Vibration v1 0

Surface Vibration Monitoring

Version 1.0 March 2001

Lewis Evans, Dave Hawker, Ruben Dario Mejia

Mission Statement To be a worldwide leader in providing drilling and geological monitoring solutions to the oil and gas industry, by utilizing innovative technologies and delivering exceptional customer service.

DATALOG: VIBRATION MONITORING MANUAL, Version 1.0, issued March 2001

Surface Vibration Monitoring, Detection and Analysis

CONTENTS 1 INTRODUCTION ............................................................................................................................. 2 2 VIBRATION TYPES......................................................................................................................... 3 2.1 TORSIONAL VIBRATION ................................................................................................................. 3 2.1.1 Problems ................................................................................................................................ 4 2.1.2 Typical Occurrences .............................................................................................................. 5 2.1.3 Contributing Factors ............................................................................................................. 5 2.1.4 Identification .......................................................................................................................... 6 2.1.5 Remedies ................................................................................................................................ 6 2.2 AXIAL VIBRATION ......................................................................................................................... 8 2.2.1 Problems ................................................................................................................................ 8 2.2.2 Typical Occurrences .............................................................................................................. 9 2.2.3 Contributing Factors ............................................................................................................. 9 2.2.4 Identification ........................................................................................................................ 10 2.2.5 Remedies .............................................................................................................................. 10 2.3 LATERAL VIBRATION................................................................................................................... 12 2.3.1 Problems .............................................................................................................................. 12 2.3.2 Typical Occurrences ............................................................................................................ 13 2.3.3 Contributing Factors ........................................................................................................... 13 2.3.4 Identification ........................................................................................................................ 14 2.3.5 Remedies .............................................................................................................................. 14 3 VIBRATION ANALYSIS – STICK SLIP SOFTWARE ............................................................. 16 3.1 OPERATION PRINCIPLES ............................................................................................................... 16 3.2 QLOG COMMANDS...................................................................................................................... 17 3.3 TORQUE ANALYSIS ...................................................................................................................... 18 3.3.1 Waveform Sampling, Filtering and Analysis........................................................................ 18 3.4 SOFTWARE DISPLAY .................................................................................................................... 20 3.4.1 Real Time Output ................................................................................................................. 20 3.4.2 Parameters and Scales......................................................................................................... 21 3.5 STICK SLIP PREDICTION ............................................................................................................... 23 3.5.1 Analysis and Alarm Setup .................................................................................................... 23 3.6 DATA STORAGE AND OUTPUT ..................................................................................................... 25 4 RESPONSIBILITIES ...................................................................................................................... 27 4.1 CORRECT USE OF THE SOFTWARE................................................................................................ 27 4.2 RESPONSE TO ALARM .................................................................................................................. 28 5 VIBRATION EXAMPLE ............................................................................................................... 29 6 QUALITY ASSURANCE ............................................................................................................... 32 APPENDIX A – TRAFFIC LIGHT WIRING .................................................................................. 37

DATALOG: VIBRATION MONITORING MANUAL, Version 1.0, issued March 2001

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DATALOG: VIBRATION MONITORING MANUAL, Version 1.0, issued March 2001

1 INTRODUCTION The relationship between friction and vibration is a commonly recognised source of inefficiency in all mechanical systems. The nature of this relationship is very dependent on the dynamic environment present, and its control requires an understanding of its cause. In the drilling industry, it is now a widely accepted fact that downhole vibrations can cause premature wear, or even failure of the drill string and bit. Recently, this understanding has been expanded to encompass the relationship between certain discrete types of downhole vibration and specific equipment damage. Vibration detection with mud logging systems such as the Datalog Stick Slip system have revealed that vibrations are always present to some degree, but can be especially bad in difficult drilling environments (e.g. hard formations, steep angle wells) and this is a major cause of bit and drill string failure. This phenomenon is made more critical by the ease in which vibration can be induced and its persistence once initiated. Downhole torque variations, caused by nothing more than changing the RPM or WOB, can trigger torsional oscillations and stick slip behavior with a PDC bit. Likewise, the initial axial vibration phase in the string is produced by nothing more than tagging bottom. A combination of mud logging data acquired at a sufficient frequency, coupled with a high resolution realtime graphical display, is all that is required to provide a detection capability for drillstring vibrations. There are three principal types of drill string vibration: –

Torsional Vibration -

variable pipe rotation, torque vs RPM stick slip in severe cases

Axial Vibration -

up and down the string bit bounce

Lateral Vibration -

off centre rotation, side to side bit whirl BHA whirl

In addition, all three vibration types are able to combine to produce a more severe effect.

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DATALOG: VIBRATION MONITORING MANUAL, Version 1.0, issued March 2001

2 VIBRATION TYPES 2.1 Torsional Vibration Torsional vibration occurs, fundamentally, when the rotation of the drill string is slowed down (or stopped) at the bottom and released when the torque overcomes the friction resisting string rotation. The main effect, as seen at surface, is an opposite variation of torque and RPM readings; in other words, high torque – low RPM, low torque – high RPM. The significance of this relationship is the accompanying alternation of acceleration and deceleration of the BHA and bit, and repeated twisting of the more flexible drillpipe section. The most severe form of this vibration produces “stick slip” behavior of the BHA and bit. This is defined as the bit and BHA coming to a complete halt, until the twisting of the drillpipe section by the surface drive motor produces sufficient force, as torque, to overcome the resistance, as friction, to bit and BHA rotation. The bit then spins free at a vastly accelerated rate to that seen at surface, before slowing back down to the observed speed as the its energy is dissipated. Laboratory tests on PDC bits being rotated backwards show remarkable similarities to the damage suffered by bits after periods of cyclic torque. This suggests that occasional periods of backward rotation may be occurring at the bit and BHA during extreme cases of slip stick, after the bit has halted on bottom. It is then released with such force, as the torque of the motor overcomes the friction of the hole, that after spinning freely in the direction of rotation, it has “wound itself up” and must spin in the opposite direction to dissipate its energy. Some degree of torsional vibration is unavoidable as it begins as soon as the string begins to rotate. During lowering of the assembly to bottom, the drive system (TDS or rotary table) generates a torsional wave that propagates to the bit. Depending on the time for the bit to impact on bottom, the torsional disturbance reflects (often more than once) from the bit which is undergoing a steady acceleration. These reflections cause propagating torque pulses along the string. Once bit contact is made with bottom, the bit RPM decelerates and a much more severe torque pulse travels to the top, where an RPM decrease can be observed.

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DATALOG: VIBRATION MONITORING MANUAL, Version 1.0, issued March 2001

2.1.1 Problems • • • • • •

Damage to, or fatigue failure of bit cutting elements through variable RPM and cutter load. Reduced ROP Connection fatigue and premature failure of drillstring, BHA and downhole tools. Washouts, twist-offs Fishing trips and replacements. Increased Costs!

The problem is seen at its most severe whilst using PDC bits, due to the lack of moving parts (i.e. cones and bearings). It is therefore much easier to generate the frictional forces needed to enter the “stick” phase of the stick slip phenomena. Fluctuating bit speeds (up to three or four times the surface RPM) during torsional vibration events and therefore cutter loads on fixed cutter bits, will cause rapid fatigue failure of the inserts or teeth of the bit. A bit which is run at significantly higher rotary speeds should last up to four times longer than one which has been run at a lower speed and been subjected to vibration. The disappearance of torsional vibration above certain rotary speeds is a documented fact, and this critical speed, with a PDC bit is normally in the area of 150-220 RPM. The elimination of stick slip above these speeds explains the disappearance of torsional vibration and bit impact damage in certain situations where PDC bits are run with motors or turbines, giving very high bit RPM compared to surface RPM. Worn or damaged cutting elements will obviously slow down the ROP as it becomes more difficult for the bit to cut the formation. It is also feasible for uneven bit wear to be a cause of downhole vibration, exacerbating the problem. Drilling with a blunt cutter bit allows the torsional stick slip amplitudes to grow and tends to accelerate axial vibration problems as well. Measurements of vibration using downhole tools indicate that sensitive components such as MWD tools can be damaged, or connections backed off in extreme cases involving occasional bursts of backwards BHA rotation that can occur during stick slip – this has been thought to be the cause of wash outs and twist-offs if the phenomena is sufficiently severe in its amplitude and frequency. The cost of torsional vibration causing problems such as twist-offs can be enormous. This is not only due to the replacement costs of downhole tools, but the unnecessary fishing trips required to recover them, possible damage to casing or the formation during fishing operations, and even the loss of a section of the well if the tool is not recovered. These problems can be effectively eliminated with the correct use of the Datalog Stick Slip programme.

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DATALOG: VIBRATION MONITORING MANUAL, Version 1.0, issued March 2001

2.1.2 Typical Occurrences • • •

Hard drilling regions Hard, abrasive lithologies High angle, deviated wells

Torsional vibration always occurs at some level throughout the drilling of a well. It can be especially prevalent in hard drilling regions however. When the lithology is hard, and/or abrasive, the problem can be exacerbated as the critical friction required to enter the “stick” phase of the cycle is more easily achieved. High angle, deviated wells can also be a problem, as can deep vertical holes. A long section of drill pipe is always going to be more limber than the BHA, due to its low torsional stiffness, and the longer the drillpipe section, the larger the rotational displacements, which initiate the stick slip cycle.

2.1.3 Contributing Factors • • • •

Bit type (PDC) Hole angle BHA weight and stability Mud lubricity

The bit type is a very important factor in predicting torsional vibration problems, and it is sometimes necessary to select the correct bit for the job with this in mind. By far the worst type of bit to induce torsional vibration is a PDC, since it has no moving parts such as cones or bearings. It is therefore easier to generate the critical level of friction required to initiate the “stick” phase. The angle of the hole can also excite vibration - generally the higher the angle, the more pronounced the oscillations and the worse the problem. As we have mentioned before however, stick slip can and does occur frequently in vertical wells. Both the mass of the BHA and the drill pipe size and length have to be considered, since these factors control the fundamental torsional mode of the string. This natural resonance of the drillstring occurs at a frequency close to that which is seen as the oscillations of torque at surface; in other words, the torsional vibrations. The amplitude of these vibrations, natural or induced, is dependant on the nature of the frictional torque downhole, i.e. the formation characteristics, and the properties of the surface drive system , i.e. the power of the top drive or rotary table. Increasing mud lubricity in the well can dramatically lower the friction profile at the bit, making it harder to induce the “stick” phase and easier to induce the “slip”. This modification to the friction profile corresponds to a smaller difference between the static and dynamic friction on bottom, and should be part of the well planning phase.

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DATALOG: VIBRATION MONITORING MANUAL, Version 1.0, issued March 2001

2.1.4 Identification • •

High amplitude torque oscillations coupled with high frequency (0.5-3.0 Hz) RPM fluctuates inversely to torque

The following diagram shows torque oscillating at a frequency of 1.7-1.9 Hz, amplitude 200-300A. Continual RPM adjustments are masking the oscillations.

2.1.5 Remedies There are two instant remedies for severe torsional vibration once it has been initiated: • •

Increase RPM, either at surface or downhole (motor or turbine). Reduce WOB, and subsequently ROP.

For any given setting of initial conditions, such as lithology type, top drive torque and friction forces at the bit, self-sustaining torsional vibration, leading to the stick slip condition, can be excited. The two most important parameters that affect the nature of torsional vibration are the bit RPM and the motion of the top of the drill string, i.e. the WOB and subsequent ROP. There is a critical rotary speed, at the bit, above which self-sustaining torsional vibration becomes minimal. When drilling with PDC bits, that critical rotary speed lies in the range of 150-220 RPM, a difficult to achieve value without the use of downhole motors or turbines.

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DATALOG: VIBRATION MONITORING MANUAL, Version 1.0, issued March 2001

The RPM should be increased incrementally, until the condition has been eradicated. Short bursts of increased RPM, returning afterwards to its original value are less effective than a sustained stepped increase. The increments should also be greater in harder lithologies and where dynamic viscous forces are large e.g. in cold mud. Increasing the top tension of the string has the obvious effect of decreasing the dynamic WOB, and also has the effect of increasing the cutting power of the bit. A WOB increase decreases the cutting power of the bit, which is equivalent to using a blunt cutter bit, further exciting the amplitude of the torsional oscillations. Backing off with the bit weight will therefore reduce the amplitude of these oscillations. It is recommended to attempt to reduce the amplitude and frequency of the torsional stick slip oscillations, first, by increasing the RPM. Reducing WOB usually has the effect of reducing ROP, whereas increasing rotary speed does not have any negative implications for penetration rate. Both methods have been shown, however, to be equally effective, and it may even be necessary to adjust both parameters in a serious situation. Even a slight increase in ROP, once stick slip has been induced, can lead to a significant lengthening of the “stick” phase of the cycle, causing the slip phase to produce very high bit-release RPM, as the stored energy is dissipated. Subsequent stick slip oscillations can rarely be eliminated by jerky motions of the string, such as raising and lowering it over short periods of time (working the pipe). This is therefore not a recommended course of action in this situation. Another method of commencing the drilling operation is to lower the bit to bottom while it is not rotating (rather than having an accelerating angular bit speed). This is also not recommended since, unless extreme care is taken to ensure that the static WOB is below a critical value, torsional vibration or stick slip is almost guaranteed to be induced as the angular bit speed will be sub critical. In effect, a “stick” phase will have been initiated if the static WOB is sufficient to generate enough friction to overcome the initial torque pulse as the rotation begins at surface. As torque then overcomes friction, the “slip” phase is initiated and the bit accelerates to a speed much greater than that indicated by surface.

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DATALOG: VIBRATION MONITORING MANUAL, Version 1.0, issued March 2001

2.2 Axial Vibration Axial vibration appears during the drilling operation in two forms: •

Vertical vibration while the bit is still in contact with the formation.

•

Bit bounce when contact is repeatedly lost as the bit bounces on and off bottom.

Like torsional vibrations, axial vibrations are present during all phases of the drilling operation. The axial vibration phase in the drillstring is produced by the initial impact of the bit with the formation on bottom. The amplitude of these initial vibrations generally decreases to a background level unless it is interrupted by bit bouncing or another such disturbance. The initial bit bounce is triggered by an excessive impact speed when lowering to bottom. Its amplitude can therefore be lowered considerably by simply lowering the bit to bottom more smoothly. It can, however, be triggered by a change in lithology (which could give rise to impulsive forces on the bit), excessive or uneven bit wear, or torsional and lateral vibration exciting the situation.

2.2.1 Problems • • •

Broken or rapidly worn bits, BHA failures Reduced ROP Impact inducing other vibration modes

While drilling the first 100ft of a well in Colombia, boulders in a conglomerate formation caused such a degree of axial vibration, seen as bit bounce, that the crown depth sensor came loose from its mountings. This bit bounce reduced the ROP to 10 ft/hr, when 100+ ft/hr was expected, and after reaching the casing point, the bit, hole opener and drill collars were scrapped. There was, however, no remedy for this situation. It is a fact that torsional stick slip will trigger axial vibration in harder lithologies. Increases in axial vibration are often accompanied by stick slip, sudden increases in changes in WOB and rapid increases in bit RPM. During periods of torsional stick slip, the frequency of axial vibrations increases in the order of 0.2 to 0.8, depending on the hardness of the lithology. The harder the formation, then generally the higher

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DATALOG: VIBRATION MONITORING MANUAL, Version 1.0, issued March 2001

the frequency of axial vibration at the bit. Impulses sent through the drillstring generate correspondingly higher amplitudes of axial vibration energy.

2.2.2 Typical Occurrences • • •

Hard drilling regions Vertical wells More common with tri-cone bits

As mentioned before, some degree of axial vibration is both unavoidable and natural. However, it is likely to be a problem in the same kind of harsh drilling environments as torsional vibration. Generally speaking, when using a PDC bit, increasing the cutting power of the bit (increasing top tension) increases the energy of axial vibrations in the presence of torsional stick slip. This is because increasing the cutting power during the high bit RPM generated during the “slip” phase enhances cutting, reduces the dynamic WOB and allows a larger elastic deformation in the string to be induced. These increases then subside during the “stick” phase, only to be triggered again during the next “slip” phase. In a soft lithology, such as poorly consolidated sandstone, this effect is less pronounced, and strong axial vibrations are more easily triggered by sudden changes in WOB. As the WOB increases, both their amplitude and frequency increase. This enhanced vibration is maintained until the WOB is decreased again, the only method of control in this situation. Axial vibration is more likely to be excited in a vertical well as this makes it easier for the energy to propagate along the drillstring. The damping effect of the wellbore is also less than would be found in a deviated well, where much of the drillstring is in contact with the annulus. It is also more common with tri-cone bits due to their smaller contact area with the formation, and the fact that they possess moving parts, exiting small scale vibrations. However, the selection of a PDC bit could make torsional vibration more likely to occur.

2.2.3 Contributing Factors • • • • •

Lithology hardness Bit type (tri-cone) Hole angle BHA length Fluid viscosity

Lithology is an important factor with axial vibration. As mentioned before, a clastic formation can induce bit bounce resulting in serious damage, both to downhole and surface systems. In hard formations, axial vibration will be triggered automatically if DATALOG: VIBRATION MONITORING MANUAL, Version 1.0, issued March 2001

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DATALOG: VIBRATION MONITORING MANUAL, Version 1.0, issued March 2001

torsional stick slip is induced and in softer formations strong axial vibrations are more easily triggered by changes, especially increases, in the WOB. Bit type is a factor as problematic axial vibration is exacerbated with the use of tricone bits. The use of PDC bits does not eradicate axial vibration, but will help to minimize it. Hole angle is also a factor, as the amount of contact between the wellbore and drillstring determines the amount of vibration damping downhole, transferring the axial energy into the well walls instead of propagating up to the surface. The BHA length is proportional to the top tension, i.e. how easy it is to control the dynamic WOB, and how much can be run in tension below the jars. Whereas both RPM and WOB control torsional vibration, mainly WOB controls the severity of axial excitement. A drilling system is subject to hydrodynamic forces and torque from the drilling fluid, both inside and outside the drill pipe. Under theoretical steady conditions, the mud flow gives a steady torque in the drillstring and hydraulic upthrust at the bit. When axial motion is present however, stresses will be generated between fluid flow and the drillstring. The effect of these stresses combined with the viscoelasticity in the drillstring, and the contact of the string with the wellbore is to act as a dampener for the propagating axial vibrations. This dampening increases dramatically with the viscosity of the drilling mud.

2.2.4 Identification • • •

Erratic WOB, amplitude increasing with the severity of vibration. Obvious surface vibration or shaking. During bit bounce, variations with SPP as the bit loses and regains contact with bottom with high frequency.

The following diagram shows axial vibration combined with torsional stick slip. The bit bounce is causing variations in the WOB (green trace) between 80-105 Klbs. Note, the time scale is 60 seconds between horizontal lines.

2.2.5 Remedies • • •

Lower the bit to bottom slowly and smoothly. Reduce WOB, adjust RPM. Use of PDC bits, shock subs.

To minimize the initial phase of axial vibration, the bit must make contact with the formation as smoothly as possible. Although some axial stress must be induced, no matter how smooth the contact, the lower the amplitude of the initial phase, the faster it’s energy will dissipate.

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DATALOG: VIBRATION MONITORING MANUAL, Version 1.0, issued March 2001

WOB is the first parameter that should be adjusted in the modification of the vibration profile, and this in turn is dependant on the formation type. In a soft formation such as sandstone, increasing the WOB even slightly will increase the amplitude and frequency of axial vibration. Increasing RPM will have the effect of reducing the severity of any torsional vibration, which may be present concurrently with the axial. This would be effective as it is often torsional behavior that induces axial vibration in the first place, notably in harder lithologies. The use of PDC bits will reduce axial excitement when compared with the use of a tricone bit. This is a minor reduction however, and is nowhere near as effective as the use of a shock sub. This should be installed just behind the bit (and motor if present), and acts very successfully in the control of axial vibration, functioning like a suspension system for the drillstring. Being installed at the bottom of the BHA, it is positioned to protect sensitive downhole equipment such as MWD tools.

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DATALOG: VIBRATION MONITORING MANUAL, Version 1.0, issued March 2001

2.3 Lateral Vibration The theoretical “spinning string” of a perfect drilling assembly in a vertical hole, is known as axisymmetric motion, i.e. symmetrical motion around an axis. Lateral vibration is contrary to this and is defined as noncentral rotation of the bit, and/or BHA, causing lateral impacts with the sides of the wellbore. The rotation of the drillstring generates and maintains this motion. The resulting eccentricity causes a dynamic imbalance which generates torsional, axial, and lateral vibration. It can take three forms, each more severe than the previous: •

Bit Whirl describes an off-centre bit rotation, which is especially common with PDC bits.

•

Forward BHA Whirl describes off-centre BHA rotation, with its centre line rotating in the same direction as the drillstring rotation, i.e. clockwise.

•

Backward BHA Whirl occurs where the borehole wall friction causes the centre line rotation to become anti-clockwise, opposite to the rotation of the drillstring.

When trying to visualise the mechanism of lateral vibration, known in drilling circles simply as “whirl”, it is necessary to remember that the ratio of the transverse to longitudinal dimension of a typical drillstring is less than that of a long human hair. A popular analogy is that of the motion of a skipping rope held and turned in a vertical position, but this motion would obviously be greatly exaggerated due to the constriction of the wellbore. Initiation of lateral vibration normally takes higher loads and stresses than would be necessary to induce torsional or axial vibrations. It is thought, however, that lateral vibration is initiated by either torsional or axial vibration, and can eventually be more destructive than both of them, a fact exacerbated by its difficulty to detect.

2.3.1 Problems • • • • • •

Reduced ROP Premature bit wear Uneven string/stabilizer wear BHA washouts and twist offs Borehole enlargement, hole instability, casing damage Lateral impacts inducing other vibrations

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DATALOG: VIBRATION MONITORING MANUAL, Version 1.0, issued March 2001

Most of the problems have been discussed already with the other forms of vibration. The uneven wear of the drill string components is a new concept however, as previously, only damage due to stress had been noted. With lateral vibration, the stress problems of vibration are still evident, but there is also the danger of abrading away the metal of the tools against the wellbore or casing. Being struck at high frequency against the annulus is obviously extremely detrimental to downhole tools as well. The damage to the hole and casing in these circumstances is also significant, as they are often less resistant to impact and wear than the drillstring components. Unwanted wellbore enlargements due to impact damage are a problem, and this can be worsened in interbedded lithologies, especially where hard and soft formations are alternated. Uneven damage in the wellbore can lead to hole instability, even sticking, with the loss of the filter cake leading to other well control problems.

2.3.2 Typical Occurrences • • •

PDC bits Alternating lithologies Vertical wells

The problem with PDC bits again comes from the way they are used to cut, their lack of moving components making it easier for them to be directed from a centre line and begin a non-axisymmetric motion. Alternating lithologies make it necessary to continually adjust parameters to combat torsional and axial vibration, but with lateral vibration being almost impossible to detect discretely from the other types, if they are not significant at the time, whirl of one type or another could be happening downhole. Axisymmetric motion is easier to stimulate in a vertical well, as the walls of the wellbore are exerting equal forces on each side. In a deviated well, gravity would tend the string towards the low side making whirl a virtual impossibility

2.3.3 Contributing Factors • • • •

Bit type BHA stability and centralization Lithology (alternating hardness) Bit profiling

So called “anti-whirl” bits are now in use in the oilfield, and it is becoming increasingly necessary to use these bit types in lateral vibration prone areas. They have been modified for both enhanced stability and direction. It is also worth noting DATALOG: VIBRATION MONITORING MANUAL, Version 1.0, issued March 2001

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DATALOG: VIBRATION MONITORING MANUAL, Version 1.0, issued March 2001

that if bit whirl has been the major cause of impact damage to a PDC bit in a hard formation, the use of a higher rotary speed would worsen the damage sustained. This is contrary to everything we have so far learned, but even so, the use of higher rotary speeds tends to improve PDC performance more often than it worsens it. The extra life that is afforded by extinguishing excessive torsional and axial vibration more than makes up for the increase in abrasive wear to the cutters found with high RPM. A drill collar is far less likely to display lateral motion than heavy weight drill pipe due to its less flexible nature. In a whirl prone area, it is also necessary to use centralizers to keep the drillstring on the centre line, especially in a vertical well.

2.3.4 Identification • • •

More difficult detection, often occurring in association with other vibration types. High erratic torque will be seen, but torque oscillations may not be as regularly cyclic as torsional stick slip. Combination of torsional and axial vibrations may indicate whirl.

Lateral vibration should appear as high frequency hookload variations coupled with torque oscillations. The hookload variations would be derived from axial vibration linearly associated with bending stresses, and the torque cycles would be due to the friction of the drillstring against the wellbore. Vibration periods would be much shorter and less cyclic than in torsional stick slip. If these conditions were being met, whirling would be suggested. In essence, lateral vibration, in whichever form it takes, is more difficult to detect and analyze than either torsional or axial vibration, both of which have a defining parameter (cyclic torque oscillations and erratic hookload readings respectively). It is fair to assume though, that in a vertical well, with little centralization of the BHA, using a PDC bit through alternating lithologies and with torsional and axial vibration present, that whirl is present at the bit and/or BHA.

2.3.5 Remedies • • •

Reduce RPM, change WOB (increase for forward BHA whirl, decrease for backward BHA whirl). Anti-whirl bits Packed assemblies and centralization of the BHA.

In order to eliminate lateral vibration, action would have to be taken to reduce both torsional and axial vibration. Whirling does not seem to occur unless these vibrations are first being generated.

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DATALOG: VIBRATION MONITORING MANUAL, Version 1.0, issued March 2001

2.4 All Vibrations Different vibration mechanisms may be associated with and induce other vibration modes – this can complicate the analysis of vibration through surface monitoring alone. Check with the MWD company whether they are running downhole vibration monitoring tools, and if so, corroborate data. Vibrations in the drillstring can be initiated by shock, so routine drilling practices should include smooth, even changes of drilling parameters, maintaining constant parameters where possible and tagging bottom with a smooth application of WOB.

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DATALOG: VIBRATION MONITORING MANUAL, Version 1.0, issued March 2001

3 VIBRATION ANALYSIS – STICK SLIP SOFTWARE 3.1 Operation Principles Although the occurrence and detection of all vibration types has been discussed, the stick slip program is only designed to monitor, analyse and inform the user of torsional vibration. Only the torque signal is processed to activate the alarm system. The other parameters can be used to identify axial or lateral vibration, but only oscillations of torque can trigger the alarms. The Datalog stick slip programme is QNX based and run through windows from a dedicated network node. It is necessary to run the time database to sample every 10 seconds while using the stick slip programme, which has the following attributes: •

Rapid signal processing in order to detect the high frequency cyclic behaviour associated with torsional and other vibrations.

•

It is based on the analysis, within set limits, of the surface frequency and amplitude of wave oscillations produced by the vibrations. o Cycle frequency largely depends on drillpipe size, length, and BHA weight. o Amplitude is a combination of downhole torque and the surface drive system.

•

Alarm triggering to alert personnel to the event. The alarm will be audible in the unit, and in the form of a traffic light on the drill floor.

•

Response and action, in terms of changing drilling parameters, until the conditions no longer exist for vibration behaviour to continue. Communication and understanding is essential between ourselves, the operator and the contractor.

Four parameters are recorded and displayed by the stick slip programme on a graphical plot. They are: •

Torque, Hookload and Pump Pressure

•

RPM - 1 second acquisition time whether a digital or analogue system.

- 0.1 second acquisition time

All data is displayed and stored, in a data file and on a graphical plot respectively, but only Torque is processed and analysed. DATALOG: VIBRATION MONITORING MANUAL, Version 1.0, issued March 2001

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DATALOG: VIBRATION MONITORING MANUAL, Version 1.0, issued March 2001

3.2 QLOG commands Four administrators need to be started to run the stick slip programme correctly. They are: To run

To slay

stkslpacq & stkslpanl & stkslpdbase & stkslpwin &

dau_kill stkslpac dau_kill stkslpan dau_kill stkslpdb dau_kill stkslpwn

As the programme is QNX-windows based, it is better to run the entire system through the same dedicated windows node. Start up: • • • •

From QNX windows, open a shell, and enter the four administrators from a command line. Each administrator will then be given a Tid number. To see which program administrators are running, it is necessary to use the tsk na command. Immediately after entering stkslpwin &, the graphical plot should automatically open and begin displaying the four parameters.

Shutting down: • • •

Simply quitting the graphical display is usually enough to kill the stkslpwin administrator. Other administrators will need to be closed down manually using the dau_kill command. If the dedicated stick slip node has locked up or frozen, it may be necessary to shut down stkslpwin with the dau_kill command from another node.

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3.3 Torque Analysis The surface torque signal is sampled at 10 times per second. The raw data is displayed real-time, stored and available for historical plots and data dumps. The data is also stored in the QLOG depth and time databases with the resolution they are set at (e.g. 1 ft, 10 seconds). The parameter names and locations should have been preset in the workshop, and are (usually) as follows: -

Maximum Torque Minimum Torque Average Torque Sigma Torque Stick Slip Trend

MAX TRQ MIN TRQ AVE TRQ SIGMA TQ STKSLP

These parameters are from the stick slip program, therefore the readings from the QLOG Torque (channel au in the database) will not correspond to the stick slip torque values. Their cell references vary, but they are normally found in the user defined columns.

3.3.1 Waveform Sampling, Filtering and Analysis Waveform analysis is performed by looking at the maximum and minimum values every sample in order to determine the cyclic frequency and amplitude of oscillation. The cyclic frequency will commonly lie between 0.5 –3.0 Hz (1 cycle every 2 seconds – 3 cycles per second), but the amplitude may vary by over 300 Amps peak to trough.

amplitude

Cyclic frequency (0 5 - 3 Hz) Due to the speed of sampling (10 times per second), spikes in the raw cycle can complicate the waveform analysis when trying to determine the maximum and minimum values of a cycle. Because of this, filtering is required to reduce noise outside of the desired bandwidth. DATALOG: VIBRATION MONITORING MANUAL, Version 1.0, issued March 2001

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A rolling average is performed on the basis of a user-defined frequency, e.g. 2 Hz (the High Hz value in the alarm shell). The number of samples averaged is determined by an empirically derived formula: 2 Hz – 5 samples 1 Hz – 6 samples 0.5 Hz – 9 samples Using 2 Hz as an example, every 1/10th of a second the previous 5 samples are averaged. This has the effect of smoothing the signal. This user-defined frequency is the highest frequency of oscillation that can be measured, however, and still indicate a stick slip condition. If the well was experiencing very high frequency oscillations, this number would have to be changed. If the High Hz is set at 2 Hz (standard) and the torsional oscillations are 2.5 Hz, the stick slip condition will not be indicated. It would become necessary to change the High Hz value to >2.5 Hz. The filtering smoothes the raw torque data, allowing the maximum and minimum points, together with cyclic frequency to be determined. The filtering also results in an offset delay corresponding to the filtering frequency (the High Hz value). The lower the frequency (and greater number of samples) selected for filtering, the more the averaging and delay to the filtered wave. It is therefore important to try and keep the filtering frequency high. Exact wave amplitude is determined by referencing (by the known offset) the filtered maximum and minimum points to the raw data.

maximum

minimum

frequency

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DATALOG: VIBRATION MONITORING MANUAL, Version 1.0, issued March 2001

3.4 Software Display 3.4.1 Real Time Output

Stick Slip Time Base 5.0 minute

150 100 2000 100

Torque (A) 250 Hookload (Klb) 150 SPP (psi) 3000 RPM (/min) 200

StkSlp Trend 0

11:55 14/07/00

100 A=30

11:51 14/07/00

11:52 14/07/00

11:53 14/07/00

11:54 14/07/00

11:55 14/07/00

Frequency = 0.52

Amplitude = 24

The raw data is displayed, at 0.1 second acquisition rate, with a continual update. The length of time displayed on the screen is a user-defined parameter. 2 minutes displayed time of data on screen is normal, but with high frequency oscillations it may be necessary to zoom in on the screen to display 1 minute or even 30 seconds of data.

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The less time on the screen, the less likely a trend is to be observed, and in certain cases, a 30 minute displayed time screen might be necessary, e.g. if vibration had been detected already and was taking a long while to eradicate. If necessary, the user can also scroll back through the last 30 minutes of historical data. All characteristics (i.e. colours as well as display length) of the real-time display are user selectable. By clicking (left hand mouse) on the “Time Base” part of the display screen, the following “Time Chart Setup” window can be modified: -

Time Chart Display length (min): Chart Color Border Color Grid Color Screen Color Select Font

5.0

8 x 14 font

Memory Used (k): 393.5 Accept

Cancel

3.4.2 Parameters and Scales Scales for torque, RPM, hookload and SPP are also user defined. The “Data Chart Setup” window is brought up by clicking (left mouse) on the parameter/scale section of the main display screen. It is necessary to have torque displayed in Amps rather than Kft/lbs, as the minimum amplitude setpoint is asking for a value in Amps. Both frequency and amplitude of torque are reported numerically in real time across the bottom of the screen. If a stick slip condition has been indicated by the alarm system, the corresponding depth will also be displayed across the bottom of the screen.

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DATALOG: VIBRATION MONITORING MANUAL, Version 1.0, issued March 2001

Data Chart Setup Torque

150

250

Hookload

100

150

SPP

2000

3000

RPM

100

200

Chart Color Border Color Grid Color Accept

Cancel

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3.5 Stick Slip Prediction For a stick slip condition to exist for a cycle, three user defined conditions have to be met: Minimum wave amplitude

For example, set at 20% of the average torque

Low Hz cycle frequency

Typically set at between 0.5 and 2.0 Hz

High Hz cycle frequency filter

Typically set at 2.0 - 3.0 Hz

A stick slip condition is said to exist for a given cycle if: •

The amplitude is greater than the minimum amplitude setpoint.

•

The frequency is greater than the low frequency setpoint and less than the high frequency setpoint.

3.5.1 Analysis and Alarm Setup In order for the software to have functionality, the criteria for which to confirm the presence of stick slip behaviour have to be specified. This is done by clicking on the “StkSlp Trend” chart on the main display screen, which brings up the “Stkslp Analysis Setup” window. Here, the waveform frequency and amplitude conditions can be set, along with an alarm timeout function.

Stkslp Analysis Setup Alarm Timeout:

20

High (Hz):

2.0

(cps): 0.5

Low (Hz):

0.5

(cps): 2.0

Minimum Amplitude

Accept

25

Cancel

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DATALOG: VIBRATION MONITORING MANUAL, Version 1.0, issued March 2001

The severity (0-100%) in stick slip prediction depends on the duration of the condition, which is regulated by the Time Out function. The stick slip factor is based on the percentage of the time out setting where a stick slip condition exists: • •

If the time out is set at 30 seconds, the stick slip condition must exist for 30 seconds for the stick slip trend to indicate 100%. If the condition exists about half of the time, the stick slip factor will vary around 50%.

The audible alarm and the traffic light system are based on the stick slip factor, and the user defined set points for amber and red lights. The alarm setpoint is a percentage value of the stick slip factor, and breaching this value will sound the alarm and cause the red warning on the traffic light to be lit. It is again user defined, and should be set with regard to the typical occurrence of stick slip. For example: •

If a stick slip condition is present 30% of the time, the alarm should be set at around 50% which indicates a change in conditions for the worst.

•

If no stick slip is present, the alarm should be set at 10-20% to indicate its first appearance.

The alarm level is set in the “StkSlp Trend Setup” window, which is brought up by clicking on the “Stkslp Trend” title on the main display screen.

StkSlp Trend Setup Stick Slip Factor Alarm Level:

30

Chart Color Border Color Grid Color

Accept

Cancel

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DATALOG: VIBRATION MONITORING MANUAL, Version 1.0, issued March 2001

3.6 Data Storage and Output All recorded data is stored in a daily time based database file found in: 3:/datalog/stkslp/data/dd_mm_yyyy.dat These files are very large (over 17,000 blocks) and are created at full size automatically before midnight every day that the programme is running. Due to the known problem with extents in the QNX system, the file is written in full and filled with zeros. These are then overwritten throughout the next 24 hours. Even when the .dat files are zoo’d up, they are still too large to fit on a 1.44 Mb floppy disc, so it is normal practice to send than via a Qterm Hyperlink to the DOS report computer where they can be periodically backed up to CD. By left clicking on the date/time chart on the main display screen, the “Report Control Setup” window can be accessed. Here, historical plots and ASCII data dumps can be configured. The data files (i.e. which day) to access, are specified by clicking on the start/end day buttons and ticking the appropriate boxes.

Report Control Setup Start Time 14/06/99 11: 50 Start Day Which Day 01 05 2000.dat 02 05 2000.dat 03 05 2000.dat

End Time 14/06/99 12: 00 End Day Graphical Plot: Min Per Page:

5

ASCII Data Dump: Accept

Cancel

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HPGL historical plots can also be created and are stored as: 3:/datalog/stkslp/yymmdd_hhmm.n where hhmm is the start time of the plot, and n is the plot number. The user simply selects the particular data file, the date and time interval to plot and the time scale for the plot (minutes per page).

Data dumps can also be done in an ASCII data format. The user simply selects the data interval required and the file is stored as: 3:/datalog/stkslp/yymmdd_hhmm.d

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4 Responsibilities 4.1 Correct Use of the Software Theoretically, we should confirm the desired control settings for frequency, amplitude, timeout and alarm setpoint with the Company Representative. However, many operators may not be fully aware of the principles involved, so it may require education, from the Datalog operator, to inform the Company Representative of the basic principles of vibration monitoring and the functionality of the programme. If an agreement over initial settings is impractical, use the default values of: High Hz Low Hz Minimum Amplitude Alarm Timeout

2.0 0.2 50 10

Hz Hz Amps Seconds

These settings should indicate the first onset of cyclic torque, and as soon as it has made a first appearance, the operator can customise the parameters to suit the situation. There must be a visual conformation of torque cycling by the operator, noting any changes in string behaviour that may require the settings to be changed. There must be correct use of the realtime scales of the four displayed parameters so that the relationship between torque, RPM, hookload and SPP can clearly be seen and the vibration type identified e.g. stick slip, lateral, bit bounce etc.

*N.B. The system is only designed to work when rotary drilling* The scales only have to be sufficient to be on screen when rotary drilling – if the plot goes off scale during a connection or while backreaming it is irrelevant. It is also worth checking with the MWD crew to find out if downhole vibration monitoring tools are being run. This is becoming increasingly common as unnatural vibration is being accepted as a common cause of downhole tool failure.

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4.2 Response to Alarm •

Our primary responsibility is to notify the Driller and Company Representative to the event and its severity, duration etc. After notifying the relevant personnel, attempt to determine the mode of vibration from our own data and any downhole data that is available. It is also hoped that offset data will become more easily accessible as vibration monitoring becomes a standard mudlogging service.

•

It then becomes the responsibility of the Driller to adjust the parameters as per vibration type. Increasing RPM should always be the first option, but if this is not possible, the dynamic WOB (and therefore the ROP) must be reduced. It is hoped that in the future, Drillers will be issued with a sheet of recommended actions to aid in the reduction or eradication of harmful vibrations.

•

Incremental changes in parameters should be held for several minutes, allowing stored energy to dissipate completely from the string and determine whether the condition has been successfully removed.

•

If requested, the operator should be ready to give HPGL plots to the Company Representative or Drilling Engineer for confirmation of our evaluation.

•

The data files and historical plots must be retained for reference of parameter settings that lead to and remove the stick slip condition.

•

It is also wise to keep a note in the unit diary at what time of day the vibration occurred, hole size, the hole angle, mud type, weight and viscosity, bit type and some BHA details. The information is also far more presentable if the results of the bit run are known, i.e. hours on bottom, footage and dull grading. This will all be necessary for the End of Well Report.

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5 Vibration Example • • •

Hole Diameter Mud system Bit Type

• • •

Stick Slip occurred at 6040ft, after 1100 ft and 40 hours of drilling. The bit was pulled due to torque after a total of 1879ft/60.5hrs Grading 5-5-CT-A-E-I-NO-TQ

- 17 1/2” – Water Based, PHPA, 9.8ppg – Reed EMS11GC

STICK SLIP 1.7 – 1.9 Hz Amplitude 200 – 280A

POSSIBLE BIT BOUNCE

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DATALOG: VIBRATION MONITORING MANUAL, Version 1.0, issued March 2001

STICK SLIP Freq = 0.6 Hz Amp = 200 – 300A

Freq = 0.23 Hz Amp = 40 – 50A

STICK SLIP Freq = 0.6 Hz Amp = 200 – 300A

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DATALOG: VIBRATION MONITORING MANUAL, Version 1.0, issued March 2001

Lower Frequency Vibration Frequency = 0.18 Hz Amplitude = 150 – 180 A

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DATALOG: VIBRATION MONITORING MANUAL, Version 1.0, issued March 2001

6 Quality Assurance Vibration Analysis is a new procedure to many people, not only Datalog engineers, but to many drilling and company personnel. A procedural form has been created to help the Datalog engineer understand the vibrational software and use it effectively. By using the material available in this manual and by following the procedures and guidelines, you can have confidence in the analysis you are performing and that you are using the software correctly and to full advantage. Just as importantly, you will be able to impart this confidence to the drilling representatives. It is crucial that they understand what vibrational analysis is all about: • How torsional vibration occurs • The potential problems, and costs, created when it occurs • How it can be recognised • How our particular system operates and how everyone can utilize it to maximize cost savings for the operation.

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1. OBJECTIVE This document is designed as a guideline or checklist to establish the general procedures for installation and operation of the Datalog Stick Slip program. It does not replace the manual. 2. DEFINITIONS AND ABBREVIATIONS !"

STICK SLIP is Datalog´s real time vibration monitoring program. If set up and used correctly, it can quickly detect and identify undesirable vibrational occurrences that adversely affect the drillstring. This is achieved through high frequency analysis of torque cycles together with user interpretation of RPM, pump pressure and hookload fluctuations. This program is eventually to be implemented in each mudlogging unit, where it is recommended to have a separate node and screen specifically for running this program.

!"

TRAFFIC LIGHT: This is located on the rig floor and is a warning system for the driller, indicating the degree of stick slip present at any given moment. It consists of green, amber and red lights, sequentially activated by the duration of the stick slip condition.

3. UTILIZATION Datalog intends on installing and implementing this service in each unit and impressing on operators its potential importance in helping to avoid wasted operational time and reduce operational and equipment costs. However, the programme is very dependant on the skills of the Datalog operator in identifying and classifying vibrational phenomena. 4. SUPPORT DOCUMENTS !"

In the Datalog office and in each unit, there will be up to date Stick Slip manuals and additional information such as technical papers and references.

5. PRODUCTS RPM, pump pressure and hookload are recorded at high frequency and graphically displayed on screen while the programme is running. The torque signal, as well as being displayed with the other parameters, is analysed by the programme, using the Datalog Stick Slip algorithm. Cycle frequency and amplitude are considered within the analysis. Torque, SPP and hookload are all sampled at 10 times per second and RPM once per second. Data is stored in a daily database format. Any information can be displayed with hard copy graphical or ASCII data dump format.

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6. PROCEDURAL GUIDE

WHO

ACTIVITY

1

Data Engineer Logging Geologist

Check verbally with the technical department that the latest version has been installed or is present from stkslp.zoo. Configure the dedicated Stick Slip node for running the QNX-windows based program.

2

Data Engineer Logging Geologist

If the latest version has not been installed, use the following procedure:

3

1.

Copy the latest version of stkslp.zoo into the temporary directory, 3:/tmp. It is a small file, which can comfortably be sent via email or hand carried on a floppy.

2.

List the contents of the stkslp.zoo file. (zoo l stkslp.zoo) Any files present in the system already with the same name, i.e. older versions, will be overwritten by the new version at the operators command.

3.

Once this is done, extract the contents of the stkslp.zoo file (zoo xS stkslp.zoo).

4.

Shut down the administrators with dau_kill +a, and restart. Run the Stick Slip administrators from the dedicated node after starting QNX Windows. Open a shell and start them from there.

5.

To run the program, we need the following administrators $ stkslpacq & $ stkslpanl & $ stkslpdbase & $ stkslpwin &

Logging Geologist Data Engineer

Always start stkslpwin last, as this command opens the window to the programme. If the other administrators are not running, the programme will not run. 6.

A window should open titled “Stick Slip”. This window, when left-clicked, will display:

Time Chart Setup -Display length: Time interval in which torque, RPM, SPP and hookload are displayed. A two minute resolution is recommended to start with. Data Chart Setup -Parameter scales: The most narrow scale practical, whilst rotary drilling, is necessary for each of the four displayed parameters. For example, if we have an average torque reading of 15,000 ftlb, our range on the scale should be 10,000 to 20,000 ftlb rather than 0 to 30,000 ftlb. This allows a much easier interpretation of data, and is far more presentable graphically. Stkslp Trend Setup -SS factor alarm level: The user defined value used to indicate the presence of stick slip. It is based on the alarm timeout function, which is the amount of time that a stick slip condition is said to DATALOG: VIBRATION MONITORING MANUAL, Version 1.0, issued March 2001

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exist. If the user has decided that a stick slip condition must exist for 60 seconds before the stick slip trend graph reaches 100%, then after 30 consecutive seconds of a stick slip condition, the graph will show a stick slip trend of 50%. The system alarm and rig floor traffic light are based on the SS factor, with user defined points for the amber and red levels of stick slip. The traffic light is not an indicator of severity, it is an indicator of duration – in order for it to pass from green to yellow or red, the predetermined levels of amplitude and frequency will already have been met. Report Control Setup -Start/End Time: Any given time period can be displayed in graphical format, presented as an historical HPGL plots. After setting the time desired, the written file will be displayed in 3:/datalog/stkslp/yymmdd_hhmm.n. These files can then be converted into PCX or PDF format for presentation. -Minutes per page: The time interval desired for each page. If a graphic is requested between the interval 10:00 to 10:30 and 10 minutes per page is selected, three files will be created: 3:/datalog/stkslp/20000205_1000.1...From 10:00 to 10:10 3:/datalog/stkslp/20000205_1000.2...From 10:10 to 10:20 3:/datalog/stkslp/20000205_1000.3...From 10:20 to 10:30 -ASCII dump: This is numeric information in ASCII code, corresponding to the selected time period, e.g. from 10:00 to 10:30. It is written to, and found in: 3:/datalog/stkslp/yymmdd_hhmm.d For the date 27 March 2001 starting at 10am, the file would appear as 3:/datalog/stkslp/010327_1000.d Stkslp Analysis Setup The correct setting of these parameters is vital if a stick slip condition is going to be identified. -Alarm timeout: The duration that a stick slip condition must exist before the unit alarm will sound. -High Hz: The upper limit of torque frequency in Hertz (cycles pre second), normally set at 1.5 to 2.0 hz. The frequency of the torque signal oscillations must be lower than this number. If they are not, change it. -Low Hz: The lower limit of torque frequency in Hertz, set anywhere from 0.2 to 1.0 Hz. Torque must be cycling at a higher rate than this to indicate a stick slip condition. -Minimum amplitude setpoint: The lower limit for the amplitude of the torque oscillations, measured peak to trough of one cycle. The amplitude must be higher than this value to indicate a stick slip condition. Setting this value is dependant on the local conditions. -Amber and Red alarm levels: These are the values for the traffic light indicators. A recommended level to use is 30% for the amber alarm and 80% for the red. If the stick slip trend has been set at 60 seconds for a 100% stick slip condition, this would warn the driller with an amber light if a stick slip condition exists for around 20 DATALOG: VIBRATION MONITORING MANUAL, Version 1.0, issued March 2001

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DATALOG: VIBRATION MONITORING MANUAL, Version 1.0, issued March 2001

secs, and a red light if the condition has been present for 50 secs. The setting of these parameters is entirely dependant on local conditions. 7.

The information recorded daily is stored in:

3:/datalog/stkslp/data/dd_mm_yyyy.dat This is a very big file (17000+ blocks) and must be zoo´ed or zipped and saved on another computer (either another node or sent to the DOS computer via Hyperlink). It is recommended to do this daily to avoid filling the hard disk on node 1.

Logging Geologist Data Engineer

4

When a cyclic torque condition is first observed, our primary responsibility is to inform the driller and company man. We must then try to analyse and understand the situation and endeavour to help find the solution. After the resolution of the situation, check with the operator as to what kind of data presentation is required. It is suggested that with a graphical plot, the 10 minutes before and after the event are also displayed for comparison.

7. CHECK RECORD No. Check 1 2

Check Date February 2001 March 2001

Authority

Author

Check Details

Rubén Mejía Lewis Evans

Creation of first format Update and conversion to English.

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Appendix A – Traffic Light Wiring

The current limiting resistor value is determined by the supply voltage and the LED Bias Current, typically 20mA. Therefore, connected to a 12V supply rail, the resistor value will be: 12 / 20mA = 600 Ohms

R1, R2 and R3 are pull-up resistors, with typical values 4K7 or 10Kohm. The purpose of the pull-up resistor is to prevent fault conditions.

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