Sonic method and apparatus for freeing a stuck drill string

A mechanical oscillator which employs unbalanced rotors is coupled to the top end of a drill string which has become stuck in a bore hole. The rotors are driven at a rotation speed such as to generate high level energy at a sonic frequency, this energy being coupled to the drill string. A low impedance load for the oscillator and the top of the drill string is created by supporting the oscillator from the support structure, which may comprise a derrick or the like, by means of a highly elastic support which may include elastomeric stretch bands, springs or other elastomeric support having a linear constant spring rate. In addition, hydraulic cylinders which may be servo driven are used to keep the drive motors for the oscillators in one place as the oscillator structure vibrates. The oscillators are driven at a high energy level to effect high displacement of the top of the drill string which presents a low impedance load to this energy in view of the highly elastic support provided. The drill string acts as an acoustic lever which translates the high displacement at the top of the string into a very high force at the point along the drill string which is stuck in the bore hole where a high impedance load is presented. The frequency of the oscillator may be adjusted to provide resonant vibration of the drill string to effect a reflected wave at the stuck point with a resultant increased cyclic force at this point.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to oil well drilling and more particularly to a 
method and apparatus employing sonic energy for freeing a section of drill 
string which has become stuck in a bore hole. 
2. Description of the Related Art 
An ongoing problem in the drilling of oil wells is the sticking of the 
drill string against the bore hole wall. This problem becomes accentuated 
where the sticking occurs at a great distance below the surface where the 
freeing of the string becomes increasingly difficult. Methods and 
apparatus have been devised in the prior art for employing sonic energy 
which is fed to the stuck section of the drill pipe to effect its freeing. 
Such prior art methods and apparatus generally employ mechanical 
oscillators which are operated at a frequency such as to effect resonant 
standing wave vibration of the drill string. In U.S. Pat. No. 3,168,140 
issued Feb. 2, 1985 to Albert G. Bodine and U.S. Pat. No. 3,155,163 issued 
Nov. 3, 1964 to Albert G. Bodine, methods and apparatus are described for 
freeing a drill string in which sonic energy is applied to the top of the 
drill string to effect resonant standing wave vibration of the string 
which vibrational energy is fed down the string to the point of sticking. 
In U.S. Pat. No. 4,667,742 issued May 26, 1987 to Albert G. Bodine, a 
method and apparatus is described in which the oscillator is lowered down 
the bore hole and connected to the down hole stuck pipe section and the 
string then resonantly vibrated to effect the desired freeing action. 
While some effort is made in all of these prior art devices to 
vibrationally isolate the drill string from the suspension hardware, 
either in the form of springs, or by adjusting the frequency of vibration 
so that a node of the resonant wave appears at the point of connection to 
the support members, there is no suggestion in these prior art patents of 
suspending the top of the drill string with a highly elastic support so as 
to make for very low impedance to the vibrational energy generated by the 
oscillator at this point. Further, there is no disclosure in such prior 
art directed to utilizing the drill string as an acoustic lever with the 
top end of the string being driven with a very high displacement and this 
high displacement being reflected at the sticking point as a high force 
vibration. 
SUMMARY OF THE INVENTION 
The basic feature of the present invention is the utilization of the stuck 
drill string as an acoustic lever to multiply the cyclic output of the 
mechanical oscillator employed so as to develop a very high force down 
hole where the string is stuck in the bore hole. The principles of 
acoustical leverage are aptly described on pages 418-422 of A Textbook of 
Sound by A. W. Wood, published in 1957 by G. Bell and Sons Ltd., London, 
England. As pointed out in this treatise, there is a close analogy between 
a static lever and an acoustic lever such that a small vibratory effort at 
one end of the acoustic lever exerted over a long distance(lever arm) will 
produce a large force acting over a short distance. In implementing such 
operation in the apparatus and method of the present invention, a low 
impedance condition is created at the top end of the drill string where 
the sonic energy is coupled thereto from a mechanical oscillator formed by 
unbalanced rotors which are rotatably driven to generate such energy. This 
low impedance condition is achieved by supporting the oscillator structure 
on a highly elastic support system which has constant linear properties. 
In the preferred embodiment such support system is formed by elastomeric 
bands through which the oscillator structure and the drill string to which 
the oscillator is connected are suspended from a lift system which may 
comprise a derrick. The drill string is thus acoustically free at its top 
end, presenting a low impedance to the acoustical output of the 
oscillator. The rotors are driven by a high power motor to effect high 
vibratory displacement of the top end of the drill string(typically of the 
order of two inches). The frequency of the oscillator is adjusted to 
provide a resonant standing wave vibration of the drill string with a node 
of the velocity wave(antinode of the force wave) at the point where the 
string is stuck and a velocity antinode at the free top end of the string. 
The elastic suspension system is designed so that it is as linear in 
response as possible so as to minimize the generation of harmonics or 
overtones in the vibration system and the resultant energy losses 
engendered by such harmonics. The drill string thus operates in the nature 
of a lever, the "handle" of which moves freely with high displacement to 
provide amplified force by virtue of the lever action down hole at the 
portion of the string stuck in the bore hole. 
In one embodiment of the invention, means which may comprise a hydraulic 
servo are employed to keep the oscillator drive motors fixed in position 
and thus isolated from the high displacement vibration of the oscillator 
rotors. 
In another embodiment of the invention, a second column made of twisted 
steel strand cable is inserted within the drill string, the strands of the 
cable being adapted to slip on each other when bending of the cable 
occurs. This inner cable by virtue of the friction between the coacting 
strands thereof operates to damp lateral vibrations which may be induced 
due to misalignment of the string or other unbalances which may be 
present. 
In another aspect of the invention, a portion of the longitudinal energy 
generated in the drill string is converted to lateral vibratory energy 
which can be highly useful in freeing the string. This end result is 
achieved by releasing the pull of the derrick on the string allowing the 
weight of the string and the equipment attached thereto to provide a 
compressional force on the stuck portions of the string. This tends to 
engender a lateral vibrational mode to effect a torsional force on the 
string at its point of lodgement which is highly effective in freeing the 
string. 
It is therefore an object of the invention to facilitate the freeing of 
drill strings and the like which may become stuck in a bore hole; 
It is a further object of this invention to provide an acoustic lever for 
multiplying sonic energy generated on the free end of a drill string to a 
high force level at a downhole point on the string which is stuck in a 
borehole; 
Other objects of the invention will become apparent from the following 
description taken in connection with the accompanying drawings.

DETAILED DESCRIPTION OF THE INVENTION 
Referring now to FIG. 1, the system of the invention is schematically 
illustrated. Pull beam 56 is suspended from a derrick through link 40. 
Vibrating beam 42 is suspended from pull beam 56 by means of "spring" 
members 30 which in the preferred embodiment are formed by a plurality of 
elastomeric bands which provide a "soft" connection between the two beams. 
These bands may be fabricated of a material such as woven rubber strands 
covered by cross sectional braided yarn. Typically such bands have a 
diameter of 13/16" and a ring diameter of 20". Bands of this type are 
termed "shock rings" and are commercially available from Thomas Taylor and 
Sons, Hudson, Mass. The bands should have a highly linear spring rate with 
a large vibration stroke which is typically two inches or more in the 
preferred embodiment. A pair of orbiting mass oscillators 10 having 
semi-circular rotors 48 are mounted on vibrating beam 42 and are rotatably 
driven in opposite directions by means of associated motors 20 through 
drive belts 16. Motors 20 are typically high power hydraulic motors. The 
rotors are phased in the preferred embodiment to generate longitudinal 
vibrational energy, i.e. along the longitudinal axis of vibrating stem 44, 
transverse vibrational energy being cancelled out. To accomplish this end 
result, the rotors are phased as shown in the drawing so that when they 
are ninety degrees from where shown, one rotor will be generating an 
outward force while the other rotor will be generating an equal and 
opposite inward force. 
Motors 20 are kept centered during the high stroke vibration of vibrating 
beam 42 by means of a hydraulic servo system which employs a pair of 
centering cylinders 80, the drive shafts 86 of which are connected to 
support or pull beam 56, the opposite ends of the cylinders being 
connected to the casing of motors 20. Link arm 18 is pivotally supported 
on the casing of motor 20 on one end and on vibrating beam 42 on the other 
end. With pivotal motion of the link arm, piston rod 82 is driven in and 
out of valve body 75 to cause the valve body to deliver hydraulic pressure 
fluid in response to such motion to either lines 88 or 90 to effect the 
desired centering action. The details of the operation of this hydraulic 
servo will be described in detail further on in the specification. If so 
desired, a simple pair of cylinders which operate without a servo system 
may be employed, these cylinders having their shafts coated with a 
material such as a suitable polymer material to provide viscous damping 
action to the vibratory energy which damping tends to keep the motors 
centered. 
Stem 44 is suspended from vibrating beam 42 by virtue of flanged end 44A 
which is supported on mating flange 42A. A swivel 57 is provided between 
stem 44 and the drill string 41, the string fitting through a conventional 
rotary table 45 mounted on derrick floor 47. 
In operation, rotors 48 are driven in opposite directions by means of 
motors 20 to generate vibratory energy in drill string 41 at a sonic 
frequency which is typically about 5-10 Hertz. This frequency is adjusted 
to produce standing wave vibration of the drill string with a longitudinal 
mode of vibration; (as already noted, the lateral vibrations are cancelled 
out). The frequency is adjusted by changing the speed of the motors and 
thus the rotors so that the standing waves 64 which are generated have an 
antinode of vibration at the bottom of stem 44 and a node of vibration at 
the point 65 at which the string is stuck in the bore hole. Thus, the 
displacement of the vibration is at its highest point in the vicinity of 
the oscillator and its lowest point at the stuck point, this providing the 
desired force lever action, with optimum force being developed at the 
stuck point. 
An insert column 60 which is attached at its top end to pull beam 56 and at 
its bottom end to drill stem 44 may be employed to damp out lateral 
parasitic vibrations which may be generated due to misalignments in the 
drill string or unbalances in the mechanism. This column may be made of 
twisted steel strand cable, the strands of the cable slipping on each 
other when bending of the cable occurs. Thus with bending of the main 
drill string, mutual bending vibration of cable 60 occurs. This mutual 
bending vibration causes slippage between the strands which are also in 
the contact regions between the cable and the drill string with consequent 
mutual slipping like a leaf spring. The inter layer slipping involves 
friction which tends to damp the unwanted lateral vibrational energy. This 
frictional damping can be enhanced by applying non-hardening adhesive or 
mastic to the surfaces of cable 60 which contact the inner surfaces of the 
stem 44. The intimacy of this contact can be enhanced by bending cable 60 
at several points along its length. 
Referring now to FIGS. 2-4, a preferred embodiment of the invention is 
illustrated. Like numerals have been used to identify corresponding 
components schematically illustrated in FIG. 1. 
Pull beam 56 is supported from a derrick(not shown) through a pair of 
derrick links 40. A pair of vibrating beams 42 are resiliently supported 
from pull beam 56 by means of a plurality of highly elastic stretch bands 
30 which may be fabricated of a material such as woven rubber strands and 
typically have a ring diameter of 20" and a cross sectional diameter of 
3/16". Bands 30 are capable of considerable extension and in operation 
typically extend about thirty inches when a pull of four hundred thousand 
pounds is applied through derrick links 40. In an operative embodiment, 
about 360 bands are employed, such bands being supported on slats 59a and 
59b which are carried in slots 58a and 58b formed in the pull and 
vibrating beams respectively. Bands 30 provide a "soft" connection between 
beams 42 and 56 effectively vibrationally isolating the pull beam 56 from 
the vibrating beams. 
A pair of hydraulic motors 20 which typically, for example, may be series 
F-11 motors manufactured by Volvo Hydraulics, Sweden are each employed to 
rotatably drive one of rotors 48 of paired oscillators 10. The motors are 
coupled to the rotors through gear trains 100 via drive shafts 101 and 
U-joints 102. As described in connection with FIG. 1 the rotors are driven 
in opposite directions and phased to generate vibratory energy along the 
longitudinal axis of shaft 44 with lateral vibrations effectively being 
cancelled out. The motors are supported on support cage 105. Support cage 
in turn is supported from pull beam 56 by means of springs 22 and viscous 
damped cylinders 80, these cylinders having a loosely fitted shaft 80a 
supported in cylinder housing 80b, as shown in FIG. 2A. The cylinder 
housing is filled with a viscous damping fluid such as a polymer butene. 
The support cage 105 is thus vibrationally isolated from the vibrational 
energy generated by the oscillator and coupled to the inner drive shafts 
of the motor. In this manner, the motor is kept "floating" (with the help 
of U-joints 102) so that it does not follow the high displacement 
vibration of the oscillator. As described in connection with FIG. 1 and as 
further to be described in connection with FIG. 5, a hydraulic servo 
system may also be employed to achieve this end result. 
In operation, the rotors 48 of oscillators 10 are driven in opposite 
directions to produce longitudinal resonant vibration of stem 44 and the 
drill string to which stem 44 is connected, as described in connection 
with FIG. 1. The speed of rotation of the drive motors 20 is adjusted to 
produce a resonant mode of vibration which provides an antinode of the 
standing wave at the oscillator housing and a node of such standing wave 
at the point at which the drill string is stuck in the bore hole. This 
results in a lever action with the high displacement at the oscillator 
resulting in a high force at the point of sticking. The highly elastic 
support bands 30 enable a linear vibratory displacement of the vibrating 
beam 42 on which the oscillators are mounted which typically is 1-2 
inches. Such linear displacement, which is enabled by the elastic support 
bands, avoids the generation of undesirable harmonics in the vibration 
system with its resultant wasted energy and affords high "Q" efficient 
operation of the system. 
In practicing the method of the invention, a portion of the longitudinal 
vibration cycle can be converted into lateral vibrations to improve the 
action in freeing the drill string. This end result can be accomplished by 
making a portion of the drill string buckle somewhat This can be achieved 
by locating a stress or force antinode of the standing wave in a region of 
the drill string which is bent slightly due to hole deviation. The 
longitudinal push-pull cyclic force tends to enhance the bending as the 
result of a toggle effect which tends to buckle the pipe in the nature of 
an over-loaded compression column. This end result can be monitored by 
selecting an overtone vibration frequency at which a moderate amount of 
lateral vibration travels up the string which can be observed as lateral 
vibration at the oscillator. If the string is differentially stuck such as 
by the hydrostatic pressure of the mud column holding the string laterally 
against the bore hole wall, the superposition of lateral vibrational 
energy can shake the pipe away from the wall and allow the mud liquid to 
flow therebetween and free up the string. 
Another technique which may be employed is to reduce the upward pull of the 
derrick so that the drill string starts to buckle sidewise under the 
compressional force thus induced, thus developing lateral vibrational 
energy down hole from the longitudinal energy. 
A further technique for engendering lateral vibration down hole is to 
introduce torquing force to the drill string while it is being 
longitudinally vibrated. This produces a diagonal vibrational component 
with this component combining with the longitudinal force to provide a net 
force vector which is tilted or slopes relative to the longitudinal axis 
of the string. The torquing force is best provided to the drill string by 
utilizing the conventional rotary drilling system available on the rig 
such as a standard rotary table which turns a Kelly bar. This avoids the 
need for turning the entire mechanism. 
As explained in connection with FIG. 1, a twisted wire cable 60 may be 
installed within the drill string to damp out parasitic lateral vibrations 
resuling from misalignment of the system. 
Referring now to FIG. 5, a hydraulic servo system which may be used to keep 
the motors in place with the vibrational motion of the oscillators is 
shown. This servo system is schematicallly illustrated in FIG. 1. 
Hydraulic positioning valve 75 has a piston rod 82 extending therefrom 
which is pivotally connected to link arm 18 which in turn is connected 
between the oscillator and motor housings(see FIG. 1). Hydraulic cylinder 
80 is pivotally attached at its botttom end to the housing of motor 20 
while cylinder piston 86 is pivotally attached to pull bar 56. Attached to 
piston rod 82 is valve assembly 75a of hydraulic positioning valve 75. 
Fluid line 90 is connected to one end of cylinder 80 while line 88 is 
connected to the opposite end thereof. Hydraulic fluid is pumped into 
positioning valve 75 from a hydraulic motor(not shown) through line 110 
with a return to the hydraulic motor being provided by means of line 111. 
When the drive motor starts to fall below a predetermined "neutral" 
position, the housing of positioning valve 75, which as shown in FIG. 1 is 
supported on the motor housing, moves in the direction indicated by arrow 
115. Valve assembly 75a will resultantly move to the left in the 
positioning valve casing. This will open the fluid path between the valve 
casing and line 88, providing hydraulic pressure to the top of cylinder 
80, thus causing the motor to be driven back to its initial position. 
Conversely should the motor move above its predetermined neutral position, 
the valve casing will move in the direction indicated by arrow 116 so as 
to cause the valving of fluid pressure through line 90 so as to drive the 
motor downwardly. The valve provides rapid response to any changes in 
motor position from neutral such that incipient changes in position are 
sensed and immediately compensated for so as to effectively maintain the 
motor in its predetermined neutral position at all times and thus 
effectively isolated from the vibrational displacement of the oscillator. 
While the invention has been described and illustrated in detail, it is to 
be clearly understood that this is to be taken by way of illustration and 
example only and not by way of limitation, the spirit and scope of the 
invention being limited only by the terms of the following claims.