Drill String Dynamics Closed Loop

Copyright 2003, SPE/IADC Drilling Conference This paper was prepared for presentation at the SPE/IADC Drilling Conference held in Amsterdam, The Netherlands, 1921 February 2003. This paper was selected for presentation by an SPE/IADC Program Committee following review of information contained in an abstract submitted by the author(s). Contents of the paper, as presented, have not been reviewed by the Society of Petroleum Engineers or the International Association of Drilling Contractors and are subject to correction by the author(s). The material, as presented, does not necessarily reflect any position of the SPE, IADC, their officers, or members. Electronic reproduction, distribution, or storage of any part of this paper for commercial purposes without the written consent of the Society of Petroleum Engineers or the International Association of Drilling Contractors is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of where and by whom the paper was presented. Write Librarian, SPE, P.O. Box 833836, Richardson, TX 75083-3836, U.S.A., fax 01-972-952-9435.

Abstract This paper presents a newly developed intelligent system designed to avoid resonance and to reduce vibrations. The system integrates real-time BHA dynamics software and real-time downhole vibration data to provide accurate modeling results and data analyses. Unlike conventional BHA dynamics software that is run for well planning or post-run analysis, this system uses real-time data (e.g., WOB, inclination, DLS) to produce real-time updates of critical rotary speeds. The updates are then displayed along with the rotary speed to show if the rotary speed is too close to one of the predicted critical rotary speeds. In addition, the modeling results can be compared with actual real-time downhole vibration data to corroborate the actual downhole condition. Field data have shown that the new system is effective in identifying the vibration mechanism and avoiding harmful vibrations.

Introduction Severe vibrations have been shown to be harmful to downhole equipment. Among them, lateral vibrations (particularly backward whirl) are commonly associated with drillstring fatigue failure (wash-outs, twist-offs), excessive bit wear, and MWD tool failure1-5. Lateral vibrations are caused by one common reason - mass imbalance through a variety of sources: bit-formation interaction, mud motor, and drillstring mass imbalance, etc. A rotating body is unbalanced when its center of gravity does not coincide with the axis of rotation. Due to the crookedness or mass imbalance, centrifugal forces are generated while rotating the unbalanced drillstring. The magnitude of the centrifugal force depends on its mass, the eccentricity and the rotary speed. In general, the higher the rotary speed, the larger the centrifugal force. Thus, the common practice is to lower the rotary speed when severe lateral vibration occurs.

However, vibration will not be reduced if the lower rotary speed results in a resonant condition in the assembly. A resonant condition occurs when the frequency of any one of the excitation mechanisms matches the natural frequencies of the BHA (often called the critical rotary speeds). Under a resonant condition, the BHA has a tendency to vibrate laterally with continuously increasing amplitudes, resulting in severe vibration and causing drillstring and MWD failures.

Thus, it is important to identify and avoid critical rotary speeds during drilling operation. A number of finite element based computer programs have been developed to predict critical rotary speeds. However, the accuracy of their predictions is often limited due to the uncertainties in the input data and boundary conditions. Conventional BHA dynamics software is usually run during well planning or sometimes at the rig when the BHA is made up. And a set of predicted critical speeds, (CRPM), is provided to the driller to be avoided. Common operational difficulties with this approach are: (i) complex BHA modeling and results; (ii) inaccurate results due to incorrect input data ;( iii) modeling results not being used in conjunction with the real-time vibration data to optimize the drilling process.

To provide accurate modeling results on a timely basis that are easy to understand, an integrated drilling dynamics system has been developed. The system combines real-time modeling with downhole MWD vibration data. While running the real-time mode, real-time data (e.g., WOB, inclination, DLS, etc.) are used to produce real-time updates of critical rotary speeds. The updates are then displayed along with the rotary speed to show if the rotary speed is too close to one of the predicted critical rotary speeds. The modeling results are confirmed by actual real-time downhole vibration data for accurate vibration diagnosis. To integrate the real-time modeling and measurements, an integrated dynamics system has been developed for data acquisition, display, diagnosis, and optimization. The Integrated Dynamics System This integrated dynamics system consists of (1) a real-time BHA dynamics software, (2) a MWD downhole vibration sensor, and (3) an integrated rigsite information system.

SPE/IADC 79888

Integrated Drilling Dynamics System Closes the Model-Measure-Optimize Loop in Real Time David C-K Chen, Mark Smith, and Scott LaPierre, Halliburton Sperry-Sun

2 D. C-K CHEN, M. SMITH, AND S. LAPIERRE SPE/IADC 79888

Real-time BHA dynamics software - WHIRL In mid 1990s Sperry-Sun developed a BHA dynamics program WHIRL for predicting the critical rotary speeds. The program consists of three parts: (i) a BHA static analysis using a semi-analytical method to predict the upper boundary condition, (ii) a finite element based program to calculate the natural frequencies6, and (iii) proprietary methods to calculate the critical rotary speeds. The WHIRL software has been upgraded to run in real-time mode by using the data supplied from mud logging and MWD data. Conventional mud logging data used in the model are BHA configuration, WOB, RPM, and mud weight, etc. These can be obtained from an integrated surface system, or via WITS transfer from third party mud logging or other digital rig monitoring systems commonly employed by drilling contractors. MWD data used for the modeling are inclination, DLS and hole size if the AcoustiCaliper is run. Fig. 1 shows the flow chart of the WHIRL program and Fig. 2 shows the display of the real-time WHIRL using the MWD and surface drilling data.

MWD downhole vibration sensor - DDS Real-time downhole vibration data are supplied by Sperry-Suns DDS (Drillstring Dynamics Sensor) developed in early 90s7. The DDS is located in the existing MWD tool such as the Gamma Ray sub (see Fig. 3). Three mutually orthogonal accelerometers are used to measure three axes of accelerations: X, Y, and Z. The X-axis is used to measure both lateral and radial accelerations. The Y-axis is used to measure both lateral and tangential accelerations, and the Z-axis is used to measure axial accelerations.

The signal from each axis is conditioned using three different methods: average, peak and instantaneous (burst). The average measurement ranged of 0 to 45 gs represents the average acceleration over the sampled period. The peak measurement ranged of 0 to 200 gs represents the highest acceleration which has occurred over the sampled period. The instantaneous (burst) measurement records high frequency data for frequency analysis. Details of the DDS design can be found in the SPE paper #26341. Using three different accelerations and measurements, various modes of downhole dynamics (e.g. bit and BHA whirl, bit bounce and stick-slip, etc.) can be detected using proprietary methods. Indications of destructive vibration mode(s) are then transmitted to the surface. A traffic light display is used to indicate the vibration severity (low, medium, high, and very high). Recommendations are made to correct various modes of downhole vibration that can be identified by the tool. Note: a next generation of DDS is currently being developed which will enhance the capability and also simplify the diagnosis process.

Integrated rigsite information system - INSITE The system will not work without an integrated rigsite information system. Sperry-Suns Window NT based INSITE (Integrated System for Information Technology and

Engineering) is used to acquire the mud logging and downhole data, run the engineering software, and process and display the data in real-time8. The integrated information is derived by intelligent combination of the various data into useable information and is displayed in an informative manner such as the displays shown in Fig. 4. This information may be viewed by any workstation on the network, which may include locations such as the drilling office, rig floor, geologists and company mans workstations. Real-time satellite or network links can make the displays available in the operators shore based office as well. Fig. 5 shows the network of INSITE system that allows the rig information be shared in any place in the world.

Field Tests Three case studies presented in this paper involved the use of bi-centered drilling assemblies in the Gulf of Mexico. Due to their inherently high imbalance force, bi-centered assemblies are prone to vibrations resulting in many MWD and BHA failures. The first run shows a case without the integrated dynamics system resulting in a parted motor. It illustrates that vibration data alone may not be adequate to comprehend the onset of bit whirl. The second and third examples show how the integrated dynamics system can help to optimize the drilling operation. Case #1: Uncontrolled vibrations resulted in broken motor A straight mud motor assembly with a 14.5 x 17.5 bi-center bit was used to drill a vertical section in the Gulf of Mexico. This section was drilled without the integrated dynamics system. The run ultimately ended early as a result of a parted mud motor. The DDS vibration data collected did not show high magnitude of vibrations. The average lateral accelerations (X and Y) were about 2 to 3 gs indicating a low to medium severity. The axial accelerations (Z) were very low. See Fig. 6 for the DDS log with Gamma Ray and ROP data. The majority of the vibrations from this run occurred while drilling in sand as shown by the Gamma Ray data. While the magnitudes of the lateral vibrations appear benign, frequency analyses of the high frequency DDS burst data (recorded) reveals sustained vibrations at a frequency about 8.3 Hz (see Fig. 7). The vibration frequency matched to the motor rotor speed suggesting that motor vibration was responsible for the parting of the mud motor. However, the majority of vibration energy was absorbed by the motor itself, thus was not picked up by the vibration sensor located at the MWD tool. Case #2: Integrated system run to verify the modeling A rotary assembly with a 14 x 17.5 bi-center bit with two 14 stabilizers was used to drill a vertical section in the GOM. Rotary speed was operated close to and away from critical RPMs (CRPMs) to assess validity of the model using DDS information. Good correlation between CRPM and the increased lateral vibrations were seen as shown in Fig. 8. This time-based plot shows drilling parameters, including rotary speed in the 4th column along with calculated CRPMs #3 to #5. In the 5th column accelerations (in gs) show the peak measurements of each of the three accelerometers of the DDS

SPE/IADC 79888 INTEGRATED DRILLING DYNAMICS SYSTEM CLOSES THE MODEL-MEASURE-OPTIMIZE LOOP IN REAL TIME 3

tool. These curves are shaded according to the severity of vibration present; green for low, yellow for medium, and red for high severity. Recommended practice is to never operate a drilling assembly under high severity vibration, since this commonly results in component failure. During the period between 01:00 and 01:10 the rotary RPM had been reduced until, at around 105 RPM, it was close to the 4th CRPM which triggered high severity vibrations as indicated by the DDS. At 01:10 the rotary speed was increased to 130, placing it between the 4th and 5th CRPMs, (a safe zone), and the downhole vibration reduced immediately. Note that the normal recommendation in the event of high severity vibration is to stop drilling and shut down the rotary to allow string oscillations to dissipate, then resuming drilling with a lower rotary speed. In this case, according to the WHIRL prediction, the RPM was increased to remove the resonant excitation and the vibration stopped. In order to further verify the predictions of the Whirl model, the rotary speed was increased at 01:17 to around 155 RPM, close to the 5th CRPM. As before, there was an increase in vibration severity as a result of resonance in the BHA, which continued until the RPM was reduced at 01:32. This time the reduction in RPM did not completely cure the vibration, only reduce its severity. The downhole shocks only reduced to a benign level after the RPM was further reduced at 01:48 when the string was picked up to back-ream prior to making a connection. This observation proves that it is sometimes necessary to stop drilling to fully eliminate resonant vibration. Frequency analysis of some of the high frequency sampled burst files taken around the first of these periods shows bit whirl at a frequency around 4.9 Hz (or 294 RPM). See Fig. 9 for the frequency plots of the two lateral (X and Y) accelerations. The vibration frequency of 249 RPM was very close to twice the rotary RPM suggesting a correlation to the bi-centered bit. Case #3: Integrated system run to control vibrations A straight mud motor assembly with a 14 PDC bit combined with a bi-centre reamer tool that opened the hole to 17. This BHA incorporated three 14.5 stabilizers and utilized real-time vibration sensor to monitor vibration because of difficulties on previous bit runs. Due to the salt formation, DDS data indicate sustained high magnitude lateral vibrations. Fig. 10 shows the time based plot of the rotary speed (green) along with calculated critical RPMs #2 and #3. For the first 30 minute interval the rotary speed was close to the 3rd CRPM. After the connection at 00:00, drilling was resumed with a rotary speed close to the 2nd CRPM. At 01:25 the speed was increased again to be right on the 3rd CRPM. Throughout this period there was medium to high severity vibration detected by the MWD tool, but the vibration often interfered with MWD detection, resulting in no real-time data. Thus, real-time vibration data should not be the only means to detect vibrations as it could become unavailable during high vibrations when it is most needed.

At 01:45 the rotary speed was reduced to be less than the 2nd CRPM, but as with the previous example, reduction did not immediately eliminate the downhole dynamics. Vibration severity remained medium severity until after 02:15, when a slight reduction in WOB decreased the vibration to a benign level. Note that the lower vibration coincided with an increase in ROP suggesting that reducing vibrations not only prolong bit and BHA life but also can improve the ROP. Fig. 11 shows a scatter plot of DDS data versus operating rotary speeds. The higher severity of vibrations was associated with 43 RPM (2nd CRPM) and 60 RPM (3rd CRPM), whereas other rotary speeds produced a distribution across the range of severities. Post-run frequency analyses of the DDS burst data confirmed this motion to be whirl with a frequency at about 9 Hz.

Conclusions 1. Resonance is an important cause of BHA and bit whirl.

Good correlation between the critical rotary speed predictions and the onset of BHA and bit whirl confirm its importance.

2. Frequency analyses of the high frequency burst vibration data are effective in identifying the vibration mechanisms and supporting the accuracy of the modeling.

3. Because the input parameters are constantly being updated, real-time modeling is more accurate than conventional pre-run modeling.

4. BHA instability due to an enlarged hole is an important cause of BHA and bit whirl. Wells drilled by bi-centered bits or by downhole reamers commonly used in the Gulf of Mexico frequently produce BHA and bit whirl. Available field data show that avoiding critical RPMs mitigates the vibration created by BHA instability.

5. Combining the real-time modeling and real-time downhole vibration data in an integrated system is effective in identifying the vibration mechanism and in avoiding harmful vibrations, even for unstable BHA such as those in the enlarged wellbore.

Acknowledgements The authors wish to thank the management of Halliburton Company for permission to publish this paper. References 1. Allen M.B., "BHA Lateral Vibrations: Case Studies and

Evaluation of Important Parameters, SPE/IADC paper #16110, presented at the 1987 Drilling Conference in New Orleans, Louisiana.

2. Mitchell, R.F. and Allen M.B., "Case Studies of BHA Vibration Failure, SPE paper #16675, presented at the 1987 Annual Technical Conference and Exhibition, Dallas, Texas.

3. Close, D.A., Owens, S.C., and MacPherson, J. D., Measurement of BHA Vibration Using MWD", IADC/SPE paper #17273, presented at the 1988 Drilling Conference, Dallas, Texas.

4. Vandiver, J.K., Nicholson, J.W., and Shyu, R.J., Case Studies of the Bending Vibration and Whirling Motion of Drill Collars, SPE/IADC paper #18652 presented at the


1989 Drilling Conference in New Orleans, Louisiana. 5. Dykstra, M.W., Chen, D. C-K, Warren, T.M., and Azar,

J.J., Drillstring Component Mass Imbalance: A Major Source of Downhole Vibrations, SPE/IADC paper #29350 presented at the 1995 SPE/IADC Drilling Conference in Amsterdam.

6. Dykstra, M. W.,Nonlinear Drill String Dynamics, Ph. D. Dissertation, The University of Tulsa, Oklahoma, 1996.

7. Zannoni, S.A., Cheatham, C.A., Chen, D. C-K., and Golla, C.A., Development and Field Testing of a New Downhole MWD Drillstring Dynamics Sensor, SPE paper #26341 presented at the 1993 SPE Annual Technical Conference and Exhibition in Houston.

8. Hudson, P., Riley, E.D., and Gidley, J.K.,A New Model for Integrity in Management Systems, SPE paper 46694, presented at the 1998 SPE International Conference on Health, Safety and Environment in Oil and Gas Exploration and Production held in Caracas, Venezuela.


Fig. 2 The real-time WHIRL display shows the real-time input data on the right. The program is automatically rerun every 30 seconds to 3 minutes (selected by the user) using the updated data. The main screen shows the critical RPM (red lines) to be avoided and the operating rotary speed (yellow line).

Mud Logging Data (WOB, RPM Mud Wt.)

MWD Data (Inc., DLS, hole size, etc.)

Real-Time WHIRL Module: (1) Static BHA analysis to calculate the upper boundary condition. (2) Finite element based program to calculate the natural freqs and mode shapes (3) Proprietary methods to calculate the critical rotary speeds

Drillstring Data Display input data and the current RPM with the predicted RPMs

Fig. 1 Flow Chart of Real-Time WHIRL program

Fig. 3 The Drillstring Dynamics Sensor (DDS) is located at the MWD tool like the Gamma Ray sub. Three accelerometers (X, Y, Z) are used to measure lateral, axial, and torsional vibrations.

Fig. 4 The display of the integrated drillstring dynamics system. The intelligent information is derived from raw data and is displayed in an informative manner. It includes the real-time critical RPMs vs. the operating RPM, the downhole vibration data (severity and mechanisms) with remedy recommendations, and time and depth based vibration log.


Fig. 5 Layout of the Integrated Rigsite Information System INSITE. INSITE can be used to link information between rig floor and offices outside the rig using Internet, satellite or direction connection.

Fig. 6 Depth-based vibrations from the DDS show lateral vibrations occurring while drilling sands.


Fig. 7 Frequency analyses of the DDS burst File clearly show motor vibration at a frequency of 8.3 Hz, close to the motor rotor speed. The motor vibration resulted in a parted motor.


Fig. 8 Time-based drilling parameters and MWD vibration data compared to real-time Whirl Critical RPM predictions (CRPM). The results show that critical rotary speeds correlated well with high downhole vibrations.


Fig. 9 Frequency analyses of the DDS burst File clearly show bit whirl with lateral vibrations at a frequency of 4.88 Hz, close to twice the rotary RPM of around 150, which is close to the 5th CRPM


Fig. 10 Time-based drilling parameters and MWD vibration compared to real-time Whirl Critical RPM predictions (CRPM). The results verify the modeling when drilling with mud motor coupled with a bit and a simultaneous reaming device.


Fig. 11 Cross-plot of MWD vibration versus rotary RPM shows higher severity vibrations occurred when rotating at predicted critical rotary speeds of around 43, and 60 RPM

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Drill String Dynamics Closed Loop