1. Modes Of Vibration
MECHANISMS MODE OF VIBRATION FREQUENCY
BIT BOUNCE AXIAL 1 - 10 Hz
BIT CHATTER LATERAL 50 - 350+ Hz
BIT WHIRL LATERAL / TORSIONAL 5 - 100 Hz
BHA WHIRL LATERAL / TORSIONAL 5 - 20 Hz
SLIP STICK TORSIONAL 0 - 5 Hz
MODAL COUPLING AXIAL,LATERAL,TORSIONAL 0 - 20 Hz
Bit Bounce
• Causes
– Bit/Formation Interaction
– Multilobe Pattern of RC Bits
– WOB Variation
• Consequences
– Impact Loading of Cutters, Seals and Bearings
– Drill String Can Flex Causing Lateral Shocks
• Detection
– Axial movement of String at Surface at Shallow Depths
– Shaking of Hoisting Equipment at Shallow Depths
– Use Downhole Shock Sensor mounted for Z-Axis (DDS)
– Broken Teeth / Inserts and Ball Race Indention's on RC Bits
– Broken Nose Cutters on PDC Bits
• Corrective Actions
– Decrease WOB and/or Decrease RPM
– If Persists Stop, Pick up and Restart with Low RPM and WOB
until Bit is bedded in.
• Other Solutions (Post Run)
– Less Aggressive Bit
– Use Shock Sub if run with correct RPM, WOB and ROP
Bit Bounce
Backward Whirl
n Bit or BHA Whirl
– Eccentric rotation
about a point other than
geometric center
– Caused by Bit or BHA
/ Wellbore gearing
– Results in backward
rotation of
circumferential points
(e.g. blade tip)
Backward Whirl
Potential Consequences
• Bit damage
• MWD / BHA Tool component failures
• Localized Tooljoint / Stabilizer Wear
(Flat Spots)
• Connection fatigue cracks leading to washout
and / or twist off
• Excessive torque
• Enlarged / spiral wellbore profile
Bit Damage
Resulting From Whirl
• Impact damage
on rear of blades
due to backward
rotation, resulting
in broken blades
Forward Whirl
• Eccentric rotation about
a point other than
geometric center
– May occur with bent
housing motors
– Results in bit ‘rolling
around’ wellbore
synchronously with
direction of rotation
– Not generally damaging
Bit Whirl
Bottom hole patterns obtained from roller cone bits with 3
(left) and 2 (right) cones. (Ref SPE paper 28323)
Lateral Vibration
• Sideways movement of BHA
• Random (chaotic) forward & backward
whirl
• Caused by imbalance BHA components
• DDS indications:
– Medium to high Px & Py
– Low to medium Ax & Ay
• Consequences similar to whirl
Bit Chatter
2. • High frequency resonance of Bit and BHA caused
by impacts of individual cutter blades or
individual cutters
• Occurs with PDC bits in high compressive
strength rocks
• Consequences
– Bit cutter damage
– Failure of electronic components and solder joints
Bit Chatter
• Detection
– DDS high peak x and y accelerations
– DDS high average x and y accelerations
– Burst data shows high frequency peak in x and y axes
• Corrective action
– Adjust RPM and WOB down away from resonant
condition
• Other Solutions (Post Run)
– Consider a bit more suitable to drill the high
compressive strength rock or stringers
Slip-Stick
• Torsional Vibration of the Drill String
• Initiated by mechanical or frictional resistance to
Rotation
– Non-Uniform Rotation of the Drill String
– Repeated Stalling of the Bit, Twisting and Build up of
Energy in the Drill Pipe then Freeing of the Bit Causing
Energy Transfer from the Drill String to the BHA.
– Rotary Drive Reacts to RPM Changes by Increasing or
Decreasing the Torque Causing a Self Perpetuating
Cycling
• Consequences
– Torque Cycling Over or Under Torques Connections
– Fatigue Rates are Increased Through High Amplitude
Shear Stress Cycling
– Extreme Levels Cause the String to Rotate Backwards
at the End of the “Slip” Phase Damaging PDC cutting
Structures
– Extreme Levels Can Back off Connections
– Lateral Shocks are Associated with the Free Spinning in
the Slip Phase Stressing Collar Connections and
Damaging Downhole Electronics
Slip-Stick
• Detection
– Surface Torque and RPM Oscillations : “Slip-Stick”
monitoring Systems or DrilSaver
– Downhole Sensors : DDS, 4D
– Surface Instrumented Subs : Adams
• Corrective Actions
– Increase RPM, Decrease WOB
– If Vibration Persists Pick up and Restart Drilling with
Higher RPM.
Slip-Stick
• Other Solutions
– Less Aggressive PDC bit, Select Rock Bit
– Good Drilling Practices to Create Smooth Well Profile
Slip-Stick
Slip-Stick
Torsional Vibration Example
‘FLAT-TOP’ TRQ TRACE
DUE TO LIMIT BEING
Coupling of Vibration Mechanisms
Coupling means that one vibration
mechanism can induce another
SLIP-STICK
HIGH RPM
During slip phase
BIT WHIRL
AXIAL BIT WHIRL
D WOB
&
Bending
• Hole Angle
– Lateral Vibrations are More Likely in Vertical
Wells
– In Directional and High Angle Holes Gravity
Damps Lateral Displacement
– Deviated and High angle Holes are More Likely
to Induce Torsional Vibration
– High Angle, Tortuous Holes and Large Dogleg
Severities Increase Frictional Torque
Factors Effecting Downhole Vibration
• BHA Design
3. – Use of a Downhole Motor Reduces Energy
Interactions Between BHA and Wellbore
– Packed Assemblies are less Susceptible to
Vibration than Slick Assemblies
– Under Gauge Stabilisers are more likely to
generate Whirl
– Surround MWD with Full Gauge Stabilisers to
protect the Tool
Factors Effecting Downhole Vibration
• Drill Bits
– PDC bits tend to Whirl at High RPM in Hard
formations
– Dull or Under Gauge PDC bits can generate
“Slip-Stick” Torsional Vibrations
– Too High WOB and Too Low RPM creates
Torsional Vibration and “Slip-Stick”
– Roller Cone bits can bounce, Axial Vibration,
in Hard Formations or if too low WOB is
applied
Factors Effecting Downhole Vibration
Factors Effecting Downhole Vibration
• Lithology
– Vibration Increases with Formation Strength
– Vibrations are Particularly Associated with Zones of High and
Low Compressive Strength
• Hole Size
– Over Gauge Hole leads to Lateral Shocks, BHA Whirl
and Bit Whirl
– Under Gauge Hole Creates Torsional Vibration
• Drilling Mud
– Mud Condition and type can Influence Vibration levels
Vibration Mechanisms
• Each Mechanism Describes the Behaviour
of the Drill String During Vibration
– Bit Bounce
– Slip-Stick
– Bit Whirl
– BHA Whirl
– Modal Coupling
– Bit Chatter
– Lateral Shock Motion
Methods of Reducing Vibration Damage
• Bit Design
– F.A.S.T., SE3000,
• Planning - BHA design
– WHIRL
• Real-time vibration monitoring
– DDS, DrilSaver, Instrumented Bit, VSS,
AcoustiCaliper, WOB/TOB
• Improved Tool Reliability
• Reduced Vibration Drilling systems
– SlickBore, Geo-Pilot
Force Balancing
Penetration and Drag Cutter Forces
Fvertical = WOB
Mcenterline = TORQUE
Fradial = 0
M M M
Engagement
PDC Cutter
F
P
WOB
TORQUE
• Symmetric blades create a lobed
bottom hole pattern in a regular
periodic manner which can lead to
self-regenerative whirl
• Asymmetric blade layout upsets the
periodic pattern thus disrupting the
harmonic effects of a lobed bottom
hole pattern
Asymmetric Blade Layout
0 0°
248°
105°
Asymmetric
Symmetric
0°
120°
240°
Symmetric
4. Predicted Lobe Generation
for Symmetric & Asymmetric
• Symmetric
Asymmetric
120°
0°
240°
0°
105°
248°
Low Torque Gauge Pads
• Removal of sharp edges
and aggressive wear
surface reduces ability of
gauge pad to bite into
borehole wall.
• Creates less drag
between the bit and the
borehole wall reducing
any tendency to pivot
about a gauge pad.
Spiraled Blades
As with asymmetry, spiraled cutter
layouts and gauge pads help break up
any regenerative lobed cutting patterns
by:
• Lessening probability of a new center of
rotation occurring along a cutter blade or
gauge pad.
• Effectively increases resistance of the gauge
pad from biting into the borehole wall.
• When spiraled, the release of a blade happens
in closer proximity with the contact of the
next, passing the load to the next blade more
smoothly.
Trac-Set Cutting Structure
Unlike the standard layout, the ridged pattern created by a
Trac-Set cutting structure resists the bits tendency to move
laterally by stabilizing on formation ridges.
Standard Layout Trac-Set
Restoration Forces
Standard Set
13mm PDC. 50 FPH. 120 RPM
No Offset
Total Vertical Fv = 408 lbf
= = Net Radial Force Fr = 0.0 lbf = =
Outside Radial
Fr1 = 38.4 lbf
Fr1/Fv = 0.094
Inside Radial
Fr2 = 38.4 lbf
Fr2/Fv = 0.094
13 mm PDC. 50 FPH. 120 RPM
< - Offset 0.025”
Total Vertical Fv = 408 lbf
< = = Net Radial Force Fr = 8.48 lbf < = =
Outside Radial
Fr1 = 35.7 lbf
Fr1/Fv = 0.088
Inside Radial
Fr2 = 44.1 lbf
Fr2/Fv = 0.108
Restoration Forces
Trac Set
13mm PDC. 50 FPH. 120 RPM
No Offset
Total Vertical Fv = 451 lbf
= = Net Radial Force Fr = 0.0 lbf = =
13 mm PDC. 50 FPH. 120 RPM
< - Offset 0.025”
Total Vertical Fv = 454 lbf
= = > Net Radial Force Fr = 35.8 lbf = = >
Outside Radial
Fr1 = 81.0 lbf
Fr1/Fv = 0.180
Inside Radial
Fr2 = 81.0 lbf
Fr2/Fv = 0.180
Outside Radial
Fr1 = 96.4 lbf
Fr1/Fv = 0.212
Inside Radial
5. Fr2 = 60.6 lbf
Fr2/Fv = 0.180
Impact Arrestors
Impact Arrestors act to dampen chaotic
vibrations that typically exist downhole by
providing stabilization to both the axial and
lateral modes of vibration.
Impact
Arrestor PDC
Impact Arrestors
Standard W / Impact Arrestors
ROP
Time >
Axial Vibration Reduction
ROP
DDS Tool
The DDS Sensor
consists of tri-axial
accelerometers
mounted on the
DGR gamma
sensor electronics
insert
Records
accelerations in
g as Average,
Peak, and Burst
Vibrations
DrilSaver
• DrilSaver is an advanced “Slip-Stick” monitoring
system
– Uses Fourier analysis to breakdown the torque signal,
providing a frequency domain spectrum of the of the
torque oscillation
– Sinusoidal oscillation in the torque signal is
characteristic of torsional vibration present in the
drillstring
– DrilSaver provides a method of quantifying torsional
vibration (KT magnitude)
Whirl Program
• Based on the DYNAMICS program developed by Boeing
• Further developed by Dykstra at Tulsa University and Amoco
• Incorporated into Sperry-Sun’s well planning software (PLANIT)
• Uses Jacobi and finite element analysis to predict critical rotary
speeds for excitation of fundamental lateral vibrations (resonance) of
BHA in drilling fluid in a straight hole (vertical or inclined)
Slickbore Bit
Slickbore Bit on Geopilot
Slickbore Bit
• End of story
