The invention relates to a device for preventing torsion in a tool string from exceeding a predetermined threshold. When performing work in a wellbore, for example during completion of the well, tools or equipment are often run into the wellbore using a tool string. In some situations, for exam ple due to challenging wellbore conditions, it may be useful to rotate the tool or equipment to easier run it into the wellbore. Such a rotation may be performed using a motor at the surface, e.g. a top drive, to rotate the upper end of the tool string, and the rotation is then transmitted to the tool at the downhole end via the tool string. However, some tools and equipment cannot tolerate high levels of torsion. If the downhole tool gets stuck, the tor sion in the tool or equipment may become too large if the entire torque is transmitted to the tool or equipment. This could damage the tool, or possibly even break the tool string. Therefore, devices can be used which are located between the top drive and the down- hole tool, typically close to the tool, to prevent the torsion in the tool or tool string from exceeding a predetermined threshold. Such devices are often referred to as safety subs.

These devices, which are typically based on shear pins, allow transmission of torque/rotation up to a certain pre-set torque level. When the torque exceeds this level, the pins shear, and the device is basically a swivel afterwards. This has the disadvantage that the shear pins only function once, whereafter they must be replaced or repaired if torque is to be transferred to the downhole tool again. Other types of devices comprise a mechanism for turning the swivel feature on and off, e.g. mechanically or hydraulically, so that the device either transmit all the torque to the tool, or no torque at all. Such a device still has the disadvantage that torsion in the tool may be too large when all torque is trans- ferred to the tool.

A solution to this problem is proposed in US8852004B2, which discloses a downhole torque-limiting assembly for a tool string. If the pre-set torsion threshold is exceeded, the assembly disclosed therein prevents torque from being transmitted from the motor to the tool. When the torsion is decreased to a level below the threshold, the assembly will transmit the torque again without the need of replacement of any parts. However, as the assembly includes protrusions sliding along inner surfaces of corresponding receptacles, the protrusions and receptacles suffer from a significant degree of wear. The invention has for its object to remedy or to reduce at least one of the drawbacks of the prior art, or at least provide a useful alternative to prior art. The object is achieved through features which are specified in the description below and in the claims that follow. The invention is defined by the independent patent claim, while the dependent claims de fine advantageous embodiments of the invention. More specifically, the invention relates to a device for preventing torsion in a tool string from exceeding a predetermined threshold, wherein the device comprises: an inner man drel for transmitting torque between said inner mandrel and a first portion of the tool string, the inner mandrel comprising at least one flexible member; an outer pipe for transmitting torque between said outer pipe and a second portion of the tool string; and a cage ar- ranged between the inner mandrel and the outer pipe, the cage comprising rollers for no slip rolling on the outer pipe; wherein the flexible member is configured to form a flexible wedge trap for trapping one of the rollers when the torsion between the inner mandrel and outer pipe is below a predetermined threshold, and for the flexible member to flex and allow the roller to roll past the flexible member and escape the flexible wedge trap when the pre-determined torsion threshold is exceeded.

The outer pipe may typically be connected to the second portion of the tool string, but alternatively the tool string may be utilized as the outer pipe so that the outer pipe is part of the second portion of the tool string. Either way torque will be transmitted to the second portion of the tools string from the outer pipe. The inner mandrel may typically be con- nected to an upper portion of a tool string, i.e. transmitting torque between the inner man drel and the upper portion of the tool string, while the outer pipe may typically be connect ed to a lower portion of the tool string. The device connected to a tool string will thereby prevent that torsion in the tool string or connected equipment or tools exceed a predeter mined threshold value. If the value is exceeded, for example because the tool gets stuck while rotating, the flexible member will flex enough for the roller to escape the wedge trap, whereby the inner mandrel and outer pipe will rotate relative to each other without being damaged. This mechanism is herein referred to as a clutch feature. The device will in this way be able to unload torque-transmission multiple times. The free relative rotation be tween the inner mandrel and outer pipe will persist until the flexible member meets anoth- er roller, typically an adjacent roller. If the torsion has by then decreased, for example if the tool is loose again, this roller will be trapped inside the flexible wedge trap and trans mit torque between the inner mandrel and the outer pipe. The inner mandrel and the outer pipe can be rotationally connected to each other using any suitable mechanism which can withstand compression and tension, for example comprising a thrust bearing.

Since the rollers roll without slip on both the outer pipe and the flexible members, there is no or little sliding between different portions of the device, which minimizes the wear. The device will therefore function for a longer period, even if the clutch feature is activated regularly. The rollers may typically be distributed evenly in the circumferential direction, whereby the wear is evenly distributed, and the device is stabilized. The device may be configured so that the rollers are constantly in contact with both the outer pipe and the inner mandrel including flexible members for further stabilization of the device. At least three evenly distributed rollers are required for the rollers to function as a bearing, but more may typically be desired to decrease the wear on each roller.

The flexible member may be a flexible blade, or it may be an assembly comprising a flexi ble part and a rigid part, for example a spring with a rigid blade. The important aspect is that the flexible member forms a flexible wedge trap which traps a roller when the torsion is below a predetermined threshold but flexes and allows the roller to escape when the torsion is above the predetermined threshold. The level of the threshold depends on the flexibility of the flexible member. The flexible member may be slight curved at the radially outermost side which faces the rollers, so that this side of the flexible member to a larger degree follows the curvature of the inner mandrel. In this way it may be assured that the rollers roll on the entire surface of the flexible member, which will stabilize the rollers and the device and ensure that the cage is connected with both the outer pipe and the inner mandrel at all time.

The inner mandrel may comprise a plurality of flexible members configured to form a plu rality of flexible wedge traps distributed evenly in the circumferential direction of the inner mandrel, wherein the number of rollers divided by the number of flexible wedge traps is equal to a positive integer. When the rollers and flexible wedge traps are evenly distribut ed and satisfy this condition, the positions of the wedge traps relative to the rollers will be the same for all wedge traps. Thus, in torque-transmitting mode, each wedge trap will have trapped a roller, whereby the torque transmitted between the inner mandrel and the outer pipe will be divided between the different pairs of rollers and blades. This makes the device more stable and decreases the wear on each roller and flexible member even fur ther. The rollers and flexible members may for example be equally distributed in the cir- cumferential direction. When more flexible members are used, the risk of backspin of the device is also decreased. For example, when the device is positioned deep within a well bore and being rotated from the surface, the tool string may continue to rotate a short dis tance after the tool gets stuck downhole, and torque will be build up in the tool string. When the torque becomes too high and the roller escapes the flexible wedge trap, the build-up torque in the tool string will cause one portion, typically the inner mandrel, to start rotating with a high rotation speed until the next pair of flexible wedge trap and roller meet. This is referred to as backspin. Depending on the torque at this next pair, the roller may get trapped or may escape this flexible wedge trap as well. The more pairs of flexible wedge traps and rollers, the more the backspin will be slowed down. The risk of injuring any portion of the tool string due to a high degree of backspin is thereby decreased.

The distribution and numbers of rollers and flexible members may also be chosen such that there will be more than one roller engaging the flexible member at any position. This may have the effect that the rollers stabilize the flexible members to ensure that they flex as desired and do not experience undesired strain. Such stabilization of the flexible mem bers by more than one roller may also put less requirement on the construction and at tachment of the flexible members. For example, if the flexible members are positioned at least partly in corresponding pockets in the inner mandrel, the more than one roller may additionally function to hold the flexible members in place in the pockets, whereby the flexible members are not required to be fastened any further to the inner mandrel. This may make both the production and maintenance of the device easier, and thereby more cost-effective. More than one flexible member may engage a respective roller, for exam ple two flexible blades positioned in a pocket in the inner mandrel. By using more than one flexible blade with the same spring constant for engaging a roller, the flexible blades may provide the same force as for a single blade but a larger radial displacement at the transition point. This may make the clutch feature more reliable as there are less require ments on the accuracy of the positions of the flexible members relative to the rollers. Even if the radial position of one wedge trap is slightly different than the radial positions of other wedge traps, it will provide substantially the same force to the rollers as the others.

It has been observed that a circumferential distribution of seven flexible members and twenty-one rollers provides a good performance and stability.

In one embodiment, the device may comprise a plurality of layers of flexible wedge traps in an axial direction and a cage for each layer of flexible wedge traps. In this way the transmitted torque may be divided into even more pairs of flexible members and rollers. The cages in each layer may preferably be interlocked along a longitudinal direction of the device so that all cages will rotate at the same time and speed. This will ensure that all flexible wedge traps will trap a roller at the same time so that the torque will be divided into all possible pairs of flexible members and rollers. The cages may for example be in terlocked by means of protrusions and receptacles with complementing shapes, for ex ample a sawtooth structure on adjacent cages.

In one embodiment, the device may be fluid-tight and comprise a lubricating fluid to lubri cate the device to further reduce wear of the moving parts. The lubricating fluid may also help to distribute any heat, which may build up at specific locations, for example around the wedge traps, to other part of the device. This may ensure that the device does not overheat, thereby further prolonging the life span of the device. The device may advanta geously comprise a compensation piston which is configured to balance the pressure be tween the inside and the outside of the device. Since the device is intended for use in a wellbore, the pressure surrounding the device may in use in the wellbore be significantly higher than atmospheric pressure. By balancing the pressure with a compensation piston, a large difference in differential pressure across the outer pipe may be avoided, thereby putting less requirements on the strength on this outer pipe. This may make the device less costly to manufacture. Additionally, the outer pipe may be thinner, whereby the flexi ble wedge traps and rollers may be positioned further out from the centre of the device. The device may thereby be able to transmit more torque.

In one embodiment, the device may comprise a spline sleeve for locking rotation of the inner mandrel and outer pipe relative to each other. This has the advantage that a higher torque may be transmitted via the device if required. The spline sleeve may for example comprise a locking mechanism which activates upon increase in pressure. The spline sleeve can for example have a ball seat valve which allows pressure to be increased above the ball seat if a ball or dart is dropped into the tool string to seal the seat. If the spline sleeve is connected to the inner mandrel via a spline connection and comprises shear pins for maintaining the position of the spline sleeve relative to the inner mandrel, an increase in pressure may cause the shear pins to shear and allow the spline sleeve to move axially within the inner mandrel. As the spline sleeve slides downwards, a portion of the splines of the spline sleeve may engage with internal splines on the outer pipe. In this way the inner mandrel and outer pipe are locked relative to each other. Darts for blocking the ball seat may comprise a hole sealed by a membrane which can withstand the pres sure needed to engage the locking mechanism, but which will rupture if the pressure is increased further above a certain value. In this way fluid flow through the device may be continued after the device has been locked. In the following is described an example of a preferred embodiment illustrated in the ac companying drawings, wherein:

Fig. 1 shows a cross-sectional view of a device according to the invention, where in the device is cut perpendicular to the axis of the tool string; and Fig. 2 shows a cross-sectional view of a device according to the invention, where in the device is cut along the axis of the tool string.

In the figures, the reference numeral 1 indicates a device according to the invention. Iden tical reference numerals indicate identical or similar features. The drawings are shown in a schematic manner and are not necessarily drawn to scale. Figure 1 shows a cross-sectional view of a device 1 according to the invention, wherein the device 1 is cut perpendicular to the axis of a tool string. The device 1 comprises an inner mandrel 3, which is the main internal part of the device 1. The inner mandrel 3 has a circular opening 4 for being connected to the tool string above the device 1. The inner mandrel 3 comprises flexible blades 5 as flexible members. The flexible blades 5 are posi- tioned in complementary pockets 7 in the inner mandrel 3. In the shown embodiment each pocket 7 houses two blades 5, but one or more blades 5 may be used depending on the shape and flexibility of the blades 5. The flexible blades 5 are shaped to form a flexible wedge trap 9 for rollers 11. An advantage of using more than one blade 5 may be that the radial displacement of the blades 5 at the transition point 8 can be larger while the force provided by the flexible blade 5 is the same. The flexible blades 5 are slightly curved at their radially outermost side to ensure that the rollers 11 will be in contact with the flexible blades 5. The rollers 11 are positioned in a cage 15 with equal circumferential distance between each roller 11. The rollers 11 are in constant contact with an outer pipe 13 in a no-slip rolling condition. The flexible blades 5 and the rollers 11 are positioned such that the relative positions of different rollers 11 in different wedge traps 9 are the same. In this way the rollers 11 transmit torque from the inner mandrel 3 via the blades 5 to the outer pipe 13. In the shown embodiment, the inner mandrel 3 is rotated in a clockwise direction and transmitting torque to the outer pipe 13. The outer pipe 13 is connected to the tool string below the device 1. If the torsion increases, for example due to the tool getting stuck, the flexible blades 5 flex inwards and move over the rollers 11, whereby said rollers 11 will escape the flexible wedge trap 9 without transmitting torque to the outer pipe 13. If the torsion decreases again, for example due to the tool not being stuck anymore, the rotation of the inner mandrel 3 relative to the outer pipe 13 will cause rollers 11 to be trapped in the flexible wedge traps 9 again, whereby the device 1 resumes the transmis sion of torque from the inner mandrel 3 to the outer pipe 13 via the flexible members 5 and the rollers 11. The shown embodiment with twenty-one rollers 11 and seven pockets 7 with flexible blades 5 per axial layer has been shown to provide a good and stable solu tions. The torque-transfer is distributed well around the circumference, and the flexible blades 5 are stabilized by at least two rollers 11 per pocket 7 at every rotational position. Such stabilization of the flexible blades 5 by a plurality of rollers 11 puts less requirements on the fastening of the flexible blades 6 to the inner mandrel 3.

Figure 2 shows a cross-sectional view of the device 1 according to the invention, wherein the device 1 is cut along the axis of the tool string. The shown embodiment comprises two layers of flexible blades 5 and correspondingly two cages 15 comprising rollers 11, but even more layers of flexible blades 5 and cages 15 can be included to increase the stabil ity and reduce the load on each roller 11 and blade 5. The shown embodiment additionally comprises a spline sleeve 17 which connects the inner mandrel 3 and the outer pipe 13. The spline sleeve 17 is connected to the inner mandrel 3 with a spline connection 19. Shear pins 21 ensure that the inner mandrel 3 and the spline sleeve 17 cannot move rela tive to each other. A ball seat 23 provides a mechanism for shearing the shear pins 21. A ball or dart can be pumped from the top of the tool string to the ball seat 23, where it stops and creates a sealed barrier for fluid, which will allow pressure to be increased above the ball seat 23. At high enough pressure the shear pins 21 will shear and allow the spline sleeve 17 to move relative to the inner mandrel 3. When the shear pins 21 shear, the spline sleeve 17 will move downwards and engage in the spline connection 25 (indicated with dashed lines) with the outer pipe 13. The downward movement of the spline sleeve may be stopped by e.g. a shoulder (not shown) inside the outer pipe 13. When the spline sleeve 17 is engaged with both the inner mandrel 3 and the outer pipe 13 through the spline connections 19, 25, relative rotation between the inner mandrel 3 and the outer pipe 13 is denied. In this way the device 1 may be locked to allow higher torque to be transmitted to the tool. A screw 27 ensures that the spline sleeve 17 and the inner man drel 3 are connected at all time so they do not slide apart if there is tension across the device 1.

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 embodi ments without departing from the scope of the appended claims. In the claims, any refer ence 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 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.