U.S. patent number 3,658,138 [Application Number 05/030,534] was granted by the patent office on 1972-04-25 for process for optimizing the penetration speed of a drilling tool driven by a motor whose torque decreases with an increasing running speed and apparatus therefor.
This patent grant is currently assigned to Institut Francais du Petrole des Carburants et Lubrifiants. Invention is credited to Jean Charles Gosselin.
United States Patent |
3,658,138 |
Gosselin |
April 25, 1972 |
PROCESS FOR OPTIMIZING THE PENETRATION SPEED OF A DRILLING TOOL
DRIVEN BY A MOTOR WHOSE TORQUE DECREASES WITH AN INCREASING RUNNING
SPEED AND APPARATUS THEREFOR
Abstract
A process for optimizing the penetration speed of a drilling
tool driven by a motor whose torque decreases with an increasing
running speed and vice-versa, comprising the steps of alternately
increasing and decreasing the load on the tool about its optimum
value by acting on the tensile stress exerted on the drill string,
thereby defining periods of increase and periods of decrease of the
ratio -(.DELTA.Va/T ) which is, with a changed sign the ratio
between the variation of the penetration speed of the tool and the
corresponding variation of the tensile stress exerted on the drill
string, of controlling the passage from a tensile stress-increasing
period, at the latest when the ratio -(.DELTA.Va/T) attains, while
decreasing, a lower limit value and of controlling the passage from
a tensile stress-decreasing period to a tensile stress-increasing
period at the latest when said ratio attains, while increasing, an
upper limit-value, said lower and upper limit values being
preselected and adjustable.
Inventors: |
Gosselin; Jean Charles
(Versailles, FR) |
Assignee: |
Institut Francais du Petrole des
Carburants et Lubrifiants (Rueil Malmaison (Hauts de Seine),
FR)
|
Family
ID: |
9033369 |
Appl.
No.: |
05/030,534 |
Filed: |
April 21, 1970 |
Foreign Application Priority Data
|
|
|
|
|
Apr 30, 1969 [FR] |
|
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6913967 |
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Current U.S.
Class: |
173/1; 173/6;
175/103; 175/26 |
Current CPC
Class: |
E21B
19/08 (20130101); E21B 19/22 (20130101) |
Current International
Class: |
E21B
19/22 (20060101); E21B 19/00 (20060101); E21B
19/08 (20060101); E21b 003/12 () |
Field of
Search: |
;173/1,4,6
;175/26,103 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Purser; Ernest R.
Claims
What I claim is :
1. A process for optimizing the penetration speed of a drilling
tool driven by a motor whose torque decreases with an increasing
running speed and vice-versa, comprising the steps of alternately
increasing and decreasing the load on the tool about its optimum
value by acting on the tensile stress exerted on the drill string,
thereby defining periods of increase and periods of decrease of the
ratio - (.DELTA. Va/.DELTA.T) which is, with a changed sign the
ratio between the variation of the tensile stress exerted on the
drill string, of controlling the passage from a tensile
stress-increasing period, at the latest when the ratio - (.DELTA.
Va/.DELTA.T) attains, while decreasing, a lower limit-value and of
controlling the passage from a tensile stress-decreasing period to
a tensile stress-increasing period at the latest when said ratio
attains, while increasing, an upper limit-value, said lower and
upper limit values being preselected and adjustable.
2. A process according to claim 1 wherein said ratio - (.DELTA.
Va/.DELTA.T) is determined from average values of the penetration
speed Va and tensile stress T over time intervals equal to the
pulsing period of the drilling fluid circulation pumps.
3. Automatic apparatus for optimizing the penetration rate of a
drilling tool driven by a motor whose torque decreases with an
increasing running speed and vice-versa, comprising in combination
means for progressively varying the weight on the tool, means for
reversing the direction of said weight variation, means for
measuring the algebraic value of the ratio - (.DELTA. Va/.DELTA.T)
which is, with a changed sign, the ratio between the variation of
the penetration speed of the tool and the corresponding variation
of the tensile stress exerted on the drill string, means for
setting up an upper limit-value and a lower limit-value of said
ratio, and means for comparing the value of said ratio with said
limit-values, said means for comparing being connected to said
means for measuring said ratio and to said means for reversing the
direction of variation of the weight on the tool and being adapted
to initiate a decrease in this weight at the latest when the value
of said ratio attains said lower limit-value and an increase of
this weight when said ratio attains said upper limit-value.
4. Apparatus according to claim 3 further comprising means for
comparing the value of at least one other drilling parameter with a
preset safety limit, said means being also connected to said means
for reversing the direction of variation of the weight on the tool,
so as to actuate the latter when the value of said parameter
attains said limit.
5. A process for controlling the penetration speed of a drilling
tool driven by a motor, the torque of which varies in inverse
proportion to the running speed, comprising the steps of :
measuring the penetration speed of the drilling tool ;
detecting a change in said penetration speed;
measuring the tensile stress exerted on the drilling tool;
detecting the change in said tensile stress;
determining the ratio of a change in said penetrating speed for a
change in said tensile stress negative with respect thereto;
and
maintaining said ratio within prescribed maximum and minimum limits
by controlling the load on the drilling tool in response to said
ratio.
6. A method according to claim 5, wherein said step of controlling
the load on the drill tool comprises the steps of alternately
increasing the load on the tool until said ratio reaches said
prescribed minimum limit and then decreasing the load exerted on
the tool until said ratio reaches said prescribed maximum
limit.
7. A method according to claim 5, wherein said ratio is determined
from the average values of the penetrating speed and tensile stress
over time intervals equal to the pulsing period of the fluid
circulation pumps for the drilling fluid .
8. A method according to claim 7, wherein said step of increasing
the load on the tool includes the step of driving the drill string
downwardly with a feed break greater than the maximum penetrating
speed of the drilling tool.
9. A method according to claim 7, wherein said step of decreasing
the load exerted on the tool comprises the step of terminating the
displacement of the drill string to thereby release the load on the
drill tool.
10. A method according to claim 7, wherein said step of controlling
the load on said drill tool further includes comparing said ratio
with each of said prescribed maximum and minimum limits.
11. A method according to claim 10, wherein said step of driving
said drill string comprises the step of driving said drill string
at a linear speed substantially equal to about twice the
penetration speed of the drill tool.
12. An apparatus for controlling the penetration rate of a drilling
tool by a motor , the torque of which varies inversely with respect
to the running speed, comprising:
first means for controlling the penetration speed of the drill
tool;
second means, responsive to said first means, for detecting a
change in said penetrating speed;
third means for measuring the tensile stress exerted on the drill
string;
fourth means, responsive to said third means for detecting a change
in said tensile stress ;
fifth means, responsive to said second and fourth means, for
determining the rate of a change in said penetrating speed for a
change in said tensile stress negative with respect thereto;
and
sixth means, responsive to said fifth means, for maintaining said
ratio within prescribed maximum and minimum limits by controlling
the load on the drill tool in response to said ratio .
13. An apparatus according to claim 12, wherein said sixth means
comprises means for alternately increasing the load on the drill
tool until said ratio reaches said predetermined minimum limit and
means for decreasing the load exerted on the tool until said ratio
reaches said predetermined maximum limit.
14. An apparatus according to claim 13, wherein said means for
increasing the load exerted on the drill tool comprises means for
driving the drill string downwardly with a feed rate greater than
the maximum penetration speed of the drilling tool.
15. An apparatus according to claim 14, wherein said means for
decreasing the load exerted on the drill tool comprises means for
terminating the displacement of the drill string to thereby release
the load on the drill tool.
16. An apparatus according to claim 12, wherein said sixth means
includes means for comparing said ratio with each of said
prescribed maximum and minimum limits.
17. An apparatus according to claim 16, wherein said driving means
comprises means for driving said drill string at a linear speed
substantially equal to about twice the penetration speed of said
drill tool.
18. An apparatus according to claim 16, wherein said comparing
means includes means for comparing the value of at least one other
drilling parameter with a preset safety limit, being connected to
said displacement terminating means, so as to actuate said
terminating means when the value of said parameter attains said
limit.
Description
This invention relates to a process and an apparatus for optimizing
the penetration speed of a drilling tool driven by a motor giving a
torque which decreases with an increasing running speed and
vice-versa, so that the speed of penetration of said tool in the
ground will be as high as possible, for any power supply,
irrespective of the type of bit used and the traversed
formations.
The bottom motors such as turbines used in the turbodrilling
process and the series wound or compound electric motors used in
the so-called electrodrilling process have this characteristic
relationship between torque and motor running speed.
It must be understood however that the scope of the invention is
not limited to the case where the driving motor of the tool is
directly coupled therewith at the lower end of the drill string,
but also includes the case where the tool is driven by rotation of
the drill string through a surface motor provided that it
communicates to the tool a torque which is a decreasing function of
the rotation speed thereof.
With such a torque-speed relationship, it is observed that, when
starting from a low value of the thrust load or weight on the
drilling tool and progressively increasing said weight, the
penetration rate begins to increase up to a maximum value and then
decreases with still increasing weight on the tool, until stalling
of the tool occurs.
The optimum value of the weight on the tool, to which corresponds
the maximum penetration speed, varies according to the nature of
the formations traversed by the bore hole.
The essential object of the present invention is to provide a
process and an apparatus for a regulation, optionally completely
automatic, of the weight on the drilling tool, so as to keep the
penetration speed of the tool as close as possible to its maximum
value for each of the geological strata traversed by the bore
hole.
The process of the invention, whereby this object is achieved, is
characterized in that the value of the load on the tool is
alternatively decreased and increased about its optimum value, by
correspondingly increasing and decreasing the tensile stress
exerted on the drill string and accordingly the ratio -(dVa/dT)
which is the ratio with a changed sign, of the variation of the
penetration speed of the tool to the corresponding variation of the
tensile stress at a given point of the drill string, in that the
passage from a tensile stress increasing phase to a tensile stress
decreasing phase is initiated at the latest when the value of said
ratio -(dVa/dT) during its decreasing phase has reached a lower
limit-value and the passage from a decreasing to an increasing
phase for said tensile stress is initiated at the latest when the
value of said ratio has reached an upper limit-value, said two
limit-values being preselected and adjustable.
The reversing of the direction of variation of the tensile stress
exerted on the drill string may be effected before the ratio
-(dVa/dT) reaches any one of said limit-values, in the case where
at least another operating parameter of the drilling apparatus,
such as the tensile stress T exerted on the drill string or the
rotation speed of the tool, will reach a predetermined safety
limit.
An automatic apparatus for carrying out the process of the
invention is characterized by the combination of means for
gradually varying the weight on the tool, means for reversing the
direction of variation of said weight, means for measuring the
algebraic value of -(dVa/dT) which is the ratio with a changed sign
of the variation of the penetration speed of the tool to the
corresponding variation of the tensile stress exerted on the drill
string at a point thereof, means for setting an upper limit-value
and a lower limit-value of said ratio, means for comparing the
value of said ratio with said limit-values, said means for
comparing being connected to said means for measuring said ratio
and to said means for reversing the direction of variation of the
weight on the tool and being adapted to initiate a decrease in this
weight at the latest when the value of said ratio reaches said
lower limit-value and an increase in this weight when the value of
said ratio reaches said upper limit value.
The apparatus may be provided with means for comparing at least one
other drilling parameter, such as the tensile stress of the drill
string and the rotation speed of the tool, with a preset safety
limit, said means being also connected to said means for reversing
the direction of the variation of the weight on the tool, so as to
actuate this means when the value of said parameter reaches said
safety limit.
One particular embodiment of the invention, given by way of
illustrative example, is hereafter described more in detail with
reference to the accompanying drawing wherein :
FIG. 1 illustrates, in the case of drilling with use of a bottom
motor, the law of variation of the bit speed of penetration in
terms of the weight applied thereto,
FIGS. 2 and 2A diagrammatically show two examples of apparatus
according to the invention,
FIG. 3 shows, facing each other, a curve of variation of the
penetration speed of the bit versus time and the corresponding
variation of pressure of the drilling fluid as measured at the
surface,
FIG. 4 shows the law of variation of the ratio (.DELTA.Va/.DELTA.T)
versus time, when using the process of the invention,
FIG. 5 shows the corresponding law of variation of the feeding rate
of the drill string at the surface, and
FIGS. 6 and 7 respectively show the corresponding variations of the
speed of penetration of the drilling tool and of the weight exerted
thereon.
In FIG. 1, wherein curve 1 and curve 2 represent, for two ground
layers, respectively, the variation of the penetration speed of the
tool versus the thrust load W or weight exerted thereon, points
M.sub.1 and M.sub.2 represent the respective operating points for
these two ground layers whose abscissae correspond to the weight Wf
applied on the tool and the ordinates to the corresponding
penetration speed through the considered ground layer. It can be
seen that, when increasing the weight Wf from the operating point
M.sub.1 or M.sub.2, this operating point moving to the right of the
figure on any of the two curves, the speed of penetration Va of the
tool increases and attains a maximum at point S.sub.1 (or S.sub.2),
the curve portion described by the operating point corresponding to
stable operating conditions. Beyond the vertex of the operation
curve the operating conditions are unstable and the penetration
speed decreases with an increasing weight on the tool until
stalling of the bottom motor.
As shown in FIG. 3, the penetration speed of the tool undergoes
fluctuations of time variation about an average curve 3. These
fluctuations are of a frequency associated with the pulsing rate of
the drilling mud circulation pump and cannot be filtered without
introducing a time constant which is incompatible with the response
time required for an automatic device for optimizing the
penetration speed of the drill bit, which device must prevent any
possibility of stalling of the bit.
This inconvenience can be avoided by taking in consideration the
average penetration speed of the tool over a time interval equal to
the pulsing period of the mud pumps or to a multiple thereof.
The instants t.sub.n.sub.-1 and t.sub.n, which are the limits of
such period intervals (FIG. 3) may be selected as being the
instants at which the alternating component of the pressure P.sub.5
of the drilling fluid, as measured at the surface, passes through
the value zero, such pressure value oscillating about an average
value corresponding to the straight line 4, at the same frequency
as the penetration speed, but with a time lag .phi. corresponding
to the lag between the respective pressures of the drilling fluid
at the surface and on the hole bottom.
The average penetration speed on the hole bottom between instants
t.sub.n.sub.-1 and t.sub.n is given by the formula :
wherein (Vd) is the average value of the linear speed of the drill
string as measured at the surface between the two instants under
consideration, .alpha. the lengthening coefficient of the drill
string under the effect of a tensile stress, and T.sub.n.sub.-1 and
T.sub.n are the respective tensile stresses to which the drill
string is subjected at instant t.sub.n.sub.-1 and instant
t.sub.n.
FIGS. 2 and 2 A diagrammatically illustrate two optional
embodiments of apparatus for carrying out the invention; FIG. 2
illustrates the case of a flexible drill string unwound from a
storing reel and FIG. 2 A the case of a drill string formed of
rigid elements.
On these figures reference 5 indicates the drilling tool suspended
from the drill string 7 and driven by the bottom motor 6
consisting, for example, of a drilling turbine fed with hydraulic
energy from the surface, reference 8 indicating the bore hole.
The drill string is lowered in the bore hole through a handling
device which may consist of one or more caterpillar chains carrying
jaws or clamping shoes in the case of FIG. 2 and of a winch 33 with
a cable 34 wound thereon and supporting a pulley block 32 in the
case of use of rigid drill pipes forming the drill string 7 (FIG. 2
A). This handling device is driven by a motor 10, through an
irreversible coupling device 11, which can be engaged or disengaged
at will by means of electric signals transmitted through control
cable 12.
A digital computer in actual time 13, receives the measuring value
of the tensile stress T applied to the drill string, which
measuring value can be supplied, in the case of the embodiment of
FIG. 2, through electric conductors 14, by a device 15 comprising
at least one strain gage placed at the lower end of the drill
string 7 or at 35, on the dead end of the pulley block 31, at the
surface, in the case of embodiment of FIG. 2 A.
A device 16 is used for measuring the length L of the drill string
suspended from the surface through device 9.
This device may consist, for example, of a roller in contact with
the drill string 7 and driven in rotation by the linear
displacement thereof, said roller driving in turn in rotation the
emitter of a synchromechanism well known in the art as "Selsyn" and
whose receiver, to which it is electrically connected through cable
18, actuates a revolution counter device providing, in the form of
a series of electric pulses, a digital measuring value of length L,
supplied to computer 13 through cable 19.
The feed rate Vd of the drill string, at the surface, can be
measured by devices 20 and 21, respectively similar to devices 16
and 17, the digital value of said speed being supplied to the
digital computer 13 through cable 22.
For this measurement of Vd it is preferred to use devices separate
from those used for measuring L, so as to determine with a
sufficient accuracy the linear speed of the drill string.
The value of the elasticity coefficient of the drill string 7 is
set up at 23 in the computer and there is also supplied to said
computer, through conductor 24, a signal representing the digital
value of the alternating component of the pressure Ps of the
drilling mud at the surface, this pressure being measured by any
suitable pressure sensor not shown in the figure, placed in the mud
circuit and supplying a measuring signal filtered through device
25, which keeps only the alternating component of this signal
supplied to the computer.
The numerical value of .alpha. will be preferably measured
periodically in the hereunder stated manner, after stopping the
drilling operation, the average value of the coefficient .alpha.
corresponding to the depth reached.
This measurement can be effected by raising the drill bit over a
few meters and then lowering it again so as to rest it on the well
bottom without drilling.
By observing during this lowering of the drill bit the variations
in the tensile stress T on the drill string, of the linear speed Vd
of the drill string at the surface and in the penetration speed Va,
it will be seen that the tensile stress T progressively decreases
from instant t.sub.A until the bit reaches the bottom of the bore
hole, in proportion to the elastic shortening of the drill
string.
This shortening is terminated at instant t.sub.B when the feed rate
at the surface becomes zero.
The relationship :
shows that, between instants t = t.sub.A and t = t.sub.B the value
of V.sub.a given by the computer 13 will be different from zero if
the value of .alpha. which is used is not the proper one.
More precisely the value of V.sub.a will be kept positive between
instant t.sub.A and instant t.sub.B, when the value of .alpha.
which is used by the computer 13 is too low since, in such case,
the absolute value of the negative term
will be too low for having the right-hand side of equation 1 equal
to zero, the value of (Vd) thus being the greater in said
right-hand side and resulting in a positive value of (V.sub.a)
.
On the contrary, if the value of .alpha. is too high, the absolute
value of the negative term will be greater than that of (Vd) and
will accordingly result in a negative value of (V.sub.a) determined
by the computer 13.
The exact value of .alpha., measured in situ for the drilled depth,
may be calculated by computer 13, using the fact that if t.sub.k
and t.sub.k.sub.-1 indicate two instants within the time interval
between t.sub.A and t.sub.B, the following relationship is
applicable :
(Vd) (t.sub.k - t.sub.k.sub.-1) = - .alpha. L (T.sub.k -
T.sub.k.sub.-1) 2.
wherein L is the drill string length supported from the surface,
T.sub.k.sub.-1 and T.sub.k are the respective tensile stresses
applied to said drill string at instants t.sub.k.sub.-1 and
t.sub.k.
The digital computer 13 is adapted to determine the value of the
ratio
wherein :
respectively represent the average values of the penetration speed
of the tool and of the tensile stress on the drill string, as
determined by the computer 13 between instants t.sub.n.sub.-2 and
t.sub.n.sub.-1 and between instants t.sub.n.sub.-1 and t.sub.n as
hereabove defined and indicated at 24 on the computer as explained
above.
The computer 13 determines the values of (Va) and (Va) by using the
above formula (1).
In this embodiment such values are successively displayed by the
computer on device 27 to which it is connected through cable
28.
The ratio - (.DELTA. V.sub.a /.DELTA. T) is equal to the ratio
(.DELTA. Va/.DELTA. W) when the mud flow rate Q is constant and
accordingly corresponds substantially, on the operating curve 1 or
2 of FIG. 1, to the slope of the tangent at the operating point M,
or M.sub.2, this slope being equal to zero when the penetration
speed attains its maximum value for the considered ground
layer.
The computer 13 is supplied, by setting up at 29 and 30
respectively, a negative limit-value (- .DELTA.Va).sup.-/.DELTA.T
and a positive limit-value (- .DELTA. Va).sup.+/.DELTA.T of the
calculated ratio - (.DELTA. Va/.DELTA. T), these two values
corresponding respectively to a positive limit-value .theta..sub.1
and a negative limit-value .theta..sub.2 (FIG. 1) of the angle of
inclination, with respect to the abscissae axis, of the tangent to
the operating curve (1 or 2) at the operating point.
The apparatus works as follows : the device 9 drives the drill
string 7 downwardly with a feed rate higher than the maximum
penetration speed (Va max) of the drilling tool 5 (FIG. 5); e.g
twice such a speed, which results in an increase of the weight W on
the tool and makes the operating point M.sub.1 (or M.sub.2)
describe the operating curve 1 (or 2) towards the right side of the
FIG. 1. The penetration speed Va first increases with the weight W
on the tool (FIGS. 6 and 7), reaches a maximum and then decreases,
while - (.DELTA. Va/.DELTA. T) decreases.
The computer 13 compares the calculated value of the ratio -
(.DELTA. Va/.DELTA. I) with each of the aforesaid limit-values :
when this value becomes at instant t.sub.o (FIG. 4) lower than the
negative limit-value (- .DELTA. Va).sup.-/.DELTA. T the computer 13
delivers a pulse controlling the disengaging of device 11 and, the
device 9 being no longer driven, the displacement of the drill
string is stopped (FIG. 5), thereby releasing the load exerted on
the tool (FIG. 7) which disengages itself.
The operating point M.sub.1 (or M.sub.2) now describes leftwards
the operating curve (FIG. 1) and the penetration speed again begins
to increase up to a maximum value and then decreases again while -
(.DELTA. Va/.DELTA. T) increases (FIG. 4).
When at the instant t.sub.1 the calculated value of ratio -(.DELTA.
Va/.DELTA. T) becomes greater than the positive limit-value (-
.DELTA. Va).sup.+/.DELTA. T, the computer 13 actuates through
conductor 12 the clutch of device 11, thereby starting again the
displacement of the drill string (FIG. 5), which results in an
increase of the weight on the tool. A new cycle, identical to the
preceding one, can then start again and so on indefinitely.
It is apparent that this results in a continuous oscillation about
its optimum value, of the weight on the tool, said oscillation
resulting in the maintenance of the average penetration speed at a
value very close to the maximum value, the difference being
dependent on values (- .DELTA. Va).sup.+/.DELTA. T and (- .DELTA.
Va).sup.-/.DELTA. T which can be selected at will and as close to
each other as permitted by the drilling procedure and the control
apparatus.
The rapid control of the weight on the tool obtained through
actuation of an engaging or a disengaging system, avoids any
possible stalling, when, beyond the maximum of the operating curve
(FIG. 1) the operating point enters the zone of unstable operating
conditions.
The above-mentioned selection method of instants t.sub.n.sub.-1,
t.sub.n, t.sub.n.sub.+1 , etc . . . delimiting the time intervals
in which is determined the average penetration speed of the drill
bit, gives the penetration speed with the required accuracy for a
proper operation of the apparatus without incurring the risk of
improper actuations of the engaging or disengaging system of device
10 which might result in alternative changes of the penetration
speed Va at constant weight on the tool (FIG. 3) if the
instantaneous value of said speed were supplied to the computer
13.
In order that the time intervals during which the device 10 is
disengaged be substantially equal to those during which it is
engaged (so that the computer may have the maximum efficiency in
case of change of ground layer), it is advantageous to have a
linear speed Vd at the surface equal to about twice the penetration
speed Va of the bottom tool. This can be obtained by controlling
the driving speed of device 9 by means of the maximum value read at
27 (FIG. 2). This control, which can be effected by the computer
13, does not need a high accuracy and allows a relatively long time
constant, due to inertia of motor 10 (FIG. 2).
The apparatus according to the invention automatically adjusts
itself to changes in the nature of the drilled formations. Such
changes result in fact in a mere change of operating curve (FIG.
1), point M.sub.1 being shifted to M.sub.2.
The negative threshold (-.DELTA.Va).sup.+/.DELTA.T being reached
later than if the operating point had been kept on curve 1, it
results that the weight on the tool increases more, the penetration
speed Va attains a greater value and consequently the feed rate Vd,
which is controlled by the value 2 Va.sub.max, will increase
accordingly, thereby achieving the adaptation to the new ground
layer traversed by the drill bit.
The adjustment of the feed rate Vd of the drill string might also
be achieved by hand, in accordance with the indications read at 27
(FIG. 2).
It is of course also possible, without departing from the scope of
the invention, to assign to the apparatus through the computer,
more complex orders for controlling the engaging and disengaging
systems, so as to obtain a very safe operation.
For example, the actuation of the disengaging system, the
triggering of which was supposed to take place only when the actual
operating value of the ratio - (.DELTA. Va/.DELTA.T) runs past the
negative threshold of the calculated value - (.DELTA.Va/.DELTA.T),
will be also triggered, by means of any logical system such as
electric circuits of the "OR gate" type, not only when -
(.DELTA.Va/.DELTA.T) runs past the preselected negative threshold,
but also when the weight W on the tool attains a preset maximum
value, or if the speed of the turbine falls below a predetermined
threshold etc . . .
Similarly the actuation of the engaging system, so as to increase
the weight on the tool, may be triggered not only when -
(.DELTA.Va/.DELTA.T) runs past the preselected positive threshold,
but also when the weight W on the tool is reduced in a
predetermined proportion, or when the running speed of the turbine
exceeds a predetermined limit-value and so on .
In these conditions the triggering of the engaging or the
disengaging system is initiated at the latest when -
(.DELTA.Va/.DELTA.T) attains one or the other of said negative or
positive threshold (or limit-values) which have been selected for
it, said triggering being optionally initiated sooner when at least
one of the other drilling parameters such as the weight on the tool
or the running speed of the turbine exceeds a predetermined safety
limit.
* * * * *