GB2289911A - Torsional damper - Google Patents

Abstract

A torsional damper is described primanly for use in an oil well tool. The oil well tool comprises mandrels (41, 42) contained within a housing body (44). Rotational torque is transmitted between the mandrels (41, 42) and the housing body (44) by elastomeric materials (64, 65). Drilling fluid flowing down the bore of the mandrels (41, 42) is prevented from communicating into the workings of the tool by seals (52, 53) which are mounted on a flexible seal carrier (55). <IMAGE>

Description

"Torsional Damper" This invention relates to a torsional damper, and in particular it relates to a torsional damper for use in oil-well drilling equipment.

In the process of downhole drilling where the drill bit is rotated and connected to the surface via a drill string, and where an axial load is applied to the drill bit via the bottom hole assembly, a number of dynamic forces are created. The superimposition of these forces can have damaging effects on the downhole drilling system and can lead to equipment failure such as NWD tools, drill bits and other drilling components and devices. These forces generally exhibit themselves in the way of forced vibration. The elements comprise of, but not limited to, harmonic loading due to the rotating effect of the system, transient and continuous loading as a result of the drilling action and the applied axial load (or WOB) onto the bit.

The problem often arises of how to control forced vibration for an existing system. It not only depends upon the physical characteristics of the system: mass, stiffness and damping but also on the relationship between the driving frequency and the natural frequency. Usually damping has the strongest influence and in most cases is the simplest controlling factor and can have a beneficial effect for harmonic excitation within a certain range of frequencies. The forces generated internally by the rotating parts and internal impacts are often transmitted over the entire drilling system. The extent of this transmission governs not only the noise but also the damage to the connecting parts of the rotating drilling system.

In this field it is already known that such devices, namely referred to as shock subs or tools, are readily available for use downhole. These tools are available having a variety of combinations of mechanical springs, elastomeric materials and hydraulic fluid to provide spring stiffness and damping resistance for axial load variations.

But this has the disadvantage since the method in which the rotary drive is transferred through these tools does not provide for any resilience/flexibility.

Conventionally splines are used between the mandrel means and the housing. Direct spline coupling does not provide for any compliance in the system when the drive is subjected to forces which create erratic rotary motion. Phenomena such as torque oscillations and drill string wind up are well known. These tools therefore have the disadvantage that they are limited in their extent in only being able to combat impact loads or shocks and have no compliance and or damping in any other plane.

Also these tools were designed at a time when large axial loads were being applied to the drill string due to the nature of the drilling technology. However, invariably these shock tools are used outside their useful operating range due to higher pump opening forces (POF) and lower drilling weights (WOB). This has resulted from the change in the design of present day drill bits.

The significance of the above and in particular POF & BR< WOB on these tools may be best described as follows.

The designs of these tools involves a single mandrel sliding inside a housing. The mandrel is splined to the housing to prevent independent rotation of the two main parts. As the mandrel slides into the housing its axial travel is limited by the resistive forces inside the housing of the spring/damper arrangement. This is sandwiched between the mandrel and the housing. As more weight is applied (WOB) the more active are these resistive forces. However, the use of smaller bit nozzles and higher pumping/circulating rates causes a much larger/higher hydraulic force (POF) in the bore of the mandrel as the mud flows down to the bit. This force acts on the mandrel in the opposite direction to the mechanical force or weight (WOB) along the axes of the tool. It is therefore possible if the tool is not specifically calibrated for a particular requirement, for the POF to be larger than the WOB.This would leave the mandrel basically pumped out against the outer stops on the housing. In this condition the tool would not be operative and serve no purpose.

According to the present invention there is provided a torsional damper comprising a first member and a second member concentric with the first member, said first and second members being mutually relatively rotatable, said first member having a non-constant external radius, said second member having a non-constant internal radius, and said torsional damper further comprising elastomeric means interposed between said first and second members to provide torsionally damped torsional coupling therebetween.

Said first member may be a shaft and said second member may be a sleeve surrounding at least part of the shaft, mutually facing surfaces of said shaft and said sleeve preferably each being formed with one or more lobes.

Said lobes may be dimensioned such that a respective lobe on each of the shaft and sleeve can undergo mutually interfering contact in the absence of said elastomeric means to rotationally link the shaft and sleeve upon such absence.

Said first and second members may be mutually relatively axially displaceable, and axial damping means and/or axial spring means may be interposed between said first and second members to provide nonrigid axial coupling therebetween.

A further shaft may be coupled to the sleeve preferably by a further elastomeric means.

Preferably the torsional damper is interposed between the first and second ends of an oil well tool, the oil well tool typically having a flow channel extending between the first and second ends, the first and second ends of the oil tool preferably being capable of connection in a tool string.

While further modifications and improvements may be made without departing from the scope of this invention, embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings in which: Fig. la and Fig. 1b together form a cross sectional view of a cross sectional view of a drill string connector forming an embodiment of the invention; Fig. 2a is a transverse cross section of the connector of Fig. la and Fig. 1b on the line A-A of Fig. la.

Fig. 2b is a transverse cross section of the connector of Fig. la and Fig. 1b on the line B-B of Fig. lb.

Fig. 2c is a transverse cross section of the mandrel of Fig. la on the line C-C.

Fig. 3 is a cross section of a further embodiment.

Fig. 4 is a transverse cross section of the tool of Fig. 3 on the line A-A.

Embodiments of the invention such as shown in the figures are used to cushion both axial and torsional oscillations in the drill string and bottom hole assembly. In addition there is elastomeric material to provide for flexibility in the bending plane along the axis of the tool.

Fig. la and Fig. 1b together show a drill string connector comprising mandrels 41, 42 which are contained within a housing body 44 and are retained by split nuts 45, 46. Springs and/or elastomeric material 47, 48 provide flexible and resistive force to the outward sliding movement of the mandrels 41, 42 from the housing body 44 by butting up against the split nuts 45, 46. A spring and/or elastomeric material 49 provides the resistive compressive force between the mandrels 41, 42 in the middle of the housing body 44, whilst spacers 50, 51 provide a uniform force on the spring and/or elastomeric material 49 from the mandrel end. The drilling fluid flowing down the bore of the mandrel is prevented from communicating into the workings of the tool by seals 52, 53 which are mounted on a flexible seal carrier 55.Vibration isolating cushions 54, 56 provide support in the bending plane of the mandrels 41, 42, and are shown in more detail in Fig. 2c. The rotary drive is taken through elastomeric materials 64, 65 and is described in more detail below in Fig. 2a.

Fig. 2a provides a view in section A-A, showing the rotary drive arrangement between the mandrel and housing. This arrangement replaces the splines as normally used on drilling tools. An internal cam geometry 62 of the housing bore is such that the maximum diametrical dimension of the external cam geometry or lobes 60 of the mandrel is greater than the smallest diametrical dimension of the internal cam geometry 62.

Fig. 2b shows that if the elastomeric material 64, 65 were to fail, the invention has the advantage that rotational torque would continue to be transmitted between the mandrels 41, 42 and the housing body 44.

Fig. 2c shows that the vibration isolating cushions 54, 56 are secured to the mandrels 41, 42 by means of screws 59.

Fig. 3 shows another embodiment of the invention, a drill string connector incorporating a single mandrel shaft 1 which is contained in a housing body 2. The mandrel shaft 1 is allowed to slide inwards compressing an arrangement of onbottom springs 3, 3a and 4 into an elastomeric material 5 via spacers 6. The mandrel shaft 1 is also allowed to move in the opposite or outward direction by compressing an elastomeric material 7 and spring 8 via spacers 9, 10 onto a retaining nut 11. The retaining nut 11 provides the means of preloading the springs 3, 3a, 4, 8 and elastomeric materials 5, 7 on assembly of the mandrel shaft 1 into the tool and thereafter prevents the mandrel shaft 1 from sliding completely out of the housing body 2. Within the retaining nut 11 are seals 12 and packing 15 preventing wellbore fluids from contaminating the inside workings of the tool.The seals 12 once installed are maintained in position by a spacer 13 and fasteners 14. As the mandrel shaft 1 slides relative to the housing body 2, the change in the internal volume is compensated by a piston 16. The piston 16 contains seals 17, 18, 19, 20 which prevent drilling fluids flowing down through the bore of the mandrel shaft 1 from communicating with the internal workings of the tool. The piston 16 is prevented from being separated from the mandrel shaft 1 by a second retaining nut 21 which is threaded onto the mandrel tail section. A crossover sub 22 is used to allow the tool to be readily assembled into the drill string.

In Fig. 4 a separate cylinder can 23 contains an internal cam geometry 24 which accommodates elastomeric material 25 and an external mandrel lobe 26. The separate can 23 slides together with the mandrel shaft 1 inside the housing body 2. The separate cylinder can 23 is prevented from rotating independently from the housing body 2 via drive keys 27 mounted into the housing body 2 and retained by fasteners 28. In this embodiment the relative rotary movement is contained between the mandrel shaft 1 and separate cylinder can 23 whilst there is a spline drive between the separate cylinder can 23 and the housing body 2.

If the elastomeric material were to fail, the invention has the advantage that rotational torque would continue to be transmitted between the mandrel shaft 1 and the housing body 2, on the basis that the maximum diametrical dimension of the external mandrel lobe 26 is greater than the smallest diametrical dimension of the internal cam geometry 24.

The invention may be incorporated in other forms of tool, such as downhole stabilisers and downhole roller reamers.

The invention introduces into the downhole drilling system a mechanism which acts as a vibration isolator.

The device can be configured so as not to be effected by hydraulic forces. This device is interposed between the drill bit and drill string and minimises the vibrational forces transmitted during the drilling process.

Claims (7)

1. A torsional damper for use in oil-well drilling equipment comprising a first member and a second member concentric with the first member, said first and second members being mutually relatively rotatable, said first member having at least a portion which is of a nonconstant external radius, said second member having at least a portion which is of a non-constant internal radius, and said torsional damper further comprising elastomeric means interposed between said first and second members to provide torsionally damped torsional coupling therebetween.

2. A torsional damper according to claim 1 wherein the first member is a shaft and said second member is a sleeve surrounding at least part of the shaft, mutually facing surface portions of said shaft and said sleeve each being formed with one or more lobes.

3. A torsional damper according to either of the preceding claims wherein the lobes are dimensioned such that a respective lobe on each of the shaft and sleeve can undergo mutually interfering contact in the absence of said elastomeric means to rotationally link the shaft and sleeve upon such absence.

4. A torsional damper according to any of the preceding claims wherein the first and second members are mutually relatively axially displaceable, and axial damping means and/or axial spring means are interposed between said first and second members to provide nonrigid axial coupling therebetween.

5. A torsional damper according to claim 2, including a further shaft coupled to the sleeve by a further elastomeric means.

6. A torsional damper as substantially hereinbefore described with reference to and as shown in Figs. 1 and 2 or Figs. 3 and 4 of the accompanying drawings.

7. An oil well tool having first and second ends for connection in a tool string and a flow channel extending between the first and second ends, and in which a torsional damper according to any preceding claim is interposed between the first and second ends.