TECHNICAL FIELD The present invention relates to a downhole apparatus recovery and in particular to the recovery of apparatus, for example drill string, that is stuck in a well.

BACKGROUND Drill strings are used in the creation of wells and comprise multiple components. At the lower end of the drill string is a bottom hole assembly (BHA), which typically includes a drill bit, drill collars, reamers, and stabilisers. The drill sting may comprise subs which are small components located in the drill string. An example of a sub is a crossover sub, which is used to crossover from one connection size to another, allowing different types of equipment to attach to the drill string. The BHA typically connects to a transition pipe, which connects to a drill pipe. The drill pipe extends up to a derrick, which is located at the surface of the well for onshore drilling, and a rig for offshore drilling. The derrick includes a hoist for lowering and raising the drill string into the well. The derrick may also include a top drive for transmitting torque to the drill string and mud pumps for pumping circulation fluid down the well.

During the drilling process, a part of the drill string may become stuck. This is commonly known as "going stuck", and in such situations it can be challenging to free the drill sting and retrieve equipment attached to the drill string. The drill string can become stuck for many reasons. For example, the well may collapse around the BHA during drilling or collapse around completion equipment during completion. Alternatively, the shape and size of sections of the well may be such that the equipment cannot be pulled past such sections. In situations where the drill string or equipment becomes stuck and is unable to be retrieved, it is usually necessary to cut the drill string at a point above the section that is stuck, leaving a portion of the drill string and equipment downhole. This may be done by using a wireline cutting assembly or by using explosives run down the well on a wireline. In a liner drilling scenario (also known as drilling with liner scenario), retrieving a stuck drill string is even more complicated. During liner drilling, a pilot BHA is connected to an inner string, which is run inside a liner such that the pilot BHA protrudes from the liner. However, as the pilot BHA is smaller in diameter than the liner, the hole drilled by the drill bit of the pilot BHA is too small for the liner to fit through. In order to address this, an underreamer can be used to expand the diameter of the hole such that the liner may enter the hole. The underreamer has extendable arms having cutters which, having been run through the liner, are extended and used to increase the hole size. The underreamer arms are retracted when the inner string and pilot BHA are to be pulled out of the well. However, if the underreamer's extendable arms fail to retract, it will not be possible to pull the underreamer past the liner and out of the well.

In situations where the pilot BHA becomes stuck, the drill pipe must be cut above the liner since restrictions at the top of the liner drilling system prevent wireline equipment from passing through the top of the liner assembly. Furthermore, when drilling with liner it is impossible to pull the liner or cut and pull part of the liner out without first retrieving the inner string. If the inner string has become stuck, due to the BHA becoming stuck, it is not possible to pull the liner or a part of the liner out of the well, and so a sidetrack becomes impossible and the well might be lost Present retrieval means can result in an excessive amount of equipment being lost. Drilling equipment is expensive and so losing equipment down a well can have serious financial consequences. Therefore, it is desirable to minimise the amount of equipment that is lost down a well. SUMMARY

It is an object of the present invention to provide a more efficient way of retrieving a stuck drill string from a well. According to an aspect of the present invention there is provided a sub for connection into a drill string. The sub comprises an upper body and a lower body, and is configured so that torque can be transferred from the upper body to the lower body. The upper and lower bodies are axially coupled by means of an axial coupling that is arranged to separate when placed under a predetermined tension. Once separated, the upper body can be separated from the lower body. The axial coupling is substantially isolated from said torque.

Advantageously, this allows for an efficient way to retrieve part of a stuck drill string without having to run a wireline cutting assembly down the well. Furthermore, because the axial coupling is arranged such that it is substantially isolated from the torque, it is easier to construct an axial coupling that fractures when placed under the predetermined tension, i.e. shear stress does not need to be taken into account when designing or selecting the axial coupling.

As an option, the separation is caused by a fracturing of the axial coupling.

As an option, the torque is transferred from the upper body to the lower body by means of one or more interlocking features extending generally axially on the upper and lower body. The interlocking features may comprise fingers that axially extend from the upper or lower body and mate with corresponding recesses located on the other of the upper or lower body.

As an option, the axial coupling means comprises a sleeve extending around the upper and lower bodies. The sleeve is arranged to inhibit axial separation of the upper and lower bodies whilst not inhibiting their relative rotation. The sleeve and one of the upper and lower body may comprise complementary screw threads. The threads are arranged to allow the sleeve to be connected to one of the upper and lower body. As an option, the sleeve is connected to one of the upper and lower body by means of sliding and abutting shoulders on the sleeve and on the body.

As an option, the sleeve comprises a material that is weak relative to the material of a drill string such that the sleeve fractures when placed under a predetermined tension. This arrangement ensures that the sleeve fractures before the drill string or other components of the drill string.

As an option, a thickness of a wall of the sleeve is such that the sleeve fractures when placed under a predetermined tension. Since the maximum predetermined tension that the sleeve can withstand is a function of the wall thickness, selection of the appropriate wall thickness allows for the selection of a sleeve that will fracture when subjected to the predetermined tension.

As an option, the sleeve comprises a substantially circumferential groove along which the sleeve is arranged to fracture. This groove provides a weaker section of the sleeve which will fracture before a further section of the sleeve.

As an option, the predetermined tension is less than the maximum tension that the drill string can withstand, such that the sub fractures before the dill string.

As an option, the upper and lower bodies are substantially cylindrical.

As an option, the upper and lower bodies comprise screw threads for screwing into respective parts of a drill string.

As an option, the upper and lower bodies comprise a through bore arranged to allow circulation fluid to flow through the sub.

According to a second aspect of the present invention there is provided a sub for connection into a drill string. The sub comprises an upper body and a lower body. The upper or lower body comprises one or more fingers that axially extend from the upper or lower body and mate with corresponding recesses located on the other of the upper or lower body such that torque can be transferred between the two bodies. A sleeve extends around the upper and lower bodies, wherein the sleeve is connected to one of the bodies by way of abutment between a shoulder on an inner surface of the sleeve and a shoulder on an outer surface of the body. The sleeve is connected to the other body by way of complementary screw threads located on the inner surface of the sleeve and the outer surface of the other body. The sleeve is arranged to fracture when placed under a predetermined tension, allowing axial separating of the upper body from the lower body.

According to a second aspect of the present invention there is provided a method of recovering a stuck drill string from a well, wherein the drill string comprises a sub according to any one of the first and second aspects. A detection is made that the drill string has become stuck in the well. A predetermined axial force is applied to the drill string so as to create the predetermined tension in the sub. The predetermined tension fractures the axial coupling of the sub, allowing recovery of a part of the drill string located above the lower body. BRIEF DESCRIPTION OF DRAWINGS

Figure 1 shows a partial cross-section through a sub according to an embodiment of the invention;

Figure 2 shows a partial cross-section through an upper body of the sub;

Figure 3 shows a partial cross-section through a lower body of the sub; and

Figure 4 is a flow diagram according to an embodiment of the invention.

DETAILED DESCRIPTION With reference to the Figures, there is now described a sub, herein referred to as a weak link sub. The weak link sub is designed to be a "weak link" in the drill string, i.e. by having a lower tensile strength then the rest of the drill string. If the drill string becomes stuck, by pulling upward on the drill string with a predetermined force, the weak link sub will fracture, separating the drill string into two parts. This allows equipment above the weak link sub to be retrieved without having to use wireline cutting assemblies or explosives.

Figure 1 shows a weak link sub 1 according to a first embodiment of the invention. The sub is shown in partial cross-section as will be understood from the following discussion. The weak link sub 1 comprises an upper body 2 and a lower body 3 shown separately in Figures 2 and 3. The upper body 2 and lower body 3 are connected so as to rotate together by torque fingers 4 that extend from the upper body 2 and mate with grooves 5 in the lower body 3. This connection allows torque, supplied from a top drive, to be transferred between the two bodies and on towards the drill bit. The grooves 5 are arranged around an outer circumferential surface of the lower body 3.

The upper body 2 has a box connection 6 for receiving a pin connection of a device to be placed above the weak link sub. A part of the drill string such as the drill pipe (not shown) may connect to the upper body box connection 6. The lower body has a pin connection 7 for connecting with a box connection of a further device, such as the drill pipe or BHA. The upper and lower bodies may connect to the drill string or equipment in the drill sting by way of screw threads.

The upper 2 and lower 3 bodies are axially held together by a sleeve 8, which is coaxially located around the two bodies 2, 3. The maximum tension that the sleeve 8 can withstand before fracturing is chosen to be less than the tension required to fracture any other part of the drill sting. When the drill sting becomes stuck or trapped, an upwards force is applied to the drill string by the hoist at the derrick. As the drill string is fixed at a point down the well, the force applied by the hoist creates tension within the drill sting. Once this tension exceeds the maximum tension that the sleeve 8 can withstand, the sleeve fractures. Since the fingers 4 and recesses 5 do not provide a substantial axial coupling, the upper body 2 is axially separated from the lower body 3, and so anything above the upper body 2, including the upper body 2, can be pulled from the well.

The ultimate tensile strength of the sleeve 8 is a function of the material used in the sleeve, and of the thickness of a wall of the sleeve 8. Therefore, by selecting a particular material for the sleeve 8, and/or selecting an appropriate wall thickness, the sleeve can be designed to fracture when exposed to a predetermined axial force. A circumferential groove may be applied to a surface of the sleeve, where the remaining wall thickness of the groove is chosen such that the sleeve fractures along the groove when subjected to the predetermined tension. The sleeve may have a section that is treated such that the treated section is weaker than the rest of the sleeve, and is arranged to fracture when subjected to the predetermined tension.

The sleeve 8 is connected to the lower body 3 via complementary acme screw threads 10 located on an inner surface of the sleeve 8 and an outer surface of the lower body 3. The sleeve 8 extends over the outer surface of the upper body 2, and is mounted on the upper body 2 by way of abutting shoulders 9. This arrangement is achieved by proving a first shoulder arranged around the outer circumference of the upper body 2 and a second shoulder arranged around the inner surface of the sleeve 8. The second shoulder is arranged above the first shoulder so as to prevent the sleeve 8 from falling off the bottom of the upper body 2. This arrangement also allows the sleeve and upper body 2 to rotate relative to one another. The connection means between the sleeve and bodies may be reversed (for example, the acme threads may be used between the upper body and sleeve).

Alternatively, the sleeve may screw into threads located on both the upper 2 and lower 3 bodies, or the sleeve may be welded to both the upper and lower bodies. These solutions would rely upon the fingers and grooves preventing any relative rotation of the bodies sufficient to weaken or break the joints.

By using the arrangement described, torque can be transferred from the upper to lower body without transferring substantial torque to the sleeve 8. In other words, the sleeve 8 is substantially isolated from the torque. The only significant force that acts on the sleeve 8 is in the axial direction, and so the sleeve can be designed and manufactured with this in mind. The weak link sub 1 has a through-bore (not shown) for allowing circulation fluid from the drill pipe to flow through the weak link sub 1 . The diameter of the through bore is similar to the internal diameter of the drill pipe in order to help avoid large pressure differentials, and to allow drop balls to pass through. Where the upper body 2 and lower body 3 connect, a sealing element 1 1 , such as an O-ring, is provided to prevent loss of the circulating fluid as it is pumped through the weak link sub 1 .

With reference to Figure 4, where will now be described a method of using the weak link sub 1 . S1 . The positioning of the weak link sub 1 in the drill string is decided. This may be decided based on a number of factors such as by determining the likelihood of each part of the drill sting becoming stuck. The weak link sub 1 may be located at any suitable point along the drill string. During liner drilling, the weak link sub 1 is typically placed inside the liner as low as possible, usually just above the pilot BHA. If the pilot BHA becomes stuck, the positioning of the weak link sub 1 means that the inner string is still retrievable. Multiple weak link subs may be used in the same drill string. In this example, the weak link sub is placed above the BHA.

S2. A value of the maximum tension that the weak link sub 1 can withstand is selected. When selecting the value of the maximum tension that the sleeve 8 can withstand before fracturing, the expected maximum tensile load of the drill string and of any equipment to be positioned above the weak link sub 1 will be taken into account. This is to make sure that it is the weak link sub that fractures, and not a part of the drill string. The axial force that can be supplied to the drill string when downhole may also be taken into consideration when selecting the maximum tension. An estimation on a likely position that a drill string will become stuck may also factor into the selection of the maximum tension. This is because a drill string of greater length will be heavier than a drill string of shorter length. A longer (heavier) drill sting will require a greater upwards force to impart the same tension in the weak link sub 1 as compared to a shorter (lighter) drill string, as some of the force will be used to overcome the additional weight of the longer string. Once the value of the maximum tension is selected, a specific weak link sub is either manufactured, modified, or selected from a plurality of weak link subs, which meet the maximum tension criterion. S3. The weak link sub 1 is incorporated into the drill string, and the drill sting is run into the well from a derrick located on a rig to commence drilling.

54. A part of the well collapses around the BHA. The BHA cannot be pulled past the blockage, and so the drill string cannot be pulled out of the well.

55. Once it has been detected that the drill string has become stuck, an upwards force is applied to the drill string from a hoist at the rig. As the drill string is stuck and unable to move, tension builds up within the drill string. The upwards force supplied by the hoist is predetermined so as to create sufficient tension in the drill string and weak link sub 1. Once sufficient tension has been reached, the sleeve 8 of the weak link sub 1 fractures.

56. The drill string above the fracture point can be now be recovered from the well. Once the upper and lower bodies have been separated, cement can be pumped through the separated weak link sub 1 prior to pulling to surface, if required. In prior art systems, applying a lifting force to the drill string would not free the drill string, or cause the drill string to fracture. Instead, a wireline cutting assembly or explosives run on a wireline would need to be used in order to cut though the drill string. The weak link sub 1 described above improves operational efficiency when equipment becomes stuck down a well. This is because further equipment such as wireline cutting assemblies are not required to be run into the well in order to cut and free the drill string. Furthermore, due to the difficulty of running wireline cutting assemblies down wells, particularly when liner drilling, the wireline cutting assembly may not be able to reach the section of drill string that has become stuck. This can result in unnecessary sections of the drill string being left down the well. The weak link sub 1 addresses this issue as it does not need to be run down the well after the drill string becomes stuck. Therefore, the weak link sub 1 minimises the equipment lost in a well when the drill string becomes stuck.

It will be appreciated by the person of skill in the art that various modifications may be made to the above described embodiments without departing from the scope of the present invention. For example, a set of complementary interlocking teeth may be arranged on each body to replace the torque fingers and grooves. While the sleeve is described as being screwed into the lower body, and connected to the upper body by way of abutting shoulders, it will be appreciated that any suitable connection means may be used to attach the sleeve to the upper and lower bodies.