NO343669B1

NO343669B1 - A torsional shock absorber and a method of using same

Info

Publication number: NO343669B1
Application number: NO20180821A
Authority: NO
Inventors: Nils Reimers; Jone Mellemstrand
Original assignee: Tomax As
Priority date: 2018-06-13
Filing date: 2018-06-13
Publication date: 2019-05-06
Also published as: NO20180821A1

Description

A TORSIONAL SHOCK ABSORBER AND A METHOD OF USING SAME

This invention relates to a damper for use in a drill string. More particularly, this invention relates to a torsional shock absorber for absorbing or dampening rotary shock loading caused by torsional fluctuations which may cause damage to downhole drilling equipment used for drilling boreholes for example in connection with the production of petroleum.

Downhole drilling equipment may typically be the drill string which transfers energy from a drive assembly on a drilling rig to the drill bit. The lower part of the drill string being closest to the bit is made up of extra heavy string components to provide extra mass and gravity force to load up the bit. This part of the string, commonly named the Bottom Hole Assembly (BHA), also holds sensitive electronic guiding systems, formation sensors and additional motor and stabilization systems, as will be appreciated by a person skilled in the art. In the following, the complete assembly of such drilling equipment will be denoted drill string.

During drilling of such boreholes, or wells, oscillations in the form of torque variations will be generated as a result of parts of the drill-string acting as a "torsional spring” in connection with the transfer of energy to the bit.

Oscillations in the range of 0.1 to 5 Hz, in this document denoted low frequency oscillations, are usually generated as a function of the spring constant of the total drill string, while oscillations in the range of 5 to 500 Hz, in this document denoted high frequency oscillations, occur primarily in the stiffer part of the drill string formed by drill collars at the bottom of the drill string.

The highest frequencies are damped relatively quickly in amplitude compared to the lower frequencies. The highest frequencies, typically above 100 Hz, are due to friction from the wall of the borehole, material properties and geometry of the drill string, damped typically within 10-30m along the length of the drill string. The lower frequencies may propagate throughout the length of the drill string. Consequently, the high frequency oscillations exist typically in limited intervals or ranges of the drill string. The high frequency oscillations may therefore be difficult to monitor and consequently can cause major damage through rapid material fatigue in portions of the drill string where the amplitude is high. This is called critical zones. The amplitude is determined by the amount of energy that is deferred from the rock cutting process to instead be consumed by the oscillations. This is uncontrolled energy typically originating from friction; either from the drill bit at the bottom of the borehole or from the drill string abutting the wall of the borehole.

There is therefore a need for a damper that is capable of absorbing or damping high frequency oscillations in the critical portions of the drill string.

It is generally known to absorb or dampen high frequency oscillations or vibrations by means of an element or "vibration barrier" providing a big contrast in modulus or elongation compared with the rest of the drill string. The contrast makes it difficult for an established oscillation to propagate from a first material to a second material being different from the first material, because it is very unlikely there is a common, natural harmonic frequency between the two.

Various industries use different materials for such oscillation or vibration barriers, but couplings comprising an elastomeric material such as natural and synthetic rubber are most common within a temperature range limited upwards to about 150 °C and a stress below 10 MPa. In conjunction with higher temperatures and higher stresses, alternative systems based on mechanical springs and dynamic damping are used. Rubber has an advantage over mechanical spring systems in that it has a greater natural frequency range, typically from 10 to 1000 Hz compared with engineered, mechanical systems that are typically adjusted within frequency ranges less than 100 Hz, for example 40-130 Hz, 200-280 Hz, etc.

Publication US 3,323,326 A, application filed in 1965, discloses a well drilling shock absorber including two telescopically arranged parts. Each part has a means at one end thereof for attachment to parts of a well drill string. The parts have vertically opposed helical ribs thereon disposed in spaced relationship to each other, and vertically extending splines also disposed in spaced relationship to each other. A deformable, elastic rubber-like material fills the spaces between the ribs and between the splines through which torsional and axial stresses can be transmitted from one part to the other.

Publication US 6,808,455 B1, application filed in 2000, discloses a device for absorbing and dampening rotary shock loading on drill bits, drill string tubulars or other component assemblies used in rotary drilling applications. The device comprises an outer housing provided with internal drive splines in a lower portion thereof, and a mandrel wherein an upper portion thereof is located within the outer housing. The mandrel has drive splines on an outer surface thereof that fit within spaces between the drive splines of the lower housing to receive rotary energy therefrom. A plurality of elastomeric dampening bars are positioned between the splines of the housing and the splines of the mandrel. The dampening bars are replaceable so that the device may be adapted to different drilling conditions by using bars having a greater or lesser hardness or different shapes.

There exist also systems that are based on oil chamber and springs. In such systems, load capacity can be controlled better through elastic elements or common spiral springs. The disadvantage of these systems is a specific operational interval in the ranges as narrow as 50 Hz compared with an elastomeric material, such as for example rubber, that provides for a range of almost 1000 Hz.

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 torsional shock absorber according to the present invention is based on a combination of common constructional elements combined in a new way that at least reduces the problem of prior art.

The torsional shock absorber according to the invention is typically arranged in a lowermost portion of the drill string and inserted above the drill bit preferably by means of standard pipe couplings used in the petroleum industry.

The invention is defined by the independent patent claims. The dependent claims define advantageous embodiments of the invention.

In a first aspect of the invention there is provided a torsional shock absorber for dampening rotary shock loading on a drill string, comprising:

- a housing having an upper end portion for connection to a drill string, and a lower end portion provided with housing drive splines;

- a mandrel arranged coaxially with the housing, the mandrel having a portion provided with mandrel drive splines fitting within spaces in the housing drive splines of the housing such that a gap is provided between the housing drive splines and the mandrel drive splines, and a lower end portion for connection to the drill string;

- an elastomeric material filling the gap;

- a connection means for axially connecting the mandrel to the housing.

The torsional shock absorber is further provided with a torsion spring for rotating the housing an angle with respect to the mandrel and thereby prestressing the elastomeric material within the gap.

In one embodiment there is provided a torsional shock absorber for dampening rotary shock loading on a drill string, comprising:

a housing having an upper end portion adapted to be connected to a drill string, and a lower end portion provided with housing drive splines, the housing comprising an axial bore extending through it;

a mandrel having a portion provided with mandrel drive splines fitting within spaces in the housing drive splines of the housing such that a gap is provided between the housing drive splines and the mandrel drive splines, and a lower end portion adapted to be connected to the drill string, the mandrel comprising an axial bore extending through it;

an elastomeric material filling the gap; and

a connection means for transferring axial load from the mandrel to the housing.

The torsional shock absorber is further provided with a torsion spring having a first end portion and a second end portion, wherein the first end portion is arranged in the upper end portion of the central bore of the housing, and the second end portion of the torsion spring is in engagement with the mandrel.

When potentially damaging high frequency torsional oscillations arise downhole, the driving moment, i.e. the general level of stress, is already high. The purpose of the torsion spring is to provide a pre-stressing, or preloading moment, of the elastomeric material. If a moment of point of fixation, i.e. a preloading moment, of the torsion spring is opposite to the driving moment of the drill string, the elastomeric material will be in a compressed state when high frequency oscillations are absent or low. When the driving moment increases, the compression of the elastomeric material will decrease. This has the effect that the elastomeric material may be substantially uncompressed or "neutral” when the potentially damaging shock loadings such as high frequency oscillations occur during a drilling operation. An elastomeric material as used in the present invention has a best absorbing or damping effect when being uncompressed.

A high general level of stress caused by the driving moment means that the elastomeric material disclosed in US 3,323,326 and US 6,808,455 has already been compressed. A compressed elastomeric material has a considerably reduced capacity for absorbing high frequency oscillations. Thus, in the devices disclosed in US 3,323,326 and US 6,808,455, the capacity of absorbing high frequency oscillations abates proportionally to an increased load or stress.

Although providing a preloading moment of the torsion spring being opposite to the driving moment of the drill string is the primary situation, it may in some situations be desirable to provide a preloading moment of the torsion spring being in the same direction as the driving moment of the drill string. The torsion spring will then facilitate compression of the elastomeric element. An example of a situation wherein it may be advantageous to provide a preloading moment of the torsion spring being in the same direction as the driving moment of the drill string, is in operation with low torsional force on the drill-bit and so the elastomer may become relatively stiff compared with the general level of torsion and the peaks of torsional oscillations. This situation might occur in situations requiring restricted drilling speed or also in situations with low temperature, so the elastomer of the elastomeric element becomes stiff as a result of its characteristics versus temperature.

The torsion spring may be adapted for selective prestressing or -loading. By selective prestressing is meant prestressing to a desired moment, and in a desired direction. This has the effect of tailor or optimise the compression of the elastomeric material and thus the absorbing or dampening effect of the torsional shock absorber, to different drilling conditions.

In one embodiment, a top portion of the torsion spring is provided with a coupling adapted for receiving a complementary prestressing tool such that the prestressing moment of the torsion spring can be altered to desired needs in situ on for example an offshore drilling rig when making up the drilling string. Thus, there is no need for sending the torsional shock absorber to a workshop for altering its characteristics.

The housing may comprise an upper housing secured to a lower housing by threads. The housing drive splines is in such an embodiment arranged in a lower portion of the lower housing. The advantage of making a housing comprising an upper housing secured to a lower housing, as compared with a "one-piece housing" is that the manufacturing process is easier as will be understood by a person skilled in the art.

In a preferred embodiment the housing drive splines are arranged on an inner surface of the housing. Consequently, the drive splines of the mandrel are in such an embodiment arranged on an outer surface of the mandrel.

The connection means may typically comprise a lock nut secured to the mandrel by threads. A portion of such a lock nut abuts an upwards facing shoulder in the housing so that the lock nut and thus the mandrel is free to rotate with respect to the housing. A rotation of such a connection means is limited by the spline arrangement comprising the elastomeric material.

The torsional shock absorber may further comprise a thrust bearing for facilitating relative rotation between the housing and the mandrel. A thrust bearing will reduce the friction between the housing and the mandrel even when subject to large compression forces. Such large compression forces typically arise from the drill bit when this is urged towards the bottom of the borehole. Again, the relative rotation is limited by the spline arrangement comprising the elastomeric material.

A section of the housing of the torsional shock absorber may be provided with a reduced crosssectional area arranged above an interface between the housing and the mandrel. An effect of such a reduced cross-sectional area is that any bending forces are absorbed substantially by the reduced cross-section area and not by the "active" parts of the torsional shock absorber which could result in wedging and malfunction of the torsional shock absorber. By active parts are meant the portion comprising the spline arrangement, connection means, and a possible thrust bearing.

In a second aspect of the invention there is provided a drill string comprising the torsional shock absorber according to the first aspect of the invention.

In a third aspect of the present invention, there is provided a method of dampening rotary shock loading in a drill string, comprising the steps of:

a) providing a torsional shock absorber according to the first aspect of the invention;

b) inserting the torsional shock absorber in a portion of the drill string above a drill bit; and d) initiate the drilling operation.

The method may further comprise, between step a) and step b) above, prestressing the elastomeric material filling the gap between the housing drive splines and the mandrel drive splines by prestressing the torsion spring of the torsional shock absorber to a predetermined level in a desired direction.

In the following is described an example of a preferred embodiment illustrated in the accompanying drawings, wherein:

Fig. 1 is a principle drawing showing a drill string connected to a top drive, wherein the torsional shock absorber according to the invention is arranged between drill collars and a drill bit;

Fig. 2 shows in a larger scale the torsional shock absorber according to the present invention;

Fig. 3 shows a sectional view through A-A of fig. 2;

Fig. 4 shows a sectional view through A-A of fig. 3 rotated an angle around a longitudinal axis;

Fig. 4a shows in a larger scale detail 4A of fig. 4;

Fig. 5 shows a sectional view through B-B of fig. 3;

Fig. 6 shows a sectional view through C-C of fig. 3; and

Fig. 7 shows in larger scale a perspective view of a tool for operating the torsional shock absorber.

In the figures, same or corresponding elements are indicated by the same reference numerals.

Positional specifications, such as for example above, between, inner, and outer, refer to the positions that are shown in the figures.

For illustrative reasons, the relative proportions of some of the elements may be somewhat distorted.

For clarity, some of the elements shown may in some of the figures be without reference numerals.

In the figures 1 to 6, reference numeral 1 denotes a torsional shock absorber according to the present invention.

Fig. 1 , which is highly distorted, shows in principle an arrangement wherein the torsional shock absorber 1 according to the invention is arranged near a bottom portion of a drill string 100. The torsional shock absorber 1 will hereinafter also be denoted apparatus 1.

The apparatus 1 is arranged in series between a drill bit 102 and drill collars 104. The drill collars 104 and drill bit 102 form part of the drill string 100 comprising a series of drill pipes 106 extending to a surface drilling rig 108. The drilling rig 108 is provided with a top drive 110 for rotating the drill string 100 and thus the drill bit 102, as indicated by arrow R. A borehole provided by the drill bit 102 is not shown.

The apparatus 1 is connected to the drill string 100 by means of standard pin/box coupling means 112, 114 as will be appreciated by a person skilled in the art.

The torsional shock absorber or apparatus 1 is shown in larger scale in figures 2, 3 and 4.

The apparatus 1 comprises a housing 3, 3a having an upper end portion 5 adapted to be connected to the drill string 106 shown in fig. 1. As mentioned above, the drill string 106 comprises drill collars 104 to which the apparatus 1 is connected by means of standard pipe coupling 112.

In the embodiment shown, the housing 3, 3a comprises an upper housing 3 secured to a lower housing 3a by threads. A lower end portion 7 of the lower housing 3a is provided with housing drive splines 9 as best seen in figures 4 and 5. The upper housing 3 has an axial bore 11 for receiving a torsion spring 60 as will be discussed below.

Drill fluid to the drill bit 102 is communicated through an axial bore 61 through the torsion spring 60.

The lower housing 3a has a bore 11’ for receiving a mandrel 30 as will be discussed below.

The apparatus 1 further comprises a mandrel 30 having a portion 32 (see fig. 4) provided with mandrel drive splines 34. As seen in fig. 5, the mandrel drive splines 34 fits within spaces 10 in the housing drive splines 9. However, the mandrel drive splines 34 is smaller, both with respect to breadth and radial extent, than said spaces 10 such that a gap is provided between the housing drive splines 9 and the mandrel drive splines 34. The gap is filled with a suitable elastomeric material such as for example rubber 12 shown dotted in fig. 5. In the embodiment shown, the elastomeric material 12 is bonded to all surfaces defined by the gap, i.e. the internal surface of the splined portion of lower housing 3a and the external surface of the splined portion of the mandrel 30. Torsional forces in the apparatus 1 will thus be carried by the elastomeric material 12 and the torsion spring 60 mentioned above, and which will be discussed below.

As shown in figures 3 and 4, a lower end portion 36 of the mandrel 30 is provided with a standard thread coupling 1 14 used in the industry, here shown as a pin coupling 114, for mating with a box coupling as will be appreciated by a person skilled in the art.

The mandrel 30 further comprises a central bore 40 for communicating drilling fluid to the drill bit 102.

The apparatus 1 further comprises a rotatable connection means 50 for transferring axial load from the mandrel 30 to the housing 3, 3a. In the embodiment shown, the connection means is in the form of a lock nut 50.

As seen best in fig. 4a, the lock nut 50 is secured to the mandrel 30 by threads 52. The lock nut 50 is provided with a downward facing shoulder 54 abutting an upward facing shoulder 4 in the lower housing 3a.

The shoulder 4 is in the shown embodiment provided by an upper face of the lower housing 3a.

As seen in figures 3 and 5, the housing drive splines 9 are arranged in a lower portion of the lower housing 3a.

The apparatus 1 is further provided with a torsion spring 60. The torsion spring 60 has a first end portion 62 and a second end portion 64 and is provided with an axial bore 61 as mentioned above.

The first end portion 62 of the torsion spring 60 is provided with an inclined shoulder 66 resting against a mating seat 68 of the upper portion 5 of the housing 3, 3a. When prestressing the torsion spring 60, the shoulder 66, and thus the torsion spring 60, rotates with respect to the seat 68. The second end portion 64 of the torsion spring 60 is in engagement with an upper portion 33 of the mandrel 30 such that the torsion spring 60 is prevented from rotating. The engagement is provided by means of external threads on the second end portion 64 of the torsion spring 60 engaging mating internal threads in a top portion 33 of the mandrel 30. In the embodiment shown, a counter nut 69 is provided for securing the threaded connection of the torsion spring 60 and the mandrel 30.

After prestressing or -loading the torsion spring 60, it is prevented from "spinning out " due to friction lock between the tapered lower shoulder 66 and the mating seat 68.

As shown in figures 3 and 4, the apparatus further comprises a thrust bearing 70 for reducing an undesired friction between the housing 3, 3a and the mandrel 30 even when subject to large compression force. Such large compression forces typically arise from the drill bit 102 when this is urged towards the bottom of a borehole, while at the same time a weight of the drill string 100 including the drill collars 104 provides a downward force on the apparatus 1. Thus, in order at least to reduce the effect of such compression forces within the apparatus 1, and thus increasing the desired dampening effect of the apparatus 1 provided by the torsion spring 60 and the spline connection 9, 34, 12, the thrust bearing 70 is preferred, but not mandatory.

As best seen in fig. 4a, seals 90, here in the form of O-rings, are provided for sealing between the upper housing 3 and the lower housing 3a, between the lock nut 50 and the lower housing 3a and the mandrel 2.

In the embodiment shown in figures 1, 2, 3 and 4, the upper housing 3 is provided with a section 8 having a reduced cross-sectional area. The purpose of the section 8 is to provide a section being weaker than the rest of the apparatus 1 , such that possible bending forces that are likely to arise in a wellbore wherein the trajectory of the wellbore in three-dimensional space changes rapidly, for example in a so-called dog leg. Bending forces will be absorbed substantially by the section 8, and thereby avoiding bending forces acting on the "active” parts of the apparatus 1 as discussed above.

Referring now to fig. 6, the first end portion 62 of the torsion spring 60 is provided with a connection means, here in the form of a splines 63 for engagement with splines 82 arranged in an inner end portion of a tool 80, shown in fig. 7, for prestressing the torsion spring 60.

The splines 82 of the inner end portion of the tool 80 is for engaging the splines 63 of the torsion spring 60 shown in fig 6.

It should be noted that the connection means of the first end portion 62 of the torsion spring 60, and consequently the tool 80, may have other engagement means than the splines 63, 82 shown. The engagement means may for example have an octagonal form or another form for providing rotational engagement.

The tool 80 may be operated on a rig floor by a standard rig tongs that are used for making up and breaking out tubular goods such as the drill string, or by a suitable torque tool in a workshop.

From the above, it should be understood that the dampening of undesired and potentially detrimental high frequency oscillations is provided primarily by means of the elastomeric material 12 within the spline connection 9, 34 and the torsion spring 60 for optimising the effect of the elastomeric material 12. Thus, a standard type or standard types of torsional shock absorber(s) or apparatus(es) 1 may be adapted to required need in situ, for example on an offshore drilling rig, prior to or during making up of the drill string 100.

Different types of torsional shock absorbers 1 may be provided by one or both of different characteristics of the torsion spring 60 and elastomeric material 12.

The torsional shock absorber 1 according to the invention has advantages over prior art shock absorbers. One advantage is the possibility of prestressing the elastomeric material in a desired direction with respect to the rotation of the drill string 100 such that the effect of the elastomeric material 12 for dampening high frequency oscillations may be optimized. Another advantage is that the torsional shock absorber 1 may be prestressed for example on an offshore drilling rig by means of standard rig equipment and the tool 80. Thus, a “standard” torsion shock absorber 1 according to the invention may be optimized for a specific drilling operation in short notice “from the shelf” without any dismantling for customizing shock absorption rates. Further, the torsional shock absorber 1 of the invention is compatible with directional orientation requirements and may be adapted for multiple uses including drill string protection, drilling rig equipment protection, or instrument protection.

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 not exclude the presence of a plurality of such elements.

Claims

C l a i m s

A torsional shock absorber (1 ) for dampening rotary shock loading on a drill string (100), comprising:

- a housing (3, 3a) having an upper end portion (5) for connection to a drill string (100), and a lower end portion (7) provided with housing drive splines (9);

- a mandrel (30) arranged coaxially with the housing (3, 3a), the mandrel (30) having a portion (32) provided with mandrel drive splines (34) fitting within spaces (10) in the housing drive splines (9) of the housing (3, 3a) such that a gap (13) is provided between the housing drive splines (9) and the mandrel drive splines (34), and a lower end portion (36) for connection to the drill string (100);

- an elastomeric material (12) filling the gap (13);

- a connection means (50) for axially connecting the mandrel (30) to the housing (3, 3a); c h a r a c t e r i s e d i n that the torsional shock absorber (1) is further provided with a torsion spring (60) for rotating the housing (3, 3a) an angle with respect to the mandrel (30) and thereby prestressing the elastomeric material (12) within the gap (13).

The torsional shock absorber (1) according to claim 1, wherein the torsion spring (60) is configured for being selectively prestressed to a desired moment, and in a desired direction.

The torsional shock absorber (1) according to claim 1 or 2, wherein the housing (3, 3a) comprises an upper housing (3) secured to a lower housing (3a) by threads, the housing drive splines (9) arranged in a lower portion of the lower housing (3a).

The torsional shock absorber (1) according to claim 1, 2 or 3, wherein the housing drive splines (9) are arranged on an inner surface of the housing (3, 3a).

The torsional shock absorber (1) according to any of the preceding claims, wherein the connection means comprises a lock nut (50) secured to the mandrel (30) by threads, a portion of the lock nut (50) abutting an upwards facing shoulder (4) in the housing (3, 3a).

The torsional shock absorber (1) according to any of the preceding claims, further comprising a thrust bearing (70) for facilitating relative rotation between the housing (3, 3a) and the mandrel (30).

The torsional shock absorber (1) according to any of the preceding claims, wherein a section (8) of the housing (3, 3a) is provided with a reduced cross-sectional area arranged above an interface between the housing (3, 3a) and the mandrel (30).

8. A drill string (100) comprising the torsional shock absorber (1 ) according to any of the preceding claims.

9. A method of dampening rotary shock loading in a drill string, comprising the steps of:

a) providing a torsional shock absorber (1 ) according to any of claims 1 - 7;

b) inserting the torsional shock absorber (1 ) in a portion of the drill string (100) above a drill bit (102); and

c) initiate the drilling operation.

10. The method according to claim 9, further comprising, between step a) and step b) prestressing the elastomeric material (12) filling the gap between the housing drive splines (9) and the mandrel drive splines (34) by prestressing the torsion spring (60) of the torsional shock absorber (1) to a predetermined level in a desired direction.