 is configured for holding the correct pressure on the first hydraulic supply line 200 as earlier discussed. Both the sequence valve 112 and the relief valve 114 are connected to tank 99 as illustrated. The manifold block 110 further comprises a further relief valve 116 and a further sequence valve 118 connected to the second hydraulic supply line 300 as illustrated. Both the further relief valve 116 and the further sequence valve 118 are connected to tank 99 as illustrated.

(20) FIG. 2 shows an embodiment of a downhole tractor 10 in accordance with the invention.

(21) The downhole tractor 10 comprises a plurality of modules that are connected together, which is a common way downhole tractors are made. It must be stressed, however, that the invention is not limited to modularly-built downhole tractors. The downhole tractor comprises a top module 11 (also called top sub). This top module 11 forms the interface with the wireline (not shown) to the surface. The top module 11 is connected to a telemetry module 13, which is configured for the telemetric communication with the surface through the wireline (including the transmission of sensor data to the surface). The telemetry module 13 is connected to hydraulic power pack 100a, which comprises a motor module and a motor controller. As will be discussed later in the detailed description, the hydraulic power pack 100a of the invention may be adapted in comparison with the hydraulic power pack 100 of the prior art as shown in FIG. 1.

(22) The hydraulic power pack 100a is connected to a series connection of two drive modules (sections) 150-1, 150-2, also being referred to as the upper drive sections. The second upper drive sections 150-2 is connected to a valve module 400, which is a new feature of the downhole tractor 10 of the invention that has not been reported in the prior art before. That is to say a valve module in between drive sections. The valve module 400 is further connected to a series connection of two further drive modules (sections) 150-3, 150-4, also being referred to as the lower drive sections. The lowest drive section 150-4 is connected to a terminal module 18 (also called compensator sub), which on its turn is connected to a bottom module 19 (or bottom sub). This downhole tractor 10 comprises a hydraulic system that extends from the hydraulic power pack 100a all the way to the compensator sub 18.

(23) FIG. 3 shows the hydraulic system 10s of the downhole tractor of FIG. 2. The hydraulic system 10s comprises an adapted hydraulic power pack 100a that is connected to two (modified) upper drive sections 150a-1, 150a-2. The upper drive sections 150a-1, 150a-2 on their turn are connected to a (specially designed) valve module 400, which on its turn is connected to two (not necessarily modified) lower drive sections 150a-3, 150a-4. The chain of (hydraulic) modules is closed by the terminal module 18. It must be noted that a different number of upper and lower drive sections could be chosen, for example six or eight. Each drive section 150a-1, 150a-2, 150-3, 150-4 comprises a respective hydraulic cylinder 160-1 . . . 160 . . . 4, a respective hydraulic motor 170-1 . . . 170-4, and a respective tractor wheel 180-1 . . . 180-4, similar to the drive sections as known from the prior art. It must be also noted that for the lower drive sections 150-3, 150-4 of the downhole tractor 10, the modified drive sections 150a-1, 150a-2 (with a hydraulic bypass supply line) could be used. However, in that case the termination module 18 needs to be adapted in that it also properly terminates the hydraulic supply line 250.

(24) The upper drive sections 150a-1, 150a-2 have been amended in that these comprise a hydraulic bypass supply line 250. The hydraulic bypass supply line 250 is a new feature that has not been reported in the prior art before. It is this extra line 250, which in combination with the valve module 400 opens up a tremendous amount of extra options.

(25) The valve module 400 effectively divides the first hydraulic supply line 200 in a first part 200-1 (within the upper drive sections 150a-1, 150a-2) and a second part 200-2 (within the lower drive sections 150-3, 150-4). Similarly, the valve module 400 effectively divides the second hydraulic supply line 300 in a first part 300-1 (within the upper drive sections 150a-1, 150a-2) and a second part 300-2 (within the lower drive sections 150-3, 150-4). Consequently, within the valve module 400 there is respective parts 200a, 300a of the respective hydraulic supply lines 200, 300 that connected the respective first parts 200-1, 300-1 with the respective second parts 200-2, 300-2.

(26) The hydraulic bypass supply line 250 is fed into an inlet 401 of the valve module 400, which then leads this line towards a feeding point FP on the respective part 200a of the first hydraulic supply line 200 as illustrated.

(27) FIG. 3 further shows that the adapted hydraulic power pack 100a comprises an adapted manifold block 110a and motor/pump block 105, which may be the same as in the prior art, comprising one motor M, an axle, two pumps P driven by the motor M, and two hydraulic power supply lines 200, 300 driven by said pumps P as illustrated. The adapted manifold block 110a comprises a first controllable switch V1 coupled between the first hydraulic supply line 200 and tank 99. A similar controllable switch V2 is coupled between the second hydraulic supply line 300 and tank 99. In an embodiment the first switch V1 and the second switch V2 are 2-position, 2-port normally-closed type valves (for example solenoid valves in order to define clearly what is meant with a valve being open or closed, the following definition is given for this specification. When a valve is open it allows flow through it. In a three-port valve this means that the flow is not connected to return/tank. When a valve is closed this means that the valve does not allow flow through it OR it is connected to return/tank.

(28) The purpose of the first and second controllable switches V1, V2 is to be able to selectively dump the respective hydraulic fluid in the respective hydraulic supply lines 200, 300 to tank for the purpose of changing the operational mode of the tractor as will be explained later.

(29) The adapted manifold block 110a further comprises a rerouting of the first hydraulic supply line 200 to the hydraulic bypass supply line 250 in the upper drive sections 150a-1, 150a-2. Furthermore, the manifold block 110a comprises a termination TM for terminating the first hydraulic supply line 200. This termination is illustrated in the manifold block 110a, but it might also have been placed within the most upper drive section 150a-1.

(30) The adapted manifold block 110a further comprises a check valve CV in the second hydraulic supply line 300 as illustrated. The function of the check valve CV is to allow hydraulic fluid to flow from the left to the right, but not in the opposite direction. The purpose of this check valve will be explained with reference to FIGS. 4 to 9.

(31) The main purpose of the valve module 400 is to receive the hydraulic fluid from the hydraulic bypass supply line 250 and to distribute it for the respective parts 200-1, 200-2, 300-1, 300-2 of the hydraulic supply lines 200, 300. However, a further purpose is to be able to individually control a flow of hydraulic fluid in each of said parts 200-1, 200-2, 300-1, 300-2. In order to achieve that an implementation of the valve module 400 as shown in FIG. 3 may be chosen. The valve module 400 comprises a plurality of controllable switches V3, V4, V5, V6 (which may be 2-position, 3-port valves in an embodiment) as shown. Furthermore, the valve module 400 comprises two piloted check-valves PCV1, PCV2 in the second hydraulic supply line 300 as shown. These piloted check-valves PCV1, PCV2 are the components that enables or disable hydraulic fluid flow in the respective parts 300-1, 300-2 of the second hydraulic supply line 300. The piloted check-valves PCV1, PCV2 are controlled by respective first and second pilot lines PL1, PL2 as illustrated. Two of said earlier mentioned controllable switches V4, V6 have been provided in these pilot lines PL1, PL2 as illustrated. These controllable switches in V4, V6 in the pilot lines PL1, P12, enable an extra set of operational modes of the downhole tractor 10 as will be illustrated in the discussion of FIGS. 4 to 9. In addition, a pressure-setting valve 115 is provided in between the first hydraulic supply line 200 and the second hydraulic supply line 300, which is configured for feeding excess hydraulic fluid in the first hydraulic supply line 200 to the second hydraulic supply line 300. It must be stressed that the embodiment in FIG. 3 is just one of many implementations of the function to be able to individually control hydraulic fluid flow in each of the respective parts 200-1, 200-2, 300-1, 300-2 of the hydraulic supply lines. The valve modules 400 further comprises a relief valve 116 between the second hydraulic supply line 300 and tank 99 as illustrated. The purpose of this valve 116 is to regulate the pressure in the second hydraulic supply line 300. In the prior art solutions this valve is placed in the manifold block 110. In the illustrated embodiment of the invention this valve 116 is to be placed in the valve module 400 in between said drive sections 150a-1, 150a-2, 150-3, 150-4.

(32) FIGS. 4 to 9 illustrate what happens in the hydraulic system in different operational modes of the downhole tractor. These figures are similar to FIG. 3, with the exception that the respective switches are in different positions. Some reference numerals have been left out to facilitate reading of the figures.

(33) FIG. 4 shows the standard tractor mode. Switch (valve) V1 and switch (valve) V2 are closed, allowing the hydraulic fluid to flow at normal pressure into hydraulic bypass supply line 250 and the first part 300-1 of the second hydraulic supply line 300. Switch (valve) V3 and switch (valve) V5 are open, allowing the hydraulic fluid to flow from the hydraulic supply line 250 to the feeding point FP into and into the first part 200-1 and the second part 200-2 of the first hydraulic supply line 200. Switch V4 and V6 are also open, allowing the hydraulic fluid to activate the piloted check-valves PCV1, PCV2 through the pilot lines PL1, PL2, thereby opening said check-valves. Consequently, the hydraulic fluid in the second hydraulic supply line 300 can flow through the first part 300-1 through the valve module 400 into the second part 300-2 of the second hydraulic supply line 300. The pressure-setting valve 115 feeds possible excess hydraulic fluid (pressure) towards the second hydraulic supply line 300, which increases the conveyance speed of the tractor. The arrows in FIG. 4 illustrate the flow direction of hydraulic fluid. One important footnote FIGS. 4 to 9 is that the first hydraulic supply line 200 may typically be bidirectional hydraulic supply line, which means that the flow in that line may be reverse, when the hydraulic cylinders are being emptied to tank 99.

(34) FIG. 5 shows the standard tractor low-speed (low power) mode. The difference between this mode and the standard tractor mode in FIG. 4 is that the second switch V2 is now open, dumping the hydraulic fluid to tank 99. Consequently, the second hydraulic supply line 300 is now only fed through the pressure-setting valve 115. The arrows in FIG. 5 illustrate the flow direction of hydraulic fluid. The check valve CV is now blocking the hydraulic fluid in the second hydraulic supply line 300 to flow back to the pump. The flow in the second hydraulic supply line 300 is now lower than in FIG. 4, which sets the tractor in low-speed (or low-power) mode.

(35) FIG. 6 shows the tandem tractor upper high-speed mode. The difference between this mode and the standard tractor mode in FIG. 4 is that the switch valve V5 is now closed, i.e. dumping to tank 99. Consequently, both the second part 200-2 of the first hydraulic supply line 200 as well as the second pilot line PL2 are shut-off. The latter also causing the second part 300-2 of the second hydraulic supply line 300 being shut-off, i.e. deactivating the lower drive sections 150-3, 150-4.

(36) FIG. 7 shows the tandem tractor upper low-speed mode. The difference between this mode and the standard tractor mode in FIG. 6 is that the second switch valve V2 is now open, i.e. dumping the hydraulic fluid to tank 99. Consequently, the second hydraulic supply line 300 is now only fed through the pressure-setting valve 115. The arrows in FIG. 7 illustrate the flow direction of hydraulic fluid. The check valve CV is now blocking the hydraulic fluid in the second hydraulic supply line 300 to flow back to the pump. The flow in the second hydraulic supply line 300 is now lower than in FIG. 6, which sets the tractor in low-speed (or low-power) mode.

(37) FIG. 8 shows the tandem tractor lower (low-speed) mode. The difference between this mode and the low-speed tractor mode of FIG. 5 is that the third switch valve V3 is now closed, dumping the hydraulic fluid to tank 99. The first piloted check-valve PCV1 is closed. Consequently, the upper drive sections 150a-1, 150a-2 are deactivated.

(38) FIG. 9 shows the rolling anchor mode, wherein the wheels 180-1 . . . 180-4 in the driving sections 150a-1, 150a-2, 150-3, 150-4 are not driven, while all arms are pressed out. In order to achieve this mode, the controllable switch valves V4, V6 in the pilot lines PL1, PL2 are required, when piloted check-valves PCV1, PCV2 are used as in FIG. 9. Both controllable switch valves V4, V6 are closed, i.e. dumping to tank 99, while valves V3 and V5 are open. In addition, the second controllable valve V2 in the manifold block needs to be dumping to tank 99, otherwise wheels 180-1, 180-2 of the upper drive sections 150a-1, 150a-2 would still be driven.

(39) As discussed in view of the figures, the downhole tractor in accordance with the invention has upper and lower drive sections separated by a valve module, wherein the number of upper and lower sections is flexible (for instance two as illustrated).

(40) As has been explained, the downhole tractor can run as a normal 4-section tractor, and using the adapted manifold (part of hydraulic power pack) it has become very flexible.

(41) The wireline tractor of the invention can also run with just the upper sections powered and the lower sections closed (tandem tractor upper mode). This gives the benefit of extra speed but it also means that the downhole tractor can push the closed lower sections past impassable obstructions (for example the Side Pocket challenge), where after the downhole tractor may be switched to the tandem tractor lower mode (lower sections powered and upper sections closed). The length of the valve module 400 actually is a benefit to ensure the lower sections have passed the obstacle.

(42) If needed the downhole tractor of the invention could be fitted in the lower sections with reverse arms so that the tractor goes forward with the upper sections powered or in reverse with the lower sections powered.

(43) In the table below there the various operational modes (including the ones that have been discussed with reference to FIGS. 4 to 9) that are possible with the embodiment of FIG. 3 are listed. This list is not exhaustive, but rather illustrative for what possibilities the invention creates. A 1 indicates that the valve is dumping to tank and a 0 indicates the other mode. The five modes mentioned at the bottom are modes that become possible if the lower drive sections are hard-wired such that the motors are in (permanent) reverse mode.

(44) TABLE-US-00001 TABLE Operational modes of an embodiment of the downhole tractor in accordance with the invention. Hydraulic power pack Valve section Operational Modes V1 V2 V3 V4 V5 V6 All drive section forward Motor Turning, No arm force, No wheel drive 1 1 0 0 0 0 Arms out, No wheel drive (rolling anchor) 0 1 0 1 0 1 (FIG. 9) Standard Tractor Full Flow 0 0 0 0 0 0 (FIG. 4) Standard Tractor Low speed (driven from arm force) 0 1 0 0 0 0 (FIG. 5) All Arms Open, Upper sections driving, Hi Speed 0 0 0 0 0 1 All Arms Open, Upper sections driving, Lo Speed 0 1 0 0 0 1 All Arms Open, Lower sections driving, Lo Speed 0 1 0 1 0 0 Tandem-Upper Sections Driven (lower closed), Hi speed 0 0 0 0 1 0 (FIG. 6) Tandem-Upper Sections Driven (lower closed), Lo speed 0 1 0 0 1 0 (FIG. 7) Tandem-Lower Sections Driven (upper sections closed) 0 1 1 0 0 0 (FIG. 8) Lower drive sections in reverse (hard-wired) Tandem-Upper Sections Driven (lower closed), Hi speed 0 0 0 0 1 0 Tandem-Upper Sections Driven (lower closed), Lo speed 0 1 0 0 1 0 Tandem-Lower Sections Driven Reverse (upper closed) 0 1 1 0 0 0 All Arms Open, Upper Driving, Lo Speed Shuttle Forward 0 1 0 0 0 1 All Arms Open, Lower Driving, Lo Speed Shuttle Reverse 0 1 0 1 0 0

(45) Furthermore, there are many variations possible with respect to the example embodiments here discussed. For example, the rerouting and the termination TM in FIG. 3 could be replaced by a 3-port valve and a termination allowing the hydraulic fluid to flow either in the first hydraulic supply line 200-1 (normal tractor operation) or the hydraulic bypass supply line 250. In another embodiment the relief valve controlling pressure in the secondary hydraulic line 300a can be an electrically controlled proportional relief valve. This adds even more flexibility and functionality (i.e. Weight on bit control) allowing the tractor drive pull to be remotely controlled.

(46) In a first variant the downhole tractor only has one hydraulic supply line instead of two (as is the case in the detailed description and the claims) in addition to the hydraulic bypass supply line. The bypass effect of the invention still applies in such embodiment.

(47) In a further variant the invention can be further extended in that at least one of the respective drive sections (comprising the hydraulic cylinders and the hydraulic motors may be replaced by other types of hydraulically-actuated equipment that can be turned on and off. Such piece of equipment may be placed both upstream as well as downstream of the valve section in order to benefit from the invention (i.e. to become individually controllable). Examples of such equipment are further hydraulic operated modules, pistons, motors, anchors, centralizers, actuators, and the like.

(48) The claims and description are focussed on implementing a first hydraulic bypass supply line to run from the hydraulic power pack to the valve section. However, in the same line of thought a second hydraulic bypass supply line may be implemented, whenever there is place for such line in the downhole tractor. This then opens up the possibility to apply the bypass feature similar to the second hydraulic supply line as it is done for the first hydraulic supply line as shown in the examples. This means that the second bypass supply line is then connected to a further inlet coupled to a further feeding point in the second hydraulic supply line within the valve section. This renders the check valve in the second hydraulic supply line optional as the first part of the second hydraulic supply line now always can be fed via the valve block.

(49) In a further variant the invention may be further extended in that there is provided two or more valve sections distributed over the length of the downhole tractor, each valve section being configured similar to the valve section as discussed in the figure description. The hydraulic bypass supply line in this embodiment is connected to inlets of the respective valve sections in order to provide hydraulic fluid directly thereto. The respective hydraulic supply lines may be provided with termination in order to divide said supply lines in part, such that they can be fed through the respective valve section that are connected to them.

(50) In view of the above-mentioned variations the invention, in accordance with a second aspect, also relates to a downhole tractor comprising a hydraulic system for driving a plurality of hydraulic components (such as hydraulic cylinders and hydraulic motors), wherein the hydraulic system comprises: a hydraulic power pack; a hydraulic supply line for supplying hydraulic fluid to the plurality of hydraulic components; a valve section comprising respective parts of said hydraulic supply line, and an inlet for receiving the hydraulic fluid, and a set of valves, and a hydraulic bypass supply line coupled to the hydraulic power pack for supplying the hydraulic fluid directly to the inlet of the valve section bypassing at least part of the hydraulic supply line; wherein the hydraulic supply line comprises a first part and a second part that are connected via the valve section, wherein the first part of the hydraulic supply line is connected to a sub-set of the plurality of hydraulic components, wherein the second part of the hydraulic supply line is connected to a further sub-set of the plurality of hydraulic components, wherein the valve section is configured for individually controlling flow of the hydraulic fluid into each respective part of the hydraulic supply line.

(51) The downhole tractor in accordance with the second aspect benefits from similar embodiments as mentioned for the downhole tractor in accordance with the first aspect.

(52) The particular embodiments disclosed above are illustrative only, as the invention may be modified and practiced in different but equivalent manners apparent to those skilled in the art having the benefit of the teachings herein. For example, the method steps set forth above may be performed in a different order. Furthermore, no limitations are intended to the details of construction or design herein shown, other than as described in the claims below. It is therefore evident that the particular embodiments disclosed above may be altered or modified and all such variations are considered within the scope of the invention. Accordingly, the protection sought herein is as set forth in the claims below.

(53) It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. Use of the verb comprise and its conjugations does not exclude the presence of elements or steps other than those stated in a claim. The article a or an preceding an element does to not exclude the presence of a plurality of such elements. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. In the device claim enumerating several means, several of these means may be embodied by one and the same item of hardware.