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Downhole drilling tool

Downhole drilling tool description/claims

This application is a divisional of U.S. patent application Ser. No. 12/079,550, entitled “DOWNHOLE DRILLING TOOL”, as filed Mar. 27, 2008, which is a divisional of U.S. patent application Ser. No. 11/033,576, entitled “DOWNHOLE DRILLING TOOL”, as filed Jan. 12, 2005.

The invention relates to a downhole drilling toot for drilling an oil well, and in particular to a downhole drilling tool that can transmit a high torque without slipping in a downhole drilling operation.

When drilling a well bore using a downhole drilling tool including a downhole motor, the drilling head can become snagged or stuck on the rock formation. In order to free the drilling head it is necessary to apply a very large torque to the drilling head, which is much larger than the torque available from the downhole motor. Therefore the torque must be applied to the drilling head from the surface through the drillstring.

If this is not possible, or the drilling head cannot be freed, then the downhole motor and drilling head have to be abandoned in the well bore, and the direction of the well bore diverted around the abandoned drilling head. This is extremely time consuming and expensive.

It is a problem that in a typical downhole motor, of the turbine type, there is no convenient way of transferring the torque of the drillstring to the drilling head, save for effectively jamming the mud motor or turbine shaft to the motor housing, which is connected to the drillstring. The housing of the turbine is mounted to the end of the drillstring, and therefore torque applied to the drillstring can be passed through the housing of the turbine. In one known method, metal balls or spheres are pumped down the centre of the drillstring to the turbine in the pressurized mud flow, which on reaching the turbine, lodge in cavities between the housing and the drive shaft of the turbine. The cavities are positioned and shaped such that when the housing is rotated in the drilling direction relative to the drilling head, the balls are forced into locking engagement with the housing and shaft, thus locking them together. The torque of the drillstring applied at the surface is therefore passed through the shaft to the drilling head and the required passage of fluid through the drillstring is not compromised.

A disadvantage of this method is that the torque from the drillstring is passed substantially through the whole length of the shaft of the turbine, which is primarily designed to transfer the torque generated by the turbine alone. The torque passed through the drillstring is significantly greater, and this can cause the shaft to fail, if the drilling head proves unmovable.

A further disadvantage of this method is that even if the drilling head is released from the rock formation, then the turbine and drilling head must be withdrawn to the surface to facilitate removal of the locking balls, before drilling can be resumed.

According to a first aspect of the present invention, there is provided a downhole drilling tool comprising a drillstring and a drilling head, wherein a freewheel for transferring drive from the drillstring to the drilling head is positioned between the drillstring and the drilling head.

The freewheel may be positioned at any point in the drive train between the drillstring and the drilling head. Preferably, the freewheel is provided in a downhole motor assembly which transfers drive to the drilling head.

A freewheel device links a driving member, that is, a member providing a driving force or torque, to a driven member. A freewheel device is designed so that, when the rotational speed of the driving member is greater than the rotational speed of the driven member, the freewheel locks the driving and driven members together so the torque driving the driving member is transmitted to the driven member. When the rotational speed of the driving member is less than the rotational speed of the driven member, the freewheel allows the driven member to rotate freely of the driving member. Requirements of a freewheel device of the invention are that the freewheel locks quickly when a sufficient torque is applied, and that the freewheel does not slip when subjected to very high torques.

In some types of freewheel devices, the locking and unlocking action is achieved by jamming one or more rollers or ball bearings between the driving and driven members, and by releasing them respectively. The rollers or ball beatings are located between a pair of races with one race being formed on the driving member and the other race being formed on the driven member. One of the races is designed so that, when a sufficient torque is applied to the driving member, sloped faces on the race bear against the rollers or ball bearings, so that they are jammed between the sloped faces and the race formed on the driven member. Torque is therefore transmitted through the rollers or ball bearings from the driving member to the driven member. When the driving force or torque reduces, the rollers or ball bearings are released, allowing the driven member to rotate freely of the driving member.

However, if the driving force or torque is particularly large, for example in a downhole turbine or other downhole drilling tool, the rollers or ball bearings may slip from their jammed positions, causing the freewheel to unlock. Although the freewheel will re-lock when the rollers or ball bearings again jam between the sloped faces and the race formed on the driven member, the slipping of the freewheel may result in excessive wear or damage to the freewheel or to the driving or driven machinery.

There is therefore a need for a freewheel device that locks quickly when a driving torque is applied, and which is less susceptible to the locking elements slipping at high torques. There is also a need to allow the free passage of fluid through the device.

Therefore in a second aspect of the present invention, there is provided a freewheel, comprising:

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Previous Patent Application:
Drilling fluid flow facilitation
Next Patent Application:
Downhole assembly and cutter assembly
Industry Class:
Boring or penetrating the earth

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