Patent Application
20090133867
The present invention relates, in general, to systems and apparatus for generating electrical power within a borehole, and, more particularly, to a downhole alternator for generating power upon rotation of the portion of the drillstring in which it is mounted.
When drilling boreholes in the earth's crust for such purposes as oil and gas exploration, tunneling under obstacles for the placement of cables, or for a wide variety of other purposes, it is generally necessary to provide drilling tools in the drillstring which incorporate various sensors and control instrumentation for guiding the direction of drilling, detecting the conditions at the drill head, and transmitting data and control signals between the drill head and the earth's surface. Such downhole instrumentation requires a reliable source of electrical power for operation, and over the years a great deal of effort has been put into developing such a source. For example, attempts have been made to supply electrical power to downhole instruments from a surface source, as by way of wires or cables extending the length of the drillstring. However, the use of such wires or cables in rotating drillstrings has been unsatisfactory, because of the difficulty in maintaining secure connections at each joint in the drillstring.
Downhole battery supplies have generally replaced surface power sources for downhole circuitry, but they, too, have limitations in that they have a limited life, requiring periodic replacement that results in costly down time for the drilling operation.
Various attempts have been made to overcome these problems by introducing a downhole electrical generator to either recharge batteries or to provide power directly to the downhole instruments. Such generators have been powered by the flow of drilling fluid in the borehole, but difficulties have been encountered in maintaining reliable operation in the hostile environment of downhole drilling. The abrasive, high-pressure drilling fluid flowing through and around the drillstring is destructive of generator moving parts, and is particularly hard on rotating seals, while the placement of such generators in the flow path of the drilling fluid interferes with the flow and prevents access to the drillstring below the generator location.
The present invention overcomes the foregoing problems by providing a downhole source of electrical power for drilling tools that is driven by the rotation of the drillstring or, if the drillstring is not rotating, by the rotation of the drilling sub, which is the portion of the drillstring that carries the drill bit. The source consists of an alternator having a rotor made up of a series of coils that are mounted on, and spaced around the circumference of, a rotating drilling sub, and a stator made up of a multiplicity of permanent magnets also spaced around the circumference of the drilling sub. The stator is mounted on the drilling sub by means of bearings and incorporates a counterweight that holds the rotor relatively stationary with respect to the sub and with respect to the coils, so that rotation of the sub, either by rotation of the drillstring or by rotation of a drill motor in the drilling sub, produces relative rotation of the rotor and stator to generate an AC electrical output. The rotor and stator surround an axial fluid passage through the center of the alternator so that they do not impede the flow of the drilling fluid, and the motor is totally sealed to prevent damage to the bearings.
The present alternator structure requires inclination of the borehole to enable the counterweight to function to produce relative rotation of the rotor and the stator, but vertical boreholes are rare, so this is generally not an issue. The present invention is extremely valuable for directional drilling, since that type of drilling requires reliable downhole instrumentation. The alternator of the present invention relies on rotation of that portion of the drillstring where it is located, and thus normally will be near the drill bit. The bit conventionally located in a drilling head or sub that is driven by drillstring rotation or by a drilling motor in the drilling sub. The alternator stator is an annular permanent magnet structure which surrounds the drilling axis, and which may include two sets of rare earth disc magnets spaced axially to form an axial gap, with the counterweight holding the magnets stationery with respect to the alternator coils. The coil structure is also annular and also surrounds the drilling axis, with the coil structure preferably including two sets of offset coils positioned to rotate in the gap between the permanent magnets. The two sets of coils make up a two-phase system; if desired, a single set of coils may be used to provide a single-phase system.
The objects, features and advantages of the present invention will become apparent to those of skill in the art from the following detailed description of preferred embodiments thereof, taken with the accompanying drawings, in which:
Turning now to a more detailed description of the invention,
In accordance with the invention, power is supplied to the tool 28 by means of a downhole alternator 40, illustrated in greater detail in
The cylindrical inner housing wall 43 provides an axial passageway 54 through the length of the alternator to provide an unobstructed flow path for drilling fluid that flows down the drillstring to the drill bit. The wall 43 is spaced radially inwardly from sleeve 48 and is coaxial to provide an annular chamber 56 in which the alternator components are mounted.
The alternator incorporates a semicylindrical counterweight 60 that is mounted on a cylindrical stator 62 which surrounds, and is spaced from, the inner housing wall 43 and is mounted for relative rotation with respect to the wall, and thus with respect to the alternator housing 50. Bearings 65 and 66 support the opposite ends of the stator 62 on the housing wall portion 43 to allow relative rotation. The counterweight 60 preferably is of lead, and is of sufficient axial length, radial thickness, and arcuate length around the axis 64 of the alternator to ensure that it remains on the low side of the inclined borehole as the sub and the housing rotate with the drive motor or with the drillstring. As illustrated in
Also mounted on the stator 62, preferably near one end of it, are a multiplicity of sets of permanent magnets 70 and 72 for use in generating an output current. As illustrated, the permanent magnet sets 70 and 72 may be rare earth disc magnets secured to opposing surfaces of a pair of radially outwardly-extending, thin annular discs or plates 74 and 76, respectively, secured to the stator 62 and thus to the counterweight 60. As illustrated in
An annular rotor 82 is secured to the housing 50, for example to an end wall 84 of the annular chamber 56, for rotation with the housing, and thus for relative rotation with respect to the stator and counterweight. The rotor includes a radially inwardly-extending annular coil support disc 86 which extends into the gap 80 between stator magnets 70 and 72, and which carries, in the preferred form of the invention, two offset layers 90 and 92 of flat, or “pancake” coils. Each layer includes a plurality of individual spirally-wound pancake coils such as the coil 96, with each coil having an inner lead 98 from the center of the coil and an outer lead 100 from the outer edge of the coil. In the preferred form of the invention the individual coils of layer 90 are interconnected to form a first output current having a first phase, while the individual coils of layer 92 are interconnected to form a second output current having a second phase, as rotation of housing 50 causes the rotor to spin the coils through the gap 80 between the relatively stationery permanent magnets on the stator. The output leads from the coils are connected to the downhole drilling tool 28 to provide two phase electrical power.
In the preferred form of the invention, the coils of layer 90 are connected in series so that the voltage generated by each coil is additive to produce the output voltage of phase 1. Similarly, the coils of layer 92 are connected in series to produce the output voltage of phase 2.
The counterweight 60 is suspended on the bearings 64, 66 for free rotation within cavity 56 about the inner wall portion 43, so that as the drillstring travels through the earth (in a non-vertical direction) the counterweight remains on the low side of the borehole. The cavity 56 is sealed by O-ring seals 110 and 112 so that the drilling fluid, flowing down through the center of the drillstring and back to the surface around the outside of the drillstring, does not interfere with the operation of the alternator, even as the alternator housing 50 rotates to produce relative rotation between the spaced stator permanent magnet sets 70 and 72 and the rotor coil sets 90 and 92 located in the gap 80. The lead counterweight, in one embodiment of the invention, was 12 inches long, with a six-inch outer diameter and a 3-inch inner diameter and extending 180 degrees around the axis, as illustrated in
It is noted that the coil sets 90 and 92 preferably do not utilize iron cores in order to reduce the start-up torque of the alternator; air core coils produce insignificant induced magnetism so they will not drag the permanent magnets and the counterweight with them as the housing is rotated. However, if an excessive amount of current is drawn from the coils, the counterweight can be carried around the axis, and this limits the available alternator output power. This limit can be increased by using a larger counterweight. Alternatively, additional electrical capacity can be obtained by using multiple alternators along the axis of the drillstring.
The twisting force for generating the needed electrical power is derived from either the downhole drilling motor 24, which may be driven by the drilling fluid in conventional manner, or from rotation of the drillstring by surface motors at the drillrig 18, and this mechanical force is carried by the outer sleeve 48, which is secured to the body portion 42 by any suitable means, such as bolts, welding, structural adhesives, or the like. The alternator stator is isolated from the mechanical drilling stresses, with the O-rings 110 and 112 permanently sealing the interior cavity 56.
The coils 96 in the coil sets 90 and 92 are flat, wound coils, and are potted in epoxy for protection. The two-phase connection discussed above is preferred, not only because it provides a smoother torque, and because the overlapping structure (illustrated in
Although the invention has been described in terms of preferred embodiments, it will be understood that modifications and variations may be made without departing from the true spirit and scope thereof, as described in the following claims.
