Mud pressure control system with magnetic torque transfer

A down-hole signal generator for a mud-pulse telemetry system comprises a flow constrictor defining a throttle orifice for the mud passing along a drill string, a throttling member displaceable with respect to the throttle orifice to modulate the mud pressure for the purpose of transmitting measurement data up the drill string, and a turbogenerator. The turbogenerator incorporates an annular impeller surrounding a casing and arranged to be driven by the mud passing along the drill string, and a rotatable magnet assembly disposed in a mud-free environment within the casing. The impeller includes an electrically conductive drive ring and the rotatable magnet assembly includes rare earth magnets, so that, when the impeller is rotated by the mud flow, eddy currents are induced in the drive ring by the magnetic field associated with the magnets and the magnet assembly is caused to rotate with the impeller by virtue of the interaction between the magnetic field associated with the magnets and the magnetic field associated with the induced currents. In this manner torque may be imparted to an electrical generator within the casing without a rotating seal having to be provided between the impeller and the generator.

CROSS-REFERENCE TO RELATED APPLICATION 
Applicant claims priority for this application from British Patent 
Application No. 8037213 filed on 20 Nov. 1980. 
BACKGROUND OF THIS INVENTION 
This invention relates to apparatus for signalling within a borehole while 
drilling, and is more particularly concerned with a down-hole signal 
transmitter for a mud-pulse telemetry system. 
Various types of measurements-while-drilling (MWD) systems have been 
proposed for taking measurements within a borehole while drilling is in 
progress and for transmitting the measurement data to the surface. 
However, to date only one type of system has enjoyed commercial success, 
that is the so-called mud-pulse telemetry system. In that system the mud 
stream, which passes down the drill string to the drill bit and then back 
up the annular space between the drill string and the bore wall with the 
object of lubricating the drill string and carrying away the drilling 
products, is used to transmit the measurement data from a down-hole 
measuring instrument to a receiver and data processor at the surface. This 
is achieved by modulating the mud pressure in the vicinity of the 
measuring instrument under control of the electrical output signal from 
the measuring instrument, and sensing the resultant mud-pulses at the 
surface by means of a pressure transducer. 
Current mud-pulse telemetry systems utilize a down-hole signal transmitter 
which is built into the drill collar. These systems therefore suffer from 
the disadvantage that, in the event of instrumentation failure in the 
transmitter, the complete drill string must be withdrawn to enable the 
faulty part to be replaced. Moreover the combined transmitter/drill collar 
is very costly to produce. 
One such system comprises a turbine which is driven by the mud flow and 
drives an electrical generator for supplying the measuring instrument with 
power. The turbine also drives a hydraulic pump for displacing a 
throttling member to produce the required mud pulses. The displacement of 
the throttling member is determined by the electrical output of the 
measuring instrument. However, it is of the utmost importance that the mud 
should not penetrate into the housing containing the electrical generator 
and associated mechanism, and accordingly a rotating seal surrounds the 
shaft coupling the turbine to the generator. Such a seal is difficult to 
manufacture and prone to failure leading to the complete drill string 
having to be withdrawn and the drill collar having to be replaced. 
It is an object of the invention to provide a generally improved down-hole 
signal transmitter for a mud-pulse telemetry system. 
SUMMARY OF THE INVENTION 
According to the invention there is provided a down-hole signal transmitter 
for a mud-pulse telemetry system, comprising a flow constrictor defining a 
throttle orifice for the mud passing along a drill string, a throttling 
member displaceable with respect to the throttle orifice to vary the 
throughflow cross-section of the throttle orifice, control means for 
displacing the throttling member to modulate the mud pressure, and a 
turbo-generator having an impeller arranged to be driven by the mud 
passing along the drill string and an electrical generator disposed in a 
mud-free environment within a casing, the impeller being magnetically 
coupled to the electrical generator to impart driving torque thereto. 
Such an arrangement is particularly convenient as it not only generates the 
electrical power required for operating the measuring instrument and/or 
other devices, but also enables the generator to be maintained in a 
clean-fluid environment within the casing without a rotating seal having 
to be provided between the impeller and the generator. 
Preferably the turbogenerator includes a rotatable magnet assembly within 
the casing adapted to rotate with the impeller and coupled to the 
generator, and the impeller comprises an electrically conductive ring 
surrounding the casing in the vicinity of the rotatable magnet assembly 
such that, when the impeller is rotated by the mud flow, eddy currents are 
induced in the electrically conductive ring by the magnetic field 
associated with the magnet assembly and the magnet assembly is caused to 
rotate with the impeller by virtue of the interaction between the magnetic 
field associated with the magnet assembly and the magnetic field 
associated with the induced currents. The electrically conductive ring 
preferably comprises an annulus of material, such as copper, of high 
electrical conductivity within which eddy currents may be induced 
surrounded by an annulus of highly magnetisable material, such as steel, 
which may provide a return path for the magnetic flux. 
Alternatively the impeller may comprise a magnetisable ring surrounding the 
casing in the vicinity of the rotatable magnet assembly such that, when 
the impeller is rotated by the mud flow, the magnet assembly is caused to 
rotate with the impeller by virtue of the magnetic attraction between the 
magnet assembly and the magnetised ring. The magnetisable ring is 
preferably a hysteresis ring, that is a ring of ferromagnetic material, 
such as 35% cobalt-steel, having a high coercivity and therefore a 
hysteresis loop of large area, since the magnitude of the torque which may 
be transferred by the ring to the magnet assembly is dependent on the area 
of the hysteresis loop. 
The magnet assembly is preferably a rare earth magnet assembly, that is a 
magnet assembly employing magnets, such as samarium-cobalt magnets, which 
incorporate rare earth elements. Such magnets have a very high coercivity 
so that, even when used in an open loop configuration, the magnets may be 
capable of inducing appreciable eddy currents in the electrically 
conductive ring or of magnetically saturating the magnetisable ring. 
Furthermore such magnets are almost impossible to demagnetise.

DETAILED DESCRIPTION OF THE INVENTION 
The signal transmitter 1 illustrated in the drawings is installed in use 
within a non-magnetic drill collar and coupled to a measuring instrument 
disposed in an instrument pressure casing installed within the drill 
collar immediately below the transmitter 1. The drill collar is disposed 
at the end of a drill string within a borehole during drilling, and the 
measuring instrument may serve to monitor the inclination of the borehole 
in the vicinity of the drill bit during drilling, for example. The signal 
transmitter 1 serves to transmit the measurement data to the surface in 
the form of pressure pulses by modulating the pressure of the mud which 
passes down the drill string. The transmitter 1 is formed as a 
self-continued unit and is installed within the drill collar in such a 
manner that it may be retrieved, in the event of instrumentation failure 
for example, by inserting a wireline down the drill string and engaging 
the wireline with a fishing neck on the transmitter, for example by means 
of a per se known gripping device on the end of the wireline, and drawing 
the transmitter up the drill string on the end of the wireline. 
Referring to FIGS. 1 to 3, the transmitter 1 includes a duct 2 provided, at 
its upper end, with an annular flow constrictor 4 defining a throttle 
orifice 6 for the mud passing down the drill string in the direction of 
the arrow 8. Within the duct 2 is an elongate casing 10 bearing at its 
upper end, in the vicinity of the throttle orifice 6, a throttling member 
12 which is displaceable with respect to the casing 10 in the direction of 
the axis of the duct 2 to vary the throughflow cross-section of the 
throttle orifice 6. The throttling member 12 is provided with a shaft 14 
which extends into the casing 10, the space within the casing 10 being 
filled with hydraulic oil in order to ensure hydrostatic pressure balance 
and being sealed at its upper end by a Viton diaphragm 16 extending 
between the inside wall of the casing 10 and the shaft 14. The casing 10 
is rigidly mounted within the duct 2 by three upper support webs 18 and 
three lower support webs 20 extending radially between the casing 10 and 
the duct 2, so as to provide an annular gap between the casing 10 and the 
duct 2 for mud flow. 
An annular impeller 22 having a series of blades 24 distributed around its 
periphery and angled to the mud flow surrounds the casing 10, and is 
carried on a shoulder 26 of the casing 10 by means of a filled PTFE 
(polytetrafluoroethylene) thrust bearing 28. The blades 24 are mounted on 
a magnetisable steel boss 30 which surrounds a copper drive ring 32. A 
rare earth magnet assembly 34 is carried by an annular shaft 36 rotatably 
mounted within the casing 10 by means of bearings such as 38, and 
incorporates six Sm Co (samarium-cobalt) magnets 40 distributed about the 
periphery of the shaft 36. Three of the magnets 40 have their North poles 
facing radially outwardly and a further three of the magnets 40, 
alternating with the previous three magnets 40, have their South poles 
facing radially outwardly. As the impeller 22 rotates in the mud flow, 
eddy currents will be induced in the copper drive ring 32 by the intense 
magnetic field associated with the six Sm Co magnets 40, the magnetisable 
steel boss 30 providing return paths for the magnetic flux, and the magnet 
assembly 34 and hence the shaft 36 will be caused to rotate with the 
impeller 32 by virtue of the interaction between the magnetic field 
associated with the magnets 40 and the magnetic field associated with the 
eddy currents induced in the drive ring 32. 
The annular shaft 36 drives a rotor 42 of an electrical generator 44 for 
supplying power to the measuring instrument by way of a circular 
escapement plate 46, pivotally mounted within the shaft 36 by pivot pins 
47, and a torque drive arm 48 (see FIG. 4) attached to the periphery of 
the plate 46 and arranged to engage a drive pin 50 attached to the 
periphery of the rotor 42. In addition the annular shaft 36 drives a 
hydraulic pump 52 by way of an angled swashplate 54 and an associated 
piston thrust plate 56 provided with a bearing race 57. 
The hydraulic pump 52 comprises eight cylinders 58 extending parallel to 
the axis of the casing 10 and arranged in an annular configuration, and a 
respective piston 60 associated with each cylinder 58. The lower end of 
each piston 60 is permanently biased into engagement with the thrust plate 
56 by a respective piston return spring 62, so that rotation of the 
swashplate 54 with the shaft 36 will cause the pistons 60 to axially 
reciprocate within their cylinders 58, the eight pistons 60 being 
reciprocated cyclically so that when one of the pistons is at the top of 
its stroke the diametrically opposing pistons will be at the bottom of its 
stroke and vice versa. In addition the pump 52 comprises a rotary valve 
member 64 mounted on bearings 65 and intended to rotate in synchronism 
with the swashplate 54 so as to supply the output from each cylinder 58 in 
turn to one side of a double-acting ram 66 disposed within a cylinder 68. 
The double-acting ram 66 is coupled to the shaft 14 of the throttling 
member 12 by an output shaft 70, so that the throttling member 12 may be 
displaced by the pump 52 to vary the throughflow cross-section of the 
throttle orifice 6. 
More particularly the hydraulic oil which fills the casing 10 and which is 
supplied to each of the cylinders 58 from one side of the double-acting 
ram 66 is forced by the associated piston 60 into a respective axial bore 
72 in a valve housing 74 which surrounds the rotary valve member 64 on the 
upstroke of the piston 60. Each of the axial bores 72 is crossed by a 
respective lower radial bore 78. The rotary valve member 64 is provided 
with an upper peripheral recess 80 which opens out at the periphery of the 
valve member 64 over approximately 180.degree. of arc and which also opens 
at the top of the valve member 64 into the lower part 82 of the cylinder 
68 below the ram 66, and a lower peripheral recess 84 (shown in FIG. 2 in 
broken lines) which opens out at the periphery of the valve member 64 over 
approximately 180.degree. of arc on the opposite side of the valve member 
64 to the upper peripheral recess 80 and which also opens at its upper 
region into a central annular recess 86 formed in the valve member 64. The 
central annular recess 86 is permanently maintained in fluid communication 
with an annular passage 88 surrounding the cylinder 68 and valve housing 
74 by radial passages (not shown) extending through the valve housing 74. 
The annular passage 88 is itself in fluid communication with the upper 
part 90 of the cylinder 68 above the ram 66. 
There are two possible phases of rotation of the rotary member 64 with 
respect to the rotation of the swashplate 54, namely a first phase of 
rotation in which the upper peripheral recess 80 communicates with the 
upper radial bores 76 on the upstroke of the associated pistons 60 and the 
lower peripheral recess 84 communicates with the lower radial bores 78 on 
the downstroke of the associated pistons 60, and a second phase of 
rotation in which the upper peripheral recess 80 communicates with the 
upper radial bores 76 on the downstroke of the associated pistons 60 and 
the lower peripheral recess 84 communicates with the lower radial bores 78 
on the upstroke of the associated pistons 60. Thus, during the first phase 
of rotation of the valve member 64, the input of the pump 52 will be 
connected to the upper part 90 of the cylinder 68 and the output of the 
pump 52 will be connected to the lower part 82 of the cylinder 68, so that 
the ram 66 and hence the throttling member 12 will be displaced upwardly. 
Conversely, during the second phase of rotation of the valve member 64, 
the input of the pump 52 will be connected to the lower part 82 of the 
cylinder 68 and the output of the pump 52 will be connected to the upper 
part 90 of the cylinder 68, so that the ram 66 and the throttling member 
12 will be displaced downwardly. 
The rotary valve member 64 is coupled to a torque-sensitive actuator, 
comprising a circular drive plate 92 disposed opposite the escapement 
plate 46, by a drive shaft 94 rotatably mounted within the annular shaft 
36 by bearings 96. The drive plate 92 is provided with a driven pin 98 at 
its periphery which is engaged by a first escapement pin 100 at a first 
rotational position at the periphery of the escapement plate 46 in order 
to cause the valve member 64 to be driven by the shaft 36 with the first 
phase of rotation or alternatively by a second escapement pin 102 (see 
FIG. 4), which is disposed at a second rotational position offset by 
180.degree. with respect to the first rotational position at the periphery 
of the escapement plate 46, in order to cause the valve member 64 to be 
driven by the shaft 36 with the second phase of rotation. 
As shown clearly in FIG. 4, which shows a section taken along the line 
IV--IV in FIG. 3 but with the casing 10 and the duct 2 omitted, the 
escapement plate 46 is capable of being tilted about a tilt axis defined 
by the pivot pins 47 between a first angled position (shown in solid lines 
in FIG. 4) and a second angled position (shown in broken lines in FIG. 4). 
A tension spring 104 biases the escapement plate 46 into its first angled 
position. For relatively low electrical loads applied to the output of the 
generator 44, the escapement plate 46 will drive the drive plate 92 with 
the first phase of rotation by means of the first escapement pin 100 and 
will also drive the rotor 42 of the generator 44 by way of the torque 
drive arm 48. However, if the generator load increases to a point where 
the torque required to drive the rotor 42 is sufficient to overcome the 
bias of the spring 104, the torque drive arm 48 will be caused to tilt the 
escapement plate 46 into its second angled position against the action of 
the spring 104. This will cause the first escapement pin 100 to be brought 
out of engagement with the driven pin 98 of the drive plate 92, and the 
second escapement pin 102 to be engaged with the driven pin 98 after the 
escapement plate 46 has rotated through 180.degree. with respect to the 
drive plate 92. This will cause the drive plate 92 to be driven with the 
second phase of rotation by means of the second escapement pin 102, and 
the supply of hydraulic fluid from the pump 52 to the double-acting ram 66 
will be reversed. Of course, if the generator load subsequently decreases 
to a sufficient extent, the spring 104 will tilt the escapement plate 46 
back into its first angled position, and the drive plate 92 will again be 
driven with the first phase of rotation. 
It will therefore be appreciated that, if the measurement data from the 
measuring instrument is arranged to suitably vary the electrical load of 
the generator 44, the phase of rotation of the rotary valve member 64, and 
hence the direction of displacement of the double-acting ram 66, will vary 
with the output of the measuring instrument. This will in turn cause the 
throttling member 12 to be displaced with respect to the throttle orifice 
6 to modulate the pressure of the mud flow upstream of the throttle 
orifice 6, and will produce a series of pressure pulses corresponding to 
the measurement data which will travel upstream in the mud flow and may be 
sensed at the surface by a pressure transducer in the vicinity of the 
output of the pump producing the mud flow. This arrangement therefore 
enables data in digital form to be transmitted to the surface.