GB2332690A - Mechanical oscillator and methods for use - Google Patents

Abstract

A mechanical oscillator 24 is used for imparting vibrations to a drill string. The oscillator includes at least one pair of flywheels 28a and 28b, each pair being contra-rotated to provide the oscillations. The apparatus further includes an isolator 22 e.g. a fluid spring, which ensures that the vibrations, caused by the oscillator, do not affect those string elements situated above said oscillator. Applications include downhole operations such as drilling, cementing liners, fishing and freeing stuck tubulars.

Description

apparatus and Methods Relating to Downhole Operations" The present invention relates to apparatus and methods relating to downhole operations, and particularly, but not exclusively, to an apparatus for and methods of fishing and retrieval of components stuck downhole.

For clarification purposes, the term fishing is commonly used in the art to define downhole operations to retrieve any item that becomes accidentally or intentionally lost or stuck down the hole. Such items include downhole tools and lengths of drill string and must be retrieved before normal drilling can resume.

Such stuck items are generally referred to as fish.

One of the principal sources of problems for oil operators in general is where oil field tubulars become stuck in a well bore. Over the past few decades, a large number of new and innovative tools and procedures have been developed to improve the success and efficiency of so-called fishing operations. Such fishing operations can take extensive periods of time before a result is achieved. However, the result is not always guaranteed to be favourable. Fishing operations are very expensive in terms of time spent in actually performing the operations, and also in terms of rig down-time, and loss of previously drilled hole.

According to a first aspect of the present invention there is provided a method of freeing a stuck string from a well bore, the method comprising the steps of connecting a mechanical oscillator to the string; providing a fluid circulation means in the string, the circulation means being in fluid communication with the string; oscillating the mechanical oscillator to oscillate the string, preferably on a longitudinal axis thereof; and passing fluid through the string.

Energy transmitted from the oscillating string (preferably oscillating at a resonant frequency) to the medium in which the string is stuck preferably has the effect of enabling the sticking medium to behave like a fluid.

Typically, the fluid circulating means is located below the mechanical oscillator in the drill string.

Typically, the fluid is circulated through the end of the string and back up the annulus between tubulars in the string and the hole, increasing the fluid pressure below the stuck portion of the tubular etc, whilst the mechanical oscillator oscillates the string. The vibration of the string excites the medium around the stuck portion causing the medium to fluidise allowing the circulation of fluid to break down the medium, thereby freeing the stuck portion of the string.

The fluid typically enters the tubular using a circulating fluid sub, positioned in the string. The sub (or other fluid circulating means) can be located above or below the oscillator. When positioned above the oscillator that component can be provided with a conduit to allow fluid to pass through the oscillator direct from a top drive system.

The string is preferably a drill string.

According to a second aspect of the present invention, there is provided a method of freeing a stuck string from a well bore, the method comprising the steps of connecting a mechanical oscillator to the string; providing a rotational bearing means in the string; applying torque to the string; and oscillating the mechanical oscillator to oscillate the string, preferably on a longitudinal axis thereof.

The torque is preferably maintained while the string is being oscillated, and is preferably applied at or near the top of the string.

The rotational bearing means is typically located below the mechanical oscillator in the string.

Typically also, the rotational bearing means allows the tubular to rotate whilst keeping the drill string immediately above the bearing means stationary. The bearing means may typically comprise a swivel.

Typically, the torque is maintained by tying off a wire connected to the tubular to a static point. This is commonly known as a deadline. Alternatively torque may be generated by a top drive system, and can be applied with or without the circulation of fluid.

Typically, the rotational torque is held whilst the tubular is vibrated. Keeping the rotational torque constant whilst vibrating the tubular means that the tubular is subjected to a constant rotational force while the applied vibrations simultaneously free the stuck portion of tubular, thus assisting the stuck portion to roll away from the medium to (or in) which it is stuck.

According to a third aspect of the present invention there is provided a method of fishing using a wireline in a well bore, the method comprising the steps of connecting a mechanical oscillator to a string; providing a wireline entry means in the string; passing the wireline down the well bore towards the fish; oscillating the mechanical oscillator to oscillate the string, preferably on a longitudinal axis thereof; and obtaining data from, or manipulating, the fish.

The acquisition of data is preferable but not essential. The wireline may be adapted to perforate the fish or to conduct some other remedial action, such as retrieval of the fish from the stuck position. The wireline may be reeled in to recover the data.

Typically, the wireline entry means is located below the mechanical oscillator in the string. Typically also, the wireline entry means is a wireline entry sub.

The wireline typically has a fishing tool attached to the lower end for retrieving the fish. Alternatively, the wireline may have any type of tool attached thereto, depending upon the application. Examples include strain gauges and perforators or other tools for manipulation of the fish.

According to a fourth aspect of the present invention there is provided a method for cementing a string in a well bore, the method comprising the steps of connecting a mechanical oscillator to the string; providing a circulation means in fluid communication with the string; circulating cement in the well bore between the string and the wall of the bore; and oscillating the mechanical oscillator to oscillate the string, preferably on a longitudinal axis thereof.

The circulation means is typically located below the mechanical oscillator in the string. Typically, the cement is circulated in the well bore whilst the mechanical oscillator oscillates the string. The vibration of the string allows the cement to move more freely, also allows it to settle into place, and reduces the likelihood of the packing of solids which are fluidised by the oscillating tubular.

In addition, the vibration provides an even distribution of cement leading to higher integral compressive strength and improved zonal isolation.

The cement typically enters the tubular using a circulating fluid sub, positioned in the string.

According to a fifth aspect of the present invention there is provided a method for reducing friction in a string being moved in a well bore, the method comprising the steps of connecting a mechanical oscillator to the string; and oscillating the mechanical oscillator to oscillate the string, preferably on a longitudinal axis thereof, whilst the string is moved in the hole.

Typically, the string may have a tool attached to it, such as a drill bit, and may be a drill string.

Alternatively, the method may be used when inserting a liner into a pre-bored well or the like.

Friction-reducing means may be included in the string to enhance the oscillations.

According to a sixth aspect of the present invention there is provided a method for drilling a borehole, preferably for recovery of hydrocarbons, the method comprising the steps of connecting a mechanical oscillator to a drill string; and oscillating the mechanical oscillator to oscillate the drill string, preferably on a longitudinal axis thereof, during rotation of the drill bit.

Typically, the drill string is oscillated simultaneously with the rotation of the drill string (from the surface or with a downhole motor). This provides an impact and shear drilling at the drill face, as the oscillations give a larger impact due to the longitudinal movement of the drill string.

According to a seventh aspect of the present invention there is provided an isolating device for use with an oscillator for a drill string, the isolating device comprising a housing, a reciprocating piston, and a fluid spring for isolating longitudinal movement of the piston from the housing.

The fluid spring typically comprises a compressible portion and a non-compressible portion.

In a preferred embodiment, the non-compressible portion is typically located in a chamber. The chamber is typically located below the piston. The chamber is typically filled with a fluid. The fluid is typically a hydraulic/oil or water/oil emulsion, and preferably a water/oil emulsion. Water/oil or hydraulic/oil emulsions typically comprise ratios of water to oil.

Ratios in the range of 5% to 35% of oil in water may be used. The ratio is preferably in the range of 25% to 35% oil in water, and most preferably, the ratio is 70% water and 30% diesel, although variations of this ratio may also be used. The formulation of the hydraulic/oil or water/oil emulsion produces minimum compressibility characteristics, which reduces temperature variations during the adiabatic pressure cycling. In addition, the formulation ensures that load changes are communicated directly to a section of the compressible portion with minimum hysteresis lag.

The compressible portion typically comprises two sections. The first section typically comprises a bladder which contains a first compressible fluid. In a preferred embodiment, the bladder is contained within the chamber of the non-compressible portion. The second section typically contains a second compressible fluid. The first and second compressible fluids may be the same fluid.

The first compressible fluid is typically a gas which is maintained at a substantially constant and predefined pressure. The fluid may be supplied from a group manifold and accumulator system. The manifold and accumulator system typically maintains the fluid at the predefined pressure.

The second section of the compressible portion is typically located above the piston. Typically, the second compressible fluid is a gas, which is preferably air. The second section is typically vented to the atmosphere by a venting means. The venting means typically comprises a free flow valve. This avoids pressure and temperature biasing during operation of the isolation means.

The first and/or second compressible fluids may be another gas, oil, water or the like. In these cases, the first and second sections of the compressible portion are typically in fluid communication with an appropriate fluid supply.

The housing is typically a tubular member. The tubular member typically comprises a high strength tube.

Preferably, an inner surface of the tube is plated with a wear-resistant metal, the metal most preferably being chrome. This gives a smooth finish to the interior surface for improved sealing characteristics, and also provides a friction reducing surface.

According to an eighth aspect of the present invention, there is provided a mechanical oscillator comprising means for attachment to a drill string, a conduit for circulation of fluids through the string and an oscillation means.

Typically, the mechanical oscillator is suspended from a derrick structure, positioned over the well bore.

The mechanical oscillator is typically isolated from the derrick structure by an isolation means.

Preferably, the isolation means comprises the isolating device according to the seventh aspect of the present invention. Alternatively, the isolation means comprises at least two helical springs, mounted on the hook or travelling arrangement of a derrick.

Typically, the means for attachment to a drill string comprise male and female threaded portions. As conventional in the art, the male portion typically comprises a pin connection and the female portion typically comprises a box connection. The oscillator is preferably coupled directly to a conventional top drive. Alternatively, the oscillator may be coupled to a kelly.

The conduit for circulation of fluids typically comprises a conduit through the mechanical oscillator for the transfer of fluids from above the oscillator to below the oscillator. The conduit is typically in fluid communication with the drill string through the means for attachment to a drill string.

The oscillating means typically comprises at least one pair of flywheels, preferably two pairs. The flywheels typically include offset weights. Each pair of flywheels are preferably synchronised to rotate in opposite directions with respect to one another.

Preferably, the pairs of flywheels are rotated by a respective double-shafted motor. Preferably, each motor is driven by a single power means. This ensures that the flywheels operate in synchronisation. Where size limitations exist, the double-shafted motors may be replaced with four single ended motors.

The flywheels are typically connected to the power means by respective shafts, each shaft being provided with a driver gear which interengages with the flywheel.

The flywheels are typically dynamically balanced to match within 0.5%.

The mechanical oscillator typically includes at least two, and preferably four, isolating devices according to the seventh aspect of the present invention. The isolating device is typically connected at a first end to a static yoke and at a second end to a dynamic yoke.

Typically, the oscillating means is provided on the dynamic yoke. The dynamic yoke is typically connected to the static yoke by a bumper sub.

Typically, the string is tensioned during the time the mechanical oscillator is activated. This keeps the string taught and allows the vibrations from the oscillator to propagate down the string easily and efficiently. Alternatively, the string may be held in a state of compression or neutral tension.

In one particular embodiment of the invention, where the depth of the bore is approximately 5490 metres (18000 feet) and the drill strings use drill pipe having an outside diameter in the order of either 163mm (6 inches), 140mm (5 inches) or 127mm (5 inches), the static workload is typically 750 kLb, the dynamic load capability is typically around 500-600 lbs, the frequency range is typically 5 to 45 Hz and the stroke of the bumper sub is typically 203mm (8 inches).

In this case, the input power requirements would typically be 130 kW to overcome the friction acting on a workstring of 163 mm outside diameter drill pipe at a depth of 4000 metres in a "standard" wellbore.

The mechanical oscillator typically generates a longitudinal excitation of the workstring and substantially obviates or mitigates all lateral forces.

The frequency of the mechanical oscillator is typically controllable so that the oscillator may be tuned to a particular resonant frequency.

The mechanical oscillator typically generates sufficient power to overcome initial friction effects and internal losses at least until resonance is achieved. The oscillator preferably generates sufficient power to perform work on the stuck section.

The mechanical oscillator (and preferably the drill string) is typically isolated from the top drive, kelly or the like (by the isolation means for example), to avoid damaging the top drive etc by the vibrations or oscillations caused.

The mechanical oscillator typically includes means for allowing pressure, torque and the like to be transmitted to the drill string, which may include the bumper sub. This is advantageous in most stuck pipe situations.

The mechanical oscillator is typically provided with means for monitoring at least one system parameter.

The monitoring means typically includes means for measuring pressure and flow rate. The pressure and flow rate is typically generated from a hydraulic skid which is the base for a hydraulic power pack.

In addition, the monitoring means typically includes means for measuring the acceleration of the tool.

Typically, the acceleration monitoring means comprises at least one, and preferably two, accelerometers. The accelerometers are typically of the tri-axial type.

Tri-axial accelerometers typically measure the acceleration of the oscillator on the three conventional axes and in addition, the acceleration of the drill string.

The present invention will now be described, by way of example only, with reference to the accompanying drawings in which: Fig. 1 is a schematic view of a derrick structure above a well bore which is unlined; Fig. 1b is a schematic representation of a circulating sub for use with certain embodiments of the present invention; Fig. lc is a schematic representation of a swivel for use with certain embodiments of the present invention; Fig. 1d is a schematic representation of a wireline entry sub for use with certain embodiments of the present invention; Fig. 2 is a schematic view of a well bore with a casing/liner therein; Fig. 3 is a perspective view of a preferred oscillator for use with the invention; Fig. 4 is a schematic side view of another embodiment of an oscillator; Fig. 5 is a plan view of a top yoke of the Fig 4 oscillator; Fig. 6 is a front elevation of an oscillator according to an eighth aspect of the present invention; Fig. 7 is a side elevation of the oscillator of Fig. 6; Fig. 8 is a plan view of the oscillator of Figs 6 and 7; Fig. 9 is a schematic sectional view of an isolating means according to a seventh aspect of the present invention; Fig. 10 is schematic view showing the operating cycle of the isolating means of Fig. 9; Fig. 11 is a schematic diagram showing fluid line connections to the oscillator of Figs 6 to 8; and Fig. 12 is a schematic view of part of the oscillator of Figs 6 to 8 showing the load characteristics.

Resonance is generally defined as the state of a system during vibration at one or more of a family of characteristic frequencies. In resonant states, the rate of decay of energy is lower than at non-resonant frequencies as no energy is lost to the internal reactance of the system. Resonance is observed in many physical systems, such as stringed musical instruments and electronic resistor/capacitor networks for example.

One benefit of using resonance in downhole applications is that it allows for exploitation of the efficient transfer of acoustic energy in the resonant state, as only frictional losses need be overcome to transmit useful energy.

In some embodiments of the invention, the vibration is axial acting on a free-fixed system. Free-fixed systems are defined as having one end constrained so that it cannot move and the other end free to move. In particular embodiments, the top of the string is free to oscillate whereas the lower end is constrained at the stuck point. Free-fixed systems resonate at each odd harmonic due to this constraint, similar to the effect when standing sound waves are generated in a closed end vessel.

Referring firstly to Fig. 1 there is shown a derrick structure which has suspended therefrom a hook/block 12. The derrick structure 10 is located above a bore hole 14 in which a drill string 16 is positioned. At the bottom of the drill string is a drill bit 18 which may optionally have a stabiliser 20 above it.

Suspended from the hook 12 of the derrick 10 is a pair of isolating springs 22. The springs 22 are large, helical springs, having 2.5in diameter wire with approximately 80001b/in load rate. Below the springs 22 is a mechanical oscillator 24 (Fig. 3).

The oscillator 24 consists of a pair of hydraulic motors 26 (only one motor is shown in Fig. 1); one motor (not shown) at the rear and one motor 26 at the front. Each hydraulic motor 26 has a flywheel 28 suspended from it, the flywheels 28 in the front and rear pairs being offset and synchronised with respect to each other.

The hydraulic motors 26 may be powered by a hydraulic power pack 30, for example. The pair of motors 26 is driven in synchronised rotation. However, in any given pair, the flywheels 28 will be rotating in opposite directions. For example, flywheel 28a may be rotating in a clockwise direction as shown by arrow 32a, whereas flywheel 28b will be rotating in an anti-clockwise direction as shown by arrow 32b.

The rotational motion of the flywheels 28 induces an axial vibration on the drill string 16 below. If the frequency of the sinusoid can be tuned to the resonant frequency of the system, the energy developed at the oscillator 24 will be more efficiently transmitted to drill string 16.

Located in the drill string 16 below the oscillator 24 is a swivel 34. The swivel 34 is a series of bearings which allows the drill string 16 below the swivel 34 to rotate, whilst allowing the drill string 16 above it to remain stationary.

Having described the basic apparatus which is employed in the present invention, the various aspects will now be described. Note that the apparatus as described above will be substantially the same in all of the applications, but drill strings, work strings or any other collection of tubulars for any purpose can be subjected to the method.

CIRCULATION When retrieving downhole tools and, for example, liners from a borehole 14, it is often found in practice that blockages or pack-offs 36 prevent the retraction of the liner 46 from the hole 14. When a medium such as soil particles which make up the blockage 36 are excited by a vibrational force, the granular material behaves like a fluid. In this state, the soil or the like offers less resistance to the movement of bodies through it.

To aid the clearing of the blockage 36, a circulating sub 38 (Fig. lb) is located on the drill string 16 beneath the swivel 34. Before activating the oscillator 24, the tension in the drill string can be increased by retracting the hook 12. This exerts a tensioning force on the isolating springs 22 and tensions the drill string 16. In this state, the vibrational forces imparted by the oscillator 24 can be transmitted down the string 16 easily and with greater efficiency. It will be appreciated that the string 16 could be compressed rather than tensioned.

Once the tension (or compression) on the string 16 has been increased, the oscillator 24 induces an axial sinusoidal vibration on the drill string 16, whilst attempting to circulate fluid in the bore hole 14. In conventional manner, the fluid enters at the circulating sub 38 and is pumped down the central bore 16b of the drill string 16. The fluid exits the central bore 16b at the drill bit 18 and is forced back towards the surface. As the fluid is forced upwards, it increases the fluid pressure below the blockage 36 and this force, in combination with the vibration of the drill string 16 acting on it, begins to loosen the blockage 36.

The frequency of the oscillations imparted to the drill string 16 may be adjusted to achieve resonance. Two fundamental terms require to be briefly introduced in relation to resonance systems: forced vibration and natural frequency. Forced vibration is the name given to the periodic application of a force to a body which is capable of vibrating, by some external source. The natural frequency is the frequency of the system having free vibration without friction.

Resonance of a system occurs when vibrations of maximum velocity amplitude occur. This happens when a vibrating system is driven by a periodic driving force, which applies a forced vibration, at or near the natural frequency of the system. Under these conditions, minimal mechanical or acoustical impedance is experienced.

To operate efficiently, the system as described above is tuned to the resonant frequency by adjusting the frequency of the oscillator 24 which is applying the forced vibrations. Normally, the frequency of the vibrations is in the order of 15 to 25 Hz. When the frequency of the oscillator 24 is tuned to be at or near the natural frequency of the system, the energy developed by the oscillator 24 is efficiently transmitted to the blockage 36 with minimal losses, any such losses being due to frictional resistance.

To tune the system effectively, the oscillator 24 is driven at a relatively low frequency and the amplitude of the vibrations is monitored. The amplitude is continually monitored as the frequency is slowly increased. At a certain point, the amplitude of the vibrations will reach a maximum and will begin to decrease if the frequency is increased further. At this point of maximum amplitude, the system is said to be resonating and is therefore operating at maximum efficiency.

As will be appreciated, the above method of achieving resonance will be used in all of the following methods, as the system works most efficiently under resonance conditions.

The apparatus is capable of exerting tension of lMlbs on the drill string.

ROTATION The rotation method is used generally where differential sticking has occurred. The term differential sticking relates to the hydraulic sticking of a pipe to the side of the bore hole 14.

The mechanism which is used gradually works the contact area off the wall from the top down. In addition, the dilation of the tubular due to the Poisson effect may also assist in breaking the bond.

The apparatus used in this method is generally the same as that for the previous method, with one exception; the circulating sub 38 is replaced with a swivel 40 (Fig. lc). The swivel 40 is typically a series of bearings which allows the drill string 16 below the swivel 40 to rotate, whilst allowing the drill string 16 above it to remain stationary.

In this particular application, torque is applied to the tubular which has become hydraulically stuck. The tubular is turned a number of times, normally in the order of 5 to 15 times, to create rotational torque on the stuck tubular. The torque is then maintained on the pipe by attaching one end of a static line (not shown) to the tubular and the other end of the line to a static point (not shown).

When the tubular is held under rotational torque, vibrations are applied to the system using the oscillator to generate the axial vibrations. Using the rotational torque in combination with the vibration from the oscillator 24, encourages the pipe to roll-off the wall of the bore hole 14.

WIRELINE The use of a wireline is widespread in many operations.

However, it has particular applications when used for fishing for example, to determine data regarding the stuck point or to perforate the drillstring to assist circulation.

To retrieve a fish, a wireline has a tool such as a strain gauge connected to the lower end and is then spooled down the hole. It is often a difficult task to retrieve fish which are stuck and the use of vibration increases the chances of a successful fishing operation.

The apparatus required is generally the same as the first aspect with the exception that the circulating sub 38 is replaced by a wireline entry sub 42 (Fig. lid). A wireline entry sub 42 is a sub which has a male and female connection at either end to allow the sub 42 to be located within the drill string 16. A side entry port 44 allows the wireline which is stored on a large drum reel (not shown) to be fed down the central bore of the drill string 16.

The wireline is fed down until the fishing tool catches on the fish. At this stage, the oscillator 24 vibrates the drill string to cause axial vibrations thereof.

The wireline is then reeled in and the combination of the pull of the wireline and the vibrational force applied by the oscillator 24 generally increases the chances and speed of retrieval of the fish.

The method allows the entry or extraction of wireline without the need for lengthy expensive rigging and derigging operations.

CEMENTING A further use of the vibrational force produced by the oscillator is to aid in the cementing operation. Large diameter holes must be lined as soon as possible with a steel casing or liner. This prevents the wall of the bore 48 (Fig. 2) from caving in and also prevents fluid from entering the bore 14 or drilling mud permeating out of the bore 14 into the surrounding formations.

Referring to Fig. 2, it is often necessary to cement a liner 46 into the bore 14. The apparatus used is again the same as before, with a circulating sub 38 (Fig. lb) being used as in the first aspect.

During cementing operations, cement is not poured in from above, but rather pushed up from the bottom using a rubber plug as a plunger. A cement slurry is fed into the central bore of the liner 46 and a rubber plug (not shown) is positioned directly above it. Drilling fluid is then pumped down onto the plug, which then acts like a piston or plunger, forcing the cement out of the open-ended liner casing 46 and up the gap 50 between the liner 46 and the outer bore 48.

When the rubber plug reaches the open (lower) end of the liner 46, pumping is stopped and the oscillator 24 is activated. The longitudinal vibrations of the oscillator 24 allow the cement to spread and settle more evenly and, in addition, allows the flow of cement to enter into the more difficult to access areas. This increases the efficiency of the cementation by enhancing the strength of the bond between the casing and the wall of the length. Traditional drilling methods encounter difficulties in drilling at such lengths due to the high degree of skin friction between the drill string and the bore.

Predominantly, this method could be used to reduce skin friction wherever it occurs, such as when inserting liners, drill string and the like.

Any particular apparatus may be used, in addition to that as previously described, depending upon the particular application.

IMPACT DRILLING The rate of drilling depends upon the hardness of the rock face encountered. For very hard rock faces, the speed at which drilling can progress is reduced. With the high running costs involved with the day-to-day operation of a drilling rig, such reduction in progress is unwanted.

To counteract this speed reduction and increase the efficiency of the drill bit when harder rock is encountered, the vibrations from the oscillator 24 may be used to increase the impact force and/or enhance the drilling action which the bit has on the rock face.

The apparatus can be the same as that used in the previous aspects. However, the oscillator 24 would be run at the resonant frequency to give large amplitude vibrations at a constant rate. Such vibrations would act like a hammer drill, causing the drill bit to penetrate deeper and hence speed up the rate at which the drill bit penetrates the harder rock.

With reference to Figs 4 and 5, there is shown one embodiment of an oscillator for use with the invention.

The oscillator has a conduit 60 in communication with the string 16 to allow passage of fluids through the tool, and can be attached directly to a top drive system. Thus the invention also provides a mechanical oscillator for a string in a well bore, the oscillator comprising means for attachment to a string, a port for circulation of fluid through the string, and an oscillation means, the oscillator having a conduit 60 therethrough to allow circulation of fluids direct from the circulation means to the string. This allows the advantage of coaxial supply of fluids though the apparatus, and direct connection of the apparatus to a top drive system.

The oscillator shown in Figs 4 and 5 comprises a top yoke 65 and a bottom yoke 66, interconnected by isolation pods 68 and a bumper sub 69, and having hydraulic (or other) motors 70 to power the rotation of flywheels 75 (or other oscillating devices). The isolation pods 68 are similar to the isolating springs 22 in function, but comprise a shaft 68s housing a piston 68p between which are located a number of elastomeric "donuts" 68d, which absorb vibration of the lower yoke 66 by the flywheels, and prevent the transfer of vibration to the upper yoke 65. The bumper sub 69 accommodates the relative movement of the upper and lower yokes 65,66 and maintains a conduit 69c therethrough through which fluids can flow. The apparatus is attached in use to a conventional top drive system (not shown) through which the fluids required for circulation can be pumped into the apparatus at a position in the string which is isolated from the oscillation.

The top and bottom yokes 65, 66 are provided with sensors which measure various parameters of movement of the yokes 65, 66. Typically, the sensors are accelerometers (not shown) which measure the acceleration of the yokes 65, 66. More specifically, the accelerometer(s) may be of the tri-axial type, which measures the acceleration forces of the yokes 65, 66 on the three conventional axes and in addition, measures the work string acceleration.

The accelerometer(s) provide a measure of the excitation applied to the yokes 65, 66 so that parameters such as frequency and amplitude of the tool may be tuned to an optimum level for different conditions.

Figs 6 to 8 show an alternative mechanical oscillator tool 100. The tool 100 is broadly similar to that shown in Figs 4 and 5 and like reference numerals have been used to designate like parts.

The basis of the operation of the tool 100 is an unbalanced or eccentric flywheel 75 which is rotated to produce a centripetal force on the driving shaft. A single flywheel will produce an approximately equal force in all directions during a single revolution, with any variations being due to gravity. A pure sinusoidal force in a given direction is produced when two flywheels 75a, 75b of the same form and mass are coupled within the plane of rotation and are contrarotated. The plane formed by the axes of two parallel driving shafts controls the direction of the resultant force vector. The resulting moment of the force vector across the tool 100 is balanced by the provision of a second pair of flywheels 75a, 75b on the opposing side (see Fig 8).

The flywheels 75 are dynamically balanced to match within 0.5%. This provides benefits in that pure sinusoids can be generated with a high signal to noise ratio. The cross synchronisation of the flywheels 75 further reduces the signal noise. These features contribute to a smooth running tool which can be effectively tuned to a string harmonic frequency, and more importantly, remain at that point, thus delivering maximum effective energy to the system.

Appendix A shows calculated results from an embodiment of the invention. The results show forces and energy generated by a rotating, out-of-balance mechanism, where two identical, in-phase, but contra-rotated flywheels are used. The data shown in Appendix A allows for the following:a) calculation of the total energy yield of any mechanical oscillator system; b) observations of the work rates required to power any system (assuming accepted levels of efficiency during energy conservation); c) determination of the required size and form of any flywheel system for the purpose of application in downhole methods; and d) determination of the peak forces generated by any eccentric system.

The tool 100 is provided with means to attach the tool 100 to a drill string, the means forming part of a bumper sub 110, circulating slip or the like. Sub 110 is provided with a female box connection 112 and a male pin connection 114. Alternatively, sub 110 can be attached directly to a top drive system, kelly, working single or the like, using a standard rotary shoulder connection (RSC) box, for example. Sub 110 is also provided with a central conduit (not shown) for the passage of drilling fluids from above tool 100 to below tool 100, as in the embodiments shown in Figs 4 and 5.

Both of these features offer the advantage of co-axial supply of drilling fluids through the tool 100 and also direct connection of tool 100 to a top drive, kelly, working single or the like.

Tool 100 is provided with a top or static yoke 65 and a bottom or dynamic yoke 66, which are interconnected by four pods 200 (best shown in Fig 9) which will be described hereafter. The pods 200 are similar to pods 68.

The upper part 116 of bumper sub 110 is secured into the static yoke 65 from which the four pods 200 are suspended. The dynamic yoke 66 is suspended from the lower part 118 of the sub 110 and the pods 200. Bumper subs are conventionally used as tubular motion compensators incorporated in the drill string of a floater to eliminate bit movement caused by the vessel's heave.

Sub 110 is provided with a slip joint. Lower part 118 of the sub 110 may slide within the upper part 116 on internal bearings. The sub 110 accommodates the relative movement of the upper 65 and lower yokes 66.

Mounted on the dynamic yoke 66 are two hydraulic (or any other type) motors 70. The motors 70 are preferably matched, double-shafted hydraulic motors.

As shown in Fig. 8, each motor 70 has a synchronising shaft 80 on either side. Each shaft 80 rotates on a bearing 82 and terminates in a driver gear 84. Each flywheel 75 and driver gear 84 has an outer face which is provided with gear teeth, the gear teeth facilitating interengagement between the driver gear 84 and the flywheel 75.

Thus, when the motors 70 are actuated, shafts 80 are rotated and consequently the flywheels 75 are driven by the interengagement of the gear teeth of the flywheels 75 and driver gears 84.

The interengagement of the driver gears 84 and the flywheels 75 offer two main design benefits. Firstly, the start-up torque requirement is reduced due to the reduction gearing effect. Secondly, no bending forces are transmitted to the drive shafts. This load is taken on the synchronising shafts 80, and transmitted to the dynamic yoke 66 through heavy duty bearing packages 82.

In use, however, the flywheels 75a and 75b best shown in Fig. 6, rotate in opposite directions (ie flywheel 75a rotates counterclockwise as shown by arrow 86, and flywheel 75b rotates clockwise as shown by arrow 88).

The gear teeth on the outer surface of flywheels 75a, 75b also interengage. This ensures that the flywheels 75 rotate (in opposite directions) in synchrony. The synchronising shafts 80 couple opposing flywheels 75 also to ensure synchronised rotation in conjunction with the intermeshed gears.

When the motors 70 are actuated, the shafts 80 rotate each pair of flywheels 75a, 75b in synchrony. Taking the starting position of the flywheels 75a, 75b as shown in Fig. 6, weights 76, mounted eccentrically, are both at the bottom of their cycle. The flywheels 75a, 75b will then begin to rotate in respective opposite directions as shown by arrows 86, 88. The weights 76 will thus meet in the middle and continue up to the top of their cycle. At the top, the weights 76 will cause the dynamic yoke 66 to move upwards. The lower part 118 of bumper sub 110 will slide within the upper part 116 to facilitate this upwards motion.

The weights 76 will then rotate away from each other and will end their revolution at the bottom of their cycle again. At this point the dynamic yoke 66 will move downwards creating a longitudinal oscillation by the upward-downward motion of the dynamic yoke 66.

Thus, if the motors 70 are rotated, the dynamic yoke 66 moves upward and downward causing longitudinal oscillations of the drill string below the tool 100.

The frequency of the oscillations is typically in the range of 5 to 45 Hz, although values outside this may also be used.

To give a better frequency response and avoid the above problems where higher load tools are used, an alternative isolation means which may be used in place of the coil springs 22 has been developed. As shown in Fig. 9, the isolation means 200 is in the form of a pod 200 which has an outer tubular member 210. A piston 212 is mounted within the outer member 210 for reciprocal movement therein.

The inner surface of the tubular member 210 may be honed and chromed to give good sealing characteristics and also to reduce friction between the piston 212 and the tubular member 210.

The piston 212 creates two chambers within the tubular member 210; an upper chamber 214 and a lower chamber 216. The upper chamber 214 is filled with a fluid which is advantageously air, and is open to the atmosphere using a full flow valve 218. This avoids temperature and pressure biasing during the operating cycle.

The lower chamber 216 houses an elastomeric annular bladder 220. The bladder 220 is isolated from the reciprocating piston 212 by an isolation sleeve 222, and contains a fluid which is charged to a preselected pressure. The fluid, such as air, nitrogen or the like, is supplied from a group manifold and accumulator system (Fig. 11), as will be described hereafter. An equalising port 230 is used to allow the pressure within the bladder 220 to be set initially and controlled thereafter. The air in the bladder 220 will be referred to as the gaseous phase.

The remainder of lower chamber 216 contains an hydraulic/oil or water/oil emulsion. The formulation of the emulsion produces minimum compressibility characteristics, due to the smaller compression of water which forms the majority of the emulsion. The oil is used to reduce friction. A conventional emulsion comprises a ratio of water to oil, where the proportion of water is greater than the proportion of oil. A typical ratio for the emulsion is 70% water and 30% diesel, although other ratios may be used. Typical values would be from 5% to 35% oil in water.

The use of a water/oil emulsion is preferred, as this allows raw air to be used as the gaseous phase in the bladder 220. Raw air may be used as the gaseous phase, as the liquid (ie water) is not prone to explode under pressure. Although the two phases are separated by the bladder 220, it would be safer to use Nitrogen if a hydraulic/oil emulsion was used. The emulsion serves to reduce temperature variations during adiabatic pressure cycling and also ensures that load changes are communicated directly to the gas-filled bladder 220 with minimum hysteresis lag.

The design of the pods 200 contribute to improved frequency response. The oscillator is operated at frequencies in the range of 5 to 45 Hz, thus good frequency response is required.

The fluid within the pod 200 preferably has the lowest compressibility possible. It is required to act like a "solid coupling", transferring the load onto the gaseous phase. A water/oil emulsion is a good transmitter of such forces, since the compressibility of water is very low due to hydrogen bonding effects.

This may be less than 10% of the compressibility of some oils. The oil part is present primarily to give lubrication to the surfaces of the moving parts, thus reducing friction.

It follows from low compressibility that the fluid will not absorb any significant energy as the pressure forces passes through. Temperature variations will thus be reduced due to the Coulomb effect, as will hysteresis lag.

There is therefore provided a direct transmission of pressure changes (due to the oscillating load working on the piston 212) to the gaseous phase in the bladder 220. Due to this efficient transfer of pressure changes, the frequency response of the system is improved; that is there is very little delay between the reaction of the gaseous phase (ie to reduce or increase in volume) to changes in pressure.

An upper mount assembly 224 and a lower mount assembly 226 are used to mount the pod 200 to the tool, as will be described. The lower mount assembly 226 includes a centralising guide bushing 228 which centralises the reciprocating piston 212 during operation. To protect the upper mount assembly 224, a piston stop ring 234 is provided as part of the assembly 224 to restrict the upward movement of the piston 212.

To isolate the bladder 220 from the piston, a chevron packing 232 is used. Chevron packings are circular seals which are normally placed around a reciprocating shaft, such as piston 212. The chevron packing 232 allows good sealing capability whilst allowing maximum freedom of movement for the piston 212. In section the packings 232 form a "V" (or chevron), with the two limbs of the V facing the pressure side (ie towards the bladder 220). Thus, the packing 232 is activated by the pressure it is required to contain (that is the expansion of the bladder 220, whilst maintaining a minimum contact area on the piston 212. This gives less resistance to movement of the piston 212.

Referring now to Fig. 10, there is shown a typical cycle of the pod 200 under load conditions. The lower end of the piston 212 is coupled to the dynamic yoke 66.

Fig. 10a shows the pod 200 in the neutral position or at the centre of its stroke, with the pod 210 approximately at the centre of its stroke. The flywheel 75 (and weight 76) is rotating in a clockwise direction as shown by arrow 77.

In Fig. 10b, the flywheel 75 has turned sufficiently to be at its lowest point of travel. At this point, the dynamic yoke 66 approaches its lowest point of travel and the pod 200 has stroked downward in response to the higher apparent hookload. As the flywheel rotates in the direction of arrow 77, the piston 212 moves downward, thereby compressing the emulsion in the chamber 216 and the air in the bladder 220. The air in the bladder 220 is vented off into the accumulator system (described hereafter) via equalising port 230.

In Fig. 10c, the piston 212 is at the top of its cycle, as is the flywheel 75. Chamber 214 has been compressed and the air contained within has been vented outwards through the full flow port 218. The air within bladder 220 has increased in volume via the port 230 and the accumulator system (Fig. 11).

The stroking of the pod 200 is achieved by a reduction in the volume of the bladder 220, but without a significant pressure increase. When any given system is pressurised, its volume will decrease. A feature of the pod 200 is that a change in volume of the bladder 220 does not result in a large increase in pressure as this will be the equivalent of a change in load seen by the hoisting system (ie energy has been passed through instead of being isolated by compliance). The term compliance may be generally defined in this context as a measure of the ability of a mechanical system (ie the oscillator 100) to respond to an applied force, expressed as the reciprocal of the system's stiffness.

In theory, no gaseous system can reduce in volume without increasing pressure, but the increase can be made very small in relation to the total volume.

The design of the pod 200 provides a system which will support the maximum static and oscillating loads while imparting not more than 5% of the oscillating load to the hoisting system (ie the top drive, kelly or the like). The degree of compliance to the oscillating load depends upon the pressure change in the bladder 220 at the extremes of the amplitude cycle. At constant temperat re, this is given by Boyle's law.

Hence, for the parameters defined previously we can calculate the total system volume required (Vt) as follows: Maximum oscillating load transfer = 0.05*60,000 = 3000 lbs Total stroke length = 12 inches (approximately 300mm) Cylinder inside diameter = 12.415 inches (315mm) Shaft outside diameter = 4 inches (102mm) Effective piston area = 108.5 square inches Bladder precharge pressure at 600 kLbs static load = 1382 psi Maximum oscillating load transfer expressed as a pressure = 3000/108.5 (ie maximum allowable pressure increase = 27.64 psi) Swept volume during the full stroke cycle = 12*108.5 = 1302 cubic inches So swept volume for half-cycle (ie static load to plus 60 kLb) = 651 cubic inches Total required system gas volume is expressed by: Vt * Pstatic = (Vt - swept volume) * (Pstatic + Pincrement) Vt * 1382 = (Vt - 651) * (1382 + 27.64) Hence Vt = 19.21 cubic feet of gas reservoir per pod 220.

This is equivalent to 13.6 barrels of available gas for a 4-pod system, including that present in the pods 200.

No assumption has been made for gas flow friction losses, and 2 inch (50 mm) internal bore lines are typically used to ensure these are minimised.

Additionally, the volume of gas within and in close proximity to the pods 200 will be maximised for the same reasons. The sizing of a gas reservoir system 350 (shown in Fig. 11) will be in excess of these requirements to allow for further reduction of transmitted oscillating load. Also, the extra volume will allow greater variation in static load working before changes to overall system pressure is required.

Referring to Fig. 11 there is shown a schematic of the system including a hydraulic power pack 300 and an isolator gas accumulator system 350.

The bladder 220 is supplied with air from the accumulator system 350 through an equalising conduit 352. A first end of conduit 352 is connected to the equalising ports 230. The conduit 352 is preferably at least 51mm (approximately 2 inches) in diameter.

Conduit 352 is connected at a second end to a plurality of gas volume reservoir tanks 354. The reservoir tanks 354 have a back-up facility which may be a compression skid 356 which will supply the air in the event that the reservoir tanks 354 fail.

The power for the hydraulic motors 70 is provided by a hydraulic skid 300. To keep the motors 70 running synchronously, they are both fed from the same power source so that the incoming power is matched as closely as possible. The projected maximum power input to the workstring is 300 kW (403 HP). To ensure that sufficient excess is available to provide for start-up torque requirements, impedance matching losses and efficiency power losses, the hydraulic skid 300 is typically rated to 5000 psi at 500 gallons per minute (gpm) operating flow rate, generating a total of 1460 HHP. The skid 300 is self-contained with an integral diesel-powered prime mover and the output is preferably variable over the design range. The hydraulic motors 70 of the tool 100 will be sized to operate within these design limits and loads.

The system is controllable by a control unit 360.

Feedback in the form of pressure and flow rate data from the hydraulic skid 300 and the gas accumulator system 350 is used to control the system and measure performance.

A pair of intrinsically safe accelerometers, similar to those described in the previous embodiment, are provided. One is mounted on the dynamic yoke 66 and the other on the static yoke 65. The accelerometers provide a measure of the acceleration of the yokes 65, 66 on the three conventional axes and the workstring.

These transducers output an electrical signal, which is an analogue equivalent of the acceleration experienced by the accelerometer. This signal can therefore be integrated once to provide velocity and a second time to produce the displacement value.

The signal from the accelerometer on the static yoke 65 will relate to the magnitude of the transmitted oscillating load, and can be used to adjust the volume of the gas reservoir 350 on line to the pods 200. The accelerometer mounted on the dynamic yoke 66 may indicate the applied force, frequency and amplitude levels of the yoke 66.

Both signals may be displayed during operation on a dual-trace digital sampling oscilloscope. This allows direct display of current frequency and amplitude levels, and so is an indicator of when a resonant frequency has been achieved.

All data may be recorded on multi-track data loggers for post job analysis. A later development may be the integration of the data signal into a computerised tool/workstring monitoring system.

Digital pressure gauges and flow metres as required perform data feedback from the hydraulic skid 300 and the gas reservoir/compressor skid 350. This data may also be recorded for presentation in a post job analysis report.

The yokes 65, 66 may be constructed from 50mm (2 inch) forged, tempered and relieved plate. An upper mandrel of the circulating slip joint 110 carries static load to the hoisting system using an integral collar 111 located in contact with the lower face of the static yoke 65, as best shown in Fig. 12. Transmitted oscillating components are constrained by a keyed locking collar 113 on the upper face.

The pod 200 is required to support static load from the upper face to the lower face and hence the integral collar 119 and keyed collar 123 connected to the static yoke 65 is the inverse of the connections at the circulating slip joint 110. That is, integral collar 113 on the circulating sub 110 in on the lower face of the static yoke 65, whereas the integral collar 119 on the pod 200 is on the lower face of yoke 65. These connections are again inverted in the connection to the dynamic yoke 66, with an integral collar 125 on the upper face of the dynamic yoke 66, and a keyed collar 121 on the lower face.

Fig. 12 is a schematic of the load path during operation of the tool 100. Note that only one pod 200 and the circulating sub 110 are shown as the tool 100 is substantially symmetrical about a longitudinal axis.

The pods 200 are also affixed to the static and dynamic yokes 65, 66 using integral load collars 119, 121 and keyed collars 123, 125.

The weight of the drill string below tool 100 acts in the direction of arrow 130. Arrow 132 shows the transfer of load from the static yoke 65 to the upper mandrel and hoisting system. As previously stated, this load transfer should be in the order of 5% of the oscillating load.

Arrow 134 shows the load transfer from the dynamic yoke 66 to the pod 200. Note that this will be replicated at the other side of the circulating slip joint 110 also. Arrow 136 shows the load transfer from the pod 200 to the static yoke 65.

Note that the stroke of the slip joint 110 will be slightly less than the maximum extension of the pod 200 (when bled down to below applied load using the accumulator system 350). This will ensure that the load path during overpull etc is via the slip joint (rated to 1.4MMLb) and not the internal mechanism of the pod 200.

The system is generally controlled using three parameters : a) hydraulic flow rate b) pod gas pressure and c) gas reservoir volume on line to the system.

Normal operations are performed by adjusting each parameter whilst observing the effect upon the feedback system.

Hydraulic flow rate may be adjusted using a throttle control to the hydraulic prime mover located as part of the hydraulic skid 300. Since the hydraulic motors 70 are not positive displacement motors, this cannot be accurately calibrated to provide a frequency of oscillation. This data will be obtained from the feedback system as described.

Pod gas pressure will be monitored from the head of the gas reservoir bank 350. Bleed off pressure will be performed by manifold manipulation, as will the volume of gas on line to the system from the accumulator bank 350.

In the case where the workstring is stuck at shallow depth, yet requires high energy levels to free it, it may be important to shut the tool 100 down quickly as the amplitude capacity of the pods 200 may be exceeded.

A feature of this design is a system which allows this to be performed quickly and safely. The operator, by actuating a double circuit, four-way valve, simultaneously dumps power fluid via a fixed choke to prevent motor overspeed and directs the return fluid flow to an open variable choke. This choke is then gradually closed to exert a hydraulic braking effect on the motors and thus the rotating flywheels 75.

Thus, the present invention teaches novel applications and methods for a resonant vibratory system. Such methods include friction reduction by fluidising the soil particles in contact with tubulars or freeing hydraulically stuck tubulars.

The present invention also provides a novel mechanical oscillator for generating the resonance and an isolator for use with the oscillator which substantially prevents damage to the top drive, kelly or whatever the mechanical oscillator is connected to.

Modifications and improvements may be made to the foregoing without departing from the scope of the present invention.

APPENDIX A Calculation and Analysis of Force, Energy and Power Effects with Unbalanced Rotating Bodies.

Engineering Basis: Angular Velocity (w) = radians/sec Period (T) = 2PI/w Centripetal Acceleration (a) = w^2.r OR a = V^2.r where v is the constant tangential velocity of the mass.

Rotational Kinetic Energy (Er) = 0,5*1.w^2 Moment Of Inertia (I) = Integral r^2.dm Flywheel Form SEMICIRCULAR+BAR Note: Form is of radius r, offset to semicircle of r from point of rotation.

Flywheel Radius 0.3 m Number of Flywheels in the system= 4 Flywhell Thickness 0.025 m Material Density 7880 kg/m^3 Centre Of Mass 1.0301 * r 0.31 m FW Volume 5.53E-03 m^3 FW Mass 43.61 kg 95.94 Lb Moment of Inertia 4.16 kgm^2 5.90E-03 Lb.in^2

CPS of Flywheel w (rad.s^-1) Centripetal a (m.s^2) Centripetal Force (N) Rot. K.E. (Joules) Centripetal Force (Lbf) Rot R.E. (Ft.Lbf) Output H.P.

(Per Flywheel) (Multiple FW's) (Sum of FW's) (Multiple FW's) 0 0.0000 0.00 0.00 0.00 0.00 0.00 0.00 5 31.4159 305.00 13301.13 8220.90 11960.86 6063.41 44.44 10 62.8319 1220.00 53204.53 32883.59 47843.43 24253.66 177.75 15 94.2478 2745.00 119710.20 73988.09 107647.72 54570.73 399.94 20 125.6637 4880.01 212818.14 131534.38 191373.73 97014.63 711.00 25 157.0796 7625.01 332528.34 205522.46 299021.45 151585.35 1110.93 30 188.4956 10980.01 478840.81 295952.35 430590.89 218282.91 1599.74 35 219.9115 14945.02 651755.54 402824.03 586082.05 297107.30 2177.43 Frequency Of Rotation 20 Hz Centripetal Force Per Flywheel 212818 N Workstring 6.5/8"28.5# S135 DP ID (in)= 5,761 Area (in^2)= 8.406

Angular Position A.P. (Rad) Vertical Force Component Vert.Component Pipe Stress (tube) From Horizontal Multiple FW - (N) Multiple FW (LbF) (psi) 0 0.00 0.000 0.00 0.00 20 0.35 291152.171 65453.63 7786.54 40 0.70 547187.094 123012.58 14633.90 60 1.05 737223.178 165734.41 19716.20 80 1.40 838339.266 188466.21 22420.44 100 1.75 838339.266 188466.21 22420.44 120 2.09 737223.178 165734.41 19716.20 140 2.44 547187.094 123012.58 14633.90 160 2.79 291152.171 65453.63 7786.54 180 3.14 0.000 0.00 0.00 200 3.49 -291152.171 -65453.63 -7786.54 220 3.84 -547187.094 -123012.58 -14633.90 240 4.19 -737223.178 -165734.41 -19716.20 260 4.54 -838339.266 -188466.21 -22420.44 280 4.89 -838339.266 -188466.21 -22420.44 300 5.24 -737223.178 -165734.41 -19716.20 320 5.59 -547187.094 -123012.58 -14633.90 340 5.93 -291152.171 -65453.63 -7786.54 360 6.28 0.000 0.00 0.00 Force and Stress Analysis Lbf and PSI

Angular offset Peak Force (Lbf) vs Frequency

Frequency (Hz)

Claims (74)

1. CLAIMS 1. A mechanical oscillator comprising means for attachment to a drill string, a conduit for circulation of fluids through the string, and an oscillation means.
2. 2. A mechanical oscillator according to claim 1, wherein the oscillating means comprises at least one pair of flywheels.
3. 3. A mechanical oscillator according to claim 2, wherein the flywheels include offset weights.
4. 4. A mechanical oscillator according to either claim 2 or claim 3, wherein each pair of flywheels are synchronised to rotate in opposite directions with respect to one another.
5. 5. A mechanical oscillator according to any one of claims 2 to 4, wherein the flywheels are dynamically balanced to match within 0.5%.
6. 6. A mechanical oscillator according to any one of claims 2 to 5, wherein the pairs of flywheels are rotated by a respective double-shafted motor.
7. 7. A mechanical oscillator according to claim 6, wherein each motor is driven by a single power means.
8. 8. A mechanical oscillator according to claim 7, wherein the flywheels are connected to the power means by respective shafts, each shaft being provided with a driver gear which interengages with the flywheel.
9. 9. A mechanical oscillator according to any preceding claim, wherein the mechanical oscillator includes isolating means, the isolating means being used to isolate vibrations caused by the oscillator from the drill string above the oscillator.
10. 10. A mechanical oscillator according to claim 9, wherein the isolation means comprises at least two isolating devices according to any one of claims 24 to 39.
11. 11. A mechanical oscillator according to claim 9, wherein the isolation means comprises at least two helical springs.
12. 12. A mechanical oscillator according to any one of claims 9 to 11, wherein the isolating means is connected at a first end to a static yoke and at a second end to a dynamic yoke.
13. 13. A mechanical oscillator according to claim 12, wherein the oscillating means is provided on the dynamic yoke.
14. 14. A mechanical oscillator according to either claim 12 or claim 13, wherein the dynamic yoke is coupled to the static yoke by a bumper sub.
15. 15. A mechanical oscillator according to any preceding claim, wherein the means for attachment to a drill string comprises male and female threaded portions.
16. 16. A mechanical oscillator according to any preceding claim, wherein the oscillator is coupled to a top drive system.
17. 17. A mechanical oscillator according to any preceding claim, wherein the conduit for circulation of fluids comprises a conduit through the mechanical oscillator for the transfer of fluids from above the oscillator to below the oscillator.
18. 18. A mechanical oscillator according to any preceding claim, wherein the conduit for circulation of fluids is in fluid communication with the drill string through the means for attachment to a drill string.
19. 19. A mechanical oscillator according to any preceding claim, wherein the oscillator includes an oscillating frequency controller whereby the frequency of the mechanical oscillator is controllable.
20. 20. A mechanical oscillator according to any preceding claim, wherein the mechanical oscillator is provided with means for monitoring at least one system parameter.
21. 21. A mechanical oscillator according to claim 20, wherein the monitoring means includes means for measuring the acceleration of the tool.
22. 22. A mechanical oscillator according to claim 21, wherein the acceleration monitoring means comprises at least one accelerometer.
23. 23. A mechanical oscillator according to claim 22, wherein the accelerometers are tri-axial accelerometers.
24. 24. An isolating device for use with a drill string oscillator, the isolating device comprising a housing, a reciprocating piston, and a fluid spring for isolating longitudinal movement of the piston from the housing.
25. 25. An isolating device according to claim 24, wherein the fluid spring comprises a compressible portion and a non-compressible portion.
26. 26. An isolating device according to claim 25, wherein the non-compressible portion is located in a chamber.
27. 27. An isolating device according to claim 26, wherein the chamber is located below the piston.
28. 28. An isolating device according to either claim 26 or claim 27, wherein the chamber is filled with a fluid.
29. 29. An isolating device according to claim 28, wherein the fluid is a hydraulic/oil or water/oil emulsion.
30. 30. An isolating device according to claim 29, wherein the ratio of water to oil is in the range of 5% to 35% of oil in water.
31. 31. An isolating device according to any one of claims 25 to 30, wherein the compressible portion comprises first and second sections.
32. 32. An isolating device according to claim 31, wherein the first section comprises a bladder which contains a first compressible fluid.
33. 33. An isolating device according to claim 32 when dependent upon any one of claims 26 to 31, wherein the bladder is contained within a chamber of the noncompressible portion.
34. 34. An isolating device according to any one of claims 31 to 33, wherein the second section contains a second compressible fluid.
35. 35. An isolating device according to any one of claims 32 to 34, wherein the first compressible fluid is a gas which is maintained at a substantially constant and predefined pressure.
36. 36. An isolating device according to any one of claims 31 to 35, wherein the second section of the compressible portion is located above the piston.
37. 37. An isolating device according to any one of claims 34 to 36, wherein the second compressible fluid is a gas.
38. 38. An isolating device according to claim 37, wherein said gas is air.
39. 39. An isolating device according to any one of claims 31 to 38, wherein the second section is vented to the atmosphere by a venting means.
40. 40. A method of freeing a stuck string from a well bore, the method comprising the steps of connecting a mechanical oscillator to the string; providing a fluid circulation means in the string, the circulation means being in fluid communication with the string; oscillating the mechanical oscillator to oscillate the string, and passing fluid through the string.
41. 41. A method according to claim 40, wherein energy transmitted from the oscillating string to the medium in which the string is stuck fluidises the sticking medium.
42. 42. A method according to either claim 40 or claim 41, wherein the fluid circulating means is located below the mechanical oscillator in the drill string.
43. 43. A method according to any one of claims 40 to 42, wherein the fluid is circulated through the end of the string and back up an annulus, the annulus being formed between tubulars in the string and/or the hole.
44. 44. A method according to any one of claims 40 to 43, wherein the method includes the additional step of increasing the fluid pressure below the stuck portion of the tubular, whilst the mechanical oscillator oscillates the string.
45. 45. A method according to any one of claims 40 to 44, wherein the fluid circulating means comprises a circulating fluid sub, positioned in the string.
46. 46. A method according to any one of claims 40 to 45, wherein the string is a drill string.
47. 47. A method of freeing a stuck string from a well bore, the method comprising the steps of connecting a mechanical oscillator to the string; providing a rotational bearing means in the string; applying torque to the string; and oscillating the mechanical oscillator to oscillate the string.
48. 48. A method according to claim 47, wherein the torque is maintained while the string is being oscillated.
49. 49. A method according to either claim 47 or claim 48, wherein the bearing means comprises a swivel.
50. 50. A method according to any one of claims 47 to 49, wherein torque is maintained by tying off the tubular to a static point.
51. 51. A method according to any one of claims 47 to 50, wherein torque is generated by a top drive system.
52. 52. A method according to any one of claims 47 to 51, wherein the method comprises the additional steps of providing a fluid circulation means in the string, the circulation means being in fluid communication with the string, and passing fluid through the string.
53. 53. A method of fishing using a wireline in a well bore, the method comprising the steps of connecting a mechanical oscillator to a string; providing a wireline entry means in the string; passing the wireline down the well bore towards the fish; oscillating the mechanical oscillator to oscillate the string; and manipulating the fish.
54. 54. A method according to claim 53, wherein the wireline is adapted to perforate the fish and/or to retrieve the fish from the stuck position.
55. 55. A method according to either claim 53 or claim 54, wherein the wireline entry means is located below the mechanical oscillator in the string.
56. 56. A method according to any one of claims 53 to 55, wherein the wireline entry means is a wireline entry sub.
57. 57. A method for cementing a string in a well bore, the method comprising the steps of connecting a mechanical oscillator to the string; providing a circulation means in fluid communication with the string; circulating cement in the well bore between the string and the wall of the bore; and oscillating the mechanical oscillator to oscillate the string.
58. 58. A method according to claim 57, wherein the circulation means is located below the mechanical oscillator in the string.
59. 59. A method according to either claim 57 or claim 58, wherein the cement is circulated in the well bore whilst the mechanical oscillator oscillates the string.
60. 60. A method according to any one of claims 57 to 59, wherein the circulation means comprises a circulating fluid sub, positioned in the string.
61. 61. A method for reducing friction in a string being moved in a well bore, the method comprising the steps of connecting a mechanical oscillator to the string; and oscillating the mechanical oscillator to oscillate the string, whilst the string is moved in the hole.
62. 62. A method according to claim 61, wherein the method is used when inserting a liner into a pre-bored well or the like.
63. 63. A method according to either claim 61 or claim 62, wherein the string includes friction-reducing means to enhance the oscillations.
64. 64. A method for drilling a borehole, the method comprising the steps of connecting a mechanical oscillator to a drill string; and oscillating the mechanical oscillator to oscillate the drill string, during rotation of the drill bit.
65. 65. A method according to claim 64, wherein the drill string is oscillated simultaneously with rotation of the drill string.
66. 66. A method according to any one of claims 40 to 65, wherein the string is oscillated on a longitudinal axis thereof.
67. 67. A method according to any one of claims 40 to 66, wherein the oscillations are at a resonant frequency of the string.
68. 68. A method according to any one of claims 40 to 67, wherein the string is tensioned during the time the mechanical oscillator is activated.
69. 69. A method according to any one of claims 40 to 68, wherein the string is held in a state of compression or neutral tension during the time the oscillator is activated.
70. 70. A method according to any one of claims 40 to 69, wherein the mechanical oscillator generates a longitudinal excitation of the drill string.
71. 71. A method according to any one of claims 40 to 70, wherein the mechanical oscillator comprises an oscillator according to any one of claims 1 to 23.
72. 72. A mechanical oscillator substantially as hereinbefore described, with reference to Figs 3 to 8, 11 and 12 of the drawings.
73. 73. An isolating device substantially as hereinbefore described, with reference to Figs 9 to 12 of the drawings.
74. 74. A method substantially as hereinbefore described, with reference to the accompanying drawings.