Background:
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The invention relates to a horizontal directional drilling.
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Horizontal directional drilling is a method that is used in
the laying of underground cables or pipes. The horizontal directional
drilling method does not need trenches. A bore hole is made in
horizontal direction at the position where the underground cable,
pipe, etcetera has to be layed. In this way cables, pipes, etcetera
are layed crossing railways, highways, waterways, etcetera. Drilling
starts at a drilling starting point outside the bore hole to be made.
Horizontal directional drilling systems need a guidance system to
guide the drill head to a desired position. Two types of guidance
systems are known. A first known guidance system is integrated in the
drill head. A second known guidance system is the so-called walk-over
system in which at the surface provisions are made to locate the drill
head. A typical example of a walk-over system is known as the
TruTracker system in which an artificial magnetic field is created at
the surface and detected by magnetometers in or near the drill head.
The reference point for the actual drill head position is the drilling
starting point. The position of the drill head is continuously
estimated, based on the actual drill head angles and the starting
point by dead reckoning. Guidance systems integrated in the drill head
assembly are based on sensors, which are moving down-hole together
with the drill head. Such sensors measure the direction of the drill
head in space, i.e. the azimuth angle (yaw), the tilt angle (roll) and
the inclination angle (pitch). Present sensors, available on the
market, are magnetometers, accelerometers and mechanical gyroscopes.
The azimuth angle is measured by a magnetometer which uses the earth
magnetic field to determine the azimuth angle relative to the earth
magnetic field. The tilt angle and the inclination angle are measured
by accelerometers. Accelerometers measure the earth gravity. When the
sensor direction is parallel to the gravity field a value of 9.8 m/s2
is measured. When the sensor direction is perpendicular to the gravity
field a value of 0 m/s2 is measured. An output of an accelerometer
varies with the angle with respect to the earth gravity field
according to the sine of the angle between the sensor and the earth
gravity field. In this way the tilt angle and the inclination angle of
the drill head can be measured. Traditional accelerometers have
resolutions in the magnitude of 5 micro g. (1 g equals 9.8 m/s2) and
temperature coefficients in the magnitude of 75 micro g/degr.
Centigrade. A typical guidance system integrated in the drill head
assembly therefore comprises a magnetometer to determine the azimuth
angle and two accelerometers to determine the tilt angle and
inclination angle. Magnetometer readings of the azimuth angle are not
always correct. In areas with underground magnetic constructions or
electric power cables, underground or at the surface a lot of magnetic
interference exists disturbing a correct azimuth angle reading by the
magnetometer. Walk-over systems, such as the TruTracker system
mentioned above, may avoid the consequences of magnetic interference.
For example, the TruTracker system induces a magnetic field by wires
at the surface. Thereby an artificial magnetic field is created
overcoming many interferences. A disadvantage of walk-over systems is
that they can only be applied when there is sufficient access to the
surface overhead the drill head. Such sufficient access for example is
not available when the bore hole has to be drilled under rivers with
heavy ship traffic, highways or railways. Guidance systems in which
use is made of mechanical gyroscopes do not suffer from the
disadvantages mentioned herein before related to the use of
magnetometers. A disadvantage of mechanical gyroscopes presently
available is that they have relatively large dimensions. Because of
those large dimensions and further because of their need for placement
on a stabilized platform mechanical gyroscopes cannot be used for
guidance in the initial bore hole. Consequently mechanical gyroscopes
are only used for survey activities after the bore hole has been
drilled already. Moreover mechanical gyroscopes are not suited for the
harsh environment during drilling. In horizontal directional drilling
accuracies of better than 30 centimeters for crossings with a length
of 400 meters are required. The above described present guidance
systems cannot achieve such accuracy. Magnetometers (when not
interfered) and mechanical gyroscopes have an accuracy in the
magnitude of 0.5 degrees. However in order to reach an accuracy of
better than 30 centimeters over 400 meters an azimuth accuracy in the
magnitude of 0.03 degrees is required. Not only the guidance systems
do not achieve the required accuracies but also the skills of the
personnel controlling the horizontal directional drilling systems
plays an important role. Especially in magnetically contaminated
areas, i.e. areas with a lot of magnetic interference, highly
experienced personnel is required to achieve an acceptable level of
accuracy. But even with highly experienced personnel in magnetically
contaminated areas location errors in the magnitude of 10 to 50 meters
over a distance of 400 meters are not uncommon. This not only results
in additional costs, but sometimes also in (near) environmental
disasters. Such disasters may happen when the drilling takes place in
the vicinity of underground electrical cables or oil and gas pipes. In
view of the above mentioned disadvantages of present day guidance
systems there exists a need to improve the horizontal directional
drilling accuracy to become more reliable, more accurate, more easy to
use, immune for magnetic interference and to have measurement data
continuously available even while drilling.
Summary of the invention
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It is an object of the invention to provide a guidance
system for a horizontal directional drilling system comprising sensors
at the drill head, which sensors comprise at least one of the sensors:
fiber optic gyroscope, ring laser gyroscope, micro-electro-mechanical
system, rate sensor.
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A further object of the invention is to provide such a
guidance system further comprising in microcontroller for receiving
and processing data from sensors at the drill head.
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A still further object of the invention is to provide such
a guidance system in which the microcontroller comprises neural fuzzy
control logic for processing the data from the sensors at the drill
head.
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A still further object of the invention is to provide such
a guidance system in with the microcontroller further comprises means
for application of model-based deterministic and stochastic,
respectively, filtering techniques to the data from the sensors at the
drill head. Thereby a magnetic interference canceling adaptive filter
is obtained.
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A still further object of the invention is to provide such
a system in which the sensor further comprises at least one of a
magnetometer and an accelerometer and at least one rate sensor and
means for integrating signal from the rate sensor.
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Further object of the invention is to provide such a system
comprising a magnetometer and a rate sensor measuring rate of change
of azimuth and further comprising means for controlling the system in
dependence on an integrated change of rate of azimuth signal when a
magnetic interference is present.
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A still further object of the invention is to provide such
a system comprising at least one accelerometer and at least one rate
sensor for measuring a rate of change of the same quantity that is
measured by the at least one accelerometer and further comprising
means for from time to time resetting the rate sensor and/or an
integrated rate of change of the relative quantity signal.
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It is also an object of the invention to provide such a
system further comprising transmitting means for transmitting data
from an output of the microcontroller to a surface device, which
surface device comprises a computer and display device and which
computer is programmed with a user interface to display at least one
of azimuth, tilt and inclination angles of the drill head on the
display device.
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An other object of the invention is to provide such a
system further comprising at least one rate sensor and transmitting
proces data from an output of the microcontroller to a surface device,
means for determining a reliability figure for data dependent on
integrated signals from the rate sensor, which surface device
comprises a computer and a display device and which computer is
programmed with a user interface to display the reliability figure.
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A further object of the invention is to provide such a
guidance system in which the computer is further programmed to display
guidance instructions in case the reliability figure is smaller than a
predetermined minimum.
Brief description of the drawings
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The invention will now be described in greater detail below
with reference to the drawings in which:
- Figure 1 shows schematically a down-hole probe near a drill
head with various elements of a guidance system according to the
invention;
- Figure 2 shows a vector exemplary of the direction of the
earth magnetic field relative to the earth surface;
- Figure 3 shows a first embodiment of a sensor unit;
- Figure 4 shows a second embodiment of a sensor unit;
- Figure 5 shows a third embodiment of a sensor unit;
- Figure 6 shows a fourth embodiment of a sensor unit;
- Figure 7 shows a fifth embodiment of a sensor unit.
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Detailed description of the invention
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Referring to Figure 1 an earth surface 1 is shown. Beyond
the surface 1 and in the earth 2 a bore hole is being made according
to the horizontal directional drilling method. A drill head (not
shown) is provided with a probe, schematically indicated by the
reference number 3, which is part of a guidance system guiding the
drill head through the earth 2 and, since it is at the front of the
bore hole, is called a down-hole probe. At the earth surface 1 the
guidance system comprises a computer 4 with a keyboard 5 and a monitor
6. The down-hole probe 3 comprises a power source 7, a transmitter 8,
a microcontroller 9 and a sensor unit 10 comprising one or more
sensors. The power source 7 may comprise a battery and/or electric DC
power supplies. Power is supplied to the transmitter 8, the
microcontroller 9 and the sensor unit 10. Signals from sensor unit 10
are input tot microcontroller 9 as indicated by arrow 11. Output
signals from microcontroller 9 are input to transmitter 8, as
indicated by arrow 12. Arrow 13 indicates a connection between
transmitter 8 and computer 4. The connection between transmitter 8 and
computer 4 may be a wire connection but preferably is a radio-wave
connection. Computer 4 calculates, based upon signals transmitted by
transmitter 8, signals which are data and other information generated
by microcontroller 9, guidance signals for the horizontal directional
drilling system of which the down-hole probe 3 is a part. The
outputting of guidance signals by computer 4 to the horizontal
directional drilling system, schematically indicated by the reference
number 14, has been schematically indicated by arrow 15.
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As has been described before traditional horizontal
directional drilling systems 14 comprise as sensors a magnetometer 16
and an accelerometer 17 for measuring the tilt angle and an
accelerometer 18 for measuring the inclination angle ( see Figure 3),
all inside down-hole probe 3.
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In order to achieve the objects of the present invention
use is being made of various elements, some of which are already known
as such, but have never been used or proposed for use in horizontal
directional drilling systems. Such elements comprise fiber optic
gyroscopes, ring laser gyroscopes, micro-electro-mechanical systems,
rate sensors and fuzzy logic. Those elements either alone or in
combination with each other and/or in combination with magnetometers
and/or accelerometers are able to make a horizontal directional
drilling system achieve the requirements mentioned hereinbefore
relating to reliability, accuracy, ease of use, immunity to magnetic
field interferences and continuity of availability of measurement
data, even while drilling.
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Fiber optic gyroscopes and ring laser gyroscopes are
elements in which two lightbeams travel in opposite directions around
a common path. When the plane of the path rotates a relative phase
shift will occur between the two lightbeams travelling in opposite
directions. In a ring laser gyroscope the phase shift is measured
which is due to an inherent change in oscillation frequency. In a
fiber optic gyroscope the phase shift is measured by interference
techniques. Both types of gyroscopes allow to measure yaw-rate, pitch-rate
and roll-rate. When such gyroscopes include integration circuits
output signals of such gyroscopes deliver output signals that are
representative of for example an azimuth angle, a tilt angle, or an
inclination angle. External dimensions of fiber optic gyroscopes and
ring laser gyroscopes are substantially smaller than corresponding
dimensions of mechanical gyroscopes. Ring laser gyroscopes and fiber
optic gyroscopes are sufficiently small to be integrated in a sensor
package 10 of a drill head. They also have as advantages over
mechanical gyroscopes no run up time, higher accuracy and far higher
reliabilities. Ring laser gyroscopes and fiber optic gyroscopes are
able to operate in a rotating drill head guidance assembly, whereas
mechanical gyroscopes are not suited for such harsh environments.
Accuracy of a fiber optic gyroscope can be in the magnitude of 0.01
degree for ambient temperature ranges from -40 to +80 degrees
Centigrade. As is to be expected fiber optic gyroscopes and ring laser
gyroscopes are insensitive to magnetic interference.
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Rate sensors as such are available in the market place and
are based on principles that range from Coriolis fork gyro to hybrid
solutions. Rate sensors are sensors that deliver output signals that
are representative of changes of a measured quantity per unit of time.
In order to obtain an integrated value such output signals have to be
integrated over time. When for example a rate sensor is used to
determine the rate at which an inclination angle changes with the time
the inclination angle at a certian point of time is obtained by
integrating the rate signal. Generally the integrated signal will
slowly walk away, depending on resolution, temperatue sensitivity
etcetera of the relative rate sensor. Typical resolutions achievable
by rate sensors are in the magnitude of 0.01 degree per sec to 1
degree per hour. The external dimensions of rate sensors generally are
sufficiently small for integrating such rate sensors in a down-hole
probe of a horizontal directional drillign system.
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Drill head angles can be calculated from the signals
delivered by the beforementioned magnetometers, accelerometers, fiber
optic gyroscopes, ring laser gyroscopes, micro-electro-mechanical
systems and rate sensors. Data from all those sensors must be
intelligently combined to achieve a reliable output for the drill head
angles, regardless of magnetic interference, or other disturbing
circumstances. All these calculations can be very complicated. Good
results can be achieved when these calculations are being carried out
by using so called neural fuzzy control methods. Preferably these
calculations are carried out by a microcontroller 9 which is part of
the down-hole probe. In that case the connections between the sensors
and the calculating logic are very short and chances are minimal for
the sensor signals to be contaminated with noise signals from other
sources. By carrying out calculations on the signals delivered by the
sensors to the microcontroller 9 through line 11 and by applying
deterministic and stochastic, respectively, filtering techniques a
magnetic interference canceling adaptive filter is obtained.
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Various combinations of sensors in the sensor unit 10 will
now be described.
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Figure 4 shows a sensor unit 10 comprising three fiber
optic gyroscopes 19, 20 and 21. It is to be noted that instead of
fiber optic gyroscopes the gyroscopes 19, 20 and 21 may also be ring
laser gyroscopes, the only difference being the physics way in which a
phase shift is measured. Unless otherwise mentioned any time that a
fiber optic gyroscope is mentioned it is to be noted that in place of
a fiber optic gyroscope a ring laser gyroscope could be used in that
same place. The fiber optic gyroscopes 19, 20 and 21 are placed each
in a plane from which can be measured the azimuth angle, the pitch
angle and the inclination angle, respectively. For example, fiber
optic gyroscope 19 may measure azimuth angle, fiber optic gyroscope 20
may measure tilt angle and fiber optic gyroscope 21 may measure
inclination angle. Since fiber optic gyroscopes measure angles by
integrating rate of change of angle offset values have to be input
into the control system. After the offset angles have been input into
the control system the fiber optic gyroscopes 19, 20 and 21 deliver
the required angle values. Those angle values are sent over line 11,
which of course may be a multiple line, to the microcontroller 9 for
calculating purposes. Thereafter calculated values are sent over line
12 to transmitter 8. The calculated values that are input into
transmitter 8 via line 12 are transmitted, for example by radio-signal,
over line 13 to computer 4. Computer 4 may be a regular
personal computer with a keyboard 5 an a monitor 6. Due to the
accuracy of fiber optic gyroscopes the sensor unit 10 in principle
does not need any more sensors than the three fiber optic gyroscopes
19, 20 and 21.
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Figure 5 shows a further embodiment of the sensor unit 10.
The sensor unit 10 comprises again three fiber optic gyroscopes 19, 20
and 21 and in addition thereto accelerometers 22 and 23. Accelerometer
22 measures a tilt angle and accelerometer 23 measures an inclination
angle of the drill head. The signals from the accelerometers 22 and 23
can be used in the microcontroller 9 to determine offset values for
the fiber optic gyroscopes, for example fiber optic gyroscopes 20 and
21 that measure tilt angle and inclination angle, respectively.
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Figure 6 shows a further embodiment of sensor unit 10. The
sensor unit 10 shown in Figure 6 comprises a magnetometer 24, a yaw-rate
sensor 25, a roll-rate sensor 26 and a pitch-rate sensor 27. It
also comprises an accelerometer 22 and an accelerometer 23.
Magnetometer 24 and yaw-rate sensor 25 cooperate. When there is no
magnetic interference magnetometer 24 may determine the azimuth angle.
However when there is magnetic interference the magnetometer output
will drift away. Such drift will be communicated through line 11 to
microcontroller 9 and from microcontroller 9 to line 12 to transmitter
8 and from transmitter 8 to line 13 to computer 4. Computer 4 will use
the data generated by microcontroller 9 and based upon output signals
from the magnetometer 24, which output signals have drifted away to
control through line 15 the drill head 16. The drift in output signal
of the magnetometer will result in drifting away in direction of the
drill head 16. Such drifting away of the drill head 16 will be sensed
by rate sensor 25. Microcontroller 9 will determine that rate sensor
25 generates a signal where it should not generate a signal and passes
this information to computer 4. That will determine that rate sensor
25 has sensed an ongoing change in azimuth angle whereas magnetometer
24 has not sensed such change and computer 4 will determine that a
drift is present in the output signal of the magnetometer which should
not be translated into a change in the azimuth angle of the drill head
16. Most underground magnetic interference is due to various
materials. These various materials normally have influence on the
horizontal directional drilling system and its sensor during a limited
period of time. Those magnetic interferences therefore also are very
much locally. During those periods of magnetic interference the
control of the drill head 16 will not be based upon output signals of
the magnetometer 24 but on integrated output signals of yaw-rate
sensor 25.
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Nevertheless integrating the signals from the yaw-rate
sensor 25 will lead to slowly walking away of the integrated signals,
depending upon resolution, temperature sensitivity etcetera of the
rate sensor 25. Therefore control of the direction of the drill head
16 by computer 4 based upon signals from yaw-rate sensor 25 should
only be done for a limited period of time. Present yaw-rate sensors
limit such period to a maximum of about half an hour when the
resolution of the rate sensor is 1 degree per hour. Offset of the yaw-rate
sensor, which takes place for example in the microcontroller 9,
may for example be based upon the reading of the magnetometer at the
point of time that it is decided to take over the directional control
of the drill head 16 from the signals from the magnetometer 24 to
control based upon the signals from the yaw-rate sensor 25.
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The rate signals generated by the rate sensors 26 and 27
may be integrated to provide inclination angle and tilt angle of the
drill head. As with rate sensor 25 the integrated signals of the rate
sensors 26 and 27, which integration may take place in the
microcontroller 9, will slowly walk away depending upon the
resolution, temperature sensitivity etcetera. These walk away effects
can be compensated by use of the accelerometers 22 and 23. Each time
that drill head rotation is stopped to steer the drill head in a
certain direction, which stopping happens periodically, the
accelerometers will give accurate values for the tilt and inclination
angles. The results of these measurements of tilt- and inclination
angle can be used to, automatically, offset the rate sensors 26 and
27.
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Hereinbefore it has been assumed that at a certain position
of the drill head it is known that magnetic interference exists at
that location. The existence of such magnetic interference is not
detected by the magnetometer 24 itself. However two methods will be
described hereinafter to determine the presence of magnetic
interference, i.e. the presence of a magnetic field of sufficient
strenght to make the magnetometer measure a value and direction of a
magnetic field that is not identical to the value and direction of the
earth magnetic field at that location.
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Figure 2 shows a coordinate system in which the drill head
is considered to be in the origin and the earth magnetic field is
expressed as a vector 28. One of the axes, indicated by the letter N,
is directed to true North. Angle δ indicates the deviation of the
magnetic North MN from the true North N and angle 29 indicates the
angle of dip of the earth magnetic field relative to the surface of
the earth which corresponds to the x-y plane of the coordinate system
shown in figure 2.
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A first device to determine the presence of magnetic
interference comprises two magnetometers at a few meters distance from
each other in the down-hole probe. Preferably those magnetometers are
3-axis magnetometers measuring components of the earth magnetic field
in three mutually orthogonal directions, but this is not a necessity
in this first embodiment. In first instance both magnetometers
determine the azimuth angle at their locations a few meters apart. In
case the magnetometers give the same output signal it can be assumed
that there is no magnetic interference at that location. In case the
magnetometers give different outputs at least one of the two
manetometers is in a location in which there is magnetic interference.
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A second device for determining the presence of magnetic
interference comprises one or even two 3-axis magnetometers. 3-axis
magnetometers are able to determine not only the direction of the
magnetic North, i.e. the azimuth angle, but also the angle of dip 29.
The angle of dip 29 is known as such for all locations in the world. A
single measurement with a 3-axis magnetometer may suffice to determine
the angle of dip. As the angle of dip measured by a single
magnetometer differs from the angle of dip that should be present due
to the location on the earth where the drilling takes place, that is
an indication that there is magnetic interference. In case two 3-axis
magnetometers are being used a comparison can be made between the true
directions of the vectors 28, one measured by each magnetometer. In
case a difference in direction exists between the vector 28 as
measured by a first 3-axis magnetometer from the vector 28 as measured
by a second 3-axis magnetometer that is a very strong indication that
a magnetic interference exists at that location.
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A sensor unit 10 that may be used to determine the presence
of magnetic interference is shown in figure 7 and comprises two
magnetometers 24a en 24b. As hereinbefore the sensor unit 10 also
comprises rate sensors 25, 26 and 27 and accelerometers 22 and 23 for
the purposes described in relation to figure 6. When no magnetic
interference is present either one of the magnetometers 24a or 24b can
be used in a same way as magnetometer 24 was used in the system
described in relation with figure 6. Both embodiments shown in figures
6 and 7 have the possibility of having the yaw-rate sensor 25 being
automatically off-set from time to time by the magnetometer 24 and
24a, 24b respectively. In that way, when a magnetic interference comes
up and control of the drill head has to be based on integration of the
signal from the yaw-rate sensor 25 that signal can be used reliably.
The reliability of the signal from the yaw-rate sensor 25 decreases
with increasing periods of time since the last point of time that it
was reset by the signal from magnetometer 24 or one of the
magnetometers 24a and 24b.
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As described hereinbefore rate sensors have offsets and
after integration may show drift in their output signals. Contrary
thereto accelerometers show a stable output as function of time. A
gravity angle determined from an accelerometer can therefore be used
to compensate a rate sensor. That results in a drift free rate
measurement. Such a drift free rate measurement again can be used to
correct an output of a magnetometer in case of magnetic interference.
However not in all circumstances traditional accelerometers can be
used to achieve this result.
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Traditional accelerometers have resolutions in the
magnitude of 5 micro g. and temperature coefficients in the magnitude
of 75 micro g per degree Centigrade. A typical time constant of a
traditional accelerometer is 0.13 seconds. A rotational speed of a
drill head typically is approximately 20 RPM, which equals 120 degrees
per second. Therefore typically traditional accelerometers have a time
constant that is too large to be used in magnetometer compensation in
case of magnetic interference. Present micro-electro-mechanical system
sensors do show time constants in the magnitude of 1 millisecond.
Hence these sensors can be used to enhance the magnetometer response
and accuracy.
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It is extremely difficult to employ skilled staff having
sufficient knowledge about the measurement techniques and their inter
relationship with the directional control of the drill head. User
interfaces, i.e. computer programs on the computer 4 that allow an
operator to enter correct commands through the keyboard or other data
entry elements such as a mouse, should therefore be simple and easy to
understand. For example in an abnormal situation, such as magnetic
interference, easy to understand guidance must be given to the
operator.
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Raw data are sent by microcontroller 9 to computer 4 once
every so many seconds. A program in computer 4 translates those raw
data into signals for the monitor 6 to display the azimuth, tilt, and
inclination angle. It also shows the position of the toolface, which
is of importance while steering. In general these presentations of
data require various skilled staff for interpretation. Improvement is
required. For example when applying fiber optic gyroscopes as sensors
it is easy, due to their high accuracy, to display reliable true North
data and to calculate, by dead-reckoning, the precise position of the
drill head. It is also possible then to show on the monitor 6, by
means of a suitable programme, both the actual and the desired track.
This may in particular be of importance when underground curves are
being made by the drill head. With the accurate sensor systems
described hereinbefore for use in a horizontal directional drilling
system, it is possible to steer the drill head at desired locations to
make underground curves.
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In certain of the embodiments described hereinabove rate
sensors were applied. It is known that integrated rate sensor signals
will slowly walk away depending on resolution, temperature sensitivity
etcetera. Therefore the more time has elapsed between the last point
of time that the integrated value of a rate sensor was reset the less
reliable is a present value of the integrated signal. A program
displaying integrated signals of rate sensors, or of other signals
that depend on integrated signals from rate sensors, will therefore be
displayed on monitor 6 together with a reliability figure. The
reliability figure informs an operator of the measure of reliability
of the displayed figures. In a situation in which one or more of the
figures displayed on monitor 6 are displayed with a reliability figure
that is out of range, i.e. the reliability figure shows that the
reliability is below a certain minimum reliability figure then the
operator should switch to another method of control of steering the
drill head, for example by following instructions generated by the
computerprogram and displayed on the screen of the monitor 6.
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After the above descriptions various modifications and
alterations will become clear to a person skilled in the art. Such
modifications and alterations are considered to be within the scope of
the appended claims.
