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BACKGROUND OF THE INVENTION
This invention relates to the field of borehole telemetry. More
particularly, this invention relates to the field of rotation sensors for
borehole telemetry whereby borehole parameters are sensed and telemetered
to the surface only when the drill string has ceased rotation or reached a
predetermined low rate of rotation.
In the field of borehole drilling, particularly oil and gas well drilling,
the usefulness of a system capable of detecting certain parameters at the
bottom of a drill string and transmitting such data to the surface during
the course of drilling has long been recognized. Several systems have been
proposed for accomplishing sensing and data transmission. One of the
principal types of such systems is the mud pulse telemetry system wherein
pulses ar generated in the mud column in the drill string for transmission
of the data to the surface. The present invention is particularly adapted
for use in mud pulse transmission systems.
In the case of several classes of data, it is quite unnecessary to obtain
readings more frequently than once every 30 feet or so of depth of the
well. This corresponds to readings every 1/4 to 11/2 hours at typical
penetration rates of 120 feet per hour to 20 feet per hour. It, therefore,
becomes desirable to turn off the downhole parameter sensing equipment
during long periods of drilling thereby minimizing wear which would
otherwise result from continuous operation of the parameter sensors.
SUMMARY OF THE INVENTION
The present invention senses the state of absence of rotation of the drill
string, and the condition of no rotation is used as a signal to activate
the parameter sensing apparatus in the system.
The present invention is particularly suitable for use in a downhole
telemetry system which contains a turbine driven by the mud. Rotation of
the turbine shaft drives an electrical generator which powers the
telemetry equipment. The downhole parameter sensing equipment may include
sensors which detect the magnetic heading and inclination of the borehole
with respect to the vertical. To take accurate measurements, it is
necessary for the instruments to temporarily come to rest, i.e., the drill
string must be held stationary. In normal rotary drilling, the drill
string is rotated at a speed of from 40 to 160 rpm, and mud is circulated
downward through the inside of the drill string. To obtain a reading in
the present invention, mud flow is maintained, but rotation is stopped.
The rotation sensor detects the "no-rotation"condition for a preset length
of time. This permits the long pendulous drill string to come fully to
rest. Once the no rotation state has been sensed, the parameter sensors
are given the command to obtain readings, and the readings are then
transmitted to the surface in the form of pulses in the mud column. As
long as the drill pipe is held stationary, repeat readings may be taken.
A magnetic detecting device, in the form of a ring core flux gate
magnetometer, constitutes the rotation sensor. This sensor operates by
interaction with the earth's magnetic field. Thus, the sensor must be
housed within a non-magnetic housing. This rotation sensor contains no
moving parts, and therefore, unlike many other motion sensors which may
contain moving elements, offers high reliability while exposed to
mechanical shocks and vibrations. Another important feature to be noted is
that the rotation sensor is controllable at the surface by the driller.
That is, since the driller controls rotation, the driller can be sure that
telemetering will not be initiated at inconvenient or unwanted times,
since the driller has direct command of the rotation sensor which, in
turn, controls sensing of the downhole parameters and generation of the
telemetry signals.
The phase angle of the second harmonic of the output, which varies as a
function of the rotation of the magnetometer, is detected and compared to
a reference to generate a signal of varying frequency which is then
delivered as the input to zero crossing detector. The zero crossing
detector produces an output pulse each time the phase angle between the
second harmonic and the reference is at a zero value. The pulses generated
by the zero crossing detector are then delivered to a digital filter where
they are compared with the output of a clock. The digital filter generates
a first output level when the drill string is rotating and a second output
level when rotation of the drill string has ceased. The output level
commensurate with a cessation of rotation is then used to activate the
parameter sensing apparatus.
BRIEF DESCRIPTION OF THE DRAWINGS
In the drawings, wherein like elements are numbered alike in the several
figures:
FIG. 1 is a generalized schematic view of a borehole and drilling derrick
showing the environment for the present invention.
FIG. 2 is a view of a section of the drill string of FIG. 1 showing, in
schematic form, the drill string environment of the present invention.
FIG. 3 is a view, partly in section, of a detail of FIG. 2.
FIG. 4 is a view of the flux magnetometer of the rotation sensor.
FIG. 5 is a block diagram of the rotation sensor.
FIG. 5A is a schematic showing of the digital filter of FIG. 10B.
FIGS. 6A, 6B and 6C are curves showing outputs at various stages of the
rotation sensor of FIG. 5.
FIG. 7 is a schematic representation of the sensor device for determining
inclination, reference and azimuth angles.
FIG. 8 is a representative curve of the output of one of the accelerometers
of FIG. 7.
FIG. 9 is a representative curve of the output of the magnetometer of FIG.
7.
FIGS. 10A and 10B constitute a block diagram of the control system.
FIGS. 11A, 11B and 11C are a schematic of the control system shown in block
diagram in FIGS. 10A and 10B.
FIG. 12 is a schematic showing of the initiation control of FIG. 10B.
FIG. 13 is a schematic showing of the master clock of FIG. 10B.
FIG. 13A shows the output pulses of the master clock and divider circuit.
FIG. 14A shows the output from the summer of FIG. 10A which is delivered to
the sign and magnitude detector.
FIGS. 14B, 14C, 14D and 14E show outputs from the sign detector of FIG.
10A.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring now to FIG. 1, the general environment is shown in which the
present invention is employed. It will, however, be understood that the
generalized showing of FIG. 1 is only for the purpose of showing a
representative environment in which the present invention may be used, and
there is no intention to limit applicability of the present invention to
the specific configuration of FIG. 1.
The drilling apparatus shown in FIG. 1 has a derrick 10 which supports a
drill string or drill stem 12 which terminates in a drill bit 14. As is
well known in the art, the entire drill string may rotate, or the drill
string may be maintained stationary and only the drill bit rotated. The
drill string 12 is made up of a series of interconnected segments, with
new segments being added as the depth of the well increases. The drill
string is suspended from a movable block 16 of a winch 18, and the entire
drill string is driven in rotation by a square kelly 20 which slidably
passes through but is rotatably driven by the rotary table 22 at the foot
of the derrick. A motor assembly 24 is connected to both operate winch 18
and rotatably drive rotary table 22.
The lower part of the drill string may contain one or more segments 26 of
larger diameter than other segments of the drill string. As is well known
in the art, these larger segments may contain sensors and electronic
circuitry for sensors, and power sources, such as mud driven turbines
which drive generators, to supply the electrical energy for the sensing
elements. A typical example of a system in which a mud turbine, generator
and sensor elements are included in a lower segment 26 is shown in U.S.
Pat. No. 3,693,428 to which reference is hereby made.
Drill cuttings produced by the operation of drill bit 14 are carried away
by a large mud stream rising up through the free annular space 28 between
the drill string and the wall 30 of the well. That mud is delivered via a
pipe 32 to a filtering and decanting system, schematically shown as tank
34. The filtered mud is then sucked by a pump 36, provided with a
pulsation absorber 38, and is delivered via line 40 under pressure to a
revolving injector head 42 and thence to the interior of drill string 12
to be delivered to drill bit 14 and the mud turbine if a mud turbine is
included in the system.
The mud column in drill string 12 also serves as the transmission medium
for carrying signals of down the well drilling parameters to the surface.
This signal transmission is accomplished by the well known technique of
mud pulse generation whereby pressure pulses are generated in the mud
column in drill string 12 representative of sensed parameters down the
well. The drilling parameters are sensed in a sensor unit 44 (see also
FIG. 2) in a drill collar unit 26 near or adjacent to the drill bit.
Pressure pulses are established in the mud stream in drill string 12, and
these pressure pulses are received by a pressure transducer 46 and then
transmitted to a signal receiving unit 48 which may record, display and/or
perform computations on the signals to provide information of various
conditions down the well.
Referring briefly to FIG. 2, a schematic system is shown of a drill string
segment 26 in which the mud pulses are generated. The mud flows through a
variable flow orifice 50 and is delivered to drive a turbine 52. The
turbine powers a generator 54 which delivers electrical power to the
sensors in sensor unit 44. The output from sensor unit 44, which may be in
the form of electrical or hydraulic or similar signals, operates a plunger
56 which varies the size of variable orifice 50, plunger 56 having a valve
driver 57 which may be hydraulically or electrically operated. Variations
in the size of orifice 50 create pressure pulses in the mud stream which
are transmitted to and sensed at the surface to provide indications of
various conditions sensed by sensor unit 44. Mud flow is indicated by the
arrows.
For several classes of data or parameters to be sensed at the bottom of a
well, it is quite unnecessary to sense the data and obtain readings more
frequently than once every thirty feet or so of depth. This corresponds to
readings every one quarter hour to one and one-half hour at typical
drilling rates of one hundred twenty feet per hour to twenty feet per
hour. It therefore becomes desirable to turn off the down hole sensing
equipment during long periods of drilling, thereby minimizing wear of the
sensors, transmitter and other parts of the telemetry system which would
otherwise result from continuous operation. The invention shown in FIGS
3-6 is directed to this feature of turning off the parameter sensing
equipment by sensing and distinguishing between periods of rotation and
absence of rotation of the drill string. The invention requires a rotation
sensor to detect drill string rotation and interrupt the delivery of
electrical power to the well parameter sensors when the drill string is
rotated, and, conversely, to permit the delivery of power to the well
parameter sensors when the drill string is not rotated. A magnetic
detecting device which senses the earth's magnetic flux is used as a
rotation sensor to detect the presence or absence of rotation of the drill
string. This rotation sensor contains no moving parts, and, therefore,
unlike other motion sensors which may contain moving elements, offers high
reliability notwithstanding exposure to mechanical shocks and vibrations.
Referring now to FIGS. 2 and 3, some details of a drill string segment 26
are shown housing the rotation sensor 58 in accordance with this
invention. Since both the rotation sensor and one or more other sensors in
sensor unit 44 are magnetically sensitive, the particular drill string
segment 26A which houses the rotating sensor of this invention and the
other sensor elements must be a non-magnetic section of the drill string,
preferably of stainless steel or monel. The rotation sensor 58 may be
incorporated in sensor unit 44 or may be separately packaged, and for the
sake of convenience it is shown as part of sensor unit 44 in FIG. 3.
Sensor unit 44 is further encased within a non-magnetic pressure vessel 60
to protect and isolate the sensor unit from pressure down in the well.
Referring to FIG. 4, the rotation sensor 58 is a ring-core fluxgate
magnetometer which is used to determine the direction of the earth's
magnetic field. Although theoretically many other kinds of flux detecting
devices could be used, the ring-core fluxgate magnetometer is used because
of its low power consumption and its rugged physical construction.
Operation of the ring-core fluxgate magnetometer is based on the nonlinear
or asymmetric characteristics of the magnetically saturable transformer
which is used in the sensing element. As seen in FIG. 4, the device has a
toroidal or annular core 62 which is appropriately wound (winding details
not shown), an input or primary winding 64 and an output or secondary or
sensing winding 66. Core 62 is made of a material with a square B-H
hysteresis curve such as permalloy. The characteristic of this device is
such that when the core is saturated by appropriate AC energizing of the
primary winding in the absence of an external magnetic field, the output
of the secondary windings, i.e. the voltage induced in the secondary
windings is symmetrical, i.e. contains only odd harmonics of the
fundamental of the driving current. However, in the presence of an
external magnetic signal field such as the earth's magnetic field, the
output voltage of the secondary windings becomes asymmetrical with second
and other even harmonics of the primary frequency appearing at the output
of the secondary windings. This asymmetry is related in direction and
magnitude to the signal field and can be detected by several known
techniques. Discussions of such fluxgate magnetometers can be found in the
article by Gordon and Brown, IEEE Transactions on Magnetics, Vol. Mag-8,
No. 1, Mar. 1972, and the article by Geyger, Electronics, June 1, 1962 and
in the article by R. Munoz, AA-3.3, 1966 National Telemetering Conference
Proceedings, to all of which reference is made for incorporation herein of
a more detailed discussion of construction and theory of operation of the
magnetometer.
As employed in the present invention, the input to the primary windings 64
drives core 62 to saturate twice for each cycle of the primary winding
input. The moment in time that the core saturates is related to the
ambient external magnetic field that biases the drive field in the core.
That is, saturation of the core varies as a function of the intensity and
direction of the earth's magnetic field, which field is indicated
diagrammatically by the flux lines in FIG. 4.
Sensor 58 is physically supported on a shaft 68 which is fixed in drill
string segment 26A and is on or parallel to the axis of rotation of drill
string segment 26A. While the drill string is being rotated, rotation
sensor 58 is also being rotated in the ambient magnetic field of the
earth. As rotation sensor 58 is rotated, the combined action of the input
to primary windings 64 and the ambient magnetic field of the earth result
in a varying phase shift in the second harmonic output at secondary
windings 66.
Referring now to FIG. 5, a block diagram of the rotation sensor output
signal processing is illustrated. The input to primary winding 64 emanates
from an oscillator 61, the output frequency of which is divided in half by
divider 63 and then delivered to amplifier 65 and then delivered to
primary winding 64. The output from secondary windings 66, which is tuned
to the second harmonic of the primary winding input by capacitor 67, is
delivered to a buffer amplifier 69 and then to phase detector 70A of
detector 70. Detector 70 also includes low pass filter 70B and amplifier
70C. The output of oscillator 61 (which is equal in frequency to the
second harmonic output of secondary winding 66) is also delivered to phase
detector 70A. The phase angle of the second harmonic output of secondary
windings 66 is a function of the rate of rotation of magnetometer 58, and
that phase angle varies as a function of changes in the rate of rotation
of magnetometer 58. The output of secondary windings 66 is compared with
the output of oscillator 61 in phase detector 70A, where the difference in
phase between the two is detected and delivered to low pass filter 70B.
The output from filter 70B (when the drill string is rotating) is an
alternating signal which varies in frequency as a function of the rate of
change of the phase angle of the second harmonic output of secondary
winding 66; i.e. the output of filter 70B varies in frequency as a
function of changes in the rate of rotation of the drill string. The
output from filter 70B is amplified in amplifier 70C and is then delivered
to a zero crossing detector 72 which produces an output pulse each time
the alternating signal from detector 70 crosses through the zero value.
The pulses generated by crossing detector 72 (which are also a function of
the rate of rotation of the drill string) are delivered to a digital
filter 74 which produces output signals commensurate with states of
rotation and no rotation.
Referring also to FIG. 5A, digital filter 74 includes a counter-divider 75,
an S-R type flip flop 76, J-K type flip-flops 77 and 78, and an AND gate
79 connected as shown. The output pulses from zero crossing detector 72
are delivered to the C input of counter-divider 75. Assuming the drill
string is normally rotating, the pulses delivered to counter 75 cause
counter 75 to overflow before being reset by a clock pulse CPN (which may
be any selected subdivision of a clock pulse commensurate with a
predetermined minimum rate of rotation), whereby the Q output of counter
75 goes high. The Q output of counter 75 is connected to the S input of
flip-flop 76 and the high state of the Q output of counter 75 sets
flip-flop 76, whereby the Q output of flip-flop 76 goes high and the Q
output goes low. The Q output of flip-flop 76 is connected to the J input
of flip-flop 77. Flip-flop 77 is initially cleared by a reset pulse ICLEAR
which may be obtained from any convenient place in the system upon the
initiation of power in the control system. The J input of flip-flop 77 is
examined by the leading edge of each pulse CPN delivered to the C input of
flip-flop 77 whereby the J input is delivered to the Q output. Thus, when
the drill string is normally rotating, counter 75 repeatedly overflows and
is then reset by clock pulses CPM; flip-flop 76 is repeatedly set by the Q
output from counter 75 and reset by the upper level of clock pulses CPN;
and the J input of flip-flop 77 is low each time it is examined by the
leading edge of the CPN pulse at the C input of flip-flop 77. The Q output
of flip-flop 77 is thus also low when the drill string is normally
rotating, and a first output level indicating rotation is delivered from
filter 74 (see Level X, FIG. 6C).
Referring again to FIG. 6, the various signals discussed above are shown
graphically. The abscissa in each graph is time, and the ordinate in each
graph is signal amplitude. FIG. 6A shows the second harmonic output of
detector 70, FIG. 6B shows the pulse output from zero crossing detector
72, and FIG. 6C shows the outputs from digital filter 74. From time
T.sub.1 to T.sub.2 in all the graphs, the drill string is rotating at
constant speed. As the drill string slows down when approaching a state of
no rotation (after time T.sub.2), the frequency of the alternating output
of detector 70 decreases, thus resulting in a lower frequency output from
zero crossing detector 72.
When the rotation of the drill string ceases, or the rate of rotation drops
to a very low rate on the way to a state of no rotation, the pulses from
zero crossing detector 72 drop below a predetermined minimum frequency
corresponding to a predetermined low rate of rotation of the drill. since
the angular velocity of the drill string must go through decreasing levels
in going from normal to zero rotation, a predetermined low rate (on the
order of 3 rpm or less) can be used as a signal of no rotation, in that
rotation is about to cease and will have ceased within the time required
to initiate operation of desired sensors which operate when rotation has
ceased.
When rotation ceases or drops below the predetermined low rate, which
signals the imminence of the state of no rotation, counter 75 does not
overflow before being reset by the clock pulse CPN. Thus, the Q output of
counter 75 stays low, and flip-flop 76 does not get set. Since flip-flop
76 does not set, the Q output of flip-flop 76 is high and the J input of
flip-flop 77 is high. The leading edge of clock pulse CPN then sets
flip-flop 77 whereby the Q output of flip-flop 77 is high (see level Y of
FIG. 6C) indicating the state of no rotation. Thus, when the predetermined
minimum frequency output from zero crossing detector 72 is maintained for
a given time period from T.sub.2 to T.sub.3 (e.g. ten seconds), the
digital filter output (i.e. the Q level of flip-flop 77) is switched, as
shown in FIG. 6C, to a second level indicating a state of no rotation (see
level Y of FIG. 6C). This second output level, commensurate with a
condition of no rotation, is then used as a control signal for arming or
powering the other sensor elements in sensor unit 44. Prior to generation
of this control signal, the other sensor elements in unit 44 are not
powered. The control signal (i.e. the second output level from digital
filter 74) is used as a signal to arm or deliver the power from generator
54 to valve driver 57 and to those other sensor elements, such as by
operating flip-flops or arming gates to enable power to be delivered to
the other sensor elements in sensor unit 44 or in any other desired
fashion to that end.
Referring now to FIG. 7, the invention of the parameter sensing elements in
sensor unit 44 and operation thereof are shown, i.e. the sensor units for
sensing the various down the well parameters which are to be sensed after
rotation has ceased and transmitted to the surface periodically to provide
a measurement and indication of certain directional characteristics at the
bottom of the well.
The characteristics to be measured and determined in the present invention
are directional characteristics of the drilling line, especially a
drilling line which is slanted either from its point of origin or from an
intermediate point in the well. As is known in the art (for example see
U.S. Pat. No. 3,657,637 to Claret), the parameters of inclination angle,
azimuth angle and reference angle must be known in order to have total
information about the position and direction of a drilling line. For
purposes of clarification, the following definitions of the several angles
are presented:
1. Inclination angle (i) is the angle of inclination of the drill axis with
respect to the vertical (V) where both the drill axis and the vertical are
contained in a common vertical plane. Referring to FIG. 7, the drilling
axis is X'X, and I = angle XOV.
2. azimuth (A) is a magnetic azimuth. It is defined as the dihedral angle
formed by the vertical plane which contains the horizontal projection of
the drill axis and the vertical plane containing the horizontal projection
of the local terrestral magnetic field. Referring to FIG. 7, it is the
angle A as shown in connection with the ring core fluxgate magnetometer.
3. The reference angle R is the dihedral angle defined by the intersection
between a first plane containing the drill axis and a line (commonly
referred to as the scribe line) on the drill string parallel to the drill
axis and a second plane containing the drill axis and the vertical
projection of the drilling axis. The reference angle R is shown at the top
of the unit in FIG. 7.
Generally speaking, the sensor system, shown in FIG. 7, includes:
1. A mechanical device with three axes for determining
a. A vertical plane, using the force of gravity as a reference, and
b. A horizontal plane, using the force of gravity as a reference, and
c. The north direction, using the earth's magnetic field as a reference.
2. A motor drive system to drive parts of the mechanism to desired
positions about the axes.
3. Error transducers to determine deviation from the desired positions
about the axes and provide feedback to the motor drive system.
4. A control and a measuring system to measure the total movement of the
motor drive system required to eliminate the error.
FIG. 7 schematically shows the mechanism of the system and the interaction
with the motor drives and error transducers. The sensor is a multi-axis or
multi-gimbal system servo controlled by error transducers. More
specifically, the sensor consists of a three gimbal system, servo
controlled by two error transducing accelerometers and one error
transducing magnetometer. The accelerometers are used to establish
horizontal and vertical planes, and the magnetometer is used to establish
a direction of magnetic north in a horizontal plane.
The sensor includes an outer frame 100 which is rotatably mounted in sensor
unit 44 in pressure vessel 60 with non-magnetic drill collar section 26A
(see FIG. 3). Frame 100 is rotatably mounted on axis 102 which is the axis
of the drill string at the bottom of the well, or frame 100 may be mounted
for rotation about an axis parallel to axis 102. Frame 100 is mounted for
such rotation by shafts 104 and 106 which extend from opposite ends of the
frame and are mounted in bearings 108 and 110, respectively, which are, in
turn, connected to sensor housing 44 by supports 112 and 114. Frame 100 is
shown as a rectangular structure with sides parallel to axis 102 and ends
perpendicular to axis 102; however, the frame can be of any shape
symmetric about axis 102 or could be a surface of revolution about axis
102. Thus, in the embodiment being discussed, the axis of the frame, which
is the axis of rotation of the frame, coincides with or may be parallel to
drill string axis 102. Frame 100 constitutes a first gimbal in the system.
A first accelerometer 116 (sometimes referred to as the reference
accelerometer) is mounted on a platform 118 between the sides of frame 100
with its sensitive axis perpendicular to the direction of drill string
axis 102 (as used throughout this specification, the term "perpendicular"
as used with lines or axes will be understood to mean a right angle
relationship regardless of whether the lines or axes intersect in a common
plane or are in different planes. By definition, the sensitive axis is the
axis along which gravity forces will generate an output. Accelerometer 116
is an error transducing device of the type whose output go | | |
