Systems and methods to reduce oscillations in magnetic couplings

BACKGROUND

p-00021. Field of the Invention

p-0003The present invention generally relates to magnetic couplings and, in particular, to reducing rotational variations due to self oscillations.

p-00042. Description of the Related Art

p-0005Magnetic couplings can be used to transmit rotary motion from one rotatable element to another. A typical magnetic coupling includes two movers. The first mover surrounds a portion of the second mover. The first and second movers both include magnets in the region where they overlap. As is known in the art, the magnets are arranged such that rotation of one of the movers causes the other mover to rotate due to attraction and repulsive forces between the magnets.

p-0006One advantage of magnetic couplings is that they can transmit rotary motion from one mover to another without the two movers mechanically contacting each other. This can be useful in situations where a shaft or other mover located in a sealed environment needs to be rotated. An example of such a case can occur in context of drilling a borehole into the earth. In such a case, a bottom hole assembly (BHA) of drill string may require power. The power can be generated by an alternator in the BHA. Given the harsh conditions that exist in a borehole, it is desirable that the alternator be protected from drilling fluid and enclosed in a sealed environment. To this end, a magnetic coupling can be attached to the shaft of the alternator. The magnetic coupling includes an inner rotor having magnets surrounded by an outer housing that also includes magnets. The outer housing can be coupled to the alternator such that the combination forms a sealed environment. The outer housing is fixedly coupled to a turbine. Drilling mud is pumped through the turbine causing it, the outer housing of the alternator to rotate. The magnets in the outer housing and the magnets on the rotor interact such that the rotation of the outer housing causes the rotor to rotate. The rotation can be used to generate electricity for the BHA. As is known in the art, rather than being coupled to an alternator, the magnetic coupling can be attached to any shaft driven devices such as, for example, a pump.

BRIEF SUMMARY

p-0007Disclosed is drilling system that includes a magnetic coupling and an oscillation absorber. The magnetic coupling includes a rotor having a plurality of inner magnets disposed thereon in circular arrangement in an inner magnet region, a separator layer surrounding the inner magnet region, and an outer housing surrounding the inner magnet region and separated from the inner magnet region by the separator layer and including outer magnets and arranged such that rotation of the outer housing causes the rotor to rotate about an axis of rotation. The oscillation absorber is in operable communication with the magnetic coupling and includes an outer layer coupled to the separator layer to form an enclosed area, an absorber shaft at least partially within the enclosed area that is coupled to the rotor and that includes inner absorber shaft magnets coupled to it, and an outer mover having outer mover magnets disposed therein and arranged such that rotation of the absorber shaft causes the outer mover to rotate due to interaction of the inner absorber shaft magnets and the outer mover magnets.

p-0008Also disclosed is a drilling system that includes a turbine and turbine a magnetic coupling coupled to the turbine. The magnetic coupling includes a rotor having a plurality of inner magnets disposed thereon in circular arrangement in an inner magnet region and separator layer surrounding the inner magnet region. The magnetic coupling also includes an outer housing surrounding the inner magnet region and separated from the inner magnet region by the separator layer and including outer magnets and arranged such that rotation of the outer housing causes the rotor to rotate about an axis of rotation and a first electrically conductive layer disposed on the rotor between the inner magnets and the separator layer.

p-0009In addition, a drilling system that includes a turbine and a magnetic coupling coupled to the turbine is disclosed. The magnetic coupling includes a rotor having a plurality of inner magnets disposed thereon in circular arrangement in an inner magnet region, a separator layer surrounding the inner magnet region and an outer housing surrounding the inner magnet region and separated from the inner magnet region by the separator layer and including outer magnets and arranged such that rotation of the outer housing causes the rotor to rotate about an axis of rotation. The drilling system further includes an alternator coupled to the magnetic coupling that provides electricity at an output, a load coupled to the output and an electrical damping circuit coupled in parallel with the load and configured to cancel oscillations in the electricity at a self excitation frequency of the magnetic coupling.

BRIEF DESCRIPTION OF THE DRAWINGS

p-0010The following descriptions should not be considered limiting in any way. With reference to the accompanying drawings, like elements are numbered alike:

p-0011<figref idrefs="DRAWINGS">FIG. 1</figref> is a partial cut-away perspective view of a magnetic coupling that provides energy from the motion of drilling mud to an alternator;

p-0012<figref idrefs="DRAWINGS">FIG. 2</figref> is a cut-away side view of a drilling system that includes a magnetic coupling coupled to an oscillation absorber;

p-0013<figref idrefs="DRAWINGS">FIGS. 3A-3C</figref> illustrate equivalents of the drilling system illustrated in <figref idrefs="DRAWINGS">FIG. 2</figref>;

p-0014<figref idrefs="DRAWINGS">FIG. 4</figref> is a cross-section of the dampener included in <figref idrefs="DRAWINGS">FIG. 2</figref>;

p-0015<figref idrefs="DRAWINGS">FIG. 5</figref> is a partial cross-sectional view of a magnetic coupling according to an alternative embodiment; and

p-0016<figref idrefs="DRAWINGS">FIG. 6</figref> illustrates an electrical damping circuit connected to the output of an alternator.

DETAILED DESCRIPTION

p-0017A detailed description of one or more embodiments of the disclosed apparatus and method presented herein is by way of exemplification and not limitation with reference to the Figures.

p-0018<figref idrefs="DRAWINGS">FIG. 1</figref> shows an assembly <b>20</b> that includes an energy transfer device <b>22</b> coupled to an alternator <b>24</b>. The assembly <b>20</b> can be utilized, for example, to create electricity for a bottom hole assembly (BHA) (not shown) of a drill string utilized to drill a borehole into the earth. In general, the energy transfer device <b>22</b> converts the flow of a fluid (e.g., drilling mud) into rotational energy. As illustrated in <figref idrefs="DRAWINGS">FIG. 1</figref>, the rotational energy generated by the energy transfer device <b>22</b> is used to drive an alternator <b>24</b> to create electrical energy. Of course, the rotational energy could be provided to any type of shaft driven device such as, for example, a pump.

p-0019The illustrated energy transfer device <b>22</b> includes a turbine <b>29</b> operably connected to a magnetic coupling <b>25</b>. In relation to <figref idrefs="DRAWINGS">FIG. 1</figref>, assume that a fluid such as drilling mud is being pumped by a mud pump (not shown) in the direction shown by arrow A. The drilling mud is forced between a blade section <b>30</b> and a turbine casing <b>32</b> of the turbine <b>29</b> and causes the blade section <b>30</b> to rotate as in known the art.

p-0020The magnetic coupling <b>25</b> includes an outer housing <b>28</b> that surrounds an inner mover illustrated as rotor <b>26</b>. The outer housing <b>28</b> includes outer magnets <b>34</b> that surround inner magnets <b>27</b> coupled to the rotor <b>26</b>. The outer housing <b>28</b> is rigidly coupled to or integral with the blade section <b>30</b> such that it rotates as the blade section <b>30</b> rotates. The rotation of the outer housing <b>28</b> causes the rotor <b>26</b> to rotate due to attractive/repulsive forces between the outer magnets <b>34</b> and the inner magnets <b>27</b>. The rotor <b>26</b> is coupled to the alternator rotor <b>40</b> of the alternator <b>24</b>. Magnets <b>42</b> coupled to the alternator rotor <b>40</b> interact with the stator <b>50</b> in a known manner to create electricity.

p-0021It is important that fluids (e.g., drilling mud) or solids liberated during drilling do not enter the alternator <b>24</b>. Accordingly, a separator layer <b>46</b> is coupled to an outer casing <b>52</b> of the alternator <b>24</b> such that debris cannot enter the alternator <b>24</b>. The separator layer <b>46</b>, like stator <b>50</b>, does not rotate and, in combination, the separator layer <b>46</b> and the outer casing <b>52</b> form a sealed environment for the rotor <b>26</b> and the alternator rotor <b>40</b>. Of course, as is known in the art, the separator layer <b>46</b> includes a portion disposed between the inner magnets <b>27</b> and the outer magnets <b>34</b>. In this manner, rotation energy can be passed from a harsh environment (e.g., from outside the outer housing <b>28</b>) into a sealed environment formed at least in part by the separator layer <b>46</b> without physical mechanical contact.

p-0022In one embodiment, the separator layer <b>46</b> is formed of a material that is neither magnetically or electrically conductive. Examples of suitable materials include, for example, ceramics and advanced materials like (carbon) fiber materials. In another embodiment, the separator layer <b>46</b> is formed of an electrically conductive material such as Inconel. Regardless of the particular material used, the separator layer <b>46</b> should be formed such that it can withstand hydrostatic pressures that can exist in a down-hole drilling environment. As will be appreciated, the thicker the illustrated separator layer <b>46</b> is, the more it leads to a performance degradation (loss of torque) of the magnetic coupling <b>25</b> due to increased gap distance between the inner and outer magnets <b>27</b>, <b>34</b>. In addition, as the separator layer <b>46</b> is increased in thickness, the magnitude of efficiency-reducing eddy currents increases. It shall be understood that the same considerations can apply to any separator layer described herein.

p-0023In operation, the relative position of the outer housing <b>28</b> and the rotor <b>26</b> can vary. In particular, the rotor <b>26</b> can alternate between leading and lagging relative to the outer housing <b>28</b> due to self-excitation. Measuring the distance in the circumferential direction between two points, one on the outer housing <b>28</b> and one on the rotor <b>26</b>, forms a time varying plot having a frequency referred to as the Eigen, or self oscillation, frequency. In more general terms, the rate of rotation of the rotor <b>26</b> increases and decreases in normal operation at a particular frequency. The variations in rotation rate can cause, in some cases, damage to elements connected to the rotor <b>26</b>. For example, variations in rotor speed can cause the alternator <b>24</b> to produce voltage fluctuations and harm electronic components powered by the alternator <b>24</b>. In extreme cases, the self-oscillation can cause the inner and outer magnets <b>27</b>, <b>34</b> to become decoupled rendering the energy transfer device <b>22</b> inoperable.

p-0024One approach taken in the prior art to reducing the effects of self-excitation involves the use of complicated electronics to dissipate over-voltages produced by the alternator <b>24</b> in a resistor. The complexity of the electronics, the fact that such systems cannot typically account for under-voltages and that such systems cannot usually be utilized for high power (greater than 1400 Watts) renders such an approach less than ideal.

p-0025<figref idrefs="DRAWINGS">FIG. 2</figref> is a partial cut-away side view of a drilling system <b>70</b> that includes an oscillation absorber <b>71</b> according to one embodiment. The drilling system <b>70</b> includes a magnetic coupling <b>25</b> having a shaft (rotor) <b>26</b>. As described above, an external force causes rotation of the outer housing <b>28</b>. The outer housing <b>28</b> includes outer magnets <b>34</b> disposed within or coupled to it. The outer housing <b>28</b> is separated from the rotor <b>26</b> by a separator layer <b>78</b>. Due to the interaction between outer magnets <b>34</b> and inner magnets <b>27</b> coupled to the rotor <b>26</b>, the rotor <b>26</b> rotates. However, as described above, the relative position between the rotor <b>26</b> and the outer housing <b>28</b> can vary in a roughly sinusoidal manner about a base rotational rate. That is, in some cases, the rotor <b>26</b> rotates faster than the outer housing <b>28</b> and sometimes it rotates slower than the outer housing <b>28</b>.

p-0026Referring now to <figref idrefs="DRAWINGS">FIG. 3</figref><i>a</i>, the magnetic coupling <b>25</b> can be modeled as a spring-mass system. The outer housing <b>28</b> is modeled as the ground, the inner and outer magnets <b>17</b>, <b>34</b> are modeled as spring c₁and the rotor <b>26</b> can be thought of as an oscillating mass having an angular frequency w₀. In this case, w₀is equivalent to the Eigen frequency described above.

p-0027Referring again to <figref idrefs="DRAWINGS">FIG. 2</figref>, the magnetic coupling <b>25</b> is attached to the oscillation absorber <b>71</b>. In general, the oscillation absorber <b>71</b> includes a magnetic spring (i.e., another magnetic coupling) arranged and configured to absorb or otherwise dampen the variations in the rotational rate of the rotor <b>26</b>. In particular, the oscillation absorber <b>71</b> provides a counterweight in the form of outer mover <b>80</b> that makes it more difficult for the rotor <b>26</b> to vary in speed. As illustrated, the oscillation absorber <b>71</b> is between the magnetic coupling <b>25</b> and a shaft driven device <b>92</b> that utilizes rotational energy provided to input shaft <b>88</b> to produce a desired result. Of course, the absorber could be located on an opposite side of the device <b>92</b> than the magnetic coupling <b>25</b>. It shall be understood thathe device <b>92</b> could be an alternator that produces electricity from the rotation of input shaft <b>88</b> or a pump that is driven by input shaft <b>88</b> or any other shaft driven device.

p-0028The oscillation absorber <b>71</b> includes an outer layer <b>82</b> that mates either directly or indirectly with the separator layer <b>78</b> to form an enclosed area <b>73</b> that is sealed from a formation <b>62</b> or drilling mud that is external to the drilling system <b>70</b>. An absorber shaft <b>86</b> is coupled to the rotor <b>26</b> at coupling <b>74</b>. Thus, rotor <b>26</b> and absorber shaft <b>86</b> form a generally unitary shaft that drives the input shaft <b>88</b> through coupling <b>90</b>.

p-0029The absorber shaft <b>86</b> includes inner absorber shaft magnets <b>83</b> coupled to it in the same or similar manner as the inner magnets <b>27</b> are coupled to the rotor <b>26</b>. One or more bearings <b>96</b> are disposed between the absorber shaft <b>86</b> and the outer mover <b>80</b> and hold them in a concentric relationship to one another. Both the absorber shaft <b>86</b> and the outer mover <b>80</b> are arranged such that they can generally rotate freely within the outer layer <b>82</b>. The outer mover <b>80</b> includes outer absorber magnets <b>84</b> coupled to or disposed within it. Absent outer absorber magnets <b>84</b>, rotation of the absorber shaft <b>86</b> would not generally cause outer layer <b>82</b> to rotate except due to friction that can exist in the bearing <b>96</b>. However, the outer absorber magnets <b>84</b> can be arranged such that, in combination with inner absorber magnets <b>83</b> they form a magnetic spring that behaves in the same manner as a magnetic coupling. That is, in general, as the absorber shaft <b>86</b> rotates, so does the outer mover <b>80</b>.

p-0030As described above, rotor <b>26</b> can experience rotational rate variations. As the rate increases, the weight of the outer mover <b>80</b> will oppose such an increase due to its magnetic coupling to absorber shaft <b>86</b>. With reference now to <figref idrefs="DRAWINGS">FIG. 3B</figref>, the system <b>70</b> as described to this point can be modeled by adding a representation of the absorber <b>71</b> to the representation of the magnetic coupling <b>25</b> illustrated in <figref idrefs="DRAWINGS">FIG. 3A</figref>. In particular, the weight of the outer mover <b>80</b> a second mass and the inner and outer absorber magnets <b>83</b>, <b>84</b> are modeled as spring c₂. As can be seen easily from <figref idrefs="DRAWINGS">FIG. 3A</figref>, the outer mover <b>80</b> serves to oppose the rotation of the rotor <b>26</b>. In this example, the weight of the absorber shaft <b>86</b> has been omitted but could be included as part of rotor <b>26</b> if greater accuracy is desired. The mass of the outer mover <b>80</b> can be selected such that it opposes the Eignen frequency of the magnetic coupling <b>25</b>.

p-0031Optionally, the absorber <b>71</b> can also include a dampening device <b>76</b>. The dampening device <b>76</b> can further dampen the rotation rate variations. In one embodiment, and as best illustrated in <figref idrefs="DRAWINGS">FIG. 4</figref>, which a cross-section taken along line B-B of <figref idrefs="DRAWINGS">FIG. 2</figref>, the dampening device <b>76</b> includes a rotor portion <b>90</b> coupled to absorber rotor <b>86</b> and that includes a plurality of radially extending rotor fins <b>92</b>. A stator portion <b>94</b> is coupled to the outer mover <b>80</b> and includes stator fins <b>94</b> that extend radially inward between the rotor fins <b>92</b> towards the absorber rotor <b>86</b>. A fluid such as oil can be disposed in the areas <b>98</b> between the stator fins <b>94</b> and the rotor fins <b>92</b>. It shall be understood that the dampening device <b>76</b> can dampen the rotational rate variation at a different frequency than the magnetic spring formed by inner and outer dampener magnets <b>83</b>, <b>84</b>. <figref idrefs="DRAWINGS">FIG. 3C</figref> illustrates a model of the system <b>70</b> that includes damping device <b>76</b>. As illustrated in <figref idrefs="DRAWINGS">FIG. 3C</figref>, the damping device <b>76</b> is connected in parallel with the spring c<b>2</b>. Of course, is coupled in series with it. It shall be understood that the damping device <b>76</b> could be implemented in other manners. For example, the damping device could be implemented as described below and could be used alone or in combination with the spring c<b>2</b>.

p-0032According to another embodiment of the present invention rotational rate variations can be resisted by simply surrounding the inner magnets <b>27</b> with a conductor such as copper. In addition, the inside diameter of the outer magnets <b>34</b> can also becovered with the same or a similar conductor. <figref idrefs="DRAWINGS">FIGS. 5A-5C</figref> illustrate cross-sectional views of a magnetic coupling <b>200</b> with different relative orientations of the inner and outer magnets <b>208</b>, <b>210</b> according to such an embodiment. In particular, in <figref idrefs="DRAWINGS">FIG. 5A</figref> the inner magnets <b>208</b> lag the outer magnets <b>210</b>, in <figref idrefs="DRAWINGS">FIG. 5B</figref> the inner magnets <b>208</b> are in phase with the outer magnets <b>210</b> and in <figref idrefs="DRAWINGS">FIG. 5C</figref> the inner magnets lead <b>208</b> the outer magnets <b>210</b>. The magnetic coupling includes a rotor <b>202</b> and an outer mover <b>204</b>. In one embodiment, the outer mover <b>204</b> can be coupled to a turbine <b>29</b> as illustrated in <figref idrefs="DRAWINGS">FIG. 1</figref>. The rotor <b>202</b> includes inner magnets <b>208</b> and the outer mover <b>204</b> includes outer magnets <b>210</b>.

p-0033According to one embodiment, a first conductive layer <b>220</b> surrounds the inner magnets <b>208</b>. Similarly, a second conductive layer <b>222</b> is disposed on an inner circumference of the outer magnets <b>210</b>. In one embodiment, the first and second conductive layers <b>220</b>, <b>222</b> are formed of an electrically conductive material such as, for example, copper. In one embodiment, the first conductive layer <b>220</b> is disposed between some or all of the inner set of magnets <b>208</b> and the separator layer <b>206</b>. Likewise, in one embodiment, the second conductive layer <b>222</b> is disposed between the separator layer <b>206</b> and the outer set of magnets <b>210</b>.

p-0034In ideal operation, the rotor <b>202</b> and the outer mover <b>204</b> move synchronously with one another. However, as discussed above, during normal operation, the relative positions of the rotor <b>202</b> and the stator <b>204</b> can vary. As indicated in <figref idrefs="DRAWINGS">FIGS. 5A-5C</figref>, such variation can cause the bending in the flux lines <b>240</b> between the inner and outer magnets <b>208</b>, <b>210</b>. Such bending causes small movements of the magnetic flux in the first and second conductive layers <b>220</b>, <b>222</b>. The movements result in the formation of currents in the first and second conductive layers <b>220</b>, <b>222</b> that serve to oppose movement of the movement of the rotor <b>202</b> and the outer mover <b>204</b> relative to one another.

p-0035According to another embodiment, and referring now to <figref idrefs="DRAWINGS">FIG. 6</figref>, fluctuations in the output current of an alternator <b>312</b> driven by a magnetic coupling <b>300</b> caused by relative motion between a rotor <b>302</b> and stator <b>304</b> can be damped by applying an electrical damping circuit <b>308</b> varies the load <b>314</b> driven by the alternator <b>312</b>. As illustrated, the load <b>314</b> includes the electrical dampening circuit <b>308</b> and a real load <b>316</b> such as a pump, sensor or other machine that consumes either AC or DC power.

p-0036As discussed above, the magnetic coupling <b>300</b> will have an Eigen frequency. After the Eigen frequency is determined, the electrical dampening circuit <b>308</b> can be designed to oscillate at the same frequency, but 180 degrees out of phase. In its simplest form, and as illustrated in <figref idrefs="DRAWINGS">FIG. 6</figref>, the electrical dampening circuit <b>308</b> includes a resistor <b>350</b>, a capacitor <b>352</b> and an inductor or coil <b>354</b> serially connected and arranged in parallel with the load <b>314</b>. In some cases, the output of alternator <b>312</b> is rectified either in the alternator itself or in an external rectifier (not shown).

p-0037Elements of the embodiments have been introduced with either the articles “a” or “an.” The articles are intended to mean that there are one or more of the elements. The terms “including” and “having” are intended to be inclusive such that there may be additional elements other than the elements listed. The conjunction “or” when used with a list of at least two terms is intended to mean any term or combination of terms. The terms “first,” “second,” and “third” are used to distinguish elements and are not used to denote a particular order.

p-0038It will be recognized that the various components or technologies may provide certain necessary or beneficial functionality or features. Accordingly, these functions and features as may be needed in support of the appended claims and variations thereof, are recognized as being inherently included as a part of the teachings herein and a part of the invention disclosed.

p-0039While the invention has been described with reference to exemplary embodiments, it will be understood that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications will be appreciated to adapt a particular instrument, situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.