SOFT TORQUE ROTARY SYSTEM REDUCES DRILLSTRING FAILURES

Kazem Javanmardi, Debbie Gaspard
Shell Offshore Inc.
New Orleans

The use of the soft torque system has significantly reduced torque fluctuations (up to 80%), torsional drillstring vibrations, and bit slip-stick conditions to help reduce drillstring failures and improve penetration rates in deep directional wells.

Shell Offshore Inc. (SOI) successfully installed and operated the soft torque rotary system on two rigs drilling in Mobile Bay.

The system was instrumental in eliminating expensive ($5-10 million) drillstring failures on Well SL 531 No. 3, a complex directional well in Mobile Bay. The soft torque rotary system attenuates and interrupts the torsional oscillations of the drillstring and thus prevents the buildup of energy in torsional waves that are reflected back and forth between the bit and the rotary table.

The soft torque rotary system can be installed on any rig equipped with an independent electronically driven rotary table or top drive. The system is relatively inexpensive and easy to install.

SOI has recently installed the soft torque rotary system on a Sonat Offshore Drilling Inc. semisubmersible, the Sonat George Richardson, for the Auger predrill program on Garden Banks 426. The soft torque rotary system was also recently installed on Marine Drilling Co.'s Marine 200 jack up for drilling on Eugene Island 100.

TORSIONAL VIBRATIONS

Torsional vibrations which normally result from slip-stick motion of the bottom hole assembly (BHA) have been studied in detail since 1986 by Halsey, et al. 1 2 Unlike most types of drillstring vibrations, torsional vibrations are not significantly influenced by rotary speed or weight on bit.

Therefore, avoiding the critical rotary speeds may not reduce or eliminate slip-stick motion of the BHA.

Downhole conditions, such as significant drag, tight hole, severe doglegs, and key seats, can cause the bit to stall in the formation while the rotary table continues to rotate. When the trapped torsional energy (similar to a wound-up spring) reaches a level that the bit can no longer resist, the bit suddenly comes loose, rotating and whipping at very high speeds.

This slip-stick behavior can generate a torsional wave that travels up the drillstring to the rotary table. Because of the high inertia of the rotary table, it acts like a fixed end to the drillstring and reflects the torsional wave back down the drillstring to the bit. The bit may stall again, and the torsional wave cycle repeats.

This cyclic loading can lead to drill pipe fatigue problems. The whipping and high speed rotations of the bit in the slip phase can generate severe axial and lateral vibrations in the BHA. These axial and lateral vibrations can cause drillstring connection failures, reduced penetration rates, excessive bit wear, and failures with measurement-while-drilling tools.

A conventional rotary table keeps the surface rotational speed as constant as possible and independent of the torque load. Thus, the rotary table acts as an effective reflector for torsional waves generated downhole.

Halsey, et al., proposed, the torque feedback concept to help stop the flow of torsional energy up and down the drillstring." The torque feedback system controls the rotary table motion; thus, vibrations in the drillstring are dampened instead of reflected. The dynamic character of the rotary table (i.e., variable instead of constant rotary speed) can be changed by use of torque values at the rotary table and by use of the torque signal to control the rotary speed.

Thus, the rotary table or the top drive end of the drillstring can be forced to react in a "soft" manner rather than as a fixed heavy flywheel. The torsional waves arriving at the rotary table are then absorbed, breaking the harmful cycle of torsional drillstring vibration.

The torque feedback concept was tested on a full-scale research drilling rig. 1 2 A computer generates the control voltage representing the desired speed signal. After the computer reads, filters, and digitizes the torque signal, it corrects the desired speed signal according to the filtered torque signal. By dictating higher rotary speed fluctuations, the soft torque system minimizes fluctuations in torque. This torque feedback system requires measurement of torque at the rig floor.

SOFT TORQUE

A modified version of the torque feedback system was developed and field tested by Koninklijke/Shell Exploratie en Produktie Laboratorium and Deutsche Tiefbohr AG (Deutag) in Germany.

This soft torque system relies on a minor modification to the electronic speed control system of the rotary table. Because the current of the rotary drive motor is a measure of the torque at the rotary table, the current can be directly used to control the motor's speed. The need for torque measurement at the rig floor is therefore eliminated.

The soft torque rotary system can be installed on rigs with silicon-controlled rectifier systems, including electric top drives.

Several successful field tests were performed in Germany, Holland, and Oman. 3 Tests conducted in Holland showed a reduction in torque fluctuations of up to 95%, and the tests in other areas have yielded similar results.

MOBILE BAY

The hard rock formations drilled in Mobile Bay include chalk, carbonate, anhydrite, and limestone. Most of the wells are designed to develop the deep upper Jurassic/Norphlet sands at about 21,500 ft (6,553 m) true vertical depth. Fig. 1 shows a well schematic for a typical well drilled in the area.

Many of the wells must be drilled directionally because of the proximity of the shipping fairway. Three of SOI's five wells in the area are directional. High torque is typically expected in most directional wells that extend beyond 21,500 ft true vertical depth. Moreover, the torque can become particularly troublesome in the highly active, unforgiving formations in Mobile Bay.

The high torque in this area often causes stuck pipe or drill pipe and BHA failures (Table 1). The excessive wear on bits and stabilizers often results in undergauge holes, which further contribute to stuck pipe and drillstring failures. These problems have resulted in extensive fishing jobs or costly sidetracks for virtually every operator in the area.

SOI had a costly (about $7.5 million) and catastrophic drillstring failure on its second directional well (Well SL 532 No. 1) in Mobile Bay. An analysis of the failure revealed that adverse downhole conditions, such as high doglegs and excessive torque and drag, helped generate torsional vibration energy which may have become trapped in the drillstring by the conventional rotary table acting as a heavy flywheel.

Excessive torque fluctuations, slip-stick motion of the BHA, and repeated stalling of the rotary table just prior to the failure also indicated that the trapped torsional vibration energy may have reached a level that caused the drillstring failure (Fig. 2). In addition, the high speed of the bit during the slip mode (300 rpm bit speed compared to 50 rpm rotary table speed) may have whipped the BHA, producing harmful axial and lateral vibrations.

The analysis indicated that the reduction or elimination of torsional vibrations should reduce the possibility of these types of catastrophic drillstring failures.

While drilling its third directional well (Well SL 532 No. 3) in Mobile Bay in 1990, SOI decided to use the soft torque rotary system, which had become recently available. The soft torque rotary system was installed on both rigs working for SOI in Mobile Bay at the time. One rig was drilling a directional hole and the other a vertical hole.

The vertical hole was drilled with no excessive torque fluctuations or any indication of slip-stick conditions with the soft torque system on or off. On the directional well, however, the soft torque system significantly reduced torque fluctuations and slip-stick conditions.

SURFACE ROTARY SPEED

Fig. 3 shows a portion of the torque and rotary speed readings taken from Marine Drilling's Marine 300 rig while it drilled the directional well (SL 531 No. 3). The rotary amps (right column) are directly proportional to the rotary torque. The bottom part of the figure corresponds to drilling with the soft torque off, indicating high torque variations from 280 to 535 amps. The upper portion shows a dramatic reduction in dynamic torque variations (400-480 amps) which occurs when the soft torque system is activated. The increase in rotary speed fluctuations corresponds to the decrease in torque variation when the soft torque is on.

Conventional rotary tables and electric top drives typically produce minimal rotary speed fluctuations. Thus, SOI was concerned about the adverse effect of the observed surface rotary speed variations on the drilling efficiency (penetration rates) when the soft torque rotary table was activated on the well.

An analysis of the drilling operations with and without the soft torque system indicated no detrimental changes in ROP. Moreover, the use of the soft torque system may have been a factor in increasing the penetration rate (Fig. 4).

As shown in Fig. 3, the use of the soft torque system represents a compromise between two conflicting operations: maintaining a desired rotary speed and keeping the torque fluctuations to a minimum. In conventional drilling, maintaining the desired rpm helps avoid the critical rotary speed to prevent other damaging downhole resonant vibrations.

With the soft torque rotary system activated, the driller can still avoid the critical rotary speed by changing the speed (as in conventional drilling) or by readjusting the soft torque rotary system controls. But, because the soft torque system eliminates torsional vibrations and as a result significantly reduces harmful axial and lateral vibrations, avoiding the critical speed may not be an important requirement.

Unfortunately, available research does not adequately address this issue. Additional field and laboratory testing are necessary to determine the importance of avoiding the critical speed when torque-feedback or soft torque rotary systems are used.

OTHER APPLICATIONS

The soft torque rotary system has been successfully used on several top drive rigs outside the U.S. Fig. 5 shows a geolograph recording which indicates the effectiveness of the system installed on a Deutag drilling rig (T-28) equipped with a top drive. The well was drilled with a polycrystalline diamond bit and a turbine.

Deutag drilling has successfully used the soft torque rotary system on several drilling operations with downhole motors, turbines, and steerable systems.

Because different sizes of drill pipe exhibit different vibrational characteristics, the soft torque system has to be tuned for each drill pipe size used on the rig. Separate soft torque rotary systems with a changeover switch need to be installed if different sizes of drill pipe are regularly used on a particular rig (i.e., 5 in. and 3/2 in.).

For tapered drillstrings, (e. g., 3-1/2 in. x 5 in.), more vibrational modeling and analysis are needed to ensure all different drill pipe sizes are protected from damaging vibrations.

Well SL 531 No. 3 was drilled with a 4-1/2-in. x 5-in. tape red drillstring. Because of the small difference in drill pipe sizes, the soft torque system was tuned and operated with no problems.

ACKNOWLEDGMENT

The authors would like to thank Shell Offshore Inc. for permission to publish this article.

REFERENCES

Halsey, G.W., Kyllingstad, A., and Kylling, A., "Torque Feedback Used to Cure Slip-Stick Motion," SPE paper 18049 presented at the SPE Annual Technical Conference and Exhibition in Houston, Oct. 2-5, 1988.
Kyllingstad, A., and Halsey, G.W., "A Study of Slip-Stick Motion of the Bit," SPE paper 16659 presented at the SPE Annual Technical Conference and Exhibition in Dallas, Sept. 27-30, 1987.